Catalyst for exhaust gas purification and method for exhaust gas purification using the same

Abstract

Disclosed is a means for improving catalytic performance in an exhaust gas purification catalyst. The present invention relates to an exhaust gas purification catalyst which is formed by sequentially stacking a first catalyst layer and a second catalyst layer on a three-dimensional structure, the first catalyst layer containing rhodium, the second catalyst layer containing palladium, strontium, and alumina, the content of the strontium in the second catalyst layer (converted into strontium oxide) being more than 0 g / L and less than 15 g / L with respect to 1 liter of the three-dimensional structure, and the exhaust gas purification catalyst having a pore volume of more than 0.26 mL / g for pores having a pore diameter of 2 nm or more and 40 nm or less in a pore volume distribution measured by a nitrogen adsorption method.

Description

Technical Field

[0001] This invention relates to catalysts for waste gas purification and waste gas purification methods using the same. Background Technology

[0002] Various technologies have been proposed for treating exhaust gases from internal combustion engines. In particular, various technologies have been proposed for treating exhaust gases from gasoline engines, with the aim of removing hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) contained in the exhaust gases. For example, as a catalyst for purifying exhaust gases, a three-way catalyst has been proposed, which consists of a first layer containing palladium, aluminum oxide, and cerium oxide-zirconia composite oxide, and a second layer containing rhodium, aluminum oxide, and cerium oxide-zirconia composite oxide, stacked sequentially on a three-dimensional structure (Japanese Patent Application Publication No. 2009-541041 (corresponding to U.S. Patent Application Publication No. 2010 / 0263357)). Summary of the Invention

[0003] However, in recent years, given the increasingly stringent restrictions on automobile exhaust emissions, there has been a demand for further improvements in the catalytic performance of catalysts used for exhaust purification.

[0004] The present invention was made in view of the above circumstances, and its object is to provide a means for improving the catalytic performance of a catalyst for exhaust gas purification.

[0005] The inventors conducted in-depth research to solve the aforementioned problems. As a result, they discovered that by sequentially forming a catalyst layer containing rhodium and a catalyst layer containing palladium and a specific amount of strontium on a three-dimensional structure, and by increasing the volume of pores with a specific pore size, the aforementioned problems can be solved, thus completing the present invention.

[0006] In detail, the first aspect of the present invention relates to a catalyst for purifying exhaust gas, which is formed by sequentially stacking a first catalyst layer and a second catalyst layer on a three-dimensional structure. The first catalyst layer contains rhodium, and the second catalyst layer contains palladium, strontium, and aluminum oxide. The content of strontium in the second catalyst layer (converted to strontium oxide) is more than 0 g / L and less than 15 g / L relative to 1 liter of the three-dimensional structure. In the pore volume distribution determined by nitrogen adsorption, the pore volume of pores with a pore size of 2 nm or more and 40 nm or less exceeds 0.26 mL / g.

[0007] A second aspect of the present invention relates to a method for purifying exhaust gas from a gasoline engine, the method comprising the step of treating exhaust gas discharged from a gasoline engine using an exhaust gas purification catalyst of the present invention. Attached Figure Description

[0008] Figure 1This is a bar chart representing the T50 (°C) of carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) for catalysts A to M. Detailed Implementation

[0009] The first aspect of the present invention relates to a catalyst for purifying exhaust gas, which is formed by sequentially stacking a first catalyst layer and a second catalyst layer on a three-dimensional structure. The first catalyst layer contains rhodium, and the second catalyst layer contains palladium, strontium, and aluminum oxide. The content of strontium in the second catalyst layer (converted to strontium oxide) is more than 0 g / L and less than 15 g / L relative to 1 liter of the three-dimensional structure. In the pore volume distribution determined by nitrogen adsorption, the pore volume of pores with a pore size of 2 nm or more and 40 nm or less exceeds 0.26 mL / g.

[0010] A second aspect of the present invention relates to a method for purifying exhaust gas from a gasoline engine, the method comprising the step of treating exhaust gas discharged from a gasoline engine using an exhaust gas purification catalyst of the present invention.

[0011] According to the present invention, the catalytic performance of catalysts used for waste gas purification can be improved.

[0012] In this specification, the pore volume distribution determined by nitrogen adsorption will be referred to simply as "pore volume distribution". Furthermore, the pore volume of pores with a diameter of 2 nm or more and less than 40 nm in the pore volume distribution determined by nitrogen adsorption will also be referred to simply as "pore volume of pores with a diameter of 2 nm or more and less than 40 nm" or "V". 2-40nm In addition, the strontium content (converted to strontium oxide) relative to 1 liter of three-dimensional structure will also be referred to as "strontium content", "Sr content", "strontium content of the present invention" or "Sr content of the present invention".

[0013] In this specification, the description of one aspect of the invention described above can be appropriately modified to apply in other ways.

[0014] The following describes embodiments of the present invention. It should be noted that the present invention is not limited to the following embodiments, and various modifications can be made within the scope of the claims. Throughout this specification, unless otherwise stated, singular expressions should be understood to include their plural forms. Therefore, unless otherwise stated, singular articles (e.g., "a," "an," "the," etc. in the English context) should also be understood to include their plural forms. Furthermore, the terms used in this specification, unless specifically mentioned, should be understood to be used in their commonly understood meaning in the art. Therefore, unless otherwise defined, all technical and technical terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. In cases of conflict, this specification (including definitions) takes precedence.

[0015] Furthermore, in this specification, the range "X~Y" includes both X and Y, meaning "above X and below Y". In this specification, "A and / or B" refers to at least one of A and B, including both A and B, or either A or B. Additionally, unless otherwise specified, operations and measurements of physical properties are performed at room temperature (20°C to 25°C).

[0016] Catalysts for Waste Gas Purification

[0017] The catalyst of the present invention is characterized by having two catalyst layers, wherein the first catalyst layer (the lower layer on the three-dimensional structure side) comprises rhodium; the second catalyst layer (the upper layer on the side in contact with the exhaust gas) comprises palladium, and contains more than 0 g / L and less than 15 g / L (SrO conversion) of strontium and alumina relative to 1 liter of the three-dimensional structure; and the pore volume (V) of the pores having a pore size of 2 nm or more and 40 nm or less is... 2-40nm (Exceeding 0.26 mL / g)

[0018] The catalyst described in Japanese Patent Application Publication No. 2009-541041 (corresponding to U.S. Patent Application Publication No. 2010 / 0263357) has the reverse stacking order of the catalyst of the present invention (i.e., the first catalyst layer contains palladium, strontium, and alumina; the second catalyst layer contains rhodium). In this case, by placing the rhodium-containing layer on the side in contact with the exhaust gas, the NO-CO reaction (the reduction reaction of NOx) and the CO-O2 reaction (the oxidation reaction of CO) take place on the surface, thus NOx and CO are efficiently purified. However, in the above configuration, oxygen is consumed due to the CO-O2 reaction in the second catalyst layer, so oxygen cannot sufficiently reach the first catalyst layer (the lower layer on the three-dimensional structure side). Therefore, HC cannot be sufficiently purified (comparison of catalyst A and catalyst L below).

