Zirconia as a support for catalysts. Evolution of the texture and structure on calcination in air

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_BET of 111 m²g^-1 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 850°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.