Patent Description:
Oxy-combustion consists in the use of a high-purity O<NUM> stream as comburent instead of air, as is done in conventional combustion processes, thereby achieving higher flame temperatures with lower fuel consumption and thus improving the combustion. The use of oxygen-rich comburents makes it possible to obtain combustion gases with a composition consisting mainly of CO<NUM> and water steam. The high CO<NUM> concentration of the exhaust gases in the oxy-combustion process facilitates the potential separation of the same. In fact, this oxy-combustion process in thermal or intensive energy plants makes it possible to produce electrical energy or industrial products from fossil fuels by minimizing the emission of CO<NUM>, being technologically and economically feasible thanks to its integration with capture and CO<NUM> storage technologies. These processes have high energy efficiencies, what reduces fuel consumption and reduces the size of industrial units and equipment. Likewise, in the case of combustion only with oxygen instead of air, and not feeding N<NUM> to the furnace, reactor or boiler, it is possible to considerably reduce the NOx emissions. Therefore, this process has the advantage of facilitating the CO<NUM> separation and capture, which can subsequently be liquefied, conveyed and stored, or used in other industrial processes. This combustion process concept allows to minimize CO<NUM> and NOx emissions, as well as to substantially increase the energy efficiency of the process. Examples of intensive industries in the use of energy that require the use of oxygen are the glass industry, incinerator plants, frits manufacturing, glazes, paining industry and metallurgy, steel, chemical, refining and petrochemical industries, enamels and ceramic colors. One of the industrial sectors in which the use of oxygen makes oxy-combustion possible is the melting of glass and the manufacture of frits, enamels and ceramic colors. In this type of industries, the need to reach temperatures above <NUM> inside the furnaces, in order to melt the mixture of raw materials that is introduced, is achieved using oxygen instead of air in the natural gas burners.

Essentially, oxy-combustion consists of the combustion of a fuel with oxygen. The oxygen which is fed to the furnace, the reactor or the boiler, under conditions of high purity, is previously obtained by a separation process of oxygen from the air.

Oxygen membranes can also be applied in air enrichment, therefore the oxygen concentration of <NUM>% is raised to higher values, typically above <NUM>%. This increase in concentration is necessary in certain combustion or chemical conversion processes in which the calorific power of the product to be treated, generally a fuel, is insufficient to maintain adequate operating conditions. A typical example of enrichment is the use in cement plants that use alternative fuels or that incinerate residues during the clinker manufacturing.

Oxy-combustion aims to be one of the most economical technologies for the capture of CO<NUM>, being its main drawback the need of high O<NUM> demand and the cost involved in obtaining this O<NUM>. The great challenge of this technology lies in the O<NUM> production in order to achieve the supply of the high amounts thereof that are required.

Currently, the only available technologies on an industrial scale capable of producing large O<NUM> volumes are the cryogenic distillation of air, and absorption facilities in columns of solid absorbers (PSA-VPSA), the later with lower production and generation capacities, with lower oxygen purities, generally less than <NUM>% by volume. The drawback of cryogenic distillation of air is its high energy consumption. In the case of a thermal power plant, this consumption can reach <NUM>% of its electricity production, penalizing the overall efficiency of the plant by <NUM>%. A very interesting alternative with which it is expected to reduce the overall loss of efficiency in the oxycombustion plant up to <NUM>% is the use of dense ceramic membranes based on conductive materials of oxygen ion, that allow to separate oxygen at high temperature with a <NUM>% theoretical purity.

The transport of oxygen through the membranes requires temperatures above <NUM> to achieve technically competitive permeabilities. The transport of the oxygen ion is simultaneous to the transport of electrons or electron gaps (electronic carriers), consequently the material must possess sufficient electronic conductivity under the operating conditions of the membrane. The driving force responsible for transporting oxygen through the membrane is the difference in oxygen partial pressure between both sides of the membrane. Thus, oxygen flow through a membrane is determined by the temperature and the partial pressure difference of the oxygen in addition to the thickness of the membrane.

