A dense oxygen separation membrane with a layered morphologic structure

A configuration of dense mixed ionic and electronic conducting (MIEC) membrane with layered morphological structure for oxygen separation, which combines the benefits of high oxygen permeation flux of cobalt-based membrane, high chemical stability of iron-based perovskite and high mechanical strength of thick membrane, was studied. The membrane is normally composed of two layers; each layer is a dense MIEC oxide. The substrate layer is a thick dense membrane with high oxygen permeability but relatively lower chemical stability. The feasibility of dense thick Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF5582) membrane as the substrate layer and Ba_0.5Sr_0.5Co_0.2Fe_0.8O_3−δ (BSCF5528) as the thin-film layer was mainly experimentally investigated. Both the BSCF5582 and the BSCF5528 show the same cubic perovskite structure and the similar lattice constant with no detrimental reaction products formed. By optimizing fabrication parameters of a simple dry pressing process, dual-layered membrane, free of cracks, was successfully fabricated. The oxygen permeation flux of a dual-layered membrane with the thin-film BSCF5528 layer facing to the sweep gas reached 2.1 mL cm⁻² min⁻¹ [STP] (1.56 × 10⁻⁶ mol cm⁻² s⁻¹) at 900 °C, which is about 3.5 times higher than that of the BSCF5528 membrane (0.6 mL cm⁻² min⁻¹, [STP] (4.46 × 10⁻⁷ mol cm⁻² s⁻¹) under the same conditions.     

Oxygen permeation and oxidative coupling of methane in membrane reactor: A new facile synthesis method for selective perovskite catalyst

A dense membrane of La_0.6Sr_0.4Co_0.8Fe_0.2O_3−δ (LSCF) perovskite was prepared by a new chelating agent, ethylene diamine N,N,N′,N′-tetra N-acetyl diamine (EDTNAD). As a potent ligand, EDTNAD provided a facile one-step method to form complexes of the four metal ions, simultaneously. The oxygen permeation flux through the pure perovskite LSCF dense membrane was measured over temperature range of 1073–1223 K, thickness of 0.7–1.0 mm and oxygen partial pressure of 0.1–1.0 bar. Oxidative coupling of methane (OCM) reaction using LSCF disk in the atmospheric membrane reactor and over the temperature range of 1073–1173 K showed a C₂ selectivity of 100% and C₂ yield of 5.01% at 1153 K. Furthermore, OCM reaction data of the membrane reactor were discussed and compared with those of the fixed bed using the same perovskite powder as the catalyst.     

Grain boundaries as barrier for oxygen transport in perovskite-type membranes

Perovskite-type membranes of (Ba_0.5Sr_0.5)(Co_0.8Fe_0.2)O_3−δ (BSCF) and (Ba_0.5Sr_0.5)(Fe_0.8Zn_0.2)O_3−δ (BSFZ) were successfully prepared via liquid-phase sintering using BN as sintering aid. The obtained membranes were examined via powder X-ray diffraction pattern (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and oxygen permeation experiments. It has emerged that the use of BN as sintering aid lowers sintering temperatures in order to obtain dense membranes with relative densities in the range of 93–96% as proven by the Archimedes method. It was further shown that the perovskite structure could be maintained after sintering with BN. Additionally, BN was completely removed from the sintered membranes. Investigation of the microstructure revealed that the average grain size of the membranes was influenced by the amount of BN added prior the sintering process. It was found that large amounts of BN effectively lower the average grain size. Oxygen permeation experiments have shown that the lower the average grain size the lower the oxygen permeation performance, particularly in the case of BSCF. Transmission electron microscopy revealed that no evidence for an amorphous layer or any other interfacial phase in the grain boundary is present.     

