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

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 (O-2(center dot-)), hydroxyl radical (center dot 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, tertbutyl 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 center dot OH radicals are the major reactive species responsible for DCA degradation and no significant degradation is due to O-2(center dot-)-radicals. Nevertheless, different center dot OH generation pathways were found, depending on the nature of the catalysts. Degradation using TiO2 was conducted mainly via center dot OH radicals generated in the positive holes, while noble metal-doped TiO2 catalysts generated center dot OH radicals through the transformation of O-2(center dot-) radicals.