The Preparation of High-Surface-Area Zirconia: II. Influence of Precipitating Agent and Digestion on the Morphology and Microstructure of Hydrous Zirconia

High-surface-area zirconia can be prepared by precipitation from zirconium salts. The initial product from precipitation is a hydrous zirconia of composition ZrO(OH)2. The properties of the final product, zirconia, are affected by digestion of the hydrous zirconia. Hence, the aim of this study is to investigate the effects of digestion on this hydrous zirconia. It was found that hydrous oxides digested at 100°C were thermally more stable than undigested ones or samples aged in the mother liquor at room temperature. Therefore, the high surface area of the digested hydrous oxides could be retained in the resulting zirconia. The digested samples had larger pores and higher porosity than the undigested hydrous oxide. Digestion at high pH results in thicker walls in the mesoporous material, which enhances thermal stability. The higher solubility of the hydrous zirconia at pH 14 as compared to pH 9 results in faster Ostwald ripening, by which the initially amorphous samples were transformed into a crystalline material. Upon digestion at pH 14, the hydrous oxide became more dehydrated with only 0.3 to 0.5 moles of water per ZrO2. Samples digested at pH 9 did not lose structural water. The water loss leads to a material with considerable (long range) order. The increased order manifests itself in the absence of a «glow exotherm» that is otherwise observed when the energy-rich amorphous hydrous oxide transforms on heating into a crystalline material.