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Homogeneous for the fuel cycle COMMENTARY Yanming Liua,b and Thomas J. Meyera,1

The nitrogen cycle is an important biogeochemical cycle in which nitrogen, a building block of proteins and nucleic acids, is chemically activated. In the cycle, atmospheric dinitrogen is converted into as our primary source of bioavailable nitrogen. Ammonia is a carbon-free energy carrier and widely used as a chemical feedstock. Ammonia oxidation is also a critical step in global nitrogen cycling. Harnessing the energy stored in ammonia involves oxidization to Fig. 1. Simplified reaction scheme illustrating an ammonia/N fuel cycle in which dinitrogen and hydrogen, NH3 → 1/2N2 + 3/2H2, with 2 NH is electrochemically oxidized to N and H ,NH →1/2N + 3/2H , with H the hydrogen produced utilized in hydrogen-based 3 2 2 3 2 2 2 utilized in a hydrogen/O2 fuel cell. In a reverse cycle, N2 is electrochemically fuel cells (Fig. 1). Ammonia can be also easily lique- reduced to NH3 by nitrogen fixation (1/2N2 +3/2H2O → NH3 + 3/4O2). In a fied. As a hydrogen carrier, it provides a strategy for complete ammonia fuel cycle, both electrochemical ammonia oxidation and the efficient storage, transport, and utilization of re- nitrogen fixation would be driven by renewable energy sources. newable hydrogen on large scales, and it is not sur- prising that ammonia oxidation has attracted great of ammonia to dinitrogen under mild conditions, as interest in clean energy and environmental pollution shown in Fig. 1. control. Electrochemical oxidation of ammonia occurs Ammonia oxidation has been extensively studied in at room temperature under ambient pressures (1, 2) biological systems and in heterogeneous systems based on thermal conversion, photocatalysis, or electrocatalysis with an interest in active electrocatalysts with low (4, 5). Biological ammonia oxidation is driven by bacteria overpotentials and high faradaic efficiencies for its oxi- and archaea, and they transform ammonia to dinitrogen dization to hydrogen to dinitrogen. by anaerobic ammonium oxidation, or oxidize ammonia A study in PNAS by Habibzadeh et al. (3) reports to nitrite or nitrate by nitrification(5,6)or,asinFig.1,to that polypyridyl complexes can be used dinitrogen as a product. as electrocatalysts for the oxidation of ammonia to Electrolysis of ammonia to dinitrogen and dihy- dinitrogen and dihydrogen at ambient pressures drogen should be thermodynamically accessible un- and temperature. The results from Habibzadeh et al. der mild conditions with renewable energy sources. (3) are an important extension of the remarkable high- An ideal electrocatalyst for ammonia oxidation should oxidation-state chemistry of these complexes with RuII have appropriate binding energies for ammonia and and OsII. Known examples in this chemistry include cat- for the key oxidation intermediates that interconvert alysts for oxidation, catalytic organic oxidations, the two (7). There are obvious problems, including and a remarkable series of inorganic reactions such as − stable coordination of ammonia in the first place and the oxidation of NH3 to NO3 , all in polypyridyl coordi- accessing the higher oxidation states at the nation environments. while avoiding competing decomposition pathways. The discovery reported here is an important ex- such as Pt, Ir, Ru, and Au and their alloys tension. It exploits the reactivity of these complexes to are active heterogeneous catalysts for electrochem- a reaction of potential interest in energy application: ical oxidation of ammonia to dinitrogen (1, 2, 8). homogenous electrochemical oxidation of ammonia Among these electrocatalysts, Pt and Ir alloys have to N2. Exploitation of the reaction could provide a been considered as the most effective for ammonia basis for efficient molecular catalysis in the oxidation oxidation, with their low overpotentials and high

aDepartment of Chemistry, The University of North Carolina at Chapel Hill, Chapel Hill, NC 27599; and bSchool of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China Author contributions: Y.L. and T.J.M. wrote the paper. The authors declare no conflict of interest. Published under the PNAS license. See companion article 10.1073/pnas.1813368116. 1To whom correspondence should be addressed. Email: [email protected].

