Morphology and catalysis of size-selected Pt clusters tuned by interfacial interaction with metal oxide support

Metal clusters supported on oxide materials are widely-used for heterogeneous catalysts. One of the most important roles of an oxide support is to make active metal clusters stable and dispersive to decrease the amount of costly metal. Another important role is to modify the catalytic activities via the metal-support interactions.
We combine scanning tunneling microscopy, infrared reflection absorption spectroscopy, and temperature-programed desorption to investigate the adsorption states of size-selected Pt_n clusters on TiO₂(110) and Al₂O₃/NiAl(110).
For Pt_n /TiO₂(110), the transition from a planar structure to a three-dimensional (3D) structure occurred at the cluster size of 8 atoms. CO oxidation activity shifted to a more active state at the same cluster size at which planar-to-3D transition occurred¹ For Pt_n /Al₂O₃/NiAl(110), the transition from a planar structure to a 3D structure occurred at the cluster size of 19 atoms.
Two ontop CO species were assigned as adsorbed CO on neutral and slightly cationic Pt atoms. Despite of the first layer Pt atoms, the Pt clusters are composed of the two kinds of Pt atoms. With increasing cluster size, the number of neutral Pt atoms increases. CO prefers to adsorb on the cationic Pt atoms.² Oxygen prefers to adsorb on the neutral Pt atoms. Two kinds of Pt sites would expect to be mixed in good proportion to proceed the reaction cycles of CO oxidation. The interfacial interaction would control the cluster morphology and the ratio of the number of the neutral Pt site to the cationic one. The perturbation of the electronic states of Pt clusters by interacting via the metal-oxide interface is found to be one of the most important roles of a support. Our findings provide novel insights into the material design of a metal supported catalyst.
References
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2. A. Beniya, N. Isomura., H. Hirata, Y. Watanabe, Phys. Chem. Chem. Phys.16, 26485-26492 (2014).