Exhaust gas purification catalyst

Abstract

Provided is an exhaust gas purification catalyst having improved NOx-occluding performance under a lean atmosphere. An exhaust gas purification catalyst 100 disclosed herein is provided with a base material 10 and a catalyst layer 20. The catalyst layer 20 includes a bottom layer 22, a middle layer 24, and a top layer 26. The top layer 26 contains Rh. The middle layer 24 at least includes Pt and a NOx-occluding material. The bottom layer 22 has a bottom layer front part 22a containing Pd and a bottom layer rear part 22b containing Pd. A Pd content (CF) in the bottom layer front part 22a is greater than a Pd content (CR) in the bottom layer rear part 22b.

Description

Exhaust gas purification catalyst

[0001] The present invention relates to an exhaust gas purification catalyst.

[0002] Exhaust gases emitted from internal combustion engines such as automobile engines contain hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NO ｘ ) and other harmful components. Conventionally, in order to remove these harmful components, an exhaust gas purification catalyst comprising a substrate and a catalyst layer containing a catalytic metal has been used. Exhaust gas supplied to the exhaust gas purification catalyst comes into contact with the catalyst layer, and the harmful components are purified. For example, HC and CO in the exhaust gas are oxidized to water (H ２ O) and carbon dioxide (CO ２ ), and NOx in the exhaust gas is reduced to nitrogen (N ２ ) is converted (purified).

[0003] However, when an internal combustion engine is started, the exhaust gas purification catalyst is not sufficiently warmed up, and the activity of the catalytic metal is low. Therefore, there is a risk that exhaust gas containing residual harmful components will be emitted until the catalytic metal reaches a predetermined activation temperature. Therefore, CO and HC can be reduced by leaning the air-fuel ratio (A / F) of the mixture supplied at the start of the internal combustion engine and controlling the internal combustion engine in a lean (excess oxygen) state, so-called lean start control. However, in a lean atmosphere, it is difficult to extract oxygen from NOx, which poses a problem of not being able to purify NOx.

[0004] Therefore, in order to suppress NOx emissions during the warm-up process during lean start control, NOx storage-reduction (NSR) catalysts containing NOx storage materials are widely used (see Patent Documents 1 and 2). For example, Patent Document 1 discloses an NSR catalyst having a three-layer catalyst layer structure, in which the lower layer contains Pt and / or Pd, the middle layer contains Pt and / or Pd and a NOx storage material, and the upper layer contains Rh.

[0005] Japanese Patent Application Publication No. 2018-143935 Japanese Patent Application Publication No. 2016-93760

[0006] In the catalyst layer containing the NOx storage material as disclosed in Patent Documents 1 and 2, the NOx storage reaction occurs in a lean atmosphere. That is, since NOx in exhaust gas is mostly NO, NOx is generally first oxidized by a catalytic metal such as Pt, and then NOx is removed. ２ is generated. ２ reacts with the NOx storage material (e.g., alkaline earth metal) to form nitrates, which are temporarily captured in the NOx storage material. This suppresses the emission of NOx. ２ When the control is switched from stoichiometric (theoretical air-fuel ratio) to rich (excess fuel) atmosphere, the carbon monoxide is desorbed from the catalyst layer and reduced on the catalyst metal by using reducing gases such as HC and CO as reducing agents. As a result, the NOx components in the exhaust gas are converted to nitrogen (N ２ ) is converted (purified).

[0007] In recent years, exhaust gas regulations have tended to become even stricter. Furthermore, for example, in eco-cars equipped with energy-saving mechanisms, the internal combustion engine may be frequently stopped and started while driving. Therefore, there is a demand for exhaust gas purification catalysts equipped with NOx storage materials to further improve NOx storage performance in lean atmospheres, i.e., to further reduce NOx emissions.

[0008] The present inventors have attempted to improve the NOx storage performance in a lean atmosphere by taking a different approach than conventional approaches. More specifically, they have attempted to improve the NOx storage performance in a lean atmosphere by causing the NOx storage reaction, particularly the NO oxidation reaction, to occur early.

[0009] That is, in order to cause the above-mentioned NOx absorption reaction, NO is oxidized to NO ２ However, according to the study by the present inventors, if CO coexists with NO at this time, the oxidation reaction of CO occurs preferentially, and the oxidation reaction of NO is inhibited. ２ It has been newly discovered that this makes it difficult for NOx to be generated, delaying the onset of the NOx storage reaction.

[0010] CO is primarily purified by Pd, but in a catalyst layer such as that disclosed in Patent Document 1, in which a NOx storage material is contained in the middle layer and Pd is contained in the lower layer, CO is particularly likely to remain in the middle layer, which tends to inhibit the oxidation reaction of NO. Therefore, the inventors of the present invention have considered that, in order to improve the NOx storage performance in a lean atmosphere in a catalyst layer containing a NOx storage material in the middle layer and Pd in ​​the lower layer, it is important to purify CO as quickly as possible during the warm-up process to promote the oxidation reaction of NO and to quickly initiate the NOx storage reaction. The present invention was completed based on this finding.

[0011] The exhaust gas purifying catalyst [1] disclosed herein is an exhaust gas purifying catalyst that is disposed in an exhaust path of an internal combustion engine and that purifies exhaust gas emitted from the internal combustion engine, and includes a substrate and a catalyst layer formed on the substrate. The catalyst layer includes a lower layer located on the substrate side, an upper layer located on the surface layer side, and a middle layer located between the lower layer and the upper layer. The upper layer contains Rh. The middle layer contains at least Pt and a NOx storage material. The lower layer has a lower layer front portion located upstream in the exhaust gas flow direction when disposed in the exhaust path, and a lower layer rear portion located downstream in the exhaust gas flow direction, and the lower layer front portion and the lower layer rear portion each contain Pd. The Pd content (C Ｆ ) is the Pd content (C Ｒ ) is larger than

[0012] In the exhaust gas purifying catalyst [1], the lower layer containing Pd has an upstream lower layer front portion and a downstream lower layer rear portion, and the Pd content (C Ｆ ) is the Pd content of the lower layer rear part (C Ｒ ) is larger than C Ｆ >C ＲNormally, when an internal combustion engine is started, the lower front portion of the catalyst layer is likely to be heated by exhaust gas. Therefore, by increasing the Pd content in the lower front portion, which is likely to be heated during startup (in other words, by biasing the Pd concentration upstream), the CO purification reaction can be actively initiated. This allows CO to be purified quickly, making it less likely that the NO oxidation reaction will be inhibited. Therefore, the NO oxidation reaction can be smoothly initiated, and the NOx storage reaction can be initiated early.

[0013] Furthermore, the heat of reaction (heat capacity) generated during the CO purification reaction is transferred downstream with the flow of exhaust gas, improving the warm-up performance of the entire catalyst layer. As a result, the catalyst metal can be quickly heated to its activation temperature (e.g., 200 to 250°C). Therefore, the exhaust gas purification catalyst [1] can improve the NOx storage performance in a lean atmosphere, especially during the start-up of an internal combustion engine.

[0014] The exhaust gas purifying catalyst [2] disclosed herein is the exhaust gas purifying catalyst [1], wherein the C Ｒ The above C Ｆ The ratio (C Ｆ / C Ｒ ) satisfies the following formula: 1.5≦(C Ｆ / C Ｒ )≦3.0; This makes it possible to improve the NOx purification performance in a stoichiometric to rich atmosphere, and reduce NOx emissions in a wide range of atmospheres from lean to rich.

[0015] The exhaust gas purifying catalyst [3] disclosed herein is the exhaust gas purifying catalyst [1] or [2], in which the total amount of Pd contained in the entire lower layer per 1 L of the substrate is 3.0 g / L-cat or less. In such cases, application of the technology disclosed herein is particularly effective.

