CATALYTICALLY ACTIVE PARTICLE FILTER WITH A HIGH DEGREE OF FILTRATION EFFICIENCY

Abstract

The present invention relates to a wall-flow filter for removing particles from the exhaust gas of combustion engines, comprising a wall-flow filter substrate of length L and coatings Z and F that differ from one another, wherein the wall-flow filter substrate has channels E and A, which extend in parallel between a first and a second end of the wall-flow filter substrate, are separated by porous walls and form 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, and wherein the coating Z is located in the porous walls and/or on the surfaces O.sub.A, but not on the surfaces O.sub.E, and comprises palladium and/or rhodium a cerium/zirconium mixed oxide, characterized in that the coating F is located in the porous walls and/or on the surfaces O.sub.E, but not on the surfaces O.sub.A and comprises a particulate metal compound and no noble metal.

Claims

1. Wall-flow filter for removing particles from the exhaust gas of combustion engines, comprising a wall-flow filter substrate of length L and coatings Z and F that differ from one another, wherein the wall-flow filter substrate has channels E and A, which extend in parallel between a first and a second end of the wall-flow filter substrate, are separated by porous walls and form 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, and wherein the coating Z is located in the porous walls and/or on the surfaces O.sub.A, but not on the surfaces O.sub.E, and comprises palladium and/or rhodium and a cerium/zirconium mixed oxide, and that the coating F is located in the porous walls and/or on the surfaces O.sub.E, but not on the surfaces O.sub.A, and comprises a particulate metal compound and no noble metal, characterized in that the wall-flow filter substrate has a coating Y which is different from the coatings Z and F, which comprises platinum, palladium or platinum and palladium, which contains no rhodium and no cerium/zirconium mixed oxide and which is located in the porous walls and/or on the surfaces O.sub.E, but not on the surfaces O.sub.A.

2. Wall-flow filter according to claim 1, characterized in that coating Z is located on the surfaces O.sub.A of the wall-flow filter substrate and extends from the second end of the wall-flow filter substrate to 50 to 90% of the length L.

3. Wall-flow filter according to claim 1, characterized in that coating Z is located in the porous walls of the wall-flow filter substrate and extends from the first end of the wall-flow filter substrate to 50 to 100% of the length L.

4. Wall-flow filter according to claim 1, characterized in that coating Z contains palladium and rhodium.

5. Wall-flow filter according to claim 1, characterized in that coating Z does not contain platinum.

6. Wall-flow filter according to claim 1, characterized in that the cerium/zirconium mixed oxide of coating Z contains one or more rare earth metal oxides.

7. Wall-flow filter according to claim 6, characterized in that the rare earth metal oxide is lanthanum oxide, yttrium oxide, praseodymium oxide, neodymium oxide and/or samarium oxide.

8. Wall-flow filter according to claim 1, characterized in that coating Z comprises lanthanum-stabilized aluminum oxide, rhodium, palladium or palladium and rhodium, and a cerium/zirconium/rare earth metal mixed oxide containing yttrium oxide and lanthanum oxide as rare earth metal oxides.

9. Wall-flow filter according to claim 1, characterized in that coating Z comprises lanthanum-stabilized aluminum oxide, rhodium, palladium or palladium and rhodium, and a cerium/zirconium/rare earth metal mixed oxide containing praseodymium oxide and lanthanum oxide as rare earth metal oxides.

10. Wall-flow filter according to claim 1, characterized in that coating F consists of one or more particulate metal compounds.

11. Wall-flow filter according to claim 10, characterized in that the particulate metal compound of the coating F is cerium oxide, titanium dioxide, zirconium dioxide, silicon dioxide, aluminum oxide, or mixtures or mixed oxides thereof.

12. Wall-flow filter according to claim 1, characterized in that it has an increasing concentration gradient of the coating F in the longitudinal direction of the filter from its first to its second end.

13. Wall-flow filter according to claim 1, characterized in that coating Y extends over a length of 50 to 100% of the length L.

14. Method for producing a wall-flow filter according to claim 1, characterized in that the channels E of the dry wall-flow filter substrate already coated with coating Z and optionally coating Y are impinged on by a dry powder/gas aerosol, wherein the powder contains a particulate metal compound.

15. A method of reducing harmful exhaust gases of an internal combustion engine, comprising passing the harmful exhaust gases of the internal combustion engine through a wall-flow filter according to claim 1.

Description

[0153] The advantages of the invention are explained using examples below.

Comparative Example 1: Coating Z Only

[0154] 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 for coating a commercially available wall-flow filter substrate, the coating being introduced over 80% of the substrate length on the surfaces O.sub.A. The total load of this filter amounted to 75 g/l; the total noble 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. It is hereinafter referred to as VGPF1.

Example 1 According to the Invention: Coating Z in Combination with Coating F

[0155] 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 for coating a commercially available wall-flow filter substrate, the coating being introduced over 80% of the substrate length on the surfaces O.sub.A. The total load of this filter amounted to 75 g/l; the total noble 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. Subsequently, the filter was impinged on by a dry powder/gas aerosol, wherein 7 g/L of a metal oxide was introduced into channels E. It is hereinafter referred to as GPF1.

