Direct Electrochemical Conversion of Nitrogen to Ammonia from Air and Water on Nitride Electro-Catalysts at Ambient Conditions
Authors: Abghoui Younes , Skulason Egill 
Published: February 2020

The use of ammonia as an energy carrier, chemical feedstock and specially fertilizer has triggered increased interest in developing alternative methods for ammonia synthesis in pursuit of decentralized and environmentally friendly approach. Although Ru and Fe within the Haber-Bosch process show good activity for catalysing nitrogen conversion to ammonia, its expense, scarcity, harsh experimental conditions (extremely high temperature and pressure) and sophisticated industrial setup are prohibitive to its decentralization complicating ammonia production in regions with relatively undeveloped infrastructure such as developing countries. Besides, the reliance of this process on natural gases for supplying its necessary hydrogen feedstock not only promotes severe CO2 emission to the environment but it also makes the production of ammonia and accordingly food and biofuels vulnerable to natural gas price fluctuations and political conflict in the oil-rich regions. Nature, on the other hand, reduces atmospheric nitrogen to ammonia by solvated protons and electrons at very moderate conditions, but engineering man-made ammonia to duplicate this in a small-scale approach still remains one of the challenges in modern chemistry. Nonetheless, electro-catalytic ammonia synthesis is a particularly feasible method due to the potential of utilizing renewable energy sources and mild operating conditions. Solid-state electrolytes and polymeric membrane assemblies are the most promising electrochemical approaches developed thus far and have exhibited great success with respect to increased yield of ammonia at ambient conditions. Nevertheless, the yield still remains rather low and the complexity of systems of this sort necessitates a simpler approach for realistic commercialization. In our previous work (Abghoui et al., Physical Chemistry Chemical Physics, 17 (2015) 4909), first-principles (DFT) calculations are used to identify transition metal nitride catalysts that show evidence of being able to produce ammonia electrochemically under small over-potential at ambient conditions. Among a wide range of transition metal nitrides, the (100) facets of rocksalt structure of VN and ZrN were explored as nitride (electro-) catalysts, which are able to suppress the hydrogen evolution reaction and most likely to increase the yield of ammonia production. In the current work and with similar approach, the catalytic activity of the (111) facets of mono-nitrides is investigated through Mars-van Krevelen (MVK) mechanism. This introduces new (electro-) catalysts for direct conversion of nitrogen to ammonia electrochemically at applied potentials of -0.60 V to -1.0 V vs. SHE. Possibility of ammonia formation is also explored via associative, dissociative and mixed associative-dissociative mechanisms on the clean surface of all these nitrides. Besides, stability of these catalysts against poisoning and decomposition is investigated. Upon exposure of the catalyst to atmosphere or electrochemical media, there is a possibility of oxide formation on the surface. The potential required to reduce this oxide layer to water is estimated as well. Results presented here show that both associative and dissociative pathways for ammonia formation is deactivated at the surface of these nitrides due to extremely weak adsorption of N2 on the surface and very high activation barriers of above 2.0 eV for nitrogen splitting, respectively. Therefore, the MVK mechanism is the dominant pathway for effective reduction of nitrogen to ammonia at ambient conditions on these nitride (electro-) catalysts. Hence, with the high abundance of atmospheric nitrogen and by using water as a proton source these promising catalysts could facilitate low-cost, high-yield ammonia production at ambient conditions, with significant benefits to the environment and economy.