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Photothermal and Photochemical Nanostructured Catalyst Engineering Towards Efficient Solar Fuel Production
by
Jia Jia
A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy
Department of Materials Science and Engineering University of Toronto
Copyright by Jia Jia 2017
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Photothermal and Photochemical Nanostructured Catalyst
Engineering Towards Efficient Solar Fuel Production
Jia Jia
Doctor of Philosophy
Department of Materials Science and Engineering
University of Toronto
2017
Abstract
With global energy demand rising alongside the advancement of climate change, the conversion
of greenhouse gas carbon dioxide into value-added chemicals and fuels is attracting increasing
attention. The crux for the successful development of this promising technology is the
exploration and discovery of highly active, selective, and stable catalyst materials. Herein we
demonstrate that the reverse water gas shift (RWGS) reaction can be driven by Nb2O5 nanorod-
supported Pd nanocrystals, without external heating, using visible and near infrared (NIR) light.
By measuring the dependence of the RWGS reaction rates on the intensity and spectral power
distribution of filtered light incident on the nanostructured Pd@Nb2O5 catalyst, we determine the
RWGS reaction to be initiated by heat generated from thermalized charge carriers in the Pd
nanocrystals that are excited by inter-band and intra-band absorption of visible and NIR light.
We also demonstrate that the catalytic activity and selectivity of CO2 reduction to CO and CH4
products can be systematically tailored, by varying the size of the Pd nanocrystals, to acheive
champion turnover frequencies (0.61 s1) and efficient conversion of solar energy to stored
chemical energy. The remarkable control over the catalytic performance of Pd@Nb2O5 stems
from a combination of photothermal, electronic, and size effects. The insight gleaned from this
detailed experimental-theoretical study provides a blueprint for how to tailor the performance
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metrics of earth-abundant, low-cost metal-metal oxide (M@M'Ox) analogues. Finally, we report
a lattice strain and defect controlled strategy that enables the high-performance of low-cost
photocatalystic materials. Lattice compressed ultrafine nonstoichiometric indium oxide dots,
In2O3-x(OH)y, grown on the surface of niobium pentoxide nanorods were fabricated; the
optimized hybrid structure exhibits 44-fold increase in efficiency compared with pristine In2O3-
x(OH)y, along with extremely long-term operational stability, potentially originating from the
increased number of active oxygen vacancies, prolonged excited-state lifetimes, and enhanced
photo-generated carrier energies.
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Acknowledgments
I thank my supervisors, Prof. Geoffrey Ozin, Prof. Doug Perovic and Prof. Nazir Kherani for
their strong support and insightful guidance and for the great degree of scientific freedom that
they has afforded me throughout my PhD studies. I would also like to thank my committee
members, Profs. Chandra Veer Singh, Robert Morris, Yujie Xiong, Zheng-Hong Lu, and Dwight
Seferos, for their advice and encouragement over the course of this project.
I am very grateful to have been able to work with so many excellent collaborators. In particular, I
would like to thank all the engineers in the Solar Fuel ClusterDr. Thomas Wood, Dr. Paul
OBrien, and Amit Sandhel, without whom this project would not have been possible, Dr. Paul
OBrien for all of many great discussions and for all of his hard work, Dr. Hong Wang and Dr.
Miaomiao Ye for the synthetic insights and great collaboration, and Prof. Amr Helmy for the
help and guidance with Raman, and Joller Lu, Mireille Ghoussou, Dr. Kulbir Kaur Ghuman for
all of the computational insights, and Dr. Qiao Qiao for her hard work on the high resolution
imaging, and Dr. Lu Wang and Abdinoor Jelle for the help with XPS analysis presented herein,
and Dr. Paul Duchesne for the help with XAS analysis, and Peicheng Li for UPS analysis, and
Yuchan, Mireille Ghoussou as well as Dr. Laura Reyes for BET, TGA, and DRIFT
measurements, and Teng Fei, Ziqi Zheng as well as Yue Shao for their excellent undergraduate
research.
I am also extremely grateful to my colleague in the Solar Fuel Cluster both past and present for
all of their camaraderie and support, over the past 5 years. In particular, I would like to thank Sue
Mamiche for always keeping the group running so smoothly, Dr. Mohammed Hmadeh, Dr. Paul
OBrien, Dr. Le He, Dr. Miaomiao Ye, Dr. Hong Wang, and Dr. Lu Wang, Dr. Benoit Mahler
for their constant supply of helpful and entertaining discussions as senior colleague, Dr. Wei Sun,
Dr. Chenxi Qian, Dr. Laura Hock, Dr. Jon Moir, Mireille Ghoussou, Yuchan Dong, Meikun Xia,
Annabelle Wong, Kenny Chen, Dr. Laura Reyes, Dr. Melanie Matronardi, Dr. Navid Soheilnia,
Dr. Daniel Faulkner, Dr. Liwei Wang, Prof. Changlong Chen, and Prof. Dongzhi Chen, for
encouragement, advice, and friendship.
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I also appreciate all the help from the administrative staff in the department, Maria Fryman,
Fanny Strumas-Manousos, Krista Haldenby, and Luke Y. H. Ng, for their help with my life and
study in Canada.
Financial support from the Doctoral Completion Award, the Atsumi Ohno Scholarship, the
Lachlan Gilchrist Fellowship Fund, the Nanotechnology Network Graduate Student
Enhancement Award, and the Department of Materials Science and Engineering, and University
of Toronto is gratefully acknowledged.
Finally, I would like to thank my family, my boyfriend Wei Zhao, and my friends for their
continued love and support in everything I choose to do. I would certainly not be where I am
today without them and knowing that no matter where they are they will always be proud of me.
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Table of Contents
Acknowledgments .......................................................................................................................... iv
Table of Contents ........................................................................................................................... vi
List of Tables ................................................................................................................................. ix
List of Figures ................................................................................................................................ xi
Copyright Acknowledgements ..................................................................................................... xxi
Chapter 1 Introduction and Background ......................................................................................... 1
1 Introduction and Background ..................................................................................................... 1
1.1 Introduction ......................................................................................................................... 1
1.2 Methanation ........................................................................................................................ 2
1.3 CO Production .................................................................................................................... 4
1.4 Methanol Synthesis ............................................................................................................. 9
1.5 C2 C11 ............................................................................................................................. 11
1.6 The Goals of this Doctoral Thesis Work .......................................................................... 13
Chapter 2 Photothermal Catalysed Hydrogenation of Gaseous CO2 over Nanostructured
Pd@Nb2O5 ............................................................................................................................... 15
2 Photothermal Catalysed Hydrogenation of Gaseous CO2 over Nanostructured Pd@Nb2O5... 15
2.1 Abstract ............................................................................................................................. 15
2.2 Introduction ....................................................................................................................... 16
2.3 Experimental ..................................................................................................................... 17
2.3.1 Synthesis of Nb3O7(OH) Nanorods ...................................................................... 17
2.3.2 Synthesis of Nanostructured Pd@Nb2O5 .............................................................. 18
2.3.3 Physical Characterization ...................................................................................... 18
2.3.4 Gas Phase Catalytic Measurements ...................................................................... 20
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2.4 Results and Discussion ..................................................................................................... 22
2.4.1 Characterization of Nanostructured Pd@Nb2O5 ................................................... 22
2.4.2 Investigation of Photothermal RWGS Catalysis on Nanostructured Pd@Nb2O5. 31
2.4.3 Investigating the Effect of Spectral Distribution and Light Intensity on the
Photothermal Catalytic Activity ........................................................................... 32
2.4.4 Investigating the Effect of the Temperature Dependence of RWGS Reaction
Rates over Pd@Nb2O5 in the Dark ....................................................................... 35
2.4.5 Raman Investigation of the Visible Light Induced Photothermal Effect .............. 37
2.4.6 Investigating In-situ Generated Oxygen Vacancies and Reduced Oxidation
State Surface Niobium Sites in the Photothermal Catalytic CO2 Reduction of
Nanostructured Pd@Nb2O5................................................................................... 44
2.5 Conclusions ....................................................................................................................... 47
Chapter 3 Photothermal Catalyst Engineering with High Activity and Tailored Selectivity ....... 49
3 Photothermal Catalyst Engineering with High Activity and Tailored Selectivity ................... 49
3.1 Abstract ............................................................................................................................. 49
3.2 Introduction ....................................................................................................................... 50
3.3 Experimental ..................................................................................................................... 51
3.3.1 Preparation of Pd@Nb2O5 Heteronanostructure with Different Loadings of Pd.. 51
3.3.2 Characterization .................................................................................................... 51
3.3.3 Gas Phase Catalytic Measurements ...................................................................... 53
3.4 Results and Discussion ..................................................................................................... 54
3.4.1 Synthesis and Characterization of Pd@Nb2O5 Heteronanostructures .................. 54
3.4.2 Photothermal Materials Engineering of the Gas-Phase CO2 Catalytic
Reduction .............................................................................................................. 63
3.5 Conclusion ........................................................................................................................ 68
Chapter 4 Mechanistic Insights into the Photocatalytic Reduction of CO2 on Nanostructured
Pd@Nb2O5 ............................................................................................................................... 69
4 Mechanistic Insights into the Photocatalytic Reduction of CO2 on Nanostructured
Pd@Nb2O5 ............................................................................................................................... 69
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4.1 Experimental ..................................................................................................................... 69
4.1.1 Physical Characterization ...................................................................................... 69
4.1.2 Theoretical Methods ............................................................................................. 70
4.2 Investigating the RWGS Reaction Pathway over Pd@Nb2O5 .......................................... 72
4.3 Investigating the Size Effect of Pd Nanocrystals on the CO2 to CO versus CH4
Selectivity and the Reaction Mechanisms. ....................................................................... 78
Chapter 5 Lattice Strain Engineering of Photocatalyst for Highly-Efficient Gas-Phase CO2
conversion ................................................................................................................................ 89
