Catalytically active particle filter with a high degree of filtration efficiency

Abstract

The invention relates to a wall-flow filter as a particle filter with catalytically active coatings in the channels which are closed in a gas-tight manner at the opposing closed ends of the channels A at the first end, wherein the inlet region of the filter is additionally supplied with a dry powder-gas aerosol which contains metal compounds with a high melting point (such as the metal oxides Al2O3, SiO2, FeO2, TiO2, ZnO2, etc. for example) and which is to simultaneously improve the catalytic activity and the degree of filtration efficiency with respect to the exhaust gas back-pressure.

Claims

1. A catalytically active wall-flow filter for removing particles from the exhaust gas of internal combustion engines, which comprises a wall-flow filter of length L and at least one catalytically active coating Y and/or 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 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, the coating Y, when present, is located in the channels E on the walls of the surfaces O.sub.E, wherein the coating Y on the walls of the surface O.sub.E extends from the first end of the wall-flow filter to a length of less than the length L, the coating Z, when present, is located in the channels A on the walls of the surfaces O.sub.A, wherein the coating Z on the walls of the surface O.sub.A extends from the second end of the wall-flow filter to a length of less than the length L, wherein an inlet region of channels E of the wall-flow filter has additionally been impinged with at least one dry powder-gas aerosol, and wherein the coating Y and/or Z has a thickness gradient over the length L such that the least thickness of the coating Y and/or Z prevails at the ends of the wall-flow filter.

2. The catalytically active wall-flow filter according to claim 1, wherein the inlet region of channels E of the wall-flow filter is impinged with the at least one dry powder-gas aerosol such that the powder precipitates in the pores of the wall-flow filter walls and fills the pores as far as the walls of the surfaces O.sub.E.

3. The catalytically active wall-flow filter according to claim 1, wherein the coating Y extends from the first end of the wall-flow filter to a length of up to 90% of the length L.

4. The catalytically active wall-flow filter according to claim 1, wherein the coating Z extends from the second end of the wall-flow filter to a length of up to 90% of the length L.

5. The catalytically active wall-flow filter according to claim 1, wherein the coating Y and/or Z is at least 1.25 cm long.

6. The catalytically active wall-flow filter according to claim 1, further comprising a coating X that is located in the walls of the wall-flow filter and the coating X extends from the first or the second end of the wall-flow filter to a length of 100% of the length L.

7. The catalytically active wall-flow filter according to claim 1, wherein each of the coatings Y and Z are present, and the coatings Y and Z respectively contain one or more noble metals, which are fixed on one or more carrier materials, along with one or more oxygen storage components.

8. The catalytically active wall-flow filter according to claim 1, wherein each of the coatings Y and Z are present and contain a cerium/zirconium/rare earth metal mixed oxides as an oxygen storage component.

9. The catalytically active wall-flow filter according to claim 1, wherein the amount of powder in the wall-flow filter is less than 50 g/l.

10. The catalytically active wall-flow filter according to claim 1, wherein the powder is selected from the group consisting of silicon dioxide, aluminum oxide, titanium dioxide, zirconium dioxide, cerium oxide, iron oxide, zinc oxide, zeolites, metal-substituted zeolites or mixtures thereof.

11. The catalytically active wall-flow filter according to claim 1, wherein the powder precipitates in the pores of the filter walls and does not form a cohesive layer on the walls of the wall-flow filter.

12. The catalytically active wall-flow filter according to claim 1, wherein a tapped density of the powder is at most 900 kg/m3.

13. A method for producing the catalytically active wall-flow filter according to claim 1, that comprises providing the catalytically active coating Y and/or Z to the wall-flow filter.

14. The method for producing the catalytically active wall-flow filter according to claim 13, wherein the method includes, prior to impinging the wall-flow filter with the powder-gas aerosol, first dispersing the powder in a gas as to create a powder and gas dispersion combination, the powder and gas dispersion combination is then fed to a gas stream and is then subsequently sucked through the wall-flow filter and wherein the powder-gas aerosol is sucked through the wall-flow filter at a rate of 5 m/s to 50 m/s.

15. The method for producing the catalytically active wall-flow filter according to claim 13, wherein the powder has a moisture content of less than 20% at the time of impingement on the wall-flow filter.

16. A method of reducing harmful exhaust gases comprising passing the harmful exhaust gases through the catalytically active wall-flow filter according to claim 1.

17. The method according to claim 14, wherein, the powder and gas dispersion combination is sucked initially through an inlet region of the channels E of the wall-flow filter.

