C–C Bond Formation during Electrochemical CO₂Reduction on Pristine Cu(100) Unlikely to Involve Adsorbed CO at Any Potential

Formation of hydrocarbons containing two or more carbon atoms (C₂₊) during heterogeneous electrochemical CO and CO₂ reduction (ECOR and ECO₂R) only occurs, among pure metals, on Cu electrodes. Moreover, the activity and selectivity is facet dependent, with Cu(100) generally preferentially forming ethylene over methane. Previously, we found via quantum-mechanics-based modeling that, unlike standard density functional theory, more accurate correlated wavefunction methods predict that non-electroactive coupling pathways involving two adsorbed COs (*CO) or a *CO and a *COH to form C–C bonds on Cu(100) are kinetically inhibited, with the former also thermodynamically unfavorable. Here, we extend that embedded complete active space second order perturbation theory (ECASPT2) study, further showing that electrochemical coupling of two *COs to form an anionic dimer [OC*–*CO]^(1+δ)–, followed by protonation to form [OC*–*COH]^δ−, is not kinetically competitive with the reduction of *CO to *COH at relevant ECO/CO₂R potentials. Our simulations therefore suggest that the ability of Cu(100) to electrochemically synthesize C₂₊ molecules from CO and CO₂ is unlikely to be via *CO, at least on pristine Cu(100). Instead, hydrogenated CO species (*COH, *CH_xOH, or *CH_x) are most likely to be the key intermediates in C–C bond formation.