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Ronny Neumann
Weizmann Institute of Science
Clean and Renewable Energy Technologies via the Chemical Route Bengaluru India- November 27-December 2, 2017
Photochemical Electrochemical and Photo-electrochemical Reduction of CO2 and the Use of
Polyoxometalates as Electron “Shuttles”
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Solar Fuels – Where are we and where are we headed?Why do we have a problem?
Resources versus population and standard of living
Until mid 1800’s, wood for heating and transportation by animals, ships driven by wind energy
Coal – mid 1800’s – the beginning of the industrial revolution, the steam engine. First power stations - late 19th century
Petroleum – beginning of the 20th century - the transportationage. Mobility – cars. Higher energy density, advantage of liquids
Nuclear fission– mid 20th century – very high energy density (106 vscarbon fuels. An endless resource? Dreams of fusion energy
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Power (electricity)
Fuels (transportation, industry) > 98% fossil fuel
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Why do we have a problem?Resources versus climate change and the
future of solar
Fossil fuels (coal, petroleum, natural gas) are more plentiful and therefore much cheaper than anticipated. Oil is 1/3 the price it was in 1974 (adjusted to inflation)
There is no real ability to estimate the ramifications or economic cost of climate change and polution. There is no sense of urgency in the general public!!
Therefore: the cost of solar power which is technologically viable (but not optimal) through photovoltaic cells (solar panels) is too high?!
Solar fuels – yet no real scientific/technological viability
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Capture and Transformation of Carbon Dioxide Back to Fuel
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Capture of Carbon Dioxide
•Concentration in atmosphere - < .5 ‰•Capture at the source is much easier- Power plant? Vehicle?
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Is CO2 an Inert Molecule? No?
CO2 is an acid (forms carbonic acid in water) - reacts with bases
CO2 is a strong electrophile - reacts easily with nucleophiles
Note, CO2 is linear with short/strong C-O bonds but easily gives trigonal/tetrahedral transition states or intermediatesWhich are non-linear and have weaker C-O bonds
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HCO3– + H2 HCO2
– + H2O
CO2 + H2 HCOOH
CO2 + H2 CO + H2O
CO2 CO + O2
CO2 + 2 H2 HCHO + H2O
CO2 + 3 H2 CH3OH + H2O
CO2 + 4 H2 CH4 + 2 H2O
DGaq = -4.59 kcal/mol
DGg,l = +5.07 kcal/mol; DGaq
= -1.17 kcal/mol
DGg,l = +4.86 kcal/mol; DGaq
= +2.64 kcal/mol
DGg,l = +61.45 kcal/mol
DGg,l = +11.3 kcal/mol
DGg,l = -0.57 kcal/mol
DGg,l = -27.17 kcal/mol
Two electron reduction
Four electron reduction
Six electron reduction
Eight electron reduction
Thermodynamics of CO2 ReductionIs CO2 an Inert Molecule? Yes and No?
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Manipulating the Hammond Postulate: From a Thermochemical Reaction. . . . . .
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Manipulating the Hammond Postulate: . . . . . . to a Photo- or Electrochemical Reaction
Electrochemical – R is a metal ionthat changes its charge
Photochemical – R is a colored compound that absorbs light
Photovoltaic cells can be used to turn light into electricity;therefore can be used forelectrochemical reactions
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From Photosynthesis . . . . . . . .
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. . . . . . . .to Artificial Photosynthesis
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Hydrogen and the Hydrogenation Approach
1. CO2 can be hydrogenated to formic acid (formate) especially under basic conditions, BUT, formic acid has little energy content by weight and is difficult to reduce further.
2. CO be used to make H2 by the gas-water shift reaction - an establishedProcess - BUT the reverse reaction becomes thermodynamically feasible onlyat high temperature.
