Sulfur uptake and exchange, HDS activity and structure of sulfided, Al2O3 supported MoOx, PdMoOx and PtMoOx catalysts

The bulk composition, structure and valence state of an alumina supported PdMoOx catalyst have been studied by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), energy dispersive X-ray (EDX) analysis, X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS) before and after sulfiding. Results are compared with data from a previous study of PtMoOx. Temperature programmed reduction (TPR) data indicate that both catalysts contain Pd and Pt in metallic form with high metal dispersion. Pd4S and Pd16S7 are formed upon sulfidation of PdMoOx opposite to PtS formation in PtMoOx. The sulfur uptake by alumina supported MoOx, PdMoOx and PtMoOx was determined by using H235S tracer and compared with XPS data on the sulfur content of the sulfided catalysts. EM data indicate, that MoS2 forms patches on the support surface. The mean sulfur to metal ratio in the catalysts, as calculated from sulfur uptakes, measured by radiotracer experiments, is equal with that measured by XPS for the topmost layers in the case of MoOx, substantially lower in the case of PdMoOx and somewhat lower in the case of PtMoOx. The extent of sulfur heteroexchange and the ratio of mobile sulfur is much higher in the case of molybdena alone than in the case of metal doped catalysts.