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New Plasmonic Photocatalysts for Fine
Organic Synthesis
Submitted in fulfilment of the requirements for the degree
of
Doctor of Philosophy
Yiming Huang
B. Eng.; M. A. Sci.
School of Chemistry, Physics and Mechanical Engineering
Queensland University of Technology
2018
New Plasmonic Photocatalysts for Fine Organic Synthesis I
Keywords
Metallic photocatalysis; LSPR effect; Plasmonic metal; Visible-light; Gold nanoparticles;
Gold-Palladium Alloy; Air-stable copper nanoparticles, Organic synthesis; Unsaturated
Aromatic Hydrogenation; Aromatic alcohol oxidation; Epoxidation; Action spectra;
Photoexcited electron; Hybridised molecular orbitals
New Plasmonic Photocatalysts for Fine Organic Synthesis
II
Abstract
Photocatalysis is a rapidly growing research field aiming to direct utilisation of abundant,
non-polluting and renewable solar energy, preferably in the visible light range, as a driven force
to trigger organic synthesis. Metallic photocatalysis is a new class of photocatalysis which has
been developed in the last decade. Nanoparticles of transition metal nanoparticles (Gold and
Copper) as well as their alloy nanoparticles were found strongly adsorbing light energy within
the visible light range owing to the LSPR effect, and thus these three transition metals were
labelled as plasmonic metals. The unique optical property of plasmonic metals allows them to
harvest and transfer photonic energy into chemical energy, which is driving increasing research
interest in their application for the synthesis of fine chemicals. Challenges lie in the
development of new metallic photocatalysts with high catalytic efficiency for important
organic synthesis reactions. As a result, this thesis focuses on three types of metallic
photocatalysts and their applications in important organic synthesis reactions as well as
mechanism studies.
In the first part of this thesis, a new reaction system utilising supported Au nanoparticles
(NPs) was developed for the selective hydrogenation of C=C, C≡C, C=O, N=O and C=N bonds
in the presence of the aromatic ring. Reaction systems were designed with green chemistry
principles that operated in an aqueous solution under mild reaction conditions, and formic acid
was applied as the reductive agent without further additives. The reaction system exhibited
excellent photocatalytic activity with high substituent tolerance. The reaction selectivity is
tuneable by manipulating the incident light wavelength. The mechanism study revealed that
the LSPR induced photoexcited electrons are responsible for the reaction activity. Furthermore,
the cooperation of formic acid and water in the reaction system was investigated using the
New Plasmonic Photocatalysts for Fine Organic Synthesis
III
isotope techniques. The result indicated that water participated in the reaction and acting as a
hydrogen source. Thus, a water related hydrogen evolution route was proposed, such theory
could potentially be beneficial to future researchers.
In order to further develop the photocatalytic applications in organic synthesis, Au was
alloyed with another metal forming an alloy NP photocatalyst. Palladium (Pd) was selected
due to its excellent catalytic activity for a wide range of organic reactions, thus alloying Pd
with Au to form a new type of photocatalyst could enlarge application range of photocatalysis
in organic synthesis. In the alloy photocatalytic system, Au predominantly played the role of
the light harvesting site while Pd is the main catalytic active site. The high photocatalytic
activity of Au-Pd alloy NP was observed for dehydrogenation of aromatic alcohols to
corresponding aldehydes at ambient temperatures under visible light irradiation. The molar
ratio of Au to Pd was found to be critical to the photocatalytic performance, and further
theoretical simulations suggested a surface charge heterogeneity in the alloy NPs owing to the
charge re-distribution between Au and Pd atoms. The DFT simulation results made a
compelling case that the catalytic performance is coupled with surface charge heterogeneity
which is determined by Au to Pd ratio. Moreover, the experimentally optimised Au to Pd ratio
in terms of best photocatalytic performance is in good agreement with the theoretical
simulation, and such agreement confirmed the critical role of surface charge heterogeneity in
the alloy NP based photocatalysis. This work not only presents a photolytic dehydrogenation
reaction system over Au-Pd alloy nanoparticle photocatalyst but also provides useful
information for the future design of plasmonic-transition metal alloy photocatalyst.
In addition to Au metal, the other two plasmonic metals (Ag and Cu) are promising
candidates for organic synthesis applications. Cu, in particular, is known to be a versatile
catalyst, yet it is the least studied plasmonic metal for photocatalysis due to the instability of
New Plasmonic Photocatalysts for Fine Organic Synthesis
IV
Cu NPs in an oxidative environment. In this study, it has been demonstrated that titanium
nitride (TiN) support material can effectively prevent the attached Cu NPs from being oxidised
by air and therefore demonstrate the first air-stable TiN supported Cu photocatalyst. This
stability may be attributed to the significant charge exchange between Cu NPs and TiN support,
which was revealed by density functional theory (DFT) calculation. Selective epoxidation of
alkenes to corresponding epoxides is a class of important but difficult reaction because the
alkenes can be easily over oxidised to aldehydes. The air stable Cu photocatalyst was
successfully applied in the selective epoxidation of alkenes using molecular oxygen (O2) under
mild reaction conditions and exhibiting good to high photocatalytic activity with excellent
product selectivity. The Cu NPs were found to mostly remain in the metallic state after seven
reaction cycles and the recovered photocatalyst was easily reactivated without significant loss
of activity or selectivity using hydrogenation gas reductive treatment. In this work, the
plasmonic catalysis has been extending into the previously unachievable use of readily oxidised
metals. The stabilisation strategy could provide a solution for practical applications of many
other non-precious metal nanoparticles.
Finally, a mechanistic physical chemistry study was performed for metallic
photocatalysis to foster better scientific understanding. The action spectra of several organic
reactions were investigated using several different types of metallic photocatalysts. By
analysing the different trend in the action spectra, a photon energy threshold was proposed
existing in metallic photocatalysis. A photocatalytic chemical reaction can only be triggered,
when the energy level of the incident photon is higher than the energy level of the lowest
unoccupied molecular orbital. The predominant role of direct photo-electron excitation in
metallic photocatalysis is demonstrated which provide an answer to a debate in the metallic
New Plasmonic Photocatalysts for Fine Organic Synthesis
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photocatalysis research that whether photoexcitation or photothermal is the main reason for
metallic photocatalysis.
In summary, this thesis shows three new photocatalytic systems with a different type of
metals to expand the applications of metallic photocatalysis in organic synthesis. A mechanistic
study for general metallic photocatalysis was performed illuminating the energy transfer and
evolution between light, metal NPs and the organic molecules. Lastly, the perspective is
expressed to a greener process of organic synthesis via metallic photocatalysis based.
New Plasmonic Photocatalysts for Fine Organic Synthesis
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List of Publications
Publications presented in this Thesis:
(1) Huang, Y.; Liu, Z.; Gao, G.; Xiao, G.; Du, A.; Bottle, S.; Sarina, S.; Zhu, H. Stable Copper
Nanoparticle Photocatalysts for Selective Epoxidation of Alkenes with Visible Light. ACS
Catalysis, 2017, 7, 4975-4985.
(2) Huang, Y.; Liu, Z.; Gao, G.; Xiao, Q.; Martens, W.; Du, A.; Sarina, S.; Guo, C.; Zhu, H.
Visible light-driven selective hydrogenation of unsaturated aromatics in an aqueous solution
by direct photocatalysis of Au nanoparticles. Catalysis Science & Technology, 2018, 8, 726-
734.
(3) Sarina, S.; Jaatinen, E. A.; Xiao, Q.; Huang, Y.; Christopher, P.; Zhao, J.; Zhu, H., Photon
Energy Threshold in Direct Photocatalysis with Metal Nanoparticles: Key Evidence from
Action Spectrum of the Reaction. The Journal of Physical Chemistry Letters, 2017, 8, 2526-
2534.
(4) Sarina, S.; Bai, S.; Huang, Y.; Chen, C.; Jia, J.; Jaatinen, E.; Ayoko, G. A.; Bao, Z.; Zhu,
H., Visible light enhanced oxidant free dehydrogenation of aromatic alcohols using Au/Pd
alloy nanoparticle catalysts. Green Chemistry, 2014, 16, 331-341.
Publications not presented in this thesis:
(1) Zhao, J.; Zheng, Z.; Bottle, S.; Chen, C.; Huang, Y.; Sarina, S.; Chou, A.; Zhu, H., Factors
influencing the photocatalytic hydroamination of alkynes with anilines catalyzed by
supported gold nanoparticles under visible light irradiation. RSC Advances, 2016, 6, 31717-
31725.
New Plasmonic Photocatalysts for Fine Organic Synthesis
VII
(2) Zhao, J.; Ke, X.; Liu, H.; Huang, Y.; Chen, C.; Bo, A.; Sheng, X.; Zhu, H., Comparing the
Contribution of Visible-Light Irradiation, Gold Nanoparticles, and Titania Supports in
Photocatalytic Nitroaromatic Coupling and Aromatic Alcohol Oxidation. Particle & Particle
Systems Characterization, 2016, 33, 628-634.
(3) Zavahir, S.; Xiao, Q.; Sarina, S.; Zhao, J.; Bottle, S.; Wellard, M.; Jia, J.; Jing, L.; Huang,
Y.; Blinco, J. P.; Wu, H.; Zhu, H.-Y., Selective Oxidation of Aliphatic Alcohols using
Molecular Oxygen at Ambient Temperature: Mixed-Valence Vanadium Oxide
Photocatalysts. ACS Catalysis, 2016, 6, 3580-3588.
(4) Tana, T.; Guo, X.-W.; Xiao, Q.; Huang, Y.; Sarina, S.; Christopher, P.; Jia, J.; Wu, H.; Zhu,
H., Non-plasmonic metal nanoparticles as visible light photocatalysts for the selective
oxidation of aliphatic alcohols with molecular oxygen at near ambient conditions. Chem.
Commun., 2016, 52, 11567-11570.
(5) Liu, Z.; Huang, Y.; Xiao, Q.; Zhu, H., Selective reduction of nitroaromatics to azoxy
compounds on supported Ag-Cu alloy nanoparticles through visible light irradiation. Green
Chemistry, 2016, 18, 817-825.
(6) Xiao, Q.; Sarina, S.; Bo, A.; Jia, J.; Liu, H.; Arnold, D. P.; Huang, Y.; Wu, H.; Zhu, H.,
Visible Light-Driven Cross-Coupling Reactions at Lower Temperatures Using a
Photocatalyst of Palladium and Gold Alloy Nanoparticles. ACS Catalysis, 2014, 4, 1725-
1734.
(7) Sarina, S.; Zhu, H.-Y.; Xiao, Q.; Jaatinen, E.; Jia, J.; Huang, Y.; Zheng, Z.; Wu, H., Viable
Photocatalysts under Solar-Spectrum Irradiation: Nonplasmonic Metal Nanoparticles.
Angewandte Chemie International Edition, 2014, 53, 2935-2940.
New Plasmonic Photocatalysts for Fine Organic Synthesis
VIII
Conferences and Presentations
(1) Oral presentation: Pacifichem 2015, Hawaii, Dec. 2015.
Presentation title: Novel Photocatalysed Aqueous Hydrogenation System for Unsaturated
Aromatics using Formic Acid.
(2) Oral presentation: NMS HDR symposium, Brisbane, Feb. 2015.
Presentation title: Supported Metallic Photocatalysts for Organic Synthesis.
(3) Oral presentation: 8th Pacific Symposium on Radical Chemistry (PSRC-8), Brisbane,
July 2017.
Presentation tile: Free Radical Induced Selective Oxidation of Alkyl Aromatics under
Sunlight
New Plasmonic Photocatalysts for Fine Organic Synthesis
IX
Table of Contents
Keywords ................................................................................................................................... I
Abstract ..................................................................................................................................... II
List of Publications .................................................................................................................. VI
Table of Contents ..................................................................................................................... IX
List of Figures .......................................................................................................................... XI
List of Abbreviations ............................................................................................................. XII
Statement of Original Authorship ......................................................................................... XIII
Acknowledgements ............................................................................................................... XIV
Chapter 1 Introduction .......................................................................................................... - 1 -
Research Background .................................................................................................. - 2 -
Research Problems ...................................................................................................... - 5 -
Research Objectives and Aims .................................................................................... - 7 -
Significance and Contribution to the Knowledge Base .............................................. - 9 -
Thesis Outline and Linkage between Chapters ......................................................... - 11 -
Chapter 2 Literature Review ............................................................................................... - 12 -
Early Study of Photochemistry and Photocatalysis ................................................... - 12 -
LSPR Effect and Plasmonic Metallic Photocatalyst ................................................. - 13 -
Gold NPs Photocatalyst for Organic Synthesis ......................................................... - 19 -
2.3.1 Introduction to gold NPs Photocatalyst .............................................................. - 19 -
2.3.2 Au NPs Photocatalysed Organic synthesis ......................................................... - 20 -
2.3.3 Hydrogenation of unsaturated aromatics. ........................................................... - 22 -
Au-Pd alloy NPs Photocatalysed Oxidation of Aromatic Alcohols .......................... - 23 -
2.4.1 Introduction to Au-Pd alloy NPs Photocatalyst .................................................. - 23 -
2.4.2 Au-Pd alloy NPs photocatalysed organic synthesis reactions ............................ - 23 -
New Plasmonic Photocatalysts for Fine Organic Synthesis
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2.4.3 Oxidation of Aromatic Alcohols ........................................................................ - 25 -
Stabilised Copper NPs Photocatalyst ........................................................................ - 26 -
2.5.1 Introduction to Cu NPs photocatalyst ................................................................. - 26 -
Reference ............................................................................................................................ - 29 -
Chapter 3 Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous
Solution over Supported Au Nanoparticle under Visible Light.......................................... - 42 -
Introductory Remarks ................................................................................................ - 42 -
Article 1 ..................................................................................................................... - 44 -
Chapter 4 Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy
Nanoparticle Catalysts ........................................................................................................ - 79 -
Introductory Remarks ................................................................................................ - 79 -
Article 2 ..................................................................................................................... - 81 -
Chapter 5 Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper
Nanoparticles as Visible Light Photocatalysts .................................................................. - 125 -
Introductory Remarks .............................................................................................. - 125 -
Article 3 ................................................................................................................... - 127 -
Chapter 6 In-depth Mechanism Study of Metallic Nanoparticle Based Photocatalyst ..... - 222 -
Introductory Remarks .............................................................................................. - 222 -
Article 4 ................................................................................................................... - 224 -
Chapter 7 Conclusions and Future Perspective ................................................................ - 259 -
Conclusions ............................................................................................................. - 259 -
Future Perspective ................................................................................................... - 263 -
Bibliography ..................................................................................................................... - 265 -
New Plasmonic Photocatalysts for Fine Organic Synthesis
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List of Figures
Figure 2-1. Mechanism of a semiconductor photocatalysis. .............................................. - 13 -
Figure 2-2. The Localised Surface Plasmon Resonance (LSPR) effect. ........................... - 14 -
Figure 2-3. (a) Metal nanoparticles absorbing light from its vicinity area owing to the
surrounded electric field of metal nanoparticles, (B) the demonstration of extinction cross
section of an isolated metal nanoparticle at LSPR wavelength. ......................................... - 15 -
Figure 2-4. Schematic illustration of three surface plasmon dephasing processes. ........... - 16 -
Figure 2-5. Schematic of photoexcitation of adsorbate-metal complexes on metal NPs
surface. ................................................................................................................................ - 18 -
Figure 2-6. UV-Vis absorption spectrum of plasmonic metal nanoparticles. .................... - 19 -
Figure 2-7. The diagram of band structures of supported Au-NPs and the proposed
mechanism for photocatalysis using supported Au-NPs. ................................................... - 20 -
Figure 2-8. The mechanism for the photocatalytic reduction of nitroaromatic compounds. - 21
-
Figure 2-9. Reaction mechanism for Au-Pd alloy NP photocatalysed Suzuki reaction. ... - 24 -
Figure 2-10. One pot synthesis of ester from aliphatic alcohols. ....................................... - 25 -
Figure 2-11 TEM images (A and B; the scale bars are 30 and 10 nm respectively; the scale
bar in the inset of (B) is 2 nm) and XPS profile(C) of 5wt% Cu/graphene, and UV/Vis
absorption spectra of Cu/graphene photocatalysts with different Cu loadings (D). ........... - 27 -
New Plasmonic Photocatalysts for Fine Organic Synthesis
XII
List of Abbreviations
CID Chemical Interface Damping
DFT Density Function Theory
FDTD Finite Difference Time-Domain
HOMO Highest Occupied Molecular Orbital
LSPR Localised Surface Plasmon Resonance
LUMO Lowest Unoccupied Molecular Orbital
NPs Nanoparticles
XPS X-ray Photoelectron Spectroscopy
New Plasmonic Photocatalysts for Fine Organic Synthesis
XIII
Statement of Original Authorship
The work contained in this thesis has not been previously submitted to meet
requirements for an award at this or any higher education institution. To the best of my
knowledge and belief, the thesis contains no material previously published or written by
another person except where due reference is made.
Signature:
Date: February 2018
QUT Verified Signature
New Plasmonic Photocatalysts for Fine Organic Synthesis
XIV
Acknowledgements
Here I would like to express my sincere gratitude to the following persons and
organisations that accompanied my PhD study.
First and foremost, to my principle supervisors: Prof. Huaiyong Zhu for his far-sighted
academic guidance, financial support and patience throughout the past few years. My progress
would never have been achieved without his drive and warm encouragement. The methodology
of doing research, he passed to me is the precious gift that would benefit my whole future
academic career. It was my privilege to complete PhD study under his supervision.
I would like to deeply thank my associate supervisor Prof. Steven Bottle, for his
excellent guidance, patience and valuable criticism towards the completion of my research and
publication.
Special thanks to senior colleagues from our research group: Dr Zhanfeng Zheng, Dr
Sarina Sarina, Dr Xingguang Zhang, Dr Jian Zhao, Dr Qi Xiao and Dr Sifani Zavahir for their
willing support and suggestions.
My gratefulness also goes to associate Prof. Aijun Du from QUT and his PhD student
Mr Guoping Gao for providing kind assistance to my research and paper editing.
I am grateful for the technical support from Central Analytic Research Facility (CARF)
of QUT and their friendly and professional technicians: Dr Jamie Riches, Dr Peter Hines, Dr
Sanjleena Singh, Mr Tony Raftery and Dr Hui Diao. Many thanks to the technicians who run
our research lab and common analytic instruments: Dr Lauren Butler, Mr Peter Hegarty, Mrs
Leonora Nebwby and the late Dr Chris Carvalho for the supportive environment they created.
I also offer gratitude to Prof. Godwin Ayoko, A/Prof. Esa Jaatinen, Dr Wayde Martens,
Dr Hongwei Liu and collaborators outside QUT: Prof. Chen Guo, Dr Gang Xiao for their
assistance in data analysis and paper revisions.
Thank you to my dear fellow HDR students: Arixin Bo, Gallage Sunari Peiris, Zhe Liu,
Pengfei Han, Erandi Sakunthala Peiris Prangige, Xiayan Wu, Chenhui Han, Tana Tana, Fan
New Plasmonic Photocatalysts for Fine Organic Synthesis
XV
Wang for your support and the friendly atmosphere you created in the daily research
environment.
Lastly, the greatest gratitude to my parents for their love which has been accompanying
me throughout the good and bad days of this journey.
Introduction - 1 -
Chapter 1 Introduction
This thesis focuses on the fabrication of new plasmonic metal nanoparticle photocatalysts
and their applications in visible-light driving photocatalytic fine organic synthesis and the
accompanying photoexcitation mechanism.
This chapter briefly outlines the Research Backgrounds (Section 1.1); Research Problems
(Section 1.2); Research Objectives and Aims (Section 1.3) and Research Contributions to
Knowledge (Section 1.4). The linkage chart for this thesis is outlined in Section 1.5.
Introduction - 2 -
Research Background
Chemistry is one of the few essential pillars of the science and industry with wide
influence in almost every aspect of modern society. Wide applications of chemistry are
associated with the great energy consumption. According to data from U.S. energy information
administration, the energy consumption in the chemical industry accounts for 29% out of all
industrial manufacturing and only 15.17% of the energy consumed originated from the
renewable source.[1] The traditional approach to chemicals synthesise is thermal activation,
which is normally associated with the consumption of fossil fuel.[2] Thus, a long standing
challenge in chemistry and chemical engineering is to drive chemical transformations with
green energy sources: clean, renewable and ideally abundant.[3]
Solar energy is an abundant, non-polluting and renewable energy source and direct use
of solar energy to initiate chemical reactions is a promising research field that seeks to
eventually employ solar energy in the chemical industry. Long before chemists introduced solar
energy into their labs, photosynthesis was recognised as a light induced chemical process. In
general, plants, bacteria and other organisms convert light energy into chemical energy and
generates carbohydrate and oxygen from carbon oxide and water. Such a process is so powerful
that it helped shape the nature before the human race.[4]
Since the first introduction of photochemistry in 1912 by Giacomo Ciamician, [5] a door
was opened to the direct utilisation of light energy in the organic synthesis, the organic
molecules were found activated by short wavelength UV light. In 1972, Fujishima and Honda’s
reported a representative photocatalysis work over semiconductor photocatalyst. They reported
a water splitting over TiO2 photocatalyst under UV irradiation.[6] Great efforts have been made
in the semiconductor photocatalysis ever since.[7-10] However, the drawbacks of semiconductor
photocatalysts are the low quantum efficiency and low thermal resistance. Additionally, the
Introduction - 3 -
affinity of semiconductors to many organic molecules is relatively weak, which results in the
limited application in organic synthesis.[2] Regarding the drawbacks of conventional
photochemistry and semiconductor photocatalysis, the developments of new photocatalysts
which can efficiently utilize visible light and catalyse various organic reactions are in demand.
Gold metal nanoparticles were found to exhibit non-linear optical properties and the UV-
Vis spectrum of Au nanoparticles (NPs) suggests a strong light absorption in the visible and
near UV range which peaked at around 500-600 nm due to the localised surface plasmon
resonance (LSPR) effect.[11-13] The initial applications of Au nanoparticles were reported as an
enhancer to the performance of semiconductor photocatalysts in order to create a visible range
active photocatalyst, yet it does not play a leading role in the photocatalysis.[14,15] However, Au
nanoparticles were reported to be catalytically active in various organic reactions.[16] As a result,
the photocatalytic activity of Au nanoparticles under visible light irradiation was discovered
and the first application was reported by our group regarding the photocatalytic oxidation of
organic pollutants and selective reduction of the aromatic nitro group.[17,18]. Following this,
silver and copper nanoparticles were found to exhibit similar light absorption properties in the
visible range and therefore showed potential in photocatalysis.[19] Thus, three metals (Au, Ag
and Cu) were categorised as plasmonic metals and represented a new area of metallic
photocatalysts other than tradition semiconductors.
Among the three plasmonic metals (Au, Ag and Cu), Cu metal is the least studied
plasmonic metal compared with Au and Ag. The reason for this is attributed to the instability
of Cu in the presence of oxygen gas or other oxidants especially when Cu in the forms of the
nanostructure. Cu, in different forms, is a highly active element for a large number of catalytic
organic syntheses and enzymatic reactions., The history of Cu based catalyst in thermal-driven
organic synthesis and biological applications is extremely rich due to the good association
between Cu and various organic compounds.[20-23] The distinctive catalytic activity combining
Introduction - 4 -
with strong light absorption allow Cu NPs as a promising candidate in the direct photocatalysis.
However, from the material science point of view, previous studies and applications of CuNPs
are strictly limited because CuNPs can be readily oxidized by air, oxidizing support materials
and oxidants in the reaction environment, yet there is evidence that the metallic state of Cu is
crucial to its non-linear optical properties and catalytic selectivity in many cases.[24] Normally,
Polymer stabilizers and/or inert atmospheres have been employed to maintain Cu in the
metallic state.[25] Thus, developing a convenient and practical method to stabilise Cu
nanoparticles for the resistance of oxygen gas or other oxidative environment could largely
expand the applicability of Cu nanoparticles as plasmonic metal photocatalysts.
Numbers of organic synthesis reactions were reported to be catalysed by plasmonic
metals, however, the versatile applications of the metallic photocatalyst are restricted by the
limited numbers of plasmonic metals. There are numerous transition metals, which are
catalytically active to a wide range of organic reactions. Nevertheless, those metals show poor
light absorption property in the visible range.[26] Introducing other non-plasmonic but
catalytically active metals into an alloy of plasmonic metals, using the plasmonic metal as the
light harvesting site and the non-plasmonic metal as the catalytic active site, is able to enhance
the photocatalytic performance and applicability of plasmonic metal photocatalysts and
significantly promote the metallic photocatalysis to a broader field of fine organic synthesis.[27]
In addition, metal alloys exhibit various forms of nanostructure. Au-Pd core-shell
nanostructure, for instance, can alter their optical property and eventually influence the
photocatalytic performance.[28] The vast combinations of metal alloys and correspondingly
different nanostructures imply a promising opportunity for the metallic photocatalyst.
Apart from the research of metallic photocatalyst from a material point of view, the other
promising research focus is to explore the metallic photocatalysts in organic synthesis.[2] To
Introduction - 5 -
date, metallic photocatalysts were reported active in several types of fine organic synthesis
reactions including redox oxidation, degradation, cross coupling and addition reactions.[29]
The development of metallic photocatalysts and their applications in organic synthesis is
growing rapidly and that demands a comprehensive mechanism study for a better
understanding of the photocatalysis and illumination for future research. The investigation of
the energy transfer on the surface of metal nanoparticles at electron-scale provides fundamental
insights from both a physical and a chemical point of view. It is now well accepted that photon
excited electrons from the surface of metal nanoparticles are responsible for the initiation of
photocatalytic chemical reactions.[30,31] Further study reveals the adsorption of reactant
molecular on metal nanoparticles results in hybridised adsorbate-metal bonding and
antibonding states, the direct photoexcitation of electrons within such states is the major reason
for the photocatalysis.[32] [33] Nevertheless, it is also critical to investigate the behaviours of
different types of metallic photocatalysts under controlled light irradiance.[34] This research
contributed knowledge into the field of both photocatalysts design and organic synthesis
optimisation of metallic photocatalysis.
Research Problems
Photocatalytic organic synthesis over metal nanoparticle photocatalysts is a relatively new
class of photocatalysis, which differs from traditional semiconductor based photocatalysis.
Despite the increasing interest in this research direction, there is only a small proportion of
organic reactions that have been accomplished with metal nanoparticles under visible light
irradiation. Thus, great opportunities are still open for researchers to enrich its applicability in
fine organic synthesis. An intriguing direction is to develop new reaction system, this includes
the fabrication of novel metallic nanostructure photocatalysts and searching for high
demanding organic reactions.
Introduction - 6 -
Firstly, since the Au based metallic photocatalyst has been recognised, expanding its
application in visible light driven organic synthesis is an attractive research direction. Thus,
locating a new high demanding organic reactions over Au based photocatalyst is considered
long term research task. More importantly, a nature of photocatalysis is to build eco-friendly
organic process, therefore developing a green reaction system over Au based photocatalyst, for
example avoiding hazardous reaction solvent, additives and side products, would be a
promotion to metallic photocatalysis. Moreover, the application scope of Au based
photocatalyst can be widely spread by introducing other catalytic active metal into the Au-M
alloy nanoparticles. This is a promising strategy for the development of metallic photocatalysts
family.
In addition to Au based photocatalysts, Cu is the least studied plasmonic metal due to its
intrinsic instability in an oxidative environment. Fabrication of versatile air-stable Cu
nanoparticle photocatalysts is an appealing research focus and the achievement of high
photocatalytic activity in organic synthesis with Cu nanoparticles is expected. The Cu
nanoparticle stabilisation mechanism is to be thoroughly studied in order to propose a strategy
for the versatile applications of Cu nanoparticles photocatalyst.
Secondly, the reaction pathways and mechanisms for each photocatalyst and reaction
system are investigated by the mechanistic study. This part of the study will illustrate metallic
photocatalysis in detail including: (i) the energy transfer and evolution of light energy to
chemical energy through metallic photocatalysts and (ii) the behaviour of metallic
photocatalysts and the targeted reactant is being intensively studied, yet a widely accepted
consensus has not been reached. Thus, the in-depth investigation of such subjects can provide
insights into metallic photocatalysis from a scientific point of view.
Introduction - 7 -
Given the current research progress, the research gaps that require fulfilling are listed as
below:
What are the important reactions, which can be photocatalysed by Au nanoparticles
under visible light irradiation?
What efforts can be made to further promote Au nanoparticle photocatalysed organic
reaction toward green chemistry?
How does an Au based alloy nanoparticle photocatalyst perform in organic synthesis?
What are the roles of Au and the other alloyed transition metal in a photocatalytic
organic synthesis and their influence on each other?
How to fabricate air-stable Cu nanoparticles and what is the stabilisation mechanism?
What is the photocatalytic performance of Cu nanoparticle photocatalysts in organic
synthesis?
What is the energy alignment pattern in the plasmonic metals and their alloy
nanoparticle photocatalysed organic reactions?
Research Objectives and Aims
Firstly, photocatalytic organic synthesis reaction over Au nanoparticle photocatalyst is
to be further explored with respect to green chemistry. The research objective and aims to this
part as listed as below:
1. Synthesis of Au nanoparticles supported by inert materials.
2. Apply supported metal nanoparticles in the photocatalytic hydrogenation of
unsaturated aromatics and evaluate the photocatalytic performance.
3. Optimise the hydrogenation reaction system toward green chemistry: (1) visible light
source: (2) moderate reaction temperature; (3) aqueous reaction system; (4) eco-
friendly hydrogen source; (5) additive-free reaction system.
Introduction - 8 -
4. Investigate the reaction mechanism, including photocatalysis mechanism and organic
reaction path.
The next part of the thesis aims to expand the applications of supported Au nanoparticle
photocatalysts by alloying Pd with Au to form Au/Pd alloy nanoparticle photocatalysts.
1. Synthesis of supported Au/Pd alloy nanoparticles with different Au-Pd ratios.
2. Appling supported Au/Pd alloy nanoparticles in the dehydrogenation of aromatic
alcohols and evaluate the photocatalytic performance.
3. Investigate the influence of Au-Pd ratio on the photocatalytic activity and optimise
the Au-Pd alloy nanoparticle photocatalyst.
4. Investigate the role of Au and Pd in the photocatalytic dehydrogenation respectively
and their possible interactions.
5. Investigate the reaction mechanism, including photocatalysis mechanism and organic
reaction path.
In the third part of the thesis, the research focused on the fabrication of air stable Cu
nanoparticle photocatalysts and their application in the epoxidation of various alkenes.
1. Synthesis of supported Cu nanoparticles that can resist oxidation from the air in the
ambient environment.
2. Apply supported Cu nanoparticles in the epoxidation of various alkenes and evaluate
the photocatalytic performance.
3. Investigate the stabilisation mechanism for supported air stable Cu nanoparticles.
4. Investigate the reaction mechanism, including photocatalysis mechanism and organic
reaction path.
Lastly, this thesis investigated the mechanism of photocatalysis by exploring the action
spectra (the relationship between the irradiation wavelength and apparent quantum efficiency
Introduction - 9 -
of reactions under constant irradiance) of different reactions to clarify the photon-electron
excitations process in the direct photocatalysis of metallic photocatalysis.
1. Investigate the action spectra of Au and Au-Pd alloy NPs for the same reaction.
2. Investigate the action spectra of Au-Pd alloy NPs for different reactions.
3. Investigate the action spectra of non-plasmonic metal NPs for different
reactions.
4. Investigate the action spectra of Au NPs for different reaction temperature.
5. Data collection and analysis, computational calculation.
6. Propose overall mechanism.
Significance and Contribution to the Knowledge Base
This thesis contributes to the following knowledge gaps:
The first application of a supported Au nanoparticle photocatalyst in the
hydrogenation of a series of unsaturated aromatics. The reaction system is designed
in the principle of green chemistry that using solar energy as driving force, water as
a solvent and formic acid as the reductive agent without other additives. This
photocatalytic system produces only carbon dioxide and water as by-products.
Formic acid was found to cooperate with water, yielding an intermediate orthofomic
acid as a hydrogen source. The hydrogenation mechanism demonstrated in this work
provides a theoretical basis for future hydrogenation applications involving formic
acid aqueous solutions.
The first report of photocatalytic oxidation of aromatic alcohols over supported Au-
Pd alloy nanoparticles. The metallic photocatalysts have been extended by alloying a
transition metal (Pd) with a plasmonic metal (Au) for the photocatalytic oxidation of
an aromatic alcohol to the corresponding aldehyde. The surface charge heterogeneity
Introduction - 10 -
caused by bi-metallic nanostructure was found to control the photocatalytic
performance. Such an understanding may enlighten designs and applications of future
alloy photocatalysis.
