Patent Description:
Catalysts for purifying vehicle exhaust gas are now almost mandatory in order to meet pollutants regulations that are in force in most of the regions worldwide (e.g. Europe, USA, Japan, China, Korea, India,. The function of the catalyst is to remove pollutants like CO, unburnt particulate matter (e.g. soot), unburnt hydrocarbons (HC), nitrogen oxides NO and NO<NUM> (referenced as NOx) that are noxious for health and for the environment.

Regulations are now becoming more and more challenging and are still expected to be stricter in the near future: see Euro <NUM> in Europe, SULEV <NUM> in the USA or China 6b in China. Emissions in real conditions are now also controlled like in Europe with RDE limits (Real Driving Emission) which add another challenge as the catalysts have to be efficient in a large variety of driving conditions. As a consequence, there is a requirement for having more and more efficient catalysts.

For gasoline vehicles, the control of the emissions is achieved using the so-called 'three way' catalyst (TWC) which can simultaneously decrease the amounts of hydrocarbons, CO and NOx. Natural gas vehicles may also generally rely on TWC catalysts when the engine is running in stoechiometric mode. Yet, the same challenge as for gasoline engines is expected. In addition, the fuel to air ratio is expected to be broader which would require larger Oxygen Storage Capacity (OSC).

Gasoline vehicles are also equipped in several regions with gasoline particulate filters (GPF), the function of which is to reduce the release of particulate matter emitted in particular, but not only, for gasoline direct injection engine technologies. The GPF is based on a TWC catalyst coated on the filter and having an increased OSC to promote the additional need of oxygen to burn the particulate matter. There is therefore a general need for catalysts exhibiting an improved OSC.

For diesel engines, several catalysts can be used like the Diesel Oxidation Catalyst (DOC) to control the oxidation functions of CO, HC but also to some extent particulate matter and NOx. Diesel engines are now also in general fitted with Diesel Particulate Filters (DPF), the function of which is to filter and burn the particulate matter. So the oxidation function is key for DOC and DPF and this is important to keep oxidation function after thermal stress due to soot oxidation during DPF regeneration. NOx emissions for diesel engines are either managed by Selective Catalytic Reduction (SCR) catalyst where the NOx reduction is obtained through the reaction between NOx and NH<NUM> or by NOx trapping or adsorption on Lean NOx Trap (LNT) or Partial NOx Absorber (PNA). In both cases, oxidation function is needed to efficiently remove the NOx.

The above-disclosed catalysts also require in all cases (except for SCR) the presence of at least one noble metal (e.g. Pt, Pd and Rh) aka platinum group metal (or PGM). Rh and Pd are generally more expensive than Pt, so that Pt tends to be now more commonly used. Due to the price of the PGMs, there is a general requirement to minimize their content in the catalysts. The PGMs, notably Pd and Pt, are dispersed on the surface of alumina and known to be stabilized by cerium oxide.

All the above-disclosed catalysts comprise alumina which is either used to disperse the PGM(s) (TWC, GPF, DOC, DPF, LNT, PNA) or mixed with a SCR catalyst (e.g. in general zeolite based or vanadium oxide associated with titanium oxide). All of them also comprise a ceria-based material which provides the Oxygen Storage Capacity (OSC) or assists the oxidation. Thermal stability is required for both alumina and the ceria-based material.

There is therefore a need for a thermally resistant alumina-based support which can be used for the preparation of a catalyst containing at least one PGM and which exhibits an OSC and helps stabilize the PGM(s) present in the catalyst. In the context of the present application, the term 'thermally resistant' is used to characterize a support which is able to maintain a high specific surface area and/or a small particle size after a heat treatment at high temperature. A simple and common way of characterizing the thermal stability of the support consists in measuring its specific surface area after a calcination in air at high temperature. Another way is to measure the particle size of the particles by X-ray diffraction (XRD) after the same treatment. The expression 'high temperature' depends on the nature of the catalyst used: in general the temperature of calcination is around <NUM> maximum for a diesel catalyst (like a DOC, DPF, LNT or SCR) and around <NUM>, even sometimes <NUM>, for a gasoline or natural gas catalyst (like a TWC or GPF).

In addition, the preparation of a catalyst generally involves coating of a suspension of a catalytic composition composed of inorganic materials onto a substrate or a monolith. It is of course more convenient for the preparation to use a highly concentrated suspension exhibiting a low viscosity. There is therefore also a need for a thermally resistant support which can be easily handled and used for the preparation of a highly concentrated and with low viscosity suspension.

The composition of the invention aims at solving these technical problems.

<CIT> discloses a porous inorganic composite oxide, comprising oxides of aluminum and cerium, or oxides of aluminum and zirconium, or oxides of aluminum, cerium, and zirconium, and, optionally, one or more oxides of dopants selected from transition metals, rare earths, and mixtures thereof. There is no disclosure of the composition as in claim <NUM>.

<CIT> discloses an emission treatment system comprising: a catalyst including a catalyst composition comprising ceria-alumina particles having a ceria phase present in a weight percent of the particles in the range of about <NUM>% to about <NUM>% on an oxide basis, an alkaline earth metal component supported on the ceria-alumina particles, wherein the CeO<NUM> is present in the form of crystallites that are hydrothermally stable and have an average crystallite size of less than <NUM> after aging at <NUM> for <NUM> hours in <NUM>% O<NUM> and <NUM>% steam in N<NUM>. There is no disclosure of the composition as in claim <NUM>. <NPL>; <NPL>, and <CIT> all disclose further CeO<NUM>/Al<NUM>O<NUM> materials and methods for making them.