[0016] The exhaust gas purifying catalyst [4] disclosed herein is any one of the exhaust gas purifying catalysts [1] to [3], wherein the lower layer front section and the lower layer rear section each contain an OSC material having oxygen storage capacity and a non-OSC material not having oxygen storage capacity, and the content of the non-OSC material in the lower layer front section is greater than the content of the non-OSC material in the lower layer rear section, and the content of the OSC material in the lower layer front section is less than the content of the OSC material in the lower layer rear section, thereby enabling excellent exhaust gas purifying performance to be exhibited over a long period of time.

[0017] The exhaust gas purifying catalyst [5] disclosed herein is any one of the exhaust gas purifying catalysts [1] to [4], in which the coating length of the lower layer front portion in the exhaust gas flow direction is shorter than the coating length of the lower layer rear portion in the exhaust gas flow direction, thereby enabling the effects of the technology disclosed herein to be exerted at a high level and further improving NOx occlusion performance in a lean atmosphere.

[0018] The exhaust gas purifying catalyst [6] disclosed herein is any one of the exhaust gas purifying catalysts [1] to [5], wherein the coating length of the lower layer front portion in the exhaust gas flow direction is 30% to 60% of the total length of the substrate, and the coating length of the lower layer rear portion in the exhaust gas flow direction is 60% to 90% of the total length of the substrate, thereby achieving both improved NOx storage performance in a lean atmosphere and improved NOx purification performance in a stoichiometric to rich atmosphere at a stable and high level.

[0019] FIG. 1 is a perspective view schematically showing an exhaust gas purifying catalyst according to one embodiment. FIG. 2 is a partial cross-sectional view of the exhaust gas purifying catalyst of FIG. 1 cut in the cylinder axis direction. FIG. 3 is a cross-sectional view schematically showing the configuration of the lower layer in Test I. FIG. 4 is a graph showing the relationship between the Pd content ratio (C Ｆ / C Ｒ 5 is a graph showing the relationship between the Pd content ratio (C Ｆ / C Ｒ6 is a graph showing the relationship between the total amount of Pd in ​​the lower layer and the time to start occluding NOx. FIG. 7 is a graph showing the relationship between the total amount of Pd in ​​the lower layer and the time to start occluding NOx. FIG. 8 is a graph showing the relationship between the total amount of Pd in ​​the lower layer and the time to start occluding NOx.

[0020] Preferred embodiments of the present invention will be described below with reference to the drawings. Matters necessary for implementing the present invention other than those specifically mentioned in this specification (e.g., a general method for manufacturing an exhaust gas purification catalyst) can be understood as design matters for a person skilled in the art based on the prior art in the relevant field. The present invention can be implemented based on the contents disclosed in this specification and the technical common sense in the relevant field. Furthermore, in the following drawings, components and parts that perform the same function are designated by the same reference numerals, and redundant explanations may be omitted or simplified. Dimensional relationships (e.g., length, width, thickness) in each figure do not necessarily reflect actual dimensional relationships. Furthermore, in this specification, the notation "A to B" (A and B are arbitrary numbers) indicating a range includes not only the meaning of A or more and not more than B, but also the meanings of "greater than A (exceeding A)" and "smaller than B (less than B)."

[0021] FIG. 1 is a schematic diagram of an exhaust gas purification catalyst 100. The exhaust gas purification catalyst 100 is disposed in the exhaust path of an internal combustion engine to purify exhaust gas emitted from the internal combustion engine. The exhaust gas purification catalyst 100 includes a substrate 10 and a catalyst layer 20 (see FIG. 2) formed on the substrate 10. In FIG. 1 and other figures, arrow F indicates the flow direction of exhaust gas when the catalyst is disposed in the exhaust path. Arrow X indicates the cylindrical axis direction of the substrate 10. X1 indicates the upstream side (front side) in the exhaust gas flow direction F, and X2 indicates the downstream side (rear side) in the exhaust gas flow direction F.

[0022] The exhaust gas purifying catalyst 100 can be disposed in the exhaust system (exhaust pipe) of various internal combustion engines, particularly automobile engines, by appropriately selecting, for example, the type and shape of the substrate 10 and the design of the catalyst layer 20, which will be described later. In the following, the exhaust gas purifying catalyst 100 will be described on the assumption that the internal combustion engine is an automobile gasoline engine, but it is not intended that the exhaust gas purifying catalyst 100 be limited to such an application.

[0023] <Substrate 10> The substrate 10 constitutes the skeleton of the exhaust gas purification catalyst 100. As the substrate 10, various materials and shapes conventionally used for this type of application can be adopted. For example, ceramics such as cordierite, aluminum titanate, and silicon carbide are suitable as materials because of their high heat resistance. Alternatively, a substrate made of an alloy (stainless steel, etc.) can be used.

[0024] In terms of configuration, the substrate 10 has a honeycomb structure having a plurality of cells 12 regularly arranged along the cylinder axis direction X (the flow direction of exhaust gas) and rib walls 14 that separate the plurality of cells 12. The cells 12 are through-holes that function as exhaust gas passages. The rib walls 14 are partition walls that separate the individual cells 12. The cross-sectional shape of the cells 12 is rectangular here. However, the cross-sectional shape of the cells 12 may be other shapes (e.g., circular, triangular, hexagonal, etc.).

[0025] The outer shape of the substrate 10 is cylindrical here. However, the outer shape of the substrate 10 may be other shapes (e.g., elliptical cylinder, polygonal cylinder, etc.). The substrate 10 is honeycomb-shaped here. However, the substrate 10 may also be foam-shaped, pellet-shaped, etc. The volume of the substrate 10 is typically about 0.1 to 10 L, for example, 0.5 to 5 L. The overall length L (see FIG. 2 ) of the substrate 10 in the cylindrical axis direction X is typically about 10 to 500 mm, for example, 50 to 300 mm. In this specification, the volume of the substrate 10 refers to the bulk volume (apparent volume) including the net volume of the substrate 10 as well as the volume of internal voids such as the cells 12.

[0026] 1 is a so-called straight-flow type substrate in which the inlet and outlet openings of the cells 12 are not blocked. However, the substrate 10 may also be a so-called wall-flow type (also called wall-through type) substrate in which the inlet and outlet openings of a large number of cells 12 are alternately blocked, and exhaust gas flows from one cell (inlet cell) through the rib wall 14 to the adjacent cell (outlet cell).

[0027] <Catalyst Layer 20> Fig. 2 is a partial cross-sectional view of the exhaust gas purification catalyst 100 taken along the cylinder axis direction X. As shown in Fig. 2, the catalyst layer 20 is formed on the substrate 10 (specifically, the rib walls 14). However, a portion of the catalyst layer 20 may penetrate into the rib walls 14. The catalyst layer 20 constitutes the main part of the exhaust gas purification catalyst 100, serving as a site for purifying exhaust gas. The exhaust gas supplied to the exhaust gas purification catalyst 100 comes into contact with the catalyst layer 20 while flowing (passing) through the flow paths (cells 12) of the substrate 10. Harmful components in the exhaust gas come into contact with the catalyst layer 20 and are purified.

[0028] As shown in FIG. 2 , the catalyst layer 20 has a multi-layer structure, specifically a three-layer structure. The catalyst layer 20 includes a lower layer 22 located on the substrate 10 side in the thickness direction, an upper layer 26 located on the surface layer side of the catalyst layer 20, and a middle layer 24 located between the lower layer 22 and the upper layer 26. However, the catalyst layer 20 may include layers other than the lower layer 22, middle layer 24, and upper layer 26 as long as the effects of the present invention are not significantly impaired. That is, the catalyst layer 20 may have a stacked structure of four or more layers. In this case, the lower layer 22 and the middle layer 24 are preferably in contact with each other in the thickness direction. Furthermore, the middle layer 24 and the upper layer 26 are preferably in contact with each other in the thickness direction.

[0029] The coating amount of the entire catalyst layer 20 can be appropriately determined depending on the type of substrate 10 and the like, and is not particularly limited, but is, for example, 150 to 450 g / L-cat, preferably 200 to 400 g / L-cat, and more preferably 250 to 350 g / L-cat per 1 L of volume of substrate 10. In this specification, the unit [g / L-cat] indicates the content contained per unit volume of one substrate.

