Chemisorption and dissociation of O2, on Ag(110)

Using an Ag₂₄ cluster, we have calculated the potential energy surface of an O₂ molecule chemisorbing and dissociating on the Ag(110) surface, with its molecular axis parallel to the grooves in the 1̄10] direction. For molecular chemisorption, we find equilibrium positions and vibrational frequencies in good agreement with experiment and with earlier calculations; the chemisorption energy, ∼ 0.1 eV, may be somewhat too low. Although both our calculated and the experimental vibrational frequencies suggest a chemisorbed O²⁻₂ ion, our calculated charge transfer to the O₂ molecule is a little less than one electron. We can explain the calculated low OO strength frequency by changing occupations of OO bonding and antibonding states with changing OO separation. These results show that one has to be cautious in directly relating observed vibrational frequency lowerings of chemisorbed molecules to their charge states. The dissociation path is completely along the direction of the OO stretch; we find a barrier height of ∼ 0.2 eV. The atomic equilibrium distance, vibrational frequency, and binding energy are in reasonable agreement with experiment. At large OO separation, the energy still decreases with increasing OO distance, indicative of OO Coulomb repulsion.