Exhaust System

Abstract

An exhaust system for an internal combustion engine, the exhaust system comprising, a lean NO.sub.x trap, and a wall flow monolithic substrate having a pre-coated porosity of 40% or greater, and comprising an oxidation catalytic zone, the oxidation catalytic zone comprising a platinum group metal loaded on a first support, the first support comprising at least one inorganic oxide and a zinc compound.

Claims

1. An exhaust system for an internal combustion engine, the exhaust system comprising, a. a lean NO.sub.x trap, and b. a wall flow monolithic substrate having a pre-coated porosity of 40% or greater, and comprising an oxidation catalytic zone, the oxidation catalytic zone comprising a platinum group metal loaded on a first support, the first support comprising at least one inorganic oxide and a zinc compound.

2. The exhaust system according to claim 1, wherein the zinc compound is selected from zinc oxide, zinc nitrate, zinc carbonate, zinc hydroxide or a mixture of two or more thereof.

3. The exhaust system according to claim 1, wherein the inorganic oxide comprises cerium oxide.

4. The exhaust system according to claim 1, wherein the first support further comprises alumina and/or an aluminate.

5. The exhaust system as claimed in claim 1, wherein the first support comprises alumina and ceria-zirconia mixed oxide.

6. The exhaust system according to claim 1, wherein the zinc compound has a particle size according to d.sub.90 in the range 1 to 25 μm.

7. (canceled)

8. The exhaust system according to claim 1, wherein the platinum group metal is selected from the group consisting of platinum, palladium, rhodium, and a mixtures of any two or more thereof.

9. The exhaust system according to claim 8, wherein the platinum group metal comprises a mixture of platinum and palladium in a Pt:Pd weight ratio in the range 0.5:1 to 7:1.

10. (canceled)

11. The exhaust system according to claim 1, wherein the pre-coated porosity of the wall flow monolithic substrate is 41% or greater.

12. (canceled)

13. (canceled)

14. (canceled)

15. The exhaust system according to claim 1, wherein the oxidation catalyst zone is applied to be in a single layer.

16. (canceled)

17. The exhaust system according to claim 1, further comprising a selective catalytic reduction zone on a monolithic substrate, the selective catalytic reduction zone comprising copper or iron loaded on a second support, the second support comprising a molecular sieve.

18. The exhaust system according to claim 17, wherein the molecular sieve is selected from the group consisting of a beta zeolite (BEA), a faujasite (FAU), an L-zeolite, a chabazite, a ZSM zeolite, a small pore molecular sieve having a maximum pore opening of eight tetrahedral atoms, an SSZ-zeolite, a ferrierite (FER), a mordenite (MOR), an offretite (OFF), a clinoptilolite (HEU), a silicalite, an aluminophosphate molecular sieve, a mesoporous zeolite, and/or mixtures of any two or more thereof.

19. (canceled)

20. The exhaust system according to claim 17, wherein the oxidation catalyst zone is on a first wall flow monolithic substrate and the selective catalytic reduction zone is on a second monolithic substrate.

21. The exhaust system according to claim 17, wherein the oxidation catalyst zone and the selective catalytic reduction zone are each on portions of the same wall flow monolithic substrate.

22. The exhaust system according to claim 21, wherein the oxidation catalyst zone is disposed in channels of the wall flow monolithic substrate from one end thereof and the selective catalytic reduction zone is disposed in channels of the wall flow monolithic substrate from the other end thereof.

23. The exhaust system according to claim 22, wherein the oxidation catalyst zone extends over between 10% and 90% of the axial length of the monolithic substrate and the selective catalytic reduction zone extends over between 90% and 10% of the axial length of the monolithic substrate.

24. (canceled)

25. The exhaust system according to claim 1, wherein the wall flow monolithic substrate comprises pores having a diameter, the pores of the wall flow monolithic substrate have a diameter in the range 9 μm to 25 μm.

26. The exhaust system according to claim 1, wherein the wall flow monolithic substrate comprises an inlet end having inlet channels and an outlet end having outlet channels and the oxidation catalyst zone is on and/or within the walls of the inlet channels of the inlet end of the monolithic substrate and/or within the walls of the outlet channels of the outlet end of the monolithic substrate.

27. A catalytic wall flow monolithic substrate, the wall flow monolithic substrate having a oxidation catalyst zone thereon, the wall flow monolithic substrate having a pre-coated porosity of 40% or greater, the oxidation catalyst zone comprising a platinum group metal loaded on a first support, the first support comprising at least one inorganic oxide, and a zinc compound.

28. (canceled)

29. A method of making a catalysed monolithic substrate, the method comprising: a. providing a wall flow monolithic substrate, the wall flow monolithic substrate having a pre-coated porosity of 40% or greater, b. preparing a oxidation catalyst zone washcoat comprising a source of a platinum group metal, a first support comprising at least one inorganic oxide and a source of a zinc compound, and c. applying the oxidation catalyst zone washcoat to a first portion of the monolithic substrate.