[0030] The coating length of the multi-layer structure (particularly a three-layer structure) portion of the catalyst layer 20, i.e., the average coating length in the cylindrical axis direction X of the substrate 10, may be, for example, 60% or more of the total length L in the cylindrical axis direction X of the substrate 10, and may be 70% or more, 80% or more, 90% or more, or 100% of the total length L in the cylindrical axis direction X of the substrate 10. Note that, although the lower layer 22, the middle layer 24, and the upper layer 26 are shown in Fig. 2 to have the same coating length, the lower layer 22, the middle layer 24, and the upper layer 26 may have different coating lengths as long as they are stacked in three layers in at least a portion of the cylindrical axis direction X.

[0031] The coating thickness of the multilayer structure (particularly a three-layer structure) portion of the catalyst layer 20, i.e., the average thickness in the direction perpendicular to the cylinder axis direction X, may be, for example, 3 to 300 μm, 5 to 200 μm, or 10 to 100 μm. While FIG. 2 illustrates the lower layer 22, middle layer 24, and upper layer 26 as having the same thickness, the lower layer 22, middle layer 24, and upper layer 26 may have different thicknesses. While not particularly limited, the coating thickness of the lower layer 22 is, for example, 10 to 30 μm, preferably 15 to 25 μm. The coating thickness of the middle layer 24 is, for example, 40 to 60 μm, preferably 45 to 55 μm. The coating thickness of the upper layer 26 is, for example, 20 to 40 μm, preferably 25 to 35 μm. Furthermore, the coating thicknesses of the lower layer 22, middle layer 24, and upper layer 26 do not have to be uniform. For example, the lower layer 22 may have a thicker coating thickness in the central portion in the axial direction X of the cylinder.

[0032] The catalyst layer 20 contains at least (1) a catalytic metal and (2) a NOx storage material. Preferably, the catalyst layer 20 further contains (3) an oxygen storage material (OSC material) having oxygen storage capacity, and / or (4) a non-OSC material not having oxygen storage capacity. The OSC material and / or the non-OSC material may be contained in the catalyst layer 20 as a carrier that supports the catalytic metal, or may be contained in the catalyst layer 20 in a form that does not support the catalytic metal. Furthermore, the catalyst layer 20 may be composed mainly of the OSC material and / or the non-OSC material (a component that accounts for 50 mass% or more of the total; the same applies hereinafter).

[0033] (1) The catalytic metal essentially contains three platinum group precious metals (PGMs): rhodium (Rh), palladium (Pd), and platinum (Pt). In addition to the other three types, the catalytic metal may further contain various metal species capable of functioning as oxidation catalysts and / or reduction catalysts in purifying harmful components in exhaust gas. Examples of PGMs include other PGMs, such as ruthenium (Ru), osmium (Os), iridium (Ir), iron group metals such as iron (Fe), cobalt (Co), and nickel (Ni), as well as gold (Au), silver (Ag), and copper (Cu). The catalytic metal is typically supported on a support (e.g., an OSC material and / or a non-OSC material, as described below). The catalytic metal is preferably contained in each of the lower layer 22, middle layer 24, and upper layer 26. The lower layer 22, middle layer 24, and upper layer 26 preferably contain different types of catalytic metals as the main component. The relationship between each layer and the type of catalytic metal will be described later.

[0034] From the viewpoint of increasing the contact area with exhaust gas, the catalytic metal is preferably fine particles with a sufficiently small particle size. The average particle size of the catalytic metal is, for example, 1 to 15 nm, preferably 1 to 10 nm, and more preferably 1 to 5 nm. The average particle size of the catalytic metal can be determined by acquiring a transmission electron microscope (TEM) image of the catalytic metal and averaging the particle sizes of 50 or more catalytic metal particles selected arbitrarily from the image.

[0035] The total amount of catalytic metal contained in the entire catalyst layer 20 can be appropriately determined depending on, for example, the amount of exhaust gas, the application, the type of catalytic metal, etc., and is not particularly limited, but may be, for example, 0.5 g / L-cat or more, preferably 1.0 g / L-cat or more, and more preferably 2.0 g / L-cat or more per 1 L of the volume of the substrate 10. On the other hand, the total amount of catalytic metal contained in the entire catalyst layer 20 may be, for example, 8.0 g / L-cat or less, preferably 7.0 g / L-cat or less, and more preferably 6.0 g / L-cat or less per 1 L of the volume of the substrate 10.

[0036] (2) As the NOx storage material, known materials conventionally used for this type of application can be used. The NOx storage material typically contains at least one of an alkali metal element and an alkaline earth metal element as a NOx storage element. Examples include compounds containing the at least one NOx storage element in the form of a carbonate or an oxide, or a combination thereof. Examples of alkali metal elements include Li, K, and Cs, with K being preferred. Examples of alkaline earth metal elements include Mg, Sr, and Ba, with Ba being preferred. The NOx storage material is essentially contained in the middle layer 24.

[0037] The total amount of NOx storage material contained in the entire catalyst layer 20 is not particularly limited, but is preferably 100 g / L-cat or less per liter of the volume of the substrate 10, for example, 10 to 50 g / L-cat.

[0038] (3) As the OSC material, known materials conventionally used for this type of application can be used. For example, ceria (cerium oxide, CeO ２ ) and composite oxides containing ceria, for example, composite oxides containing ceria and zirconia (ceria-zirconia composite oxides, so-called CZ composite oxides (also referred to as ZC composite oxides)). From the viewpoint of suppressing thermal degradation of ceria, the OSC material preferably contains a CZ composite oxide, and more preferably consists essentially of a CZ composite oxide (accounting for 95 mass% or more of the total; the same applies hereinafter). From the viewpoint of preventing oxygen depletion, etc., the OSC material is preferably contained in each of the lower layer 22, middle layer 24, and upper layer 26.

[0039] The OSC material (e.g., CZ composite oxide) may contain other additive components in addition to ceria and zirconia as the main components, from the viewpoint of improving heat resistance and oxygen absorption / release characteristics. Examples of additive components that may be contained in the OSC material include oxides containing rare earth elements, alkali metal elements, alkaline earth metal elements, transition metal elements, Si, Al, etc. Examples of rare earth elements include Sc, Y, La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, etc. The oxide of a rare earth element is preferably Pr.２ O ３ , Nd ２ O ３ , La ２ O ３ , and Y ２ O ３ The additive component may be a polycrystalline substance or a single crystal.

[0040] When the OSC material contains ceria, the ceria content in the entire OSC material is preferably 10% by mass or more, more preferably 25% by mass or more, from the viewpoint of fully exhibiting its oxygen storage capacity. On the other hand, the ceria content is preferably 90% by mass or less, more preferably 75% by mass or less, from the viewpoint of appropriately suppressing the basicity of the OSC material.

[0041] The total amount of the OSC material contained in the entire catalyst layer 20 is not particularly limited, but is preferably 45 to 250 g / L-cat, and more preferably 80 to 200 g / L-cat per 1 L of the volume of the substrate 10.

[0042] (4) As the non-OSC material, known materials conventionally used for this type of application can be used. Among them, from the viewpoint of supporting the catalytic metal in a highly dispersed state, inorganic porous materials having a relatively large specific surface area and excellent heat resistance can be preferably used. As an example, alumina (Al ２ O ３ , aluminum oxide), titania (TiO ２ , titanium oxide), zirconia (ZrO ２ , zirconium oxide), silica (SiO ２ , silicon oxide), etc. Among these, the non-OSC material preferably contains alumina, and more preferably consists essentially of alumina, because it has particularly excellent heat resistance. From the viewpoint of improving heat resistance and durability, etc., the non-OSC material is preferably contained in each of the lower layer 22, the middle layer 24, and the upper layer 26.

