Catalytic Currents 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. 

For the catalytic electrode mechanism, the total surface concentration of R plus O is conserved throughout the voltammetric experiment. As a consequence, the position and width of the net response are constant over entire range of values of the parameter e. Figure 2.35 shows that the net peak current increases without limit with e. This means that the maximal catalytic effect in particular experiment is obtained at lowest frequencies. Figure 2.36 illustrates the effect of the chemical reaction on the shape of the response. For log(e) < -3, the response is identical as for the simple reversible reaction (curves 1 in Fig. 2.36). Due to the effect of the chemical reaction which consumes the O species and produces the R form, the reverse component decreases and the forward component enhances correspondingly (curves 2 in Fig. 2.36). When the response is controlled exclusively by the rate of the chemical reaction, both components of the response are sigmoidal curves separated by 2i sw on the potential axes. As shown by the inset of Fig. 2.36, it is important to note that the net currents are bell-shaped curves for any observed kinetics of the chemical reaction, with readily measurable peak current and potentials, which is of practical importance in electroanalytical methods based on this electrode mecharusm. 

The effect of the volume and the surface catalytic reaction is sketched in Figs. 2.80 and 2.81, respectively. Obviously, the voltammetric behavior of the mechanism (2.188) is substantially different compared to the simple catalytic reaction described in Sect. 2.4.4. In the current mechanism, the effect of the volume catalytic reaction is remarkably different to the surface catalytic reaction, revealing that SWV can discriminate between the volume and the surface follow-up chemical reactions. The extremely high maxima shown in Fig. 2.81 correspond to the exhaustive reuse of the electroactive material adsorbed on the electrode surface, as a consequence of the synchronization of the surface catalytic reaction rate, adsorption equilibria, mass transfer rate of the electroactive species, and duration of the SW potential pulses. These results clearly reveal how powerful square-wave voltammetry is for analytical purposes when a moderate adsorption is combined with a catalytic regeneration of the electroactive material. This is also illustrated by a comparative analysis of the mechanism (2.188) with the simple surface catalytic reaction (Sect. 2.5.3) and the simple catalytic reaction of a dissolved redox couple (Sect. 2.4.4), given in Fig. 2.82. 

Catalytic current — is a -> faradaic current that is obtained as a result of a catalytic electrode mechanism (see - electrocatalysis) in which the catalyst (Cat) is either dissolved in the bulk solution or adsorbed or immobilized at the electrode surface, or it is electrochemi-cally generated at the electrode-electrolyte solution interface [i]. The current obtained in the presence of the catalyst and the substrate (S) must exceed the sum of the currents obtained with Cat and S separately, provided the currents are measured under identical experimental conditions. The catalytic current is obtained in either of the two following general situations ... 

Catalytic current — Figure. Reaction scheme of a reductive regenerative catalytic electrode mechanism (the charge of the species is omitted)... 

Attempts to support models of the catalytic activity and the operative mechanism with results of theoretical considerations have been reported for the oxygen reduction [iii] and hydrogen oxidation [iv]. Electrocatalytic electrodes are indispensable parts of fuel cells [v]. A great variety of electrocatalytic electrodes has been developed for analytical applications [vi]. See also electro catalysis, catalytic current, -> catalytic hydrogen evolution, catalymetry. 

Figure 30. Current-reversal chronopotentiometry working curve for the catalytic eC mechanism. The parameter tg on the figure corresponds to tp in the text. (From Ref. 234.)...

The term electrocatalysis will be used in the following for designing electrochemical processes involving the oxidation or reduction of a substrate species, S, whose reaction rate is varied in the presence of a given catalytic species. Cat. The effect of electrocatalysis is an increase of the standard rate constant of the electrode reaction resulting in a shift of the electrode reaction to a lower overpotential at a given current density and a current increase. The faradaic current resulting from the occurrence of a catalytic electrode mechanism is called catalytic current. For a positive electrocatalysis, the current obtained in the presence of the catalyst must exceed the sum of the currents obtained for the catalyst and the substrate, separately, under selected experimental conditions (Bard et al., 2008). Three possible situations can be discerned ... 

Figure 4-1. Protein film voltammetry as a technique for studying redox enzyme mechanisms. The catalytic current-potential profile provides information on the rate-defining catalytic processes occurring within the enzyme. It is important that interfacial electron transfer is facile and information is not masked by limitations due to tlie transport of substrate and product for this reason the rotating disc electrode is an important tool in these studies.

In this system the catalytic current is shown to vary with N2O as expected for the above mechanism. The concentration of N2O can also be measured in the presence of oxygen. This is achieved by exploiting the reduction of oxygen by the anthraguinone dianion ... 

The above mechanism can be deduced since a smaller catalytic current than would be expected for a simple CE process is observed. This occurs because when more of the reactive species, F, is added the equilibrium is shifted to the left. Thus less F is recycled. 

The equilibrium potential for a aingle cell, given by equation (11), for the cathodic and anodic reactions (5) and (8), is -406mV for a process gas containing 2000 ppm HgS and an anode product of pure sulfur vapor. To this must be added the overpotentials needed for both electrode reactions and ohmic loss. The electrode reactions have been studied in free electrolyte on graphite electrodes . Potential-step experiments showed very rapid kinetics, with exchange currents in both cathodic and anodic direction near 40 mA/cm . Cyclic voltammetry verified a catalytic reaction mechanism with disulfide as the electro-active species. At the cathode ... 

A number of other features follow from the relatively large size of the diffusion zone when compared to the electrode radius. One of these is quite significant to biosensors based on enzyme systems. The large diffusion zone radidly dilutes the products of the electrode reaction, thus catalytic mechanisms of the type shown in Table 8.2, mechanisms 3-5, are not perceived at microelectrodes, unless the rate constant for the reaction is fast. It can be shown that for a reversible electrode reaction, the ratio of the catalytic current (i kc) to the faradaic current obtained in the absence of the homogeneous reaction is given by... 

Here, we have demonstrated the differences between turnover and nontumover CVs for G. sulfurreducens biofilms. Turnover and nonturnover CVs can be used to correlate catalytic current under turnover conditions to redox peaks under nonturnover conditions. Scan rate analysis can provide a qualitative understanding of the biofilm electron-transfer mechanisms occurring within a biofilm. However, caution should be used when applying the Randles-Sevcik criterion to biofilms, as it was derived for a single-step, reversible electron-transfer step for diffusing mediators. 

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