Hydrogen Evolution at Certain Liquid Alloys

It has been mentioned above that the isotope effect is quite sensitive to a change in the proton tunneling probability. This allows us to use this effect for analyzing the cases where it is not clear whether the observed effect is caused by a change in the tunneling probability or by some other phenomena. 

During an investigation of the temperature dependence of the rate of hydrogen evolution at a liquid mercury-gallium alloy (containing 2.1 at. % Hg), very high values of the preexponential factor were observed in acidic as well as alkaline solutions. At least in acidic solutions, the experimental value (log k = 7.6) exceeded any reasonable theoretical estimates of the upper limits of the preexponential factor by several orders of magnitude[262]. Hence, it was naturally assumed that the experimental value was considerably distorted due to the dependence of the composition of the alloy surface on temperature. This assumption was verified with the help of the kinetic isotope effect. 

The high value of the preexponential factor may be explained by a considerable change in the properties of the surface layer with temperature. An alloy containing 2.1 at.% Hg is nearly saturated at room temperature, while at a temperature of about 80°C, the solubility of mercury increases about 1.5 times[403]. Consequently, at increased temperatures, the alloy is far from saturation. We can therefore expect a lower surface concentration for mercury and higher for gallium. The enrichment of the surface by a metal with a lower overpotential increases (in absolute magnitude) the temperature coefficient of the overpotential, i.e. increases the activation energy and the preexponential factor. 

It is interesting to compare these data with the data for thallium amalgam, for which the change in solubility of T1 with temperature is small[403], and hence we can expect that the distortions in the temperature dependence are small. Indeed, it was described in section 4.4 that the preexponential factor and the isotope effect for such a cathode are in good agreement with each other and with the results for several pure metals. 

So far, it has not been possible to provide a proper explanation for the appearance of a maximum on the curve representing the dependence of S on the alloy composition. This phenomenon can be explained tentatively as follows. 

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