EP4313366A1

EP4313366A1 - Method for increasing the fresh filtration of gasoline particle filters

Info

Publication number: EP4313366A1
Application number: EP22720306.4A
Authority: EP
Inventors: Jan Schoenhaber; Naina DEIBEL; Joerg-Michael Richter; Michael Schiffer
Original assignee: Umicore AG and Co KG
Current assignee: Umicore AG and Co KG
Priority date: 2021-03-23
Filing date: 2022-03-22
Publication date: 2024-02-07
Also published as: US20240159175A1; WO2022200311A1; DE102021107130B4; DE102021107130A1; CN117083118A

Abstract

The present invention relates to a wall-flow filter. Said wall-flow filter contains a powder coating which increases the filtration efficiency only in the fresh state. An exhaust gas system comprising such a wall-flow filter is also claimed.

Description

Process for increasing the fresh filtration of gasoline particle filters

description

The present invention is directed to a wall flow filter. This contains a powder coating, which only increases the filtration efficiency when it is fresh. Also claimed is an exhaust system which has such a wall-flow filter.

The exhaust gas from internal combustion engines in motor vehicles typically contains the pollutant gases carbon monoxide (CO) and hydrocarbons (HC), nitrogen oxides (NO _x ) and possibly sulfur oxides (SO _x ), as well as particles, which largely consist of solid carbonaceous particles and possibly adhering organic agglomerates . These are referred to as primary emissions. CO, HC and particles are products of the incomplete combustion of fuel in the engine's combustion chamber. Nitrogen oxides are formed in the cylinder from nitrogen and oxygen in the intake air when the combustion temperatures exceed 1200°C. Sulfur oxides result from the combustion of organic sulfur compounds, which are always present in small amounts in non-synthetic fuels. Compliance with future legal exhaust emission limits for motor vehicles in Europe, China, North America and India requires the extensive removal of the pollutants mentioned from the exhaust gas. To remove these emissions, which are harmful to the environment and health, from the exhaust gases of motor vehicles, a large number of catalytic exhaust gas cleaning technologies have been developed, the basic principle of which is usually based on the fact that the exhaust gas to be cleaned is passed through a flow (flow-through) or a wall flow (wall flow) flow) honeycomb body is passed with a catalytically active coating applied thereto. The catalytic converter promotes the chemical reaction of various exhaust gas components with the formation of harmless products such as carbon dioxide, water and nitrogen.

The flow-through or wall-flow honeycomb bodies just described are also referred to as catalyst carriers, carriers or substrate monoliths, since they carry the catalytically active coating on their surface or in the walls forming this surface. The catalytically active coating is often applied to the catalyst support in a so-called coating process in the form of a suspension. Many such processes are in the past of automotive catalytic converter manufacturers been published on this (EP1064094B1, EP2521618B1, W010015573A2,

EP1136462B1, US6478874B1, US4609563A, WO9947260A1, JP5378659B2,

EP2415522A1, JP2014205108A2).

The operating mode of the internal combustion engine is decisive for the possible methods of pollutant conversion in the catalytic converter. Diesel engines are usually operated with excess air, most petrol engines with a stoichiometric mixture of intake air and fuel. Stoichiometric means that on average there is just as much air available to burn the fuel in the cylinder as is required for complete combustion. The combustion air ratio l (A/F ratio; air/fuel ratio) relates the air mass mi_,tats actually available for combustion to the stoichiometric air mass mi_,st:

If l < 1 (e.g. 0.9) this means "lack of air", one speaks of a rich exhaust gas mixture, l > 1 (e.g. 1.1) means "excess air" and the exhaust gas mixture is referred to as lean . The statement l = 1.1 means that 10% more air is present than would be necessary for the stoichiometric reaction.

