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 O-2 on a (101)-oriented PdO thin film grown on Pd(111) in ultrahigh vacuum (UHV), using an oxygen atom beam. Our results show that the molecular chemisorption of O-2 is facile on the PdO film at 85 K, producing a saturation O-2 coverage of 0.27 ML (monolayers). Experiments with co-adsorbed O-16(2) and O-18(2) further reveal that molecularly chemisorbed O-2 dissociates negligibly on the PdO(101) surface under the conditions examined. The O-2 TPD, spectrum from the PdO(101) surface at O-2 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 O-2. Comparison with O-2 TPD obtained from clean Pd(111) demonstrates that a large fraction of the O-2 molecules on the PdO(101) surface are more strongly bound than O-2 chemisorbed on the metallic surface at 85 K and saturation of the respective O-2 layers. We find that O-2 molecules chemisorb only in small quantities (< 0.03 ML) on the p(2 x 2) and two-dimensional Pd5O4 phases of atomic oxygen on Pd(111), indicating that these phases have much weaker binding affinities toward O-2 than the PdO(101) surface generated in our experiments. Finally, temperature programmed reaction spectra with co-adsorbed O-18(2) and CO demonstrate that both PdO and molecularly adsorbed O-2 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 O-2 on PdO and observe that these molecularly adsorbed species are active in CO oxidation.