Synthesis of High Surface Area Molybdenum Nitride in Mixtures of Nitrogen and Hydrogen

The production of high surface area (150 m2/g, passivated) unsupported γ-Mo2N by temperature programmed reaction of MoO3 powder with mixtures of H2 and N2 is reported. The addition of 1290 ppm H20 or more to the synthesis gases leads to reduced product surface areas by either hydrothermal sintering or lattice fluidization mechanisms. Reduced surface areas in syntheses with lower N2/H2 space velocities and higher temperature ramping rates are attributed to increased concentrations of H2O evolved by reaction. Elevated H2O concentrations increase the temperature required for solid reduction. Observed reaction intermediates include MoO2, Mo, and an unidentified molybdenum oxide, hydroxide, or hydrate. Intermediates in topotactic syntheses exhibited intermediate surface areas (up to 60 m2/g). A thermodynamic analysis indicates that, in most cases, the intermediate solids are not in equilibrium with the gas phase and that solids may be reduced completely to Mo before nitridation. It is concluded that the rate of the gas/solid reaction rate is determined primarily by the rate of oxygen and nitrogen diffusion in the solid lattice but that competitive adsorption of H2O and H2 also influences the rate of the gas/solid reaction.