Insofar as lean-burn motor vehicle engines are mentioned in the present text, reference is hereby mainly made to diesel engines and predominantly lean-burn Otto engines on average. The latter are predominantly gasoline engines operated with an average lean A/F (air/fuel) ratio. On the other hand, most gasoline engines are mainly operated with a combustion mixture that is stoichiometric on average. The expression "on average" takes into account the fact that modern petrol engines are not operated statically at a fixed air/fuel ratio (A/F ratio; l value). Rather, the engine control system specifies a mixture with a discontinuous course of the air ratio l around l=1.0, resulting in a periodic alternation of oxidizing and reducing exhaust gas conditions. This change in the air ratio l is essential for the exhaust gas cleaning result. For this purpose, the l value of the exhaust gas is regulated around the value l = 1.0 with a very short cycle time (approx. 0.5 to 5 Hertz) and an amplitude Dl of 0.005 < Dl < 0.07. On average, in such operating states, the exhaust gas can therefore be described as “on average” stoichiometric. So that these deviations do not have a negative impact on the exhaust gas cleaning result when the exhaust gas is transferred via the Affect three-way catalyst, the oxygen storage materials contained in the three-way catalyst compensate for these deviations by absorbing oxygen from the exhaust gas as required or releasing it into the exhaust gas (R. Heck et al., Catalytic Air Pollution Control - Commercial Technology, Wiley, 2nd edition 2002, page 87). Due to the dynamic mode of operation of the engine in the vehicle, however, further deviations from this state occur at times. For example, when accelerating sharply or when overrun, the engine and thus the exhaust gas operating conditions can be set, which can be over- or under-stoichiometric on average. Otto engines that burn stoichiometrically therefore have an exhaust gas which burns predominantly, ie for most of the time during combustion operation, at an air/fuel ratio that is on average stoichiometric.

The pollutant gases carbon monoxide and hydrocarbons can be rendered harmless from a lean exhaust gas by oxidation on a suitable oxidation catalytic converter. In a stoichiometrically operated internal combustion engine, all three pollutant gases (HC, CO and NOx) can be eliminated via a three-way catalytic converter. The reduction of nitrogen oxides to nitrogen ("denitrification" of the exhaust gas) is more difficult due to the high oxygen content of a lean-burn engine. A well-known method here is the selective catalytic reduction of nitrogen oxides (Selective Catalytic Reduction; SCR) on a suitable catalyst, called SCR catalyst for short. This method is currently considered to be preferred for the denitrification of lean-burn engine exhaust gases. The nitrogen oxides contained in the exhaust gas are reduced in the SCR process with the aid of a reducing agent dosed into the exhaust line from an external source. Ammonia is used as a reducing agent, which converts the nitrogen oxides in the exhaust gas to nitrogen and water in the SCR catalytic converter. The ammonia used as a reducing agent can be made available by metering an ammonia precursor compound, such as urea, ammonium carbamate or ammonium formate, into the exhaust system and subsequent hydrolysis.

Diesel particle filters (DPF) or petrol particle filters (GPF)/otton particle filters (OPF) with and without an additional catalytically active coating are suitable units for removing particle emissions. In order to meet the legal standards, it may be desirable for current and future applications for exhaust gas aftertreatment of internal combustion engines to combine particle filters with other catalytically active functionalities for cost reasons but also for space reasons. The use A particulate filter - whether catalytically coated or not - leads to a noticeable increase in exhaust back pressure compared to a flow carrier of the same dimensions and thus to a reduction in engine torque or possibly increased fuel consumption. In order not to further increase the exhaust gas back pressure, the quantities of oxidic support materials for the catalytically active elements of the catalyst or oxidic catalyst materials are generally applied in smaller quantities in a filter than in a flow carrier. As a result, the catalytic effectiveness of a coated particle filter is often inferior to that of a flow monolith of the same size.

Some efforts have already been made to provide particle filters which have good catalytic activity due to an active coating and yet have the lowest possible exhaust gas back pressure. On the one hand, it has proven to be advantageous if the catalytically active coating is not located as a layer on the wall of a porous wall-flow filter, but instead permeates the wall of the filter with the catalytically active material (WO2005016497A1, JPH01-151706, EP1789190B1). For this purpose, the particle size of the catalytic coating is chosen so that the particles penetrate into the pores of the wall flow filter and can be fixed there by calcination.

Another functionality of the filter that can be improved by a coating is its filtration efficiency, ie the filter effect itself. Increasing the filtration efficiency of catalytically inactive filters is described in WO2012030534A1. A filtration layer (“discriminating layer”) is created on the walls of the flow channels on the inlet side by depositing ceramic particles via a particle aerosol. The layers consist of oxides of zirconium, aluminum or silicon, preferably in the form of fibers from 1 nm to 5 μm and have a layer thickness of more than 10 μm, generally 25 μm to 75 μm. After the coating process, the applied powder particles are calcined in a heat process.

