EP2723495A1 - Device for purifying exhaust gases from a heat engine, comprising a catalytic ceramic support comprising an arrangement of essentially identical crystallites - Google Patents

Abstract

The invention relates to a device for purifying exhaust gases from 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 essentially the same size, same isodiametric morphology and same chemical composition, wherein each crystallite is in contact at certain points, or almost in contact at certain points, with surrounding crystallites, and whereon at least one active phase is deposited for the chemical destruction of impurities present in the exhaust gas.

Description

 "Device for cleaning the exhaust gases of a heat engine comprising a catalytic ceramic support comprising an arrangement

 The invention relates to a device for cleaning the exhaust gases of a heat engine, in particular for a motor vehicle, comprising a support on which at least one catalyst is deposited for the chemical destruction of impurities of the exhaust gas, commonly called "catalytic converter". Such a device has the function of removing at least part of the pollutant gases contained in the exhaust gases, in particular carbon monoxide, hydrocarbons and nitrogen oxides, by transforming them by reduction or reduction reactions. 'oxidation.

 In particular, the invention proposes exhaust gas purification devices comprising oxide ceramic supports adapted to heterogeneous catalysis, the structural characteristics of which give higher performances than those of conventional catalyst oxide supports.

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 closest to that of a fully loaded engine is the SMR (Steam Methane Reforming) process in terms of temperature and gaseous compositions (CH ₄ , H ₂ O, CO ₂ , CO ₂₎ . .). This is particularly true for catalytic materials on the choice aspects of the active phases (noble metals, Ni, ...), degradation of the oxide supports and / or active phases, temperature zones (600-1000 ° C.) and in one embodiment. to a certain extent, 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 may be involved not only from a physical point of view (high pore volume and BET surface area to improve the dispersion of the phases active) but also chemical (accelerate for example the dissociation and diffusion of such or such molecules). The catalyst participates in the conversion by returning to its original state at the end of each cycle throughout its lifetime. A catalyst modifies / accelerates the reaction mechanism (s) and the associated reaction kinetics without changing the thermodynamics thereof.

 In order to maximize the conversion rate of 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, reacts at the surface of the catalyst to form a gas of species B.

 The set of elementary steps are:

a) Transport of reagent A (volume diffusion), through a layer of gas, to the external surface of the catalyst,

b) Diffusion of species A (volume or molecular diffusion (Knudsen)), 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 surface of the catalyst e) Desorption of the 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 number of reactive molecules converted into product (s) within a defined time interval is directly related to the accessibility and the number of catalytic sites (s) available. It is therefore necessary to initially increase as much as possible the number of available active sites per unit area. To do this, it is necessary to reduce the size of the metal nanoparticles (from 1.5 to 3 nm) and maximize the dispersion of said active nanoparticles on the surface of the support. In order to reduce the average size of the active phase particles and to maximize the dispersion of the latter, it is necessary to provide a support having itself a maximum specific surface area and a suitable pore volume.

The active species in the context of the automobile depollution reaction and of the steam reforming reaction can be noble metal (s) (Ruthenium, Rhenium, Rhodium, Palladium, Osmium, Iridium, Platinum) or an alloy between one, two or three of these noble metals or a transition metal and a two or three noble metals. Nickel, silver, gold, copper, zinc and cobalt are mentioned as transition metals. The ideal is to disperse nanometric active phases (<5 nm) on the surface of a ceramic support in general. The smaller the catalytic particle, the greater the surface-to-volume ratio and the greater the surface area developed per unit mass (for active phases, MSA: Metallic Surface Area expressed as surface area per unit mass such as m for example, for catalytic ceramic substrates, it is referred to as BET surface and / or porous volume). Another consequence is obviously the reduction of costs, especially that related to the impact of the price of raw materials (noble metals). Control of the process of preparation of the support (s) and its chemical stability must not only maximize the dispersion and the size of the active phase (s) (metal (with the noble (s) associated or not with transition metals) but also reduce the amount of active phase (s) used, and therefore the associated cost, directly related to the prices of raw materials and their availability.

 By definition, a ceramic surface receiving energy (for example heat) will always tend to minimize its energy. The two main barriers to the development of ceramic substrates with high specific surfaces and porous volumes are:

 Sintering, a natural phenomenon appearing in temperature; and

 - The crystalline phase change: a phase change is usually accompanied by a destructuration.

