In situ UV‐VIS diffuse reflectance spectroscopy of reduction–reoxidation of heteropoly compounds by methanol and ethanol: a correlation between spectroscopic and catalytic data

The degree of reduction/oxidation of H₄PVMo₁₁O₄₀ (HPA) and Cs₂H₂PVMo₁₁O₄₀ (CsPA) was studied during reduction and reoxidation by methanol, ethanol and mixtures with oxygen, respectively. The peak intensity of the intervalence charge transfer (IVCT) band, the apparent band gap energy (E _g ^* ) and catalytic data were obtained by in situ UV‐VIS diffuse reflectance spectrosccopy (UV‐VIS‐DRS) and on‐line gas chromatography (GC), respectively. The peak intensity of the IVCT band and E _g ^* increase during the reduction of heteropoly compounds by the alcohols. The spectroscopic and catalytic data (conversion, selectivity) correlate in the transient state during the reoxidation process. It is shown that isolated Keggin anions act as precursors for the active states of the catalysts, which molecular structure cannot be deduced from UV‐VIS spectroscopy alone. UV‐VIS spectroscopy, however, can serve as a tool to determine the degree of reduction in future combined in situ UV‐VIS/Raman/XRD studied. This revised version was published online in July 2006 with corrections to the Cover Date.     

A study of the increase in selective catalytic reduction (SCR) activity of the V/TiO2 catalyst due to the addition of monoethanolamine (MEA)

Experiments were conducted to evaluate the effect of adding ethanol amine during the preparation of a V/TiO₂ catalyst to remove nitrogen oxide (NO _x ) by selective catalytic reduction (SCR). The catalyst added monoethanolamine (MEA) had the highest NO _x conversion among all the neutralization reagents tested, and the optimum MEA concentration was determined to be 10%. The catalyst-added MEA had a large amount of lattice oxygen, which was determined in the O₂ on/off experiment. In addition, it also displayed a high reoxidation rate in the O₂ reoxidation experiment. In the XPS analysis, the superior redox properties of the catalyst-added MEA were shown to be caused by the presence of Ti⁺³, a non-stoichiometric species.     

Comparative Study of the Physico-Chemical Properties of Nanocrystalline CuO–ZnO–Al2O3 Prepared from Different Precursors: Hydrogen Production by Vaporeforming of Bioethanol

The physico-chemical and catalytic properties of CuO–ZnO–Al₂O₃, synthesised by sol–gel process (SG), impregnation method (IMP) and a combination of both preparative procedures (ISG), were comparatively studied. Samples were characterised with thermogravimetric-differential thermal analysis (TG–DTA), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS) techniques and oxygen chemisorption. XPS study was not consistent with the bulk findings and revealed the presence of Cu²⁺, Cu⁺ and/or Cu⁰ species at the catalysts surface. Surface analysis revealed also that copper enrichment occurred mainly at the surface of SG and IMP solids. The reducibility of the mixed oxides catalysts was always modified with respect to that of pure copper oxides phases and the reduction of CuO was markedly affected by the presence of ZnO–Al₂O₃. Temperature programmed reduction (H₂-TPR) analysis showed that the temperature corresponding to maximum reduction rate of copper oxide was ca. 256 °C for IMP sample and ca. 296 °C for both SG and ISG solids. These latter showing a high resistance to reduction suggest a strong interaction of copper species with ZnO–Al₂O₃, limiting thus copper particles sintering. CuO particle size was found to be ca. 20 nm for both SG and ISG solids and ca. 40 nm for IMP catalysts. Besides, at 300 °C SG and ISG samples showed superior amount of reversible O₂ uptake with respect to IMP solids. Catalytic activity of CuO–ZnO–Al₂O₃ was measured with bio-ethanol steam reforming reaction. SG catalysts exhibited both high selectivity to hydrogen and high stability with time on stream than IMP and ISG catalysts. This was attributed both to the particles size of copper species, their amount on the catalytic surface and to their strong interaction with ZnO–Al₂O₃.     

