Ammonia is a promising carbon-free based fuel for storing the power generated by renewable energy technologies. Ammonia can be easily transported using existing infrastructure, and the stored energy can be regenerated in the form of electricity in ammonia and hydrogen fuel cells. To produce electricity from its chemical bonds, ammonia can be electrocatalytically oxidized to nitrogen, protons, and electrons. The most relevant steps of this reaction are the cleavage of its N-H bonds and N-N bond formation to generate N2. Transition metal molecular complexes, such as metalloporphyrins, represent an ideal platform to elaborate fundamental design principles for electrocatalytic conversion of ammonia to nitrogen. To develop catalyst design principles and identify promising catalysts, we used DFT to assess the ability of (TPP)M(NH3)2 complexes (where M = Cr, Mn, Fe, Co, Ni, Mo, Ru, W, and Os and TPP = tetraphenylporphyrin) to weaken the N-H bonds of ammonia, as well as their reactivity towards N-N bond formation. We investigated N-N bond formation through the bimolecular coupling of M-NHx (x = 0, 1, 2) intermediates and through the addition of NH3 to M-NHx intermediates. While we found (TPP)Mo(NH3)2 and (TPP)W(NH3)2 to perform best in weaking all three N-H bonds of ammonia, our results show that these complexes form stable nitride species that hinder their reactivity towards N-N bond formation. N-N bond formation can alternatively occur through a “hydrazine pathway”. In this pathway, N-N bond formation can occur via the bimolecular coupling of M-NH2 intermediates to form a bridged hydrazido species, or by reacting with NH3 to form a M-N2H4 species. Our results show that (TPP)Fe(NH3)2 favors N-N bond formation through bimolecular coupling of Fe-NH2 intermediates. Additionally, our investigation of other possible N-N bond formation pathways by reacting with NH3 led us to identify a FeIII-NH species with significant radical character that promotes homolytic N-H bond cleavage of ammonia and results in the formation of Fe-N2H4 species. These results indicate the need to inspect the radical character of M-NH species and their reactivity towards N-H bond cleavage of ammonia prior to N-N bond formation. Understanding the various mechanisms via which ammonia oxidation can occur is essential for learning ways in which transition metal complexes can be tuned and improved for the ammonia oxidation reaction.

Computational investigation of metalloporphyrins for ammonia oxidation

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