Photocatalytic decomposition of benzene over TiO2 in the gas phase at room temperature was studied with a fixed-bed flow reactor. In a humidified airstream ([H2O] = 2.2%), benzene was efficiently decomposed to CO2 and CO with the selectivities of 93 and 7%, respectively. The selectivities were almost independent of the benzene conversion, indicating that CO is not the intermediate of CO2 in the reaction. The selectivity of CO was in the range of 7-10% with varying concentration of O2, H2O, and benzene. The formation of phenol and brownish carbonaceous matter attributable to polymeric products was observed on the catalyst surface. In the absence of O2, benzene oxidation did not proceed at all, showing that O2 is essential for the reaction. The presence of H2O not only suppressed the formation of the carbon deposits on the catalyst surface, but also accelerated the decomposition of them to CO2 and CO. Diffuse reflectance IR study showed that the presence of H2O regenerated the surface hydroxyl groups of TiO2 which were consumed in the photoreaction. With increase in the benzene concentration, the benzene conversion was decreased and the amount of carbon deposits on the catalyst surface was increased.
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