Gasoline particulate filter

Abstract

A catalytic wall-flow monolith filter having three-way catalytic activity for use in an emission treatment system of a positive ignition internal combustion engine comprising a porous filter substrate having a first face and a second face defining a longitudinal direction there between and first and second pluralities of channels extending in the longitudinal direction, wherein the first plurality of channels is open at the first face and closed at the second face and the channels of the first plurality of channels are defined in part by channel wall surfaces, wherein the second plurality of channels is open at the second face and closed at the first face and the channels of the second plurality of channels are defined in part by channel wall surfaces and wherein channel walls between the channel wall surfaces of the first plurality of channels and the channel wall surfaces of the second plurality of channels are porous, wherein a first on-wall coating comprising catalytic material having a layer thickness is present on at least the channel wall surfaces of the first plurality of channels, wherein the catalytic material on channel wall surfaces of the first plurality of channels comprises one or more platinum group metal selected from the group consisting of (i) rhodium (Rh) only; (ii) palladium (Pd) only; (iii) platinum (Pt) and rhodium (Rh); (iv) palladium (Pd) and rhodium (Rh); and (v) platinum (Pt), palladium (Pd) and rhodium (Rh) and a refractory metal oxide support, wherein: (i) an amount by weight of the one or more platinum group metal, per unit volume of the on-wall coating present on channel wall surfaces of the first plurality of channels varies continually along the longitudinal direction; and/or (ii) the layer thickness of the on-wall coating present on channel wall surfaces of the first plurality of channels varies continually along the longitudinal direction.

Claims

1. A catalytic wall-flow monolith filter having three-way catalytic activity for use in an emission treatment system of a positive ignition internal combustion engine, wherein the wall-flow monolith filter comprises a porous filter substrate, the porous filter substrate having a first face and a second face defining a longitudinal direction therebetween and first and second pluralities of channels extending in the longitudinal direction, wherein the first plurality of channels is open at the first face and closed at the second face and the channels of the first plurality of channels are defined in part by channel wall surfaces, wherein the second plurality of channels is open at the second face and closed at the first face and the channels of the second plurality of channels are defined in part by channel wall surfaces and wherein channel walls between the channel wall surfaces of the first plurality of channels and the channel wall surfaces of the second plurality of channels are porous, wherein a first on-wall coating comprising catalytic material having a layer thickness is present on at least the channel wall surfaces of the first plurality of channels, wherein the catalytic material on channel wall surfaces of the first plurality of channels comprises one or more platinum group metal selected from the group consisting of (i) rhodium (Rh) only; (ii) palladium (Pd) only; (iii) platinum (Pt) and rhodium (Rh); (iv) palladium (Pd) and rhodium (Rh); and (v) platinum (Pt), palladium (Pd) and rhodium (Rh) and a refractory metal oxide support, wherein: the layer thickness of the on-wall coating present on channel wall surfaces of the first plurality of channels varies continually along the longitudinal direction and the layer thickness of the on-wall coating is from 10 to 150 microns.

2. The catalytic wall-flow monolith filter according to claim 1, wherein a maximum layer thickness present on channel wall surfaces of the first plurality of channels is at the open end of the first plurality of channels.

3. The catalytic wall-flow monolith filter according to claim 1, wherein the catalytic material on the channel wall surfaces of the first plurality of channels comprises Pd:Rh at a ratio of 1:1 or higher.

4. The catalytic wall-flow monolith filter according to claim 1, wherein the catalytic material on the channel wall surfaces of the first plurality of channels comprises Pd as the only platinum group metal.

5. The catalytic wall-flow monolith filter according to claim 1, wherein the catalytic material on the channel wall surfaces of the first plurality of channels comprises Rh as the only platinum group metal.

6. The catalytic wall-flow monolith filter according to claim 1, wherein the catalytic material on channel wall surfaces of the first plurality of channels comprises an oxygen storage component (OSC).

7. The catalytic wall-flow monolith filter according to claim 6, wherein the OSC comprises ceria; a mixed oxide comprising ceria; a mixed oxide of cerium and zirconium; a mixed oxide of cerium, zirconium, and neodymium; a mixed oxide of praseodymium and zirconium; a mixed oxide of cerium, zirconium and praseodymium; or a mixed oxide of praseodymium, cerium, lanthanum, yttrium, zirconium and neodymium.

