Niobium oxide catalysts

Datka, J. Turek, A.M. Jehng, J.M. Wachs, I.E. Acidic properties of supported niobium oxide catalysts An infrared spectroscopy investigation. J. Catal. 1992, 35, 186-199. 

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. 

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%... 

Supported niobium oxide catalysts have recently been shown to be e-f-fective catalysts Tor many catalytic reactions pollution abatement, selective oxidation,... 

Niobium ethoxide [Nb(0C2H5)g] has traditionally been used as a precursor Tor the preparation oT supported niobium oxide catalysts. This non-aqueous preparation... 

The crystalline Nb205 phase in the supported niobium oxide catalysts was detected by an APD 3600 automated X-... 

Improve activity and test stability of platinum-niobium oxide catalysts. 

Figure 1.3 Left. Detailed view of the Nb K-edge XANES data of a pyridine salt of niobium-exchanged molybdo(vanado)phosphoric acid (NbPMo fVJpry) as a function of temperature [31]. A change in niobium oxidation state, from Nb5+ to Nb4+, is identified between 350 and 420°C by a relative increase in absorption about 19.002 keV, and can be connected with the activation of the catalyst for light alkane oxidation. Right. Radial Fourier-transform EXAFS function for the NbPMo (V)pyr sample heated to 420°C [31 ]. The two peaks correspond to the Nb-O (1.5 A) and Nb-Mo (3 A) distances in the heteropolymolybdate fragments presumed to be the active phase for alkane oxidation. (Reproduced with permission from Elsevier.)...

VV Bhat, A. Rougier, L. Aymard, G.A. Nazii, J.-M. Tarascon, High surface area niobium oxides as catalysts for improved hydrogen sorption properties of ball milled MgH, J. Alloys Compd. 460 (2008) 507-512. 

Niobium Products Co., 50 m /g). Many different synthesis methods have been used to prepare supported metal oxide catalysts. In the case of supported vanadium oxide catalysts, the catalysts were prepared by vapor phase grafting with VOCI3, nonaqueous impregnation (vanadium alkoxides), aqueous impregnation (vanadium oxalate), as well as spontaneous dispersion with crystalline V2O5 [4]. No drastic reduction of surface area of the catalysts was observed. 

Niobium and titanium incorporation in a molecular sieve can be achieved either by hydrothermal synthesis (direct synthesis) or by post-synthesis modification (secondary synthesis). The grafting method has shown promise for developing active oxidation catalyst in a simple and convenient way. Recently, the grafting of metallocene complexes onto mesoporous silica has been reported as alternate route to the synthesis of an active epoxidation catalyst [21]. Further the control of active sites, the specific removal of organic material (template or surfactant) occluded within mesoporous molecular sieves during synthesis can also be important and useful to develop an active epoxidation catalyst. Thermal method is quite often used to eliminate organic species from porous materials. However, several techniques such as supercritical fluid extraction (SFE) and plasma [22], ozone treatment [23], ion exchange [24-26] are also reported. 

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]. 

P oly 0 xome tall ate s, derived from both isopoly adds and heteropoly adds, are important homogeneous oxidation catalysts. The metals involved are vanadium, niobium, tantalum, molybdenum, and tungsten. The reactions involved are the oxidation of a wide range of organic compounds by hydrogen peroxide or organic hydroperoxide. 

LEIS is most often carried out with a beam of helium ions, but when a catalyst contains heavier elements, it may be advantageous to use neon ions to distinguish better between the elements (see Fig. 4.13). Of course, elements with a mass smaller than neon cannot be detected in that case. Figure 4.20 illustrates this with He- and Ne-LEIS spectra of a multicomponent oxide catalysts containing vanadium, molybdenum, niobium and tellurium. These mixtures are of inter-... 

Preparation of metal oxide thin film by means of stepwise absorption of metal alkoxide has been carried out in the past for the activation of heterogeneous catalysts [13]. For example, Asakura et al. prepared one-atomic layer of niobium oxide by repeating chemisorption of Nb(OEt)5 on silica beads. The catalyst obtained by immobilizing platinum particles on a niobum oxide layer showed improved reactivity for hydrogenation of ethylene in comparison with... 

Hence, the challenge remains to fully analyze the surface layers of bulk oxide catalysts. Recently, Zhao et al. succeeded for the first time in detecting surface MoO and VOv species on molybdenum-niobium and vanadium-niobium mixed metal oxides by Raman spectroscopy (Zhao et al., 2003). 

Flowever, a true catalytic effect is most probably present in transition-metal doped magnesium. A proof of this is the fact that nanostructured catalyst gives enhanced sorption properties compared to its micro-sized counterparts [226-229]. Hanada et al. also showed that after milling the catalyst is homogeneously distributed on a nanometer scale [230]. A possible interpretation of the catalytic effect may be the appearance of ternary magnesium-niobium oxides, which was evidenced by TEM [229, 231] and neutron diffraction [232]. Despite the abundant literature on... 

A series of supported niobium oxide on alumina catalysts, 0-45% Nb205/Al203, were further characterized by XRD, XPS, CO2 chemisorption, as well as Raman spectroscopy in order to determine the monolayer content of the Nb205/Al203 system. The transition from a two-dimensional metal oxide overlayer to three-dimensional metal oxide particles can be detected by monitoring the... 

A similar technique has been used to determine the acidic character of niobium oxide and niobyl phosphate catalysts in different solvents (decane, cyclohexane, toluene, methanol and isopropanol) using aniline and 2-phenyl-ethylamine as probe molecules [27, 28]. The heat evolved from the adsorption reaction derives from two different contributions the exothermic enthalpy of adsorption and the endothermic enthalpy of displacement of the solvent, while the enthalpy effects describing dilution and mixing phenomena can be neglected owing to the differential design and pre-heating of the probe solution. 

Catalysts containing niobia supported on various oxides have been the subject of considerable recent interest [1-4]. The molecular structures and reactivity of niobium oxides supported on alumina, titania, zirconia and silica have been intensively investigated over the last few years. Niobia supported on silica has been shown to be active for the dehydrogenation and dehydration of alcohols, photo-oxidation of propene and oxidative decomposition of methyl tertiary butyl ether. Titania supported niobia is active for the selective catalytic reduction (SCR) of NO by NH3. 

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