Catalyst for exhaust gas purification

Abstract

The present invention addresses the problem of providing an exhaust gas purification catalyst which does not undergo the deterioration in exhaust gas purification performance due to phosphorus poisoning. The present invention provides an exhaust gas purification catalyst (1A) provided with a base material (10) and first and second catalyst layers (20, 30) both arranged on the base material (10), in which the second catalyst layer (30) is provided above or on the upstream side of the first catalyst layer (20), the first catalyst layer (20) contains at least one element selected from Pd and Pt, the second catalyst layer (30) contains Rh, Al and barium sulfate, the mole-based ratio of the content of Al in the second catalyst layer (30) to the total content of all of the metal elements in the second catalyst layer (30) is 0.55 to 0.85 inclusive, and the mole-based ratio of the content of Rh in the second catalyst layer (30) to the content of Ba in the second catalyst layer (30) is 0.0040 to 0.040 inclusive.

Description

Technical Field

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

Background Art

[0002] Exhaust gas discharged from internal combustion engines such as automobiles and motorcycles contains harmful components such as hydrocarbons (THC), carbon monoxide (CO), and nitrogen oxides (NOx). A three-way catalyst is used for the purpose of purifying these harmful components and rendering them harmless. As the three-way catalyst, a catalyst containing noble metal elements such as Pt, Pd, and Rh is used.

[0003] Pt and Pd are easily poisoned by phosphorus. For this reason, a second catalyst layer containing Rh is provided on the upper side or upstream side of a first catalyst layer containing Pd and / or Pt, and a phosphorus scavenger is included in the second catalyst layer, thereby preventing poisoning of Pd and / or Pt in the first catalyst layer. Techniques are known.

[0004] As the phosphorus scavenger, for example, alkaline earth metal elements such as Ba are used.

[0005] However, when a water-soluble barium salt such as barium acetate is used, the dispersibility of Ba in the catalyst layer is high, so it is easy to contact Rh, and Rh is easily stabilized in the oxidized state, thereby preventing Rh from becoming a metallic state. As a result, the exhaust gas purification performance of Rh contained in the second catalyst layer deteriorates.

[0006] On the other hand, techniques are known in which a water-insoluble barium salt such as barium sulfate is used as the Ba source (for example, Patent Documents 1 and 2). When a water-insoluble barium salt such as barium sulfate is used as the Ba source, the dispersibility of Ba in the catalyst layer is low, so a decrease in the exhaust gas purification performance of Rh contained in the second catalyst layer due to stabilization of the oxidized state of Rh is prevented, but the phosphorus capture ability of the second catalyst layer decreases. As a result, the exhaust gas purification performance of Pt and / or Pd contained in the first catalyst layer deteriorates due to phosphorus poisoning.

Prior Art Documents

Patent Document

[0007]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0008] Therefore, the present invention provides an exhaust gas purification catalyst including a base material and first and second catalyst layers provided on the base material, wherein the second catalyst layer is provided on the upper side or upstream side of the first catalyst layer, the first catalyst layer contains at least one selected from Pd and Pt, and the second catalyst layer contains Rh and barium sulfate, and aims to provide an exhaust gas purification catalyst capable of preventing a decrease in exhaust gas purification performance due to phosphorus poisoning.

Means for Solving the Problems

[0009] In order to solve the above problems, the present invention provides an exhaust gas purification catalyst including a base material and first and second catalyst layers provided on the base material, wherein the second catalyst layer is provided on the upper side or upstream side of the first catalyst layer, the first catalyst layer contains at least one selected from Pd and Pt, the second catalyst layer contains Rh, Al, and barium sulfate, the ratio of the content of Al in the second catalyst layer to the total content of all metal elements in the second catalyst layer is 0.55 or more and 0.85 or less in terms of molar ratio, and the ratio of the content of Rh in the second catalyst layer to the content of Ba in the second catalyst layer is 0.0040 or more and 0.040 or less in terms of molar ratio.

Effects of the Invention

[0010] According to the present invention, there is provided an exhaust gas purification catalyst capable of preventing a decrease in exhaust gas purification performance due to phosphorus poisoning.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Mode for Carrying Out the Invention

[0012] ≪Explanation of Terms≫ Hereinafter, terms used in this specification will be explained.

[0013] <BET Specific Surface Area> The BET specific surface area is measured according to the "one-point method" in "(3.5) One-point method" in "6.2 Flow method" of JIS R1626 "Method for Measuring Specific Surface Area of Fine Ceramic Powders by Gas Adsorption BET Method". As the gas, a nitrogen-helium mixed gas containing 30% by volume of nitrogen as the adsorption gas and 70% by volume of helium as the carrier gas is used. As the measuring device, "BELSORP-MR6" manufactured by Microtrac Bell is used.

[0014] <Rare Earth Element> Examples of rare earth elements include Ce, Y, Pr, Sc, La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, etc. In this specification, rare earth elements may be denoted as "Ln".

[0015] <Oxide> The oxide of Al is Al2O3, the oxide of Si is SiO2, the oxide of B is B2O3, the oxide of Zr is ZrO2, the oxide of Cr is Cr2O3, the oxide of Mg is MgO, the oxide of Ca is CaO, the oxide of Sr is SrO, the oxide of Ba is BaO, the oxide of Fe is Fe3O4, the oxide of Mn is Mn3O4, and the oxide of Ni is NiO. The oxides of rare earth elements mean sesquioxides (Ln2O3) except for the oxides of Ce, Pr and Tb. The oxide of Ce is CeO2, the oxide of Pr is Pr6O 11 , and the oxide of Tb means Tb4O7.

[0016] <Al-based oxide> Al-based oxide means an oxide containing Al.

[0017] Al-based oxides are, for example, particulate.

[0018] Al-based oxides are used as carriers for catalytic active components. From the viewpoint of improving the supportability of catalytic active components, Al-based oxides are preferably porous. Al-based oxides are distinguished from alumina used as a binder (hereinafter referred to as "alumina binder"). Alumina binder is derived from alumina sol used as a catalytic layer forming material.

[0019] Al-based oxides may or may not contain elements other than Al and O.

[0020] Elements other than Al and O can be selected, for example, from B, Si, rare earth elements (such as Y, Ce, La, Nd, Pr, etc.), Zr, Cr, alkaline earth metal elements (such as Mg, Ca, Sr, Ba, etc.). From the viewpoint of improving the heat resistance of Al-based oxides, it is preferable to select from Y, Ce, La, Nd, Pr, etc.

[0021] Examples of the Al-based oxide include alumina (oxide consisting only of Al and O), an oxide obtained by modifying the surface of alumina with elements other than Al and O, an oxide obtained by solid-solubilizing elements other than Al and O in alumina, and the like. Specific examples of the Al-based oxide containing elements other than Al and O include alumina-silica, alumina-silicate, alumina-zirconia, alumina-chromia, alumina-ceria, alumina-lanthana, and the like.