A coating within the pores of a wall flow filter unit by means of atomizing dry particles is described in US Pat. No. 8,388,721 B2. Here, however, the powder should penetrate deep into the pores. 20% to 60% of the surface of the wall should be accessible to soot particles, i.e. remain open. Depending on the flow rate of the powder-gas mixture, a more or less strong powder gradient can be set between the inlet and outlet side. Also, the introduction of the powder into the pores, z. B. using an aerosol generator, described in EP2727640A1. Here a non-catalytically coated wall flow filter with a z. B. aluminum oxide particles containing gas stream Derge stalt coated that the complete particles having a particle size of 0.1 pm to 5 pm deposited as a porous filling in the pores of the wall flow filter who the. The particles themselves can implement a further functionality of the filter in addition to the filter effect. For example, these particles are deposited in the pores of the filter in an amount of more than 80 g/l based on the filter volume. They fill 10% to 50% of the volume of the filled pores in the canal walls. Both with soot and without soot, this filter has an improved filtration efficiency compared to the untreated filter with a lower exhaust back pressure of the filter loaded with soot.

In EP2502661A1 and EP2502662B1 further processes for the application of powder coating to filters are mentioned. Corresponding apparatuses for subjecting the filter to a powder-gas aerosol are also shown there, in which the powder applicator and the wall-flow filter are each separated in such a way that air is sucked in through this space during coating. Another method in which a membrane (“trapping layer”) is produced on the surfaces of the inlet channels of filters to increase the filtration efficiency of catalytically inactive wall-flow filters is described in patent specification US8277880B2. The filtration membrane on the surfaces of the inlet channels is realized by sucking through a gas flow loaded with ceramic particles (e.g. silicon carbide, cordierite). After the filter layer has been applied, the honeycomb body is fired at temperatures above 1000°C in order to increase the adhesive strength of the powder layer on the channel walls.

WO2011151711 A1 describes a method with which a non-coated or catalytically coated filter is exposed to a dry aerosol. The aerosol is provided by dispersing a powdered high-melting metal oxide with an average particle size of 0.2 μm to 5 μm and guided by means of a gas stream over the inlet side of a wall-flow filter. The individual particles agglomerate to form a bridged network of particles and are deposited as a layer on the surface of the individual inlet channels running through the wall flow filter. The typical loading of a filter with the powder is between 5 g and 50 g per liter of filter volume. It is expressly pointed out that that it is not desirable to coat the pores of the wall-flow filter with the metal oxide.

Due to the introduction of particulate limit values and the simultaneous implementation of the real driving emissions test as part of the type approval process (RDE), there is an acute need for gasoline particulate filters (GPF, gasoline particulate filters; OPF), which are characterized by a particularly high fresh filtration efficiency. Such Fil ter usually have a particularly high exhaust back pressure, which - as ge says - can lead to reduced engine performance and / or increased fuel consumption. Oil ash and soot particles that are deposited during operation inevitably lead to a further increase in exhaust back pressure and filtration performance (Fig. 2 for stable powder).

The object of the present invention was therefore to provide particle filters with a sufficiently high fresh filtration efficiency, which, however, have little or no increase in the exhaust gas back pressure during proper operation. These and other objects, which are obvious to a person skilled in the art from the prior art, are solved by a wall-flow filter according to present claim 1. Claim 8 relates to an exhaust system according to the invention. The subclaims dependent on these claims are directed to preferred embodiments of the wall-flow filter or the exhaust gas system according to the invention.

By providing a wall-flow particle filter for cleaning the exhaust gases of an Otto engine, in which it contains a thermolabile powder on and/or in its input surface, which increases the filtration efficiency of the filter when it is fresh and its surface or volume when properly Operation of the filter decreases in such a way that an increase in the exhaust back pressure compared to a filter that has not been treated with the thermolabile powder by a maximum of 10% after an equivalent exposure to particulate exhaust gas components is recorded, this can be achieved very easily and elegantly, but not for that less surprising to solve the task. The thermal load on the filter in normal operation and during regeneration phases reduces the surface area and volume of the thermolabile powder, so that the proportion of the powder coating in the total back pressure slowly decreases over time. Since the filter at the same time, for example If oil ash accumulates, the filtration performance remains largely unchanged, despite the decrease in surface area and volume of the thermolabile powder (Fig. 1/2).

