These conditions provide an easy control of the particle size and allow the synthesis of uniformly dispersed metal particles characterizing by uniformity of their surface properties. In w/o method, all processes including the chemical reduction of metal ions, nucleation and growth of the metal particles take place within the confined space of aqueous droplets dispersed in a continuous oil phase stabilized by surfactant molecules. The colloid-based preparation method, namely a reverse “water-in-oil” microemulsion (w/o), was also reported to be effective in the synthesis of alloyed nanoparticles such as Pd–Pt, Pd–Au, Pt–Au, Pd–Ni, C–Ni and Au–Ag. Both the complex deposition and the growth pattern of these nanostructures driven by the reaction kinetics have been invoked to explain the formation of these structures by colloidal chemical reduction. In the case of completely immiscible Au and Rh metals, the nanostructures of a different segregation ranging from Au(core)–Rh(shell) to alloyed AuRh were identified. The formation of alloy nanostructures has been related to the fact that the reduction of the metals ions in the confined spaces (within micelles) facilitated co-nucleation via spatial sequestration. The Au–Rh alloy nanoparticles over the entire composition range across the miscibility gap were synthesized by the room-temperature borohydride co-reduction of aqueous metal salts solutions in the presence of variety of surface stabilizers (polyvinyl alcohol, polyvinylpyrrolidone, citrate, CTAB). The colloidal strategy was found to be effective in the synthesis of alloy nanoparticles of metals with miscibility gap in bulk-scale systems like Au–Rh, Au–Pt, Pt–Rh, Pd–Rh Steric stabilizers like polyvinylpyrrolidone (PVP) are usually used during the synthesis in order to protect nanoparticles from the excessive growth. The colloidal techniques have been reported to be more advantageous as they allowed for synthesis of size- and structure-controlled metal nanoparticles of almost uniform size. Such a procedure usually results in a poor control over the particles size, shape and nanoscale phase structures of the particles, surface segregation of the components and alloying degree or atomic distribution, while these properties are of crucial importance in determining the catalytic properties of bimetallic systems. Most of the literature studies on bimetallic systems preparation have focused on the traditional co-impregnation/reduction method. The Pd 85Ir 15/C catalyst of the Pd–Ir alloyed and the surface composition expressed by the Pd/Ir atomic ratio near to 6 displayed the highest activity which was 2.9-times higher relative to that of Pd. The catalytic studies involving formic acid electrooxidation reaction showed the activity enhancement for the L series catalysts with respect to monometallic Pd/C (twofold TOF increase) and H series counterparts. At the lowest bulk Ir content, 5 mol%, the particles have Ir-rich surface regardless of the preparation method. In contrast to the largely alloyed PdIr nanoparticles with the Pd-rich surface formed at low concentration of the metals precursors (0.02 M), the particles of almost closed surface and bulk Pd/Ir ratios composed mostly of randomly distributed single-phase domains were formed at high concentration (0.2 M). 4 nm), close to that of Ir (3.8 nm) and below that of Pd (6.2 nm) were formed regardless of the Pd/Ir proportion and the concentration of the metals precursors in the w/o. The Pd–Ir nanoparticles of similar average size (ca. Electrochemical characterization allowed estimating the PdH formation ability and the surface composition of Pd–Ir particles what was confirmed by XPS data. Structural characterization was performed using XRD, SEM and HRTEM techniques. The bimetallic particles were examined in terms of nanoscale phase properties (extent of Pd–Ir alloying, phase separation), surface composition (Pd and Ir fractions) and electrocatalytic performance for the formic acid oxidation reaction. Carbon (Vulcan XC-72)-supported bimetallic Pd–Ir catalysts with different Pd/Ir proportions (5–50 mol% Ir, 2 wt% Pd) were prepared by “water-in-oil” microemulsion method (w/o) using solutions of low (0.02 M, L series) and high concentration (0.2 M, H series) of the metals precursors (PdCl 2 and IrCl 3).
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |