AMMONIA SLIP CATALYST FILTER

Abstract

A catalytic wall-flow filter for the treatment of an exhaust gas is disclosed. The catalytic wall-flow filter comprise at least a first, a second and a third catalytic layers: the first catalytic layer extends from the inlet-end of the substrate and comprises a first SCR composition; the second catalytic layer is provided in or on the walls of the inlet channels, extending from the inlet-end of the substrate, and comprises a PGM-containing composition; the third catalytic layer is provided in or on the walls of the outlet channels, extending from the outlet-end of the substrate, and comprises a second SCR composition.

Claims

1. A catalyst article for the treatment of an exhaust gas, the catalyst article comprising: a wall-flow filter substrate having inlet channels open at an inlet-end of the substrate and closed at an outlet-end of the substrate, the inlet channels adjacent outlet channels which are closed at the inlet-end of the substrate and open at the outlet-end of the substrate, the wall-flow filter comprising at least first, second and third catalytic layers, wherein: (i) the first catalytic layer extends from the inlet-end of the substrate and comprises a first SCR composition; (ii) the second catalytic layer is provided in or on the walls of the inlet channels, extending from the inlet-end of the substrate, and comprises a PGM-containing composition, wherein: when the second catalytic layer is provided on the walls of the inlet channel, the first catalytic layer is provided on the second catalytic layer, and when the second catalytic layer is provided in the walls of the inlet channels, the first catalytic layer is provided on the walls of the inlet channels; and (iii) the third catalytic layer is provided in or on the walls of the outlet channels, extending from the outlet-end of the substrate, and comprises a second SCR composition; wherein a ratio of the PGMs in the second catalytic layer in g per ft.sup.3 to the total amount of the first and second SCR compositions in the first and third catalytic layers in g per in.sup.3 is from 1.5:8 to 5:8.

2. The catalyst article according to claim 1, wherein the second catalytic layer is provided in the walls of the inlet channels and/or the third catalytic layer is provided on the walls of the outlet channels.

3. The catalyst article according to claim 1, wherein: (i) the first catalytic layer extends up to 100% of a longitudinal length of the substrate extending from the inlet-end to an outlet-end; and/or (ii) the second catalytic layer extends up to 100% of a longitudinal length of the substrate extending from the inlet-end to an outlet-end; and/or (iii) the third catalytic layer extends 20 to 90% of a longitudinal length of the substrate extending from the outlet-end to an inlet-end.

4. The catalyst article according to claim 3, wherein all three of the first, second and third catalytic layers extend 70 to 90% of the longitudinal length.

5. The catalyst article according to claim 1, wherein the second and third catalytic layers do not overlap and together extend 100% of a longitudinal length of the substrate extending from an outlet-end to an inlet end, preferably wherein the second catalytic layer is provided in the walls of the inlet channels and wherein the third catalytic layer is provided in the walls of the outlet channels.

6. The catalyst article according to claim 1, wherein the ratio of the PGMs in the second catalytic layer in g per ft.sup.3 to the total amount of the first and second SCR compositions in the first and third catalytic layers in g per in.sup.3 is about 5:16.

7. The catalyst article according to claim 1, wherein the PGM-containing composition contains Pt.

8. The catalyst article according to claim 1, wherein the PGM-containing composition comprises alumina as a support for the PGMs.

9. The catalyst article according to claim 1, wherein a catalytically active SCR components of the first and/or second SCR compositions comprises one or more metal-exchanged zeolites.

10. The catalyst article according to claim 9, wherein the metal-exchanged zeolite is a small pore zeolite having a CHA framework structure.

11. The catalyst article according to claim 1, wherein a weight ratio of the first and second SCR compositions in the first and third catalytic layers is from 2:3 to 3:2.

12. The catalyst article according to claim 1, further comprising means for electrically heating the catalyst article.

13. An exhaust-gas treatment system comprising the catalyst article according to claim 1.

14. The exhaust-gas treatment system according to claim 13, comprising in order, a catalysed soot filter (CSF), means for injecting a nitrogenous reductant, an SCR catalyst article and the catalyst article according to claim 1, preferably wherein the system further comprises a Diesel Oxidation Catalyst (DOC) upstream of the CSF.

