Cyclic voltammetry irreversible systems

Although cyclic voltammetry could fruitfully be applied to the kinetic analysis of these catalytic systems, it has mostly been investigated by means of rotating disk electrode voltammetry (Section 1.3.2). The simplest case is that of an irreversible catalytic reaction at a monolayer coating. The next section is devoted to the analysis of these systems by the two techniques. 

As already stated, other electrochemical techniques have been used to derive thermodynamic data, some of them considered to yield more reliable (reversible) redox potentials than cyclic voltammetry. This is the case, for instance, of second harmonic alternating current voltammetry (SHACV) [219,333], Saveant and co-workers [339], however, concluded that systems that appear irreversible in slow-scan CV are also irreversible in SHACV experiments. We do not dwell on these matters, important as they are. Instead, we concentrate on a different methodology to obtain redox potentials, which was developed by Wayner and colleagues [350-352]. 

The cyclic voltammetry behavior of the Cu(II) rotaxane, 4(5)2+ (Fig. 14.8b), is very different from that of 4, t l +. The potential sweep for the measurement was started at - 0.9 V, a potential at which no electron transfer should occur, regardless of the nature of the surrounding of the central Cu(II) center (penta- or tetracoordinate). Curve i shows two cathodic peaks a very small one, located at + 0.53 V, followed by an intense one at —0.13V. Only one anodic peak at 0.59 V appears during the reverse sweep. If a second scan ii follows immediately the first one i, the intensity of the cathodic peak at 0.53 V increases noticeably. The main cathodic peak at —0.15 V is characteristic of pentacoordinate Cu(II). Thus, in 4(5)2+ prepared from the free rotaxane by metalation with Cu(II) ions, the central metal is coordinated to the terdentate terpyridine of the wheel and to the bidentate dpp of the axle. On the other hand, the irreversibility of this peak means that the pentacoordinate Cu(I) species formed in the diffusion layer when sweeping cathodically is transformed very rapidly and in any case before the electrode potential becomes again more anodic than the potential of the pentacoordinate Cu2 + /Cu+ redox system. The irreversible character of the wave at —0.15 V and the appearance of an anodic peak at the value of + 0.53 V indicate that the transient species, formed by reduction of 4(5)2 +, has undergone a complete reorganization, which leads to a tetracoordinate copper rotaxane. The second scan ii, which follows immediately the first one i, confirms this assertion. 

It will be clear that cyclic voltammetry is a powerful tool for a first analysis of an electrochemical reaction occurring at the surface of an electrode because it will reveal reversibility. Depending on whether the system is reversible, information will be obtained about half wave potential, number of electrons exchanged in the reaction, the concentration and diffusion coefficient of the electroactive species. However, these data can also be obtained for an irreversible system1113 but, in this case, the equations describing the current-potential curves differ somewhat from Equations 2.21 to 2.27. 

Electrochemical properties were examined to gain more quantitative insight into the redox properties of this system. Cyclic voltammetry on bis(dithiazole) 23 in acetonitrile (with Pt electrodes and 0.1 M -Bu4NPF6 as supporting electrolyte) reveals a reversible oxidation wave with i/2(°x) = 0.93 V and a second, irreversible oxidation process... 

Fig. 9.4. Linear sweep voltammogram for an irreversible system (O + ne —> R). In cyclic voltammetry, on inverting the sweep direction, one obtains only the continuation of current decay (-------------------).

Eirrev diagnostics in cyclic voltammetry — For electrochemical systems with kinetic constraints in the heterogeneous electron transfer reaction (- irreversibility)... 

There is additionally the important problem involved in choosing the reduction or oxidahon potential of the electrolyte solutions from either cyclic voltammetry (CV) or linear sweep voltammetry (LSV). Since the oxidation or reduction reachon of cations or anions contained in the RTILs are electrochemically irreversible in general [8-10], the corresponding reduction or oxidation potential cannot be specifically obtained, unlike the case of the redox potential for an electrochemically reversible system. Figure 4.1 shows the typically observed voltammogram (LSV) for RTILs. Note that both the reduchon and oxidation current monotonically increase with the potential sweep in the cathodic and anodic directions, respectively. Since no peak is observed even at a high current density (10 mA cm ), a certain... 

Since the resulting radical cations proved to be highly reactive with lifetimes much below 10 s in acetonitrile (results from fast scan cyclic voltammetry [64]), only irreversible oxidation potentials of the enols and ketones were obtained. Therefore, the data can only be viewed as a good estimate (Table 4). Nevertheless, in agreement with gas-phase results, it is evident that, in solution as well, enol radical cations are more stable than the corresponding ketone ions. Unfortunately, no solution data are so far available for simple aliphatic systems. 

All potentials refer to SCE, except for Pd(5-H ) for which the potential refers to Fc" /Fc. The systems are reversible except in few cases indicated by qrev (quasi-reversible) or irr (irreversible). The potentials have been determined by cyclic voltammetry on both Pt and Hg electrodes, leading to identical values, unless otherwise noted. 

Redox potential (V) measured in CH3CN versus the saturated calomel electrode (SCE), determined by cyclic voltammetry at the platinum electrode, 0.1 M (nC4H9)4N. BF4 room temperature under argon scan rate, 100 mV s" the systems are reversible except in a few cases indicated by qrev (quasi reversible) and irr (irreversible). 

Since the Co(II) to Co(III) complexes were redox active, an electrochemical method of analysis seemed viable for the quantification of the two species in the reaction. The specific electrochemical technique developed to monitor the activation reaction allowed the simultaneous quantitative measurement of (salen)Co(II) and (salen)Co(III) species in the medium. The principle of the method is based on the electro-oxidation of both species on a platinum-rotating electrode linearly polarized with respect to a standard electrode [7]. The electrochemical reactions operative with this cyclic voltammetry technique involve the single electron oxidation of each species and occur at the revolving surface of the electrode. With this salen ligand system, the Co(II) to Co(III) transformation was determined as being fully reversible, while the Co(III) to Co(IV) reaction was irreversible. 

Since a is usually between 0.3 and 0.7, both the wave slope and the Tomes criterion for a totally irreversible system are normally significantly larger than for a reversible system. These figures of merit are not without ambiguity, however. Consider the predicted wave slope of 63.8 mV for a = 0.85. Within the precision of normal measurements, one could diagnose the system as either reversible or irreversible. It is always a good idea to examine reversibility by a method, such as cyclic voltammetry, that allows a view of the electrode reaction in both directions. 

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