Hydrated Niobium Oxide Research Articles

Niobium Oxide Nanorods Obtained by Hydrothermal Synthesis—Structure, Morphology, and Electrochemical Detection of Oxygen Via Oxygen Reduction Reaction

Niobium oxides are promising materials for applications within various research fields, especially as electrocatalysts for various chemical reactions. The tuning of the synthetic parameters can achieve a successful compromise between morphology and structure, aiming to obtain certain properties. Hence, this study aimed to investigate the influence of hydrothermal synthesis parameters on the morphology and structure of niobium oxide growth on a niobium metallic plate. The effect of annealing on the material performance was also evaluated. Afterward, the most crystalline sample was tested for the electrochemical determination of dissolved oxygen, a fundamental reaction in corrosion, biomedicine, and environmental monitoring. This is the first work using this material configuration as an electrochemical sensor. The hydrothermal synthesis produced nanorods formed by poorly crystalline, acidic, hydrated Nb2O5. Increasing the mineralizer concentration could increase the crystallinity and the nanorod growth rate, but it could also promote a lack of structural and morphological uniformity throughout the surface. Heat treatment allowed the increase in crystallinity and favored orthorhombic Nb2O5. Raman spectroscopy revealed that, at the first moment, acidic, hydrated niobium oxide structures were formed as precursors of crystalline niobium oxide that would be developed with longer reaction times and a higher mineralizer concentration. The obtained niobium oxide showed electrocatalytic activity toward the oxygen reduction reaction, with comparable performance between the samples with and without heat treatment. At all analyzed pH values, the amperometric response was linearly correlated with the dissolved oxygen concentration. pH influenced the sensitivity of the material; a maximum sensitivity of 0.0417 mA/cm2·mg/L O2 was achieved at pH = 6. The participation of the acidic functionalities of the surface in the ORR reaction was confirmed by Raman spectroscopy.

SiO2 supported niobium oxides with active acid sites for the catalytic acetalization of glycerol

The silica supported niobium oxide catalysts with abundant acid active sites were successfully fabricated and demonstrated catalytic activity in the acetalization of glycerol with acetone to produce solketal. Although Niobium pentachloride which was used as the niobium precursor preferred to aggregate on the surface, the chemical vapor deposition method was introduced to remove the surface chloride. The synthesis parameter of the material affects the structure and the type of the supported niobium oxide on the carrier, and directly changes the amounts of catalytic sites. The effect of calcination temperature/time and loading amount of hydrated niobium oxide on the catalytic performance was investigated. The properties of acid sites on catalysts were characterized, and the catalytic process was directly observed by operando Raman. These in-situ experimental trials could provide deep insight to understand the catalytic mechanism.

