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

The colloidal dispersions of the poly(N-vinyl-2-pyrrolidone)-protected gold/palladium bimetallic clusters were prepared by simultaneous reduction of HAuC14 and PdCl2 in the presence of poly(N-vinyl-2-pyrrolidone). The bimetallic clusters were relatively uniform in size (e.g., 1.6 nm in average diameter) and were used as a catalyst for the selective partial hydrogenation of 1,3-cyclooctadiene at 30-degrees-C under 1 atm of hydrogen. The initial rate of the hydrogenation depended on the metal composition of the cluster catalyst, and the maximum catalytic activities were achieved at Au/Pd - 1/4. The catalytic activities of the mixtures of the monometallic Au and Pd clusters were higher than those expected for the two kinds of the monometallic clusters without reciprocal actions. The structures of the bimetallic clusters were studied using extended X-ray absorption fine structure (EXAFS). The structural parameters were determined by fitting the experimental data for the two absorption edges simultaneously using improved computer program. As a consequence of this improvement, the coordination numbers of Au atoms around the Pd atom and of Pd atoms around the Au atom could be determined with greater confidence. The EXAFS data of the Au/Pd(1/4 and 1/1) bimetallic clusters prepared by simultaneous reduction indicate that the surface of the particle entirely consists of palladium atoms and the core of gold atoms (an Au core structure).