Exhaust gas purification catalyst

Abstract

This exhaust gas purification catalyst has a base material and a catalyst layer provided on the base material. The catalyst layer includes a layer a, a layer b, and a layer c. The layer a is provided on an upstream side of the layer b in a flow direction of exhaust gas. The layer c is laminated on at least one of the layer a and the layer b. In the laminated portion of the layer c and the at least one of the layer a and the layer b, the layer c is provided at a position further from the base material than the at least one of the layer a and the layer b. The layer a contains Pd. The layer b contains Pt. CPd / CPt, which is the ratio of the mass content CPd of the Pd contained in the layer a to the mass content CPt of the Pt contained in the layer b, exceeds 1. The layer c contains Rh, a Ce-Zr-based composite oxide, and a Zr-based oxide. In the layer c, at least a portion of the Rh is supported by the Zr-based oxide.

Description

exhaust gas purification catalyst

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

[0002] Exhaust gas (also referred to as exhaust gas) emitted from internal combustion engines of automobiles, motorcycles, and the like contains harmful components such as HC (hydrocarbons), CO (carbon monoxide), and NOx (nitrogen oxides). Conventionally, three-way catalysts have been used to purify and neutralize these harmful components. Known three-way catalysts use precious metals such as Pt (platinum), Pd (palladium), and Rh (rhodium), with Pt and Pd primarily acting to oxidize and purify HC and CO, and Rh primarily acting to reduce and purify NOx. In recent years, due to stricter exhaust gas regulations and growing environmental awareness, attempts have been made to improve the exhaust gas purification performance of such precious metal catalysts.

[0003] Patent Document 1 describes an exhaust gas purification catalyst having a substrate and a catalyst coating layer formed on the substrate, in which the catalyst coating layer contains Rh fine particles and a promoter made of a Ce-Zr-based composite oxide and a Zr-based composite oxide not containing Ce oxide, the Rh fine particles having an average particle size and a standard deviation of particle size within a specific range, and the Rh fine particles are supported on each of the Ce-Zr-based composite oxide and the Zr-based composite oxide not containing Ce oxide.

[0004] Patent Documents 2 and 3 describe an exhaust gas purification catalyst that is disposed in an exhaust path of an internal combustion engine and purifies exhaust gas emitted from the internal combustion engine, the exhaust gas purification catalyst comprising: a substrate that defines cells through which exhaust gas flows; and a catalyst layer provided on the surface of the substrate, the catalyst layer including: a palladium layer that extends from a first end that is an end of the cell where exhaust gas flows in toward a second end that is an end of the cell where exhaust gas flows out, and contains palladium; a platinum layer that extends from the second end toward the first end and contains platinum; and a rhodium layer that is laminated on both the palladium layer and the platinum layer and contains rhodium. Patent Document 4 also describes a trimetallic layered catalyst article, comprising: a. an upper layer including platinum supported on at least one of an oxygen storage component, a zirconia component, and an alumina component, and rhodium supported on the oxygen storage component; A trimetallic layered catalyst article is described, comprising: (a) a lower layer including a front zone and a rear zone, wherein the front zone comprises palladium supported on an oxygen storage component and an alumina component, and the rear zone comprises platinum supported on at least one of the alumina component, the ceria component, and the oxygen storage component; and (b) a substrate, wherein the weight ratio of palladium to platinum is in the range of 1.0:0.4 to 1.0:2.0.

[0005] Japanese Patent Publication No. 2021-104472 Japanese Patent Publication No. 2020-157262 Japanese Patent Publication No. 2020-157263 International Publication No. 2020 / 190999

[0006] In recent years, there has been a demand for improved NOx purification performance for exhaust gas purification catalysts immediately after starting an internal combustion engine such as an engine (immediately after starting the internal combustion engine). Patent Document 1 describes that by supporting Rh particulates on a Ce-Zr-based composite oxide and a Zr-based composite oxide, NOx can be purified even at low temperatures (low-temperature activity is improved). However, the present inventors have found that the exhaust gas purification catalyst described in Patent Document 1 has a problem in that it is inferior in purification performance for non-methane hydrocarbons (NMHCs), which are hydrocarbons other than methane.

[0007] Therefore, an object of the present invention is to provide an exhaust gas purification catalyst that can achieve both NOx purification performance immediately after starting an internal combustion engine and NMHC purification performance immediately after starting an internal combustion engine.

[0008] The gist of the present invention is as follows.

[0009] [1] An exhaust gas purification catalyst having a substrate and a catalyst layer provided on the substrate, wherein the catalyst layer includes a layer a, a layer b, and a layer c, the layer a is provided upstream of the layer b in a flow direction of exhaust gas, the layer c is laminated with at least one of the layer a and the layer b, and in a laminated portion of the layer c and at least one of the layer a and the layer b, the layer c is provided at a position farther from the substrate than at least one of the layer a and the layer b, the layer a contains Pd, the layer b contains Pt, and a mass content C of Pt contained in the layer b is Ｐｔ The mass content C of Pd contained in the layer a relative to Ｐｄ C is the ratio of Ｐｄ / C Ｐｔ [2] The layer c does not contain Al, or when it contains Al, the Al content of Al is ２ O ３ The exhaust gas purifying catalyst according to [1], wherein the mass M of the Ce—Zr-based composite oxide contained in the layer c is 22 mass % or less based on the mass of the layer c. ＣＺ The mass M of the Zr-based oxide contained in the layer c ＺｒＯ２ M is the ratio of ＺｒＯ２ / M ＣＺ [4] The exhaust gas purifying catalyst according to [1] or [2], wherein the mass content C of the Zr-based oxide contained in the layer c is 1.5 or more and 6.0 or less. ＺｒＯ２ The mass content C of Pd contained in the layer a relative to Ｐｄ C is the ratio of Ｐｄ / C ＺｒＯ２[5] The exhaust gas purifying catalyst according to any one of [1] to [3], wherein the mass content C of the Zr-based oxide contained in the layer c is 0.015 or more and 0.120 or less. ＺｒＯ２ The mass content C of Pt contained in the layer b relative to Ｐｔ C is the ratio of Ｐｔ / C ＺｒＯ２ is 0.001 or more and 0.008 or less, [1] to [4], the exhaust gas purification catalyst according to any one of [1] to [4].

[0010] According to the present invention, it is possible to provide an exhaust gas purification catalyst that can achieve both NOx purification performance immediately after starting an internal combustion engine and NMHC purification performance immediately after starting an internal combustion engine.

[0011] Schematic diagram of an embodiment of an exhaust gas purification catalyst of the present invention. ...

[0012] Hereinafter, embodiments of the present invention will be described in detail.

[0013] The exhaust gas purifying catalyst of the present invention is an exhaust gas purifying catalyst having a substrate and a catalyst layer provided on the substrate, wherein the catalyst layer includes a layer a, a layer b, and a layer c, the layer a is provided upstream of the layer b in the flow direction of exhaust gas, the layer c is laminated with at least one of the layer a and the layer b, and in a laminated portion of the layer c and at least one of the layer a and the layer b, the layer c is provided at a position farther from the substrate than at least one of the layer a and the layer b, the layer a contains Pd, the layer b contains Pt, and the mass content C of Pt contained in the layer b is Ｐｔ The mass content C of Pd contained in the layer a relative to Ｐｄ C is the ratio of Ｐｄ / C Ｐｔis greater than 1, the layer c contains Rh, a Ce—Zr-based composite oxide, and a Zr-based oxide, and in the layer c, at least a part of the Rh is supported on the Zr-based oxide.

