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”

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

Power (electricity)

Fuels (transportation, industry) > 98% fossil fuel

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

Capture and Transformation of Carbon Dioxide Back to Fuel

Capture of Carbon Dioxide

•Concentration in atmosphere - < .5 ‰•Capture at the source is much easier- Power plant? Vehicle?

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

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?

Manipulating the Hammond Postulate: From a Thermochemical Reaction. . . . . .

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

From Photosynthesis . . . . . . . .

. . . . . . . .to Artificial Photosynthesis

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.

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

Do we want to be bio-inspired? Probably NotCarbon Dioxide Hydrogenase Enzymes

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

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

General Principles of a Catalytic Cycle with Artificial Donor

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

Hybrid Rhenium Phenanthroline – Polyoxometalate Complex

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.

Proposed Mechanism

X-band EPR showing Re(0) and W(V)

Shift of CO stretch as a result of an electron Re(0)

Computed Reaction Mechanism (DFT and TDDFT)

ACS Catalysis, 2016. 6, 6422–6428

The Photoelectrochemical Concept

Chem. Eur. J. 2017, 23, 92–95

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

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.

Photocatalyzed Electron Transfer from the ReducedPolyoxometalate to the Rhenium Compound

60 W Tungsten

or Red LED

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.

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

The O2 Evolving Complex of PSII and the Kok Cycle

High Valent Mn Oxo and Hydroxo Species:Polyoxometalates and Reactions in Dense Phases

JACS, 2015, 137, 8738–8748.

X-ray Photoelectron and X-ray Absorption Spectroscopy

Magnetic Susceptibility – All compounds are high spin

*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

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

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

Li Na

K Cs

Cryogenic Scanning Transmission Electron Microscopy

Cryogenic Transmission Electron Microscopy

- O

- W

- Na

- F

- Mn

Intermolecular Mn-Mn Distances in the Crystal StructureFour Mn(IV)OH moieties are Viable for O2 Formation

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

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)