Tafel-Volmer 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... 

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. 

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 ... 

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. 

On certain noble metals of the platinum group, however, the so-called Volmer-Tafel mechanism applies ... 

The steady-state conditions for the HER mechanisms where the electroadsorption step is coupled to one of the subsequent steps lead to pseudoequilibriiun adsorption isotherms [26,28,61], It is seen in Table 2 that for a coupled Volmer-Heyrovsky mechanism, the dependence ofXjj on AG is slight if P + 8 1. Only for the coupled Volmer-Tafel mechanism may X depend significantly on AG, imless y + p/2 1 for 0jj 1 or P + 2y 3/2 for 0jj Vi. Figure 10a and b show the theoretical variations of log and Xjj versus... 

AG /RT for an HER mechanism with the Volmer step in quasi-equilibrium and for die coupled Volmer-Tafel mechanism. From Table 2, the maximum of the HER current occurs at 0jj = Vi if the S5mmetiy factors are equal to Vi or for the two coupled mechanisms if P = 8 or P = y. For the coupled Volmer-Tafel mechanism. Figure 10b shows a case where X is minimum for 0jj V2 and maximum for extreme values of coverage (0jj 1 or 0jj 1), i.e., of the adsorption fiiee energy. 

The preceding analysis provides no explanation for ASB promoting effects on H absorption observed in cases where the HER is not controlled by a coupled Volmer-Tafel mechanism, e.g., on noble metals such as Pt and Pd [73,74,101,113]. On... 

Hydrogen evolution occurs by the Volmer-Tafel mechanism if Steady-state conditions are ... 

Figure 2.10a and b show the theoretical variations of log 4 (here 4 = x her), 0H/ and Xjj versus -AG JRT for a HER mechanism with the Volmer step in quasiequilibrium and for the coupled Volmer-Tafel mechanism. The symmetry factors are taken equal to 2, and a Langmuir adsorption model is taken, where AG ds is not coverage dependent. 

The preceding analysis provides no explanation for ASB promoting effects on H absorption observed in cases where the HER is not controlled by a coupled Volmer-Tafel mechanism, for example, on noble metals such as Pt and Pd [77,78,108,122]. On these metals, the electroadsorption step in the HER is in quasiequilibrium at low cathodic overpotentials (chemical combination being the rate-determining step) when the overpotential increases, it becomes coupled to the electrocombination step [129]. So H absorption promotion by ASB on these metals can be explained only if the surface-bulk transfer step is not in equilibrium, that is. Equations 2.12a and 2.38 do not apply, at least on the clean surface, and the ASB increases the kinetics of this step [60,68]. 

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