Another crucial step in the process of oxygen separation in ionic transport membranes is the gas exchange. As it was mentioned, the transport through the selective separation layer consists of the diffusion of oxygen ions and electronic carriers. Therefore, two surface reactions are necessary, a first one in which the oxygen gas is adsorbed and transformed into oxygen ions on the surface of the membrane exposed to the feed gases, generally compressed air, and a second one, in which the oxygen ions are transformed into molecular oxygen and desorbed. For various reasons, these transport steps may be limiting and cause a decrease in permeation flux across the membrane. Among the different possible reasons, we can highlight: (<NUM>) the thickness of the selective separation layer is very small, so diffusion through the solid is much faster than the gas exchange. Typically, this critical dimension is called the "characteristic length" and is the quotient between the diffusion coefficient and the kinetic constant of the surperficial gaseous exchange reaction, under the conditions of operation and composition of gases in contact with the membrane surface. (<NUM>) The membrane surface does not have significant catalytic activity for the oxygen activation reaction. (<NUM>) The gaseous atmospheres in contact with the membrane surface or surfaces disfavor the adsorption/desorption of the molecular oxygen and its evolution through the reaction O<NUM> + 2e- ↔ O-<NUM>. In processes that are relevant from an industrial point of view, both the permeate and the feed tend to have significant amounts of acid gases such as CO<NUM> and SO<NUM>, which make the reaction difficult because they passivate or inactivate the surface and compete with the adsorption and reaction centers involved in the gaseous oxygen exchange reaction. This damaging effect is accentuated as the operating temperature of the process is lowered, especially below <NUM>, and when the concentration of SO<NUM> and CO<NUM> is increased. The effect of SO<NUM> gas is especially negative is, since concentrations above <NUM> ppm produce severe effects on membrane permeation and produce irreversible degradations in many materials, which result in the permanent and irreparable damage of the membrane.

The pressure difference between both sides of the membrane can be achieved by two actions: (a) increasing the air pressure through compression stages; and/or (b) by lowering the oxygen partial pressure, what is possible by applying vacuum or by diluting the oxygen in the permeate by an entrainment gaseous stream. This last option usually consists of recirculating the exhaust gases from the furnace or combustion boiler, increasing at the same time the operating temperature. Also, in line with the second option, it is possible to pass a reducing gas (usually methane or other hydrocarbons) which consumes the oxygen permeating through the membrane to give complete or partial combustion products and to directly release heat in contact with the ceramic membrane.

Sulfur is present to a greater or lesser extent in practically all fuels, depending on their origin, nature and previous refining and/or purification. Its combustion produces SO<NUM> in variable concentrations and it is usually in contact - in one way or another - with one or both surfaces of the membranes in combustion processes using membranes. Therefore, to ensure the stability and effectiveness in the oxygen production of these membranes in atmospheres containing SO<NUM>, is a technological aspect to be taken into account for their industrial use.

<FIG> shows a diagram of a process having an oxy-combustion furnace and membrane module wherein oxygen is separated from compressed air by applying vacuum to the permeate part. In this process it is necessary to make a fuel supply to the compressed air to reach the necessary temperature for the compressed air to reach the necessary temperature for the operation of the module operation. In this process configuration it is necessary that the membrane operates properly in contact with the combustion gases (mainly CO<NUM>, H<NUM>O and SO<NUM>) mixed with air. In other processes, part of the combustion gases from the furnace, boiler or reactor are used as entrainment gas of the permeated oxygen, and in this case the CO<NUM>, H<NUM>O and SO<NUM> concentration of the gases in contact with the membrane may be higher.

