Hydrogen evolution reaction mechanism

Hydrogen evolution reaction, mechanism, 1135, 1151, 1163, 1164, 1189 catalytic pathway, 1163,1194, 1255 electrocatalysis, 1280 Frumkin-Temkin isotherm, 1194 Langmuir isotherm, 1194 Hydrogen coadsorption... 

The standard electrode trotential, Ep, 2+ Pb = —Q.126V . shows that lead is thermodynamically unstable in acid solutions but stable in neutral. solutions. The exchange current for the hydrogen evolution reaction on lead is very small (-10 - 10"" Acm ), but control of corrosion is usually due to mechanical passivation of the local anodes of the corrosion cells as the majority of lead salts are insoluble and frequently form protective films or coatings. 

Table 20.3 Mechanism of the hydrogen evolution reaction at different metals (data after...

Table 5.5 Constants a and b of the Tafel equation and the probable mechanism of the hydrogen evolution reaction at various electrodes with H30+ as electroactive species (aH3o+ ) (According to L. I. Krishtalik)... 

Classically, processes involving surface intermediates were investigated primarily by methods (2) (4) above and in particular by measuring current as a function of concentration of reagents and electrode potential. A familiar example is the hydrogen evolution reaction, which may proceed by one of two possible mechanisms, both of which share a common first step ... 

Schuldiner (1959) studied the effect of H2 pressure on the hydrogen evolution reaction at bright (polished) Pt in sulphuric acid. The mechanism of the reaction was assumed to be as in equations (3.3) and (3.4). The step represented by equation (3.3) was assumed to be at equilibrium at all potentials and equation (3.4) represented the rate-determining step. The potentials were measured as overpotentials with respect to the hydrogen potential, i.e. the potential of the H +/H2 couple in the solution (0 V vs. RHE). 

This result represents the first use of FTIR measurements to obtain information about the hydrogen evolution reaction on iron. It also represents one of the first uses of FTIR to study the mechanism of the electrode kinetic reaction (14). 

Atomic hydrogen formed as an intermediate in the hydrogen evolution reaction is adsorbed to the surface of the membrane. Molecular hydrogen is formed by one of at least two mechanisms, but parallel to this, a fraction of the atomic hydrogen is absorbed by the metal, eventually leading to an equilibrium between adsorbed and absorbed atomic hydrogen. In the absorbed state, the hydrogen atoms are able to diffuse as interstitials in the metal lattice. 

Pathway of reaction, 1167 consecutive reactions, 1259 definition, 1259 and FTTR spectroscopy, 1259 hydrogen evolution reaction, 1259 isotopic analysis. 1259 mechanism, 1259 parallel reactions, 1259 Permitivity of free space. 875 Perez, 1519... 

If these conditions are not satisfied, some process will be involved to prevent accumulation of the intermediates at the interface. Two possibilities are at hand, viz. transport by diffusion into the solution or adsorption at the electrode surface. In the literature, one can find general theories for such mechanisms and theories focussed to a specific electrode reaction, e.g. the hydrogen evolution reaction [125], the reduction of oxygen [126] and the anodic dissolution of metals like iron and nickel [94]. In this work, we will confine ourselves to outline the principles of the subject, treating only the example of two consecutive charge transfer processes O + n e = Z and Z 4- n2e — R. 

This section differs from the previous one in that the materials considered here are obtained by combining two (or more) metals in well defined proportions corresponding to the appearance of a new phase. Since the electronic structure of the component metals is drastically changed in intermetallic compounds, it is expected that the mechanism of the hydrogen evolution reaction can change as well. The majority of... 

Reaction mechanism graphs may be useful in studying reaction mechanisms even when there is no species cycling through the reaction. To see this, consider the simple example of the electrochemical hydrogen evolution reaction (HER) mechanism. Here the overall reaction is 2H20 +2e / 20H + H2. The elementary reaction steps that combined to yield HER (cf. Fishtik etal., 2005)26 are... 

