Catalysts for Solar FuelsBryan D. Stubbert, Bert T. Lai, and Harry B. Gray

Division of Chemistry and Chemical Engineering, California Institute of Technology

Selected Nonaqueous H2 Evolution Studies

Electrocatalytic H2 evolution occurs near Co2+/1+ coupleSimulations and thermodynamics favor bimetallic pathway

J. L. Dempsey, J. R. Winkler, H. B. Gray manuscript in preparation.Hu, X.; Brunschwig, B. S.; Peters, J. C. J. Am. Chem. Soc. 2007, 129, 8988-8998.Connolly, P.; Espenson, J. H. Inorg. Chem. 1986, 25, 2684-2688.

AcknowledgementsTeam GCEP: Kyle M. Lancaster,

Keiko Yokoyama, A. Katrine Museth, Rose Bustos

Bruce Brunschwig & Jay Winkler

Jillian Dempsey & Lionel Cheruzel

Gray and Lewis research groups

Additional funding:

Aqueous Co(P) Electrocatalysts

Aqueous electrocatalysis (>90% Faradaic H2 yield) at moderate overpotentials (<0.6 V)Catalysis likely occurs at CoII/I interface (limited mechanistic details reported)Several electronic absorptions in UV-visible region: oxidation state sensitive photoprobes

Kellet, R.; Spiro, T. G. Inorg. Chem. 1985, 24, 2373-2377.

K. E. Plass, M. A. Filler, J. M. Spurgeon, B. M. Kayes, S. Maldonado, B. S. Brunschwig, H. A. Atwater, N. S. Lewis Adv. Mater 2009, 21, 325-328

Covalently tethered or adsorbed electrocatalyst on a light-absorbing nanostructured cathode stable to (moderately) reducing conditions

Nanostructured anode or adsorbed thin film electrocatalyst stable to strongly oxidizing conditions

NSF Center for Chemical Innovation: CCI SolarInterdisciplinary collaboration focused on

building and understanding a self-contained water splitting system powered by the sun as a

source of clean, sustainable energy

Thermodynamic ConsiderationsBimetallic pathway strongly favored under most conditions

Strong acid and/or less positive E° (Co3+/2+) favor monometallic routeBoth can be competitive under intermediate conditions

Hu, X.; Brunschwig, B. S.; Peters, J. C. J. Am. Chem. Soc. 2007, 129, 8988-8998.

Kellett, R. M.; Spiro, T. G. Inorg. Chem. 1985, 24, 2373-2377.

Chao, T.-H.; Espenson, J. H. J. Am. Chem. Soc. 1978, 100, 129-133.

Tk

IIICoEEeK H

B

'0'0

bi

))/((2exp 2

Tk

IIICoEEIIIIICoEEe

Tk

IIICoEIIIIICoEEeK

B

HH

B

Hmono

))/(())/((exp

))/(())/((2exp

'0'0'0'0

'0'0'0

22

2

Flash-quench laser photolysis studies (pH 5)Data consistent with CoIP generation [Ru(bpy)3]2+ bleach convolutes data (480 nm)

Reductive Quenching: [Ru(bpy)3]1+ from MeODMA

Ongoing Flash-Quench Spectroscopic Studies

Juris; Balzani; Barigeletti; Campagna; Belser; von Zelewsky Coord. Chem. Rev. 1988, 84, 85-277.Hoffman, M. Z.; Bolletta, F.; Moggi, L.; Hug, G. L. J. Phys. Chem. Ref. Data 1989, 18, 219-543.

Spectroelectrochemistry: CoI(TMPyP)┐3+ in CH3CN

SEC-derived difference spectrum (blue; Pt mesh, 0.1 M TBAH in MeCN)UV-vis absorption spectrum of [CoII(TM4PyP)]4+ in MeCN (orange)

CO2 Reduction Catalysts: Very High

Fisher, B.; Eisenberg, R. J. Am. Chem. Soc. 1980, 102, 7361.Bhugun, I.; Lexa, D.; Saveant, J.-M. J. Am. Chem. Soc. 1996, 118, 1769.Grodkowski, J.; Neta, P.; Fujita, E.; Mahammed, A.; Simkhovich, L.; Gross, Z. J. Phys. Chem. A 2002, 106, 4772.Benson, E. E.; Kubiak, C. P.; Sathrum, A. J.; Smieja, J. M. et al. Chem. Soc. Rev. 2009, in press.

