Nb2O5 Catalyst Research Articles

Photocatalytic Selective Oxidation of 5-Hydroxymethylfurfural to 2,5-Diformylfuran over Nb2O5 under Visible Light

The photocatalytic selective oxidation of biomass derivate compound 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran is an environmentally friendly and energy-saving process. Nb2O5 was prepared and applied in the oxidation of HMF under visible light, although its bandgap energy is higher than 3.2 eV. The reaction process follows a special mechanism over a Nb2O5 catalyst different from the normal oxide semiconductor. The effect of calcination temperature, incident wavelength, and reaction time on photocatalytic performance was investigated. Nb2O5-800 exhibits the best performance in the presence of O2 with benzotrifluoride as a solvent under visible light irradiation.

Lactic Acid Production from Glucose Over a Novel Nb2O5 Nanorod Catalyst

A novel Nb2O5 catalyst with the highly single crystallinity and well aligned nanorods was successively synthesized by hydrothermal method and presented promising catalytic behavior for lactic acid synthesis from glucose. Notably, the sample was easily separated and reused without the obvious deactivation. It might shed light on an alternative process for lactic acid production instead of conventional fermentation.

Solar photocatalytic degradation of textile effluent with TiO2, ZnO, and Nb2O5 catalysts: assessment of photocatalytic activity and mineralization.

The photocatalytic degradation of textile effluent was investigated using TiO2, ZnO, and Nb2O5 catalysts under solar irradiation. The procedures were carried out at ambient conditions in April 2014, with pH3.0 and catalyst concentration of 0.250gL-1. The photocatalytic activity of the oxides was evaluated by means of kinetic efficiency (rate constant and half-life time), chemical oxygen demand reduction, and absorbance reduction at 228, 254, 284, 310, 350, 500, and 660nm (λmáx). Mineralization in terms of the formation of inorganic ions and toxicity reduction using bioassays with Artemia salina were performed. TiO2 reduced the absorbance at 660nm (λmax) after 300min of solar irradiation around 94 and 93%; and 68 and 60% of COD, respectively. ZnO showed lower photocatalytic activity giving 64 and 42% of absorbance and COD reduction, respectively. The photocatalytic activity of Nb2O5 was very close to TiO2-P25. In this sense, Nb2O5 becomes a promising alternative to replace the commercial TiO2-P25. Bioassays confirmed the efficacy of treatment, increasing the lethal concentration of 27.59 (in natura) to 131.95% in the presence of Nb2O5.

Influence of the metallic content on Pt-Ir/Nb2O5 catalysts for decalin selective ring opening

Ring opening of naphthenic rings on Pt-Ir catalysts supported on Nb2O5 was studied in this work. The catalysts were prepared by impregnation. In the case of the bimetallic catalyst, the total metallic load was kept constant while varying the individual quantity of impregnated Ir and Pt. The samples were characterized by transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, N2 sorption, NH3 TPD and model reactions (cyclohexane dehydrogenation and cyclopentane hydrogenolysis). The decalin ring opening capacity of the catalysts was evaluated. Monometallic Ir and Pt catalysts as well as bimetallic catalysts were active towards cyclohexane dehydrogenation and cyclopentane hydrogenolysis. Regarding the bimetallic catalysts, an increase on the Pt content promoted the cyclohexane dehydrogenation while hindering the hydrogenolytic activity. In the selective ring opening of decalin, the bimetallic catalysts presented activities comparable to the monometallic catalysts. Nonetheless, Ir(0.3%)/Nb2O5 catalyst presented the highest selectivity towards ring opening products with the lowest selectivity towards dehydrogenated products.

Hydrodeoxygenation of butyric acid at multi-functional Nb2O5 catalyst: A density functional theory study

Fatty acids are important reaction intermediates in catalytic reduction of triglycerides. In this work, we conducted a detailed investigation of the adsorption and hydrodeoxygenation of butyric acid at Nb2O5(001) surface to understand the performance of Nb based catalytic support in biomass conversion by using density functional theory calculations. It has been found that the hydrodeoxygenation process starts with an adsorbed butyrate species in bidentate configuration at both clean and reduced surface with oxygen vacancy. The CO bond activation facilitated by the interaction of the molecule and the surface Lewis acid can largely suppress the CC bond cleavage, resulting in a dominant reductive deoxygenation pathway to generate butane. The unique surface structures of Nb2O5 are also found to play an important role in promoting the dissociative adsorption and further hydrogenation of the butyric acid through release of the surface tension.

Alternative route for the synthesis of high surface-area η-Al2O3/Nb2O5 catalyst from aluminum waste

This paper describes an alternative route for the production of a high-surface-area ɳ-Al2O3/Nb2O5 catalyst synthesized from aluminum waste and niobium ammonium oxalate (NH4H2[NbO(C2O4)3]·3H2O). The effects of thermal treatment on the morphology and crystal structure were examined by X-ray powder diffraction (XPD), surface area measurements (BET), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray fluorescence, dynamic scanning calorimetry (DSC) and thermogravimetry (TG) measurement. The catalysts were evaluated in the glycerol dehydration reaction. Catalytic tests were carried out with reactants in gas-phase with a fixed-bed reactor at 300° and 400 °C.

Direct Synthesis of Cyclic Imides from Carboxylic Anhydrides and Amines by Nb2O5 as a Water‐Tolerant Lewis Acid Catalyst

AbstractIn the 20 types of heterogeneous and homogenous catalysts screened, Nb2O5 showed the highest activity for the synthesis of N‐phenylsuccinimide by dehydrative condensation of succinic anhydride and aniline. Nb2O5 was used in the direct imidation of a wide range of carboxylic anhydrides with NH3 or amines with various functional groups and could be reused. Kinetic studies showed that the Lewis acid Nb2O5 catalyst was more water tolerant than both the Lewis acidic oxide TiO2 and the homogeneous Lewis acid ZrCl4, which resulted in higher yields of imides through the use of Nb2O5.

Amidation of Carboxylic Acids with Amines by Nb2O5 as a Reusable Lewis Acid Catalyst

AbstractAmong 28 types of heterogeneous and homogenous catalysts tested, Nb2O5 shows the highest yield for direct amidation of n‐dodecanoic acid with a less reactive amine (aniline). The catalytic amidation by Nb2O5 is applicable to a wide range of carboxylic acids and amines with various functional groups, and the catalyst is reusable. A comparison of the results of the catalytic study and an infrared study of the acetic acid adsorbed on the catalyst suggests that activation of the carbonyl group of the carboxylic acid by Lewis acid sites on Nb2O5 is responsible for the high activity of the Nb2O5 catalyst. Kinetic studies show that Lewis acid sites on Nb2O5 are more water‐tolerant than conventional Lewis acidic oxides (Al2O3, TiO2). In comparison with the state‐of‐the‐art homogeneous Lewis acid catalyst for amidation (ZrCl4), Nb2O5 undergoes fewer negative effects from basic additives in the solution, which indicates that Nb2O5 is a more base‐tolerant Lewis acid catalyst than the homogeneous Lewis acid catalyst.

Synthesis, characterisation and photocatalytic activity of N-doped TiO2–Nb2O5 mixed oxides

Nitrogen-doped TiO2 and TiO2–Nb2O5 catalysts were prepared by sol–gel technique using titanium isopropoxide (TIP) and potassium niobate. The obtained samples were subsequently calcined at 380 and 500°C. Undoped TiO2 and TiO2–Nb2O5 samples were also prepared by way of comparison. The photocatalytic activities of the synthesised catalysts were evaluated on the photodegradation of benzene, toluene and xylenes (BTX) solution under UVA and visible light. The level of nitrogen incorporated on N-doped TiO2–Nb2O5 catalyst was found to be 0.49 at%. Concentrations of BTX were analysed by gas chromatography applying the headspace technique. The N-doped TiO2–Nb2O5 catalyst calcined at 500°C led to reductions of benzene, toluene and xylenes greater than 65wt% after 150min of irradiation. The photocatalytic reactions followed the first order kinetics.

