Metal dusters spectroscopy

Valence photoelectron spectroscopy has also been used to follow the evolution of the electronic structure as the size of the metal duster increases. For instance, the Au 5d binding energies of [Au,i(PPh3)703], [Auss(PPh3)]206] and Au metal... 

EXAFS spectroscopy was already discussed for metal carbonyl dusters in Section 4.2.2.S. Since metal dusters are simpler in composition than metal carbonyl dusters, the EXAFS method [72, 73, 137] can provide more information about the former than about the latter. In particular, information about the structure of the... 

Zeolite supported metal dusters have been characterized by several kinds of NMR spectroscopy. Xe NMR spectroscopy provides a sensitive probe of the contents of the cages and NMR of spin active metals provide evidence for the various metals in zeolites. Spectra of the sorbed xenon give information about the chemical shifts assodated with the collisions of the xenon atoms with the cage walls and with the encaged spedes. Hence, the method can be used to estimate the average number of metal atoms per duster. [148] The line shape and the chemical shift are sensitive to the metal spedes present within the cages or channels. For example, zeolite supported metal dusters show much larger chemical shifts than the zeolite alone under the same experimental conditions. 

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. 

EXAFS spectroscopy is one of the most powerful methods for determining the structures of zeolite entrapped metal carbonyl dusters because it gives quantitative structural information. EXAFS spectroscopy can, in prindple, be used to characterize samples in reactive atmospheres. However, most of the reported results have been obtained for samples in unreactive atmospheres, and usually at liquid nitrogen temperature where the signal-to-noise ratio is markedly greater than that at room temperature. 

Many metal complexes and clusters are colored and have distinctive ultraviolet-visible spectra. [80] The method offers the advantage of ease of application, but it has been used only seldom in the characterization of zeolite entrapped oigano-metallics. The spectra may provide evidence of metal-metal bonds, as has been shown for carbonyl clusters of Fe, Ru, and Os, [81, 82] but there are hardly any data for zeolite entrapped clusters. The absorption bands of dusters are shifted to lower energy as the cluster nudearity increases. [83] Ultraviolet-visible spectroscopy has been used to detect the formation of [HFe3(CO)n] in NaY zeolite [50] and of clusters suggested to be [Pt,(CO),g] in NaY zeolite. [40-42] Since the spectra do not provide highly spedfic structural information, the method is of secondary importance. 

Electron paramagnetic resonance (EPR) [or electron spin resonance (ESR)] spectroscopy is useful for the characterization of spedes having unpaired electrons. Since most molecular dusters are diamagnetic, the application of this technique is limited. However, paramagnetic spedes may be formed under certain conditions when metal carbonyl clusters are formed in zeolite cages. Due to the... 

X-ray photoelectron spectroscopy (XPS) provides an indication of the oxidation states of the metals in dusters by comparisons of their binding energies with those of standards. Typically, the determinations are not exact and need further confirmation by other methods. XPS is espedally useful for detedion of changes in oxidation states. Since the technique requires ultrahigh vacuum, instability and volatility of the samples are often complications. This technique has been used to characterize the formation of Rh(CO)2 in NaY zeolite. [85]... 

The other vibrational spectroscopies, although less easily applied, may provide complementary structural information. Raman spectroscopy has been used to detect metal-metal bonds in metal oxide supported osmium [86] and iridium [87] clusters. This method might be expected to find application in the study of zeolite supported metal carbonyl dusters, but it is still far from routine since samples are subject to destruction by laser beams, and fluorescence often prevents measurement of useful spectra. 

Researchers have attempted to drive off the carbonyl ligands of molecular metal carbonyl clusters in the hope of preparing naked clusters of the same nuclearity. It is now evident, although this assertion contradicts some of the claims in the literature, that most of the attempts have failed and have instead led to increases in cluster nuclearity and loss of structural simplicity. Usually, the lack of sufficient characterization has prohibited the determination of the nudearities of all the resultant spedes, and often only the larger spedes (dusters) have been detected. With the increasing availability of techniques such as high resolution transmission electron microscopy and EXAFS spectroscopy this field is expected to develop rapidly. 

Vibrations in zeolite frameworks are observed in the range 250-400 cm , [161] whereas those of metal atoms, ions, and dusters within the framework are expected in the range 30-250 cm" . [161, 162] Far infrared spectroscopy is therefore an informative method for determining the locations of metal ions in zeolites. The method has been used to characterize various cations (e.g. Na, Cs, Co ) in zeolite Y, and has provided information about the cations location, population, and distribution. [163, 164] The method is sometimes more effective than X-ray diffraction to determine cation locations. 

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. 

Metal clusters in zeolites are catalysts for a number of reactions, including alkene hydrogenation and alkane hydrocracking. The former is an example of shape selective catalysis, whereby straight diain alkenes can enter the zeolite pores and react but branched alkenes cannot enter and so do not substantially react. The latter have been apidied commerdally. Pt dusters in the zeolites KL and BaKL are remarkably selective catalysts for the dehydroi dization of n-hexane to give benzene, and they are now applied commerdally. The origin of the selectivity is still not fully understood, but it may be primarily a consequence of the smallness of the Pt clusters, which consist of only about S or 6 atoms on average, as determined by EXAFS spectroscopy, H2 chemisorption, and electron microscopy. 

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