WO2019043346A1

WO2019043346A1 - Cerium- and zirconium-based mixed oxide

Info

Publication number: WO2019043346A1
Application number: PCT/FR2018/052141
Authority: WO
Inventors: Laure Jeanne Simone BISSON
Original assignee: Rhodia Operations
Priority date: 2017-09-01
Filing date: 2018-09-03
Publication date: 2019-03-07

Abstract

The present invention concerns a mixed oxide of zirconium, cerium, lanthanum and at least one oxide of a rare earth other than cerium and lanthanum that comprises two crystalline phases and that has a large specific surface area. The invention also concerns the method for preparing same and the use thereof in catalysis.

Description

MIXED OXIDE BASED ON CERIUM

AND ZIRCONIUM

This application claims priority to French patent application No. 14 17581 filed on September ^1, 2017 and whose contents are fully incorporated by reference. In case of inconsistency between the text of the present application and the text of the French patent application which affects the clarity of a term or expression, reference will be made to this application only.

Technical area

The present invention relates to a mixed oxide of zirconium, cerium, lanthanum and at least one oxide of a rare earth other than cerium and lanthanum which comprises two crystalline phases and which has a high specific surface area. The invention also relates to its preparation process and its use in catalysis.

Technical problem

At the present time, so-called multifunctional catalysts are used for the treatment of the exhaust gases of internal combustion engines (automotive post-combustion catalysis). Multifunctional means catalysts capable of operating not only the oxidation in particular of carbon monoxide and hydrocarbons present in the exhaust gas but also the reduction in particular nitrogen oxides also present in these gases (catalysts "three ways").

The catalyst results from the interaction of a precious metal (eg Pd, Pt, Rh) with a mixed oxide of cerium and zirconium often mixed with alumina. The mixed oxide must have a high thermal stability to avoid sintering. It is also preferable that it has a suitable porosity that allows interaction between the gases and the catalytic sites.

The mixed oxide according to the invention as described in claim 1 aims at such a compromise.

Other characteristics, details and advantages of the invention will appear even more completely on reading the description and the appended figures. figures

Fig. 1 is a diagram of the reactor which was used for the preparation of the mixed oxides of the examples.

Fig. 2a and FIG. 2b show the diffractogranne RX of the mixed oxide of Example 1 after calcination at 1000 ° C for 4 h.

Fig. 3a and FIG. 3b show the X-ray diffractograms of the mixed oxide of Example 1 after calcination at 900 ° C., 1000 ° C. and 1100 ° C. for 4 hours.

Fig. 4a and FIG. 4b show the X-ray diffractograms of the mixed oxide of Example 6 after calcination at 900 ° C., 1000 ° C. and 1100 ° C. for 4 hours. Fig. 5a and FIG. 5b show the X-ray diffractograms of the mixed oxide of Example 9 after calcination at 900 ° C., 1000 ° C. and 1100 ° C. for 4 hours.

Fig. 6a and Figs. 6b show the X-ray diffractograms of the mixed oxide of Example 11 after calcination at 900 ° C., 1000 ° C. and 1100 ° C. for 4 hours.

Fig. 7a and 7b show the X-ray diffractograms of the mixed oxide of Example 15 after calcination at 900 ° C., 1000 ° C. and 1100 ° C. for 4 hours.

The diffractograms are represented in a conventional manner with the angle 2Θ in ° on the abscissa and the intensity of the detector in the ordinate. When the diffractograms are superimposed, the bottom diffractogranne corresponds to a calcination at 900 ° C., that of the medium to a calcination at 1000 ° C. and that of the top to a calcination at 1100 ° C. Fig. 8 represents the porogram of the mixed oxide of Example 1 after calcination at 900 ° C. and 1100 ° C. for 4 hours.

Fig. 9 shows the porogram of the mixed oxide of Example 9 after calcination at 900 ° C. and 1100 ° C. for 4 hours.

Technical background

US 2016/0279607 discloses a mixed oxide comprising two different types of crystallites, A and B, one of which contains a precious metal (Pd, Rh, Pt). The process The preparation consists of joining the precursor precipitates of A and B and calcining the whole under air.

US 2010/0004123 discloses an OSC-like composition comprising a 1 ^st particle based on cerium, zirconium or based on cerium, zirconium and a rare earth and a 2 ^nd particle based on a rare earth, d an alkaline earth element, zirconium and a precious metal. The composition therefore results from a physical mixture. EP 2374525 discloses an exhaust gas treatment catalyst comprising a plurality of catalytically active layers arranged on a support. One of the layers comprises a precious metal attached to activated alumina particles or cerium oxide. Another layer comprises a precious metal attached to activated alumina particles or cerium oxide.

EP 2404668 describes the physical mixing of solids of different compositions.

US 2006/089256 discloses a composition comprising particles of activated alumina or cerium-zirconium mixed oxide to which platinum is attached; rare earth doped zirconia particles to which rhodium is attached; and particles of a mixed cerium-zirconium oxide without precious metal

WO 03/035256 discloses a catalyst comprising a plurality of catalytically active layers.

WO 2005/102933 discloses a core-shell particle comprising a metal oxide core and a bark of another metal oxide, the core and the bark being in the form of aggregated primary particles. The examples describe particles with a zirconia core and a bark in a mixed cerium-zirconium-barium oxide.

detailed description

In the value ranges of the present application, the terminal values are included. In addition, unless otherwise indicated, calcinations are performed under air. Porosity characteristics (such as total pore volume or pore size) are obtained by mercury porosimetry. This technique is based on measuring the amount of mercury that can be inserted into the pores of a solid at different pressures (mercury intrusion). As regards the mixed oxide according to the invention, this is a mixed oxide based on zirconium, cerium, lanthanum and possibly at least one rare earth other than cerium and lanthanum (denoted TR). the proportions by weight of these elements, expressed as oxide equivalents relative to the total weight of the mixed oxide, being as follows:

^■ between 25.0% and 45.0% cerium;

^■ between 1.0% and 10.0% of lanthanum;

^■ between 0 and 15.0% of the rare earth other than cerium and lanthanum;

■ the zirconium supplement.

The proportions are given in weight of oxide unless otherwise indicated. It is considered for the calculation of these proportions that the cerium oxide is in the form of ceric oxide (CeO 2), the oxides of the other rare earths are in TR2O3 form, TR denoting the rare earth (with the exception of the praseodymium expressed in the form Pr ₆ On) and that the oxides of zirconium and hafnium are respectively in ZrO 2 and HfO 2 form.

The aforementioned elements Ce, La, TR and Zr are generally present in the form of oxides. However, it is not excluded that they may be present at least partly in the form of hydroxides or oxyhydroxides. The proportions of these elements can be determined using the usual analysis techniques in laboratories, especially X-ray fluorescence, for example using the PANalytical Axios-Max spectrometer. The proportions of these elements are given by weight in oxide equivalent relative to the total weight of the mixed oxide.

The mixed oxide comprises the aforementioned elements in the proportions indicated but it may also comprise other elements in low proportions, designated by the term "impurity". The impurities can come from raw materials or starting reagents used. The total proportion of impurities is generally less than or equal to 0.5%, or even less than or equal to 0.3%, expressed by weight relative to the total weight of the mixed oxide. According to one embodiment, the mixed oxide does not comprise precious metal, in particular chosen from the list formed by the following elements: Pd, Rh, Pt, Au, Os, Ir and / or does not include aluminum. The mixed oxide may also comprise hafnium which is generally present in association with zirconium in natural ores. The proportion of hafnium relative to zirconium depends on the ore from which the zirconium is extracted. The mass proportion Zr / Hf in certain ores can thus be of the order of 50/1. Thus, for example, baddeleyite contains about 98% zirconium oxide for 2% hafnium oxide. Like zirconium, hafnium is usually present as an oxide. However, it is not excluded that it may be present at least partly in the form of hydroxide or oxyhydroxide. The proportion by weight of hafnium in the mixed oxide is less than or equal to 2.5%, or even 2.0%, expressed as oxide equivalent relative to the total weight of the mixed oxide. The proportions of the impurities can be determined using Inductively Coupled Plasma Spectrometry (ICP-MS).

According to one variant, for which the characteristics and embodiments described below also apply, the mixed oxide consists of a mixture of oxides of zirconium, cerium, lanthanum, and possibly at least one rare earth other than cerium and lanthanum (TR) and optionally hafnium, the proportions by weight deducted oxides being the following:

Between 25.0 and 45.0% of cerium oxide;

· Between 1.0% and 10.0% lanthanum oxide;

Between 0% and 15.0% of at least one rare earth oxide other than cerium and lanthanum;

Between 0% and 2.5% of hafnium oxide;

• the zirconium oxide supplement,

The proportion by weight of cerium is between 25.0% and 45.0%, more particularly between 28.0% and 42.0%.

