Nb Species Research Articles

Catalytic hydrolysis of HCN on ZSM-5 modified by Fe or Nb for HCN removal: surface species and performance

Interactions present between Fe and Nb species resulted in larger amounts of isolated Fe3+, Nb5+, oligomeric FexOy and NbxOy clusters on zeolite catalysts, resulting in improved HCN hydrolysis.

The role of metallic modifiers of SBA-15 supports for propyl-amines on activity and selectivity in the Knoevenagel reactions

A series of bifunctional heterogeneous catalysts based on SBA-15 supports modified with Zr, Nb and Mo species functionalized by aminopropyl siliceous species has been used in the Knoevenagel condensation. The results indicated that the acid–base synergetic effects of the centres present on these catalysts seem to have significant effect on the Knoevenagel condensation of benzaldehyde with active methylene compounds. Basic sites and Lewis acid sites play different roles depending on the methylene compound used in the Knoevenagel condensation. Depending on the chemical composition of the catalysts, the reaction can proceed on basic sites exclusively or on the pairs of acidic-basic centres. The nature of metal species strongly influences the activity in the Knoevenagel condensations but does not affect the selectivity.

Mesoporous niobium-silicates prepared by evaporation-induced self-assembly as catalysts for selective oxidations with aqueous H2O2

Hydrothermally stable mesoporous niobium-silicates Nb-MMM-E have been first prepared following the convenient and versatile evaporation-induced self-assembly (EISA) methodology. The new materials have been characterized by elemental analysis, XRD, low-temperature N2 adsorption, SEM, XPS, DRS UV–vis, and Raman techniques. DR UV–vis spectroscopy enabled differentiation between isolated and oligomerized Nb centers on the silica surface. Use of niobium(V) ethoxide modified with acetylacetone (acac) as a Nb source favored the formation of dimeric and/or small oligomeric Nb centers on the surface of mesoporous silica while ammonium niobate(V) oxalate coupled with acac gave mostly site-isolated Nb species. The Nb-MMM-E materials catalyzed selectively oxidation of various unsaturated compounds of interest for fine and specialty chemistry using the green oxidant – aqueous hydrogen peroxide. Epoxidation of both electron rich and electron deficient CC bonds could be accomplished over Nb-MMM-E. While catalysts with isolated Nb centers were preferable for the selective formation of epoxides sensitive to ring opening and overoxidation, both single site and oligomerized Nb centers were equally effective for the production of relatively stable epoxides. The truly heterogeneous nature of the catalysis over Nb-MMM-E and absence of Nb leaching into solution have been proved by hot filtration tests coupled with elemental analysis. The catalysts could be easily recovered by filtration and reused several times without significant deterioration of the catalytic performance.

Sorption of niobium on boreal forest soil

Abstract The sorption of niobium (Nb) was investigated on humus and mineral soil samples taken from various depths of a four-metre deep forest soil pit on Olkiluoto Island, southwestern Finland. Mass distribution coefficients, K d, were determined in batch sorption tests. The steady state of Nb sorption was observed in the mineral soil samples already after one week of equilibration, and sorption decreased with depth from a very high value of 185000 mL/g at 0.7 m to 54000 mL/g at 3.4 m. The reason behind this decrease is probably the tenfold reduction in the specific surface area of the soil at the same depth range. Distribution coefficients were clearly lower in the humus layer (1000 mL/g). The K d values determined in pure water at a pH range of 4.7–6.5 were at a high level (above 55000 mL/g), but decreased dramatically above pH 6.5, corresponding to the change in the major Nb species from the neutral Nb(OH)5 to the low-sorbing anionic Nb(OH)6 − and Nb(OH)7 2−. However, the K d values in the model soil solution were in the slightly alkaline range an order of magnitude higher than in pure water, which is probably caused by the formation of calcium niobate surface precipitate or electrostatic interaction between surface-sorbed calcium and solute Nb. Among nine soil constituent minerals kaolinite performed best in retaining Nb in both pure water and model soil solution at pH 8, whereas potassium feldspar showed the poorest sorption. The K d value for kaolinite was above 500000 mL/g in both solutions, while the respective potassium feldspar values were in the range of 120–220 mL/g.

