Exhaust gas purification catalyst

Abstract

The present invention relates to an exhaust gas purification catalyst having improved NOx occlusion performance in a lean-burn atmosphere. This exhaust gas purification catalyst (100) is provided with a substrate (10) and a catalyst layer (20). The catalyst layer (20) includes a lower layer (22), an intermediate layer (24), and an upper layer (26). The upper layer (26) contains Rh. The intermediate layer (24) contains at least Pt and a NOx storage material. The lower layer (22) has a lower layer front portion (22a) containing Pd and a lower layer rear portion (22b) containing Pd, and the content (CF) of Pd per 1 L of the substrate in the lower layer front portion (22a) is greater than the content (CR) of Pd per 1 L of the substrate in the lower layer rear portion (22b).

Description

Technical Field

[0001] The present invention relates to a catalyst for purifying exhaust gas.

[0002] In addition, the present application claims priority based on Japanese Patent Application No. 2022-161921 filed on October 6, 2022, the entire contents of which are incorporated herein by reference. Background Art

[0003] The exhaust gas discharged from internal combustion engines such as automobile engines contains 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 having a substrate and a catalyst layer containing a catalyst metal is used. The exhaust gas supplied to the exhaust gas purification catalyst contacts the catalyst layer, thereby purifying the harmful components. For example, HC and CO in the exhaust gas are oxidized and converted (purified) into water (H 2 O) and carbon dioxide (CO 2 ), NO in exhaust gas x is reduced and converted (purified) into nitrogen (N 2 ).

[0004] However, when the internal combustion engine is started, the exhaust gas purification catalyst is not fully preheated, and the activity of the catalyst metal is low. Therefore, there is a concern that the exhaust gas will be discharged with residual harmful components before the catalyst metal reaches the specified active temperature. Therefore, the air-fuel ratio (A / F) of the mixed gas supplied when the internal combustion engine is started is made lean, and the control of the internal combustion engine is set to a lean burn (excess oxygen) state, so-called lean burn start control, thereby reducing CO and HC. However, it is difficult to reduce NO in a lean burn atmosphere. x Seize oxygen elements, there is no way to purify NO x topic.

[0005] Therefore, in order to suppress NOx during the warm-up process during lean start control, x The emission of NO is widely adopted x NO2 Occlusion Materials x Occlusion reduction (NSR: NO x Storage-Reduction catalyst (see Patent Documents 1 and 2). For example, Patent Document 1 discloses an NSR catalyst, in which the catalyst layer is set as a three-layer structure, the lower layer contains Pt and / or Pd, the middle layer contains Pt and / or Pd and NO x Occlusion material, the upper layer contains Rh.

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent Application Laid-Open No. 2018-143935

[0009] Patent Document 2: Japanese Patent Application Laid-Open No. 2016-93760 Summary of the Invention

[0010] In the catalyst layer containing a NO x occlusion material as disclosed in Patent Documents 1 and 2, a NO x occlusion reaction occurs in a lean burn atmosphere. That is, since NO occupies most of the NO x in the exhaust gas, usually NO is first oxidized by a catalyst metal such as Pt to form NO 2 . The generated NO 2 reacts with the NO x occlusion material (such as an alkaline earth metal) to form a nitrate form, and is thus temporarily absorbed by the NO x occlusion material. Thereby, the emission of NO x is suppressed. The NO x absorbed by the NO 2 occlusion material is detached from the catalyst layer when controlling the switching from a stoichiometric (theoretical air-fuel ratio) to a rich burn (fuel-rich) atmosphere, and is reduced on the catalyst metal using a reducing gas such as HC or CO as a reducing agent. Thus, the NO x component in the exhaust gas is converted (purified) to nitrogen gas (N 2 ).

[0011] In recent years, exhaust gas regulations have tended to be further strengthened. Moreover, for example, in an environmentally friendly vehicle equipped with an energy-saving component, the engine as an internal combustion engine frequently stops and starts repeatedly during driving. Therefore, in an exhaust gas purification catalyst having a NO x occlusion material, it is necessary to further improve the NO x occlusion performance in a lean burn atmosphere, that is, to further reduce the NO x emission amount (discharge amount).

[0012] The inventors of the present invention explored ways to improve the NO x occlusion performance in a lean burn atmosphere from a different approach from the prior art. More specifically, it was conceived to improve the NO x occlusion performance in a lean burn atmosphere by causing the NO x occlusion reaction, particularly the oxidation reaction of NO, to occur earlier.

[0013] That is, in order for the above-mentioned NO x occlusion reaction to occur, it is necessary to oxidize NO to NO 2The state. However, according to the inventors' research, if CO and NO coexist at this time, the oxidation reaction of CO occurs preferentially, hindering the oxidation reaction of NO. As a result, it is difficult to generate NO 2 , delaying NO x The occurrence of the occlusion reaction.

[0014] CO is mainly purified by Pd, but in the catalyst layer disclosed in, for example, Patent Document 1, in the catalyst layer structure where the NO x occlusion material is included in the intermediate layer and Pd is included in the lower layer, CO is particularly likely to remain in the intermediate layer, easily hindering the oxidation reaction of NO. Therefore, the inventors believe that in the catalyst layer with a structure where the NO x occlusion material is included in the intermediate layer and Pd is included in the lower layer, in order to improve the NO x occlusion performance in a lean combustion atmosphere, it is important to purify CO as early as possible during the preheating process and carry out the oxidation reaction of NO, and initiate the NO x occlusion reaction as early as possible. The present invention has been completed based on this idea.

[0015] The exhaust gas purification catalyst [1] disclosed herein is disposed in the exhaust path of an internal combustion engine for purifying the exhaust gas discharged from the internal combustion engine, and includes a substrate and a catalyst layer formed on the substrate. The catalyst layer includes a lower layer on the substrate side, an upper layer on the surface layer side, and an intermediate layer between the lower layer and the upper layer. The upper layer contains Rh. The intermediate layer contains at least Pt and NO x occlusion material. The lower layer has a lower layer front part located on the upstream side in the exhaust gas flow direction and a lower layer rear part located on the downstream side in the exhaust gas flow direction when disposed in the exhaust path, and the Pd in the lower layer front part and the Pd in the lower layer rear part contain Pd respectively. The content (C F ) of Pd in the lower layer front part per 1 L of the substrate is greater than the content (C R ) of Pd in the lower layer rear part per 1 L of the substrate.

[0016] In the exhaust gas purification catalyst [1], the lower layer containing Pd has a lower layer front part on the upstream side and a lower layer rear part on the downstream side, and the Pd content (C F ) in the lower layer front part is greater than the Pd content (C R ) in the lower layer rear part. That is, it satisfies C F > C R. Generally, when an internal combustion engine is started, due to the exhaust gas, it is easy to heat up starting from the front part of the lower layer of the catalyst layer. Therefore, by increasing the Pd content in the front part of the lower layer that is prone to heat up during startup (in other words, by making the Pd concentration biased towards the upstream side), the purification reaction of CO can occur actively. As a result, CO can be purified as early as possible, so it is not easy to hinder the oxidation reaction of NO. Therefore, the oxidation reaction of NO can occur smoothly, and the NO x occlusion reaction can be initiated as early as possible.

[0017] In addition, the reaction heat (heat capacity) during the above purification reaction of CO is conducted to the downstream side along with the flow of the exhaust gas, thereby enabling the overall preheating of the catalyst layer to be improved. As a result, the catalyst metal can be heated to the active temperature (for example, 200 - 250 °C) as early as possible. Therefore, according to the exhaust gas purification catalyst [1], in particular, the NO x occlusion performance during the startup of the internal combustion engine in a lean burn atmosphere can be improved.

[0018] The exhaust gas purification catalyst [2] disclosed herein, in the above exhaust gas purification catalyst [1], the C F and the above C R ratio (C F / C R ) satisfies the following formula: 1.5 ≤ (C F / C R ) ≤ 3.0. Thus, the NO x purification performance in the stoichiometric - rich burn atmosphere can also be improved, and the NO x emission can be reduced in a wide range of lean burn - rich burn atmospheres.

[0019] The exhaust gas purification catalyst [3] disclosed herein, in the above exhaust gas purification catalyst [1] or [2], the total amount of the above Pd contained in the entire lower layer per 1 L of the substrate is 3.0 g / L or less. At this time, the application of the technology disclosed herein will be particularly effective.

[0020] The exhaust gas purification catalyst [5] disclosed herein, in any one of the above exhaust gas purification catalysts [1] - [3], the front part of the lower layer and the rear part of the lower layer respectively contain an OSC material having oxygen occlusion ability and a non - OSC material not having oxygen occlusion ability, the content of the non - OSC material in the front part of the lower layer per 1 L of the substrate is greater than the content of the non - OSC material in the rear part of the lower layer per 1 L of the substrate, and the content of the OSC material in the front part of the lower layer per 1 L of the substrate is less than the content of the OSC material in the rear part of the lower layer per 1 L of the substrate. Thereby, excellent exhaust gas purification performance can be maintained for a long time.

[0021] The catalyst for purifying exhaust gas disclosed herein, among any one of the catalysts for purifying exhaust gas [1] to [4] above, the coating length in the exhaust gas flow direction at the front part of the lower layer is shorter than the coating length in the exhaust gas flow direction at the rear part of the lower layer. Thus, the technical effects disclosed herein can be exerted at a high level, and the NO occlusion performance in a lean combustion atmosphere can be better improved. x Occlusion performance.

[0022] The catalyst for purifying exhaust gas [7] disclosed herein, among any one of the catalysts for purifying exhaust gas [1] to [5] above, the coating length in the exhaust gas flow direction at the front part of the lower layer is 30% or more and 60% or less of the total length of the substrate, and the coating length in the exhaust gas flow direction at the rear part of the lower layer is 60% or more and 90% or less of the total length of the substrate. Thus, it is possible to stably and highly balance the improvement of NO occlusion performance in a lean combustion atmosphere and the improvement of NO purification performance in a stoichiometric to rich combustion atmosphere. x Occlusion performance improvement and NO x Purification performance improvement. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a three-dimensional schematic view of the catalyst for purifying exhaust gas of one embodiment.

[0024] Figure 2 It is a partial sectional view of the catalyst for purifying exhaust gas cut along the cylinder axis direction Figure 1 .

[0025] Figure 3 It is a sectional schematic view showing the structure of the lower layer of Test I.

[0026] Figure 4 It is a graph showing the relationship between the Pd content ratio (C F / C R ) and the CO 50% purification time.

[0027] Figure 5 It is a graph showing the relationship between the Pd content ratio (C F / C R ) and the NO occlusion start time.

[0028] Figure 6 It is a graph showing the relationship between the CO 50% purification time and the NO occlusion start time.

[0029] Figure 7 It is a graph showing the relationship between the total amount of Pd in the lower layer and the CO 50% purification time.

[0030] Figure 8 It is a graph showing the relationship between the total amount of Pd in the lower layer and the NO occlusion start time. DETAILED DESCRIPTION OF THE INVENTION

[0031] The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. In addition, matters required for the implementation of the present invention other than those specifically mentioned in this specification (for example, the manufacturing method of a general exhaust gas purification catalyst, etc.) can be grasped as design matters of those skilled in the art based on the prior art in this field. The present invention can be implemented based on the content disclosed in this specification and the common general knowledge in this field. Also, in the drawings, components and parts having the same function are denoted by the same reference numerals, and repeated descriptions may sometimes be omitted or simplified. The dimensional relationships (length, width, thickness, etc.) in each drawing do not always reflect the actual dimensional relationships. In addition, the expression "A to B" (A and B are arbitrary values) indicating a range in this specification includes the meaning of A or more and B or less, as well as the meanings of "greater than A (higher than A)" and "less than B (lower than B)".

[0032] Figure 1 FIG. is a schematic view of an exhaust gas purification catalyst 100. The exhaust gas purification catalyst 100 is disposed in the exhaust path of an internal combustion engine and purifies the exhaust gas discharged from the internal combustion engine. The exhaust gas purification catalyst 100 includes a substrate 10 and a catalyst layer 20 formed on the substrate 10 (see Figure 2 ). In addition, in Figure 1 and the like, the arrow F indicates the flow direction of the exhaust gas when disposed in the exhaust path. The arrow X indicates the cylindrical axis direction of the substrate 10. X1 represents the upstream side (front side) of the flow direction F of the exhaust gas, and X2 represents the downstream side (rear side) of the flow direction F of the exhaust gas.

[0033] The exhaust gas purification catalyst 100 can appropriately select the design of the catalyst layer 20, etc. according to, for example, the type and shape of the substrate 10 described later, and is disposed in the exhaust systems (exhaust pipes) of various internal combustion engines, particularly automotive engines. Hereinafter, on the premise that the internal combustion engine is a gasoline engine of an automobile, the exhaust gas purification catalyst 100 will be described, but it is not intended to limit the exhaust gas purification catalyst 100 to such uses.

[0034] <Substrate 10>

[0035] The substrate 10 constitutes the skeleton of the exhaust gas purification catalyst 100. As the substrate 10, various raw materials and forms used in the past for such uses can be adopted. For example, as raw materials, ceramics such as cordierite, aluminum titanate, and silicon carbide are preferred because of their high heat resistance. Or a substrate made of an alloy (such as stainless steel) can also be used.

[0036] Regarding the form, here, the substrate 10 has a honeycomb structure, which has a plurality of chambers 12 regularly arranged along the cylinder axis direction X (the flow direction of the exhaust gas), and rib walls 14 that separate the plurality of chambers 12. The chambers 12 are through-holes that function as channels for the exhaust gas. The rib walls 14 are partition walls that separate the respective chambers 12. The cross-sectional shape of the chambers 12 is a quadrilateral here. However, the cross-sectional shape of the chambers 12 can also be other shapes (such as circular, triangular, hexagonal, etc.).

[0037] The outer shape of the substrate 10 is a cylindrical shape here. However, the outer shape of the substrate 10 can also be other shapes (such as an elliptical cylinder shape, a multi-sided cylinder shape, etc.). In addition, the substrate 10 is a honeycomb shape here. However, the substrate 10 can also be a foam shape, a granular shape, etc. The volume of the substrate 10 is typically about 0.1 to 10 L, for example, 0.5 to 5 L. In addition, the total length L of the substrate 10 in the cylinder axis direction X (refer to Figure 2 ) is typically about 10 to 500 mm, for example, 50 to 300 mm. In addition, in this specification, the volume of the substrate 10 refers to the apparent volume (apparent volume) that includes the internal voids such as the chambers 12 in addition to the pure volume of the substrate 10.

[0038] Figure 1 The shown substrate 10 is a so-called direct-flow type substrate in which the inlet side opening and the outlet side opening of the chambers 12 are not closed. However, the substrate 10 can also be a so-called wall-flow type (also called through-wall type) substrate in which the inlet side openings and the outlet side openings of the plurality of chambers 12 are alternately closed, and the exhaust gas flows from one chamber (inlet side chamber) to the adjacent chamber (outlet side chamber) through the rib wall 14.

[0039] <Catalyst layer 20>

[0040] Figure 2 It is a partial cross-sectional view of the exhaust gas purification catalyst 100 cut along the cylinder axis direction X. As Figure 2 shown, here the catalyst layer 20 is formed on the substrate 10 (specifically on the rib wall 14). However, a part of the catalyst layer 20 can also penetrate into the inside of the rib wall 14. The catalyst layer 20 forms the main body of the exhaust gas purification catalyst 100 as a place for purifying the exhaust gas. The exhaust gas supplied to the exhaust gas purification catalyst 100 contacts the catalyst layer 20 during its flow (passage) in the flow path (inside the chamber 12) of the substrate 10. The harmful components in the exhaust gas are purified by contacting the catalyst layer 20.