[0014] The exhaust gas purification catalyst (also simply referred to as "catalyst") of the present invention, due to the above-mentioned configuration, exhibits the effect of being able to achieve both NOx purification performance immediately after the start of the internal combustion engine and NMHC purification performance immediately after the start of the internal combustion engine. The mechanism by which the above-mentioned effect is achieved by the present invention is not completely clear, but the inventors have speculated as follows. However, the present invention is not limited in any way by the speculated mechanism below. In general, Rh is thought to mainly contribute to the purification of NOx, and in order to fully exhibit the NOx purification performance of Rh, it is thought that an oxygen storage component (also referred to as "OSC material") that has the ability to mitigate fluctuations in oxygen concentration in exhaust gas (OSC ability) is effective. In the present invention, a Ce-Zr based composite oxide (also referred to as "CZ") that can function as an OSC material is used. CZ functions not only as an OSC material but also as a carrier, and the CeO in CZ ２ has the property of oxidizing Rh and reducing its activity. Therefore, it is considered to use not only CZ but also a carrier other than CZ for the catalyst layer containing Rh. The most common carrier other than CZ is Al ２ O ３ The catalyst is a catalyst whose main component is Al. Since the exhaust gas purification reaction is a chemical reaction, the higher the temperature, the faster the reaction proceeds, resulting in a higher purification rate. Immediately after the internal combustion engine is started, the catalyst temperature is low, resulting in a low purification rate. In order to improve the NOx purification performance immediately after the internal combustion engine is started, it is effective to quickly raise the temperature of the catalyst after the internal combustion engine is started. However, Al ２ O ３ Since Al has a large specific heat, it rises in temperature slowly, which is considered to be disadvantageous in improving the purification performance immediately after starting the internal combustion engine. ２ O ３A Zr-based oxide, which has a smaller specific heat capacity than CZ and does not have the property of oxidizing Rh like CZ, is used in combination with CZ. This is thought to have improved the NOx purification performance immediately after starting the internal combustion engine. In order to suppress the phenomenon in which Rh is oxidized by CZ and its activity decreases, it is necessary for at least a portion of Rh to be supported on the Zr-based oxide. Incidentally, Rh is thought to contribute not only to the purification of NOx but also to the purification of non-methane hydrocarbons (NMHC). However, the inventors' studies have shown that Zr-based oxides have the property of assisting the purification action of Rh against NMHC, which is due to the Al content of Al. ２ O ３ It was found that the NMHC purification performance immediately after starting the internal combustion engine was weaker than that of the layer b. Therefore, it is believed that the present invention can improve the NMHC purification performance immediately after starting the internal combustion engine by particularly adopting the following configurations (I) and (II): (I) A layer a containing Pd and a layer b containing Pt are provided such that the layer a is located upstream of the layer b in the flow direction of the exhaust gas; (II) The mass content C of Pt contained in the layer b is Ｐｔ The mass content C of Pd contained in layer a Ｐｄ C is the ratio of Ｐｄ / C Ｐｔis greater than 1. Compared to Pd, Pt is more advantageous for purifying saturated hydrocarbons among NMHCs. Compared to Pd, Pd is more advantageous for purifying unsaturated hydrocarbons. Therefore, by using Pd and Pt in combination, the overall NMHC emissions can be reduced compared to using either one alone. Pt also contributes to purifying NOx. However, compared to Pd, Pt has the property that, when exposed to a high-temperature environment, its particles move and agglomerate, making it more likely to reduce active sites. The exhaust gas temperature is higher upstream in the exhaust gas flow direction, and lower downstream in the exhaust gas flow direction than the upstream side. Therefore, in order to arrange the Pt-containing layer downstream in the exhaust gas flow direction where the temperature is lower, the present invention employs the above-mentioned configuration (I), in which the Pd-containing layer a is provided upstream in the exhaust gas flow direction and the Pt-containing layer b is provided downstream in the exhaust gas flow direction. Furthermore, the above-mentioned configuration (II) allows a large amount of catalytically active components to be arranged upstream in the flow direction of exhaust gas, which is closer to the internal combustion engine (e.g., engine) and where the temperature is likely to rise. As a result, even if the total mass of the catalytically active components is the same, the NMHC purification performance immediately after starting the internal combustion engine can be improved. For the above reasons, it is believed that the exhaust gas purification catalyst of the present invention is able to achieve both NOx purification performance immediately after starting the internal combustion engine and NMHC purification performance immediately after starting the internal combustion engine.

[0015] <Substrate> The exhaust gas purification catalyst of the present invention has a substrate. As the substrate, a substrate used in a conventionally known exhaust gas purification catalyst can be used, and for example, a substrate having partition walls made of a porous material and exhaust gas flow channels (spaces between the partition walls) partitioned by the partition walls can be suitably used. As the shape of the substrate, known substrates such as honeycomb, DPF (diesel particulate filter), GPF (gasoline particulate filter) and the like can be suitably used. In addition, as the material of the partition walls of the substrate, for example, alumina (Al ２ O ３ ), mullite (3Al ２ O ３ -2SiO ２ ), cordierite (2MgO-2Al ２ O ３ -5SiO２ ), aluminum titanate (Al ２ TiO ５ ), ceramics such as silicon carbide (SiC), and metal materials such as stainless steel.

[0016] <Catalyst Layer> The exhaust gas purification catalyst of the present invention has a catalyst layer. The catalyst layer is provided on a substrate. The catalyst layer may be provided directly on at least a portion of the surface of the substrate (so as to be in contact with at least a portion of the surface of the substrate) or may be provided via another layer. The catalyst layer includes a layer a, a layer b, and a layer c.

[0017] [Layer a] Layer a contains Pd. Pd is a catalytically active component. Mass content C of Pd contained in layer a Ｐｄ is preferably 0.010% by mass or more, more preferably 0.050% by mass or more, and even more preferably 0.10% by mass or more. Ｐｄ is preferably 15% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less. Ｐｄ is the ratio (mass %) of the mass of Pd contained in layer a to the mass of layer a.

[0018] The content (mass%) of each element, including Pd, in layer a can be determined by a conventional method such as scanning electron microscope-energy dispersive X-ray analysis (SEM-EDX). Specifically, the method is as follows. Elemental analysis is performed on a sample obtained from layer a using a conventional method such as SEM-EDX to identify the types of constituent elements of the entire sample and to determine the content (mass%) of each identified metal element. The content (mass%) of each metal element is determined for each of 10 SEM fields, and the average of the content (mass%) of each metal element in the 10 fields is taken as the content (mass%) of each metal element in layer a. When calculating the content (mass%) using the above method, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au are considered to be in a metallic state, and the other metal elements are considered to be in an oxidized state. Specific oxide forms for each element are as follows. Oxides of rare earth elements other than Ce, Pr, and Tb are considered to be sesquioxides (M２ O ３ , M represents a rare earth element other than Ce, Pr, and Tb), and the oxide of Ce is CeO ２ , Pr oxide is Pr ６ O １１ , the oxide of Tb is Tb ４ O ７ , the oxide of Al is Al ２ O ３ , Zr oxide is ZrO ２ , and the oxide of Si is SiO ２ , the oxide of B is B ２ O ３ , Mg oxide is MgO, Ca oxide is CaO, Sr oxide is SrO, Ba oxide is BaO, Fe oxide is Fe ３ O ４ , Mn oxide is Mn ３ O ４ , Cu oxide is CuO, Ti oxide is TiO ２ Zn oxide is ZnO, Sn oxide is SnO ２ is.

[0019] Layer a may contain, in addition to Pd, a catalytically active component other than Pd. Examples of catalytically active components other than Pd include conventionally known components, such as Rh, Pt, gold (Au), silver (Ag), iridium (Ir), ruthenium (Ru), and osmium (Os). Layer a preferably does not contain any catalytically active components other than Pd, or, if it contains any catalytically active components other than Pd, the proportion of such components is preferably small. Specifically, the mass of the catalytically active components other than Pd contained in layer a is preferably 30% by mass or less, more preferably 20% by mass or less, and even more preferably 10% by mass or less, based on the total mass of Pd and the catalytically active components other than Pd contained in layer a.

[0020] Layer a may further contain components other than those described above. For example, layer a may contain a Ce—Zr-based composite oxide. The term "Ce—Zr-based composite oxide" refers to a composite oxide containing Ce and Zr, and the Ce in the composite oxide is expressed as CeO. ２ The content of Zr in the composite oxide in terms of ZrO is 5 mass % or more and 95 mass % or less with respect to the mass of the composite oxide. ２The Ce-Zr-based composite oxide refers to a material in which the content, calculated as a percentage of the total mass of the composite oxide, is 5% by mass or more and 95% by mass or less. Ce-Zr-based composite oxides are usually in the form of particles. Whether layer a contains a Ce-Zr-based composite oxide can be determined by a conventional method such as SEM-EDX. Specifically, the method is as follows. First, a sample containing layer a is analyzed by SEM-EDX, and particles containing the two elements Ce and Zr are identified by the obtained elemental mapping. Next, a composition analysis (elemental analysis) is performed on the identified particles, and if the results of the composition analysis are within the above range, the material can be determined to be a Ce-Zr-based composite oxide.

[0021] The Ce—Zr-based composite oxide can function as an oxygen storage component (OSC material). The Ce—Zr-based composite oxide can also function as a support component that supports the above-mentioned catalytically active component. Supporting the catalytically active component refers to a state in which the catalytically active component is physically or chemically adsorbed or held on the outer surface or the inner surface of the pores. Specifically, the support component can be determined to "support" the catalytically active component by, for example, confirming that the support component and the catalytically active component are present in the same region in elemental mapping obtained by analyzing a cross section of the exhaust gas purification catalyst using SEM-EDX.

[0022] CeO of Ce in the Ce-Zr based composite oxide that may be contained in layer a ２ The converted content is preferably 10% by mass or more and 70% by mass or less, more preferably 20% by mass or more and 60% by mass or less, and even more preferably 30% by mass or more and 50% by mass or less, based on the mass of the Ce-Zr-based composite oxide.

[0023] ZrO of Zr in the Ce-Zr based composite oxide that may be contained in layer a ２ The converted content is preferably 10% by mass or more and 80% by mass or less, more preferably 20% by mass or more and 75% by mass or less, and even more preferably 30% by mass or more and 70% by mass or less, based on the mass of the Ce—Zr-based composite oxide.

[0024] The Ce—Zr-based composite oxide that may be contained in layer a may contain a rare earth element other than Ce or an alkaline earth metal element such as barium (Ba), strontium (Sr), or calcium (Ca). Examples of rare earth elements other than Ce include scandium (Sc), yttrium (Y), lanthanum (La), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). These rare earth elements may be added to the Ce—Zr-based composite oxide, for example, as oxides. The rare earth element and / or alkaline earth metal element contained in the Ce—Zr-based composite oxide may be one type or two or more types.

[0025] When layer a contains a Ce—Zr-based composite oxide, the content of the Ce—Zr-based composite oxide contained in layer a is preferably 40 mass % or more, more preferably 50 mass % or more, and even more preferably 60 mass % or more, relative to the mass of layer a. Furthermore, the content of the Ce—Zr-based composite oxide contained in layer a is preferably 90 mass % or less, more preferably 85 mass % or less, and even more preferably 80 mass % or less, relative to the mass of layer a.

