EP2419211A1

EP2419211A1 - Honeycomb catalyst substrate and method for producing same

Info

Publication number: EP2419211A1
Application number: EP10723674A
Authority: EP
Inventors: Philippe Auroy; Ahmed Marouf; Damien Philippe Mey
Original assignee: Saint Gobain Centre de Recherche et dEtudes Europeen SAS
Current assignee: Saint Gobain Centre de Recherche et dEtudes Europeen SAS
Priority date: 2009-04-16
Filing date: 2010-04-14
Publication date: 2012-02-22
Also published as: CN102395429A; US20120021895A1; KR20110138241A; JP2012523954A; MX2011010797A; WO2010119226A1

Abstract

The invention relates to a catalyst substrate made of a porous inorganic material for treating exhaust gas, having a honeycomb structure, one of the surfaces of the structure enabling the intake of exhaust gas to be treated and the other surface enabling the discharge of the treated exhaust gas, and comprising, between said intake and discharge surfaces, an assembly of pipes or adjacent channels with parallel axes therebetween, separated by porous walls, said substrate being coated on at least a portion of the inner surface thereof with at least one polymer or a vinylpyrrolidone copolymer.

Description

CATALYST SUBSTRATE IN HONEYCOMB AND METHOD THEREOF

OBTAINING

The invention relates to the field of catalyst substrates made of porous inorganic material for the treatment of exhaust gases, in particular from internal combustion engines, particularly motor vehicles, for example diesel engines. These substrates have a honeycomb structure, one of the faces of the structure allowing the admission of the exhaust gas to be treated and the other side the evacuation of the treated exhaust gases, and comprise, between these faces, intake and discharge a set of ducts or adjacent channels axes parallel to each other, separated by porous walls. The channels may be alternately closed at one or the other end of the structure so as to allow the filtration of soot or particles contained in the exhaust gas. A filtering structure commonly known as a particle filter is thus obtained.

Some inorganic materials, such as for example aluminum titanate (Al ₂ TiO ₅ ) or cordierite, have up to temperatures of about 800 ^° C. a very low thermal expansion. This advantageous characteristic is due to the presence of microcracks in the ceramic grains. During heating, the intrinsic expansion of the material leads first to close the microcracks, but without macroscopic expansion of the substrate. Thanks to this low thermal expansion, it is possible to use substrates or monolithic filters, that is to say made of a single ceramic block.

However, the deposition of catalytic coatings on the surface of the porous walls of the honeycombs, generally leads to closing these microcracks, so that the thermal expansion of the substrate or the filter is increased. The presence of the catalyst will indeed prevent microcracks to close.

Several solutions have been proposed to this problem, but none are without drawbacks. These solutions consist in depositing on the surface of the substrate polymeric compounds before the deposition of the catalytic coating, a technique described as "passivation".

US application 2006/183632 thus proposes to passivate the surface of the substrate with the aid of gelatin or copolymers of vinyl alcohol with vinylamines or vinylformamides. Crosslinking agents are usually added. The passivation layer is then calcined at the same time as the catalytic coating. This solution, however, leads to a low affinity of the catalytic coating for the substrate, and therefore reduces the amount of catalyst that can be fixed on the substrate. In addition, the calcination of crosslinking agents generates gaseous effluents often toxic that must be reprocessed. The application DE 10 2007 023120 proposes to deposit silanes which will be converted into silicones by crosslinking. However, the decomposition of silicones during calcination generates a lot of gaseous effluents and creates silica which closes the microcracks, resulting in an increase in the coefficient of thermal expansion.

The object of the invention is to obviate these various disadvantages by proposing a passivation method that is more respectful of the environment. Another goal of the invention is to obtain a better affinity (before and after calcination) between the substrate or the passivation layer and the catalytic coating that is deposited after the passivation step. An object of the invention is also to limit the increase in the coefficient of macroscopic expansion of the substrate provided with its catalytic coating.

For this purpose, the subject of the invention is a catalyst substrate made of porous inorganic material for the treatment of exhaust gases, the structure of which is in honeycomb, one of the faces of the structure allowing the admission of the gases of exhaust to be treated and the other face the evacuation of the treated exhaust gases, and comprises, between these intake and discharge faces, a set of adjacent ducts or channels of axes parallel to each other, separated by porous walls, said substrate being coated on at least a portion of its inner surface with at least one vinylpyrrolidone polymer or copolymer.

