Formation of Pd-group VIII bimetallic nanoparticles by the “water-in-oil” microemulsion method

Abstract

Pd-based bimetallic catalysts PdM-0.1/C where M=Au, Pt, Ru, Ir with total metal loading of 2wt % and a Pd: M molar ratio of 10: 1 were synthesized by the “water-in-oil” microemulsion method. Metal particles are prepared by directly adding the reducing agent (NaBH4) as a powder to the metal salts − containing microemulsion (water/Triton X-114/cyclohexane). The monometallic Pd/C, Pt/C, Au/C, Ru/C and Ir/C catalysts were prepared with the same procedure. Physicochemical (XRD, XPS, SEM, HRTEM, EDS) and electrochemical (cyclic voltammetry) methods were applied to characterize the structure, morphology and surface properties of the synthesized catalysts. The microscopic results show that the mono- and bimetallic particles are monodisperse and well distributed on the carbon support. It is suggested that nucleation processes are mainly responsible for the metal particle formation if one uses the single-microemulsion procedure. The average size of the monometallic particles increased from Ir to Au. The size of all bimetallic PdM particles is smaller than that of Pd and they differ in the extent of surface segregation. The segregation is most pronounced in the PdAu and PdPt systems, resulting in a surface enrichment with Pd and Pt, respectively, whereas the PdRu and PdIr systems, in contrast, exhibit practically homogeneous alloy microstructure. It has been shown that these microstucture-induced surface effects are related to the catalytic properties of the PdM-0.1/C samples in the cinnamaldehyde hydrogenation reaction.