CO + H2O CO2 + H2
DG (300K) = -6.81 Kcal/mol
DG (700K) = -3.05 Kcal/mol
3. There are heterogeneous catalysts for high temperature (450-500K)hydrogenation of CO2 to methane and to methanol (the latter usually inthe presence of CO), BUT catalyst efficiency is still (too) low.
4. Methanol is made from CO and 2 H2.
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US Department of Energy report prepared in 2013“The major obstacle preventing conversion of CO2 into
energy bearing products is the lack of catalysts. . .”
•Molecular Compounds – Selective, Low TOF (slow), Stability?, Excellent Reproducibility, High Potentials/Need Blue or UV Light
•Metal Oxide an Metals – Not selective, Higher TOF, Stability?Poor Reproducibility, Very High Potentials/Need UV light
Most Progress on Conversion to CO and HCOOH, but these are the highest energy reactions that will typically require 4-5 V
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Do we want to be bio-inspired? Probably NotCarbon Dioxide Hydrogenase Enzymes
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Some Early Landmarks in CO2 Photo Reduction Catalysis
1970’s
CO2– + OHsurf + H+
CO2 + H2O
CO + 2OHsurf
CO + H2O2
CO + H2O + 1/2O2~105 Kcal/mol
62 Kcal/mol
91 Kcal/mol
Ti+IV - O2– –––> Ti+III - O1- One Electron Reduction
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CO2 + 2 H+ + 2 e– CO + H2O
The half cell methodology; photogeneration of protons andelectrons from sacrificial reducing agent (tertiary amine)
A two electron molecular approach
(CH3CH2)3N (CH3CH2)3NPShn
+ e–H2O
(CH3CH2)2NH + CH3CHO + H+
CO2 + 2 e– + 2 H+ CatalystCO + H2O
•Catalyst and Photosensitizer (Separate and/or Same)
Photosensitizer – visible light, Ru and Ir complexesCatalysts – Co, Fe-porphyrins, Ni-cyclams, Re-Bipyridines
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General Principles of a Catalytic Cycle with Artificial Donor
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Replacement of tertiary amine sacrificial
reducing agent by H2
H3PWVI12O40 + H2
Pt/C H5PWV2WVI
10O40
N NRe
OC
OCCl
COCO2 + Et3N
hnCO + Et2NH + MeCHO
J. Am. Chem. Soc., 2011, 132, 188-190
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Hybrid Rhenium Phenanthroline – Polyoxometalate Complex
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Structure Solution - NMR, MS, IR, UV-vis
Reactivity
Reaction conditions: catalyst (0.5 µmol), DMA (0.5 mL), 20 µg Pt/C, CO2 (1 bar), H2 (2 bar), 20 °C, 14 h under irradiation with a 150 W Xe lamp with cutoff filter at 300 nm. ND- not detected.
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Proposed Mechanism
X-band EPR showing Re(0) and W(V)
Shift of CO stretch as a result of an electron Re(0)
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Computed Reaction Mechanism (DFT and TDDFT)
ACS Catalysis, 2016. 6, 6422–6428
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The Photoelectrochemical Concept
Chem. Eur. J. 2017, 23, 92–95
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CV of H3PW12O40 in DMA Re Cmpd-H3PW12O40 in DMA/CO2
DMA, 100 mV/s . 0.1M TBAPF6
WE: GC CE: Pt wire RE: Ag wire.
Electrochemical Properties of Re Cmpd and H3PW12O40
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Photocatalyzed Electron Transfer from the ReducedPolyoxometalate to the Rhenium Compound
Compoun
d
Ered1(V) λedge (nm) EHOMO (eV) ELUMO (eV) ΔEgap (eV)
Re-Cmpd-1.001 435 -6.22 -3.39 2.85
Reduced
H3PW12O40
-1.127 909 -4.27 -2.90 1.36
Ered1 is the onset value of the first reduction peak in the CV scan. ΔEgap = 1239.8/λedge (nm).ELUMO (eV) = – (Ered1(versus SCE) +4.4), EHOMO (eV) = ELUMO – ΔEgap.