The structure of vanadium oxide species on γ-alumina; an in situ X-ray absorption study during catalytic oxidation

The mechanism of catalytic oxidation reactions was studied using in situ X-ray absorption spectroscopy (XAFS) over a 17.5 wt% V₂O⁵/Al₂O₃ catalyst, i.e., at reaction temperatures and in the presence of reactants. It was found that X-ray absorption near-edge structure (XANES) is a powerful tool to study changes in the local environment and the oxidation state of the vanadium centres during catalytic oxidation. At 623 K, the catalyst follows the associative mechanism in CO oxidation. XAFS revealed that the Mars–van Krevelen mechanism is operative at 723 K for CO oxidation. The extended X-ray absorption fine structure (EXAFS) results showed that the structure of the supported V₂O⁵ phase consists of monomeric tetrahedral (Al–O)₃–V=O units after dehydration in air at 623 K. However, the residuals of the EXAFS analysis indicate that an extra contribution has to be accounted for. This contribution probably consists of polymeric vanadate species. The structure remains unchanged during steady-state CO oxidation at 623 and 723 K. Furthermore, when oxygen was removed from the feed at 623 K, no changes in the spectra occurred. However, when oxygen is removed from the feed at 723 K, reduction of the vanadium species was observed, i.e., the vanadyl oxygen atom is removed. The V³⁺ ion subsequently migrates into the γ-Al₂O₃ lattice, where it is positioned at an Al³⁺ octahedral position. This migration process appears to be reversible; so the (Al–O)₃–V=O units are thus restored by re-oxidation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Fe‐substituted Ba‐hexaaluminates oxygen carrier for carbon dioxide capture by chemical looping combustion of methane

Fe‐substituted Ba‐hexaaluninates (BFA‐x (x = 1–3), x indicates Fe content) oxygen carrier (OC) were found to exhibit excellent sintering‐resistance under cyclic redox atmosphere at 800°C thanks to the reservations of the structure during the CH₄ reduction step, thus preventing the agglomeration of particles during the subsequent reoxidation step. Lattice oxygen highly active for the total combustion of CH₄ was observed in the hexaaluminate structure and its chemical state was influenced by Fe content. The highest amount of active O coordinated with Fe³⁺ in the mirror plane (O‐Fe³⁺(M)) for the total combustion was reacted (0.77 mmol/g) for BaFe₃Al₉O₁₉ hexaaluminate OC. As a result, it exhibited the best reactivity with the CH₄ conversion of 83% and CO₂ selectivity of 100%. Moreover, superior regeneration and recyclability was also obtained, which originated from the fully recovery of O‐Fe³⁺(M) in the hexaaluminate structure. © 2015 American Institute of Chemical Engineers AIChE J, 62: 792–801, 2016     

Microstructure,kinetics and mechanisms of CO2 catalytic decomposition over freshly reduced nano-crystallite CuFe2O4 at 400–600 °C

The decomposition of CO₂ was investigated as a process of both industrial and environmental importance. Copper ferrite was obtained by the thermal decomposition of acetate precursors. CuFe₂O₄ were isothermally reduced in H₂ flow at 400–600 °C, the isothermal reduction profiles obtained in this study show that a topochemical mode of reduction is done by which the reduction process proceeds. The nano-wires metallic phase of iron (106 nm) and copper (56 nm), produced from the complete reduction of CuFe₂O₄, were subjected to the direct reoxidation in CO₂ flow at 400–600 °C. The reoxidation process was found to be controlled by both the reduction and reoxidation temperatures. CO₂ decomposes to carbon nano-tubes during the reoxidation of the freshly reduced CuFe₂O₄. The prepared, completely reduced and reoxidized CuFe₂O₄ compacts, were characterized by XRD, SEM, TEM and reflected light microscope. For the reoxidation process, it is found that at the initial stages the reaction is controlled by the interfacial chemical reaction mechanism with some contribution to the gaseous diffusion mechanism. On the other hand at the intermediate and final stages the mechanism by which the reoxidation process proceeds was found to be the solid-state diffusion.     

