Synthesis and characterization of high surface area molybdenum nitride

摘要

The synthesis of high surface area γ-Mo_2N materials using the nitridation of oxide precursors MoO_3, H_2MoO_5, and H_2MoO_5·H_2O with ammonia at 650°C is described. H_2MoO_5 and its hydrated form were obtained from the reaction of MoO_3 and diluted H_2O_2. The materials were characterized by means of X-ray powder diffraction, thermal analysis and nitrogen physisorption. Directly after the preparation, the nitride materials were subjected to different processing conditions: (1) contact to air, (2) inert gas or (3) treated with 1% O_2(g)/N_2(g) gas mixture (Passivation). The synthesis and passivation conditions critically affect the specific surface area of the final product. By means of XRD a minor quantity of MoO_2 was detected in most of the products. The highest specific surface area of the nitrides was 158.4 m~(2)/g for γ-Mo_2N materials using H_2MoO_5·H_2O as the precursor. The high specific surface area corresponds to an average particle diameter of 4 nm, assuming a cubic morphology of the nanocrystals (d p = 6/ρSBET, ρ = 9.5 g/cc). The nitrogen physisorption isotherms of γ-Mo2N are of type IV, but pore sizes and diameters differ significantly depending on the synthesis conditions due to different defect structures of the intermediates generated in the course of the topotactic transformation of the oxides to nitrides.