www.pnas.org/cgi/doi/10.1073/pnas.1822090116 PNAS Latest Articles | 1of2 Downloaded by guest on September 29, 2021 current densities (2). The results of experiments on catalyst structure the reaction of ammonia splitting to dinitrogen and dihydrogen (1:3). 15 and composition have been used to lower overpotential and en- The results of NH3 labeling experiments also show that dinitrogen is hance the kinetics and efficiency for ammonia electrooxidation. In generated from ammonia oxidation. There are and iron heterogeneous catalysis, a major concern for metal heterogeneous complex catalysts that are active for fixing dinitrogen to ammonia in catalysis is electrode-surface poisoning by adsorbed intermediates. nitrogen fuel cycles (13, 14), but this is a molecular catalyst for the One answer is homogeneous electrocatalysis at inert, conducting electrochemical oxidation ammonia to dinitrogen at low overpotential. electrodes to overcome activity loss of the catalyst. As noted above, given their background chemistry, A study in PNAS by Habibzadeh et al. reports 2+ = ′ ′ ′′ polypyridyl complexes like [Ru(trpy)(bpy)(NH3)] (trpy 2,2 :6 ,2 - that ruthenium polypyridyl complexes can be terpyridine, bpy = 2,2′-bipyridine) are of particular interest in this chemistry. Mechanistic studies have shown that they undergo used as electrocatalysts for the oxidation of single-electron oxidation from RuII to RuIII, followed by oxidation ammonia to dinitrogen and dihydrogen at and proton loss to RuIV = NH, with the latter undergoing nucleo- ambient pressures and temperature. philic attack on the imido N to give RuII-hydroxylamines (9). There are related results on the oxidation of coordinated ammonia li- Mechanistic details were also investigated by cyclic voltam- gands to N2 in dinuclear ruthenium complexes, mononuclear os- metry and additional studies in solution. In THF, the rate of – mium complexes, and on a series of molybdenum complexes (10 ammonia oxidation was first order in 1a, suggesting that single- 12). These observations provide a basis for dinitrogen generation electron oxidation of RuII to RuIII occurs before the rate-limiting 2+ by oxidation of ammonia in complexes like [Ru(trpy)(bpy)(NH3)] , step. Based on the results of previous studies on a series of transition III II but E1/2 for the Ru /Ru couple was 0.15 V more positive than the metal complexes, including examples from ruthenium, osmium, and onset potential for ammonia oxidation at glassy carbon electrodes. molybdenum, the overall reaction presumably proceeds through A key to driving the reaction has come from the results of Habib- hydrazine or nitride pathways (11, 12, 15). In the hydrazine pathway, zadeh et al. (3) by exploiting the background synthetic chemistry – 2+ deprotonation is followed by N N bond formation to give the in- to prepare the modified complex [Ru(trpy)(dmabpy)(NH )] [trpy = + 3 termediate hydrazine complex, RuII(H N-NH )2 , with the latter gen- 2,2′:6′,2′′-terpyridine, dmabpy = 4,4´-bis(dimethylamino)-2,2′- 2 2 erated by nucleophilic attack of the amine on a transient imido bipyridine] (catalyst 1a in ref. 3). They demonstrate 1a to be an intermediate. In the nitride pathway, N–Ncouplingoccursbetween efficient homogenous catalyst for electrochemical oxidation of a nitride, RuV(N), and NH . ammonia to dinitrogen at room temperature and ambient pres- 3 III II Experimental evidence was found for the hydrazine inter- sure. The introduction of the dmabpy ligand reduces the Ru /Ru + mediate [(trpy)(dmabpy)RuIII(N H )]2 (5a in ref. 3) by heating oxidation potential from 1.03 to 0.68 V vs. normal hydrogen elec- 2 4 ∼ [(trpy)(dmabpy)Ru(Cl)][Cl] in a hydrazine hydrate solution satu- trode (NHE), which is 0.19 V below the potential for direct oxi- 1 dation at a glassy carbon electrode. rated with [NH4][PF6]. The was characterized by HNMR 1 The reactivity of catalyst 1a toward ammonia oxidation was and cyclic voltammetry. H NMR spectra of reaction from 1a, 5a, III first shown by the appearance of a significantly enhanced current or [(trpy)(dmabpy)Ru (N2H4)][PF6]2, with and without added NH3, – density with added ammonia after oxidation of RuII to RuIII in are all consistent with N N bond formation through the hydrazine 3+ [Ru(trpy)(dmabpy)(NH3)] . Compared with uncatalyzed ammonia complex as an intermediate. oxidation on glassy carbon electrodes, catalyst 1a lowers the over- The mechanistic details revealed by Habibzadeh et al. (3) will be potential for ammonia oxidation by ∼0.3 V in THF solution. Of useful for defining conditions for maximizing the oxidation of ammo- significant interest here is that dinitrogen and dihydrogen are pro- nia to dinitrogen in catalytic systems in the future. The key features II III duced from ammonia electrolysis in the solvent with high faradaic emerging here show that Ru oxidation to Ru is followed by depro- efficiencies of 86% and 78% at 0.73 V vs. NHE, showing that the tonation and N–N bond formation through a hydrazine intermediate. catalyst converts ammonia to dinitrogen and dihydrogen. The ratio The challenge now will be to identify conditions that enhance re- of dinitrogen to dihydrogen is 1:2.74, close to the stoichiometry for activity further at larger scales forthedesignofammoniafuelcycles.