Structure and redox properties of CexPr1−xO2−δ mixed oxides and their catalytic activities for CO, CH3OH and CH4 combustion

A series of Ce_xPr_1−xO_2−δ mixed oxides were synthesized by a sol–gel method and characterized by Raman, XRD and TPR techniques. The oxidation activity for CO, CH₃OH and CH₄ on these mixed oxides was investigated. When the value x was changed from 1.0 to 0.8, only a cubic phase CeO₂ was observed. The samples were greatly crystallized in the range of the value x from 0.99 to 0.80, which is due to the formation of solid solutions caused by the complete insertion of Pr into the CeO₂ crystal lattices. Raman bands at 465 and 1150 cm⁻¹ in Ce_xPr_1−xO_2−δ samples are attributed to the Raman active F_2g mode of CeO₂. The broad band at around 570 cm⁻¹ in the region of 0.3 ≤ x ≤ 0.99 can be linked to oxygen vacancies. The new band at 195 cm⁻¹ may be ascribed to the asymmetric vibration caused by the formation of oxygen vacancies. The TPR profile of Pr₆O₁₁ shows two reduction peaks and the reduction process is followed: . The reduction temperature of Ce_xPr_1−xO_2−δ mixed oxides is lower than those of Pr₆O₁₁ or CeO₂. TPR results indicate that Ce_xPr_1−xO_2−δ mixed oxides have higher redox properties because of the formation of Ce_xPr_1−xO_2−δ solid solutions. The presence of the oxygen vacancies favors CO and CH₃OH oxidation, while the activity of CH₄ oxidation is mostly related to reduction temperatures and redox properties.     

Survey of the quasi-ternary system La0.8Sr0.2MnO3–La0.8Sr0.2CoO3–La0.8Sr0.2FeO3

An overview on the variation of the thermal expansion, the electrical conductivity as well as non-stoichiometry of the oxide content as a function of composition within the quasi-ternary system La_0.8Sr_0.2MnO_3−δ–La_0.8Sr_0.2CoO_3−δ–La_0.8Sr_0.2FeO_3−δ in air is presented. The various powders were synthesized under identical conditions. The DC electrical conductivity values of the compositions at 800 °C in air vary in a wide range from 15 to 1338 S cm⁻¹. The magnitude of electrical conductivity of the perovskites is mainly determined by the percentage of cobalt in the compositions. A similar behaviour was observed for the measured thermal expansion coefficients between room temperature and 1000 °C in air, increasing from 10.9 to 19.4 × 10⁻⁶ K⁻¹ as a function of cobalt content. Changes in the oxygen stoichiometry of the materials were characterized by temperature-programmed oxidation measurements.     

Oxygen permeation study in a tubular Ba0.5Sr0.5Co0.8Fe0.2O3-δ oxygen permeable membrane

Dense tubular Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCFO) membranes were successfully prepared by the plastic extrusion method. The oxygen permeation flux was determined at different oxygen partial pressures in the shell side and different temperatures between 700 and 900 °C. The oxygen vacancy diffusion coefficients (D_v) at different temperatures were calculated from the dependence of oxygen permeation flux on the oxygen partial pressure term based on the surface current exchange model. No unsteady-state of oxygen permeation flux was observed at the initial stage in our experiments. The reason is the equilibrium time is too short (less than 10 min) to observe the unsteady-state in time. The increase of the helium flow rate can increase the oxygen permeation flux, which is due to the decrease of the oxygen partial pressure in the tube side with increasing of the helium flow rate. The oxygen permeation flux can also be affected by the air flow rate in the shell side when the air flow rate is lower than 150 ml/min. But the oxygen permeation flux is insensitive to the air flow rate when the air flow is higher than 150 ml/min. The membrane tube was operated steadily for 150 h with oxygen permeation flux of 1.12 ml/(cm² min) at 875 °C. X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) analysis showed that both the surface exposed to air and the surface exposed to helium of the BSCFO membrane tube after permeation for 150 h are similar to the fresh membrane tube in composition and structure. These results indicated that the membrane tube exhibits high structure stability.     

