Photocatalytic oxidation of gaseous toluene on anatase TiO₂ catalyst: Mechanistic aspects and FT-IR investigation

Photo-oxidation of toluene has been carried out in gas-solid regime by using polycrystalline anatase TiO₂ as the catalyst. The reacting gaseous mixture was toluene, air and water in various molar ratios. A pyrex fixed-bed continuous photoreactor irradiated by a medium pressure Hg lamp was used for performing the photoreactivity experiments. The main oxidation product was benzaldehyde but benzene, benzyl alcohol and traces of benzoic acid, phenol and unidentified compounds were also detected. The molar conversion to benzaldehyde with respect to the initial amount of toluene was ca. 20% in the best experimental conditions. Benzene was produced only as a transient product during the first 3-4 h of irradiation. The dependence of feed composition and flow rate on the toluene fractional conversion was investigated. Selected experiments were performed by using N₂ or CO₂ instead of air. The presence of oxygen was essential for the occurrence of the photoreaction while water played an important role in order to maintain the catalyst activity. Moreover, a Fourier-transform infrared spectroscopy (FT-IR) investigation was carried out simulating the experimental conditions used during the photoreactivity experiments. The results indicated that toluene is weakly stabilised on the hydrated TiO₂ particles by hydrogen-bonding with surface hydroxyl groups, and that it is photo-oxidised to benzaldehyde only in the presence of surface OH groups.