[0019] In contrast, in the catalyst of the present invention, the first catalyst layer (the lower layer on the three-dimensional structure side) contains rhodium, and the second catalyst layer (the upper layer on the side in contact with the exhaust gas) contains palladium, strontium, and aluminum oxide. By placing the second catalyst layer containing palladium on the side in contact with the exhaust gas in this way, HC-O2 reactions (the oxidation reaction of HC) and the like take place on the surface, thus hydrocarbons (HC) are efficiently purified. In addition, NO and CO that are not completely purified in the second catalyst layer diffuse to the first catalyst layer containing rhodium (the lower layer on the three-dimensional structure side) and are purified by NO-CO reactions and the like. Here, the catalyst of the present invention has a large pore volume with a pore size of 2 to 40 nm (pores with a pore size of 2 to 40 nm are abundant). Exhaust gas molecules pass through the pores while colliding with the inner walls of the pores, thus the exhaust gas is moderately retained in the pores. Therefore, efficient contact with precious metals (rhodium or palladium) in each catalyst layer can effectively purify the exhaust gas. On the other hand, pores with diameters greater than 40 nm are less likely to retain NO, CO, and HC compared to pores with diameters of 2–40 nm, thus contributing less to the effective purification of exhaust gases. Furthermore, the pores of refractory inorganic oxides such as alumina typically shrink due to heat, leading to a reduction in pore volume when exposed to high temperatures. However, in this invention, by coexisting strontium and alumina in the second catalyst layer in contact with the high-temperature exhaust gases, the pore volume can be maintained. Therefore, the catalyst of this invention can become a three-way catalyst exhibiting high catalytic performance (especially capable of efficiently purifying hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) contained in exhaust gases from gasoline engines even after high-temperature heat treatment). The above effect is achieved by increasing the pore volume (V) of the second catalyst layer containing pores with diameters of 2 nm or more and 40 nm or less. 2-40nm It is particularly effective when the alumina has a macroporous volume of 0.60 mL / g or higher.

[0020] It should be noted that the above mechanism is speculative, and the present invention is not limited by the above speculation.

[0021] The catalyst of the present invention, in the pore volume distribution determined by nitrogen adsorption method, has pore volumes (V) of pores with pore sizes of 2 nm or more and 40 nm or less. 2-40nm The concentration is greater than 0.26 mL / g, preferably greater than 0.31 mL / g, and more preferably greater than 0.36 mL / g. Here, if V 2-40nm If the pore volume is below 0.26 mL / g, the exhaust gas cannot be sufficiently retained in the catalyst layer. Therefore, sufficient catalytic performance cannot be achieved (see catalyst F below). On the other hand, if the pore volume is within the preferred range described above, the exhaust gas can be retained more efficiently within the pores, thus enabling more efficient contact with precious metals (rhodium, palladium) and further improving catalytic performance. It should be noted that the pore volume (V) of pores with a diameter of 2 nm or more and 40 nm or less is considered to be... 2-40nmThe higher the value, the better; therefore, there is no particular upper limit, typically less than 0.75 mL / g, and preferably less than 0.61 mL / g. The pore volume (V) of pores with a pore size of 2 nm or more and less than 40 nm in the pore volume distribution determined by nitrogen adsorption is defined above. 2-40nm ")" is a value relative to the catalyst before use (before contact with exhaust gas).

[0022] The total pore volume of the catalyst of the present invention is, for example, greater than 0.41 mL / g, preferably greater than 0.45 mL / g, more preferably greater than 0.48 mL / g, even more preferably greater than 0.51 mL / g, and particularly preferably greater than 0.51 mL / g. With such a total pore volume, both the diffusion of exhaust gas into the catalyst layer and the retention of the catalyst layer in the refractory three-dimensional structure can be achieved simultaneously. It should be noted that there is no particular upper limit to the total pore volume of the present invention; it is generally less than 0.96 mL / g, preferably less than 0.78 mL / g. The above-mentioned "total pore volume" is a value relative to the catalyst before use (before contact with exhaust gas).

[0023] The catalyst of the present invention has a pore volume ratio (%) of pores with a pore size of 2 nm or more and 40 nm or less relative to the total pore volume [=(pore volume of pores with a pore size of 2 nm or more and 40 nm or less)×100 / (total pore volume)] preferably of 65.0% or more, more preferably of 70.0% or more, and particularly preferably more than 71.0%. This allows exhaust gas to remain within the pores for a longer period, thus enabling more efficient purification of the exhaust gas. It should be noted that a higher ratio (%) of pore volume ratio (pores with a pore size of 2 nm or more and 40 nm or less relative to the total pore volume is preferable; therefore, an upper limit of 100% is acceptable, but 90% or less is permissible, preferably 80.0% or less. The above-mentioned "pore volume ratio (%)" is a value relative to the catalyst before use (before contact with exhaust gas).

[0024] In this specification, the pore volume and total pore volume of pores with a pore size of 2 nm or more and 40 nm or less in the pore volume distribution determined by nitrogen adsorption method are calculated based on the pore volume distribution (pore size distribution) using the following method.

[0025] -Pore volume distribution (pore size distribution) determination-

[0026] Pore ​​volume distribution (pore size distribution), pore size (pore diameter), and pore volume can be determined by known methods, preferably by nitrogen adsorption. The determination based on nitrogen adsorption is performed according to ISO 15901-3:2007. It should be noted that, in order to distinguish the pores of the catalyst from the pores of three-dimensional structures such as cordierite supports, after coating the three-dimensional structure with the catalyst component (i.e., in the state of the catalyst as the final product of supporting noble metals, alumina, cerium oxide-zirconia composite oxides, etc., on the three-dimensional structure), the coated catalyst component (catalyst layer) is peeled off, and the peeled powder is measured as the Fresh state (before heat treatment). Alternatively, in the determination of "pore retention rate of the catalyst after heat treatment" below, the coated catalyst component (catalyst layer) is peeled off from the heat-treated catalyst, and the peeled powder is measured as the Post-Heat-Treatment state (after heat treatment). The pore size of the three-dimensional structure is typically 500 nm or larger; therefore, the presence or absence of the three-dimensional structure does not affect the pore volume distribution with a pore size of 40 nm or smaller. More specifically, pore size distribution, pore volume, and total pore volume were determined by the following methods.

[0027] The catalyst was subjected to reduced pressure treatment at 250°C for 2 hours to prepare a sample. Using an automated specific surface area / pore distribution measuring device (Tristar IIPlus 3020, manufactured by Shimadzu Corporation), liquid nitrogen was introduced into the sample, and the adsorption-desorption isotherms of nitrogen were measured at -196°C when the relative pressure (P / saturated vapor pressure) was 0–1. Based on the obtained adsorption-desorption isotherms and the amount of nitrogen adsorbed, the pore size distribution and total pore volume of each catalyst were determined using the Barrett-Joyner-Halenda method (BJH method).

[0028] Based on the obtained pore size distribution and total pore volume (mL / g), the pore volume (V) for pores with a pore size greater than 2 nm and less than 40 nm is calculated. 2-40nm Additionally, based on the pore volume (V) of pores with a diameter of 2 nm or more and 40 nm or less. 2-40nm ) and total pore volume, calculate the pore volume (V) of pores with a diameter greater than 2 nm and less than 40 nm. 2-40nm The proportion (%) of (V) 2-40nm ()×100 / (total pore volume)).

[0029] The catalyst for exhaust gas purification of the present invention preferably maintains a pore volume (V) of pores with a pore size of 2 nm or more and 40 nm or less, even after prolonged exposure to high-temperature exhaust gas from an internal combustion engine. 2-40nmThis allows the exhaust gas to remain within the pores for a longer period, thus increasing the contact between the exhaust gas and the precious metal, and enabling more efficient purification of the exhaust gas. Specifically, the pore retention rate of the catalyst after heat treatment is preferably 42.0% or more and less than 80.0%, more preferably more than 46.0% and less than 75.0%, further preferably more than 55.0% and less than 75.0%, more preferably more than 60.0% and less than 72.0%, and particularly preferably more than 60.0% and less than 69.0%.

[0030] In this specification, the pore retention rate of the catalyst after heat treatment is determined by the method described above, wherein the catalyst is treated at 1000°C in a nitrogen stream containing 5% by volume hydrogen (hereinafter referred to as heat treatment).

