Heterogeneous reaction mechanism, anodic current

Application of equation (4) to the organic cocktail oxidation profiles, given in Fig. 6(a), yields straight line plots. Two of these kinetic plots for current densities of 0.14 and 0.40 A cm are presented in Fig. 8. The linear relationships obtained demonstrate that pseudo first-order kinetics are obeyed. This relationship directly supports the heterogeneous bimolecular reaction mechanism proposed for the electrochemical oxidation of organics in the electrochemical reactor. The slopes of the linear plots yield the pseudo first-order rate constants, which are summarized in Table 2 for each value of the current density (i.e. the anodic electrode potential) used. It can be seen from the table that, with increasing current... 

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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