Patent Description:
For internal combustion engines including gasoline engines, gasoline direct injection engines (hereinafter also referred to as GDI engines) are now widely used in order to comply with fuel economy standards becoming stricter year by year. It is known that GDI engines have low fuel consumption and provide high output, but that the amount of particulate matter (hereinafter also referred to as PM; including soot) emitted in exhaust gas is <NUM> to <NUM> times or more higher than that of conventional port fuel injection engines. In order to comply with environmental restrictions for PM emission, vehicles mounted with a gasoline engine such as a GDI engine are also required to include a filter that has a function for collecting PM (gasoline particulate filter, hereinafter also referred to as GPF), as with diesel engine-mounted vehicles.

In general, the space for installing an exhaust gas purifying catalyst in vehicles is limited, and thus, in view of space-saving, an exhaust gas purifying catalyst has come into use in recent years that includes a filter as described above and a noble metal three-way catalyst component such as Pd, Pt, or Rh supported on the filter to collect PM and purify nitrogen oxide (NOx), carbon monoxide (CO), hydrocarbon (HC), and the like.

For example, Patent Literature <NUM> discloses a treatment system for a gasoline engine exhaust gas stream including a particulate filter, the particulate filter including a particulate filter substrate, an inlet layer disposed on the exhaust gas inlet surface of the filter substrate, and an outlet layer disposed on the exhaust gas outlet surface of the filter substrate, wherein the inlet layer includes Rh and/or Pd and the outlet layer includes Rh and/or a zeolite. <CIT> Al discloses exhaust gas purification catalyst.

However, for a conventional filter catalyst that supports a noble metal three-way catalyst component, there is demand for a technique that enables even better exhaust gas purification performance while reducing the amount of noble metal used.

It is an object of the present invention to provide an exhaust gas purifying catalyst of wall-flow type that has better exhaust gas purification performance than a conventional exhaust gas purifying catalyst.

The inventors of the present invention have conducted in-depth studies on the configuration of a filter catalyst that has a wall flow structure. As a result, they have found that a filter catalyst that has better exhaust gas purification performance than a conventional filter catalyst can be obtained by arranging an oxidizing catalyst layer and a reducing catalyst layer under specific conditions on an inlet side and an outlet side of a substrate.

The present invention has been made based on the findings described above, and provides an exhaust gas purifying catalyst including a substrate and catalyst portions provided in the substrate,.

According to the present invention, a filter catalyst is provided that has superior exhaust gas purification performance to that of a conventional filter catalyst.

Hereinafter, the present invention will be described by way of preferred embodiments thereof, but the present invention is not limited to the embodiments given below.

<FIG> show an example of an exhaust gas purifying catalyst <NUM> according to the present embodiment. The drawings merely show a schematic example of an exhaust gas purifying catalyst, and are not intended to limit the present invention in any way.

The exhaust gas purifying catalyst <NUM> is provided in an exhaust path of an internal combustion engine such as a gasoline engine, in particular, a GDI engine for vehicles. The exhaust gas purifying catalyst <NUM> is used as, for example, a GPF.

As shown in <FIG>, the exhaust gas purifying catalyst <NUM> includes a substrate <NUM> that has a so-called wall flow structure. As the substrate <NUM>, a substrate made of any material can be used. For example, a substrate formed of ceramic such as cordierite or silicon carbide (SiC) can be favorably used. Usually, the substrate has a column-like outer shape as shown in <FIG>, and is disposed in the exhaust path of the internal combustion engine such that the axis direction of the column-like outer shape substantially matches an exhaust gas flow direction X. <FIG> shows a substrate that has a cylindrical column-like outer shape. However, the outer shape of the substrate as a whole may be an elliptic column-like shape or a polygonal column-like shape, instead of a cylindrical column-like shape.

As shown in <FIG>, the substrate <NUM> includes inflow-side cells <NUM> and outflow-side cells <NUM>. Each inflow-side cell <NUM> is a space, the space extending in the exhaust gas flow direction X and having an open end on the inflow side thereof and a closed end on the outflow side thereof in the flow direction X. Each outflow-side cell <NUM> is a space, the space extending in the flow direction X and having a closed end on the inflow side thereof and an open end on the outflow side thereof in the flow direction X.

The inflow-side cell <NUM> and the outflow-side cell <NUM> have the shape of a hole with a bottom. The inflow-side cell <NUM> is closed by a sealing portion <NUM> at the end on the exhaust gas outflow-side in a downstream end portion R2 in the exhaust gas flow direction X, but is open at the end on the exhaust gas inflow-side in an upstream end portion R1. The outflow-side cell <NUM> is closed by a sealing portion <NUM> at the end on the exhaust gas inflow-side in the upstream end portion R1, but is open at the end on the exhaust gas outflow-side in the downstream end portion R2. The inflow-side cell <NUM> and the outflow-side cell <NUM> are configured such that a gas, a liquid, and the like can flow through an opening end (hereinafter also referred to as "opening"), but the flow of exhaust gas is blocked at the sealing portion <NUM> and the sealing portion <NUM>, which are closed portion. The inflow-side cell <NUM> and the outflow-side cell <NUM> are each a space having the shape of a hole with a bottom and extending in the axis direction of the substrate <NUM>. The cross-sectional shape of each of the inflow-side cell <NUM> and the outflow-side cell <NUM> on a cross section perpendicular to the axis direction of the substrate <NUM> may be any geometric shape such as a quadrilateral including a square, a parallelogram, a rectangle, and a trapezoid, a polygon including a triangle, a hexagon, and an octagon, a circular shape, and an elliptic shape.

An inflow-side cell <NUM> and an outflow-side cell <NUM> that is provided adjacent to the inflow-side cell <NUM> are separated by a porous partition wall <NUM>. The partition wall <NUM> serves as a side wall of the inflow-side cell <NUM> and the outflow-side cell <NUM>. The partition wall <NUM> has a porous structure to allow a gas such as exhaust gas to pass therethrough. The thickness of the partition wall <NUM> is preferably <NUM> to <NUM>, for example. As used herein, the term "thickness" refers to the thickness of a thinnest portion when the partition wall <NUM> between the inflow-side cell <NUM> and the outflow-side cell <NUM> does not have a uniform thickness.

In the substrate <NUM>, the opening of one inflow-side cell <NUM> at the inflow-side end portion R1 and the opening of one outflow-side cell <NUM> at the outflow-side end portion R2 may have the same area or different areas. As used herein, the area of the opening refers to the area on a plane that is perpendicular to the axis direction of the substrate <NUM>.

In the substrate <NUM>, catalyst portions containing a catalytically active component are provided. As shown in <FIG>, the catalyst portions include: first catalyst portions <NUM>, each first catalyst portion <NUM> being in the form of a layer and provided at least on the upstream side in the exhaust gas flow direction X (hereinafter also referred to as "X direction") on the surface of the partition wall <NUM> that faces the inflow-side cell <NUM> (herein, the first catalyst portions are also collectively referred to as group A), and second catalyst portions <NUM>, each second catalyst portion <NUM> being in the form of a layer and provided at least on the downstream side in the exhaust gas flow direction X on the surface of the partition wall <NUM> that faces the outflow-side cell <NUM> (herein, the second catalyst portions are also collectively referred to as group B). The hatching in <FIG> does not limit the positions of an oxidizing catalyst layer and a reducing catalyst layer, which will be described later.

