Hydrogen Volmer-Tafel mechanism

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. 

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. 

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

The electrochemical hydrogen permeation technique has proved to be a valuable tool in the study of these reaction mechanisms. This is mainly due to the ability to estimate the amount of an intermediate (Hads) in the reaction scheme. Such studies have been presented, for example, by Devanathan and Stachurski, by Bockris et and by Iyer et The applicability of the Volmer-Tafel reaction scheme can be evaluated by considering the kinetic expressions for reactions (22) and (23), together with equilibrium in the absorption process (25)... 

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 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 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 ... 

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]. 

The adsorption of hydrogen on metal electrodes such as platinum has been studied extensively in electrochemical systems over the last several decades. The mechanism for the hydrogen oxidation reaction on a Pt electrode in an acid electrolyte proceeds through two pathways, Tafel-Volmer and Heyrosky-Volmer, both of which involve the adsorption of molecular hydrogen followed by a... 

Hydrogen under electrochemical conditions was investigated very recently [222, 223]. Santana et al. investigated the electro-oxidation of molecular hydrogen at the Pt(110)-water interface [222]. The Tafel-Volmer mechanism with a homolytic H-H bond cleavage followed by the formation of adsorbed terminal hydrogen atoms and further oxidation of the H atoms was observed by the authors. Furthermore, Santana et al. found the potential dependent activation energies for this process to be in accordance with experimental results. 

Erdey-Gruz and Volmer and Butler/ to whom we owe a rate expression for charge transfer processes. Later, Frumkin " took into account the potential distribution at the electrode-electrolyte interface and its influence upon the process of discharge of ions. Experimentally observable quantities other than the Tafel constants have also been employed since then in elucidating the mechanism. For example, experimental observations of and attempts of theoretical interpretation of the electrolytic hydrogen-deuterium separation factor were presented by Topley and Eyring, Horiuti, " and others, " as briefly reviewed elsewhere. ... 

HOR studies on smooth noble metal surfaces in acidic media show that platinum is the most active. The reaction mechanism on bulk polycrystalline platimun electrode is usually assumed to proceed through a Tafel-Volmer mechanism via reactions (4) and (5), the dissociative adsorption of hydrogen being the rate-determining step (r.d.s.) [51]. In alkaline electrolytes, HOR on polycrystalline Pt follows a Tafel-Volmer sequence and the r.d.s. is the dissociative hydrogen adsorption given by reactions (4) and (6) [52]. 

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