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Electrocatalytic Reduction of Carbon Dioxide Using Molecular Iron Carbonyl Clusters

Introduction

Natalia D. Loewen, Atefeh Taheri, Louise A. Berben* Department of Chemistry; University of California Davis, One Shields Ave, Davis, CA 95616

Berben Group UCDAVIS

Acknowledgements We thank Dr. J. C. Fettinger and the current and previous

members of Berben group, especially Emily Thompson, Thomas Myers, Michael Kagan. We thank the University of California Davis, the NSF CAREER Award (CHE-1055417), and the Alfred P. Sloan foundation for funding. N. D. L. thanks the UC Davis Chemistry Department for a Bradford-Borge Fellowship and the US Department of Education for a GAANN Fellowship.

Thermochemical measurements Mechanistic studies of H+ and CO2 reduction •! Infrared spectroelectrochemistry (IR-SEC) was used

to determine pKa and "GH- of hydrides

Reaction E°(V) pH 7

E° (V) pH 0

CO2 + 2H+ + 2e- ! CO + H2O -0.76 -0.34 V CO2 + H+ + 2e- ! HCOO- -0.67 -0.46

CO2 + 2H+ + 2e- ! HCOOH -0.85 -0.35 V CO2 + 6H+ + 6e- ! CH3OH + H2O -0.62 -0.21 V

CO2 + 8H+ + 8e- ! CH4 + H2O -0.48 -0.07 V

Multielectron Reduction of CO2 (vs. SCE in water):

Electrocatalytic formate production from CO2 in aqueous solutions:

Catalyst E a % b j c stability d

IrHL(S)2e -1.65 93 0.6 25 hr

IrHL(S)2 CNTf -1.65 83 3.5 2

Pb -1.4 98 0.3 < 4 Pd on Pt -0.83 100 0.3 nr

10% Pd on C -0.77 99 4 < 3

SnO2 NC g -1.8 93 5.5 18

[Fe4N(CO)12]- -1.2 96 3.8 > 24 a Applied potential (V vs. SCE), b Faradaic yield, formate, c Current density (mAcm-2), d duration of reported controlled potential electrolysis experiment (hr) e L: PNP pincer ligand, S: MeCN f CNT: Carbon nanotubes, g nanocrystals,

[Fe4C(CO)12]2-

Iron Cluster Electrocatalysts for CO2 and H2 reduction:.

Compound pKa

MeCN H2O !G˚H- (kcal mol-1) MeCN H2O

[HFe4C(CO)12]2- 24.6 ± 1 < 13 44 < 15 [HFe4N(CO)12]- 20.3 13 ± 1 49 15.5

[HFe4N(CO)11(PPh3)]- 23.7 nd 45-46 nd H2 55.5 25.1 76 34.2

HCOO- - - 44 24.1

Figure 4. (Left) IR-SEC spectrum at -1.4 V in MeCN/H2O (95:5) showing reduction of [Fe4N(CO)12]– to afford hydride. Inset: Normalized difference spectra. (Right) IR-SEC spectra in MeCN at -1.4 V showing reduction of [Fe4N(CO)12]– with benzoic acid present. Inset: Normalized difference spectra.

Fe

N

FeFe Fe

OC

OC

LCOOC

COOC CO

OCOC CO

CO

Fe

N

FeFe Fe

OC

OC

LCO

COOC CO

OCOC CO

CO

Fe

N

FeFe Fe

OC

OC

LCOOC

COOC CO

OCOC CO

CO

1. 1e2. H+

1e

CO2HCO2-

1-

COH

1-

L: CO, PPh3

Figure 1. CVs of [Fe4N(CO)11(PPh3)]– in MeCN under 1 atm N2 (black), with 5% H2O (red), and sparged with CO2 (blue).

Figure 2. CVs of [Fe4N(CO)11(Ph2P(CH2)2OH)]- in MeCN under 1 atm N2 (black), with 5% H2O (red), and sparged with CO2 (blue).

•! CV studies probed the kinetics of CO2 and H+ reduction by [Fe4N(CO)11(PPh3)]– and [Fe4N(CO)11(Ph2P(CH2)2OH)]-.

•! Order in catalyst, H+, and CO2 were

used to propose mechanisms

Figure 3. CVs of [Fe4N(CO)11(Ph2P(CH2)2OH)]- with increasing catalyst conc. demonstrate 1st order behavior under 1 atm N2.

[Fe4N(CO)12]- [Fe4N(CO)11(PPh3)]- [Fe4N(CO)11(Ph2P(CH2CH2OH)]-

Thermochemical properties in MeCN and H2O:

References

Fe

N

FeFe Fe

OC

OC

OCCOOC

LOC CO

OCOC CO

CO

1-

Fe

C

FeFe Fe

OC

OC

OCCOOC

CO

OC CO

OCOC CO

CO

2-

L: CO, PPh3, Ph2P(CH2)2OH

Fe

N

FeFe Fe

OC

OC

PPh2

COOC

COOC CO

OCOC CO

CO

Fe

N

FeFe Fe

OC

OC

CO

COOC CO

OCOC CO

CO

1. 1e2. H+

1e

H+HH

1-

COH

1-

Fe

N

FeFe Fe

OC

OC

COOC

COOC CO

OCOC CO

CO

1-HO

PPh2HOPPh2HO