In situ UV-Vis and ex situ IR spectroelectrochemical investigations of amorphous and crystalline electrochromic Nb2O5 films in charged/discharged states

Niobium oxide (Nb₂O₅) films and powders have been obtained via the sol-gel route from an NbCl₅ precursor. XRD spectra revealed that films with pseudohexagonal (TT-phase) and orthorhombic (T-phase) structure were formed at 500  °C and 800  °C, respectively, while at 300  °C films were amorphous. Infrared (IR) and Raman spectra of powders and films of Nb₂O₅ in different polymorphic forms were detected, and vibrational band assignments were made. Electrochromic properties of amorphous films and films with the TT-phase were established from in situ ultraviolet visible (UV-Vis) spectroelectrochemical measurements and correlated with ex situ IR transmission spectra of charged films. Ex situ IR spectra revealed that charging of amorphous films is accompanied by variations of the Nb-O stretching mode intensity, while, for films with the TT- and T-phase, splitting of the Nb₃-O stretching modes and the appearance of polaron absorption were noted with Li⁺ ion insertion. Ex situ X-ray diffraction (XRD) spectra of charged films with the TT-phase showed changes of the unit cell dimensions with charging. The influence of the polaron absorption on the ex situ near-grazing incidence angle (NGIA) IR reflection-absorption spectra of charged/discharged films is discussed in detail. Received: 21 August 1997 / Accepted: 9 October 1997     

Photomineralization of phenol on Al<Subscript>2</Subscript>O<Subscript>3</Subscript>: synergistic photocatalysis by semiconductors

Phenol gets adsorbed on Al₂O₃ and mineralizes under UV light in the presence of dissolved O₂. The degradation exhibits first-order kinetics and its rate increases linearly with the light intensity and decreases with pH. 2,4-Diphenoxycyclohexanone and 2,6-diphenoxycyclohex-3-ene-1-ol are the intermediates of the reaction. While particulate TiO₂, ZnO, ZnS, Fe₂O₃, CuO, CdO, and Nb₂O₅ individually photocatalyze the degradation, each semiconductor exhibits synergistic photocatalysis, an enhanced photodegradation, when present along with Al₂O₃, indicating electron abstraction by illuminated semiconductors from the phenol adsorbed on Al₂O₃.     

Na3Al2Nb34O64 und Na (Si,Nb) Nb10O19: Clusterverbindungen mit isolierten Nb6-Oktaedern

Na₃Al₂Nb₃₄O₆₄ and Na (Si, Nb) Nb₁₀O₁₉. Cluster Compounds with Isolated Nb₆-Octahedra Hexagonal ormolu coloured plates of the new compounds Na₃Al₂Nb₃₄O₆₄ ( I ) and Na(Si, Nb)Nb₁₀O₁₉ ( II ) were prepared by heating pellets of NaF, Al₂O₃, NbO₂ and NbO (3:1:8:2) and NaF, NbO₂ and NbO (1:4:2), respectively, at approx. 850°C. I was contained in a sealed gold capsule, II in a silica tube. The Si incorporated in II originates from the container material. Both compounds crystallize in R 3 , I with a = 784.4(1), c = 7065(1) pm, Z = 3 and II with a = 784.1(1), c = 4221.8(5) pm, Z = 6. I and II represent new structure types. They contain the same characteristic structural units, namely discrete Nb₆O₁₂ clusters (d_Nb–Nb = 283 ± 4 pm) and Nb₂O₁₀ units with Nb–Nb dumbells (d_Nb–Nb ≈? 269 pm) in edgesharing coordination octahedra. In addition NbO₆ octahedra containing Nb in the oxidation state + 5 and NaO₁₂ cube-octahedra occur in both compounds besides AlO₄ and SiO₄ tetrahedra in I and II , respectively. The structures can be described in terms of a common closepacking of O and Na atoms together with Nb₆ octahedra.     