Electrochemical synthesis of Nb3Sn coatings on Cu substrates

This work aims at contributing to the development of superconducting Nb3Sn thin films for possible applications, as for instance in superconducting radio frequency (SRF) cavities. The synthesis of Nb–Sn coatings was carried out on copper substrates by electrodeposition from 1-butyl-3-methylimidazolium chloride (BMIC) ionic liquids containing SnCl2 and NbCl5. Cyclic voltammetric curves were recorded to identify the reduction potentials of Nb and Sn ionic species. Electrodeposition was performed at 40 and 400mA/cm2 and 130°C. The CV demonstrated that BMIC has a suitable potential window for co-deposition of Nb and Sn. The electrodeposited coatings showed a cubic Nb3Sn phase with (211) preferred orientation, a disordered orthorhombic NbSn2 phase and Sn–Cu phases. Film thickness was from 200 to 750nm. These results suggest that electrodeposition of Nb–Sn coatings on copper substrates could be a suitable route to one day replace the current expensive Nb SRF cavities.

Niobium-Doped Hydroxyapatite Bioceramics: Synthesis, Characterization and In Vitro Cytocompatibility.

Doping calcium phosphates with ionic species can play an important role in biological responses promoting alkaline phosphatase activity, and, therefore inducing the generation of new bone. Thus, in this study, the synthesis of niobium-doped hydroxyapatite (Nb-HA) nanosize particles obtained by the precipitation process in aqueous media followed by thermal treatment is presented. The bioceramics were extensively characterized by X-ray diffraction, wavelength dispersive X-ray fluorescence spectrometry, Fourier transform infrared spectroscopy, scanning electron microscopy/energy dispersive X-ray spectroscopy analysis, transmission electron microscopy, atomic force microscopy and thermal analysis regarding their chemical composition, structure and morphology. The results showed that the precipitate dried at 110 °C was composed of amorphous calcium phosphate and HA, with polidisperse particles ranging from micro to nano dimensions. After the thermal treatment at 900 °C, the bioceramic system evolved predominantly to HA crystalline phase, with evident features of particle sintering and reduction of surface area. Moreover, the addition of 10 mol% of niobium salt precursor during the synthesis indicated the complete incorporation of the Nb(V) species in the HA crystals with detectable changes in the original lattice parameters. Furthermore, the incorporation of Nb ions caused a significant refinement on the average particle size of HA. Finally, the preliminary cytocompatibility response of the biomaterials was accessed by human osteoblast cell culture using MTT and resazurin assays, which demonstrated no cytotoxicity of the Nb-alloyed hydroxyapatite. Thus, these findings seem promising for developing innovative Nb-doped calcium phosphates as artificial biomaterials for potential use in bone replacements and repair.

Cooperative Effect of Monopodal Silica-Supported Niobium Complex Pairs Enhancing Catalytic Cyclic Carbonate Production.

Recent discoveries highlighted the activity and the intriguing mechanistic features of NbCl5 as a molecular catalyst for the cycloaddition of CO2 and epoxides under ambient conditions. This has inspired the preparation of novel silica-supported Nb species by reacting a molecular niobium precursor, [NbCl5·OEt2], with silica dehydroxylated at 700 °C (SiO(2-700)) or at 200 °C (SiO(2-200)) to generate diverse surface complexes. The product of the reaction between SiO(2-700) and [NbCl5·OEt2] was identified as a monopodal supported surface species, [≡SiONbCl4·OEt2] (1a). The reactions of SiO(2-200) with the niobium precursor, according to two different protocols, generated surface complexes 2a and 3a, presenting significant, but different, populations of the monopodal surface complex along with bipodal [(≡SiO)2NbCl3·OEt2]. (93)Nb solid-state NMR spectra of 1a-3a and (31)P solid-state NMR on their PMe3 derivatives 1b-3b led to the unambiguous assignment of 1a as a single-site monopodal Nb species, while 2a and 3a were found to present two distinct surface-supported components, with 2a being mostly monopodal [≡SiONbCl4·OEt2] and 3a being mostly bipodal [(≡SiO)2NbCl3·OEt2]. A double-quantum/single-quantum (31)P NMR correlation experiment carried out on 2b supported the existence of vicinal Nb centers on the silica surface for this species. 1a-3a were active heterogeneous catalysts for the synthesis of propylene carbonate from CO2 and propylene oxide under mild catalytic conditions; the performance of 2a was found to significantly surpass that of 1a and 3a. With the support of a systematic DFT study carried out on model silica surfaces, the observed differences in catalytic efficiency were correlated with an unprecedented cooperative effect between two neighboring Nb centers on the surface of 2a. This is in an excellent agreement with our previous discoveries regarding the mechanism of NbCl5-catalyzed cycloaddition in the homogeneous phase.