[0156] The two filters VGPF1 and GPF1 thus obtained were measured on a cold-blast test bench in order to determine the pressure loss over the respective filter. At room temperature and a volumetric flow rate of 900 m.sup.3/h of air, the back pressure is 111 mbar for the VGPF1 and 122 mbar for the GPF1. As already described, the filtration coating F only leads to a moderate increase in back pressure.

[0157] Furthermore, the two filters were investigated with respect to the back pressure after soot loading. For this purpose, both filters were sooted on an engine test bench with a direct-injection turbocharged engine. The final soot loading was about 3 g. FIG. 13 shows that the GPF1 according to the invention, despite the higher back pressure in the clean state, has a lower back pressure after soot loading than the comparative filter VGPF1.

[0158] At the same time, fresh VGPF1 and GPF1 filters were investigated in the vehicle in terms of their particle filtration efficiency. For this purpose, the filters were measured in an RTS-95, also known as RTC-aggressive, driving cycle in a position close to the engine between two particle counters. In both cases, a three-way catalyst was located upstream in the exhaust tract, through which the lambda control of the vehicle was effected. Here the filter GPF1 according to the invention has a filtration efficiency of 84%, calculated from the particle values of the two particle counters, while the comparative filter VGPF1 achieves a filtration efficiency of only 55.5%. Overall, it can be seen that the combination of filtration coating F and the three-way coating Z is particularly advantageous in terms of back pressure after soot loading and filtration efficiency.

Comparative Example 2: Coating Z Only

[0159] 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 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 75 g/l; the total noble metal load amounted to 1.24 g/l with a ratio of palladium to rhodium of 6:1. The coated filter thus obtained was dried and then calcined. It is hereinafter referred to as VGPF2.

Example 2 According to the Invention: Coating Z in Combination with Coating F

[0160] 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 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 75 g/l; the total noble metal load amounted to 1.24 g/l with a ratio of palladium to rhodium of 6:1. The coated filter thus obtained was dried and then calcined. Subsequently, the filter was impinged on by a dry powder/gas aerosol, wherein 10 g/L of a metal oxide was introduced into the channels E. It is hereinafter referred to as GPF2.

[0161] The two filters VGPF2 and GPF2 were investigated in the vehicle in terms of their particle filtration efficiency. For this purpose, the filters were measured in a WLTP driving cycle in a position close to the engine between two particle counters. In both cases, a three-way catalyst was located upstream in the exhaust tract, through which the lambda control of the vehicle was effected. Here the filter GPF2 according to the invention has a filtration efficiency of 85%, calculated from the particle values of the two particle counters, while the comparative filter VGPF2 achieves a filtration efficiency of only 65%.

[0162] Furthermore, the filters VGPF2 and GPF2 were aged 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 76 hours (see Motortechnische Zeitschrift, 1994, 55, 214-218). The catalytically active particulate filters were then tested in the aged state on an engine test bench in the so-called light-off test and in the lambda sweep test to compare their catalytic activity. 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 ? (?C=0.999 with ?3.4% amplitude).

[0163] Table 1 below contains the temperatures Tso at which 50% of the considered components are respectively converted.

TABLE-US-00001 TABLE 1 T.sub.50 HC T.sub.50 CO T.sub.50 NOx stoichiometric stoichiometric stoichiometric VGPF2 366 373 372 GPF2 366 373 373

[0164] The dynamic conversion behavior of the particulate filters was determined in a lambda sweep test in a range from ?=0.99-1.01 at a constant temperature of 510? C. The amplitude of ? in this case was ?6.8%. Table 2 shows the conversion at the intersection of the CO and NO.sub.x conversion curves, along with the associated HC conversion of the aged particulate filters.

TABLE-US-00002 TABLE 2 CO/NOx conversion at the HC conversion at ? of the point of intersection CO/NOx point of intersection VGPF2 93 92 GPF2 92 93

[0165] From these tests it is evident that the filtration coating F is particularly suitable for significantly increasing the filtration efficiency of the filter GPF2 according to the invention without influencing the catalytic activity of the filter.

Comparative Example 3: Coating Z Only

[0166] 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 for coating a commercially available wall-flow filter substrate, the coating being introduced over 60% of the substrate length on the surfaces O.sub.A. The total load of this filter amounted to 75 g/l; the total noble metal load amounted to 0.88 g/l with a ratio of palladium to rhodium of 4:1. The coated filter thus obtained was dried and then calcined. It is hereinafter referred to as VGPF3.

Comparative Example 4: Coating Z Only

[0167] 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, a platinum nitrate solution and a rhodium nitrate solution with constant stirring. The resulting coating suspension was used directly for coating a commercially available wall-flow filter substrate, the coating being introduced over 60% of the substrate length on the surfaces O.sub.A. The total load of this filter amounted to 75 g/l; the total noble metal load amounted to 0.88 g/l with a ratio of platinum to palladium to rhodium of 92:108:50. The coated filter thus obtained was dried and then calcined. It is hereinafter referred to as VGPF4.

