2D model catalysts studied by scanning tunneling microscopy: Rh, Ir supported on TiO2(110)-(1x2)

Preparation and properties of noble metal clusters (Rh, Ir) on TiO2(110)-(1 x2) surface are studied by scanning tunneling microscopy and spectroscopy. Formation of 1-2 nm of 3D clusters begins already at 300 K for 1 % monolaye r (ML) noble metal coverage. The terrace edges are not preferred centres in this case. By enhancement of the deposited metal up to 1-2 ML the cluster size increases above 5 nm. The effect of annealing below 700 K results in a moderate increase of the cluster site. Above 900 K, however, the metal clu sters agglomerate and collapse resulting in separated larger crystallites, which are bonded mainly at the crosspoints of terrace edges. By exploiting the large difference in the diffusion coefficient between noble metal atoms and crystallites larger than 1-2 nm, we suggest a method (called "seeding + growing") by which an independent control of average cluster size and int erparticle distance is attainable. The applicability of this method in the studies of some important processes (cluster size dependence of the treatme nts in different gases, decoration phenomena, etc.) in the field of heterog eneous catalysis are demonstrated in this work.