FR2822722A1 - Automotive exhaust particle filter has nitrate-forming coating applied to porous substrate - Google Patents

Abstract

An internal combustion engine has an exhaust filter that oxidizes trapped particles. The filter is coated with a nitrate-forming substance. The coating is a silver-based compound e.g. Ag, Ag2O, AgOH, AgNO3, AgCO3 or a mixture of these, including alkaline metals, alkaline earth metals, rare earths, or a mixture of these. The coating is applied to a finely-pored base. The coating is in elemental form, or as an oxide, hydroxide, carbonate or nitrate. The porous substrate is zirconium oxide, zirconium oxide with La, aluminum oxide, Mg-Al mixed oxides, La-aluminum oxide, Ceroxide, Titanium oxide, silicon oxide, Si-Al mixed oxide, a zeolite, or a mixture of these.

Description

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 The invention relates to a filter for removing particles from the exhaust gases of internal combustion engines, the particles trapped in the filter being oxidized.

 Particle filters for use in engine exhaust gases are usually constructed in the following manner, namely that the channel ends of a porous ceramic honeycomb body with a parallel channel structure , are closed alternately on one or the other side. FIG. 1 shows, in sectional representation, a particle filter in which one half of the channels is of the open type on the side of the gas inlet and the closed type on the side of the gas outlet, and the other half half of the channels is of the closed type on the side of the gas inlet and the open type on the side of the gas outlet. The gas flowing through the particulate filter is conveyed through the porous walls of the channels, whereby the particles contained in the gas are filtered. Particle filters are usually made of SiC or cordierite.

 A problem that may arise for the operation of such filters when they are used for engine exhaust gases is insufficient regeneration of the filter. The particles trapped in the filter during the running operation of the engine progressively lead to clogging of the filter, resulting in an increase in the exhaust gas back pressure and a drop in engine power. Regeneration of the filter is necessary at regular time intervals. Regeneration is performed at high exhaust gas temperatures (> 500 C),

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gaz d'échappement, à des températures en-dessous de 5000C [1, 2]. oxidation of soot with the oxygen contained in the exhaust gas, carbon dioxide. Since high exhaust temperatures are only rarely attained during the normal operation of a vehicle, attempts are made to contribute to the fact that oxidation is already occurring at lower temperatures (5000C). It is known from the state of the art to achieve this goal by the following measures: 1. Additive to the fuel: an additive is added to the fuel, which is for example an organic compound with a Ce or Fe content. This additive contributes to the combustion of the soot particles contained in the

exhaust gas at temperatures below 5000C [1, 2].

2. Catalytic coating of the particulate filter: The particulate filter is coated with a catalytically active compound, which accelerates the oxidation of soot trapped in the filter. As a catalyst, for example, compounds containing Pt (platinum) or also with a content of a transient metal are used. The use of catalysts allows the oxidation of soot at temperatures below 500 [3, 4].

3. Soot oxidation with NO2 (CRT = Continuously
Regenerating Trap, filter with continuous regeneration): in this process the NO2 oxidation force is used for the combustion of soot. For this purpose, upstream of the particulate filter is mounted a Pt-containing oxidation catalyst, which oxidizes NO contained in the exhaust gases to NO2. The NO2-containing gas then flows through the particulate filter and oxidizes the soot to form CO2. This reaction is already taking place at temperatures below 3000C [5-7].

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 The object of the invention is to provide an improved particulate filter for motor vehicles, by which the filtered soot particles are oxidized at low temperatures, thus making possible a more efficient regeneration of the particulate filter during the course of the process. running operation of the motor vehicle.

 According to the invention, this object is achieved for a particulate filter of the type mentioned in the introduction, in that the filter is coated with a solid material forming nitrates.

 With the aid of this nitrate-forming compound, on the particulate filter, the nitrogen oxides contained in the exhaust gases are stored as nitrate during the running operation of the vehicle, in particular when operating at poor mixture. These stored nitrates produce, from temperatures of 300oC, the oxidation of the soot particles trapped in the particulate filter, in CO2. On this occasion, the nitrate is reduced chemically to NO and NO2. It is thus possible to lower the temperature necessary for the oxidation of soot and to obtain a regeneration of the filter.

 The particle filter according to the invention can advantageously be used in automotive vehicles with internal combustion engines operating with poor mixtures, including diesel engines or Beau de Rochas cycle engines lean mixture.