The proportion by weight of lanthanum is between 1.0% and 10.0%, more particularly between 2.0% and 9.0%. The mixed oxide may also comprise between 0% and 15.0% by weight of at least one rare earth other than cerium or lanthanum (TR). The rare earth may be selected from yttriunn, neodymium or praseodymium. The proportion by weight of lanthanum and rare earth (TR) is advantageously greater than or equal to 4.0%, or even 6.0% or even 8.0%.

The mixed oxide also comprises zirconium. The proportion by weight of zirconium is in addition to 100% of the other elements of the mixed oxide. According to one embodiment, zirconium is, apart from oxygen, the majority element, that is to say the proportion by weight of oxide equivalent is greater than the proportion by weight in equivalent of oxide of each of the other constituent elements of the mixed oxide (that is to say Ce, La and optionally TR, Hf). The proportion by weight of zirconium may be strictly greater than 45.0%, or even greater than or equal to 50.0%. This proportion can be between 45% and 65%.

The mixed oxide may also optionally comprise at least one rare earth other than cerium and lanthanum (denoted TR). Rare earth means yttrium or an element of the Periodic Table with an atomic number inclusive of between 57 and 71. TR may be, for example, yttrium, praseodymium or neodymium. According to one embodiment, the oxide does not include rare earth other than cerium and lanthanum. According to this embodiment, the proportion by weight of cerium can be between 30.0 and 40.0% and that of lanthanum between 3.0 and 6.0%. The zirconium balance can range from 54.0 to 67.0% by weight. According to another embodiment, the mixed oxide comprises only yttrium as a rare earth other than cerium and lanthanum. According to another embodiment, the mixed oxide comprises only two rare earths other than cerium and lanthanum which may be yttrium and neodymium or else yttrium and praseodymium. Thus, the following constituent elements of the mixed oxide may be the following:

zirconium and optionally hafnium, cerium, lanthanum, yttrium; or

- zirconium and optionally hafnium, cerium, lanthanum, yttrium, neodymium; or

zirconium and optionally hafnium, cerium, lanthanum, yttrium, praseodymium; or

- zirconium and possibly hafnium, cerium, lanthanum.

An important feature of the mixed oxide is that it is in the form of a mixture of two crystalline phases both comprising cerium and zirconium. It is possible to identify these crystalline phases by the X-ray diffraction technique.

In addition, after the mixed oxide of the invention has been calcined under air

at a temperature of 900 ° C. for 4 hours; or

at a temperature of 1000 ° C. for 4 hours; or at a temperature of 1100 ° C. for 4 hours;

the diffractograms of the mixed oxide obtained by X-ray diffraction can be distinguished by two distinct peaks attributed to these two crystalline phases:

o the ^1st peak is positioned at a 2Θ value of between 47.6 ° and 49.0 °

(value included);

o ^2nd peak is set to a value between 49.0 ° 2Θ range (exclusive) and 50.1 °;

o the intensity of peak ¹ (li _st peak) being less than or equal to the ^2nd peak

The mixed oxide may have these characteristics after calcination in air at 900 ° C. for 4 hours. The mixed oxide may also have these characteristics after calcination in air at 1000 ° C. for 4 hours.

¹ peak is positioned at a 2Θ value of between 47.6 ° and 49.0 ° (value included), more particularly between 47.9 ° and 48.8 °. The ^2nd peak is positioned at a 2Θ value of between 49.0 ° (exclusive) and 50,1 °, more particularly between 49.4 ° and 49.9 °. After the mixed oxide has been calcined in air at 4 to 900 ° C or 1000 ° C or 1100 ° C, the diffractogram of the mixed oxide may comprise only two distinct peaks in the range 2Θ between 47.6. ° and 50.1 °.

¹ peak can be attributed to a cubic or tetragonal crystal phase. The ^2nd peak can be attributed to a tetragonal phase. The intensities of ¹ and

2 ^e peak are more particularly in a ratio R = 1 _{er P} ic / beme _P ic between 0.50 and 1, 00, more particularly between 0.70 and 1 00. Note that the ratio R is likely to vary according to the calcination temperature (900 ° C;

1000 ° C or 1100 ° C). It will also be noted that the two distinct peaks may already be observed for the mixed oxide at the end of the calcination step (a5) depending on the temperature at which the solid of step (a3) or (a4) has been calcined, for example from a calcination temperature of 900 ° C.

The mixed oxide is in the form of a mixture of two crystalline phases both comprising cerium and zirconium. According to one embodiment, after calcination in air for 4 hours at 900 ° C. or at 1000 ° C. or at 1100 ° C., the mixed oxide comprises only these two phases. According to another embodiment, after calcination for 4 hours, at 900 ° C. or at 1000 ° C. or at 1100 ° C., the mixed oxide comprises these two phases as majority phases. Therefore, on the diffractogram, in addition to the peaks attributed to the two crystalline phases, it is possible to observe additional peaks attributable to at least one other crystalline phase, these being of lower intensity relative to the two peaks positioned respectively between 47.6 ° and 49.0 ° (value included) and between 49.0 ° (excluding value) and 50.1 °. More particularly, these additional peaks may each have an intensity I such that the ratio I / I ₂ th peak is less than or equal to 10%.

After calcination under air for 4 hours, at 900 ° C. or at 1000 ° C. or at 1100 ° C., the diffractogram can also have two distinct peaks attributed to the two crystalline phases:

a peak being positioned at a value of 2Θ between 28.5 ° and 29.5 ° (inclusive);

o another peak being positioned at a value 2Θ between 29.5 ° (excluded) and 30.5 °.

The intensities are determined on the diffractograms with respect to a baseline taken over the range of angle θ between 5.0 ° and 90.0 °.

The mixed oxide is also characterized by a high specific surface, in particular because of its specific porosity. By specific surface is meant the BET specific surface area determined by nitrogen adsorption on a powder according to the known Brunauer-Emmett-Teller method (BET method). This method is described in ASTM D 3663-03 (re-approved 2015). This method is also described in "The Journal of the American Chemical Society, 60, 309 (1938)". The specific surface is determined automatically using a Tristar Il 3020 Micromeritics device in accordance with the indications recommended by the manufacturer. Prior to any measurement, the sample in powder form is heat-treated to remove the adsorbed species. The abbreviation Sr (° c) / x (h) is used to denote the specific surface area of a composition, obtained by the BET method as described above, after calcination under air of the composition at a temperature T expressed in ° C for a period of x hours. For example, 100 ° C./4 h denotes the BET specific surface area of a composition after calcination thereof at 1000 ° C. for 4 hours. Thus, the mixed oxide has a surface Snoo ° c / 4 h of at least 28 m ² / g, more particularly at least 30 m ² / g, or even at least 32 m ² / g. This specific surface is generally at most 40 m ² / g, or even at most 35 m ² / g, or even at most 33 m ² / g. This specific surface is generally between 28 m ² / g and 40 m ² / g.

The mixed oxide also has a surface area of 100 ° C./4 hr of at least 40 m ² / g, more particularly at least 45 m ² / g. This specific surface is generally at most 70 m ² / g, or even at most 60 m ² / g, or even at most 57 m ² / g. This specific surface is generally between 40 and 70 m ² / g.

The surface Sgoo-cw h may be at least 50 m ² / g, more particularly at least 55 m ² / g. This specific surface is generally at most 90 m ² / g, or even at most 85 m ² / g, or even at most 82 m ² / g. This specific surface is generally between 50 and 90 m ² / g.

Moreover, the mixed oxide may also be characterized by a specific porosity which is induced by the texturing agent which is added in step (a3). The porosities shown are measured by mercury intrusion porosimetry in accordance with ASTM D 4284-83 ("Standard method for determining the volume distribution of catalysts by mercury intrusion porosimetry"). M icromeritics Autopore IV 9500 can be used with a powder penetrometer in accordance with the instructions given by the manufacturer. Thus, after calcination at a temperature of 900 ° C. for 4 hours, the mixed oxide may have a total pore volume greater than or equal to 1.00 ml / g, or even greater than or equal to 1.10 ml / g, or even greater than or equal to 1, 20 mL / g. This pore volume can reach 2.50 ml / g. After calcination at 900 ° C. for 4 h, the mixed oxide may have a pore volume in the pore area of diameter less than or equal to 200 nm which is greater than or equal to 0.50 ml / g, or even greater than or equal to 0.55 mL / g or even greater than or equal to 0.60 mL / g. After calcination at 900 ° C. for 4 hours, the mixed oxide may have a pore volume in the pore area of diameter less than or equal to 100 nm which is greater than or equal to 0.50 ml / g. The derivative curve (dV / dlogD) obtained by mercury porosimetry on the mixed oxide after calcination at a temperature of 900 ° C. for 4 hours, has in the range of pores with a diameter of less than or equal to 200 nm, a peak whose maximum is a diameter of pore of between 20 and 40 nm, more particularly between 20 and 35 nm, V and D respectively denoting the pore volume and the pore diameter.