5 Lattice Strain Engineering of Photocatalyst for Highly-Efficient Gas-Phase CO2
Conversion ............................................................................................................................... 89
5.1 Abstract ............................................................................................................................. 89
5.2 Introduction ....................................................................................................................... 90
5.3 Synthesis and Characterization of Lattice Strained In2O3-x(OH)y grown on Nb2O5
nanorods. ........................................................................................................................... 91
5.4 Photocatalytic Performance of Lattice Strained In2O3-x(OH)y grown on Nb2O5
nanorods. ......................................................................................................................... 101
5.5 Proposed Mechanism ...................................................................................................... 102
5.6 Conclusion ...................................................................................................................... 103
Chapter 6 Conclusion and Future Work ..................................................................................... 104
6 Conclusion and Future Work ................................................................................................. 104
6.1 Conclusion ...................................................................................................................... 104
6.2 Future Work .................................................................................................................... 105
References ................................................................................................................................... 106
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List of Tables
Chapter 2
Table 2.1. Estimation of reaction temperatures over Pd@Nb2O5 under irradiation from the 300 W
Xe lamp using different cut-off filters for batch reactions A through F and without any filter at
different light intensities for batch reactions I through V. ............................................................ 36
Table 2.2. Summary of the Raman band frequency for the Nb=O stretching vibrations of
Pd@Nb2O5 and Nb2O5 at different power levels using a 633 nm laser. Temperature estimation
was performed using the slope of the dependence of the Raman band shift on temperature in
prior work.92 .................................................................................................................................. 39
Table 2.3. Estimation of the temperature of Pd@Nb2O5 and Nb2O5 under the power level of 24
mW using a laser pump wavelength of 785 nm. The signals of the Raman Stokes and anti-Stokes
scattering at around 640 cm-1 are obtained from the deconvolution of the spectra by LabSpec
software, as shown in Figure 17. The estimation of temperatures is calculated using the ratio of
Stokes signals to anti-Stokes signals and the band position of the Raman modes. ...................... 43
Chapter 3
Table 3.1. Elemental analysis of different Pd loading samples .................................................... 56
Table 3.2. Parameter values obtained from Pd K-edge EXAFS fitting. Values marked with an
asterisk indicate that these parameters were correlated during EXAFS fitting. ........................... 61
Chapter 4
Table 4.1. The surface areas for the different Pd loading samples ............................................... 75
Table 4.2. CO chemisorption, the amount of exposed Pd surface atoms, apparent metal
dispersion, CO production rates, CH4 production rates, Nb2O5 active sites, and turnover
frequency (TOF) for 3% Pd@Nb2O5 and 10% Pd@Nb2O5 under 4.2 W cm-2 in light-batch
system and for 10% Pd@Nb2O5 under 2.1 W cm-2 in light-flow system ..................................... 79
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Table 4.3. The energy conversion efficiency in light-batch system over 0.1% Pd@Nb2O5 under
4.2 W cm-2 and in light-flow system over 10% Pd@Nb2O5 under 2.1 W cm-2 ............................ 80
Table 4.4. Locations of surface sites used in DFT calculations .................................................... 85
Table 4.5. Adsorption energies of relevant intermediates on different reactive sites.
Corresponding site locations are shown in in Table 4. Values marked in bold denote the
adsorption energy of the most stable intermediate. Note that for the *CO, *CHO, and *CH
intermediates on Pd(111), only the fcc site has been calculated since previous studies have shown
fcc to be the most stable adsorption site in all three of these cases. 128, 129 ................................... 86
Chapter 5
Table 5.1. PXRD and ICP-AES results of In2O3-x(OH)y and In2O3-x(OH)y/Nb2O5 hybrids ......... 93
Table 5.2. BET surface area of different samples ....................................................................... 100
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List of Figures
Chapter 1
Figure 1.1 Methanation of Carbon Dioxide. a) Visualizing strong metal support interaction,
SMSI, states by in situ scanning transmission electron microscope (STEM) images of sintered
6% Rh/TiO2 after treatment in 5% H2 and 95% N2 at 550 C for 10 min. b) Visualizing
adsorbate-mediated SMSI (A-SMSI) states by in situ STEM images of sintered 6% Rh/TiO2 after
treatment in 20CO2:2H2 at 250C for 3 h. c) Infrared analysis of selectivity switch for thermally
reduced (red), 20CO2:2H2-treated (blue) and re-reduced (black) catalysts. Reprinted with
permission.3 Copyright 2016, Nature Publishing Group. d) Illustration of charge transfer cross
the surface facet homojunction of CeO2. Reprinted with permission.5 Copyright 2015, American
Chemical Society.e) Isolation of Cu atoms in Pd nanoparticle lattice for highly selective
conversion of CO2 to CH4. 6 f) Average production rates of CH4 and CO in photocatalytic CO2
reduction with H2O by bare TiO2, Pd1Cu1TiO2, Pd3Cu1TiO2, Pd5Cu1TiO2, Pd7Cu1TiO2,
Pd9Cu1TiO2,Pd11Cu1TiO2, and PdTiO2 hybrid structures under UV-light (
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Academy of Sciences. f) Nanostructured indium oxide coated silicon nanowire arrays as a hybrid
photothermal-photochemical approach to solar fuels. 26 g) The absorption spectrum of
In2O3x(OH)y nanoparticles and the photon utilization of the solar irradiance for
In2O3x(OH)y/SiNW hybrid materials. h) 13CO production rates of evenly coated
In2O3x(OH)y/SiNW, bilayer In2O3x(OH)y/SiNW, and In2O3x(OH)y/glass in the dark and under
illumination, with or without external heating. Reprinted with permission.26 Copyright 2016,
American Chemical Society. .......................................................................................................... 6
Figure 1.3. Reactor design. a) Schematic of the solar reactor configuration for splitting CO2 into
separate streams of CO2 and O2 via a 2-step thermochemical redox cycle. b) Photographs of the
solar reactor, showing the front face of the solar reactor with the windowed aperture and its
interior containing the octagonal reticulated porous ceramics (RPC) structure lined with alumina
thermal insulation.28 Copyright 2017, by The Royal Society of Chemistry, under the Creative
Commons Attribution-NonCommercial 3.0 License.c) Photograph of a photoreactor in operation
Reprinted with permission.9 Copyright 2014, American Chemical Society.d-e) Setup for
photothermal conversion of CO2 in d) batch system and e) flow system. Reprinted with
permission. 7, 29 Copyright 2014, Wiley. Copyright 2016, Wiley. ................................................. 8
Figure 1.4. Methanol synthesis from CO2. a) Theoretical activity volcano plot for CO2
hydrogenation to methanol. b) Zn 2p3/2 XPS binding energies measured after performing the
hydrogenation of CO2 on the Zn/Cu(111) catalyst. Reprinted with permission.31 Copyright 2014,
Nature Publishing Group. c) Potential energy diagram for the hydrogenation of CO2(g) to
CH3OH(g) on ZnO/Cu(111) via the RWGS + CO-hydroxycarbonyl and formate pathways. (Inset)
Structures of *HCOO on ZnO/Cu(111). Cu, brown; Zn, blue; O, red; H, white; C, gray. d)
Coverage of surface reaction intermediates on ZnCu(211) and ZnO/Cu(111) under reaction
conditions. Reprinted with permission.33 Copyright 2017, American Association for the
Advancement of Science. e) Ex situ MAS-NMR 1H-13C HETCOR spectra of Cu/ZrO2 reacted
with H2/13CO2 (3:1) at 230C for 12 h at 5 bars. f) Reaction scheme derived from the
spectroscopic measurements. Reprinted with permission.34 Copyright 2017, Wiley. .................. 10
Figure 1.5. FT synthesis. a) CO conversion and product distribution at different H2/CO ratios in
syngas over a catalyst with a mass ratio of ZnCrOx/mesoporous SAPO zeolite (MSAPO) = 1.4 at
a space velocity of 4800ml/hgcat b) A stability test of a composite catalyst with
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ZnCrOx/MSAPO ratio = 0.9 at 6828 ml/hgcat and H2/CO of 2.5. Reprinted with permission.35
Copyright 2016, American Association for the Advancement of Science. c) The
CeO2Pt@mSiO2Co tandem catalyst. d) CO2 conversion and hydrocarbon distribution at
different H2/CO2 ratios over the tandem catalyst at 250 C. Reprinted with permission.36
Copyright 2017, American Chemical Society. e) Reaction scheme for CO2 hydrogenation to
gasoline-range hydrocarbons.37 Copyright 2017, Nature Publishing Group, under the Creative
Commons Attribution 4.0 International License .......................................................................... 12
Chapter 2
Figure 2.1. a) Schematic illustration of the photothermal-reactor system used for gas-phase
catalysis testing. b) Film fabrication process. c) The spectral output of the 300W xenon lamp. . 21
Figure 2.2. Synthesis, morphology and structure of Nb3O7(OH) nanorods and Pd@Nb2O5
nanocrystal-nanorod samples. a) Scheme of the synthesis of Nb3O7(OH) and Pd@Nb2O5 nanorod
samples. b) TEM images of Nb3O7(OH) nanorod sample. c) TEM images of Pd@Nb2O5
nanocrystal-nanorod sample. ........................................................................................................ 22
Figure 2.3. a) HAADF image of the Nb3O7(OH) nanorods at low magnification. b) HAADF
image taken from a nanorod in its [001] direction. The preferred growth direction (longitudinal)
of the nanorod is [010]. c) FFT of (b) showing (200), (110) and (020) spots. d) Atomic model
indicated by the red rectangle in Figure 3b, where the dotted blue rectangle indicates a
Nb3O7(OH) unit cell. ..................................................................................................................... 23
Figure 2.4. Powder X-ray diffraction patterns of Nb2O5, Nb3O7(OH), 0.5% Pd@Nb2O5 (after pre-
treatment) and 0.5% Pd@Nb3O7(OH) (before pre-treatment). ..................................................... 24
Figure 2.5. TEM images of a) Nb3O7(OH) nanorods and b) Nb2O5 nanorods. ............................ 25
Figure 2.6. a) HAADF image of a Pd nanocrystal decorated Nb2O5 nanorod. The cyan arrow
indicates the [010] growth direction of the nanorod, and the dashed yellow rectangle marks the
area where EELS elemental mapping was taken. Scale bar, 5 nm. b) Enlarged image showing
nanocrystalline Pd viewed from its [112] direction. The d-spacing of the (111) and (022) planes
were measured. c) Inverse FFT by masking spots only contributed from Pd, as indicated in the
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inserted figure by the red circles. The inserted figure is the FFT of a), showing an overlap of
contributions from Pd and Nb2O5. d) EEL spectra from the Pd nanocrystal (red), Nb2O5 nanorod
(cyan), and interface between the Pd and Nb2O5 (blue). The Pd M4,5-edge and Pd M3-edge are
visible in the red spectrum; the Nb M3-edge, Nb M2-edge and O K-edge are visible in the cyan
spectrum; a superposition of these edges are exhibited in the blue spectrum. e) HAADF image
taken while simultaneously collecting the EELS map. f) Fitting coefficient of Pd showing the
spatial distribution of the Pd signal. g) Fitting coefficient of Nb2O5 showing the spatial
distribution of the Nb2O5 signal. ................................................................................................... 27
Figure 2.7. High resolution XPS spectra of the a) Nb3d, b) O1s. c) and d) Pd 3d regions taken on
Nb2O5, Pd@Nb2O5, 0.5% Pd@Nb2O5 and 1% Pd@Nb2O5, respectively. ................................... 28
Figure 2.8. a) Diffuse reflectance spectra of Nb3O7(OH), Nb2O5 and Pd@Nb2O5 films dispersed
on a borosilicate filter. b) The band energy diagram of Nb2O5 in comparison with the work
function of Pd. ............................................................................................................................... 29
Figure 2.9. Estimation of the electronic band gap of Nb2O5. By using a modified Kubelka-Munk
function, (F(R)*h)n is plotted as a function of photon energy for Nb2O5 where F(R) = (1-R)2/2R.
