Sulphated zirconia by sol–gel route. The effects of the preparative variables

A single step sol–gel reaction starting from Zr alkoxides, in water–propanol mixtures, was employed to obtain ZrO2–SO4 powders; sulphuric acid was used both as the sulphating agent and as the catalyst of the polycondensation reaction in the gel formation. The water/alkoxide ratio and the acid catalyst amounts were varied in the starting mixtures. Samples prepared by using nitric acid as the reaction catalyst and subsequently impregnated with sulphuric acid were also obtained for the sake of comparison. All the samples were calcined at three temperatures (470, 550, and 630 C) for the same time length (5 h). The different characterizations evidence the role played by the sol–gel reaction conditions in affecting the physico-chemical bulk and surface properties of the powders. Particularly, the water/alkoxide ratio, while not influencing the final structure of the catalyst, is important in the development of a high surface area and in the retention of sulphur after the calcination step. The sulphur content, instead, is critical for the crystal phase development. Surface segregation of sulphate groups is associated with the development of well-defined crystallites as calcination temperature grows.