Julio Garcia-Fayos

Enhancing oxygen permeation through hierarchically-structured perovskite membranes elaborated by freeze-casting

Innovative asymmetric oxygen-transport membrane architectures were prepared by combining freeze-casting and film deposition techniques. Freeze-casting enabled the optimization of the gas transport through the support by creating a hierarchical porosity while a dense top-layer of 30 μm was coated over this support by screen printing. The versatility of this technique was demonstrated by preparing highly porous bodies made of fast ionic conductors, e.g. perovskites and doped ceria fluorites, with a large number of applications in catalysis, electrochemistry and gas separation. Permeation tests using an all-La0.6Sr0.4Co0.2Fe0.8O3−δ asymmetric membrane proved the beneficial effect of such porous supports over the O2 fluxes with a maximum value of 6.8 mL min−1 cm−2 at 1000 °C, markedly above the results achieved so far with conventional preparation techniques. Gas permeance study through the porous freeze-cast support showed that the particular pore structure allows the gaseous transport resistance to be minimized. The related pressure drop is found to be very low in comparison with conventional porous supports, e.g. tape-cast supports, with for example only 0.59 bar mm−1 with argon at 800 °C for an inlet flow of 400 mL min−1 cm−2. Finally, the stability of the asymmetric membrane has been evaluated under CO2 atmosphere during 48 hours and at 900 °C. The membrane is found to be stable without deactivation nor decrease in the O2 permeation flux.

Enhanced Oxygen Separation through Robust Freeze‐Cast Bilayered Dual‐Phase Membranes

Dual-phase oxygen-permeable asymmetric membranes with enhanced oxygen permeation were prepared by combining freeze-casting, screen-printing, and constraint-sintering techniques. The membranes were evaluated under oxyfuel operating conditions. The prepared membranes are composed of an original ice-templated La(0.6)Sr(0.4)Co(0.2)Fe(0.8)O(3-δ) support with hierarchically oriented porosity and a top fully densified bilayered coating comprising a 10 μm-thick La(0.6)Sr(0.4)Co(0.2)Fe(0.8)O(3-δ) layer and a top protective 8 μm-thick layer made of an optimized NiFe2O4/Ce(0.8)Tb(0.2)O(2-δ) composite synthesized by the one-pot Pechini method. Preliminary analysis confirmed the thermochemical compatibility of the three involved phases at high temperature without any additional phase detected. This membrane exhibited a promising oxygen permeation value of 4.8 mL min(-1)  cm(-2) at 1000 °C upon using Ar and air as the sweep and feed gases, respectively. Mimicking oxyfuel operating conditions by switching argon to pure CO2 as a sweep gas at 1000 °C and air as feed enabled an oxygen flux value of 5.6 mL min(-1)  cm(-2) to be reached. Finally, under the same conditions and increasing the oxygen partial pressure to 0.1 MPa in the feed, the oxygen permeation reached 12 mL min(-1)  cm(-2). The influence of CO2 content in the sweep gas was studied and its reversible and positive effect over oxygen permeation at temperatures equal to or above 950 °C was revealed. Finally, the membrane stability over a period of 150 h under CO2-rich sweep gas showed a low degradation rate of 2.4×10(-2)  mL min(-1)  cm(-2) per day.

Ethylene Production by ODHE in Catalytically Modified Ba0.5Sr0.5Co0.8Fe0.2O3−δ Membrane Reactors†

