Ammonia is a promising carbon-free based fuel for storing the power generated by renewable energy technologies. 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 density functional theory (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-NH_x 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.
We investigated N-N bond formation via NH₃ nucleophilic attack on M-NH and M-N intermediates, compared to bimolecular coupling of M-NH_x intermediates. We evaluated the reactivity of (TPP)Fe(NH₃)₂ towards N-N bond formation via a hydrazine pathway, and found amide-amide coupling to form a bridged hydrazido complex to be the most favorable pathway for N-N bond formation. Further investigation of possible N-N bond formation pathways by reaction with NH₃ led us to identify a possible Fe^III-NH species with significant aminyl character that bypasses the nucleophilic attack of NH₃ and that promotes homolytic N-H bond cleavage of ammonia. 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.

Margareth Mbea

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