Hidden Complexity in the Chemistry of Ammonolysis-Derived “γ-Mo2N”: An Overlooked Oxynitride Hydride

Abstract

Molybdenum nitrides have been employed in a variety of applications. For use in catalysis, the cubic γ phase with the nominal stoichiometry Mo₂N and the space group Fm3?m is typically prepared by high-temperature reaction of MoO₃ with NH₃. The literature presents conflicting reports of the possible presence of residual oxygen from typical ammonolysis reactions and whether such species influence the crystal structure and morphology. With the aim of resolving these open questions, a comprehensive study of the chemistry, crystal structure, and electronic structure of molybdenum nitride materials prepared by ammonolysis has been undertaken here, with particular focus on the role of reaction temperature. Ammonolysis of MoO₃ was carried out at 973 and 1073 K and yielded single-phase cubic products. Using electron energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis, prompt gamma-ray neutron activation analysis, and combustion analysis, significant concentrations of oxygen and, to a lesser extent, hydrogen were found in both materials. The crystal structure of each phase was refined by Rietveld analysis using combined synchrotron X-ray diffraction and neutron diffraction data. The structures were found to be derivatives of the B1 rock salt (halite) structure, as is often reported for “γ-Mo₂N.” However, both materials adopt the space group Pm3?m, as opposed to the typically presumed space group of Fm3?m, and both have much higher anion content than implied by the stoichiometry Mo₂N. Ordering of cation vacancies and of anion species is responsible for the loss of the translational symmetry expected for the space group Fm3?m. X-ray absorption spectroscopy studies, along with the EELS and XPS results, showed the Mo oxidation state to be diminished with higher temperature synthesis, consistent with the retention of a lower concentration of anions and in particular oxygen. The difficulty in differentiating oxygen and nitrogen and the impossibility of detecting hydrogen by X-ray and electron diffraction methods, especially in the presence of the heavy element Mo, have likely inhibited accurate identification of Mo_1–x(N_1–yO_y)H_z as the product of MoO₃ ammonolysis. The findings reported here offer a critical assessment for understanding the properties of molybdenum “nitrides” in electronic and catalytic applications.