Catalyst-containing oxygen transport membrane

Abstract

A method is described of producing a catalyst-containing composite oxygen ion membrane and a catalyst-containing composite oxygen ion membrane in which a porous fuel oxidation layer and a dense separation layer and optionally, a porous surface exchange layer are formed on a porous support from mixtures of (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3 and a doped zirconia. Adding certain catalyst metals into the fuel oxidation layer not only enhances the initial oxygen flux, but also reduces the degradation rate of the oxygen flux over long-term operation. One of the possible reasons for the improved flux and stability is that the addition of the catalyst metal reduces the chemical reaction between the (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3 and the zirconia phases during membrane fabrication and operation, as indicated by the X-ray diffraction results.

Claims

1. A method of producing an oxygen ion composite membrane comprising: forming a first layer on a porous support containing a first mixture of particles of (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3, doped zirconia, catalyst metal M, and pore formers, where Ln is La, Y, Pr, Ce or Sm, A is Ca or Sr, B is Fe, Mn, Co, Ni, Al, Ti or combinations thereof, w is from about 0.9 to about 1.0, x is from about 0.1 to about 0.3, y is from about 0.1 to about 0.7, and is a value that renders the composition charge neutral, catalyst metal M is a catalyst metal or an oxide, carbonate or nitrate of a catalyst metal, wherein said catalyst metal is Ru: the first mixture containing the (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3, the doped zirconia and the catalyst metal M such that when sintered, the first layer will contain from about 20 vol. % to about 70 vol. % of the (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3, from about 30 vol. % to about 80 vol. % of the doped zirconia, and from about 0.1 vol. % to about 20 vol. % of the catalyst metal M, based on the volume percentage of the total solid mass; forming a second layer on the first layer that contains a second mixture of particles of (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3 and the doped zirconia and that does not contain pore formers, where Ln is La, Y, Pr, Ce or Sm, A is Ca or Sr, B is Fe, Mn, Co, Ni, Al, Ti or combinations thereof, w is from about 0.9 to about 1.0, x is from about 0.1 to about 0.3 and y is from about 0.1 to about 0.7 and is a value that renders the composition charge neutral; the second mixture containing the (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3 and the doped zirconia such that when sintered, the second layer will contain from about 20 vol. % to about 70 vol. % of the (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3 and from about 30 vol. % to about 80 vol. % of the doped zirconia, based on the volume percentages of the total solid mass; heating the first layer, the second layer and the porous support so that said first layer partially sinters into a porous mass containing the first mixture of particles, thereby to provide a porous fuel oxidation layer and the second layer fully sinters into a densified mass containing the second mixture of particles, thereby to provide a dense separation layer.

2. The method of claim 1, wherein: prior to heating the first layer, the second layer and the porous support in nitrogen atmosphere a third layer is formed on the second layer containing a third mixture of particles of (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3, the doped zirconia and pore formers, where Ln is La, Y, Pr, Ce or Sm, A is Ca or Sr, B is Fe, Mn, Co, Ni, Al, Ti or combinations thereof, w is from about 0.9 to about 1.0, x is from about 0.1 to about 0.3 and y is from about 0.1 to about 0.7 and is a value that renders the composition charge neutral; the third mixture having a third volume ratio of the (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3 and the doped zirconia such that when sintered, the third layer will contain from about 20 vol. % to about 70 vol. % of the (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3 and from about 30 vol. % to about 80 vol. % of the doped zirconia, based on the volume percentages of the total solid mass; followed by heating the first layer, the second layer, the third layer and the porous support, wherein the first layer partially sinters into a porous mass containing the first mixture of particles, thereby to provide a porous fuel oxidation layer and the second layer fully sinters into a densified mass containing the second mixture of particles, thereby to provide a dense separation layer and the third layer is heated so that said third layer partially sinters into a porous mass containing the third mixture of particles, thereby to provide a porous surface exchange layer.

3. The method of claim 2, wherein the doped zirconia is 10mol % scandia and 1 mol % yttria doped zirconia (10Sc1YSZ) or 10 mol % scandia and 1 mol % ceria doped zirconia (10Sc1CeSZ) or 10 mol % scandia and 1 mol % ceria and 1 mol % yttria doped zirconia (10Sc1Ce1YSZ10Sc1YSZ).

