Heterogeneous electron transfer kinetics

Morris studied the aqueous solution voltammetric behavior of some uranyl coordination complexes to learn how changes in the ligand environment influence the redox potentials and heterogeneous electron-transfer kinetic parameters for the single-electron transfer... 

From Eq. (9.6), mo D/d if L<1. This suggests the usefulness of SECM for the study of rapid heterogeneous electron transfer kinetics, the largest ks values that can be determined being of the order of D/d. The largest ks values measurable by voltammetry at UMEs are of the order of D/a. It is easier to get a small L value with SECM than to prepare a UME of very small a value. For example, ks for the Fc+/Fc couple in AN has been determined by SECM to be 3.7 cm s-1 [29]. 

When the electrolysis process is irreversible (in a thermodynamic sense) but still limited by linear diffusion, the potential-time relationship takes a form that includes the heterogeneous electron-transfer kinetic parameters. For a cathodic process the relation is... 

The variation in the peak-separation values (AEp) for the cyclic voltammetric data of Table 9.9 may be interpreted in terms of heterogeneous electron-transfer kinetics, but the most reasonable explanation is uncompensated resistance (especially for py and MeCN) and surface reactions (especially for the metal electrodes). 

The two models most commonly applied to the heterogeneous electron transfer kinetics are the Butler-Volmer model, which is primarily a macroscopic approach... 

Miller, C.J. (1995). Heterogeneous electron transfer kinetics at metallic electrodes, in Physical Electrochemistry Principles, Methods and Applications, I. Rubinstein (Ed.), Marcel Dekker, New York, pp. 27-80. 

Refs. [i] Born M (1920) Z Phys 1 45 [ii] Marcus Y (1997) Ion properties. Marcel Dekker, New York [iii] Rashin AA, Honig B (1985) ] Phys Chem 89 5588 [iv] Stilly WC, TempczykA, Hawley RC, Hendrickson TA (1990) ] Am Chem Soc 112 6127 [v] Rashin AA (1990) / Phys Chem 94 1725 [vi] Shoichet BK, Leach AR, Kuntz ID (1999) Proteins 34 4 [vii] Marcus RA (1977) Theory and application of electron transfer at electrodes and in solutions. In Rock PA (ed) Special topics in electrochemistry. Elsevier, Amsterdam, pp 61 [viii] Millery C/ (1995) Heterogeneous electron transfer kinetics at metallic electrodes. In Rubinstein I (ed) Physical electrochemistry. Marcel Dekker, New York, pp 46-47... 

If the potential applied in CA is not polarized sufficiently to fulfil the condition Co(0, r) = 0, the current also depends on the heterogeneous electron-transfer kinetics. The Cotrell expression given in Eq. 39 should for a reversible process then be modified according to Eq. 51, where z cot denotes the Cottrel current. 

Kempton, J. H., Lindberg, R. D., and Runnels, D. D. (1988) Numerical Modeling of Pt- n Measurements by Using Heterogeneous Electron Transfer Kinetics. In Chem-... 

DO Wipf, EW Kristensen, MR Deakin, RM Wightman. Fast-scan cyclic voltammetry as a method to measure rapid heterogeneous electron-transfer kinetics. Anal Chem 60 306-310, 1988. 

One can see from Eq. (36) that at L 1, mo D/a (as for a microdisk electrode alone), but at L 1, mo D/d, i.e., a thin-layer cell (TLC)-type behavior. This suggests that the SECM should be useful for studying rapid heterogeneous electron transfer kinetics. By decreasing the tip/substrate distance, the mass-transport rate can be increased sufficiently for quantitative characterization of the electron-transfer kinetics, preserving the advantages of steady-state methods, i.e., the absence of problems associated with ohmic drop, adsorption, and charging current. 

The alternative approach to examine heterogeneous electron transfer kinetics on the substrate is to hold the tip at a potential where the reaction is mass transfer controlled and study the approach curve as a function of substrate potential. For example, one can generate iron(II) by reduction of iron(III) at the tip and study the oxidation of iron(II) at the substrate. One... 

Numerical Modeling of Platinum Eh Measurements by Using Heterogeneous Electron-Transfer Kinetics... 

This research evaluates the measurement of the "master" Eh of solutions in terms of heterogeneous electron-transfer kinetics between aqueous species and the surface of a polished platinum electrode. A preliminary model is proposed in which the electrode/solution interface is assumed to behave as a fixed-value capacitor, and the rate of equilibration depends on the net current at the interface. Heterogeneous kinetics at bright platinum in 0.1 m KCl were measured for the redox couples Fe(III)/Fe(II), Fe(CN)53-/Fe(CN)6, Se(VI)/Se(IV), and As(V)/As(Iin. Of the couples considered, only Fe(III)/Fe(II) at pH 3 and Fe(CN)g37Fe(CN)g at pH 6.0 were capable of imposing a Nemstian potential on the platinum electrode. 

In this study, heterogeneous electron-transfer kinetics were measured for the following Se(VI)/Se(IV), As(V)/As(III), Fe(CN)5 /Fe(CN)5 ", and Fe(III)/Fe(II). All experiments were done at pH 6.0 with the exception of the iron couple, which was done at pH 3.0. Using electron-transfer kinetic constants, aqueous diffusion coefficients, aqueous concentrations, starting potentials, and a constant double-layer capacitance model, values for the change of EMF as a function of time for a platinum electrode were calculated numerically. The result of this simulation was then compared to the observed potentiometric response for a solution of the same concentration. 

Table H. Heterogeneous electron-transfer kinetics at the platinum disk electrode at 298 K. All values determined from linear portions of Tafel plots at rates of rotation extrapolated to infinity. Reaction of Se and As too slow for observation...

The previous sections dealt with a generalized theory of heterogeneous electron-transfer kinetics based on macroscopic concepts, in which the rate of the reaction was expressed in terms of the phenomenological parameters, and a. While useful in helping to organize the results of experimental studies and in providing information about reaction mechanisms, such an approach cannot be employed to predict how the kinetics are affected by such factors as the nature and structure of the reacting species, the solvent, the electrode material, and adsorbed layers on the electrode. To obtain such information, one needs a microscopic theory that describes how molecular structure and environment affect the electron-transfer process. 

In the intermediate and late time regimes, the current density at a UME disk is intrinsically nonuniform because the edges of the electrode are more accessible geometrically to the diffusing electroreactant (17). This non-uniformity affects the interpretation of phenomena that depend on local current density, such as heterogeneous electron-transfer kinetics or the kinetics of second-order reactions involving electroactive species in the diffusion layer. 

However, the mass-transfer coefficient can be increased above this level by introducing convective flow, for example, by rotating the electrode or rapidly flowing a solution to it. This might be useful in measurements of rapid heterogeneous electron-transfer kinetics, where the radial diffusion alone is insufficient to remove mass-transfer limitations. 

Heterogeneous Electron Transfer Kinetic Parameters of Redox Proteins by Single Potential Step Chronoabsorptometry... 

E. E. Bancroft, H. N. Blount, and F. M. Hawkridge, in Spectroelectrochemical Determination of Heterogeneous Electron Transfer Kinetic Parameters (K. M. Kadish, ed.). Advances in Chemistry Series No. 201, Chap. 2, American Chemical Society, Washington,... 

The material of this section will focus on results of homogeneous electron transfer kinetic studies involving reactions between electrochemically generated redox reactants and on direct heterogeneous electron transfer kinetic studies. An overview of the work in this area concludes this chapter. 

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