Direct Observation of Surface-Bound Intermediates in CO/CO2 Reduction on a Polycrystalline Cu Electrode Using Broad-Band SFG Spectroscopy

Interface-specific detection of adsorbed key intermediates and product species on the Cu electrode is critical for unraveling the electrocatalytic mechanism of CO2 reduction. In this study, Cu-catalyzed electrocatalysis of CO and CO2 reduction (COR and CO2R, respectively) is investigated using in situ broad-band sum-frequency generation (BB-SFG) spectroscopy. By analysis of the potential-dependent SFG spectra of surface-bound CO (*CO) and possible intermediates in the C–H stretching vibrational range, valuable insights into the electrocatalysis of CO2R are obtained. The SFG spectra of the C–H vibrations reveal two distinct adsorption configurations of surface-bound ethoxy (“standing” and “lying”) on Cu, which provides new molecular evidence to support the presence of C2 intermediates with the atoms of O and C at both ends attached to the Cu surface in the pathway leading to ethanol formation. Moreover, we observed distinct potential-dependent SFG features of *CO in COR and CO2R, while the ethoxy intermediates and product distributions are nearly identical, suggesting that the dimerization of *CO might not be the rate-determining step for the C–C coupling to form ethanol. With these findings, we present a mechanistic scheme for the formation of C1 and C2 products with surface-bound CO and ethoxy intermediates for the formation of ethanol.