Catalyst article and the use thereof for filtering fine particles

Abstract

The present invention provides catalyst article, and its use in an exhaust system for internal combustion engines, is disclosed. The catalyst article catalyst article comprises: a substrate which is a wall-flow filter having an inlet end and an outlet end and an axial length L therebetween, a plurality of inlet channels extending from the inlet end and a plurality of outlet channels extending from the outlet end, wherein the plurality of inlet channels comprise a first catalyst composition extending from the inlet or outlet end for at least 50% of L and the plurality of outlet channels comprise a second catalyst composition extending from the outlet or inlet end for at least 50% of L, wherein the first and second catalyst compositions overlap by at most 80% of L, and wherein the first and second catalyst compositions each independently comprise a particulate oxygen storage component (OSC) having a first D90 and a particulate inorganic oxide having a second D90 and: i) the first D90 is less than 1 micron and the second D90 is from 1 to 20 microns; or ii) the second D90 is less than 1 micron and the first D90 is from 1 to 20 microns.

Claims

1. A catalyst article for treating an exhaust gas from a positive-ignition internal-combustion engine, the article comprising: a substrate which is a wall-flow filter having an inlet end and an outlet end and an axial length L therebetween, a plurality of inlet channels extending from the inlet end and a plurality of outlet channels extending from the outlet end, wherein the plurality of inlet channels comprise a first catalyst composition extending from the inlet or outlet end for at least 50% of L and the plurality of outlet channels comprise a second catalyst composition extending from the outlet or inlet end for at least 50% of L, wherein the first and second catalyst compositions overlap by at most 80% of L, and wherein the first and second catalyst compositions each independently comprise a particulate oxygen storage component (OSC) having a first D90 and a particulate inorganic oxide having a second D90 and: i) the first D90 is less than 1 micron and the second D90 is from 1 to 20 microns; or ii) the second D90 is less than 1 micron and the first D90 is from 1 to 20 microns.

2. The catalyst article according to claim 1, wherein the particulate OSC has a first D10and the particulate inorganic oxide has a second D10, wherein: i) when the first D90 is less than 1 micron and the second D90 is from 1 to 20 microns, the first D10 is at least 100 nm and the second D10 is at least 500 nm; or ii) when the second D90 is less than 1 micron and the first D90 is from 1 to 20 microns, the second D10 is at least 100 nm and the first D10 is at least 500 nm.

3. The catalyst article according to claim 1, wherein: i) the first D90 is less than 1 micron and the second D90 is from 5 to 20 microns, or ii) the second D90 is less than 1 micron and the first D90 is from 5 to 20 microns.

4. The catalyst article according to any of claim 1, wherein the first and second catalyst compositions overlap by at most 20% of L.

5. The catalyst article according to claim 1, wherein the particulate OSC is selected from the group consisting of cerium oxide, zirconium oxide, a ceria-zirconia mixed oxide, and an alumina-ceria-zirconia mixed oxide.

6. The catalyst article of claim 1, wherein particulate inorganic oxide is selected from the group consisting of alumina, magnesia, silica, lanthanum, neodymium, praseodymium, yttrium oxides, and mixed oxides or composite oxides thereof.

7. The catalyst article of claim 1, wherein the first and/or second catalyst composition further comprises a first platinum group metal (PGM) component selected from the group consisting of platinum, palladium, rhodium, and a mixture thereof.

8. The catalyst article of claim 1, wherein the first and/or second catalyst composition further comprises barium or strontium.

9. The catalyst article according to claim 1, wherein the first and second catalyst compositions are the same.

10. The catalyst article according to claim 1, wherein the first and/or second catalyst composition have a ratio by weight of the particulate OSC to the particulate inorganic oxide of from 3:1 to 1:3.

11. The catalyst article of claim 1, wherein the first and/or second catalyst compositions are provided directly on the substrate.

12. An exhaust gas treatment system comprising the catalyst article according to claim 1 in fluid communication with an exhaust manifold of a positive ignition engine.

13. A method of treating exhaust gas emitted from a positive ignition internal combustion engine, the method comprising contacting the exhaust gas with the catalyst article of claim 1.

14. Use of the catalyst article according to claim 1 in an exhaust gas treatment system for reducing the emission of particles smaller than 23 nm.

15. The use according to claim 14, wherein the use reduces the emission of particles smaller than 23 nm by at least 20% by number.

Description

(1) FIG. 1 shows a schematic of a wall-flow filter as described herein.

(2) FIG. 2 shows a schematic of a wall-flow filter as described herein.

(3) FIG. 3 shows particulate emissions and specifically those between 10-23 nm for exemplary catalyst articles, in particular, simulated WLTC engine bench data for 10-23 nm Particulate count in #/km.

(4) FIG. 1 shows a cross-section through a wall-flow filter 1 made of a porous cordierite substrate. The wall-flow filter 1 has a length L extending between the inlet end 5 and the outlet end 10. The wall-flow filter 1 comprises a plurality of inlet channels 15 and a plurality of outlet channels 20 (only one shown). The plurality of inlet channels 15 and outlet channels 20 are plugged at their respective ends with plugs 21.

(5) The internal surfaces 25 of the inlet channels 15 have been coated with an on-wall coating 30 comprising a first catalyst composition. The coating 30 extends at least 50% of L from the inlet end 10.

(6) The internal surfaces 35 of the outlet channels 20 have been coated with an on-wall coating 40 comprising a second catalyst composition. The coating 40 extends at least 50% of L.

