Heterogeneous electron transfer potential-dependent

The experimentally obtained curves do not always look like the ideal curves this can be caused by several factors. One can be that the rate constant of the heterogeneous electron transfer, which depends on the potential, is not high compared to the time scale of the experiment. This means that it is necessary to apply a more negative potential than in the ideal case to... 

In a direct electrolysis, the electron is exchanged between the electrode and the substrate, and the rate of the reaction depends on the electrode potential and the rate constant of the heterogeneous electron-transfer reaction. In an indirect electrolysis, the electron is primarily exchanged with a substance (a mediator) that exchanges the electron with the substrate in a chemical reaction, and the rate does not depend on the ability of the substrate to exchange an electron with the electrode. 

SEV is an effective means of probing homogeneous chemical reactions that are coupled to electrode reactions, especially when it is extended to cyclic voltammetry as described in the next section. Considerable information can be obtained from the dependence of ip and Ep on the rate of potential scan. Figure 3.20 illustrates the behavior of ip and Ep with variation in scan rate for a reversible heterogeneous electron transfer reaction that is coupled to various types of homogeneous chemical reactions. The current function j/p is proportional to ip according to the equation... 

There has been keen interest in determination of activation parameters for electrode reactions. The enthalpy of activation for a heterogeneous electron transfer reaction, AH X, is the quantity usually sought [3,4]. It is determined by measuring the temperature dependence of the rate constant for electron transfer at the formal potential, that is, the standard heterogeneous electron transfer rate constant, ks. The activation enthalpy is then computed by Equation 16.7 ... 

The sensitivity of the heterogeneous electron transfer rate constant to the overpotential depends on the extent of electronic coupling between the reactant and the electrode [19]. For strongly coupled reactants, electron transfer occurs predominantly through states near the Fermi level of the electrode and the adiabatic potential-dependent rate constant is given by the product of the frequency factor, vn, and the density of acceptor states in the molecule, Dox, according to the following ... 

If the potential applied in CA is not polarized sufficiently to fulfil the condition Co(0, r) = 0, the current also depends on the heterogeneous electron-transfer kinetics. The Cotrell expression given in Eq. 39 should for a reversible process then be modified according to Eq. 51, where z cot denotes the Cottrel current. 

If, during the chemical step, a product is formed that undergoes further electron transfer at the applied potential, as for example in the eCe and eCeh-type mechanisms (Scheme 2), or if the electroactive substrate is regenerated in a catalytic process (Scheme 7), the value of n pp/n depends on k even under conditions in which the heterogeneous electron transfer is diffusion controlled [68,133]. It is easily understood that iiapp/n approaches unity when k decreases toward zero, and a higher value, depending on the mechanism, when k increases. 

In general, electrochemical systems are heterogeneous and involve at least one (or both) of the fundamental processes - mass transport and an electron-transfer reaction. Moreover, electrochemical reactions involve charged species, so the rate of the electron-transfer reaction depends on the electric potential difference between the phases (e.g. between the electrode surface and the solution). The mass transport processes mainly include diffusion, conduction, and convection, and should be taken into account if the electron-transfer reaction properties are to be extracted from the experimental measurements. The proper control of the mass transport processes seems to be one of the main problems of high-temperature electrochemical studies. 

Temperature-dependent measurements of potential and electron-transfer rate constant are reported for the electrochemical reduction of a series of [MoO(Tp )(X,Y)], where X,Y is a series of bidentate 1,2-disubstituted aliphatic or aromatic ligands in which oxygen donors are replaced sequentially by sulfur.97 Solution redox potentials and heterogeneous electron transfer rate constants for [(Tp )Mo(E)(SS)] [E = O, NO SS = dithiolate] are also reported.98... 

Mo(Tp )(E)(bdt)j, [Mo(Tp )(E)(tdt)], [Mo(Tp )(E)(bdtCl2)]82 83 (E = O, NO), and [MoO(qdt)(Tp )]84 have been investigated as models for various pyranopterin Mo enzyme active sites, including sulfite oxidase. Solution redox potentials and heterogeneous electron transfer rate constants for these species have been also reported.85 The interactions between the sulfur tt-orbitals of arene dithiolates and high-valent Mo in [MoO(Tp )(bdt)] have been investigated by gas-phase photoelectron spectroscopy and DFT methods in order to understand the properties of the active site of pyranopterin Mo-W enzymes.86 Temperature-dependent measurements of potential and electron-transfer rate constants are also reported for electrochemical reduction of a series of [MoO(Tp )(X,Y)] complexes.87 The molecular and electronic structures of the SO active site [MoO(Tp )(bdt)] have been also reported.88... 

In heterogeneous electron transfer experiments, the gold electrode with a monolayer film is placed in contact with the solution containing a redox couple [such as Fe(CN)63-/Fe(CN)64- etc.]60-63. The shape of the cyclic voltammogram depends on how effectively the monolayer blocks access for the redox probe to the electrode surface. This method is therefore invaluable for permeability studies. Absence of redox waves close to the formal potential of the probe indicates that the monolayer is completely impermeable for redox probe species. On the other hand, the presence of a redox wave shows either that the monolayer is loosely packed and easily penetrated by external molecules, or the presence of numerous defects and pin-holes in the monolayer. 

This research evaluates the measurement of the "master" Eh of solutions in terms of heterogeneous electron-transfer kinetics between aqueous species and the surface of a polished platinum electrode. A preliminary model is proposed in which the electrode/solution interface is assumed to behave as a fixed-value capacitor, and the rate of equilibration depends on the net current at the interface. Heterogeneous kinetics at bright platinum in 0.1 m KCl were measured for the redox couples Fe(III)/Fe(II), Fe(CN)53-/Fe(CN)6, Se(VI)/Se(IV), and As(V)/As(Iin. Of the couples considered, only Fe(III)/Fe(II) at pH 3 and Fe(CN)g37Fe(CN)g at pH 6.0 were capable of imposing a Nemstian potential on the platinum electrode. 

With respect to the heterogeneous electron-transfer process, reversible (nemstian) systems are always at equilibrium. The kinetics are so facile that the interface is governed solely by thermodynamic aspects. Not surprisingly, then, the shapes and positions of reversible waves, which reflect the energy dependence of the electrode reaction, can be exploited to provide thermodynamic properties, such as standard potentials, free energies of reaction, and various equilibrium constants, just as potentiometric measurements can be. On the other hand, reversible systems can offer no kinetic information, because the kinetics are, in effect, transparent. 

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