The first report of fabrication of TiN support material stabilised Cu nanoparticles that
can resist oxidation from the air. The TiN supported Cu nanoparticles were found to
be stable after a few reaction cycles and can be easily reactivated even after catalyst
passivation. Revealing the stabilisation mechanism brings knowledge into the future
practical applications of Cu nanoparticles which is not limited to photocatalysis. The
first report of epoxidation of alkenes over Cu nanoparticle photocatalyst using
molecular oxygen as an oxidant in the assistance of cyclic ether solvent. The oxygen
activation process has been investigated to creating a versatile photocatalytic
oxidative reaction pattern.
This thesis also contributes to the fundamental and comprehensive mechanism
research of general metallic photocatalysis. The photo-electron excitation was
demonstrated as the dominant driving force and a photon energy threshold was first
proposed existing in general metallic photocatalysis. The understanding of the
interaction between photons, metal electrons and targeted organic reactions is crucial
to the guidance of future metallic photocatalysis.
Introduction - 11 -
Thesis Outline and Linkage between Chapters
The following figure shows that how this thesis is constructed
Literature Review - 12 -
Chapter 2 Literature Review
Early Study of Photochemistry and Photocatalysis
Large energy consumption and its associated environmental issues is one of the major
concerns in traditional chemical industry, thus continuous attention has been attracted to seek
alternative energy sources for chemical industry with minimised environmental and economic
impacts. Sunlight, a natural energy source, is an inexpensive, non-polluting, abundant and
endlessly renewable source of clean energy.[35] Therefore, direct and efficient utilisation of
solar energy in chemical applications could be a promising solution to reduce the current
energy consumption.[36]
The earliest study of photochemistry can be traced back to 1912 by Dr Giacomo
Ciamician who is well-known as a pioneer in this field. He described a new concept for organic
chemistry associated with solar energy in his famous paper “The Photochemistry of the
Future”.[5] However, at Dr Ciamician’s time and the subsequent few decades, the
photochemistry was limited to the direct but inefficient interaction between light and organic
compounds.[37,38] One fundamental impediment of the early photochemistry was that only
ultraviolet light with very short wavelength can activate organic molecules, yet the proportion
of such ultraviolet light in solar spectrum is low.
In 1972, a breakthrough was published by Dr Fujishima and Honda reporting splitting of
water into hydrogen and oxygen gas over ultraviolet radiation illuminated titanium dioxide
(TiO2). The TiO2 photocatalysed water splitting was known as a representative application for
semiconductor based photocatalysis. Semiconductors can absorb certain wavelength range of
light, usually in the ultraviolet range depending on the width of its band gap, and activate
Literature Review - 13 -
reactant molecules due to the interband excitation effect as shown in Fig. 2-1.[39] This study of
heterogeneous photocatalysts has enlightened photocatalysis for the past forty years.[6]
Following Fujishima and Honda’s work, extensive researches were conducted on classic
semiconductors such as TiO2, ZnO and CdS. In the meantime, various semiconductor
photocatalysts had been developed for wide applications other than the classic water splitting.[7-
10] Despite the significant progress in semiconductor based photocatalysis, the catalytic
property of semiconductor photocatalysts still suffers from low quantum efficiency and low
thermal resistance owing to their intrinsic nature. Moreover, the applications of semiconductor
photocatalysts in organic synthesis are rarely reported due to the relatively weak affinity of
semiconductors to organic molecules.[2] Regarding the drawbacks of conventional
photochemistry and semiconductor photocatalysis, the developments of new types of
photocatalysts which can efficiently utilize visible light and catalyse various organic reactions
are in demand.
Figure 2-1. Mechanism of a semiconductor photocatalysis.[39]
LSPR Effect and Plasmonic Metallic Photocatalyst
Localised Surface Plasmon Resonance (LSPR) effect is a non-linear optical effect, it is
mostly observed on the metal nanostructure surface when irradiating with a certain wavelength
Literature Review - 14 -
of light.[40] The collective oscillation of surface electrons from metal nanostructure stimulated
by incident light is the source of the LSPR effect, and it reaches the maximum when the
frequency of incident light is resonant with the inherent oscillating frequency of surface
electrons against the restoring force of positive nuclei (Figure 2-2).[30] The resonance effect
results in a high light absorption and eventually produces a high concentration of energetic
electrons at the surface of the nanostructure.[41,42]
Figure 2-2. The Localised Surface Plasmon Resonance (LSPR) effect.[30]
The LSPR effect can further allow the metal nanostructures to absorb incident light not
limited to its cross section but rather to a larger vicinity area at the LSPR wavelength. Garcia
found that when exposed to light irradiation, the conduction electrons of metal nanoparticles
resonance with the electromagnetic field of incident light resulting in an electric field
surrounding the metal nanoparticles that opposite to that of the incident light. [43] As illustrated
in Fig. 2-3A, a metal nanoparticle is surrounded by the enhanced electric field. Such electric
field can influence the light passing its vicinity, resulting in an extinction cross section larger
than the physical cross section of metal nanoparticles. Moreover, when a plasmonic metal
nanoparticle is irradiated with light at its LSPR wavelength, which is 530 nm for Au
nanoparticles, the conduction electron resonance reaches its maximum and thusly create the
largest electromagnetic field region reflected by the huge extinction cross section as shown in
Figure 2-3B. Incident light from wavelength other than LSPR wavelength, on the other hand,
Literature Review - 15 -
cannot results in an expanded extinction cross section due to the weak LSPR effect. This light
energy concentration effect is one of the reasons that metallic photocatalysts are superior to
other photocatalysts including semiconductors.
Figure 2-3. (a) Metal nanoparticles absorbing light from its vicinity area owing to the
surrounded electric field of metal nanoparticles, (B) the demonstration of the extinction cross
section of an isolated metal nanoparticle at LSPR wavelength.[43]
In addition, the proximity of plasmonic metal NPs significantly enhances their
electromagnetic field. When two metal NPs are close, on the order of several nanometers, a hot
spot with over 106 times enhanced electromagnetic field is created between these two metal
NPs revealed by Finite-Difference Time-Domain (FDTD) simulations.[40,44,45] However, such
enhancement decays rapidly with increasing distance of two nanoparticles.[46]
The LSPR effect generates energetic electrons and transfers energy to adsorbed organic
molecules via three dephasing processes to initiate organic reactions: (i) Elastic radiative
reemission of photons; (ii) Non-radiative Landau damping and (iii) Chemical Interface
Literature Review - 16 -
Damping (CID) effect.[32] In the first process, the plasmonic nanostructures reradiate intensive
photons which are adsorbed by organic molecules, the energy from reradiated photons can be
transfer to organic molecular through a vibronic energy exchange follow the Franck-Condon
principle. This energy transfer route is particularly similar to the surface-enhanced Raman
spectroscopy (SERS) except SERS does not aim to initiate chemical reactions. The second
proposed dephasing process through Landau damping relies on the plasmon generated
energetic charge carriers transfer from metal nanostructure to the unoccupied orbitals of
adsorbed molecules. In this theory, it is proposed that metal electrons below Fermi level are
excited to an energy level above Fermi level and results in electron/hole pairs in metal
nanostructures, the energetic electrons with sufficient energy are scattered into the molecular
orbitals of adsorbed molecules through transient charge transfer to trigger chemical reactions.
The third plasmon decay mechanism is similar to the Landau damping process in terms of
energetic electrons interact with adsorbate orbitals, the difference is whether energetic
electrons are directly injected (fast) or scattered (slow) into adsorbate orbital. The CID theory
takes particular adsorbate into account that the energetic electrons instantly injected into the
adsorbate orbitals, this dephasing process is ultrafast and influenced by the interaction between
plasmonic nanostructures and the particular adsorbate.
Figure 2-4. Schematic illustration of three surface plasmon dephasing processes.[32]
Literature Review - 17 -
Recently, Linic et al proposed a new theory base on CID damping and it was further
comprehensively described by Kale et al, this theory suggested two possible photoexcitation
method for metallic photocatalyst to activate target organic reactants.[33,47] They categorised
metallic photocatalysis into three models: one indirect photoexcitation and two direct
photoexcitations distinguished by the degree of reactant chemisorbed onto metal NP surface.
In the indirect photoexcitation, the reactant is not chemically bonded to the metal NP surface,
which limited their energy transfer efficiency. The energy transfer occurs through the relatively
slow migration of energetic electrons to the unoccupied state of reactant (Fig. 2-5a). In the
contrast, the direct photoexcitation occurs when there is a chemisorption between metal NPs
and the reactant, such interaction can significantly enhance the energy transfer efficiency, and
it leads to the instant photoexcitation between Highest Occupied Molecular Orbital (HOMO)
and Lowest Unoccupied Molecular Orbital (LUMO) of the reactant (Fig. 2-5b) which mush
faster than that of indirect photoexcitation. In the third scenario, if the chemisorption between
metal NPs and the reactant is rather strong that a metal-adsorbate complex is created, then the
molecular orbitals of metal and adsorbate could be hybridised to form a reduced energy gap
between hybridised HOMO and LUMO (Fig. 2-5c). The photoexcitation in this scenario is
considered the most efficient.
Literature Review - 18 -
Figure 2-5. Schematic of photoexcitation of adsorbate-metal complexes on metal NPs
surface.[47]
Based on the above knowledge, the LSPR effect could be introduced into the
photocatalytic organic synthesis, many studies have already been reported focused on
fabricating plasmonic photocatalysts and applying them in important organic reactions. There
are three well-known plasmonic transition metals: gold (Au), silver (Ag) and copper (Cu), Fig.
2-6 displays the their light adsorption for ~ 5 nm nanoparticles and LSPR peak are located at
530, 400 and 580 nm respectively, it is noteworthy that the LSPR wavelength varies with the
size and shape of plasmonic metal nanostructure.[48-51] Due to the outstanding visible light
absorption and stable chemical and physical properties of noble metals, the application of gold
and silver nanoparticles in photocatalysis was soon discovered and many articles have been
published on gold and silver NPs since 1990.[49] On the other hand, copper NPs can be readily
oxidised by air, therefore the applications of Cu NP photocatalysts has significantly lagged
behind. As the plasmonic metals are being studied in depth, the high cost of a noble metal is
not to be neglected, and thus researchers have started to search for low cost replacements. Many
notable materials have been discovered such as Graphene, nitride ceramics and tungsten oxide
materials etc.[52-56] Nevertheless, the LSPR absorption of most newly uncovered plasmonic
materials lies on infrared range, where the photon energy is insufficient to trigger organic
reaction. Therefore, the potentials of new plasmonic materials have yet to be realised.
Literature Review - 19 -
Figure 2-6. UV-Vis absorption spectrum of plasmonic metal nanoparticles.[49]
Gold NPs Photocatalyst for Organic Synthesis
2.3.1 Introduction to gold NPs Photocatalyst
Au NPs have been long applied to adorn glass windows of medieval churches due to its
brilliant colours without knowing the mechanism.[57] Recently, the distinctive light absorption
was found responsible for the various colours of Au NPs.[2] As a consequence, the optical
property of Au NPs came into scientific sight and had been widely studied, researchers found
that the light absorption of Au NPs is a result of the LSPR effect. As shown in Figure 2-7, when
irradiated with visible light, electrons in the 6sp band of Au NPs are excited to high energy
level due to the LSPR effect. The adsorbed molecules gain energy from those excited electrons,
it results in chemical reactions which has been discussed in section 2.2. Meanwhile, the Au
NPs can additionally absorb UV light to trigger chemical reactions. The electrons from 5d band
of Au NPs can directly adsorb photons from UV light and be excited to higher energy level.
Similar to LSPR induced energetic electrons, those UV photon excited electrons are also
capable of initiating chemic reactions. Thus, Au NPs can efficiently utilise light energy from
the visible range and UV range for possible photocatalytic reactions. Moreover, the
nanostructure of Au was found effective in many thermally catalytic reactions at elevated
Literature Review - 20 -
temperatures including the oxidation of various substrates and the reduction of nitrobenzene.[58-
60] Therefore, the combination of the optical properties and catalytic properties of gold NPs
results in a promising photocatalyst candidate. [30,43]
Figure 2-7. The diagram of band structures of supported Au-NPs and the proposed mechanism
for photocatalysis using supported Au-NPs.[2]
It has been experimentally demonstrated that Au NPs are capable of activating steady-
state photocatalytic reactions under low intensity visible light. In addition, they demonstrated
the mechanism by FDTD simulations and found that the electric field intensity at the surface
of an isolated particle is around 1000 times greater than the incoming photon flux. The field
enhancement results in high energy state electrons at the surface and the excited electrons are
able to active organic molecules and initiate chemical reactions.[31]
2.3.2 Au NPs Photocatalysed Organic synthesis
Au NPs photocatalysts were firstly applied by our group in the photocatalytic reduction
of nitrobenzene. Nitrobenzene reduction attracted wide research interest because the reductive
products of nitrobenzene are feedstock in the chemical and food industry.[61] It was reported
Literature Review - 21 -
that inert ZrO2 nanopowder supported Au NPs can selectively convert nitrobenzene to
azobenzene under the irradiation of visible light and UV light. Isopropanol was found to
provide hydrogen atoms to bond with gold NPs and form an Au-H species, such species are
capable of acquiring oxygen atoms from nitrobenzene and releasing oxygen gas, the reaction
path of which is shown in Fig. 2-8.[17]
Figure 2-8. The mechanism for the photocatalytic reduction of nitroaromatic compounds.[17]
Au NP photocatalysts have also been employed in the photocatalytic oxidations. Zhang
et al. reported the use of zeolite material supported Au NPs as photocatalysts for the oxidation
of benzyl alcohol to benzaldehyde at ambient temperature under O2 atmosphere.[62] Further, in
2014, Zhang et al.reported another application of supported Au NPs in the esterification of
benzaldehyde with alcohol under visible light irradiation.[63] The mechanism study suggested
a hemiacetal intermediate is formed between benzaldehyde and alcohol.
In addition to photocatalytic redox reaction, Au NP photocatalysts have also been applied
in other types of organic syntheses, such as hydroamination reaction, delivers a convenient tool
for the C-N bond coupling yet usually requires high reaction temperature due to the high energy
barrier.[64]
Literature Review - 22 -
2.3.3 Hydrogenation of unsaturated aromatics.
The photocatalytic reduction of nitrobenzene to azobenzene was reported as mentioned
above. On the other hand, a full reduction of nitroaromatics results in aromatic amine
compounds, and it also attracts particular interest because they are essential industrial
intermediates for pharmaceuticals, polyurethanes, herbicides, agricultural products and other
fine chemicals.[61,65] In general, synthesis strategies fall into two main categories, one is direct
amination of aryl compounds and the other is catalytic hydrogenation of aryl nitro
compounds.[66,67]
The direct amination of aryl compounds normally uses ammonia as a nitrogen source for
direct amination of aryl compounds.[68,69] The direct coupling of ammonia and aryl halides
process has been discovered over a century ago and was named the Ullmann-type aryl
amination.[70] However, the Ullmann-type aryl amination was greatly limited by its harsh
reaction conditions due to the inactivity of primary aryl compounds.[71]
Given the drawbacks of direct amination, there is a strong incentive to discover other
synthetic pathways. Nitroaromatic compounds are normally resistant to oxidation and to
hydrolysis owing to the electron withdrawing effect of the nitro group. However, reduction of
the nitro group to low valence state such as an amino group is practically achievable.[72] As a
result, hydrogenation of nitro-substituted aromatic compounds provides researchers with
another powerful tool for the synthesis of aromatic amines.[73] Current commercial synthesis
of aniline through nitrobenzene reduction is a typical direct hydrogenation process, and it is
carried out in the gas phase where hydrogen gas acts as a reducing agent and hydrogen source
and various metal based catalysts are applied.[74] Although high conversion yield is achieved,
it still requires a temperature over 200 oC and a pressure over 4 bar.[75-77] The harsh reaction
conditions block the application of nitroaromatic compounds reduction in fine chemical
Literature Review - 23 -
synthesis field such as pharmaceutical industry which requires the mild condition to prevent
the destruction of other sensitive functional groups.[78] In a laboratory scale, various catalysts,
as well as reductive agents, were utilized to avoid the harsh reaction conditions. However, the
progress on thermal catalysis is slow and most reductive agents such as hydroboron, low
valence state metals, hydrosulfite salt and hydrazine are harmful to the environment.[79-90]
Moreover, the selective hydrogenation of various unsaturated aromatics is a class of
reaction fundamental in organic synthesis. Traditionally they are attained with pressurised
hydrogen gas or hazardous reductive agents under elevated reaction temperatures. Thus, there
is a strong motivation to design a new reaction system with minimized energy consumption
and eco-friendly reductive agent.[91] In this thesis, one of the projects aimed to use supported
Au NPs to photocatalyse hydrogenation of unsaturated aromatics in an environmental-friendly
system where water is used as a solvent and formic acid as a reductive agent. The system yields
carbon dioxide and water as the only by-products. The project is introduced in Chapter 3.
Au-Pd alloy NPs Photocatalysed Oxidation of Aromatic Alcohols
2.4.1 Introduction to Au-Pd alloy NPs Photocatalyst
Despite the reported progress of Au NPs in organic synthesis, the total number of
chemical reactions that can be catalysed by Au NPs is still small, especially compared with
reactions thermally catalysed by non-plasmonic metals such as Palladium (Pd).[92-94] Au can be
alloyed with Pd to form a bimetallic NP where light energy absorbed by Au to enhance the
catalytic activity of Pd. This strategy greatly expands the application of Au NPs based
photocatalyst to a wide range of organic synthesis.
2.4.2 Au-Pd alloy NPs photocatalysed organic synthesis reactions
Owing to the significant catalytic activity of Pd, Au-Pd alloy NP photocatalysts have
been applied in multiple photocatalytic reactions. For example, Pd metal is known as an
Literature Review - 24 -
excellent catalytic site for various C-C cross-coupling reactions. Thus, Kevin et al reported the
photocatalytic application of Au-Pd alloy NPs in coupling reactions, such as Suzuki-Miyaura
coupling, Stille coupling, Sonogashira coupling and Buchwald-Hartwig coupling.[27] The
reaction mechanism is shown in Fig. 2-9.
Figure 2-9. Reaction mechanism for Au-Pd alloy NP photocatalysed Suzuki reaction.[27]
Moreover, the Au-Pd alloy photocatalysts have been employed in more challenging
organic synthesis reaction, which is the one-pot esterification of aliphatic alcohols.[95] The
activation of aliphatic alcohols is considered difficult. However, with visible light irradiated
Au-Pd alloy photocatalyst, 94% conversion was achieved at ambient temperature. The
illustration of the esterification reaction pathway is shown in Fig. 2-10.
Literature Review - 25 -
Figure 2-10. One pot synthesis of the ester from aliphatic alcohols.[95]
2.4.3 Oxidation of Aromatic Alcohols
A typical example of a Pd catalysed characteristic reaction is the selective oxidation of
aromatic alcohols to corresponding aldehydes. This oxidative reaction is one of the most
fundamental transformations in synthetic organic chemistry since the carbonyl products can
serve as important and versatile intermediates in the fine chemical industry. Traditional
aromatic alcohol oxidation involves toxic oxidative agents such as permanganate and
dichromate which cause serious environmental issues.[96] In 1977, Blackburn and Schwartz
firstly used palladium to accelerate this reaction and many studies were subsequently focused
on this subject.[97-100] Supported Pd NPs were reported in the thermal catalytic oxidation of
aromatic alcohols where elevated temperature and/or high pressure were applied.[101-103] As a
result, utilising solar energy to enhance the oxidation of aromatic alcohol can promote this
reaction to a new level of green chemistry. Thus, Au-Pd alloys NPs catalysts were designed to
fulfil this task. This project is presented in Chapter 4.
Literature Review - 26 -
Stabilised Copper NPs Photocatalyst
2.5.1 Introduction to Cu NPs photocatalyst
Copper is a transition metal with excellent electrical and thermal conductivity and is
relatively cheap compared with noble metals. Initially, copper was found to be widely
distributed in the enzymatic systems, and by studying the role copper played in enzymes,
researchers found copper exhibits promising catalytic potential.[104] Following this lead, a series
of copper based catalysts were developed with high catalytic efficiency and wide applicability.
First of all, , Cu salt such as CuI can be directly applied as catalyst in organic synthesis reactions
such as amidation of aryl halides.[21, 105] Secondly, the copper based complex is a large class of
catalyst for organic synthesis, for example Zall et al fabricated Cu(I) complex LCu(MeCN)PF6
successfully applied it in the hydrogenation of CO2 to formate.[106] In addition, the Cu metal
nanostructure catalyst was also developed for many reductive reactions such as the methanation
of CO2,[107] however the application of Cu nanostructures in oxidative reaction have been
overlooked and it will be discussed later. Moreover, the formation of C-C and C-N bonds are
fundamental reactions that of great importance in organic synthetic. This type of reaction is
normally catalysed by Pd catalysts, however cupric iodide associate with ligands and base
additives and was found exhibiting excellent catalytic performance. Hence, Cu catalysts appear
to be desirable alternatives for Pd especially considering the low cost of copper metal.[105]
Literature Review - 27 -
Figure 2-11 TEM images (A and B; the scale bars are 30 and 10 nm respectively; the scale bar
in the inset of (B) is 2 nm) and XPS profile(C) of 5wt% Cu/graphene, and UV/Vis absorption
spectra of Cu/graphene photocatalysts with different Cu loadings (D).[110]
Copper NPs are promising plasmonic materials similar to gold and silver, which exhibit
strong light absorption at around ~580 nm wavelength depending on its size.[51,108] Therefore,
it is attractive to combine the optical and catalytic properties of Cu to develop an efficient and
low cost photocatalyst. However, copper NPs could be readily oxidised in atmospheric
conditions and oxidative reaction environment. Therefore, most of the current applications of
copper NPs require the association of a polymer stabiliser or noble gas protection.[109] As a
result, the study of copper NP photocatalysts was significantly restricted, thus there remains a
strong incentive to fabricate an air-stable copper photocatalyst. Recently, an inspiring work has
been reported that graphene can stabilise metallic state copper NPs (Fig. 2-11) [110] The copper
nanoparticles can be observed in TEM images and their metallic state is demonstrated by X-
ray Photoelectron Spectroscopy (XPS) technique. Moreover, the graphene supported copper
NPs were successfully applied in the oxidative coupling of nitroaromatics. However, the
Literature Review - 28 -
stabilisation mechanism for Cu NPs is not clear and limits the applicability of this
photocatalytic system. Furthermore, graphene is a relatively expensive material, which cannot
be extended to large scale application. This thesis tends to create an efficient copper NPs
photocatalyst with other novel support and clarify the stabilisation mechanism. This work is
described in Chapter 5.
Reference - 29 -
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Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
Supported Au Nanoparticle under Visible Light
- 42 -
Chapter 3 Photocatalytic Selective Hydrogenation of
Unsaturated Aromatic in Aqueous Solution over
Supported Au Nanoparticle under Visible Light
Introductory Remarks
This chapter presents Article 1 (published in Catalysis Science & Technology, 2018, 8,
726-734.) reporting the direct photocatalytic hydrogenation of unsaturated aromatics over
supported Au nanoparticles under visible light irradiation. This work focus on the research
problem that applying Au nanoparticle photocatalyst for a new class of hydrogenation, the
reaction system was designed for a green process including using eco-friendly hydrogen source,
using water as solvent, avoid of additives and undesired side products. The mechanistic study
in this work deposited knowledge into both Au based photocatalysis process and hydrogenation
of unsaturated aromatics.
The selective hydrogenation of unsaturated aromatics is a type of fundamental reaction
among organic synthesis that traditionally is attained with pressurised hydrogen gas and an
elevated reaction temperature. In this paper, we use supported Au nanoparticles for the
photocatalyst selective hydrogenation of C=C, C≡C, C=O, N=O and C=N bonds in the
presence of aromatic rings under mild reaction conditions. To make this process greener, we
created the reaction system using formic acid as an environmentally friendly hydrogen donor
to avoid the suppressed hydrogen gas or pollutant hydrogen source. The formic acid can be
commercially produced from biomass and generates only carbon dioxide and water as oxidized
products. Furthermore, the hydrogenation reaction is taking place in an aqueous system where
Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
Supported Au Nanoparticle under Visible Light
- 43 -
only water is used as solvent without any other additives. The photocatalytic hydrogenation
reaction system exhibits excellent photocatalytic activity with high substituent tolerance. The
reaction selectivity was found to be tunable by varying the irradiated light wavelength. The
photocatalytic activity is owing to the photoexcited electrons from supported Au nanoparticles.
The mild reaction temperature and pressure are the reason to the high selectivity control. The
reaction pathway was studied with the assistance of the isotope tracking technique We
discovered that the cooperation of water and formic acid as hydrogen source, formic acid react
with water forming an intermediate named orthoformic acid which directly deliver hydrogen
to the Au nanoparticles surface yield active Au-H species that can reduce the unsaturated
functional groups. This work illustrates an example for a green photocatalytic process that
using solar energy as driving force, water as reaction solvent without other additives and
releasing only carbon dioxide and water as by-products. It also reveals that formic acid and
water cooperating together as hydrogen donor, which bring knowledge into the future
hydrogenation applications involving formic acid
Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
Supported Au Nanoparticle under Visible Light
- 44 -
Article 1
Statement of Contribution of Co-Authors
Publication title and date of publication or status:
Visible light-driven selective hydrogenation of unsaturated aromatics in an aqueous
solution by direct photocatalysis of Au nanoparticles
Yiming Huang, Sarina Sarina, Qi Xiao, Wayde Martens, Zhe Liu, Cheng Guo,* and
Huaiyong Zhu*
Manuscript published in Catalysis Science and Technology
Contributor Statement of contribution
Student Author:
Yiming Huang
Wrote the manuscript, experimental design,
conducted experiments and data analysis.
Signature
Date
Dr Sarina Sarina Corresponding author, aided experimental
design, data analysis.
Dr Qi Xiao Aided experimental design, data analysis.
Dr Wayde martens Aided experimental design, data analysis.
Ms Zhe Liu Conducted experiments, data analysis.
Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
Supported Au Nanoparticle under Visible Light
- 45 -
Prof. Dr Cheng Guo Aided data analysis
Prof. Dr Huaiyong Zhu The corresponding author, aided
experimental design, data analysis.
Principal Supervisor Confirmation
I have sighted email or other correspondence from all Co-authors confirming their certifying
authorship.
____________ _____________ ________________
Name Signature Date
Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
Supported Au Nanoparticle under Visible Light
- 46 -
Visible light-driven selective hydrogenation of unsaturated aromatics in an
aqueous solution by direct photocatalysis of Au nanoparticles
Yiming Huang,[a] Sarina Sarina,[a] Qi Xiao,[b] Wayde Martens,[a] Zhe Liu,[a] Cheng Guo,*[c]
and Huaiyong Zhu*[a]
[a] School of Chemistry, Physics and Mechanical, Faculty of Science and Technology,
Queensland University of Technology, Brisbane, Queensland 4001, Australia
[b] Ian Wark Laboratory, Commonwealth Scientific and Industrial Research Organisation,
Bayview Ave, Clayton, Victoria 3168, Australia
[c] College of Science, Nanjing University of Technology, Nanjing, Jiangsu 211800, China
*E-mail: [email protected]; [email protected]
Abstract: Herein we report a new visible light driven efficient and eco-friendly selective
hydrogenation of C=C, C≡C, C=O, N=O and C=N bonds over supported gold nanoparticle
(AuNP) photocatalyst under mild reaction condition. The reaction system exhibits high
substituent tolerance and tuneable selectivity by light wavelength. The photocatalytic
mechanism is proposed that photoexcited hot-electrons are the driven force for the
hydrogenation reaction. The hydrogenation pathway is investigated with isotope tracking
technique. We reveal the cooperation of water and formic acid (FA) as a hydrogen source and
its hydrogenation route through Au-H species on the Au NP surface.
Introduction
Photocatalysis using plasmonic metal (Au, Ag, Cu and Al) nanoparticles (NPs) has
attracted much attention as they directly utilize solar power and are usually associated with
mild reaction conditions.[1-8] The NPs of these metals strongly absorb visible light via the
Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
Supported Au Nanoparticle under Visible Light
- 47 -
localized surface plasmon resonance (LSPR) effect which is the light-induced collective
oscillation of the metal conduction electrons, established when the incident light frequency is
resonant with the frequency of metal electron oscillation in response to the restoring force of
the positive nuclei, it results in energetic electrons (so-called hot-electrons) from metal NPs.[9-
11] Chemical transformations can thus be mediated due to the direct charge excitation within
the metal -adsorbed organic molecular system.[12] In addition, metal electrons can directly
absorb photons of ultraviolet (UV) and infrared irradiation to be excited to higher energy levels
due to the continuous electron energy levels of metals, thus making the utilization of most solar
spectrum possible.[13,14]
The selective hydrogenation of unsaturated organics is a fundamental reaction in organic
synthesis that is traditionally attained with the assistance of redundant hydrogen gas under high
pressures, and elevated reaction temperatures (over 100oC).[15] Elevated temperature in
traditional hydrogenation may accelerate undesired side reactions, such as hydrogenation of
the aromatic rings of the reactants, wasting the reduction agent resulting in poor economic
viability.[16,17] In an attempt to moderate the reaction conditions, various homogeneous or
heterogeneous transition metal based catalysts including gold, nickel, ruthenium, cobalt,
copper or palladium are widely employed in the hydrogenation of unsaturated bonds of C=C,
C≡C, C=O, N=O and C=N toward benign processes.[18-21] However, challenges remain in
energy efficiency, product selectivity and catalyst recyclability.[22] Thus, the hydrogenation
reactions can be greatly promoted by visible light driven mild photocatalysis. Meanwhile, the
high corrosivity of hydrogen gas at high pressure and temperature causes hydrogen
embrittlement of pressurized metal reactors resulting in high safety risks.[23,24] In contrast, with
another reducing agent such as formic acid (FA), the necessity of high-pressure vessels, as well
as corresponding risks, can be avoided.[25] FA is an environmental-friendly liquid reducing
Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
Supported Au Nanoparticle under Visible Light
- 48 -
agent readily produced from biomass, is also a convenient hydrogen donor and it only generates
carbon dioxide and water as oxidized products.[26] Commercially supplied FA is always in the
form of an aqueous solution. In organic synthesis, however, water is not a widely used solvent
despite its low cost and low environmental toxicity.[27,28] For this reason, there have been few
reports involving an aqueous solution of FA in organic hydrogenation reactions, to the best of
our knowledge.[29-31]
In this work, we developed an environmental friendly selective hydrogenation system for
unsaturated aromatics. The reaction is driven by a renewable energy source visible light
irradiation, FA aqueous solution is employed as a green hydrogen source and the reaction takes
place in water solvent. The supported AuNP photocatalyst is prepared by the impregnation-
reduction method. Five representative types of hydrogenation reactions were investigated
under mild temperatures in aqueous FA solution without other additives. The reaction
mechanism is investigated by isotope study where we found that water plays more of a role
than simply being a solvent but also acts as a hydrogen source in cooperation with FA.
Results and Discussion
The photocatalytic hydrogenation reaction system was tested with five types of
unsaturated aromatics, the catalytic activities of model reactions as well as control reactions,
presented with product yields, are shown in Table 1. The significant difference in the yields
between the reactions under irradiation and in the dark clearly demonstrates the contribution
of light. The explanation is that the hot electrons on the AuNP surface, generated by light
irradiation, are able to provide the activation energy necessary for a reaction of the reactant
adsorbed onto the AuNPs. Such a contribution is quantitatively reflected by the 73% decrease
of apparent activation energy between light irradiated styrene hydrogenation and that of the
Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
Supported Au Nanoparticle under Visible Light
- 49 -
same reaction in the dark. In the case of hydrogenation, the decrease was 35.3% decrease
(Figure S1).
The broad functional group tolerance of the new photocatalytic system is demonstrated in Table
1. The irradiation promotes the hydrogenation with excellent tolerance to various functional
groups, especially reducible substituents such as ketones, aldehydes and alkenes. It is worth
noting that FA itself, having a carbonyl group, should be active for the condensation with
amines and alkenes at elevated temperatures (over 70 °C).32 The low operating temperature
allows our photocatalytic reaction system to effectively avoid such side reactions and achieving
high chemo-selectivity.
Table 1. Performance of Au@ZrO2 catalyst for five hydrogenation reactions. The red numbers
are the product yield under visible light irradiation, and the black numbers in the parentheses
are the product yield for the control reaction in the dark.
Hydrogenation of Aromatic Olefins[a]
Hydrogenation of Aromatic Nitro-compounds
Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
Supported Au Nanoparticle under Visible Light
- 50 -
Hydrogenation of Aromatic Aldehydes[a]
Hydrogenation of Aromatic Alkyne[d]
Hydrogenation of Aromatic Imine[a]
Reaction conditions: 50 mg catalysts, 1 mmol reactant, 2 mL of 85% FA mixed with 2 mL
deionized water as solvent, light intensity 0.5 W/cm2, 40°C, 1 atm argon, reaction time 8 h. [a]
reaction time 16 h. [b] 80°C, reaction time 24 h. [c] UV light with a peak wavelength at 365
Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
Supported Au Nanoparticle under Visible Light
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nm, 24 h. [d] 60 °C, 16 h. [e] combined yield of both styrene and ethylbenzene. The products
were identified by mass spectrometry (MS) and yields were measured by gas chromatography
(GC) using external standard.
