Evaluation of mixed‐conducting lanthanum‐strontium‐cobaltite ceramic membrane for oxygen separation

In this study, La_0.4Sr_0.6CoO_3‐δ (LSC) oxide was synthesized via an EDTA‐citrate complexing process and its application as a mixed‐conducting ceramic membrane for oxygen separation was systematically investigated. The phase structure of the powder and microstructure of the membrane were characterized by XRD and SEM, respectively. The optimum condition for membrane sintering was developed based on SEM and four‐probe DC electrical conductivity characterizations. The oxygen permeation fluxes at various temperatures and oxygen partial pressure gradients were measured by gas chromatography method. Fundamental equations of oxygen permeation and transport resistance through mixed conducting membrane were developed. The oxygen bulk diffusion coefficient (D_v) and surface exchange coefficient (K_ex) for LSC membrane were derived by model regression. The importance of surface exchange kinetics at each side of the membrane on oxygen permeation flux under different oxygen partial pressure gradients and temperatures were quantitatively distinguished from the oxygen bulk diffusion. The maximum oxygen flux achieved based on 1.6‐mm‐thick La_0.4Sr_0.6CoO_3‐δ membrane was ～4.0 × 10^?7 mol cm^?2 s^?1at 950°C. However, calculation results show theoretical oxygen fluxes as high as 2.98 × 10^?5 mol cm^?2 s^?1 through a 5‐μm‐thick LSC membrane with ideal surface modification when operating at 950°C for air separation. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Oxygen permeability and structural stability of a novel tantalum‐doped perovskite BaCo0.7Fe0.2Ta0.1O3−δ

Dense BaCo_0.7Fe_0.2Ta_0.1O_3?δ (BCFT) perovskite membranes were successfully synthesized by a simple solid state reaction. In situ high‐temperature X‐ray diffraction indicated the good structure stability and phase reversibility of BCFT at high temperatures. The thermal expansion coefficient (TEC) of BCFT was determined to amount 1.02 × 10^?5 K^?1, which is smaller than those of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ (BSCF) (1.15 × 10^?5 K^?1), SrCo_0.8Fe_0.2O_3?δ (SCF) (1.79 × 10^?5 K^?1), and BaCo_0.4Fe_0.4Zr_0.2O_3?δ (BCFZ) (1.03 × 10^?5 K^?1). It can be seen that the introduction of Ta ions into the perovskite framework could effectively lower the TEC. Thickness dependence studies of oxygen permeation through the BCFT membrane indicated that the oxygen permeation process was controlled by bulk diffusion. A membrane reactor made from BCFT was successfully operated for the partial oxidation of methane to syngas at 900°C for 400 h without failure and with the relatively high, stable oxygen permeation flux of about 16.8 ml/min cm². © 2009 American Institute of Chemical Engineers AIChE J, 2010     

High CO2 resistance of indium-doped cobalt-free 60 wt.%Ce0.9Pr0.1O2-δ-40 wt.%Pr0.6Sr0.4Fe1-xInxO3-δ oxygen transport membranes

The oxygen transport membrane (OTM) has huge application prospects in gas separation and carbon neutralization based on oxygen enriched combustion. In this paper, the family 60 wt.%Ce_0.9Pr_0.1O_2-δ-40 wt.%Pr_0.6Sr_0.4Fe_1-xIn_xO_3-δ (CPO-PSF_1-xI_xO, x = 0.01, 0.025, 0.05, 0.075, 0.1) cobalt-free dual-phase MIEC OTMs doped with indium have been successfully prepared by Pechini method. The phase structure, surface morphology, element distribution, oxygen permeability, and long-term operation stability of these OTMs are systematically explored. Among these OTMs, the champion oxygen permeable flux of CPO-PSF_0.99I_0.01O reaches 1.07 mL min^?1·cm^?2 and 0.80 mL min^?1·cm^?2 at 1000 °C under air/He gradient and air/CO₂ gradient. Meanwhile, CPO-PSF_0.99I_0.01O maintains the value of 0.80 mL min^?1·cm^?2 steadily at 1000 °C for 100 h when pure CO₂ as the sweep gas. The surface element distribution and phase structure of the OTMs after long-term oxygen permeability reaction are investigated by XRD, SEM combining with EDS, where the spent membranes retain the same structure and component as the fresh membranes, demonstrating that the In-doped OTMs have an excellent CO₂ tolerance. Suitable indium substitution for iron of these OTMs not only improves the oxygen permeability, but also maintains the long-term reaction stability of the material.     