The invention relates to a composition as defined in any one of claims <NUM> to <NUM>. This composition is according to two main embodiments:.

The invention also relates to a catalytic composition as defined in any one of claims <NUM> to <NUM>, and also to the use of the composition as defined in claims <NUM> and <NUM>. The invention relates to a process for preparing the composition as defined in claim <NUM>.

All these objects are now defined in greater details.

In the present patent application, it is specified, for the continuation of the description, that, unless otherwise indicated, in the ranges of values which are given, the values at the limits are included. It is also specified that the calcinations are performed in air. "wt%" is a % by weight.

In the context of the invention, the term "particle" means an agglomerate formed from primary particles. The particle size is determined from a particle size distribution by volume obtained by means of a laser particle size analyzer. The particle size distribution is characterized by means of the parameters D10, D50 and D90. These parameters have the usual meaning in the field of measurements by laser diffraction. Dx thus denotes the value which is determined with regard to the particle size distribution by volume for which x% of the particles have a size less than or equal to this value Dx. D50 thus corresponds to the median value of the distribution. D90 corresponds to the size for which <NUM>% of the particles have a size which is less than D90. D10 corresponds to the size for which <NUM>% of the particles have a size which is less than D10. The measurement is generally performed on a dispersion of the particles in water.

In the context of the invention, a rare earth element means an element of the group comprising yttrium and the elements of the Periodic Table with an atomic number between <NUM> and <NUM> inclusive.

The porosity data are obtained via the mercury porosimetry technique. This technique makes it possible to define the pore volume (V) as a function of the pore diameter (D). Use may be made of a Micromeritics Autopore <NUM> machine equipped with a powder penetrometer in accordance with the instructions recommended by the manufacturer. The procedure of ASTM D <NUM>-<NUM> may be followed. By means of these data, it is possible to determine the pore volume in the range of pores whose size is between <NUM> and <NUM> (denoted PV<NUM>-<NUM>), the pore volume in the range of pores whose size is between <NUM> and <NUM> (denoted PV<NUM>-<NUM>) and the total pore volume (denoted TPV).

The term "specific surface area" means the BET specific surface area determined by nitrogen adsorption determined by means of the Brunauer-Emmett-Teller method. This method was described in the journal "<NPL>)". The recommendations of the standard ASTM D3663 - <NUM> may be followed. Unless otherwise indicated, the calcinations for a given temperature and a given duration correspond to calcinations in air at a steady temperature stage over the duration indicated.

Furthermore, it is specified that the concentrations of the solutions or the proportions in the composition of the elements Al, Ce and La (if any) are given as weight percentages of oxide equivalents. The following oxides are thus retained for the calculation of these concentrations or proportions: Al<NUM>O<NUM> for Al, CeO<NUM> for Ce and La<NUM>O<NUM> for La. For example, an aqueous aluminum sulfate solution with an aluminum concentration of <NUM> wt% corresponds to a solution containing <NUM>% by weight of Al<NUM>O<NUM> equivalent. Similarly, a composition with <NUM> wt% of Al and <NUM> wt% of Ce corresponds to <NUM> wt% of Al<NUM>O<NUM> and <NUM> wt% of CeO<NUM>.

The composition is based on Al and Ce in the form of oxides (composition C1) or Al, Ce and La in the form of oxides (composition C2). According to an embodiment, composition C1 consists of oxides of Ce and Al. According to another embodiment, composition C2 consists of oxides of Ce, Al and La.

First of all, the composition is defined by the proportions of its constituents. Their proportions are given by weight relative to the total weight of the composition. Thus, for compositions C1 and C2, the proportion of CeO<NUM> is between <NUM> wt% and <NUM> wt%, or between <NUM> wt% and <NUM> wt% or even between <NUM> wt% and <NUM> wt%. This proportion may be between <NUM> wt% and <NUM> wt% or even between <NUM> wt% and <NUM> wt%. The proportion of CeO<NUM> may be lower than <NUM> wt%.

For composition C2, the proportion of La is between <NUM> wt% and <NUM> wt% or even between <NUM> wt% and <NUM> wt% or even between <NUM> wt% and <NUM> wt%, this proportion being expressed as weight of La<NUM>O<NUM> relative to the total weight of the composition.

The proportion of Al<NUM>O<NUM> corresponds to the remainder to <NUM>%. For composition C1, the proportion of Al<NUM>O<NUM> is between <NUM> wt% and <NUM> wt%. For composition C2, the proportion of Al<NUM>O<NUM> is between <NUM> wt% and <NUM> wt%. Al<NUM>O<NUM> is the major constituent of the composition.

Then, the composition is also defined by a compromise between physico-chemical properties. The thermal stability of an alumina-based product is generally partly linked to its pore volume. By increasing the total pore volume, the thermal stability is generally increased. Yet, the increase in the total pore volume usually brings about a significant lowering of the bulk density and also an increase in the viscosity of a suspension of the product in an aqueous medium. It has been discovered that by combining specific properties such as a particular porosity and a high bulk density, it is possible to have a thermally-stable support which can be easily handled as a powder and used for the preparation of a highly-loaded and with low viscosity suspension. Thus, the composition is characterized by the following porosity profile:.

The composition may also be defined by the following porosity profile:.

Moreover, the composition may exhibit a high specific surface area. It may have a BET specific surface area of between <NUM> and <NUM><NUM>/g, more particularly between <NUM> and <NUM><NUM>/g. This specific surface area may be greater than or equal to <NUM><NUM>/g. This specific surface may also be between <NUM> and <NUM><NUM>/g.

