Surface Science 421 (1999) 59–72 An embedded atom approach to underpotential deposition phenomena C.G. Sa ´ nchez, M.G. Del Po ´ polo, E.P.M. Leiva * Unidad de Matema ´ tica y Fı ´sica, Facultad de Ciencias Quı ´micas, Universidad Nacional do Co ´rdoba, Agencia Postal 4, C.C. 61, 5000 Cordoba, Argentina Received 5 August 1998; accepted for publication 8 October 1998 Abstract We have performed embedded atom calculations for a number of systems of electrochemical interest involving a metallic single- crystal substrate and a metallic adsorbate. Different thermodynamic contributions to the so-called underpotential shift are calculated and analyzed comparatively, drawing some general trends. The metal pairs considered involve silver, gold, platinum, palladium and copper. We consider the possibility of underpotential deposition through the excess of binding energy, arriving at two novel conclusions. First, for some systems consisting of metal M 1 and M 2 , underpotential deposition should be energetically possible in both cases, that is M 1 on M 2 and M 2 on M 1 . Second, anions may play a decisive role in changing the energetics of some systems. In particular cases like copper on Au(111) they may be responsible to a large extent for the existence of an underpotential deposition. Entropic contributions were neglected in the present analysis. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Adsorption; Embedded atom method; Monolayer; Underpotential deposition 1. Introduction was developing at that time. Pauling’s model of chemical binding for diatomic molecules, which should in principle account for covalent and ionic Since the very beginning of the recognition of underpotential deposition ( UPD), electrochemists contributions to the binding energy, could not account for the experimental results [3] and an have been trying to rationalize its occurrence in thermodynamic terms [1]. On the other hand, alternative model was formulated by Kolb et al. [3] in terms of the work functions of the metals considerations in purely geometrical terms like the lattice dimensions of deposited metal and substrate involved in the UPD process. According to this, the experimentally determined energy gain of the have also been made in an attempt to explain this phenomenon in terms of some simple experimental adsorbate with respect to its bulk state (usually denoted as the underpotential shift) should be parameters [2]. The status of this filed prior to the 1980s has been compiled by Kolb [3], with infor- linearly related to the work function difference between adsorbate and substrate. Furthermore, mation mainly related to polycrystalline surfaces, UPD should only occur when the work function since electrochemistry on single-crystal surafces of the bulk adsorbate metal is lower than that of the substrate. Similar predictions arose from a * Corresponding author. Fax: +54 51 334174; e-mail: eleiva@fcq.unc.edu.ar model for UPD developed by Trasatti from 0039-6028/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0039-6028(98)00818-8