DEVELOPMENT OF RARE EARTH BASED OXIDES FOR AUTOMOTIVE EXHAUST CATALYSTS

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In this work we aimed development of rare-earth based oxides of high thermal stability and high catalytic activity. Especially we focused on ceria-zirconia mixed oxides which were applied as promoters in the automotive exhaust three-way catalysts.Nanosize Ceria and ceria-zirconia ultrafine particles prepared using a reaction within reversed micelles were characterized by HREM. The catalytic activities of the ceria-zirconia ultrafine particles supported on γ-alumina powders were higher than those given by the conventional catalysts prepared by the co-precipitation method, even after the heat treatment at 1273 K for 5 h in air.Ceria-zirconia solid solutions were also prepared by thermal decomposition of cerium zirconyl oxalate in an argon flow at 1273 K followed by oxidation at 673-873 K in air. The reduction behavior of the obtained powders was investigated and compared with the samples of high redox activity prepared by the conventional method. The ceria-zirconia samples prepared from the oxalate showed better reduction behavior, in spite of their very low surface areas. The reason for the behavior was attributed to both the appearance of a cubic phase with a different oxygen arrangement and the reduction by carbon formed in the sample preparation process.Surface modification of the ceria-zirconia solid solutions was carried out using formation and transport of volatile vapor complexes such as AlClィイD23ィエD2-CeClィイD23ィエD2 and AlClィイD23ィエD2-ZrClィイD24ィエD2, This process was named as "chemical filing." Reduction temperature of the sample after the chemical filing was lower than that of the fresh sample, and further decreased by a reduction/reoxidation cycle. The redox property of the chemically filed sample was maintained even after the redox cycles at high temperatures, indicating its durability compared to the solid solution prepared by a conventional procedure.