Benzene and toluene gas-phase photocatalytic degradation over H2O and HCL pretreated TiO2: by-products and mechanisms

Photocatalytic oxidations of toluene and benzene in air were carried out over water (TiO2/H2O) and HCl (TiO2/HCl) pretreated TiO2 as this latter pretreatment enhances the toluene removal rate. The main purpose was to identify intermediate products. No gas-phase by-products were detected by direct GC/FID analysis under our conditions despite the high aromatic concentration (50 mg m−3) and the short contact time (ms); this result illustrates an attractive capability of this air purification method. Adsorbed intermediate products recovered by extraction from the used photocatalysts were the same over TiO2/H2O and TiO2/HCl; i.e. no chlorinated products were found in this latter case. Benzoic acid, benzaldehyde, and benzyl alcohol were three major toluene intermediate products identified; trace derivatives of these products and of toluene that were monohydroxylated on the ring were also detected in catalyst extract samples. The benzene major by-product was phenol which was accompanied by hydroquinone and 1,4-benzoquinone. Acetic and formic acids were also formed from both benzene and toluene. For both aromatics, a water extraction of the used photocatalysts permitted separation of a yellow viscous material that settled between water and TiO2 after centrifugation. We have not identified the products contained in this material but believe them to be polymeric products which may be at the origin of the decreased photocatalytic activity of used TiO2 with respect to fresh TiO2. Finally, we discuss the photocatalytic oxidation pathways.