1
Catalysts for Solar Fuels Bryan D. Stubbert, Bert T. Lai, and Harry B. Gray Division of Chemistry and Chemical Engineering, California Institute of Technology Selected Nonaqueous H 2 Evolution Studies Electrocatalytic H 2 evolution occurs near Co 2+/1+ couple Simulations 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. Acknowledgemen ts Team 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 H 2 yield) at moderate overpotentials (<0.6 V) Catalysis likely occurs at Co II/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 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 Thermodynamic Considerations Bimetallic pathway strongly favored under most conditions Strong acid and/or less positive E° (Co 3+/2+ ) favor monometallic route Both 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. T k I II Co E E e K H B ' 0 ' 0 bi )) / ( ( 2 exp 2 T k I II Co E E II III Co E E e T k I II Co E II III Co E E e K B H H B H mono )) / ( ( )) / ( ( exp )) / ( ( )) / ( ( 2 exp ' 0 ' 0 ' 0 ' 0 ' 0 ' 0 ' 0 2 2 2 Flash-quench laser photolysis studies (pH 5) Data consistent with Co I P 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: Co I (TMPyP) ┐3+ in CH 3 CN SEC-derived difference spectrum (blue; Pt mesh, 0.1 M TBAH in MeCN) UV-vis absorption spectrum of [Co II (TM 4 PyP)] 4+ in MeCN (orange) CO 2 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 >10 6 × CO 2 :H 2 O selectivity!! HN NH NH HN Ni 2+ (cyclam) CO 2 H 2 O (pH 5) Ni 2+ + CO + H 2 O N N N N Ni 2+ H H H H 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 CO 2 Reduction: [NiL 4 ] 2+ N N N N Ni 2+ H H H H N-substitution induces conformational change: counterintuitive decrease in overpotential, but diminished reactivity N N N N Ni 2+ Me H Me H Roles of Ligand-N H–Bonding Fujita, E.; Creutz, C.; Sutin, N. Brunschwig, B. S. Inorg. Chem. 1993, 32, 2657-2662. Plausible CO 2 Reduction Mechanism at Ni Ethylene-bridged bis(cyclam)Ni 2 4+ lowers overpotential Modest gain possibly at the expense of selectivity Exogenous Lewis bases (e.g., pyridine) lower overpotential Reduced pyridinium catalysis not observed CO Dehydrogenase Ni( 1 -CO 2 ) interaction H-bond stabilization Fe for “O” transfer Jeoung & 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< (Ni 2+ and Ni 3+ ) Ni–N(R)< cleavage to Ni–N< (3° N L < and Ni 1+ ) 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 Å 3.77 Å 4.46 Å 6.55 Å Distance (C60 and C5) = 38.4 Å Currently investigating timescale to confirm reduction at Co II site in [Co(TMPyP)] 4+ Reductive quenching of [Ru II (bpy) 3 ] 2+ promising for Co I (P) generation in situ Efforts to expand range of conditions to pH 5-8 continue Bulk photolysis experiments ongoing to confirm H 2 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 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
