Metal encaged clusters

The radial electron distribution (RED) determined from X-ray diffraction data has been frequently used to characterize the structures of encaged metal clusters. In contrast to EXAFS spectroscopy, the RED gives only metd-metal distances, not metal-support and metal-adsorbate distances. 

Zeolite supported metal clusters are important catalysts. The zeolite matrix not only imposes steric constraints on reacting molecules (shape selective catalysis) and provides addic sites, but it also apparently affects the electronic properties of the encaged metal clusters. 

Up to 1999, only metal atoms [1-5], metal clusters [6,7], metal nitrides [55-57], and noble gas atoms [58-60] were observed to be encaged inside C60, C70, or various sizes of higher fullerenes. The experimental evidence for carbon atoms or metal-carbon compounds (carbides) being encapsulated inside fullerenes had not yet been observed. In 2000, Shinohara et al. succeeded in the first production, isolation, and spectroscopic characterization of a scandium carbide endohedral fullerene (Sc2C2) C84. Following this, the first experimental evidence based on synchrotron X-ray diffraction was presented and revealed that the Sc carbide is encapsulated in the form of a lozenge-shaped Sc2C2 cluster inside the D2d-C84 fullerene [8]. 

As clarified later, such a classification is reasonable and useful because electron transfers exist from the encaged metallic species to the fullerene cages so that the structures and properties depend strongly on the encapsulated atom(s). Particularly, cluster metallofullerenes show different properties from those containing only metals (mono-metallofullerenes and di-metallofullerenes), which, in return, strongly affects the synthesis and extraction processes, structures, chemical reactivities, and their applications. Consequently, we must, to a certain degree, address cluster metallofullerenes separately in the following text. 

With the confinement of CNTs, direct observations of the dynamic motions of EMFs and the encaged metallic species were also achieved using high-resolution transmission electronic microscopy (HRTEM). Figure 7.18 shows that the motions of the fullerene cage and the encaged ErsN cluster are clearly resolved [185],... 

For PtCu/NaY, however, most of the coke is presumably not in intimate contact with metal clusters, its oxidation therefore takes place at a higher temperature. Since splll -over of active oxygen from the oxidized metal over finite distance can be excluded, the oxidation of this coke will not be catalyzed by Pt or Cu. In the reduced form of the catalysts the Ft ensembles at the surface of the encaged bimetal particles are diluted with Cu and additional protons of hi Brensted acidity were formed in the reduction of each Cu ion to Cu°. The combination of both phenomena, small Pt ensembles and hl concentration of protons of strong Brensted acidity, results in an Increase of the RE/RO activity ratio as observed and reported(S). It is therefore reasonable to attribute the TPO peak at higher temperature to the coke deposited on the acid sites of the zeolite via carbenium ion formation and polymerization. The results are thus quite similar to those observed and... 

Transition metal clusters in zeolite cages form another important class of supported species. Zeolites are very suitable supports/hosts for small metal particles because the dimensions of their cavities affect the formation of encaged moieties of nanometer and sub-nanometer scale, thus stabilizing clusters of desired sizes and shapes. Experimental investigations in this direction resulted in preparation procedures that yield nearly uniform small encapsulated metal moieties in zeolites [18-20]. 

The encaged clusters considered in this chapter are almost exclusively metal carbonyls and metals (including bimetallics). llte encaged metal carbonyls that have been most investigated include [Rh (CO)i6], [Ir4(CO)i2], and the isomers of [Ir (CO),d the crystal structures of the iridium clusters are shown in Figure 4-6. Some of the most thoroughly characterized encaged metal dusters have been made from these metal carbonyls. Brief mention is made of metal oxide and also nonmetal clusters ionic dusters are scarcely considered. Synthesis, characterization, reactivity, and catalytic and other properties are considered for these materials. 

An important consequence of the effects of the adduct formation on the Vco infrared spectra of encaged metal carbonyl clusters is that the spectra of dusters with bridging carbonyl groups, which are rather strongly basic, are significantly shifted from those of the clusters in neutral solvents, and identification by comparison with the spectra of the same dusters in solution is usually not straightforward. This point has not always been appredated in the literature. 

Characterization is relatively simple since the dusters contain only metal atoms, and usually of only a single element. However, there is one problem asso-dated with the characterization of metal clusters in cages. In contrast to the situation for metal carbonyl dusters, there is no base data set for these metal clusters themselves in a pure state to be used for comparison. This means that spectra of encaged metal dusters cannot be compared with those of their analogues in the liquid or solid state because they simply are not known. Thus the basis for structure determination is in a sense weaker than that for metal carbonyl dusters. 

