Environ. Sci. Technol. 2001, 35, 1880-1884
Complete Oxidation of Benzene in Gas Phase by Platinized Titania Photocatalysts HISAHIRO EINAGA,* SHIGERU FUTAMURA, AND TAKASHI IBUSUKI National Institute of Advanced Industrial Science and Technology, AIST Tsukuba West, 16-1 Onogawa, Tsukuba, Ibaraki, 305-8569, Japan
Photocatalytic oxidation of benzene in gas phase was carried out with a flow reactor at room temperature. In a humidified airstream ([H2O] ) 2.2%), benzene was quantitatively decomposed to CO2 over UV-irradiated 1.0 wt %-Pt/TiO2 catalyst. When the benzene conversion was decreased, the selectivity to CO2 was decreased, while that to CO was increased. As the amount of Pt loaded on the TiO2 catalyst was increased, the rate of the CO photooxidation was increased, while that of benzene photooxidation was almost unchanged. These findings showed that the photooxidation of benzene to CO2 over Pt/ TiO2 catalyst proceeded by the two sequential steps: (i) benzene was decomposed to CO2 and CO with the selectivities of 94% and 6%, respectively, and (ii) CO was subsequently oxidized to CO2. The rate of CO photooxidation over Pt/ TiO2 catalyst was greatly decreased by the presence of benzene in the reaction gas stream. The complete oxidation of benzene to CO2 could be also achieved by using the hybrid catalysts comprising pure TiO2 and platinized TiO2.
Introduction Air pollution with volatile organic compounds (VOCs) has been a serious environmental problem for these decades. It is known that VOCs are toxic in themselves and they react with NOx in air to form ozone, which is the cause of the smog conditions in urban air. They are also indirectly responsible for the green house effect since they inhibit the degradation of methane in troposphere by lowering the atmospheric hydroxyl radical concentrations. Hitherto, heterogeneous photocatalytic oxidation systems using TiO2 have been extensively studied for the removal of various VOCs (1). Irradiation of TiO2 with UV or near UV light results in formation of highly reactive electron-hole pairs and initiates redox reactions that decompose VOCs. The photocatalytic oxidation system is effective for the destruction of dilute VOCs under ambient conditions and it does not emit hazardous byproducts such as NOx and ozone at all. Among the VOCs, benzene is an important chemical feedstock and a gasoline ingredient. However, it should be removed from exhaust gases emitted from petrochemical plants, petroleum tanks, coke ovens, printing offices, distillation towers, and the installations in which benzene is used as a solvent, due to its carcinogenicity. Photocatalytic oxidation of benzene in air has been extensively studied by * Corresponding author phone: +81(298)61-8260; fax +81(298)61-8258; e-mail:
[email protected]. 1880
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 9, 2001
FIGURE 1. Schematic of the photocatalytic reactor. many researchers (2-10). Anderson and co-workers reported the complete oxidation of benzene in air to CO2 for the first time (3). They have used the 0.1 wt % Pt loaded TiO2 catalyst and conducted the photoreaction at the temperature above 100 °C. In the previous paper, we carried out the benzene photooxidation with pure TiO2 at ambient temperature and showed that benzene was readily decomposed in humidified air to form CO2 and CO with the selectivity of 93 and 7%, respectively (10). The CO selectivity was almost independent of the benzene conversion when the conversion was changed from 13 to 100%, indicating that CO is not the intermediate of CO2. To our knowledge, there has been no report on the complete oxidation of benzene to CO2 in air under ambient conditions. In this study, we carried out the heterogeneous photocatalytic oxidation of benzene in air at ambient temperature and reported that benzene was completely oxidized to CO2 without CO formation in the humidified air by using the 1.0 wt % Pt loaded TiO2 as the photocatalyst. This paper also describes the reaction mechanism and the optimum conditions for the complete oxidation of benzene.
Experimental Section Materials. TiO2 powders (P-25) were mainly used as the catalyst and the precursor. For comparison, anatase and rutile powders (purity 99.9%; particle diameter
