CATALYTICALLY ACTIVE PARTICULATE FILTER

Abstract

The present invention relates to a particulate filter which comprises a wall-flow filter of length L and two different catalytically active coatings Y and Z, 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 which form the 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. The invention is characterized in that the coating Y is located in the channels E on the surfaces O.sub.E and the coating Z is located in the porous walls.

Claims

1. A particulate filter for removing particles, carbon monoxide, hydrocarbons and nitrogen oxides from the exhaust gas of combustion engines fueled by stoichiometric air-fuel mixtures, which filter comprises a wall-flow filter of length L and two different coatings Y and Z, 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 which form the 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, wherein 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 over a length of 51 to 90% of the length L with a thickness between 5-250 μm and coating Z is located in the porous walls and extends from the second end of the wall-flow filter over a length of 60 to 100% of the length L such that from 10% to 49% of the porous walls of channels E are exposed as to enable exhaust gas contact with coating Y followed by exhaust gas flow to the exposed porous walls in channels E and into contact with coating Z.

2. The particulate filter in accordance with claim 1, wherein the coating Y extends from the first end of the wall-flow filter over 51 to 80% of the length L of the wall-flow filter such that from 20% to 49% of the porous walls of channels E are exposed.

3. The particulate filter in accordance with claim 2, wherein the coating Y extends from the first end of the wall-flow filter over 57 to 65% of the length L of the wall-flow filter such that 35% to 43% of the porous walls of channels E are exposed.

4. The particulate filter in accordance with claim 1, wherein the coating Y has a thickness between 10-200 μm.

5. The particulate filter in accordance with claim 1, wherein each of the coatings Y and Z contains one or more noble metals fixed to one or more substrate materials, and one or more oxygen storage components, with each of coatings Y and Z containing at least some common material in each category of noble metals, substrate materials, and oxygen storage components.

6. (canceled)

7. The particulate filter in accordance with claim 5, wherein each of the coatings Y and Z contains the noble metals palladium, rhodium or palladium and rhodium.

8. The particulate filter in accordance with of claim 5, wherein the substrate materials for coatings Y and Z are the same and are metal oxides with a BET surface area of 30 to 250 m.sup.2/g (determined according to DIN 66132—newest version on the date of application), and wherein the oxygen storage materials for coatings Y and Z share at least one difference in material composition.

9. The particulate filter in accordance with claim 5, wherein the substrate materials for the noble metals are selected from the series consisting of aluminum oxide, doped aluminum oxide, silicon oxide, titanium dioxide and mixed oxides of one or more of these.

10. The particulate filter in accordance with claim 5, wherein the coatings Y and Z contain a cerium/zirconium/rare earth metal mixed oxide as oxygen storage component.

11. The particulate filter in accordance with claim 10, wherein the cerium/zirconium/rare earth metal mixed oxides contain lanthanum oxide, yttrium oxide, praseodymium oxide, neodymium oxide and/or samarium oxide as rare earth metal oxide.

12. The particulate filter in accordance with claim 10, wherein the cerium/zirconium/rare earth metal mixed oxides contain lanthanum oxide and yttrium oxide, yttrium oxide and praseodymium oxide or lanthanum oxide and praseodymium oxide as rare earth metal oxide.

13. The particulate filter in accordance with claim 5, wherein the coatings Y and Z both comprise lanthanum-stabilized aluminum oxide, palladium, rhodium or palladium and rhodium and an oxygen storage component comprising a zirconium oxide, cerium oxide, yttrium oxide and lanthanum oxide and/or a zirconium oxide, cerium oxide, praseodymium oxide and lanthanum oxide.

