Patent ID: 12239968

DESCRIPTION OF PREFERRED EMBODIMENTS

An embodiment of the present invention will be described with reference to the drawings. Matters not specifically mentioned in the description but required for carrying out the invention (e.g., general information on arrangement of a particulate filter in an exhaust passage) can be understood as matters of design variation of a person skilled in the art based on related art in the field. The present invention can be carried out on the basis of the contents disclosed in the description and common general knowledge in the field. The expression “A to B” representing a numerical value range herein refers to “A or more and B or less.”

A. Overall Configuration

First, an overall configuration of an exhaust gas purification device using a particulate filter according to a preferred embodiment will be describedFIG.1schematically illustrates an exhaust system in which a particulate filter according to this preferred embodiment is disposed. In the exhaust system illustrated inFIG.1, an exhaust gas purification device1is disposed in an exhaust passage of an internal combustion engine2.

The internal combustion engine2is supplied with an air-fuel mixture including oxygen and a fuel gas. The internal combustion engine2converts thermal energy obtained by combustion of this air-fuel mixture to kinetic energy. Then, an exhaust gas generated by combustion of the air-fuel mixture is emitted to an exhaust passage constituted by an exhaust manifold3and an exhaust pipe4, as indicated by an arrow inFIG.1. In this specification, a side toward the internal combustion engine2in a flow direction of an exhaust gas will be referred to as an upstream side, and a side away from the internal combustion engine2will be referred to as a downstream side, for convenience of description.

The exhaust gas purification device1purifies an exhaust gas emitted into the exhaust passage. The exhaust gas purification device1includes an engine control unit (ECU)7and a sensor8. The sensor8detects information on components and temperature of an exhaust gas. The ECU7receives detection results of the sensor8as information for controlling operation of the internal combustion engine2. In addition, the exhaust gas purification device1illustrated inFIG.1includes a catalyst portion5and a filter portion6.

The catalyst portion5is disposed inside the exhaust pipe4. The catalyst portion5can use an exhaust gas purifying catalyst for purifying ternary components (NOx, HC, and CO) in an exhaust gas. A specific structure of the exhaust gas purifying catalyst used in the catalyst portion5is not a feature of the present invention, and thus, detailed description thereof is omitted. In the exhaust gas purification device1illustrated inFIG.1, the catalyst portion5is disposed upstream of the filter portion6, but the location of the catalyst portion is not specifically limited. For example, the catalyst portion may be disposed downstream of the filter portion, or a pair of catalyst portions may be respectively disposed at the upstream side and the downstream side of the filter portion.

The filter portion6collects particulate matter (PM) in an exhaust gas to thereby purify the exhaust gas. The particulate filter according to this preferred embodiment can be used for the filter portion6of the exhaust gas purification device1. In other words, the particulate filter according to this preferred embodiment serves as a component of the exhaust gas purification device1and is disposed in an exhaust passage (exhaust pipe4) of the internal combustion engine2.

B. Particulate Filter

A particulate filter according to this preferred embodiment will be hereinafter described with reference toFIGS.2through5. Character X inFIGS.2and3denotes a “distribution direction of an exhaust gas.” Character X1denotes an “exhaust gas inflow side (upstream side)” and character X2denotes an “exhaust gas outflow side (downstream side).”FIG.2is a perspective view schematically illustrating the particulate filter according to this preferred embodiment.FIG.3schematically illustrates a cross section of the particulate filter illustrated inFIG.2taken along an extension direction (exhaust gas distribution directions X).FIG.4schematically illustrates a cross section taken along a radial direction of the particulate filter (direction perpendicular to the exhaust gas distribution directions X).FIG.5is a cross-sectional schematic view illustrating a region V inFIG.3in an enlarged manner.

As illustrated inFIGS.2through5, a particulate filter100according to this preferred embodiment includes a base material10having a wall-flow structure, and wash-coating layers20formed in partition walls16of the base material10. These components will be described below.

1. Base Material

As illustrated inFIG.2, the particulate filter100according to this preferred embodiment uses the cylindrical base material10extending along the exhaust gas distribution directions X. As described above, the base material10has a wall-flow structure. Specifically, the base material10has a honeycomb structure including a plurality of hollow portions (cells12and14). These cells12and14extend along the exhaust gas distribution directions X. As illustrated inFIGS.3and4, the cells12and14of the base material10in this preferred embodiment are constituted by inlet cells12and outlet cells14adjacent to the inlet cells12. The inlet cells12and the outlet cells14are separated from one another by the porous partition walls16. As the material of the base material10, various materials that can be used for conventional applications of this type may be used without particular limitation. Examples of the material of the base material10include ceramics such as cordierite and silicon carbide (SiC) and alloys (e.g., stainless). In this preferred embodiment, although the cylindrical base material10is used, the outer shape of the base material is not specifically limited, and may be an oval or a polygonal prism, for example.

