WO2019042911A1

WO2019042911A1 - Use of a mixed oxide to absorb nox

Info

Publication number: WO2019042911A1
Application number: PCT/EP2018/072963
Authority: WO
Inventors: Fabien OCAMPO; Barry William Luke Southward; Naotaka Ohtake
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 the use of a mixed oxide based upon cerium, an element which is Al or Si and a base metal (BM) as to absorb NOx. The invention also relates to a process for the treatment of an exhaust gas using said mixed oxide.

Description

USE OF A MIXED OXIDE TO ABSORB NO_x

The present application claims the priority of European patent application EP 17306104 filed on 29 August 201 7, 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. The present invention relates to the use of a mixed oxide based upon cerium, an element which is Al or Si and a base metal (BM) as to absorb NO_x. The invention also relates to a process for the treatment of an exhaust gas using said mixed oxide. Technical context

The use of catalysts for purifying vehicle exhaust gas is now very common to remove pollutants like CO, unburnt particulate matter (e.g. soot), unburnt hydrocarbons (HCs), nitrogen oxides NO and NO₂ (NO_x) that are noxious for the health and for the environment. The ever stricter regulations {e.g. regulation (EC) n° 71 5/2007 of the European Parliament) require that the catalysts are more and more efficient. For gasoline engine vehicles, the control of the emissions is achieved using the so-called 'three way' catalyst which can simultaneously decrease the amounts of hydrocarbons, CO and NO_x. In the case of diesel engine vehicles, the situation is rather more complex and the emissions trains comprises a series of catalyst monoliths, each having a very specific functionnality. For example, the Diesel Oxidation Catalyst (DOC) is an ubiquitous implemented technology employed for the control of CO and HC emissions. However, the DOC does not address the challenge of conversion of NO_x to N₂ since the excess oxygen present in the exhaust gas with respect to the standard stoichiometric point of conventional gasoline engines results in the preferential conversion of reductants such as CO or H₂ by O₂ resulting in a complete depletion of the reductant species required for the catalytic conversion of NO to nitrogen. There are two main technologies developed to address the issue of the NO_x reduction : the Selective Reduction Catalyst (or SCR) and the Lean NO_x Trap (on LNT). Both approaches require a material that stores the NO_x. For both approaches, the current and future regulations like Euro 6b or Euro 6c require a ΝΟχ reduction at 'low' temperatures. A higher thermal stability is also required in more severe conditions like hydrothermal conditions and/or lean/rich conditions. Problem to be solved

Hence there is still a need to provide a mixed oxide that exhibits an efficient NO_x absorption, even at low temperatures, and a good thermal stability, along with good DeSOx properties. This need is solved by the mixed oxide according to the invention.

Technical background

WO 2007/131 902 discloses a composition that can be used as a lean NO_x trap which is based upon alumina, cerium and a metal element M selected from the group consisting of barium, strontium and a combination of these two elements.

US 9,51 7,448 discloses materials based on cerium as LNTs. The materials may be cerium oxide, cerium-zirconium oxide, cerium-lanthanum oxide, cerium-yttrium oxide, cerium-zirconium-lanthanum oxide, cerium-zirconium-yttrium oxide, ceriumlanthanum-yttrium oxide, or cerium-zirconium-lanthanumyttrium oxide.

US 201 0/0197479 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 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 platinum group metal (PGM) catalyst dispersed on particles of aluminum oxide (AI₂O₃). The combination requires the use of zirconium.

Journal of Catalysis 201 0, 272, 1 09-1 20 discloses in general terms a catalyst CeO₂-CuO on a γ-ΑΙ₂Ο₃ prepared by impregnation with precursors Cu(NO₃)2 and Ce(N0₃)₄ on Al₂0₃ as a support. The proportion of aluminium exceeds the proportion of claim 1 .

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 29%, higher than the claimed content of 15.0%.

Applied Catalysis B: environmental 2006, 65, 101 -109 discloses gold supported on CeO₂-AI₂O₃ for NO_x reduction by CO.

J. Haz. Mat. 201 1 , 187, 283-290 discloses mixed oxides of Mn, Ce and Al. Figures

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

Fig. 2 provides the TPR curves of the mixed oxides of comparative example 8 (5.0% CuO on CZ-1 ) and of comparative example 10 (5.0% CuO on CZ-2). Fig. 3 provides the diffractograms of the mixed oxides of example 2 (5.0% CuO on CeAI), of comparative example 6 (5.0% CuO on CeO₂) and of comparative example 8 (5.0% CuO on CZ-1 ) after aging under hydrothermal conditions (800°C / 1 6 h). Fig. 4 provides the diffractograms of the mixed oxides of example 2 (5.0% CuO on CeAI), of comparative example 6 (5.0% CuO on CeO₂) and of comparative example 8 (5.0% CuO on CZ-1 ) after aging under lean/rich conditions (720°C / 8 h). Fig. 5 provides the absorption of NO_x under the conditions given in example 12 at two temperatures (200°C and 250°C) for the mixed oxides of example A (CeAI), of example 2 (5.0% CuO on CeAI) and of example 4 (10.0% CuO on CeAI).