Synthesis, structure and oxygen nonstoichiometry of La0.4Sr0.6Co1−yFeyO3−δ

The samples of La_0.4Sr_0.6Co_1−yFe_yO_3−δ (y = 0.2 and 0.4) were prepared using both conventional ceramic technique and nitrate–citrate precursors technique. The phase identification was made by X-ray diffraction method. The refinement of structural parameters from the XRD and neutron diffraction measurements was performed by full profile Rietveld analysis. Neutron diffraction showed that both samples possess distorted perovskite-type structure. Oxygen nonstoichiometry was measured by chemical analysis and thermogravimetry (TG) analysis in the range 20 ≤ T/°C ≤ 900 and 2E-5 ≤ pO₂/atm ≤ 4E-1. TG-experiments indicate a relatively fast and reversible oxygen exchange at pO₂ > 1E-2 atm. Mass saturation occurs at T < 300 °C upon cooling. The absolute value of oxygen nonstoichiometry was determined by iodometric titration measurements. It was found that both samples have practically stoichiometric composition at 300 °C in air and δ increases with increasing temperature and decreasing oxygen partial pressure.     

Oxygen permeation through a Ce0.8Sm0.2O2−δ–La0.8Sr0.2CrO3−δ dual-phase composite membrane

A composite of oxygen ion conducting oxide Ce_0.8Sm_0.2O_2−δ (60 vol.%) and electron conducting oxide La_0.8Sr_0.2CrO_3−δ was prepared by sintering a powder compact at a temperature of 1550 °C. No significant reaction between the two constituent oxides was observed under preparation and oxygen permeation conditions. Appreciable oxygen permeation fluxes through the composite membrane were measured at elevated temperatures with one side of it exposed to the ambient air and the other side to a flowing helium gas stream. The oxygen flux initially increased with time, and took a long time to reach a steady value. A steady oxygen permeation flux as high as 1.4 × 10⁻⁷ mol cm⁻² s⁻¹ was obtained with a 0.3 mm thick membrane at 950 °C under a relatively small oxygen partial pressure difference of 0.21 bar/0.0092 bar. It was revealed that the overall oxygen permeation process was mainly limited by the transport in the bulk of the membrane in the range of the membrane thickness greater than 1.0 mm, and the limitation by the surface oxygen exchange came into play at reduced thickness of 0.6 mm.     

Mechanical stability and transport properties of the Sn-promoted SrCo0.8Fe0.2O3−δ ceramic membrane

In this article, the phase compositions, thermal, mechanical and transport properties of both the SrCo_0.8Fe_0.2O_3−δ (SCF) and the SrCo_0.8Fe_0.1Sn_0.1O_3−δ (SSCF) ceramic membranes were investigated systematically. As compared with the SCF membrane, the SSCF one had a more promoted thermal shock resistance, which related to its small thermal expansion coefficient between them and an enhanced composite structure for it. For the SCF membrane, a permeation rate of 1.9 × 10⁻⁶ mol cm⁻² s⁻¹ was obtained at 1000 °C and under the oxygen partial pressure gradient of P_O2 (h)/P_O2 (l) = 0.209 atm/0.012 atm; however, the permeation rate was 2.5 × 10⁻⁶ mol cm⁻² s⁻¹ for the SSCF one in the same measuring condition. In addition, both peak values of total electrical conductivity (σ_e) for SSCF sample appeared with increasing temperature. The second peak value of σ_e for SSCF one was regarded as the contribution from its minor phase, which appeared with the mixed conducting behavior resulting from partly Co-dissolving into its lattice.     

Synthesis and low temperature fluorination of the alkaline-earth palladates Ba2−xSrxPdO3 (x = 0–2)

Since the discovery of superconductivity in Sr₂CuO₂F_2+δ there has been an increased interest in ternary oxide-fluorides. Sr₂CuO₂F_2+δ is prepared via low temperature (T = 220 °C) reaction routes. Low temperature fluorination induces an interesting structural rearrangement in the parent compound Sr₂CuO₃, which is a one-dimensional material containing linear chains of vertex sharing CuO₄ squares along the crystallographic b axis. Upon fluorination, one oxide is substituted by two fluorides and Cu²⁺ becomes octahedrally coordinated by four oxides and two fluorides. The fluorinated compound Sr₂CuO₂F_2+δ displays the T-type structure (La₂CuO₄). Insertion of excess fluorine, δ, also takes place and this fluorine occupies interstitial sites in the T structure. Although the starting material Ca₂CuO₃ is isostructural to Sr₂CuO₃, Ca₂CuO₂F_2+δ displays the T′ (Nd₂CuO₄) structure due to the smaller radius of Ca²⁺ compared to that of Sr²⁺.

