Bimetallic colloidal structure

This survey focuses on recent developments in catalysts for phosphoric acid fuel cells (PAFC), proton-exchange membrane fuel cells (PEMFC), and the direct methanol fuel cell (DMFC). In PAFC, operating at 160-220°C, orthophosphoric acid is used as the electrolyte, the anode catalyst is Pt and the cathode can be a bimetallic system like Pt/Cr/Co. For this purpose, a bimetallic colloidal precursor of the composition Pt50Co30Cr20 (size 3.8 nm) was prepared by the co-reduction of the corresponding metal salts [184-186], From XRD analysis, the bimetallic particles were found alloyed in an ordered fct-structure. The elecbocatalytic performance in a standard half-cell was compared with an industrial standard catalyst (bimetallic crystallites of 5.7 nm size) manufactured by co-precipitation and subsequent annealing to 900°C. The advantage of the bimetallic colloid catalysts lies in its improved durability, which is essential for PAFC applicabons. After 22 h it was found that the potential had decayed by less than 10 mV [187],... 

Wang Y, Toshima N (1997) Preparation of Pd-Pt bimetallic colloids with controllable core/ shell structures. J Phys Chem B 101 5301-5306... 

Bimetallic particles with layered structures have opened fascinating prospects for the design of new catalysts. Schmid et al. [10m] have applied the classical seed-growth method [20] to synthesize layered bimetallic Au/Pd and Pd/Au colloids in the size range of 20-56 nm. The sequential reduction of gold salts and palladium salts with sodium citrate allows the gold core to be coated with Pd. This layered bimetallic colloid is stabilized by trisulfonated triphenylphosphane and sodium sulfanilate. More than 90% metal can be isolated in the solid state and is redispersable in water in high concentrations. 

This very versatile preparation route for nano structured mono- and bimetallic colloids has been further developed by Reetz and his group since 1994 [2e,12a,b]. The overall process of electrochemical synthesis [Eq. (7)] can be divided into six elemental steps (see Figure 6). 

Figure 4. High-resolution electron microscopy (HRTEM) of bimetallic, shell structured gold-palladium colloids. The 18-nm gold cores (dark areas) are covered by a Pd shell ca 4-5 nm thick. J...

In view of the extensive literature covering the syntheses and structures of bimetallic metal carbonyl clusters, it is even more surprising that few attempts to use the fadle decomposition of these compounds in the preparation of bimetallic colloids have been reported. Although this would be an unnecessarily complicated route to colloids of misdble metals if the salts of these metals could be coreduced to the same end, there remains the possibility of preparing bimetallic partides of immisdble metals starting from well defined molecular bimetallic clusters of immisdble metals. Such dusters are known to exist desjnte the immis-dbility of their constituent metals in the bulk, and there is reason to believe that bulk immisdbility might not apply to very small partides due to the importance of surface effects. 

Reduction of two different precious metal ions by refluxing in ethanol/water in the presence of PVP gave a colloidal dispersion of core/shell structured bimetallic nanoparticles. In the case of Pd and Au ions, e.g., the colloidal dispersions of bimetallic nanoparticles with a Au core/Pd shell structure are produced. In contrast, it is difficult to prepare bimetallic nanoparticles with the inverted core/shell (in this case, Pd-core/Au-shell) structure. The sacrificial hydrogen strategy was used to construct the inverted core/shell structure, where the colloidal dispersions of Pd-cores are treated with hydrogen and then the solution of the second element, Au ions, is slowly added to the dispersions. This novel method, developed by us, gave the inverted core/shell structured bimetallic nanoparticles. The Pd-core/Au-shell structure was confirmed by FT-IR spectra of adsorbed CO [144]. 

