| Summary: | CO[subscript 2] reduction in aqueous electrolytes suffers efficiency losses because of the simultaneous reduction of water to H[subscript 2]. We combine in situ surface-enhanced IR absorption spectroscopy (SEIRAS) and electrochemical kinetic studies to probe the mechanistic basis for kinetic bifurcation between H[subscript 2] and CO production on polycrystalline Au electrodes. Under the conditions of CO[subscript 2] reduction catalysis, electrogenerated CO species are irreversibly bound to Au in a bridging mode at a surface coverage of ∼0.2 and act as kinetically inert spectators. Electrokinetic data are consistent with a mechanism of CO production involving rate-limiting, single-electron transfer to CO[subscript 2] with concomitant adsorption to surface active sites followed by rapid one-electron, two-proton transfer and CO liberation from the surface. In contrast, the data suggest an H[subscript 2] evolution mechanism involving rate-limiting, single-electron transfer coupled with proton transfer from bicarbonate, hydronium, and/or carbonic acid to form adsorbed H species followed by rapid one-electron, one-proton, or H recombination reactions. The disparate proton coupling requirements for CO and H[subscript 2] production establish a mechanistic basis for reaction selectivity in electrocatalytic fuel formation, and the high population of spectator CO species highlights the complex heterogeneity of electrode surfaces under conditions of fuel-forming electrocatalysis. MIT International Science and Technology Initiatives MISTI (Hayashi Seed Fund) United States. Air Force Office of Scientific Research (Award FA9550-15-1-0135) Massachusetts Institute of Technology. Department of Chemistry National Science Foundation (U.S.). Graduate Research Fellowship Program
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