Here R is the diffuse reflectance of the films loaded onto the borosilicate sample supports and n
was set to 2 for Nb2O5. The linear portion of the plot was extrapolated and its intercept with the
abscissa is considered to be the band-gap. .................................................................................... 29
Figure 2.10. XPS Spectra of Nb2O5 a) secondary electron cut-off region. b) Valence band region
(3.3 eV is relative to the work function.) ...................................................................................... 30
Figure 2.11. Mass spectrometry of photo-thermally generated 13CO from 13CO2. The 28 AMU
mass fragment peak at approximately 1.32 min corresponds to N2 and the 29 AMU mass
fragment peak at approximately 1.345 min corresponds to 13CO. The fact that there is no peak in
the vicinity of 1.345 min retention time for the 28 AMU curve shows that there is no 12CO in the
products that could have been generated from sources of adventitious 12C. ................................ 32
Figure 2.12. Photothermal catalytic performance of the nanostructured Pd@Nb2O5 samples. a)
CO production rates over Pd@Nb2O5 in the dark at room temperature (RT), under irradiation
from a 300 W Xe lamp, and in the dark at a reaction temperature of 160 C. b) Spectral
irradiance incident onto the Pd@Nb2O5 catalyst with different cut-off filters for batch reaction
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tests A through F. c) Spectral irradiance incident onto the Pd@Nb2O5 catalyst for batch reactions
I through V. d) The RWGS reaction rates plotted as a function of absorbed power for the series
of batch reactions A - F (red line) and I V (black line). ............................................................. 33
Figure 2.13. TEM image of Pd@Nb2O5 after gas-phase catalytic testing. ................................... 35
Figure 2.14. a) RWGS reaction rate measured in the dark as a function of reaction
temperature b) The effective reaction temperature as a function of radiant power absorbed
by the Pd@Nb2O5 catalyst. The equation used to estimate Te is provided in Figure 5b. .. 37
Figure 2.15. Dependence of the Raman frequency from 100 cm-1 to 1500 cm-1 for Nb=O
stretching vibrations of a) Pd@Nb2O5 and b) Nb2O5 nanorods at different power levels. ........... 40
Figure 2.16. Dependence of the Raman frequency for Nb=O stretching vibrations of a)
Pd@Nb2O5 and b) Nb2O5 nanorods at different power levels. c) Estimated temperatures for
Pd@Nb2O5 and Nb2O5 at different power levels. ......................................................................... 41
Figure 2.17. Selected wavelength Stokes and anti-Stokes Raman spectra between 500 cm 1 and
900 cm1 with representative deconvolution of the spectra analyzed by LabSpec software for a)
Pd@Nb2O5 Stokes spectra; b) Pd@Nb2O5 anti-Stokes spectra; c) Nb2O5 Stokes spectra and d)
Nb2O5 anti-Stokes spectra. ............................................................................................................ 42
Figure 2.18. Photothermal catalysis by nanostructured Pd@Nb2O5 (reduced) sample. a) Without
exposure to air, the production rates of CO over Pd@Nb2O5 kept monotonically increasing from
the 1st run to the 4th run. When the sample was exposed to air for 24 h, the production rate of the
5th run dropped to that of the 1st. b) Diffuse reflectance spectra and appearance of (i) pristine
Pd@Nb2O5 and (ii) reduced Pd@Nb2O5 nanostructured films. c) Schematic illustration of in-situ
generation of reduced surface niobium sites and/or oxygen vacancies during photothermal
reaction. ......................................................................................................................................... 45
Figure 2.19. EPR spectra of Nb2O5 before (black) and after (red) being illuminated under a 300
W Xe lamp for 2 hours, recorded at 20 K. .................................................................................... 46
Chapter 3
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Figure 3.1. Schematic illustration of the growth of Pd nanocrystals on Nb2O5 nanorods a, low
loading of Pd precursor and b, high loading of Pd precursor. ...................................................... 55
Figure 3.2. TEM bright field images of a) 0.1%Pd@Nb2O5, b) 0.5% Pd@ Nb2O5, c) 1% Pd@
Nb2O5, d) 2% Pd@ Nb2O5, e) 3% Pd@ Nb2O5, f) 5% Pd@ Nb2O55, g) 10% Pd@ Nb2O5 and h)
15% Pd@ Nb2O5. Scale bar: 20 nm. ............................................................................................. 56
Figure 3.3. Morphology and structural analysis of Pd@Nb2O5 samples. Bright field TEM
images of a, 0.1% Pd@Nb2O5 sample; b, 3% Pd@Nb2O5 sample; c, 10% Pd@Nb2O5 sample;
Scale bar: 20 nm. HRTEM image of d, 0.1% Pd@Nb2O5 sample; e, 3% Pd@Nb2O5 sample; f,
10% Pd@Nb2O5 sample; Scale bar: 2 nm. g, Pd K-edge XANES spectra from the supported Pd
nanocrystals. h, Pd K-edge FT-EXAFS spectra from the supported Pd nanocrystals. ................. 57
Figure 3.4. Pd particle size distribution determined for a) 10% Pd@Nb2O5, b) 3% Pd@Nb2O5,
and c) 0.1% Pd@Nb2O5 from TEM images. ................................................................................ 58
Figure 3.5. a) HRTEM images of Nb2O5 nanorod viewed from its [[ 0 4] direction, showing
(010) and (401) planes. b) HRTEM image of 10% Pd-Nb2O5 interface, where the (111) plane of
Pd and (401) plane of Nb2O5 are visible. c-d) HRTEM images of 3% Pd-Nb2O5 interface. For the
3% sample, Pd particles are round with no obvious facets. .......................................................... 59
Figure 3.6. Powder X-ray diffraction patterns of Nb2O5, 0.1% Pd@Nb2O5, 0.5% Pd@Nb2O5, 1%
Pd@Nb2O5, 2% Pd@Nb2O5, 3% Pd@Nb2O5, 5% Pd@Nb2O5, 10% Pd@Nb2O5 and 15%
Pd@Nb2O5..................................................................................................................................... 60
Figure 3.7. Optical and Raman characterization of Pd@Nb2O5 samples. a, Digital photographs
of pristine Nb2O5 and Pd@Nb2O5 samples with different loadings of Pd. b, Diffuse reflectance
spectra of pristine Nb2O5 and different Pd@Nb2O5 samples dispersed on borosilicate filter films.
Nb=O stretching vibrations of c, 0.1% Pd@Nb2O5, d,
0.5% Pd@Nb2O5 and e, 5% Pd@Nb2O5 at different incident light power levels. f, The estimated
temperatures of 0.1% Pd, 0.5% Pd, and 5% Pd loaded onto Nb2O5 nanorods at different incient
light power levels .......................................................................................................................... 62
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Figure 3.8. Fragmentation patterns measured by GC-MS for different products. On the top is CH4
and on the bottom is CO. (For CO,the mass of 13CO is 29; the mass of 12CO is 28.). ................. 64
Figure 3.9. Photothermal catalytic performance of Pd@Nb2O5 samples. a, CO production rates
over Pd@Nb2O5 film samples with different Pd loadings under irradiation from a 300 W Xe
lamp. b, CH4 selectivities over Pd@Nb2O5 film samples with different Pd loadings. c, CH4
selectivities over 10% Pd@Nb2O5 with different sample weights. d, Comparison of TOFPd methane
and TOFPd CO for 3% Pd@Nb2O5 and 10% Pd@Nb2O5 in a batch photoreactor under 4.2 W cm-2,
and for 10% Pd@Nb2O5 in flow photoreactor under 2.1 W cm-2. ................................................ 65
Figure 3.10. CH4 production rates over Pd@Nb2O5 film samples with different Pd loadings under
irradiation from a 300 W Xe lamp. ............................................................................................... 66
Figure 3.11. CO and CH4 production rates over 10% Pd@Nb2O5 under different light intensities.
