Stabilized tetragonal zirconium oxide as a support for catalysts Evolution of the texture and structure on calcination in static air

Single-phase tetragonal zirconium oxides have been made by the incorporation of 5.4 mol-% of Y³⁺ or La³⁺ in ZrO₂ to form solid solutions. The samples were prepared by controlled coprecipitation from aqueous solutions of the respective metal chlorides at room temperature and at a constant pH of 10, followed by calcination at 500°C (in the case of the Y³⁺ -doped sample) or 600°C (in the case of the La³⁺ -doped sample) to effectuate the crystallization into the tetragonal phase. The process of crystallization of the hydrous zirconia precursor was found to be retarded by the incorporation of Y³⁺ or La³⁺, the latter giving the greater effect. Upon crystallization, stabilized tetragonal samples were obtained with high specific surface areas (S_BET ca. 88 m² g⁻¹ for both the samples) and well-developed mesoporous textures but without any microporosity. Both the Y³⁺ - and the La³⁺ -alloyed ZrO₂ samples were found to fully retain the tetragonal phase upon calcination over the entire range of temperatures studied (up to 900°C). The thermal stability of the texture of zirconia was found to be considerably improved, in comparison with the undoped monoclinic material, by the stabilization of the crystal structure in the defect tetragonal form. In particular, incorporation of 5.4 mol-% of La³⁺ resulted in a support material which had a remarkable thermal stability. It is shown that the improvements in the thermal stability are derived from a strong inhibition of the processes of crystallite growth and the accompanying intercrystallite sintering and thus of the process of mass transport; the mass transport probably occurs by a mechanism of surface diffusion.