4. The method of claim 3, wherein: the (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3 within the first mixture of particles is (La.sub.1xSr.sub.x).sub.wCr.sub.1yFe.sub.yO.sub.3, where w is 0.95, x is 0.2 and y is 0.3; the (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3 within the second mixture of particles is (La.sub.1xSr.sub.x).sub.wCr.sub.1yFe.sub.yO.sub.3, where w is 0.95, x is 0.2 and y is 0.3; the (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3 within the third mixture of particles is (La.sub.1xSr.sub.x).sub.wCr.sub.1yFe.sub.yO.sub.3, where w is 0.95, x is 0.2 and y is 0.3; and the sintered porous support is formed from doped zirconium oxide or a mixture of MgO and MgAl.sub.2O.sub.4.

5. The method of claim 4, wherein the first layer contains 20-60 vol. % of (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3, 40-80 vol. % of the doped zirconia, and 1-15 vol. % of the catalyst metal M, the second layer contains 20-60 vol. % of (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3 and 40-80 vol. % of the doped zirconia, the third layer contains 20-60 vol. % of (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3 and 40-80 vol. % of the doped zirconia, based on the volume percentages of the total solid mass.

6. The method of claim 5, wherein the porous support is of tubular or planar configuration.

7. The method of claim 1, wherein the porous support is formed of 4YSZ and is optionally heated at a temperature ranging from about 950 to about 1200 C., so that it is not fully sintered prior to forming the first layer on the porous support; the first layer after having been formed on the porous support is dried at ambient temperature prior to coating the second layer on the first layer; and the first layer, the second layer and the porous support are heated at a temperature of from about 1350 C. to about 1450 C.

8. The method of claim 7, wherein the third layer is heated at a temperature of from about 1250 C. to about 1350 C.

9. The method of claim 2, wherein the porous support is formed of 4YSZ and is optionally heated at a temperature ranging from about 950 to about 1200 C., so that it is not fully sintered prior to forming the first layer on the porous support, wherein the first layer, the second layer, the third layer and the porous support are heated at a temperature of from about 1350 C. to about 1450 C. in an inert atmosphere, and wherein said first layer, second layer and/or said third layer are optionally formed by slurry coating.

10. The method of claim 9, wherein the doped zirconia is 10Sc1YSZ or 10Sc1CeSZ or 10Sc1Ce1YSZ.

11. The method of claim 10, wherein the porous support is 4YSZ.

12. An oxygen ion composite membrane comprising: first and second layers on a porous support providing a porous fuel oxidation layer and a dense separation layer, respectively, for the oxygen ion composite membrane; the first of the layers containing a mixture of (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3, doped zirconia, and catalyst metal M, where Ln is La, Y, Pr, Ce or Sm, A is Ca or Sr, B is Fe, Mn, Co, Ni, Al, Ti or combinations thereof, w is from about 0.9 to about 1.0, x is from about 0.1 to about 0.3 and y is from about 0.1 to about 0.6, M is Ru; the second of the layers containing a mixture of (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3 and doped zirconia, where A is Ca or Sr, and B is Fe, Mn, Co, Ni, Al, Ti or combinations thereof, w is from about 0.9 to about 1.0, x is from about 0.1 to about 0.3 and y is from about 0.1 to about 0.7; the first of the layers containing from about 30% to about 70% of the (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3, from about 30 vol. % to about 70 vol. % of the doped zirconia, and from about 0.1 vol. % to about 20 vol. % of the catalyst metal M; and the second of the layers containing from about 30 vol. % to about 70 vol. % of the (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3 and from about 30 vol. % to about 70 vol. % of the doped zirconia, based on the volume percentages of the total solid mass.

13. The oxygen ion composite membrane of claim 12, wherein: a third layer is situated on the second layer to form a porous surface exchange layer and that also contains the mixture of (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3 and the doped zirconia, where Ln is La, Y, Pr, Ce or Sm, A is Ca or Sr, B is Fe, Mn, Co, Ni, Al, Ti or combinations thereof, w is from about 0.9 to about 1.0, x is from about 0.1 to about 0.3 and y is from about 0.1 to about 0.7; and the third layer containing the (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3 and the doped zirconia in a third volume percentage of (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3 of from about 20 vol. % to about 70 vol. % of the total solid mass.

