Synthesis of Ceria-Zirconia Nanocrystals with Improved Microstructural Homogeneity and Oxygen Storage Capacity by Hydrolytic Sol-Gel Process in Coordinating Environment

Ceriazirconia solid solution nanocrystals, (1-x)CeO2xZrO2, 0 = x = 1, are prepared by solgel processing in dodecylamine of solutions obtained by forced hydrolysis of inorganic salts. The as-prepared nanoparticles have a ceria cubic structure, up to x = 0.35, or are amorphous. Heat-treatment is carried out at temperatures ranging from 500 to 800 degrees C, the latter temperature begin suitable to obtain solid solutions throughout the composition range. For all the heating temperatures and x values, the fluorite cubic structure of pure CeO2 transforms to a mixture (c') of the cubic c and tetragonal t? phases for x = 0.35, and to tetragonal t phase only for x = 0.8 at 650 degrees C, x = 0.65 at 800 degrees C, and, to a very limited extent, x = 0.5 at 1000 degrees C. No evidence is obtained at low x values of the t phase, which is detrimental to the oxygen storage capacity. Prolonged heating at 1000 degrees C demonstrates that only for x = 0.65 a limited separation of CeO2-rich nanocrystals occurs. The samples undergo the same transition without simultaneous occurrence of different phases, apart for the two mentioned limited cases. This result is attributed to the intimate mixing of the metal cations even in the early stages of processing. In as-prepared samples the Zr distribution becomes inhomogeneous when going from x = 0.2 to x = 0.35, but no early phase separations appear. The oxygen storage capacity is favorably influenced by the persistence of the cubic c' phase.