The alkaline-earth palladates with the general formula A₂PdO₃ (A = Ba, Sr) are isostructural with the A₂CuO₃(A = Ca, Sr) materials. We prepared the Ba_2−xSr_xPdO₃ (x = 0–2) series and performed low temperature fluorination, which led to the synthesis of the series Ba_2−xSr_xPdO₂F_2+δ (0 ≤ x ≤ 1.5). All the compounds in the Ba_2−xSr_xPdO₂F_2+δ series show T′ structure (Ca₂CuO₂F_2+δ). Similarities and differences with Sr₂CuO₂F_2+δ and Ca₂CuO₂F_2+δ will be discussed.     

Supported Pd-perovskite catalyst for CNG engines'' exhaust gas treatment

Perovskite-type oxides of the series La_1−xA_xMn_1−yB_yO₃ (A = Sr; B = Fe or Co) were prepared by solution combustion synthesis and characterized by X-ray diffraction, specific surface analysis, transmission electron microscopy and field emission scanning electron microscopy techniques. Their activity towards the combustion of methane was evaluated in a temperature programmed combustion microreactor. The LaMn_0.9Fe_0.1O₃ catalyst was found to provide the best performance. The half-conversion temperature of methane over the LaMn_0.9Fe_0.1O₃ catalyst was 398 °C with a W/F = 0.12 g s/cm³ and a methane feed concentration of 0.4 vol% under oxygen excess. Via temperature programmed oxygen desorption (TPD) analysis as well as catalytic combustion runs, the prevalent activity of the LaMn_0.9Fe_0.1O₃ catalyst could be explained by its higher and increased capability to desorb suprafacial, weakly chemisorbed oxygen species. Further catalyst development allowed to maximise the catalytic activity of this compound by promoting it with CeO₂ (1:1 molar ratio) and with 1 wt% Pd. This promoted catalyst was lined on cordierite monoliths in a γ-Al₂O₃-supported form (catalyst weight percentage: 15 wt%) and then tested in a lab-scale test rig under realistic conditions for compressed natural gas-vehicles' exhaust gas treatment. Half methane conversion was achieved at 340 °C (gas high space velocity = 10 000 h⁻¹), nearly the same but with a fourfold lower amount of the expensive noble metal than that used in commercial 4wt%Pd–γ-Al₂O₃ catalysts.     

Phase equilibria and crystal structure of the solid solution LaFe1−xNixO3−δ (0 ≤ x ≤ 1)

Phase equilibria in the LaFeO₃–“LaNiO₃” were studied at 1100 °C in air. The samples were synthesized by standard ceramic and/or solution route via nitrate or citrate precursors. According to the results of XRD it was found that the homogeneity ranges of LaFe_1−xNi_xO_3−δ solid solution lay within 0.0 ≤ x ≤ 0.4 (sp.gr. Pbnm) and 0.6 ≤ x ≤ 0.8 (sp.gr. ). The structural parameters (bond lengths, atom coordinates) for the single-phase samples were refined using Rietveld analysis. The unit cell parameters versus LaFe_1−xNi_xO_3−δ composition are presented.     

Properties of oxygen permeation and partial oxidation of methane in La0.6Sr0.4CoO3−δ (LSC)–La0.7Sr0.3Ga0.6Fe0.4O3−δ (LSGF) membrane

The oxygen separation membrane having perovskite structure for the partial oxidation of methane to synthesis gas was prepared. La_0.7Sr_0.3Ga_0.6Fe_0.4O_3−δ (LSGF) perovskite membrane coated with La_0.6Sr_0.4CoO_3−δ (LSC) (M1), and the one side of M1 membrane coated with NiO (M2) was prepared to examine the partial oxidation of methane. The single oxygen permeations of the LSC + LSGF (M1) membrane and NiO coated membrane (M2) were measured. The oxygen permeation flux in M1 membrane was higher than that of M1 membrane at 850 °C.

