Preparation of alkaline solutions of niobium(V)

Alkaline niobium(V) solutions containing up to 190 g l⁻¹ of niobium oxide were prepared by sintering niobium(V) oxide with potassium carbonate and following leaching of the sinters with water.     

Electrochemical deposition of electrochromic niobium oxide films from an acidic solution of niobium peroxo complexes

Deposition of electrochromic niobium(V) oxide films from an acidic solution of niobium peroxo complexes on a transparent conducting cathode in the form of an SnO₂ film on glass was studied. With an increase in the negative potential of the deposition of niobium(V) oxide films from a solution of niobium peroxo complexes at pH 2.5, the structure and composition of the films changed. A study of the electrochromic properties of Nb₂O₅ films revealed broadening of the bands in the electrochromic coloration spectrum with an increase in the negative potential of the deposition.     

Cation Exchange in Oxohydroxide Matrices of Niobium(V)

Exchange of lithium, sodium, and potassium cations for hydrogen ions in oxohydroxide matrices of niobium(V), with alkali metal to niobium ratio of 1, was studied potentiometricaly. The possibility is considered of predicting the content of various singly charged cations in the case of their simultaneous presence in a complex hydrated oxide based on niobium(V).     

Novel sulfur-doped niobium pentoxide nanoparticles: fabrication, characterization, visible light sensitization and redox charge transfer study

Novel sulfur-modified niobium(V) oxide nanoparticles (SNON) that firstly exhibited good visible light sensitization were fabricated by a modified sol–gel technique using a very stable sol containing niobium(V) chloride, oxalic acid, isopropanol as chelating agent and thiourea as sulfur source. The resulting S-doped Nb₂O₅ nanomaterials were characterized by cyclic voltammetry (CV), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDAX), scanning electron microscope (SEM), ultra-violet diffuse reflectance (UV-DRS) and thermogravimetry thermal Analysis (TG-DTA). As against the response of unmodified niobium(V) oxide nanoparticles (UNON), the doped samples show different electrochemical response indicating an induced charge transfer across the niobium pentoxide/solution interface, thus forming two anodic peaks and a cathodic peak. This important observation was confirmed by UV-DRS in terms of band bending due to sulfur doping. Upon sulfur-modification, the absorption edge extends into the visible light region. The SEM observation shows that the SNPN existed in the mode of polycrystalline structure and the average grain size 63 nm. The EDAX analysis of undoped Nb₂O₅ and sulfur doped Nb₂O₅ shows the Nb₂O₅ (98%) and S (2%) content of nanopowder. These SNON nanoparticles are expected to be suitable candidates as visible light niobium(V) oxide nanoparticles sensitization.     

Electrochemical study of niobium fluoride and oxyfluoride complexes in molten LiF–KF–K2NbF7 bath

The reduction of niobium in an equimolar molten mixture of LiF and KF has been investigated using a cyclic voltammetric technique. A two-step mechanism of reduction of niobium in LiF–KF melt is proposed. The influence of oxide ions on the reduction has been studied and it was found to strongly influence the redox properties and structure of niobium ions in the melt. The presence of oxide ions caused the formation of the mono-oxyfluoride complex [Nb(V)OF₅]²⁻. The formation of the mono-oxyfluoride complex was not complete at 720°C. Free oxide ions caused an appearance of di-oxyfluoride complex. In the redundant oxide ions the di-oxyfluoride complex was converted to [Nb(IV)O₂F]⁻ by a redox reaction.     

The preparation of new oxoniobium(V) complexes from hydrated Niobium(V) Oxide: the crystal and molecular structure of Oxotris(2-pyridinolato-<Emphasis Type="Italic">N</Emphasis>-oxide)niobium(V)

Three oxoniobium(V) complexes ONbL₃ with HL = tropolone (1), 2-pyridinol-N-oxide (2) or, 3-hydroxy-1,2-dimethyl-4(1H)-pyridone (3) are obtained in good yield by one step reactions from commercially available hydrated niobium(V) oxide in aqueous media. The products are characterized by elemental analysis, FT-IR spectroscopy, NMR spectroscopy (¹H- and ¹³C-) and thermogravimetry. The crystal structure of oxotris(2-pyridinolato-N-oxide)niobium(V) is determined by single crystal X-ray diffraction, showing discrete molecules with pentagonal bipyramidal coordinated niobium. The ¹H-NMR spectra of CDCl₃ solutions indicate a fluxional behaviour for the three complexes.     

