(748e) Ammonia Synthesis: Development of Novel Heterogeneous Catalysts Operating Via Mars-Van Krevelen Type Mechanisms

The success of the chemical industry to sustain the ever-growing demand on energy and for chemical products in our modern society can only be assured by the development of more cost and energy efficient processes. Since most important industrial reactions are catalytic, design of new catalytic materials presenting higher catalytic activity, selectivity and stability is the most effective means to improve existing industrial processes. Catalytic production of ammonia from N₂ and H₂ realized under intensive consuming energy conditions (T = 400-500°C and P = 150-200 atm)¹is an important industrial reaction where any improvement in catalytic activity, selectivity and poison tolerance could have a major economical and environmental impact as it consumes 1-2% of the world’s annual energy production.²

In recent years, interstitial nitrides, in particular Co₃Mo₃N have been shown to possess higher catalytic activity for the Haber – Bosch process, than the commonly used iron-based catalyst.^1,3 It has been suggested that the origin of the high catalytic activity of this system is due to the intermediate binding energy for N₂, close to that of Ru – the optimal catalyst – associated with the (111) plane. The role of N species in this structure was proposed to be limited to ensuring the correct crystallographic ordering.⁴ However, recent investigations conducted by Hargreaves et al have suggested the possible reactivity of "lattice nitrogen" in the Co₃Mo₃N system under ammonia synthesis conditions.⁵ These results raise the possibility that ammonia synthesis on this material occurs via a Mars-van Krevelen type mechanism in which lattice nitrogen is being the active species, reacting with H₂ to yield ammonia. Therefore, controlling the "lattice N" reactivity in nitride materials is a key for the preparation of highly active materials.

In the present work, tantalum nitride material is chosen as a basis to develop new highly active materials for ammonia synthesis under mild conditions. Preliminary tests conducted on Ta₃N₅ samples demonstrated the ability of lattice nitrogen to be reactive to H₂ to produce ammonia. At high temperature, T= 700°C, 19% of the lattice nitrogen of Ta₃N₅ forms ammonia under Ar/H₂ atmosphere as depicted in eqn (1): Ta₃N₅ + 3/2δH₂ → Ta₃N_5-δ + δNH₃. Upon reaction at 700°C for 7h, the production of NH₃ was observed to be ca. 410 μmol g^-1. N post-reaction analysis confirmed that the N content was reduced from 11.15 to 6.71 wt%. In view of this, Ta₃N₅ could show significant promise as new nitrogen transfer material operating with a Mars-van Krevelen type mechanism, wherein the “lattice” N can be directly used as an activated form of nitrogen.

In order to further improve the system and to enhance the lattice nitrogen reactivity, we have explored the role of Mⁿ⁺ doping in Ta₃N₅. For this, a series of Ta_3-xM_xN_y (M = Ru, Co, Fe, x = 0, 0.5, 1, 1.5) nitrides were prepared by high temperature ammonolysis of various precursors. In this presentation, the effect of such cation insertion upon the reactivity of lattice nitrogen as well as the ammonia synthesis performance will be outlined in detail. Furthermore, the control of textural properties of the resultant materials by variation starting precursor will be described.

In addition to the potential development of new and improved ammonia synthesis catalysts, the control of the reactivity of lattice nitrogen may affording interesting opportunities for the development of novel nitrogen transfer pathways.

Reference

(1) Jacobsen, C. J. H. Chemical Communications 2000, 1057.

(2) Ritter, S. K. Chem. Eng. News 2008, 86, 53.

(3) (a) Kojima, R.; Aika, K. I. Applied Catalysis A: General 2001, 215, 149(b) Kojima, R.; Aika, K. I. Applied Catalysis A: General 2001, 218, 121.

(4) Jacobsen, C. J. H.; Dahl, S.; Clausen, B. S.; Bahn, S.; Logadottir, A.; Nørskov, J. K. Journal of the American Chemical Society 2001, 123, 8404.

(5) (a) Hunter, S. M.; Gregory, D. H.; Hargreaves, J. S. J.; Richard, M.; Duprez, D.; Bion, N. ACS Catalysis 2013, 3, 1719(b) Hunter, S. M.; McKay, D.; Smith, R. I.; Hargreaves, J. S. J.; Gregory, D. H. Chemistry of Materials 2010, 22, 2898(c) Gregory, D. H.; Hargreaves, J. S. J.; Hunter, S. M. Catal Lett 2011, 141, 22.