Facile Synthesis of Nb2O5 as a Visible Light-Sensitive Photocatalyst

Niobium pentoxide (Nb2O5) is a promising semiconductor in catalysis due to its chemical and physical properties, availability, and non-toxicity. However, because of its large bandgap energy (Eg= 3.1 – 4.0 eV), Nb2O5 does not work as a photocatalyst under visible light, which makes its use of solar light limited. Even though Nb2O5 presents this drawback, its photocatalytic activity under visible light radiation can be enhanced due to the presence of surface groups, known as niobium peroxo complexes, formed by the treatment with hydrogen peroxide. In general, the modified niobia is synthetized in multi-step processes which allow the generation of strongly reactive oxygen species (ROS) (i.e. peroxo and superoxo) on its surface. In contrast, in this study, the peroxo-niobium oxide was prepared by a facile combination of the oxidant peroxide method (OPM) and hydrothermal treatment. The ability of Nb2O5 to form the ROS using this method was investigated in respect to the different Nb:H2O2 molar ratio (1:2 or 1:10) added in the metal oxide synthesis. For this purpose, ammonium niobium oxalate was dissolved in distilled water and hydrogen peroxide was then added to the reaction medium. The niobium precursor in contact with H2O2 forms instantaneously the niobium peroxo complex that was crystallized at 120 ⁰C with a heating ramp of 1 oC/min, for 12h in a home-made teflon lined autoclave. When the desired temperature was reached, the internal pressure of the reactor varied from ca. 10 to 27 atm, depending upon the amount of hydrogen peroxide used to prepare each material. The oxidative properties of the reactive oxygen species formed on the surface of metal oxides were tested in photodegradation of caffeic acid (CA) under visible light. In addition, reuse tests were performed in order to investigate the loss of photoactivity of the best photocatalyst in the CA degradation. XRD patterns revealed that the crystallographic structures of the samples Nb2H and Nb10H contain the Nb2O5 pseudohexagonal-type phase (JCPDS 28-317) and hydrated niobium oxide. Although no changes in the crystalline structure of both materials were observed, the presence of peroxo and superoxo species in the photocatalysts was confirmed by XPS and FTIR. The composites Nb2H and Nb10H presented band gap energies of 3.20 and 3.16 eV, respectively. However, Nb10H sample absorbs visible light at more than 400 nm, suggesting that the synthesis of the Nb2O5 with excess of H2O2 shifts the absorption edge of this material to the visible range, as well as reduces the energy required to the photoactivation of the catalyst. It was found that the photocatalytic activity increased with increasing oxidant concentration in the synthesis medium due to formation of a higher amount of oxygen reactive species on the surface of niobia. The peroxo-niobium groups on the Nb2O5 surface allowed the photocatalysts to be activated by visible light, and the superoxo groups formed can be also be responsible to improve its photocatalytic efficiency. The reuse tests of the Nb10H oxide indicated that the CA removal capacity remained practically constant even after four consecutive cycles. Therefore, the possibility of using solar irradiation to supply energy to active the photocatalyst make this material promising for use in low-cost photocatalytic systems for treatment of wastewater.

Methylene blue photodegradation employing hexagonal prism-shaped niobium oxide as heterogeneous catalyst: Effect of catalyst dosage, dye concentration, and radiation source

This paper evaluates the feasibility of using niobium-based catalysts for the photodegradation of organic dyes. The metal oxides were prepared using niobium scraps and chips as precursors. The semiconductors were characterized by diffuse reflectance spectroscopy, scanning electron microscopy (SEM), X-ray diffraction, infrared and Raman spectroscopy. The as-prepared anhydrous niobium oxide has a pseudohexagonal structure, whereas the hydrated niobium oxide is an amorphous material. The specific surface area of the hydrated niobium oxide is found to be double the area of its calcined counterpart. The photocatalytic efficiency of the materials was evaluated by methylene blue (MB) decomposition, measured by UV-visible spectroscopy. The effects of catalyst dosage and initial dye concentration were investigated in both the adsorption and photocatalysis processes. Increasing the initial MB concentration leads to increase the amount of adsorbed MB but to decrease the photocatalytic efficiency for all materials. In contrast, both the amount of adsorbed dye and the photocatalytic efficiency increase with increasing the catalyst dosage (0.5–2 g L−1). The highest photodegradation efficiency is achieved using UVC radiation. The specific surface area as well as the amount of acid sites, adsorbed water, and OH− surface groups on the catalyst surface demonstrates to be fundamental to the photocatalytic properties of the materials. Furthermore, the photocatalytic mechanism is controlled by superoxide and singlet oxygen species for the hydrated material, whereas the hydroxyl radical is the main active species in the photodegradation employing the anhydrous oxide. The hydrated material achieved a complete degradation of the methylene blue.

Synergic antimony–niobium pentoxide nanomeshes for high-rate sodium storage

Synergic antimony–niobium pentoxide nanomeshes are fabricated on a large scaleviathe controllable decomposition of SbNbO4nanosheets, which are obtained from ultrathin hydrated niobium oxide (Nb2O5·nH2O) layers. Synergizing the merits of highly active Sb and structurally stable Nb2O5, the nanomeshes exhibit enhanced charge-transfer kinetics, thus leading to a high capacity and good high-rate performance for sodium storage.

