Composition containing zirconium, cerium and yttrium oxides having a high reducibility, method for preparing same and use thereof in catalysis

Abstract

The invention relates to a composition containing zirconium, cerium and yttrium oxides with a cerium oxide proportion of between 3% and 15%, and yttrium oxide proportions corresponding to the following conditions: 6% at most if the cerium oxide proportion is between 12% excluded and 15% included; 10% at most if the cerium oxide proportion is between 7% excluded and 12% included; 30% at most if the cerium oxide proportion is between 3% and 7% included; the balance consisting of zirconium oxide. The composition may optionally include an oxide of a rare earth selected from lanthanum, neodymium and praseodymium. The composition can be used for processing the exhaust gases of a vehicle.

Claims

1. A catalyst composition comprising oxides of zirconium, cerium and yttrium, in the following proportions by weight relative to the total weight of the composition: a proportion of cerium oxide of between 3 and 15%; a proportion of yttrium oxide which meets one of the following conditions: a proportion of yttrium oxide of at most 6% if the proportion of cerium oxide is between 12% exclusive and 15% inclusive; a proportion of yttrium oxide of at most 10% if the proportion of cerium oxide is between 7% exclusive and 12% inclusive; or a proportion of yttrium oxide of at most 30% if the proportion of cerium oxide is between 3 and 7% inclusive; and the remainder as zirconium oxide; wherein the composition, after calcination at 1000 C. for 4 hours, exhibits: a degree of reducibility of at least 90%; a maximum reducibility temperature of at most 550 C.; and a specific surface area of at least 40 m.sup.2/g.

2. The catalyst composition as claimed in claim 1, the content of yttrium is at least 3%.

3. The catalyst composition as claimed in claim 1, wherein the composition has a degree of reducibility of at least 94%, measured on the composition calcined at 1000 C. for 4 hours.

4. The catalyst composition as claimed in claim 1, wherein the composition has a maximum reducibility temperature of at most 530 C.

5. A catalytic system comprising a catalytic composition as claimed in claim 1.

6. A process for the treatment of exhaust gases from internal combustion engines, the process comprising contacting the exhaust gas with a catalyst composition as claimed in claim 1.

7. The catalyst composition according to claim 1, wherein the composition has a specific surface area of at least 10 m.sup.2/g after calcination at 1100 C. for 4 hours.

8. The catalyst composition according to claim 1, wherein the composition has a specific surface area of at least 15 m.sup.2/g after calcination at 1100 C. for 4 hours.

9. The catalyst composition according to claim 1, wherein the composition has a specific surface area of at least 20 m.sup.2/g after calcination at 1100 C. for 4 hours.

10. The catalyst composition according to claim 1, wherein the composition has a specific surface area of at least 2 m.sup.2/g after calcination at 1200 C. for 4 hours.

11. A catalyst composition comprising oxides of zirconium, cerium, yttrium and at least one oxide of a rare earth metal selected from lanthanum, neodymium and praseodymium, in the following proportions by weight relative to the total weight of the composition: a proportion of cerium oxide of between 3 and 15%; a proportion of yttrium oxide which meets one of the following conditions: a proportion of yttrium oxide of at most 6% if the proportion of cerium oxide is between 12% exclusive and 15% inclusive; a proportion of yttrium oxide of at most 10% if the proportion of cerium oxide is between 7% exclusive and 12% inclusive; or a proportion of yttrium oxide of at most 30% if the proportion of cerium oxide is between 3 and 7% inclusive; a proportion of the at least one oxide of a rare earth metal that meets one of the following conditions: a proportion of oxide of said rare earth metal of at most 10% if the proportion of cerium oxide is between 12% exclusive and 15% inclusive; a proportion of oxide of said rare earth metal of at most 18% if the proportion of cerium oxide is between 7% exclusive and 12% inclusive; or a total proportion of yttrium oxide and of oxide of said rare earth metal of at most 30% if the proportion of cerium oxide is between 3 and 7% inclusive; and the remainder as zirconium oxide; wherein the composition, after calcination at 1000 C. for 4 hours, exhibits: a degree of reducibility of at least 90%; a maximum reducibility temperature of at most 550 C.; and a specific surface area of at least 40 m.sup.2/g.