All ceramic materials customary in the prior art can be used as wall-flow monoliths or wall-flow filters. Porous wall flow filter substrates made of cordierite, silicon carbide or aluminum titanate are preferably used. These wall-flow filter substrates have inflow and outflow channels, with the outflow-side ends of the inflow channels and the inflow-side ends of the outflow channels being offset from one another and sealed with gas-tight “plugs”. The exhaust gas to be cleaned, which flows through the filter substrate, is forced to pass through the porous wall between the inflow and outflow channels, which results in an excellent particle filter effect. The filtration properties for particles can be designed through the porosity, pore/radius distribution and thickness of the wall. The porosity of the uncoated wall flow filter is usually more than 40%, generally from 40% to 75%, especially from 50% to 70% [measured according to DIN 66133 - latest version on the filing date]. The average pore size of the uncoated filters is at least 7 µm, e.g. B. from 7 pm to 34 pm, preferably more than 10 pm, in particular more preferably from 10 pm to 25 pm or very preferably from 15 pm to 20 pm [measured according to DIN 66134 latest version on the filing date]. The finished filters with a pore size of typically 10 μm to 20 μm and a porosity of 50% to 65% are particularly preferred.

In the context of the invention, the term “thermolabile” means the property of exhibiting instability under the influence of elevated temperatures. Before lying a thermolabile powder is used. According to the invention, the powder therefore consists of a solid which undergoes a transformation under the action of sufficient heat energy in such a way that its density increases. In particular, the volume and surface area of the powder decreases as a result of this heat effect. One can speak of sintering of the thermolabile powder. As a result, after the heat has been applied, the powder presents less volume or surface area to the incoming exhaust gas. Therefore, the exhaust back pressure of the filter decreases as described above. This decrease in exhaust back pressure is compensated for by the oil ash that accumulates in the filter over time and cannot be removed. By selecting a suitable powder, its original filtration efficiency and its exhaust back pressure can ideally be kept constant within certain ranges during the entire operation of the filter. The thermolability - ie the speed at which the powder loses its volume and surface - should correspond to this ideal as closely as possible. The thermolabile powder should therefore preferably show a reduction in surface area of 15-50%, preferably 20-40% and most preferably 25-35% after aging for 6 hours in the oven in the presence of air at 1000°C.

The use of a thermolabile powder in the present invention runs counter to the trend that normally high surface area carrier substances for catalytically active metals are used in car exhaust gas catalysts, which have a surface which is as thermostable as possible. Because the more stable the surface, the less subject a catalyst is to thermally induced aging through sintering of the carrier oxide. In the present case, however, it is a question of allowing such a high-surface area powder to age thermally in a targeted manner. In principle, the same substances can be used for the present inventions that also serve as normal carrier oxides in car exhaust gas catalysts, provided they are in a form that has a thermal stability as characterized above. The powder is preferably high-surface oxides of metals, for example those selected from the group consisting of aluminum oxide, silicon dioxide, cerium oxide, zirconium oxide, titanium dioxide or mixtures or mixed oxides (solid solutions) thereof. The oxides are preferably not doped with other metals, which leads to better stability. The thermolabile powder is more preferably aluminum oxide, very preferably an undoped aluminum oxide or mixed oxides of aluminum oxide and silicon oxide, such as zeolites. In particular, zeolites can be selected or synthesized in a way that is tailor-made for the problem at hand in terms of their structure. If the present invention refers to oxides with a high surface area, this means oxides with a BET surface area of more than 10 m ² /g, preferably more than 30 m ² /g and very particularly preferably more than 50 m ² /g . The person skilled in the art knows how to obtain such oxides.

