Electronic structure calculations of polyatomic oxyanions adsorbed on metal surfaces.

P. PAREDES OLIVERA; E. M. PATRITO; HARRELL SELLERS; ANDRZEJ WIECKOWSKI

Intefacial electrochemistry: theory, experiment and applications

Marcel Dekker

Lugar: New York; Año: 1999; p. 63 - 81

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 thanvia only one. For bisulfate and sulfuric acid we obtained binding energiesof 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 H2SO4, HSO4- and SO42- and the square of their effective chargeon the surface. This behavior is characteristic of image charge interactions between ions and metal surfaces. The calculated binding energies for theanions allowed us to establish appropriate thermodynamic cycles which showthat 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 between5 and 10 kcal/mol.