The Structure of Highly Dispersed SiO2-Supported Molybdenum Oxide Catalysts during Sulfidation

The structure of sulfided Mo-catalysts and their oxidic precursors has been abundantly studied, but the genesis\nof the active phase has remained much less investigated. The sulfidation (in H2S/H2 atmosphere) of a series\nof MoO3/SiO2 catalysts has been examined by means of temperature-programmed sulfidation, X-ray absorption\nfine structure, and transmission electron microscopy.The oxidic, oligomeric clusters in a 5.6 wt 5% MoO3/Si02\ncatalyst are transformed into partly sulfided particles (MOOS,,) by 0-S exchange at room temperature. A\nmolybdenum sulfide species the structure of which resembles the MoS, structure is formed during sulfidation\nat 423 K. The MoS2 phase is formed at temperatures between 523 and 573 K, depending on the dispersion\nof the initial Moo3 phase. The transition of MoS3 into MoS2 can be monitored by the evolution of H2S from\nthe catalyst with a simultaneous consumption of H2. The two-dimensional size of the MoS2 slabs can be derived\nfrom the EXAFS Mo-Mo coordination number by means of a theoretical model. TEM is required to elucidate\nthe stacking height of the slabs.