[0026] The content of the Ce—Zr-based composite oxide contained in layer a can be determined by a conventional method such as SEM-EDX. Specifically, the method is as follows: (1) A sample obtained from layer a is subjected to elemental analysis using a conventional method such as SEM-EDX to identify the types of constituent elements of the entire sample and to determine the content (mass%) of each identified element. (2) A sample obtained from layer a is subjected to elemental mapping using a conventional method such as SEM-EDX to identify the types of particles contained in the sample (e.g., Ce—Zr-based composite oxide particles and, in some cases, other particles). (3) For each type of particle, a plurality of arbitrarily selected particles (e.g., 50 particles) are subjected to elemental analysis using SEM-EDX to identify the types of constituent elements of the particles and to determine the content (mass%) of each identified element. The average content (mass%) of each element is determined for each type of particle. (4) An equation is created and solved to represent the relationship between the content (mass%) of each element in the sample, the content (mass%) of each element in each type of particle, and the content (mass%) of each type of particle in the sample, thereby calculating the content (mass%) of each type of particle in the sample, and this is set as the content (mass%) of each type of particle in layer a.

[0027] Layer a may contain an OSC material other than a Ce—Zr-based composite oxide. The OSC material can be any metal oxide that undergoes a change in the valence of its constituent elements under the operating conditions of the exhaust gas purification catalyst and has the ability to store oxygen, and examples of such an OSC material include oxides of elements that are likely to undergo a change in valence state under the operating conditions of the catalyst, such as manganese (Mn), iron (Fe), and copper (Cu), as well as composite oxides containing these elements.

[0028] The layer a may contain an inorganic oxide other than the oxygen storage component as a carrier. The inorganic oxide other than the oxygen storage component may be a metal oxide other than the oxygen storage component described above, such as Al. ２ O ３ , ZrO ２ , SiO ２ , TiO ２ , La ２ O ３ Rare earth oxides such as zeolite (aluminosilicate), MgO, ZnO, SnO ２Examples of the inorganic oxides include oxide materials based on aluminum (Al), zirconium (Zr), silicon (Si), titanium (Ti), rare earth elements, magnesium (Mg), zinc (Zn), and the like, as well as oxide materials obtained by compounding these materials together. Phosphates and borates of inorganic oxides other than the oxygen storage component are also included. Porous inorganic oxides are preferred. The porous material should have a BET specific surface area of ​​30 m or less. ２ / g or more 600m ２ / g or less.

[0029] Furthermore, from the viewpoint of suppressing a decrease in catalytic activity due to phosphorus poisoning and improving heat resistance, layer a may contain an alkaline earth metal compound. Preferred alkaline earth metal elements include Sr and Ba. Examples of alkaline earth metal compounds include oxides and carbonates. Layer a may contain a binder. The binder can be selected from, for example, alumina, zirconia, titania, silica, ceria, etc.

[0030] The mass of layer a per unit volume of the portion of the substrate where layer a is formed is preferably 20 g / L or more and 200 g / L or less, more preferably 50 g / L or more and 180 g / L or less, and even more preferably 70 g / L or more and 160 g / L or less.

[0031] [Layer b] Layer b contains Pt. Pt is a catalytically active component. Mass content C of Pt contained in layer b Ｐｔ is preferably 0.010% by mass or more, more preferably 0.050% by mass or more, and even more preferably 0.10% by mass or more. Ｐｔ is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 3% by mass or less. Ｐｔ is the ratio (mass %) of the mass of Pt contained in layer b to the mass of layer b. The Pt content in layer b can be determined in the same manner as the Pd content in layer a described above.

[0032] Layer b may contain, in addition to Pt, a catalytically active component other than Pt. Conventional catalytically active components other than Pt can be used, such as Rh, Pd, Au, Ag, Ir, Ru, and Os. Layer b does not contain any catalytically active component other than Pt, or if it contains a catalytically active component other than Pt, the proportion of such a component is preferably small. Specifically, the mass of the catalytically active component other than Pt contained in layer b is preferably 30 mass% or less, more preferably 20 mass% or less, and even more preferably 10 mass% or less, based on the total mass of Pt and the catalytically active component other than Pt contained in layer b.

[0033] Layer b may further contain components other than those described above. For example, layer b may contain a Ce—Zr-based composite oxide. The Ce—Zr-based composite oxide can function as an OSC material. Furthermore, the Ce—Zr-based composite oxide can function as a support component that supports the above-mentioned catalytically active component. Whether layer b contains a Ce—Zr-based composite oxide can be determined in the same manner as whether layer a described above contains a Ce—Zr-based composite oxide.

[0034] CeO of Ce in the Ce-Zr based composite oxide that may be contained in layer b ２ The converted content is preferably 10% by mass or more and 70% by mass or less, more preferably 20% by mass or more and 60% by mass or less, and even more preferably 30% by mass or more and 50% by mass or less, based on the mass of the Ce-Zr-based composite oxide.

[0035] ZrO of Zr in the Ce—Zr-based composite oxide that may be contained in layer b ２ The converted content is preferably 10% by mass or more and 80% by mass or less, more preferably 20% by mass or more and 75% by mass or less, and even more preferably 30% by mass or more and 70% by mass or less, based on the mass of the Ce—Zr-based composite oxide.

[0036] The Ce—Zr-based composite oxide that may be contained in layer b may contain a rare earth element other than Ce or an alkaline earth metal element such as Ba, Sr, or Ca. Examples of the rare earth element other than Ce include those similar to those described above for layer a.

[0037] When layer b contains a Ce—Zr-based composite oxide, the content of the Ce—Zr-based composite oxide in layer b is preferably 40 mass % or more, more preferably 50 mass % or more, and even more preferably 60 mass % or more, relative to the mass of layer b. Furthermore, the content of the Ce—Zr-based composite oxide in layer b is preferably 90 mass % or less, more preferably 85 mass % or less, and even more preferably 80 mass % or less, relative to the mass of layer b. The content of the Ce—Zr-based composite oxide in layer b can be determined in the same manner as the content of the Ce—Zr-based composite oxide in layer a described above.

[0038] Layer b may contain an OSC material other than a Ce—Zr-based composite oxide, such as the same OSC materials as those described above for layer a.

[0039] Layer b may contain, as a carrier, an inorganic oxide other than the oxygen storage component. Examples of the inorganic oxide other than the oxygen storage component include those similar to those described above for layer a.

[0040] Furthermore, from the viewpoints of suppressing a decrease in catalytic activity due to phosphorus poisoning and improving heat resistance, layer b may contain an alkaline earth metal compound. Preferred alkaline earth metal elements include Sr and Ba. Examples of alkaline earth metal compounds include oxides and carbonates. Layer b may contain a binder. The binder can be selected from, for example, alumina, zirconia, titania, silica, ceria, etc.

[0041] The mass of layer b per unit volume of the portion of the substrate where layer b is formed is preferably 20 g / L or more and 200 g / L or less, more preferably 50 g / L or more and 180 g / L or less, and even more preferably 70 g / L or more and 160 g / L or less.

[0042] [Layer c] Layer c contains Rh, a Ce—Zr-based composite oxide, and a Zr-based oxide. Rh is a catalytically active component. The mass content C of Rh contained in layer c is Ｒｈis preferably 0.0010% by mass or more, more preferably 0.0050% by mass or more, and even more preferably 0.010% by mass or more. Ｒｈ is preferably 5% by mass or less, more preferably 3% by mass or less, and even more preferably 1% by mass or less. Ｒｈ is the ratio (mass %) of the mass of Rh contained in layer c to the mass of layer c. The Rh content in layer c can be determined in the same manner as the Pd content in layer a described above.

[0043] Layer c may contain, in addition to Rh, a catalytically active component other than Rh. Examples of catalytically active components other than Rh include conventionally known components, such as Pt, Pd, Au, Ag, Ir, Ru, and Os. When layer c contains Pt, the content of Pt in layer c relative to the total mass of Rh and Pt in layer c is preferably 10% by mass or more and 60% by mass or less, more preferably 20% by mass or more and 50% by mass or less, and even more preferably 30% by mass or more and 40% by mass or less.

[0044] Layer c contains a Ce—Zr-based composite oxide. The Ce—Zr-based composite oxide can function as an OSC material. Furthermore, the Ce—Zr-based composite oxide can function as a support component that supports the above-mentioned catalytically active component. Whether layer c contains a Ce—Zr-based composite oxide can be determined in the same manner as whether layer a above contains a Ce—Zr-based composite oxide.

[0045] CeO of Ce in the Ce-Zr composite oxide contained in layer c ２ The converted content is preferably 5% by mass or more and 40% by mass or less, more preferably 8% by mass or more and 30% by mass or less, and even more preferably 10% by mass or more and 20% by mass or less, based on the mass of the Ce—Zr-based composite oxide.

[0046] ZrO of Zr in the Ce-Zr composite oxide contained in layer c ２The converted content is preferably 20% by mass or more and 95% by mass or less, more preferably 30% by mass or more and 90% by mass or less, and even more preferably 40% by mass or more and 85% by mass or less, based on the mass of the Ce—Zr-based composite oxide.

[0047] The Ce—Zr-based composite oxide contained in layer c may contain rare earth elements other than Ce, or alkaline earth metal elements such as Ba, Sr, and Ca. Examples of rare earth elements other than Ce include those similar to those described above for layer a.