[0041] As Figure 2As shown, the catalyst layer 20 here has a multi-layer structure, specifically a three-layer structure. The catalyst layer 20 has a lower layer 22 on the substrate 10 side in the thickness direction, an upper layer 26 on the surface layer portion side of the catalyst layer 20, and an intermediate layer 24 between the lower layer 22 and the upper layer 26. However, within the range that does not significantly hinder the effects of the present invention, the catalyst layer 20 may also include layers other than the lower layer 22, the intermediate layer 24, and the upper layer 26. That is, the catalyst layer 20 may also be a laminated structure of four or more layers. At this time, it is preferred that the lower layer 22 and the intermediate layer 24 are in contact with each other in the thickness direction. And it is preferred that the intermediate layer 24 and the upper layer 26 are in contact with each other in the thickness direction.

[0042] The coating amount of the entire catalyst layer 20 can be appropriately determined according to the type of the substrate 10 and the like, and there is no particular limitation. For example, it is 150 to 450 g / L, preferably 200 to 400 g / L, and more preferably 250 to 350 g / L with respect to each 1 L volume of the substrate 10.

[0043] The coating length of the multi-layer structure (especially the three-layer structure) portion of the catalyst layer 20, that is, the average coating length in the cylinder axis direction X of the substrate 10, is preferably, for example, 60% or more of the full length L of the cylinder axis direction X of the substrate 10, and can be 70% or more, 80% or more, 90% or more, or 100% of the full length L of the cylinder axis direction X of the substrate 10. In addition, although the lower layer 22, the intermediate layer 24, and the upper layer 26 are shown with the same coating length in Figure 2 , as long as the lower layer 22, the intermediate layer 24, and the upper layer 26 are laminated in at least a part of the cylinder axis direction X in three layers, they may also have different coating lengths.

[0044] The coating thickness of the multi-layer structure (especially the three-layer structure) portion of the catalyst layer 20, that is, the average thickness in the direction perpendicular to the cylinder axis direction X, can be, for example, 3 to 300 μm, 5 to 200 μm, or 10 to 100 μm. In addition, although the lower layer 22, the intermediate layer 24, and the upper layer 26 are shown with the same thickness in Figure 2 , the lower layer 22, the intermediate layer 24, and the upper layer 26 may also have different thicknesses. Although there is no particular limitation, 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 intermediate 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. In addition, the coating thicknesses of the lower layer 22, the intermediate layer 24, and the upper layer 26 may be uneven. For example, the coating thickness of the lower layer 22 may become thicker at the central portion in the cylinder axis direction X.

[0045] The catalyst layer 20 contains at least (1) a catalyst metal and (2) NO xOcclusion material. The catalyst layer 20 preferably further contains (3) an oxygen occlusion material (OSC material) having an oxygen storage capacity, and / or (4) a non-OSC material having no oxygen storage capacity. The OSC material and / or the non-OSC material may be included in the catalyst layer 20 as a carrier for supporting the catalyst metal, or may be included in the catalyst layer 20 in a form not supporting the catalyst metal. In addition, the catalyst layer 20 may also adopt a configuration with the OSC material and / or the non-OSC material as the main body (a component accounting for more than 50% by mass of the whole, the same hereinafter).

[0046] (1) Among the catalyst metals, as essential components, it contains three kinds of noble metals (PGM) belonging to the platinum group, namely rhodium (Rh), palladium (Pd), and platinum (Pt). In addition to the above three kinds, the catalyst metal may further contain various metal species that function as an oxidation catalyst and / or a reduction catalyst in the purification of harmful components in the exhaust gas. As an example, PGM other than the above, that is, ruthenium (Ru), osmium (Os), iridium (Ir), and metals belonging to the iron group such as iron (Fe), cobalt (Co), nickel (Ni), gold (Au), silver (Ag), copper (Cu), etc. can be cited. The catalyst metal is typically supported on a carrier (such as the OSC material and / or the non-OSC material described later). The catalyst metals are preferably included in each of the lower layer 22, the intermediate layer 24, and the upper layer 26. It is preferred that the types of the main catalyst metals in each of the lower layer 22, the intermediate layer 24, and the upper layer 26 are different. In addition, the correspondence between each layer and the type of catalyst metal will be described later.

[0047] From the viewpoint of increasing the contact area with the exhaust gas, the catalyst metal is preferably fine particles with a sufficiently small particle size. The average particle size of the catalyst metal is, for example, 1 to 15 nm, preferably 1 to 10 nm, more preferably 1 to 5 nm. In addition, the average particle size of the catalyst metal can be obtained as the average value of the particle sizes of 50 or more catalyst metals arbitrarily selected in the transmission electron microscope (TEM) image after obtaining the TEM image of the catalyst metal.

[0048] The total amount of the catalyst metal contained in the entire catalyst layer 20 can be appropriately determined according to, for example, the exhaust gas volume, use, type of catalyst metal, etc. Although not particularly limited, for example, it can be 0.5 g / L or more, preferably 1.0 g / L or more, more preferably 2.0 g / L or more per 1 L volume of the substrate 10. On the other hand, the total amount of the catalyst metal contained in the entire catalyst layer 20 can be, for example, 8.0 g / L or less, preferably 7.0 g / L or less, more preferably 6.0 g / L or less per 1 L volume of the substrate 10.

[0049] (2) As for the NO x occlusion material, a known material used for such purposes in the past can be used. NOx In the occluding material, as NO x occluding element, typically includes at least one of an alkali metal element and an alkaline earth metal element. As an example, there can be mentioned a compound containing at least one of such NO x occluding element and being in the form of a carbonate or an oxide, or a combination thereof. As examples of the alkali metal element, there can be mentioned Li, K, Cs, etc., among which K is preferred. As examples of the alkaline earth metal element, there can be mentioned Mg, Sr, Ba, etc., among which Ba is preferred. In the intermediate layer 24, NO x occluding material must be included.

[0050] The total amount of NO x occluding material contained in the entire catalyst layer 20 is not particularly limited, typically below the content of the OSC material and / or non-OSC material. Specifically, per 1 L volume of the substrate 10, it is preferably 100 g / L or less, for example, 10 - 50 g / L.

[0051] (3) As the OSC material, known materials used for such purposes in the past can be used. As an example, there can be mentioned cerium dioxide (cerium oxide, CeO 2 ), and composite oxides containing cerium dioxide, such as composite oxides containing cerium dioxide and zirconium dioxide (cerium dioxide - zirconium dioxide composite oxide, so-called CZ composite oxide (also called ZC composite oxide)). From the viewpoint of suppressing the thermal deterioration of cerium dioxide, the OSC material preferably contains a CZ composite oxide, and more preferably consists essentially of a CZ composite oxide (more than 95% by mass of the whole, the same below). From the viewpoints such as preventing oxygen depletion, it is preferred to include the OSC material in each of the lower layer 22, intermediate layer 24, and upper layer 26 respectively.

[0052] From the viewpoints of improving heat resistance, oxygen absorption and release characteristics, etc., the OSC material (such as a CZ composite oxide) can be mainly composed of cerium dioxide and zirconium dioxide, and also contain other additive components. As examples of the additive components that can be included in the OSC material, there can be mentioned oxides containing rare earth elements, alkali metal elements, alkaline earth metal elements, transition metal elements, Si, Al, etc. As examples of the rare earth elements, there can be mentioned Sc, Y, La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, etc. As oxides of rare earth elements, Pr 2 O 3 、Nd 2 O 3 、La 2 O 3 and Y 2 O 3 are preferred. The additive components can be polycrystals or single crystals.

