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 Å 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 H2 at 450°C led to the formation of platinum clusters about 10 Å 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 Ir4, were fragmented by treatment in O2, but treatment of the oxidized iridium clusters with H2 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 Å), 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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry