From poison to promoter: The multifunctional role of surface grafted SO42- in enhancing CH3SH photocatalytic removal over Bi2WO6

Odor pollution caused by low-concentration volatile organic sulfur compounds (VOSCs) pose a persistent challenge in air quality management, and their complex interactions with co-existing air pollutants or oxidation products during photocatalytic process remained underexplored. In this work, SO42-, a common particulate species and typical oxidation product of VOSCs, was deliberately introduced via impregnation onto B2WO6 (BWO), and significantly enhanced CH3SH photocatalytic removal was obtained. The improvement followed a volcano-type dependence on sulfate loading, with the optimal 0.48-BWO achieving 78% efficiency under simulated solar irradiation, representing a 26% increase over pristine BWO. Mechanistic investigations revealed SO42- pre-adsorption on BWO was thermodynamically spontaneous, and induced electron accumulation, which markedly facilitated CH3SH adsorption and activation, improved light absorption, accelerated charge separation, and triggered downward band bending. These effects synergistically enhanced the production of •O2- and •OH radicals. More importantly, both pre-adsorbed SO42- and in situ-formed sulfate species could be dynamically activated to generate surface-bound •SO4- radicals, which participated in reactions in a surface-confined manner within the immediate vicinity of the catalyst surface. While •O2- served as the dominant reactive species in both catalysts, secondary contributions came from •SO4- in 0.48-BWO and •OH in BWO. In-situ diffuse reflectance infrared Fourier transform spectra and density functional theory calculations further revealed that pre-adsorbed SO42- preferentially lowered the energy barriers of the rate-limiting *CH3SH dissociation step and its subsequent conversion into key intermediates like HCOOH, CH3COOH and S0, thereby kinetically steering the entire oxidation pathway toward deep mineralization. This work overturns the conventional perception of sulfate as a catalyst poison and opens new avenues for designing efficient and durable photocatalysts for VOSCs purification.