WO2019042910A1

WO2019042910A1 - Mixed oxide with enhanced redox properties

Info

Publication number: WO2019042910A1
Application number: PCT/EP2018/072962
Authority: WO
Inventors: Fabien OCAMPO; Naotaka Ohtake; Barry William Luke Southward
Original assignee: Rhodia Operations
Priority date: 2017-08-29
Filing date: 2018-08-27
Publication date: 2019-03-07

Abstract

The present invention relates to a mixed oxide based upon cerium, an element which is Al or Si and a base metal (BM). The invention also relates to the process of preparation of said mixed oxide and to the use of said mixed oxide for the preparation of a catalytic converter. The invention also relates to a process for the treatment of an exhaust gas released by the internal combusion engine of a vehicle, using said mixed oxide.

Description

MIXED OXIDE WITH ENHANCED

REDOX PROPERTIES

The present application claims the priority of European patent application EP 17306103 filed on 29 August 2017, the content of which being entirely incorporated herein by reference for all purposes. In case of any incoherency between the present application and the EP application that would affect the clarity of a term or expression, it should be made reference to the present application only.

Technical context

Catalysts for purifying vehicle exhaust gases are composed of a catalytic metal selected from one or a Platinum Group Metal (or PGM) which is dispersed on a co-catalytic support material to enhance catalytic function. In the art, typical support materials include, but are not limited to, high surface area, thermally stable oxides including (doped) alumina, silica, zeolites and (doped) ceria or (doped) ceria-zirconia solid solutions. This PGM promoted support oxide is in turn mixed with promoters or other functional materials to yield the fully formulated catalyst washcoat which is then deposited upon an appropriate ceramic substrate e.g. cordierite, SiC or metallic form. The ceramic or metallic form is then canned and incorporated in the exhaust train to provide the required catalytic conversion of pollutants.

One of the support oxides of choice in washcoat formulations for emissions control comprises cerium oxide-containing materials. These materials may be present as pure, high surface area cerium oxide e.g. as described in US 7,361 ,322 B2 or as a (doped) ceria-zirconia (CeO₂-ZrO₂) solid solution e.g. as described in US 7,939,040 or US 8,956,994. Cerium oxide is a ubiquitous component of aftertreatment catalysts for gasoline vehicles due to its ability to 'buffer' the active components in the catalyst against local fuel rich (reducing) or fuel lean (oxidising) conditions. Ceria facilitates this buffering or reduction- oxidation (hereafter redox) chemistry by utilisation of the Ce -» Ce redox couple, with the oxidation state of Ce depending upon local O₂ content. Thus Ce⁴⁺ releases active oxygen from its 3-D structure in a rapid and reproducible manner under oxygen-depleted transients, regenerating this 'lost' oxygen by adsorption from the gaseous phase when oxygen rich conditions arise. This high availability of oxygen is critical for the promotion of generic oxidation / reduction chemistries e.g. CO oxidation or NO reduction of the gasoline three-way catalyst.

A key challenge the function of such cerium oxide-containing co-catalyst material to keep the co-catalyst at a sufficient minimum temperature to activate the required redox chemistry. Hence at low temperature of the exhaust gas, as is typical of engine start-up aka the so-called 'cold start' condition, there is low or no activation of the lattice oxygen and hence no buffering of air fuel transients and thus low catalytic performance / conversion efficiency. Vehicle manufacturers are presently trying to solve this problem by placing the catalyst system close to the engine for introducing hot exhaust gas right after its emission from the engine into the catalyst system. However, despite this trend of the so-called 'close- coupled' catalyst there is still a pressing and unmet need for co-catalyst materials that are activated at lower temperatures to fulfil the highly demanding emissions targets required by forthcoming legislation e.g. RDE ("real driving emissions").

The vehicle manufacturers are therefore under pressure to find technical solutions to this problem in view of the stricter legislations to be put in place (e.g. in Europe, the 'real driving emission' or RDE legislation). Moreover, the regulation on CO₂ emissions which is getting more stringent, is also leading to a decrease in the temperature of the exhaust gas. One solution consists in placing the catalyst system close to the engine to benefit from the energy of the hot exhaust gas right after its emission ('close-coupled' catalyst system), but this may not be sufficient in 'cold start' conditions. Another solution consists in increasing the amount of the PGM, however reliable this solution may be, it may prove to be very expensive.

Problem to be solved

Hence there is still a need to provide a mixed oxide having improved low temperature redox function, enhanced catalytic function for the conversion of pollutants such as CO and/or unburnt hydrocarbons (HCs) without any PGM or with a reduced amount of PGM whilst also demonstrating high heat resistance and oxygen absorbing / desorbing capability to render said mixed oxide useful as a catalytic or a co-catalytic material suitable for a catalyst, such as a material for the purification of exhaust gas from an internal combustion engine. This need is solved by the mixed oxide according to the invention.

Technical background

US 2010/0197479 discloses the ion exchange of a base metal on a mixed oxide of formula Ce-Zr-O_x or Ce-Zr-RE-O_x (RE = a rare-earth element). The base metal may be Fe, Cu or Ag. US 2009/0257935 discloses a cerium-zirconium-based mixed oxide modified by a base metal. The compositions disclosed comprise zirconium.

US 8,858,903 discloses a layer (overcoat 106) which includes at least copper oxide, ceria, alumina, and optionally one oxygen storage material. The amount of ceria is at most 50 wt%.

US 7,329,627 discloses a mixed oxide based on cerium, lanthanum, copper and manganese. Cu can be partially substituted with Co, Fe, Ni and/or Zn. The compositions disclosed do not comprise Al or Si.

US 8,479,493 discloses the combination of particles of a mixed oxides of Ce, Zr, and Cu with particles of at least one PGM catalyst dispersed on particles of aluminum oxide (AI2O3). The combination requires the use of zirconium. Journal of Catalysis 2010, 272, 109-120 discloses in general terms a catalyst CeO2-CuO on a γ-ΑΙ₂03 prepared by impregnation with precursors Cu(NOs)2 and Ce(NO3)₄ on AI2O3 as a support. The proportion of aluminium exceeds the proportion of claim 1 . WO 2013/163536 discloses in example 1 of Table 1 an impregnated catalyst containing an equivalent of 5 wt% MnO₂, 10 wt% CuO on ceria and 3 wt% of an alumina binder. There is no disclosure of a specific surface area of at least 35 m²/g after being ageing at 800°C for 16 hours under a atmosphere composed of 10.0% O₂ / 10.0% H₂O / balance of N₂.

Green and Sustainable Chemistry 2015, vol. 5(1 ), 15-24 discloses a catalyst for the conversion of cellulose to sorbitol in the form of a CuO on CeO₂-AI₂O3 or CeO₂-SiO₂. The aluminium or silicon content is higher than 20.0%. Cat. Com. 2008, 9, 2428-2432 discloses a mixed oxide Cu-Ce-AI for the removal of soot and NO with the following molar ratio (Cu+Ce):AI = 2:1 which corresponds to a CuO content of approximately 29%, higher than the claimed content of 15.0%.

Figures

Fig. 1 provides the TPR curves of the mixed oxides of example 6 (5.0% CuO on CeO₂) and of example 2 (5.0% CuO on CeAI).

Fig. 2 provides the TPR curves of the mixed oxides of example 8 (5.0% CuO on CZ-1 ) and of example 10 (5.0% CuO on CZ-2).

Fig. 3 provides the diffractograms of the mixed oxides of example 8 (5.0% CuO on CZ-1 ), of example 6 (5.0% CuO on CeO₂) and of example 2 (5.0% CuO on CeAI) after aging under hydrothermal conditions (800°C / 16 h).

Fig. 4 provides the diffractograms of the mixed oxides of example 8 (5.0% CuO on CZ-1 ), of example 6 (5.0% CuO on CeO₂) and of example 2 (5.0% CuO on CeAI) after aging under lean/rich conditions (720°C / 8 h).

Description of the invention

about the mixed oxide of the invention

The invention relates to a mixed oxide of cerium, of an element which is aluminium or silicon and a base metal (BM) selected from Fe, Co, Ni or an element of groups 5, 6, 7 and 1 1 of the periodic table, with the following proportions:

^■ aluminium or silicon: up to 20.0%;

^■ base metal: up to 15.0%;

■ the remainder as cerium;

these proportions being given by weight of the corresponding oxide (CeO₂, AI₂O3 or SiO₂, oxide of the BM) with respect to the total weight of the mixed oxide.

