Insights into Nonelectroactive C-C Bond Formation on Cu(100) during Electrochemical CO₂ Reduction from...

Carbon-carbon (C-C) bond formation is necessary for hydrocarbon (and oxygenate) synthesis beyond methane (and formate/formic acid) during electrochemical CO and CO₂ reduction (ECOR and ECO₂R). Cu has notable ability to form hydrocarbons compared to other pure metals. In particular, the (100) facet of face-centered cubic Cu forms ethylene competitively with H₂ and methane during both ECOR and ECO₂R. Past simulations based on density functional theory (DFT) with standard exchange-correlation functional approximations predict fast nonelectroactive C-C bond formation channels involving adsorbed (*) CO together with another *CO, formyl (*CHO), or hydroxy-methylidyne (*COH), forming OC*-*CO, OC*-CHO*, and OC*-*COH, respectively. Such simulations support the prevailing hypothesis that emergence of C₂ products is kinetically determined at the early stages of the reduction chemistry. Here we show, via simulations with more accurate many-body, i.e., "correlated", wavefunction theory (enabled by an embedding scheme), that the coupling of *CO with a *CO or a *COH (previously predicted at the same level of theory to kinetically dominate over *CHO as the one-electron reduction product of *CO) is highly activated (kinetically impeded), with free energy barriers >1 eV, in contradiction to previous DFT-based simulations. Intriguingly, we find that the coupling of two adjacent *COHs incurs only a small barrier (<0.3 eV) and is exoergic (< -1 eV); however, given the predicted low surface mobility of *COH, the emergence of HOC*-*COH is also improbable, at least at low *COH coverages. We therefore conclude that it is highly unlikely for *CO to participate in nonelectroactive C-C bond formation on pristine Cu(100), contrary to conventional wisdom, and that the energetically favorable *COH dimerization may occur only after substantial buildup of *COH on the surface.