....................................................................................................................................................... 66
Figure 3.12. CO and CH4 production stability on the presence of light for 3% Pd@Nb2O5 sample,
where each test lasted for 30 minutes. .......................................................................................... 67
Chapter 4
Figure 4.1. Atomic models used to simulate the Pd(111) and Pd(211) surfaces and Pd
nanoparticle in DFT calculations. ................................................................................................. 72
Figure 4.2. Possible reaction pathway for the reverse water gas shift reaction (RWGS) on the
Pd@Nb2O5 hybrid catalyst............................................................................................................ 73
Figure 4.3. CO production rate from CO2 in the presence and absence of H2 over Pd@Nb2O5. . 74
Figure 4.4. The dependence of the CO2 adsorption capacity on the Pd loading in hybrids. The
CO2 adsorption capacity is normalized to the surface area of each sample. ................................. 76
Figure 4.5. Thermogravimetric analysis (TGA) and differential thermogravimetric analysis of the
fresh catalyst and used catalyst after catalytic reaction tests of 40 h. ........................................... 77
Figure 4.6. High resolution Pd 3d XPS spectra ............................................................................ 81
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Figure 4.7. XPS, UPS and in situ DRIFT analysis. a, Pd 3d5/2 binding energy position as a
function of Pd loading. b, The Fermi-level position of pristine Nb2O5 in comparison with that of
0.1% Pd@Nb2O5 and 10% Pd@Nb2O5 measured by UPS (Fermi-levels are referenced to the
vacuum level, i.e., the materials work functions). c,d, In situ DRIFT spectra of 0.1% Pd@Nb2O5
under a gas mixture of H2 and CO2 (1:1 ratio) at 240 C. e,f, In situ DRIFT spectra of 10%
Pd@Nb2O5 under a gas mixture of H2 and CO2 (1:1 ratio) at 240 C. .......................................... 82
Figure 4.8. Density functional theory analysis and reaction mechanism on low Pd loading and
high Pd loading samples. a, Calculated free energy diagram for *CO intermediate reduction to
CH4 on Pd(111), Pd(211) and Pd55. b, Pd(111), stepped Pd(211), and Pd55 computational
models for representing surface terrace sites, edge and corner sites. c, Calculated activation
barrier for the rate determine step on Pd(111), Pd(211) and Pd55. d, Schematic illustration
depicts the weak CO bonding of small Pd nanocrystals in low Pd loading sample. .................... 84
Figure 4.9. The weight fraction of adsorption sites (step, corner, terrace and bulk sites) on ideal
octahedral Pd NPs as a function of the cluster diameter. .............................................................. 87
Figure 4.10. TEM images of 1%Pd@Nb2O5 a) before gas-phase catalytic testing, b) after 10
consecutive gas-phase catalytic tests, where each test lasted for 30 minutes. Scale bar: 100 nm. 88
Chapter 5
Figure 5.1. TEM image of Nb2O5 nanorods. ................................................................................ 91
Figure 5.2. Schematic illustration of the preparation of In2O3-x(OH)y via slowly decomposition of
urea in the H2O/ethanol mixture solvent and how the lattice strain induced to In2O3-x(OH)y. (A)
The growth of lattice strained In2O3-x(OH)y on the surface of Nb2O5 and synthesis of pristine
In2O3-x(OH)y as reference. (B) PXRD patterns of pristine In2O3-x(OH)y, Nb2O5 and their hybrids.
....................................................................................................................................................... 92
Figure 5.3. Thermal analysis of In(OH)3, S3 and Nb2O5. (A) TGA and (B) DTA curves ........... 93
Figure 5.4.. PXRD patterns of pristine In(OH)3, Nb2O5 and their hybrids. These samples are
denoted as S1-S4, labeled with gradually increasing In(OH)3 contents. It can be seen clearly
that there is no XRD peaks shifts of In(OH)3 from S1 to S4 samples. ....................................... 94
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Figure 5.5. Representative enlarged PXRD patterns of lattice compressed In2O3-x(OH)y along (A)
(211), (B) (222) and (C) (411) facets, respectively. ..................................................................... 95
Figure 5.6. XPS spectrum of pristine In2O3-x(OH)y, physical mixture and S3 sample. (A) Full
spectrum of pristine In2O3-x(OH)y, S3 sample and physical mixture. (B) C1S (284.6 eV)
spectrum which serve as reference for calibration. (C) Enlarged In3d5/2 and In3d3/2 peaks of
pristine In2O3- x(OH)y, physical mixture and S3 sample. (D) Fitting curves of the In3d5/2 and
In3d3/2 peaks of pristine In2O3-x(OH)y, physical mixture and S3 sample. ..................................... 95
Figure 5.7. Structural and component characterizations of In2O3-x(OH)y/Nb2O5 hybrid (S3
sample). a-c, TEM and HRTEM images of S3. D-E, HRTEM image of pristine and compressed
In2O3-x(OH)y with (222) lattice fringe are aligned for the direct evidence of lattice constraint,
scale bar 0.5 nm. (i) TEM image and corresponding elemental (Nb, In, O) mappings. ............... 97
Figure 5.8. EDX Spectrum. The energy dispersive X-ray (EDX) spectrum of the ultrafine In2O3-
x(OH)y nanodots with a diameter of ~5 nm uniformly grown on the surface of Nb2O5 nanorods.
....................................................................................................................................................... 98
Figure 5.9. TEM image of Nb2O5 nanorods and In2O3-x(OH)y composite prepared with ammonia
method. Clearly, the Nb2O5 nanorods and In2O3-x(OH)y species are separated existing in the
composite. ..................................................................................................................................... 98
Figure 5.10. PXRD pattern of In2O3-x(OH)y/Nb2O5 composite prepared with ammonia method.
The 2 angle of the peaks of (211), (222) located at 25.0o and 31.6o, indicating this method could
not lead to lattice compressed In2O3-x(OH)y in In2O3-x(OH)y/Nb2O5 composite. ......................... 99
Figure 5.11. Pore size distribution of In2O3-x(OH)y, Nb2O5 and their hybrids. ............................. 99
Figure 5.12. N2 absorptiondesorption isotherms of In2O3-x(OH)y, Nb2O5 and their hybrids. ... 100
Figure 5.13. Photocatalytic performance of as-synthesized samples. (A) S3 deposited on the
borosilicate filter film. (B) Photocatalytic performance of various photocatalysts. (C) Long-term
stability measurements of S3 sample. ......................................................................................... 101
Figure 5.14. Mass spectrometry of S3 sample generated 13CO from 13CO2. The 28 AMU mass
fragment peak at approximately 1.32 min corresponds to N2 and the 29 AMU mass fragment
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xx
peak at approximately 1.345 min corresponds to 13CO. The fact that there is no peak in the
vicinity of 1.345 min retention time for the 28 AMU curve shows that there is no 12CO in the
products that could have been generated from sources of adventitious 12C. .............................. 102
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Copyright Acknowledgements
The thesis has been compiled using excerpts with permissions from the following published and
unpublished work:
(a) J. Jia, N. P. Kherani, D. D. Perovic, G. A. Ozin, Recent developments in gaseous
carbon dioxide conversion powered by light and heat. In preparation .
(b) J. Jia, H. Wang, Z. Lu, P. G. O'Brien, M. Ghoussoub, P. Duchesne, Z. Zheng, P. Li, Q.
Qiao, L. Wang, A. Gu, A. A. Jelle, Y. Dong, Q. Wang, K. K. Ghuman, T. Wood, C. Qian,
Y. Shao, M. Ye, Y. Zhu, Z. H. Lu, P. Zhang, A. S. Helmy, C. V. Singh, N. P. Kherani, D.
D. Perovic, and G. A. Ozin. Photothermal Catalyst Engineering: Hydrogenation of
Gaseous CO2 with High Activity and Tailored Selectivity Adv. Sci. 2017, 1700252.
Copyright 2017, published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
(c) J. Jia, P. G. O'Brien, L. He, Q. Qiao, T. Fei, L. M. Reyes, T. E. Burrow, Y. Dong, K.
Liao, M. Varela, S. J. Pennycook, M. Hmadeh, A. S. Helmy, N. P. Kherani, D. D.
Perovic, G. A. Ozin, Visible and Near Infrared PhotothermalCatalysed Hydrogenation
of Gaseous Carbon Dioxide over Nanostructured Pd@Nb2O5 Adv. Sci. 2016, 3,
1600189-n/a. Copyright 2016, published by WILEY-VCH Verlag GmbH & Co. KGaA,
Weinheim
(d) M. Ye, J. Jia, Z. Wu, C. Qian, R. Chen, P. G. O'Brien, W. Sun, Y. Dong, G. A. Ozin,
Synthesis of Black TiOx Nanoparticles by Mg Reduction of TiO2 Nanocrystals and their
Application for Solar Water Evaporation Adv. Energy Mater. 2017, 7, 1601811-n/a.
Copyright 2017, published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
(e) H., Wang*, J. Jia*(equal contribution), G. Casillas, M.Wei, M. Liu, Y. Dong, L. Wang,
M. Ghoussoub, S. Ye, Z. Zheng, Q. Wang, N. P. Kherani, D. D. Perovic, A. Walsh, G. A
Ozin. Lattice Strain Engineering of Photocatalyst for Highly-efficient CO2 Conversion.
Submitted. .
