Tailoring Cu valence and oxygen vacancy in Cu/TiO2 catalysts for enhanced CO2 photoreduction efficiency

[Display omitted] ► Cu/TiO2 photocatalysts are fabricated for CO2 photoreduction with water vapor. ► Cu valence and oxygen vacancy are tailored by thermal treatment in He or in H2. ► Photo-activity in the order: H2-pretreated>He pretreated>unpretreated. ► Mixed Cu+/Cu0 promote electron–hole separation more effectively than Cu+ and Cu2+. ► CO2− species is a CO2 reduction intermediate according to in situ DRIFTS. The incorporation of Cu species in TiO2 photocatalysts is critical in photocatalytic CO2 reduction to fuels, but the effect of Cu valence is poorly understood. In this work, Cu/TiO2 (P25) nanoparticle catalysts were prepared by a simple precipitation and calcination method. The as-prepared Cu/TiO2 sample was dominated by Cu2+ species. Thermal pretreatment of the as-prepared samples in He and H2 atmosphere resulted in the transition to a surface dominated by Cu+ and mixed Cu+/Cu0, respectively, confirmed by in situ X-ray photoelectron spectroscopy (XPS) and diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS) analyses. These thermal pretreatments in reducing atmospheres also induced the formation of defect sites such as oxygen vacancies and Ti3+. The various Cu/TiO2 catalysts were tested in CO2 photoreduction with water vapor under simulated solar irradiation, and their activities were in the order of as-prepared (unpretreated)