The ECE mechanism

Strictly, this is a logical extension of the EE mechanism with stable intermediate, considered in Sect. 5. The reduction product of the first electrode reaction, Z, is supposed to be non-reducible itself, but can react to yield a compound X that can be reduced to R. 

Many variations on this theme are conceivable (see refs. 153 and 154 in which references to practical examples have been collected). 

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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. 

By employing an extinction coefficient for the anion radical obtained from the pulse radiolysis experiments, the concentration of the radical could be calculated, and plotted against /c. The straight line plot so obtained was taken as strong evidence for the ECE mechanism, i.e. the solution phase attack of C02 on C02, thus fully resolving the controversy over the identity and state of the intermediate. From the slope of the plot the authors obtained the rate constant k2 as 7.5 x I03dm3mol 1 s 1. 

Since the occurrence of the ECE mechanism implies that yD , B, it follows that Kd 1, meaning that the disproportionation reaction is strongly exergonic. Since we have assumed that the two electrode electron transfer reactions are fast, the same is true for the disproportionation... 

Once a DISP mechanism has been recognized, the procedures for determining the rate constant of the follow-up reaction and the standard potential of the A/B couple from peak current and/or peak potential measurements are along the same lines as the procedures described above for the ECE mechanism. A distinction between the ECE and DISP mechanisms cannot be made when the pure kinetic conditions are achieved since the peak height, peak width, and variations of the peak potential with the scan rate and rate constant are the same, and so is its independence vis-a-vis the concentration of substrate. The only difference is then the absolute location of the peak, which cannot be checked, however, unless the standard potential of the A/B couple and the follow-up rate constant are known a priori. 

The fact that the normalized current ratio becomes negative at intermediate values of X with the ECE mechanism and not with the DISP mechanism stems from the same phenomenon as the one causing the tracecrossing behavior in cyclic voltammetry (Figure 2.9) (i.e., continuation of the reduction of C during the anodic scan). 

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... 

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. 

Amongst the ECE mechanisms, this one is perhaps the most commonly encountered in inorganic electrochemistry. 

Both the rhodium atoms assume a tetrahedral geometry with respect to the RI12P2 plane (the TTD label derives from the tetrahedral-tetrahedral geometry of the two rhodium atoms in the dianion). On this basis, the overall electrode process involves the ECE mechanism illustrated in Scheme 4, where TPA = tetrahedral-planar monoanion, TTA = tetrahedral-tetrahedral monoanion. 

The ECE mechanism [54] unifies the previously elaborated EC and CE mechanisms. It is represented by the following scheme ... 

The experimental model used to illustrate the ECE mechanism was the reduction of p-nitrosophenol at a mercury electrode, in which the chemical step is dehydration [54]. The experimental data have been analyzed by best-fitting curve pro-... 

As can be seen from the model, each redox reaction is attributed with different set of kinetic parameters. The current contributions are designated with I and h-As for the ECE mechanism considered in Sect. 2.4.3, the total current that can be experimentally observed is a sum of distinct current contributions, / = I +I2. Solutions for the surface concentration of the electroactrve species are given by ... 

The ECE mechanism may be distinguished from the simple EC scheme by the dependence of the peak height on v CV is, however, not well suited to distinguish ECE from DISP-1 here another technique, e.g., double-step potentiometry, may be used. 

According to the ECE mechanism proposed, the first E step would be an... 

This reaction was studied by Amatore and Sav ant [88] using DPSC. Pseudo-first-order kinetics for the reaction in the presence of excess phenol was implied from the observation of dEp/d log v close to 30 mV decade-1 during LSV analysis. The objective of the study was to show that the eCeh, where the subscript refers to a homogeneous reaction, rather than the eCe mechanism is followed. The substrate and phenol concentrations were invariant at 10-3 and 10-2 M, respectively. A more extensive study using both DPSC and DCV by Ahlberg and Parker [50] supported the mechanism assignment. Thus, at that point it appeared that the data could be accounted for by the mechanism... 

Further guidelines for the interpretation of chronopotentiometric results can be found in refs. 21 and 22. Its diagnostic value, especially, should be emphasized, as well as its sensitivity to a wide range of chemical rate constants, because its time window can be adapted by choosing proper values for the controlled current density j. The ECE mechanism, left out of the discussion here, is treated in detail in ref. 22. 

By combination of eqns. (194) and (208) and using Table 8, it will also be possible to obtain explicit expressions for the four surface concentrations involved in the ECE mechanism, see Sect. 7.2.2(e). 

