Homogeneous catalysis for the nitrogen fuel cycle

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 ammonia 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 dinitrogen and hydrogen, NH3 → 1/2N2 + 3/2H2, with the hydrogen produced utilized in hydrogen-based fuel cells (Fig. 1). Ammonia can be also easily liquefied. As a hydrogen carrier, it provides a strategy for the efficient storage, transport, and utilization of renewable hydrogen on large scales, and it is not surprising that ammonia oxidation has attracted great interest in clean energy and environmental pollution control. Electrochemical oxidation of ammonia occurs at room temperature under ambient pressures (1, 2) with an interest in active electrocatalysts with low overpotentials and high faradaic efficiencies for its oxidization to hydrogen to dinitrogen. Fig. 1. Simplified reaction scheme illustrating an ammonia/N2 fuel cycle in which NH3 is electrochemically oxidized to N2 and H2, NH3 →1/2N2 + 3/2H2, with H2 utilized in a hydrogen/O2 fuel cell. In a reverse cycle, N2 is electrochemically reduced to NH3 by nitrogen fixation (1/2N2 +3/2H2O → NH3 + 3/4O2). In a complete ammonia fuel cycle, both electrochemical ammonia oxidation and nitrogen fixation would be driven by renewable energy sources. A study in PNAS by Habibzadeh et al. (3) reports that ruthenium polypyridyl complexes can be used as electrocatalysts for the oxidation of ammonia to dinitrogen and dihydrogen at … [↵][1]1To whom correspondence should be addressed. Email: tjmeyer{at}unc.edu. [1]: #xref-corresp-1-1