CATALYTICALLY ACTIVE PARTICULATE FILTER

Abstract

The present invention relates to a particulate filter for removing particles, carbon monoxide, hydrocarbons and nitrogen oxides from the exhaust gas from internal combustion engines operated with a stoichiometric air-fuel mixture. Two coatings Y and Z are located in the porous walls and are present from the first end of the wall-flow filter over the entire length L of the particulate filter. Both contain active alumina, at least one cerium-zirconium-rare earth metal mixed oxide and at least one platinum group metal.

Claims

1. Particulate filter for removing particles, carbon monoxide, hydrocarbons, and nitrogen oxides out of the exhaust gas of combustion engines operated with a stoichiometric air/fuel mixture, 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 which extend in parallel between a first and a second end of the wall-flow filter and are separated by porous walls which form surfaces OE or OA, and wherein the channels E are closed at the second end and the channels A are closed at the first end, characterized in that the two coatings Y and Z are located in the porous walls and extend from the first end of the wall-flow filter over the entire length L and both comprise active alumina, at least one oxygen storage material, and at least one platinum group metal.

2. Particulate filter according to claim 1, characterized in that in coating Z comprises two different cerium-zirconium-rare earth metal mixed oxides.

3. Particulate filter according to claim 2, characterized in that the weight ratio of active alumina to the sum of the two cerium-zirconium-rare earth metal mixed oxides is in the range from 10:90 to 60:40.

4. Particulate filter according to claim 1, characterized in that coating Z comprises two different oxygen storage components, wherein the weight ratio of the first cerium-zirconium-rare earth metal mixed oxide to the second cerium-zirconium-rare earth metal mixed oxide is in the range from 4:1 to 1:4.

5. Particulate filter according to claim 1, characterized in that coating Z comprises a first and a second cerium-zirconium-rare earth metal mixed oxide, wherein the first cerium-zirconium-rare earth metal mixed oxide preferably has a higher zirconium oxide content than the second cerium-zirconium-rare earth metal mixed oxide.

6. Particulate filter according to claim 1, characterized in that coating Z comprises a first and a second cerium-zirconium-rare earth metal mixed oxide, wherein the first cerium-zirconium-rare earth metal mixed oxide preferably has a lower cerium oxide content than the first cerium-zirconium-rare earth metal mixed oxide.

7. Particulate filter according to claim 1, characterized in that the weight ratio of active alumina to the sum of the preferably one cerium-zirconium-rare earth metal mixed oxide in coating Y is in the range from 25:75 to 75:25.

8. Particulate filter according to claim 1, characterized in that the first cerium-zirconium-rare earth metal mixed oxide of coating Z has a cerium oxide to zirconium oxide weight ratio of 0.1 to 0.7, which is smaller than in the second cerium-zirconium-rare earth metal mixed oxide, which has a cerium oxide to zirconium oxide weight ratio of 0.5 to 1.5.

9. Particulate filter according to claim 1, characterized in that the cerium-zirconium-rare earth metal mixed oxide of coating Y has a cerium oxide to zirconium oxide weight ratio of 0.1 to 0.7.

10. Particulate filter according to claim 1, characterized in that all cerium-zirconium-rare earth metal mixed oxides used are doped with lanthanum oxide.

11. Particulate filter according to claim 1, characterized in that, in addition to lanthanum oxide, the first cerium-zirconium-rare earth metal mixed oxide of coating Z is doped with 2% to 25% yttrium oxide based on the weight of the first cerium-zirconium-rare earth metal mixed oxide.

12. Particulate filter according to claim 1, characterized in that, in addition to lanthanum oxide, the second cerium-zirconium-rare earth metal mixed oxide of coating Z is doped with 2% to 10% praseodymium oxide based on the weight of the second cerium-zirconium-rare earth metal mixed oxide.

13. Particulate filter according to claim 1, characterized in that, in addition to lanthanum oxide, the cerium-zirconium-rare earth metal mixed oxide of coating Y is doped with 2% to 25% yttrium oxide based on the weight of the first cerium-zirconium-rare earth metal mixed oxide yttrium oxide.

14. Particulate filter according to claim 13, characterized in that the mass fraction of yttrium oxide is greater in the coating Y than in the coating Z.

15. Particulate filter according to claim 1, characterized in that all cerium-zirconium-rare earth metal mixed oxides of coating Z are activated with palladium, platinum, or palladium and platinum.

16. Particulate filter according to claim 1, characterized in that the the cerium-zirconium-rare earth metal mixed oxide of the coatings Y and Z are each activated with palladium or rhodium, or palladium and rhodium.

