Revisiting the Role of Peroxymonosulfate in TiO₂-Mediated Photocatalytic Oxidation: Dependence of Kinetic Enhancement on Target Substrate and Surface Platinization

An enhancing effect of peroxymonosulfate (PMS) on photocatalytic activity has previously been demonstrated, but the research focus was concentrated primarily on the extent to which treatment efficiency was improved. To further comprehend the role of PMS, this study examined the TiO₂ photocatalytic systems in the presence of PMS for the oxidation of diverse organics and showed that the extent of performance enhancement via PMS addition was substrate-dependent. This resulted from the involvement of multiple oxidants owing to the dual role of PMS as an electron acceptor and a radical precursor, confirmed in the open circuit potential shifts and electron paramagnetic resonance spectra. Surface platinization, typically adopted to modulate charge carrier dynamics in photocatalysis, caused the nonphoton-driven degradative pathway, i.e., mediated electron transfer involving PMS as a two-electron acceptor, and promoted the one-electron reduction of PMS to sulfate radicals (SO₄^•-). The contribution of SO₄^•- was assessed based on efficiencies for decarboxylation and hydroxylation, isomer distribution in the hydroxylated intermediates, the quenching effect of alcohols, and net conversion of methanol to formaldehyde. Relying on the selective reactivity of Pt-activated PMS, Pt-TiO₂/PMS outperformed bare TiO₂/PMS in oxidatively treating the binary mixtures of organics and enabled effective organic degradation under periodic light irradiation.