18. A catalytically active wall-flow filter for removing particles from the exhaust gas of internal combustion engines, which comprises a wall-flow filter of length L and at least one catalytically active coating Y and/or 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 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, the coating Y, when present, is located in the channels E on the walls of the surfaces O.sub.E, wherein the coating Y on the walls of the surface O.sub.E extends from the first end of the wall-flow filter to a length of less than the length L, the coating Z, when present, is located in the channels A on the walls of the surfaces O.sub.A, wherein the coating Z on the walls of the surface O.sub.A extends from the second end of the wall-flow filter to a length of less than the length L, wherein an inlet region of channels E of the wall-flow filter has additionally been impinged with at least one dry powder-gas aerosol, wherein the ratio of the average particle diameter (Q3 distribution; measured according to the most recent ISO13320 on the date of application) d50 in the dry powder aerosol and the average pore diameter of the wall-flow filter after coating with a washcoat (measured according to DIN 66134, latest version on the date of application) is between 0.03 and 2; and wherein the coating Y and/or Z has a thickness gradient over the length L such that the least thickness of the coating Y and/or Z prevails at the ends of the wall-flow filter.

19. The catalytically active wall-flow filter according to claim 18, wherein, the powder precipitates in the pores of the walls, but the powder does not form a cohesive layer on the walls of the wall-flow filter.

20. A method of reducing harmful exhaust gases comprising passing the harmful exhaust gases through the catalytically active wall-flow filter according to claim 18.

21. The catalytically active wall-flow filter according to claim 18, wherein each of coatings Y and Z are present.

Description

(1) FIG. 1 shows a light microscopy image of a filter impinged with powder. The photograph shows a top view of a plurality of channel walls in a region of the filter where no catalytically active on-wall layer is located. The powder is selectively deposited in the pores of the wall and fills them.

(2) FIGS. 2-8 show different coating arrangements in catalytically active particulate filters according to the invention. The following designations are used therein:

(3) (E) the input channel/inflow channel of the filter

(4) (A) the output channel/outflow channel of the filter

(5) (L) the length of the filter wall

(6) (X) catalytic in-wall coating

(7) (Y) and (Z) catalytic on-wall coating

(8) (P) regions of the filter wall impinged with powder

(9) FIG. 2 schematically shows a filter which has on the inlet side (Y) and outlet side (Z) one zone each of an on-wall layer with a zone length of approx. 60% of the total length and is impinged with powder (P) at the end in the uncoated part of the inlet channel (E).

(10) FIG. 3 shows the same structure of the filter as FIG. 2, with the difference that the two on-wall layers (Y, Z) have a coating gradient from the end faces toward the center of the carrier.

(11) FIG. 4 shows a filter with an on-wall layer (Y) in the inlet region E, which is impinged with a powder (P) in the inlet channel (E).

(12) FIG. 5 shows a filter with an on-wall layer (Z) in the outlet region A and a zone with an in-wall coating (X), with which the powder (P) precipitates in the region of highest permeability.

(13) FIG. 6 shows a filter with which the walls of the channels are completely activated with an in-wall coating (X) and which on the inlet side (E), has a zone with an on-wall layer (Y) and was impinged with powder (P).

(14) FIG. 7 shows the same filter as FIG. 5, but without catalytically active in-wall coating (X).

(15) FIG. 8 shows schematically a filter with an on-wall layer each in the input channel (Y) and output channel (Z) with a length of approximately 30% of the total length L and a region in the input channel (E) impinged with powder (P).

(16) FIG. 9 shows the results of the investigations on the filtration effect of all described particulate filters according to the invention and particulate filters according to the prior art.

(17) FIG. 10 shows a photograph of an opened filter with a zone of an on-wall layer in the inlet channels (gray, left side) and the region impinged with powder (right side).

(18) FIG. 11 shows enlargements of the filter of FIG. 10 with photos of the input (1), the center (2) and the output (3) of the filter.

(19) FIG. 12 shows a schematic drawing of an advantageous device for impinging the filters with a powder. Together with the gas, the powder 420 or 421 is mixed with the gas stream 454 under pressure 451 by the atomizer nozzle 440 in the mixing chamber and then sucked or pushed through the filter 430. The particles that passed through are filtered out in the exhaust gas filter 400. The blower 410 provides the necessary volumetric 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.

