Exhaust system

Abstract

An exhaust system for an internal combustion engine, the exhaust system comprising, a lean NO.sub.x trap, a NO.sub.x storage and reduction zone on a wall flow monolithic substrate having a pre-coated porosity of 50% or greater, the NO.sub.x storage and reduction zone comprising a platinum group metal loaded on one or more first support, the or each first support comprising one or more alkaline earth metal compound, and 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.

Claims

1. An exhaust system for an internal combustion engine, the exhaust system comprising, a. a lean NO.sub.x trap, b. a NO.sub.x storage and reduction zone on a wall flow monolithic substrate having a pre-coated porosity of 50% or greater, the NO.sub.x storage and reduction zone comprising a platinum group metal loaded on one or more first support, the or each first support comprising one or more alkaline earth metal compound, and c. 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; wherein the NO.sub.x storage and reduction zone and the selective catalytic reduction zone are each on portions of the same wall flow monolithic substrate; wherein the NO.sub.x storage and reduction 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; wherein the NO.sub.x storage and reduction 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; and wherein an axial length of the NO.sub.x storage and reduction zone and an axial length of the selective catalytic reduction zone overlap by 20% or less of a total axial length of the monolithic substrate.

2. An exhaust system according to claim 1, wherein the NOx storage and reduction zone is on a first wall flow monolithic substrate and the selective catalytic reduction zone is on a second monolithic substrate.

3. An exhaust system according to claim 1, wherein the pre-coated porosity of the wall flow monolithic substrate is 52% or greater.

4. An exhaust system according to claim 1, wherein the pores of the wall flow monolithic substrate have a diameter in the range 12 m to 25 m.

5. An exhaust system according to claim 1, wherein the NO.sub.x storage and reduction zone is on and/or within the walls of the inlet channels of the inlet end of the monolithic substrate and the selective catalytic reduction zone is on and/or within the walls of the outlet channels of the outlet end of the monolithic substrate.

6. An exhaust system according to claim 1, wherein the or each first support comprises a cerium compound.

7. An exhaust system according to claim 1, wherein the or each alkaline earth metal compound comprises an oxide, carbonate and/or hydroxide of magnesium, calcium, strontium or barium or a mixture of any two or more of these compounds.

8. An exhaust system according to claim 1, wherein the molecular sieve is selected from 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, or mixtures of any two or more thereof.

9. An exhaust system according to claim 1, wherein the platinum group metal is platinum, palladium, rhodium, or mixtures of any two or more thereof.

10. An exhaust system according to claim 9, wherein the platinum group metal comprises a mixture of platinum and palladium in a Pt:Pd weight ratio in the range 2:1 to 7:1.

11. An exhaust system according to claim 1 wherein the platinum group metal contains substantially no Rh.

12. An exhaust system according to claim 1, wherein the NO.sub.x storage and reduction zone is upstream of the selective catalytic zone.

13. 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.

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

15. A vehicle comprising a compression ignition engine and an exhaust system according to claim 14.

16. A method of making a catalysed monolithic substrate, the method comprising: a. providing a wall flow monolithic substrate having a pre-coated porosity of 50% or greater, b. preparing a NO.sub.x storage and reduction zone washcoat comprising a source of a platinum group metal and an alkaline earth metal compound, c. applying the NO.sub.x storage and reduction zone washcoat to a first portion of the monolithic substrate, d. preparing a selective catalytic reduction zone washcoat comprising a molecular sieve and a source of copper or a source of iron, and e. applying the selective catalytic reduction zone washcoat to a second portion of the monolithic substrate, wherein the NO.sub.x storage and reduction zone and the selective catalytic reduction zone are each on portions of the same wall flow monolithic substrate; wherein the NO.sub.x storage and reduction 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; wherein the NO.sub.x storage and reduction 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; and wherein an axial length of the NO.sub.x storage and reduction zone and an axial length of the selective catalytic reduction zone overlap by 20% or less of a total axial length of the monolithic substrate.

Description

(1) In order that the present invention may be better understood, reference is made to accompanying drawings, in which:

(2) FIG. 1 illustrates schematically a first exhaust system according to the present invention, and

(3) FIG. 2 illustrates schematically a second exhaust system according to the present invention.

(4) 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 may be other catalytic zones or the exhaust gases may be released to atmosphere.

(5) The second monolithic substrate 6 is a filter, in particular a wall flow monolith substrate having 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 two zones, a NO.sub.x storage and reduction (NSC) zone of a platinum group metal and a first, first support of a Mg-doped alumina coated with ceria (derived from a soluble cerium source e.g. a cerium salt and so comprising nano-size ceria crystals supported on the Mg-doped alumina) and a second, first support of ceria and barium (in the form of particulate ceria supporting a barium compound e.g. impregnated from a soluble barium salt) provided on and/or within the walls of the inlet channels at the inlet end of the second monolithic substrate 6 and a selective catalytic reduction (SCR) zone provided on and/or within the walls of the outlet channels at the outlet end of the second monolithic substrate 6. The exhaust system of FIG. 1 may be formed as described below in the Examples.

