CO 2 reduction in aqueous electrolytes suffers efficiency losses because of the simultaneous reduction of water to H 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 2 and CO production on polycrystalline Au electrodes. Under the conditions of CO 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 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 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 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.carbon dioxide reduction | catalyst selectivity | in situ spectroscopy | proton-coupled electron transfer P roduct selectivity is a principal design consideration for the development of practical catalysts. Catalyst selectivity is dictated by (i) the relative free energy barriers for progress along competing reaction pathways and (ii) the relative rates of reactant delivery to active sites (1). Enzymes fine tune these parameters with exquisite precision to achieve selectivity (2). Nature augments the coordination environment of metallocofactor active sites to optimize the binding strengths of reaction partners and preorganizes reaction participants toward low-barrier pathways (3). Additionally, many active sites reside at the terminus of molecular channels that gate the coordinated delivery of substrates (4, 5) required for selective transformations. Efforts to prepare artificial catalysts with product selectivities rivaling that of nature require a detailed understanding of these factors.Currently, our understanding of how to systematically modulate selectivity in heterogeneous catalysts remains poor (6-8). Unlike (bio)molecular catalysts, which ideally consist of a uniform ensemble of active sites, heterogeneous catalysts consist of a nonuniform distribution of surface sites (9), requiring an understanding of which are active and which are dormant. The surface site distribution is strongly dependent on the surface nanostructure, oxidation state, and degree of restructuring (7). Superimposed on this distribution are the rate-limiting elementary reaction steps that dictate kinetic branching ratios at surface active sites (1...

Rational design of selective CO2-to-fuels electrocatalysts requires direct knowledge of the electrode surface structure during turnover. Metallic Cu is the most versatile CO2-to-fuels catalyst, capable of generating a wide array of value-added products, including methane, ethylene, and ethanol. All of these products are postulated to form via a common surface-bound CO intermediate. Therefore, the kinetics and thermodynamics of CO adsorption to Cu play a central role in determining fuel-formation selectivity and efficiency, highlighting the need for direct observation of CO surface binding equilibria under catalytic conditions. Here, we synthesize nanostructured Cu films adhered to IR-transparent Si prisms, and we find that these Cu surfaces enhance IR absorption of bound molecules. Using these films as electrodes, we examine Cu-catalyzed CO2 reduction in situ via IR spectroelectrochemistry. We observe that Cu surfaces bind electrogenerated CO, derived from CO2, beginning at −0.60 V vs RHE with increasing surface population at more negative potentials. Adsorbed CO is in dynamic equilibrium with dissolved 13CO and exchanges rapidly under catalytic conditions. The CO adsorption profiles are pH independent, but adsorbed CO species undergo a reversible transformation on the surface in modestly alkaline electrolytes. These studies establish the potential, concentration, and pH dependencies of the CO surface population on Cu, which serve to maintain a pool of this vital intermediate primed for further reduction to higher order fuel products.

Despite the significance of phosphate buffer solutions in (bio)electrochemistry, detailed adsorption properties of phosphate anions at metal surfaces remain poorly understood. Herein, phosphate adsorption at quasi-Au(111) surfaces prepared by a chemical deposition technique has been systematically investigated over a wide range of pH by surface-enhanced infrared absorption spectroscopy in the ATR configuration (ATR-SEIRAS). Two different pH-dependent states of adsorbed phosphate are spectroscopically detected. Together with DFT calculations, the present study reveals that pKa for adsorbed phosphate species at the interface is much lower than that for phosphate species in the bulk solution; the dominant phosphate anion, H2PO4(-) at 2 < pH < 7 or HPO4(2-) at 7 < pH < 12, undergoes deprotonation upon adsorption and transforms into the adsorbed HPO4 or PO4, respectively. This study leads to a conclusion different than earlier spectroscopic studies have reached, highlighting the capability of the ATR-SEIRAS technique at electrified metal-solution interfaces.

We investigated lattice strain on alloyed surfaces using ∼10 nm core–shell nanoparticles with controlled size, shape, and composition.