The increasing concentration of greenhouse gases in the atmosphere is an inevitable consequence of an energy infrastructure that relies on the combustion of fossil fuels. Thus, finding solutions that reduce or eliminate emissions of CO2 is desirable. Renewable energy sources such as wind and solar are promising solutions; however, storing the intermittently generated energy and distributing the energy for use on demand remains a challenge. Converting energy to high energy-density liquid fuels is preferable for ease of distribution. Ammonia is an attractive fuel option because it is produced from nitrogen which is the most abundant molecule in the atmosphere. Since the reaction of N2 and H2 to produce ammonia is effectively thermoneutral at ambient temperature and pressure, the thermodynamic penalties for storing H2 in ammonia, transporting, and then regenerating H2 at distribution points are acceptable. This would close a zero-carbon fuel cycle. There are two methods for converting ammonia to N2 and H2. The first one is the thermal cracking of ammonia. However, most catalysts have high activation energies and are only effective if the process is run continuously. The second method is electrolysis of ammonia which includes oxidation of ammonia at the anode and reduction of proton at cathode electrodes. One of the main issues in ammonia electrolysis is a requirement of very high potential (1 V) compared to the thermodynamically determined one, in order to drive anodic and cathodic half-reactions at typical electrodes. This discrepancy between two potentials is referred to overpotential (η) which is needed to drive a reaction at a specific rate. The overpotential can decrease by employing suitable catalysts. The focus of this study is on homogeneous catalysts that can facilitate the ammonia oxidation half-reaction. In a homogeneous electrocatalytic system, a transition metal complex, which dissolves in ammonia solution, is oxidized to a higher oxidation state intermediate by applying potential on an anode. Then, the intermediate oxidizes NH3 in the bulk solution (at the redox potential (E1/2) of the metal complex) and undergoes reduction to the original oxidation. If the E1/2 of the metal complex is lower than the onset potential of ammonia, it can catalyze the oxidation reaction by lowering the overpotential of ammonia. Thus, by designing catalysts with low E1/2, we can decrease the overpotential of ammonia oxidation toward its thermodynamic limit which is 0.1 V vs NHE. In this regard, several well-defined homogeneous catalytic systems for ammonia oxidation have been reported. For example, Habibzadeh et al showed [Ru(trpy)(dmabpy)(NH3)][PF6]2 (1b), which contains a single NH3 ligand, along with tridentate terpyridine (trpy) ligand and bidentate 4,4 ́-bis(dimethylamino)-2,2 ́-bipyridine ligands (dmabpy) can catalyze ammonia oxidation to dinitrogen by reducing the overpotential of ammonia over 300 mV. Relative to the parent complex, [Ru(trpy)(bpy)(NH3)][PF6]2 (1a, bpy = 2,2’-bipyridine), substituting H at the 4 and 4 ́ positions of bpy with NMe2 lowered E1/2 by 350 mV. In this work, a series of Ru complexes are synthesized with phpy (2-phenyl pyridine), which bonded to the Ru metal center with the carbon and nitrogen of phenyl and pyridine rings, respectively. Because of the introduction of phpy, which is a negatively charged substituent, the net charge of the Ru complexes lowered by one in comparison with 1a and 1b complexes. In this new system, we evaluated the effects of lowering RuII/III E1/2 values by replacing bpy with an electron-donating substituent (phpy) in parent complex 1a. The structure, coordination chemistry and mechanistic implication of this new Ru chemistry in N2 evolution reactions will be discussed.
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