[0022] In the Al-based oxide, elements other than Al and O may form a solid solution phase together with Al and O, may form a single phase that is a crystalline phase or an amorphous phase (for example, an oxide phase of elements other than Al and O), or may form both a solid solution phase and a single phase.

[0023] From the viewpoint of improving the heat resistance of the Al-based oxide, the content of Al in terms of oxide in the Al-based oxide is preferably 50% by mass or more, more preferably 60% by mass or more, still more preferably 70% by mass or more, still more preferably 80% by mass or more, still more preferably 90% by mass or more, still more preferably 95% by mass or more, based on the mass of the Al-based oxide. The upper limit is 100% by mass.

[0024] When the Al-based oxide contains elements other than Al and O, from the viewpoint of improving the heat resistance of the Al-based oxide, the content of the elements other than Al and O in terms of oxide in the Al-based oxide is preferably 0.1% by mass or more and 50% by mass or less, more preferably 0.1% by mass or more and 40% by mass or less, still more preferably 0.1% by mass or more and 30% by mass or less, still more preferably 0.2% by mass or more and 20% by mass or less, still more preferably 0.3% by mass or more and 10% by mass or less, still more preferably 0.5% by mass or more and 5% by mass or less, based on the mass of the Al-based oxide. The "content of elements other than Al and O in terms of oxide" means the total content of the two or more elements other than Al and O in terms of oxide when the Al-based oxide contains two or more elements other than Al and O.

[0025] <Ce-based oxide> The Ce-based oxide means an oxide containing Ce. In addition, for an oxide containing Al and Ce, when the content rate in terms of the oxide of Al is the same as or greater than the content rate in terms of the oxide of Ce, it shall be regarded as an Al-containing oxide, and when the content rate in terms of the oxide of Al is less than the content rate in terms of the oxide of Ce, it shall be regarded as a Ce-containing oxide.

[0026] The Ce-based oxide is, for example, particulate.

[0027] The Ce-based oxide is used as a carrier for a catalytic active component. From the viewpoint of improving the supportability of the catalytic active component, the Ce-based oxide is preferably porous. The Ce-based oxide is distinguished from ceria (hereinafter referred to as "ceria binder") used as a binder. The ceria binder is derived from a ceria sol used as a catalytic layer forming material.

[0028] The Ce-based oxide may or may not contain elements other than Ce and O.

[0029] Elements other than Ce and O can be selected, for example, from rare earth elements other than Ce, alkaline earth metal elements (for example, Mg, Ca, Sr, Ba, etc.), Fe, Mn, Ni, Zr, Al, etc.

[0030] Examples of the Ce-based oxide include ceria (an oxide composed only of Ce and O), an oxide obtained by modifying the surface of ceria with elements other than Ce and O, an oxide obtained by dissolving elements other than Ce and O in ceria, and the like.

[0031] In the Ce-based oxide, elements other than Ce and O may form a solid solution phase together with Ce and O, may form a single phase (for example, an oxide phase of elements other than Ce and O) that is a crystalline phase or an amorphous phase, or may form both a solid solution phase and a single phase.

[0032] In a Ce-based oxide according to one embodiment (hereinafter referred to as "the first Ce-based oxide"), the content of Ce in terms of oxide is preferably more than 90% by mass, more preferably 95% by mass or more, still more preferably 99% by mass or more, based on the mass of the first Ce-based oxide. The upper limit is 100% by mass.

[0033] When the first Ce-based oxide contains elements other than Ce and O, from the viewpoint of improving the oxygen storage capacity, the content of elements other than Ce and O in the first Ce-based oxide in terms of oxide is preferably more than 0% by mass and less than 10% by mass, more preferably more than 0% by mass and 5% by mass or less, still more preferably more than 0% by mass and 1% by mass or less, based on the mass of the first Ce-based oxide. The content of elements other than Ce and O in terms of oxide may be, for example, 0.1% by mass or more, 0.2% by mass or more, 0.3% by mass or more, etc. "The content of elements other than Ce and O in terms of oxide" means the total content in terms of oxide of two or more elements other than Ce and O when the first Ce-based oxide contains two or more elements other than Ce and O.

[0034] In a Ce-based oxide according to another embodiment (hereinafter referred to as "the second Ce-based oxide"), the content of Ce in terms of oxide is preferably 1% by mass or more and 90% by mass or less, more preferably 2% by mass or more and 50% by mass or less, still more preferably 3% by mass or more and 45% by mass or less, based on the mass of the second Ce-based oxide.

[0035] From the viewpoint of improving the heat resistance of the Ce-based oxide, the content of elements other than Ce and O in the second Ce-based oxide in terms of oxide is preferably 10% by mass or more and 99% by mass or less, more preferably 50% by mass or more and 98% by mass or less, still more preferably 55% by mass or more and 97% by mass or less, based on the mass of the second Ce-based oxide. "The content of elements other than Ce and O in terms of oxide" means the total content in terms of oxide of two or more elements other than Ce and O when the second Ce-based oxide contains two or more elements other than Ce and O.

[0036] <Ce-Zr-based oxide> The Ce-Zr-based oxide is a kind of Ce-based oxide (preferably a kind of the second Ce-based oxide), and means a composite oxide containing Zr in addition to Ce.

[0037] The Ce-Zr-based oxide is, for example, particulate.

[0038] The Ce-Zr-based oxide is used as a carrier for a catalytic active component. From the viewpoint of improving the supportability of the catalytic active component, the Ce-Zr-based oxide is preferably porous.

[0039] In the Ce-Zr-based oxide, Ce may form a solid solution phase together with Zr and O, may form a single phase (for example, a CeO2 single phase) that is a crystalline phase or an amorphous phase, or may form both a solid solution phase and a single phase, but at least a part of Ce preferably forms a solid solution phase.

[0040] In the Ce-Zr-based oxide, Zr may form a solid solution phase together with Ce and O, may form a single phase (for example, a ZrO2 single phase) that is a crystalline phase or an amorphous phase, or may form both a solid solution phase and a single phase, but at least a part of Zr preferably forms a solid solution phase.

[0041] The Ce-Zr-based composite oxide has the ability to store oxygen when the oxygen concentration in the exhaust gas is high and release oxygen when the oxygen concentration in the exhaust gas is low (that is, oxygen storage capacity), and relaxes the fluctuation of the oxygen concentration in the exhaust gas to expand the operating window of the catalytic active component. Therefore, when the Ce-Zr-based composite oxide is used, the exhaust gas purification ability of the exhaust gas purification catalyst is improved. Zr mainly contributes to the improvement of the heat resistance of the Ce-Zr-based oxide, and Ce mainly contributes to the improvement of the oxygen storage capacity of the Ce-Zr-based oxide.