In 1989, we developed colloidal dispersions of Pt-core/ Pd-shell bimetallic nanoparticles by simultaneous reduction of Pd and Pt ions in the presence of poly(A-vinyl-2-pyrrolidone) (PVP) [15]. These bimetallic nanoparticles display much higher catalytic activity than the corresponding monometallic nanoparticles, especially at particular molecular ratios of both elements. In the series of the Pt/Pd bimetallic nanoparticles, the particle size was almost constant despite composition and all the bimetallic nanoparticles had a core/shell structure. In other words, all the Pd atoms were located on the surface of the nanoparticles. The high catalytic activity is achieved at the position of 80% Pd and 20% Pt. At this position, the Pd/Pt bimetallic nanoparticles have a complete core/shell structure. Thus, one atomic layer of the bimetallic nanoparticles is composed of only Pd atoms and the core is completely composed of Pt atoms. In this particular particle, all Pd atoms, located on the surface, can provide catalytic sites which are directly affected by Pt core in an electronic way. The catalytic activity can be normalized by the amount of substance, i.e., to the amount of metals (Pd + Pt). If it is normalized by the number of surface Pd atoms, then the catalytic activity is constant around 50-90% of Pd, as shown in Figure 13. 

After our success in preparation of the colloidal dispersions of Pt-core/Pd-shell bimetallic nanoparticles by simultaneous reduction of PdCl2 and H2PtCl6 in refluxing ethanol/water in the presence of poly(V-vinyl-2-pyrroli-done) [15,16] several reports have appeared on the formation of the core/shell-structured bimetallic nanoparticles by simultaneous reactions [5,52,68,183]. 

Coreduction of Mixed Ions. Coreduction of mixed ions is the simplest method to synthesize bimetallic nanoparticles. However, this method cannot be always successful. Au/Pt bimetallic nanoparticles were prepared by citrate reduction by Miner et al. from the corresponding two metal salts, such as tetrachloroauric(III) acid and hexachloroplatinic(IV) acid (24). Reduction of the metal ions is completed within 4 h after the addition of citrate. Miner et al. studied the formation of colloidal dispersion by ultraviolet-visible (UV-Vis) spectrum, which is not a simple sum of those of the two monometallic nanoparticles, indicating that the bimetallic nanoparticles have an alloy structure. The average diameter of the bimetallic nanoparticles depends on the metal composition. By a similar method, citrate-stabilized Pd/Pt bimetallic nanoparticles can also be prepared. 

An alcohol reduction method has been applied to the synthesis of polymer-stabilized bimetallic nanoparticles. They have been prepared by simultaneous reduction of the two corresponding metal ions with refluxing alcohol. For example, colloidal dispersions of Pd/Pt bimetallic nanoparticles can be prepared by refluxing the alcohol-water (1 1 v/v) mixed solution of palladium(II) chloride and hexachloro-platinic(IV) acid in the presence of poly(/V-vinyl-2-pyrrolidone) (PVP) at about 90-95°C for 1 h (Scheme 9.1.5) (25). The resulting brownish colloidal dispersions are stable and neither precipitate nor flocculate over a period of several years. Pd/ Pt bimetallic nanoparticles thus obtained have a so-called core/shell structure, which is proved by an EXAFS technique (described in Section 9.1.3.3). 

Coreduction of Au and Pt ions by refluxing alcohol in the presence of PVP gives the colloidal dispersions of Au-core/Pt-shell structured bimetallic nanoparticles, as mentioned before. The formation of this bimetallic nanoparticles was traced by in situ UV-Vis spectra (68). The spectral change is shown in Figure 9.1.15, in which the peaks ascertained to be the metal ions disappear at first, and then the broad tailing peaks due to the colloidal dispersions appear. More precisely speaking, the tetrachloroauric(III) acid (at —320 nm) is reduced first, followed by reduction of hexachloroplatinic(IV) acid (at —265 nm). This order of reduction is consistent with the standard redox potential of the two metal ions. After the reduction of two... 

Colloidal dispersions of fine metal particles have a long history. Metal nanoparticles are now in the spotlight because of recent developments in nanometer-scale science and technology. Especially the precise structure of the monodispersed bimetallic nanoparticles has become clear quite recently, thanks to the development of EXAFS technology. The mechanism of formation, growth, and structure control is not completely clear yet. In some parts, especially in Section 9.1.4, the discussion may be speculative but is based on the experience of the present author for over 20 years. 

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