Highly selective phenol production from benzene on a platinum-loaded tungsten oxide photocatalyst with water and molecular oxygen: selective oxidation of water by holes for generating hydroxyl radical as the predominant source of the hydroxyl group

Abstract

Particles of tungsten oxide loaded with nanoparticulate platinum (Pt/WO₃) photocatalytically produced phenol from benzene with high selectivity (e.g., 74% at 69% of benzene conversion) in water containing molecular O₂; the selectivity for phenol was much higher than that on conventional titanium oxide (TiO₂) photocatalysts (both the unmodified and Pt-loaded) that generated CO₂ as a main product. Results confirmed that photoexcited electrons on the Pt/WO₃ photocatalysts mainly generated H₂O₂ from molecular O₂ through a two-electron reduction; the H₂O₂ generated did not significantly contribute to the undesirable peroxidation of the phenol produced. In contrast, the oxygen radical species, such as ˙O₂⁻ or ˙HO₂, generated on TiO₂ photocatalysts partially contributed to the successive oxidation of phenol and other intermediates to reduce the selectivity for phenol. More importantly, the reactions using ¹⁸O-labeled O₂ and H₂O clearly revealed that the holes generated on Pt/WO₃ react primarily with H₂O molecules, even in the presence of benzene in aqueous solution, selectively generating ˙OH radicals that subsequently react with benzene to produce phenol. In contrast, a portion of the holes generated on TiO₂ photocatalysts reacts directly with benzene molecules, which are adsorbed on the surface of TiO₂ by strong interaction with surface hydroxyl groups. This direct oxidation of substances by holes undoubtedly enhanced non-selective oxidation, consequently lowering the selectivity for phenol by TiO₂. The two unique features of Pt/WO₃, the absence of reactive oxygen radical species from O₂ and the ability to selectively oxidize water to form ˙OH, are the most likely reasons for the highly selective phenol production.