Vanadium oxide characterization

The feasibility of synthesizing oxovanadium phthalocyanine (VOPc) from vanadium oxide, dicyanobenzene, and ethylene ycol using the microwave synthesis was investigated by comparing reaction temperatures under the microwave irradiations with the same factors of conventional synthesis. The efficiency of microwave synthesis over the conventional synthesis was illustrated by the yield of crude VOPc. Polymorph of VOPc was obtained ttough the acid-treatment and recrystallization step. The VOPos synthesized in various conditions were characterized hy the means of an X-ray dif actometry (XRD), a scanning electron microscopy (SEM), and a transmission electron Microscopy (TEM). 

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... 

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. 

The surface structure and reactivity of vanadium oxide monolayer catalysts supported on tin oxide were investigated by various physico-chemical characterization techniques. In this study a series of tin oxide supported vanadium oxide catalysts with various vanadia loadings ranging from 0.5 to 6. wt.% have been prepared and were characterized by means of X-ray diffraction, oxygen chemisorption at -78°C, solid state and nuclear magnetic resonance... 

The technique of solid-state NMR used to characterize supported vanadium oxide catalysts has been recently identified as a powerful tool (22, 23). NMR is well suited for the structural analysis of disordered systems, such as the two-dimensional surface vanadium-oxygen complexes to be present on the surfaces, since only the local environment of the nucleus under study is probed by this method. The nucleus is very amenable to solid-state NMR investigations, because of its natural abundance (99.76%) and favourable relaxation characteristics. A good amount of work has already been reported on this technique (19, 20, 22, 23). Similarly, the development of MAS technique has made H NMR an another powerful tool for characterizing Br 6nsted acidity of zeolites and related catalysts. In addition to the structural information provided by this method direct proportionality of the signal intensity to the number of contributing nuclei makes it a very useful technique for quantitative studies. 

The influence of the specific oxide support phase upon the structure and reactivity of the surface vanadia species was also recently investigated.54 A series of titania-supported vanadia catalysts were synthesized over a series of Ti02 supports possessing different phases (anatase, rutile, brookite and B). Raman and solid state vanadium-51 characterization studies revealed that the same surface vanadia species were present in all the different V20/ri02 catalysts54. The reactivity of the surface vanadia species on the different oxide supports was probed by methanol oxidation and the TOFs are shown in Figure 6 (all the catalysts contained 1% V205)... 

Analysis of structure-activity relationships shows that various species characterized by different reactivities exist on the surface of vanadium oxide-based catalysts.339 The redox cycle between V5+ and V4+ is generally accepted to play a key role in the reaction mechanism, although opposite relationships between activity and selectivity, and reducibility were established. More recent studies with zirconia-supported vanadium oxide catalysts showed that vanadium is present in the form of isolated vanadyl species or oligomeric vanadates depending on the loading.345,346 The maximum catalytic activity was observed for catalysts with vanadia content of 3-5 mol% for which highly dispersed polyvanadate species are dominant. 

Characterization, and Battery Applications of Silver Vanadium Oxide Materials... 

Leising, R.A. and E.S. Takeuchi. 1994. Solid-state synthesis and characterization of silver vanadium oxide for use as a cathode material for lithium batteries. Chem. Mater. 6 489 195. 

Crespi, A.M., PM. Skarstad, and H.W. Zandbergen. 1995. Characterization of silver vanadium oxide cathode material by high-resolution electron microscopy. J. Power Sources. 54 68-71. 

The results of the above characterization studies indicate that also in titania-supported catalysts the vanadium oxide layer slightly sinters. Since the vanadium oxide dispersion strongly effects the activity of the catalyst [16], it is likely that this sintering process is causing the deactivation observed in Fig. 3. The TPR and TPD results show that also some carbonaceous deposits are formed under reaction conditions, but these deposits are only present in low concentrations and, therefore, not likely to cause the deactivation of the catalyst. 