8. The catalytic wall-flow monolith filter according to claim 7, wherein the praseodymium is present at 2-10 wt % based on the total content of the mixed oxide.

9. The catalytic wall-flow monolith filter according to claim 1, wherein an on-wall coating comprising catalytic material having a layer thickness is further provided on the wall surfaces of the second plurality of channels, wherein an on-wall coating comprising catalytic material having a layer thickness is present on the channel wall surfaces of the second plurality of channels, wherein the catalytic material on channel wall surfaces of the second plurality of channels comprises one or more platinum group metal selected from the group consisting of (i) rhodium (Rh) only; (ii) palladium (Pd) only; (iii) platinum (Pt) and rhodium (Rh); (iv) palladium (Pd) and rhodium (Rh); and (v) platinum (Pt), palladium (Pd) and rhodium (Rh) and a refractory metal oxide support, and wherein: the layer thickness of the on-wall coating present on channel wall surfaces of the second plurality of channels varies continually along the longitudinal direction and the layer thickness of the on-wall coating is from 10 to 150 microns.

10. The catalytic wall-flow monolith filter according to claim 9, wherein a maximum layer thickness present on channel wall surfaces of the second plurality of channels is at the open end of the second plurality of channels.

11. The catalytic wall-flow monolith filter according to claim 9, wherein the catalytic material on the channel wall surfaces of the second plurality of channels, the second on-wall coating of the first plurality of channels or the in-wall coating comprises Pd:Rh at a ratio of 1:1 or higher.

12. The catalytic wall-flow monolith filter according to claim 9, wherein the catalytic material on the channel wall surfaces of the second plurality of channels, in the second on-wall coating of the first plurality of channels or in the in-wall coating comprises Pd as the only platinum group metal and is different from the platinum group metal or combination of platinum group metals in the first plurality of channels or in the first on-wall coating of the first plurality of channels.

13. The catalytic wall-flow monolith filter according to claim 9, wherein the catalytic material on the channel wall surfaces of the second plurality of channels, in the second on-wall coating of the first plurality of channels comprises Rh as the only platinum group metal and is different from the platinum group metal or combination of platinum group metals in the first plurality of channels or in the first on-wall coating of the first plurality of channels.

14. The catalytic wall-flow monolith filter according to claim 9, wherein a composition of the catalytic material in the first plurality of channels is the same as that in the second plurality of channels, in the second on-wall coating in the first plurality of channels or in the in-wall coating.

15. The catalytic wall-flow monolith filter according to claim 9, wherein the catalytic material on channel wall surfaces of the second plurality of channels or the in-wall coating comprises an oxygen storage component (OSC).

16. The catalytic wall-flow monolith filter according to claim 15, wherein the OSC is ceria, a mixed oxide comprising ceria; a mixed oxide of cerium and zirconium; a mixed oxide of cerium, zirconium, and neodymium; a mixed oxide of praseodymium and zirconium; a mixed oxide of cerium, zirconium and praseodymium; or a mixed oxide of praseodymium, cerium, lanthanum, yttrium, zirconium and neodymium.

17. The catalytic wall-flow monolith filter according to claim 1, wherein the refractory metal oxide support comprises alumina or doped alumina.

18. The catalytic wall-flow monolith filter according to claim 1, wherein the layer thickness of the on-wall coating is from 10 to 100 microns.

19. The catalytic wall-flow monolith filter according to claim 1, wherein the layer thickness of the on-wall coating is from 20 to 50 microns.

20. An emission treatment system for treating a flow of a combustion exhaust gas from a positive ignition internal combustion engine, the system comprising the catalytic wall-flow monolith filter according to claim 1, wherein the first face is disposed upstream from the second face.