The inlet cells12refer to cells that are open only at the exhaust gas inflow side X1(seeFIG.3) among the cells12and14formed in the base material10. Specifically, the inlet cells12are open to the outside of the filter with the ends at the exhaust gas inflow side X1serving as gas inflow ports12a, and with the ends at the exhaust gas outflow side X2sealed by sealing portions12b. On the other hand, the outlet cells14refer to cells that are open only at the exhaust gas outflow side X2. Specifically, the ends of the outlet cells14at the exhaust gas inflow side X1are sealed by sealing portions14a, and the ends of the outlet cells14at the exhaust gas outflow side X2serve as gas outflow ports14band are open to the outside of the filter.

The shapes and sizes of the inlet cells12and the outlet cells14may be suitably changed in consideration of a flow rate and components of an exhaust gas supplied to the particulate filter100. For example, as illustrated inFIG.4, in this preferred embodiment, the cells12and14have square cross-sectional shape in a cross section orthogonal to the extension directions X of the base material10(cross section along the radial direction of the base material10). The cross-sectional area of the inlet cells12and the cross-sectional area of the outlet cells14may be approximately the same, or may be different in consideration of a flow rate of an exhaust gas. The cross-sectional shapes of the cells12and14are not limited to the square as described in this preferred embodiment, and general shapes such as a parallelogram, a rectangle, a trapezoid, a triangle, a pentagon, or a circle may be employed without any particular limitation.

As illustrated inFIGS.2and4, the base material10is formed such that the inlet cells12and the outlet cells14are adjacent to one another. The base material10of this preferred embodiment are formed such that the inlet cells12and the outlet cells14having square cross sections are arranged in a checkerboard pattern.

As described above, the inlet cells12and the outlet cells14are partitioned from one another by the partition walls16. In this preferred embodiment, each of the partition walls16formed in an array extends along the exhaust gas distribution directions X, and spaces surrounded by the partition walls16serve as the cells12and14(seeFIGS.3and4). The partition walls16have a porous structure having a plurality of pores. Specifically, wall bodies17of the partition walls16have a plurality of pores18as illustrated inFIG.5, and inlet cells and outlet cells communicate with one another through some of the pores18. Accordingly, as indicated by arrows inFIG.3, an exhaust gas that has flowed into the inlet cells12passes through the partition walls16and flows out to the outlet cells14. The thickness T and the overall length L of the partition walls16are preferably adjusted from the viewpoint of obtaining both PM collection performance and pressure loss suppression performance. For example, the thickness T of the partition walls16is preferably about 0.2 mm to about 1.6 mm. The overall length L of the partition walls16is preferably about 50 mm to about 500 mm (more preferably 100 mm to 200 mm).

From the viewpoint of suppressing an increase in pressure loss, a pore ratio of the partition walls16is preferably 40% or more, more preferably 45% or more, much more preferably 50% or more, and especially preferably 55% or more. On other hand, from the viewpoint of maintaining mechanical strength of the base material10, the upper limit of the pore ratio of the partition walls16is preferably 80% or less, more preferably 75% or less, much more preferably 70% or less, and especially preferably 65% or less. The “pore ratio of the partition wall” herein refers to the proportion of the volume of the pores18to the total volume (total volume of the wall bodies17and the pores18) of the partition walls16of the base material10, and is a value measured by mercury intrusion porosimetry.

From the viewpoint of enhancing pressure loss suppression performance, the average pore size of the pores18is preferably 1 μm or more, more preferably 5 μm or more, much more preferably 7 μm or more, and especially preferably 10 μm or more. On the other hand, from the viewpoint of enhancing PM collection performance, the upper limit of the average pore size of the pores18is preferably 50 μm or less, more preferably 40 μm or less, much more preferably 30 μm or less, and especially preferably 25 μm or less. The “average pore size of the pores18” herein refers to an average value in pore distribution obtained by mercury intrusion porosimetry.

2. Wash-Coating Layer

As illustrated inFIG.3, the wash-coating layers20are coating layers formed inside the partition walls16of the base material10. Specifically, as illustrated inFIG.5, the wash-coating layers20are porous heat-resistance layers formed on the wall surfaces of the pores18(i.e., surfaces of the wall bodies17in contact with the pores18). The wash-coating layers20can have the function of enhancing stability at high temperatures and absorbing property, for example. The wash-coating layers20can contribute to enhancement of PM collection performance achieved by increasing the surface area and reducing the size of the pores18. The wash-coating layers20in this preferred embodiment can use materials known to date without any particular limitation. The wash-coating layers20are typically made of a heat-resistant material as a main component. Typically, the content of the heat-resistant material in the wash-coating layers20is preferably 50% by mass or more and more preferably 85% by mass or more. As the heat-resistant material, a refractory defined by JIS R2001 can be used. Examples of the refractory include neutral refractories such as alumina (Al2O3), acid refractories such as silica (SiO2) and zirconia (ZrO2), and basic refractories such as magnesia (MgO) and calcia (CaO). Among these refractories, alumina (preferably activated alumina) is preferable. The heat-resistant material of the wash-coating layers20may be constituted by only one of the refractories listed above, or a mixture (or a complex) of two or more of these refractories. Examples of the complex include ceria-zirconia complex oxide. The wash-coating layers20may include other materials (typically an inorganic oxide) as accessory ingredients. Examples of the accessory ingredients include rare earth metal oxides such as yttria (Y2O3) and alkaline earth metal oxides such as barium oxide (BaO).

