EC mechanisms

The electrode reaction (2.46) is followed by a first-order homogeneous chemical reaction (2.47), in which the product of the electrode reaction O is converted to a final electroinactive product Y. By analogy with the CE mechanism, the chemical step can proceed as  

The meaning of all symbols is equivalent as for CE mechanism (Sect. 2.4.1). Combining (2.54) and (2.55) with the Nemst equation (1.8) yields an integral equation, as a general solution for a reversible EC mechanism. The numerical solution reads  

The variation of the peak current with the electrode kinetic parameter k and chemical kinetic parameter e is shown in Fig. 2.31. When the quasireversible electrode reaction is fast (curves 1 and 2 in Fig. 2.31) the dependence is similar as for the reversible case and characterized by a pronounced minimum If the electrode reaction is rather slow (curves 3-5), the dependence A fJ, vs. log( ) transforms into a sigmoidal curve. Although the backward chemical reaction is sufficiently fast to re-supply the electroactive material on the time scale of the reverse (reduction) potential pulses, the reuse of the electroactive form is prevented due to the very low kinetics of the electrode reaction. This situation corresponds to the lower plateau of curves 3-5 in Fig. 2.31. 

The validity of the theoretical predictions is yet not experimentally rigorously confirmed by a model experimental system, although the theory has a safe background in the theory and experiments of similar potential ptrlse techniques as well as cyclic staircase voltammetry. 

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Chemical reducing conditions and flow geometries are optimized to minimize EC. Reducing conditions especially must not be too severe (as may happen with excess hydrazine) to prevent autocat-alytic EC mechanisms proliferating. 

For example, when the redox system is perturbed by a following chemical reaction, that is, an EC mechanism,... 

A special case of the EC mechanism is the catalytic regeneration of O during the chemical step ... 

Exp lam and demonstrate clearly how spectroelectrochemistry can provide useful information about a reaction mechanism involving a redox process followed by a chemical reaction (EC mechanism), involving decomposition of the reaction product. Draw an absorbance-time plot for different rate constants of the decomposition reaction. 

EC mechanism, 34, 42, 113 E. Coli, 186 Edge effect, 129 Edge orientation, 114 Electrical communication, 178 Electrical double layer, 18, 19 Electrical wiring, 178 Electrocapillary, 22 Electrocatalysis, 121 Electrochemical quartz crystal, microbalance, 52 Electrochemihuiiinescence, 44 Electrodes, 1, 107... 

Reactions (6.8)-(6.10) corresponding to the proposed EC mechanism, and not taking into account the initiation step (which involves a small fraction of the charge), it is possible to obtain an analytical expression for the voltammetric CO stripping peak [Herrero et al., 2004] ... 

In an EC mechanism the ratio of the forward and backward reaction rates is decisive for k/ d in , the chemical follow-up reaction has no influence here, so that for a sufficiently rapid electron transfer step the limiting current remains diffusion controlled.)... 

The natural assumption made by a large number of researchers in the field of electrochemical C02 reduction was that the intermediate was C02, as postulated by Haynes and Sawyer (1967). The observation of oxalate as a major product in addition to, or in competition with, the formation of CO, CO, HCOj and HCOO , increased the attention focused on the reactive intermediate and the mechanisms by which it reacted. However, controversy has arisen over whether the subsequent reaction of the CO 2 was via dimerisation (the EC mechanism) or via attack on another C02 molecule (the ECE mechanism). In addition, the existence of such species as CO 2 (ads) and HCOO (ads) have also been suggested but, as we shall see, these are not now thought to play a major role on simple metals. 

NO can be oxidized or reduced on an electrode surface. Since the reduction potential of NO is close to that of oxygen which causes huge interference NO measurement, therefore, usually oxidation of NO is used for measurement of NO. NO oxidation on solid electrodes proceeds via an EC mechanism electrochemical reaction [22] followed by chemical reaction [23], First, one-electron transfer from the NO molecule to the electrode occurred and resulted in the formation of a cation ... 

Hogenauer C, Hammer HF, Krejs GJ, Reisin-ger EC Mechanisms and management of anti-biotic-associated diarrhea. Clin Infect Dis 1998 27 702-710. 

The EC mechanism (Scheme 2.1) associates an electrode electron transfer with a first-order (or pseudo-first-order) follow-up homogeneous reaction. It is one of the simplest reaction schemes where a heterogeneous electron transfer is coupled with a reaction that takes place in the adjacent solution. This is the reason that it is worth discussing in some detail as a prelude to more complicated mechanisms involving more steps and/or reactions with higher reaction orders. As before, the cyclic voltammetric response to this reaction scheme will be taken as an example of the way it can be characterized qualitatively and quantitatively. 

FIGURE 2.7. Double potential step chronoamperometry for an EC mechanism with an irreversible follow-up reaction, a Potential program with a cyclic voltammogram showing the location of the starting and inversion potentials to avoid interference of the charge transfer kinetics, b Example of chronoamperometric response, c Variation of the normalized anodic-to-cathodic current ratio, R, with the dimensionless kinetic parameter X. 

Under these conditions, the wave is exactly twice the wave pertaining to the corresponding EC mechanism, with the and the same peak potential same peak width [equations (2.6) and (2.7)]. 

The simplest electrodimerization mechanism occurs when the species formed as the result of a first electron transfer reaction reacts with itself to form a dimer (Scheme 2.7). This mechanism is usually termed radical-radical dimerization (RRD) because the most extensive studies where it occurs have dealt with the dimerization of anion and cation radicals formed upon a first electron transfer step as opposed to the case of radical-substrate dimerizations, which will be discussed subsequently. It is a bimolecular version of the EC mechanism. The bimolecular character of the follow-up reaction leads to nonlinear algebra and thus complicates slightly the analysis and numerical computation of the system. The main features of the cyclic voltammetric responses remain qualitatively similar, however. Unlike the EC case, however, the dimensionless parameter,... 