It is specified, for the continuation of the description, that, unless otherwise indicated, in all ranges of values which are given, the values at the limits are included. The mixed oxide of the invention is based upon the above mentioned elements with the above mentioned proportions. The mixed oxide may thus be described by the generic formula Cei-(_y+Z) (Al or Si)_y (BM)_Z O_a wherein:

^■ y is the proportion of Al or Si;

■ z is the proportion of the base metal;

^■ a is the stoechiometric balance in oxygen in the mixed oxide;

these proportions being given by weight of the corresponding oxide (CeO2, AI2O3 or S1O2, oxide of the BM) with respect to the total weight of the mixed oxide. The base metal may more particularly be Fe, Co, Ni or Cu. A particular preferred base metal is Cu. More particularly, when the base metal is Cu, no other base metal like from Fe, Co, Ni or an element of groups 5, 6, 7 and 1 1 is present in the mixed oxide. In particular, when the base metal is Cu, the mixed oxide does not comprise Mn and/or Fe.

The above mentioned elements Ce, Al, Si, BM that constitute the mixed oxide are present in the mixed oxide as oxides. According to an embodiment, they may nonetheless be also partially present in the form of hydroxides or oxyhydroxides. Their proportions are given by weight of the corresponding oxide (CeO2, AI2O3 or S1O2, oxide of the base metal) with respect to the total weight of the mixed oxide and are expressed as wt%. The proportions of each of these elements are determined by the usual analytical methods like X-ray fluorescence which are known to a person skilled in the art. An XRF spectrometer PANalytical Axios-Max may for instance be used. When the base metal affords different oxides, the oxide to be retained for the calculation of the proportions corresponds to the most common and thermodynamically stable oxide at ambiant temperature and ambiant pressure of the highest oxidation number. Examples of such oxides are given: Fe₂O₃, Co₃O₄, MnO₂, CuO, NiO.

The mixed oxide contains aluminium or silicon which are present respectively in the form of aluminium oxide or of silicon oxide, y is up to 20.0%. y may more particularly be between 0.1 % and 19.8%, more particularly between 1 .0% and 10.0%, even more particularly between 2.0% and 6.0% or between 4.0% and The mixed oxide contains also a base metal which is selected from Fe, Co, Ni or an element of group 5, 6, 7 and 1 1 of the periodic table, more particularly Cu. z is up to 15.0%. z may be between 0.5% and 10.0%, more particularly between 1 .0% and 8.0%, even more particularly between 4.0% and 8.0% or between 4.0% and 6.0%.

The mixed oxide contains also cerium which is present in the form of cerium oxide. Cerium oxide is the predominant oxide of the mixed oxide. This is to say that the proportion by weight of cerium oxide is superior to each proportion by weight of each other oxide. The proportion of cerium (or 1 -(y+z) in the above formula) in the mixed oxide may be at least 68.0%, more particularly of at least 80.0%, even more particularly of at least 85.0% or of at least 90.0%. The proportion of cerium in the mixed oxide may be between 68.0% and 98.9%, more particularly between 86.0% and 94.0% or between 88.0% and 94.0%.

A mixed oxide according to the invention may thus comprise:

^■ aluminium or silicon: between 2.0% and 6.0%;

^■ the base metal: between 4.0% and 8.0%, more particularly between 4.0% and 6.0%;

■ cerium: between 86.0% and 94.0%, more particularly between 88.0% and 94.0%.

The mixed oxide may also comprise:

^■ aluminium or silicon: between 2.0% and 6.0%;

■ the base metal: between 4.0% and 6.0%;

^■ cerium: between 88.0% and 94.0%.

The mixed oxide of the invention may also additionally comprise impurities. The impurities may stem from the raw materials or starting materials used in the process of preparation of the mixed oxide. The total proportion of the impurities is generally lower than 0.3% by weight, more particularly lower than 0.1 %, with respect to the mixed oxide as a whole. The proportions of the impurities may be determined with an Inductively Coupled Plasma Mass Spectrometry. The mixed oxide of the invention may also consist of cerium oxide, of an oxide of an element which is aluminium or silicon and an oxide of a base metal (BM) selected from Fe, Co, Ni or an element of groups 5, 6, 7 and 1 1 of the periodic table, and optionally of impurities, with the following proportions:

^■ aluminium or silicon: up to 20.0%;

^■ base metal: up to 15.0%;

■ the remainder as cerium;

these proportions being given by weight of the corresponding oxide (CeO2, AI2O3 or S1O2, oxide of the BM) with respect to the total weight of the mixed oxide.

The mixed oxide of the invention may also consist essentially of cerium oxide, of an oxide of an element which is aluminium or silicon and an oxide of a base metal (BM) selected from Fe, Co, Ni or an element of groups 5, 6, 7 and 1 1 of the periodic table, with the following proportions:

^■ aluminium or silicon: up to 20.0%;

^■ base metal: up to 15.0%;

- the remainder as cerium;

The mixed oxide of the invention may be prepared by impregnation of a mixed oxide of cerium and aluminium (CeAI) or of cerium and silicon (CeSi) with an aqueous solution of a precursor of the oxide of the base metal. This technique consists in filling the porous volume of the mixed oxide CeAI or CeSi with an aqueous solution of a precursor of the oxide of the base metal, drying the resulting mixture and calcining in air the resulting mixture at a temperature of at least 400°C, more particularly between 400°C and 800°C. The temperature calcination temperature may be comprised between 400°C and 800°C. The temperature of calcination should be high enough to convert the elements Ce, Al, Si and BM into oxides. The duration of the calcination may be comprised between 0.5 h and 15 h. The precursor of the oxide of the base metal may be a salt or a coordination compound of the base metal which is soluble in water. The salt of the base metal may for instance be a chloride, a nitrate or an acetate. The coordination compound of the base metal may be an acetonate. An example of salt is copper (II) nitrate trihydrate. The impregnation technique may be more particularly an incipient wetness impregnation technique (also known as 'dry impregnation'). This technique is well-known to a person skilled in the art. It consists in (i) filling the pores of the mixed oxide CeAI or CeSi in the form of a powder with an aqueous solution of the precursor of the oxide of the base metal; (ii) in drying the solid obtained at the end of step (i) and (iii) in calcining in air the solid obtained at the end of step (ii) at a temperature of at least 400°C. The temperature of calcination of step (iii) should be high enough to convert the elements Ce, Al, Si and BM into oxides. The volume of the aqueous solution of the precursor having the appropriate concentration is equal or slightly lower than the pore volume of the mixed oxide CeAI or CeSi. According to an embodiment, the volume of the aqueous solution is substantially the same as the volume of water added to the mixed oxide CeAI or CeSi to be impregnated before it visually appears wet. The impregnation of the solid and the determination of the water added are performed with a continuous addition of respectively the aqueous solution of the base metal or of water.

The incipient wetness impregnation technique may be performed according to the method which involves the following steps and which is used for the preparation of the mixed oxide of examples 1 -5:

1 ) determination of the fire loss of the mixed oxide CeAI or CeSi: the fire loss which allows to determine the weight of the adsorbed species (mainly water) at the surface of the mixed oxide is calculated by the formula: fire loss = 1 - (weight of mixed oxide after calcination / weight of mixed oxide prior to calcination). The calcination used for the determination of the fire loss is performed at 160°C.

2) determination of the retention volume of water Vr of the mixed oxide CeAI or CeSi: this step and the impregnation step detailed below need to be done strictly under identical conditions (same temperature at 20-25°C, same product lot number, same conditioning). The retention volume of water of the mixed oxide CeAI or CeSi is the volume of water that the mixed oxide can adsorb inside its porosity before liquid water can be observed between the particles of the powder. The retention volume is usually close to the porous volume. The retention volume is given in ml_ of water per g of mixed oxide CeAI or CeSi and is determined at 20°C / 1 atm.

- the mixed oxide CeAI or CeSi to be impregnated is weighed. The weight m may be from 10.0 g to 30.0 g;

- water is poured drop by drop onto the solid while ensuring the good homogeneity of the deposit by mixing the mixture with a spatula; - the aspect of the solid changes when in contact with water. The addition of water is stopped when the change in aspect is even over the entire sample;

- the mixture is kept being homogeneized during 30 min at room temperature;

- the volume v (in ml_) of water that has been added is recorded;

- one or two extra drops of water are added to confirm water is not adsorbed any more. The retention volume of water V_r (in ml_ / g) is given by the formula below:

V_r = v / m

3) preparation of an aqueous solution of the precursor of the oxide of the base metal: the aqueous solution is characterized by a concentration C of the base metal so that after last step 4), the proportion of the base metal in the mixed oxide genuinely corresponds to the desired proportion (P%) of the base metal in the mixed oxide:

C = cP% / V_r

cP% is the corrected proportion which is given by the formula below:

cP% = P% x (1 - fire loss)

The volume of the aqueous solution of the precursor to be used for the impregnation is V, (ml_) given by the formula below:

Vi = M x V_r

M is the weight of the mixed oxide CeAI or CeSi to be impregnated.

