GB2545747A

GB2545747A - Gasoline particulate filter

Info

Publication number: GB2545747A
Application number: GB1522917.2A
Authority: GB
Inventors: Clowes Lucy; Destecroix Oliver; Benjamin Goodwin John; Anthony Howard Michael; Lakadamyali Fezile; Robson Chris
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 2015-12-24
Filing date: 2015-12-24
Publication date: 2017-06-28
Also published as: BR112018012782B1; KR20180098600A; CN108602050A; BR112018012782A2; DE112016005997T5; WO2017109514A1; RU2018126874A; GB201522917D0; EP3393654B1; RU2732400C2; GB2558332B; US20190009254A1; RU2018126874A3; GB2558332A; GB201604915D0; CN108602050B; JP6920304B2; EP3393654A1; US10625243B2; GB201707959D0

Abstract

A catalytic wall-flow monolith for use in an emission treatment system comprise a porous substrate and a three-way catalyst, wherein the three-way catalyst is distributed substantially throughout the porous substrate and where the three-way catalyst comprises: alumina; one or more platinum group metals; and an oxygen storage component, wherein the oxygen storage component comprises ceria or one or more mixed oxides comprising cerium and is present in a ratio by weight of oxygen storage component to alumina of 65:35-85:15. The weight ratio of oxygen storage component to alumina may be 75:25. The oxygen storage component may be a ceria zirconia mixed oxide. A method for the formation of the catalytic wall-flow monolith is also disclosed.

Description

GASOLINE PARTICULATE FILTER

Field of the Invention

The present invention relates to a particulate filter, in particular a catalyst-coated monolith for use in an emission treatment system of a positive ignition, e.g. spark ignition, gasoline-fuelled internal combustion engine, preferably a gasoline direct ignition engine. The monolith provides an effective method of remediating engine exhaust streams.

Background of the Invention

Gasoline engines produce combustion exhaust streams containing hydrocarbons, carbon monoxide, and oxides of nitrogen in conjunction with particulates. It is known to treat the gases with a three-way catalyst composition, and it is known to recover the particulates in particulate traps such as soot filters.

Historically, gasoline engines which are operated predominantly stoichiometrically have been designed such that low levels of particulates were formed. However, gasoline direct injection (GDI) engines, which are finding increasing application due to their fuel efficiency, can have lean burn conditions and stratified combustion resulting in the generation of particulates. Particulate emissions for engines fuelled by gasoline fuel, such as gasoline direct injection engines, are being subject to regulations and existing after-treatment systems for gasoline engines are not suitable for achieving the proposed particulate matter standard.

In contrast to particulates generated by diesel lean burning engines, the particulates generated by gasoline engines tend to be finer and at lower levels. This is due to the different combustion conditions of a diesel engine as compared to a gasoline engine. For example, gasoline engines run at a higher temperature than diesel engines.

Also, the resultant hydrocarbon components are different in the emissions of gasoline engines as compared to diesel engines.

Emission standards for unburned hydrocarbons, carbon monoxide and nitrogen oxide pollutants continue to become more stringent. In order to meet such standards, catalytic converters containing a three-way catalyst (TWC) are located in the exhaust gas line of gasoline-fuelled internal combustion engines. Such catalysts promote the oxidation by oxygen and oxides of nitrogen in the exhaust gas stream of unburned hydrocarbons and carbon monoxide, as well as the concomitant reduction of nitrogen oxides to nitrogen.

Emission legislation in Europe from 1st September 2014 (Euro 6) requires control of the number of particles emitted from both diesel and gasoline (positive ignition) passenger cars. For gasoline EU light duty vehicles the allowable limits are: 1000mg/km carbon monoxide; 60mg/km nitrogen oxides (NOx); 100mg/km total hydrocarbons (of which < 68mg/km are non-methane hydrocarbons); and 4.5mg/km particulate matter ((PM) for direct injection engines only). The Euro 6 PM standard will be phased in over a number of years with the standard from the beginning of 2014 being set at 6.0 x 1012 per km (Euro 6) and the standard set from the beginning of 2017 being 6.0 x 1011 per km (Euro 6c). In a practical sense, the range of particulates that are legislated for are between 23 nm and 3 pm.