(f) H. Wang, J. Jia, P. Song, Q. Wang, D. Li, S. Min, C. Qian, L. Wang, Y. F. Li, C. Ma, T.
Wu, J. Yuan, M. Antonietti, G. A. Ozin. Efficient Electrocatalytic Reduction of CO2 by
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Nitrogen-Doped Nanoporous Carbon/Carbon Nanotube Membranes: A Step Towards the
Electrochemical CO2 Refinery. Angew. Chem. Int. Ed. 2017. Copyright 2017,
published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Chapter 1 Introduction and Background
This chapter has been compiled using excerpts from previously published work and unpublised
work:
(a) Recent developments in gaseous carbon dioxide conversion powered by light and heat.
In preparation.
1 Introduction and Background
1.1 Introduction
Global warming, caused by greenhouse gases, has a significant effect on climate and
environment. For decades, the concentration of CO2, mainly originating from the combustion of
fossil fuels, continuously grows in the atmosphere. Another serious issue associated with fossil
fuels is their unsustainability.
Because of climate change and the energy crisis, researchers have proposed using CO2 as a
feedstock to store the energy from renewable energy sources in the form of chemical energy. By
converting CO2 into chemicals and fuels, the energy from renewable sources can be stored in
chemical bonds. Upon combustion, the energy of these renewable fuels can power our industries,
transportation systems, buildings and homes around the world. CO2 will be recycled in this
sustainable carbon-neutral carbon-cycle. If the existing infrastructure in the chemical and
petrochemical industry can be developed into CO2 refineries, together with other approaches,
they will play a vital role to facilitate greenhouse gas reduction and ameliorate climate change.
It has been reported that CO2 can be converted into a variety of value-added chemicals and fuels,
including carbon monoxide, methane, ethylene, methanol, ethanol, urea, dimethyl ether and other
higher hydrocarbons. Among different CO2 conversion approaches, gas-phase heterogeneous
catalytic reduction of CO2, driven by renewable forms of energy, is considered as a promising
strategy, benefiting from its potential to be easily integrated into existing chemical and
petrochemical industry infrastructure.1, 2 However, there are still remaining challenges to be
overcome. Currently, there is an urgent need to discover and design highly active, selective and
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2
stable heterogeneous catalysts for the production of fuels powered by light and/or heat from the
largest renewable energy resource, the sun.
In this chapter, we present a summary of advances specifically for the heterogeneous catalytic
conversion of gaseous carbon dioxide into different classes of chemicals and fuels.
1.2 Methanation
Research on the thermal reduction of CO2-to-CH4 has yielded experimental and theoretical
mechanistic insights into the activity and selectivity of the reaction, with a focus on different
reaction pathways and the formation, binding and chemistry of intermediate species. In the case
of oxide-supported Rh catalysts, it has been demonstrated that the thermal treatment of TiO2
supported Rh nanoparticles in a H2 atmosphere at 500 C creates a strong metal support
interaction (SMSI) encapsulated state, as shown in Figure 1.1a, which facilitates the selective
production of CH4. 3 An adsorbate-mediated SMSI (A-SMSI) overlayer can be formed in CO2
H2 environments at temperatures of 150300 C, as shown in Figure 1.1b, which causes a
selectivity switch from CH4 to CO production. Probed by in situ spectroscopy, the high coverage
of adsorbates (HCOx) on the A-SMSI overlayer, illustrated in Figure 1.1c, enables oxygen
vacancy formation and the selectivity switch. Theoretically, it has been demonstrated by Density
Functional Theory (DFT) calculations that the difference in selectivity of metal metal oxide
catalysts is associated with different reaction pathways and the binding strength of reaction
intermediates. In the case of PtCo bimetallics supported on reducible metal oxides for the CO2
hydrogenation, by changing the metal oxide support, selectivity could be modified as a result of
different modes of binding of C and O-bound and O-bound species at the metal metal oxide
interface.4
For photochemical reduction of CO2 to CH4, several distinct materials chemistry strategies have
been devised for increasing the yield and lifetime of charge carriers or improving adsorption and
activation of CO2, to tailor the reactivity of heterogeneous catalysts. The effect of creating
homojunctions between different facets of CeO2 has been explored.5 Intentionally formed
homojunctions are found to enable the efficient separation and fast transfer of photoinduced
charge carriers, as a means for optimizing the photocatalytic performance, as illustrated in
Figure 1.1d. In the case of Cu atoms in a Pd catalyst, the observed correlation between adjacent
Cu-Pd sites and reduction of CO2 to CH4, as illustrated in Figure 1.1e-f, implies they are
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3
responsible for enhanced CO2 adsorption.6 Based on synchrotron radiation characterizations and
theoretical simulations, the Cu atoms in the Pd lattice can play dual roles: (1) providing the
paired CuPd sites for enhanced CO2 adsorption and suppressed H2 evolution; and (2) elevation
of the Cu d-band in Cu-Pd, considered to be the reason for the improved activation of CO2.
Figure 1.1 Methanation of Carbon Dioxide. a) Visualizing strong metal support interaction,
SMSI, states by in situ scanning transmission electron microscope (STEM) images of sintered
6% Rh/TiO2 after treatment in 5% H2 and 95% N2 at 550 C for 10 min. b) Visualizing
adsorbate-mediated SMSI (A-SMSI) states by in situ STEM images of sintered 6% Rh/TiO2 after
treatment in 20CO2:2H2 at 250C for 3 h. c) Infrared analysis of selectivity switch for thermally
reduced (red), 20CO2:2H2-treated (blue) and re-reduced (black) catalysts. Reprinted with
permission.3 Copyright 2016, Nature Publishing Group. d) Illustration of charge transfer cross
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4
the surface facet homojunction of CeO2. Reprinted with permission.5 Copyright 2015, American
Chemical Society.e) Isolation of Cu atoms in Pd nanoparticle lattice for highly selective
conversion of CO2 to CH4. 6 f) Average production rates of CH4 and CO in photocatalytic CO2
reduction with H2O by bare TiO2, Pd1Cu1TiO2, Pd3Cu1TiO2, Pd5Cu1TiO2, Pd7Cu1TiO2,
Pd9Cu1TiO2,Pd11Cu1TiO2, and PdTiO2 hybrid structures under UV-light (
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5
thermal CO2-to-CO conversion, as illustrated in Figure 1.2a-c.4, 10-15 In situ diffuse reflectance
infrared Fourier transform (DRIFT) spectroscopy has been widely used to investigate details of
the reaction mechanism. By changing the oxide support, the metal-metal oxide heterogeneous
catalysts can be tuned to selectively produce CO, which is related to the binding strength of
intermediate species.4 Alternative catalysts to precious metals, such as molybdenum carbide
(Mo2C), have been demonstrated to be highly active for selective reduction of CO2 to CO.16
Photochemical catalytic conversion of CO2 involves light-induced electron-hole pair generation,
charge carrier seperation and transfer, activation of H2 and CO2 by charge carriers, and CO2-to-
CO conversion. A class of materials capable of photocatalytically reducing CO2 are oxygen
deficient metal oxides. For example, defect laden In2O3-xOHy[O]z was discovered to be an
efficient heterogeneous photocatalyst for the hydrogenation of CO2.17 Detailed experimental and
theoretical studies on In2O3-xOHy[O]z have revealed the function of oxygen vacancies and
hydroxyls, the Lewis acidity and Lewis basicity of surface active sites, and kinetics and
mechanistic insights into the photochemical CO2-to-CO conversion. Following band gap
excitation, oxygen vacancies and hydroxyls, the former located below the conduction band and
the latter above the valence band, serve as traps for photo-generated electrons and holes
respectively, as illustrated in Figure 1.2e.18. This has both the effect of slowing down carrier
relaxation times and enhancing the Lewis acidity and Lewis basicity of the defect centres in the
excited state. These two effects work synergistically and result in the reduction of the activation
energy for the rate determining carbon dioxide hydrogenation step by 20 kJ/mole, in the light
compared to the dark,19 thereby enhancing the rates for the reverse water gas shift reaction.
In2O3-xOHy[O]z also served as an archetype for tailoring the effect of defects on its photocatalytic
properties.18-24 Recent work uncovered the role of defects in affecting electronhole separation at
the atomic level to achieve highly efficient solar CO2 reduction, as illustrated in Figure 1.2d.25
Verified by DFT and ultrafast transient absorption spectroscopy, the presence of zinc vacancies
increased charge density, carrier transport and separation rates. Together with increased
photoabsorption, higher CO2 adsorption capacity, and more pronounced surface hydrophilicity,
the zinc vacancy rich compared to vacancy poor, single-unit-cell thick ZnIn2S4 layers boost the
carbon monoxide formation rate by roughly 3.6 times.
Powered by both the heat and light from the sun, the photothermal strategy is extremely
appealing for the development of highly efficient and stable gas-phase CO2 reduction catalysts.26
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6
An example is a hybrid catalyst consisting of a vertically aligned silicon nanowire (SiNW)
support evenly coated by In2O3x(OH)y nanoparticles that utilizes the vast majority of the solar
irradiance to simultaneously produce both the photogenerated charge carriers and heat required
to reduce CO2 to CO at a rate of 22.0 molgcat1h1, illustrated in Figure 1.2f-g. Attributed to
reduced reflection losses and improved light harvesting efficiency within the SiNW array, a
nearly 6-fold enhancement over identical In2O3x(OH)y films on roughened glass substrates has
been observed. The even distribution of the In2O3x(OH)y nanoparticles on the SiNW array also
facilitate heat transfer from the SiNW support to the In2O3x(OH)y nanoparticles. This strategy of
utilizing nanostructured photothermal supports to provide a direct source of heating and light
absorption of photocatalytic materials is demonstrated to be an effective way for improving the
conversion rates for gas-phase reactions that require both light and heat energy.
