Oxidation catalyst

Abstract

A catalysed soot filter comprises an oxidation catalyst for oxidizing NO to NO.sub.2 and/or oxidizing CO to CO.sub.2 and/or HC to CO.sub.2 and H.sub.2O disposed on a wall flow filter monolithic substrate, the oxidation catalyst comprising: a platinum group metal component, and a pre-calcined support material comprising a mixed magnesium aluminium metal oxide having a magnesium content, calculated as Mg, of 15 wt % Mg or lower.

Claims

1. A catalysed soot filter comprising an oxidation catalyst for oxidizing NO to NO.sub.2 and/or oxidizing CO to CO.sub.2 and/or HC to CO.sub.2 and H.sub.2O disposed on a wall flow filter monolithic substrate, the oxidation catalyst comprising: a platinum group metal component, and a pre-calcined support material comprising a mixed magnesium aluminum metal oxide having a magnesium content, calculated as Mg, of 15 wt % Mg or lower, wherein the pre-calcined support material comprising a mixed magnesium aluminum metal oxide comprises a magnesium deficient spinel.

2. A catalysed soot filter according to claim 1, wherein the pre-calcined support material has a specific surface area of 250 m.sup.2g.sup.?1 or lower.

3. A catalysed soot filter according to claim 1, wherein the mixed magnesium aluminum metal oxide has a magnesium content, calculated as Mg, of 0.1 wt % to 12 wt % Mg.

4. A catalysed soot filter according to claim 1, wherein the porosity of the wall flow filter monolithic substrate is 40% or greater.

5. A catalysed soot filter according to claim 1, wherein the pores of the wall flow filter monolithic substrate have a mean diameter in the range 10 ?m to 25 ?m.

6. A catalysed soot filter according to claim 1, wherein the wall flow filter monolithic substrate comprises inlet channels at the inlet end thereof and outlet channels at the outlet end thereof and wherein the oxidation catalyst is disposed on or in at least the walls of the inlet channels.

7. A catalysed soot filter according to claim 1, wherein the oxidation catalyst is disposed on the wall flow filter monolithic substrate so that it extends over between 10% and 90% of the axial length of the filtering monolithic substrate.

8. A catalysed soot filter according to claim 1, wherein the platinum group metal component comprises platinum, palladium, rhodium, or mixtures of any two or more thereof.

9. A catalysed soot filter according to claim 1, wherein the platinum group metal component comprises a mixture of platinum and palladium in a Pt:Pd weight ratio in the range 20:1 to 2:1.

10. A catalysed soot filter according to claim 1, wherein the total platinum group metal loading in the oxidation catalyst is in the range 5 to 50 gft.sup.?3.

11. A catalysed soot filter according to claim 1, wherein the washcoat loading of the oxidation catalyst is in the range 0.1 to 2.0 gin.sup.?3.

12. An exhaust system for an internal combustion engine, the exhaust system comprising a catalysed soot filter according to claim 1.

13. An exhaust system according to claim 12, wherein the oxidation catalyst is in a zone disposed on the wall flow filter monolithic substrate, the wall flow filter monolithic substrate comprising an active or a passive selective catalytic reduction catalyst in a zone downstream of the oxidation zone.

14. An exhaust system according to claim 12, comprising an active or a passive selective catalytic reduction catalyst on a separate substrate monolith downstream of the catalysed soot filter.

15. An exhaust system according to claim 12, wherein the selective catalytic reduction catalyst comprises a molecular sieve and a copper or an iron promoter.

16. An exhaust system according to claim 12 comprising a separate monolithic substrate comprising a diesel oxidation catalyst upstream of the catalysed soot filter.

17. An exhaust system according to claim 16, wherein the diesel oxidation catalyst is disposed on a flow through honeycomb monolithic substrate.

18. An exhaust system according to claim 12 wherein the oxidation catalyst is in a zone disposed on the wall flow filter monolithic substrate, which wall flow filter monolithic substrate comprising a lean NO.sub.x trap catalyst in a zone upstream or downstream of the oxidation zone.

19. A vehicle comprising an internal combustion engine and an exhaust system according to claim 12.

20. A method of treating exhaust gases from an internal combustion engine comprising flowing the exhaust gases through a catalysed soot filter according to claim 1.