The partial oxidation experiment of methane using the prepared membranes was examined at 850 °C. The value of CH₄ conversion and CO selectivity of M2 membrane was higher than that of M1 membrane.

NiO/NiAl₂O₄ catalyst was used to improve the methane conversion, and the partial oxidation experiment of methane with M1 membrane was examined at 850 °C. The CH₄ conversion was 88%, and CO selectivity was 100%.     

Properties of Ce–Zr–La–O nano-system with ruthenium modified surface

Structural peculiarities of Ce–Zr–La–O and Ce–Zr–La–O/Ru samples in mean of catalytic properties are compared. The samples (Ce:Zr = 1:1, La = 10÷30 mol.%, Ru = 1.5 wt.%) were obtained by sol–gel method (X-samples) and co-precipitation (P-samples). It is shown that Ce_0.45Zr_0.45La_0.1O_2−δ/Ru X-samples are characterized by high thermal stability and the highest catalytic activity in partial methane oxidation reaction. According to XRD, BET, FTIR, EPR and XPS data it is concluded that the difference in the samples catalytic activity is caused by various disposition of Ru-containing phase on the support surface. The distinction in the dimension of Ru-containing particles (3D or 2D) is conditioned by structural peculiarities of Ce_0.45Zr_0.45La_0.1O_2−δ and Ce_0.35Zr_0.35La_0.3O_2−δ P- and X-samples.     

Relationship between temperature-programmed reduction profile and activity of modified ferrite-based catalysts for WGS reaction

A series of modified ferrites were prepared by doping iron oxide with various transition/non-transition/inner-transition metal ions [M = Cr, Mn, Co, Ni, Cu, Zn and Ce] in situ during synthesis. All the modified ferrites thus obtained exhibit remarkably high surface areas, greater than that of pure iron oxide (Fe₂O₃) sample. The efficacy of the dopant ions in modifying the resultant specific surface area, could be directly related to variations in the rate of crystal growth. The nature and concentration of the foreign cations present in the system govern this variation. Interestingly all the modified ferrites, exhibit a narrow pore size distribution in the range of 4.9–25 nm. XRD analysis revealed the existence of hematite (Fe₂O₃) phase in all the as-prepared samples. The X-ray diffraction experiments performed on activated catalysts, confirmed the existence of magnetite (Fe₃O₄) phase with a nominal composition of Fe_2.73M_0.27O₄. These inverse or mixed spinels with general formula A_(1−δ)B_δ[A_δB_(2−δ)]O₄, possess highly facile Fe³⁺  Fe²⁺ redox couple, the degree of facileness depends on the extent of synergistic interaction between iron and the other substitutent metal ion. The rapid electron hopping between Fe³⁺  Fe²⁺ in the Fe₃O₄ lattice system is essential to catalyze WGS reaction. From TPR it was observed that, incorporation of metal cations into the hematite (-Fe₂O₃) crystal structure alters the reducibility of the hematite particles, which in turn depends on the nature of the incorporated metal cation. A plausible explanation for the WGS activity over various modified ferrites has been attempted with the help of TPR analysis.     

Photodecomposition of molybdenum(II) and tungsten(II) carbonyl complexes with triazole, benz-imidazole, and oxadiazole acetylinic derivatives

Photodecomposition of 10 different molybdenum and tungsten mixed carbonyl complexes, [M(CO)₃(B)(A)]I₂ where B=o-phenanthroline or bipyridyl, A=3-(2-propynyl)thio-4,5-diphenyl-4H-1,2,4-triazole (TRZA) or S-propynyl-2-thio benz-imidazole (BIMDA) and 2(2-propynyl-thio(5-phenyl)-1,3,4-oxadiazole (OXA). M(CO)₃(TRZA)I₂, [M(CO)₂(PPh₃)_X(TRZA)I_Y]I_Z where M=Mo, X, Y and Z=1 and M=W, X and Z=2, Y=0, have been performed at 365 nm in oxygen saturated chloroform at 25 °C. The absorbance spectrum of these complexes have been recorded with the time of irradiation in order to examine the kinetics of photodecay.

