Size-selected, supported clusters: the interaction of carbon monoxide with nickel clusters

When investigating size-dependent properties of particles consisting of only few atoms on surfaces, it is necessary to be able to deposit mass-filtered particles of nanometer-size on well-characterized supports. So far the investigated systems have consisted of size-distributed nanoparticles supported on metal or oxide substrates [1]. Here we present infrared and thermal desorption spectroscopic studies of CO adsorbed on size-selected nickel clusters composed of up to 30 atoms and supported on ultrathin magnesium oxide films. We report on the size-dependent chemical reactivity of nickel clusters with up to 30 atoms. Monodispersed Ni-30 clusters show a higher reactivity for CO dissociation than Ni-11 and Ni-20 Under our experimental conditions the smallest nickel clusters (Ni-x, x < 4) produce nickelcarbonyl complexes. These results demonstrate that such small clusters are unique for catalytic reactions not only due to their high surface-to-volume ratio but also essentially because of the distinctive properties of different cluster sizes. In addition thermal desorption spectroscopy of CO shows that on average four molecules are weakly adsorbed per Ni-11 at saturation coverage. Using an isotopic mixture of (CO)-C-12 and (CO)-C-13, infrared spectroscopy reveals the existence of a vibrational coupling interaction between the four COs. A semi-classical model of interacting dipoles is 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.