Activity and active sites of nitrogen-doped carbon nanotubes for oxygen reduction reaction

Nitrogen-doped carbon (CNx) nanotubes were synthesized by thermal decomposition of ferrocene/ethylenediamine mixture at 600–900 °C. The effect of the temperature on the growth and structure of CNx nanotubes was studied by transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. With increasing growth temperature, the total nitrogen content of CNx nanotubes was decreased from 8.93 to 6.01 at.%. The N configurations were changed from pyrrolic-N to quaternary-N when increasing the temperature. Examination of the catalytic activities of the nanotubes for oxygen reduction reaction by rotating disk electrode measurements and single-cell tests shows that the onset potential for oxygen reduction in 0.5 M H₂SO₄ of the most effective catalyst (CNx nanotubes synthesized at 900 °C) was 0.83 V versus the normal hydrogen electrode. A current density of 0.07 A cm^?2 at 0.6 V was obtained in an H₂/O₂ proton-exchange membrane fuel cell at a cathode catalyst loading of 2 mg cm^?2.     

Structural evolution of H4PVMo11O40⋅xH2O during calcination and isobutane oxidation: New insights into vanadium sites by a comprehensive in situ approach

H₄PVMo₁₁O₄₀⋅8H₂O was studied during thermal activation as well as during isobutane oxidation by simultaneous in situ-EPR/UV–vis/Raman spectroscopy, in situ-FTIR spectroscopy, and quasi-in situ-¹H and -⁵¹V-MAS-NMR. In as-synthesized form, most V sites are pentavalent, octahedrally coordinated, and located within the intact Keggin anions. Stepwise dehydration in N₂ up to 350 °C leads to partial reduction and disintegration of the V sites from the Keggin units, followed by their condensation on the outer surface of the latter in square-pyramidal form. In water-free H₄PVMo₁₁O₄₀, only V⁵⁺ (not V⁴⁺) is stable inside the Keggin unit. Thus, disintegration of V from the latter is favored by its reduction to the tetravalent state and thus depends on the redox properties of the atmosphere. Active sites in isobutane oxidation are most likely composed of single O₄V^4+/5+O species connected to Mo⁶⁺ via oxygen bridges. Partial deactivation occurs by formation of carbon-containing deposits.     

Preparation of Mesoporous V–Ce–Ti–O for the Selective Oxidation of Methanol to Dimethoxymethane

Mesoporous V–Ce–Ti–O oxides were synthesized through the combination of sol–gel and hydrothermal methods and were characterized by different techniques. N₂ adsorption showed that the mesoporous oxides with 0–20 wt.% V₂O₅ possessed the surface areas of about 160 m² g^?1 with narrow pore size distribution centered around 4–5 nm. Vanadium species were highly dispersed in the samples, as confirmed by the wide angle XRD and Raman spectroscopy. The surface acidity of the materials was determined by the microcalorimetric adsorption of NH₃. Temperature programmed reduction and O₂ chemisorption were used to probe the redox property of the materials. It was found that the mesoporous V–Ce–Ti–O possessed bifunctional characters of acidic and redox properties that catalyzed the oxidation of methanol to dimethoxymethane (DMM). These bifunctional characters were further enhanced by the addition of V₂O₅ and SO₄ ^2? onto V–Ce–Ti–O simultaneously. Such supported catalysts exhibited excellent performance for the selective oxidation of methanol to DMM. Specifically, 72% conversion of methanol with 85% selectivity to DMM was achieved at 423 K over a SO₄ ^2?–V₂O₅/V–Ce–Ti–O catalyst.     

