FILTER FOR THE AFTERTREATMENT OF EXHAUST GASES OF INTERNAL COMBUSTION ENGINES

Abstract

The present invention relates to a wall-flow filter for removing particles from the exhaust gas of an internal combustion engine, which comprises a coating F, which comprises a sintered material S, wherein material S comprises an oxide, oxide-hydroxide, carbonate, sulphate, silicate, phosphate, mixed oxide, composite oxide, molecular sieve or a mixture comprising two or more of these materials.

Claims

1. Wall-flow filter for removing particles from the exhaust gas of an internal combustion engine, which comprises a wall- flow filter substrate of length L and coating F, wherein the wall-flow filter substrate has channels E and A extending in parallel between a first and a second end of the wall-flow filter substrate, which are separated by porous walls and form surfaces O.sub.E and O.sub.A, and wherein the channels E are closed at the second end and the channels A are closed at the first end, and wherein coating F is located on the surfaces O.sub.E, optionally in the porous walls but not on the surfaces O.sub.A, characterized in that coating F comprises a sintered material S, wherein material S comprises an oxide, oxide-hydroxide, carbonate, sulphate, silicate, phosphate, mixed oxide, composite oxide, molecular sieve or a mixture comprising two or more of these materials, wherein coating F is applied onto the surfaces O.sub.E by depositing material S in powder form onto the channels E, followed by a thermal treatment to cause sintering of material S, and the heat treatment comprises heating the wall-flow filter substrate which carries material S on its surfaces O.sub.E to a temperature T and maintaining this temperature for a period of time M, and the wall-flow filter substrate is heated with a heating rate of 100 to 1000 K/h to a temperature T of 100 to 1350? C. and maintained at that temperature for a period of time M of 1 second to twelve hours.

2. Wall-flow filter according to claim 1, characterized in that material S comprises an oxide, oxide-hydroxide, carbonate, sulphate, silicate or phosphate of aluminum, calcium, silicon, titanium, zirconium or cerium or is a molecular sieve of the framework type CHA or FAU.

3. Wall-flow filter according to claim 1, characterized in that material S comprises ceria, calcium sulphate, aluminum silicate, aluminum phyllosilicate or a molecular sieve of the framework type CHA or FAU.

4. (canceled)

5. Wall-flow filter according to claim 1, characterized in that material S in powder form has a particle size distribution with a d.sub.50 value of between 1 and 30 ?m and a D.sub.90 value of between 3 and 150 ?m, wherein the d.sub.50 and the d.sub.90 value of the particle size distribution of material S means that 50% and 90%, respectively, of the total volume of the material contains only particles whose diameter is less than or equal to the value specified as d.sub.50 and d.sub.90 respectively.

6. Wall-flow filter according to claim 1, characterized in that material S is applied onto the surfaces O.sub.E of the wall-flow filter substrate in that the channels E of the dry wall flow filter are exposed to a dry powder-gas-aerosol, wherein the powder is material S in powdery form.

7. (canceled)

8. (canceled)

9. Wall-flow filter according to claim 1, characterized in that it comprises in addition to coating F coating Z, which is coated on the surfaces O.sub.E, the surfaces O.sub.A and/or within the porous walls and comprises palladium and/or rhodium and a cerium/zirconium mixed oxide.

10. Wall-flow filter according to claim 1, characterized in that it comprises in addition to coating F coating Y, which is different from coatings F and Z and is coated on the surfaces O.sub.E and/or within the porous walls but not in the surfaces O.sub.A and which comprises platinum, palladium or platinum and palladium and no rhodium and no cerium/zirconium mixed oxide.

11. Wall-flow filter according to claim 1, characterized in that in contains in addition to coating F coating X, which is different from coatings F, Z and Y, is coated on the surfaces O.sub.E, O.sub.A and/or within the porous walls and comprises a SCR catalyst.

12. Wall-flow filter according to claim 1, characterized in that it carries either coating F alone, or coatings F and Z, or coatings F and Y or coatings F and X or coatings F, Z and Y or coatings F, Y and Z.

13. Use of a wall-flow filter according to claim 1 for reducing harmful exhaust gases of an internal combustion engine, wherein the gas enters the wall-flow filter through channels E and leaves it through channels A.

14. Exhaust gas cleaning system comprising a wall-flow filter according to claim 1 and at least one further catalyst.

15. Exhaust gas cleaning system according to claim 14, characterized in that the further catalyst is a three-way catalyst or an oxidation catalyst or a NOx-storage catalyst or a SCR catalyst.

Description

[0114] FIG. 1 shows a schematic drawing of an advantageous device for applying powder to the wall-flow filter. The powder (420) or (421) is mixed with the gas under pressure (451) through the atomizing nozzle (440) in the mixing chamber with the gas stream (454) and then sucked or pushed through the filter (430). The particles that have passed through are filtered out in the exhaust gas filter (400). Blower (410) provides the necessary volume flow. The exhaust gas is divided into an exhaust gas (452) and a warm cycle gas (453). The warm cycle gas (453) is mixed with the fresh gas (450).

EXAMPLE 1

[0115] a) A wall-flow filter substrate of cordierite with a diameter of 4.66, a length of 6, a cell density of 300 CPSI and a wall thickness of 8.5 mils was coated on its inlet channels with 28 g/l of a zeolite of the structure type CHA with a SAR of 13, and a particle size distribution with d.sub.50=2.6 ?m and d.sub.90=5.4 ?m.

[0116] The coating process used was a dry coating process using air as gas for producing the powder-gas-aerosol and for introducing it into the inlet channels of the wall-flow filter substrate.

