Photocatalytic oxidation of gaseous toluene on polycrystalline TiO₂: FT-IR investigation of surface reactivity of different types of catalysts

Commercial TiO₂ Merck and TiO₂ Degussa P25 powders were employed as the catalysts for the photo-oxidation of toluene. By using TiO₂ Merck benzaldehyde was found in gas phase as the main product of the toluene partial oxidation. After an initial transient period, this catalyst exhibited a high stability in the presence of water vapour in the gaseous mixture, whereas the photoproduction of benzaldehyde sharply decreases after removal of H₂O from the feed. IR spectra of the used catalyst revealed that in the absence of water vapour benzaldehyde is molecularly held on the catalyst surface. This feature was confirmed by co-adsorbing benzaldehyde and water on the fresh catalyst. By contrast, when toluene photo-oxidation was carried out on TiO₂ Degussa P25 no products in the gas phase were detected. In this case benzaldehyde adsorption, monitored by IR spectroscopy, mainly resulted in the formation of benzoate-like species, strongly adsorbed on the catalyst surface.