As illustrated inFIG.3, each of the wash-coating layers20in this preferred embodiment includes an inlet layer22and an outlet layer24. The thicknesses and lengths of the inlet layer22and the outlet layer24are set such that the inlet layer22and the outlet layer24partially overlap with each other. These layers will be described below.

(1) Inlet Layer

The inlet layer22is a wash-coating layer formed in a region including the partition wall16near the gas inflow port12a. Specifically, the inlet layer22is formed to have a predetermined thickness TAfrom the surface of the partition wall16in contact with the inlet cell12toward the inside of the partition wall16, and has a predetermined length from a vicinity of an end at the exhaust gas inflow side X1along the extension directions of the partition wall16(exhaust gas distribution directions X). From the viewpoint of enhancing PM collection performance, the thickness TAof the inlet layer22is preferably 50% or more of the thickness T of the partition wall16, more preferably 60% or more, much more preferably 70% or more, and especially preferably 75% or more. The upper limit of the thickness TAof the inlet layer22is not specifically limited, and may be 100% or less of the thickness T of the partition wall16, may be 95% or less, or may be 90% or less. As described above, a region where only the inlet layer22is formed will be hereinafter referred to as an “inlet region.”

In the particulate filter100according to this preferred embodiment, the inlet layer22contains substantially no noble metal catalyst. Although specifically described later, a noble metal catalyst is a catalytic material having the function of promoting PM combustion, and includes a noble metal such as gold (Au), silver (Ag), palladium (Pd), rhodium (Rh), platinum (Pt), ruthenium (Ru), iridium (Ir), or osmium (Os). The presence of the inlet layer22containing substantially no noble metal catalyst can suppress combustion of PM accumulated in the inlet region. Thus, in the particulate filter100according to this preferred embodiment, PM collection performance can be easily enhanced because of an increase in the PM accumulation amount in the inlet region.

The expression “containing substantially no noble metal catalyst” herein means that components that can be interpreted as a noble metal catalyst is not added intentionally. Thus, the concept of “containing substantially no noble metal catalyst” herein includes a case where a trace amount of a component that can be interpreted as a noble metal catalyst is included from, for example, a raw material or a manufacturing process and a case where a trace amount of a noble metal catalyst has moved from another catalyst layer into the inlet layer (e.g., a case where a noble metal catalyst has moved from the outlet layer into the inlet layer). For example, the state of “containing substantially no noble metal catalyst” is established in a case where the content (g/L) of a noble metal catalyst with respect to the volume of the inlet layer22is 0.05 g/L or less (preferably 0.03 g/L or less, more preferably 0.01 g/L or less, much more preferably 0.005 g/L or less, and especially preferably 0.001 g/L or less).

(2) Outlet Layer

The outlet layer24is a wash-coating layer formed in a region including the partition wall16near the gas outflow port14b. Specifically, the outlet layer24is formed to have a predetermined thickness TB from the surface of the partition wall16in contact with the outlet cell14toward the inside of the partition wall16and have a predetermined length LBfrom a vicinity of an end at the exhaust gas outflow side X2along the extension directions of the partition wall16(exhaust gas distribution directions X). From the viewpoint of enhancing PM collection performance, the thickness TB of the outlet layer24is preferably 50% or more of the thickness T of the partition wall16, more preferably 60% or more, much more preferably 70% or more, and especially preferably 75% or more. The upper limit of the thickness TB of the outlet layer24is not specifically limited, and may be 100% or less of the thickness T of the partition wall16, may be 95% or less, may be 90% or less, and may be 85% or less. A region where the outlet layer24is formed herein will be referred to as an “outlet region.” This “outlet region” includes a region where the inlet layer22and the outlet layer24overlap with each other.

In the particulate filter100according to this preferred embodiment, unlike the inlet layer22described above, the outlet layer24contains a noble metal catalyst. As described above, the noble metal catalyst is a catalytic material having the function of promoting PM combustion, and contains at least one of noble metal elements such as Au, Ag, Pd, Rh, Pt, Ru, Ir, and Os. Among these elements, platinum group elements such as Pt, Pd, and Rh have excellent function of promoting PM combustion, and thus, are especially preferable as noble metals contained in the outlet layer24. In addition to the noble metal described above, the noble metal catalyst may include a carrier supporting this noble metal. Examples of a material for the carrier include alumina (Al2O3), rare earth metal oxides, alkali metal oxides, alkaline earth metal oxides, zirconia (ZrO2), ceria (CeO2), silica (SiO2), magnesia (MgO), and titania (TiO2).

The presence of the noble metal catalyst in the outlet layer24can promote combustion of PM accumulated in the outlet region. Accordingly, in the particulate filter100according to this preferred embodiment, clogging of the pores18of the partition wall16in the outlet region with PM can be prevented so that high gas distributability can be maintained in the outlet region.

