Highly Efficient Catalysts For Chlorinated Volatile Organic Compounds (CVOCs) Combustion

Catalytic oxidation is one of the most widely used techniques to control CVOCs emissions. However, several challenges specific to the oxidation of chlorinated VOCs (CVOCs) have been identified recently, including:deactivation; hazardous byproduct formation; and the fact that higher temperature are required. Al-based catalysts and Cr-based catalysts have been widely applied in abatement of chlorinated volatile organic compounds, due to the surface acidity of Al-based catalysts and the reducibility of Cr-based catalysts, which is useful to the adsorption and oxdation of CVOCs. Wheras, pure Al-based catalysts require higher reaction temperature and Cr-based catalysts is potential for the formation of toxic Cr2O2Cl2, which leads to the loss of Cr and the pollution of the environment. Hence, screening high activity, cheap and green catalysts for oxidation of low concentration of influent CVOCs and exploring the reaction mechanism is the objective of this thesis. Research contents and results of this thesis are as follows:1. A series of K-promoted Pt/Al2O3 catalysts were prepared by an incipient wetness impregnation method and tested for oxidation of dichloromethane (DCM). It was found that the activity was greatly enhanced by the modification of K, which depended on the K content in the catalyst. The T50 temperature (at which DCM conversion was 50%) on a 0.42K-2Pt/Al2O3 catalyst was 270 ℃, which was much lower than that on a K-free 2Pt/Al2O3 catalyst (400 ℃). The remarkable improvement of activity was contributed to the enhanced catalyst reducibility, by the generation of Pt-O-Kx (x≈2) surface species through an intimate interaction between K and Pt. The presence of such species could significantly accelerate the decomposition of formate intermediates formed on Al2O3 surface and thus the overall reaction, as evidenced by the in-situ Fourier transform infrared spectroscopic results.2. A series of spinel type CoCr2O4 catalysts calcined at different temperatures were prepared and tested for oxidation of dichloromethane (CH2CI2). These catalysts were active for this reaction. The final products in the reaction were COx, HC1 and Cl2, without the formation of toxic Cl-containing organics. The bes tperformance was obtained on a catalyst calcined at 400 ℃ (CoCr2O4-4), with a Tso of 218 ℃ and a T90 of 257 ℃, which was mainly due to the highest surface area of this catalyst (91.3 m2 · g-1). Detailed quantitative analyses revealed that the catalytic behavior was synergistically governed by surface acidity and reducibility of the catalyst, as evidenced by ammonia temperature-programmed desorption and hydrogen temperature-programmed reduction results, respectively. Kinetic studies revealed similar activation energies (124.5-155.5 kJ·mol-1) on these catalysts, implying the reaction might follow the same pathways on these catalysts. More importantly, in situ Fourier transform infrared spectroscopic investigation of the reaction revealed that formate species were the main reaction intermediates, which could befurther oxidized to CO and CO2.