Strong Support Effects on the Insulator to Metal Transition in Supported Pt Clusters as Observed by X-ray Absorption Spectroscopy

The development of a new in situ probe of metallic character in supported metal clusters utilizing X-ray absorption spectroscopy is described. The technique is based on the extent of screening of the core-hole left in the neutral final state after the X-ray absorption. The technique allows for the clear differentiation between local interatomic charge transfer and more delocalized metallic screening. The particle size at the metal−insulator transition is found to depend strongly on the electron richness of the support oxygen atoms (i.e., ionic vs covalent oxides). Pt particles on supports with electron poor oxygen atoms (covalent) show metallic screening for sizes as small as 12 Å in diameter. In contrast, on supports with electron rich oxygen atoms (ionic) the Pt particles do not show metallic behavior until around 20 Å. The wide variation of previously reported estimates of the particle size at which the insulator to metal transition occurs is explained, giving a consistent picture for the onset of metallic character, and the reasons for the strong support effect.