Vanadate site nature

It must also be pointed out that other basic materials have been synthesized which present no surface oxygens and hydroxyls, but other types of active sites whose exact nature remains controversial. These type of solids are, for example, impregnated imides and nitrides on zeolites and alumina, amorphous oxynitrides obtained by treatment with ammonia or aluminium orthophosphate, zirconium phosphate, aluminium vanadate or galloaluminophosphate, and KF supported on alumina/1,3,41 One of the main advantages of these solids with respect to basic oxides is their resistance to carbon dioxide or water. 

The nature of the binding sites for vanadyl and vanadate in Tf has been inferred from EPR and NMR spectroscopy, respectively. Figure 5.22 provides selected spectral... 

The phosphate site. The phosphate site is highly regular and can be substituted by vanadate, chromate, arsenate, carbonate, silicate and sulfate, but only vanadate, arsenate and carbonate solid solution is widespread in natural apatites. Charge-coupled substitution schemes to aid incorporation involve the substitution of silicate for phosphate and REE for Ca, and carbonate or sulfate for phosphate and Na for Ca. Numerous oxyanions may act as activators and sensitizers, with the most widespead being molybdate and tungstate, but vanadate and niobate can also activate luminescence (Blasse and Grabmaier 1994). Silicate is a strong absorber of UV radiation and can serve as a... 

Environmental studies [17] of urban airsheds in several areas of the country have shown that high levels of atmospheric vanadium oxide are associated with industrialized areas, especially those areas where fossil fuels are burned or where vanadate steel is being produced. In addition, vanadium has been shown to exhibit increased tissue levels in fish and other marine animals associated with oil rigs in the Santa Barbara basin of the United States [18]. Blotcky et al. [19] determined the vanadium content in shrimp, crab, and oyster from four ocean sites off and near Galveston Island, Texas. They found that the vanadium content was greater in marine biological samples taken in waters near industrialized areas as compared to samples taken in waters near the nonindustrialized sections. Speciation of the vanadium is very important since the two oxidation states, i.e., IV or V, have different nutritional and toxic properties [14]. Orvini et al. [20] applied a preirradiation speciation method to freshwaters from the Italian Ticino and Po rivers and found out that vanadium was present in various tetravalent cationic and pentavalent anionic as well as in natural complexed forms. 

The number and nature of the active surface sites and the catalytic activity of bulk metal molybdates and vanadates were also investigated through methanol chemisorption [51-53]. These materials proved to be equally or more active and stable than the industrial catalyst Mo03/Fe2(Mo04)3 in formaldehyde production [54-56],... 

The observation that bulk metal vanadates posses a high selectivity toward formaldehyde strongly suggests that the surface of bulk metal vanadates is composed of vanadium oxide sites with redox properties that cover the counter-cation sites (Mg, Ni, Mn, Cr, Co, Zn, Al, Nb, Fe, Cu, and Ag) thus, inhibit methanol total oxidation. The TOF values of the bulk metal vanadates are similar indicating that there is no significant influence of the specific nature of the metal oxide counter-cation on the catalytic behavior. Moreover, there are significant differences between the TOF values of bulk metal vanadates and pure metal oxides. 

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