Nb Oxide Research Articles

Kinetics and Mechanism of Ethane Oxidation to Acetic Acid on Catalysts Based on Mo−V−Nb Oxides

Kinetic and isotopic studies showed that C−H bond activation in ethane by surfaces essentially saturated with lattice oxygens is the sole kinetically relevant step in ethane oxidation on Mo−V−NbOx mixed oxides. These conclusions are consistent with the dependence of oxidation rates on O2 and C2H6 pressures, with H/D exchange and kinetic isotope effects, and with the preferential initial incorporation of 16O atoms from the oxide lattice into products formed from 18O2−C2H6 mixtures. The precipitation of active components (Mo0.61V0.31Nb0.08Ox) in the presence of colloidal TiO2 led to 10-fold increases in all rate constants (per active component), consistent with higher dispersion of active components resembling in structure and surface reactivity those prevalent in bulk powders. The concurrent presence of PdOx cocatalyst, even as a separate solid, markedly increased all rate constants for oxidation of ethene intermediates and specifically that for ethene oxidation to acetaldehyde molecules, which are rapidly converted to acetic acid on active Mo−V−NbOx sites. Water, whether formed as a byproduct or added with C2H6−O2 reactants, increases acetic acid selectivities by promoting the desorption of adsorbed acetate species as acetic acid. Ethene molecules, formed as reactive intermediates, inhibit ethane oxidation rates by depleting surface lattice oxygen atoms in fast oxidation reactions, thus decreasing the number of sites available at steady state for the kinetically relevant C−H bond activation step required for ethane conversion.

Influence of oxygen ion irradiation on the corrosion aspects of Ti-5%Ta-2%Nb alloy and oxide coated titanium

The corrosion resistance of Ti-5%Ta-2%Nb alloy and DOCTOR (double oxide coating on titanium for reconditioning) coated titanium by O 5+ ion irradiation were compared and investigated for their corrosion behaviour. O 5+ ion irradiations were carried out at a dose rate of 1 × 10 17, 1 × 10 18 and 1 × 10 19 ions/m 2 at 116 MeV. The surface properties and corrosion resistance were evaluated by using scanning electron microscopy (SEM), atomic force microscopy (AFM), energy dispersive X-ray (EDX), glancing-angle X-ray diffraction (GXRD) and electrochemical testing methods. The results of electrochemical investigations in 11.5 N HNO 3 indicated that the open circuit potential (OCP) of DOCTOR coated titanium is nobler than Ti-5%Ta-2%Nb alloy. The potentiodynamic polarization study of Ti-5%Ta-2%Nb alloy and DOCTOR coated specimen indicated decrease in passive current density with increase in ion doses (1 × 10 17 to 1 × 10 19 ions/m 2) indicating decrease in anodic dissolution. Nyquist arc behaviour in the electrochemical impedance study substantiated the enhancement in oxide layer stability by O 5+ ion irradiation. AFM results revealed that the DOCTOR coated Ti surface was dense without gross voids, and the surface roughness decreased by O 5+ ion irradiation, but increased after corrosion test. EDX and GXRD patterns of DOCTOR coated Ti sample indicated that the coating was mainly composed of rutile TiO 2. Based on the above results, the O 5+ ion irradiation effect on corrosion behavior of Ti-5%Ta-2%Nb alloy and DOCTOR coated titanium are discussed in this paper.

VIGEZZITE AND ASSOCIATED OXIDES OF Nb-Ta FROM EMERALD-BEARING PEGMATITES OF THE VIGEZZO VALLEY, WESTERN ALPS, ITALY

Two sequences of crystallization involving oxides of Nb–Ta occurred during the albitization process of three emerald-bearing pegmatites that cut serpentinites at Mount Pizzo Marcio and Alpe Rosso, in Vigezzo Valley, Western Alps, Italy: 1) a higher-temperature sequence, characterized by an alkaline micro-environment [high a (Na + )], allowing the crystallization of Mg-rich columbite, vigezzite, thorian vigezzite and U-rich microlite, and 2) a lower-temperature sequence, characterized by high a (Ca 2+ ), producing replacements of tapiolite-(Fe) and ferrowodginite by microlite, of tantalite-(Mn) by microlite and (later) fersmite, and of columbite-(Mn) by fersmite. Chromium-bearing beryl crystals (emerald) occur along the exocontacts characterized by a green to grey, sheared micaceous unit composed of phlogopite + talc + tremolite. Beryl crystals commonly show a green Cr-rich vitreous rim and a whitish milky core, partially replaced by beryllium silicates such as bavenite, bertrandite and bityite. Tourmaline crystals vary from schorl–dravite to Fe-rich dravite. In the model of evolution envisaged, we propose an anatectic origin of the pegmatite-forming melts during the latest stages of the Alpine tectonometamorphic event. The emplacement of the pegmatitic dikes in the serpentinites was followed by intense circulation of supercritical to hydrothermal fluids enriched in Mg, B and Be, and depleted in F, producing albitization of pegmatites and generating the aforementioned mineral reactions and replacements.

Cross-correlative TEM and atom probe analysis of partial crystallisation in NiNbSn metallic glasses

Devitrification in a Ni60Nb35Sn5 bulk metallic glass alloy is investigated below the glass transition temperature using cross-correlative TEM and atom probe tomography. As cast and post-anneal (500°C for 2 h) specimens were analysed in both the TEM and the atom probe to determine the extent and nature of any phase separation in the as cast material or devitrification in the annealed material. Furthermore, it was of interest to determine the accuracy of the atom probe reconstructions of both types of material. No significant compositional partitioning was observed in the as cast state, suggesting an amorphous material, and the atom probe reconstructions of these specimens were found to be consistent with the TEM data. The annealed specimens contained Nb and Nb oxide rich regions, Ni3Sn nanocrystals, and a significant build-up of oxygen at the boundaries between these nanocrystals and the remaining amorphous matrix regions. These oxygen enriched regions are shown to result in anomalous atom probe reconstructions that are inconsistent with the TEM analyses. It is theorised that the initial crystallisation of Nb and Nb oxide rich regions results in NiSn rich regions, thereby resulting in the formation of Ni and Ni3Sn crystalline precipitates.

Experimental study of loparite formation in complex fluid-magmatic systems

Loparite is a mineral-concentrator of Nb, Ti, and REE. Its general formula is (Ce, Na, Ca) 2 (Ti, Nb) 2 O 6 [1]. Loparite is the major mineral of the REE‐Nb ore in the Lovozero peralkaline pluton in the Kola Peninsula. The ore mineralization is largely related to units of urtite, malignite, and less frequent lujavrite of the differentiated complex. The deposit has a long history of its geological investigation, including experimental works. The experiments on the loparite‐nepheline [2] and the lujavrite melt‐loparite [4] systems have established a wide field of loparite crystallization from the peralkaline melt. However, the studies were performed without the involvement of volatile components, although the fluid regime often is an important factor of magmatic evolution and the related ore formation [5‐7]. Only few experiments were carried out at T = 850 and 750 ° C and P = 1 kbar of water pressure [4]. Our experimental investigations were conducted with the aim of approaching natural conditions of the evolution of magmatism and ore formation closely related to fluid impact on magmatic systems. Therefore, crystallization of loparite was studied in complex silicate‐salt systems [8, 9] that contain sodium carbonate, chloride, fluoride, or sulfate. The runs were performed on a high-pressure apparatus with external heating and cold seal ( T = 400 and 450 ° C, P = 1 kbar, during 30 days; T = 840 ° C, P = 1.5 kbar, during 5‐7 days) and in high gas pressure vessel ( T = 1200 ° C, P = 2 kbar, during 6 h or 1 day) with subsequent quenching. The charge consisted of an eutectic mixture of albite and nepheline gels with the addition of oxides of Nb, La, Ce, and Y; carbonates of Ca and Sr; and excess of titanium oxide, which was commonly placed on the ampule bottom. The respective salt or a combination of salts was also added. The runs were performed in the

Structural and reactive relevance of [formula omitted] coverage on alumina of V [sbnd]Nb [sbnd]O/Al 2O 3 catalytic systems

Vanadium and niobium species (together and separately) were loaded on gamma alumina, and the resulting catalysts were run in the methanol conversion. This reaction was studied by both GC analysis and FTIR study in the flow system. The catalytic properties are discussed based on the combined FTIR and 27Al, 51V and 1H MAS NMR studies. The NMR studies revealed a different mechanism of interaction between Nb and Al 2O 3 than that between V and Al 2O 3. This predetermines the structure of vanadium sites in bimetallic VNb/Al samples. The effect of coverage was considered for various metal loadings ranging from below to above monolayer. One of our most interesting findings is that the surface Nb oxide species exhibited a redox character below monolayer but were acidic above monolayer. 27Al MAS NMR revealed a strong alumina–Nb interaction that may account for its redox performance. Moreover, the role of sulfate from vanadium precursor is evidenced.