14. The oxygen ion composite membrane of claim 13, wherein the doped zirconia is the doped zirconia is 10 mol % scandia and 1 mol % yttria doped zirconia (10Sc1YSZ) or 10 mol % scandia and 1 mol % ceria doped zirconia (10Sc1CeSZ) or 10 mol % scandia and 1 mol % ceria and 1 mol % yttria doped zirconia (10Sc1Ce1YSZ10Sc1YSZ).

15. The oxygen ion composite membrane of claim 14, wherein: the (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3 within the first layer is (La.sub.1xSr.sub.x).sub.wCr.sub.1yFe.sub.yO.sub.3, where w is 0.95, x is 0.2 and y is 0.3; the (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3 within the second layer is (La.sub.1xSr.sub.x).sub.wCr.sub.1yFe.sub.yO.sub.3, where w is 0.95, x is 0.2 and y is 0.3; the (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3 within the third layer is (La.sub.1xSr.sub.x).sub.wCr.sub.1yFe.sub.yO.sub.3, where w is 0.95, x is 0.2 and y is 0.3; and the sintered porous support is formed from stabilized zirconia oxide or a mixture of MgO and MgAl.sub.2O.sub.4.

16. The oxygen ion composite membrane of claim 15, wherein the first of the layers contains about 30 vol. % of (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3, about 70 vol. % of doped zirconia; the second of the layers contains about 30 vol. % of (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3 and about 70 vol. % of doped zirconia; and the third of the layers contains about 30 vol. % of (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3 and about 70 vol. % of doped zirconia, based on the volume percentages of the total solid mass.

17. The oxygen ion composite membrane of claim 16, wherein the porous support is of tubular or planar configuration and is formed from 4 mol % yttria stabilized zirconia (4YSZ).

18. The method of claim 1 wherein said heating step is conducted in a nitrogen atmosphere, air, forming gas atmosphere, CO.sub.2, argon, or mixtures thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) While the specification concludes with claims distinctly pointing out the subject matter that the inventors regard as their invention, it is believed that the invention will be better understood when taking in connection with the accompanying drawings in which:

(2) FIG. 1 is a cross-sectional SEM micrograph image of a composite oxygen ion transport membrane.

(3) FIG. 2 is a comparison of oxygen flux between membranes with and without Ru in the fuel oxidation layer.

(4) FIG. 3 is a comparison of X-ray diffraction patterns of powder mixtures of (La.sub.0.8Sr.sub.0.2).sub.0.95Cr.sub.0.5Fe.sub.0.5O.sub.3 and 10Sc1YSZ with and without Ru after exposure in reducing atmosphere at 1200 C. for 100 hours.

DETAILED DESCRIPTION

(5) FIG. 1 illustrates a cross-sectional micrograph of a composite oxygen transport membrane 1 of the invention. Composite oxygen transport membrane 1 has a porous support layer 10. Applied to the porous support layer 10 is a first layer 12, a second layer 14 and a third layer 16. The composite oxygen transport membrane is specifically designed to function in an environment in which a fuel or other combustible substance is introduced to the porous support layer 10, on the side opposite to the first, second and third layer 12, 14 and 16, and subjected to combustion supported by permeated oxygen to both provide the partial pressure difference necessary to drive oxygen ion transport and also to heat the membrane to an operational temperature at which oxygen ion transport will occur. In this regard, the term fuel when used in connection with this layer, both herein and in the claims, is not intended to be limiting, but rather, to indicate and include any substance that can be oxidized through permeation of oxygen through the membrane. The second layer 14 is the active layer at which oxygen ion transport principally occurs and as such, serves as dense separation layer that is impervious to gas, but allows oxygen ion transport. The third layer 16 serves to initially reduce the oxygen and thus serves as a porous surface activation layer. Each of the first layer 12, the second layer 14 and the third layer 16 after heating and sintering will preferably each have a thickness of between about 10 micron and about 50 micron.

(6) The porous support layer 10 could be formed preferably from partially stabilized zirconia oxide e.g. from about 3 to about 7 mol % yttria stabilized zirconia. Partially doped zirconia with yttria content lower than 4 mol % tends to experience a tetragonal-to-monoclinic phase transformation at ambient temperature, especially when under stress or in the presence of water vapor. The tetragonal-to-monoclinic phase transformation is accompanied by about 5% volume increase and results in cracking of the porous support or delamination of the coating layers from the porous support. Although not part of the present invention, as would be appreciated by those skilled in the art, porous support layer 10 should provide as open an area as possible while still being able to be structurally sound in its supporting function.

