Electrochemical ammonia production on molybdenum nitride nanoclusters

Abstract

Theoretical investigations of electrochemical production of ammonia at ambient temperature and pressure on nitrogen covered molybdenum nanoparticles are presented. Density functional theory calculations are used in combination with the computational hydrogen electrode approach to calculate the free energy profile for electrochemical protonation of N2 and N adatoms on cuboctahedral Mo13 nanoparticles. Pathways for electrochemical ammonia production via direct protonation of N adatoms and N2 admolecules with an onset potential as low as -0.5 V and generally lower than -0.8 V on both a nitrogen covered or a clean Mo nanoparticle. Calculations presented here show that nitrogen dissociation at either nitrogen vacancies on a nitrogen covered molybdenum particle or at a clean molybdenum particle is unlikely to occur under ambient conditions due to very high activation barriers of 1.8 eV. The calculations suggest that the nitrogen will be favored at the surface compared to hydrogen even at potentials of -0.8 V and the Faradaic losses due to HER should be low.

Progress in the Electrochemical Synthesis of Ammonia

“…The significant results of theoretical studies in ammonia production systems have also been expanded to electrocatalytic systems at low temperature and pressure [2,9,[76][77][78][79][80][81][82]. The scientific interest in the electrochemical synthesis increased considerably because ammonia production under mild conditions could reduce the overall cost of the process [1].…”

Electrochemical synthesis of ammonia via Mars-van Krevelen mechanism on the (111) facets of group III–VII transition metal mononitrides

“…In many cases, N 2 adsorption to the N-vacant site(s) becomes an exothermic process on TMNs [4] compared to when N 2 needs to be adsorbed on pure transition metals [9,12] or on clean TMNs [13], which is usually endothermic. Therefore, it is interesting to investigate catalytic reactions like nitrogen activation on these TMN surfaces via a MvK mechanism where its feasibility has recently been reported on different TMNs [3,4,10,14]. All these characteristics mentioned above, together with high resistance against corrosion and high http://dx.doi.org/10.1016/j.cattod.2016.06.009 0920-5861/© 2016 Elsevier B.V. All rights reserved.…”

“…Therefore, TMNs have evolved a potential candidate for the noble metal material catalyst and consequently represented better activity i.e. for electrochemical ammonia synthesis [3][4][5] and solar thermochemical ammonia production [6][7][8] when compared with pure metals [9]. For catalytic nitrogen reduction to ammonia, these TMNs have the extra benefit over pure transition metals since nitrogen atoms are already incorporated in their structure.…”

“…However, many of these metals require a relatively large overpotential of up to −1.5 V vs. standard hydrogen electrode (SHE) to activate nitrogen and form ammonia in 1 M aqueous electrolyte (pH = 0) at room temperature [9], where the equilibrium potential for ammonia formation is around +0.06 V. In addition to susceptibility of some of these pure metals to readily form oxides, the formation of H 2 (g) is very fast on most of them and severely hinder the production of ammonia on those surfaces. Recent theoretical studies revealed the possibility of employing under-coordinated molybdenum nanoclusters, with [3] and without [16] a nitrogen skin, for nitrogen activation and electrochemical ammonia formation. There it was found that ammonia can be formed at −0.6 V vs. SHE but the N-vacant sites that are created in that process get poisoned with oxygen from the electrolyte [17].…”

Simultaneous electrocatalytic reduction of dinitrogen and carbon dioxide on conducting polymer electrodes

“…When this essential chemical process occurs in nature it is based -like almost all natural processes-on a sensitive dynamic equilibrium. However, this process is too slow for use at a technical scale and a large amount of energy is needed for the reduction due to the strong triple bond in the N 2 molecule [1][2][3][4].…”