Rate-limiting step in environmental Bi2WO6 solar light photoelectrocatalysis

Abstract

Bi2WO6 / solar light photocatalysis was used to probe the rate - limiting step (rls) on this Aurivillius oxide. An unusually large solvent isotope effect has been measured during glucose oxidation (kH2O/kD2O around 7). The large value come from the contribution of the equilibrium L2O ⇄ L + + OL− previous to the rls and the kinetic one proton transfer involved at the transition state. The rls is the 1e − Volmer type electrosorption of H+ on the Bi2WO6 surface (Bi2WO6/Bi2WO6‐H, −0.6 V vs. NHE). Subsequently, O2 reduction to L2O, as driving force, occurs at the catalyst conduction band (CB). Kinetic constant measurements as a function of the solution pH, photoluminescence spectroscopy in presence of different electron acceptor (O2, H2O2 and S2O8 2- ) and electrochemical experiments at diverse type of irradiation (dark, sunlight: UV-Vis and Vis), support these results. In fact, positive conduction band potential of Bi2WO6 (+0.5 V vs. NHE) establishes the difference with other semiconductors where oxidation (L2O/L+ , OL•) instead of reduction has been proposed as rls in photo - oxidation. Strategies to enhance environmental photocatalysis by matching the redox assistance at the semiconductor CB with the appropriate photon flux have been envisaged.