After calcination at a temperature of 1100 ° C. for 4 h, the oxide may have a total pore volume greater than or equal to 0.70 ml / g, or even greater than 0.80 ml / g. After calcination at 1100 ° C. for 4 hours, the mixed oxide may have a pore volume in the range of pores with a diameter of less than or equal to 200 nm which is greater than or equal to 0.28 ml / g, or even greater than or equal to at 0.30 mL / g. After calcination at 1100 ° C. for 4 hours, the mixed oxide may have a pore volume in the range of pores with a diameter of less than or equal to 100 nm which is greater than or equal to 0.28 ml / g. The derivative curve (dV / dlogD) obtained by mercury porosimetry on the mixed oxide after calcination at a temperature of 1100 ° C. for 4 hours, has in the range of pores with a diameter of less than or equal to 200 nm, a peak of which the maximum corresponds to a pore diameter of between 30 and 60 nm, V and D respectively denoting the pore volume and the pore diameter.

In the field of pores with a diameter of less than or equal to 100 nm, the derived curve (dV / dlogD) may comprise only one peak either after calcination at 900 ° C./4 h or at 1100 ° C./4 h.

As regards the process for preparing the mixed oxide according to the invention, this comprises the following steps:

- (a1) is simultaneously introduced into a stirred vessel containing a basic aqueous solution, two aqueous solutions noted (A) and (B) comprising cerium nitrate and zirconium nitrate and may optionally also include lanthanum nitrate and / or nitrate from rare earth (s) other than cerium and lanthanum;

- (a1 ') in the case where lanthanum and / or rare earth (s) other than cerium and lanthanum are not present in solutions (A) and (B), then introduced into the mixture formed at the end of step (a1) maintained with stirring, an aqueous solution (C) comprising lanthanum nitrate and / or rare earth nitrate (TR) possibly present in the mixed oxide;

- (a2) the suspension obtained at the end of step (a1) or of step (a1 ') is heated with stirring;

- (a3) is then introduced to the suspension obtained in the preceding step a texturing agent;

- (a4) optionally, the suspension is filtered and the precipitate is washed; - (a5) the solid obtained after step (a3) or (a4) is calcined in air at a temperature between 700 ° C and 1100 ° C to give the mixed oxide;

- (a6) the mixed oxide obtained in step (a5) may be optionally ground; characterized in that the two solutions (A) and (B) respectively have mass ratios (Ce / Zr) _A and (Ce / Zr) _B which are different.

In step (a1), two aqueous solutions noted (A) and (B) containing cerium and zirconium nitrate and possibly also possibly containing the lanthanum nitrate and / or the nitrate of the earth (s) are used. ) rare other than cerium and lanthanum. To prepare these solutions, crystallized zirconyl nitrate can be dissolved in water. It is also possible to advantageously use an aqueous solution formed by dissolving basic zirconium carbonate or zirconium hydroxide with nitric acid. This acid attack may be preferably conducted with a molar ratio NO3 ^~ / Zr of between 1.3 and 2.3. In the case of zirconium carbonate, this ratio can be between 1, 3 and 2.0. Examples of usable zirconium nitrate solutions resulting from this attack are described in the examples. For the source of cerium can be used for example a salt of this ^lv as nitrate or ceric ammonium nitrate, which are particularly well suited. Preferably, ceric nitrate is used. An aqueous solution of ceric nitrate may, 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 preferable to use a solution of ceric nitrate obtained by the electrolytic oxidation method of a cerous nitrate solution as described in document FR-A-2570087, and which constitutes here an interesting raw material. It is possible to obtain with this process a solution of ceric nitrate with a molar ratio Ce ^lv / total amount of Ce greater than or equal to 0.90, which may constitute an interesting raw material. Examples of cerium nitrate solutions that can be used are described in the examples. Solutions (A) and (B) may also contain lanthanum nitrate and / or nitrate from the rare earth (s) other than cerium and lanthanum. The aqueous solutions (A) and (B) may have some initial free acidity which may be adjusted by the addition of a base or an acid. It is indeed as possible to implement solutions that actually have a certain free acidity, as solutions that have been previously neutralized more or less extensively. This neutralization can be done by adding a basic compound so as to limit this acidity but while avoiding precipitation. This basic compound may be for example a solution of ammonia or an alkali hydroxide (sodium, potassium, ...). A solution of ammonia can advantageously be used. Examples of aqueous solutions (A) or (B) that can be used are described in the examples. It will be noted that when the starting mixture contains Ce '', it is preferable to use an oxidizing agent, for example hydrogen peroxide, in the course of the process.This oxidizing agent can be directly added to solutions (A) and ( B).

In general, it is advantageous to use purity salts of at least 99.5% by weight and more particularly at least 99.9% by weight.

Solutions (A) and (B) can be obtained by mixing the nitrate solutions in any order.

In step (a1), the two solutions (A) and (B) are simultaneously introduced into a stirred tank containing a basic aqueous solution so as to initiate the precipitation.

The basic compound dissolved in the basic aqueous solution may be a hydroxide, for example an alkali or alkaline earth hydroxide. It is also possible to use secondary, tertiary or quaternary amines as well as ammonia. As in the examples, an aqueous ammonia solution can be used advantageously because the ammonium nitrate which forms can be removed more easily than a nitrate of an alkaline or alkaline earth element. As in Example 1, it is possible to use an aqueous solution of ammonia, for example of concentration 12 mol / l. The mixed oxide according to the invention is therefore advantageously obtained using ammonia as the basic aqueous solution.

The basic compound is advantageously used with a stoichiometric excess to ensure optimum precipitation of all cations. The stoichiometric excess is preferably at least 40 mol% relative to all the cations from solutions (A), (B) and optionally (C) which are introduced into the stirred tank. Both solutions (A) and (B) are introduced directly into the basic aqueous solution at two points which are hereinafter referred to as pA and pB. This can be achieved using two tubes (or tubes), denoted respectively tA and tB, which dive directly into the basic aqueous solution. pA and pB are advantageously located in two places where the characteristics of the mechanical agitation are substantially identical. Thus, to do this, according to one embodiment, in the case of a vessel having an axis of symmetry, such as for example a vessel having a cylindrical or substantially cylindrical geometry, pA and pB may be two symmetrically opposite points with respect to the axis of symmetry of the tank. Thus, in the examples except for Example 8, pA and pB are located at two symmetrically opposite points (see Fig. 1). According to another embodiment, pA and pB are located in almost the same place. Thus, in Example 8, the two pipes tA and tB are adjacent.

In the case where the two solutions (A) and (B) do not include lanthanum nitrate and / or nitrate of the rare earth other cerium and lanthanum, the two elements La and / or TR are then introduced into the mixture formed in step (a1) by means of an aqueous solution (C) containing lanthanum nitrate and / or rare earth nitrate (TR). It is possible to introduce the solution (C) through one of the two pipes used in step (a1) to introduce the two solutions (A) and (B).

At the end of step (a1) and of step (a1 '), a suspension of a precipitate is obtained. In order to prepare the mixed oxide according to the invention, it should be noted that the concentrations of the constituent elements of the mixed oxide in solutions (A) and (B) and, if appropriate, in solution (C), are subject to the following constraints. It is indeed necessary on the one hand that these concentrations make it possible to obtain the mixed oxide having the desired overall proportions in each of the constituent elements of the mixed oxide. On the other hand, the two solutions (A) and (B) must have respective mass ratios (Ce / Zr) _A and (Ce / Zr) _B that are different. (Ce / Zr) _A and (Ce / Zr) _B which are respectively greater than 1.0 and less than 1.0 and which are sufficiently different to obtain the two crystalline structures characterized by two distinct peaks.

The ratio (Ce / Zr) _A may advantageously be between 1.00 and 4.00. The ratio (Ce / Zr) _B may advantageously be between 0.05 and 0.30. In addition, the ratio (Ce / Zr) _A / (Ce / Zr) _B may advantageously be greater than 9.0, or even greater than 1 1, 0. It will be noted that it is not possible to obtain the mixed oxide according to the invention by using a solution (A) which comprises only cerium nitrate and not zirconium nitrate or a solution (B) which does not would include only zirconium nitrate and no cerium nitrate.