[0031] The total pore volume and the pore volume of pores with a diameter of 2 nm or more and less than 40 nm after 10 hours (pore volume after heat treatment). Next, the ratio (%) of the pore volume of pores with a diameter of 2 nm or more and less than 40 nm (pore volume after heat treatment) of the catalyst before heat treatment (before use after manufacturing) to the pore volume of pores with a diameter of 2 nm or more and less than 40 nm (pore volume before heat treatment) [=(V after heat treatment] 2-40nm (pore volume) × 100 / (V before heat treatment) 2-40nm [Pore volume]. It should be noted that exhaust gas purification catalysts are sometimes exposed to exhaust gases at temperatures exceeding 1000°C. Even after exposure to such high-temperature exhaust gases, it is necessary to maintain the exhaust gas purification performance. Therefore, to simulate the catalyst after exposure to high-temperature exhaust gases, the exhaust gas purification performance of the catalyst after heat treatment at 1000°C in a nitrogen stream containing 5% by volume hydrogen for 10 hours can be evaluated. In addition, the pore size, pore volume, and pore size distribution of the heat-treated catalyst can be measured.

[0032] The exhaust gas purification catalyst of the present invention can be used for purifying exhaust gases emitted from diesel engines or gasoline engines, but is particularly preferred for purifying exhaust gases emitted from gasoline engines. It should be noted that exhaust gases emitted from gasoline engines differ significantly from those emitted from diesel engines in many aspects: they substantially lack particulate matter (PM), have different compositions of exhaust gases (e.g., CO, NOx, HC), and contain shorter chains of HC compared to those emitted from diesel engines. Therefore, even a catalyst with excellent purification capabilities for exhaust gases emitted from diesel engines may not be excellent for purifying exhaust gases emitted from gasoline engines.

[0033] [First catalyst layer]

[0034] The catalyst for purifying exhaust gas of the present invention is formed by sequentially stacking a first catalyst layer (the lower layer on the side of the three-dimensional structure) and a second catalyst layer (the upper layer on the side in contact with the exhaust gas) on a three-dimensional structure. Here, the first catalyst layer can be directly disposed on the three-dimensional structure, or it can be disposed on the three-dimensional structure with other layers in between. Preferably, the first catalyst layer is directly disposed on the three-dimensional structure.

[0035] In addition, the first catalyst layer contains rhodium (Rh), but may further contain other components. These other components include, for example, alumina, cerium oxide-zirconia composite oxide, refractory inorganic oxides, and co-catalysts. From the viewpoint of further improving exhaust gas purification efficiency (catalytic performance), the first catalyst layer preferably further contains cerium oxide-zirconia composite oxide, and more preferably further contains alumina and cerium oxide-zirconia composite oxide. Furthermore, the first catalyst layer preferably does not substantially contain combustion improvers containing Group II elements such as magnesium (Mg), calcium (Ca), and barium (Ba) (e.g., barium sulfate (BaSO4), barium oxide (BaO)) (e.g., the content of Group II elements in the first catalyst layer is less than 0.1 g relative to 1 liter of three-dimensional structure, preferably 0 g).

[0036] (Precious metals)

[0037] The first catalyst layer contains rhodium (Rh). In addition to rhodium, the first catalyst layer may also contain platinum (Pt) as a noble metal, but preferably the only noble metal contained in the first catalyst layer is rhodium.

[0038] There is no particular limitation on the amount (load) of rhodium (Rh) used. However, considering the purification capacity of exhaust gas (especially NOx), the amount is preferably 0.001 to 20 g relative to 1 liter of three-dimensional structure when converted to precious metals, more preferably 0.01 to 5 g, and most preferably 0.05 to 3 g.

[0039] Furthermore, when the first catalyst layer also contains platinum, the amount of platinum (Pt) used (loading) is not particularly limited. However, considering the exhaust gas purification capacity, it is preferably 0.01 to 20 g relative to 1 liter of three-dimensional structure in precious metal terms, more preferably 0.05 to 10 g, and most preferably more than 0.5 g and less than 5 g. In this case, the mixing ratio of rhodium to platinum (rhodium:platinum (mass ratio)) is preferably, for example, 1:30 to 1:1.1, more preferably 1:20 to 1:1.3, and particularly preferably 1:5 to 1:1.5. As the mixing ratio of rhodium to platinum falls within the above-mentioned preferred range, the exhaust gas purification efficiency can be improved.

[0040] As a raw material for precious metals (rhodium, platinum), nitrates, acetates, amines, ammonium salts, etc. can be used, with nitrates being more preferred.

[0041] There is no particular limitation on the amount of the precious metal source (rhodium source, platinum source), but it is preferable to have the content (loading) of each precious metal as described above. It should be noted that when using two or more precious metal sources in combination, it is preferable that the total amount of the precious metal sources is the content (loading) of the aforementioned precious metals.

[0042] (Cerium oxide-zirconia composite oxide)

[0043] The first catalyst layer preferably comprises a cerium oxide-zirconia composite oxide.

[0044] Here, when the first catalyst layer contains a cerium oxide-zirconia composite oxide, the cerium oxide-zirconia composite oxide (CeO2-ZrO2) functions as an oxygen uptake and storage material, particularly possessing the advantage of a rapid oxygen uptake and release rate. This oxygen uptake and storage material (also called an "oxygen uptake and release substance") functions to: uptake and store oxygen in an oxidizing atmosphere (lean) and release oxygen in a reducing atmosphere (rich) according to the change in the air-fuel ratio (A / F) corresponding to the operating conditions, thereby ensuring the stable conduct of the redox reaction.

[0045] Cerium oxide-zirconia composite oxides may contain at least one metal selected from the group consisting of lanthanum (La), yttrium (Y), neodymium (Nd), and praseodymium (Pr). Specifically, examples include cerium oxide-zirconia-lanthanum oxide composite oxides and cerium oxide-zirconia-lanthanum oxide-yttrium oxide composite oxides.

[0046] There is no particular limitation on the BET specific surface area of ​​cerium oxide-zirconia composite oxides, but it is preferably 20–150 m². 2 / g, more preferably 50-90mg 2 / g. The average secondary particle size of the cerium oxide-zirconia composite oxide is preferably 1–50 μm, more preferably 5–20 μm. It should be noted that the average secondary particle size of the cerium oxide-zirconia composite oxide refers to the average particle size (D50) measured using a laser diffraction / scattering particle size distribution measuring device.

[0047] The content (loading; oxide conversion) of the cerium oxide-zirconia composite oxide in the first catalyst layer is not particularly limited, but is preferably 5 to 200 g relative to 1 liter of three-dimensional structure, more preferably 5 to 100 g, and even more preferably 10 to 90 g. The cerium (Ce) content in the cerium oxide-zirconia composite oxide (oxygen absorption and storage material), converted to oxide (CeO2), is preferably 1 to 60% by mass, more preferably 10 to 50% by mass, and even more preferably 20% by mass or more and less than 50% by mass. Furthermore, the zirconium (Zr) content in the cerium oxide-zirconia composite oxide, converted to oxide (ZrO2), is preferably 30 to 90% by mass, more preferably 40 to 80% by mass, and even more preferably more than 40% by mass and less than 70% by mass. It should be noted that when the first catalyst layer contains two or more cerium oxide-zirconia composite oxides, the above-mentioned cerium oxide-zirconia composite oxide content refers to their total amount. In addition to cerium and zirconium, the cerium oxide-zirconia composite oxide may also contain at least one element selected from the group consisting of lanthanum (La), yttrium (Y), neodymium (Nd), and praseodymium (Pr). In this case, the content of lanthanum (La), yttrium (Y), neodymium (Nd), and praseodymium (Pr) can be appropriately selected from the viewpoint of redox reaction stability.

[0048] In addition, the crystal structure of cerium oxide-zirconia composite oxide (oxygen absorption and storage material) includes cubic crystal, tetragonal crystal, monoclinic crystal, orthorhombic crystal, etc., preferably cubic crystal, tetragonal crystal or monoclinic crystal, more preferably cubic crystal or tetragonal crystal.