The best way to control the nuclearity of metal clusters is to trap them in the calibrated pores of zeolites. The production of clusters encaged in zeolites has been described in a recent review [5]. Metal loading is carried out either by ion exchange and reduction or by adsorption and decomposition of organometallic complexes. If these treatments are performed under proper conditions, the clusters can be obtained at precise locations in the zeolite micropores, which impose an upper limit on the cluster size. Thus in faujasite-type zeolites, the supercages can accommodate up to 60 metal atoms. 

Figure 29. Water clusters with H30+ or metal ions (depicted by a shaded circle) encaged inside the clathrate (bfeOho. Taken with permission from Int. J. Mass Spectrom. Ion Proc. 1994, 131, 233-264.

This principle appears amenable to generalization active sites and catalyst promoters can be positioned in the same cage in order to systematically study catalyst promoter effects due to direct interaction of metal particles and metal ions. Quantum chemical calculations by van Santen et al. have resulted in detailed predictions, e.g., of the effects of Mg ions, that are in direct contact with zeolite-encaged Ir4 tetrahedra, on the adsorption of H2 (i72) or CO 373) on these clusters. These theoretical results should be verified experimentally, as they could form a basis for general predictions on the action of ionic promoters on chemisorbing transition metals. 

The IR spectra of endohedral Li3 xMxN C80, where 0 < x < 3, M = Sc, Y, Tb, Ho or Er, include vibrations of an encaged trimetal nitride cluster.2 The Raman spectra of the D2d symmetry species M2 C84, where M = Sc, Y or Dy, show three groups of metal-cage modes in the range 35-200 cm-1. Variable-temperature data for M = Y shows that there is an order/disorder transition near 150 K.3... 

Ousters adsorbed on the outside surfaces of zeolites can often be easily extracted with neutral solvents or with salt solutions which remove the cluster ions by cation metathesis (ion exchange). Comparison of the infrared spectra of the extracted spedes and those of known spedes helps identify the encaged spedes. When treatment of a sample with such solutions fails to remove sorbed dusters, they are inferred to be trapped within the cages. The inference is supported when the same clusters adsorbed on the surface of a large pored material such as an amorphous metal oxide are removed by extraction. 

Extraction metal carbonyl clusters internal or external location of metal carbonyl cluster in zeolite Carbonyl cluster encaged in the zeolite cages cannot diffuse through the zeolite aperture and cannot be extracted out effective for anionic dusters but not effective for some neutral carbonyl clusters that are difficult to dissolve in solvent. 

These results also illustrate the advantages of using zeolite encaged dusters as precursors for well defined metal catalysts. Some encaged clusters appear to be stable to cycling through oxidation and reduction without forming crystallites on the external zeolite surface. 

Metal NMR spectroscopy is also beginning to gain favor as a technique for the characterization of encaged clusters. [156] For example, Zhang et al. [157] used Co spin echo NMR spectroscopy to characterize the size and location of the Co dusters in the cages of NaY zeolite. 

The measurement of the sorption of CO and of NO in combination with infrared spectroscopy gives valuable structural information about encaged dusters. Primet et al. [166] showed that the smaller the Pt duster, the higher the vibrational frequency Vnq of NO sorbed on the metal. A similar trend is expected for CO. When using this method one must be aware of the possibility that the CO or NO may cause oxidative fragmentation of the clusters or lead to cluster agglomeration. 

Since the oxygen content of the clusters can be quantitatively adjusted by a thermal vacuum induced reversible reductive-elimination oxidative-addition of dioxygen, the electronic properties of these oxide clusters can be easily manipulated as a result of their facile redox interconvertibility. Ozin et al. [230] discovered how to alter local electrostatic fields experienced by the zeolite encaged tungsten oxide by varying the ionic potential of the constituent supercage ions across the alkali metal series. This method provides the first opportunity to fine tune the band gap of tungsten oxide clusters. 

There is a rapidly developng literature describing other metal oxides in zeolites, with Ti02 being well represented. [231-234] The characterization of these materials is largely based on ultraviolet-visible spectroscopy, and the structures are less than well understood. Whether the encaged spedes are clusters and whether they are uniform in structure remains to be determined. Similar statements can also be made about the materials described in the following sections (4.6.2.3 and 4.6.2.4). 

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