Electrochemical Production of Nitrite and Nitrate via Cathodic Oxygen Activation in Liquefied Ammonia
Authors: Krebs Moritz Lukas , Schüth Ferdi 
Published: December 2024

Nitrite and nitrate play pivotal roles in industrial applications, being utilized in the synthesis of pharmaceuticals, nylon precursors, explosives, and especially as fertilizer compounds. However, their synthesis primarily relies on the energy-intensive Ostwald process, which operates at high temperatures. Recently, there has been growing interest in producing nitrite and nitrate through the electrooxidation of ammonia. This method shows promise for enabling decentralized small-scale production under mild conditions while directly utilizing renewable energy sources.[1] However, production is currently restricted to low ammonia concentrations (typically 0.1 M) in aqueous systems while requiring high cell voltages.[2,3] An alternative yet unexplored concept is the electrooxidation of liquefied ammonia gas with molecular oxygen. This concept would enable nitrite and nitrate production at lower cell voltages without being restricted to low ammonia concentrations. In fact, the pioneering studies of Allen Bard have demonstrated that cathodic activation of gaseous oxygen in liquefied ammonia is possible.[4] Nonetheless, the possibility of producing nitrite and nitrate in liquefied ammonia has yet to be investigated.

By using an autoclave for electrochemistry experiments, we are able to investigate the possibility of nitrite and nitrate synthesis at ammonia equilibrium pressures, elevated temperatures, and different oxygen partial pressures.[5] In a two-electrode configuration, a Pt coil and a Pt sheet (1cm x 1cm) are used as the cathode and anode, respectively. In this work, we utilize the high overpotential of Pt electrodes in the hydrogen evolution reaction (HER) in liquefied ammonia to selectively form active oxygen species via the competitive oxygen reduction reaction (ORR). The so-formed oxygen species then react with the ammonium-containing electrolyte to form nitrogen oxides.

By introducing 30 bars of pressure of O2 (4%) in Ar, we observed a reduction in the overall cell voltage by approximately 800 mV at current densities of 20 mA cm-2. After transferring 100 C of charge, ion chromatography (IC) confirms the formation of both nitrite and nitrate with formal faradaic efficiencies of 15% and 41%, respectively. Additional headspace analysis via diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) disproved the formation of significant amounts of gaseous NOx species. Via GC analysis, we confirmed that the ORR successfully suppresses the HER. At the same time, the Faraday efficiency towards the anodic nitrogen evolution (NER) is unchanged, indicating a solely cathodically driven reaction mechanism. To determine the role of in-situ formed water in the reaction mechanism, we conducted experiments examining the impact of water addition. These investigations revealed that adding water is advantageous in the presence of oxygen. However, when water was the exclusive source of oxygen, only trace amounts of nitrite were observed, and no nitrate was formed. This indicates that in-situ-formed water molecules are not the primary active species responsible for the production of nitrite and nitrate. Currently, we further study the reaction mechanism and the influence of different electrolyte salts and ammonium-ion concentrations on the nitrite and nitrate yield. In addition, experiments at various reaction temperatures have been conducted, further improving nitrite and nitrate efficiency.

In summary, we present a novel method for the electrochemical formation of nitrite and nitrate in liquefied ammonia under equilibrium pressures, which, to the best of our knowledge, is the first study of this kind. By exploring various reaction parameters, we additionally probe the impact of water content, reaction temperature, and different electrolyte salts on the yields of the nitrogen oxides, thereby advancing our understanding of the underlying reaction mechanism.

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[2] Bunce, N. J.; Bejan, D. Mechanism of electrochemical oxidation of ammonia. Electrochim. Acta
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[3] Tian, Y.; Mao, Z.; Wang, L.; Liang, J. Green Chemistry: Advanced Electrocatalysts and System Design for Ammonia Oxidation. Small Structures
2023, 4 (6), B83. DOI: 10.1002/sstr.202200266.

[4] Uribe, F. A.; Bard, A. J. Electrochemistry in liquid ammonia. 5. Electroreduction of oxygen. Inorg. Chem.
1982, 21 (8), 3160–3163. DOI: 10.1021/ic00138a048.

[5] Britschgi, J.; Kersten, W.; Waldvogel, S. R.; Schüth, F. Electrochemically Initiated Synthesis of Methanesulfonic Acid. Angew. Chem. In. Ed.
2022, 61 (41), e202209591. DOI: 10.1002/anie.202209591.

Figure 1