Preparation of mesoporous In–Nb mixed oxides and its application in photocatalytic water splitting for hydrogen production

A series of mesoporous In–Nb mixed oxides was synthesized using NbCl 5 and In 2O 3 as the starting material and triblock copolymer P123 as template. We investigated the influence of indium content on the synthesis and characteristics of the mesoporous In–Nb mixed oxides, and their photocatalytic activities for water splitting. The materials were characterized by small angle X-ray scattering, powder X-ray diffraction, extended X-ray absorption fine structure, transmission electron microscopy, scanning electron microscopy, energy dispersive spectrometer, N 2 sorption and UV–vis spectroscopy. The surface area of mesoporous In–Nb mixed oxides was greater than 90 m 2/g with a wormhole framework. The optimization of synthesis condition of the mesoporus In–Nb oxides catalyst contained a small fraction of highly dispersed indium (In/Nb = 0.13) species intercalated into the framework of mesoporous niobium oxides and exhibited a high photocatalytic activity for water splitting reaction which was about 2.7 times as compared to mesoporous Nb 2O 5 and was about 19 times higher than commercial bulk Nb 2O 5.

Erosion corrosion properties of HVOF coatings for municipal solid waste incinerator protection

Municipal solid waste incinerators (MSWI) usually have erosion–corrosion (EC) problems. A possible solution for the damage caused by EC processes in the heat exchanger zone has been found in thermal spray coatings with Ni based and Fe based high velocity oxygen fuel (HVOF) coatings showing promising results. High velocity oxygen fuel coatings generally have a low porosity and good adherence. These properties help to prevent damage to exchanger tubes. In the present paper Ni and Fe alloys were compared under different laboratory tests and in service inside a MSWI heat exchanger. Coatings were also characterised by standardised tribological and microhardness tests. A eutectic mixture of KCl–ZnCl2 found on the boiler ashes was used as a corrosion agent in the laboratory tests. Scanning electron microscopy (SEM) images of sprayed coating structures before and after the tests were compared. Energy dispersive spectroscopy (EDS) analysis and X-ray diffraction (XRD) were used to characterise the coatings and corrosion products. Studies on bulk materials with similar composition help to understand some steps in the corrosion processes that take place during incineration. It is concluded that the low level of protection provided by Fe based alloy HVOF coating in these conditions may be attributed to the role of non-protective chromium oxides, and the good behaviour of Ni based alloy HVOF coating may be attributed to the formation of more protective Ni, Mo and Nb oxides.

Synergistic Effects of TiO2 and Palladium‐Based Cocatalysts on the Selective Oxidation of Ethene to Acetic Acid on Mo–V–Nb Oxide Domains

Synergistic Effects of TiO₂ and Palladium‐Based Cocatalysts on the Selective Oxidation of Ethene to Acetic Acid on Mo–V–Nb Oxide Domains

Selective Catalytic Oxidation of Ethanol to Acetic Acid on Dispersed Mo‐V‐Nb Mixed Oxides

The direct oxidation of ethanol to acetic acid is catalyzed by multicomponent metal oxides (Mo-V-NbO(x)) prepared by precipitation in the presence of colloidal TiO(2) (Mo(0.61)V(0.31)Nb(0.08)O(x)/TiO(2)). Acetic acid synthesis rates and selectivities (~95 % even at 100 % ethanol conversion) were much higher than in previous reports. The presence of TiO(2) during synthesis led to more highly active surface areas without detectable changes in the reactivity or selectivity of exposed active oxide surfaces. Ethanol oxidation proceeds via acetaldehyde intermediates that are converted to acetic acid. Water increases acetic acid selectivity by inhibiting acetaldehyde synthesis more strongly than its oxidation to acetic acid, thus minimizing prevalent acetaldehyde concentrations and its intervening conversion to CO(x). Kinetic and isotopic effects indicate that C-H bond activation in chemisorbed ethoxide species limits acetaldehyde synthesis rates and overall rates of ethanol conversion to acetic acid. The VO(x) component in Mo-V-Nb is responsible for the high reactivity of these materials. Mo and Nb oxide components increase the accessibility and reducibility of VO(x) domains, while concurrently decreasing the number of unselective V-O-Ti linkages in VO(x) domains dispersed on TiO(2).