[0043] Non-OSC materials (e.g., alumina) may contain the above-mentioned additive components, such as Pr, from the viewpoint of improving heat resistance. ２ O ３ , Nd ２ O ３ , La２ O ３ , Y ２ O ３ The oxides of rare earth elements such as Cr, Ni, and Ni may also be included.

[0044] The total amount of non-OSC materials contained in the entire catalyst layer 20 is not particularly limited, but is preferably 100 to 370 g / L-cat, and more preferably 170 to 300 g / L-cat per 1 L of the volume of the substrate 10.

[0045] In addition to the above (1) to (4), the catalyst layer 20 may further contain an auxiliary material. The auxiliary material is intended, for example, to suppress sintering or poisoning of the catalyst metal or to improve the oxygen storage capacity of the OSC material. Known materials conventionally used for this type of application can be used as the auxiliary material. Examples include simple metals, alloys, and compounds (e.g., oxides, sulfates, carbonates, nitrates, and chlorides) containing one or more metal elements selected from alkali metal elements, alkaline earth metal elements, rare earth metal elements, and transition metal elements. Layers that may contain auxiliary materials will be described later.

[0046] The lower layer 22, the middle layer 24, and the upper layer 26 included in the catalyst layer 20 will be described in detail below.

[0047] <Lower Layer 22> As shown in Figure 2, the lower layer 22 is formed directly on the surface of the substrate 10. In this embodiment, the lower layer 22 has a lower-layer front portion 22a located on the upstream side X1 in the exhaust gas flow direction F, and a lower-layer rear portion 22b located on the downstream side X2 in the exhaust gas flow direction F relative to the lower-layer front portion 22a. The lower-layer front portion 22a is provided from an end of the substrate 10 on the upstream side X1 along the cylindrical axis direction X. The lower-layer rear portion 22b is provided from an end of the substrate 10 on the downstream side X2 along the cylindrical axis direction X. The lower layer 22, particularly the lower-layer front portion 22a, functions as a CO purification layer in a lean atmosphere, for example, during lean start control.

[0048] The lower-layer front portion 22a and the lower-layer rear portion 22b each contain Pd as a catalytic metal. Pd has particularly high oxidation activity among catalytic metals and is particularly excellent in CO purification performance. By disposing Pd in ​​the lower layer 22, which is easily heated during startup, the CO purification reaction can be favorably initiated during lean start control. Furthermore, oxidation catalysts such as Pd are easily affected by poisoning substances (e.g., sulfur) contained in exhaust gas. Therefore, by disposing Pd in ​​the lower layer 22 away from the upper layer 26, where poisoning substances are likely to adhere, contact between Pd and poisoning substances can be suppressed.

[0049] The total amount of Pd contained in the entire lower layer 22 is not particularly limited, but may be, for example, 0.1 g / L-cat or more, 0.5 g / L-cat or more, preferably 1.0 g / L-cat or more, and more preferably 2.0 g / L-cat or more per 1 L of the volume of the substrate 10. This allows the CO purification reaction to occur more actively during lean start control, allowing the effects of the technology disclosed herein to be exerted to a high level. On the other hand, the total amount of Pd contained in the entire lower layer 22 may be, for example, 5.0 g / L-cat or less, preferably 4.0 g / L-cat or less, more preferably 3.0 g / L-cat or less, or even 2.9 g / L-cat or less, or 2.5 g / L-cat or less per 1 L of the volume of the substrate 10. In such cases, the CO purification reaction becomes slow during lean start control, and the oxidation reaction of NO, in particular, is likely to be inhibited. Therefore, applying the technology disclosed herein is particularly effective.

[0050] In this embodiment, the Pd content (C Ｆ ) is the Pd content (C Ｒ ) is larger than C Ｆ >C Ｒ In other words, the Pd content (C Ｒ ) the Pd content of the lower layer front portion 22a relative to the Ｆ ) ratio (C Ｆ / C Ｒ ) satisfies the following formula: 1<(C Ｆ / C Ｒ) is satisfied. By increasing the Pd content in the lower layer front portion 22a, which is easily heated during startup, the CO purification reaction can be actively initiated. This allows CO to be purified quickly and the NO oxidation reaction to be started smoothly. In addition, the reaction heat (heat capacity) generated during the CO purification reaction is transferred downstream along with the flow of exhaust gas, improving the warm-up properties of the entire catalyst layer. As a result, the catalyst metal can be quickly heated to its activation temperature. Therefore, it is possible to prevent NOx from passing through the exhaust gas purification catalyst 100 and being released as emissions, particularly during startup of the internal combustion engine.

[0051] In this specification, the term "content" refers to the content (g / L) of a specific substance per unit volume of the substrate in the portion coated with the catalyst layer. Hereinafter, the Pd content (C Ｆ ) as an example, the measurement procedure will be described. In this measurement, first, a scanning electron microscope (SEM) is used to identify the lower layer front portion 22a, and its total length (coat length) La is calculated. Next, only the lower layer front portion 22a is cut out to a predetermined length L1 and pulverized into powder. This powder is subjected to inductively coupled plasma analysis (ICP) to measure the weight w1 (g) of Pd per predetermined length L1. Then, the weight W (g) of Pd contained in the entire lower layer front portion 22a is calculated using the following formula: W = (w1 × La) / L1. Next, the substrate volume Va (L) of the portion coated with the lower layer front portion 22a is determined. The substrate volume Va of the portion coated with the lower layer front portion 22a is calculated by multiplying the substrate volume V (L) by the ratio of the coat length La of the lower layer front portion 22a to the substrate length L (i.e., substrate volume V × coat length La / substrate length L). Therefore, the Pd content of the lower front portion 22a (C ＦThe Pd content (unit: g / L) is calculated by dividing the "weight W (g) of Pd contained in the entire lower layer front portion 22a" by the "volume Va (L) of the lower layer front portion 22a." The substrate volume V is calculated from the substrate length x the substrate cross-sectional area, and the substrate cross-sectional area is calculated from pi x the square of the substrate radius. In the following description, when simply referring to the "content," it refers to a value measured according to the same procedure as the above-mentioned "Pd content in the lower layer front portion 22a." In other words, in this specification, the unit [g / L] indicates the amount per unit volume of the substrate in the portion coated with the catalyst layer.

[0052] The above ratio (C Ｆ / C Ｒ From the viewpoint of achieving the effects of the technology disclosed herein at a high level, the ratio (C Ｆ / C Ｒ Although the upper limit of the ratio (C) is not particularly limited, when the ratio becomes large to a certain extent, the effect of the technology disclosed herein reaches a plateau. Ｆ / C Ｒ ) is generally 10 or less, 5.0 or less, preferably 4.0 or less, more preferably 3.5 or less, and even more preferably 3.0 or less.

[0053] Among them, the above ratio (C Ｆ / C Ｒ ) satisfies the following formula: 1.2≦(C Ｆ / C Ｒ )≦3.5; and it is preferable that the following formula is satisfied: 1.5≦(C Ｆ / C Ｒ )≦3.0; and more preferably satisfies the following formula: 1.5≦(C Ｆ / C Ｒ )≦2.0 is particularly preferred. This makes it possible to improve the NOx purification performance in a stoichiometric to rich atmosphere. Therefore, NOx emissions can be reduced in a wide range of atmospheres from lean to rich.

[0054] Pd content of the lower layer front portion 22a (C Ｆ) is not particularly limited, but is preferably about 0.1 g / L or more, 0.5 g / L or more, preferably 1.0 g / L or more, more preferably 2.0 g / L or more, and particularly preferably 2.5 g / L or more. This can further improve the NOx occlusion performance in a lean atmosphere, and the effects of the technology disclosed herein can be exerted at a high level. The Pd content (C Ｆ If the content of the catalytic metal is too high, the effect of the technology disclosed herein may plateau or the catalytic metal may be prone to grain growth. Therefore, the catalytic metal content may be 6.0 g / L or less, preferably 5.0 g / L or less, more preferably 4.0 g / L or less, and even 3.5 g / L or less.