The composition moreover has high thermal stability. It may have a BET specific surface area after calcination in air at <NUM> for <NUM> hours which is between <NUM><NUM>/g and <NUM><NUM>/g, preferably between <NUM><NUM>/g and <NUM><NUM>/g. This specific surface area is usually strictly lower than <NUM> x (Al<NUM>O<NUM>) + <NUM><NUM>/g wherein (Al<NUM>O<NUM>) corresponds to the proportion of Al<NUM>O<NUM> in wt% in the composition. As an example, for a proportion of Al<NUM>O<NUM> of <NUM> wt%, the calculation leads to a value of: <NUM> x <NUM>% + <NUM> = <NUM><NUM>/g.

The composition moreover has high thermal stability. It may have a BET specific surface area after calcination in air at <NUM> for <NUM> hours of between <NUM> and <NUM><NUM>/g.

The composition generally has a total pore volume which is generally strictly greater than <NUM>/g. This total pore volume may advantageously be at least <NUM>/g, or even at least <NUM>/g. This total pore volume is generally not more than <NUM>/g, or even no more than <NUM>/g.

The composition may have a bulk density of between <NUM>/cm<NUM> and <NUM>/cm<NUM>, more particularly between <NUM>/cm<NUM> and <NUM>/cm<NUM>. This bulk density of the powder corresponds to the weight of a certain amount of powder relative to the volume occupied by this powder:<MAT>.

This bulk density may be determined by the method described below. Firstly, the volume of a measuring cylinder of about <NUM> with no spout is determined precisely. To do this, the empty measuring cylinder is weighed (tare T). Distilled water is then poured into the measuring cylinder up to the rim but without exceeding the rim (no meniscus). The measuring cylinder filled with distilled water is weighed (M). The mass of water contained in the measuring cylinder is thus: <MAT>.

The calibrated volume of the measuring cylinder is equal to Vmeasuring cylinder = E/(density of water at the measurement temperature). The density of the water is, for example, equal to <NUM>/mL for a measurement temperature of <NUM>.

The composition in the form of powder is carefully poured into the empty and dry measuring cylinder using a funnel until the rim of the cylinder is reached. The excess powder is levelled off using a spatula. The powder must not be compacted or tamped down during the filling. The measuring cylinder containing the powder is then weighed. bulk density (g/mL) = (mass of the measuring cylinder containing the alumina powder - <MAT>.

The composition may have a D50 of between <NUM> and <NUM>. It may have a D90 of less than or equal to <NUM>, more particularly less than or equal to <NUM>. It may have a D10 of greater than or equal to <NUM>.

It will also be noted that the composition is crystalline. This may be demonstrated by means of an X-ray diffractogram. The composition comprises a crystalline phase based on alumina. Such phase may be a delta phase, a theta phase, a gamma phase or a mixture of at least two of these phases.

Composition C2 preferably does not comprise any XRD diffraction pattern of a phase containing lanthanum, in particular any of the following phases LaAlO<NUM> or LaAl<NUM>O<NUM>. These last two phases may be identified by the following respective references of the International Centre for Diffraction Data: ICDD <NUM>-<NUM>-<NUM> and ICDD <NUM>-<NUM>-<NUM>.

The composition also comprises a crystalline phase based on cerium oxide. This phase may correspond to pure CeO<NUM> or to CeO<NUM> containing lanthanum. This phase exhibits a diffraction line 2θ between <NUM>° and <NUM>°.

From the diffraction peak [<NUM>] of the cubic phase corresponding to cerium oxide, which is present at 2θ between <NUM>° and <NUM>°, it is possible to determine the mean size of the crystallites. The composition exhibits indeed:.

The mean size D<NUM>-<NUM> is generally at least <NUM>. Likewise, the mean size D<NUM>-<NUM> is generally at least <NUM>.

The mean size of the crystallites D is measured by X-ray diffraction. It corresponds to the size of the coherent domain calculated from the width of the diffraction line 2θ between <NUM>° and <NUM>° and using the Scherrer equation. According to the Scherrer equation, D is given by formula (I): <MAT>.

One usually takes into account the broadening due to the instrument to determine B.

In formula (II), the broadening due to the instrument is Binstr and <MAT>.

The composition is generally in the form of powder. As a powder, the composition may be characterized according to two specific embodiments:.

D50 and D90 are determined by means of a laser particle size analyzer from a particle size distribution by volume, the measurement being performed on a dispersion of the particles in water.

The composition may also comprise some residual components. The composition may comprise residual sodium. The content of residual sodium may be less than or equal to <NUM>% by weight, or even less than or equal to <NUM>% by weight. The sodium content may be greater than or equal to <NUM> ppm. This content may be between <NUM> and <NUM> ppm, or even between <NUM> and <NUM> ppm. This content is expressed as weight of Na<NUM>O relative to the total weight of the composition. Thus, for a composition having a residual sodium content of <NUM>%, it is considered that there is, per <NUM> of mixed oxide, <NUM> of Na<NUM>O. The method for determining the sodium content within this concentration range is known to those skilled in the art. For example, the composition can be digested in acidic conditions, the digestion being optionally assisted by microwaves and once the composition is fully dissolved, the acidic solution is titrated by inductively coupled plasma spectroscopy technique.

