Effect of Ag cocatalyst on highly selective photocatalytic CO2 reduction to HCOOH over CuO/Ag/UiO-66 Z-scheme heterojunction

CuO/Ag/UiO-66 catalysts were applied in photocatalytic CO2 reduction, and the optimal catalyst exhibited superior selectivity of HCOOH as high as 95.9%. The reasons for the high selectivity were investigated in depth, especially the effect of Ag cocatalyst. Z-scheme heterojunction was formed between CuO and UiO-66, with Ag as the bridge to promote the charge transfer and separation. The in-situ FT-IR and theoretical calculation results confirmed that CuO is the real site for activation of CO2, while Ag nanoparticles possess strong adsorption capacity for *H protons, which is conducive to the combination of *H with adsorbed *OCO on adjacent CuO to form *OCOH intermediates and generate HCOOH subsequently. This result validated that Ag could promote the generation of two-electron reduction products by controlling the reaction intermediates and the degree of CO2 reduction, and might provide a universal approach to achieve the regulation of product selectivity of photocatalytic CO2 reduction.