[0055] Pd content of the lower rear portion 22b (C Ｒ ) is not particularly limited, but is preferably 0.1 g / L or more, 0.5 g / L or more, and more preferably 1.0 g / L or more. This can improve the NOx purification performance in a stoichiometric to rich atmosphere. Ｒ If the content of the catalytic metal is too high, it is disadvantageous in terms of cost, and so on. Therefore, the catalytic metal content may be 4.0 g / L or less, preferably 3.0 g / L or less, more preferably 2.0 g / L or less, for example, 1.5 g / L or less.

[0056] The lower-layer front portion 22a and / or the lower-layer rear portion 22b may contain a catalytic metal other than Pd. Examples include the platinum group noble metals (PGMs) described above. Among these, Pt, which has high oxidation activity, is preferred. In the lower-layer front portion 22a and the lower-layer rear portion 22b, the Pd content of the entire catalytic metal is preferably 80 mass% or more, more preferably 90 mass% or more. It is particularly preferred that the catalytic metal consists essentially of Pd.

[0057] Preferably, the lower-layer front portion 22a and the lower-layer rear portion 22b each contain a non-OSC material. The non-OSC material may be a carrier supporting a catalytic metal such as Pd, or may be in a form that does not support a catalytic metal such as Pd. The non-OSC material may be the first component (the component with the highest content by mass; the same applies hereinafter) of the lower-layer front portion 22a and the lower-layer rear portion 22b. The content of the non-OSC material in the lower-layer front portion 22a is preferably greater than the content of the non-OSC material in the lower-layer rear portion 22b. This can further improve the heat resistance and durability of the lower-layer front portion 22a.

[0058] The content of the non-OSC material in the lower-layer front portion 22a is not particularly limited, but is preferably 40 to 110 g / L, more preferably 50 to 100 g / L, for example 60 to 90 g / L. The content of the non-OSC material in the lower-layer front portion 22a is preferably 10 g / L or more, even more preferably 20 g / L or more, greater than the content of the non-OSC material in the lower-layer rear portion 22b. The content of the non-OSC material in the lower-layer rear portion 22b is not particularly limited, but is preferably 20 to 90 g / L, more preferably 30 to 80 g / L, for example 40 to 70 g / L.

[0059] Preferably, the lower-layer front portion 22a and the lower-layer rear portion 22b each contain an OSC material. The OSC material may be a carrier that supports a catalytic metal such as Pd, or may be in a form that does not support a catalytic metal such as Pd. Contrary to non-OSC materials, the content of the OSC material in the lower-layer front portion 22a is preferably smaller than the content of the OSC material in the lower-layer rear portion 22b. This can further improve exhaust gas purification performance in stoichiometric to rich atmospheres. The content of the OSC material in each layer can be measured according to the same procedure as the "Pd content in the lower-layer front portion 22a" described above.

[0060] The content of the OSC material in the lower-layer front portion 22a is not particularly limited, but is preferably 1 to 60 g / L, more preferably 5 to 50 g / L, for example 10 to 40 g / L. The content of the OSC material in the lower-layer front portion 22a is preferably smaller than the content of the OSC material in the lower-layer rear portion 22b by at least 10 g / L, at least 20 g / L, or even at least 30 g / L. The content of the OSC material in the lower-layer rear portion 22b is not particularly limited, but is preferably 20 to 90 g / L, more preferably 30 to 80 g / L, for example 40 to 70 g / L.

[0061] The lower layer front portion 22a and / or the lower layer rear portion 22b may further contain the auxiliary materials and / or NOx storage material described above. For example, by containing an alkaline earth element (such as Ba), poisoning of the catalytic metal, particularly an oxidation catalyst such as Pd, can be suppressed. Furthermore, the dispersibility of the catalytic metal can be improved, thereby suppressing sintering of the catalytic metal. Furthermore, by containing an alkaline earth element together with the OSC material, the amount of oxygen stored in the OSC material can be improved in a lean atmosphere. Furthermore, by containing a NOx storage material, the amount of NOx stored can be improved in a lean atmosphere.

[0062] The content of the auxiliary material and / or NOx occluding material in the lower layer front portion 22a and / or the lower layer rear portion 22b is not particularly limited, but is preferably 30 g / L or less, more preferably 20 g / L or less, for example, 10 g / L or less. The contents of the auxiliary material and / or NOx occluding material in the lower layer front portion 22a and the lower layer rear portion 22b may be substantially the same (generally within ±10%, for example, within ±5%). The contents of the auxiliary material and / or NOx occluding material in each layer can be measured according to the same procedure as described above for the "Pd content in the lower layer front portion 22a."

[0063] The coating amounts of the lower front portion 22a and the lower rear portion 22b are not particularly limited, but are preferably, for example, 50 to 250 g / L, more preferably 70 to 200 g / L, and even more preferably 80 to 150 g / L, respectively.

[0064] The coating length La of the lower-layer front portion 22a in the exhaust gas flow direction (cylinder axis direction X) is typically shorter than the overall length L of the substrate 10 in the extension direction (cylinder axis direction X). The coating length La of the lower-layer front portion 22a is not particularly limited, but is preferably 20 to 70% of the overall length L of the substrate 10, more preferably 30 to 60%, and even more preferably 40 to 50%, for example, less than 50%. The coating length Lb of the lower-layer rear portion 22b in the exhaust gas flow direction (cylinder axis direction X) is typically shorter than the overall length L of the substrate 10 in the extension direction (cylinder axis direction X). The coating length Lb of the lower-layer rear portion 22b is preferably 50% or more of the overall length L of the substrate 10, more preferably 60 to 90%, and even more preferably 70 to 80%. This allows for both improved NOx storage performance in lean atmospheres and improved NOx purification performance in stoichiometric to rich atmospheres to a high level.

[0065] The coat length La of the lower layer front portion 22 a is preferably shorter than the coat length Lb of the lower layer rear portion 22 b. That is, La<Lb is preferable. This allows the Pd concentration to be significantly different between the upstream and downstream sides, concentrating Pd on the upstream side, thereby better achieving the above-mentioned effects.

[0066] In the embodiment shown in FIG. 2 , La + Lb ≈ L, and the lower layer front portion 22a and the lower layer rear portion 22b are in contact in the cylindrical axis direction X. However, the lower layer front portion 22a and the lower layer rear portion 22b may be spaced apart in the cylindrical axis direction X. Furthermore, the sum (La + Lb) of the coat length La of the lower layer front portion 22a and the coat length Lb of the lower layer rear portion 22b preferably satisfies L≦(La + Lb), for example, L≦(La + Lb)≦1.5L. That is, the lower layer front portion 22a and the lower layer rear portion 22b may partially overlap in the center in the cylindrical axis direction X, for example, due to reasons such as the manufacturing method using a slurry. In this case, it is preferable to position the lower layer rear portion 22b on the substrate 10 side.

[0067] <Middle layer 24> The middle layer 24 contains Pt as a catalytic metal and the above-mentioned NOx storage material, and functions as a NOx storage layer in a lean atmosphere, such as during lean start control. The middle layer 24 can also function as a three-way catalyst layer in a stoichiometric to rich atmosphere, for example. Because the middle layer 24 is sandwiched between the lower layer 22 and the upper layer 26, exhaust gas flowing in from the upper layer 26 side or harmful components contained in the exhaust gas tend to remain. For this reason, by including a NOx storage material in the middle layer 24, NOx can be removed in a lean atmosphere. ２ In addition, in a stoichiometric to rich atmosphere, NO desorbed from the NOx storage material ２ However, it is more likely to come into contact with reducing gases such as HC and CO. Therefore, NOx emissions can be reduced in a wide range of atmospheres, from lean to rich.

