(Invited) Electrocatalytic Ammonia Oxidation

Abstract

Much of the global effort toward producing solar fuels has involved splitting water to obtain molecular hydrogen. Unfortunately, hydrogen gas has a relatively low energy-per-volume, and extreme cryogenic temperature and pressure is needed to bring hydrogen to a condensed state. An interesting choice for a hydrogen storage medium that circumvents these setbacks is ammonia. Synthesis of ammonia (largely via the Haber-Bosch process) is already the second-largest chemical industrial process in the world, and as a result the infrastructure for ammonia storage and transport already exists. Ammonia can be easily liquefied under 10 bar of pressure which leads to a three-fold higher energy density than hydrogen gas in state-of-the-art carbon fiber cylinders. One significant bottleneck limiting the use of ammonia for hydrogen storage is the inability to efficiently extract hydrogen from ammonia on-demand.Electrolysis offers a convenient route to decompose ammonia into nitrogen and hydrogen, which potentially only requires 0.1 V. However, there is a lack of fundamental understanding of the reactions involved in the electrolysis. In this talk, the electrolysis of liquid ammonia with heterogeneous catalysts, as well as ammonia dissolved in non-aqueous solvents, using two and three electrode measurements, will be presented.1,2 The possible mechanisms of the anodic and cathodic reactions will be discussed, as well as the cause and magnitude of the related overpotentials which limit the efficiency. Further, while there have been multiple reports on heterogeneous catalysts for electrocatalytic oxidation of NH3, with limited success, homogenous molecular catalysts have been non-existent until now.3 A transition metal complex, [Ru(tpy)(dmabpy)NH3]2+ (tpy = 2,2′:6′,2′′-terpyridine, dmabpy = 4,4'-dimethylamino-2,2'-bipyridine), will be presented which shows the first example of electrocatalytic activity in presence of ammonia at room temperature and ambient pressure. Gas chromatography measurements reveals production of N2 and H2 in a 1:3 ratio following bulk electrolysis of ammonia. Absorption measurements following reaction of [Ru(tpy)(dmabpy)NH3]3+ with one equivalent of ammonia clearly show regeneration of [Ru(tpy)(dmabpy)NH3]2+ which provides evidence of redox disproportionation to produce a Ru(IV) imido species as an intermediate in formation of the N-N bond. Nitrogen isotope labeling experiments reveal incorporation of the coordinated amine with free ammonia in solution to produce dinitrogen. Results from cyclic voltammetry, stopped-flow, NMR and rotating ring-disc measurements will also be presented. From these combined results, a tentative mechanism will be presented and the implications discussed.1) Little, D.J., Smith, M.R., Hamann, T.W.; “Electrolysis of Liquid Ammonia for Hydrogen Generation” Energy & Environmental Science, 2015, 8, 2775 – 27812) Little, D.J., Edwards, D., Smith, M.R., Hamann, T.W.; “As Precious as Platinum: Iron Nitride for Electrocatalytic Oxidation of Liquid Ammonia” ACS Applied Materials & Interfaces 2017, 9 (19), 16228 – 162353) Habib-Zadeh, F., Miller, S.L., Hamann, T.W., Smith, M.R.; “Homogenous Electro-Catalytic Oxidation of Ammonia to N2 Under Mild Conditions” Proceedings of the National Academy of Science 2019, 116 (8) 2849 – 2853