Hydrogen Volmer-Heyrovsky mechanism

More recently, a theoretical and experimental investigation has been carried out on the hydrogen evolution process, assuming a Volmer-Heyrovsky mechanism and following the technique proposed by Tyagai [7]. It was found that the apparent electron number values ranged between one and two, depending on the applied current [22]. 

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... 

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... 

This is a so-called Volmer-Heyrovsky type of mechanism, although it was originally found for the hydrogen evolution reaction. 

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]. 

In particular, we shall present results for the two electrochemical steps of the hydrogen reaction mechanism (Volmer and Heyrovsky) on plain and nanostructured electrodes, and illuminate the role of the reaction intermediate. We also discuss in detail the influence of the position of the metal d band and its interaction with the hydrogen orbital on the catalytic activity. 

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... 

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