Vanadium oxide structure

The surface phase diagram of vanadium oxides on Rh(l 11) has been investigated in a series of papers of our group [4, 18, 19, 90, 101-103]. It is characterized by pronounced polymorphism and many different oxide structures have been detected as a function of coverage and growth temperature. The vanadium oxide structures for coverages up to the completion of the first monolayer formed on Rh(l 11) under the different preparation conditions may be subdivided into highly oxidized phases... 

The reaction of pure silica MCM-48 with dimethyldichlorosilane and subsequent hydrolysis results in hydrophobic materials with still a high number of anchoring sites for subsequent deposition of vanadium oxide structures. The Molecular Designed Dispersion of VO(acac)2 on these silylated samples results in a V-loading of 1.2 mmol/g. Spectroscopic studies evidence that all V is present as tetrahedral Vv oxide structures, and that the larger fraction of these species is present as isolated species. These final catalysts are extremely stable in hydrothermal conditions. They can withstand easily hydrothermal treatments at 160°C and 6.1 atm pressure without significant loss in crystallinity or porosity. Also, the leaching of the V in aqueous conditions is reduced with at least a factor 4. 

Second, the Tw of the reduction peak is shifted from 383 K for the fresh catalyst to 413 K for the spent catalyst. In our previous work [16] we found that the peak position in TPR strongly depends on the vanadium oxide structure and that the reduction peak shifts to higher temperatures when the degree of polymerization of the vanadium oxide species increases. Therefore, the shift in reduction temperature in Fig. 4 is attributed to an increase in the degree of polymerization of the vanadium oxide species under reaction conditions. 

The supported vanadium oxide on 7-AI2O3 is present as a well dispersed phase Tor 3-20% V2O5/AI2O3 (Figure 4). The Raman spectra oT V205/7-A1203 have been discussed beTore (11), and the vanadium oxide structure has been... 

The effect of the impurities besides sodium do not seem to play an important role in determining the structure of the surface vanadium oxide species. Flourine is the only other impurity whose concentration varies appreciably in the different alumina phases. Flourine cannot be responsible for influencing the vanadium oxide structures since flourine is present in similar concentrations in a and 6,0-Al2O3 while the vanadium oxide structures are changing. It may therefore be concluded that surface impurities other than sodium oxide do not contribute significantly to the change in the structure of the surface vanadium oxide species on the different alumina phases. 

Starting with the sample of low oxygen content, the vanadium oxide structure that is obtained after vanadium-oxidation by substrate annealing... 

The pyromellitic dianhydride is itself obtained by vapour phase oxidation of durene (1,2,4,5-tetramethylbenzene), using a supported vanadium oxide catalyst. A number of amines have been investigated and it has been found that certain aromatic amines give polymers with a high degree of oxidative and thermal stability. Such amines include m-phenylenediamine, benzidine and di-(4-amino-phenyl) ether, the last of these being employed in the manufacture of Kapton (Du Pont). The structure of this material is shown in Figure 18.36. 

The flexibility of polyoxovanadate species [VxOy]" to adopt a wide variety of structural configurations, such as rings, chains, and layers, is reflected in their predominance in a wide variety of hybrid solids. For example, approximately sixteen distinct hybrid vanadium oxides based solely on V205 layers, with about nine different layer types, have currently been synthesized [34—47]. Layered structures of... 

In the bulk form, vanadium oxides display different oxidation states and V—O coordination spheres and exhibit a broad variety of electronic, magnetic, and structural properties [96, 97], which make these materials attractive for many industrial applications. Prominent examples range from the area of catalysis, where V-oxides are used as components of important industrial catalysts for oxidation reactions [98] and environment pollution control [99], to optoelectronics, for the construction of light-induced electrical switching devices [100] and smart thermo-chromic windows. In view of the importance of vanadium oxides in different technological applications, the fabrication of this material in nanostructured form is a particularly attractive goal. 

We refrain here from giving an extensive overview of studies on the surface structure of vanadium oxide nanolayers, as this has already been done for up to year 2003 in our recent review [97]. Instead, we would like to focus on prototypical examples, selected from the V-oxide-Rh(l 1 1) phase diagram, which demonstrate the power of STM measurements, when combined with state-of-the-art DFT calculations, to resolve complex oxide nanostructures. Other examples will highlight the usefulness of combining STM and STS data on a local scale, as well as data from STM measurements, and sample area-averaging spectroscopic techniques, such as XPS and NEXAFS, to derive as complete a picture as possible of the investigated system. 

The growth of vanadium oxide overlayers on Rh(l 11) converges after a number of intermediate stages to the formation of a three-dimensional bulk-like epitaxial V203 film [90], which is oriented with the (0 0 01) plane of its corundum structure parallel to the Rh(l 1 1) substrate surface. The V203 phase is the thermodynamically stable... 

