Electroreduction of N₂ to Ammonia at Ambient Conditions on Mononitrides of Zr, Nb, Cr, and V: A DFT Guide for Experiments

Abstract

A rapid and facile reduction of nitrogen to achieve sustainable and energy-efficient production of ammonia is critical to its use as a hydrogen storage medium, chemical feedstock, and especially for manufacturing inorganic fertilizers. For a decentralization of catalytic ammonia production, small-scale N2 reduction devices are required that are equipped with the most stable, selective, and active catalysts that operate at low temperature and ambient pressure. Here, we report the development of new and cost-efficient catalysts, transition metal nitrides, which enable electrochemical reduction of molecular nitrogen to ammonia in aqueous media at ambient conditions with only a low applied bias. The most promising catalysts are VN, ZrN, NbN, and CrN, which are identified among a range of transition metal nitride surfaces through a comprehensive density functional theory based analysis. All four nitrides are found to be more active toward nitrogen reduction than toward the competing hydrogen evolution reaction, in contrast to pure metal catalysts, which largely evolve hydrogen. Furthermore, their stability against poisoning and possible decomposition under operating conditions is also studied. Particular single-crystal surfaces are needed for ZrN, NbN, and CrN because polycrystalline surfaces may result in decomposition of the whole catalyst. Polycrystalline surfaces of VN may, however, be used since the rocksalt (100) facet is predicted to produce ammonia via a Mars–van Krevelen mechanism with only a −0.5 V overpotential, thereby avoiding decomposition. We suggest that this is a promising step toward the development of a method for synthesizing ammonia cheaply, to prepare high-value-added nitrogenous compounds directly from air, water, and electricity at ambient conditions. An additional benefit to the present analysis is that the method used in this work may be applicable to other aqueous phase catalytic reactions, where a Mars–van Krevelen mechanism is operative and product selectivity and activity are key catalytic criteria.

Recent progress towards the electrosynthesis of ammonia from sustainable resources

“…56 NbN and CrN may also act in a similar fashion. 57 As yet, it would again seem that no such nitride materials have been tested experimentally as cathodes for nitrogen electroreduction, and so it would be very interesting to see if they performed as anticipated in real-world situations. It would also be very useful if the same kind of theoretical investigations that have been performed on these simple nitrides could be extended to binary nitrides.…”

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

“…The single N-vacancy approach studied for the (100) facets [4,14] is not applicable here due to the closely packed structure of the (111) surfaces where N atoms are protruded from the surface. Hydrogenation of the surface is carried out by adding H atoms one-by-one to the surface to represent a proton from the solution and an electron from the electrode.…”

“…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.…”

“…There it was shown that the energy lowest surface facets of the earlier TMNs are the (100) facets of the rocksalt (RS) structures whereas the (110) facets of the zincblende (ZB) structures were energy lowest for the later TMNs when comparing the energetics of RS(100), RS(111), ZB(110) and ZB(100) facets using electronic structure calculations [4]. The most active and selective nitride surfaces that catalyze nitrogen activation to ammonia electrochemically at ambient conditions and relatively small overpotentials turned out to be the RS(100) facets [4,14]. However, the RS(111) facets of earlier TMNs are also one of the most prevalent texture orientations when characterizing the manufactured TMN surfaces [18][19][20][21] and those deserve a detailed theoretical investigation for their NER activity.…”

A Computational Study of the Heterogeneous Synthesis of Hydrazine on Co3Mo3N

“…In particular, we found that N 2 can be activated at a surface cavity where N 2 is bound side-on a Co 8 cluster at the 16e Wyckoff site or it can be activated at surface 3f-nitrogen vacancies where N 2 is activated in an end-on configuration [12]. There are other DFT studies that have shown that N-vacancies can participate in the mechanism for ammonia synthesis, such as in the electrochemical reduction of ammonia on Zr, Nb, Cr, V mononitrides [14][15][16] and in the two-phase solarenergy driven ammonia synthesis on metal-nitrides [17,18]. End-on activated N 2 also exists in Fe(0) complexes (i.e.…”