Exhaust gas purification catalyst

Abstract

This invention provides an exhaust gas purification catalyst in which the alloying of Pt-Rh is suppressed. [Solution] The exhaust gas purification catalyst 100 disclosed herein comprises a substrate 10 and a coating layer 20. The coating layer 20 comprises a Pt layer 22 located on the substrate 10 side and containing Pt, an Rh layer 23 located on the surface layer side and containing Rh, and an alloying suppression layer 24 interposed between the Pt layer 22 and the Rh layer 23 and containing an Al-containing oxide.

Description

[Technical Field]

[0001] This invention relates to a catalyst for purifying exhaust gas. [Background technology]

[0002] Exhaust gases emitted from internal combustion engines such as vehicle engines contain harmful components such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). Conventionally, exhaust gas purification catalysts have been used to remove these harmful components by oxidation or reduction reactions. A typical exhaust gas purification catalyst comprises a base material and a catalyst layer containing a catalytic metal (see, for example, Patent Documents 1-3).

[0003] For example, Patent Document 1 discloses an exhaust gas purification catalyst having a Pt layer provided on a substrate and containing Pt (platinum) as a catalytic metal, and an Rh layer laminated on the Pt layer and containing Rh (rhodium) as a catalytic metal. Patent Document 1 states that by including Pt and Rh in separate layers, the decrease in catalytic activity can be suppressed. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2020-157262 [Patent Document 2] Japanese Patent Publication No. 2006-346661 [Patent Document 3] International Publication No. 2015 / 076403 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] According to the inventors' findings, even if Pt and Rh are contained in separate layers, if the Pt layer and the Rh layer are in contact, Pt and / or Rh may migrate between layers (typically by evaporation) in an environment exposed to high-temperature (e.g., 500°C or higher) exhaust gas, potentially leading to Pt-Rh alloying. This results in a decrease in catalytic activity and a deterioration of exhaust gas purification performance. In particular, the inherent activity of Rh is lost, which tends to reduce NOx purification performance. Therefore, there is a need to suppress Pt-Rh alloying to a higher level. [Means for solving the problem]

[0006] The exhaust gas purification catalyst [1] disclosed herein is an exhaust gas purification catalyst for which exhaust gas is disposed in the exhaust path of an internal combustion engine and for purifying exhaust gas discharged from the internal combustion engine, and comprises a substrate and a coating layer provided on the substrate. The coating layer comprises a Pt layer located on the substrate side and containing Pt as a catalytic metal, an Rh layer located on the surface layer side and containing Rh as a catalytic metal, and an alloying suppression layer interposed between the Pt layer and the Rh layer, containing an Al-containing oxide and not containing Pt or Rh.

[0007] In the exhaust gas purification catalyst [1], by placing the Pt layer on the substrate side, the evaporation of Pt can be suitably suppressed in high-temperature environments. Furthermore, by placing an alloying suppression layer containing Al, which has excellent heat resistance, between the Pt layer and the Rh layer, the occurrence of Pt-Rh alloying can be suitably suppressed even in high-temperature environments. The combined effects of these factors allow for suitable suppression of the decrease in catalyst activity. Consequently, the initial exhaust gas purification performance can be stably maintained over a long period of time.

[0008] The exhaust gas purification catalyst [2] disclosed herein is an exhaust gas purification catalyst [1] wherein the alloying suppression layer has a ceria content of 6 g / L or less per liter of substrate. This makes it possible to more effectively suppress the decrease in catalytic activity and to exhibit the effects of the disclosed technology at a higher level.

[0009] The exhaust gas purification catalyst [3] disclosed herein, in the exhaust gas purification catalyst [1] or [2] above, the alloying suppression layer contains a catalyst metal (excluding Pt and Rh). Thereby, while suppressing the alloying of Pt-Rh, the catalytic performance can be improved by the activity of the added catalyst metal (for example, Pd).

[0010] The exhaust gas purification catalyst [4] disclosed herein, in any one of the exhaust gas purification catalysts [1] to [3] above, 90% by mass or more of the alloying suppression layer is composed of the Al-containing oxide. The Al-containing oxide is excellent in heat resistance and durability. Therefore, by setting the proportion of the Al-containing oxide to a predetermined value or more, a stronger alloying suppression layer can be formed, and the effects of the technology disclosed herein can be exerted at a higher level.

[0011] The exhaust gas purification catalyst [5] disclosed herein, in any one of the exhaust gas purification catalysts [1] to [4] above, the Pt layer and the Rh layer each contain an Al-containing oxide. Thereby, sintering of the catalyst metal in each layer can be preferably suppressed. Also, the integrity and durability of the coat layer can be enhanced.

[0012] The exhaust gas purification catalyst [6] disclosed herein, in the exhaust gas purification catalyst [5] above, when the content of the Al-containing oxide per 1 L of the substrate in the Pt layer is C2 and the content of the Al-containing oxide per 1 L of the substrate in the alloying suppression layer is C4, the following relationship: C4 < C2; is satisfied. Thereby, for example, interlayer migration of the noble metal evaporated from the Pt layer can be preferably suppressed, and the effects of the technology disclosed herein can be stably exerted at a high level over a long period.

[0013] The exhaust gas purification catalyst [7] disclosed herein is the exhaust gas purification catalyst [5] in which, when the content of the Al-containing oxide per liter of the substrate in the Rh layer is C3 and the content of the Al-containing oxide per liter of the substrate in the alloying suppression layer is C4, the following relationship: C3 < C4; is satisfied. Thereby, for example, the interlayer movement of the noble metal evaporated from the Rh layer can be suitably suppressed, and the effects of the technology disclosed herein can be stably exhibited at a high level over a long period of time.

[0014] The exhaust gas purification catalyst [8] disclosed herein is any one of the exhaust gas purification catalysts [1] to [7] in which the coating amount per liter of the substrate in the Rh layer is less than the coating amount per liter of the substrate in the Pt layer. Thereby, the exhaust gas easily diffuses to the lower layer (Pt layer), and the utilization efficiency of the catalyst can be improved.

[0015] The exhaust gas purification catalyst [9] disclosed herein is any one of the exhaust gas purification catalysts [1] to [8] in which, when the content of Pt per liter of the substrate in the Pt layer is X (g / L) and the specific surface area of the Al-containing oxide per liter of the substrate in the alloying suppression layer is Z (m 2 / L), the following formula: Z ≧ 745.69X + 2138.1; is satisfied. Thereby, Pt evaporated from the Pt layer can be effectively captured, and the interlayer movement of Pt can be suppressed at a particularly high level.

[0016] The exhaust gas purification catalyst

[10] disclosed herein is any one of the exhaust gas purification catalysts [1] to [9] in which, when the coating length in the flow direction of the exhaust gas of the Pt layer is 100%, the coating length in the flow direction of the exhaust gas of the alloying suppression layer is 75% or more. Thereby, the surface of the Pt layer can be widely covered with the alloying suppression layer to reduce the contact between the Pt layer and the Rh layer, and the effects of the technology disclosed herein can be exhibited at a higher level.

[0017] The exhaust gas purification catalyst

[11] disclosed herein, in any one of the exhaust gas purification catalysts [1] to

[10] , when the Pt layer is disposed in the exhaust passage, it is located on the downstream side in the flow direction of the exhaust gas, and when disposed in the exhaust passage, it is located on the upstream side in the flow direction of the exhaust gas than the Pt layer, and further includes a Pd layer containing Pd as a catalyst metal. By disposing the Pd layer on the upstream side, the lean exhaust gas can be suitably adjusted to a stoichiometric state by an oxidation reaction and supplied to the Pt layer. Therefore, in various exhaust gas conditions (e.g., a wide A / F region) that vary depending on the control of the internal combustion engine, the effects of the technology disclosed herein can be exhibited at a high level.

[0018] The exhaust gas purification catalyst

[12] disclosed herein, in the exhaust gas purification catalyst

[11] , the alloying suppression layer has a coat length in the flow direction of the exhaust gas shorter than that of the base material, and on the upstream side, the Rh layer and the Pd layer are in contact with each other in the thickness direction. Thereby, the catalyst performance (especially warm-up performance) immediately after starting the internal combustion engine can be improved, and excellent purification performance can be realized from the start.

Brief Description of the Drawings

[0019] <s [Figure 1] FIG. 1 is a perspective view schematically showing an exhaust gas purification catalyst according to an embodiment. [Figure 2] FIG. 2 is a partial cross-sectional view of the exhaust gas purification catalyst of FIG. 1 cut in the cylinder axis direction. [Figure 3] FIG. 3 is an exploded view schematically showing the coat length of each layer in Example 1. [Figure 4] FIG. 4 is a graph showing the relationship between the coat thickness of the alloying suppression layer and the NOx purification performance. [Figure 5] FIG. 5 is a graph showing the relationship between the coat length of the alloying suppression layer and the NOx purification performance. [Figure 6] FIG. 6 is a graph showing the relationship between the coat amount of the alloying suppression layer and the NOx purification performance when the Pt content of the Pt layer is changed. [Figure 7]Figure 7 is a graph showing the relationship between the Pt content of the Pt layer and the specific surface area of ​​the Al-containing oxide in the alloying-suppressing layer. [Modes for carrying out the invention]

[0020] Preferred embodiments of the present invention will be described below with reference to the drawings. Matters other than those specifically mentioned herein that are necessary for carrying out the present invention (for example, a general method for manufacturing exhaust gas purification catalysts) can be understood as design matters for those skilled in the art based on the prior art. The present invention can be carried out based on the contents disclosed herein and the common technical knowledge in the art. In addition, in the following drawings, the same reference numerals are used for members and parts that perform the same function, and redundant explanations may be omitted or simplified. The dimensional relationships (length, width, thickness, etc.) in each figure do not necessarily reflect the actual dimensional relationships. Also, in this specification, the notation "A~B" (A, B are arbitrary numerical values) indicating a range includes not only the meaning of A or more and B or less, but also the meaning of "greater than A" and "less than B".

[0021] ≪Exhaust gas purification catalyst 100≫ Figure 1 is a schematic diagram of the exhaust gas purification catalyst 100. Figure 2 is a partial cross-sectional view of the exhaust gas purification catalyst 100 cut along the cylinder axis X. The exhaust gas purification catalyst 100 is placed in the exhaust path of an internal combustion engine to purify the exhaust gas discharged from the internal combustion engine. The exhaust gas purification catalyst 100 can be placed in the exhaust system (exhaust pipe) of various internal combustion engines, especially vehicle engines.

[0022] As shown in Figure 1, the exhaust gas purification catalyst 100 comprises a base material 10 and a coating layer 20 (see Figure 2) provided on the base material 10. In Figure 1 and other figures, the symbol F indicates the direction of exhaust gas flow when the exhaust gas purification catalyst 100 is placed in the exhaust path. The symbol X indicates the cylindrical axis direction of the base material 10. The symbol X1 indicates the upstream side (front side) in the exhaust gas flow direction F, and the symbol X2 indicates the downstream side (rear side) in the exhaust gas flow direction F.

[0023] <Base material 10> The base material 10 constitutes the framework of the exhaust gas purification catalyst 100. Various materials and forms conventionally used for this type of application can be used for the base material 10. For example, as a material, ceramics such as cordierite, aluminum titanate, and silicon carbide are preferred due to their high heat resistance. Alternatively, a base material made of an alloy (such as stainless steel) can be used. Regarding the form, the base material 10 here has a honeycomb structure with a plurality of cells 12 regularly arranged along the cylindrical axis X (exhaust gas flow direction F) and rib walls 14 that partition the plurality of cells 12. The cells 12 are through-holes that function as passages for exhaust gas. The rib walls 14 are partition walls that separate each cell 12. The cross-sectional shape of the cells 12 here is rectangular. However, the cross-sectional shape of the cells 12 may be other shapes (e.g., circular, triangular, hexagonal, etc.).

[0024] The external shape of the base material 10 is cylindrical in this example, but the external shape of the base material 10 may be other shapes (e.g., elliptical cylinder, polygonal cylinder, etc.). Also, the base material 10 is honeycomb in this example, but the base material 10 may be foam, pellet, etc. The total length L (see Figure 2) of the base material 10 in the direction of the cylindrical axis X is typically 10 to 500 mm, for example 50 to 300 mm. The volume of the base material 10 is typically approximately 0.1 to 10 L, for example 0.5 to 5 L. In this specification, the volume of the base material 10 refers to the bulk volume (apparent volume) which includes the volume of internal voids such as cells 12 in addition to the net volume of the base material 10.

[0025] The substrate 10 shown in Figure 1 is a so-called straight-flow type substrate in which the openings X1 on the upstream side and X2 on the downstream side of the cell 12 are not closed. However, the substrate 10 may also be a so-called wall-flow type (also called a wall-through type) substrate in which the openings X1 on the upstream side and X2 on the downstream side of a number of cells 12 are alternately closed, and exhaust gas flows from the first cell (inlet cell) through the rib wall to the adjacent second cell (outlet cell).

[0026] <Coat layer 20> As shown in Figure 2, the coating layer 20 is provided on the substrate 10 (more specifically, the rib wall 14). The coating layer 20 has a multilayer structure in which multiple layers with different compositions are stacked in the thickness direction T, which is perpendicular to the cylindrical axis direction X. In the thickness direction T, the coating layer 20 includes a Pt layer 22 located on the substrate 10 side and containing Pt as a catalytic metal, an Rh layer 23 located on the surface side of the coating layer 20 and containing Rh as a catalytic metal, and an alloying suppression layer 24 interposed between the Pt layer 22 and the Rh layer 23. However, the coating layer 20 may further include layers other than the Pt layer 22, the Rh layer 23, and the alloying suppression layer 24, to the extent that it does not significantly impede the effects of the present invention.

[0027] In this embodiment, the Pt layer 22 is located downstream X2 in the exhaust gas flow direction F. The coating layer 20 is located upstream X1 in the exhaust gas flow direction F from the Pt layer 22 and further comprises a Pd layer 21 containing Pd as a catalytic metal. The Pd layer 21, Pt layer 22, and Rh layer 23 are catalytic layers containing catalytic metals. The exhaust gas purification catalyst 100 can function as a three-way catalyst by comprising these Pd layer 21, Pt layer 22, and Rh layer 23. The exhaust gas supplied to the exhaust gas purification catalyst 100 flows (passes) through the flow path of the catalytic layers (more specifically, within the cell 12), coming into contact with each catalytic layer and having harmful components purified.

[0028] By providing the Pd layer 21 upstream of the Pt layer 22 on X1, the effects of the technology disclosed herein can be realized at a high level. For example, high exhaust gas purification performance can be achieved under various exhaust gas conditions that fluctuate due to the control of the internal combustion engine (e.g., a wide A / F range). Furthermore, the Pt layer 22 can exhibit excellent catalytic activity under exhaust gas conditions that are mitigated by the Pd layer 21 (e.g., close to a stoichiometric atmosphere). However, the Pd layer 21 is not essential and can be omitted in other embodiments.

[0029] In each catalyst layer, Pd layer 21, Pt layer 22, and Rh layer 23, it is preferable that the type of catalyst metal that constitutes the main component (accounting for the highest proportion by mass) is different from that of the other. In addition to the main catalyst metal, each of these catalyst layers may also contain other types of catalyst metals known to function as oxidation catalysts and / or reduction catalysts in the purification of exhaust gas. For example, it may contain noble metals other than the main component, metals belonging to the iron group such as iron (Fe), cobalt (Co), and nickel (Ni), gold (Au), silver (Ag), and copper (Cu).

[0030] As shown in Figure 2, the Pd layer 21 is provided here on the surface of the substrate 10. The Pt layer 22 is provided here on the surface of the substrate 10. Parts of the Pd layer 21 and / or the Pt layer 22 may penetrate into the interior of the rib wall 14. The alloying suppression layer 24 is provided here on the surface of the Pt layer 22 and part of the Pd layer 21. The Rh layer 23 is provided here on the surface of the alloying suppression layer 24 and part of the Pd layer 21.

[0031] In this embodiment, the alloying suppression layer 24 has a coat length L4 in the exhaust gas flow direction F that is shorter than that of the base material 10, and the Pd layer 21 and the Rh layer 23 are in contact in the thickness direction T at the upstream side X1 in the cylindrical axis direction X. That is, at the upstream end X1 in the cylindrical axis direction X, the alloying suppression layer 24 is not interposed between the Pd layer 21 and the Rh layer 23. At the upstream end X1 in the cylindrical axis direction X, the Pd layer 21 and the Rh layer 23 constitute a two-layer structure. On the other hand, at the downstream end X2 in the cylindrical axis direction X, the Pt layer 22, the alloying suppression layer 24, and the Rh layer 23 constitute a three-layer structure.

[0032] In Figure 2, the Pd layer 21, Pt layer 22, Rh layer 23, and alloying suppression layer 24 are shown with approximately the same thickness, but the Pd layer 21, Pt layer 22, Rh layer 23, and alloying suppression layer 24 may have different thicknesses. Also, the coating thickness does not have to be uniform in the cylindrical axis direction X. In the following explanation, "coating thickness" refers to the thickness of the thinnest part of each layer.

[0033] The total coating amount of the coating layer 20 can be appropriately determined according to the type of the base material 10, the use of the exhaust gas purification catalyst 100, etc., and is not particularly limited. However, per liter of the base material 1L, for example, it is 150 to 600 g / L, preferably 200 to 500 g / L, and more preferably 300 to 450 g / L. Hereinafter, each of the Pd layer 21, the Pt layer 22, the Rh layer 23, and the alloying inhibition layer 24 that constitute the coating layer 20 will be described in order.

[0034] <Pd layer 21> The Pd layer 21 is a catalyst layer that essentially contains palladium (Pd) as a catalyst metal. Pd has particularly high activity as an oxidation catalyst and excellent purification performance for CO and HC. Also, by disposing the Pd layer 21 on the upstream side X1, the lean exhaust gas can be suitably adjusted to a stoichiometric state by an oxidation reaction and supplied to the Pt layer 22. The Pd layer 21 preferably contains a non-OSC material having no oxygen storage capacity (Oxygen Storage Capacity: OSC), and more preferably further contains an oxygen storage material (OSC material) having an oxygen storage capacity in addition to the non-OSC material.

[0035] The catalyst metal of the Pd layer 21 is preferably mainly composed of Pd (occupying 50% by mass or more. The same applies hereinafter), preferably Pd occupies 80% by mass or more of the total catalyst metal of the Pd layer 21, more preferably 90% by mass or more, and particularly preferably consists essentially of Pd (95% by mass or more of the total catalyst metal of the Pd layer 21 is Pd). The Pd layer 21 may further contain a metal species other than Pd as a catalyst metal. The Pd layer 21 preferably does not contain a reduction catalyst (for example, Rh) as a catalyst metal (the content of the reduction catalyst per liter of the base material is less than 0.1 g / L). Thereby, alloying of the catalyst metal can be better suppressed.

[0036] The content of catalyst metal (typically Pd) per liter of substrate in the Pd layer 21 can be appropriately determined according to, for example, the amount of exhaust gas and the application of the exhaust gas purification catalyst 100, and is not particularly limited, but is preferably 0.5 g / L or more, preferably 1.0 g / L or more, and more preferably 2.0 g / L or more. This allows the above-mentioned effects to be exhibited at a higher level. On the other hand, the content of catalyst metal (typically Pd) per liter of substrate in the Pd layer 21 is preferably 5.0 g / L or less, preferably 4.0 g / L or less, and more preferably 3.0 g / L or less, from the viewpoint of suppressing sintering of the catalyst metal.

