Preparation Of Ceria-zirconia-alumina Complex Oxides And Its Three-way Catalytic Activity

It is well-known that gasoline engine exhaust is one of the main sources for air pollution.Three-way catalystic converter equipped on vehicles can reduce emissions of CO, HC(hydrocarbons) and NOx, simultaneously. Ceria-zirconia complex oxides(CZ) have drawngreat attention as an outstanding oxygen storage material in three-way catalysts (TWCs).Their specific surface area and catalytic performance of Ceria-zirconia complex oxides wouldbe significantly reduced at high temperature. However, the decrease in the specific surfacearea of CZA was suppressed as a result of growth inhibition of CZ by the Al2O3diffusionbarrier. In this paper, the ceria-zirconia-alumina complex oxides(CZA) were prepared byco-precipitation method to enhance the thermal stability of CZ at1000°C. Then, thermaltreatments under oxidizing and reducing conditions were carried out for CZ and CZA. Thethree-way catalysts with1wt.%Pd loaded were also prepared by chemical reduction method.X-ray powder diffraction (XRD), N2adsorption/desorption, X-ray absorption fine structure(XAFS), and temperature-progranuned reduction (TPR) were used to characterize the CZ andCZA complex oxides. The effect of the thermal treatments on the structural and redoxproperty of the complex oxides were investigated, and the dynamic oxygen storage capacity(DOSC), total oxygen storage capacity (TOSC) and their three-way catalytic activity werealso tested.The fresh and aged CZ samples presented t-Ce0.5Zr0.5O2crystal phase and the fresh andaged CZA samples consisted with γ-Al2O3or δ-Al2O3and t-Ce0.5Zr0.5O2phases under the hightemperature treatment in air. However, under high-temperature reduction treatment (1000℃in H2atmosphere), the complex oxides presented pyrochlore crystal phase. Under thehigh-temperature reduction treatment with a mild-temperature reoxidation (500℃in air), theCZ and CZA presented k-CeZrO4crystal phase. The high-temperature reduction treatmentwith a mild-temperature reoxidation can significantly improve the thermal stability,low-temperature reducibility, DOSC and TOSC of the complex oxides. This could beattributed to the large amount of Ce3+produced during the high-temperature reduction andmild-temperature oxidation treatment, which was characterized by XAFS. The Pd/50CZA-ro exhibited higher DOSC and TOSC than those of Pd/50CZA-f andPd/50CZA-a. The Pd/50CZA-ro presented excellent three-way catalytic activity and the wideoperation window in comparison with Pd/50CZA-f and Pd/50CZA-a.