[0049] (alumina)

[0050] The first catalyst layer preferably replaces the cerium oxide-zirconia composite oxide or contains alumina in the cerium oxide-zirconia composite oxide, and more preferably contains alumina in the cerium oxide-zirconia composite oxide. This allows rhodium (the catalyst component) to be loaded onto alumina, increasing the contact area between rhodium and the waste gas or increasing the adsorption of reactants. As a result, the overall reactivity of the catalyst can be further improved.

[0051] That is, in one embodiment of the present invention, the first catalyst layer comprises rhodium, and at least one of cerium oxide-zirconia composite oxide and aluminum oxide. In one embodiment of the present invention, the first catalyst layer comprises rhodium, cerium oxide-zirconia composite oxide, and aluminum oxide. In one embodiment of the present invention, the first catalyst layer is composed of rhodium, cerium oxide-zirconia composite oxide, and aluminum oxide.

[0052] Here, when the first catalyst layer contains alumina, there are no particular limitations on the alumina that can be used as long as it contains an aluminum oxide. Examples include active alumina such as γ-, δ-, η-, and θ-alumina, lanthanum-containing alumina, silica-containing alumina, silica-titanium dioxide-containing alumina, and silica-titanium dioxide-zirconia-containing alumina. These alumina can be used alone or in combination of two or more. From the viewpoint of high dispersion of precious metals, γ-, δ-, or θ-alumina and lanthanum-containing alumina are preferred. Furthermore, in the case of lanthanum-containing alumina, the lanthanum oxide content is preferably 0.5 to 8% by mass, more preferably 1 to 5% by mass. It should be noted that, in this specification, alumina containing X refers to alumina containing aluminum in more than half of its total composition (molar ratio in metal terms) and X in the remaining proportion. For example, the molar ratio of aluminum (Al) in lanthanum oxide-containing alumina to the total molar ratio of lanthanum (La) and aluminum (Al) constituting lanthanum oxide-containing alumina [=Al / (La+Al)] exceeds 0.5.

[0053] The pore volume distribution of the alumina used in the first catalyst layer is not particularly limited, but alumina with a pore structure different from the macroporous alumina described later is preferred. In the pore volume distribution of the alumina used in the first catalyst layer, as determined by nitrogen adsorption, the pore volume of pores with a diameter of 2 nm or more and 40 nm or less is preferably less than 0.98 mL / g, more preferably less than 0.70 mL / g, and particularly preferably less than 0.60 mL / g. By using alumina with such pore volume, the waste gas flowing into and diffusing into the second catalyst layer can be effectively purified within the second catalyst layer. It should be noted that the lower limit of the pore volume of the alumina used in the first catalyst layer with a pore size of 2 nm or more and 40 nm or less is not particularly limited, but is, for example, 0.01 mL / g or more, preferably more than 0.1 mL / g.

[0054] Furthermore, the total pore volume of the alumina is, for example, 0.2 to 1.0 mL / g, preferably 0.4 to 0.8 mL / g. By using alumina with such a pore volume, the waste gas flowing into and diffusing into the second catalyst layer can be effectively purified within the second catalyst layer.

[0055] Furthermore, there is no particular limitation on the BET specific surface area of ​​alumina; from the viewpoint of supporting catalyst composition, it is preferably 50 to 350 m². 2 / g, more preferably 80-200m 2 / g, particularly preferably 100-180mg 2 / g. With such a specific surface area, a sufficient amount of noble metal (catalyst component) can be loaded onto alumina, increasing the contact area between the catalyst component and the waste gas, or adsorbing reactants. As a result, the reactivity of the catalyst as a whole can be further improved.

[0056] The shape of alumina is not particularly limited; for example, it can be any shape, such as granular, microparticle, powder, cylindrical, conical, prismatic, cubic, pyramidal, or irregular. Granular, microparticle, or powder is preferred, and powder is even more preferred. The average primary particle size of the alumina (in the first catalyst layer) in granular, microparticle, or powder form is preferably 5–20 nm, and more preferably 5–10 nm. Within this range, the catalyst components can be efficiently loaded onto the alumina surface. It should be noted that, in this specification, the average primary particle size of alumina (including macroporous alumina described below), cerium oxide-zirconia composite oxide, and refractory inorganic oxides can be measured using a transmission electron microscope (TEM). In this specification, "primary particle size" refers to the maximum distance between any two points on the outline of the granular, microparticle, or powder sample. Furthermore, the average secondary particle size of the alumina (before pulverization) is preferably 20–150 μm, and more preferably 30–90 μm. If the range is within this range, the catalyst components can be efficiently loaded onto the alumina surface. It should be noted that, in this specification, the average secondary particle size of alumina (including macroporous alumina described below), cerium oxide-zirconia composite oxide, and refractory inorganic oxides can be determined using a laser diffraction / scattering particle size distribution measuring device.

[0057] The content (loading) of alumina in the first catalyst layer is not particularly limited, but is preferably 10 to 300 g, more preferably 50 to 200 g, relative to a 1L three-dimensional structure. If the alumina content is 10 g or more relative to a 1L three-dimensional structure, rhodium can be sufficiently dispersed in the alumina, resulting in a catalyst with greater durability. On the other hand, when the alumina content is 300 g or less, the contact between rhodium and the exhaust gas becomes better, allowing for more efficient exhaust gas purification. It should be noted that when the first catalyst layer contains two or more types of alumina, the above-mentioned alumina content refers to their total amount.

[0058] Furthermore, when the first catalyst layer comprises alumina and cerium oxide-zirconia composite oxide, the mixing ratio (mass ratio) of alumina to cerium oxide-zirconia composite oxide is preferably 10:1 to 1:10, more preferably 10:2 to 5:10, and most preferably 10:3 to 10:10. With such a ratio, a sufficient amount of rhodium (catalyst component) can be loaded onto the alumina, increasing the contact area between the catalyst component and the waste gas, and enabling the cerium oxide-zirconia composite oxide to adequately adsorb hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) in the waste gas. As a result, the overall reactivity of the catalyst can be further improved, and the waste gas purification performance can be further enhanced.

[0059] (Refractory inorganic oxides)

[0060] The first catalyst layer may further comprise refractory inorganic oxides (other refractory inorganic oxides) other than alumina and cerium oxide-zirconia composite oxides. Here, the refractory inorganic oxides have a high specific surface area, which, by loading the catalyst components onto them, increases the contact area between the catalyst components and the exhaust gas, or adsorbs reactants. As a result, the reactivity of the catalyst as a whole can be further improved.

[0061] Examples of refractory inorganic oxides include zeolites, titanium dioxide, zirconium oxide, and silicon dioxide. These refractory inorganic oxides can be used alone or in combination of two or more.

[0062] [Second catalyst layer]

[0063] The catalyst for purifying exhaust gas of the present invention has a second catalyst layer (the upper layer on the side in contact with the exhaust gas) formed on a first catalyst layer formed on a three-dimensional structure. Here, the second catalyst layer can be directly disposed on the first catalyst layer, or it can be disposed on the first catalyst layer with other layers in between. Preferably, the second catalyst layer is directly disposed on the first catalyst layer. Alternatively, other layers can be disposed on the second catalyst layer, but it is preferred that the second catalyst layer is the outermost layer (in contact with the exhaust gas).

[0064] (Precious metals)

[0065] The second catalyst layer contains palladium (Pd). In addition to palladium, the second catalyst layer may also contain platinum (Pt) as a noble metal, but preferably, the noble metal contained in the second catalyst layer is only palladium.