The non-porous selective separation layer in this type of dense ceramic membranes is generally composed of a mixed conductive material of electrons and solid-state oxygen ions with a structure of the perovskite family, including alkaline earth elements, rare earths and transition metals, such as iron and cobalt, in its crystalline structure. These oxides have oxygen deficiency in their structure and it is just the presence of vacant oxygen positions in their network what makes the mechanism of diffusion of the oxygen ion through the crystalline structure possible. The most commonly used materials for this application currently have crystalline structures of perovskite type, with compositions such as Lao. <NUM>Sr<NUM>Feo. <NUM>O<NUM>-δ or Bao. <NUM>Fe<NUM>Coo. <NUM>O<NUM>-δ. However, the big problem of this type of materials is their low stability when being subjected for long time periods to oxygen concentration gradients and, mainly, to being subjected to the presence of CO<NUM> under the operating conditions, generally producing alkaline earth elements carbonates (carbonation phenomenon). Another type of ionic ceramic membranes are those formed by the mixture of two types of crystalline phases, one that predominantly carries oxygen ions and the other one which predominantly carries electrons or electron voids. For example, it has recently been reported that the combination of gadolinium-doped cerium oxide with a spinel, free of cobalt and alkaline-earth metal, such as Fe<NUM>NiO<NUM>, has given rise to a promising material with regard to its oxygen flow (<NPL>.

Other document of the state of the art are:
<NPL>, that discloses a porous NFO-CTO support as Example <NUM> of the present application, and different analytical methods applied to it, such as Raman, SEM analysis, XRD and EDS to proof the behavior of this material.

<NPL>, that discloses the preparation and properties of the perovskite oxide La<NUM>Sr<NUM>Co<NUM>Fe<NUM><NUM><NUM>-δ (LSCF6428), an electronic-ionic conductor that could be used as cathode of solid oxide fuel cell (SOFC).

<CIT> that refers to a catalyst suitable for the treatment of automotive engine exhaust exhibiting enhanced oxygen storage capacity. The catalyst composition contains, in addition to a catalytic material such as one or more of platinum, rhodium and palladium dispersed on an activated alumina support, an oxygen storage component ("OSC") which is an intimately mixed oxide of ceria and praseodymia having a Pr:Ce atomic ratio in the range of about <NUM>:<NUM> to <NUM>:<NUM> and optionally containing one or more other rare earth metal oxides.

<CIT>, that discloses a composite membrane for selective gas separation, comprising a layer system having a through-and-through porous, mechanically stable carrier layer, which has an average pore size in the [mu]m range, further having at least one through-and-through porous intermediate layer, which is disposed on the carrier layer and has an average pore size in the range between <NUM> and <NUM>, and further having a gas-tight functional layer, which is disposed on the intermediate layer and is made of mixed-conductive material having a maximum layer thickness of <NUM> [mu]m. This functional layer comprises a perovskite, a fluorite, or a material having a K<NUM>NiF<NUM> structure, such as La<NUM>-xSrxCo<NUM>-yFeyO<NUM>-δ.

The document<NPL>" discloses a catalytic mixed ionic-electronic conducting (MIEC) membranes. The surface of the membrane made of BSCFO is activated using different porous catalytic layers based on rare earth-doped cerias (fluorite structure) while the porous catalytic coating was deposited by screen printing (coating around <NUM>). However, in addition to other differences, the CTO portion disclosed by this document does not have the properties of the CTO portion according to the present invention either acting as porous support structure or acting as catalytic coating. For their practical use, oxygen separation membranes at high temperature through ion transport are generally formed by the following components:.

<FIG> shows a diagram of a membrane wherein the architecture and sequence among (i), (ii) and (iii) are illustrated. The geometry of the membrane in the final module can be flat, tubular or any other complex geometry that enhances the performance of the module, i.e. thermo fluid dynamics, pressure resistance, heat exchange and proper sealing of the system.

