FR2900920A1 - Composition useful to treat exhaust gases of internal combustion engines, comprising zirconium oxide, cerium oxide, lanthanum oxide and other rare earth oxides, exhibits defined specific surface of lanthanum oxide - Google Patents

Abstract

Composition (A) comprising zirconium oxide, cerium oxide, lanthanum oxide and oxide of another rare earth e.g. yttrium, samarium and gadolinium, presents a specific surface of >=40 m 2>/g after calcination for 4 hours at 1000[deg]C, a specific surface of >=15 m 2>/g after calcination for 10 hours at 1150[deg]C, a specific surface of >=7 m 2>/g after calcination for 10 hours at 1200[deg]C and a content of lanthanum oxide and oxide of other rare earth metals at >=15%. Independent claims are included for: (1) preparation of (A) comprising preparing a mixture in a liquid medium containing zirconium, cerium, lanthanum and other rare earth compounds, contacting the mixture with basic compound to obtain a precipitate, heating the precipitate in a liquid medium, adding additives such as to the obtained precipitate anionic surfactants, non-ionic surfactants, polyethyleneglycols, carboxylic acids and their salts or ethoxylate type surfactant of fatty carboxymethyl alcohols, washing the obtained precipitate and calcining the obtained precipitate; (2) a catalytic system, comprising (A); and (3) treatment of exhaust gases from internal combustion engines, using the catalytic system.

Description

1 COMPOSITION BASED ON OXIDES OF ZIRCONIUM, CERIUM, LANTHAN AND

OF YTTRIUM, GADOLINIUM OR SAMARIUM, WITH A SPECIFIC STABLE SURFACE, METHOD OF PREPARATION AND USE AS A CATALYST The present invention relates to a composition based on a zirconium oxide, a cerium oxide, a lanthanum oxide. and an oxide of another rare earth chosen from yttrium, gadolinium and samarium, with a stable specific surface, its preparation process and its use in the treatment of automobile exhaust gases. So-called multifunctional catalysts are currently used for the treatment of exhaust gases from internal combustion engines (automotive post-combustion catalysis). The term “multifunctional” is understood to mean catalysts capable of carrying out not only the oxidation in particular of carbon monoxide and the hydrocarbons present in the exhaust gases but also the reduction in particular of the nitrogen oxides also present in these gases (catalysts " three-way "). Zirconium oxide and cerium oxide appear today as two particularly important and interesting constituents of materials entering into the composition of this type of catalyst. To be effective in such a use, these materials must have a specific surface area which remains sufficiently large even at high temperature. It is known to use in combination with zirconium and cerium other rare earths to obtain materials having a satisfactory specific surface. However, in some cases these are complex combinations because they include up to three other rare earths in addition to the elements zirconium and cerium. In addition, efforts are always made to improve this surface stability, that is to say to obtain even greater surface values at the same temperature or always sufficiently large at even higher temperatures. There is therefore a need for materials with a specific surface area with increased surface stability and, if possible, the compositions of which are not too complex. The invention relates to products which meet this need.

For this purpose and according to a first embodiment, the composition according to the invention consists essentially of a zirconium oxide, a cerium oxye, a lanthanum oxide and an oxide of another rare earth chosen from yttrium, gadolinium and samarium, characterized in that it has the following characteristics: a specific surface area after calcination for 4 hours at 1000 ° C. of at least 40 m2 / g; - a specific surface after calcination for 10 hours at 1150 C of at least 15 m2 / g; - a specific surface after calcination for 10 hours at 1200 ° C. of at least 7 m2 / g, - a content of lanthanum oxide and of oxide of the other rare earth of at least 15%. According to a second embodiment of the invention, the composition consists essentially of a zirconium oxide, a cerium oxide, a lanthanum oxide and an oxide of another rare earth chosen from yttrium, gadolinium and samarium. , characterized in that it has the following characteristics: - a specific surface after calcination for 4 hours at 1000 C of at least 40 m2 / g; - a specific surface after calcination for 10 hours at 1150 C of at least 15 m2 / g; the composition at the end of this calcination being in the form of a pure solid solution; - a specific surface after calcination for 10 hours at 1200 C of at least 5 m2 / g; - a lanthanum oxide and other rare earth oxide content of at least 15%.

