Nanoassembled Model Catalysts: Changing Efficiency and Selectivity Atom-by-Atom
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Nanoassembled Model Catalysts: Changing Efficiency and Selectivity Atom-by-Atom U. Heiz1, S. Abbet1, H. Häkkinen2 and U. Landman2 1 University of Ulm, Institute of Surface Chemistry and Catalysis, D-89069 Ulm, Germany 2 Georgia Institute of Technology, School of Physics, Atlanta, Georgia 30332-0430 ABSTRACT Cluster-assembled materials open fascinating new routes for tuning physical and chemical properties by changing cluster size and often these materials behave completely differently than their bulk analogues. By depositing gas phase cluster cations on various surfaces, cluster model catalysts are fabricated, which exhibit remarkable catalytic activity. While inert as bulk material, gold cluster catalysts oxidize carbon monoxide with Au8 to be the smallest active size. In contrast to the high selectivity of Pd(111) surfaces for the cyclotrimerization of acetylene, small supported Pdn clusters reveal a strongly size-dependent selectivity for the polymerization of acetylene and catalyze the cyclotrimerization as well as the hydrogenation of the formed intermediate C4H4.
INTRODUCTION The preparation of a collection of supported metallic particles that are truly monodisperse, i.e. they all have exactly the same size, has long considered to be virtually impossible [1]. As a consequence it has been difficult to recognize size effects for small supported clusters, as size distributions have been broad. For clusters in the gas phase, however, strong size-dependent chemical properties have been discovered during the last two decades and the obtained results lead to new concepts for understanding their chemical properties [2-7]. Connections of these observations with real catalysis has often been stressed and suggestions for tuning efficiency and selectivity of a certain catalytic process by simply changing cluster size were made already in early days. Efficient and selective conversion is indeed important in catalysis, as most catalytic surfaces assist a variety of reactions. It is therefore of interest to study the factors affecting the size-dependent selective and efficient behavior of catalytic systems. We succeeded to prepare model catalysts consisting of a collection of metal particles of a single size, which are deposited on oxide supports and observed a strongly size-dependent efficiency and selectivity for the oxidation of CO on small gold clusters [8] and the polymerization of acetylene on supported palladium clusters [9], respectively.
EXPERIMENTAL The experimental setup has been described in detail elsewhere [10] and only a brief description is given here. The clusters are prepared in a high frequency laser evaporation source. The produced cluster ions are guided with ion optics through differentially pumped vacuum chambers. A quadrupole mass filter is used for the mass-selection. The mass-selected cluster beam is then softlanded on thin MgO(100) films under UHV conditions. The chemical reactivity P9.1.1
of the clusters is studied by Temperature Programmed Reaction (TPR) spectroscopy, where the desorbing prod
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