Synergistic Effects of TiO2 and Palladium‐Based Cocatalysts on the Selective Oxidation of Ethene to Acetic Acid on Mo–V–Nb Oxide Domains

Synergistic Effects of TiO₂ and Palladium‐Based Cocatalysts on the Selective Oxidation of Ethene to Acetic Acid on Mo–V–Nb Oxide Domains

Compositional Inhomogeneity in Li‐ and Ta‐Modified (K, Na)NbO3 Ceramics

Compositional inhomogeneity is examined in Li‐ and Ta‐modified potassium sodium niobate (K,Na)NbO3 perovskite ceramics. The inhomogeneous distribution of the A‐site (K and Na) and B‐site (Nb and Ta) cations is found to be correlative. The inhomogeneity could not be eliminated by prolonged high‐temperature annealing in samples prepared by direct mixing of alkaline carbonates and Ta and Nb oxides. In contrast, the precursor method, where a (NbxTa1−x)2O5 solid solution was first formed, led to a considerably improved compositional homogeneity and appreciably enhanced dielectric, ferroelectric, and piezoelectric properties. Our results suggest that more attention needs to be paid toward controlling the compositional fluctuation of this complex solid solution system.

Determination of long-lived Nb isotopes in nuclear power plant wastes

94Nb and 93mNb are long-lived radionuclides, produced by thermal and fast neutrons from 93Nb that is a major component of the Zr alloys used in nuclear reactors. A radiochemical method for the determination of these nuclides has been developed. The separation is based on the insolubility of Nb oxides and the retention of the fluoric complexes on anion exchange resin. The Nb sources are detected by γ- and X-ray spectrometries. Activity concentrations determined in radioactive waste samples of a nuclear power plant are presented.

In Situ Energy-Dispersive XAS and XRD Study of the Superior Hydrogen Storage System MgH2/Nb2O5

By in situ energy-dispersive X-ray absorption spectroscopy (EDXAS) and X-ray diffraction (XRD), we analyzed the evolution of niobium in a MgH2/Nb2O5 system based on high-energy ball milling during hydrogen cycling. The high time resolution of the EDXAS method allowed us to monitor fast sample changes during this process. Thereby, we demonstrated that the Nb2O5 is already partially reduced during the milling process with the MgH2. Further reduction occurs during the heating and cycling processes, in which a lower limit of oxidation state is reached. Hereby, a reaction between the niobium oxide and the Mg/MgH2 leads to a decrease of crystalline Nb2O5 and the formation of a ternary oxide phase MgxNbyO. During the cycling processes a repetitive Nb oxidation−reduction process was observed, which may indicate hydrogen diffusion along the ternary oxide by the formation of metastable niobium hydrides. This points to a mechanism of kinetic sorption improvement by diffusion of hydrogen through pathways of ternary Mg−Nb oxides, which may also reduce the activation energy of the Mg−MgH2 transition.

Reduction of oxide layer on various metal surfaces by atomic hydrogen treatment

The reduction of various metallic oxides was examined. Atomic hydrogen generated on a heated tungsten catalyzer was used for reduction. It was found that Cu, Ru, Nb, Mo, Rh, Pd, Ir and Pt oxides can be reduced by irradiation with atomic hydrogen. The activation energy for oxide removal was examined and it was found that the values were very small, 10 − 2 to 10 − 4 eV.

MINERALOGICAL AND GEOCHEMICAL STUDY OF THE TRUE BLUE AQUAMARINE SHOWING, SOUTHERN YUKON

Aquamarine of distinctly dark blue color was discovered during the summer of 2003 in the Pelly Mountains, southern Yukon Territory, Canada. The beryl is found within quartz veins that fill sigmoidal tension gashes, which cut a syenite of Mississippian age. The True Blue showing is differentiated from other beryl occurrences in the northern Cordillera by the color of the beryl, the host rock, mineral associations, timing, and mineralizing fluid. The syenite was emplaced within an extensional setting into undeformed Paleozoic sediments of the Cassiar Platform and felsic volcanic rocks of the Pelly Mountain Volcanic Belt. Post-late-Triassic tectonics resulted in a number of northeasterly directed thrust panels that were subsequently cut by Cretaceous granitic magmatism. Accessory minerals in the veins include siderite, ankerite, allanite-(Ce), fluorite, and minor albite, sulfides, and Fe–Ti–Nb oxides. Electron-microprobe analyses of beryl ( n = 192) revealed that FeO values range up to 5.92 wt.%, Na 2 O up to 2.66 wt.%, MgO up to 3.42 wt.%, CaO up to 0.11 wt.%, and H 2 O (calculated) up to 3.10 wt.%, whereas little to no Cr or V was detected. The darkest blue examples of beryl also have the highest concentrations of FeO. The allanite-(Ce) contains up to 26 wt.% REE 2 O 3 , and exhibits Fe 2+ > Fe 3+ . The fluorite that coprecipitated with beryl from several veins has been dated using Sm–Nd geochronology at 171.4 ± 4.8 Ma. In situ and whole-mineral δ 18 O values of the beryl and whole-mineral δ 18 O values of the quartz are variable; temperature estimates derived from these data suggest fluid temperatures between ~275 and ~400°C. Fluid-inclusion data from quartz, beryl, and fluorite suggest variable but high salinity (~6 to 24 wt.% NaCl equivalent) and CH 4 -absent mineralizing fluids. Conventional models to explain the formation of gem beryl, and consequently exploration parameters, applied in Yukon involve late-stage magmatic fluids. Evidence gathered in this study points to a metamorphic origin for the mineralizing fluid and a local derivation of vein constituents, which distinguish the fluids at True Blue from other intrusion- related beryl-forming fluids in the northern Cordillera.

