Rapid Aqueous Ammonia Oxidation to N₂ Using a Molecular Ru Electrocatalyst

Abstract

The storage of renewable energy in ammonia (NH3) is a promising alternative to hydrogen (H2) for our transition from fossil fuels. Solid oxide fuel cells (SOFCs) are the leading technology for direct ammonia fuel cells (DAFCs); however, their high temperature of operation makes them unsuitable for light-duty vehicles. As such, there has been growing interest in aqueous-fed DAFCs. Such technologies face challenges with respect to reaction selectivity, precious metal catalyst loading, and stability; however, there is a dearth of reported molecular catalysts to address such issues. Ruthenium bipyridinedicarboxylate complexes are known for catalyzing water oxidation at rapid rates and were recently reported to catalyze the oxidation of ammonia, i.e., nitrogen evolution reaction (NER) in acetonitrile, albeit at sluggish rates. Herein, we present our use of the known complex [Ru(bipyridinedicarboxylate)(4-methylpyridine)2] (RuBda, 1) to electrocatalyze ammonia oxidation of aqueous NH3 to N2 at high faradaic efficiencies (>80%), unprecedented rates (turnover frequency ≈ 3757 s–1), and high turnover. Our kinetic analyses suggest that the catalyst operates via a unimolecular mechanism, which is highly applicable for commercially viable fuel cells.

Molecular Complexes for Catalytic Ammonia Oxidation to Dinitrogen and the Cleavage of N−H Bonds

“…The use of 15 A significant advancement in electrocatalytic oxidation of NH 3 to N 2 was reported in 2023 by Sepunaru, Ménard, and coworkers where the complex [Ru(bipyridinedicarboxylate)(4methylpyridine) 2 ] (discussed in the next section; catalyst depicted in Figure 43) was subjected to aqueous reaction conditions. [89] In controlled potential electrolysis experiments with an applied potential of 0.685 V (vs NHE), glassy carbon rod working electrode, 0.5-1.0 mM Ru catalyst, 0.1 M NaOTf, 1.0 M NH 4 OTf and a commercial grade aqueous 14.3 M NH 3 solution, afforded Faradaic efficiencies for N 2 production up to 89 %. Voltammetry experiments suggested a catalytic mechanism with a rate-determining step of the nucleophilic attack of NH 3 at a high-valent Ru-nitride species that resulted in a catalyst turnover frequency (TOF) of 3757 s À 1 .…”

Electrocatalytic Ammonia Oxidation with a Tailored Molecular Catalyst Heterogenized via Surface Host–Guest Complexation

“…Beiler et al reported a heterogenized approach anchoring a Ru-bda oligomer as the catalyst to the electrode via π-π interactions, albeit with relatively low Faradaic efficiencies . A clear benefit of the anchored catalyst is that electrocatalysis can be conducted in an aqueous medium.…”

Improving Molecular Iron Ammonia Oxidation Electrocatalysts via Substituent Effects that Modulate Standard Potential and Stability

“…Ammonia oxidation (AO) and its mediation by transition-metal catalysts have emerged as burgeoning research topics across industry and academia. − Catalysts such as platinum and other precious metals degrade due to metal-nitride formation. , There is hence motivation to study new catalysts that are highly active, selective, and robust, including molecular systems that are amenable to systematic structure–function studies. , This goal has sparked recent research efforts, with the initial reports of authentic AO catalysis mediated by molecular systems appearing as recently as 2019. − To date, a variety of catalyst structures have been reported, featuring metals including ruthenium, iron, manganese, nickel, and copper supported by a variety of auxiliary ligands. − The pursuit of efficient AO catalysis encompasses various strategies, including weak, leveled N–H bond strengths; the promotion of early N–N bond formation to hydrazine; or the promotion of intermolecular nitride homocoupling. These strategies can call for unique catalyst designs. ,,− As yet, no single strategy prevails, underscoring an ongoing need for systematic comparative studies.…”