(184h) Molecular Chemisorption of O2 on a Pdo(101) Thin Film on Pd(111)

We used temperature programmed desorption (TPD) to investigate the molecular chemisorption of O2 on a PdO(101) thin film grown on Pd(111) in ultrahigh vacuum using an oxygen atom beam. Our results show that the molecular chemisorption of O2 is facile on the PdO film at 85 K, producing a saturation O2 coverage of 0.27 ML (monolayers). Experiments with co-adsorbed 16O2 and 18O2 further reveal that molecularly chemisorbed O2 dissociates negligibly on the pristine PdO(101) surface under the conditions examined. The O2 TPD spectrum from the PdO(101) surface at O2 saturation exhibits two main features centered at 117 K and 227 K, as well as smaller features at 275 K and 315 K, associated with the desorption of molecularly chemisorbed O2. Comparison with O2 TPD obtained from clean Pd(111) demonstrates that a large fraction of the O2 molecules on the PdO(101) surface are more strongly bound than O2 chemisorbed on the metallic surface at 85 K and saturation of the respective O2 layers. We find that O2 molecules chemisorb only in small quantities (< 0.03 ML) on the p(2×2) and 2D Pd5O4 phases of atomic oxygen on Pd(111), indicating that these phases have much weaker binding affinities toward O2 than the PdO(101) surface generated in our experiments. Finally, temperature programmed reaction spectra with co-adsorbed 18O2 and CO demonstrate that both PdO and molecularly adsorbed O2 actively participate in the oxidation of CO, with the atomic and molecular species exhibiting similar activities for the conditions studied. The results of this study may have implications for understanding Pd oxidation catalysis at high pressures given that we find relatively strong binding states of O2 on PdO and observe that these molecularly adsorbed species are active in CO oxidation.