Structure-reactivity correlations for oxide-supported metal catalysts: newperspectives from STM

Deposition of metals onto planar oxide supports provides a convenient metho dology for modeling important aspects of supported metal catalysts. In this work, scanning tunneling microscopy (STM), in conjunction with traditional surface-science techniques, is used to monitor the morphological changes o f oxide-supported metal clusters upon exposure to reactants at elevated pre ssures. Of special concern is the relationship between catalytic activity/s electivity and surface structure, e.g., metal-support interaction and intri nsic cluster size effects. Au and Ag clusters were vapor-deposited onto TiO 2(110) under ultrahigh vacuum (UHV) conditions. Characterization of cluster size and density as a function of metal coverage is correlated with cataly tic reactivity. Oxygen-induced cluster ripening occurs upon exposure of Au/ TiO2(110) and Ag/TiO2(110) to 10.00 Torr O-2. The morphology of the metal c lustering induced by O-2 exposure implies the chemisorption of O-2 onto the metal clusters and the TiO2 substrate at room temperature. Ag and Au clust ers exhibited a bimodal size distribution following O-2 exposure due to Ost wald ripening, i.e., some clusters increased in size while other clusters s hrank. A volatile oxide species is proposed to form at high oxygen pressure s, accelerating intercluster atom transport. The oxide substrate was found to play a role in the kinetics of cluster ripening. STM shows that oxide-su pported metal clusters are very reactive to O-2 and that nanoclusters are p articularly susceptible to adsorbate-induced restructuring. (C) 2000 Elsevi er Science B.V. All rights reserved.