Zirconia as a support for catalysts: influence of additives on the thermal stability of the porous texture of monoclinic zirconia

A single-phase monoclinic zirconia (the thermodynamically stable modification up to a temperature of 1170°C), having a specific surface area of 67 m2g¿1 and a well-developed mesoporous texture, has been prepared by gel-precipitation followed by calcination at 450°C. A commercially available high-surface area monoclinic zirconia powder (SBET=71 m2g¿1) has also been studied. It was found that the specific surface area and pore volume of monoclinic zirconia both decreased markedly on increasing the calcination temperature; despite the fact that the crystal structure was that of the stable modification, this did not seem to impart any substantial resistance to thermal sintering. The thermal stability of monoclinic zirconia could however be improved significantly by addition (by an impregnation technique) of various oxides: CaO, Y2O3, La2O3 all led to an improvement in the thermal stability up to 900°C while MgO exhibited stabilizing properties only up to 700°C; the best results were obtained with La2O3. All the additives investigated other than MgO were found to bring about a partial transition of the monoclinic to a fluorite-like phase of zirconia upon heat treatment; this phase has been shown in the case of the CaO-doped sample to be cubic zirconia and in the cases of the Y2O3- and La2O3-doped samples to be tetragonal zirconia. As little as 20¿50% of a theoretical monolayer quantity of La2O3 was sufficient to give satisfactory thermal stability. The results can be explained by a model involving mass transport by a surface diffusion mechanism

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 (SBET 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

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 (SBET 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