The composition may comprise residual sulfate. The content of residual sulfate may be less than or equal to <NUM> wt%, or even less than or equal to <NUM> wt%, or else even less than or equal to <NUM> wt%. The sulfate content may be greater than or equal to <NUM> ppm. This content may be between <NUM> and <NUM> ppm, or even between <NUM> and <NUM> ppm. This content is expressed as weight of sulfate relative to the total weight of the composition. Thus, for a composition having a residual sulfate content of <NUM> wt%, it is considered that there is, per <NUM> of the composition, <NUM> of SO<NUM>. The method for determining the sulfate content within this concentration range is known to those skilled For instance, the same method as for the sodium titration can be applied.

The composition may contain impurities other than sodium and sulfate, for example impurities based on silicium, titanium or iron. The proportion of each impurity is generally less than <NUM> wt%, or even less than <NUM> wt%.

The composition of the invention may be used in the field of catalysis and in particular in the preparation of a catalyst used to purify vehicle exhaust gas. The composition according to the invention may be used in the preparation of a catalytic converter, the role of which is to treat motor vehicle exhaust gases. The catalytic converter comprises at least one catalytically active coating layer prepared from the composition and deposited on a support. The role of the catalytic converter is to convert, by chemical reactions, certain pollutants of the exhaust gas, in particular carbon monoxide, unburnt hydrocarbons and nitrogen oxides, into products which are less harmful to the environment.

The composition of the invention may also be used for the preparation of a catalytic composition. A catalytic composition generally comprises:.

According to an embodiment, the catalytic composition:.

According to another embodiment, the catalytic composition:.

The inorganic material other than the composition of the invention is selected in the group consisting of zeolites; alumina-based materials; ceria-based materials; zirconia-based materials; mixed oxides comprising oxides of cerium and zirconium; mixed oxides comprising oxides of aluminium, cerium and zirconium; and combinations thereof.

The inorganic material may be a zeolite. The zeolite may be selected in the group consisting of AEI, AFT, AFV, AFX, AVL, CHA, DDR, EAB, EEI, ERI, IFY, IRN, KFI, LEV, LTA, LTN, MER, MWF, NPT, PAU, RHO, RTE, RTH, SAS, SAT, SAV, SFW, TSC, UFI and combinations thereof. The zeolite may be ion-exchanged with at least one catalytic metal, such as Cu, Fe, Ce and combinations thereof.

The inorganic material may be a ceria-based material. The ceria-based material may be selected in the group consisting of cerium oxide; mixed oxides of cerium and at least one rare earth element other than cerium and composite oxides of cerium and at least one alkaline earth element. The proportion of the rare earth element or of the alkaline earth element is usually between <NUM> wt% and <NUM> wt%, this proportion being expressed as oxides relative to the ceria-based material as a whole. Examples of ceria-based materials may be found in the following references: <CIT>, <CIT>, <CIT>, <CIT>.

The inorganic material may be an alumina-based material. The alumina-based material may be selected in the group consisting of alumina; alumina stabilized by at least one oxide of an element selected in the group consisting of silicon, zirconium, rare earth metals, alkaline earth metals; aluminium hydrate; and combinations thereof. The alumina-based material may more particularly be an alumina or an alumina stabilized by lanthanum oxide. The proportion of the stabilizing element is usually between <NUM> wt% and <NUM> wt%, this proportion being expressed as oxides relative to the alumina-based material as a whole. Examples of alumina-based materials may be found in the following references: <CIT>, <CIT>.

The inorganic material may be a zirconia-based material. The zirconia-based material may be selected in the group consisting of zirconia and zirconia stabilized by at least one oxide of an element selected in the group consisting of yttrium, lanthanum, praseodymium, cerium and combinations thereof. Examples of zirconia-based materials may be found in the following references: <CIT>, <CIT> and <CIT>.

The inorganic material may be a mixed oxide comprising oxides of cerium and zirconium. The mixed oxide comprising oxides of cerium and zirconium may be selected in the group of mixed oxides of cerium and zirconium and mixed oxides of cerium, zirconium and at least one rare earth element other than cerium. Examples of mixed oxides comprising oxides of cerium and zirconium may be found in the following references: <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>.

The inorganic material may be mixed oxide comprising oxides of aluminium, cerium and zirconium. The mixed oxide comprising oxides of aluminium, cerium and zirconium may be selected in the group of mixed oxides of aluminium, cerium and zirconium and mixed oxides of aluminium, cerium, zirconium and at least one rare earth element other than cerium. Examples of such mixed oxides may be found in the following references: <CIT>, <CIT>, <CIT>.

The platinum group metal (PGM) is an element selected in the group of group VIII of the periodic table. The platinum group metal may more particularly be and is usually selected in the group consisting of Pt, Pd, Rh and combinations thereof.

A catalytic composition is prepared by the usual techniques in the field known to the skilled person. For example, a process of preparation of said catalytic composition comprises the following steps: (a) preparing a suspension in an aqueous medium containing the composition of the invention and the inorganic material other than the composition of the invention, (b) wet-milling of the suspension of step (a), (c) optionally bringing into contact the suspension with an aqueous solution of at least one PGM, (d) coating the obtained suspension onto a support e.g. a monolith or a filter, (e) drying and/or calcining in air. As an alternative, the process does not comprise step (c) and the PGM is introduced onto the composition of the invention before step (a) by any known technique such as incipient wetness impregnation method.

An example of preparation of a catalytic composition according to this process corresponds to example <NUM> of <CIT>: the composition of the invention is impregnated with a Pd nitrate solution using standard incipient wetness techniques. The Pd impregnated powder is placed into deionized water, and a Pt nitrate solution is added. After reducing the pH to <NUM> by addition of acid, the slurry is milled and the milled slurry is dried and calcined at <NUM> for <NUM> hours in air. Another example of a preparation is provided in example <NUM> of <CIT>.

