title: Oxidation state and symmetry of magnesia-supported Pd13O(x) nanocatalysts influence activation barriers of CO oxidation.
abstract: Combining temperature-programmed reaction measurements, isotopic labeling experiments, and first-principles spin density functional theory, the dependence of the reaction temperature of catalyzed carbon monoxide oxidation on the oxidation state of Pd(13) clusters deposited on MgO surfaces grown on Mo(100) is explored. It is shown that molecular oxygen dissociates easily on the supported Pd(13) cluster, leading to facile partial oxidation to form Pd(13)O(4) clusters with C(4v) symmetry. Increasing the oxidation temperature to 370 K results in nonsymmetric Pd(13)O(6) clusters. The higher symmetry, partially oxidized cluster is characterized by a relatively high activation energy for catalyzed combustion of the first CO molecule via a reaction of an adsorbed CO molecule with one of the oxygen atoms of the Pd(13)O(4) cluster. Subsequent reactions on the resulting lower-symmetry Pd(13)O(x) (x < 4) clusters entail lower activation energies. The nonsymmetric Pd(13)O(6) clusters show lower temperature-catalyzed CO combustion, already starting at cryogenic temperature.
authors.affiliation: Freiburg Materials Research Center, University of Freiburg, Stefan-Meier-Strasse 21, 79104 Freiburg, Germany. michael.moseler@iwm.fraunhofer.de
authors.forename: Michael
authors.initials: M
authors.lastname: Moseler
authors.forename: Michael
authors.initials: M
authors.lastname: Walter
authors.forename: Bokwon
authors.initials: B
authors.lastname: Yoon
authors.forename: Uzi
authors.initials: U
authors.lastname: Landman
authors.forename: Vahideh
authors.initials: V
authors.lastname: Habibpour
authors.forename: Chris
authors.initials: C
authors.lastname: Harding
authors.forename: Sebastian
authors.initials: S
authors.lastname: Kunz
authors.forename: Ueli
authors.initials: U
authors.lastname: Heiz