What Is the Best Size of Subnanometer Copper Clustersfor CO 2 Conversion to Methanol at Cu/TiO 2 Interfaces?A Density Functional Theory Study

The activation and hydrogenation of CO2 at the Cu/TiO2 interfaces that are formed by depositing subnanometer Cu-n (n = 1-8) clusters on TiO2(110) surfaces have been systematically investigated using density functional theory calculations. The most stable structures with a bent CO2 delta- configuration at the Cu-n/TiO2 interfaces are determined, which indicate that the binding strength of CO2 on the Cu-n/TiO2 (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 CO2 activation, and the strongest binding interaction between CO2 and Cu-4/TiO2 (110) is mainly ascribed to the formation of the strong Cu-C and Ti-O adsorption bonds. The reaction mechanisms of CO2 conversion to CH3OH at the Cu-4/TiO2 (110) interface via the formate and the reverse water gas shift (RWGS) + CO-hydrogenation pathways are further investigated by microkinetic simulations. The production of CH3OH over Cu-4/TiO2 is mainly via the RWGS pathway to yield CO followed by the formation of H3CO* as the most stable intermediate, while the formate pathway is not efficient enough because of the higher apparent activation energy of CH3OH generation and the overly strong binding of HCOO* species at the interface. Compared with other Cu-n/TiO, interfaces, the TiO2 (110) surface-supported size-selected Cu(4 )cluster exhibits the highest CO2 hydrogenation activity. The findings obtained in the present work provide useful insight to design Cu/oxide interfaces with high activity toward methanol synthesis from CO2 hydrogenation by precisely controlling the size of copper clusters.