[0042] From the perspective of improving heat resistance and oxygen storage capacity, the total content of Ce and Zr in terms of oxides in the Ce-Zr based oxide is preferably 60% by mass or more, more preferably 70% by mass or more, still more preferably 80% by mass or more, based on the mass of the Ce-Zr based oxide. The upper limit is 100% by mass.

[0043] From the perspective of improving oxygen storage capacity, the content of Ce in terms of oxide in the Ce-Zr based oxide is preferably 5% by mass or more and 90% by mass or less, more preferably 7% by mass or more and 60% by mass or less, still more preferably 10% by mass or more and 50% by mass or less, based on the mass of the Ce-Zr based oxide.

[0044] From the perspective of improving heat resistance, the content of Zr in terms of oxide in the Ce-Zr based oxide is preferably 10% by mass or more and 95% by mass or less, more preferably 40% by mass or more and 95% by mass or less, still more preferably 50% by mass or more and 90% by mass or less, based on the mass of the Ce-Zr based oxide.

[0045] The Ce-Zr based oxide may contain rare earth elements (Ln) other than Ce, alkaline earth metal elements (e.g., Mg, Ca, Sr, Ba, etc.), Fe, Mn, Ni, Al, etc.

[0046] When the Ce-Zr based oxide contains Ln other than Ce, from the perspective of improving the heat resistance of the Ce-Zr based oxide, Ln other than Ce is preferably selected from La, Nd, Pr, Y, Gd, Sm, etc., and more preferably selected from La, Nd, Pr, Y, etc.

[0047] When the Ce-Zr based oxide contains Ln other than Ce, Ln other than Ce may form a solid solution phase together with Zr and / or Ce and O, or may form a single phase in the crystalline phase or amorphous phase (e.g., a single phase of an oxide of Ln other than Ce), or may form both a solid solution phase and a single phase, but at least a part of Ln other than Ce preferably forms a solid solution phase.

[0048] When the Ce-Zr-based oxide contains Ln other than Ce, from the viewpoint of improving heat resistance, the content of Ln other than Ce in the Ce-Zr-based oxide in terms of the oxide is preferably 1% by mass or more and 40% by mass or less, more preferably 3% by mass or more and 30% by mass or less, and even more preferably 5% by mass or more and 20% by mass or less based on the mass of the Ce-Zr-based oxide. When the Ce-Zr-based oxide contains two or more kinds of Ln other than Ce, the "content of Ln other than Ce in terms of the oxide" means the total content of the two or more kinds of Ln other than Ce in terms of the oxide.

[0049] When the Ce-Zr-based oxide contains Al, Al may form a solid solution phase with Zr and / or Ce and O, or may form a single phase (for example, a single phase of an Al oxide) that is a crystal phase or an amorphous phase, or may form both a solid solution phase and a single phase, but at least a part of Al preferably forms a solid solution phase.

[0050] When the Ce-Zr-based oxide contains Al, from the viewpoint of improving the heat resistance of the Ce-Zr-based oxide, the content of Al in the Ce-Zr-based oxide in terms of the oxide is preferably 3% by mass or more and 40% by mass or less, more preferably 5% 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 oxide.

[0051] ≪First Embodiment≫ Hereinafter, based on FIGS. 1 to 4, the exhaust gas purification catalyst 1A according to the first embodiment of the present invention will be described.

[0052] As shown in FIG. 1, the exhaust gas purification catalyst 1A is disposed in an exhaust passage in the exhaust pipe P of an internal combustion engine. The internal combustion engine is, for example, a gasoline engine or the like. The exhaust gas discharged from the internal combustion engine flows through the exhaust passage in the exhaust pipe P from one end to the other end of the exhaust pipe P and is purified by the exhaust gas purification catalyst 1A provided in the exhaust pipe P. In the drawing, the exhaust gas flow direction is indicated by the symbol X. In this specification, the upstream side in the exhaust gas flow direction X may be referred to as the "exhaust gas inflow side", and the downstream side in the exhaust gas flow direction X may be referred to as the "exhaust gas outflow side".

[0053] In the exhaust passage in the exhaust pipe P, other exhaust gas purification catalysts may be arranged together with the exhaust gas purification catalyst 1A. For example, the exhaust gas purification catalyst 1A may be arranged on the upstream side of the exhaust passage in the exhaust pipe P, and other exhaust gas purification catalysts may be arranged on the downstream side of the exhaust passage in the exhaust pipe P.

[0054] As shown in FIGS. 2 to 4, the exhaust gas purification catalyst 1A includes a substrate 10, a first catalyst layer 20 provided on the substrate 10, and a second catalyst layer 30 provided on the substrate 10.

[0055] <substrate> Hereinafter, the substrate 10 will be described.

[0056] The material constituting the substrate 10 can be appropriately selected. The material constituting the substrate 10 is preferably a material that can stably maintain the shape of the substrate even when the substrate is exposed to high-temperature exhaust gas. Examples of the material constituting the substrate 10 include ceramic materials and metal materials. Examples of the ceramic materials include cordierite, silicon carbide, aluminum titanate, etc. Examples of the metal materials include alloys such as stainless steel.

[0057] In this specification, "high temperature" preferably means a temperature of 850 °C or higher, more preferably 900 °C or higher.

[0058] As shown in FIGS. 2 to 4, the substrate 10 has a cylindrical portion 11, a partition portion 12 provided in the cylindrical portion 11, and cells 13 partitioned by the partition portion 12. The substrate 10 is preferably a honeycomb structure.

[0059] As shown in FIG. 2, the shape of the cylindrical portion 11 is cylindrical, but it may be other shapes such as elliptical cylindrical or polygonal cylindrical.

[0060] As shown in FIGS. 2 to 4, a partition portion 12 exists between adjacent cells 13, and the adjacent cells 13 are partitioned by the partition portion 12. The thickness of the partition portion 12 is, for example, 20 μm or more and 1500 μm or less. The partition portion 12 may have a porous structure through which exhaust gas can pass.

[0061] As shown in FIG. 4, the cell 13 extends in the exhaust gas flow direction X and has an end portion on the exhaust gas inflow side and an end portion on the exhaust gas outflow side.

[0062] As shown in FIG. 4, both the end portion on the exhaust gas inflow side and the end portion on the exhaust gas outflow side of the cell 13 are open. Therefore, the exhaust gas flowing in from the end portion (opening) on the exhaust gas inflow side of the cell 13 flows out from the end portion (opening) on the exhaust gas outflow side of the cell 13. Such a mode is called a flow-through type.

[0063] As shown in FIGS. 2 and 3, the planar shape of the end portion (opening) on the exhaust gas inflow side of the cell 13 is a quadrilateral, but it may be other shapes such as a hexagon or an octagon. The same applies to the planar shape of the end portion (opening) on the exhaust gas outflow side of the cell 13.

