Filter substrate comprising three-way catalyst

Abstract

A catalysed filter for a positive ignition internal combustion engine comprises a porous filtering substrate having a total substrate length coated with a three-way catalyst washcoat composition comprising at least one precious metal selected from the group consisting of rhodium and one or both of platinum and palladium supported on a high surface area oxide, and an oxygen storage component, which composition being axially shared by a first zone comprising inlet surfaces of a first substrate length<total substrate length and a second zone comprising outlet surfaces of a second substrate length<total substrate length, wherein a sum of the substrate length in the first zone and the substrate length in the second zone≧100% and wherein one or both of the following applies: a washcoat loading in the first zone>second zone; and a total precious metal loading in the first zone>second zone.

Claims

1. A positive ignition engine comprising an exhaust system comprising a catalysed filter for filtering particulate matter from exhaust gas emitted from the engine, the filter comprising a porous substrate having a total substrate length and having inlet surfaces and outlet surfaces, wherein the inlet surfaces are separated from the outlet surfaces by a porous structure containing pores of a first mean pore size, wherein the porous substrate is coated with a three-way catalyst washcoat comprising a plurality of solid particles and at least one precious metal, wherein the porous structure of the washcoated porous substrate contains pores of a second mean pore size, wherein the second mean pore size is less than the first mean pore size, which three-way catalyst washcoat being axially arranged on the porous substrate between a first zone comprising the inlet surfaces of a first substrate length less than the total substrate length and a second zone comprising the outlet surfaces of a second substrate length less than the total substrate length, wherein the sum of the substrate length in the first zone and the substrate length in the second zone>100%, the first zone is disposed upstream of the second zone and wherein: i. a washcoat loading in the first zone>second zone; ii. a total precious metal loading in the first zone>second zone; or iii. both a washcoat loading and a total precious metal loading in the first zone>second zone.

2. The positive ignition engine according to claim 1, wherein the washcoat loading in the first zone is>1.60 g in.sup.−3.

3. The positive ignition engine according to claim 1, wherein a substrate length in the first zone is different from that of the second zone.

4. The positive ignition engine according to claim 3, wherein the substrate length in the first zone is<the substrate length in the second zone.

5. The positive ignition engine according to claim 3, wherein the substrate zone length in the first zone is<45% of the total substrate length.

6. The positive ignition engine according to claim 1, wherein (ii) the total precious metal loading in the first zone is greater than in the second zone or (iii) both the washcoat loading and the total precious metal loading in the first zone are greater than in the second zone, and the total precious metal loading in the first zone is greater than 50 gft.sup.−3.

7. The positive ignition engine according to claim 1, comprising a surface washcoat, wherein a washcoat layer substantially covers surface pores of the porous structure and the pores of the washcoated porous substrate are defined in part by spaces between the particles in the washcoat.

8. The positive ignition engine according to claim 7, wherein a D90 of solid washcoat particles is in the range 0.1 to 20 μm.

9. The positive ignition engine according to claim 1, wherein the mean size of the solid washcoat particles is in the range 1 to 40 μm.

10. The positive ignition engine according to claim 1, wherein the porous substrate is a wall-flow filter.

11. The positive ignition engine according to claim 1, wherein the uncoated porous substrate has a porosity of>40%.

12. The positive ignition engine according to claim 1, wherein a first mean pore size of the porous structure of the porous substrate is from 8 to 45 μm.

13. A method of simultaneously converting carbon monoxide, hydrocarbons, oxides of nitrogen and particulate matter in the exhaust gas of a positive ignition internal combustion engine, which method comprising the step of contacting the gas with a catalysed filter comprising a porous substrate having a total substrate length and having inlet surfaces and outlet surfaces, wherein the inlet surfaces are separated from the outlet surfaces by a porous structure containing pores of a first mean pore size, wherein the porous substrate is coated with a three-way catalyst washcoat comprising a plurality of solid particles and at least one precious metal, wherein the porous structure of the washcoated porous substrate contains pores of a second mean pore size, wherein the second mean pore size is less than the first mean pore size, which three-way catalyst washcoat being axially arranged on the porous substrate between a first zone comprising the inlet surfaces of a first substrate length less than the total substrate length and a second zone comprising the outlet surfaces of a second substrate length less than the total substrate length, wherein the sum of the substrate length in the first zone and the substrate length in the second zone≧100%, the first zone is disposed upstream of the second zone and wherein: i. a washcoat loading in the first zone>second zone; ii. a total precious metal loading in the first zone>second zone; or iii. both a washcoat loading and a total precious metal loading in the first zone>second zone.