15. A fuel combustion and exhaust treatment system comprising an engine and the exhaust-gas treatment system according to claim 13.

16. A vehicle comprising the fuel combustion and exhaust treatment system according to claim 15, wherein at least the catalyst article according to claim 1 is located in an underfloor location and/or encounters, in normal use, exhaust gases at a temperature of from 270 to 350? C.

17. A method for the treatment of an exhaust gas, the method comprising passing an exhaust gas through the catalyst article according to claim 1.

Description

[0077] The invention will now be described further in the following figures. In which:

[0078] FIG. 1 shows a diagrammatic cross-section of a portion of a catalyst article described herein.

[0079] FIG. 2 shows a diagrammatic cross-section of a portion of a catalyst article described herein.

[0080] FIG. 3 shows a configuration of components of an exhaust-gas treatment system downstream of an engine.

[0081] FIG. 4 shows charts of NH.sub.3 conversion, N.sub.2O and NOx production for a comparative flow-through ASC against ASCF with different ratios of PGM to SCR loading (NH.sub.3-only500 ppm SCAT SV=90 k).

[0082] FIG. 5 shows charts of NH.sub.3 conversion, N.sub.2O and NOx production for a comparative flow-through ASC against ASCF with different ratios of PGM to SCR loading (NOx-biased: 500 ppm NH.sub.3 and 500 ppm NO, SCAT SV=90 k).

[0083] FIGS. 6A, 6B and 6C show engine testing data as discussed below.

[0084] FIGS. 7A-C show three preferred embodiments of the present invention.

[0085] FIGS. 8-10 show testing data when comparing the embodiments of FIGS. 7A-C against a standard.

[0086] FIG. 1 shows a diagrammatic portion of a catalyst article 1 as described herein. In particular, the portion shown focuses on a single inlet channel 5 and a corresponding single outlet channel 10. The inlet channel 5 is separated from the outlet channel 10 by a porous wall 15. The inlet channel 5 is open at an inlet face 20 of the catalyst article 1 and blocked with a plug 25 at an outlet face 30 of the catalyst article 1. The outlet channel 10 is open at the outlet face 30 of the catalyst article 1 and blocked with a plug 25 at an inlet face 20 of the catalyst article 1.

[0087] There is provided a first catalytic layer 35 on a first wall surface 40 on the inlet channel 5. The first catalytic layer 35 is provided as a washcoat and comprises an SCR composition such as a Cu-CHA washcoat composition, together with conventional binders and processing aids. The first catalytic layer 35 extends from the inlet face 20 at least 70% of a total length 45 of the catalyst article 1.

[0088] There is provided a second catalytic layer 50 in the porous wall 15. The second catalytic layer 50 is provided as a washcoat and comprises a PGM-containing composition such Pt supported on alumina, together with conventional binders and processing aids. The second catalytic layer 50 extends from the inlet face 20 at least 70% of a total length 45 of the catalyst article 1. The second catalytic layer 50 is preferably a little shorter than the first catalytic layer 35.

[0089] There is provided a third catalytic layer 55 on a second wall surface 60 on the outlet channel 10. The third catalytic layer 55 is provided as a washcoat and comprises an SCR composition such as a Cu-CHA washcoat composition, together with conventional binders and processing aids. The third catalytic layer 55 extends from the outlet face 30 at least 70% of a total length 45 of the catalyst article 1.

[0090] In use, exhaust gases entering the catalyst article 1 through the inlet face 20 pass into the inlet channel 5, through the porous wall 15, into the outlet channel 10 and out of the article 1. This route is shown by the large arrows. The gases pass through the first, second and third catalytic layers (35, 50, 55) in order. Ammonia is stored on the first and third catalytic layers (35, 55) until it can be used to perform an SCR reaction to decompose NO.sub.x produced on the second catalytic layer 50. Particulate matter from the ammonia/urea accumulates in the inlet channel 5 rather than being released to the atmosphere. The particulate matter can be combusted, if necessary, to reduce undue build-up and this can be assisted, if necessary, but localised resistive heating using an electrical element (not shown).

[0091] FIG. 2 shows an alternative configuration of the first, second and third catalytic layers (35, 50, 55) in the catalyst article 1. All reference numerals refer to the same components and all of the compositional details remain the same.

[0092] The second catalytic layer 50 is provided on the wall 40 of the inlet channel 5. The first catalytic layer 35 is provided on the second catalytic layer 50 and, since it is a little longer, provided overlapping a small portion of the wall 40 of the inlet channel 5. The third catalytic layer 55 is provided in the porous wall 10, although it could instead be provided on the surface 60 of the outlet channel 10.

[0093] FIG. 3 shows an exhaust gas treatment system comprising, downstream of an engine 65, a diesel oxidation catalyst (DOC) 70, then a catalysed soot filer (CSF) 75, then a means for the injection of a nitrogenous reductant 80, then an SCR 85 and finally a catalyst article 1 as described herein, i.e., an ASCF. The CSF 75 can be a diesel particulate filter (DPF). It is desirable that there is a CSF 75 or DPF upstream of the catalyst article 1, since it is difficult to regenerate such a component and the build-up of soot on the catalyst article 1 (prevented by the CSF 75 or DPF) would lead to unacceptable back-pressures. The difficulty in regenerating the catalyst article 1 were soot to build up is exacerbated by the end-of-system location (such as underfloor) where the lower temperatures do not suffice.

Examples

[0094] The invention will now be described further in relation to the following non-limiting examples.

[0095] Exemplary catalyst articles were produced and tested as explained below.

Engine Testing

[0096] The inventive examples used a wall-flow filter substrate of 10.5?6.0. A reference filter was used which was 10.5?4.0 but coated with the same total mass (g) of washcoat as on the coated filter (varied on the filter as discussed below).

SCR In-Wall CoatingCoating 1:

[0097] A slurry of spray-dried Cu(3.3 wt %) Chabazite (SAR 20) was milled to target particle size distribution characterised by D90=3.8-3.9 ?m and stirred overnight. The milled slurry was adjusted to target pH 9.9-10.2 by addition of tetraethylammonium hydroxide aqueous solution and stirred for 5 minutes. Finally, a low particle size mixed oxide containing Al.sub.2O.sub.3 (91.2 wt %)/La.sub.2O.sub.3 (4.8 wt %)/Nd.sub.2O.sub.3 (4.1 wt %), with particle size distribution characterised by D90=3.0-5.0 ?m, was added under high-speed stirring to target 11 wt % with respect to CuChabazite calcined weight. The washcoat with the composition above was stirred overnight, then re-adjusted to pH 9.9-10.2 by addition of tetraethylammonium hydroxide aqueous solution. The pH-adjusted washcoat was stirred for 30 minutes and coated from the rear end of a suitable DPF, to a target washcoat loading of 0.8 g/in.sup.3 with reference to the volume of the ASCF brick.

[0098] The block was dried at 110? C. for 30 minutes and calcined at 500? C. for 2 hours.

PGM In-Wall CoatingCoating 2:

[0099] Succinic acid was dissolved in demineralised water to target 40 g/ft.sup.3 and the solution was stirred for 5 minutes. To this solution, platinum(IV)nitrate was added slowly to target 0.5 g/ft.sup.3 of platinum, and the mixture was stirred for 5 minutes. Finally, low particle size alumina P0 (previously milled to target particle size distribution characterised by D90=4.8-5.0 ?m) was added as a slurry, to target 0.06 g/in.sup.3. The washcoat with the composition above was stirred for 3 h and coated from the front end of the mid-processed DPF containing the calcined SCR Coating 1 (described above), then dried at 115? C. for 30 minutes and calcined at 500? C. for 2 hours.

[0100] All the targets above refer to final loadings with reference to the volume of the ASCF brick.

SCR Porous On-Wall CoatingCoating 3:

[0101] A slurry of spray dried Cu(3.3 wt %) Chabazite (SAR 20) was milled to target particle size distribution characterised by D90=3.8-3.9 ?m and stirred overnight. The slurry was transferred in a chiller tank, and colloidal aluminium oxide hydroxide (boehmite) was added under high-speed stirring to target 18 wt % with respect to CuChabazite calcined weight. The mixture was stirred for 30 minutes, then a cellulose pore former (Arbocel UFC100) was added under high-speed stirring to target 54 wt % with respect to CuChabazite calcined weight. The mixture was stirred for 30 minutes, then a cellulose thickener was added under high-speed stirring to target 0.2 wt % with respect to full weight of the wet washcoat. The mixture was stirred for 30 minutes at high speed, then the washcoat was stored in a sealed container for 2 days. The washcoat was then stirred for 3 minutes at high speed, and coated from the front end of the mid-processed DPF containing the calcined SCR Coating 1 and the calcined PGM Coating 2 (described above), to a target washcoat loading of 0.8 g/in.sup.3 with reference to the volume of the finished catalyst. The brick was dried at 110? C. for 45 minutes and then calcined at 500? C. for 2 hours.

