Bulk and surface analysis of a supported Pt[ndash ]MoOx–Al2O3model system in the fresh and sulfided state

Abstract

The bulk composition, structure and valence state of a model catalyst containing 3% Pt and 12% MoO₃ supported on Al₂O₃ 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). Electron microscopy (EM) indicated that molybdenum oxide was likely to be present as a thin surface layer of Mo^VI as shown by EDX and XPS. Mo^VI was not quantitatively reduced to Mo^V and Mo^IV upon H₂ treatment at 673 K. Sulfidation by H₂S at 673 K produced more Mo^IV, mainly as MoS₂ spread over the surface and/or in the pores of alumina. A constant in-depth Mo ISS signal indicated that there were larger Mo particles. Sulfur, in turn, was removed by ISS, pointing to the presence of S adspecies other than MoS₂, in agreement with XPS. Pt was distributed as separate small crystallites. Reduction created almost clean metallic Pt sputtered away by ISS, indicating its very disperse character. EM, XRD and XPS confirmed that some Pt transformed to PtS upon H₂S–H₂ treatment, larger PtS crystallites coexisting with smaller Pt particles. A reduced catalyst was very active in cyclohexene hydrogenation; much lower activity with increased benzene selectivity was observed after sulfidation.