Bimetallic metal clusters

Although the mechanism of the photo-induced generation of mono- and bimetallic metal clusters, except for the photographic application (Section 20.6), has been studied with considerably less detail than for the radiolytic route, some stable clusters, mostly of noble metals (Ag, Au, Pt, Pd, Rh), have also been prepared by UV excitation of metal ion solutions [129-141]. Generally, halides and pseudo-halides counter anions are known to release, when excited, solvated electrons, which reduce the metal ions up to the zerovalent state. Oxalate excitation yields the strong reducing carbonyl radical COO [30]. Photosensitizers are likewise often added [142]. Metal clusters are photo-induced as well at the surface of photo-excited semiconductors in contact with metal ions [143,144]. 

Cryophotochemical techniques have been developed that (i) allow a controlled synthetic approach to mini-metal clusters 112), Hi) have the potential for "tailor-making small, bimetallic clusters (mini-alloy surfaces) 114,116), Hi) permit the determination of relative extinction-coefficients for naked-metal clusters 149), and iv) allow naked-cluster, cryophotochemical experiments to be conducted in the range of just a few atoms or so 112,150,151). 

A method has recently been described for wrapping polymers around metal atoms and very small metal clusters using both matrix and macroscale metal vapor-fluid polymer synthetic techniques. Significant early observations are that (i) the experiments can be entirely conducted at, or close to room temperature, (ii) the resulting "pol5aner stabilized metal cluster combinations are homogeneous liquids which are stable at or near room temperature, and (,iii) the methodology is easily extended to bimetallic and trimetallic polymer combinations. ... 

There has been great interest in the preparation of bimetallic transition metal cluster complexes containing palladium.899-902 Bimetallic palladium-ruthenium clusters have been shown to be good precursors to supported bimetallic catalysts.903,904... 

The well-defined large bimetallic Au—Ag cluster as [(Ph3P)i2AuigAg2oCli4] can achieve nearly two orders of magnitude of attenuation of high-intensity laser power and, consequently opens the door to a new class of optical materials based on nanosized metal clusters 3269... 

Recently, metal cluster catalysts composed of two different metallic elements are of interest from both scientific and technological points of view because of their interesting physiochemical properties [46, 47]. Bimetallic clusters are known to exhibit specific reactivity. Their catalytic efficiency is also controlled by their size. 

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. 

Molecular precursors for tailored metal catalysts, 38 283-392 see also Bimetallic catalysts, cluster-derived Zeolites carbon-supported, 38 389-390 chemical interaction between clusters and supports, 38 295-296... 

Bimetallic metal particles are important materials because their characteristics, especially their catalytic properties, are often quite different from those of pure metal particles. Dendrimer-encapsulated bimetallic clusters can be synthesized by any of three methods (Fig. 17) (1) partial displacement of the dendrimer-encapsidated cluster, (2) simultaneous co-complexation of two different metal ions followed by reduction, or (3) sequential loading and reduction of two different metal ions. 

Preparation of mixed-metal intradendrimer clusters by partial displacement is a straightforward extension of the complete displacement approach for forming single-metal clusters described in the previous section. If less than a stoichiometric amount of Ag+, Au +, Pd +, or Pt + is added to a Gh-OHlCujj) solution, or if less than a stoichiometric amount of Au +,Pd +, or Pt + is added to G6-OH(Agiio) solution, it is possible to form Ag/Cu,Au/Cu (Au/Ag),Pd/Cu (Pd/Ag), and Pt/Cu (Pt/Ag) bimetallic clusters inside dendrimers. 

Ni and Co or of oxophilic metals, for example. Re, is still poorly studied the surface-mediated synthesis of bimetallic carbonyl clusters is limited to a few examples the surface-mediated synthesis of metal compounds without carbonyl ligands has just begun with the silica-mediated synthesis of [RhH2(PMe3)4] by treatment of bis (allyl) rhodium with PMe3 followed by H2 [121] the silica-mediated synthesis of tantalum clusters has been investigated recently but the products were not extracted from the surface-for example, treatment of silica physisorbed Ta(CH2Ph)5 in H2 at 523 K for 20 h led to tri-tantalum clusters, as shown by EXAFS spectroscopy [122]. 

The mobility of metal atoms in bare metal clusters and small metallic nanoparticles (NPs) is of fundamental importance to cluster science and nanochemistry. Atomic mobility also has significant implications in the reactivity of catalysts in heterogeneous transformation [6]. Surface restmcturing in bimetallic NP and cluster catalysts is particularly relevant because changes in the local environment of a metal atom can alter its chemical activity [7, 8]. 

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 facile loss of carbonyl groups in the mass spectra of metal carbonyls permits the generation of novel bare metal cluster ions Mj in the mass spectra of polynuclear metal carbonyls of the type Mx(CO)y. Thus in the mass spectra of Mn2(CO)io and Co2(C0)s all carbonyl groups are lost before rupture of the metal-metal bond resulting in the production of the bimetallic ions Mn and Co 2, respectively 14>. 

In the preparation and activation of a catalyst, it is often the case that the chemical form of the active element used in the synthesis differs from the final active form. For example, in the preparation of supported metal nanoclusters, a solution of a metal salt is often used to impregnate the oxide support. The catalyst is then typically dried, calcined, and finally reduced in H2 to generate the active phase highly dispersed metal clusters on the oxide support. If the catalyst contains two or more metals, then bimetallic clusters may form. The activity of the catalyst may depend on the metal loading, the calcination temperature, and the reduction temperature, among others. 

It is not currently possible to model the full thermal decomposition of a large number of carbonyl ligands from relatively large bimetallic clusters followed by the subsequent electronic and atomic relaxation using only DFT. To attempt to smdy metal clusters derived from organometallic precursors therefore, we need a rapid approximate method for generating plausible models of the final denuded state of the clusters. 

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