Compared to aromatic aldehyde (Table 1, entry 21), we found a poor yield (17%) for
aliphatic aldehyde at 40 °C whereas negligible activity was observed in the dark (Table 1, entry
27). The hydrogenation of aliphatic aldehyde demands greater reduction power since the
reduction potential of aliphatic aldehydes is generally more negative than that of aromatic
aldehydes. For example, the reduction potential of acetaldehyde is -1.7 eV and that of
benzaldehyde is -1.36 eV (Table S1). When the hydrogenation of aliphatic aldehyde was
conducted under UV irradiation (365 nm) at 40 °C, a much higher yield of 53% was achieved.
Apart from LSPR excitation, electrons from Au NPs can be directly excited by UV light
through a photon excitation process which also results in energetic electrons in Au NPs, the
energy of UV photons is greater than that of visible photons, meaning the UV photons are able
to generate hot electrons with higher energies than the ones generated by LSPR absorption of
Au NPs in the visible range. These higher energy hot electrons are capable of reducing
compounds with more negative reduction potentials.33 In addition, when the reaction
temperature was raised to 80 °C and the reaction was prolonged to 24 h, the yields for visible
light irradiated reaction and reaction in the dark were 42% and 4%, respectively. Therefore, we
conclude that AuNPs can combine photonic energy (light) and thermal energy (heat or IR
irradiation) because metals have a continuous electron energy level. Above results reveal an
important feature that the reduction power of the reported system is tuneable by regulating the
irradiation wavelength and reaction temperature.
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An unexpected finding is that the ratio of water to FA in the FA aqueous solution has a
significant impact on the reductive activity of the photocatalytic system. For example, in our
reaction system, phenylacetylene was reduced to styrene and ethylbenzene along with side
product of acetophenone, which is attributed to the hydration of phenylacetylene in the
presence of a hydroxyl group.34 As shown in Table 2, increasing water/FA ratio can
significantly enhance the yields of reductive products, while the production of acetophenone is
inhibited. In the cases of benzaldehyde and styrene hydrogenation (Table S2), increasing
water/FA ratio can accelerate the reaction rate. These results imply that the role of water is
more than merely acting as a solvent.
Table 2. Impact of water/FA ratio on the reductive performance of phenylacetylene
hydrogenation
Entry H2O/FA
(Volume ratio)
Conversion
%
Selectivity
a b c
1 1.5:8.5 100 0 0 100
2 3.5:6.5 100 0 10 90
3 6:4 100 25 59 15
4 8:2 100 22 73 5
Reaction conditions: 50 mg Au@ZrO2 photocatalyst, 0.3 mmol phenylacetylene as the
reactant, formic acid mixed with H2O as a solvent, 0.5 W/cm2 irradiance, 60 °C, 1 atm argon
gas and 16 h.
Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
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- 53 -
Prior to the mechanism investigation, the Au@ZrO2 photocatalyst was characterized.
Figure 1a indicates 3 wt% AuNPs, with a mean size of 7 nm (Figure 1d), are well dispersed on
the ZrO2 surface and predominantly exhibit crystal face (111) (Figure 1b). The light absorption
peak of Au@ZrO2 at 520 nm (Figure 1c) is attributed to the LSPR effect of AuNPs as ZrO2
support exhibits negligible visible light absorption. AuNPs also absorb UV light because
electrons of metal can be directly excited by photons from the ground state to high energy states.
Figure 1. Characterization of Au@ZrO2 photocatalyst. (a) TEM image of Au@ZrO2, (b) High-
resolution TEM image of Au@ZrO2, (c) Diffuse reflectance UV-Visible spectra of Au@ZrO2
and ZrO2; (d) Particle size distribution of Au@ZrO2.
To clarify the reaction pathway, isotope tracking was applied by using D2O and H218O.
We found the H-D ratio in the product ethylbenzene was 28% to 72%, which is close to 1:2.5.
Such an H-D ratio in the product cannot be rationalized by cation exchange between FA and
water (the excessive amount of water in FA solution should causing H-D ratio 1:4 in a typical
reaction). Pure water was found no activity in the hydrogenation. Hence, we hypothesise that
hydrogen atoms are transferred from water to the reactant with the assistance of FA. The role
of water could be overlooked although FA aqueous solution was previously reported as
Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
Supported Au Nanoparticle under Visible Light
- 54 -
hydrogen donor.35 Within gaseous products, C18O2 was detected with a 16O:18O ratio
approximately 2:1 when H218O was used. A rational deduction is that FA reacts with H2
18O
producing an intermediate which provides hydrogen to the styrene hydrogenation and itself is
oxidized to C18O2 and H2O. The possible intermediate is orthoformic acid which has the
formula HC(OH)3.36 It is an unstable hydrate, consisting of one water and one FA molecule,
and has not been isolated to date.
The proposed complete route is illustrated in Scheme 1. The hydration of FA (step 1,)
yields orthoformic acid, which is then oxidized on the surface of AuNPs (step 2), yielding H-
Au surface species.37 The orthoformic acid is oxidized possibly following the same principle
as the oxidation of a primary alcohol to carboxylic acid and eventually to carbon dioxide and
water.38 H-Au surface species are capable of reducing olefins by adding two hydrogen atoms
into the C=C double bond basically following Horiuti-Polanyi Mechanism (step 3 and 4,),39
the reaction equation is as follow:
These two reduction steps consume 2 H-Au species and restore the AuNP surface for
subsequent catalytic cycles (step 5,). This inference is supported by the fact that the reduction
is inhibited when removing these surface species by adding 0.1 equiv. of 2, 2’, 6, 6’-
tetramethylpiperidine N-oxyl (TEMPO) to the photocatalytic reaction system, which can
abstract hydrogen atoms from the AuNP surface,40 and found only trace amounts of
ethylbenzene in the product.
Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
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Scheme 1. Mechanism for the photocatalytic hydrogenation of styrene.
The proposed reaction mechanism is supported by step to step isotope study. According
to step 1 in Scheme 1, the H-Au (or D-Au) surface species form on the AuNP surface with the
H (or D) atoms from the orthoformic acid and the H or D atom should be eventually found in
the product ethylbenzene. When D2O was used, the orthoformic acid formed in the reaction
has two D atoms and one H atom available to yield the H-Au surface species (product of step
2). Therefore, the ratio of H to D in the product ethylbenzene should be 1:2, which is very close
to our observation mentioned above. Similarly, when H218O was used, the content of 18O atoms
in the product CO2, is in agreement with the experimental observation.
Table 3. Isotope abundance of deuterium and 18O in the products of nitrobenzene
hydrogenation.
Entry Water Conversion (%) Product Abundance (%)
1 D2O 87 PhNH2 H D
Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
Supported Au Nanoparticle under Visible Light
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34 66
2 H218O 74 CO2 16O 18O
63 37
O2 100 0
Reaction conditions: 50 mg Au@ZrO2, 1mmol nitrobenzene, 2 ml Formic acid mixed with 2
ml water as a solvent, light intensity 0.5 W/cm2, 40 ℃, 1 atm Argon, 8 h. The products were
identified by mass spectrometry MS and yields were measured by GC using external standard.
We also verified the proposed mechanism with the hydrogenation of nitrobenzene.
Similar results were received, shown in Table 3, except pure 16O2 was observed in gaseous
product suggesting a source differ from H218O. We investigated the compositional evolution of
the reaction (Figure 2a). In the first two hours, the nitrobenzene was consumed rapidly and
aniline formed correspondingly. A notable side-product formanilide was detected after 2 hours,
resulting from the condensation of aniline and FA and it was confirmed by a control experiment
where aniline was used as an initial reactant. To our surprise, no azo or azoxy compounds were
detected, suggesting that our reaction system does not strictly follow the Haber’s mechanism.
In the Haber’s mechanism, the nitro group is reduced stepwise first to nitrosobenzene and then
the hydroxylamine, hydroxylamine can easily react with nitrosobenzene yielding
azoxybenzene and then further reduce to azobenzene and eventually aniline, details see Figure
S2. However, when nitrosobenzene was employed as reactant in our system, azobenzene was
produced predominantly along with the consumption of substrate in the first 5 hours, it was
gradually converted to aniline in the rest of reaction period without detection of other
intermediates (Figure 2b), and such process is in harmony with Haber’s mechanism. In addition,
we received a similar result in the reduction of hydroxylamine and azobenzene as shown in
Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
Supported Au Nanoparticle under Visible Light
- 57 -
Table S3. Thus, we can conclude that the reduction of nitrobenzene directly yields aniline
instead of flowing a stepwise reduction process, accordingly a tentative mechanism for the
photocatalytic reduction of nitrobenzene is proposed in Figure S3.
The influence of light wavelength on photocatalysis was studied by investigating action
spectra of hydrogenations of styrene, nitrobenzene and benzaldehyde. In an action spectrum,
the photocatalytic efficiency is plotted against light wavelength. Quantum yield (Φ) which was
converted from reaction rate is used to present the photocatalytic efficiency. It was calculated
as follows:
Φ = [the number of converted reactant molecules×100]/[the number of incident photons].
We observed two types of action spectrum as shown in Figure 3. The result indicates that
AuNPs can most efficiently drive the hydrogenation of nitrobenzene by the LSPR effect as the
highest Φ value was observed at 530 nm (Figure 3c). For light spectrum other than LSPR
wavelength, the photocatalytic activity is attributed to the hot electrons directly excited by
photons.
Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
Supported Au Nanoparticle under Visible Light
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Figure 2. Time-conversion plot. (a) nitrobenzene hydrogenation and (b) nitrosobenzene
hydrogenation. Reaction condition: 200 mg Au@ZrO2 dispersed into a solution of 15 mL of
85% formic acid and 15 mL deionized water, light intensity 0.5 W/cm2, 40 ℃, 1 atm argon,
reaction time 8 h, 0.5 mL specimen was taken every hour and analysed by mass spectrometry
(identifying the species) and GC (determining the concentration of the species using external
standard).
Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
Supported Au Nanoparticle under Visible Light
- 59 -
Figure 3. Action spectra of selected hydrogenations. Hydrogenation of styrene (a),
benzaldehyde (b) and nitrobenzene (c) catalysed by Au@ZrO2. The purple line represents the
diffuse reflectance UV-vis spectrum of Au@ZrO2, blue marks represent the quantum yield of
each wavelength.
These hot electrons produced by the photons of shorter wavelength possess higher energy
levels but less population. It explains the relatively high quantum yield of 400 nm in the
hydrogenation of nitrobenzene. The light of 590 nm and 620 nm are neither triggering LSPR
Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
Supported Au Nanoparticle under Visible Light
- 60 -
effect nor delivering photons with sufficient energy, therefore resulting in low reaction rate. In
the styrene and benzaldehyde’s case (Figure 3a-b), the reaction demands more reduction power,
meaning hot-electrons with higher energy, due to their much more negative reduction potential
(Table S1). Although a strong LSPR absorption can generate a large population of hot
electrons,[20] many of them do not have sufficient energy to enable absorbed styrene to cross
the activation energy barrier. On the other hand, photons of 400 nm wavelength can generate
hot electrons with sufficient energy to drive the reduction of styrene and benzaldehyde. More
hot electrons generated by LSPR absorption have sufficient energy to drive the benzaldehyde
reduction compared with those in the styrene reduction as the reduction potential of
benzaldehyde is higher than that of styrene.
The dependence of the reaction yield on the irradiance was investigated as well. As
shown in Figure 4, the yield of hydrogenation increases with increasing irradiance for all four
reactions. It is direct evidence that irradiation can significantly promote the hydrogenation
reactions. In addition, the relative contribution of the irradiation in comparison to the thermal
contribution for the hydrogenation reactions is clarified. The grey part of a column is the yield
of the reaction in the dark at the same reaction temperature, which represents the contribution
of thermal effect. When irradiation was applied, the yield increased significantly and almost
linearly. It is also noted that at higher irradiance, the contribution of the irradiation to the overall
reaction rate is greater. For example, when the irradiance was 0.1 W/cm2 the contribution of
light to the yield of nitrobenzene hydrogenation was merely 25.6%, while the contribution was
79% when irradiance was raised up to 0.5 W/cm2. These results indicate that the irradiation is
the predominating force driving the hydrogenation reactions. Generally, higher irradiance can
produce more hot electrons resulting in a higher reaction rate. The hot electrons that have
insufficient energy to induce the reduction will relax and release their energy to heat the lattice
Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
Supported Au Nanoparticle under Visible Light
- 61 -
of the AuNP. Such a photo-thermal effect also accelerates the reaction. Therefore, higher
reaction rates are always observed at higher irradiance.
Figure 4. Dependence of the photocatalytic activity of Au@ZrO2 for hydrogenation on
irradiance. The grey part of a column represents the contribution of thermal effect and green
column represents the contribution of light. Hydrogenation of nitrobenzene (a), styrene (b),
benzyl aldehyde (c) and imine (d).
Experimental Section
Materials. Zirconium(IV) oxide (ZrO2, particle size <100 nm, TEM), gold (III) chloride
trihydrate (HAuCl4·3H2O, ≥99.9% trace metal basis), sodium borohydride powder (NaBH4,
≥98.0%), Deuterium oxide (D2O, 99.9 atom% D), Nitrosobenzene (C6H5NO, ≥97%),
azobenzene (C12H10N2, ≥98%), styrene (C8H8, ≥99.9%), N-(phenylmethylene)- Benzenamine
Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
Supported Au Nanoparticle under Visible Light
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(C13H11N, ≥ 96%) and L-lysine (2,6-diaminocaproic acid, ≥ 97%), were purchased from
Sigma-Aldrich (Australia). H218O (≥96 atom% 18O) was purchased from HuaYi Isotopes Co.
(China). Formic acid solution (HCOOH, 85%), ethanol (CH3OH, ≥95%) and dichloromethane
(CH2Cl2) were of analytical grade. All chemicals were used as received without further
purification unless otherwise noted. The water used in all experiments was prepared by an
Ultrapure Water System from Merck Millipore Co.
Synthesis of Au@ZrO2 photocatalyst. ZrO2 supported AuNPs (Au@ZrO2) was prepared by
the impregnation-reduction method. For example, to prepare 3wt% Au@ZrO2, ZrO2 powder
(2.0 g) was dispersed into an aqueous solution of HAuCl4 (32.7 mL, 0.01 M) under magnetic
stirring at room temperature, followed by addition of a lysine aqueous solution (10 mL, 0.53
M) while it was vigorously stirred for 30 min. The pH value of the mixture was 8-9. To this
suspension, a freshly prepared aqueous NaBH4 (10 mL, 0.35 M) was added dropwise. The
mixture was aged overnight, and then the solid was separated by centrifugation, washed with
water (three times), ethanol (once), and was dried at 60 °C in a vacuum oven for 24 h.
Characterization. The morphology and elemental composition of photocatalysts were studied
using a JEOL 2100 transmission electron microscopy (TEM) coupled with an energy
dispersion X-ray (EDX) spectrometer (X-MAXN 80TLE, OXFORD Instruments). The
accelerating voltage of TEM was 200 KV. Diffuse reflectance UV-visible spectra of the
catalysts were collected with a Varian Cary 5000 spectrometer with BaSO4 as a reference.
General procedure for photocatalytic reactions. In a typical activity test, a 25 mL Pyrex
round-bottom flask was used as a container, and after 1 mmol reactants and 50 mg catalyst had
been added, the flask was filled with 1 atm argon gas and sealed in order to isolate the reaction
from the air. The flask was then stirred magnetically and irradiated with a halogen lamp (from
Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
Supported Au Nanoparticle under Visible Light
- 63 -
Nelson, 500W, and wavelength in the range of 400-750 nm). The irradiance was 0.5 W/cm2
unless otherwise specified. The reaction temperature was carefully controlled at 40 oC by using
an air conditioner unless otherwise specified. The reaction system in the dark was conducted
using a water bath placed above a magnetic stirrer, and the reaction flask was wrapped with
aluminium foil to isolate the contents from the influence of light. The reaction temperature was
maintained at the same temperature as the corresponding reaction under irradiation. At the end
of reaction time, the product was firstly extracted with an equivalent amount of
dichloromethane, and then 2 mL aliquots were collected and filtered through a Millipore filter
(pore size of 0.45 µm) to remove particulate matter. The clear liquid-phase products were
analysed with an Agilent 6980 gas chromatography (GC) using a HP-5 column to analyse the
change in the concentrations of reactants and products. An Agilent HP5973 mass spectrometer
was used to identify the products. In the analysis of gaseous products, prior to the photocatalytic
reaction, the reaction tube was purged with argon gas and sealed to isolate the reaction from
the air. After the reaction, a 1 mL gas sample was taken from the atmosphere above the reaction
suspension and analysed using mass spectrometry.
Action spectrum Test. An action spectrum indicates the dependence of reaction rate on the
wavelength of irradiation, which provides evidence for the mechanism of how the photocatalyst
responses to different light wavelengths and activates the reactant molecules. Action spectrum
experiments were conducted with light emitting diode (LED) lamps (Tongyifang, Shenzhen,
China) with wavelengths of 400 ± 5 nm, 470 ± 5 nm, 530 ± 5 nm, 590 ± 5 nm, and 620 ± 5 nm.
The light intensity was measured to be 0.50 W/cm2 using an energy meter (CEL-NP2000) from
AULTT Company and other reaction conditions were maintained identical to those of typical
reaction procedures.
Acknowledgements
Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
Supported Au Nanoparticle under Visible Light
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Authors gratefully acknowledge financial support from the Australia Research Council.
(DP150102110).
Keywords: Photocatalysis; Visible light; Plasmonic metal nanoparticles, Selective
hydrogenation; Formic acid
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Supporting Information
Visible light driven selective hydrogenation of unsaturated aromatics in aqueous
solution by direct photocatalysis of supported Au nanoparticle
Yiming Huang,[a] Sarina Sarina,[a] Qi Xiao,[b] Wayde Martens,[a] Zhe Liu,[a] Cheng Guo,*[c] and
Huaiyong Zhu*[a]
[a] School of Chemistry, Physics and Mechanical, Faculty of Science and Technology,
Queensland University of Technology, Brisbane, Queensland 4001, Australia
[b] Ian Wark Laboratory, Commonwealth Scientific and Industrial Research Organisation,
Bayview Ave, Clayton, Victoria 3168, Australia
[c] College of Science, Nanjing University of Technology, Nanjing, Jiangsu 211800, China
*E-mail: [email protected]; [email protected]
Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
Supported Au Nanoparticle under Visible Light
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Table of contents:
1. Apparent activation energy of nitrobenzene hydrogenation
2. Reduction potential of relevant substrate
3. Impact of formic acid-water ratio on the photocatalytic performance
4. Scheme of Haber’s mechanism
5. Time resolved evolution of hydroxylamine and azobenzene hydrogenation
6. Mechanism for the photocatalytic hydrogenation of nitrobenzene
7. Reference
Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
Supported Au Nanoparticle under Visible Light
- 69 -
1. Apparent activation energy of light irradiated and dark hydrogenation reaction
Figure S1-1. The apparent activation energy of styrene hydrogenation. 50 mg
Au@ZrO2 photocatalyst, 1 mmol nitrobenzene as reactant, 2 ml formic acid mixed with
2 ml H2O as a solvent, 0.5 W/cm2 irradiance, 1 atm argon gas and 16 h. The apparent
activation energy of catalytic styrene hydrogenation was estimated by using the
Arrhenius equation and kinetic data at different temperatures: 40 °C, 50 °C, 60 °C and
70 °C.
Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
Supported Au Nanoparticle under Visible Light
- 70 -
Figure S1-2. The apparent activation energy of nitrobenzene hydrogenation. 50 mg
Au@ZrO2 photocatalyst, 1 mmol nitrobenzene as reactant, 2 ml formic acid mixed with
2 ml H2O as a solvent, 0.5 W/cm2 irradiance, 1 atm argon gas and 14 h. The apparent
activation energy of catalytic styrene hydrogenation was estimated by using the
Arrhenius equation and kinetic data at different temperatures: 30 °C, 40 °C, 50 °C and
60 °C.
Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
Supported Au Nanoparticle under Visible Light
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2 Reductive potential of relevant substrate
Table S1 Reductive potential of relevant substrates
Entry Substrate Reduction potential (eV)
11 Acetaldehyde -1.7
21 Benzaldehyde -1.36
32 Styrene -2.65
43 Nitrobenzene -0.356
Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
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3 Impact of water/formic acid ratio on photocatalytic performance
Table S2-1. Impact of water/formic acid ratio on the photocatalytic performance of
benzaldehyde hydrogenation.
Entry H2O:Formic Acid* Conversion
Volume ratio %
1 1.5:8.5 8
2 3.5:6.5 11
3 6:4 28
4 8:2 42
5 10:0 0
Reaction condition: 50 mg Au@ZrO2 photocatalyst, 1 mmol benzaldehyde as a reactant,
formic acid mixed with H2O as a solvent, 0.5 W/cm2 irradiance, 70 ℃, 1 atm argon gas and
16 h.
Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
Supported Au Nanoparticle under Visible Light
- 73 -
Table S2-2. Impact of water/formic acid ratio on the photocatalytic performance of styrene
hydrogenation.
Entry H2O:Formic Acid* Conversion
Volume ratio %
1 1.5:8.5 10
2 3.5:6.5 30
3 6:4 44
4 8:2 41
5 10:0 0
Reaction condition: 50 mg Au@ZrO2 photocatalyst, 1 mmol styrene as a reactant, formic acid
mixed with H2O as a solvent, 0.5 W/cm2 irradiance, 50 ℃, 1 atm argon gas and 5 h.
Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
Supported Au Nanoparticle under Visible Light
- 74 -
4 Scheme of Haber’s mechanism
Figure S2. Haber’s mechanism for nitrobenzene reduction
According to Haber’s mechanism,4 the nitro group is reduced stepwise first to
nitrosobenzene and then the hydroxylamine. Hydroxylamine can easily react with
nitrosobenzene yielding azoxybenzene, which is further reduce to azobenzene and
eventually aniline.
Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
Supported Au Nanoparticle under Visible Light
- 75 -
5 Time resolved evolution of hydroxylamine and azobenzene hydrogenation
Table S3. Time resolved performance of the hydrogenation using hydroxylamine and
azobenzene.
Reactant Time Conv. %
Select. %
Aniline Azobenzene
Hydroxylamine
1h 100 54 46
2h 100 50 50
4h 100 52 48
Azobenzene 1h 68 100 -
2h 100 100 -
4h 100 100 -
Reaction condition: 50 mg Au@ZrO2 dispersed into a solution of 4 mL of 85% formic acid and
15 mL deionised water, light intensity 0.5 W/cm2, 40 °C, 1 atm argon, reaction time 4 h, 1 mL
specimen was taken every hour and analysed by MS (identifying the species) and GC
(determining the concentration of the species).
Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
Supported Au Nanoparticle under Visible Light
- 76 -
6 Mechanism for the photocatalytic hydrogenation of nitrobenzene
Figure S3. Mechanism for the photocatalytic hydrogenation of nitrobenzene.
The first step is that orthoformic acid forms via the hydration of formic acid (step 1), which
can provide hydrogen to yield H-Au species (step 2). This step should appear in all the reactions
in the present study. These H-Au surface species are capable of interacting with the oxygen
atom of the N-O bonds (step 3), yielding OH-Au species on the surface of AuNPs while a
hydrogen atom from another H-Au species is added to the nitrogen atom forming a N-H bond
(step 4). Further reaction of the intermediate with H-Au surface species could yield aniline and
HO-Au surface species (step 5)
In the previously reported photocatalytic system of AuNP catalyst and KOH in isopropanol,[5]
which has weaker reduction power, the one N-O bond in the -NO2 group of nitrobenzene react
with the H-Au surface species yielding nitrosobenzene. The subsequent conversion of
Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
Supported Au Nanoparticle under Visible Light
- 77 -
nitrosobenzene to azoxybenzene, azoxybenzene to azobenzene are much easier than that from
azobenzene to aniline, and azoxybenzene was observed. As we did not observe the Ph-(HNO)
and other intermediates in the present study, we cannot exclude that the two N-O bonds in -
NO2 group react simultaneously with the H-Au surface species. In a difference from the
hydrogenation of styrene, this reduction of a nitrobenzene molecule consumes 4 H-Au species
and produces two OH-Au species on the surface of AuNPs. The two OH-Au species release an
oxygen molecule (oxygen gas was detected in the reaction system) and yield two H-Au species
in the subsequent process (step 6). As expected, the addition of TEMPO also stopped this
reaction, further supporting the role of H-Au species in the catalytic cycle.
Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over
Supported Au Nanoparticle under Visible Light
- 78 -
Reference
1. R. L. Guimarães, D. J. Lima, M. E. S. Barros, L. N. Cavalcanti, F. Hallwass, M. Navarro,
L. W. Bieber, I. Malvestiti, Molecules 2007, 12, 2089-2105.
2. R. S. Ruoff, K. M. Kadish, P. Boulas, E. C. M. Chen, J. Phys. Chem. 1995, 99, 8843-8850.
3. A.-J. Wang, H.-Y. Cheng, B. Liang, N.-Q. Ren, D. Cui, N. Lin, B. H. Kim, K. Rabaey,
Environ. Sci. Tech. 2011, 45, 10186.
4. Haber, Elektrochem 1892, 22, 506.
5. H. Zhu, X. Ke, X. Yang, S. Sarina, H. Liu, Angew. Chem. Int. Ed. 2010, 49, 9657.
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 79 -
Chapter 4 Photocatalytic Dehydrogenation of
Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts
Introductory Remarks
This chapter presents Article 2 (published in Green Chemistry, 2014, 16, 331-341) first
reporting the dehydrogenation of aromatic alcohols using Au-Pd alloy nanoparticle catalyst
under visible light irradiation. Following the work of article 1, in this work, the applications of
Au based photocatalysts are further expanded to selective dehydrogenation of aromatic
alcohols by alloying Au nanoparticles with Pd. This work responds to the research problem
which is the development of new metallic photocatalyst design strategy. Introducing Pd, which
is non-plasmonic metal, into the visible light driven photocatalysis is a step towards the better
applicability of metallic photocatalysts in organic synthesis. The mechanistic study in this work
reveals the roles of each metal component in photocatalysis, investigation of the interaction
between two metal component help understand the bi-metallic photocatalysts and bring
knowledge that would benefit future photocatalysts design.
The selective oxidation of alcohols to the corresponding aldehydes is an essential reaction
type from organic synthesis, however, an Au nanoparticle photocatalyst was found to be
ineffective in the activation of alcohol molecules. Thus, we introduced the transition metal Pd
into the photocatalysts to create supported Au-Pd alloy nanoparticle photocatalysts in which
the Au metal predominantly played the role of light harvesting site whereas the Pd metal was
mainly the catalytic active site. In this work, we applied Au-Pd alloy nanoparticles
photocatalysts in the oxidant-free dehydrogenation of substituted aromatic alcohols at
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 80 -
moderate reaction condition (45 oC, 1 atm Argon) under visible light irradiation. The Au:Pd
ratio was found to be critical to the photocatalytic performance, optimised Au-Pd molar ratio
was determined to be 1:1.186. By using density functional theory (DFT) simulation technique,
we revealed that the surface charge heterogeneity on the Au-Pd alloy particle is the dominant
factor which can enhance the chemical adsorption of reactant and alloy nanoparticles. The
theoretical simulation results match the experimental results regarding the optimised Au-Pd
molar ratio. The photoexcited electrons from alloy nanoparticles were the direct energy source
to this photocatalytic action, meanwhile, the strong binding of reactant onto alloy nanoparticles
ensured the energy transfer. This work extended the plasmonic metals to alloy nanoparticle
photocatalyst and thus represents a step toward versatile applications. In addition, the proposed
theory of surface charge heterogeneity in Au-Pd alloy nanoparticle is useful for the future
design of plasmonic metal with other transition metal alloy nanoparticle photocatalysts for
broader applicability.
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 81 -
Article 2
Statement of Contribution of Co-Authors
Publication title and date of publication or status:
Visible light enhanced oxidant free dehydrogenation of aromatic alcohols using Au–Pd alloy
nanoparticle catalysts
Sarina Sarina, Sagala Bai, Yiming Huang, Chao Chen, Jianfeng Jia, Esa Jaatinen, Godwin A.
Ayoko, Zhaorigetu Bao* and Huaiyong Zhu*
Published in Green Chemistry, 2014, 16, 331-341.
Contributor Statement of contribution
Student Author:
Yiming Huang
Fabrications of catalyst, characterisations
including UV-Vis, TEM analysis, EDX
mapping, XRD analysis. Operation of
photocatalytic activity test and followed
data analysis including light wavelength and
intensity dependence experiments.
Participated in the manuscript drafting and
revision related to the conducted
experiments.
Signature
Date
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 82 -
Dr Sarina Sarina First author, wrote the manuscript,
experimental design and theoretical
calculation.
Dr Sagala Bai Revise the manuscript and data analysis.
Mr Chao Chen Conducted experiments
A/Prof. Esa Jaatinen Aided experimental design, data analysis.
Prof. Godwin Ayoko Aided experimental design, data analysis.
Prof. Zhaorigetu Bao Aided experimental design, data analysis.
Prof. Huaiyong Zhu The corresponding author, aided
experimental design, data analysis.
Principal Supervisor Confirmation
I have sighted email or other correspondence from all Co-authors confirming their certifying
authorship.
____________ _____________ ________________
Name Signature Date
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 83 -
Visible light enhanced oxidant free dehydrogenation of aromatic alcohols
using Au–Pd alloy nanoparticle catalysts
Sarina Sarinaa, Sagala Baib, Yiming Huanga, Chao Chena, Jianfeng Jiac, Esa Jaatinena,
Godwin A. Ayokoa, Zhaorigetu Bao*b and Huaiyong Zhu*a
a School of Chemistry, Physics and Mechanical Engineering, Queensland University of
Technology, Brisbane, QLD4001, Australia
b School of Chemistry, Inner Mongolia Normal University, Hohhot, China.
c School of Chemical and Material Science, Shanxi Normal University, Linfen 041004, China.
*E-mail: [email protected]; [email protected]
Abstract: We find that visible light irradiation of gold-palladium alloy nanoparticles supported on
photocatalytically inert ZrO2 significantly enhances their catalytic activity for oxidant-free
dehydrogenation of aromatic alcohols to the corresponding aldehydes at ambient temperatures.
Dehydrogenation is also the dominant process in the selective oxidation of the alcohols to the
corresponding aldehydes with molecular oxygen. The alloy nanoparticles strongly absorb light and
exhibit superior catalytic and photocatalytic activity when compared to either pure palladium or gold
nanoparticles. Analysis with a free electron gas model for the bulk alloy structure reveals that the
alloying increases the surface charge heterogeneity on the alloy particle surface, which enhances the
interaction between the alcohol molecules and the metal NPs. The increased surface charge
heterogeneity of the alloy particles is confirmed with density functional theory applied to small alloy
clusters. Optimal catalytic activity was observed with an Au : Pd molar ratio of 1 : 186, which is in
good agreement with the theoretical analysis. The rate-determining step of the dehydrogenation is
hydrogen abstraction. The conduction electrons of the nanoparticles are photo-excited by the
incident light giving them the necessary energy to be injected into the adsorbed alcohol molecules,
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 84 -
promoting the hydrogen abstraction. The strong chemical adsorption of alcohol molecules facilitates
this electron transfer. The results show that the alloy nanoparticles efficiently couple thermal and
photonic energy sources to drive the dehydrogenation. These findings provide useful insight into the
design of catalysts that utilize light for various organic syntheses at ambient temperatures.
Introduction
Photocatalytic processes are generally able to drive reactions at ambient temperature and
pressure. As a result, product selectivity is improved and unstable intermediates of thermal reactions
may be produced as products of photocatalytic reactions. However, the bulk of all reported
photocatalytic studies focus on semiconductor photocatalysts such as TiO2, ZnO and CdS and their
application in decomposing organic pollutants,1 the development of new solar cells and the
production of hydrogen and oxygen from water.2 To date, only limited progress has been reported
in applying TiO2 and Nb2O5 semiconductor photocatalysts to synthesize organic chemicals.3–5 This
is largely because of the limited light absorption and low efficiency of these photocatalysts. Due to
their band structure, the well-known TiO2 photocatalysts only show significant light absorption of
ultraviolet (UV) light.1,2,6,7 Consequently, the efficiency of the semiconductor photocatalysts,
expressed as photon yield, is typically low.7 To overcome this, various methods have been
developed to produce efficient visible light photocatalysts from semiconducting materials.8 In recent
years, photocatalysis with nanoparticles (NPs) of plasmonic metals, such as gold (Au), silver (Ag)
and copper (Cu), has attracted significant interest because of their exceptional absorption of visible
light and thus their potential as visible light photocatalysts.9–17 Plasmonic metal nanoparticles exhibit
strong visible-light absorption due to the localised surface plasmon resonance (LSPR).18–20 In
addition they also strongly absorb ultraviolet (UV) light due to inter-band electron transitions (e.g.
for Au, between the 5d and 6sp bands).21–23 The NP conduction electrons gain the irradiation energy,
resulting in high energy electrons at the surface. These energetic electrons, which remain in an
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 85 -
excited ‘hot’ state for up to 0.5–1 ps,24 can promote chemical reactions of molecules on the surface
of the NPs.25,26
The underpinning photocatalytic mechanisms of the Au and Ag NPs supported by insulating
solids with very wide band gaps are quite distinct from those of semiconducting photocatalysts.9–
12,25,26 These highly efficient NP photocatalysts have several important advantages over their
extensively studied semiconductor counterparts. (1) The NP conduction electrons gain light energy,
resulting in high energy electrons at the NP surface which is advantageous for activating molecules
adsorbed on the NPs for chemical reactions. (2) Since both light harvesting and the catalysing
reaction take place on the NPs, the photon efficiency is not significantly affected by photo-excited
charge migration. (3) The density of the conduction electrons at the NP surface is much higher than
that at the surface of any semiconductor, so once photo-energized more electrons are available to
drive reactions. (4) The metal NPs have a strong affinity for some reactants, such as CO and organic
compounds, making the NPs superior photocatalysts for organic synthesis reactions compared to
semiconductor photocatalysts.
