Kinetic studies electroactive species

Adsorption can be studied by many electrochemical methods, as can adsorption kinetics. When electroactive species are adsorbed, reagents or products of electrode reactions or both, a significant change in voltam-metric response can occur. Adsorption of non-electroactive species can inhibit the electrode reaction. These processes depend on electrode material as well as on solution composition. 

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. 

Cyclic voltammetry and other electrochemical methods offer important and sometimes unique approaches to the electroactive species. Protein organization and kinetic approaches (Correia dos Santos et al. 1999, Schlereth 1999) can also be studied by electrochemical survey. The electron transfer reaction between cytochrome P450scc is an important system for... 

Of great interest in lots of domains, oxygen has been studied by voltammetry under very different experimental conditions. As its reaction is under kinetic control, the electrode reaction is influenced by various factors, in particular by the nature of the electrode and of the solution. The metal, through its chemical properties and surface states, which influence the adsorption-desorption of electroactive species or... 

In electrode kinetic studies, reactant concentrations are, in general, in the millimolar range and double layer contributions for such low ionic concentrations may become very important. If excess of inert or supporting electrolyte is used, the relative variation in the ionic concentration at the double layer due to the electrochemical reaction is at a minimum at high concentration of an inert z z electrolyte, most of the interfacial potential drop corresponds to the Helmholtz inner layer and variations of A02 with electrode potential are small (Fig. 3). In addition, use of supporting electrolyte prevents the migration of electroactive ionic species from becoming important and also reduces the ohmic overpotential. 

Chlorobenzonitrile and adrenaline, our second example, both give electrode products that are unstable with respect to subsequent chemical reaction. Because the products of these homogeneous chemical reactions are also electroactive in the potential range of interest, the overall electrode reaction is referred to as an ECE process that is, a chemical reaction is interposed between electron transfer reactions. Adrenaline differs from/ -chlorobenzonitrile in that (1) the product of the chemical reactions, leucoadrenochrome, is more readily oxidized than the parent species, and (2) the overall rate of the chemical reactions is sufficiently slow so as to permit kinetic studies by electrochemical methods. As a final note before the experimental results are presented, the enzymic oxidation of adrenaline was known to give adrenochrome. Accordingly, the emphasis in the work described by Adams and co-workers [2] was on the preparation and study of the intermediates. 

The chronocoulometry and chronoamperometry methods are most useful for the study of adsorption phenomena associated with electroactive species. Although less popular than cyclic voltammetry for the study of chemical reactions that are coupled with electrode reactions, these chrono- methods have merit for some situations. In all cases each step (diffusion, electron transfer, and chemical reactions) must be considered. For the simplification of the data analysis, conditions are chosen such that the electron-transfer process is controlled by the diffusion of an electroactive species. However, to obtain the kinetic parameters of chemical reactions, a reasonable mechanism must be available (often ascertained from cyclic voltammetry). A series of recent monographs provides details of useful applications for these methods.13,37,57... 

Electrochemical studies on SAMs have proven invaluable in elucidating the impact of various molecular parameters such as bridge structure, molecular orientation or the distance between the electroactive species and electrode surface. As described above in Section 5.2.1, the kinetics of heterogeneous electron transfer have been studied as a function of bond length for many systems. Similarly, the impact of bridge structure and inter-site distances have been studied for various supramolecu-lar donor-acceptor systems undergoing photoinduced electron transfer in solution. In both types of study, electron transfer is observed to increase as the distance between the donor and acceptor decreases. As discussed earlier in Chapter 2, the functional relationship between the donor-acceptor distance and the electron transfer rate depends on the mechanism of electron transfer, which in turn depends on the electronic nature of the bridge. 

Impedance methods have been more useful in studying electron-transfer kinetics in electroactive monolayers in the absence of an electroactive solution species (71-73), such as alkylthiol layers with tethered electroactive groups (Section 14.5.2). The equivalent circuit adopted is shown in Figure 14.3.18, where the adsorbed layer is represented by Cads = (F AT)/4RT and the electron-transfer kinetics by = (2RT)/F ATkf, so that... 