Synthesis and characterization of a synthetic heme‐based oxygen carrier

We have prepared a novel oxygen carrier composed of Starburst dendrimer 3 hematoporphyrin iron(III) di‐1‐(3‐aminopropyl)imidazole di‐succinic acid (SDHAPI). This compound has a molecular weight of ～12.7 kD. O₂ and CO binding to SDHAPI was demonstrated by UV‐vis spectroscopy. Oxygenated SDHAPI had an estimated auto‐oxidation half‐life of 24 min. The oxygen stoichiometry (～6 mol of O₂ mol^?1 SDHAPI) and the k_obs(on+off) of 0.084 s^?1 and k_obs(off) of 0.009 s^?1 were estimated with an oxygen electrode. Copyright © 2005 Society of Chemical Industry     

A kinetic study of the interactions of trans-polyacetylene (CH)x with oxygen

The kinetics of doping and degradation of all trans-polyacetylene by oxygen, have been studied over the temperature range 70°–110°C. Doping and degradation have first-order kinetics with the observed Arrhenius parameters E = 11.52, E = 13.45 kcal mol^?1 and log A (s^?1 = 4.19, log A (s^?1) = 4.49 respectively. The observations support the view that the reactions proceed via an intermediate-complex mechanism. Initially oxygen reduced the resistivity of (CH)_x by formation of a charge transfer complex and subsequently reacted with (CH)_x with an increase in resistivity.     

SrCo0.9Sc0.1O3−δ perovskite hollow fibre membranes for air separation at intermediate temperatures

SrCo_0.9Sc_0.1O₃ (SCSc) perovskite powders with sub-micron particle size were synthesized by a modified Pechini method combined with a post-treatment of sintering and ball-milling. From the prepared powders, the SCSc hollow fibre membranes with asymmetric structure and gas-tight property were fabricated by spinning a polymer solution containing 58.4 wt% SCSc followed by sintering at 1200 °C for 5 h. The oxygen permeation properties of the obtained SCSc fibres were measured under air/He gradients at 500–800 °C. This showed the oxygen flux of 1 mL cm^?2 min^?1 at 750 °C and 4.41 mL cm^?2 min^?1 at 900 °C. Modeling analysis reveals that the oxygen permeation process is predominated by oxygen surface exchange kinetics with an activation energy of 95.0 kJ mol^?1. The SCSc membranes showed excellent oxygen permeation performance while exhibiting high structural and permeating stability at intermediate temperatures (500–800 °C).     

Profiles of xylose reductase,xylitol dehydrogenase and xylitol production under different oxygen transfer volumetric coefficient values

BACKGROUND: Xylitol is a sugar alcohol (polyalcohol) with many interesting properties for pharmaceutical and food products. It is currently produced by a chemical process, which has some disadvantages such as high energy requirement. Therefore microbiological production of xylitol has been studied as an alternative, but its viability is dependent on optimisation of the fermentation variables. Among these, aeration is fundamental, because xylitol is produced only under adequate oxygen availability. In most experiments with xylitol‐producing yeasts, low oxygen transfer volumetric coefficient (K_La) values are used to maintain microaerobic conditions. However, in the present study the use of relatively high K_La values resulted in high xylitol production. The effect of aeration was also evaluated via the profiles of xylose reductase (XR) and xylitol dehydrogenase (XD) activities during the experiments. RESULTS: The highest XR specific activity (1.45 ± 0.21 U mg_protein^?1) was achieved during the experiment with the lowest K_La value (12 h^?1), while the highest XD specific activity (0.19 ± 0.03 U mg_protein^?1) was observed with a K_La value of 25 h^?1. Xylitol production was enhanced when K_La was increased from 12 to 50 h^?1, which resulted in the best condition observed, corresponding to a xylitol volumetric productivity of 1.50 ± 0.08 g_xylitol L^?1 h^?1 and an efficiency of 71 ± 6.0%. CONCLUSION: The results showed that the enzyme activities during xylitol bioproduction depend greatly on the initial K_La value (oxygen availability). This finding supplies important information for further studies in molecular biology and genetic engineering aimed at improving xylitol bioproduction. Copyright © 2008 Society of Chemical Industry     

