Ta Mixed Oxide Research Articles

Evaluation of the effect of the fluxing agent over the purification of Columbite-Tantalite ore for the synthesis of Nb,Ta mixed oxides

In this paper, the effect of alkaline fusion associated with acid leaching for the purification process of tantalum-niobium ore (Fe,Mn)(Ta1−xNbx)2O6 was investigated. The milled, screened, and sieved ore was melted upon the use of alkaline fluxing agents: potassium pyrosulfate, an equimolar mixture of potassium pyrosulfate and potassium bisulfite, and potassium bisulfate. The fused and crushed samples were subject to hot HCl leaching under several experimental conditions. The retrieved solid was then washed with DI water and dried so that XRF analysis could be performed to assess the attained purification of the ore. It was found that the use of potassium bisulfate as an alkaline fluxing agent was the most efficient of the three, and that total purification (>97 %wt. of Nb and Ta) was attained after the 15th round of acid leaching with HCl producing a nanoscale mixed and amorphous Nb,Ta oxide powder as the final product.

Syntheses and structural understanding of a Ti–Ta alloy-based nanotubular oxide photocatalyst

Ti–Ta mixed oxide freestanding nanotubes obtained by means of anodic oxidation of a homemade Ti–Ta (50–50 at%) alloy.

Effect of vanadium admixing on the surface structure of TiO2(110) under non-oxidizing conditions

Single crystalline Ti+V mixed oxide layers have been prepared by doping vanadium into TiO2(110) thin films on TiO2(110) single crystal substrates with a Ti+Ta mixed oxide interlayer between the film and the substrate. The interlayer prevents the diffusion of vanadium into the substrate and also the diffusion of Ti3+ between substrate and overlayer. Mixing vanadium into the TiO2 lattice increases the reducibility of the host oxide as concluded from an appreciable degree of reduction produced by comparatively mild annealing. A high density of bridging oxygen vacancies was identified at the surface of films with a low vanadium content (2%) while a (1×2) reconstruction as also known for massively reduced TiO2(110) was observed for layers with 8% of vanadium. Studies of methanol adsorption indicate that the vanadium atoms are mostly located below the surface since there is no indication of a vanadium-methanol interaction. We provide evidence that the reducibility of the vanadium ions in the thin film is higher than that of the titanium ions and we suggest that this is the origin of the increased reducibility of the mixed oxide.

Decoupling a Thin Well-Ordered TiO2(110) Layer from a TiO2(110) Substrate with a Ti + Ta Mixed Oxide Interlayer

A well-ordered TiO2(110) thin film has been successfully prepared on a rutile TiO2(110) single crystal substrate with a Ti + Ta mixed oxide interlayer. LEED and STM experiments show that the surface structure of the TiO2 thin film is the same as that of a regular TiO2(110) single crystal sample, and methanol TPD studies indicate that the chemical activities of the surfaces are also essentially identical. The Ti + Ta oxide interlayer hinders Ti diffusion, and therefore the TiO2 film can quickly be reduced and oxidized due to the small layer volume without changing the state of the substrate notably. This permits to study fully oxidized defect-free electrically insulating TiO2(110) surfaces with methods employing charged particles without surface charging. Also, strongly reduced layers can be prepared without substrate reduction.

A novel Ce–Ta mixed oxide catalyst for the selective catalytic reduction of NOx with NH3

A series of novel CeaTabOx catalysts prepared by a co-precipitation method was investigated on selective catalytic reduction (SCR) of NOx with NH3. It was found that the Ce1Ta1Ox catalyst showed excellent activity, N2 selectivity and strong resistance to H2O and SO2 through a relatively wide temperature range. The CeaTabOx catalysts were characterized using XRD, BET, XPS, H2-TPR, NH3-TPD, NO+O2-TPD and in situ DRIFTS. The characterization results indicated that Ta addition could increase the BET surface area and the surface acidity of the catalysts, lead to the appearance of higher levels of Ce3+ and chemisorbed oxygen species, influence the reducibility of the catalysts, and promote the presence of nitrite and nitrate species. According to the in situ DRIFTS investigations, the reactivity of surface-adsorbed NH3 and NOx species depended on the temperature.

CO oxidation on Ta-Modified SnO2 solid solution catalysts

Co-precipitation method was adopted to prepare Sn–Ta mixed oxide catalysts with different Sn/Ta molar ratios and used for CO oxidation. The catalysts were investigated by N2-Brunauer–Emmett–Teller (N2-BET), X-ray diffraction patterns (XRD), H2-temperature programmed reduction (H2-TPR), Thermal Gravity Analysis – Differential Scanning Calorimetry (TGA–DSC) techniques. It is revealed that a small amount of Ta cations can be doped into SnO2 lattice to form solid solution by co-precipitation method, which resulted in samples having higher surface areas, improved thermal stability and more deficient oxygen species on the surface of SnO2. As a result, those Sn rich Sn–Ta solid solution catalysts with an Sn/Ta molar ratio higher than 4/2 showed significantly enhanced activity as well as good resistance to water deactivation. It is noted here that if tantala disperses onto SnO2 surface instead of doping into its lattice, it will then have negative effect on its activity.