The ECE mechanism involves electrochemical generation of a species that then reacts with some other component in solution. The product of this reaction is more easily oxidized/reduced than the starting material and so is immediately electrolyzed. The sequence of reactions can be summarized as follows ... 

Figure 3.40 Variation in apparent n as a function of angular velocity of an RDE for the ECE mechanism.

The simulation of the ECE mechanism may also employ the double-potential-step technique, but a working curve can be constructed from single-potential-step data also. This is because some of the current that passes, as A is converted to B, is due to the electrolysis of C, the decomposition product of B. The greater the decomposition rate of B, the more current flows, approaching the rate of... 

Thus one may obtain kt by multiplying the quantity previously referred to as the dimensionless time by k,tf, the dimensionless rate constant. This is particularly useful in constructing the single-potential-step working curve for the ECE mechanism mentioned earlier. This parametric substitution allows the experimental time to be rendered dimensionless by the inverse of the rate constant instead of by some known time tk. 

This parametric substitution allows the current to be rendered dimensionless by a quantity that is independent of the time of the physical experiment (all the factors of nFACk11/2D,/2 are constant for a given physical system). A typical working curve for the ECE mechanism obtained using these parametric substitutions is illustrated in Figure 20.7. 

Figure 20.7 Working curve for the ECE mechanism obtained by parametric substitution. Note that this represents the current-time behavior over five time decades. The slope of the dashed line is -1/2, and the separation between it and the parallel working curve immediately above it is 0.301 logarithmic units.

Figure 23.18 Experimental CV scans to two different switching potentials for a complex reducing by the ECE mechanism of Eq. 23.24-23.26 with a rapid chemical reaction (Eq. 23.25). The system is [Fe(CN)5NO]2- in CH2C12 (Eq. 23.27-23.29), and the anodic feature at approximately -1.0 V arises from a product not referred to in Eq. 23.27-23.29. v = 0.08 V/s. Reprinted from Ref. 23 with permission.

Nicholson and Shain [24] have shown that when the same number of electrons is transferred in each step of the ECE mechanism, Equation 23.33 holds, in which Xc and XD are the current functions in the presence and absence, respectively, of the intervening chemical reaction (XD is the hypothetical value of the current function under diffusion-controlled conditions, i.e., if the C step were not present) ... 

As indicated in Sect. 3.4.8, by considering that the two-electron transfers behave as reversible, the ECE mechanism can be written [27-30] ... 

From Fig. 6.14, it can be deduced that for A Ef = —142.4 mV, the two mechanisms show distinctly different voltammograms for small values of %cw(= (k 1 + ki) ja), whereas the responses become more similar as the chemical kinetics is faster. Thus, for// v >100 the voltammetric signal of the ECE mechanism is equivalent to that of an EE mechanism where the half-wave potentials correspond to those of the EC (first electron transfer) and CE (second electron transfer) mechanisms under fully labile conditions (Eqs. (3.201c) and (3.221b) for <5, > 0, respectively). 

Fig. 7.41 Square Wave Voltammetry. Net currents corresponding to ECE and EE mechanisms at planar electrodes calculated by using the numerical procedure described in [66, 67] (ECE) and Eq. (7.65) (EE). The values of fw for the ECE mechanism are 0.01 (solid lines), 0.05 (dashed lines), 0.5 (white circles), 10 (dashed-dotted lines), and 100 (dotted lines). The curves corresponding to the EE mechanism appear with black circles. The values of Ain mV appear in the figures. w = 50mV, AEs = 5mV, T = 298 K...

The effect of complexation on redox properties was studied by cyclic voltammetry. Unbound flavin, dissolved in an aprotic solvent (dichloromethane), undergoes a two electron reduction perfectly explained by the ECE mechanism. Upon addition of cyclene ligand and coordination of flavin to the zinc ion complex, the flavohydroquinone redox state was stabilised. 

The most interesting for chemists probably is the ECE mechanism, which involves a chemical step between two electron-transfer reactions ... 

Since this paper ended with a claim of rather universal validity of the ECE mechanism in anodic addition and substitution reactions, cf. the mechanism proposed for the solvolysis of Th + in homogeneous medium, namely, disproportionation, it started a controversy. We shall not go into detail with this story (Hammerich and Parker, 1972 Marcoux, 1971, 1972 Parker, 1972 Parker and Eberson, 1970b), since it encompasses a lot of rather involved argumentation which... 

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