Role of reactive oxygen species on the activity of noble metal-doped TiO2 photocatalysts

Highlights: • The potential carcinogenic disinfection by-product DCA is removed by photocatalysis. • DCA degradation was achieved using TiO₂/Pt 0.5 wt% photocatalyst. • Role of O₂⁻, OH_ads, OH_free and h⁺_VB radicals is determined in DCA abatement. • DCA decomposition is conducted mainly by OH_free radicals using TiO₂ as catalyst. • OH_free radicals from O₂⁻ play a key role in DCA removal using TiO₂/NM catalysts. -- Abstract: Modified TiO₂ catalysts are of interest in environmental water remediation since they can lead to efficient electron-hole separation and greatly enhance the photocatalytic properties of TiO₂. Reactive oxygen species (ROS), such as the superoxide radical (O₂⁻), hydroxyl radical (OH), and positive valence band holes (h⁺_VB), have been reported as the main oxidative species involved in photocatalytic degradation processes. In this work, the role of these species using TiO₂, TiO₂/Pt 0.5 wt%, and TiO₂/Ag 10 wt% has been examined in order to clarify the oxidation pathways. For this purpose, the contribution of the main oxidative species was analyzed in the photocatalytic degradation of dichloroacetic acid (DCA) solutions using specific scavengers (benzoquinone, tert-butyl alcohol, and formic acid). Moreover, the hydroxyl radicals were quantitatively determined in order better understand the results. Regardless of the catalyst used, it is concluded that OH radicals are the major reactive species responsible for DCA degradation and no significant degradation is due to O₂⁻ radicals. Nevertheless, different OH generation pathways were found, depending on the nature of the catalysts. Degradation using TiO₂ was conducted mainly via OH radicals generated in the positive holes, while noble metal-doped TiO₂ catalysts generated OH radicals through the transformation of O₂⁻ radicals.