These two phenomena are related to one another and result in a reduction in the specific surface area of the material under consideration, a collapse of the associated pore volume and a redistribution of pore sizes with appearance of macroporosity at the expense of micro and mesoporosity. Take the example of the transformation of alumina γ into alumina a spontaneously occurring above 1100 ° C in air (from 800-900 ° C under conditions SMR). The specific surface area of a γ-alumina can be up to several hundred m ² / g while a standard alumina has a specific surface area of less than about 10 m ² / g. Alumina γ is conventionally used especially in automotive pollution control as a catalytic support stabilized or not with lanthanum, cerium, zirconium ... In all cases however, after a few automobile stop-start cycles, the specific surface of stabilized or non-stabilized gamma alumina collapses inducing / promoting migration of active particles resulting in coalescence of the latter. Catalyst manufacturers deposit larger quantities of noble metals in order to avoid catalytic performance deactivation too quickly in order to minimize the impact related to the degradation of the structural properties of the ceramic support.

 Several ceramic supports with high specific surface area and high pore volume have already been synthesized.

 Silica is the first mesoporous material to have been synthesized in 1992. US2003 / 0039744A1 discloses from the method of self-assembly induced by evaporation how to obtain a mesoporous silica support.

 Documents Crepaldi, E.L. , et al. , Nanocrystallised titania and zirconia mesoporous thin films exhibiting enhanced thermal stability, New Journal of Chemistry, 2003. 27 (1): p. 9-13 and Wong, M.S. and J.Y. Ying, Amphiphilic Templating of Mesostructured Zirconium Oxide, Chemistry of Materials, 1998. (8): p. 2067-2077, describe the synthesis of mesoporous zirconia. As with most mesoporous materials, thermal stability is assured only up to 500 ° C-600 ° C. For higher temperatures, structures collapse by sintering or phase change.

A review by Kaspar, J. et al., Nanostructured materials for advanced automotive-pollution catalysts, Journal of Solid State Chemistry 171 (2003): p 19-29 presents the state of the art in the search for nanostructured materials for optimizing the oxide supports of the TWC: Three Way Catalysts 3-way catalysts in the automotive industry. The synthetic methods identified as the most promising are co-precipitation and frost sol. Catalyst supports three current paths are composed of a mixture of gamma alumina generally (γ-Α1 ₂ 0 _3), ceria (CE02) and zirconia (Zr02). The article concludes on the need to develop new synthesis methods to stabilize nanomaterials under catalytic converter operating conditions. The main problem is the non-stability under operating conditions of the synthesized support materials related to thermal cycling (300-1000 ° C.) and atmosphere containing a mixture of exhaust gases (CO, H ₂ O, NO, N ₂ , C _× H). _y , 0 ₂ , N ₂ 0 ...). It collapsed the specific surface of the oxide support, from 50 to 200 m 2 / g to less than 10 m 2 / g after a few thermal cycles (see Table 1: effect of the calcination temperature on the BET surface). oxides).

On the basis of this, a problem that arises is to provide a device for cleaning the exhaust gases of a thermal engine comprising a catalytic ceramic support having good physicochemical stability under the severe operating conditions (ie amplitude of the changes in temperature and atmosphere modification)

 A solution of the invention is a device for cleaning the exhaust gas 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 of substantially the same size, even Isodiametric morphology and same chemical composition in which each crystallite is in point or near-point contact with crystallites surrounding it, and on which is deposited at least one active phase for the chemical destruction of impurities in the exhaust gas.

Note that the catalytic ceramic support implemented in the purification device according to the invention has the first advantage of developing a large available surface area, typically greater than or equal to 20 m ² / g and up to several hundred m ² /boy Wut. Furthermore, it is stable in terms of specific surface area at least up to 1000 ° C under an atmosphere containing an exhaust gas mixture (CO, H ₂ O, NO, N ₂ , C _x H _y , O ₂ , N ₂ 0 ...).

Figure la) schematically shows a catalytic support according to the state of the art. It is more precisely a mesoporous structure. Figure lb) schematically shows a catalytic support implemented in the purification device according to the invention. In this figure each crystallite is in contact with 6 other crystallites in a plane (ie compact stack).