When the proportion of cerium is between 35% and 45%, the ratio (Ce / Zr) _A is advantageously between 1.80 and 4.00 and the ratio (Ce / Zr) _B is advantageously between 0.10 and 0.30. When the proportion of cerium is between 25% and 35%, the ratio (Ce / Zr) _A is advantageously between 1.00 and 1.60 and the ratio (Ce / Zr) _B is advantageously between 0.05 and 0.20.

The fact that the two solutions have different ratios (Ce / Zr) makes it possible to obtain an intimate mixture of aggregates and agglomerates of particles comprising both cerium and zirconium, of different compositions, with different ratios (Ce / Zr). / Zr) different. This has been observed on mixed oxides prepared by microscopy with EDX mapping which makes it possible to visualize areas richer in cerium element and others richer in zirconium element, of similar size. These domains have a size of the order of 50 nm to 500 nm for a mixed oxide calcined at 900 ° C. These domains have a size of the order of 200 nm to micron sizes for a mixed oxide calcined at 1100 ° C. This was also confirmed by EDS analysis in transmission electron microscopy (-130 points analyzed with a probe diameter of 12 nm).

The two solutions (A) and (B) may have the same overall concentration in each of their constituent elements, expressed in g of oxide / L, and may be introduced with the same volume flow rates.

Note that when both solutions (A) and (B) are introduced into the basic aqueous solution, it is preferable that the stirring rate is not too high (see Comparative Example 3). Similarly, it is preferable to introduce two solutions (A) and (B) with feed rates that are not too high (see Examples 4 and 5). In the case of the configuration selected in the examples (tank of about 1 L, moving at 45 ° inclined blades), good results are obtained with a stirring speed of between 200 and 400 revolutions / min and an introduction of solutions (A) and (B) at a rate between 0.1 and 0.25 L / h.

The next step (a2) of the process is the step of heating the precipitate suspension obtained in step (a1) or (a1 '). This heating can be carried out directly on the reaction mixture obtained at the end of step (a1) or (a1 ') or on a suspension obtained after separation of the precipitate from the reaction mixture of step (a1) or (a1' ), washing or preferably successive successive washings of the precipitate and put into water thereof. The suspension may be heated to a temperature of at least 100 ° C and even more preferably at least 130 ° C, or even at least 150 ° C. The temperature may be between 100 ° C and 200 ° C, more particularly between 130 ° C and 200 ° C. It can be for example between 100 ° C and 160 ° C. The heating operation can be conducted by introducing the liquid medium into a closed reactor (autoclave type). Under the conditions of the temperatures given above, and in 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 (10 ⁵ Pa) and 165 bar (1 bar). , 65 × 10 ⁷ Pa), preferably between 5 bar (5.10 ⁵ Pa) and 165 bar (1.65 × 10 ⁷ Pa). It is also possible to carry out heating in an open reactor for temperatures close to 100 ° C. The heating may be conducted either in air or in an atmosphere of inert gas, preferably nitrogen.

The duration of the heating can vary within wide limits, for example between 1 and 48 hours, preferably between 1 and 24 hours. Similarly, the rise in temperature 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 30 minutes and 4 hours, these values being given for all purposes. indicative fact.

It is possible to do several heats. Thus, the precipitate obtained after the heating step and possibly a washing can be resuspended in water, and then another heating of the medium thus obtained can be carried out. This other heating is done under the same conditions as those described for the first. The next step (a3) of the process consists in adding to the precipitate from the previous step a texturing agent whose function is to control the porosity of the mixed oxide. A texturizing agent includes polar chemical groups that interact with the chemical groups on the surface of the precipitate. The texturizing agent is subsequently removed in the calcination step.

The texturing agent may be chosen from anionic surfactants, nonionic surfactants, polyethylene glycols and carboxylic acids and their salts, as well as surfactants of the ethoxylate type of carboxymethylated fatty alcohols. Regarding this additive, reference may be made to the teaching of the application WO-98/45212 and use the surfactants described herein. 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.

As nonionic surfactants, there may be mentioned acetylenic surfactants, alcohol ethoxylates, alkanolamides, amine oxides, ethoxylated alkanolamides, long chain ethoxylated amines, ethylene oxide / propylene oxide copolymers, sorbiatan derivatives, ethylene glycol, propylene glycol, glycerol, polyglyceryl esters and their ethoxylated derivatives, alkylamines, alkylimidazolines, ethoxylated oils and alkylphenol ethoxylates. These include in particular the products sold under the trademark IGEPAL ^®, DOWANOL ^®, ^® and Rhodamox® Alkamide ^®.

As regards the carboxylic acids, it is possible to use, in particular, aliphatic mono- or dicarboxylic acids and, among these, more particularly saturated acids. These include formic, acetic, propionic, butyric, isobutyric, valeric, caproic, caprylic, capric, lauric, myristic and palmitic acids. As dicarboxylic acids, there may be mentioned oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid and sebacic acid. It is also possible to use fatty acids and more particularly saturated fatty acids. It can be in particular linear and saturated acids of formula CH ₃ - (CH ₂ ) _m - COOH, m being an integer between 6 and 20, more particularly between 9 and 15. The salts of all the acids mentioned can also be used, especially the ammoniacal salts. By way of example, there may be mentioned more particularly lauric acid and ammonium laurate.

Finally, it is possible to use a surfactant which is chosen from those of the type ethoxylates of carboxymethylated fatty alcohols. The term carboxymethylated fatty alcohol ethoxylates is understood to mean products consisting of ethoxylated or propoxylated fatty alcohols comprising at the end of the chain a CH ₂ --COOH group. These products may have the formula: R-O- (CR2R3-CR ₄ R ₅ -O) _n - CH ₂ -COOH wherein R means a carbon chain, saturated or unsaturated, whose length is generally at most 22 carbon atoms, preferably of at least 12 carbon atoms; R2, R3, R4 and R ₅ can be identical and represent hydrogen, or R2 may be CH ₃ and R _3, R and R ₅ are hydrogen; n is a non-zero integer of up to 50 and more particularly between 5 and 15, these values being included. It will be noted that a surfactant may consist of a mixture of products of the above formula for which R 1 may be saturated and unsaturated respectively or products comprising both -CH ₂ -CH ₂ -O groups. and -C (CH ₃ ) -CH ₂ -O-. The addition of the texturing agent can be done in two ways. It can be added directly to the suspension resulting from step (a3). In this case, it is preferably added to a suspension whose temperature is at most 60 ° C. It may also be added to the solid precipitate after separation thereof by any known means from the medium in which the heating took place.

The amount of texturizing agent used, expressed as a percentage by weight of texturing agent relative to the mixed oxide, is generally between 5% and 100%, more particularly between 15% and 60%. Step (a4) is optional and consists in filtering the suspension and washing precipitated. This step may comprise a washing or several successive washes. In the case where there are several washes, the solid which has been washed is resuspended in water. Each washing can be done with water, preferably with water at basic pH, for example with ammonia water.

In the case where the basic compound used for the precipitation of step (a1) is an alkali or alkaline earth hydroxide, it may be necessary to conduct a thorough washing in step (a2) or to step (a4) or in both steps (a2) and (a4) so as to remove any trace of or reduce the content of the alkaline or alkaline earth element, which is likely to be detrimental to obtaining a specific surface area raised to 1100 ° C. The mixed oxide of the invention thus preferably has a weight content of alkaline or alkaline earth element, for example Na or K element, less than or equal to 200 ppm, or even less than or equal to 100 ppm, this content being expressed in terms of weight of the oxide of said element (eg Na 2 O or K ₂ O) based on the total weight of the mixed oxide. In the presence of several alkaline and / or alkaline earth elements, this content corresponds to the total content of these elements.

In step (a5), the precipitate recovered is then calcined in air to give the mixed oxide according to the invention. This calcination makes it possible to develop the crystallinity of the product formed. The specific surface of the product is even lower than the calcination temperature used is higher. The calcination is generally carried out under air, but a calcination carried out for example under an inert gas or under a controlled atmosphere (oxidizing or reducing) is not excluded. The calcining temperature is generally between 700 ° C and 1100 ° C. The duration of the calcination is not critical and depends on the temperature. As a guide only, it may be at least 2 hours, more particularly between 2 and 4 hours. It was possible to obtain the mixed oxide according to the invention with a calcination in air at 900 ° C. for 4 hours.

During a step (a6), the mixed oxide which is obtained in step (a5) may be optionally milled to obtain powders of the desired particle size. For example, a hammer mill may be used.