The filter according to the invention can be manufactured according to methods which are described above as prior art. For this purpose, a metal oxide powder, for example, is generally mixed with a gas (http://www.tsi.com/Aerosolgeneratoren-und-disperqierer/; https://www.palas.de/de/product/aerosolgeneratorssolidparticles). This mixture of the gas and the powder produced in this way is then advantageously conducted via a gas flow into the inlet side of the wall-flow filter. The part of the filter formed by the inflow channels/inlet channels is seen on the inlet side. The entrance surface is defined by the wall surfaces of the inflow ducts/entrance ducts formed on the inlet side of the wall flow filter. The same applies to the outlet side.

All gases suitable for the person skilled in the art for the present purpose can be used as gases for the production of the aerosol and for the introduction into the filter. The use of air is very particularly preferred. However, other reaction gases can also be used which can develop either an oxidizing (e.g. O2, NO2) or a reducing (e.g. H2) activity towards the powder used. The use of inert gases (e.g. N2) or noble gases (e.g. He) can also prove advantageous for certain powders. Mixtures of the listed gases are also conceivable.

In order to be able to separate the powder sufficiently well on the surface of the filter wall on the inlet side of the filter, a certain suction power is required. The specialist can form his own picture here in orienting tests for the respective filter and the respective powder. It has been found that the aerosol (powder-gas mixture) preferably has a speed of 5 m/s to 50 m/s, more preferably 10 m/s to 40 m/s and very particularly preferably 15 m/s up to 35 m/s is sucked through the filter. This also achieves advantageous adhesion of the applied powder.

A wall-flow filter manufactured according to the method outlined above should preferably have the powder in the large pores, since these are mainly responsible for the poor filtration efficiency of the filter. For this purpose it is preferred if the powder does not fall below a certain particle size (measured according to the latest ISO 13320-1 on the filing date). The D50 values of the powder are usually between 1 and 5 μm, preferably between 2 and 4 μm and very particularly preferably around 3 μm. This preferably blocks the large pores of the filter, so that it has a significantly increased filtration performance, but also a greater back pressure than the raw substrate.

The filtration performance or, to put it another way, the filtration efficiency of the filter containing powder should correspond as closely as possible to that which results after the oil ash has been deposited after proper operation. As a rule, the filtration efficiency of the filter containing powder in the fresh state is between 85% and 99.9%, preferably >87% and most preferably >90%. Those skilled in the art know how to determine filtration efficiency. Another key factor in how this filtration efficiency can be achieved is the amount of powder to be separated in the wall flow filter. It should not be too high in order not to create excessive exhaust gas back pressure of the filter when fresh, but it should be high enough to achieve the targeted fresh filtration efficiency. For the wall flow filter considered here, the powder should be applied to the filter in an amount of 1-40 g/l, preferably 1.5-30 g/l and very preferably 2-25 g/l.

The comparison envisaged according to the invention with regard to the exhaust gas back pressure of a wall flow filter treated according to the invention with thermolabile powder and an untreated filter of the same type in which the exhaust gas back pressure increases by a maximum of 10%, preferably a maximum of 7% and particularly preferably a maximum of 5% increase should be done after a period of proper operation of the filter. The filter has then already undergone several filter regenerations and the applied powder should no longer change its volume or its surface due to the effect of heat at this point. The filter regenerations can also be artificially simulated in appropriate systems. Before geous way, the increase in the exhaust back pressure for the specified comparison is determined immediately after 10 active soot regenerations. Temperatures of approx. 700 - 800 °C act on the filter for 5 - 10 minutes during each regeneration. This should be sufficient to allow maximum sintering of powder in the pores of the wall flow filter. The test to be assessed here is advantageously based on 10 filter regenerations, each lasting 10 minutes and during which the filter is exposed to a temperature of at least 800°C for 5 minutes. In the case of artificially induced tests, the amount of ash must therefore be dimensioned accordingly so that such a temperature curve can be ensured.

