Some Electrocatalytic Reactions

However, for some electrocatalytic reactions, such as the electrooxidation of alcohols, aldehydes or acides, and also the electro reduction of oxygen, lead adatoms can exhibit a promoting effect (3-7). Moreover, lead can change the selectivity in the case of electrocatalytic reductions of nitrocompounds (8), whereas it inhibits the adsorption of hydrogen on platinum (9,10),... 

Although some electrocatalytic reactions are first order in a key reactant [e.g., oxygen reduction (77)], several reactions of organic species exhibit other orders. Thus oxidation of hydrocarbons has a small fractional order in reactant and a negative order in H concentration (78). Alkene reduction is... 

For some electrocatalytic reaction (e.g. fuel cell), parts of energy accompanied with chemical reaction can be used as electric energy for external application. Moreover, by reverse reaction, electric energy can be converted to chemical energy and can be stored (secondary cell and electrol3dic synthesis etc). 

In this section we treat some electrochemical reactions at interfaces with solid electrolytes that have been chosen for both their technological relevance and their scientific relevance. The understanding of the pecularities of these reactions is needed for the technological development of fuel cells and other devices. Investigation of hydrogen or oxygen evolution reactions in some systems is very important to understand deeply complex electrocatalytic reactions, on the one hand, and to develop promising electrocatalysts, on the other. 

The amount of oxygen adsorbed in the three-phase region has been found to depend linearly on the exchange current density for different catalyst-electrodes under similar conditions.31,32 This indicates that the electrocatalytic reaction takes place at the three-phase boundary. Vayenas and co-workers pointed out that for less porous electrodes the charge-transfer reaction at the two-phase boundary might become important and that under some conditions oxygen on the electrolyte surface itself might play a role. 

An electrocatalytic reaction is an electrode reaction sensitive to the properties of the electrode surface. An electrocatalyst participates in promoting or suppressing an electrode reaction or reaction path without itself being transformed. For example, oxygen reduction electrode kinetics are enhanced by some five orders of magnitude from iron to platinum in alkaline solutions or from bare carbon to carbon electrodes modified with Fe phthalocyanines or phenylporphyrins. For a comprehensive discussion of the subject, the reader is referred to refs. (76, 95, and 132-136). 

R. Garjonyte and A. Malinauskas, Electrocatalytic reactions of hydrogen peroxide at carbon paste electrodes modified by some metal hexacyano-ferrates, Sens. Actuators B, 46 (1998) 236-241. 

The aim of this chapter is to show that the choice of a catalyst formulation leading to increase the activity and the selectivity of a given electrochemical reaction involved in a fuel cell can only be achieved when the mechanism of the electrocatalytic reaction is sufficiently understood. The elucidation of the mechanism caimot be obtained by using only electrochemical techniques (e.g. cyclic voltammetry, chronopotentiometry, chrono-amperometiy, coulo-metry, etc.), and usually needs a combination of such techniques with spectroscopic and analytical techniques. A detailed study of the reaction mechanism has thus to be carried out with spectroscopic and analytical techniques under electrochemical control. In short, the combination of electrochemical methods with other physicochemical methods cannot be disputed to determine some key reaction steps. For this purpose, it is then necessary to be able to identify the nature of adsorbed intermediates, the stractuie of adsorbed layers, the natirre of the reaction products and byproducts, etc., and to determine the amormt of these species, as a fimction of the electrode potential and experimental conditions. 

The principal aims of this review are to indicate the role of chemisorbed intermediates in a number of well-known electrocatalytic reactions and how their behavior at electrode surfaces can be experimentally deduced by electrochemical and physicochemical means. Principally, the electrolytic gas evolution reactions will be covered thus, the extensive work on the important reaction of O2 reduction, which has been reviewed recently in other literature, will not be covered. Emphasis will be placed on methods for characterization of the adsorption behavior of the intermediates that are the kinetically involved species in the main pathway of the respective reactions, rather than strongly adsorbed by-products that may, in some cases, importantly inhibit the overall reaction. The latter species are, of course, also important as they can determine, in such cases, the rate of the overall reaction and its kinetic features, even though they are not directly involved in product formation. 

This article concentrates on principles and methodologies for examination and interpretation of experiments and behavior of some selected electrocatalytic reactions, rather than providing an exhaustive catalog of the very many works that have been published in this fleld. Such a review would take much more space than is allocated for this article. Several other relevant reviews are to be noted, as follows, in the references indicated Sakellaropoulos (4), Trasatti and Lodi (5), Conway (6), Conway and Angerstein-Kozlowska (7), Yeager (S) and others on the O2 reduction reaction, Jaksic (9), O Sullivan... 

An important point to monitor the catalytic properties of modified ECP is to understand the mechanism of an electrocatalytic reaction, i.e., to identify the adsorbed intermediate species. During an electrocatalytic reaction the key steps always involve species coming from the dissociative chemisorption of reactants (organic molecules and solvent). These species can react at the electrode surface to give the reaction products (generally also adsorbed), but some of them can remain... 

In the case of well-known electrochemical reactions, as well as for electrolyses where larger scales are involved, two-electrode cells (connected to a galvanostat) can be used with continuous feed of the reactant to the working electrode. This type of electrolysis is suitable for industrial purposes where specific devices and cells are utilized. Since electrodes of large areas are necessary, the distance between the anode and the cathode is small and determines the cell geometry (e.g. capillary-gap cell or filter-press cell). The use of cells equipped of porous electrodes (materials like graphite or carbon moss, platinum, nickel) have also been developed to perform electrocatalytic reactions at very large surfaces. Some typical cells used in the laboratory and in industry are presented at the end of this review. 

Electrocatalytic reactions often involve several elementary steps some of which are not necessarily electrochemical. The transfer coefficient is, then, related to the symmetry factor of an electron transfer rate-determining step (rds) through (74)... 

Another important reaction is the coupling of two carbon dioxide molecules at the carbon atoms to give oxalate. This process appears to be a side reaction in some electrocatalytic reductions yet it has... 

Some Examples of Electrocatalytic Reactions Based on EHMO Calculations... 

In most of the cases, the main electrochemical or electrocatalytic reaction is coupled with homogeneous chemical reactions that can be defined either in the bulk of the solution or in a Nemstian heterogeneous film. Whether one or the other takes place, the result is the modification of the kinetic constant of the main process. The topic is so general that we can only discuss some... 

The design of the electrochemical reactor, in the case of a fuel cell, is not yet totally solved as classical heterogeneous chemical reactors do not meet the requirements of the triphasic interface anode and the cathode binary system. Some papers [1-3] have considered the problem at the cathode and at the anode independently. However, the electrocatalytic reactions on both the electrodes produce a single chemical reaction, which is the chemical outlet of the energy conversion process. 

In Table I, the various stages in the determination of the mechanism of an electrocatalytic reaction and the methods used to make such determinations are outlined. The techniques used to determine the over-all reaction are simple except in the case of some reactions, as for example anodic oxidation of certain organic compounds, where several reactions may occur in parallel. The analysis of the over-all reaction also gives the number of electrons transferred per molecule. As seen from Table I, a determination of the reaction path and rate-determining step involves ... 

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