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

In thermodynamics, the one-electron reduction of O2 by the conduction band electrons of Bi2WO6 or WO3 is not allowed. However, many studies have reported that Bi2WO6 is photocatalytically active for organic degradation in aerated aqueous suspension. In this work, the photocatalytic activities of Bi2WO6 and WO3 under visible light have been compared by using phenol degradation as a model reaction. In aerated aqueous solution, Bi2WO6 and WO3 were indeed active and inactive, respectively, as reported. However, by using Pt as a catalyst for O2 reduction, or by using H2O2 as an electron scavenger, Bi2WO6 became much less active than WO3. Similar results were also obtained in the production of H2O2 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 H2O2, the EPR signal due to •OH radical was reduced, but not disappeared. These observations indicate that the irradiated Bi2WO6 is not only active for water oxidation to •OH but also active for the two-electron reduction of O2 to H2O2, the latter of which hardly occurs with the irradiated WO3.