CATALYTICALLY ACTIVE PARTICLE FILTER WITH A HIGH DEGREE OF FILTERING EFFICIENCY

Abstract

The invention relates to a wall flow filter for removing particulate matter from the exhaust of internal combustion engines, comprising a wall flow filter substrate having a length L, and different coatings Z and F, the wall flow filter substrate being provided with channels E and A which run parallel between a first end and a second end of the wall flow filter substrate, are separated by porous walls, and form surfaces O.sub.E and O.sub.A, respectively; channels E are closed at the second end, and channels A are closed at the first end; coating Z is disposed in the porous walls and/or on surfaces O.sub.A, but not on surfaces O.sub.E, and contains palladium and/or rhodium and a cerium/zirconium mixed oxide; coating F is disposed mainly on surfaces O.sub.E, but not on surfaces O.sub.A, and comprises a membrane and no precious metal. The wall flow filter is characterized in that the mass ratio of coating Z to coating F ranges from 0.1 to 25.

Claims

1. Wall-flow filter for removing particles from the exhaust gas of combustion engines, comprising a wall-flow filter substrate of length L and coatings Z and F that differ from one another, wherein the wall-flow filter substrate has channels E and A, which extend in parallel between a first and a second end of the wall-flow filter substrate, are separated by porous walls and 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, and wherein the coating Z is located in the porous walls and/or on the surfaces O.sub.A, but not on the surfaces O.sub.E, and comprises palladium and/or rhodium and a cerium/zirconium mixed oxide, and wherein the coating F is located mainly on the surfaces O.sub.E, but not on the surfaces O.sub.A, and comprises a membrane and no noble metal, characterized in that the mass ratio of coating Z to coating F ranges from 0.1 to 25 and coating F consists of a cohesive membrane on the surfaces O.sub.E.

2. Wall-flow filter according to claim 1, characterized in that coating Z is located on the surfaces O.sub.A of the wall-flow filter substrate and extends from the second end of the wall-flow filter substrate over 50 to 90% of the length L or is located in the porous walls of the wall-flow filter substrate and extends from the first end of the wall-flow filter substrate over 50 to 100% of the length L.

3. Wall-flow filter according to claim 1, characterized in that the cerium/zirconium mixed oxide of the coating Z contains one or more rare earth metal oxides.

4. Wall-flow filter according to claim 1, characterized in that coating Z comprises lanthanum-stabilized aluminum oxide, rhodium, palladium or palladium and rhodium, and a cerium/zirconium/rare earth metal mixed oxide containing yttrium oxide and lanthanum oxide or praseodymium oxide and lanthanum oxide as rare earth metal oxides.

5. Wall-flow filter according to claim 1, characterized in that 55 to 100% of the total mass of the coating F is located on the surfaces O.sub.E.

6. Wall-flow filter according to claim 1, characterized in that the membrane of the coating F contains a particulate oxide of an element selected from the group consisting of silicon, aluminum, titanium, zirconium, cerium, yttrium, praseodymium, strontium, bismuth, neodymium, lanthanum and barium, or a mixture of two or more of said oxides.

7. Wall-flow filter according to claim 1, characterized in that the membrane of the coating F comprises a component A which comprises aluminum oxide or silicon oxide or titanium oxide and has a proportion of more than 50% of the total mass of the coating F, and a component B which comprises an oxide of the elements cerium, zirconium, barium or lanthanum or a mixture of two or more of said oxides and has a proportion of less than 50% of the total mass of the coating F.

8. Wall-flow filter according to claim 1, characterized in that the ratio of the wall thickness of the wall-flow filter substrate to the thickness of the coating F ranges from 0.8 to 400.

9. Wall-flow filter according to claim 1, characterized in that the average pore size d.sub.50 of the membrane of the coating F is at least 50 nm, wherein the d.sub.50 value of the pore size distribution is understood to mean that 50% of the total pore volume determinable by mercury porosimetry is formed by pores whose diameter is less than or equal to the value specified as d.sub.50.

10. Wall-flow filter according to claim 1, characterized in that the average pore size d.sub.50 of the membrane of the coating F is smaller than the average pore size d.sub.50 of the wall-flow filter substrate.

11. Wall-flow filter according to claim 1, characterized in that the membrane contains large particles with a d.sub.50 of 1 to 15 m and additionally small particles on a sub-micron scale.