14. The method according to claim 13, wherein the washcoat loading in the first zone is>1.60 g in.sup.−3.

15. The method according to claim 13, wherein a substrate length in the first zone is different from that of the second zone.

16. The method according to claim 15, wherein the substrate length in the first zone is<the substrate length in the second zone.

17. The method according to claim 16, wherein the substrate zone length in the first zone is<45% of the total substrate length.

18. The method according to claim 13, wherein (ii) the total precious metal loading in the first zone is greater than in the second zone or (iii) both the washcoat loading and the total precious metal loading in the first zone are greater than in the second zone, and the total precious metal loading in the first zone is greater than 50 gft.sup.−3.

19. The method according to claim 13, wherein the filter comprises a surface washcoat layer, wherein the washcoat layer substantially covers surface pores of the porous structure and the pores of the washcoated porous substrate are defined in part by spaces between the particles in the washcoat.

20. The method according to claim 13, wherein the mean size of the solid washcoat particles is in the range 1 to 40 μm.

21. The method according to claim 20, wherein a D90 of solid washcoat particles is in the range 0.1 to 20 μm.

22. The method according to claim 13, wherein the porous substrate is a wall-flow filter.

23. The method according to claim 13, wherein the uncoated porous substrate has a porosity of>40%.

24. The method according to claim 13, wherein a first mean pore size of the porous structure of the porous substrate is from 8 to 45 μm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) 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;

(2) FIG. 2 is a schematic drawing of an embodiment of a washcoated porous filter substrate according to the invention; and

(3) FIG. 3 is a schematic drawing of an embodiment of an exhaust system according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

(4) According to one aspect, the invention provides a catalysed filter for filtering particulate matter from exhaust gas emitted from a positive ignition internal combustion engine, which filter comprising a porous substrate having a total substrate length and having inlet surfaces and outlet surfaces, wherein the inlet surfaces are separated from the outlet surfaces by a porous structure containing pores of a first mean pore size, wherein the porous substrate is coated with a three-way catalyst washcoat comprising a plurality of solid particles and at least one precious metal, wherein the porous structure of the washcoated porous substrate contains pores of a second mean pore size, wherein the second mean pore size is less than the first mean pore size, which three-way catalyst washcoat being axially arranged on the porous substrate between a first zone comprising the inlet surfaces of a first substrate length less than the total substrate length and a second zone comprising the outlet surfaces of a second substrate length less than the total substrate length, wherein the sum of the substrate length in the first zone and the substrate length in the second zone≧100% and wherein: (iv) a washcoat loading in the first zone>second zone; (v) a total precious metal loading in the first zone>second zone; or

(5) both a washcoat loading and a total precious metal loading in the first zone>second zone.

(6) For the washcoat loading and total precious metal loading in features (i) and (ii) but not specifically mentioned in the definition of feature (i) or (ii), such feature is homogeneously applied between the inlet and outlet surfaces. So, for example, since feature (i) defines only the washcoat loading, the total precious metal loading is substantially the same (homogeneous) in both the first zone and the second zone. Similarly, in feature (ii), the total precious metal loading is defined. Therefore, the washcoat loading is homogeneously applied between the first zone and the second zone.

(7) Positive ignition internal combustion engines, such as spark ignition internal combustion engines, for use in this aspect of the invention can be fuelled by gasoline fuel, gasoline fuel blended with oxygenates including methanol and/or ethanol, liquid petroleum gas or compressed natural gas.

(8) Mean pore size can be determined by mercury porosimetry.