However, the total number of chemical reactions that can be catalysed by the three plasmonic
metals is relatively few when compared to those thermally catalysed by non-plasmonic transition
metals. To develop catalysts for light driven synthesis of a broad range of organic chemicals, we
presented a unique but viable approach: alloying gold and a transition metal such as Pd, which is
thermally catalytically active for many reactions.13 Thus, the light energy absorbed by the gold can
enhance the intrinsic catalytic activity of Pd at moderate temperatures. A typical example is the
selective (or partial) oxidation of aromatic alcohols to the corresponding aldehydes, which can be
catalysed by Pd catalysts but not Au catalysts. Here we show that Au–Pd alloy NPs exhibit superior
catalytic activity when exposed to visible light irradiation at ambient temperatures than that
displayed by NPs made from either pure component metals.
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 86 -
Selective oxidation of alcohols is widely considered one of the most fundamental
transformations in both laboratory and industrial synthetic chemistry because the product carbonyl
compounds can serve as important and versatile intermediates for fine chemical synthesis.27–30
Permanganate and dichromate have been traditionally employed31,32 but they are expensive and/or
toxic. For environmental and economic reasons, metal-catalysed reactions using molecular oxygen
as an oxidant are particularly attractive. Many of the reactions are conducted under heating and/or
high pressure to achieve a better reaction efficiency. Since the first successful example of palladium-
catalysed aerobic oxidation of alcohols in 1977 by Blackburn and Schwartz,33subsequent efforts
have extended the substrate scope and efficiency of palladium catalysts. However, the reported Pd(II)
salt based homogeneous catalysts34–37 require high catalyst concentration and an excess of ligands
or bases. On an industrial scale, the problems related to corrosion and plating out on the reactor wall,
handling, recovery, and reuse of the catalyst represent serious limitations of these processes.38 Solid
catalysts active in the liquid phase under mild conditions have a much broader application range.
Relatively few heterogeneous Pd (supported Pd nanoparticles and Pd II)) are available. For example,
Pd on hydrotalcite,39,40 carbon,39,41 Al2O3,39 SiO2,39 pumice,42 SiO2–Al2O3 mixed oxide,43 TiO244 and
polymer supported Pd.45 Those catalysts, both in the metallic NPs or immobilized ionic state, can
catalyse the benzyl alcohol oxidation at elevated temperatures and/or high pressure. Recently it was
reported that activation of molecular oxygen is the key step in the selective oxidation of aromatic
alcohols using TiO2 as photocatalysts under UV irradiation.
The use of Au and Pd alloy NPs as visible light photocatalysts for the selective oxidation of
aromatic alcohols will have an underlying mechanism different from those aforementioned
processes. It is known that during selective oxidation, the aromatic alcohols may undergo abstraction
of a hydrogen atom bonded to the α-carbon atom (the carbon atom of the methylene group bonded
to the hydroxyl group), which is denoted as α-H, followed by abstraction of the hydrogen atom from
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 87 -
the hydroxyl group.46,47 If the first hydrogen abstraction is the rate-determining step and can take
place via a photocatalytic process, high reaction temperature and high oxygen pressure are not
necessary for selective oxidation. In the present study, we find that light irradiation can drive the α-
H abstraction of aromatic alcohols with Au–Pd alloy NPs and thus the transformation from alcohol
into the corresponding aldehyde can be achieved in an oxidant free environment at ambient
temperatures. The α-H abstraction is the rate-determining step of the selective oxidation. Theoretical
calculations by two independent methods show that the alloying of gold and palladium enhances the
interaction between the alcohol molecules and the alloy NPs. The strong interaction facilitates the
transfer of light excited electrons of the alloy NPs to the alcohol molecules adsorbed on the NPs,
and such an electron transfer enables the hydrogen abstraction under moderate conditions.
Understanding this mechanism is useful for developing photocatalytic processes for other important
syntheses.
Results and discussion
Performance of the photocatalysts
In the present study, Au and Pd alloy NPs with varying relative ratios were prepared on
a ZrO2 support (abbreviated as Au–Pd@ZrO2, and details are given in ESI†). The metal
nanoparticles on the support were well dispersed avoiding aggregation of the particle. ZrO2 has
a wide band gap (5 eV) and exhibits no visible light absorption. Hence, the support does not
contribute to the photocatalytic activity. Fig. 1 shows the photocatalytic performance of the
Au–Pd@ZrO2 photocatalysts with various Au : Pd mass ratios in catalysing the
dehydrogenation of aromatic alcohols. All experiments were conducted under an Ar gas
atmosphere after a strict freeze–pump–thaw degassing process to remove O2. The conversion
rates achieved under light irradiation (in blue) are compared with those obtained in the dark (in
black).
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 88 -
Fig. 1 Dehydrogenation of aromatic alcohols with the Au–Pd@ZrO2 catalysts of various Au : Pd
mass ratios under visible light irradiation (blue bar) and in the dark (black bar). All of the data
represent the averages of triplicate runs with a mean variation of less than ±3%. The quantum yield
(Q.Y., %) and its calculation method are given in ESI.† A(1) Benzyl alcohol dehydrogenation using
the catalyst of 3% Au–Pd@ZrO2; the reaction proceeded for 5 h. A(2) 4-Methyloxy benzyl alcohol
dehydrogenation, reaction for 2 h. A(3) 1-Phenylethanol dehydrogenation, reaction for 22 h.
Reaction conditions: 2 mmol of the reactant, 50 mg of catalyst in trifluorotoluene solvent at 45 °C
and under 1 atm of Ar after O2 removal using freeze–pump–thaw degassing. B(1) Benzyl alcohol
dehydrogenation, 2% Au–Pd@ZrO2, reaction for 16 h. B(2) 2-Phenylethanol dehydrogenation, 2%
Au–Pd@ZrO2, reaction for 16 h. B(3) 3-Phenylpropanol dehydrogenation, 2% Au–Pd@ZrO2,
reaction for 16 h. Reaction conditions: 1 mmol of the reactant, 50 mg of catalyst in trifluorotoluene
solvent at 45 °C and under 1 atm of Ar after O2 removal using freeze–pump–thaw degassing.
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 89 -
The results in Fig. 1 demonstrate that visible light irradiation increased the product yield of
the dehydrogenation product of aromatic alcohols – corresponding aldehydes. For example, the
conversion rate of benzyl alcohol with the Au–Pd@ZrO2catalyst with an Au : Pd mass ratio of 1 :
1 is 100%. In contrast, the rate is 43% when the reaction was conducted in the dark. Similar trends
are observed in other reactants, for example, the conversion rate of 4-methoxybenzyl alcohol is 99%
under light irradiation but 54% in the dark at the same reaction temperature. Blank experiments
without metal NPs were also conducted with just the photocatalytically inactive ZrO2 supports
dispersed in a toluene solution of benzyl alcohol. Under otherwise identical conditions, no alcohol
conversion was observed under visible light irradiation or in the dark. We found that at identical
reaction temperatures and identical reaction periods, the conversion rates of the oxygen-free
dehydrogenation of aromatic alcohols (under an argon atmosphere) are comparable to those of the
selective oxidation conducted under an oxygen atmosphere. This demonstrates that the rate-
determining step of the transformation from alcohols into the corresponding aldehydes with Au–
Pd@ZrO2 photocatalysts is dehydrogenation, and light irradiation significantly enhances the
dehydrogenation.
Compared to the Au–Pd@ZrO2 catalyst containing 3% of alloy NPs, the Au–Pd@ZrO2
catalyst with lower metal loading (2%), the reaction time required to achieve the same conversion
rate with identical reaction conditions was longer, as shown in Fig. 1B. The impact on the
photocatalytic activity of varying mass ratios of the two metals with 2% metal loading has the same
trend as that observed with the catalysts with 3% metal loading.
As can be seen in Fig. 1, the alloy NP catalysts exhibited significantly higher activity than
pure Au NPs (Au : Pd ratio of 1 : 0) or pure Pd NPs (Au : Pd ratio of 0 : 1) for the dehydrogenation
of alcohols under visible light irradiation (>420 nm). This fact implies that Au–Pd@ZrO2 catalysts
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 90 -
are not collections of Au NPs and Pd NPs; instead, Au and Pd exist as binary alloy particles in these
samples.
Transmission electron microscopy (TEM) analysis of the NPs (Fig. 2) shows that the mean
diameters of the Au–Pd alloy NPs are less than 10 nm. Fig. 2B is a line profile of the energy
dispersion X-ray (EDX) spectrum of a typical Au–Pd alloy NP in Fig. 2A, which shows the
elemental distribution along the radial direction of the metal NP. The line profile indicates that the
NP consists of both Au and Pd distributed spherically around a common centre. This means that the
two metals exist as binary alloy NPs in this sample. No diffraction peaks corresponding to either
metallic Au or metallic Pd were observed from X-ray diffraction (XRD) patterns of the Au, Pd and
Au–Pd samples (not shown) because of the low metal content of the samples.
Fig. 2 (A) TEM image of 1.5% Au–1.5% Pd@ZrO2 catalyst. The arrows indicate Au–Pd NPs. (B)
High resolution TEM images of an alloy particle in the catalyst. (C) EDX spectrum line profile
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 91 -
analysis of a typical Au–Pd NP indicated along the blue arrow in Fig. 2A, providing information of
the elemental composition and distribution of the NP.
The formation of Au–Pd alloy NPs is also supported by the significant change of light
absorption properties of the sample as shown in Fig. 3.48–50 Therefore, the catalytic properties of this
sample resulted from Au–Pd alloy NPs rather than a collection of discrete Au NPs and Pd NPs.
Fig. 3 Diffuse reflectance UV-visible (DR-UV-vis) spectra of the photocatalysts and ZrO2 support.
ZrO2 has a band gap of about 5 eV,51 and exhibits weak visible light absorption with no charge
carriers generated under irradiation of light with wavelengths above 400 nm (Fig. 3). Therefore, the
ZrO2 support by itself does not contribute to photocatalytic activity. The absorption peak at 520 nm
in the spectrum of the pure Au (3 wt%) sample is due to the LSPR absorption of the Au NPs.21,25,26,48–
50,52–55 The presence of the support and its interaction with the Au NP can shift the resonance to
longer wavelengths and broaden the LSPR absorption peak. The LSPR absorption band of Pd NPs
is in the UV range at a wavelength of 330 nm.18 However, this absorption is not observed in the
extinction spectrum of the pure Pd NPs on ZrO2.
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 92 -
Interestingly, we observed a clear light absorption near the Pd absorption band (350 nm) in
the spectrum of Au–Pd alloy particles on ZrO2 (trace b, Fig. 4B), which is believed to be associated
with Au–Pd alloy NPs.18 In the spectrum of the alloy sample, the characteristic LSPR absorption
peak of Au NPs at 520 nm is much weaker when compared to the spectrum of the pure Au sample,
but larger than the absorption observed for the pure Pd sample.
Fig. 4 (A) The dependence of the catalytic activity of the Au–Pd@ZrO2 catalyst for the benzyl
alcohol dehydrogenation on the wavelength of light irradiation. Both the light-driven reaction and
the reaction in the dark were conducted at 45 °C ± 1 °C. (B) Action spectrum based on the data in
(A), and the light absorption spectra of Au–Pd@ZrO2 and Au@ZrO2 are given for comparison.
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 93 -
The influence of light irradiation
The wavelength and intensity are important irradiation energy parameters and, hence, they are
the critical factors influencing the performance of photocatalysts in reactions. The dependence of
the photocatalytic dehydrogenation of benzyl alcohol on the wavelength and intensity was
investigated. The results are illustrated in Fig. 4.
Optical filter glasses were applied to block irradiation below specific cut-off wavelengths.
Therefore, we were able to tune the wavelength of the light to clarify the influence of the wavelength
range on the catalytic activity of the Au–Pd@ZrO2 catalyst for benzyl alcohol oxidation. When light
with wavelengths in the full 400 nm to 800 nm range irradiated the reaction system a 100% reaction
conversion was observed. Applying a filter that blocked wavelengths below 490 nm resulted in a
decrease in the conversion of the reaction to 83%. Increasing the cut-off wavelength to 550 nm and
then 600 nm resulted in the conversion decreasing to 65% and 46%, respectively. Given that the
thermal conversion rate at this temperature is 44%, the contribution of light irradiation in the
wavelength range of 400–800 nm to the overall catalytic activity is 56%.
From the results acquired from the irradiation of tuned wavelength we can estimate the
contribution of the light in a narrow wavelength range. For example, the yield of the reaction is 83%
when the light with wavelengths below 490 nm was cut off. Since the reaction temperature was held
constant, the contribution of the thermal reaction remains constant regardless of the filter used.
Therefore, the observed decrease in the yield of 17% (= 100%–83%) can be attributed to light
irradiation by wavelengths in the 400 nm–490 nm range. The enhancement in the yield due to the
light irradiation by wavelengths in the 490 nm–800 nm range is 39% (= 83%–44%), which accounts
for 47% of the overall yield achieved (83%). When the system was irradiated with light with
wavelengths in the 600 nm–800 nm range, the enhancement in the yield decreased to 4%. As shown
by the results in Fig. 4A, light irradiation in the wavelength range of 490 nm–600 nm results in the
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 94 -
largest enhancement in yield, accounting for 66% of the total light irradiation enhancement
(conversion rate difference between photocatalytic reaction and that observed in the dark), 32%
results from the light irradiation with wavelengths between 490 nm and 550 nm, 34% from light
between 550 nm and 600 nm, while the light with wavelengths between 400–490 nm and 600–800
nm account for the remaining 34%. The total light energy absorbed by the NPs was estimated from
the overlap of the absorption spectrum of the Au–Pd@ZrO2 catalyst, and the spectral distribution of
the light irradiated. It was found that 36.2% of the total light energy absorption by the NPs was in
the 490 nm–600 nm wavelength range. Plotting the enhancement caused by light irradiation from
the different wavelength ranges (the vertical axis on the right hand side) against the light wavelength
reveals the action spectrum (Fig. 4B). It shows which light wavelengths are most effectively used in
specific chemical reactions. Given that the LSPR peak of Au NPs is in the wavelength range
between 500 nm and 600 nm, these results suggest that for Au–Pd@ZrO2 photocatalysts, it is the
gold that harvests visible light and that the gold nanostructure's LSPR plays an important role in
enhancing the reaction yield in alloy NP catalysed reactions.
A different wavelength range is found to produce the most significant enhancement in
performance of the pure Au NP photocatalyst from that observed for Au–Pd@ZrO2. In our recent
study on selective reactions catalysed by the Au NPs under visible light, acetophenone
hydrogenation to 1-phenyl ethyl alcohol and styrene oxide reduction to styrene,14,15 light irradiation
in the wavelength range between 490 nm and 550 nm made the most important contribution to
driving the reaction. The contributions from light in this wavelength range for the two reactions are
41% and 65%, respectively. The contribution of light in the wavelength range of 550–600 nm is
much less (24% and 0% for the two reactions, respectively), while for the Au–Pd@ZrO2catalyst in
the present study, the contribution of light within the wavelength range of 550–600 nm is even more
significant than that of light in the range of 490–550 nm. In other words, the effective wavelength
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 95 -
range of Au–Pd alloy NPs is broader (490–600 nm) than that of Au NPs. This phenomenon can be
attributed to the formation of the alloy NPs, and it also reveals that Au–Pd alloy NPs have the
potential to utilize light energy from a wider portion of the visible light spectrum for enhancing
reactions than pure Au NPs.
The impact of the light intensity on the catalyst performance was investigated while keeping
other experimental conditions unchanged. Fig. 5 shows the rate of benzyl alcohol dehydrogenation
over the Au–Pd@ZrO2 catalyst with an Au : Pd molar ratio of 1 : 1 as a function of light intensity at
45 °C ± 1 °C and 60 °C ± 1 °C, respectively. When the light intensity increased (the reaction
temperature of the reaction mixture was controlled at 45 °C; the only parameter changed is light
intensity), the conversion of benzyl alcohol oxidation increased linearly up to a light intensity of 0.8
W cm−2. Further increase in light intensity results in much greater rate increases and the relation
between light intensity and reaction rate becomes nonlinear. This is a feature of the chemical
processes driven by the light excited electrons of metals.16 It is also possible that when the light
intensity is very high, multi-photon absorption occurs, increasing the number of excited metal
electrons with sufficient energy to drive the reactions.
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 96 -
Fig. 5 Light intensity dependent photocatalytic activity of Au–Pd alloy NPs for the transformation
from benzyl alcohol into benzaldehyde at 45 °C (A) and 60 °C (B).
The light induced enhancement on the conversion was calculated by subtracting the observed
conversion of a reaction performed in the dark from the conversion observed under light irradiation
performed at the same temperature. This allows the photo-induced and thermal contributions to the
conversions to be determined and expressed as a percentage for each process, as shown in Fig. 6. It
shows clearly that higher light intensities result in a larger light enhanced contribution to the total
conversion rate.
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 97 -
Fig. 6 Light intensity dependent activity of Au–Pd@ZrO2 photocatalysts on oxidant free
dehydrogenation of benzyl alcohol.
The influence of reaction temperature
Increasing operating temperature was observed to increase the photocatalytic rate of benzyl
alcohol oxidation. Fig. 7 shows that at a constant light intensity (we applied 0.4 W cm−2 as an
example), increasing operating temperature results in a significant increase of the photocatalytic
reaction rate. There are two critical aspects of the temperature effect on the photocatalytic rate. First,
the relative population of the adsorbed reactant molecule with excited states increases according to
the Bose–Einstein distribution at higher temperatures, which means that the reactant molecule
(benzyl alcohol or oxygen molecule) will require less energy to overcome the activation barrier, and
this “less energy” could be provided by light irradiation.16,56 Second, at higher temperature more
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 98 -
electrons of the alloy NPs are in higher energy levels which can be excited by light irradiation
yielding more electrons with sufficient energy to induce reactions in the alcohol molecules adsorbed
on the alloy NPs.
Fig. 7 Dependence of the photocatalytic rate of benzyl alcohol oxidation on reaction temperature
at a constant light intensity.
The influence of the Au : Pd component ratio
As shown by the results in Fig. 1, the Au : Pd mass ratio of the alloy particles is a key
influencing factor on the catalytic performance of the oxidant free dehydrogenation of the aromatic
alcohols both under light illumination and in the dark. All reactions achieved the highest yield of
target products when the Au : Pd mass ratio of the alloy NPs was 1 : 1 (molar ratio of 1 : 1.86). Alloy
NPs with other Au : Pd mass ratios proved to be much less active. When the reactions were
performed in the dark, the same dependence on the Au : Pd mass ratio was observed but with much
lower conversion efficiencies. Catalytic processes driven by heating Au–Pd alloy catalysts have
been reported in the literature. It was found that Au–Pd alloy NPs could catalyse the hydrogenation
of cinnamaldehyde to cinnamyl alcohol.57 Toshima et al. reported that Au–Pd alloy NPs were more
active in catalysing the hydrogenation of 1,3-cyclooctadiene than either gold or palladium alone.58,59
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 99 -
A possible explanation for the superior catalytic activities of Au–Pd alloy NPs to NPs
consisting of either a pure component is that Au can isolate active Pd sites within bimetallic
systems.60 In the present study, to confirm the significant role that the Pd sites at the alloy interface
play in the catalytic processes, a sample of NPs with a Pd core and an Au shell was prepared by
reducing HAuCl4 on the Pd-ZrO2 support with H2. This sample has almost no Pd sites at the NP
surface and exhibited low activity for selective oxidation of benzyl alcohol (10% conversion
compared to 100% achieved by the Au–Pd alloy NPs with a similar Au : Pd ratio). However, the
existence of Pd sites at the alloy surface does not explain why the optimal catalytic activity was
observed when the Au : Pd molar ratio is 1 : 1.86. The underlying cause of this dependence on alloy
composition is believed to be related to the higher surface charge heterogeneity of the alloy NPs,
compared with those of either pure component NPs.13,61 Pure palladium has a slightly larger work
function (ΦPd ∼ 5.6 eV) than pure gold (ΦAu ∼ 5.3 eV), so once the two metals are in contact,
electrons will redistribute between gold and palladium until equilibrium is reached and the chemical
potentials are equal everywhere in the NPs (see Fig. 8). This electron redistribution between Au and
Pd enhances surface charge heterogeneity of the NPs (the surface charge of an NP of pure metal is
not homogeneous). The greater surface charge heterogeneity results in an enhanced interaction
between the alcohol and the NP. The enhanced interaction may lower the activation energy of the
oxidation and thus enhance the catalytic activity. Furthermore, the Fermi level in alloy NPs, Φalloy,
is higher than that in pure Pd NPs, ΦPd, so that the transfer of electrons at the Fermi level of the alloy
NPs to the benzyl alcohol molecule adsorbed on the NPs is easier compared with that from the Fermi
level of pure Pd NPs to the adsorbed molecule. The electron transfer causes the transformation of
benzyl alcohol molecules as discussed later. The light absorption of gold results in energetic
conduction electrons, which are in an even higher energy level Φalloy* and have a better ability to
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 100 -
migrate to the Pd sites on the surface. This further increases the possibility of electron transfer from
the alloy NPs to the reactant molecules.
Fig. 8 Schematic profiles showing the impact of alloying and visible light irradiation of an Au–Pd
alloy NP. (A) The surface electronic properties of the alloy NPs are different from those of pure gold
NPs as there are Pd islets on the alloy NP surface. The Pd sites are electron-rich because Pd has a
slightly larger work function than gold and electrons will flow from gold to palladium until
equilibrium is reached (the chemical potentials of the electrons are equal in the two metals, being
Φalloy). (B) The light absorption by gold results in energetic conduction electrons, which migrate to
the Pd sites on the surface. The Fermi level of the alloy NPs under light irradiation is higher (Φalloy*)
than that without irradiation (Φalloy), which increases the charge transfer possibility from the NP to
the reactant molecule. Thus the surface Pd sites with energetic electrons could exhibit significantly
enhanced catalytic activity even at ambient temperatures. Since in such an alloy NP structure the
energy of incident light is very efficiently utilised to enhance the intrinsic catalytic activity of
palladium, efficient photocatalysts may be developed from it for the synthesis of organic chemicals.
(C) Electron transfer from gold to palladium in the alloy NPs, expressed as ΔN, varies with the
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 101 -
composition of the alloy NPs (the curve). ΔN reaches a maximum when the atomic ratio of Au and
Pd in alloy NPs is 1 : 1.86 (mass ratio 1 : 1). The information on gold content of the alloy NPs
(horizontal axis) and photocatalytic conversion (vertical axis on the right hand side) of the reactions
in the present study is also given in the figure (the symbols). The photocatalytic efficiency of the
alloy NP photocatalysts is strongly correlated to the electron transfer ΔN.
Since the increased surface charge heterogeneity of the alloy NPs is due to the electron
redistribution between Au and Pd, we estimated the increase (details are provided in ESI†) by using
a free electron gas model.51 The analysis reveals that the number of electrons transferred between
the two metals, (ΔN), is maximum when the molar ratio of the two metals in the alloy NPs is 1 :
1.86. A plot of the electron transfer (ΔN; refer to the axis on the left hand side) predicted by the
model as a function of the gold mole fraction (%) in the Au–Pd alloy NPs is shown in Fig. 8C. The
catalytic conversion rates (refer to the axis on the right hand side) of the photocatalysts for oxidant
free dehydrogenation of benzyl alcohol are also given (the symbols; refer to the axis on the right
hand side). The results in Fig. 8C demonstrate a strong correlation between ΔN and the catalytic
conversion efficiency of reactants over the alloy NP catalysts. Note that this electron transfer ΔN
analysis uses parameters of bulk particles and is independent of the size of alloy particles.
We also carried out simulations using the density functional theory (DFT) for electron states
with and without light irradiation; the irradiation wavelength range between 532 and 535 nm is
chosen, which is around the LSPR absorption of Au. Calculation capacity limitations of our DFT
simulation necessitated the examination of a Pd32, Au32, and Au12Pd20 cluster. The Au : Pd ratio of
the Au12Pd20 cluster is 1.67 close to the ratio of 1 : 1.86 for the optimal Au–Pd@ZrO2 photocatalyst.
The detailed calculation method and the calculated Mulliken charge distributions are given in ESI.†
The DFT simulation results confirm that charge heterogeneity exists even in the monometallic Pd
clusters and monometallic Au clusters (Fig. 9), and the alloy structure of Au and Pd increases the
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 102 -
charge heterogeneity of the NP surface. This result is consistent with that of the free electron gas
model analysis and previous reports.62,63 Furthermore, the comparison of simulation results without
irradiation (blue lines) with the results under irradiation (red lines) suggests that light irradiation also
promotes the charge heterogeneity in Au–Pd alloy NPs.
Fig. 9 The optimized geometry and the natural charge distributions of the Au32 cluster (A) and
Au12Pd20 clusters (B) in the ground state and the considered excited state. (C) Apparent activation
energy reduction of benzyl alcohol dehydrogenation caused by the light irradiation on the Au–Pd
alloy NP photocatalyst.
The apparent activation energies of the oxidant free dehydrogenation of benzyl alcohol under
light irradiation and in the dark were derived from the kinetic data of the reaction at different
temperatures (details are provided in ESI†) using the Arrhenius equation. The difference between
the activation energies under light irradiation and in the dark (ΔEa) indicates the contribution of the
light irradiation to reducing the apparent activation energy.14,15 For example, as illustrated in Fig.
9C, the apparent activation energy for the dehydrogenation in the dark is ∼74.5 kJ mol−1 and it is
∼58.7 kJ mol−1 for the reaction under visible light illumination. The apparent activation energy in
the dark is in agreement with those reported by Bavykin et al.64 (79 kJ mol−1 for the ruthenium-
catalysed oxidation of benzyl alcohol), and by Ilyas et al.65 (77.8 kJ mol−1 for a Pt/ZrO2 catalyst
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 103 -
system). Visible light irradiation reduces the activation energy of the partial oxidation by 15.8 kJ
mol−1, which represents 21% of the activation energy.
It was reported that the first step of alcohol oxidation with a Pd catalyst was alcohol
dehydrogenation, which resulted in adsorbed hydrogen on the Pd surface and the formation of
aldehyde.66 The rate-determining step of the alcohol oxidation on gold catalysts is believed to be the
hydrogen abstraction from alcohol and the formation of Au–H species; the role of oxygen in this
reaction is to remove hydrogen from the gold surface, leading to the catalytic cycle.67,68 In order to
clarify the reaction mechanism we investigated the hydrogen abstraction from alcohol to the surface
of Au–Pd NPs. 2,2′,6,6′-Tetramethylpiperidine N-oxyl (TEMPO) is a good hydrogen abstractor, and
can abstract hydrogen from the surface of metals to form hydroxylamine but not from alcohol
molecules.69 If the addition of TEMPO can result in alcohol dehydrogenation in the absence of
oxygen, then it proves that hydrogen transfer from the alcohol to Au–Pd NPs occurs in the reaction
because the TEMPO can only abstract the hydrogen on the NP surface. Here TEMPO plays a role
similar to oxygen: removing hydrogen from the NP surface to complete the catalytic cycle so that
the catalytic conversion of alcohol to the corresponding aldehyde could proceed.
Benzyl alcohol dehydrogenation with Au–Pd@ZrO2 (Au : Pd ratio of 1 : 1.86) catalyst at
45 °C under light irradiation was conducted under a N2 atmosphere with 300 mg of TEMPO added
to the reaction system. After visible light irradiation, 21% of the benzyl alcohol was converted to
benzaldehyde. No conversion was observed in the dark at 45 °C. When the reaction was conducted
at 80 °C in the dark, 30% of the benzyl alcohol was converted by the TEMPO. Evidently, in the
absence of oxygen, TEMPO could abstract hydrogen atoms and drive alcohol oxidation either under
visible light irradiation or in the dark at a higher temperature. Blank experiments of TEMPO addition
without metal NPs (ZrO2 support only) were also conducted at both 45 °C and 80 °C. No conversion
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 104 -
was observed even under visible light, indicating that TEMPO could not abstract hydrogen atoms
directly from the alcohol but could capture them from the surface of Au–Pd NPs.
It is known that light excited electrons of plasmonic metal NP can populate unoccupied
orbitals of the molecules adsorbed on the NPs yielding a transient anion species.16,25,70 Results of a
DFT simulation (the detailed calculation method is given in ESI†) show that in a benzyl alcohol
molecule, the distances between the α-C and the two H atoms are 1.098 and 1.100 Å, respectively,
while in the transient benzyl alcohol anion species one C–H distance remains at 1.100 Å while the
other elongates to 1.104 Å. The energy required to break one of the C–H bonds at the α-C in the
molecule is 371 kJ mol−1, but only 242 kJ mol−1 is required to break the longer C–H bond of the α-
C in the transient anion species. Hence the light irradiation can facilitate the hydrogen abstraction
from the α-C through the excitation of NP electrons to the benzyl alcohol molecules adsorbed on
them. Therefore, the irradiation reduces the apparent activation energy of the reaction.
On the basis of these facts, a tentative mechanism for selective benzyl alcohol oxidation is
proposed as depicted in Scheme 1. The results of the influence of light intensity and the action
spectrum indicate that the rate determining step of the partial oxidation takes place on the surface of
the supported alloy NPs. The first step should be the abstraction of α-H atoms from the alcohol
molecules. Once this abstraction is completed the subsequent abstraction of the hydrogen atom from
the hydroxyl group of the transient anion species proceeds readily producing aldehyde as the final
product while the negative charge of the transient anions returns to the alloy NPs. The charge
injection to the reactant on the plasmonic metal NPs and the charge return to the NPs after reaction
are known processes.25,26 Oxygen or TEMPO takes away the hydrogen on the NP surface yielding
water or hydroxylamine, respectively, and thus, the NPs’ ability for dehydrogenation of alcohol is
restored.
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 105 -
Scheme 1 Proposed mechanism of aromatic alcohol oxidation over Au–Pd alloy NPs under
visible light irradiation.
The conduction electrons of the alloy NPs can gain the energy of the incident visible light via
the LSPR effect and inter-band transition. These energetic electrons are available to Pd sites at the
NP surface because of electron collisions and electron redistribution between Au and Pd. The Pd
sites have good affinity to the aromatic alcohol molecules and the interaction between the alcohol
molecules and the NP surface is enhanced by the surface charge heterogeneity of the alloy NPs. The
strong interaction facilitates the transfer of the light excited electrons of the NPs to the adsorbed
alcohol molecules. As a result, the catalytic activity of alloy NPs is significantly enhanced at ambient
temperatures, which allows the alloy NPs to efficiently catalyse the selective oxidation of aromatic
alcohols to the corresponding aldehydes and ketones. This explains our observations that the
conversion of a reaction under light irradiation is always higher than that of the corresponding
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 106 -
reaction in the dark, and that the selective oxidation of benzyl alcohol could proceed at moderate
temperatures (e.g. <45 °C) under irradiation but not in the dark.
Conclusions
In summary, it was found for the first time that visible light could significantly enhance the
performance of Au–Pd alloy NPs supported on ZrO2 for the dehydrogenation of aromatic alcohols
to yield the corresponding aldehydes in the absence of oxygen at ambient temperature. The rate
determining step of the dehydrogenation is the abstraction of α-H atoms from the alcohol molecules.
