Electrocatalytic reactions involving hydrogen

Many reactions of industrial importance are electrocatalytic, i.e., they involve the specific adsorption of intermediates, for example hydrogen, chlorine, and oxygen evolution, oxygen reduction, and methanol or ethanol oxidation in fuel cells. Many different electrochemical techniques were used to study these reactions, and EIS is one of them, providing interesting kinetic and surface information. Certain model reactions will be presented in what follows with a detailed method of relating impedance parameters with mechanistic and kinetic equations. 

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Based on that knowledge, the impedance of electrode processes in the presence of diffusion in various geometries and adsorption is mathematically developed. This leads to the general method of determining the impedances of complex mechanisms. As an illustration, the impedance of electrocatalytic reactions involving hydrogen adsorption, absorption, and evolution is presented. 

Electrocatalytic reactions involving adsorbed species By far the most extensively studied electrode reaction involving adsorbed species on electrode surfaces is the hydrogen electrode reaction (30),... 

In the field of electrocatalysis the situation seems to be somewhat better as far as the problems of clean surfaces and the existence of chemical effects are concerned. Unfortunately, so far only a few reactions have been studied. Thus, much more systematic fundamental research has to be done. From the results already available one can extract the hope that ion-bombardment will be of future importance for fields involving electrocatalytical reactions such as, for example, hydrogen technology, energy conversion, fuel cells and electrochemical redox reactions. 

The hydrogen evolution reaction is an example where its electrocatalytic character shows that it is necessary for both the description of the adsorption process and the knowledge of the kinetic parameters. Most analysis of the electrocatalytic properties involve correlations from the estimated exchange current densities with characteristic electric potentials, free energies of adsorption, enthalpy of sublimations for the metal electrode, etc. [60]. 

Electrode processes in experiments with redox indicators involved one or a few electrons and were, therefore, inherently low-yield reactions. Recently catalytic processes have been used to collect as many electrons as possible ([327, 454, 455] E. Palecek, M. Fojta, and L. Havran, unpublished). Thorp [327] used a soluble mediator that moved close to G residues present only in target DNA (but absent in the probe) and shuttled electrons to the polymer-modified ITO electrode. The reduced form of the mediator [Ru(bipy)3] + was oxidized by holding the electrode at a sufficiently positive potential. The oxidized form of the mediator removed electrons from G residues, generating reduced [Ru(bipy)3] + and completing a catalytic cycle. About 100 electrons per hybridized G could be collected under favorable conditions. Horseradish peroxidase coupled to target DNA was applied to detect the hybridization by electrocatalytic reduction of hydrogen peroxide... 

The anthraquinone moieties on the surface of electrode show excellent electrocatalytic ability towards oxygen. The reaction involves the reduction of anthraquinone to dihydranthraquinone via two electrons and two protons. Afterwards, dihydranthraquinone reacts with oxygen and converts it to hydrogen peroxide while anthraquinone is regenerated. 

In the present chapter we want to look at certain electrochemical redox reactions occurring at inert electrodes not involved in the reactions stoichiometrically. The reactions to be considered are the change of charge of ions in an electrolyte solution, the evolution and ionization of hydrogen, oxygen, and chlorine, the oxidation and reduction of organic compounds, and the like. The rates of these reactions, often also their direction, depend on the catalytic properties of the electrode employed (discussed in greater detail in Chapter 28). It is for this reason that these reactions are sometimes called electrocatalytic. For each of the examples, we point out its practical value at present and in the future and provide certain kinetic and mechanistic details. Some catalytic features are also discussed. 

Electrocatalytic Reduction of Dioxygen The electrocatalytic reduction of oxygen is another multi-electron transfer reaction (four electrons are involved) with several steps and intermediate species [16]. A four-electron mechanism, leading to water, is in competition with a two-electron mechanism, giving hydrogen peroxide. The four-electron mechanism on a Pt electrode can be written as follows ... 

BDD anodes without impurities are not electrocatalytically active because water electrolysis is characterised by the formation of OH radicals (Marselli et al. 2003), ozone (Cho et al. 2005) and hydrogen peroxide (Drogui et al. 2001). One can conclude from radical chemistry that other radicals have to be expected in the anodic reaction layer and, maybe, in the bulk. Foerster and co-workers compared active chlorine formation on Pt and BDD anodes (Foerster et al. 2002). Formation of active chlorine was explained by a mechanism involving the formation of Cl radicals (Ferro et al. 2000) ... 

F — F), SO that the whole acting force of a photoelectrode is concentrated on the anodic partial reaction taking place with minority carriers (holes) being involved. It is therefore convenient to carry out the cathodic partial reaction (e.g., hydrogen evolution) on a metal electrode, which possesses good electrocatalytic properties for this reaction. 

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