Catalysis on Pd/WO3 and Pd/WO2: Effect of the Modifications of the Surface States Due to Redox Treatments on the Skeletal Rearrangement of Hydrocarbons: Part I. Physical and Chemical Characterizations of Catalysts by BET, TPR, XRD, XAS, and XPS

Bulk and surface properties of Pd/WO3 and Pd/WO2 catalysts were studied using temperature programmed reduction, X-ray diffraction, X-ray absorption, and X-ray photoelectron spectroscopies. Special attention was paid to the oxidation state of tungsten upon reduction under hydrogen at 350 and 600°C. The influence of oxidation treatments was also studied. Our results showed that reduction at 350°C led to the tranformation of bulk WO3 into the stable W20O58 phase. At higher reduction temperature and also at 350°C, if the support WO3 was calcined before impregnation, metallic tungsten was detected and we postulated that this transformation starts with the formation of the metastable W3O phase via W20O58. All these transformations concern the surface as well as the bulk for Pd/WO3 catalysts and the WO3 contamination layer only for Pd/WO2 catalyst. WO2 seems more difficult to reduce than WO3 even if the latter becomes less reducible after oxidation treatment. Autoreduction of the palladium salt was suggested on these catalysts, resulting both from the pretreatment of the supports and the nature of the Pd salt. Addition of a metal to bulk oxides facilitates their reducibility and allows one to obtain a stable surface state.