21. A method of manufacturing a catalytic wall-flow monolith filter according to claim 1, comprising: providing a porous filter substrate having a first face and a second face defining a longitudinal direction therebetween and first and second pluralities of channels extending in the longitudinal direction, wherein the first plurality of channels is open at the first face and closed at the second face and the channels of the first plurality of channels are defined in part by channel wall surfaces, wherein the second plurality of channels is open at the second face and closed at the first face and the channels of the second plurality of channels are defined in part by channel wall surfaces and wherein channel walls between the channel wall surfaces of the first plurality of channels and the channel wall surfaces of the second plurality of channels are porous, contacting the first face of the porous filter substrate with a liquid slurry washcoat containing a catalytic material; drawing the liquid slurry washcoat into the first plurality of channels by application of a vacuum, wherein at least one of: a liquid catalytic washcoat solids content; a liquid catalytic washcoat rheology; a porosity of the porous filter substrate; a mean pore size of the porous filter substrate; a liquid catalytic washcoat volumetric mean particle size; and a liquid catalytic washcoat D90 (by volume), is pre-selected so that at least some of the catalytic material remains on channel wall surfaces of the first plurality of channels or both remains on channel wall surfaces of the first plurality of channels and permeates channel walls of the first plurality of channels; and drying and calcining the coated filter substrate, wherein the catalytic material in the liquid slurry washcoat comprises one or more platinum group metal selected from the group consisting of (i) rhodium (Rh) only; (ii) palladium (Pd) only; (iii) platinum (Pt) and rhodium (Rh); (iv) palladium (Pd) and rhodium (Rh); and (v) platinum (Pt), palladium (Pd) and rhodium (Rh) and a refractory metal oxide support, such that: the layer thickness of the on-wall coating present on channel wall surfaces of the first plurality of channels varies continually along the longitudinal direction and the layer thickness of the on-wall coating is from 10 to 150 microns.

22. The method according to claim 21, wherein the first face is disposed uppermost, the liquid slurry washcoat is applied to the first face and the vacuum is applied from the second face.

23. A method of treating a combustion exhaust gas from a positive ignition internal combustion engine containing oxides of nitrogen (NO.sub.x), carbon monoxide (CO) unburned hydrocarbon fuel (HC) and particulate matter (PM), which method comprising contacting the exhaust gas with the catalytic wall flow filter according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described in relation to the following non-limiting figures, in which:

(2) FIG. 1 is a graph showing the size distributions of PM in the exhaust gas of a diesel engine. For comparison, a gasoline size distribution is shown at FIG. 4 of SAE 1999-01-3530;

(3) FIG. 2A is a perspective view that schematically shows a wall flow monolith filter 1 according to the present invention;

(4) FIG. 2B is an A-A line cross-sectional view of the wall flow monolith filter 1 shown in FIG. 1A;

(5) FIG. 3 shows a schematic diagram of an exhaust gas treatment system for a gasoline engine;

(6) FIG. 4 shows a schematic diagram of varying layer thicknesses according to the invention; and

(7) FIG. 5 shows a schematic diagram of another arrangement according to the invention having varying layer thicknesses.

DETAILED DESCRIPTION OF THE INVENTION

(8) A wall flow monolith 1 according to the present invention is shown in FIG. 2A and FIG. 2B. It includes a large number of channels arranged in parallel with each other in the longitudinal direction (shown by a double-sided arrow “a” in FIG. 2A) of the monolith 1. The large number of channels includes a first subset of channels 5 and a second subset of channels 10.

(9) The channels are depicted such that the second subset of channels 10 is narrower than the first subset of channels 5. This has been found to provide an increased ash/soot storage capacity in the filter. However, alternatively, the channels may be substantially the same size.

(10) The first subset of channels 5 is open at an end portion on a first end face 15 of the wall flow monolith 1 and is sealed with a sealing material 20 at an end portion on a second end face 25.

(11) On the other hand, the second subset of channels 10 is open at an end portion on the second end face 25 of the wall flow monolith 1 and is sealed with a sealing material 20 at an end portion on the first end face 15.

(12) The wall flow monolith 1 is provided with a catalytic material within pores of the channels walls 35. This may be provided with a washcoat application method, as is known in the art and is discussed elsewhere in the specification.

(13) Therefore, when the wall flow monolith is used in an exhaust system, exhaust gases G (in FIG. 2B “G” indicates exhaust gases and the arrow indicates a flowing direction of exhaust gases) introduced to the first subset of channels 5 will pass through the channel wall 35 interposed between the channel 5a and the channels 10a and 10b, and then flow out from the monolith 1. Accordingly, particulate matter in exhaust gases is captured by the channel wall 35.