A Solid‐State 17O NMR Study of Local Order and Crystallinity in Amine‐Templated Mesoporous Nb Oxide

Order locally: 17O magic-angle-spinning NMR spectroscopy of bulk sol–gel-prepared Nb2O5 and mesoporous, template-free Nb2O5 (see picture) shows very high local order in the mesoporous sample. The oxygen atoms are coordinated only as ONb2 in contrast with bulk phases in which the oxygen atoms are always present in a mixture of ONb2 and ONb3 coordination environments.

A Solid‐State 17O NMR Study of Local Order and Crystallinity in Amine‐Templated Mesoporous Nb Oxide

Lokal geordnet: 17O-NMR-Spektroskopie unter dem magischen Winkel an mit einem Sol-Gel-Verfahren hergestelltem Nb2O5 und an mesoporösem, templatfreiem Nb2O5 (siehe Bild) ergibt eine hohe lokale Ordnung in der mesoporösen Probe. Die Sauerstoffatome liegen nur in ONb2-Koordination vor, während bei den nichtporösen Phasen immer eine Mischung aus ONb2- und ONb3-Koordination existiert.

Copper oxide and niobium pentoxide supported on silica-alumina: Synthesis, characterization, and application on diesel soot oxidation

Copper oxide and niobium pentoxide were supported on silica-alumina by two methods: co-impregnation and sequential impregnation, with CuO:Nb 2O 5 ratios of 1:10 and 1:1. These new materials showed different characteristics according to the preparation method. The copper and niobium precursors totally decomposed after treatment at 300 °C for 6 h. The materials prepared by sequential impregnation exhibited lower thermal stability than the analogous ones prepared by co-impregnation based on the Nb 2O 5 transition phase. Whereas that transition occurs at about 1356 °C in the absence of CuO, it occurs at 1340 and 1290 °C using the co-impregnation and sequential processes, respectively. In addition, the materials with a CuO:Nb 2O 5 ratio of 1:1 and copper oxide content >2 wt% did not present that transition phase. This indicates a strong influence of CuO over Nb 2O 5 crystallites. XRD studies confirmed this interaction, showing the patterns of Cu and Nb oxides (for most of the samples prepared by both methods) and the presence of a third component (copper niobium oxide) on the samples with CuO content ⩾10 wt% when calcined at 800 °C for 6 h. FTIR, DRIFTS, and FT-Raman studies provided additional evidence of that interaction, which occurs not only on the surface hydroxyl groups of silica-alumina, but also on the surface hydroxyl groups of niobium pentoxide. The application of CuO/Nb 2O 5/SiO 2-Al 2O 3 materials in the oxidation of diesel soot particulates is promising; mixtures of the catalyst with a model particulate (Printex ® U) lowered the oxidation temperature from 622 to 518 °C.

A new molecular precursor route for the synthesis of Bi–Y, Y–Nb and Bi-doped Y–Nb oxides at moderate temperatures

Yttrium-based multimetallic oxides containing bismuth and/or niobium were prepared by a method starting from pre-isolated stable water-soluble precursors which are complexes with the ethylenediaminetetraacetate ligand (edta). The cubic Bi 1− x Y x O 1.5 ( x=0.22, 0.25 and 0.3) and Y 3NbO 7 oxides were obtained in a pure form in a range of moderate temperatures (600–650 °C). This preparation method also allowed to stabilize at room temperature, without quenching, the tetragonal YNbO 4 oxide in a distorted form (T′-phase) by calcining the precursor at 800 °C. When heated up to 1000 °C, this metastable T′-phase transforms into the metastable “high-temperature” T oxide, which converts on cooling down to room temperature into the thermodynamically stable monoclinic M oxide. Doping the YNbO 4 oxide with Bi 3+ cations (0.5% and 1% Bi with respect to total Bi+Y amount) led at 800 °C to a mixture of the T′-phase and the thermodynamically stable monoclinic one. At 900 °C, the almost pure monoclinic structure was obtained.