14. A particulate filter, comprising a wall-flow filter of length L and two different coatings Y and Z, 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 which form the 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, wherein 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 over 57 to 65% of the length L and contains aluminum oxide in an amount of 35 to 60% by weight, based on the total weight of the coating Y, palladium, rhodium or palladium and rhodium and an oxygen storage component in an amount of 40 to 50% by weight, based on the total weight of the coating Y, wherein the oxygen storage component comprises zirconium oxide, cerium oxide, lanthanum oxide and yttrium oxide or zirconium oxide, cerium oxide, lanthanum oxide and praseodymium oxide, and coating Z is located in the porous walls and extends from the second end of the wall-flow filter over 90 to 100% of the length L and contains aluminum oxide in an amount of 25 to 50% by weight, based on the total weight of the coating, palladium, rhodium or palladium and rhodium and two oxygen storage components in a total amount of 50 to 80% by weight, based on the total weight of the coating Z, wherein one oxygen storage component contains zirconium oxide, cerium oxide, lanthanum oxide and yttrium oxide and the other contains zirconium oxide, cerium oxide, lanthanum oxide and praseodymium oxide.

15. A method for removing particles, carbon monoxide, hydrocarbons, and nitrogen oxides from the exhaust gas of combustion engines fueled by a stoichiometric air-fuel mixture, wherein the exhaust gas is conducted through a particulate filter in accordance with claim 1.

16. The particulate filter of claim 8, wherein each of coatings Y and Z have the same noble metal material.

17. The particulate filter of claim 1, wherein each of coatings Y and Z are three-way catalytically active, and each comprise one or more noble metals, at least one substrate material for noble metal support, and oxygen storage components, with the weight ratio of substrate material to oxygen storage components for the coating Z being less than that of coating Y.

18. The particulate filter of claim 1, wherein each of coatings Y and Z are three-way catalytically active, and each comprise one or more noble metals, at least one substrate material for noble metal support, and oxygen storage components, and each of the substrate material and oxygen storage components in coating Y and Z support noble metals.

19. The particulate filter of claim 18, wherein the noble metal load of coating Z is greater than that of coating Y.

20. The particulate filter of claim 18, wherein the noble metal load of coating Z is less than that of coating Y.

21. The particulate filter of claim 1, wherein channels A are free of a coating layer such that the exhaust gas travels along exposed porous surfaces of channel A after passing through coating Z.

22. The particulate filter of claim 1, wherein coating Y has a washcoat load of 33 to 125 g/1 based on the volume of the wall-flow filter, and coating Y has a washcoat load that is less than that of coating Z.

23. The particulate filter of claim 1, wherein coating Y has a washcoat load of 33 to 125 g/l based on the volume of the wall-flow filter and coating Y has a washcoat load that is more than that of coating Z.

24. The particulate filter of claim 1, wherein both coating Y and coating Z have a common first oxygen storage component composition and coating Z has a second oxygen storage component composition that is in addition to the first oxygen storage component composition, with the second oxygen storage component composition having a higher, by weight, relative cerium oxide content than the first oxygen storage component composition.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0063] FIG. 1 shows a particulate filter in accordance with the invention.

[0064] FIG. 2 shows backpressure evaluation results relative to different layer thicknesses and zone lengths.

[0065] FIG. 3 shows CO/NOx conversion results for lambda sweep testing.

[0066] FIG. 1 shows a particulate filter in accordance with the invention, comprising a wall-flow filter of length L (1) with channels E (2) and channels A (3), which 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) forming surfaces O.sub.E (7) and O.sub.A (8), respectively, and wherein channels E (2) are closed at the second end (5) and channels A (3) are closed at the first end (4). Coating Y (9) is located in the channels E (2) on the surfaces O.sub.E (7) and coating Z (10) is located in the porous walls (6).

[0067] The invention is explained in more detail in the following examples.

Comparative Example 1

[0068] 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 then 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, wherein the coating was introduced into the porous filter wall over 100% of the substrate length. The total load of this filter amounted to 100 g/l; the total noble metal load amounted to 0.44 g/l with a ratio of palladium to rhodium of 8:3. The coated filter thus obtained was dried and then calcined. It is hereinafter referred to as VGPF 1.

Example 1

[0069] a) Application of the In-Wall Coating:

[0070] 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 then 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, wherein the coating was introduced into the porous filter wall over 100% of the substrate length. The load of this filter amounted to 100 g/l; the noble metal load amounted to 0.34 g/l with a ratio of palladium to rhodium of 16:3. The coated filter thus obtained was dried and then calcined.