The catalytic composition may also comprise at least one element selected in the group consisting of the alkali metals and the alkaline earth metals. This element is usually in the form of an oxide. The element may be an alkali metal such as sodium or potassium. The element may also be an alkaline earth metal such as magnesium, barium or strontium. The element may be present in different forms in the catalytic composition:.

As examples and without limitations, below are provided examples of catalytic compositions :.

The invention also relates to a process for preparing the composition of the invention comprising the following steps:.

In step (a), the following are introduced with stirring into a tank initially containing an acidic aqueous solution with a pH of between <NUM> and <NUM>, or even between <NUM> and <NUM>:.

The acidic aqueous solution initially contained in the tank has a pH of between <NUM> and <NUM>, or even between <NUM> and <NUM>. This solution may consist of a dilute aqueous solution of a mineral acid, for instance sulfuric acid, hydrochloric acid or nitric acid.

The acidic aqueous solution may also consist of an aqueous solution of an acidic aluminum salt such as aluminum nitrate, chloride or sulfate. Preferably, the aluminum concentration of this solution is between <NUM> wt% and <NUM> wt%, or even between <NUM> wt% and <NUM> wt%, or else even between <NUM> wt% and <NUM> wt%. Preferably, the acidic aqueous solution is an aqueous solution of aluminum sulfate. This solution is prepared by dissolving aluminum sulfate in water or by diluting preformed aqueous solution(s) in water. The pH of the aqueous solution developed by the presence of the aluminum sulfate is generally between <NUM> and <NUM>, or even between <NUM> and <NUM>.

Step (a) is performed according to two embodiments (a1) or (a2). According to embodiment (a1), an aqueous solution of sodium aluminate is introduced with stirring. According to embodiment (a2), (i) an aqueous solution of aluminum sulfate and (ii) an aqueous solution of sodium aluminate are introduced simultaneously with stirring.

Preferably, the aqueous solution of sodium aluminate does not contain any precipitated alumina. The sodium aluminate preferably has an Na<NUM>O/Al<NUM>O<NUM> ratio of greater than or equal to <NUM>, for example between <NUM> and <NUM>.

The aqueous solution of sodium aluminate may have an aluminum concentration of between <NUM> wt% and <NUM> wt%, more particularly between <NUM> wt% and <NUM> wt%, or even between <NUM> wt% and <NUM> wt%. The aqueous solution of aluminum sulfate may have an aluminum concentration of between <NUM> wt% and <NUM> wt%, more particularly between <NUM> wt% and <NUM> wt%.

On conclusion of step (a), the aluminum concentration of the reaction mixture is between <NUM> wt% and <NUM> wt%.

In this step (a), the time of introduction of the solution(s) is generally between <NUM> and <NUM>.

In step (a), the introduction of the aqueous solution of sodium aluminate has the effect of increasing the pH of the reaction mixture.

In particular for embodiment (a1), the aqueous solution of sodium aluminate may be introduced directly into the reaction medium, for example via at least one introduction cannula. In particular for embodiment (a2), the two solutions may be introduced directly into the reaction medium, for example via at least two introduction cannulas. For these two embodiments (a1) and (a2), the solution(s) are preferably introduced into a well-stirred zone of the reactor, for example into a zone close to the stirring rotor, so as to obtain efficient mixing of the solution(s) introduced into the reaction mixture. For embodiment (a2), when the solutions are introduced via at least two introduction cannulas, the points of injection via which the two solutions are introduced into the reaction mixture are distributed so that the solutions become efficiently diluted in said mixture. Thus, for example, two cannulas may be arranged in the tank so that the points of injection of the solutions into the reaction mixture are diametrically opposed.

In step (b), an aqueous solution of aluminum sulfate and an aqueous solution of sodium aluminate are introduced simultaneously, the rates of introduction of which solutions are regulated so as to maintain a mean pH of the reaction mixture within the pH range targeted in step (a). Thus, the target value of the mean pH is between:.

The term "mean pH" means the arithmetic mean of the pH values of the reaction mixture which are recorded continuously during step (b).

Preferably, the aqueous solution of sodium aluminate is introduced at the same time as the aqueous solution of aluminum sulfate at a flow rate that is regulated so that the mean pH of the reaction mixture is equal to the target value. The flow rate of the aqueous solution of sodium aluminate which serves to regulate the pH may fluctuate in the course of step (b).

The time of introduction of the two solutions may be between <NUM> minutes and <NUM> hours, or even between <NUM> minutes and <NUM> minutes. The flow rate of introduction of the solution or of the two solutions may be constant.

It is necessary for the temperature of the reaction mixture for steps (a) and (b) to be at least <NUM>. This temperature may be between <NUM> and <NUM>. To do this, the solution initially contained in the tank in step (a) may have been preheated before the start of introduction of the solution(s). The solutions that are introduced into the tank in steps (a) and (b) may also be preheated beforehand.

In step (c), the pH of the reaction mixture is optionally adjusted to a value of between <NUM> and <NUM>, or even between <NUM> and <NUM> or between <NUM> and <NUM>, by adding a basic or acidic aqueous solution.

The acidic aqueous solution that may be used for adjusting the pH may consist of an aqueous solution of a mineral acid, for instance sulfuric acid, hydrochloric acid or nitric acid. The acidic aqueous solution may also consist of an aqueous solution of an acidic aluminum salt such as aluminum nitrate, chloride or sulfate.