 According to an advantageous configuration of the invention, the nitrate-forming solid material has a content of Ag and contains in particular Ag

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élémentaire, Ag20, AgOH, AgNO3, AgCO3 ou un mélange des composés évoqués.

elemental, Ag20, AgOH, AgNO3, AgCO3 or a mixture of the evoked compounds.

 According to another advantageous configuration of the invention, the nitrate-forming solid material is a compound of alkali metals, alkaline earth metals, rare earths or a mixture thereof and of transition metals, main groups and a mixture of these. The alkali metals, alkaline earth metals, rare earths or their mixtures have a nitrating action here, while the transient elements, the main group elements or their mixtures act as oxidation catalyst for the formation of nitrates. Advantageously, the compounds mentioned are in the form of element, oxide, hydroxide, carbonate or nitrate.

 The nitrate-forming solid is advantageously applied to a finely porous support substance, such as, for example, zirconium oxide, La (lanthanum) -circular zirconium oxide, aluminum, a mixed oxide of Mg-Al, aluminum oxide with a content of La, cerium oxide, titanium oxide, silicon oxide, a mixed oxide of Si-Al , a zeolite or a mixture of several of these compounds. The BET surface of the support substance is in particular between 10 and 1000 m 2 / g.

 According to a particular characteristic of the invention, at least two compounds contained in the solid material forming nitrates are a mixed oxide.

 For its use in vehicles, the solid material forming nitrates, in the form of

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 powder, is applied to a ceramic or metallic honeycomb body, also hereinafter referred to as a geometric support that stands out from the finely porous support substance.

 According to a configuration of the invention, the compounds of the nitrate-forming solid can be in the form of an atomic mixture on a finely porous support substance. These compounds of the nitrate-forming solid may also be individually or individually on the same or different finely porous support substances. On the other hand, the compounds of the nitrate-forming solid material being respectively individually on the same or different finely porous support substances, can be applied as a powder mixture on a geometrical support, or can be applied on a support geometrically being arranged respectively in layers.

 Thus, the compounds of the nitrate-forming solid can be combined in the following manner: Atomic mixing: The individual compounds of the nitrate-forming solid are located side by side on a finely porous support substance, which for its part is applied to the geometric support.

- Powder mixture: the individual compounds of the nitrate-forming solid are respectively individually on finely porous support substances, which are applied as a powder mixture on the geometrical support.

- Layering arrangement: the individual compounds of

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 the nitrate-forming solid are respectively individually on finely porous support substances, which are arranged in layers on the geometrical support.

 The nitrate-forming solid may for example be applied to the finely porous carrier material by the following methods: - Impregnation - Sol-gel process - Chemical wet precipitation, eg hydroxide precipitation - Ion exchange in a zeolite.

 In the following, the invention will be explained in more detail with reference to two experiments and the accompanying drawings which show: FIG. 1 the principle of a method of construction of a particulate filter for use in engine exhaust, FIG. 2 the law of variation of the oxidation of soot as a function of temperature, for a mixture of AgNO 3 powder and soot and comparatively for soot alone, FIG. 3 the law of variation of the soot oxidation for a geometrical support coated with a nitrate-forming solid material, and comparatively for a geometrical support on which there are only soot.

 To demonstrate the action of nitrates on the combustion of soot, the following tests were carried out.

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Test 1 In this test 267 mg of silver nitrate (AgNO3) in powder form was mixed with 132 mg of soot. The powder mixture was then slurried with 3600 mg SiC, in order to lower the temperature rise by the exothermicity of the reaction. In a reaction tube, the powder mixture was then heated in a stream of air. The volume flow rate of the air was 10 Nl / min, the air being heated with a temperature ramp of 15 K / min. At the outlet of the reaction tube, the CO 2 concentration was recorded as a function of temperature and used as an indicator of the initiation of soot oxidation.

Test 2 In this test, a cordierite support coated with a mixture of powder Ag (25% by weight) / Al 2 O 3 and soot was brought into a stream of air (T = 4500 C) with an NO concentration of 1000 ppm. The powder mixture was made with 1213 mg Ag (25% by weight) / Al 2 O 3 and 150 mg of soot. In the production of Ag (25% by weight) / Al 2 O 3, the following procedure was carried out: Al 2 O 3 with a BET surface area of 180 m 2 / g was impregnated with a solution of AgNO 3. Then, A1203 was dried and calcined at a temperature of 650oC for a period of 5 hours.

