Effects of inorganic oxidants on kinetics and mechanisms of WO₃-mediated photocatalytic degradation

This study evaluates the capacity of various inorganic oxidants (IO₄^-, HSO₅^-, S₂O₈^2-, H₂O₂, and BrO₃^-) to act as alternative electron acceptors for WO₃-mediated photocatalytic oxidation. Combination with IO₄^- drastically increased the rate of photocatalytic degradation of 4-chlorophenol by WO₃, while the other oxyanions only negligibly improved the photocatalytic activity. The extent of the photocatalytic performance enhancement in the presence of inorganic oxidants correlated well with the efficiencies for: (1) hydroxylation of benzoic acid as an OH probe, (2) dechlorination of dichloroaceate as a hole scavenger, and (3) water oxidation with O₂ evolution. The results suggest that the promoted charge separation primarily causes kinetic enhancement in photocatalytic degradation using the WO₃/IO₄^- system. In marked contrast to the substrate-dependent activity of the photochemically activated IO₄^- (generating selective IO₃), the efficiency of the WO₃/IO₄^- system for photocatalytic degradation did not sensitively depend on the type of target organic compound, which implies the existence of a minor contribution of the photocatalytic reduction pathway associated with the production of IO₃ as a secondary oxidant. On the other hand, the insignificant inhibitory effect of methanol as an OH quencher may reveal the possible involvement of SO₄^- in the improved photocatalytic activity of the WO₃/HSO₅^- system. The alternative use of platinized WO₃, where the interfacial electron transfer occurs in a concerted step, achieved a highly accelerated photocatalytic oxidation in the presence of HSO₅^- and polyoxometalates as electron scavengers. In particular, the surface loading of nanoscale platinum appeared to retard the reaction route for SO₄^- generation associated with a one-electron transfer.