The precursor is weighted and dissolved in water to obtain the aqueous solution of the precursor. When the precursor is commercialized as a solution, the volume of the solution is determined and the solution is mixed with water to obtain the aqueous solution of the precursor. In that case, it is preferable to use a solution, the concentration of which is certified.

4) impregnation of the mixed oxide CeAI or CeSi with the aqueous solution of the base metal and drying: the impregnation is performed in conditions that are identical to the conditions of step 2):

- the aqueous solution of the precursor (volume Vi) is added drop by drop onto the mixed oxide (weight M) CeAI or CeSi to be impregnated; - the mixture is kept being homogeneized during 30 min at room temperature;

- the solid is then dried in an oven at 120°C overnight;

- the dried solid is then calcined in air in a furnace at 500°C for 2 hours.

The incipient wetness impregnation may also be performed automatically using an automatic appliance developed by Chemspeed Technologies AG (see https://w ww.chemspeed.com/videopaae/incipient-wetness-impreanation/). In that case, steps 1 ) to 3) disclosed above may be used similarly with the automatic appliance.

Preferably, the mixed oxide CeAI or CeSi used exhibits a total pore volume of at least 0.30 mL/g. The total pore volume may be comprised between 0.30 and 2.0 mL/g, more particularly between 0.50 and 1 .5 mL/g. The porosity is measured by mercury intrusion according to the well-known techniques in the field. The total pore volume may be determined using a Micromeritics Autopore IV 9500 comprising a powder penetrometer according to the guidelines of the constructor. The method is based on the determination of the pore volume as a function of the pore size (V=f(d), V denoting the pore volume and d denoting the pore diameter). From the data, it is possible to obtain a curve (C) giving the derivative dV/dlogD. From the curve (C) , the total pore volume is determined.

Preferably also, the mixed oxide CeAI or CeSi is an intimate mixture of the oxides of Ce and Al or of Ce and Si. The process of preparation disclosed below makes it possible to obtain such an intimate mixture on an atomic level.

With the impregnation method, the oxide of the base metal is dispersed on the surface of the mixed oxide CeAI or CeSi. The invention thus also more particularly relates to a mixed oxide of cerium and aluminium or of cerium and silicon on which the oxide of the base metal is dispersed. The proportions of cerium and aluminium or silicon in the mixed oxide CeAI or CeSi may be the following :

^■ aluminium or silicon: up to 20.0%;

^■ the remainder as cerium;

these proportions being given by weight of the corresponding oxide (CeO2, AI2O3 or S1O2) with respect to the total weight of the mixed oxide CeAI or CeSi. The above proportion of aluminium or of silicon may range from 0.1 % to 20.0% and the proportion of cerium may range from 80.0% to 99.9%. The mixed oxide CeAI or CeSi may also additionaly comprise impurities. These impurities may stem from the raw materials or starting materials used in the process of preparation of the mixed oxide CeAI or CeSi. The total proportion of the impurities is generally lower than 0.3 wt%, more particularly lower than 0.1 wt%, with respect to the mixed oxide CeAI or CeSi as a whole. The proportions of the impurities may be determined with an Inductively Coupled Plasma Mass Spectrometry.

The mixed oxide of the invention is characterized by its thermal resistance in classical conditions. It may thus exhibit a specific surface area of at least 70 m²/g, more particularly of at least 80 m²/g after calcination in air at 800°C for 2 hours. The specific surface area after calcination in air at 800°C for 2 hours may range from 70 to 130 m²/g, more particularly from 80 to 130 m²/g. It may also exhibit a specific surface area of at least 40 m²/g, more particularly of at least 60 m²/g after calcination in air at 900°C for 5 hours. The specific surface area after calcination in air at 900°C for 5 hours may range from 40 to 85 m²/g, more particularly from 60 to 85 m²/g. It may also exhibit a specific surface area of at least 20 m²/g, more particularly of at least 40 m²/g after calcination in air at 1000°C for 5 hours. The specific surface area after calcination in air at 1000°C for 5 hours may range from 20 to 55 m²/g, more particularly from 40 to 55 m²/g.

The mixed oxide may be also characterized by its resistance in more severe conditions ('hydrothermal' conditions and/or 'lean/rich' conditions) which are now detailed below.

'hydrothermal' conditions

To evaluate the resistance in 'hydrothermal' conditions, the mixed oxide is aged at 800°C for 16 hours in an atmosphere composed of 10.0 vol% O2 / 10.0 vol% H₂O / the balance being N₂. The expression 'balance being N₂' means that the balance to 100.0 vol% of the atmosphere is made of N₂. This means that this atmosphere is thus of the following composition: 10.0 vol% O₂ / 10.0 vol% H₂O and 80.0 vol% N₂. These conditions or similar conditions are known in the field: see "Multi-component zirconia-titania mixed oxides: catalytic materials with unprecedented performance in the selective catalytic reduction of NO_x with NH₃ after harsh hydrothermal ageing", Appl. Catal. B 201 1 , 105, 373-376. After such treatment, the mixed oxide exhibits a specific surface area of at least 35 m²/g, more particularly of at least 40 m²/g. The hydrothermal conditions may be as detailed in the examples. The specific surface under these conditions may range from 35 to 55 m²/g, more particularly from 48 to 55 m²/g.

The specific surface areas are determined by adsorption of nitrogen by the Brunauer-Emmett-Teller method (BET method). The method is disclosed in standard ASTM D 3663-03 (reapproved 2015). The method is also described in the periodical "The Journal of the American Chemical Society, 60, 309 (1938)". The specific surface areas are expressed for specific conditions of temperature, time and atmosphere. The specific surface areas are determined with an appliance Flowsorb II 2300 of Micromeritics according to the guidelines of the constructor. Prior to the measurement, the samples are degassed under static air by heating at a temperature of at most 200°C to remove the adsorbed species. Conditions may be found in the examples. 'lean-rich' conditions

The mixed oxide is also resistant under 'lean-rich' conditions. Thus, to evaluate the resistance in 'lean/rich' conditions, the mixed oxide is aged at 720°C for 8 hours in an atmosphere which is alternatively:

^■ an atmosphere A1 composed of 2.7 vol% O2 10.0 vol% H₂O / the balance being N₂ and applied for 90 seconds; then

^■ an atmosphere A2 composed of 2.7 vol% CO / 10.0 vol% H₂O / the balance being N₂ and applied for another 90 seconds;

^■ the cycle of the alternating atmosphere A1 -A2 being repeated for the whole 8 hours of the aging.

In other words, the mixed oxide is placed at 720°C for 8 hours in an atmosphere which alternates between A1 for 90 s and A2 for 90 s. The cycle is applied during 8 hours and is thus the following : A1 (90 s), A2 (90 s), A1 (90 s), A2 (90 s),... (note that the cycle may start either with A1 or with A2 without any impact). By doing so, the atmosphere switches alternatively from A1 to A2 and vice versa. The cycle is operated for 8 hours. The 'lean/rich' conditions which are applied may be as detailed in the examples. After such treatment under the cycles A1 - A2, the mixed oxide exhibits a specific surface area of at least 35 m²/g, more particularly of at least 40 m²/g. The specific surface under these conditions may range from 35 to 55 m²/g, more particularly from 40 to 55 m²/g. more severe 'lean-rich' conditions The mixed oxide is also resistant under an even more severe 'lean-rich' aging. Indeed, when the mixed oxide is aged at 800°C for 16 hours in an atmosphere which is alternatively:

^■ the cycle of the alternating atmosphere A1 -A2 being repeated for the whole 16 hours of the aging,

it may exhibit a specific surface area of at least 8 m²/g.

The conditions of the method of preparation of the mixed oxide and used in the examples have made it possible to obtain a good dispersion of the oxide of the base metal on the CeAI or CeSi mixed oxide. The oxide of the base metal is such that is not detectable by X-ray powder diffraction (XRD).

More particularly, when the base metal is copper, the diffractogram of the mixed oxide obtained by XRD does not exhibit any reflexion in the range 2Θ between 38.0° and 40.0°. This means that the intensity of the signal of the detector is of the same order as the intensity of signal of the baseline.

Without being bound by any theory, these data suggest that the oxide of the base metal is dispersed on the surface of the mixed oxide of cerium and aluminium or of cerium and silicon in the form of a substoichiometric oxide of the base metal. For instance, when the base metal is copper, the oxide of copper may be described by formula CuO_p with β<1 .

