Model catalysts prepared by size-selected nanocluster deposition

Catalytic activity and product selectivity of supported metal catalysts strongly depend on the size of metal particles, support materials, and preparation methods. A novel instrument was employed to investigate electronic structure, morphology, and chemical properties of the model supported catalysts.{09}Size- and energy-selected metal clusters containing fewer than 30 atoms on TiO ₂ (110) were characterized by X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), ion scattering spectroscopy (ISS), and temperature programmed desorption (TPD). The performance of the instrument was checked with investigating the oxidation of vanadium and niobium clusters supported on TiO₂ (110).; Ni clusters are in the zero oxidation state on the support at low impact energies. Oxidation, however, occurs either when increasing the impact energy or when chemisorbed oxygen being available at oxygen defect sites of TiO ₂. The small clusters bind preferentially to oxygen sites. The large clusters appear to retain some three dimensional structure on the support. For these clusters, no obvious desorption features were observed in the temperature range above 140K. The lack of CO desorption is interpreted in terms of strong Ni cluster-TiO₂ binding. It seems that the nickel clusters are sintering and/or are encapsulated during the TPD experiments.; Ir clusters are also in the zero oxidation state on the support irrespective of the cluster size and the impact energies. At low impact energies the clusters stay more or less intact on the support. The clusters embed themselves into the support at higher impact energies. The threshold energy for the embedding is lower for the larger clusters. This system shows pronounced substrate-mediated adsorption (SMA) of CO at low doses, with the effect varying inversely with cluster size. CO adsorbed via SMA at low CO dose is bound differently from that at high CO dose. In experiments with sequential C16O and C18O doses, C16O → C18O exchange was observed for all cluster sizes, and the amount of exchange increased with the C18O dose. The peak CO desorption temperature was found to decrease with increasing cluster size. In addition, the CO desorption intensity decreases with increasing deposition energy, consistent with the ISS data showing that the iridium clusters tend to embed into the support at elevated impact energies.