In some cases, there is an additional porous catalytic layer (iv) between the porous support (i) and the non-porous separation layer (ii) which has the function of improving the gas exchange stages, especially when the porous support (i) does not have catalytic activity nor does it allow to carry out the transport of oxygen ions or electronic carriers. Generally, the properties of layer (iii) and layer (iv) are quite similar, although generally the surface specific area of layer (iii) is larger. <FIG> shows a diagram of a membrane wherein the architecture and sequence between (i), (ii), (iii) and (iv) are illustrated.

Optionally another additional non-porous layer (v) may also be required. This layer would be located between the non-porous layer (ii) and the porous layer (iii), and would serve to protect layer (ii) against possible degradation interactions or degradation reactions in contact with layer (iii) or with operation gases in contact with the layer (iii). Layer (v) must allow the transport of oxygen ions and oxygen carriers while being thermo-chemically compatible with the adjacent layers and with the gases it is in contact with. <FIG> shows a diagram of a membrane wherein the architecture and sequence between (i), (ii), (iii), (iv) and (v) are illustrated.

<FIG> shows a scanning electron microscope image of an oxygen-permeable ceramic membrane having components (i), (ii) and (iii), as shown schematically in <FIG>. <FIG> shows a scanning electron microscopy image of an oxygen-permeable ceramic membrane having components (i), (ii), (iii) and (v), the latter component being a composite material made of two crystalline phases, one conducting mainly oxygen ions and another one that drives mainly electronic carriers.

The present invention relates to a novel catalytic activation layer and to its incorporation into a catalytically activated oxygen membrane, thanks to said catalytic layer which allows efficient operation in the presence of acid gases such as CO<NUM> and SO<NUM>. Thus, the present invention provides a solution for improving the oxygen permeation (permeate flow) of a membrane under severe operating conditions, similar to those of oxy-combustion, and therefore to overcome the disadvantages of the prior art; also employing materials which have a high chemical stability in contact with said gaseous streams.

The present invention relates to a catalytic activation layer for its use in oxygen-permeable membranes, which may comprise at least one porous structure formed by particles of ceramic oxides, said particles linked to each other, conducting oxygen ions and electronic carriers, wherein the surface of said particles exposed to the pores is coated with nanoparticles made from a catalyst.

Considering the neutral electric charge of the compound, the oxygen anion content is necessary to compensate for the positive charge of the sum of all remaining metal cations.

Also, the particles forming the porous structure may have an average grain size, preferably comprised between <NUM> and <NUM>.

By way of example, the following combinations of metals, Pr-Al and Ce-Pr have shown to be especially active. Other possible examples would be combinations such as Pr-Ga, Pr-Nb, Pr-W, Pr-Mo, Ce-Al, Ce-Y, Ce-Pr-AI, Cs-Sm-AI, Ce-Sm-Ga, etc..

According to a preferred embodiment of the invention, the porous structure may be formed of mixtures of particles having two different compositions and crystalline phases.

According to a particular embodiment, the porous structure is formed by mixtures of particles having two different crystalline compositions and phases:.

In another particular embodiment of the invention, the porous structure is formed of mixtures of particles having two different compositions and crystalline phases:.

The present invention also relates to a process for the preparation of the porous catalytic layer described above. According to a preferred embodiment, the process of obtaining the catalytic activation layer may comprise at least one step of incorporating the catalyst into the surface of the porous structure particles by a technique selected from among impregnation or infiltration of liquid solutions of precursors of the metals comprised in the final catalyst composition; infiltration of a nanoparticle dispersion of the catalyst; deposition in vapor phase by PVD or CVD techniques, and combinations thereof.

This stage of catalyst incorporation could be carried out in <NUM> steps, i.e. introducing a first element (A), and then introducing at least one second element (B) or two further elements (B) and (C) using the techniques described above. It is common practice to carry out a heat treatment after incorporating the first element (A) and before incorporating another element (B). Making it in <NUM> steps may be advantageous in some cases since it may allow to modify or preferably promote the surface of nanoparticles of the A-based compound without producing any effect in the interior of said nanoparticles.

