Multiple-Electron Transfer Processes

It is frequent that electroactive molecules exchange more than one electron in successive transfer steps, giving rise to multi-E mechanisms. This situation is commonly found in the electrochemistry of organic and organometal-lic compounds [3] and biological molecules [11], In order to show how to tackle this problem, we consider a three-electron process involving four electroactive species, although the same treatment is applicable to any number of consecutive electrochemical processes  

The surface conditions associated with the kinetics of the different electron transfers can be expressed as 

Traditionally, for the sake of accuracy, the calculation of the current is carried out from the surface fluxes of the electroactive species such that the product of large and small numbers is avoided and higher-order approximations for the first derivative at the electrode surface can be employed 

In general, for a series of nr consecutive electron transfers in the form given by (5.51), the current response can be calculated from 

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The standard electrode potentials are far more anodic than that of one-electron transfer process, -0.284 V (SHE) and the visible-light photocatalytic activity of platinum-loaded tungsten(VI) oxide could be interpreted by enhanced multiple-electron transfer process by deposited platimun (45), since it is well known that platinum and the other noble metals catalyze such multiple-electron transfer processes. Similar phenomena, cocatalyst promoted visible-light photocatalytic activity, have been reported with palladium 46) and copper oxide (47). Thus, change of reaction process seems beneficial to realize visible-light photocatalytic activity. 

On a glassy carbon electrode, the Tafel slope was observed to be 60 mV/dec in alkaline solutions, and at pH < 10, the Tafel slope was 120 mV/dec. These values are in accordance with the proposed mechanisms. In the case of 120 mV/dec, the first electron transfer is the rate determining step. In the case of 60 mV/dec, the current-potential relationship observed from the multiple-electron transfer process of ORR on carbon electrodes was expressed as Equation 2.27, given by Taylor and Humffray [14, 17] ... 

Multiple electron transfer processes with an intervening reversible chemical reaction E CrEr. ... 

The nature of electrode processes can, of course, be more complex and also involve phase fonnation, homogeneous chemical reactions, adsorption or multiple electron transfer [1, 2, 3 and 4],... 

Peroxyoxalate chemiluminescence is the most efficient nonenzymatic chemiluminescent reaction known. Quantum efficiencies as high as 22—27% have been reported for oxalate esters prepared from 2,4,6-trichlorophenol, 2,4-dinitrophenol, and 3-trif1uoromethy1-4-nitropheno1 (6,76,77) with the duorescers mbrene [517-51-1] (78,79) or 5,12-bis(phenylethynyl)naphthacene [18826-29-4] (79). For most reactions, however, a quantum efficiency of 4% or less is more common with many in the range of lO " to 10 ein/mol (80). The inefficiency in the chemiexcitation process undoubtedly arises from the transfer of energy of the activated peroxyoxalate to the duorescer. The inefficiency in the CIEEL sequence derives from multiple side reactions available to the reactive intermediates in competition with the excited state producing back-electron transfer process. 

Reinmuth notation. In the electrochemical world, the sequence of electrode and/or chemical reactions that occur are described by a simple shorthand code. Simple electron-transfer reactions are called E reactions. In the same shorthand system, a multiple electron-transfer reaction such as Fe " Fe " -> Fe is an EE reaction , i.e. the product of an electron-transfer process itself undergoes a second electron-transfer process. (Note that the two electron-transfer processes might occur at the same time, in which case it is merely an E reaction.) The vanadium pentoxide system illustrated in Figure 6.14 is another example of an EE system. 

The subject matter covered below is divided into sections according to the structure of the redox unit(s). This review is restricted primarily to materials for which well-defined redox behavior has been repiorted, usually involving cyclic voltammetric studies and other electrochemical techniques in solution. Unraveling the electron transfer processes in laiger macromolecules which contain multiple redox sites can be very challenging, thus for some systems model branched oligomers have been studied in detail, and this work will be discussed. Selected synthetic schemes are included to acquaint the reader with the building blocks which are available for the construction of new derivatives, and with the synthetic steps involved. 