21. A method of making a catalytic monolith, the method comprising providing a calcined support material component comprising a mixed magnesium aluminum metal oxide having a magnesium content, calculated as Mg, of 15 wt % Mg or lower, preparing a washcoat comprising a platinum group metal component and the calcined support material component, and applying the washcoat to a wall flow monolithic substrate.

Description

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

(2) FIG. 1 illustrates schematically an exhaust system for a compression ignition (diesel) engine, and

(3) FIG. 2 illustrates schematically an exhaust system for a compression ignition (diesel) engine.

(4) FIG. 3 is a powder XRD pattern of a magnesium-alumina sample containing 3 wt % Mg in its fresh condition as discussed in Example 2.

(5) FIG. 4 is a powder XRD pattern of a magnesium-alumina sample containing 3 wt % Mg after calcination/ageing in an oven at 810? C. for 2 hours as discussed in Example 3.

(6) FIG. 1 shows schematically an exhaust system 2 of the present invention. The exhaust system 2 comprises a first monolithic flow through honeycomb substrate 4 coated with a diesel oxidation catalyst (DOC). The exhaust gases from the engine (not shown) upstream of the first monolithic substrate/DOC 4 enter the first monolithic honeycomb substrate 4 through inlet 10 and exit the first monolithic substrate 4 through first conduit 8. The exhaust gases then enter a second monolithic substrate 6 before exiting through second conduit 12, entering a third monolithic substrate 14 before exiting through outlet 20. Downstream of outlet 20 there may be other catalytic zones or the exhaust gases may be released to atmosphere.

(7) The second monolithic substrate 6 is a wall flow monolith filter 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 an inlet channel 22 from the inlet, then flow through the porous channel walls, and exit the filter from an outlet channel 26 leading to the outlet. The second monolithic substrate 6 contains an oxidation catalyst 24 comprising a catalytic composition as indicated in Example 1, below, according to the invention provided on the walls of the inlet channels 22 of the second monolithic substrate 6. Thus, the second monolithic substrate 6 is a catalysed soot filter (CSF)

(8) The third monolithic substrate 14 is a flow through honeycomb substrate with a selective catalytic reduction (SCR) catalyst provided on the walls of the channels of the substrate.

(9) FIG. 2 shows schematically an exhaust system 32 of the present invention. The exhaust system 32 comprises a first, flow through, monolithic honeycomb substrate 34 coated with a diesel oxidation catalyst (DOC). As in FIG. 1, the exhaust gases from the engine (not shown) upstream of the first monolithic substrate/DOC 34 enter the first monolithic substrate 34 through inlet 40 and exit the first monolithic substrate 34 through first conduit 48. The exhaust gases then enter a second monolithic substrate 36 before exiting through second conduit 42, which is attached to a nitrogenous reductant injection system 60, and then to a third monolithic substrate 44 and then through outlet 50. Downstream of outlet 50 there may be other catalytic zones or the exhaust gases may be released to atmosphere.

(10) Generally as in FIG. 1, the second monolithic substrate 36 is a filter, 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 an inlet channel 52 from the inlet, then flow through the porous channel walls, and exit the filter from a different outlet channel 56 leading to the outlet. The second monolithic substrate 36 contains an oxidation catalyst 54 comprising a catalytic composition as indicated in the Example, below, according to the invention provided on the walls of the inlet channels 52 of the second monolithic substrate 36. Thus, the second monolithic substrate 36 is a catalysed soot filter (CSF).

(11) Leading into the second conduit 42 is a connection through valve 62 to an injector system 60 for injecting a nitrogenous reductant (e.g. urea or ammonia) into the exhaust gas stream. The third monolithic substrate 44 is a flow through honeycomb substrate with a SCR catalyst (for example Fe/beta zeolite) provided on the walls of the channels of the substrate.

(12) The invention is further illustrated by the following Examples, which are provided by way of illustration only.