Figure 1.2 Reverse Water Gas Shift (RWGS) Reaction CO2 + H2 CO + H2O. a)
Heterogeneous catalysis of CO2 reduction on supported metals. b) CO2 conversion on Pd/Al2O3
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7
at different loadings and temperatures. Reprinted with permission.11 Copyright 2013, American
Chemical Society. c) DRIFT spectrum obtained from a saturated layer of CO adsorbed at 300 K
on 4% Rh/TiO2. Reprinted with permission.13 Copyright 2015, American Chemical Society.d)
Scheme for the photoreduction CO2 into CO on the VZn-rich one-unit-cell ZIS layers. Reprinted
with permission.25 Copyright 2017, American Chemical Society.e) Schematic illustrating the
different charge carrier relaxation processes in In2O3x(OH)y[O]z.18 Copyright 2016, National
Academy of Sciences. f) Nanostructured indium oxide coated silicon nanowire arrays as a hybrid
photothermal-photochemical approach to solar fuels. 26 g) The absorption spectrum of
In2O3x(OH)y nanoparticles and the photon utilization of the solar irradiance for
In2O3x(OH)y/SiNW hybrid materials. h) 13CO production rates of evenly coated
In2O3x(OH)y/SiNW, bilayer In2O3x(OH)y/SiNW, and In2O3x(OH)y/glass in the dark and under
illumination, with or without external heating. Reprinted with permission.26 Copyright 2016,
American Chemical Society.
The goal to create CO2 refineries will also require the establishment of laboratories that
specialize in the design and development of catalyst reactors and operando analytical
instrumentation, specifically for the purpose of evaluating the efficacy, durability and
reproducibility of catalytic materials able to convert gaseous CO2 into chemicals and fuels. In
this context, a solar thermal reactor has been designed for converting CO2 to CO.27 As shown in
Figure 1.3a, the reactor consists of ceria catalysts, gas-collection gap, thermal insulation layer,
quartz window, gas inlet, and gas outlet. Concentrated solar radiation passes through the quartz
window and is focused on the chemical reaction chamber. Powered by thermal energy from the
concentrated light, the temperature of the reactor can be as high as 1500 C. Porous ceria
catalysts convert CO2 to CO efficiently and selectively at such elevated temperatures. The solar
reactor consists of a 100 mm-inner diameter and a 75 mm-depth cylindrical cavity-receiver,
demonstrating the potential for the development of large scale CO2 conversion demonstrators.
There have been recent advances in the design of photothermal reactors for CO2 conversion as
well. Instead of generating high uniform temperatures throughout the entire catalyst sample,
photothermal catalysts can generate high local temperatures at the nanoscale of the catalyst,
without causing significant temperature increase in the environment. A photothermal reactor
system has been designed with a quartz window using a solar simulator coupled with an AM1.5
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Figure 1.3. Reactor design. a) Schematic of the solar reactor configuration for splitting CO2 into
separate streams of CO2 and O2 via a 2-step thermochemical redox cycle. b) Photographs of the
solar reactor, showing the front face of the solar reactor with the windowed aperture and its
interior containing the octagonal reticulated porous ceramics (RPC) structure lined with alumina
thermal insulation.28 Copyright 2017, by The Royal Society of Chemistry, under the Creative
Commons Attribution-NonCommercial 3.0 License.c) Photograph of a photoreactor in operation
Reprinted with permission.9 Copyright 2014, American Chemical Society.d-e) Setup for
photothermal conversion of CO2 in d) batch system and e) flow system. Reprinted with
permission. 7, 29 Copyright 2014, Wiley. Copyright 2016, Wiley.
filter as the light source (Figure 3c).9 Two other kinds of photothermal reactors based on batch
and a flow system (Figure 3d,e), have also been developed.7, 30 When the illumination from the
lamp is focused on the metal-metal oxide catalysts, it will absorb the incident radiation and
convert it into heat locally thereby elevating the temperature of these catalysts by a few hundred
degrees celsius. It has been demonstrated that the mechanism of photothermal reactions may be
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9
identical with thermal analogues. The difference is the distinct ways of heat generation and
transfer to the catalytically active site. Photothermally induced local heating to drive CO2
heterogeneous catalytic hydrogenations opens new doors in the design of catalysts and reactors.
1.4 Methanol Synthesis
If methanol could be efficiently obtained from captured carbon dioxide, solar-generated
hydrogen, renewable forms of energy and produced economically competitive with methanol
made from non-renewable fossil fuels, then a transition from an unsustainable carbon-positive to
a sustainable carbon-neutral future will be brought closer to reality. Recent advances in synthetic,
characterisation and computational techniques provide new approaches to the rational design of
heterogeneous catalysts for methanol synthesis.
A Ni-Ga catalyst which reduces CO2 to methanol at ambient pressure has been discovered.
Identified through a combination of a descriptor-based analysis of the process and computational
methods, the Ni-Ga catalyst demonstrates the same or better methanol synthesis activity
compared to conventional Cu/ZnO/Al2O3 catalysts, as illustrated in Figure 1.4a. 31 The power of
materials theory and simulation has been demonstrated by insightful mechanistic studies,
identification of active sites, the synergy effect of Cu and ZnO at the interface and catalyst
optimization. 32, 33 By combining X-ray photoemission spectroscopy, DFT calculations, and
kinetic Monte Carlo simulations, ZnCu and ZnO/Cu model catalysts have been identified and
characterized for methanol synthesis, as illustrated in Figure 1.4b-d. 33 Both experimental and
theoretical results indicate that ZnCu undergoes surface oxidation under the reaction conditions
and surface Zn transforms into ZnO with activity comparable to ZnO/Cu with the same Zn
coverage. Additonally, an in-depth mechanistic study of the hydrogenation of CO2 on Cu/ZrO2
has been performed using a combination of kinetics, in situ IR and NMR spectroscopies, isotopic
labeling strategies, and DFT calculations, as illustrated in Figure 1.4e-f. 34 It is confirmed by
NMR spectroscopic investigations that the initially formed formate intermediate species
adsorbed on zirconia converts successively to acetal, methoxy and finally methanol.
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Figure 1.4. Methanol synthesis from CO2. a) Theoretical activity volcano plot for CO2
hydrogenation to methanol. b) Zn 2p3/2 XPS binding energies measured after performing the
hydrogenation of CO2 on the Zn/Cu(111) catalyst. Reprinted with permission.31 Copyright 2014,
Nature Publishing Group. c) Potential energy diagram for the hydrogenation of CO2(g) to
CH3OH(g) on ZnO/Cu(111) via the RWGS + CO-hydroxycarbonyl and formate pathways. (Inset)
Structures of *HCOO on ZnO/Cu(111). Cu, brown; Zn, blue; O, red; H, white; C, gray. d)
Coverage of surface reaction intermediates on ZnCu(211) and ZnO/Cu(111) under reaction
conditions. Reprinted with permission.33 Copyright 2017, American Association for the
Advancement of Science. e) Ex situ MAS-NMR 1H-13C HETCOR spectra of Cu/ZrO2 reacted
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with H2/13CO2 (3:1) at 230C for 12 h at 5 bars. f) Reaction scheme derived from the
spectroscopic measurements. Reprinted with permission.34 Copyright 2017, Wiley.
1.5 C2 C11
Recent progress has been made in the hydrogenation of gaseous CO2 to C2 C11 hydrocarbons
via Fischer-Tropsch (FT) synthesis by either an indirect or a direct route. The indirect route is
often performed using different catalysts, reactors and conditions with CO and H2. The CO can
be produced from hydrogenation of CO2 through the reverse water-gas shift (RWGS) reaction.
Since it was developed almost a century ago, Fischer-Tropsch (FT) synthesis has played a vital
role as it is the only effective technology to date for direct conversion of synthesis gas (syngas)
to C2 C11 hydrocarbons, producing synthetic lubricants and synthetic fuels from coal, natural
gas, or biomass. However, the bottomneck of this technology is low hydrocarbon selectivity and
high methane selectivity, as well as carbon coking. The key challenges of selective formation of
hydrocarbon from FT synthesis are the precise control of CC coupling, the degree of
hydrogenation, and the suppression of methane formation. Rising to this challenge, a
bifunctional catalyst has been designed, which separates CO activation and CC coupling onto
two different types of active sites with complementary properties. This ZnCrOx/ mesoporous
SAPO zeolite (MSAPO) catalyst enabled activation of CO2 to CO on the surface of the reduced
oxide (ZnCrOx) and C-C coupling within the acidic pores of zeolites. As a result, C2-C4
selectivity as high as 80% with long-term stability was achieved (Figure 1.5a-b).35
The direct CO2 hydrogenation is usually a two-step process, including the reduction of CO2 to
CO via reverse water-gas shift (RWGS) reaction and subsequent hydrogenation of CO to
hydrocarbons via FT synthesis. A model catalyst that enables the direct route is a well-defined
nanostructured catalyst CeO2Pt@meso SiO2Co, which functions as a tandem catalysis for the
conversion of CO2 to C2C4 hydrocarbons (Figure 1.5c-d). The reaction mechanism involves the
conversion of CO2 and H2 to CO at the Pt/CeO2 interface, and a subsequent production of C2C4
hydrocarbons through a FischerTropsch process at the neighboring Co/meso SiO2 interface. 36
Another interesting recent development concerns studies that are designed to achieve direct
conversion of CO2 to gasoline-range (C5C11) hydrocarbons with selectivity up to 78%, driven
thermochemically under industrial relevant conditions, as illustrated in Figure 1.5e.37 In this
context, detailed experimental studies have revealed that the high selectivity is achieved by a
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multifunctional catalyst consisting of three types of active sites (Fe3O4, Fe5C2 and Brnsted acid
sites), which cooperatively catalyse a tandem reaction. The appropriate proximity of three types
of active sites is essential to enable the successive and synergetic catalytic conversion of CO2 to
C5C11 hydrocarbons. Remarkably, the multifunctional catalyst exhibits a stability for 1,000 h
on stream.37
Figure 1.5. FT synthesis. a) CO conversion and product distribution at different H2/CO ratios in
syngas over a catalyst with a mass ratio of ZnCrOx/mesoporous SAPO zeolite (MSAPO) = 1.4 at
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a space velocity of 4800ml/hgcat b) A stability test of a composite catalyst with
ZnCrOx/MSAPO ratio = 0.9 at 6828 ml/hgcat and H2/CO of 2.5. Reprinted with permission.35
Copyright 2016, American Association for the Advancement of Science. c) The
CeO2Pt@mSiO2Co tandem catalyst. d) CO2 conversion and hydrocarbon distribution at
different H2/CO2 ratios over the tandem catalyst at 250 C. Reprinted with permission.36
Copyright 2017, American Chemical Society. e) Reaction scheme for CO2 hydrogenation to
gasoline-range hydrocarbons.37 Copyright 2017, Nature Publishing Group, under the Creative
Commons Attribution 4.0 International License
1.6 The Goals of this Doctoral Thesis Work
Among different approaches that can convert CO2 into value-added products, gas-phase light-
assisted reduction of CO2 is a practical option, since gas phase processes can be easily scaled and
integrated with existing chemical and petrochemical industry infrastructure.1 The activity of
numerous semiconductor photocatalyts towards the light-assisted reduction of CO2 to useful
fuels has been studied.38-49 Photothermal and photochemical approaches in which gas-phase
light-assisted hydrogenation of CO2 to useful fuels over supported metal or metal oxide
nanostructured catalysts have been investigated.