12. Wall-flow filter according to claim 1, characterized in that the wall-flow filter substrate has a coating Y which is different from the coatings Z and F, which comprises platinum, palladium or platinum and palladium, which contains no rhodium and no cerium/zirconium mixed oxide and which is located in the porous walls and/or on the surfaces O.sub.E, but not on the surfaces O.sub.A.

13. Wall-flow filter according to claim 1, characterized in that the coating Z is located in the porous walls and/or on the surfaces O.sub.A, but not on the surfaces O.sub.E, extends from the second end over 60 to 100% percent of the substrate length L and comprises lanthanum-stabilized aluminum oxide, rhodium, palladium or palladium and rhodium, and a cerium/zirconium/rare earth metal mixed oxide containing yttrium oxide or neodymium oxide or praseodymium oxide and lanthanum oxide as rare earth metal oxides, the coating F is located mainly on the surfaces O.sub.E, but not on the surfaces O.sub.A, comprises a membrane and no noble metal, has a layer thickness of 1 to 150 m and extends from the first end over a length of 80 to 100% of the substrate length L, wherein the mass ratio of coating Z to coating F ranges from 0.15 to 15.

14. Method for producing a wall-flow filter according to claim 1, characterized in that the channels E of the dry wall-flow filter substrate already coated with coating Z and optionally coating Y are coated with the coating F, in that a suspension containing the constituents of the coating F is first pumped into the channel E, is then suctioned off against the pumping-in direction with a first suction pulse and then, after the wall-flow filter substrate has been inverted, is suctioned off in the pumping-in direction with a second suction pulse, characterized in that the second suction pulse, measured in mbar negative pressure, is greater than or equal to the first suction pulse, wherein the ratio of the pressures of the first to the second suction pulse is preferably in the range from 0.1 to 1.

15. A method for reducing harmful exhaust gases of an internal combustion engine, comprising passing the harmful exhaust gases of the internal combustion engine through a wall-flow filter according to claim 1.

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

Description

[0104] FIG. 1 relates to a wall-flow filter according to the invention in which the coating Z is located in the channels A on the surfaces O.sub.A and extends from the second end of the wall-flow filter substrate over 50% of the length L. The coating F is located in the channels E and extends over the entire length L.

[0105] FIG. 2 also relates to a wall-flow filter according to the invention in which the coating Z is located in the channels A on the surfaces O.sub.A. Starting from the second end of the wall-flow filter substrate, however, it extends over 80% of the length L. The coating F is located in the channels E and extends over the entire length L.

[0106] FIG. 3 relates to a wall-flow filter according to the invention in which the coating Z is located in the porous walls and extends over the entire length L. The coating F is located in the channels E and likewise extends over the entire length L.

[0107] FIG. 4 relates to a wall-flow filter according to the invention in which the coating Z is located in the porous walls and extends from the second end of the wall-flow filter substrate over 50% of the length L. The coating F is located in the channels E and extends over the entire length L.

[0108] FIG. 5 relates to a wall-flow filter according to the invention which differs from that of FIG. 4 in that coating Z is located over 50% of the length L on the surfaces O.sub.A and additionally coating Y is located in the porous walls over the entire length L. The coating F is located in the channels E and extends over the entire length L.

[0109] FIG. 6 relates to a wall-flow filter according to the invention which differs from that of FIG. 4 in that coating Z extends over 50% of the length L on the surfaces O.sub.A and additionally coating Y extends in the porous walls, starting from the first end of the wall-flow filter substrate, over 50% of the length L. The coating F is located in the channels E and extends over the entire length L.

[0110] FIG. 7 relates to a wall-flow filter according to the invention in which the coating Z is located in the channels A on the surfaces O.sub.A and extends over 50% of the length L. In addition, coating Y is located in the channels E on the surfaces O.sub.E and extends from the first end of the wall-flow filter substrate over 50% of the length L. The coating F is located the channels E and extends from the second end of the wall-flow filter substrate over 50% of the length L.

[0111] FIG. 8 relates to a wall-flow filter according to the invention in which the coating Z is located in the porous walls and extends over the entire length L. In addition, coating Y is located in the channels E on the surfaces O.sub.E and extends from the first end of the wall-flow filter substrate over 50% of the length L. The coating F is located the channels E and extends from the second end of the wall-flow filter substrate over 50% of the length L.