[0048] The content of the Ce—Zr-based composite oxide contained in layer c is preferably 1 mass % or more, more preferably 5 mass % or more, even more preferably 10 mass % or more, and particularly preferably 12 mass % or more, relative to the mass of layer c. Furthermore, the content of the Ce—Zr-based composite oxide contained in layer c is preferably 90 mass % or less, more preferably 50 mass % or less, and even more preferably 20 mass % or less, relative to the mass of layer c. The content of the Ce—Zr-based composite oxide contained in layer c can be determined in the same manner as the content of the Ce—Zr-based composite oxide contained in layer a described above.

[0049] Layer c may contain an OSC material other than a Ce—Zr-based composite oxide. Examples of OSC materials include those similar to those described above for layer a. However, from the viewpoint of suppressing a decrease in activity due to oxidation of Rh, layer c does not contain an OSC material other than a Ce—Zr-based composite oxide, or if layer c contains an OSC material other than a Ce—Zr-based composite oxide, the content of the OSC material other than a Ce—Zr-based composite oxide contained in layer c is preferably 50 mass % or less, more preferably 30 mass % or less, more preferably 20 mass % or less, even more preferably 10 mass % or less, still more preferably 5 mass % or more, and particularly preferably 1 mass % or less, relative to the mass of layer c.

[0050] The layer c includes a Zr-based oxide. The term "Zr-based oxide" refers to an oxide containing Zr, and Zr in the oxide is expressed as ZrO. ２The content of Ce in the oxide is 50 mass% or more based on the mass of the oxide, and the Ce content is CeO ２ The term "layer c" refers to a layer in which the content, calculated as a Zr-based oxide, is less than 5% by mass relative to the mass of the oxide. The Zr-based oxide can function as a support component that supports the catalytically active component. Whether layer c contains a Zr-based oxide can be determined in the same manner as whether layer a contains a Ce-Zr-based composite oxide.

[0051] ZrO of Zr in Zr-based oxides contained in layer c ２ The converted content is preferably 50% by mass or more and 98% by mass or less, more preferably 60% by mass or more and 95% by mass or less, and even more preferably 70% by mass or more and 90% by mass or less, based on the mass of the Zr-based oxide.

[0052] CeO of Ce in Zr-based oxide contained in layer c ２ The converted content is preferably less than 3 mass % relative to the mass of the Zr-based oxide, and more preferably 0 mass % (i.e., no Ce is contained).

[0053] The Zr-based oxide contained in layer c may contain a rare earth element other than Ce or an alkaline earth metal element such as Ba, Sr, or Ca. Examples of rare earth elements other than Ce include Sc, Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

[0054] Mass content C of Zr-based oxide contained in layer c ＺｒＯ２ is preferably 40% by mass or more, more preferably 50% by mass or more, even more preferably 60% by mass or more, and particularly preferably 70% by mass or more. ＺｒＯ２ is preferably 95% by mass or less, more preferably 90% by mass or less, and even more preferably 85% by mass or less. ＺｒＯ２ is the ratio (mass %) of the mass of the Zr-based oxide contained in layer c to the mass of layer c. The content of the Zr-based oxide contained in layer c can be determined in the same manner as the content of the Ce—Zr-based composite oxide contained in layer a described above.

[0055] From the viewpoint of improving the OSC performance and suppressing the decrease in activity due to the oxidation of Rh, the mass M of the Ce-Zr composite oxide contained in layer c is ＣＺ The mass M of Zr-based oxide contained in layer c ＺｒＯ２ M is the ratio of ＺｒＯ２ / M ＣＺ is preferably 1.0 or more and 7.0 or less, more preferably 1.5 or more and 6.0 or less, even more preferably 2.0 or more and 6.0 or less, particularly preferably 3.0 or more and 6.0 or less, and most preferably 4.0 or more and 5.5 or less. In particular, M ＺｒＯ２ / M ＣＺ When the ratio is 1.5 or more, the decrease in activity due to oxidation of Rh can be effectively suppressed, and when the ratio is 6.0 or less, the OSC performance can be further improved.

[0056] Layer c may further contain components other than those described above. Layer c may contain an inorganic oxide other than the oxygen storage component and the Zr-based oxide as a carrier. Examples of the inorganic oxide other than the oxygen storage component and the Zr-based oxide include Al ２ O ３ In addition, ZrO ２ Other examples include the same.

[0057] As mentioned above, Al ２ O ３ Since Al has a large specific heat, it improves the NOx purification performance immediately after starting the internal combustion engine. For this reason, the layer c does not contain Al, or if it contains Al, the Al content of Al is ２ O ３ The content of Al in terms of Al is preferably 30 mass % or less with respect to the mass of the layer c. ２ O ３ The content in terms of Al converted is more preferably 22 mass % or less, further preferably 15 mass % or less, and particularly preferably 13 mass % or less, relative to the mass of layer c. When the binder contained in layer c contains Al element, the Al content of Al contained in layer c is ２ O ３ The converted content includes Al derived from the binder.

[0058] Furthermore, from the viewpoint of suppressing a decrease in catalytic activity due to phosphorus poisoning and improving heat resistance, layer c may contain an alkaline earth metal compound. Preferred alkaline earth metal elements include Sr and Ba. Examples of alkaline earth metal compounds include oxides and carbonates. Layer c may contain a binder. The binder can be selected from, for example, alumina, zirconia, titania, silica, ceria, etc.

[0059] In layer c, at least a portion of Rh is supported on a Zr-based oxide. When Rh is supported on a Zr-based oxide, the phenomenon of Rh being oxidized by the Ce-Zr-based composite oxide and decreasing in activity is suppressed. On the other hand, when Rh is supported on a Ce-Zr-based composite oxide, the activity of Rh decreases, but the OSC function of the Ce-Zr-based composite oxide is increased, thereby improving the purification performance of layer c as a whole. From the above, the mass percentage of Rh supported on the Zr-based oxide in layer c, based on the total mass of Rh contained in layer c, is preferably 50 mass% or more and 90 mass% or less, more preferably 60 mass% or more and 87 mass% or less, and even more preferably 70 mass% or more and 85 mass% or less. The mass percentage of Rh supported on the Ce—Zr-based composite oxide in layer c, based on the total mass of Rh contained in layer c, is preferably 10 mass % or more and 50 mass % or less, more preferably 13 mass % or more and 40 mass % or less, and even more preferably 15 mass % or more and 30 mass % or less. The mass percentage of Rh supported on the Zr-based oxide or Ce—Zr-based composite oxide in layer c, based on the total mass of Rh contained in layer c, can be adjusted, for example, by adjusting the mass ratio of the Ce—Zr-based composite oxide to the Zr-based oxide when mixing the Rh source, Ce—Zr-based composite oxide, Zr-based oxide, and solvent during preparation of a slurry for forming layer c, which will be described later.

[0060] The mass of layer c per unit volume of the portion of the substrate where layer c is formed is preferably 40 g / L or more and 160 g / L or less, more preferably 60 g / L or more and 140 g / L or less, and even more preferably 80 g / L or more and 120 g / L or less.

[0061] [C Ｐｄ / C ＰｔIn the exhaust gas purifying catalyst of the present invention, the mass content C of Pt contained in the layer b Ｐｔ The mass content C of Pd contained in layer a Ｐｄ C is the ratio of Ｐｄ / C Ｐｔ is greater than 1. Ｐｄ / C Ｐｔ is preferably 1.5 or more, more preferably 2.0 or more, and even more preferably 2.5 or more. Ｐｄ / C Ｐｔ may be 50.0 or less, 30.0 or less, 10.0 or less, or 5.0 or less.

[0062] [C Ｐｄ / C ＺｒＯ２ Pd has an excellent purification effect on unsaturated hydrocarbons among NMHCs, but from the viewpoint of cost reduction, it is preferable to use an amount corresponding to the Zr-based oxide. In the exhaust gas purification catalyst of the present invention, the mass content C of the Zr-based oxide contained in the layer c is ＺｒＯ２ The mass content C of Pd contained in layer a Ｐｄ C is the ratio of Ｐｄ / C ＺｒＯ２ is preferably 0.015 or more and 0.120 or less, more preferably 0.015 or more and 0.080 or less, and even more preferably 0.015 or more and 0.050 or less.

[0063] [C Ｐｔ / C ＺｒＯ２ ] Pt has an excellent purification effect on saturated hydrocarbons among NMHCs. On the other hand, Pt has a tendency to aggregate when exposed to a high-temperature environment, and if Pt is excessive, the distance between Pt particles becomes smaller, making them more likely to aggregate. Therefore, it is preferable to use Pt in an amount corresponding to the Zr-based oxide. In the exhaust gas purification catalyst of the present invention, the mass content C of the Zr-based oxide contained in layer c is ＺｒＯ２ The mass content C of Pt contained in layer b Ｐｔ C is the ratio of Ｐｔ / C ＺｒＯ２is preferably 0.001 or more and 0.008 or less, more preferably 0.002 or more and 0.007 or less, and even more preferably 0.003 or more and 0.006 or less.