Other characteristics, details and advantages of the invention will emerge even more fully on reading the description which follows, given in particular with reference to the appended drawings in which: FIG. 1 is a diffractogram obtained by X-ray diffraction on a composition of the invention; - Figure 2 is a diffractogram obtained by X-ray diffraction on another composition of the invention. For the remainder of the description, the term “specific surface area” means the specific surface area B.E.T. determined by nitrogen adsorption in accordance with the ASTM D 3663-78 standard established from the BRUNAUER-EMMETT-TELLER method described in the periodical "The Journal of the American Chemical Society, 60, 309 (1938)".

It is specified for the remainder of the description that, unless otherwise indicated, in the ranges of values which are given, the values at the limits are included. The contents or rates of elements are given in mass of oxide of these elements unless otherwise indicated, these oxides for the expression of these contents being considered in the form of ceric oxide for cerium, in the form Ln2O3 for the others. rare earth. The specific surface area values which are indicated for a given temperature and time correspond, unless otherwise indicated, to calcinations in air at a temperature level over the indicated time. The compositions according to the invention consist essentially of a mixture of four oxides of the elements zirconium, cerium, lanthanum and another rare earth which is either yttrium, or samarium, or gadolinium. By consists essentially, it is meant that the composition in question contains only the oxides of the four aforementioned elements and that it does not contain an oxide of another element, for example of another rare earth, capable of having an influence. positive on the stability of the specific surface of the composition. On the other hand, the composition may contain elements such as impurities which may in particular originate from its method of preparation, for example from the raw materials or from the starting reagents used. The compositions of the invention have a content for the lanthanum oxide / oxide of the other rare earth unit which is at least 15%. This content can be between 15% and 35%. Beyond 35% the effect on the stability of the surface may no longer be noticeable. It will be noted that if the content of lanthanum oxide and of oxide of the other rare earth is at least 30%, in particular in the range of 30% to 35%, it may be preferable to have an oxide content of cerium not more than 30%. It is preferable that the compositions have a lanthanum content of at least 2%. The relative proportions between the lanthanum and the other rare earth can vary over a wide range. A ratio of the lanthanum content to that of the other rare earth (mass ratio of lanthanum oxide / mass of oxide of the other rare earth) of less than 1.5, or even less than 1, may be preferable for obtain the highest surface products. The cerium content is generally between 10% and 60%, preferably between 20% and 50% and even more preferably between 30% and 45%. The compositions of the invention are moreover characterized by the specific surfaces which they exhibit at different temperatures.

As indicated above, they first of all exhibit a specific surface after calcination for 4 hours at 1000 ° C. of at least 40 m2 / g. Preferably, this surface can be at least 45 m2 / g and even more preferably at least 50 m2 / g. By way of specific example only, it can be mentioned that surfaces of about 60 m2 / g can be achieved. The compositions of the invention then exhibit a specific surface area after calcination for 10 hours at 1150 ° C. of at least 15 m 2 / g, this surface possibly even being at least 18 m 2 / g. By way of specific example only, it can be mentioned that surfaces of about 23 m2 / g can be achieved. It should be noted here that the description which has just been given and in particular all the characteristics which have been given apply to the compositions of the invention regardless of the embodiment. However, according to a particular embodiment, the compositions may, after calcination for 10 hours at 1150 ° C., be in the form of a pure solid solution. The diffractograms obtained by X-ray diffraction on these compositions reveal in particular, within the latter, the existence of a single clearly identifiable phase of cubic crystalline symmetry of the fluorite type, thus translating the existence of a pure solid solution. cerium, lanthanum, zirconium and the other rare earth. The compositions of the invention then exhibit a specific surface area after calcination for 10 hours at 1200 ° C. which varies according to the embodiment considered. [) In the case of a first embodiment, this surface area is at least 7 m2 / g. In the case of another embodiment, that is to say that which has just been described above and for which the composition is in the form of a solid solution after calcination at 1150 ° C., this surface is of at least 5 m2 / g. According to a variant of this second embodiment, this surface can also be at least 7 m2 / g, still after calcination for 10 hours at 1200 C. Advantageously, and this whatever the embodiment, the specific surface after calcination 10. hours at 1200 C is at least 10 m2 / g. By way of specific example only, it can be mentioned that surfaces of about 15 m2 / g can be achieved. The process for preparing the compositions of the invention will now be described. This process is characterized in that it comprises the following steps: - (a) a mixture is formed in a liquid medium comprising compounds of zirconium, cerium, lanthanum and the other rare earth; - (b) said mixture is brought into contact with a basic compound, whereby a precipitate is obtained; - (C) said precipitate is heated in a liquid medium; - (d) an additive is added to the precipitate obtained in the preceding step, chosen from anionic surfactants, nonionic surfactants, polyethylene glycols, carboxylic acids and their salts and surfactants of the carboxymethylated fatty alcohol ethoxylate type ; - (e) the precipitate is washed at the end of step (d) - (f) the precipitate thus obtained is calcined.

