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*-*COHδ-, 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, *CHxOH, or *CHx) are most likely to be the key intermediates in C-C bond formation.
Martirez et al. (Thu,) studied this question.