Catalytic layer and use thereof in oxygen-permeable membranes

Abstract

The invention relates to a catalytic activation layer for use in oxygen-permeable membranes, which can comprise at least one porous structure formed by interconnected ceramic oxide particles that conduct oxygen ions and electronic carriers, where the surface of said particles that is exposed to the pores is covered with nanoparticles made from a catalyst, the composition of which corresponds to the following formula: A.sub.1-x-yB.sub.xC.sub.yO.sub.R where: A can be selected from Ti, Zr, Hf, lanthanide metals and combinations thereof; B and C are metals selected from Al, Ga, Y, Se, B, Nb, Ta, V, Mo, W, Re, Mn, Sn, Pr, Sm, Tb, Yb, Lu and combinations of same; and A must always be different from B. 0.01<x<0.5; 0<y<0.3.

Claims

1. A catalytic activation layer in an oxygen-permeable membrane, wherein the catalytic activation layer 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 which has a composition with the following formula:
A.sub.1-x-yB.sub.xC.sub.yO.sub.R wherein A is selected from Ti, Zr, Hf, lanthanide metals 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, 0.01?x?0.5; 0?y?0.3; and wherein R represents the molar content of oxygen, which is determined by the molar composition of the remaining metal elements of the composition; and wherein said catalytic activation layer has a thickness comprised between 5 and 100 ?m, a porosity comprised between 10 and 60% and pores with an average size comprised between 0.1 and 5 ?m and a content of supported catalyst on the porous structure between 0.5 and 10% by weight of the porous structure.

2. The catalytic activation layer of claim 1, wherein the porous structure is made of mixtures of particles having two different compositions and crystalline phases: a first phase which is made of cerium oxide partially substituted by one element selected from the group consisting of Zr, Gd, Pr, Sm, Nd, Er, Tb and combinations thereof, and has crystalline structure of the fluorite type, and has an ionic conductivity greater than 0.001 S/cm under operating conditions; a second phase comprising a mixed oxide with a spinel type structure, comprising at least one metal selected from the group consisting of Fe, Ni, Co, Al, Cr, Mn and combinations thereof, and has a total conductivity greater than 0.05 S/cm under operating conditions.

3. The catalytic activation layer according to claim 1, wherein the porous structure consists of mixtures of particles having two different compositions and crystalline phases: a first phase comprising cerium oxide partially substituted by an element selected from the group consisting of Zr, Gd, Pr, Sm, Nd, Er, Tb and combinations thereof, has a crystalline structure of the fluorite type, and has ionic conductivity greater than 0.001 S/cm under operating conditions; a second phase comprising a mixed oxide with perovskite type structure comprising at least one metal selected from the group consisting of lanthanides, Fe, Ni, Co, Cr, Mn and combinations thereof and has a total conductivity greater than 0.05 S/cm under operating conditions.

4. A process for producing a catalytic activation layer described in claim 1 comprising at least one step of incorporating the catalyst into the particles surface of the porous structure by a technique selected from the group consisting of: impregnation of liquid solutions or precursors of metals comprised in the catalyst composition of formula A.sub.1-x-yB.sub.xC.sub.yO.sub.R; infiltration of liquid solutions of precursors of the metals comprised in the catalyst composition of formula A.sub.1-x-yB.sub.xC.sub.yO.sub.R; infiltration of a nanoparticle dispersion of the catalyst; deposition in vapor phase by physical vapor deposition techniques; deposition in vapor phase by chemical vapor deposition techniques; and combinations thereof.

5. A process for producing the catalytic activation layer according to claim 4, further comprising a thermal treatment stage at temperatures comprised between 650 and 1100? C. after the incorporation of the catalyst at least in the catalytic activation layer (iii).

6. An oxygen-permeable membrane, comprising, at least: a porous ceramic or metallic support (i) with a porosity between 20 and 60%, and a thickness of less than 2 mm; a non-porous layer (ii) with a thickness of less than 150 ?m made of an oxide or mixtures of oxides that allows the simultaneous transport of oxygen ions and electronic carriers through it; said catalytic activation layer (iii), and produced according to the process of claim 4.