[0053] When the OSC material contains cerium dioxide, from the viewpoint of fully exerting its oxygen storage capacity, the content ratio of cerium dioxide in the whole OSC material is preferably 10% by mass or more, more preferably 25% by mass or more. On the other hand, from the viewpoint of moderately controlling the alkalinity of the OSC material, the content ratio of cerium dioxide is preferably 90% by mass or less, more preferably 75% by mass or less.

[0054] The total amount of the OSC material contained in the whole catalyst layer 20 is not particularly limited, and is preferably 45 to 250 g / L, more preferably 80 to 200 g / L, per 1 L volume of the substrate 10.

[0055] (4) As the non-OSC material, known materials used in such applications in the past can be used. Among them, from the viewpoints such as highly dispersedly supporting the catalyst metal, an inorganic porous body having a relatively large specific surface area and excellent heat resistance is preferably used. As an example, alumina (Al 2 O 3 , aluminum oxide), titanium dioxide (TiO 2 , titanium oxide), zirconium dioxide (ZrO 2 , zirconium oxide), silicon dioxide (SiO 2 , silicon oxide), etc. can be mentioned. Among them, due to particularly excellent heat resistance and other reasons, it is preferred that the non-OSC material contains alumina, and more preferably it is substantially composed of alumina. From the viewpoints of improving heat resistance, durability, etc., it is preferred that the non-OSC material is included in each of the lower layer 22, the intermediate layer 24, and the upper layer 26.

[0056] From the viewpoints of improving heat resistance, etc., the non-OSC material (such as alumina) may contain the above-mentioned additive components, such as oxides of rare earth elements such as Pr 2 O 3 , Nd 2 O 3 , La 2 O 3 , Y 2 O 3 and so on.

[0057] The total amount of the non-OSC material contained in the whole catalyst layer 20 is not particularly limited, and is preferably 100 to 370 g / L, more preferably 170 to 300 g / L, per 1 L volume of the substrate 10.

[0058] In addition to the above (1) to (4), the catalyst layer 20 may further contain auxiliary materials. The auxiliary materials are, for example, materials for suppressing sintering and poisoning of the catalyst metal or increasing the oxygen storage amount of the OSC material. As the auxiliary materials, known materials conventionally used for such purposes can be used. As an example, simple substances, alloys, and compounds (such as oxides, sulfates, carbonates, nitrates, and chlorides) of metals containing one or more metal elements among alkali metal elements, alkaline earth metal elements, rare earth metal elements, and transition metal elements can be cited. In addition, layers that can contain auxiliary materials will be described later.

[0059] Hereinafter, the lower layer 22, the intermediate layer 24, and the upper layer 26 included in the catalyst layer 20 will be described in detail, respectively.

[0060] <Lower layer 22>

[0061] As Figure 2 shown, here, the lower layer 22 is directly formed on the surface of the substrate 10. In the present embodiment, the lower layer 22 has a lower front portion 22a located on the upstream side X1 in the exhaust gas flow direction F and a lower rear portion 22b located on the downstream side X2 in the exhaust gas flow direction F compared to the lower front portion 22a. Here, the lower front portion 22a is provided along the cylinder axis direction X from the end portion on the upstream side X1 of the substrate 10. Here, the lower rear portion 22b is provided along the cylinder axis direction X from the end portion on the downstream side X2 of the substrate 10. The lower layer 22, particularly the lower front portion 22a, functions as a CO purification layer in a lean combustion atmosphere such as during lean combustion start control.

[0062] Both the lower front portion 22a and the lower rear portion 22b contain Pd as the catalyst metal. Among the catalyst metals, Pd has particularly high oxidation activity and particularly excellent CO purification performance. By disposing Pd in the lower layer 22 that easily gets hot during startup, the purification reaction of CO can occur well during lean combustion start control. In addition, oxidation catalysts such as Pd are easily affected by toxic substances (such as sulfur components) contained in the exhaust gas. Therefore, by disposing Pd in the lower layer 22 separated from the upper layer 26 where toxic substances are likely to adhere, contact between Pd and toxic substances can be suppressed.

[0063] The total amount of Pd contained in the lower layer 22 as a whole is not particularly limited, and can be, for example, 0.1 g / L or more, 0.5 g / L or more, preferably 1.0 g / L or more, and more preferably 2.0 g / L or more, relative to each 1 L volume of the substrate 10. As a result, the purification reaction of CO can be actively caused during lean start control, and the effect of the technology disclosed herein can be exerted at a high level. On the other hand, the total amount of Pd contained in the lower layer 22 as a whole can be, for example, 5.0 g / L or less, preferably 4.0 g / L or less, more preferably 3.0 g / L or less, and further preferably 2.9 g / L or less, 2.5 g / L or less, relative to each 1 L volume of the substrate 10. In this case, the purification reaction of CO becomes slow during lean start control, and the oxidation reaction of NO is particularly likely to be hindered. Therefore, it is particularly effective to adopt the technology disclosed herein.

[0064] In this embodiment, the content of Pd in ​​the lower front portion 22a per 1L of the base material (C F ) is greater than the content of Pd in ​​the lower rear portion 22b per 1L of substrate (C R ). That is, satisfying C F >C R In other words, the Pd content (C F ) and the Pd content (C R ) ratio (C F / C R ) satisfies the following formula: 1<(C F / C R ). By increasing the Pd content in the lower front portion 22a, which is easily heated at startup, the CO purification reaction can be actively generated. This allows CO to be purified as early as possible, and the NO oxidation reaction to start smoothly. In addition, the reaction heat (heat capacity) during the CO purification reaction is conducted to the downstream side along with the flow of exhaust gas, thereby improving the preheating property of the catalyst as a whole. As a result, the catalyst metal can be heated to the active temperature as early as possible. Therefore, NO can be particularly suppressed. x When the internal combustion engine is started, the gas flows out from the exhaust gas purifying catalyst 100 and is released as exhaust.

[0065] From the viewpoint of achieving the effect of the technology disclosed herein at a high level, the above ratio (C F / C R ) is preferably 1.2 or more, more preferably 1.5 or more. F / C R ) is not particularly limited, but when the above ratio is too large, the effect of the technology disclosed herein reaches a stagnation state. F / C R) can be about 10 or less, 5.0 or less, preferably 4.0 or less, more preferably 3.5 or less, and still more preferably 3.0 or less.

[0066] Among them, the above ratio (C F / C R ) preferably satisfies the following formula: 1.2 ≤ (C F / C R ) ≤ 3.5, more preferably satisfies the following formula: 1.5 ≤ (C F / C R ) ≤ 3.0, and particularly preferably satisfies the following formula: 1.5 ≤ (C F / C R ) ≤ 2.0. Thereby, the NO purification performance in the stoichiometric - rich combustion atmosphere can be improved. Therefore, NO emissions can be reduced in a wide range of lean - rich combustion atmospheres. x x

[0067] The Pd content (C F ) of the front part 22a of the lower layer is not particularly limited, and can be 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 per 1 L volume of the substrate 10. Thereby, the NO occlusion performance in the lean combustion atmosphere can be better improved, and the technical effects disclosed herein can be exerted at a high level. When the content of the catalyst metal is too high, the technical effects disclosed herein reach a stagnant state, or particle growth of the catalyst metal is likely to occur. Therefore, the Pd content (C x ) of the front part 22a of the lower layer can be 6.0 g / L or less, preferably 5.0 g / L or less, more preferably 4.0 g / L or less, and still more preferably 3.5 g / L or less per 1 L volume of the substrate 10. F

[0068] The Pd content (C R ) of the rear part 22b of the lower layer is not particularly limited, and can be about 0.1 g / L or more, 0.5 g / L or more, preferably 1.0 g / L or more per 1 L volume of the substrate 10. Thereby, the NO purification performance in the stoichiometric - rich combustion atmosphere can be improved. Due to reasons such as being disadvantageous in terms of cost when the content of the catalyst metal is too high, the Pd content (C x ) in the rear part 22b of the lower layer can 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 per 1 L volume of the substrate 10. R

[0069] The lower front part 22a and / or the lower rear part 22b may also contain a catalyst metal other than Pd. As an example, a noble metal (PGM) belonging to the platinum group as described above can be cited. Among them, Pt with high oxidation activity is preferred. In the lower front part 22a and the lower rear part 22b, the content ratio of Pd in the whole catalyst metal is preferably 80% by mass or more, more preferably 90% by mass or more, and particularly preferably the catalyst metal is substantially composed of Pd.