 Depending on the case, the catalytic ceramic support implemented in the purification device according to the invention may have one or more of the following characteristics:

 the crystallite arrangement is a hexagonal compact or cubic face-centered stack in which each crystallite is in point or almost point contact with at most 12 other crystallites in a 3-dimensional space.

said arrangement is made of alumina (Al ₂ O ₃ ), or of cerine (CeO ₂ ) stabilized or not with gadolinium oxide, or of zirconia (ZrO ₂ ) stabilized or not with yttrium oxide or in phase spinel or lanthanum oxide (La ₃ O ₃ ) or in a mixture of one or more of these compounds.

 the crystallites are of substantially spherical shape.

 the crystallites have a mean equivalent diameter of between 2 and 20 nm, preferably between 5 and 15 nm.

 said support comprises a substrate and a film on the surface of said substrate comprising said arrangement of crystallites.

 said ceramic support comprises granules comprising said arrangement of crystallites.

 the granules are of substantially spherical shape.

 The catalytic ceramic support implemented in the purification device according to the invention can be deposited (washcoated) on a ceramic and / or metal substrate optionally coated with ceramic of various architectures such as honeycomb structures, barrels, monoliths. , honeycomb structures, spheres, multi-scale structured reactors-reactors (reactors) ...

 The present invention also relates to a method of 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 gasoline or diesel engine.

We will now see in detail how are synthesized catalytic ceramic supports implemented in the purification device according to the invention. According to a first synthetic process, the following steps are carried out to synthesize the catalytic ceramic support:

 a) Preparation of a sol comprising salts of nitrate and / or aluminum carbonate and / or magnesium and / or cerium and / or zirconium and / or yttrium and / or gadolinium and / or lanthanum a surfactant and solvents such as water, ethanol and ammonia; b) Soaking a substrate in the soil prepared in step a);

 c) drying the soil impregnated substrate to obtain a gelled composite material comprising a substrate and a gelled matrix; and

 d) Calcining the gelled composite material of step c) at a temperature between 500 ° C and 1000 ° C, preferably between 700 ° C and 900 ° C, more preferably at a temperature of 900 ° C.

 Preferably the substrate used in this first synthesis process is dense alumina or cordierite or mullite or silicon carbide.

 According to a second synthesis method, the following steps are carried out to synthesize the catalytic ceramic support:

 a) Preparation of a sol comprising salts of nitrate and / or aluminum carbonate and / or magnesium and / or cerium and / or zirconium and / or yttrium and / or gadolinium and / or lanthanum a surfactant and solvents such as water, ethanol and ammonia; b) Atomization of the soil in contact with a stream of hot air so as to evaporate the solvent and form a micron powder;

 c) calcination of the powder at a temperature between 500 ° C and 1000 ° C, preferably between 700 ° C and 900 ° C, more preferably at a temperature of 900 ° C.

 The two methods of synthesis of the catalytic ceramic supports, mentioned above, may have one or more of the following characteristics:

 the soil prepared in step a) is aged in a ventilated oven at a temperature of between 15 and 35 ° C. for a period of 24 hours.

 - Step d) calcination is carried out under air and for a period of 4 hours.

 The soil prepared in the two processes for synthesizing the ceramic supports mentioned above preferably comprises four main constituents:

Inorganic precursors: for reasons of cost limitation, it has been chosen to use nitrates of magnesium, aluminum, cerium, zirconium, yttrium, gadolinium, lanthanum. The stoichiometry of these nitrates can be verified by Induced Coupled Plasma (ICP), before their solubilization in osmosis water. Any other chemical precursor (carbonate, chloride, etc.) can be used in the production process.

 - The surfactant otherwise called surfactant. It is possible to use a Pluronic F 127 triblock copolymer of the EO-PO-EO 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.

 An example of molar ratios between these different constituents is given in the table below (Table 1):

The soil preparation process is described in Figure 2.

 In the following paragraph the quantities between -parents correspond to a single example.

 The first step is to solubilize the surfactant (0.9g) in absolute ethanol (23 mL) and in an ammoniacal solution (4.5 mL). The mixture is then refluxed for 1 hour. Then, the nitrate solution previously prepared (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, soaking consists in immersing a substrate in the soil and removing it at a constant speed. The substrates used in the context of our study are sintered alumina plates at 1700 ° C. for 1 h 30 in air (relative density of the substrates = 97% relative to the theoretical density). 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:

with K deposition constant depending on the viscosity and 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 spraying fine droplets of soil 3, in a chamber 4 in contact with a stream of hot air 2 in order to evaporate the solvent. The powder obtained is entrained 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" brand Buchi.

 The powder recovered after the atomization is dried in an oven at 70 ° C and then calcined.

 Also, in both processes, the precursors, i.e. in this example magnesium and aluminum nitrate salts, are partially hydrolysed (Equation 2).