(9) The porous substrate is preferably a monolith substrate and can be a metal, such as a sintered metal, or a ceramic, e.g. silicon carbide, cordierite, aluminium nitride, silicon nitride, aluminium titanate, alumina, mullite e.g., acicular mullite (see e.g. WO 01/16050), pollucite, a thermet such as Al.sub.2O.sub.3/Fe, Al.sub.2O.sub.3/Ni or B.sub.4C/Fe, or composites comprising segments of any two or more thereof. In a preferred embodiment, the filter is a wallflow filter comprising a ceramic porous filter substrate having a plurality of inlet channels and a plurality of outlet channels, wherein each inlet channel and each outlet channel is defined in part by a ceramic wall of porous structure, wherein each inlet channel is separated from an outlet channel by a ceramic wall of porous structure. This filter arrangement is also disclosed in SAE 810114, and reference can be made to this document for further details. Alternatively, the filter can be a foam, or a so-called partial filter, such as those disclosed in EP 1057519 or WO 01/080978.

(10) It is a particular feature of the present invention that washcoat loadings used in the first, upstream zone can be higher than the previously regarded highest washcoat loadings, e.g. those disclosed in the Examples in WO 2010/097634. In a particular embodiment, the washcoat loading in the first zone is1.60 g in.sup.−3, and in preferred embodiments the washcoat loading in the first zone is>2.4 g in.sup.−3.

(11) In the catalysed filter according to the invention, the sum of the substrate length in the first zone and the substrate length in the second zone≧100%, i.e. there is no gap in the axial direction, or there is axial overlap, between the first zone on the inlet surface and the second zone on the outlet surface.

(12) The length of axial overlap between inlet and outlet surface coatings can be>10%, e.g. 10-30%, i.e. the sum of the substrate length in the first zone and the substrate length in the second zone >110%, e.g. 110-130%.

(13) The substrate length in the first zone can be the same as or different from that of the second zone. So, where the first zone length is the same as the second zone length the porous substrate is coated in a ratio of 1:1 between the inlet surface and the outlet surface. However, in one embodiment, the substrate length in the first zone<the substrate length in the second zone.

(14) In embodiments, the substrate length in the first zone<the substrate length in the second zone, e.g.<45%. In preferred embodiments, the substrate zone length in the first zone is<40%, e.g. <35% of the total substrate length.

(15) In the catalysed filter of feature (ii) or (iii), the total precious metal loading in the first zone>the total precious metal loading in the second zone. In particularly preferred embodiments, the total precious metal loading in the first zone is>50 gft.sup.−3, but is preferably between 60-250 gft.sup.−3, and is typically from 70-150 gft.sup.−3. Total precious metal loadings in the second zone can be e.g.<50 gft.sup.−3, e.g.<30 gft.sup.−3 such as<20 gft.sup.−3.

(16) In preferred embodiments, the first and second zones comprise a surface washcoat, wherein a washcoat layer substantially covers surface pores of the porous structure and the pores of the washcoated porous substrate are defined in part by spaces between the particles (interparticle pores) in the washcoat. Methods of making surface coated porous filter substrates include introducing a polymer, e.g. poly vinyl alcohol (PVA), into the porous structure, applying a washcoat to the porous filter substrate including the polymer and drying, then calcining the coated substrate to burn out the polymer. A schematic representation of the first embodiment is shown in FIG. 2.

(17) Methods of coating porous filter substrates are known to the skilled person and include, without limitation, the method disclosed in WO 99/47260, i.e. a method of coating a monolithic support, comprising the steps of (a) locating a containment means on top of a support, (b) dosing a pre-determined quantity of a liquid component into said containment means, either in the order (a) then (b) or (b) then (a), and (c) by applying pressure or vacuum, drawing said liquid component into at least a portion of the support, and retaining substantially all of said quantity within the support. Such process steps can be repeated from another end of the monolithic support following drying of the first coating with optional firing/calcination.