[0070] Preferably, the lower front part 22a and the lower rear part 22b each contain a non-OSC material. The non-OSC material may be a carrier carrying a catalyst metal such as Pd, or may be in a form that does not carry a catalyst metal such as Pd. The non-OSC material may also be the first component (the component with the highest content ratio based on mass, the same below) of the lower front part 22a and the lower rear part 22b. Preferably, the content of the non-OSC material in the lower front part 22a per 1 L of the substrate is greater than the content of the non-OSC material in the lower rear part 22b per 1 L of the substrate. Thereby, the heat resistance and durability of the lower front part 22a can be better improved.

[0071] The content of the non-OSC material in the lower front part 22a is not particularly limited. Per 1 L volume of the substrate 10, it is preferably 40 to 110 g / L, more preferably 50 to 100 g / L, for example, 60 to 90 g / L. It is preferred that the content of the non-OSC material in the lower front part 22a is 10 g / L or more higher than the content of the non-OSC material in the lower rear part 22b, and more preferably 20 g / L or more higher. The content of the non-OSC material in the lower rear part 22b is not particularly limited. Per 1 L volume of the substrate 10, it is preferably 20 to 90 g / L, more preferably 30 to 80 g / L, for example, 40 to 70 g / L.

[0072] Preferably, the lower front part 22a and the lower rear part 22b each contain an OSC material. The OSC material may be a carrier carrying a catalyst metal such as Pd, or may be in a form that does not carry a catalyst metal such as Pd. Contrary to the non-OSC material, it is preferred that the content of the OSC material in the lower front part 22a per 1 L of the substrate is less than the content of the OSC material in the lower rear part 22b per 1 L of the substrate. Thereby, the exhaust gas purification performance in the stoichiometric to rich combustion atmosphere can be better improved.

[0073] The content of the OSC material in the lower front part 22a is not particularly limited, and is preferably 1 to 60 g / L, more preferably 5 to 50 g / L, for example 10 to 40 g / L, per 1 L volume of the substrate 10. It is preferred that the content of the OSC material in the lower front part 22a is 10 g / L or more, 20 g / L or more lower than the content of the OSC material in the lower rear part 22b, and more preferably 30 g / L or more lower. The content of the OSC material in the lower rear part 22b is not particularly limited, and is preferably 20 to 90 g / L, more preferably 30 to 80 g / L, for example 40 to 70 g / L, per 1 L volume of the substrate 10.

[0074] The lower front part 22a and / or the lower rear part 22b may also contain the auxiliary material and NO x occlusion material as described above. For example, by containing an alkaline earth metal element (such as Ba), poisoning of the catalyst metal, especially an oxidation catalyst such as Pd, can be inhibited. The dispersibility of the catalyst metal can also be improved, and sintering of the catalyst metal can be inhibited. In addition, for example, by containing an alkaline earth metal element together with the OSC material, the oxygen occlusion amount to the OSC material can be increased in a lean combustion atmosphere. Furthermore, by containing NO x occlusion material, the NO x occlusion amount can be increased in a lean combustion atmosphere.

[0075] In the lower front part 22a and / or the lower rear part 22b, the content of the auxiliary material and / or the NO x occlusion material is not particularly limited, and is preferably 30 g / L or less, more preferably 20 g / L or less, for example 10 g / L or less, per 1 L volume of the substrate 10. In the lower front part 22a and the lower rear part 22b, the content of the auxiliary material and / or the NO x occlusion material per 1 L of the substrate may also be substantially the same (within about ±10%, for example within ±5%).

[0076] The coating amounts of the lower front part 22a and the lower rear part 22b are not particularly limited, and are respectively preferably, for example, 50 to 250 g / L, more preferably 70 to 200 g / L, and further preferably 80 to 150 g / L, per 1 L volume of the substrate 10.

[0077] The coating length La in the exhaust gas flow direction (cylinder axis direction X) of the front part 22a of the lower layer is typically shorter than the total length L in the extending direction (cylinder axis direction X) of the substrate 10. The coating length La of the front part 22a of the lower layer is not particularly limited, preferably 20 to 70% of the total length L of the substrate 10, more preferably 30 to 60%, still more preferably 40 to 50%, for example, less than 50%. The coating length Lb in the exhaust gas flow direction (cylinder axis direction X) of the rear part 22b of the lower layer is typically shorter than the total length L in the extending direction (cylinder axis direction X) of the substrate 10. The coating length Lb of the rear part 22b of the lower layer is preferably 50% or more of the total length L of the substrate 10, more preferably 60 to 90%, still more preferably 70 to 80%. Thus, it is possible to achieve a high level of balance between the improvement of NO x occlusion performance in a lean combustion atmosphere and NO x purification performance improvement in a stoichiometric to rich combustion atmosphere.

[0078] Preferably, the coating length La of the front part 22a of the lower layer is shorter than the coating length Lb of the rear part 22b of the lower layer. That is, preferably La < Lb. Thus, it is possible to make the Pd concentrations on the upstream side and the downstream side significantly different, with Pd being biased towards the upstream side, and the above effects can be better exerted.

[0079] In addition, in the Figure 2 shown manner, La + Lb ≈ L, and the front part 22a of the lower layer and the rear part 22b of the lower layer are in contact with each other in the cylinder axis direction X. However, the front part 22a of the lower layer and the rear part 22b of the lower layer may also be separated from each other in the cylinder axis direction X. In addition, the sum (La + Lb) of the coating length La of the front part 22a of the lower layer and the coating length Lb of the rear part 22b of the lower layer is preferably L ≤ (La + Lb), for example, L ≤ (La + Lb) ≤ 1.5L. That is, for reasons such as the manufacturing method of the slurry, etc., the front part 22a of the lower layer and the rear part 22b of the lower layer may also partially overlap in the central part in the cylinder axis direction X. At this time, it is preferable to dispose the rear part 22b on the side of the substrate 10.

[0080] <Intermediate layer 24>

[0081] The intermediate layer 24 contains Pt as a catalyst metal and the above-mentioned NO x occlusion material, and functions as a NO x occlusion layer in a lean combustion atmosphere such as during lean combustion start control. The intermediate layer 24 can also function as a three-way catalyst layer in a stoichiometric to rich combustion atmosphere. Since the intermediate layer 24 is sandwiched between the lower layer 22 and the upper layer 26, it is easy to retain the exhaust gas flowing in from the upper layer 26 side or the harmful components contained in the exhaust gas. Therefore, by making the intermediate layer 24 contain a NO x occlusion material, it is easy to occlude NO 2 . Moreover, in a stoichiometric to rich combustion atmosphere, from NO xOccluded material-detached NO 2 It is liable to come into contact with reducing gases such as HC and CO. Therefore, it can reduce the emission of NO in a wide range of lean-burn to rich-burn atmospheres. x

[0082] Among catalyst metals, Pt has particularly high oxidation performance, especially excellent NO oxidation performance. Therefore, by containing Pt in the intermediate layer 24, NO can be oxidized to generate NO near the occluded material in a lean-burn atmosphere. x 2 Moreover, Pt has a high reactivity with paraffin-based HC. Therefore, by containing Pt in the intermediate layer 24, HC and CO can be efficiently purified in a stoichiometric to rich-burn atmosphere, for example. The Pt content in the intermediate layer 24 is not particularly limited, and is preferably 0.1 to 5 g / L, more preferably 0.5 to 3 g / L, per 1 L volume of the substrate 10. Thereby, it is possible to achieve a high level of balance between the improvement of NO occlusion performance in a lean-burn atmosphere and the improvement of HC purification performance in a stoichiometric to rich-burn atmosphere. x