Alumina and Alumina–Baria Supported Cobalt Catalysts for DeNO x : Influence of the Support and Cobalt Content on the Catalytic Performance

Co/Al₂O₃ and Co/Al₂O₃–BaO catalysts with low cobalt loading (0.1, 0.3 and 1 wt%) for the selective catalytic reduction (SCR) of NO _x by C₃H₆ were prepared. The distribution of cobalt species was investigated by UV–vis diffuse reflectance spectroscopy and by H₂-TPR in order to identify the active cobalt species in hydrocarbons (HC)-selective catalytic reduction (SCR). It was found that the nature of cobalt species strongly depends on the cobalt loading as well as on the properties of the support. The barium addition to the alumina slows down solid state diffusion processes, improving the thermal stability of the support and preventing diffusion of cobalt into the bulk. Highly dispersed surface Co²⁺ species over alumina were identified as active sites in the NO-SCR process. Accordingly, a high concentration of surface Co²⁺ sites in Co 1 wt%/Al₂O₃ improves the catalytic performance in NO-SCR, the long term stability as well as the water tolerance. On the contrary, the formation of Co₃O₄ particles in Co 1 wt%/Al₂O₃–BaO promotes the propylene oxidation by oxygen, decreasing the activity and selectivity of the catalyst in NO reduction.     

A novel CeO2–xSnO2/Ce2Sn2O7 pyrochlore cycle for enhanced solar thermochemical water splitting

A novel CeO₂–xSnO₂/Ce₂Sn₂O₇ pyrochlore stoichiometric redox cycle with superior H₂ production capacities is identified and corroborated for two‐step solar thermochemical water splitting (STWS). During the first thermal reduction step (1400°C), a reaction between CeO₂ and SnO₂ occurred for all the CeO₂–xSnO₂ (x = 0.05–0.20) solid compounds, forming thermodynamically stable Ce₂Sn₂O₇ pyrochlore rather than metastable CeO_2‐δ. Consequently, substantially higher reduction extents were achieved owing to the reduction of Ce^IV to Ce^III. Moreover, in the subsequent reoxidation with H₂O (800°C), H₂ production capacities increased by a factor of 3.8 as compared to the current benchmark material ceria when x = 0.15, with the regeneration of CeO₂ and SnO₂ and the concomitant reoxidation of Ce^III to Ce^IV. The H₂O‐splitting performance for CeO₂–0.15SnO₂ was reproducible over seven consecutive redox cycles, indicating the material was also robust. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3450–3462, 2017     

Structural dependence of microwave dielectric properties in ilmenite-type Mg(Ti1-xNbx)O3 solid solutions by Rietveld refinement and Raman spectra

Mg(Ti_1-xNb_x)O₃ (x = 0–0.09) ceramics were prepared by the conventional solid-state reaction method. The phase composition, sintering characteristics, microstructure and dielectric properties of Ti⁴⁺ replacement by Nb⁵⁺ in the formed solid solution Mg(Ti_1-xNb_x)O₃ (x = 0–0.09) ceramics were systematically studied. The structural variations and influence of Nb⁵⁺ doping in Mg(Ti_1-xNb_x)O₃ were also systematically investigated by X-ray diffraction and Raman spectroscopy, respectively. X-ray diffraction and its Rietveld refinement results confirmed that Mg(Ti_1-xNb_x)O₃ (x = 0–0.09) ceramics crystallised into an ilmenite-type with R-3 (148) space group. The replacement of the low valence Ti⁴⁺ by the high valence Nb⁵⁺ can improve the dielectric properties of Mg(Ti_1-xNb_x)O₃ (x = 0–0.09). This paper also studied the different sintering temperatures for Mg(Ti_1-xNb_x)O₃ (x = 0–0.09) ceramics. The obtained results proved that 1350 °C is the best sintering temperature. The permittivity and Q × f initially increased and then decreased mainly due to the effects of porosity caused by the sintering temperature and the doping amount of Nb₂O₅, respectively. Furthermore, the increased Q × f is correlated to the increase in Ti–O bond strength as confirmed by Raman spectroscopy, and the electrons generated by the oxygen vacancies will be compensated by Nb⁵⁺ to a certain extent to suppress Ti⁴⁺ to Ti³⁺, which was confirmed by XPS. The increase in τ_f from ?47 ppm/°C to ?40.1 ppm/°C is due to the increment in cell polarisability. Another reason for the increased τ_f is the reduction in the distortion degree of the [TiO₆] octahedral, which was also confirmed by Raman spectroscopy. Mg(Ti_0.95Nb_0.05)O₃ ceramics sintered at 1350 °C for 2 h possessed excellent microwave dielectric properties of ε_r = 18.12, Q × f = 163618 GHz and τ_f = ?40.1 ppm/°C.     

