MOLYBDENUM NITRIDE CATALYSTS .1. INFLUENCE OF THE SYNTHESIS FACTORS ON STRUCTURAL-PROPERTIES

Effects of the synthesis parameters on the structural properties of molybdenum nitride catalysts, prepared by the temperature-programmed reaction of MoO3 with NH3, have been examined. Molybdenum trioxide was heated in flowing NH3 through two linear heating segments (623 to 723 K then 723 to 973 K) with different space velocities in a 2(3) factorial design. The temperature limits for these heating segments were defined based on the results of in situ X-ray diffraction analysis of the gas-solid reaction. The resulting catalysts were characterized using BET surface area analysis, environmental scanning electron microscopy, ex situ X-ray diffraction, and oxygen chemisorption. The primary bulk phase present was gamma-Mo2N. Some of the lower surface area catalysts also contained MoO2 and Mo, but there was no evidence of nitrides other than gamma-Mo2N. The catalysts consisted of micrometer-sized, plate-like aggregates of nanometer-sized crystallites, and possessed surface areas ranging up to almost-equal-to 140 m2/g depending on the synthesis and reduction conditions employed. Statistical analysis of the results revealed that the space velocity individually and the heating rates combined had the most significant effects on the structural properties. The production of catalysts with surface areas in excess of 50 m2/g required the use of slow heating rates during the first segment and high space velocities. We concluded that the key to producing the highest surface area Mo nitrides was channeling the reaction through HxMoO3 (X less-than-or-equal-to 0.34) and gamma-Mo2OyN1-y intermediates. Passivation of the materials immediately following synthesis appeared to produce an oxynitride at the surface. Reduction of the passivated materials in H-2 at temperatures up to 673 K caused a significant increase in the surface area and O2 uptake. The O2 uptake for the low and medium surface area catalysts varied linearly with the BET surface area and corresponded to an O:Mo stoichiometry of approximately 1:5. The oxygen site density for the highest surface area nitride was lower than those for the lower surface area catalysts, presumably due to differing surface structUreS. (C) 1994 Academic Press, Inc.