Binding and orientations of O 2 on Ag(100) and Pb/Ag(100) . Relationships to O 2 reduction by UPD lead on a silver electrode

Molecular orbital calculations using the atom superposition and electron delocalization method predict that O ₂ favors binding parallel to the Ag(100) surface on the high-coordinate 4-fold site with its axis oriented in such a way that the O-ends point toward the neighboring 2-fold surface sites. In this orientation, the filled O ₂ 5σ and π _u Orbitals donate to the Ag sd-hybrid orbitals and the surface back-donates to the half-filled O ₂ π _g orbitals. For the Pb-covered Ag(100) surface, the calculations show that the favored binding site and orientation for O ₂ depend on the adatom coverage. At low coverages (θ _pb≈ {1}/{4}) , O ₂ is predicted to bind perpendicular to the surface on the top of a Pb atom, whereas at higher coverages (θ _pb≳ {1}/{2}) , a lying-down O ₂ with its axis bridging two nearest-neighbor Pb atoms on the surface is favored. These results for O ₂ adsorption are related to the electrochemical results for O ₂ reduction on the bare and UPD Pb-covered silver electrodes.