CATALYTICALLY ACTIVE PARTICLE FILTER HAVING A HIGH DEGREE OF FILTERING EFFICIENCY

Abstract

The invention relates to a wall flow filter for removing particulate matter from the exhaust of internal combustion engines, comprising a wall flow filter substrate having a length L, and different coatings Z and F, the wall flow filter substrate being provided with channels E and A which run parallel between a first end and a second end of the wall flow filter substrate, are separated by porous walls, and form surfaces O.sub.E and O.sub.A, respectively; channels E are closed at the second end, and channels A are closed at the first end; coating Z is disposed in the porous walls and/or on surfaces O.sub.A, but not on surfaces O.sub.E, and contains palladium and/or rhodium and a cerium/zirconium mixed oxide. The wall flow filter is characterized in that coating F is disposed in the porous walls and/or on surfaces O.sub.E, but not on surfaces O.sub.A, and comprises a mineral material and no precious metal.

Claims

1. Wall-flow filter for removing particles from the exhaust gas of combustion engines, comprising a wall-flow filter substrate of length L and coatings Z and F that differ from one another, wherein the wall-flow filter substrate has channels E and A, which extend in parallel between a first and a second end of the wall-flow filter substrate, are separated by porous walls and form surfaces O.sub.E and O.sub.A respectively, and wherein the channels E are closed at the second end and the channels A are closed at the first end, and wherein the coating Z is located in the porous walls and/or on the surfaces O.sub.A, but not on the surfaces O.sub.E, and comprises palladium and/or rhodium and a cerium/zirconium mixed oxide, wherein the coating F is located in the porous walls and/or on the surfaces O.sub.E, but not on the surfaces O.sub.A, and comprises a mineral material and no noble metal, characterized in that the mineral material is a silicate selected from the group consisting of island silicates, group silicates, ring silicates, layered silicates, chain silicates, amorphous silicates and technical silicate.

2. Wall-flow filter according to claim 1, characterized in that coating Z is located on the surfaces O.sub.A of the wall-flow filter substrate and extends from the second end of the wall-flow filter substrate to 50 to 90% of the length L.

3. Wall-flow filter according to claim 1, characterized in that coating Z is located in the porous walls of the wall-flow filter substrate and extends from the first end of the wall-flow filter substrate to 50 to 100% of the length L.

4. Wall-flow filter according to claim 1, characterized in that coating Z contains palladium and rhodium and no platinum.

5. Wall-flow filter according to claim 1, characterized in that the cerium/zirconium mixed oxide of the coating Z contains one or more rare earth metal oxides.

6. Wall-flow filter according to claim 5, characterized in that the rare earth metal oxide is lanthanum oxide, yttrium oxide, praseodymium oxide, neodymium oxide and/or samarium oxide.

7. Wall-flow filter according to claim 1, characterized in that coating Z comprises lanthanum-stabilized aluminum oxide, rhodium, palladium or palladium and rhodium, and a cerium/zirconium/rare earth metal mixed oxide containing yttrium oxide and lanthanum oxide as rare earth metal oxides.

8. Wall-flow filter according to claim 1, characterized in that coating Z comprises lanthanum-stabilized aluminum oxide, rhodium, palladium or palladium and rhodium, and a cerium/zirconium/rare earth metal mixed oxide containing praseodymium oxide and lanthanum oxide as rare earth metal oxides.

9. Wall-flow filter according to claim 1, characterized in that coating F consists of one or more mineral materials.

10. Wall-flow filter according to claim 1, characterized in that the mineral material contains one or more elements selected from the group consisting of silicon, aluminum, titanium, zirconium, cerium, iron, zinc, magnesium, calcium, potassium and sodium.

11. Wall-flow filter according to claim 1, characterized in that the mineral material has a fibrous structure.

12. Wall-flow filter according to claim 1, characterized in that it has an increasing concentration gradient of the coating F in the longitudinal direction of the filter from its first to its second end.

13. Wall-flow filter according to claim 1, characterized in that the wall-flow filter substrate has a coating Y which is different from the coatings Z and F, which comprises platinum, palladium or platinum and palladium, which contains no rhodium and no cerium/zirconium mixed oxide and which is located in the porous walls and/or on the surfaces O.sub.E, but not on the surfaces O.sub.A.

14. Wall-flow filter according to claim 13, characterized in that coating Y extends over a length of 50 to 100% of the length L.

