Viola von Oeynhausen

Publications (3)8.06 Total impact

• Article: Surface photovoltage of Ag nanoparticles and Au chains on Si(111)

  Kristian Sell, Ingo Barke, Stefan Polei, Christian Schumann, Viola von Oeynhausen, Karl‐Heinz Meiwes‐Broer
  physica status solidi (b) 04/2010; 247(5):1087 - 1094. · 1.32 Impact Factor
• Article: Combined temperature-programmed reaction and in situ x-ray scattering studies of size-selected silver clusters under realistic reaction conditions in the epoxidation of propene

  Stefan Vajda, Sungsik Lee, Kristian Sell, Ingo Barke, Armin Kleibert, Viola von Oeynhausen, Karl-Heinz Meiwes-Broer, Arantxa Fraile Rodríguez, Jeffrey W. Elam, Michael M. Pellin, Byeongdu Lee, Sönke Seifert, Randall E. Winans
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  ABSTRACT: The catalytic activity and dynamical shape changes in size-selected nanoclusters at work are studied under realistic reaction conditions by using a combination of simultaneous temperature-programmed reaction with in situ grazing-incidence small angle x-ray scattering. This approach allows drawing a direct correlation between nanocatalyst size, composition, shape, and its function under realistic reaction conditions for the first time. The approach is illustrated in a chemical industry highly relevant selective partial oxidation of propene on a monodisperse silver nanocatalyst. The shape of the catalyst undergoes rapid change already at room temperature upon the exposure to the reactants, followed by a complex evolution of shape with increasing temperature. Acrolein formation is observed around 50 °C while the formation of the propylene oxide exhibits a sharp onset at 80 °C and is leveling off at 150 °C. At lower temperatures acrolein is produced preferentially to propylene oxide; at temperatures above 100 °C propylene oxide is favored.
  The Journal of Chemical Physics 09/2009; 131(12):121104-121104-4. · 3.33 Impact Factor
• Source

  Available from: Alessandro Fortunelli

  Article: Size-dependent selectivity and activity of silver nanoclusters in the partial oxidation of propylene to propylene oxide and acrolein: A joint experimental and theoretical study

  Luis M. Molina, Sungsik Lee, Kristian Sell, Giovanni Barcaro, Alessandro Fortunelli, Byeongdu Lee, Soenke Seifert, Randall E. Winans, Jeffrey W. Elam, Michael J. Pellin, [......], Viola von Oeynhausen, Yu Lei, Randall J. Meyer, Julio A. Alonso, Arantxa Fraile Rodríguez, Armin Kleibert, Suzanne Giorgio, Claude R. Henry, Karl-Heinz Meiwes-Broer, Stefan Vajda
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  ABSTRACT: Model silver nanocatalysts between 9 and 23 nm in size were prepared by size-selected cluster deposition from a free cluster beam on amorphous alumina films and their size-dependent catalytic performance studied in the partial oxidation of propylene under realistic reaction conditions. Smaller clusters preferentially produced acrolein, while the 23 nm particles were considerably more selective towards the formation of propylene oxide, at reaction rates far exceeding those previously reported for larger silver particles. The activity of clusters dropped significantly with increasing particle size. First-principle calculations, of the activation energies for oxygen adsorption and its dissociation, at variable surface coverage yielded surface energies which resulted in particle shapes resembling the experimentally observed shapes of partially oxidized silver clusters. The calculated activation barriers for propylene oxide and acrolein formation on various facets and on the edges of the nanoparticles provided detailed information about the energetics of the competing reaction pathways. The size- and corresponding morphology dependent theoretical activity and selectivity are in good accord with experimental observations.
  Catalysis Today 160(1):116-130. · 3.41 Impact Factor