Mechanistic Study of CO2 Photoreduction with H2O on Cu/TiO2 Nanocomposites by in Situ X-ray Absorption and Infrared Spectroscopies

Cu/TiO 2 composites are extensively studied for photocatalytic reduction of CO 2 with H 2 O, but the roles of Cu species (Cu 2+, Cu +, or Cu 0 ) is not well understood, and the photocatalyst deactivation mechanism is seldom addressed. In this work, we have employed in situ techniques, i.e., X-ray absorption spectroscopy (XAS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), to explore the surface chemistry of Cu/TiO 2 composites under CO 2 photoreduction environment. We found that the air-calcined Cu/TiO 2 (Cu/Ti(air)) surface was dominated by isolated Cu 2+ sites, while the one post-treated with H 2 at 200 °C (Cu/Ti(H 2 )) was rich in Cu + and oxygen vacancy (V O ). Cu/Ti(H 2 ) showed more than 50% higher activity than Cu/Ti(air) for CO 2 photoreduction to CO, mainly resulting from the synergy of Cu +, OH groups, and V O that could scavenge holes to enhance electron transfer, provide CO 2 adsorption sites, and facilitate the activation and conversion of the adsorbed CO 2 (HCO 3 – and CO 2 – ). Meanwhile, the consumption of OH groups and Cu + active sites by holes may result in the deactivation of Cu/Ti(H 2 ). Moreover, in situ XAS results directly demonstrated that (1) the photoinduced oxidation of Cu + to Cu 2+ changed the surrounding environments of Cu by increasing the coordination number; (2) thermal treatment by H 2 could not fully recover the OH and Cu + sites to their original states; and (3) adding hole scavengers (e.g., methanol) maintained or even increased the more active Cu + species from the photoreduction of Cu 2+, thus leading to a higher and more stable CO 2 reduction activity. Findings in this work and the application of in situ XAS technique will help develop a more efficient photocatalyst for CO 2 photoreduction and advance the understanding of the reaction mechanism and surface chemistry.