(7) A stabilized zirconia, namely, Zr.sub.1xyA.sub.xB.sub.yO.sub.2 is a common material in all three active membrane layers, namely, the first layer 12, the second layer 14 and the third layer 16. As mentioned above in all of these layers oxygen ion transport occurs and as such, are active. In order to generate industrially relevant levels of oxygen ion conductivity, A and B are typically Sc, Y, Ce, Al, Yb or Ca. Preferably, such stabilized zirconia has a composition given by the formula: Zr.sub.0.802Sc.sub.0.180Y.sub.0.018O.sub.2 or Zr.sub.0.809Sc.sub.0.182Ce.sub.0.009O.sub.2, often denoted as 10Sc1YSZ or 10Sc1CeSZ, respectively, in literature associated with this class of membrane. However it should be noted that many different combinations of Sc, Y, Ce, Al, Yb, Ca or other elements can be substituted to achieve the same end.

(8) Turning first to the first layer 12, this layer is formed from a first mixture of particles of (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3, 10Sc1YSZ, metal M, and pore formers. In this layer, Ln is La, Y, Pr, Ce or Sm, A is Ca or Sr, B can be Mn, Fe, Co, Ni, Al, Ti or combinations thereof, w is from about 0.9 to about 1.0, x is from about 0.1 to about 0.3 and y is from about 0.1 to about 0.6. The metal M can be added in the form of elemental state or compounds, including but not limited to oxides, carbonates or nitrates. The preferred composition of the perovskite material for this layer is (La.sub.0.8Sr.sub.0.2).sub.0.95Cr.sub.0.7Fe.sub.0.3O.sub.3. The first layer contains from about 20 vol. % to 70 vol. % of (La.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3, from 30 vol. % to 80 vol. % of the doped zirconia, and optionally from 0.1 vol. % to 20 vol. % of the metal M, all in volume percentage of the total sintered mass. In one embodiment, the first layer contains 28-30 vol. % of (La.sub.1 xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3, 67-70 vol. % of the doped zirconia, and optionally 5 vol. % of the metal M, all in volume percentage of the sintered mass. In another embodiment, the first layer contains 28-30 vol. % of (La.sub.0.8Sr.sub.0.2).sub.0.95Cr.sub.0.5Fe.sub.0.5O.sub.3, 67-70 vol. % of 10Sc1YSZ, and optionally 5 vol. % of Ru, all in volume percentage of the sintered mass.

(9) The second layer 14 is formed of a second mixture of particles of (Ln.sub.1 xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3 and the doped zirconia. The function of the second layer 14 is to be a gas separation layer that is impervious to gas molecules but should be conductive to oxygen ions and electrons. In this layer, Ln is La, Y, Pr, Ce or Sm, A is Ca or Sr, B is Mn, Fe, Co, Ni, Al, Ti or combinations thereof, x is from about 0.1 to about 0.3, y is from about 0.1 to about 0.7 and w is from about 0.9 to about 1.0. In one embodiment, the preferred compositions of material for this layer are (La.sub.0.8Sr.sub.0.2).sub.0.95Cr.sub.0.7Fe.sub.0.3O.sub.3 and 10Sc1YSZ. Within the second mixture of particles, the (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3 and the doped zirconia should be present within a second volume percentage of (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3 of from about 20 vol. % to about 70 vol. % of the total sintered mass. In one embodiment, the second volume percentage is about 30 vol. % of (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3 and 70 vol. % of the doped zirconia.

(10) The third layer 16, that serves as the porous surface exchange layer, is formed of a third mixture of particles of (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3 and the doped zirconia. In this layer, Ln is La, Y, Pr, Ce or Sm, A is Ca or Sr, B can be Mn, Fe, Co, Ni, Al, Ti or combinations thereof; w is from about 0.9 to about 1.0, x is from about 0.1 to about 0.3 and y is from about 0.1 to about 0.7. In one embodiment, the compositions of material for this layer are (La.sub.0.8Sr.sub.0.2).sub.0.95Cr.sub.0.7 Fe.sub.0.3 O.sub.3 and 10Sc1YSZ. The (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3 and the doped zirconia should be present within a third volume percentage of (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3 of from about 20 vol. % to about 70 vol. % of the total sintered mass. In one embodiment, the third volume percentage of (Ln.sub.1xA.sub.x).sub.wCr.sub.1yB.sub.yO.sub.3 is about 30 vol. %.