[0037] Furthermore, as described above, the function of the Pd layer 21 is important for the Pt layer 22 to perform its function effectively. Therefore, in some embodiments, the ratio (R1 / R2) of the Pd content R1 per liter of substrate in the Pd layer 21 to the Pt content R2 per liter of substrate in the Pt layer 22 is preferably 0.8 or higher, and more preferably 1 or higher. The above ratio is generally preferably 3 or lower, more preferably 2 or lower, and even more preferably 1.5 or lower.

[0038] The non-OSC material has at least one of the following functions: improving the heat resistance and durability of the Pd layer 21, and suppressing the peeling of the Pd layer 21 from the substrate 10. Preferably, the non-OSC material is included in the Pd layer 21 as a carrier for supporting the catalyst metal. However, the non-OSC material may be included in the Pd layer 21 in whole or in part in a form that does not support the catalyst metal. As the non-OSC material, known materials conventionally used in this type of application can be used. As an example, from the viewpoint of supporting the catalyst metal in high dispersion, inorganic porous materials with a relatively large specific surface area, such as alumina (Al2O3, aluminum oxide), titania (TiO2, titanium oxide), zirconia (ZrO2, zirconium oxide), silica (SiO2, silicon oxide), etc., can be preferably used. Among these, metal oxides containing alumina (Al-containing oxides) that have high heat resistance and durability are preferred. Alumina has excellent heat resistance and durability, so it can suitably suppress sintering of the catalyst metal (typically Pd). Furthermore, it enhances the integration and durability with the base material 10. The non-OSC material preferably contains Al-containing oxides, and more preferably consists substantially of Al-containing oxides (95% or more by mass of the entire non-OSC material is Al-containing oxide).

[0039] The Al-containing oxide may be alumina, or a composite oxide of alumina and a metal oxide other than alumina (e.g., rare earth metal oxide). From the viewpoint of improving heat resistance and durability, the Al-containing oxide preferably includes, for example, a composite oxide of alumina and lanthanum oxide (Al2O3-La2O3 composite oxide, LA composite oxide). The LA composite oxide may be La-rich or Al-rich. In some embodiments, the Al-containing oxide preferably does not contain the element Ce. In some embodiments, from the viewpoint of suppressing deterioration over time with use, the mixing ratio of the metal oxide other than alumina (e.g., rare earth metal oxide) is preferably less than 50% by mass, for example, 0.1 to 20% by mass, when the total LA composite oxide is considered to be 100% by mass.

[0040] The content of non-OSC material (preferably Al-containing oxide) per liter of substrate in the Pd layer 21 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. In the Pd layer 21, the content of non-OSC material (preferably Al-containing oxide) per liter of substrate may be approximately the same as that of the Pt layer 22 described later (generally within ±10%, for example within ±5%).

[0041] OSC material has the function of absorbing oxygen in a lean atmosphere. By including OSC material, oxidation of the catalyst metal (typically Pd) can be suppressed in a lean atmosphere. Preferably, the OSC material is included in the Pd layer 21 as a support for the catalyst metal. However, the OSC material may be included in the Pd layer 21 in whole or in part in a form that does not support the catalyst metal. As the OSC material, known materials conventionally used for this type of application can be used. As an example, metal oxides containing ceria (cerium oxide, CeO2) with high oxygen absorption capacity (Ce-containing oxides) can be used.

[0042] The Ce-containing oxide may be ceria, or a composite oxide of ceria and a metal oxide other than ceria. In some embodiments, the Ce-containing oxide is preferably a composite oxide containing at least one of Zr and Al, from the viewpoint of improving heat resistance and durability, and more preferably a ceria (CeO2)-zirconia (ZrO2) composite oxide (CZ composite oxide). It is more preferable that the Ce-containing oxide does not contain the element Al. From the viewpoint of improving heat resistance, the CZ composite oxide may be, for example, Nd2O3, La2O3, Y2O3, Pr6O 10 It may further contain rare earth metal oxides such as the above. The CZ composite oxide may be Ce-rich or Zr-rich. In some embodiments, the mixing ratio of ceria is preferably about 10 to 90% by mass, for example 15 to 70% by mass, when the total CZ composite oxide is considered to be 100% by mass.

[0043] The content of OSC material (e.g., CZ composite oxide) per liter of substrate in the Pd layer 21 is not particularly limited, but is typically less than the content of non-OSC material per liter of substrate in the Pd layer 21, preferably 1 to 60 g / L, more preferably 5 to 50 g / L, for example 10 to 40 g / L. The content of OSC material per liter of substrate in the Pd layer 21 is preferably less than the content of OSC material per liter of substrate in the Pt layer 22, which will be described later.

[0044] The Pd layer 21 is preferably composed of a non-OSC material as the first component (the component that accounts for the largest proportion by mass; the same applies hereinafter). The Pd layer 21 is preferably composed mainly of a non-OSC material. The proportion of non-OSC material (preferably Al-containing oxide) in the Pd layer 21 is preferably 55 to 90% by mass, and more preferably 60 to 80% by mass. The proportion of OSC material (e.g., CZ composite oxide) in the Pd layer 21 is typically less than that of the non-OSC material, for example less than 50% by mass, preferably 1 to 40% by mass, and more preferably 10 to 30% by mass. The proportion of catalyst metal in the Pd layer 21 is preferably 0.1 to 10% by mass, and more preferably 1 to 5% by mass.

[0045] The Pd layer 21 may further contain auxiliary materials. These auxiliary materials are, for example, used to suppress sintering and poisoning of the catalyst metal, improve the oxygen storage capacity of the OSC material, or have NOx storage capacity to absorb NOx in a lean atmosphere. As auxiliary materials, known materials conventionally used for this type of application can be used. Examples include elemental metals, alloys, and compounds (e.g., oxides, sulfates, carbonates, nitrates, chlorides) containing one or more metal elements from alkali metal elements, alkaline earth metal elements, rare earth metal elements, and transition metal elements. For example, including alkaline earth elements (such as Ba) can suppress poisoning of the catalyst metal. It can also improve the dispersibility of the catalyst metal and suppress sintering of the catalyst metal. Furthermore, for example, including alkaline earth elements together with the OSC material can improve the oxygen storage capacity of the OSC material in a lean atmosphere.

[0046] The coating amount of the Pd layer 21 is not particularly limited, but per liter of the base material 1L, for example, 50 to 300 g / L is preferable, 70 to 250 g / L is more preferable, and 100 to 200 g / L is even more preferable. The coating amount of the Pd layer 21 is preferably less than the coating amount of the Pt layer 22 described later. Thereby, the pressure loss can be reduced. Also, the heat capacity of the upstream side X1 can be decreased, and the catalyst performance (warming-up property) immediately after starting the internal combustion engine can be improved.

[0047] In the embodiment of FIG. 2, the Pd layer 21 is provided along the cylinder axis direction X from the end of the upstream side X1 to the downstream side X2 of the base material 10. The coating length L1 of the Pd layer 21 in the exhaust gas flow direction (cylinder axis direction X) is shorter than the total length L of the base material 10 in the stretching direction (cylinder axis direction X). The coating length L1 of the Pd layer 21 is not particularly limited, but 20 to 70% of the total length L of the base material 10 is preferable, 30 to 60% is more preferable, and less than 50%, for example, 40 to 50% is even more preferable. Thereby, high exhaust gas purification performance can be realized under various exhaust gas conditions (for example, a wide A / F region) that vary depending on the control of the internal combustion engine. The coating length L1 of the Pd layer 21 is preferably shorter than the coating length L2 of the Pt layer 22 described later. That is, L1 < L2 is preferable. Thereby, the effects of the technology disclosed herein can be exhibited at a high level. However, in other embodiments, the coating length L1 of the Pd layer 21 may be the same as or longer than the coating length L2 of the Pt layer 22.

[0048] In the present embodiment, a part of the surface of the Pd layer 21 is covered with the alloying inhibition layer 24. However, in other embodiments, the entire surface of the Pd layer 21 may be completely covered with the alloying inhibition layer 24, or conversely, the entire surface may not be covered with the alloying inhibition layer 24 at all. The entire surface of the Pd layer 21 may be covered with the Rh layer 23. Also, although not particularly limited, the coating thickness of the Pd layer 21 is, for example, 10 to 100 μm, preferably 20 to 60 μm.

[0049] <Pt layer 22> The Pt layer 22 is a catalyst layer that essentially contains platinum (Pt) as a catalytic metal. Pt has particularly high activity as an oxidation catalyst and excellent purification performance of HC and HC. However, according to the inventors' studies, Pt is more prone to evaporation than, for example, Rh in high-temperature environments. By arranging the Pt layer 22 on the substrate 10 side, the movement of Pt in the thickness direction T in high-temperature environments can be suitably suppressed. The Pt layer 22 preferably contains a non-OSC material, and more preferably contains an OSC material in addition to the non-OSC material.

[0050] The catalyst metal of the Pt layer 22 is preferably mainly Pt, and in some embodiments, Pt preferably accounts for 80% by mass or more of the total catalyst metal of the Pt layer 22, more preferably 90% by mass or more, and is particularly preferably substantially Pt (95% by mass or more of the total catalyst metal of the Pt layer 22 is Pt). The Pt layer 22 may further contain metal species other than Pt as catalyst metal. Pd is one example. The Pt layer 22 preferably does not contain a reducing catalyst (e.g., Rh) as catalyst metal (the content of reducing catalyst per 1L of substrate is less than 0.1g / L). This can better suppress alloying of the catalyst metal.

