The molecular structures of the WO,/support (Al,O,, TiO,, Nb,O,, ZrO,, SO,, 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 WO,/support catalysts, with the exception of the WO,/SiO, and WO,/MgO catalysts, possess a highly distorted, octahedrally coordinated surface tungsten oxide species with one short W=O bond (mono-oxo tungsten oxide species) at high surface coverages. The WO,/SiO, catalysts exhibit strong Raman features of crystalline WO, particles due to the relative low density and reactivity of the surface hydroxyl groups. The WO,/MgO catalysts possess non-stoichiometric compounds, MgS(WO,), and Ca,(WO,),, at low tungsten oxide contents and crystalline MgWO, and CaWO, 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 WOi-with Mg(OH), and Ca(OH),. 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.