| With the development of energy and chemical industry, the total energy consumption in China is rising up year by year. Thus the environmental pollution caused by the heavy use of fossil fuels is becoming more and more serious, especially the volatile organic contaminant s (VOCs), which are hazardous to human health and ecosystem safety. Due to the urgency of environmental situation, more and more countries and districts pay attention to the removal of volatile organic compounds, and some of them have made strict standards to restrict the emission of VOCs. As it is difficult to fundamentally suppress the release of VOCs, more effective treatment of VOCs is in urgent need. With the advantages of low energy consumption and wide application, the photo-catalytic degradation technology based on TiO2 has attracted more and more attention in recent years.Currently, light utilization efficiency of the modified TiO2 can be further improved to some extent. Moreover, the detailed degradation ways and mechanism of photo-catalytic reaction of VOCs is still under a lot of controversies. Therefore, this article built a batch-type platform for photolysis and a continuous platform for photo-catalytic degradation to study the degradation disciplines of toluene under different conditions. A reasonable reaction mechanism of the toluene degradation based on the analysis results was proposed.During the photolysis and photo-catalytic degradation reaction, the conversion of toluene decreased slowly with the increase of the initial concentration of toluene; while it increased with the light intensity rising up; in the dynamic analysis, the photolysis and photo-catalytic degradation of toluene were in accordance with pseudo first-order kinetics equation.Through photolysis degradation of toluene on batch-type platform, intermediates during the photolysis of toluene mainly included benzene, benzoic acid, ethanol, acetic acid, etc.; With the increase of illumination intensity, the relative content of ethanol increased from 38.7% to 43.19%, whereas that of aromatic compounds decreased from 58.05% to 42.01%. With the increase of irridation time, toluene can be degraded further and these intermediates can be further degraded into carbon dioxide and water.Photocatalytic degradation experiments with catalysts TiO2 doped with different proportion of La and thiourea were carried out on the continuous photocatalytic degradation platform. The TiO2 catalysts modified by La were prepared through Sol-gel method. La/TiO2 catalysts showed high catalytic activity after calcination at 550 ℃; when the content of La was 2.5%(catalyst LMT-2.5-550), the conversion of toluene reaches the highest (31.7%),11.83% higher than that of the commercial P25. When the addition of thiourea reached 25%(catalyst SMT-25-550), the optimal catalytic degradation performance was observed with 55.4% of toluene conversion, which was 24.23% and 36.06% higher than those of LMT-2.5-550 and P25 under the same conditions, respectively.Compounds, involving benzoic acid and benzyl alcohol which were generated from the reaction of toluene and hydroxyl radical, were detected on the surface of TiO2 undoped. Moreover, methyl benzoate was formed from the reaction of degraded products of benzaldehyde and methanol. The conversion of toluene was relatively high using LMT-2.5-550, and the products identified on the surface were mainly benzoic acid, phenylcarbinol and benzaldehyde dimethyl acetal generated by the reaction of benzaldehyde ad methanol. The toluene conversion reached 55.40% under the effect of SMT-25-550, but its degraded products were pretty complex, including benzoic acid ethyl ester, and hydroxylated isomers of methyl benzoate and benzoic acid. The formation of hydroxylated isomers reduced the activation energy of aromatic ring opening, indicating that the toluene conversion degree was higher. |
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