30. (canceled)

31. A method of treating exhaust gases from an internal combustion engine, the method comprising flowing the exhaust gas through an exhaust system according to claim 1, wherein the exhaust gas comprises a lean exhaust gas intermittently becoming rich.

32. A compression ignition engine fitted with an exhaust system according to claim 1.

33. A vehicle comprising a compression ignition engine fitted with an exhaust system according to claim 32.

Description

[0065] In order that the present invention can be better understood, reference is made to accompanying drawings, in which:

[0066] FIG. 1 illustrates schematically a first exhaust system according to the present invention.

[0067] FIG. 2 illustrates schematically a second exhaust system according to the present invention.

[0068] FIG. 3 shows a graph of CO conversion as function of inlet temperature for an oxidation catalyst according to the Examples.

[0069] FIG. 4 graph of HC conversion as function of inlet temperature for an oxidation catalyst according to the Examples.

[0070] FIG. 1 shows schematically a first exhaust system 2 of the present invention. The exhaust system 2 comprises a first monolithic substrate 4 which forms a lean NO.sub.x trap (LNT) catalyst. The exhaust gases from the engine (not shown) upstream of the first monolithic substrate/lean NO.sub.x trap 4 enter the first monolithic substrate 4 through inlet 10 and exit the first monolithic substrate 4 through pipe 8. The exhaust gases then enter a second monolithic substrate 6 before exiting through outlet 12. Downstream of outlet 12 there can be other catalytic zones or the exhaust gases can be released to atmosphere.

[0071] The second monolithic substrate 6 is a SiC wall-flow filter substrate of 2.5 litre volume having 300 cells per square inch, a wall thickness of 10 Mil (thousands of an inch) and 42% porosity. The wall-flow substrate has a honeycomb structure with many small, parallel thin-walled channels running axially through the substrate, with the channels of the wall flow substrate being alternately blocked, which allows the exhaust gas stream to enter a channel from the inlet, then flow through the porous channel walls, and exit the filter from a different channel leading to the outlet. The second monolithic substrate 6 contains an oxidation catalytic zone of a platinum group metal and a support of alumina and ceria-zirconia mixed oxide milled to a d.sub.90 particle size of <15 micron and zinc oxide provided on both the walls of the inlet channels at the inlet end of the second monolithic substrate 6 and on the walls of the outlet channels at the outlet end of the second monolithic substrate 6. The exhaust system of FIG. 1 can be produced using a wall flow monolithic oxidation catalyst produced as described below in Example 2.

[0072] FIG. 2 shows schematically a second exhaust system 13 of the present invention. The exhaust system 13 comprises a first monolithic substrate 14 which forms a lean NO.sub.x trap catalyst. As in FIG. 1, the exhaust gases from the engine (not shown) upstream of the first monolithic substrate/lean NO.sub.x trap 14 enter the first monolithic substrate 14 through inlet 20 and exit the first monolithic substrate 14 through pipe 18. The exhaust gases then enter a second monolithic substrate 16 before exiting through pipe 19 to a third monolithic substrate 17 and then through outlet 22. Downstream of outlet 22 there can be other catalytic zones or the exhaust gases can be released to atmosphere.

[0073] The second monolithic substrate 16 is a filter, wall flow monolithic substrate having an oxidation catalytic zone provided on the walls of the channels. The third monolithic substrate 17 is a flow through monolithic substrate having a uniform coating throughout of a selective catalytic reduction zone.

[0074] The following Examples are provided by way of illustration only.

COMPARATIVE EXAMPLE 1

[0075] A slurry was prepared using alumina and ceria-zirconia mixed oxide milled to a d90 particle size of <15 micron. Appropriate amounts of soluble Pt and Pd salts were added to give a final coated catalyst loading of 10 g/ft.sup.3 with a Pt:Pd weight ratio of 2:1 and the mixture stirred to homogenise. The coating slurry was applied to the entire volume of a 2.5 litre volume SiC wall-flow filter substrate having 300 cells per square inch, a wall thickness of 10 Mil (thousands of an inch) and 42% porosity. The coating was dried using forced air flow and calcined at 500° C.

EXAMPLE 2

ZnO

[0076] A slurry was prepared using alumina and ceria-zirconia mixed oxide milled to a d90 particle size of <15 micron. Appropriate amounts of soluble Pt and Pd salts were added to give a final coated catalyst loading of 10 g/ft.sup.3 with a Pt:Pd weight ratio of 2:1. Zn oxide was added to the slurry and the mixture stirred to homogenise. The coating slurry was applied to the entire volume of a 2.5 litre volume SiC wall-flow filter substrate having 300 cells per square inch, a wall thickness of 10 Mil (thousands of an inch) and 42% porosity. The coating was dried using forced air flow and calcined at 500° C. The coated filter had a zinc loading of 250 g/ft.sup.3.

