Bimetallic system

Scheme 5.1. Alloy formation and segregation in bimetallic systems with one of the metals present as a minority. The scheme qualitatively predicts whether two elements form a surface alloy or a solid solution. The results are valid in vacuum. As soon as an adsorbing gas is...

Extended X-ray absorption fine structure (EXAFS) studies have been very useful for obtaining structural information on bimetallic cluster catalysts. The application to bimetallic systems is a particularly good one for illustrating the various factors which have an influence on EXAFS. Moreover, the applicability of EXAFS to this area has been very timely, in view of the enormous interest in bimetallic systems in both catalytic science and technology. 

From the viewpoint of size control, bimetallic systems are usually very convenient to produce monodispersed metal nanoparticles [49]. Although the exact reason is not clear yet, this is probably attributed to the redox equilibrium between the two elements. 

In 1993, we examined formation processes of PVP-protected AuPt bimetallic nanoparticles by in-situ UV-Vis spectroscopy during the reduction [53]. Figure 8 shows the in-situ UV-Vis spectra during the simultaneous reduction of Au(III) and Pt(IV) ions. In the case of PVP-protected AuPt bimetallic system, Au(III) ions are... 

Figure 8. In-situ UV-Vis spectra during the formation of PVP-protected AuPt (1 1) bimetallic system in the region of l<350nm (a) and 1>350 nm (b). (a) Sampling from the solution of the metal ions in ordinary conditions for preparation at 100 °C with refluxing (b) the solution of the metal ions in a quartz UV cell was heated up to 80 °C without refluxing. (Reprinted from Ref [53], 1993, with permission from Elsevier.)...

Figure 9. Proposed formation process of PVP-protected AuPt bimetallic system. (Reprinted from Ref [53], 1993, with permission from Elsevier.)...

This means that the improvement of catalytic activity of Pd nanoparticles by involving the Pt core is completely attributed to the electronic effect of the core Pt upon shell Pd. Such clear conclusion can be obtained in this bimetallic system only because the Pt-core/Pd-shell structure can be precisely analyzed by EXAFS and Pd atoms are catalytically active while Pt atoms are inactive. 

Greeley J, Kibler L, El-Aziz AM, Kolb DM, N0rskov JK. 2006b. Hydrogen evolution over bimetallic systems understanding the trends. Chem Phys Chem 7 1032-1035. 

The latter report demonstrated the unique ability of this technique to resolve surface structure as well as surface composition at the electrified solid-liquid interfaces. In particular, STM has become an important tool for ex situ and in situ characterization of surfaces at the atomic level, in spite its significant limitations regarding surface composition characterization for bimetallic systems, such as the lack of contrast for different elements and the scanned surface area being too small to be representative for the entire surface. To avoid these limitations, STM has been mostly used as a complementary tool in surface characterization. 

In our early work with bimetallic systems, we noticed that, depending on the preparation procedure in UHV, different surface compositions could be produced over the same bulk material owing to the phenomenon of surface segregation [Stamenkovic et al., 2002]. It was essential, then, to establish a methodology for transferring a well-defined bimetallic system into an electrochemical environment for further electrochemical characterization. 

Overall, this chapter aimed to emphasize and demonstrate the great potential of utilizing a multidisciplinary approach to bimetallic systems that combines computational methods with a number of highly sophisticated in situ and ex situ surface-sensitive techniques at electrified solid-liquid interfaces. Advances in the understanding of fundamental properties that govern catalytic processes at well-defined multimetallic... 

GroB A. 2006. Reactivity of bimetallic systems studied from first principles. Top Catal 37 29. 

Information on the chemical state of iridium on going from the molecular precursors, and its adsorption on the surface of the support can be obtained by Ir Mossbauer spectroscopy. It allows to estimate the composition of the Ir-containing alloys that are possibly formed during the activation treatment of supported bimetallic systems. The main results obtained in the application of Ir Mossbauer spectroscopy to characterize two Ir-containing bimetallic supported nanoparticles, i.e., Pt-Ir on amorphous silica and Fe-Ir on magnesia are presented and discussed... 

Related bimetallic systems were also prepared with M = Ni, Pd, Pt or Ag [3-5]. This unprecedented Tl2- t2-SiO bonding mode is also found in the siloxane-substituted complex 4 [6], and may assist the rapid c-Ji-isomerization observed in the allylic complex 5. 

An explanation for this increase in selectivity with the addition of aluminum could be related to the better dispersion of iron metallic clusters, which could be anchored to the acidic sites on the mesoporous support, as observed by Lim et al [13] for bimetallic systems in MCM41. 

In addition to palladium catalysts, Co(OAc)2 shows a catalytic activity for the arylation of heterocycles, including thiazole, oxazole, imidazole, benzothiazole, benzoxazole, and benzimidazole.78 As shown in Scheme 6, the catalytic system Co(OAc)2/9/Cs2C03 gives G5 phenylated thiazole, while the bimetallic system Co(OAc)2/CuI/9/Cs2C03 furnishes the G2 phenylated thiazole. The rhodium-catalyzed reaction of heterocycles such as benzimidazoles, benzoxazole, dihydroquinazoline, and oxazoline provides the arylation product with the aid of [RhCl(coe)]2/PCy3 catalyst.79 The intermediacy of an isolable A-heterocyle carbene complex is proposed. 

Ffirai and Toshima have published several reports on the synthesis of transition-metal nanoparticles by alcoholic reduction of metal salts in the presence of a polymer such as polyvinylalcohol (PVA) or polyvinylpyrrolidone (PVP). This simple and reproducible process can be applied for the preparation of monometallic [32, 33] or bimetallic [34—39] nanoparticles. In this series of articles, the nanoparticles are characterized by different techniques such as transmission electronic microscopy (TEM), UV-visible spectroscopy, electron diffraction (EDX), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) or extended X-ray absorption fine structure (EXAFS, bimetallic systems). The great majority of the particles have a uniform size between 1 and 3 nm. These nanomaterials are efficient catalysts for olefin or diene hydrogenation under mild conditions (30°C, Ph2 = 1 bar)- In the case of bimetallic catalysts, the catalytic activity was seen to depend on their metal composition, and this may also have an influence on the selectivity of the partial hydrogenation of dienes. 

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