The dispersion remains stable even after the 'hydrothermal' conditions (800°C / 16 hours / atmosphere composed of 10.0 vol% O₂ / 10.0 vol% H₂O / the balance being N₂) or the 'lean/rich' conditions (720°C / 8 hours / atmosphere alternating between A1 and A2) that are detailed above. This means that the diffractogram of the mixed oxide does not exhibit any reflexion (or peak) in the range 2Θ between 38.0° and 40.0° after these agings under either the 'hydrothermal' or 'lean/rich' conditions.

The mixed oxide of the invention may also exhibit good reducibility properties. These properties such as T_max and T_end are determined by the temperature- programmed reduction (TPR) measurement method. This method consists in measuring the consumption of hydrogen of the mixed oxide of the invention while being heated, as a function of temperature. The hydrogen consumption is measured with a conductivity thermal detector (TCD) while the mixed oxide is heated from 20°C to 900°C with an increase ramp of 10°C/min under a reducing atmosphere composed of Ar (90.0 vol%) and H₂ (10.0 vol%). The measurement can be performed with a Micromeritics Autochem 2920 machine. The TPR curve gives the intensity of the signal (y axis) of the TCD as a function of the temperature of the sample (x axis). The TPR curve is the curve from 20°C to 900°C. The baseline used for the TPR corresponds to the line of equation y=y₀ for which y₀ is the intensity of the signal at 50°C. Examples of TPR curves are given on Fig. 1 and 2.

Tmax is defined as the temperature for which the intensity of the signal of the TCD is maximum on the TPR curve. On this regard, the raw signal of the TCD is negative, it is standard to plot the opposite of the raw signal so that the TPR curve contains peaks with maximum values. The mixed oxide of the invention may exhibit a T_max ranging from 1 15°C to 140°C. Without being bound by any theory, the T_max observed may be assigned to a species based on the BM.

Tend is defined as the temperature defined by the following characteristics :

^■ Tend is superior to T_max; and

^■ Tend corresponds to the first temperature at which the signal of the TCD returns to the baseline.

The mixed oxide of the invention may exhibit a T_end lower than 215°C, more particularly lower than 200°C. In some cases, the intensity of the signal of the TCD may not return to the value yo. In this case, the signal is nonetheless considered to return to the baseline when the intensity of the signal of the TCD returns to a value equal to yo+5%.

It is also observed that for the mixed oxide of the invention, the width at half height of the peak of the signal at T_max is generally lower than 30°C, more particularly lower than 25°C, even more particularly lower than 15°C. It is generally considered that the more narrow the peak at T_max, the better the dispersion of the reducible species. It is also observed that the average size t of the crystallites of cerium oxide may be lower than 20 nm, more particularly lower than 19 nm after the mixed oxide is aged at 800°C for 16 hours in a atmosphere composed of 10.0 % O₂ / 10.0 % H₂O / balance of N₂. The average size t of the crystallites of cerium may also be lower than 20 nm, more particularly lower than 19 nm after the mixed oxide is aged at 720°C for 8 hours in an atmosphere which is alternatively:

^■ an atmosphere A1 composed of 2.7 vol% O₂ / 10.0 vol% H₂O / the balance being N₂ and applied for 90 seconds; then

This average size t is determined with the use of the Scherer formula from an X- ray powder diffraction pattern obtained by standard techniques. The Scherer formula below gives the aver

k: form factor taken as 0.9;

λ (lambda) : wavelength of the radiation used; it is usually and more particularly a copper source (λ=1 .5406 Angstrom);

H: width of the peak at half height of the considered reflexion;

s: instrumental width which depends on the appliance used and of the angle Θ;

Θ: Bragg angle. The average size t is based on the most intense reflexion. A reflexion located at 2 Θ = 28.8° ±1 ° is usually selected. It corresponds to the (1 1 1 ) phase of the cerium oxide.

The mixed oxide CeAI may be prepared by the method disclosed in example 6 of EP 444470. The mixed oxide CeSi may be prepared by the method disclosed in US 5,529,969. The mixed oxide CeAI or CeSi may also be prepared by the preferred method disclosed below:

- step (a): an aqueous solution comprising Ce^lv, optionally Ce'", H⁺ and NO₃ ^" with a molar ratio Ce^lv/total cerium of at least 0.9 is heated at a temperature between 60°C and 170°C, more particularly between 90°C and 160°C, to obtain a suspension comprising a liquid medium and a precipitate; - step (b): the solid of the suspension obtained at the end of step (a) is allowed to settle and the liquid on the top is partly removed, a precursor of aluminium oxide or of silicon oxide is added to the suspension and water is optionally added to adjust the total volume;

- step (c): the mixture obtained at the end of step (b) is heated ;

- step (c'): an aqueous solution of a basic compound may optionally be added to adjust the pH of the mixture to at least 7.0, more particularly between 8.0 and 9.0;

- step (d): a precipitate is recovered from the mixture obtained at the end of step (c) or at the end of step (c') and it is optionally dried to remove partly or completely the water which is present;

- step (e): the solid obtained at the end of the step (d) is calcined in air at a temperature between 300°C and 800°C;

- step (g): the solid obtained from step (f) is optionally ground to reduce the size of the particles;

wherein the process is characterized by a decrease ratio (DR) between 10% and 90%, more particularly between 30% and 50%, even more particularly between 35% and 45%, DR being given by formula (I) :

DR = [NO₃ ^"]step b / [NO₃ ^"]step a X 100 (I)

where [NO3^"]_ste_P a is the concentration in mol/L of the nitrate anions in the aqueous solution of cerium used in step (a) ;

and [NO3^"]ste_P b is concentration in mol/L of the nitrate anions in the liquid medium of the suspension obtained at the end of step (b). The preferred method involves the use of an aqueous solution of Ce^lv and optionally Ce'" cations characterized by a molar ratio Ce^lv/total cerium of at least 0.9. This ratio may be 1 .0. It is advantageous to use a salt of cerium with a purity of at least 99.5%, more particularly of at least 99.9%. An aqueous eerie nitrate solution can for instance be obtained by reaction of nitric acid with an hydrated eerie oxide prepared conventionally by reaction of a solution of a cerous salt and of an aqueous ammonia solution in the presence of aqueous hydrogen peroxide to convert Ce'" cations into Ce^lv cations. It is also particularly advantageous to use a eerie nitrate solution obtained according to the method of electrolytic oxidation of a cerous nitrate solution as disclosed in FR 2570087.

The water used to prepare the aqueous solution is preferably deionized water. The concentration of cerium expressed in terms of cerium oxide in the aqueous solution of cerium may be comprised between 5 and 150 g/L, more particularly between 30 and 60 g/L. As an example of concentration in terms of oxide, a concentration of 225 g/L of nitrate of cerium corresponds to 100 g/L of CeO2. The amount of H⁺ in the aqueous solution of cerium of Ce^lv and optionally Ce'" cations used in step (a) may be between 0.01 and 1 .0 N, more particularly between 0.05 and 0.20 N. The acidity of the aqueous solution used in step (a) may be adjusted by the addition of HNO3 and/or NH₃ (in the case of the addition of NH3, the quantity of base added is just necessary to adjust the acidity without there being any precipitation). The aqueous solution used in step (a) may exhibit an acidity around 0.12 N, as in example A.

Thus a typical aqueous solution of cerium contains Ce^lv, optionally Ce'", H⁺ and NO3^". The aqueous solution may be obtained by mixing the appropriate quantities of nitrate solutions of Ce^lv and Ce'" and by optionally adjusting the acidity.

In the first step (a), the aqueous solution of cerium is heated and held at a temperature between 60°C and 170°C, more particularly between 90°C and 160°C, so as to obtain a suspension comprising a liquid medium and a precipitate. The precipitate may be in the form of cerium hydroxide. Any reaction vessel may be used in step (a) without critical limitation, and either a sealed vessel or an open vessel may be used. Specifically, an autoclave reactor may preferably be used. The duration of the heat treatment is usually between 10 min and 48 hours, preferably between 30 min and 36 hours, more preferably between 1 hour and 24 hours. Without wishing to be bound by any particular theory, the function of this heating step is to improve the crystallinity of the precipitate which results in a better heat resistance of the mixed oxide. The conditions used in the examples may be applied. In step (b), the concentration of the nitrate anions NO3^" present in the liquid medium is decreased by removing a part of the liquid medium from the suspension obtained at the end of step (a). This is performed by leaving the solid of the suspension settle and by removing partly the liquid on the top. After that, the precursor of aluminium oxide or of silicon oxide is added to the suspension. The precursor for CeAI may be aluminium nitrate such as e.g. aluminum nitrate nonahydrate. The precursor for CeSi may be silicon dioxide. The silicon dioxide may in the form of a colloidal dispersion of silicon dioxide in water. The primary particles in the colloidal dispersion may exhibit a size lower than 15 nm. The secondary particles in the colloidal dispersion may exhibit a size lower than 30 μιτι. An example of suitable colloidal dispersion is Aedlite AT-20Q commercialized by Adeka Chemical.