Nanostructured catalysts for oxygen electroreduction based on bimetallic monoethanolamine complexes of Co(III) and Ni(II)

Oxygen reduction electrocatalysts based on the monoethanolmine complexes {[CoEtm]₂(μ-Etm)₄Ni(NO₃)₂} and {[CoEtm]₂(μ-Etm)₄Ni(NO₃)₂} + activated carbon AG-3 have been obtained by high-temperature synthesis. The nature of active centers on the synthesized electrocatalysts was described. Using potentiostatic and cyclic potentiodynamic voltammetry, the kinetic characteristics of catalysts in the oxygen electroreduction reaction were determined. Thermal decomposition of the thermally unstable complexes was described and character of the active centers formed was discussed. The optimal synthesis temperature of electrocatalysts is 600 °C in an inert atmosphere. The calculated exchange current densities for the oxygen electroreduction reaction at the catalysts in 1 M KOH at 20 °C was j ₀  = 1.01 × 10^?3 A g^?1–3.3 × 10^?3 A g^?1. The Tafel slopes of stationary polarization curves are 0.054–0.063 V for b ₁ and 0.106–0.125 V for b ₂ . The prepared electrocatalysts can be recommended only for electrochemical systems with alkaline electrolyte.     

Enlargement of the micropores of a caking bituminous coal by controlled oxidation

In a study of the enlargement of pores of coals it has been found that treatment of a bituminous coal (PSOC No. 371, from the Pennsylvania State University Coal Section) with a 5:95 O₂:N₂ stream 4 h at 400 °C increases the surface area as measured by nitrogen adsorption at 77K by a factor of at least 50 to a value 52 m² g^?1. The increase in pore size was accompanied by a 9.7% weight loss. Simultaneously, the area as measured by carbon dioxide at 195K increased from 61 to 136 m² g^?1 and that measured by carbon dioxide at room temperature increased from 125 to 237 m² g^?1. Attempts to enlarge the pores by oxidation with hydrogen peroxide or ozone were unsuccessful. A Pittsburgh coal subject to a small percentage of oxygen in nitrogen or steam at 300 to 400 °C showed a surface area as measured by nitrogen adsorption of less than 1 m² g^?1 both before and after such pretreatment. This same coal with a 5:95 O₂:N₂ stream for 4 h at 450 °C showed a surface area of 110 m² g^?1 measured by nitrogen adsorption at 77K.     

Towards the design of non-amyloidogenic proteins

Glucose oxidase was immobilized onto poly(2-hydroxyethyl methacrylate) (pHEMA) membranes by two methods: by covalent bonding through epichlorohydrin and by entrapment between pHEMA membranes. The highest immobilization efficiency was found to be 17.4% and 93.7% for the covalent bonding and entrapment, respectively. The K_m values were 5.9 mmol dm^?3, 8.8 mmol dm^?3 and 12.4 mmol dm^?3 for free, bound and entrapped enzyme, respectively. The V_max values were 0.071 mmol dm^?3 min^?1, 0.067 mmol dm^?3 min^?1 and 0.056 mmol dm^?3 min^?1 for free, bound and entrapped enzyme. When the medium was saturated with oxygen, K_m was not significantly altered but V_max was. The optimum pH values for the free, covalently-bound and entrapped enzyme were determined to be 5, 6, and 7, respectively. The optimum temperature was 30°C for free or covalently-bound enzyme but 35°C for entrapped enzyme. The deactivation constant for bound enzyme was determined as 1.7 × 10^?4 min^?1 and 6.9 × 10^?4 min^?1 for the entrapped enzyme.     

