WO3-CeO2 and Pd/WO3-CeO2 as potential catalysts for reforming applications1. Physicochemical characterization study

WO3-CeO2 (9.1 wt% WO3) and Pd/WO3-CeO2 materials were prepared with a tungs ten 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 sh ow 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 suppo rt would prevent the condensation of monomeric WO42-, avoiding the formatio n of polymeric species. We have proposed the existence of various WO42- spe cies (monomeric or dimeric), as a function of the adsorption mode for which the tungsten is maintained in the +VI oxidation state. Dehydration treatme nt leads to WO42- species linked to a pair of hydroxyl groups which induces structure distortions. Some WO3 crystallites are observed after a reductio n at 350 degreesC of the WO3-CeO2 material which are no longer observed aft er calcination. The tungstate species are strongly bound to the support and thus hardly reducible. They are reduced around 900 degreesC in one step gi ving tungsten metal. (C) 2001 Academic Press.