The subject of the invention is also a process for obtaining a porous inorganic material catalyst substrate according to the invention, comprising a step in which a polymer or copolymer of vinylpyrrolidone is deposited on said substrate, followed by a drying step.

The use as a passivation material of polyvinylpyrrolidone (PVP) -based polymers has several advantages.

No crosslinking agent or polymerizing agent is necessary because these polymers self-crosslink during drying. The process is therefore faster and less expensive, and also more environmentally friendly since it leads to the use of non-toxic products and reduces the problems of gaseous effluents during calcination. The chemical affinity between the catalytic coating and the substrate is further improved over the solutions of the prior art. This better affinity makes it possible subsequently to fix a greater surface quantity of catalyst and to obtain a more homogeneous catalytic coating (washcoat), better distributed on the surface, and thus a greater catalytic efficiency for the same substrate surface.

The polymers based on polyvinylpyrrolidone are particularly suitable for passivating a substrate on which a catalytic coating would then be deposited which, after calcination, crystallites of very small size, in particular of size less than 20 nm, in order to increase the catalytic performance of this coating. This type of coating, for example deposited as boehmite, indeed has the disadvantage of infiltrating easily into the microcracks of the substrate.

Polyvinylpyrrolidone-based polymers have also been found to be better passivating materials than those known from the prior art. Deposited on the substrate before depositing a catalytic coating, they make it possible to limit the increase in the coefficient of thermal expansion due to the infiltration of the catalyst into the microcracks of the ceramic grains of the support. The channels are preferably alternately closed at one or the other end so as to allow the filtration of soot or particles contained in the exhaust gas. The resulting substrate is then a particulate filter with a catalytic component, allowing for example the removal of pollutants of the type NO _x , carbon monoxide (CO) or unburned hydrocarbons (HC). The porous inorganic material is preferably selected from aluminum titanate, cordierite and mullite. Other materials may also be used, such as silicon carbide or sintered metals. By the term "aluminum titanate" is meant not only the aluminum titanate itself, of the formula Al ₂ TiO ₅ , but also any material based on aluminum titanate, in particular any material comprising at least one less than 70%, even 80% and even 90% of an aluminum titanate phase, the titanium and aluminum atoms possibly being partially substituted, in particular by silicon, magnesium or zirconium atoms. By way of example, the aluminum titanate may contain a minority phase of the mullite type, as taught in the application WO 2004/011124, or of the feldspar type, as taught in the application EP 1 559 696. Examples of materials are also given in WO 2009/156652, WO 2010/001062, WO 2010/001064, WO 2010/001065 or WO 2010/001066.

The vinylpyrrolidone polymer or copolymer is preferably chosen from polyvinylpyrrolidone, copolymers of vinylpyrrolidone and of vinyl acetate, copolymers of vinylpyrrolidone and vinylimidazole or copolymers of vinylpyrrolidone and vinylcaprolactam, or any of their mixtures. Preferably, no crosslinking agent is added.

The substrate according to the invention may also be coated on at least part of its internal surface with at least one silane compound, in particular of the silane type comprising at least one carbon chain which has at least one nucleophilic group. This compound is generally deposited at the same time as the polymer or copolymer of vinylpyrrolidone. It allows a better grafting of the polymer or copolymer of vinylpyrrolidone on the porous ceramic substrate. When adding silane, the alkoxide groups of the latter are hydrolyzed by the hydroxyl groups present on the surface of the substrate and bind to this surface. Silanes which comprise at least one carbon chain having at least one nucleophilic group can bind the other end of the grafted silane to the polymer or copolymer of vinylpyrrolidone by reaction with the carbonyl groups thereof. The silane comprising at least one carbon chain which has at least one nucleophilic group is in particular of the Nu-Ri-Si- (OR 2) 3 type in which R 1 and R 2 are alkyl radicals and the nucleophilic group Nu can be chosen from the NH 2 groups. , SH, OH. The silane may be added in the aqueous solution of polymer or copolymer or in a mixture of water and alcohol to promote its dispersion and limit its hydrolysis.