Niobium(V) oxide coated on thin glass-ceramic rod as a solid phase microextraction fiber

The efficiency of niobium(V) oxide as a sorbent phase for solid phase microextraction (SPME) was investigated. The thin glass-ceramic rod was coated with niobium(V) oxide using chemical vapor deposition and Nb₂O₅ as a chemical precursor. Optimum conditions for the preparation and conditioning of the fibers are presented. The fibers were used for the extraction of a mixture of alcohols and a mixture of phenols from the headspace samples. The results obtained proved the suitability of niobium(V) oxide as a new SPME fiber. The calibration graphs for alcohols and phenols in a concentration range of 50-1000 μg l⁻¹ were linear (r > 0.995) and the detection limits were below 0.8 μg l⁻¹ level. The repeatability for one fiber (n = 6) under similar conditions was between 3 and 10.4%. The fiber-to-fiber reproducibility (n = 6) was between 5 and 15%.     

Formation and distribution of neutral vanadium, niobium, and tantalum oxide clusters: single photon ionization at 26.5 eV

Neutral vanadium, niobium, and tantalum oxide clusters are studied by single photon ionization employing a 26.5 eV/photon soft x-ray laser. During the ionization process the metal oxide clusters are almost free of fragmentation. The most stable neutral clusters of vanadium, niobium, and tantalum oxides are of the general form (MO2)0,1(M2O5)y. M2O5 is identified as a basic building unit for these three neutral metal oxide species. Each cluster family (Mm, m=1,...,9) displays at least one oxygen deficient and/or oxygen rich cluster stoichiometry in addition to the above most stable species. For tantalum and niobium families with even m, oxygen deficient clusters have the general formula (MO2)2(M2O5)y. For vanadium oxide clusters, oxygen deficient clusters are detected for all cluster families Vm (m=1,[ellipsis (horizontal)],9), with stable structures (VO2)x(V2O5)y. Oxygen rich metal oxide clusters with high ionization energies (IE>10.5 eV, 118 nm photon) are detected with general formulas expressed as (MO2)2 (M2O5)y O1,2,3. Oxygen rich clusters, in general, have up to three attached hydrogen atoms, such as VO3H1,2, V2O5H1,2, Nb2O5H1,2, etc.     

Characterization of niobium(v) oxide received from different sources

The characterization of three commercial powders of niobium(V) oxide received from two producers was made. The thermal behavior of Nb₂O₅ up to melting point and its microstructure were studied using X-ray powder diffraction, thermoanalytical methods (DSC/TG), infrared spectroscopy (IR) and scanning microscopy. Analysis of the obtained results revealed that the starting structure of niobium(V) oxide and its thermal behavior depend on the origin of niobia. Depending on the origin of the powder and of its thermal treatment, three polymorphs of Nb₂O₅ can be observed. Sintering of powders above 1200 °C results in the formation of single phase, H-Nb₂O₅.     

Niobium-containing catalysts for oxydehydrogenation of hydrocarbons and alcohols

The first methods are developed for introducing niobium(V) into Mg-Al hydrotalcites used as precursors of oxide catalysts for oxydehydrogenation (OD) of alkanes and alcohols. Samples of niobium(V)-containing oxide catalysts are synthesized. Their catalytic properties are studied in oxydehydrogenation of ethane and ethylbenzene to styrene, oxidation dehydrocyclization of octane into ethylbenzene and styrene, and oxydehydrogenation of sec-butanol to ketone (octane-(2)-one). It is ascertained that ethane transformation into ethylene is highly a selective highly process (92–97%) at low temperatures (450–500°C) in the presence of a niobium-containing catalyst; the catalyst is appreciably efficient in ethylbenzene transformation to styrene and dehydrocyclization of n-octane to ethylbenzene and styrene, and in oxydehydrogenation of secbutanol to octane-(2)-one. All the catalysts studied operate stably in OD reactions; no decrease in their activity or selectivity was detected after 50 h operation.     

