Characterization of Cobalt Molybdenum Nitrides for Thiophene HDS by XRD, TEM, and XPS

The activities of the CoMo nitride catalysts, with and without sulfidation, were studied based on the rates of thiophene HDS per catalyst weight and turnover frequency (thiophene converted per irreversibly adsorbed CO). The surface properties, such as morphology, composition, and Co and Mo oxidation state numbers of the catalysts without exposure to air after nitriding were characterized by XRD, TEM, and temperature-programmed reduction. The CoMo nitride catalysts were used after nitriding during the temperature-programmed reaction of mixtures of MoO3 or (NH4)6Mo7O24·4H2O with CoO or Co(NO3)2·H2O with Co/(Co+Mo)=0, 0.25, and 0.5 in a stream of ammonia. Co3Mo3N was not produced by nitridation of a mixture of CoO and MoO3 but was produced from CoMoO4. The TEM analysis showed that Co3Mo3N particles (20 to 30 nm) were surrounded by γ-Mo2N particles (ca. 4 nm) in the catalyst nitrided at 1023 K. An XPS study without argon etching showed that the catalysts nitrided at 773 and 873 K contained Mo2+ and Co2+, while the catalysts nitrided at 973–1073 K mainly consisted of Mo0 and Co2+. The higher activity of the catalyst nitrided at 973 K compared to the other nitrided catalyst is probably due to the formation of a small amount of Co3Mo3N before the reaction and the obvious formation after the reaction. The sulfidation of the oxidic catalysts and the catalyst nitrided at 973 K produces stronger active sites of CoMo sulfide rather than Co3Mo3N for HDS.