Characterization and hydrodesulfurization activity studies of unpromoted molybdenum sulfides prepared by elemental solid state reaction

Abstract

Solid state reactions of elemental molybdenum and sulfur were used to synthesize unpromoted, model HDS catalysts. The samples studied had synthesis stoichiometries of MoS2 MoS1.95, and MoS1.975. Catalytic activity for the hydrodesulfurization of thiophene was tested in a flow reactor at temperatures ranging from 523 to 673 K and I atm of pressure. Characterization of these materials was carried out using X-ray diffraction, electron microscopy, O2 and CO chemisorption, and X-ray photoelectron spectroscopy. Our activity measurements indicate that increased HDS activity coincides with the presence of a nonstoichiometric molybdenum sulfide phase as determined by XRD and electron microscopy. The chemisorption studies revealed that a small reduction in a catalyst's sulfur content could cause a 20-fold increase in BET surface area and a 50-fold increase in O2 or CO uptake. Normalization of the HDS activity based on chemisorption data using O2 and CO as probe molecules proved to be a meaningful method for assessing HDS activities. On the stoichiometric MoS2 sample, XPS showed the presence of significant amounts of surface oxygen even after prolonged treatment in H2/H2S while surface oxygen was completely removed from the nonstoichiometric samples after reduction under similar conditions. This resulted in greater Mo and S concentrations at the surface of the nonstoichiometric materials, improved the surface sulfiding, and may be linked to higher HDS activity. Additional work is necessary to establish the link between the observed increased in catalytic activity, increased H2 dissociation capability, and the specific surface reaction mechanism.