SIZE-SELECTED, SUPPORTED CLUSTERS - THE INTERACTION OF CARBON-MONOXIDE WITH NICKEL CLUSTERS

When investigating size-dependent properties of particles consisting o f only few atoms on surfaces, it is necessary to be able to deposit ma ss-filtered particles of nanometer-size on well-characterized supports . So far the investigated systems have consisted of size-distributed n anoparticles supported on metal or oxide substrates [1]. Here we prese nt infrared and thermal desorption spectroscopic studies of CO adsorbe d on size-selected nickel clusters composed of up to 30 atoms and supp orted on ultrathin magnesium oxide films. We report on the size-depend ent chemical reactivity of nickel clusters with up to 30 atoms. Monodi spersed Ni-30 clusters show a higher reactivity for CO dissociation th an Ni-11 and Ni-20 Under our experimental conditions the smallest nick el clusters (Ni-x, x < 4) produce nickelcarbonyl complexes. These resu lts demonstrate that such small clusters are unique for catalytic reac tions not only due to their high surface-to-volume ratio but also esse ntially because of the distinctive properties of different cluster siz es. In addition thermal desorption spectroscopy of CO shows that on av erage four molecules are weakly adsorbed per Ni-11 at saturation cover age. Using an isotopic mixture of (CO)-C-12 and (CO)-C-13, infrared sp ectroscopy reveals the existence of a vibrational coupling interaction between the four COs. A semi-classical model of interacting dipoles i s applied to correlate the observed vibrational frequency shifts with the arrangement of the COs on the cluster. This simple analysis favors a three-dimensional structure for the deposited clusters.