What is the mechanism of this phenomenon Very early during investigations of this field, it was realized that metals become embrittled because at some stage of their career, their surface was the scene of a hydrogen-evolution reaction either because the metal was deliberately used as an electron-source electrode in a substance-producing cell or because parts of the metal became electron-source areas in a corrosion process. In fact, the phenomenon has come to be known as hydrogen embrittlement. 

In heterogeneous electro catalysis, the catalyst is immobilized on the electrode surface, or the electrode itself plays the role of a catalyst. Catalytic effects of various electrode materials on the hydrogen evolution reaction are typical examples of heterogeneous electro -catalysis [iii]. Further examples are electrode mechanisms involving hydrogen evolution at a mercury electrode catalyzed by adsorbed organic bases, microparti-... 

The Tafel slope for this mechanism is 2.3RT/PF, and this is one of the few cases offering good evidence that P = a, namely, that the experimentally measured transfer coefficient is equal to the symmetry factor. A plot of log i versus E for the hydrogen evolution reaction (h.e.r.), obtained on a dropping mercury electrode in a dilute acid solution is shown in Fig. 4F. The accuracy shown here is not common in electrode kinetics measurements, even when a DME is employed. On solid electrodes, one must accept an even lower level of accuracy and reproducibility. The best values of the symmetry factor obtained in this kind of experiment are close to, but not exactly equal to, 0.500. It should be noted, however, that the Tafel line is very straight that is, P is strictly independent of potential over 0.6-0.7 V, corresponding to five to six orders of magnitude of current density. 

Which metals are "similar" to mercury in this respect It turns out that most of the soft metals in group 5B and 6B of the periodic table (including Pb, Bi, Cd, In and Sn) behave rather similarly to mercury in respect to the hydrogen-evolution reaction. It would be presumptuous to claim that we could have predicted this similarity from theory, but being confronted with the facts, we can reasonably well explain this result on the basis of the catalytic activity of these metals (or rather the lack of it), as shown in Section 15.7. The hydrogen-tritium separation factor on mercury and the other "soft" metals is low (5 = 6). This low value is believed to be cliaracteris-tic of the mechanism just presented. 

Although many metal hydrides decompose at temperatures well below their melting points [1], there have been comparatively few studies of the kinetics and mechanisms of these hydrogen evolution reactions. Much of the interest in solid hydrides has been concerned with their thermodynamic properties, as hydrogen sources in fuel cells, or as reducing agents, or for technological applications in nuclear processes. 

Electrochemical reduction of CO2 in nonaqueous solutions is significant from the following viewpoints Firstly, hydrogen evolution reaction can be suppressed. Secondly, the concentration of water as a reagent can be accurately regulated and the reaction mechanism may be more easily studied. Thirdly, the solubility of CO2 in organic solvents is much liigher than in water. Various metal electrodes have been tested for CO2 reduction in some nonaqueous solvents, such as propylene carbonate (PC), acetonitrile (AN), DMF, and dimethyl sulfoxide (DMSO), as tabulated in Table 5. Methanol is also used for CO2 reduction, and mentioned in the next Section. 

In electrochemical proton transfer, such as may occur as a primary step in the hydrogen evolution reaction (h.e.r.) or as a secondary, followup step in organic electrode reactions or O2 reduction, the possibility exists that nonclassical transfer of the H particle may occur by quantum-mechanical tunneling. In homogeneous proton transfer reactions, the consequences of this possibility were investigated quantitatively by Bernal and Fowler and Bell, while Bawn and Ogden examined the H/D kinetic isotope effect that would arise, albeit on the basis of a primitive model, in electrochemical proton discharge and transfer in the h.e.r. 

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 ... 

These descriptions about the rate of the mechanism of the hydrogen evolution reaction and how the mechanism changes with increase of M-H are rather simple, but they are consistent with the observations and the proposed change of r.d.s. can be regarded as probable. In fact, over the 78 years that have passed, it is still current thinking. 

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]. 

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