Fig. 6 provides the release of NO_x under the conditions given in example 13 at for the mixed oxides of example A (CeAI) and of example 2 (5.0% CuO on CeAI). Fig. 7 provides the absorption of NO_x under the conditions given in example 14 for the mixed oxide of example 2 (5.0% CuO on CeAI) and for two catalysts comprising Pt (Ref. 1 and Ref. 2). Fig. 8 provides the release of NO_x under the conditions given in example 14 for the mixed oxide of example 2 (5.0% CuO on CeAI) and for two catalysts comprising Pt (Ref. 1 and Ref. 2).

Fig. 9 and Fig. 10 provide the absorption of NO_x under the conditions given in example 15 for the mixed oxide of example 2 (5.0% CuO on CeAI) and for a catalyst comprising Pt (Ref. 2) for respectively two durations of absorption (30 s and 5 min).

Fig. 11 provides the release of SO_x under the conditions given in example 1 6 for the mixed oxides of example A (CeAI), of example 2 (5.0% CuO on CeAI) and of example 5 (5.0% CuO on CeSi) and for a catalyst comprising Pt (Ref. 2).

Description of the invention

The invention relates to the use of a mixed oxide based upon cerium, an element which is Al or Si and a base metal (BM) to absorb NO_x. The invention also relates to the use of a composition comprising said mixed oxide to absorb NO_x. NO_x means NO and/or NO₂. The mixed oxide of the invention exhibits a capacity for efficiently absorbing NO_x at a temperature lower than 400°C, more particularly lower than 250°C as is documented in the examples.

The ability of the mixed oxide to absorb the NO_x is useful in two competing technologies that are currently used to reduce the amounts of NO_x in an exhaust gas released by the engine of a vehicle by converting NO_x into N₂. The details of these two technologies (LNT and SCR), which are well known in the field of treatment of exhaust gases, are more precisely given below. Both technologies operate under the same general method of treatment of an exhaust gas released by the combustion engine of a vehicle by which: (a) the NO_x contained in the exhaust gas are absorbed by the mixed oxide or a composition comprising the mixed oxide; (b) the nitrogen-containing compounds that are released from the mixed oxide are converted into N₂. The term 'absorb' used herein refers to the phenomenon by which the NO_x are physically (adsorption) and/or chemically (chemisorption) captured by the mixed oxide. As the identification of (i) the compounds that are present in/on the mixed oxide following the absorption of the NO_x and (ii) the compounds that are released from the mixed oxide following the absorption of the NO_x has not been performed and is difficult in view of the complex chemistry involved (the exhaust gas is composed of many molecules, the amount of oxygen in the fuel/air mixture varies over time, the temperature may also vary,...), the term 'nitrogen-containing compounds' is used to designate these compounds. Without being bound by any theory, these compounds are likely to be NO and NO₂. Yet, as previous scientific studies revealed for other compositions useful as LNT, nitrites and/or nitrates may also be absorbed by said other compositions and may be released (see e.g. J. Phys. Chem. 2001 , 12732-12745 for BaO/AI₂O₃ or Pt-BaO/AI₂O₃ catalysts; Phys. Chem. Chem. Phys. 2003, 4435-4440 for Pt-Rh/Ba/AI₂O₃ catalyst; J. Catal. 2004, 377-388 for Pt-Ba/alumina catalysts; Catal. Today 1 19 2007, 73-77; ChemCatChem 2012, 55-58 for Pt-Ba/AI₂O₃ catalyst).

LNT technology

The 1 ^st technology is the Lean NO_x Trap technology (or LNT). In this technology, step (a) is performed under lean exhaust conditions and step (b) is performed under rich exhaust conditions. In step (b), the nitrogen-containing compounds that are released are converted into N₂ in the presence of at least one reductant, the chemical conversion into N₂ being catalyzed by at least one reduction catalyst. The reduction catalyst may be based on at least one PGM, such as e.g. Rh. PGM designates a platinum group metal which is a chemical element selected in the following list: Ru, Rh, Pd, Os, Ir, Pt. The reductant which reacts with a nitrogen-containing compound may be e.g. CO, hydrocarbons (HCs), H₂ or any other chemical compound which is present in the exhaust gas. The periodic switch from lean exhaust conditions to rich exhaust conditions is operated by modifying the regime of the running combustion engine so as to increase the richness of the fuel/air mixture. The quality of the fuel/air mixture is usually described by a factor λ. λ, which has the classical meaning, is defined by the following formula:

λ = (A/F)_mj_xture / (A/F)_stoichiometric

wherein:

^■ (A/F)_mixture is the weight of air to the weight of the fuel ratio of the fuel/air mixture used to operate the engine; (A/F)_stoichiom_etric is the ratio corresponding to a stoichiometric combustion of a hydrocarbon fuel.

