Proton-coupled electron-transfer disproportionation

The reported work by Nocera and coworkers concerning the dissolution and redeposition of MnO to form self-healing stable electrocatalysts under acidic, neutral, and basic conditions not only supported that manganese(iii) underwent disproportionation in acidic medium, whereas the one-electron-one-proton PCET (proton-coupled electron transfer) pathway is dominant under alkaline conditions, but resulted in a Tafel slope of 60 mV dec in alkaline pH regime.Apart from the usually discussed overpotential of electrochemical water oxidation, the kinetics of the reaction has also been considered for an efficient electrocatalyst. It is well known that the higher the Tafel slope value, the higher the energy needed to reach maximum current density. 

Protonation of dinitrogen or hydrazido(2-) ligands to yield ammonia [reactions (44)—(50)] is not coupled to electron transfer from an external reductant. Electrons for (new) N—H electron pair bonds must therefore come from the metal as is the case in reaction (46), or from ligand-centered oxidation or disproportionation reactions, as appears to be the case in reaction (49). 

Reoxidation of the cosubstrate at an appropriate electrode surface will lead to the generation of a current that is proportional to the concentration of the substrate, hence the coenzyme can be used as a kind of mediator. The formal potential of the NADH/NAD couple is - 560 mV vs. SCE (KCl-saturated calomel electrode) at pH 7, but for the oxidation of reduced nicotinamide adenine dinucleotide (NADH) at unmodified platinum electrodes potentials >750 mV vs. SCE have to be applied [142] and on carbon electrodes potentials of 550-700 mV vs. SCE [143]. Under these conditions the oxidation proceeds via radical intermediates facilitating dimerization of the coenzyme and forming side-products. In the anodic oxidation of NADH the initial step is an irreversible heterogeneous electron transfer. The resulting cation radical NADH + looses a proton in a first-order reaction to form the neutral radical NAD, which may participate in a second electron transfer (ECE mechanism) or may react with NADH (disproportionation) to yield NAD [144]. The irreversibility of the first electron transfer seems to be the reason for the high overpotential required in comparison with the enzymatically determined oxidation potential. 

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