(18) Alternatively, the method disclosed in WO 2011/080525 can be used, i.e. comprising the steps of: (i) holding a honeycomb monolith substrate substantially vertically; (ii) introducing a pre-determined volume of the liquid into the substrate via open ends of the channels at a lower end of the substrate; (iii) sealingly retaining the introduced liquid within the substrate; (iv) inverting the substrate containing the retained liquid; and (v) applying a vacuum to open ends of the channels of the substrate at the inverted, lower end of the substrate to draw the liquid along the channels of the substrate.

(19) In this preferred embodiment, an average interparticle pore size of the porous washcoat is 5.0 nm to 5.0 μm, such as 0.1-1.0 μm.

(20) As explained hereinabove, TWC composition generally comprises one or both of platinum and palladium in combination with rhodium, or even palladium only (no rhodium), supported on a high surface area oxide, e.g. gamma alumina, and an oxygen storage component, e.g. comprising a mixed oxide comprising cerium. In embodiments, the mean size (D50) of the solid washcoat particles is in the range 1 to 40 μm. In practice, the oxygen storage components may have a different particle size from the high surface area oxide. So, an OSC may have a D50 between 1-10 μm, such as from 4 and 6 μm; and a high surface area oxide may have a D50 of between 1-10 μm, such as from 4 and 6 μm.

(21) In further embodiments, the D90 of solid washcoat particles is in the range of from 0.1 to 20 μm. Again, the D90 of the OSC may be different from that of the high surface area oxide. So, the D90 of the OSC can be<18 μm and the D90 of the high surface area oxide can be<20 μm.

(22) Preferably, the porous substrate is a monolith substrate. In particularly preferred embodiments, the porous substrate for use in the present invention is a ceramic wall flow filter made from e.g. cordierite, or silicon carbide or any of the other materials described hereinabove. However, substrate monoliths other than flow-through monoliths can be used as desired, e.g. partial filters (see e.g. WO 01/080978 or EP 1057519), metal foam substrates etc.

(23) The cell density of diesel wallflow filters in practical use can be different from wallflow filters for use in the present invention in that the cell density of diesel wallflow filters is generally 300 cells per square inch (cpsi) or less, e.g. 100 or 200 cpsi, so that the relatively larger diesel PM components can enter inlet channels of the filter without becoming impacted on the solid frontal area of the diesel particulate filter, thereby caking and fouling access to the open channels, whereas wallflow filters for use in the present invention can be up to 300 cpsi or greater, such as 350 cpsi, 400, cpsi, 600 cpsi, 900 cpsi or even 1200 cpsi.

(24) An advantage of using higher cell densities is that the filter can have a reduced cross-section, e.g. diameter, than diesel particulate filters, which is a useful practical advantage that increases design options for locating exhaust systems on a vehicle.

(25) It will be understood that the benefit of filters for use in the invention is substantially independent of the porosity of the uncoated porous substrate. Porosity is a measure of the percentage of void space in a porous substrate and is related to backpressure in an exhaust system: generally, the lower the porosity, the higher the backpressure. However, the porosity of filters for use in the present invention are typically>40% or>50% and porosities of 45-75% such as 50-65% or 55-60% can be used with advantage. The mean pore size of the washcoated porous substrate is important for filtration. So, it is possible to have a porous substrate of relatively high porosity that is a poor filter because the mean pore size is also relatively high.

(26) In embodiments, the first mean pore size e.g. of surface pores of the porous structure of the porous filter substrate is from 8 to 45 μm, for example 8 to 25 μm, 10 to 20 μm or 10 to 15 μm. In particular embodiments, the first mean pore size is>18 μm such as from 15 to 45 μm, 20 to 45 μm e.g. 20 to 30 μm, or 25 to 45 μm.

(27) According to a second aspect, the present invention provides an exhaust system for a positive ignition internal combustion engine comprising a catalysed filter according to the first aspect of the present invention, wherein the first zone is disposed upstream of the second zone.

(28) In a preferred embodiment, the exhaust system comprises a flow through monolith substrate comprising a three-way catalyst composition disposed upstream of the catalysed filter.

(29) According to a third aspect, the invention provides a positive ignition engine comprising an exhaust system according to the second aspect of the present invention.