Chelate membrane from poly(vinyl alcohol)/poly(n-salicylidene allyl amine) blend. II. Effect of co(II) content on oxygen/nitrogen separation

Facilitated transport of oxygen was performed through chelate membranes containing cobalt with selective oxygen binding ability as a fixed oxygen carrier. Chelate membranes were obtained from Schiff base membranes after treating a poly(allyl amine) (PAAm) and poly(vinyl alcohol) (PVA) blend with salicylaldehyde. It is confirmed that the O? O stretching peak through a frequency change in FTIR could be seen at 1150 cm^?1 between cobalt in the membrane and incoming oxygen. The permeability of oxygen through Schiff base membranes was 2.01?2.98 × 10^?13 [cm³ (STP) cm²/cm s cmHg] and oxygen permselectivity was in the range of 1.83?3.27. For chelate membranes, both the permeability of oxygen and oxygen selectivity increased to 2.15?2.82 × 10^?12 [cm³ (STP) cm²/cm s cmHg] and around 8, respectively. Permselectivity of chelate increased as a result of facilitation of O₂ and inhibition of N₂ transport. Detailed results and the mechanism of facilitation of oxygen are discussed on the basis of molecular interactions. © 1995 John Wiley & Sons, Inc.     

The diffusivity of oxygen in dilute alkaline solution from 0° to 65°C

Diffusion coefficients for oxygen in 0·1 M sodium hydroxide solution have been determined from 0° to 65°C using a rotating platinum electrode. The results may be represented with a standard deviation of approximately 1% by the expression:

where D_o = 8·03 × 10^?3cm²s^?1 and the apparent activation energy, Q_D, is 3·49 kcal mol^?1. Between 25° and 65°C the product of the diffusivity and the solution viscosity is essentially constant:

Values for diffusion coefficients for oxygen in pure water have been derived from these data with an absolute error probably less than 4%.Difficulties in obtaining reproducible results at a rotating platinum electrode are attributed to deactivation of the electrode by reduction of a surface oxygen phase. Results at the higher temperatures indicate that methods of determining diffusivities by diffusion through a stagnant layer of solution involve an increasing indeterminate error as the temperature rises, due to convective mass transport.     

High oxide ion conduction in sintered oxides of the system Bi2O3-Y2O3

The ionic conduction in sintered Bi₂O₂-Y₂O₃ was investigated by measuring the conductivity and the emf of an oxygen concentration cell using the specimen tablet as electrolyte. The face centred cubic phase in this system was found to show high oxide ion conduction accompanied by a little electronic conduction when exposed to air. This phase was stable with a composition of 25 ～ 43 mol % Y₂O₃ over a wide range of temperatures, and the oxide ion conductivity increased with decrease in Y₂O₃. The conductivities of (Bi₂O₃)_0.75 (Y₂O₃)_0.25 were 1.6×10^?1 Ω^?1 cm^?1 at 700°C and 1.2×10^?2 Ω^?1 cm^?1 at 500°C values which are many times higher than those of stabilized zirconia (ZrO₂)_0.90(Y₂O₃)_0.10 at corresponding temperatures. Specimens containing less than 25 mol % Y₂O₃ showed a phase transition at 700 ～ 580°C and the conductivities decreased remarkably below these temperatures. High oxide ion conduction in the fcc phase is attributed to the migration of oxide ion vacancies which were present in an appreciable amount.     

Oxygen transport,thermal and electrochemical properties of NdBa0.5Sr0.5Co2O5+δ cathode for SOFCs

In this study, the crystal structure, thermal, oxygen transport, electrical conductivity and electrochemical properties of the perovskite NdBa_0.5Sr_0.5Co₂O_5+δ (NBSC55) are investigated. In the temperature range of 250 °C–350 °C, the weight loss upon heating was due to a partial loss of lattice oxygen and along with a reduction of Co⁴⁺ to Co³⁺. The tend of weight-loss slows down as temperature increased above 350 °C indicating a reduction of Co³⁺ to Co²⁺ during this stage. The oxygen migration is dominated by surface exchange process at high temperature range (650-800 °C); however, the bulk diffusion process prevails at low temperature range (500–600 °C). For long-term testing, the polarization resistance of NBSC55 increases gradually form 3.13 Ω cm² for 2 h to 3.34 Ω cm² for 96 h at 600 °C and an increasing-rate for polarization resistance is around 0.22% h^?1. The power density of the single cell with NBSC55 cathode reached 341 mW cm^?2 at 800 °C.     

