Temperature Dependence of the Heterogeneous Ferrous-Ferric Electron Transfer Reaction Rate: Comparison of Experiment and Theory

L. A. Curtiss, Z. Nagy, R. M. Yonco, J. W. Halley, Y. J. Rhee, J. Hautman, N. C. Hung

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Abstract

We describe experiments on the temperature dependence of the rate of the ferrous-ferric electron transfer reaction at a gold electrode and compare them with a detailed molecular dynamics simulation which is used to predict the rate. We find from the experiments that the temperature dependence of the rate has the Arrhenius form over the temperature range from 25 to 275°C, and that the transfer coefficient is independent of temperature in this range. The molecular dynamics simulations are used in two ways to extract activation energies and transfer coefficients for comparison with experiment. In one of these methods, we assume parabolic dependence of the energies for the product and reactant in a reaction coordinate which is not specified a priori. In the other method, we use a quantum mechanical calculation extrapolated from the very short molecular dynamics time scale to times characteristic of the electron transfer rate. The assumption of parabolic dependence of the energies gives an estimate for the activation energy which is consistent with experiment. The transfer coefficient calculated using this assumption is also consistent with experiment. The activation energy and the transfer coefficient from the quantum mechanical calculation are both lower than the experimental values. The quantum mechanical method, together with a molecular orbital calculation of the electron transfer matrix element, permits a theoretical estimate of the absolute value of the rate, which is also compared with the experimental result. These results show that the ferrous-ferric reaction, which is a single-step outer-sphere charge-transfer reaction, follows the classical Butler-Volmer equation at temperatures up to 275°C, and that earlier results on other reactions giving a temperature dependent transfer coefficient are likely to arise from elementary steps other than outer-sphere charge transfer.

Original languageEnglish (US)
Pages (from-to)2032-2041
Number of pages10
JournalJournal of the Electrochemical Society
Volume138
Issue number7
DOIs
StatePublished - Jul 1991

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