Preferential oxidation of CO in excess of H2 on Pt/CeO2–Nb2O5 catalysts

A series of CeO2–Nb2O5 mixed oxides with different Nb content, as well as the pure oxides, have been synthesized by co-precipitation with excess urea. These materials have been used as supports for platinum catalysts, with [Pt(NH3)4](NO3)2 as precursor. Both supports and catalysts have been characterized by several techniques: N2 physisorption at 77K, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, UV–vis spectroscopy, scanning electron microscopy, transmission electron microscopy, temperature-programmed reduction and temperature-programmed desorption (CO and H2), and their catalytic behaviour has been determined in the PROX reaction, both with an ideal gas mixture (CO, O2 and H2) and in simulated reformate gas containing CO2 and H2O. Raman spectroscopy analysis has shown the likely substitution of some Ce4+ cations by Nb5+ to some extent in supports with low niobium contents. Moreover, the presence of Nb in the supports hinders their ability to adsorb CO and to oxidize it to CO2. However, an improvement of the catalytic activity for CO oxidation is obtained by adding Nb to the support, although the Pt/Nb2O5 catalyst shows very low activity. The best results are found with the Pt/0.7CeO2–0.3Nb2O5 catalyst, which shows a high CO conversion (85%) and a high yield (around 0.6) after a reduction treatment at 523K. The effect of the presence of CO2 and H2O in the feed has also been determined.

Synthesis of catalytically active rock salt structured MgxNb1−xO nanoparticles for MgH2 system

This communication deals with the ex-situ synthesis of rock salt type MgxNb1−xO whose structural characteristics are closely related with MgO. XRD examination of 30 h ball milled MgH2 + Nb2O5 confirms the formation of a rock salt product MgxNb1−xO, which is comparable to the recently reported active catalyst MgxNb1−xO formed in-situ in MgH2 milled with 8 mol.% Nb2O5. It is shown that MgH2 catalyzed with the pre-made 2 wt.% MgxNb1−xO desorbs hydrogen at least 50 °C lower than the in-situ 2 wt.% Nb2O5 catalyzed MgH2 with improved reversible absorption. This result highlights that the proposed pathway mechanism on the basis of Nb2O5 catalyst may need further verification and that the addition of the MgxNb1−xO catalyst in a pre-reduced state can offer distinct performance advantages over its in-situ preparation.

Photocatalytic treatment of wastewater containing Rhodamine B dye via Nb2O5 nanoparticles: effect of operational key parameters

Heterogeneous photocatalysis has proved to be a useful tool for the degradation of water pollutants over the past 30 years. In this work, the photocatalytic decolorization of Rhodamine B (RhB) dye as a model compound has been studied by using nano-sized Nb2O5 semiconductor as a photocatalyst. Structural and textural features of the Nb2O5 sample were characterized by X-ray diffraction, scanning electron microscope, UV–vis DRS, and Brunauer–Emmett–Teller techniques. As a result, the orthorhombic Nb2O5 particles displayed nearly spherical shape with the average crystalline size and the band gap energy of 43.7 nm and 2.9 eV, respectively. Different parameters of decolorization reaction like dye concentration, catalyst amount, pH, and H2O2 concentration have been studied. The optimum conditions for the photo treatment of the dye were initial concentration 10 mg L−1 of dye, catalyst concentration 0.5 g L−1, and pH 4. The optimum value of H2O2 concentration for the conditions used in this study was 700 mg L−1. Moreover, the efficiency of photo-Fenton process either alone or in conjunction with Nb2O5 and a dark-Fenton process as a control experiment were evaluated. The combination of photo-Fenton and Nb2O5 catalysts was found to be most effective for the treatment of such type of wastewaters.

Mesoporous Nb2O5 as solid acid catalyst for dehydration of d-xylose into furfural

The acid-catalyzed dehydration of d-xylose to furfural has been investigated in a biphasic water–toluene system, using a mesoporous Nb2O5 catalyst prepared by a neutral templating route. The catalytic behavior was compared with a commercial Nb2O5. Materials were characterized by XRD, XPS, TEM, NH3-TPD, Raman spectroscopy and N2 sorption. The d-xylose conversion and furfural yield over the mesoporous niobia were found to increase with reaction temperature and time, in such a way that at 170°C and 90min, a d-xylose conversion and a furfural yield were higher than 90% and 50%, respectively. However, the commercial crystalline niobia displayed a low activity. The stability of the mesoporous catalyst has been demonstrated by XRD and N2 sorption, and corroborated by the absence of significant niobium leaching in solution.

