Heterogeneous electron transfer reactions applications

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. 

Refs. [i] Samec Z (1979) J Electroanal Chem 99 197 [ii] Samec Z, Marecek V, Weber J (1979) J Electroanal Chem 96 245 [iii] Chen QZ, Iwamoto K, Seno M (1991) Electrochim Acta 36 291 [iv] Wei C, Bard Aj, Mirkin MV (1995) J Phys Chem 99 16033 [v] Shi C, Anson FC (1998) I Phys Chem B 102 9850 [vi] Geblewicz G, Schiffrin Df (1988) ] Electroanal Chem 244 27 [vii] Marcus RA (1990) J Phys Chem 94 1050 [viii] Barker AL, Unwin PR, Amemiya S, Zhou JF, Bard AJ (1999) JPhys Chem B 103 7260 [ix] Fermln DJ, Lahtinen R (2001) Dynamic aspects of heterogeneous electron-transfer reactions at liquid-liquid interfaces. In Volkov AG (ed) Liquid interfaces in chemical, biological, and pharmaceutical applications. Marcel Dekker, New York, pp 179-227 [x] Cheng Y, Schiffrin DJ (1996) J Chem Soc Faraday Trans 92 3865 [xi] Fermin DJ, Doung H, Ding Z, Brevet PF, Girault HH (1999) Electrochem Com-mun 1 29... 

The above analysis also shows that for almost all applications of fast CV employing V > 1 kV s , the quasi-reversible or irreversible nature of heterogeneous electron transfer reactions must be considered. In particular, this becomes important when fast CV is used in a kinetic analysis of fast homogeneous follow-up reactions. The extraction of the relevant rate constants is complicated by the mixed kinetic control of the electrode process and the chemical reaction. As a result, the number of parameters involved in the fitting procedures is increased considerably and with it the possibility of introducing errors. 

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... 

Application of electrochemical methods can be suitable for certain oxidation-reduction reactions. Both homogeneous and heterogeneous electron transfer reactions have been investigated electrochemically. The technique and apparatus can range from reasonably standard cyclic voltammetry and rapid scan voltammetry to quite specialized arrangements.In the latter case, the method often needs to be adapted to the context of a particular reaction. An example of high-pressure electrochemistry apparatus is shown in Figure 6. 

Fermin, D.J. and R. Lahtinen (2001). Dynamic aspects of heterogeneous electron transfer reactions at liquid/Uquid interfaces. In A. Volkov, (ed). Liquid Interfaces in Chemical, Biological, and Pharmaceutical Applications. Marcel Dekker, Boca Raton, pp. 179-228. 

Beyond the ability to probe heterogeneous electron transfer reactions that occur on the micro, or even nano timescale, high-speed cyclic voltammetry has found application in the investigation of complex reaction mechanisms. An example of this is the following chemical (EC) reaction. The oxidation or reduction (E reaction) of an electroactive species can produce a chemically unstable product that undergoes a chemical reaction (C reaction) to form an electroinactive product. Electron transfer reactions can also lead to isomerizations and ligand loss in organometallic compounds, such as... 

Varco Shea T, Bard AJ (1987) Digital simulation of homogeneous chemical reactions coupled to heterogeneous electron transfer and applications at platinum/mica/platinum ultramicroband electrodes. Anal Chem 59 2101-2111... 

Fermin, D. J. and R. Lahtinen,Dynamic aspects of heterogeneous electron-transfer reactions at liquid-liquid interfaces, in Liquid interfaces in chemical, biological, and pharmaceutical applications f Volkov, A. G. Eds, Marcel Dekker, New York, 2001, p. 179. 

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. ... 

One of the most intriguing aspects of electrochemistry involves the homogeneous chemical reactions that often accompany heterogeneous electron-transfer processes occurring at the electrode-solution interface. The addition or removal of an electron from a molecule generates a new redox state, which can be chemically reactive. A variety of mechanisms, some of which involve complicated sequences of electrode and chemical reactions, have been characterized. Several of the more common mechanisms with examples of applicable chemical systems are described next. More examples are given in Chaps. 21 and 23. 

The application of electrochemical methods for the study of the kinetics and mechanisms of reactions of electro chemically generated intermediates is intimately related to the thermodynamics and kinetics of the heterogeneous electron transfer process and to the mode of transport of material to and from the working electrode. For that reason, we review below some basics, including the relationship between potential and current (Section 6.5.1), the electrochemical double layer and the double layer charging current (Section 6.5.2), and the... 

