Mechanism hydrogen evolution

Krishtalik LI (1957) A contribution to the slow discharge theory, Zh Fiz Khim 31 2403-2413 (1959) Velocities of the elementary stages of the hydrogen evolution mechanism on the cathode I, Zh Fiz Khim 33 1715-1725... 

Cathodic reactions (1.58) and (1.59) can occur at potentials ( = -1.114 V and = -1.256 V respectively) which are more positive than reaction (1.53). When the cathodic polarization potential is not too negative, the cathodic hydrogen process may follow the MgH2-catalyzed hydrogen evolution mechanism in addition to the normal cathodic hydrogen evolution mechanism. However, four electrons are involved in reactions (1.58) and (1.59) and as such, the probability for these reactions is very low in practice. 

In theory, all three cathodic hydrogen evolution mechanisms, i.e., normal, Mg -catalyzed and MgH2-catalyzed processes which all have the same overall hydrogen reactions, can result in the Tafel behavior. However, the latter two processes are relatively unlikely to dominate the cathodic process, since they have equilibrium potentials much more negative than the normal hydrogen equilibrium potential and since the four electron reactions (1.58) and (1.59) very rarely occur. Therefore, the most practical hydrogen evolution mechanism should be the normal hydrogen evolution process. 

Hydrogen Evolution Mechanism at the B-doped Diamond Electrode... 

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...

The (photo)electrochemical behavior of p-InSe single-crystal vdW surface was studied in 0.5 M H2SO4 and 1.0 M NaOH solutions, in relation to the effect of surface steps on the crystal [183]. The pH-potential diagram was constructed, in order to examine the thermodynamic stability of the InSe crystals (Fig. 5.12). The mechanism of photoelectrochemical hydrogen evolution in 0.5 M H2SO4 and the effect of Pt modification were discussed. A several hundred mV anodic shift of the photocurrent onset potential was observed by depositing Pt on the semiconductor electrode. 

The mechanism of anodic chlorine evolution has been studied by many scientists. In many respects this reaction is reminiscent of hydrogen evolution. The analogous pathways are possible. The most probable one is the second pathway, in which the adsorbed chlorine atoms produced are eliminated by electrochemical desorption, but sometimes the first pathway is also possible. As a rule the first step, which is discharge of the chloride ion, is the slow step. 

In contrast to metal ion discharge, hydrogen evolution according to reaction (15.4) causes a pH increase of the solution layer next to the cathode. At a certain value of pH in this layer, hydroxides or basic salts of the metal start to precipitate, which affects the mechanism of further metaf deposition and also the structure and properties of the deposit produced. 

A nonuniform distribution of the reactions may arise when the metal s surface is inhomogeneous, particularly when it contains inclusions of other metals. In many cases (e.g., zinc with iron inclusions), the polarization of hydrogen evolution is much lower at the inclusions than at the base metal hence, hydrogen evolution at the inclusions will be faster (Fig. 22.3). Accordingly, the rate of the coupled anodic reaction (dissolution of the base metal) will also be faster. The electrode s OCP will become more positive under these conditions. At such surfaces, the cathodic reaction is concentrated at the inclusions, while the anodic reaction occurs at the base metal. This mechanism is reminiscent of the operation of shorted galvanic couples with spatially separated reactions Metal dissolves from one electrode hydrogen evolves at the other. Hence, such inclusions have been named local cells or microcells. 

Until the advent of modem physical methods for surface studies and computer control of experiments, our knowledge of electrode processes was derived mostly from electrochemical measurements (Chapter 12). By clever use of these measurements, together with electrocapillary studies, it was possible to derive considerable information on processes in the inner Helmholtz plane. Other important tools were the use of radioactive isotopes to study adsorption processes and the derivation of mechanisms for hydrogen evolution from isotope separation factors. Early on, extensive use was made of optical microscopy and X-ray diffraction (XRD) in the study of electrocrystallization of metals. In the past 30 years enormous progress has been made in the development and application of new physical methods for study of electrode processes at the molecular and atomic level. 

Hydrogen evolution at metal electrodes is one of the most important electrochemical processes. The mechanisms of the overall reaction depend on the nature of the electrode and solution. However, all of them involve the transfer of proton from a donor molecule in the solution to the adsorbed state on the electrode surface as the first step. The mechanism of the elementary act of proton transfer from the hydroxonium ion to the adsorbed state on the metal surface is discussed in this section. 

Bockris, J. O M. and Potter, E. C. (1952) The mechanism of hydrogen evolution at nickel cathodes in aqueous solutions. 

Solid metal electrodes are usually polished mechanically and are sometimes etched with nitric acid or aqua regia. Purification of platinum group metal electrodes is effectively achieved also by means of high-frequency plasma treatment. However, electrochemical preparation of the electrode immediately prior to the measurement is generally most effective. The simplest procedure is to polarize the electrode with a series of cyclic voltammetric pulses in the potential range from the formation of the oxide layer (or from the evolution of molecular oxygen) to the potential of hydrogen evolution (Fig. 5.18F). 

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)... 

Feldstein and Lancsek [30] measured plating rate, potential, and hydrogen evolution rate during the reduction of Ni2 + and Co2 + with H2P02 in the presence of various additives. They concluded that the deposition process could be described by a modified hydride mechanism. The basic steps of the process were identified as follows ... 

The stereochemistry of electrochemical reduction of acetylenes is highly dependent upon the experimental conditions under which the electrolysis is carried out. Campbell and Young found many years ago that reduction of acetylenes in alcoholic sulfuric acid at a spongy nickel cathode produces cis-olefins in good yields 126>. It is very likely that this reduction involves a mechanism akin to catalytic hydrogenation, since the reduction does not take place at all at cathode substances, such as mercury, which are known to be poor hydrogenation catalysts. The reduction also probably involves the adsorbed acetylene as an intermediate, since olefins are not reduced at all under these conditions and since hydrogen evolution does not occur at the cathode until reduction of the acetylene is complete. Acetylenes may also be reduced to cis olefins in acidic media at a silver-palladium alloy cathode, 27>. 

The reaction is carried out in a 500-ml. three-necked flask equipped with a reflux condenser, mechanical stirrer, heating mantle, and nitrogen inlet. The equipment is similar to that pictured in Fig. 11, except that an addition funnel is not required. In the reaction flask 20 g. (0.36 mol, 100% excess) of potassium hydroxide is dissolved in 300 ml. of absolute ethanol. The spare neck is closed with a ground-glass stopper, and the solution is stirred until it reaches room temperature. Addition of the carborane to the warm basic solution may result in an initial vigorous reaction. To this solution is added 30.0 g. (0.175 mol) of solid dimethylcarborane. The solution is stirred for one hour at room temperature and is then heated at the reflux temperature for 14 hours or until hydrogen evolution has stopped. 

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). 

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