A computational study of the electrochemical cyanide reduction for ambient ammonia production on a nickel cathode. Issue 16 (21st July 2021) - Brennan, Kevin
Abstract : Electroreduction of cyanide can proceed through one of two pathways. The first one results in the generation and desorption of methylamine as major product, whereas the second path the formation of this species, yielding methane and ammonia. Abstract : The dependence of the global population on ammonia means that the Haber–Bosch process is one of the most studied and optimised processes in chemistry. Despite this focus, the reaction still requires harsh conditions, which has prompted substantial research efforts to enable the electroreduction of N2 at ambient conditions. The literature regarding this reaction is plagued with reports of competition with the hydrogen evolution reaction and large potentials required for N2 dissociation. Cyanide, isoelectronic with N2, has a significantly lower bond dissociation energy which could enable a route to sustainable ammonia production. Herein, we present the first mechanistic study of the cyanide electroreduction on a Ni cathode, experimentally shown to produce methylamine as a major product, and CH4 and NH3 as minor products. Theoretical calculations reveal that the dominance of methylamine in the products is due to the weak binding of this species, which desorbs from the surface preventing further hydrogenation. We also present an alternative CNRR pathway which involves the hydrogen atoms comprising the predicted surface coverage at relevant reaction conditions. Interestingly, this new pathway circumvents the formation of methylamine, exclusively producing CH4 and NH3, although it is hindered by the presence of a large energy barrier. Strategies to favour this alternative pathway and the prospects of CNRR for the sustainable production of ammonia under ambient conditions are also outlined.