(25i) Computational Screening of Transition Metal Nitride Catalysts for Electrochemical Ammonia Formation

With an increasing global population, one of the most pressing issues facing mankind at present is increasing the world’s food supply. To this end, cheaper and more versatile production of fertilizer is vital. The most critical step in the production of fertilizer is the synthesis of ammonia (NH₃). Since the early 20^th century, ammonia has been primarily synthesized via the Haber-Bosch process, in which gaseous nitrogen (N₂) and hydrogen (H₂) are passed over Ru or Fe-based catalysts at high temperatures and pressures. Due to the industrial nature of the Haber-Bosch process, the synthesis of NH₃ is conducted in only a few large-scale chemical plants around the world.  In addition, gaseous H₂ is required, the production of which is rather expensive.

In stark contrast to the harsh conditions required by the Haber-Bosch process, the enzyme nitrogenase catalyzes the formation of NH₃ at ambient temperature and pressure by reacting gaseous N₂ with protons (H⁺) from solution and electrons (e^-) from the enzyme. This electrochemical approach thereby avoids the need for costly production of H₂ as well as requiring only mild conditions. It is thus an alluring prospect to try and mimic the enzyme process in an alternative synthesis of NH₃.

There have been several attempts at electrochemical NH₃ synthesis using simple transition metal catalysts, however these have had only limited success. One of the biggest problems with catalysts identified thus far is that they bind H⁺ ions more strongly than N₂ and thus preferentially form and evolve H₂ [1]. In the present project we investigate the use of transition metal nitride catalysts for electrochemical ammonia synthesis. These nitrides offer the advantage of being able to form NH₃ through reduction of an N atom in the surface layer of the nitride itself, rather than relying on binding N₂ to the surface. The resulting vacancy is later repaired by gaseous N₂, thus completing the catalytic cycle.

We have used density functional theory (DFT) to calculate the energetics of each of the elementary steps in the catalytic cycle on a large number of transition metal nitride surfaces and have identified several potential surfaces that could be used to electrochemically synthesize NH₃ at ambient conditions. We envisage that such catalysts could be used in cost-efficient, environmentally-friendly fertilizer production, using atmospheric nitrogen and water.

References:

[1] E. Skúlason, T. Bligaard, S. Gudmundsdóttir, F. Studt, J. Rossmeisl, F. Abild-Pedersen, T. Vegge, H. Jónsson and J. K. Nørskov, Phys. Chem. Chem. Phys., 14, 1235, (2012).