Cyclic voltammetry redox catalysis

For the sake of comparison and mutual validation of methods for measuring large follow-up reaction rate constants, it is interesting to apply different methods to the same system. Such a comparison between high-scan-rate ultramicroelectrode cyclic voltammetry, redox catalysis, and laser flash photolysis has been carried out for the system depicted in Scheme 2.25, where methylacridan is oxidized in acetonitrile, generating a cation radical that is deprotonated by a base present in the reaction medium.20... 

As shown in Section 2.2.7, chemical reactions may be triggered by electrons or holes from an electrode as illustrated by SrnI substitutions (Section 2.5.6). Instead of involving the electrode directly, the reaction may be induced indirectly by means of redox catalysis, as illustrated in Scheme 2.15 for an SrnI reaction. An example is given in Figure 2.30, in which cyclic voltammetry allows one to follow the succession of events involved in this redox catalysis of an electrocatalytic process. In the absence of substrate (RX) and of nucleophile (Nu-), the redox catalysis, P, gives rise to a reversible response. A typical catalytic transformation of this wave is observed upon addition of RX, as discussed in Sections 2.2.6 and 2.3.1. The direct reduction wave of RX appears at more negative potentials, followed by the reversible wave of RH, which is the reduction product of RX (see Scheme 2.21). Upon addition of the nucleophile, the radical R is transformed into the anion radical of the substituted product, RNu -. RNu -... 

FIGURE 2.30. Redox catalysis induction of Srn1 reactions. Cyclic voltammetry in liquid ammonia + 0.1 M KC1 at —40°C of (a) redox catalyis of the reductive cleavage of 2-chlorobenzonitrile, RX, by 4-cyanopyridine, P. The dotted reversible cyclic voltammogram corresponds to P in the absence of RX. The solid line shows the catalytic increase of the current, (b) Transformation of the voltammogram upon addition of the nucleophile PhS. Adapted from Figure 1 in reference 23, with permisison from the American Chemical Society. 

FIGURE 4.3. Redox and chemical homogeneous catalysis of trans-1,2 dibromocyclohexane. a cyclic voltammetry in DMF of the direct electrochemical reduction at a glassy carbon electrode (top), of redox catalysis by fhiorenone (middle), of chemical catalysis by an iron(I) porphyrin, b catalysis rate constant as a function of the standard potential of the catalyst couple aromatic anion radicals, Fe(I), a Fe(0), Co(I), Ni(I) porphyrins. Adapted from Figures 3 and 4 of reference lb, with permission from the American Chemical Society. 

Figure 4-3. Electrochemical techniques and the redox-linked chemistries of an enzyme film on an electrode. Cyclic voltammetry provides an intuitive map of enzyme activities. A. The non-turnover signal at low scan rates (solid lines) provides thermodynamic information, while raising the scan rate leads to a peak separation (broken lines) the analysis of which gives the rate of interfacial electron exchange and additional information on how this is coupled to chemical reactions. B. Catalysis leads to a continual flow of electrons that amphfles the response and correlates activity with driving force under steady-state conditions here the catalytic current reports on the reduction of an enzyme substrate (sohd hne). Chronoamperometry ahows deconvolution of the potenhal and hme domains here an oxidoreductase is reversibly inactivated by apphcation of the most positive potential, an example is NiFe]-hydrogenase, and inhibition by agent X is shown to be essentially instantaneous.

A more complicated variation of the EC scheme, largely studied by voltammetry, is the situation where reaction (12.3.28) is reversible, but the product Y is unstable and undergoes a fast following reaction (Y X). This instability of Y tends to drive reaction (12.3.28) to the right, so the observed behavior resembles that of the Ei-Cj scheme. In this case, the 0/R couple mediates the reduction of species Z, with the ultimate production of species X, and the process is called redox catalysis. By selecting a mediator couple whose lies positive of that of the Z/Y couple and noting changes in the cyclic voltammetric response with v and the concentration of Z, one can find the rate constant for the decomposition of Y to X, even if it is too rapid to measure by direct electrochemistry of Z (i.e., as an EC reaction) (8, 9). 

Another application of electrochemistry to heterogeneous catalysis is cyclic voltammetry, which is an important electroanalytical technique. Cyclic voltammograms (CV) trace the transfer of electrons during an oxidation-reduction (redox) reaction (Figure 7.11). 

The temperature dependence of the catalyst activity of an iron fluoro-porphyrin-coated graphite electrode was studied by RDE coupled with the surface cyclic voltammetry. The purpose was to investigate the surface adsorption and reaction, O2 reduction catalysis kinetics, and especially the temperature effect on the catalyst activity. Figure 7.11(A) shows the surface CVs of 5,10,15,20-Tetrakis(pentafluorophenyl)-21H,23H-porphine iron (III) chloride (abbreviated as Fe TPFPP)-coated graphite electrode, recorded in a pH 1.0 Ar-saturated solution at different potential scan rates. The 1-electron reversible redox peak of approximately 0.35 V can be seen, which has a peak current increased linearly with increasing the potential scan rate, indicating the electrochemical behavior of this peak follows the feature of a reversible redox reaction of an adsorbed species on the electrode surface. 

When the redox activity of an adsorbed protein is coupled to catalytic redox transformation of a molecule in solution, termed the substrate, then catalytic voltammetry is observed. Electrons may be relayed between the active site and electrode by the action of one or more ancillary redox centers, as implied by the cartoon in Fig. 1. Alternatively, direct electron exchange between the electrode and the catalytic center may occur. In either case, when the electrode potential provides sufficient driving force for catalysis, there will be a sustained flow of current due to the net transfer of electrons between the electrode and molecules in solution. Fig. 4. This situation is in contrast to the discrete peaks that arise from cyclic voltammetry of non-tumover events. Figs. 2 and 3. A negative (positive) catalytic current describes catalytic reduction (oxidation). The catalytic current magnitude quantifies the rate of electron transfer through the adsorbed enzyme and so the catalytic rate. Voltammetry of a solution of the substrate should always be performed in the absence of enzyme to confirm that the catalytic currents arise from the activity of the enzyme rather than direct catalytic transformation of the substrate by the electrode surface. 

Catalytic processes have been studied to great advantage by cyclic voltammetry. These include catalysis of chemical reactions initiated by electron transfer at the electrode and catalysis of redox processes by solution chemical reactions. 

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