Forms of chemisorbed oxygen on Ag(111): NDDO/MC semiempirical investigation

The atomic and molecular adsorption of oxygen on Ag(lll) is calculated by t he NDDO/MC semiempirical method. The Ag-38 cluster with a rhombic 16-atomic upper (active) face is employed. To model the metallic properties of silve r, we used the self-consistent procedure of equalization of the HOMO of the cluster to the experimental Fermi level. Two surface and three subsurface stable sites of atomic adsorption of oxygen are found. According to the res ults of calculations, molecular adsorption of oxygen is energetically unfav orable but corresponds to the local minimum on the potential energy surface . The role of subsurface oxygen in the activation of the oxygen atom on the surface is discussed.