CATALYTICALLY ACTIVE PARTICULATE FILTER

Abstract

The invention relates to a particulate filter for removing particles, carbon monoxide, hydrocarbons and nitrogen oxides out of the exhaust gas of combustion engines operated with stoichiometric air/fuel mixture, comprising a wall flow filter with length L and a coating Z, wherein the wall flow filter includes 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, characterised in that coating Z is located in the porous walls and extends from the first end of the wall flow filter over the entire length L, and includes active aluminum oxide, two different cerium/zirconium/rare earth metal mixed oxides 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 stoichiometric air/fuel mixture, comprising a wall flow filter with length L and a coating Z, wherein the wall flow filter includes 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 coating Z is located in the porous walls and extends from the first end of the wall flow filter over the length L, and includes active aluminum oxide, at least two different cerium/zirconium/rare earth metal mixed oxides and at least one platinum group metal, and the first cerium/zirconium/rare earth metal mixed oxide is doped with yttrium oxide in addition to lanthanum oxide.

2. Particulate filter according to claim 1, characterized in that the weight ratio of aluminum oxide to the sum of the two cerium/zirconium/rare earth metal mixed oxides is in the range from 10:90 to 60:40.

3. Particulate filter according to claim 1, characterized in that 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.

4. Particulate filter according to claim 1, characterized in that the first cerium/zirconium/rare earth metal mixed oxide has a higher zirconium oxide content than the second cerium/zirconium/rare earth metal mixed oxide.

5. Particulate filter according to claim 1, characterized in that the first cerium/zirconium/rare earth metal mixed oxide has a cerium oxide to zirconium oxide weight ratio of 0.7 to 0.1, 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.

6. Particulate filter according to claim 1, characterized in that the first cerium/zirconium/rare earth metal mixed oxide has a cerium oxide content of 10% to 40% based on the weight of the first cerium/zirconium/rare earth metal mixed oxide.

7. Particulate filter according to claim 1, characterized in that the first cerium/zirconium/rare earth metal mixed oxide has a zirconium oxide content of 40% to 90% based on the weight of the first cerium/zirconium/rare earth metal mixed oxide.

8. Particulate filter according to claim 1, characterized in that the second cerium/zirconium/rare earth metal mixed oxide has a cerium oxide content of 25% to 60% based on the weight of the second cerium/zirconium/rare earth metal mixed oxide.

9. Particulate filter according to claim 1, characterized in that the second cerium/zirconium/rare earth metal mixed oxide has a zirconium oxide content of 20% to 70% based on the weight of the second cerium/zirconium/rare earth metal mixed oxide.

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

11. Particulate filter according to claim 1, characterized in that the lanthanum oxide content is >0% to 10% based on the weight of the particular cerium/zirconium/rare earth metal mixed oxide.

12. Particulate filter according to claim 1, characterized in that the yttrium oxide content of the first cerium/zirconium/rare earth metal mixed oxide is 2% to 25% based on the weight of the first cerium/zirconium/rare earth metal mixed oxide.

13. Particulate filter according to claim 1, characterized in that the second cerium/zirconium/rare earth metal mixed oxide is doped not only with lanthanum oxide but also with a further metal oxide from the group of rare earth metal oxides, preferably with praseodymium.

14. Particulate filter according to claim 1, characterized in that the content of the second rare earth metal of the second cerium/zirconium/rare earth metal mixed oxide is 2% to 15% based on the weight of the second cerium/zirconium/rare earth metal mixed oxide.

15. Particulate filter according to claim 1, characterized in that the catalytically active coating of one of the platinum group metals contains platinum, palladium, rhodium or mixtures thereof.

16. Particulate filter according to claim 1, characterized in that both cerium/zirconium/rare earth metal mixed oxides are activated with palladium and rhodium, platinum and rhodium or platinum, palladium and rhodium.

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

Description

EXAMPLES

[0060] Four 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 used as filter substrates. Each filter was provided with a coating of 76.27 g/based on the filter volume.

Comparative Example 1

[0061] Aluminum oxide stabilized with lanthanum oxide was suspended in water together with an oxygen storage component containing 40% by weight cerium oxide, 50% by weight zirconium oxide, 5% by weight lanthanum oxide and 5% by weight praseodymium oxide. The weight ratio of aluminum oxide to 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, the coating being introduced into the porous filter wall over 100% of the substrate length. The total load of this filter amounted to 76.27 g/l; the precious metal load amounted to 1.271 g/l having a palladium to rhodium ratio of 5:1. The coated filter thus obtained was dried and then calcined.

