The structure and surface acidity of zirconia-supported tungsten oxides

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.