Niobium-nitrides derived from nitrogen splitting.

The easy-to-prepare Nb(V) aryloxide complex [(ArO)2Nb(μ-Cl)Cl2]2 (OAr = 2,6-bis(diphenylmethyl)-4-tert-butylphenoxide) is a precursor to both Nb(IV), [trans-(ArO)2NbCl2(THF)2], and Nb(III), K3[(ArO)4Nb2(μ-Cl)3Cl2], molecules. The Nb(IV) and (V) complexes readily split atmospheric nitrogen at room temperature and 1 atmosphere, under reducing conditions, to produce the low-coordinate nitride dimer [(ArO)2Nb(μ-N)]2 and its radical anion, K[(ArO)2Nb(μ-N)]2.

A one step synthesis ofcaprolactam out of cyclohexanone by combinded ammoximation and Beckmann rearrangement over Nb-MCM-41 catalysts

Caprolactam was synthesized directly in one step out of cyclohexanone via combined liquid phase ammoximation and Beckmann rearrangement using H2O2 and NH3 in presence of Nb-MCM-41 catalysts and ethanol as solvent. Thereby the Beckmann rearrangement of the oxime to caprolactam occurs simultaneously with the ammoximation of cyclohexanone. Nb-incorporated MCM-41 molecular sieves (Si/Nb- 16, 32, 64 and 128) have been synthesized hydrothermally using tetradecyltrimethylammoniumbromide as template in absence of auxiliary organics. Synthesized Nb-MCM-41 samples were characterized using various physical-chemical techniques such as X-ray diffraction, N2-physisorption, Atomic Absorption Spectroscopy (ICP-AAS), Thermal Gravimetric Analysis (TGA-DTA), Raman spectroscopy and H2-Temperature Programmed Reduction (TPR). All spectroscopic characterization techniques revealed that two types of Nb species are present in the samples. One is tetrahedral coordinated framework Nb species and the other type extra framework Nb2O5 species. All these Nb-MCM-41 catalysts were employed to catalyze the ammoximation reaction of cyclohexanone with H2O2 and NH3 and the simultaneously occurring Beckmann rearrangement reaction. The influence of essential reaction parameters such as temperature, NH3/cyclohexanone ratios and Si/Nb ratios were investigated. A caprolactam selectivity of 66% was obtained at a cyclohexanone conversion of 7.3% at NH3/cyclohexanone molar ratio=1.5, H2O2: cyclohexanone molar ratio=1.32 over Nb-MCM-41 (Si/Nb-32) at 80°C.

Unraveling the Cooperative Mechanism of Visible-Light Absorption in Bulk N,Nb Codoped TiO2 Powders of Nanomaterials

N,Nb-codoping has recently been proposed as a promising strategy to enhance the activity of nanostructured TiO2 under visible irradiation. Here, we suggest a possible electronic mechanism to account for the observed visible absorption improvement. The effects of N and Nb species on the electronic, crystallographic, and morphological properties of TiO2 were deeply investigated both experimentally (HR-XRPD, EXAFS, EDX, BET, SEM, EPR, and DRS) and theoretically (DFT). We found a significant synergism between N and Nb species, while EXAFS, HR-XRPD, and DFT simulations provided compelling evidence for the Nb substitutional position in anatase. At variance with interstitial, substitutional Nb can transfer an electron to low-energy valence states of the N codopant near the valence band. This intrinsic charge compensation mechanism is substantiated by EPR, which shows a reduction of the paramagnetic bulk N species signal in N,Nb-codoped samples. DRS analysis of N,Nb-codoped samples shows a slight reduction of the...

