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.

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 at least one platinum group metal comprises at least one of platinum, palladium, or rhodium.

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

17. Particulate filter according to claim 1, wherein the second cerium/zirconium/rare earth metal mixed oxide is doped not only with lanthanum oxide but also with praseodymium.

18. Particulate filter according to claim 1, wherein coating Z does not contain a zeolite or a molecular sieve.

19. Particulate filter according to claim 1, wherein >95% of coating Z is present in the porous walls.

20. 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

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 shows a schematic view of a particulate filter according to the invention.

(2) Surprisingly, it has been found that a combination of different cerium/zirconium/rare earth metal mixed oxides can bring about a greatly improved conversion of gaseous pollutants after hard aging. Lanthanum oxide and/or yttrium oxide are particularly preferred as rare earth metals in this context, and lanthanum oxide and yttrium oxide, yttrium oxide and praseodymium oxide, and lanthanum oxide and praseodymium oxide are more particularly preferred.

(3) In embodiments of the present invention, the oxygen storage components are free from neodymium oxide.

(4) In embodiments of the present invention, in coating Z, 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, preferably in the range from 20:80 to 50:50 and particularly preferably in the range from 25:75 to 35:65. In preferred embodiments, coating Z comprises in each case lanthanum-stabilized aluminum oxide in amounts of 10 to 60% by weight, preferably 20 to 50% by weight, particularly preferably 25 to 35% by weight, and oxygen storage components in amounts of 40 to 90% by weight, preferably 50 to 80% by weight, particularly preferably 85 to 75% by weight, in each case based on the sum of the weights of aluminum oxide and oxygen storage components in coating Z.

(5) In embodiments, coating Z preferably comprises two oxygen storage components different from one another, 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, preferably in the range from 3:1 to 1:3 and particularly preferably in the range from 2:1 to 1:2.

(6) In embodiments of the present invention, coating Z comprises a first and a second oxygen storage component, wherein the first oxygen storage component has a higher zirconium oxide content than the second oxygen storage component.

(7) In accordance with the invention, the cerium oxide to zirconium oxide ratio in the cerium/zirconium/rare earth metal mixed oxides can vary within wide limits. It amounts to, for example, 0.1 to 1.5, preferably 0.2 to 1.25, more preferably 0.3 to 1. It is furthermore preferred for the first oxygen storage component to have 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. Other more preferred embodiments include a first oxygen storage component having a cerium oxide to zirconium oxide weight ratio of 0.6 to 0.2 and a second oxygen storage component having a cerium oxide to zirconium oxide weight ratio of 0.6 to 1.2. Still other most preferred embodiments include a first oxygen storage component having a cerium oxide to zirconium oxide weight ratio of 0.5 to 0.3, and the second oxygen storage component has a cerium oxide to zirconium oxide weight ratio of 0.7 to 1.0.

(8) In a preferred embodiment, the particulate filter according to the invention is designed such 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, more preferably of 15% to 35% and very particularly preferably of 20% to 30% based on the weight of the first cerium/zirconium/rare earth metal mixed oxide.

(9) In contrast, the zirconium oxide content in the first cerium/zirconium/rare earth metal mixed oxide is 40% to 90% based on the weight of the first cerium/zirconium/rare earth metal mixed oxide. It is advantageous if the zirconium oxide content in the first cerium/zirconium/rare earth metal mixed oxide is between 50% and 75%, very preferably 55% to 65%, based on the weight of the first cerium/zirconium/rare earth metal mixed oxide.

(10) Likewise, a cerium oxide content of 25% to 60% based on the weight of the second cerium/zirconium/rare earth metal mixed oxide should prevail in the second cerium/zirconium/rare earth metal mixed oxide. It is more advantageous if in the second cerium/zirconium/rare earth metal mixed oxide there is a cerium oxide content of 30% to 55%, very preferably 35% to 50%, based on the weight of the second cerium/zirconium/rare earth metal mixed oxide.

(11) In a further preferred embodiment, 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. It is more preferred here if the second cerium/zirconium/rare earth metal mixed oxide has a zirconium oxide content of 30% to 60% very particularly preferably of 40% to 55% based on the weight of the second cerium/zirconium/rare earth metal mixed oxide.

(12) It is preferred according to the invention if both cerium/zirconium/rare earth metal mixed oxides are doped with lanthanum oxide, so that the content of lanthanum oxide is preferably >0% to 10% based on the weight of the cerium/zirconium/rare earth metal mixed oxide. Particularly advantageously, these lanthanum oxide-containing oxygen storage components have a lanthanum oxide to cerium oxide mass ratio of 0.05 to 0.5.