The preparation of the mixed oxide according to the invention may be based on the conditions of the examples which are given below. The invention also relates to a mixed oxide obtainable by the process just described.

As regards the use of the mixed oxide according to the invention, this falls within the field of automobile pollution remediation. The mixed oxide according to the invention can be used in the manufacture of a catalytic converter ("catalytic converter") whose function is to treat automotive exhaust. The catalytic converter comprises a catalytically active coating layer prepared from the mixed oxide and deposited on a solid support. The The purpose of the coating layer is to transform certain pollutants of the exhaust gas, in particular carbon monoxide, unburned hydrocarbons and nitrogen oxides, into chemical products that are less harmful to the environment.

The chemical reactions involved may be the following:

2 CO + O ₂ 2 CO ₂

2 NO + 2 CO N ₂ + 2 CO ₂

4 C _x H _y + (4x + y) O ₂ 4x CO ₂ + 2yH ₂ O

The solid support may be a metal monolith, for example FerCralloy, or ceramic. The ceramic may be cordierite, silicon carbide, alumina titanate or mullite. A commonly used solid support consists of a generally cylindrical monolith comprising a multitude of small parallel porous wall channels. This type of support is often cordierite and presents a compromise between a large specific surface area and a limited pressure drop.

On the surface of the solid support, the coating layer commonly called "washcoat" is deposited. The coating layer is formed from a composition comprising the mixed oxide in admixture with at least one inorganic material. The inorganic material may be chosen from alumina, titanium oxide, cerium oxide, zirconium oxide, silica, spinels, zeolites, silicates, crystalline silicoaluminium phosphates, calcium phosphates and the like. crystalline aluminum. The composition may also comprise other additives which are specific to each formulator: H ₂ S trap, organic or inorganic modifier whose function is to facilitate the coating, colloidal alumina, etc. The coating layer thus comprises such composition. Alumina is a mineral material commonly used, this alumina possibly being doped, for example by an alkaline earth metal such as barium. The coating layer also comprises at least one precious metal (such as for example Pt, Rh, Pd) dispersed. The amount of precious metal is generally between 1 and 400 g, based on the volume of the monolith expressed in ft ³ . The precious metal is catalytically active.

To disperse the precious metal, a salt of the precious metal may be added to a suspension of the mixed oxide or mineral material or the mixture of the mixed oxide and the inorganic material. The salt can be for example a chloride or a nitrate of the precious metal (eg Rh ¹ nitrate) The water is removed from the suspension to fix the precious metal, the solid is dried and calcined in air at a temperature generally between 300 and 800 ° C. An example of a precious metal dispersion can be found in Example 1 of US 7,374,729.

The coating layer is obtained by applying the suspension to the solid support. The coating layer therefore has a catalytic activity and can serve as a depollution catalyst. The depollution catalyst can be used to treat the exhaust gases of internal combustion engines. The catalytic systems and mixed oxides of the invention can finally be used as NO _x traps or to promote the reduction of NO _x even in an oxidizing medium.

Therefore, the invention also relates to a method for treating the exhaust gases of internal combustion engines which is characterized in that a catalytic converter comprising a coating layer as described is used.

Examples

BET specific surfaces: The specific surfaces are determined automatically using a Tristar II 3020 Micromentics device on samples in accordance with the manufacturer's instructions. The samples are pretreated in vacuo for 15 minutes at 300 ° C. It is necessary to desorb the species possibly adsorbed on the surface. The measurement is made on 5 points in the range of relative pressures p / po ranging from 0 to 0.3 inclusive. The equilibrium time for each point is 5 s. porosities: for mercury porosity measurements, a Micromeritics Autopore IV 9500 device was used with a powder penetrometer according to the instructions recommended by the manufacturer. The following parameters can be used: penetrometer used: 3.2 ml; capillary volume: 0.412 ml; head pressure: 4.68 psi; contact angle: 130 °; superficial mercury tension: 485 dynes / cm; Mercury density: 13.5335 g / ml. At the beginning of the measurement, a vacuum of 50 mmHg is applied to the sample for 5 minutes. The equilibrium times are as follows: low pressure range (1, 3-30 psi): 20 s - high pressure range (30-60,000 psi): 30 s. Prior to the measurement, the samples are degassed in an oven at 100 ° C for at least 15 min. X-ray diffraction: X-ray diffraction is obtained with a copper source (CuKal, λ = 1.5406 Angstrom). An X Pert Pro gyroometer was used in Panalytical's Bragg Brentano geometry with a copper source, 0.04 rad Soller slots, a spinner sample holder, and a X Celerator 1 D detector with an angular width of 2.122. °. The unit is equipped with a nickel filter on the back and programmable slots to illuminate a constant square area of 10 mm side. An angle pitch of 29 = 0.017 ° and a recording time of 40 seconds per step was used and an instrumental width s equal to 29 = 0.07 ° in the range of angles 29 from 28 to 32 ° was determined. . The acquisition is performed on a range of angles 29 between 5 ° and 90 °.

The intensities are determined on the diffractograms with respect to a baseline taken on the angle range 29 between 5.0 ° and 90.0 ° (see baseline in Fig. 2a). The baseline is determined automatically from the diffractogram data analysis software.

The compositions are given in percentages by weight of the ZrO ₂ , CeO ₂ , TR ₂ O ₃ oxides (for example La ₂ O ₃ , Y ₂ O ₃ , Nd ₂ O ₃ ) and Pr ₂ O ₆ .

Example 1 (according to the invention): Preparation of 60.0 g of mixed ZrO ₂ (50%) - CeO ₂ (40%) oxide - ₂ O ₃ (5%) - Y ₂ O ₃ (5%)

A solution of nitrates of cerium and zirconium (solution A) is prepared by introducing into a beaker 123 ml of water, 28 ml of a solution of zirconium nitrate obtained by attacking zirconium carbonate with nitric acid in a NO ₃ 7Zr ratio of 1.4; [ZrO ₂ ] = 288 g / L; density d = 1.388) as well as 74 mL of a solution of ceric nitrate ([CeO ₂ ] = 256 g / L, d = 1.438, free acidity 0.61 N). A solution of nitrates of cerium and zirconium (solution B) is also prepared by introducing into a beaker 123 ml of water, 81 ml of a solution of zirconium nitrate obtained by attacking zirconium carbonate with nitric acid. in a NO ₃ 7Zr ratio of 2.0 ([ZrO ₂ ] = 268 g / L, density d = 1.415) and 21 mL of the ceric nitrate solution.

Finally, a solution (C) of nitrates of lanthanum and yttrium is prepared by introducing into another vessel 29.3 ml of water, 6.4 ml of a nitrate solution. of lanthanum ([La ₂ O ₃ ] = 472.5 g / L) and 14.4 ml of a solution of yttrium nitrate ([Y ₂ O ₃ ] = 208.5 g / L).

A reactor of about 1 liter equipped with a 45 ° inclined paddle stirrer shown schematically in FIG. 1 was used. 12 N ammonia solution (144 ml) is introduced with stirring into the reactor and then it is made up with distilled water so as to obtain a total volume of 500 ml of basic solution at 3.45 mol / l. This makes it possible to ensure a stoichiometric excess of ammonia of 40 mol% relative to the cations which are present in the three solutions described above.

The three solutions previously prepared are stirred constantly. The two solutions (A) and (B) are introduced simultaneously in 54 min into the stirred reactor which contains the ammonia solution. Stirring is set at a speed of 400 rpm. The two solutions (A) and (B) are introduced into the stirred tank using 2 pipes of internal diameter 1, 6 mm positioned vertically and rigid. The exit points pA and pB are diametrically opposed and located at a distance of 5 mm from the mobile and 3 mm above the mobile (see Fig. 1). Once the two solutions (A) and (B) introduced, the solution (C) is introduced into the reactor in 6 min via one of the introduction rods, stirring being maintained at 400 revolutions / min. A suspension of precipitate is obtained.

The suspension is poured into a stainless steel autoclave equipped with a stirrer. The suspension is heated with stirring at 150 ° C for 2 hours. 19.8 g of lauric acid are then added to the suspension. The suspension is stirred for 1 hour.

The suspension is then filtered and the precipitate is washed with ammonia water pH = 9 at a rate of once the volume of the mother liquors filtration (~ 1 L). The solid product obtained is then calcined in air at 900 ° C. for 4 hours to recover about 60 g of mixed oxide.

