BROWNMILLERITE-BASED POLYCRYSTALLINE FUSED PRODUCT

Abstract

A polycrystalline fused product based on brownmillerite, includes, for more than 95% of its weight, of the elements Ca, Sr, Fe, O, M and M′, the contents of the elements being defined by the formula X.sub.yM.sub.zFe.sub.tM′.sub.uO.sub.2.5, wherein the atomic indices are such that 0.76≤y≤1.10, z≤0.21, 0.48≤t≤1.15 and u≤0.52, 0.95≤y+z≤1.10, and 0.95≤t+u≤1.10, X being Ca or Sr or a mixture of Ca and Sr, M being an element chosen from the group formed by La, Ba and mixtures thereof, M′ being an element chosen from the group formed by Ti, Cu, Gd, Mn, Al, Sc, Ga, Mg, Ni, Zn, Pr, In, Co, and mixtures thereof, the sum of the atomic indices of Ti and Cu being less than or equal to 0.1.

Claims

1. A polycrystalline fused product based on brownmillerite, consisting, for more than 95% of its weight, of the elements Ca, Sr, Fe, O, M and M′, the contents of said elements being defined by the formula X.sub.yM.sub.zFe.sub.tM′.sub.uO.sub.2.5, wherein the atomic indices are such that 0.76≤y≤1.10, z≤0.21, 0.48≤t≤1.10 and u≤0.52, 0.95≤y+z≤1.10, and 0.95≤t+u≤1.10, X being Ca or Sr or a mixture of Ca and Sr, M being an element chosen from the group formed by La, Ba and mixtures thereof, M′ being an element chosen from the group formed by Ti, Cu, Gd, Mn, Al, Sc, Ga, Mg, Ni, Zn, Pr, In, Co, and mixtures thereof, the sum of the atomic indices of Ti and Cu being less than or equal to 0.1.

2. The fused product as claimed in claim 1, wherein 0.85≤y≤1.05 and/or z≤0.15 and/or 0.75≤t≤1.05 and/or u≤0.25.

3. The fused product as claimed in claim 1, wherein the content of brownmillerite phase is greater than 50%.

4. The fused product as claimed in claim 1, wherein z=0.

5. The fused product as claimed in claim 1, wherein u=0.

6. The fused product as claimed in claim 1, wherein z=0 and u=0.

7. The fused product as claimed in claim 1, wherein the element M′ is chosen from Ti, Cu, Ni, Co, Mn and mixtures thereof.

8. The fused product as claimed in claim 1, of formulation X.sub.yM.sub.zFe.sub.tM′.sub.uO.sub.2.5, wherein, if x represents the proportion of Sr and (1−x) the relative proportion of Ca in the formula (Ca.sub.(1-x)Sr.sub.x).sub.yM.sub.zFe.sub.tM′.sub.uO.sub.2.5, 0<x≤0.1, 0.9≤y≤1.05, 0.1≥z≥0.01, y+z≤1.1, t≥0.8, 0.01≤u≤0.2, t+u≤1.1, and the total weight content of elements other than Ca, Sr, Fe, M, M′ and O is less than 3% as a percentage on the basis of the weight of the product.

9. The fused product as claimed in claim 1, of formulation X.sub.yM.sub.zFe.sub.tM′.sub.uO.sub.2.5, wherein, if x represents the proportion of Sr and (1−x) the relative proportion of Ca in the formula (Ca.sub.(1-x)Sr.sub.x).sub.yM.sub.zFe.sub.tM′.sub.uO.sub.2.5, x=0, 0.9≤y≤1.05, 0.1≥z≥0.01, y+z≤1.1, t≥0.8, 0.01≤u≤0.2, t+u≤1.1, and the total weight content of elements other than Ca, Sr, Fe, M, M′ and O is less than 3% as a percentage on the basis of the weight of the product.

10. The fused product as claimed in claim 1, wherein, if x represents the proportion of Sr and (1−x) the relative proportion of Ca in the formula (Ca.sub.(1-x)Sr.sub.x).sub.yM.sub.zFe.sub.tM′.sub.uO.sub.2.5, 1>x≥0.9, 0.9≤y≤1.05, 0.1≥z≥0.01, y+z≤1.1, t≥0.8, 0.01≤u≤0.2, t+u≤1.1, and the total weight content of elements other than Ca, Sr, Fe, M, M′ and O is less than 3% as a percentage on the basis of the weight of the product.