The apparent rate constant (K_d) for the first-order reaction have been calculated and found to be (3.32–4.79)×10⁻⁵ s⁻¹. The primary quantum yields (Φ) has also been calculated and were in the range (8.33–12.1)×10⁻⁴. The mechanism of the photodecomposition has been suggested according to the kinetic, and photoproduct analysis data, and is similar to reaction of photo-oxidative degradation of polluted molecules in the water.     

Far-UV laser flash photolysis in solution. A study of the chemistry of 1,1-dimethylsilene in hydrocarbon solvents

The far-UV (193 nm) laser flash photolysis of nitrogen-saturated isooctane solutions of 1,1-dimethylsiletane allows the direct detection of 1,1-dimethylsilene as a transient species, which (at low laser intensities) decays with pseudo-first-order kinetics (τ 10 μs) and exhibits a UV absorption spectrum with λ_max 255 nm. Characteristic rapid quenching is observed for the silene with methanol (k_McOH = (4.9 ± 0.2) × 10⁹ M⁻¹ s⁻¹), tert-butanol (k_BuOH = (1.8 ± 0.1) × 10⁹ M⁻¹ s⁻¹) and oxygen (k_O2 = (2.0 ± 0.5) × 10⁸ M⁻¹ s⁻¹). The Arrhenius activation parameters for the reaction with methanol have been determined to be E_a = −2.6 ± 0.6 kcal mol⁻¹ and log A = 7.7 ± 0.3.     

Influence of composition and thermal treatment on the properties of Cu2+xTa4O12+δ

Polycrystalline samples of Cu_2+xTa₄O_12+δ were prepared by solid-state reactions. Copper tantalate shows a remarkable compositional flexibility with respect to both the copper and oxygen stoichiometry. Single phase compounds could be synthesised for 0.125 ≤ x ≤ 0.5. Slowly cooled samples are green and possess a pseudo-tetragonal unit cell, which changes to a pseudo-cubic symmetry for x ≥ 0.45. Rapidly cooled aliquots are brown and have a (pseudo-) cubic structure. For both the slow-cooled and quenched samples a linear increase of the oxygen content with x was observed, the values of δ for the latter being significantly smaller. Magnetic measurements reveal a ferrimagnetic transition at 12.5 K, the strength of which is strongly reduced both by increasing the copper content and by quenching.     

Influence of the size of doping ion on phase stability and oxygen permeability of SrCo0.8Fe0.2O3−δ oxide

The compounds of SrCo_0.8Fe_0.2O_3−δ (SCF) doped with Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Ti⁴⁺ and Zr⁴⁺ are synthesized by conventional solid-state reaction. The phase stabilities and oxygen permeabilites of these synthesized oxides are investigated. It is found that the doping Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺ totally dissolve into the SCF unit cell, while SrTiO₃ and SrZrO₃ phases form when SCF is doped with Ti⁴⁺ and Zr⁴⁺, respectively. A possible mechanism of the doping behavior is proposed to interpret the structure transformation of the SCF unit cell. Phase stability increases directly with the size of the doping ion when SCF is doped with the ions in the same group. However, the influence of the size of the doping ion on the oxygen permeation behavior is negligible for the membranes doped with the ions in the same group.     

Synthesis and thermochemical study of 1 : 2 : 4 phases in the (Y,Gd)–Ba–Cu–O system

Synthesis of the Y_xGd_1−xBa₂Cu₄O₈ phases (x=1; 0.5; 0.75) by special method from nitrates is described in this paper. Dissolution enthalpies of YBa₂Cu₄O₈, Y_0.5Gd_0.5Ba₂Cu₄O₈, Y_0.75Gd_0.25Ba₂Cu₄O₈, Y₂O₃, Gd₂O₃, CuO, BaCO₃ were measured in 6 N HCl at 323 K. On the basis of obtained experimental data, the enthalpies of some reactions with Y_xGd_1−xBa₂Cu₄O₈ were determined. It was established that the above-mentioned 1 : 2 : 4 superconductors were thermodynamically more favourable than mixtures including CuO, YBa₂Cu₃O_x. It was also established that, according to the obtained data, these phases can react with CO₂.