(14) The catalyst supported in the channel wall 35 of the monolith 1 functions as a catalyst for treating the exhaust gases from a gasoline engine including oxides of nitrogen (NOx), carbon monoxide (CO) unburned hydrocarbon fuel (HC) and particulate matter (PM).

(15) In the embodiment of the exhaust gas treatment system 100 shown in FIG. 3 the flow of exhaust gas 110 passes through the wall flow monolith 1. The exhaust gas 110 is passed from the engine 115 through ducting 120 to the exhaust system 125. The exhaust system can comprise additional components, such as a TWC composition applied to a honeycomb monolith (flow through) substrate and disposed either upstream or downstream of the catalytic wall flow monolith filter according to the invention. In the upstream or downstream TWC, unburned gaseous and non-volatile hydrocarbons (volatile organic fraction) and carbon monoxide are largely combusted to form carbon dioxide and water. In addition, nitrogen oxides are reduced to form nitrogen and water. Removal of substantial proportions of the VOF using the oxidation catalyst, in particular, helps to prevent too great a deposition of particulate matter (i.e., clogging) on the downstream filter according to the present invention.

(16) As desired, after leaving the filter of the present invention, optionally the exhaust gas stream can next be conveyed via an appropriate exhaust pipe to a downstream NOx trap for adsorbing any remaining NOx emission contaminants in the exhaust gas stream. From the NOx trap through a further exhaust pipe, a SCR catalyst can be disposed to receive the outlet of the NOx trap to provide further emissions treatment of any ammonia generated by the NOx trap with a selective catalytic reduction catalyst for reducing oxides of nitrogen to form nitrogen and water using the ammonia as reductant. From the SCR catalyst, an exhaust pipe can lead to the tail pipe and out of the system.

(17) FIG. 4 shows an embodiment of the invention wherein the coating provided on the channel walls 35 of the first plurality of channels 5 is formed of a first layer 43. The first layer 43 comprises a Pd/Rh mixture, a ceria-zirconia mixed oxide as an OSC and lanthana-stabilised alumina. The second layer 44 on the channel walls of the second plurality of channels 6 has the same composition as the first layer. In FIG. 4 the first layer 43 and the second layer 44 both vary continuously along the longitudinal direction (such as from 0 to 50 microns and vice versa).

(18) FIG. 5 shows another embodiment of the invention wherein the coating provided on the channel walls 35 of the first plurality of channels 5 is formed of a first layer 43. The first layer 43 comprises a Pd/Rh mixture, a ceria-zirconia mixed oxide as an OSC and lanthana-stabilised alumina. The second layer, also on the channel walls of the first plurality of channels 5 has the same composition as the first layer 43. In FIG. 5 the first layer 43 and the second layer 54 both vary continuously along the longitudinal direction (such as from 0 to 50 microns and vice versa). Associated with the second layer 54 is an identical in-wall coating in the porous channel walls 35 corresponding to a wall portion having the same coating depth (longitudinal direction) as the second layer 54.

(19) It should be noted that the wall flow monolith is described herein as a single component. Nonetheless, when forming an emission treatment system, the monolith used may be formed by adhering together a plurality of channels or by adhering together a plurality of smaller monoliths as described herein. Such techniques are well known in the art, as well as suitable casings and configurations of the emission treatment system.

(20) The catalytic wall-flow monolith will now be described further in relation to the following non-limiting examples.

Example 1

(21) A wall-flow filter was prepared based on a substrate having dimensions of 4.66 inches (diameter)×4.5 inches (length), a cell density/wall thickness 300/8 (cells per square inch/mils (thousandths of an inch channel wall thickness)), and a fully formulated three-way catalyst washcoat comprising Pd/Rh at 10:1 weight ratio at 22 g/ft.sup.3 also comprising a ceria-zirconia based mixed oxide OSC and an alumina-based refractory oxide support at a loading of 2.4 g/in.sup.3 (146.5 g/l) split 50:50 between a first plurality of channels and a second plurality of channels. The washcoat comprised a D50 of 4-6 microns and a D90 of <20 microns. The washcoat solids used was 32% which was thickened using a thickening agent as known to the skilled person to a target viscosity of 2200-2300 cP as measured at 20° C. on a Brookfield RV DVII+ Extra Pro viscometer using a SC4-27 spindle at 50 rpm spindle speed. The coating method used was according to the first method disclosed in UK patent publication no. 2524662, i.e. introducing a pre-determined amount of a liquid into a containment means at an upper end of the filter substrate; and applying a vacuum to a lower end of the filter substrate. The vacuum used is as described in the description above, but a combination at shorter duration and lower vacuum strength was used. The coated product was dried and calcined in the usual way.