(7) The overlap region 45 between coatings 30 and 40 is less than or equal to 20% of L.

(8) In use, an exhaust gas flow 50 (shown with the arrows) passes into the inlet channels 15, through the walls of the wall-flow filter 1, and out of the outlet channels 20.

(9) FIG. 2 shows a cross-section through a wall-flow filter 1 made of a porous cordierite substrate. The wall-flow filter 1 has a length L extending between the inlet end 5 and the outlet end 10. The wall-flow filter 1 comprises a plurality of inlet channels 15 and a plurality of outlet channels 20 (only one shown). The plurality of inlet channels 15 and outlet channels 20 are plugged at their respective ends with plugs 21.

(10) The internal surfaces 25 of the inlet channels 15 have been coated with an on-wall coating 30 comprising a first catalyst composition. The coating 30 extends at least 50% of L from the outlet end 10. The coating 30 is wedge-shaped with a thicker amount at the outlet end 10. The coating 30 is applied in accordance with the method of WO 99/47260.

(11) The internal surfaces 35 of the outlet channels 20 have been coated with an on-wall coating 40 comprising a second catalyst composition. The coating 40 extends at least 50% of L from the inlet end 5. The coating 40 is wedge-shaped with a thicker amount at the inlet end 5. The coating 40 is applied in accordance with the method of WO 99/47260.

(12) The overlap region 45 between coatings 30 and 40 is less than or equal to 20% of L.

(13) In use, an exhaust gas flow 50 (shown with the arrows) passes into the inlet channels 15, through the walls of the wall-flow filter 1, and out of the outlet channels 20.

(14) The invention will now be described in relation to the following non-limiting examples.

(15) Materials

(16) All materials are commercially available and were obtained from known suppliers, unless noted otherwise.

EXAMPLE 1

(17) A three way catalyst washcoat was prepared at a washcoat loading of 1.6 g/in.sup.3 (1.2 g/in.sup.3 CeZr mixed oxide and 0.4 g/in.sup.3 alumina) and a PGM loading of 22 g/ft.sup.3 (Pt:Pd:Rh, 0:20:2): comprised of a Rare Earth Oxide (REO) Oxygen Storage Component (OSC) (weight ratio of ZrO.sub.2 to CeO.sub.2 is about 2:1) with a D90 of <1 μm and a La-stabilized alumina component which was wet milled to a D90 of 12 μm.

(18) The completed washcoat was adjusted to a suitable final washcoat solids content in order to coat onto the GPF substrate using Johnson Matthey's precision coating process described in WO 99/47260. The substrate used was a commercially available cordierite GPF substrate of a nominal 63% porosity and 17.5 μm mean pore size and of dimensions 4.66 inch diameter by 6 inch in length, 300 cells per square inch and a channel wall thickness of 8 thousandths of an inch. The coating was applied from each end of the substrate with each application covering a length between 50 and 65% of to achieve a fully coated final product with no uncoated region. The coated part was then dried and calcined in the normal way known to the art.

EXAMPLE 2

(19) Example 2 was prepared in the same way as Example 1 but the alumina component was wet milled further to a D90 of 5 μm.

COMPARATIVE EXAMPLE 3

(20) A reference coated GPF was prepared by applying a current state of art TWC coating developed for GPF substrate. The washcoat loading was prepared at 1.6 g/in.sup.3 (1.2 g/in.sup.3 CeZr mixed oxide and 0.4 g/in.sup.3 alumina) and the PGM loading 22 g/ft.sup.3 (Pt:Pd:Rh, 0:20:2). In contrast to Examples 1 and 2, here both the OSC component and alumina component have a D90 of 7 μm.

(21) Uncoated 220/6 Reference GPF

(22) For the purposes of further comparison to the invention, an uncoated GPF reference has been used. This is a commercially available GPF substrate which may be considered for use in such aftertreatment systems for Euro 6 PN limit of 6×10.sup.11. The same catalyst diameter and length and therefore catalyst volume was selected as Examples 1-3, 4.66″ diameter and 6″ length. However, to achieve the required filtration efficiency, such bare filters require lower nominal porosity and mean pore size. Since this can cause an undesirable increase in backpressure, the cells per square inch and wall thickness are typically lower than the substrates which are designed to be coated with washcoat. The uncoated reference GPF was of 220 cells per square inch and a wall thickness of 6 thousandths of an inch.

(23) Filtration Performance Evaluation

(24) The above examples were evaluated for particulate emissions and specifically those between 10-23 nm. The evaluation was carried out over a simulated WLTC performed on an engine bench equipped with a 2.0 L Gasoline Direct Injection engine. The particles are measured using a Combustion DMS500 particle analyser.

(25) The results are shown in FIG. 3 and were an average of 3 WLTC tests completed for each example. The results clearly show the benefit of Examples 1 and 2 in comparison to the uncoated low porosity 220/6 GPF and Comparative Example 3. Particularly, Example 1 shows a much improved benefit over the currently available alternatives.

(26) Table 1 below demonstrates additional reduced backpressure benefit of Example 1 and Example 2 vs. Comparative Example 3.

(27) TABLE-US-00001 TABLE 1 Cold Flow BP at Sample 600 m.sup.3/hr (mbar) Bare 220/6 44.1 Comparative 65.7 Example 3 Example 1 63.0 Example 2 56.4
Unless otherwise stated, all percentages herein are by weight.

(28) Although preferred embodiments of the invention have been described herein in detail, it will be understood by those skilled in the art that variations may be made thereto without departing from the scope of the invention or of the appended claims.