Molybdenum nitrides and oxynitrides, and in particular cubic γ-Mo_1-x(N_1-yO_y)H_z, have been increasingly realized as (electro)catalysts for a variety of reactions. Synthesis of γ-Mo_1-x(N_1-yO_y)H_z is often realized from ammonolysis of MoO₃ at temperatures of 650°C – 800°C. These high reaction temperatures are necessary in order to fully react the stable MoO₂ intermediate that forms from the reduction of the MoO₃ precursor, limiting the specific surface area of the final product. With the aim of circumventing the MoO₂ intermediate and enabling low-temperature synthesis of γ-Mo_1-x(N_1-yO_y)H_z, ammonolysis of two hydrogen molybdenum bronzes (H_1.7MoO₃ and H₂MoO₅) was undertaken in this work. Utilizing in situ powder X-ray diffraction, we find that, as desired, the MoO₂ intermediate did not form during the reactions when using these precursors. Instead, the γ-Mo_1-x(N_1-yO_y)H_z was phase pure at the lowest maximum reaction temperatures reported in literature (380°C for the H₂MoO₅ and 480°C for the H_1.7MoO₃), and similarly the specific surface areas amongst the highest in the literature, 152 m²/g and 205 m²/g, respectively. The in situ studies reveal a pathway for formation from H₂MoO₅ that includes an amorphous intermediate, not seen in the case of the H_1.7MoO₃, and is evidently responsible for the lower surface area in spite of the lower synthesis temperature. Characterization of these oxynitrides as catalysts for hydrogen evolution revealed comparable macroscopic activity, despite differences in surface area. We attribute to the moderate activity of the high surface area material (synthesized at lower temperature) to the higher oxidation state of Mo in this material. This work reveals the high degree of tunability of γ-Mo_1-x(N_1-yO_y)H_z possible through control of the reaction pathways for its synthesis, opening the door for further optimization to balance the surface area and composition.

Matthew Sweers

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