Comparative Example 5: Coating Y in Combination with Coating Z

[0168] Stabilized aluminum oxide was suspended in water. The suspension thus obtained was subsequently mixed with a palladium nitrate solution and a platinum 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 10 g/l; the total noble metal load amounted to 0.35 g/l with a ratio of palladium to rhodium of 12:1. The coated filter thus obtained was dried and then calcined. Aluminum oxide stabilized with lanthanum oxide was then 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 for coating a commercially available wall-flow filter substrate, the coating being introduced over 60% of the substrate length on the surfaces O.sub.A. The total load of this filter amounted to 85 g/l; the total noble metal load amounted to 0.88 g/l with a ratio of platinum to palladium to rhodium of 92:108:50. The coated filter thus obtained was dried and then calcined. It is hereinafter referred to as VGPF5.

[0169] The filters VGPF3, VGPF4 and VGPF5 produced in this way were initially loaded with a defined amount of soot and subsequently investigated in a soot burn-off test on the engine test bench with a lean exhaust gas composition and constant temperature of 450? C. upstream of the catalyst inlet to determine their soot oxidation properties. It is found that the combination of coating Y and coating Z in VGPF5 is most suitable for completely regenerating the particulate filter. Considering the time required for the back pressure of the soot-loaded filter to drop to 25% of the original back pressure without soot (defined as p25), it can be seen in FIG. 14 that the combination of coating Y and coating Z in VGPF5 reaches the p25 value after about 1700 sec, while VGPF3 and VGPF4 reach the p25 value only after a time of 4100 and 2250 sec, respectively.

Comparative Example 6: Coating Y in Combination with Coating Z

[0170] Stabilized aluminum oxide suspended in water. The suspension thus obtained was subsequently mixed with a palladium nitrate solution and a platinum 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 10 g/l; the total noble metal load amounted to 0.28 g/l with a ratio of palladium to rhodium of 12:1. The coated filter thus obtained was dried and then calcined. Aluminum oxide stabilized with lanthanum oxide was then 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 55:45. 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, the coating being introduced over 80% of the substrate length on the surfaces O.sub.A. The total load of this filter amounted to 60 g/l after the second step; the total noble metal load amounted to 0.48 g/l with a ratio of palladium to rhodium of 738:262:359. The coated filter thus obtained was dried and then calcined. It is hereinafter referred to as VGPF6.

Example 3 According to the Invention: Coating Y in Combination with Coating Z and Coating F

[0171] Stabilized aluminum oxide suspended in water. The suspension thus obtained was subsequently mixed with a palladium nitrate solution and a platinum 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 10 g/l; the total precious metal load amounted to 0.28 g/l with a ratio of palladium to rhodium of 12:1. The coated filter thus obtained was dried and then calcined. Aluminum oxide stabilized with lanthanum oxide was then 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 55:45. 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, the coating being introduced over 80% of the substrate length on the surfaces O.sub.A. The total load of this filter amounted to 60 g/l after the second step; the total noble metal load amounted to 0.48 g/l with a ratio of palladium to rhodium of 738:262:359. The coated filter thus obtained was dried and then calcined. Subsequently, the filter was impinged on by a dry powder/gas aerosol, wherein 4 g/L of a metal oxide was introduced into the channels E. It is hereinafter referred to as GPF3.

[0172] The two filters VGPF6 and GPF3 were investigated in the vehicle with regard to their particle filtration efficiency. For this purpose, the filters were measured in a WLTP driving cycle in a position close to the engine between two particle counters. In both cases, a three-way catalyst was located upstream in the exhaust tract, through which the lambda control of the vehicle was effected. Here the filter GPF3 according to the invention has a filtration efficiency of 96%, calculated from the particle values of the two particle counters, while the comparative filter VGPF6 achieves a filtration efficiency of only 71%.

[0173] In addition, the two filters thus obtained were measured on a cold-blast test stand in the fresh state in order to determine the pressure loss over the respective filter. At room temperature and a volumetric flow rate of 300 m.sup.3/h of air, the back pressure is 69 mbar for the VGPF6 and 122 mbar for the GPF3. As already described, the filtration coating F only leads to a moderate increase in back pressure.

[0174] Furthermore, the two filters were investigated with respect to the back pressure after soot loading. For this purpose, both filters were sooted on an engine test bench with a direct-injection turbocharged engine. For a soot amount of 3 g, the GPF3 according to the invention, despite the higher back pressure in the clean state, has a lower back pressure than the comparative filter VGPF6 (Table 3).

TABLE-US-00003 TABLE 3 Back pressure in the clean state Back pressure with 3 g soot VGPF6 69 244 GPF3 122 205

[0175] Overall, it can be seen that the additional regeneration coating Y can be optionally combined with the filtration coating F and three-way coating Z, wherein the advantages in terms of back pressure behavior after soot loading and in terms of filtration efficiency still remain in comparison with a filter without a coating F.