Zirconia-supported tungsten oxide samples, prepared by the
equilibrium adsorption method in the pH range
2−13 by using as a support either hydrous
[ZrO2(383)] or thermally pretreated
[ZrO2(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 ZrO2(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
concentrations and of polymeric species at high loading. The
uptake of the tungsten species depends on the
morphology and the texture of the support.
The formation of solid solution and ZrO2 phase stabilization were investigated by Raman spectroscopy and X-ray diffraction (XRD) in calcium-containing and cadmium-containing zirconium oxide samples heated at 1073 K in air. The adopted preparation procedure led to the incorporation of calcium and cadmium in solid solution into the zirconia structure. The solid solution favored the tetragonal and cubic zirconia phases at the expense of the thermodynamically stable monoclinic modification. Combined macro-and micro-Raman spectroscopy disclosed that instead of forming a homogeneous phase t '', intermediate between the tetragonal t' and the cubic phase, the tetragonal and cubic phases coexisted in the range 9.49-13.89 mol% for Ca and 11.88-17.23 mol% for Cd. At higher dopant contents the cubic form stabilized. The impurity content necessary to stabilize the high-symmetry phases was defined. Copyright (C) 2007 John Wiley & Sons, Ltd
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