Characterization of Thermally Stable Brønsted Acid Sites on Alumina‐Supported Niobium Oxide after Calcination at High Temperatures

Thermally stable Brønsted acid sites were generated on alumina‐supported niobium oxide (Nb₂O₅/Al₂O₃) by calcination at high temperatures, such as 1123 K. The results of structural characterization by using Fourier‐transform infrared (FTIR) spectroscopy, TEM, scanning transmission electron microscopy (STEM), and energy‐dispersive X‐ray (EDX) analysis indicated that the Nb₂O₅ monolayer domains were highly dispersed over alumina at low Nb₂O₅ loadings, such as 5 wt %, and no Brønsted acid sites were presents. The coverage of Nb₂O₅ monolayer domains over Al₂O₃ increased with increasing Nb₂O₅ loading and almost‐full coverage was obtained at a loading of 16 wt %. A sharp increase in the number of hydroxy groups, which acted as Brønsted acid sites, was observed at this loading level. The relationship between the acidic properties and the structure of the material suggested that the bridging hydroxy groups (Nb? O(H)? Nb), which were formed at the boundaries between the domains of the Nb₂O₅ monolayer, acted as thermally stable Brønsted acid sites.     

A new approach to chemical concentration in activation analysis for some noble and rare elements

The possibility of concentrating Pt, Ir, Au, Ag, Re and some other elements from samples of chromites, sulfide ores, laterites, shales, titanium magnetites, and ultrabasic rocks was studied. A new simple procedure is based on sublimation of elements to be determined in air stream at 1200 °C in the presence of some powdered reagents /e.g., TiO₂, Nb₂O₅, Nb/ to enhance the yield, and on the use of chemical filters /CaO, MgO, TiO₂, Al₂O₃, Nb₂O₅/ absorbing the interfering volatile elements from the gas phase. Neutron activation and X-ray fluorescence isotopic excitation were used to analyze the obtained concentrates.     

Effect of Pt and Pd additives on the state of the surface layer and catalytic activity of niobium oxides in the oxidation of hydrogen

Catalysts prepared by the hydrogen reduction of Nb₂O₅ in the presence of Pt or Pd have specific surface much greater than for the starting oxide and their catalytic activity in the oxidation of hydrogen is much greater than the activity of Pt/Al₂O₃ or Pd/Al₂O₃. X-ray phase analysis and X-ray photoelectron spectroscopy were used to establish the existence of Nb₂O_5–x nonstoichiometric oxides in the catalyst, which enhances the catalytic activity of the surface. The kinetic behavior of the oxidation of hydrogen on these catalysts is explained in the framework of the Eley–Riedel mechanism.     

Studies on the effect of milling on reduction of niobium pentaoxide with aluminium

The effect of milling on the aluminothermic reduction of niobium pentaoxide was investigated. Charges of Nb₂O₅ and Al (containing 5 % excess Al) were milled for different time (2, 5 and 10 h). XRD profile of milled samples indicated no phase formations during milling; only peak broadening were seen. Milled and unmilled charges were heated in a thermal analyser up to 1,400 °C. Products of milled charges showed formation of Nb, NbO and Al₂O₃; whereas unmilled hand mixed charges showed formations of Nb₃Al along with Nb and Al₂O₃. The tendency of milled charges towards Nb formations without the presence of aluminides was explained from the increase in surface area of charges caused by particle reduction.     

High-resolution electron microscopy of defects in W4Nb26O77

Several different kinds of planar defects have been observed by means of high-resolution electron microscopy in W₄Nb₂₆O₇₇, such as disordered intergrowth of WNb₁₂O₃₃ and W₃Nb₁₄O₄₄ structural slabs, locally ordered intergrowth with a sequence of AABAAB, two separate microdomains of WNb₁₂O₃₃ and W₃Nb₁₄O₄₄ coexisting with W₄Nb₂₆O₇₇ and a complicated intergrowth of W₄Nb₂₆O₇₇, NNb₂O₅, W₃Nb₁₄O₄₄, and Nb₃₁O₇₇F types of structure.     

