Photocatalytic oxidation of toluene on platinum/titania catalysts

Abstract

During heterogeneous photocatalytic oxidation (PCO) at room temperature, aromatics are not as readily oxidized as other organics, and intermediates accumulate that deactivate the catalyst. Toluene was used as a model aromatic on TiO 2 and Pt/TiO 2 catalysts. Transient isothermal reaction and reaction of a continuous flow of toluene were used to study reaction steps during PCO. Oxygen uptake was rapid for transient PCO of adsorbed toluene, but the rate of CO 2 formation was much slower. The Pt/TiO 2 catalyst deactivated within 7 h during PCO of a continuous flow of unhumidified toluene. With the addition of water, the rate of CO 2 production increased, and the catalyst was still active after 42 h. Less-reactive intermediates accumulated on the surface during PCO, and they were more strongly bound than toluene, which desorbed at relatively low temperatures. These intermediates were characterized by temperature-programmed hydrogenation (TPH). The strongly bound intermediates either thermally decomposed or oxidized during temperature-programmed desorption (TPD) or oxidation (TPO). However, these intermediates were hydrogenated to more weakly bound species that desorbed during TPH. The TPH spectra indicated that the aromatic ring remains intact during the first steps in toluene PCO. The transient photocatalytic oxidation of benzaldehyde was also investigated, but benzaldehyde does not appear to be the main reaction intermediate formed during toluene PCO.