[0066] There is no particular limitation on the amount (load) of palladium (Pd) used. If the purification capacity of exhaust gas (especially HC) is taken into consideration, the amount is preferably 0.01 to 20 g relative to 1 liter of three-dimensional structure in terms of precious metal conversion, more preferably 0.1 to 15 g, and most preferably 1 to 7 g.

[0067] Furthermore, when the second catalyst layer also contains platinum, the amount of platinum (Pt) used (loading) is not particularly limited. Considering the exhaust gas purification capacity, the amount is, for example, 0.1 to 15 g (converted to precious metals) per liter of three-dimensional structure, preferably 1 to 6 g. In this case, the mixing ratio of palladium to platinum (palladium:platinum (mass ratio)) is, for example, 0.1:1 to 10:1, preferably 0.5:1 to 5:1. As the mixing ratio of palladium to platinum falls within the above-mentioned preferred range, the exhaust gas purification efficiency can be improved.

[0068] As a raw material for precious metals (palladium, platinum), nitrates, acetates, amines, ammonium salts, etc. can be used, with nitrates being more preferred.

[0069] There is no particular limitation on the amount of the precious metal source (palladium source, platinum source), but it is preferably the amount of each precious metal content (loading) as described above. It should be noted that when using two or more precious metal sources in combination, it is preferable that the total amount of the precious metal sources is the amount of the precious metal content (loading) described above.

[0070] (alumina)

[0071] The second catalyst layer contains alumina. This allows palladium (the catalyst component) to be supported on the alumina, increasing the contact area between palladium and the exhaust gas or adsorbing reactants. Consequently, the overall reactivity of the catalyst can be further improved.

[0072] Here, there are no particular limitations on the alumina that can be used in the second catalyst layer as long as it contains aluminum oxide. Examples include active alumina such as γ-, δ-, η-, and θ-alumina, alumina containing lanthanum oxide, alumina containing silica, alumina containing silica-titanium dioxide, and alumina containing silica-titanium dioxide-zirconia. These alumina can be used alone or in combination of two or more. From the viewpoint of high-temperature durability and high specific surface area, γ-, δ-, or θ-alumina, and alumina containing lanthanum oxide are preferred. Furthermore, in the case of alumina containing lanthanum oxide, the lanthanum oxide content in the alumina containing lanthanum oxide is preferably 0.5 to 8% by mass, more preferably 1 to 5% by mass. It should be noted that, in this specification, alumina containing X refers to alumina containing aluminum in more than half of its total composition (molar ratio in metal terms), and the remaining proportion containing component X. For example, the molar ratio of aluminum (Al) in lanthanum oxide-containing alumina to the total molar ratio of lanthanum (La) and aluminum (Al) constituting lanthanum oxide-containing alumina [=Al / (La+Al)] exceeds 0.5.

[0073] The pore volume distribution of alumina is not particularly limited, but the alumina preferably contains macroporous alumina. This macroporous alumina, in a pore volume distribution determined by nitrogen adsorption, has a pore volume of 0.60 mL / g or more for pores with a diameter of 2 nm or more and 40 nm or less, more preferably containing macroporous alumina with a pore volume exceeding 0.70 mL / g, even more preferably containing macroporous alumina with a pore volume exceeding 0.8 mL / g, and particularly preferably containing macroporous alumina with a pore volume exceeding 0.98 mL / g. By using macroporous alumina, the Vo of the catalyst can be more easily achieved. 2-40nm Therefore, catalytic performance can be further improved. It should be noted that the upper limit of the pore volume of macroporous alumina with pore sizes of 2 nm to 40 nm is, for example, 5 mL / g or less, preferably less than 2.1 mL / g, and more preferably less than 1.5 mL / g. Furthermore, it is particularly preferable to configure macroporous alumina in the second catalyst layer, and to configure the first catalyst layer with pores having pore sizes of 2 nm to 40 nm and small pore volumes (preferably V...). 2-40nm = 0.70 mL / g or less, V is particularly preferred 2-40nm Alumina with a pore volume less than 0.60 mL / g. According to this method, the retention of waste gas within the pores is promoted in the second catalyst layer (thus, the waste gas comes into contact with palladium), enabling more efficient purification of waste gas (especially hydrocarbons in the waste gas). Simultaneously, NO and CO that are not completely purified in the second catalyst layer diffuse to the first catalyst layer (the lower layer on the three-dimensional structure side) and are purified by utilizing the NO-CO reaction with rhodium. Therefore, the catalyst of this method effectively purifies hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) in waste gas, and is particularly suitable as a three-way catalyst. The pore volume (V) of commercially available alumina with pore sizes of 2 nm to 40 nm can be measured using the macroporous alumina described above. 2-40nm Choose alumina with the desired pore volume. Alternatively, macroporous alumina can be produced by the hydrolysis of alkoxides.

[0074] Furthermore, the total pore volume of the macroporous alumina is, for example, 0.6 to 2.1 mL / g, preferably 0.8 to 1.6 mL / g. By using alumina with such a pore volume, the waste gas flowing into and diffusing into the second catalyst layer can be effectively purified within the second catalyst layer.

[0075] In this specification, in the pore volume distribution determined by the nitrogen adsorption method, alumina with pores having a diameter of 2 nm or larger and a pore volume of 0.60 mL / g or larger is also referred to as "macroporous alumina".

[0076] When the second catalyst layer contains macroporous alumina, the proportion of macroporous alumina in the total alumina in the second catalyst layer is preferably 50% by mass or more, more preferably more than 50% by mass, further preferably more than 80% by mass (upper limit: 100% by mass), and particularly preferably 100% by mass. That is, in a preferred embodiment of the invention, the second catalyst layer contains macroporous alumina at a proportion of 50% by mass or more relative to the total mass of alumina. In a more preferred embodiment of the invention, the second catalyst layer contains macroporous alumina at a proportion of more than 50% by mass relative to the total mass of alumina. In a further preferred embodiment of the invention, the second catalyst layer contains macroporous alumina at a proportion of more than 80% by mass relative to the total mass of alumina. In a particularly preferred embodiment of the invention, the second catalyst layer contains macroporous alumina at a proportion of 100% by mass relative to the total mass of alumina (i.e., the second catalyst layer contains only macroporous alumina as alumina). By using macroporous alumina in such an amount, the V of the catalyst can be further increased. 2-40nm This further improves catalytic performance. It should be noted that the first catalyst layer may contain macroporous alumina, but the proportion of macroporous alumina in the total alumina in the first catalyst layer is less than 50% by mass, preferably less than 10% by mass, more preferably less than 5% by mass (lower limit: 0% by mass), and particularly preferably 0% by mass (i.e., the first catalyst layer does not contain macroporous alumina).

[0077] It should be noted that, in the case where the second catalyst layer contains alumina other than macroporous alumina, the alumina specified in the alumina of the first catalyst layer described above can be used.

[0078] Furthermore, when the second catalyst layer contains macroporous alumina, the proportion of macroporous alumina in the second catalyst layer relative to the total mass of the second catalyst layer is preferably more than 20% by mass, more preferably more than 40% by mass and less than 70% by mass, even more preferably more than 45% by mass and less than 60% by mass, and particularly preferably more than 48.0% by mass and less than 55.0% by mass. By using macroporous alumina in such a proportion, the V of the catalyst can be improved. 2-40nmThat is, in a preferred embodiment of the invention, macroporous alumina is included in a proportion exceeding 20% ​​by mass relative to the total mass of the second catalyst layer. In a more preferred embodiment of the invention, macroporous alumina is included in a proportion exceeding 40% by mass and less than 70% by mass relative to the total mass of the second catalyst layer. In a further preferred embodiment of the invention, macroporous alumina is included in a proportion of 45% by mass or more and less than 60% by mass relative to the total mass of the second catalyst layer. In a particularly preferred embodiment of the invention, macroporous alumina is included in a proportion exceeding 48.0% by mass and less than 55.0% by mass relative to the total mass of the second catalyst layer.