In the United States, on 22nd March 2012, the State of California Air Resources Board (CARB) adopted new Exhaust Standards from 2017 and subsequent model year “LEV III” passenger cars, light-duty trucks and medium-duty vehicles which include a 3mg/mile emission limit, with a later introduction of 1mg/mi possible, as long as various interim reviews deem it feasible.

The new Euro 6 (Euro 6 and Euro 6c) emission standard presents a number of challenging design problems for meeting gasoline emission standards. In particular, how to design a filter, or an exhaust system including a filter, for reducing the number of PM gasoline (positive ignition) emissions, yet at the same time meeting the emission standards for non-PM pollutants such as one or more of oxides of nitrogen (NOx), carbon monoxide (CO) and unburned hydrocarbons (HC), all at an acceptable back pressure, e.g. as measured by maximum on-cycle backpressure on the EU drive cycle.

It is known in gasoline systems to provide a three-way catalyst (TWC) located on a substrate carrier, such as a flow-through monolith. It is also known to combine the TWC and particulate removal functions in a single device by coating a TWC onto a wall-flow monolith (particulate filter). An example is described in US2009/0193796.

Accordingly, it is desirable to provide an improved particulate filter and/or tackle at least some of the problems associated with the prior art or, at least, to provide a commercially useful alternative thereto. US 2010/275579A1 discloses a catalytically active particulate filter, an exhaust gas cleaning system and a process for cleaning the exhaust gases of predominantly stoichiometrically operated internal combustion engines, which are said to be suitable for removing particulates from the exhaust gas, as well as the gaseous CO, HC and NOx pollutants also. The particulate filter comprises a filter body and a catalytically active coating consisting of two layers. The first layer is in contact with the incoming exhaust gas, the second layer with the outgoing exhaust gas. Both layers contain alumina. The first layer contains palladium. The second layer contains, in addition to rhodium, an oxygen-storing cerium/zirconium mixed oxide. US 2009/087365A1 discloses a catalytically active particulate filter, an exhaust gas cleaning system and a process for cleaning the exhaust gases of predominantly stoichiometrically operated internal combustion engines, which are said to be suitable for removing particulates from the exhaust gas, as well as the gaseous CO, HC and NOx pollutants also. The particulate filter comprises a filter body and a catalytically active coating consisting of two layers. Both layers contain alumina. The first layer contains palladium. The second layer contains rhodium. The second layer is disposed above the first layer. WO2011133503 discloses exhaust systems and components suitable for use in conjunction with gasoline engines to capture particulates in addition to reducing gaseous emission such as hydrocarbons, nitrogen oxides, and carbon monoxides.

Exhaust treatment systems comprising a three-way conversion (TWC) catalyst located on a particulate filters are provided. Coated particle filters having washcoat loadings in the range of 1 to 4 g/ft3 are said to result in minimal impact on back pressure while simultaneously providing TWC catalytic activity and particle trapping functionality sufficient to meet Euro 6 emission standards. Relatively high levels of oxygen storage components (OSC) are said to be delivered on and/or within the filter. However, it is not possible from the information provided to determine a weight ratio of OSC:alumina, except in one embodiment, wherein the TWC catalytic material is substantially free of alumina, i.e. the ratio of OSC:alumina is °°. The filters can have a coated porosity that is substantially the same as its uncoated porosity. The TWC catalytic material can comprise a particle size distribution such that a first set of particles has a first d90 particle size of 7.5pm or less and a second set of particles has a second d90 particle size of more than 7.5 pm.

Summary of the Invention

According to a first aspect there is provided a catalytic wall-flow monolith for use in an emission treatment system, wherein the monolith comprises a porous substrate and a three-way catalyst (TWC), wherein the TWC is distributed substantially throughout the porous substrate and wherein the TWC comprises: (i) alumina; (ii) one or more platinum group metals; and (iii) an oxygen storage component (OSC), wherein the OSC comprises ceria or one or more mixed oxides comprising cerium and is present in a ratio by weight of OSC to alumina of from 65:35 to 85:15.

The present invention will now be further described. In the following passages different aspects of the invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

The present invention relates to a catalytic wall-flow monolith for use in an emission treatment system, such as an automobile exhaust. Wall-flow monoliths are well-known in the art for use in particulate filters. They work by forcing a flow of exhaust gases (including particulate matter) to pass through walls formed of a porous material.