In the exhaust gas purifying catalyst <NUM>, exhaust gas flows into the inflow-side cell <NUM> of the substrate <NUM>, as shown in <FIG>. The exhaust gas flowing into the inflow-side cell <NUM> passes through the porous partition wall <NUM> and reaches the outflow-side cell <NUM>. In <FIG>, a path along which the exhaust gas flowing into the inflow-side cell <NUM> passes through the partition wall <NUM> and reaches the outflow-side cell <NUM> is indicated by an arrow. Since the partition wall <NUM> has a porous structure, PM is collected on the surfaces of the partition wall <NUM> and in the pores inside the partition wall <NUM> while the exhaust gas passes through the partition wall <NUM>. Moreover, while the exhaust gas passes through the inside of and the surfaces of the partition wall <NUM>, the exhaust gas comes into contact with a catalytically active component of the first catalyst portion <NUM> and the second catalyst portion <NUM>, and harmful components in the exhaust gas are thus purified. The exhaust gas passing through the partition wall <NUM> and reaching the outflow-side cell <NUM> is then discharged from the opening of the outflow-side cell <NUM> to the outside of the exhaust gas purifying catalyst <NUM>.

In the present embodiment, each catalyst portion of one of group A (the first catalyst portions <NUM>) and group B (the second catalyst portions <NUM>) includes at least one oxidizing catalyst layer and at least one reducing catalyst layer, and each catalyst portion of the other of group A and group B includes either one or both of at least one oxidizing catalyst layer and at least one reducing catalyst layer.

An oxidizing catalyst layer refers to a catalyst layer that contains mainly an oxidation catalyst among catalytically active components. Here, the oxidation catalyst is a catalyst having properties such that its catalytic effect of oxidizing hydrocarbon (HC) and carbon monoxide (CO) is greater than its catalytic effect of reducing NOx (nitrogen oxide). One or two selected from palladium (Pd) and platinum (Pt) may be used as the oxidation catalyst. On the other hand, a reducing catalyst layer refers to a catalyst layer that contains mainly a reduction catalyst among catalytically active components. Here, the reduction catalyst is a catalyst having properties such that its catalytic effect of reducing NOx (nitrogen oxide) is greater than its catalytic effect of oxidizing hydrocarbon (HC) and carbon monoxide (CO). Rhodium (Rh) may be used as the reduction catalyst.

The oxidizing catalyst layer may also contain a reduction catalyst and other catalytically active components in addition to an oxidation catalyst, but the amount of the oxidation catalyst is larger than the total amount of the reduction catalyst and the other catalytically active components. The "amount" here refers to an amount in terms of a metal element on a mass basis. In particular, the proportion of the sum of the amounts of Pd and Pt as the oxidation catalyst is more preferably higher than <NUM> mass% based on the total amount of catalytically active components contained in the oxidizing catalyst layer. The other catalytically active components as used herein refer to components other than the oxidation catalyst and the reduction catalyst described above, and may be one or more selected from ruthenium (Ru), iridium (Ir), osmium (Os), gold (Au), and silver (Ag).

The reducing catalyst layer may also contain an oxidation catalyst and other catalytically active components in addition to a reduction catalyst, but the amount of the reduction catalyst is larger than the total amount of the oxidation catalyst and the other catalytically active components. The "amount" as used herein refers to an amount in terms of a metal element on a mass basis. In particular, the proportion of the amount of Rh as the reduction catalyst is more preferably higher than <NUM> mass% based on the total amount of catalytically active components contained in the reducing catalyst layer. The other catalytically active components as used herein include the above-described components as well as Pt and Pd.

In the present embodiment, each catalyst portion of one of group A (the first catalyst portions <NUM>) and group B (the second catalyst portions <NUM>) includes at least one oxidizing catalyst layer and at least one reducing catalyst layer, and each catalyst portion of the other of group A and group B includes either one or both of at least one oxidizing catalyst layer and at least one reducing catalyst layer. That is to say, in the exhaust gas purifying catalyst <NUM> of the present embodiment, each catalyst portion of one of group A and group B includes both an oxidizing catalyst layer and a reducing catalyst layer, whereas each catalyst portion of the other of group A and group B includes at least one selected from the group consisting of an oxidizing catalyst layer and a reducing catalyst layer. By such a configuration, the exhaust gas purifying catalyst <NUM> allows exhaust gas to come into contact with a total of three or more of the oxidizing catalyst layer and the reducing catalyst layer. Although the mechanism of improving exhaust gas purification performance by such a configuration is not clear, the inventors of the present invention consider that one of the reasons might be that the increase in probability of exhaust gas coming into contact with an oxidizing catalyst layer or a reducing catalyst layer activates the oxidation/reduction reaction in the oxidizing catalyst layer or/and the reducing catalyst layer.

In the exhaust gas purifying catalyst <NUM> of the present embodiment, when an oxidizing catalyst layer and a reducing catalyst layer are stacked, the following configuration may be adopted: an oxidizing catalyst layer is first formed on the partition wall <NUM> and a reducing catalyst layer is then formed on the oxidizing catalyst layer; or alternatively, a reducing catalyst layer is first formed on the partition wall <NUM> and an oxidizing catalyst layer is then formed on the reducing catalyst layer. The oxidizing catalyst layer or the reducing catalyst layer that is formed on the partition wall <NUM> may be formed on the partition wall <NUM> so as to be in direct contact with the partition wall <NUM> or may be formed on the partition wall <NUM> via an intermediate layer. The intermediate layer does not contain a catalytically active component and may be, for example, a layer that is formed mainly of metal oxide particles, which will be described later. As described above, the expression "a predetermined catalyst layer is formed on the partition wall <NUM>" as used herein includes a case where the catalyst layer is formed on a surface of the partition wall <NUM> and a case where the catalyst layer is formed on an outer side of the partition wall <NUM> on a surface side of the partition wall <NUM>. Likewise, the expression "a catalyst layer B is stacked on a catalyst layer A" includes a case where the catalyst layer B is formed on a surface of the catalyst layer A and a case where the catalyst layer B is formed on an outer side of the catalyst layer A on a surface side of the catalyst layer A. Hereinafter, in a stacking structure composed of an oxidizing catalyst layer and a reducing catalyst layer that are provided on the partition wall <NUM>, a layer that is located closer to the partition wall <NUM> will also be referred to as a "lower layer", and a layer that is formed on the other side of the "lower layer" than the partition wall <NUM>-side will also be referred to as an "upper layer". In general, an oxidizing catalyst layer or a reducing catalyst layer that is provided in direct contact with the partition wall <NUM> may be present inside the partition wall <NUM> or may be present on the surface of the partition wall <NUM>, depending on the particle diameter and the other features of constituent particles of that catalyst layer. In a stack of an oxidizing catalyst layer and a reducing catalyst layer in the first catalyst portion <NUM>, the boundary between the oxidizing catalyst layer and the reducing catalyst layer is preferably located over a surface of the partition wall <NUM> that faces the inflow-side cell <NUM>, rather than being located inside the partition wall <NUM>. Likewise, in a stack of an oxidizing catalyst layer and a reducing catalyst layer in the second catalyst portion <NUM>, the boundary between the oxidizing catalyst layer and the reducing catalyst layer is preferably located over a surface of the partition wall <NUM> that faces the outflow-side cell <NUM>, rather than being located inside the partition wall <NUM>.

