FR2976823A1 - EXHAUST GAS PURIFYING DEVICE OF A THERMAL MOTOR COMPRISING A CERAMIC SUPPORT AND AN ACTIVE PHASE MECHANICALLY ANCHORED IN THE SUPPORT - Google Patents

Abstract

Apparatus for purifying the exhaust gases of a heat engine comprising: - a catalytic ceramic support (s) comprising an arrangement of crystallites of the same size, same isodiametric morphology and same chemical composition or of substantially the same size, same isodiametric morphology and same chemical composition in which each crystallite is in point or almost punctual contact with the surrounding crystallites, and - an active phase (s) for the chemical destruction of impurities in the exhaust gas comprising metal particles mechanically anchored in said catalytic support in such a way that the coalescence and mobility of each particle are limited to a maximum volume corresponding to that of a crystallite of said catalytic ceramic support.

Description

The invention relates to a device for cleaning the exhaust gases of a heat engine, commonly called a "catalytic converter", in particular for a motor vehicle, comprising a support on which at least one catalyst is deposited for the chemical destruction of The function of such a device is to eliminate, at least in part, the polluting gases contained in the exhaust gases, in particular carbon monoxide, hydrocarbons and nitrogen oxides, by transforming by reduction or oxidation reactions. The invention particularly provides exhaust gas cleaning devices comprising oxide ceramic supports and active metal particles whose structural characteristics and the anchoring of the particles in the support lead to higher performances than those of the catalyst oxide supports. conventional. Synergies between various chemical and petrochemical industrial applications and the operating conditions of an automotive engine have been observed. It can be seen that the process that is closest to that of a fully loaded engine is SMR (Steam Methane Reforming) in terms of temperature and gaseous atmosphere (Hz, CO, CO2, residual CH4, H2O). This is particularly true for catalytic materials on the choice aspects of the active phases (noble metals, Ni, ...), for the degradation mechanisms of the oxide supports and / or the active phases, for the operating temperature zones (600 -1000 ° C) and to a certain extent on space velocities, particularly in the context of SMR structured exchange reactors. The consequence is in particular phenomena of physical degradation (temperature inducing coalescence of nanoparticles, delamination of deposits, ...) very close. A heterogeneous gas-solid catalyst is generally an inorganic material consisting of at least one oxide or non-oxide ceramic support on which is dispersed one or more active phases which convert reagents into products through repeated and uninterrupted cycles of elementary phases (adsorption, dissociation, diffusion, reaction-recombination, diffusion, desorption). In some cases, the support can intervene not only from a physical point of view (high pore volume and high BET surface to improve the dispersion of the active phases) but also chemical (accelerate for example adsorption, dissociation, diffusion and desorption of such and such molecules). The catalyst participates in the conversion by returning to its original state at the end of each cycle throughout its life. A catalyst modifies / accelerates the reaction mechanism (s) and the kinetics of the associated reaction (s) without changing the thermodynamics thereof. In order to maximize the conversion rate by the supported catalysts, it is essential to maximize reagent accessibility to the active particles. In order to understand the interest of a support such as that developed here, let us first recall the main steps of a heterogeneous catalysis reaction. A gas composed of molecules A passes through a catalytic bed and reacts at the surface of the catalyst to form a gas of species B. The set of elementary steps are: a) Transport of the reagent A (diffusion by volume), through a layer of gas, up to the outer surface of the catalyst, b) Diffusion of species A (volume or molecular diffusion (Knüdsen)), through the porous network of the catalyst, to the catalytic surface, c) Adsorption of species A on the catalytic surface, d) Reaction of A to form B on the catalytic sites present on the catalyst surface, e) Desorption of product B from the surface, f) Diffusion of species B through the porous network (g) Transport of product B (volume diffusion) from the outer surface of the catalyst through the gas layer to the gas flow. The European standard EURO 5 applicable since September 1, 2009 (and soon EURO 6 which will be applicable on September 1, 2014) obliges motor vehicle manufacturers to drastically limit the release of toxic gases (CO, NOx, unburnt hydrocarbons). . However, the optimization of catalytic converters is today largely linked to that (efficiency, lifetime) of the catalysts. As a reminder, a catalytic converter consists of a stainless conversion chamber in which the exhaust gas is introduced. These gases pass through a ceramic structure generally consisting of a ceramic honeycomb substrate oxide (cordierite, mullite, ...). On the walls of the ceramic substrate (honeycomb) is deposited a so-called three-way catalyst (TWC: Three Ways Catalysts). The catalyst accelerates the conversion kinetics of the reagents into products. In catalytic converters, the objective is to limit the release of toxic gases (CO, NOx and unburned hydrocarbons) by transforming them mainly into water, COz and nitrogen.