(30) The filter according to the invention could obviously be used in combination with other exhaust system aftertreatment components to provide a full exhaust system aftertreatment apparatus, e.g. a low thermal mass TWC upstream of the filter and/or downstream catalytic elements, e.g. NO.sub.x trap or SCR catalyst, according to specific requirements. So, in vehicular positive ignition applications producing relatively cool on-drive cycle exhaust gas temperatures, we contemplate using a low thermal mass TWC disposed upstream of the filter according to the invention. For vehicular lean-burn positive ignition applications, we envisage using a filter according to the invention upstream or downstream of a NO.sub.x trap. In vehicular stoichiometrically-operated positive ignition engines, we believe that the filter according to the present invention can be used as a standalone catalytic exhaust system aftertreatment component. That is, in certain applications the filter according to the present invention is adjacent and in direct fluid communication with the engine without intervening catalysts therebetween; and/or an exit to atmosphere from an exhaust gas aftertreatment system is adjacent to and in direct fluid communication with the filter according to the present invention without intervening catalysts therebetween.

(31) An additional requirement of a TWC is a need to provide a diagnosis function for its useful life, so called “on-board diagnostics” or OBD. A problem in OBD arises where there is insufficient oxygen storage capacity in the TWC, because OBD processes for TWCs use remaining oxygen storage capacity to diagnose remaining catalyst function. However, if insufficient washcoat is loaded on the filter such as in the specific Examples disclosed in US 2009/0193796 and WO 2009/043390, there may not be enough OSC present to provide an accurate OSC “delta” for OBD purposes. Since the present invention enables washcoat loadings approaching current state-of-the-art TWCs, the filters for use in the present invention can be used with advantage in current OBD processes.

(32) According to a fourth aspect, the invention provides a method of simultaneously converting carbon monoxide, hydrocarbons, oxides of nitrogen and particulate matter in the exhaust gas of a positive ignition internal combustion engine, which method comprising the step of contacting the gas with a catalysed filter according to the first aspect of the present invention.

(33) In order that the invention may be more fully understood, reference is made to the accompanying drawings wherein:

(34) 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;

(35) FIG. 2 is a schematic drawing of an embodiment of a washcoated porous filter substrate according to the invention; and

(36) FIG. 3 is a schematic drawing of an embodiment of an exhaust system according to the invention.

(37) FIG. 2 shows a cross-section through a porous filter substrate 10 comprising a surface pore 12. FIG. 2 shows an embodiment, featuring a porous surface washcoat layer 14 comprised of solid washcoat particles, the spaces between which particles define pores (interparticle pores). It can be seen that the washcoat layer 14 substantially covers the pore 12 of the porous structure and that a mean pore size of the interparticle pores 16 is less than the mean pore size 12 of the porous filter substrate 10.

(38) FIG. 3 shows an apparatus 11 according to the invention comprising a vehicular positive ignition engine 13 and an exhaust system 15 therefor. Exhaust system 15 comprises a conduit 17 linking catalytic aftertreatment components, namely a Pd—Rh-based TWC coated onto an inert cordierite flowthrough substrate 18 disposed close to the exhaust manifold of the engine (the so-called close coupled position). Downstream of the close-coupled catalyst 18 in turn is a zoned Pd—Rh-based TWC coated onto a cordierite wall-flow filter 20 having a total length and comprising inlet channels coated to a length of one third of the total length measured from an upstream or inlet end of the wall-flow filter with a washcoat loading of 2.8 gin.sup.−3 comprising a relatively high precious metal loading of 85 gft.sup.−3 (80Pd:5Rh), which coating defining a first zone 22. The outlet channels are coated with a Pd—Rh-based TWC coated on two thirds of the total length of the wall-flow filter measured from the downstream or outlet end of the wall-flow filter with a washcoat loading of 1.0 gin.sup.−3 comprising a relatively low precious metal loading of 18 gft.sup.−3 (16Pd:2Rh), which coating defining a second zone 24.

EXAMPLES

(39) In order that the invention may be more fully understood the following Examples are provided by way of illustration only. The washcoat loadings quoted in the Examples were obtained using the method disclosed in WO 2011/080525.