The first step (a) of the process therefore consists in preparing a mixture in a liquid medium of the compounds of the constituent elements of the composition, ie zirconium, cerium, lanthanum and the other rare earth. The mixing is generally done in a liquid medium which is preferably water.

The compounds are preferably soluble compounds. They can in particular be salts of zirconium, cerium, lanthanum and the other rare earth. These compounds can be chosen from nitrates, sulphates, acetates, chlorides, cerium-ammoniacal nitrates. By way of examples, mention may thus be made of zirconium sulfate, zirconyl nitrate or zirconyl chloride. Zirconyl nitrate is most commonly used. Mention may also be made in particular of cerium IV salts such as cerium ammoniacal nitrates or nitrates, for example, which are particularly suitable here. Preferably, ceric nitrate is used. It is advantageous to use salts with a purity of at least 99.5% and more particularly of at least 99.9%. An aqueous solution of ceric nitrate can, for example, be obtained by reacting nitric acid with a hydrated ceric oxide prepared in a conventional manner by reacting a solution of a cerous salt, for example cerous nitrate, and an ammonia solution in the presence of hydrogen peroxide. It is also possible, preferably, to use a ceric nitrate solution obtained according to the electrolytic oxidation process of a cerous nitrate solution as described in document FR-A-2 570 087, and which constitutes here an interesting raw material. . It will be noted here that the aqueous solutions of cerium salts and of zirconyl salts can exhibit a certain initial free acidity which can be adjusted by the addition of a base or of an acid. It is however as much possible to use an initial solution of cerium and zirconium salts effectively exhibiting a certain free acidity as mentioned above, as solutions which will have been neutralized beforehand to a greater or lesser extent. This neutralization can be done by adding a basic compound to the aforementioned mixture so as to limit this acidity. This basic compound can be, for example, a solution of ammonia or alternatively of alkali metal hydroxides (sodium, potassium, etc.), but preferably an ammonia solution. Finally, it will be noted that when the starting mixture contains cerium essentially in form III, it is preferable to involve in the course of the process an oxidizing agent, for example hydrogen peroxide. This oxidizing agent can be used by being added to the reaction medium during step (a) or during step (b) or at the end of the latter. It is also possible to use a sol as the starting compound for zirconium or cerium. The term sol denotes any system consisting of fine solid particles of colloidal dimensions, that is to say dimensions of between approximately 1 nm and approximately 500 nm, based on a zirconium or cerium compound, this compound generally being an oxide and / or a hydrated oxide of zirconium or cerium, in suspension in an aqueous liquid phase, said particles possibly additionally possibly containing residual amounts of bound or adsorbed ions such as, for example, nitrates, acetates, chlorides or ammoniums. It will be noted that in such a sol, the zirconium or the cerium can be either completely in the form of colloids, or simultaneously in the form of ions and in the form of colloids. The mixture can be obtained without distinction either from compounds initially in the solid state which will subsequently be introduced into a starter of water for example, or also directly from solutions of these compounds then mixing, in any order of said solutions. In the second step (b) of the process, said mixture is brought into contact with a basic compound. Products of the hydroxide type can be used as the base or basic compound. Mention may be made of alkali metal or alkaline earth hydroxides. It is also possible to use secondary, tertiary or quaternary amines. However, amines and ammonia may be preferred insofar as they reduce the risks of pollution by alkali or alkaline earth cations. C) n can also mention urea. The basic compound can more particularly be used in the form of a solution.