By definition, a 3-way catalyst is capable of performing 3 types of reactions simultaneously: - a reduction of nitrogen oxides to nitrogen and carbon dioxide: 2N0 + 2CO -> Nz + 2COz - an oxidation of carbon monoxides in carbon dioxide: 2C0 + Oz -> 2CO2 and - oxidation of unburned hydrocarbons (HC) to carbon dioxide and water: 4CXHy + (4x + y) 02 -> 4xC0z + 2yH20 Oxidation reactions (requiring pressure partial high oxygen) and reduction (low oxygen partial pressure) add constraints. They require a very precise amount of air to add to the fuel. A lambda probe placed on the exhaust measures the amount of oxygen output. A servo loop makes it possible to control the air / fuel ratio very precisely by keeping it at an ideal value. It should be noted that: - The catalytic converter is only effective from around 250-300 ° C. This is why small trips are problematic. - The following parasitic reaction is likely to occur at high temperatures: 2N0 + CO -> N2O + CO2 The ceramic architectures of catalytic converters for automotive pollution control are generally substrates in honeycombs and are for the most part made of cordierite (2 MgO-2 Al2O3-5 SiO2) or mullite. These architectures develop a low specific surface area (a few m2 / g) with a volume porosity of 20% to 40%. The supports of the classical active phases are oxides: alumina for its low-temperature thermo-chemical stability (800 ° C.), ceria for its redox properties with respect to oxygen and zirconia for its chemical affinity with rhodium. For a long time, specific surface development has been sought from alumina in its forms y, o and O (from 50 to 250 m 2 / g). Since then, supports ceria and zirconia developing from 20 to 100 m2 / g have been made. However, in all cases, the support collapses thermally after a few cycles inducing a drop in the specific surface, a drop in the pore volume and an acceleration of migration / diffusion / coalescence phenomena of the metal nanoparticles. In order to minimize these phenomena of thermal collapse under operating conditions of the oxide supports, the latter have been stabilized by additions of elements such as yttrium, gadolinium, lanthanum, etc. La-Al 2 O 3, CeGdO, ZrYO are thus used. , CeZrYO, ... This limits their thermal collapses but does not minimize the phenomena of migration / sintering of metal particles. The deactivation of the 3-way catalysts has been the subject of numerous studies which do not take into account the mechanical problems of resistance of the cordierite structure (fracture due to vibrations). The deactivation phenomena can be classified according to Figure 1. Reversible deactivation phenomena appear at low temperatures "300 ° C): - Physisorption of products and reagents for example CO2 - Chemisorption of products and reagents (for example, the oxide of suffers on an oxide) The phenomena of deactivation which appear at the high high temperatures (600-1000 ° C) are irreversible, it is often reactions between: - The elements of the oxide (s) supporting the active phases - The noble metals causing the formation of unwanted alloys - The noble metals and oxide support active phases (eg the migration of the Rh3 + ion into a structure y Al2O3) However, as in the case of methane steam reforming process (SMR: Steam Methane Reforming), the phenomena that have the greatest impact on the performance of high temperature catalysts are (i) the sintering of the support oxide of the phase (s) active (s) and (ii) the coalescence of the metal particles of the active phases (phenomenon of diffusion / segregation / coalescence of the nanoparticles), the second phenomenon being accelerated by the first. Therefore, a problem that arises is to provide a device for cleaning the exhaust gas of a heat engine comprising an improved catalyst capable of stabilizing under conditions similar to those encountered during methane steam reforming, nanometric particles of active phases so as to improve its performance. A solution of the invention is a device for cleaning the exhaust gases of a heat engine comprising: a catalytic ceramic support comprising an arrangement of crystallites of the same size, same isodiametric morphology and same chemical composition or substantially the same size, same isodiametric morphology and same chemical composition in which each crystallite is in point or almost punctual contact with the surrounding crystallites, and - an active phase for the chemical destruction of impurities in the exhaust gas comprising anchored metallic particles mechanically in said catalytic support such that the coalescence and mobility of each particle is limited to a maximum volume corresponding to that of a crystallite of said catalytic ceramic support. The first advantage of the proposed solution relates to the ultra-divided mesoporous catalytic ceramic support of active phase (s). Indeed, it develops a large available surface area greater than or equal to 20 m2 / g, due to the size of its nanoscale particles which constitutes it and their respective arrangement. Furthermore, the support is stable under the operating conditions of the catalytic converters; in other words, the support is stable at temperatures of between 600 ° C. and 1000 ° C. in an atmosphere containing a mixture of exhaust gases (CO, H 2 O, NO, N 2, CXH 2, Oz, N 2 O, etc.). This thermal stability is directly related to the microstructure of the synthesized material (arrangement of crystallites of the same size, same isodiametric morphology and same chemical composition or substantially of the same size, same isodiametric morphology and same chemical composition in which each crystallite is in point contact or almost punctual with surrounding crystallites) and associated synthesis method (s). The particular architecture of the catalytic support has a direct influence on the stability of the metal nanoparticles. The arrangement of the crystallites and the porosity makes it possible to develop a mechanical anchoring of said metallic nanoparticles on the surface of the support. In parallel, the excellent dispersion of the active phases thus obtained makes it possible to envisage a significant reduction in the amount of noble metals employed without loss of catalytic performance.

