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

Abstract WO 3 –CeO 2 (9.1 wt% WO 3 ) and Pd/WO 3 –CeO 2 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 (Na 2 WO 4 ) on the structural evolution of W species. Na + strongly bound to the support would prevent the condensation of monomeric WO 4 2− , avoiding the formation of polymeric species. We have proposed the existence of various WO 4 2− 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 WO 4 2− species linked to a pair of hydroxyl groups which induces structure distortions. Some WO 3 crystallites are observed after a reduction at 350°C of the WO 3 –CeO 2 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°C in one step giving tungsten metal.