The manner of bringing the mixture into contact with the basic compound, that is to say the order of introduction of the latter, is not critical. However, this bringing together can be done by introducing the mixture into the basic compound in the form of a solution. This variant is preferable in order to obtain the compositions of the invention in the form of a pure solid solution. The bringing together or the reaction between the mixture and the basic compound, in particular the addition of the mixture into the basic compound in the form of a solution, can be carried out all at once, gradually or continuously, and it is preferably carried out with stirring. It is preferably carried out at room temperature. The next step (c) of the process is the step of heating the precipitate in an aqueous medium.

This heating can be carried out directly on the reaction medium obtained after reaction with the basic compound or on a suspension obtained after separation of the precipitate from the reaction medium, optional washing and placing the precipitate back in water. The temperature to which the medium is heated is usually at least 100 ° C. and it is generally between 100 ° C. and 160 ° C. The heating operation can be carried out by introducing the liquid medium into a closed chamber (closed reactor of the type autoclave). Under the temperature conditions given above, and in an aqueous medium, it is thus possible to specify, by way of illustration, that the pressure in the closed reactor can vary between a value greater than 1 Bar (105 Pa) and 165 Bar (1, 65.107 Pa), preferably between 5 Bar (5. 105 Pa) and 165 Bar (1.65.107 Pa). The heating can also be carried out in an open reactor for temperatures close to 100 C. The heating can be carried out either in air or under an inert gas atmosphere, preferably nitrogen.

The duration of the heating can vary within wide limits, for example between 30 minutes and 48 hours, preferably between 1 and 5 hours. Likewise, the temperature rise is carried out at a speed which is not critical, and it is thus possible to reach the reaction temperature set by heating the medium, for example between 3 ° C) minutes and 4 hours, these values being given as a whole. quite indicative. It is possible to make several heaters. Thus, it is possible to resuspend in water the precipitate obtained after the heating step and optionally one or more washings and then carry out another heating of the medium thus obtained. This further heating is carried out under the same conditions as those which have been described for the first. The next step (d) of the process consists in adding to the precipitate resulting from the previous step an additive which is chosen from anionic surfactants, nonionic surfactants, polyethylene glycols and carboxylic acids and their salts as well as surfactants of the carboxymethylated fatty alcohol ethoxylate type. As regards this additive, reference may be made to the teaching of application WO-93/45212 and use the surfactants described in this document.

Mention may be made, as surfactants of the anionic type, of ethoxycarboxylates, ethoxylated fatty acids, sarcosinates, phosphate esters, sulphates such as alcohol sulphates, ether alcohol sulphates and sulphated alkanolamide ethoxylates, sulphonates such as sulphosuccinates. , alkyl benzene or alkyl naphthalene sulfonates.

