SYNTHESIS OF ULTRAFINE PARTICLES OF RARE EARTH OXIDES

本论文以汽车尾气三效催化剂中的CeO 2和CeO 2-ZrO 2体系为研究对象，采用反胶束反应--微乳液法制备了CeO 2和Y2 O3超微粒子。用高分辨电子显微镜（HREM）观察了由含稀土离子的W/O型中乳与氢氧化铵的中乳混合得到的超微粒子。HREM显微照片表明，制备出了纳米级的颗粒，颗粒形状较好。在该方法中获得的氧化铈和氧化钇的最小平均粒径分别为2.6和4.0 nm。采用微乳液法制备了纳米CeO_2-ZrO_2超微粒子，用高分辨电子显微镜（HREM）对其进行了表征，并与传统的共沉淀法制备的CeO_2-ZrO_2超微粒子对CO氧化反应的催化活性进行了比较。微乳液法制备的Ce0.2Zr0.8O2/Al 2 O3催化剂经1273 K空气热处理5 h后，仍具有较高的活性。这一发现表明，本研究中提出的制备方法是非常有用的合成高活性和高耐热性的催化剂。

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.