CORE-SHELL INVERSION DURING NUCLEATION AND GROWTH OF BIMETALLIC PT RUNANOPARTICLES/

The reductive condensation of a carbon-supported molecular cluster pre cursor, PtRu5C(CO)is, into a bimetallic nanoparticle has been followed by using in situ extended X-ray absorption fine structure spectroscop y, temperature-programmed desorption, and scanning transmission electr on microscopy. The data reveal that during activation in hydrogen the metal centers associated with the molecular precursor lose the stabili zing CO shell and assume. an increasingly metallic electronic characte r. This support-mediated condensation process is highly activated. The incipient Pt-Ru nanoparticles initially form a disordered structure a t 473 K in which Pr is found preferentially at the core of the condens ing particle. After further high-temperature treatment to 673 K, the n anoparticles adopt an inverted structure in which Pt appears preferent ially at the surface of the equilibrated bimetallic nanoparticle.