Volmer-Heyrovsky-Tafel mechanism

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... 

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... 

As far as the chl.e.r. mechanism is concerned, the same, previously described, investigation has been performed and Figures 24 and 25 respectively report the polarization curve and the Tafel plot (currents normalized to the number of active sites at the electrode surface), for the case of a 1 M NaCl/3 M NaC104/0.01 M HCIO4 test solution. The measured Tafel slope has a value of 0.149 V, and the reaction order with respect to CP is about 0.7 the values of b and R both agree well with a Volmer-Heyrovsky mechanism [24], with a rate-determining electrochemical desorption, provided a value of about 0.7 is assumed for the coverage by the intermediate chlorine radicals [28] ... 

The hydrogen oxidation reaction (HOR) at the anode proceeds on Pt-based catalysts and is one of the simplest reaction systems. ° Nonetheless, fundamental information of the mechanism and kinetics of HOR is still lacking. The most common mechanisms are the so-called Heyrovsky-Volmer and Tafel-Volmer mechanisms involving the following steps ... 

More detailed analyses were conducted later, both for the Volmer-Tafel and the Volmer-Heyrovsky reaction routes, with allowance for comparable magnitudes of exchange c.d. s of the constituent steps. It became clear that such analyses of the so-called linked/coupled t mechanism could well provide an interpretation of some experimental data, as will be discussed later. Meanwhile, some comments will be made concerning the definition of the now widely accepted terms rds and other related topics. 

Reaction steps (4) and (5) or (6), the two latter in acid and alkaline media, respectively, correspond to the Tafel-Vohner (T-V) mechanism, whereas reaction steps (7), in acid, and (8), in alkaline media, are known as Heyrovsky-Volmer (H-V) mechanism. 

If the first step would limit the reaction rate, the cathodic Tafel coefficient would be identical to that of the Volmer-Heyrovsky mechanism (the anodic Tafel coefficient would be different though). On the other hand, if the second step is rate determining the cathodic Tafel slope differs. The rate of the reaction determining step, Vn, is given by... 

The presence of H2S catalyzes the cathodic formation of hydrogen. In weakly acidic solutions, the decomposition of H2S instead of water is kinetically favored. This follows a sequence of reaction steps similar to the Volmer-Heyrovski (Reactions (1-77) and (1-78)) or Volmer-Tafel (Reactions (1-77) and (1 -79)) mechanism. As discussed earlier, the promoting property of HS favors the penetration of adsorbed hydrogen atoms into the metal according to Reaction (1-80). 

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... 

The subscripts which distinguish the steps honor, respectively, Tafel, Volmer and Heyrovsky. Unlike the MCFC cathodic reaction mechanisms, however, these steps combine pairwise to yield the overall reaction. The reaction mechanism graphs for each of the three reaction mechanisms are shown in Figure 6. Notice that it is not possible to represent the entire mechanism by a single reaction mechanism graph. This is because, unlike in the MCFC case, there are now independent full reaction routes which yield the over all reaction. In both of the MCFC examples, there was only one. Still the three separate graphs do clearly convey the three HER reaction routes. 

Any combination of two or three elementary pathways will give the overall mechanism of the hydrogen evolution reaction. Thus, the electrochemical Volmer discharge of the proton, the electrodesorption Heyrovsky step, and the chemical Tafel recombination of the H adatoms can serve as a combination for the hydrogen evolution process. The electrochemical rate constants can be estimated through different experimental conditions, such as their exchange current densities 7o,i = 10 1, yo2 10 4 and jo 3 = 10 2 A cm-2 at V= 0 V where AGads = 0 with 0H 1/2 [7,48]. 

Based on the Tafel-Heyrovsky-Volmer mechanism [5-7], there are three elementary reaction steps for the reaction, H2 2H -1- 2e , on Pt catalysts ... 

As discussed above, the mechanism for the HOR on a platinum electrode in acid electrolyte proceeds through two pathways, Tafel-Volmer and Heyrovsky-Volmer, both of which involve the adsorption of molecular hydrogen (Had), followed by a fast charge-transfer step ... 

The thermodynamics of the reactions was established as a function of voltage by calculating the stability of the reaction intermediates and the overpotential of the reaction could be linked directly to the proton and electron transfer (Norskov et al, 2004). A Tafel-Heyrovsky-Volmer three-step reaction model (Wang et al, 2006) for the hydrogen oxidation is considered at the anode, while a Damjanovic three-step reaction model (Malek et al, 2008) for the oxygen reduction mechanism is assumed at the cathode. Assuming non-interaction between adsorbed intermediate species and... 

The HOR in acid solution is considered to proceed through either the Tafel-Volmer or Heyrovsky-Volmer mechanisms, depending on the nature of the adsorption step. If it is a purely chemical process, the mechanism is Tafel-Volmer and if it is a combined chemical and electrochemical process, it is Heyrovsky-Vohner [3]. 

The mechanism of HOR is thought to follow a similar Tafel-Volmer or Heyrovsky-Volmer mechanism, as discussed in Section 1.2.3, except with OH -mediated reactions ... 

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