Thermodynamic and ab initio calculations of properties of chemisorbed ions and application to adsorbed sulfur oxide anions

Abstract

We discuss thermodynamic conditions for the chemisorption of ions and show that, in the low coverage limit, chemisorbed cationic and anionic specie having the same stoichiometric formulas produce surface adsorbates that are virtually indistinguishable from each other and from the chemisorbed neutral species. This is also observed from high quality ab initio (RECP HF + correlation) calculations of small ions chemisorbed on silver surfaces including H +, H −, OH +, OH −, SO 2− 3 and SO 2− 4. The additional demand on the cluster model of the surface in ab initio calculations due to the charge of the ion is investigated. For ionic adsorbates bigger clusters are required than in the case of neutral adsorbates. The structure of adsorbed sulfate was optimized at the Hartree-Fock level and the geometry changes upon chemisorption were found to be consistent with the principle of bond order conservation. The nature of the chemisorbed ions is discussed together with the implications that this has in the modeling of surface reactions and, in particular, in the chemistry of sulfur oxides on metal surfaces.