Water may also be added to adjust the total volume. The amount of the precursor added depends on the targeted composition of the mixed oxide. It must be noted that, when removing some of the liquid from the suspension, some of the solid may be removed as well. In that case, the amount of the precursor to be added is calculated by mass balance to take into account the amount of solid (and hence of cerium) having been so removed.

The method is characterized by the decrease of the concentration of nitrate anions NO3^" in the liquid medium of the suspension obtained at the end of step (b) (that is after the removal of part of the liquid medium and the addition of the precursor and optionally water) in comparison to the concentration of NO3^" in the aqueous solution of cerium used in step (a). The decrease may be characterized by a decrease ratio (DR) between 10% and 90%, more particularly between 30% and 50%, even more particularly between 35% and 45%, wherein DR is given by formula (I) :

DR = [NO₃ ^"]step b / [NO₃ ^"]step a X 100 (I)

[NO3^"]ste_P a : concentration in mol/L of the nitrate anions in the aqueous solution of cerium used in step (a) ;

[NO3^"]ste_P b : concentration in mol/L of the nitrate anions in the liquid medium of the suspension obtained at the end of step (b) (that is after the removal of part of the liquid medium and the addition of the precursor and optionally of water).

To be more specific, DR is given by (F/G)/(D/E) x 100.

1 . Calculation of D which is the total number of nitrate anions NO3^" (mol) in the aqueous solution of cerium used in step (a).

2. Calculation of the concentration of the nitrate anions NO3^" (mol/L) in the aqueous solution of cerium used in step (a) : [NO₃ ^"]_step a = D/E wherein E is the total volume (L) of the aqueous solution of cerium used in step (a).

3. Calculation of F which is the total number of nitrate anions NO3^" (mol) in the liquid medium of the suspension obtained at the end of step (b) that is after the removal of part of the liquid medium and the addition of the precursor and optionally water. To calculate F, the amount of liquid removed in step (b) is taken into account.

4. Calculation of the concentration of nitrate anions NO3^" (mol/L) in the suspension obtained at the end of step (b) : [NO3^~]_ste_P b = F/G wherein G is the total volume (L) of the solution at the end of step (b) after the addition of the precursor and optionally of water. DR is usually calculated by mass balance taking into account the exact quantities of products added or removed from the tank.

More precisely,

D (mol) = A/172.12 x [B/100 x 4+(100-B)/100 x 3] + C

wherein:

- A is the quantity of cerium cations in terms of CeO₂ (gram);

- B is the percentage of tetravalent cerium cations per total cerium cations;

- C is the quantity of nitrate anions NO₃ ^" (mol) from other sources than cerium nitrate(s) e.g. the nitrate anions associated with the H⁺ in the solution.

F (mol) = (D x removal ratio of the liquid medium) + anions NO₃ ^" (mol) from the nitrate of aluminium.

The volumes E and G may be measured by a gauge.

In step (c), the mixture obtained at the end of step (b) is heated. In this step, the temperature of the mixture may be from 100°C to 300°C, more particularly from 1 10°C to 150°C. The duration of the heating may be from 10 min to 40 h, more particularly from 30 min to 36 h. The conditions of example A can be applied.

Then, in step (c'), an aqueous solution of a basic compound may be added to adjust the pH of the mixture to at least 7.0, more particularly between 8.0 and 9.0. An aqueous ammonia solution may be added.

In step (e), the precipitate which is recovered from the mixture obtained at the end of step (c) or at the end of step (c') is optionally dried to remove partly or completely the water which is present. The recovery may be performed by separation of the liquid medium from the precipitate. Filter pressing, decantation or Nutsche filter may be used. The precipitate may optionally be washed with water, preferably water at a pH > 7. An aqueous ammonia solution may be used.

In step (f), the solid obtained at the end of the step (e) is calcined in air at a temperature between 300°C and 800°C. The duration of the calcination may vary from 1 to 20 hours. For instance, the solid may be calcined at a temperature of 500°C for 10 hours.

In step (g), the solid (mixed oxide) from step (f) may be ground to reduce the size of the solid particles. Use can be made of a hammer mill. The powder may exhibit an average particle size d₅o between 0.05 and 50.0 μιτι. d₅o is obtained from a distribution in volume which is measured by laser diffraction. The distribution may be obtained for instance with a LA-920 particle size analyzed commercialized by Horiba, Ltd.

The preparation of CeAI or CeSi may be as disclosed in Examples A or B. A skilled person may adapt the recipes disclosed in these two examples to prepare mixed oxides CeAI or CeSi that exhibit other contents of Ce and Al or Ce and Si. about the use of the mixed oxide of the invention

The mixed oxide of the invention may be used in the field of exhaust gas treatment. The mixed oxide of the invention may be used to reduce the amounts of CO and/or unburnt hydrocarbons (HCs) in an exhaust gas released by the internal combusion engine of a vehicle. It may be more particularly used in the preparation of a catalytic converter which is used to treat exhaust gases released by the internal combusion engine of a vehicle.

The catalytic converter comprises at least one catalytically active layer prepared with the mixed oxide and deposited on a solid support. The function of the layer is to chemically convert some pollutants of the exhaust gas, more particularly CO or unburnt hydrocarbons, into products that are less harmful to the environment. The chemical reactions involved with CO and HCs may be the following :

2 CO + O₂ 2 CO₂

4 C_xHy + (4x+y) O₂ 4x CO₂ + 2y H₂O

The solid support may be a monolith made of ceramic, for example of cordierite, of silicon carbide, of alumina titanate or of mullite, or of metal, for example Fecralloy. The support is usually made of cordierite exhibiting a large specific surface area and a low pressure drop.

On the surface of the solid support, the catalytically active layer (also called 'washcoat') is deposited. This layer is made from a composition comprising the mixed oxide of the invention. The composition usually also comprises at least one inorganic material other than the mixed oxide. The inorganic material other than the mixed oxide may be for instance selected in the group of alumina, titanium oxide, zirconium oxide, silicas, spinels, zeolithes, silicoaluminium phosphates, crystalline aluminium phosphates. The composition may also comprise other ingredients like a H₂S trap; any organic or inorganic modifier to help deposit the composition on the support; colloidal alumina;... Alumina is very often used as one inorganic material. Alumina may optionally comprise a doping element like e.g. lanthanum.

Due to the presence of the base metal, the mixed oxide of the invention is already catalytically active without any PGM (see examples 12 and 13). It may be used to chemically convert some pollutants of the exhaust gas, such as CO or HC, into products that are lass harmful to the environment. This means that according to an embodiment, the composition comprising the mixed oxide of the invention may be used to prepare the catalytically active layer without the need to disperse any PGM on said layer.

Yet, according to another embodiment, to optimize the reduction of the amounts of both CO and HCs present in the exhaust gas, the catalytically active layer may also comprise a DOC in addition to the mixed oxide of the invention. The function of the DOC is to convert CO and HCs into CO₂ and to convert NO into NO₂. A DOC usually consists of at least one PGM, such as e.g. Pt, dispersed on an refractory oxide selected from the group consisting of alumina, silica, titania, zirconia, ceria, silica-alumina, titania-alumina, zirconia-alumina, ceria-alumina, titania-silica, zirconia-silica, zirconia-titania, ceria-zirconia and alumina- magnesium oxide. PGM designates a platinum group metal which is a chemical element selected in the following list: Ru, Rh, Pd, Os, Ir, Pt. The refractory oxide is usually an optionally doped alumina such as La-doped alumina. A combination of two PGMs, such as a combination of Pt and Pd, may also be dispersed on the alumina. Thus, the invention also relates to a composition comprising the mixed oxide of the invention and a DOC. The dispersion of the PGM on the refractory oxide is well-known to a person skilled in the art. An aqueous solution of a salt of the PGM, such as a chloride or a nitrate of the PGM, e.g. Pd(ll) tetraamine nitrate or Pt(ll) tetraamine hydroxide may be added to an aqueous dispersion of the refractory oxide and water is removed to fix the salt on the surface of the refractory oxide. The resulting mixture is then dried and calcined in air at a temperature between 300°C and 800°C. An example of such a method of dispersion of the PGM is disclosed in example 1 of US 7,374,729.

According to another embodiment, to optimize the reduction of the amounts of both CO and HCs, the catalytic converter comprises the catalytically active layer without any PGM and an additional catalytically active layer comprising the DOC.