The content of the noble metal catalyst in the outlet layer24(the content g of the noble metal catalyst with respect to a volume of 1 L of the outlet layer24) is preferably 0.1 g/L or more. Accordingly, higher gas distributability can be maintained in the outlet region. From the viewpoint of maintaining higher gas distributability in the outlet region, the content of the noble metal catalyst in the outlet layer24is more preferably 0.5 g/L or more, much more preferably 0.7 g/L or more, and especially preferably 1 g/L or more. The upper limit of the noble metal catalyst in the outlet layer24is not specifically limited, and may be 20 g/L or less. From the viewpoint of easily forming the outlet layer24, the content is preferably 10 g/L or less, more preferably 7 g/L or less, much more preferably 5 g/L or less, and especially preferably 2 g/L or less.

(3) Relationship Between Inlet Layer and Outlet Layer

As described above, in the particulate filter100according to this preferred embodiment, the inlet layer22and the outlet layer24partially overlap with each other in the extension directions of the partition walls16(exhaust gas distribution directions X), as illustrated inFIG.3. This can ensure prevention of the presence of a region where no wash-coating layer is formed in the partition wall16. At this time, the length LAof the region where only the inlet layer22is formed (i.e., the inlet region including no noble metal catalyst) is preferably 30% or more of the overall length L (100%) of the partition wall16, more preferably 35% or more, and much more preferably 40% or more. As described above, by securing the length LAof the inlet region at the given level or more, an exhaust gas G1in the initial stage of operation is allowed to pass through the inlet region appropriately. On the other hand, the upper limit of the length LAof the inlet region is preferably 70% or less, more preferably 65% or less, and much more preferably 60% or less. Accordingly, the length LBof the outlet region can be sufficiently long so that an exhaust gas G2in the intermediate stage of operation is allowed to pass through the outlet region appropriately. A preferred example of the length LAof the inlet layer is 55%.

In the particulate filter100according to this preferred embodiment, the inlet layer22contains substantially no noble metal catalyst, and the outlet layer24contains a noble metal catalyst. Accordingly, high levels of PM collection performance and pressure loss suppression performance can be achieved.

Specifically, as illustrated inFIG.3, the exhaust gas G1in the initial stage of operation flows at a low flow rate, and thus, easily passes through the inlet region of the partition wall16. Although the exhaust gas G1in the initial stage of operation contains a relatively large amount of PM, since PM collection performance can be easily enhanced in the inlet region where substantially no noble metal catalyst is present in the particulate filter100according to this preferred embodiment, PM can be appropriately removed from the exhaust gas G1in the initial stage of operation. In addition, the flow rate of the exhaust gas G1in the initial stage of operation is relatively small, and thus, even when the exhaust gas G1passes through the inlet region where the pores18are maintained in a relatively small size, an abrupt increase of a pressure loss does not easily occur.

On the other hand, since the flow rate of the exhaust gas G2in the intermediate stage of operation is high, the exhaust gas G2passes through the outlet region of the partition wall16. In the particulate filter100according to this preferred embodiment, since high gas distributability is maintained in the outlet region where the noble metal catalyst is present, even when the exhaust gas in the intermediate stage of operation at a high flow rate is applied, an abrupt increase in pressure loss can be appropriately suppressed. In addition, since the exhaust gas G2in the intermediate stage of operation has a small PM content, PM can also be sufficiently removed from the outlet region where the pores18are maintained in a relatively large size.

(4) Total Coating Amount of Wash-Coating Layer

In the particulate filter100according to this preferred embodiment, the total coating amount of the wash-coating layers20can affect both pressure loss suppression effect and PM collection performance. Thus, the total coating amount of the wash-coating layers20is preferably appropriately adjusted as necessary. The “total coating amount of the wash-coating layers” herein refers to a total weight of the wash-coating layers20including the inlet layers22and the outlet layers24. As the total coating amount increases, the thickness of the wash-coating layers20adhered to the wall surfaces of the pores18increases, and thus, the size of the pores18decreases so that PM collection performance can be thereby enhanced (seeFIG.5). On the other hand, when the total coating amount excessively increases, wash-coating layers are formed on the outside of the pores18(i.e., surfaces16aof the partition walls16adjacent to the cells12and14). In this case, a pressure loss might increase abruptly, since the cells12and14are blocked by the wash-coating layers. As a result of experiments conducted by the inventors in consideration of these points, it was found that favorable PM collection performance is obtained by setting a ratio (W/V) of the total coating amount W of the wash-coating layers20to an effective volume V of the pores18of the partition walls16in the entire base material10at 200 g/L or more (preferably 250 g/L or more). It was also found that an abrupt increase of a pressure loss is prevented by setting the upper limit of the ratio W/V at 400 g/L or less (preferably 350 g/L or less).

The “effective volume V of pores of the partition walls in the entire base material” described above is obtained by multiplying the “pore ratio of the partition walls” by the “volume of the effective partition walls.” As described above, the “pore ratio of the partition walls” refers to the ratio of the volume of the pores18to the total volume of the partition walls16of the base material10(total volume of the wall bodies17and the pores18). On the other hand, the “volume of the effective partition walls” refers to the volume of partition walls through which an exhaust gas can pass. Specifically, an exhaust gas does not pass through regions16bwhere the partition walls16intersect with each other inFIG.4and regions16cin contact with the sealing portions12band14ainFIG.3. The “volume of the effective partition walls” refers to the volume of the partition walls16excluding the regions through which an exhaust gas does not pass. The “volume of the effective partition walls” is obtained by subtracting the volume of the cells12and14from the volume of the base material10. At this time, the “volume of the cells12and14” can be calculated by multiplying the sum of the opening areas of the cells12and14in a front view as illustrated inFIG.4by the overall length L of the base material10.