Mention may be made, as nonionic surfactant, of acetylenic surfactants, alcohol ethoxylates, alkanolamides, amine oxides, ethoxylated alkanolamides, long chain ethoxylated amines, ethylene oxide / propylene oxide copolymers, derivatives. sorbiatan, ethylene glycol, propylene glycol, glycerol, polyglyceryl esters and their ethoxylated derivatives, alkylamines, alkylimidazolines, ethoxylated oils and alkylphenol ethoxylates. Mention may in particular be made of the products sold under the brands IGEPAL, DOWANOL, RHODAMOX and ALKAMIDE. As regards the carboxylic acids, use may in particular be made of aliphatic mono- or dicarboxylic acids and, among these, more particularly saturated acids. It is also possible to use fatty acids and more particularly saturated fatty acids. Mention may thus be made in particular of formic, acetic, propionic, butyric, isobutyric, valeric, caproic, caprylic, capric, lauric, myristic and palmitic acids. As dicarboxylic acids, mention may be made of oxalic, malonic, succinic, glutaric, adipic, pimelic, suberic, azelaic and sebacic acids. The salts of the carboxylic acids can also be used, in particular the ammoniacal salts. By way of example, mention may more particularly be made of lauric acid and ammonium laurate. Finally, it is possible to use a surfactant which is chosen from those of the ethoxylates type of carboxymethylated fatty alcohols. The term “carboxymethylated fatty alcohol ethoxylates” type product is understood to mean products consisting of ethoxylated or propoxylated fatty alcohols comprising at the end of the chain a CH2-000H group. These products can correspond to the formula: RI-O- (CR2R3-C; Rc.R5-0) n-CH2-000H in which RI denotes a carbon chain, saturated or unsaturated, the length of which is generally at most 22 carbon atoms, preferably at least 12 carbon atoms; R2, R3, R4 and R5 may be identical and represent hydrogen or else R2 may represent a CH3 group and R3, R4 and R5 represent hydrogen; n is a non-zero integer which can range up to 50 and more particularly between 5 and 15, these values being included. It will be noted that a surfactant can consist of a mixture of products of the above formula for which RI can be saturated and unsaturated respectively or else products comprising both -CH2-CH2-O- groups and -C (CH3) -CH2-O-. The addition of the surfactant can be done in two ways. It can be added directly to the suspension of precipitate resulting from the previous heating step (c). It can also be added to the solid precipitate after separation of the latter by any known means from the medium in which the heating has taken place. The amount of surfactant used, expressed as a percentage by mass of additive relative to the mass of the composition calculated as oxide, is generally between 5% and 100%, more particularly between 15% and 60%.

According to an implementation variant of the process, it is possible to subject the suspended precipitate to medium energy grinding by subjecting this suspension to shearing, for example by using a colloid mill or a stirring turbine. At the end of step (d), the precipitate is washed after having separated it from the medium in which it was in suspension. This washing can be done with water, preferably with water at basic pH, for example ammonia water. In a last step of the process according to the invention, the precipitate recovered is then calcined. This calcination makes it possible to develop the crystallinity of the product formed and it can also be adjusted and / or chosen as a function of the subsequent use temperature reserved for the composition according to the invention, and this taking into account that the specific surface of the product is lower the higher the calcination temperature used. Such calcination is generally carried out in air, but calcination carried out for example under inert gas or under a controlled atmosphere (oxidizing or reducing) is obviously not excluded. In practice, the calcination temperature is generally limited to a range of values between 500 and 1000 C.

The compositions of the invention as described above or as obtained in the process studied above are in the form of powders but they can optionally be shaped to be in the form of granules, beads, cylinders or nests. bee of variable dimensions. The compositions of the invention can be used as catalysts or catalyst supports. Thus, the invention also relates to catalytic systems comprising the compositions of the invention. For such systems, these compositions can thus be applied to any support usually used in the field of catalysis, ie in particular thermally inert supports. This support can be chosen from alumina, titanium oxide, cerium oxide, zirconium oxide, silica, spinels, zeolites, silicates, crystalline silicoaluminum phosphates, and phosphates. crystalline aluminum.

The compositions can also be used in catalytic systems comprising a wash coat with catalytic properties and based on these compositions, on a substrate of the type, for example metal monolith or ceramic. The coating may also include a support of the type of those mentioned above. This coating is obtained by mixing the composition with the support so as to form a suspension which can then be deposited on the substrate. These catalytic systems and more particularly the compositions of the invention can find very many applications. They are thus particularly well suited to, and therefore usable in the catalysis of various reactions such as, for example, dehydration, hydrosulfurization, hydrodenitrification, desulfurization, hydrodesulfurization, dehydrohalogenation, reforming, reforming to steam, cracking, hydrocracking, hydrogenation, dehydrogenation, isomerization, disproportionation, oxychlorination, dehydrocyclization of hydrocarbons or other organic compounds, oxidation and / or reduction reactions, Claus reaction, treatment of internal combustion engine exhaust gases, demetallation, methanation, shift conversion, catalytic oxidation of soot emitted by internal combustion engines such as diesel or gasoline engines operating at lean speed . Finally, the catalytic systems and the compositions of the invention can be used as NOx traps or to promote the reduction of NOx even in an oxidizing medium.