[0064] <Layer Structure of Exhaust Gas Purification Catalyst> In the exhaust gas purification catalyst of the present invention, the layer a is provided upstream of the layer b in the exhaust gas flow direction (hereinafter, when simply referred to as "upstream" and "downstream," they mean upstream and downstream, respectively). Here, "the layer a is provided upstream of the layer b" means that the upstream end of the layer a is located upstream of the upstream end of the layer b, and the downstream end of the layer a is located upstream of the downstream end of the layer b. As described above, this allows the Pt-containing layer b to be disposed downstream in the exhaust gas flow direction, which is at a lower temperature, and is thought to suppress aggregation of Pt particles and reduce the decrease in active sites. Furthermore, the layer c is laminated with at least one of the layers a and b, and in the laminated portion of the layer c and at least one of the layers a and b, the layer c is provided farther from the substrate than at least one of the layers a and b. As a result, in the laminated portion of the layer c and at least one of the layers a and b, the Rh-containing layer c comes into contact with the exhaust gas first. It is believed that layer c can be heated more quickly by first contacting the high-temperature exhaust gas, thereby improving the NOx purification performance immediately after starting the internal combustion engine in the present invention.

[0065] FIG. 1 shows a schematic diagram of one embodiment of the exhaust gas purification catalyst of the present invention. The exhaust gas purification catalyst 10 in FIG. 1 has a substrate S1 and a catalyst layer 1 provided on the substrate S1. The catalyst layer 1 includes a layer a (designated by the symbol a), a layer b (designated by the symbol b), and a layer c (designated by the symbol c). The layer a is provided upstream of the layer b in the flow direction X of the exhaust gas. The layer c is stacked with the layers a and b. In the stacked portion of the layer c, the layer a, and the layer b, the layer c is provided farther from the substrate S1 than the layers a and b.

[0066] In the exhaust gas purification catalyst 10 of Fig. 1, layer a and layer b are provided on one substrate, but in the present invention, layer a and layer b may be provided on separate substrates. Fig. 2 shows a schematic diagram of one embodiment of the exhaust gas purification catalyst of the present invention. The exhaust gas purification catalyst 11 of Fig. 2 has substrates S1 and S2 as substrates. The substrate S1 and the substrate S2 are separate substrates. The exhaust gas purification catalyst 11 has a catalyst layer 2 formed on the substrate S1 and consisting of layer a and layer c1, and a catalyst layer 3 formed on the substrate S2 and consisting of layer b and layer c2. Both layer c1 and layer c2 correspond to layer c.

[0067] In the exhaust gas purification catalyst of the present invention, one of the layer a and the layer b may cover at least a part of the other. Figure 3 shows a schematic diagram of one embodiment of the exhaust gas purification catalyst of the present invention. In the exhaust gas purification catalyst 12 of Figure 3, the layer a covers a part of the layer b. The exhaust gas purification catalyst 12 is the same as the exhaust gas purification catalyst 10 of Figure 1 except that the layer a covers a part of the layer b. Furthermore, in the exhaust gas purification catalyst 12 of Figure 3, the layer a covers a part of the layer b, but in the exhaust gas purification catalyst of the present invention, the layer b may cover a part of the layer a.

[0068] In the exhaust gas purification catalyst 10 of Figure 1, layer a and layer b are in contact with each other, but in the exhaust gas purification catalyst of the present invention, layer a and layer b may be in contact with each other (layer a and layer b may be formed with no gap between them in the exhaust gas flow direction), or may be separated from each other. An embodiment in which layer a and layer b are separated from each other includes an embodiment in which layer a and layer b are provided on separate substrates, as in the exhaust gas purification catalyst 11 of Figure 2 described above, or an embodiment in which layer a and layer b are provided on a single substrate so as to be separated from each other. Figure 4 shows a schematic diagram of one embodiment of the exhaust gas purification catalyst of the present invention. In the exhaust gas purification catalyst 13 of Figure 4, layer a and layer b are provided on a single substrate so as to be separated from each other. Layer c is present between layer a and layer b. In the exhaust gas purification catalyst 13, there is a layer c between the layer a and the layer b, but in the exhaust gas purification catalyst of the present invention, when the layer a and the layer b are separated, there may be a layer c between the layer a and the layer b, or another layer (a layer other than the layer a, the layer b, and the layer c) may be present, or none of the layers may be present (there may be a gap between the layer a and the layer b). The exhaust gas purification catalyst 13 has the same configuration as the exhaust gas purification catalyst 10 of FIG. 1 except for the above-mentioned points.

[0069] In the exhaust gas purification catalyst 10 of FIG. 1, layer c is laminated over the entire layers a and b, but in the exhaust gas purification catalyst of the present invention, layer c may be laminated over at least a portion of layers a and b. FIGS. 5 to 8 each show a schematic diagram of an embodiment of the exhaust gas purification catalyst of the present invention. In the exhaust gas purification catalyst 14 of FIG. 5, layer c is laminated over a portion of layer a. In the exhaust gas purification catalyst 15 of FIG. 6, layer c is laminated over a portion of layer b. In the exhaust gas purification catalyst 16 of FIG. 7, layer c is laminated over the entire layer a and a portion of layer b. In the exhaust gas purification catalyst 17 of FIG. 8, layer c is laminated over the entire layer b and a portion of layer a. Note that the exhaust gas purification catalysts 14 to 17 have the same configuration as the exhaust gas purification catalyst 10 of FIG. 1 except as described above. Furthermore, in the exhaust gas purification catalyst of the present invention, layer c may be laminated over a portion of layer a and a portion of layer b.

[0070] In the exhaust gas purification catalyst of the present invention, layer c is laminated with at least one of layer a and layer b. However, in the laminated portion, layer c may or may not be in contact with at least one of layer a and layer b. When layer c is not in contact with at least one of layer a and layer b in the laminated portion, another layer (a layer other than layer a, layer b, and layer c) may be present between layer c and at least one of layer a and layer b. FIG. 9 is a schematic diagram of one embodiment of the exhaust gas purification catalyst of the present invention. In the exhaust gas purification catalyst 18 of FIG. 9, another layer, layer d, is present between layer c and layers a and b in the laminated portion. The exhaust gas purification catalyst 18 has a catalyst layer 4 consisting of layers a, b, d, and c. Note that layer d may, for example, contain Rh and have a different Rh content than layer c. Note that the exhaust gas purification catalyst 18 has the same configuration as the exhaust gas purification catalyst 10 of FIG. 1 except for the above.

[0071] In the exhaust gas purification catalyst of the present invention, the substrate and at least one of layer a and layer b may or may not be in contact with each other. When the substrate and at least one of layer a and layer b are not in contact with each other, another layer (a layer other than layer a, layer b, and layer c) may be present between the substrate and at least one of layer a and layer b. FIG. 10 is a schematic diagram of one embodiment of the exhaust gas purification catalyst of the present invention. In the exhaust gas purification catalyst 19 of FIG. 10, another layer, layer d, is present between the substrate S1 and layer b. The exhaust gas purification catalyst 19 has a catalyst layer 5 consisting of layers a and c provided on the substrate S1, and a catalyst layer 6 consisting of layers d and b provided on the substrate S1. An example of layer d is a layer containing Pt, the Pt content of which is different from that of layer b. The exhaust gas purification catalyst 19 has the same configuration as the exhaust gas purification catalyst 10 of FIG. 1 except for the above.

[0072] From the viewpoint of purifying saturated hydrocarbons and unsaturated hydrocarbons, in each embodiment, the length of layer a is preferably 10% to 60% when the length of the substrate is 100%, more preferably 20% to 50% and even more preferably 30% to 40%. From the viewpoint of purifying saturated hydrocarbons and unsaturated hydrocarbons, in each embodiment, the length of layer b is preferably 40% to 90% when the length of the substrate is 100%, more preferably 50% to 80% and even more preferably 60% to 70%. From the viewpoint of NOx purification, in each embodiment, the length of layer c is preferably 60% or more when the length of the substrate is 100%, more preferably 70% or more and even more preferably 80% or more. The upper limit is 100% or less. Note that the length of the substrate or catalyst layer refers to the dimension of the substrate or catalyst layer along the flow direction X of the exhaust gas. Furthermore, when the substrate is separate as shown in FIG. 2, the length of the substrate above refers to the sum of the lengths of the individual substrates.

[0073] The exhaust gas purification catalyst of the present invention is capable of efficiently purifying NOx and NMHC as an exhaust gas purification catalyst for an internal combustion engine. Therefore, an exhaust gas purification system can be provided in which exhaust gas from an internal combustion engine is introduced into the exhaust gas purification catalyst in an exhaust path connected to the internal combustion engine without passing through another exhaust gas purification catalyst. Note that internal combustion engines that emit NOx and NMHC include gasoline engines, engines fueled by compressed natural gas, and engines fueled by gasoline containing any amount of ethanol (flex fuel).

[0074] <Production of catalyst> The exhaust gas catalyst of the present invention can be produced by forming layers a and b on a substrate, and then forming layer c on at least one of layers a and b, provided that the mass content C of Pt contained in layer b is Ｐｔ The mass content C of Pd contained in layer a Ｐｄ C is the ratio of Ｐｄ / C Ｐｔ Adjust so that is greater than 1.

[0075] Layer a can be formed by mixing a Pd source (e.g., Pd nitrate, ammine complex salt, acetate, chloride, etc.) and, if necessary, other components (e.g., OSC material such as Ce—Zr composite oxide, inorganic oxide other than OSC material, binder, solvent, etc.) to prepare a layer a-forming slurry, applying the layer a-forming slurry onto a substrate, drying, and firing.