[0068] Among catalytic metals, Pt has particularly high oxidation performance, and is particularly excellent in NO oxidation performance. Therefore, when the middle layer 24 contains Pt, NO is oxidized in the vicinity of the NOx storage material in a lean atmosphere, and NO is oxidized. ２ Furthermore, Pt has high reactivity with paraffinic HC. Therefore, when the middle layer 24 contains Pt, it can efficiently purify HC and CO, for example, in a stoichiometric to rich atmosphere. The Pt content in the middle layer 24 is not particularly limited, but is preferably 0.1 to 5 g / L, and more preferably 0.5 to 3 g / L. This allows for a high level of both improved NOx storage performance in a lean atmosphere and improved HC purification performance in a stoichiometric to rich atmosphere.

[0069] The middle layer 24 may further contain a catalytic metal other than Pt. An example is the platinum group noble metal (PGM) as described above. Among these, Pd, which has high oxidation activity, is preferred. In the middle layer 24, the content of Pt in the entire catalytic metal is preferably 80 mass % or more, more preferably 90 mass % or more, and it is particularly preferred that the catalytic metal consists essentially of Pt. In the case where the middle layer 24 does not substantially contain Pd (the content in the entire layer is approximately 5 mass % or less, preferably 1 mass % or less, more preferably 0.1 mass % or less; the same applies below), or in the case where the middle layer 24 contains Pd and the content of Pd in ​​the middle layer 24 (C ｍ) is the Pd content (C Ｒ ), CO is likely to remain in the middle layer 24, and the oxidation reaction of NO is likely to be inhibited. Therefore, applying the technology disclosed herein is particularly effective.

[0070] The NOx storage material preferably contains an alkaline earth metal element as a NOx storage element, and particularly preferably contains Ba. The NOx storage material is particularly preferably one compound selected from the group consisting of Ba-containing oxides and Ba-containing carbonates. The content of the NOx storage material in the middle layer 24 is not particularly limited, but is preferably 1 to 60 g / L, more preferably 10 to 50 g / L, for example 20 to 40 g / L.

[0071] Preferably, the middle layer 24 further contains an OSC material. More preferably, the middle layer 24 contains an OSC material and a non-OSC material. The OSC material and / or the non-OSC material may be a support supporting a catalytic metal such as Pt, or may be in a form that does not support a catalytic metal such as Pt. The OSC material may be the first component of the middle layer 24. The content of the OSC material in the middle layer 24 is not particularly limited, but is preferably 60 to 120 g / L, more preferably 70 to 110 g / L, for example, 80 to 100 g / L. The content of the non-OSC material in the middle layer 24 is not particularly limited, but is preferably 1 to 60 g / L, more preferably 10 to 50 g / L, for example, 20 to 40 g / L.

[0072] The coating amount of the middle layer 24 is not particularly limited, but is preferably 120 to 200 g / L, more preferably 130 to 180 g / L, and even more preferably 140 to 170 g / L.

[0073] <Upper Layer 26> The upper layer 26 contains Rh as a catalytic metal. The upper layer 26 can function as a three-way catalyst layer. The upper layer 26 functions as a NOx reduction layer, for example, in a stoichiometric to rich atmosphere. That is, the NOx stored in the middle layer 24 is reduced. ２ When Rh turns into a gas and is desorbed from the NOx occlusion material, it tends to move toward the surface layer side. ２When the NOx is desorbed from the NOx storage material, it can be efficiently removed.

[0074] Among catalytic metals, Rh is particularly ２ The upper layer 26 has a high production capacity and a high three-way performance (especially NOx purification performance). Therefore, for example, in a stoichiometric to rich atmosphere, it can efficiently purify NOx. The Rh content in the upper layer 26 is not particularly limited, but is preferably 0.01 to 1.0 g / L, and more preferably 0.05 to 0.5 g / L.

[0075] The upper layer 26 may further contain a catalytic metal other than Rh. Examples include the platinum group noble metals (PGMs) described above. Among these, Pd and Pt, which have high oxidation activity, are preferred. In the upper layer 26, the content of Rh relative to the total catalytic metal is preferably 30% by mass or more, and more preferably 50% by mass or more.

[0076] Preferably, the upper layer 26 further contains a non-OSC material. More preferably, the upper layer 26 contains both a non-OSC material and an OSC material. The OSC material and / or the non-OSC material may be a support supporting a catalytic metal such as Rh, or may be in a form that does not support a catalytic metal such as Rh. The non-OSC material may be the first component of the upper layer 26. The content of the non-OSC material in the upper layer 26 is not particularly limited, but is preferably 30 to 100 g / L, more preferably 40 to 90 g / L, for example, 50 to 80 g / L. The content of the OSC material in the upper layer 26 is not particularly limited, but is preferably 1 to 50 g / L, more preferably 5 to 40 g / L, for example, 10 to 30 g / L.

[0077] The upper layer 26 may further include the auxiliary materials and NOx storage materials described above. In a preferred embodiment, the upper layer 26 is substantially free of NOx storage materials, auxiliary materials containing alkali metal elements, and auxiliary materials containing alkaline earth metal elements.

[0078] The coating amount of the upper layer 26 is not particularly limited, but is preferably 50 to 120 g / L, more preferably 60 to 110 g / L, and even more preferably 70 to 100 g / L.

[0079] <<Method of Manufacturing Exhaust Gas Purifying Catalyst 100>> The exhaust gas purifying catalyst 100 can be manufactured, for example, by the following method, although not particularly limited thereto. First, a substrate 10 and a catalyst layer forming slurry for forming the catalyst layer 20 are prepared. The catalyst layer forming slurry can be prepared, for example, by mixing a catalytic metal source (e.g., a solution containing catalytic metal ions) and other components (e.g., NOx storage material, non-OSC material, OSC material, binder, various additives, etc.) in a dispersion medium. As the dispersion medium, for example, water or a mixture of water and a water-soluble organic solvent, etc., can be used.

[0080] The properties of the slurry (e.g., viscosity, solid content, etc.) can be appropriately determined depending on the size of the substrate 10 used, the shape of the cells 12 or rib walls 14, the properties required for the catalyst layer 20, etc. The use of a thickener is advantageous for adjusting the viscosity of the slurry. Examples of thickeners that can be used include cellulose-based polymers such as carboxymethyl cellulose (CMC), methyl cellulose (MC), hydroxypropyl methyl cellulose (HPMC), hydroxyethyl methyl cellulose (HEMC), hydroxyethyl cellulose (HEC), and HEC citric acid crosslinked product obtained by crosslinking HEC with citric acid.

[0081] Specifically, first, a lower layer slurry containing a Pd source, a middle layer slurry containing a Pt source and a NOx storage material, and an upper layer slurry containing a Rh source are prepared. At this time, as the lower layer forming slurries, a lower layer front portion forming slurry having a relatively high Pd content and a lower layer rear portion forming slurry having a relatively low Pd content are prepared.

[0082] Next, using a known coating method (e.g., suction coating, air blowing, wash coating, etc.), the lower layer rear portion forming slurry is coated from the end of the downstream side X2 of the substrate 10 to a predetermined position according to a known method, and then dried. Subsequently, the lower layer front portion forming slurry is coated from the end of the upstream side X1 of the substrate 10 to a predetermined position according to a known method, and then dried. This is then fired to form the lower layer 22. Note that, for example, with the suction coating method, the coating length of each layer can be precisely adjusted by immersing one end of the substrate in the slurry and sucking the slurry from the other end.

[0083] Next, a slurry for forming a middle layer is coated onto the lower layer 22 by a known coating method, dried, and then fired to form the middle layer 24. Here, the middle layer 24 is formed over the entire length of the substrate. Furthermore, a slurry for forming an upper layer is coated onto the middle layer 24 by a known coating method, dried, and then fired to form the upper layer 26. Here, the upper layer 26 is formed over the entire length of the substrate. The drying conditions for the slurry are typically 70 to 150°C, for example 90 to 130°C, for about 1 to 10 hours. The firing conditions are typically 300 to 800°C, for example 400 to 500°C, for about 1 to 4 hours.