Example 1

(11) In a first example of forming the composite oxygen ion transport membrane 1, the perovskite material (La.sub.0.8Sr.sub.0.2).sub.0.95Cr.sub.0.Fe.sub.0.5O.sub.3 (LSCrF55) for the second layer 14, the gas separation layer, can be obtained from NexTech Materials, Ltd., Lewis Center, Ohio and Zr.sub.0.802Sc.sub.0.180Y.sub.0.018O.sub.2 (10Sc1YSZ) can be obtained from Daiichi Kigenso Kagaku Kogyo Co., Ltd, Osaka, Japan through their US agent Sojitz, Ltd, in New York, United States of America. The perovskite phase LSCrF55 can have a particle size d.sub.50 in the range of from about 0.3 to about 0.5 micron, the 10Sc1YSZ should have a d.sub.50 of less than 0.6 micron. In order to fabricate a 70 gram batch of gas separation layer slurry, 36.75 gram of LSCrF55 are mixed with 33.25 gram of 10Sc1YSZ, 36 gram Ferro B73210 binder, 170 gram toluene and 1200 gram of 1.5 millimeter diameter YSZ milling media in a 500 milliliter Teflon bottle. The mixture is milled until the particle size of the mixture is in the range of from about 0.3 to 0.5 micron. The perovskite material (La.sub.0.8Sr.sub.0.2).sub.0.95Cr.sub.0.5Fe.sub.0.5O.sub.3 (LSCrF55) for the first layer 12, the fuel oxidation layer, is also obtained from NexTech Materials, Ltd., Lewis Center, Ohio and the 10Sc1YSZ can also be obtained from Daiichi Kigenso Kagaku Kogyo Co. Ltd, Osaka, Japan through their US agent Sojitz, Ltd, in New York. Ru is added in the form of RuO.sub.2 which is procured from Johnson Matthey, West Deptford, N.J. and has a particle size d.sub.50 of 1.3 micron. The perovskite phase LSCrF55 is specified as having a particle size d.sub.50 in the range of from about 0.3 to about 0.5 micron, the 10Sc1YSZ has a particle size d.sub.50 of less than 0.6 m as received. In order to prepare a 60 gram batch of fuel oxidation layer slurry, 30 gram of LSCrF55, 18.09 gram of 10Sc1YSZ, 2.91 gram of RuO.sub.2, 100 gram of toluene, 20 gram of Ferro B73210 binder, and 500 gram of 1.5 millimeter diameter YSZ grinding media are added in a 250 milliliter Teflon bottle. The mixture is then milled for about 6 hours to form a slurry having a particle size d.sub.50 of from about 0.3 to about 0.5 micron. About 9 gram of carbon black having a particle size of about d.sub.50 of 0.8 micron and 1.2 gram of surfactant KD-1 are added to the slurry and milled for additional 2 hours. To prepare the surface exchange layer slurry, 80 gram of the electronic and ionic mixture having (La.sub.0.8Sr.sub.0.2).sub.0.95Cr.sub.0.3Fe.sub.0.7O.sub.3 (LSCrF37) and 10Sc1YSZ is prepared so that the mixture contains about 60% of LSCrF37 and about 40% of 10Sc1YSZ by volume. To the mixture, 28.8 gram of toluene, 19.2 gram of ethyl alcohol, 16 gram of the same Ferro binder mentioned above, 1.6 gram of surfactant KD-1, and about 500 gram of 1.5 millimeter diameter YSZ grinding media are added and the resultant mixture is milled for about 2 hours to form a slurry having a particle size d.sub.50 of from about 0.3 to about 0.5 micron. About 12 gram of carbon black are added to the slurry and it is milled for additional 2 hours.

