Identification of Active Sites for CO2 Reduction on Graphene-Supported Single-Atom Catalysts

Transition metal- and nitrogen-codoped graphene (referred to as M-N-G, where M is a transition metal) has emerged as an important type of single-atom catalysts with high selectivities and activities for electrochemical CO2 reduction (CO2R) to CO. However, despite extensive previous studies on the catalytic origin, the active site in M-N-G catalysts remains puzzling. In this study, density functional theory calculations and computational hydrogen electrode model is used to investigate CO2R reaction energies on Zn-N-G, which exhibits outstanding catalytic performance, and to examine kinetic barriers of reduction reactions by using the climbing image nudged elastic band method. We find that single Zn atoms binding to N and C atoms in divacancy sites of graphene cannot serve as active sites to enable CO production, owing to *OCHO formation (* denotes an adsorbate) at an initial protonation process. This contradicts the widely accepted CO2R mechanism whereby single metal atoms are considered catalytic sites. In contrast, the C atom that is the nearest neighbor of the single Zn atom (C-NN) is found to be highly active and the Zn atom plays a role as an enhancer of the catalytic activity of the C-NN. Detailed analysis of the CO2R pathway to CO on the C-NN site reveals that *COOH is favorably formed at an initial electrochemical step, and every reaction step becomes downhill in energy at small applied potentials of about -0.3 V with respect to reversible hydrogen electrode. Electronic structure analysis is also used to elucidate the origin of the CO2R activity of the C-NN site.