METHOD FOR COATING A WALL-FLOW FILTER

Abstract

The present invention relates to a method for producing a coated wall-flow filter. The wall-flow filter is coated with a powder aerosol.

Claims

1. Method for producing a wall-flow filter for reducing the harmful substances in the exhaust gas of an internal combustion engine, wherein a dry filter is selectively impinged on its input surface with a dry powder/gas aerosol which has at least one high-melting compound, such that the powder precipitates in the pores of the filter walls, characterized in that the powder is dispersed in the gas, then guided into a gas stream, and drawn into the inlet side of the filter without further supply of a gas.

2. Method according to claim 1, characterized in that the dispersion of the powder is effected by at least one of the following measures: Dispersion by means of compressed air Dispersion by ultrasound Dispersion by sieving Dispersion by “in-situ milling” Dispersion by blower Dispersion by expansion Dispersion in the fluidized bed.

3. Method according to claim 1, characterized in that the powder has a moisture content of less than 20% at the time of impingement on the wall-flow filter.

4. Method according to claim 1, characterized in that the amount of powder remaining in the filter is below 50 g/l.

5. Method according to claim 1, characterized in that the powder coating has an increasing concentration gradient over the length of the filter from the inlet side to the outlet side.

6. Method according to claim 1, characterized in that the aerosol is a mixture of air and a high-melting metal oxide, metal sulfate, metal phosphate, metal carbonate, or metal hydroxide powder or mixtures thereof.

7. Method according to claim 1, characterized in that the filter was catalytically coated prior to impingement with the powder/gas aerosol.

8. Method according to claim 1, characterized in that the powder is likewise catalytically active with regard to the reduction of the harmful substances in the exhaust gas of an internal combustion engine.

9. Method according to claim 1, characterized in that the powder/gas aerosol is sucked through the filter at a rate of 5 m/s to 50 m/s.

10. Method according to claim 1, characterized in that at least one partial gas stream is extracted downstream of the suction device and, before the powder addition, is added to the gas stream which is sucked through the filter.

11. Method according to claim 1, characterized in that a defined powder distribution over the filter cross section is set by an accelerated flow upstream of the filter.

12. Method according to claim 1, characterized in that the powder is vortexed before flowing into the filter in such a way that deposits of powder on the input plugs of the wall-flow filter are avoided as far as possible.

13. Method according to claim 1, characterized in that said filter has an increase in filtration efficiency of at least 5% at a relative increase in the exhaust-gas back pressure of at most 40% compared to a filter not treated with powder.

14. Device for producing a wall-flow filter for reducing the pollutants in the exhaust gas of an internal combustion engine, characterized in that said device has a unit for dispersing powder in a gas; a unit for mixing the dispersion with an existing gas stream; a filter-receiving unit designed to allow the gas stream to flow through the filter without further supply of a gas; a suction-generating unit that maintains the gas stream through the filter; and optionally, a unit for generating vortices upstream of the filter so that a deposition of powder on the input plugs of the filter is prevented as much as possible.

Description

FIGURES

[0082] FIG. 1: image of a preferred apparatus for carrying out the method according to the invention

[0083] FIG. 2: image of a wall-flow filter wall powder-sprayed according to the invention

[0084] FIG. 3: increase in exhaust-gas back pressure by powder-spraying in a catalytically precoated filter

[0085] FIG. 4: increase in filtration efficiency by the powder-spraying according to the invention in a catalytically precoated filter

[0086] FIG. 5: section through a powder-sprayed wall of a wall-flow filter and graphical analysis of points of powder-spraying

[0087] FIG. 6: increase in exhaust-gas back pressure by powder-spraying in a non-catalytically precoated filter

[0088] FIG. 7: increase in filtration efficiency by the powder-spraying according to the invention in a non-catalytically precoated filter

[0089] FIG. 8: dispersion by ultrasound; powder in free fall

[0090] FIG. 9: dispersion by ultrasound; powder in free fall; with sieve

[0091] FIG. 10: dispersion by ultrasound; powder in free fall; predispersed with sieve

[0092] FIG. 11: dispersion by an ultrasonic sieve

[0093] FIG. 12: dispersion energies of a pin mill or jet mill

[0094] FIG. 13: dispersion by a blower

EXAMPLES WITH CATALYTICALLY PRECOATED FILTERS

[0095] Cordierite wall-flow filters with a diameter of 11.8 cm and a length of 13.5 cm were in-wall coated in order to produce the VGPF, GPF1, GPF2, and GPF3 particulate filters described in the examples and comparative examples. The wall-flow filters had a cell density of 46.5 cells per square centimeter at a wall thickness of 0.203 mm. The average pore size of the filters was 20 μm, with the porosity of the filters being about 65%.

