CATALYTIC ACTIVITY AND STRUCTURAL-ANALYSIS OF POLYMER-PROTECTED AU/PD BIMETALLIC CLUSTERS PREPARED BY THE SUCCESSIVE REDUCTION OF HAUCL4 AND PDCL2

Colloidal dispersions of the poly(N-vinyl-2-pyrrolidone)-protected gold/palladium bimetallic clusters were prepared by successive reduction in order to compare the catalytic activity and the structure of the clusters. The catalytic activity of the colloidal dispersions of the Au/Pd(1/4) clusters for the selective partial hydrogenation depends on the reduction order in the preparation of the clusters. The clusters prepared by successive reduction starting from the reduction of gold ions (Au --> Pd) have higher activity than those prepared by the reverse order (Pd --> Au). The EXAFS measurements indicate that the Au/Pd(1/4 and 1/1) bimetallic clusters prepared by the successive (Pd --> Au) reduction have the cluster-in-cluster structure. In the case of the successive (Au --> Pd) reduction, however, the dispersions contain both large monometallic Au clusters and monometallic Pd clusters, not Au/Pd bimetallic clusters. Mixing the dispersions of the Au and Pd monometallic clusters changes their structures at room temperature in solution, resulting in bimetallic alloy structures in the course of time, which is suggested by both the TEM observation and the electronic spectra.