Palladium Doped In Ceria-zirconia Mixed Oxides Directly Washcoated Monolithic Catalysts For Catalytic Combustion

Volatile organic compounds (VOCs) are known as one of the main air pollutants, which results in a series of serious harms to pepole's health and damages to the ecological environment. The catalytic combustion technology has been identified as one of the most effective measures to destroy VOCs, whose core is the efficient catalyst. CeO₂-ZrO₂ mixed oxides have been widely investigated due to their excellent oxygen storage capacity, high thermal stability, enhanced redox, while the Pd-based catalyst has been applied to catalytic combustion due to its high activity. According, the aim of the thesis is to develop novel CeO₂-ZrO₂ mixed oxides washcoat on the cordierite monolith, which is used as the washcoat of the Pd-based monolithic catalyst. The catalytic activity behavior and thermal stability of the monolithic catalysts for toluene combustion were investigated. Besides, the results of catalyst characterization were correlated to the catalytic activity reasonably.1. A novel Ce_xZr_1-xO₂ washcoat has been prepared by an impregnation method, which acts as a host for the active Pd component to prepare a series of the Pd-Ce_xZr_1-xO₂/substrate (x=1, 0.8, 0.6 and 0.5) monolithic catalysts for toluene combustion. The results indicate that the washcoat shows excellent adhesion to the substrate according to an ultrasonic test. The order of the catalytic activity in terms of x is 0.8 > 0.6 > 0.5≈1. There is strong interaction among palladium, CeO₂-ZrO₂ solid solution and the substrate. The interaction changes the redox property and influences the catalytic activity of the catalysts.2. The effect of calcination temperature on the Pd-Ce_0.8Zr_0.2O₂/substrate monolithic catalyst for toluene combustion was investigated. The results indicate that when the calcination temperature is around 400℃, the uniform and densely packed particles deposit onto the surface of the substrate, implying that the washcoat is well distributed on the substrate. The homogeneous washcoat is very favorable for the dispersion of the active phase and the improvement of the catalytic activity. The Pd-Ce_0.8Zr_0.2O₂/substrate monolithic catalyst calcined at 400℃exhibits the highest catalytic activity, over which toluene conversion is up to 97% at a temperature as low as 210℃. The catalytic activities of the monolithic catalysts calcined at different temperatures decrease with raising calcination temperature. This result may be attributed to the sintering of the PdO particles and CeO₂-ZrO₂ mixed oxides particles and the decrease in the surface area. In addition, the thermal stability may be associated with the formation of CeO₂-ZrO₂ solid solution and the stable presence of PdO phase at high temperatures.3. The Pd-Ce_0.4Zr_0.6O₂/substrate monolithic catalyst with the component of Zr-rich has been prepared similarly. The catalytic activity behavior and thermal stability of the monolithic catalysts for toluene combustion has been investigated. The results indicate that the washcoat could also tightly anchor onto the substrate and has strong vibration-shock resistance. The Pd-Ce_0.4Zr_0.6O₂/substrate monolithic catalyst calcined at 400℃shows 95% toluene conversion at a temperature as low as 210℃. Furthermore, the lowest temperature for 95% toluene conversion was increased by 40℃after the catalyst calcined at 900℃, indicating that the catalyst had good thermal stability. The stable presence of PdO phase at high temperatures is primarily responsible for the good thermal stability of the monolithic catalyst.