Direct coupling of two inert CO₂ molecules to form a C-C bond on the Cu⁰ atomic interfaces of the nitrogen-doped graphene-supported Cu₄ cluster

The electrochemical reduction reaction of CO2 (CO2RR) to C2+ products is strongly related to the C-C coupling reaction. In general, deoxidation productions of CO2 have been assigned as the active species for the C-C coupling reaction. For example, the direct coupling of two *CO, *CO and *CHO, and two *CHO species are always proposed to be the three main pathways for the formation of the C-C bond in CO2RR. In this case, a direct coupling of two inert CO2 molecules to form a CO2 dimer with strong C-C bond over the Cu-0 atomic interfaces of the Cu4 cluster, which was anchored on a nitrogen-doped graphene support (Cu-4/N(3)GN), has been proposed based on our periodic density functional theory (DFT) calculations. The mechanistic investigation shows that the atomic interface formed by the three Cu-0 species of Cu-4/N(3)GN can simultaneously adsorb two CO2 molecules, and two adsorbed CO2 molecules could be reduced to a CO2-center dot anionic radical with [O = C-center dot=O](-) configuration via an electron-transfer process from the three Cu0 species to the two adsorbed CO2 molecules. Furthermore, the direct coupling of two O-2(-center dot) anionic radicals results in the formation of a C-C bond. The calculated free energy profiles reveal that the direct coupling of two CO2 molecules should be the main reaction relevant to the coupling of *OCHO and CO, *COOH and *CO, and two CO molecules over the catalyst surface. Electronic structural analysis indicates that the bent arrangement of two adsorbed CO2 molecules is a key factor in the determination of the formation of the C-C bond in the CO2 dimer via effective orbital interactions. In addition, the CO2 dimer could be reduced to ethane in the series of electrochemical elementary steps. The high ethane selectivity of the Cu-4/N(3)GN catalyst studied here mainly arises from the strong Cu-O bonding interaction. Regarding the direct coupling of two inert CO2 molecules over the atomic interface of the Cu-4 cluster, these findings would be very useful to guide the search for potential catalysts for the formation of the C-C bond in CO2RR into the subnanometer cluster with various active sites.