Example 2 (according to the invention): conditions identical to those of Example 1 except the agitation which is set at 200 revolutions / min. Example 3 (comparative): conditions identical to those of Example 1 except the agitation which is set at 800 revolutions / min. In this case, the specific surface Snoo ° c / 4 h is lower (27 m ² / g). Example 4 (according to the invention): conditions identical to those of Example 1 except that the rate of introduction of the two solutions (A) and (B) is 0.1 L / h (introduction in 135 min instead 54 min)

Example 5 (according to the invention): conditions identical to those of Example 1 except that the rate of introduction of the two solutions (A) and (B) is 0.4 L / h (introduction in 34 min instead 54 min)

Example 6 (according to the invention): preparation of 60 g of mixed ZrO ₂ (50%) - CeO ₂ (40%) oxide - ₂ O ₃ (5%) - Y ₂ O ₃ (5%)

In this case, solutions (A) and (B) comprising the four constituent elements of the mixed oxide (Zr, Ce, La, Y) have been used. A solution of nitrates of cerium, zirconium, lanthanum and yttrium (solution A) is prepared by introducing into a beaker 138 ml of water, 28 ml of a solution of zirconium nitrate obtained by etching of sodium carbonate. zirconium with nitric acid in a ratio N0 ₃ 7Zr of 1, 4 ([ZrO ₂ ] = 288 g / L, density d = 1.388), 74 ml of a solution of ceric nitrate ([CeO ₂ ] = 256 g / L, d = 1.438, free acidity 0.61M), 3.2 mL of a solution of lanthanum nitrate ([La ₂ O ₃ ] = 472.5 g / L) and 7.2 mL of yttrium nitrate solution ([Y ₂ O ₃ ] = 208.5 g / L). A solution of nitrates of cerium, zirconium, lanthanum and yttrium (solution B) is also prepared by introducing into a beaker 138 ml of water, 81 ml of a solution of zirconium nitrate obtained by attacking carbonate. of zirconium with nitric acid in a ratio NO ₃ 7Zr of 2.0 ([ZrO ₂ ] = 268 g / L, density d = 1.415), 21 mL of the ceric nitrate solution, 3.2 mL of the lanthanum nitrate solution and 7.2 ml of the yttrium nitrate solution.

In the reactor previously described, a solution of 12 N ammonia (144 ml) is introduced with stirring and then it is made up with distilled water so as to obtain a total volume of 500 ml of basic solution at 3.45 mol / l. L. This makes it possible to ensure a stoichiometric excess of ammonia of 40 mol% relative to the cations which are present in the two solutions described above. The two solutions (A) and (B) are kept under constant stirring and introduced simultaneously over 60 min into the stirred reactor which contains the ammonia solution and whose stirring is set at a speed of 400 rpm. The rate of introduction is 0.25 L / h, and it is carried out using 2 pipes of internal diameter 1, 6 mm introduced into the reactor via vertical and rigid introduction rods. The latter arrive at the end of the stirring blades and are positioned diametrically opposite in the reactor. A suspension of precipitate is obtained. The suspension is poured into a stainless steel autoclave equipped with a stirrer. The suspension is heated with stirring at 150 ° C for 2 hours. 19.8 g of lauric acid are then added to the suspension. The suspension is stirred for 1 hour. The suspension is then filtered and the precipitate is washed with ammonia water pH = 9 at a rate of once the volume of the mother liquors filtration (~ 1 L). The solid product obtained is then calcined in air at 900 ° C. for 4 hours to recover about 60 g of mixed oxide. Example 7 (according to the invention):

The conditions of Example 1 are repeated except that the two solutions (A) and (B) are prepared as follows:

(A) 122 mL of water, 19 mL of the zirconium nitrate solution obtained by etching zirconium carbonate with nitric acid in a NOsVZr ratio of 1.4; [ZrO ₂ ] = 288 g / L; density d = 1.388), as well as 84 mL of the ceric nitrate solution of Example 1.

(B) 124 mL of water, 91 mL of zirconium nitrate solution obtained by zirconium carbonate etching with nitric acid in a NOsVZr ratio of 2.0 ([ZrO ₂ ] = 268 g / L; d = 1.415) as well as 11 mL of the ceric nitrate solution of Example 1.

Example 8 (according to the invention):

The conditions of Example 1 are repeated except that the two insertion rods are positioned adjacent (touching) and not diametrically opposed. The injection points pA and pB are therefore located at the same location at a distance of 5 mm from the mobile and 3 mm above the mobile. Example 9 (according to the invention):

The conditions of Example 1 are repeated for the preparation of 60 g of mixed ZrO ₂ (60%) - CeO ₂ (30%) - ₂ O ₃ (5%) - Y ₂ O ₃ (5%) oxide. with both solutions (A) and (B) and the basic solution prepared as follows:

(A) 125 mL of water, 41 mL of the zirconium nitrate solution obtained by attacking zirconium carbonate with nitric acid in a NOsVZr ratio of 1.4; [ZrO ₂ ] = 288 g / L; density d = 1.388) as well as 59 mL of the ceric nitrate solution of Example 1.

(B) 124 mL of water, 91 mL of the zirconium nitrate solution obtained by zirconium carbonate etching with nitric acid in a NOsVZr ratio of 2.0 ([ZrO ₂ ] = 268 g / L; d = 1.415) as well as 11 mL of the ceric nitrate solution of Example 1.

Basic solution: 137 ml of a solution of 12 N ammonia then added with distilled water so as to obtain a total volume of 500 ml of basic solution at 3.29 mol / l.

Example 10 (according to the invention): preparation of 60 g of mixed oxide composition ZrO ₂ (60%) - CeO ₂ (30%) - ₂ O ₃ (5%) - Y ₂ O ₃ (5%)

The conditions of Example 9 are used with two solutions (A) and (B) prepared as follows:

(A) 139 mL of water, 41 mL of the zirconium nitrate solution obtained by attacking zirconium carbonate with nitric acid in a NOsVZr ratio of 1.4; [ZrO ₂ ] = 288 g / L; density d = 1.388), 59 ml of a ceric nitrate solution of Example 1, 3.2 ml of the lanthanum nitrate solution of Example 1 and 7.2 ml of the nitrate solution of yttrium of Example 1.

(B) 138 mL of water, 91 mL of the zirconium nitrate solution obtained by zirconium carbonate etching with nitric acid in a NOsVZr ratio of 2.0 ([ZrO ₂ ] = 268 g / L; d = 1, 415), 1 1 ml of a ceric nitrate solution of Example 1, 3.2 ml of the lanthanum nitrate solution of Example 1 and 7.2 ml of the nitrate solution of yttrium of Example 1.

Example 11 (comparative):

The conditions of Example 1 are repeated except that the two solutions (A) and (B) have the same ratio (Ce / Zr). The two solutions are prepared with 126 ml of water, 52 ml of the zirconium nitrate solution of Example 1 (NO.sub.2 Zr ratio of 1.4) and 47 ml of the ceric nitrate solution of Example 1. It is found that the mixed oxide obtained does not have the two characteristic peaks. Example 12 (comparative):

(A) 124 mL of water, 38 mL of the zirconium nitrate solution of Example 1 (NO ₃ VZr ratio of 1.4) as well as 63 mL of the ceric nitrate solution of Example 1;

(B) 123 mL of water, 71 mL of the zirconium nitrate solution of Example 1 (NO ₃ VZr ratio of 2) and 32 mL of the ceric nitrate solution of Example 1.

Example 13 (Comparative): Preparation of 60 g of mixed ZrO ₂ (60%) - CeO ₂ (30%) oxide - ₂ O ₃ (5%) - Y ₂ O ₃ (5%)

(A) 126 mL of water, 51 mL of the zirconium nitrate solution obtained by etching zirconium carbonate with nitric acid in a NOsVZr ratio of 1.4; [ZrO ₂ ] = 288 g / L; density d = 1.388) as well as 49 mL of the ceric nitrate solution of Example 1.

(B) 123 mL of water, 81 mL of the zirconium nitrate solution obtained by zirconium carbonate etching with nitric acid in a NOsVZr ratio of 2.0 ([ZrO ₂ ] = 268 g / L; d = 1.415) as well as 21 mL of the ceric nitrate solution of Example 1. Example 14 (comparative): preparation of 60 g of mixed oxide composition ZrO ₂ (60%) - CeO ₂ (30%) - ₂ O ₃ (5%) - Y ₂ O ₃ (5%)

This mixed oxide was prepared by coprecipitation of all the elements. A solution of nitrates of cerium, zirconium, lanthanum and yttrium is prepared by introducing into a beaker 275 ml of water, 134 ml of a solution of zirconium nitrate obtained by etching zirconium carbonate with the acid. nitric acid in a NO ₃ VZr ratio of 2.0 ([ZrO ₂ ] = 268 g / L, density d = 1.415), 70 mL of a solution of ceric nitrate ([CeO ₂ ] = 256 g / L, d = 1.388, free acidity 0.61 N), 6.4 mL of a solution of lanthanum nitrate ([La ₂ O ₃ ] = 472.5 g / l) and 14.4 mL of a nitrate solution of yttrium ([Y ₂ O ₃ ] = 208.5 g / l).