(A/F)_stoichiom_etric depends on the relative proportions of carbon and hydrogen in the fuel composed of hydrocarbons. For instance, gasolines correspond to an average hydrocarbon of formula CH-i .ss and the (A/F)_st0ichiom_etric is generally equal to 14.6. For diesels, the (A/F)_st0ichiom_etric is generally equal to 14.5. Lean exhaust conditions are obtained when the engine runs with a fuel/air mixture defined by λ > 1 . Rich exhaust conditions are obtained when the engine runs with a fuel/air mixture defined by λ < 1 . Different operations, alone or in combination, such as changing the amount of air and / or EGR (Exhaust Gas Recycling) admitted into the combustion chamber of the engine and / or an additional fuel injection in the combustion chamber, are used to obtain the rich exhaust conditions. The main purpose of these operations is to consume the oxygen in the exhaust gas.

The release of the nitrogen-containing compounds is triggered by the switch to lean exhaust conditions to rich exhaust conditions. The rich exhaust conditions lead also to an increase of the temperature of the exhaust gas in the exhaust line which also helps in desorbing the nitrogen-containing compounds.

The LNT technology is based on a complex catalytic system. The term 'catalytic system' is used to designate the system which is present between the engine and the exhaust pipe of a vehicle and which is used to chemically treat the exhaust gas to get rid of its pollutants. Pollutants are inter alia CO, unburnt particulate matter such as soot, unburnt hydrocarbons and NO_x. The catalytic system comprises the mixed oxide, the function of which is to absorb the NO_x. Another function of the mixed oxide is to convert or catalyze the conversion of NO to NO₂. The catalytic system may further comprise a DOC, the function of which 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. The catalytic system also comprises at least one reduction catalyst, the function of which is to catalyze the conversion of the nitrogen-containing compounds into N₂. The catalytic system is part of an automotive catalytic converter which comprises a solid support on which at least one layer (or washcoat) is applied. 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. The solid support may be of any generally suitable form. For example, the support may comprise a filter, a flow through monolith, such a ceramic, honeycomb or an extruded structure. The solid support is typically made of a refractory ceramic having a honeycomb structure. The solid support may have a plurality of fine, parallel gas flow passages extending there through from an inlet or an outlet and the passages are open to fluid flow. The passages are usually thin-walled channels and may be of any suitable cross-sectionnal shape and size such a trapezoidal, rectangular, square, sinusoidal, hexagonal, oval, circular, etc.

The layer is made from a composition comprising the mixed oxide. 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 layer may be prepared according to well-known methods for a person skilled in the art. One of these methods involves coating a dispersion of the composition in water on the support, drying and calcining the resulting product. Methods of applying a layer on the support may for instance be found in WO 201 1 /080525 or in example 7 of US 2013/0089481 .

The catalytic system may be arranged according to several embodiments. According to an embodiment, the catalytic system comprises a layer being made of a composition comprising altogether the mixed oxide, the reduction catalyst and optionally the additional oxydation catalyst. According to another embodiment, the catalyst system comprises superposed layers, one layer being made from a composition comprising the mixed oxide and optionally the additional oxidation catalyst and another layer being made from a composition comprising the reduction catalyst. According to another embodiment, the catalytic system comprises a layer segmented into several parts arranged in the longitudinal direction of the support, one part of the layer being made from a composition comprising the mixed oxide and optionally the additional oxidation catalyst and another part of the layer being made from a composition comprising the reduction catalyst. Whatever the spatial arrangement, the nitrogen-containing compounds released need to be in contact with the the reduction catalyst and with the reductant.

SCR technology

The 2^nd technology is the Selective Catalytic Reduction (or SCR). This technology is based on the catalytic reaction of ammonia with the NO_x in the presence of an SCR catalyst. Ammonia may be directly injected in the exhaust line but a precursor of ammonia which decomposes into ammonia at high temperature is more conveniently injected. The precursor of ammonia may be for instance urea or an ammonium carbamate. Urea is the most common precursor of ammonia as it is conveniently stored and decomposes smoothly to ammonia. It is moreover already commercialized under the trademark Adblue™. The chemical equations involved in the SCR technology are given below when the nitrogen-containing compounds are NO and NO₂:

Urea decomposes to ammonia at a temperature which is higher than 180°C which explains that the SCR technology requires the use of high temperatures to be efficient (usually higher than 180°C). Moreover, high temperatures are also preferred to avoid the clogging of the lines or of the injector of urea by solid urea. Although the SCR catalyst is promoted by the presence of NO₂, there is also usually limited NO₂ present at low temperatures either due to the low NO oxidation activity of the DOC or the NO₂ that gets out of the engine is reduced to NO by HCs or CO on the DOC, such the NO_x is predominantly present as NO. Finally, it is to be noted that the current SCR technologies based on urea implemented for meeting Euro 6 emissions require that the temperature is at least 180°C before urea can be dosed and used for NO_x conversion. Because of these constraints, the SCR technology is relatively inefficient at 'low' temperatures, for instance when the vehicle has just started running (the 'cold start' period) or when the vehicle makes frequent stops. Because of its ability to absorb the NO_x, the mixed oxide can be used in the SCR technology to store the NO_x at low temperatures when the SCR catalyst is not sufficiently warm to function efficiently. The nitrogen-containing compounds are then released at a higher temperature to be reduced by the SCR catalyst in the presence of ammonia. At this higher temperature, the SCR catalyst is sufficiently warm to function efficiently. The combination of the absorption of NO_x at low temperature and of the SCR technology is already known for other materials used to absorb NO_x as is documented for instance in a communication titled "Al₂0₃-based passive NO_x adsorbers for low temperature applications" by Mark Crocker presented in the 8^th International Conference on Environmental Catalysis held on 24-27 August 2014 in North Carolina (USA). The composition used to absorb the NO_x at low temperatures is described usually as a passive NO_x adsorber (or PNA).

