Novel quantification of formation trend and reaction efficiency of hydroxyl radicals for investigating photocatalytic mechanism of Fe-doped TiO2 during UV and visible light-induced degradation of acid orange 7

A detailed mechanistic investigation of the hydroxyl radical (center dot OH) formation and organic pollutant degradation over transition metal-doped and undoped TiO2 photocatalysts was performed by the quantitative measurement of center dot OH and the identification of intermediate products under various experimental conditions. The Fe-doped TiO2 as a typical subject was prepared, characterized and used to degrade an azo dye Acid Orange 7 (AO7). It is indicated that the enhanced photocatalytic activity of Fe-doped TiO2 for AO7 degradation was attributed to the increase in surface area, the facilitated charge transfer via Fe-dopant, and a red shift of absorbable wavelength, maintaining a great formation of center dot OH under visible irradiation. The oxidation of H2O by holes was estimated as the major pathway of center dot OH formation rather than the reduction of dissolved O-2 by electrons, and their formation trends reached to approximately 75% and 25%, respectively. Meanwhile the synergistic effect of Fe-dopant produced nearly 10% of extra center dot OH by visible light photoactivation. The intermediate products and pathways of AO7 degradation varied greatly with different photocatalysts and conditions of the process, involving several reaction mechanisms such as the azo bond cleaving, naphthalene oxidation, desulfonation, and hydroxylated products generation. Through the quantification of center dot OH-reacted efficiency we proposed, a stoichiometry of center dot OH affecting overall reaction mechanisms in the TiO2-assisted photodegradation of AO7 was further established. This study can provide new insights on how to better clarify the variation regularity of organic pollutant degradation from different treatments of the center dot OH-based advanced oxidation processes. (C) 2020 Elsevier Ltd. All rights reserved.