PARTICLE FILTER FOR EXHAUST GAS OF GASOLINE ENGINES

Abstract

The present invention is directed to a wall-flow filter which is used in particular in exhaust gas systems of vehicles driven by gasoline engines. The filter has three-way activity and filters fine particles, which result from the combustion of gasoline, from the exhaust gas stream. The invention also relates to a method for producing a corresponding filter and to the preferred use thereof.

Claims

1. Wall-flow filter for reducing particle emissions in the exhaust gas of gasoline engines having the length L, wherein the wall-flow filter comprises channels E and A that extend in parallel between a first and a second end of the wall-flow filter and are separated by porous walls forming surfaces OE and OA, respectively, and wherein the channels E are closed at the second end and the channels A are closed at the first end, characterized in that said wall-flow filter has two catalytically active coatings applied to the surfaces OE and OA in separate coating steps, wherein a first coating extends from the first end of the wall-flow filter over 55 to 96% of the length L and a second coating extends from the first end of the wall-flow filter over 10 to 40% of the length L and wherein a third coating extends from the second end of the wall-flow filter over 55 to 96% of the length L and a fourth coating extends from the second end of the wall-flow filter over 10 to 40% of the length L.

2. Wall-flow filter according to claim 1, characterized in that a first coating extends from the first end of the wall-flow filter over 55 to 80% of the length L and a second coating extends from the first end of the wall-flow filter over 20 to 40% of the length L and wherein a third coating extends from the second end of the wall-flow filter over 55 to 80% of the length L and a fourth coating extends from the second end of the wall-flow filter over 20 to 40% of the length L.

3. Wall-flow filter according to claim 1, characterized in that the ratio of the amount of catalytically active coating in g/l between the first coating and the second coating or between the third and fourth coating is less than or equal to 1:1 and greater than or equal to 1:3.

4. Wall-flow filter according to claim 1, characterized in that the coatings on the surface OE and OA are different in each case.

5. Wall-flow filter according to claim 1, characterized in that first the first coating and the third coating are applied to the wall-flow filter before the second and the fourth coating are applied.

6. Wall-flow filter according to claim 1, characterized in that at least one of the coatings 1-4 contains stabilized aluminum oxide in an amount from 20 to 70% by weight based on the total weight of the coating, rhodium, palladium or palladium and rhodium and one or more oxygen storage components in an amount from 30 to 80% by weight based on the total weight of the respective coating.

7. Method for producing a wall-flow filter according to claim 1, characterized in that it has the following steps: i) a first coating suspension is introduced in excess via the first end by applying a pressure difference into the wall-flow filter via the vertically locked wall-flow filter, and a pressure difference reversal removes an excess of the first coating suspension from the wall-flow filter; ii) a third coating suspension is introduced in excess via the second end by applying a pressure difference into the wall-flow filter via the vertically locked wall-flow filter, and a pressure difference reversal removes an excess of the third coating suspension from the wall-flow filter; iii) a second coating suspension is introduced into the wall-flow filter via the first end by applying a pressure difference across the vertically locked wall-flow filter; iv) a fourth coating suspension is introduced into the wall-flow filter via the second end by applying a pressure difference across the vertically locked wall-flow filter.

8. Method according to claim 7, characterized in that the coating suspensions in steps iii) and iv) are introduced into the wall-flow filter without an excess of coating suspension.

9. Method according to claim 7, characterized in that no separate drying of the wall-flow filter takes place between the coating steps.

10. Use of a wall-flow filter according to claim 1 in an exhaust gas system for gasoline engine exhaust gases.

11. Use according to claim 10, characterized in that exhaust gas reduction units selected from the group consisting of three-way catalysts, SCR catalysts, nitrogen oxide storage catalysts, hydrocarbon traps, nitrogen oxide traps are present as further exhaust gas reduction units in the exhaust gas system.