[0076] Layer b can be formed by mixing a Pt source (e.g., Pt nitrate, ammine complex salt, acetate, chloride, etc.) and, if necessary, other components (e.g., OSC material such as Ce—Zr composite oxide, inorganic oxide other than OSC material, binder, solvent, etc.) to prepare a layer b-forming slurry, applying the layer b-forming slurry onto the substrate, drying, and firing.

[0077] Layer c can be formed by mixing a Rh source (e.g., Rh nitrate, ammine complex salt, acetate, chloride, etc.), a Ce—Zr-based composite oxide, a Zr-based oxide, and optionally other components (e.g., an OSC material other than a Ce—Zr-based composite oxide, an inorganic oxide other than an OSC material, a binder, a solvent, etc.) to prepare a layer c-forming slurry, and then applying the layer c-forming slurry onto at least one of layer a and layer b, drying, and firing.

[0078] Next, the embodiments of the present invention will be specifically described with reference to the following examples, but the present invention is not limited to these examples.

[0079] Example 1 (1) Preparation of Slurry for Forming Layer a In pure water, 1.2 parts by mass of an aqueous solution of palladium nitrate in terms of Pd metal, 1.2 parts by mass of Ce—Zr-based composite oxide (CeO ２ Conversion content: 40 mass% ZrO ２ 70.4 parts by mass of one or more rare earth elements other than Ce in terms of oxides: 10% by mass), La ２ O ３ Modified alumina (La ２ O ３ 14.2 parts by mass of barium hydroxide (1% by mass), 6.1 parts by mass of barium hydroxide in terms of oxide, and a binder containing alumina sol (Al in the solid content ２ O３ The mixture was mixed and stirred to prepare a slurry for forming layer a.

[0080] (2) Preparation of Slurry for Forming Layer b: 0.4 parts by mass of dinitrodiamine platinum (II) solution in terms of Pt metal, 0.4 parts by mass of Ce-Zr based composite oxide (CeO ２ Conversion content: 40 mass% ZrO ２ 70.4 parts by mass of one or more rare earth elements other than Ce in terms of oxides: 10% by mass), La ２ O ３ Modified alumina (La ２ O ３ 15.0 parts by mass of barium hydroxide (1% by mass), 6.1 parts by mass of barium hydroxide in terms of oxide, and a binder containing alumina sol (Al in the solid content ２ O ３ The mixture was mixed and stirred to prepare a slurry for forming layer b.

[0081] (3) Formation of Layer a and Layer b As a flow-through type substrate, a cordierite honeycomb porous substrate (manufactured by Nippon Gaishi Co., Ltd., axial length: 80 mm, outer diameter: 93 mm, volume: 0.54 L, cell density: 900 cells / inch) having cells extending in the axial direction and partitioned by partition walls having a thickness of 50 to 70 μm was used. ２ ) was prepared.

[0082] The layer a-forming slurry was applied to the upstream side of the flow-through substrate from the upstream end of the flow-through substrate in the exhaust gas flow direction X by immersing the layer a-forming slurry in the layer a-forming slurry. Next, the layer b-forming slurry was applied to the downstream side of the flow-through substrate from the downstream end of the flow-through substrate in the exhaust gas flow direction X by immersing the layer b-forming slurry in the layer a-forming slurry in the layer b-forming slurry. Next, the flow-through substrate coated with the layer a-forming slurry and the layer b-forming slurry was dried at 90 ° C. and then fired at 450 ° C. In this way, layers a and b were formed on the flow-through substrate without gaps in the exhaust gas flow direction X. The mass of layer a per unit volume (washcoat amount) of the flow-through substrate in the portion where layer a was formed was 127.8 g / L. The mass of layer b per unit volume (washcoat amount) of the flow-through substrate in the portion where layer b was formed was 127.8 g / L. The washcoat amount means the mass of the catalyst layer after calcination.

[0083] (4) Preparation of Slurry for Forming Layer c: 0.1 parts by mass of dinitrodiamine platinum (II) solution in terms of Pt metal, 0.2 parts by mass of rhodium nitrate aqueous solution in terms of Rh metal, and 0.1 parts by mass of Ce-Zr based composite oxide (CeO ２ Conversion content: 15 mass% ZrO ２ 15.0 parts by mass of Zr-based oxide (CeO ２ Conversion content: 0 mass%, ZrO ２ 74.4 parts by mass of neodymium nitrate in oxide equivalent: 83% by mass, 17% by mass of one or more rare earth elements other than Ce in oxide equivalent: 17%, 0.3 parts by mass of neodymium nitrate in oxide equivalent, a binder containing alumina sol (Al in the solid content ２ O ３ The slurry for forming layer c was prepared by adding 10 parts by mass of Al (content: 100% by mass) converted to solid content, mixing and stirring. ２ O ３ This was calculated to be 10.0% by mass.

[0084] (5) Formation of Layer c The entire flow-through substrate on which layers a and b were formed was immersed in the layer c-forming slurry, and the layer c-forming slurry was applied to the entire flow-through substrate on which layers a and b were formed. Next, the flow-through substrate on which the layer c-forming slurry was applied was dried at 90°C and then fired at 450°C. In this way, layer c was formed on layers a and b. The mass (washcoat amount) of the upper catalyst layer per unit volume of the portion of the substrate on which layer c was formed was 100g / L. The washcoat amount means the mass of the catalyst layer after firing.

[0085] In this manner, an exhaust gas purification catalyst of Example 1 was produced, which included a flow-through type substrate, a layer a provided on the upstream side of the flow-through type substrate, a layer b provided on the downstream side of the flow-through type substrate, and a layer c provided on the upper sides of layers a and b. The schematic diagram of the exhaust gas purification catalyst of Example 1 is the same as that of exhaust gas purification catalyst 10 in FIG.

[0086] Hereinafter, in the exhaust gas purification catalysts of Examples and Comparative Examples, layer a and a layer located corresponding to layer a will also be referred to as a "lower-layer upstream catalyst layer." In the exhaust gas purification catalysts of Examples and Comparative Examples, layer b and a layer located corresponding to layer b will also be referred to as a "lower-layer downstream catalyst layer." In the exhaust gas purification catalysts of Examples and Comparative Examples, layer c and a layer located corresponding to layer c will also be referred to as an "upper catalyst layer." Furthermore, in the exhaust gas purification catalyst of Comparative Example 1, the layer provided between the substrate and the upper catalyst layer (layer c) will also be referred to as a "lower catalyst layer."

[0087] In the exhaust gas purification catalyst of Example 1, the mass content C of Pt contained in layer b Ｐｔ Mass content C of Pd contained in layer a Ｐｄ C is the ratio of Ｐｄ / C Ｐｔ The mass M of the Ce—Zr-based composite oxide contained in layer c was 3.0. ＣＺ The mass M of Zr-based oxide contained in layer c ＺｒＯ２ M is the ratio of ＺｒＯ２ / M ＣＺ The mass content C of the Zr-based oxide contained in the layer c was 5.0. ＺｒＯ２The mass content C of Pd contained in layer a Ｐｄ C is the ratio of Ｐｄ / C ＺｒＯ２ The mass content C of the Zr-based oxide contained in the layer c was 0.0161. ＺｒＯ２ The mass content C of Pt contained in layer b Ｐｔ C is the ratio of Ｐｔ / C ＺｒＯ２ The mass percentage of Rh supported on the Zr-based oxide in layer c, based on the total mass of Rh contained in layer c, was 74.8 mass %. The mass percentage of Rh supported on the Ce—Zr-based composite oxide in layer c, based on the total mass of Rh contained in layer c, was 15.1 mass %.

[0088] Example 2 (6) Preparation of Slurry for Forming Layer c A dinitrodiamine platinum (II) solution of 0.1 parts by mass in terms of Pt metal, a rhodium nitrate aqueous solution of 0.2 parts by mass in terms of Rh metal, a Ce-Zr composite oxide (CeO ２ Conversion content: 15 mass% ZrO ２ 45.0 parts by mass of Zr-based oxide (CeO ２ Conversion content: 0 mass%, ZrO ２ 44.4 parts by mass of neodymium nitrate in oxide equivalent: 83% by mass, 17% by mass of one or more rare earth elements other than Ce in oxide equivalent: 17%, 0.3 parts by mass of neodymium nitrate in oxide equivalent, a binder containing alumina sol (Al in the solid content ２ O ３ The slurry for forming layer c was prepared by adding 10 parts by mass of Al (content: 100% by mass) converted to solid content, mixing and stirring. ２ O ３ This was calculated to be 10.0% by mass.

[0089] An exhaust gas purifying catalyst of Example 2 was produced in the same manner as in Example 1, except that the slurry for forming layer c produced in the above (6) was used. In the exhaust gas purifying catalyst of Example 2, the mass content C of Pt contained in layer bＰｔ Mass content C of Pd contained in layer a Ｐｄ C is the ratio of Ｐｄ / C Ｐｔ The mass M of the Ce—Zr-based oxide contained in layer c was 3.0. ＣＺ The mass M of Zr-based oxide contained in layer c ＺｒＯ２ M is the ratio of ＺｒＯ２ / M ＣＺ The mass content C of the Zr-based oxide contained in the layer c was 1.0. ＺｒＯ２ The mass content C of Pd contained in layer a Ｐｄ C is the ratio of Ｐｄ / C ＺｒＯ２ The mass content C of the Zr-based oxide contained in the layer c was 0.0270. ＺｒＯ２ The mass content C of Pt contained in layer b Ｐｔ C is the ratio of Ｐｔ / C ＺｒＯ２ The mass percentage of Rh supported on the Zr-based oxide in layer c, based on the total mass of Rh contained in layer c, was 44.7 mass%. The mass percentage of Rh supported on the Ce—Zr-based composite oxide in layer c, based on the total mass of Rh contained in layer c, was 45.3 mass%.