The basic aqueous solution that may be used for adjusting the pH may consist of an aqueous solution of a mineral base, for instance sodium hydroxide, potassium hydroxide or aqueous ammonia. The basic aqueous solution may also consist of an aqueous solution of a basic aluminum salt such as sodium aluminate. An aqueous solution of sodium aluminate is preferably used.

Preferably, the pH is adjusted by stopping:.

According to one embodiment, the introduction of the aqueous solution of aluminum sulfate is stopped and the introduction of the aqueous solution of sodium aluminate is continued until a target pH of between <NUM> and <NUM> and preferably between <NUM> and <NUM> is reached. The duration of step (c) may be variable. This duration may be between <NUM> and <NUM>.

In step (d), the reaction mixture is filtered. The reaction mixture is generally in the form of a slurry. The solid recovered on the filter may be washed with water. To do this, use may be made of hot water having a temperature of at least <NUM>.

In step (e), a dispersion in water of the solid recovered on conclusion of step (d) undergoes a mechanical or ultrasonication treatment so as to reduce the particle size of the dispersion. The pH of this dispersion before milling may optionally be adjusted to between <NUM> and <NUM>. A nitric acid solution may be used, for example, to do this.

The D50 of the particles of the dispersion before the mechanical or ultrasonication treatment is generally between <NUM> and <NUM>, or even between <NUM> and <NUM>. The D50 of the particles of the solid after the mechanical or ultrasonication treatment is preferably between <NUM> and <NUM>, or even between <NUM> and <NUM>.

The mechanical treatment consists in applying a mechanical stress or shear forces to the dispersion so as to fractionate the particles. The mechanical treatment may be performed, for example, by means of a ball mill, a high-pressure homogenizer or a milling system comprising a rotor and a stator. On a laboratory scale, use may be made of a Microcer or Labstar Zeta ball mill, both of which are sold by the company Netzsch (for further details, see: https://www. netzsch-grinding. com/fr/produits-solutions/broyage-humide/broyeurs-de-laboratoire-serie-mini/). A milling system as described in the examples may be used. In the case of a ball mill, use may be made, for example, of yttrium-stabilized zirconium oxide beads. ZetaBeads Plus <NUM> balls may be used, for example.

The ultrasonication treatment consists, for its part, in applying a sound wave to the dispersion. The sound wave which propagates through the liquid medium induces cavitation which enables the particles to be fractionated. On a laboratory scale, use may be made of an ultrasonication system with a Sonics Vibracell VC750 generator equipped with a <NUM> probe. The duration and the power applied are adjusted so as to achieve the targeted D50.

The mechanical or ultrasonication treatment may be performed in batch mode or continuously.

In step (f), at least one salt of cerium is added to the dispersion obtained at the end of step (e). It may also be envisaged to add in this step an aqueous ammonia solution to raise the pH, preferably to a value of between <NUM> and <NUM>. The salt of cerium may be selected in the group consisting of cerium chloride, cerium acetate and cerium nitrate. The salt of cerium is preferably a salt of Ce (III).

The salt of cerium may also be added before step (d), for instance in step (a), e.g. in the solution containing aluminium sulfate. The proportion α of the salt of cerium added in step (f) is between <NUM>% and <NUM>%, preferably between <NUM>% and <NUM>%, α being calculated by the following formula: α = amount added in step (f) / total amount of cerium added x <NUM>. Preferably, α = <NUM>% which means that the salt of cerium is added only in step (f) to the dispersion obtained at the end of step (e).

For composition C2, at least one salt of lanthanum may also be added before step (d) or at step (f) to the dispersion obtained at the end of step (e).

The salts of cerium and of lanthanum are conveniently introduced in the form of aqueous solutions.

In step (g), the dispersion from step (f) is dried, preferably by spraying.

Spray-drying has the advantage of giving particles with a controlled particle size distribution. This drying method also offers good production efficiency. It consists in spraying the dispersion as a mist of droplets in a stream of hot gas (for example a stream of hot air) circulating in a chamber. The quality of the spraying controls the size distribution of the droplets and, consequently, the size distribution of the dried particles. The spraying may be performed using any sprayer known per se. Two main types of spraying devices exist: turbines and nozzles. Regarding the various spraying techniques that may be implemented in the present process, reference may be made notably to the standard manual by <NPL>on). The operating parameters which a person skilled in the art can modify are notably the following: the flow rate and the temperature of the dispersion entering the sprayer; the flow rate, pressure, humidity and temperature of the hot gas. The inlet temperature of the gas is generally between <NUM> and <NUM>. The outlet temperature of the gas is generally between <NUM> and <NUM>.

The D50 of the powder recovered on conclusion of step (g) is generally between <NUM> and <NUM>. This size is linked to the size distribution of the droplets leaving the sprayer. The evaporation capacity of the atomizer is generally linked to the size of the chamber. Thus, on a laboratory scale (Büchi B <NUM>), the D50 may be between <NUM> and <NUM>. On a larger scale, the D50 may be between <NUM> and <NUM>.

In step (h), the solid obtained from step (g) is calcined in air. The calcination aims at converting the ingredients added in the previous steps into oxides and at developing the crystallinity of the composition. The calcination temperature is generally between <NUM> and <NUM>, more particularly between <NUM> and <NUM>. The calcination time is generally between <NUM> and <NUM> hours.

The balance between the calcination temperature and the calcination time should be adapted to convert the ingredients into oxides and to develop the crystallinity of the composition having in mind that too high a calcination temperature affects both the specific surface area and the mean crystallite size. The calcination conditions given in example <NUM> may be used as they offer such a good balance.

