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

Abstract

The bulk composition, structure and valence state of an alumina supported PdMoO_x 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 PtMoO_x. 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 PdMoO_x opposite to PtS formation in PtMoO_x. The sulfur uptake by alumina supported MoO_x, PdMoO_x and PtMoO_x 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 MoO_x, substantially lower in the case of PdMoO_x and somewhat lower in the case of PtMoO_x. 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.