In the technology which combines the PNA and the SCR, step (a) is performed when the temperature of the exhaust gas in contact with the mixed oxide or the composition comprising said mixed oxide is T_a and step (b) is performed when the temperature of the exhaust gas in contact with the mixed oxide or the composition comprising said mixed oxide is T_b which is higher than T_a. In step (b), the nitrogen-containing compounds that are released are converted into N₂ in the presence of ammonia, the chemical conversion into N₂ being catalyzed by at least one SCR catalyst. Ammonia which reacts with the nitrogen-containing compounds may be added in the exhaust gas as such or may be the result of the thermal decomposition of a precursor of ammonia, such as urea. T_a and T_b depends on several factors like the arrangement of the catalytic converter, the SCR catalyst and the type of vehicle (passenger car, light-duty diesel vehicle, heavy-duty diesel vehicle,...). As detailed above, T_b is preferably higher than 180°C and/or T_a is lower than 180°C. The release of the nitrogen-containing compounds may be performed at a temperature T_b between 180°C and 400°C, more particularly between 200°C and 400°C.

The SCR catalyst is a catalyst that catalyze the conversion of the nitrogen- containing compounds into N₂ in the presence of ammonia. The SCR catalyst may be a zeolite containing a metal or an oxide of V, W, Ti, Fe, Cu, Co, Mn, Cr or mixtures thereof. A zeolite is a crystalline material having rather uniform pore sizes which, depending upon the type of zeolite and the type and amount of cations included in the zeolite lattice, range from about 3 to 10 Angstroms in diameter. Zeolites generally comprise silica to alumina molar ratios of 2 or greater. The zeolite may more particularly be an 8-member ring zeolite. The metal may be Fe or Cu. The 8-member ring zeolite may have a CHA, SAPO or AEI structure. The SCR catalyst may also consist of vanadium supported on a metal oxide such as alumina, silica, zirconia, ceria and combinations thereof. An example of a SCR catalyst comprises Ti0₂ on to which W0₃ and V₂0₅ have been dispersed at concentrations ranging from 5.0 to 20.0 wt% and from 0.5 to 6.0 wt%. Typical SCR catalysts are described in US 4,010,238 and US 4,085,193. Other examples of SCR catalysts are disclosed in US 5,300,472; US 8,367,578; US 4,961 ,917 and US 5,51 6,497. Examples of SCR catalyst include VNX™ commercialized by BASF, Cu/ZSM-5, Fe/WO_x/ZrO₂, vanadia-based catalysts such as V₂O₅/TiO₂ or V2O5/WO3/T1O2.

The association of the PNA with the SCR technology is based on a complex catalytic system. The catalytic system comprises the mixed oxide the function of which is to absorb the NO_x. Apart from this function, the mixed oxide is also able to convert some of the NO to NO₂. The catalytic system also comprises at least one SCR catalyst, the function of which is to catalyze the conversion of the nitrogen-containing compounds into N₂ in the presence of ammonia.

The catalytic system is part of an automotive catalytic converter which comprises a solid support on which at least one layer (or washcoat) is applied. 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. The solid support may be of any generally suitable form. For example, the support may comprise a filter, a flow through monolith, such a ceramic, honeycomb or an extruded structure. The solid support is typically made of a refractory ceramic having a honeycomb structure. The solid support may have a plurality of fine, parallel gas flow passages extending there through from an inlet or an outlet and the passages are open to fluid flow. The passages are usually thin-walled channels and may be of any suitable cross-sectionnal shape and size such a trapezoidal, rectangular, square, sinusoidal, hexagonal, oval, circular, etc.

