15 Heterogeneous H2 Formation Catalyzed by Economical Molecular Coordination Compounds Anchored to High-Surface-Area Metal-Oxide Thin Films

Tuesday, June 23, 2009: 11:15 AM
2101/2103/2105 (Marriott Inn and Conference Center University Of MD University College)
Shane Ardo , Department of Chemistry, Johns Hopkins University, Baltimore, MD
Siah Hong Tan , Department of Chemistry, Johns Hopkins University, Baltimore, MD
Gerald J. Meyer , Department of Chemistry, Johns Hopkins University, Baltimore, MD
There is general agreement within the scientific community that climate change promoted by anthropogenic greenhouse gases seriously threatens the sustainability of life on Earth. Immediate steps toward worldwide implementation of carbon-neutral power are necessary. The required power input will undoubtedly come from renewable sources; however, the intermittency and locality of most preclude their direct use. By mimicking photosynthesis, an effective solution can be envisioned whereby transient renewable power is transduced into energetic reducing equivalents that are then stored in stable chemical bonds. Whether or not H2 is the ultimate fuel or an intermediate, there is no mistaking the benefits of its relatively simplistic overall formation reaction. With that being said, efficient carbon-neutral H2 production often requires bulk platinum catalysts, even though reaction specificity and optimization are often better achieved via molecular catalysis. In this study, two classes of cobalt-based molecular coordination catalysts are investigated for H2 production when bound to high-surface-area electrodes capable of storing multiple reducing equivalents. Synthesis of a novel glyoxime-based catalyst from highly abundant tartaric acid was necessary in order to achieve surface binding to nanocrystalline thin films. Overall two-electron reduction to the Co(I) state poises the compounds for controlled, inner-sphere reactivity via a concerted proton-coupled two-electron-transfer mechanism. Towards this end, the Co(I) state of the compounds is accessible at moderate applied potentials, i.e. as favorable as -90 mV vs. NHE. By binding said catalysts to anatase TiO2 an additional attribute often results, tunable non-Nernstian redox behavior. Relationships to account for this phenomenon have been developed based on the Gouy-Chapman-Stern model. This feature will be employed to fine-tune the reduction potentials of each catalyst, without demanding synthetic modification, in order to maximize the rate of H2 production as assessed by spectroscopic chronoamperometry and/or gas chromatography with a thermal-conductivity detector (GC-TCD).