[0064] The cell density per square inch of the substrate 10 is, for example, 100 cells or more and 1000 cells or less. The cell density per square inch of the substrate 10 is the total number of cells 13 per square inch in the cross section obtained by cutting the substrate 10 in a plane perpendicular to the exhaust gas flow direction X.

[0065] The volume of the substrate 10 is, for example, 0.1 L or more and 20 L or less. The volume of the substrate 10 means the apparent volume of the substrate 10. When the substrate 10 is cylindrical, if the outer diameter of the substrate 10 is 2r and the length of the substrate 10 is L, the volume of the substrate 10 is expressed by the formula: volume of the substrate 10 = π × r 2 × L.

[0066] The base material 10 may be provided with a first sealing portion that seals the exhaust gas outflow side end portions of some of the cells 13 and a second sealing portion that seals the exhaust gas inflow side end portions of the remaining cells 13. As a result, some of the cells 13 become inflow side cells in which the end portion on the exhaust gas inflow side is open and the end portion on the exhaust gas outflow side is blocked by the first sealing portion, and the remaining cells 13 become outflow side cells in which the end portion on the exhaust gas inflow side is blocked by the second sealing portion and the end portion on the exhaust gas outflow side is open. A plurality (for example, four) of outflow side cells are arranged around one inflow side cell, and the inflow side cell and the outflow side cells arranged around the inflow side cell are partitioned by a porous partition wall portion 12. The exhaust gas that has flowed in from the end portion (opening portion) on the exhaust gas inflow side of the inflow side cell flows out from the end portion (opening portion) on the exhaust gas outflow side of the outflow side cell 13b through the porous partition wall portion 12. Such a mode is called a wall flow type.

[0067] <The first catalyst layer> Hereinafter, the first catalyst layer 20 will be described.

[0068] As shown in FIGS. 3 and 4, the first catalyst layer 20 is provided on the cell 13 side surface of the partition wall portion 12 of the base material 10. The "cell 13 side surface of the partition wall portion 12" means the outer surface of the partition wall portion 12 extending in the exhaust gas flow direction X. The first catalyst layer 20 may be provided directly on the cell 13 side surface of the partition wall portion 12 of the base material 10, or may be provided via another layer, but usually it is provided directly on the cell 13 side surface of the partition wall portion 12 of the base material 10.

[0069] As shown in FIG. 4, the first catalyst layer 20 extends along the exhaust gas flow direction X from the end portion on the exhaust gas inflow side of the partition wall portion 12 to the end portion on the exhaust gas outflow side of the partition wall portion 12. The first catalyst layer 20 may extend along the exhaust gas flow direction X from the end portion on the exhaust gas inflow side of the partition wall portion 12 so as not to reach the end portion on the exhaust gas outflow side of the partition wall portion 12, or may extend along the direction opposite to the exhaust gas flow direction X from the end portion on the exhaust gas outflow side of the partition wall portion 12 so as not to reach the end portion on the exhaust gas inflow side of the partition wall portion 12.

[0070] From the perspective of achieving a good balance between exhaust gas purification performance and cost, the mass of the first catalyst layer 20 per unit volume of the substrate (mass after drying and firing) is preferably 50 g / L or more and 150 g / L or less, more preferably 70 g / L or more and 120 g / L or less, and even more preferably 80 g / L or more and 100 g / L or less.

[0071] The first catalyst layer 20 contains at least one selected from Pd and Pt.

[0072] Pd and Pt are each contained in the first catalyst layer 20 in a form that can function as a catalytic active component, for example, in the form of metallic Pd, metallic Pt, an alloy containing Pd or Pt, a compound containing Pd or Pt (for example, an oxide of Pd or Pt), etc. The catalytic active component is, for example, in particulate form.

[0073] From the perspective of achieving a good balance between exhaust gas purification performance and cost, the content of Pd in the first catalyst layer 20, in terms of the content of Pd per unit volume of the substrate 10, is preferably 0.1 g / L or more and 2.5 g / L or less, more preferably 0.2 g / L or more and 2.1 g / L or less, and even more preferably 0.3 g / L or more and 1.6 g / L or less. From the same perspective, the content of Pt in the first catalyst layer 20, in terms of the content of Pt per unit volume of the substrate 10, is preferably 0.0050 g / L or more and 0.15 g / L or less, more preferably 0.010 g / L or more and 0.12 g / L or less, and even more preferably 0.015 g / L or more and 0.090 g / L or less. Note that the contents of Pd and Pt are each the content in terms of metal conversion.

[0074] The contents of Pd and Pt in the first catalyst layer 20 can be measured in the same manner as the content of Al in the second catalyst layer 30 described later.

[0075] From the perspective of improving exhaust gas purification performance, the first catalyst layer 20 preferably contains a carrier, and the catalytic active component is preferably supported on the carrier.

[0076] "The catalyst active component is supported on the carrier" means a state in which the catalyst active component is physically or chemically adsorbed or retained on the outer surface or inner pore surface of the carrier. Whether the catalyst active component is supported on the carrier can be confirmed, for example, by using a scanning electron microscope - energy dispersive X-ray analyzer (SEM-EDX) or the like.

[0077] Examples of the carrier include inorganic oxides and the like. The inorganic oxide is, for example, in a particulate form. Examples of the inorganic oxide include Al-based oxides, Ce-based oxides, and the like.

[0078] The first catalyst layer 20 may contain two or more carriers. For example, the first catalyst layer 20 may contain at least one Al-based oxide and at least one Ce-based oxide.

[0079] The first catalyst layer 20 may contain a binder. Examples of the binder include inorganic binders such as alumina, zirconia, titania, silica, and ceria.

[0080] <The second catalyst layer> Hereinafter, the second catalyst layer 30 will be described.

[0081] As shown in FIGS. 3 and 4, the second catalyst layer 30 is provided above the first catalyst layer 20. Therefore, the exhaust gas flowing in from the end portion (opening) on the exhaust gas inflow side of the cell 13 contacts the second catalyst layer 30 and then contacts the first catalyst layer 20. When the exhaust gas contacts the second catalyst layer 30, phosphorus in the exhaust gas is captured by barium sulfate in the second catalyst layer 30. Thereby, phosphorus poisoning of Pd and / or Pt contained in the first catalyst layer 20 is prevented.

[0082] The expression "the second catalyst layer 30 is provided above the first catalyst layer 20" means that part or all of the second catalyst layer 30 is present on the main surface of the first catalyst layer 20 that is opposite to the main surface on the side of the partition portion 12 of the base material 10 among the two main surfaces of the first catalyst layer 20. The "main surface of the first catalyst layer 20" means the outer surface of the first catalyst layer 20 extending in the exhaust gas flow direction X. The second catalyst layer 30 may be provided directly on the main surface of the first catalyst layer 20 or may be provided via another layer, but usually it is provided directly on the main surface of the first catalyst layer 20.