COMPARATIVE EXAMPLE

(13) Silica-alumina powder was slurried in water and milled to a d.sub.90<8 ?m. Soluble platinum and palladium salts were added and the slurry stirred to homogenise. The resulting washcoat was applied to a 3.0 liter silicon carbide wall-flow filter substrate with 42% porosity having 300 cells per square inch and wall thickness of 12 thousands of an inch using established coating techniques. The filter was then dried and calcined at 500? C. The resulting catalysed soot filter had a total PGM loading of 20 g ft.sup.?3 and a Pt:Pd weight ratio of 10:1. The coating loading was 0.3 g in.sup.?3.

EXAMPLE 1

(14) Magnesium-alumina powder comprising 3 wt % magnesium was calcined at 850? C. for 3 hours then slurried in water and milled to a d.sub.90<8 micron. Soluble platinum and palladium salts were added and the slurry stirred to homogenise. The resulting washcoat was applied to a 3.0 liter silicon carbide wall-flow filter substrate with 42% porosity having 300 cells per square inch and wall thickness of 12 thousands of an inch using established coating techniques. The filter was then dried and calcined at 500? C. The resulting catalysed soot filter had a total PGM loading of 20 g ft.sup.?3 and a Pt:Pd weight ratio of 10:1. The coating loading was 0.3 g in.sup.?3.

(15) Characterisation of Support Material

(16) Nitrogen physisorption (at ?196? C.) was used to determine the specific surface area and pore characteristics of samples of the support material in the Examples.

(17) Table 2, below, indicates the surface area (N.sub.2), total pore volume and average pore diameter as determined for a fresh support material (uncalcined) of magnesium-alumina powder comprising 3 wt % magnesium (M3), and the same material after calcination at 850? C. for 4 hours.

(18) X-ray diffraction (XRD) analysis refers to a technique for identifying crystalline materials. XRD patterns were measured on a PANalytical EMPYREAN powder diffractometer using Cu K-alpha radiation in the 2-theta range of about 10?-90?. The resulting diffraction patterns are analysed by comparison to known references in the International Centre for Diffraction Data pdf4+ database.

EXAMPLE 2

(19) A powder XRD sample was prepared using a magnesium-alumina sample containing 3 wt % Mg in its fresh condition. The XRD pattern of this sample is shown in FIG. 3.

EXAMPLE 3

(20) A powder XRD sample was prepared using a magnesium-alumina sample containing 3 wt % Mg after calcination/ageing in an oven at 810? C. for 2 hours. The XRD pattern is shown in FIG. 4.

(21) In FIG. 3 the XRD pattern for Example 2 shows peaks assigned to boehmite (AlO(OH)). No peaks are assigned to a Mg spinel material. In FIG. 4, the XRD pattern for Example 3 shows peaks assigned to a Mg deficient spinel group Mg.sub.0.4Al.sub.2.4O.sub.4. The material as in Example 3 is thus magnesium deficient.

(22) Oxidation Performance

(23) The catalyst Comparative Example and Example 1 were hydrothermally aged (with water) in an oven at 800? C. for 16 hours. They were fitted to a 2.0 liter turbo charged diesel bench mounted engine. The catalytic activity was tested by stepwise increasing the load on the engine to increase the exhaust gas temperature. Concentrations of the exhaust gas pollutants were measured both pre- and post-catalyst. The oxidation activity for CO and HC is determined by the light off temperature whereby 50% conversion is achieved (T50). The NO oxidation activity is determined as the percentage conversion at 270? C. Activity results for the Comparative Example and Example 1 is reported in table 1.

(24) TABLE-US-00001 TABLE 1 NO oxidation T50 CO T50 HC performance at 270? C. light off light off (NO.sub.2/NO.sub.x %) (? C.) (? C.) Comparative 23 229 244 Example Example 1 33 224 237

(25) Results in table 1 show that the NO oxidation activity of Example 1 is greater than that of the Comparative Example. Example 1 comprises the magnesium-alumina support material. The magnesium deficient spinel structure used in Example 1 shows improved NO oxidation performance. Example 1 also has a lower T50 light off temperature for CO and HC than the Comparative Example. Example 1 comprises the magnesium deficient spinel and has improved CO and HC activity.

(26) TABLE-US-00002 TABLE 2 Surface Area Total Pore Average Pore (m.sup.2/g) Volume Diameter Material (Nitrogen adsorption) (cm.sup.3/g) (nm) M3 (fresh) 408 0.549 5.4 M3 (calcined) 207 0.553 10.7