FIG. 2 illustrates the mechanical blocking of the metal particles by the catalytic ceramic support. Firstly, it is clear that the elementary active particles will be at most the size of a support crystallite. Second, their movement under the combined effect of a high temperature and an atmosphere rich in water vapor is still limited to potential wells that represents the space between two crystallites. The arrows represent the only possible movement of the metal particles.

Finally, note that the mechanical blocking produced by the catalytic ceramic support limits the possible coalescence of the active particles. Depending on the case, the device according to the invention may have one or more of the following characteristics: - said arrangement is alumina (Al2O3) stabilized or not stabilized with lanthanum, cerium or zirconium, or cerine (CeO2) stabilized or not gadolinium oxide, or zirconia (ZrO 2) stabilized or non-stabilized with yttrium oxide or spinel phase or lanthanum oxide (La 2 O 3) or a mixture of one or more of these compounds. the metal particles are chosen: (i) between the noble metals chosen from Ruthenium, Rhodium, Palladium, Silver, Osmium, Iridium, Platinum or an alloy between one, two or three of these noble metals, or (ii) between transition metals selected from Nickel, Silver, Gold, Cobalt, and Copper or an alloy between one, two or three of these transition metals, or (iii) between an alloy between one, two or three of these noble metals and one, two or three of these transition metals. the crystallites have a mean equivalent diameter of between 2 and 20 nm, preferably between 5 and 15 nm, and the metal particles have a mean equivalent diameter of between 2 and 20 nm, preferably less than 10 nm. the phase-active crystallite arrangement (s) active is at best a hexagonal compact or cubic face-centered stack in which each crystallite is in point contact or almost punctual with at most 12 other crystallites in a 3-dimensional space . Preferably, the catalytic assembly (substrate + catalyst) used in the purification device according to the invention may comprise a substrate of various architectures such as honeycomb structures, barrels, monoliths, honeycomb structures. bees, spheres, multi-scale structured reaction heat exchangers (μreactors), ... of a ceramic or metallic or metallic nature coated with ceramics, and on which the active phase support (s) can be deposited (washcoat) . The present invention also relates to a process for cleaning the exhaust gas of a heat engine in which said exhaust gas is circulated through a device according to the invention.