15. Method for producing a wall-flow filter according to claim 1, characterized in that the channels E of the dry wall-flow filter substrate already coated with coating Z and optionally coating Y are impinged on by a dry powder/gas aerosol, wherein the powder contains a mineral material.

16. A method for reducing harmful exhaust gases of an internal combustion engine, comprising passing the harmful exhaust gases of the internal combustion engine through a wall-flow filter according to claim 1.

Description

[0082] FIG. 1 relates to a wall-flow filter according to the invention in which the coating Z is located in the channels A on the surfaces O.sub.A and extends from the second end of the wall-flow filter substrate over 50% of the length L. The coating F is located in the channels E and extends over the entire length L.

[0083] FIG. 2 also relates to a wall-flow filter according to the invention in which the coating Z is located in the channels A on the surfaces O.sub.A. Starting from the second end of the wall-flow filter substrate, however, it extends over 80% of the length L.

[0084] The coating F is located in the channels E and extends over the entire length L.

[0085] FIG. 3 relates to a wall-flow filter according to the invention in which the coating Z is located in the porous walls and extends over the entire length L. The coating F is located in the channels E and likewise extends over the entire length L.

[0086] FIG. 4 relates to a wall-flow filter according to the invention in which the coating Z is located in the porous walls and extends from the second end of the wall-flow filter substrate over 50% of the length L. The coating F is located in the channels E and extends over the entire length L.

[0087] FIG. 5 relates to a wall-flow filter according to the invention which differs from that of FIG. 4 only in that additionally coating Y is located in the porous walls over the entire length L. The coating F is located in the channels E and extends over the entire length L.

[0088] FIG. 6 relates to a wall-flow filter according to the invention which differs from that of FIG. 4 only in that, in addition, coating Y extends in the porous walls, starting from the first end of the wall-flow filter substrate and extending over 50% of the length L. The coating F is located in the channels E and extends over the entire length L.

[0089] FIG. 7 relates to a wall-flow filter according to the invention in which the coating Z is located in the channels A on the surfaces O.sub.A and extends over 50% of the length L. In addition, coating Y is located in the channels E on the surfaces O.sub.E and extends from the first end of the wall-flow filter substrate over 50% of the length L. The coating F is located the channels E and extends from the second end of the wall-flow filter substrate over 50% of the length L.

[0090] FIG. 8 relates to a wall-flow filter according to the invention in which the coating Z is located in the porous walls and extends over the entire length L. In addition, coating Y is located in the channels E on the surfaces O.sub.E and extends from the first end of the wall-flow filter substrate over 50% of the length L. The coating F is located the channels E and extends from the second end of the wall-flow filter substrate over 50% of the length L.

[0091] FIG. 9 relates to a wall-flow filter according to the invention in which the coating Z is located in the porous walls and extends from the second end of the wall-flow filter substrate over 50% of the length L. In addition, coating Y is located in the channels E on the surfaces O.sub.E and extends from the first end of the wall-flow filter substrate over 50% of the length L. The coating F is located the channels E and extends from the second end of the wall-flow filter substrate over 50% of the length L.

[0092] FIG. 10 relates to a wall-flow filter according to the invention in which the coating Z is located in the channels A on the surfaces O.sub.A and extends from the second end of the wall-flow filter substrate over 80% of the length L. In addition, coating Y is located in the porous walls and extends over the entire length L. The coating F is located the channels E and extends over the entire length L.

[0093] The wall-flow filter according to the invention can be produced by applying the coatings Z, F and, if present, Y to a wall-flow filter substrate. In this case, the catalytic activity is provided as specified by the person skilled in the art by coating the wall-flow filter substrate with coating Z and, if present, with coating Y.

[0094] The term coating is accordingly to be understood to mean the application of catalytically active materials to a wall-flow filter substrate. The coating assumes the actual catalytic function. In the present case, the coating is carried out by applying a correspondingly low-viscosity aqueous suspension of the catalytically active components, also referred to as a washcoat, into or onto the wall of the wall-flow filter substrate, for example in accordance with EP1789190B1. After application of the suspension, the wall-flow filter substrate is dried in each case and, if applicable, calcined at an increased 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 (coating Z or sum of the coatings Z and Y). The most suitable amount of loading of a filter coated in the wall depends on its cell density, its wall thickness, and the porosity.