COMPARATIVE EXAMPLE 3

MnO.SUB.2

[0077] A slurry was prepared using alumina and ceria-zirconia mixed oxide milled to a d90 particle size of <15 micron. Appropriate amounts of soluble Pt and Pd salts were added to give a final coated catalyst loading of 10 g/ft.sup.3 with a Pt:Pd weight ratio of 2:1. Mn dioxide was added to the slurry and the mixture stirred to homogenise. The coating slurry was applied to the entire volume of a 2.5 litre volume SiC wall-flow filter substrate having 300 cells per square inch, a wall thickness of 10 Mil (thousands of an inch) and 42% porosity. The coating was dried using forced air flow and calcined at 500° C. The coated filter had a manganese loading of 250 g/ft.sup.3.

COMPARATIVE EXAMPLE 4

Mn(NO.SUB.3.).SUB.2

[0078] A slurry was prepared using alumina and ceria-zirconia mixed oxide milled to a d.sub.90 particle size of <15 micron. Appropriate amounts of soluble Pt and Pd salts were added to give a final coated catalyst loading of 10 g/ft.sup.3 with a Pt:Pd weight ratio of 2:1. Manganese nitrate was added to the slurry and the mixture stirred to homogenise. The coating slurry was applied to the entire volume of a 2.5 litre volume SiC wall-flow filter substrate having 300 cells per square inch, a wall thickness of 10 Mil (thousands of an inch) and 42% porosity.

[0079] The coating was dried using forced air flow and calcined at 500° C. The coated filter had a manganese loading of 250 g/ft.sup.3.

H.SUB.2.S Simulated Catalyst Activity Testing (SCAT) Procedure

[0080] The H.sub.2S controlling performance of the coated filters was determined using a laboratory reactor and a simulated exhaust gas. Lean and rich exhaust gas mixtures were used to represent those produced during the desulphation of a lean NO.sub.x trap. All samples were previously aged under hydrothermal conditions of 800° C. for 16 hours. Core samples were taken and tested on the lab reactor. The reactor was heated to the first evaluation temperature and a lean gas mix was passed through the sample for 20 seconds. The gas mix was then switched to a rich gas mix for 20 seconds. This cycle of alternating lean and rich gas mixes was repeated during the test. Gas mix concentrations are given in Table 1, with the balance being nitrogen.

TABLE-US-00001 TABLE 1 Lean gas mix Rich gas mix CO.sub.2  14%   14% HC 120 ppm (C.sub.1) 2000 ppm (C.sub.1) O.sub.2 1.7% 0 H.sub.2O   5%   5% H.sub.2 0 0.07% CO 0 0.24% H.sub.2S 0 500 ppm

[0081] The concentration of H.sub.2S downstream of the filter sample was continuously measured and the peak concentration of H.sub.2S determined. This value is termed H.sub.2S slip. The average H.sub.2S slip measured over 5 cycles of lean/rich operation is shown in Table 2 for each of the catalysts as a function of inlet temperature.

HC/CO Oxidation SCAT Procedure

[0082] The catalysed substrate monoliths of Example 1, Example 2, Example 3 and Example 4 were tested for CO and HC oxidation performance. The aged cores were tested in a simulated catalyst activity testing (SCAT) gas apparatus using the inlet gas mixtures shown in Table 3, the balance is nitrogen. The results for the tests for each Example are shown in FIGS. 3 and 4.

TABLE-US-00002 TABLE 2 Inlet Average Peak H.sub.2S slip (ppm) Temperature Example Example 2 Example 3 Example 4 (° C.) 1 (ZnO) (MnO.sub.2) (Mn(NO.sub.3).sub.2) 300 446 164 239 189 400 430 162 105 177 500 402 116 102 165 600 362 54 3 150 650 417 52 1 138

TABLE-US-00003 TABLE 3 Quantity in Inlet Gas Gas Component Mixture Composition CO 1500 ppm HC (as C.sub.1)  430 ppm NO  100 ppm CO.sub.2  4% H.sub.2O  4% O.sub.2 14% Space velocity 55000/hour

[0083] The light off temperatures for Example 1, Example 2, Example 3 and Example 4 for HC and CO oxidation are shown in Table 4, below.

TABLE-US-00004 TABLE 4 HC T50 CO T50 Example 1 211° C. 206° C. Example 2 (ZnO) 224° C. 220° C. Example 3 (MnO.sub.2) 281° C. 246° C. Example 4 (Mn(NO.sub.3).sub.2) 321° C. 289° C.