Visible Light-Induced Degradation of Phenol over Iodine-Doped Titanium Dioxide Modified with Platinum: Role of Platinum and the Reaction Mechanism

Abstract

The role of platinum deposited on iodine-doped titanium dioxide (Pt/I-TiO2) catalyst during photodegradation of phenol under irradiation with visible light was investigated. The results of this study demonstrate that the Pt/I-TiO2 photocatalyst enhanced the phototransformation of phenol, with a negligible increase upon photomineralization. The results of the phototransformation of para substiuted phenols (p-methylphenol, phenol, p-chlorophenol, and p-nitrophenol) show that the phenol−TiO2 interaction, influenced by the Hammett constants, has the principal role in photodegradation. Furthermore, the action of scavengers (iodide ion, tert-butyl alcohol, fluoride ion, and persulfate ion), as well as N2 purge on the photodegradation of phenol prove that the phototransformation of phenol occurs on the surface of the photocatalyst and is initiated mostly by a valence band hole (hvb+), and the degradation process is limited by the keto−enol tautomeric equilibrium between hydroquinone and quinone. On the basis of these experimental results, we conclude that the role of platinum on the I-TiO2 surface is to inhibit the recombination of electron−hole pairs and promote a more efficient diffusion of the hvb+, which was propitious to the phototransformation of phenol. Nonetheless, the tautomeric equilibrium restrains the degree of mineralization. Additionally, identification of the main products showed that more quinone, formic acid, and oxalic acid were found in solution after reaction for 4 h using the Pt/I-TiO2 photocatalyst in comparison to the I-TiO2 photocatalyst, which could further explain the insignificant action of platinum for the photomineralization of phenol.