The monolith preferably has a first face and a second face defining a longitudinal direction therebetween. In use, one of the first face and the second face will be the inlet face for exhaust gases and the other will be the outlet face for the treated exhaust gases.

As is conventional for a wall-flow monolith, it has first and second pluralities of channels extending in the longitudinal direction. The first plurality of channels is open at the first face and closed at the second face. The second plurality of channels is open at the second face and closed at the first face. The channels are preferably parallel to each other to provide a constant wall thickness between the channels. As a result, gases entering one of the plurality of channels cannot leave the monolith without diffusing through the channel walls into the other plurality of channels. The channels are closed with the introduction of a sealant material into the open end of a channel. Preferably the number of channels in the first plurality is equal to the number of channels in the second plurality, and each plurality is evenly distributed throughout the monolith.

Preferably the hydraulic channel diameter of the first and second pluralities of channels is 1 to 1.5 mm, defined as four times the flow area divided by the wetted perimeter. The channels may be of a constant width and each plurality of channels may have a uniform channel width. Preferably, however, the plurality of channels that serves as the inlet in use has a greater mean cross-sectional width than the plurality of channels that serves as the outlet. Preferably, the difference is at least 10%. This affords an increased ash storage capacity in the filter, meaning that a lower regeneration frequency can be used. The wetted perimeter of the channels can be measured using known microscopic techniques such as SEM or TEM.

Preferably the mean minimum thickness of the substrate between adjacent channels is from 8 to 20 mil (where a “mil” is 1/1000 inch) (0.02 to 0.05 cm). This can be measured using known microscopic techniques such as SEM or TEM. Since the channels are preferably parallel and preferably have a constant width, the minimum wall thickness between adjacent channels is preferably constant. As will be appreciated, it is necessary to measure the mean minimum distance to ensure a reproducible measurement. For example, if the channels have a circular cross-section and are closely packed, then there is one clear point when the wall is thinnest between two adjacent channels.

Preferably within a plane orthogonal to the longitudinal direction, the monolith has from 100 to 500 channels per square inch (cpsi), preferably from 200 to 400 cpsi. For example, on the first face, the density of open first channels and closed second channels is from 200 to 400 channels per square inch. The channels can have cross sections that are rectangular, square, circular, oval, triangular, hexagonal, or other polygonal shapes.

In order to facilitate the passage of gases to be treated through the channel walls, the monolith is formed out of a porous substrate. The substrate also acts as a support for holding catalytic material. Suitable materials for forming the porous substrate include ceramic-like materials such as cordierite, silicon carbide, silicon nitride, zirconia, mullite, spodumene, alumina-silica-magnesia or zirconium silicate, or of porous, refractory metal. Wall-flow substrates may also be formed of ceramic fiber composite materials. Preferred wall-flow substrates are formed from cordierite and silicon carbide. Such materials are able to withstand the environment, particularly high temperatures, encountered in treating the exhaust streams and can be made sufficiently porous. Such materials and their use in the manufacture of porous monolith substrates is well known in the art.

Preferably the porous substrate prior to coating has a porosity of 30-70%, such as 40-65%, most preferably >50% such as >55% e.g. 55-70% Suitable techniques for determining porosity are known in the art and include mercury porosimetry and x-ray tomography.

Preferably, the porous substrate can comprise a mean pore size in the range of 10-30pm, such as 13-25pm, 18-23pm, 15-25pm, 16-21 pm or 13-23pm. Suitable techniques for determining mean pore size of a porous substrate are known in the art and include mercury porosimetry.

The present invention relates to exhaust systems and components suitable for use in conjunction with gasoline engines, particularly gasoline direct injection (GDI) engines but also predominantly stoichiometrically operated gasoline engines to capture particulates in addition to treating gaseous emissions such as hydrocarbons, nitrogen oxides, and carbon monoxides. In particular, it relates to exhaust treatment systems comprising a three-way catalyst (TWC) and a particulate trap. That is, the particulate trap is provided with TWC catalyst compositions therein.