Effect of niobium addition in support catalysts applied in satellite propulsion

Catalysts composed of iridium as the active phase dispersed in aluminum oxide (Ir/Al2O3) are used in propulsion systems that employ hydrazine as monopropellant in the control of satellite orbit and attitude. The aluminum oxide (Al2O3) utilized as support must present high values of specific surface area, pore volume, and crush strength. The niobium effect was evaluated in this work, in its oxide form (Nb2O5), by 3 different methods: with the employment of a NbCl5 precursor solution, by wet impregnation and dry impregnation of an alumina obtained from a mixture of gibbsite and boehmite and by physical mixing of gibbsite and hydrated niobium oxide, both autoclaved separately. Aluminum oxides were prepared in both cases containing Nb2O5 contents of 10, 20, and 30% w/w. The acid impregnating NbCl5 solution in the wet impregnation method caused a strong attack to the Al2O3 support, altering and compromising its initial structure and morphology. This process did not occur in the supports prepared by dry impregnation. However, results indicated that the use of this methodology with Nb2O5 contents of 20% and 30%, caused an extensive coverage of the support by Nb2O5, modifying the nature and amount of alumina sites responsible for anchorage of the iridium precursor. In the case of supports prepared through physical mixture (Nb2O5-Al2O3) from aluminum hydroxide and niobium acid precursor compounds, with both being previously autoclaved separately, the 20% and 30% Nb2O5 contents presented the most promising properties, since the binder effect caused by amorphous Nb2O5 increased the crush strength of the support, without compromising the aluminum oxide morphology and texture. Despite of existence of stronger acid sites due to the addition of niobium oxide to aluminum oxide, no increase in the acidity of the materials was observed due to the presence of niobium oxide. This can be attributed to the high calcination temperature used, as well as, to the procedures used for the preparation of such materials.

Soybean oil transesterification: Study of using Nb2O5.xH2O as catalyst in biodiesel production

Economic and environmental reasons show a trend towards replacing fossil fuels with biofuels such as those from triglycerides. Biodiesel can be obtained from vegetable oils and animal fat through several processes such as transesterification, esterification, usually with methanol, ethanol or through pyrolysis, all of them in the presence of an acid or basis catalyst. The use of solid catalysts in biodiesel production has the following advantages: easy recovery and reuse, thus decreasing process costs and amount of waste generated. 1 Some of the problems in the use of solid catalysts are: low concentration of active sites, microporosity, and leaching of active sites. 2 Studies aiming at developing methodologies involving hydrated niobium oxide as catalyst in biodiesel production have been carried out by our research group. 3,4 Parameters such as the use of assistant solvent to increase the boiling point of the mixture (toluene, ethylene glycol, and DMSO), pre-thermal treatment (calcinations) and catalyst molar concentration were initially assessed in esterification, oleic acid, and methanol reactions. From these studies we could observe that high temperatures and excessive alcohol favor esterification reactions. The best reaction conditions were then used as models and employed in transesterification reactions of soybean oil. DMSO (Dimethyl sulfoxide) was the solvent used to increase the reaction medium temperature without evaporating all the methanol. Transesterification reactions were carried out with soybean oil (0.5 g), methanol (0.85 g), DMSO (2.50 ml), and hydrated niobium oxide as catalyst in ratios of 20% and 100% (in relation to oil mass). Catalyst was employed without pretreatment and after pretreatment at 115 °C, 300 °C, and 500 °C. The reactions occurred at 170 °C, under reflux for 48 hours. A reaction without a catalyst was also carried out. All the reactions have shown conversion using CCD and they have been determined by 1 H NMR spectroscopy.