[0095] The coating F is applied to the wall-flow filter substrate in particular by impinging a dry powder/gas aerosol on the channels E of the dry wall-flow filter substrate already coated with coating Z and optionally coating Y, wherein the powder contains a mineral material and in particular consists of a mineral material.

[0096] By impinging a dry powder/gas aerosol on a wall-flow filter substrate which has been wet-coated in the conventional manner with coating Z and optionally Y dried and optionally calcined, a wall-flow filter according to the invention is obtained which has extremely good filtration efficiency and only slightly increased exhaust gas back pressure and, at the same time, excellent catalytic efficiency.

[0097] The wall-flow filters which are catalytically coated according to the invention and then impinged on by powder, differ from those that are produced in the exhaust system of a vehicle by ash deposition during operation. According to the invention, the catalytically active wall-flow filter substrates are selectively powder-sprayed with a specific, dry powder. As a result, the balance between filtration efficiency and exhaust-gas back pressure can be adjusted selectively right from the start. Wall-flow filters in which undefined ash deposits have resulted from combustion of fuel, e.g., in the cylinder during driving operation or by means of a burner, are therefore not included in the present invention. When the dry powder/gas aerosol is impinged on the wall-flow filter substrates considered here, the powder particles are deposited in the pores of the wall-flow filter substrate and, optionally, on the surfaces O.sub.E following the flow of the gas. In the process, the different wall permeability of the wall-flow filter substrate (e.g., due to inhomogeneities of the filter wall itself or different coating zones) leads to selective deposition of the powder in the pores of the wall or on the surfaces O.sub.E where the flow is the greatest. This effect also results in, for example, cracks or pores in the washcoat layer being filled up by the porous powder due to coating defects, such that the soot particles in the exhaust gas are later increasingly retained as the exhaust gas passes through the filter. A better filtration efficiency is consequently the result.

[0098] According to the invention, the dry wall-flow filter substrate coated with coating Z and optionally coating Y is covered with a powder starting from its first and in the direction of its second end (i.e. with respect to the intended use in the direction of exhaust gas flow) in such a way that the cell wall regions through which the flow is strongest are covered with loose, inherently porous powder accumulations in the porous walls and/or also on the surfaces O.sub.E in order to obtain a desired increased filtration efficiency. In the process, the formation of the intrinsically porous powder accumulations surprisingly leads to a relatively low increase in back pressure. In a preferred embodiment, the wall-flow filter substrate is impinged with a powder/gas aerosol in such a way that during impingement, the powder is deposited in the pores of the porous wall and on the surfaces O.sub.E and builds up a cohesive layer here. In a further most preferred embodiment, the wall-flow filter is impinged with a powder/gas aerosol such that during impingement, the powder precipitates in the pores of the filter walls and fills them as far as the surfaces O.sub.E and thereby does not form a cohesive layer on surfaces O.sub.E.

[0099] In order for the powder of the powder/gas aerosol to deposit sufficiently well in the pores of the wall-flow filter substrate coated with coating Z and, optionally, coating Y, or to adhere to the surfaces O.sub.E, the particle diameter in the aerosol should be at least smaller than the pores of the wall-flow filter substrate. This can be expressed by the ratio of the average particle diameter (Q.sub.3 distribution, measured according to the most recent ISO 13320 on the date of application) d.sub.50 in the dry aerosol and the average pore diameter of the wall-flow filter after coating (measured according to DIN 66134, latest version on the date of application) being 0.03 to 2, preferably 0.05 to 1.43 and very particularly preferably 0.05 to 0.63. As a result, the particles of the powder in the aerosol, following the gas flow, can precipitate in the pores of the walls of the wall-flow filter substrate. A suitable powder has in particular a specific surface area of at least 100 m.sup.2/g and a total pore volume of at least 0.3 ml/g.

[0100] For a powder suitable for producing the wall-flow filters according to the invention, an optimization between the largest possible surface area of the powder used, the crosslinking, and the adhesive strength is advantageous. During operation in the vehicle, small particles follow the flow lines approximately without inertia due to their low particle relaxation time. A random trembling movement is superimposed on this uniform, convection-driven movement. Following this theory, the largest possible flowed-around surfaces should be provided for a good filtration effect of a wall-flow filter impinged on by powder. The powder should therefore have a high proportion of fines, since with the same total volume of mineral material, small particles offer significantly larger surfaces. At the same time, however, the pressure loss must only increase insignificantly. This requires a loose crosslinking of the powder. For a filtration efficiency-enhancing coating, it is preferable to use powders having a tapped density of between 50 g/l and 900 g/l, preferably between 200 g/l and 850 g/l and very preferably between 400 g/l and 800 g/l.