In a preferred embodiment, the filter can have been catalytically coated before being subjected to the powder/gas aerosol. In the present context, catalytic coating means the ability to convert harmful components of the exhaust gas from internal combustion engines into less harmful ones. In particular, the exhaust gas components NOx, CO and HC as well as particles should be mentioned here. According to the expert, this catalytic activity is provided by coating the wall-flow filter with a catalytically active material. The term coating is accordingly understood to mean the application of catalytically active materials to the wall flow filter. The coating takes on the actual catalytic function. In the present case, the coating is carried out by applying a correspondingly low-viscosity aqueous suspension—also called a washcoat—or a solution of the catalytically active components to the wall-flow filter, see e.g. B. according to EP1789190B1. After the suspension/solution has been applied, the wall-flow filter is dried and, if necessary, calcined at elevated temperature. The catalytically coated filter preferably has a loading of 20 g/l to 200 g/l, preferably 30 g/l to 150 g/l. The most suitable loading level of a wall-coated filter depends on its cell density, wall thickness and porosity. In the case of common medium-porous filters (<60% porosity) with, for example, 200 cpsi cell density and 8 mil wall thickness, the preferred load is 20 g/l to 50 g/l (related to the outer volume of the filter substrate). Highly porous filters (>60% porosity) with, for example, 300 cpsi and 8 mil have a preferred loading amount of 25 g/l to 150 g/l, particularly preferably 50 g/l to 100 g/l.

In principle, all coatings known to those skilled in the art for the automotive exhaust sector are suitable for the present invention. The catalytic coating of the filter can preferably be selected from the group consisting of a three-way catalytic converter, SCR catalytic converter, nitrogen oxide storage catalytic converter, oxidation catalytic converter, and soot ignition coating. With regard to the individual possible catalytic activities and their explanation, reference is made to the statements in WO2011151711A1. Particularly advantageously, this has a catalytically active coating comprising at least one metal ion-exchanged zeolite, cerium/zirconium mixed oxide, aluminum oxide and palladium, rhodium or platinum or combinations of these noble metals.

The present invention also relates to an exhaust system having a wall flow filter according to the invention and at least one other unit for reducing harmful exhaust gas components selected from the group consisting of oxidation catalyst, three-way catalyst, SCR catalyst, hydrocarbon trap and ammonia blocking catalyst. The use of an exhaust gas system is particularly preferred which has a three-way catalytic converter close to the engine and a wall-flow filter according to the invention, also positioned close to the engine and provided with a three-way catalytic coating. It is also preferred if the exhaust gas system has a wall flow filter according to the invention provided with a three-way catalytic coating located in the underbody of the vehicle downstream of the close-coupled three-way catalytic converter. Where the word underbody (uf) is mentioned in the text, this refers in connection with the present invention to an area in the vehicle in which the catalytic converter is at a distance of 0.2 - 3.5 m, more preferably 0.5 - 2 m and very particularly preferably 0.7-1.5 m after the end of the first close-coupled catalytic converter of the at least 2 catalytic converters, preferably under the driver's cab (FIG. 1).

In the context of this invention, close-coupled (cc) is an arrangement of the catalytic converter at a distance from the exhaust gas outlet of the cylinder of the engine of less than 120 cm, preferably less than 100 cm and very particularly preferably less than 50 cm. The catalytic converter close to the engine is preferably arranged directly after the exhaust manifold is merged into the exhaust pipe.

The invention is explained in more detail in the following examples.

Comparative example VGPF1 not according to the invention:

Alumina stabilized with lanthana was combined with a first oxygen storage component comprising 40% by weight ceria, zirconia, lanthana and praseodymia and a second oxygen storage component comprising 24% by weight ceria, zirconia, lanthana and yttria. suspended in water. Both oxygen storage components were used in equal parts. The weight ratio of alumina and oxygen storage component was 30:70. A palladium nitrate solution and a rhodium nitrate solution were then added to the suspension thus obtained, with constant stirring. The resulting coating suspension was used directly for coating a commercially available wall-flow filter substrate, with the coating being introduced into the porous filter wall over 100% of the substrate length. The total loading of this filter was 75 g/l, the total precious metal loading was 1.986 g/l with a ratio of palladium to rhodium of 5:1. The coated filter obtained in this way was dried and then calcined. It is hereinafter referred to as VGPF1.