Structure and viscosity of CaO–Al2O3–B2O3–BaO slags with varying mass ratio of BaO to CaO

The structure of CaO–Al₂O₃–B₂O₃–BaO glassy slags with varying mass ratio of BaO to CaO has been investigated by Raman spectroscopy, ¹¹B and ²⁷Al magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy and atomic pair distribution function (PDF). ¹¹B MAS-NMR spectra reveal the dominant coordination of boron as trigonal. Both simulations on ¹¹B MAS-NMR spectra and Raman spectroscopy indicate the presence of orthoborate as the primary borate group with a few borate groups with one bridging oxygen and minor four-coordinated boron sites. ²⁷Al MAS-NMR and PDF show the Al coordination as tetrahedral. Raman spectral study shows that the transverse vibration of Al^IV–O–Al^IV and Al^IV–O–B^III, stretching vibration of aluminate structural units and vibration of orthoborate and pyroborate structural groups. A broader distribution of Al–O bond lengths in PDF also supports the enhanced network connectivity. Viscosity measurements show the increase in viscosity of molten slags with increasing mass ratio of BaO to CaO, which further attributes to the enhanced degree of polymerization of the aluminate network.     

The effect of the Ce content on the oxidative dehydrogenation of propane over CrOy-CeO2/γ-Al2O3 catalysts

A series of CrO_y (17.5 wt%)-CeO₂ (X wt%)/γ-Al₂O₃ catalysts (X = 0, 0.5, 2, 5, 8) with various Ce contents were prepared by a wetness impregnation method and were applied to the dehydrogenation of propane to propylene at 550 °C and 0.1 MPa. The prepared catalysts were characterized by BET, H₂-TPR, O₂-TPD, XPS, XRD, SEM-EDS and Raman spectroscopy. Among the prepared catalysts, the 17.5Cr-2Ce/Al catalyst with the largest amount of lattice oxygen exhibited the best catalytic performance for the dehydrogenation of propane to propylene with lattice oxygen. The decreased presence of oxygen defects and reducibility were the factors responsible for the improved dehydrogenation activity of the catalysts. The CeO₂ layer could inhibit the evolution of lattice oxygen (O²⁻) to electrophilic oxygen species (O₂⁻), and the oxygen defects on the catalyst surface were reduced. The inhibited lattice oxygen evolution prevented the deep oxidation of propane or propylene, the average CO_x selectivity decreased from 24.41% (17.5Cr/Al) to 5.71% (17.5Cr-2Ce/Al), and the average propylene selectivity increased from 60.15% (17.5Cr/Al) to 85.05% (17.5Cr-2Ce/Al).     

Effect of Surface Tungstate W<Superscript>5+</Superscript> Species on the Metathesis Activity of W-Doped Spherical Silica Catalysts

The high surface area W-doped spherical silica (SSP) catalysts were prepared with different sequences of W and Si addition (W–Si_(Alt), Si₁–W_2, and W₁–Si₂) by the sol–gel method with CTAB as a structure directing agent and compared with the impregnated one (W/SSP). All the catalysts exhibited high specific surface area (～?1100 m² g^?1) with a closely perfect spherical shape. The presence of surface/sub-surface tungstate W⁵⁺ species, crystalline bulk WO₃, and tetrahedral tungsten oxide species on the prepared catalysts was investigated by means of X-ray photoelectron spectroscopy depth profile analysis, X-ray diffraction, and Raman spectroscopy. Without in situ reduction by the reactants/products, tungstate W⁵⁺ species was found on the top surface of the as-prepared W–Si_(Alt) whereas for the Si₁–W₂, W/SSP, and W₁–Si₂, the W⁵⁺ appeared only on the sub-surface of the catalysts after 5 and 15 s Ar⁺ etching. The abundance of surface W⁵⁺ species is suggested to facilitate the establishment of the active tungsten carbenes and was correlated well to the catalytic activity in propene metathesis. The surface W⁵⁺-activity relationship of the WO₃-based metathesis catalysts is useful especially when the catalyst activity did not depend solely on the amount of active tetrahedral coordinated tungsten oxides.     