Influence of operational key parameters on the photocatalytic decolorization of Rhodamine B dye using Fe2+/H2O2/Nb2O5/UV system

The present research deals with the development of a new heterogeneous photocatalysis and Fenton hybrid system for the removal of color from textile dyeing wastewater as Rhodamine B (RhB) solutions by using Fe(2+)/H2O2/Nb2O5 as a photocatalytic system. The application of this photocatalytic system for the decolorization of dye contaminants is not reported in the literature yet. Different parameters like dye concentration, Nb2O5/Fe(2+) catalyst amount, pH, and H2O2 concentration have been studied. The optimum conditions for the decolorization of the dye were initial concentration of 10 mg L(-1) of dye, pH 4, and Nb2O5/Fe(2+) catalyst concentration of 0.5 g L(-1)/50 mg L(-1). The optimum value of H2O2 concentration for the conditions used in this study was 700 mg L(-1). Moreover, the efficiency of the Nb2O5/photo-Fenton hybrid process in comparison to photo-Fenton alone and a dark Fenton process as a control experiment to decolorize the RhB solution has been investigated. The combination of photo-Fenton and Nb2O5 catalysts has been proved to be the most effective for the treatment of such type of wastewaters. The results revealed that the RhB dye was decolorized in a higher percent (78 %) by the Nb2O5/photo-Fenton hybrid process (Fe(2+)/H2O2/Nb2O5/UV) than by the photo-Fenton process alone (37 %) and dark Fenton process (14 %) after 120 min of treatment. Moreover, the Nb2O5 catalyst as a heterogeneous part of the photocatalytic system was demonstrated to have good stability and reusability.

The effect of PO4 to Nb2O5 catalyst on the dehydration of glycerol

The liquid-phase dehydration of glycerol to acrolein over PO4/Nb2O5 catalysts was performed to investigate the effect of various amounts of PO4 supported and the regeneration of used catalysts on catalytic performance. The amount of acid sites and strength of the surface acidity increased with the amount of PO4 supported. Thus, these results seem to suggest that the amount of produced acrolein in the dehydration of glycerol is relatively influenced by the surface acidity of the catalysts used. Catalytic activity was restored by burning out of the surface deposited carbons in the used PO4/Nb2O5 catalysts at 500°C in air. However, it was not completely restored after regeneration for the 50wt% PO4/Nb2O5 catalyst due to the incomplete recovery of the drastically reduced surface acidity by the loss of the much resolved PO4 during the reaction.

Nanorod Niobium Oxide as Powerful Catalysts for an All Vanadium Redox Flow Battery

A powerful low-cost electrocatalyst, nanorod Nb2O5, is synthesized using the hydrothermal method with monoclinic phases and simultaneously deposited on the surface of a graphite felt (GF) electrode in an all vanadium flow battery (VRB). Cyclic voltammetry (CV) study confirmed that Nb2O5 has catalytic effects toward redox couples of V(II)/V(III) at the negative side and V(IV)/V(V) at the positive side to facilitate the electrochemical kinetics of the vanadium redox reactions. Because of poor conductivity of Nb2O5, the performance of the Nb2O5 loaded electrodes is strongly dependent on the nanosize and uniform distribution of catalysts on GF surfaces. Accordingly, an optimal amount of W-doped Nb2O5 nanorods with minimum agglomeration and improved distribution on GF surfaces are established by adding water-soluble compounds containing tungsten (W) into the precursor solutions. The corresponding energy efficiency is enhanced by ∼10.7% at high current density (150 mA·cm(-2)) as compared with one without catalysts. Flow battery cyclic performance also demonstrates the excellent stability of the as prepared Nb2O5 catalyst enhanced electrode. These results suggest that Nb2O5-based nanorods, replacing expensive noble metals, uniformly decorating GFs holds great promise as high-performance electrodes for VRB applications.