The heat engine is preferably a motor vehicle engine, in particular a diesel engine or a gasoline engine. We will now see in detail how is synthesized ceramic support - active phases (catalyst) implemented in the purification device according to the invention. A method of preparing a ceramic support (s) - active phase (s) may comprise the following steps: a) preparation of a catalytic ceramic support comprising an arrangement of crystallites of the same size, even morphology and same chemical composition or substantially the same size, same morphology and same chemical composition in which each crystallite is in point contact or almost punctual with the crystallites that surround it, b) impregnation of the catalytic ceramic support with a precursor solution of the metallic active phases; c) calcining in air the impregnated catalyst at a temperature between 350 ° C and 1000 ° C, preferably at a temperature of between 450 ° C and 700 ° C, still more preferably at a temperature of 500 ° C to obtain an active phase (s) oxidized (s) deposited on the surface (s) support (s) ceramic (s) catalytic (s); and d) optionally reducing the oxidized active phase (s) between 300 ° C and 1000 ° C, preferably at a temperature of between 300 ° C and 600 ° C, even more preferably at a temperature of 300 ° C. Note that this process may comprise one or more of the following characteristics: the impregnation stage b) is carried out under vacuum for a period of between 5 and 60 minutes; in step b), the active phase solution (s) is a solution of rhodium nitrate (Rh (NO 3) 3, 2H 2 O) or a solution of nickel nitrate (Ni (NO 3) 2, 6H 2 O ) or palladium ((Pd (NO3) 3,2 H2O) or platinum ((Pt (NO3M, yH2O)) or a mixture of these solutions, precursors carbonate (s), chloride (s), ... or a mixture of various precursors (nitrates, carbonates, ...) containing noble metals (Rh, Pt, Ir, Ru, Re, Pd) and / or transition metals (Ni, Cu, Co,. ..) - said method comprises after step d) optionally a step e) aging under operating conditions or close to the operating conditions of the catalyst The first operating cycle (stop / start) can be considered as an aging step. The catalytic ceramic support described in step a) of the process for the preparation of the ceramic support assembly - active phase (s) implemented in the purification device according to the invention. The invention can be prepared by two methods. A first method will result in a catalytic ceramic support comprising a substrate and a film on the surface of said substrate comprising an arrangement of crystallites of the same size, same isodiametric morphology and same or substantially the same chemical composition, same isodiametric morphology and same chemical composition in which each crystallite is in point or almost punctual contact with the crystallites which surround it. A second method will result in a catalytic ceramic support comprising granules comprising an arrangement of crystallites of the same size, same isodiametric morphology and same or substantially the same chemical composition, same isodiametric morphology and same chemical composition in which each crystallite is in point contact or almost punctual with the crystallites that surround it. Note that the granules are substantially spherical. The first process for preparing the catalytic ceramic support comprises the following steps: i) Preparation of a sol comprising salts of nitrates and / or carbonates of aluminum and / or magnesium and / or cerium and / or zirconium and and / or yttrium and / or gadolinium and / or lanthanum, a surfactant and solvents such as water, ethanol and ammonia; ii) Soaking a substrate in the soil prepared in step i); iii) drying the soil impregnated substrate to obtain a gelled composite material comprising a gelled film coated substrate; and iv) calcining the gelled composite material of step iii) at a temperature typically between 500 ° C and 1000 ° C in air. Preferably, the substrate used in this first process for preparing the catalytic ceramic support is of dense alumina.

