Surface-adsorbed CO is generally considered a reactive on-pathway intermediate in the aqueous electrochemical reduction of CO2 on Cu electrodes. Though CO can bind to a variety of adsorption sites (e.g., atop or bridge), spectroscopic studies of the Cu/electrolyte contact have mostly been concerned with atop-bound CO. Using surface-selective infrared (IR) spectroscopy, we have investigated the reactivities and coverages of atop- and bridge-bound CO on a polycrystalline Cu electrode in contact with alkaline electrolytes. We show here that (1) a fraction of atop-bound CO converts to bridge-bonded CO when the total CO coverage drops below the saturation coverage and (2) unlike atop-bound CO, bridge-bonded CO is an unreactive species that is not reduced at a potential of -1.75 V vs SHE. Our results suggest that bridge-bonded CO is not an on-pathway intermediate in CO reduction. Using density functional theory (DFT) calculations, we further reveal that the activation barrier for the hydrogenation of bridge-bonded CO to surface-adsorbed formyl on Cu(100) is higher than that of the reduction of atop-bound CO, in qualitative agreement with our experimental findings. The possible modulation of the catalytic properties of the interface by the electrochemically inert bridge-bonded CO population should be considered in future studies involving CO2 or CO reduction on Cu under alkaline conditions.
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