12. The catalyst composition as claimed in claim 11, wherein the composition is in the form of particles which exhibit a concentration gradient for the yttrium and for the rare earth metal.

13. A process for the preparation of the catalyst composition of claim 11, the process comprising: (a) contacting at least a portion of a basic compound and compounds of zirconium and cerium with a portion of at least one of a compound of yttrium or compounds of the at least one rare earth metal selected from lanthanum, neodymium and praseodymium, in a liquid medium to form a precipitate; (b) heating the precipitate in the liquid medium; (c) adding, after stage (b), remaining basic compound, if any, and remaining compounds of yttrium and of the rare earth metal to the liquid medium; (d) adding an additive selected from anionic surfactants, nonionic surfactants, polyethylene glycols, carboxylic acids and their salts, and surfactants of the carboxymethylated ethoxylates of fatty alcohols type, to the precipitate obtained in stage (c); (e) calcining the precipitate obtained in stage (d).

14. The catalyst composition according to claim 11, wherein the composition has a specific surface area of at least 45 m.sup.2/g after calcination at 1000 C. for 4 hours.

15. The catalyst composition according to claim 11, wherein the composition has a specific surface area of at least 50 m.sup.2/g after calcination at 1000 C. for 4 hours.

16. The catalyst composition according to claim 11, wherein the composition has a specific surface area of at least 20 m.sup.2/g after calcination at 1100 C. for 4 hours.

17. The catalyst composition according to claim 11, wherein the composition has a specific surface area of at least 25 m.sup.2/g after calcination at 1100 C. for 4 hours.

18. The catalyst composition according to claim 11, wherein the composition has a specific surface area of at least 30 m.sup.2/g after calcination at 1100 C. for 4 hours.

19. The catalyst composition according to claim 11, wherein the composition has a specific surface area of at least 2 m.sup.2/g after calcination at 1200 C. for 4 hours.

20. A process for the preparation of a catalyst composition, the process comprising: (a) contacting a basic compound with compounds of zirconium, cerium, and yttrium, in a liquid medium to form a precipitate; (b) heating the precipitate in the liquid medium; (c) adding an additive selected from anionic surfactants, nonionic surfactants, polyethylene glycols, carboxylic acids and their salts, and surfactants of the carboxymethylated ethoxylates of fatty alcohols type, to the precipitate-obtained in stage (b); (d) calcining the precipitate obtained in stage (c), wherein the catalyst composition comprises oxides of zirconium, cerium and yttrium, in the following proportions by weight relative to the total weight of the composition: a proportion of cerium oxide of between 3 and 15%; a proportion of yttrium oxide which meets one of the following conditions: a proportion of yttrium oxide of at most 6% if the proportion of cerium oxide is between 12% exclusive and 15% inclusive; a proportion of yttrium oxide of at most 10% if the proportion of cerium oxide is between 7% exclusive and 12% inclusive; or a proportion of yttrium oxide of at most 30% if the proportion of cerium oxide is between 3 and 7% inclusive; and the remainder as zirconium oxide, and wherein the composition, after calcination at 1000 C. for 4 hours, exhibits: a degree of reducibility of at least 90%; a maximum reducibility temperature of at most 550 C.; and a specific surface area of at least 40 m.sup.2/g.

21. The process as claimed in claim 20, wherein stage (a) comprises: contacting the compounds of zirconium and of cerium with a portion or all of the basic compound and, the compounds of yttrium or of the rare earth metal, and adding remaining basic compound, if any, and any remaining compound of yttrium.

22. The process as claimed in claim 20, wherein the compounds of zirconium, cerium, and yttrium and comprise the nitrates, sulfates, acetates and/or chlorides thereof.