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Photocatalyzed Electron Transfer from the ReducedPolyoxometalate to the Rhenium Compound
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60 W Tungsten
or Red LED
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TOF (sec-1) Conditions Catalyst
2.13 CO2 Re Cmpd + POM
2.67 CO2, 0.4M TFE Re Cmpd + POM
4 CO2, 0.3M PhOH Re Cmpd + POM
- N2, 0.3M PhOH Re Cmpd + POM
- CO2,0.3M PhOH, dark Re Cmpd + POM
- CO2,0.3M PhOH (bpy)Re(CO)3Cl +POM
Photoelectrochemical Reduction - Results
• Electron transfer between the reduced POM and the Rheniumcatalyst.
• Polyoxometalate as electron shuttle which can lower thereduction potential by 400 mV.
• Photoreduction reaction with visible light without usingadditional photosensitizers.
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Summary on CO2 Reduction
and Perspectives
Polyoxometalates are can be used as electron “shuttles” and
act as cofactors for two-electron reduction of CO2.
Reduced polyoxometalates can transfer electrons
photochemically through an intervalence charge transfer.
Prospect – more multi-electron transformations to methanol
and ethanol – but how do we increase rates
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The O2 Evolving Complex of PSII and the Kok Cycle
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High Valent Mn Oxo and Hydroxo Species:Polyoxometalates and Reactions in Dense Phases
JACS, 2015, 137, 8738–8748.
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X-ray Photoelectron and X-ray Absorption Spectroscopy
Magnetic Susceptibility – All compounds are high spin
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*F was fitted as O due to the similar atomic mass
Terminal O(H)x
x = 0-2
FMn
Bridging O
Bridging O
EXAFS – Mn(IV) and Mn(V) have terminal hydroxo ligands
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Mn(V)-OH-PFOM+
4Mn(IV)-OH-PFOM+Highconcentra on
orhightemperature
2H2O
4Mn(III)-OH-PFOM+O2
Slope=2.2
Slope=4.1
*4.5mMMn(IV)PFOM,70mMMCl,80°C
Kine csofO2Forma oninWaterInthepresenceofCs+theReac onis4thorderinMn(IV)OH
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ΔS‡=-117 ± 9
ΔH‡=61 ± 4
ΔS‡=-173 ± 6
ΔH‡=40 ± 2
KCl RbCl
*4.5mM Mn(IV)PFOM, 70mM MCl
Transition State Energies for the O2 Formation Reaction
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Li Na
K Cs
Cryogenic Scanning Transmission Electron Microscopy
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Cryogenic Transmission Electron Microscopy
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- O
- W
- Na
- F
- Mn
Intermolecular Mn-Mn Distances in the Crystal StructureFour Mn(IV)OH moieties are Viable for O2 Formation
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n Mn(IV)-OH-PFOM ⇌ → + O2
→→→
⇌
(n-m) Mn(IV)-OH-PFOM + m Mn(III)-OH2-PFOM
+ (m/4-1) O2
A Dense Phase Mechanism Can Explain the Reaction
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Acknowledgments
Research Group
Dr. Alex Khenkin
Bo Chen
Bidyut-Bikash Sarma
*Jessica Ettedgui (CO2)
*Huijun Yu (CO2)
*Roy Schreiber (Dense Phases)
Marco Bugnola
Miriam Somekh
*Eynat Haviv (CO2)
Kaiji Shen
Dima Azaiza
Yehonatan Kaufman
Funding
Israel Science Foundation
X-ray
Dr. Linda Shimon
Dr. Gregory Leitus
MS
Dr. Aryeh Tishbee
EPR
Prof. Daniella Goldfarb
Dr. Raanan Carmeli
DFT
Dr. Irena Efremenko
Prof. Josep Poblet (Tarragona)
XAS
Prof. Larry Que (Minnesota)