Reactions Volmer

This process is often called the Volmer reaction (I). In the second step, adsorbed hydrogen is removed from the electrode, either in a chemical reaction... 

It was demonstrated by R. Parsons and H. Gerischer that the adsorption energy of the hydrogen atom determines not only the rate of the Volmer reaction (5.7.1) but also the relative rates of all three reactions (5.7.1) to (5.7.3). The relative rates of these three reactions decide over the mechanism of the overall process of evolution or ionization of hydrogen and decide between possible rate-determining steps at electrodes from different materials. 

The effect of adsorption on the electroreduction of hydrogen ions, i.e. the Volmer reaction, is strongly affected by the potential difference in the diffuse electrical layer (Eq. 5.3.20). 

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 ... 

On mercury and gold the Volmer reaction is rate determining Tafel behavior is observed, but the apparent transfer coefficients depend on temperature [1],... 

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] ... 

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)... 

It can be assumed that the current representing the Volmer discharge reaction (26) equals the current representing the Tafel reaction (27), i.e., only a minor part of the discharge current (iV) results in hydrogen atoms being absorbed into the membrane the reverse Volmer reaction is taken to be negligible so that the observed current, I obs = iv = h It follows from (27) that... 

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... 

Volmer turned his attention to processes at - nonpo-larizable electrodes [iv], and in 1930 followed the famous publication (together with - Erdey-Gruz) on the theory of hydrogen - overpotential [v], which today forms the background of phenomenological kinetics of electrochemistry, and which resulted in the famous - Butler-Volmer equation that describes the dependence of the electrochemical rate constant on applied overpotential. His major work, Kinetics of Phase Formation , was published in 1939 [v]. See also the Volmer reaction (- hydrogen), and the Volmer biography with selected papers [vi]. 

The rate-determining step varies depending on the specific catalysts and the reaction conditions. For a PEM fuel cell with a Pt anode, the HOR process involves only the Tafel and Volmer reactions, with the Tafel reaction being the rate-determining step." The rate of the overall HOR process can be expressed in the Butler-Vol-mer form (equation 12.1) ... 

Figure 28. Adiabatic potential energy surfaces in thermodynamic equilibrium as a function of the solvent coordinate q and the separation distance r-ro between the atoms of the molecule for the overall reaction (a) and as a function of the solvent coordinate q and the distance dei of the hydrogen atom to the surface for the Volmer reaction (b). At the bottom are shown the contour projection of the 3D-surfaces.

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. 

We note in passing, that there is no particle size effect on the H2-oxidation rate over Pt, although the Volmer reaction, the non-limiting step, is faster on the larger crystallites. 

Electrochemical reactions are usually complicated by several intermediate processes. As an example let us first discuss the hydrogen evolution reaction. It should be noted that although the hydrogen evolution reaction is probably one of the most studied, its mechanism is still not yet clearly established. In alkaline solutions it is generally believed that the production of hydrogen proceeds by chemisorption of water molecules on free electrode sites M, through the so-called Volmer reaction ... 

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