The peak current is proportional to the substrate concentration and to the square root of the scan rate as for a simple diffusion-controlled wave. The proportionality coefficient is slightly larger, 0.527 instead of 0.446. Correspondingly, the wave is thinner, in the ratio 1.51/1.86. As with the EC mechanism, the peak potential is more sensitive to the follow-up reaction. It varies linearly with the logarithm of the scan rate, of the rate constant of the dimerization reaction, and of the substrate concentration. The rates of these variations are summarized in Table 2.1, where they can be compared to the values characterizing other mechanisms, hence serving as diagnostic... 

As with the other reaction schemes involving the coupling of electron transfer with a follow-up homogeneous reaction, the kinetics of electron transfer may interfere in the rate control of the overall process, similar to what was described earlier for the EC mechanism. Under these conditions a convenient way of obtaining the rate constant for the follow-up reaction with no interference from the electron transfer kinetics is to use double potential chronoamperometry in place of cyclic voltammetry. The variations of normalized anodic-to-cathodic current ratio with the dimensionless rate parameter are summarized in Figure 2.15 for all four electrodimerization mechanisms. 

FIGURE 2.21. Homogeneous catalytic EC mechanism. Passage from control by forward electron transfer to control by follow-up reaction upon increasing the mediator concentration. y = 5, k-e/kc = 1000M TZTke/Fv = 100AT1. 

Two-Electron Catalytic Reactions In a number of circumstances, the intermediate C formed upon transformation of the transient species B is easily reduced (for a reductive process, and vice versa for an oxidative process) by the active form of the mediator, Q. This mechanism is the exact counterpart of the ECE mechanism (Section 2.2.2) changing electron transfers at the electrode into homogeneous electron transfers from Q, as depicted in Scheme 2.9. In most practical circumstances both intermediates B and C obey the steady-state approximation. It follows that the current is equal to what it would be for the corresponding EC mechanism with a... 

When pc —> oo, the catalytic loop is complete. The reaction sequence and the current-potential responses are the same as in the two-electron ECE homogeneous catalytic mechanism analyzed in the preceding subsection. When pc —> 0, deactivation prevails, and if the first electron transfer and the deactivation steps are fast, the same irreversible current-potential responses are obtained as in a standard EC mechanism. 

The transition between the two limiting situations is a function of the parameter (k-e/kc)Cp. The ratio between the catalytic peak current, ip, and the peak current of the reversible wave obtained in the absence of substrate, Pp, is thus a function of one kinetic parameter (e.g., Xe) of the competition parameter, (k e/A c)c and of the excess ratio y = C /Cp, where and Cp are the bulk concentrations of the substrate and catalyst, respectively. In fact, as discussed in Section 2.6, the intermediate C, obtained by an acid-base reaction, is very often easier to reduce than the substrate, thus leading to the redox catalytic ECE mechanism represented by the four reactions in Scheme 2.13. Results pertaining to the EC mechanism can easily be transposed to the ECE mechanism by doubling the value of the excess factor. 

The governing dimensionless partial derivative equations are similar to those derived for cyclic voltammetry in Section 6.2.2 for the various dimerization mechanisms and in Section 6.2.1 for the EC mechanism. They are summarized in Table 6.6. The definition of the dimensionless variables is different, however, the normalizing time now being the time tR at which the potential is reversed. Definitions of the new time and space variables and of the kinetic parameter are thus changed (see Table 6.6). The equation systems are then solved numerically according to a finite difference method after discretization of the time and space variables (see Section 2.2.8). Computation of the... 

The main difference with the EC mechanism (Section 6.2.1) is that C is reduced as soon as it reaches the electrode hence the replacement of the boundary condition (QCc/Qx)x=0 = 0 by the condition (Cc)x=0 = 0. A second difference is the contribution to the current provided by the reduction of C. Introduction of the same normalized variables and parameters as in Sections 6.1.2 and 6.2.1 leads to... 

Homogeneous Catalytic EC Mechanism The system is governed by the following dimensionless equations (we need not consider equations involving p, since as in all preceding cases, p = 1 — q), where two additional normalized rate parameters are introduced ... 

Comparison of equations (6.85) and (6.86) with equations (6.83) and (6.84) shows that the previous analysis of the catalytic EC mechanism is applicable to the catalytic ECE mechanism after replacement of y by 2y. 

Data were obtained in aqueous solution containing 0.2 mol dm"3 KC1 as supporting electrolyte. Solutions were 3 X 10 3 mol dm"3 in compound and potentials were determined with reference to SCE at 21 1°C at 50 mV s"1 scan rate. The solution pH was adjusted with 0.5 mol dm"3 KOH and 0.1 mol dm"3 HC1. 6For [28], [29], [30] and [31] the CVs were reversible one electron oxidations at pH < 6. At pH = 11, an EC mechanism was observed for [28], [29] and [31]. Minor oxidation waves of the amino groups appeared after that of the Fc+/Fc couple at slow scan rate. The CV of [32] was a one-electron reversible oxidation wave, less dependent on the solution pH, and showed no oxidation of the amino groups in the pH range explored. Anodic shifts of anodic current peak potential of the Fc+/Fc couple produced by the presence of metal cations (1 or 2 equiv added as their perchlorate salts). 

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