Metal-alloy clusters

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]. 

Yamada Y, Castlemaim AW (1992) The magic numbers of metal tmd metal alloy clusters. J... 

Among large metal clusters there are also some heteronuclear or mixed-metal clusters that have been mentioned as metal-alloy clusters . The bimetallic Au-Ag clusters (p-Tol3P)i2Aui8Ag2oCli4 and [(p-Tol3P)i2Aui8Agi9Brn], whose metal frameworks are illustrated in Fig. 2.39, belong to this class. A very... 

Perspectives for fabrication of improved oxygen electrodes at a low cost have been offered by non-noble, transition metal catalysts, although their intrinsic catalytic activity and stability are lower in comparison with those of Pt and Pt-alloys. The vast majority of these materials comprise (1) macrocyclic metal transition complexes of the N4-type having Fe or Co as the central metal ion, i.e., porphyrins, phthalocyanines, and tetraazaannulenes [6-8] (2) transition metal carbides, nitrides, and oxides (e.g., FeCjc, TaOjcNy, MnOx) and (3) transition metal chalcogenide cluster compounds based on Chevrel phases, and Ru-based cluster/amorphous systems that contain chalcogen elements, mostly selenium. 

Pd trimethylphosphine carbonyl clusters, CO FTIR spectra, 39 174-175 PdjWjCpjfCOl fPPh,), 38 350-351 Pdo.2,Zro glassy metal alloy, 38 71 Pebbly surface model, 39 114 Peclet number, 27 80, 81... 

We have overviewed some strategies for the surface-mediated fabrication of metal and alloy nanoscale wires and particles in mesoporous space, and their structural characterization and catalytic performances. Extension of the present approaches for metal/alloy nanowires may lead to the realization of the prospechve tailored design of super active, selective and stable catalysts applicable in industrial processes. The organometallic clusters and nanowires offer exciting and prospechve opportunities for the creahon of new catalysts for industry. Various metal/ alloy nanowires and nanoparhcles in the anisotropic arrangement in porous supports would help in understanding the unexpected electronic and optic properties due to the quantum effect, which are relevant to the rational design of advanced electronic and optic devices. 

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]. 

Figure 9.61 ToF mass spectrum of metal-carbon cluster ions (TiC2+ and ZrnCm+ cluster ions) using a titanium-zirconium (50 50) mixed alloy rod produced in a laser vaporization source (Nd YAG, = 532 nmj and ionization by a XeCI excimer laser (308 ). ( . M. Davis, S. J. Peppernick and A. W Castleman, J. Chem. Phys., 124, 164304(2006). Reproduced by permission of American Institute of Physics.)...

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... 

There has been interest in using metal cluster compounds as precursors to heterogeneous catalysts on oxide supports.21 It is already known that metal alloys dispersed on supports are heterogeneous catalysts and they are widely used in petroleum refining. 

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