Segregation and phase dynamics in supported bimetallic nanoparticle

A set of supported bimetallic catalysts, designated $rmlbrack Resb7Ir$-$rm Nrbrack, lbrack Resb7Ir$-$rm Prbrack, lbrack Resb5IrResb2$-$rm Prbrack,$ and $rmlbrack Resb5IrResb2$-$rm Prbrack,$ has been prepared from two structural isomers of the cluster compound (Z) $rmsb2lbrack Resb7IrC(CO)sb{23}rbrack (Zsp+=NEtsb4sp+, N(PPhsb3)sb2sp+)$ by deposition onto high surface area alumina $({le}1$% Re) and activation in H$sb2$ at 773 K. The more active catalysts $rm(lbrack Resb7$Ir-N) and $rmlbrack Resb5IrResb2$-$rm Nrbrack)$ are modeled by a hemisphere of close-packed (hcp) metal atoms (avg. diameter 1 nm) with Ir at the core. On the other hand the less active catalysts $rm(lbrack Resb7Ir$-$rm Prbrack$ and $rmlbrack Resb5IrResb2$-$rm Prbrack)$ are better described as two-dimensional layer structures.;Supported bimetallic particles were obtained by reduction of the neutral molecular carbonyl cluster precursor $rm PtRusb5C(CO)sb{16}$ 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 (STEM), microprobe energy dispersive x-ray (EDX) analysis, and electron microdiffraction. These experiments show that the bimetallic nanoparticles have a Pt:Ru composition of 1:5, an average diameter of ca. 1.5 nm, and adopt a face-centered cubic (fcc) closest packing structure. The local metal coordination environment, revealed by multiple scattering analysis of the EXAFS data, shows Pt segregation to the particle surfaces under an ambient H$sb2$ atmosphere. The incipient $rmlbrack PtRusb5rbrack$ nanoparticles were found to nucleate from a disordered structure where Pt is found in highly coordinating environments (i.e., the core) at temperatures as low as 473 K. This structure inverts to form the structure with surface segregated Pt.;The reaction between a silicon monolayer deposited on Pt(111) by chemical vapor deposition (CVD) using silane (SiH$sb4)$ is described. Using Auger electron spectroscopy (AES), Si was found to readily diffuse into the Pt substrate and sequentially form at least two unique intermetallic Pt-Si surface structural phases with $(surd7$ x $surd7)Rpm19.1spcirc$ and $(surd19$ x $surd19)Rpm23.4spcirc$ real space unit cells as characterized by low energy electron diffraction (LEED). The reactive properties of each overlayer were studied, with respect to the clean Pt(111) surface, using CO as a chemisorption probe.