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

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) < He-pretreated < H2-pretreated. Compared with unpretreated TiO2 (P25), the H2-pretreated Cu/TiO2 demonstrated a 10-fold and 189-fold enhancement in the production of CO and CH4, respectively. This significant enhancement was mainly attributed to the synergy of the following two factors: (1) the formation of surface defect sites promoting CO2 adsorption and subsequent charge transfer to the adsorbed CO2; (2) the existence of Cu+/Cu0 couples that facilitate electron and hole trapping at different sites.