It may be envisaged to perform the two steps (g) and (h) in the same equipment in which the dispersion obtained from step (f) undergoes a heat treatment for performing both drying and calcination.

Preferably, the mixed oxide that is recovered on conclusion of step (h) (i.e. at the end of the calcination) has a D50 generally between <NUM> and <NUM>. It generally has a D90 of less than or equal to <NUM>, more particularly less than or equal to <NUM>.

According to a first embodiment, at the end of step (h), the D50 may be between <NUM> and <NUM>, or even between <NUM> and <NUM>. The D90 may be between <NUM> and <NUM>, or even between <NUM> and <NUM>. This embodiment may rather be performed when step (f) is performed on a laboratory scale using, for example, a Büchi B <NUM> atomizer.

According to a second embodiment, at the end of step (h), the D50 may be between <NUM> and <NUM>, or even between <NUM> and <NUM>. The D90 may be between <NUM> and <NUM>, or even between <NUM> and <NUM>. This embodiment may rather be performed when step (f) is performed on a larger scale.

The process may also comprise a final step via which the solid obtained in the preceding step undergoes milling so as to adjust the particle size of the solid. Use may be made of a knife mill, an air jet mill, a hammer mill or a ball mill. Preferably, the milled product has a D50 generally between <NUM> and <NUM>. The D90 may be between <NUM> and <NUM>, or even between <NUM> and <NUM>.

The composition of the invention is in the form of a powder.

Further details for the preparation of the composition of the invention will be found in the following illustrative examples.

For the continuation of the description, the term "specific surface area" means the BET specific surface area determined by nitrogen adsorption in accordance with the standard ASTM D <NUM>-<NUM> established from the Brunauer-Emmett-Teller method described in the journal "<NPL>)". The specific surface area is determined automatically using, for example, a Tristar II <NUM> machine from Micromeritics in accordance with the instructions recommended by the manufacturer. The samples are pretreated at <NUM> for <NUM> under vacuum (for example to reach a pressure of <NUM> mmHg). This treatment makes it possible to remove the physisorbed volatile species at the surface (for instance H<NUM>O, etc.).

The measurement is performed using a mercury porosimetry machine. In the present case, use was made of a Micromeritics Autopore IV <NUM> machine equipped with a powder penetrometer, in accordance with the instructions recommended by the manufacturer. The following parameters were used: penetrometer used: <NUM> (Micromeritics reference: penetrometer type No. <NUM>); capillary volume: <NUM>; max. pressure ("head pressure"): <NUM> psi; contact angle: <NUM>°; surface tension of the mercury: <NUM> dynes/cm; density of the mercury: <NUM>/ml. At the start of the measurement, a vacuum of <NUM> mmHg is applied to the sample for <NUM>. The equilibrium times are as follows: low pressure range (<NUM>-<NUM> psi): <NUM> - high pressure range (<NUM>-<NUM><NUM> psi): <NUM>. Prior to the measurement, the samples are treated at <NUM> for <NUM> to remove the physisorbed volatile species at the surface (for instance H<NUM>O, etc). From this measurement, the pore volumes may be deduced.

X-ray diffraction: use was made of an x-ray diffractometer X'Pert Pro with a copper source (CuKα1, λ=<NUM> Angstrom).

To perform the particle size measurements, use is made of a Malvern Mastersizer <NUM> or <NUM> laser diffraction particle size analyzer (further details regarding this machine are given here: https://www. malvernpanalytical. com/fr/products/product-range/mastersizer-range/mastersizer-<NUM>). The laser diffraction technique used consists in measuring the intensity of the light scattered during the passage of a laser beam through a sample of dispersed particles. The laser beam passes through the sample and the intensity of the scattered light is measured as a function of the angle. The diffracted intensities are then analyzed to calculate the particle size using the Mie scattering theory. The measurement makes it possible to obtain a volume-based size distribution, from which the parameters D10, D50 and D90 are deduced.

<NUM> of deionized water are introduced into a stirred reactor and heated to <NUM>. This temperature is maintained throughout steps (a) to (c). <NUM> of an aluminum sulfate solution with a concentration of <NUM> wt% of alumina (Al<NUM>O<NUM>) are introduced at a flow rate of <NUM> of solution/min via an introduction cannula close to the stirring rotor. At the end of the introduction, the pH in the reactor is close to <NUM> and the aluminum concentration is <NUM>% by weight of alumina (Al<NUM>O<NUM>). The introduction of the aluminum sulfate solution is then stopped.

step (a): a sodium aluminate solution with a concentration of <NUM> wt% of alumina (Al<NUM>O<NUM>) and a Na<NUM>O/Al<NUM>O<NUM> molar ratio of <NUM> is introduced at a flow rate of <NUM> of solution/min via a second introduction cannula close to the stirring rotor, until a pH of <NUM> is reached. The introduction is then stopped. The aluminum concentration of the reaction mixture is then <NUM> wt% of alumina (Al<NUM>O<NUM>).

In step (b), the introduction of the aluminum sulfate solution is again started at a flow rate of <NUM> of solution/min and the sodium aluminate solution is simultaneously introduced into the stirred reactor at a regulated flow rate so as to maintain the pH at a value of <NUM>. This step lasts <NUM> minutes.

In step (c), the introduction of the aluminum sulfate solution is stopped and the addition of the sodium aluminate solution is continued at a flow rate of <NUM> of solution/min until a pH of <NUM> is reached. The addition of the sodium aluminate solution is stopped.

