Cluster of molybdenum

In the preparation of Mo/KL, the addition of 1 was stopped when pH of the solution was lowered to about 3. The resulting Mo/KL contained only 2.1 wt% (74% of added molybdenum clusters) of molybdenum. Chlorine was absent, which indicates that the cluster 1 acted as a tetravalent cation. The LTL structure is characterized by a monodimensional system of channels, whose diameter (0.70 nm) [14] is close to the size of the cluster 1. It is conceivable that, once the cluster 1 was incorporated into an LTL main channel and present at a site near the external surface, the sites deep in the channel are no more accessible to another cluster. 

Figure If shows the molecular structure of the [Tc6Br6(/t3-Br)5]2- anion. In general, the structure of this anion is similar to the structure of the well-known octahedral halogenide clusters of molybdenum and tungsten [M6X8]4 + (X = Cl,Br, I) [5,8]. The principal difference is that the eight equivalent positions of bridging bromine atoms in the technetium clusters are not fully occupied.

We were quite optimistic in the beginning as the second reduction process corresponds to the formation of a black deposit which was potentially the first electrochemical route to make thick tantalum layers. After having washed off all ionic liquid from the sample we were already a bit sceptical as the deposit was quite brittle and did not look metallic. The SEM pictures and the EDX analysis supported our scepticism and the elemental analysis showed an elemental Ta/Cl ratio of about 1/2. Thus, overall we have deposited a low oxidation state tantalum choride. Despite the initial disappointment we were still eager to obtain the metal and found some old literature from Cotton [122], in which he described subvalent clusters of molybdenum, tungsten and tantalum halides. In the case of tantalum the well-defined Ta6Cli22+ complex was described with an average oxidation number of 2.33 and thus with a Ta/Cl molar ratio very close to 1/2. Such clusters are depicted in Figure 4.15. 

ELI Reactivity of dinuclear and tetranuclear clusters of molybdenum and chromium... 

Thus it is positioned, that at interaction MoClj with acetylene in not polar mediums there is allocation FICl, downturn of a degree of oxidation of molybdenum and formation metallo-oiganic nanoclusters. Two distances Mo-Mo are found out in these nanoclusters by method EXAFS spectroscopy. In coordination sphere. Mo there are two nonequivalent atom of chlorine and atom of carbon. On the basis of results MALDI-TOF mass spectrometry, the conclusion is made that cluster of molybdenum has 12- or the 13-nuclear metal skeleton and its structure can be expressed by formulas[MOj,Cl,(q j)] or [MOj3CyCj3H,)]. ... 

Mitchell S A, Llan L, Rayner D M and Hackett P A 1995 Reaction of molybdenum clusters with molecular nitrogen J. Chem. Rhys. 103 5539... 

D. gigas formate dehydrogenase seems to be quite different in terms of subunit composition. It does not contain a y subunit and no heme c was detected (225). Also, two MGD were identified, but surprisingly, the enzyme contains tungsten instead of molybdenum. Mossbauer and EPR studies confirmed the presence of two [4Fe-4S] + + clusters with similar properties to the ones found in D. desulfuricans FDH (247). 

MgO-supported model Mo—Pd catalysts have been prepared from the bimetallic cluster [Mo2Pd2 /z3-CO)2(/r-CO)4(PPh3)2() -C2H )2 (Fig. 70) and monometallic precursors. Each supported sample was treated in H2 at various temperatures to form metallic palladium, and characterized by chemisorption of H2, CO, and O2, transmission electron microscopy, TPD of adsorbed CO, and EXAFS. The data showed that the presence of molybdenum in the bimetallic precursor helped to maintain the palladium in a highly dispersed form. In contrast, the sample prepared from the monometallie precursors was characterized by larger palladium particles and by weaker Mo—Pd interactions. ... 

Fig. 6.1 The fundamental structural unit found in the Chevrel phases (cluster MoeXg full circles Mo atoms) displayed in three ways to emphasize different views of the connectivity. In (a) an octahedron of molybdenums (Mo-Mo = 2.7 A) is encased in a cube of chalcogens (Mo-S 2.45 or Mo-Se 2.6 A). Scheme (b) exhibits the same cluster as consisting of an octahedron with its triangular faces capped by chalcogenides. In (c), the cluster has been reoriented so that a threefold axis is vertical. (Reproduced from [10])...

This paper describes the successful incorporation of molybdenum and molybdenum-nickel clusters into zeolites with 12-membered ring by aqueous ion exchange and application of the resulting materials to HDS reaction of benzothiophene. Stoichiometry of the ion exchange was examined by elemental analysis. UV-visible spectroscopy and EXAFS measurements were carried out to investigate the structure of molybdenum species loaded on zeolites. 

The cluster 2 was also introduced into NaY by ion exchange. After the ion exchange, 99% of molybdenum and nickel were loaded on NaY the Ni/Mo ratio did not change. The Cl/Mo ratio suggests that excess chlorine was present on the catalyst. 

Taking into account the high Mo content (5 - 7%wt) of V-Mo-ZSM-5 samples and to the conclusions of the different characterisations, we may assume that die most of molybdenum and vanadium are well dispersed on die surface of ZSM-5 in form of small oxide clusters. 

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