[0083] The intermediate layer 24 may also contain a catalyst metal other than Pt. As an example, noble metals (PGM) belonging to the platinum group as described above can be cited. Among them, Pd with high oxidation activity is preferred. In the intermediate layer 24, the content ratio of Pt in the overall catalyst metal is preferably 80 mass% or more, more preferably 90 mass% or more, and particularly preferably the catalyst metal consists substantially of Pt. In the case where the intermediate layer 24 substantially does not contain Pd (the content in the entire layer is about 5 mass% or less, preferably 1 mass% or less, more preferably 0.1 mass% or less, the same below), and in the case where the intermediate layer 24 contains Pd and the content of Pd in the intermediate layer 24 per 1 L of the substrate (C m ) is less than the content of Pd in the rear part 22b of the lower layer per 1 L of the substrate (C R ), CO is particularly likely to remain in the intermediate layer 24 and is likely to hinder the NO oxidation reaction. Therefore, the technology disclosed herein is particularly effective.

[0084] NO x The occluded material preferably contains an alkaline earth metal element as the NO x occluding element, and particularly preferably contains Ba. The NO x occluded material is particularly preferably one compound selected from Ba-containing oxides and Ba-containing carbonates. The content of the NO x occluded material in the intermediate layer 24 is not particularly limited, and is preferably 1 to 60 g / L, more preferably 10 to 50 g / L, for example 20 to 40 g / L, per 1 L volume of the substrate 10.

[0085] ​​​The intermediate layer 24 preferably further contains an OSC material. More preferably, the intermediate layer 24 contains an OSC material and a non-OSC material. The OSC material and / or the non-OSC material may be a carrier supporting a catalyst metal such as Pt, or may be in a form not supporting a catalyst metal such as Pt. The OSC material may also be the first component of the intermediate layer 24. The content of the OSC material in the intermediate layer 24 is not particularly limited, and is preferably 60 to 120 g / L, more preferably 70 to 110 g / L, for example 80 to 100 g / L, per 1 L volume of the substrate 10. In addition, the content of the non-OSC material in the intermediate layer 24 is not particularly limited, and is preferably 1 to 60 g / L, more preferably 10 to 50 g / L, for example 20 to 40 g / L, per 1 L volume of the substrate 10.

[0086] The coating amount of the intermediate layer 24 is not particularly limited, and is preferably, for example, 120 to 200 g / L, more preferably 130 to 180 g / L, and still more preferably 140 to 170 g / L, per 1 L volume of the substrate 10.

[0087] <Upper layer 26>

[0088] The upper layer 26 contains Rh as a catalyst metal. The upper layer 26 can function as a three-way catalyst layer. In, for example, a stoichiometric to rich combustion atmosphere, the upper layer 26 functions as a NO x reduction layer. That is, NO occluded in the intermediate layer 24 2 becomes a gas and is easily moved to the surface layer side when it detaches from the NO x occlusion material. Therefore, by containing Rh in the upper layer 26 at a position closer to the surface layer side than the intermediate layer 24, NO can be efficiently removed when it detaches from the NO 2 occlusion material. x When the NO occlusion material detaches, it can be efficiently removed.

[0089] Among the catalyst metals, the H 2 generation ability of Rh is particularly high, and the three-way catalytic performance (especially the NO x purification performance) is also high. Therefore, for example, in a stoichiometric to rich combustion atmosphere, NO can be efficiently purified x . The content of Rh in the upper layer 26 is not particularly limited, and is preferably 0.01 to 1.0 g / L, more preferably 0.05 to 0.5 g / L, per 1 L volume of the substrate 10.

[0090] The upper layer 26 may further contain a catalyst metal other than Rh. As an example, noble metals (PGM) belonging to the platinum group as described above can be cited. Among them, Pd and Pt with high oxidation activity are preferred. In the upper layer 26, the content ratio of Rh in the total catalyst metal is preferably 30% by mass or more, more preferably 50% by mass or more.

[0091] The upper layer 26 preferably further contains a non-OSC material. More preferably, the upper layer 26 contains a non-OSC material and an OSC material. The OSC material and / or the non-OSC material may be a carrier supporting a catalyst metal such as Rh, or may be in a form that does not support a catalyst metal such as Rh. The non-OSC material may also 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, and is preferably 30 to 100 g / L, more preferably 40 to 90 g / L, for example 50 to 80 g / L, per 1 L volume of the substrate 10. The content of the OSC material in the upper layer 26 is not particularly limited, and is preferably 1 to 50 g / L, more preferably 5 to 40 g / L, for example 10 to 30 g / L, per 1 L volume of the substrate 10.

[0092] The upper layer 26 may also further contain an auxiliary material and NO x occlusion material as described above. In a preferred embodiment, the upper layer 26 substantially does not contain NO x occlusion material, an auxiliary material containing an alkali metal element, and an auxiliary material containing an alkaline earth metal element.

[0093] The coating amount of the upper layer 26 is not particularly limited, and is preferably, for example, 50 to 120 g / L, more preferably 60 to 110 g / L, and further preferably 70 to 100 g / L, per 1 L volume of the substrate 10.

[0094] Method for Manufacturing Exhaust Gas Purification Catalyst 100

[0095] The exhaust gas purification catalyst 100 can be manufactured, for example, by the following method, but is not particularly limited. 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 catalyst metal source (for example, a solution containing a catalyst metal in an ionic form) and other components (for example, NO x occlusion 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 can be used.

[0096] The properties of the slurry (such as viscosity, solid content, etc.) can be appropriately determined according to the size of the substrate 10 used, the shape of the chamber 12 or the rib wall 14, the required properties of the catalyst layer 20, etc. The use of a thickener is beneficial for adjusting the viscosity of the slurry. As the thickener, for example, 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 crosslinking product obtained by crosslinking HEC with citric acid can be used.

[0097] Specifically, first, prepare a slurry for forming the lower layer containing a Pd source, a slurry for forming the intermediate layer containing a Pt source and a NO x occlusion material, and a slurry for forming the upper layer containing a Rh source. At this time, as the slurry for forming the lower layer, prepare a slurry for forming the front part of the lower layer with a relatively high Pd content and a slurry for forming the rear part of the lower layer with a relatively low Pd content.

[0098] Next, by a known coating method (such as suction coating method, blowing method, dip coating method, etc.), apply the slurry for forming the rear part of the lower layer from the end of the downstream side X2 of the substrate 10 to a specified position according to a known method, and dry it. Next, apply the slurry for forming the front part of the lower layer from the end of the upstream side X1 of the substrate 10 to a specified position according to a known method, and dry it. Then, fire it to form the lower layer 22. In addition, for example, in the suction coating method, the coating length of each layer can be accurately adjusted by sucking the slurry from the other end while immersing one end of the substrate in the slurry.

[0099] Next, apply the slurry for forming the intermediate layer on the lower layer 22 by a known coating method, dry it and then fire it to form the intermediate layer 24. Further, apply the slurry for forming the upper layer on the intermediate layer 24 by a known coating method, dry it and then fire it to form the upper layer 26. In addition, the drying conditions of the slurry can typically be carried out at a temperature of 70 - 150 °C, for example, 90 - 130 °C for about 1 - 10 hours. In addition, the firing conditions can typically be carried out at a temperature of about 300 - 800 °C, for example, 400 - 500 °C for about 1 - 4 hours.

