Catalytic Combustion Of CH₄and Cvocs Over Perovskite Catalysts

Chlorinated volatile organic compounds (CVOCs) are hazardous to the environment and public health. CH4as an important component of natural gas, and is the most difficult oxidation of organic matter, if a highly active catalyst is prepared, will also have good catalytic effect on other organic waste gas. Catalytic combustion is recognized as one of the most promising technologies in the abatement of CVOCs and CH4. The most commonly employed catalysts include noble metal catalysts and transition metal oxides. Although noble metals are effective for CVOCs combustion, the high costs limit their practical applications. Therefore, more attention has been paid to tranisition metal oxides, which are stable and active in the reaction process. Among the transition metal oxides, Perovskite-type oxides (ABO3) have many advantages such as low cost and high temperature stability, which make them widely applied in catalytic combustion. The main role of A-site cation is to keep the catalyst stability, while the B-site cation usually determines the activity of the catalyst. In this thesis, various Perovskite-type oxides were prepared and tested for catalytic combustion of CH4and CVOCs. The influences of preparation parameters and cation-substitution on the catalyst structure and their catalytic performance were investigated. The main contents of the work are as follows:1. A series of LaM03(M=Mn, Fe, Cr) catalysts were prepared by a sol-gel method, and tested for CH4and CH2Cl2catalytic combustion. The results revealed that LaMnO3catalyst were effective for both CH4and CH2Cl2combustion. The effect of calcination on the catalytic performance was also studied and it was found that high temperature calcination (700℃) was beneficial to the activity. In addition, Al-substitution was found to be helpful in the enhancement of the activity. Various characterization results showed that the catalytic performance was mainly determined by surface area and reducibility of the catalyst.2. A series of Al-substituted AlxLa1-xMnO3(x=0-0.3) catalysts were prepared using a sol-gel method and tested for1,2-dichloroethane oxidation in order to investigate the effects of Al-doping and thermal treatment on the catalyst structures and catalytic behaviors. It was found that both Al-doping and thermal treatment had great influence on the activities of the catalysts, and the highest activity was obtained on a Al0.2La0.8MnO3catalyst calcined at700℃, with a T5o of295℃. The enhanced activity could be attributed to synergetic effects of its high surface area, high reducibility and surface acidity, as evidenced by various characterizations such as H2temperature-programmed reduction and NH3temperature-programmed desorption. However, comparisons of the areal specific reaction rates revealed that the catalysts calcined at900℃had much higher rates than the ones calcined at500and700℃, suggesting structural properties of the catalysts exert stronger influence on the catalytic performance than the surface area does. It was also evidenced that the reducibility of the catalyst had a leading role in determining the intrinsic activity, which was closely related to the presence of high valent Mn4+species, induced by the introduction of Al in the catalyst.