[0090] Comparative Example 1 (7) Preparation of Slurry for Forming Lower Catalyst Layer In pure water, 0.7 parts by mass of an aqueous solution of palladium nitrate in terms of Pd metal, 0.7 parts by mass of a Ce—Zr-based composite oxide (CeO ２ Conversion content: 40 mass% ZrO ２ 70.4 parts by mass of one or more rare earth elements other than Ce in terms of oxides: 10% by mass), La ２ O ３ Modified alumina (La ２ O ３ 14.7 parts by mass of barium hydroxide (1% by mass), 6.1 parts by mass of barium hydroxide in terms of oxide, and a binder containing alumina sol (Al in the solid content ２ O ３ The mixture was mixed and stirred to prepare a slurry for forming a lower catalyst layer.

[0091] (8) Formation of Lower Catalyst Layer A honeycomb porous substrate made of cordierite similar to that in Example 1 was prepared as a flow-through type substrate.

[0092] The entire flow-through substrate was immersed in the lower catalyst layer forming slurry, and the lower catalyst layer forming slurry was coated on the entire flow-through substrate. The flow-through substrate coated with the lower catalyst layer forming slurry was then dried at 90°C and calcined at 450°C. In this manner, a lower catalyst layer was formed on the flow-through substrate in the exhaust gas flow direction X. That is, the lower catalyst layer of Comparative Example 1 consisted of only one layer, and not two layers, a lower upstream catalyst layer and a lower downstream catalyst layer, as in the Examples and other Comparative Examples. The mass of the lower catalyst layer per unit volume (washcoat amount) of the flow-through substrate in the portion where the lower catalyst layer was formed was 127.8 g / L.

[0093] An exhaust gas purifying catalyst of Comparative Example 1 was produced in the same manner as in Example 1, except that the lower catalyst layers were formed as in (7) and (8) above instead of the layers a and b.

[0094] Comparative Example 2 (9) Preparation of Slurry for Forming Lower Downstream Catalyst Layer In pure water, 0.4 parts by mass of an aqueous solution of palladium nitrate in terms of Pd metal, 0.4 parts by mass of a Ce—Zr-based composite oxide (CeO ２ Conversion content: 40 mass% ZrO ２ 70.4 parts by mass of one or more rare earth elements other than Ce in terms of oxides: 10% by mass), La ２ O ３ Modified alumina (La ２ O ３ 15.0 parts by mass of barium hydroxide (1% by mass), 6.1 parts by mass of barium hydroxide in terms of oxide, and a binder containing alumina sol (Al in the solid content ２ O ３ The mixture was mixed and stirred to prepare a slurry for forming a lower downstream catalyst layer.

[0095] An exhaust gas purifying catalyst of Comparative Example 2 was produced in the same manner as in Example 1, except that the slurry for forming the lower downstream catalyst layer produced in (9) above was used instead of the slurry for forming layer b. In the exhaust gas purifying catalyst of Comparative Example 2, the mass content D of Pd in ​​the lower downstream catalyst layer was Ｐｄ The mass content C of Pd contained in layer a Ｐｄ C is the ratio of Ｐｄ / D Ｐｄ The mass content C of the Zr-based oxide contained in the layer c was 3.0. ＺｒＯ２ The mass content C of Pd contained in layer a Ｐｄ C is the ratio of Ｐｄ / C ＺｒＯ２ was 0.0161.

[0096] Comparative Example 3 (10) Preparation of Slurry for Forming Upper Catalyst Layer In pure water, 0.1 parts by mass of dinitrodiamine platinum (II) solution in terms of Pt metal, 0.2 parts by mass of rhodium nitrate aqueous solution in terms of Rh metal, and 0.1 parts by mass of Ce-Zr based composite oxide (CeO ２ Conversion content: 15 mass% ZrO ２ 45.0 parts by mass of one or more rare earth elements other than Ce in terms of oxides: 15% by mass; ２ O ３ Modified alumina (La ２ O ３ 44.4 parts by mass of 1% by mass of neodymium nitrate in terms of oxide, 0.3 parts by mass of a binder containing alumina sol (Al in the solid content ２ O ３ The slurry for forming the upper catalyst layer was prepared by adding 10 parts by mass of Al (content: 100% by mass) converted to solid content, mixing and stirring. ２ O ３ This was converted to 54.0% by mass.

[0097] An exhaust gas purifying catalyst of Comparative Example 3 was produced in the same manner as in Example 1, except that the slurry for forming the upper catalyst layer produced in (10) above was used instead of the slurry for forming the layer c. In the exhaust gas purifying catalyst of Comparative Example 3, the mass content C of Pt contained in the layer b ＰｔThe mass content C of Pd contained in layer a Ｐｄ C is the ratio of Ｐｄ / C Ｐｔ The mass percentage of Rh supported on the Zr-based oxide in layer c, based on the total mass of Rh contained in layer c, was 0 mass %. The mass percentage of Rh supported on the Ce—Zr-based composite oxide in layer c, based on the total mass of Rh contained in layer c, was 45.3 mass %.

[0098] Comparative Example 4 (11) Preparation of Slurry for Forming Upper Catalyst Layer In pure water, 0.1 parts by mass of dinitrodiamine platinum (II) solution in terms of Pt metal, 0.2 parts by mass of rhodium nitrate aqueous solution in terms of Rh metal, and 0.1 parts by mass of Zr-based oxide (CeO ２ Conversion content: 0 mass%, ZrO ２ 45.0 parts by mass of one or more rare earth elements other than Ce in terms of oxides: 17% by mass; ２ O ３ Modified alumina (La ２ O ３ 44.4 parts by mass of 1% by mass of neodymium nitrate in terms of oxide, 0.3 parts by mass of a binder containing alumina sol (Al in the solid content ２ O ３ The slurry for forming the upper catalyst layer was prepared by adding 10 parts by mass of Al (content: 100% by mass) converted to solid content, mixing and stirring. ２ O ３ This was converted to 54.0% by mass.

[0099] An exhaust gas purifying catalyst of Comparative Example 4 was produced in the same manner as in Example 1, except that the slurry for forming the upper catalyst layer produced in (11) above was used instead of the slurry for forming the layer c. In the exhaust gas purifying catalyst of Comparative Example 4, the mass content C of Pt contained in the layer b Ｐｔ The mass content C of Pd contained in layer a Ｐｄ C is the ratio of Ｐｄ / C Ｐｔ The mass content D of the Zr-based oxide contained in the upper catalyst layer was 3.0. ＺｒＯ２The mass content C of Pd contained in layer a Ｐｄ C is the ratio of Ｐｄ / D ＺｒＯ２ The mass content D of the Zr-based oxide contained in the upper catalyst layer was 0.0267. ＺｒＯ２ The mass content C of Pt contained in layer b Ｐｔ C is the ratio of Ｐｔ / D ＺｒＯ２ The mass percentage of Rh supported on the Zr-based oxide in layer c, based on the total mass of Rh contained in layer c, was 45.3 mass%. The mass percentage of Rh supported on the Ce—Zr-based composite oxide in layer c, based on the total mass of Rh contained in layer c, was 0 mass%.

[0100] Comparative Example 5 (12) Preparation of Slurry for Forming Upper Catalyst Layer In pure water, 0.1 parts by mass of dinitrodiamine platinum (II) solution in terms of Pt metal, 0.2 parts by mass of rhodium nitrate aqueous solution in terms of Rh metal, and 0.1 parts by mass of Ce-Zr based composite oxide (CeO ２ Conversion content: 15 mass% ZrO ２ 89.4 parts by mass of neodymium nitrate in oxide equivalent: 70% by mass, 15% by mass of oxide equivalent of one or more rare earth elements other than Ce, 0.3 parts by mass of neodymium nitrate in oxide equivalent, a binder containing alumina sol (Al in the solid content ２ O ３ The slurry for forming the upper catalyst layer was prepared by adding 10 parts by mass of Al (content: 100% by mass) converted to solid content, mixing and stirring. ２ O ３ This was calculated to be 10.0% by mass.

[0101] An exhaust gas purifying catalyst of Comparative Example 5 was produced in the same manner as in Example 1, except that the slurry for forming the upper catalyst layer produced in (12) above was used instead of the slurry for forming the layer c. In the exhaust gas purifying catalyst of Comparative Example 5, the mass content C of Pt contained in the layer b Ｐｔ The mass content C of Pd contained in layer a Ｐｄ C is the ratio of Ｐｄ / C ＰｔThe mass percentage of Rh supported on the Zr-based oxide in layer c, based on the total mass of Rh contained in layer c, was 0 mass %. The mass percentage of Rh supported on the Ce—Zr-based composite oxide in layer c, based on the total mass of Rh contained in layer c, was 89.9 mass %.