The polymer or copolymer of vinylpyrrolidone is preferably deposited by impregnation with a liquid solution or dispersion, in particular aqueous dispersion. The weight content of polymer or copolymer of vinylpyrrolidone in the solution or dispersion is advantageously between 1 and 30%, preferably between 5 and 15%. The average molar mass of the polymer or copolymer of vinylpyrrolidone, especially at the time of deposition, is preferably between 10,000 and 1,000,000 g / mol, especially between 15,000 and 500,000 g / mol, or between 15,000 and 400,000 g / mol, or between 15,000 and 300,000 g / mol or even between 20,000 and 100,000 g / mol. These different parameters, weight content in the solution or the dispersion, average molar mass, make it possible to adjust the viscosity of the solution or dispersion, and thus the penetration of the polymer in the microcracks of the substrate. It has been observed that for high molar masses, typically of 1,000,000 or more, the The amount of catalytic coating that can subsequently be attached to the substrate decreases substantially. The average molar mass of the polymer or copolymer of vinylpyrrolidone is therefore preferably less than 1,000,000 g / mol.

The impregnation may be carried out in particular by soaking the substrate and / or vacuum impregnation. In the latter case, the substrate can be placed in a desiccator under a pressure of 25 mbar or less and the polymer solution or dispersion deposited on the substrate.

After impregnation, the excess of solvent, in particular water, can be removed, for example by blowing a gas such as air, or by applying a vacuum at one end of the substrate, for example a pressure of less than 100 mbar.

In order to optimize the adhesion to the substrate of the catalytic coating, the drying step is preferably carried out at a temperature of at least 100 ^° C., in particular between 130 and 170 ^° C., or even between 130 and 160 ^° C. For lower temperatures, the adhesion of the polymer to the substrate is reduced. The polymer is more soluble in water and may dissolve upon deposition of the catalytic coating. Too high temperatures, especially greater than 180 ⁰ C or 190 ⁰ C, may stiffen the polymer and create mechanical stresses within the substrate, particularly during the deposition of the catalytic coating. It has furthermore been observed that these high drying temperatures have the effect of reducing the amount of catalytic coating that can be subsequently fixed on the substrate.

The substrate according to the invention is preferably coated on at least a part of its surface with a catalytic coating. This coating is deposited on the surface of the walls of the substrate or filter after the passivation step. It preferably comprises a base material and a catalyst. The base material is usually an inorganic material with a high specific surface area

(typically of the order of 10 to 100 m ² / g) ensuring the dispersion and stabilization of the catalyst. The base material is advantageously chosen from alumina, zirconia, titanium oxide, rare earth oxides such as cerium oxide and alkali or alkaline earth metal oxides. The catalyst is preferably based on a noble metal, such as platinum, palladium or rhodium, or based on transition metals.

The size of the base material particles on which the catalyst particles are arranged is generally of the order of a few nanometers to a few tens or exceptionally a few hundred nanometers.

The process according to the invention is therefore preferably followed by a step of depositing a catalytic coating and then a calcination step, typically in air and between 300 and 900 ^° C., preferably between 400 and 600 ^° C. The invention also relates to a catalyst substrate that can be obtained by this preferred method. Before calcination, the substrate according to the invention has at its surface a polymer layer (the polymer or copolymer of vinylpyrrolidone). This polymer layer is removed during calcination. Its presence, however, makes it possible to obtain a calcined substrate different from the substrates known from the prior art.

The polymer layer may in particular be identified, before calcination, according to the following two methods: by thermogravimetric analysis coupled to a mass spectrometer to identify the decomposition products of the deposited polymer, by extraction, for example by leaching, followed by chromatographic analysis possibly coupled to a mass spectrometer.

The catalytic coating is typically deposited by impregnating a solution comprising the base material or its precursors and a catalyst or precursor thereof. In general, the precursors used are in the form of salts or organic or inorganic compounds, dissolved or suspended in an aqueous or organic solution. The impregnation is followed by a thermal calcination treatment aimed at obtaining the final deposition of a solid and catalytically active phase in the porosity of the substrate or of the filter.