Bemerkungen zur Anodentemperatur eines spektrographischen Kohledauerbogens

Zusammenfassung Eine Spektralanalyse nichtleitender Substanzen kann so durchgeführt werden, daß man das Analysengut, gemischt mit Graphit, in die Bohrung einer Kohleanode stopft. Die Temperatur am Boden dieser Aushöhlung, die zum Teil die sich dort abspielenden chemischen Erscheinungen bedingt, wurde bei verschiedenen Stromstärken gemessen: sie steigt von 900 auf 2000°C bei Dauerbögen von 4–13 A. Es war nicht möglich, diese Temperatur eindeutig mit den Eigenschaften der eingesetzten Elemente in Beziehung zu bringen. Es besteht allerdings ein enger Zusammenhang zwischen dem Dampfdruck dieser Elemente und der Anstiegsgeschwindigkeit der Temperatur. Folgende Oxide wurden untersucht: Ag₂O, Al₂O₃, Fe₂O₃, Mn₃O₄, Nb₂O₅, SnO₂, WO₃ und ZrO₂.

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Remarks on the anode temperature of a spectrographic d.c. carbon arc

In the anodes used by analysis of nonconducting materials with arc emission spectroscopy, a cavity is bored in which a mixture of graphite and the substances to be analysed are introduced. The temperature at the bottom of this cavity is responsible, for a part, for the chemical processes occurring between the substances present. It has been measured at different current intensities: it increases from 900° to 2000° C for d.c. arcs from 4 to 13 A. This temperature cannot be correlated with the properties of the used elements. However, there is a relation between vapour pressure of these elements and the rate of temperature increase. The following oxides have been examined: Ag₂O, Al₂O₃, Fe₂O₃, Mn₃O₄, Nb₂O₅, SnO₂, WO₃, ZrO₂.

     

Catalytic transfer hydrogenation of 2-butanone over oxide catalysts

Catalytic transfer hydrogenation of 2-butanone with 2-propanol was studied in gas phase over a series of oxides of different acid-base properties. Although the basic oxides (MgO, La₂O₃) gave high initial conversions, these oxides underwent deactivation during the reaction. This deactivation could be partially prevented by a previous treatment with chloroform of the oxide. The amphoteric oxides (TiO₂, ZrO₂, Al₂O₃) were also active in this reaction. Increasing the acidic character of the catalyst (Nb₂O₅, WO₃) led to a pronounced dehydration of 2-propanol. The results obtained over a series of rare earth oxides (La₂O₃, Sm₂O₃, Gd₂O₃, Dy₂O₃, Er₂O₃) revealed that beside the role of basic and acid sites a correlation seems to exist between the number of unpaired electrons of the metal ion and the catalytic activity, indicating the role of one electron donor sites.     

Beiträge zur Untersuchung anorganischer nichtstöchiometrischer Verbindungen. XXXIII. Darstellung und elektronenmikroskopische Untersuchung von Hf2Nb20O54, M3Nb44O116 und MNb24O62 (M = Zr,Hf)

Contributions to the Investigation of Inorganic Non-stoichiometric Compounds. XXXIII. Preparation and Electron Microscopical Investigation of Hf₂Nb₂₀O₅₄, M₃Nb₄₄O₁₁₆, and MNb₂₄O₆₂ (M = Zr, Hf) The phases Hf₂Nb₂₀O₅₄, Zr₃Nb₄₄O₁₁₆, Hf₃Nb₄₄O₁₁₆, and β-HfNb₂₄O₆₂ have been prepared for the first time. The oxides MO₂ (M = Zr, Hf) and Nb₂O₅ were fused with (NH₄)₂SO₄ and their mixed precipitations were heated at 1350°C. In the same way we also obtained pure β-ZrNb₂₄O₆₂. The new compounds have block structures, as the structure investigation with HRTEM shows. Hf₂Nb₂₀O₅₄ is isostructural with Nb₂₂O₅₄, the same is valid for Zr₃Nb₄₄O₁₁₆ and Hf₃Nb₄₄O₁₁₆ with respect to Nb₄₇O₁₁₆. β-HfNb₂₄O₆₂ has the same structure as β-ZrNb₂₄O₆₂ and β-Nb₂₅O₆₂.     

