Silver–Gold Bimetal-Loaded TiO2 Photocatalysts for CO2 Reduction

Bimetallic Au–Ag/TiO2 materials have been investigated as promising catalysts for the photocatalytic conversion of CO2 with water as the reducing agent. The deposition of both metals on the TiO2 surface was performed using a deposition method in independent steps. Characterization studies reveal a parallel distribution of both metals, with a preferential surface exposure of silver nanoparticles (NPs). The close contact between metal NPs and TiO2 allows an efficient charge transfer between both phases upon excitation, which results in a distinct reaction mechanism under ultraviolet (UV) and visible illumination. Under UV light, bimetallic catalysts are able to shift the reaction selectivity toward methane, compared to the solely syngas (CO + H2) production over bare TiO2. This effect is ascribed to the capacity of the metal NPs to extract photoexcited electrons. Under visible light, this selectivity modification is not observed, whereas bimetallic catalysts are active toward syngas production, significantly improving the residual activity of TiO2. Under these conditions, an additional strong interfacial interaction between metal NPs and TiO2 allows the photoexcitation of electrons from surface states in the band gap, leading to the occurrence of near-surface electron–hole pairs.