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

The bimetallic catalysts Pt-Au/C (graphite) were prepared by selectively depositing Au on supported monometallic Pt/C catalysts by means of the reduction 'in situ' of AuCl₄^-. The parent metal (Pt) was used as the reducing agent for the direct redox reactions ('DR'), while the second redox method used was the refilling method ('RE'), which consisted in adsorbing hydrogen first on the parent metal (Pt) and subsequently reducing the AuCl₄^- species by contact with the Pt-H interface at low temperature. The catalysts Pt-Au/C were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and high-resolution electron microscopy (HREM). The interaction between Pt and Au was explored at the 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 the solids 'as-prepared' (i.e., DR-A and RE-B) indicated the presence of both metals, Pt and Au. However, a thermal reduction at 300°C under H₂ 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 substantially; Au migrates toward the small particles (i.e., 5 ≤ D ≤ 7 nm), while the bigger particles (i.e., D ≥ 10 nm) show almost pure Pt. A partial Au coating of certain Pt facets is probable, which indicates that the coating mechanism is selective and could influence the catalytic properties of the bimetallic Pt-Au/C catalysts.