The dehydrogenation is also the rate determining step of selective oxidation of the alcohols to the
corresponding aldehydes with molecular oxygen. The results of both the free electron gas model
analysis and DFT simulation indicate that the alloy structure of Au and Pd increases the charge
heterogeneity of the NP surface, which enhanced interaction between the alloy NPs and the alcohol
molecules adsorbed on the NPs. The combination of light absorption of alloy NPs, the enhanced
interaction and the intrinsic catalytic activity of the transition metal leads to a unique structure where
the absorption of visible and UV radiation can yield energetic electrons available at catalytically
active transition metal sites on the NP surface promoting the reaction of the molecules adsorbed on
the NPs. The optimal activity for the alloy NPs was observed with an Au : Pd molar ratio of 1 : 1.86,
being in good agreement with the simulation results. The Au–Pd@ZrO2 is an example of the alloy
NPs formed by gold and a catalytically active transition metal, which can be used as a new superior
catalyst for fine organic chemical synthesis under light irradiation. Since little input energy is
consumed by other components of the reaction system, such as the solvent, support of the NPs, the
atmosphere or container, this catalyst structure is highly efficient for driving various chemical
reactions with sunlight. The knowledge acquired in this study is useful for designing suitable
photocatalysts made from gold alloyed with other transition metals and may inspire further studies
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 107 -
on new efficient photocatalysts of gold and other transition metals for a wide range of organic
synthesis driven by sunlight.
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Electronic Supplementary Information for
Visible light enhanced oxidant free dehydrogenation of aromatic alcohols
using Au–Pd alloy nanoparticle catalysts
Sarina Sarinaa, Sagala Baib, Yiming Huanga, Chao Chena, Jianfeng Jiac, Esa Jaatinena,
Godwin A. Ayokoa, Zhaorigetu Bao*b and Huaiyong Zhu*a
a School of Chemistry, Physics and Mechanical Engineering, Queensland University of
Technology, Brisbane, QLD4001, Australia
b School of Chemistry, Inner Mongolia Normal University, Hohhot, China.
c School of Chemical and Material Science, Shanxi Normal University, Linfen 041004, China.
*E-mail: [email protected]; [email protected]
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 113 -
Table of Content
1. Experimental Section
2. Table S1. A, Dehydrogenation of aromatic alcohols with the 3% Au-Pd@ZrO2
catalysts of various Au:Pd mass ratios under visible light in air and in argon gas
atmosphere. B, Dehydrogenation of aromatic alcohols with the 2% Au-Pd@ZrO2
catalysts of various Au:Pd mass ratios under visible light in argon gas atmosphere.
3. Text S1. The calculation method of quantum yield.
4. Text S2. Estimation of Au-Pd alloy NPs’ ionic property by free gas model.
5. Text S3. Density function theory (DFT) calculation of charge distribution in Au - Pd
alloy nanoparticle.
6. Text S4. Calculation of apparent activation energy for all reactions.
7. Text S5. DFT simulation of PhCH2OH- transient anion.
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 114 -
Experimental Section
Materials and Methods
Photocatalyst preparation: Catalysts with 3wt% of pure gold nanoparticles on ZrO2
(labelled 3%Au), 3wt% of pure palladium nanoparticles on ZrO2 (3%Pd) and three Au-
Pd@ZrO2 photocatalysts with different Au:Pd ratios on ZrO2 were prepared by
impregnation-reduction method. For example, 1.5%Au-1.5%Pd/ZrO2 was prepared by the
following procedure: 2.0 g ZrO2 powder was dispersed into 15.2 ml of 0.01 M HAuCl4
aqueous solution and 28.3 ml of 0.01 M NaPdCl3 aqueous solution (0.05g of PdCl2 was
dissolved in 28.3ml of 0.02M NaCl solution under stirring) were added while magnetically
stirring. 20 mL of 0.53 M lysine was then added into the mixture with vigorous stirring for 30
min. To this suspension, 10 mL of 0.35 M NaBH4 solution was added dropwise in 20 min,
followed by an addition of 10 mL of 0.3 M hydrochloric acid. The mixture was aged for
overnight and then the solid was separated, washed with water and ethanol, and dried at
60 °C. The dried solid was used directly as catalyst. Catalysts with other Au:Pd ratios were
prepared in a similar method but using different quantities of HAuCl4 aqueous solution or
NaPdCl3 aqueous solution.
Catalyst Characterization: TEM study and Line profile analysis by energy dispersion X-ray
spectrum technique of the photocatalysts were carried out on a Philips CM200 TEM with
an accelerating voltage of 200 kV. The Au and Pd content of the prepared catalysts were
determined by EDX technology using the attachment to a FEI Quanta 200 Environmental
SEM. The element line scanning was conducted on a Bruker EDX scanner attached to JEOL-
2200FS TEM with scanning beam diameter down to 1.0 nm. X-ray diffraction (XRD)
patterns of the sample powders were collected using a Philips PANalytical X’pert Pro
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 115 -
diffractometer. CuKα radiation (λ= 1.5418 Å) and a fixed power source (40 kV and 40 mA)
were used. DR-UV-vis spectra of the sample powders were examined by a Varian Cary 5000
spectrometer.
Activity Test: The information of reaction system is given briefly as footnotes of Figure1.
The suspension of catalyst powder, solvent and the reactant was placed in a chamber in which
a 500 W Halogen lamp (from Nelson, wavelength in the range 400–750 nm) was used as a
light source and the light intensity was usually 0.40 W/cm2 (except for the experiments
investigating the impact of the intensity), 50W high power LED lamps are applied as high
intensity light source in the experiment of investigating impact of light intensity (0.6~1.2
W/cm2). At given irradiation time intervals, 2 ml aliquots were collected, centrifuged, and then
filtered through a Millipore filter (pore size 0.45 μm) to remove the catalyst particulates. The
filtrates were analysed in a Gas Chromatography (HP6890 Agilent Technologies) with a HP-5
column to measure the concentration change of alcohol and products.
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 116 -
Table S1. A . Dehydrogenation of aromatic alcohols with the Au-Pd@ZrO2 catalysts of
various Au:Pd mass ratios under visible light at oxygen gas atmosphere and argon gas
atmosphere.
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 117 -
Table S1. B, Dehydrogenation of aromatic alcohols with the 2% Au-Pd@ZrO2 catalysts of
various Au:Pd mass ratios under visible light in argon gas atmosphere.
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 118 -
Text S1. The calculation method of quantum yield
The light intensity measured at the reaction system was 0.30 W/cm2 (which included
both the absorbed and scattered light). The overall energy of the photons of the irradiation on
the reaction system was derived from the product of the light intensity and section area of
the reactor under irradiation. The overlap of the light source and the absorption spectrum of
catalysts provide the distribution of the absorbed photons over the wavelength range between
400 nm and 800 nm, as shown in figure below. We could estimate the mean wavelength of
the absorbed photons from the distribution (after being normalized). The mean energy of the
photons could be calculated from the mean wavelength. The number of the photons
introduced in the reaction system in our study was calculated from the ratio of the overall
energy of the photons and mean energy of the photons. The number of molecules formed
was determined during the reaction course. Thus the apparent quantum yield was from the
ratio of the number of molecules formed to the number of the photons introduced in the
reaction system.
Figure. Absorption intensity of Au-Pd alloy NPs on ZrO2 (a) and irradiation intensity of
incandescent light (b). The overlapped area indicated the distribution of the absorbed
photons.
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 119 -
Text S2. Estimation of Au-Pd alloy NPs’ ionic property by free gas model.
The electron redistribution of the Au-Pd bond is dependent on the magnitude of the
electron transferred between the two metals. An estimate of magnitude of the charge
transferred can be obtained with the free electron gas model (1), with the change in the
number of electrons given by:
(1)
where D(εF) is the density of electron states at the Fermi energies for the two metals and:
(2a)
(2b)
(2c)
where ФPd and ФAu are the work functions of pure palladium and gold, respectively, and
Ф* is the work function of the alloy once charge equilibrium is reached.(see Scheme 2).
Effectively Δa and Δb give the shift in Fermi level (chemical potential) of the two metals at
their interface upon contact. The density of states of a free electron gas at the Fermi level is
given by (3):
𝐷(𝜀F,Au ) =3𝑁𝐴𝑢
2𝜀F,Au
where N is the number of electrons, so for the two metals the densities are:
𝐷(𝜀F,Au ) =3𝑁𝐴𝑢
2𝜀F,Au (3a)
𝐷(𝜀F,Pd ) =3𝑁𝑃𝑑
2𝜀F,Pd (3b)
Combining Equations 3a and 3b with Equation 1 the ratio Fermi level shift is given by:
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 120 -
∆𝑎
∆𝑏=
𝑁𝑃𝑑
𝑁𝐴𝑢 𝜀F,Au ,
𝜀F,Pd (4)
In the alloy systems in this study, the relative concentration of Pd and Au is varied. If
the relative concentration of Pd in the alloy is x, then that of Au will be 1-x and Equation 4
becomes:
∆𝑎
∆𝑏=
𝑥
1−𝑥 𝜀F,Au ,
𝜀F,Pd
(5)
By combining Equation 5 with Equations 2c and 1, the total change in electron
concentration can be evaluated:
(6)
where K is a constant of proportionality. Therefore, the net increase in electron concentration
on the Pd outer-shell of the nanoparticle will be:
(7)
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 121 -
Text S3. Density function theory (DFT) calculation of charge distribution in Au - Pd alloy
nanoparticle.
An Au32 cluster and a corresponding Au12Pd20 alloy cluster were constructed to mimic
the Au and Au-Pd nanoparticles. The geometry of the Au32 and Au12Pd20 were optimized by
PBE1 method of density functional theory implemented in CP2K2 code. The Molecular
optimized double zeta-valence Shorter- Range basis sets3 with a polarization function was
used to describe the valance orbitals and Goedecker-Teter-HutterPseudo-potential4 was used
to describe the core electrons. The excited state calculations on as optimized structures were
performed in the framework of Time-Dependent density functional theory with B3LYP5,6
functional provided by Gaussian09 package7. In this stage, Lanl2dz basis set8was selected
to describe the atomic orbital of Au and Pd atoms. The excited states with excited
wavelength of 534 nm for Au32 and 532 nm for Au12Pd20were considered in our
calculations. The optimized geometry of the Au32 and Au12Pd20 clusters and the natural
charge distributions9 of them in ground state and considered excited state were depicted in
Figure 8.
References:
1. Perdew, J. P; Burke, K; Ernzerhof, M., Physical Review Letters, 77 (18), 3865-3868 (1996).
2. CP2K version 2.4, the CP2K developers group (2013), http://www.cp2k.org/.
3. D. Becke, Density-functional thermochemistry. III. The role of exact exchange, J. Chem.
Phys.1993, 98, 5648- 5652.
4. VandeVondele, J; Hutter, J. J. Chem. Phys., 127 (11), 114105 (2007).
5. Krack, M., Theoretical Chemistry Accounts, 114 (1-3), 145-152 (2005).
6. Lee, C., Yang, W., and Parr, R. G., Phys. Rev. B, 1998, 3, 785-789.
7. Hay, P. J. and Wadt, W. R., J. Chem. Phys.1985, 82, 299-310.
8. Frisch, M. J., Trucks, G. W., Schlegel, H. B., G. E. Scuseria, M. A. Robb, J. R.
Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji,
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 122 -
M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L.
Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T.
Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E.
Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N.
Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S.
S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B.
Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev,
A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V.
G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels,
O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, and D. J. Fox, Gaussian 09,
Revision C.01, Gaussian, Inc., Wallingford CT, 2010.
9. Reed, A. E., Weinstock, R. B., and Weinhold, F., J. Chem. Phys., 83 (1985) 735-46.
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 123 -
Text S4. Calculation of apparent activation energy
Panel a shows the Arrhenius plots for benzyl alcohol dehydrogenation in dark (Thermal
reaction) and under light irradiation (Photo-reaction).1,2 The conversion rates of the catalytic
oxidation were used for the calculation of the reaction rate k. Arrhenius equation is applied
for calculating apparent activation energy based on reaction rate k: k=Ae-Ea/RT. Panel b
schematically illustrates the difference in activation energy between the dark reaction and
the reaction under light irradiation.
Reference
1. K. Yamada, K. Miyajima, F. Mafune, J. Phys. Chem. C 111, 11246 (2007)
2. Kittle, Introduction to Solid State Physics, 8th ed. Wiley and Sons, New York (2005).
Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle
Catalysts - 124 -
Text S5. DFT simulation of PhCH2OH- transient anion:
To model the PhCH2OH- transient anion, all the associated species were optimized at the
level of density functional theory (DFT) with Becke’s1 three-parameter exchange and Lee-
Yang-Parr correlation functional2 implemented in Gaussian 09 package3. 6-311++G(d,p) basis
set was employed to describe the orbital of all atoms involved. The energy to break the bond
between C and α-H in PhCH2OH was calculated favouring the reaction of PhCH2OH =
PhCHOH + H, while in PhCH2OH- favouring the reaction of PhCH2OH- = PhCHOH + H-.
References
1. A. D. Becke, J. Chem. Phys. 98, 5648 (1993).
2. C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 37, 785 (1988).
3. M. J. Frisch, et al., Gaussian 09, C.01, Gaussian, Inc., Wallingford CT (2010).
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 125 -
Chapter 5 Epoxidation of Alkenes with Molecular
Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts
Introductory Remarks
This chapter presents Article 3 (accepted manuscript on ACS Catalysis, 2017, DOI:
10.1021/acscatal.7b01180), in which supported Cu NPs are employed for the highly selective
epoxidation of various alkenes. This work expands the perspective of metallic photocatalysis
from tradition Au metal to another plasmonic metal and also introduced a new type of organic
into photocatalysis. Moreover, this study overcomes an intrinsic and long-standing challenge
of Cu NPs photocatalysts, which is its instability in the oxidative environment. The mechanistic
study of Cu NPs photocatalysts and the epoxidation revealed a new oxygen activation process
on the Cu NPs in the assistance of light irradiation, and therefore illustrated the potential of
photocatalysis in a wide range of oxidative organic synthesis.
Selective epoxidation of alkenes is a difficult and important reaction in organic synthesis
requiring fine catalyst design and reaction condition control to avoid the over oxidation of
alkenes to aldehydes. Copper based catalysts have been proven active for this reaction and the
metallic state of Cu is found crucial to the epoxidation selectivity. However, the instability of
Cu metal in an oxidative environment hinders the practical application of Cu metal based
catalysts. Meanwhile, metallic state Cu nanoparticles are strong light absorbers in the visible
range due to the LSPR effect. Therefore, in this part of the thesis, we studied the least
investigated plasmonic metal - Cu nanoparticle photocatalysts. We managed to stabilise Cu
nanoparticles by using titanium nitride (TiN) support material and the Cu nanoparticles were
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 126 -
found to be stable when exposed to air in the absence of additional polymer stabilisers. Through
DFT simulation technique, we illustrated that the metallic state of Cu nanoparticles is
maintained owing to the significant charger transfer loop between Cu and TiN. Next, the air
stable Cu nanoparticle photocatalyst was employed in the epoxidation of various alkenes using
oxygen gas or even air as a benign oxidant in the cyclic ether solvent. A good to high yield and
excellent selectivity were received with multiple types of alkenes. The photocatalysis
mechanism study reveals the strong chemical adsorption of alkene to Cu nanoparticles resulting
in Cu-alkene surface complexes. Such complexes can be activated under light irradiation. The
further investigation of epoxidation path tells us that the cyclic ether solvent plays a key role
in the reaction cycle. When illuminated with light, the ether interacts with O2 on the Cu
nanoparticles surface to yield active oxygen adatoms able to convert alkenes to corresponding
epoxides. The reusability test of TiN supported Cu nanoparticles suggested a long-lasting
stabilisation effect after several cycle runs of epoxidation experiments. Moreover, the
passivated Cu nanoparticles after several reaction cycles can be easily recovered by reductive
hydrogen gas treatment without significant activity or selectivity loss. In summary, this paper
introduced a convenient method for fabrication of metallic state Cu nanoparticles and reported
a successful application in the photocatalytic epoxidation of alkenes. Apart from photocatalysis,
it also could benefit future applications that require the metallic state of Cu nanostructure.
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 127 -
Article 3
Statement of Contribution of Co-Authors
Publication title and date of publication or status:
Stable Copper Nanoparticle Photocatalysts for Selective Epoxidation of Alkenes with Visible
Light
Yiming Huang, Zhe Liu, Guoping Gao, Gang Xiao, Aijun Du, Steven Bottle, Sarina Sarina*,
and Huaiyong Zhu*,
Accepted manuscript on ACS Catalysis, 2017, DOI: 10.1021/acscatal.7b01180
Contributor Statement of contribution
Student Author:
Yiming Huang
Wrote the manuscript, experimental design,
conducted experiments, and data analysis.
Signature
Date
Ms Zhe Liu Aided experimental design, conducted
experiments and data analysis.
Mr Guoping Gao Aided experimental design, conducted
computational simulation.
Dr Gang Xiao Aided experimental design.
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 128 -
A/Prof. Dr. Aijun Du Aided experimental design, conducted
computational simulation.
Prof. Dr Steven Bottle Aided experimental design and data
analysis.
Dr Sarina Sarina Corresponding author, aided experimental
design and data analysis.
Prof. Dr Huaiyong Zhu Corresponding author, aided experimental
design and data analysis.
Principal Supervisor Confirmation
I have sighted email or other correspondence from all Co-authors confirming their certifying
authorship.
____________ _____________ ________________
Name Signature Date
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 129 -
Stable Copper Nanoparticle Photocatalysts for Selective Epoxidation of Alkenes with Visible
Light
Yiming Huang,† Zhe Liu,† Guoping Gao,† Gang Xiao,‡ Aijun Du,† Steven Bottle,† Sarina
Sarina*,† and Huaiyong Zhu*,†
†School of Chemistry, Physics and Mechanical Engineering, Faculty of Science and
Engineering, Queensland University of Technology, Brisbane, QLD 4001, Australia
‡Key Laboratory of Bioprocess, Beijing University of Chemical Technology, Beijing,
100029, P. R. China.
*E-mail: [email protected]; [email protected]
Abstract: Selective epoxidation of various alkenes with molecular oxygen (O2) under mild
conditions is a long standing challenge in achieving syntheses of epoxides. Cu based catalysts
have been found to be catalytically active for selective epoxidations. However, the application
of copper nanoparticles (CuNPs) for photocatalysed epoxidations is encumbered by the
instability of CuNPs in air. Herein we report that CuNPs supported on titanium nitride (TiN)
without additional stabilizers, not only are stable in air but also can catalyse selective
epoxidation of various alkenes with O2 or even air as benign oxidant under light irradiation.
CuNPs remain in the metallic state due to the significant charge transfer that occurs between
CuNPs and TiN. The epoxidation is driven by visible light irradiation at moderate temperatures,
achieving good-to-high yields and excellent selectivity. The photocatalytic process is
applicable to the selective epoxidation of various alkenes. In this photocatalytic system,
reactant alkenes chemically adsorb on CuNPs forming Cu-alkene surface complexes and light
irradiation can activate the complexes for reaction. The cyclic ether solvent also plays a key
role, reacting with O2 on the surface of CuNPs under light irradiation, yielding oxygen adatoms.
The activated surface complexes react with the adatoms, yielding corresponding epoxides.
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 130 -
Analysis of the influence of irradiation wavelength and intensity on the epoxidation suggests
that light-excited electrons of CuNPs drive the reaction. The adatoms formed react with alkenes
producing the final product epoxides. We also observed interesting product stereo-selectivity
predominantly generating the trans-isomers for the epoxidation of stilbene (up to 97%). The
findings reported here not only provide an effective and selective reaction system for alkene
epoxidations but also are a step towards demonstrating the practical use of CuNPs as
photocatalysts for various applications.
Introduction
Selective epoxidation of alkenes is a reaction of great interest because epoxides are
versatile building blocks in organic syntheses, the pharmaceutical industry and in materials and
life sciences.1-3 At the forefront of existing challenges is the development of processes that use
a benign oxidant, have broad substrate scope while maintaining high selectivity, high atom
economy and reduced environmental impact.4
The use of metal-based homogeneous catalysts has a rich history and has been
comprehensively reviewed,5 however, such catalysts can be costly and not eco-friendly.6,7 In
the last decade, heterogeneous metal catalysts from group IB (Au, Ag, Cu etc.) have been found
to be highly effective in epoxidations in the presence of activated oxygen atoms. It is generally
accepted, although the exact details remain to be determined, that an oxametallacyclic (OME)
intermediate is the key species in the catalytic cycle that drives the epoxidation.8-13
In the past decade, the direct utilization of solar energy over various forms of
photocatalyst to promote organic synthesis has been recognized as prospective strategies.14-17
Among such, Au, Ag, Cu and Al, as plasmonic metals, their nanoparticles (NPs) exhibit strong
visible light absorption due to the localized surface plasmon resonance (LSPR) effect.18-22 Free
electrons of such NPs can also absorb ultraviolet, visible and infrared photons because these
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 131 -
metals have continuous electron energy levels. Thus NPs of these metals can utilize light
energy arising from most of the solar spectrum for catalysis.23 The light irradiation generates
photoexcited electrons and these electrons can transfer energy to molecules absorbed on the
metal surface, inducing a range of chemical transformations.24-32
Consequently, the NPs of group IB metals represent excellent candidates for the
development of new photocatalysts for epoxidations. Copper has the lowest cost among the IB
metals, whereas it also exhibits the highest selectivity.13 Studies and applications of CuNPs are,
however, strictly limited because CuNPs can be readily oxidized by air, oxidizing support
materials and oxidants in the reaction environment, yet there is evidence that the metallic state
of Cu is critical for both the optical properties of the catalyst and the epoxidation selectivity.33
Polymer stabilizers and/or inert atmospheres have been employed to maintain copper in the
metallic state.34 Nitrobenzene reduction over a Cu@graphene photocatalyst has been
reported,35 in which CuNPs stay in the metallic state on the graphene support. As yet there has
been no in-depth study to establish the basis of the stabilization effect. In this work, we
demonstrate that CuNPs can be prevented from oxidation when titanium nitride (TiN) is used
as a support, on which copper is dispersed as nanoparticles. Density function theory (DFT)
calculations suggest that the significant charge transfer loop between CuNPs and TiN support
provides the resistance of CuNPs towards oxidation.
Another challenge for selective epoxidation is using molecular oxygen as the oxidant for
the epoxidation of alkenes at mild temperatures. Oxygen, especially if delivered in air,
represents the ideal oxidant, superior even to the well-known “green” oxidant hydrogen
peroxide.36,37
In addition, low reaction temperatures are usually employed for photocatalysis processes,
which not only reduces the energy consumed by the reaction but also allow the process to be
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 132 -
applied to temperature sensitive substrates. Despite these attractions, the use of O2 as a selective
oxidant and Cu-based catalysts under mild conditions has been faced with some challenges.38,39
In this study, we overcame many of these obstacles by using a novel copper supported
photocatalyst (Cu@TiN) and a cyclic ether solvent, successfully achieving epoxidation of a
range of alkenes with molecular oxygen under visible light irradiation at moderate reaction
temperatures. Preliminary investigations of the photocatalytic epoxidation mechanism suggest
a process involving adsorbed oxygen interacting with solvent to transfer oxygen atoms to the
alkene.
Results and discussion
Metallic state of air-stable CuNPs. The photocatalyst is readily prepared by
impregnation-reduction methodology to load CuNPs onto TiN powder (with particle size ~30
nm). In the as-prepared sample, copper exists at mixed oxidation states according to the XPS
spectrum (shown in Figure S1a in Supporting Information, SI). This sample was labelled as
CuO/Cu@TiN. The CuO/Cu@TiN was further treated under a hydrogen atmosphere at 300°C
to secure the NPs in metallic state (Cu@TiN), this is confirmed by the XPS result shown in
Figure 1). The binding energies at around 952.0 eV and 932.5 eV are indicative of the existence
of Cu(0), and peaks for other oxidation stated are notably absent.
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 133 -
Figure 1. XPS spectrum of Cu@TiN photocatalyst suggests the metallic state of air-stable
CuNPs. Air-stable metallic CuNPs supported by TiN substrate.
Transmission electron microscopy (TEM) images indicate that well-dispersed CuNPs
(with a mean size of 4 nm) are distributed on the TiN surface (Figure 2a). Scanning electron
microscopy (SEM) images and energy dispersive spectrometer (EDX) mapping shown in
Figure S3 confirm the elemental composition of Cu@TiN is as designed. A representative
single-crystal Cu NP with (111) lattice planes predominantly exposed is illustrated in Figure
2b. It has previously been reported that the (111) planes of Cu are favoured for the absorption
of reactants such as styrene.40 O2 can also be absorbed easily onto the surface of the (111) plane
and this likely to represent the first step of oxygen activation.13 XRD patterns indicate that
CuNPs have negligible impact on the TiN crystal structure presumably due to the low copper
loading (Figure 2c).
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 134 -
Figure 2. TEM and XRD characterizations of Cu@TiN. (a) TEM image of Cu@TiN, CuNPs
are well dispersed on TiN substrate. (b) High-resolution TEM image, single crystal CuNP was
form on the surface of TiN substrate with clear (111) index face. (c) XRD patterns of the
Cu@TiN, CuO/Cu@TiN and TiN. Cu@TiN exhibits identical XRD peaks compared with TiN
substrate indicating negligible influence of CuNPs loading on the TiN substrate.
The light absorption of TiN and Cu@TiN were examined by diffuse reflectance UV-Vis
(DR-UV/Vis) spectra (Figure 3). Distinctly different light absorption in the visible range is
observed between TiN and Cu@TiN material. We further analyzed the UV-Vis spectrum of
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 135 -
Cu@TiN by using TiN spectrum as background and obtained a spectrum attributed to isolated
CuNPs as shown in the insert in Figure 3. This indicates that CuNPs exhibit strong visible light
absorption with peak at around 580 nm being the characteristic LSPR absorption of CuNPs.35,41
Figure 3. DR-UV-Vis spectra of Cu@TiN, TiN and isolated CuNPs (insert); Cu loading has a
significant impact on the optical properties of the TiN substrate; the isolated spectrum of
CuNPs was obtained by measuring Cu@TiN using TiN as background, peak at 580 nm is
attributed to the LSPR peak of CuNPs.
To provide an in-depth understanding of the stabilization mechanism for CuNPs
supported on the TiN substrate, we have carried out systematic DFT calculations for the
Cu/TiN interface. Figure 4 presented a side view of 3D plot of charge density difference for
CuNPs on the TiN surface. Yellow and cyan iso-surfaces represented charge accumulation and
depletion in the 3D space with an iso-surface value of 0.005 e/Å3.
300 400 500 600 700
450 600 750
No
rma
lize
d a
bs
orb
an
ce
(a
.u.)
TiN
Cu@TiN
Wavelength (nm)
580 nm
Cu NPs
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 136 -
Figure 4. Structure and charge density difference plot at the CuNPs/TiN interface simulated
using DFT method. Charges are transferred from Cu atoms to N atoms and a similar degree
of charge transfer from Ti atoms back to Cu results in the stabilization of CuNPs on the TiN
substrate. The oxidation state of Cu@TiN was calculated to be +0.02.
Clearly, there is a significant charge transfer from the CuNPs to the N atoms of the TiN
substrate, which can be the major reason for CuNPs being more stable. Importantly, an equal
amount of charge is donated back from the Ti atoms of TiN to the CuNPs. Such a charge
exchange between the CuNPs and the TiN support consequently gives rise to a slightly
positively charged state (+0.02) for the CuNPs, thus suggesting a negligible oxidation state of
Cu atom. The stabilization of CuNPs is also confirmed by the large negative formation energy
(-4.08 eV) of Cu@TiN.
In the investigation of photocatalytic performance of Cu@TiN for epoxidation of alkenes.
We found that the new Cu@TiN photocatalyst exhibits excellent activity for selective
epoxidation of various alkenes using air or molecular oxygen as the oxidant under visible light
irradiation at 60°C. Table 1 shows the results of the representative epoxidation of styrene,
trans-stilbene and norbornene (Table 1, entries 1, 4 and 5). The photocatalytic epoxidation
using air as an oxidant proceeds smoothly over the Cu@TiN photocatalysts under
moderate/mild reaction conditions. The significant difference in both conversion and
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Visible Light Photocatalysts - 137 -
selectivity between light irradiated reactions and the same reactions in the dark (numbers in
parentheses) confirms that light absorption is the major driving force for the reactions.
We also observed only a certain degree of photocatalytic-performance from the TiN
support material alone (Table S1). The TiN support exhibits light adsorption (Figure 3) and
results in 40% conversion for oxidizing styrene to benzaldehyde yet no epoxides product was
detected, which is a critical issue in the epoxidation of terminal alkenes. In the contrast, with
CuNPs loaded TiN photocatalyst, we observed 100% conversion with 89% selectivity towards
epoxides product, demonstrating the support makes little contribution to the overall conversion
and selectivity of the Cu@TiN catalyst.
The epoxidation over Cu@TiN is inhibited when air or pure O2 is replaced by argon
(Table 1, entry 3; Table S2) indicating that molecular oxygen is required as the oxidant in the
process. It also reveals the fact that Cu@TiN can cooperate with O2 while maintaining its
metallic state.
The important role of the metallic state CuNPs in epoxidation is also demonstrated in
Table 1. The precursor powder CuO/Cu@TiN comprises both metallic and oxidized Cu, as
confirmed by XPS measurements (Figure S1a). We applied CuO/Cu@TiN in photocatalytic
epoxidations as well as CuO@ZrO2 (XPS spectrum in Figure S1b), commercial Cu oxides and
salts for comparison. It should be noted that Cu oxides are p-type semiconductors with narrow
band gaps and can also absorb visible light (see the reflectance UV-Vis extinction spectra in
Figure S2a).42 Thus, as expected, we observed some light induced conversion with
CuO/Cu@TiN, CuO@ZrO2 and commercial CuO and Cu2O powder (with much higher Cu
content); although none of these materials proved as effective as metallic Cu@TiN (Table 1,
entries 6-11). It has been previously reported that copper salts, complexes or oxides exhibit
catalytic activity for epoxidation of alkenes in the assistance of stronger oxidants than
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molecular oxygen.43,44 This may explain the very low conversion that we observed with copper
oxide catalyst using molecular oxygen in the dark. Thus, on the basis of the results in Table 1,
we conclude that metallic state CuNP plays an important role in the activation of molecular
oxygen, whereas copper oxides are much less effective.
Table 1. High performance epoxidation of alkenes over Cu@TiN.[a]
Entry Catalyst Atmosphere Substrate
Conv.
[%]
Select. [%]
2 3
1 Cu@TiN Air[b] 1a 100(6) 89(n.d) 11(100)
2 Cu@TiN O2 1a 100(5) 81(n.d) 19(100)
3 Cu@TiN Argon 1a n.r. n.d. n.d.
4[c] Cu@TiN Air[b] 1b 100(14) 100(100) n.d(n.d)
5[d] Cu@TiN Air[b] 1c 100(23) 100(100) n.d(n.d)
6 CuO/Cu@TiN O2 1a 85(13) 44(19) 56(81)
7[c] CuO/Cu@TiN O2 1b 36(4) 100(100) n.d(n.d)
8[d] CuO/Cu@TiN O2 1c 72(20) 100(100) n.d(n.d)
9 CuO@ZrO2 O2 1a 74(25) 32(20) 68(80)
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10 CuO[e] O2 1a 87(7) 53(n.d) 47(100)
11 Cu2O[e] O2 1a 79(4) 54(n.d) 46(100)
12 Cu(NO3)2[e]
O2 1a 7(3) n.d(n.d) 100(100)
13 None O2 1a 6 n.d 100
Reaction conditions: 20 mg catalyst, 0.1 mmol substrate and 3 mL 1,4-dioxane as solvent.
Selected gas was bubbled for 5 min and then reaction tube was sealed. The reaction mixture
was stirred under visible light irradiation (0.5 W/cm2) at 60°C for 4 h. Conversion were
determined by using an Agilent 6980 gas chromatography coupling with an Agilent HP5973
mass spectrometer equipped with a HP-5 column (GCMS). Numbers in parentheses are results
of reactions in the dark. [a] (n.r.= no reaction; n.d.=not detected); [b] air was bubbled for 5 min
prior to the start of reaction and every hour after; [c] reaction for 8 h; [d] reaction for 16 h; [e]
the Cu loading of the catalyst is 40 times of that in Cu@TiN.
More importantly, in the epoxidation of terminal alkenes, where aldehyde can be easily
formed as by-product, the work of Marimuthu et al had proved that metallic state CuNPs are
superior to copper oxides in respect to epoxidation selectivity,33 a possible reason is the direct
electron photoexcitation between hybridized orbitals of metal-organic as such effect does not
occur on copper oxides (comparing the results of entries 2, 6 and 9 in Table 1), this theory will
be further discussed later. Cu(NO3)2 showed negligible activity in both light irradiated and dark
reactions (Table 1, entry 12) demonstrating the ineffectiveness of Cu2+ ions in epoxidation of
alkenes. The blank reaction of styrene epoxidation was also tested in the absence of any catalyst
showing a 6% conversion with only benzaldehyde product as shown in (Table 1, entry 13).
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As shown in Table 2, Cu@TiN photocatalyst exhibits wide substrate scope while
maintaining excellent selectivity, being active on linear, aromatic, terminal, electron deficient,
electron rich and conjugated alkenes. For example, good to excellent yields were achieved for
epoxidation of terminal alkenes. This is of particular interest as the 1,2-epoxide products are
often key intermediates in organic synthesis (Table 2, entries 1, 2, 4 and 5 etc.).45-47 Epoxidation
of electron-deficient alkenes, which is regarded to require strong oxidants,5 was achieved with
excellent selectivity using Cu@TiN (Table 2, entries 7 and 8).