Wet oxidation pretreatment of lignocellulosic residues of sugarcane,rice, cassava and peanuts for ethanol production

Wet oxidation (WO) pretreatment of sugarcane bagasse, rice hulls, cassava stalks and peanut shells was investigated. WO was performed at 195 °C for 10 min, with 2 g kg^?1 of Na₂CO₃ and under either 3 or 12 bar of oxygen. Oxygen pressure and the type of raw material used had a major effect on the fractionation of the materials, formation of sugars and by‐products, and cellulose enzymatic convertibility. Cellulose content in the solid fraction increased after pretreatment of all materials, except rice hulls. The greatest increase, from 361 g kg^?1 to almost 600 g kg^?1, occurred for bagasse. The solubilisation of individual components was different for each material. Bagasse xylan was solubilised to a large extent, and 45.2% of it was recovered as xylose and xylo‐oligosaccharides in the liquid fraction. In the prehydrolysates of rice hulls around 40% of the original glucan was recovered as gluco‐oligosaccharides, due to hydrolysis of starch contained in grain remains. The formation of by‐products was modest for all the materials, but increased with increasing oxygen pressure. The highest yield of acetic acid (34–36 g kg^?1 of raw material) and furfural (0.7–1.8 g kg^?1) occurred for bagasse. The pretreatment enhanced the enzymatic convertibility of cellulose giving the best result (670.2 g kg^?1) for bagasse pretreated at the highest oxygen pressure. However, for the other materials the pretreatment conditions were not effective in achieving cellulose conversions above 450 g kg^?1. Some enzymatic conversion of xylan was observed. Copyright © 2007 Society of Chemical Industry     

Ammonium fumarate production by free or immobilised Rhizopus arrhizus in bench‐ and laboratory‐scale bioreactors

Ammonium fumarate production from glucose‐based media by Rhizopus arrhizus NRRL 1526 with mycelial growth controlled by phosphorus limitation exhibited mixed‐growth‐associated product formation kinetics, with growth‐associated production related to secondary mycelial growth only. The contribution of the primary mycelial growth phase was minimised by resorting to prolonged batch production using free mycelia under intermittent glucose feeding or repeated batch production using immobilised mycelia. The metabolic activity of free or immobilised mycelia was limited by fumarate accumulation or by oxygen diffusion phenomena, respectively. For batch cultures in a 15 dm³ stirred bioreactor the peripheral impeller speed (v_I) was increased from 1.88 to 3.3 m s^?1, and the fumarate yield coefficient on glucose increased from 0.25 ± 0.01 to 0.42 ± 0.02 g g^?1, while the malate yield coefficient on fumarate (Y_M/F) reduced from 0.46 ± 0.01 to 0.14 ± 0.01 g g^?1. With a net increase in the fumarate‐to‐malate ratio from 2 to 6.5, a v_I value of 3.3 m s^?1 gave the best fermentation performance and provided a basis for further scale‐up studies. © 2002 Society of Chemical Industry     

