XANES study of polyaniline-V2O5 and sulfonated polyaniline-V2O5 nanocomposites

In this work, two materials for secondary lithium battery cathodes formed by polyaniline-V₂O₅ and sulfonated polyaniline-V₂O₅, which have a higher charge capacity than the V₂O₅ xerogel, were synthesized. X-ray absorption and Fourier transform infrared spectroscopies were employed to analyze the short-range interactions in these materials. Based on these experiments, it was possible to observe significant differences in the symmetry of the VO₅ units, and this was attributed to the intimate contact between V₂O₅ and the polymers, and to some flexibility of the VO₅ square pyramids due to the low range order of the nanocomposites.     

SO2 and NO removal from flue gas over V2O5/AC at lower temperatures — role of V2O5 on SO2 removal

Supporting V₂O₅ onto an activated coke (AC) has been reported to significantly increase the AC's activity in simultaneous SO₂ and NO removal from flue gas. To understand the role of V₂O₅ on SO₂ removal, V₂O₅/AC is studied through SO₂ removal reaction, surface analysis, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) techniques. It is found that the main role of V₂O₅ in SO₂ removal over V₂O₅/AC is to catalyze SO₂ oxidation through a VOSO₄-like intermediate species, which reacts with O₂ to form SO₃ and V₂O₅. The SO₃ formed transfers from the V sites to AC sites and then reacts with H₂O to form H₂SO₄. At low V₂O₅ loadings, a V atom is able to catalyze as many as 8 SO₂ molecules to SO₃. At high V₂O₅ loadings, however, the number of SO₂ molecules catalyzed by a V atom is much less, due possibly to excessive amounts of V₂O₅ sites in comparison to the pores available for SO₃ and H₂SO₄ storage.     

Gas-phase elemental mercury capture by a V2O5/AC catalyst

Gas-phase elemental mercury (Hg⁰) capture by an activated coke (AC) supported V₂O₅ (V₂O₅/AC) catalyst was studied in simulated flue gas and compared with that by the AC. The study on the influences of V₂O₅ loading, temperature, capture time and flue gas components (O₂, SO₂, H₂O and N₂) shows that the Hg⁰ capture capability of V₂O₅/AC is much higher than that of AC. It increases with an increase in V₂O₅ loading and is promoted by O₂, which indicates the important role of V₂O₅ in Hg⁰ oxidation and capture; it is promoted slightly by SO₂ but inhibited by H₂O; it increases with an increase in temperature up to 150 °C when Hg desorption starts. X-ray photoelectron spectroscopy analysis and sequential chemical extraction experiments indicate that the main states of Hg captured on V₂O₅/AC are HgO and HgSO₄. Temperature programmed desorption experiments were also made to understand the stability of the Hg captured.     

Low-temperature firing and microwave dielectric properties of 16CaO–9Li2O–12Sm2O3–63TiO2 ceramics with V2O5 addition

The sintering behaviors and microwave dielectric properties of the 16CaO–9Li₂O–12Sm₂O₃–63TiO₂ (abbreviated CLST) ceramics with different amounts of V₂O₅ addition had been investigated in this paper. The sintering temperature of the CLST ceramic had been efficiently decreased by nearly 100 °C. No secondary phase was observed in the CLST ceramics and complete solid solution of the complex perovskite phase was confirmed. The CLST ceramics with small amounts of V₂O₅ addition could be well sintered at 1200 °C for 3 h without much degradation in the microwave dielectric properties. Especially, the 0.75 wt.% V₂O₅-doped ceramics sintered at 1200 °C for 3 h have optimum microwave dielectric properties of Kr = 100.4, Q × f = 5600 GHz, and TCF = 7 ppm/°C. Obviously, V₂O₅ could be a suitable sintering aid that improves densification and microwave dielectric properties of the CLST ceramics.     

