Aggregation state of Pt-Au/C bimetallic catalysts prepared by surface redox reactions

The bimetallic catalysts Pt-Au/C (graphite) were prepared by selectively de positing Au on supported monometallic Pt/C catalysts by means of the reduct ion "in situ" of AuCl4-. The parent metal (Pt) was used as the reducing age nt for the direct redox reactions ("DR"), while the second redox method use d was the refilling method ("RE"), which consisted in adsorbing hydrogen fi rst on the parent metal (Pt) and subsequently reducing the AuCl4- species b y contact with the Pt-H interface at low temperature. The catalysts PtAu/C were characterized by X-ray diffraction (XRD), transmission electron micros copy (TEM), energy dispersive spectroscopy (EDS), and high-resolution elect ron microscopy (HREM). The interaction between Pt and Au was explored at th e nanometer scale by means of EDS and HREM, with the aim of characterizing the aggregation state of the metals in the Pt-Au/C systems prepared by the novel redox methods. The composition of the individual metal particles of t he solids "as-prepared" (i.e., DR-A and RE-B) indicated the presence of bot h metals, Pt and Au. However, a thermal reduction at 300 degrees C under Ha seems to redistribute the Au phase in the DR-A solids, then the particles remain bimetallic, but in the RE-B solids the Au concentration increases su bstantially; Au migrates toward the small particles (i.e., 5 less than or e qual to D less than or equal to 7 nm), while the bigger particles (i.e., D greater than or equal to 10 nm) show almost pure Pt. A partial Au coating o f certain Pt facets is probable, which indicates that the coating mechanism is selective and could influence the catalytic properties of the bimetalli c Pt-Au/C catalysts.