11. The fused product as claimed in claim 1, wherein, if x represents the proportion of Sr and (1−x) the relative proportion of Ca in the formula (Ca.sub.(1-x)Sr.sub.x).sub.yM.sub.zFe.sub.tM′.sub.uO.sub.2.5, x=1, 0.9≤y≤1.05, 0.1≥z≥0.01, y+z≤1.1, t≥0.8, 0.01≤u≤0.2, t+u≤1.1, and the total weight content of elements other than Ca, Sr, Fe, M, M′ and O is less than 3% as a percentage on the basis of the weight of the product.

12. A powder comprising more than 90% by weight of particles in a fused product as claimed in claim 1.

13. The powder as claimed in claim 12, having a median size D.sub.50 of greater than 0.1 μm and less than 4 mm.

14. A process for producing a fused product as claimed in claim 1, comprising: a. mixing starting materials so as to form a starting feedstock suitable for obtaining, at the end of step c), said fused produce, b. melting the starting feedstock until a liquid mass is obtained, c. cooling until complete solidification of said liquid mass, so as to obtain said fused product.

15. A catalytic support or a catalyst comprising or consisting of a fused product as claimed in claim 1.

16. An oxygen separation membrane comprising or consisting of a fused product as claimed in claim 1.

17. An electrode for a solid oxide fuel cell SOFC comprising or consisting of a fused product as claimed in claim 1.

Description

EXAMPLES

[0180] The following examples are given for illustrative purposes and do not limit the invention.

[0181] The products of examples 1 and 2, which are comparative examples, are produced chemically in the following way.

[0182] For each of examples 1 and 2, the mixture of salts that is defined in the following table 1 is introduced into distilled water, and the pH is kept below 1 by addition of anhydrous citric acid.

TABLE-US-00001 TABLE 1 Example Ca(NO.sub.3).sub.2 .Math. 4H.sub.2O Sr(NO.sub.3).sub.2 Fe(NO.sub.3).sub.3 .Math. 9H.sub.2O 1(*) 1.76 — 3.04 2(*) — 1.15 2.20 (*)outside the invention

[0183] After 3 hours of stirring using a magnetic stirrer, the pH is brought to a value equal to 3 by addition of aqueous ammonia, then the mixture is brought to 110° C. so as to evaporate off the water. When the gel begins to form, 1 ml of ethylene glycol is added. After a stirring time of 30 minutes using a magnetic stirrer, the gel formed is brought to a temperature equal to 370° C. for a holding time at this temperature equal to 2 hours by means of a hotplate. The powder thus obtained is then calcined in an electric furnace at 600° C. for 6 hours, under air, with a temperature increase rate to this temperature equal to 100° C./h.

[0184] The fused products of examples 3 and 4 were produced in the following way.

[0185] The following initial starting materials were first intimately mixed in a mixer: [0186] for examples 3 and 4, an iron oxide powder comprising more than 99% by weight of Fe.sub.2O.sub.3, the median size of which is equal to 0.34 μm; [0187] for example 3, a powder comprising more than 99% by weight of calcium carbonate CaCO.sub.3, the median size of which is equal to 1.85 μm; [0188] for example 4, a powder comprising more than 99% by weight of strontium carbonate SrCO.sub.3, the median size of which is equal to 4.3 μm.

[0189] For each of examples 3 and 4, the starting feedstock is defined in the following table 2, as weight percentages:

TABLE-US-00002 TABLE 2 Example CaCO.sub.3 SrCO.sub.3 Fe.sub.2O.sub.3 3 55.6 — 44.4 4 — 64.9 35.1

[0190] For each example, the starting feedstock having a weight of 5 kg was poured into a Heroult arc melting furnace. It was then melted according to melting with a voltage of 120 volts and an applied energy substantially equal to 2000 kWh/T, in order to completely and uniformly melt all the mixture.

[0191] Then, when the melting was complete, the molten liquid was cast so as to form a thin stream.