The layer is made from a composition comprising the mixed oxide. 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 layer may be prepared according to well-known methods for a person skilled in the art. One of these methods involves coating a dispersion of the composition in water on the support, drying and calcining the resulting product. Methods of applying a layer on the support may for instance be found in WO 201 1 /080525 or in example 7 of US 2013/0089481 . The catalytic system may be arranged according to several embodiments. In one embodiment, the catalytic system comprises in the direction from the engine to the exhaust pipe: a DOC in combination with the mixed oxide (PNA); a particulate filter (PF); a SCR catalyst (SCR). This embodiment is represented by: engine -> DOC+PNA -> PF -> SCR -> exhaust pipe. The DOC reduces the amounts of CO and HCs that are present in the exhaust gas and also converts NO into NO₂. The DOC is well-known to a person skilled in the art. It is usually based on a PGM such as Pt. An example of DOC consists of Pt dispersed on an optionally doped alumina such as La-doped alumina. The PNA absorbs the NO_x when the temperature of the exhaust gas is low. The nitrogen-containing compounds that are released from the PNA when the temperature of the exhaust gas is high reacts then with the SCR catalyst in the presence of ammonia. The function of the PF is to reduce the amount of particulate matter.

Other embodiments are given below with the same representation:

2^nd embodiment: engine -> DOC+PNA -> SCR -> PF -> exhaust pipe

3^rd embodiment: engine -> DOC+PNA -> SDPF -> exhaust pipe

SDPF (acronym used for SCR-Catalysed Diesel Particulate Filter) designates a particulate filter that comprises in its porosity a SCR catalyst. The SDPF has thus a dual function of reducing the amount of particulate matter and of reducing the amount of NO_x. The efficiency of the reduction of the NO_x may not be sufficient so that another SCR may be added between the SDPF and the exhaust pipe in the 3^rd embodiment. In these embodiments, the DOC is in combination with the mixed oxide (PNA), which means the DOC and the mixed oxide are comprised in the same composition whereas the SCR catalyst is comprised in another composition. The compositions are usually arranged in the form of layers on the solid support. According to a variant to all embodiments 1 to 3, the DOC and the mixed oxide are separated and the DOC is present before the mixed oxide. about the mixed oxide of the invention

The mixed oxide used in the present invention is 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 is based upon the above mentioned elements with the above mentioned proportions. The mixed oxide may thus be described by the generic formula Ce-i -(_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;

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. The proportions of these elements are given by weight of the corresponding oxide (Ce0₂, Al₂0₃ or Si0₂, 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 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: Fe₂0₃, Co₃0₄, Mn0₂, 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 6.0%.

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 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 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 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 (Ce0₂, Al₂0₃ or Si0₂, 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 used in 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 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 1 60°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://www.chemspeed.com/videopaqe/incipient-wetness-impreqnation/). In that case, steps 1 ) to 3) disclosed above may be used similarly with the automatic appliance.

Preferably, the mixed oxide CeAI or CeSi 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. Thus, use may also be made of 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 (Ce0₂, Al₂0₃ or Si0₂) 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 used 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 used 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 used is aged at 800°C for 1 6 hours in an atmosphere composed of 10.0 vol% 0₂ / 10.0 vol% H₂0 / 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% 0₂ / 10.0 vol% H₂0 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 used 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% C½ / 10.0 vol% H₂0 / 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 used 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:

^■ 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 1 6 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 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% 0₂ / 10.0 vol% H₂0 / 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 used may also exhibit good reducibility properties. These properties 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.

T_end is defined as the temperature defined by the following characteristics

■ T_end is superior to T_max; and ^■ T_end corresponds to the first temperature at which the signal of the TCD returns to the baseline.

The mixed oxide used 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 y₀. 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 y₀+5%. It is also observed that for the mixed oxide used, 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 1 6 hours in a atmosphere composed of 10.0 % 0₂ / 10.0 % H₂0 / 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% 0₂ / 10.0 vol% H₂0 / 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 average size t:

k ^■ λ

t = _

-ff²— s^{2 ■} cos Θ

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 N0₃ ^" 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 = [N0₃ ^"]step b [N0₃ ^"]st_ep a X 100 (I)

where [N0₃ ^"]_step a is the concentration in mol/L of the nitrate anions in the aqueous solution of cerium used in step (a) ;

and [N0₃ ^"]_step 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 and optionally Ce¹" 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 Ce0₂. 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 HN0₃ 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 N0₃ ^". 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 N0₃ ^" 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 μηι. 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 N0₃ ^" 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 N0₃ ^" 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 = [N0₃ ^"]_s,ep b / [N0₃-]_step a X 100 (I)

[N0₃ ^"]_step a : concentration in mol/L of the nitrate anions in the aqueous solution of cerium used in step (a) ; [N0₃ ^"]step 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 N0₃ ^" (mol) in the aqueous solution of cerium used in step (a).

2. Calculation of the concentration of the nitrate anions N0₃ ^" (mol/L) in the aqueous solution of cerium used in step (a) : [N0₃ ^"]_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 N0₃ ^" (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 N0₃ ^" (mol/L) in the suspension obtained at the end of step (b) : [N0₃ ^"]_step 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 Ce0₂ (gram);

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

- C is the quantity of nitrate anions N0₃ ^" (mol) from other sources than cerium nitrate(s) e.g. the nitrate anions associated with the FT in the solution. F (mol) = (D x removal ratio of the liquid medium) + anions N0₃ ^" (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 μηι. 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. 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

Diffractograms were obtained on powders with a copper source (CuKal, λ=1 ,54 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 / 1 6 hours / atmosphere composed of 10.0 vol% O / 10.0 vol% HPO / the balance being N?) or HT 800°C/1 6 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: 1 6h

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 μηι and retained with a sieve of 125 μηι.