Whether or not a catalyst layer is in direct contact with the partition wall <NUM> can be checked in the following manner: the catalyst <NUM> is cut along a cross section that is perpendicular to the exhaust gas flow direction; the exposed cross section is observed using a scanning electron microscope (for example, "JEM-ARM200F" available from JEOL, Ltd. ); and also energy dispersive X-ray spectrometry (EDS) is performed thereon to line-analyze the distribution of elements (for example, Si and Mg) that are present only in the substrate and the distribution of elements (for example, Pd, Pt, and Rh) that are present only in the catalyst layer. Alternatively, that can also be checked by analyzing the cross section using an electron probe micro analyzer (EPMA). The catalyst including the substrate is cut using a band saw or the like. The catalyst that has been cut is embedded in an epoxy resin to prepare a sample in which the above-described cross section is exposed on the surface thereof. It can be checked in the same manner as above whether or not the layer boundary between an oxidizing catalyst layer and a reducing catalyst layer in a stack of the oxidizing catalyst layer and the reducing catalyst layer is located over a surface of the partition wall <NUM> that faces the cell <NUM> or <NUM> rather than being located inside the partition wall <NUM>.

In order to locate the boundary between the lower layer and the upper layer over a surface of the partition wall <NUM> that faces the cell <NUM> or <NUM>, metal oxide particles having a particle diameter that makes it difficult for the particles to pass through the pores of the partition wall can be used as catalyst-supporting metal oxide particles for the lower catalyst layer, or a pore-forming material having a particle diameter that makes it difficult for the material to pass through the pores of the partition wall can be included in a slurry for forming the lower layer.

In a stack of an oxidizing catalyst layer and a reducing catalyst layer, the total number of oxidizing catalyst layers and reducing catalyst layers is <NUM> or more. The total number of oxidizing catalyst layers and reducing catalyst layers in the first catalyst portion <NUM> and the second catalyst portion <NUM> is preferably <NUM> or less, and more preferably <NUM> or less, in view of the production cost of the exhaust gas purifying catalyst <NUM> and the prevention of pressure loss. If oxidizing catalyst layers are stacked with no reducing catalyst layer provided therebetween, the number of such oxidizing catalyst layers is counted as <NUM>, and not as <NUM> or more. Likewise, if reducing catalyst layers are stacked with no oxidizing catalyst layer provided therebetween, the number of such reducing catalyst layers is counted as <NUM>, and not as <NUM> or more. Herein, one catalyst layer (also referred to as "a single (catalyst) layer") does not need to have a uniform composition. When viewed in the thickness direction, a single catalyst layer may include portions containing different catalytically active components, as long as these portions have the same tendency in terms of which of the oxidizing properties and the reducing properties are the stronger.

In the present invention one of the first catalyst portion <NUM> and the second catalyst portion <NUM> includes a stack of an oxidizing catalyst layer and a reducing catalyst layer, and the other of the first catalyst portion <NUM> and the second catalyst portion <NUM> is a single oxidizing catalyst layer, a single reducing catalyst layer, or a stack of an oxidizing catalyst layer and a reducing catalyst layer.

The number of layers of a stacking structure in each of the first catalyst portion <NUM> and the second catalyst portion <NUM> can be determined by analyzing the distributions of the reduction catalyst component(s) and the oxidation catalyst component(s) through line-analysis (Pd, Pt, Rh, and the like) using EDS or the like.

In a preferred embodiment of the present invention, the total number of the oxidizing catalyst layer(s) and the reducing catalyst layer(s) included in the first catalyst portion <NUM> is different from the total number of the oxidizing catalyst layer(s) and reducing catalyst layer(s) included in the second catalyst portion <NUM>. When the first catalyst portion <NUM> (or the second catalyst portion <NUM>) includes only a single oxidizing catalyst layer or only a single reducing catalyst layer, the total number of layers herein is <NUM>.

When the total number of the oxidizing catalyst layer(s) and the reducing catalyst layer(s) included in the first catalyst portion <NUM> is different from the total number of the oxidizing catalyst layer(s) and the reducing catalyst layer(s) included in the second catalyst portion <NUM>, the catalyst portion including the smaller number of layers contributes to reducing pressure loss, while the catalyst portion including the larger number of layers contributes to improving exhaust gas purification performance, and it is therefore easy to provide both a reduction in pressure loss and an improvement in purification performance. A preferred configuration in this case may be as follows: one of the first catalyst portion <NUM> and the second catalyst portion <NUM> consists of a single reducing catalyst layer or a single oxidizing catalyst layer and the other of the first catalyst portion <NUM> and the second catalyst portion <NUM> is a stack of two or more layers including an oxidizing catalyst layer and a reducing catalyst layer. <FIG> and <FIG> show examples of such a configuration.

In a configuration shown in <FIG>, the first catalyst portion <NUM> consists of a single reducing catalyst layer or a single oxidizing catalyst layer and the other catalyst portion is a stack of two or more layers including an oxidizing catalyst layer and a reducing catalyst layer. When adopting this configuration, both the exhaust gas (<NUM>) passing through the first catalyst portion <NUM> and the gas (<NUM>) passing through the partition wall <NUM> downstream of the first catalyst portion <NUM> in the X direction can come into contact with the second catalyst portion <NUM>, which is a stack of two or more layers. Accordingly, this configuration can provide an increased probability of contact of exhaust gas with both an oxidizing catalyst layer and a reducing catalyst layer. Also, a configuration shown in <FIG>, in which both an oxidizing catalyst layer and a reducing catalyst layer are disposed on the inflow side, can provide an increased probability of exhaust gas with an oxidizing catalyst layer or a reducing catalyst layer, compared with a conventional configuration, in which only one catalyst layer is disposed on each of the inflow side and the discharge side.

When the total number of the reducing catalyst layer(s) and the oxidizing catalyst layer(s) included in the first catalyst portion <NUM> is different from the total number of the reducing catalyst layer(s) and the oxidizing catalyst layer(s) included in the second catalyst portion <NUM>, the total number of the layers of the second catalyst portion <NUM> is preferably larger than the total number of the layers of the first catalyst portion <NUM>. The reason for this is that exhaust gas purification performance can be even more readily improved with limited amounts of the catalytically active components, and also that an increase in pressure loss can also be readily suppressed. Moreover, the advantage of suppressing an increase in pressure loss after PM and ash have accumulated can also be provided.

In another preferred embodiment of the present invention, both the total number of the reducing catalyst layer(s) and the oxidizing catalyst layer(s) included in the first catalyst portion <NUM> and the total number of the reducing catalyst layer(s) and the oxidizing catalyst layer(s) included in the second catalyst portion <NUM> are <NUM> or more. <FIG> shows an example of such a configuration. In an embodiment shown in <FIG>, an oxidizing catalyst layer and a reducing catalyst layer are present in both the first catalyst portion <NUM> and the second catalyst portion <NUM>. Accordingly, this configuration can provide an increased probability of contact of exhaust gas with both an oxidizing catalyst layer and a reducing catalyst layer, and the purification performance for the gas species NOx, HC, and CO can be even more readily improved. Moreover, this configuration can readily improve the PM collection rate.