17. Particulate filter according to claim 1, characterized in that the ratio of the applied quantity of coating Z to the applied quantity of coating Y is in the range from 3:1 to 1:3.

18. Particulate filter for removing particles, carbon monoxide, hydrocarbons and nitrogen oxides out of the exhaust gas of combustion engines operated with a stoichiometric air-fuel mixture, 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 which extend in parallel between a first and a second end of the wall-flow filter and are separated by porous walls which form surfaces OE and OA, and wherein the channels E are closed at the second end and the channels A are closed at the first end, characterized in that the two coatings Y and Z are located in the porous walls and extend from the first end of the wall-flow filter over the entire length L and both comprise active alumina, at least one cerium-zirconium-rare earth metal mixed oxide and at least one platinum group metal, wherein layer Z is free of rhodium, and that the ratio of the applied quantity of coating Z to the applied quantity of coating Y is in the range from 3:1 to 1:3.

19. Method for removing particles, carbon monoxide, hydrocarbons and nitrogen oxides out of the exhaust gas of combustion engines operated with a stoichiometric air/fuel mixture, characterized in that the exhaust gas is passed over a particulate filter according to claim 1.

Description

[0057] FIG. 1 shows a particulate filter comprising a wall-flow filter of length L (1) and two different coatings Y and Z, wherein the wall-flow filter comprises channels E (2) and A (3) which extend in parallel between a first (4) and a second end (5) of the wall-flow filter and are separated by porous walls (6) which form surfaces O.sub.E (7) or O.sub.A (8), and wherein the channels E are closed at the second end and the channels A are closed at the first end, characterized in that the two coatings Y (9) and Z (10) are located in the porous walls and extend from the first end of the wall-flow filter over the entire length L.

[0058] The invention is explained in more detail in the following examples.

EXAMPLES

[0059] Five filters each were provided with different catalytically active coatings. Ceramic wall-flow filters of highly porous cordierite having a diameter of 11.84 cm and a length of 15.24 cm and a cell density of 300 cpsi (46.5 cells per cm.sup.2) and a wall thickness of 8.5 mil, i.e., 0.02 mm, were in each case used as filter substrates. Each filter was provided with a coating of 100 g/l based on the filter volume.

Comparative Example 1

[0060] Alumina stabilized with lanthanum oxide was suspended in water together with a first oxygen storage component comprising 40% by weight cerium oxide, 50% by weight zirconium oxide, 5% by weight lanthanum oxide and 5% by weight praseodymium oxide, and a second oxygen storage component comprising 24% cerium oxide, 60% by weight zirconium oxide, 3.5% by weight lanthanum oxide and 12.5% by weight yttrium oxide. Both oxygen storage components were used in equal parts. The weight ratio of alumina to oxygen storage components 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, wherein the coating was introduced into the porous filter wall over 100% of the substrate length from both directions (outlet and inlet). The total loading of this filter was 100 g/l; the precious metal loading was 1.589 g/l having a palladium to rhodium ratio of 3.5:1. The coated filter thus obtained was dried and subsequently calcined.

Comparative Example 2

[0061] In embodiments of the present invention, two different coatings Y and Z are introduced into the porous filter wall over 60% of the substrate length in each case. First, alumina stabilized with lanthanum oxide was suspended in water together with a first oxygen storage component comprising 40% by weight cerium oxide, 50% by weight zirconium oxide, 5% by weight lanthanum oxide and 5% by weight praseodymium oxide, and a second oxygen storage component comprising 24% cerium oxide, 60% by weight zirconium oxide, 3.5% by weight lanthanum oxide and 12.5% by weight yttrium oxide. Both oxygen storage components were used in equal parts. The weight ratio of alumina to oxygen storage components was 30:70. The suspension thus obtained was subsequently mixed with a palladium nitrate solution under constant stirring. The resulting coating suspension Z was used directly for coating a commercially available wall-flow filter substrate, wherein the coating took place over 60% of the substrate length starting from the inlet channel into the porous filter wall. The loading of coating Z was 50 g/l. The coated filter thus obtained was dried and subsequently calcined.

In coating Y, was alumina stabilized with lanthanum oxide was suspended in water together with a first oxygen storage component containing 24% by weight cerium oxide, 60% by weight zirconium oxide, 3.5% by weight lanthanum oxide and 12.5% by weight yttrium oxide. The weight ratio of alumina to oxygen storage components was 55:45. The suspension thus obtained was subsequently mixed with a rhodium nitrate solution under constant stirring. The resulting coating suspension was coated onto a commercially available wall-flow filter substrate already containing coating Z, wherein the coating was introduced into the porous filter wall over 60% of the substrate length starting from the outlet channel. The loading of coating Y was 50 g/l. The total loading of this filter was 100 g/l; the precious metal loading was 1.589 g/l having a palladium to rhodium ratio of 3.5:1. The coated filter thus obtained was dried and subsequently calcined.