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 1 6 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

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 μηι and retained with a sieve of 125 μηι. 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 0₂ 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 / 1 6 hours / atmosphere alternating between A1 and A2) or LR 800°C/1 6 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(NO₃)3, 9H₂O (5.2 g in terms of AI₂O₃) 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 ^"]_step 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)7(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 Ce0₂ 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 Ce0₂ 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 Si0₂) 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 (%) = [N0₃ ^"]_step b / [N0₃ ^"]_step 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 (Ce0₂) and silicon oxide (Si0₂) 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 already disclosed. The fire loss of support A was determined at 1 60°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 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 Ce0₂ 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 oxides, 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.

Table I

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: determination of the NO_x absorption at two temperatures (200°C and 250°C) of three mixed oxides aged under 'hydrothermal' conditions (800°C / 16 h)

Three materials were compared:

- the mixed oxide CeAI of example A (comparative example);

- the mixed oxide of example 2 (invention);

- the mixed oxide of example 4 (invention). conditions of aging of the mixed oxides

The mixed oxide is compacted in the form of pellets with a diameter of 32 mm, by applying a pressure of 8 tons on the powder during 2 min. The pellet is then deagglomerated in a mortar and the obtained powder is sieved in order to conserve the particles which size is contained in the range 125-250 μηι. Hydrothermal aging was performed for the powder catalysts (2.2 g) under an atmosphere composed of 10.0 vol.% 0₂, 10.0 vol.% H₂0 and balance N₂ (total flow rate = 24 L/h) at 800°C/1 6 h. These conditions are selected to simulate severe lean conditions.

Conditions of the test

The aged mixed oxide (250 mg) was introduced in a quartz U-shaped down-flow reactor (length: 225 mm, internal diameter: 6 mm). The compositions of the gases used for NO_x absorption and for NO_x release are given in Table V below. The gas is introduced in the reactor and flows though the mixed oxide at a total flow-rate of 250 mL/min so as to obtain a GHSV (gas hourly space velocity) of 80,000 h^"1. The NO_x absorption was determined when the material is at 200°C or 250°C after contact with the gas for a period of 60 s. The composition of the gases getting out of the reactor was monitored online by Fourier Transform InfraRed (FTIR) spectrometer.

Table V

The ΝΟχ absorption is calculated by measuring the amount of the NO_x in the gas getting out of the reactor:

NOx absorption (%) = (NO_x entry - NO_{x exi}t) / NO_x entry x 100

The results are given on Fig. 5. As the gas getting in the reactor is essentially composed of NO (the amount of NO₂ in equilibrium with NO is negligible), the NO_x absorption corresponds to the amount of NO absorbed by the material. From these results, one can conclude that in the conditions of the test:

- the introduction of copper oxide leads to an improvement of the absorption of NO;

- the mixed oxide with 5.0% of copper provides a better absorption of NO than the two other mixed oxides. Example 13: determination of the NO_x release of two mixed oxides aged under 'hydrothermal' conditions (800°C / 1 6 h)

Two mixed oxides were compared:

- the mixed oxide CeAI of example A (comparative example);

- the mixed oxide of example 2 (invention).

NOy absorption

The conditions of the test are the same as in example 12 except that:

- the composition of the gas used for NO_x absorption and containing a stoichiometric amount of NO and NO₂ is given in Table VI below ;

- the gas is introduced in the reactor and flows though the material (150 mg)at a total flow-rate of 500 mL/min so as to obtain a GHSV of 250,000 h^"1. Table VI

NOx release

The contact between the materials and the gas with NO and NO₂ is performed during 90 min under the conditions above (120°C; GHSV= 250,000 h^"1). Then, the ΝΟχ release was determined by switching the gas containing NO and NO₂ to the gas with no NO and no NO₂ (see Table VI) and by raising the temperature of the material from 120°C up to 600°C (ramp=10°C/min). It is then possible to obtain the amount of NO_x released as a function of temperature (Fig. 6). The results on Fig. 6 show that:

- copper is benefitial to the release of NO_x;

- the temperature at which the release of NO_x is maximum is decreased by 75°C;

- the temperature at which the release of NO_x starts is decreased by 130°C. Example 14: determination of the NO_x absorption and NO_x release at 'low' temperature (120°C) of three materials aged under 'hydrothermal' conditions (700°C / 4 h)

Three materials were compared:

- the mixed oxide of example 2;

- a material consisting of Pt (1 wt%) dispersed on a mixed oxide of Ce and Ba (Ref. 1 );

- a material consisting of Pt (1 wt%) dispersed on a mixed oxide of Ce, Ba and Al (Ref. 2). conditions of aging

The material is compacted in the form of pellets with a diameter of 32 mm, by applying a pressure of 8 tons on the powder during 2 min. The pellet is then deagglomerated in a mortar and the obtained powder is sieved in order to conserve the particles which size is contained in the range 125-250 μηι.

