Chemical reactions coupled to electron transfer

The Effect of Chemical Reactions Coupled to Electron Transfers... 

Commonly, in the description of chemical reactions coupled to electron transfer, the homogeneous chemical reaction is indicated by C and the heterogeneous electron transfer by E. The order of C with respect to E then follows the chronological order in which the two events occur. Furthermore, while Ox and Red indicate the electro active species, other non-electro active species which result from the coupled chemical complications are indicated by Y, Z, W, etc. 

Bott, A.W., Practical problems in voltammetry. 3 Reference electrodes for voltammetry . Current Separations, 14, 64-68 (1995) is an excellent first stop for the novice, as is Bott, A. W Characterization of chemical reactions coupled to electron-transfer reactions using cyclic voltammetry . Current Separations, 18, 9-16 (1999), which also introduces simulations. In addition, the article by Hitchman and Hill in Chemistry in Britain (see above) contains a low-level general introduction to cyclic voltammetry for analyses. 

Modern electrochemical methods provide the coordination chemist with a powerful means of studying chemical reactions coupled to electron transfer and exploiting such chemistry in electrosynthesis. In addition, the electrochemical generation of reactive metallo intermediates can provide routes for the activation of otherwise inert molecules, as in the reduction of N2 to ammonia,50 and for electrocatalyzing redox reactions, such as the reduction of C02 to formate and oxalate,51 the oxidation of NH3 to N02-,52 and the technologically important oxidation of water to 02 or its converse, the reduction of 02 to water.53 Electrochemical reactions involving coordination compounds and organometallic species have been extensively reviewed.54-60... 

Other chemical reactions coupled to electron transfer are summarized for several of the most important types. 

Firstly, we should define the types of complexity which need to be considered when dealing with homogeneous chemical reactions coupled to electron transfer. The most common one is that the conversion of primary intermediates into final product is, in fact, a sequence of several, maybe four or five, elementary steps. In addition to defining the reaction pathway, it is necessary to decide which step is the rate determining one and also to consider the possibility that two steps have approximately the same rate, or that the r.d.s. changes, say with concentration of electroactive species. It is, however, also common in organic electrochemistry to find that the electrode reaction leads to a mixture of products and this is a clear indication of a branch mechanism where two competing reactions have comparable rates branch mechanisms can even lead to the same product. A further uncertainty arises as to the source of electrons does the second... 

Quantitative studies using LSV and CV can be carried out for both heterogeneous charge transfer kinetics and the kinetics of homogeneous chemical reactions coupled to charge transfer at electrodes. These methods should continue to play a major role in the study of electron transfer reactions. 

A systematic description of all possible combinations of homogeneous chemical processes coupled to electron transfer at an electrode surface is impossible because an infinite range of theoretically possible reaction schemes can be constructed. Unfortunately, a consistent form of nomenclature for defining the possible web of reaction pathways has not yet been invented. However, the lUPAC nomenclature [89] is of assistance with respect to simple reaction schemes. In this article, the commonly employed descriptors for electron transfer (E) and chemical (C) sequences of reaction steps, e.g. ECEC, will be used for a sequence of reactions involving electron transfer-chemical process-electron transfer-chemical process. Reaction schemes involving branching of a reaction pathway will be considered later. 

In this section, the effect of chemical reactions coupled with electron transfer processes studied by three pulse methods is discussed, namely in normal pulse (NP), differential pulse (DP), and square-wave polaro-graphic/voltammetric techniques. These methods, especially DPV, belong to the most frequently employed voltammetric methods in contemporary analytical practice. In recent years, criteria for elucidation of electrode mechanisms have been also developed for these techniques. Under favorable conditions (in pure kinetic zone), the electrode mechanisms for simple reaction systems can be established without difficulties. 

Discrimination of various mechanisms This method (DPP) seems to be an efficient tool in elucidating various chemical reactions coupled with electron transfer. This view may be supported not only by earlier papers [119-122], but also by recent works published in the 1990s, e.g. [123]. Two time scales (time windows) are inherent in DPP the mercury drop time (usually 0.2 <1 5 s) and the pulse width tp<50ms situated near the end of the drop time. 

At higher current densities, the primary electron transfer rate is usually no longer limiting instead, limitations arise tluough the slow transport of reactants from the solution to the electrode surface or, conversely, the slow transport of the product away from the electrode (diffusion overpotential) or tluough the inability of chemical reactions coupled to the electron transfer step to keep pace (reaction overpotential). 

Other reaction mechanisms can be elucidated in a similar fashion. For example, for a CE mechanism, where a slow chemical reaction precedes the electron transfer, the ratio of is generally larger than unity, and approaches unity as the scan rate decreases. The reverse peak is usually not affected by the coupled reaction, while the foiward peak is no longer proportional to the square root of the scan rate. 

Voltcoulommetry (DSCVC) and Square Wave Voltcoulommetry (SWVC) are also considered, since they are very valuable tools for the analysis of fast electrochemical reactions between surface-confined molecules. First, a simple mono-electronic electrochemical reaction is analyzed and, after that, the cases corresponding to multi-electron electrochemical reactions and chemical reactions coupled to the surface charge transfer, including electrocatalytic processes, are discussed. 

In this section we examine the relationship between current and potential in the case where the primary product of the electrode reaction is stable in solution, that is, there are no chemical reactions coupled to the electron transfer reaction. 

Voltammetric methods provide ways to study mechanistically complex electrode reactions in which chemical reactions accompany the electron transfer. Chemical reactions can be coupled to electron transfer, either preceding or following it. A nomenclature that aids in cataloging coupled chemical reactions denotes E as an electron-transfer step and C a chemical reaction. Thus, EC refers to a chemical reaction following electron transfer. Even a simple mechanism, such as CE, can be complex owing to such variables as the reversibility of E, the rate and equilibrium constants of C and the time scale of the electrochemical experiment. Our discussion is restricted to the limiting kinetic cases for each mechanism. 

We have seen in this section how microelectrodes can be used in a variety of ways to study both coupled homogeneous chemical reactions and heterogeneous electron transfers. We have also seen one example of a study of metal deposition. The development of microelectrodes has made possible... 

In this section we are going to consider the implementation of first-order heterogeneous chemical reactions coupled to the electron transfer. In these processes the electroactive species are transformed in a surface-catalysed chemical process that can be characterised through a first-order rate constant fchet (m s ) that is independent of the applied potential. Although a more detailed description of these systems may need to consider the possible adsorption/desorption of the species involved in the heterogeneous chemical transformation, the formalism presented in this section enables a first, simpler description with only one additional unknown variable fchet [13]. 

Already in the mid-1960s, there was rich potential of applying such experiments to the determination of concentrations but even more to the elucidation of reaction mechanisms and kinetics coupled to electron transfer at an electrode was recognized. Today the resulting Knear sweep or cyclic voltammetries are employed as simple, flexible routine techniques in particular as sophisticated means to solve chemical and mechanistic problems. The combination with computer control, ultramicroelectrodes, and digital simulation has further contributed to their success. 

The use of well-known model systems has been undertaken to confirm the validity and define the limits of the treatment of sonovoltammetry in line with the model of a uniformly accessible electrode and also to assess the influence of ultrasound on homogeneous chemical reactions coupled to the electron transfer. The uniformly accessible electrode model allows the introduction of a reaction layer, which has also been successfully employed for rotating disc voltammetry, in studies using channel electrodes [64] and in a slightly more complex form for studies in turbulent voltammetry [65]. 

A second voltammetric approach to probing the dynamics of homogeneous chemical reactions that are coupled to electron transfer is to exploit the... 

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