From the viewpoint of ensuring prevention of an abrupt increase in a pressure loss, the ratio W/V is preferably adjusted such that substantially no wash-coating layers20are present on the surfaces of the partition walls16. The expression “substantially no wash-coating layers are present on the surfaces of the partition walls” means that, supposing the total coating amount W is 100%, the amount of coating present inside the pores18of the partition walls16is 90% or more (preferably 95% or more).

(5) Other Materials

The wash-coating layers of the particulate filter disclosed here can be supplemented with other materials as long as the essence of the present invention is not impaired. Examples of materials that can be added to the wash-coating layers include a material having an oxygen storage/release capacity (OSC material), a NOx absorbent, and a selective catalytic reduction (SCR) catalyst.

The OSC material is a material that stores oxygen when the oxygen concentration in an exhaust gas is high (i.e., when the air-fuel ratio is lean) and releases oxygen when the oxygen concentration in an exhaust gas is low (i.e., when the air-fuel ratio is rich). Examples of the OSC material include a material based on cerium oxide (ceria: CeO2). Examples of the material based on CeO2include a CZ-based composite material (CeO2—ZrO2composite oxide). The CZ-based composite material is polycrystal or single crystal including CeO2and ZrO2as main components. Various additional components may be added to the CZ-based composite material. Examples of the additional components include rare-earth oxides, alkaline earth metal oxides, transition metals, alumina, and silica.

The OSC material has the function of maintaining an oxygen gas passing through the partition walls16in an oxidizing atmosphere, and thus, can exhibit the function of promoting PM combustion. Thus, even in the case of using the OSC material, the inlet layers22preferably contain substantially no OSC material. In this case, higher levels of pressure loss suppression performance and PM collection performance can be achieved. The expression that “the inlet layers contain substantially no OSC material” means that components that can be interpreted as the OSC materials are not intentionally added, similarly to the noble metal catalyst described above. That is, the case where “the inlet layers contain substantially no OSC material” is established when the ratio (g/L) of the content g of the OSC material to a volume of 1 L of the inlet layers22is 5 g/L or less (preferably 3 g/L or less, more preferably 2 g/L or less, much more preferably 1 g/L or less, and especially preferably 0.5 g/L or less).

The NOxabsorbent is a material that stores NOxin an oxygen gas when the air-fuel ratio of the exhaust gas is in a lean state where oxygen is excessive, and releases NOxwhen the air-fuel ratio changes to a rich state. As the NOxabsorbent, a basic material including one or more of metals that can provide electrons to NOxcan be preferably used. For example, alkali metals such as potassium (K), sodium (Na), and cesium (Cs), alkaline earth metals such as barium (Ba) and calcium (Ca), rare earths such as lanthanoid, and metals such as silver (Ag), copper (Cu), iron (Fe), and iridium (Ir) are preferably used. Among these materials, barium compounds (e.g., barium sulfate) having high NOxstorage capacity are preferable.

The SCR catalyst only needs to purify nitrogen oxide (NOx) in an exhaust gas. The SCR catalyst is not limited to a specific material, and may be β-zeolite or silicoaluminophosphate (SAPO)-based zeolite, for example, may be used. Examples of the SAPO include SAPO-5, SAPO-11, SAPO-14, SAPO-17, SAPO-18, SAPO-34, SAPO-39, SAPO-42, and SAPO-47. The SCR catalyst may include any metal component. Examples of the metal component include copper (Cu), iron (Fe), sodium (Na), potassium (K), magnesium (Mg), calcium (Ca), cobalt (Co), nickel (Ni), zinc (Zn), silver (Ag), lead (Pb), vanadium (V), chromium (Cr), molybdenum (Mo), yttrium (Y), cerium (Ce), neodymium (Nd), tungsten (W), indium (In), and iridium (Ir). The presence of the metal in the SAPO can purify NOx more efficiently. In the case where the wash-coating layers20contain the SCR catalyst, a reducing agent supply unit that supplies a reducing agent (e.g., urea water) for generating ammonia is preferably disposed upstream of the particulate filter (e.g., upstream of the filter portion5inFIG.1).

C. Applications

As described above, the particulate filter100according to this preferred embodiment can be disposed, as the filter portion5for removing PM in an exhaust gas, in the exhaust passage of the internal combustion engine2(seeFIG.1). However, the particulate filter disclosed here is not limited to this example, and can be used for various applications. For example, the particulate filter disclosed here contains the noble metal catalyst in the outlet layer, and thus, can serve as a three-way catalyst for purifying hydrocarbon (HC), carbon monoxide (CO), and nitrogen oxide (NOx). Thus, the particulate filter disclosed here can be used as an exhaust gas purifying catalyst having the functions of both the catalyst portion5and the filter portion6inFIG.1. In the case of using the particulate filter disclosed here as a three-way catalyst, the OSC material or the NOx absorbent described above, for example, is preferably added to the wash-coating layers.