[0112] FIG. 9 relates to a wall-flow filter according to the invention in which the coating Z is located in the porous walls and extends from the second end of the wall-flow filter substrate over 50% of the length L. In addition, coating Y is located in the channels E on the surfaces O.sub.E and extends from the first end of the wall-flow filter substrate over 50% of the length L. The coating F is located the channels E and extends from the second end of the wall-flow filter substrate over 50% of the length L.

[0113] FIG. 10 relates to a wall-flow filter according to the invention in which the coating Z is located in the channels A on the surfaces O.sub.A and extends from the second end of the wall-flow filter substrate over 80% of the length L. In addition, coating Y is located in the porous walls and extends over the entire length L. The coating F is located the channels E and extends over the entire length L.

[0114] The wall-flow filter according to the invention can be produced by applying the coatings Z, F and, if present, Y to a wall-flow filter substrate.

[0115] In this case, the catalytic activity is provided as specified by the person skilled in the art by coating the wall-flow filter substrate with the coating Z and, if present, with the coating Y.

[0116] The term coating is accordingly to be understood to mean the application of catalytically active materials to a wall-flow filter substrate. The coating assumes the actual catalytic function. In the present case, the coating is carried out by applying a correspondingly low-viscosity aqueous suspension of the catalytically active components, also referred to as a washcoat, into or onto the wall of the wall-flow filter substrate, for example in accordance with EP178919061. After application of the suspension, the wall-flow filter substrate is dried in each case and, if applicable, calcined at an increased temperature. The catalytically coated filter preferably has a loading of 20 g/l to 200 g/l, preferably 30 g/l to 150 g/l (coating Z or sum of the coatings Z and Y). The most suitable amount of loading of a filter coated in the wall depends on its cell density, its wall thickness, and the porosity.

[0117] The coating F can likewise be applied to the surfaces O.sub.E by the coating method described above, i.e., by means of a wet-chemical coating step.

[0118] Thus, the coating F can first be coated onto the surfaces O.sub.E and subsequently, after calcining, the coatings Y, if present, and Z can be applied.

[0119] Alternatively, the coatings Y, if present, and Z, can first be applied and then the coating F can be coated onto the surfaces O.sub.E.

[0120] Generally, the suspensions required for coating are obtained by mixing the constituents and then grinding them with the aid of an appropriate mill to the desired particle size and setting the viscosity. If coating F is to contain sub-micron particles, these are added in particular after the grinding step and before the viscosity is set.

[0121] For the coating of the coating F, the suspension is generally first pumped into channel E at a pumping-in speed of 25 to 500 ml/s. Subsequently, the suspension is suctioned off against the pumping-in direction with a first suction pulse and then, after being inverted, is suctioned off again with a second suction pulse in the pumping-in direction.

[0122] According to the invention, the negative pressure of the second suction pulse, measured in mbar, is greater than or equal to the negative pressure of the first suction pulse, wherein the ratio of the pressures of the first to the second suction pulse is 0.1 to 1, preferably 0.15 to 0.8 and particularly preferably 0.2 to 0.75.

[0123] The first suction pulse extends in particular over a period of 0.5 to 15 seconds. The second suction pulse also extends over a period of 0.5 to 15 seconds. The first suction pulse can be longer than the second suction pulse or the second suction pulse can be longer than the first suction pulse.

[0124] After the channels E have been completely filled and before the first suction pulse is applied, a dwell time of 0 to 200 seconds can be applied while the coating suspension remains in the channels E.

[0125] The suspension for producing the coating F has a certain viscosity which is influenced using a plurality of commercially available additives. These are well known to a person skilled in the art.

[0126] The viscosity of the suspension for producing the coating F is preferably set in a range from 0.01 to 10 Pa s.sup.1, preferably from 0.02 to 7.5 Pa s.sup.1 and particularly preferably in a range from 0.03 to 5 Pa s.sup.1, measured at a shear rate of 1000 s.sup.1 and a temperature of 23 C.

[0127] The mass of the coating F is in particular 3 to 75 g/L, based on the volume of the wall-flow substrate, preferably 5 to 60 g/L.

[0128] The applied mass of the coating F can be varied depending on the wall-flow filter substrate used. It is thus advantageous if the ratio of the mass of the coating F to the average pore diameter of the wall-flow filter substrate, measured in m, is 0.25 to 8, preferably 0.5 to 6.