[0101] The aerosol consisting of the gas and the powder may be produced in accordance with the requirements of the person skilled in the art or as illustrated below. For this purpose, a powder is usually mixed with a gas (http://www.tsi.com/Aerosolqeneratoren-und-disperqierer/; https://www.palas.de/de/product/aerosolqeneratorssolidparticles). This mixture of gas and powder produced in this way is then advantageously fed into the channels E of the wall-flow filter substrate via a gas stream.

[0102] All gases considered by the person skilled in the art for the present purpose can be used as gases for producing the aerosol and for introduction into the wall-flow filter substrate. The use of air is very particularly preferred. However, it is also possible to use other reaction gases which can develop either an oxidizing (e.g., O.sub.2, NO.sub.2) or a reducing (e.g., H.sub.2) activity with respect to the powder used. With certain powders, the use of inert gases (e.g., N.sub.2) or noble gases (e.g., He) may also prove advantageous. Mixtures of the listed gases are also conceivable. In order to be able to deposit the powder to a sufficient depth into the channels E and with good adhesion, a certain suction power is needed. In orientation experiments for the respective wall-flow filter and the respective powder, the person skilled in the art can form their own idea in this respect. It has been found that the aerosol (powder/gas mixture) is preferably sucked through the wall-flow filter at a velocity of 5 m/s to 60 m/s, more preferably 10 m/s to 50 m/s, and very particularly preferably 15 m/s to 40 m/s. This likewise achieves an advantageous adhesion of the applied powder.

[0103] Dispersion of the powder in the gas for establishing a powder/gas aerosol takes place in various ways. Preferably, the dispersion of the powder is generated by at least one or a combination of the following measures: compressed air, ultrasound, sieving, in situ milling, blowers, expansion of gases, fluidized bed. Further dispersion methods not mentioned here can likewise be used by the person skilled in the art. In principle, the person skilled in the art is free to select a method for producing the powder/gas aerosol. As already described, the powder is first converted by dispersion into a powder/gas aerosol and then guided into a gas stream. This mixture of the gas and the powder thus produced is only subsequently introduced into an existing gas stream, which carries the finely distributed powder into the channels E of the wall-flow filter substrate. This process is preferably assisted by a suction device which is positioned in the pipeline on the outflow side of the filter. This is in contrast to the device shown in FIG. 3 of U.S. Pat. No. 8,277,880B, in which the powder/gas aerosol is produced directly in the gas stream. The method according to the invention allows a much more uniform and good mixing of the gas stream with the powder/gas aerosol, which ultimately ensures an advantageous distribution of the powder particles in the filter in the radial and axial direction and thus helps to make uniform and control the deposition of the powder particles on the filter.

[0104] The powder is dry when the wall-flow filter substrate is impinged on in the sense of the invention. The powder is preferably mixed with ambient air and applied to the filter. By mixing the powder/gas aerosol with particle-free gas, preferably dry ambient air, the concentration of the particles is reduced to such an extent that no appreciable agglomeration takes place until deposition in the wall-flow filter substrate. This preserves the particle size in the aerosol adjusted during the dispersion.

[0105] A preferred device for producing a wall-flow filter according to the invention is schematically illustrated in FIG. 12. Such a device is characterized in that [0106] at least one unit for dispersing powder in a gas; [0107] a unit for mixing the dispersion with an existing gas stream; [0108] at least two filter-receiving units designed to allow the gas stream to flow through the filter without further supply of a gas; [0109] a suction-generating unit that maintains the gas stream through the filter; [0110] optionally, a unit for generating vortices upstream of the filter so that deposition of powder on the inlet plugs of the filter is prevented as much as possible; [0111] and optionally, a unit by which at least one partial gas stream is extracted from the outflow side of the suction device and, before the powder addition, is added to the gas stream which is sucked through the filter;
are available.

