Coated wall-flow filter

Abstract

The present invention relates to a catalytically coated wall-flow filter, to a method for the production thereof and to the use thereof in order to reduce harmful exhaust gases of an internal combustion engine.

Claims

1. Catalytically active wall-flow filter for reducing the harmful substances in the exhaust gas of an internal combustion engine, wherein the wall-flow filter comprises walls with pores, and the wall-flow filter is coated with catalytically active material in the walls, with the wall-flow filter being selectively impinged on an inlet side with a dry powder/gas aerosol which has a high-melting compound in such a way that the powder is deposited in the pores of the filter walls and does not form a continuous layer on the walls of the filter.

2. Catalytically active wall-flow filter 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.

3. Catalytically active wall-flow filter according to claim 1, characterized in that the catalytically active coating of the filter is selected from the group consisting of three-way catalyst, SCR catalyst, nitrogen oxide storage catalyst, oxidation catalyst, soot-ignition coating, hydrocarbon storage.

4. Catalytically active wall-flow filter according to claim 3, characterized in that the catalytically active coating of the filter consists of at least one metal-ion-exchanged zeolite, cerium/zirconium mixed oxide, aluminum oxide and palladium, rhodium, or platinum, or combinations of these noble metals.

5. Catalytically active wall-flow filter according to claim 1, characterized in that said filter has a loading with the catalytic coating of 20 g/l to 200 g/l based on the volume of the filter.

6. Catalytically active wall-flow filter according to claim 1, characterized in that the ratio of average particle diameter (measured according to the most recent ISO 13320 on the date of application) d50 in the dry aerosol and the average pore diameter of the filter after coating with washcoat (measured according to DIN 66134, latest version on the date of application) is between 0.03 and 2.

7. Catalytically active wall-flow filter according to claim 1, characterized in that the loading of the filter with the powder is not more than 50 g/l based on the volume of the filter.

8. Catalytically active wall-flow filter according to claim 1, characterized in that the powder coating has an increasing gradient from the inlet end to the outlet end.

9. Catalytically active wall-flow filter according to claim 1, characterized in that the powder is also catalytically active with regard to reducing the harmful substances in the exhaust gas of an internal combustion engine.

10. Catalytically active wall-flow filter 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 10% compared to a fresh filter coated with catalytically active material but not treated with powder.

11. Method for producing a catalytically active wall-flow filter according to claim 1, characterized in that a carrier gas is charged with a powder and sucked into a filter.

12. Method for producing a catalytically active wall-flow filter according to claim 11, characterized in that the aerosol is sucked through the filter at a rate of 5 m/s to 50 m/s.

13. Method for producing a catalytically active wall-flow filter according to claim 11, characterized in that the powder has a moisture content of less than 20% at the time of impingement on the wall-flow filter.

14. A method of reducing harmful exhaust gases of an internal combustion engine, comprising passing the exhaust gas through a catalytically active wall-flow filter according to claim 1 in order to reduce harmful exhaust gases produced by the internal combustion engine.

Description

FIGURES

(1) FIG. 1: Image of a wall-flow filter powder-sprayed according to the invention (color photography)

(2) FIG. 2: Increase in exhaust-gas back pressure as a result of the powder-spraying

(3) FIG. 3: Increase in filtration efficiency as a result of the powder-spraying according to the invention

(4) FIG. 4: Section through a powder-sprayed wall of a wall-flow filter and graphical analysis of points of powder-spraying (color photography)

EXAMPLES

(5) 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%.

(6) 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.

Example 1

(7) 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

(8) 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

(9) 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.

(10) 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. 2.

(11) 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 catalytic converter. This three-way catalytic converter 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 catalytic converter and downstream of the particulate filter. FIG. 3 shows the results of the filtration efficiency measurement in the WLTP.

(12) FIG. 3 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. 2) of only about 9% can be achieved.

(13) 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.

(14) Catalytic Characterization

(15) 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%.

(16) 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

(17) GPF4: The open pores of an in-wall-coated filter were coated with 10 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

(18) 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.

(19) 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.

(20) 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 λ changed. In addition, the filters were subjected to a “lambda sweep test.”

(21) 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.

(22) 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.

(23) TABLE-US-00001 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

(24) TABLE-US-00002 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

(25) TABLE-US-00003 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

(26) 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%.

(27) Table 4 contains the data for the fresh filters, Table 5 the data after Aging 1, and Table 6 the data after Aging 2.

(28) TABLE-US-00004 TABLE 4 CO/NOx HC conversion at conversion at the the λ of the CO/NOx intersection intersection VGPF2 99% 99% GPF4 99% 99% GPF5 99% 99%

(29) TABLE-US-00005 TABLE 5 CO/NOx HC conversion at conversion at the the λ of the CO/NOx intersection intersection VGPF2 98% 97% GPF4 98% 97% GPF5 98% 97%

(30) TABLE-US-00006 TABLE 6 CO/NOx HC conversion at conversion at the the λ of the CO/NOx intersection intersection VGPF2 79% 94% GPF4 80% 94% GPF5 83% 95%

(31) 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.