In this report, we describe the use of several analytical techniques, including X-ray absorption spectroscopy (XAS), electron microscopy, and electron diffraction, as tools for characterizing the structural dynamics of supported Pt nanoscale particles. We examined several carbon-supported samples. Electron microscopy shows that the particles in these samples (S1−S3) have average particle diameters of roughly 20, 40, and 60 Å respectively, while electron microdiffraction data for these particles provided evidence of long-ranged ordering in the form of face centered cubic structures. This study highlights the use of advanced synchrotron X-ray absorption spectroscopies (XAS), in particular extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES), as powerful tools for studying the structural habits and dynamics of these prototypical nanoscale materials. Using state-of-the-art methods of measurement and computational modeling, we demonstrate that it is possible to develop a detailed understanding of the shape and morphology of the nanoscale clusters. We use these techniques to provide information about the nature of their surface texturing, establishing that they preferentially adopt oblate (“hemispherical”) cuboctahedra cluster shapes truncated along the [111] basal plane. We further describe the use of temperature-dependent EXAFS measurements to investigate the nature of bond relaxation phenomenon occurring within the small metallic nanoparticles. To evaluate these complex structural behaviors, the disorder parameters are calculated from temperature-dependent EXAFS data and then subsequently compared to simple molecular graphics simulations of mechanisms involving either full cluster or surface relaxations. The average bond length and static disorder obtained by experiment appear to best fit a model involving dominant contributions made by surface atom bond relaxation.

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.

We describe in this report the preparation, structural characterization, and phase behaviors exhibited by supported metallic and bimetallic nanoparticles. Homometallic nanoparticles of either Pt or Ru were synthesized by the reduction of various precursors ((CH 3 ) 2 Pt(COD), H 2 PtCl 6 , and RuCl 3 ) onto different carbon supports: Vulcan XC-72 (VXC) and Shawinigan Acetylene Black (SAB). The choice of precursor has a large structural influence on the reductive condensation of the Pt metal particles. All of the various precursors and supports produced particles with very similar size distributions, with the exception of (CH 3 ) 2 Pt(COD), which formed a complex distribution of small (20 Å) and large (>50 Å) particles. The centerpiece of this study is the characterization of the growth behaviors seen in the synthesis of binary Pt-Ru nanoparticles. These heterometallic particles were synthesized via a seeded reductive condensation of one metal precursor onto pre-supported nanoparticles of a second metal; the latter serve as nucleating sites for the growth of the binary phase. As shown via data from X-ray photoelectron spectroscopy (XPS), electron microscopy, and energydispersive X-ray analysis (EDX), this growth technique yields fully alloyed metallic nanoparticles, albeit ones of varying size and compositional distributions depending on the specific conditions used. Generally we found that the particles had a wide composition distribution. The nature of this distribution and the correlations between the nanoparticle sizes, compositions, and structures embedded in it were characterized in depth by scanning transmission electron microscopy (STEM). These results are used to establish an apparent size correlated binary phase diagram of the bimetallic (Pt-Ru) nanoparticles. The structural properties of the supported bimetallic nanoclusters are different from that of the bulk, as evidenced by the presence of strongly persistent metastable structures that are not found in the bulk phase diagram.