[0051] The content of catalyst metal (typically Pt) per liter of substrate in the Pt layer 22 can be appropriately determined according to, for example, the amount of exhaust gas and the application of the exhaust gas purification catalyst 100, and is not particularly limited, but is preferably 0.5 g / L or more, preferably 1.0 g / L or more, and more preferably 2.0 g / L or more. In such cases, applying the technology disclosed herein is particularly effective. On the other hand, the content of catalyst metal (typically Pt) per liter of substrate in the Pt layer 22 is preferably 5.0 g / L or less, preferably 4.0 g / L or less, and more preferably 3.0 g / L or less, from the viewpoint of suppressing sintering of the catalyst metal and from the viewpoint of exhibiting the effects of the technology disclosed herein at a high level.

[0052] As described above, the non-OSC material has at least one of the following functions: improving the heat resistance and durability of the Pt layer 22, and suppressing the peeling of the Pt layer 22 from the substrate 10. Preferably, the non-OSC material is included in the Pt layer 22 as a carrier for supporting the catalyst metal. However, the non-OSC material may be included in the Pt layer 22 in whole or in part in a form that does not support the catalyst metal. As the non-OSC material, materials such as those exemplified for use in the Pd layer 21 can be used. In particular, it is preferable to include an Al-containing oxide, and more preferable to include an LA composite oxide as described above. This allows the effects of the technology disclosed herein to be demonstrated at a high level. It also enhances the integration with the substrate 10 and durability. Preferably, the non-OSC material includes an Al-containing oxide, and more preferably consists substantially of an Al-containing oxide (95% by mass or more of the non-OSC material in the Pt layer 22 is an Al-containing oxide).

[0053] The content of non-OSC material (preferably Al-containing oxide) per liter of substrate in the Pt layer 22 is not particularly limited, but is preferably 40 to 110 g / L, more preferably 50 to 100 g / L, and even more preferably 50 to 75 g / L.

[0054] As described above, the OSC material has the function of absorbing oxygen in a lean atmosphere. It is preferable that the OSC material be included in the Pd layer 21 as a carrier for supporting the catalyst metal. However, the OSC material may be included in the Pd layer 21 in whole or in part in a form that does not support the catalyst metal. As the OSC material, materials such as those exemplified as usable in the Pd layer 21 can be used. In particular, from the viewpoint of exhibiting the effects of the technology disclosed herein at a high level, it is preferable to include a Ce-containing oxide, and more preferable to include the CZ composite oxide described above.

[0055] The content of OSC material per liter of substrate in the Pt layer 22 is not particularly limited, but is typically less than the content of non-OSC material per liter of substrate in the Pt layer 22, preferably 20 to 90 g / L, more preferably 30 to 80 g / L, and for example 40 to 70 g / L.

[0056] The Pt layer 22 is preferably composed of a non-OSC material as the first component. The proportion of the non-OSC material (preferably Al-containing oxide) in the Pd layer 21 is preferably 20 to 70% by mass, and more preferably 40 to 60% by mass. The proportion of the OSC material (e.g., CZ composite oxide) in the Pt layer 22 is typically less than that of the non-OSC material, preferably 20 to 70% by mass, and more preferably 40 to 60% by mass. The proportion of the catalyst metal in the Pt layer 22 is preferably, for example, 0.1 to 10% by mass, and more preferably 1 to 5% by mass. The Pt layer 22 may further contain auxiliary materials. As auxiliary materials, materials such as those exemplified for use in the Pd layer 21 can be used.

[0057] The amount of Pt layer 22 coating is not particularly limited, but is preferably, for example, 50 to 300 g / L per liter of substrate, more preferably 70 to 250 g / L, and even more preferably 100 to 200 g / L. As mentioned above, since Pt is prone to evaporation in high-temperature environments, the amount of Pt layer 22 coating is preferably greater than the amount of Rh layer 23 and / or alloying suppression layer 24 coating, which will be described later.

[0058] In the embodiment shown in Figure 2, the Pt layer 22 is provided along the cylindrical axis X from the downstream end X2 of the base material 10 toward the upstream end X1. The coat length L2 of the Pt layer 22 in the exhaust gas flow direction (cylindrical axis X) is shorter here than the total length L of the base material 10 in the stretching direction (cylindrical axis X). The coat length L2 of the Pt layer 22 is not particularly limited, but is preferably 50% or more of the total length L of the base material 10, more preferably 60-90%, and even more preferably 70-80%. This makes it possible to achieve high exhaust gas purification performance under various exhaust gas conditions that vary by the control of the internal combustion engine (e.g., a wide A / F range).

[0059] In the schematic diagram shown in Figure 2, the sum of the coating length L1 of the Pd layer 21 and the coating length L2 of the Pt layer 22 (L1+L2) is the same as the total length L of the substrate 10, and the Pd layer 21 and the Pt layer 22 are in contact in the cylindrical axis direction X. However, in some embodiments, the sum of the coating length L1 of the Pd layer 21 and the coating length L2 of the Pt layer 22 (L1+L2) is preferably L≦(L1+L2), for example, L≦(L1+L2)≦1.5L. That is, the Pd layer 21 and the Pt layer 22 may partially overlap in the central part in the cylindrical axis direction X, for example, to prevent a gap from forming in the cylindrical axis direction X due to manufacturing reasons such as using a slurry.

[0060] In this embodiment, the entire surface of the Pt layer 22 is covered with an alloying-inhibiting layer 24. However, the Pt layer 22 may not have a portion of its surface covered with the alloying-inhibiting layer 24, as long as this does not significantly impair the effects of the technology disclosed herein. In other words, the Pt layer 22 may have a portion of its surface in direct contact with the Rh layer 23. Furthermore, although not particularly limited, the coating thickness of the Pt layer 22 is, for example, 10 to 100 μm, preferably 20 to 60 μm.

[0061] <Alloying suppression layer 24> The alloying suppression layer 24 is a layer sandwiched between the Pt layer 22 and the Rh layer 23 in the thickness direction T, and is a layer that separates the Pt layer 22 and the Rh layer 23 in the thickness direction T (increases their physical distance). By providing the alloying suppression layer 24, even if, for example, the Pt contained in the Pt layer 22 moves towards the surface layer, or the Rh contained in the Rh layer 23 moves towards the substrate 10, it becomes difficult for the Pt and Rh to come into contact. The alloying suppression layer 24 can also function as a trapping layer that traps the Pt and Rh that have moved in the thickness direction T. Due to these effects, alloying of Pt and Rh can be suitably suppressed.

[0062] The alloying suppression layer 24 is a layer that contains Al-containing oxides and does not contain Pt and Rh. In this specification, "free of Pt and Rh" of the alloying suppression layer 24 means, for example, that in the state before use (before durability) of the exhaust gas purification catalyst 100, the total content of Pt and Rh per liter of substrate in the alloying suppression layer 24 is less than 0.01 g / L. However, as described above, since the alloying suppression layer 24 can also function as a trapping layer that traps Pt and Rh that have moved in the thickness direction T, in the state after use (after durability), Pt and Rh that have moved from the Pt layer 22 and / or the Rh layer 23 may be included in the alloying suppression layer 24.

[0063] As the Al-containing oxide, materials such as those exemplified in the Pd layer 21 as usable for non-OSC materials can be used. In particular, it is preferable to include the LA composite oxide (a composite oxide of alumina and lanthanum oxide) as described above. LA composite oxide has superior heat resistance and durability compared to pure alumina. Therefore, by including the LA composite oxide, a stronger alloying suppression layer 24 can be formed, and the effects of the technology disclosed herein can be exhibited at a higher level. The alloying suppression layer 24 may further contain Al-containing oxides other than the LA composite oxide. Specific examples include pure alumina and composite oxides of alumina and rare earth metal oxides other than lanthanum oxide. In some embodiments, it is preferable that the Al-containing oxide does not contain the element Ce.

[0064] In some embodiments, the Al-containing oxide is in powder form. 2 Preferably 50m / g or more. 2 More preferably 80m 2 A value of 1 / g or more is even more preferable. By setting the specific surface area to a predetermined value or higher, the catalyst metal that has migrated between layers can be more easily trapped within the alloying suppression layer 24, and the effects of the technology disclosed herein can be demonstrated at a high level. The specific surface area of ​​the Al-containing oxide is approximately 200 m². 2 Preferably less than / g, 150m 2It may also be below / g. In this specification, the "specific surface area" refers to a value obtained by analyzing the gas adsorption amount measured by a gas adsorption method (constant volume adsorption method) using nitrogen (N2) gas as an adsorbate by the BET method (for example, the BET multi-point method).

[0065] The content C4 of the Al-containing oxide per 1 L of the substrate in the alloying suppression layer 24 is not particularly limited, but is preferably 10 to 130 g / L, more preferably 20 to 120 g / L, for example, 40 to 80 g / L.

[0066] In some embodiments, when the Pt layer 22 contains an Al-containing oxide, the content of the Al-containing oxide per 1 L of the substrate in the Pt layer 22 is designated as C2, and the content of the Al-containing oxide per 1 L of the substrate in the alloying suppression layer 24 is designated as C4, it is preferable that C2 and C4 satisfy the following relationship: C4 < C2. Thereby, for example, the interlayer movement of the noble metal evaporated from the Pt layer 22 can be suitably suppressed by the alloying suppression layer 24, and the effects of the technology disclosed herein can be stably exhibited at a high level over a long period.

[0067] In some embodiments, when the Rh layer 23 contains an Al-containing oxide, the content of the Al-containing oxide per 1 L of the substrate in the Rh layer 23 is designated as C3, and the content of the Al-containing oxide per 1 L of the substrate in the alloying suppression layer 24 is designated as C4, it is preferable that C3 and C4 satisfy the following relationship: C3 < C4. Thereby, for example, the interlayer movement of the noble metal evaporated from the Rh layer 23 can be suitably suppressed by the alloying suppression layer 24, and the effects of the technology disclosed herein can be stably exhibited at a high level over a long period.

