TECHNICAL: Surface composition and structure are responsible for the catalytic properties of bimetallic nanoparticles. Industrial bimetallic catalysts exhibit particle populations with a range of particle sizes and compositions that frustrate characterization of surface segregation by conventional surface analysis methods. However, direct analysis of surface species in broad nanoparticle populations has just become possible with the advent of spherical aberration- corrected scanning transmission electron microscopes (Cs-corrected STEMs). Moreover, local conditions of temperature and gas atmosphere can induce changes in nanoparticle surface composition, even against normal equilibrium tendencies. When fully understood, this gas-adsorption effect could provide a method for controlled tailoring of bimetallic nanoparticle surfaces. A Cs-corrected STEM combined with x-ray emission spectrometry and electron energy- loss spectrometry has both the spatial resolution, ~0.2 nm, and the signal generation capability for analyzing the surface layers of nanoparticles. Lehigh University recently acquired two Cs-corrected STEM instruments thanks to a combination of several NSF grants. These instruments will allow quantitative compositional analysis of sub-5-nm particles and direct analysis of nanoparticle surfaces, atomic layer by atomic layer. Surface compositions in bimetallic nanoparticles are affected by the following: surface energy differences of the metals, changes in particle composition or particle size, temperature changes, phase transformations, metal-support interactions, effects within particles related to specific crystallographic configurations, and reactive atmospheres above particles that strongly adsorb one of the elements. In this project heat treatments in highly adsorbing gases are used to tailor the concentration of active metal species on the surfaces of nanoparticles. Similar surface energies for Pt and Rh make this alloy system a good candidate for controlling the composition of the segregated surface layer. Other candidate second metals are Fe, Co, and Ni which all have surface energies nearly the same as Pt. The limits of surface composition tailoring are examined in the Pt-Pd and Pt-Ru systems. For single-phase nanoparticle populations STEM surface composition measurements can be compared with results from other analysis methods; however, measurements on nanoparticle populations that have separated into two phases can only be made by STEM. NONTECHNICAL: Bimetallic catalysts play a significant part in the $600 billion slice of U.S. industry related to catalysis; thus, even small improvements arising from this research would have significant commerical impact. Beyond catalysis, an understanding of the microstructure and phase stability of nanoparticles may have implications for other nanotechnology devices. Nanoparticle structures are discussed in several courses of the Lehigh Microscopy School (150 participants/year). Graduate courses in nanotechnology and nanocharacterization will highlight results from this work as part of Lehigh University.s Certificate Program in Nanomaterials. Some of these courses are offered via Internet2 to other Pennsylvania universities, and certain underrepresented universities have expressed interest in connecting to this service.
