The structure and surface acidity of zirconia-supported tungsten oxides

Abstract

The structure and surface acidity of zirconia-supported tungsten oxide are studied by using BET, XRD, FT Raman, Tian–Calvet microcalorimetry as well as XPS. Three different crystal forms of ZrO₂ and their hydroxide precursors were prepared in a controllable way. It was found that the starting material and preparing conditions used have a significant effect on the structure of ZrO₂ obtained. The tungstate species on zirconium hydroxide generally promote the transformation of the hydroxide precursor into tetragonal ZrO₂, while the same species on the pre-calcined ZrO₂ has less effect during calcination. A bulk-phase WO₃ existed in the samples prepared by initially impregnating the monoclinic, tetragonal and cubic ZrO₂ or the hydroxide precursors of m- and t-ZrO₂, while no crystalline WO₃ was present in the samples prepared by initially impregnating the hydroxide precursor of c-ZrO₂. Raman results revealed that the surface tungstate(s) on three different zirconium hydroxides or their respective zirconia have similar structural features before calcination. The surface tungsten oxides and some WO₃ bulk phase are the detectable tungsten species in the final samples, the ratio of each component is strongly dependent on the preparation history and the nature of the support. Creation of very strong acidic sites on the zirconia-supported tungsten oxide is related to the crystal form of zirconia itself, the type of tungsten oxide species, and the cooperation between the surface tungsten oxide overlayer and zirconia. The strong acidic sites can be completely poisoned by the remaining sodium ion impurities, but have not been appreciably influenced by added yttrium component. The crystalline WO₃ seems of little importance in building up the strong surface acidity.