Preferably, a reducing catalyst layer containing Rh is included in each of the first catalyst portion <NUM> and the second catalyst portion <NUM>. In this case, the proportion of portions where a reducing catalyst layer containing Rh is disposed in the substrate increases when viewed in the X direction, and therefore, the probability of contact of exhaust gas with a reducing catalyst layer containing Rh is effectively increased, so that NOx purification performance can be significantly improved. Since NOx emission restrictions have become especially strict in recent years, the exhaust gas purifying catalyst <NUM> in this case has especially high industrial applicability.

Preferred configurations in the case where a reducing catalyst layer is included in each of the first catalyst portion <NUM> and the second catalyst portion <NUM> are configurations (I) and (II) below. According to the embodiment (I) or (II) below, exhaust gas comes into contact with a reducing catalyst layer, an oxidizing catalyst layer, and a reducing catalyst layer in this order. Thus, the supply of a reduction substance to the reducing catalyst layers is facilitated, and the amount of reduction substance that is to be purified through an oxidation reaction decreases. Therefore, oxidation/reduction of exhaust gas proceeds smoothly. These are particularly effective in improving the NOx reduction efficiency.

In the configurations (I) and (II), it is preferable that each oxidizing catalyst layer contain Pd and that each reducing catalyst layer contain Rh, because exhaust gas purification performance can be improved even more. The reason for this is probably because oxidation of HC and CO by Pd and reduction of NOx by Rh can be performed in a favorably balanced manner due to the effects of Pd and Rh. Moreover, in the configurations (I) and (II), since a reducing catalyst layer is included in each of the first catalyst portion <NUM> and the second catalyst portion <NUM>, the reducing catalyst layers are present over a wide area in the exhaust gas purifying catalyst <NUM> in the X direction. Therefore, NOx purification can be performed efficiently, and excellent NOx purification performance is obtained particularly during high speed driving. Thus, the amount of NOx emitted can be effectively reduced by installing the exhaust gas purifying catalyst <NUM> in a vehicle.

As described above, an oxidizing catalyst layer or a reducing catalyst layer formed on the partition wall <NUM> may be formed on the partition wall <NUM> so as to be in direct contact with the partition wall <NUM>, or may be formed on the partition wall <NUM> via an intermediate layer. However, the oxidizing catalyst layer or reducing catalyst layer is preferably formed on the partition wall <NUM> so as to be in direct contact therewith. In particular, in order for an oxidation/reduction reaction to be performed in a balanced manner, it is preferable that an oxidizing catalyst layer that is in direct contact with the partition wall <NUM> and contains Pd and a reducing catalyst layer that is in contact with the partition wall <NUM> and contains Rh be arranged with the partition wall <NUM> therebetween, as in the configuration A or B below.

A: The first catalyst portion <NUM> includes a reducing catalyst layer that is in direct contact with the partition wall <NUM> and contains Rh, and the second catalyst portion <NUM> includes an oxidizing catalyst layer that is in direct contact with the partition wall <NUM> and contains Pd.

B: The first catalyst portion <NUM> includes an oxidizing catalyst layer that is in direct contact with the partition wall <NUM> and contains Pd, and the second catalyst portion <NUM> contains a reducing catalyst layer that is in direct contact with the partition wall <NUM> and contains Rh.

The configuration A or B is preferable because the configuration allows an efficient purification reaction in the exhaust gas flow direction of the catalyst <NUM> throughout.

In order to further improve exhaust gas purification performance, the length L1 (see <FIG>) of the first catalyst portion <NUM> in the exhaust gas flow direction is preferably <NUM>% or greater based on the length L (see <FIG>) of the substrate in the same direction. The length L2 (see <FIG>) of the second catalyst portion <NUM> in the exhaust gas flow direction is preferably <NUM>% or greater based on the length L of the substrate in the same direction. Furthermore, it is preferable that L1 be <NUM>% or less based on L and that L2 be <NUM>% or less based on L, because the amounts of catalytically active components and the others can be reduced. In view of these, it is more preferable that L1 be <NUM>% to <NUM>% based on L, and that L2 be <NUM>% to <NUM>% based on L.

The lengths of the first catalyst portion <NUM> and the second catalyst portion <NUM> can be measured by the following preferable method: the exhaust gas purifying catalyst <NUM> is cut in the axis direction of the substrate <NUM> along a cross section containing the central axis thereof; the central axis portion of the cross section is visually observed to identify the boundary of the first catalyst portion <NUM> and the boundary of the second catalyst portion <NUM>; and the length of the first catalyst portion <NUM> and the length of the second catalyst portion <NUM> are measured. In this case, it is preferable to measure the length of the first catalyst portion <NUM> and the length of the second catalyst portion <NUM> at, for example, <NUM> arbitrarily selected positions on the exhaust gas purifying catalyst <NUM>, and take the average values as the length of the first catalyst portion <NUM> and the length of the second catalyst portion <NUM>. In the case where it is not possible to determine the boundaries of the first catalyst portion <NUM> and the second catalyst portion <NUM> in the exhaust gas flow direction through visual observation, the composition is analyzed at a plurality of (for example, <NUM> to <NUM>) positions in the exhaust gas flow direction of the exhaust gas purifying catalyst, and the lengths of the first catalyst portion <NUM> and the second catalyst portion <NUM> can be determined on the basis of the content of a catalytically active component in the composition at each position. The content of a catalytically active component at each position can be determined through, for example, X-ray fluorescence analysis (XRF) or ICP emission spectroscopic analysis (ICP-AES).

The first catalyst portion <NUM> is preferably formed so as to extend from the upstream end portion R1 of the substrate <NUM> in the X direction toward the downstream side in view of both ease of production of the catalyst portion and exhaust gas purification performance. Likewise, the second catalyst portion <NUM> is preferably formed so as to extend from the downstream end portion R2 of the substrate <NUM> in the X direction toward the upstream side.

In view of further improving the exhaust gas purification performance of the exhaust gas purifying catalyst <NUM>, the amount of the catalytically active component(s) contained in the first catalyst portion <NUM> (the total amount of an oxidation catalyst, a reduction catalyst, and other catalytically active components) is generally preferably <NUM> or more, and more preferably <NUM> or more per liter of volume of the substrate. The amount of the catalytically active component(s) contained in the second catalyst portion <NUM> (the total amount of an oxidation catalyst, a reduction catalyst, and other catalytically active components) is generally preferably <NUM> or more, and more preferably <NUM> or more per liter of volume of the substrate. For the upper limit, the amount of the catalytically active component(s) contained in the first catalyst portion <NUM> and the second catalyst portion <NUM> is preferably <NUM> or less per liter of volume of the substrate, and may be <NUM> or less, or <NUM>/L or less in some cases. As used herein, the volume of the substrate refers to an apparent volume that includes not only the volume of the substrate portion but also the volumes of the first catalyst portion <NUM>, the second catalyst portion <NUM>, the pores of the partition wall <NUM>, and the spaces in the cells <NUM> and <NUM>.

