(495a) Chemical Looping Synthesis of Ammonia over Cobalt Molybdenum Nitride: Effects of Surface Hydrogen on Productivity

Ammonia synthesis currently proceeds by the Haber-Bosch process which requires high pressures and temperatures. Unconventional methods have been increasingly explored to try to overcome the ammonia equilibrium limit and to operate at lower pressures Ammonia synthesis is fundamental to the global supply chain and the Haber-Bosch process consumes 1-2% of global energy [1]. This approach enables activation of N₂ at lower pressure [1-4].

Cobalt molybdenum nitride was identified as a useful ammonia catalyst by Kojima and Aika in 2001 [3]. It was also explored as a nitrogen transfer agent by Mckay et. al. where it could be used as a reversible nitrogen store [2]. It was theorized that a Mars–van Krevelen mechanism of reaction occurs with cobalt molybdenum nitride catalysts that involves the both bulk and the surface [4]. This work will explore the effects of hydrogen on the cobalt molybdenum catalyst system.

Ammonia synthesis over Co₃Mo₃N can proceed by two different methods: hydrogenation of the stored nitride with hydrogen and as a traditional ammonia synthesis catalyst using a 3:1 H₂:N₂ gas.

It was found that ammonia synthesis under pure N₂ was at times more productive than under H₂ gas. It is theorized that metal hydrides, oxynitrides or oxyhydrides may play a role in enhanced ammonia production. Chemical looping cycle times and temperatures also effect crystal structure and regeneration of the catalyst surface.

References

1. Michalsky, R., Avram., A. M., Peterson, B. A., Pfromm, P. H., and Peterson, A. A., Sci. 6, 3965, (2015)
2. Mckay, D., Gregory, D. H., Hargreaves, J. S. J., Hunter, S. M., and Sun, X. Commun., 3051 (2007)
3. Kojima, R., Aika, K., Catal. A: Gen., 215, 149 (2001)
4. Hunter, S. M., Gregory D. H., Hargreaves J. S. J., Richard, M. Duprez, D., and Bion, N., ACS Catal. 1719 (2013)