[0068] The alloying suppression layer 24 is preferably composed of an Al-containing oxide (particularly an LA composite oxide) as the first component. The alloying suppression layer 24 is preferably composed mainly of an Al-containing oxide (particularly an LA composite oxide). In some embodiments, the alloying suppression layer 24 is preferably composed of 80% by mass or more of Al-containing oxide, more preferably 90% by mass or more of Al-containing oxide, and particularly preferably 95% by mass or more of Al-containing oxide (substantially composed of Al-containing oxide). This makes it possible to form an alloying suppression layer 24 with high heat resistance and durability and a stronger structure, thereby demonstrating the effects of the technology disclosed herein at a high level.

[0069] The alloying suppression layer 24 may contain catalyst metals other than Pt and Rh (e.g., Pd), as long as this does not significantly impair the effects of the technology disclosed herein. This allows for improved catalytic performance due to the activity of the added catalyst metal (e.g., Pd) while suppressing Pt-Rh alloying. The content of catalyst metals (excluding Pt and Rh) per liter of substrate in the alloying suppression layer 24 is typically less than the content of catalyst metals per liter of substrate in the Pd layer 21 and / or Pt layer 22, preferably 3 g / L or less, for example, 0.1 to 2 g / L or 0.5 to 2 g / L. The proportion of catalyst metals (e.g., Pd) in the alloying suppression layer 24 is preferably 10% by mass or less, and more preferably 5% by mass or less.

[0070] Furthermore, the alloying suppression layer 24 may also contain auxiliary materials, similar to the Pd layer 21 and Pt layer 22 described above. As auxiliary materials, materials such as those exemplified for use in the Pd layer 21 can be used. The proportion of auxiliary materials in the alloying suppression layer 24 is preferably 10% by mass or less, and more preferably 5% by mass or less.

[0071] In some embodiments, the alloying suppression layer 24 preferably has a low OSC content. The ceria content per liter of substrate in the alloying suppression layer 24 is preferably less than, for example, the OSC content per liter of substrate in the Pd layer 21 and / or Pt layer 22, more preferably 10 g / L or less, and even more preferably 6 g / L or less. This prevents the catalyst metal trapped in the alloying suppression layer 24 from solid-solving with ceria in the alloying suppression layer 24, and allows the effects of the disclosed technology to be exerted at a high level. From this viewpoint, it is particularly preferable that the alloying suppression layer 24 does not contain Ce-containing oxides, such as ceria or CZ composite oxides (the Ce-containing oxide content per liter of substrate is less than 0.1 g / L).

[0072] In some other embodiments, the alloying inhibition layer 24 contains ceria as an OSC material (in other words, in a form other than the Al-containing oxide), and the ceria content per liter of substrate satisfies the above range (e.g., 6 g / L or less). By including a small amount of OSC material in the alloying inhibition layer 24, evaporated Pt and Rh are more likely to remain on the Al-containing oxide, and an anchoring effect can be exerted. As a result, Pt and Rh are less likely to move within the alloying inhibition layer 24, and the time until alloying can be increased. Therefore, durability can be improved. In addition, since the catalyst as a whole can contain a large amount of OSC material, it becomes easier to accommodate a wide purification window (A / F region).

[0073] The amount of alloying suppression layer 24 coated is not particularly limited, as it can vary depending on, for example, the content of the catalyst metal in the Pt layer 22 and / or the Rh layer 23. However, it is preferably 10 to 150 g / L per liter of substrate, more preferably 20 to 100 g / L, and even more preferably 30 to 80 g / L. The amount of alloying suppression layer 24 coated is preferably less than the amount of Pt layer 22 and / or the Rh layer 23 described later, and more preferably less than the amount of coating of both the Pt layer 22 and the Rh layer 23.

[0074] In some embodiments, when the Pt content per liter of the substrate 1L in the Pt layer 22 is X (g / L), and the coating amount per liter of the substrate 1L in the alloying inhibition layer 24 (or the content of the Al-containing oxide) is Y (g / L), it is preferable that X and Y satisfy the following formula: Y ≧ 6.779X + 19.437;. Thereby, Pt evaporated from the Pt layer 22 can be effectively captured by the Al-containing oxide of the alloying inhibition layer 24, and the interlayer movement of Pt that is likely to evaporate can be suppressed at a higher level.

[0075] In some embodiments, when the Pt content per liter of the substrate 1L in the Pt layer 22 is X (g / L), and the specific surface area of the Al-containing oxide per liter of the substrate 1L in the alloying inhibition layer 24 is Z (m 2 / L), it is preferable that X and Z satisfy the following formula: Z ≧ 745.69X + 2138.1;. Thereby, Pt evaporated from the Pt layer 22 can be particularly effectively captured by the alloying inhibition layer 24, and the interlayer movement of Pt that is likely to evaporate can be suppressed at a particularly high level. The specific surface area of the Al-containing oxide per liter of the substrate 1L in the alloying inhibition layer 24 (Z (m 2 / L) can be obtained by multiplying the content of the Al-containing oxide per liter of the substrate 1L in the alloying inhibition layer 24 (g / L) by the specific surface area of the Al-containing oxide (m 2 / g).

[0076] In the embodiment shown in Figure 2, the alloying suppression layer 24 is provided along the cylindrical axis X from the downstream end X2 of the base material 10 toward the upstream end X1. The coat length L4 of the alloying suppression layer 24 in the exhaust gas flow direction (cylindrical axis X) is shorter than the total length L of the base material 10 in the stretching direction (cylindrical axis X). From the viewpoint of improving warm-up performance, the coat length L4 of the alloying suppression layer 24 is preferably 90% or less of the total length L of the base material 10, and more preferably 80% or less. However, in other embodiments, the coat length L4 of the alloying suppression layer 24 may be the same as the total length L of the base material 10. In this case, the coat length L4 of the alloying suppression layer 24 is shorter than the coat length L3 of the Rh layer 23. The coat length L4 of the alloying suppression layer 24 is not particularly limited, but it is preferably the same as or longer than the coat length L2 of the Pt layer 22. That is, L2 ≤ L4 is preferred. The coating length L4 of the alloying suppression layer 24 is preferably 50% or more of the total length L of the base material 10, more preferably 60% or more, and even more preferably 70% or more. However, in other embodiments, the coating length L4 of the alloying suppression layer 24 may be shorter than the coating length L2 of the Pt layer 22.

[0077] In some embodiments, the coating length L4 of the alloying suppression layer 24 in the exhaust gas flow direction (cylinder axis direction X) is more preferably 50% or more, more preferably 75% or more, even more preferably 80% or more, and particularly preferably 100% or more, when the coating length L2 of the Pt layer 22 is set to 100%. This reduces the contact area between the Pt layer 22 and the Rh layer 23, thereby suppressing Pt-Rh alloying to a higher level.

[0078] When the coating length L4 of the alloying suppression layer 24 is shorter than the coating length L2 of the Pt layer 22, the Pt layer 22 and the Rh layer 23 can typically come into direct contact in the thickness direction T in the central portion of the exhaust gas flow direction F. In this case, the length of the exhaust gas flow direction F in which the Pt layer 22 and the Rh layer 23 are in direct contact (contact length) is preferably less than 50%, more preferably 30% or less, even more preferably 15% or less, and particularly preferably 10% or less, when the coating length L2 of the Pt layer 22 in the exhaust gas flow direction F is taken as 100%.

[0079] The coating thickness of the alloying inhibition layer 24 is not particularly limited because it can vary depending on, for example, the content of the catalytic metal in the Pt layer 22 and / or the Rh layer 23. However, from the perspective of sufficiently separating the Pt layer 22 and the Rh layer 23 in the thickness direction T, 2 μm or more is preferable, 5 μm or more is more preferable, 10 μm or more is further preferable, and 15 μm or more is particularly preferable. Also, since the effect levels off when exceeding a certain thickness, the coating thickness of the alloying inhibition layer 24 is preferably approximately 80 μm or less, more preferably 50 μm or less, further preferably 30 μm or less, and particularly preferably 20 μm or less. The coating thickness of the alloying inhibition layer 24 may be thinner than the Pd layer 21 and / or the Pt layer 22. Thereby, the effects of the technology disclosed herein can be exhibited at a high level. Also, the pressure loss can be reduced.

[0080] <Rh layer 23> The Rh layer 23 is a catalyst layer that essentially contains rhodium (Rh) as a catalytic metal. Rh has particularly high activity as a reduction catalyst and, for example, has excellent NOx purification performance in a stoichiometric to rich atmosphere. The Rh layer 23 preferably contains a non-OSC material, and more preferably further contains an OSC material in addition to the non-OSC material. The Rh layer 23 is the layer that constitutes the outermost surface of the coating layer 20 here.

[0081] The catalytic metal of the Rh layer 23 preferably has Rh as the main component, and it is preferable that Rh occupies 80 mass% or more of the total catalytic metal of the Rh layer 23, more preferably 90 mass% or more, and particularly preferably consists substantially of Rh (95 mass% or more of the total catalytic metal of the Rh layer 23 is Rh). In some embodiments, the Rh layer 23 preferably contains Pd. In some other embodiments, the Rh layer 23 preferably does not contain an oxidation catalyst (e.g., Pt or Pd) as a catalytic metal (the content of the oxidation catalyst per substrate 1L is less than 0.1 g / L). Thereby, alloying of the catalytic metal can be better suppressed.