SCAT Testing

[0102] Testing was performed with SCAT-testing. This type of testing does not include any other catalysts upstream or downstream, but artificially provides the core with a gas composition that is representative of specific operation conditionsnamely 350 ppm CO/500 ppm NH.sub.3/0 ppm NO.sub.x for NH.sub.3-rich conditions, or 350 ppm CO/500 ppm NH.sub.3/500 ppm NO for NO.sub.x-biased conditions.

[0103] All the examples used a wall-flow filter substrate of 1?5.53. The space velocity was the same in all tests.

[0104] Example ASCFs were manufactured having an on-wall SCR washcoat in the inlet channels (0.8 g/in.sup.3); a PGM-containing in-wall composition applied from the inlet side (Pt on alumina with varying loadings); and an in-wall SCR washcoat in the outlet channel (0.8 g/in.sup.3). A comparative flow-through filter was provided with an overlying SCR layer (2.4 g/in.sup.3) and an underlying PGM-containing layer with 3 g/ft.sup.3 of Pt. The PGM coating was applied to the rear 50% of the brick.

[0105] The PGM levels were 1 g/ft.sup.3, 0.75 g/ft.sup.3, 0.5 g/ft.sup.3 and 0.25 g/ft.sup.3. This gives ratios with the SCR material (1.6 g/in.sup.3) as discussed herein of 10:16, 7.5:16, 5:16 and 2.5:16. The SCR material here is the copper exchanged zeolite and binder of the washcoat layers. These compare with a standard ASC which had in the examples a ratio of 30:24. This is representative of a conventional configuration.

[0106] In further SCAT testing the results show that NH.sub.3 conversion was comparable for Pt 1 g/ft.sup.3, 0.75 g/ft.sup.3, 0.5 g/ft.sup.3, but much worse for 0.25 g/ft.sup.3. N.sub.2O production was substantially the same for Pt 0.5 g/ft.sup.3 as on the standard flow-through, but much worse for those examples with higher Pt loading.

[0107] The ASCF catalysts showed improved filtration efficiency in preliminary measurements when compared with a std-ASC on FT. In particular, PN10 emissions were 70-90% lower when using an ASCF catalyst in place of a std-ASC on FT.

[0108] The values around Pt 0.5 g/ft.sup.3 (paired with SCR of 1.6 g/in.sup.3) therefore provides a sweet-spot where the oxidation performance matches the SCR performancethe NH.sub.3 conversion is sufficient without undue N.sub.2O make and at the same time the filtration performance addresses the reductant-derived particulate matter. This is observed ideally at and around a ratio of 5:16.

[0109] In FIG. 4, for NH.sub.3 conversion, the lines at 300? C., in order from top to bottom: the Std, 1, 0.75, 0.5 and 0.25 g/ft.sup.3. For N.sub.2O production, the lines peak in order from top to bottom: 1, the STD, 0.75, 0.5, and 0.25 g/ft.sup.3. For NOx production, the lines at 400? C. in order from top to bottom: 1 g, 0.75 g, 0.5 g, 0.25 g, and STD.

[0110] As shown in these examples, a three-layer ASCF has been developed with the aim of translating the ASC-concept from flow through to filter substrates. The three-layer ASCF approach broadens the variety of feasible coating designs, when compared with a standard two-layer ASC, while also reducing the emission of particulate matter significantly. In particular, the use of a three-layer ASCF can exploit different combinations of in-wall and on-wall coatings, thus allowing for a fine control of ASC activity, backpressure and filtration efficiency.

