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

McKay, D., Hargreaves, J.S.J. , Rico, J.L., Rivera, J.L. and Sun, X.L. (2008) The influence of phase and morphology of molybdenum nitrides on ammonia synthesis activity and reduction characteristics. Journal of Solid State Chemistry, 181(2), pp. 325-333. (doi: 10.1016/j.jssc.2007.12.001)

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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 °C and ambient pressure using a 3:1 H<sub>2</sub>:N<sub>2</sub> mixture. Amongst the ternary nitrides, the mass normalised activity is in the order Co<sub>3</sub>Mo<sub>3</sub>N>Fe<sub>3</sub>Mo<sub>3</sub>N⪢Ni<sub>2</sub>Mo<sub>3</sub>N. For the binary molybdenum nitrides, the ammonia synthesis activity is significantly lower than that of Co<sub>3</sub>Mo<sub>3</sub>N and Fe<sub>3</sub>Mo<sub>3</sub>N and varies in the order <i>γ</i>-Mo<sub>2</sub>N∼β-Mo<sub>2</sub>N<sub>0.78</sub>⪢δ-MoN. Nanorod forms of β-Mo2N<sub>0.78</sub>and <i>Y</i>-Mo<sub>2</sub>N 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 H<sub>2</sub>:Ar mixture at temperatures up to 700 °C have been performed. For all materials studied, the predominant form of nitrogen lost was N<sub>2</sub>, with smaller amounts of NH<sub>3</sub> being formed. Post-reaction powder diffraction analyses demonstrated lattice shifts in the case of Co<sub>3</sub>Mo<sub>3</sub>N and Ni<sub>2</sub>Mo<sub>3</sub>N upon temperature programmed reaction with H<sub>2</sub>/Ar. Incomplete decomposition yielding mixtures of Mo metal and the original phase were observed for Fe<sub>3</sub>Mo<sub>3</sub>N and <i>γ</i>-Mo<sub>2</sub>N, whilst β-Mo<sub>2</sub>N<sub>0.78</sub> transforms completely to Mo metal and δ-MoN is converted to <i>γ</i>-Mo<sub>2</sub>N.

Item Type:Articles
Glasgow Author(s) Enlighten ID:Hargreaves, Professor Justin
Authors: McKay, D., Hargreaves, J.S.J., Rico, J.L., Rivera, J.L., and Sun, X.L.
College/School:College of Science and Engineering > School of Chemistry
Journal Name:Journal of Solid State Chemistry
ISSN (Online):1095-726X
Published Online:08 December 2007

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Project CodeAward NoProject NamePrincipal InvestigatorFunder's NameFunder RefLead Dept
370401Fundamental studies as a route towards greener, nitrogen insertion chemistryJustin HargreavesEngineering & Physical Sciences Research Council (EPSRC)GR/S87300/01Chemistry