As is disclosed in example 12, the mixed oxide is already catalytically active at low temperatures which is of interest for the chemical conversion of the pollutants at 'low' temperatures. Indeed, in the 'cold start' conditions when the engine has just started running or in city-driving conditions when the engine runs at low speed and is frequently stopped (jams, traffic lights etc), the exhaust gas which enters the catalytic converter is at low temperature and the temperature may not be high enough to activate the conversion of the pollutants. The cold start challenge is especially important in the field of diesel emissions control as the lean compression-ignition cycle of the diesel engine results in significantly lower temperatures of the exhaust gas (e.g. 200°C or more) compared to a conventional 3-way / stoichiometric gasoline exhaust. The invention also relates to a process of treatment of an exhaust gas released by the internal combusion engine of a vehicle, comprising contacting the exhaust gas with the mixed oxide or with a composition comprising the mixed oxide of the invention and optionally at least one inorganic material other than the mixed oxide. The process makes it possible to convert the CO and/or the unburnt hydrocarbons into CO₂. The exhaust gas may more particularly be produced by a diesel engine. The temperature at which the process operates should preferably be higher than 130°C. Good conversions into CO₂ may be observed in the range 130-200°C. Examples

BET The specific surface areas were determined automatically on a Flowsorb II 2300. Prior to any measurement, the samples are carefully degassed to desorb the adsorbed species. To do so, the samples may be heated at 200°C for 2 hours in a stove, then at 300°C for 15 min in the cell of the appliance.

Diffractoqrams obtained by XRD

Diffractograms were obtained on powders with a copper source (CuKal, λ=1 .5406 Angstrom) using an X-ray diffraction apparatus X'Pert Pro MPD system commercialized by PANanalytical. Data were collected with a Bragg Brentano geometry and 0.04 rad soller slit, a X celerator 1 D detector with a 2.122 length. The system is equipped with a Ni° filter and programmable slits to provide a beam on a square surface with 10 mm sides. A step size of 0.017° and a count time of 40 s per step were used.

TPR

The experimental protocol used for the TPR consists in weighing out around 200.0 mg. The sample is then introduced into a quartz cell containing quartz wool at the bottom. The sample is finally covered with quartz wool and placed in the oven of the appliance (Micromeritis Autochem 2920 machine). The temperature programme used is as follows: temperature rise from 20°C to 900°C with an increase ramp of 10°C/min. The reducing atmosphere is composed of Ar (90.0 vol%) and H₂ (10.0 vol%). During this programme, the temperature of the sample is measured using a thermocouple placed in the quartz cell, just above the sample. The hydrogen consumption during the reduction phase is deduced by means of the calibration of the variation in thermal conductivity of the gas stream measured at the cell outlet using a TCD.

The hydrogen consumption is measured between 20°C and 900°C. The higher this hydrogen consumption, the better the reducibility properties of the sample (redox properties). The TPR curve makes it possible to detect the reductible species that may be present in the mixed oxide.

Conditions used for the aging The agings are aimed to reproduce the severe conditions that a catalyst or a mixed oxide faces when in contact with the hot gases in the exhaust line. The aging is operated according to three different protocols that are detailed below. The synthetic gas mixtures are obtained by mixing different gases, and the content of each gas in the total mixture is controlled by mass flow meters.

'Hvdrothermal' conditions (800°C / 16 hours / atmosphere composed of 10.0 vol% O? 1 10.0 vol% H?O / the balance being N?) or HT 800°C/16 h

conditions of the aging: 2.2 g of solid ; atmosphere: 10 vol.% O2, 10 vol.% H₂O and balance N₂ (total flow rate = 24 L/h) ; temperature: 800°C; duration: 16h

10.0 g of the solid to be aged (in the powder form) are compacted in the form of a pellet with a diameter of 32 mm by applying a pressure of 8 tons for 2 minutes. The pellet so obtained is deagglomerated in a mortar to provide a fine powder which is sieved so as to retain only the fraction of the powder which passes through a sieve of 250 μιτι and retained with a sieve of 125 μιτι.

The fraction of the powder (2.2 g) is then placed in a fixed bed reactor and aged at a temperature of 800°C during 16 hours in an atmosphere composed of 10.0 vol% O2 / 10.0 vol% H₂O / the balance being N₂ flowing through the sample with a volumic flow-rate of 24 L/h (measured at 20°C and 1 atm).

'lean/rich' conditions (720°C / 8 hours / atmosphere alternating between A1 and A2) or LR 720°C/8 h

conditions of the aging: 0.8 g of solid ; atmosphere: alternating between A1 and A2 (total flow rate = 24 L/h) ; temperature: 720°C; duration : 8h

The fraction of powder (0.8 g) is placed in a fixed bed reactor and then aged for 8 hours at a temperature of 720°C in an atmosphere which alternates between A1 (90 s) and A2 (90 s). The atmospheres A1 and A2 are obtained by mixing alternatively measured quantities of O₂ or of CO in a gas composed of H₂O and N₂. The total volumic flow-rate of A1 or of A2 is 24 L/h (20°C / 1 atm).

A1 : 2.7 vol% O₂ / 10.0 vol% H₂O / the balance being N₂

A2 : 2.7 vol% CO / 10.0 vol% H₂O / the balance being N₂. more severe 'lean/rich' conditions (800°C / 16 hours / atmosphere alternating between A1 and A2) or LR 800°C/16 h

conditions of the aging: 2.2 g of solid ; atmosphere: alternating between A1 and A2 (total flow rate = 24 L/h) ; temperature: 800°C; duration : 16 h

The conditions are the same as for LR 720°C/8 h above except for the temperature and the duration of the aging.

Example A: preparation of a mixed oxide of Ce and Al (CeAI)

This example describes a mixed oxide of cerium and aluminum at 95:5 by mass in terms of oxides. An aqueous solution was prepared by mixing 100.0 g in terms of CeO₂ of a eerie nitrate solution with a Ce^lv / total Ce ratio > 90 mole % in water and the total volume was adjusted to 2 L with pure water. The acidity of the aqueous solution after the addition of pure water was 0.12 N. The solution was heated to 100°C, held at this temperature for 30 min and allowed to cool down to the room temperature, to thereby obtain a suspension. After the mother liquor was removed from the suspension thus obtained (by doing so, 1 .2 g of cerium in terms of CeO₂ were removed at the same time), 38.6 g of aluminum nitrate nonahydrate of formula AI(NOs)3, 9H₂O (5.2 g in terms of AI₂Os) was added, and the total volume was adjusted to 2 L with pure water.

A DR of 58% was used. This value is based on the following calculations.

The removal ratio is 0.46 (0.92 L removed from 2 L).

DR (%) = [NO₃ ^"]step b [NO₃ ^"]ste_P a X 100 = (F/G)/(D/E) x 100

D (mol) = 100/172.12 x [92.3/100 x 4+(100-92.3)/100 x 3] + 0.24 = 2.52 0.24 mol stem from the nitrate anions associated with the H⁺ in the solution.

F (mol) = (2.52 x 0.46) + 0.3 = 1 .46

E = G = 2 L

DR = (1 .46/2)/(2.52/2) x100 = 58%. Then the suspension containing the precursor of aluminum oxide was held at 120°C for 2 hours, allowed to cool, and neutralized to pH 8.5 with aqueous ammonia to confirm precipitation. The obtained slurry was subjected to solid-liquid separation by Nutsche filtering to obtain a filter cake, which was calcined at 500°C for 10 hours in air to obtain a mixed oxide of Ce and Al in the form of a powder.

This powder was subjected to quantitative analysis by ICP to determine its composition, which was cerium oxide and aluminum oxide at 95:5 by mass. The results of the characterization of the mixed oxide are shown below :

after calcination in air :

^■ at 500°C / 10 h : S = 135 m²/g;

^■ at 800°C / 2 h : S = 81 m²/g and crystallite size in the (1 1 1 ) plane at 28.6° = 14 nm;

^■ at 900°C / 5 h : S = 49 m²/g;

^■ at 1000°C / 5 h : S = 31 m²/g and crystallite size in the (1 1 1 ) plane at 28.6° = 29 nm. Example B: preparation of a mixed oxide of Ce and Si (CeSi)

This example describes a mixed oxide of cerium and silicon at 95:5 by mass in terms of oxides. An aqueous solution was prepared by mixing 100 g in terms of CeO2 of a eerie nitrate solution (Ce^lv / total Ce > 90 mole %) in water and the total volume was adjusted to 2 L with pure water. The acidity of the aqueous solution after the addition of pure water and HN0₃ was 0.12 N. The solution was heated to 100°C, held at this temperature for 30 min, and allowed to cool down to the room temperature, to thereby obtain a suspension. After the mother liquor was removed from the suspension thus obtained (by doing so, 1 .2 g of cerium in terms of CeO2 were removed at the same time), 25.4 g of a dispersion of colloidal silica Adelite AT-20Q supplied by ADEKA chemical (5.2 g in terms of S1O2) was added, and the total volume was adjusted to 2 L with pure water.