[0111] Subsequent testing was performed using engine testing data to show that the same benefits can be achieved under genuine use conditions. In this case, either the ASCFs or the SCR/ASC bricks are downstream to another SCR flow-through brick (VSCR, in this case). Further information is shown in FIG. 6. Here a standard ASCF is coated with the same mass (grammes) of SCR- and PGM-catalysts as the standard flow-through SCR/ASC, while the lower PGM-variant has just half the PGM amount but the same SCR washcoat amount. The steady state (SS) engine testing data was performed at three different temperatures (270, 320, 370 C), where the low-PGM variant is much better for secondary emissions, while still being comparable with the SCR/ASC reference in terms of NH.sub.3-conversionthis gives a better trade-off between NH.sub.3-conversion and secondary emissions for the low-PGM variant.

[0112] In each cluster of results, the left-hand column is a conventional SCR/ASC, the middle column is an ASCF provided with comparable levels of SCR and PGMs to the conventional SCR/ASC and the right-hand column has half the level of PGMs as the middle column (0.5 g/ft.sup.3 of PGMs and 1.6 g/in.sup.3 of SCR total).

[0113] FIG. 6A shows that the inventive ASCF achieves sufficiently good NH.sub.3 slip control. FIG. 6B demonstrates an improvement in reduced N.sub.2O production levels. FIG. 6C demonstrates an improvement in reduced NO.sub.x production levels. The SS numbers in each chart reflect the testing temperature under steady state.

[0114] FIGS. 7A-7C use the same reference numerals as FIGS. 1 and 2 for the same component parts.

[0115] FIG. 7A resembles FIG. 2, but this is an embodiment where the second catalytic layer 50 has a shorter length, extending about 50% of the total length 45 of the catalyst article 1. The first catalytic layer extends about 80% of the total length 45 of the catalyst article 1. Coating the PGM layer as a porous coating on-wall in 7A was an attempt to prevent/minimise close contact with the SCR catalyst particles (which are dispersed within the wall volume and then fundamentally segregated from the PGM on-wall)

[0116] FIGS. 7B and 7C are embodiments where the second catalytic layer 50 does not overlap the third catalytic layer 55. Both of these layers (50,55) are provided as in-wall washcoats and together coat the total length 45 of the catalyst article 1. In FIG. 7B the second catalytic layer 50 is about 25% of the total length 45 and the third catalytic layer 55 is about 75% of the total length 45. In FIG. 7C the second catalytic layer 50 is about 50% of the total length 45 and the third catalytic layer 55 is about 50% of the total length 45.

[0117] As supported in the embodiments 7B and 7C, avoiding overlap between these layers avoids close contact between the platinum group metals and the SCR catalysts.

[0118] FIG. 8 shows a comparison of the embodiments 7A-7C against a comparative in which the second catalytic layer 50 is about 70% of the total length 45 (in-wall) and the third catalytic layer 55 is about 80% of the total length 45 (in-wall). The provision of these overlapping in-wall coating means that there is overlap in the wall of the platinum group metals and the SCR catalyst. The columns from left to right are: comparative, 7C, 7B and 7A respectively. FIG. 8 shows that the lowest back pressure is obtained for the embodiment of FIGS. 7C (62) and 7B (63).

[0119] FIG. 9 shows NH.sub.3 conversion at different temperatures. Performance is consistently highest for the embodiment of FIG. 7C. The embodiment of FIG. 7B has the worst performance. The columns from left to right are: comparative, 7C, 7B and 7A respectively.

[0120] FIG. 10 shows NH.sub.3, N.sub.2O and NOx output at different temperatures and ANR ratios. The columns from left to right at each set of conditions are: comparative, 7C, 7B and 7A respectively.

[0121] The term comprising as used herein can be exchanged for the definitions consisting essentially of or consisting of. The term comprising is intended to mean that the named elements are essential, but other elements may be added and still form a construct within the scope of the claim. The term consisting essentially of limits the scope of a claim to the specified materials or steps and those that do not materially affect the basic and novel characteristic(s) of the claimed invention. The term consisting of closes the claim to the inclusion of materials other than those recited except for impurities ordinarily associated therewith.

[0122] The foregoing detailed description has been provided by way of explanation and illustration and is not intended to limit the scope of the appended claims. Many variations in the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art and remain within the scope of the appended claims and their equivalents.

[0123] For the avoidance of doubt, the entire contents of all documents acknowledged herein are incorporated herein by reference.