[0071] b) Coating the Input Channels

[0072] Aluminum oxide stabilized with lanthanum oxide was suspended in water with an oxygen storage component which comprised 24% by weight cerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide. The weight ratio of aluminum oxide and oxygen storage component was 56:44. The suspension thus obtained was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly to coat the wall-flow filter substrate obtained under a), wherein the filter walls of the substrate were coated in the input channels to a length of 38% of the filter length. The load of the input channel amounted to 54 g/l; the noble metal load amounted to 0.27 g/l with a ratio of palladium to rhodium of 2.6:5. The coated filter thus obtained was dried and then calcined. The total load of this filter thus amounted to 121 g/l; the total noble metal load amounted to 0.44 g/l with a ratio of palladium to rhodium of 8:3. It is hereinafter referred to as GPF 1.

Catalytic Characterization

[0073] The particulate filters VGPF1 and GPF1 were aged together in an engine test bench aging process. This aging process consists of an overrun fuel 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 9.5 hours (see Motortechnische Zeitschrift, 1994, 55, 214-218).

[0074] The catalytically active particulate filters were then tested in the aged state at an engine test bench in the so-called “light-off test” and in the “lambda sweep test.” In the light-off test, the light-off performance is determined in the case of a stoichiometric exhaust gas composition with a constant average air ratio λ (λ=0.999 with ±3.4% amplitude).

[0075] Table 1 below contains the temperatures T.sub.50 at which 50% of each of the considered components is converted.

TABLE-US-00001 TABLE 1 T.sub.50 HC T.sub.50 CO T.sub.50 NOx stoichiometric stoichiometric stoichiometric VGPF1 418 430 432 GPF1 377 384 387

[0076] The dynamic conversion behavior of the particulate filters was determined in a lambda sweep test in a range from λ=0.99-1.01 at a constant temperature of 510° C. The amplitude of λ in this case amounted to ±6.8%. Table 2 shows the conversion at the intersection of the CO and NOx conversion curves, along with the associated HC conversion of the aged particulate filters.

TABLE-US-00002 TABLE 2 HC CO/NOx conversion conversion at λ of the at the CO/NOx point of point of intersection intersection VGPF1 79% 94% GPF1 83% 95%

[0077] The particulate filter GPF1 according to the invention shows a marked improvement in light-off performance and dynamic CO/NOx conversion in the aged state compared with VGPF1.

Comparative Example 2

[0078] a) Application of the In-Wall Coating: 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 then 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, wherein the coating was introduced into the porous filter wall over 100% of the substrate length. The total load of this filter amounted to 75 g/l; the noble metal load amounted to 0.71 g/l with a palladium to rhodium ratio of 3:1. The coated filter thus obtained was dried and then calcined.

[0079] b) Coating the Input Channels

[0080] Aluminum oxide stabilized with lanthanum oxide was suspended in water with an oxygen storage component which comprised 24% by weight cerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide. The weight ratio of aluminum oxide and oxygen storage component was 56:44. The suspension thus obtained was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly to coat the wall-flow filter substrate obtained under a), wherein the filter walls of the substrate were coated in the input channels to a length of 25% of the filter length. The load of the input channel amounted to 50 g/l; the noble 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.

[0081] c) Coating the Output Channels

[0082] Aluminum oxide stabilized with lanthanum oxide was suspended in water with an oxygen storage component which comprised 24% by weight cerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide. The weight ratio of aluminum oxide and oxygen storage component was 56:44. The suspension thus obtained was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly to coat the wall-flow filter substrate obtained under b), wherein the filter walls of the substrate were coated in the output channels to a length of 25% of the filter length. The load of the output channel amounted to 50 g/l; the noble 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. The total load of this filter thus amounted to 100 g/l; the total noble metal load amounted to 1.77 g/l with a ratio of palladium to rhodium of 4:1. It is hereinafter referred to as VGPF2.

Example 2

[0083] a) Application of the In-Wall Coating:

[0084] 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 then 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, wherein the coating is introduced into the porous filter wall over 100% of the substrate length.

[0085] The load of this filter amounted to 50 g/l; the noble metal load amounted to 0.71 g/l with a ratio of palladium to rhodium of 3:1. The coated filter thus obtained was dried and then calcined.