[0117] b) The wall-flow filter obtained according to a) above was heated with a heating rate of 150 K/h to 1100? C. and kept at this temperature for 10 h. This treatment caused the zeolite to sinter.

[0118] c) The wall-flow filter obtained according to b) above was coated on the surfaces of its outlet channels over 80% of its length starting from the outlet end with 61 g/l of a washcoat comprising 45 g/ft.sup.3 of palladium and rhodium in a weight ratio of 7:2. The process used was a conventional wet coating process. Subsequently, the filter was dried.

EXAMPLE 2

[0119] a) A wall-flow filter substrate of cordierite with a diameter of 4.66, a length of 6, a cell density of 300 CPSI and a wall thickness of 8.5 mils was coated on its inlet channels with 28 g/l of a zeolite of the structure type FAU with a SAR of 5, and a particle size distribution with d.sub.50=2.2 ?m and d.sub.90=5.9 ?m.

[0120] The coating process used was a dry coating process using air as gas for producing the powder-gas-aerosol and for introducing it into the inlet channels of the wall-flow filter substrate.

[0121] b) The wall-flow filter obtained according to a) above was heated with a heating rate of 150 K/h to 900? C. and kept at this temperature for 1 h. This treatment caused the zeolite to sinter.

[0122] The product obtained is subsequently called F2.

COMPARISON EXAMPLE 1

[0123] Example 2 was repeated with the exception that in step b) the filter was heated up to 700? C. This temperature did not cause the zeolite to collapse.

[0124] The product obtained is subsequently called CF1.

Comparison of F2 and CF1

[0125] a) The back pressures of wall-flow filters F2 and CF1 were determined (in mbar at 600 Nm.sup.3/h) and subsequently, they were subjected to a water soaking test as follows: [0126] 1. Measure back pressure [0127] 2. Fill a glass beaker with an amount of water corresponding to the water uptake of the used ceramic substrate (e.g. 300 ml). [0128] 3. Place the part in the beaker and let water being soaked until the wall-flow substrate is completely wet [0129] 4. Dry the part at 120? C., followed by 350? C. in the direction of gas flow [0130] 5. Cool to Room temperature and measure back pressure

[0131] b) After the water soaking, the back pressures of F2 and CF1 were determined again. The results are shown in FIGS. 2 and 3. The back pressure of wall-flow filter F2 was unchanged compared to before the soaking with water, indicating that the zeolite was sintered and formed a coating which is stable against liquid water. In contrast to that, CF1 suffered a tremendous loss of the back pressure indicating that the zeolite was not sintered and was not stable against liquid water.

EXAMPLE 3

[0132] Example 2 was repeated with the difference that instead of a zeolite commercially available kaolin, a naturally occurring product based on alumina silicate, was used. The kaolin had a d.sub.50 of 5 ?m and a d.sub.90 of 17 ?m. It turned out that the treatment of the wall-flow filter obtained by dry coating with this product for 10 h at 1100? C. resulted in a back-pressure decrease, indicating that sintering occurred. In addition, back pressure did not change after the water soaking test described above. The results are also confirmed by a stable filtration efficiency.

EXAMPLE 4

[0133] Example 2 was repeated with the difference that instead of a zeolite a commercially available product based on calcium sulphate (trade name Uniflott) was used. The product had a d.sub.50 of 3 ?m and a d.sub.90 of 6.5 ?m.

[0134] It turned out that the treatment of the wall-flow filter obtained by dry coating with this product for 1 h at 900? C. resulted in a back pressure decrease, indicating that sintering occurred. In addition, back pressure did not change after the water soaking test described above. The results are also confirmed by a stable filtration efficiency.

EXAMPLE 5

[0135] Example 2 was repeated with the difference that instead of a zeolite commercially available Bentonite was used. The product had a d.sub.50 of 3 ?m and a d.sub.90 of 6.0 ?m.

[0136] It turned out that the treatment of the wall-flow filter obtained by dry coating with this product for 1 h at 900? C. resulted in a back pressure decrease, indicating that sintering occurred. In addition, back pressure did not change after the water soaking test described above. The results are also confirmed by a stable filtration efficiency.

[0137] The data for Examples 1 to 5 and comparison Example 1 are given in Table 1

TABLE-US-00001 TABLE 1 T M Example Material [? C.] [hours] BP0 BP1 BP2 BP3 1 (Filter F1) Zeolite 1100 10 53.9 71.2 57.4 57.4 (CHA; SAR = 13) 2 (Filter F2) Zeolite 900 1 54.9 80.3 68.3 68.2 (FAU; SAR = 5) Comparison Zeolite 700 1 55.2 81.4 81.3 57.8 Example 1 (FAU; (Filter CF1) SAR = 5) 3 Kaolin 1100 10 55.0 64.1 58.3 58.3 4 Uniflott 900 1 54.3 61.2 59.0 59.0 5 Bentonite 900 1 54.6 68.4 59.6 59.6

[0138] In Table 1 BP0 represents the back pressure measured at a flow of 600 Nm.sup.3/h over the uncoated substrate, BP1 represents the back pressure measured at a flow of 600 Nm.sup.3/h over the sample after being coated with the powder material, BP2 represents the back pressure measured at a flow of 600 Nm.sup.3/h over the sample after being subject to a thermal treatment at temperature T for a duration M (as given in Table 1) and BP3 represents the back pressure measured at a flow of 600 Nm.sup.3/h over the sample after being subject to the water soaking test as described in the following.

[0139] A difference between BP1 and BP2 (with BP2<BP1) indicates the sintering of material S.

[0140] A difference between BP2 and BP3 (with BP3<BP2) indicates the absence of stability against liquid water.