Example 1 According to the Invention

[0062] In embodiments of the present invention, two different coatings Y and Z are introduced into the porous filter wall over 100% of the substrate length. First, alumina stabilized with lanthanum oxide was suspended in water together with a first oxygen storage component comprising 40% by weight cerium oxide, 50% by weight zirconium oxide, 5% by weight lanthanum oxide and 5% by weight praseodymium oxide, and a second oxygen storage component comprising 24% cerium oxide, 60% by weight zirconium oxide, 3.5% by weight lanthanum oxide and 12.5% by weight yttrium oxide. Both oxygen storage components were used in equal parts. The weight ratio of alumina to oxygen storage components was 30:70. The suspension thus obtained was subsequently mixed with a palladium nitrate solution under constant stirring. The resulting coating suspension was used directly for coating a commercially available wall-flow filter substrate, wherein the coating took place over 100% of the substrate length into the porous filter wall. The loading of coating Z was 50 g/l. The coated filter thus obtained was dried and subsequently calcined.

In coating Y, was alumina stabilized with lanthanum oxide was suspended in water together with a first oxygen storage component containing 24% by weight cerium oxide, 60% by weight zirconium oxide, 3.5% by weight lanthanum oxide and 12.5% by weight yttrium oxide. The weight ratio of alumina to oxygen storage components was 55:45. The suspension thus obtained was subsequently mixed with a rhodium nitrate solution under constant stirring. The resulting coating suspension was coated onto a commercially available wall-flow filter substrate already containing coating Z, wherein the coating was introduced into the porous filter wall over 100% of the substrate length. The loading of coating Y was 50 g/l. The total loading of this filter was 100 g/l; the precious metal loading was 1.589 g/l having a palladium to rhodium ratio of 3.5:1. The coated filter thus obtained was dried and subsequently calcined.

[0063] Example 1 according to the invention and Comparative Example 1 exhibit similar dynamic pressures, while Comparative Example 2 exhibits a higher dynamic pressure, particularly at 600 m.sup.3/h.

TABLE-US-00001 TABLE 1 Dynamic pressure measured at 300 m.sup.3/h and 600.sup.3 m/h. 300 m.sup.3/h 600 m.sup.3/h Comparative Example 1 .sup. 11 mbar ± 0.3 mbar 35.5 mbar ± 0.3 mbar Comparative Example 2 12.4 mbar ± 0.4 mbar 38.0 mbar ± 0.5 mbar Example 1  110 mbar ± 0.5 mbar .sup. 36 mbar ± 0.6 mbar

[0064] In order to determine the catalytic properties of the filter according to the invention, a filter each of Comparative Example 1 and Example 1 was aged in an engine test bench aging process. The aging process consists of an overrun cut-off aging process at an exhaust gas temperature of 950° C. before the catalyst input (maximum bed temperature of 1030° C.). The aging time was 38 hours. Subsequently, an engine test bench was used to test the light-off performance at a constant average air ratio λ and the dynamic conversion with a change of λ.

[0065] Table 2 contains the temperatures T.sub.50 at which 50% of the considered component are in each case converted. Here, the light-off performance with a stoichiometric exhaust gas composition (λ=0.999 with ±3.4% amplitude) was determined.

TABLE-US-00002 TABLE 2 Results of the light-off performance after aging for Example 1 and Comparative Examples 1 and 2 T.sub.50 HC T.sub.50 CO T.sub.50 NOx stoichio- stoichio- stoichio- metric metric metric Comparative Example 1 385 391 392 Comparative Example 2 363 373 366 Example 1 355 359 356

[0066] Example 1 according to the invention exhibits a significant improvement in temperatures T.sub.50 by 30° C. for all components considered (HC, CO, and NOx).

[0067] Furthermore, the filters were subjected to a so-called amplitude test which provides information about the dynamic oxygen storage capacity. Here, the lambda is acted on by three different amplitudes of 2, 3.4 and 6.8% and the respective damping by the catalyst is determined. Table 2 shows the damping behavior of the three examples.

TABLE-US-00003 TABLE 3 Results of the amplitude test after aging for Example 1 and Comparative Examples 1 and 2 2% 3.4% 6.8% Comparative Example 1 0.10 0.11 0.19 Comparative Example 2 0.18 0.14 0.12 Example 1 0.07 0.07 0.08

[0068] Example 1 according to the invention exhibits a significantly stronger damping of the lambda amplitude and thus a higher dynamic oxygen storage capacity than the two Comparative Examples 1 and 2.