Description

FIGURES

[0040] FIG. 1 shows a particle filter of the prior art not according to the invention which comprises a wall-flow filter of length L (1) having channels E (2) and channels A (3) that extend in parallel between a first end (4) and a second end (5) of the wall-flow filter and are separated by porous walls (6), which form surfaces OE (7) and OA (8), respectively, and wherein the channels E (2) are closed at the second end (5) and the channels A (3) are closed at the first end (4). A first coating (9) is located in the channels E (2) on the surfaces OE (7) and a second coating (10) is located in the channels A (3) on the surfaces OA (8). This filter is an intermediate product after the first and third coating have been applied.

[0041] FIG. 2a-d show schematic architectures according to the invention. The layout of FIG. 1 is to be considered in accordance with FIG. 2a-d. The same reference signs apply. The sizes of the rectangles (I, II, III, IV) symbolize the respective amounts of coating. Therefore, the embodiments advantageous according to the invention are to be made clear with a corresponding coating ratio between I/II and III/IV.

[0042] FIG. 3 shows the improved catalytic activity of the embodiments according to the invention of FIG. 2a=[1]; 2b=[2]; 2c=[3]; 2d=[4] in comparison to the reference according to FIG. 1.

[0043] FIG. 4 shows the exhaust gas back pressure of the embodiments of FIG. 2a=[1]; 2b=[2]; 2c=[3]; 2d=[4] in comparison to the reference according to FIG. 1.

[0044] FIG. 5 shows the filtration efficiency of the embodiments according to the invention of FIG. 2a=[1]; 2b=[2]; 2c=[3]; 2d=[4] in comparison to the reference according to FIG. 1.

EXAMPLES

Reference

[0045] 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 to coat a commercially available wall flow filter substrate. The coating suspension was coated onto the filter walls of the substrate, first in the input channels to a length of 60% of the filter length. The load of the inlet channel amounted to 83.33 g/l; the precious metal load amounted to 1.06 g/l with a ratio of palladium to rhodium of 5:1. The coated filter thus obtained was dried and then calcined. Then, the output channels of the filter were coated to a length of 60% of the filter length with the same coating suspension. The coated filter thus obtained was dried again and then calcined. The total load of this filter thus amounted to 100 g/l; the total precious metal load amounted to 1.27 g/l with a ratio of palladium to rhodium of 5:1. It is hereinafter referred to as reference.

Example 1

[0046] 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 to coat a commercially available wall flow filter substrate. The coating suspension was coated onto the filter walls of the substrate, first in the input channels to a length of 25% of the filter length (II). The load of this zone amounted to 100 g/l; the precious metal load amounted to 1.27 g/l with a ratio of palladium to rhodium of 5:1. The coated filter thus obtained was dried and then calcined. Then, the output channels of the filter were coated to a length of 25% of the filter length (IV) with the same coating suspension. The coated filter thus obtained was dried again and then calcined. In the third step, the coating suspension was coated in the input channels to a length of 60% of the filter length (l). The load of this zone amounted to 41.67 g/l; the precious metal load amounted to 0.53 g/l with a ratio of palladium to rhodium of 5:1. The coated filter thus obtained was dried and then calcined. In the last step, the output channels of the filter were coated to a length of 60% of the filter length (III) with the same coating suspension. The coated filter thus obtained was dried again and then calcined. The total load of this filter thus amounted to 100 g/l; the total precious metal load amounted to 1.27 g/l with a ratio of palladium to rhodium of 5:1. It is hereinafter referred to as 1.

Example 2

[0047] 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 to coat a commercially available wall flow filter substrate. The coating suspension was coated onto the filter walls of the substrate, first in the input channels to a length of 25% of the filter length (II). The load of this zone amounted to 66.67 g/l; the precious metal load amounted to 0.85 g/l with a ratio of palladium to rhodium of 5:1. The coated filter thus obtained was dried and then calcined. Then, the output channels of the filter were coated to a length of 25% of the filter length (IV) with the same coating suspension. The coated filter thus obtained was dried again and then calcined. In the third step, the coating suspension was coated in the input channels to a length of 60% of the filter length (l). The load of this zone amounted to 55.56 g/l; the precious metal load amounted to 0.71 g/l with a ratio of palladium to rhodium of 5:1. The coated filter thus obtained was dried and then calcined. In the last step, the output channels of the filter were coated to a length of 60% of the filter length (III) with the same coating suspension. The coated filter thus obtained was dried again and then calcined. The total load of this filter thus amounted to 100 g/l; the total precious metal load amounted to 1.27 g/l with a ratio of palladium to rhodium of 5:1. It is hereinafter referred to as 2.