The silver obtained on this occasion after calcination is in the form of elemental silver, silver oxide or silver carbonate. The powder mixture was then applied to the monolithic ceramic support with 62 cells / cm 2 (400 cpsi: cells per square inch, number of cells per unit area).

 Figure 1 shows the principle method of construction of a particulate filter for use in engine exhaust, as already mentioned in the introduction. The particle filter

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 is usually made of porous SiC or cordierite. It is constructed such that the channel ends of a honeycomb body 1 with parallel channel structure 2 are alternately closed on one side and the other. Half of the filter channels are of the open type on the gas inlet side and the closed type on the gas outlet side. Correspondingly, the other half of the channels is of the closed type on the side of the gas inlet and the open type on the gas outlet side. The gases flowing through the filter are thus forced to pass through the porous walls 3 of the channels, thus filtering the particles contained in the gases.

 FIG. 2 shows, according to test 1, the law of variation as a function of temperature, of soot oxidation for a mixture of AgNO 3 powder and soot, and comparatively for soot alone.

The powder mixture and the soot are, as described in Test 1, introduced into a reaction tube and subjected to a stream of air. Curve A shows here the law of variation of the oxidation of soot of the powder mixture, and the curve B the law of variation of the oxidation of soot for soot alone. As an indicator for the oxidation of soot, the concentration of CO2, which is measured at the outlet of the gases of the reaction tube, has been taken here.

atteint un maximum et est achevée à 8400C. Il est à noter que le taux d'échauffement (15 K/min dans la présente expérience) influence dans certaines limites la position et la largeur du pic de CO2. If soot without addition of nitrate (curve B) is heated, the formation of CO2 begins at about 650 C. At a temperature of 7500C the formation of CO2

reaches a maximum and is completed at 8400C. It should be noted that the rate of heating (15 K / min in the present experiment) influences within certain limits the position and the width of the CO2 peak.

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d'argent selon les relations suivantes : 3 C + 4 AgNO3--+ 3 CO2 + 2 Ag2O + 4 NO
C + AgNO3--+ CO2 + Ag + NO. If silver nitrate is added to the soot (A curves), the formation of CO2 starts at significantly lower temperatures. There are two striking CO2 peaks here. The first peak Pl appears in the temperature range between 2800C and 350 C. This formation of CO2 is to be attributed to the oxidation of soot (carbon C) by nitrate

of silver according to the following relations: 3 C + 4 AgNO3 - + 3 CO2 + 2 Ag2O + 4 NO
C + AgNO3 - + CO2 + Ag + NO.

avec un maximum à environ 5000C. Celui-ci est à mettre sur le compte de l'oxydation catalytique des suies sur Ag à savoir Ag20, qui se trouve encore dans le mélange de matière solide en tant que produit de décomposition de l'oxydation des suies assistée par les nitrates. From this the silver nitrate is reduced to Ag2O, Ag and NO. Due to the limited amount of silver nitrate, only a portion of the soot can be oxidized after this reaction. The residual carbon (soot) undergoes combustion above 4000C. There appears a second peak P2 of CO2 between 4000C and 5800C

with a maximum at about 5000C. This is due to the catalytic oxidation of soot on Ag, namely Ag20, which is still in the solid mixture as a decomposition product of nitrate-assisted soot oxidation.

(first peak of CO2).

 Under the given reaction conditions, it is possible, by adding nitrate to the soot, to obtain a lowering of the soot oxidation temperature to about 2500C (first peak of CO2 P1) and respectively to about 4300C (second peak of CO2 P2).

 FIG. 3 shows the variation law as a function of time of soot oxidation (COx concentration) for a geometrical support coated according to the invention with a nitrate-forming solid material (curve A) and a support

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 Geometric coated with soot (curve B).

 In this case, according to test 2, a ceramic support was coated with a mixture of Ag / Al 2 O 3 powder and soot. The ceramic support was introduced into a reaction tube and subjected to a stream of air containing NO. The NO concentration was 1000 ppm. The temperature of the air stream was set at 4500C and the concentration of COx (sum of COs and CO) of the air at the outlet of the reaction tube was determined as a function of time. Soot oxidation was observed for about 40 minutes. Curve A shows that after about 12 minutes, the COx concentration reaches a maximum of 1200 ppm. The carbon balance of COx formed and the carbon initially supplied (in the form of soot) gives a total combustion of the soot. For comparison (curve B), a support only coated with soot at 5000C was studied.

