SYNTHESIS OF RARE EARTH OXIDES HAVING NEW FORMS

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In this work we aimed development of preparation methods of rare earths oxides having new forms, and clarified the properties with the change of the forms. Especially we focused on the redox properties of the surface-modified ceria-zirconia mixed oxides and optical properties of the surface coated cerium oxide fine particles. These materials are applied as promoters in the automotive exhaust three-way catalysts and sunscreens against UV light, respectively.Surface modification of the ceria-zirconia solid solutions has been carried out using chlorine gas, ammonium chloride, or alkali-metal chlorides. The formed volatile species were stripped off the surface of the ceria-zirconia solid solutions by the chemical vapor transportation. This process was named as "chemical filing." Reduction temperature of the samples after the chemical filing process were lower than those of the fresh samples. In addition, the redox properties were maintained even after the aging at high temperatures, indicating its durability compared to the solid solution prepared by the conventional procedures. Using Raman scattering, X-ray photoelectron spectroscopy, and high-resolution TEM measurements, it was identified that the change of structures, compositions, and forms is responsible for these properties.Also, a new UV-blocking material consisting of ceria and turbostratic boron nitride (t-BN) has been synthesized. The thickness of the t-BN layer estimated by high resolution transmission electron microscopy was ca. 10 nm. The BN-coating was very effective in reducing the catalytic activity of the ceria and the resulting material showed good light scattering and absorption of UV light. BN-coated ceria particles included in an organic thin film showed higher transparency and blocked UV light more effectively than conventional micronized titanium dioxide and zinc oxide particles, two inorganic materials in common use in cosmetics and in many other applications as UV blocking materials.