[0082] The content of catalyst metal (typically Rh) per liter of substrate in the Rh layer 23 can be appropriately determined according to, for example, the amount of exhaust gas and the application of the exhaust gas purification catalyst 100, and is not particularly limited, but is preferably 0.01 g / L or more, preferably 0.05 g / L or more, and more preferably 0.1 g / L or more. On the other hand, the content of catalyst metal (typically Rh) per liter of substrate in the Rh layer 23 is preferably 1.0 g / L or less, preferably 0.5 g / L or less, and more preferably 0.4 g / L or less. The content of catalyst metal (typically Rh) per liter of substrate in the Rh layer 23 is typically less than the content of catalyst metal per liter of substrate in the Pd layer 21 and / or Pt layer 22.

[0083] As described above, the non-OSC material has at least one of the following functions: improving the heat resistance and durability of the Rh layer 23, and suppressing the peeling of the Rh layer 23. Preferably, the non-OSC material is included in the Rh layer 23 as a carrier for supporting the catalyst metal. However, the non-OSC material may be included in the Rh layer 23 in whole or in part in a form that does not support the catalyst metal. As the non-OSC material, materials such as those exemplified for use in the Pd layer 21 can be used. In particular, from the viewpoint of exhibiting the effects of the technology disclosed herein at a high level, it is preferable to include an Al-containing oxide, and more preferable to include the LA composite oxide as described above. This allows the effects of the technology disclosed herein to be exhibited at a high level. It also enhances the integration with the substrate 10 and durability. Preferably, the non-OSC material contains an Al-containing oxide, and more preferably consists substantially of an Al-containing oxide (95% by mass or more of the total non-OSC material in the Rh layer 23 is an Al-containing oxide).

[0084] The content of non-OSC material (preferably Al-containing oxide) per liter of substrate in the Rh layer 23 is not particularly limited, but is preferably 5 to 100 g / L, more preferably 10 to 80 g / L, and more preferably 15 to 65 g / L. In some embodiments, the content C3 of Al-containing oxide per liter of substrate in the Rh layer 23 is preferably smaller than the content C2 of Al-containing oxide per liter of substrate in the Pt layer 22. This reduces pressure drop and improves catalytic performance.

[0085] As described above, the OSC material has the function of absorbing oxygen in a lean atmosphere. It is preferable that the OSC material be included in the Rh layer 23 as a support for the catalyst metal. However, the OSC material may be included in the Rh layer 23 in whole or in part in a form that does not support the catalyst metal. As the OSC material, materials such as those exemplified for use in the Pd layer 21 can be used. In particular, from the viewpoint of exhibiting the effects of the technology disclosed herein at a high level, it is preferable to include a Ce-containing oxide, and more preferable to include a CZ composite oxide as described above.

[0086] The content of OSC material per liter of substrate in the Rh layer 23 is not particularly limited, but is preferably 1 to 50 g / L, more preferably 5 to 40 g / L, and for example, 10 to 30 g / L. The content of OSC material per liter of substrate in the Rh layer 23 is preferably less than the content of OSC material per liter of substrate in the Pt layer 22.

[0087] The Rh layer 23 is preferably composed of a non-OSC material as the first component. The Rh layer 23 is preferably composed mainly of a non-OSC material. The proportion of the non-OSC material (preferably Al-containing oxide) in the Rh layer 23 is preferably 55 to 90% by mass, and more preferably 60 to 80% by mass. The proportion of the OSC material (e.g., CZ composite oxide) in the Rh layer 23 is typically less than that of the non-OSC material, for example less than 50% by mass, preferably 1 to 45% by mass, and more preferably 10 to 40% by mass. The proportion of the catalyst metal in the Rh layer 23 is preferably 0.1 to 10% by mass, and more preferably 1 to 5% by mass.

[0088] The Rh layer 23 may further contain auxiliary materials. These auxiliary materials may be those exemplified for use in the Pd layer 21. In some embodiments, it is preferable that the Rh layer 23 is substantially free of auxiliary materials containing alkali metal elements or alkaline earth metal elements (e.g., less than 1 g / L).

[0089] The amount of coating on the Rh layer 23 is not particularly limited, but is preferably, for example, 30 to 120 g / L per liter of substrate, more preferably 50 to 100 g / L, and even more preferably 60 to 80 g / L. The amount of coating on the Rh layer 23 is preferably less than the amount of coating on the Pd layer 21 and / or Pt layer 22.

[0090] In some embodiments, it is preferable that the coating amount per liter of substrate in the Rh layer 23 is less than the coating amount per liter of substrate in the Pt layer 22. This reduces pressure loss and improves catalyst performance. Therefore, the effects of the technology disclosed herein can be realized at a high level.

[0091] In the embodiment shown in Figure 2, the Rh layer 23 is provided along the cylindrical axis X of the base material 10. The coat length L3 of the Rh layer 23 in the exhaust gas flow direction (cylindrical axis X) is the same as the total length L of the base material 10 in the stretching direction (cylindrical axis X). However, in other embodiments, the coat length L3 of the Rh layer 23 may be shorter than the total length L of the base material 10. Preferably, the coat length L3 of the Rh layer 23 is longer than the coat length L1 of the Pd layer 21. Preferably, the coat length L3 of the Rh layer 23 is longer than the coat length L1 of the Pt layer 22. Preferably, the coat length L3 of the Rh layer 23 is longer than the coat length L4 of the alloying suppression layer 24. The coat length L3 of the Rh layer 23 is not particularly limited, but is preferably 50% or more of the total length L of the base material 10, more preferably 80% or more, and even more preferably 90% or more. This makes it possible to achieve high exhaust gas purification performance under various exhaust gas conditions (e.g., A / F range) that vary depending on the control of the internal combustion engine.

[0092] While not particularly limited, the coating thickness of the Rh layer 23 is, for example, 10 to 100 μm, preferably 20 to 60 μm. In some embodiments, the coating thickness of the Rh layer 23 is preferably thinner than that of the Pd layer 21 and / or Pt layer 22. This reduces pressure drop and improves catalytic performance. Therefore, the effects of the technology disclosed herein can be demonstrated at a high level.

[0093] With the exhaust gas purification catalyst 100 having the above configuration, by arranging the Pt layer 22 on the substrate 10 side, the evaporation of Pt in a high-temperature environment can be suitably suppressed. Furthermore, by arranging an alloying suppression layer 24 containing Al, which has excellent heat resistance, between the Pt layer 22 and the Rh layer 23, even if Pt and / or Rh migrate in the thickness direction T in a high-temperature environment, the occurrence of Pt-Rh alloying can be suitably suppressed. The combined effects of these factors can suitably suppress the decrease in catalyst activity. Consequently, the excellent initial exhaust gas purification performance (e.g., NOx purification performance) can be stably exhibited over a long period of time.

[0094] ≪Manufacturing method for exhaust gas purification catalyst 100≫ While not particularly limited, the exhaust gas purification catalyst 100 can be manufactured by, for example, the following methods. First, a base material 10 and several slurries for forming the various parts of the coating layer 20 are prepared. Of these, the slurry for forming the catalyst layer can be prepared, for example, by mixing a catalyst metal source (for example, a solution containing catalyst metal as ions) and other components (for example, non-OSC material, OSC material, binder, various additives, etc.) in a dispersion medium. The slurry for forming the alloying suppression layer can be prepared, for example, by mixing an Al-containing oxide and other components (for example, a binder, various additives, etc.) in a dispersion medium.

[0095] As the dispersion medium, for example, water or a mixture of water and a water-soluble organic solvent can be used. As the binder, for example, alumina sol or silica sol can be used. The properties of the slurry (e.g., viscosity, solid content, etc.) can be adjusted as appropriate depending on the size of the substrate 10 used, the shape of the cells 12 or rib walls 14, the required characteristics for the coating layer 20, etc. The average particle size of the particles in the slurry can be approximately 1 μm or more, preferably 2 μm or more, more preferably 3 μm or more, and preferably approximately 30 μm or less, preferably 20 μm or less, and more preferably 10 μm or less.

[0096] Specifically, first, a slurry for forming a Pd layer containing a Pd source, a slurry for forming a Pt layer containing a Pt source, a slurry for forming an Rh layer containing an Rh source, and a slurry for forming an alloying-suppressing layer containing an Al-containing oxide are prepared.

[0097] Next, a Pt layer-forming slurry is coated from the downstream end X2 of the substrate 10 to a predetermined position using a known coating method (e.g., suction coating method, air blow method, wash coating method, etc.), followed by drying and firing to form a Pt layer 22. For example, in 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 suctioning the slurry from the other end. 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.

[0098] Next, a slurry for forming a Pd layer is coated from the upstream end X1 of the substrate 10 to a predetermined position according to a known method, and then dried and fired to form a Pd layer 21. Next, a slurry for forming an alloying suppression layer is coated from the downstream end X2 of the substrate 10 to a predetermined position according to a known method, and then dried and fired to form an alloying suppression layer 24 that overlaps the Pt layer 22. Next, a slurry for forming an Rh layer is coated on the alloying suppression layer 24 using a known coating method, and then dried and fired to form an Rh layer 23. In this way, a coating layer 20 can be formed on the substrate 10.

[0099] <<Applications of Exhaust Gas Purification Catalyst 100>> The exhaust gas purification catalyst 100 can be suitably used to purify exhaust gases emitted from 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 lawnmowers, chainsaws, and trimmers, leisure products such as golf carts and ATVs, power generation equipment such as cogeneration systems, and internal combustion engines such as waste incinerators. In particular, it can be suitably used for vehicles such as automobiles, and especially for vehicles equipped with gasoline engines.

[0100] The following describes test examples relating to the present invention, but it is not intended to limit the present invention to those shown in the following test examples.

[0101] [Test Example I: Evaluation of the coating thickness of the alloying suppression layer and NOx purification performance] In Test Example I, the NOx purification performance, specifically the temperature at which the NOx purification rate reached 50% (NOx T-50), was evaluated by varying the coating thickness of the alloying suppression layer.