[0079] Furthermore, there is no particular limitation on the BET specific surface area of ​​macroporous alumina; from the viewpoint of supporting catalyst composition, it is preferably 80–450 μm². 2 / g, more preferably 100-250m 2 / g, particularly preferably 120-200m 2 / g. With such a specific surface area, a sufficient amount of noble metal (catalyst component) can be loaded onto macroporous alumina, increasing the contact area between the catalyst component and the waste gas, or adsorbing reactants. As a result, the reactivity of the catalyst as a whole can be further improved.

[0080] The shape of macroporous alumina is not particularly limited; for example, it can be any shape, such as granular, microparticle, powder, cylindrical, conical, prismatic, cubic, pyramidal, or irregular. Granular, microparticle, or powder form is preferred, and powder form is more preferred. When the macroporous alumina is in granular, microparticle, or powder form, the average primary particle size is preferably 5–20 nm, more preferably 5–10 nm. Within this range, the catalyst component can be efficiently loaded onto the surface of the macroporous alumina. Furthermore, the average secondary particle size of the macroporous alumina (before pulverization) is preferably 10–90 μm, more preferably 20–50 μm. Within this range, the catalyst component can be efficiently loaded onto the alumina surface.

[0081] The content (loading) of macroporous alumina is not particularly limited, but is preferably 10 to 300 g, more preferably 20 to 200 g, and particularly preferably 30 to 100 g, relative to 1 L of the three-dimensional structure. If the content of macroporous alumina relative to 1 L of the three-dimensional structure is 10 g or more, palladium can be sufficiently dispersed in the macroporous alumina, resulting in a catalyst with better durability. On the other hand, when the content of macroporous alumina is 300 g or less, the contact between palladium and the waste gas becomes better, allowing for more efficient waste gas purification. It should be noted that when the above-mentioned content of macroporous alumina is used when the second catalyst layer contains two or more types of macroporous alumina, it refers to their total amount.

[0082] (strontium)

[0083] The second catalyst layer contains strontium in addition to palladium and alumina. This second catalyst layer is positioned on the side in contact with the exhaust gas, and therefore is exposed to a higher temperature than the first catalyst layer. On the other hand, alumina, especially large-pore alumina, is prone to thermal shrinkage; therefore, the catalyst exposed to exhaust gas has a pore size of 2 nm or more and a pore volume (Vt) of 40 nm or less. 2-40nm The amount of strontium will decrease. However, the second catalyst layer of the present invention contains strontium. Strontium enters the alumina framework, inhibiting the aggregation of alumina particles. Therefore, by coexisting alumina, especially macroporous alumina, with strontium, it is possible to suppress the thermal shrinkage of alumina and maintain Vo. 2-40nm .

[0084] Strontium present in the second catalyst layer is in the form of oxides, sulfates, or carbonates. Preferably, strontium is in the form of oxides or carbonates. That is, the second catalyst layer comprises palladium, oxides, sulfates, or carbonates of strontium, and alumina. In a preferred embodiment of the invention, the second catalyst layer comprises palladium, oxides or carbonates of strontium, and alumina (particularly macroporous alumina).

[0085] The amount of strontium used (loading) relative to 1 liter of three-dimensional structure, expressed as oxide (SrO), is greater than 0 g / L and less than 15 g / L, preferably 2.5 g / L or more and less than 10.0 g / L, and most preferably 4.0 g / L or more and less than 7.5 g / L. Here, without the use of strontium (Sr usage = 0 g / L), the thermal shrinkage of alumina (especially macroporous alumina) cannot be suppressed, resulting in poor catalytic performance. On the other hand, if the amount of strontium used is 15 g / L or more, the pores are blocked by strontium, or the pore size is reduced, thus sometimes resulting in poor catalytic performance.

[0086] The mixing ratio of strontium (Sr) to palladium in the second catalyst layer is preferably 0.1 to 5.0 moles of strontium (Sr) (converted to strontium oxide) relative to palladium (Pd), more preferably 0.5 to 3.5, and particularly preferably more than 0.7 and less than 2.5. The mixing ratio of strontium (Sr) to palladium in the second catalyst layer is calculated by dividing the amount of strontium used (converted to strontium oxide) in the second catalyst layer (in moles) by the amount of palladium used (in moles) in the second catalyst layer, rounded to two decimal places, and then rounded to one decimal place.

[0087] When the second catalyst layer contains macroporous alumina, the mixing ratio of strontium to macroporous alumina in the second catalyst layer is preferably 0.01 or more and less than 0.8, more preferably 0.03 or more and less than 0.50, and particularly preferably 0.05 or more and less than 0.35. Such a mixing ratio can more effectively suppress and prevent the thermal shrinkage of the macroporous alumina. The mixing ratio of strontium to macroporous alumina in the second catalyst layer is obtained by dividing the amount of strontium used in the second catalyst layer (in moles of strontium oxide) by the amount of macroporous alumina used in the second catalyst layer (in moles), rounded to two decimal places.

[0088] When the second catalyst layer contains macroporous alumina, the product of the proportion of macroporous alumina in the total alumina in the second catalyst layer and the molar ratio of strontium (converted to strontium oxide) to macroporous alumina [=(proportion of macroporous alumina in the total alumina in the second catalyst layer (%))×(molar ratio of strontium (converted to strontium oxide) to macroporous alumina)] (“alumina2 value” in the following examples) is preferably greater than 0 and less than 32, more preferably 5.0 to 22.0, and particularly preferably 12.0 to 16.5. If such a proportion is achieved, the thermal shrinkage of macroporous alumina can be more effectively suppressed and prevented.

[0089] As a raw material for strontium (strontium source), strontium hydroxide (Sr(OH)2), strontium sulfate (SrSO4), strontium carbonate (SrCO3), strontium acetate (Sr(CH3COO)2), etc. can be used, with strontium hydroxide and strontium sulfate being preferred, and strontium hydroxide being more preferred.

[0090] There is no particular limitation on the amount of strontium source, but it is preferably the amount that results in the strontium content (loading) as described above. It should be noted that when using two or more strontium sources in combination, it is preferable that the total amount of strontium sources is the amount that results in the strontium content (loading) described above.

[0091] Furthermore, from the viewpoint of heat resistance and pore retention of macroporous alumina, the second catalyst layer preferably does not contain any Group II elements other than strontium (i.e., beryllium (Be), magnesium (Mg), calcium (Ca), barium (Ba), and radium (Ra)). Strontium is less basic than barium, therefore it is considered unlikely to disrupt the alumina structure and is more likely to stabilize it. Specifically, the content of Group II elements other than strontium in the second catalyst layer is less than 0.1 g / L relative to 1 liter of three-dimensional structure. Preferably, the content of Group II elements other than strontium in the second catalyst layer is less than 0.05 g / L relative to 1 liter of three-dimensional structure (lower limit: 0 g / L). Particularly preferably, the content of Group II elements other than strontium in the second catalyst layer is 0 g / L relative to 1 liter of three-dimensional structure (the second catalyst layer contains only strontium as a Group II element). It should be noted that the presence of Group II elements other than strontium in the second catalyst layer can be confirmed by known methods, such as fluorescence X-ray analysis.

[0092] (Cerium oxide-zirconia composite oxide)

[0093] The second catalyst layer preferably comprises a cerium oxide-zirconia composite oxide, and more preferably both the first and second catalyst layers comprise a cerium oxide-zirconia composite oxide. That is, in a preferred embodiment of the present invention, the first catalyst layer and the second catalyst layer comprise a cerium oxide-zirconia composite oxide.

