Surface properties and reactivity of supported and unsupported molybdenum nitride catalysts

A series of gamma-Al2O3 supported molybdenum nitride catalysts (nitrided Mo/gamma-Al2O3 (MNS), Co-Mo/gamma-Al2O3 (CMNS), and Ni-Mo/gamma-Al2O3 (NMNS)) and unsupported molybdenum nitride catalyst (MNUS) were prepared by the temperature programmed reaction of their corresponding molybdenum oxide precursors with NH3. The surface properties of prepared molybdenum nitride catalysts were characterized by N-2 adsorption, XRD, and XPS methods. N-2 adsorption and XRD studies show that the supported molybdenum nitride catalysts possess a highly dispersed surface molybdenum nitride species on the alumina support, while unsupported molybdenum nitride catalyst possesses a typical crystalline Mo2N (FCC) structure. The XPS data of supported molybdenum nitride catalysts reveal that Mo2+, Mo3+, and Mo4+ species were selectively produced upon nitridation. The results of dibenzothiophene (DBT) hydrodesulfurization over molybdenum nitride catalysts show that the reactivity is strongly dependent on the type of catalysts and it decreases in the order: MNS > NMNS greater than or equal to CMNS greater than or equal to MNUS. In addition, it was also found that the reactivity increases when increasing the reaction temperature and pressure and contact time. No synergistic effect for DBT hydrodesulfurization over Co (or Ni) promoted, nitrided Mo/gamma-Al2O3 catalysts is observed. The major reaction product is biphenyl and cyclohexylbenzene is next in abundance regardless of the type of catalysts and reaction conditions. The reactivity for DBT hydrodesulfurization does not correlate with amounts of CO chemisorption. The reactivity is rather well correlated with the difference in heat of adsorption between thiophene and H2S.