Heterogeneous electron transfer rate constant determination

There has been keen interest in determination of activation parameters for electrode reactions. The enthalpy of activation for a heterogeneous electron transfer reaction, AH X, is the quantity usually sought [3,4]. It is determined by measuring the temperature dependence of the rate constant for electron transfer at the formal potential, that is, the standard heterogeneous electron transfer rate constant, ks. The activation enthalpy is then computed by Equation 16.7 ... 

Heterogeneous electron transfer rate constant, see equation (8) in text, for i oxidation determined as in ref. 16. dData from ref. 16. 

Determination of Heterogeneous Electron Transfer Rate Constant... 

Fig. 9. Working curves for spectroelectrochemical determination of heterogeneous electron transfer rate constants for quasi-reversible reactions. Numerical values correspond to n T) where tj is expressed in millivolts. ...

Fig. 8. Working curve for spectroelectrochemical determination of heterogeneous electron transfer rate constants for irreversible reactions.

Then A versus log(/ f results in a single kinetic working curve, as shown in Fig. 8, and provides a convenient way of determining Xf h- A plot of log Xf h against overpotential (17) yields the formal heterogeneous electron transfer rate constant (X h) from the intercept and the value of the electrochemical transfer coefficient (a) from the slope. - ... 

Research into this area is dominated by microelectrodes. At short times, the diffusion layer thickness is much smaller than the microelectrode radius and the dominant mass transport mechanism is planar diffusion. Under these conditions, the classical theories, e.g., that of Nicholson and Shain, can be used to extract kinetic parameters from the scan rate dependence of the separation between the anodic and cathodic peak potentials. Using this approach, the standard heterogeneous electron transfer rate constant, k°, may be determined from the published working curves relating AEp to a kinetic parameter The variation of AEp with o is determined and, from this, T is calculated. k° is then determined by the following equation ... 

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