[0129] The wall-flow filters which are catalytically coated according to the invention differ from those that are produced in the exhaust system of a vehicle by ash deposition during operation. According to the invention, the catalytically active wall-flow filter substrates are selectively provided with coating F. As a result, the balance between filtration efficiency and exhaust-gas back pressure can be adjusted selectively right from the start. Wall-flow filters in which undefined ash deposits have resulted from combustion of fuel, e.g., in the cylinder during driving operation or by means of a burner, are therefore not included in the present invention.

[0130] The present invention therefore does not include wall-flow filters in which defined ash deposits are formed by dry coating of an air/powder aerosol.

[0131] In contrast to these, the catalytically coated wall flow filters according to the present invention also have a high stability in relation to condensation water, which usually collects in quantities of 10 to 1000 ml in the exhaust gas system. Unlike the filtration coatings obtained by dry coating of an air/powder aerosol, the coating F has a loss of filtration performance of only 0 to 5% after contact with more than 50 ml of water.

[0132] The wall-flow filter according to the invention exhibits an excellent filtration efficiency with only a moderate increase in exhaust-gas back pressure as compared to a wall-flow filter without the coating F in the fresh state. The wall-flow filter according to the invention preferably exhibits an improvement in soot particle deposition (filtering effect) in the filter of at least 5%, preferably at least 10% and very particularly preferably at least 20% at a relative increase in the exhaust-gas back pressure of the fresh wall-flow filter of at most 40%, preferably at most 20% and very particularly preferably at most 10% as compared to a fresh filter coated with catalytically active material but not treated with coating F. The slight increase in back pressure is probably due to the cross section of the channels on the input side not being significantly reduced by impinging, according to the invention, the filter with coating F. It is assumed that coating F forms a porous structure, which has a positive effect on the back pressure. Due to the coating F, a filter according to the invention also has a lower back pressure after soot loading than a similar filter without coating F since the latter largely prevents the soot from penetrating the porous filter wall.

[0133] Coating Z gives the wall-flow filter according to the invention excellent three-way activity, while the optional coating Y is able to reduce the soot ignition temperature and thus facilitates soot burn-off.

[0134] The present invention thus also relates to the use of a wall-flow filter according to the invention for reducing harmful exhaust gases of an internal combustion engine. The use of the wall-flow filter according to the invention for treating exhaust gases of a stoichiometrically operated internal combustion engine, i.e. in particular a gasoline-operated internal combustion engine, is preferred.

[0135] The wall-flow filter according to the invention is very advantageously used in combination with a three-way catalyst, which in particular adjoins the second end of the wall-flow filter (i.e., is arranged on the outflow side during intended use).

[0136] The preferred embodiments described for the wall-flow filter according to the invention also apply mutatis mutandis to the use mentioned here.

[0137] The present invention further relates to an exhaust gas purification system comprising a filter according to the invention and at least one further catalyst. In one embodiment of this system, at least one further catalyst is arranged upstream of the filter according to the invention. Preferably, this is a three-way catalyst or an oxidation catalyst or a NO.sub.x storage catalyst. In a further embodiment of this system, at least one further catalyst is arranged downstream of the filter according to the invention. Preferably, this is a three-way catalyst or an SCR catalyst or a NO.sub.x storage catalyst or an ammonia slip catalyst. In a further embodiment of this system, at least one further catalyst is arranged upstream of the filter according to the invention and at least one further catalyst is arranged downstream of the filter according to the invention. Preferably, the upstream catalyst is a three-way catalyst or an oxidation catalyst or a NO.sub.x storage catalyst and the downstream catalyst is a three-way catalyst or an SCR catalyst or a NO.sub.x storage catalyst or an ammonia slip catalyst.

[0138] The preferred embodiments described for the wall-flow filter according to the invention also apply mutatis mutandis to the exhaust gas purification system mentioned here.

[0139] Typically, the filter according to the invention is used primarily in internal combustion engines, in particular in internal combustion engines with direct injection or intake manifold injection. These are preferably stoichiometrically operated gasoline or natural gas engines. Preferably, these are motors with turbocharging

[0140] The requirements applicable to gasoline particulate filters (GPF) differ significantly from the requirements applicable to diesel particulate filters (DPF). Diesel engines without DPF can have up to ten times higher particle emissions, based on the particle mass, than gasoline engines without GPF (Maricq et al., SAE 1999-01-01530). In addition, there are significantly fewer primary particles in the case of gasoline engines, and the secondary particles (agglomerates) are significantly smaller than in diesel engines. Emissions from gasoline engines range from particle sizes of less than 200 nm (Hall et al., SAE 1999-01-3530) to 400 nm (Mathis et al., Atmospheric Environment 38 4347) with a maximum in the range of around 60 nm to 80 nm. For this reason, the nanoparticles in the case of GPF must mainly be filtered by diffusion separation. For particles smaller than 300 nm, separation by diffusion (Brownian molecular motion) and electrostatic forces becomes more and more important with decreasing size (Hinds, W.: Aerosol technology: Properties and behavior and measurement of airborne particles. Wiley, 2nd edition 1999).