Struktur und elektrische Leitfähigkeit von polymeren organischen Halbleitern

In this review article we want to give information about low molecular and polymer organic semiconductors, which were recently synthesized in our institute. Specific electric conductivities up to σ_298°K = 9.0 · 10^?5Ω^?1 · cm^?1 and thermic activation energies of E = 0.30 eV of polyenearylenes, respectively -heteroarylenes were measured. Polyazomethines have a maximum σ_298°K = 3.3 · 10^?9Ω^?1 · cm^?1 and E = 0.35 eV. Polymers with indophenine units have conductivities up to σ_298°K = 1.1 · 10^?4Ω^?1 · cm^?1 and E = 0.39 eV. A maximum of σ_298°K = 5.0 · 10^?2Ω^?1 · cm^?1 and E = 0.05 eV was found for bis-(1.2-dicyanoethylenedithiolo)-metal salts. Polymers with a phthalocyanine- or hemiporphyrazine-like structure achieve a conductivity of σ_298°K = 2.3 · 10^?2Ω^?1 · cm^?1 and E = 0.15eV. Coordination polymers of dimercaptomaleic acid, respectively their monoamide show a maximum of σ_298°K = 3.2 · l0^?lΩ^?l · cm^?1 and E = 0.20 ev. Polymers with σ_298°K ≤1.5 · 10^?5 Ω^?l · cm^?l and E ≥ 0.5 eV were obtained by the polymerization of succinonitrile. All the investigated substances show an electronic conductivity. The existence of an ionic conductivity could, in all cases, be excluded by using direct current measurements over a long period of time.     

The Reaction of Ozone with Bisulfide (HS−) in Aqueous Solution – Mechanistic Aspects

The ozone demand to oxidize HS^?/H₂S [pK_a(H₂S) = 6.9, k(HS^? + O₃) = 3 × 10⁹ M^?1 s^?1, k(H₂S + O₃) = 3 × 10⁴ M^?1 s^?1] to SO₄ ^2? is only 2.4 mol ozone per mol SO₄ ^2? formed, much lower than stoichiometric 4.0 mol/mol if a series of O-transfer reactions would occur. As primary step, the formation of an ozone adduct to HS^?, HSOOO^–, is suggested that decomposes into HSO^– and singlet oxygen (16%) or rearranges into peroxysulfinate ion, HS(O)OO^– (84%). Potential reactions of the above intermediates are discussed. Some of these can account for the low ozone demand.     

Negative thermal expansion properties of Cu1.5Mg0.5V2O7

Cu_1.5Mg_0.5V₂O₇ was prepared by a solid state method. Its phase, microstructure, thermal expansion property, and Raman spectra were analyzed in detail. Results show that Cu_1.5Mg_0.5V₂O₇ maintains a monoclinic crystal structure and exhibits an excellent linear negative thermal-expansion property with coefficient of thermal expansion of ?8.72?×?10^?6?K^?1 over a wide temperature range of 153–673?K. The mechanism underlying the negative thermal expansion of Cu_1.5Mg_0.5V₂O₇ involves the coupling effect of the tetrahedron caused by the lateral vibration of the bridge oxygen atom and the tensile effect of the tetrahedron, The partial collapse caused by the loss of the oxygen atoms also plays an important role in the mechanism.     

Oxygen and water vapor gas barrier poly(ethylene naphthalate) films by deposition of SiOx plasma polymers from mixture of tetramethoxysilane and oxygen

SiO_x films were deposited from a mixture of tetramethoxysilane (TMOS) and oxygen on poly(ethylene 2,6‐naphthalate) film using ion‐assisted plasma polymerization technique (Method II) and conventional plasma polymerization technique (Method I), and were compared in chemical composition and gas barrier properties. Methods I and II were different in electrical circuit between electrodes (anode and cathode) and electric power supply. In Method I, the anode electrode was grounded, and the cathode electrode was coupled to the discharge power supply. In Method II, the anode electrode was connected with the discharge power supply, and the cathode electrode was grounded. There was not large difference in SiO_x deposition rate between the plasma polymerizations by Methods I and II. Plasma polymers deposited from TMOS/O₂ mixtures by Method II possessed smaller C/Si and O/Si atomic ratios than those deposited by Method I and showed advantage in gas barrier properties. The oxygen and water vapor permeation rates were 0.08–0.13 cm³ m^?2 day^?1 atm^?1 at 30°C at 90% RH and 0.244–0.276 g m^?2 day^?1 at 40°C at 90% RH, respectively. From these results, it can be concluded that the ion‐assisted plasma polymerization is a useful technique for deposition of gas barrier SiO_x thin films. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 915–925, 2007