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 usin g polycrystalline anatase TiO2 as the catalyst. The reacting gaseous mixtur e was toluene, air and water in various molar ratios. A pyrex fixed-bed con tinuous photoreactor irradiated by a medium pressure Hg lamp was used for p erforming the photoreactivity experiments. The main oxidation product was b enzaldehyde but benzene, benzyl alcohol and traces of benzoic acid, phenol and unidentified compounds were also detected. The molar conversion to benz aldehyde with respect to the initial amount of toluene was ca. 20% in the b est experimental conditions. Benzene was produced only as a transient produ ct during the first 3-4 h of irradiation. The dependence of feed compositio n and flow rate on the toluene fractional conversion was investigated. Sele cted experiments were performed by using N-2 or CO2 instead of air. The pre sence 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 phot oreactivity experiments. The results indicated that toluene is weakly stabi lised on the hydrated TiO2 particles by hydrogen-bonding with surface hydro xyl groups, and that it is photo-oxidised to benzaldehyde only in the prese nce of surface OH groups. (C) 1999 Elsevier Science B.V. All rights reserve d.