(22) The loading was applied in a wedge shaped on-wall profile having the thick end of the wedge at the channel openings at the respective end faces of the filter to 50% of the longitudinal length in each of the first and second pluralities of channels. Measurements were taken from SEM images at regular intervals denoted A-E (5 locations along the longitudinal direction at a mid-point between corners in the square-shaped cross section of the channels). The wedge was formed in accordance with FIG. 4. As a comparison the same wall-flow substrate filter was coated with the same three-way catalyst composition at the same total washcoat loading (mass of coated substrate less mass of uncoated substrate) to a 50:50 depth in both the first and second plurality of channels using the second method disclosed in UK patent publication no. 2524662, i.e. introducing a pre-determined amount of liquid into a containment means at an upper end of the filter substrate; and draining the liquid from the containment means into the filter substrate. The resulting product is referred to as “conventional” in the Table below.

(23) TABLE-US-00001 Coating Wall washcoat Catalyst Section Method thickness (μm) 2.4 g/in.sup.3 A Conventional 30.3 B 26.7 C 71.3 D 20.3 E 26.3 2.4 g/in.sup.3 A Wedge 82.3 B 49 C 9 D 22.5 E 63.3

(24) The values at A and B represent the on-wall coating thickness in the first plurality of channels; D and E represent the on-wall coating thickness in the second plurality of channels; and the value at C is the sum of the on-wall coating thicknesses in both the first and second plurality of channels, so the arrangement is as shown schematically in FIG. 4.

(25) It can be seen from the above Table, that the product according to the invention has a tapering or wedge shaped profile compared with the conventional filter.

Example 2

(26) The conventional filter and the filter according to the invention as described in Example 1 were fitted in the exhaust system of a bench mounted laboratory V8 Land Rover gasoline turbo direct injection (GTDI) engine and aged using a proprietary test methodology involving 10 seconds of fuel cut (to simulate a driver lifting off the accelerator pedal, producing a “spike” of lean exhaust gas) followed by 180 seconds at lambda 1 (perturbated stoichiometric operation at 630° C. inlet temperature with 5% Lambda amplitude and 5 seconds switch time) repeated for 80 hours.

(27) A lambda sweep test was then conducted on the aged sampled using a 2.0 litre GTDI laboratory bench-mounted engine certificated to the Euro 5 emission standard at 450° C. filter inlet temperature and 130 kg/h mass flow, 4% lambda amplitude and lambda set point of 0.991 to 1.01. A higher value indicates better conversion activity. The results are shown in the Table below. It can be seen that the filter according to the invention has a higher CO/NOx Cross-Over Point, i.e. is more active than the conventional filter.

(28) TABLE-US-00002 Coating CO/NOx Cross-Over Point Conventional 71.50% Wedge 82.50%

Example 3

(29) A cold flow back pressure analysis of the coated and aged filters was done using a Superflow SF1020 apparatus, available commercially at http://www.superflow.com/Flowbenches/sf1020.php.

(30) At 21° C. ambient temperature and at a flow rate of 600 m.sup.3/hr, the results are as follows:
Conventional coated gasoline particulate filter (GPF)=74.6 mbar (7.46 KPa); and
Wedge coated GPF (according to the invention)=76.1 mbar (7.61 KPa)

(31) The results of Examples 2 and 3 show that the wedge shaped coating profile of the present invention has higher activity with no commensurate impact on backpressure.

