Electrode-Reaction Mechanisms

Electrocatalysis. There is a particular type of ECE electrode reaction mechanism which is designated as liCE (the arrows indicate that the second electron transfer consists of a inverse reaction with respect to the first). This process is called electrocatalysis and is of importance in inorganic chemistry.13... 

Reaction mechanisms are also defined for electroorganic reactions, induced by or including an electron transfer at an electrode. Knowledge of such electrode reaction mechanisms includes, preferably but not exclusively, the potential at which the reaction proceeds, the proof of intermediates, the electron stoichiometry, the kinetics of the various reaction steps, and the transport properties of the species involved. Recently, the terms... 

Linear sweep and cyclic voltammetry (LSV and CV) are probably the most widely used techniques to investigate electrode reaction mechanisms. They are easy to apply experimentally, readily available in... 

Qualitative and Quantitative Investigation of Electrode Reaction Mechanisms... 

Organic Electrode Reactions, Mechanisms of (Elving Pullman). 3 1... 

This chapter concerns the study of electrode reaction mechanisms of inorganic and organometallic complexes. The emphasis is on proper use of experimental measurables from cyclic voltammetry for diagnosis of common mechanisms such as E, EC, CE, and ECE reactions. We employ the standard designation of electron transfer (et) reactions as E, and other chemical reactions as C. In practice, mechanistic studies make use of an array of electrochemical and other physical and chemical methods, but space limitations restrict our attention to the powerful and versatile technique of cyclic voltammetry (CV). If necessary, the reader may review the fundamentals of this technique in Chapter 3. 

The passage of a net current through an electrode implies that the electrode is no longer at equilibrium and that a certain amount of overpotential is present at the electrode-electrolyte interface. Since the overpotential represents a loss of energy and a source of heat production, a quantitative model of the relationship between current density and overpotential is required in design calculations. A fundamental model of the current-overpotential relationship would proceed from a detailed knowledge of the electrode reaction mechanism however, mechanistic studies are complicated even for the simplest reactions. In addition, kinetic measurements are strongly influenced by electrode surface preparation, microstructure, contamination, and other factors. As a consequence, a current-overpotential relation is usually determined experimentally, and the data are often fitted to standard models. 

CELL WITH Ti02 ELECTRODES REACTION MECHANISMS AND EFFICIENCY. 

The information that can be obtained with electrochemical detectors is not restricted to quantification. Instead of the conventional use of electrochemical detectors in amperometric mode at fixed potential, electrode arrays with each electrode held at different values of fixed potential can be used, in order to build up chronovoltammograms, three-dimensional current-voltage-time profiles. A 32-microband electrode array has been described for this purpose and applied to phenolic compounds [17] and which permits studying the electrode reaction mechanism at the same time as identification and quantification are carried out. Alternatively, fast voltammetric techniques such as fast-scan cyclic voltammetry or square wave voltammetry can be used to create chronovoltammograms of the eluted components. 

In this form of voltammetry, the concentration distributions of each species in the electrode reaction mechanism are temporally invariant at each applied potential. This condition applies to a good approximation despite various processes still occurring such as mass transport (e.g. diffusion), heterogeneous electron transfer and homogeneous chemical processes. Theoretically it takes an infinite time to reach the steady state. Thus, in a practical sense steady-state voltammetric experiments are conducted under conditions that approach sufficiently close to the true steady state that the experimental uncertainty of the steady-state value of the parameter being probed (e.g. electrode current) is greater than that associated with not fully reaching the steady state. The... 

The basic concept is that the experimental voltammetric data are collected and a mechanism for the electrode reaction mechanism is postulated. The proposed mechanism may be theoretically simulated by solving the appropriate mathematical problem. Satisfactory agreement between experiment and theory is used to suggest a quantitative description for a particular mechanism, but as with most kinetic studies ideally the identity of proposed reaction intermediates must be confirmed by an independent technique, e.g. a spectroscopic technique. It is inherently dangerous to assume the structure of a reaction product or intermediate solely on the basis of a voltammetric response. 

Fig-1 Schematic diagram representing the process of examining an electrode reaction mechanism using voltammetric techniques. 

Each of the mass transport terms can be combined to give a general mass transport equation describing the temporal variation of each species in the electrode reaction mechanism i.e. (18) for species, A. 

This mechanism is denoted as an EC mechanism (Testa and Reinmuth, 1961 Bott, 1997). Thus homogeneous kinetic terms may be combined with the expressions for diffusion and convection [i.e. a modified version of (18)] to give the temporal variation of the concentration of a species in an electrode reaction mechanism. In order to model the voltammetric response associated with this mechanism, a knowledge of , a, ko and k is required, or deduced from a theoretical-experimental comparison, and the set of concentrationtime equations for species A, B and C must be solved subject to the constraints of the Butler-Volmer equation and the experimental design. Considerable simplification of the theory is achieved if the kinetics for the forward and reverse processes associated with the E step are fast, which is a good approximation for many organic reactions. Section 7 describes the approaches used to solve the equations associated with electrode reaction mechanisms, thus enabling theoretical simulation of voltammetric responses to be achieved. 

A common form of electrode reaction mechanism studied is the ECE mechanism ... 

A very common electrode reaction mechanism encountered in organic electrochemistry is the ECE mechanism. As discussed in Section 2, with this mechanism, a species generated by heterogeneous electron transfer at the... 

EXAMPLES OF ELECTRODE REACTION MECHANISMS CONSISTING OF EXTENSIVE COMBINATIONS OF E AND C STEPS... 

Conversely for slow reactions, low rates of mass transport will be required to achieve significant deviations from N fs equalling one. Consequently, it can be appreciated that it is a study of the competition between the rates of mass transport and chemical kinetics that leads to the quantitative determination of electrode reaction mechanisms in hydrodynamic voltammetry. Importantly, for each hydrodynamic technique, there is one assessable convective transport parameter that directly relates to the kinetic time-scale. 

Examples of electrode reaction mechanisms consisting of extensive combinations of E and C steps 42 Hydrodynamic voltammetry 44 Rotating-disc electrodes 46 Channel electrodes 48 Wall jet electrodes 52 Electron-transfer processes 53... 

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