Redox inner-sphere electrode reaction

In this picture, the electron transfer processes mediated by metallic electrodes (redox reactions in a heterogeneous phase) can also be classified to proceed according to outer-sphere or inner-sphere mechanisms (obviously, considering the electrode surface as a reagent). 

For the electron transfer of hydrated redox particles (the outer-sphere electron transfer), the electrode acts merely as a source or sink of electrons transferring across the compact double layer so that the nature of the electrode hardly affects the reaction kinetics this lack of influence by the electrode has been observed for the ferric-ferrous redox reaction. On the other hand, the electron transfer of adsorbed redox particles (the inner-sphere electron transfer) is affected by the state of adsorption so that the nature of the electrode exerts a definite influence on the reaction kinetics, as has been observed with the hydrogen electrode reaction where the reaction rate depends on the property of electrode. 

For the transfer of redox electrons (inner-sphere electron transfer) in which redox particles are adsorbed on a thin superficial film that covers a metal electrode, the transfer current of redox electrons is not always decreased but rather increased by the presence of the thin film. Such an increase in the reaction ciurent will occur, if the film acts as a reaction catalyst providing the adsorbed state of redox particles favorable for the redox electron transfer. For example, the anodic oxidation of carbon monoxide is catalyzed by the presence of an anodic oxide film on... 

It is important to notice that the rate of a given outer sphere electrode redox reaction should be independent of the nature of the metal electrode if allowance is made for electrostatic work terms or double layer effects which will, of course, be dependent on the nature of the electrode material. Inner sphere reactions, on the other hand, are expected to be catalytic with kinetics strongly dependent on the electrode surface due to specific adsorption interactions. 

Fig. 1. Schematic representation of the metal—electrolyte interface and reaction sites for outer-sphere (a, c) and inner-sphere (b) redox reaction paths at electrodes [12].

Redox electrode reactions on metal electrodes constitute the simpler case for a theoretical approach to the problem. In particular, outer sphere redox electrode reactions not involving specific adsorption interactions have been treated successfully in analogy with homogeneous redox reactions in solution [54, 56], Approximate extension of the theoretical approach to the case of inner sphere redox reactions at electrodes has been done [56, 57b]. 

Underpotential deposition of metal adatoms at foreign metal electrodes shows a strong effect on the kinetics of inner sphere redox reactions such as the reduction of Cr(OH2)sCl2+ [130] due to electrostatic and specific interactions. 

However, the mechanisms of conventional redox reactions and electrochemical reactions maybe quite different. Within the formalism of electron transfer theory, the electron transfer reactions at electrodes are usually of the outer-sphere type, whereas those that involve inorganic ions are often of the inner-sphere type [11]. 

The application of surface-enhanced Raman spectroscopy (SERS) for monitoring redox and other processes at metal-solution interfaces is illustrated by means of some recent results obtained in our laboratory. The detection of adsorbed species present at outer- as well as inner-sphere reaction sites is noted. The influence of surface interaction effects on the SER spectra of adsorbed redox couples is discussed with a view towards utilizing the frequency-potential dependence of oxidation-state sensitive vibrational modes as a criterion of reactant-surface electronic coupling effects. Illustrative data are presented for Ru(NH3)63+/2+ adsorbed electrostatically to chloride-coated silver, and Fe(CN)63 /" bound to gold electrodes the latter couple appears to be valence delocalized under some conditions. The use of coupled SERS-rotating disk voltammetry measurements to examine the kinetics and mechanisms of irreversible and multistep electrochemical reactions is also discussed. Examples given are the outer- and inner-sphere one-electron reductions of Co(III) and Cr(III) complexes at silver, and the oxidation of carbon monoxide and iodide at gold electrodes. 

There are several demands that must be more or less fulfilled by the mediator before a successfull amperometric detection of NADH with CMEs can be realized. Despite having a E° lower or comparable with the optimal working potential range for amperometric detection, the mediator should exhibit fast reaction rates both with the electrode proper and NADH, and also be chemically stable at any redox state. Furthermore, the redox reaction of the mediator should involve two electrons and at least one proton making possible, at least theoretically, a fast inner sphere hydride transfer in the homogeneous reaction with NADH. 

In addition to quinone reduction and hydroquinone oxidation, electrode reactions of many organic compounds are also inner-sphere. In these charge transfer is accompanied by profound transformation of the organic molecules. Some reactions are complicated by reactant and/or product adsorption. Anodic oxidation of chlorpro-mazine [54], ascorbic acid [127], anthraquinone-2,6-disulfonate [128], amines [129], phenol, and isopropanol [130] have been investigated. The latter reaction can be used for purification of wastewater. The cyclic voltammogram for cathodic reduction of fullerene Cm in acetonitrile solution exhibits 5 current peaks corresponding to different redox steps [131]. 

In the second chapter, Appleby presents a detailed discussion and review in modem terms of a central aspect of electrochemistry Electron Transfer Reactions With and Without Ion Transfer. Electron transfer is the most fundamental aspect of most processes at electrode interfaces and is also involved intimately with the homogeneous chemistry of redox reactions in solutions. The subject has experienced controversial discussions of the role of solvational interactions in the processes of electron transfer at electrodes and in solution, especially in relation to the role of Inner-sphere versus Outer-sphere activation effects in the act of electron transfer. The author distils out the essential features of electron transfer processes in a tour de force treatment of all aspects of this important field in terms of models of the solvent (continuum and molecular), and of the activation process in the kinetics of electron transfer reactions, especially with respect to the applicability of the Franck-Condon principle to the time-scales of electron transfer and solvational excitation. Sections specially devoted to hydration of the proton and its heterogeneous transfer, coupled with... 

Few standard rate constants are available for simple one-electron redox reactions known to follow inner-sphere pathways. One reason is that reactions are required that are free from coupled chemical steps (i.e., are chemically reversible) so that values of E, and hence k can be obtained. Such redox couples [e.g., Ru(III)/(II)] exhibit rapid electrode kinetics even for outer-sphere pathways, so that the inner-sphere rate constants commonly are immeasurably large. 

The redox reactions at metal electrodes can be treated in a similar way. Use of the oscillator model for the solvent implies that the solvent reorganization takes place at a fixed separation of the ion from the metal surface. With this condition all results obtained in Sec.3 2.IV for homogeneous outer-sphere and inner-sphere electron-transfer reactions can be directly applied by relating the current density (per unit ion concentration) to the expressions for the rate constant through equation (106.IV). 

For outer-sphere electron transfer, the rate equations (62.IV) and (63.IV) for non-adiabatic and adiabatic reactions may be used by introducing the electrode potential cp through the relation (107.IV) or the overvoltage through equation (114.IV), instead of the reaction heat Q. For inner-sphere redox reactions the expressions (83 IV) and (85.IV) can be used in a similar way for electronically non-adiabatic and adiabatic reactions, respectively. The conditions of validity of the Tafel equation are then given by ( 12.IV) or (116.IV). 

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