Energy Profile of Hydrogen Evolution on Metals with Consideration of Their Surface Energy

It has been established that there is a quantitative relationship between the experimental value of hydrogen overvoltage, a=eta H-2 , in the Tafel equation and the surface energy, Delta U-S , of metal with fcc and hcp structure, which is calculated theoretically in metal surface physics. An equation of slow discharge of hydrogen ions on metal is deduced, in which the a value is a linear function of the value Delta U-S/alpha F, where Delta U-S is the minimal surface energy of the specified metal, alpha = 0.5 is the transition coefficient, and F is the Faraday number. A prevailing tendency toward a decrease in hydrogen overvoltage on polycrystalline surfaces of metals with fcc, hcp, and bcc lattices with an increase in the minimal surface energy related to the appropriate (111), (0001), and (110) facets of these lattices has been observed. Results of analysis of the relationships between and the surface energy of metal are interpreted within the theory of active centers for heterogeneous catalysis with involvement of a thermodynamic model of the metal surface enriched with atom vacancies.