[0096] First, a coating suspension containing noble metal was applied to these wall-flow filters. After application of the coating suspension, the filters were dried and then calcined at 500° C. The amount of coating after calcination corresponded to 50 g/l based on the volume of the substrate. This corresponds to the preparation of the VGPF.

[0097] According to FIG. 1, 3 filters were coated with different amounts of aluminum oxide powder in the pores by means of the apparatus.

Example 1

[0098] GPF1: The open pores of an in-wall-coated filter were coated according to the invention with 3.3 g/l, based on the total filter volume, of a dry aluminum oxide. An aluminum oxide having an average particle diameter (d.sub.50) of 3.5 μm was used as the powder. This corresponds to a ratio of the average particle size of the powder used to the average pore size of the filter of 0.175.

Example 2

[0099] GPF2: The open pores of an in-wall-coated filter were coated according to the invention with 5.6 g/l, based on the total filter volume, of a dry aluminum oxide. An aluminum oxide having an average particle diameter (d.sub.50) of 3.5 μm was used as the powder. This corresponds to a ratio of the average particle size of the powder used to the average pore size of the filter of 0.175.

Example 3

[0100] GPF3: The open pores of an in-wall-coated filter were coated according to the invention with 8.6 g/l, based on the total filter volume, of a dry aluminum oxide. An aluminum oxide having an average particle diameter (d.sub.50) of 3 μm was used as the powder. This corresponds to a ratio of the average particle size of the powder used to the average pore size of the filter of 0.15.

[0101] The particulate filters GPF1, GPF2, and GPF3 according to the invention were investigated in comparison with the VGPF produced. After powder coating, the particulate filters were measured for their back pressure; as described below, filtration measurement was then carried out on the dynamic engine test bench. The back-pressure increase of the filters according to the invention is shown in FIG. 3.

[0102] The VGPF, GPF1, GPF2, and GPF3 filters described were investigated for their fresh filtration efficiency on the engine test bench in the real exhaust gas of an engine operating with an on average 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 one 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. FIG. 4 shows the results of the filtration efficiency measurement in the WLTP,

[0103] FIG. 4 shows the results of the filtration efficiency measurement. Depending on the amount of powder applied and the particle size distribution of the powder used, an improvement in the filtration efficiency by up to 20% at a maximum back-pressure increase (FIG. 3) of only about 9% can be achieved.

[0104] The measured data demonstrate that the selective coating of the open pores of an already in-wall-coated filter leads to a significant improvement in filtration efficiency with only slightly increased back pressure.

Catalytic Characterization:

[0105] The particulate filters VGPF2 as well as GPF4, GPF5 were used for catalytic characterization. The wall-flow filters had a cell density of 46.5 cells per square centimeter at a wall thickness of 0.203 mm. The average pore size of the filters was 18 μm, with the porosity of the filters being about 65%. First, a coating suspension containing noble metal was applied to these wall-flow filters. After application of the coating suspension, the filters were dried and then calcined at 500° C. The amount of coating after calcination corresponded to 75 g/l, the concentration of Pd being 1.06 g/l and concentration for Rh being 0.21 g/l. All concentrations are based on the volume of the substrate.

Example 4

[0106] GPF4: The open pores of an in-wall-coated filter were coated with g/l, based on the total filter volume, of a dry aluminum oxide, An aluminum oxide having an average particle diameter (d.sub.50) of 3.5 μm was used as the powder. This corresponds to a ratio of the average particle size of the powder used to the average pore size of the filter of 0.194.

Example 5

[0107] GPF5: The open pores of an in-wall-coated filter were coated with 15.8 g/l, based on the total filter volume, of a dry aluminum oxide. An aluminum oxide having an average particle diameter (d.sub.50) of 3.5 μm was used as the powder. This corresponds to a ratio of the average particle size of the powder used to the average pore size of the filter of 0.194.

[0108] The catalytically active particulate filters VGPF2, GPF4, and GPF5 were first tested in the fresh state and were then aged together in an engine test bench aging process. The latter consists of an overrun cut-off aging process (Aging 1) with an exhaust gas temperature of 900° C. upstream of the catalyst inlet (maximum bed temperature of 970° C.), The aging time was 19 hours. After the first aging process, the filters were examined for their catalytic activity and then subjected to a further engine test bench aging process (Aging 2). This time, the latter consists of an overrun cut-off aging process with an exhaust gas temperature of 950° C. upstream of the catalyst inlet (maximum bed temperature of 1030° C.). The filters were then tested repeatedly,

[0109] In the analysis of catalytic activity, the light-off behavior of the particulate filters was determined at a constant average air ratio λ on an engine test bench, and the dynamic conversion was checked when A changed. In addition, the filters were subjected to a “lambda sweep test.”