Niobium Speciation in NaCl-KCl Based Melts: An Electrochemical and Spectroelectrochemical Study

Molten chlorides can be used for niobium electrowinning and electrorefining. Understanding niobium electrochemical properties and speciation in chloride melts is essential for designing or optimizing the industrial electrochemical process. Apart from the technological value, the information on the kinetics of the electrode reactions can also be used for verifying theoretical predictions concerning the nature of the heterophase electrochemical reactions.In the present study a variety of electrochemical techniques (polarization curves measurements (PCM), chronoamperometry (CA), chronopotentiometry (CP), linear (LVA), cyclic (CVA) and square-wave voltammetry (SWVA), impedance spectroscopy) were applied for studying electrode processes involving niobium species in NaCl-KCl-based melts. In addition the most valuable and reliable information about niobium speciation was obtained by combining electrochemical and spectroscopic (measuring electronic absorption spectra, EAS) methods.The results of spectroelectrochemical experiments showed that the major product of niobium anodic dissolution in chloride melts at current densities up to 40 mA/cm2 were niobium(III) species, NbCl6 3-. In the potentiostatic regime of anodic dissolution of niobium metal niobium(III) complex ions were formed at potentials below -1.15 V vs.chlorine reference electrode. The polarization curves characterizing the anodic dissolution of niobium consist of three or more waves. The first wave is associated with reaching limiting current density of niobium ionization according to the three-electron scheme. Further increase of applied current leads to the oxidation of Nb(III) to Nb(IV) ions. These conclusions were confirmed by determined values of potentials of niobium or indifferent electrodes in the equilibrium with the melts containing Nb(III) and Nb(IV) ions.When studying anodic dissolution of niobium we found that at high current densities there is sudden increase of the ohmic part of voltage drop sometime accompanied by increase of the anode potential. As a result stationary and non-stationary polarization dependencies exhibit unstable behavior. This is explained by the salt passivation, SP. The product of the SP of niobium is unstable NbCl4, which decomposes to NbCl3.13 and volatile NbCl5.The results of electrochemical experiments (PCM, CVA, LVA, CA, CP and SWVA) showed that soluble niobium(V) species were formed during Nb(IV) electrooxidation and represented the highest oxidation state of niobium in NaCl-KCl melts. We noticed that peak potential value (both cathodic and anodic) on CV changes with scan rate, whereas the ip/ν1/2 value remains constant. The formation of Nb(V) species was confirmed by the disappearance of absorption bands in the visible part of the spectrum due to Nb(V) species formation during elecrooxidation of niobium-containing melts at potentials up to -0.5 V. The quasireversibility of Nb4+↔Nb5+reaction was explained by the instability of niobium(V) species.Niobium(IV) ions were formed when potentiostatic electrolysis of NbCl3-NaCl-KCl melts (prepared by anodic dissolution of niobium metal) was performed at -(1.0÷0.8) V vs.chlorine reference electrode on a glassy-carbon anode, Fig. 1.The electroreduction of niobium on a tungsten electrode in NbCl3-NaCl-KCl melts at potentials below -1.45 V led to the formation of metallic niobium (Fig. 2), no evidence of any intermediate product formation was obtained.Linear voltammetry measurements showed that intermediate products can be formed during elecroreduction of NaCl-KCl-NbCln (n=3.8-4.1) melts. Most likely the electroreduction of Nb(IV) results in the formation of niobium(III) ions. The shape of cyclic voltammograms recorded in niobium containing melts depended on the scan rate. Similar sets of cyclic voltammograms were obtained at different temperatures and niobium concentrations. SWVA measurements performed on different electrodes also confirmed that there is more than one step of electroreduction. The obtained results were explained by overlapping of the following electrode reactions: Nb(IV) → Nb(III), Nb(IV) → Nb(0) and Nb(III) → Nb(0).

Electrodeposition and Characterisation of Novel Ni-NbOx Composite Coatings as a Diffusion Barrier for Liquid Solder Interconnects - Part II: Diffusion Barrier Performance