Example 4

(32) A wall-flow filter was prepared based on a substrate having dimensions of 4.66 inches (diameter)×6 inches (length), a cell density/wall thickness 300/8 (cells per square inch/mils (thousandths of an inch channel wall thickness)), and a fully formulated three-way catalyst washcoat comprising Pd/Rh at 70:30 weight ratio at 10 g/ft.sup.3 also comprising a ceria-zirconia based mixed oxide OSC and an alumina-based refractory oxide support at a loading of 1.6 g/in.sup.3 split 50:50 between a first plurality of channels and a second plurality of channels. The washcoat comprised a D50 of 2-4 microns and a D90 of <10 microns. The washcoat solids used was 19% which was thickened using a thickening agent as known to the skilled person to a target viscosity of 900-1000 cP as measured at 20° C. on a Brookfield RV DVII+ Extra Pro viscometer using a SC4-27 spindle at 50 rpm spindle speed. The coating method used was according to the first method disclosed in UK patent publication no. 2524662, i.e. introducing a pre-determined amount of a liquid into a containment means at an upper end of the filter substrate; and applying a vacuum to a lower end of the filter substrate. The vacuum used is as described in the description above, but a combination at shorter duration and lower vacuum strength was used. The coated product was dried and calcined in the usual way.

(33) In contrast to Example 1, when the washcoat described in this Example was applied in a first step to this different substrate via the first plurality of channels using the first method disclosed in UK patent publication no. 2524662, the washcoat was “pulled through” the channel walls so that a wedge-shaped profile was observed by SEM only in that section of the substrate at the first face end of the substrate but in the second plurality of channels. The “thick end” of the wedge-shaped profile at the first face end was at the “plug end” of the second plurality of channels of the wall flow filter. It was determined also that TWC was located in-wall in the channel walls of the first approximately 50% of the longitudinal direction extending from the first face. The “thin end” of the observable on-wall coating wedge profile extended to about 50% along the second plurality of channels in the longitudinal direction from the first face towards the second face

(34) In a second step, the second plurality of channels of the substrate coated with the first “wedge” was then coated to a nominal 50% depth from the second face end using the first method disclosed in UK patent publication no. 2524662 and this resulted in a second on-wall wedge shaped coating profile to about 50% of the longitudinal direction in the second plurality of channels (with some coating also in-wall) with the thick end of the wedge at the open channel ends at the second face. The arrangement obtained is shown schematically in FIG. 5.

(35) A reference sample to Example 4 was prepared for comparison, wherein the same substrate type, coating method, precious metal and washcoat loading were used to prepare Example 4 except in that the washcoat comprised a D50 of 4-6 microns and a D90 of <20 microns. The washcoat solids used was 26.65% and the washcoat was thickened using a thickening agent as known to the skilled person to a target viscosity of 900-1000 cP as measured at 20° C. on a Brookfield RV DVII+ Extra Pro viscometer using a SC4-27 spindle at 50 rpm spindle speed.

(36) Both the sample of Example 4 and the Example 4 Reference sample (comparative) were analysed using SEM and measurements of images at three intervals denoted A-C (3 were taken at regular intervals along the longitudinal direction). The total of inlet and outlet channel on-wall washcoat thickness (correlating to a % of the total washcoat at that interval, assuming a uniform coating) was then used to infer the quantity of washcoat located in-wall. The results are shown in the Tables below. The inlet channels correspond to the first plurality of channels; and the outlet channels correspond to the second plurality of channels.

(37) TABLE-US-00003 % washcoat on inlet % % washcoat on Position channel wall washcoat in-wall outlet channel wall A (front) 0 48.3 51.8 B (middle) 10.3 75.6 14.1 C (rear) 6.1 3.5 63.4

Reference

(38) TABLE-US-00004 % washcoat on inlet % % washcoat on Position channel wall washcoat in wall outlet channel wall A (front) 17 33.6 48.9 B (middle) 11 54.9 34 C (rear) 9.1 47.7 43.2

(39) The Reference sample (comparative) was found to have a more homogeneous distribution of washcoat along the (axial) length (i.e. longitudinal direction) of the part rather than the more pronounced wedge shape seen in Example 4.

Example 5

(40) A cold flow back pressure analysis of the coated and aged filters Example 4 and its Reference (comparative) was done using the Superflow SF1020 apparatus described in Example 3 at 21° C. ambient temperature and at a flow rate of 700 m.sup.3/hr, the results are as follows:
Example 4=92.82 mbar @ 700 m.sup.3/hr (9.28 KPa); and
Example 4 Reference (comparative)=116.56 mbar @ 700 m.sup.3/hr (11.66 KPa).