It has been reported that terminal alkenes are less active in epoxidation reactions than
cyclo-alkenes,48 because the low electron density of terminal alkenes has a negative impact on
the electrophilic oxygen transfer and results in reduced reactivity.49 In the present study, poor
conversion is observed for cyclohexene and its derivative (Table 2, entries 9 and 10), while the
reaction of 1-hexene achieved a good yield (Table 2, entry 20).
We also found that increasing the strain energy of cycloalkanes can enhance the reaction
efficiency. Two examples, cyclooctene and norbornene are shown in Table 2 (entries 11 and
12). Nonetheless, low selectivity for the epoxide product is observed for propene epoxidation
over Cu@TiN although the propene conversion is high. The predominant product, in this case,
is the ketone (Table 2, entry 13) rather than the epoxide, the reason has not been understood
yet.
Cu@TiN photocatalyst also exhibited very good performance for reactants with terminal
conjugated double bonds (Table 2, entries 14-16) due to the strong adsorption of π-conjugated
molecules on to the Cu surface. Such adsorption lowers the LUMO energy of the adsorbate.50
Poor yields are observed with relatively inactivate aliphatic long-chain alkenes (Table 2, entries
18 and 19). Nevertheless, employing a stronger oxidant such as H2O2 can effectively enhance
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the reaction efficiency (see entries 18 and 19 under conditions d), indicating the Cu@TiN can
interact with other oxidants to transform less reactive alkenes to the corresponding epoxides.
Table 2. Scope of the Cu@TiN catalyzed epoxidation reaction of alkenes. The performance
is expressed in conversion of the reactant and selectivity to the corresponding epoxide
product (percentage in parentheses)
72%(83%) 100%(78%) 100%(100%) 100%(53%)
25%(63%) 3%(100%)[b] 100%(100%) 45%(100%)[a]
15%(100%)[a] 7%(100%)[a] 50%(100%)[a] 100%(100%)[b]
100%(5%)[b] 85%(100%)[b] 100%(65%)[b] 100%(100%)
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100%(15%)
32%(100%)[c]
65%(100%)[a][d]
14%(100%)[c]; 19%(100%)[a][d]
83%(100%)[a] 55%(100%)[a] 32%(100%)[a] 71%(100%)[a]
59%(100%)[a] 100%(89%)[a] 28%(100%)[a]
Reaction conditions: 20 mg Cu@TiN, 0.1 mmol reactant and 3 mL solvent. O2 was bubbled
for 5 min. and then the reaction tube was sealed. The reaction mixture was stirred under visible
light irradiation (0.5 W/cm2) at 60°C for 4 h [a] reaction time 16 h; [b] reaction temperature
40°C, reaction time 8 h; [c] reaction temperature 70°C, reaction time 48 h; [d] 2 equiv. H2O2
(0.2 mmol) was added as oxidant. Conversion (dark color numbers) and selectivity (red color
numbers) were determined by GCMS analysis. For entries 21-26, starting reactants are trans-
2-hexene, cis-2-hexene, trans-3-hexene, cis-3-hexene, trans-stilbene and cis-stilbene, for
possible enantiomer products, only one enantiomer is presented.
It is noteworthy that the photocatalytic epoxidation system favours trans-epoxide over
their cis-isomers (Table 2, entries 21-26). This stereo-selectivity is evident as the results in
Table 3 show that the trans-epoxide is predominant product no matter cis- or trans-stilbene
were used as reactant. Conversion of the cis-stilbene is substantially lower than that trans-
stilbene.
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It is known that both photo induced isomerizations from cis-stilbene to trans-stilbene and
from trans-stilbene to cis-stilbene can take place.51 But there is a larger barrier (~3 kcal/mol)
for the trans- to cis-isomerization than for the reverse reaction (the barrier is negligible). Trans-
stilbene should be less reactive than cis-stilbene from the point of view of energy. In the
contrary, results in Table 3 are against the inference. This indicates the stereo-selectivity over
Cu@TiN photocatalyst is not determined solely by bond energy. Importantly, under the
photocatalytic conditions of the present study, we did not observe direct trans- to cis-
isomerization of alkene reactants in experiments. Therefore, we deduce that the
stereoselectivity is integrated into the epoxidation process. Similar results have been reported
with Au/C catalyst, it was believed that the adsorption of olefins onto metal surface may create
steric constraints, resulting in a specific reaction pathway.52 Another possible explanation is
that the isomerization on CuNP surface is caused by light irradiation.53
Table 3. Stereoselectivity of stilbene epoxidation
Entry Substrate Light Conversion % Selectivity %
1 trans-Stilbene
Light 100 89 (trans)
Dark 12 98 (trans)
2 cis-Stilbene
Light 28 95 (trans)
Dark n.r. n.d.
3[a] mixed-Stilbene
Light 76 97 (trans)
Dark 5 99 (trans)
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Reaction conditions: 20 mg Cu@TiN, 0.1 mmol substrate, 3 ml 1,4 dioxane as solvent, O2
bubbled for 5 min, 60°C, 0.5 W/cm2, 16 h. [a] mixed stilbene was prepared by mixing trans-
stilbene and cis-stilbene in 1:1 ratio. (n.r. = no reaction; n.d. =not detected)
To study the active sites of Cu@TiN, we measured infrared emission spectra (IES) of
styrene adsorbed on Cu@TiN photocatalyst and TiN support, respectively, at stepwise elevated
temperatures. As shown in Figure 5, the characteristic peaks of the aromatic C=C stretch
located at around 1400 cm-1 can be identified for styrene adsorbed on both Cu@TiN and TiN
at 50oC. It confirms the existence of styrene on Cu@TiN photocatalyst and TiN support
material. These peaks can still be observed from the spectra of styrene adsorbed on Cu@TiN
when the sample was heated to 450oC, as shown in Figure 5a, suggesting a strong
chemisorption of styrene on the sample. In contrast, as shown in Figure 5b, the characteristic
aromatic C=C peaks from the sample of styrene adsorbed on TiN are greatly weakened with
raised temperature and nearly vanish at 300oC.
Figure 5. Infrared emission spectra of styrene adsorbed on (a) Cu@TiN photocatalyst and (b)
TiN support. The infrared spectra were measured from 50oC to 450oC at every 50oC gap.
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The results reveal that chemisorption of styrene on CuNPs is much stronger than that on
TiN metal. Thus, it is rational that CuNPs, rather than TiN support, are the active sites. It was
reported that chemisorption of styrene on Cu surface of (111) plane lowered unoccupied
molecular orbitals (LUMOs) and elevated occupied molecular orbitals (HOMOs).40 This is
caused by the hybridization of Cu d-band orbitals and styrene π1* and π2
* orbitals into bonding
and antibonding orbitals of Cu-styrene surface complexes. Electrons from such complexes can
be directly photo-excited between bonding and antibonding states, similar to the situation
reported by Christopher et al.54 Such excitation activates the double bond for epoxidation,
enhancing photocatalysis activity and well-controlled selectivity (Table 1, entry 1). This theory
can explain the fact that Cu@TiN exhibits photocatalytic performance superior to
CuO/Cu@TiN and other Cu(I) and Cu(II) photocatalysts.
The effect of light on the reaction was examined to provide insights into the
photocatalysis mechanism. Firstly, the irradiation wavelength has a crucial impact on the
photocatalytic epoxidation and can be directly reflected by action spectrum analysis, which
shows a variation of the photocatalytic activity (in quantum yield) as a function of the
irradiation wavelength.55 We found that the trend of quantum yields (orange dots in Figure 6)
does not follow the trend of light absorption shown by the DR-UV/Vis spectrum of Cu@TiN
(the dash lines in Figure 6). However, in the wavelength ranging from 365 nm to 490 nm, the
trend of the action spectra of all reactions match well to the light absorption spectrum of
isolated CuNPs (solid blue lines in Figure 6). The matching for cis-stilbene even extends to
530 nm. These results further demonstrate that CuNPs are the photocatalytic sites and TiN
surface sites have very limited contribution to the photocatalytic epoxidation. This is consistent
with reactant adsorption results obtained by IES spectroscopy. In this wavelength range,
stronger absorption by CuNPs results in a higher quantum efficiency.
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Visible Light Photocatalysts - 146 -
Figure 6. Action spectrum of (a) styrene, (b) norbornene, (c) trans-stilbene and (d) cis-stilbene
epoxidation. Reaction rates presented as quantum efficiency are plotted against wavelength of
irradiated light at 400±5 nm, 470±5 nm, 530±5 nm, 590±5 nm and 620±5 nm, 0.2 W/cm2.
Nonetheless, the CuNPs can strongly absorb light in the range from 530 nm to 630 nm
according to the DR-UV-Vis spectrum of isolated CuNPs, the quantum efficiency of Cu@TiN
in this range is rather low and does not follow the trend of the LSPR adsorption of CuNPs. This
fact clarifies two key issues.
First, the dependence on wavelength indicates that the contribution from the photo-
thermal effect to the reaction rate is not important. Absorption of light by the NPs may cause a
short term temperature increase of the NPs, leading to the so-called photo-thermal effect.32 The
elevated temperature on NP surface could enhance the catalytic reaction on particle surface,
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Visible Light Photocatalysts - 147 -
such temperature rising is stronger with longer light wavelength. However, the light source
with wavelength >530 cannot drive the epoxidation efficiently even with relatively high light
absorption at such range. For example, CuNPs exhibit strong absorption at 580 nm but give a
very low catalytic activity under irradiation at this wavelength. On the other hand, reaction
with 400 nm light source, which is weak in the photo-thermal effect, exhibits a much higher
reaction rate. Thus we conclude that the epoxidation was not driven by the photo-thermal effect
but rather by the direct photon energy.
Second, the steep drop of the catalytic performance under wavelengths >530 nm implies
that there is a threshold of photon energy required for the reaction. In Figure 6a,b and c, the
photons of long wavelengths (e.g. >570 nm) do not have sufficient energy to induce the
epoxidation; therefore negligible quantum yield was observed even though significant light
absorption occurs.56 However, in Figure 6d, we observed a higher quantum yield for cis-
stilbene epoxidation at wavelength 530 nm than that at wavelength of 470 nm. A possible
explanation for this is that the energy threshold for cis-stilbene is relatively low that photons
of 530 nm are sufficient to trigger epoxidation and, as a result, the high light adsorption at such
wavelength emerges to dominate the photocatalysis and leads to a quantum yield bounce.
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Figure 7. Dependence of photocatalytic activity on the light irradiance. 20 mg Cu@TiN, 0.1
mmol styrene and 3 mL 1,4-dioxane as solvent. Oxygen gas was bubbled for 5 mins and then
reaction tube was sealed. The reaction mixture was stirred under visible light irradiation (400-
700 nm) at 60°C for 4 h. Conversion and selectivity were determined by GCMS analysis using
HP-5 column. The reaction rate super linearly increases with the increase of light irradiance.
[a] reaction time 3 h.
The relationship between photocatalytic performance and light irradiance (that is, the
photon flux) was investigated through the epoxidation of styrene. Since the photons absorbed
by the CuNPs induce the reactions, the light absorption should be proportional to the reaction
yield. We observed a super linearly increased photocatalytic activity with the increasing light
irradiance as shown in Figure 7 and Table S3, this is proved to be one of the experimental
signatures of photo-excited electron-driven reactions.57 In general, high irradiance gives greater
conversion and product yield, presumably through the more photoexcitation of the CuNPs.
Additionally, more than one photoexcited electron may deposit their energies in one adsorbed
reactant molecule at high light irradiance causing a superlinear reaction rate increase, which is
a subsequent photoexcited electron deposits its energy in the reactant molecular before the
dissipation of molecular vibration induced by another photo-excited electron.
We also investigated the influence of reaction temperature on the photocatalysis process.
The temperature was adjusted by external heating or cooling. It was found that a moderate rise
of reaction temperature can efficiently accelerate photocatalytic epoxidation of styrene (see
Table S4). High reaction temperatures enhance the vibrational state of adsorbed reactant
molecules,57 so that less energy is required from photoexcited electron to overcome activation
barrier. As a result, the Cu@TiN photocatalyst can utilize thermal energy to promote
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photocatalytic epoxidation as well, which is an advantage to traditional semiconductor
photocatalysts.
To investigate the role of solvent for the epoxidation over Cu@TiN, we tested multiple
organic solvents listed in Table S5. First of all, the most common solvents such as toluene,
CH2Cl2, DMF and MeCN all give negative results, the results also illustrate that solvent
polarity does not exhibit any notable influence to the epoxidation. Moreover, high oxygen
solubility is not crucial for epoxidation as we do not observe notable conversion with acetone
and DMF. Solvent was found to be the determining factor to the ether structure because we
notice epoxidation over Cu@TiN only takes place in the presence of ether solvents, 1,4-
dioxane, THF and diphenyl methyl ether for instance (Table S5). Therefore, we conclude that
ethers structure, cyclic ethers, in particular, play a key role in the reaction mechanism and the
oxygen activation process as discussed in the subsequent section.
The epoxidation mechanism of the new photocatalytic system must involve two general
stages: 1) activation of molecular oxygen on CuNP surface; 2) selective epoxidation of C=C
bond. Previous work in this area has shown that oxygen adatoms (Oa) absorbed on CuNP, also
referred to as Cu-O species in many cases, have been identified, both theoretically and
experimentally, as the epoxidizing agent for alkene epoxidation.58 Adsorption of O2 on low
index faces of Cu is facile,59 60 yet it is difficult to convert the adsorbed O2 to Oa adatoms
weakly bonded onto Cu surface. This is supported by the fact that Cu@TiN cannot initiate
epoxidation in various solvents even those having high solubility of oxygen molecules (Table
S5, entries 1-5). Successful epoxidations were only observed with 1,4-dioxane and other ethers,
implying ether solvents played a crucial role in formation of Oa adatoms (Table S5, entries 6-
10).
Table 4. The formation of peroxide intermediate.
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Visible Light Photocatalysts - 150 -
Entry Catalyst Light Atmosphere Peroxide intermediate
1 Cu@TiN Light Oxygen Detected
2 Cu@TiN Light Argon Not detected
3 Cu@TiN Dark Oxygen Not detected
4 No catalyst Light Oxygen Not detected
Reaction conditions: 20 mg of 3 wt% Cu@TiN, 3 ml 1,4-dioxane as solvent, 60oC, light
irradiance 0.5 W/cm2, selected gas bubbled for 5 min, reaction time 2 h.
Taking 1,4-dioxane as an example (Table 4), GCMS analysis suggests that a seven-
membered cyclic peroxide is produced from 1,4-dioxane. We found that the cyclic peroxide
was produced within 2 h, only in the presence of both Cu@TiN catalyst and light irradiation
as shown in Table 4. It means that 1,4-dioxane interacts with the adsorbed O2 molecules on
CuNP surface yielding the peroxide, and this process is predominantly driven by light
irradiation due to the significant difference between light reaction and dark reaction.
We separated the liquid phase and solid catalyst of the system of entry 1 in Table 4, they
are labelled as 1,4-dioxane-peroxide (the liquid) and Cu@TiN-peroxide (the solid),
respectively. Both of them contain the seven-membered cyclic peroxide. Then we studied the
function of the both, separately and in combination for styrene epoxidation reaction. The results
are shown in Table 5.
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Combining Cu@TiN-peroxide with 1,4-dioxane-peroxide exhibited similar yields (Table
5, entry 2) to that of a typical reaction without the peroxide (Table 5, entry 1). When oxygen
was removed from reaction system, the typical reaction did not proceed (Table 5, entry 3).
However, 8% yield was observed with the combined system (Table 5, entry 4). This is the
evidence that the seven-membered cyclic peroxide provides the oxygen for epoxidation. We
also found that epoxidation can proceed with Cu@TiN-peroxide alone (Table 5, entries 5 and
6) and without an oxygen source (neither oxygen gas nor ether), but cannot with 1,4-dioxane-
peroxide alone (Table 5, entry 7). This means that 1,4-dioxane-peroxide does not react directly
with the C=C bond of styrene. Instead, there is an intermediate formed on the CuNPs, which is
the direct cause of epoxidation reaction. This intermediate is generated from the reaction
between from the seven-membered cyclic peroxide and the CuNP surface. Therefore, it is
rational that the peroxide on CuNPs decomposes to oxygen adatoms (Oa) and 1,4-dioxane, the
released oxygen adatoms act as an oxidant which directly react with the C=C bond of alkene
(which is activated by the photoexcitation of bonding electrons as afore discussed) as
schematically illustrated in Scheme 1. The releasing of Oa does not require light irradiation,
therefore we infer that light does not play critical role in this step.
Table 5. Roles of 1,4-dioxane-peroxide and Cu@TiN-peroxide in epoxidation reaction.
Entry Photocatalyst Solvent Atmosphere Yield %
1 Cu@TiN 1,4-dioxane Oxygen 53
2 Cu@TiN-peroxide 1,4-dioxane-peroxide Oxygen 56
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3 Cu@TiN 1,4-dioxane Argon 0
4 Cu@TiN-peroxide 1,4-dioxane-peroxide Argon 8
5 Cu@TiN-peroxide Toluene Argon 4
6 Cu@TiN-peroxide 1,4-dioxane Argon 6
7 None 1,4-dioxane-peroxide Argon 0
8 Cu@TiN Toluene Argon 0
Reaction conditions: 20 mg of photocatalyst, 3 ml solvent, 60oC, light irradiance 0.5 W/cm2,
selected gas bubbled for 5 min, reaction time 2 h.
It has been reported that styrene lays on a CuNP with C=C bond parallel to surface,12,13
and Oa reacts with the activated C=C bond causing the insertion of an O atom and producing
the -C-C-O- structure. With such a configuration, two types of surface intermediates could be
yielded according to previous studies on epoxidation.11 One is a four-member oxametallacycle
(OME-4) which includes one Cu atom bonded to both C and O atoms (see Scheme 1). The
other is a five-membered OME (OME-5) involving a Cu-Cu fragment bonded to C and O atoms.
The intermediate OME-5 would be expected to undergo a ring-opening process leading to
undesired combustion products, while the -C-C-O- structure in OME-4 yields epoxide through
a ring-closure process.11 Thus, selectivity of epoxidation is determined in this step that occurs
with lower activation energy. The overall reaction mechanism is shown in Scheme 1.
Interestingly, the selectivity also depends on the wavelength (Figure S4). When styrene
is epoxidized with irradiation at 470 nm, we observed the highest selectivity, either shorter or
longer wavelength light results in decreased selectivity towards epoxides. Photons with short
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Visible Light Photocatalysts - 153 -
wavelengths (<470 nm) are likely to lead to breakdown of the oxirane ring. This was confirmed
experimentally through irradiation of styrene oxide that was shown to be unstable using shorter
wavelength irradiation. On the other hand, photons with long wavelengths cannot deliver
sufficient energy to trigger the epoxidation process resulting decreased selectivity as well as
low conversions.
Scheme 1. Overall reaction mechanism of styrene epoxidation over Cu@TiN.
Oxygen molecules adsorb onto CuNPs first, and the consequent O2 activation process occur on
the surface of CuNPs in the assistance of cyclic ethers under visible light irradiation at mild
temperature (<60oC). The adsorbed O2 and cyclic ether yield cyclic peroxide, which then
release oxygen adatoms (Oa) on the CuNP and the ether. Styrene strongly chemisorb on the
CuNPs surface forming Cu-styrene surface complex, and the chemisorption results in
hybridization of Cu and styrene orbitals and formation of bonding and antibonding states.
Hence, the light irradiation could induce direct resonant photoexcitation of electrons from
hybrid bonding state to antibonding state to trigger the interaction between styrene and Oa
adatoms. The oxidation of styrene occurs on CuNP surface undergoes two competing reaction
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pathways through two types of OME intermediates and eventually give epoxidation product
and combustion product respectively. EF denotes the system Fermi level.
Reusability study.
We also investigated the reusability of the Cu@TiN catalyst. The used photocatalyst was
recovered, washed simply with water and ethanol and then directly applied in cycling reactions
without further treatment or regeneration. Cu@TiN exhibits good reusability as shown in
Figure 8: the reaction conversion maintained 100% for seven cycles while selectivity towards
epoxide was 85% in the first reaction run and decreased to 70%. We therefore analyzed
Cu@TiN of 4 cycles runs with XPS as shown in Figure S5. Results suggest that CuNPs remain
mostly metallic state after one cycle run, the content of CuO in Cu@TiN is increasing with the
increase of cycle run number, this is the reason to the epoxidation selectivity dropping. Next,
the Cu@TiN photocatalyst was recovered and reactivated in hydrogen gas atmosphere at 200oC
for 10 mins, we found the reaction conversion drop to 95% whereas the selectivity towards
epoxide regained to 76%. The XPS spectra (Figure S5e) confirmed that the CuNPs are
completely reformed to the metallic state. In addition, the inductively coupled plasma-atomic
emission spectroscopy, (ICP-AES) analysis of after-reaction solution indicated only 0.09% of
Cu was leached after one reaction cycle as shown in Table S6. The negligible metal loss is the
reason to the good reusability of the novel photocatalyst. Meanwhile, it also demonstrated that
the TiN support material not only effectively stabilizes CuNPs in the metallic state but also
solidly bonds the NPs avoiding the loss of CuNPs.
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 155 -
Figure 8. The reusability of the Cu@TiN. Epoxidation of styrene was used as module, used
Cu@TiN photocatalyst was recovered, washed with water and ethanol and then directly applied
in cycle reactions without further treatment or regeneration in the first seven cycle runs. Prior
to the eighth cycle run, the used Cu@TiN photocatalyst was reactivated by hydrogen gas at
200oC for 10 mins. The bar chart represents the reaction conversion and the dot line represents
the selectivity towards epoxides.
Conclusions
In summary, we have developed a stable Cu@TiN photocatalyst for selective epoxidation
of alkenes with molecular oxygen as the oxidant. Our characterization and DFT simulations
indicate that the stability of metallic state of CuNPs in air is attributed to significant charge
transfer between CuNPs and TiN substrate. Photocatalyzed epoxidation processes are driven
by visible light irradiation under mild conditions. The novel photocatalyst exhibits with wide
substrate scope with various types of alkenes. Interestingly, stereo-selectivity to trans-isomer
product was observed with stilbene epoxidation. The solvent cyclic ether facilitates the reaction
by reacting with molecular oxygen on the surface of CuNPs, yielding oxygen adatoms. This
reaction process is driven by light irradiation at mild temperatures. The reactant alkenes
1 2 3 4 5 6 7 8 9 100
20
40
60
80
100
C
on
ve
rsio
n (
%)
Runs
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 156 -
chemically adsorb on CuNPs, forming surface complex. The complex can be activated by
irradiation of visible light. The oxygen adatoms react with the activated alkene yielding final
product of corresponding epoxide. Light irradiation is the driving force of the epoxidation
process. Thus, the light intensity and wavelength, as well as the reaction temperature, are
influencing parameters on the performance of the photocatalytic epoxidation. The
photocatalyst exhibits excellent reusability and an overall reaction pathway for selective
epoxidation was proposed. Our results indicate a promising CuNP photocatalyst and
photocatalytic methodology at mild reaction conditions for alkene epoxidations by using
photocatalysis and molecular oxygen.
ASSOCIATED CONTENT
Supporting Information.
This material is available free of charge via the Internet at http://pubs.acs.org.
Calibration curve for conversion determination; Mass spectra for products characterization;
Characterization of photocatalyst: XPS, UV-Vis, SEM, EDX; Detailed reaction condition
optimization; ICP test for metal loss after reaction.
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]; [email protected]
ACKNOWLEDGMENT
Authors gratefully acknowledge financial support from the Australis Research Council.
(DP150102110)
KEYWORDS.
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 157 -
Selective epoxidation; Air-stable CuNPS; Photocatalysis; Direct electron photoexcitation;
Visible Light; Asymmetric synthesis.
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Supporting Information
Stable Copper Nanoparticle Photocatalysts for Selective Epoxidation of Alkenes with
Visible Light
Yiming Huang,† Zhe Liu,† Guoping Gao,† Gang Xiao,‡ Aijun Du,† Steven Bottle,† Sarina
Sarina*,† and Huaiyong Zhu*,†
†School of Chemistry, Physics and Mechanical Engineering, Faculty of Science and
Engineering, Queensland University of Technology, Brisbane, QLD 4001, Australia
‡Key Laboratory of Bioprocess, Beijing University of Chemical Technology, Beijing, 100029,
P. R. China
*Correspondence to: E-mail [email protected]; [email protected]
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 162 -
Table of Contents
Experimental Section
Calculation of conversion rate and calibration curve
Characterization of epoxidation products
Characterization of epoxide diastereomers
XPS analysis
UV-Vis spectra
Photocatalytic activity of TiN substrate
The influence of reaction atmosphere
SEM and EDX analysis
Dependence of selectivity on the light wavelength
Influence of irradiance
Influence of Reaction Temperature
Influence of Solvent
Meal loss in cycle reactions.
XPS Analysis of Cycled Cu@TiN Photocatalyst
Temperature Control Experiment with single wavelength light source
Photocatalyst Dose Optimization
Reference
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 163 -
Experimental Section
Materials and Chemicals
TiN nanopowder was purchased from Research Nanomaterials, Inc. USA. CuO, Cu2O
and Cu(NO3)2 were purchased from Chem-supply, were of AR grade and were used as received.
All other chemicals were purchased from Sigma-Aldrich and used without further purification.
Preparation of Catalysts. TiN supported CuNPs (Cu@TiN) were prepared by the
impregnation-reduction method. For example, to prepare 3 wt.% Cu@TiN, TiN powder (2.0
g) was dispersed in an aqueous solution of Cu(NO3)2 (47.2 mL, 0.01 M) under magnetic
stirring at room temperature, followed by addition of a lysine aqueous solution (10 mL, 0.53
M) to the suspension while it was vigorously stirred for 30 min. To this suspension, a freshly
prepared aqueous NaBH4 (20 mL, 0.7 M) was added dropwise. The mixture was aged
overnight, and then the solid was separated by centrifugation, washed with water (three times),
ethanol (once), and was dried at 60°C in a vacuum oven for 24 h. The as-prepared
CuO/Cu@TiN powder was reduced at 300°C for 20 mins in a flow of hydrogen gas under
argon gas protection; the obtained powder was labelled as Cu@TiN and used as prepared.
Characterization. The morphology and elemental composition of photocatalysts were studied
using a JEOL 2100 transmission electron microscopy (TEM) equipped with a Gatan Orius
SC1000 CCD camera, energy dispersion X-ray (EDX) spectrometer (X-MAXN 80TLE,
OXFORD Instruments) was coupled for elemental analysis. The accelerating voltage of TEM
was 200 KV. Scanning electron microscope (SEM) images and EDS mapping were obtained
with a ZEISS Sigma SEM at accelerating voltages of 20 KV. Diffuse reflectance UV-visible
spectra of the catalysts were collected with a Cary 5000 UV-Vis-NIR spectrometer from
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 164 -
Agilent company using BaSO4 as blank reference, the scanning scope was 200 nm to 800 nm.
X-ray diffraction (XRD) patterns were recorded on a Philips PANalytical X’Pert PRO
diffractometer using Cu Kα radiation (λ=1.5418 Å), the fixed power source was 40 kV and 40
mA. The diffraction data were collected from 5° to 75° at a scanning rate of 2.5o/min with
resolution of 0.01°. X-ray photoelectron spectroscopy (XPS) analysis was performed with a
Kratos Axis Ultra photoelectron spectrometer using mono Al Kα(1486.6 eV) x-ray.
Photocatalytic Activity Test. In a typical activity test, a 20 mL reaction tube was used as the
reactor, after reactant (0.1 mmol) and catalyst (20 mg) had been loaded, oxygen gas was
bubbled into the reaction solution for 5mins, the reaction tube was sealed in order to isolate the
reaction from air. The reaction tube was then stirred magnetically and irradiated with a halogen
lamp (from Nelson, 500 W, wavelength in the range of 400-750 nm). The irradiance was set to
0.5 W/cm2 unless otherwise specified. The reaction temperature was controlled by air
conditioner. The control reaction in the dark was conducted using an oil bath placed above a
magnetic stirrer, and the reaction tube was wrapped with aluminium foil to isolate the contents
from the influence of light. The temperature of dark reaction was maintained at the same
temperature as the corresponding reaction under irradiation. At the end of reaction time, 2 mL
aliquots were collected and filtered through a Millipore filter (pore size of 0.45 µm) to remove
particulate matter. The clear liquid-phase products were analyzed with an Agilent 6980 gas
chromatography (GC) using a HP-5 column to analyze the change in the concentrations of
reactants and products. An Agilent HP5973 mass spectrometer was used to identify the
products.
ICP Analysis
Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analysis was
performed using a Perkin Elmer 8300DV ICP fitted with an ESI SC-4DX auto-sampler and
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 165 -
PrepFAST 2 sample handling unit for online internal reference and auto-dilution of samples
and calibration references. Nitric acid, purified by sub-boiling distillation, was used for the
preparation of all references and blank solutions were used throughout the analysis.
IES Analysis
The IES analysis was conducted with a Digilab FTS-60A spectrometer equipped with a
TGS detector, the detector was modified by replacing the IR source with an emission cell.
Action Spectrum
Action spectrum experiments were conducted with light emitting diode (LED) lamps
(Tongyifang, Shenzhen, China) with wavelengths of 400±5 nm, 470±5 nm, 530 ± 5 nm, 590 ±
5 nm, and 620 ± 5 nm. The light intensity was measured to be 0.2 W/cm2 using an energy meter
(CEL-NP2000) from AULTT Company and other reaction conditions were identical to those
of typical reaction procedures.
The apparent quantum yield (AQY) was calculated as follows: apparent quantum yield
= [(Mlight−Mdark)/Np] × 100%, where Mlight and Mdark are the molecules of products formed under
irradiation and dark conditions, respectively, M = mole number of the reactant × conversion ×
6.02 × 1023 (Avogadro constant). Np is the number of photons involved in the reaction. Np =
Etotal/E1, Etotal (the total energy involved in the reaction irradiation) = intensity × light spot area
× reaction time, E1 (the energy of one photon) = h×c/ λ (h is Planck constant, c is light speed,
and λ is wavelength of the LED light).
Density Functional Theory (DFT) calculation
Geometry optimization and electronic structure calculations were carried out using
density functional theory plus long range dispersion correction under the TS method of DFT-
D as implemented in the Dmol3 software[1,2] with Semi-Core Pseudopots Potentials. Exchange-
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 166 -
correlation interaction is treated as generalized gradient approximation (GGA) with the Perdew,
Burke, and Ernzerhof (PBE) functional and electronic eigenfunctions are expanded in terms of
DND with a real-space cut-off of 4.4 Å. The convergence criteria for energy change, force, and
displacement during geometry optimization were set to be 2.0 × 10−5 Ha, 4.0 × 10−3 Ha/Å, and
5.0 × 10−3 Å, respectively. The differential charge density is obtained by following equation
(1):
∆𝜌 = 𝜌𝐶𝑢𝑁𝑃𝑠 𝑜𝑛 𝑇𝑖𝑁 − 𝜌𝐶𝑢𝑁𝑃𝑠 − 𝜌𝑇𝑖𝑁 (1)
where 𝜌𝐶𝑢𝑁𝑃𝑠 𝑜𝑛 𝑇𝑖𝑁 is the total electron density of the CuNPs supported on TiN, and 𝜌𝐶𝑢𝑁𝑃𝑠,
𝜌𝑇𝑖𝑁 is the eletron density of isolated CuNPs, and TiN respectively.
The formation energy of Cu13 cluster on TiN surface are calculated by following eq(1)
𝐸𝑓 = 𝐸𝐶𝑢13@𝑇𝑖𝑁 − 𝐸𝐶𝑢13 − 𝐸𝑇𝑖𝑁
Where 𝐸𝐶𝑢13@𝑇𝑖𝑁, 𝐸𝐶𝑢13,and 𝐸𝑇𝑖𝑁 is the total energies of Cu13 cluster adsorbed on the TiN
surface, Cu13 cluster and TiN surface, respectively.
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 167 -
Calculation of conversion rate and calibration curve
Conversion Calculation
In this study, the reaction conversion was determined based on the concentration
change of reactant and product. Taking styrene epoxidation as an example, the conversion
rate was calculated using followed equation:
Conv. %=(Cb-Ca)/Cb•100
where Cb is styrene concertation before the reaction, Ca is styrene concentration after
reaction. Ca and Cb were calculated using the equation obtained from calibration curve of
styrene:
C = 3-10•P + 0.05•10-2
where P is GC peak area, C is styrene concentration (mM). Reaction conversion was
calculated based on the concentration change:
Calibration curve of styrene
Concentration (mM) GC peak area
0 0
0.01 2.6E+07
0.02 5.5E+07
0.03 8.5E+07
0.04 1.22E+08
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
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0.05 1.38E+08
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 169 -
Calibration curve of styrene oxide
Concentration (mM) GC peak area
0 0
0.01 1.8E+7
0.02 3.9E+7
0.03 6.3E+7
0.04 8.9E+7
0.05 1.2E+08
Calibration curve of norbornene
Concentration (mM) GC peak area
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 170 -
0 0
0.01 2.1E+7
0.02 3.7E+7
0.03 5.6E+7
0.04 7.2E+7
0.05 9.6E+7
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 171 -
Characterization of epoxidation products
The epoxidation products were identified using an Agilent 6980 gas chromatography
(GC) coupling with an Agilent HP5973 mass spectrometer equipped with a HP-5 column. Mass
spectra of epoxides involved in this study are listed below, it should be noted that spectra of
some of products may reflect the differences in instrument and ionization methods.