Long-term degradation of Ta2O5-doped Bi2O3 systems

Bismuth oxide in δ-phase is a well-known high oxygen ion conductor and can be used as an electrolyte for intermediate temperature solid oxide fuel cells (IT-SOFCs). 5-10 mol% Ta₂O₅ are doped into Bi₂O₃ to stabilize δ-phase by solid state reaction process. One Bi₂O₃ sample (7.5TSB) was stabilized by 7.5 mol% Ta₂O₅ and exhibited single phase δ-Bi₂O₃-like (type I) phase. Thermo-mechanical analyzer (TMA), X-ray diffractometry (XRD), AC impedance and high-resolution transmission electron microscopy (HRTEM) were used to characterize the properties. The results showed that holding at 800-850 °C for 1 h was the appropriate sintering conditions to get dense samples. Obvious conductivity degradation phenomenon was obtained by 1000 h long-term treatment at 650 °C due to the formation of α-Bi₂O₃ phase and Bi₃TaO₇, and 〈1 1 1〉 vacancy ordering in Bi₃TaO₇ structure.     

The catalytic performance in oxidative coupling of methane and the surface basicity of La2O3, Nd2O3, ZrO2 and Nb2O5

The catalytic performance in oxidative coupling of methane (OCM) of unmodified pure La₂O₃, Nd₂O₃, ZrO₂ and Nb₂O₅ has been investigated under various conditions. The results confirmed that the activity of La₂O₃ and Nd₂O₃ was always much higher than that of the remaining two. The surface basicity/base strength distribution of pure La₂O₃, Nd₂O₃, ZrO₂ and Nb₂O₅ was measured using a test reaction of transformation of 2-butanol and a temperature-programmed desorption of CO₂. Both methods showed that La₂O₃ and Nd₂O₃ had high basicity and contained medium and strong basic sites (lanthanum oxide more and neodymium oxide somewhat less). ZrO₂ had only negligible amount of weak basic sites and Nb₂O₅ was rather acidic. The confrontation of the basicity and catalytic performance indicated that in the case of investigated oxides, the basicity (especially strong basic sites) could be a decisive factor in determination of the catalytic activity in OCM. Only in the case of ZrO₂ it was observed a moderate catalytic performance in spite of negligible basicity. The influence of a gas atmosphere used in the calcination of oxides (flowing oxygen, helium and nitrogen) on their basicity and catalytic activity in OCM had been also investigated. Contrary to earlier observations with MgO, no effect of calcination atmosphere on the catalytic performance of investigated oxides in OCM and on their basicity was observed.     

Preparation of columbite MgNb2O6 and ZnNb2O6 ceramics by reaction-sintering

Columbite MgNb₂O₆ (MN) and ZnNb₂O₆ (ZN) ceramics produced by the reaction-sintering process were investigated. Secondary phases Mg_0.652Nb_0.598O_2.25 and Mg_0.66Nb_11.33O₂₉ were found in MgNb₂O₆ pellets. After 1250 °C sintering for 2 h, a density 4.85 g/cm³ (97.1% of the theoretical value) was obtained in MgNb₂O₆ pellets. In ZnNb₂O₆ pellets, no secondary phase formed. The maximum density 5.55 g/cm³ (98.7% of the theoretical value) occurs at 1200 and 1180 °C sintering for 2 and 4 h, respectively.     

Effect of sintering temperature on varistor properties and aging characteristics of ZnO–V2O5–MnO2 ceramics

The microstructure, electrical properties, dielectric characteristics, and DC accelerated aging behavior of the ZVM-based varistors were investigated for different sintering temperatures of 800–950 °C. The microstructure of the ZVM-based ceramics consisted of mainly ZnO grain and secondary phase Zn₃(VO₄)₂, which acts as liquid-phase sintering aid. The Zn₃(VO₄)₂ has a significant effect on the sintered density, in the light of an experimental fact, which the decreases of the Zn₃(VO₄)₂ distribution with increasing sintering temperature resulted in the low sintered density. The breakdown field exhibited the highest value (17,640 V/cm) at 800 °C in the sintering temperature and the lowest value (992 V/cm) at 900 °C in the sintering temperature. The nonlinear coefficient exhibited the highest value, reaching 38 at 800 °C and the lowest value, reaching 17 at 850 °C. The varistor sintered at 900 °C exhibited not only high nonlinearity with 27.2 in nonlinear coefficient, but also the highest stability, in which %ΔE_1 mA = −0.6%, %Δα = −26.1%, and %Δ tan δ = +21.8% for DC accelerated aging stress of 0.85 E_1 mA/85 °C/24 h.     

