Da. Huckaby et al., CLUSTER VARIATION STUDY OF THE UNDERPOTENTIAL DEPOSITION OF CU ON AU(111) IN THE PRESENCE OF BISULFATE, The Journal of chemical physics, 109(9), 1998, pp. 3600-3606
A cluster variation method is developed to study the phase transitions
and the structures of phases which occur at the fluid-crystal interfa
ce during the underpotential deposition of a metal on an electrode in
the presence of an anion, such as bisulfate. In addition to the possib
ility of first-order condensation phase transitions occurring during t
he deposition of a metal, the steric repulsion of adsorbed anions can
also cause an order-disorder transition. Using clusters containing six
adsorption sites, the method is applied to the underpotential deposit
ion of copper on (111) gold in the presence of bisulfate. In order to
fix a constant in the expression for the entropy, the effect of the ha
rd-core exclusion of a pair of first-neighbor bisulfates, in addition
to the effect of finite interactions, is calculated exactly in the lim
it of high temperature. The cluster method yields two coupled adsorpti
on isotherms for copper and bisulfate in terms of their activities and
coverages. The resulting isotherms show an order-disorder transition
due to the hard-core exclusion of neighboring bisulfate ions, as well
as two first-order phase transitions in the copper and bisulfate cover
ages which correspond to the two spikes in the experimental voltammogr
am. The cluster method also gives the local structure of the phases wh
ich occur as the voltage is changed. (C) 1998 American Institute of Ph
ysics.