(41) From these data it can be seen that, by adjusting the D90 of the washcoat components, catalytic wall-flow filters according to the invention additionally provide the advantage of lower back pressure compared with conventional gasoline particulate filters.

Example 6 (Comparative)

(42) Four wall-flow filters (4×5″ and 600/4 cell density) were coated with TWCs having a platinum group metal (PGM) composition of 40 g/ft.sup.−3/0:9:1 [Pt:Pd:Rh weight ratio]. Each TWC comprised a different weight ratio of Al.sub.2O.sub.3 to CeZrO.sub.4.

(43) The filters were fitted in the exhaust system of a bench mounted laboratory V8 Land Rover gasoline turbo direct injection (GTDI) engine and aged using a proprietary test methodology involving 10 seconds of fuel cut (to simulate a driver lifting off the accelerator pedal, producing a “spike” of lean exhaust gas) followed by 180 seconds at lambda 1 (perturbated stoichiometric operation at 630° C. inlet temperature with 5% Lambda amplitude and 5 seconds switch time) repeated for 80 hours.

(44) A lambda sweep test was then conducted on the aged samples using a 2.0 litre GTDI (gasoline turbo direct injection) laboratory bench-mounted engine certificated to the Euro 5 emission standard at 450° C. filter inlet temperature and 130 kg/h mass flow, 4% lambda amplitude and lambda set point of 0.991 to 1.01. A higher value indicates better conversion activity. The results are shown in the Table below. It can be seen that the filter according to the invention has a higher CO/NOx Cross-Over Point, i.e. is more active than the conventional filter.

(45) The results were as follows:

(46) TABLE-US-00005 Al.sub.2O.sub.3:CeZrO.sub.4 weight ratio 1:1 1:2 1:3 1:4 Vehicle NOx Emissions (g/km) 0.049 0.052 0.053 0.057 Relative NOx Emissions 100.0% 106.1% 108.2% 116.3% NOx Conversion at lambda 1 97.1% 85.4% 85.5% 89.8% (i.e. stoichiometric point)

(47) As can be seen from the table, increasing the CeZrO.sub.4:Al.sub.2O.sub.3 ratio above 1:1 was found to be detrimental to the NOx conversion ability of the coated through-flow monolith.

Example 7

(48) Four wall-flow filters (4.66×4.5″ and 300/8 cell density) were coated with TWCs having PGM 60/0.57:3. Each TWC comprised a different weight ratio of Al.sub.2O.sub.3 to CeZrO.sub.4. The coated filters were calcined and aged (Hydrothermal, 1100° C. in air with 10% H.sub.2O added, 5 h).

(49) Using a 1.4 l GTDI test engine, the NOx emissions were measured based on a standard engine test. The results were as follows:

(50) TABLE-US-00006 Al.sub.2O.sub.3:CeZrO.sub.4 wt ratio 1:1 1:2 1:3 2:1 Vehicle NOx Emissions (g/km) 0.058 0.046 0.04 0.054 Relative NOx Emissions 145.0% 115.0% 100.0% 135.0%
As can be seen from the table, as the CeZrO.sub.4:Al.sub.2O.sub.3 wt ratio increased from 1:2 to 3:1, the relative NOx emissions decreased.

Example 8

(51) Three wall-flow filters (4.66×4.5″ and 300/8 cell density) were coated with TWCs having PGM 22/0:20:2. Each TWC comprised a different weight ratio of Al.sub.2O.sub.3 to CeZrO.sub.4. The coated filters were calcined and aged as used in Example 7.