Furthermore, according to a particular embodiment, the process of obtaining a catalytic activation layer may further comprise a second heat treatment step at temperatures comprised between <NUM> and <NUM>. As mentioned above, the catalytic activation layer of the present invention is used in oxygen-permeable membranes. Thus, another object of the present invention relates to an oxygen-permeable membrane, comprising said catalytic activation layer. It has been found that with this catalytic activation layer the membrane is especially effective because it substantially improves the gaseous exchange stages, which totally limit the permeation process in the presence of CO2 and especially of gases as SO2.

According to a preferred embodiment, the oxygen-permeable membrane may comprise, at least:.

According to a particular embodiment, the membrane may comprise, in addition to the layers described above, the following additional layers:.

The catalytic activation layer of the oxygen-permeable membrane may comprise at least one porous structure formed by particles of ceramic oxides, said particles linked to each other, conducting oxygen ions and electronic carriers, wherein the surface of said particles to the pores exposed is coated with nanoparticles made of a catalyst whose composition has the following formula:.

Said catalytic membrane activating layer has a thickness preferably comprised between <NUM> and <NUM>, a porosity preferably comprised between <NUM> and <NUM>%, and pores with an average size <NUM> and <NUM>, and a content of catalyst supported on the porous structure between <NUM> and <NUM>% by weight of said porous structure.

According to a preferred embodiment of the invention, the porous structure of the membrane catalytic activation layer may be formed of mixtures of particles having two different compositions and crystalline phases.

A particular embodiment would be a membrane whose catalytic activation layer is formed by a porous structure formed by mixtures of particles having two distinct compositions and crystalline phases:.

Another particular embodiment would be a membrane whose catalytic activation layer is formed by a porous structure formed by mixtures of particles having two distinct compositions and crystalline phases:.

The present invention further relates to the process of obtaining the oxygen-permeable membrane comprising the catalytic activation layer.

According to a preferred embodiment, the process of obtaining an oxygen-permeable membrane described above and comprising the catalytic activation layer may comprise at least the following steps:.

According to a preferred embodiment, the step of incorporating the catalyst (d) could be carried out in <NUM> steps, i.e. introducing a first element (A), and then, at least a second element (B) or two more elements (B) and (C) using the techniques described above.

In addition, according to a particular embodiment of the process of obtaining the membrane, it may further comprise a heat treatment step at temperatures between <NUM> and <NUM> between steps c and d in order to remove the organic matter present in the deposited layer in (c) and sintering and chemically connecting the ceramic particles with each other and with the underlying non-porous layer.

According to another particular embodiment, optionally, a final heat treatment step may be carried out at temperatures comprised between <NUM> and <NUM>.

According to a particular embodiment, the porous support materials (i) may have variable geometry and comprise materials resistant to high temperatures and mechanically and chemically compatible with the materials of the selective nonporous separation layer (ii). Examples of such materials may be, without limitation: magnesium oxide, aluminum and magnesium spinels, cerium oxide doped with at least one lanthanide metal, zirconium oxide doped with at least one of the following elements Y, Mg, Sc or a lanthanide metal, titanium oxide, aluminum nitride, refractory alloys / superalloys.

Also object of the present invention is the use of the catalytic activation layer described above for the manufacture of oxygen-permeable membranes.

Also object of the present invention is the use of the oxygen-permeable membranes described above which comprise the catalytic activation layer for the generation of a O<NUM>-rich stream.

These oxygen-permeable membranes are especially suitable for generating the oxygen fed in oxy-combustion processes and in contact with gases having CO<NUM> in concentrations higher than <NUM> ppm and SO<NUM> in concentrations higher than <NUM> ppm.

According to a particular embodiment, the generated stream O<NUM> may have a purity greater than <NUM>% by volume.

According to another particular embodiment, the process wherein the membrane is used comprises a entrainment gas of the permeated O<NUM>. This entrainment gas may preferably have a SO<NUM> content, preferably greater than <NUM> ppm.