[0084] <<Uses of Exhaust Gas Purification Catalyst 100>> As described above, the exhaust gas purification catalyst 100 can improve NOx storage performance in a lean atmosphere, for example, when starting an internal combustion engine. Preferably, it can also achieve excellent NOx purification performance in a stoichiometric to rich atmosphere. Therefore, the exhaust gas purification catalyst 100 can be suitably used to purify exhaust gases emitted from internal combustion engines of vehicles such as automobiles and trucks, motorcycles and mopeds, marine products such as ships, tankers, jet skis, personal watercraft, and outboard motors, gardening products such as lawn mowers, chainsaws, and trimmers, leisure products such as golf carts and all-terrain vehicles, power generation equipment such as cogeneration systems, and waste incinerators. In particular, it can be suitably used in vehicles such as automobiles, and particularly in vehicles equipped with gasoline engines.

[0085] <<Exhaust Gas Purification System and Control Method>> The exhaust gas purification system disclosed herein includes an exhaust path connected to an internal combustion engine, an exhaust gas purification catalyst 100 arranged in the exhaust path, an air-fuel ratio adjuster provided upstream of the exhaust gas purification catalyst 100 in the exhaust gas flow direction, and a control unit. The air-fuel ratio adjuster is, for example, a fuel addition device that sprays atomized fuel (HC) into the exhaust gas. The control unit is configured to control the air-fuel ratio adjuster at the start of the internal combustion engine to perform lean start control, in which the air-fuel ratio of the exhaust gas is adjusted to a lean state and then supplied to the exhaust gas purification catalyst 100. During the warm-up process during lean start control, NOx in the exhaust gas is captured by the exhaust gas purification catalyst 100 (more specifically, the NOx storage material).

[0086] In such an exhaust gas purification system, it is preferable that the control unit controls the air-fuel ratio adjusting device to perform preparatory control before starting lean start control, adjusting the air-fuel ratio of the exhaust gas to a rich state and supplying the exhaust gas to the exhaust gas purification catalyst 100. This allows the oxidation catalyst (especially Pt or Pd) to be reduced, improving the oxidation performance of the oxidation catalyst in preparation for capturing NOx in the NOx storage material. As a result, NOx becomes more easily captured in the NOx storage material, and the amount of NOx stored can be suitably increased.

[0087] Test examples relating to the present invention will be described below, but it is not intended that the present invention be limited to those shown in the following test examples.

[0088] [Test Example I: Evaluation of NOx storage performance in a lean atmosphere] In Test Example I, the Pd content of the lower front portion and the lower rear portion was changed to evaluate the NOx storage performance in a lean atmosphere, specifically, the time to achieve 50% CO purification and the time to start NOx storage.

[0089] <Test Substrate> First, a cylindrical cordierite honeycomb substrate (diameter: 118.4 mm, total length: 114.3 mm, volume: 1.26 L) was prepared. One end of this substrate was defined as the upstream side X1 end (i.e., the end on the exhaust gas inlet side), and the other end was defined as the downstream side X2 end (i.e., the end on the exhaust gas outlet side).

[0090] <Preparation of Exhaust Gas Purifying Catalyst of Example 1> In Example 1, an exhaust gas purifying catalyst was prepared in which the Pd content in the lower layer front section was higher than that in the lower layer rear section. Specifically, a lower layer 122 containing the components shown in Table 1 and having a cross-sectional shape as shown in Figure 3 was first formed. In more detail, a palladium nitrate aqueous solution as a Pd source, a CZ composite oxide powder as an OSC material, and Al as a non-OSC material were first mixed. ２ O ３ The powder, barium sulfate as an auxiliary material, and HEC as a thickener were mixed in ion-exchanged water, and then milled so that the average particle size (D50) of the powder was 5 μm, to prepare two types of slurries: a slurry for forming the lower layer front portion and a slurry for forming the lower layer rear portion.

[0091] Next, the lower layer rear section forming slurry was applied by suction coating to a region extending from the downstream end of the substrate to 75% of the substrate's total length, dried at 90°C for 1 hour, and then baked at 500°C for 1 hour to form the lower layer rear section 122b. The lower layer front section forming slurry was also applied by suction coating to a region extending from the upstream end of the substrate to 45% of the substrate's total length, dried at 90°C for 1 hour, and then baked at 500°C for 1 hour to form the lower layer front section 122a. As a result, as shown in FIG. 3 , a lower layer 122 having a lower layer front section 122a and a lower layer rear section 122b was formed on the surface of the substrate 110. The lower layer front section 122a and the lower layer rear section 122b partially overlap at the center of the substrate 110 in the axial direction (at the OL position in FIG. 3 ).

[0092] In Example 1, for example, the Pd content of the lower layer front portion 122a (C Ｆ The Pd content C of the lower layer front portion (unit: g / L) can be determined as follows. That is, first, a predetermined length L1 (for example, 10 mm) of the lower layer front portion 122a is cut out from the substrate front portion, and then crushed into powder. Assume that an inductively coupled plasma analysis is performed on this powder, and it is found that 0.271 g of Pd is present in the predetermined length L1 of the lower layer front portion. In this case, the Pd content C of the lower layer front portion is ＦThe content (g / L) of the material can be calculated as 2.46 g / L from the following formula: 0.271 g / (10 mm / substrate length 114.3 mm×substrate volume 1.26 L). The content of other materials can also be calculated using a similar method.

[0093]

[0094] Next, a middle layer containing the components shown in Table 2 was formed on the lower layer. Specifically, first, a platinum nitrate aqueous solution as a Pt source, a CZ composite oxide powder as an OSC material, and Al as a non-OSC material were mixed. ２ O ３ The powder, barium carbonate as a NOx occlusion material, and HEC as a thickener were mixed in ion-exchanged water and milled to an average particle size (D50) of 5 μm to prepare a slurry for forming the middle layer. Next, this slurry for forming the middle layer was coated on the above-formed lower layer over the entire length (100%) of the substrate by a washcoat method, dried at 90°C for 1 hour, and then fired at 500°C for 1 hour to form the middle layer.

[0095]

[0096] Next, an upper layer containing the components shown in Table 3 was formed on the middle layer. Specifically, first, an aqueous solution of rhodium nitrate as a Rh source, an aqueous solution of palladium nitrate as a Pd source, a CZ composite oxide powder as an OSC material, and Al as a non-OSC material were added. ２ O ３ The powder and HEC citric acid crosslinked product as a thickener were mixed in ion-exchanged water and milled to an average particle size (D50) of 5 μm to prepare a slurry for forming an upper layer. Next, this slurry for forming an upper layer was coated on the above-formed middle layer over the entire length (100%) of the substrate by a washcoat method, dried at 90°C for 1 hour, and then calcined at 500°C for 1 hour to form an upper layer. In this way, the exhaust gas purification catalyst of Example 1 was obtained.

[0097]

[0098] <Preparation of exhaust gas purifying catalysts of Examples 2 to 5 and Comparative Example 1> Pd content in the front portion of the lower layer (C Ｆ) and the Pd content of the lower rear part (C Ｒ ) was changed as shown in Table 4, the same procedure as in Example 1 was carried out to obtain the exhaust gas purifying catalysts of each example. Ｒ ) Pd content of the lower front layer (C Ｆ ) ratio (C Ｆ / C Ｒ ) are also shown.

[0099]

[0100] <Evaluation of CO 50% Purification Time and NOx Absorption Start Time in a Lean Atmosphere> Each example of exhaust gas purification catalyst was cooled to below 100°C and then installed in the exhaust pipe of an engine bench. Then, exhaust gas (λ value = 1.08, A / F = 15.7) whose temperature was adjusted to 300°C using a heat exchanger was allowed to flow into the exhaust gas purification catalyst. At this time, the CO concentration at a position before the catalyst flow and the CO concentration at a position after the catalyst flow were measured, and based on this, the CO 50% purification time in a lean atmosphere (A / F = 15.7) was calculated. In addition, the NOx amount at a position before the catalyst flow and the NOx amount at a position after the catalyst flow were measured, and based on this, the NOx absorption start time in a lean atmosphere (A / F = 15.7) was calculated. The results are shown in Table 4.

