Alumina, supported, clusters

The EXAFS results suggested that the iridium-rhodium clusters dispersed on alumina differed in size and/or shape from those dispersed on silica, based on the result that the total coordination nunbers of the iridium and rhodium atoms in the clusters were very different (7 and 5 in the alumina supported clusters vs. 11 and 10 in the silica supported clusters). These coordination numbers suggested that the clusters dispersed on alumina were smaller or that they were present in the form of thin rafts or patches on the support. The possibility of a "raft-like" structure in the case of the alumina supported clusters suggests an interaction between the metal clusters and the support which is much more pronounced for alumina than for silica. If the clusters on the alumina were present as rafts with a thickness of one atomic layer, one could have a situation in which the rhodium concentration at the perimeter of the raft was greater... 

Strong support effects have been observed with Au catalysts and with this in mind, Anderson s group also studied Au/ZAljOj/NiAlCllO) for n=l [33]. In contrast to the results on TiO, alumina-supported clusters did not adsorb CO at room temperature and were completely inactive for CO oxidation. Au clusters supported on alumina apparently lack the active sites (either a deficiency in then-morphology or lack of proper interaction with the support) for CO oxidation. 

The alumina-supported cluster [Al]+[HRuOs3(CO)i3] was catalytically active for 1-butene isomerization. After catalysis, [Al]+[H3RuOs3(CO)i2] was the only detectable metal carbonyl species. This supported catalyst decomposed during ethylene hydrogenation at 340 K to give catalytically active metal particles. 

Preparation of silica- and alumina-supported clusters of platinum group metals Clusters in cages... 

A comment regarding the dispersion of the Ru-Rh/Si02 and the Ru-Ir/Si02 is in order. For the case of the supported Pt-Ru catalysts. Increases in dispersion as a result of clustering were very large ( ). This effect was particularly noticeable for bimetallic particles which conform to the cherry model. Evidently, the formation of an inner core enriched in one of the two metals, followed by an outer layer enriched in the other metal, inhibits further crystal growth. For the alumina-supported Pt-Ru bimetallic clusters, the effect, although present, is considerably smaller. 

GP 1] [R 1] A comparison of four micro reactors with different Pt loadings (Pt impregnated on anodically oxidized alumina support) and different Pt structures confirmed that cluster size has an impact on the single Pt-atom activity (6 vol.-% NHj, 88 vol.-% O2, balance He 0.51 ms 260-380 °C) [28, 98]. At low Pt loadings, isolated atoms are formed. Calculated ammonia consumption rates amount to 20 s at 300 °C. At high Pt loadings, clusters are formed. Turn over frequencies (TOP) of about 40 s are determined. 

Becerra LR, Klug CA, Slichter CP, Sinfelt JH. 1993. NMR-study of diffusion of CO on alumina-supported Pt clusters. J Phys Chem 97 12014-12019. 

Alumina-supported catalysts prepared using the bimetallic carbonyl precursors showed a better performance in alkene hydroformylation than conventional Co-Rh catalysts. This was related to the presence of highly dispersed Rh-Co clusters with frames corresponding to that of the parent carbonyl-precursor that were characterized by EXAFS [140, 183]. Silica-supported bimetallic entities RhCo3,... 

PPh2 group bridges a basal edge of the cluster. The related cluster HRu5C(CO)i3 /x.-P(Ph)CH2CH2Si(OEt)3 40 may be prepared in a similar two-step method to that of 39. This phosphine was employed as the substituent group could potentially be used to attach the cluster to a silica or an alumina support. 

Budge, J. R., Scott, J. P., and Gates, B. C., Synthesis and characterization of an alumina-supported RuOs3 cluster catalyst. J. Chem. Soc., Chem. Commun. 1983,342 (1983). 

The role of iron clusters in Fischer-Tropsch catalysis has been the focus of considerable studies. Cagnoli et al. have recently studied the role of Fe clusters on silica and alumina supports for methanation.22 Chemisorption, catalysis and Mossbauer spectroscopy experiments were used to study the effect of dispersion and the role of various supports. Although several oxidation states of iron were observed, the focus of this research was on Fe clusters which were found to be on the order of 12 A crystal size. The authors proposed that metal support interactions were greater for silica than alumina supports and that selectivity differences between these catalysts were due to differences in surface properties of silica vs. alumina. Differences in selectivity for Fe/SiC>2 catalysts at different H2/CO ratios were attributed to differences in coadsorption of H2 and CO. Selectivity differences are difficult to explain in such systems even when only one metal is present. 

Figure 5. Model for the alumina-supported, Cp2M2lr2(CO)io and CpMh 3-(CO)n cluster-derived catalysts.

An example of the enhanced resolution is presented in Figure 64, where the Au L3-edge XANES of a gold foil is shown both in the normal transmission mode and in the FIERFD mode (Safonova et al., 2006). This spectral sharpening was used to determine the actual adsorption site of CO on a small alumina-supported platinum cluster (Safonova et al., 2006). The measured spectra are shown in Figure 65. Clearly, the features in the XANES spectra recorded by the use of HERFD are richer than what is determined by using total fluorescence detection. 

Judging from IR, EXAFS, and UV-Vis spectra (63), larger nuclearity Os clusters such as Ose,(CO),8 and H20sjoC(CO)24 are not fragmented on alumina and MgO even on thermal activation at 523 K in a CO -I- H2 atmosphere. The stable Osg and Os clusters are bound to one or two oxygen atoms shared with the silica or alumina support (Fig. 18), and they retain their metal framework even on hydroxyl-containing surfaces and at elevated temperatures. 

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