Effect of IrO2 crystallinity on electrocatalytic behavior of IrO2–Ta2O5/MWCNT composite as anodes in chlor-alkali membrane cell

IrO₂–Ta₂O₅ multi-wall carbon nanotube (MWCNT) composite coatings were synthesized on Ti electrodes at different calcination temperatures from 350 to 550 °C used as anodes in membrane cell for brine electrolysis. The physicochemical properties and electrochemical performance of the coatings were investigated by simultaneous differential scanning calorimetry/thermogravimetric analysis (DSC-TGA), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), cyclic voltammetry and electrochemical impedance spectroscopy (EIS) analysis. Results indicate that the degree of IrO₂ crystallinity significantly affects the coating properties. XRD pattern of the coating prepared at 350 °C has shown no reflection peaks indicating that the IrO₂ and Ta₂O₅ were amorphous. The DSC-TGA showed two major exothermic peaks at 475 and 575 °C attributed to the crystallization of IrO₂ and oxidation of Ta₂O₅ from chloride precursor solution, respectively. XPS data reveals the presence of both Ir valance states, which confirms the reversible redox transition of Ir (III)/Ir (IV). The Raman spectra of the coatings demonstrated that the MWCNT gradually loses its tube structure at the calcination temperatures of 450 and 550 °C, and they transform into a graphite-like structure by crystallization of IrO₂. However, in the coating without IrO₂, the modification of the MWCNT structure was not observed at the calcination temperature of 550 °C. The performance of calcined anodes for brine electrolysis was studied using a membrane cell, which showed that the output current density reduces with increasing calcination temperature. The results of the EIS analysis at oxygen evolution potential showed that the charge transfer resistance of IrO₂–Ta₂O₅-MWCNT composite increases from 1.1 to 18.2 (ꭥ.cm²) due to gradual IrO₂ crystallization, which illustrates a sharp reduction in the electrochemical OER activity.     

Slurry-Phase Fischer–Tropsch Synthesis Using Co/γ-Al2O3, Co/SiO2 and Co/TiO2: Effect of Support on Catalyst Aggregation

The catalytic performance of Co/γ-Al₂O₃, Co/SiO₂ and Co/TiO₂ catalysts has been investigated in a slurry-phase Fischer–Tropsch Synthesis (FTS). Although Co/SiO₂ catalyst shows higher CO conversion than the other catalysts, the intrinsic activity is much higher on Co/TiO₂ due to large pore size and low deactivation of large cobalt particles by reoxidation mechanism. Co/γ-Al₂O₃ catalyst confirms low formation rate of oxygenates and C₅₊ selectivity because of deactivation of catalyst due to catalyst aggregation and reoxidation by the in situ generated water during the FTS reaction. Long-chain hydrocarbons such as wax formed during FTS reaction generally contains water and trace amount of oxygenate which are conducive to the formation of a macro-emulsion of wax products. Formation of such macro-emulsion on the catalyst suggests that the presence of proper amount of alcohol content derived FTS reaction on large pore of catalyst inhibits the catalyst aggregation. The intrinsic activity (turn-over frequency; TOF) of cobalt-based catalysts, in a slurry-phase FTS reaction, is affected by the average pore size of catalyst, cobalt particle size, degree of reduction of cobalt species and possible reoxidation by in situ generated water.     

