We describe the preparation and structural characterization of carbon-supported Pt−Ru nanoparticles with exceptionally narrow size and compositional distributions. The supported bimetallic particles are obtained by reduction of the neutral molecular carbonyl cluster precursor PtRu5C(CO)16 with hydrogen. A detailed structural model of the nanoparticles has been deduced on the basis of studies by in situ extended X-ray absorption fine structure spectroscopy (EXAFS), scanning transmission electron microscopy, microprobe energy-dispersive X-ray analysis, and electron microdiffraction. These experiments show that the bimetallic nanoparticles have a Pt:Ru composition of 1:5 and an average diameter of ca. 1.5 nm and adopt a face-centered cubic closest packing structure. These results demonstrate a marked sensitivity of the metal particle structure to nanoscale size effects inasmuch as the thermodynamically stable phase for bulk alloys of this composition is hexagonal close-packed. The local metal coordination environment, revealed by multiple scattering analysis of the EXAFS data, shows the presence of a nonstatistical distribution of different metal atoms in the nanoparticles. Specifically, Pt shows a marked preference for segregation to the particle surfaces under an ambient H2 atmosphere. Oxidation of the alloy particle in O2 produces an outer metal oxide layer surrounding a metal-only core. This oxidation is easily reversed by exposing the nanoparticles to H2 at room temperature.

The reductive condensation of a carbon-supported molecular cluster precursor, PtRu5C(CO)16, into a bimetallic nanoparticle has been followed by using in situ extended X-ray absorption fine structure spectroscopy, temperature-programmed desorption, and scanning transmission electron microscopy. The data reveal that during activation in hydrogen the metal centers associated with the molecular precursor lose the stabilizing CO shell and assume an increasingly metallic electronic character. This support-mediated condensation process is highly activated. The incipient Pt−Ru nanoparticles initially form a disordered structure at 473 K in which Pt is found preferentially at the core of the condensing particle. After further high-temperature treatment to 673 K, the nanoparticles adopt an inverted structure in which Pt appears preferentially at the surface of the equilibrated bimetallic nanoparticle.

We describe the preparation, structural characterization, and support interactions experienced by two different compositions of Pt−Ru nanoparticles supported on several carbons (carbon black, fullerene soot, and desulfurized carbon black). The bimetallic nanoparticles, obtained by reduction of the neutral molecular precursors PtRu5C(CO)16 and Pt2Ru4(CO)18 (the latter of which lacks a central “stabilizing” carbide core) at elevated temperatures in a hydrogen atmosphere, show a structural homology, exhibiting exceptionally narrow size and compositional distributions. A detailed structural picture of the nanoparticles has been deduced on the basis of in-situ extended X-ray absorption fine structure (EXAFS), scanning transmission electron microscopy (STEM), energy-dispersive X-ray analysis (EDX), and X-ray absorption near edge structure (XANES). These techniques reveal that the bimetallic nanoparticles have Pt/Ru compositions of 1:5 and 2:4, respectively, and average diameters lying between 1.0 and 1.5 nm. The local metal coordination environments reveal a nonstatistical distribution of the two metals in the nanoparticles. Specifically, Pt shows a marked preference for segregation to the particle surfaces under an H2 atmosphere. The data also reveal a difference in the structural environment of the nanoparticles when formed on the fullerene soot support. Interactions between Ru and low-Z atoms are revealed through XANES, which, taken collectively with the other data presented, leads us to propose a possible Ru−C compound formation on this latter support phase.

A set of supported bimetallic catalysts, designated [Re7Ir−N], [Re7Ir−P], [Re5IrRe2−N], and [Re5IrRe2−P], has been prepared from two structural isomers (1 and 2) of the cluster compound [Z]2[Re7IrC(CO)23] (Z+ = NEt4 +, N(PPh3)2 +) by deposition onto high surface area alumina (≤1% Re) and activation in H2 at 773 K. The specific activities of the catalysts for ethane hydrogenolysis at 500 K vary significantly (3−63 mmol of CH4/mol of Re7Ir per s) and depend on both the metal framework structure and the counterion present in the precursor. Interpretation of EXAFS data (from both Re and Ir L3-edges) has enabled the development of specific models for the catalyst particle nanostructures that correlate with the catalytic activities. The more active catalysts ([Re7Ir−N] and [Re5IrRe2−N]) are modeled by a hemisphere of close-packed (hcp) metal atoms (average diameter 1 nm) with Ir at the core. On the other hand, the less active catalysts ([Re7Ir−P] and [Re5IrRe2−P]) are better described as two-dimensional layer structures. A combination of techniques, TPDE, IR, XANES, and EXAFS, applied under temperature-programmed conditions, has demonstrated that evolution of the final catalyst particle nanostructure depends on significant initial fragmentation of the cluster framework followed by preferential nucleation at iridium centers.

The reaction of a silicon adlayer deposited on Pt(111) by chemical vapor deposition (CVD) using silane (SiH 4 ) is described. Data from Auger electron spectroscopy (AES) reveal that Si readily diffuses into the Pt substrate and sequentially forms at least two unique intermetallic Pt-Si surface structural phases with ( 7 × 7)R19.1°and ( 19 × 19)R23.4°real space unit cells as characterized by low-energy electron diffraction (LEED). The chemisorption properties of each of the ordered overlayers were studied using CO as a molecular probe. Reflection-absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) studies indicated that CO was mostly limited to chemisorption at "atop" Pt sites in the 7 phase with a significant reduction of the heat of adsorption with respect to the Pt(111) surface. The 19 phase also showed a significant modification of the chemisorption properties although it is not as pronounced as that seen for the 7 structure. The relevance of these studies to intermetallic thin film growth is discussed.