Evaluation of photocatalytic activities of supported catalysts on NaX zeolite or activated charcoal

This study aimed to evaluate the photocatalytic activity of ZnO and Nb2O5 catalysts, both supported on NaX zeolite and activated charcoal (AC). The synergistic effect between oxide and support and the influence of solution pH (3, 7 and 9) on photocatalytic degradation of reactive blue 5G (C.I. 222) were analyzed. The catalysts Nb2O5/NaX, Nb2O5/AC and ZnO/NaX, ZnO/AC with 5 and 10% (wt%) were prepared by wet impregnation. The results showed that the catalysts exhibit quite different structural and textural properties. The synergic effect between ZnO and NaX support was higher than that with the activated charcoal, showing that these catalysts were more efficient. The most photoactive catalyst was 10% ZnO/NaX which showed 100% discoloration of the dye solution at pH 3, 7 and 9 after 0.5, 5 and 2h of irradiation, respectively. The hydrolytic nature of zeolite favored the formation of surface hydroxyl radicals, which increased the activity of the photocatalyst. Thus, catalysts supported on NaX zeolite are promising for use in photocatalysis.

Catalytic dehydration of fructose to 5-hydroxymethylfurfural over Nb2O5 catalyst in organic solvent

The catalytic dehydration of fructose to 5-hydroxymethylfurfural (HMF) in DMSO was performed over Nb2O5 derived from calcination of niobic acid at various temperatures (300–700°C). The catalysts were characterized by powder X-ray diffraction, N2 physical adsorption, temperature-programed desorption of NH3, n-butylamine titration using Hammett indicators, infrared spectroscopy of adsorbed pyridine, and X-ray photoelectron spectroscopy. It was found that both catalytic activity and surface acid sites decrease with increasing calcination temperatures. The Nb2O5 derived from calcination of niobic acid at 400°C reveals the maximum yield of HMF among all the catalysts, although the amount of acid sites on the catalyst is lower than that on the sample calcined at 300°C. The results suggest that the presence of larger amounts of strong acid sites on the surface of the Nb2O5 calcined at 300°C may promote side reactions. The Nb2O5 prepared at 400°C shows 100% fructose conversion with 86.2% HMF yield in DMSO at 120°C after 2h. The activity of the catalyst decreases gradually during recycle because of coke deposition; however, it can be fully recovered by calcination at 400°C for 2h, suggesting that this catalyst is of significance for practical applications.

Microstructure and morphology changes in MgH2/expanded natural graphite pellets upon hydrogen cycling

Compacted mixtures based on ball-milled magnesium hydride (MgH2) have gained significant research interest as suitable materials for hydrogen storage tanks. The issue related to their stability during practical service conditions is one of paramount relevance. In this work, we investigate the microstructure and morphology of pellets obtained by the compaction of ball-milled MgH2/Nb2O5 powders mixed with expanded natural graphite. The pellets are subjected to repeated hydrogen sorption cycles to measure hydrogen storage properties and its stability with cycling. Moreover, the effect of air-exposure on the hydrogen sorption behavior is studied. Electron microscopy observations of as-prepared and cycled pellets point to a dramatic modification of the material's microstructure upon repeated hydrogen cycling. In particular, the appearance of MgH2 particles depleted of the Nb2O5 catalyst and the formation of hollow MgO shells are highlighted. These findings are discussed by a simple model which takes into account the basic mechanisms intervening during the metal-hydride transformation in the pellet.

Hydrogen sorption enhancement by Nb2O5 and Nb catalysts combined with MgH2

Three Nb materials, synthesized oxide (s-Nb2O5), commercial oxide (c-Nb2O5) and metallic Nb, were applied as catalysts to promote the enhancement of hydrogen sorption in MgH2. A high specific surface area, around 100m2g−1, was obtained for both oxides used as catalysts, s-Nb2O5 and c-Nb2O5 after being heat treated at 350°C. X-ray diffraction (XRD) tests, performed prior to milling, revealed the amorphous structure of both oxides. All the catalysts were mixed for only 20min with MgH2 by mechanical milling forming composites. Differential Scanning Calorimetric curves of the s-Nb2O5 and c-Nb2O5 showed the presence of exothermic crystallization peaks. A total capacity of 5.8wt.% was attained when the hydrogen absorption kinetic tests were conducted at 350°C, with the MgH2+5wt.% s-Nb2O5 sample.