Electronics products are increasingly required to operate at high temperatures in harsh environments, for instance in oil well, geophysical drilling and aerospace applications. Conventional solder interconnects, that have been extensively utilised in consumer electronics, are no longer adequate. This is primarily due to excessive intermetallic compounds (IMCs) that can form and continuously grow during high temperature operation. With the intention of reducing such excessive IMC growth, electrically conducting niobium sub-oxide containing Ni coating has been produced which acts as a barrier layer between the solder and substrate. This then promises the possibility of developing a new solder barrier material with electrical conductivity and chemical resistance as required.The present study considers a novel electrochemical route to produce Ni-NbOx composite coatings of good uniformity, smoothness, compactness and purity. The electrolyte is derived from a Type IV eutectic solvent consisting of NbCl5 and propylene glycol (PG). The addition of a small amount of Ni ions (molar ratio: Ni : Nb=1 : 30) was found to enable the co-deposition of Nb species. The introduction of NaBH4 into the electrolyte has elevated the maximum deposit thickness (above 15 µm), although this led to a reduction in the co-deposited Nb content. The effects of cathodic current density, metal precursor and NaBH4 concentrations on the surface morphology, composition and thickness of the coatings were examined. Scanning transmission electron microscopy (STEM) revealed a nanocrystalline structure resembling that of a simple FCC structure of Ni, with a slightly fluctuating composition across the coating thickness and with Nb uniformly distributed (See Fig. 1). From X-ray photoelectron spectroscopy (XPS), a 0.7 eV shift to lower binding energy for the Nb (V) species signifies the partial chemical reduction of Nb2O5 due to possible Ni-Nb bonding. It is thus proposed that a Ni-NbOx deposit was formed by a two-step mechanism: the simultaneous electrochemical reduction of adherent Ni metal and inadherent niobium oxides on Cu, followed by nickelation of the NbOx. The composite coating’s electrical conductivity, solderability and diffusion barrier characteristics (the latter preventing excessive IMC growth between molten Sn-In solder and Cu substrate) were assessed and showed promising results. From diffusion studies, a Nb-enriched layer was observed at the solder/substrate interface after ageing at 200°C suggesting evident effectiveness of the diffusion barrier characteristics.Figure 1 - STEM cross sectional image a) of a 95Ni-5Nb coating with corresponding Energy-dispersive X-ray spectroscopy (EDX) maps of b) Ni Kα & c) Nb Lα.

Comparison of Cultivars and Seasonal Variation in Blueberry (Vaccinium Species) Leaf Extract on Adult T-Cell Leukemia Cell Line Growth Suppression.

The inhibitory effects of blueberry leaves on the proliferation of adult T-cell leukemia (ATL) cell lines have previously been reported. A comparison of blueberry leaf extracts from different cultivars and seasonal variation were investigated regarding their effects on ATL cell line proliferation. The inhibitory effects of 80% ethanol leaf extracts from different blueberry cultivars collected from April to December in 2006 or 2008 were evaluated using two ATL cell lines. The bioactivities of leaf extracts of rabbit-eye blueberry (Vaccinium virgatum Aiton; RB species), southern highbush blueberry (V. spp.; SB species), northern highbush blueberry (V. corymbosum L.; NB species), and wild blueberry (V. bracteatum Thunb.; WB species) were compared. Of these, leaves of the RB species collected in December showed a significantly stronger inhibitory effect in both cell lines than the SB, NB, or WB species. These results suggest elevated biosynthesis of ATL-preventative bioactive compounds in the leaves of the RB species before the defoliation season.

Group VB transition metal oxide clusters M4O n −/0 (M = Nb, Ta; n = 8–11): structural evolution and chemical bonding

The structural and electronic properties of two series of Group VB transition metal oxide clusters, M4O − and M4O n (M = Nb, Ta; n = 8–11), are investigated using density functional theory calculations. Generalized Koopmans’ theorem is applied to predict the vertical detachment energies and simulate the photoelectron spectra. Large highest occupied molecular orbital–lowest unoccupied molecular orbital gaps are observed for these two stoichiometric M4O10 clusters and estimated to be 3.98 and 4.38 eV for M = Nb and Ta, respectively. The M4O 10 −/0 (M = Nb, Ta) clusters are polyhedral cage structures with high symmetry (T d for the neutral and D 2d for the anion) in which each metal atom joints three bridging and one terminal O atoms. For the Nb oxide species, Nb4O 8 −/0 and Nb4O 9 −/0 can be viewed as removing two and one terminal O atoms from Nb4O 10 −/0 , respectively. The Ta species follow the same rule to the Nb species, except that the anionic Ta4O8 − is formed by removing one terminal and one bridging O atoms from Ta4O10 −. The Ta4O9 containing a localized Ta3+ site can readily react with O2 to form the Ta4O11 which can also be viewed as replacing a terminal oxygen atom in Ta4O10 by a peroxo O2 unit, whereas the added oxygen atom is found to be a bridging one in the O-rich clusters Nb4O 11 −/0 and the anionic Ta4O11 −. Molecular orbital analyses are performed to analyze the chemical bonding in the tetra-nuclear metal oxide clusters and to elucidate their structural and electronic evolution.