Determination of the chemical composition of supported vanadium-containing oxide catalysts by the differential dissolution method

The results of the application of the stoichiographic method of differential dissolution (DD) in the determination of the chemical composition of vanadium-containing catalysts are presented. In the studied catalyst series, amounts of vanadium were deposited onto TiO₂, SiO₂, Al₂O₃, ZrO₂, and Nb₂O₅. The catalysts were prepared by the impregnation method or by the spray drying method and thermally treated at different temperatures. The DD method was used for the precise correction of the phase composition of the V₂O₅/TiO₂ catalyst samples in order to determine the nature of the active component of these catalysts and obtain the correct information on their structure using the NMR method.     

Phase formation in the V<Subscript>2</Subscript>O<Subscript>5</Subscript>-Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>-MoO<Subscript>3</Subscript> system

The solid-phase interaction in the V₂O₅-Nb₂O₅-MoO₃ system has been investigated, and the formation of a solid solution bounded by the compositions MoNb₂V₄O_{18 ? δ}, Mo₂NbV₅O_{21 ? δ}, Mo₂Nb₃V₃O_{21 ? δ}, and Mo₄Nb₉V₉O_{57 ? δ} has been found (δ is nonstoichiometry). In the V₂O₅?Nb₂O₅ system, the formation of three compounds is verified, namely, VNbO₅ (tetragonal structure), VNb₉O₂₅, and V₂Nb₂₃O_62.5. The first two compounds are isostructural and form a continuous solid solution with tetragonal symmetry. A new compound of the composition Mo₃NbVO_{14 ? δ} has been synthesized. This compound is isostructural to the Mo₃Nb₂O₁₄ compound described in the literature and forms a tetragonal solid solution with it. The phase equilibria in the V₂O₅-Nb₂O₅-MoO₃ system in the subsolidus region have been determined.     

Experimental investigation of the formation of double phosphates in the LaPO4–NaF system

In the course of segregation smelting of rare-earth and rare metal raw materials with a fluxing agent (NaF), two immiscible melts form, one of which is a silicate melt and the other is a phosphate–salt melt. The silicate melt is enriched with Fe₂O₃, Al₂O₃, SiO₂, and Nb₂O₅, and the phosphate–salt melt is dominated by P₂O₅, TR₂O₃, Sc₂O₃, and Y₂O₃, and also with Ca, Sr, and Ba oxides. Chemical reactions between lanthanum orthophosphate and sodium fluoride in the LaPO₄–NaF system were studied for developing a technology for processing the phosphate–salt (rare-earth metal) melt. It was found that a metathesis reaction gives double phosphate Na₃La[PO₄]₂ and binary fluoride NaLaF₄. The products of crystallization of melts in the LaPO₄–NaF system decompose in weak mineral acids unlike those in conventional technology for processing monazite raw material.     

Hydrothermal synthesis followed by post heat-treatment: A novel route for the fabrication of KCa2Nb3O10 layered compound

Due to unique properties, KCa₂Nb₃O₁₀ compound has received great attention worldwide. High-temperature solid-state reaction is the common route for the synthesis of this compound. In order to propose a new low-temperature method (i.e. hydrothermal synthesis), which could improve the final properties of KCa₂Nb₃O₁₀, this study has been planned and performed. The preliminary experiments in KOH-Nb₂O₅ and Ca(OH)₂-Nb₂O₅ systems revealed that KCa₂Nb₃O₁₀ could be hydrothermally synthesized from a KOH-Ca(OH)₂-Nb₂O₅ system using an alkaline condition (i.e. 5 M < KOH <10 M). However, the experimental results showed that the product only consisted of KNbO₃ and Ca₂Nb₂O₇ phases. To initiate the reaction between KNbO₃ and Ca₂Nb₂O₇, the obtained KNbO₃-Ca₂Nb₂O₇ mixture was heat-treated in an air atmosphere. The results showed that KCa₂Nb₃O₁₀, with a high crystallinity and good purity, has been successfully obtained at 800 °C. This temperature is the lowest temperature, reported for the synthesis of KCa₂Nb₃O₁₀ compound so far. The SEM investigations revealed that the obtained KCa₂Nb₃O₁₀ powder has plate-like morphology due to its layered structure.     

