Electrode kinetics evolution reaction

Platinum has also had its share of attention in recent years. The effect of phosphoric acid concentration on the oxygen evolution reaction kinetics at a platinum electrode using 0-7 m-17-5 m phosphoric acid at 25°C has been studied with a rotating disc electrode . The characteristics of the ORR are very dependent on phosphoric acid concentration and H2O2 is formed as an intermediate reaction. Also, platinum dissolution in concentrated phosphoric acid at 176 and 196°C at potentials up to 0-9 (SHE) has been reported . 

For comparison we also show a cyclic voltammogram of a Au(lll) electrode (see Fig. 13.4). There is no detectable hydrogen adsorption region the hydrogen evolution reaction is kinetically hindered, and sets in with a measurable rate only at potentials well below the thermodynamic value. There is a much wider double-layer region in which other... 

This result represents the first use of FTIR measurements to obtain information about the hydrogen evolution reaction on iron. It also represents one of the first uses of FTIR to study the mechanism of the electrode kinetic reaction (14). 

CI2 evolution reaction, 38 56 electrochemical desorption, 38 53-54 electrode kinetics, 38 55-56 factors that determine, 38 55 ketone reduction, 38 56-57 Langmuir adsorption isotherm, 38 52 recombination desorption, 38 53 surface reaction-order factor, 38 52 Temkin and Frumkin isotherm, 38 53 real-area factor, 38 57-58 regular heterogeneous catalysis, 38 10-16 anodic oxidation of ammonia, 38 13 binding energy quantification, 38 15-16 Haber-Bosch atrunonia synthesis, 38 12-13... 

Wendt, H. and Plzak, V. (1990) Electrode kinetics and electrocatalysis of hydrogen and oxygen electrode reactions. 2. Electrocatalysis and electrocatalysts for cathodic evolution and anodic oxidation of hydrogen, in Electrochemical Hydrogen Technologies (ed. H. Wendt), Elsevier, Amsterdam, Chapter 1. 2. 

Chen L, Lasia A (1991) Study of the kinetics of hydrogen evolution reaction on nickel-zinc Alloy electrodes. J Electrochem Soc 138 3321-3328... 

In the following discussion, an example is given that serves to show that introducing a Frumkin-Temkin isotherm does affect the kinetic relation between the current density, i, and the corresponding overpotential. The example chosen will use the hydrogen evolution reaction once more because it is relatively simple but at the same time involves consecutive steps and alternative pathways thus it has characteristics of many practical electrode reactions likely to be met in practice.68... 

The importance of double layer structure on electrode kinetics was first shown by Frumkin for the hydrogen evolution reaction on mercury [44]. As a result of the structure of the electrochemical interface, the pre-electrode plane, i.e. the plane where the reactant undergoes electron transfer to become product, is such that the concentration of the reactant ion is different from that in the bulk solution and the corresponding potential difference with respect to the solution, (less than the applied electrode—solution potential difference ([Pg.34]

Only two general reviews [38, 39] entirely devoted to the hydrogen evolution reaction have appeared after the start of the development of cathode activation [40]. In several other cases, hydrogen evolution has been discussed within the general frame of electrocatalysis [4, 41-47] or kinetics of electrode reactions [48, 49]. However, only one of the two reviews mentioned above discusses electrocatalytic aspects with literature coverage up to the late 70 s, when the field of cathode activation was at the beginning of its development. 

Refs. [i] Koryta J, Dvorak J, Kavan L (1993) Principles of electrochemistry. Wiley, Chichester [ii] Sawyer DT, Sobkowiak A, Roberts ]L Jr (1995) Electrochemistry for chemists. Wiley, New York [iii] Calvo EJ (1986) Fundamentals. The basics of electrode reactions. In Bamford CH, Compton RG (eds) Comprehensive chemical kinetics, vol. 26. Elsevier, Amsterdam, pp 1-78 [iv] Conway BE (1999) Electrochemical processes involving H adsorbed at metal electrode surfaces. In Wieckowski A (ed) Interfacial electrochemistry, theory, experiment, and applications. Marcel Dekker, New York, pp 131-150 [v] Savadogo O (1999) Synergetic effects of surface active sites on the hydrogen evolution reaction. In Wieckowski A (ed) Interfacial electrochemistry, theory, experiment, and applications. Marcel Dekker, New York, pp 915-935... 

Owing to the success of Ru02-based DSA electrodes in the chlor-alkali industry, a significant amount of study has been carried out on the kinetics and mechanism of chlorine evolution at Ru02-based electrodes over the past 15 years or so. A considerable body of experimental data has therefore been accumulated regarding the chlorine evolution reaction at Ru02 electrodes, which includes E vs. log j plots, reaction order determinations, pH depen-... 

Lei us now return to the effect of pH on electrode kinetics, using concentrations instead of activities. Consider the hydrogen evolution reaction, and assume that it proceeds in the following two steps, with the second step being rate determining. 

The Tafel slope for this mechanism is 2.3RT/PF, and this is one of the few cases offering good evidence that P = a, namely, that the experimentally measured transfer coefficient is equal to the symmetry factor. A plot of log i versus E for the hydrogen evolution reaction (h.e.r.), obtained on a dropping mercury electrode in a dilute acid solution is shown in Fig. 4F. The accuracy shown here is not common in electrode kinetics measurements, even when a DME is employed. On solid electrodes, one must accept an even lower level of accuracy and reproducibility. The best values of the symmetry factor obtained in this kind of experiment are close to, but not exactly equal to, 0.500. It should be noted, however, that the Tafel line is very straight that is, P is strictly independent of potential over 0.6-0.7 V, corresponding to five to six orders of magnitude of current density. 

The most reliable data are from studies of hydrogen evolution on mercury cathodes in acid solutions. This reaction has been studied most extensively over the years. The use of a renewable surface (a dropping mercury electrode, in which a new surface is formed every few seconds), our ability to purify the electrode by distillation, the long range of overpotentials over which the Tafel equation is applicable and the relatively simple mechanism of the reaction in this system all combine to give high credence to the conclusion that p = 0.5. This value has been used in almost all mechanistic studies in electrode kinetics and has led to consistent interpretations of the experimental behavior. It... 

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. 

Extensive work on reaction orders in electrode kinetics, and their interpretation, have been made by Vetter (140), Yokoyama and Enyo for the Clj evolution and other reactions (141, 142, 144), and by Conway and Salomon for the HER (143). In the extensive treatment of the kinetics of O2 evolution by Bockris (145), reaction orders were derived for various possible reaction mechanisms and provide, among other factors, diagnostic criteria for the mechanisms in relation to the experimentally determined behavior, for example, pH effects in the kinetics and Tafel slope values (145). 

XVIII. Electrode Kinetic Behavior of Chlorine Evolution Reaction, and Role and Identity of Adsorbed Intermediates... 

Several reviews addressing the polarization behavior, d ion adsorption, competition between Cr adsorption and OH codeposition, oxide film formation, and cr ion discharge, as well as the kinetic aspects of the reaction on various oxide-covered and oxide-free surfaces that have been investigated during the past 15 years, have been published (55/, 333-338). Of these, particular mention should be made of Refs. 555, 335, 336, and 439-441, where the basic aspects of the properties of oxide electrodes and the kinetic aspects of oxide film formation in relation to Cl adsorption and the kinetics of Cr ion discharge were addressed. Mechanistic aspects of chlorine evolution were critically analyzed recently in an excellent article by Trasatti (338). In this article, the focus is primarily on the nature and characterization of the adsorbed intermediates partipatingin the course of CI2 evolution and their role in the electrocatalysis of the chlorine evolution reaction. As with the OER, in aqueous solutions CI2 evolution takes place on an oxidized surface of metals or on bulk oxide films, so that their surface states often have to be considered in treating the electrocatalysis of the reaction. 

Thus, the mechanistic aspects of CI2 evolution are not yet at all well clarified, although significant progress has been made during the last decade. Proper understanding of the kinetics of the O2 and CI2 evolution reactions and their relationship requires more closer scrutiny, with attention to surface characterization of electrodes. 

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