In the reactor previously described, a solution of 12 N ammonia (137 ml) is introduced with stirring and then it is brought to completion with distilled water so as to obtain a total volume of 500 ml of basic solution at 3.29 mol / l. L. This allows to ensure a stoichiometric excess of ammonia of 40 mol% relative to the cations present in the solution described above.

The previously prepared solution is kept under constant stirring and introduced into the stirred reactor which contains the ammonia solution for 1 hour and whose stirring is set at a speed of 400 rpm. The rate of introduction is 0.5 L / h, and it is carried out using only one of the two previously described pipes. The suspension is poured into a stainless steel autoclave equipped with a stirrer. The suspension is heated with stirring at 150 ° C for 2 hours. 19.8 g of lauric acid are then added to the suspension. The suspension is stirred for 1 hour. The suspension is then filtered and the precipitate is washed with ammonia water pH = 9 at a rate of once the volume of the mother liquors filtration (~ 1 L). The solid product obtained is then calcined in air at 900 ° C. for 4 hours to recover about 60 g of mixed oxide. Example 15 (comparative): preparation of 60 g of mixed ZrO ₂ (50%) - CeO ₂ (40%) - ₂ O ₃ (5%) - Y ₂ O ₃ (5%) oxide

The conditions of Example 1 are repeated except that the two solutions (A) and (B) are not introduced simultaneously but sequentially in the following order: solution (B) introduced in 27 min at a flow rate of 0, 5 L / h, solution (A) introduced in 27 min at a rate of 0.5 L / h, and finally, solution (C) introduced in 6 min at a rate of 0.5 L / h. Stirring is maintained at 400 rpm in the reactor.

Example 16 (comparative): preparation of 60 g of mixed ZrO ₂ (50%) - CeO ₂ (40%) - ₂ O ₃ (5%) - Y ₂ O ₃ (5%) oxide

The conditions of Example 1 are repeated except that solution (A) comprises only zirconium nitrate:

(A): 92 ml of water and 75 ml of the zirconium nitrate solution of Example 1 (NO ₃ 7Zr ratio of 2.0), - introduction at a rate of 0.2 L / h;

(B): 155 mL of water, 35 mL of the zirconium nitrate solution of Example 1 (NO ₃ 7Zr ratio of 1.4) and 93 mL of the ceric nitrate solution of Example 1 - co-introduced with the solution (A) at a flow rate of 0.3 L / h. Example 17 (comparative): preparation of 60 g of mixed ZrO ₂ (50%) - CeO ₂ (40%) - ₂ O ₃ (5%) - Y ₂ O ₃ (5%) oxide

The conditions of Example 1 are repeated except that solution (A) comprises only cerium nitrate:

(A): 89 ml of water and 78 ml of the ceric nitrate solution of Example 1 - introduction at a rate of 0.2 L / h, and the conitrate

(B): 156 mL of water, 1 12 mL of the zirconium nitrate solution of Example 1 (NO3 ^~ / Zr ratio of 2.0), as well as 16 mL of the ceric nitrate solution of the Example 1 - Cointroduced with the solution (A) at a flow rate of 0.3 L / h.

The following comments can be made on the basis of the results given in Table I:

- Example 14 shows that the mixed oxide obtained by coprecipitation of a solution containing all the elements has a lower thermal stability than for the mixed oxide of the same composition prepared according to Example 1. In addition, the mixed oxide obtained does not have the two characteristic peaks.

- Example 15 shows, in comparison with Example 1, that when the two solutions are not added simultaneously, but consecutively, the mixed oxide obtained has a lower stability than for the mixed oxide of the same composition prepared according to Example 1.

- Example 11 shows that when the two solutions are of the same composition, the product obtained has a lower thermal stability than for the mixed oxide of the same composition prepared according to Example 1. In addition, the mixed oxide obtained does not have the two characteristic peaks.

examples 12 and 13 show that when the two solutions have ratios (Ce / Zr) that are not sufficiently distinct, the mixed oxides obtained do not have the two characteristic peaks.

examples 16 and 17 show that when one of the solutions contains only Ce or Zr, the mixed oxides obtained may have peaks outside the range of 47.6 ° -50.1 ° or additional peaks in this range. % weight of oxides process surface characteristics X-ray after calcination at

Ex. Of the elements ci-specific solution

1000 ° C / 4 h 1100 ° C / 4 h below (BET) 900 ° C / 4h

ω D

Zr This Y Sl100 ° C / Sl200 ° C /

(A) (B) (C)

4 h 4 h 1 ^st peak 2 ^è ™ peak R 1 ^st peak 2 ™ peak R 1 ^st peak 2 ™ peak R

1 (I) 50 40 5 5 Zr / Ce 30/70 Zr / Ce 80/20 yes 400 0.25 32 9.8

48.24 49.8 0.92 48.18 49.68 0.86 48.25 49.54 0.87

50 40 5 5 Zr / Ce 30/70 Zr / Ce 80/20

2 (l) yes 200 0.25 30 11, 1 48.13 49.7 0.92 48.17 49.67 0.86 48.29 49.62 0.89

50 40 5 5 Zr / Ce 30/70 Zr / Ce 80/20

3 (C) yes 800 0.25 27 9 48.14 49.68 0.89 48.19 49.66 0.86 48.29 49.59 0.87

50 40 5 5 Zr / Ce 30/70 Zr / Ce 80/20

4 (l) yes 400 0.10 32 9.4 48.13 49.71 0.90 48.18 49.68 0.90 48.30 49.62 0.93

50 40 5 5 Zr / Ce 30/70 Zr / Ce 80/20

5 (l) yes 400 0.40 29 9.5 48.16 49.65 0.90 48.19 49.62 0.85 48.28 49.5 0.82

50 40 5 5 Zr / Ce / La / Y Zr / Ce / La / Y

6 (l) no 400 0.25 31 8.8

27/63/5/5 72/18/5/5 48.19 49.65 0.96 48.22 49.64 0.86 48.34 49.58 0.84

50 40 5 5 Zr / Ce 20/80 Zr / Ce 90/10

yes 400 0.25 28 7.9 47.87 50.02 0.81 47.89 49.96 0.89 48.00 49.89 0.94

50 40 5 5 Zr / Ce 30/70 Zr / Ce 80/20

8 (l) ^" yes 400 0.25 33 10.8 48.17 49.72 0.91 48.19 49.68 0.86 48.30 49.61 0.89

60 30 5 5 Zr / Ce 44/56 Zr / Ce 90/10

9 (1) yes 400 0.25 32 8.3 48.59 50.00 0.65 48.64 49.98 0.67 48.71 49.89 0.74

60 30 5 5 Zr / Ce / La / Y Zr / Ce / La / Y

10 (1) no 400 6.7

39.7 / 50.3 / 5/5 81/9/5/5 0.25 33 48.55 49.86 0.73 48.71 49.90 0.69 48.77 49.84 0.73

11 50 40 5 5 Zr / Ce 55/45 Zr / Ce 55/45

(C) yes 400 0.25 27 4.5 1 single peak 1 single peak 1 single peak

12 50 40 5 5 Zr / Ce 40/60 Zr / Ce 70/30

(C) yes 400 0.25 31 6.2 1 single peak 1 shoulder 1 shoulder

13 60 30 5 5 Zr / Ce 54/46 Zr / Ce 80/20

(C) yes 400 0.25 31 7.2 1 single peak 1 single peak 1 shoulder

14 60 30 5 5 coprecipitation of all elements

(C) by introducing a single solution 400 3.6

0.50 27 1 single peak

1 single peak 1 single peak

in the tank

15 50 40 5 5 Zr / Ce 30/70 Zr / Ce 80/20

(C) introduced in introduced in yes 400 8,9

0.50 26 49.71 0.90 48.17 49.66 0.86 48.29 49.56 0.86

48.14

1 ^st

16 50 40 5 5 solution of Zr / Ce 29/71 50.19 + 50.16 + 48.21 50.12 + 1, 77

(C) Zr nitrate yes 400/27 10.6 1, 20 48.18 1, 43

48.13 49.19 49.20 49.21

17 50 40 5 5 solution of Zr / Ce 88/12 yes 400 8.4 47.47 49.82 0.98

/ 24 49.91 1, 19 47.39 49.88 1, 14

(C) nitrate of Ce 47,37

co: agitation speed in rpm of the stirring mobile during the introduction of (A) - (B) and (C); D: introduction rate of solutions (A) and (B) in L / h; Snoo-c / 4 h and Si2rwc / h in m / g

* injection with the two adjacent pipes (pA = pB)

Table II

Board

Vpt: total pore volume; Vp (<200 nm): pore volume in the pore area of diameter less than or equal to 200 nm; Vp (<100 nm): pore volume in the pore area of diameter less than or equal to 100 nm; dp: peak diameter on the derived curve

Table IV

Using the Scherer formula, the crystallite sizes of the two phases were also determined from the distinct peaks noted peak A and peak B between 28.5 ° and 30.5 °. It is observed that the crystallite sizes of the phase richest in zirconium are slightly higher when the lanthanum and the rare earth TR are introduced with solution (C) only when they are introduced together with cerium and zirconium in equivalent amount in solutions (A) and (B). On the other hand, the crystallites of the richest phase in Ce are slightly higher.