Hydrothermal aging was performed on the three materials (2.2 g) under an atmosphere composed of 10.0 vol.% 0₂, 10.0 vol.% H₂0 and balance N₂ (total flow rate = 24 L/h) at 700°C/4h. These conditions are selected to simulate severe lean conditions.

NOy absorption

The test used is the same as in example 12 except that:

- the composition of the gas used for NO_x absorption and containing a stoichiometric amount of NO and NO₂ is given in Table VI ;

- the gas is introduced in the reactor and flows though the material (150 mg) at a total flow-rate of 500 mL/min so as to obtain a GHSV of 250,000 h^"1 ;

- the ΝΟχ storage capacity was determined when the material is at 120°C after contact with the gas for 30 s and for 120 s.

The test demonstrated that NO₂ is almost completely absorbed by the 3 materials. As far as NO absorption is concerned, the results are given on Fig. 7. The mixed oxide of example 2 provides better absorption at 30 s and at 120 s than the two other materials. NOy release

The contact between the materials and the gas with NO and NO₂ is performed during 90 min under the conditions above (120°C; GHSV= 250,000 h^"1). Then, the ΝΟχ release was determined by switching the gas containing NO and NO₂ to the gas with no NO and no NO₂ (Table VI) and by raising the temperature of the material from 120°C up to 600°C (ramp=10°C/min). It is then possible to obtain the amount of NO_x released as a function of temperature (Fig. 8).

As can be seen on the curves of Fig. 8, the mixed oxide of example 2 releases more NO_x at a lower temperature than the two other materials. The release of the ΝΟχ starts at around 180°C and is completed at around 400°C.

The results show that the mixed oxide of the invention is useful for the absorption and release of NO_x at 'low' temperatures.

Example 15: determination of the NO_x absorption at 'higher' temperatures (200- 400°C) for two materials aged under 'hydrothermal' conditions (800°C / 1 6 h) The determination of NO absorption was performed with the same test as for example 12 except that the test was performed at higher temperatures (200°C; 250°C; 300°C; 400°C). The gas is introduced in the reactor and flows though the mixed oxide (150 mg) at a total flow-rate of 500 mL/min so as to obtain a GHSV of 250,000 h^"1. The NO absorption was measured at 30 s (Fig. 9) or 5 min (Fig. 10) on the materials aged under hydrothermal conditions (800°C / 1 6 h). conditions of aging of the materials

The catalyst is compacted in the form of pellets with a diameter of 32 mm, by applying a pressure of 8 tons on the powder during 2 min. The pellet is then deagglomerated in a mortar and the obtained powder is sieved in order to conserve the particles which size is contained in the range 125-250 μηι.

Hydrothermal aging was performed for the powder catalysts (2.2 g) under an atmosphere composed of 10.0 vol.% O₂, 10.0 vol.% H₂O and balance N₂ (total flow rate = 24 L/h) at 800°C/1 6h. These conditions are selected to simulate severe lean conditions. The results of Fig. 9 show that under the conditions of the test, the mixed oxide of example 2 absorbs well NO up to 250°C. At 300°C and 400°C, the absorption is nil. The results of Fig. 10 show that under the conditions of the test, the mixed oxide of example 2 absorbs well NO up to 250°C. In comparison with the results of Fig. 9, it appears that the absorption is more stable than for the material of ref. 2.

The mixed oxide of the invention is thus tailored to absorb NO_x at 'low' temperature. After a temperature comprised between 250°C and 300°C, the absorption capacity decreases strongly. At this temperature, the SCR catalyst operates well. Moreover, the collapse of the absorption capacity makes it possible to regenerate easily the mixed oxide in comparison with materials containing basic sites for which there is no such collapse at this temperature.

Example 16: determination of the S-uptake and S-release (DeSO_x capacity) Sulfation aging was performed on the powder materials (0.4 g), subsequent to hydrothermal aging treatment (800°C / 1 6 h), under a gas mixture composed of 10.0 vol.% O₂, 10.0 vol.% H₂O, 20 ppm SO₂ and balance of N₂ (total flow rate = 15 L/h) at 300°C for 12 h.

The lean desulfation capacity was then measured by DTA-TGA on a Mettler LIF 1600 Thermobalance coupled to a Mass Spectrometer (what is measured online is the intensity of the signal at m/z=64 corresponding to SO₄). To do so, the samples were analyzed under air, from 100°C up to 1200°C with a temperature ramp of 10°C/min. The results given on Fig. 1 1 show that the mixed oxide of example 2 (with Cu) exhibits better DeSOx properties than the mixed oxide of example A. The mixed oxides of the invention exhibit also better DeSOx properties than the material of Ref. 2. The mixed oxide of example 5 (with Si) provides very good resistance to sulfation. After the sulfation aging, the uptake of S was determined by dosing the elemental sulfur by microanalysis with an Horiba EMIA 320-V2, using iron and LECOCEL as combustion accelerators. The sulfur uptake is expressed in weight percentage based on the total weight of the solid (Table VII). In coherence with the data of Fig. 1 1 , these data confirm that the mixed oxide of example 2 has released more sulfur than Ref. 2.