The amount of Rh contained in a reducing catalyst layer of the first catalyst portion <NUM> is generally preferably from <NUM> to <NUM>, and more preferably from <NUM> to <NUM>, per liter of volume of the substrate. Also, the total amount of Pd and Pt contained in an oxidizing catalyst layer of the first catalyst portion <NUM> is generally preferably from <NUM> to <NUM>/L, and more preferably from <NUM> to <NUM>/L, per liter of volume of the substrate. The same applies to the amount of Rh contained in a reducing catalyst layer of the second catalyst portion <NUM> and the total amount of Pd and Pt contained in an oxidizing catalyst layer of the second catalyst portion <NUM>.

When a plurality of oxidizing catalyst layers are provided, the mass ratio between the catalytically active components in the plurality of oxidizing catalyst layers is preferably <NUM>:<NUM> to <NUM>, and more preferably <NUM>:<NUM> to <NUM>. The mass ratio here refers to the ratio of the amount of the catalytically active component in a layer that contains the catalytically active component in the smallest amount to the amount of the catalytically active component in a layer that contains the catalytically active component in the largest amount. The same is also applied mutatis mutandis to the mass ratio between the catalytically active components in the plurality of reducing catalyst layers when a plurality of reducing catalyst layers are provided.

The amount of a catalytically active component can be determined by, for example, completely dissolving a catalyst layer to obtain a solution and measuring the amount of noble metals in the solution using ICP-AES. In the case where a catalyst layer is included in the partition wall of the substrate, the amount of a catalytically active component can be determined by subtracting the amount of noble metals contained in a solution obtained by completely dissolving only the substrate from the amount of noble metals contained in a solution obtained by completely dissolving the catalyst layer and the substrate.

It is preferable that each of the first catalyst portion <NUM> and the second catalyst portion <NUM> further contain a catalyst-supporting component, which supports the catalytically active component, in view of causing the catalytically active component to efficiently exhibit exhaust gas purification performance. Metal oxide particles may be used as the catalyst-supporting component. The metal oxide for forming the metal oxide particles may be an inorganic oxide that acts as an oxygen storage component (also referred to as "OSC material"; wherein OSC stands for oxygen storage capacity), or an inorganic oxide other than the oxygen storage component. The term "metal oxide particles" used herein encompasses not only individual particles but also calcined bodies formed of metal oxide particles bonded to each other through calcining.

Herein, a state in which a catalytically active component is "supported" on metal oxide particles refers to a state in which the catalytically active component is physically or chemically adsorbed or held on the outer surfaces of the metal oxide particles or the inner surfaces of the pores of the metal oxide particles. Specifically, whether a catalytically active component is supported on metal oxide particles can be judged in the following manner, for example: a cross section of the exhaust gas purifying catalyst <NUM> is analyzed using EDS to obtain an element map, and if the presence of a metal oxide component and a catalytically active component is confirmed in the same region of the element map, it is determined that the catalytically active component is "supported" on the metal oxide particles.

As the inorganic oxide as an oxygen storage component, a metal oxide that is multivalent and is capable of storing oxygen can be used. Examples thereof include CeO<NUM> and CZ material (a ceria-zirconia composite oxide containing Ce and Zr, and a solid solution of CeO<NUM> and ZrO<NUM>), iron oxide, and copper oxide. An oxide of a rare earth element other than Ce is also preferably used in view of thermal stability. Examples of the oxide of a rare earth element other than Ce include Sc<NUM>O<NUM>, Y<NUM>O<NUM>, La<NUM>O<NUM>, Pr<NUM>O<NUM>, Nd<NUM>O<NUM>, Sm<NUM>O<NUM>, Eu<NUM>O<NUM>, Gd<NUM>O<NUM>, Tb<NUM>O<NUM>, Dy<NUM>O<NUM>, Ho<NUM>O<NUM>, Er<NUM>O<NUM>, Tm<NUM>O<NUM>, Yb<NUM>O<NUM>, and Lu<NUM>O<NUM>. CeO<NUM>-ZrO<NUM> herein refers to a solid solution of CeO<NUM> and ZrO<NUM>, and whether a solid solution of CeO<NUM> and ZrO<NUM> has been formed can be confirmed by checking whether or not a single phase derived from CeO<NUM>-ZrO<NUM> is formed, using an X-ray diffraction (XRD) apparatus. CeO<NUM>-ZrO<NUM> may be a solid solution that also contains the oxide of a rare earth element other than Ce. In particular, in view of the balance between heat resistance and OSC, the amount of CeO<NUM> contained in the first catalyst portion <NUM> or the second catalyst portion <NUM> is preferably <NUM> to <NUM> mass%. The amount of ZrO<NUM> contained in the first catalyst portion <NUM> or the second catalyst portion <NUM> is preferably <NUM> to <NUM> mass%. The preferred amounts of CeO<NUM> and ZrO<NUM> herein include the amounts of CeO<NUM> and ZrO<NUM> in the form of a solid solution, as well as the amount of Ce, in terms of CeO<NUM>, in a ceria-zirconia composite oxide and the amount of Zr, in terms of ZrO<NUM>, in a ceria-zirconia composite oxide, respectively.

The amounts of CeO<NUM> and ZrO<NUM> can be determined by, for example, completely dissolving the catalyst layer to obtain a solution, measuring the amounts of Ce and Zr contained in the solution using ICP-AES, and converting them to amounts in terms of oxide. In the case where the catalyst layer is included in the partition wall of the substrate, the amounts of CeO<NUM> and ZrO<NUM> can be determined by subtracting the amounts of Ce and Zr in a solution obtained by completely dissolving only the substrate from the amounts of Ce and Zr contained in a solution obtained by completely dissolving the catalyst layer and the substrate.

The inorganic oxide other than the oxygen storage component that can be contained in the first catalyst portion <NUM> may be a metal oxide other than the oxygen storage component. Examples thereof include alumina, silica, silica-alumina, titanium, and aluminosilicate. In particular, alumina is preferably used in view of excellent heat resistance. The amount of the inorganic oxide other than the oxygen storage component contained in the first catalyst portion <NUM> or the second catalyst portion <NUM> is preferably <NUM> to <NUM> mass%.

The amount of alumina can be determined by, for example, completely dissolving the catalyst layer to obtain a solution, measuring the amount of aluminum contained in the solution using ICP-AES, and converting it to an amount in terms of oxide. In the case where the catalyst layer is included in the partition wall of the substrate, the amount of alumina can be determined by subtracting the amount of Al contained in a solution obtained by completely dissolving only the substrate from the amount of Al contained in a solution obtained by completely dissolving the catalyst layer and the substrate.

