Vanadium clusters hydrogen

Both vanadium and niobium metals form dihydrides only at high pressures(27), and numerous phases with hydrogen compositions less than one(28). Experiments were performed to saturate vanadium clusters with deuterium. Figure 4 is a plot of the number of deuteri urn molecules found in the products. The solid straight lines are for D V ratios of 1 and 2. The corresponding curved dashed lines include corrections for some bulk atoms(2c). The best fit to the data including only surface atoms indicate a stoichiometry of 1.5. It is likely that this high surface stoichiometry is an indication of bulk i ncorporation of deuterium. 

Small ey(Ib) that cobalt and niobium clusters react in a very selective fashion with deuterium was rapidly followed by similar studies on a variety of other metal systems. Three groups simultaneously reported similar dramatic behavior for iron c usters( 2b, 3b). Di hydrogen or di deuteri urn addition reactions have been reported for vanadium( 3e), i ron(lc,2a-d,3b), cobalt(lb),... 

Figure 9. Plots of the rate constants (X) of iron, vanadium and niobium clusters reacting with di hydrogen/di denteri urn, and their respective bare cluster ionization potentials (solid lines) scaled as described in the text.

Fig. 25. A1 ternative toluene chemisorption systems involving the bipyramidal surface cluster of vanadium pentoxide. The structures (a, c, e, g, i) represent molecularly adsorbed toluene, while the remaining systems (b, d, f, h) model the dissociative adsorption, with two methyl hydrogens chemically bonded to the surface oxygens at the pyramid bases. Asterisks indicate positions which give rise to unstable MEC. The three atoms of the parallel complex (i), marked with an arrow, denote the extra instabilities appearing when the closed-system constraint (d N = 0) is imposed. The results are taken from Ref. 8. At each diagram the (I, E) stability diagnosis is also indicated (see Fig. 24)...

Such B-OH are frequently involved in strong B-OH—O-B hydrogen bonds to neighbouring clusters, and short O—O contacts are seen even in the absence of direct location of hydrogen atoms.The possibility of hydroxonium ions is harder to rule out definitively, especially as aquation in these crystals is often highly disordered. The bond valence sums for the vanadium metals are usually between 4.0-4.1. 

Fig. 12. Geometric structure of (a) the V17O42H16 cluster representing a section at the VO2(011) surface and (b) the V11O33H33 cluster representing a section at the V203(0001) surface. The V (O) atoms are shown as large (small) shaded balls while very small while balls refer to hydrogen atoms used to saturate peripheral oxygen atoms. The differently coordinated surface oxygen centers, 0(2), 0(3), and the 5- and 6-fold coordinated vanadium centers V(5), V(6) are labeled accordingly.

Redox properties of the catalysts can be determined, on the contrary, by submitting them to TPR (Thermal Programmed Reduction) with hydrogen (see for example Fig.l). Reduced catalysts can be reoxidized with pulses of oxygen so determining also the catalyst dispersion. These determinations have been made for vanadium based catalysts. It is very interesting to observe, for example, that clusters of vanadia of different sizes, corresponding to different dispersion indexes, can show very different redox properties, as it can be seen in Table 4 and Fig. 1. 

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