EXAMPLES

(20) Producing and Testing the Particulate Filters

(21) Conventional high-porosity cordierite filters having a round cross section were used to produce the catalytically active particulate filters described in examples and comparative examples. The wall-flow filter substrates had a cell density of 46.5 cells per square centimeter at a cell wall thickness of 0.22 mm. They had a porosity of 65% and an average pore size of 18 μm.

(22) In Comparative Example 1, a coating suspension was applied in the outflow channels. In Comparative Example 2, coating suspensions were applied in two steps in the inflow channels and outflow channels. The filter described in Comparative Example 3 contains both a coating arranged in the wall and a coating suspension applied in the inflow channels. After the application of each coating suspension, the wall-flow filters were dried and calcined at 500° C. for the duration of 4 hours. The coating suspension was applied according to the requirements of the person skilled in the art (as described in DE102010007499A1).

(23) In the case of the particulate filters according to the invention (Examples 1 to 5), the filters from Comparative Examples 1-3 were additionally impinged with a powder in the input channels.

(24) The catalytically active filters thus obtained were investigated for their fresh filtration efficiency on the engine test bench in the real exhaust gas of a motor operated predominantly (>50% of the operating time) and on average (mean lambda over the running time) with a stoichiometric air/fuel mixture. A globally standardized test procedure for determining exhaust emissions, or WLTP (Worldwide harmonized Light vehicles Test Procedure) for short, was used here. The driving cycle used was WLTC Class 3. The respective filter was installed close to the engine immediately downstream of a conventional three-way catalyst. This three-way catalyst was the same for all filters measured. Each filter was subjected to a WLTP. In order to be able to detect particulate emissions during testing, the particle counters were installed upstream of the three-way catalyst and downstream of the particulate filter.

(25) Some catalytically active particulate filters were also additionally subjected to engine test bench aging. The aging process consisted of an overrun cut-off aging process with an exhaust gas temperature of 950° C. before the catalyst input (maximum bed temperature of 1030° C.). The aging time was 38 hours. After aging, the filters were investigated for their catalytic activity.

(26) In the analysis of catalytic activity, the light-off behavior of the particulate filters was determined at a constant average air ratio A on an engine test bench, and the dynamic conversion was checked when A changed.

Comparative Example 1

(27) On wall-flow filters with a diameter of 118 mm and a length of 152 mm, a noble metal-containing coating suspension containing a cerium/zirconium mixed oxide, a lanthanum-doped aluminum oxide and barium sulfate was applied to 80% of the length of the output channel of the filter and subsequently calcined at 500° C. The grain size of the oxides of the coating suspension was selected such that the suspension was applied predominantly to the filter wall (only a small amount of the fraction of ultra-fine coating particles penetrates into the pores of the wall; less than 10%). After calcination, the coating amount of the VGPF1 corresponded to 67 g/l based on the volume of the substrate.

Comparative Example 2

(28) On wall-flow filters with a diameter of 118 mm and a length of 118 mm, a noble metal-containing coating suspension containing a cerium/zirconium mixed oxide, a lanthanum-doped aluminum oxide and barium sulfate was applied in a first step to 60% of the length of the input channel of the filter and subsequently calcined. In a second coating step, a further noble metal-containing coating suspension was applied to 60% of the length of the output channel and subsequently calcined. The coating suspension used in the second coating step also contained a cerium/zirconium mixed oxide, a lanthanum-doped aluminum oxide and barium sulfate. The grain size of the oxides of the coating suspension was selected such that the suspension was applied predominantly to the filter wall. The amount of coating of the VGPF2 after calcination corresponded to 100 g/l based on the volume of the substrate.

Comparative Example 3

(29) On a highly porous wall-flow filter with a diameter of 132 mm and a length of 102 mm, a noble metal-containing coating suspension containing a cerium/zirconium mixed oxide, a lanthanum-doped aluminum oxide and barium sulfate was applied in a first step to the entire length of the filter and subsequently calcined. The grain size of the oxides of the coating suspension was selected such that the suspension is predominantly located in the filter wall (>90%). The amount of coating after calcination corresponded to 100 g/l based on the volume of the substrate.

(30) In a second coating step, a further noble metal-containing coating suspension was applied to 60% of the input channel and subsequently calcined. The coating suspension used in the second coating step also contained a cerium/zirconium mixed oxide, a lanthanum-doped aluminum oxide and barium sulfate. The grain size of the oxides of the coating suspension was selected such that the suspension was applied predominantly to the filter wall. The amount of coating of the VGPF3 after calcination corresponded to 132 g/l based on the volume of the substrate.