[0094] Specifically, in one embodiment of the present invention, the second catalyst layer comprises palladium, strontium, and alumina, wherein the alumina comprises macroporous alumina. In another embodiment of the present invention, the second catalyst layer comprises palladium, strontium, alumina, and a cerium oxide-zirconia composite oxide, wherein the alumina comprises macroporous alumina. In yet another embodiment of the present invention, the second catalyst layer is composed of palladium, strontium, macroporous alumina, and a cerium oxide-zirconia composite oxide.

[0095] In one embodiment of the present invention, the first catalyst layer comprises rhodium, and at least one of cerium oxide-zirconia composite oxide and alumina; the second catalyst layer comprises palladium, strontium, and alumina, wherein the alumina in the second catalyst layer comprises macroporous alumina in a proportion of 50% by mass or more relative to the total mass of the alumina in the second catalyst layer. In another embodiment of the present invention, the first catalyst layer comprises rhodium, cerium oxide-zirconia composite oxide, and alumina; the second catalyst layer comprises palladium, strontium, alumina, and cerium oxide-zirconia composite oxide, wherein the alumina in the second catalyst layer comprises macroporous alumina in a proportion of more than 80% by mass relative to the total mass of the alumina in the second catalyst layer. In another embodiment of the present invention, the first catalyst layer is composed of rhodium, cerium oxide-zirconia composite oxide, and alumina; the second catalyst layer is composed of palladium, strontium, macroporous alumina, and cerium oxide-zirconia composite oxide.

[0096] Here, when the second catalyst layer contains a cerium oxide-zirconia composite oxide, the cerium oxide-zirconia composite oxide (CeO2-ZrO2) functions as an oxygen uptake and storage material. This oxygen uptake and storage material (also called an "oxygen uptake and release material") functions to uptake and store oxygen in an oxidizing atmosphere (lean) and release oxygen in a reducing atmosphere (rich) according to the change in the air-fuel ratio (A / F) corresponding to the operating conditions, thereby stabilizing the redox reaction.

[0097] Unless otherwise specified, the definition of cerium oxide-zirconia composite oxide that can be used in the second catalyst layer is the same as that in the first catalyst layer described above, and therefore the description is omitted here.

[0098] The content (loading; oxide conversion) of the cerium oxide-zirconia composite oxide in the second catalyst layer is not particularly limited, but is preferably 5 to 200 g, more preferably 5 to 100 g, and even more preferably 10 to 90 g, relative to 1 liter of three-dimensional structure. Furthermore, when the second catalyst layer contains two or more cerium oxide-zirconia composite oxides, the above-mentioned content of the cerium oxide-zirconia composite oxide refers to its total amount. The cerium oxide-zirconia composite oxide can be the same composite oxide as that in the first catalyst layer.

[0099] Furthermore, when the second catalyst layer comprises alumina and cerium oxide-zirconia composite oxide, the mixing ratio (mass ratio) of alumina to cerium oxide-zirconia composite oxide is preferably 10:1 to 1:10, more preferably 10:2 to 5:10, and most preferably 10:3 to 10:10. With such a ratio, a sufficient amount of palladium (catalyst component) can be loaded onto the alumina, increasing the contact area between the catalyst component and the exhaust gas, and enabling the cerium oxide-zirconia composite oxide to adequately adsorb hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) in the exhaust gas. As a result, the overall reactivity of the catalyst can be further improved, and the exhaust gas purification performance can be further enhanced.

[0100] (Refractory inorganic oxides)

[0101] The second catalyst layer may further comprise refractory inorganic oxides other than alumina (especially macroporous alumina) and cerium oxide-zirconia composite oxides (other refractory inorganic oxides). Here, the refractory inorganic oxide has a high specific surface area, which, by loading the catalyst components onto it, increases the contact area between the catalyst components and the exhaust gas, or adsorbs reactants. As a result, the overall reactivity of the catalyst can be further improved. When the second catalyst layer comprises other refractory inorganic oxides, the same materials as those used in the first catalyst layer described above can be used.

[0102] [3D Structure]

[0103] The catalyst of the present invention is formed by stacking (loading) the first and second catalyst layers on a three-dimensional structure.

[0104] Here, there are no particular limitations on the three-dimensional structure, and the fire-resistant three-dimensional structures commonly used in this field can be used in the same way. For example, a heat-resistant carrier with triangular, quadrilateral, or hexagonal honeycomb carriers for the through-holes (gas passages, compartment shapes) can be used as a three-dimensional structure. The three-dimensional structure is preferably a monolithic three-dimensional structure (three-dimensional monolithic structure, monolithic structure), for example, a one-piece carrier, a metal honeycomb carrier, or perforated metal is preferred.

[0105] As a monolithic carrier, any carrier known as a ceramic honeycomb carrier is generally acceptable, with carriers made of materials such as cordierite, mullite, silicon carbide, and silicon nitride being particularly preferred, especially cordierite carriers (cordierite carriers). Alternatively, materials that utilize oxidation-resistant and heat-resistant metals, such as stainless steel or Fe-Cr-Al alloys, to form a monolithic structure can also be used.

[0106] These integral carriers are manufactured using methods such as extrusion molding or winding and fixing sheet-like elements. The shape of the through-hole (gas passage, compartment shape) can be any of hexagonal (honeycomb), quadrilateral, triangular, or corrugated (pleated). A compartment density (number of compartments / unit cross-sectional area) of 100 to 1200 compartments / square inch is sufficient, preferably 200 to 1000 compartments / square inch, and more preferably 300 to 900 compartments / square inch (1 inch = 25.4 mm).

[0107] <Manufacturing Method of Waste Gas Purification Catalyst>

[0108] The catalyst for exhaust gas purification of the present invention can be manufactured by appropriately referring to known methods. As described above, by using rhodium to form a first catalyst layer and palladium, strontium, and alumina (especially macroporous alumina) to form a second catalyst layer, a pore volume (V) of pore size of 2 nm or more and 40 nm or less can be achieved. 2-40nm ).

[0109] Hereinafter, as a preferred embodiment of the present invention, there is a first catalyst layer formation step (A) including a slurry preparation step (a-1), a slurry coating step (a-2), a drying step (a-3), and a calcination step (a-4); and a second catalyst layer formation step (B) including a slurry preparation step (b-1), a slurry coating step (b-2), a drying step (b-3), and a calcination step (b-4).

[0110] [Formation process of the first catalyst layer (A)]

[0111] (a-1) Slurry preparation process

[0112] After dispersing the rhodium raw material and the raw materials described in the above-mentioned [first catalyst layer] as needed in an aqueous medium (e.g., water), wet pulverization is performed to prepare a slurry. Here, wet pulverization can be carried out, for example, by a known method using a ball mill or the like. In addition, there are no particular limitations on the wet pulverization conditions, and ordinary conditions can be appropriately selected. For example, it is preferable to perform wet pulverization using a ball mill at a speed of, for example, 50 rpm to 5000 rpm for 5 minutes to 5 hours.

[0113] (a-2) Slurry coating process

[0114] In this process, the slurry obtained in the slurry preparation step (a-1) above is coated onto the three-dimensional structure. The method for coating the slurry onto the three-dimensional structure can appropriately employ known methods such as washing. Furthermore, the coating amount of the slurry can be appropriately set by those skilled in the art based on the amount of solids in the slurry and the thickness of the formed catalyst layer. Preferably, the coating amount of the slurry is the amount (loading) of rhodium, alumina, and cerium oxide-zirconia composite oxide, and (if applicable) any other additives, as described above.

[0115] (a-3) Drying process

[0116] The drying process is the process of drying the slurry applied to the three-dimensional structure in the slurry coating process to form a coating film.

[0117] In the drying process, the coating (slurry coating) applied to the three-dimensional structure is dried in air, preferably at a temperature of 50°C to 170°C, more preferably 70°C to 150°C, for 5 minutes to 10 hours, preferably 15 minutes to 3 hours.