1 Little DJ, Smith MR, III, Hamann TW (2015) Electrolysis of ammonia for hydrogen generation. Energy Environ Sci 8:2775–2781.

2 Sacr ´eN, et al. (2018) Tuning Pt–Ir interactions for NH3 electrocatalysis. ACS Catal 8:2508–2518. 3 Habibzadeh F, Miller SL, Hamann TW, Smith MR, 3rd (2019) Homogeneous electrocatalytic oxidation of ammonia to N2 under mild conditions. Proc Natl Acad Sci USA, 10.1073/pnas.1813368116. 4 Chen JG, et al. (2018) Beyond fossil fuel-driven nitrogen transformations. Science 360:eaar6611.

5 Pratscher J, Dumont MG, Conrad R (2011) Ammonia oxidation coupled to CO2 fixation by archaea and bacteria in an agricultural soil. Proc Natl Acad Sci USA 108:4170–4175. 6 Canfield DE, Glazer AN, Falkowski PG (2010) The evolution and future of Earth’s nitrogen cycle. Science 330:192–196. 7 Katsounaros I, et al. (2018) On the mechanism of the electrochemical conversion of ammonia to dinitrogen on Pt(100) in alkaline environment. J Catal 359:82–91. 8 Xu W, et al. (2018) Electrodeposited NiCu bimetal on carbon paper as stable non-noble anode for efficient electrooxidation of ammonia. Appl Catal B 237:1101–1109. 9 Thompson MS, Meyer TJ (1981) Oxidation of coordinated ammonia to nitrate. J Am Chem Soc 103:5577–5579. 4+ 10 Ishitani O, Ando E, Meyer TJ (2003) Dinitrogen formation by oxidative intramolecular N—N coupling in cis,cis-[(bpy)2(NH3)RuORu(NH3)(bpy)2] . Inorg Chem 42:1707–1710. 11 Coia GM, et al. (1994) Preparation of osmium hydrazido complexes by interception of an osmium(IV) imido intermediate. J Am Chem Soc 116:3649–3650. 12 Bezdek MJ, Guo S, Chirik PJ (2016) Coordination-induced weakening of ammonia, water, and hydrazine X-H bonds in a molybdenum complex. Science 354:730–733. 13 Arashiba K, Miyake Y, Nishibayashi Y (2011) A molybdenum complex bearing PNP-type pincer ligands leads to the catalytic reduction of dinitrogen into ammonia. Nat Chem 3:120–125. 14 Macleod KC, Holland PL (2013) Recent developments in the homogeneous reduction of dinitrogen by molybdenum and iron. Nat Chem 5:559–565.

15 Bhattacharya P, et al. (2017) Ammonia oxidation by abstraction of three hydrogen atoms from a Mo-NH3 complex. J Am Chem Soc 139:2916–2919.

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