title: Pronounced Size Dependence in Structure and Morphology of Gas-Phase Produced, Partially Oxidized Cobalt Nanoparticles under Catalytic Reaction Conditions.
abstract: It is generally accepted that optimal particle sizes are key for efficient nanocatalysis. Much less attention is paid to the role of morphology and atomic arrangement during catalytic reactions. Here, we unravel the structural, stoichiometric, and morphological evolution of gas-phase produced and partially oxidized cobalt nanoparticles in a broad size range. Particles with diameters between 1.4 and 22 nm generated in cluster sources are size selected and deposited on amorphous alumina (Al2O3) and ultrananocrystalline diamond (UNCD) films. A combination of different techniques is employed to monitor particle properties at the stages of production, exposure to ambient conditions, and catalytic reaction, in this case, the oxidative dehydrogenation of cyclohexane at elevated temperatures. A pronounced size dependence is found, naturally classifying the particles into three size regimes. While small and intermediate clusters essentially retain their compact morphology, large particles transform into hollow spheres due to the nanoscale Kirkendall effect. Depending on the substrate, an isotropic (Al2O3) or anisotropic (UNCD) Kirkendall effect is observed. The latter results in dramatic lateral size changes. Our results shed light on the interplay between chemical reactions and the catalyst's structure and provide an approach to tailor the cobalt oxide phase composition required for specific catalytic schemes.
authors.affiliation: †Institut für Physik, Universität Rostock, Universitätsplatz 3, D-18051 Rostock, Germany.
authors.forename: Stephan
authors.initials: S
authors.lastname: Bartling
authors.forename: Chunrong
authors.initials: C
authors.lastname: Yin
authors.affiliation: †Institut für Physik, Universität Rostock, Universitätsplatz 3, D-18051 Rostock, Germany.
authors.forename: Ingo
authors.initials: I
authors.lastname: Barke
authors.affiliation: †Institut für Physik, Universität Rostock, Universitätsplatz 3, D-18051 Rostock, Germany.
authors.forename: Kevin
authors.initials: K
authors.lastname: Oldenburg
authors.affiliation: †Institut für Physik, Universität Rostock, Universitätsplatz 3, D-18051 Rostock, Germany.
authors.forename: Hannes
authors.initials: H
authors.lastname: Hartmann
authors.affiliation: †Institut für Physik, Universität Rostock, Universitätsplatz 3, D-18051 Rostock, Germany.
authors.forename: Viola
authors.initials: V
authors.lastname: von Oeynhausen
authors.affiliation: ¶Leibniz-Institut für Katalyse e.V. an der Universität Rostock (LIKAT), Albert-Einstein-Strasse 29a, D-18059 Rostock, Germany.
authors.forename: Marga-Martina
authors.initials: MM
authors.lastname: Pohl
authors.affiliation: §Laboratory of Solid-State Physics and Magnetism, KU Leuven, Celestijnenlaan 200d, Box 2414, 3001 Leuven, Belgium.
authors.forename: Kelly
authors.initials: K
authors.lastname: Houben
authors.forename: Eric C
authors.initials: EC
authors.lastname: Tyo
authors.forename: Sönke
authors.initials: S
authors.lastname: Seifert
authors.affiliation: §Laboratory of Solid-State Physics and Magnetism, KU Leuven, Celestijnenlaan 200d, Box 2414, 3001 Leuven, Belgium.
authors.forename: Peter
authors.initials: P
authors.lastname: Lievens
authors.affiliation: †Institut für Physik, Universität Rostock, Universitätsplatz 3, D-18051 Rostock, Germany.
authors.forename: Karl-Heinz
authors.initials: KH
authors.lastname: Meiwes-Broer
authors.affiliation: #Department of Chemical and Environmental Engineering, Yale University, 10 Hillhouse Avenue, New Haven, Connecticut 06520, United States.|@Institute for Molecular Engineering, The University of Chicago, 5801 South Ellis Avenue, Chicago, Illinois 60637, United States.
authors.forename: Stefan
authors.initials: S
authors.lastname: Vajda
keywords: Co clusters and nanoparticles; Kirkendall effect; cluster source; nanocatalysis; oxidation; oxidative dehydrogenation of cyclohexane; surface deposition