Example 1

(40) Two cordierite wall-flow filters of dimensions 4.66×5.5 inches, 300 cells per square inch, wall thickness 13 thousandths of an inch and having a mean pore size of 20 μm and a porosity of 65% were each coated with a TWC composition in a different configuration from the other. In each case, the TWC composition was milled to a d90<17 μm) so that the coating when applied would be expected preferentially to locate more at the surface of a wallflow filter wall (“on-wall”).

(41) A first filter (referred to in Table 1 as having a “Homogeneous” washcoat loading) was coated in channels intended for the inlet side of the filter with a TWC washcoat zone extending for a targeted 33.3% of the total length of the filter substrate measured from the open channel ends with a washcoat comprising a precious metal loading of 85 g/ft.sup.3 (80Pd:5Rh) and at a washcoat loading of 2.4 g/in.sup.3. The outlet channels were coated to a length of 66.6% of the total length of the filter substrate measured from the open channel ends with a washcoat comprising a precious metal loading of 18 g/ft.sup.3 (16Pd:2Rh) at a washcoat loading also of 2.4 g/in.sup.3. X-ray imaging was used to ensure that an overlap occurred in the longitudinal plane between the inlet channel zone and the outlet channel zone. So, the washcoat loading was homogeneous between the first and second zones, but the platinum group metal loading in the first zone>second zone. That is, the first filter is according to claim 1, feature (ii).

(42) A second filter (referred to in Table 1 as having a “Zoned” washcoat loading) was coated in the inlet channels with a TWC washcoat zone extending for a targeted 33.33% of the total length of the filter substrate measured from the open channel ends with a washcoat comprising a precious metal loading of 85 g/ft.sup.3 (80Pd:5Rh) and at a washcoat loading of 2.8 g/in.sup.3. The outlet channels were coated to a length of 66.66% of the total length of the filter substrate measured from the open channel ends with a washcoat comprising a precious metal loading of 18 g/ft.sup.3 (16Pd:2Rh) at a washcoat loading of 1.0 g/in.sup.3. X-ray imaging was used to ensure that an overlap occurred in the longitudinal plane between the inlet channel zone and the outlet channel zone. So, both the washcoat loading and the platinum group metal loading in the first zone>second zone. That is, the second filter is according to claim 1, feature (iii).

(43) The total precious metal content of the first and second filters was identical.

(44) Each filter was hydrothermally oven-aged at 1100° C. for 4 hours and installed in a close-coupled position on a Euro 5 passenger car with a 2.0 L direct injection gasoline engine. Each filter was evaluated over a minimum of three MVEG-B drive cycles, measuring the reduction in particle number emissions relative to a reference catalyst. The reference catalyst was a TWC coated homogeneously onto a 600 cells per square inch cordierite flowthrough substrate monolith having the same dimensions as the first and second filters and at a washcoat loading of 3 gin.sup.−3 and a precious metal loading of 33 gft.sup.−3 (30Pd:3Rh). The backpressure differential was determined between sensors mounted upstream and downstream of the filter (or reference catalyst).

(45) In Europe, since the year 2000 (Euro 3 emission standard) emissions are tested over the New European Driving Cycle (NEDC). This consists of four repeats of the previous ECE 15 driving cycle plus one Extra Urban Driving Cycle (EUDC) with no 40 second warm-up period before beginning emission sampling. This modified cold start test is also referred to as the “MVEG-B” drive cycle. All emissions are expressed in g/km.

(46) The Euro 5/6 implementing legislation introduces a new PM mass emission measurement method developed by the UN/ECE Particulate Measurement Programme (PMP) which adjusts the PM mass emission limits to account for differences in results using old and the new methods. The Euro 5/6 legislation also introduces a particle number emission limit (PMP method), in addition to the mass-based limits.

(47) The results of the tests are shown in Table 1, from which it can be seen that the filter washcoated in the zoned configuration shows improved back pressure and has good (though moderately lower) levels of particle number reduction relative to the homogeneously washcoated filter. Despite the moderate reduction in lower particle number reduction, the second filter would still meet the full Euro 6+(2017) standard limit.

