Supported clusters

To search for the forms of potentials we are considering here simple mechanical models. Two of them, namely cluster support algorithm (CSA) and plane support algorithm (PSA), were described in details in [6]. Providing the experiments with simulated and experimental data, it was shown that the iteration procedure yields the sweeping of the structures which are not volumetric-like or surface-like, correspondingly. While the number of required projections for the reconstruction is reduced by 10 -100 times, the quality of reconstruction estimated quantitatively remained quite comparative (sometimes even with less artefacts) with that result obtained by classic Computer Tomography (CT). 

Recently we reported EXAFS results on bimetallic clusters of iridium and rhodium, supported on silica and on alumina (15). The components of this system both possess the fee structure in Efie metallic state, as do the components of the platinum-iridium system. The nearest neighbor interatomic distances in metallic iridium and rhodium are not very different (2.714A vs. 2.690A). From the results of the EXAFS measurements, we concluded that the interatomic distances corresponding to the various atomic pairs (i.e., iridium-iridium, rhodium-rhodium, and iridium-rhodium) in the clusters supported on either silica or alumina were equal within experimental error. Since the Interatomic distances of the pure metals differ by only 0.024A, the conclusion is not surprising. 

Keywords Clusters Clusters, metal Clusters, supported metal Clusters, ligands bonded to Clusters reactivities and catalytic activities... 

Calculations on the basis of density functional theory have been used to check the structure of Rh clusters supported on zeolite X the results indicate a slightly twisted prism, nearly matching the octahedron inferred on the basis of the EXAFS data [28,29]. (These clusters were inferred not to be bare but to have hydride ligands, as described later). 

Fig. 3 Ir4 cluster supported at the six-ring of zeolite NaX as represented by density functional theory samples were characterized by Extended X-ray absorption fine structure (EXAFS) spectroscopy and other techniques [32]...

Fig. 4 Osmium clusters supported on MgO(OOl) a OssC/MgisOs and b OS5C at a surface point Vs defect site [33] these were represented by density functional theory, and the samples were characterized by EXAFS spectroscopy, transmission electron microscopy, and other techniques [15]...

Figure 3. Example of XRPD on small Au clusters supported on silica. Total diffraction intensity has been measured with area detector (IP) on BM08-GILDA beamline at the ESRF with A = 0.6211 A and 2min exposure time. Diffraction patterns were collected on Au-supported sample (Exp) and on silica support (Support). Difference patterns, corrected for fluorescence, IP efficiency, etc., are shown (n-Au).

Meier DC, Goodman DW. 2004. The influence of metal cluster size on adsorption energies CO adsorbed on Au clusters supported on Ti02. J Am Chem Soc 126 1892-1899. 

Figure 3.26 75 nm x 75 nm in situ STM images of0.5 MLAu clusters supported ontheTi02(l 1 0) surface in the presence of 0.1 Torr CO and 02 mixture at 300 K. (a-f) are consequently taken at the same surface area. The time intervals are... 

The induction of steric effects by the pore walls was first demonstrated with heterogeneous catalysts, prepared from metal carbonyl clusters such as Rh6(CO)16, Ru3(CO)12, or Ir4(CO)12, which were synthesized in situ after a cation exchange process under CO in the large pores of zeolites such as HY, NaY, or 13X.25,26 The zeolite-entrapped carbonyl clusters are stable towards oxidation-reduction cycles this is in sharp contrast to the behavior of the same clusters supported on non-porous inorganic oxides. At high temperatures these metal carbonyl clusters aggregate to small metal particles, whose size is restricted by the dimensions of the zeolitic framework. Moreover, for a number of reactions, the size of the pores controls the size of the products formed thus a higher selectivity to the lower hydrocarbons has been reported for the Fischer Tropsch reaction. 

Because the size regime of n=l-6 atoms is of great practical significance to the spectroscopic, chemical and catalytic properties of supported metal clusters in both weakly and strongly interacting environments (28), it is important to study very small metal clusters in various types of substrate as well as in the gas phase. In this way, one can hope to develop a scale of metal cluster-support effects (guest-host interactions) and evaluate the role that they play in diverse technological phenomena. 

Gold clusters supported on a mixed Ti02-Si02 surface show activity for propylene epoxidation comparable to Ti02-supported Au catalysts but with enhanced stability.104 Therefore, it was decided... 

A.K. Santra and D.W. Goodman, Size-dependent electronic, structural, and catalytic properties of metal clusters supported on ultrathin oxide films, in Catalysis and Electrocatalysis at Nanoparticle Surfaces, eds. A. Wieckowski, E.R. Savinova, and C.G. Vayenas. Marcel Dekker, New York, 2003, pp. 281-309. 

Catalysts prepared from iridium neutral binary carbonyl compounds and several supports have been studied extensively. Small Ir (x = 4, 6) clusters supported on several oxides and caged in zeolite, and their characterization by EXAFS, have been prepared [159, 179, 180, 194-196]. The nuclearity of the resulting metallic clusters has been related with their catalytic behavior in olefin hydrogenation reactions [197]. This reaction is structure insensitive, which means that the rate of the reac-hon does not depend on the size of the metallic particle. Usually, the metallic parhcles are larger than 1 nm and consequently they have bulk-like metallic behavior. However, if the size of the particles is small enough to lose their bulk-like metallic behavior, the rate of the catalytic reaction can depend on the size of the metal cluster frame used as catalyst. 

In this section, we present XPS results [48,49] which are highly relevant to the physics of this cluster system. These results and their interpretation are in sharp disagreement with photoemission results on AU55 reported recently [47]. Both our results and those of Quinten et al. will be discussed in connection with the XPS data reported on smaller gold cluster compounds [76,153] and on bare gold clusters supported on poorly conducting substrates [74,75],... 

A standard technique for the comparison of XPS results on bare gold clusters supported on substrates [74,75] is to relate surface coverage during deposition to average diameter of the particles formed by surface diffusion after deposition. 

There is quite satisfactory agreement between our data and that for the bare gold clusters supported on a non-conducting substrate, supporting the idea that... 

As described above, it was found that physicochemical properties of the iron cluster supported on zeolite and the catalytic activity for toluene disproportionation were significantly affected by the preparation conditions. The catalyst which was prepared by modifying NH Y with 0.25M Fe(N03)3 solution at 323K showed the highest activity among the samples obtained. 

Molecular oxygen was the active oxidant for the phenol synthesis and the maximum phenol selectivity (93.9%) was achieved on the N-interstitial Re cluster supported on HZSM-5. Molecular oxygen was activated in the space between the two Re6 octahedral cores of the Re10 cluster and benzene concertedly reacted on the activated 02 with the very low activation energy of 24 kj mol-1. The pore size of HZSM-5 is 5.5 A, which is similar to the size of the Re6-octahedral cluster framework,... 

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