In view of further improving PM collecting performance and exhaust gas purification performance, the first catalyst portion <NUM> is preferably present mainly on the surface of the partition wall <NUM>, rather than the inside of the partition wall <NUM>. As used herein, the expression "the first catalyst portion <NUM> is present mainly on the surface of the partition wall <NUM>" means a state in which, in a cross section of the substrate <NUM> having the first catalyst portion <NUM>, the mass of the first catalyst portion <NUM> present on the surface of the partition wall <NUM> of the substrate <NUM> is larger than the mass of the first catalyst portion <NUM> present inside the partition wall <NUM>. For example, whether the first catalyst portion <NUM> is present mainly on the surface of the partition wall <NUM> can be checked by observing a cross section of the partition wall where the first catalyst portion <NUM> is provided, under the above-described scanning electron microscope, and performing an EDS analysis to line-analyze the boundaries between elements (for example, Si and Mg) that are present only in the substrate and elements (for example, Ce and Zr) that are present only in the catalyst layer; or by performing an analysis on a cross section of the partition wall where the first catalyst portion <NUM> is provided using an electron probe micro analyzer (EPMA). Likewise, the second catalyst portion <NUM> is preferably present mainly on the surface of the partition wall <NUM>, rather than the inside of the partition wall <NUM>. <FIG> each schematically show a state in which the first catalyst portion <NUM> is present mainly on the surface of the partition wall <NUM> and the second catalyst portion <NUM> is present mainly on the surface of the partition wall <NUM>.

In order for the first catalyst portion <NUM> to be present mainly on the surface of the partition wall <NUM> instead of inside the partition wall <NUM>, for example, metal oxide particles having a particle diameter that makes it difficult for the particles to pass through the pores of the partition wall <NUM> can be used as the catalyst-supporting metal oxide particles to be included in the first catalyst portion <NUM> and the second catalyst portion <NUM>. Moreover, the amount of a catalyst layer that is present on the surface of the partition wall <NUM> can be easily increased relative to the amount of the catalyst layer that is present inside the partition wall <NUM> by using an organic pore-forming material with a larger particle diameter than the pores of the partition wall <NUM> when forming the catalyst layer.

Next, a preferred method of producing an exhaust gas purifying catalyst according to the present invention will be described below.

A preferred method of producing the exhaust gas purifying catalyst <NUM> according to, for example, the embodiment shown in <FIG> includes the following steps (<NUM>) to (<NUM>). For other embodiments, the following method can be modified as appropriate. The step (<NUM>) and the steps (<NUM>) and (<NUM>) may be performed in any order. In the steps (<NUM>) and (<NUM>), Rh is used as a catalytically active component in one of these steps and Pd or Pt is used as a catalytically active component in the other of these steps.

As the metal oxide particles, the particles of an inorganic oxide as an oxygen storage component or an inorganic oxide other than the oxygen storage component, which have been described above as a constituent component of the first catalyst portion <NUM> and the second catalyst portion <NUM>, can be used. The catalytically active components used in the above-described steps may each be in a state of a water-soluble salt such as a nitrate, and thus be mixed with metal oxide particles to obtain a slurry for forming the first catalyst portion <NUM> and slurries for forming the second catalyst portion <NUM>, and the obtained slurries may be applied to the substrate <NUM>, and then dried or calcined. Alternatively, a catalytically active component may be supported on metal oxide particles in advance, and the resulting metal oxide particles supporting the catalytically active component thereon may be used to form a slurry. For supporting a catalytically active component on metal oxide particles in advance, a method may be used in which the metal oxide particles are impregnated with an aqueous solution of the catalytically active component in a state of a water-soluble salt, and then calcined at a temperature of <NUM> to <NUM>.

Each of the above-described slurries may contain a binder for the purpose of attaching the metal oxide particles supporting the catalytically active component to the substrate. Examples of the binder include an alumina sol, a zirconia sol, a titania sol, and a silica sol. Each of the above-described slurries may contain a pore-forming material. As the pore-forming material, cross-linked polymethyl(meth)acrylate particles, cross-linked polybutyl(meth)acrylate particles, cross-linked polystyrene particles, cross-linked polyacrylic acid ester particles, or the like can be used.

In order to apply each of the above-described slurries to the substrate <NUM>, a method may be used in which the upstream side or the downstream side of the substrate <NUM> in the exhaust gas flow direction is immersed in the slurry. The slurry may be drawn by suction from the other side of the substrate simultaneously with the immersion of the substrate <NUM>. In this manner, the slurry can be applied to a desired position. In all of the above-described steps, the temperature for drying the slurries is preferably <NUM> to <NUM>, and the temperature for calcining the slurries is preferably <NUM> to <NUM>.

In the exhaust gas purifying catalyst <NUM> according to the present embodiment, the mass of the first catalyst portion <NUM> may be tailored according to the amount of the catalytically active component. However, the mass of the first catalyst portions <NUM> on a dry mass basis is preferably <NUM> or more, and more preferably <NUM> or more per liter of volume of the substrate in a portion where the first catalyst portions <NUM> are formed. The mass of the first catalyst portions <NUM> on a dry mass basis is preferably <NUM> or less, and more preferably <NUM> or less per liter of volume of the substrate in view of reducing pressure loss and improving exhaust gas purification performance during high speed driving. The mass of the second catalyst portion <NUM> may be tailored according to the amount of the catalytically active component. However, the mass of the second catalyst portions <NUM> on a dry mass basis is preferably <NUM> or more, and more preferably <NUM> or more per liter of volume of the substrate. In order to reduce pressure loss, the mass of the second catalyst portions <NUM> on a dry mass basis is preferably <NUM> or less, and more preferably <NUM> or less per liter of volume of the substrate in a portion where the second catalyst portions <NUM> are formed.

When the first catalyst portion <NUM> or the second catalyst portion <NUM> includes two or more, in total, of an oxidizing catalyst layer and a reducing catalyst layer, the mass ratio of the mass of a layer with the largest mass to the mass of a layer with the smallest mass is preferably <NUM>:<NUM> to <NUM>, and more preferably <NUM>:<NUM> to <NUM> in view of further improving exhaust gas purification performance.

The exhaust gas purifying catalyst <NUM> produced in the manner described above can be used in various applications as an exhaust gas purifying catalyst for internal combustion engines that use fossil fuel as a power source, such as gasoline engines, by utilizing the exhaust gas purification performance thereof. The present embodiment can also provide an exhaust gas purification method that uses an exhaust gas purifying catalyst <NUM> as described above. For example, the exhaust gas purifying catalyst <NUM> may be provided in an exhaust path of an internal combustion engine such as a gasoline engine, in particular, a GDI engine in a vehicle and used as a GPF or the like, and the exhaust gas from the gasoline engine can thus be favorably purified.

<FIG> shows an example of a treatment system for purifying an exhaust gas that includes the exhaust gas purifying catalyst <NUM> of the present invention. The exhaust gas purifying catalyst <NUM> is provided in an exhaust system of an internal combustion engine. <FIG> is a diagram schematically illustrating an internal combustion engine <NUM> and the exhaust gas purifying catalyst <NUM> provided in an exhaust system of the internal combustion engine <NUM>.

