Photocurrent kinetics at electron emission from metal to electrolyte solution: Part V. Ionization rate constants of hydrogen and deuterium atoms adsorbed on mercury

Abstract

Absolute ionization rate constant values of hydrogen and deuterium atoms adsorbed on mercury were measured using the method of pulse photoelectronic emission from metal into solution. In accordance with the Tafel law, these constants decrease from 2.5×10 7s −1 to 9×10 5s −1 (Table 1) in a 1 M solution of KCl in the range −0.25 to −0.5 V SCE. The transfer coefficient is 0.33±0.03 and the isotope ratio about 2.5. Owing to specific anion adsorption, rate constants increase as their concentration increases and KBr is added to the solution. In 0.05 M solutions of HCl and H 2SO 4, transfer coefficients are 0.30±0.05. From a comparison of measured values, with the hydrogen ion discharge rate constants found by extrapolation of experimental values into the potential range mentioned (taking into account the transfer coefficient change), the change of the Gibbs free energy in the reaction H 3O ++ e Me −→H ads+H 2O was calculated and found to be 0.87–0.99 eV at the potential of the normal hydrogen electrode. Adsorption energy of the hydrogen atom from the gas phase on a mercury electrode is 1.55±0.10 eV. The volt-ampere dependence of the hydrogen ion discharge current in the range −0.25 to −2.25 V corresponding to the current change by 18 orders of magnitude, agrees with the theoretically determined values (maximum deviation in the current is less than a factor of 3) for the medium reorganization energy E r=1.75 eV. Despite constancy of the transfer coefficients of the elementary stages, in the range −0.5 V (SCE), the effective transfer coefficient of the total hydrogen evolution processes increases from 0.5 to 1.0, as the ionization rate of the adsorbed hydrogen atoms becomes greater than their electrochemical desorption rate.