Such methods, as well as the devices for their implementation, are for example described in the patent applications or patents US 2003/044520, WO 2004/091786, US 6,149,973, US 6,627,257, US 6,478,874, US 5,866,210, US 4,609,563, US 4,550,034, US 6,599,570, US 4,208,454 or US 5,422,138.

Catalyst substrates or catalytic filters according to the invention can be used in the exhaust line of an internal combustion engine, typically a diesel engine. To do this, the catalyst substrates or catalytic filters can be wrapped in a fibrous mat and then inserted into a metal casing, frequently called "canning". The fibrous mat is preferably formed of inorganic fibers to impart the thermal insulation properties required by the application. The inorganic fibers are preferably ceramic fibers, such as alumina fibers, mullite, zirconia, titanium oxide, silica, silicon carbide or nitride, or glass fibers, such as glass R. These fibers can be obtained by fiberizing from a bath of oxides, fusion, or from a solution of organometallic precursors

(sol-gel process). The fibrous mat is preferably non-intumescent. It is advantageously in the form of a needle felt.

The invention is illustrated in a nonlimiting manner by the following examples. All percentages are percentages by weight.

Porous aluminum titanate substrates are obtained by the method described hereinafter.

In a preliminary step, aluminum titanate is prepared from the following raw materials:

- about 40% by weight of alumina with a purity level Al2O3 higher than 99.5% and a median diameter d ₅ o of 90 microns, commercialized under the reference AR75 ® by Pechiney, - about 50% by weight rutile titanium oxide having more than 95% TiO 2 and about 1% zirconia, and having a median diameter dso of about 120 microns, marketed by the company Europe Minerais,

- about 5% by weight of silica with a purity level in SiO 2 more than 99.5% and a median diameter d ₅ o of the order of 210 .mu.m, marketed by Sifraco,

approximately 4% by weight of a magnesia powder with an MgO purity level of greater than 98% and of which more than 80% of particles having a diameter of between 0.25 and 1 mm, sold by the company Nedmag.

The mixture of the initial reactive oxides is melted in an electric arc furnace, under air, with a step oxidizing electric. The molten mixture is then cast into CS mold so as to obtain rapid cooling. The product obtained is crushed and sieved to obtain powders of different size fractions. More specifically, the grinding and the sieving are carried out under conditions allowing the final obtaining of two particle size fractions:

- a particle size fraction is characterized by a median diameter d ₅ o substantially equal to 50 microns, denoted by the term fat fraction,

- a particle size fraction being characterized by a median diameter d ₅ o substantially equal to 1.5 microns, referred to as the fine fraction.

For the purpose of the present description, the median diameter d ₅ o denotes the diameter of the particles, measured by sedigraphy, below which 50% of the volume of the population is located.

The microprobe analysis shows that all the grains of the melt phase thus obtained have the composition, in percentage by weight of the oxides, reproduced in Table 1:

Table 1

The grains thus obtained are then used to make monoliths (substrates) raw.

In a kneader, powders are mixed according to the following composition:

100% of a mixture of two aluminum titanate powders previously produced by electrofusion, approximately 75% of a first powder with a median diameter of 50 μm and 25% of a second powder with a median diameter of 1.5 μm.

We add, with respect to the total mass of the mixture:

- 4% by weight of an organic binder of the cellulose type,

15% by weight of a blowing agent,

5% of plasticizer derived from ethylene glycol,

- 2% lubricant (oil), - 0.1% surfactant,

approximately 20% of water so as to obtain, according to the techniques of the art, a homogeneous paste after kneading whose plasticity allows the extrusion through a die of a honeycomb structure which after cooking presents the dimensional characteristics according to Table 2.

The green microwave monoliths are then dried for a time sufficient to bring the water content not chemically bound to less than 1% by weight. The channels of both ends of the monoliths are closed according to well-known techniques, for example described in US Pat. No. 4,557,773 and with a mixture corresponding to the following formulation:

100% of a mixture of two aluminum titanate powders previously produced by electrofusion, approximately 66% of a first powder with a median diameter of 50 μm and 34% of a second powder with a median diameter of 1.5 μm .