A DR of 50% was used. This value is based on the following calculations.

The removal ratio is 0.5 (1 .0 L removed from 2 L).

DR (%) = [NO₃ ^"]ste_P b / [NO₃ ^"]ste_{P a} x 100 = (F/G)/(D/E) x 100

D (mol) = 100/172.12 x [92.3/100 x 4+(100-92.3)/100 x 3] + 0.24 = 2.52 F (mol) = (2.52 x 0.5) + 0 = 1 .26 E = G = 2 L

DR = (1 .26/2)/(2.52/2) x 100 = 50%

Then the suspension containing the precursor of silicon oxide was held at 120°C for 2 hours, allowed to cool, and neutralized to pH 8.5 with an aqueous ammonia to confirm precipitation.

The obtained slurry was subjected to solid-liquid separation by Nutsche filtering to obtain a filter cake, which was calcined at 500°C for 10 hours in air to obtain a mixed oxide CeSi in the form of a powder.

This mixed oxide powder was subjected to quantitative analysis by ICP to determine its composition, which was cerium oxide (CeO2) and silicon oxide (S1O2) at 95:5 by mass. The results are shown in below :

after calcination in air :

^■ at 500°C / 10 h : S = 195 m²/g;

^■ at 800°C / 2 h : S = 123 m²/g and crystallite size in the (1 1 1 ) plane at 28.6° = 12 nm;

^■ at 900°C / 5 h : S = 82 m²/g;

■ at 1000°C / 5 h : S = 48 m²/g and crystallite size in the (1 1 1 ) plane at 28.6° = 23 nm.

Example 1 (invention): dispersion of CuO (2.5%) on support A by the wetness impregnation technique

The mixed oxide was prepared according to the incipient wetness impregnation technique with 4 steps previously disclosed. The fire loss of support A was determined at 160°C and is equal to 1 .41 %. The retention volume V_r of support A was determined at 20°C and 1 atm by adding water drop by drop onto 20.0 g of support A while the mixture is kept being homogeneized by mixing with a spatula, until water was no longer adsorbed. V_r is equal to 0.49 mL/g. An aqueous solution of copper was prepared by dissolving the required quantity of copper (II) nitrate trihydrate in deionized water. The aqueous solution was then added drop by drop onto 20.0 g of support A while the mixture is kept being homogeneized by mixing with a spatula. The mixture was left for 30 minutes at room temperature for homogeneization. The obtained product was then dried in an oven at 120°C overnight and further calcined in a furnace at 500°C for 2 hours. Example 2 (invention): dispersion of CuO (5.0%) on support A by the wetness impregnation technique

The mixed oxide was prepared according to the incipient wetness impregnation technique with 4 steps already disclosed and with the same support A as in example 1 . An aqueous solution of copper (9.8 ml_) was prepared by dissolving copper (II) nitrate trihydrate (3.08 g) in deionized water. The aqueous solution was then added drop by drop onto 20.0 g of support A while the mixture is kept being homogeneized by mixing with a spatula. The mixture was left for 30 minutes at room temperature for homogeneization. The obtained product was then dried in an oven at 120°C overnight and further calcined in a furnace at 500°C for 2 hours.

Examples 3-5 (invention):

The mixed oxides of these examples were obtained following the same recipe as for example 2 using support A (examples 3-4) or support B (example 5).

Example 6: impregnation of CuO (5.0%) on cerium oxide (comparative example)

The recipe used in example 1 was used for preparing the mixed oxide of example 6 except that the support consists of cerium oxide prepared according to example 2 of EP 1435338 A1 . This support made of CeO2 does not contain aluminium nor silicon.

Examples 7-10: introduction of copper on mixed oxides of cerium and zirconium (comparative examples)

The introduction of copper is performed by ion exchange according to the conditions detailed in US 2010/0197479 to prepare two mixed oxides referenced as CuCZ-1 or CuCZ-2 using two types of CZ-mixed, respectively:

^■ CZ-1 refers to a mixed oxide of cerium, zirconium, lanthanum and praseodymium with the following composition (in terms of oxides): Ce

30.0%; Zr: 60.0%; La 5.0%; Y: 5.0%;

^■ CZ-2 refers to a mixed oxide of cerium, zirconium, lanthanum and praseodymium with the following composition in terms of oxides): Ce 40.0%; Zr: 50%; La 5%; Pr: 5%. for a mixed oxide with 5.0% : copper (II) nitrate trihydrate (4.0 g) is dissolved in deionized water (18.4 g). An ammonium hydroxide solution (=3.0 g; 30 wt%) is added to the solution until a blue-black copper tetramine solution is obtained. The obtained copper tetramine solution is then mixed with 25.0 g of the CZ-mixed oxide (dry powder). After mixing until a homogeneous dispersion is obtained, the solid is recovered by filtration and dried. The powder is then calcined in air at 540°C for 4 hours.

Example 11 : evaluation of the dispersion of the oxide of the base metal

Diffractograms of Fig. 3 and Fig. 4 were recorded after the aging under respectively 'hydrothermal' conditions or 'lean/rich' conditions. As can be seen, the diffractogram of the mixed oxide of example 2 according to the invention does not exhibit any reflexion in the range 2Θ between 38.0° and 40.0°. In contrast, a reflexion can be observed in this range on the diffractograms of the mixed oxides of examples 6 and 8. This is also visible with the data in Tables I and II below. These data correspond to the variations of the signal in the range 28.0-40.0°. It can be seen that the variation of the signal for example 2 is much lower than for examples 5 and 6.

The evaluation of the dispersion of the oxide of the base metal may also be performed using the scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX). This technique makes it possible to obtain high resolution images of the surface topography of the mixed oxide. The relative standard deviations of copper on the surface of the mixed oxide after aging under hydrothermal conditions could be obtained for: - the mixed oxide of example 2: 15% (see Table III below);

- the mixed oxide of example 6: 59% (see Table IV below).

Table III

Table IV

These results tend to show that copper oxide is better dispersed on the mixed oxide of example 2 (invention) than on the mixed oxide of example 6 (comparative example).

Example 12: testing of the mixed oxide of the invention : evaluation of the mixed oxides in a CO oxidation test - determination of the CO oxidation liqht-offs

The CO oxidation performance of three catalysts having been aged at 700°C for 4 hours in an atmosphere composed of 10.0 vol% O2 / 10.0 vol% H₂O / the balance being N₂ was investigated. To do so, powders of the aged catalysts were introduced into a U-shaped down-flow reactor made of quartz (length: 225 mm, internal diameter: 6 mm) and a synthetic gas containing CO (see the composition in Table V) was injected to flow through the bed of catalyst at a GHSV (gas hourly space velocity) of 100,000 h^"1 under a total flow rate of 500 mL/min. The weight of powder (350 mg or 120 mg) in the reactor depends on the density of the catalyst and is adapted to reach the targeted GHSV of 100,000 h^~1.

The composition of the gases leaving the reactor was monitored on line by Fourier Transform InfraRed (FTIR) spectrometer.

Table V

It is possible to record the curves giving the conversion of CO (%) as a function of the temperature. From the curves, the temperatures (the light-off temperatures) at which respectively 50% (T50), 60% (T60) or 90% (T90) of the CO is converted are determined.

These results demonstrate that:

- the mixed oxide of the invention is active without any PGM (see the low light-off temperatures);

- the mixed oxide of the invention is more active than traditional catalysts based on PtPd dispersed on a lanthanum-doped alumina (usually used as a DOC catalyst) or on cerium oxide.

Example 13: testing of the mixed oxide of the invention : determination of the CO oxidation light-off

The oxidation of CO for the three catalysts having been aged at 800°C for 16 hours in an atmosphere composed of 10.0 vol% O2 / 10.0 vol% H₂O / the balance being N₂ was also investigated under the same test and conditions as in example 12.

These results demonstrate that:

- despite the severe aging at 700°C or 800°C, the mixed oxide of the invention maintains a good catalytical activity (see e.g. the T90s of 139°C and 131 °C or the T50/T60 values); - the mixed oxide of the invention exhibits a better catalytical activity than mixed oxides based on Cu oxide dispersed on cerium oxide (without aluminium) or on CZ-mixed oxides (with zirconium and without aluminium);

- the mixture of the mixed oxide of example 6 and of an alumina referenced as MI-307 results in a material of similar global composition as the mixed oxide of example 2. Yet, the catalytical activity of the mixture is not as good as for the mixed oxide of example 2.

Experimental data also demonstrate that the mixed oxide of the invention exhibits good DeSOx properties.

