Bridge Sites of Au Surfaces Are Active for Electrocatalytic CO₂ Reduction

Prior in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) studies of electrochemical CO₂ reduction catalyzed by Au, one of the most selective and active electrocatalysts to produce CO from CO₂, suggest that the reaction proceeds solely on the top sites of the Au surface. This finding is worth updating with an improved spectroelectrochemical system where in situ IR measurements can be performed under real reaction conditions that yield high CO selectivity. Herein, we report the preparation of an Au-coated Si ATR crystal electrode with both high catalytic activity for CO₂ reduction and strong surface enhancement of IR signals validated in the same spectroelectrochemical cell, which allows us to probe the adsorption and desorption behavior of bridge-bonded *CO species (*CO_B). We find that the Au surface restructures irreversibly to give an increased number of bridge sites for CO adsorption within the initial tens of seconds of CO₂ reduction. By studying the potential-dependent desorption kinetics of *CO_B and quantifying the steady-state surface concentration of *CO_B under reaction conditions, we further show that *CO_B are active reaction intermediates for CO₂ reduction to CO on this Au electrode. At medium overpotential, as high as 38% of the reaction occurs on the bridge sites.