1.5% of cellulose-type organic binder, 21.4% of porogen, 0.8% of dispersant based on carboxylic acid,

approximately 55% water so as to obtain a mixture capable of closing the monoliths, one channel out of two.

The characteristics of the monoliths (substrates), after cooking under air until reaching a temperature of 1450 ⁰ C maintained for 4 hours, are shown in Table 2 below:

Table 2

The porosity characteristics were measured by high-pressure mercury porosimetry analyzes carried out using a Micromeritics 9500 porosimeter. The monoliths are then impregnated by immersion in a solution containing the polymer, and then dried. In the case of Comparative Examples C1 to C5, the polymer used is a polyvinyl alcohol marketed by Celanese Corporation under the reference Celvol 205. Its hydrolysis rate is greater than 88%. In the case of Comparative Examples C4 and C5, the polymer is crosslinked using citric acid.

Comparative example C6 corresponds to a non-passive monolith (thus without depositing any polymer).

In the case of Examples 1 and 2, the polymer is a polyvinylpyrrolidone with an average molar mass of 58000 g / mol.

In the case of Examples 3 to 7, the polymer is a polyvinylpyrrolidone having an average molar mass of

30000 g / mol. The solution used is marketed by BASF under the reference Luvitec K30. For the example

4, the solution is brought to a pH of 10 by adding NaOH.

Table 3 below shows:

the times and drying temperature, denoted respectively t and T, the concentration of the impregnation solution, denoted C and expressed as a percentage by weight of polymer relative to the quantity of solution,

the quantity of polymer (passivating material) actually deposited, in mass percentage, denoted Q,

the water intake after passivation, denoted by P, expressed as a percentage by mass,

the alumina uptake, denoted A, expressed as a percentage by mass, the average coefficient of thermal expansion of the substrate provided with its catalytic coating, denoted CTE and expressed in ^10-6 / ° C.

The water intake after passivation makes it possible to estimate the amount of catalyst that can be fixed to the substrate, and therefore the affinity between the substrate and the future catalytic coating. The measurement method involves immersing the passive substrate in water and then submitting one of its ends to a sudden suction so as to leave a film of water on the surface of the walls. A high residual water content is characteristic of a high chemical affinity between the future catalytic coating and the substrate, and hence the possibility of fixing more catalytic coating. Such a method is described in application EP 1 462 171.

The setting of alumina (A) is measured as follows: A solution of boehmite loaded at 20% by mass is prepared by putting 200 g of boehmite (Disperal® supplied by the company Sasol) in suspension in 1 liter of distilled water the acidity of the solution being obtained by adding concentrated nitric acid (52%) to pH = 2 and the dispersion being obtained by vigorous stirring for 2 hours. The monolith is then impregnated by immersion in this solution for 1 minute and the excess of solution present on the monolith is removed by blowing air under pressure. The part is then dried under air at 120 ^° C. for 2 hours and then calcined for 2 hours at 500 ^° C. in air to form an alumina deposit. The setting of alumina corresponds to the mass gain corresponding to the deposition of alumina.

The average thermal expansion coefficient (CTE) is measured between 65 ° C. and 1000 ^° C. by differential dilatometry under a rise in temperature of 5 ° C./minute according to standard NF B40-308. The specimen of material tested is obtained by cutting in the honeycomb in a plane parallel to the direction of extrusion of the monolith. Its dimensions are about 5mm * 5mm * 15mm. The measurements are made after deposition of boehmite and calcination to simulate the effect of a catalytic coating having crystallites of very small size after calcination, that is to say of the order of 10 nm.

The catches or weight losses (Q, P, A, L) are expressed in percentages by weight relative to the mass of the dry substrate before impregnation.

Table 3

These results show that the use of polyvinylpyrrolidone in place of the polyvinyl alcohol makes it possible to considerably improve the affinity between the substrate and the catalytic coating which is deposited after passivation. The water intake level of the examples according to the invention is indeed much higher than that of examples C1 to C5 and very similar to that of the non-passivated structure.