A gas mixture containing oxygen and a fuel gas is supplied to an engine, which is an internal combustion engine according to the present embodiment. The internal combustion engine burns the gas mixture and converts the combustion energy into mechanical energy. The burnt gas mixture is discharged as exhaust gas into an exhaust system. The internal combustion engine <NUM> having the configuration shown in <FIG> includes mainly a gasoline engine in an automobile. An exhaust manifold <NUM> is connected to an exhaust port (not shown) via which the internal combustion engine <NUM> is in communication with the exhaust system. The exhaust manifold <NUM> is connected to an exhaust pipe <NUM> through which the exhaust gas flows. The exhaust manifold <NUM> and the exhaust pipe <NUM> form a flow path for the exhaust gas from the internal combustion engine <NUM>. The arrow in <FIG> indicates the exhaust gas flow direction. The exhaust gas purifying catalyst <NUM> is provided in the exhaust system of the internal combustion engine <NUM>. A treatment system <NUM> for purifying an exhaust gas includes the exhaust gas purifying catalyst <NUM> according to the present embodiment and also includes another exhaust gas purifying catalyst <NUM> which is not of the present embodiment. The exhaust gas purifying catalyst <NUM> is located downstream of the other exhaust gas purifying catalyst <NUM> in the exhaust gas flow direction and provided adjacent to the other exhaust gas purifying catalyst <NUM>. The exhaust gas purifying catalyst <NUM> is configured to purify three major pollutants (NOx, HC, and CO) contained in exhaust gas. The type of the exhaust gas purifying catalyst <NUM> is not limited, and, for example, may be a catalyst that supports a noble metal such as platinum (Pt), palladium (Pd), or rhodium (Rd). A catalyst that does not have a wall flow structure can be used as the exhaust gas purifying catalyst <NUM>. The exhaust gas purifying catalyst <NUM> has PM collecting performance and acts as a GPF.

By the above-described configuration, harmful components in exhaust gas can be effectively purified due to the excellent exhaust gas purification performance of the exhaust gas purifying catalyst <NUM> without the need to increase the amount of a catalytically active component.

Hereinafter, the present invention will be described in further detail by way of examples. However, the scope of the present invention is not limited to the examples. In the following examples and comparative examples, all the drying and calcining steps were performed in an atmosphere. The amounts of catalytically active components used were such that the amounts of respective catalytically active components in catalyst layers were as shown in Table <NUM>. For an oxidizing catalyst layer that was in direct contact with the partition wall <NUM>, the particle diameters of metal oxide particles and a pore-forming material that were used for a second slurry were selected so that the mass of the oxidizing catalyst layer present on the surface of the partition wall <NUM> of the substrate <NUM> was larger than the mass of the oxidizing catalyst layer present inside the partition wall <NUM>. For a reducing catalyst layer that was in contact with the partition wall <NUM> as well, the particle diameters of metal oxide particles and a pore-forming material used for a first slurry were selected in the same manner.

In the examples below, when an oxidizing catalyst layer and a reducing catalyst layer were stacked, both ends of the stacked oxidizing catalyst layer in the X direction were located at substantially the same longitudes as both ends of the stacked reducing catalyst layer in the X direction, respectively (in other words, the oxidizing catalyst layer and the reducing catalyst layer that were stacked had substantially equal lengths).

A CeO<NUM>-ZrO<NUM> solid solution powder (the CeO<NUM>-ZrO<NUM> solid solution contained <NUM> mass% of CeO<NUM>, <NUM> mass% of ZrO<NUM>, and <NUM> mass% of an oxide of a rare earth element other than Ce) and an alumina powder were provided. The CeO<NUM>-ZrO<NUM> solid solution powder and the alumina powder were mixed at a mass ratio of <NUM>:<NUM>, and the mixture was impregnated with an aqueous solution of rhodium nitrate.

Next, the resulting mixed solution was mixed with <NUM> mass% of a pore-forming material (cross-linked polymethyl(meth)acrylate particles), <NUM> mass% of an alumina sol, and <NUM> mass% of a zirconia sol, all relative to the solid content of the mixed solution, and also water as a liquid medium, to thereby prepare a first slurry.

A CeO<NUM>-ZrO<NUM> solid solution powder (the CeO<NUM>-ZrO<NUM> solid solution contained <NUM> mass% of CeO<NUM>, <NUM> mass% of ZrO<NUM>, and <NUM> mass% of an oxide of a rare earth element other than Ce) and an alumina powder were mixed at a mass ratio of <NUM>:<NUM>, and the mixture was impregnated with an aqueous solution of palladium nitrate.

Next, the resulting mixed solution was mixed with <NUM> mass% of a pore-forming material (cross-linked polymethyl(meth)acrylate particles), <NUM> mass% of an alumina sol, <NUM> mass% of a zirconia sol, all relative to the solid content of the mixed solution, and also water as a liquid medium, to thereby prepare a second slurry.

As the substrate <NUM>, a substrate was used that had the structure shown in <FIG>, included <NUM> cells/inch<NUM> on a plane perpendicular to the axis direction, each cell being defined by partition walls with a thickness of <NUM> and extending in the axis direction, and had a volume of <NUM>. In the substrate <NUM>, the opening of a single inflow-side cell <NUM> formed in the end face on the inflow-side and the opening of a single outflow-side cell <NUM> formed in the end face on the outflow-side had roughly the same area.

The upstream end portion of the substrate <NUM> in the exhaust gas flow direction was immersed in the first slurry, and the slurry was drawn by suction from the downstream side. Then, the substrate was dried at <NUM> for <NUM> minutes. In this manner, layers (first catalyst portions before calcining) were formed of the solid content of the first slurry, each layer being provided on a surface of the partition wall <NUM> that faced the inflow-side cell <NUM>.

The downstream end portion of the dried substrate <NUM> in the exhaust gas flow direction was immersed in the second slurry, and the slurry was drawn by suction from the upstream side. Then, the substrate was dried at <NUM> for <NUM> minutes. In this manner, lower layers were formed of the solid content of the second slurry, each lower layer being provided on a surface of the partition wall <NUM> that faced the outflow-side cell <NUM>.

Next, the downstream end portion of the substrate <NUM> in the exhaust gas flow direction after the second slurry was dried was immersed in the first slurry, and the first slurry was drawn by suction from the upstream side. Then, the substrate was dried at <NUM> for <NUM> minutes. In this manner, stacks (second catalyst portions before calcining) were formed, each stack being of the lower layer and an upper layer formed of the solid content of the first slurry.

After that, the substrate <NUM> with the layers was fired at <NUM> for one hour for calcining. Accordingly, an exhaust gas purifying catalyst <NUM> of Example <NUM>, in which the first catalyst portions <NUM> and the second catalyst portions <NUM> were formed on the substrate <NUM>, was obtained. Each of the obtained first catalyst portions <NUM> was a single reducing catalyst layer. Each of the second catalyst portions <NUM> was a stack of a lower layer 15A, which was an oxidizing catalyst layer, and an upper layer 15B, which was a reducing catalyst layer stacked on the outer surface of the lower layer 15A.

In the exhaust gas purifying catalyst of Example <NUM>, each first catalyst portion <NUM> of the exhaust gas purifying catalyst <NUM> was formed on the surface of the partition wall <NUM> facing the inflow-side cell <NUM> so as to extend from the upstream end portion R1 toward the downstream side in the exhaust gas flow direction X to <NUM>% of the overall length L, and the mass of the first catalyst portions <NUM> per volume of the substrate in the portion where the first catalyst portions <NUM> were formed was <NUM>/L on a dry mass basis. In Table <NUM>, the first catalyst portion <NUM> is indicated as "lower layer" of "first catalyst portion", and the mass of the first catalyst portions <NUM> was indicated as the "amount of WC" (amount of wash coat).

