Characterization of reduced and sulfided, supported molybdenum catalysts by O2 chemisorption, X-ray diffraction, and ESCA

Abstract

Catalysts composed of molybdenum supported on Al2O3, SiO2, CeO2, and carbon were examined in calcined, reduced, and sulfided states using O2 adsorption, X-ray diffraction (XRD), electron spectroscopy for chemical analysis (ESCA), and chemical analysis. XRD and ESCA data indicate that the support greatly influences the chemical state of bulk and surface molybdenum phases in catalysts reduced at 500 °C. The principal phase in reduced Mo/SiO2 is Mo metal; MoO2 is the major bulk phase in Mo/Al2O3, Mo/CeO2, and Mo/carbon. The sulfided catalysts contain principally MoS2 at the surface although less sulfur is present in the bulk than required for the formation of MoS2. During reaction in CO and H2, much of the bulk sulfur is lost. The MoO2 and MoS2 phases in reduced and sulfided Mo/Al2O3 and Mo/CeO2 are apparently present as tiny XRD amorphous two-dimensional crystallites, while the corresponding Mo metal and MoS2 phases in Mo/SiO2 are clearly large, three-dimensional crystallites. The corresponding phases in Mo/C range in crystallite size from very small to very large but are on the average well-dispersed.