Adsorption of sulfate, bisulfate and sulfuric acid on silver surfaces: a theoretical study

Abstract

The adsorption of sulfate, bisulfate and sulfuric acid is investigated on the 111, 110 and 100 surfaces of silver by means of ab initio calculations performed at the Hartree Fock+second order perturbation level of theory. We consider the following aspects of the adsorption process: binding energies on different surfaces and coordination sites, charge transfer and adsorbate relaxation. The adsorption of sulfate is studied in more detail on the three metal surfaces. Sulfate binding energies in the range 110–180 kcal/mol are calculated for the different surfaces and coordinations investigated. The highest binding energies were obtained for the more open 110 surface. On a given surface, coordination via three oxygen atoms is more stable than via only one. For bisulfate and sulfuric acid we obtained binding energies of 58 and 27 kcal/mol on the Ag(111) and Ag(100) surfaces, respectively. The charge transferred to the metal is 0.2 electrons for bisulfate and 0.4–0.6 electrons for sulfate. A linear relation was observed between the binding energy of H 2SO 4, HSO − 4 and SO 2− 4 and the square of their effective charge on the surface. This behavior is characteristic of image charge interactions between ions and metal surfaces. The calculated binding energies for the anions allowed us to establish appropriate thermodynamic cycles which show that the acidity of bisulfate is enhanced when adsorbed on the surface. When sulfate and bisulfate are allowed to relax on the surface, the geometry of the ions changes according to the conservation of bond order on metal surfaces: while the bonds between coordinated oxygens and the metal surface strengthen, the bonds between these oxygens and sulfur weaken. The contribution of the relaxation energy to the binding energy of the anions is between 5 and 10 kcal/mol.