| 1. Carbon supported Pt nanoparticles. The structural dynamics of supported Pt nanoparticles are studied using a battery of analytical techniques, including scanning transmission electron microscopy (STEM), electron diffraction, and most notably X-ray absorption spectroscopy (XAS). 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.; 2. Pt-Ru phase dynamics. Metallic and bimetallic nanoparticles are synthesized by depositing several metallic salt precursors (H2PtCl 6, RuCl3, (COD)Pt(CH3)2) on two different carbon supports (Vulcan XC-72, Shawinigan Acetylene Black). The correlations between size, composition, and structure are used to establish the apparent binary phase diagram of the bimetallic (Pt-Ru) nanoparticles. Secondly, a template effect is apparent in the nanophase diagram, when using pre-supported Pt particles.; 3. Carbon supported Pt-Ru nanoparticles. Supported bimetallic nanoparticles are synthesized through the reductive condensation of molecular precursors (PtRu5C(CO)16 and Pt2Ru4 (CO)18) supported on different carbon supports (Vulcan XC-72 and fullerene soot). A detailed structural picture of the nanoparticles has been deduced on the basis of X-ray absorption spectroscopy (XAS), scanning transmission electron microscopy (STEM), and energy-dispersive X-ray analysis (EDX). These techniques reveal that the bimetallic nanoparticles have compositions of 1:5 and 2:4, respectively, and average diameters between 1.0 and 1.5 nm. 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, which taken collectively with the other data presented, leads us to propose a possible Ru-C compound formation on this latter support phase.; 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 stabilizing CO shell is lost, and the metal centers assume an increasingly metallic electronic character. 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. (Abstract shortened by UMI.)... |