The monolith comprises a three-way catalyst (TWC). TWCs are well known in the art. The TWC is distributed substantially throughout the porous substrate. TWC compositions are generally provided in washcoats. Layered TWC catalysts can have different compositions for different layers. Traditionally, TWC catalysts can comprise washcoat layers having loadings of up to 2.5 g/in3and total loadings of 5 g/in3or more. For use with particulate traps, due to backpressure constraints, TWC catalyst washcoat layer are preferably from 1 g/in3 to 0.1 g/in3, preferably from 0.7 g/in3 to 0.25 g/in3, and most preferred from 0.6 g/in3 to 0.5 g/in3. This provides sufficient catalyst activity to oxidize carbon dioxide and hydrocarbons, as well as reduce nitrogen oxides (NOx).

The application may be characterised as “on wall” application or “in wall” application. The former is characterised by the formation of a coating layer on a surface of a channel. The latter is characterised by the infiltration of catalytic material into the pores within the porous material. The techniques for “in wall” or “on wall” application can depend on the viscosity of the material applied, the application technique (spraying or dipping, for example) and the presence of different solvents. Such application techniques are known in the art. The viscosity of the washcoat is influenced, for example, by its solids content. It is also influenced by the particle size distribution of the washcoat - a relatively flat distribution will give a different viscosity to a finely milled washcoat with a sharp peak in its particle size distribution - and rheology modifiers such as guar gums and other gums. Suitable coating methods are described in WO2011/080525, W01999/047260, WO2014/195685 and WO2015/145122, which are incorporated herein by reference.

The monolith described herein includes catalytic material distributed throughout the porous substrate. This material is included in the pores of the substrate, such as by infiltration with a washcoating method. This coats the pores and holds catalytic material therein, while maintaining sufficient porosity for the gases to penetrate through the channel walls.

The TWO comprises alumina, preferably gamma-alumina. This is an advantageous carrier material since it has a high surface area and is a refractory metal oxide. In other words, the alumina serves as a refractory support material. This lends the filter good thermal capacity which is required for the high-temperature conditions encountered.

The TWC also comprises one or more platinum group metals (e.g., platinum, palladium, rhodium, rhenium and iridium). These exhibit good activity and long life. Preferably the one or more platinum group metals is selected from Pt, Pd and Rh, or combinations of two or more thereof. The platinum group metals serve to catalyse the reactions required to remediate the exhaust gases.

Preferably the platinum group metal is Pt, Pd and Rh; Pd and Rh; or Pd only; or Rh only. ΝΟχ reduction is most effective in the absence of O2, whereas the abatement of CO and hydrocarbons requires O2. In order to convert all three components, the exhaust gas entering the TWC must be close to the “stoichiometric point” (14.7:1 air-to-fuel (AFR) mass ratio). The air-fuel equivalence ration, λ (lambda), is the ratio of actual AFR to stoichiometry for a given air/fuel mixture, λ =1.0 at stoichiometry, rich mixtures (mixtures generating reducing species such as unburned hydrocarbons (HCs) and CO in excess of oxidising species) λ < 1.0, i.e. an AFR of <14.7:1 and lean mixtures λ > 1, i.e. an AFR >14.7:1 .There is only a narrow window where simultaneous catalytic conversion of all three of NOx, CO and HC occurs.

While oxygen sensors provide feedback to keep the AFR within the desired window as much as possible, the the feedback causes perturbation about the stoichiometric point, meaning that the catalyst will alternately see slightly rich and slightly lean conditions. During dynamic driving, e.g. hard accelerations can enrich the exhaust gas before the feedback mechanism can regain control. Similarly, fuel cuts when a driver lifts off the accelerator can result in excessively lean exhaust gas. Therefore, in order to achieve as complete three-way conversion activity as possible, when operating rich, there is a need for the TWC to provide a small amount of O2 to consume the unreacted CO and HC. Conversely, when the exhaust becomes slightly oxidising, the excess O2 needs to be consumed.

