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

Abstract

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 Cu⁰ atomic interfaces of the Cu₄ cluster, which was anchored on a nitrogen-doped graphene support (Cu₄/N₃GN), has been proposed based on our periodic density functional theory (DFT) calculations. The mechanistic investigation shows that the atomic interface formed by the three Cu⁰ species of Cu₄/N₃GN can simultaneously adsorb two CO₂ molecules, and two adsorbed CO₂ molecules could be reduced to a CO₂⁻˙ anionic radical with [OC˙O]⁻ configuration via an electron-transfer process from the three Cu⁰ species to the two adsorbed CO₂ molecules. Furthermore, the direct coupling of two CO₂⁻˙ 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 Cu₄/N₃GN 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 Cu₄ 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.