Exhaust System

Abstract

An exhaust system for an internal combustion engine, the exhaust system comprising, a lean NO.sub.x trap (LNT), a wall flow monolithic substrate having a NO.sub.x storage and reduction zone thereon, the wall flow monolithic substrate having a pre-coated porosity of 40% or greater, the NO.sub.x storage and reduction zone comprising a platinum group metal loaded on a first support, the first support comprising one or more alkaline earth metal compounds, a mixed magnesium/aluminium oxide, cerium oxide, and at least one base metal oxide selected the group consisting of copper oxide, manganese oxide, iron oxide and zinc oxide.

Claims

1. An exhaust system for an internal combustion engine, the exhaust system comprising, a) a lean NO.sub.x trap, b) a wall flow monolithic substrate having a NO.sub.x storage and reduction zone thereon, the wall flow monolithic substrate having a pre-coated porosity of 40% or greater, the NO.sub.x storage and reduction zone comprising a platinum group metal loaded on a first support, the first support comprising one or more alkaline earth metal compounds, a mixed magnesium/aluminium oxide, cerium oxide, and at least one base metal oxide selected the group consisting of copper oxide, manganese oxide, iron oxide and zinc oxide.

2. The exhaust system according to claim 1, wherein the base metal oxide comprises zinc oxide.

3. The exhaust system according to claim 1, wherein the first support comprises 1 wt % or less zirconia.

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

5. The exhaust system as claimed in claim 1, wherein the mixed magnesium/aluminium oxide comprises magnesium doped alumina.

6. The exhaust system according to claim 1, wherein the mixed magnesium/aluminium oxide comprises ceria spray-dried on to magnesium doped alumina.

7. The exhaust system according to claim 1, wherein the mixed magnesium/aluminium oxide comprises magnesium in the amount of 0.1 wt % to 12 wt % based on the weight of the mixed magnesium/aluminium oxide.

8. The exhaust system according to claim 1, wherein the mixed magnesium/aluminium oxide comprises a magnesium aluminate spinel.

9. The exhaust system according to claim 1, wherein the first support comprises the alkaline earth metal at a loading in the range of 90 to 200 g/ft.sup.3 based on the weight of the alkaline earth metal.

10. The exhaust system according to claim 1, wherein the first support comprises the base metal oxide at a loading in the range of 100 to 300 g/ft.sup.3, based on the weight of the metal.

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

12. The exhaust system according to claim 11, wherein the platinum group metal comprises a mixture of platinum and palladium in a Pt:Pd weight ratio in the range 2:1 to 8:1.

13. (canceled)

14. The exhaust system according to claim 1, wherein the pre-coated porosity of the wall flow monolithic substrate is 40% or greater.

15. The exhaust system according to claim 1, wherein the NO.sub.x storage and reduction zone is applied to be in a single layer.

16. (canceled)

17. The exhaust system according to clam 1, wherein the wall flow monolithic substrate comprises pores having a diameter, the pores of the wall flow monolithic substrate have a pre-coated mean pore diameter in the range 9 μm to 25 μm.

18. 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 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/or is on and/or within the walls of the outlet channels of the outlet end of the monolithic substrate.

19. A catalytic wall flow monolithic substrate, the wall flow monolithic substrate having a NO.sub.x storage and reduction zone thereon, the wall flow monolithic substrate having a pre-coated porosity of 40% or greater , the NO.sub.x storage and reduction zone comprising a platinum group metal loaded on a first support, the first support comprising an alkaline earth metal compound a mixed magnesium/aluminium oxide, cerium oxide, and a base metal oxide selected from copper oxide, manganese oxide, iron oxide or zinc oxide.

20. 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 preparing a NO.sub.x storage and reduction zone washcoat comprising a source of a platinum group metal, a source of an alkaline earth metal compound and a mixed magnesium/aluminium oxide, cerium oxide, and at least one base metal oxide selected from the group consisting of copper oxide, manganese oxide, iron oxide and zinc oxide, and b) applying the NO.sub.x storage and reduction zone washcoat to at least a first portion of the monolithic substrate.

21. 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.

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

23. A vehicle comprising a compression ignition engine according to claim 22.

Description

EXAMPLE 1

[0068] Ce/magnesium-aluminate spinel was slurried in water and milled to a d.sub.90 of less than 10 micron. Water soluble salts of Pt and Pd were added followed by cerium oxide and barium acetate. The mixture was stirred to homogenise and form a coating slurry. The coating slurry was applied to a 3.0 litre 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.

