Cerium(IV) 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 water-in-oil microemulsions containing cerium nitrate solution with those of ammonium hydroxide, were characterized by high-resolution electron microscope (HREM) observations. HREM micrographs revealed that nanometer-sized CeO 2 particles were formed, and their shapes were uniform. Selected-area electron diffraction patterns of the particles were completely indexed as those of cerium(IV) oxide with the cubic fluorite structure, and the lattice constant calculated from radii of Debye-Sherrer rings was 0.541 nm. Most of the particles were distributed between 2 and 6 nm, and the mean particle size obtained in this method was between 2.6 and 4.1 nm. The size of 2.6 nm is the smallest among the values ever reported for CeO 2 particles. Both direct and indirect optical energy gaps of the particles were independent of the mean particle size, and their values were almost the same as that of the bulk material, suggesting that there was no quantum size effect of the CeO 2 ultrafine particles. However, the alumina-supported ultrafine CeO 2 catalysts prepared by a microemulsion method showed much higher CO oxidation activities than those prepared by a coprecipitation method.

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