STRUCTURAL-ANALYSIS OF POLYMER-PROTECTED PALLADIUM RHODIUM BIMETALLIC CLUSTERS USING EXAFS SPECTROSCOPY

The structures of the colloidal dispersions of the Pd/Rh bimetallic clusters, prepared by refluxing the solution of their salts in water/ethanol in the presence of Poly(N-vinyl-2-pyrrolidone), were investigated by means of an extended X-ray absorption fine structure (EXAFS) technique. On the basis of the EXAFS measurements of the Pd/Rh(1/1) alloy foil, the Pd-Rh bond distance in this alloy foil is about 2.70 angstrom, different neither from the Pd-Pd bond distance in the Pd foil nor the Rh-Rh bond distance in the Rh foil. Comparison of the difference of the peak height between the Pd foil and the Pd/Rh(1/1) alloy foil with that between the Rh foil and the same alloy foil has also shown that the disorder in the Pd foil or in the Rh foil is nearly the same as that in the Pd/Rh(1/1) alloy foil, even at room temperature. Under the condition that each contribution has the corresponding bond distance with the same disorder, the contributions of Pd-Pd, Pd-Rh, Rh-Rh, and Rh-Pd bonds can be completely distinguished from each other in the Pd/Rh bimetallic cluster. From the electron microprobe analysis and the obtained coordination numbers of the colloidal dispersions of the Pd/Rh(1/1 and 1/2) bimetallic clusters, it can be concluded that the small particles (less than 20 angstrom in diameter) mainly consist of Rh clusters and the large particles (more than 35 angstrom in diameter) of Pd clusters. The bimetallic alloy clusters have diameters in the range 25-35 angstrom. The small sum of coordination numbers around the Rh atom of the dispersions suggests that the Rh microcluster particles coexist in the colloidal dispersions and that the Rh atoms of the bimetallic alloy clusters are highly dispersed on the surface.