[0102] <Test substrate> First, a cylindrical cordierite honeycomb substrate (diameter Φ: 118 mm, total length L: 114 mm, volume: 1.2 L) was prepared. One end of this substrate was defined as the upstream end X1 (i.e., the end on the exhaust gas inlet side), and the other end was defined as the downstream end X2 (i.e., the end on the exhaust gas outlet side).

[0103] <Preparation of the exhaust gas purification catalyst in Example 1> In Example 1, a coating layer with the configuration shown in Figure 3 was formed on a substrate. Specifically, first, an aqueous platinum nitrate solution as a Pt source, LA composite oxide powder (Al-containing oxide) as a non-OSC material, CZ composite oxide powder as an OSC material, barium sulfate as an auxiliary material, and an alumina-based binder were mixed in ion-exchanged water. The mixture was then pulverized in a ball mill so that the average particle size (D50) of the powder was 1 to 10 μm to prepare a slurry for Pt layer formation containing the components shown in Table 1. Next, with the downstream end X2 of the substrate facing upward, the slurry for Pt layer formation was coated from the downstream end X2 of the substrate to 70% of the total length of the substrate by suction coating. After drying at 110°C, the Pt layer was formed by firing at 500°C for 1 hour.

[0104] Next, an aqueous solution of palladium nitrate as a Pd source, LA composite oxide powder (Al-containing oxide) as a non-OSC material, CZ composite oxide powder as an OSC material, barium sulfate as an auxiliary material, and an alumina-based binder were mixed in ion-exchanged water. The mixture was then pulverized in a ball mill to achieve an average particle size (D50) of 1 to 10 μm, preparing a slurry for Pd layer formation containing the components shown in Table 1. Next, with the upstream end X1 of the substrate facing upward, the slurry for Pd layer formation was coated by suction coating from the upstream end X1 of the substrate to 45% of the total length of the substrate. After drying at 110°C, the Pd layer was formed by firing at 500°C for 1 hour. The Pd layer partially overlaps the previously formed Pt layer in the central part of the substrate in the direction of the cylindrical axis.

[0105] Next, LA composite oxide powder (Al-containing oxide, specific surface area: 110 m²) 2A slurry for forming an alloying-suppressing layer containing the components shown in Table 1 was prepared by mixing the alloying-suppressing layer slurry with an alumina-based binder in ion-exchanged water and then grinding it in a ball mill so that the average particle size (D50) of the powder was 1 to 10 μm. Next, with the downstream end X2 of the substrate facing upward, the slurry for forming the alloying-suppressing layer was coated by suction coating from the downstream end X2 of the substrate to 75% of the total length of the substrate. After drying at 110°C, it was fired at 500°C for 1 hour to form an alloying-suppressing layer (coat thickness: 15 μm).

[0106] Next, an aqueous rhodium nitrate solution as an Rh source, LA composite oxide powder (Al-containing oxide) as a non-OSC material, CZ composite oxide powder as an OSC material, and an alumina-based binder were mixed in ion-exchanged water. The mixture was then pulverized in a ball mill to achieve an average particle size (D50) of 1-10 μm, and a slurry for Rh layer formation containing the components shown in Table 1 was prepared. Next, with the upstream end X1 of the substrate facing upward, the slurry for Rh layer formation was coated by suction coating from the upstream end X1 to 75% of the total length of the substrate, and dried at 110°C. Then, with the downstream end X2 of the substrate facing upward, the slurry for Rh layer formation was coated by suction coating from the downstream end X2 to 45% of the total length of the substrate, and dried at 110°C. Finally, the mixture was fired at 500°C for 1 hour to form the Rh layer. As described above, the exhaust gas purification catalyst of Example 1 was obtained.

[0107] [Table 1]

[0108] <Preparation of exhaust gas purification catalysts for Examples 2-7 and Comparative Example 1> Except for changing the content of the Al-containing oxide in the alloying suppression layer (= the coating amount of the alloying suppression layer) as shown in Table 2, exhaust gas purification catalysts of Examples 2 to 7 were obtained in the same manner as in Example 1. As shown in Table 2, in the exhaust gas purification catalysts of Examples 2 to 7, the coating thicknesses of the alloying suppression layer are different from each other within the range of 2 to 30 μm. The coating thickness of the alloying suppression layer was measured as the thickness of the thinnest part by a scanning electron microscope (SEM). Further, an exhaust gas purification catalyst of Comparative Example 1 was obtained in the same manner as in Example 1 except that the alloying suppression layer was not formed.

[0109] <Evaluation of NOx purification performance> The exhaust gas purification catalyst of each example was installed in the exhaust pipe of an engine bench of a V-type 8-cylinder (displacement: 4600 cc). Then, the engine was operated at an average engine speed of 3000 rpm, and a simulated exhaust gas in a stoichiometric state to a lean atmosphere was alternately circulated through the exhaust gas purification catalyst at a constant time interval for 45 hours at a catalyst inlet gas temperature of 950 °C to conduct a durability test. The exhaust gas purification catalyst after the durability test was installed in the exhaust pipe of an engine bench of an L-type 4-cylinder (displacement: 2000 cc). Then, while supplying a simulated exhaust gas with an A / F (air-fuel ratio, Air / Fuel ratio) = 14.6, the catalyst inlet gas temperature was raised from room temperature (25 °C) to 500 °C at a heating rate of 20 °C / min. At this time, the NOx purification rate was measured from the ratio of the NOx concentration of the inflow gas to the exhaust gas purification catalyst and the NOx concentration of the outflow gas from the exhaust gas purification catalyst. And the temperature (NOx T-50) at which the NOx purification rate became 50% was determined. The results are shown in Table 2 and FIG. 4.

[0110]

Table 2

[0111] As shown in Table 2 and Figure 4, Examples 1-7, which had an alloying suppression layer, all exhibited higher NOx purification performance after high-temperature endurance compared to Comparative Example 1, which did not have an alloying suppression layer. Furthermore, a comparison of Examples 1-7 revealed that when the catalytic metal (here, Pt) content in the Pt layer is approximately 3 g / L, the coating thickness of the alloying suppression layer is preferably 2 μm or more, and more preferably 10 μm or more. This is thought to be because a thicker alloying suppression layer increases the physical distance between the Pt layer and the Rh layer, and makes it easier to trap the catalytic metal that has moved in the thickness direction within the alloying suppression layer. Additionally, when the catalytic metal (here, Pt) content in the Pt layer is approximately 3 g / L, the effect plateaus above 10 μm, indicating that the coating thickness of the alloying suppression layer can generally be 30 μm or less, for example, 20 μm or less.

[0112] [Test Example II: Evaluation of the coating length of the alloying suppression layer and NOx purification performance] In Test Example II, the coating length of the alloying inhibition layer was varied, and the NOx purification performance was evaluated in the same manner as in Test Example I. Specifically, the exhaust gas purification catalysts of Examples 8 to 12 were prepared in the same manner as in Example 1 of Test Example I, except that the coating length of the alloying inhibition layer relative to the coating length of the Pt layer (70% of the total length of the substrate) was changed as shown in Table 3, and the NOx purification performance (NOx T-50) after high-temperature endurance was evaluated in the same manner as in Test Example I. Comparative Example 1 is the same as in Test Example I. The results are shown in Table 3 and Figure 5.

[0113] [Table 3]

[0114] As shown in Table 3 and Figure 5, a comparison of Examples 8 to 12 revealed that, when the coating length of the Pt layer is set to 100%, the coating length of the alloying suppression layer is preferably 50% or more, more preferably 75% or more, and even more preferably 100% or more. This is thought to be because the longer the coating length of the alloying suppression layer, the wider the surface of the Pt layer can be covered.

[0115] [Test Example III: Evaluation of catalyst metal species and NOx purification performance in the Pt layer 1] In Test Example III, the catalyst metal species in the Pt layer was changed, and the NOx purification performance was evaluated in the same manner as in Test Example I. Specifically, the exhaust gas purification catalyst of Example 13 was prepared in the same manner as in Example 1 of Test Example I, except that Pt and Pd were used as the catalyst metals in the Pt layer in a 1:1 ratio (1.481 g each). The NOx purification performance (NOx T-50) after high-temperature endurance was evaluated in the same manner as in Test Example I. The results are shown in Table 4.

[0116] [Table 4]

[0117] As shown in Table 4, since the NOx purification performance of Example 13 after high-temperature endurance was the same as that of Example 1, it was found that the effects disclosed herein can be obtained even when Pd is added to the Pd layer. This is thought to be because Pd is a catalytic metal that is relatively resistant to migration (evaporation).

[0118] [Test Example IV: Evaluation of the composition of the alloying suppression layer and its NOx purification performance 1] In Test Example IV, a catalyst metal was added to the alloying inhibition layer, and the NOx purification performance was evaluated in the same manner as in Test Example I. Specifically, the exhaust gas purification catalyst of Example 14 was prepared in the same manner as in Example 13 of Test Example I, except that 1.481 g of Pd as a catalyst metal was added to the slurry for forming the alloying inhibition layer. The NOx purification performance (NOx T-50) after high-temperature endurance was evaluated in the same manner as in Test Example I. The results are shown in Table 5.

[0119] [Table 5]

[0120] As shown in Table 5, the NOx purification performance of Example 14 after high-temperature endurance was approximately the same as that of Example 13. This indicates that the effects disclosed herein can be obtained even when Pd is added to the alloying suppression layer instead of the Pt layer. This is thought to be because Pd is a catalytic metal that is relatively less prone to migration (evaporation).