The passivating effect of the polyvinylpyrrolidone, illustrated by Example 7, is particularly advantageous, since the coefficient of thermal expansion of the passive substrate and then provided with its catalytic coating is reduced by more than 40% relative to a non-passive substrate before deposition. catalytic coating (example C6). The passivating effect of the polyvinylpyrrolidone is also better than that of the polyvinyl alcohol (example C3).

Table 4 below illustrates the influence of the drying temperature on the adhesion of the polymer to the substrate. Unlike Example 7, Examples 9 and 11 were dried at 170 and 190 ^° C., respectively.

In contrast to Examples 7 and 9, in Examples 8 and 10 respectively, 3-aminopropyl trimethoxysilane (99% purity provided by Sigma Aldrich) is added to the solution at a rate of 5% by weight relative to the mass. of polyvinylpyrrolidone.

In addition to the parameters already described, Table 4 reports the parameter denoted L, which corresponds to the loss of mass after immersion of the dried substrate in water for one minute at ambient temperature and drying at 105 ^° C. in air. Table 4

These results show that a higher drying temperature leads to better adhesion of the passivating polymer layer to the substrate because the weight loss after immersion of the substrate (L) decreases as the drying temperature increases. However, this also causes a decrease in the affinity with the future catalytic coating for the highest drying temperatures because the water intake after passivation (P) also decreases as the drying temperature increases. Consequently, a drying temperature of between 130 and 170 ^° C., or even between 130 and 160 ^° C., constitutes an optimum.

Comparing Examples 8 and 10 with Examples 7 and 9 respectively shows that the addition of a small amount of silane further improves the adhesion of the polymer layer to the substrate.

Claims

1. Catalyst substrate made of porous inorganic material for the treatment of exhaust gases, the structure of which is honeycombed, one of the faces of the structure allowing admission of the exhaust gases to be treated and the other side the evacuation of the treated exhaust gases, and comprises, between these intake and discharge faces, a set of adjacent ducts or channels with axes parallel to each other, separated by porous walls, said substrate being coated on a at least part of its internal surface of at least one vinylpyrrolidone polymer or copolymer.

2. Substrate according to the preceding claim, wherein the channels are alternately closed at one or the other end so as to filter the soot or particles contained in the exhaust gas.

3. Substrate according to one of the preceding claims, such that the porous inorganic material is selected from aluminum titanate, cordierite and mullite.

4. Substrate according to one of the preceding claims, such that the polymer or copolymer of vinylpyrrolidone is chosen from polyvinylpyrrolidone, copolymers of vinylpyrrolidone and vinyl acetate, copolymers of vinylpyrrolidone and vinylimidazole or copolymers of vinylpyrrolidone and vinylcaprolactam, or any of their mixtures.

5. Substrate according to one of the preceding claims, coated on at least part of its inner surface with at least one compound of the silane type, especially the silane type comprising at least one carbon chain which has at least one nucleophilic group.

6. Substrate according to one of the preceding claims, coated on at least a portion of its surface with a catalytic coating.

7. Substrate according to the preceding claim, such that the catalytic coating comprises a base material and a catalyst.

8. Substrate according to the preceding claim, such that the base material is an inorganic material whose specific surface is of the order of 10 to 100 m ² / g.

9. Process for obtaining a porous inorganic material catalyst substrate according to one of the preceding claims, comprising a step in which is deposited on said substrate a polymer or copolymer of vinylpyrrolidone, then a drying step.

10. Method according to the preceding claim, wherein the polymer or copolymer of vinylpyrrolidone is deposited by impregnation of a liquid solution or dispersion, in particular aqueous.

11. Method according to the preceding claim, such that the weight content of polymer or copolymer of vinylpyrrolidone in the solution or the dispersion is between 1 and 30%, preferably between 5 and 15%.

12. Process according to one of the preceding process claims, such that the average molar mass of the polymer or copolymer of vinylpyrrolidone is between 10,000 and 1,000,000 g / mol, in particular between 20,000 and 100,000 g / mol.

13. Method according to one of the preceding method claims, such that the drying step is carried out at a temperature of at least 100 ⁰ C, especially between 130 and 170 ⁰ C.

14. Method according to one of the preceding process claims, followed by a step of depositing a catalytic coating and then by a calcination step.

15. Catalyst substrate obtainable by the method according to the preceding claim.