Effects Of Preparation Methods On Structures And Oxygen Storage/release Capacities Of Ca_yCe_xZr_1-x-yO₂ Materials

CeO₂-ZrO₂ solid solution with its own special oxygen storage/release capacities（OSC） is usually used as catalysts or supports, which have been widely applied to many aspects of the redox systems, such as automobile exhaust catalysts, two-step thermal decomposition of CO₂, CH₄ combustion, CO oxidation. Pros and cons of OSC of cerium-zirconium solid solution directly affect the overall efficiency of the reaction system, and the OSC have a close relationship with its chemical and physical structures, so the study of physical and chemical structures of the cerium-based materials have been one of the hot research. The preparation method and Ce/Zr ratio are two important factors of cerium-zirconium solid solution structure, it is necessary to systematically study actions of these factors in order to see how they affect the structure and OSC of cerium-zirconium solid solution. The third element（such as rare-earth metals, alkaline earth metals） which was introduced into the structure of cerium-zirconium solid solution can be also further improved OSC of solid solution. In this paper, Ca was chosen for doping of cerium-zirconium solid solution and its effect on the structures and properties of cerium-zirconia solid solution was studied. XRD, N₂ adsorption-desorption, Raman, XPS, H₂-TPR and other characterization techniques and CO-CO₂ cycle assessment method were used to systematically investigate the effects of preparation method, Ce/Zr ratio and Ca doping on the structures and OSC of cerium-zirconium solid solutions. The main conclusions were as followed:（1） The effects of urea combustion method and citric acid method on Ce_xZr_1-xO₂ solid solutions with different Ce/Zr ratio were investigated. The results showed that the OSC of Ce_xZr_1-xO₂ solid solutions with different Ce/Zr ratio from two methods were not higher than that of pure CeO₂ in CO-CO₂ cycle test. The OSC of samples were significantly different when the Zr doping ratio exceeds 20%. All cerium-zirconium solid solutions could maintain thermal stability for a long time in the 1000 oC. Under the same Ce/Zr ratio, the crystallite sizes of samples prepared by citric acid method were less than that of samples prepared by urea combustion method, and the specific surface areas of samples prepared by citric acid method were higher than that of samples prepared by urea combustion method. Samples prepared by citric acid method formed more oxygen vacancies than samples prepared by urea combustion method, which was more conducive to migration of lattice oxygen. Particle surfaces of samples prepared by citric acid method were more regular than samples prepared by urea combustion method. Considered it from both OSC and thermal stability, Ce_0.8Zr_0.2O₂ prepared by citric acid method had the best performance in CO-CO₂ cycle test.（2） The effects of Ca doped on the structure, OSC, thermal stability of Ce_0.8Zr_0.2O₂ solid solution were investigated. Ca-doped ways in 20% of Zr fixed content and 80% of Ce fixed content samples had diffierent effects on the oxygen storage/release capacities and thermal stabilities of Ca-Ce-Zr-O solid solutions. The lattice oxygen release capacities of samples with 20% Zr fixed content had been improved greatly, but lattice oxygen storage capacities of reduced state of the sample were less than that of undoped sample. The OSC change of samples with 80% Ce fixed content were not obvious. Samples with 20% Zr fixed content and samples with 80% Ce fixed content at 1000 oC temperature could remain relatively stable [O]out and [O]in, but their values were lower than those of Ce_0.8Zr_0.2O₂ sample. Ca-Ce-Zr-O ternary solid solutions of the above-described two different Ca doping way could maintain cubic fluorite structures in CO-CO₂ cycle test. The crystallinities of samples after circular reaction were better than that of sample before circular reaction.（3） The oxygen storage/release capacities of four types of materials in two-step thermal decomposition of CO₂ were investigated, and these materials included AR CeO₂, sample by urea combustion method, sample by citric acid method and Ca-Ce-Zr-O solid solutions. AR CeO₂ showed a faster rate of O₂ and CO release in two-step thermal decomposition of CO₂, but severe sintering resulted in some loss of lattice oxygen makes the reaction difficultly continue. The OSC of U0.8 and U0.7 by urea combustion method were superior to that of pure CeO₂ in two-step thermal decomposition of CO₂. After high-temperature deoxygenation, lattice oxygen of C_0.8and C_0.6 by citric acid method were difficult to be recharged, resulted in that the reaction cannot process. All Ca-Ce-Zr-O solid solutions participated in the two-step thermal decomposition of CO₂, and part lattice oxygen of samples could be refilled after high-temperature deoxygenation, and their [O]out decreased and [O]in increased with increasing Ca content. Among them, the incremental [O]in of samples with 80% Ce fixed content were more obvious.