(13) In embodiments of the present invention, coating Z comprises lanthanum-stabilized aluminum oxide as well as rhodium, palladium or palladium and rhodium and two different oxygen storage components comprising zirconium oxide, cerium oxide, lanthanum oxide and yttrium oxide or praseodymium oxide.

(14) The first cerium/zirconium/rare earth metal mixed oxide is preferably doped with yttrium oxide in addition to lanthanum oxide. A preferred particulate filter has an yttrium oxide content in the first cerium/zirconium/rare earth metal mixed oxide of 2% to 25% based on the weight of the first cerium/zirconium/rare earth metal mixed oxide. More preferably, the yttrium content of the first cerium/zirconium/rare earth metal mixed oxide is between 4% and 20%, very preferably 10% to 15%, based on the weight of the first cerium/zirconium/rare earth metal mixed oxide.

(15) An embodiment in which 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, is also advantageous.

(16) In embodiments of the present invention, the zirconium oxide content of the yttrium oxide-containing oxygen storage component is greater in coating Z than the zirconium oxide content of the praseodymium oxide-containing oxygen storage component, in each case based on the respective oxygen storage component.

(17) The content of the second rare earth metal of the second cerium/zirconium/rare earth metal mixed oxide may be between 2% and 15% based on the weight of the second cerium/zirconium/rare earth metal mixed oxide. It is more advantageous if the content of the second rare earth metal of the second cerium/zirconium/rare earth metal mixed oxide is 3% to 10%, very preferably 4% to 8%, based on the weight of the second cerium/zirconium/rare earth metal mixed oxide.

(18) In coating Z, the yttrium oxide content of the first oxygen storage component is in particular 5 to 15% by weight based on the weight of the oxygen storage component. The lanthanum oxide to yttrium oxide weight ratio is in particular 0.1 to 1, preferably 0.15 to 0.8 and very preferably 0.2 to 0.5.

(19) In coating Z, the praseodymium content of the second oxygen storage component is in particular 2 to 10% by weight based on the weight of the oxygen storage component. The lanthanum oxide to praseodymium oxide weight ratio is in particular 0.1 to 2.0, preferably 0.2 to 1.8 and very preferably 0.5 to 1.5.

(20) In one embodiment, coating Z contains precious metals as catalytically active elements.

(21) Platinum, palladium and rhodium or mixtures thereof are particularly suitable for this purpose, palladium, rhodium, palladium and rhodium, or platinum, palladium and rhodium being preferred, and palladium and rhodium being particularly preferred. Furthermore, both cerium/zirconium/rare earth metal mixed oxides may be activated with palladium and rhodium, platinum and rhodium or platinum, palladium and rhodium.

(22) It is preferred for the catalytically active coating to be located in the pores of the porous wall of a wall flow filter. Only small parts can be present on the wall due to the coating process. According to the invention, coating Z is present in the pores of the wall by >95%.

(23) The precious metals are usually used in quantities of 0.15 to 5 g/l based on the volume of the wall flow filter. In a preferred embodiment, the precious metals are present both on the aluminum oxide and on the oxygen storage components.

(24) All materials familiar to the person skilled in the art for this purpose may be used as substrate materials for the precious metals. Such materials are in particular metal oxides with a BET surface area of 30 to 250 m.sup.2/g, preferably 100 to 200 m.sup.2/g (determined according to DIN 66132—newest version on the date of application).

(25) Particularly suitable carrier materials for the precious 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. Doped aluminum oxides are, for example, aluminum oxides doped with lanthanum oxide, barium oxide, zirconium oxide and/or titanium oxide. Lanthanum-stabilized aluminum oxide is advantageously used, wherein lanthanum is used in quantities of 1 to 10% by weight, preferably 3 to 6% by weight, in each case calculated as La.sub.2O.sub.3 and based on the weight of the stabilized aluminum oxide.

(26) Coating Z usually contains oxygen storage components in amounts of 15 to 120 g/l based on the volume of the wall flow filter.

(27) The mass ratio of carrier materials and oxygen storage components in coating Z is usually 0.2 to 1.5, for example 0.3 to 0.8.

(28) In embodiments of the present invention, coating Z contains one or more alkaline earth compounds such as strontium oxide, barium oxide or barium sulfate. The amount of barium sulfate per coating is, in particular, 2 to 20 g/l volume of the wall flow filter.

(29) Coating Z contains, in particular, strontium oxide or barium oxide.