(12) In order to form a composite oxygen transport membrane 1 from these slurries the slurries are deposited on a porous support 10 by slurry coating followed by firing in nitrogen. The porous support 10 can be tubular and fabricated by an extrusion process. Although the porous support 10 can be fully sintered, it can first be fired at a low temperature e.g. at about 1050 C. after green forming such that some residual shrinkage remains when the coated substrate is fired again at higher temperatures. The first layer 12 is then deposited on the surface of the porous support layer 10 and the proper thickness is controlled by the speed at which the supporting substrate is dipped into the slurry. The first layer 12 is allowed to dry at ambient temperature. The second layer 14 is then applied on top of the first layer 12 by dipping the component into the gas separation slurry and allowed to dry. The coating process is repeated two to three times to achieve the desirable thickness. The coated tube is slowly heated in flowing nitrogen to a temperature of from about 1350 C. to about 1450 C. and held at the same temperature for about 6 hours for the membrane to sinter completely. During sintering, the oxygen partial pressure of the atmosphere in the furnace is controlled below 20 Pa. The tube is then cooled in nitrogen to complete the sintering process. The sintered tube is checked for flow coefficient, Cv, as defined below:

(13) $\mathrm{Cv}=\frac{q}{0.471N_2{p}_1\sqrt{\frac{1}{G_g{T}_1}}}$

(14) where q is the flow rate, N.sub.2 is a constant, p.sub.1 is the inlet pressure, G.sub.g is the gas specific gravity, and T.sub.1 is the absolute upstream temperature. The Cv of a sintered 2-foot long tube should not exceed 1.510.sup.5. After densification of the separation layer 14, the third layer 16 is applied by slurry coating the sintered three layer membrane structure and firing at a temperature of from 1250 C. to about 1350 C. in air. The third layer 16 could also be applied after drying of the dense layer, 14 and then all three active layers 12, 14, 16 are co-fired together in one high temperature sintering step at a temperature of about 1430 C. in a nitrogen atmosphere. Combining the high temperature sintering steps for these three layers leads to lower manufacturing costs than can be achieved when using separate high temperature sintering steps for each of the three layers. The Cv of the tube is checked again after the sintering of the surface exchange layer to ensure no significant change has occurred.

(15) The resultant tube has the preferred thickness, pore size and porosity within the ranges, namely, the fuel oxidation layer 12 has a thickness of from about 10 microns to about 50 micron, an average pore size of from about 0.1 micron to about 1 micron and a porosity of from about 25 percent to about 50 percent. The porous support layer 10 has a thickness of about 1.3 millimeter, an average pore size of from about 0.5 micron to about 3 micron and a porosity of from about 25 to 45 percent. The surface exchange layer 16 has a thickness of from about 10 micron to about 50 micron, an average pore size from about 0.1 micron to about 1 micron and a porosity of from about 25 percent to about 50 percent. The separation layer 14 has a thickness of from about 10 micron to about 50 micron, with substantially no connected porosity; in another embodiment with no connected porosity.

(16) The oxygen flux of the tube with all three active layers and Ru catalyst in the fuel oxidation layer is tested in a single tube reactor at a high temperature for over 1000 hours. A tube with the same active layers but without catalyst addition in the fuel oxidation layer is also tested for comparison. FIG. 2 shows the normalized oxygen flux of the two tubes with and without Ru catalyst in the fuel oxidation layer. It can be appreciated that the tube with 5% Ru in the fuel oxidation layer exhibits a higher initial oxygen flux than the tube without Ru catalyst and does not show any noticeable degradation in flux over the whole testing period of time. The tube without Ru catalyst in the fuel oxidation layer shows a lower initial oxygen flux and degrades rapidly for the first 100 hours before it reaches a stable state. The higher initial flux might be attributed to the catalytical activity of the Ru metal in the fuel oxidation layer; however, it is unexpected that the addition of the Ru catalyst decreases the degradation rate as well.