[0086] b) Coating the Input Channels

[0087] Aluminum oxide stabilized with lanthanum oxide was suspended in water with an oxygen storage component which comprised 24% by weight cerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide. The weight ratio of aluminum oxide and oxygen storage component was 56:44. The suspension thus obtained was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly to coat the wall-flow filter substrate obtained under a), wherein the filter walls of the substrate were coated in the input channels to a length of 60% of the filter length. The load of the input channel amounted to 83.3 g/l; the noble metal load amounted to 1.77 g/l with a ratio of palladium to rhodium of 42:8. The coated filter thus obtained was dried and then calcined. The total load of this filter thus amounted to 100 g/l; the total noble metal load amounted to 1.77 g/l with a ratio of palladium to rhodium of 4:1. It is hereinafter referred to as GPF2.

Catalytic Characterization

[0088] The particulate filters VGPF2 and GPF2 were aged together in an engine test bench aging process. This aging process consists of an overrun fuel 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 58 hours (see Motortechnische Zeitschrift, 1994, 55, 214-218). The catalytically active particulate filters were then tested in the aged state at an engine test bench in the so-called “light-off test” and in the “lambda sweep test.” In the light-off test, the light-off performance is determined in the case of a stoichiometric exhaust gas composition with a constant average air ratio λ (λ=0.999 with ±3.4% amplitude).

[0089] Table 3 below contains the temperatures T.sub.50 at which 50% of each of the considered components is converted.

TABLE-US-00003 TABLE 3 T.sub.50 HC T.sub.50 CO T.sub.50 NOx stoichiometric stoichiometric stoichiometric VGPF2 356 360 365 GPF2 351 356 359

[0090] The dynamic conversion behavior of the particulate filters was determined in a lambda sweep test in a range from λ=0.99-1.01 at a constant temperature of 510° C. The amplitude of λ in this case amounted to ±6.8%. Table 4 shows the conversion at the intersection of the CO and NOx conversion curves, along with the associated HC conversion of the aged particulate filters.

TABLE-US-00004 TABLE 4 HC CO/NOx conversion conversion at λ of the at the CO/NOx point of point of intersection intersection VGPF2 79% 96% GPF2 86% 97%

[0091] The particulate filter GPF2 according to the invention shows a marked improvement in light-off performance and dynamic CO/NOx conversion in the aged state compared with VGPF2.

Comparative Example 3

[0092] a) Application of the In-Wall Coating:

[0093] 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 then 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, wherein the coating was introduced into the porous filter wall over 100% of the substrate length.

[0094] The total load of this filter amounted to 100 g/l; the noble metal load amounted to 2.60 g/l with a palladium to rhodium ratio of 60:13.75. The coated filter thus obtained was dried and then calcined.

[0095] b) Coating the Input Channels

[0096] Aluminum oxide stabilized with lanthanum oxide was suspended in water with an oxygen storage component which comprised 40% by weight cerium oxide, zirconium oxide, lanthanum oxide and praseodymium oxide. The weight ratio of aluminum oxide and oxygen storage component was 50:50. The suspension thus obtained was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly to coat the wall-flow filter substrate obtained under a), wherein the filter walls of the substrate were coated in the input channels to a length of 25% of the filter length. The load of the input channel amounted to 58 g/l; the noble metal load amounted to 2.30 g/l with a ratio of palladium to rhodium of 10:3. The coated filter thus obtained was dried and then calcined.

[0097] c) Coating the Output Channels

[0098] Aluminum oxide stabilized with lanthanum oxide was suspended in water with an oxygen storage component which comprised 24% by weight cerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide. The weight ratio of aluminum oxide and oxygen storage component was 56:44. The suspension thus obtained was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly to coat the wall-flow filter substrate obtained under b), wherein the filter walls of the substrate were coated in the output channels to a length of 25% of the filter length. The load of the outlet channel amounted to 59 g/l; the noble metal load amounted to 1.06 g/l with a ratio of palladium to rhodium of 1:2. The coated filter thus obtained was dried and then calcined. The total load of this filter thus amounted to 130 g/l; the total noble metal load amounted to 3.44 g/I with a ratio of palladium to rhodium of 10:3. It is hereinafter referred to as VGPF3.