[0083] As shown in FIG. 4, the second catalyst layer 30 extends along the exhaust gas flow direction X from the end on the exhaust gas inflow side of the partition portion 12 to the end on the exhaust gas outflow side of the partition portion 12. The second catalyst layer 30 may extend along the exhaust gas flow direction X from the end on the exhaust gas inflow side of the partition portion 12 so as not to reach the end on the exhaust gas outflow side of the partition portion 12, or may extend along the direction opposite to the exhaust gas flow direction X from the end on the exhaust gas outflow side of the partition portion 12 so as not to reach the end on the exhaust gas inflow side of the partition portion 12.

[0084] From the viewpoint of achieving a good balance between exhaust gas purification performance and cost, the mass of the second catalyst layer 30 per unit volume of the base material (mass after drying and firing) is preferably 50 g / L or more and 150 g / L or less, more preferably 70 g / L or more and 120 g / L or less, and even more preferably 80 g / L or more and 100 g / L or less.

[0085] The second catalyst layer 30 contains Rh, Al, and barium sulfate.

[0086] Rh is contained in the second catalyst layer 30 in a form that can function as a catalytic active component, for example, in the form of metallic Rh, an alloy containing Rh, a compound containing Rh (for example, an oxide of Rh), etc. The catalytic active component is, for example, in particulate form.

[0087] From the viewpoint of achieving a good balance between the exhaust gas purification performance and cost, the content of Rh in the second catalyst layer 30, in terms of the content of Rh per unit volume of the substrate, is preferably 0.0050 g / L or more and 0.15 g / L or less, more preferably 0.010 g / L or more and 0.12 g / L or less, and even more preferably 0.015 g / L or more and 0.090 g / L or less. Note that the content of Rh is the content in terms of the metal conversion.

[0088] The fact that the second catalyst layer 30 contains barium sulfate can be confirmed, for example, by performing elemental mapping of the cross-section of the second catalyst layer 30 using an electron probe microanalyzer (EPMA), a scanning electron microscope - energy dispersive X-ray analyzer (SEM-EDX), etc., based on the overlap of the distribution of Ba and the distribution of S.

[0089] From the viewpoint of achieving a good balance between the phosphorus trapping ability and the exhaust gas purification performance of Rh, the content of barium sulfate in the second catalyst layer 30, based on the unit volume of the substrate, is preferably 1.0 g / L or more and 15 g / L or less, more preferably 4.0 g / L or more and 12 g / L or less, and even more preferably 7.0 g / L or more and 9.0 g / L or less.

[0090] The second catalyst layer 30 may contain a Ba source other than barium sulfate. Examples of the Ba source other than barium sulfate include water-soluble Ba salts, Al-based oxides containing Ba, Ce-based oxides containing Ba, etc.

[0091] The ratio of the content of Rh in the second catalyst layer 30 to the content of Ba in the second catalyst layer 30 is preferably 0.0040 or more and 0.040 or less, more preferably 0.0045 or more and 0.020 or less, and even more preferably 0.0050 or more and 0.010 or less in terms of the molar ratio.

[0092] When the ratio of the content of Rh in the second catalyst layer 30 to the content of Ba in the second catalyst layer 30 is within the above range, the contact frequency between Rh and Ba becomes appropriate, and thereby, a decrease in the exhaust gas purification performance of Rh contained in the second catalyst layer due to the stabilization of the oxidation state of Rh is prevented.

[0093] From the viewpoint of more effectively preventing a decrease in the exhaust gas purification performance of Rh contained in the second catalyst layer due to the stabilization of the oxidation state of Rh, the ratio of the content of Ba in the second catalyst layer 30 to the total content of all metal elements in the second catalyst layer 30 is preferably 0.005 or more and 0.060 or less, more preferably 0.015 or more and 0.045 or less, still more preferably 0.025 or more and 0.035 or less in terms of molar ratio. The "content of Ba in the second catalyst layer 30" means the content of Ba derived from the one kind of Ba source when the second catalyst layer 30 contains one kind of Ba source, and means the total content of Ba derived from the two or more kinds of Ba sources when the second catalyst layer 30 contains two or more kinds of Ba sources.

[0094] From the viewpoint of more effectively preventing a decrease in the exhaust gas purification performance of Rh contained in the second catalyst layer due to the stabilization of the oxidation state of Rh, the proportion of the content of Ba derived from barium sulfate in the content of Ba in the second catalyst layer 30 is preferably 80 mol% or more, more preferably 90 mol% or more, still more preferably 95 mol% or more. The upper limit is 100 mol%. Note that the fact that the ratio is 100 mol% means that barium sulfate is the only Ba source contained in the second catalyst layer 30.

[0095] The proportion of the content of Ba derived from barium sulfate in the content of Ba in the second catalyst layer 30 can be measured as follows.

[0096] First, the contents of Ba and S in the second catalyst layer 30 are determined. At this time, the contents of Ba and S in the second catalyst layer 30 can be measured in the same manner as the content of Al in the second catalyst layer 30 described later, respectively.

[0097] Barium sulfate is the only S source contained in the second catalyst layer 30. Therefore, when the content of S ≥ the content of Ba, the proportion of Ba derived from barium sulfate in the content of Ba in the second catalyst layer 30 is 100 mol%.

[0098] When the content of S < the content of Ba, the proportion of Ba derived from barium sulfate in the content of Ba in the second catalyst layer 30 is calculated based on the following formula. Proportion of Ba derived from barium sulfate (mol%) = (content of S / content of Ba) × 100

[0099] When the ratio of the content of Rh in the second catalyst layer 30 to the content of Ba in the second catalyst layer 30 is within the above range, there is a possibility that the content of Rh in the second catalyst layer 30 becomes insufficient. From the viewpoint of compensating for the insufficient content of Rh and improving the exhaust gas purification performance, it is preferable that the second catalyst layer 30 further contains at least one selected from Pd and Pt.

[0100] Pd and Pt are each contained in the second catalyst layer 30 in a form that can function as a catalyst active component, for example, in the form of metallic Pd, metallic Pt, an alloy containing Pd or Pd, a compound containing Pd or Pt (for example, an oxide of Pd or Pt), etc. The catalyst active component is, for example, particulate.

[0101] From the viewpoint of achieving a good balance between exhaust gas purification performance and cost, the content of Pd in the second catalyst layer 30, in terms of the content of Pd per unit volume of the substrate 10, is preferably 0.0050 g / L or more and 0.10 g / L or less, more preferably 0.010 g / L or more and 0.20 g / L or less, and even more preferably 0.015 g / L or more and 0.35 g / L or less. From the same viewpoint, the content of Pt in the second catalyst layer 30, in terms of the content of Pt per unit volume of the substrate 10, is preferably 0.0050 g / L or more and 0.15 g / L or less, more preferably 0.010 g / L or more and 0.12 g / L or less, and even more preferably 0.015 g / L or more and 0.090 g / L or less. Note that the contents of Pd and Pt are each the content in terms of metal conversion.