[0112] In this preferred embodiment of the method according to the invention, as shown in the drawing of FIG. 11, at least a partial gas stream is extracted from the outflow side of the suction device and added, before the powder addition, back to the gas stream that is sucked through the filter. The powder is thereby metered into an already heated air stream. The suction blowers for the necessary pressures generate approximately 70 C. exhaust air temperature since the installed suction power is preferably >20 kW. In an energetically optimized manner, the waste heat of the suction blower is used to heat the supply air in order to reduce the relative humidity of the supply air. This in turn reduces the adhesion of the particles to one another and to the input plugs. The deposition process of the powder can thus be better controlled. In the present method for producing a wall-flow filter according to the invention, a gas stream is impinged on by a powder/gas aerosol and sucked into a wall-flow filter substrate. This ensures that the powder can be distributed sufficiently well in the gas stream for it to be able to penetrate into the channels E. Homogeneous distribution of the powder in the gas/air requires intensive mixing. For this purpose, diffusers, venturi mixers, and static mixers are known to the person skilled in the art. Particularly suitable for the powder coating process are mixing devices that avoid powder deposits on the surfaces of the coating system. Diffusers and venturi tubes are thus preferably used for this process. The introduction of the dispersed powder into a fast-rotating rotating flow with a high turbulence has also proven effective. In order to achieve an advantageous uniform distribution of the powder over the cross section of the wall-flow filter substrate, the gas transporting the powder should have a piston flow (if possible, the same velocity over the cross section) when impinging on the filter. This is preferably adjusted by an accelerated flow upstream of the filter. As is known to the person skilled in the art, a continuous reduction of the cross section without abrupt changes causes such an accelerated flow, described by the continuity equation. Furthermore, it is also known to the person skilled in the art that the flow profile is thus more closely approximated to a piston profile. For the targeted change of the flow, built-in components, such as sieves, rings, disks, etc., can be used below and/or above the filter.

[0113] In a further advantageous design of the present method, the apparatus for powder coating has one or more devices (turbulators, vortex generators) with which the gas stream carrying the powder/gas aerosol can be vortexed prior to impingement on the filter. As an example in this respect, corresponding sieves or grids can be used which are placed at a sufficient distance on the inflow side of the wall-flow filter substrate. The distance should not be too large or small so that sufficient vortexing of the gas stream directly upstream of the wall-flow filter substrate is achieved. The person skilled in the art can determine the distance in simple experiments. The advantage of this measure is explained by the fact that powder constituents do not deposit on the plugs of the channels A and all the powder can penetrate into the channels E. Accordingly, it is preferred according to the invention if the powder is vortexed before flowing into the filter in such a way that deposits of powder on the plugs of the wall-flow filter substrate are avoided as far as possible. A turbulator or turbulence generator or vortex generator in aerodynamics refers to equipment which causes an artificial disturbance of the flow. As is known to the person skilled in the art, vortices (in particular microvortices) form behind rods, gratings, and other flow-interfering built-in components at corresponding Re numbers. Known are the Karman vortex street (H. Benard, C. R. Acad. Sci. Paris. Ser. IV 147, 839 (1908); 147, 970 (1908); T von Karman, Nachr. Ges. Wiss. Gottingen, Math. Phys. KI. 509 (1911); 547 (1912)) and the wake turbulence behind airplanes which can cover roofs. In the case according to the invention, this effect can be intensified very particularly advantageously by vibrating self-cleaning sieves (so-called ultrasonic screens) which advantageously move in the flow. Another method is the disturbance of the flow through sound fields, which excites the flow to turbulences as a result of the pressure amplitudes. These sound fields can even clean the surface of the filter without flow. The frequencies may range from ultrasound to infrasound. The latter measures are also used for pipe cleaning in large-scale technical plants.

[0114] The preferred embodiments for the wall-flow filter apply mutatis mutandis also to the method. Reference is explicitly made in this respect to what was said above about the wall-flow filter.

[0115] Dry in the sense of the present invention means exclusion of the application of a liquid, in particular water. In particular, the production of a suspension of the powder in a liquid for spraying into a gas stream should be avoided. A certain moisture content may possibly be tolerable both for the filter and for the powder, provided that achieving the objective, i.e., the most finely distributed deposition of the powder in the porous walls and/or the surfaces O.sub.E of the wall-flow filter substrate possible, is not negatively affected. As a rule, the powder is free-flowing and dispersible by energy input. The moisture content of the powder or of the wall-flow filter substrate at the time of being impinged on by the powder should be less than 20%, preferably less than 10%, and very particularly preferably less than 5% (measured at 20 C. and normal pressure, ISO 11465, latest version on the filing date).

