Heterogeneous electron transfer reactions studies

Cyclic voltammetry is the most widely used technique for acquiring qualitative information about electrochemical reactions. The power of cyclic voltammetry results from its ability to rapidly provide considerable information on the thermodynamics of redox processes, on the kinetics of heterogeneous electron-transfer reactions, and on coupled chemical reactions or adsorption processes. Cyclic voltammetry is often the first experiment performed in an electroanalytical study. In particular, it offers a rapid location of redox potentials of the electroactive species, and convenient evaluation of the effect of media upon the redox process. 

By the use of various transient methods, electrochemistry has found extensive new applications for the study of chemical reactions and adsorption phenomena. Thus a combination of thermodynamic and kinetic measurements can be utilized to characterize the chemistry of heterogeneous electron-transfer reactions. Furthermore, heterogeneous adsorption processes (liquid-solid) have been the subject of intense investigations. The mechanisms of metal ion com-plexation reactions also have been ascertained through the use of various electrochemical impulse techniques. 

Activation volume — As in case of homogeneous chemical reactions, also the rate of heterogeneous electron transfer reactions at electrode interfaces can depend on pressure. The activation volume AVZ involved in electrochemical reactions can be determined by studying the pressure dependence of the heterogeneous -> standard rate constant ks AVa = -RT j (p is the molar - gas constant, T absolute temperature, and P the pressure inside the electrochemical cell). If AI4 is smaller than zero, i.e., when the volume of the activated complex is smaller than the volume of the reactant molecule, an increase of pressure will enhance the reaction rate and the opposite holds true when A14 is larger than zero. Refs. [i] Swaddle TW, Tregloan PA (1999) Coord Chem Rev 187 255 [ii] Dolidze TD, Khoshtariya DE, Waldeck DH, Macyk J, van Eldik R (2003) JPhys Chem B 107 7172... 

Studies on the electrochemical behavior of ferrocene encapsulated in the hemi-carcerands 61 and 62, indicated that encapsulation induces substantial changes in the oxidation behavior of the ferrocene subunit [98]. In particular, encapsulated ferrocene exhibits a positive shift of the oxidation potential of c. 120 mV, probably because of the poor solvation of ferrocenium inside the apolar guest cavity. Lower apparent standard rate constants were found for the heterogeneous electron transfer reactions, compared to those found in the uncomplexed ferrocene under identical experimental conditions. This effect may be due to two main contributions (i) the increased effective molecular mass of the electroactive species and (ii) the increased distance of maximum approach of the redox active center to the electrode surface. 

SECM has been used to probe heterogeneous electron transfer reaction kinetics on semiconductor electrodes, such as WSe2 (29). In these studies, as... 

This chapter reviews in detail the principles and applications of heterogeneous electron transfer reaction analysis at tip and sample electrodes. The first section summarizes the basic principles and concepts. It is followed by sections dedicated to one class of sample material glassy carbon, metals and semiconductors, thin layers, ion-conducting polymers, and electrically conducting polymers. A separate section is devoted to practical applications, in essence the study of heterogeneous catalysis and in situ characterization of sensors. The final section deals with the experiments defining the state of the art in this field and the outlook for some future activities. Aspects of heterogeneous electron transfer reactions in more complex systems, such as... 

In the study of chemical kinetics, one can often simplify the prediction and analysis of behavior by recognizing that a single step of a mechanism is much more sluggish than all the others, so that it controls the rate of the overall reaction. If the mechanism is an electrode process, this rate-determining step (RDS) can be a heterogeneous electron-transfer reaction. 

The charge balance in solution is maintained by the release of a cation (M ). Mineral systems such as biotite and vermiculite have been observed in laboratory studies to mediate the reduction of hexachloroethane and carbon tetrachloride (Kriegman-King, 1991, 1992). The heterogeneous electron transfer reaction between hexachloroethane (HCA) and the sheet silicates, resulting in the formation of tetrachloroethylene (PCE), is proposed to occur by the following mechanism ... 

Nevertheless, voltammetric techniques have long been applied to the study of direct electron transfer processes of biological molecules. Such early voltammetric studies revealed a high degree of electrochemical irreversibility in the direct heterogeneous electron transfer reactions between electrodes and biological molecules. The irreversible nature of direct electron transfer observed in these early studies precluded the accurate and precise characterization of the electron transfer stoichiometry and thermodynamics of biological molecules. The models alluded to above were often used to account for the irreversible electron transfer kinetics observed in these studies. 

