Heterogeneous reaction, electrochemical experiments

Electrochemical processes are always heterogeneous and confined to the electrochemical interface between a solid electrode and a liquid electrolyte (in this chapter always aqueous). The knowledge of the actual composition of the electrode surface, of its electronic and geometric structure, is of particular importance when interpreting electrochemical experiments. This information cannot be obtained by classical electrochemical techniques. Monitoring the surface composition before, during and after electrochemical reactions will support the mechanism derived for the process. This is of course true for any surface sensitive spectroscopy. Each technique, however, has its own spectrum of information and only a combination of different surface spectroscopies and electrochemical experiments will come up with an almost complete picture of the electrochemical interface. XPS is just one of these techniques. 

Saveant s electrochemical work on reductive CX bond cleavage is highlighted in Section 10.3, and Chapter 9 (Volume I, Part 1) discusses other heterogeneous electron-transfer experiments. Only homogeneous reactions will be discussed here. 

The electrochemical redox reaction of a substrate resulting from the heterogeneous electron transfer from the electrode to this substrate (cathodic reduction) or the opposite (anodic oxidation) is said to be electrochemically reversible if it occurs at the Nernstian redox potential without surtension (overpotential). This is the case if the heterogeneous electron transfer is fast, i.e. there must not be a significant structural change in the substrate upon electron transfer. Any structural change slows down the electron transfer. When the rate of heterogeneous electron transfer is within the time scale of the electrochemical experiment, the electrochemical process is fast (reversible). In the opposite case, it appears to be slow (electrochemically irreversible). Structural transformations are accompanied by a slow electron transfer (slow E), except if this transformation occms after electron transfer (EC mechanism). 

Thus far we have considered only the case of planar macroelectrodes. Although these are widely used for electrochemical experiments, they have some drawbacks mainly due to the distorting effects arising from their large capacitance and ohmic drop. In addition, mass transport in linear diffusion is quite inefficient such that in the case of fast homogeneous and heterogeneous reactions, the response is diffusion-limited and therefore it does not provide kinetic information. 

The peak-to-peak potential splittings (A p values) observed in the presence of the calixarene host clearly depart from the theoretical 57-mV value that would be expected for a reversible redox couple. This finding clearly indicates that the complexation reactions interfere with the heterogeneous electron transfer process. We are currently carrying out further electrochemical experiments to improve our understanding of the mechanism for the oxidation of the calixarene-bound ferrocene guests. 

Cyclic voltammetry (CV) is a potential-controlled reversal electrochemical experiment. A cyclic potential sweep is imposed on an electrode and the current response is observed. Analysis of the current response can give information about the thermodynamics and kinetics of electron transfer at the electrodesolution interface, as well as the kinetics and mechanisms of solution chemical reactions initiated by the heterogeneous electron transfer. This chapter examines fundamental experimental and theoretical aspects of the CV expjeriment (Figure 2-1). 

Cyclic voltammetry is the most widely used technique for acquiring qualitative information about electrochemical reactions. The power of cyclic voltammetry results from its ability to rapidly provide considerable information on the thermodynamics of redox processes, on the kinetics of heterogeneous electron-transfer reactions, and on coupled chemical reactions or adsorption processes. Cyclic voltammetry is often the first experiment performed in an electroanalytical study. In particular, it offers a rapid location of redox potentials of the electroactive species, and convenient evaluation of the effect of media upon the redox process. 

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