In this case, no COx was measured in the air stream, which means that the soot was not burnt at this temperature. If the temperature is subsequently raised, the total oxidation of the soot is at temperatures above 6000 ° C (not shown).

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Catalysts thooxydation of sootfrom diesel exhaust gases 7. An exploratory study Appl. Catal. B : Enviromenta 1 8 (1996) 57-78 [5] P. Hawker, N. Myers, G. Hüthwohl, H. Th. Vogel, B. Bates, L. Magnusson, P. Bibliography [1] SL Jelles, M. Makkee, JA Moulijn, GJK Acres, JD Peter-Hoblyn Diesel particulate control Application of an active particle in combination with fuel additives at an ultra low rate SAE Technica 1 Paper Series 1999-01-0113 , pp. 69-74 [2] G. Lepperhoff, H. Lüders, P. Barthe, J. Lemaire Quasi-continuous particle trap regeneration by cerium additives SAE Technica 1 Paper Series No. 950369,1995 [3] R. Krabetz, WD Mross Heterogene Katalyse und Katalysatoren Ullmann Enzyklopadie, 13th edition (1967) pp. 513-566 [4] John PA Neeft, Michiel Makkee, Jacob A. Moulijn

Catalysts thooxidation of sootfrom diesel exhaust gases 7. An exploratory study Appl. Catal. B: Enviromenta 1 8 (1996) 57-78 [5] P. Hawker, N. Myers, G. Huthwohl, H. Th. Vogel, B. Bates, L. Magnusson, P.

 Bronnenberg Experience with a new particle trap technology in Europe SAE Technica 1 Paper Series NO 970479, 1997 [6] JP Warren et al Effect of particulate matter when using the Continuously Regeneration Trap (crude) IMechE Seminar Diesel Engine-Particulate Control, 23 Nov. 1998, London [7] MV Twigg Advanced Exhaust Emissions Platinum Metals Rev., 2000,44, (2) 67-71

Claims (11)

CLAIMS. 1. Filter for removing particles from the exhaust gases of internal combustion engines, the particles trapped in the filter being oxidized, characterized in that the filter is coated with a solid material forming nitrates.  2. Particle filter according to claim 1, characterized in that the solid material forming nitrates has a content of Ag and contains in particular elemental Ag, Ag2O, AgOH, AgNO3, AgCO3 or a mixture of the compounds mentioned.  3. Particle filter according to claim 1, characterized in that the nitrate-forming solid is a compound of alkali metals, alkaline earth metals, rare earths or a mixture thereof and of transition metals, elements of main groups and a mixture of these.  Particle filter according to Claim 2 or 3, characterized in that the nitrate-forming solid is applied to a finely porous carrier material.  Particle filter according to Claim 3, characterized in that the alkali metal and / or the alkaline earth metal and / or the rare earths and / or the transition metal and / or the main group element are in the form of element, oxide, hydroxide, carbonate or nitrate.  Particle filter according to Claim 4, characterized in that the carrier substance <Desc / Clms Page number 13>  finely porous contains zirconium oxide, L (lanthanum) zirconium oxide, aluminum oxide, Mg-Al mixed oxide, aluminum oxide La, cerium oxide, titanium oxide, silicon oxide, a mixed oxide of Si-Al, a zeolite or a mixture thereof.  7. Particle filter according to one of the preceding claims, characterized in that at least two compounds contained in the solid material forming nitrates are a mixed oxide.  Particle filter according to one of the preceding claims, characterized in that the compounds of the nitrate-forming solid are in the form of an atomic mixture on a finely porous support substance.  9. Particle filter according to one of the preceding claims, characterized in that the compounds of the nitrate-forming solid are respectively individually on identical or different finely porous carrier substances.  Particle filter according to Claim 9, characterized in that the compounds of the nitrate-forming solid material, which are respectively individually on the same or different finely porous carrier substances, are applied as a powder mixture to one another. geometric support.  Particle filter according to Claim 9, characterized in that the compounds of the nitrate-forming solid material are <Desc / Clms Page number 14>  respectively individual on identical or different finely porous support substances, are applied on a geometric support being arranged respectively in layers.