Hydrogen electrode process

The determination of the stoichiometric number was made by Horiuti and Ikushiraa (102) for the hydrogen electrode process on platinum and more recently by Horiuti and Enomoto (103) for the ammonia synthesis... 

For the hydrogen electrode process, H2 = 2 H+ -f- 2e, particular emphasis was laid by the former workers on the point that either of the following two mechanisms is fitting ... 

The Hydrogen Electrode Process. There are three oxidation states of hydrogen, as shown in Table 4.1.4. The hydrogen-water couple in solution at pH = 0, in contact with hydrogen gas at a partial pressure of 1 atm at a given temperature, is called the SHE. The electrode reaction is given by Eq. (36) ... 

Aluminum is preferentially leached out from NiaAls and NisAl by hot caustic solutions. The mixture of these compounds, applied by the plasma spraying technique, was found to be active for the hydrogen electrode process, the best results being observed at Ni/Al = 65/35 [33,34]. 

It is clear from Table 14.6 that the reversible potential for the hydrogen electrode process (reaction 6) is more positive than that for the iron electrode process. Therefore, the overall reaction (reaction 4) will proceed on the surface of iron immersed in dilute HCl because of the driving force arising from the potential difference between the two electrode processes. Since the two reactions proceed on the same location with opposing charge transfer, both electrodes are polarized in opposite directions to reach... 

The measurement of a from the experimental slope of the Tafel equation may help to decide between rate-determining steps in an electrode process. Thus in the reduction water to evolve H2 gas, if the slow step is the reaction of with the metal M to form surface hydrogen atoms, M—H, a is expected to be about If, on the other hand, the slow step is the surface combination of two hydrogen atoms to form H2, a second-order process, then a should be 2 (see Ref. 150). 

Although Table 2.16 shows which metal of a couple will be the anode and will thus corrode more rapidly, little information regarding the corrosion current, and hence the corrosion rate, can be obtained from the e.m.f. of the cell. The kinetics of the corrosion reaction will be determined by the rates of the electrode processes and the corrosion rates of the anode of the couple will depend on the rate of reduction of hydrogen ions or dissolved oxygen at the cathode metal (Section 1.4). 

Participation in the electrode reactions The electrode reactions of corrosion involve the formation of adsorbed intermediate species with surface metal atoms, e.g. adsorbed hydrogen atoms in the hydrogen evolution reaction adsorbed (FeOH) in the anodic dissolution of iron . The presence of adsorbed inhibitors will interfere with the formation of these adsorbed intermediates, but the electrode processes may then proceed by alternative paths through intermediates containing the inhibitor. In these processes the inhibitor species act in a catalytic manner and remain unchanged. Such participation by the inhibitor is generally characterised by a change in the Tafel slope observed for the process. Studies of the anodic dissolution of iron in the presence of some inhibitors, e.g. halide ions , aniline and its derivatives , the benzoate ion and the furoate ion , have indicated that the adsorbed inhibitor I participates in the reaction, probably in the form of a complex of the type (Fe-/), or (Fe-OH-/), . The dissolution reaction proceeds less readily via the adsorbed inhibitor complexes than via (Fe-OH),js, and so anodic dissolution is inhibited and an increase in Tafel slope is observed for the reaction. 

These primary electrochemical steps may take place at values of potential below the eqnilibrinm potential of the basic reaction. Thns, in a solntion not yet satnrated with dissolved hydrogen, hydrogen molecnles can form even at potentials more positive than the eqnilibrinm potential of the hydrogen electrode at 1 atm of hydrogen pressnre. Becanse of their energy of chemical interaction with the snbstrate, metal adatoms can be prodnced cathodically even at potentials more positive than the eqnilibrinm potential of a given metal-electrolyte system. This process is called the underpotential deposition of metals. 

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. 

Considerable practical importance attaches to the fact that the data in Table 6.11 refer to electrode potentials which are thermodynamically reversible. There are electrode processes which are highly irreversible so that the order of ionic displacement indicated by the electromotive series becomes distorted. One condition under which this situation arises is when the dissolving metal passes into the solution as a complex anion, which dissociates to a very small extent and maintains a very low concentration of metallic cations in the solution. This mechanism explains why copper metal dissolves in potassium cyanide solution with the evolution of hydrogen. The copper in the solution is present almost entirely as cuprocyanide anions [Cu(CN)4]3, the dissociation of which by the process... 

In electrode processes, the overall ( brutto ) reaction must be distinguished from the actual mechanism of the electrode process. For example, by a cathodic reaction at a number of metal electrodes, molecular hydrogen is formed, leading to the overall reaction... 

The basic characteristics of electrocatalysis will be demonstrated on several examples, in the first place on the electrode processes of hydrogen,... 

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. 

Electrode processes can be retarded (i.e. their overpotential is increased) by the adsorption of the components of the electrolysed solution, of the products of the actual electrode reaction and of other substances formed at the electrode. Figure 5.43 depicts the effect of the adsorption of methanol on the adsorption of hydrogen at a platinum electrode (see page 353). 

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