1. Styrene oxide. Table 1 entry 1, m/z for C8H8O is 120.15.
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
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2. Oxirane, 2-(4-methyl phenyl)-. Table 2 entry 1, m/z for C9H10O is 134.17.
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 173 -
3. Oxirane, 2-(3-methyl phenyl)-. Table 2 entry 2, m/z for C9H10O is 134.17.
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
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4. Oxirane, 2-methyl-3-phenyl-. Table 2, entry 3, m/z for C9H10O is 134.17.
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 175 -
5. Oxirane, 2-methyl-2-phenyl-. Table 2, entry 4, m/z for C9H10O is 134.17.
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
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6. Oxirane, 2-(2-methylphenyl)-. Table 2, entry 5, m/z for C9H10O is 134.17.
The cluster of peaks of Oxirane, 2-(2-methylphenyl)- ~ 120 m/z which is not present in
the experimental spectrum may reflect different fragmentation pathways to those generated
using the Agilent HP5973 mass spectrometer. However, this fragmentation is unusual
compared with similar compounds such as Oxirane, 2-(3-methylphenyl)- and Oxirane, 2-(4-
methylphenyl)-.
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7. Oxirane, 2-ethyl-3-phenyl-. Table 2, entry 6, m/z for C10H12O is 148.20.
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 178 -
8. 2-oxiranecarboxaldehyde, 3-phenyl-. Table 2, entry 7, m/z for C9H8O2 is 148.05.
The peaks in the experimental spectrum roughly match with the peaks in the reference
spectrum however weak ionization gives a noisy spectrum.
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Visible Light Photocatalysts - 179 -
9. Oxirane, 2-(3-nitrophenyl)-. Table 2, entry 8, m/z for C8H7NO3 is 165.04.
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
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10. 1,2-Cyclohexene oxide. Table 2, entry 9, m/z for C6H10O is 98.13.
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11. 1,2-Epoxy-3,4-cyclohexene. Table 2, entry 10, m/z for C6H8O is 96.13.
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
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12. 1,2-Epoxycyclooctane. Table 2, entry 11, m/z for C8H14O is 126.20.
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 183 -
13. 2,3-Epoxynorbornane. Table 2, entry 12, m/z for C7H10O is 110.15.
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
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14. Propylene oxide. Table 2, entry 13, m/z for C3H6O is 58.08.
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 185 -
15. Oxirane, 2-methyl-2-(1-methylethenyl)-. Table 2, entry 14, m/z for C6H10O is 98.14.
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 186 -
16. Oxirane, 2-(1-methylethenyl)-. Table 2, entry 15, m/z for C5H8O is 84.11.
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Visible Light Photocatalysts - 187 -
17. Oxirane, 2,2-dimethyl-3-(2-methyl-1-propen-1-yl)-, table 2 entry 16, m/z for C8H14O is
126.20.
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Visible Light Photocatalysts - 188 -
18. 2,3-Oxiranedimethanol. Table 2, entry 17, m/z for C4H8O3 is 104.10.
The peaks in the experimental spectrum match with the peaks in the reference spectrum
at high m/z, however, the experimental result involves a noisier spectrum below 70 m/z which
may arise from the different sensitivity of the Agilent HP5973 mass spectrometer used for the
measurements.
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Visible Light Photocatalysts - 189 -
19. Oxirane, 2-hexyl-. Table 2, entry 18, m/z for C8H16O is 128.21.
The peaks in the experimental spectrum match with the peaks in the reference spectrum
at high m/z. However, the experimental result involves a noisier spectrum below 70 m/z which
may arise from the different sensitivity of the Agilent HP5973 mass spectrometer used for the
measurements. The additional reference spectrum from MassHunter gives well match to the
experimental spectrum.
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Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
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20. Oxirane, 2-decyl-. Table 2, entry 19, m/z for C12H24O is 184.32.
In experimental spectrum, the peaks above 110 m/z are too weak to be observed, possibly
due to the poor ionization of the compound in the instrument. Additional reference spectrum
from MassHunter is provided for comparison:
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Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
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21. Oxirane, 2-butyl-. Table 2, entry 20, m/z for C6H12O is 100.16.
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22. Oxirane, 2-methyl-3-propyl-. Table 2, entry 21, m/z for C6H12O is 100.16.
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
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23. Oxirane, 2-methyl-3-propyl-.Table 2, entry 22, m/z for C6H12O is 100.16.
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24. Oxirane, 2,3-diethyl-. Table 2, entry 23, m/z for C6H12O is 100.16.
The peaks in the experimental spectrum match with the peaks in the reference spectrum
at high m/z. However, the experimental result involves a noisier spectrum below 60 m/z which
may arise from the different sensitivity of the Agilent HP5973 mass spectrometer used for the
measurements. Additional reference spectrum from MassHunter is provided for comparison:
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 197 -
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 198 -
25. Oxirane, 2,3-diethyl-.Table 2, entry 24, m/z for C6H12O is 100.16.
The peaks in the experimental spectrum match with the peaks in the reference spectrum
at high m/z. However, the experimental result involves a noisier spectrum below 70 m/z which
may arise from the different sensitivity of the Agilent HP5973 mass spectrometer used for the
measurements.
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26. Oxirane, 2,3-diphenyl-.Table 2, entry 25, m/z for C14H12O is 196.24.
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
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27. Oxirane, 2,3-diphenyl-, (2R,3S)-. Table 2, entry 6, m/z for C14H12O is 196.24.
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
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Characterization of Epoxide Diastereomers
The epoxidation product diastereomers were identified using an Agilent 6980 gas
chromatography (GC) coupling with an Agilent HP5973 mass spectrometer equipped with a
HP-5 column. In this study, the difference in melting point of the two pairs of enantiomers,
trans-stilbene /cis-stilbene and trans-stilbene oxide/cis-stilbene oxide, are remarkable, as
shown below:
Entry Enantiomers Melting Point Boiling Point
1 trans-stilbene 124 °C [3] 307 °C [3]
2 cis-stilbene 5-6 °C [4] No experimental data
3 trans-stilbene oxide 67-69 °C [5] No experimental data
4 cis-stilbene oxide 38-40 °C [5] No experimental data
Thus, GCMS equipped with HP-5 column (non-chiral column) is capable of
distinguishing trans-stilbene/cis-stilbene and trans-stilbene oxide/cis-stilbene oxide. Two GC
spectra are shown below as examples.
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Visible Light Photocatalysts - 202 -
1. GC spectrum of mixed-stilbene which is prepared by mixing trans-stilbene and cis-stilbene
in 1:1 ratio.
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2. GC spectrum of epoxidation products from mixed-stilbene.
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Trans-stilbene and cis-stilbene give identical mass spectrum.
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XPS analysis
Figure S1. XPS analysis. XPS spectra of (a) CuO/Cu@TiN and (b) CuO@ZrO2.
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 206 -
UV-Vis Spectra
Figure S2. UV-Vis spectra. UV-Vis spectra of isolated (a) CuNPs and Cu oxides; (b)
CuO/Cu@TiN and TiN.
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Visible Light Photocatalysts - 207 -
Table S1. Photocatalytic activity of TiN substrate.[a]
Entry Catalyst Atmosphere Substrate Conv. [%]
Select.
2 [%] 3 [%]
1 TiN O2 1a 40
(12)
n.d
(n.d)
100
(100)
2[b] TiN O2 1b 11
(n.r)
100
(n.d)
n.d
(n.d)
3 [c] TiN O2 1c 15
(2)
n.d
(100)
100
(n.d)
Reaction conditions: 20 mg catalyst, 0.1 mmol substrate and 3 mL 1,4-dioxane as solvent were
added. Selected gas was bubbled for 5 mins and then reaction tube was sealed. The reaction
mixture was stirred under visible light irradiation (0.5 W/cm2) at 60°C for 4 h. Numbers in
parentheses are results of reactions in the dark. [a] (n.r.= no reaction; n.d.=not detected); [b] 8
h. [c] 16 h.
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Table S2. The influence of reaction atmosphere. The influence of different atmosphere on
the epoxidation of styrene over different photocatalyst
Entry Catalyst Atmosphere Conv. Select.
1 Cu@TiN O2 100 81
2 Cu@TiN Air 64 72
2 Cu@TiN Air[a] 100 89
3 Cu@TiN Argon n.r. n.d.
4 CuO/Cu@TiN Argon n.r. n.d.
5 TiN Argon n.r. n.d.
6 Cu@TiN Argon n.r. n.d.
7 Cu@ZrO2 Argon n.r. n.d.
8 CuO[b] Argon n.r. n.d.
17 Cu2O[b] Argon n.r. n.d.
Reaction conditions: 20 mg catalyst with 3 wt% Cu loading, 3ml 1,4-dioxane, 0.1 mmol styrene,
halogen lamp 0.5 W/cm2, selected gas was bubbled for 5 mins, 60 °C, 4h. Conversion and
selectivity were determined by GCMS analysis. [a] air was bubbled for 5 mins at the start of
reaction and every hour after. [b] 40 equiv. Cu loading compared with Cu@TiN.
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
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SEM and EDX analysis
Figure S3. SEM and EDX analysis. Scanning electron microscopy (SEM) images and
corresponding energy dispersive spectrometer (EDX) mapping of (a) Cu@TiN and (b)
CuO/Cu@TiN.
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Visible Light Photocatalysts - 210 -
Dependence of selectivity on the light wavelength
Figure S4. Dependence of selectivity on the light wavelength.
Reaction conditions: 20 mg 3 wt% Cu@TiN, 3ml 1,4-dioxane as solvent, 0.1 mmol styrene,
60oC, oxygen gas bubbled for 5 min, then reaction proceeded for 4 h. Light emitting diode
(LED) lamps (Tongyifang, Shenzhen, China) with wavelengths of 400±5 nm, 470±5 nm, 530
± 5 nm, 590 ± 5 nm, and 620 ± 5 nm were used as light source. The light intensity was measured
to be 0.2 W/cm2 using an energy meter (CEL-NP2000) from AULTT Company and other
reaction conditions were identical to those of typical reaction procedures, the reaction
selectivity was determined by product distribution which was measured with an Agilent 6980
gas chromatography (GC) equipped with an Agilent HP5973 mass spectrometer using a HP-5
column.
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 211 -
Table S3. Influence of irradiance. Influence of light irradiance on the styrene epoxidations.
Entry Irradiance (W/cm2) Conv. (%) Select. (%) Yield (%) Reaction
rate
(µmol/min)
1 0.30 39 21 8 0.1625
2 0.45 47 35 16 0.19
3 0.60 92 77 71 0.38
4 0.75 100 77 77 0.41
5[a] 0.90 90 75 67.5 0.50
Reaction conditions: 20 mg 3 wt% Cu@TiN, 3ml 1,4-dioxane as solvent, 0.1 mmol styrene,
halogen lamp, 60oC, oxygen gas bubbled for 5 min., reaction time 4 h, [a] reaction time 3 h.
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
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Table S4. The Influence of Reaction Temperature
Entry Temperature(oC) Reaction time Light Conv. (%) Select. (%)
1 30 24h Light 32 26
2 24h Dark 0 0
3 40 24h Light 28 24
4 24h Dark 2 0
5 50 4h Light 25 23
6 8h Light 54 44
7 24h Light 99 68
8 24h Dark 0 0
9 55 2h Light 48 43
10 4h Light 63 52
11 8h Light 95 66
12 16h Light 100 62
13 24h Light 100 67
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 213 -
14 8h Dark 0 0
15 60 2h Light 36 43
16 4h Light 100 89
17 24h Light 100 74
18 4h Dark 5 0
19 65 2h Light 54 75
20 4h Light 100 77
21 5h Light 100 74
22 8h Light 100 76
23 4h Dark 0 0
24 70 2h Light 100 70
25 4h Light 100 67
26 24h Light 100 49
27 2h Dark 0 0
28 4h Dark 13 11
Reaction conditions: 20 mg 3 wt% Cu@TiN, 3ml 1,4-dioxane as solvent, 0.1 mmol styrene,
halogen lamp 0.5 W/cm2, oxygen or argon gas bubbled for 5 min. Numbers in red are data
from light reaction and those in dark are data from dark reaction at the same temperature.
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 214 -
Table S5. The influence of solvent. The influence of solvent on the epoxidation of styrene
Entry Solvent Light Conversion (%) Selectivity (%)
1 Acetone
Light 0 0
Dark 0 0
2 MeCN
Light Trace 0
Dark Trace 0
3 DMF
Light 0 0
Dark 0 0
4 Toluene
Light 0 0
Dark 0 0
5 DMSO
Light 4.3 0
Dark Trace 0
6 CH2Cl2
Light 0 0
Dark 0 0
7
Light 100 81
Dark 5 0
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 215 -
8
Light 70 80
Dark 8 0
9[a]
Light 30 50
Dark 12 0
10[a]
Light 0 0
Dark 0 0
11a]
Light 20 41
Dark 3 0
12[b]
Light 2 0
Dark 0 0
13[b]
Light Trace 0
Dark 0 0
14[a]
Light 62 0
Dark 20 0
15
Light 0 0
Dark 0 0
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 216 -
16 Ethyl acetate
Light 12 0
Dark 0 0
Reaction conditions: Cu@TiN. 20 mg 3 wt% Cu@TiN catalyst, 0.1 mmol styrene, 3 mL
solvent. Oxygen was bubbled into solvent for 5 mins. Halogen lamp 0.5 W/cm2, 60°C, 4 h.
Conversion and selectivity were determined by GCMS analysis. [a] 16 h, [b] 24 h.
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 217 -
Meal Loss in Cycle Reactions.
Table S6 ICP data
Sample Cu mg/L
Reaction solution after cycle one 0.181
Sample was collected after one reaction cycle and filtrated with 0.45 μm polymer filter.
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 218 -
XPS Analysis of Cycled Cu@TiN Photocatalyst
Figure S5. XPS spectra of Cu@TiN recovered from styrene epoxidation cycle run test, (a) 1
cycle; (b) 2 cycles; (c) 3 cycles; (d) 4 cycles and (e) reactivated Cu@TiN after 8 cycles using
H2 flow at 200 oC.
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 219 -
Figure S6. The temperature control experiment (50 oC, 60 oC, 70 oC) using the light source
with wavelength > 530 nm.
We conducted the styrene epoxidation at different temperature (50 oC and 70oC) using
light source 530 nm, 590 nm and 630 nm. The reaction rate shows a positive relationship to
temperature indicating a positive impact of system temperature on the yield.
520 540 560 580 600 620
0
10
20
30
70 oC
60 oC
50 oC
Yie
ld (
%)
Wavelength (nm)
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 220 -
Photocatalyst Dose optimization
3wt% Cu@TiN dose Conv. (%) Select. (%)
1 mg 13 65
10 mg 82 87
20 mg 100 90
50 mg 100 88
Reaction conditions: 20 mg 3 wt% Cu@TiN, 3ml 1,4-dioxane as solvent, 0.1 mmol styrene,
halogen lamp 0.5 W/cm2, oxygen gas bubbled for 5 min, 60 oC, 6 h.
The result suggests 20 mg Cu@TiN gives 100% conversion with 90% selectivity to
epoxides. On the other hand, 82% conversion was obtained with 10 mg Cu@TiN. 50 mg
Cu@TiN can also completely convert styrene with 88% selectivity to epoxide, however owing
to the catalyst efficiency reaction, 20 mg is the optimal amount.
Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as
Visible Light Photocatalysts - 221 -
Reference
[1] B. Delley, J. Chem. Phys. 1990, 92, 508.
[2] B. Delley, J. Chem. Phys. 2000 113, 7756.
[3] J. Laane, K. Haller, S. Sakurai, K. Morris, D. Autrey, Z. Arp, W.-Y. Chiang, A. Combs, J.
Mol. Struct. 2003, 650, 57.
[4] D. S. Brackman, P. Plesch, J. Chem. Soc. (Resumed) 1952, 2188.
[5] A. C. Cope, P. A. Trumbull, E. R. Trumbull, J. Am. Chem. Soc. 1958, 80, 2844.
In-depth Mechanism Study of Metallic Nanoparticle Based Photocatalyst
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Chapter 6 In-depth Mechanism Study of Metallic
Nanoparticle Based Photocatalyst
Introductory Remarks
In this chapter presented is Article 4 (Published in The Journal of Physical Chemistry
Letters, 2017, 8, 2526-2534.) reporting fundamental mechanism research by analysing action
spectra (the photocatalytic performance of a metallic photocatalyst plot against the incident
light wavelength) that bring insights into the metallic photocatalysis.
In the past decades, the transition metal based photocatalysts have been realised as a new
class of promising photocatalysis candidates. Many organic synthesis reactions have been
reported over different types of transition metal and their alloy involved photocatalysts. Despite
the increasing effects being made in this area, little is known fundamentally regarding the
metallic photocatalysis mechanisms. The understanding of light induced chemical conversion
associated with energy transfer and evolution had been through the path of photothermal effect,
photoexcited charger carriers transfer (indirect photoexcitation), direct photoexcitation of
intramolecular orbitals and direct photoexcitation of the hybridised molecular orbitals of the
metal-substrate complex. Consensus has yet to be reached and proposed theories are still under
debate and being examined from both theoretical and experimental aspects. In this part of the
thesis, we investigated a number of action spectra obtained from multiple organic reactions
photocatalysed by different classes of transition metal or metal alloy NP photocatalysts. By
comparing the different action spectra trends, we reveal the versatile existence of photon
threshold energy in photoexcited electron induced metallic photocatalytic reactions. In addition,
we have further demonstrated that direct photon-electron excitation rather than photothermal
In-depth Mechanism Study of Metallic Nanoparticle Based Photocatalyst
- 223 -
effect is the dominant driving force for the initiation of metallic photocatalytic reactions. In
summary, this work contributes to the construction of a comprehensive theory to the metallic
photocatalysis and could benefit the future work in this field.
In-depth Mechanism Study of Metallic Nanoparticle Based Photocatalyst
- 224 -
Article 4
Statement of Contribution of Co-Authors
Publication title and date of publication or status:
Photon Energy Threshold in Direct Photocatalysis with Metal Nanoparticles: Key
Evidence from the Action Spectrum of the Reaction
Sarina Sarina, Esa Jaatinen, Qi Xiao, Yi Ming Huang, Philip Christopher, Jin Cai Zhao and Huai
Yong Zhu
Published in The Journal of Physical Chemistry Letters, 2017, 8, 2526-2534.
Contributor Statement of contribution
Student Author:
Yiming Huang
Conducted catalyst fabrication and
characterisations, conducted action spectra
experiments of Au-Pd alloy nanoparticles
and responsible for related data collection
and calculation, participated in the overall
data analysis and mechanism development,
revised the manuscript to improve the
logical chain.
Signature
Date
In-depth Mechanism Study of Metallic Nanoparticle Based Photocatalyst
- 225 -
Dr Sarina Sarina First author, wrote the manuscript,
experimental design, conducted experiments
and data analysis.
A/Prof. Dr. Esa Jaatinen Aided experimental design, data analysis.
Dr. Qi Xiao Aided experimental design, data analysis.
Dr Philip Christopher Aided experimental design, data analysis.
Prof. Dr Jin Cai Zhao Aided experimental design, data analysis.
Prof. Dr Huai Yong Zhu The corresponding author, aided
experimental design, data analysis.
Principal Supervisor Confirmation
I have sighted email or other correspondence from all Co-authors confirming their certifying
authorship.
____________ _____________ ________________
Name Signature Date
In-depth Mechanism Study of Metallic Nanoparticle Based Photocatalyst
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Photon Energy Threshold in Direct Photocatalysis with Metal Nanoparticles: Key
Evidence from the Action Spectrum of the Reaction
Sarina Sarina,† Esa Jaatinen,† Qi Xiao,†‡ Yi Ming Huang,† Philip Christopher,§ Jin Cai Zhao∥
and Huai Yong Zhu*†
†School of Chemistry, Physics and Mechanical Engineering, Queensland University of
Technology, Brisbane, Queensland 4001, Australia
‡CSIRO Manufacturing, Bayview Avenue, Clayton, Victoria 3168, Australia
§Department of Chemical & Environmental Engineering, University of California, Riverside,
Riverside, California 92521, United States
∥Key Laboratory of Photochemistry, Institute of Chemistry, The Chinese Academy of
Sciences, Beijing 100190, China
*E-mail: [email protected]
Abstract: By investigating the action spectra (the relationship between the irradiation
wavelength and apparent quantum efficiency of reactions under constant irradiance) of a
number of reactions catalysed by nanoparticles including plasmonic metals, nonplasmonic
metals, and their alloys at near-ambient temperatures, we found that a photon energy threshold
exists in each photocatalytic reaction; only photons with sufficient energy (e.g., higher than
the energy level of the lowest unoccupied molecular orbitals) can initiate the reactions. This
energy alignment (and the photon energy threshold) is determined by various factors, including
the wavelength and intensity of irradiation, molecule structure, reaction temperature, and so
forth. Hence, distinct action spectra were observed in the same type of reaction catalysed by
the same catalyst due to a different substituent group, a slightly changed reaction temperature.
In-depth Mechanism Study of Metallic Nanoparticle Based Photocatalyst
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These results indicate that photon–electron excitations, instead of the photothermal effect, play
a dominant role in direct photocatalysis of metal nanoparticles for many reactions.
Nanoparticles (NPs) of transition metals, such as Au, Ag, Cu;(1-7) Pd, Pt, Rh, Ir, Ru, and
their alloys,(8-10) dispersed on optically and catalytically inert materials (ZrO2, Al2O3, etc.) have
been found as efficient photocatalysts. The NPs exhibit strong optical absorption over the entire
solar spectrum and efficiently channel the photon energy into molecules that are adsorbed on
their surfaces and initiate chemical transformations.(4, 11) This process can significantly promote
catalytic activity at near-ambient temperature and pressure.(5) Direct photocatalysis by metal
NPs has inspired rapid expansion of the field of green photocatalysis, which is a promising
alternative to the many heat-driven reactions currently using thermal catalysts.(1-18) As a result,
it is of great interest to fully understand the mechanisms underpinning direct photocatalysis on
metal NP surfaces.
When the incident photon energy is less than the work function of the metal, Φ, the
incident light excites conduction electrons in metal NPs to higher energy levels by two
pathways depending on the wavelength. Single-photon excitation (one incident photon excites
one metal electron–hole pair) occurs in all metal particles. The excited metal electrons will
rapidly thermalize by successive electron–electron scattering with the high density of free
electrons in the metal.(9, 19) This results in an initial distribution of hot electrons with energies
in the range of EFermi < Ee< EFermi + Φ, where EFermi is the Fermi level of the metal. Single-
photon excitation and subsequent electron–electron scattering occur regardless of the metal
NP’s physical properties (particle size, morphology, etc.) due to the near-continuum of electron
energy levels possessed by the metal. The maximum energy reached by the hot electrons and
the rate at which the Fermi–Dirac electron distribution cools via electron–electron scattering,
however, is determined by the incident photon energy or wavelength. Irradiation can also
In-depth Mechanism Study of Metallic Nanoparticle Based Photocatalyst
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induce a collective oscillation of the conduction electrons in some metal NPs when the light
frequency is resonant with the metal’s plasma oscillation frequency, with the electrons gaining
light energy through the localized surface plasmon resonance (LSPR) effect.(1-7) The LSPR
absorption depends strongly on the properties of metal NPs, such as the dielectric coefficient
and particle morphology.(20) NPs made from Au, Ag, Cu, and Al strongly absorb visible light
through the LSPR effect, characterized by a strong absorption peak, and as a result, these metals
are often referred to as plasmonic metals.(18, 20-27) Following a very short coherency lifetime of
the LSPR, plasmons will decay, resulting in electron excitations, similar to the single-photon
excitation process described above, albeit with much higher cross sections.
Over time, the hot free electron gas relaxes to lower energy levels through electron–
phonon scattering that distributes the absorbed energy to the larger thermal mass of the NP
lattice. When high-intensity light is applied, plasmonic metal NPs absorb a significant amount
of light energy and the resulting hot NP has advantageous uses in some applications such as
the photothermal effect, plasmonic photothermal therapy, and plasmonic autoclaves.(22, 27-29)
Initially, the photothermal effect was considered as the responsible or sole driving mechanism
for direct photocatalysis involving metal NPs.(30) In our earlier study, we also assumed that
reactions on Au NPs could have been driven predominantly by a photothermal effect.(1)
However, further analysis reveals that when a metal NP 5 nm in diameter is exposed to
moderate light intensities (∼0.5 W·cm–2, about five times of mean irradiance of sunlight, which
is usually applied to photocatalytic reactions) and assuming that the whole photon energy
absorbed by this NP is converted to heat, the resulting temperature increase is about 1 K.(31, 32)
This suggests that the photothermal effect plays a minimal role in direct photocatalysis of metal
NPs when using low-intensity continuous-wave excitation sources.
In-depth Mechanism Study of Metallic Nanoparticle Based Photocatalyst
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It is often noted that the photocatalytic performance of metal NPs depends exponentially
on reaction temperature,(4, 5, 8, 15, 31) while semiconductor photocatalysts generally do not show
this dependence. Temperature can positively influence catalysis by providing an increased
driving force to overcome activation barriers by populating higher-lying vibrational states
along the reaction coordinate. For semiconductors, the minimal influence of temperature is
because increasing the temperature also increases the rate of electron–hole pair
recombination.(33) The direct photocatalysis by supported metal NPs is only influenced by
temperature through increased population of high-lying vibrational states because higher
temperature does not significantly modify hot electron and hole lifetimes, thus minimizing the
required photocatalytically mediated non-adiabatic energy gain required to drive the
reaction.(15) Nevertheless, the temperature dependence of photocatalytic performance of metal
NPs is often confused with the photothermal effect, causing difficulty in understanding the
mechanism of photocatalysis of the metal NPs.
While it is known that direct photocatalysis of metal NPs is influenced by multiple
effects,(31) the underlying mechanisms are not fully understood, and significant confusion exists.
Experimentally, distinguishing the role of the photothermal effect, hot electron transfer, and
photon–electron excitation of hybridized metal–molecule states will help shed light on the
reaction mechanisms but is a challenging undertaking.
It is known that the hot electrons at higher energy levels of the Fermi–Dirac distribution
or nonthermal distribution at very short time scales after photoexcitation of the metal could
transfer the necessary energy to the adsorbed species on metal NPs and initiate reactions of the
species.(2, 4, 8, 12, 15) We note that energy alignment is required for the reactions mediated by the
hot electrons. Such an energy alignment requirement is different from reaction to reaction and
depends closely on reaction conditions that influence the rate-limiting step of the reaction, for
In-depth Mechanism Study of Metallic Nanoparticle Based Photocatalyst
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which the energy state population will drive the system along the reaction coordinate. Because
the energy state of the photoexcited electrons is mainly determined by the irradiation
wavelength, it is possible to gain insight into the mechanism of direct photocatalysis of metal
NPs from analysis of the impact of the irradiation wavelength on the photocatalytic reactions
catalysed by metal NPs.
In this study, we attempt to clarify the situation by investigating the dependence of
photocatalytic activity on the irradiation wavelength for many reactions directly photocatalysed
by a variety of metal NP photocatalysts strictly under the regime of low-intensity continuous-
wave excitation.
We analyse the dependence of photocatalytic activity in many photocatalytic reaction
systems under illumination of different wavelengths, using six catalysts in total (NPs made
from six different metals, including Au, Pd, Pt, Rh, Ir, and Au–Pd alloy). The performances
and structure information on some of the catalysts were reported in our previous studies.(8, 34,
35) To avoid interference from light absorption by the support solids, metal NPs were supported
on photocatalytically inert support materials (such as ZrO2, etc.) that have negligible light
absorption at the wavelengths used.
The metal NP catalysts were prepared by reducing the corresponding metal salt with
NaBH4 in the presence of the support solids, abbreviated as M@support. Light-emitting diodes
(LEDs) with narrow emission bands (365 ± 5, 400 ± 5, 470 ± 5, 530 ± 5, 590 ± 5, and 620 ± 5
nm) are used as the light sources for all reactions. The reaction temperature and irradiation
intensities are kept constant for the reactions to ensure that the impact of the external heating
on each reaction remains identical.
We use action spectra (the wavelength dependence of the photocatalytic apparent
quantum efficiency, AQE) in the present study to analyse the contribution of the different
In-depth Mechanism Study of Metallic Nanoparticle Based Photocatalyst
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effects. By measuring the wavelength dependence of the AQE, we reveal the photon energy
threshold, above which significant photocatalytic activity is observed. Analysis of the action
spectra and the shape of the spectral dependence allows us to distinguish reactions mediated
by energetic electrons and those mediated by plasmon-induced heating, as the spectral
dependence of heating-induced catalytic processes should be identical regardless of the
reaction or conditions. The action spectra shapes also shed light on the nature of how energy is
transferred to molecular species.
The photocatalytic reaction rates under illumination are converted into the AQE. The
AQE is the number of reactant molecules converted by each photon absorbed by the metal NPs,
expressed as a percentage at each wavelength
Where Ylight and Ydark are the number of reactants converted with and without light irradiation,
respectively. This metric directly measures the influence of the light only as the number of
reactants converted in the dark at the same temperature is deducted.
Action Spectra of Plasmonic Metal NPs. It is generally expected that the photocatalytic
performance of a metal NP catalyst will follow its light absorption spectrum: the greater the
light absorption, the higher the photocatalytic activity. However, we found that the action
spectra of one reaction can differ due to small changes in moieties on the reactants. Figure 1
shows the action spectra of Sonogashira cross-coupling reactions of phenylacetylene and
iodobenzene with different substituent groups.
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Figure 1. Different action spectra shown on Au−Pd alloy NP photocatalysts. The dashed line
is the light absorption of the Au−Pd alloy NP photocatalyst, and the solid line is the absorption
of Au NPs dispersed on the same ZrO2 support. Left axes are the normalized absorption intensity
of metal NP photocatalysts (lines), and the right axes are the normalized AQE of reactions
(symbols). (a) Sonogashira coupling of phenylacetylene with 1-iodo-4-methoxybenzene (4-
OCH3−PhI) catalyzed by Au−Pd alloy NPs showing an action spectrum that follows the light
absorption of Au− Pd alloy NPs. (b) Sonogashira coupling of phenylacetylene with 1-iodo-4-
nitrobenzene (4-NO2−PhI) catalyzed by Au−Pd alloy NPs that exhibits an action spectrum that
does not follow the light absorption of Au−Pd alloy NPs but follows absorption of the Au NP.
Reaction conditions are listed in the Experimental Section in the Supporting Information.
In these two reactions, light absorption by the catalysts (Au–Pd alloy NPs) is the same
(the dashed line) and all other reaction conditions are identical, but the reactant aryl iodides
possess different substituent groups. When we use 1-iodo-4-methoxybenzene, the action
spectrum matches to the light absorption of Au–Pd alloy NPs (dashed line, Figure 1a).
Nonetheless, when 1-iodo-4-nitrobenzene instead of 1-iodo-4-methoxybenzene was the
reactant, the action spectrum did not follow the light absorption spectrum of the Au–Pd NPs
(dashed line in Figure 1b). The highest AQE was observed at 530 nm, even though light
absorption of the Au–Pd alloy NP (dashed line) at this wavelength was weaker than that at
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shorter wavelengths. We found that this action spectrum matches the light absorption of Au
NPs instead of the Au–Pd alloy, which shows an intensive LSPR absorption peak at about 530
nm. Evidently, in the Sonogashira coupling of 1-iodo-4-nitrobenzene (Figure 1b), the LSPR
absorption of the catalyst plays a more dominant role than the absorption at shorter wavelengths
(365 and 400 nm). In contrast, the shorter-wavelength photons more effectively drive the
reaction when 1-iodo-4-methoxybenzene is used as the reactant (Figure 1a).
The mismatch of AQE spectral dependences with the most intensive light absorption of
metal NP catalysts demonstrates directly that the major driving force of direct photocatalysis
in these cases is not a simple photothermal effect. The −NO2 group is an electron-withdrawing
group that increases the activity of iodobenzene in reaction with the nucleophilic reagent alkyne,
and −OCH3 is an electron-donating group that decreases the activity. We deduce that the
different action spectra are result of the existence of an energy threshold for inducing the
carbon–iodine bond cleavage, and the particular substituent group of the reactants influences
the threshold. Scheme 1 illustrates reactant molecules for the Sonogashira coupling with
reactants of 1-iodo-4-nitrobenzene and 1-iodo-4-methoxybenzene, respectively.
