Generation of H₂O₂ and OH Radicals on Bi₂WO₆ for Phenol Degradation under Visible Light

In thermodynamics, the one-electron reduction of O 2 by the conduction band electrons of Bi 2 WO 6 or WO 3 is not allowed. However, many studies have reported that Bi 2 WO 6 is photocatalytically active for organic degradation in aerated aqueous suspension. In this work, the photocatalytic activities of Bi 2 WO 6 and WO 3 under visible light have been compared by using phenol degradation as a model reaction. In aerated aqueous solution, Bi 2 WO 6 and WO 3 were indeed active and inactive, respectively, as reported. However, by using Pt as a catalyst for O 2 reduction, or by using H 2 O 2 as an electron scavenger, Bi 2 WO 6 became much less active than WO 3 . Similar results were also obtained in the production of H 2 O 2 under visible light, and in the generation of • OH radicals under UV light, measured by a spin-trapping electron paramagnetic resonance (EPR) spectroscopy. Moreover, in the presence of catalase to completely remove H 2 O 2, the EPR signal due to • OH radical was reduced, but not disappeared. These observations indicate that the irradiated Bi 2 WO 6 is not only active for water oxidation to • OH but also active for the two-electron reduction of O 2 to H 2 O 2, the latter of which hardly occurs with the irradiated WO 3 .