Electrodeposition and Characterization of Nanocrystalline Ni-NbOx Composite Coatings from Glycol-based Electrolytes for High Temperature Electronics Packaging

The present investigation demonstrates a novel electrochemical route for the production of bright, compact, dense and high purity Ni-NbOx nano-composite coatings on Cu, from glycol-based electrolytes consisting of NbCl5, NiCl2·6H2O and propylene glycol. The effects of both cathodic current density and metal electrodeposition bath concentrations on the surface morphology and composition of the coating were examined. The highest relative metal percentage of co-deposited Nb was 14.3 at% obtained at a current density of 400 mA/cm2. Scanning transmission electron microscopy (STEM) revealed a nanocrystalline FCC Ni structure without any significant lattice distortion, as well as an as-yet-unidentified compound. STEM X-ray mapping suggests Nb and O were uniformly distributed in the Ni matrix, without any significant phase segregation, which is consistent with XPS compositional depth profiling. Also, according to the XPS spectra, a 0.7 eV shift to lower binding energy for the Nb (V) species may suggest the partial chemical reduction of Nb2O5 due to possible Ni-Nb bonding. It is thus proposed that a Ni-NbOx deposit was formed by two parallel electrochemical reduction routes: 1) deposition of Ni metal; 2) deposition of a Ni-Nb-O compound as nano-clusters.

Preparation, characterization of N–I co-doped TiO2 and catalytic performance toward simultaneous Cr(VI) reduction and benzoic acid oxidation

Four N–I co-doped TiO2 catalysts having Ti:N/I molar ratios of 1:1, 1:3, 1:5 and 1:10 were prepared via a sol–gel method using NH4I as N–I dopant precursor. A pure TiO2 (undoped) sample was also prepared by the same method for comparison. The catalysts were evaluated for the simultaneous photocatalytic reduction of Cr(VI) and oxidation of benzoic acid (BA). TiO2 anatase phase was formed for all N–I co-doped catalysts as shown by XRD. UV–vis diffuse reflectance spectra showed that N–I co-doping resulted in increased absorption at visible wavelengths and a decrease of the band gap energy (Eg). The smallest Eg value of 2.34eV was observed for the 1:5 Ti:N/I molar ratio. The structure and photodynamics of the TiO2 catalysts was investigated in detail by Electron Paramagnetic Resonance (EPR) spectroscopy. The EPR data showed: [i] formation of non-photoactive NO centers and photoinduced Nb paramagnetic species as a result of N doping, [ii] photoinduced Ti3+ surface ions, and [iii] formation of surfacial oxygen O2− radical ions and trapped holes TiO4+–O−. The Nb species act upon the narrowing of the band gap and the production of photogenerated electrons, i.e. Ti3+ surface ions. These Ti3+ surface ions have a key role, capturing gas O2 and supporting both reduction and oxidation process. Cr(VI) reduction by the N–I co-doped catalysts followed the trend: TNI5>TNI10>TNI1>TNI3>TiO2[undoped] which correlates with the concentration of Nb species formed and the narrowing of Eg values. Oxidation of benzoic acid (BA) followed a more complex trend as follows: TNI1>TNI3>TNI10>TNI5 which is the reverse of the trend for trapped holes TiO4+–O−. Finally progressive microwave EPR saturation experiments show that Nb species are located deeper in the TiO2 lattice in particles with narrower Eg values.

Electronic Absorption Spectra of Niobium Species in Halide Melts

The results of spectroscopic studies of Nb anodic dissolution, dissolution of NbCl5 and reaction of various niobium oxides with HCl in different chloride melts at 450-7500C are presented. Nb(III) species, NbCl63-, is the major product of niobium anodic dissolution at current densities up to 40 mA/cm2. Addition of NbCl5 to alkali chloride melts predominantly leads to the formation of Nb(V) species, NbCl6-. Upon increasing Nb(V) concentration NbCl6- tend to decompose to Nb(IV) and Cl2. Reaction of NbO and NbO2 with HCl proceeds via an intermediate oxygen-containing Nb(IV) complexes. Oxygen-free Nb(IV) ions, NbCl62-, were obtained by exchange reactions between metallic niobium and melts containing Bi(III), Ag(I) or Ni(II) chloro-ions. Electroreduction of Nb(III) on a tungsten working electrode below -1.45 V vs. Cl2/Cl- led to the formation of metallic niobium, no evidence of intermediate reduction products was obtained.

