Catalysts for Solar Fuels
Bryan D. Stubbert, Bert T. Lai, and Harry B. Gray
Division of Chemistry and Chemical Engineering, California Institute of Technology
NSF Center for Chemical Innovation: CCI Solar
Interdisciplinary collaboration focused on
building and understanding a self-contained
water splitting system powered by the sun as a
source of clean, sustainable energy
Covalently tethered or adsorbed electrocatalyst on a lightabsorbing nanostructured cathode stable to (moderately)
reducing conditions
Ongoing Flash-Quench Spectroscopic Studies
Aqueous Co(P) Electrocatalysts
Selected Nonaqueous H2 Evolution Studies
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
Electrocatalytic H2 evolution occurs near Co2+/1+ couple
Simulations and thermodynamics favor bimetallic pathway
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.
Kellet, R.; Spiro, T. G. Inorg. Chem. 1985, 24, 2373-2377.
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.
Reductive Quenching: [Ru(bpy)3]1+ from MeODMA
Thermodynamic Considerations
Bimetallic pathway strongly favored under most conditions
Strong acid and/or less positive E° (Co3+/2+) favor monometallic route
Both can be competitive under intermediate conditions

0'
0'

 2e EH 2  E (Co( II / I ))
K bi  exp 
kBT


Nanostructured anode or adsorbed thin film electrocatalyst
stable to strongly oxidizing conditions





 e 2 EH0'2  E 0' (Co( III / II ))  E 0 ' (Co( II / I ))
K mono  exp 
k BT


 
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
Very Long Range Membrance Electron Transfer
CO2 Reduction Catalysts: Very High h
in CH3CN



 e EH0'2  E 0' (Co( III / II ))  EH0 '2  E 0 ' (Co( II / I ))
 exp 
k BT

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.
Spectroelectrochemistry:
┐3+
I
Co (TMPyP)
Flash-quench laser photolysis studies (pH 5)
Data consistent with CoIP generation
[Ru(bpy)3]2+ bleach convolutes data (480 nm)



SEC-derived difference spectrum (blue; Pt mesh, 0.1 M TBAH in MeCN)
UV-vis absorption spectrum of [CoII(TM4PyP)]4+ in MeCN (orange)
Non-rigidity and Preferred Conformations
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 continue
Bulk photolysis experiments ongoing to confirm H2 evolution via homogeneous catalyst
Roles of Ligand-N H–Bonding
Plausible CO2 Reduction Mechanism at Ni
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+)
Goal: understand tryptophan electron
transfer through OmpA as model
membrane protein for PSII
Photosystem II
Fujita, E.; Creutz, C.; Sutin, N. Brunschwig, B. S. Inorg. Chem. 1993, 32, 2657-2662.
OmpA
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.
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.
Pautsch, A.; Schulz, G. E. Nat. Struct. Biol. 1998, 5, 1013-1017.
OmpA ET Pathway: Follow the Hopping Hole
6.55 Å
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
W7
Jeoung & Dobbek Science 2007, 318, 1461
8.35 Å
F40W
6.27 Å
>106 × CO2:H2O selectivity!!
Gray and Lewis research groups
R60C-Re
Distance (C60 and C5) = 38.4 Å
Electron transfer rates: hopping mechanism >> tunneling mechanism
Tryptophan residues provide launch pads and landing sites
OmpA has several well-positioned residues for long range ET
ET dynamics investigated with time resolved laser flash photolysis
Team GCEP: Kyle M. Lancaster,
Keiko Yokoyama, A. Katrine
Museth, Rose Bustos
Jillian Dempsey & Lionel Cheruzel
6.98 Å
10.42 Å
Shih, C.; et al. (2008) Science 320, 1760-1762.
Acknowledgements
Bruce Brunschwig & Jay Winkler
Y55W
W57
Currently moving towards heterobimetallic ligands to facilitate oxygen
atom or hydroxyl group transfer in a two electron process
Targeting Cooperative C–O Cleavage
CO Dehydrogenase
Ni(h1-CO2) interaction
H-bond stabilization
Fe for “O” transfer
N5C-Ru
4.46 Å Y43W
3.77 Å
Y8W
Catalytic CO2 Reduction: [NiL4]2+
Single pendant arm donors afford similar results: diminished
reactivity and minor decrease in overpotential
Beley, M.; Cellis, J.-P.; Ruppert, R.; Sauvage, J.-P. J. Am. Chem. Soc. 1986, 108, 7641
N-substitution induces conformational change: counterintuitive
decrease in overpotential, but diminished reactivity
Ethylene-bridged bis(cyclam)Ni24+ lowers overpotential
Modest gain possibly at the expense of selectivity
Exogenous Lewis bases (e.g., pyridine) lower overpotential
Reduced pyridinium catalysis not observed
Additional funding: