Tafel mechanism

We discuss acid solutions in greater detail. Two different mechanisms have been established. The first is the Volmer- Tafel mechanism, which consists of a proton-transfer step followed by a chemical recombination reaction ... 

Both schemes have been observed in various systems. We consider hydrogen evolution on platinum from an aqueous solution in greater detail. In this system the Volmer-Tafel mechanism operates, the Volmer reaction is fast, the Tafel reaction is slow and determines the rate. Let us denote the rate constant for the Volmer reaction as ki(rj), that of the back reaction as k i(rj). Since the Volmer reaction is fast and in quasiequilibrium, we have ... 

The two main reaction mechanisms are analogous to the mechanisms for hydrogen evolution. The equivalent scheme to the Volmer-Tafel mechanism is ... 

According to the Volmer-Tafel mechanism, the discharge of a proton forms an adsorbed hydrogen atom (Volmer reaction). Two such adsorbed hydrogen atoms then combine to form molecular hydrogen (Tafel reaction)... 

The mechanism of anodic hydrogen oxidation is much simpler than that of oxygen reduction. Reaction pathways would be the reverse of the Volmer-Heyrovsky or the Volmer-Tafel mechanism, that is, with anodic adsorption... 

For instance, the Tafel mechanism for the evolution of hydrogen is an EC mechanism with reaction (25) occurring twice for each time reaction (26) takes place... 

According to the International Union of Pure and Applied Chemistry (IUPAC), the stoichiometric number is a positive integer that indicates the number of identical activated complexes formed and destroyed in the completion of the overall reaction as formulated with the charge number, n [8, 9], The stoichiometric number is introduced to allow for the possibility that the rate-determining step occurs more or less than once in the overall stoichiometric reaction for instance in the Tafel mechanism for the reduction of proton according to eqns. (25) and (26), reaction (25) occurs twice each time a hydrogen molecule is formed. 

A generalized Volmer-Heyrovsky-Tafel mechanism explained the problem of the intermediate and product adsorption with a nearly zero coverage at potentials approximately equal to that of the net hydrogen evolution process [51]. The electrochemical reaction rates of the individual processes are... 

Mechanism and reaction intermediates of the hydrogen oxidation reaction on platinum have been studied with SEIRAS [324]. A band observed around 2090 cm was assigned to adsorbed hydrogen species and the band height dependency on electrode potential and hydrogen overpressure was found to match values predicted on the basis of the Volmer-Tafel mechanism. 

A further conclusive support of the Vohner-Tafel mechanism is given by the log-log plot of the current density of HER (i) versus the band intensity shown in Fig. 8.22 B. The slope of 2 indicates iccff, which is the well-known rate equation of the Tafel step at limiting low 0 (i.e., in the framework of the Langmuir isotherm of adsorption) [103-105]. 

The Tafel plot suggests that the Volmer-Tafel mechanism holds over the potential range examined however, the slope in the semi-log plot of the band intensity deviates from 60 mV/decade at <0.02 V, where the intensity is proportional to (Fig. 8.21 B). The reason for the inconsistency between the SEIRAS... 

The Volmer-Tafel mechanism corresponds to the case where the discharge reaction is the rate-determining step, while the recomhination step is fast. 

FIGURE 20.13 Reaction schematic of cathodic hydrogen formation (a) by the Volmer-Tafel mechanism and (b) by the Volmer-Heyrowsky mechanism. (H q) MS means hydronium ion (H3O+) on metal surface. 

For the reduction of hydrogen ions two mechanisms are discussed. According to the Volmer-Tafel mechanism the following two reactions take place ... 

A-1. The slowt Volmer-rapidt Tafel mechanism (the slow-discharge mechanism)... 

If we go back to mechanism (149) and consider step IV [2 H(a") -> H2(a" )] as rate-determining, we are dealing with the so-called Tafel mechanism. We easily see that a mechanism of the type (158), but with all i s except A4 equal to zero, and a mechanism of the type (149), but with all y s except A4 equal to zero, give the same expression for the current ... 

The reaction kinetics on a given metal has been found to follow the Vobner-Tafel mechanism with the Tafel reaction being rate-limiting. Show that if the surface coverage of adsorbed hydrogen remains low, the Tafel slope is equal to = 30 mV (25 °C, base-10 logarithm). 

Concerning the CER kinetics, it was accordingly concluded by the pioneer in this field, L. I. Krishtalik that the CER proceeds on a DSA as a barrier-less reaction at low over potentials. Taking into account all the possibilities, the most probable mechanism follows the so-called Volmer-Krishtalik-Tafel mechanism ... 

More detailed experiments enabled slight modifications of the Volmer-Krishtalik-Tafel mechanism to i... 

In the past, the majority of the basic laws and concepts in electrode kinetics were developed and verified by Tafel [11], Volmer [12], and Frumkin [13] using the hydrogen electrode. Two important reaction mechanisms are well recognized and experimentally validated. The first is the Volmer-Tafel mechanism, shown in Equations 3.11-3.13. The other, which is more important for the hydrogen electrode, is the Hyrovsky-Volmer mechanism, expressed in Equations 3.14-3.17. 

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