Example 3

[0048] 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 to coat a commercially available wall flow filter substrate. The coating suspension was coated onto the filter walls of the substrate, first in the input channels to a length of 60% of the filter length (l). The load of this zone amounted to 41.67 g/l; the precious metal load amounted to 0.53 g/l with a ratio of palladium to rhodium of 5:1. The coated filter thus obtained was dried and then calcined. Then, the output channels of the filter were coated to a length of 60% of the filter length (III) with the same coating suspension. The coated filter thus obtained was dried again and then calcined. In the third step, the coating suspension was coated in the input channels to a length of 25% of the filter length (II). The load of this zone amounted to 100 g/l; the precious metal load amounted to 1.27 g/l with a ratio of palladium to rhodium of 5:1. The coated filter thus obtained was dried and then calcined. In the last step, the output channels of the filter were coated to a length of 25% of the filter length (IV) with the same coating suspension. The coated filter thus obtained was dried again and then calcined. The total load of this filter thus amounted to 100 g/l; the total precious metal load amounted to 1.27 g/l with a ratio of palladium to rhodium of 5:1. It is hereinafter referred to as 3.

Example 4

[0049] 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 to coat a commercially available wall flow filter substrate. The coating suspension was coated onto the filter walls of the substrate, first in the input channels to a length of 60% of the filter length (l). The load of this zone amounted to 55.56 g/l; the precious metal load amounted to 0.71 g/l with a ratio of palladium to rhodium of 5:1. The coated filter thus obtained was dried and then calcined. Then, the output channels of the filter were coated to a length of 60% of the filter length (III) with the same coating suspension. The coated filter thus obtained was dried again and then calcined. In the third step, the coating suspension was in the input channels to a length of 25% of the filter length (II). The load of this zone amounted to 66.67 g/l; the precious metal load amounted to 0.85 g/l with a ratio of palladium to rhodium of 5:1. The coated filter thus obtained was dried and then calcined. In the last step, the output channels of the filter were coated to a length of 25% of the filter length (IV) with the same coating suspension. The coated filter thus obtained was dried again and then calcined. The total load of this filter thus amounted to 100 g/l; the total precious metal load amounted to 1.27 g/l with a ratio of palladium to rhodium of 5:1. It is hereinafter referred to as 4.

[0050] Catalytic Characterization

[0051] All five particle filters were aged together in an engine test bench aging process. This aging process consists of an overrun cut-off aging process with an exhaust gas temperature of 950? C. before the catalyst inlet (maximum bed temperature of 1030? C.). The aging time was 38 hours (see Motortechnische Zeitschrift, 1994, 55, 214-218). The catalytically active particle filters were then tested in the aged state on an engine test bench in the so-called light-off test. In the light-off test, the light-off behavior is determined in the case of a stoichiometric exhaust gas composition with a constant average air ratio ? (?=0.999 with ?3.4% amplitude). The results are shown in FIG. 3. The particle filters 1-4 and in particular 1 and 3 according to the invention show a significant improvement in light-off behavior compared to the reference.

[0052] Physical Characterization

[0053] All five particle filters were compared with respect to the exhaust gas back pressure on the engine test bench. The average exhaust gas back pressure in a WLTC driving cycle is shown in FIG. 4. As expected, the changed distribution of the coating suspension results in a slight increase in back pressure compared to the reference. In addition, all five particle filters were tested on the engine test bench for filtration efficiency in WLTC driving cycles. FIG. 5 shows the filtration efficiency in the first WLTC cycle. Filtration efficiency was improved for all particle filters 1-4 according to the invention compared to the reference, particularly for 2 and 4.