What Is the Best Size of Subnanometer Copper Clusters for CO2 Conversion to Methanol at Cu/TiO2 Interfaces? A Density Functional Theory Study

The activation and hydrogenation of CO 2 at the Cu/TiO 2 interfaces that are formed by depositing subnanometer Cu n ( n = 1–8) clusters on TiO 2 (110) surfaces have been systematically investigated using density functional theory calculations. The most stable structures with a bent CO 2 δ− configuration at the Cu n /TiO 2 interfaces are determined, which indicate that the binding strength of CO 2 on the Cu n /TiO 2 (110) surface can be tuned by controlling the size of the deposited Cu cluster. It is interesting that the copper cluster with a specific size of Cu 4 exhibits a distinct preference for CO 2 activation, and the strongest binding interaction between CO 2 and Cu 4 /TiO 2 (110) is mainly ascribed to the formation of the strong Cu–C and Ti–O adsorption bonds. The reaction mechanisms of CO 2 conversion to CH 3 OH at the Cu 4 /TiO 2 (110) interface via the formate and the reverse water gas shift (RWGS) + CO-hydrogenation pathways are further investigated by microkinetic simulations. The production of CH 3 OH over Cu 4 /TiO 2 is mainly via the RWGS pathway to yield CO followed by the formation of H 3 CO* as the most stable intermediate, while the formate pathway is not efficient enough because of the higher apparent activation energy of CH 3 OH generation and the overly strong binding of HCOO* species at the interface. Compared with other Cu n /TiO 2 interfaces, the TiO 2 (110) surface-supported size-selected Cu 4 cluster exhibits the highest CO 2 hydrogenation activity. The findings obtained in the present work provide useful insight to design Cu/oxide interfaces with high activity toward methanol synthesis from CO 2 hydrogenation by precisely controlling the size of copper clusters.