title: Comparison of the reactivity of different Pd-O species in CO oxidation.
abstract: The reactivity of several Pd-O species toward CO oxidation was compared experimentally, making use of chemically, structurally and morphologically different model systems such as single-crystalline Pd(111) covered by adsorbed oxygen or a Pd(5)O(4) surface oxide layer, an oriented Pd(111) thin film on NiAl oxidized toward PdO(x) suboxide and silica-supported uniform Pd nanoparticles oxidized to PdO. The oxygen reactivity decreased with increasing oxidation state: O(ad) on metallic Pd(111) exhibited the highest reactivity and could be reduced within a few minutes already at 223 K, using low CO beam fluxes around 0.02 ML s(-1). The Pd(5)O(4) surface oxide on Pd(111) could be reacted by CO at a comparable rate above 330 K using the same low CO beam flux. The more deeply oxidized Pd(111) thin film supported on NiAl was already much less reactive, and reduction in 10(-6) mbar CO at T > 500 K led only to partial reduction toward PdO(x) suboxide, and the metallic state of Pd could not be re-established under these conditions. The fully oxidized PdO nanoparticles required even rougher reaction conditions such as 10 mbar CO for 15 min at 523 K in order to re-establish the metallic state. As a general explanation for the observed activity trends we propose kinetic long-range transport limitations for the formation of an extended, crystalline metal phase. These mass-transport limitations are not involved in the reduction of O(ad), and less demanding in case of the 2-D Pd(5)O(4) surface oxide conversion back to metallic Pd(111). They presumably become rate-limiting in the complex separation process from an extended 3-D bulk oxide state toward a well ordered 3-D metallic phase.
authors.affiliation: Institut für Physikalische Chemie, Universität Innsbruck, A-6020 Innsbruck, Austria.
authors.forename: Harald
authors.initials: H
authors.lastname: Gabasch
authors.forename: Axel
authors.initials: A
authors.lastname: Knop-Gericke
authors.forename: Robert
authors.initials: R
authors.lastname: Schlögl
authors.forename: Marta
authors.initials: M
authors.lastname: Borasio
authors.forename: Christian
authors.initials: C
authors.lastname: Weilach
authors.forename: Günther
authors.initials: G
authors.lastname: Rupprechter
authors.forename: Simon
authors.initials: S
authors.lastname: Penner
authors.forename: Bernd
authors.initials: B
authors.lastname: Jenewein
authors.forename: Konrad
authors.initials: K
authors.lastname: Hayek
authors.forename: Bernhard
authors.initials: B
authors.lastname: Klötzer
mesh_terms: D002248:Carbon Monoxide / Q000737:chemistry*; D002384:Catalysis; D003198:Computer Simulation; D007700:Kinetics; D008956:Models, Chemical; D008958:Models, Molecular; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D010165:Palladium / Q000737:chemistry*
chemical_list: D010165:Palladium; D002248:Carbon Monoxide; D010100:Oxygen