[0102] The contents of Rh, Pd, Pt, and Ba in the second catalyst layer 30 can be measured in the same manner as the content of Al in the second catalyst layer 30 described below, respectively.

[0103] From the viewpoint of improving the exhaust gas purification performance, the second catalyst layer 30 preferably includes a carrier, and the catalytically active component is supported on the carrier. The meaning of "supported", the method for confirming that it is supported, and specific examples of the carrier are the same as above.

[0104] The second catalyst layer 30 may contain two or more kinds of carriers. For example, the second catalyst layer 30 may contain at least one kind of Al-based oxide and at least one kind of Ce-based oxide.

[0105] The second catalyst layer 30 may contain a binder. Examples of the binder include inorganic binders such as alumina, zirconia, titania, silica, and ceria.

[0106] The second catalyst layer 30 contains one kind or two or more kinds of Al sources.

[0107] The Al source is an oxide containing Al. Examples of the oxide containing Al include Al-based oxides, Ce-based oxides containing Al, and alumina binders.

[0108] The second catalyst layer 30 preferably contains at least one kind of Al-based oxide as the Al source. The second catalyst layer 30 may further contain at least one kind selected from Ce-based oxides containing Al and alumina binders as the Al source. In one embodiment, the second catalyst layer 30 contains at least one kind of Al-based oxide and an alumina binder as the Al source.

[0109] The ratio of the content of Al in the second catalyst layer 30 to the total content of all metal elements in the second catalyst layer 30 is preferably 0.55 or more and 0.85 or less, more preferably 0.65 or more and 0.85 or less, and even more preferably 0.75 or more and 0.80 or less, in terms of molar ratio. "The content of Al in the second catalyst layer 30" means the content of Al derived from the single Al source when the second catalyst layer 30 contains one type of Al source, and means the total content of Al derived from the two or more Al sources when the second catalyst layer 30 contains two or more types of Al sources.

[0110] The oxide containing Al has a larger specific surface area compared to other materials contained in the second catalyst layer 30. Further, the oxide containing Al has high heat resistance and the specific surface area is maintained even after exposure to a high-temperature environment. Therefore, when the ratio of the content of Al in the second catalyst layer 30 to the total content of all metal elements in the second catalyst layer 30 is within the above range, the dispersibility of barium sulfate in the second catalyst layer 30 is improved. Also, the dispersibility of barium sulfate is maintained even after exposure to a high-temperature environment. As a result, the phosphorus capture ability of the second catalyst layer is improved, and a decrease in the exhaust gas purification performance of Pt and / or Pd contained in the first catalyst layer due to phosphorus poisoning is prevented.

[0111] The second catalyst layer 30 preferably contains a γ-alumina phase. The γ-alumina phase is derived from an oxide containing Al as an Al source, preferably at least one type of Al-based oxide. When the oxide containing Al contains a γ-alumina phase, the specific surface area of the oxide containing Al increases, and the dispersibility of barium sulfate in the second catalyst layer 30 is improved. As a result, the phosphorus capture ability of the second catalyst layer is improved, and a decrease in the exhaust gas purification performance of Pt and / or Pd contained in the first catalyst layer due to phosphorus poisoning is prevented.

[0112] The second catalyst layer 30 may contain other alumina phases (for example, α-alumina phase, θ-alumina phase, etc.). The other alumina phases are derived from an oxide containing Al as an Al source, preferably at least one type of Al-based oxide.

[0113] When the composition of the raw material used for forming the second catalyst layer 30 is known, the Al content in the second catalyst layer 30 can be calculated from the composition of the raw material used for forming the second catalyst layer 30.

[0114] In addition, the Al content in the second catalyst layer 30 can be measured using conventional methods such as scanning electron microscope - energy dispersive X-ray spectroscopy (SEM-EDX), X-ray fluorescence analysis (XRF), inductively coupled plasma atomic emission spectroscopy (ICP-AES). Specifically, it is as follows.

[0115] First, after pulverizing a test piece cut out from the exhaust gas purifying catalyst 1, it is analyzed by XRF or ICP-AES, and 20 metal elements with a large content are specified in descending order. Al is included in the specified 20 metal elements. The test piece may include a portion derived from the substrate 10 and / or a portion derived from the first catalyst layer 20 as long as it includes a portion derived from the second catalyst layer 30. When the test piece includes a portion derived from the substrate 10 and / or a portion derived from the first catalyst layer 20, the specified 20 metal elements may include the metal elements constituting the substrate 10 and / or the metal elements constituting the first catalyst layer 20.

[0116] Next, the second catalyst layer 30 is analyzed by SEM-EDX. In SEM-EDX, the 20 metal elements specified above are the analysis targets, and for each of the 10 fields of view of the SEM, the total mole% of the 20 metal elements = 100 mole% is set, and the mole% of each metal element is analyzed. The average value of the mole% of Al in the 10 fields of view is taken as the Al content in the second catalyst layer 30.

[0117] Note that the content of other metal elements in the second catalyst layer 30 can also be calculated or measured in the same manner as the Al content in the second catalyst layer 30.

[0118] Of the content of Al in the second catalyst layer 30, the proportion occupied by the content of Al derived from the Al-based oxide is preferably 80% by mass or more, more preferably 90% by mass or more, still more preferably 95% by mass or more. The upper limit is 100% by mass. The "content of Al derived from the Al-based oxide" means the content of Al derived from the one kind of Al-based oxide when the second catalyst layer 30 contains one kind of Al-based oxide, and means the total content of Al derived from the two or more kinds of Al-based oxides when the second catalyst layer 30 contains two or more kinds of Al-based oxides.

[0119] The second catalyst layer 30 preferably contains Ce.

[0120] When the second catalyst layer 30 contains Ce, the second catalyst layer 30 contains one kind or two or more kinds of Ce sources.

[0121] The Ce source is an oxide containing Ce. Examples of the oxide containing Ce include Al-based oxides containing Ce, Ce-based oxides, ceria binders, etc. Note that the Ce source does not necessarily have to be a different oxide from the Al source, and it may be the same oxide as the Al source. For example, Al-based oxides containing Ce and Ce-based oxides containing Al are each both an Al source and a Ce source.