Mixed oxides Ca2Fe2O5 and Ca2Co2O5 as anode materials for Li-ion batteries

The electrochemical performance of mixed oxides, Ca₂Fe₂O₅ and Ca₂Co₂O₅ for use in Li-ion batteries was studied with Li as the counter electrode. The compounds were prepared and characterized by X-ray diffraction and SEM. Ca₂Fe₂O₅ showed a reversible capacity of 226 mAh/g at the 14th cycle and retained 183 mAh/g at the end of 50 cycles at 60 mA/g in the voltage window 0.005-2.5 V. A reversible capacity in the range, 365-380 mAh/g, which is stable up to 50 charge-discharge cycles is exhibited by Ca₂Co₂O₅ in the voltage window, 0.005-3.0 V and at 60 mA/g. This corresponds to recycleable moles of Li of 3.9±0.1 (theoretical: 4.0). Significant improvement in the cycling performance and attainable reversible capacity were noted for Ca₂Co₂O₅ on cycling to an upper cut-off voltage of 3.0 V as compared to 2.5 V. Coulombic efficiency for both compounds is >98%. Electrochemical impedance spectroscopy (EIS) data clearly indicate the reversible formation/decomposition of polymeric surface film on the electrode surface of Ca₂Co₂O₅ in the voltage window, 0.005-3.0 V. Cyclic voltammetry results compliment the galvanostatic cycling data.     

Selective oxidation of methanol to dimethoxymethane over acid-modified V2O5/TiO2 catalysts

Selective oxidation of methanol to dimethoxymethane (DMM) was conducted in a fixed-bed reactor over an acid-modified V₂O₅/TiO₂ catalyst. The influence of the acid modification on its structure, redox and acidic properties, and catalytic performance for methanol oxidation were investigated. The results indicated that the content of vanadia in the catalyst exhibits a vital influence on the dispersion of vanadium species, while the acid modification can enhance its surface acidity. Proper amounts of the acid (W() = 15%) and V₂O₅ (W(V₂O₅) = 15%) components loaded in the acid-modified V₂O₅/TiO₂ catalyst are able to build a bi-functional circumstance that is favorable for the formation of DMM with high activity and selectivity. As a result, for the selective oxidation of methanol, the H₂SO₄-modified V₂O₅/TiO₂ catalyst gives a much higher DMM yield at 150 °C than the unmodified one.     

Enhancement of the visible light photocatalytic performance of C-doped TiO2 by loading with V2O5

V₂O₅ was loaded on the surface of C-doped TiO₂ (C-TiO₂) by incipient wetness impregnation in order to enhance the visible light photocatalytic performance. The physicochemical properties of the C-TiO₂/V₂O₅ composite were characterized by XRD, Raman, TEM, XPS, UV–vis diffuse reflectance spectra, and PL in detail. The result indicated that a heterojunction between C-TiO₂ and V₂O₅ was formed and the separation of excited electron–hole pairs on C-TiO₂/V₂O₅ is greatly promoted. Thus, this composite photocatalyst exhibited enhanced visible light photocatalytic activity in degradation of gas-phase toluene compared with the pristine C-TiO₂.     

Matrix–filler interactions in polysilazane-derived ceramics with Al2O3 and ZrO2 fillers

Interactions between a poly(vinyl)silazane and Al₂O₃ or Y₂O₃-stabilised ZrO₂ fillers were studied during the fabrication of polysilazane-derived bulk ceramics in order to investigate the influence of oxide fillers on resulting properties. Specimens were produced by coating of the filler powders with the polysilazane, warm-pressing of the resulting composite powders, and pyrolytic conversion in flowing N₂ at various temperatures between 1000 °C and 1400 °C. Significant differences in densification were observed, depending on the filler used. Reactions between the polysilazane-derived matrix and Al₂O₃ or ZrO₂ at temperatures ≥1300 °C resulted in the formation of Si₅AlON₇ or ZrSiO₄, respectively. Reactivity in the polysilazane-derived component was a result of SiO₂ contamination caused primarily by adsorbed species on the filler particle surface. Knowledge of polysilazane/filler interface processes is found to be decisive for the prediction of properties such as shrinkage and porosity, which heavily influence performance of a material.     