[0141] FIGS. 1 to 10 show the different coating arrangements of wall-flow filters according to the invention, which are already described in more detail above. The following designations are used therein: [0142] (E) the inlet channel/inflow channel of the wall-flow filter [0143] (A) the outlet channel/outflow channel of the wall-flow filter [0144] (O.sub.E) the surfaces formed by the inlet channels (E) [0145] (O.sub.A) the surfaces formed by the outlet channels (A) [0146] (L) the length of the filter wall [0147] (Z) the coating Z [0148] (Y) the coating Y [0149] (F) the coating F

[0150] The advantages of the invention are explained using examples below.

Comparative Example 1: Coating Z Only

[0151] Aluminum oxide stabilized with lanthanum oxide was suspended in water with a first oxygen storage component, which comprised 40% by weight cerium oxide, zirconium oxide, lanthanum oxide and praseodymium oxide, and a second oxygen storage component, which comprised 24% by weight cerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide. Both oxygen storage components were used in equal parts. The weight ratio of aluminum oxide and oxygen storage component was 30:70. The suspension thus obtained was subsequently mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly for coating a commercially available wall flow filter substrate, the coating being introduced into the porous filter wall over 100% of the substrate length. The total loading of this filter was 25 g/l, and the total noble metal loading was 0.166 g/l, with only palladium used as the noble metal species. The coated filter thus obtained was dried and then calcined. It is hereinafter referred to as VGPF1.

Example 1 According to the Invention: Coating Z in Combination with Coating F

[0152] Aluminum oxide stabilized with lanthanum oxide was suspended in water with a first oxygen storage component, which comprised 40% by weight cerium oxide, zirconium oxide, lanthanum oxide and praseodymium oxide, and a second oxygen storage component, which comprised 24% by weight cerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide. Both oxygen storage components were used in equal parts. The weight ratio of aluminum oxide and oxygen storage component was 30:70. The suspension thus obtained was subsequently mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly for coating a commercially available wall flow filter substrate, the coating being introduced into the porous filter wall over 100% of the substrate length. The total loading of this filter was 25 g/l, and the total noble metal loading was 0.166 g/l, with only palladium used as the noble metal species.

[0153] The coated filter thus obtained was dried and then calcined. The filter was then coated on the surfaces O.sub.E with a wet-chemical filtration efficiency-increasing membrane consisting of aluminum oxide. For this purpose, a suspension of a metal oxide with an average particle size of 5.7 m was coated in a wet-chemical process. The suspension was first pumped into the substrate and then emptied with a weak suction pulse counter to the pumping-in direction. Subsequently, the filter was again suctioned with a second, stronger suction pulse in the pumping-in direction. The coated filter thus obtained was dried and then calcined. The total loading of this filter was thus 55 g/l, with 25 g/L attributed to coating Z and 30 g/L to coating F. It is hereinafter referred to as GPF1.

[0154] The two filters thus obtained were subsequently measured on a cold-blast test bench in order to determine the pressure loss across each filter. At room temperature and a volumetric flow rate of 600 m.sup.3/h of air, the back pressure is 16 mbar for the VGPF1 and 50 mbar for the GPF1. As already described, the filtration coating F only leads to a moderate increase in back pressure.

[0155] At the same time, fresh VGPF1 and GPF1 filters were investigated in the vehicle in terms of their particle filtration efficiency. For this purpose, the filters were measured in a RTS aggressive driving cycle in a position close to the engine between two particle counters. Here the filter GPF1 according to the invention has a filtration efficiency of 88.3%, calculated from the particle values of the two particle counters, while the comparative filter VGPF1 achieves a filtration efficiency of only 68.4%. Overall, it can be seen that the combination of filtration coating F and the three-way coating Z is particularly advantageous in terms of filtration efficiency.