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Zirconia samples, prepared by precipitation from a solution of zirconyl chloride at a constant pH of 10, were calcined in flowing air at temperatures up to 850' C in order to study the development and stability of the porous texture in conjunction with the development of the structure of the resulting materials as a function of calcination temperature. The gel precipitation technique employed yields a high surface area zirconia (s IBET of 111 m2 gg ' after calcination at 450 ' C) with a well-developed mesoporous texture. The porous texture is, however. unstable under the experimental conditions employed, the initial high specific surface area being lost quite rapidly with increase in calcination temperature; calcination at 85O'C brings about a reduction of the (BET) specific surface area by approximately 97% Two process were identified as being responsible for the changes in pore structure and surface area: (i) crystallite growth and an accompanying phase transformation; and (ii) inter-crystallite sintering (neck-formation and growth); both these phenomena probably occur via a mechanism of surface diffusion. The inter-crystallite sintering process becomes more pronounced at higher calcination temperatures.

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Single-phase tetragonal zirconium oxides have been made by the incorporation of 5.4 mol-% of Y3+ or La3+ in ZIG, to form solid solutions Tbe 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 precurso r 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 (Sn, ca. 88 m g -' for both the samples) and well-developed mesoporous textures but without any microporosity. Both the Y3+-and the La3'-alloyed ZrG2 samples were found to fully retain the tetragonal phase upon calcination over the entire range of temperatures studied (up to 9OO'C). The thermal stability of the texture of zirconia was found to bs 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 La'+ resulted in a support material which bad a remarkable thermal stability. It is shown that the improvements in the thermal stability are derived from a strong inhibition of the processe s of crystallite growth and the accompanying intercrystallite sinking and thus of the process of mass transport; the mass transport probably occurs by a mechanism of surface diffusion.