Active Cu0-Cuσ+ Sites for the Hydrogenation of Carbon-Oxygen Bonds over Cu/CeO2 Catalysts

CeO2-supported copper species have been reported as an active catalyst for the hydrogenation of carbon-oxygen bonds (CO, CO2, furfural, esters, etc.). However, the identification of active sites remains challenging. Herein, we prepared a series of rod-shaped ceria-supported copper catalysts with different copper sizes (single-atom, 1.4 nm nanoclusters, 3.0 nm and 6.8 nm nanoparticles) and applied them for methyl acetate (MA) hydrogenation. The structure and chemical environment of copper species were detected, and the surface Cu-0 and Cu sigma+ species and defects (oxygen vacancy and M- [O-x]-Ce solid solution) were quantitatively measured. To identify the active sites for MA hydrogenation, we also prepared contrast samples with increased surface defects or with reduced Cu-0-Cu sigma+ species. It is demonstrated that the Cu-0-Cu sigma+ species rather than oxygen vacancies or M-[O-x]-Ce solid solution are the primary active sites for MA hydrogenation. From the results of in situ experiments and various chemisorption and density functional theory calculations, the Cu-0-Cu sigma+ interface located at the surface Cu deposits is evidenced to play the key role in enhancing the adsorption and activation of MA. The turnover frequency of Cu/CeO2 catalysts for MA hydrogenation is linearly increased with the increase of the Cu-0-Cu sigma+ interfacial perimeter. This insight into active sites for carbon-oxygen bond hydrogenation may provide guidance for high-performance catalyst design.