Cluster size effects on sintering, CO adsorption, and implantation in Ir/SiO(2)

A series of planar model catalysts have been prepared via deposition of Ir(n) (+) on thermally grown amorphous SiO(2)/Si(100) and ion scattering spectroscopy was used to probe surface structure as a function of cluster size, impact energy, and surface temperature. Deposition of Ir(2) or Ir(10) at low energies and room temperature results in stable clusters forming one- or two-dimensional single layer islands on the oxide surface. Heating the samples to 750 K leads to agglomeration, forming multilayer structures on the surface. Ir(1) deposited under similar conditions sinters into large clusters at room temperature. Deposition at 110 K at least partially stabilizes the Ir atoms with respect to diffusion and sintering. At higher deposition energies, partial implantation into the surface is observed, but this appears to be insufficient to stabilize the clusters against sintering at elevated temperature. At low temperatures, substrate-mediated adsorption of CO is found to be highly efficient, leading to near saturation coverages of CO bound atop the Ir(n) clusters. The CO can be removed by careful He(+) sputtering. The deposition/binding behavior of Ir(n) on SiO(2) is quite different from Ir(n)/TiO(2)(110), for which the clusters bind in three-dimensional morphology, starting at Ir(5). That system also shows substrate-mediated adsorption of CO, but the CO preferentially binds at the periphery of the clusters rather than on top.