Determination of ultratrace impurity elements in high purity niobium materials by on-line matrix separation and direct injection/inductively coupled plasma mass spectrometry

The determination of 52 impurity elements in niobium materials (niobium metal, niobium oxide (V), and niobium pentaethoxide) was performed by inductively coupled plasma mass spectrometry (ICP-MS) with on-line anion exchange matrix separation as well as direct nebulization. Niobium material samples were decomposed with a mixture of hydrofluoric acid and nitric acid to prepare 10% niobium solutions. In the on-line anion exchange matrix separation/ICP-MS, the niobium and hydrofluoric acid concentrations in sample solution were adjusted to 5% and ca. 8 M, respectively. The solution was then injected into the carrier stream from the sample loop of injection valve to pass through an anion exchange resin column. In the anion exchange separation, niobium in the fluoro-complex form was adsorbed on the resin, while impurity elements were eluted. The eluted elements were introduced into ICP-MS for the determination of 25 impurity elements. On the other hand, 27 impurity elements could not be separated well from niobium matrix under the above anion exchange conditions, and then the sample solution with the niobium concentration of max. 0.2% containing internal standard elements was injected from the sample loop of injection valve directly to introduce into ICP-MS. As a result, 52 impurity elements in three kinds of niobium materials could be determined at the ng g⁻¹ level.     

Extractive Recovery of Tantalum(V) and Niobium(V) with Octanol from Hydrofluoric Acid Solutions Containing Large Amounts of Titanium(IV)

Extractive recovery with n-octanol of tantalum(V) and niobium(V) from hydrofluoric acid solutions containing large amounts of titanium (up to 2-3 M) was studied. The conditions were found for separation of tantalum(V) and niobium(V) from titanium(IV), allowing recovery of 95.7 and 84.1% of tantalum and niobium fluoride complexes, respectively, in one extraction cycle, with 2.6% recovery of titanium.     

外延定向生长氧化铌模型催化剂的制备

为用现代表面科学技术研究金属氧化物催化剂，在Pt（111）上于超高真空系统中原位蒸镀制备了NbO、NbO2、NbO2、单晶薄膜（＞2nm）．通过AES、ISS、LEED、ILS等手段研究了单晶薄膜的成长模式、化学计量和几何结构．表明通过选择合适的废物和控制制备条件，可制备出确定结构的金属氧化物单晶薄膜表面作为体相氧化物催化剂的模型表面．这种方法克服了电子能谱技术研究金属氧化物表面的困难，为研究金属氧化物催化剂的表面化学物理性质提供了方法     

Tantalum(V) or niobium(V) catalyzed oxidation of sulfides with 30% hydrogen peroxide

Tantalum(V) and niobium(V) are effective catalysts for the oxidation of sulfides with 30% hydrogen peroxide. The reaction of sulfides with 30% hydrogen peroxide catalyzed by tantalum(V) chloride or niobium(V) chloride in acetonitrile, i-propanol or t-butanol selectively provided the corresponding sulfoxides in high yields. The corresponding sulfones are efficiently obtained from the reaction of sulfides with 30% hydrogen peroxide in methanol catalyzed by tantalum(V) or niobium(V).     

Extraction and complex formation of niobium(V) with 3-hydroxy-2-methyl-1-(4-tolyl)-4-pyridone

The extraction of niobium(V) from aqueous hydrochloric and sulphuric acid solutions with 3-hydroxy-2-methyl-1-(4-tolyl)-4-pyridone (HY) dissolved in chloroform is described. Niobium(V) can be quantitatively extracted with HY in the form of two different complexes depending on the chloride ion concentration in the aqueous phase. At a low chloride concentration or without chloride in the aqueous phase niobium(V) is extracted with HY in the form of Nb(OH)₃Y₂ and at a high chloride concentration as a mixed Nb(OH)₃ClY complex. Niobium extraction with HY enables the separation of niobium(V) from zirconium(IV) and hafnium(IV). The formation of a mixed chloro-4-pyridone complex is also applicable for the spectrophotometric determination of niobium in the organic phase at the maximum absorption at 350 nm.     

