Coated wall-flow filter

Abstract

The present invention relates to a wall-flow filter, to a method for the production and the use thereof in order to reduce harmful exhaust gases of an internal combustion engine. The wall-flow filter was produced by applying a powder-gas aerosol to the filter, whereby the powder was deposited in the pores of the wall-flow filter.

Claims

1. A wall-flow filter for the reduction of harmful substances in the exhaust gas of an internal combustion engine, wherein a dry, non-catalytically coated filter substrate is selectively impinged on an input surface of the filter substrate with a dry powder-gas aerosol, which has at least a high-melting compound, in such a way that powder of the dry powder-gas aerosol is deposited in pores in walls of the filter substrate and fills the pores up to the input surface of the filter substrate, wherein no continuous layer forms on the walls of the filter substrate, and an amount of the powder remaining in the wall-flow filter is below 50 g/l and a powder coating has an increasing concentration gradient over a length of the wall-flow filter from an inlet side to an outlet side of the wall-flow filter.

2. The wall-flow filter according to claim 1, characterized in that the powder coating has an increasing concentration gradient over the length of the wall-flow filter from the inlet side to the outlet side, which is such that in an area near the inlet side and in an area in the middle of the wall-flow filter, less than 40% of a wall surface of an input channel is respectively coated with powder, while in an area near the outlet side, more than 40% of the wall surface of the input channel is coated with powder.

3. The wall-flow filter according to claim 1, characterized in that the filter substrate has square input channels, and wherein the powder coating in the vicinity of the corners of the input channels is thicker than a corresponding center of the input surface of the input channels.

4. The 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, or mixtures thereof.

5. The wall-flow filter according to claim 4, characterized in that the high-melting powder has an outer surface of at least 5 m.sup.2 per liter of filter substrate volume.

6. The wall-flow filter according to claim 1, characterized in that a) a d90 value of a volume-related q3 particle size distribution of the powder used is less than or equal to 60% of the average volume-related q3 pore size (d50) of the filter substrate used, b) an average volume-related q3 particle size of the powder (d50) is 5% to 30% of the average volume-related q3 pore size (d50) of the filter substrate used, and c) a d10 value of the volume-related q3 particle size distribution of the powder is at least 20% to 60% of the average volume-related q3 particle size (d50) of the powder used, and d) a d10 value of the number-related q0 particle size distribution of the powder used is greater than 0.05 μm.

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

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

9. A method for producing the wall-flow filter according to claim 1, characterized in that a carrier gas is charged with the powder and sucked into the filter substrate.

10. The method for producing the wall-flow filter according to claim 9, characterized in that the aerosol is sucked through the filter substrate at a rate of 5 m/s to 50 m/s.

11. A method of reducing harmful exhaust gases of an internal combustion engine, comprising: passing exhaust gases of the internal combustion engine through the wall-flow filter according to claim 1 in order to reduce harmful exhaust gases generated by the internal combustion engine.

12. The method according to claim 11, characterized in that the wall-flow filter is used in an exhaust system together with one or more catalytically active aggregates selected from the group consisting of a nitrogen oxide storage catalyst, an SCR catalyst, a three-way catalyst, and a diesel oxidation catalyst.

Description

FIGURES

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

(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 the points of powder-spraying

(5) FIG. 5: Powder-spraying result in the corner of a filter wall

(6) FIG. 6: Schematic drawing of the filling of a pore with particles

EXAMPLES

(7) 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%.

(8) An air-powder aerosol composed 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 to coat the filters according to the invention. 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%.

(9) As comparative example VGPF, an untreated filter as described above was used.

Example 1

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

Example 2

(11) GPF2: The open pores of a filter were coated with 11.7 g/l of the dry aluminum oxide, based on the total filter volume.

(12) The particulate filters GPF1 and GPF2 according to the invention were examined 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. 2.

(13) 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 filters were installed 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 catalytic converter and downstream of the particulate filter. FIG. 3 shows the results of the filtration efficiency measurement in the WLTP.

(14) FIG. 3 shows the results of the filtration efficiency measurement. Depending on the amount of powder applied, an improvement in the filtration efficiency of up to 10% is to be already observed in the first WLTP cycle with a slight back pressure increase (FIG. 2).

(15) 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.