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 are protected against agglomeration by the surrounding tetraalkylammonium ions. The mean particle size of the isolated metal aggregates, as determined by TEM, is 3 nm and below. The use of two different metal halides simultaneously, allows coreduction to occur, giving an X-ray amorphous bimetallic product. In order to determine whether the resulting colloid is a mixture of the two individual metals or contains a bimetallic alloy, the environment of each metal in a Rh0.44Pt0.56 product was examined using EXAFS spectroscopy at LURE, France (Rh K- and Pt L(3)-edges). In each case, mixed-metal shells were observed, compatible with the formation of intermetallic aggregates. The environment around the Rh absorber consists of 3.5 Rh atoms at a distance of 2.72 Angstrom 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 Angstrom (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 indicating 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 implications of this finding in catalysis are discussed.