Characterization of Adsorbed Species on TiO2 after Photocatalytic Oxidation of Toluene

During the initial step of photocatalytic oxidation (PCO) of toluene on TiO2 at room temperature, strongly bound intermediates form that are much less reactive than toluene. Temperature-programmed hydrogenation (TPH) is shown to be an effective technique to characterize these intermediates. Titania is a hydrogenation catalyst at elevated temperatures, and the intermediates hydrogenate to benzene and toluene, which are more weakly bound. Platinum was added to TiO2 to increase the rate of hydrogenation through spillover of hydrogen so that the intermediates were removed at lower temperatures, and the Pt also increased the rate of PCO at room temperature. More than one intermediate may be on the surface, and the intermediates have intact aromatic rings. Similarities in TPH spectra indicate that toluene reacted through benzaldehyde, but benzaldehyde oxidized quickly to some less reactive intermediate. Benzaldehyde adsorbed in more than one form on TiO2, and these forms had different reactivities during PCO. Benzoic acid is not the less reactive intermediate formed during benzaldehyde PCO.