ESR study of the initial stages of the photocatalytic oxidation of toluene over TiO2 powders

In the present work, the initial stages of the photocatalytic oxidation of toluene over two commercial TiO₂ powders were investigated at the molecular level by ESR spectroscopy. UV-irradiation of the TiO₂ samples, which present different phase composition and surface area, gave rise to several oxygenated radicals like O⁻, O₂⁻ and O₃⁻, as well as Ti³⁺ centers. The proportion of these species generated depends on the gaseous environment (vacuum or oxygen), and the structural and morphological characteristic of the TiO₂ samples. In contrast, co-adsorption of toluene and oxygen on any of the TiO₂ samples studied yields upon UV illumination slightly different ESR signals, which have been assigned to the formation of benzylperoxy radicals, Ph-CH₂OO, adsorbed on the semiconductor. Such species are only detected when TiO₂ is exposed to toluene–oxygen mixtures enriched in the organic, and under these conditions neither of the other oxygenated radicals is formed. The reasons for this behavior are discussed on the basis of the mechanism for the photocatalytic oxidation of toluene.