[0100] "Use of the Exhaust Gas Purification Catalyst 100"

[0101] As described above, the exhaust gas purification catalyst 100 can, for example, improve the NO x occlusion performance in a lean burn atmosphere during the start-up of an internal combustion engine. Moreover, it is preferably capable of achieving excellent NO x purification performance in a stoichiometric - rich burn atmosphere. Therefore, the exhaust gas purification catalyst 100 can be applied to the purification of exhaust gas discharged from vehicles such as cars and trucks, motorcycles, motorized bicycles, and marine products such as ships, tankers, jet skis, personal watercraft, and outboard motors, gardening products such as lawn mowers, chain saws, and trimmers, leisure products such as golf carts and all-terrain vehicles, power generation equipment such as combined heat and power systems, and internal combustion engines such as waste incinerators. Among them, it can be applied to vehicles such as cars, and particularly to vehicles equipped with gasoline engines.

[0102] "Exhaust Gas Purification System and Control Method"

[0103] The system for purifying exhaust gas disclosed by the present invention includes: an exhaust path connected to an internal combustion engine; an exhaust gas purification catalyst 100 disposed in the above exhaust path; an air-fuel ratio adjustment device disposed upstream of the exhaust gas purification catalyst 100 in the exhaust gas flow direction; and a control unit. The air-fuel ratio adjustment device is, for example, a fuel addition device that sprays atomized fuel (HC) into the exhaust gas. The control unit is configured to be able to perform the following lean combustion start control, that is, when the internal combustion engine starts, control the air-fuel ratio adjustment device to adjust the air-fuel ratio of the exhaust gas to a lean combustion state and then start supplying it to the exhaust gas purification catalyst 100. During the preheating process in the lean combustion start control, NO in the exhaust gas x is absorbed by the exhaust gas purification catalyst 100 (specifically, the NO x occlusion material).

[0104] In such an exhaust gas purification system, it is preferable that the control unit performs the following preparation control, that is, before starting the lean combustion start control, control the air-fuel ratio adjustment device to adjust the air-fuel ratio of the exhaust gas to a rich combustion state and then supply it to the exhaust gas purification catalyst 100. Thereby, the oxidation catalyst (especially Pt, Pd) can be reduced, and as a preparation for absorbing NO with the NO x occlusion material, the oxidation performance of the oxidation catalyst can be improved. As a result, the NO x occlusion material can easily absorb NO x , and the NO x occlusion amount can be increased well. x

[0105] Hereinafter, test examples related to the present invention will be described, but the present invention is not intended to be limited to the forms shown in the following test examples.

[0106] [Test Example I: Evaluation of NO x Occlusion Performance in Lean Combustion Atmosphere]

[0107] In Test Example I, the Pd content in the front part of the lower layer and the rear part of the lower layer was changed, and the NO x occlusion performance in the lean combustion atmosphere was evaluated. Specifically, the time to achieve 50% CO purification and the NO x occlusion start time were evaluated.

[0108] <Test Substrate>

[0109] First, a cylindrical cordierite honeycomb substrate (diameter: 118.4 mm, total length: 114.3 mm, volume: 1.26 L) was prepared. One end of the substrate was defined as the end of the upstream side X1 (i.e., the end where the exhaust gas flows in), and the other end was defined as the end of the downstream side X2 (i.e., the end where the exhaust gas flows out).

[0110] <Preparation of Exhaust Gas Purification Catalyst for Example 1>[[]]​

[0111] As in Example 1, a catalyst for purifying exhaust gas is produced in which the Pd content in the front part of the lower layer is higher than that in the rear part of the lower layer. Specifically, first, a lower layer 122 is formed which contains the components shown in Table 1 and has the Figure 3 cross-sectional shape shown. In detail, first, an aqueous palladium nitrate solution as a Pd source, a CZ composite oxide powder as an OSC material, Al 2 O 3 powder as a non-OSC material, barium sulfate as an auxiliary material, and HEC as a thickener are mixed in ion-exchanged water, and then ground so that the average particle diameter (D50) of the powder becomes 5 μm, to prepare two kinds of slurries: a slurry for forming the front part of the lower layer and a slurry for forming the rear part of the lower layer.

[0112] Next, the slurry for forming the rear part of the lower layer is applied by a suction coating method to an area of 45% of the full length of the substrate from the end on the downstream side of the substrate, dried at 90°C for 1 hour, and then fired at 500°C for 1 hour to form the rear part 122b of the lower layer. In addition, by the suction coating method, the slurry for forming the front part of the lower layer is applied to an area of 75% of the full length of the substrate from the end on the downstream side of the substrate, dried at 90°C for 1 hour, and then fired at 500°C for 1 hour to form the front part 122a of the lower layer. Thus, as Figure 3 shown, a lower layer 122 having a front part 122a and a rear part 122b of the lower layer is formed on the surface of the substrate 110. In addition, the front part 122a and the rear part 122b of the lower layer partially overlap at the central part in the cylinder axis direction of the substrate 110 ( Figure 3 the OL part in

[0113] [Table 1]

[0114] Table 1 Composition of the lower layer

[0115]

[0116] Next, an intermediate layer containing the components shown in Table 2 is formed on the lower layer. Specifically, first, an aqueous platinum nitrate solution as a Pt source, a CZ composite oxide powder as an OSC material, Al 2 O 3 powder as a non-OSC material, barium carbonate as a NO x occlusion material, and HEC as a thickener are mixed in ion-exchanged water, and then ground so that the average particle diameter (D50) of the powder becomes 5 μm, to prepare a slurry for forming the intermediate layer. Then, the slurry for forming the intermediate layer is applied to the entire length (100%) of the substrate on the lower layer formed above by a wash coating method, dried at 90°C for 1 hour, and then fired at 500°C for 1 hour, thereby forming the intermediate layer.

[0117] [Table 2]

[0118] Composition of the intermediate layer in Table 2

[0119]

[0120] Next, an upper layer containing the components shown in Table 3 is formed on the intermediate layer. Specifically, first, an aqueous rhodium nitrate solution as the Rh source, an aqueous palladium nitrate solution as the Pd source, CZ composite oxide powder as the OSC material, Al 2 O 3 powder as the non-OSC material, and HEC citric acid crosslink as the thickener are mixed in ion-exchanged water, and then ground so that the average particle size (D50) of the powder becomes 5 μm to prepare a slurry for forming the upper layer. Then, the slurry for forming the upper layer is applied to the entire length (100%) of the substrate on the above-formed intermediate layer by the wash coating method, dried at 90°C for 1 hour, and then fired at 500°C for 1 hour to form the upper layer. The exhaust gas purification catalyst of Example 1 is obtained as described above.

[0121] [Table 3]

[0122] Composition of the upper layer in Table 3

[0123]

[0124] <Preparation of exhaust gas purification catalysts of Examples 2 to 5 and Comparative Example 1>

[0125] Except that the Pd content (C F ) in the front part of the lower layer and the Pd content (C R ) in the rear part of the lower layer are changed as shown in Table 4, the same operations as in Example 1 are performed to obtain the exhaust gas purification catalysts of each example. In addition, the ratio (C F ) of the Pd content (C R ) in the front part of the lower layer to the Pd content (C F ) in the rear part of the lower layer (C R ) is also shown in Table 4.

[0126] [Table 4]

[0127] Table 4

[0128]

[0129] ※Lean burn atmosphere (A / F = 15.7)

[0130] <Evaluation of CO 50% purification time and NO x occlusion start time under lean burn atmosphere>

[0131] After cooling the exhaust gas purification catalyst for each example to below 100°C, it was installed in the exhaust pipe of the engine bench. Then, exhaust gas (λ value = 1.08, A / F = 15.7) whose temperature was adjusted to 300°C by a heat exchanger was made to flow into the exhaust gas purification catalyst. At this time, the CO concentration at the position before the catalyst inflow and the CO concentration at the position after the catalyst outflow were measured, and based on this, the CO 50% purification time in the lean combustion atmosphere (A / F = 15.7) was calculated. In addition, the NO x amount at the position before the catalyst inflow and the NO x amount at the position after the catalyst outflow were measured, and based on this, the NO x occlusion start time in the lean combustion atmosphere (A / F = 15.7) was calculated. The results are shown in Table 4.

