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

Cu/TiO2 composites are extensively studied for photocatalytic reduction of CO2 with H2O, but the roles of Cu species (Cu2+, 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/TiO2 composites under CO2 photoreduction environment. We found that the air-calcined Cu/TiO2 (Cu/Ti(air)) surface was dominated by isolated Cu2+ sites, while the one post-treated with H-2 at 200 degrees 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 CO2 photoreduction to CO, mainly resulting from the synergy of Cu+, OH groups, and V-o that could scavenge holes to enhance electron transfer, provide CO2 adsorption sites, and facilitate the activation and conversion of the adsorbed CO2 (HCO3- and CO2). 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 Cu2+ changed the surrounding environments of Cu by increasing the coordination number; (2) thermal treatment by Hy 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 Cu2+ thus leading to a higher and more stable CO2 reduction activity. Findings in this work and the application of in situ XAS technique will help develop a more efficient photocatalyst for CO2 photoreduction and advance the understanding of the reaction mechanism and surface chemistry.