(6) 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 may be other catalytic zones or the exhaust gases may be released to atmosphere.

(7) The second monolithic substrate 16 is a filter, wall flow monolithic substrate having a NO.sub.x storage and reduction 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.

(8) The following Examples are provided by way of illustration only.

EXAMPLE 1

(9) A standard lean NO.sub.x trap (LNT) catalyst was prepared on a 1.4 liter volume ceramic substrate having 400 cells per square inch. The catalyst had a total PGM loading of 118 g ft.sup.3 and a Pt:Pd:Rh weight ratio of 94:19:5.

EXAMPLE 2

(10) Preparation of PGM/NOx Storage and Reduction Catalytic Zone (NSC) Coating

(11) Ce/magnesium-aluminate spinel was slurried in water and milled to d90 of less than 10 micron. Soluble salts of Pt and Pd were added followed by cerium oxide and barium acetate. The slurry was stirred to homogenise and applied to the inlet channels of a 3.0 liter volume SiC wall-flow filter substrate having 300 cells per square inch, a wall thickness of 12.5 Mil (thousands of an inch) and 63% porosity. The coating was dried using forced air flow. The coating depth was 55% of the total substrate length when measured from the inlet side.

(12) Preparation of 3 wt % Cu/CHA Zeolite Coating as Passive Selective Catalytic Reduction (SCR) Catalyst

(13) Commercially available aluminosilicate CHA zeolite (a chabazite) was added to an aqueous solution of Cu(NO.sub.3).sub.2 with stirring. The slurry was filtered, then washed and dried. The procedure can be repeated to achieve a desired metal loading. The final product was calcined. After mixing, binders and rheology modifiers were added to form a washcoat composition. This washcoat was applied to the outlet end of the SiC filter substrate. It was then dried and calcined at 500 C. The coating depth was 55% of the total substrate length when measured from the outlet side.

(14) The finished catalyst coating on the filter had a Pt:Pd weight ratio of 5:1 and total PGM loading of 24 g ft.sup.3.

EXAMPLE 3

(15) Ce/magnesium-aluminate spinel was slurried in water and milled to d90 of less than 10 micron. Soluble salts of Pt and Pd were added followed by cerium oxide and barium acetate. The slurry was stirred to homogenise and applied to the entire volume of a 3.0 liter volume SiC wall-flow filter substrate having 300 cells per square inch, a wall thickness of 12.5 Mil (thousands of an inch) and 63% porosity. The coating was dried using forced air flow and calcined at 500 C.

(16) The finished catalyst coating on the filter had a Pt:Pd weight ratio of 5:1 and total PGM loading of 48 g ft.sup.3.

EXAMPLE 4

(17) Commercially available aluminosilicate CHA zeolite (a chabazite) was added to an aqueous solution of Cu(NO.sub.3).sub.2 with stirring. The slurry was filtered, then washed and dried. The procedure can be repeated to achieve a desired metal loading. The final product was calcined. After mixing, binders and rheology modifiers were added to form a washcoat composition. This washcoat was applied to a 1.25 liter volume ceramic flow through substrate having 350 cells per square inch. The coating was dried and calcined at 500 C.

(18) Engine Testing

(19) The catalyst from Example 1 was hydrothermally aged at 800 C. for 5 hours. The catalysts from Example 2 and Example 3 were hydrothermally aged at 800 C. for 16 hours. The catalyst from Example 4 was hydrothermally aged at 750 C. for 24 hours. The aged catalysts of Examples 1, 2, 3 and 4 were tested on a 1.6 liter engine employing low pressure exhaust gas recirculation, running simulated Common ARTEMIS (Assessment and Reliability of Transport Emissions Models and Inventory Systems) Driving Cycles (CADC). The engine was enabled to run rich purges at fixed points over the cycle. A total of 12 rich purges were performed over the complete cycle. The % NO.sub.x conversion over the CADC cycle is shown in Table 1 for Examples 1 and 2. Also included in Table 1 are results for a system of Examples 1, 3 and 4, where the c PGM/NOx storage and reduction coating (NSC) is on Example 3 and the SCR coating is on Example 4 as a separate monolith downstream of the PGM/NOx storage and reduction coating as shown in FIG. 2.

(20) TABLE-US-00001 TABLE 1 NO.sub.x conversion over CADC System % NO.sub.x conversion Example 1 38 Example 1 and Example 2 51 Example 1 + Example 3 + Example 4 76

(21) The results in Table 1 show that Example 2 provides significant additional NOx conversion when tested in combination with Example 1. Further additional NOx conversion is achieved from the system combining Examples 1, 3 and 4 where the SCR coating is located on a separate monolith downstream of the PGM/NO.sub.x storage and reduction coating.