The photothermal approach to the catalytic hydrogenation of CO2 has now achieved conversion
rates as high as mmol to mol gcat1 h1.7-9 This tactic exploits the transformation of light into heat,
generating high local temperatures in nanostructured catalysts that drive CO2 reduction reactions.
The mechanistic details of photothermal catalysis remain to be clarified for different classes of
materials and supports.
For the case of metal nanocrystals, the photothermal effect can arise in different ways that
include non-radiative relaxation of photoexcited surface plasmons, and intra-band or inter-band
non-radiative relaxation of photoexcited charge carriers. The plasmonic effect occurs when
absorbed radiant energy is stored in the collective resonant oscillations of conduction electrons.[6,
10-12] Inter-band and intra-band absorption commonly occur in transition metals when electrons
are photoexcited to higher energetic states within the same electronic band or between different
bands, respectively.[13, 14] By identifying the key factors which underpin the photothermal effect
that promotes catalytic CO2 reduction reactions, it will prove possible to design and make
nanoscale catalysts with compositions and structures, chemical and physical properties that serve
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to maximize the rate of reduction of gaseous CO2 to chemicals and fuels, using both the heat and
light from sunlight.
The catalyst support may also influence the photothermal effect. Recent efforts towards the
development of photothermal catalysts for gas-phase CO2 reduction have focused on metal
nanocrystals dispersed on insulating metal oxide supports.[9] Photothermal catalysts based on
metal nanocrystals deposited on metal oxide semiconductor supports, especially reducible metal
oxide supports, have received much less attention. It is therefore essential to identify the roles
played by the catalyst supports, since redox activity in semiconductor supports may influence the
catalytic reactions through different mechanisms.
Another important factor is the size and shape of the nanocrystals.[6] However, this has not been
explored in the photothermal catalytic hydrogenation of CO2. We aim at investigating the effect
of size and shape of nanostructured catalysts in the photothermal hydrogenation of CO2. A goal
is to pinpoint the mechanistic details that underpin the size and shape dependence of the activity
and selectivity of the photothermal hydrogenation process, experimentally and theoretically,
using a powerful combination of in situ diffuse reflectance infrared Fourier transform (DRIFT)
spectroscopy, Density Functional Theory (DFT) calculations, X-ray photoemission spectroscopy
(XPS) and ultra-violet photoemission spectroscopy (UPS), thermogravimetric analysis (TGA),
and chemisorption analysis.
Semiconducting metal oxides have also been widely investigated as photocatalysts for CO2
reduction.50-54 We have shown that intentionally engineered defects in non-stoichiometric indium
oxide, In2O3-x(OH)y, are active for the photocatalytic hydrogenation of CO2-to-CO.17, 20 Despite a
significant amount of effort, developing CO2 photocatalysts with high activity, selectivity and
stability and at low cost, remains a great challenge. We aim to explore new strategies to achieve
high-performance yet low-cost photocatalysts for reduction of gaseous CO2 to CO.
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Chapter 2 Photothermal Catalysed Hydrogenation of Gaseous CO2 over
Nanostructured Pd@Nb2O5
This chapter has been compiled using excerpts from previously published work:
(a) J. Jia, P. G. O'Brien, L. He, Q. Qiao, T. Fei, L. M. Reyes, T. E. Burrow, Y. Dong, K.
Liao, M. Varela, S. J. Pennycook, M. Hmadeh, A. S. Helmy, N. P. Kherani, D. D.
Perovic, G. A. Ozin, Ambient Temperature Visible and Near Infrared
PhotothermalCatalysed Hydrogenation of Gaseous Carbon Dioxide over Nanostructured
Pd@Nb2O5 Adv. Sci., 3, 1600189
(b) M. Hmadeh, V. Hoepfner, E. Larios, K. Liao, J. Jia, M. Jose-Yacaman, and G. A. Ozin,
ChemSusChem, 2014, 7, 2104-2109.
2 Photothermal Catalysed Hydrogenation of Gaseous CO2 over Nanostructured Pd@Nb2O5
2.1 Abstract
The reverse water gas shift (RWGS) reaction driven by Nb2O5 nanorod-supported Pd
nanocrystals without external heating using visible and near infrared (NIR) light is demonstrated.
By measuring the dependence of the RWGS reaction rates on the intensity and spectral power
distribution of filtered light incident onto the nanostructured Pd@Nb2O5 catalyst, it is determined
that the RWGS reaction is activated photothermally. That is the RWGS reaction is initiated by
heat generated from thermalization of charge carriers in the Pd nanocrystals that are excited by
inter-band and intra-band absorption of visible and NIR light. Taking advantage of this
photothermal effect, a visible and NIR responsive Pd@Nb2O5 hybrid catalyst that efficiently
hydrogenates CO2 to CO at an impressive rate as high as 1.8 mmol gcat-1 h1 is developed. The
mechanism of this photothermal reaction involves H2 dissociation on Pd nanocrystals and
subsequent spillover of H to the Nb2O5 nanorods whereupon adsorbed CO2 is hydrogenated to
CO. This work represents a significant enhancement in our understanding of the underlying
mechanism of photothermally driven CO2 reduction and will help guide the way towards the
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development of highly efficient catalysts that exploit the full solar spectrum to convert gas-phase
CO2 to valuable chemicals and fuels.
2.2 Introduction
Solar fuels are attracting increasing attention owing to their potential of being a viable alternative
to fossil fuels. One appealing route to the production of solar fuels is to use renewable sources of
hydrogen to reduce the greenhouse gas CO2 to value-added chemicals and fuels.55 If realized in
practice at a technologically significant efficiency, economically competitive cost and
industrially relevant scale, this could simultaneously address several significant global
challenges: climate change, renewable energy, and protection of the environment.38, 39, 45, 47, 56-60
For the case of metal nanocrystals, the photothermal effect can arise in different ways that
include Surface Plasmon Resonance (SPR) non-radiative relaxation, and intra-band or inter-band
non-radiative relaxation of photoexcited charge carriers. The SPR effect occurs when absorbed
radiant energy is stored in the collective resonant oscillations of conduction electrons.61, 62 On the
other hand inter-band and intra-band absorption commonly occur in transition metals when
electrons are photoexcited to higher energetic states within the same electronic band or between
different bands, respectively.63, 64 For the case of Pd nanocrystals below 10 nm in size, the SPR is
around 200-250 nm and these nanocrystals exhibit strong ultraviolet (UV) light absorption.65-68
Furthermore, visible and near infrared (NIR) light absorption in these Pd nanocrystals is
dominated by inter-band electron transitions (between the d band and s-p conduction band) and
intra-band transitions (between filled and empty states in the d and s-p bands).61, 62, 64, 69
The catalyst support may also influence the photothermal effect. Recent efforts towards the
development of photothermal catalysts for gas-phase CO2 reduction have focused on metal
nanocrystals dispersed on insulating metal oxide supports.7 Photothermal catalysts based on
metal nanocrystals deposited on metal oxide semiconductor supports, especially reducible metal
oxide supports, have received much less attention. Intuitively, such photoactive reducible
semiconductor supports may further accelerate the catalytic reactions through different
mechanisms. For example, defects in metal oxide semiconductors, such as oxygen vacancies
hydroxyl groups and reduced or co-ordinately unsaturated metal sites, can promote the affinity of
CO2 to the surface of the catalysts, prolong photo-generated charge-carrier lifetimes and enhance
the reactivity of CO2.45, 70-74
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Herein we report the reverse water gas shift (RWGS) reaction driven with visible and near-
infrared photons at ambient temperature over hybrid catalysts comprised of palladium
nanocrystals dispersed on Nb2O5 nanorods denoted Pd@Nb2O5. Relevant prior work on Pd
nanocrystals includes electrocatalytic,75, 76 photocatalytic,77 and thermocatalytic hydrogenation of
CO2,11, 78-82 as well as CO2 photothermal methanation.