[0116] The wall-flow filter according to the invention exhibits an excellent filtration efficiency with only a moderate increase in exhaust-gas back pressure as compared to a wall-flow filter in the fresh state that has not been impinged on by powder. The wall-flow filter according to the invention preferably exhibits an improvement in soot particle deposition (filtering effect) in the filter of at least 5%, preferably at least 10% and very particularly preferably at least 20% at a relative increase in the exhaust-gas back pressure of the fresh wall-flow filter of at most 40%, preferably at most 20% and very particularly preferably at most 10% as compared to a fresh filter coated with catalytically active material but not treated with powder. The slight increase in back pressure is probably due to the cross section of the channels on the inlet side not being significantly reduced by impinging, according to the invention, the filter with a powder. It is assumed that the powder in itself forms a porous structure, which has a positive effect on the back pressure. For this reason, a wall-flow filter according to the invention should also exhibit better exhaust-gas back pressure than those of the prior art, with which a powder was deposited on the walls of the inlet side of a filter or a traditional coating using wet techniques was chosen.

[0117] Coating Z gives the wall-flow filter according to the invention excellent three-way activity, while the optional coating Y is able to reduce the soot ignition temperature and thus facilitates soot burn-off.

[0118] The present invention thus also relates to the use of a wall-flow filter according to the invention for reducing harmful exhaust gases of an internal combustion engine. The use of the wall-flow filter according to the invention for treating exhaust gases of a stoichiometrically operated internal combustion engine, i.e. in particular a gasoline-operated internal combustion engine, is preferred. The wall-flow filter according to the invention is very advantageously used in combination with at least one three-way catalyst. In particular, it is advantageous if a three-way catalyst is located in a position close to the engine on the inflow side of the wall-flow filter according to the invention. It is also advantageous if a three-way catalyst is located on the outflow side of the wall-flow filter according to the invention. It is also advantageous if a three-way catalyst is located on the inflow side and on the outflow side of the wall-flow filter. The preferred embodiments described for the wall-flow filter according to the invention also apply mutatis mutandis to the use mentioned here. The preferred embodiments described for the wall-flow filter according to the invention also apply mutatis mutandis to the use mentioned here.

[0119] The present invention further relates to an exhaust gas purification system comprising a filter according to the invention and at least one further catalyst. In one embodiment of this system, at least one further catalyst is arranged upstream of the filter according to the invention. Preferably, this is a three-way catalyst or an oxidation catalyst or a NOx storage catalyst. In a further embodiment of this system, at least one further catalyst is arranged downstream of the filter according to the invention. Preferably, this is a three-way catalyst or an SCR catalyst or a NOx storage catalyst or an ammonia slip catalyst. In a further embodiment of this system, at least one further catalyst is arranged upstream of the filter according to the invention and at least one further catalyst is arranged downstream of the filter according to the invention. Preferably, the upstream catalyst is a three-way catalyst or an oxidation catalyst or a NOx storage catalyst and the downstream catalyst is a three-way catalyst or an SCR catalyst or a NOx storage catalyst or an ammonia slip catalyst. The preferred embodiments described for the wall-flow filter according to the invention also apply mutatis mutandis to the exhaust gas purification system mentioned here.

[0120] Typically, the filter according to the invention is used primarily in internal combustion engines, in particular in internal combustion engines with direct injection or intake manifold injection. These are preferably stoichiometrically operated gasoline or natural gas engines. Preferably, these are motors with turbocharging.

[0121] The requirements applicable to gasoline particulate filters (GPF) differ significantly from the requirements applicable to diesel particulate filters (DPF). Diesel engines without DPF can have up to ten times higher particle emissions, based on the particle mass, than gasoline engines without GPF (Maricq et al., SAE 1999-01-01530). In addition, there are significantly fewer primary particles in the case of gasoline engines, and the secondary particles (agglomerates) are significantly smaller than in diesel engines. Emissions from gasoline engines range from particle sizes of less than 200 nm (Hall et al., SAE 1999-01-3530) to 400 nm (Mathis et al., Atmospheric Environment 38 4347) with a maximum in the range of around 60 nm to 80 nm. For this reason, the nanoparticles in the case of GPF must mainly be filtered by diffusion separation. For particles smaller than 300 nm, separation by diffusion (Brownian molecular motion) and electrostatic forces becomes more and more important with decreasing size (Hinds, W.: Aerosol technology: Properties and behavior and measurement of airborne particles. Wiley, 2nd edition 1999).