The second method for preparing the catalytic ceramic support comprises the following steps: i) Preparation of a sol comprising salts of nitrates and / or carbonates of aluminum and / or magnesium and / or cerium and / or zirconium and and / or yttrium and / or gadolinium and / or lanthanum, a surfactant and solvents such as water, ethanol and ammonia; ii) Atomizing the soil in contact with a hot air stream so as to evaporate the solvent and form a micron powder; iii) calcination of the powder at a temperature between 500 ° C and 1000 ° C. The soil prepared in the two processes for preparing the catalytic ceramic support preferably comprises four main constituents: inorganic precursors: for reasons of cost limitation, we have chosen to use magnesium and aluminum nitrates, cerium, zirconium, yttrium or a mixture of these nitrate salts. Other inorganic precursors can be used (carbonates, sulfonates, chlorides, ...) alone or mixed in the process. The stoichiometry of the nitrates, in the context of the example, can be verified by ICP (Inductively Coupled Plasma) before their solubilization in osmosis water. 15 - The surfactant otherwise called surfactant. It is possible to use a Pluronic F127 triblock copolymer of the EO-PO-E0 type. It has two hydrophilic blocks (EO) and a hydrophobic central block (PO). - The solvent (absolute ethanol). NH 3 H 2 O (28% by weight). The surfactant is solubilized in an ammoniacal solution which makes it possible to create hydrogen bonds between the hydrophilic blocks and the inorganic species. The first step is to solubilize the surfactant (0.9 g) in absolute ethanol (23 mL) and in an ammoniacal solution (4.5 mL). The mixture is then refluxed for 1 hour. Then, the previously prepared nitrate solution (20 mL) is added dropwise to the mixture. The whole is refluxed for 1 h and then cooled to room temperature. The soil thus synthesized is aged in a ventilated oven whose ambient temperature (20 ° C.) is precisely controlled. In the case of the first synthetic process, dipping involves dipping a substrate into the soil and removing it at a constant rate. The substrates used in the context of our study are sintered alumina plates at 1700 ° C. for 1h30 in air (relative density of the substrates = 97% relative to the theoretical density). This invention applies to substrates of various architectures such as honeycomb structures, barrels, monoliths, honeycomb structures, spheres, multi-scale structured reactor-exchangers (μrefactors), etc. of ceramic or metallic nature, or ceramic coated metal, and on which said support is removable (wash coat).

When removing the substrate, the movement of the substrate causes the liquid forming a surface layer. This layer divides in two, the inner part moves with the substrate while the outer part falls into the container. The progressive evaporation of the solvent leads to the formation of a film on the surface of the substrate. It is possible to estimate the thickness of the deposit obtained as a function of the viscosity of the soil and the drawing speed (Equation 1): Equation 1: e oc x v 2/3 with x deposition constant depending on the viscosity and the density of the soil and the liquid-vapor surface tension. v is the drawing speed. Thus, the higher the pulling speed, the greater the thickness of the deposit.

The quenched substrates are then baked at 30 ° C to 70 ° C for a few hours. A gel is then formed. Calcination of substrates under air eliminates nitrates but also decomposes the surfactant and thus release porosity. In the case of the second synthesis process, the atomization technique makes it possible to transform a sol into a solid dry form (powder) by the use of a hot intermediate (FIG. 3).

The principle is based on the spraying of fine droplets soil 3, in a chamber 4 in contact with a hot air stream 2 in order to evaporate the solvent. The powder obtained is driven by the heat flow 5 to a cyclone 6 which will separate the air 7 from the powder 8. The apparatus that can be used in the context of the present invention is a reference commercial model "190 Mini Spray Dryer by Büchi brand.

