Studies of molybdena-alumina catalysts: IV. Rates and stoichiometry of sulfidation

Abstract

Molybdena-alumina catalysts were sulfided with H 2 S H 2 blends either directly from the calcined (oxidized) state or after prereduction with H 2. The course of the reaction was followed gravimetrically; sulfur analyses at the end of a run permitted material balance calculations to be made on the end state of the catalyst. Sulfided and reduced catalysts were additionally characterized by NH 3 adsorption and D 2 exchange. Catalyst sulfiding occurred readily above 300 °C. Extent of sulfiding increased with temperature. However, a limiting catalyst sulfur content was obtained at a given temperature; neither increase in H 2S partial pressure nor time much affected the sulfur level. The predominant reaction was exchange of oxygen associated with the molybdena (reactive oxygen) for sulfur, with formation of water. At higher temperatures, some additional reactive oxygen was lost due to a reductive reaction (also forming water), which presumably created anion vacancies. Prereduced catalysts sulfided to a lesser extent, even though some sulfur apparently added to anion vacancies present after the prereduction. Ammonia adsorption on sulfided and reduced catalysts showed a correlation with anion vacancy concentration. Exchange measurements with D 2 revealed that the sulfided catalyst irreversibly retained appreciable hydrogen, probably as −SH groups on the surface; the magnitude of the retained H was far greater than that found for reduced catalysts. The Al 2O 3 portion of the catalyst appeared to contain less hydrogen than that characteristic of the pure Al 2O 3 base. A model of the catalyst surface, consisting of one-dimensional, chain-like groupings of MoO 2 over the Al 2O 3 substrate, is proposed to explain the results.