[0122] The second catalyst layer 30 preferably contains at least one kind of Ce-based oxide as the Ce source, and more preferably contains at least one kind of second Ce-based oxide (particularly Ce-Zr-based oxide) as the Ce source. The second catalyst layer 30 may further contain at least one selected from Al-based oxides containing Ce and ceria binders as the Ce source. In one embodiment, the second catalyst layer 30 contains at least one kind of Ce-based oxide (preferably the second Ce-based oxide, particularly Ce-Zr-based oxide) and a ceria binder as the Ce source.

[0123] The ratio of the content of Ce in the second catalyst layer 30 to the total content of all metal elements in the second catalyst layer 30 is preferably 0.020 or more and 0.050 or less, more preferably 0.020 or more and 0.040 or less, still more preferably 0.025 or more and 0.030 or less, in terms of molar ratio. The "content of Ce in the second catalyst layer 30" means the content of Ce derived from the single Ce source when the second catalyst layer 30 contains one kind of Ce source, and means the total content of Ce derived from the two or more Ce sources when the second catalyst layer 30 contains two or more Ce sources.

[0124] When the ratio of the content of Ce in the second catalyst layer 30 to the total content of all metal elements in the second catalyst layer 30 is within the above range, the contact frequency between Rh and Ce becomes appropriate, whereby the decrease in the exhaust gas purification performance of Rh contained in the second catalyst layer due to the stabilization of the oxidation state of Rh is prevented, and the exhaust gas purification performance can be improved. Further, when the second catalyst layer 30 contains at least one selected from Pd and Pt, when the ratio of the content of Ce in the second catalyst layer 30 to the total content of all metal elements in the second catalyst layer 30 is within the above range, the contact frequency between at least one selected from Pd and Pt and Ce becomes appropriate, whereby the oxygen storage capacity by Ce is effectively exhibited, and the exhaust gas purification performance can be improved.

[0125] The proportion of the content of Ce derived from the Ce-based oxide in the content of Ce in the second catalyst layer 30 is preferably 80% by mass or more, more preferably 90% by mass or more, still more preferably 95% by mass or more. The upper limit is 100% by mass. The "content of Ce derived from the Ce-based oxide" means the content of Ce derived from the single Ce-based oxide when the second catalyst layer 30 contains one kind of Ce-based oxide, and means the total content of Ce derived from the two or more Ce-based oxides when the second catalyst layer 30 contains two or more Ce-based oxides.

[0126] <Method for manufacturing exhaust gas purification catalyst> The exhaust gas purification catalyst 1A can be manufactured by forming the first catalyst layer 20 on the substrate 10 and then forming the second catalyst layer 30 on the upper side of the first catalyst layer 20.

[0127] The first catalyst layer 20 can be formed by mixing a source of noble metal elements (e.g., noble metal salts, etc.) and other components (e.g., inorganic oxides such as Al-based oxides, Ce-based oxides, binders, solvents, etc.) to prepare a first slurry, applying the first slurry onto the partition portion 12 of the substrate 10, drying, and firing.

[0128] The second catalyst layer 30 can be formed by mixing a source of noble metal elements (e.g., noble metal salts, etc.) and other components (e.g., inorganic oxides such as Al-based oxides, Ce-based oxides, barium sulfate, binders, solvents, etc.) to prepare a second slurry, applying the second slurry onto the first catalyst layer 20, drying, and firing.

[0129] From the viewpoint of improving the supportability of the catalyst active component, the inorganic oxide used as the carrier is preferably porous. The BET specific surface area of the Al-based oxide is preferably 50 m 2 / g or more and 200 m 2 / g or less, more preferably 80 m 2 / g or more and 150 m 2 / g or less. The BET specific surface area of the Ce-based oxide is preferably 20 m 2 / g or more and 100 m 2 / g or less, more preferably 30 m 2 / g or more and 90 m 2 / g or less.

[0130] Examples of the binder include metal oxide sols such as alumina sol, zirconia sol, titania sol, silica, and ceria sol. Examples of the solvent include water and organic solvents.

[0131] The drying temperature is, for example, 60°C or higher and 120°C or lower, and the drying time is, for example, 0.1 hour or longer and 1 hour or shorter. The firing temperature is, for example, 300°C or higher and 700°C or lower, and the firing time is, for example, 1 hour or longer and 10 hours or shorter. The firing can be carried out, for example, in an air atmosphere.

[0132] <<Second Embodiment>> Hereinafter, based on FIG. 5, the exhaust gas purification catalyst 1B according to the second embodiment of the present invention will be described. In the exhaust gas purification catalyst 1B, the same members as those in the exhaust gas purification catalyst 1A are denoted by the same reference numerals as those in the exhaust gas purification catalyst 1A. Unless otherwise described below, the above description regarding the exhaust gas purification catalyst 1A is also applicable to the exhaust gas purification catalyst 1B.

[0133] As shown in FIG. 5, the second catalyst layer 30 is provided on the upstream side of the first catalyst layer 20. That is, the second catalyst layer 30 is provided on the inflow side of the partition wall portion 12 of the base material 10, and the first catalyst layer 20 is provided on the outflow side of the partition wall portion 12 of the base material 10. The exhaust gas purification catalyst 1B is different from the exhaust gas purification catalyst 1A in that the second catalyst layer 30 is provided on the upstream side of the first catalyst layer 20. The exhaust gas purification catalyst 1B is the same as the exhaust gas purification catalyst 1A in other respects and exhibits the same operational effects as the exhaust gas purification catalyst 1A.

[0134] The expression "the second catalyst layer 30 is provided on the upstream side of the first catalyst layer 20" means that a part or all of the second catalyst layer 30 exists in a region on the upstream side in the exhaust gas flow direction X of the region where the first catalyst layer 20 is provided, among the cell 13 - side surfaces of the partition wall 12 of the base material 10. The "cell 13 - side surface of the partition wall portion 12" means the outer surface of the partition wall portion 12 extending in the exhaust gas flow direction X.

[0135] As shown in FIG. 5, the first catalyst layer 20 extends along a direction opposite to the exhaust gas flow direction X from the end portion on the exhaust gas outflow side of the partition portion 12 so as not to reach the end portion on the exhaust gas inflow side of the partition portion 12, and the second catalyst layer 30 extends along the exhaust gas flow direction X from the end portion on the exhaust gas inflow side of the partition portion 12 so as not to reach the end portion on the exhaust gas outflow side of the partition portion 12.

[0136] In the exhaust gas purification catalyst 1B, since the second catalyst layer 30 is provided upstream of the first catalyst layer 20, the exhaust gas flowing in from the end portion (opening) on the exhaust gas inflow side of the cell 13 contacts the second catalyst layer 30 and then contacts the first catalyst layer 20. When the exhaust gas contacts the second catalyst layer 30, phosphorus in the exhaust gas is captured by barium sulfate in the second catalyst layer 30. Thereby, phosphorus poisoning of Pd and / or Pt contained in the first catalyst layer 20 is prevented.

