FORMATION OF A SMALL PALLADIUM CLUSTER BY ANCHORING ON A MULTIVALENT CATION IN A ZEOLITE SUPERCAGE STUDIED BY XENON ADSORPTION

Abstract

A palladium cluster has been supported on X and Y zeolites of various cationic forms by using a scheme consisting of Pd(NH3)42+ exchange into the zeolite, activation in O2, and subsequent reduction with H-2. Xenon has been found to be adsorbed onto the Pd cluster much more strongly than the zeolite support. The amount of xenon adsorbed on the Pd cluster has been determined from the xenon adsorption isotherm and then used as a sensitive probe for the study of the cluster size variation. The chemical shift in Xe-129 NMR of the adsorbed xenon gas and transmission electron microscopy have also been used to investigate the Pd cluster size dependence on the cation in the zeolite. The results indicate that small Pd cluster of about 1-nm size, which is stable up to 673 K, can be prepared by anchoring on multivalent cations such as Ca2+ and Y3+ located in the supercage. Otherwise, the cluster grows to the size of above 2 nm upon heating to 673 K.