SYNTHESIS OF RARE EARTH OXIDES HAVING NEW FORMS

本工作旨在发展具有新形态的稀土氧化物的制备方法，并阐明其随形态变化的性质。重点研究了表面改性氧化铈-氧化锆混合氧化物的氧化还原性能和表面包覆氧化铈细颗粒的光学性能。这些材料分别作为促进剂应用于汽车尾气三元催化剂和紫外线防晒剂中。用氯气、氯化铵或碱金属氯化物对铈锆固溶体进行了表面改性。形成的挥发性物质通过化学蒸汽输送从氧化铈-氧化锆固溶体表面剥离。这个过程被命名为“化学归档”。化学归档后样品的还原温度低于新鲜样品的还原温度。此外，即使在高温老化后，其氧化还原性能仍保持不变，这表明与传统方法制备的固溶体相比，其耐久性更好。利用拉曼散射、x射线光电子能谱和高分辨率透射电镜测量，确定了结构、成分和形式的变化是这些特性的原因。同时，合成了一种由氧化铈和涡层氮化硼（t-BN）组成的新型防紫外线材料。高分辨透射电镜测得的t-BN层厚度约为10 nm。bn涂层能有效地降低氧化铈的催化活性，并具有良好的光散射和紫外光吸收性能。包含在有机薄膜中的bn涂层的二氧化铈颗粒比传统的微粉二氧化钛和氧化锌颗粒具有更高的透明度和更有效地阻挡紫外线，这两种无机材料通常用于化妆品和许多其他应用中作为阻挡紫外线的材料。

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.