Process intensification by the integration of membranes and high-temperature reactors offers several advantages with regard to conventional process schemes, that is, energy saving, safe operation, reduced plant/unit size, and higher process performance, for example, higher productivity, catalytic activity, selectivity, or stability. We present the study of oxidative dehydrogenation of ethane at 850 °C on a catalytic membrane reactor based on a mixed ionic-electronic conducting membrane. The surface of the membrane made of Ba(0.5)Sr(0.5)Co(0.8)Fe(0.2)O(3-δ) has been activated by using different porous catalytic layers based on perovskites. The layer was deposited by screen printing, and the porosity and thickness was studied for the catalyst composition. The different catalyst formulations are based on partial substitution of A- and B-site atoms of doped strontium ferrite/cobaltites (A(0.6)Sr(0.4)Co(0.5)Fe(0.5)O(3-δ) and Ba(0.6)Sr(0.4)BO(3-δ)) and were synthesized by an ethylenediaminetetraacetic acid-citrate complexation route. The use of a disk-shaped membrane in the reactor enabled the direct contact of gaseous oxygen and hydrocarbons to be avoided, and thus, the ethylene content increased. High ethylene yields (up to ≈81 %) were obtained by using a catalytic coating based on Ba(0.5)Sr(0.5)Co(0.8)Fe(0.2)O(3-δ), which included macropores produced by the addition of graphite platelets into the screen-printing ink. The promising catalytic results obtained with this catalytically modified membrane reactor are attributed to the combination of 1) the high activity, as a result of the high temperature and oxygen species diffusing through the membrane; 2) the control of oxygen dosing and the low concentration of molecules in the gas phase; and 3) suitable fluid dynamics, which enables appropriate feed contact with the membrane and the rapid removal of products.

Rare Earth‐doped Ceria Catalysts for ODHE Reaction in a Catalytic Modified MIEC Membrane Reactor

An intensification process for the selective oxidation of hydrocarbons integrates a catalytic reactor and an oxygen separation membrane. This work presents the study of oxidative dehydrogenation of ethane at 1123 K in a catalytic membrane reactor based on mixed ionic‐electronic conducting (MIEC) membranes. The surface of a membrane made of Ba0.5Sr0.5Co0.8Fe0.2O3−δ has been activated using different porous catalytic layers based on rare earth‐doped cerias (fluorite structure) and the porous catalytic coating was deposited by screen printing (coating around 15 μm). The different catalyst formulations were developed by partial substitution of Ce and were synthesized by co‐precipitation followed by cobalt impregnation when required. Specifically, seven different catalysts based on the system Ce1−xLnxO2−δ (x=0.1 or 0.2; Ln=Tb, Pr, Er, Gd, and Tb+Er), including the effect of cobalt addition (2 % molar) in Ce0.8Tb0.2O2−δ, were studied. The ceria catalysts were studied by XRD, SEM, DC‐conductivity as a function of oxygen partial pressure, and the high‐temperature stability in a CO2 environment was assessed using thermogravimetry. Then, the influence of the ceria catalytic coating on the oxygen permeation flux through the MIEC membrane was studied using argon and methane as the sweep gas in the permeate side. Finally, oxidative dehydrogenation of ethane reaction tests were performed at 1123 K, as a function of the ethane concentration in the feed. The use of a disk‐shaped membrane in the reactor made it possible to prevent the direct contact of gaseous oxygen and hydrocarbons and thus to increase the ethylene yield. High ethylene yields (up to ≈84 %) were obtained using a catalytic coating based on 20 % Tb‐doped ceria including macropores produced by the addition of graphite platelets in the screen printing ink. The high yields obtained in this kind of catalytic membrane are attributed to the combination of: the high catalytic activity; the control of the oxygen concentration in the gas phase (reaction chamber); and the appropriate fluid dynamics, enabling the fast ethylene evacuation.

Dual‐Phase Oxygen Transport Membranes for Stable Operation in Environments Containing Carbon Dioxide and Sulfur Dioxide