[0132] Figure 4 Indicates the relationship between the Pd content ratio (C F / C R ) and the CO 50% purification time. Figure 5 Indicates the relationship between the Pd content ratio (C F / C R ) and the NO occlusion start time. Figure 6 Indicates the relationship between the CO 50% purification time and the NO occlusion start time. As shown in Table 4 and Figure 4 , Figure 5 shown, the larger the Pd content ratio (C F / C R ), the earlier the CO 50% purification time and the NO occlusion start time. In addition, as Figure 6 shown, it was confirmed that there was a positive correlation between the CO 50% purification time and the NO occlusion start time. From the above results, it can be seen that in the lean combustion atmosphere, as a result of promoting the CO purification reaction by increasing the Pd content in the front part of the lower layer, the NO oxidation reaction becomes likely to occur. Therefore, with the exhaust gas purification catalyst disclosed herein, it is possible to reduce the emission of NO x especially during the start of an internal combustion engine.

[0133] [Test Example II: Evaluation of NO x Purification Performance in Rich Combustion Atmosphere]

[0134] In Test Example II, the exhaust gas purification catalysts of Examples 1 to 5 in which the NO x occlusion start time was 40 seconds or less in Test Example I were evaluated for the NO x purification rate in a rich combustion atmosphere (A / F = 14.45).

[0135] <Evaluation of NO x Purification Rate in Rich Combustion Atmosphere>

[0136] Place the exhaust gas purification catalyst for each example in the exhaust pipe of the engine bench, and let the exhaust gas whose temperature is adjusted to 460 °C by the heat exchanger flow into the exhaust gas purification catalyst. At this time, change the A / F of the inflowing exhaust gas, and measure the NO amount before the catalyst inflow and the NO amount after the catalyst outflow when A / F = 14.45. Based on this, calculate the NO purification rate (%) in the rich combustion atmosphere (A / F = 14.45). The results are shown in Table 5. x amount and the NO x amount after the catalyst outflow, and calculate the NO x purification rate (%) in the rich combustion atmosphere (A / F = 14.45) based on this. The results are shown in Table 5.

[0137] [Table 5]

[0138] Table 5

[0139]

[0140] ※Rich combustion atmosphere (A / F = 14.45)

[0141] As shown in Table 5, when the ratio of Pd content (C F / C R ) is 3.5, the NO x purification rate in the rich combustion atmosphere is the lowest. It can be considered that this is because the Pd content in the front part of the lower layer is too large, so the NO that is not completely reacted in the rich combustion atmosphere x flows through the exhaust gas purification catalyst and is discharged.

[0142] On the other hand, when the ratio of Pd content (C F / C R ) is 1.5 - 3, the NO x purification rate is a high value of 90% or more. Especially when the ratio of Pd content (C F / C R ) is 1.5 - 2.5, the NO x purification rate is 92% or more, and when the ratio of Pd content (C F / C R ) is 1.5 - 2, the NO x purification rate is 94% or more, showing a significantly high value. From the above results, it can be seen that by setting the ratio of Pd content (C F / C R ) within the above range, not only can the NO x occlusion performance in the lean combustion atmosphere be improved, but also the NO x purification performance in the rich combustion atmosphere can be improved.

[0143] [Test Example III: Study on the total amount of Pd and the NO x occlusion performance]

[0144] In Test Example III, fix the NO xOcclusion performance and NO under rich combustion atmosphere x The ratio of Pd content (C F / C R ) = 1.5, and the NO x occlusion performance was evaluated when the total amount of Pd in the lower layer was changed. Specifically, in addition to changing the Pd content in the front part and the rear part of the lower layer as shown in Table 6, the same operations as in Example 1 of Test Example I were performed to fabricate the exhaust gas purification catalysts of Examples 6 to 8, and the time to achieve 50% purification of CO and the NO x occlusion start time were evaluated in a lean combustion atmosphere. The results are shown in Table 6.

[0145] [Table 6]

[0146] Table 6

[0147]

[0148] Figure 7 Shows the relationship between the total amount of Pd in the lower layer and the time to achieve 50% purification of CO. Figure 8 Shows the relationship between the total amount of Pd in the lower layer and the NO occlusion start time. As shown in Table 6 and Figure 7 、 Figure 8 shown, when the ratio of Pd content (C F / C R ) = 1.5, starting from around when the total amount of Pd in the lower layer exceeds about 3, specifically around exceeding 2.9, the effects of the technology disclosed here become smaller and reach a stagnant state. It is considered that this is because when the amount of Pd used increases, there is a sufficient amount of Pd in the front part of the lower layer. Therefore, it can be known that when the total amount of Pd in the lower layer is 3.0 g / L or less, further 2.9 g / L or less, 2.5 g / L or less, which is less, the effects of the technology disclosed here can be exerted at a particularly high level.

[0149] The specific examples of the present invention have been described in detail above, but these are only examples and are not intended to limit the scope of the claims. The technology described in the scope of the claims includes various modified and changed forms of the specific examples illustrated above.

[0150] Symbol description

[0151] 10, 110: Substrate; 12: Chamber; 14: Rib wall; 20: Catalyst layer; 22, 122: Lower layer; 22a, 122a: Front part of the lower layer; 22b, 122b: Rear part of the lower layer; 24: Intermediate layer; 26: Upper layer; 100: Exhaust gas purification catalyst.

Claims

1. A catalyst for exhaust gas purification, disposed in the exhaust path of an internal combustion engine, for purifying the exhaust gas discharged from the internal combustion engine. It is characterized in that: It includes a substrate and a catalyst layer formed on the substrate. The catalyst layer includes a lower layer on the substrate side, an upper layer on the surface layer side, and an intermediate layer between the lower layer and the upper layer. The upper layer contains Rh. The intermediate layer contains at least Pt and NO x occlusion material The lower layer has a lower layer front part located on the upstream side in the exhaust gas flow direction and a lower layer rear part located on the downstream side in the exhaust gas flow direction when disposed in the exhaust path. The lower layer front part and the lower layer rear part each contain Pd. The content C of Pd in the front part of the lower layer per 1 L of the substrate F is greater than the content C of Pd in the rear part of the lower layer per 1 L of the substrate R .

2. The catalyst for exhaust gas purification according to claim 1. It is characterized in that: The said C F and the said C R The ratio of C F / C R satisfies the following formula: 1.5 ≤ (C F / C R ) ≤ 3.

0.

3. The catalyst for exhaust gas purification according to claim 1 or 2. It is characterized in that: The total amount of Pd per 1 L of the substrate contained in the entire lower layer is 3.0 g / L or less.

4. The catalyst for exhaust gas purification according to claim 1 or 2. It is characterized in that: The lower layer front part and the lower layer rear part 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 per 1 L of the substrate in the lower layer front part is greater than the content of the non-OSC material per 1 L of the substrate in the lower layer rear part. And, the content of the OSC material per 1 L of the substrate in the lower layer front part is less than the content of the OSC material per 1 L of the substrate in the lower layer rear part.

5. The catalyst for exhaust gas purification according to claim 1 or 2. It is characterized in that: The coating length of the lower layer front part in the exhaust gas flow direction is shorter than the coating length of the lower layer rear part in the exhaust gas flow direction.

6. The catalyst for exhaust gas purification according to claim 5. It is characterized in that: The coating length of the lower layer front part in the exhaust gas flow direction is 30% or more and 60% or less of the total length of the substrate. The coating length of the lower layer rear part in the exhaust gas flow direction is 60% or more and 90% or less of the total length of the substrate.

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