Self-assembled nanorods of vanadium oxide bundles were synthesized by treating bulk V2O5 with high intensity ultrasound [34]. By prolonging the duration of ultrasound irradiation, uniform, well defined shapes and surface structures and smaller size of nanorod vanadium oxide bundles were obtained. Three steps which occur in sequence have been proposed for the self-assembly of nanorods into bundles (1) Formation of V2O5 nuclei due to the ultrasound induced dissolution and a further oriented attachment causes the formation of nanorods (2) Side-by-side attachment of individual nanorods to assemble into nanorods (3) Instability of the self-assembled V2O5 nanorod bundles lead to the formation of V2O5 primary nanoparticles. It is also believed that such nanorods are more active for n-butane oxidation. 

XRD patterns of the prepared samples V-Mo-Zeolite are similar to that of zeolites which suggests that the metal species (i.e. oxide, cations,...) are well dispersed through the zeolites structure and the absence of bulk phases in the XRD patterns implies that for these samples the molybdenum and vanadium oxides are present in either a nanocrystalline state or as a small crystallites which measured less than 4 nm in diameter. Furthermore, XRD and FTIR (1500-400 cm 1) showed no significant damage of the zeolite host structure after exchange and thermal treatment except for the sample V2MoMor. 

Murphy et al. made an extensive study of a number of vanadium oxides and discovered the excellent electrochemical behavior of the partially reduced vanadium oxide, VeOis, which reacts with up to 1 LiA/. They also recognized that the method of preparation, which determines the V 0 ratio, critically controls the capacity for reaction with lithium. The structure consists of alternating double and single sheets of vanadium oxide sheets made up of distorted VOe octahedra. A variety of sites are available for lithium intercalation, which if filled sequentially would lead to the various steps seen in the discharge curve. The lattice first expands along the c-axis and then along the b-axis. Thomas et ai 87 91 an in-depth study of the complex... 

Another vanadium oxide that has received much attention is LiVaOs, which has a layer structure composed of octahedral and trigonal bipyramidal ribbons that can be swelled just like other layered compounds and can intercalate lithium. Here again, the method of preparation is important to its electrochemical characteristics. West et al. made a systematic study of the impact of synthesis technique on capacity and cycling and showed that amorphous material increased the capacity above 2 V from 3—4 lithium per mole of LiVsOs at low current drains, 6—200 fiAlcm. ... 

Double-Sheet Structures Xerogels, d-Vanadium Oxides, and Nanotubes... 

Although much of the V NMR has been performed on model systems or catalytic materials containing vanadium, 29 >30 compounds such as V2O5 or VOPO4 are used in both the catalysis and lithium battery fields, and many of the results can be used to help elucidate the structures of vanadium-containing cathode materials. V NMR spectra are sensitive to changes in the vanadium coordination number and distortions of the vanadium local environments from regular tetrahedra or octahedra. >33 5>V isotropic chemical shifts of between —400 and —800 ppm are seen for vanadium oxides, and unfortunately, unlike... 

Lithium intercalation in VeOis has been studied by Stallworth et al. ° Variable-temperature Li NMR indicated considerable mobility for Li+ in the intercalated materials. The Li NMR data were compared with ESR spectra and near-edge X-ray absorption fine structure (NEXAFS) data on the same materials, and a correlation between vanadium oxidation state (from NEXAFS data) and NMR shift was observed. The authors explained the shifts in terms of different coupling mechanisms between the and shifts. The shifts were, however, extracted from static NMR experiments, and it is possible that some of the different local environments, typically revealed in a MAS spectrum, were not seen in this study. 

Simple Binary and Related Compounds.—Oxides. The information presently available concerning the crystal structures and properties of vanadium oxides has been tabulated. 

Vanadium Oxides The stmcture of silica-supported vanadium oxides, which can catalyze the selective oxidation of NO and hydrocarbons [117, 118], has been assigned to tetrahedral oxovanadium(V) structures like [(=SiO)3VO], through inter alia Raman and NMR data [117], rather than to octahedraUy coordinated decavan-... 

Ti02 nanotubes were used to support M0O3 observing a spontaneous dispersion of molybdenum-oxide on the surface of nanotubes, which was different from that observed on titania particles.Supporting tungsten oxides a preferential orientation of the (002) planes was observed. Vanadium-oxide in the form of nanorods could be prepared using the titania nanotube as structure-directing template under hydrothermal... 

Another way of investigating structure is through the classical method on metals of varying catalyst particle size. The key to this method is to measure active catalyst surface areas in order to determine changes in turnover rates with ensemble size. In recent years several chemisorption techniques have been developed to titrate surface metal centers on oxides (25). In this volume Rao and Narashimha and Reddy report on the use of oxygen chemisorption to characterize supported vanadium oxide. 

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