Oxide surface, size-selected clusters

With the advent of synthetic methods to produce more advanced model systems (cluster- or nanoparticle-based systems either in the gas phase or on planar surfaces), we come to the modern age of surface chemistry and heterogeneous catalysis. Castleman and coworkers demonstrate the large influence that charge, size, and composition of metal oxide clusters generated in the gas phase can have on the mechanism of a catalytic reaction. Rupprechter (Chap. 15) reports on the stmctural and catalytic properties of planar noble metal nanocrystals on thin oxide support films in vacuum and under high-pressure conditions. The theme of model systems of nanoparticles supported on planar metal oxide substrates is continued with a chapter on the formation of planar catalyst based on size-selected cluster deposition methods. In a second contribution from Rupprecther (Chap. 17), the complexities of surface chemistry and heterogeneous catalysis on metal oxide films and nanostructures, where the extension of the bulk structure to the surface often does not occur and the surface chemistry is often dominated by surface defects, are discussed. 

The examples presented above showed that size-selected clusters on surfaces reveal remarkable size-effects, however, they have been studied so far only by one-cycle experiments. Thus an experimental proof that these systems are active for catalytic processes, e.g. several cycles of a catalytic reaction are promoted without destruction of the catalyst, is still missing. In the following we present an experimental scheme to study catalytic processes of clusters on surfaces with high sensitivity and we report turn-over frequencies TOFs for the oxidation of CO on size-distributed supported Pd clusters. 

FTIR has been successfully applied for studying chemical properties of size-selected clusters on oxide surfaces [39,192,244,252]. In these experiments, the clusters were deposited onto thin oxide films grown on a metal single crystal. In this approach, the metal is used as the mirror in the optical setup the oxide films ought to be thinner than the wavelength of the IR radiation. In this way, vibrations of adsorbates on size-selected clusters with densities as low as 0.2% ML could be detected. Mechanistic information of chemical reactions... 

A new experimental setup has recently been designed to study the chemical properties of size-selected metal clusters deposited on oxide substrates [210,211], Pd clusters have been produced by a laser evaporation source, ionized, then guided by ion optics through differentially pumped vacuum chambers and size-selected by a quadrupole mass spectrometer [210-212], The monodispersed clusters have been deposited with low kinetic energy (0,l-2eV) onto an MgO thin-film surface. The clusters-assembled materials obtained in this way exhibit peculiar activity and selectivity in the polymerization of acetylene to form benzene and aliphatic hydrocarbons [224], Figure 6 shows the temperature-programmed reaction (TPR) spectra for the cyclotrimerization of acetylene on supported Pd (1 30)... 

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