The membrane feed streams that are used are oxygen rich ones. A non-limiting example of the feed stream may be compressed air.

According to a particular embodiment, said feed stream has a SO<NUM> content greater than <NUM> ppm.

According to a preferred embodiment, the membranes described can be used in an integrated manner in an oxy-combustion system or systems comprising oxygen-enriched combustion stages, as described in <FIG>.

Throughout the description and claims the word "comprises" and its variants are not intended to exclude other technical features, additive components or steps. Other objects, advantages and features of the invention will be apparent to those skilled in the art in part from the description and in part from the practice of the invention. The following examples are provided by way of illustration, and are not intended to be limiting of the present invention.

Example <NUM> A Fe<NUM>NiO<NUM>-Ce<NUM>Tb<NUM>O<NUM>-δ (NFO-CTO) composite material in a <NUM>:<NUM> volumetric ratio between both crystalline phases is prepared by the method called Pechini. This method consists in dissolving the precursors of the metals, in this case nitrates, in an aqueous solution containing citric acid in a <NUM>:<NUM> molar ratio with respect to the metallic cations. The water in the solution is evaporated and the resulting residue is calcined at <NUM> in air. NFO-CTO is a composite material that has mixed conductivity of ions and electronic carriers. The material obtained is used to prepare serigraphic inks containing terpineol and ethylcellulose. Subsequently, two layers of NFO-CTO composite are deposited on discs of an ionic conductor (Ce<NUM>Gd<NUM>O<NUM>, CGO) by calcining at <NUM> and obtaining porous structures composed of NFO-CTO perfectly adhered to the surface of the CGO disc. Said disc is obtained by uniaxial pressing of commercial powder (Treibacher, Austria) and later calcined at <NUM>, and the disc is given a final flat shape by sanding and polishing.

Example <NUM>. Sample prepared in the same manner as Example A, but to which an aqueous solution of Ce precursors (nitrates) has been infiltrated after calcination of the NFO-CTO porous substrate. Such infiltration is performed by adding a specific volume of the precursor solution in each porous layer so that the added catalyst charge is known. Subsequent to the infiltration, it is calcined at <NUM> in such a way that the catalyst is deposited in its active form.

Example <NUM>. Sample prepared in the same manner as Example B, but infiltrating on this occasion a precursor solution of Pr nitrate.

Example <NUM>. Sample prepared in the same manner as Example B, but infiltrating on this occasion a Sm nitrate precursor solution.

Example <NUM>. Sample prepared in the same manner as Example B, but infiltrating on this occasion a precursor solution of Tb nitrate.

Example <NUM>. Sample prepared in the same manner as Example B, but infiltrating on this occasion a precursor solution of Co nitrate.

Example <NUM>. Sample prepared in the same manner as Example B, but infiltrating on this occasion a precursor solution of Nb oxalate.

Example <NUM>. Sample prepared in the same manner as Example B, but infiltrating on this occasion an ammonium heptamolybdate precursor solution.

Example <NUM>. Sample prepared in the same manner as Example B, but infiltrating on this occasion a precursor solution of zirconyl nitrate.

Example <NUM>. Sample prepared in the same manner as Example B, but infiltrating on this occasion an Al nitrate precursor solution.

Example <NUM>. Sample prepared in the same manner as Example C, but adding, on this occasion, to this solution <NUM>% by volume of the precursor solution of Example J, consequently adding a molar charge of <NUM>% of Pr and <NUM>% Al.

Example <NUM>. Sample prepared in the same manner as Example B, but adding, on this occasion, to this solution <NUM>% by volume of the precursor solution in Example B, consequently adding a molar charge of <NUM>% of Pr and <NUM>% Ce.

In order to carry out the electrochemical study on each of the materials of the previous examples, an experimental assembly constructed in quartz, capable of resisting the high temperatures of the study (<NUM>) has been arranged.

