Alloy clusters

Interesting nanostructures, that may present an interaction among nanoclusters, with consequent increase of local field enhancement factor are obtained by irradiating AuCu alloy clusters with Ne ions at 190 keV [30]. 

Watanabe M, Uchida M, Motoo S. 1987. Preparation of highly dispersed Pt-I-Ru alloy clusters and the activity for the electrooxidation of methanol. J Electroanal Chem 229 395-406. 

R. W. Murray, Stable, monolayer-protected metal alloy clusters, J. Am. Chem. Soc. 120,9396-9397 (1998). 

In heterogeneous catalysis by metal, the activity and product-selectivity depend on the nature of metal particles (e.g., their size and morphology). Besides monometallic catalysts, the nanoscale preparation of bimetallic materials with controlled composition is attractive and crucial in industrial applications, since such materials show advanced performance in catalytic processes. Many reports suggest that the variation in the catalyst preparation method can yield highly dispersed metal/ alloy clusters and particles by the surface-mediated reactions [7-11]. The problem associated with conventional catalyst preparation is of reproducibility in the preparative process and activity of the catalyst materials. Moreover, the catalytic performances also depend on the chemical and spatial nature of the support due to the metal-support interaction and geometrical constraint at the interface of support and metal particles [7-9]. 

In some other cases, the intermetal electron transfer does not occur even during hour-long irradiations. The initial simultaneous reduction reactions of and M are followed by mixed coalescence and association of atoms and clusters with ions, homolog or not. Besides the dimerization of atoms of the same metal into M2 and M 2, coalescence of both types of atoms occurs twice more frequently. Subsequently, mixed coalescence and reduction reactions progressively build bimetallic alloyed clusters according to the statistics of encounters, therefore to the relative initial ion abundance [53]. 

The possible formation of an alloyed or a core-shell cluster depends on the kinetic competition between, on one hand, the irreversible release of the metal ions displaced by the excess ions of the more noble metal after electron transfer and, on the other hand, the radiation-induced reduction of both metal ions, which depends on the dose rate (Table 5). The pulse radiolysis study of a mixed system [66] (Fig. 7) suggested that a very fast and total reduction by the means of a powerful and sudden irradiation delivered for instance by an electron beam (EB) should prevent the intermetal electron transfer and produce alloyed clusters. Indeed, such a decisive effect of the dose rate has been demonstrated [102]. However, the competition imposed by the metal displacement is more or less serious, because, depending on the couple of metals, the process may not occur [53], or, on the contrary, may last only hours, minutes, or even seconds [102]. 

For example, when the mixed solution of Ag(CN)2 and Au(CN)2 is irradiated by y-radiolysis at increasing dose, the spectrum of pure silver clusters is observed first at 400 nm, because Ag is more noble than Au due to the CN ligand. Then, the spectrum is red-shifted to 500 nm when gold is reduced at the surface of silver clusters in a bilayered structure [102], as when the cluster is formed in a two-step operation [168] (Table 5). However, when the same system is irradiated at a high dose rate with an electron beam, allowing the sudden (out of redox thermodynamics equilibrium) and complete reduction of all the ions prior to the metal displacement, the band maximum of the alloyed clusters is at 420 nm [102]. 

Similarly, at moderate dose rate for the couple Au Cl4, Ag, gold initially appears at 520 nm. Therefore, Ag ions essentially act as an electron scavenger, and as an electron relay toward more noble gold ions as far as gold ions are not totally reduced. Then silver-coated gold clusters are formed and the maximum is shifted to 400 nm, which is that of silver (Fig. 11) [102]. But at higher y- or EB dose rate (irradiation time of a few seconds), the electron transfer is too slow to compete with coalescence and the spectrum of alloyed clusters... 

The same contrast between bilayered and alloyed clusters of Au-Pd has been observed at low- and high-dose rate, respectively [180], by using spectrophotometry, ED AX, and XPS analysis at partial and complete reduction. 

Clusters and alloys are molecular species that may show different catalytic activity, selectivity and stability than bulk metals and alloys. Small metal clusters and alloy clusters have been studied reeendy for potential use as catalysts, ceramic precursors, and as thin films. Several fundamental questions regarding such clusters are apparent. How many atoms are needed before metallic properties are observed How are steric and electronic properties related to the number, type and structure of such clusters Do mixed metal clusters behave like bulk alloy phases ... 

Temperature Programmed Reduction. Temperature-programmed reduction (TPR), one of the indirect analysis methods, yielded data that suggested that Sn was not reduced to zero-valent state (10,16). Burch (15) has reviewed early work on the characterization of this type of catalyst. Lieske and Volter (21) reported, based on the results obtained from TPR studies, that a minor part of the tin is reduced to the metal, and this Sn(O) combined with Pt to form "alloy clusters" but the major portion of the tin is reduced to only the... 

Fused catalysts allow the combination of compounds and elements in atomic dispersions which do not mix either in solution (e.g. oxides) or in the solid state. Melting provides the necessary means to generate an intimate, eventually atomically disperse distribution a carefully controlled solidification can preserve the mctastablc situation in the melt down to operation temperature. In the melt the preformation of molecules such as oxo complexes or alloy clusters can occur. The final short-range order of the catalyst is predetermined. Examples are alloys of noble metals with elements located in the main group sections or in... 

Ferrando R, Jellinek J, Johnston RL (2008) Nanoalloys from theory to applications of alloy clusters and nanoparticles. Chem Rev 108 845-910... 

While 7-radiolysis at relatively low dose rates enabled the synthesis of a few alloyed clusters such as Ag-Pd nanoparticles (also chemically synthesized), radiolysis at very high dose rates (by electron beams) led to the synthesis, at room temperature, of a lot of new alloys such as Au-Pd bimetallic nanoparticles/ Like in the case of Ag-Au system, at low dose rate (7-irradiation), bilayered Au -Pd.i,ii nanoparticles were obtained. However, at high dose rates (electron beams), the reduction is faster than the possible inter-metal electron transfer, then alloyed clusters were prepared. Moreover, since the radio-induced reduction of metal ions is faster at high dose rates, the synthesized particles are, in these conditions, always smaller with a narrow distribution in size. 

Silicon-based polymers form a dimensional hierarchy from disilanes, to crystal silicon, and through polysilanes, ladder polymers, siloxenes, polysilane alloys, clusters, and amorphous silicons and include unsaturated systems, such as polysilenes, hexasilabenzenes, and so on. Their properties depend basically on the network dimensions and can vary from conducting (metallic) and semiconducting to insulating. 

Fig. 35. Preparation of tailored Rhlr alloy clusters inside NaY from [Rh6 ,Ir,(CO)i6]-NaY x = 0-6) by the ship-in-bottle technique.

Studies on unsupported, bulk Pt-Sn alloys have been helpful in assessing the properties of Pt-Sn alloy clusters formed in catalysts [15, 16], A large number of surface science studies of Pt-Sn alloys have also been conducted the most extensive of these concerned the oxidation of PtjSn, in which much work was done by Hoflund and coworkers (as reviewed by Unger and Marton [17]). Bulk alloy samples can be problematic in fundamental chemisorption and catalytic reaction studies because of significant differences between the composition and structure of the surface and bulk. For example, exposure to or annealing in vacuum causes Sn... 

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