Crystallization and relative stabilities of Polymorphs of Niobium(V) Oxide under hydrothermal conditions

The TT-, T-, B-, P-, and poorly ordered M-forms of niobium(V) oxide were crystallized hydrothermally by treating niobic acid with pure water and acids such as hydrochloric acid, nitric acid, and sulfuric acid at 250–750°C and 15–100MPa. Triniobium chloride heptaoxide was hydrolyzed in pure water and acids solutions at 250–500°C and 15–98MPa, producing the P-, R-, and B-forms. The formation of the above polymorphs involved either dissolution-precipitation or the solid-phase rearrangement of the structures of amorphous solids. The B-form was shown to be the most stable phase below 760°C by an isothermal-conversion method. The relative stabilities of seven polymorphs of niobium(V) oxide were estimated by observing the rates of conversion of metastable forms into the B-forms in 1.2 mol/dm³ hydrochloric acid.     

N-acetylsalicyloyl-N-phenylhydroxylamine as an analytical reagent Determination of niobium and tantalum in the presence of each other

N-Acetylsalicyloyl-N-phenylhydroxylamme is proposed for the separation of niobium(V) and tantalum(V) and their gravimetric determination. Niobium is precipitated at pH 5.5-6.5 by the reagent and the complex is weighed directly. Tantalum is precipitated from 1-2M hydrochloric acid solutions and the complex is ignited to tantalum pentoxide. The method is fairly selective. In the presence of thiocyanate the reagent forms an extractable complex with niobium. The reaction forms the basis of a selective and sensitive spectrophotometric determination of niobium.     

The separation of niobium from tantalum by extraction with tributyl phosphate and determination of niobium as the thiocyanate complex

A method is proposed for the rapid extraction and separation of microgram amounts of niobium(V). The niobium is extracted quantitatively by 100 % TBP from 7.7-9.4 M (initial) hydrochloric acid and determined spectrophotometrically as the thiocyanate in TBP-acetophenone solution. Beer's Law is obeyed at 430 mmu over the range 0.8-9.0 mug ml . The system is stable for 72 hr. Caesium, calcium, strontium, barium, aluminium, titanium(IV), zirconium(IV), cerium(TV), fluoride, thiocyanate and oxalate do not interfere (1 mg). Niobium(V) can be determined in a niobium(V)-tantalum(V) mixture. The method is accurate and reproducible to within +/-2%.     

Liquid-liquid extraction of niobium(V) in the presence of other metals with high molecular mass amines and ascorbic acid

A novel method is proposed for the solvent extraction of niobium(V). A 0.1M solution of Aliquat 336S in xylene quantitatively extracts microgram quantities of niobium(V) from 0.01M ascorbic acid at pH 3.5-6.5. Niobium from the organic phase is stripped with 0.5M nitric acid and determined spectrophotometrically in the aqueous phase as its complex with TAR. The method permits separation of niobium not only from tantalum(V) but also from vanadium(IV), titanium(IV), zirconium(IV), thorium(IV), chromium(III), molybdenum(VI), uranium(VI), iron(III), etc. Niobium from stainless steel was determined with a precision of 0.42%.     

Electrochemical synthesis of niobium methylate

The formation of niobium(V) methylate in methanol against the background of lithium chloride is studied. It is found that, in the anodic dissolution of niobium in the diaphragmless electrolyzer, formed niobium(V) methylate is partially reduced to the four-valent state at the cathode. In order to suppress the reduction of Nb(V), an electrolyzer was designed, which enabled one to separate the anolyte from the catholyte using the difference between their densities. Niobium methylate was not found in the anolyte; however, it forms as a result of mixing the anolyte with the catholyte after completion of electrolysis. The current efficiency of niobium methylate of 96–97% was achieved. Possible mechanism of the process is discussed.