[0122] FIGS. 1 to 10 show the different coating arrangements of wall-flow filters according to the invention, which are already described in more detail above. The following designations are used therein: [0123] (E) the inlet channel/inflow channel of the wall-flow filter [0124] (A) the outlet channel/outflow channel of the wall-flow filter [0125] (O.sub.E) the surfaces formed by the inlet channels (E) [0126] (O.sub.A) the surfaces formed by the outlet channels (A) [0127] (L) the length of the filter wall [0128] (Z) the coating Z [0129] (Y) the coating Y [0130] (F) the coating F

[0131] FIG. 12 shows a schematic drawing of an advantageous device for impinging the filters with a powder. The powder 420 or 421 is mixed with the gas under pressure 451 through the atomizer nozzle 440 in the mixing chamber with the gas stream 454 and then is sucked or pushed through the filter 430. The particles that have penetrated are filtered out in the exhaust filter 400. The blower 410 provides the necessary volumetric flow. The exhaust gas is divided into an exhaust 452 and a warm cycle gas 453. The warm cycle gas 453 is mixed with the fresh gas 450.

[0132] The advantages of the invention are explained using examples below.

Comparative Example 1: Coating Z Only

[0133] Aluminum oxide stabilized with lanthanum oxide was suspended in water with a first oxygen storage component, which comprised 40% by weight cerium oxide, zirconium oxide, lanthanum oxide and praseodymium oxide, and a second oxygen storage component, which comprised 24% by weight cerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide. Both oxygen storage components were used in equal parts. The weight ratio of aluminum oxide and oxygen storage component was 30:70. The suspension thus obtained was subsequently mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly for coating a commercially available wall-flow filter substrate, the coating Z being introduced into the porous filter wall over 100% of the substrate length. The total load of this filter amounted to 75 g/l; the total precious metal load amounted to 2.12 g/l with a ratio of palladium to rhodium of 5:1. The coated filter thus obtained was dried and then calcined. It is hereinafter referred to as VGPF1.

Example 1 According to the Invention: Coating Z in Combination with Coating F

[0134] Aluminum oxide stabilized with lanthanum oxide was suspended in water with a first oxygen storage component, which comprised 40% by weight cerium oxide, zirconium oxide, lanthanum oxide and praseodymium oxide, and a second oxygen storage component, which comprised 24% by weight cerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide. Both oxygen storage components were used in equal parts. The weight ratio of aluminum oxide and oxygen storage component was 30:70. The suspension thus obtained was subsequently mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly for coating a commercially available wall-flow filter substrate, the coating Z being introduced into the porous filter wall over 100% of the substrate length. The total load of this filter amounted to 75 g/l; the total precious metal load amounted to 2.12 g/l with a ratio of palladium to rhodium of 5:1. The coated filter thus obtained was dried and then calcined. Subsequently, the filter was impinged on by a dry powder/gas aerosol, wherein 4 g/L of a mineral material was introduced into the channels E. It is hereinafter referred to as GPF1.

[0135] The two filters thus obtained were subsequently measured on a cold blast test bench in order to determine the pressure loss over the respective filter. At room temperature and a volumetric flow rate of 600 m.sup.3/h of air, the back pressure is 29 mbar for the VGPF1 and 39 mbar for the GPF1. As already described, the filtration coating F only leads to a moderate increase in back pressure. Furthermore, the two filters were investigated with respect to the back pressure after soot loading. For this purpose, both filters were sooted on an engine test bench with a direct-injection turbocharged engine. The final soot loading was about 3 g. Under these conditions, the soot back pressure of the VGPF1 is 73 mbar and that of the GPF1 only 44 mbar. At the same time, fresh VGPF1 and GPF1 filters were investigated in the vehicle in terms of their particle filtration efficiency. For this purpose, the filters were measured in an RTS-95, also known as RTC-aggressive, driving cycle in a position close to the engine between two particle counters. In both cases, a three-way catalyst was located upstream in the exhaust tract, through which the lambda control of the vehicle was effected. Here the filter GPF1 according to the invention has a filtration efficiency of 96%, calculated from the particle values of the two particle counters, while the comparative filter VGPF1 achieves a filtration efficiency of only 42%. Overall, it can be seen that the combination of filtration coating F and the three-way coating Z is particularly advantageous in terms of back pressure after soot loading and filtration efficiency.