The Preparation Of Spinel CoCr₂O₄ Catalyst And Their Performance On Volatile Organic Compounds （CVOCs） Combustion

Catalytic combustion is one of the most widely used techniques to the abatement of CVOCs. Cr-based catalysts are active and selective for the reaction due to their good reducibility, which is beneficial to the adsorption and oxdation of CVOCs. However, the easy leakage of the active Cr species during the reaction results in severe pollution of the environment. Therefore, the main objective of this work is the synthesis of highly active and stable Cr-based catalysts.for catalytic combustion of low-concentration of CVOCs, as well as the investigation on the reaction mechanism. The main contents of this thesis are as follows:1. A series of spinel type CoCr2O4 catalysts calcined at different temperatures were prepared by a NH4HCO3 co-precipitation method and tested for catalytic combustion of dichloromethane (CH2Cl2). These catalysts were active for this reaction. Detailed quantitative analyses revealed that the catalytic behavior was synergistically governed by surface acidity and redox ability of the catalyst. The best performance was obtained on a catalyst calcined at 600 ℃ (CoCr2O4-6), with a T50 of 201 ℃ and a T90 of 278 ℃, which was mainly due to the higher surface acidity (0.46 mmol·gcat-1) and the higher concentration of surface absorbed oxygen (Oads/ Olat=0.55). The catalysts contained a mixture of spinel CoCr2O4 and crystalline Cr2O3, with the former being the main active component for observed activity. The surface acidic sites in the spinel CoCr2O4 provided centers for adsorption and activation of CH2Cl2.2. Pure spinel CoCl2O4 catalysts were successfully synthesized by cnontrolling the Co/Cr ratio in the preparation procedure. The CoCr2O4 spinel catalysts were highly active and stable for CH2Cl2 combustion, which shows promising potential in practical application. It was found that the best performance was obtained on a catalyst calcined at 600 ℃ (with an areal specific reaction rate of 3.41×10-8molCH2Cl2 s-1m-2 at 220 ℃). The final products in the reaction were COx, HC1 andCl2, without the formation of any other Cl-containing organic byproducts, indicating the high selectivity. Quantitative analyses revealed that high-temperature calcination led to partial substitution of Co3+ cations by Cr3+/Cr6+ cations in the octahedral sites of the spinel oxide and thus to enhanced reducibility and surface acidity of the oxide, which synergistically governed the observed catalytic behaviors. Moreover, it was found that high valent Cr species (Cr6+) played very important role in the reaction, with a much higher turnover frequency (2.2 × 10-3 s-1) than that of the Cr3+(0.56 × 10-3 s-1).