EC catalytic mechanism

We finally consider the EC catalytic mechanism in which the product of the electrode reaction transforms back to the initial electroactive reactant by means of a pseudo first-order chemical reaction [ 15,53,55]  

Note that the chemical step (2.75) is totally irreversible, attributed with a pseudo first-order rate constant (s ) defined as Atc =, rCx, where cx has the same meaning as for the CE and EC mechanisms (Sect. 2.4.1). Although this is the simplest case of an electrode mechanism involving chemical reaction, it has particular analytical utihty [53]. The mass transport of the redox species is described by the following model  

By substituting (2.81) and (2.82) into the Nemst equation (1.8), one obtains an integral equation, as a solution for a reversible catalytic mechanism. The numerical solution for the reversible case reads  

The theory for catalytic reaction has been verified by studying the reductions of Ti + in presence of NH2OH and CIO and the reduction of Fe + in presence of NH2OH. In these stndies the mercury electrode has been applied [53]. The properties of the experimental voltammograms confirm the theoretical predic- 

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A typical result for DPV In Fig. 4a shows the presence of two redox couples with peak potentials of 0.25 V and 0.19 V ( lOmV). Similar results have also been obtained with SWV. The relative Intensities of the two peaks vary from sample to sample but are always present with activated electrodes. The similarities between the potentials found for the surface species and for the oxidation of ascorbic acid suggest that an ec catalytic mechanism may be operative. The surface coverage of the o-qulnone Is estimated to be the order of 10 mol cm . It Is currently not possible to control the surface concentration of the o-qulnone-llke species or the oxygen content of the GCE surface. 

Simulation of the Cyclic Voltammetric Characteristics of a Second Order EC Catalytic Mechanism... 

More recently Andrieux et. al. (5a,5b) have described a procedure for computer simulation of a second-order ec catalytic mechanism. In their work cyclic voltammetric data were calculated while changing the rate and reversibility of the follow-up reaction. Using the implicit finite-difference method... 

The work of Nicholson and Shain (7) is also a convenient resource for the study of the pseudo first-order ec catalytic mechanism. Pseudo first-order conditions were simulated in the present study by making the reactant concentration (Cg) greater than the catalyst concentration (CM>... 

Figure 2. Effects of kt and k9 on the CV characteristics of an ec catalytic mechanism for large values of kf. Key A, normalized current of the catalyzed wave versus potential B, normalized current contribution of the reactant (i.e. (i — i0)/iVc,o) versus potential and C, normalized current of the catalyzed wave plotted versus potential.

Figure 9. CV i-E scans pertinent to oxygen reduction catalysis. Key a, simulated, reversible, four electron reduction of oxygen to water b, simulated ec catalytic mechanism, N — 4 with Eu° = 1.23 volts c, experimental oxygen reduction on glassy carbon d, experimental FeTMPyP reduction on glassy carbon and e, experimental oxygen reduction catalyzed by FeTMPyP (on glassy carbon).

Three different ec catalytic mechanisms are considered in the simulation of the i-E curves for oxygen reduction catalyzed by FeTMPyP. These are ... 

At high overpotential electrodes, e.g. vitreous carbon, chlorine reduction is at more negative potentials (0.85 V vs. NHE). In the presence of CIO2, the reduction wave for chlorine is not observed. This is explained by an EC catalytic mechanism in which the C102 formed electrochemically is re-oxidised near the electrode by chlorine in solution ... 

Nolan JE, Plambeck JA (1990) The EC-catalytic mechanism at the rotating disk electrode. Part II. Comparison of approximate theories for the second-order case and application to the reaction of bipyridinium cation radicals with dioxygen in non-aqueous solutions. J Electroanal Chem 294 1-20... 

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