7. A process for producing the oxygen-permeable membrane of claim 6, comprising at least the following steps: a) forming the porous support (i) by a technique selected from the group consisting of uniaxial pressing, isostatic pressing, extrusion, calendering, tape casting, slip casting, dip coating, spin coating, roller coating, silk-screen printing, physical vapor deposition, spraying of suspensions, 3D printing, stereolithography, injection and combinations thereof; b) forming the non-porous layer (ii) by a technique selected from the group consisting of uniaxial pressing, isostatic pressing, extrusion, calendering, tape casting, slip casting, dip coating, spin coating, roller coating or silk-screen printing, physical vapor deposition, spraying of suspensions, 3D printing, stereolithography, injection, inkjet printing and combinations thereof; c) coating the surface of the non-porous separation layer (ii) with a material comprising ceramic oxide particles which conduct oxygen ions and electronic carriers by a technique selected from the group consisting of nebulization, atomization, thermal atomization, pyrolytic atomization, airbrushing, dip coating, spin coating, roller coating, silk screen printing, technique of chemical vapor deposition, physical vapor deposition, printing by inkjet and thermal spraying, and combinations thereof; d) incorporating the catalyst into the particles surface of the porous structure that covers the non-porous separation layer (ii) by a technique selected from the group consisting of: impregnation of liquid solutions of precursors of metals comprised in the catalyst composition of formula A.sub.1-x-yB.sub.xC.sub.yO.sub.R; infiltration of liquid solutions of precursors of metals comprised in the catalyst composition of formula A.sub.1-x-yB.sub.xC.sub.yO.sub.R; infiltration of a nanoparticle dispersion of the catalyst; deposition in vapor phase by physical vapor deposition techniques; deposition in vapor phase by chemical vapor deposition techniques; and combinations thereof.

8. The process for producing an oxygen-permeable membrane of claim 7, further comprising a thermal treatment step at temperatures between 900 and 1250? C. between steps c and d.

9. The process for producing an oxygen-permeable membrane of claim 7 further comprising a last step of thermal treatment at temperatures comprised between 650 and 1100? C.

10. A method of preparing an oxygen-permeable membrane comprising the step of providing said catalytic activation layer produced according to the method of claim 4, and producing oxygen-permeable membranes.

11. A method of generating an O.sub.2 rich stream comprising: providing the membrane of claim 6 or the membrane being produced by the process of claim 7 and generating an O2 rich stream.

12. The method of claim 11, wherein the generated O.sub.2 stream has a purity greater than 99% by volume.

13. The method of claim 11, wherein the membrane comprises an entrainment gas for the permeated O.sub.2.

14. The method of claim 13, wherein the entrainment gas has an SO.sub.2 content greater than 5 ppm.

15. The method of claim 11, wherein the membrane feed stream has an SO.sub.2 content greater than 5 ppm.

16. The method of claim 11, wherein the membrane is integrated in an oxy-combustion system or systems which comprise oxygen enriched combustion stages.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1. Shows an oxy-combustion system provided with a O.sub.2 production unit by means of ion transport membranes. The system uses vacuum in the permeate chamber of the module and different heat exchangers (1), (2) and (3)

(2) FIG. 2 shows a simplified representation of a membrane with (i) a porous support, (ii) a selective separation non-porous layer and (iii) a catalytic layer;

(3) FIG. 3 shows a simplified representation of a membrane with (i) a porous support, (iv) an intermediate catalytic layer, (ii) a selective separation nonporous layer and (iii) an upper catalytic layer;

(4) FIG. 4 shows a simplified representation of a membrane with (i) a porous support, (iv) an intermediate catalytic layer, (ii) a non-porous selective separation layer, (v) a second nonporous separation and protection layer and (iii) an upper catalytic layer;

(5) FIG. 5 shows a scanning electron microscope image of a cross-section of an oxygen-permeable ceramic membrane having (i) a porous support, (ii) a selective separation nonporous layer and (iii) a catalytic layer;

(6) FIG. 6 shows a scanning electron microscope image of a cross-section of an oxygen-permeable ceramic membrane having (i) a porous carrier, (ii) a non-porous selective separation layer, (v) a second nonporous separation and protection layer and (iii) a catalytic layer;

EXAMPLES

Example 1

(7) A Fe.sub.2NiO.sub.4Ce.sub.0.8Tb.sub.0.2O.sub.2-? (NFO-CTO) composite material in a 1:1 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 2:1 molar ratio with respect to the metallic cations. The water in the solution is evaporated and the resulting residue is calcined at 800? C. 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.sub.0.2Gd.sub.0.2O.sub.1.9, CGO) by calcining at 1000? C. 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 1500? C., and the disc is given a final flat shape by sanding and polishing.

Example 2

(8) 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 700? C. in such a way that the catalyst is deposited in its active form.

Example 3

(9) Sample prepared in the same manner as Example B, but infiltrating on this occasion a precursor solution of Pr nitrate.

Example 4

(10) Sample prepared in the same manner as Example B, but infiltrating on this occasion a Sm nitrate precursor solution.

Example 5

(11) Sample prepared in the same manner as Example B, but infiltrating on this occasion a precursor solution of Tb nitrate.

Example 6

(12) Sample prepared in the same manner as Example B, but infiltrating on this occasion a precursor solution of Co nitrate

Example 7

(13) Sample prepared in the same manner as Example B, but infiltrating on this occasion a precursor solution of Nb oxalate.