In the case of these uses in catalysis, the compositions of the invention are used in combination with precious metals, they thus play the role of support for these metals. The nature of these metals and the techniques for incorporating them into support compositions are well known to those skilled in the art. For example, the metals can be platinum, rhodium, palladium or iridium; they can in particular be incorporated into the compositions by impregnation. Among the uses cited, the treatment of exhaust gases from internal combustion engines (automotive post combustion catalysis) constitutes a particularly interesting application. As a result, the invention also relates to a process for treating the exhaust gases of internal combustion engines which is characterized in that a catalytic system as described above or a composition according to Catalyst is used as catalyst. invention and as described above.

Examples will now be given.

EXAMPLE 1 This example relates to a composition containing 40% zirconium, 40% cerium, 6% lanthanum and 14% yttrium, these proportions being expressed as 20 percentages by weight of the oxides ZrO2, CeO2, La2O3 and Y2O3. 152 ml of zirconium nitrate (264 g / 1 in ZrO2), 163.9 ml of cerium nitrate (244 g / I in CeO2, 6.8% of the total cerium in Ce3 + form), the remaining cerium in Ce4 + form and 0.6 mol / l of free acidity), 13.2 ml of lanthanum nitrate (454 g / l as La2O3) and 36.6 ml of yttrium nitrate (382 g / II in Y2O3). It is then made up with distilled water so as to obtain 1 liter of a nitrate solution. 253.4 ml of an ammonia solution (12 mol / l) are introduced into a stirred reactor and the mixture is then made up with distilled water so as to obtain a total volume of 1 liter. The nitrate solution is introduced into the reactor with constant stirring. The solution obtained is placed in a stainless steel autoclave equipped with a stirring mobile. The temperature of the medium is brought to 150 ° C. for 2 hours with stirring. 33 grams of lauric acid are added to the suspension thus obtained. The suspension is kept under stirring for 1 hour.

The suspension is then filtered on a Büchner funnel, then the filtered precipitate is washed with ammonia-containing water. The product obtained is then brought to 700 ° C. for 4 hours at a level.

EXAMPLES 2A8 The procedure is the same as in Example 1. The compositions and amounts of reagents used are indicated in Tables 1 and 2 respectively below. For example 6, the gadolinium nitrate solution has a concentration of 380 g / l in Gd2O3. For Example 7, the samarium nitrate solution has a concentration of 369 g / I in Sm2O3.

Table 1 of the compositions The proportions are expressed as percentages by weight of the oxides Example% Zr% Ce% La% Y% Gd% Sm 2 45 40 2.8 12.2 - - 3 37ä9 40 10.7 11.4 - - 4 40 40 11.8 8.2 - - 5 45 40 7.3 7.7 - - 6 37.9 40 10.7 - 11.4 - 7 37.9 40 10.7 - - 11.4 8 48 30 11 11 - - Table 2 of the quantities of reagents The quantities are expressed in volume (ml) of the ammonia or nitrate solutions of the elements concerned Example Zr Ce La Y Gd / Sm ammonia 2 170 163.9 6.2 31.9 - 250.8 3,144 163.9 23.6 29.8 -251.2 4,152 163.9 26 21.5 - 247.8 5 170 163.9 16.1 20.2 - 246.5 6 144 163, 9 23.6 - 30 251.2 7 144 163.4 23.6 - 30.9 251.2 8 182 122.9 24.2 28.8 - 241 In order to determine their thermal stability, the compositions were subjected to calcinations in air at different temperatures. The specific surfaces measured after these heat treatments are reported in Table 3 below. The values are expressed in m2 / g. 20 Table 3 Example 4h / 1000 C 10h / 1150 C 10h / 1200 C _ 1 54 20 10 2 54 15 8 _ 3 _ 56 20 12 56 20 7 55 16 5 6 49 15 7 7 47 16 7 8 53 15 6 5 After a heat treatment for 10 hours at 1150 ° C. in air, the X-ray analysis of these compositions systematically reveals a single phase of cubic symmetry. The analyzes are carried out on powder using a Panalytical diffractometer equipped with an X'Celerator type multichannel detector and a KJ3 / Ka monochromator. The data is collected in 20 minutes between 20 = 10 and 20 = 95 with a step of 0.017 mm. FIGS. 1 and 2 are the diffractograms obtained by X-ray diffraction on the compositions of Examples 1 and 6 respectively.