The material of this section will focus on results of homogeneous electron transfer kinetic studies involving reactions between electrochemically generated redox reactants and on direct heterogeneous electron transfer kinetic studies. An overview of the work in this area concludes this chapter. 

Duong, H.D., P.R Brevet, and H.H. Girault (1998). Heterogeneous electron transfer reactions at liquid/liquid interfaces studied by time resolved absorption spectroscopy. J. Photochem. Photobiol. A 117, 27-33. 

In 2002, Uhneanu et al. used a thin organic layer that is supported by a porous hydrophobic membrane such as porous Teflon or poly vinylidenedifluoride (PVDF), or sandwiched between two aqueous dialysis membranes [295]. With this setup, they showed that the transfer of highly hydrophilic ions at one interface can be studied by limiting the mass transfer of the other ion-transfer reaction at the other interface. They have also shown that cyclic voltammetry for coupled ion-transfer reactions at the two interfaces in series is analogous to cyclic voltammetry for electron-transfer reactions studied by Stewart et al. [207], as the diffusion equations of the reactants and products are analogous, and as the overall Nernst equation for the coupled ion transfer equal to the two individual Nemst equations for ion distribution is also analogous to the Nemst equation for the heterogeneous ET. 

Activation volume — As in case of homogeneous chemical reactions, also the rate of heterogeneous electron transfer reactions at electrode interfaces can depend on pressure. The activation volume AV involved in electrochemical reactions can be determined by studying the pressure dependence of the heterogeneous standard rate constant ks AV = R is the molar... 

Heterogeneous electron transfer reactions between electrodes and c-type cytochromes in the bulk of solution have been extensively studied. Cytochrome c exhibits voltammetric responses ranging from reversible to kinetically irreversible at various electrodes , 71 0 electrode reactions of cytochrome cs have been found to exhibit reversible heterogeneous electron transfer at various electrodes without mediators . It has been shown that both cytochrome c and cytochrome C3 adsorb strongly on various electrode surfaces from aqueous solutions. The voltammetric behavior of these cytochromes in solution is strongly influenced by the nature of the adsorbed films on the electrode . 

Direct electron transfer between redox proteins and metal electrodes has many advantages with respect to analytical applications. Hill and coworkers have pointed out the similarities between heterogeneous electron transfer reactions of proteins at electrodes and catalysis. The sequence of events at the electrode include 1) diffusion of reactant protein to the electrode surface 2) adsorption of the protein in an orientation suitable for electron transfer 3) electron transfer 4) dissociation of the protein from the electrode surface and 5) diffusion of the protein away from the surface. If all these requirements are not met, well behaved redox activity will not be observed. There have been various approaches to accomplishing reversible redox reactions in proteins based upon these requirements. Hill and coworkers have focused on the second step and have shown by their elegant promoter studies that the correct orientation of the protein at the electrode is crucial for rapid electron transfer. Others have utilized mediator-type electrodes or chemically modified proteins. ... 

The electron formed as a product of equation (2.5) will usually be received (or collected ) by an electrode. It is quite common to see the electrode described as a sink of electrons. We need to note, though, that there are two classes of electron-transfer reaction we could have considered. We say that a reaction is heterogeneous when the electroactive material is in solution and is electro-modified at an electrode which exists as a separate phase (it is usually a solid). Conversely, if the electron-transfer reaction occurs between two species, both of which are in solution, as occurs during a potentiometric titration (see Chapter 4), then we say that the electron-transfer reaction is homogeneous. It is not possible to measure the current during a homogeneous reaction since no electrode is involved. The vast majority of examples studied here will, by necessity, involve a heterogeneous electron transfer, usually at a solid electrode. 

The validity of an electroanalytical measurement is enhanced if it can be simulated mathematically within a reasonable model , that is, one comprising all of the necessary elements, both kinetic and thermodynamic, needed to describe the system studied. Within the chosen model, the simulation is performed by first deciding which of the possible parameters are indeed variables. Then, a series of mathematical equations are formulated in terms of time, current and potential, thereby allowing the other implicit variables (rate constants of heterogeneous electron-transfer or homogeneous reactions in solution) to be obtained. 

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