Current, catalytic

In many other cases (by a change in experimental conditions, faster chemical reaction) the value of the catalytic current may be governed by the SET rate (see reaction 20). The value of k1 may be found and its variation as a function of the nature of the mediator (with several values for °j) leads by extrapolation (when k2 can be assumed to be diffusion-controlled) to the thermodynamical potential °RS02Ar which is somewhat different from the reduction potentials of overall ECE processes observed in voltammetry. 

However, in all cases it appears important to check that the increase of current for the mediator is determined by the SET (catalytic current) and not by the protonation of the mediator anion radical e.g., in cases of acidic sulphones acting as proton donors. 

Fewer examples are reported for organic electrode reactions some alkyl halides were catalytically reduced at electrodes coated with tetrakis-p-aminophenylporphy-rin carboxylate ions are oxidized at a triarylamine polymer and Os(bipy)3 in a Nafion film catalytically oxidizes ascorbic acid Frequently, modified electrodes fail to give catalytic currents for catalyst substrate combinations that do work in the homogeneous case even when good permeability of the film is proven... 

One also needs to be careful when using the slope of the Koutecky-Levich plot to determine av of the catalytic film. Examples of metaUoporphyrin-catalyzed ORR have been reported where, above a certain value of the electrode rotational frequency, the catalytic currents became independent of Koutecky-Levich model, either because the rate of charge or substrate transfer within the film became rate-limiting or the catalyst became partially samrated with O2 [Boulatov et al., 2002 Song et al., 1998 CoUman et al., 1980]. In other cases, the versus graphs may remain mostly linear within the experimental... 

Coordination of NO to the divalent tetrasulfonated phthalocyanine complex [Co(TSPc)]4 results in a complex formally represented as [(NO )Coin(TSPc)]4 kf= 142M-1s-1, KA 3.0 x 105 M-1). When adsorbed to a glassy carbon electrode, [Co(TSPc)]4- catalyzes the oxidation and reduction of NO with catalytic currents detectable even at nanomolar concentrations. Electrochemistry of the same complex in surfactant films has also been studied.905 Bent nitrosyl complexes of the paramagnetic trivalent tropocoronand complex Co(NO)(TC) ((189), R = NO) have also been reported.849... 

Figure 3.55(b) shows the first sweep of the voltammogram obtained in the presence of C02 and also in Ar saturated solution for comparison. The authors concluded that the reductive scan showed that little or no catalytic enhancement occurs at the Bipy-based reduction potential and that the onset of the catalytic current occurs at -1.4V continuing through the region characteristic of the metal-based reduction process (and the 2e reduction of the dimer) in direct contrast to the results obtained by the majority of other workers (see below). 

In redox mediation, to have an effective electron exchange, the thermodynamic redox potentials of the enzyme and the mediator have to be accurately matched. For biocatalytic electrodes, efficient mediators must have redox potentials downhill from the redox potential of the enzyme a 50 mV difference is proposed to be optimal [1, 18]. The tuning of these potentials is a compromise between the need to have a high cell voltage and a high catalytic current. Furthermore, an obvious requirement is that the mediator must be stable in the reduced and oxidized states. Finally, for operation of a membraneless miniaturized biocatalytic fuel cell, the mediators for both the anode and the cathode must be immobilized to prevent power dissipation by solution redox reactions between them. 

FIGURE 17.4 Relationship of catalytic current, obtained with CYP2B4 biosensor after addition of ami-nopyrine, with increasing concentrations of aminopyrine. (From [222], with permission.)... 

Cyclic voltammetric responses corresponding to the simple catalytic scheme in Figure 4.1 and to more complex schemes were discussed in detail in Section 2.2.6. The parameters that control the catalytic current have been identified and their effects quantified. Applications of homogeneous redox catalysis to the characterization of short-lived intermediates and the determination of their redox properties have been discussed in Sections 2.3 and 2.6.4. 

It should be noted that in most practical circumstances, the Nernstian current can be neglected in front of the catalytic current. 

If substrate concentration in the bulk is large enough, it remains constant toward time and space ([S] c=0 = C ), and the catalytic current is controlled by the enzymatic reaction. Then... 

This is no longer true when substrate diffusion interferes in the kinetic control as discussed next. From equations (5.15) to (5.18), one obtains a relationship between the concentration of substrate at the electrode surface and the catalytic current ... 

The steady-state approximation applied to all enzyme forms leads to the following expression of the catalytic current ... 

All these experiments were carried out at such low scan rates that the outside diffusion layer of the cosubstrate (on the order of 105 A) is much larger than the film thickness. An experimental test for knowing whether this condition is fulfilled is that the plateau of S-shaped catalytic current then observed is much larger than the reversible cosubstrate peak observed in the absence of substrate i icat. Under these conditions, the concentration profiles within the film (bottom of Figure 5.30) do not depend on time. 

The dimensionless expression of the catalytic current may be recast from Section 6.5.1 as... 

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