DFT study of Cl/MgO(1 0 0), influence of the stoichiometry M. M en etrey, A. Markovits, C. Minot * Laboratoire de Chimie Th eorique, Universit e P. et M. Curie, 4 Place Jussieu case 137, 75252 Paris Cedex 05, France Available online 15 June 2004 Abstract We have performed DFT–GGA periodic calculations for Cl/MgO(1 0 0). Chlorine is an example of electron-acceptor radical adsorbate. The adsorption on a perfect stoichiometric surface (one atom per unit cell) takes place at the O site. It is weak and explained by a three-electron interaction, preserving the radical character. The adsorption on the oxygen- defective surface is much stronger, Cl being bound as Cl  to the Mg 2þ ions in the vacancy or at the surface. This occurs through a redox process (electron transfer from the reduced cations of the lattice to the Cl atoms). The ratio of two chlorine atoms per O vacancy is optimal and restores the energy gap of the perfect metal oxide. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Chlorine; Magnesium oxides; Chemisorption; Density functional calculations 1. Introduction Atomic chlorine is a common species found on metal-oxide surfaces. For instance, it is present on TiO 2 catalysts when synthesized by oxidizing TiCl 4 precursors. Cl atoms might be present as impurity or produced by the degradation of pollutants. It might as well be a by-product of the surface reaction when TiO 2 is used as a photocatalyst [1]. Chlorine also modifies the conductivity of the material [2]. It is important for the corrosion. Localized interaction of Cl  with the passivated surface (NiO surface) can cause corrosion (pitting) [3,4]. The mechanism involves the adsorption of Cl  on the passive film surface followed by the penetration of Cl  into the oxide layer [5,6]. The Cl atom is a radical; the adsorption of a single radical does not lead to electron pairs insuring for stoichiometric oxides an energy gap and a good stability. The electron count is thus more favorable to the adsorption of pairs of radi- cals (dimers or a pair of ions). The adsorption on non-stoichiometric oxides implying a redox mech- anism is another possibility. The Cl atom being electronegative, the electron transfer from the metal oxide should prevail contrary to NO adsorption [7]. The understanding of the influence of the number of electron monitoring the interaction during the adsorption is precisely the goal of this paper. MgO(1 0 0) is probably the simplest metal-oxide surface. The (1 0 0) surface is stable, non-polar, classified of type I according to Tasker [8]. The clean surface is easy to prepare with well defined stoichiometry [9] and does not undergo large relaxation. Mg 2þ , the metal cation, being very electropositive, MgO is an irreducible oxide. Oxygen vacancies leave electrons trapped in the * Corresponding author. Tel.: +33-0144272505; fax: +33- 0144274117. E-mail address: minot@lct.jussieu.fr (C. Minot). 0039-6028/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2004.06.001 Surface Science 566–568 (2004) 693–697 www.elsevier.com/locate/susc