Accordingly, the TWC also comprises an oxygen storage component (OSC). This is an entity that has multi-valence state and can actively react with oxidants such as oxygen or nitrous oxides under oxidative conditions, or reacts with reductants such as carbon monoxide (CO) or hydrogen under reducing conditions. Examples of suitable oxygen storage components include ceria, which is preferably stabilised with one or more additional oxides in a mixed oxide or composite oxide therewith. Praseodymia can also be included as an OSC. Delivery of an OSC to the washcoat layer can be achieved by the use of, for example, mixed oxides. For example, ceria can be delivered by a mixed oxide of cerium and zirconium, and/or a mixed oxide of cerium, zirconium, and neodymium. For example, praseodymia can be delivered by a mixed oxide of praseodymium and zirconium, and/or a mixed oxide of praseodymium, cerium, lanthanum, yttrium, zirconium, and neodymium.

The OSC comprises or consists of one or more mixed oxides. Preferably the OSC comprises a ceria and zirconia mixed oxide optionally including one or more rare earth element.

The ratio by weight of OSC to alumina is from 65:35 to 85:15, more preferably from 70:30 to 80:20 and most preferably about 75:25. Whereas it is conventional to provide the alumina and the OSC in a roughly 50:50 ratio when the TWC is provided as a separate unit, the inventors have found that when providing a combined particulate filter and catalytic treatment unit, the ratio of about 75:25 provided far better process efficiency. In particular, the additional oxygen storage capacity allows the device to operate across a range of conditions from start-up to full temperature, without having insufficient thermal mass to function or inability to adhere to the substrate. As a result of the improved oxygen storage capacity, NOx conversion under most conditions is improved. It has been found that above an upper limit of 85:15 OSC:alumina, the coating is too thermally unstable to function effectively.

Preferably the TWC is homogenous throughout the porous substrate. That is, the relative concentration of the components within the TWC are preferably constant throughout the pores of the porous substrate.

The TWC is preferably further present as a coating on a surface of at least one of the first and second plurality of channels. Preferably the TWC is coated on both the inlet side and the outlet side of the particulate trap.

According to a further aspect there is provided an emission treatment system for treating a flow of a combustion exhaust gas, the system comprising the catalytic wall-flow monolith as disclosed herein.

According to a further aspect there is provided a method for the manufacture of a catalytic wall-flow monolith, comprising: providing a porous 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 wherein the second plurality of channels is open at the second face and closed at the first face; infiltrating the porous substrate with a washcoat containing a catalytic material; and calcining the catalytic material within the porous substrate, wherein the catalytic material comprises: (i) alumina; (ii) one or more platinum group metals; and (iii) an oxygen storage component (OSC), wherein the OSC comprises ceria or one or more mixed oxides comprising cerium and is present in a ratio by weight of OSC to alumina of from 65:35 to 85:15.

Preferably the catalytic wall-flow monolith manufactured according to the foregoing method is the monolith as described herein. That is, all features of the first aspect may be freely combined with the further aspects described herein.

Preferably preparation of the washcoat before infiltrating the porous substrate includes mixing Pt, Pd and/or Rh with alumina and calcining to form a first portion of the catalytic material. The inventors have found that this treatment serves to pre-fix the platinum group metal.

Preferably preparation of the washcoat before infiltrating the porous substrate includes mixing Rh with the OSC and calcining to form a second portion of the catalytic material. The inventors have found that this treatment similarly serves to pre-fix the platinum group metal. This prevents the deactivation of the rhodium which can be observed when it is contacted with alumina.

The present invention permits the provision of the TWC in a single application step, rather than the multiple layers of prior art methods. This therefore avoids process complexity and high back pressures.

Brief Description of the Drawings

The invention will now be described in relation to the following non-limiting figures, in which: FIG. 1A is a perspective view that schematically shows a wall flow monolith filter 1 according to the present invention. FIG. 1B is an A-A line cross-sectional view of the wall flow monolith filter 1 shown in FIG. 1A. FIG. 2 shows a schematic diagram of an exhaust gas treatment system for a gasoline direct injection engine.

Detailed Description of the Invention A wall flow monolith 1 according to the present invention is shown in FIG. 1A and FIG. 1B. 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. 1A) of the monolith 1. The large number of channels includes a first subset of channels 5 and a second subset of channels 10.

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, the channels may alternatively be substantially the same size.

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.

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.

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.

Therefore, when the wall flow monolith is used in an exhaust system, exhaust gases G (in FIG. 1B “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.

The catalyst supported in the channel wall 35 of the monolith 1 functions as a catalyst for treating the exhaust fumes.

In the embodiment of the exhaust gas treatment system 100 shown in FIG. 2 an 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.

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.

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

Examples

Example 1 (comparative)

Four wall-flow filters (4 x 5” and 600/4 cell density) were coated with TWCs having a platinum group metal (PGM) composition of 40 [g/ft'3]/0:9:1 [Pt:Pd:Rh weight ratio]. Each TWC comprised a different weight ratio of AI2O3 to CeZrCU.

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. 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.

The results were as follows:

As can be seen from the table, increasing the CeZrO^AI2O3 ratio above 1:1 was found to be detrimental to the NOx conversion ability of the coated through-flow monolith.

Example 2

Four wall-flow filters (4.66x4.5” and 300/8 cell density) were coated with TWCs having PGM 60/0.57:3. Each TWC comprised a different weight ratio of AI2O3 to

CeZrCU. The coated filters were calcined and aged (Hydrothermal, 1100°C in air with 10% H20 added , 5h).

Using a 1.41 GTDI test engine, the NOx emissions were measured based on a standard engine test. The results were as follows:

As can be seen from the table, as the CeZrO^A^Os wt ratio increased from 1:2 to 3:1, the relative NOx emissions decreased.

Example 3

Three wall-flow filters (4.66x4.5” and 300/8 cell density) were coated with TWCs having PGM 22/0:20:2. Each TWC comprised a different weight ratio of AI2O3 to CeZr04. The coated filters were calcined and aged as in Example 2.

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

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.

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.

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.

Claims

CLAIMS:

1. A catalytic wall-flow monolith for use in an emission treatment system, wherein the monolith comprises a porous substrate and a three-way catalyst (TWC), wherein the TWC is distributed substantially throughout the porous substrate and wherein the TWC comprises: (i) alumina; (ii) one or more platinum group metals; and (iii) an oxygen storage component (OSC), wherein the OSC comprises ceria or one or more mixed oxides comprising cerium and is present in a ratio by weight of OSC to alumina of from 65:35 to 85:15

2. The catalytic wall-flow monolith according to claim 1, wherein the OSC comprises a ceria and zirconia mixed oxide.

3. The catalytic wall-flow monolith according to claim 1 or claim 2, wherein the ratio by weight of OSC to alumina is about 75:25.

4. The catalytic wall-flow monolith according to any of the preceding claims, wherein the one or more platinum group metals is selected from Pt, Pd and Rh, or combinations of two or more thereof.

5. The catalytic wall-flow monolith according to any of the preceding claims, wherein the TWC is homogenous throughout the porous substrate.

6. The catalytic wall-flow monolith according to any of the preceding claims, wherein the monolith has 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 wherein the second plurality of channels is open at the second face and closed at the first face, and wherein the TWC is further present as a coating on a surface of at least one of the first and second plurality of channels.

7. A catalytic wall-flow monolith according to claim 6, wherein the mean minimum thickness of the substrate between adjacent channels is from 8 to 20 mil (0.02 to 0.05 cm).

8. An emission treatment system for treating a flow of a combustion exhaust gas, the system comprising the catalytic wall-flow monolith according to any of the preceding claims.

9. A positive ignition engine comprising an emission treatment system according to claim 8.

10. An automobile comprising an engine according to claim 9.

11. A method for the manufacture of a catalytic wall-flow monolith, comprising: providing a porous 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 wherein the second plurality of channels is open at the second face and closed at the first face; infiltrating the porous substrate with a washcoat containing a catalytic material; and calcining the catalytic material within the porous substrate, wherein the catalytic material comprises: (i) alumina; (ii) one or more platinum group metals; and (iii) an oxygen storage component (OSC), wherein the OSC comprises ceria or one or more mixed oxides comprising cerium and is present in a ratio by weight of OSC to alumina of from 65:35 to 85:15.

12. The method according to claim 11, wherein preparation of the washcoat before infiltrating the porous substrate includes: mixing Pt and/or Pd with alumina and calcining to form at a first portion of the catalytic material.

13. The method according to claim 11 or claim 12, wherein preparation of the washcoat before infiltrating the porous substrate includes: mixing Rh with the OSC and calcining to form a second portion of the catalytic material.

14. The method according to any of claims 11 to 13, wherein the catalytic wall-flow monolith is the monolith according to any of claims 1 to 7.