7 The thermally driven systems promote
CO2 hydrogenation at high rates but require high operating temperatures (523 K 900 K) and
high pressures.11, 78-82
In this study we use nanostructured Pd@Nb2O5 as a model hybrid catalyst system to demonstrate
the potential of driving the RWGS reaction using light, without providing external heating to the
catalyst. Specifically, we observe CO2-to-CO hydrogenation rates as high as 5 mmol gcat-1 h1
over these hybrid Pd@Nb2O5 catalysts, normalized to the weight of the Pd nanocrystals, when
subjected to light emitted from a Xe lamp at 25 Suns intensity. This is only about two orders of
magnitude less than a rate of technological significance. Notably, the CO2 hydrogenation
reaction proceeded at a rate of 2 mmol gcat-1h-1 using light in the visible and NIR spectral region,
normalized to the weight of the Pd nanocrystals. We then attempt to pinpoint the key factors
responsible for the photothermal hydrogenation of CO2-to-CO over Pd@Nb2O5 by investigating
the reaction rate dependence on the spectral power distribution and intensity of the incident light.
We also utilize variable temperature Raman spectroscopy measurements to investigate the
photothermal effect on the local temperature of individual and assemblies of the nano-
components that comprise our Pd@Nb2O5 hybrid catalysts. Additionally, we have observed an
enhancement of the CO2 conversion rate that is connected to the in situ generation of Nb4+ and/or
O vacancy defects. This effect more than doubles the RWGS rate.
2.3 Experimental
2.3.1 Synthesis of Nb3O7(OH) Nanorods
Niobium powder (680 mg of Nb, 325 Mesh, Aldrich) was dissolved in a hydrochloric acid
solution (9 mL HCl, SigmaAldrich; 10 mL deionized water) in a pyrex beaker. The aqueous
solution was ultra-sonicated for half an hour and then stirred for 15 min. The solution was
subsequently placed in a Teflon-lined stainless steel autoclave with 100 mL capacity. The
hydrothermal reaction was performed at T=200 C for 24 h.83 After cooling to room temperature,
the white product was collected through a centrifugation process and washed three times with
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deionized water to remove non-reacted residues. Finally, the sample was dried in a vacuum oven
at T=70 C for 12 h.
2.3.2 Synthesis of Nanostructured Pd@Nb2O5
Nanostructured Pd@Nb2O5 was synthesized via a microwave-assisted reaction. Typically, 50 mg
of Nb3O7(OH) nanorods were suspended in anhydrous ethanol (20 mL) in a pyrex vessel with 40
mL capacity. A stock solution of Na2PdCl4 (39.3 mg, Na2PdCl43H2O, Alfa Aesar) was prepared
in anhydrous ethanol (20 mL), the concentration of which is 1 mg/mL. 0.25 mL of Pd precursor
solution, equivalent to 0.5 wt%, was added to the dispersion of 50 mg Nb3O7(OH) under
sonication. After 30 min of sonication, the vessel was capped and transferred to the microwave
reactor (CEM Discover, 220 W, 220 psi, T=150 C, 20 min). After centrifugation and being
washed with deionized water, the sample was placed into a vacuum oven at T=70 C for 24 h.
The dried Pd@Nb3O7(OH) sample was then treated at T=120 C in air for 24 h to obtain the final
Pd@Nb2O5 nanostructured samples ready for characterization and catalytic testing.
2.3.3 Physical Characterization
Sample morphology was determined using a Hitachi H-7000 transmission electron microscopy
(TEM) at 100 kV. The scanning transmission electron microscopy (STEM), high angle annular
dark field (HAADF) imaging and electron energy-loss spectroscopy (EELS) analysis were
performed using an aberration-corrected Nion UltraSTEM-200, equipped with a cold-field
emission gun. The Nion UltraSTEM-200 was operated at 200 kV when performing HAADF
imaging and EELS mapping. EEL spectra were collected using a Gatan Enfinium Dual EELS
spectrometer. Principal component analysis (PCA) was performed on the EELS data to remove
random noise.84
Powder X-ray diffraction (PXRD) was performed on a Bruker D2-Phaser X-ray diffractometer,
using Cu K radiation at 30 kV.
The surface area of each sample was determined through volumetric nitrogen adsorption at 77 K
using a Quantachrome Autosorb-1-C and calculated using Brunauer-Emmet-Teller (BET) theory.
Samples were outgassed overnight at 80C prior to being analysed.
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Diffuse reflectance spectra of the samples were measured using a Lambda 1050 UV/VIS/NIR
spectrometer from Perkin Elmer equipped with an integrating sphere with a diameter of 150 mm.
The film samples were prepared by drop-casting aqueous dispersions of Pd@Nb2O5 and Nb2O5
samples onto borosilicate glass microfiber filters.
X-ray photoelectron spectroscopy (XPS) was performed using a Perkin Elmer Phi 5500 ESCA
spectrometer. The spectrometer uses an Al K X-ray source to generate X-rays with an energy of
1486.7 eV. The samples used in XPS analyses were prepared by drop-casting aqueous
dispersions of Pd@Nb2O5 and Nb2O5 samples onto p-doped Si(100) wafers. All measurements
were conducted in an ultrahigh vacuum chamber with a base pressure of 1x10-9 Torr. Data
analyses were performed using the Multipak program and all binding energies were referenced to
the NIST-XPS database and the Handbook of X-ray Photoelectron Spectroscopy.85, 86
CO2 adsorption capacity was determined using thermogravimetric analysis (Discovery TGA, TA
Instruments) with the following procedure. Each sample (approximately 5-10 mg) was first
heated at a rate of 10C/min to a final temperature of 120C, under a nitrogen flow of 100
mL/min. This temperature and nitrogen flow was maintained for 1 hour to remove any adsorbed
moisture. The gas flow was then switched to carbon dioxide, maintaining the same flow rate and
temperature for 2 hours. The observed weight gain between the N2 and CO2 gas streams was
directly used to calculate CO2 capacity, and normalized against the surface area of each sample.
TGA was performed to determine whether or not carbon deposits reside on the used catalyst. 35,
87, 88 These experiments were carried out using a Discovery TGA (TA Instruments). A catalyst
film sample was prepared on frosted glass substrates (Erie Scientific) and tested for a duration of
40 h under testing conditions. After the catalytic tests, the used sample was scraped off the
frosted glass substrate for TGA measurements. Two catalyst samples, one fresh sample and one
used sample, were each heated under flowing CO2, at a rate of 100 mL/min. The samples were
heated at a rate of 1C/min to 700C. The same TGA experiments were also performed using air
since it is known to efficiently remove carbon deposits.
The Raman spectra were obtained using the Horiba Jobin Yvon confocal system; namely a
LabRAM HR 800 spectrometer equipped with a continuous wave HeNe laser (=633 nm, with
r=400 nm) and a CW diode laser (=785 nm, with r=500 nm). The maximum laser power
available using the HeNe laser was 12 mW. The samples were illuminated using a 100x
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objective. For the Raman spectra, which were obtained as a function of laser power, the exposure
time varied from 1s to 40s, with averaging over 4 spectra depending on the power used. The
exposure time was increased as the incident power was reduced to maintain a signal to
background ratio conducive to accurate peak fitting. Prior to fitting the Raman peaks, the spectra
were identified and isolated from the measured data by subtracting a polynomial curve fit to the
background signal. The stokes/anti-stokes spectra were obtained using the 785 nm laser with a
power of 24 mW. The samples for Raman measurements were prepared by drop-casting aqueous
dispersions of Pd@Nb2O5 and Nb2O5 samples onto borosilicate glass microfiber filters.
Electron paramagnetic resonance (EPR) measurements were performed at 20K using a Bruker
ECS-EMX X-band EPR spectrometer equipped with an ER4119HS cavity. An Oxford ESR 900
helium cryostat controlled by an Oxford ITC503 temperature controller was utilized. Typical
operating parameters were as follows: microwave frequency 9.381799030 Ghz, microwave
power 21.63 mW, modulation amplitude 4 G, sweep width 512 G centred at 3455.45 G, time
constant 0.01 ms, total sweep time 512 s, 4096 points were acquired. The EPR analysis was
applied to nanostructured Nb2O5 samples, which were sealed in the EPR tubes in the glove-box
under a N2 gas atmosphere.
2.3.4 Gas Phase Catalytic Measurements
For gas-phase photothermal catalytic testing, samples were prepared by drop casting
nanostuctured Nb2O5 and Pd@Nb2O5 from an aqueous dispersion onto 1 inch by 1 inch binder
free borosilicate glass microfiber filters (Whatman, GF/F, 0.7 m).
The gas phase photothermal catalytic measurements were conducted in a custom-built 1.8-mL
stainless steel batch reactor with a fused silica view port sealed with a Viton O-ring (see Figure
2.1). The reactor was evacuated using an Alcatel dry pump prior to being purged with H2
(99.9995%) at a flow rate of 20 mL/min. After purging, the reactor was infiltrated with H2 and
CO2 gas at a 1:1 ratio to a total pressure of 27 psi prior to being sealed. The pressure inside the
reactor was monitored during the reaction using an Omega PX309 pressure transducer. Reactors
were irradiated with a 300W Xe lamp for a duration of 3 hours. The spectral output from the
300W Xe lamp was measured using a StellarNet Inc spectrophotometer and the power of the
incident irradiation was measured using a Spectra-Physics Power meter (model 407A) (see
Figure 2.1). The radiant power absorbed by the sample for each run was determined using the
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equation ; where P and t, are the power emitted by the Xe lamp and the fraction
of light transmitted through the quartz window as well as any filters used in the measurement,
respectively. The light absorbed by the sample was calculated using the equation a = 1-R,
where R is the diffuse reflectance measured at wavelength . The reactors were not connected
to a heating source for the photothermal tests, although the reactor temperature increased to 50C
over the 3 hour duration of the test due to the heat generated from the incident irradiation. For
thermal tests in the dark, the reactor temperature was controlled by an OMEGA temperature
controller combined with a thermocouple placed in the reactor. Product gases were analysed with
a flame ionization detector (FID) insta