Niobium oxide species

The aqueous preparation oT supported niobium oxide catalysts was developed by using niobium oxalate as a precursor. The molecular states oT aqueous niobium oxalate solutions were investigated by Raman spectroscopy as a -function o-f pH. The results show that two kinds o-f niobium ionic species exist in solution and their relative concentrations depend on the solution pH and the oxalic acid concentration. The supported niobium oxide catalysts were prepared by the incipient wetness impregnation technique and characterized by Raman, XRD, XPS, and FTIR as a -function o-f niobium oxide coverage and calcination temperature. The Raman studies reveal that two types o-f sur-face niobium oxide species exist on the alumina support and their relative concentrations depend on niobium oxide coverage. Raman, XRD, XPS, and FTIR results indicate that a monolayer oT sur-face niobium oxide corresponds to 19%... 

Antonelli and co-workers have recently demonstrated that room temperature stoichiometric ammonia synthesis is possible with their mesoporous titanium and niobium oxide catalysts. In this study, they proposed that the ammonia species are formed via the reaction activated nitrogen with the underlying moisture of the support. Reversible, inter-conversion of and NH2 species via exposure to moist air for aluminophosphate oxynitride catalysts has been observed by FTIR and XPS by Marquez and co-workers. There has been a lot of interest in the literature in the development of novel routes for the low temperature stoichiometric conversion of nitrogen to ammonia, e.g.. However, in principle this could be realised by the nitridation of Li, followed by hydrolysis, although the kinetics would be very slow. 

Depending on niobium location, the Nb-containing catalysts can reveal Bronsted acid, Lewis acid, or redox properties. Niobium oxide cationic species (NbOn(5-2n)+), which occupy the extra lattice cation positions, play the role of the Lewis acid sites and may exhibit the redox properties. Nb localized in the framework of mesoporous MCM-41 sieves provides the Lewis acidity [3,4] and the oxidizing properties [5,12]. 

The Raman investigation of niobium species in aqueous solutions of niobium oxalate (Jehng and Wachs, 1991) nicely showed the dependence of their constitution on pH and concentration. The PZC theory was successfully applied to predict the hydrated, molecular structures of multicomponent supported metal oxide species, such as iron-molybdenum, iron-vanadium, molybdenum-vanadium, tungsten-vanadium, and sodium-vanadium oxide species (Vuurman et al., 1991 Wachs et al., 1993). 

It is known that the niobium oxide complexes in oxalic acid aqueous solutions display an equilibria between two ionic species containing 2 or 3 oxalate... 

However, evacuation at 525K removed this species, and a band at 1446 cm discovered. The latter has been assigned to pyridine adsorbed on niobium oxide [14]... 

Niobium oxides, provided by the ore treatment, are dissolved in chloride melts (NaCl-KCl or LiCl-KQ) in the form of NbCls [7], and Nb metal is produced by electroreduction in the molten chloride solution. A French company, Cezus, developed this process at the industrial scale in the 1990s [8] for Nb and other refractory metals. Nevertheless, the cathodic process in pure chloride melts was proved to be too complex to be industrially valid, with a series of intermediate steps [8, 9], and provides nonadherent or powder metal layers with a low current efficiency. Better results are obtained in chloride melts containing fluoride ions because of the complexation of Nb in NbF7 ions which are reduced in only two steps Nb —> Nb Nb [10,11], with current efficiencies less than 100 % since Nb reacts readily with Nb cathodic product (proportionation reaction) for giving the intermediate species, Nb. ... 

The data concerning the oxidation state of niobium and the coordination properties of its species in molten halides are incomplete and often contradictory. There is no doubt about the existence of niobium(IV) and (V) species in molten niobium-containing alkali chloride-based mixtures. The only question concerns the stability of NbClg" complex ions under an inert atmosphere. The other disputed moment involves the value of the lowest niobium oxidation state stable in chloride melts. According to the different points of view niobium-containing melts held in contact with the metal can contain Nb ", Nb + or Nb" + ions [1]. 

Further experiments were performed to ascertain the nature and to characterise the intermediate product of chlorination. The spectra recorded were similar for different temperatures and various types of alkali chloride mixtures (Figure 4.4.6). The results of oxidimetric titrations indicate the predominant formation of niobium(IV) species. Deviations of oxidation state to the higher values result from the partial oxidation of Nb(IV) into Nb(V) by HCI. Oxidation state values lower than four, obtained in the case of NbO chlorination, most likely were caused by niobium(II) oxide particles captured during melt sampling procedure and trapped in the quenched melt samples subjected to the analysis (Table 4.4.3). 

Additional experiments are required to determine the potential range values where particular niobium oxidation state species can be stabilised. 

This system typically uses sulfuric acid as the electrolyte with a proton exchange membrane. While a porous separator could be used, for high efficiency operation, ion-selective membranes are generally preferred as vanadium crossover leads to losses in coulombic efficiency. At present, Nafion is the membrane of choice as V(V) is a powerful oxidizing agent, which can attack cheaper hydrocarbon-based ion selective membranes [21]. The redox reactions of different vanadium species have displayed reversibility and high activity on carbon based electrodes. Moreover, Li et al. discovered the catalytic effects of bismuth nanoparticles on V(II)/V(III) [51] and of niobium oxide nanorods on both V(II)Af(lII) and V(IV)Af(V) [52], which have been shown to further enhance the energy efficiency of the VRB by more than 10 %. 

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