Hydrothermal synthesis of NaNbO3-morphology change by starting compounds-

Perovskite-type sodium niobate, NaNbO3 was prepared by hydrothermal reaction using an amorphous hydrated niobium oxide (a-Nb2O5). A single phase of perovskite-type NaNbO3 was obtained under the condition of 160°C and 1.0 M NaOH solution, and at higher temperature (≥180°C) cubic shape particles with the size of 5–10 µm were produced. These cubic shape particles had well developed 110 and 001 planes. When other starting compounds, Nb2O5, HNbO3·nH2O and HNb3O8·nH2O were used, well crystallized particles were not obtained. The particles prepared from HNbO3·nH2O and HNb3O8·nH2O had irregular shape and cubic-like shape with the size of 1–5 µm. In the case of Nb2O5 fine particles with the submicron size of ∼0.5 µm were obtained.

Facile preparation of SBA-15-supported niobic acid (Nb 2O 5· nH 2O) catalyst and its catalytic activity

Hydrated niobium oxide (niobic acid) was effectively supported on mesoporous silica SBA-15 by HCl treatment of colloidal niobium oxide solution. The obtained Nb/SBA-15 exhibited high thermal stability, with no crystallization of niobium oxide. The number of generated Brönsted and Lewis acid sites increased with the Nb 2O 5 content. The catalytic activity of Nb/SBA-15 was evaluated to catalyze the hydrolysis of sucrose; Nb/SBA-15 exhibited higher sucrose conversion and TON than both bulk niobic acid and Nb/SBA-15-Oxa prepared according to a previously reported method. Moreover, we found that the choice of acid to be used in the preparation process influences catalytic activity. Nb/SBA-15 prepared by H 3PO 4 treatment yielded the highest sucrose conversion.

Low temperature synthesis of lithium niobate powders based on water-soluble niobium malato complexes

Pure lithium niobate (LN) powders were prepared by a wet-chemical method. Hydrated niobium oxide was dissolved in a hot aqueous solution of oxalic acid, followed by the precipitation of niobic acid by the addition of ammonia solution until pH=11. Niobic acid precipitated was dissolved in an aqueous solution of dl-malic acid. A small amount of the Nb–malic complex synthesized was isolated and characterized by thermogravimetry (TG) and infrared (IR) spectroscopy, and the molecular composition of the complex was determined as H[Nb 2O(H 3C 4O 5) 3]. The exact content of niobium in the stable solution of Nb–malic solution was determined by gravimetric analysis. Stoichiometric amount of lithium carbonate was added into Nb–malic solution. After the solvent elimination, the remaining solid powder was calcined at 350 °C, ground and calcined over a temperature range from 300 to 900 °C. Calcined powders were characterized by X-ray diffraction (XRD) and Raman spectroscopy. Crystalline LN phase was identified at 500 °C/2 h, although the total elimination of the inorganic carbon occurred only at 650 °C/2 h.

Recent topics of research and development of catalysis by niobium and tantalum oxides

Current topics of catalysts containing niobium and tantalum, especially in the field of solid acid catalysis and selective oxidation of hydrocarbons are reviewed. Hydrated niobium oxide and hydrated tantalum oxide are highly acidic. Hydrated niobium oxide is active for the hydration of ethene to ethanol, and Nb–W mixed metal oxide is more active for the reaction. Acid properties of tantalum oxide are changed by being supported on SiO 2. Ta oxide/SiO 2, prepared by the chemical reaction between tantalum alkoxide and surface hydroxyl groups of SiO 2, is active and selective for the gas phase Beckmann rearrangement of cyclohexanoneoxime to caprolactam. Niobium oxide and tantalum oxide easily react with many other oxides to form mixed metal oxide phases with complex structure. Mixed metal oxide catalysts, containing molybdenum, vanadium, certain elements together with niobium are active for the selective oxidation of hydrocarbons. Especially, the selective oxidation of propane by such mixed metal oxide catalysts has been paid attention. Additionally, recent progress of environmental catalysts, promoted by niobium and tantalum compounds, namely catalysts for the pollution abatement is reviewed.