The powder recovered after the atomization is dried in an oven at 70 ° C and then calcined. Calcination at 900 ° C destroys the mesostructuration of the deposit which was present at 500 ° C. The crystallization of the phase (spinel in this example) causes a local disorganization of the porosity. This nevertheless results in a catalytic ceramic support according to the invention, in other words an ultra-divided and highly porous deposit with quasi-spherical particles in point contact with each other (FIG. 4). Figure 4 corresponds to 3 high resolution SEM micrographs of the catalytic support with 3 different magnifications.

These particles support phase (s) active (s) of a size of about ten nanometers display a very narrow particle size distribution centered around 12 nm. The average spinel crystallite size in this example is 12 nm (measured by small angle X-ray diffraction, Figure 5). This size corresponds to that of the elementary particles observed in scanning electron microscopy indicating that the elementary particles are monocrystalline. X-ray diffraction at small angles (angle values between 0.5 and 6 °): this technique allowed us to determine the size of the crystallites of the catalyst support. The diffractometer used in this study, based on a Debye-Scherrer geometry, is equipped with a localized curved detector (Inel CPS 120) at the center of which the sample is positioned. The latter is a monocrystalline sapphire substrate on which the soil has been dip-coated. Scherrer's formula makes it possible to relate the width at mid-height of the diffraction peaks to the size of the crystallites (Equation 2). Equation 2: D corresponds to the size of the crystallites (nm) is the wavelength of the Cu Ka line (1.5406 A) R corresponds to the width at mid-height of the line (in rad) 0 corresponds to the diffraction angle. In the process for preparing the catalyst according to the invention, the catalytic ceramic support is then impregnated with a solution of precursor Rh, and / or Pt, and / or Pd and / or Ni. The catalyst studied is the 3-way catalyst for use in catalytic converters. In the case of an active phase comprising rhodium supported by a spinel support (catalyst named AlMg + Rh), the impregnation is carried out under vacuum for 15 minutes. Rh (Rh (NO 3) 3, 2H 2 O) nitrate was retained as the inorganic precursor of Rh. The concentration of Rh in the nitrate solution was set at 0.1 g / L. After impregnation, the catalyst is calcined in air at 500 ° C. for 4 hours. At this point, we have a rhodium oxide deposited on the surface of the ultra-divided mesoporous support. The reduction of the active phase is carried out under Ar-Hz (3% vol) at 300 ° C. for 1 hour. D = 0.9x Pcose In order to observe the size and the metal dispersion at the surface of the support, observations by transmission electron microscopy were made (FIG. 6a). The latter reveal the presence of particles of Rh in the elemental state of a size of the order of one nanometer. These small particles are concentrated around the spinel particles of the support.

After aging simulating the conditions in a catalytic converter of this catalyst (900 ° C., 48 h), the Rh particles coalesce to a size of 5 nm (FIG. 6b). At this point, a Rh particle is stabilized on a spinel support particle, which greatly reduces the possibility of future coalescence of the metal particles during catalyst operation.

In the case of an active phase comprising nickel (catalyst named AlMg + Ni), the impregnation of the support is carried out with a solution of Ni nitrate (Ni (NO3) 2, 6H20). The Ni concentration in this solution can be set at 5g / L. After impregnation, the catalyst can be calcined under air at 500 ° C. for 4 hours and then reduced under Ar-Hz (3% vol) at 700 ° C. for 2 hours.

Results similar to those obtained with the AlMg + Rh catalyst are obtained with the AlMg + Ni catalyst. In the case of active phase (s) comprising rhodium, platinum and palladium (catalyst named AlMg + RhPtPd), the impregnation of the support is carried out with a solution of nitrates containing said elements).

It should be noted in the case of the ultra-divided mesoporous ceramic support, the study conducted solely on the spinel MgAl204. The 2 methods of synthesis of the support described can be for example extrapolated to gadolinium-doped or non-doped ceria, or else to zirconia doped or not with yttrium oxide. The stability over time of a catalyst according to the invention has been achieved.