The electrochemical characterization by impedance spectroscopy allows to know the effectiveness in the activation of gaseous oxygen, under severe conditions close to its use in oxygen membranes in oxy-combustion processes, of the catalytic layer prepared according to the previous examples. This electrochemical characterization consists of the analysis of the resistive characteristics of the materials by means of the voltammetric impedance spectroscopy method. With this analysis it is possible to characterize the electrochemical properties under different temperature conditions, and under different atmospheres (in the presence of CO<NUM> and SO<NUM>).

For this purpose, a ceramic material that is conductor of oxygen-ions is arranged in the form of a disk, on which mixed porous layers of oxygen ions and electron carriers with thicknesses around <NUM> have been deposited, and on which different catalysts have been infiltrated, said catalyst being object of study. To carry out the measurements, the sample is placed inside the quartz assembly, connecting to each side, and in such a way that current collectors of a highly conductive material are in contact with the porous catalytic layers.

Two catalytic porous layers of the reference material are applied on each side of the ceramic disc by a silk screen printing technique. Subsequently, it is calcined at <NUM> to consolidate the bonding of the layers to the disc and for the support porous structure to remain stable.

For each characterization the addition of a catalyst to the porous structure is taken into account, said addition is made by infiltration of the considered elements, from solutions of precursor compounds. Said infiltrations are performed by adding a specific volume to each porous substrate, being this volume the same for each of the catalysts studied, so that the same charge of matter is always added. Subsequently a calcination of the precursors is carried out, in such a way that the catalysts are infiltrated into their active forms (usually oxides or elemental species).

The results of the study are shown in Table <NUM>, which shows the polarization resistance in ohms per square centimeter (Ω-cm<NUM>) obtained for each of the examples at <NUM> after a stabilization of <NUM> hours under each condition depending on the atmospheres to which it has been subjected, including the study in air, CO<NUM> with (<NUM>%) O<NUM>, and CO<NUM> with (<NUM>%) O<NUM> and <NUM> ppm of SO<NUM>. The catalytic activity is better the lower the polarization resistance. While different examples show an improvement over the non-infiltrated porous structure (Example A) under conditions of absence of SO<NUM>, only in three compositions is it possible to obtain an improvement, in some cases a substantial one, over Example A. Said examples, according to the present invention are those wherein the catalyst consists of Ce, but especially those wherein two metals were combined in the catalyst, examples K (Pr-AI) and L (Ce-Pr). In the case of the Pr-Al combination, a metal with high redox catalytic activity was combined under conditions of absence of SO<NUM> and a promoter of this activity and that had acidity under operating conditions, and allowed to decrease the adsorption of SO<NUM> and its consequent damaging effect in the catalytic activity. The case of the combination Ce-Pr is analogous, the Pr with high catalytic activity in air and Ce with more acidity and also relevant catalytic activity were combined. Other possible examples of catalysts following this concept would be combinations such as Pr-Ga, Pr-Nb, Pr-W, Pr-Mo, Ce-Al, Ce-Y, Ce-Pr-Al, Cs-Sm- Sm-Ga, etc..

Thus, it has been possible to obtain a high catalytic activity for the activation of gaseous oxygen in oxygen transport membranes in gases containing <NUM> ppm of SO<NUM> using the catalytic layer composed of (<NUM>) a porous structure made of a ceramic material having mixed conductivity with oxygen ions and electronic carriers, with adequate porosity and connection between its particles and the underlying membrane and with chemical stability against SO<NUM> under the described operating conditions, and (<NUM>) a catalyst in the form of nanoparticles dispersed on the surface of the prior porous structure, having a composition as described in the preceding paragraph.

