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

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 (HMoO₃) 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 HMoO₃-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 HMoO₃-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.