[0102] Comparative Example 6 (13) Preparation of Slurry for Forming Upper Catalyst Layer A dinitrodiamine platinum (II) solution of 0.1 parts by mass in terms of Pt metal, a rhodium nitrate aqueous solution of 0.2 parts by mass in terms of Rh metal, a Zr-based oxide (CeO ２ Conversion content: 0 mass%, ZrO ２ 89.4 parts by mass of neodymium nitrate in oxide equivalent: 83% by mass, 17% by mass of one or more rare earth elements other than Ce in oxide equivalent: 17%, 0.3 parts by mass of neodymium nitrate in oxide equivalent, a binder containing alumina sol (Al in the solid content ２ O ３ The slurry for forming the upper catalyst layer was prepared by adding 10 parts by mass of Al (content: 100% by mass) converted to solid content, mixing and stirring. ２ O ３ This was calculated to be 10.0% by mass.

[0103] An exhaust gas purifying catalyst of Comparative Example 6 was produced in the same manner as in Example 1, except that the slurry for forming the upper catalyst layer produced in (13) above was used instead of the slurry for forming the layer c. In the exhaust gas purifying catalyst of Comparative Example 6, the mass content C of Pt contained in the layer b Ｐｔ The mass content C of Pd contained in layer a Ｐｄ C is the ratio of Ｐｄ / C Ｐｔ The mass content D of the Zr-based oxide contained in the upper catalyst layer was 3.0. ＺｒＯ２ The mass content C of Pd contained in layer a Ｐｄ C is the ratio of Ｐｄ / D ＺｒＯ２ The mass content D of the Zr-based oxide contained in the upper catalyst layer was 0.0134. ＺｒＯ２ The mass content C of Pt contained in layer b Ｐｔ C is the ratio ofＰｔ / D ＺｒＯ２ The mass percentage of Rh supported on the Zr-based oxide in layer c, based on the total mass of Rh contained in layer c, was 89.9 mass %. The mass percentage of Rh supported on the Ce—Zr-based composite oxide in layer c, based on the total mass of Rh contained in layer c, was 0 mass %.

[0104] (Durability Treatment) Each of the exhaust gas purification catalysts of Examples 1 and 2 and Comparative Examples 1 to 6 was mounted in an exhaust pipe, and this exhaust pipe was set in a gasoline engine. The engine speed / torque, etc. were adjusted so that the catalyst temperature would reach 950°C, and durability treatment was carried out for 50 hours.

[0105] (Evaluation of Exhaust Gas Purification Performance) A vehicle equipped with the durability-treated exhaust gas purification catalyst was driven under the driving conditions of the World Wide Harmonized Exhaust Gas Test Mode (WLTC). The emissions (emission values) of non-methane hydrocarbons (NMHC) and nitrogen oxides (NOx) in the exhaust gas that passed through the exhaust gas purification catalyst were measured, and the NMHC and NOx emissions per unit driving distance (g / km) were calculated. A gasoline-powered vehicle was used, and certification test fuel was used as the gasoline. An exhaust gas measurement device manufactured by Horiba, Ltd. was used. Table 1 shows the NMHC emissions (g / km) and NOx emissions (g / km) per unit driving distance at 100 seconds into the test mode. The NMHC emissions and NOx emissions at 100 seconds into the test mode are considered to represent the NMHC emissions and NOx emissions, respectively, immediately after the internal combustion engine is started.

[0106] In Table 1, "precious metal contained in lower-layer downstream catalytic layer" refers to the precious metal that is a catalytically active component contained in the lower-layer downstream catalytic layer. Although the exhaust gas purification catalyst of Comparative Example 1 does not have a lower-layer downstream catalytic layer, Table 1 lists the precious metal contained in the lower catalytic layer for convenience. In Table 1, "(A)" represents the ratio of the mass content of the precious metal contained in the lower-layer upstream catalytic layer to the mass content of the precious metal contained in the lower-layer downstream catalytic layer. Since the exhaust gas purification catalyst of Comparative Example 1 does not have a lower-layer upstream catalytic layer or a lower-layer downstream catalytic layer, "(A)" cannot be calculated. However, since the exhaust gas purification catalyst of Comparative Example 1 has a lower catalytic layer consisting of only one type of layer, "(A)" for Comparative Example 1 was set to 1.0 for convenience. In Table 1, "(B)" represents the Al content of Al contained in the upper catalytic layer relative to the Al content of the upper catalytic layer. ２ O ３ The mass content of the Pd in ​​the lower upstream catalytic layer is the mass ratio (mass %) of the Zr-based oxide in the upper catalytic layer to the mass of the Ce—Zr-based composite oxide in the upper catalytic layer. In Table 1, "(C)" represents the ratio of the mass of the Zr-based oxide in the upper catalytic layer to the mass of the Ce—Zr-based composite oxide in the upper catalytic layer. In Table 1, "(D)" represents the ratio of the mass of the Pd in ​​the lower upstream catalytic layer to the mass of the Zr-based oxide in the upper catalytic layer. The mass content of the Zr-based oxide in the upper catalytic layer is the mass ratio (mass %) of the Zr-based oxide in the upper catalytic layer to the mass of the upper catalytic layer. The mass content of the Pd in ​​the lower upstream catalytic layer is the mass ratio (mass %) of the Pd in ​​the lower upstream catalytic layer to the mass of the lower upstream catalytic layer. In Table 1, "(E)" represents the ratio of the mass content of the Pt in the lower downstream catalytic layer to the mass content of the Zr-based oxide in the upper catalytic layer. In Table 1, "(F)" represents the NMHC emissions per unit driving distance (g / km) at 100 seconds into the test mode. In Table 1, "(G)" represents the amount of NOx emissions per unit driving distance (g / km) at 100 seconds during the test mode. In Table 1, "(H)" represents the mass percentage of Rh supported on the Zr-based oxide in the upper catalyst layer, based on the total mass of Rh contained in the upper catalyst layer. In Table 1, "(I)" represents the mass percentage of Rh supported on the Ce-Zr-based composite oxide in the upper catalyst layer, based on the total mass of Rh contained in the upper catalyst layer.

[0107]

[0108] From Table 1, it was found that the exhaust gas purification catalysts of Examples 1 and 2 had small "(F)" values ​​and "(G)" values, and were able to achieve both NOx purification performance immediately after starting the internal combustion engine and NMHC purification performance immediately after starting the internal combustion engine. In particular, Example 1 had even smaller "(F)" and "(G)" values ​​than Example 2, and was found to be particularly excellent in NOx purification performance and NMHC purification performance immediately after starting the internal combustion engine. The exhaust gas purification catalysts of Comparative Examples 1 to 6 had larger "(G)" values ​​than the Examples, and were found to have lower NOx purification performance immediately after starting the internal combustion engine. Furthermore, it was found that the exhaust gas purification catalysts of Comparative Examples 1, 5, and 6 also had larger "(F)" values ​​than the Examples, and were also found to have lower NMHC purification performance immediately after starting the internal combustion engine.

[0109] According to the present invention, it is possible to provide an exhaust gas purification catalyst that can achieve both NOx purification performance immediately after starting an internal combustion engine and NMHC purification performance immediately after starting an internal combustion engine.

[0110] Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. This application is based on a Japanese patent application (Patent Application No. 2023-204822) filed on December 4, 2023, the contents of which are incorporated herein by reference.

[0111] S1, S2 substrate a layer a b layer b c, c1, c2 layer c d other layers 1, 2, 3, 4, 5, 6 catalyst layer 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 exhaust gas purification catalyst X flow direction of exhaust gas

Claims

1. An exhaust gas purification catalyst having a substrate and a catalyst layer provided on the substrate, the catalyst layer including a layer a, a layer b, and a layer c, the layer a being provided upstream of the layer b in a flow direction of exhaust gas, the layer c being laminated with at least one of the layers a and b, the layer c being provided at a position farther from the substrate than the layer a and at least one of the layers b in a laminated portion of the layer c and the layer a and at least one of the layers b, the layer a including Pd, the layer b including Pt, and a mass content C of Pt included in the layer b. Ｐｔ The mass content C of Pd contained in the layer a Ｐｄ C is the ratio of Ｐｄ / C Ｐｔ is greater than 1, the layer c contains Rh, a Ce—Zr-based composite oxide, and a Zr-based oxide, and in the layer c, at least a part of the Rh is supported on the Zr-based oxide.

2. The layer c does not contain Al, or if it contains Al, the Al content of Al is ２ O ３ 2. The exhaust gas purifying catalyst according to claim 1, wherein the content in terms of the converted amount is 22 mass % or less based on the mass of said layer c.

3. The mass M of the Ce-Zr based composite oxide contained in the layer c ＣＺ The mass M of the Zr-based oxide contained in the layer c ＺｒＯ２ The ratio of M ＺｒＯ２ / M ＣＺ The exhaust gas purifying catalyst according to claim 1 or 2, wherein the ratio of the stoichiometric ratio to the total stoichiometric ratio is 1.5 or more and 6.0 or less.

4. Mass content C of Zr-based oxide contained in the layer c ＺｒＯ２ The mass content C of Pd contained in the layer a Ｐｄ C is the ratio of Ｐｄ / C ＺｒＯ２ The exhaust gas purifying catalyst according to claim 1 or 2, wherein is 0.015 or more and 0.120 or less.

5. Mass content C of Zr-based oxide contained in the layer c ＺｒＯ２ The mass content C of Pt contained in the layer b Ｐｔ C is the ratio of Ｐｔ / C ＺｒＯ２ The exhaust gas purifying catalyst according to claim 1 or 2, wherein is 0.001 or more and 0.008 or less.

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