Corrosion mechanisms of Al2O3/MgAl2O4 by V2O5, NiO, Fe2O3 and vanadium slag

The effects of V₂O₅, NiO, Fe₂O₃ and vanadium slag on the corrosion of Al₂O₃ and MgAl₂O₄ have been investigated. The specimens of Al₂O₃ and MgAl₂O₄ with the respective oxides above mentioned were heated at 10 °C/min from room temperature up to three different temperatures: 1400, 1450 and 1500 °C. The corrosion mechanisms of each system were followed by XRD and SEM analyses. The results obtained showed that Al₂O₃ was less affected by the studied oxides than MgAl₂O₄. Alumina was only attacked by NiO forming NiAl₂O₄ spinel, while the MgAl₂O₄ spinel was attacked by V₂O₅ forming MgV₂O₆. It was also observed that Fe₂O₃ and Mg, Ni, V and Fe present in the vanadium slag diffused into Al₂O₃. On the other hand, the Fe₂O₃ and Ca, S, Si, Na, Mg, V and Fe diffused into the MgAl₂O₄ structure. Finally, the results obtained were compared with those predicted by the FactSage software.     

Adsorption and oxidation of NH3 over V2O5/AC surface

V₂O₅/AC has been reported to be active for selective catalytic reduction (SCR) of NO with NH₃ at around 200 °C and resistant to SO₂ deactivation. To elucidate its SCR mechanism, adsorption and oxidation of NH₃ over V₂O₅/AC are studied in this paper using TG, MS and DRIFTS techniques. It is found that the adsorption and oxidation of NH₃ take place mainly at VO bond of V₂O₅. A higher V₂O₅ loading results in more NH₃ adsorption on the catalyst. V₂O₅ contains both Brnsted and Lewis acid sites; NH₄⁺ on Brnsted acid sites is less stable and easier to be oxidized than NH₃ on Lewis acid sites. Gaseous O₂ promotes interaction of NH₃ with AC and oxidation of NH₃ over V₂O₅/AC. NH₃ is oxidized into NH₂ and acylamide structures and then to isocyanate species, which is an intermediate for N₂ formation.     

Mechanochemical synthesis of FeSbO4-based materials from FeOOH and Sb2O5 powders

Iron antimony oxide (FeSbO₄) with specific surface area (SSA) over 50 m²/g was synthesized mechanochemically by milling a mixture of iron oxy-hydroxide (FeOOH) and antimony pentoxide (Sb₂O₅) using a planetary ball mill at room temperature. The mechanochemical reaction proceeds with an increase in milling time and has been completed by 120 min. The prepared product powders are in the state of agglomerates consisted of fine particles of several dozen nanometers. This method has been extended to synthesis of FeSbO₄-based materials with different Fe/Sb atomic ratios (1 ≤ Fe/Sb ≤ 4). The SSA value of these prepared samples is in the range of 50 to 65 m²/g.     

Phase diagram of the Al2O3-HfO2-Y2O3 system

The phase diagram of the Al₂O₃-HfO₂-Y₂O₃ system was first constructed in the temperature range 1200-2800 °C. The phase transformations in the system are completed in eutectic reactions. No ternary compounds or regions of appreciable solid solution were found in the components or binaries in this system. Four new ternary and three new quasibinary eutectics were found. The minimum melting temperature is 1755 °C and it corresponds to the ternary eutectic Al₂O₃ + HfO₂ + Y₃Al₅O₁₂. The solidus surface projection, the schematic of the alloy crystallization path and the vertical sections present the complete phase diagram of the Al₂O₃-HfO₂-Y₂O₃ system.     