Scheme 1. Energy Levels of Hot Electrons Generated by a Short-Wavelength and a
Long-Wavelength Excitation of the Au−Pd Alloy NP, LUMO and HOMO Orbitals of 1-
Iodo-4- methoxybenzene (left hand side) and 1-Iodo-4-nitrobenzene (right hand side)
Moleculesa
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a Longer-wavelength irradiation (at the LSPR absorption range, blue area) can drive hot
electron transfer to the LUMO orbitals of 1-iodo-4-nitrobenzene only, while shorter
wavelengths are required to produce hot electrons with sufficient energy to transfer to the
LUMO orbitals of 1-iodo-4-methoxybenzene. As the adsorption of reactant molecules reduces
the energy gap between their LUMO and HOMO orbitals,(22) the relative positions of the
LUMO and HOMO with respect to the Fermi level are only qualitative and schematically show
the relative positions for the reactants.
The energy of the lowest unoccupied molecular orbital (LUMO) of 1-iodo-4-
methoxybenzene is ∼2.1 eV higher than that of 1-iodo-4-nitrobenzene, and thus, light with
shorter wavelengths are required to efficiently drive the reaction with 1-iodo-4-
methoxybenzene. This indicates that shorter wavelengths are required to generate a significant
population of hot electrons that have sufficient energy to transfer to the LUMO of 1-iodo-4-
methoxybenzene. The result shown in Figure 1 demonstrates that the rate of direct
photocatalytic conversions at each different wavelength is determined not only by the
absorption spectrum of the photocatalyst but also by the energy level of the relevant molecular
orbitals of the reactants, which in turn are dependent on the chemical bonds being activated.
The photon energy threshold in reactions involving hot electron transfer depends on energy
level alignment between hot electron energy (and therefore photon energy) and molecular
orbitals of reactants.
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Scheme 2. Hot Electron Distribution of a Plasmonic Metal or Its Alloy NPs under
Irradiation of Different Wavelengths and Their Contribution to AQE in Different
Reactionsa
(a) Irradiation with Light of 400 nm wavelength: only hot electrons located above the LUMO
level (area between two red dash line) are able to contribute to AQE. (b) Irradiation with light
of 530 nm wavelength (plasmonic wavelength of Au NPs in this study): the hot electron
distribution area above LUMO level is much smaller than that in (a). (c) Irradiated with a 400
nm wavelength on the NP and a reactant molecule with a lower LUMO: more hot electrons can
contribute to AQE. (d) Irradiated with a 530 nm wavelength on the NP and a reactant molecule
with a lower LUMO: the hot electrons able to contribute to AQE are the area above the LUMO,
involving hot electrons excited by both wavelengths.
The plasmonic metal NP catalysed photocatalytic reactions include two light absorption
mechanisms: (1) short-wavelength (e.g., 400 nm) absorption via single-electron excitation
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(also called interband excitation); and (2) LSPR absorption (e.g., the adsorption at 530 nm for
Au NPs).(36) Reactant molecules on the metal NP surface with different LUMO energy levels
are indicated in Scheme 2: high-energy LUMO (a,b) and low-energy LUMO (c,d). When
irradiated with a shorter wavelength (e.g., 400 nm), single-photon excitation generates hot
electrons. Nonetheless, the number of hot electrons with sufficient energy to inject into the
higher LUMO (Scheme 2a) is much less than that able to inject into a lower LUMO (Scheme
2c). This is the simplest situation involving only light absorption of short wavelength by single-
electron excitation.
When irradiated with longer wavelengths that drive LSPR excitation in the metal NP, the
population of hot electrons is significantly increased, as shown in Scheme 2b,d, compared with
that produced from absorption of non-LSPR wavelength light (even at short wavelengths)
(Scheme 2a,c). The LSPR excitation is the collective excitation of the conduction electrons of
metal NPs by the resonant incident light. The number of hot electrons generated by the LSPR
can be very different from the number of photons of incident light. It is rational that more hot
electrons are generated by the same quantity of light energy absorbed by the LSPR effect than
that generated by single-photon excitation, but the hot electrons generated by the LSPR effect
are at lower energy levels and can only induce reactions with lower-energy thresholds. Hence,
in a photocatalytic reaction with a lower LUMO (Scheme 2d), hot electrons produced from
LSPR decay can effectively drive the reaction, and in this case, the action spectra of plasmonic
metal NPs are expected to align well with the light absorption spectra, with higher AQE values
observed at LSPR wavelengths, such as in Figure 1b. If the reactant molecule has a relatively
higher energy LUMO, as shown in Scheme 2b, most of the hot electrons excited by LSPR
absorption are unable to transfer to the LUMO as they have insufficient energy. As a result, we
observe a lower AQE at the long wavelength, as shown in Figure 1a. Therefore, for reactions
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with a high photon energy threshold, light at shorter wavelengths is more effective in driving
the reactions than light at the LSPR peak wavelength as most of the hot electrons yielded
though LSPR absorption have energies below the threshold. For reactions with a low photon
energy threshold, the observed AQE at the LSPR peak wavelength dominates because the
strong absorption yields a large number of hot electrons that have sufficient energy for transfer
to the reactant. Consistent with this, Toker et al. reported that excitation with the LSPR
wavelength on the Ag NP cannot increase photocatalytic activity to activation of the C–Cl bond
in ethyl chloride, while the short wavelength (266 nm for example) can.(37) This is due to the
fact that the ethyl chloride LUMO level is relatively high and able to be activated by the high
energetic electrons (excited by short wavelength) only. We achieved a much lower conversion
rate of bromobenzene and chlorobenzene on the Au–Pd alloy NP photocatalyst due to the
significant higher bond energies of C–Cl and C–Br bonds than the C–I bond. To investigate
the impact of irradiation wavelength on the reaction clearly, a considerable conversion rate of
reactant is necessary; thus, we chose iodobenzene as a model reactant.
In addition to the reactions shown in Figure 1, another pair of comparable reactions is
shown in Figure 2. The action spectra for Au–Pd alloy NP@ZrO2-catalyzed Sonogashira
coupling and Suzuki coupling (coupling reactions of iodobenzene and phenylboronic acid) are
distinctly different even though the reactant is 3-methyl-iodobenzene for both reactions.
Sonogashira coupling has a lower photon energy threshold for activation of the reaction. This
is attributed to the other reactant, phenylacetylene, having a strong affinity with Au.(13) Thus,
both reactants have an easier activation on Au–Pd alloy NPs. In comparison, the other reactant
of the Suzuki reaction, phenylboronic acid, has a relatively weak affinity to the NPs, and thus,
a higher energy threshold for activating this reaction is expected. The results indicate that the
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threshold may depend on multiple factors, which are directly related to the energetic position
of the adsorbate orbital involved in charge transfer.
Figure 2. Different action spectra shown on Au−Pd alloy NP photocatalysts. (a) Sonogashira
coupling shows an action spectrum that follows plasmonic absorption of Au NPs (solid line).
(b) Suzuki coupling shows an action spectrum that follows light absorption of Au−Pd alloy
NPs (dashed line). Left axes are the normalized absorption intensity of metal NP
photocatalysts (lines), and right axes are the normalized AQE of reactions (symbols). Reaction
conditions are listed in the Experimental Section in the Supporting Information.
In a recent study of Au–Cu alloy NP-mediated photocatalytic reduction of nitrobenzene
and 4-CH2OH-substituted nitrobenzene (4-nitrobenzyl alcohol),(35) we also found that the
action spectra can be distinctly different even though the same catalyst was used. The above
argument on energy alignment requirement is applicable to these cases. The LUMO of 4-
CH2OH-substituted nitrobenzene is higher than that of nitrobenzene, resulting in a higher
threshold for activating the molecules. For reactions driven by the photothermal effect, there
should not be such a threshold because metal NPs have continuous electronic energy levels and
can absorb irradiation of all wavelengths to be heated.
We found that a photon energy threshold exists in many reactions. Data summarized in
Table 1show the wavelength dependence of AQE for several other reactions catalysed by Au
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NPs and Au–Pd alloy NPs. Irradiation with a 590 nm wavelength is not able to activate Cl-
substituted benzylamine for oxidation (entry 1) but is able to activate benzylamine (entry 2);
benzyl alcohol can be activated by the longest considered wavelength, 620 nm (entry 3), while
benzylamine cannot (entry 2). Furthermore, the highest AQE of benzyl alcohol oxidation (entry
3) and benzylamine oxidation (entry 2) is found at the LSPR wavelength (530–590 nm), while
that of Cl-substituted benzylamine is found at the short wavelength only (365 nm, see entry 1).
Table 1. AQE at Different Wavelengths in LSPR-Based Photocatalytic Reactions
Catalysed by Au−Pd Alloy and Au NPs
Entry Photocatalysts and reactions
AQE at different wavelengths (%)
365
± 5 400 ± 5
470 ± 5
530 ± 5
590 ± 5
620 ± 5
1 Au−Pd alloy @ ZrO2 benzylamine oxidative coupling (−Cl)
0.02 0.02 0.01 0.01 0 0
2 Au−Pd alloy @ ZrO2
benzylamine oxidative coupling 0.02 0.01 0.02 0.03 0.03 0
3 Au−Pd alloy @ ZrO2
benzyl alcohol dehydrogenation 1.12 0.56 0.98 1.00 1.05 0.26
4 Au NP @ CeO2
acetophenone hydrogenation 0.21 0.12 0.09 0.16 0.07 0
5 Au NP @ CeO2
styrene hydrogenation 0.15 0.11 0.05 0.17 0 0
For reactions that have a low photon energy threshold and are catalysed by Au NPs, as
shown in entries 4–5, the reaction rates under short wavelengths (365 and 400 nm) are relatively
high (entry 4) but comparable to that under the LSPR absorption wavelength, 530 nm (entry
5). This series of results is a clear demonstration that a complete understanding of the energy
of targeted molecular orbitals associated with driving rate-limiting steps in the reaction must
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be achieved before photocatalytically enhancing the processes. Christopher et al. theoretically
predicted a similar phenomenon in a small gas molecule–metal NP surface adsorbate system.(38)
These cases demonstrate convincingly that catalytic reactions are not driven by a
photothermal effect of the catalyst as the effect depends on the light absorption of
photocatalysts. If the pair of reactions was driven by the photothermal effect, the action spectra
should have been the same. Similarly, these cases directly show that the shape of action spectra
reflects the photocatalytic light absorption and the energetic position of the molecular orbital
that is accepting the hot electron.
Action Spectra of Nonplasmonic Metal NPs. Visible light irradiation also significantly
enhances the catalytic performance of nonplasmonic metal NPs,(8) and the wavelength
dependences of AQE for reactions catalysed by nonplasmonic metal NPs (Table 2) share a
common feature, which differentiates them from the plasmonic NPs: higher AQEs are always
achieved with shorter wavelengths. Light absorption cross sections of nonplasmonic metal NPs
vary only slightly in the visible light range as the LSPR absorption for these metal NPs is in
the UV region.(39) This implies that when exposed to low-intensity visible light, reactions on
nonplasmonic NPs are driven by neither LSPR light absorption nor the photothermal effect. As
shown in Table 2 (entries 1–4), the AQE values when irradiated with wavelengths of 530 nm
or longer are negligible. Appreciable reactions only take place when the irradiation
wavelengths are shorter than 530 nm. There exists a photon energy threshold for each
photocatalytic reaction, for example, the AQE values for reactions given by entries 2–4 at a
wavelength of 470 nm are two to seven times greater than that observed at 530 nm.
Table 2. AQE at Different Wavelengths in Various Photocatalytic Reactions Catalysed by
Nonplasmonic Metal NPsa
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Entry Photocatalysts and reactions
AQE at different wavelengths (%)
365 ±
5 400 ± 5
470 ± 5
530 ± 5
590 ± 5
620 ± 5
1
Pd@ZrO2
benzyl alcohol
dehydrogenation
0.17 0.08 0.07 0.03 0.02 0
2
Pt@ZrO2
benzyl amine oxidative
coupling
0.39 0.28 0.19 0.04 0.03 0
3
Rh@ZrO2
benzyl amine oxidative
coupling
0.50 0.15 0.14 0.02 0 0
4
Ir@ZrO2
benzyl alcohol
dehydrogenation
0.25 0.18 0.12 0.04 0 0
5 Pd@ZrO2
Heck coupling 0.15 0.09 0.07 0.06 0.04 0.03
a Reproduced with permission of ref 8.
Consider a reaction driven by hot electron transfer, which occurs when the interaction
between the reactant and metal NPs is relatively weak. The electron energy distribution of the
hot electrons of nonplasmonic metal NPs soon after photon absorption is schematically
depicted in Scheme 3 for two different photon energies. Hot electron transfer can be very rapid
and can take place in tens of femtoseconds (fs),(9, 19, 27, 40-43) which is much shorter than the
length of time that the population of electrons remains “hot”. Whether an electron transfer
occurs will depend on whether there are enough electrons with sufficiently high energies to
make the transition possible. As previously discussed, absorption of higher-energy incident
photons will increase the number of electrons that have energies aligned to the molecular
orbitals during the time that the electrons remain hot and thereby increases the probability of
electron transfer.(44, 45) Only hot electrons with energies higher than the LUMO of the reactant
molecule (area between two red dashed lines) are able to induce a reaction and contribute to
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the AQE. For equivalent light intensities, a shorter wavelength (400 nm for example, as shown
in Scheme 3a) is able to excite more hot electrons to higher energy levels than the longer
wavelength (530 nm, Scheme 3b). Hence, when the LUMO energy of the reactant molecule is
relatively high, the hot electrons excited by shorter-wavelength photons are more effective at
activating the reactant.
Scheme 3. Hot Electron Distribution of Nonplasmonic Metal NPs under Different
Wavelength Irradiation and Their Contribution to AQEa
a(a) Irradiation with light of 400 nm wavelength: only the hot electrons located above the
LUMO of the reactant molecule (area between two red dashed line) are able to contribute to
AQE. (b) Irradiation with light of 530 nm wavelength: the hot electron distribution area above
LUMO is much smaller than that in (a).
This phenomenon occurs in all metal NPs, plasmonic and nonplasmonic, as all metals
have conduction electrons (including hot electrons) distributed over continuous electron energy
levels. Combined with the information given in Table 2, irradiation with longer wavelengths
does not induce measurable reaction activity, although the irradiation should cause a similar
photothermal effect on the metal NPs with shorter wavelengths. Evidently, the photothermal
effect is not the driving force for these reactions.
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It is also important to note that the relative distribution of hot electrons is likely very
different comparing plasmonic (Ag, Au, Cu, and Al) metals and nonplasmonic metals.
Plasmonic metals have filled d bands that sit well below EFermi and most of the time below the
LSPR energy. This suggests that hot electron production from plasmon decay will occur by
intraband transitions within the sp band of the metal, producing a flat probability distribution
in the range EFermi < Ee < EFermi + Φ. On the other hand, nonplasmonic metals are characterized
by unfilled d bands, meaning that the EFermi cuts through the d band. This suggests that
photoexcitation of nonplasmonic metals will mostly produce hot holes and electrons with
energies only slightly above EFermi, where the empty d band density is positioned. This may
explain why long-wavelength excitation produces so little photocatalytic activity because the
hot electrons are not very hot and situated only just above EFermi.
Other Factors Influencing Action Spectra. We also noted that photon energy thresholds
in the reactions are influenced not only by the molecular structure of the reactants but also by
other factors such as temperatures. Figure 3 shows one case where the highest reaction rate was
observed at short wavelengths even if light absorption at the short wavelength was not as
intense as that at the LSPR absorption wavelength of the Au NPs. When the reaction
temperature was raised from 30 to 45 °C, the highest AQE of nitrobenzene (Ph-NO2) reduction
shifted from 365 nm (in the ultraviolet, UV, range) to 530 nm (typical LSPR absorption peak
of the supported Au NPs). The difference in reaction temperature of this reaction system is
sufficient to cause profound changes in the action spectrum. At lower temperature (30 °C), the
AQE does not match to the light absorption spectrum of Au NPs, while it matches at higher
temperatures (≥45 °C). This means that at 30 °C the light energy absorbed by the Au NPs
through LSPR absorption (peaked at 530 nm) is insufficient to surmount the activation barrier
of the reaction. As mentioned, the changes in molecule vibration states resulted from a rise of
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reaction temperature and contribute to reduce the photon energy threshold. This inference can
explain spectra in Figure 3a,b. As the temperature rises, the energy threshold is reduced; at
45 °C, the hot electrons generated by LSPR absorption are able to induce this reaction.
Figure 3. Action spectra of nitrobenzene reductive coupling catalysed by Au NPs. (a) Catalysis
by Au NPs at 30 °C exhibits an action spectrum that does not follow plasmonic absorption of
Au NPs. (b) Reactions catalysed by Au NPs at 45 °C show an action spectrum following
plasmonic absorption of Au NPs.
In summary, analysis of the wavelength dependence of AQE indicates that there is a
photon energy threshold for many reactions photocatalysed directly by metal NPs. The
threshold is a feature of the photoinduced electron excitation-driven reactions. The reactions
are predominantly driven through interaction between the hot metal electrons with sufficient
energy and reactant molecules adsorbed on the metal NPs or by exciting the hybridized states
of metal electron states and orbitals of the reactant molecules strongly adsorbed on the metal
NPs. By changing the position of the orbital that must be populated to drive the chemical
reaction, the threshold varies and the action spectra are significantly modified. Thus, one can
see a molecular imprint on the action spectra. The contribution to the observed photocatalysis
from the photoinduced heating is not dominant in most cases when using low-intensity light
sources. The photon–electron excitation mechanisms are important as they can efficiently
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channel the photon energy into the chemical bonds to be activated. They also reveal
opportunities to achieve efficient chemical transformations at near-ambient conditions by
tuning the wavelength and intensity of the irradiation as well as the reaction temperature. Such
photocatalysis may lead to discoveries in the selective organic synthesis reactions driven by
irradiation of the solar spectrum.
ASSOCIATED CONTENT
*Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at
DOI: 10.1021/acs.jpclett.7b00941. Experimental methods, TEM images of photocatalysts
(Figure S1), and photograph of the LED lamp setup (Figure S2)
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected].
Notes
The authors declare no competing financial interest.
Acknowledgment
The authors gratefully acknowledge financial support from the Australian Research
Council (ARC DP110104990 and DP150102110). Q.X. acknowledges an Office of the Chief
Executive (OCE) Postdoctoral Fellowship from CSIRO. P.C. acknowledges funding from the
U.S. Army Research Office through Grant No. W911NF-14-1-0347. Electron microscopy
analysis was performed through a user project supported by the Central Analytical Research
Facility (CARF), Queensland University of Technology.
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Supporting Information
Photon Energy Threshold in Direct Photocatalysis with Metal Nanoparticles –Key
Evidence from Action Spectrum of the Reaction
Sarina Sarina,a Esa Jaatinen,a Qi Xiao,a,b Yi Ming Huang,a Philip Christopher,c Jin Cai
Zhao,d Huai Yong Zhua,*
a School of Chemistry, Physics and Mechanical Engineering, Queensland University of
Technology, Brisbane, QLD4001, Australia
b CSIRO Manufacturing, Bayview Avenue, Clayton, VIC 3168, Australia
c Department of Chemical & Environmental Engineering, University of California, Riverside,
Riverside, California 92521, United States
d Key Laboratory of Photochemistry, Institute of Chemistry, the Chinese Academy of Sciences,
Beijing 100190, China.
*Corresponding author: Prof. Huai-Yong Zhu, E-mail: [email protected]
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Contents
Experimental section
Figure S1 TEM images of metal nanoparticle photocatalysts supported on ZrO2
Figure S2 The digital photograph of the LED lamps setup used in the wavelength
experiments
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Experimental Preparation of catalysts
Au NP@ZrO2: ZrO2 powder (1.0 g) was dispersed in a HAuCl4 (13mL, 0.01 M)
aqueous solutions under magnetic stirring at room temperature. An aqueous solution of
lysine (3 mL, 0.1 M) was then added to the mixture with vigorous stirring for 30 min,
and the pH was 8−9. To this suspension was added a freshly prepared aqueous solution of
NaBH4 (2 mL, 0.35 M) dropwise. The mixture was aged for 24 h, and then the solid was
separated by centrifugation, washed with water (three times) and ethanol (once), and
dried at 60°C in a vacuum oven for 24 h.
The same procedure is applied to other monometallic NPs, Ag, Pt, Pd, Rh and Ir onto
ZrO2 powder by reducing the corresponding metal salt with NaBH4 in the presence of ZrO2
powder.
Au-Pd alloy NP@ZrO2: 2.0 g ZrO2 powder was dispersed into 15.2 mL of 0.01 M
HAuCl4 aqueous solution and 28.3 mL of 0.01 M PdCl2 aqueous solution were added
while magnetically stirring. A total of 20 mL of 0.53 M lysine was then added into the
mixture with vigorous stirring for 30 min. To this suspension, 10 mL of 0.35 M NaBH4
solution was added dropwise in 20 min, followed by an addition of 10 mL of 0.3 M
hydrochloric acid. The mixture was aged for 24 h and then the solid was separated, washed
with water and ethanol, and dried at 60°C.
Characterization of Catalysts
The sizes, morphologies, and compositions of the catalyst samples were
characterized by TEM using a JEOL 2100 transmission electron microscope equipped
with a Gatan Orius SC1000 CCD camera and an Oxford X-Max EDS instrument. TEM
images are provided in Figure S1.
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Photocatalytic Reaction conditions
For all reactions: A 20 ml Pyrex glass tube was used as the reaction container with
seal of a rubber septum cap. The reaction mixture was stirred magnetically and irradiated
using Light-emitting diode (LED) lamps (Tongyifang, Shenzhen, China, setting image is
given in Figure S2) with wavelengths of 365±5, 400±5, 470±5, 530±5, 590±5, and 620±5
nm. After reaction 0.5 ml aliquots were collected at given irradiation time intervals and
filtered through a Millipore filter (pore size 0.45 µm) to remove the catalyst particulates.
The flask was purged with argon again for more than 3 min to remove air and then sealed.
The filtrates were analysed by an Agilent 6890 gas chromatograph with HP-5 column.
An Agilent HP5973 mass spectrometer was used to determine and analyse the product
compositions.
Reaction condition of data collection in Figure 1 and 2. Aryl iodide (1 mmol),
alkyl alkyne (1.2 mmol), photocatalysts (50 mg), cetyltrimethylammonium bromide
(CTAB) (1 mmol), and K3PO4 (2 mmol) were added to 10 mL of H2O. The reaction
temperature was 45±2 °C, under a 1 atm argon atmosphere, with a reaction time of 24 h.
Reaction condition of data collection in Table 1. Entry 1 and 2: 1 mmol of reactant,
50 mg (containing 3% of metals) of catalyst in CH3CN solvent at 45°C and 1 atm of O2,
reaction time 48 h. Entry 3: 2 mmol of the reactant, 50 mg (containing 3% of metals) of
catalyst in trifluorotoluene solvent at 45°C and 1 atm of O2, reaction time 5h. Entry 4
and 5: 1 mmol nitrobenzene in 2 ml isopropyl alcohol (IPA), 0.3 mmol KOH, and 50
mg of catalyst were added to the reaction tube, and the reaction was run at a required
temperature under a 1atm argon atmosphere for a reaction time of 18 h.
Reaction condition of data collection in Table 2. Entry 1 and 4: 100 mg catalyst,
the metal content is 3 wt%; 0.5 mmol reactant in 5 ml triflourotoluene solvent; 1atm Ar
In-depth Mechanism Study of Metallic Nanoparticle Based Photocatalyst
- 256 -
atmosphere. Oxygen was removed from the reaction mixture prior to introducing Ar and
the reaction proceeded 48 h for all catalysts under irradiation of various light and in the
dark at 45±2 ºC. Entry 2 and 3: 1 mmol of reactant, 50 mg (containing 3% of metals) of
catalyst in CH3CN solvent at 45 °C and 1 atm of O2, reaction time 24 h. Entry 5 and 6:
catalyst 50 mg, iodobenzene 0.1 mmol, styrene 0.12 mmol, N,N-Dimethylformamide
(DMF) 2 mL, 1 atm Ar, sodium acetate (AcONa) 50 mg, reaction time 17 h, temperature
for (a) 50 ± 2 °C.
In-depth Mechanism Study of Metallic Nanoparticle Based Photocatalyst
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Figure S1 TEM images of metal nanoparticle photocatalysts supported on ZrO2.
In-depth Mechanism Study of Metallic Nanoparticle Based Photocatalyst
- 258 -
Figure S2 The digi ta l photograph of the LED lamps se tup used in the wavelength
experiments.
Conclusions and Future Perspective - 259 -
Chapter 7 Conclusions and Future Perspective
Conclusions
Overall, this thesis focus on the comprehensive study of new metallic photocatalysis
system for organic synthesis. Three types of organic reactions photocatalysed by different types
of metal NP photocatalysts under visible light irradiation are developed. This thesis also
includes an in-depth mechanism study to describe the photo-electron excitation scheme that in
general application to all metallic photocatalysis.
Through the direct utilisation of solar energy, the photocatalysis is capable of reducing
energy consumption in organic synthesis and therefore attracted interests, great efforts were
made to develop new photocatalysts, yet most researchers stopped at this point and omitted
further attempt towards a greener process. This project suggests another direction that one can
promote the application of a relatively traditional photocatalyst, Au nanoparticles for example,
with respect to sustainable chemistry. Chapter 3 presents a novel photocatalytic system for the
hydrogenation of five types of unsaturated aromatics using aqueous solutions of FA as the
reducing agent over a visible light irradiated supported Au nanoparticle photocatalyst at
ambient temperatures and pressures. The visible light induced LSPR effect of Au nanoparticles
provides driving force to the hydrogenation reaction. The photocatalytic system exhibits high
catalytic efficiency and broad substituent tolerance. One can tune the reduction power of this
system by regulating the irradiation wavelength. We have found that water plays an active role
in enhancing the reductive efficiency of FA. Water reacts with FA to yield orthoformic acid
which provides the hydrogen to yield H-Au surface species. These H-Au species react with the
groups to be reduced in the hydrogenation. This finding will improve the understanding of the
selective reductions and inspire the future application of FA as a hydrogen source. Another
important finding is that the hydrogenation of nitroaromatic compounds follows a mechanism
Conclusions and Future Perspective - 260 -
distinct from the well-known Haber mechanism, yielding corresponding anilines directly due
to the strong reduction power of the new photocatalytic system. This photocatalytic process is
green, in terms of reduction agent, and is feasible for a wide range of molecules.
Apart from pure Au nanoparticle photocatalyst, this thesis introduced an alloy
photocatalyst of Au and Pd metal and its applications in the dehydrogenation of aromatic
alcohols yielding corresponding aldehydes at ambient temperature. The wide applications of
LSPR effect induced photocatalysis in organic synthesis are restricted by the number of
plasmonic metals and their limited catalytic potential. This thesis proposes a strategy that
alloying plasmonic metal with other catalytically active metal to establish alloy nanoparticles
for photocatalysis. The alloy nanoparticles, owing to their bi-metallic structure, are found to be
efficient in both visible light absorption and affinity to organic compounds. The driving force
is also the photoexcited electrons, the energy level and the population of those electrons
determines the reaction rate, which is reflected by the influence of incident light wavelength
and irradiation. The optimal photocatalytic activity to aromatic alcohol dehydrogenation is
observed with Au to Pd molar ratio 1:1.186. As a result, we proposed that the photocatalytic
performance is controlled by different surface charge heterogeneity caused by different Au to
Pd molar and its enhancement to the metal-substrate chemisorption. This theory is supported
by the agreement of experimental results and free electron gas model analysis and DFT
simulations. In addition, this project indicates that a great number of organic reaction thermally
catalysed by metals can be photocatalysed by alloying the catalytic active metals with
plasmonic metals, and thus the current chemical industry to extent could be reshaped. The
knowledge that we acquired from this work is beneficial to the design and application of future
plasmonic-transition metal alloy photocatalysts.
In addition to Au based photocatalysts, this thesis also turned to the Cu based
nanoparticle photocatalyst and its application in organic synthesis. Cu is a promising
Conclusions and Future Perspective - 261 -
photocatalyst candidature with both strong LSPR effect and significant catalytic potential, yet
it is the least studied plasmonic metal compared with Au and Ag, its practical applications were
confined due to its low resistance to oxygen and oxidative environment. For the first time, we
fabricated a TiN supported Cu NP photocatalyst, in which TiN support material plays a critical
role in the stabilisation of Cu NPs when exposing in the atmosphere of air. DFT calculation
results revealed a considerable charge exchange loop between TiN support and Cu NP which
is the main reason to Cu nanoparticle stabilisation. The stability of supported Cu nanoparticles
was further tested in cycled reactions and showed good re-usability. Furthermore, the
passivated Cu nanoparticles can be reactivated after a convenient reductive treatment. Highly
selective epoxidation of various alkenes using molecular oxygen under mild reaction condition
was achieved with the new TiN supported Cu photocatalyst with the assistance of cyclic ether
solvent. The reaction system showed good to high reaction conversion depending on the
different alkenes substrate. Light irradiance induced electron excitation is the driving force for
the photocatalytic epoxidation reaction. Cyclic ether solvent participates in the oxygen
activation step, which transfers molecular oxygen to activated oxygen adatom on the surface
of Cu nanoparticles when illuminated with visible light. The oxygen adatoms can effectively
oxidise alkenes, which chemosorbed onto the Cu nanoparticles surface, and the different
chemisorption pattern of alkenes with Cu is the determining factor to epoxidation selectivity.
This work has the potential to extend the field of plasmonic catalysis into the previously
unachievable use of readily oxidised metals. Meanwhile, the concept on the method to inhibit
the oxidation of Cu nanoparticles with TiN may be applicable to many other non-precious
metal nanoparticles not limited to plasmonic metals such as Fe, Co, Ni and etc.
Finally, the energy transfer pattern and charge carrier flow route in plasmonic effect
induced photocatalysis have yet been clearly understood. Currently reported theories were
normally based on theoretical calculation and supported by molecular level experiments. In
Conclusions and Future Perspective - 262 -
this thesis, we managed to reveal the in-depth mechanism by comprehensive investigation of
action spectra of multiple photocatalytic systems. By comparing the action spectra trend in
different cases. It is proposed for the first time that there is a threshold that exists in the metallic
photocatalysis. Photoexcited electrons with energy level higher than LUMO of chemosorbed
organic molecular can trigger a photocatalytic reaction. Overall, the predominant role of
photoexcited electrons in metallic photocatalysis is further demonstrated. The finding extends
current understanding of energy transfer and charge carrier flow route
Conclusions and Future Perspective - 263 -
Future Perspective
Metallic photocatalysis is a rapidly growing research field. Despite the considerable
progress to date, continuous efforts are still being made by researchers to create a various
efficient photocatalytic system and reveal their physical and chemical mechanism. Based on
the results of this thesis as well as other’s work, the perspective to the future work of metallic
photocatalysis can be categorised into several potential directions as followed:
I. Development of new materials for photocatalysis. The first expected research focus
lies in the development of well-designed photocatalysts using novel materials. The
candidates for new metallic photocatalyst can be expanded from plasmonic metals to a
wide range of transition and non-transition metals. New forms of nanostructures
consisting of more than one metal component (plasmonic, transition or non-transition)
are promising research directions, the alloy combinations are vast and alloys can exhibit
various forms of nanostructure that facilitate the design of promising photocatalysts. In
addition, a combination of metal with other forms of catalysts such as functional
organic reagents is another field worth entering. In addition, the metallic photocatalyst
can be promoted by the modification of other catalytic components such as functional
organic reagents. Lastly, a new research direction could be developing non-metal
materials for LSPR induced photocatalysis. A series of ceramic materials and carbon
materials were found exhibiting LSPR in the visible and near-infrared region.
Introducing them into photocatalysis could extend the applications of metallic
photocatalysis.
II. Photocatalytic applications in organic synthesis and greener chemical processes.
The family of metallic photocatalysed organic reactions is still young, therefore the
application of metallic photocatalysts in more important organic synthesis reactions is
another major part of photocatalysis research, and in most cases, it is associated with
Conclusions and Future Perspective - 264 -
the development of new material photocatalysts. More importantly, tuning the
photocatalytic selectivity by manipulating the photocatalyst construction or
wavelength/irradiance of incident light is an intriguing and very attractive direction.
The metallic photocatalysis represents research efforts towards a greener organic
synthetic process.
III. Development of a comprehensive theoretical model for metal photocatalysis. The
metallic photocatalysis has been realised for less than one decade, thus the proposed
photocatalytic mechanism is still under broad debate and there are also unrevealed light
induced physical and chemical processes in quantum scale. More theoretical efforts, as
well as experimental illustration, are desired to further clarify the picture of
photoexcitation and resultant energy transfer and evolution in the photocatalysts-
substrate system. Research in this area is crucial for the future design of efficient
metallic photocatalyst and their applications in organic synthesis.
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