Hydrogen-evolution reaction on mercury

Fig. 7.73 The reaction order of the hydrogen-evolution reaction (on mercury) in respect to a hydrogen ion is equal to 1, as seen from the slope of the straight line.

The importance of double layer structure on electrode kinetics was first shown by Frumkin for the hydrogen evolution reaction on mercury [44]. As a result of the structure of the electrochemical interface, the pre-electrode plane, i.e. the plane where the reactant undergoes electron transfer to become product, is such that the concentration of the reactant ion is different from that in the bulk solution and the corresponding potential difference with respect to the solution, (less than the applied electrode—solution potential difference ([Pg.34]

We recall that the Tafel equation was originally observed for the hydrogen evolution reaction on mercury. We should therefore have an equation similar to Eq. 41E to describe the current-potential relationship. For a single cathodic step we wrote ... 

It cannot be overemphasized that one can measure only the transfer coefficient a, not the symmetry factor p. The latter can be inferred from the former by making a suitable set of assumptions. For example, for the hydrogen evolution reaction on mercury, it is commonly assumed that a = 6. One often tends to refer in this case to the measurement... 

For the hydrogen evolution reaction on platinum, for example, k 10 A/cm, but on mercury, io A/cm. It is, in fact, these values of io that allow us to use platinum as the electron collector for a reversible hydrogen electrode and prevent our using mercury for this purpose. 

Figure 5. Experimental dependence of islf rjc) on rj for the hydrogen evolution reaction at mercury in various acid solutions. , 1, lofa and Stepanova (l.OA/ HCl), recalculated x, 2, Bockris and Parsons (O.lAf HCl), recalculated o, 3, Post and Hiskey (0.1 A/ HCl), recalculated +, 4, Tsionskii and Kriksunov (0.3M HCl), recalculated A, 5, Dekker et (l.OM HCl), recalculated , 6, Potapova et alP (0.1 A/ HCIO4 + 0.9M NaC104), original data.

We have just mentioned that one reason for a limited range of potentials in a particular SSE is the reactivity of the components of the SSE toward oxidation and reduction. It is also obvious that the limiting cathodic process in protic solvents, nos 1-9 in Table 4, must be reduction of protons or the equivalent, the proton donor. The unfavourable cathodic limit for reduction of protons can, however, be vastly improved by the use of mercury as the cathode material and a tetraalkylammonium salt as SSE (nos. 1 and 3). The reason for mercury being such a favourable material is its large overpotential (see Section 10) for the reduction of protons (hydrogen evolution reaction). We have already commented (p. 24) on the fact that the reduction of protons occurs many orders of magnitude faster on certain metals than on others, and this manifests itself by the overpotential, i.e., in order to make the reaction go at a measurable rate one has to increase the electrode potential from the equilibrium potential. Table 6 shows overpotentials for hydrogen evolution and... 

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. 

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