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

The production of high surface area (150 m 2/g, passivated) unsupported γ-Mo 2N by temperature programmed reaction of MoO 3 powder with mixtures of H 2 and N 2 is reported. The addition of 1290 ppm H 20 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 N 2/H 2 space velocities and higher temperature ramping rates are attributed to increased concentrations of H 2O evolved by reaction. Elevated H 2O concentrations increase the temperature required for solid reduction. Observed reaction intermediates include MoO 2, Mo, and an unidentified molybdenum oxide, hydroxide, or hydrate. Intermediates in topotactic syntheses exhibited intermediate surface areas (up to 60 m 2/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 H 2O and H 2 also influences the rate of the gas/solid reaction.