[0121] [Test Example V: Evaluation of the composition of the alloying-inhibiting layer and its NOx purification performance 2] In Test Example V, a Ce-containing oxide was added to the alloying inhibition layer, and the NOx purification performance was evaluated in the same manner as in Test Example I. Specifically, as shown in Table 6, CZ composite oxide powder was added to the slurry for forming the alloying inhibition layer, and the amount of LA composite oxide powder (Al-containing oxide) was reduced accordingly to keep the coating amount constant. Except for these differences, the exhaust gas purification catalysts of Examples 15 and 16 were prepared in the same manner as in Example 1 of Test Example I, and the NOx purification performance (NOx T-50) after high-temperature endurance was evaluated in the same manner as in Test Example I. The results are shown in Table 6.

[0122] [Table 6]

[0123] As shown in Table 6, Examples 15 and 16, which had Ce-containing oxides added, showed higher NOx purification performance after high-temperature endurance compared to Comparative Example 1, which did not have an alloying suppression layer. However, their NOx purification performance after high-temperature endurance was lower compared to Example 1. From a comparison of Examples 1, 15, and 16, it was found that the ceria content of the alloying suppression layer is preferably 6 g / L or less, and more preferably substantially absent (e.g., less than 1 g / L). This is thought to be because Rh trapped in the alloying suppression layer dissolved in the Ce-containing oxide in the alloying suppression layer, reducing catalytic activity.

[0124] [Test Example VI: Evaluation of Coat Length and Warming Performance of Alloy Formation Inhibiting Layer] In Test Example VI, the warm-up properties were evaluated by changing the coating length of the alloying inhibition layer. Specifically, the exhaust gas purification catalyst of Example 17 was prepared in the same manner as Example 8 of Test Example I, except that the coating length of the alloying inhibition layer relative to the total length of the substrate was changed to 100%.

[0125] <Evaluation of warm-up performance> The exhaust gas purification catalysts for each example were installed in the exhaust pipe of an L-type 4-cylinder engine (displacement: 2000cc) engine bench, and simulated exhaust gas at 500°C was passed through them. The time (T50) until the HC purification rate reached 50% was measured. The results are shown in Table 7.

[0126] [Table 7]

[0127] As shown in Table 7, when the coating length of the alloying suppression layer was 100% (the same length as the total length of the substrate), the warm-up performance decreased significantly. Therefore, it was found that the coating length of the alloying suppression layer should preferably be less than 100% of the total length of the substrate. This is thought to be due to the increased heat capacity caused by the increased coating amount on the upstream side. Although detailed results are not shown here, the NOx purification performance (NOx T-50) after high-temperature endurance was the same for Examples 8 and 17.

[0128] [Test Example VII: Evaluation of Pt content in the Pt layer and coating amount of the alloying-suppressing layer] In Test Example VI, the NOx purification performance was evaluated in the same manner as in Test Example I by changing the Pt content in the Pt layer and the Al-containing oxide content in the alloying suppression layer. Specifically, the exhaust gas purification catalysts for Comparative Examples 2, 3 and Examples 18-25 were prepared in the same manner as Comparative Example 1 and Examples 1-7 (see Table 2) of Test Example I, except that the Pt content in the Pt layer was changed to 0.1 g / L and 6 g / L, and the Al-containing oxide content in the alloying suppression layer was changed as shown in Table 8. The NOx purification performance (NOx T-50) after high-temperature endurance was evaluated. The results are shown in Table 8 and Figure 6.

[0129] [Table 8]

[0130] As shown in Table 8 and Figure 6, it was found that the optimal amount of Al-containing oxide (or coating amount of the alloying suppression layer) in the alloying suppression layer differs depending on the Pt content in the Pt layer. Specifically, from a comparison of Examples 18 to 21, it was found that when the Pt content in the Pt layer is approximately 0.1 g / L, the effect plateaus when the Al-containing oxide content in the alloying suppression layer exceeds 10 g / L, so it was found that an Al-containing oxide content of 10 g / L or more is preferable in the alloying suppression layer. Furthermore, from a comparison of Examples 22 to 25, it was found that when the Pt content in the Pt layer is approximately 6 g / L, the effect plateaus when the Al-containing oxide content in the alloying suppression layer exceeds 60 g / L, so it was found that a coating amount of 60 g / L or more is preferable. In addition, from a comparison of Examples 1 to 7, it was found that when the Pt content in the Pt layer is approximately 3 g / L, the effect plateaus when the Al-containing oxide coating amount in the alloying suppression layer exceeds 40 g / L, so it was found that a coating amount of 40 g / L or more is preferable.

[0131] Furthermore, as shown in Table 9, from the above results, it was found that when the Pt content of the Pt layer is X (g / L) and the Al-containing oxide content (or coating amount of the alloying suppression layer) in the alloying suppression layer is Y (g / L), it is preferable that X and Y satisfy the following equation: Y ≥ 6.779X + 19.437.

[0132] Furthermore, as shown in Table 9 and Figure 7, let X (g / L) be the Pt content of the Pt layer, and let Z (m²) be the specific surface area of ​​the Al-containing oxide in the alloying suppression layer. 2 When we set / L), it was found that it is more preferable for X and Z to satisfy the following equation: Z ≥ 745.69X + 2138.1;

[0133] [Table 9]

[0134] Although specific examples of the present invention have been described in detail above, these are merely illustrative and do not limit the scope of the claims. It does not do so. The technology described in the claim includes various modifications of the specific examples exemplified above. This includes the changes. [Explanation of Symbols]

[0135] 10 Base material 12 cells 14 Rib wall 20 coat layers 21 Pd layer 22 Pt layer 23 Rh layer 24 Alloying suppression layer 100 Catalysts for exhaust gas purification

Claims

1. An exhaust gas purification catalyst, which is placed in the exhaust path of an internal combustion engine and purifies the exhaust gas discharged from the internal combustion engine, The invention comprises a base material and a coating layer provided on the base material, The aforementioned coating layer is Located on the substrate side, a Pt layer containing Pt as a catalytic metal, Located on the surface side, the Rh layer contains Rh as a catalytic metal, An alloying-suppressing layer is interposed between the Pt layer and the Rh layer, and contains an Al-containing oxide but does not contain Pt or Rh. Equipped with, The alloying-suppressing layer is composed of 90% by mass or more of the Al-containing oxide and contains ceria as an OSC material, with a ceria content of 6 g / L or less per liter of substrate. Catalyst for exhaust gas purification.

2. An exhaust gas purification catalyst, which is placed in the exhaust path of an internal combustion engine and purifies the exhaust gas discharged from the internal combustion engine, The invention comprises a base material and a coating layer provided on the base material, The aforementioned coating layer is Located on the substrate side, a Pt layer containing Pt as a catalytic metal, Located on the surface side, the Rh layer contains Rh as a catalytic metal, An alloying-suppressing layer is interposed between the Pt layer and the Rh layer, and contains an Al-containing oxide but does not contain Pt or Rh. Equipped with, The alloying-suppressing layer contains ceria as an OSC material, and the ceria content per liter of substrate is 6 g / L or less. The Pt layer and the Rh layer each contain an Al-containing oxide. When the content of the Al-containing oxide per liter of substrate in the Rh layer is C3, and the content of the Al-containing oxide per liter of substrate in the alloying suppression layer is C4, then C3 and C4 satisfy the following relationship: C3 < C4; Catalyst for exhaust gas purification.

3. The alloying suppression layer contains a catalyst metal (excluding Pt and Rh). The exhaust gas purification catalyst according to claim 1 or 2.

4. The Pt layer and the Rh layer each contain an Al-containing oxide. The exhaust gas purification catalyst according to claim 1.

5. Let C2 be the amount of the Al-containing oxide per liter of substrate in the Pt layer. When the content of the Al-containing oxide per liter of substrate in the alloying suppression layer is C4, The above C2 and C4 satisfy the following relationship: C4 < C2; The exhaust gas purification catalyst according to claim 2 or 4.

6. Let C3 be the amount of the Al-containing oxide per liter of substrate in the Rh layer. When the content of the Al-containing oxide per liter of substrate in the alloying suppression layer is C4, The above C3 and C4 satisfy the following relationship: C3 < C4; The exhaust gas purification catalyst according to claim 4.

7. The amount of coating per liter of substrate in the Rh layer is less than the amount of coating per liter of substrate in the Pt layer. The exhaust gas purification catalyst according to claim 1 or 2.

8. Let X (g / L) be the Pt content per liter of substrate in the Pt layer, and let Z (m²) be the specific surface area of ​​the Al-containing oxide per liter of substrate in the alloying suppression layer. ２ When / L), X and Z satisfy the following equation: Z ≥ 745.69X + 2138.1; The exhaust gas purification catalyst according to claim 1 or 2.

9. When the coat length of the Pt layer in the exhaust gas flow direction is set to 100%, the coat length of the alloying suppression layer in the exhaust gas flow direction is 75% or more. The exhaust gas purification catalyst according to claim 1 or 2.

10. An exhaust gas purification catalyst, which is placed in the exhaust path of an internal combustion engine and purifies exhaust gas discharged from the internal combustion engine, The invention comprises a base material and a coating layer provided on the base material, The aforementioned coating layer is Located on the substrate side, a Pt layer containing Pt as a catalytic metal, Located on the surface side, the Rh layer contains Rh as a catalytic metal, An alloying-suppressing layer is interposed between the Pt layer and the Rh layer, and contains an Al-containing oxide but does not contain Pt or Rh. Equipped with, The Pt layer is located downstream in the direction of exhaust gas flow when placed within the exhaust path. When placed within the exhaust path, it is located upstream of the Pt layer in the direction of exhaust gas flow, and further comprises a Pd layer containing Pd as a catalytic metal, The alloying suppression layer has a coat length in the exhaust gas flow direction that is shorter than that of the substrate. In the upstream side, the Rh layer and the Pd layer are in contact in the thickness direction. Catalyst for exhaust gas purification.

11. The alloying-suppressing layer contains ceria as an OSC material, and the ceria content per liter of substrate is 6 g / L or less. The exhaust gas purification catalyst according to claim 10.

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