Evaporation of the solvents (ethanol and water) allows the gel solids to cross-link around the surfactant micelles by forming bonds between the hydroxyl group of one salt and the metal of another salt (Equations 3 and 4). .

The control of these reactions related to the electrostatic interactions between the inorganic precursors and the surfactant molecules allows a cooperative assembly of the organic and inorganic phases, which generates micellar aggregates of surfactants of controlled size within an inorganic matrix.

Indeed, the surfactants used, nonionic, are copolymers which have two parts of different polarities: a hydrophobic body and hydrophilic ends. These copolymers are part of the family of block copolymers consisting of poly (alkylene oxide) chains. An example is the copolymer (EO) n- (PO) m- (EO) n, constituted by the chain of polyethylene oxide (EO), hydrophilic at the ends and in its central part propylene oxide (PO), hydrophobic. The polymer chains remain dispersed in solution at a concentration below the critical micelle concentration (CMC). CMC is defined as the limiting concentration beyond which occurs the phenomenon of self-arrangement of surfactant molecules in the solution. Beyond this concentration, the chains of the surfactant tend to be grouped by hydrophilic / hydrophobic affinity. Thus, the hydrophobic bodies are grouped together and form spherical micelles. The ends of the polymer chains are pushed outwardly of the micelles, and associate during the evaporation of the volatile solvent (ethanol) with the ionic species in solution which also have hydrophilic affinities.

 This phenomenon of self-arrangement occurs during c) drying stages of the synthesis processes of the ceramic supports mentioned above.

 Let us now consider the advantages of a calcination at a temperature of between 500 ° C. and 1000 ° C.

 Firstly, the substrate coated with a thin film was calcined under air at 500 ° C. for 4 hours, with a temperature rise rate of 1 ° C./min.

The sample is observed using a high-resolution scanning electron microscope (SEM-FEG) and an Atomic Force Microscope (AFM). The Atomic Force microscope allows to account for the surface topography of a sample with an ideally atomic resolution. The principle consists in sweeping the surface of the sample with a tip whose end is of atomic dimension, while measuring the interaction forces between the end of the tip and the surface. By keeping the interaction constant, it is possible to measure the topography of the sample.

 The AFM images produced over a surface of 500 nm (FIG. 4) and the SEM-FEG micrographs (FIG. 5) reveal the formation of a mesostructured deposit at this calcination temperature. Figure 4a) is a topography image while Figure 4b is an auto-correlation image.

 The mesostructuration of the material is due to a progressive concentration within the deposition of the aluminum and magnesium precursors, as well as the surfactant, to a micellar concentration greater than the critical concentration, which results from the evaporation of the solvents.

On the other hand, at this calcination temperature (500 ° C.-4h), the spinel phase is not completely formed and the compound is amorphous (FIG. 6). The diffractogram was made on powder obtained by atomization of the soil. Therefore, we chose to increase the calcination temperature of materials at 900 ° C.

At this temperature, the spinel phase (MgAl ₂ O ₄ ) is perfectly crystallized (FIG. 7). Calcination at 900 ° C destroys the mesostructuration of the deposit which was present at 500 ° C. Crystallization of the spinel phase results in local disorganization of the porosity. This nevertheless results in a catalytic ceramic support implemented in the purification device according to the invention, in other words an ultra-divided and highly porous deposit with quasi-spherical particles in point or almost punctual contact with each other ( Figure 8). FIG. 8 corresponds to 3 SEM-FEG micrographs of the catalytic support with 3 different magnifications.

 These particles display a very narrow particle size distribution centered on 12 nm (average size of spinel crystallites measured by X-ray diffraction at small angles, Figure 9). This size corresponds to that of the elementary particles observed in scanning electron microscopy indicating that the elementary particles are mono-crystalline.

 X-ray diffraction at small angles (values of the angle 2Θ 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) in the center of which the sample is positioned. The latter is a monocrystalline sapphire substrate on which was deposited by soaking-drawing the soil. The Scherrer formula makes it possible to connect the width at half height of the diffraction peaks to the size of the crystallites (Equation 5).

 Equation 5:

 D is the size of the crystallites (nm)

λ is the wavelength of the Cu Ka line (1.5406 Å)

β corresponds to the width at mid-height of the line (in rad)

 Θ corresponds to the diffraction angle.

The atomization of the soil, followed by calcination of the powder at 900 ° C., produces spherical granules with a diameter of less than 5 μm and preferably in a range of between 10 nm and 2 μm (FIG. 10). The microstructure of this powder is identical to that obtained on the deposit, namely an ultra-divided and porous micro structure with a crystallite size of the same order of magnitude.

