You Cong

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

Dense tubular Ba0.5Sr0.5Co0.8Fe0.2O3-delta (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 degreesC. The oxygen vacancy diffusion coefficients (Dv) 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 It with oxygen permeation flux of 1.12 ml/(cm(2) min) at 875 degreesC. 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

High selectivity of oxidative dehydrogenation of ethane to ethylene in an oxygen permeable membrane reactor

An oxygen permeable membrane based on Ba0.5Sr0.5Co0.8Fe0.2O3-delta is used to supply lattice oxide continuously for oxidative dehydrogenation of ethane to ethylene with selectivity as high as 90% at 650 degrees C.

Stabilization of Low‐Temperature Degradation in Mixed Ionic and Electronic Conducting Perovskite Oxygen Permeation Membranes

Mixed ionic and electronic conducting (MIEC) oxides are multifunctional materials used as catalysts, superconductors, fuel-cell and battery electrodes, as well as membranes for gas separation. Among the applications, MIEC membranes with fast transport of oxygen ions at elevated temperature have received increasing attention because the membranes can potentially be used for highly pure oxygen production with low costs; they are highly efficient as membrane reactors for the selective oxidation of C1 and C2 molecules, [4g,h] water splitting for hydrogen production reactions, and CO2 capture integrated with oxy-fuel combustion technology. Despite their widespread use, the hightemperature (usually 800–1000 8C) operation of the membrane modules requires special sealants sustaining highpressure gradients and special stainless steel resisting high temperatures and oxidation, which have become the bottleneck technology of the application of MIEC membrane technology. Great energy consumption and high investment on high-temperature membrane modules count against the reduction of the costs for oxygen production. The bottleneck technology is easy to be overcome, as well as the energy consumption and the investment can be significantly reduced at low-temperature (LT; 350–650 8C) operation. Furthermore, many catalytic oxidation reactions of C3–C6 organic molecules can be performed in the MIEC membrane reactors with high efficiency at LT. However, most MIEC materials suffer from large and irreversible performance losses during long-term operation at LT that limit their commercial application. This problem is particularly acute for many of the recent higher-performance perovskite-based MIEC materials such as Ba0.5Sr0.5Co0.8Fe0.2O3 d (BSCF) [4a,9] and La1 xSrxCo1 yFeyO3 d (LSCF, 0 x 1, 0 y 1) that have been designed for low-temperature fuel cells, gas separations, and catalytic processes. Up to now, it is still difficult to stabilize oxygen permeation fluxes of MIEC membranes at LT. In general, degradation processes of materials become more severe with increasing temperature. Thus, the fact that MIEC membrane degradation becomes more severe at lower temperatures stands in stark contrast to the conventional wisdom. Twomain mechanisms have been proposed to account for the LT degradation in perovskite MIEC membranes. The first is that oxygen vacancy ordering occurs in these materials during sustained operation at LT, which severely decreases their ionic conductivity; the second is that LT operation introduces thermodynamic driving forces that favor kinetic demixing in these materials under oxygen permeation conditions, producing new phases with low ionic conductivity or undesirable changes to the surface elemental composition and morphology. However, these two proposed mechanisms cannot explain why degradation is observed even in many otherwise structure-stable membranes, for example BaZrxCoyFe1 x yO3 d. [4b,12]

Continuous Oxygen Ion Transfer Medium as a Catalyst for High Selective Oxidative Dehydrogenation of Ethane

An oxygen permeable mixed ion and electron conducting membrane (OPMIECM) was used as an oxygen transfer medium as well as a catalyst for the oxidative dehydrogenation of ethane to produce ethylene. O2- species transported through the membrane reacted with ethane to produce ethylene before it recombined to gaseous O2, so that the deep oxidation of the products was greatly suppressed. As a result, 80% selectivity of ethylene at 84% ethane conversion was achieved, whereas 53.7% ethylene selectivity was obtained using a conventional fixed-bed reactor under the same reaction conditions with the same catalyst at 800 °C. A 100 h continuous operation of this process was carried out and the result indicates the feasibility for practical applications.

Partial oxidation of ethane to syngas in an oxygen-permeable membrane reactor

A perovskite-type oxide of Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCFO) with mixed electronic and oxygen ionic conductivity at high temperatures was used as an oxygen-permeable membrane. A tubular membrane of BSCFO made by extrusion method has been used in the membrane reactor to exclusively transport oxygen for the partial oxidation of ethane (POE) to syngas with catalyst of LiLaNiO/gamma-Al2O3 at temperatures of 800-900 degreesC. After only 30 min POE reaction in the membrane reactor, the oxygen permeation flux reached at 8.2 ml cm(-2) min(-1). After that, the oxygen permeation flux increased slowly and it took 12 h to reach at 11.0 ml cm(-2) min(-1). SEM and EDS analysis showed that Sr and Ba segregations occurred on the used membrane surface exposed to air while Co slightly enriched on the membrane surface exposed to ethane. The oxygen permeation flux increased with increasing of concentration of C2H6, which was attributed to increasing of the driving force resulting from the more reducing conditions produced with an increase of concentration of C2H6 in the feed gas. The tubular membrane reactor was successfully operated for POE reaction at 875 degreesC for more than 100 h without failure, with ethane conversion of similar to 100%, CO selectivity of >91% and oxygen permeation fluxes of 10-11 ml cm(-2) min(-1)

Oxidative dehydrogenation of propane in a dense tubular membrane reactor

Oxidative dehydrogenation of propane (ODP) to propylene was investigated in a dense tubular membrane reactor made of Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) at 700oC and 750oC. The propylene selectivity in the membrane reactor (44.2%) is much higher than that in the fixed-bed reactor (15%) at the similar propane conversion (23-27%). Higher propylene selectivity in the membrane reactor was attributed to the lattice oxygen (O2-) supplied through the membrane.

