Vanadium clusters oxygen

Theory has been used predominantly to probe the nature of the sites on vanadium clusters and model vanadium oxide surfaces. Cluster and p>eriodic DFT calculations [68,69] have been carried out in order to imderstand the electronic and structural properties of the exposed (100) surface of (VO)2P207. Both cluster and slab calculations reveal that surface vanadium sites can act as both local acid and base sites, thus enhancing the selective activation of n-butane as well as the adsorption of 1-butene. Vanadium accepts electron density from methylene carbon atoms and, thus aids in the subsequent activation of other C-H bonds. Calculations reveal that that the terminal P=0 bonds lie close to the Fermi level and thus present the most nucleophihc oxygen species present at the surface for both the stoichiometric as well as phosphate-terminated surfaces. These sites may be involved in the nucleophilic activation of subsequent CCH bonds necessary in the selective oxidative conversion of butane into maleic anhydride. Full relaxation of the surface, however, tends to lead to a contraction of the terminal P=0 bonds and a lengthening of the P V bonds. This pushes the P V states, initially centered on the oxygen atoms, higher in energy and thus increases their tendency to be involved in nucleophilic attack . 

Figure I. Oxygen uptake by supported clusters and vanadium oxide. Samples were pre-reduced and re-oxidized at the temperatures indicated on the abscissa. Silica-supported polyoxometalates PVl ( ), PV3 (A), PVI4 ( ). Bulk V2O5 (+, after [10])...

Figure 6.10 STS differential conductance spectra taken in constant height (left, blue curves) and constant current (right, blackcurves) modes from the (a) ( j7 x 7)R19.1°, (b) (5 x /3)-rect, (c) (9 x 9) structures, and (d) from the star cluster. The DFT-calculated DOS for the oxygen (red) and vanadium (green) atoms in the star cluster are shown in (d) for comparison. (Reproduced with permission from Ref. [23].)...

The reactions of the vanadium oxide cluster cations with CCI4 were of three types (248). The small cluster ions reacted by transfer of a chloride ion. The larger clusters starting with the V4OJ+ series reacted by the addition of a chlorine atom to the cluster or the loss of one oxygen atom and the addition of two chlorine atoms. 

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)...

Agaskar et al. 28) proposed that the catalyst surface contains clusters of four active dimeric sites that can each exist in one of four states. These states differ in the number of oxygen atoms associated with them and in the oxidation state of the vanadium ions present (Figure 23). The V " " site [So] acts as an oxygen acceptor. It can either react with gaseous O2 to give... 

Grasselli et al. [75] based their model on four vanadium dimers within each cluster that can assume one of the four distinct states, 80 83, shown Figure 10. The original state of the vanadyl dimer (Si) is that in the bulk (200) plane of vanadyl pyrophosphate. The Si site is transformed into S3 after molecular oxygen forms peroxo or superoxo species on the coordinatively unsaturated in the dimer. Two additional states are possible, denoted by So and S2, with zero and two vanadyl oxygens, respectively. 

In addition to bulk material, vanadium oxide cluster ions, such as V20(4.e/, V30(6 8), V40(8-ii/, VsOdi.n), V60(i3.i5), and V70(i6.i8)" [78, 79], can be prepared in gas phase. The chemical reactivity of these species shows a distinct dependence on cluster size. While the smaller clusters are reacting quite easily with other molecules, the reactivity of the larger systems is decreased with the exception of oxygen-rich clusters, which can release molecular oxygen upon collision with reactant gas. 

Theoretical studies on various physical and chemical parameters of vanadium oxide surfaces have been performed using both repeated slab as well as local cluster models. However, the vast majority of studies has focused on the (010) surface of the pentoxide, V2O5. This is a result of rather little experimental information on details of V Oy surface systems other than V205(010) that has attracted great interest due to its possible importance in catalytic applications. Further, most of the theoretical work has been based on cluster type studies where local surface behavior, in particular near surface oxygen sites, is discussed in detail. This will be reflected in the following discussion. 

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