Comparative example GPF1 according to the invention:

Alumina stabilized with lanthana was combined with a first oxygen storage component comprising 40% by weight ceria, zirconia, lanthana and praseodymia and a second oxygen storage component comprising 24% by weight ceria, zirconia, lanthana and yttria. suspended in water. Both oxygen storage components were used in equal parts. The weight ratio of alumina and oxygen storage component was 30:70. A palladium nitrate solution and a rhodium nitrate solution were then added to the suspension thus obtained, with constant stirring. The resulting coating suspension was used directly for coating a commercially available wall-flow filter substrate, with the coating being introduced into the porous filter wall over 100% of the substrate length. The total loading of this filter was 75 g/l, the total precious metal loading was 1.986 g/l with a ratio of palladium to rhodium of 5:1. The coated filter obtained in this way was dried and then calcined. This filter was then sprayed with an aerosol (powder-gas mixture) coated, in which 7 g/l aluminum oxide were deposited on the filter. This filter is hereinafter referred to as GPF1.

The VGPFl and the GPFl were then characterized with regard to their physical properties, filtration efficiency and back pressure behavior. First, the back pressure of the two filters was measured on the cold gas test stand at a volume flow of 600 m ³ /h. The filter VGPF1 had a pressure loss of 36.4 mbar, while the filter GPF1 according to the invention had a correspondingly higher back pressure of 42 mbar. This difference corresponds to a 15% increase in the back pressure of GPF1 over VGPF1, which is due to the deposition of the alumina. The two filters were then examined on the engine test bench with regard to their filtration performance. For this purpose, the filters were installed in the exhaust system in a position close to the engine, on the downstream side of a conventional three-way catalytic converter, and measured between two particle counters in the so-called WLTP cycle. Here, the filter VGPF1 showed a filtration efficiency of 60%, while the filter according to the invention had a filtration efficiency of 76% due to the filtration efficiency-increasing coating.

The filter GPF1 was then tempered for 10 hours at 1100°C in an air atmosphere and then measured again. It was found that after the temperature exposure on the cold gas test stand, the filter had a back pressure of only 37.1 mbar with the same volume flow as before. This corresponds to a back pressure increase of only 2% compared to the VGPF1. Although the back pressure of the filter has decreased after the temperature treatment, the filter still has an unchanged high filtration performance. This method is therefore ideally suited for providing filters that have an initially increased filtration performance and maintain this during continuous operation and at the same time have an ever-decreasing back pressure during operation due to the sintering of the filtration efficiency material.

Claims

patent claims

1. Wall-flow particle filter for cleaning the exhaust gases of a gasoline engine, characterized in that it contains a thermolabile powder on and/or in its input surface, which increases the filtration efficiency of the filter when fresh and whose surface or volume during proper operation of the fil ters decreases in such a way that an increase in the exhaust gas back pressure compared to a filter not treated with the thermolabile powder after an equivalent exposure to particulate exhaust gas components by a maximum of 10% is recorded.

2. Wall-flow filter according to claim 1, characterized in that the thermolabile powder shows a reduction in surface area by 15-50% after aging for 6 hours in an oven at 1000°C.

3. Wall-flow filter according to one of the preceding claims, characterized in that an undoped metal oxide selected from the group consisting of aluminum oxide, silicon dioxide, cerium oxide, zirconium oxide, titanium dioxide or mixtures or mixed oxides (solid solutions) thereof is used as the thermolabile powder.

4. Wall-flow filter according to one of the preceding claims, characterized in that the filtration efficiency of the filter containing powder in the fresh state is between 85-99.9%.

5. Wall-flow filter according to one of the preceding claims, characterized in that the powder is applied to the filter in an amount of 1-40 g/l.

6. Wall-flow filter according to one of the preceding claims, characterized in that the increase in the exhaust gas back pressure for the specified comparison is determined after 10 active soot regenerations.

7. Wall-flow filter according to one of the preceding claims, characterized in that the filter has been catalytically coated before being acted upon with the thermolabile powder.

8. Exhaust system having a wall flow filter according to one of the preceding claims and at least one further unit for reducing harmful exhaust gas components selected from the group consisting of oxidation catalytic converter, three-way catalytic converter, SCR catalytic converter, hydrocarbon trap and ammonia blocking catalytic converter.Exhaust gas system according to claim 8, characterized in that this has a close-coupled three-way catalytic converter and a wall-flow filter according to one of claims 1 - 7 which is located in the underbody of the vehicle and is provided with a three-way catalytic coating.