Effect of cobalt oxide on surface structure of alumina supported molybdena catalysts studied by in situ Raman spectroscopy

A set of Co promoted 10% Mo/Al₂O₃ samples have been characterized by means of Raman spectroscopy under ambient as well as in situ dehydrated conditions. Under ambient conditions, the degree of the polymerization of surface molybdenum oxide species decreases with increasing Co loading. Under dehydrated conditions, the polymeric molybdenum oxide species is absent with the addition of only 0.2% Co. At low Co loadings (2%), before the formation of CoMoO₄ compound, the spectral features are very similar under ambient conditions. Dehydration causes the upward shift of the Mo=O symmetric stretching mode. A broad band around 920–930 cm^–1 was thus observed. This band has been suggested to be associated with the Co-Mo interaction species. In contrast to crystalline CoMoO₄, this species shows a reversibility on H₂ reduction-O₂ reoxidation treatments. From the results obtained, it is proposed that cobalt oxide interacts with the most polymerized molybdenum oxide species to form Co-Mo interaction species and/or crystalline CoMoO₄; therefore, the amount of the surface molybdenum oxide species decreases with a change in the molecular structure as a function of the Co concentration.     

A core-shell structured diffusion-bubbling membrane for efficient oxygen separation: Formation and transport properties

The newly developed core-shell structured molten oxide membranes with fast combined diffusion-bubbling oxygen mass transfer and theoretically infinite selectivity are of technological interest because of their high separation efficiency. In this article, a core-shell structured molten V₂O₅–Cu₂O- based diffusion-bubbling membrane was prepared by one-step thermal treatment of initial CuO–25 wt.% Cu₅V₂O₁₀ ceramic composite in a chemical field (under an oxygen partial pressure difference across the composite) above copper vanadate peritectic transformation temperature (816°C). Oxygen fluxes through the membrane were measured at 830°C, using either gas mixtures (O₂ + N₂) with different oxygen concentrations or air as feed gas at the shell of the membrane and helium (He) as sweep gas. Oxygen flux through the membrane with a shell thickness of 0.15–0.61 mm was 3.8·10^–8–1.4·10^–7 mol/cm²/s under an oxygen partial pressure difference of 0.1 –0.75 atm, respectively. The effect of oxygen partial pressure on the thickness of the membrane shell is found. The relationship between membrane shell thickness, oxygen partial pressure difference across the membrane, and oxygen permeation flux through the membrane is established. Oxygen permeation flux through the dual-phase MIEC membrane shell is described in terms of the diffusion model. Oxygen permeation flux through the membrane core is described both within the framework of the stationary model and nonstationary model for uniform (the membrane thickness is much larger than the characteristic distance of bubble dynamic relaxation) and accelerated (the membrane thickness is comparable to the characteristic distance of bubble dynamic relaxation) bubble motion in a viscous oxide melt, respectively.     

Second-harmonic generation and structural rearrangements in multicomponent antimonite glasses by electrothermal poling

Electrothermal poling is shown here to effectively induce second-order nonlinear effects in heavy-metal oxide antimonite glasses. In M₂O–PbO–WO₃–Sb₂O₃ (M = Li, Na, K) glasses, the poling-induced second-harmonic generation intensity is five times larger than in silica (Infrasil) for M = Na, twice as large as in silica for M = Li, and smaller than in silica for M = K. X-ray photoelectron spectroscopy suggests that antimony ions exist predominantly in the trivalent oxidation state in the studied glass samples. Raman and infrared spectroscopy confirm that the glass network is comprised of SbO₃, WO₄, WO₆, and PbO₄ units—with some SiO₄ moieties due to leaching from the silica crucible. The WO₄ units appear to exist in two distinct sites, as evidenced by comparison of the vibrational spectra of alkali–tungsten–antimonite glasses with those of previously reported crystalline tungstate phases. The alkali type influences the equilibrium between tetrahedral tungstate anions, [WO₄]²⁻, and the isomeric partially polymerized octahedral tungstate units, [WØ₄O₂]²⁻ (Ø denotes a bridging oxygen). Raman spectroscopy line scans were used to track near-surface structural changes on the anode side of poled glasses. They reveal that the tungstate equilibrium is also affected by poling. At the anode side, the population of partially polymerized [WØ₄O₂]²⁻ species increases at the expense of anionic [WO₄]²⁻ species. This yields a net increase in the average bond length of the network forming constituents, which is commensurate with poling-induced structural changes observed in other systems experimentally and computationally.