The popularity of the cychc voltammetry (CV) technique has led to its extensive study and numerous simple criteria are available for immediate anal-j sis of electrochemical systems from the shape, position and time-behaviour of the experimental voltammograms [1, 2], For example, a quick inspection of the cyclic voltammograms offers information about the diffusive or adsorptive nature of the electrode process, its kinetic and thermodynamic parameters, as well as the existence and characteristics of coupled homogeneous chemical reactions [2]. This electrochemical method is also very useful for the evaluation of the magnitude of imdesirable effects such as those derived from ohmic drop or double-layer capacitance. Accordingly, cyclic voltammetry is frequently used for the analysis of electroactive species and surfaces, and for the determination of reaction mechanisms and rate constants. 

When the electroactive species or an intermediate adsorbs on the electrode surface, the adsorption process usually becomes an integral part of the charge transfer process and therefore cannot be studied without the interference of a faradaic current. In this situation, surface coverages cannot be measured directly and the role of an adsorbate must be inferred from a kinetic investigation. Tafel slopes and reaction orders will deviate substantially from those for a simple electron transfer process when an adsorbed intermediate is involved. Moreover the kinetic parameters, exchange current or standard rate constant, are likely to become functions of the electrode material and even the final products may change. These factors will be discussed further in the section on electrocatalysis (Section 1.4). 

The kinetics of c,e-type processes are also conveniently studied by means of fast-sweep cyclic voltammetry where the current measured depends on sweep-rate due to the time required for the electroactive species Ox to be produced from the initial reactant, C. Information on ks in relation to the magnitude ot k and k2 can be obtained from suitably designed experiments. 

Reinmuth has examined chronopotentiometric potential-time curves and proposed diagnostic criteria for their interpretation. His treatment applies to the very limited cases with conditions of semi-infinite linear diffusion to a plane electrode, where only one electrode process is possible and where both oxidized and reduced forms of the electroactive species are soluble in solution. This approach is further restricted in application, in many cases, to electrode processes whose rates are mass-transport controlled. Nicholson and Shain have examined in some detail the theory of stationary electrode polarography for single-scan and cyclic methods applied to reversible and irreversible systems. However, since in kinetic studies it is preferable to avoid diffusion control which obscures the reaction kinetics, such methods are not well suited for the general study of the mechanism of electrochemical organic oxidation. The relatively few studies which have attempted to analyze the mechanisms of electrochemical organic oxidation reactions will be discussed in detail in a following section. 

There are two major differences between OTE and OTTLE systems. Firstly there is considerable iR drop in the OTTLE, and therefore it is unsuitable for fast kinetic studies. Secondly it is possible to convert all of the electroactive species in the light path to product in a very short time (typically less than a minute). In view of these differences OTEs, and OTTLEs have rather different applications. 

When the above conditions of effects of convection and migration are realized, the resulting current is limited only by the diffusion-driven transport of the electroactive species to the electrode-solution interface. At the interface charge transfer reactions take place that can be studied as a function of the electrochemical potential (Fig. 4). That experimental situation contains the fundamental premise of the traditional impedance analysis - make mass-transport effect on interfacial impedance exclusively diffusion limited (Ldiff) and investigate the interfacial kinetic phenomena composed of the diffusion and electrochemical reaction (discharge or electrolysis Rqi) impedances that can be combined in the so-caUed Faradaic impedance. 

Cyclic voltammetry (CV) is perhaps the most versatile electroanalytical technique for the study of electroactive species. CV is usually used to characterize electrochemical reactions and electrode surface absorption, and has become increasingly popular in all fields of chemistry as a means of studying redox reaction rates in a wide potential range. CV is also a typical mefliod for the study of reaction mechanisms, as its mathematical description has been developed sufficiently to enable kinetic parameters to be determined. 

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