STRUCTURAL INVESTIGATION OF BIMETALLIC RH-PT NANOPARTICLES THROUGH X-RAY-ABSORPTION SPECTROSCOPY

The reduction, with tetraalkylammonium hydrotriorganoborates, of noble -metal salts suspended in tetrahydrofuran gives metal colloids which a re protected against agglomeration by the surrounding tetraalkylammoni um ions. The mean particle size of the isolated metal aggregates, as d etermined by TEM, is 3 nm and below. The use of two different metal ha lides simultaneously, allows coreduction to occur, giving an X-ray amo rphous bimetallic product. In order to determine whether the resulting colloid is a mixture of the two individual metals or contains a bimet allic alloy, the environment of each metal in a Rh0.44Pt0.56 product w as examined using EXAFS spectroscopy at LURE, France (Rh K- and Pt L(3 )-edges). In each case, mixed-metal shells were observed, compatible w ith the formation of intermetallic aggregates. The environment around the Rh absorber consists of 3.5 Rh atoms at a distance of 2.72 Angstro m and 4.7 Pt atoms at 2.74 Angstrom (total 8.2 neighbours) and, around the Pt absorber, 3.6 Rh at 2.72 Angstrom and 8.3 Pt atoms at 2.75 Ang strom (total 11.9 neighbours). The average coordination number (CN) is lower than the expected CN of 12 for a face-centred cubic structure, which is consistent with the nanoparticulate nature of the bimetallic system. Furthermore, the CN of rhodium is less than that of platinum i ndicating that, on average, rhodium is segregated from the core to the surface of the alloy nanoparticles. The system is best described as a nanoparticulate Rh/Pt alloy, surface rich in rhodium. The implication s of this finding in catalysis are discussed.