Effect Of Alkaline Earth Doping On The Structure And Oxygen Storage Capacity Of Ceria-zicornia Solid Solution

The aim of this paper is to investigate the doping effect of alkaline earth metals on chemical/structural properties and oxygen storage capacity (OSC) of CeO₂-ZrO₂ solid solution. The preparation procedure of reverse microemulison method was studied to produce ceria-based material. Also, the interaction of modified ceria-based material and Pd was studied. Further, mechanism of CO oxidation over ceria-based solid solution was studied.The quasi-ternary (cyclohexane/water/OP-10/n-hexanol) phase diagram was investigated to determine the W/O type microemulison region. The in situ XRD and TG-DTA technique were used to determine the proper calcine temperature. Some physical-chemical techniques such as XRD, BET, Raman and HRTEM are used to characterize the resultant powders. OSC (total OSC and dynamic OSC) and catalytic performance was also evaluated. The results showed that, compared with the sample with the same composition prepared by sol-gel method, the samples prepared by reversed microemulsion has higher surface area, narrower size distribution, less agglomeration, better purity in phase and better performance.To study the effect of alkaline earth metals (Mg,Ca,Sr,Ba) with the same electronic shell structure and different ionic radius on the structure/performance of Ce0.67Zr0.33O2, alkaline earth metals doped Ce0.67Zr0.33O2 was prepared via reverse microemuslsion method. Further high temperature reduction and mild temperature oxidation treatment (redox aged) was carried out on the samples. Results showed that, alkaline earth doping enhanced the phase purity, the thermal stability, which depends on the ionic radius. Ca and Sr have proper ionic radius and can create more oxygen ion vacancies and lattice defect in the fluorite lattice due to charge compensation and radius effect. So improved OSC was obtained after doping. But the ionic radius of Mg and Ba was either too small or too large for effect modification on the cell, and no positive effect was found on the improvement of OSC.The mechanism of CO oxidation under dynamic condition over ceria-based solid solution was studied using TPR, dynamic OSC and CO pulse measurement. The produced samples after measurement were characterized using FTIR, Raman and TG-DTA analysis. It was suggested that three kinds of oxygen species including surface oxygen, near surface oxygen and bulk oxygen participated the OSC process. The mechanism can be explained using L-H law and CO disproportionation (2CO=CO₂+C) reaction exist. The adsorption/desorption of CO, O2 and CO₂ also occurred with oxygen migration. Alkaline earth doping can promote the oxygen migration by the modification of surface and cell of Ce0.67Zr0.33O2 solid solution. On the other hand, the ability of CO₂ adsorption was improved due to the enhancement of alkalinity from doping.Pd(0.5wt.%)supported on the doped Ce0.67Zr0.33O2 was prepared by the impregnation method. Redox aging treatment was carried out before measurement. H2-TPR and catalytic activity of the samples was evaluated to study the interaction of Pd and OSC materials. Results showed that alkaline earth doping influenced greatly on the interaction depends on the kinds and content of dopant. Pd/CZSr has the best CO, C₃H₈ and NO conversion rate.The oxygen storage capacity (OSC) is a crucial property of CeO₂-ZrO₂ directly linked to the efficiency of TWC operating under fluctuating conditions. In this paper, an apparatus working under transient condition used for dynamic OSC measurement (less than 0.1Hz) was developed in our laboratory. Further, the quantitative and qualitative analysis method was developed to evaluate the OSC systematically.