Example <NUM>. An NFO-CTO membrane obtained by uniaxial pressing, silk screen printing and subsequent calcination at <NUM> of precursor powder obtained by the Pechini method. Subsequently, a layer of the NFO-CTO composite is deposited by silk-screen printing, calcining at <NUM> and remaining as a porous structure perfectly adhered to the surface of the non-porous layer. The obtained porous layer is identical to that obtained in Example A. The membrane obtained has a porous support (i), a non-porous separation layer of about <NUM> thickness (ii) and a upper porous catalytic layer (iii), according to the scheme shown in <FIG>.

Example <NUM>. Sample prepared in the same manner as described in Example M, but wherein the solution described in Example K has been infiltrated in both porous substrates. The porous layer obtained is identical to that obtained in Example K. The membrane obtained is in accordance with the present invention and has a porous support (i), a non-porous separation layer of about <NUM> thickness (ii) and an upper porous catalytic layer (iii), in accordance with the scheme shown in <FIG>.

In order to evaluate the oxygen separation properties of the compounds under study, an assembly built in quartz is available to analyze the behavior of different ceramic membranes.

The quartz assembly consists of a tube with two chambers separated by a ceramic membrane, there being no point of communication between the two chambers due to the density (absence of porosity) of the membrane and the sealing made with O-rings. On the one hand, a stream rich in oxygen is fed, while on the other side an entrainment gas is circulated or the vacuum is induced. This difference in oxygen content conditions serves as the driving force for oxygen diffusion from the feed-rejection side to the permeate side. Quantifying by means of a gas chromatograph the oxygen content in the permeate stream the flow of oxygen permeating through the membrane is determined under different conditions of temperature, oxygen content in the feed chamber and aggressive atmospheres in the permeate (presence of CO<NUM> and SO<NUM>).

Oxygen permeation was studied on membranes according to Examples M and N. Permeation tests and catalytic studies were carried out on disc-shaped membranes of <NUM> diameter and about <NUM> thickness. The reaction temperature is controlled by a thermocouple close to the membrane. The permeated gas stream was analyzed using a micro-GC Varian CP-<NUM> equipped with three analysis modules: Molsieve5A, PoraPlot-Q and CP-Sil. Table <NUM> shows the oxygen permeation obtained in milliliters (normal conditions) per minute and square centimeter (Nml··min-<NUM>·cm-<NUM>) for the membranes according to examples M and N in different atmospheres at <NUM> after <NUM> hour stabilization in each condition. The results show that the membrane according to the present invention (example N) has a much higher oxygen permeation than the membrane (example M) without catalyst infiltrated in layer (iii). The difference between the two membranes is much more important when the permeate contains SO<NUM>, conditions in which the oxygen gaseous exchange becomes notably difficult, and therefore the effect of an active catalyst becomes much more important.

Claim 1:
A catalytic activation layer for its use in oxygen-permeable membranes, characterized in that it comprises at least one porous structure formed by particles of ceramic oxides, said particles linked to each other, conducting oxygen ions and electronic carriers, coated with nanoparticles made of a catalyst,
- wherein the catalyst has a composition with the following formula:

        A<NUM>-x-yBxCyOR

wherein
A is selected from Ti, Zr, Hf, lanthanide metals and combinations thereof;
preferably Zr, Pr, Ce and combinations thereof
B and C are metals selected from Al, Ga, Y, Sc, B, Nb, Ta, V, Mo, W, Re, Mn, Sn, Pr, Sm, Tb, Yb, Lu and combinations thereof;
A must always be different from B <MAT>
<NUM> ≤ y ≤ <NUM>, wherein "R" represents the molar content of oxygen, which is determined by the molar composition of the remaining metal elements of the compound,
- or wherein the catalyst is an oxide of the elements Ce, Pr, Sm or Zr,
and the catalytic activation layer has a thickness comprised between <NUM> and <NUM>, a porosity comprised between <NUM> and <NUM>% and pores with an average size comprised between <NUM> and <NUM> and a content of supported catalyst on the porous structure between <NUM> and <NUM>% by weight of the porous structure.