Mechanistic Understanding on the Role of Cu Species over the CuOx/TiO2 Catalyst for CO2 Photoreduction

摘要

Incorporation of earth-abundant Cu is one of the most important approaches to improve the practicability of TiO_(2) for photoreduction of CO_(2) into value-added solar fuels. However, the molecular insight on the role of Cu is complicated and far from understood. We performed a first principle calculation on the anatase (101) surface modified by a single Cu atom deposited on the surface (Cu_(S)) or doped in the lattice (Cu_(L)). It is demonstrated the Cu_(L) is clearly more stable than the Cu_(S) and could promote the formation of oxygen vacancy (Vo) greatly. The Cu_(S) plays a role of donor, while the Cu_(L) is electronically deficient and becomes a global electron trapper. If a Vo is introduced, the excess electrons would immigrate to the empty gap state of the Cu_(L) and make it half-filled in some case, which implies its metallic characters and improved conductivity; meanwhile, the formation of Ti~(3+) is suppressed. Judging from the adsorption energies, it is the Vo that primarily improves the adsorption of CO_(2) in both linear and bent states, and the Cu_(S) could hardly stabilize CO_(2) more, while the promotion effect of Vo could even be counteracted by the Cu_(L) due to its electronic deficiency. The reduction pathways (CO_(2)* → CO* + O*) show that, with the assistance of the Cu_(S), linear CO_(2) could directly transform into the carbonate-like geometry vertically binding to the surface, and the intermediate configuration of the bent CO_(2) horizontally bridging the Vo could be successfully skipped. Therefore, the barrier of the rate-determining transformation could be lowered from 0.75 to 0.39 eV. Furthermore, it is found the strong adsorption of the produced CO by the Cu_(S) might retard the smooth going of the catalytic process.