Table VIII

(I): invention ; (C): comparative

* in the invention, the proportion of the base metal like Cu is expressed by weight of oxide with respect to the total weight of the mixed oxide; 2.5% Cu on CeAI thus means that the mixed oxide contains 2.5% of CuO

average size t of the crystallites of cerium oxide based on reflexion at 28.8°±1 °

* designates the specific surface of the mixed oxide after the calcination step at 500°C for 10 hours or at 540°C for 4 hours

Claims

1 . Mixed oxide of cerium, of an element which is aluminium or silicon and a base metal (BM) selected from Fe, Co, Ni or an element of groups 5, 6, 7 and 1 1 of the periodic table, with the following proportions:

^■ aluminium or silicon: up to 20.0%;

^■ base metal: up to 15.0%;

^■ the remainder as cerium;

these proportions being given by weight of the corresponding oxide (CeO2, AI2O3, S1O2, oxide of the BM) with respect to the total weight of the mixed oxide

2. Mixed oxide according to claim 1 of generic formula Cei-(_y+Z) (Al or Si)_y (BM)_Z O_a wherein:

■ y is the proportion of Al or Si;

^■ z is the proportion of the base metal;

^■ a is the stoechiometric balance in oxygen in the mixed oxide;

3. Mixed oxide according to claims 1 or 2 wherein the base metal is copper.

4. Mixed oxide according to any one of claims 1 to 3 wherein the the elements Ce, Al, Si, BM that constitute the mixed oxide are present in the mixed oxide as oxides.

5. Mixed oxide according to any one of claims 1 to 3 wherein the the elements Ce, Al, Si, BM that constitute the mixed oxide are present in the mixed oxide as oxides and optionally also partially in the form of hydroxides or oxyhydroxides.

6. Mixed oxide according to one of the preceding claims wherein the proportion of Al or Si is between 0.1 % and 19.8%, more particularly between 1 .0% and 10.0%, even more particularly between 2.0% and 6.0% or between 4.0% and 6.0%.

7. Mixed oxide according to one of the preceding claims wherein the proportion of the base metal is between 0.5% and 10.0%, more particularly between 1 .0% and 8.0%, even more particularly between 4.0% and 8.0% or between 4.0% and 6.0%.

8. Mixed oxide according to one of the preceding claims wherein the proportion of cerium is at least 68.0%, more particularly of least 80.0%, even more particularly of least 85.0% or of least 90.0%.

9. Mixed oxide according to claims 1 to 5 characterized by the following proportions:

^■ aluminium or silicon: between 2.0% and 6.0%;

^■ cerium: between 86.0% and 94.0%, more particularly between 88.0% and 94.0%;

or by the following proportions:

^■ aluminium or silicon: between 2.0% and 6.0%;

■ the base metal: between 4.0% and 6.0%;

^■ cerium: between 88.0% and 94.0%.

10. Mixed oxide according to one of the preceding claims consisting of cerium oxide, of an oxide of an element which is aluminium or silicon and an oxide of a base metal (BM) selected from Fe, Co, Ni or an element of groups 5, 6, 7 and 1 1 of the periodic table, and optionally of impurities.

1 1 . Mixed oxide according to claim 1 to 9 consisting essentially of cerium oxide, of an oxide of an element which is aluminium or silicon and an oxide of a base metal (BM) selected from Fe, Co, Ni or an element of groups 5, 6, 7 and 1 1 of the periodic table.

12. Mixed oxide according to any one of claims 1 to 1 1 wherein the base metal is present in the form of an oxide dispersed on a mixed oxide of cerium and aluminium or a mixed oxide of cerium and silicon.

13. Mixed oxide according to any one of claims 1 to 12 exhibiting a specific surface area of at least 35 m²/g, more particularly of at least 40 m²/g, after being aged at 800°C for 16 hours under a atmosphere composed of 10.0% O₂ / 10.0% H₂O / balance of N₂.

14. Mixed oxide according to any one claims 1 to 13 exhibiting a specific surface area of at least 35 m²/g, more particularly of at least 40 m²/g, after the mixed oxide is aged at 720°C for 8 hours in an atmosphere which is alternatively:

15. Mixed oxide according to any one of claims 1 to 14 exhibiting a specific surface area of at least 8 m²/g after the mixed oxide is aged at 800°C for 16 hours in an atmosphere which is alternatively:

^■ the cycle of the alternating atmosphere A1 -A2 being repeated for the whole 16 hours of the aging.

16. Mixed oxide according to any one of the preceding claims exhibiting a specific surface area of at least 70 m²/g, more particularly of at least 80 m²/g after calcination in air at 800°C for 2 hours.

17. Mixed oxide according to any one of the preceding claims exhibiting a specific surface area of at least 40 m²/g, more particularly of at least 60 m²/g after calcination in air at 900°C for 5 hours.

18. Mixed oxide according to any one of the preceding claims exhibiting a specific surface area of at least 20 m²/g, more particularly of at least 40 m²/g after calcination in air at 1000°C for 5 hours.

19. Mixed oxide according to any one of claims 1 to 18 wherein the base metal is Cu and the diffractogram of the mixed oxide obtained by XRD does not exhibit any reflexion in the range 2Θ between 38.0° and 40.0°.

20. Mixed oxide according to any one of the preceding claims wherein the base metal is dispersed on the surface of a mixed oxide of cerium and aluminium or of cerium and silicon in the form of a substoichiometric oxide of the base metal.

21 . Mixed oxide according to any one of claims 1 to 20 exhibiting a T_max ranging from 1 15°C to 140°C, T_max being the temperature for which the intensity of the signal of the TCD is maximum on the TPR curve.

22. Mixed oxide according to any one of claims 1 to 21 exhibiting a T_end lower than 215°C, more particularly lower than 200°C, T_end being the temperature defined by the following characteristics :

^■ Tend is superior to T_max; and

23. Mixed oxide according to any one of claims 1 to 22 wherein the width at half height of the peak of the signal at T_max is generally lower than 30°C, more particularly lower than 25°C, even more particularly lower than 15°C, T_max being the temperature for which the intensity of the signal of the TCD is maximum on the TPR curve.

24. Mixed oxide according to any one of claims 1 to 23 wherein the average size t of the crystallites of cerium oxide are lower than 20 nm, more particularly lower than 19 nm, after the mixed oxide is aged at 800°C for 16 hours in a atmosphere composed of 10% O₂ / 10% H₂O / balance of N₂.

25. Mixed oxide according to any one of claims 1 to 24 wherein the average size t of the crystallites of cerium oxide are lower than 20 nm, more particularly lower than 19 nm, after the mixed oxide is aged at 720°C for 8 hours in an atmosphere which is alternatively:

■ the cycle of the alternating atmosphere A1 -A2 being repeated for the whole 8 hours of the aging.

26. Process of preparation of a mixed oxide according to any one of claims 1 to 25 consisting in (i) filling the pores of a mixed oxide of cerium and aluminium or of cerium and silicon, in the form of a powder, with an aqueous solution of a precursor of the oxide of the base metal; (ii) drying the solid obtained at the end of step (i) and (iii) calcining in air the solid obtained at the end of step (ii) at a temperature of at least 400°C.

27. Process according to claim 26 characterized in that the mixed oxide of cerium and aluminium or of cerium and silicon exhibits a total pore volume of at least 0.30 mL/g.

28. Mixed oxide obtainable by the process according to claims 26 or 27.

29. Composition comprising the mixed oxide according to any one of claims 1 to 25 or to claim 28 and at least one inorganic material other than the mixed oxide.

30. Composition comprising the mixed oxide according to any one of claims 1 to 25 or to claim 28 and a catalyst that consists of at least one PGM dispersed on an refractory oxide selected from the group consisting of alumina, silica, titania, zirconia, ceria, silica-alumina, titania-alumina, zirconia-alumina, ceria-alumina, titania-silica, zirconia-silica, zirconia-titania, ceria-zirconia and alumina- magnesium oxide.

31 . Use of a mixed oxide according to any one of claims 1 to 25 or of claim 28 or a composition according to claims 25 or 26 to reduce the amounts of CO and/or unburnt hydrocarbons (HCs) in an exhaust gas released by the internal combusion engine of a vehicle.

32. Use of a mixed oxide according to any one of claims 1 to 25 or of claim 28 or a composition according to claims 29 or 30 in the preparation of a catalytic converter.

33. Process of treatment of an exhaust gas released by the internal combusion engine of a vehicle, notably to reduce the amount of CO and/or unburnt hydrocarbons, comprising contacting the exhaust gas with the mixed oxide according to any one of claims 1 to 25 or with a composition according to claims 29 or 30.

34. Catalytic converter comprising a mixed oxide according to any one of claims 1 to 25 or a composition according to claims 29 or 30.