Molybdenum nitride catalysts: I. Influence of the synthesis factors on structural properties

Abstract

Effects of the synthesis parameters on the structural properties of molybdenum nitride catalysts, prepared by the temperature-programmed reaction of MoO 3 with NH 3, have been examined. Molybdenum trioxide was heated in flowing NH 3 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 γ-Mo 2N. Some of the lower surface area catalysts also contained MoO 2 and Mo, but there was no evidence of nitrides other than γ-Mo 2N. The catalysts consisted of micrometersized, plate-like aggregates of nanometer-sized crystallites, and possessed surface areas ranging up to ≈140 m 2/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 m 2/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 H x MoO 3 ( x ≤ 0.34) and γ-Mo 2O y N 1- 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 O 2 uptake. The O 2 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.