[0192] Blowing of compressed dry air, at ambient temperature and at an overpressure of 3 bar, broke the thin stream and dispersed the molten liquid into droplets.

[0193] The blowing cooled these droplets and set them in the form of fused particles.

[0194] The fused particles of examples 3 and 4 were received in a container.

[0195] The chemical analyses were carried out on calcined samples for 2 hours at 1000° C.

[0196] The chemical analysis was carried out by X-ray fluorescence on a bead of the product to be analyzed, produced by fusing said product with lithium borate.

[0197] The determination of the content of brownmillerite phase was carried out on samples which had, after dry grinding, a median size of less than 40 μm.

[0198] The specific surface area is measured by the BET (Brunauer Emmet Teller) method described in the Journal of American Chemical Society 60 (1938), pages 309 to 316.

The median size is measured using a Partica LA-950 laser particle size analyzer from the company Horiba.

[0199] Tables 3 and 4 below summarize the results obtained.

TABLE-US-00003 TABLE 3 Others Ca Sr Fe M M′ (% by Ex. (1 − x) .Math. y x .Math. y t z u weight) 1(*) 0.99 0 1.00 0 0 0.00 2(*) 0 0.96 1.02 0 0 0.00 3 .sup.  0.96 0 1.02 0 0 0.11 4 .sup.  0 1.02 0.97 0 0 0.05 (*)outside the invention

TABLE-US-00004 TABLE 4 Content of brownmillerite Example phase Secondary phases 1(*) 95 CaO/Fe.sub.2O.sub.3/CaCO.sub.3 2(*) 43 SrFeO.sub.2.75/SrFeO.sub.2.875/SrCO.sub.3/Fe 3 .sup.  97 Fe.sub.2O.sub.3/FeO/Fe 4 .sup.  58 SrFeO.sub.2.75/SrFeO.sub.2.875/SrCO.sub.3/Fe (*)outside the invention

[0200] The X-ray diffraction patterns of the products of examples 3 and 4 do not show, at low angles, a halo characteristic of the presence of an amorphous phase.

[0201] The products of the examples were then ground for 72 hours in a jar mill at a rotation speed equal to 60 rpm, the volume of the jar being equal to 0.9 liter, into which were introduced 1.4 kg of cylpebs made of zirconia doped with 3% by weight of MgO having a dimension equal to 12.7 mm and 650 grams of the product of the example. After separation of the cylpebs, 40 g of the powder recovered are then ground in an attrition mill for 75 minutes at 1000 rpm, the tank with a volume equal to 0.89 liter also containing 680 g of beads of zirconia yttriated at 3 mol % having a median size equal to 0.8 mm, and 200 ml of isopropanol. The characteristics of the powders obtained are summarized in the following table 5.

TABLE-US-00005 TABLE 5 Specific surface Ex Median size (μm) area (m.sup.2/g) 1(*) 0.56 12 2(*) 0.56 13 3 .sup.  1.2 <1 4 .sup.  1 <1 (*)outside the invention

[0202] The powders of examples 1 and 2, outside the invention, and 3 and 4, according to the invention, were used in the production of catalytic systems as a catalyst support, platinum being used as catalyst, the weight amount of platinum being substantially equal to 0.99%, on the basis of the weight of the catalyst support and of the platinum.

[0203] The preparation of the catalytic systems was carried out by impregnation of the powder of each example with a solution of platinum nitrate Pt(NO.sub.3).sub.2. This method, which is simple to carry out, is well known to those skilled in the art. After suspending the powder in the platinum nitrate solution, the latter is left in an ultrasound bath for 30 minutes at ambient temperature. The various powders impregnated are dried in a rotary evaporator at 47° C. at a pressure of 130 mbar for 3 h. The various powders impregnated are then calcined under air at 500° C. for a holding time at this temperature equal to 2 hours, with a temperature increase rate equal to 10° C./min and a decrease at an uncontrolled rate.

[0204] After calcination, the powders are compacted on a hand press so as to form a pellet. The pellet is then broken using a pestle and mortar, and the powder obtained is sieved so as to recover the catalytic system corresponding to the fraction of powder which has not passed through a sieve with square meshes having an opening equal to 125 μm and which passes through a sieve with square meshes having an opening equal to 250 μm.

