Molybdenum Oxide Precursors that Promote the Low-Temperature Formation of High-Surface-Area Cubic Molybdenum (Oxy)nitride.

Abstract

Molybdenum nitrides and oxynitrides have been increasingly realized as (electro)catalysts for a variety of reactions. In this context, the cubic "γ-Mo2N", also known to contain oxygen in the bulk, is of particular interest. The γ phase is typically derived from ammonolysis of MoO3, and a high temperature is needed to fully react the stable MoO2 intermediate that often forms along the reaction pathway. In this study, ammonolysis of atypical bronze (HxMoO3) and peroxo (H2MoO5) precursors was undertaken to avoid the formation of this undesired intermediate with the aim of synthesizing "γ-Mo2N" at reduced temperatures and thus with a high surface area. It was found, using in situ powder diffraction, that, when the phase I bronze (x ≈ 0.3) served as the precursor, MoO2 formed as an intermediate and was retained in the reaction product until 700 °C. In contrast, ammonolysis of the phase III bronze (x ≈ 1.7) and of H2MoO5 circumvented the MoO2 intermediate. From these latter two precursors, "γ-Mo2N" was formed at the lowest maximum reaction temperatures reported in the literature, namely, 480 °C in the case of HxMoO3-III and 380 °C for H2MoO5. The resulting products displayed extremely high surface areas of 206 and 152 m2/g, respectively, presumably as a consequence of the low synthesis temperatures. While the HxMoO3-III precursor showed evidence of a topotactic transformation pathway, with morphological similarity between precursor and product phases, H2MoO5 transformed via amorphization. Electrochemical characterization showed moderate activity for the hydrogen evolution reaction (HER), which increased after exposure to reducing potentials and loosely scaled with the catalyst-specific surface area. This work points toward new low-temperature synthesis pathways for accessing molybdenum (oxy)nitrides with high surface areas.