(30) In further embodiments of the present invention, coating Z contains additives such as rare earth compounds, for example lanthanum oxide and/or binders such as aluminum compounds. Such additives are used in quantities that can vary within wide limits and that the person skilled in the art can determine by simple means in the specific case.

(31) According to the present invention, coating Z extends from the first end of the wan flow filter over the entire length L of the wall flow filter. The loading of the wall flow filter with coating Z preferably amounts to 20 to 125 g/l based on the volume of the wall flow filter.

(32) In embodiments of the present invention, coating Z does not contain a zeolite or a molecular sieve.

(33) Wall flow filters that can be used in accordance with the present invention are well-known and commercially available. They consist, for example, of silicon carbide, aluminum titanate or cordierite, for example having a cell density of 200 to 400 cells per square inch (cps), i.e. approximately 30 to 60 cells per cm.sup.2, and usually a wall thickness of between 6 and 12 mil, or 0.1524 and 0.305 mm.

(34) In the uncoated state, they have porosities of 50 to 80%, in particular 55 to 75%, for example. In the uncoated state, their average pore size is, for example, 10 to 25 micrometers. Generally, the pores of the wall flow filter are so-called open pores, i.e. they have a connection to the channels. Furthermore, the pores are normally interconnected with one another. This enables, on the one hand, easy coating of the inner pore surfaces and, on the other hand, easy passage of the exhaust gas through the porous walls of the wan flow filter.

(35) The particulate filter according to the invention can be produced by methods known to the person skilled in the art, for example by applying a coating suspension, which is usually referred to as washcoat, to the wall flow filter by means of one of the usual dip coating methods or pump and suction coating methods. This is usually followed by thermal post-treatment or calcination.

(36) The person skilled in the art knows that the average pore size of the wall flow filter and the average particle size of the catalytically active materials must be matched to each other in order to achieve an on-wall coating or in-wall coating. In the case of an in-wall coating, the average particle size of the catalytically active materials must be small enough to penetrate the pores of the wall flow filter. In the case of an on-wall coating on the other hand, the average particle size of the catalytically active materials must be large enough not to penetrate the pores of the wall flow filter.

(37) In embodiments of the present invention, the coating suspension for the production of coating Z is ground up to a particle size distribution of d.sub.50=1 to 2 μm and d.sub.99=6 to 7 μm.

(38) The particulate filter according to the invention is perfectly suitable for removing particles, carbon monoxide, hydrocarbons and nitrogen oxides out of the exhaust gas of combustion engines operated with stoichiometric air/fuel mixture.

(39) The present invention thus also relates to a 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 the invention.

(40) The exhaust gas can be passed over a particulate filter according to the invention in such a way that it enters the particulate filter through channels E and leaves it again through channels A.

(41) However, it is also possible for the exhaust gas to enter the particulate filter through channels A and to leave it again through channels E.

(42) FIG. 1 shows a particulate filter according to the invention which comprises a wall flow filter of length L (1) with channels E (2) and channels A (3) extending in parallel between a first end (4) and a second end (5) of the wall flow filter and separated by porous walls (6), which form surfaces O.sub.E (7) or O.sub.A (8), 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). Coating Z (9) is located in the porous walls (6).

(43) The invention is explained in more detail in the following examples.

EXAMPLES

(44) 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/l based on the filter volume.

Comparative Example 1

(45) 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 paladium to rhodium ratio of 5:1. The coated filter thus obtained was dried and then calcined.

Comparative Example 2

(46) 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 paladium 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 paladium to rhodium ratio of 5:1. The coated filter thus obtained was dried and then calcined.

Example 1 According to the Invention

(47) 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.

(48) 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

(49) 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.

(50) 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.

(51) Table 1 contains the temperatures T.sub.50 at which 50% each of the considered components are converted. Here, the light-off performance with stoichiometric exhaust gas composition (λ=0.999 with ±3.4% amplitude) was determined.

(52) 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

(53) The dynamic conversion performance was determined in a range for λ of 0.99 to 1.01 at a constant temperature of 510° C. The amplitude of λ 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.

(54) 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 λ 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%

(55) 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.

(56) OSC Properties:

(57) 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 λ before the filer is changed from rich (λ=0.96) to lean (λ=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.

(58) 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

(59) In another test, dynamic oxygen storage capability is determined. At an average value of λ=1, the exhaust gas is subjected to various λ 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.

(60) 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

(61) The example according to the invention shows both a high static and a very good dynamic oxygen storage capacity after aging.