Surface structures of supported tungsten oxide catalysts under dehydrated conditions

The molecular structures of the WO3/support (Al2O3, TiO2, Nb2O5, ZrO2, SiO2, and MgO) catalysts under in situ dehydrated conditions have been investigated by Raman spectroscopy. The series of catalysts was synthesized by the aqueous incipient wetness method. The WO3/support catalysts, with the exception of the WO3/SiO2 and WO3/MgO catalysts, possess a highly distorted, octahedrally coordinated surface tungsten oxide species with one short W=O bond (mono-ore tungsten oxide species) at high surface coverages. The WO3/SiO2 catalysts exhibit strong Raman features of crystalline WO3 particles due to the relative low density and reactivity of the surface hydroxyl groups. The WO3/MgO catalysts possess non-stoichiometric compounds, Mg-x(WO4)(y) and Ca-x(WO4)(y), at low tungsten oxide contents and crystalline MgWO4 and CaWO4 at high tungsten oxide contents. This result is attributed to the high aqueous solubility of MgO as well as the CaO impurity and the strong acid-base interaction between WO42- with Mg(OH)(2) and Ca(OH)(2), The current findings for supported tungsten oxide catalysts parallel the previous findings for supported molybdenum oxide catalysts and reflect the similar surface structural chemistry of these two oxides.