Electrocatalysis definition

Electrocatalysis Again by definition, an electrocatalyst is a solid, in fact an electrode, which can accelerate a process involving a net charge transfer, such as e.g. the anodic oxidation of H2 or the cathodic reduction of 02 in solid electrolyte cells utilizing YSZ ... 

Effective core potential, 269 Effective double layer characterization of, 189 isotherm, 306, 315 kinetic expressions, 316 observations of with STM, 259 stability of, 225, 351, 503 Effectiveness factor of promotion computation of, 505 definition of, 505 Electrocatalysis... 

Fe 2S], a [4Fe-4S] and a [3Fe-4S] center. The enzyme catalyzes the reversible redox conversion of succinate to fumarate. Voltammetry of the enzyme on PGE electrodes in the presence of fumarate shows a catalytic wave for the reduction of fumarate to succinate (much more current than could be accounted for by the stoichiometric reduction of the protein active sites). Typical catalytic waves have a sigmoidal shape at a rotating disk electrode, but in the case of succinate dehydrogenase the catalytic wave shows a definite peak. This window of optimal potential for electrocatalysis seems to be a consequence of having multiple redox sites within the enzyme. Similar results were obtained with DMSO reductase, which contains a Mo-bis(pterin) active site and four [4Fe 4S] centers. 

Erdey-Gruz, 1048, 1306 1474 Erschler, 1133, 1134, 1425 Ethylene oxidation, anodic, 1052 1258 Exchange current density, 1049, 1066 correction of, 1069 definition, 1053 electrocatalysis and, 1278 impedance and, 1136 interfacial reaction, 1047 and partly polarizable interface, 1056 Excited states, lifetime, 1478 Exothermic reaction, 1041 Ex situ techniques, 785, 788, 1146... 

The rate constants for such outer-sphere reactions can therefore differ markedly from those corresponding to true weak-overlap pathways, even after correction for electrostatic double-layer effects. This can cause some difficulties with the operational definition of inner-sphere electrocatalysis considered above, whereby outer-sphere reactions are regarded as "non-catalytic processes. In addition, there is evidence that inner- rather than outer-sphere pathways can provide the normally preferred pathways at metal-aqueous interfaces for reactants containing hydrophobic functional groups [116]. 

Although catalysis in electrochemical reactions was probably first specifically recognized by Frumkin at a conference in Leningrad in 1939, a first and perceptive definition of electrocatalysis seems to have been by Busing and Kauzmann in 1952 (72) in terms of the ability of various electrode surfaces to promote the velocity of the rate-determining step of the reaction. In this respect, their definition preceded the common use of this term in North America in the 1960s by some years, when it was applied to the activities of fuel-cell electrodes by Liebhafsky (7i). 

Electrocatalysis is the interfacing science between liquid-phase heterogeneous catalysis and electrochemistry. According to a general definition, electrocatalysis may be defined as the... 

In the fabrication of chemically modified electrodes (CMEs), one deliberately seeks, in some hopefully rational fashion, to immobilize a chemical on an electrode surface, so that the electrode thereafter displays the chemical, electrochemical, optical, and other properties of the immobilized molecule (s) (Ref. 1 and references therein). Ordinarily, electrocatalysis at a CME features a mediation of electron-transfer reactions, by the immobilized redox couple, between the electrode and some substrate that would otherwise undergo a slow electrochemical reaction at a naked electrode [1]. However, in the following, electrocatalysis will be taken in the broad sense usually attached to this term in most applications of chemically modified electrodes [2] and which can be defined as the dependence of the electrode reaction rate on the nature of the electrode material . In this section, we are not concerned about cases in which the electrode material is catalytic per se. In other words, intentional modification of the electrode composition or surface is necessary. In line with this choice, examples of electrocatal-yses triggered by deposited adatoms will be included. Selection was made of and emphasis put on examples, old and recent, that illustrate the above definitions, but the recent literature will be mostly retained. 

The term electrocatalysis is, however, more commonly applied to systems where the oxidation or reduction requires bond formation, or at least a strong interaction of the reactant, intermediates, or the product with the electrode surface. The catalyst is the electrode material itself or a species adsorbed from solution. This chapter will discuss this more limited definition of electrocatalysis (note also that simple electron transfer reactions which are pictured as occurring by an outer sphere mechanism and may have very high exchange current densities, are not normally considered within electrocatalysis — in this book they are dealt with in Chapter 3). 

The main basic parameter of catalyst evaluation is the specific exchange current density which, by definition, is normalized to the unit surface area of the electrocatalyst. This property is the target of many fundamental studies in electrocatalysis, too numerous to be listed (Adzic et al., 2007 Debe, 2013 Gasteiger and Markovic, 2009 Kinoshita, 1992 Paulus et al., 2002 Stamenkovic et al., 2007a,b Tarasevich et al., 1983 Zhang et al., 2005, 2008). 

None of these techniques, however, are particularly accurate per se as they ultimately rely on a statistical difference between two measurements, that is, the quantity of enzyme in solution before, and after, the immobilization process. Generally, the amount of immobilized enzyme is small relative to the amount of enzyme in bulk solution and thus the mathematical subtraction has significant statistical error. There is also a problem that even an accurate measurement of immobilized enzyme may not reflect the percentage of enzyme that remains active once immobilized, and more importantiy for electrocatalysis, the percentage of immobilized enzyme that is positioned preferentially to transfer electrons to the electrode. To address this issue, a more definitive measurement is one that calculates the charge transfer efficiency of bound enzyme. 

The general term electrocatalysis is many times invoked to give account for the peculiar behaviour of nanostructured surfaces with respect to bare ones. Due to the character of these electrode materials, electrocatalysis is meant here to refer to a lowering of the activation energy of the charge transfer, rather than to the involvement of a redox mediator. However, a clear definition of the mechanisms involved in the analyte/surface interaction is often challenging. The frequent occurrence of complex electrode mechanisms and the necessity to complement electrochemical investigations with microscopic and spectroscopic studies constitute main constrains... 

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