| In recent years, the rapid economic development in China has brought atmospheric pollution incidents. Many cities are facing the threat of photochemical smog, and the indoor environment is being polluted by usage of new decoration materials. Volatile organic compounds (VOCs) are precursors of photochemical smog and main pollutants of indoor air. Benzene, toluene and xylene are the main and important VOCs in indoor air. Hence, study of photo-degradation and photo-catalytic degradation of benzene, toluene and xylene is of great significance in atmospheric pollution control.In this study, influence of initial concentration and reaction system were studied during photo-degradation of benzene, toluene and xylene. Intermediate products and photo-degradation mechanism of benzene, toluene and o-xylene were studied. Photo-catalytic degradation of gas-phase benzene, toluene and xylene were studied in the conditions of no catalyst, pure anatase catalyst, pure rutile catalyst and multi-composite catalyst. The influence of initial concentration of reactants and catalyst composite on efficiency of photo-degradation was investigated in photo-catalytic degradation of gas-phase benzene, toluene and xylene. Reaction mechanism of photo-catalytic degradation of bezene, toluene and o-xylene was also studied.The key findings in this study were summarized as follows:(1) During photo-degradation and photo-catalytic degradation of benzene, toluene and xylene, the temperature of the reactor should be controlled at a value because the increase of the temperature caused by illumination would influence the adsorption equilibrium. Through experiments of adsorption equilibrium, the time needed to reach adsorption equilibrium for benzene, toluene, o-xylene, m-xylene and p-xylene was found to be 30min, 40min, 50 min, 45min and 45min, respectively.(2) During photo-degradation of benzene and toluene, the increase of initial concentration would cause the increase of reaction rates. In the same reaction condition, the relation of reaction rates for benzene series(benzene, toluene and xylene) was: m-xylene > o-xylene > p-xylene > toluene > benzene, because the introduction of methyl lowered the energy of benzene π-π- conjugated system and destroyed spatial structure of the benzene ring.(3) The photo-degradation of benzene, toluene and xylene in air were similar to nitrogen system (N2 flow for 20min). If the temperature of the system was controlled at 120℃ under N2 flow, the photo-degradation rates decreased, because H2O was source of OH·, H2O was driven off at high temperature in the reactor, and the reaction rates slowed down.(4) The main intermediate products of photo-degradation of benzene were 1-propyne, 1,3-butadiene, 2-methyl-hexane, 3-methyl-hexane and heptan. The main intermediate products of photo-degradation of toluene were 1 -propyne, benzene and benzaldehyde. The main intermediate products of o-xylene were 1-propyne, 3-methyl-butanal, m-xylene, 2-methyl-ethylbenzene, toluene and benzene. 1-propyne was the main intermediate product during photo-degradation of benzene, toluene and o-xylene and it resulted from photo-decomposition of the reactants.(5) The relation of degradation rates of benzene series in case of no catalyst, anatase and rutile was anatase > rutile > no catalyst, because the electron and hole was easy to combine to rutile.(6) A certain amount of rutile doped in anatase catalyst could improve the photo-catalytic activity. The catalyst with 80% anatase and 20% rutile showed the best photo-catalytic activity to benzene and p-xylene, and the catalyst with 90% anatase and 10% rutile gave the best photo-catalytic activity to toluene, m-xylene and o-xylene.(7) GC—MS were used to analyze the intermediate products of photo-catalytic degradation of benzene, toluene and o-xylene. In gas phase, the main product of benzene was 1-propyne, the main products of toluene were 1-propyne and benzene, the main products of o-xylene were 1-propyne, 2-methyl-ethylbenzene, m-xylene, toluene and benzene. There were more intermediate species on catalyst surface than in gas phase. The main products of photo-catalytic degradation of benzene were 3-methyl-hexane, 2,3-dimethyl-hexane, 1-methyl-2-propyl-cyclopentane, 2,4-dimethyl-hexane, 2,5-dimethyl-hexane, 3-methyl-heptane and 2,4-dimethyl-heptane. The main products of toluene were benzene, 2,3,4-trimethyl-hexane, ethylbenzene and p-xylene. The main products of o-xylene were m-xylene, 2-methyl-ethylbenzene, 2-methyl-benzaldehyde, toluene and benzene. During photo-catalytic degradation of benzene and toluene, alkane was detected in the adsorption species on catalyst surface, which indicated that hydrogen addition reaction existed during photo-catalytic degradation of benzene and toluene. Benzene, toluene and m-xylene were detected both in gas phase intermediate products and asorption species on catalyst surface during photo-catalytic degradation of o-xylene, which indicated demethylation and structure changing reaction existed during photo-catalytic degradation of o-xylene.
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