S uptake

Table VII (PpmJ

Ref 2 7650

Example A 5050

Example 2 6250

Example 5 480

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 °

Claims

1 . Use of 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 (Ce0₂, Al₂0₃, Si0₂, oxide of the BM) with respect to the total weight of the mixed oxide, or a composition comprising said mixed oxide, to absorb NO_x.

2. Use according to claim 1 of wherein the mixed oxide is of generic formula Ce-i- (y₊z₎ (Al or Si)_y (BM)z O_a, y, z and a being defined as follows:

^■ 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 (Ce0₂, Al₂0₃, Si0₂, oxide of the BM) with respect to the total weight of the mixed oxide

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

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

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

6. Use 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. Use 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. Use according to one of the preceding claims wherein the proportion of cerium is 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%.

9. Use according to any one of claims 1 to 5 wherein the mixed oxide is characterized by the following proportions:

^■ 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%; 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. Use according to one of the preceding claims wherein the mixed oxide consists 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 . Use according to claim 1 to 9 wherein the mixed oxide consists 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. Use 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. Use according to any one of claims 1 to 12 wherein the mixed oxide exhibits 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 1 6 hours under a atmosphere composed of 10.0% 0₂ / 10.0% H₂0 / balance of N₂.

14. Use according to any one claims 1 to 13 wherein the mixed oxide exhibits 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:

- an atmosphere A1 composed of 2.7 vol% C½ / 10.0 vol% H₂0 / the balance being N₂ and applied for 90 seconds; then

15. Use according to any one of claims 1 to 14 wherein the mixed oxide exhibits a specific surface area of at least 8 m²/g after the mixed oxide is aged at 800°C for 1 6 hours in an atmosphere which is alternatively:

■ an atmosphere A1 composed of 2.7 vol% C½ / 10.0 vol% H₂O / the balance being N₂ and applied for 90 seconds; then

^■ the cycle of the alternating atmosphere A1 -A2 being repeated for the whole 1 6 hours of the aging.

16. Use according to any one of claims 1 to 15 wherein the mixed oxide exhibits 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. Use according to any one of claims 1 to 1 6 wherein the mixed oxide exhibits 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. Use according to any one of claims 1 to 17 wherein the mixed oxide exhibits 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. Use 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. Use according to any one of claims 1 to 19 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 . Use according to any one of claims 1 to 20 wherein the mixed oxide exhibits a Tmax 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. Use according to any one of claims 1 to 21 wherein the mixed oxide exhibits a T_end lower than 215°C, more particularly lower than 200°C, T_end being the temperature defined by the following characteristics :

^■ T_end is superior to T_max; and

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

23. Use 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. Use according to any one of claims 1 to 23 wherein the average size t of the crystallites of cerium oxide in the mixed oxide are lower than 20 nm, more particularly lower than 19 nm, after the mixed oxide is aged at 800°C for 1 6 hours in a atmosphere composed of 10% 0₂ / 10% H₂0 / balance of N₂.

25. Use 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:

^■ an atmosphere A1 composed of 2.7 vol% 0₂ / 10.0 vol% H₂0 / 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;

26. Method of treatment of an exhaust gas released by the combustion engine of a vehicle by which: (a) the NO_x contained in the exhaust gas are absorbed by the mixed oxide as disclosed in any one of claims 1 to 25 or a composition comprising said mixed oxide; (b) the nitrogen-containing compounds that are released from the mixed oxide or from the composition are converted into N₂.

27. Method according to claim 26 wherein:

- step (a) is performed under lean exhaust conditions and step (b) is performed under rich exhaust conditions ;

- in step (b), the nitrogen-containing compounds that are released are converted into N₂ in the presence of at least one reductant, the chemical conversion into N₂ being catalyzed by at least one reduction catalyst.

28. Method according to claim 26 wherein:

- step (a) is performed when the temperature of the exhaust gas in contact with the mixed oxide or the composition comprising said mixed oxide is T_a and step (b) is performed when the temperature of the exhaust gas in contact with the mixed oxide or the composition comprising said mixed oxide is T_b which is higher than T_a ;

- in step (b), the nitrogen-containing compounds that are released are converted into N₂ in the presence of ammonia, the chemical conversion into N₂ being catalyzed by at least one SCR catalyst.

29. Use of a mixed oxide as disclosed in any one of claims 1 to 25 or a composition comprising said mixed oxide to reduce the amounts of NO_x in an exhaust gas released by the internal combusion engine of a vehicle.

30. Process of absorption of the NO_x contained in an exhaust gas released by the internal combusion engine of a vehicle comprising contacting the exhaust gas with the mixed oxide as disclosed in claims 1 to 25 or with a composition comprising said mixed oxide.