Kinetics And Mechanism On Gaseous Photo-catalytic Degradation Of Benzene Series

Volatile organic compounds (VOCs) are precursors of photochemical smogs and main pollutants of indoor air. Benzene, toluene and xylene are the main and important VOCs in air. Therefore, the study on photo-oxidated or photo-catalytic degradation of benzene, toluene and xylene is of great significance in atmospheric pollution control.In this paper, benzene, toluene and xylene were the research objects, and TiO₂ in anatase was used to be photo-catalyst. Influence of the initial concentration on the photo-oxidated and photo-catalytic degradation of benzene, toluene and xylene and kinetics were studied. Ozone, oxygen, steam and nitrogen oxides could affect the photo-degradation of benzene, toluene and xylene. The photo-oxidated and photo-catalytic degradation of benzene, toluene and xylene in various reaction conditions were investigated. Qualitative analysis of the intermediate products during photo-oxidated and photo-catalytic degradation of benzene, toluene and xylene were carried out by GC/MS. According to the intermediates detected by GC/MS, possible mechanisms for photo-oxidated and photo-catalytic degradation of gaseous benzene, toluene and xylene were proposed.The key results in this study were summarized as follows:(1) During photo-oxidated 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 affect the adsorption equilibrium. Through experiments of adsorption equilibrium, the time of adsorption equilibrium for benzene, toluene, o-xylene, m-xylene and p-xylene was 30, 30, 40, 40 and 40min, respectively.(2) The photo-oxidated degradation of benzene, toluene and xylene followed the first-order kinetics. With the increase of the initial concentration of benzene, toluene and xylene, the first-order reaction kinetic constant of photo-oxidated degradation decreased, and the initial reaction rate increased. With the increase of the initial concentration, more intermediated products were produced. The strong oxidated competition between intermediated products and initial reactants resulted in the decrease of the first-order reaction kinetic constant. In addition, with the increase of the initial concentration, the initial reaction rate of benzene, toluene and xylene increased.(3) The photo-catalytic degradation of benzene, toluene and xylene also followed the first-order kinetics. With the increase of the initial concentration of benzene, toluene and xylene, the first-order reaction kinetic constant of photo-catalytic degradation decreased. With the increase of the initial concentration, more intermediated products were produced. The strong oxidated competition between intermediated products and initial reactants on the sueface of TiO₂ resulted in the decrease of the first-order reaction kinetic constant. In addition, with the increase of the initial concentration, the initial reaction rate increased for benzene and xylene, but decreased for toluene.(4) The content of ozone, oxygen, steam and nitrogen oxides in the reaction system and the participation of TiO₂ could affect the photo-degradation of benzene, toluene and xylene. The result showed that the strong oxidation of ozone, the catalysis of TiO₂, the participation of steam and nitrogen oxides could increase the rate of photo-degradation of benzene, toluene and xylene.(5) Qualitative analysis of the intermediate products of photo-oxidated degradation process of benzene, toluene and xylene were carried out by use of GC/MS. The result showed:â‘ the main products of benzene were acrolein and furan;â‘¡the main products of toluene were 1,3-pentadiene, 1,4- pentadiene, benzene and benzaldehyde;â‘¢the main products of o-xylene were diacetyl, acetic acid, benzene, toluene and m-xylene;â‘£the main products of m-xylene were acetone, benzene, toluene, 3-methylbenzaldehyde and o-xylene;⑤the main products of p-xylene were acetic acid, benzene, toluene and o-xylene.(6) GC/MS was used to analyze the intermediate products of photo-catalytic degradation of benzene, toluene and xylene. Generally, there were more intermediate species on the surface of catalysts than in gas phase. There was no gaseous intermediate product for degradation of benzene, but the main products on the surface of catalyst were methylcyclohexane, p-xylene and cyclohexyl-one. The main gaseous products of degradation of toluene were hexane and benzene, and the main products on the surface of catalyst were benzene, heptane, and o-/m-/p-xylene. Benzene and toluene were the main gaseous products of degradation of o-xylene, and benzene, toluene, m-xylene and 2-methyl- benzaldehyde were the main products on the surface of catalyst. The main products of degradation of m-xylene in gas-phase and on the surface of catalyst were benzene, toluene, ethylbenzene and o-xylene, respectively. The main products of degradation of p-xylene were acetic acid, benzene, toluene and o-xylene in gas-phase, accompanied by benzene, toluene, ethylbenzene, o-xylene and 4- methylbenzaldehyde on the surface of catalyst.(7) Benzene was one of the main intermediate products for toluene in photo-oxidated and photo-catalytic degradation, and benzene and toluene for xylene. It was indicated that demethylation reaction played an important role in the photo-degradation. In addition, the hydroxyl radical addition played a major role in the photo-oxidated and photo-catalytic degradation of benzene, toluene and xylene.(8) Toluene and p-xylene were the main intermediate products in photo-catalytic degradation for benzene, and p-xylene the main intermediate product for toluene. It was shown that the reaction combined with methyl was carried out in photo-catalytic degradation for benzene and toluene. For o-/m-/p-xylene, isomers were found in the main intermediate products of both photo-oxidated and photo-catalytic degradation. It revealed that isomerization reaction played an important role in the photo-oxidated and photo-catalytic degradation.