Characterization of the Zirconia-Supported Tungsten Oxide System by Laser Raman and Diffuse Reflectance Spectroscopies

Zirconia-supported tungsten oxide samples, prepared by the equilibrium adsorption method in the pH range 2−13 by using as a support either hydrous [ZrO 2 (383)] or thermally pretreated [ZrO 2 ( T )] zirconium oxide, were studied by laser Raman and diffuse reflectance spectroscopies. The W uptake increased (from ∼1.0 to ∼2.8 atoms nm -2 ) as the pH decreased for the ZrO 2 ( T ) supports, whereas for hydrous zirconia support showed the opposite trend. Different tungsten species were revealed depending on the amount of the adsorbed tungsten but not on the pH of the contacting solution. The continuous frequency shift of the Raman bands with the tungsten content, from 930 to 975 cm -1, indicated the presence of monomeric species at low tungsten concentrations and of polymeric species at high loading. The uptake of the tungsten species depends on the morphology and the texture of the support.