An issue that is being considered with attention concerns the possibility to achieve multiple electron transfers on the first wave of POMs. The aim is twofold (1) save energy by favoring those electro-catalytic processes that necessitate several electron to be performed (2) avoid the... 

Last but not least, it should be noted that the description of ECL processes as a simple superposition of the two or three electron transfer channels is somewhat oversimplified from the mechanistic point of view. In real cases, the electron transfer processes are preceded and followed by the diffusion of reactants from and electron transfer products into the bulk solution, respectively. Moreover, ECL reactants and products are species with distinctly different spin multiplicities, which causes an additional kinetic complication because of spin conservation rules. Correspondingly, the spin up-conversion processes (e.g., between two forms of an activated complex 1 [A- D + ] 3 [A- D + ]) cannot be a priori excluded from the kinetic con-... 

Olson, J., Palmer, G. Concepts and approaches to the understanding of electron transfer processes in enzymes containing multiple redox centers. In Molybdenum and molybdenum-containing enzymes (Coughlan, M. P. ed.) pp. 543-568, Oxford, Pergamon Press 1980 Bray, R. C., Palmer, G., Beinert, H. J, Biol. Chem. 239, 2662 (1964)... 

There are several reasons for the appeal of polymer modification immobilization is technically easier than working with monolayers the films are generally more stable and because of the multiple layers redox sites, the electrochemical responses are larger. Questions remain, however, as to how the electrochemical reaction of multimolecular layers of electroactive sites in a polymer matrix occur, e.g., mass transport and electron transfer processes by which the multilayers exchange electrons with the electrode and with reactive molecules in the contacting solution [9]. 

The construction of an artificial protein-protein complex is an attractive subject to elucidate the electron-transfer process in biological systems. To convert Mb into an electron-transfer protein such as cytochromes, Hayashi and Ogoshi (101) prepared a new zinc Mb having a unique interface on the protein surface by the reconstitutional method as shown in Fig. 27. The modified zinc protoporphyrin has multiple functional groups, carboxylates, or ammonium groups, at the terminal of the two propionates. Thus, the incorporation of the... 

Electrode processes involving multiple electron transfer... 

Electrode processes involving multiple electron transfer 119 are two extreme cases ... 

The difference in - peak potentials observed in -> cyclic voltammetric or - AC voltammetric responses in cyclic and/or higher harmonic forms for an -> analyte in bulk solution or attached to the electrode surface. In the case of corresponding anodic and cathodic peaks, a value of AEp greater than the value predicted for reversible electron transfer may be attributed to quasireversibility of the electron transfer, to uncompensated resistance (see - uncompensated IR drop), and/or to coupled chemical processes [i—iv]. In the case of multiple peaks associated with sequential multiple electron transfers for a single analyte or for several analytes, AEp can also denote the difference in the peak potentials of individual electron-transfer steps. 

A dendrimer consisting of multiple identical and non-interacting redox units, able to reversibly exchange electrons with another molecular substrate or an electrode, can perform as a molecular battery [64, 65]. The redox-active units should exhibit chemically reversible and fast electron transfer processes at easily accessible potential difference and chemical robustness under the working conditions. 

In many cases, multiple waves were observed indicating more than one electron transfer process was oc-... 

There are several disadvantages to potential sweep methods. First, it is difficult to measure multiple, closely spaced redox couples. This lack of resolution is due to the broad asymmetric nature of the oxidation/reduction waves. In addition, the analyte must be relatively concentrated as compared to other electrochemical techniques to obtain measurable data with good signal to noise. This decreased sensitivity is due to a relatively high capacitance current which is a result of ramping the potential linearly with time. Potential sweep methods are easy to perform and provide valuable insight into the electron transfer processes. They are excellent for providing a preliminary evalnation, bnt are best combined with other complementary electrochemical techniqnes. 

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