Dipped adcluster model study for molecular and dissociative chemisorptions of O₂ on Ag surface

Chemisorption of an O₂ molecule on a Ag surface is studied theoretically with the use of the dipped adcluster model (DAM). Electron correlations in low-lying surface states and electron-transferred states from bulk metal, which are shown to be very important, are described by the symmetry adapted cluster (SAC)/SAC-configuration interaction (SAC-CI) method. Side-on geometries are used, different from the (bent) end-on geometries studied previously. Potential curves for the O₂ approaching and dissociating processes are investigated with the use of Ag₂O₂ and Ag₄O₂ adclusters. For the occurrence of chemisorption, the electron transfer from bulk metal to the adcluster, and the electrostatic image force are important, which cannot be treated by the conventional cluster model. Two different molecular adsorption states are obtained from the calculations for the Ag₂O₂ adcluster, namely Superoxide (O ₂^-) and peroxide (O₂^2-) species and the corresponding adsorption energies are calculated to be 5.5 and 17.8 kcal/mol, respectively, which compare well with the experimental value 9.2-9.3 kcal/mol. The O-O stretching frequencies of these species are in good agreement with the experimental values. From the calculations for the Ag₄O ₂ adcluster, the potential minima corresponding not only to the molecular adsorption, but also to the dissociative adsorption are obtained. The dissociative adsorption is shown to be led from the peroxide. The geometry of the dissociative O^- is at the bridge site on a silver surface and the calculated Ag-O bond distance of 2.16 Å agrees well with the experimental value 2.06-2.17 Å. The dissociative adsorption energy is estimated to be 44.0-61.4 kcal/mol, which is compared with the experimental value 40.8-44.0 kcal/mol.