A Direct Ammonia Tubular Solid Oxide Fuel Cell

Abstract A Ni-anode-supported tubular solid oxide fuel cell (SOFC) with a dense 8% yttria-stabilized zirconia (YSZ) thin film was fabricated. The YSZ film was prepared using the vacuum-assisted dip-coating method. The performance of the single SOFC-based system running on ammonia was compared with that running on hydrogen. At 800°C, the peak power densities were 202 and 200 mW/cm 2 using hydrogen and ammonia as the fuel, respectively. A GC analysis of the effluent from the anode showed considerable amounts of nitrogen and hydrogen, but no trace of NO x , when using ammonia as the fuel. The ammonia performance is comparable to hydrogen, suggesting that ammonia can be an attractive alternative fuel.

Low temperature synthesis of perovskite oxide using the adsorption properties of cellulose

La0.8Sr0.2CoO3 (LSCO) oxide powder was prepared using the adsorption properties of cellulose. The preparation process was studied by XRD, FTIR, TG-DTA and CO2-TPD techniques. The results of XRD, IR and TG-DTA testified that cellulose could successfully reserve the homogeneity of the solution system to the solid precursor. During the early stage of pyrolysis, cellulose was partially oxidized, and some COO− groups appeared in its texture, which were then complexed with the adsorbed metal ions, and effectively suppressed the aggregation of metal ions. Formation of a pure perovskite and the properties of the powder resulted were found to be significantly influenced by the cellulose to metal nitrate ratio. Also the properties of the resulting powder were greatly influenced by the calcination conditions. If the produced carbon dioxide could not be eluted in time, carbonate would be formed in the bulk. Hence, a high calcination temperature (>800°C) was needed to acquire a pure phase LSCO. At optimized conditions, nano-crystal LSCO could be obtained at as low as 500°C.

Remarkable dependence of electrochemical performance of SrCo0.8Fe0.2O3-δ on A-site nonstoichiometry

SrCo(0.8)Fe(0.2)O(3-δ) is a controversial material whether it is used as an oxygen permeable membrane or as a cathode of solid oxide fuel cells. In this paper, carefully synthesized powders of perovskite-type Sr(x)Co(0.8)Fe(0.2)O(3-δ) (x = 0.80-1.20) oxides are utilized to investigate the effect of A-site nonstoichiometry on their electrochemical performance. The electrical conductivity, sintering property and stability in ambient air of Sr(x)Co(0.8)Fe(0.2)O(3-δ) are critically dependent on the A-site nonstoichiometry. Sr(1.00)Co(0.8)Fe(0.2)O(3-δ) has a single-phase cubic perovskite structure, but a cobalt-iron oxide impurity appears in A-site cation deficient samples and Sr(3)(Co, Fe)(2)O(7-δ) appears when there is an A-site cation excess. It was found that the presence of the cobalt-iron oxide improves the electrochemical performance. However, Sr(3)(Co, Fe)(2)O(7-δ) has a significant negative influence on the electrochemical activity for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The peak power densities with a single-layer Sr(1.00)Co(0.8)Fe(0.2)O(3-δ) cathode are 275, 475, 749 and 962 mW cm(-2) at 550, 600, 650 and 700 °C, respectively, values which are slightly lower than those for Sr(0.95)Co(0.8)Fe(0.2)O(3-δ) (e.g. 1025 mW cm(-2) at 700 °C) but much higher than those for Sr(1.05)Co(0.8)Fe(0.2)O(3-δ) (e.g. only 371 mW cm(-2) at 700 °C). This remarkable dependence of electrochemical performance of the Sr(x)Co(0.8)Fe(0.2)O(3-δ) cathode on the A-site nonstoichiometry reveals that lower values of electrochemical activity reported in the literature may be induced by an A-site cation excess. Therefore, to obtain a high performance of Sr(x)Co(0.8)Fe(0.2)O(3-δ) cathode for IT-SOFCs, an A-site cation excess must be avoided.

Mixed-conducting perovskite-type SrxBi1-xFeO3_delta oxygen-permeating membranes

SrxBi1-xFeO3-delta (SBF) series mixed conductors were synthesized using standard ceramic method. The properties of such materials were characterized by XRD, O-2-TPD techniques. Abnormal crystal phenomena were found and explained and correlated with the oxygen permeation results. By analysis of the critical radius (r(c)), the degree of openness of the lattice (F-v) and the average metal-oxygen bonding energy of the perovskite lattice (ABE), it was proposed that the oxygen permeation flux is determined mainly by the oxygen diffusion rate in bulk when 1-x less than or equal to 0.5, and by the concentration of oxygen vacancy when 1-x greater than or equal to 0.5. The stability of Sr0.5Bi0.5FeO3-delta was also investigated, and the high stability of it was attributed to the stable BO6 octahedra.