The influence of phase and morphology of molybdenum nitrides on ammonia synthesis activity and reduction characteristics

The reactivities of a series of ternary and binary molybdenum nitrides have been compared. Data have been obtained for the catalytic synthesis of ammonia at 400 deg. C and ambient pressure using a 3:1 H2:N2 mixture. Amongst the ternary nitrides, the mass normalised activity is in the order Co3Mo3N>Fe3Mo3N>>Ni2Mo3N. For the binary molybdenum nitrides, the ammonia synthesis activity is significantly lower than that of Co3Mo3N and Fe3Mo3N and varies in the order γ-Mo2N∼β-Mo2N0.78>>δ-MoN. Nanorod forms of β-Mo2N0.78 and γ-Mo2N exhibit generally similar activities to conventional polycrystalline samples, demonstrating that the influence of catalyst morphology is limited for these two materials. In order to characterise the reactivity of the lattice nitrogen species of the nitrides, temperature programmed reactions with a 3:1 H2:Ar mixture at temperatures up to 700 deg. C have been performed. For all materials studied, the predominant form of nitrogen lost was N2, with smaller amounts of NH3 being formed. Post-reaction powder diffraction analyses demonstrated lattice shifts in the case of Co3Mo3N and Ni2Mo3N upon temperature programmed reaction with H2/Ar. Incomplete decomposition yielding mixtures of Mo metal and the original phase were observed for Fe3Mo3N and γ-Mo2N, whilst β-Mo2N0.78 transforms completely to Mo metal and δ-MoN is converted to γ-Mo2N. - Graphical abstract: Nanorod γ-Mo2N