[0110] The following tables contain the temperatures T.sub.50 at which 50% of the component under consideration 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.

[0111] Table 1 contains the “light-off” data for the fresh filters, Table 2 the data after Aging 1, and Table 3 the data after Aging 2.

TABLE-US-00002 TABLE 1 T.sub.50 HC stoichio- T.sub.50 CO stoichio- T.sub.50 NOx stoichio- metric metric metric VGPF2 279 277 278 GPF4 279 275 277 GPF5 278 274 277

TABLE-US-00003 TABLE 2 T.sub.50 HC stoichio- T.sub.50 CO stoichio- T.sub.50 NOx stoichio- metric metric metric VGPF2 347 351 355 GPF4 350 353 356 GPF5 349 352 355

TABLE-US-00004 TABLE 3 T.sub.50 HC stoichio- T.sub.50 CO stoichio- T.sub.50 NOx stoichio- metric metric metric VGPF2 396 421 422 GPF4 398 413 419 GPF5 394 406 412

[0112] The dynamic conversion behavior of the particulate filters 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 ±3.4%. Table 3 shows the conversion at the intersection of the CO and NOx conversion curves, along with the associated HC conversion of the aged particulate filters. The standard deviation in this test is ±2%.

[0113] Table 4 contains the data for the fresh filters, Table 5 the data after Aging 1, and Table 6 the data after Aging 2.

TABLE-US-00005 TABLE 4 CO/NOx conversion at HC conversion at the λ of the intersection the CO/NOx intersection VGPF2 99% 99% GPF4 99% 99% GPF5 99% 99%

TABLE-US-00006 TABLE 5 CO/NOx conversion at HC conversion at the λ of the intersection the CO/NOx intersection VGPF2 98% 97% GPF4 98% 97% GPF5 98% 97%

TABLE-US-00007 TABLE 6 CO/NOx conversion at HC conversion at the λ of the intersection the CO/NOx intersection VGPF2 79% 94% GPF4 80% 94% GPF5 83% 95%

[0114] In comparison to VGPF2, particulate filters GPF4 and GPF5 according to the invention show no disadvantage in catalytic activity in either the fresh or the moderately aged states. In a highly aged state, the powder-coated filters GPF4 and GPF5 even have an advantage in both CO conversion and NOx conversion and also in the dynamic CO/NOx conversion.

Examples with Non-Catalytically Precoated Filters:

[0115] Cordierite wall-flow filters with a diameter of 15.8 cm and a length of 14.7 cm were used to produce the VGPF, GPF1, and GPF2 particulate filters described in the examples and comparative examples. The wall-flow filters had a cell density of 31 cells per square centimeter at a wall thickness of 0.203 mm. The average q3 pore size (d50) of the filters was 18 μm, with the porosity of the filters being about 50%.

[0116] For coating the filters according to the invention, an air/powder aerosol of a dry aluminum oxide with a d10 value of the q3 particle size of 0.8 μm, a d50 value of the q3 particle size of 2.9 μm, and a d90 value of the q3 particle size of 6.9 μm was used. This corresponds to a ratio of the average particle size of the powder used to the average pore size of the filter of 0.16 and a ratio of d10 to d50 of 28%.

[0117] As comparative example, VGPF, an untreated filter as described above was used. The coating was carried out with an apparatus as described in FIG. 1.

Example 1

[0118] GPF1: The open pores of a filter were coated with 6 g/l, based on the total filter volume, of the dry aluminum oxide.

Example 2

[0119] GPF2: The open pores of a filter were coated with 11.7 d/l, based on the total filter volume, of the dry aluminum oxide.

[0120] The particulate filters GPF1 and GPF2 according to the invention were investigated in comparison with the conventional VGPF. After coating, the particulate filters were measured for their back pressure, after which filtration measurement was then carried out on the highly dynamic engine test bench. The increase in back pressure of the filters according to the invention, measured on a back-pressure test stand (Superflow ProBench SF1020) at room temperature with an air throughput of 600 m.sup.3/h, is shown in FIG. 6.

[0121] The VGPF, GPF1, and GPF2 filters described were investigated for their fresh filtration efficiency on the engine test bench in the real exhaust gas of an engine operating with an on average 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 cm downstream of a conventional three-way catalyst. This three-way catalyst was the same one 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. FIG. 7 shows the results of the filtration efficiency measurement in the WLTP.

[0122] FIG. 7 shows the results of the filtration efficiency measurement. Depending on the amount of powder applied, an improvement in filtration efficiency up to 10% is already observed in the first WLTP cycle with a slight back-pressure increase (FIG. 6).

[0123] The measured data demonstrate that the selective coating of the open pores of a conventional ceramic wall-flow filter leads to a significant improvement in filtration efficiency with only slightly increased back pressure.