Ni(cyclam) TON

Beley, M.; Cellis, J.-P.; Ruppert, R.; Sauvage, J.-P. J. Am. Chem. Soc. 1986, 108, 7641

>106 × CO2:H2O selectivity!!>106 × CO2:H2O selectivity!!

HNNH

NH HN

Ni2+(cyclam)CO2

H2O (pH 5)Ni2+ + CO + H2O

N N

N N

Ni2+

H H

HH

Non-rigidity and Preferred Conformations

Maimon, E.; Zilbermann, I.; Cohen, H.; Kost, D.; van Eldik, R.; Meyerstein, D. Eur. J. Inorg. Chem. 2005, 4997.Soibinet, M.; Dechamps-Olivier, I.; Guillon, E.; Barbier, J.-P.; Aplincourt, M.; Chuburu, F.; Le Baccon, M.; Handel, H. Polyhedron 2005, 24, 143-150.Ikeda, R.; Soneta, Y.; Miyamura, K. Inorg. Chem. Comm. 2007, 10, 590-592.

Catalytic CO2 Reduction: [NiL4]2+

N N

N N

Ni2+

H H

HH

N-substitution induces conformational change: counterintuitive decrease in overpotential, but diminished reactivity

N N

N N

Ni2+

Me H

MeH

Roles of Ligand-N H–Bonding

Fujita, E.; Creutz, C.; Sutin, N. Brunschwig, B. S. Inorg. Chem. 1993, 32, 2657-2662.

Plausible CO2 Reduction Mechanism at Ni

Ethylene-bridged bis(cyclam)Ni24+ lowers overpotential

Modest gain possibly at the expense of selectivityExogenous Lewis bases (e.g., pyridine) lower overpotential

Reduced pyridinium catalysis not observed

CO DehydrogenaseNi(1-CO2) interaction H-bond stabilizationFe for “O” transferJeoung & Dobbek Science 2007, 318, 1461

Targeting Cooperative C–O Cleavage

Single pendant arm donors afford similar results: diminished reactivity and minor decrease in overpotential

Currently moving towards heterobimetallic ligands to facilitate oxygen atom or hydroxyl group transfer in a two electron process

Two main isomerization routes: inversion at Ni–N< N–H deprotonation to Ni–N< (Ni2+ and Ni3+) Ni–N(R)< cleavage to Ni–N< (3° NL< and Ni1+)

Very Long Range Membrance Electron Transfer

Nelson, et al. Nat. Rev. Mol. Cell Bio. 2005, 6, 818.Babini, et al. J. Am. Chem. Soc. 2000, 122, 4532.Winkler, et al. Pure Appl. Chem. 1999, 71, 1753. Gray, et al. Annu. Rev. Biochem. 1996, 65, 537.

Photosystem II

Goal: understand tryptophan electron transfer through OmpA as model

membrane protein for PSII

R60C-Re

W57

Y55W

F40W

Y8W

W7

Y43W

N5C-Ru

10.42 Å

6.98 Å

6.27 Å

8.35 Å3.77 Å

4.46 Å

6.55 Å

Distance (C60 and C5) = 38.4 Å

Currently investigating timescale to confirm reduction at CoII site in [Co(TMPyP)]4+

Reductive quenching of [RuII(bpy)3]2+ promising for CoI(P) generation in situ Efforts to expand range of conditions to pH 5-8 continueBulk photolysis experiments ongoing to confirm H2 evolution via homogeneous catalyst

OmpA

Pautsch, A.; Schulz, G. E. Nat. Struct. Biol. 1998, 5, 1013-1017.

Shih, C.; et al. (2008) Science 320, 1760-1762.

OmpA ET Pathway: Follow the Hopping Hole

Electron transfer rates: hopping mechanism >> tunneling mechanismTryptophan residues provide launch pads and landing sitesOmpA has several well-positioned residues for long range ETET dynamics investigated with time resolved laser flash photolysis