Preparation, characterization and photocatalytic property of nanosized K–Ta mixed oxides via a sol–gel method

Pyrochlore-type K{sub 2}Ta{sub 2}O{sub 6} compound cannot be obtained by a conventional solid-state reaction. Here, we report the synthesis of K{sub 2}Ta{sub 2}O{sub 6} and tetragonal tungsten bronze (TTB)-type K{sub 6}Ta{sub 10.8}O{sub 30} nanopowders using a facile and low-cost sol-gel method at low temperature. The as-prepared samples were characterized by XRD, TG-DSC, FESEM, UV-vis diffuse reflectance spectrum and XPS. The compound K{sub 2}Ta{sub 2}O{sub 6} was formed at 700 {sup o}C and reached complete crystallization at 750 {sup o}C, and decomposed into pure K{sub 6}Ta{sub 10.8}O{sub 30} with the volatilization of K{sub 2}O at 900 {sup o}C. The transformation of K{sub 2}Ta{sub 2}O{sub 6} to K{sub 6}Ta{sub 10.8}O{sub 30} is reported for the first time. The band gap of the pure K{sub 2}Ta{sub 2}O{sub 6} and K{sub 6}Ta{sub 10.8}O{sub 30} samples was found to be about 4.16 and 3.63 eV, respectively. The as-prepared K{sub 2}Ta{sub 2}O{sub 6} sample exhibited an efficient photocatalytic activity for the decomposition of azo dye acid red G (ARG) and 4-nitrophenol (4-NP, a typical persistent organic pollutant) solution under UV irradiation. According to the experimental results, the mechanism of the photocatalytic reaction over K{sub 2}Ta{sub 2}O{sub 6} was discussed.

Nanoparticulate precursor route to fine particles of TaON and ZrO2–TaON solid solution and their photocatalytic activity for hydrogen evolution under visible light

Abstract Preparation of fine TaON particles is attempted to improve the photocatalytic activity for H2 evolution from aqueous methanol solution under visible light (λ > 420 nm). TaON is synthesized by nitriding Ta2O5 powder at 1123 K for 15 h under a flow of NH3. When nanoparticulate Ta2O5 with a primary particle size of 30–50 nm is used as the precursor, fine particles of TaON (FP-TaON) with 50–80 nm in size is obtained, but the primary particles aggregate to form larger secondary particles. Nitridation of Zr–Ta mixed oxide (Zr/Ta = 0.1) nanoparticles results in the production of a solid solution of monoclinic-ZrO2 and TaON exhibiting 30–50 nm primary particle size with less aggregation and higher activity, as compared to FP-TaON. Suppression of surface defects (reduced tantalum species) produced during nitridation from Ta2O5 to TaON is shown to be essential for achieving efficient H2 evolution using TaON-based photocatalysts.

Synthesis and characterization of homo- and heterobimetallic niobium v and tantalum v peroxo-polyaminocarboxylato complexes and their use as single or multiple molecular precursors for Nb–Ta mixed oxides

New water-soluble bimetallic peroxo complexes of niobium V and/or tantalum V with high-denticity polyaminocarboxylate ligands have been prepared, characterized from the spectroscopic point of view, and used as molecular precursors for Nb–Ta mixed oxides. Four new homobimetallic complexes, (gu) 3[Nb 2(O 2) 4(dtpaO 3)]·3H 2O 1, (gu) 3[Ta 2(O 2) 4(dtpaO 3)]·5H 2O 2, (gu) 3[Nb 2(O 2) 4(HtthaO 4)]·2H 2O 4 and (gu) 3[Ta 2(O 2) 4(HtthaO 4)]·3H 2O 5 and the corresponding heterometallic complexes, (gu) 3[NbTa(O 2) 4(dtpaO 3)]·2.5H 2O 3 and (gu) 3[NbTa(O 2) 4(HtthaO 4)]·2H 2O 6 have been obtained. In these compounds, the in situ oxidation of the nitrogen atoms of the PAC ligands into N-oxide groups has been evidenced by IR spectroscopy and mass spectrometry. The thermal treatment of the homonuclear complexes in air at 700 or 800 °C, depending on the Ta content, provided Nb 2O 5 or Ta 2O 5 while the heteronuclear compounds led to the solid solution TaNbO 5. BET and SEM measurements have been carried out and comparison of the morphology of the samples prepared from homo- and heterometallic precursors is discussed.