COMPARATIVE EXAMPLE 9 This example relates to a composition containing 50% zirconium, 40% cerium, 5% lanthanum and 5% yttrium, these proportions being expressed as percentages by mass of the oxides ZrO2, CeO2, La2O3 and Y2O3. The proportion of lanthanum and of yttrium is lower than that of these same elements in the compositions according to the invention. The amounts of reagents used for the preparation of this composition and the specific surfaces thereof at different temperatures are indicated respectively in Tables 4 and 5 below.

Table 4 The quantities are expressed in volume (ml) of the ammonia or nitrate solutions of the elements concerned Example Zr Ce La Y ammonia 9 189 163.9 11.1 13.1 243 Table 5 Values in m2 / g Example _ 4h / 1000 C 10h / 1150 C 10h / 1200 C 9 45.6 9 _ 35

Claims (13)

1- Composition consisting essentially of a zirconium oxide, a cerium oxide, a lanthanum oxide and an oxide of another rare earth chosen from yttrium, gadolinium and samarium, characterized in that it has the characteristics following: - a specific surface after calcination for 4 hours at 1000 ° C. of at least 40 2 / g; m - a specific surface after calcination for 10 hours at 1150 C of at least 15 m2 / g; - a specific surface after calcination for 10 hours at 1200 C of at least 7 m2 / g; - a lanthanum oxide and other rare earth oxide content of at least 15 15%. 2- Composition consisting essentially of a zirconium oxide, a cerium oxide, a lanthanum oxide and an oxide of another rare earth chosen from yttrium, gadolinium and samarium, characterized in that it exhibits the following characteristics: a specific surface after calcination for 4 hours at 1000 ° C. of at least 40 2 / g; m - a specific surface after calcination for 10 hours at 1150 C of at least 15 m2 / g; the composition at the end of this calcination being in the form of a pure solid solution; - a specific surface after calcination for 10 hours at 1200 C of at least 5 m2 / g; - a lanthanum oxide and other rare earth oxide content of at least 15%. 30 3- Composition according to claim 2, characterized in that it has a specific surface area after calcination for 10 hours at 1200 C of at least 7 m2 / g. 35 4- Composition according to one of the preceding claims, characterized in that it has a specific surface area after calcination for 4 hours at 1000 C of at least 45 m2 / g. 5- Composition according to one of the preceding claims, characterized in that it has a specific surface area after calcination for 4 hours at 1000 C of at least 50 m2 / g. 6- Composition according to one of the preceding claims, characterized in that it has a specific surface area after calcination for 10 hours at 1200 C of at least 10 m2 / g. 7- Composition according to one of the preceding claims, characterized in that it has a content of lanthanum oxide and of oxide of the other rare earth of between 15% and 35%. 8- Composition according to one of the preceding claims, characterized in that it has a content of lanthanum and of the other rare earth in a lower mass ratio of lanthanum oxide / mass of oxide of the other rare earth. to 1.5, more particularly less than 1. 9. Composition according to one of the preceding claims, characterized in that it has a cerium content of between 10% and 60%, more particularly between 20% and 50%. 10- Process for preparing a composition according to one of the preceding claims, characterized in that it comprises the following steps: - (a) a mixture is formed in a liquid medium comprising compounds of zirconium, cerium, lanthanum and the other rare earth; - (b) said mixture is brought into contact with a basic compound, whereby a precipitate is obtained; - (C) said precipitate is heated in a liquid medium; - (d) an additive is added to the precipitate obtained in the preceding step, chosen from anionic surfactants, nonionic surfactants, polyethylene glycols, carboxylic acids and their salts and surfactants of the fatty alcohol ethoxylate type. carboxymethyls; - (e) the precipitate is washed at the end of step (d) - (f) the precipitate thus obtained is calcined. 35 11- The method of claim 10, characterized in that the heating of the precipitate of step (c) is carried out at a temperature of at least 100 C. 12- Catalytic system, characterized in that it comprises a composition according to one of claims 1 to 9. 13- A method of treating the exhaust gases of internal combustion engines, characterized in that a catalytic system according to claim 12 or a composition according to one of claims 1 to 9 is used as catalyst.