[0137] The exhaust gas purification catalyst 1B can be manufactured by the following method. The end portion on the exhaust gas inflow side of the substrate 10 is immersed in a slurry for forming the second catalyst layer 30, the slurry is suctioned from the opposite side, dried, and a precursor layer of the second catalyst layer 30 is formed. The end portion on the exhaust gas outflow side of the substrate 10 is immersed in a slurry for forming the first catalyst layer 20, the slurry is suctioned from the opposite side, dried, and a precursor layer of the first catalyst layer 20 is formed. After the formation of the precursor layer of the first catalyst layer 20 and the precursor layer of the second catalyst layer 30, firing is performed. Thereby, the first catalyst layer 20 and the second catalyst layer 30 are formed, and the exhaust gas purification catalyst 1B is manufactured. The manufacturing conditions and the like of the exhaust gas purification catalyst 1B are the same as those of the exhaust gas purification catalyst 1A.

Example

[0138] <Examples 1 to 7 and Comparative Examples 1 to 5> (1) Formation of the lower layer A palladium nitrate solution, a Ce-Zr-based oxide, alumina, and a binder were sequentially added to a container containing pure water and sufficiently stirred and mixed to obtain a lower layer slurry.

[0139] A stainless steel metal honeycomb substrate (diameter: 40 mm, length: 120 mm, cell density: 400 cells per square inch, volume: 151 mL) was immersed in the slurry for the lower layer, and the excess slurry was removed. Then, the slurry for the lower layer was coated on the inner wall surface of the substrate. After drying the substrate coated with the slurry for the lower layer at 80 °C for 1 hour, it was fired at 500 °C for 4 hours to form a lower layer on the inner wall surface of the substrate. The mass of the lower layer per unit volume of the substrate (mass after drying and firing) was 81 g / L. The mass of Pd in the lower layer per unit volume of the substrate was 0.32 g / L in terms of metal. The total mass of the Ce-Zr based oxide and alumina in the lower layer per unit volume of the substrate was 81 g / L.

[0140] (2) Formation of the upper layer Into a container filled with pure water, rhodium nitrate solution, dinitrodiammineplatinum(II) solution, Ce-Zr based oxide, alumina (θ-alumina in Example 7, γ-alumina in other than Example 7), barium salt (barium hydroxide in Comparative Example 5, barium sulfate in other than Comparative Example 5) and binder (zirconia sol) were added in sequence and stirred well to obtain a slurry for the upper layer. The composition of the slurry for the upper layer was adjusted so that the composition of the upper layer formed from the slurry for the upper layer would be the composition shown in Table 1. In Table 1, "Rh / Ba" represents the ratio (molar ratio) of the content of Rh to the content of Ba. Note that, among the content of Al in the upper layer, the proportion occupied by the Al content derived from alumina, and among the content of Ce in the upper layer, the proportion occupied by the Ce content derived from the Ce-Zr based oxide are both 100% by mass.

[0141] The substrate with the lower layer formed was immersed in the slurry for the upper layer, and the excess slurry was removed. Then, the slurry for the upper layer was coated on the lower layer. After drying the substrate coated with the slurry for the upper layer at 80 °C for 1 hour, it was fired at 500 °C for 4 hours to form an upper layer on the lower layer. The mass of Rh in the upper layer per unit volume of the substrate was 0.018 g / L in terms of metal. The mass of Pt in the upper layer per unit volume of the substrate was 0.018 g / L in terms of metal. The mass of the upper layer per unit volume of the substrate (mass after drying and firing) was 81 g / L.

[0142] (3) Phosphorus poisoning durability test The exhaust gas purification catalyst manufactured in (2) above was mounted on the exhaust pipe, and a thermocouple was inserted into the center of the honeycomb substrate. This exhaust pipe was set in a gasoline engine (displacement: 2300 cc, fuel: gasoline added with engine oil), and the engine speed / torque, etc. were adjusted so that the temperature of the thermocouple reached a predetermined temperature of 850°C to 1000°C, and a cycle in which the A / F (air / fuel) was changed in the order of 12.5, 14.6, and 20.0 was repeated for 40 hours at a 60-second cycle.

[0143] (4) Evaluation of exhaust gas purification performance After the phosphorus poisoning durability test, the exhaust gas purification catalyst cored to 15 mL was incorporated into the muffler of a motorcycle, and the NOx emission amount (mg / km) was measured under the following conditions. The results are shown in Table 2. Vehicle in use: Single-cylinder 125 cc motorcycle Fuel: Unleaded gasoline Driving mode: WMTC Measurement method: Conforming to ISO6460

[0144]

Table 1

[0145]

Table 2

[0146] As shown in Tables 1 and 2, the exhaust gas purification catalysts of Examples 1 to 7 that satisfy the conditions that (1) the lower layer contains at least one selected from Pd and Pt, (2) the upper layer contains Rh, Al, and barium sulfate, (3) the ratio of the content of Al in the upper layer to the total content of all metal elements in the upper layer is 0.55 or more and 0.85 or less in terms of molar ratio, and (4) the ratio of the content of Rh in the upper layer to the content of Ba in the upper layer is 0.0040 or more and 0.040 or less in terms of molar ratio had higher exhaust gas purification performance (NOx purification performance) after the phosphorus poisoning durability test than the exhaust gas purification catalysts of Comparative Examples 1 to 5 that do not satisfy any one or more of (2) to (4).

Explanation of symbols

[0147] 1A, 1B ··· Exhaust gas purification catalysts 10 ··· Substrate 11 ··· Cylindrical part 12 ··· Partition part 13 ··· Cell 20 ··· First catalyst layer 30 ··· Second catalyst layer

Claims

1. An exhaust gas purification catalyst comprising a substrate and first and second catalyst layers provided on the substrate, wherein the second catalyst layer is provided above or upstream of the first catalyst layer, the first catalyst layer contains at least one selected from Pd and Pt, the second catalyst layer contains Rh, Al, and barium sulfate, the ratio of the content of Al in the second catalyst layer to the total content of all metal elements in the second catalyst layer is 0.55 or more and 0.85 or less in terms of molar ratio, the ratio of the content of Rh in the second catalyst layer to the content of Ba in the second catalyst layer is 0.0040 or more and 0.040 or less in terms of molar ratio, an exhaust gas purification catalyst.

2. The exhaust gas purification catalyst according to claim 1, wherein the second catalyst layer contains a γ-alumina phase.

3. The exhaust gas purification catalyst according to claim 1 or 2, wherein the second catalyst layer further contains at least one selected from Pd and Pt.

4. The second catalyst layer further contains Ce, the ratio of the content of Ce in the second catalyst layer to the total content of all metal elements in the second catalyst layer is 0.020 or more and 0.050 or less in terms of molar ratio, the exhaust gas purification catalyst according to any one of claims 1 to 3.

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