Bond GC, Tahir SF. Vanadium oxide monolayer catalysts Preparation, characterization and catalytic activity. Applied Catalysis. 1991 71(1) 1—31. 

A criterion for the suitability of a spectroscopy cell for investigations of working catalysts can be formulated as follows the activity or selectivity data and activation energy values have to be in agreement with the catalytic performance data measured with a conventional fixed-bed reactor. Table 1 is a comparison of the conversion and selectivity values characterizing an alumina-supported molybdenum-vanadium oxide catalyst during propane ODH obtained with a conventional fixed-bed reactor and with a spectroscopic cell that fulfills this requirement (Banares and Khatib, 2004). Similar considerations have also been reported earlier for other methods, such as X-ray diffraction (Clausen et al., 1991). 

Bruckner and Kondratenko (2006) used a similar approach to characterize VOx/Ti02 catalysts. In a separate TPR experiment carried out with a quartz reactor equipped with a UV-vis fiber optical probe, the relationship between the "absorbance" at 800 nm and the degree of reduction as determined from H2 consumption via mass spectrometry was established. The absorbance at 800 nm increased with increasing reduction of the vanadium, but not linearly. During the catalytic reaction experiment, the absorbance at 800 nm was then used to determine the average valence of vanadium. Because contributions of reduced titanium species in the analyzed spectral range could not be excluded, only a lower limit of the vanadium oxidation state could be determined, which was 4.86 at 523 K and C3H8/02 = 1 1. 

The chemistry of vanadium is characterized by multiple oxidation states (Fig. 1). The redox chemistry of this metal undoubtedly plays a role in its biochemistry. Of the four common oxidation states, only V(III), V(IV), and V(V) are important biologically, V(II) being too reducing to exist in any known organism. The best known example of the occurrence of V(III) is in the vanadocytes of the blood of tunicates22 otherwise, vanadium is largely found in the +4 and +5 oxidation states, both of which are readily accessible under physiological conditions. 

Prior to this disclosure, Trifiro (154) proposed that the active catalyst is pure vanadyl pyrophosphate and found that the catalyst was characterized by a slight increase in the vanadium oxidation state after the equilibrium period. The small increase from -1-4.00 to -h4.03 was reproducible and attributed to the formation of isolated V " surface sites. The P/V ratio was proposed to be a key characteristic in the stabilization of V + within the catalyst, as VOPO4 formation becomes very difficult at P/V ratios >2.0. Trifiro had stated that a very high surface P/V ratio is required for an active and selective catalyst, and experimentally he has found surface P/V ratios of 10 1. 

Iron vanadate, FeV04, is a prospective material for lithium rechargeable batteries and in catalysis. In [90] mechanical coactivation of iron and vanadium oxides was used to prepare intimate nanoscale mixture, similar to those prepared by soft chemistry. Reduction of this mixture at the same temperature and oxygen partial pressure conditions as of soft chemistry products (500°C and 10 Pa) leads to formation of a nanometric vanadium ferrite with the only spinel phase. The characterization of the powders thus prepared was perfomed by X-ray diffraction, SEM, IR spectrometry, thermogravimetry and colourimetry. It was shown that the homogeneity of grain size and chemical composition is achieved if the initial oxides have similar grain size. 

PREPARATION AND CHARACTERIZATION OF NANOPOWDERS AND NANOCERAMICS OF VANADIUM OXIDES... 

Recent studies of supported vanadium oxide catalysts have revealed that the vanadium oxide component is present as a two-dimensional metal oxide overlayer on oxide supports (1). These surface vanadium oxide species are more selective than bulk, crystalline V2O5 for the partial oxidation of hydrocarbons (2). The molecular structures of the surface vanadium oxide species, however, have not been resolved (1,3,4). A characterization technique that has provided important information and insight into the molecular structures of surface metal oxide species is Raman spectroscopy (2,5). The molecular structures of metal oxides can be determined from Raman spectroscopy through the use of group theory, polarization data, and comparison of the... 

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