Supported Ir and Pt clusters: Reactivity with oxygen investigated by extended X-ray absorption fine structure spectroscopy

Extended X-ray absorption fine structure (EXAFS) spectroscopy was used to investigate the effects of oxidation and reduction on supported noble metal clusters. After long-term exposure to laboratory air at room temperature, platinum clusters with an average diameter of approximately 7 Angstrom inside the pores of BaKLTL zeolite were almost completely fragmented. The EXAFS data indicate small, highly disordered platinum-platinum contributions but no bulk platinum oxides. Treatment in H-2 at 450 degrees C led to the formation of platinum clusters about 10 Angstrom in average diameter, and exposure of these to air at room temperature led to the formation of a disordered platinum-oxide-like phase covering a core of platinum atoms. The reactivity of platinum clusters with oxygen is size-dependent, and platinum cluster growth by oxidation-reduction cycles may be inhibited by the size of the zeolite pores. Similarly, iridium clusters on MgO, approximated as Ir-4, were fragmented by treatment in O-2, but treatment of the oxidized iridium clusters with H-2 at 573 K resulted in the formation of clusters with an Ir-Ir coordination number of 2.7 (and an Ir-Ir distance of 2.68 Angstrom), indicating slightly less than four Ir atoms per cluster. These data are nearly the same as those characterizing the original sample prior to oxidation, suggesting that the clusters had been reformed into almost their original state. This is the first evidence of supported metal clusters being oxidized and then regenerated nearly intact, The near reversibility of the oxidation-reduction process may be unique to iridium, being related to its slight but non-negligible oxophilicity and its resistance to sintering.