[0069] 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 2

Zinc

[0070] Ce/magnesium-aluminate spinel was slurried in water and milled to d.sub.90 of less than 10 micron. Soluble salts of Pt and Pd were added followed by cerium oxide and barium acetate. Zn oxide was added to the slurry and the mixture stirred to homogenise. The coating slurry was applied to a 3.0 litre 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.

[0071] The finished catalyst coating on the filter had a zinc loading of 250 g ft.sup.−3, a Pt:Pd weight ratio of 5:1 and total PGM loading of 48 g ft.sup.−3.

EXAMPLE 3

Manganese

[0072] Ce/magnesium-aluminate spinel was slurried in water and milled to d.sub.90 of less than 10 micron. Soluble salts of Pt and Pd were added followed by cerium oxide and barium acetate. Mn dioxide was added to the slurry and the mixture stirred to homogenise. The coating slurry was applied to a 3.0 litre 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.

[0073] The finished catalyst coating on the filter had a manganese loading of 250 g ft.sup.−3, a Pt:Pd weight ratio of 5:1 and total PGM loading of 48 g ft.sup.−3.

EXAMPLE 4

Iron

[0074] Ce/magnesium-aluminate spinel was slurried in water and milled to d.sub.90 of less than 10 micron. Soluble salts of Pt and Pd were added followed by cerium oxide and barium acetate. Ferrous hydroxide was added to the slurry and the mixture stirred to homogenise. The coating slurry was applied to a 3.0 litre 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.

[0075] The finished catalyst coating on the filter had an iron loading of 250 g ft.sup.−3, a Pt:Pd weight ratio of 5:1 and total PGM loading of 48 g ft.sup.−3.

EXAMPLE 5

Controlling H.SUB.2.S Performance

[0076] The H.sub.2S controlling performance of the coated filters was determined using a laboratory synthetic gas bench test. Core samples were taken from catalyst of each of the Examples. The cores were hydrothermally aged at 800° C. for 16 hours. Lean and rich simulated exhaust gas mixtures were used to represent those produced during the desulphation of a lean NO.sub.x trap. 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. The temperature was then increased to the next evaluation point and the lean/rich sequence repeated. Gas mix concentrations are given in Table 1, with the balance being nitrogen in both cases.

TABLE-US-00001 TABLE 1 Lean gas mix Rich gas mix CO.sub.2 14% .sup. 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  

[0077] The concentration of H.sub.2S downstream of the filter sample was continuously measured and the peak concentration of H.sub.2S was determined at temperatures of 600 and 650° C. This peak value at each temperature is termed the H.sub.2S slip at that temperature. FIG. 2 shows that Example 1 exhibits more H.sub.2S slip than Examples 2, 3 and 4 at temperatures between 600° C. and 650° C.

EXAMPLE 6

Controlling NOx Storage Performance

[0078] The NOx storage performance of the coated filters was determined using a laboratory synthetic gas bench test. Core samples were taken from catalyst examples 1, 2, 3 and 4.

[0079] The cores were hydrothermally aged at 800° C. for 16 hours. The reactor was heated to the first evaluation temperature and a lean gas mix was passed through the sample for 300 seconds. The gas mix was then switched to a rich gas mix for 16 seconds. This cycle of alternating lean and rich gas mixes was repeated a further 9 times during the test. The temperature was then increased to the next evaluation point and the lean/rich sequence repeated. Gas mix concentrations are given in Table 2, with the balance being nitrogen in both cases.

TABLE-US-00002 TABLE 2 Lean gas mix Rich gas mix CO.sub.2   6% 10.3% C.sub.3H.sub.6  45 ppm 1700 ppm O.sub.2 10.5% 1.45% H.sub.2O  6.6% .sup. 12% H.sub.2   0%  0.4% CO 0.03%   2% NO 100 ppm  200 ppm Space Velocity 62,000 h.sup.−1    52,000 h.sup.−1  

[0080] The amount of NOx stored was calculated as the mean NOx stored as NO.sub.2 in grams per litre of catalyst volume (g/L) over the 10 lean/rich cycles at each temperature evaluation point. The results are shown in FIG. 3.

[0081] FIG. 3 shows that Example 2 which comprises Zn has greater NOx storage than Examples 3 and 4, which comprise Mn and Fe respectively. The greater NOx storage from Example 2 is higher at higher temperatures (above about 300° C.).