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 H_xMoO₃-I (x ≈ 0.3), are identified, with hexagonal δ-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 H_xMoO₃-I to MoO₂ and finally to cubic γ-MoO_xN_y. There is no evidence for direct transformation from H_xMoO₃-I to γ-MoO_xN_y, 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 γ-MoO_xN_y is obtained after reaction at 800 °C. This comprehensive elucidation of the reaction pathway provides a roadmap for future tuning of the molybdenum oxynitride morphology and chemistry.