Comparative Example 2

[0062] Aluminum oxide stabilized with lanthanum oxide was suspended in water together with an 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 aluminum oxide to 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, the coating being introduced into the porous filter wall over 100% of the substrate length. The total load of this filter amounted to 76.27 g/l; the precious metal load amounted to 1.271 g/l having a palladium to rhodium ratio of 5:1. The coated filter thus obtained was dried and then calcined.

Example 1 According to the Invention

[0063] Aluminum oxide 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 aluminum oxide to oxygen storage components 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, the coating being introduced into the porous filter wall over 100% of the substrate length. The total load of this filter amounted to 76.27 g/l; the precious metal load amounted to 1.271 g/l having a palladium to rhodium ratio of 5:1. The coated filter thus obtained was dried and then calcined.

Dynamic Pressure:

[0064]

TABLE-US-00001 600 m.sup.3/h 900 m.sup.3/h Comparative 52.9 mbar 0.2 mbar 107.4 mbar 0.3 mbar Example 1 Comparative 53.3 mbar equals 0.4 mbar 107.2 mbar 0.5 mbar Example 2 Example 1 53.0 mbar 0.6 mbar 105.9 mbar 0.6 mbar

[0065] In order to determine the catalytic properties of the filter according to the invention, a filter of each of Comparative Example 1, Comparative Example 2 and Example 1 was aged in engine test bench aging. The aging process consists of overrun fuel cut-off aging with an exhaust gas temperature of 950 C. in front of the catalyst inlet (1030 C. maximum bed temperature). The aging time was 19 hours.

[0066] Subsequently, an engine test bench was used to test the light-off performance at a constant average air ratio , and the dynamic conversion with a change of .

[0067] Table 1 contains the temperatures Tso at which 50% each of the considered components are converted. Here, the light-off performance with stoichiometric exhaust gas composition (=0.999 with 3.4% amplitude) was determined.

TABLE-US-00002 TABLE 1 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 stoichiometric stoichiometric stoichlometric Cornparative Exarnple 1 391 399 406 Comparative Example 2 370 377 377 Example 1 374 379 379

[0068] The dynamic conversion performance was determined in a range for of 0.99 to 1.01 at a constant temperature of 510 C. The amplitude of in this case was 6.8%. Table 2 contains the conversion at the point of intersection of the CO and NOx conversion curves, as well as the associated HC conversion.

TABLE-US-00003 TABLE 2 Results of the dynamic conversion performance after aging for Example 1 and Comparative Examples 1 and 2 CO/NOx conversion HC conversion at of at the point of the CO/NOx point of intersection intersection Comparative Example 1 82% 96% Cornparative Example 2 81.5% 97% Example 1 90% 97%

[0069] Example 1 according to the invention shows a marked improvement in the dynamic CO/NOx conversion after aging, while the light-off performance is similarly good as in Comparative Example 2, but better than in Comparative Example 1.

OSC Properties:

[0070] The oxygen storage capacity was determined in two different experiments. Table 3 shows the values for the lambda step test which characterizes the static oxygen storage capacity. The air/fuel ratio before the filter is changed from rich (=0.96) to lean (=1.04). The stored oxygen quantity is calculated from the delay time of the post-cat lambda probe relative to the pre-cat lambda probe.

TABLE-US-00004 TABLE 3 Static oxygen storage capacity after aging for Example 1 and Comparative Examples 1 and 2 Oxygen storage capacity (mg/l) Comparative Example 1 182 Comparative Example 2 132 Example 1 194

[0071] In another test, dynamic oxygen storage capability is determined. At an average value of =1, the exhaust gas is subjected to various amplitudes with a frequency of 1 Hz. The amplitude signal of the post-cat lambda probe is divided by the amplitude signal of the pre-cat lambda probe. The smaller the value, the better the dynamic oxygen storage capacity. The results are shown in Table 4.

TABLE-US-00005 TABLE 4 Dynamic oxygen storage capacity after aging for Example 1 and Comparative Examples 1 and 2 2% 3.4% 6.8% amplitude amplitude amplitude Comparative Example 1 0.24 0.37 0.41 Comparative Example 2 0.08 0.13 0.28 Example 1 0.09 0.14 0.23

[0072] The example according to the invention shows both a high static and a very good dynamic oxygen storage capacity after aging.