SYNTHESIS OF ULTRAFINE PARTICLES OF RARE EARTH OXIDES

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This work deals with synthesis and characterization of ultrafine particles of rare earth-based oxides, especially focused on ceria and the ceria-zirconia systems which are applied as promoters in the automotive exhaust three-way catalysts.Cerium and yttrium oxide ultrafine particles were prepared using a reaction within reversed micelles, which was known as a microemulsion method. The ultrafine particles, which were obtained by mixing the W/O midroemulsions containing rare earth ions with those of ammonium hydroxide, were characterized by high-resolution electron microscope (HREM) observations. HREM micrographs showed that nanometer sized particles were prepared and their shapes were finiform. The smallest mean particle sizes of ceria and yttria obtained in this method were 2.6 and 4.0 nm, respectively. Those are the smallest among the values ever reported for ceria and yttria particles.Nanosize ceria-zirconia ultrafine particles were also prepared by the microemulsion method and were characterized by the HREM.The catalytic activities of the ceria-zirconia ultrafine particles impregnated on gamma -alumina powders for CO oxidation were compared with those of the conventional catalysts prepared by the co-precipitation method. Even after the heat treatment at 1273 K for 5 h in air, the Ce0.2Zr0.8O2/Al2O3 catalyst prepared by the microemulsion method showed a higher activity than that given by the conventional one. This finding suggests that the preparation method presented in this study is very useful to synthesize the catalysts of high activities and high heat resistance.