Abstract
The preparation of Pd clusters on Y-zeolite was investigated by the sonochemical reduction of Pd(NH_3)_4^<2+> complexes in an aqueous solution system. The measurements by UV-vis and XPS spectra showed that Pd(NH_3)_4^<2+> was found to be reduced by reducing species formed from 2-propanol sonolysis to yield zero-valent Pd. It was confirmed that the binding energy of the Pd 3d core level on the Y-zeolite was shifted to a higher value in comparison with that of Pd bulk metal. The formation of Pd metal, however, could not be observed by HRTEM and EXAFS measurements, implying the formation of ultrafine Pd clusters. On the other hand, the results of the XPS analyses for several standard Pd clusters of ca. lnm prepared by the conventional thermal treatment indicated that the peak values of the Pd core level increased with decreasing particle size: The maximum difference in binding energy between these samples was 0.6veV at the Pd 3d_<5/2> core level. This phenomenon was suggested to be attributed to a quantum size effect corresponding to the splitting of the band structure. In comparison with the standard samples, the size of sonochemically formed Pd clusters was roughly estimated to be less than 1nm.
