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

Deposition of metals onto planar oxide supports provides a convenient methodology 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 of oxide-supported metal clusters upon exposure to reactants at elevated pressures. Of special concern is the relationship between catalytic activity/selectivity and surface structure, e.g., metal-support interaction and intrinsic cluster size effects. Au and Ag clusters were vapor-deposited onto TiO2(110) under ultrahigh vacuum (UHV) conditions. Characterization of cluster size and density as a function of metal coverage is correlated with catalytic 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 clustering 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 clusters exhibited a bimodal size distribution following O-2 exposure due to Ostwald ripening, i.e., some clusters increased in size while other clusters shrank. A volatile oxide species is proposed to form at high oxygen pressures, accelerating intercluster atom transport. The oxide substrate was found to play a role in the kinetics of cluster ripening. STM shows that oxide-supported metal clusters are very reactive to O-2 and that nanoclusters are particularly susceptible to adsorbate-induced restructuring. (C) 2000 Elsevier Science B.V. All rights reserved.