CO Oxidation on Au_n/TiO_2 Catalysts Produced by Size-Selected Cluster Deposition

摘要

Since the discovery that supported gold nanoparticles show surprising catalytic activity, considerable effort has gone into probing the effects of particle size. For bulk catalysts, peak activity is typically found for particle sizes in the 2-5 nm range (hundreds to thousands of atoms), as measured by transmission electron microscopy (TEM). TEM is insensitive to particles smaller than ～1 nm and may overlook smaller active species. Indeed, Fu et al.2 recently showed that activity for the water gas shift reaction on Au/CeO_2 and Pt/CeO_2 catalysts was unchanged when the nanoparticles were removed, leaving only ionic metal species on the surface. There have been a number of planar model catalyst studies where Au particles are grown on single crystal or thin film oxide, with size characterized by STM. CO oxidation on Au/rutile TiO_2 (110) has been particularly well studied, and activities typically are found to peak for Aun in the few nanometer size range. Such experiments allow detailed probing of physical and chemical properties for nanometer size particles, but are not as well suited to studying particles in the subnanometer range. Heiz and co-workers used deposition of size-selected cluster cations to study CO oxidation on Au_n/MgO, observing significant reactivity for clusters as small as Au8. Here, we present a study of room-temperature CO oxidation on planar model Au_n/TiO_2 catalysts prepared by deposition of size-selected Au_n~+, n = 1, 2, 3, 4, 7. Activity is strongly dependent on deposited cluster size, with substantial activity for Au_n as small as three atoms.