Hydrothermal Synthesis and Characterization of Nb3O7(OH)

Triniobium hydroxide heptaoxide, Nb₃O₇(OH), was prepared hydrothermally by treating niobic acid or triniobium chloride heptaoxide with 3.0 mol/dm³ sulfuric acid at 250–350°C and 15 MPa. The hydroxide oxide was isomorphous with the low-pressure form of triniobium fluoride heptaoxide which is built up of 3 X ∞ blocks of the ReO₃ structure with crystallographic shear in one dimension. When heated in air, Nb₃O₇(OH) dehydrated up to 460°C to give poorly crystallized Nb₂O₅, which, on further heating, changed slowly into a less ordered precursor of M? Nb₂O₅(¹). Hydrothermal treatment of Nb₃O₇(OH) with pure water at 400–500°C afforded P? and R? Nb₂O₅; the conversion of Nb₃O₇(OH) is explained in terms of the close structural relation among these three forms.     

Compound and solid-solution formation in the system Li2ONb2O5TiO2

A systematic study of compound and solid-solution formation in the system Li₂ONb₂O₅TiO₂ has been made. Several solid-solution series, based on LiNbO₃, LiNb₃O₈, Li₂Nb₂₈O₇₁, Li₂TiO₃, phase M, Li₂Ti₃O₇, and TiO₂, have been characterized. In all cases, the principal solid-solution mechanism appears to involve stoichiometric formulae with constant overall cation content. One new phase, of approximate formula Li₁₃TiNb₅O₂₁, has been prepared. A subsolidus phase diagram for the ternary system is presented.     

Beiträge zur Untersuchung anorganischer nichtstöchiometrischer Verbindungen. XIII. Oxydationsprodukte von monoklinem Nb12O29 Elektronenoptische Untersuchung

Contributions to the Investigation of Inorganic Non-stoichiometric Compounds. XIII. Oxidation Products of Monoclinic Nb₁₂O₂₉, Electron Optical Investigation The electron optical investigation shows that the starting material Nb₁₂O₂₉(mon.) is well ordered and that the oxidation products Nb₂O₅(Ox1BI) and Nb₂O₅(Ox2BI) have different structures. Nb₂O₅(Ox1BI) has a similar structure as Nb₁₂O₂₉(mon.), however differs from the latter by characteristic point defects, which in the electron microscope easily disappear by reduction. Nb₂O₅(Ox2BI) has not a well ordered structure; characteristic are rows of [2×n]-blocks which on the average are separated by five [3×n]-blocks. The average block length is n = 4 octahedra. The observed composition O/Nb = 2.500 can be explained by a structure model with unoccupied tetrahedral sites of Nb.     

Die Phasen Nb3Ga2, Ta5Ga3 und Ta5Al3Bx

Zusammenfassung Die Phasen Nb₃Ga₂, Ta₅Ga₃ und Ta₅Al₃B_x werden aus den Komponenten hergestellt. Nb₃Ga₂ kristallisiert im U₃Si₂-Typ, Ta₅Ga₃ hat Cr₅Br₃-Struktur (T 2) und Ta₅Al₃B_x ist mit Mn₅Si₃ (teilweise aufgefüllt) isotyp.     

Investigation of the homogeneity of the distribution of doped elements in oxide single crystals

Summary The homogeneity of the distributions of doped V and Ti in Al₂O₃:V³⁺, Y₃Al₅O₁₂:V³⁺, Al₂O₃:Ti³⁺, Y₃Al₅O₁₂:Ti³⁺ single crystals was studied by means of laser emission microanalysis. The applied statistical methods include one-way variance analysis, two-way variance analysis, regression models and the gradient method.