The specific surface area of the powder, measured by the BET method, is 50 m ² / g.

The morphology of the powder was compared with that of a spinel phase powder of the trade name Puralox MG30, supplied by Sasol (FIG. 11). This powder has a specific surface area of 30 m ² / g.

 The particles of the commercial powder are not spherical and their particle size distribution is wide, which will potentially promote a particle enlargement (physical deactivation) during aging under automotive conditions (temperature between 300 and 1000 ° C, stop-start cycles, specific atmosphere).

Catalytic ceramic substrates obtained by dipping the soil on a substrate, in other words comprising a substrate and a film, as well as the catalytic ceramic supports obtained by atomization of the soil, in other words comprising granules, have been aged under the operating conditions of the catalytic converters, that is to say a temperature of 900 ° C for 100 h under an atmosphere containing a mixture of exhaust gases (CO, H ₂ 0, NO, N ₂ , C _x H _y , O ₂ , N ₂ O ...).

 The ultra-divided micro-structure of deposits calcined at 900 ° C evolves little during aging (Figure 12). The very high homogeneity of size, morphology and chemical composition as well as ultra-division (ie limited number of contacts between particles) considerably limit the local chemical potential gradients that constitute the driving force of the migration of species responsible for sintering. . The conservation of particle size was confirmed by the X-ray diffraction results at small angles (Figure 13). Indeed, the size of the single crystal elementary particles measured by this technique is 14 nm after aging (gray curve). It was 12nm before aging (black curve). No collapse of the structure was observed.

 The specific surface of the aged powder is 41 m 2 / g, thus showing a very low abatement of the specific surface area.

The described example (spinel support) with the associated production methods can be extended to other families of ceramic support such that said support is alumina (Al ₂ O ₃ ), or cerine (CeO ₂ ) stabilized or no to gadolinium oxide, or zirconia (ZrO ₂ ) stabilized or otherwise with yttrium oxide (such as YSZ 4 and 7-10%) or lanthanum oxide (La ₂ 0 ₃ ) or spinel phase (for example MgAl ₂ 0 ₄ ) or in a mixture of one, or two or three or four of these compounds. It is also possible to mention compounds based on alumina stabilized with cerium and / or zirconium and / or lanthanum at a level of 2-20% by mass. The microstructures obtained are identical to those described in the example detailed above.

Claims

claims

1. Apparatus for cleaning the exhaust gas of a heat engine comprising a catalytic ceramic support having a specific surface greater than or equal to 20 m ² / g and comprising an arrangement of crystallites of the same size, same isodiametric morphology and same composition chemical or substantially the same size, same isodiametric morphology and same chemical composition, the crystallites having a mean equivalent diameter of between 2 and 20 nm, in which each crystallite is in point contact or almost punctual with crystallites which surround it, and on which is deposited at least one active phase for the chemical destruction of impurities from the exhaust gas.

2. Device according to claim 1, characterized in that the arrangement of crystallites is at best a hexagonal stack compact or cubic face-centered in which each crystallite is in point contact or almost punctual with at most 12 other crystallites in a space 3 dimensions.

3. Device according to one of claims 1 or 2, characterized in that said arrangement is alumina (Al ₂ O ₃ ), or cerine (Ce0 ₂ ) stabilized or not stabilized with gadolinium oxide, or zirconia ( Zr0 ₂ ) stabilized or not with yttrium oxide or spinel phase or lanthanum oxide (La ₃ O ₃ ) or magnesium oxide or silica or a mixture of one or more of these compounds.

4. Device according to one of claims 1 to 3, characterized in that the crystallites are substantially spherical shape.

5. Device according to claim 4, characterized in that the crystallites have a mean equivalent diameter of between 5 and 15 nm.

6. Device according to one of claims 1 to 5, characterized in that said support comprises a substrate and a film on the surface of said substrate comprising said arrangement of crystallites.

7. Device according to one of claims 1 to 5, characterized in that said ceramic support comprises granules comprising said arrangement of crystallites.

8. Device according to claim 7, characterized in that the granules are substantially spherical in shape.

9. 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 8.

10. A purification process according to claim 9, characterized in that the engine is a motor vehicle engine, in particular a diesel engine.

11. A purification process according to claim 9, characterized in that the engine is a motor vehicle engine, preferably a gasoline engine.