Onset potentials for different reaction mechanisms of nitrogen activation to ammonia on transition metal nitride electro-catalysts

- The Mars-van Krevelen mechanism is the most favourable reaction mechanism for NER.
- Adsorption of N2 admolecules on the clean surface of TMNs is always endothermic.
- Adsorption of N adatoms on the clean surface of TMNs is always endothermic.
- Direct N2 dissociation encounters activation barriers of above 2 eV on TMNs.
- H-assisted N2 activation is significantly more facile than direct N2 dissociation on TMNs.

Recent theoretical calculations with DFT suggest that transition metal nitrides (TMNs) are promising materials to catalyze N2 electroreduction to ammonia at ambient conditions. To realize which mechanism is more favourable, we conduct DFT calculations to explore the catalytic activity of these materials in their most stable structures via conventional associative (AM) and dissociative (DM) mechanisms, and then compare the corresponding results with that of Mars-van Krevelen (MvK) mechanism we recently reported. The dissociation of N2 on the clean surfaces is endothermic on most of these nitrides and the activation barriers large in all cases, which is inhibitive of a DM on these materials. The onset potential predicted for ammonia formation on these TMNs is always less negative via MvK than with AM, except a few cases, where both mechanisms have similar onset potentials. In those cases, the AM is less favourable than MvK since the adsorption of N2 molecule is endothermic. Therefore, the MvK is almost always the favourable mechanism. We used the computational hydrogen electrode method and neglected any proton-electron transfer reaction barriers in this work but including those will be necessary to make a more definitive statement, which is the subject of future work.

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