Although it is not intended to limit the present invention, the particulate filter disclosed here is especially preferably used in a case where the internal combustion engine2is a gasoline engine for an automobile. An exhaust gas emitted from a gasoline engine is at a relatively high temperature, and thus, PM tends not to be easily accumulated in pores of partition walls. On the other hand, in the particulate filter disclosed here, since substantially no noble metal catalyst is present in the inlet region, PM can be suitably accumulated in the inlet region. Thus, even in the case where the particulate filter disclosed here is applied to the gasoline engine, PM collection performance can be suitably enhanced.

The particulate filter disclosed here is not necessarily applied to the gasoline engine, and may be used for purifying an exhaust gas from other engines (e.g., diesel engine). In particular, in the case where the SCR catalyst is added to the wash-coating layer and the reducing agent supply unit is disposed upstream of the particulate filter as described above, the particulate filter serves as both an SCR device for purifying NOx included in an exhaust gas from a diesel engine and a filter portion for removing PM.

D. Producing Method of Particulate Filter

An example of a method for producing the particulate filter100according to this preferred embodiment will be described. The particulate filter disclosed here is not limited to the particulate filter produced by the following method.

The particulate filter100according to this preferred embodiment can be produced by, for example, preparing a slurry including materials for the wash-coating layers20and introducing the slurry into the pores18of the partition walls16of the base material10. Process steps of the method will be described below.

(1) Preparation of Slurry

In this process step, a slurry is prepared by dispersing the materials for the wash-coating layers20described above in a predetermined dispersion medium. As the dispersion medium, any dispersion media that can be used for preparing a slurry of this type can be used without any particular limitation. For example, the dispersion medium may be a polar solvent (e.g., water) or a non-polar solvent (e.g., methanol). The slurry may include an organic component for adjusting viscosity, in addition to the materials for the wash-coating layers20and the dispersion medium described above. Examples of the organic component for adjusting viscosity include cellulose-based polymers such as carboxymethyl cellulose (CMC), methyl cellulose (MC), hydroxypropyl methyl cellulose (HPMC), and hydroxyethylmethyl cellulose (HEMC).

In producing the particulate filter100according to this preferred embodiment, two types of slurries: a slurry for the inlet layer (inlet layer slurry) containing substantially no noble metal catalyst and a slurry for an outlet layer (outlet layer slurry) containing a noble metal catalyst, are prepared. The noble metal catalyst has been already described above, and thus, will not be described in detail below. The inlet layer slurry and the outlet layer slurry may be made of different materials except for the noble metal catalyst. For example, the inlet layer slurry and the outlet layer slurry can be formed to have different viscosities by varying the amount of addition and the type of the organic components so that regions where the inlet layer and the outlet layer are formed can be thereby easily adjusted.

(2) Introduction of Slurry

In this process step, the wash-coating layers20are formed by introducing the slurry described above into the pores18of the partition walls16. The technique for introducing the slurry into the pores18is not specifically limited, and techniques known to date can be used without any particular limitation. Example of the method for introducing the slurry include an air blow technique and a suction coating technique. In the air blow technique, an end portion of the base material10is immersed in the slurry so that the slurry permeates in the cells12and14, and then the base material10is taken out and air blow is performed, thereby introducing the slurry into the pores18. On the other hand, in the suction coating technique, with an end portion of the base material10immersed in the slurry, the slurry is sucked from the other end portion of the base material10, thereby introducing the slurry into the pores18.

In the particulate filter100according to this preferred embodiment, the wash-coating layers20including the inlet layer22and the outlet layer24are formed. To form the wash-coating layers20with the suction coating technique, the gas inflow ports12aof the base material10are first immersed in the inlet layer slurry, and in this state, the slurry is sucked from the gas outflow ports14b. Accordingly, the inlet layer slurry is applied from a vicinity of an end at the exhaust gas inflow side X1to a predetermined length and a predetermined thickness, and thus, the inlet layer22is formed by drying and calcining the slurry. Next, in a state where the gas outflow ports14bof the base material10are immersed in the outlet layer slurry, the slurry is sucked from the gas inflow ports12a. Accordingly, the outlet layer slurry is applied from a vicinity of an end at the exhaust gas outflow side X2to a predetermined length and a predetermined thickness, and thus, the outlet layers24are formed by drying and calcining the slurry. At this time, slurry viscosity and a suction force of a suction coating device are adjusted such that regions to be coated with the slurries are controlled to thereby cause the inlet layers22and the outlet layers24after calcination to partially overlap with each other.

The order of forming the inlet layers22and the outlet layers24is not specifically limited. That is, the inlet layers22may be formed after the outlet layers24are formed. In the method described above, after the inlet layers22are formed by drying and calcining the inlet layer slurry, the outlet layer slurry is introduced into the base material. Alternatively, after the inlet layer slurry is dried, the outlet layer slurry may be introduced such that both the inlet layer slurry and the outlet layer slurry are calcined at the same time. In these cases, the wash-coating layers20including both the inlet layers22and the outlet layers24can also be formed.

