Mechanisms of Charge Transfer at the Chemically Derivatized Interface: The Ni/[Niii(CN)Feii/III(CN)5]2-/1- System As an Electrocatalyst

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The [Ni(NC)Fe(CN)5]2-/1- derivatized nickel electrode represents an electrocatalytic surface for a variety of oxidations and reductions. This surface exhibits a unique dependence of redox potential on supporting electrolyte cation, which allows for a direct analysis of the effect of surface redox potential on the electrocatalytic rate constant. Two electrocatalytic systems have been evaluated with respect to surface redox potential: the one-electron reduction of Fe3+(aq) and the two-electron (two-proton) oxidation of ascorbic acid. Mediated charge transfer is found to be an operational electron-transfer mechanism in both cases, with the bimolecular surface species to solution species charge transfer being rate limiting. In the case of Fe3+(aq) reduction Marcus theory is found to yield a good description of the relationship between surface redox potential and the electrocatalytic rate constant. Pseudo-first-order rate constants as large as 0.15 cm/s (in LiNO3 supporting electrolyte) have been observed for this reaction. The ascorbic acid oxidation rate constant is found to be ∼10-3 cm/s. This rate constant is independent of surface redox potential, suggesting that the transfer of the second electron is rate limiting.



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