Catalytic activity of photocharged binary TiO₂ and WO₃ membrane filters: Effect of Al₂O₃ interlayer on direct vs. mediated electron transfers

One of the challenges facing the practical application of photocatalysis is the termination of catalytic reactions in the absence of irradiation. This study synthesizes photo-charged TiO₂/WO₃ (TW) and TiO₂/Al₂O₃/WO₃ (TAW) membrane filters and compares their dark catalytic activities for Cr(VI) reduction and As(III) oxidation without any chemical reagent. The Cr(VI) reduction via direct electron transfer is 13-fold enhanced when TW is replaced with TAW. The time-resolved photoluminescence reveals that this kinetic enhancement is attributed to the prolonged lifetime of the electrons stored within the charged TAW. Nevertheless, the As(III) oxidation via mediated electron transfer with O₂^•− is significantly inhibited with the charged TAW. Alumina effectively offsets the negative surface charges of the TW, influencing the adsorption of both As(III) and As(V) and interfering with the O₂ reduction. This leads to the inhibited O₂ reduction and the retarded production of the reactive oxygen species with the photo-charged TAW.