[0101] Figure 4 shows the Pd content ratio (C Ｆ / C Ｒ 5 shows the relationship between the Pd content ratio (C Ｆ / C Ｒ 4 and 5, the relationship between the Pd content ratio (C Ｆ / C Ｒ) increased, the CO 50% conversion time and NOx absorption start time became shorter. Furthermore, as shown in FIG. 6, a positive correlation was observed between the CO 50% conversion time and the NOx absorption start time. From the above results, it can be seen that in a lean atmosphere, increasing the Pd content in the front portion of the lower layer promoted the CO purification reaction, and as a result, the NO oxidation reaction became more likely to occur. Therefore, the exhaust gas purification catalyst disclosed herein can reduce NOx emissions, particularly at the start of an internal combustion engine.

[0102] [Test Example II: Evaluation of NOx purification performance in a rich atmosphere] In Test Example II, the NOx purification rate in a rich atmosphere (A / F = 14.45) was evaluated for the exhaust gas purification catalysts of Examples 1 to 5, which had a NOx absorption start time of 40 seconds or less in Test Example I.

[0103] <Evaluation of NOx purification rate in a rich atmosphere> Each example of the exhaust gas purification catalyst was installed in the exhaust pipe of an engine bench, and exhaust gas whose temperature had been adjusted to 460°C using a heat exchanger was allowed to flow into the exhaust gas purification catalyst. At this time, the A / F of the inflowing exhaust gas was varied, and the amount of NOx before flowing into the catalyst and the amount of NOx after flowing out of the catalyst were measured when the A / F reached 14.45. Based on this, the NOx purification rate (%) in a rich atmosphere (A / F = 14.45) was calculated. The results are shown in Table 5.

[0104]

[0105] As shown in Table 5, the NOx purification rate in a rich atmosphere is proportional to the Pd content ratio (C Ｆ / C Ｒ This is thought to be because the Pd content in the front part of the lower layer was too high, and therefore, the NOx that could not react in the rich atmosphere passed through the exhaust gas purification catalyst and was emitted.

[0106] On the other hand, the ratio of Pd content (C Ｆ / C Ｒ When the Pd content ratio (C Ｆ / C Ｒ When the ratio of Pd content (CＦ / C Ｒ When the Pd content ratio (C) was 1.5 to 2, the NOx purification rate was significantly high, at 94% or more. Ｆ / C Ｒ ) in the above range, it is possible to improve the NOx absorbing performance in a lean atmosphere as well as the NOx purifying performance in a rich atmosphere.

[0107] [Test Example III: Study on the Total Amount of Pd and NOx Storage Performance] In Test Example III, the ratio of the Pd content (C Ｆ / C Ｒ The NOx storage performance was evaluated while the Pd content in the entire lower layer was changed with the Pd content in the front and rear lower layers fixed at 1.5. Specifically, the exhaust gas purifying catalysts of Examples 6 to 8 were prepared in the same manner as in Example 1 of Test Example I, except that the Pd content in the front and rear lower layers was changed as shown in Table 6. The time to achieve 50% CO purification and the time to start NOx storage in a lean atmosphere were evaluated. The results are shown in Table 6.

[0108] The total amount of Pd contained in the entire lower layer per 1 L of substrate can be calculated as follows. For example, in Example 1 having a lower layer having a configuration as shown in FIG. 3, the Pd content (C Ｆ ) is 2.46 g / L, and the coating length of the lower layer front portion 122a is 45% of the total length of the substrate. The substrate volume is 1.26 L. Therefore, the amount of Pd in ​​the lower layer front portion 122a is 2.46 [g / L] × 45 [%] × 1.26 [L] = 1.395 g. Similarly, the amount of Pd in ​​the lower layer rear portion 122b is 1.64 [g / L] × 75 [%] × 1.26 [L] = 1.550 [g]. In other words, the amount of Pd contained in the lower layer can be calculated as 1.395 [g] + 1.550 [g] = 2.945 [g]. The total amount of Pd contained in the lower layer of this substrate is 2.34 [g / L], obtained by dividing the amount of Pd contained in the lower layer (2.945 [g]) by the substrate volume (1.26 [L]). Here, to distinguish it from the above-mentioned content, the unit "-cat" is added, and it is expressed as 2.34 [g / L-cat].

[0109]

[0110] Figure 7 shows the relationship between the total amount of Pd per substrate in the lower layer and the time to 50% CO conversion. Figure 8 shows the relationship between the total amount of Pd per substrate in the lower layer and the time to start NOx occlusion. As shown in Table 6 and Figures 7 and 8, the ratio of the Pd content (C Ｆ / C Ｒ When the Pd content in the lower layer was 1.5, the effect of the technology disclosed herein decreased and plateaued when the total Pd content in the lower layer exceeded approximately 3 g / L-cat, specifically 2.9 g / L-cat. This is thought to be because a sufficient amount of Pd is contained in the front portion of the lower layer when the Pd content in the lower layer increases. Therefore, it can be seen that the effect of the technology disclosed herein is particularly high when the total Pd content in the lower layer is 3.0 g / L-cat or less, or even 2.9 g / L-cat or less, or 2.5 g / L-cat or less.

[0111] Although specific examples of the present invention have been described above in detail, these are merely examples and do not limit the scope of the claims. The technology described within the scope of the claims includes various modifications and alterations of the specific examples exemplified above.

[0112] REFERENCE SIGNS LIST 10, 110 Substrate 12 Cell 14 Rib wall 20 Catalyst layer 22, 122 Lower layer 22a, 122a Lower layer front portion 22b, 122b Lower layer rear portion 24 Middle layer 26 Upper layer 100 Exhaust gas purification catalyst

Claims

1. An exhaust gas purification catalyst disposed in an exhaust path of an internal combustion engine for purifying exhaust gas emitted from the internal combustion engine, comprising: a substrate; and a catalyst layer formed on the substrate, wherein the catalyst layer comprises a lower layer located on the substrate side, an upper layer located on a surface layer side, and a middle layer located between the lower layer and the upper layer, the upper layer containing Rh, and the middle layer containing at least Pt and a NOx storage material, the lower layer having a lower layer front portion located upstream in the exhaust gas flow direction when disposed in the exhaust path, and a lower layer rear portion located downstream in the exhaust gas flow direction, wherein the lower layer front portion and the lower layer rear portion each contain Pd, and the Pd content (C Ｆ ) is the Pd content (C Ｒ ) is a catalyst for purifying exhaust gases that is larger than the 2. Above C Ｒ The above C Ｆ The ratio (C Ｆ / C Ｒ ) satisfies the following formula: 1.5≦(C Ｆ / C Ｒ 2. The exhaust gas purifying catalyst according to claim 1, wherein the following condition is satisfied: )≦3.

0.

3. The exhaust gas purifying catalyst according to claim 1 or 2, wherein the total amount of Pd contained in the entire lower layer per 1 L of substrate is 3.0 g / L-cat or less.

4. An exhaust gas purifying catalyst according to claim 1 or 2, wherein the lower layer front portion and the lower layer rear portion each contain an OSC material having oxygen storage capacity and a non-OSC material not having oxygen storage capacity, the content of the non-OSC material in the lower layer front portion being greater than the content of the non-OSC material in the lower layer rear portion, and the content of the OSC material in the lower layer front portion being less than the content of the OSC material in the lower layer rear portion.

5. The exhaust gas purifying catalyst according to claim 1 or 2, wherein the coating length of the lower layer front portion in the exhaust gas flow direction is shorter than the coating length of the lower layer rear portion in the exhaust gas flow direction.

6. An exhaust gas purification catalyst according to claim 5, wherein the coating length of the lower layer front portion in the exhaust gas flow direction is 30% to 60% of the overall length of the substrate, and the coating length of the lower layer rear portion in the exhaust gas flow direction is 60% to 90% of the overall length of the substrate.

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