Homo- and Heterobimetallic Niobiumv and Tantalumv Peroxo-tartrate Complexes and Their Use as Molecular Precursors for Nb−Ta Mixed Oxides

New water-soluble bimetallic peroxo-tartrato complexes of niobium(V) and/or tantalum(V) have been prepared, characterized from the structural and spectroscopic point of view, and used as molecular precursors for Nb-Ta mixed oxides. Two new homometallic complexes, (gu)5[Nb2(O2)4(tart)(Htart)] x 4H2O (1a) and (gu)6[Ta2(O2)4(tart)2] x 4H2O (2a), and the corresponding heterometallic complex, (gu)5[NbTa(O2)4(tart)(Htart)] x 4H2O (3), have been obtained. The crystal structures of the homometallic compounds, (gu)5[Nb2(O2)4(tart)(Htart)] x 6H2O x 1H2O2 (1b) and (gu)6[Ta2(O2)4(tart)2] x 6H2O (2b), have been determined, showing, for both cases, two 8-fold-coordinated metal atoms, each surrounded by oxygen atoms belonging to two bidentate peroxides, two monodentate carboxylato, and two alkoxo groups from both bridging tartrato ligands. The coordination polyhedron around each metal atom is a dodecahedron. The thermal treatment of complexes 1a, 2a, and 3 in air at 700 or 800 degrees C, depending of the Ta content, provided Nb2O5, Ta2O5, and the solid solution TaNbO5, respectively. The thermal treatment of a 1:1 Nb/Ta molar ratio mixture of 1a and 2a has also been studied. BET and SEM measurements have been carried out and reveal these oxides possess relatively high specific surface areas and display a porous character. Comparison between the use of homo- and heterometallic precursors is discussed.

In Situ TEM Observation of Crystallization of Amorphous Ordered Mesoporous Nb−Ta and Mg−Ta Mixed Oxides

The crystallization of ordered mesoporous Nb−Ta and Mg−Ta mixed oxides in a vacuum is observed in situ by transmission electron microscopy (TEM). Both metal oxides become polycrystalline upon heating, yet retain the pore size and ordered structure of the amorphous precursors. The crystalline structure and meso-structure of the materials obtained by crystallization in a vacuum differ from those produced by crystallization in air, suggesting the possible role of atmospheric oxygen in the crystallization process.

Synthesis, Mesostructure, and Photocatalysis of a Highly Ordered and Thermally Stable Mesoporous Mg and Ta Mixed Oxide

An ordered (2D-hexagonal) mesoporous Mg and Ta mixed oxide (Mg−Ta oxide) is successfully prepared. The mesoporous Mg−Ta oxide is stable after calcination at 773 K for 30 h, and has a BET surface ar...

Effects of crystal structure on photolysis of H 2O on K–Ta mixed oxide

K–Ta mixed oxides with two different crystal structures, i.e. KTaO 3 and K 2Ta 2O 6, have been prepared by alkoxide and solid-state reaction routes. K 2Ta 2O 6 single phase with pyrocrhore like structure can be obtained by the alkoxide method, however, this phase cannot be obtained by a conventional solid-state reaction route. It was found that the prepared K 2Ta 2O 6 exhibits the higher activity to photolytic splitting of H 2O into H 2 than that of KTaO 3 which is reported as the active catalyst for the H 2O decomposition. Crystalline size of K 2Ta 2O 6 pseudo-pyrochlore oxide can be controlled by the hydrolysis condition for alkoxide. It was seen that the crystalline size has a large influence on the photocatalytic decomposition activity and the highest activity was obtained at the crystal size of 30 nm. Effects of metal loading were also investigated on K 2Ta 2O 6 oxide and among the examined metals, activity to photolysis of water increased by loading Rh. Formation of oxygen was also observed when 0.2 wt.% Rh was loaded.

Alkoxide Route to Mixed Oxides of Rhenium, Niobium, and Tantalum. Preparation and X-ray Single-Crystal Study of a Novel Rhenium−Niobium Methoxo Complex, Nb2(OMe)8(ReO4)2

Thermal decomposition of complex oxomethoxo compounds Nb4O2(OMe)14(ReO4)2 (I) and Ta4O2(OMe)14(ReO4)2 (II) has been investigated. The decomposition of (II) at temperatures exceeding 100 °C results in the formation of MeOMe, MeOCH2OMe, MeOH, and H2O vapor and is accompanied by sublimation at temperatures over 135 °C. Plausible mechanisms of the processes studied are discussed. A new bimetallic methoxo compound, Nb2(OMe)8(ReO4)2 (III), was synthesized by interaction between Re2O7 and Nb2(OMe)10 in toluene solution and its structure was determined by means of an X-ray single-crystal study. The structure can be considered as a product of substitution for terminal methoxo groups with two perrhenate groups in dimeric Nb2(OMe)10 molecules. Decomposition of II in an inert atmosphere at 900 °C gave a Re−Ta mixed oxide phase, supposedly based on the L-Ta2O5 structure. Decomposition of I and III under similar conditions leads to the formation of rhenium−niobium oxide phases. The results obtained provide a basis for ...