APPLICATION OF THE XENON-ADSORPTION METHOD FOR THE STUDY OF METAL CLUSTER FORMATION AND GROWTH ON Y-ZEOLITE

Abstract

Supported metal clusters of Pt, Ir, Ru, Rh, and Pd have been prepared in the supercage of Y zeolite by activating their ion-exchanged ammine complexes. Xenon adsorption isotherms obtained from these samples at temperatures ranging from 296 to 340 K and pressures up to 500 Torr, as well as the chemical shift data from the Xe-129 NMR study of the adsorbed xenon gas, indicate that the xenon adsorption can occur quite strongly, becoming saturated above ca. 50 Torr, on the metal cluster surface whereas the adsorption is so weak on the support that the adsorbed quantity increases linearly with pressure according to Henry's law. Upon chemisorption of hydrogen or oxygen, the metal clusters lose the ability to adsorb the xenon so strongly, resulting in a decrease in the xenon adsorption quantity. Such a xenon-adsorption decrease due to the chemisorption of another gas can determine the amount of xenon that can be saturated on all the metal clusters present in the sample. This quantity is sensitive to the size and the number of the supported clusters. It can then be used to probe the formation and the size variation of the clusters which are very difficult to study by other methods due to their very small size, e.g. about 1 nm. An application of such a simple technique for the study of Pt cluster formation on NaY zeolite indicates that the size of the Pt cluster formed in the supercage does not change significantly as the metal content is changed from 2 to 10%. This xenon-adsorption method has also been very useful for the study of the formation and growth of the Ru cluster on Na Y zeolite. The result indicates that very small Ru clusters are formed in the supercage by treating a precursor, which is prepared by heating Na Y zeolite in an aqueous ammonia solution of RuCl3, in vacuum at temperatures above 623 K. The clusters grow gradually at higher temperatures both in H-2 and under vacuum. The data also agree that an exposure of the supported Ru clusters in O2 at temperatures above 423 K causes excessive migration of the Ru species, resulting in large agglomeration. Another example, which can further promise wide applicability of the xenon adsorption method, is a study of the formation of Pd clusters in the supercage of various ion-exchanged Y and X zeolites. The result indicates that Ca2+, Y3+, and perhaps other multivalent cations residing in the supercage are necessary to obtain ca. 1-nm clusters whereas univalent cations give larger clusters.