Elucidating the Reaction Pathway in the Ammonolysis of MoO₃ via In Situ Powder X-ray Diffraction and Transmission Electron Microscopy

Molybdenum nitrides and oxynitrides are of interest as catalysts for a wide variety of applications. They are commonly synthesized via the high-temperature ammonolysis of MoO3 in which the oxide precursor is reacted with gas-phase ammonia at an elevated temperature. Despite the widespread adoption of ammonolysis as a synthetic approach, the reaction pathway leading to the formation of molybdenum nitrides by this method remains poorly understood. In this work, combined in situ powder X-ray diffraction (PXRD) and in situ transmission electron microscopy (TEM) are used to fully map the transformation of MoO3 to the final product and identify the structure relationships among the precursors, intermediates, and product. Two key intermediates, MoO2 and HxMoO3-I (x approximate to 0.3), are identified, with hexagonal delta-MoN occurring as a minor, short-lived, high-temperature intermediate, depending on reaction conditions. Notably, the reaction is topotactic throughout the transformation from MoO3 to HxMoO3-I to MoO2 and finally to cubic gamma-MoOxNy. There is no evidence for direct transformation from HxMoO3-I to gamma-MoOxNy, a route suggested in the prior literature. Moreover, even when the reaction follows a pathway in which the material fully transforms to MoO2 at intermediate temperatures, single-phase gamma-MoOxNy is obtained after reaction at 800 degrees C. This comprehensive elucidation of the reaction pathway provides a roadmap for future tuning of the molybdenum oxynitride morphology and chemistry.