A Scale of Absolute Surface Potentials of Metals. Part I

Abstract

The problem of the absolute electrode potentials of metals is considered as a problem that encompasses both the thermodynamics of electrode reactions on metals and the surface state of a metal. Within the framework of the physical chemistry of the surface, an adsorption model of the surface layer (SL) of a metal is developed, differing by using the value of the excessive Gibbs surface energy \(\Delta {{G}_{{\text{S}}}}.\) For the low-index facet (hkl) of the metal, the magnitude of the absolute surface potential is introduced as E_s = \({{\Delta {{G}_{{\text{S}}}}} \mathord{\left/ {\vphantom {{\Delta {{G}_{{\text{S}}}}} {zF}}} \right. \kern-0em} {zF}}\) (z is the valence of metal), which, like the magnitude, depends on the electrode potential. On the other hand, a statistical model is used that relates the autoadsorption of point defects, the surface atomic vacancies V_(S) in SL, and adatoms with the surface energy as \(\Delta {{G}_{{\text{S}}}}\) = –RTlnN_V(S)= –RTlnN_ad. For an uncharged metal, the uncharged metal value \(\Delta G_{{_{{\text{S}}}}}^{0}\) is maximum, which gives a minimum of the mole fraction of N_ad = N_V(S) and leads to a zero charge potential formula as \(E_{N}^{0}\) = –\({{\Delta G_{{\text{S}}}^{0}} \mathord{\left/ {\vphantom {{\Delta G_{{\text{S}}}^{0}} {zF}}} \right. \kern-0em} {zF}}\). The ideal polarization of the electrode relative to E_N values of \(\Delta G_{{\text{S}}}^{{}}\) and G_adsto zero that corresponds to the maximum of N_V(S) or N_ad. These extreme points of the surface activity of atoms determine the scale of absolute values of E_s calculated using known table values \(\Delta {{G}_{{\text{S}}}}\)(hkl) at T = 0 K obtained by the DFT method. When assessing the effect of temperature or potential, the change in the ΔG_S and N_V(S) (or N_ad) values in thermal and electrochemical processes is considered. In Part 2, an application of this scale to the processes of evolution of hydrogen and passivation of metals is considered.