Dual-phase membranes are appealing candidates for oxygen transport membranes owing to their unique combination of ambipolar electron-ion transport and endurance. However, O2 separation in industrial environments demands very high stability and effectiveness in the presence of CO2- and SO2-bearing process gases. Here, the composition of dual-phase membranes based on NiFe2O4-Ce(0.8) Tb(0.2)O(2-δ) (NFO-CTO) was optimized and the effective performance of catalytically-activated membranes was assessed in presence of CO2 and SO2. Further insight into the limiting mechanisms in the permeation was gained through electrical conductivity studies, permeation testing in several conditions and impedance spectroscopy analysis. The dual-phase membranes were prepared by one-pot sol-gel method and their permeability increases with increasing fluorite content. An O2 flux of 0.25 (ml min(-1)  cm(-2)) mm at 1000 °C was obtained for a thick self-standing membrane with 40:60 NFO/CTO composition. An in-depth study mimicking typical harsh conditions encountered in oxyfuel flue gases was performed on a 50:50 NFO/CTO membrane. CO2 content as well as SO2 presence in the sweep gas stream were evaluated in terms of O2 permeation. O2 fluxes of 0.13 and 0.09 mL min(-1)  cm(-2) at 850 °C were obtained for a 0.59 mm thick membrane under CO2 and 250 ppm SO2 in CO2 sweep conditions, respectively. Extended periods at work under CO2- and SO2-containing atmospheres revealed good permeation stability over time. Additionally, XRD, backscattered electrons detector (BSD)-SEM, and energy-dispersive X-ray spectroscopy (EDS) analysis of the spent membrane confirmed material stability upon prolonged exposure to SO2.

Enhancing oxygen permeation through Fe2NiO4–Ce0.8Tb0.2O2−δ composite membranes using porous layers activated with Pr6O11 nanoparticles

Fe2NiO4–Ce0.8Tb0.2O2−δ (NFO–CTO) composite membranes are of interest to separate oxygen from air. In this study, we investigate the influence of the catalytic activation of NFO–CTO membranes on the oxygen permeation rate. Specifically, the effect of activating porous NFO–CTO layers –coated on both sides of the dense NFO–CTO membrane – with Pr6O11 nanoparticles is studied. Measurements in the temperature range 850–700 °C revealed a 2–4 fold increase in the oxygen flux after coating a 30 μm-thick porous NFO–CTO layer on both membrane sides, and a 6–12 fold increase relative to the bare membrane after activating the porous layers coated on both sides of the membrane with Pr6O11 nanoparticles. No degradation of the oxygen fluxes was found in CO2-containing atmospheres. Pulse isotopic exchange measurements confirmed an increase in the oxygen surface exchange rate of more than one order of magnitude after dispersion of Pr6O11 nanoparticles on the surface of NFO–CTO composite powders. Electrochemical impedance spectroscopy measurements on symmetrical cells, using Gd-doped ceria (CGO) as the electrolyte and Pr6O11-activated NFO–CTO electrodes, showed a 10-fold decrease in the polarization resistance compared to non-infiltrated electrodes in air. Modification of porous layers by activation with Pr6O11 nanoparticles is considered a viable route to enhance the oxygen fluxes across composite membranes.

Mixed Ionic-Electronic Conduction in NiFe2O4-Ce0.8Gd0.2O2−δ Nanocomposite Thin Films for Oxygen Separation

NiFe2 O4 -Ce0.8 Gd0.2 O2-δ (NFO/CGO) nanocomposite thin films were prepared by simultaneously radio-frequency (RF) magnetron sputtering of both NFO and CGO targets. The aim is the growth of a CO2 -stable composite layer that combines the electronic and ionic conduction of the separate NFO and the CGO phases for oxygen separation. The effect of the deposition temperature on the microstructure of the film was studied to obtain high-quality composite thin films. The ratio of both phases was changed by applying different power to each ceramic target. The amount of each deposited phase as well as the different oxidation states of the nanocomposite constituents were analyzed by means of X-ray photoelectron spectroscopy (XPS). The transport properties were studied by conductivity measurements as a function of temperature and pO2 . These analyses enabled (1) selection of the best deposition temperature (400 °C), (2) correlation of the p-type electronic behavior of the NFO phase with the hole hopping between Ni3+ -Ni2+ , and (3) following the conductivity behavior of the grown composite layer (prevailing ionic or electronic character) attained by varying the amount of each phase. The sputtered layer exhibited high ambipolar conduction and surfaceexchange activity. A 150 nm-thick nanograined thin film was deposited on a 20 μm-thick Ba0.5 Sr0.5 Co0.8 Fe0.2 O3-δ asymmetric membrane, resulting in up to 3.8 mL min-1  cm-2 O2 permeation at 1000 °C under CO2 atmosphere.