Structural and optical properties of Tm:Y2O3 transparent ceramic with La2O3, ZrO2 as composite sintering aid

The paper reports the use of La₂O₃ and ZrO₂ co-doping as a composite sintering aid for the fabrication of Tm:Y₂O₃ transparent ceramics. Two groups of experiments were conducted for investigating the influences of composite sintering aids on the microstructures and the optical properties of Tm:Y₂O₃ transparent ceramics in contrast to single La³⁺ and single Zr⁴⁺ doped Tm:Y₂O₃. Samples with composite sintering aids could realize fine microstructures and good optical properties at relatively low sintering temperatures. Grain sizes around 10 μm and transmittances close to theoretical value at wavelength of 2 μm were achieved for the 9 at.% La³⁺, 3 at.% Zr⁴⁺ co-doped samples sintered at 1500-1600 °C. The influences of the composite sintering aids on the emission intensities and the phonon energies of Tm:Y₂O₃ ceramics were also investigated.     

Accommodation of impurities in α-Al2O3, α-Cr2O3 and α-Fe2O3

Atomic scale computer simulation was used to predict the mechanisms and energies associated with the accommodation of aliovalent and isovalent dopants in three host oxides with the corundum structure. Here we consider a much more extensive range of dopant ions than has previously been the case. This enables a rigorous comparison of calculated mechanism energetics. From this we predict that divalent ions are charge compensated by oxygen vacancies and tetravalent ions by cation vacancies over the full range of dopant radii. When defect associations are included in the model these conclusions remain valid. At equilibrium, defects resulting from extrinsic dopant solution dominate intrinsic processes, except for the largest dopant cations. Solution reaction energies increase markedly with increasing dopant radius. The behaviour of cluster binding energies is more complex.     

Inversion boundary induced grain growth in TiO2 or Sb2O3 doped ZnO-based varistor ceramics

In low-voltage varistor ceramics, the phase equilibrium and the temperature of liquid-phase formation are defined by the TiO₂/Bi₂O₃ ratio. The selection of a composition with an appropriate TiO₂/Bi₂O₃ ratio and the correct heating rate is important for the processing of low-voltage varistor ceramics. The total amount of added Bi₂O₃ is important as the grain growth is slowed down by a larger amount of Bi₂O₃-rich liquid phase at the grain boundaries. Exaggerated grain growth in low-voltage varistor ceramics is related to the occurrence of the liquid phase and the presence of TiO₂ which triggers the formation of inversion boundaries (IBs) in only a limited number of grains, and as a result the final microstructure is coarse grained. The Zn₂TiO₄ spinel phase only affects grain growth in compositions with a TiO₂/Bi₂O₃ ratio higher than 1.5. In high-voltage varistor ceramics, just a small amounts of Sb₂O₃ trigger the formation of IBs in practically every ZnO grain, and in compositions with a Sb₂O₃/Bi₂O₃ ratio lower than 1, grain growth that is controlled entirely by an IBs-induced grain growth mechanism results in a fine-grained microstructure. The spinel phase interferes with the grain growth only at higher Sb₂O₃/Bi₂O₃ ratios.     

Effect of an Nb2O5 nanolayer coating on ZnO electrodes in dye-sensitized solar cells

Nanostructured porous zinc oxide electrodes for use in dye-sensitized solar cells (DSSCs) were coated with thin niobium oxide layers by using sol–gel transformation of niobium pentaethoxide in air. Coating solutions were prepared by mixing niobium pentaethoxide and ethanol. A dip-coating technique was adopted at a low withdrawal speed of 100 μm s⁻¹. The coated electrodes were then heat-treated at temperatures between 400 and 600 °C. The presence of niobium in the coated electrodes was confirmed by X-ray photoelectron spectroscopy. As expected, the niobium oxide layers worked as an energy barrier between the ZnO electrode and electrolyte. Open-circuit voltage (V_OC) of the cells using the coated electrodes was then enhanced up to 0.768 V, which was attributable to the suppression of the recombination of photogenerated electrons with oxidized species in electrolytes. An additional benefit of the coating was that grain growth of ZnO particles in the electrodes was hindered and short-circuit photocurrent density (J_SC) was kept relatively high due to large amounts of adsorbed dye. An overall light-to-electricity conversion efficiency was increased to a maximum of 5.19%, indicating that the proper coating technique was the key for improving the performance of ZnO-based DSSCs.