Preparation And Performance Of CeO₂-ZrO₂Solid Solutions With High Specific Surface Area

The ecological environment and human health are suffering from damages of the pollutants in vehicle exhausts. In order to remove these pollutants, it is essential to develop advanced aftertreatment technology for the control of vehicle emission, in which three-way catalysis prevail because of its high effect on purification. CeO2have been widely used in the three-way catalysts due to its oxygen storage and release capacity. However, CeO2risks sintering and losing its activities at high temperatures. To solve this problem, adding Zr4+into CeO2can form Ce-Zr solid solution and enhance its thermal stability. The high operating temperature (the highest temperature is above1000℃) requires the high thermal stability of Ce-Zr solid solutions, which keep the high specific surface area and high oxygen storage capacity after thermal treatment. This advantage is favored for the adaptation of the catalysts to the complex conditions of vehicle and thus the more strict emission standard of vehicle can be well met.In this work, Pr, Nd and La was doped into the Ce-Zr mixed oxides, in which several synthesis methods, including coprecipitation method, microemulsion method, hydrothermal method, the combined method of coprecipitation and supercritical drying, the combined method of microemulsion and supercritical drying and the combined method of hydrothermal and supercritical drying. The final Ce-Zr solid solutions with high specific surface area were obtained after calcinations at high temperature. The structure of the as-prepared samples was characterized by X-ray diffraction (XRD), isothermal adsorption/desorption of N2and electron paramagnetic resonance (EPR). The redox ability was analyzed by temperature programmed reduction with H2(H2-TPR) and the oxygen storage capacities (OSC) were also measured using the OSC tests.The results shows that the doping of Pr and Nd as well as the adding of hydrogen peroxide can improve the thermal stability and specific surface area of Ce-Zr solid solutions. In the combined method of coprecipitation and supercritical drying with Ethanol, the specific surface area of obtained samples is above170m2/g after calcination at550℃. Furthermore, the specific surface area is still higher than40m2/g even after calcination at1000℃for12h, in which the cubic Ce-Zr solid solutions were obtained. Both Ce-Zr-Pr-Nd mixed oxides obtained from microemulsion method and hydrothermal method are the cubic Ce-Zr solid solutions. During the preparations, the specific surface area and pore distribution can be effected by the adding of surfactant and the changes in temperature and pressure. When the surfactant was added in the hydrothermal method, the obtained samples after calcination at1000℃for12h still possess the high specific surface area of is41m2/g, and the thermal stability of samples can be improved by the increase in hydrothermal temperature. The samples obtained by hydrothermal method with the adding of surfactant have the superior redox ability to the samples obtained by microemulsion method.For the supercritical drying method, the obtained sample after the calcination at1000℃for12h has the specific surface area higher than35m2/g, which also possess high thermal stability. When the microemulsion method and the supercritical drying method were combined, the specific surface area of the obtained sample is higher than those of all other samples, which are196.2and63.3m2/g after calcinations at550℃for2h and at1000℃for12h, respectively. In the combined method of coprecipitation and supercritical drying method, the specific surface area of obtained sample doped by both Pd and Nd is higher than that of obtained sample doped by all Pd, Nd and La. All samples have the superior redox ability to the samples obtained by the combined method of coprecipitation and microemulsion. The samples doped by both Pr and Nd or the samples doped by all Pd, Nd and La were characterized as the cubic Ce-Zr solid solutions.Combined with results of OSC tests, it was found that the rare earth doping Ce-Zr mixed oxides possess the superior redox ability and the larger specific surface areas and the higher OSC. In the methods of coprecipitation and microemulsion furthermore, these properties can be improved by the combination with supercritical drying. The thermal stability of the sample that was obtained using microemulsion followed by supercritical drying was better than others.