In addition, air blow may be performed after introducing the slurry and before drying the slurry. This can prevent the slurry from remaining in the cells12and14, and thus, formation of wash-coating layers on the surfaces16aof the partition walls16adjacent to the cells12and14(seeFIG.5) can be suppressed.

In the thus-produced particulate filter100, the inlet layers22contain substantially no noble metal catalyst, and the outlet layers24contain the noble metal catalyst. As described above, in the particulate filter100having the structure described above, PM collection performance can be easily enhanced in the inlet region, and a high level of pressure loss suppression performance can be maintained in the outlet region. Thus, high levels of PM collection performance and pressure loss suppression performance can be achieved as the whole of the filter.

Test Examples

Test examples concerning the present invention will be described below, but it is not intended to limit the present invention to the following test examples.

A. First Test

In this test example, a plurality of particulate filters were produced such that noble metal catalysts inside partition walls are present in different regions among the particulate filters, and PM collection performance and pressure loss suppression performance of each particulate filter were evaluated.

1. Preparation of Sample

(1) Sample 1

A palladium nitrate solution, alumina powder, barium oxide powder, and ion-exchanged water were mixed, thereby preparing a Pd-containing slurry. Then, in a state where a gas inlet port of a wall flow filter base material (made of cordierite, length: 152.4 mm, cell total volume: 1.7 L) was immersed in the Pd-containing slurry, and the slurry was sucked from the gas outflow port by using a suction coating device, thereby introducing the Pd-containing slurry to an upstream side of partitions of the base material to a predetermined length and a predetermined thickness. Thereafter, the slurry was dried and calcined to thereby form an inlet layer containing a noble metal catalyst (Pd). Subsequently, the gas outflow port was immersed in the Pd-containing slurry, and the Pd-containing slurry was sucked from the inlet port, thereby introducing the Pd-containing slurry to a downstream side of the partition walls to a predetermined length and a predetermined thickness. Thereafter, the slurry was dried and calcined to thereby form an outlet layer containing a noble metal catalyst (Pd). Table 1 shows “lengths LAand LB,” “coating amount (the amount of introduced slurry),” and “Pd content” in each of an inlet region and an outlet region.

(2) Sample 2

In this sample, a Pd-free slurry having the same composition as that of the Pd-containing slurry of sample 1 described above was prepared except that the palladium nitrate solution (precursor of a noble metal catalyst) is not contained. That is, alumina powder, barium oxide powder, and ion-exchanged water were mixed, thereby preparing a Pd-free slurry. Then, the Pd-free slurry was introduced was introduced to each of an upstream side and a downstream side of partition walls, and was dried and calcined, thereby producing a particulate filter in which none of the inlet layer and the outlet layer contains a noble metal catalyst (Pd). The other conditions were similar to those of sample 1.

(3) Sample 3

In this sample, the Pd-containing slurry described above was introduced to the upstream side of partition walls to thereby form an inlet layer containing a noble metal catalyst (Pd), and a Pd-free slurry was introduced to the downstream side of the partition walls to thereby form an outlet layer containing no noble metal catalyst. The other conditions were similar to those of sample 1.

(4) Sample 4

In this sample, the Pd-free slurry was introduced to the upstream side of partition walls to thereby form an inlet layer containing no noble metal catalyst (Pd) and the Pd-containing slurry was introduced to the downstream side of the partition walls to thereby form an outlet layer containing a noble metal catalyst. The other conditions were similar to those of sample 1.

2. Evaluation Test

(1) PM Collection Performance

In this evaluation, a PM collection ratio of each sample was measured to evaluate PM collection performance. Specifically, particulate filters of samples 1 through 4 were placed in an exhaust passage of a vehicle (2 L-gasoline engine), and the vehicle was driven in a Phase 4-mode by worldwide harmonized light duty driving test procedure (WLTP). Then, a PM emission amount X in a state where the particulate filter was installed and a PM emission amount Y in a state where the particulate filter was detached were measured, and PM collection ratio was calculated by an equation below. Table 1 shows results.
PM collection ratio (%)=[(Y−X)/Y]×100
(2) Pressure Loss Suppression Performance

A reproduction process was conducted on the particulate filter of each sample to measure a change of a pressure loss in this reproduction process. Specifically, a particulate filter in which PM was accumulated by the PM collection ratio measurement was attached to an engine bench, and a reproduction process of supplying a high-temperature exhaust gas (temperature: 500° C., air-fuel ratio: 14.7) for 60 minutes was performed. Thereafter, pressure losses (kPa) after 0 minutes (immediately after start), 30 minutes, and 60 minutes from the start of the reproduction process were measured, and a change of pressure loss suppression performance in the reproduction process was examined. In this test, in measuring a pressure loss (kPa), a PM accumulation weight (g) in the filter was also measured.FIG.6shows measurement results of the pressure loss.FIG.7shows measurement results of the PM accumulation weight.

TABLE 1PMInlet RegionOutlet RegionCol-LengthCoatingPdLengthCoatingPdlectionLAAmountContentLBAmountContentRatio(mm)(g/L)(g/L)(mm)(g/L)(g/L)(%)Sample60300.1560300.1569.91Sample6030—6030—83.92Sample60300.156030—76.43Sample6030—60300.1581.44
3. Evaluation Result

As shown in Table 1, in evaluation of PM collection performance, high PM collection ratios of 80% or more were observed in samples 2 and 4. This shows that the absence of the noble metal catalyst (Pd) at least in the inlet layer facilitates enhancement of PM collection performance of the entire filter.

