Reactions, Heyrovsky

Colloids of metals with practically zero overpotential for Hj evolution such as platinum and iridium are very efficient at catalysing production. The transferred electrons are immediately converted into adsorbed H atoms which then undergo the Heyrovsky reaction to form Hj e + H -h H2. The gold microelectrode... 

The Tafel reaction would require breaking the adsorption bonds to two hydrogen atoms strongly bound to the electrode, while the Heyrovsky reaction requires breaking only one such bond this reaction then determines the rate of the electrode process. 

The electrochemical reaction occurs at the surface of graphite anode [37 39]. At potentials lower than 1.25 V, chlorine is formed by a Volmer/Heyrovsky mechanism with the latter being the rate determining step. Chloride ions are initially discharged on surface sites that are not covered by chlorine atoms (Volmer reaction (14.4a)), followed by the discharge of chloride ions on adsorbed chlorine ions (Heyrovsky reaction (14.4b)) [39] ... 

The reaction sequence of the Volmer reaction (6a) and the Heyrovsky reaction (6b) is not the only one possible desorption of the adsorbed hydrogen may also proceed according to the so-called Tal el reaction (6c) by chemical desorptive dimerization of two adsorbed hydrogen atoms for which, however, the same fundamental considerations prevail... 

Heyrovsky-llkovic equation Ilkovic, Dionyz Heyrovsky reaction hydrogen evolution reaction... 

At the surface, the (fast) so called Volmer reaction + e -> Had takes place. Subsequently, either the homolytic Tafel reaction, 2Had -> H2, or the heterolytic Heyrovsky reaction. Had + H + e H2, may take place. Using the nudget-elastic-band approach (see, e.g., ref. 2) the authors studied the three reactions on the surfaces. They found that the Heyrovsky reaction may dominate, but the similarity of the calculated activation energies for the Heyrovsky and the Tafel reactions suggests that both reactions may take place in parallel. 

Reaction (9-3) is called the Vohner reaction or proton discharge and reaction (9-4) the Heyrovsky reaction or electrochemical desorption. Both reactions are not the only ones possible. Desorption of the adsorbed hydrogen may also proceed according to the Tafel reaction (Eq. 9-5) ... 

An alternative option is the oxidative adsorption according to the so called Heyrovsky reaction generating an adsorbed hydrogen atom, a proton and an electron. 

Hydrogen atom combination with a proton and an electron to form molecular hydrogen (Heyrovsky reaction). 

We have performed calculations for a fair amount of systems, but the representative examples given earlier should suffice. Besides the Volmer and the Heyrovsky reaction discussed earlier, there are also a few systems, for which the chemical recombination (Tafel reaction) determines the rate. Since this does not involve electron transfer, it can be calculated by standard DFT therefore, we have not discussed it in any detail. 

Distinguishability of the Kinetic Parameters of the Volmer-Heyrovsky Reaction... 

It has been found [213] that the kinetic parameters of the Volmer-Heyrovsky type of mechanism cannot be unambiguously determined. In fact, two sets of kinetic parameters can describe the experimental parameters of current and impedance, and formally these two solutions are indistinguishable and arise from the permutation of the parameters of the Volmer and Heyrovsky reactions ... 

It might be noticed that the Volmer and Heyrovsky reactions are electrochemical while the Tafel reaction is chemical, without an exchange of electrons. Assuming a Langmuir adsorption isotherm for H, the rates, Vi, are written as... 

These processes (B), (C), and (D) are often referred to, respectively, as Volmer, Tafel, and Heyrovsky reactions after the names of the investigators who were the first to distinguish and point out their significance. 

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