The application of CA for monitoring the progress of a chemical reaction following the heterogeneous electron transfer is limited by the nature of the process. It is clear from the Cottrell equation, Equation 6.33, that the only parameter that may be affected by a follow-up reaction is the number of electrons, n. Thus, important mechanisms such as eC and eC(dim) cannot be distinguished by CA. This limits the application of CA for kinetic studies. In contrast, DPSCA is a most useful method. 

The Marcus theory is the most widely applied theory used to describe electron transfer reactions and is equally applicable to photoinduced, interfacial and thermally driven electron transfers. The fundamental difference between these processes lies primarily in the nature of the driving force. In the case of heterogeneous electron... 

The Marcus theory, outlined above in Section 2.2.1 for homogeneous reactions, directly addresses these issues and is widely accepted as the most powerful and complete description of both heterogeneous and homogeneous electron transfer reactions. Its application to heterogeneous processes will be described in the following section. 

In the second chapter, Appleby presents a detailed discussion and review in modem terms of a central aspect of electrochemistry Electron Transfer Reactions With and Without Ion Transfer. Electron transfer is the most fundamental aspect of most processes at electrode interfaces and is also involved intimately with the homogeneous chemistry of redox reactions in solutions. The subject has experienced controversial discussions of the role of solvational interactions in the processes of electron transfer at electrodes and in solution, especially in relation to the role of Inner-sphere versus Outer-sphere activation effects in the act of electron transfer. The author distils out the essential features of electron transfer processes in a tour de force treatment of all aspects of this important field in terms of models of the solvent (continuum and molecular), and of the activation process in the kinetics of electron transfer reactions, especially with respect to the applicability of the Franck-Condon principle to the time-scales of electron transfer and solvational excitation. Sections specially devoted to hydration of the proton and its heterogeneous transfer, coupled with... 

Due to potential applications in biosensing and to develop a better understanding of heterogeneous biological electron transfer reactions, direct electrochemistry between adsorbed proteins and solid electrodes has been studied extensively [32,... 

Monolayer and multilayer thin films are technologically important materials that potentially provide well-defined molecular architectures for the detailed study of interfacial electron transfer. Perhaps the most important attribute of these heterogeneous systems is the ease with which their molecular architecture can be synthetically varied to tailor the properties of the ensemble. Assemblies incorporating specifically designed structures can, in principle, meet the needs of a variety of technological applications and be used as models for understanding fundamental interfacial reaction mechanisms. In fact, molecular assemblies are nearly ideal laboratories for the fundamental study of electron-transfer reactions at interfaces. In this chapter, the use of monolayer and multilayer assemblies to probe fundamental questions regarding electron transfer in surface-confined molecular assemblies will be addressed. 

Common to all the methods discussed earlier is that B is generated at the electrode surface, that is, by a direct electron exchange between the electrode and the substrate A. This approach is, however, sometimes hampered by the limitations imposed by the heterogeneous nature of the electron transfer reaction. For instance, studies of the kinetics of fast follow-up reactions may be difficult or even impossible owing to interference from the rate of the heterogeneous electron transfer process. In such cases, the kinetics of the follow-up reactions may be studied instead by an indirect method, generally known as redox catalysis [5,124-126]. Another application of redox... 

The competition between heterogeneous electron transfer (eCe) and electron transfer in solution (eCeh) in the second e step (Scheme 2) and the possibility of distinguishing between these two pathways have been analyzed in detail [207,208]. It was concluded that the eCeh pathway dominates over the eCe pathway unless the measurement time is kept below approximately 10 s. The application of chronoamperometry to determine the rate constants in more complicated reaction schemes, such as, for example, the eCeCe-type mechanism, has been addressed as well [209]. 

A major field of application of the RDE in organic electrochemistry is elucidation of the mechanistic details for reactions following the heterogeneous electron transfer. One way of using the RDE in kinetic work is based on the relationship between i m and (o. In the absence of kinetic complications, a plot of iiim versus should be a straight line [Eq. (88)]. However, if the electron transfer reaction is followed by a chemical step, the linear dependence of iiim on may change to a curved relationship. For the eCe mechanism (Scheme 2), the relationship between the apparent number of electrons, napp, iiim. and o) is given by Eq. (90) [254], where n and ium are the number of electrons and the current observed for the first electron transfer step only, i.e., in the absence of the chemical reaction. Other mechanisms have been treated as well [248,255,256]. 

When the heterogeneous electron transfer is followed by a chemical step, the registration of spectral data becomes more complicated unless the rate of the chemical reaction is very low. Rapidly reacting species—for instance, radical ions—can be characterized spectroscopically by the application of rapid scan spectroscopy (RSS), modulated specular reflectance spectroscopy (MSRS) and, more recently, diode array spectrometers. 

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