[0205] Catalytic tests were then carried out with each catalytic system, on a reaction for oxidation of carbon monoxide to carbon dioxide (CO+½ O.sub.2.fwdarw.CO.sub.2), in a quartz fixed-bed flow-through open reactor, and at atmospheric pressure, according to the following procedure: 200 mg of the catalytic system (in the case in point the powder of examples 1*, 2*, 3 and 4, impregnated with platinum) are placed in the reactor. A first step of reducing the powders consists in injecting a stream of 10 I/h of a mixture of 40% hydrogen and 60% argon by volume in a cycle having an increase rate equal to 10° C./min up to a temperature equal to 300° C. and a hold at this temperature equal to 1 hour. The temperature decrease is carried out under helium at the rate of inertia of the furnace down to ambient temperature.

[0206] A first catalytic cycle is carried out, consisting of an increase at a rate equal to 2° C./min up to a temperature equal to 350° C., a hold at 350° C. for 10 minutes, then a decrease to ambient temperature at the rate of inertia of the reactor. During the increase and the stationary phase at 350° C., a reaction mixture of 6000 ppm of CO and of 10 000 ppm of oxygen diluted in helium is injected into the reactor at an overall flow rate equal to 10 l/h. The decrease is carried out under helium, the helium flow rate being equal to 1 l/h.

[0207] The degree of conversion of the carbon monoxide to carbon dioxide, as %, is defined as the ratio of the amount of carbon monoxide having reacted to the amount of carbon monoxide introduced into the reactor, the amount of carbon monoxide having reacted being equal to the amount of carbon monoxide introduced into the reactor reduced by the amount of unreacted carbon monoxide leaving the reactor. The amount of unreacted carbon monoxide leaving the reactor and the amount of carbon dioxide leaving the reactor are measured throughout the catalytic cycle by means of micro gas chromatography with a thermal conductivity detector, equipped with two parallel columns, and at a rate of sampling every 235 seconds.

[0208] A second catalytic cycle, then a third catalytic cycle are carried out under the same conditions.

[0209] The following table 6 groups together the results obtained:

TABLE-US-00006 TABLE 6 Temperature at which Temperature at which a degree of a degree of conversion of carbon conversion of carbon Powder used as monoxide to carbon monoxide to carbon catalyst support dioxide equal to 50% dioxide equal to 50% in the catalytic is achieved in the is achieved in the system first cycle (° C.) third cycle (° C.) Ex1(*) 221 240 Ex2(*) 202 247 Ex3 .sup.  187 201 Ex4 .sup.  197 196 (*)Comparative example outside the invention

[0210] A comparison of example 1 (outside the invention) and example 3 (according to the invention), for which X═Ca, shows a degree of conversion equal to 50% achieved at a temperature of 187° C. for example 3 and of 221° C. for example 1 in the first catalytic cycle and a degree of conversion equal to 50% achieved at a temperature of 201° C. for example 3 and of 240° C. for example 1 in the third catalytic cycle.

A comparison of example 2 (outside the invention) and example 4 (according to the invention), for which X═Sr, shows a degree of conversion equal to 50% achieved at a temperature of 197° C. for example 4 and of 202° C. for example 2 in the first catalytic cycle and a degree of conversion equal to 50% achieved at a temperature of 196° C. for example 4 and of 247° C. for example 2 in the third catalytic cycle.
These results show that a high degree of conversion of carbon monoxide to carbon dioxide is achieved at a lower temperature with a catalytic system obtained from a powder derived from a polycrystalline fused product according to the invention, thus making it possible to obtain a very good conversion efficiency at lower temperatures than with the powders of the examples outside the invention.

[0211] As is clearly apparent at the current time, the powder according to the invention makes it possible to improve the catalytic performance qualities.

[0212] Of course, the present invention is not limited to the described embodiments provided by way of illustrative and nonlimiting examples.

[0213] In particular, it is obvious for those skilled in the art that the introduction, into the structures described, of another cationic element M and/or M′ according to the invention, as described above, will result in the same catalytic performance qualities as those described above, provided that the fused product obtained retains this same brownmillerite structure.