(31) In order to increase the filtration efficiency of the catalytically coated filters described in Comparative Examples 1 to 3, the inflow channels thereof were impinged with various amounts and types of powder. In this case, the coating parameters were chosen such that the powder used was deposited mainly in the region of the substrate in which there was no on-wall coating (points of highest permeability). The production of the filters GPF1 to GPF5 according to the invention is explained in the following descriptions.

(32) In the examples, the influence of the type and amount of powder used in different coating variants of the catalytic materials on filtration efficiency and catalytic activity was investigated.

Example 1

(33) In order to increase the filtration efficiency of the catalytically coated filter VGPF1 described in Comparative Example 1, the inflow channels of the filter were impinged with 4 g/l of a highly porous aluminum oxide. The relative back pressure increase of the GPF1 compared to the VGPF1 was 8.6 mbar. The filter GPF1 described in Example 1 is outlined in FIG. 7.

Example 2

(34) In order to increase the filtration efficiency of the catalytically coated filter VGPF1 described in Comparative Example 1, the inflow channels of the filter were impinged with 0.2 g/l of a pyrogenic aluminum oxide with a high melting point. The relative back pressure increase of the GPF1 compared to the VGPF1 was 5 mbar. The filter GPF2 described in Example 2 is outlined in FIG. 7.

Example 3

(35) In order to increase the filtration efficiency of the catalytically coated filter VGPF2 described in Comparative Example 2, the inflow channels of the filter were impinged with 10 g/l of a highly porous aluminum oxide. The filter GPF3 described in Example 3 is outlined in FIG. 2.

Example 4

(36) In order to increase the filtration efficiency of the catalytically coated filter VGPF3 described in Comparative Example 3, the inflow channels of the filter were impinged with 4 g/l of a highly porous aluminum oxide. The filter GPF4 described in Example 4 is outlined in FIG. 4.

Example 5

(37) In order to increase the filtration efficiency of the catalytically coated filter VGPF3 described in Comparative Example 3, the inflow channels of the filter were impinged with 7 g/l of a highly porous aluminum oxide. The filter GPF5 described in Example 5 is outlined in FIG. 4.

(38) Discussion of the Results from Filtration Efficiency Measurements of the Particulate Filters VGPF1 to VGPF3 Along with GPF1 to GPF5 Described in Comparative Examples and Examples

(39) As already described, the catalytically active filters produced in comparative examples and examples were each subjected to a WLTP on an engine test bench in order to investigate their filtering effect. The results from these investigations were shown in FIG. 9. FIG. 9 shows filtration values that have resulted from the raw particle emissions and particle emissions after the respective filter during a WLTP procedure.

(40) The advantages of the filters GPF1 to GPF5 according to the invention can be clearly observed during the filtering effect measurement. Impinging the filter with a powder results in a filtration efficiency increase of up to 20%. The desired filtration efficiency can be adjusted by the quantity of powder used.

(41) Replacing a highly porous aluminum oxide (GPF1) with a pyrogenic aluminum oxide (GPF2) reduces the amount of powder used from 4 g/l to 0.2 g/l. This leads to a saving of the powder used by 500% by weight with an unchanged filtration effect and a lower back pressure.

(42) In order to check whether the filters impinged with powder have high catalytic activity, the particulate filters GPF4 and GPF5 were subjected to engine test bench aging and a subsequent measurement of the light-off behavior.

(43) The table below contains the temperatures T.sub.50 at which 50% of the considered components are respectively converted. In this case, the light-off behavior with stoichiometric exhaust gas composition (λ=0.999 with ±3.4% amplitude) was determined. The standard deviation in this test is ±2° C.

(44) Table 1 contains the light-off data for the aged filters VGPF3, GPF4 and GPF5.

(45) TABLE-US-00001 TABLE 1 T.sub.50 HC T.sub.50 CO T.sub.50 NOx stoichiometric stoichiometric stoichiometric VGPF3 381 398 395 GPF4 386 401 401 GPF5 383 401 402

(46) As the results show, impinging the catalytically active filters with a powder leads to a significant increase in filtration efficiency with an unchanged high catalytic activity and a low back pressure rise. The choice of powder may also significantly reduce the amounts of powder used.

(47) It has been shown successfully that the catalytic activity of the zone-coated wall-flow filters, the exhaust-gas back pressure and the filtration efficiency can be adapted to the customer requirements in a targeted manner. A correspondingly produced wall-flow filter was not yet known from the prior art.