BULK AND SURFACE-ANALYSIS OF A SUPPORTED PT-MOOX-AL2O3 MODEL SYSTEM IN THE FRESH AND SULFIDED STATE

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 microsc opy (HRTEM), energy dispersive X-ray (EDX) analysis, X-ray photoelectr on spectroscopy (XPS) and ion scattering spectroscopy (ISS). Electron microscopy (EM) indicated that molybdenum oxide was likely to be prese nt 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 H-2 treatment at 67 3 K. Sulfidation by H2S at 673 K produced more Mo-IV, mainly as MoS2 s pread over the surface and/or in the pores of alumina. A constant in-d epth Mo ISS signal indicated that there were larger Mo particles. Sulf ur, in turn, was removed by ISS, pointing to the presence of S adspeci es other than MoS2, in agreement with XPS. Pt was distributed as separ ate small crystallites. Reduction created almost clean metallic Pt spu ttered away by ISS, indicating its very disperse character. EM, XRD an d XPS confirmed that some Pt transformed to PtS upon H2S-H-2 treatment , larger PtS crystallites coexisting with smaller Pt particles. A redu ced catalyst was very active in cyclohexene hydrogenation; much lower activity with increased benzene selectivity was observed after sulfida tion.