(17) To further understand the reason, powder mixtures of the fuel oxidation layer with and without the addition of the Ru catalyst are examined by X-ray diffraction to check if the two phases are chemically compatible under oxygen membrane operating conditions. In this experiment, the LSCrF55 and 10Sc1YSZ, either with or without Ru addition, are intimately mixed together by ball milling. The mixed powders are compacted into pellets and then exposed in reducing atmosphere at 1200 C. for 100 hours. The powders after exposure are examined by X-ray diffraction and the results are shown in FIG. 3. From FIG. 3, it can be easily appreciated that after high temperature exposure in reducing atmosphere, without Ru in the mixture, the LSCrF55 phase reacts with the 10Sc1YSZ phase and forms La.sub.2Zr.sub.2O.sub.7 and tetragonal ZrO.sub.2 phases, as evidenced by the appearance of additional peaks in the X-ray diffraction pattern. Formation of the La.sub.2Zr.sub.2O.sub.7 and tetragonal ZrO.sub.2 phases are detrimental to the performance of oxygen transport membrane, which is well documented in the literatures in solid oxide fuel cell community and explains the initial drop of oxygen flux for the tube without Ru in the fuel oxidation layer. The powder mixture with Ru addition, however, does not show any additional peak after high temperature exposure in reducing atmosphere, which means no new phase is formed. The lower degradation of flux of the tube with Ru in the fuel oxidation layer can be explained by the improved chemical compatibility between the LSCrF55 phase and the 10Sc1YSZ phase.

Example 2

(18) In a second example of forming the composite oxygen ion transport membrane 1, the perovskite material (La.sub.0.8Sr.sub.0.2).sub.0.95Cr.sub.0.7Fe.sub.0.3O.sub.3 (LSCrF73) for all the three active layers 12, 14 and 16, can be obtained from NexTech Materials, Ltd., Lewis Center, Ohio and Zr.sub.0.802Sc.sub.0.180Y.sub.0.018O.sub.2 (10Sc1YSZ) can be obtained from Daiichi Kigenso Kagaku Kogyo Co., Ltd, Osaka, Japan through their US agent Sojitz, Ltd, in New York, N.Y., USA. The perovskite powder LSCrF73 can have a particle size d.sub.50 in the range of from about 0.3 to about 0.5 micron, the 10Sc1YSZ powder should have a d.sub.50 of less than 0.6 micron. In order to fabricate a 70 gram batch of the gas separation layer slurry, 22.50 gram of LSCrF73 is mixed with 47.50 gram of 10Sc1YSZ, 36 gram Ferro B73210 binder, 170 gram toluene and 1200 gram of 1.5 millimeter diameter YSZ milling media in a 500 milliliter Teflon bottle. The mixture is milled until the particle size of the mixture is in the range of from about 0.3 to about 0.5 micron. The slurry was separated from the milling media and stored in a Teflon bottle. The fuel oxidation layer 12 and the surface activation layer 16 are coated from the same slurry. In order to prepare a 60 gram batch of the fuel oxidation and surface activation slurry, 16.39 gram of LSCrF73, 34.61 gram of 10Sc1YSZ, 100 gram of toluene, 20 gram of Ferro B73210 binder, and 600 gram of 1.5 millimeter diameter YSZ grinding media are added in a 250 milliliter Teflon bottle. The mixture is then milled for about 6 hours to form a slurry having a particle size d.sub.50 of from about 0.3 to about 0.5 micron. About 9 gram of carbon black having a particle size of about d.sub.50 of 0.8 micron and 1.2 gram of surfactant KD-1 are added to the slurry and milled for an additional 2 hours.

(19) In order to form a composite oxygen transport membrane 1 from these slurries the slurries are deposited on a porous support 10 by slurry coating followed by firing in nitrogen. The porous support 10 can be tubular and fabricated by an extrusion process. Although the porous support 10 can be fully sintered, it can first be fired at a low temperature, for example at about 1050 C. after green forming such that some residual shrinkage remains when the coated substrate is fired again at a higher temperature. The first layer 12 is then deposited on the surface of the porous support layer 10 and the thickness is controlled by the speed at which the supporting substrate is dipped in the slurry. The first layer 12 is allowed to dry at ambient temperature. The second layer 14 is then applied on top of the first layer 12 by dipping the component into the gas separation slurry and allowed to dry. The coating process can be repeated two or more times to achieve the desired thickness. The third layer 16 is then applied on top of the second layer 14 by dipping the component into the surface activation slurry and allowed to dry. The tube coated with all three active layers is slowly heated in flowing nitrogen to a temperature of from about 1350 C. to about 1450 C. and held at the same temperature for about 6 hours to allow the membrane to sinter completely. During sintering the partial pressure of oxygen in the atmosphere of the furnace is maintained below 20 Pa. The tube is then cooled in nitrogen to complete the sintering process.

(20) While the present invention has been described with reference to a preferred embodiment, as would occur to those skilled in the art, numerous changes, additions and omission may be made without departing from the spirit and scope of the present invention as set forth in the appended claims.