23. The process as claimed in claim 20, wherein heating the precipitate is carried out at a temperature of at least 100 C.

24. The process as claimed in claim 20, wherein the precipitate is washed before the calcination.

Description

EXAMPLE 1

(1) This example relates to a composition comprising 83% of zirconium, 5% of cerium, 2% of lanthanum, 5% of yttrium and 5% of neodymium, these proportions being expressed as percentages by weight of the oxides ZrO.sub.2, CeO.sub.2, La.sub.2O.sub.3, Y.sub.2O.sub.3 and Nd.sub.2O.sub.3.

(2) 313 ml of zirconium nitrate (266 g/l as ZrO.sub.2), 19.7 ml of cerium nitrate, 4.4 ml of lanthanum nitrate (456 g/l as La.sub.2O.sub.3), 13.1 ml of yttrium nitrate (382 g/l as Y.sub.2O.sub.3) and 9.5 ml of neodymium nitrate (524 g/l as Nd.sub.2O.sub.3) are introduced into a stirred beaker. The mixture is subsequently made up with distilled water so as to obtain 1 liter of a solution of nitrates.

(3) 203 ml of an aqueous ammonia solution (12 mol/l) are introduced into a stirred reactor and the solution is subsequently made up with distilled water so as to obtain a total volume of 1 liter.

(4) The solution of nitrates is introduced into the reactor with continual stirring.

(5) The solution obtained is placed in a stainless steel autoclave equipped with a stirrer. The temperature of the medium is brought to 150 C. for 2 hours with stirring.

(6) 33 grams of lauric acid are added to the suspension thus obtained. The suspension is kept stirred for 1 hour.

(7) The suspension is then filtered through a Bchner funnel and then aqueous ammonia solution is added to the filtered precipitate in a proportion of one times the volume of the aqueous filtration mother liquors. The product obtained is subsequently brought to 700 C. for 4 hours under stationary conditions.

EXAMPLES 2 TO 12

(8) Unless otherwise indicated below, the procedure is the same as in example 1. For example 8, the process starts with a dilute solution B containing the neodymium nitrate and with a dilute solution A containing all the other constituents. Solution B is added to the aqueous ammonia solution after solution A and then the combined mixture is heated as in example 1. For example 9, solution B contains yttrium nitrate. For example 10, solution B comprises yttrium nitrate and neodymium nitrate. For example 6, the praseodymium nitrate solution exhibits a Pr.sub.6O.sub.1 concentration of 500 g/l.

(9) The volumes of various reactant solutions used in the preparation of the compositions of the examples will be found in table 1 below. The contents of oxides of various compositions obtained on conclusion of these preparations are given in table 2. The surface values for these same compositions are shown in table 3 and their reducibility properties after calcination at 1000 C. for 4 hours are shown in table 4.

(10) The reducibility profiles drawn up during the measurement of the degree of reducibility according to the method given above for these oxides show low-temperature peaks, which means that these oxides exhibit a fraction of cerium reducible at low temperature. Thus, the temperature at which this first reduction occurs lies between 150 and 230 C. The area corresponding to this peak is equivalent to between 3 and 15% of the total area under the reducibility curve. This means that the fraction of cerium reducible at low temperature represents between 3 and 15% of the total cerium when the reducibility is 100%.

(11) In order to clearly confirm that these peaks are not due to impurities present at the surface of the oxide which are reduced during the treatment carried out during the reducibility measurement, which is sometimes the case, reducibility measurements were carried out after an in situ precalcination under air of the sample at 800 C. This pretreatment makes it possible to remove all the impurities present at the surface of the sample. The low-temperature peaks remain after pretreatment, which means that this phenomenon indeed corresponds to a reduction of the cerium.

(12) These results show that the compositions of the invention begin to exhibit oxidation/reduction properties from low temperatures, in particular in a temperature range lying between 150 C. and 230 C.