Study of the structure of niobium oxide by X-ray absorption fine structure and surface science techniques

The structure of niobium oxides was investigated by using XAFS and surface science techniques. The origin of acidic property and catalysis of niobium oxide is discussed by these results. From the EXAFS analysis, the structure of highly acidic hydrated niobium oxide is different from those of T-Nb 2O 5 and K 8Nb 6O 19, which has been previously demonstrated as the structure of hydrated niobium oxide, because NbNb and NbO distances are significantly different from each other. There are at least two types of NbO bonds, a tetrahedral form and an octahedral form are existing for hydrated niobium oxide. The excess of negative charge is present for NbO bond by applying Tanabe's hypothesis to single metal oxide consisting of at least two metal-O forms with different coordination number. That seems to be the origin of the acidity of hydrated niobium oxide. The ordered niobium oxides thin film on Pt(111) and their defect face were prepared by using UHV surface analysis apparatus. The oxygen vacancies of niobium oxide on Pt(111) were created by Ar +ion bombardment. H 2O and CH 3OH exhibit associated adsorption on the ordered NbO x and Nb 2O 5 surface and dissociative adsorption on the defect Nb 2O 5 surface at room temperature. On the ordered NbO x and Nb 2O 5 surface, no evidence of ethene adsorption at 200 K is found by HREELS, although ethene adsorbs molecularly on the defect Nb 2O 5 surface with a π-complex formation. These results imply that oxygen vacancies of NbO x surface play an important role on the adsorption of the molecules.

Preparation of highly acidic hydrated niobium oxide

In order to obtain a highly acidic hydrated niobium oxide, the preparation methods and conditions such as niobium precursors, precipitating reagents, treatment of precipitates, washing of precipitates, final pH in precipitation, and calcination temperature were systematically examined. Strong acidic sites of H 0⩽−8.2 were found to appear on hydrated niobium oxide when the oxide was prepared by boiling the precipitates, which were obtained by hydrolysis of niobium oxalate with KOH, in a diluted mild acid solution(preferably 0.1N HNO 3) followed by washing with hot water several times and heating in air at 300 °C. Final pH in the precipitation must be below 11.

Application of niobium oxides as catalysts

Characteristics of niobium oxides as a promoter of various catalysts, a catalyst support, and a unique solid acid catalyst is reviewed. The pronounced effects as a promoter and a support of catalysts for diversified reactions are demonstrated and the strongly acidic nature of hydrated niobium oxide and its application as an unusual solid acid catalyst are discussed.

Catalytic properties of hydrated tantalum oxide

The hydrated tantalum and niobium oxides display strong acid properties in their amorphous form. The H 0 value of these hydrated oxides was less than −8.2. The acid property of the hydrated tantalum oxide was maintained to higher temperatures than that of the hydrated niobium oxide. The hydrated tantalum oxide catalyzed not only some gas-phase reactions, including the esterification of methacrylic acid with methanol, the alkylation of benzene with ethene, and the isomerization of 1-butene, but also some liquid-phase reactions, such as the aldol condensation of acetone and the hydration of isobutene. The hydrated tantalum oxide showed mainly Lewis acidity in the absence of water. Brønsted acidity was regenerated by steam treatment, confirmed by infrared spectroscopy and the isomerization of 1-butene. The hydrated tantalum oxide was activated to a solid superacid by treatment with sulfuric acid.

Ultra-violet diffuse reflectance spectra of the hydrated oxides of protactinium(V) and of niobium(V) and some related compounds

Protactinium(V) and niobium(V) hydrated oxides present single absorption bands at 260 and 268 nm, respectively. The anhydrous oxides show additional lower energy bands whose relative intensities depend on their crystalline structures. The 260 and 268 nm bands are attributed to the MOM bonds in a disordered environment. Protactinium peroxide and protactinium oxalate, hydrolytic compounds where the “hydrated oxide structure” is likely to be preserved, have spectra resembling that of the hydrated oxide. On the contrary, the true complex potassium oxotrioxalatoniobate(V) has an absorption spectrum bearing little resemblance to that of the hydrated niobium oxide. The assignment of the 260 nm band to the PaOPa bonds may thus account for the absorption in the 240–300 nm region which is generally associated with the hydrolysis of Pa(V) in several aqueous media.