TEMPERATURE DEPENDENCE OF THE Pt L3-EDGE EXAFS OF PLATINUM CLUSTERS SUPPORTED ON NaY-ZEOLITE

Abstract

The temperature dependence o f the Pt L3-edge EXAFS spectra of the platinum clusters encaged in the cavities of the Nay-zeolite werz observed by use of synchrotron radiation. A contraction of Pt-Pt bond by 0.07 A from the distance in the bulk Pt metal is observed, which is likely to be associated w ith a strong electron deficiency of the platinum clusters. The temperature dependence of the mean square relative displacement (MSRD) shows that the static disorder in this system is large, while the thermal d isorder is significantly smaller than that of the bulk metal. The estimatedDebyete mperatureis 310 K, which is much higher than that of the bulk metal (240 K). This unusual phenomenom seems to be associated with the increase of the binding energy between platinum atoms and with the strong interaction between a cluster and zeolite-framework (the cage effect). EXAFS spectroscopy is a suitable technique to investigate the s tructural difference between a fine metal particle and a bulk metal. So far, much attention has been paid for the metal-metal bond distance and it has been often concluded that the bond distance becomes the shorter as the particle size is smaller. Another interesting structural aspect of a fine metal particle is the particle-size effect of thermal and/or static disorder. Recently Marques et a1 [l] measured the temperature dependence of EXAFS of supported clusters, and suggested that such an experiment is useful to investigate thermal vibrations and structural disorders as well as to determine the interatomic distance more correctly. In the present study we investigated the temperature dependence of EXAFS on platinum clusters encaged in the supercages of Nay-zeolite. The bond distance and disorder in this system are determined and discussed by comparing with those of the bulk metal and of the metal particles supported on A1203.