Next, as shown inFIGS.6and7, in sample 2, even with the reproduction process, gas distributability was not restored, and a pressure loss remained high (PM was still accumulated). On the other hand, in samples 1, 3, and 4, with the reproduction process, gas distributability was restored, and a pressure loss decreased. This shows that suppression of an increase in pressure loss requires the presence of a noble metal catalyst (Pd) in at least one of the inlet layer or the outlet layer.

These test results demonstrate that sample 4 exhibits high PM collection performance of a PM collection ratio of 80% or more and has favorable pressure loss suppression performance that restores by a reproduction process. This shows that as in sample 4, the particulate filter in which the inlet layer contains substantially no noble metal catalyst and the outlet layer contains a noble metal catalyst achieves high levels of PM collection performance and pressure loss suppression performance.

B. Second Test

In this test example, conditions for achieving high levels of PM collection performance and pressure loss suppression performance in a particulate filter in which an inlet layer contains substantially no noble metal catalyst and an outlet layer contains a noble metal catalyst were examined.

1. Preparation of Samples

First, 12 types of particulate filters (samples 5 through 16) among which the coating amount of the inlet layer, the coating amount of the outlet layer, and the type of the base material are different were produced. Table 2 shows details of the samples. In Table 2, the “base material A” is a wall flow base material (made of cordierite, length: 152.4 mm) in which the thickness of partition walls is 0.2 mm to 0.25 mm, the number of cells is 300 cpsi, the pore ratio of the partition walls is 60% to 65%, and the cell volume is 1.3 L. On the other hand, the “base material B” is a wall flow base material (made of cordierite, length: 152.4 mm) in which the thickness of partition walls is 0.2 mm to 0.25 mm, the number of cells is 200 cpsi, the pore ratio of the partition walls is 55% to 60%, and the cell volume is 1.7 L. The other conditions of samples 5 through 16 except for conditions shown in Table 1 are the same as those in sample 4 of the first test described above.

Table 2

TABLE 2Inlet RegionOutlet RegionTotalBase MaterialCoatingCoatingCoatingPdCoatingCoatingPdCoatingEffectiveAmount/EffectiveAmountWidthContentAmountWidthContentAmountVolumeVolume(g/L)(%)(g/L)(g/L)(%)(g/L)(g/L)Type(L)(g/L)Sample 500—00—0A0.280Sample 620.860—20.8600.1525A0.28152.15Sample 741.760—41.7600.1550A0.28304.31Sample 862.560—62.5600.1575A0.28456.46Sample 98360—83600.15100A0.28608.61Sample 10118.360—118.3600.15130A0.28791.19Sample 1100—00—0B0.200Sample 128.375—8.3450.1510B0.2084.230Sample 1320.875—20.8450.1525B0.20210.58Sample 1429.375—29.3450.1535B0.20294.81Sample 1541.775—41.7450.1550B0.20421.15Sample 1662.675—62.6450.1550B0.20631.73
2. Evaluation Test

Under the same conditions as those of the first test, PM collection performance evaluation and a reproduction test were conducted, and a PM collection ratio and a pressure loss after 60 minutes from start of a reproduction process were measured.FIG.8shows measurement results of the PM collection ratio.FIG.9shows measurement results of the pressure loss. InFIG.8, the ordinate represents a “PM collection ratio (%)” and the abscissa represents a “coating amount/effective volume (g/L).” InFIG.9, the ordinate represents a “pressure loss (kPa)” and the abscissa represents a “coating amount/effective volume (g/L).” Each ofFIGS.8and9shows a kinked graph connecting measurement results of samples 5 through 10 using a base material A by dotted line, and a kinked graph connecting measurement results of samples 11 through 16 using a base material B by solid lines.

3. Evaluation Result

As shown inFIG.8, either case of using any one of the base materials A and B shows a tendency in which the PM collection ratio (%) increases as the “coating amount/effective volume” increases. If the coating amount/effective volume reaches 200 g/L or more, a sufficiently high PM collection ratio is obtained. On the other hand, if the coating amount/effective volume exceeds 400 g/L, the rate of increasing the PM collection ratio becomes low. This is supposed to be because wash-coating layers with a sufficient thickness are formed in pores of partition walls.

On the other hand, as shown inFIG.9, either case of using any one of the base materials A and B shows a tendency in which a pressure loss increases as the “coating amount/effective volume” increases. It was confirmed that the pressure loss rapidly increases around a point when the coating amount/effective volume exceeds 400 g/L. This is supposed to be because wash-coating layers are formed on the surfaces of partition walls in contact with cells so that the cells were clogged.

These results show that to achieve high levels of PM collection performance and pressure loss suppression effect, the coating amount/effective volume is preferably 200 g/L or more and 400 g/L or less.

INDUSTRIAL APPLICABILITY

The present invention can provide a particulate filter capable of achieving high levels of PM collection performance and pressure loss suppression performance.