(52) Using a 2.0 l GTDI Engine Bench test engine, the NOx emissions were measured based on a standard engine test. The results were as follows:

(53) TABLE-US-00007 Al.sub.2O.sub.3:CeZrO.sub.4 wt ratio 1:3 1:4 1:5 NOx conversion at lambda 1 44.8 46.05 44.78

(54) To coat the wall flow filters with the TWC composition, porous substrates are immersed vertically in a portion of the catalyst slurry such that the top of the substrate is located just above the surface of the slurry. In this manner slurry contacts the inlet face of each honeycomb wall, but is prevented from contacting the outlet face of each wall. The sample is left in the slurry for about 30 seconds. The filter is removed from the slurry, and excess slurry is removed from the wall flow filter first by allowing it to drain from the channels, then by blowing with compressed air (against the direction of slurry penetration), and then by pulling a vacuum from the direction of slurry penetration. By using this technique, the catalyst slurry permeates the walls of the filter, yet the pores are not occluded to the extent that undue back pressure will build up in the finished filter. As used herein, the term “permeate” when used to describe the dispersion of the catalyst slurry on the filter, means that the catalyst composition is dispersed throughout the wall of the filter.

(55) The coated filters are dried typically at about 100° C. and calcined at a higher temperature (e.g., 300 to 450° C. and up to 550° C.). After calcining, the catalyst loading can be determined through calculation of the coated and uncoated weights of the filter. As will be apparent to those of skill in the art, the catalyst, loading can be modified by altering the solids content of the coating slurry. Alternatively, repeated immersions of the filter in the coating slurry can be conducted, followed by removal of the excess slurry as described above.

Example 9—Soot Combustion Tests

(56) Two ceria-zirconia mixed oxides each doped with rare earth elements and having the composition shown in the Table below were tested for their soot combustion activity using a CATLAB-PCS combined microreactor and mass spectrometer laboratory apparatus (Hiden Analytical). A cordierite sample was tested as a control. The ceria-zirconia mixed oxides and the cordierite control were pre-fired at 500° C. for 2 hours.

(57) Soot was collected from the engine of a European common rail light-duty diesel 2.2 litre capacity engine, which was certified to meet Euro IV emission standards. The exhaust system included a commercially available soot filter comprising an aluminium titanate wallflow filter. Soot was collected on the filter and the soot was removed from the filter by directing compressed air from a gun through the outlet channels of the filter.

(58) To prepare the samples, 85 mg of each sample or cordierite was mixed with 15 mg of the soot using a pestle and mortar until the mix was a uniform colour, free of lumps and streaks. Periodically the cake was scraped from the wall of the mortar. No pre-treatment was carried out.

(59) 0.1 g of each sample/soot mixture (nominally containing 15 mg soot) was placed into a CATLAB microreactor tube. It was heated in 13% O.sub.2/He with a temperature ramp rate of 10° C./min. The outlet gas was monitored by mass spectrometer.

(60) Three samples of soot ground with fine cordierite (<250 μm) taken from the same batch of grinding, were run to assess the repeatability of the method. Very good repeatability was obtained for soot oxidation peak position and the shape of the evolved CO.sub.2 profile. The reproducibility of the test methodology was investigated also by having two different scientists prepare the same mixed soot/ceria-zirconia mixed oxide material. Although differences in oxidation at higher temperatures were observed, possibly because of looser contact between the soot and the mixed oxide or unpromoted soot oxidation at 600° C., the main oxidation peak for both mixtures was sharp, well defined and at an identical temperature. Therefore, the method is reproducible and the main peak temperature is representative of the sample oxidation activity.

(61) The results for soot oxidation are set out in the Table below, from which it can be seen that Sample B comprising 5 wt % Pr.sub.6O.sub.11 has a 2.5% lower soot oxidation temperature than Sample A, despite Sample A having a similar composition to Sample B. Inventors conclude that the inclusion of Sample A in an on-wall coating on inlet channels of a filter, thereby increasing contact between the soot and the coating will beneficially promote removal of soot at lower exhaust gas temperatures.

(62) TABLE-US-00008 TABLE Soot Oxidation CeO.sub.2 ZrO.sub.2 La.sub.2O.sub.3 Nd.sub.2O.sub.3 Pr.sub.6O.sub.11 Temperature Sample % % % % % (° C.) A 45 48.5 1.5 5 0 352 B 45 45 0 5 5 343 Control n/a n/a n/a n/a n/a ≈600

(63) Although preferred embodiments of the invention have been described herein in detail, it will be understood by those skilled in the art that variations may be made thereto without departing from the scope of the invention or of the appended claims.

(64) For the avoidance of doubt, the entire contents of all documents cited herein are incorporated herein by reference.