Claims

1. Mixed oxide of zirconium, cerium, lanthanum and possibly at least one rare earth other than cerium and lanthanum (TR), the proportions by weight of these elements expressed as oxide equivalents relative to the total weight of mixed oxide being the following:

^■ between 25.0% and 45.0% cerium;

^■ between 1.0% and 10.0% of lanthanum;

■ between 0% and 15.0% of the rare earth other than cerium and lanthanum;

^■ the zirconium supplement; characterized in that after the mixed oxide has been calcined in air at a temperature of 1100 ° C for 4 hours:

^■ it is in the form of a mixture of two crystalline phases comprising both cerium and zirconium; and

^■ it has a BET surface area of at least 28 m ² / g, more particularly at least 30 m ² / g, or even at least 32 m ² / g; and

^■ the diffractogram of the mixed oxide obtained by the X-ray diffraction, shows two distinct peaks assigned to these two phases:

o ¹ peak being positioned at a 2Θ value of between 47.6 ° and 49.0 ° (value included);

o ^2nd peak being positioned at a value between 49.0 ° 2Θ range (exclusive) and 50.1 °;

2. Mixed oxide according to claim 1 characterized in that it also comprises hafnium.

3. Mixed oxide according to one of claims 1 or 2 characterized in that the proportion by weight of hafnium in the mixed oxide is less than or equal to 2.5%, or even 2.0%, expressed in equivalent d oxide relative to the total weight of the mixed oxide.

4. Mixed oxide according to one of claims 1 to 3 characterized in that the elements Ce, La, TR, Zr and optionally Hf are present in form of oxides, hydroxides or oxyhydroxides, more particularly in the form of oxides.

5. Mixed oxide according to one of the preceding claims characterized in that after the mixed oxide has been calcined in air at a temperature of 900 ° C for 4 h, the diffractogram of the mixed oxide obtained by diffraction ray X, has two distinct peaks attributed to these two phases:

6. Mixed oxide according to one of the preceding claims characterized in that after the mixed oxide has been calcined in air at a temperature of 1000 ° C for 4 h, the diffractogram of the mixed oxide obtained by diffraction ray X, has two distinct peaks attributed to these two phases:

7. Mixed oxide according to one of the preceding claims characterized in that the intensities of the ^1st and the ^2nd peak are in a ratio R = l _{P st} ic / ic Beme _P between 0.50 and 1, 00, more especially between 0.70 and 1.00.

8. Mixed oxide according to one of the preceding claims characterized in that after the mixed oxide has been calcined in air for 4 hours at 900 ° C or 1000 ° C or 1100 ° C, the diffractogram of the oxide mixed only includes two distinct peaks in the range 2Θ between 47.6 ° and 50.1 °.

9. Mixed oxide according to one of the preceding claims characterized in that after the mixed oxide has been calcined in air for 4 hours at 900 ° C, 1000 ° C or 1100 ° C, the diffractogram of the mixed oxide has two distinct peaks attributed to the two crystalline phases: a peak being positioned at a value of 2Θ between 28.5 ° and 29.5 ° (inclusive);

10. Mixed oxide according to one of the preceding claims characterized in that after the mixed oxide has been calcined in air for 4 hours at 1000 ° C, the diffractogrannn of the mixed oxide has two distinct peaks attributed to the two phases crystalline:

1 1. Mixed oxide according to one of the preceding claims, characterized in that after the mixed oxide has been calcined in air for 4 hours at 1100 ° C., the diffractogranne of the mixed oxide has two distinct peaks attributed to the two crystalline phases :

12. Mixed oxide according to one of the preceding claims having a weight content of alkaline or alkaline earth element less than or equal to 200 ppm, or even less than or equal to 100 ppm, this content being expressed by weight of the oxide of said element relative to to the total weight of the mixed oxide.

13. Mixed oxide according to one of the preceding claims characterized in that the mixed oxide has a BET specific surface area of at least 40 m ² / g, more particularly at least 45 m ² / g after calcination at a temperature 1000 ° C for 4 hours.

14. Mixed oxide according to one of the preceding claims characterized in that the mixed oxide has a BET specific surface area of at least 50 m ² / g, more particularly at least 55 m ² / g after calcination at a temperature 900 ° C for 4 hours.

15. Mixed oxide according to one of the preceding claims characterized in that after calcination at a temperature of 1100 ° C for 4 h, the mixed oxide has:

a total pore volume greater than or equal to 0.70 ml / g or even greater than 0.80 ml / g; and or

a pore volume in the pore area of diameter less than or equal to 200 nm which is greater than or equal to 0.28 ml / g, or even greater than or equal to 0.30 ml / g; and or

o the derivative curve (dV / dlogD) obtained by mercury porosimetry on the mixed oxide after calcination at a temperature of 1100 ° C. for 4 hours, having in the range of pores with a diameter of less than or equal to 200 nm, a peak the maximum of which corresponds to a pore diameter of between 30 and 60 nm, V and D respectively denoting the pore volume and the pore diameter.

16. Mixed oxide according to one of the preceding claims characterized in that after calcination at a temperature of 900 ° C for 4 h, the mixed oxide has:

a total pore volume greater than or equal to 1.00 mL / g, or even greater than or equal to 1.10 mL / g, or even greater than or equal to 1.20 mL / g; and / or o a pore volume in the pore area of diameter less than or equal to 200 nm which is greater than or equal to 0.50 mL / g, or even greater than or equal to 0.55 mL / g, or even greater than or equal to at 0.60 mL / g; and / or o the derivative curve (dV / dlogD) obtained by mercury porosimetry on the mixed oxide has in the range of pores with a diameter of less than or equal to 200 nm, a peak whose maximum corresponds to a pore diameter between 20 and 40 nm, more particularly between 20 and 35 nm, V and D denoting respectively the pore volume and the pore diameter.

17. A method for preparing a mixed oxide according to one of claims 1 to 16 comprising the following steps:

- (a1 ') in the case where lanthanum and / or rare earth (s) other than cerium and lanthanum are not present in solutions (A) and (B), we then introduced into the mixture formed at the end of step (a1) kept under stirring, an aqueous solution (C) comprising lanthanum nitrate and / or rare earth nitrate (TR) possibly present in the oxide mixed;

- (a2) the suspension obtained at the end of step (a1) or step (aV) is heated with stirring;

- (a4) optionally, the suspension is filtered and the precipitate is washed;

- (a5) the solid obtained after step (a3) or (a4) is calcined in air at a temperature between 700 ° C and 1100 ° C to give the mixed oxide;

- (a6) the mixed oxide obtained in step (a5) may be optionally ground;

characterized in that the two solutions (A) and (B) respectively have mass ratios (Ce / Zr) _A and (Ce / Zr) _B which are different.

18. Composition comprising the mixed oxide according to one of claims 1 to 16 in admixture with at least one inorganic material.

19. The composition of claim 18 wherein the inorganic material is selected from alumina, titanium oxide, cerium oxide, zirconium oxide, silica, spinels, zeolites, silicates, crystalline silicoaluminum phosphates, crystalline aluminum phosphates.

20. A catalytically active coating layer deposited on the surface of a solid support prepared from the mixed oxide as described in one of claims 1 to 16 or from a composition according to claim 18 or 19.

21. Catalytic converter for treating automobile exhaust gases having a coating layer according to claim 20.

22. Use of a mixed oxide according to one of claims 1 to 16 or the composition of claim 18 or 19 in the preparation of a catalytic converter.

23. Process for treating the exhaust gases of internal combustion engines, characterized in that a catalytic converter comprising a coating layer according to Claim 20 is used.