Vanadyl termination

Fig. 17.1 V Oj surface with different termination and degrees of perfection. Top view of stepped (top), reduced (vanadium-terminated) (middle) and oxidized (vanadyl-terminated) (bottom) V203(0001) surface. Possible configurations of CO adsorbed on different sites are included reprinted from [12] with permission. Copyright (2007) Springer...

Figure 15.18 STM images (20 x 20nm ) of the vanadyl-terminated (a) and metal-terminated (b) surface ofVj03(0001) thin films from Guimond, 2006 [75].

Figure 15.29 XPS spectra of the vanadyl- (a,b) and vanadium-terminated V20j(0001) surface (c) showing a characteristic shift of V2pj 2 emission of the vanadyl-terminated surface to higher binding energies, which indicates a change in the oxidation state. As a reference in (d), the XPS spectrum of VjOj is shown from Guimond, 2006 [75].

Figure 15.30 (a) Valence band photoemission spectra of the vanadyl-terminated... 

V20j(0001) surface below (TOOK) and above (300K) the metal-to-insulator transition temperature, (b) Comparison of the VB spectra from a vanadyl-terminated and x. 3)-reconstructed V OjfOOOl) surface taken at TOOK from Pfuner, 2005 [140],... 

These results show that for ideally terminated surfaces (we consider the vanadyl termination also as an ideal 20j(0001) termination because it represents a stable surface configuration), bulk-specific properties may also be observed on the surface. However, a perturbation of the local surface structure leads to significant changes in these properties. 

In presence of triphenyl phosphite or tri(4-methyl 6-f-butylphenyl) phosphite, (6) also acts as an oxidation inhibitor for polyenes. The complexes formed have not been studied, but the effect is attributed to termination of radical chains and the complexes are more active than the phosphites alone.29 Vanadyl acetylacetonate is also employed as a hardener for unsaturated polyesters.30... 

Agrawal and Wei (1984) isolated the metallochlorin and confirmed that the apparent fractional order kinetics resulted from a sequential mechanism much like HDS and HDN reactions. The hydrodemetallation of both nickel- and vanadyl-etioporphyrins on oxide CoMo/A1203 proceeded through two mechanistically different steps, an initial reversible hydrogenation followed by a terminal hydrogenolysis step... 

However, since the catalytic system is homogenous, carefully adjusted reaction conditions were needed to circumvent the second nonselective catalytic cycle. Slow addition of the alkene and the hydrogen peroxide was necessary to obtain good enantioselectivities (Table 6) [13]. Recently, Backvall s group reported that the Cinchona alkaloid ligand participated in the reoxidation process and took the role of NMO in the catalytic cycle [15]. Versions of the triple catalytic system with vanadyl acetylacetonate replacing the flavin analogue [16] or m-CPBA as the terminal oxidant [17] have been developed and successfully applied to racemic dihydroxylation reactions. 

Figure 6.2 V 2p and O Is region ofV203 (0001) on W(llO) in comparison with V2O5 according to Dupuis and coworkers [52]. The spectra indicate that the V2O3 (0001) surface is terminated with vanadyl groups.

Ebner and Thompson [58] proposed the termination of the surface with pendant H2P207 groups, which surround two vanadyl dimers (type B). Such surface termination may be envisioned by replacing every phosphate tetrahedron facing up in the bulk (200) planes of (V0)2P207 with pyrophosphate groups. In this type of surface termination, the pyrophosphate fence isolates pairs of vanadyl dimers from other pairs, which was proposed to be beneficial for the selectivity to maleic anhydride during n-butane oxidation. 

The adsorption of CO on pristine V=O terminated V2O3 was limited (Fig. 17.10a). The small TPD desorption peak at 105 K was most likely due to the adsorption of CO on a small number of defects (sites where the vanadyl oxygen was missing... 

The TPD/XPS results indicated that CO, propane and propene bind stronger to a reduced V-terminated 203(000 ) surface than to an oxidized V = 0 terminated surface. Nevertheless, vanadyl groups are probably required in the course of catalytic reactions. However, rates of propane oxidative dehydrogenation (ODH) to propene at atmospheric pressure are rather low and no reaction products were observed by gas chromatography, both for oxidized and reduced V Oj model surfaces at temperatures up to 500 K [12]. 

Vanabins, storage proteins for the vanadyl cation found in ascidians, and containing up to 20 VO + attached to amine terminals of lysines (section 4.1.2 and Figure 4.4). A vanadium(IV)-binding protein was also isolated from the bacterium Pseudomonas isachenkovii (Section 4.5) which utilises vanadate as terminal electron acceptor in anaerobic respiration, and appears as well to be present in the hydrogen bacterium Xanthobacter autotrophicus. ... 

During TPR water is formed and desorbs from the surface [35]. It is observed that the hydrogen consumption starts at 350 K, whereas water is formed at temperatures above 425 K. The monomeric vanadyls are restored after one cycle of reduction and oxidation. For polymeric vanadates terminal vanadyl oxygen atoms are removed leaving the structure unchanged. Upon oxidation the polymeric vanadate splits into a dimeric form and a monomeric vanadyl (Fig. 2). Oszkan et al. [36] concluded from their TPD studies that two types of ammonia species were present at the surface. One type was ammonia coordinatively bonded to surface vanadyls (Lewis sites) and a second type of ammonia species formed by the reaction of NH3 and surface hydroxyl groups [36]. 

Namely, the spectral features of the low loaded VPS 2 catalyst (Fig. 4a), consisting of a main absorption band at ca. 40,000 cm associated with the CT process in the terminal vanadyl V=0 bonds monitors in such system the presence of a monomeric tetrahedral coordinated (TJ) vanadia species (7v) [10], while the component at lower wavenumbers ( 34,000 cm ) arises from the CT in di/oligomeric structures formed by condensation of the monomeric ones,... 

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