The lower layer 15A and the upper layer 15B of each second catalyst portion <NUM> of the exhaust gas purifying catalyst <NUM> were formed on the surface of the partition wall <NUM> facing the outflow-side cell <NUM> so as to extend from the downstream end portion R2 toward the upstream side in the exhaust gas flow direction X to <NUM>% of the overall length L. The mass of the lower layers 15A per volume of the substrate in the portion where the second catalyst portions <NUM> were formed was <NUM>/L on a dry mass basis, and that of the upper layers 15B was <NUM>/L on a dry mass basis. In Table <NUM>, the lower layer 15A is indicated as "lower layer" of "second catalyst portion", the upper layer 15B is indicated as "upper layer" of "second catalyst portion", and the mass of each layer is indicated as "amount of WC" (amount of wash coat).

An exhaust gas purifying catalyst <NUM> of Example <NUM> was obtained in the same manner as in Example <NUM> except that, in "<NUM>. Formation of Second Catalyst Portion Before Calcining", the order in which the first slurry and the second slurry were applied was reversed.

An exhaust gas purifying catalyst <NUM> of Example <NUM> was obtained in the same manner as in Example <NUM> except for the following.

Formation of First Catalyst Portion Before Calcining", the upstream end portion of the substrate <NUM> in the exhaust gas flow direction was immersed in the second slurry (Pd-containing slurry) and dried. Next, the upstream end portion of the substrate <NUM> was then further immersed in the first slurry (Rh-containing slurry), the slurry was drawn by suction from the downstream side, and then, the substrate was dried at <NUM> for <NUM> minutes. In this manner, the lower layer (layer formed of the second slurry) and the upper layer (layer formed of the first slurry) were stacked on the substrate <NUM>. Also, in "<NUM>. Formation of Second Catalyst Portion Before Calcining", the downstream end portion of the substrate <NUM> in the exhaust gas flow direction was immersed in the first slurry, the slurry was drawn by suction from the upstream side, and then the substrate was dried at <NUM> for <NUM> minutes. In this manner, the lower layer was formed. The upper layer was not formed. Each layer had a mass (amount of WC) shown in Table <NUM>.

An exhaust gas purifying catalyst <NUM> of Example <NUM> was obtained in the same manner as in Example <NUM> except that "<NUM>. Formation of First Catalyst Portion Before Calcining" in Example <NUM> was changed to the same steps as those of Example <NUM>.

An exhaust gas purifying catalyst <NUM> of Comparative Example <NUM> was obtained in the same manner as in Example <NUM> except for the following.

Formation of Second Catalyst Portion Before Calcining" in Example <NUM>, the first slurry (Rh-containing slurry) and the second slurry (Pd-containing slurry) were mixed at a mass ratio of first slurry: second slurry = <NUM>:<NUM> in terms of the solid content to obtain a mixed slurry. The downstream end portion of the substrate <NUM> in the exhaust gas flow direction was immersed in the resulting mixed slurry, the substrate <NUM> having the first catalyst portions before calcining. The slurry was drawn by suction from the upstream side, and then the substrate was dried at <NUM> for <NUM> minutes. In this manner, the lower layers were formed. The mass of the lower layers was equal to the total mass of the lower layers 15A and the upper layers 15B of Example <NUM>. The upper layer was not formed.

Formation of Second Catalyst Portion Before Calcining" in Example <NUM>, the upper layer was not formed. The mass of the lower layers was equal to the total mass of the lower layers 15A and the upper layers 15B of Example <NUM>.

An exhaust gas purifying catalyst <NUM> of Comparative Example <NUM> was obtained in the same manner as in Comparative Example <NUM> except that the first catalyst portion and the second catalyst portion were interchanged.

The exhaust gas purifying catalysts of Examples and Comparative Examples were evaluated in the following manner.

A vehicle in which the exhaust gas purifying catalyst <NUM> was included was driven in accordance with the driving conditions of the Worldwide Harmonized Light Vehicles Test Cycles (WLTC). The number of PM particles contained in the exhaust gas passing through the exhaust gas purifying catalyst, PNcat, was counted for each of the following periods: a low speed driving period (from <NUM> to <NUM> seconds after the start of driving); a medium speed driving period (from <NUM> seconds to <NUM> seconds after the start of driving); a high speed driving period (from <NUM> seconds to <NUM> seconds after the start of driving); and an extra-high speed driving period (from <NUM> seconds to <NUM> seconds after the start of driving). The number of PM particles discharged directly from the engine, PNall, was also counted. The PM collection rate was determined by the following equation. The results are shown in Table <NUM>.

Each of the exhaust gas purifying catalysts of Examples and Comparative Examples was placed in an exhaust path of the engine, and the engine with the exhaust gas purifying catalyst were each exposed to the following degradation conditions for a durability test comparable to driving <NUM>,<NUM> to <NUM>,<NUM> kilometers.

After performing the durability test under the above-described conditions, each of the exhaust gas purifying catalysts that had undergone the durability test was installed in a vehicle described below. Next, the vehicle was driven in accordance with the driving conditions of the Worldwide Harmonized Light Vehicles Test Cycles (WLTC). The respective amounts emitted (emission values) of nitrogen oxide (NOx), non-methane hydrocarbon (NMHC), and carbon monoxide (CO) contained in the exhaust gas passing through the exhaust gas purifying catalyst <NUM> were measured for each of the following periods: a low speed driving period (from <NUM> to <NUM> seconds after the start of driving); a medium speed driving period (from <NUM> seconds to <NUM> seconds after the start of driving); a high speed driving period (from <NUM> seconds to <NUM> seconds after the start of driving); and an extra-high speed driving period (from <NUM> seconds to <NUM> seconds after the start of driving). The respective total amounts of NOx, NMHC, and CO emitted is shown in Table <NUM>. The amount of NOx emitted only in the extra-high speed driving period (from <NUM> seconds to <NUM> seconds after the start of driving) was also measured. The amount emitted is indicated by "NOx Ex-high" in Table <NUM>.

Claim 1:
An exhaust gas purifying catalyst comprising a substrate and catalyst portions provided in the substrate,
the substrate including:
inflow-side cells, each inflow-side cell being a space having an open end on an inflow side thereof and a closed end on an outflow side thereof in an exhaust gas flow direction;
outflow-side cells, each outflow-side cell being a space having a closed end on an inflow side thereof and an open end on an outflow side thereof in the exhaust gas flow direction; and
porous partition walls, each partition wall separating the inflow-side cell from the outflow-side cell, and
the catalyst portions including:
(group A) first catalyst portions, each first catalyst portion being provided at least on a part of a surface of the partition wall that faces the inflow-side cell; and
(group B) second catalyst portions, each second catalyst portion being provided at least on a part of a surface of the partition wall that faces the outflow-side cell,
wherein each catalyst portion of one of group A and group B includes at least one oxidizing catalyst layer and at least one reducing catalyst layer, and
each catalyst portion of the other of group A and group B includes at least one oxidizing catalyst layer and/or at least one reducing catalyst layer, and
wherein one of the first catalyst portion and the second catalyst portion includes a stack of an oxidizing catalyst layer and a reducing catalyst layer, and the other of the first catalyst portion and the second catalyst portion is a single oxidizing catalyst layer, a single reducing catalyst layer, or a stack of an oxidizing catalyst layer and a reducing catalyst layer.