Niobium–silica catalysts for the selective epoxidation of cyclic alkenes: the generation of the active site by grafting niobocene dichloride

Niobium-containing silica materials obtained by deposition via liquid-phase grafting or dry impregnation of niobocene(iv) dichloride are active and selective catalysts in the epoxidation of alkenes in the presence of aqueous hydrogen peroxide. The generation of the catalytically-active Nb species was followed step-by-step, and investigated using a combined DR-UV-Vis, NIR, Raman, XRD, XANES and EXAFS analyses. At the end of the grafting procedure, the nature of the surface active species can be described as an oxo-Nb(v) site, tripodally grafted onto the silica surface in close proximity to other Nb(v) centres. The liquid-phase methodology provides a better dispersion of the metal sites onto the siliceous support than the dry-impregnation approach. The niobium-silica catalysts were then tested in the epoxidation of cyclohexene and 1-methylcyclohexene, as model substrates.

Influence of Alloy Elements on the Osteoconductivity of Anodized Ti-29Nb-13Ta-4.6Zr Alloy

Anodizing is expected to be an effective method to improve the osteoconductivity of the Ti-29Nb-13Ta-4.6Zr (TNTZ) alloy because the bioactivity of anodized Ti is good. However, it is not known how the alloy elements influence the surface roughness, composition, hydrophilicity, and osteoconductivity of the anodized film on the Ti alloy. In this study, we investigated the effects of anodizing on the surface properties and the osteoconductivity of the anodized TNTZ alloy, focusing on the functions of the individual alloy elements. The anodized oxides of the Nb, Ta, and Zr metals were hydrophobic at all the voltages applied, in contrast to the anodized oxide of Ti. As well as pure Ti, a TiO2-based oxide film formed on TNTZ after anodizing. However, the oxide film also contained large amounts of Nb species and the molar Nb/Ti ratio in the TNTZ alloy was high, which makes the surface more hydrophobic than the anodized oxide on Ti. In vivo tests showed that the osteoconductivity of the TNTZ alloy was sensitive to both its surface roughness and hydrophilicity. When the TNTZ alloy was anodized, the process increased either the surface hydrophobicity or the surface roughness at the voltage used in this study. These changes in the surface properties did not improve its osteoconductivity.

Photocatalytic activity of N-doped and N–F co-doped TiO2 and reduction of chromium(VI) in aqueous solution: An EPR study

N-doped and N–F co-doped TiO2 catalysts were prepared via a sol–gel method using NH4Cl and NH4F as N and N–F dopant precursors, respectively, having Ti:N and/or F molar ratios of 1:1, 1:2 and 1:3. The catalysts were tested for the photocatalytic reduction of Cr(VI) in the presence of oxalate ions. XRD analysis showed the formation of TiO2 anatase phase in all cases. UV–vis DRS spectra showed that both N–F and N-doping resulted in a decrease in the band gap energy (Eg), at the values of 2.81eV and 3.01eV, respectively. Thus, N–F doped TiO2 showed enhanced absorption at visible wavelengths. The structure and photodynamics of the TiO2 catalysts was investigated in detail by electron paramagnetic resonance (EPR) spectroscopy. The EPR data showed that: [i] NO centers, Nb and O2- radicals were formed. In addition, lattice Ti3+ ions were detected in N–F co-doped solids; [ii] the Nb and Ti3+ species were photoactive, while the NO species were non-photoactive.The photocatalytic efficiency for Cr(VI) reduction in the presence of oxalate ions, followed the trend TNF1>TN1>TNF2>TN3>TN2>TNF3. Importantly, an apparent correlation between the catalytic efficiency and the concentration of Nb species was revealed by EPR. The location of Nb in the crystal lattice of TiO2 has been assessed also by measuring their microwave saturation parameters P1/2. Electron capturing by O2 and subsequent generation of O2- was favored for N-doped catalysts. In contrast, in N–F co-doped catalysts, O2 could not compete efficiently with Cr(VI) for the photogenerated electrons in energy states below the conduction band of TiO2, resulting in higher reduction efficiency for these catalysts.