In step (d), the reaction slurry is poured onto a vacuum filter. At the end of the filtration step, the cake is washed with deionized water at <NUM>.

In step (e), the cake is redispersed in deionized water to obtain a suspension having a concentration close to <NUM> wt% of oxide (Al<NUM>O<NUM>). A nitric acid solution at a concentration of <NUM>% by weight is added to the suspension so as to obtain a pH close to <NUM>. The suspension is passed through an LME20 brand ball mill from the manufacturer Netzsch. The operating conditions of the mill are adjusted so as to obtain a D50 of <NUM> microns.

In step (f), a cerium nitrate solution is prepared at a concentration in the region of <NUM>% by weight of oxide (CeO<NUM>). This solution is added with stirring to the suspension obtained from step (e) so as to obtain a CeO<NUM>/(CeO<NUM>+Al<NUM>O<NUM>) mass ratio of <NUM> wt%.

In step (g), the suspension obtained from step (f) is spray-dried to obtain a dried powder.

In step (h), the spray-dried powder is calcined in air at <NUM> for <NUM> hours (temperature increase rate of <NUM>/min). The loss of mass observed during this calcination is <NUM>% wt%.

<NUM> of deionized water are introduced into a stirred reactor and heated to <NUM>. This temperature is maintained throughout steps (a) to (c). An acid mixture consisting of <NUM> of aluminum sulfate solution with a concentration of <NUM> wt% of alumina (Al<NUM>O<NUM>) and <NUM>,<NUM> of cerium nitrate solution with a concentration of <NUM> wt% of ceria (CeO<NUM>) is made up.

<NUM> of the acid mixture are introduced at a flow rate of <NUM> of solution/min via an introduction cannula close to the stirring rotor. At the end of the introduction, the pH in the reactor is close to <NUM> and the aluminum concentration is <NUM>% by weight of alumina (Al<NUM>O<NUM>). The introduction of the acid mixture is then stopped.

step (a): a sodium aluminate solution with a concentration of <NUM> wt% of alumina (Al<NUM>O<NUM>) and a Na<NUM>O/Al<NUM>O<NUM> molar ratio of <NUM> is introduced at a flow rate of <NUM> of solution/min via a second introduction cannula close to the stirring rotor, until a pH of <NUM> is reached. The introduction is then stopped.

In step (b), the introduction of the acid mixture is again started at a flow rate of <NUM> of solution/min and the sodium aluminate solution is simultaneously introduced into the stirred reactor at a regulated flow rate so as to maintain the pH at a value of <NUM>. This step lasts <NUM> minutes.

In step (c), the introduction of the acid mixture is stopped and the addition of the sodium aluminate solution is continued at a flow rate of <NUM> of solution/min until a pH of <NUM> is reached. The addition of the sodium aluminate solution is stopped.

In step (f), a cerium nitrate solution is prepared at a concentration of <NUM>% by weight of oxide (CeO<NUM>). This solution is added with stirring to the suspension obtained from step (e) so as to obtain a CeO<NUM>/(CeO<NUM>+Al<NUM>O<NUM>) mass ratio of <NUM> wt%.

In step (h), the spray-dried powder is calcined in air at <NUM> for <NUM> hours (temperature increase rate of <NUM>/min).

<NUM> of deionized water are introduced into a stirred reactor and heated to <NUM>. This temperature is maintained throughout steps (a) to (c). An acid mixture consisting of <NUM> of aluminum sulfate solution with a concentration of <NUM> wt% of alumina (Al<NUM>O<NUM>) and <NUM> of cerium nitrate solution with a concentration of <NUM> wt% of ceria (CeO<NUM>) is made up.

In next step, the suspension obtained from step (e) is spray-dried to obtain a dried powder.

In next step, the spray-dried powder is calcined in air at <NUM> for <NUM> hours (temperature increase rate of <NUM>/min). The loss of mass observed during this calcination is <NUM> wt%.

Claim 1:
A composition
- based on Al and Ce in the form of oxides (composition C1); or
- based on Al , Ce and La in the form of oxides (composition C2),
with the following proportions:
- the proportion of CeO<NUM> is between <NUM> wt% and <NUM> wt%;
- the proportion of La<NUM>O<NUM> (for composition C2 only) is between <NUM> wt% and <NUM> wt%;
- the remainder as Al<NUM>O<NUM>;
exhibiting the following porosity profile:
- a pore volume in the range of pores with a size of between <NUM> and <NUM> which is between <NUM> and <NUM>/g; and
- a pore volume in the range of pores with a size of between <NUM> and <NUM> which is less than or equal to <NUM>/g,
these pore volumes being determined by means of the mercury porosimetry technique; and the following properties:
- a mean size of the crystallites after calcination in air at <NUM> for <NUM> hours (denoted D<NUM>-<NUM>) which is lower than <NUM>, preferably lower than <NUM> ;
- a mean size of the crystallites after calcination in air at <NUM> for <NUM> hours (denoted D<NUM>-<NUM>) which is lower than <NUM>, preferably lower than <NUM>, even more preferably lower than <NUM>; and
- an increase ΔD of the mean size of the crystallites lower than <NUM>, preferably lower than <NUM>, ΔD being calculated with the following formula: ΔD = D<NUM>-<NUM> - D<NUM>-<NUM>;
the mean size of the crystallites being being obtained by XRD from the diffraction peak [<NUM>] of the cubic phase corresponding to cerium oxide, generally present at 2θ between <NUM>° and <NUM>°.