The AlMg + Rh catalyst was aged for 20 days while being subjected to a temperature of the order of 650 ° C. and another sample was subjected to a temperature of the order of 850 ° C. The microstructure of the catalysts at the end of aging was observed by scanning electron microscopy. The images being similar for the two temperatures, we will present the characterizations of the catalysts subjected to aging at 850 ° C (Figure 7).

The atmospheres are very close to those of catalytic converters.

The ultra-divided spinel phase support (catalytic ceramic support) is preserved after aging and the magnification of the spinel particles is very limited. With regard to the metal particles, the size of the metal particles after aging generally remains less than or equal to the size of the elemental crystallites of the spinel support. The interest of developing an ultra-divided support to promote mechanical anchoring of the active phases is demonstrated on these micrographs (Figure 7a)). In fact, in this figure, we see that the metallic dispersion is better on the ultra-divided deposit than on a grain of alumina not covered with deposit, present on the left in the photograph. In places where there is no deposit, it is impossible to mechanically anchor metal particles, the coalescence is natural. Therefore, it will be possible to preferably use the catalyst according to the invention for 3-way catalysis (TWC: Three Way Catalysts) in catalytic converters for automotive pollution control. In the context of this study, the reaction concerns the pollution control of exhaust gases. This invention can be extended to various applications in heterogeneous catalysis by adaptation of the active phase (s) to the desired catalytic reaction (SMR, chemical, petrochemical, environmental reactions, etc.) on a catalytic ceramic support. ultra-divided based spinel, alumina, ceria, zirconia (stabilized with yttrium or not) or a mixture of these compounds.

Claims (8)

REVENDICATIONS1. Apparatus for purifying the exhaust gases of a heat engine comprising: - a catalytic ceramic support (s) comprising an arrangement of crystallites of the same size, same isodiametric morphology and same chemical composition or of substantially the same size, same isodiametric morphology and same chemical composition in which each crystallite is in point or almost punctual contact with the surrounding crystallites, and - an active phase (s) for the chemical destruction of impurities in the exhaust gas comprising metal particles mechanically anchored in said catalytic support in such a way that the coalescence and mobility of each particle are limited to a maximum volume corresponding to that of a crystallite of said catalytic ceramic support. 2. Device according to claim 1, characterized in that said arrangement is alumina (Al2O3), or cerine (CeO2) stabilized or not stabilized with gadolinium oxide, or zirconia (ZrO2) stabilized or not with the oxide of yttrium or spinel phase or lanthanum oxide (La203) or a mixture of one or more of these compounds. 3. Device according to one of claims 1 or 2, characterized in that the metal particles are chosen: (i) between the noble metals selected from Ruthenium, Rhodium, Palladium, Osmium, Iridium, platinum or an alloy between one, two or three of these noble metals, or (ii) between the transition metals selected from nickel, silver, gold, cobalt, and copper or an alloy between one, two or three of these transition metals or (iii) between an alloy of one, two or three of these noble metals and one, two or three of these transition metals. 4. Device according to one of claims 1 to 3, characterized in that the crystallites have a mean equivalent diameter of between 2 and 20 nm, preferably between 5 and 15 nm, and the metal particles have a mean equivalent diameter included between 2 and 20 nm, preferably less than 10 nm. 5. Device according to one of claims 1 to 4, characterized in that the arrangement of crystallites is at best a compact hexagonal or cubic face-centered stack in which each crystallite is in point contact or almost punctual with at most 12 other crystallites in a 3-dimensional space. 6. Process for cleaning the exhaust gas of a heat engine in which said exhaust gas is circulated through a device according to one of claims 1 to 5. 7. Purification process according to claim 6, characterized in that the engine is a motor vehicle engine, in particular a diesel engine. 8. Purification process according to claim 6, characterized in that the engine is a motor vehicle engine, preferably a gasoline engine.