EFFECTS OF CHEMISORBED AND SUBSTITUTIONAL 0, I, AND II-GE, SN, AND PB ON CO ADSORPTION ON PT(111) - MOLECULAR-ORBITAL THEORY

Methanol oxidation rates on Pt anodes are increased by surface Sn adatoms apparently because of faster oxidation of CO, a surface poison, to CO2. In an ASED-MO study using cluster models, Sn as well as Ge and Pb adatoms are found to reduce the chemisorption energy of CO at adjacent sites of Pt(111). Oxidation of these adatoms by OH only slightly reduces their effect on CO adsorption but oxidation by O nearly cancels it. When the atoms in their reduced and oxidized forms are placed substitutionally in the Pt surface they have very small effects on CO adsorption energies. The weakening caused by Sn adatoms stems in part from a CO4σ-Sn 5s closed-shell repulsion through the Pt surface atoms. The through-space repulsion is negligible. The weakening of the CO adsorption bond to Pt by Sn adatoms has been observed in recent high vacuum studies. During electrochemical oxidation of adsorbed CO this weakening may play a role, but other effects, such as the influence of Sn on the presence of active oxygen, need further examination. © 1990.