(48) TABLE-US-00001 TABLE 1 Effect of washcoat zoning on particle number reduction and backpressure (BP) % PN Average BP Peak BP reduction (mbar) on 70 (mbar) during Sample vs. flow kph cruise of any one filter Washcoat through MVEG-B MVEG-B properties type reference drive cycle drive cycle 20 μm, 65% Homogeneous 85 17.6 82.1 20 μm, 65% Zoned 81 12.2 59.5

Example 2

(49) Two cordierite wall-flow filters of dimensions 4.66×4.5 inches, 300 cells per square inch, wall thickness 13 thousandths of an inch, mean pore size of 20 μm and a porosity of 65% were each coated with a TWC composition in a different configuration from the other. In each case, the TWC composition was milled to a d90<17 μm) so that the coating when applied would be expected preferentially to locate more at the surface of a wallflow filter wall (“on-wall”).

(50) A third filter (referred to in Table 2 as having a “Homogeneous” platinum group metal loading (Comparative Example)) was coated in channels intended for the inlet side of the filter and outlet side of the filter with a TWC washcoat zone extending for a targeted 50% of the total length of the filter substrate measured from the open channel ends with a washcoat comprising a precious metal loading of 60 gft.sup.−3 (57Pd:3Rh) and at a washcoat loading of 2.4 g/in.sup.3.

(51) A fourth filter (referred to in Table 2 as having a “Zoned” PGM loading) was coated in channels intended for the inlet side of the filter with a TWC washcoat zone extending for a targeted 50% of the total length of the filter substrate measured from the open channel ends with a washcoat comprising 100 g/ft.sup.−3 precious metal (97Pd:3Rh) at a washcoat loading of 2.4 g/in.sup.3; and the outlet channels were coated with a TWC washcoat zone extending for a targeted 50% of the total length of the filter substrate measured from the open channel ends with a washcoat comprising 20 g/ft.sup.3 precious metal (17Pd:3Rh), also at a washcoat loading of 2.4 g/in.sup.3. That is, the fourth filter is according to claim 1, feature (ii).

(52) The total precious metal content of the third and fourth filters was identical.

(53) Each filter was hydrothermally oven-aged at 1100° C. for 4 hours and installed in a close-coupled position on a Euro 5 passenger car with a 1.4 L direct injection gasoline engine. Each filter was evaluated over a minimum of three MVEG-B drive cycles, measuring the reduction in particle number emissions relative to a reference catalyst. Peak backpressure (BP) was also evaluated in the same way as described in Example 1.

(54) Hydrocarbon light-off temperature (the temperature at which the catalyst catalyses the conversion of hydrocarbons in the feed gas at 50% efficiency or greater) was evaluated on a separate engine mounted in a laboratory test cell. This engine was a 2.0 litre turbo charged direct injection gasoline engine. The exhaust gas temperature was carefully regulated and increased from 250-450° C. over a given period of time through the use of a combination of a temperature heat sink and increasing throttle position, during which time the conversion efficiency of the catalyst was measured and reported.

(55) The results of zone coating the precious metal in the filter substrate are shown in Table 2, from which it can be seen that—as could be expected with identical washcoat loadings between the two filters—the % particle number reduction vs. the flow through reference catalyst (homogeneous 60 gft.sup.−3 precious metal content (57Pd:3Rh) at 3 gin.sup.−3 homogeneous washcoat loading on a 600 cells per square inch cordierite monolith substrate having the same dimensions as the third and fourth filters) are identical. However, the hydrocarbon light-off is higher for the Homogenous PGM configuration relative to the Zoned configuration. This can be attributed to the higher concentration of PGM on the inlet side.

(56) TABLE-US-00002 TABLE 2 Effect of PGM zoning on light-off temperature Peak BP % PN (mbar) reduction during HC vs. any one Sample light-off flow MVEG-B filter temperature through drive properties PGM zoning (° C.) reference cycle 20 μm, 65% Homogeneous 391 73 37.5 20 μm, 65% Zoned 379 73 35.8

(57) For the avoidance of any doubt, the entire contents of all prior art documents cited herein is incorporated herein by reference.