STRUCTURAL CHARACTERIZATION OF CARBON-SUPPORTED PLATINUM-RUTHENIUM NANOPARTICLES FROM THE MOLECULAR CLUSTER PRECURSOR PTRU5C(CO)(16)

We describe the preparation and structural characterization of carbon- supported Pt-Ru nanoparticles with exceptionally narrow size and compo sitional distributions. The supported bimetallic particles are obtaine d by reduction of the neutral molecular carbonyl cluster precursor PtR u5C(CO)(16) with hydrogen. A detailed structural model of the nanopart icles has been deduced on the basis of studies by in situ extended X-r ay absorption fine structure spectroscopy (EXAFS), scanning transmissi on electron microscopy, microprobe energy-dispersive X-ray analysis, a nd electron microdiffraction. These experiments show that the bimetall ic nanoparticles have a Pt:Ru composition of 1:5 and an average diamet er of ca. 1.5 nm and adopt a face-centered cubic closest packing struc ture. These results demonstrate a marked sensitivity of the metal part icle structure to nanoscale size effects inasmuch as the thermodynamic ally stable phase for bulk alloys of this composition is hexagonal clo se-packed. The local metal coordination environment, revealed by multi ple scattering analysis of the EXAFS data, shows the presence of a non statistical distribution of different metal atoms in the nanoparticles . Specifically, Pt shows a marked preference for segregation to the pa rticle surfaces under an ambient H-2 atmosphere. Oxidation of the allo y particle in O-2 produces an outer metal oxide layer surrounding a me tal-only core. This oxidation is easily reversed by exposing the nanop articles to H? at room temperature.