(13) TABLE-US-00001 TABLE 1 Amounts of reactants used, expressed as volume (ml) of the solutions of the salts of the various elements V aqueous Example V Zr V Ce V La V Y V Nd V Pr ammonia 1 313 19.7 4.4 13.1 9.5 203 2 275 19.7 4.4 13.1 28.6 204 3 256 19.7 4.4 52.4 9.5 221 4 218 19.7 4.4 52.4 28.6 222 5 266 19.7 4.4 32.4 19.1 212 6 301 19.7 26.2 10.0 208 7 294 39.4 4.4 13.1 9.5 208 8 282 39.4 4.4 20.9 9.5 211 9 313 39.4 18.3 209 10 271 39.4 20.9 19.1 212 11 256 39.4 4.4 13.1 28.6 210 12 275 59.1 4.4 13.1 9.5 213

(14) TABLE-US-00002 TABLE 2 Contents, expressed as oxides, of the various elements Example % Zr % Ce % La % Y % Nd % Pr 1 83 5 2 5 5 2 73 5 2 5 15 3 68 5 2 20 5 4 58 5 2 20 15 5 70.5 5 2 12.5 10 6 80 5 10 5 7 78 10 2 5 5 8 75 10 2 8 5 9 83 10 7 10 72 10 8 10 11 68 10 2 5 15 12 73 15 2 5 5

(15) TABLE-US-00003 TABLE 3 Specific surfaces in m.sup.2/g Example 4 h/900 C. 4 h/1000 C. 4 h/1100 C. 1 67 47 23 2 80 51 22 3 71 51 28 4 70 50 26 5 80 53 23 6 60 45 20 7 70 47 22 8 64 47 27 9 52 33 15 10 64 49 27 11 80 53 22 12 71 48 21

(16) TABLE-US-00004 TABLE 4 Reducibility properties Maximum reducibility Degree of reducibility Example temperature at 1000 C. at 1000 C. 1 516 100 2 508 100 3 515 100 4 534 100 5 514 100 6 526 100 7 534 100 8 530 98 9 544 99 10 535 93 11 526 90 12 547 94

EXAMPLES 13 AND 14

(17) Two oxides with the composition ZrO.sub.2/CeO.sub.2/Y.sub.2O.sub.3/Nd.sub.2O.sub.3 75/10/5/10 are prepared. Example 13 is carried out according to the procedure of example 1. For example 14, the process starts with a dilute solution B containing the nitrates of neodymium and of yttrium and with a dilute solution A containing all the other constituents. Solution B is added to the aqueous ammonium solution after solution A and then the combined mixture is heated as in example 1.

(18) A leaching test with dilute nitric acid makes it possible to compare the content of yttrium at the surface of the oxides prepared according to these two methods. This test is carried out in the following way: 1 gram of oxide is dispersed with stirring in 10 ml of a solution containing 0.1 mol/l of nitric acid. After stirring at 40 C. for 2 hours, the suspension is filtered. Analysis of the filtrate by the ICP technique makes it possible to determine the amounts of yttrium which are recovered. In order to determine the fractions of yttrium which are recovered, these values are respectively divided by the amount of yttrium present in the oxide sample. The results obtained for examples 13 and 14 are presented in the following table 5:

(19) TABLE-US-00005 TABLE 5 Fraction of yttrium recovered (%) Example 14 5 Example 15 19

(20) In order to characterize the crystalline phases present in the samples, an X-ray diffraction analysis is carried out. The analyses are carried out on a powder using a Panalytical diffractometer equipped with a multichannel detector of X'Celerator type and with a K/K monochromator. The data are collected in 20 minutes between 20=20 and 20=1000 with a step of 0.017 mm. All the samples exist in the form of a highly predominant phase characteristic of a solid solution of cubic or tetragonal symmetry. In particular, the presence of rare earth metal oxides by themselves is not detected, which reflects the incorporation of the rare earth metals (La, Y, Nd and/or Pr) in the CeZr matrix.