WO3-CeO2 and Pd/WO3-CeO2 as potential catalysts for reforming applications 1.: Physicochemical characterization study

WO3-CeO2 (9.1 wt% WO3) and Pd/WO3-CeO2 materials were prepared with a tungsten loading corresponding to a tungsten coverage lower than one theoretical equivalent monolayer. Physicochemical characterizations (N-2 adsorption, X-ray diffraction, Raman spectroscopy, and TPR experiments) allowed us to show that the technique used to prepare the catalyst favors the formation of a dispersed W phase containing tetrahedral tungsten species. We identified the important role of the Na remaining from the tungstate precursor (Na2WO4) on the structural evolution of W species. Naf strongly bound to the support would prevent the condensation of monomeric WO42-, avoiding the formation of polymeric species. We have proposed the existence of various WO42- species (monomeric or dimeric), as a function of the adsorption mode for which the tungsten is maintained in the +VI oxidation state. Dehydration treatment leads to WO42- species linked to a pair of hydroxyl groups which induces structure distortions. Some WO3 crystallites are observed after a reduction at 350 degreesC of the WO3-CeO2 material which are no longer observed after calcination. The tungstate species are strongly bound to the support and thus hardly reducible. They are reduced around 900 degreesC in one step giving tungsten metal. (C) 2001 Academic Press.