A general statistical mechnical model of the adsorption of ions at the
electrode interface is presented. The three-dimensional adsorption mo
del is equivalent to a two-dimensional lattice gas. The adsorption iso
therm for a single ion is given in terms of Fade approximants for the
high and low fugacity limits. This formalism is applied to the case of
the underpotential deposition of copper onto gold(lll). The spikes in
the voltammogram of this deposition are viewed as resulting from a fi
rst order phase transition on a honeycomb lattice followed by a first
order phase transition on a triangular lattice. This model is extended
by incorporating the coadsorption of(bi)sulfate with copper and by co
nsidering longer range interactions between adatoms using the mean nei
ghbor approximation. The model voltammograms agree quite well with the
experimental ones. Copyright (C) 1996 Elsevier Science