Preliminary Study On Photolytic Degradation And Photocatalytic Degradation Of Toluene

The toxic VOCs in PRD, especially BTEX, e.g. benzene, toluene, xylene, had higher concentrations than those in European and American cities due to their extensive industry sources in Pearl River Delta (PRD). In this dissertation, toluene was chosen as the representational compound of the VOCs, the influence factors and the chemical reaction mechanism of photolytic and photocatalytic degradation kinetics under different UV light irradiation were studied in details.Firstly, under illuminated by 254 run UV light, the kinetics influence factors of the toluene photolytic degradation were investigated in a homemade 160-liter chamber. It was found that the illumination intensity, the relative humidity and the initial concentration of toluene were key infactors on photolytic degradation efficiencies of toluene. The experimental results showed that the photolytic degradation efficiencies increased gradually with the increase of illumination intensity, and increased with the increase of the humidity within the low relative humidity range 20-45%, and then decreased within relative humidity range 45-75%, the best relative humidity value is 45%. The photolytic degradation efficiencies decreased with the increase of the initial concentration of toluene. A series of photochemistry reactions have occurred under irradiation by 254nm UV light, and a lot amount of degradation products were detected, such as benzene, benzaldehyde, as well as unsaturated carbonyls such as propenal, 2-butenal and 2-methyl-butenal, di-alkenes such as 1,3-butadiene, 1,3-pentadiene, 2-methyl -1,3-butadiene, and furan, 2-methyl-furan.Secondly, a ZnO-SnO₂/Al supported photocatalyst was made by immobilizing the coupled oxide ZnO-SnO₂ that was prepared by the co-precipitation method onto aluminum plate. And the supported ZnO-SnO₂/Al photocatalyst was installed in a homemade 55-liter photocatalytic reactor. Under illuminated with 365 nm UV light, the influence factor and mechanism of photocatlytic kinetics of toluene were investigated in the photocatalytic reactor. The results showed that the illumination intensity, the relative humidity, the initial concentration and oxygen concentration had great impacts on photocatalytic degradation efficiencies of toluene. It was also found that the photocatalytic degradation efficiencies decreased with the increase of the initial concentrations of toluene; the photocatalytic degradation efficiencies increased within the low relative humidity range 0-35%, and then decreased within relative humidity range 35-75%, the best relative humidity value is 35%; the photocatalytic degradation efficiencies increased gradually with the increase of illumination intensity. As a good electron trap and oxidizer, oxygen can improve photocatalytic efficiency greatly. Compared to the photolytic degradation, the photocatalytic oxidation was an advanced oxidation process. The reaction pathway was firstly preceding the intermediates benzene and benzaldehyde and then mineralizing to CO₂ and H₂O. The experimental results indicated that the ZnO-SnO₂ coupled oxide photocatalyst had great photocatalytic activity and regeneration ability.