Example 3

[0099] a) Application of the In-Wall Coating:

[0100] 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 then 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, wherein the coating was introduced into the porous filter wall over 100% of the substrate length.

[0101] The load of this filter amounted to 100 g/l; the noble metal load amounted to 2.07 g/l with a ratio of palladium to rhodium of 45:13.5. The coated filter thus obtained was dried and then calcined.

[0102] b) Coating the Input Channels

[0103] Aluminum oxide stabilized with lanthanum oxide was suspended in water with an oxygen storage component which comprised 24% by weight cerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide. The weight ratio of aluminum oxide and oxygen storage component was 56:44. The suspension thus obtained was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly to coat the wall-flow filter substrate obtained under a), wherein the filter walls of the substrate were coated in the input channels to a length of 60% of the filter length. The load of the input channel amounted to 80 g/l; the noble metal load amounted to 2.30 g/l with a ratio of palladium to rhodium of 10:3. The coated filter thus obtained was dried and then calcined. The total load of this filter thus amounted to 148 g/l; the total noble metal load amounted to 3.44 g/l with a ratio of palladium to rhodium of 10:3. It is hereinafter referred to as GPF3.

Catalytic Characterization

[0104] The particulate filters VGPF3 and GPF3 were aged together in an engine test bench aging process. This aging process consists of an overrun fuel 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 76 hours (see Motortechnische Zeitschrift, 1994, 55, 214-218). The catalytically active particulate filters were then tested in the aged state at an engine test bench in the so-called “light-off test” and in the “lambda sweep test.” In the light-off test, the light-off performance is determined in the case of a stoichiometric exhaust gas composition with a constant average air ratio λ (λ=0.999 with ±3.4% amplitude).

[0105] Table 5 below contains the temperatures T.sub.50 at which 50% of each of the considered components is converted.

TABLE-US-00005 TABLE 5 T.sub.50 HC T.sub.50 CO T.sub.50 NOx stoichiometric stoichiometric stoichiometric VGPF3 368 374 371 GPF3 341 345 340

[0106] The dynamic conversion behavior of the particulate filters was determined in a lambda sweep test in a range from λ=0.99-1.01 at a constant temperature of 510° C. The amplitude of λ in this case amounted to ±6.8%. Table 6 shows the conversion at the intersection of the CO and NOx conversion curves, along with the associated HC conversion of the aged particulate filters.

TABLE-US-00006 TABLE 6 HC CO/NOx conversion conversion at λ of the at the CO/NOx point of point of intersection intersection VGPF3 83% 97% GPF3 90% 98%

[0107] The particulate filter GPF3 according to the invention shows a marked improvement in light-off performance and dynamic CO/NOx conversion in the aged state compared with VGPF3.

[0108] It was furthermore systematically investigated what the main effects responsible for the lowest possible exhaust back pressure are. In doing so, various filters with different zone lengths (factor A) and washcoat layer thicknesses (factor B) were prepared and compared with one another. All filters had the same total washcoat load and the same noble metal content.

TABLE-US-00007 TABLE 7 Factor Name Unit Min Max A Zone length % 30 60 B Washcoat thickness g/l 50 80

[0109] The statistical evaluation shows that it is particularly advantageous to distribute the washcoat on as large a surface as possible on the filter walls with a resultant low layer thickness instead of covering only a small surface with a high layer thickness, since a high layer thickness is to be regarded as the main cause of a high exhaust back pressure (FIG. 2). In addition, the particulate filters were aged together in an engine test bench aging process. This aging process consists of an overrun fuel 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 19 hours (see Motortechnische Zeitschrift, 1994, 55, 214-218).

[0110] The catalytically active particulate filters were then tested in the aged state at an engine test bench in the so-called “lambda sweep test.” Surprisingly, the statistical evaluation of the test results also shows a significant advantage in the lambda sweep test if the catalytic coating is applied with a low layer thickness to as large a surface as possible (FIG. 3).