Craig W. Colling

Synthesis and characterization of molybdenum nitride hydrodenitrogenation catalysts

Abstract Details concerning the relationships between the structural, chemical and catalytic properties of Mo nitrides have been elucidated. A series of Mo nitride catalysts were prepared by the temperature programmed reaction of MoO3 with NH3. The structural properties of these nitrides were complex functions of the heating rates and space velocities employed. Two reaction sequences were proposed to account for the synthesis of high, medium and low surface area materials. An interesting conclusion was that the degree of reduction of the molybdate precursor or intermediate governed the structural properties of the product. Some evidence is also presented to suggest that the nucleation and growth rates involved in the transformation of the oxide to the nitride were significantly influenced by the synthesis conditions. The Mo nitrides proved to be exceptional pyridine hydrodenitrogenation catalysts. Their catalytic properties were superior to those of a commercial sulfided Co-Mo hydrotreatment catalyst, having higher activities and better C-N bond hydrogenolysis selectivities. Hydrodenitrogenation over the Mo nitrides appeared to be structure-sensitive. While detailed relationships between the catalytic activity and surface stoichiometry could not be ascertained, there did appear to be a correlation between the activity, and the particle size and grain boundary length. We proposed that at least two types of HDN sites existed on the Mo nitride surfaces; modest activity sites on the particles and high activity sites at grain boundaries. The N/Mo stoichiometry of the highest activity catalyst was near unity suggesting that MoN was present perhaps localized at the grain boundaries. Finally structures near or at the surface were markedly different from those of the bulk. While the predominant bulk phase was γ-Mo2N, the surface appeared to consist of either non-stoichiometric β-Mo16N7 or mixtures of Mo and β-Mo16N7.

Temperature-programmed desorption and decomposition of NH3 over molybdenum nitride films

The surfaces of β-Mo16N7, γ-Mo2N, and δ-MoN films were characterized using NH3 temperature-programmed desorption (TPD). Ammonia adsorption at ∼ 280 K and TPD using a heating rate of 6 K/s produced NH3 peaks at ∼ 360 K. The desorption kinetics depended on the structure and composition of the film. Ammonia desorption from the β-Mo16N7 and γ-Mo2N films was first-order; however, desorption from the δ-MoN film appeared to be second-order. Assuming a pre-exponential factor of 1013 s−1, the desorption energy for the β-Mo16N7 and γ-Mo2N films was 22 kcal/mol. The NH3 saturation capacity increased in the following order: δ-MoN < β-Mo16N7 < γ-Mo2N. This order is similar to that expected for the Mo surface atom density. Some of the NH3 decomposed into H2 and N2. Two H2 desorption peaks were produced: a low-temperature peak due to recombination of surface hydrogen and a high-temperature peak due to hydrogen that emerged from the nitride subsurface. The N2 desorption spectrum consisted of a peak at ∼ 340 K and several peaks in the range 500–900 K. 15NH3 TPD experiments indicated that the low-temperature N2 desorption peak was due to NH3 decomposition while the origin of the high-temperature peaks was the nitride itself. The amount of N2 that desorbed in this high-temperature envelope increased with increasing NH3 dose. We believe that nitrogen desorption from the nitride was induced by the presence of hydrogen which altered the MoN bonding. Ammonia desorption and decomposition spectra for the films were similar to those for a series of bulk γ-Mo2N powders. Characteristics of the γ-Mo2N film resembled those of the low-surface-area powder (< 20 m2/g), while the behavior of the β-Mo16N7 and δ-MoN films was similar to that for the higher-surface-area powders.

Surface and catalytic properties of molybdenum nitrides.

Abstract A series of Mo nitride catalysts were prepared by the temperature programmed reaction of MoO 3 with NH 3 . The structural properties of these nitrides were complex functions of the space velocities and heating rates employed. Solid-state reaction sequences have been developed to account for the structural and compositional properties of the Mo nitrides. It appeared that the degree of reduction of the molybdate precursor or intermediate significantly influenced the properties of the product. The Mo nitrides proved to be exceptional hydrodenitrogenation catalysts with catalytic properties that were superior to those of a commercial sulfided Co-Mo/γ-Al 2 O 3 hydrotreatment catalyst. Pyridine HDN appeared to be structure-sensitive over the Mo nitrides. Based on characterization using a variety of spectroscopic and microscopic techniques we concluded that at least two types of active sites were present in the Mo nitrides; modest activity sites on the particles and high activity sites at defect sites. The predominant bulk phase in the nitrides was γ-Mo 2 N, however, the surface appeared to consist of either non-stoichiometric β-Mo 16 N 7 or mixtures of Mo and β-Mo 16 N 7 . Furthermore, the surface Mo:N stoichiometry for the highest activity catalyst was near unity suggesting the presence MoN or nitrogen rich regions.