Example 8

(14) Sample prepared in the same manner as Example B, but infiltrating on this occasion an ammonium heptamolybdate precursor solution.

Example 9

(15) Sample prepared in the same manner as Example B, but infiltrating on this occasion a precursor solution of zirconyl nitrate.

Example 10

(16) Sample prepared in the same manner as Example B, but infiltrating on this occasion an Al nitrate precursor solution.

Example 11

(17) Sample prepared in the same manner as Example C, but adding, on this occasion, to this solution 20% by volume of the precursor solution of Example J, consequently adding a molar charge of 80% of Pr and 20% Al.

Example 12

(18) Sample prepared in the same manner as Example B, but adding, on this occasion, to this solution 20% by volume of the precursor solution in Example B, consequently adding a molar charge of 50% of Pr and 50% Ce.

(19) 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 (850? C.) has been arranged.

(20) 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.sub.2 and SO.sub.2).

(21) 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 30 ?m 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.

(22) 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 1000? C. to consolidate the bonding of the layers to the disc and for the support porous structure to remain stable.

(23) 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).

(24) The results of the study are shown in Table 1, which shows the polarization resistance in ohms per square centimeter (?-cm.sup.2) obtained for each of the examples at 850? C. after a stabilization of 10 hours under each condition depending on the atmospheres to which it has been subjected, including the study in air, CO.sub.2 with (5%) O.sub.2, and CO.sub.2 with (5%) O.sub.2 and 250 ppm of SO.sub.2. 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.sub.2, 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 (PrAl) and L (CePr). In the case of the PrAl combination, a metal with high redox catalytic activity was combined under conditions of absence of SO.sub.2 and a promoter of this activity and that had acidity under operating conditions, and allowed to decrease the adsorption of SO.sub.2 and its consequent damaging effect in the catalytic activity. The case of the combination CePr 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 PrGa, PrNb, PrW, PrMo, CeAl, CeY, CePrAl, CsSmSmGa, etc.

(25) TABLE-US-00001 TABLE 1 Example 21% O.sub.2 in 5% O.sub.2 in 5% O.sub.2 in 5% O.sub.2 in CO.sub.2 (catalyst) N.sub.2 N.sub.2 CO.sub.2 and 250 A (without 1.85 2.69 2.71 7.09 impregnating) B (Ce) 1.10 1.31 1.33 3.83 C (Pr) 0.18 0.88 1.15 5.27 D (Sm) 0.96 1.25 1.31 6.96 E (Tb) 0.52 0.82 0.91 8.70 F (Co) 0.47 0.76 0.84 10.34 G (Nb) 5.81 8.91 9.47 12.72 H (Mo) 16.69 22.23 23.42 24.11 I (Zr) 1.19 1.51 1.56 5.38 J (Al) 2.01 2.82 2.92 9.63 K (PrAl) 0.22 0.34 0.35 3.33 L (PrCe) 0.25 0.35 0.38 2.42

(26) Thus, it has been possible to obtain a high catalytic activity for the activation of gaseous oxygen in oxygen transport membranes in gases containing 250 ppm of SO.sub.2 using the catalytic layer composed of (1) 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.sub.2 under the described operating conditions, and (2) 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 13

(27) An NFO-CTO membrane obtained by uniaxial pressing, silk screen printing and subsequent calcination at 1400? C. of precursor powder obtained by the Pechini method. Subsequently, a layer of the NFO-CTO composite is deposited by silk-screen printing, calcining at 1000? C. 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 100 ?m thickness (ii) and a upper porous catalytic layer (iii), according to the scheme shown in FIG. 1.

Example 14

(28) 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 100 ?m thickness (ii) and an upper porous catalytic layer (iii), in accordance with the scheme shown in FIG. 1.

(29) 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.

(30) 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.sub.2 and SO.sub.2).

(31) 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 15 mm diameter and about 1 mm 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-4900 equipped with three analysis modules: Molsieve5A, PoraPlot-Q and CP-Sil. Table 2 shows the oxygen permeation obtained in milliliters (normal conditions) per minute and square centimeter (Nml.Math.min.sup.?1.Math.cm.sup.?2) for the membranes according to examples M and N in different atmospheres at 730? C. after 8 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.sub.2, conditions in which the oxygen gaseous exchange becomes notably difficult, and therefore the effect of an active catalyst becomes much more important.

(32) TABLE-US-00002 TABLE 2 Example Entrainment gas Entrainment gas Entrainment gas (catalyst) Ar 100% CO.sub.2 100% CO.sub.2 M (without 0.8 0.6 0.1 impregnating) N (PrAl) 1.6 1.3 1.0