Raman Evidence for the Mechanism of Enhanced C–C Coupling during CO 2 RR on CuSn x Bimetallic Electrocatalysts at Dilute Sn Levels

The electrochemical carbon dioxide reduction reaction (CO 2 RR) presents a promising method for converting CO 2 into valuable fuels and chemicals, but requires electrocatalysts with high selectivity. Here, we investigate CuSn x catalysts with systematically varied Sn contents to elucidate composition-selectivity relationships. Electrolyzer testing reveals that 3%Sn-doped Cu electrocatalyst selectively produces C 2 products, primarily ethanol and ethylene, whereas higher Sn contents (>3%) shift selectivity toward C 1 products, predominantly formic acid. X-ray photoelectron spectroscopy suggests the coexistence in similar proportions of near neutral (Cu) and partial positive (Cu δ + ) sites at low Sn contents. On the other hand, at high Sn contents, surface Cu skews towards monolithically Cu 2+ states. From In-situ surface-enhanced Raman spectroscopy, at 3%Sn, abundant *COOH absorbed via carbon is detected that leads to detected high *CO coverage with electrophilicity imbalance from absorbing at Cu and Cu δ + . Two electronically dissimilar *COs then promote *CO*CO dimerization favoring C 2 products. At 50% and 80% Sn, HCOO* intermediate adsorbed via oxygen is detected instead, leading to formic acid as the major product upon proton transfer. The findings experimentally validate prior computational density functional theory conclusions and provide empirical insight into the role of Sn doping in tuning the catalytic behavior of Cu for CO 2 RR.