The surface chemistries of a series of Mo nitride catalysts with surface areas ranging up to 193 m2/g were characterized using thermal desorption spectroscopies. These materials were prepared by the temperature programmed reaction of MoO3with NH3. The passivated catalysts contained up to one monolayer of oxygen on or in the surface. This oxygen was removed as H2O via two pathways during temperature programmed reduction: (a) reaction with hydrogen from decomposed NH3and hydrogen residue left on the surface after synthesis at temperatures less than ∼550 K and (b) reaction with gas phase H2at higher temperatures. Nitrogen and/or surface NHxspecies also reacted with gas phase H2producing NH3. The NH3adsorption capacities were different for each of the reduced Mo nitrides; however, other variations in the temperature programmed desorption spectra were qualitatively similar. At low coverages, all of the NH3decomposed and the products desorbed as H2at 500–700 K and N2above 600 K. Following saturation of the surface, most of the NH3desorbed molecularly at 300–500 K and the balance decomposed. The ratio of molecularly desorbed to decomposed NH3was approximately 3 : 1 for all the catalysts suggesting a connection between the desorption and decomposition of NH3. We have interpreted the results, in particular for the high surface area materials where the saturation coverage was low, in terms of localized NH3adsorption perhaps forming islands at high coverage. The pyridine HDN reaction rate increased linearly with the amount of NH3chemisorbed. The corresponding turnover frequency was 4.1×10−4s−1at 633 K. Finally, the number of types of NH3and H2desorption sites was a function of the surface area. The low surface area, high activity Mo nitrides possessed low and high temperature desorption sites. Only the low temperature sites were observed for the high surface area, low activity materials. The rate limiting step for NH3desorption from the Mo nitrides was first order with desorption energies for the low and high temperature sites of 24±4 and 32±5 kcal/mol, respectively.

A series of γ-Al2O3 supported Mo nitrides were prepared by the temperature programmed reaction of supported molybdates with NH3. Several loadings, heating rates, and space velocities were employed in an attempt to vary the properties of the nitride. The structure and composition of the supported nitrides depended on the structure of the precursor oxide and the conditions employed in nitriding the oxide. The ease with which the oxide was nitrided improved with increasing Mo loading. Nitride domains in the low-loaded materials (4 and 8 wt% Mo) were highly dispersed and X-ray amorphous while the higher-loaded catalysts (16 wt% Mo) consisted of γ-Mo2N crystallites. The response of the high-loaded materials to the various nitriding conditions was similar to that observed for unsupported γ-Mo2N. The supported Mo nitrides were active for the hydrodenitrogenation of pyridine. In fact, their activities were superior to that of a commercial sulfided NiMo hydrotreating catalyst and comparable to those of the unsupported Mo nitrides. The catalytic properties of the supported Mo nitrides depended on the size and composition of the Mo nitride domains. We believe that the most active sites were located at the perimeters of two-dimensional, raft-like domains. Regions near the perimeter also appeared to be nitrogen deficient. Lower activity sites were associated with the γ-Mo2N crystallite surfaces.

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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 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/gamma-Al2O3 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 gamma-Mo2N, however, the surface appeared to consist of either non-stoichiometric beta-Mo16N7 or mixtures of Mo and beta-Mo16N7. Furthermore, the surface Mo:N stoichiometry for the highest activity catalyst was near unity suggesting the presence MoN or nitrogen rich regions.

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.