[0118] (a-4) Firing process

[0119] In this process, the coating is fired after step (a-3) above. As a result, the catalyst components (rhodium, alumina, cerium oxide-zirconia composite oxide, etc.) adhere to the three-dimensional structure. Furthermore, residual nitrogen-containing, hydrogen-containing, and carbon-containing components in the catalyst layer are removed.

[0120] There are no particular restrictions on the firing conditions. For example, firing can be carried out in air at a temperature of 440°C to 800°C, preferably 480°C to 700°C, more preferably 500°C to 600°C, for 10 minutes to 3 hours, preferably 15 minutes to 1 hour. Under such conditions, catalyst components (rhodium, alumina, cerium oxide-zirconia composite oxide, etc.) can be efficiently attached to the three-dimensional structure.

[0121] Alternatively, firing can be carried out while air or other gases are flowing. In this case, there is no particular limitation on the speed of gas flow (gas velocity), but it is preferably 0.1 m / s or more, and more preferably 0.2 to 1.2 m / s.

[0122] Through the above process, a first catalyst layer is formed on the three-dimensional structure.

[0123] [Formation process of the second catalyst layer (B)]

[0124] (b-1) Slurry preparation process

[0125] Palladium, strontium, and alumina, along with the raw materials described in the [second catalyst layer] as needed, are dispersed in an aqueous medium (e.g., water) and then wet-milled to prepare a slurry. Here, wet milling can be performed, for example, by a known method using a ball mill or similar equipment. Furthermore, the wet milling conditions are not particularly limited; in the case of a slurry containing macroporous alumina, the particle size of the solid components in the milled slurry is preferably 2 μm to 10 μm, more preferably 3 μm to 8 μm. A particle size of 3 μm or larger allows pores of 2 nm or larger and 40 nm or smaller to exist in a predetermined proportion. Furthermore, a particle size of 8 μm or smaller improves the bonding strength to three-dimensional structures. It should be noted that the particle size of the solid components in the milled slurry can be measured using a laser diffraction / scattering particle size distribution measuring device. To prepare a slurry with such a solid component particle size, wet milling at a speed of, for example, 50 rpm to 5000 rpm for 5 minutes to 5 hours is preferred.

[0126] (b-2) Slurry coating process

[0127] In this process, the slurry obtained in the slurry preparation process (b-1) above is coated onto the first catalyst layer of the three-dimensional structure. The specific method is the same as the slurry coating process (a-2) above.

[0128] (b-3) Drying process

[0129] The drying process is the process of drying the slurry on the first catalyst layer of the three-dimensional structure coated in the slurry coating process to form a coating film. The specific method is the same as the drying process (a-3) described above.

[0130] (b-4) Firing process

[0131] In this process, the coating is fired after step (b-3) above. As a result, the catalyst components (palladium, strontium, alumina, cerium oxide-zirconia composite oxide, etc.) adhere to the three-dimensional structure. Furthermore, residual nitrogen-containing, hydrogen-containing, and carbon-containing components in the catalyst layer are removed. The specific method is the same as the firing step (a-4) above.

[0132] Thus, a second catalyst layer is formed on the first catalyst layer of the three-dimensional structure, thereby manufacturing the catalyst of the present invention.

[0133] <Methods for purifying exhaust gases>

[0134] The catalyst of the present invention exhibits high purification performance for exhaust gases (hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx)) emitted from gasoline engines. Therefore, the present invention also provides a method for purifying gasoline engine exhaust gases, comprising the step of treating exhaust gases emitted from a gasoline engine (particularly purifying hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) in the exhaust gases) using the exhaust gas purification catalyst of the present invention (second aspect of the invention). The purification rate (purification capacity) of the gasoline engine exhaust gases can be evaluated, for example, by the temperature (T50 (°C)) at which the purification rates of CO, HC, and NOx reach 50% in the following ignition (LO) test. Furthermore, the lower the T50, the higher the exhaust gas purification performance (catalytic performance) of the catalyst.

[0135] Exhaust gases typically contain HC, CO, and NOx, and may contain nitrogen oxides (e.g., NO, NO2, N2O), carbon monoxide (CO), carbon dioxide (CO2), oxygen (O2), hydrogen (H2), ammonia (NH3), water (H2O), sulfur dioxide (SO2), hydrocarbons (HC), etc. in any proportion.

[0136] The gasoline engine for which this exhaust gas purification method is applied is intended to exclude diesel engines. Besides conventional gasoline engines, it also includes, for example, gasoline-electric hybrid engines and engines using natural gas, ethanol, dimethyl ether, etc., as fuel. Among these, gasoline engines and gasoline-electric hybrid engines are preferred.

[0137] As a method for bringing the catalyst of the present invention into contact with exhaust gas, for example, a method is to place the catalyst for exhaust gas purification in the exhaust flow path of the exhaust port of a gasoline engine and allow the exhaust gas to flow into the exhaust flow path.

[0138] The exhaust gas temperature is preferably within the range of 0°C to 800°C, which is the typical exhaust gas temperature range of a gasoline engine during operation. Here, the air-fuel ratio (A / F) in the exhaust gas of a gasoline engine with a temperature of 0°C to 800°C is 10 to 30, preferably 11 to 14.7. Alternatively, as another preferred embodiment, the exhaust gas temperature can be in the high-temperature range of 800 to 1200°C. Here, the air-fuel ratio in the exhaust gas of an internal combustion engine with a temperature of 800 to 1200°C is preferably 10 to 18.6.

[0139] Furthermore, the catalyst of the present invention can exhibit excellent waste gas treatment performance even for waste gases that have been exposed to low temperatures of 50°C to 600°C for a long time (especially those containing HC, CO, NOx, water vapor, etc.) or waste gases that have been exposed to high temperatures of 650°C to 900°C (especially those containing HC, CO, NOx, water vapor, etc.) in the catalyst bed for a long time and are now at low temperatures of 50°C to 600°C.

[0140] Therefore, the catalyst of the present invention as described above, or the catalyst manufactured by the method described above, can be exposed to exhaust gas at a temperature of 650°C to 900°C, preferably 700°C to 850°C. Furthermore, the time for which the catalyst of the present invention is exposed to high-temperature exhaust gas (the time for which exhaust gas flows in) is not particularly limited, for example, 10 hours to 800 hours, preferably 16 hours to 500 hours, more preferably 40 hours to 100 hours. The catalyst of the present invention also exhibits high performance after exposure to such high-temperature exhaust gas. To investigate the exhaust gas purification performance of the catalyst after such exposure to high-temperature exhaust gas, as a heat treatment, it is effective to evaluate the exhaust gas purification performance (resistance to catalyst degradation) by exposing the catalyst to exhaust gas at, for example, 650 to 1000°C, preferably 650 to 900°C, for 10 to 300 hours.

[0141] It should be noted that, in this specification, "exhaust gas temperature" refers to the temperature of the exhaust gas at the catalyst inlet. Here, "catalyst inlet" refers to the portion extending 10 cm from the catalyst end face on the exhaust gas inflow side of the exhaust pipe where the exhaust gas purification catalyst is installed, towards the internal combustion engine side, and specifically refers to the central portion of the exhaust pipe along its length (axial direction). Furthermore, in this specification, "catalyst bed portion" refers to the central portion of the aforementioned exhaust pipe between the catalyst end face on the exhaust gas inflow side and the catalyst end face on the exhaust gas outflow side, and specifically refers to the central portion of the exhaust pipe's cross-section (or, if the exhaust pipe's cross-section is not circular, the centroid of the exhaust pipe's cross-section).

[0142] Although embodiments of the invention have been described in detail, they are illustrative and exemplary and not limiting. The scope of the invention should obviously be interpreted through the appended claims.

[0143] The present invention includes the following methods and forms.

[0144] 1. A catalyst for purifying waste gas, comprising a first catalyst layer and a second catalyst layer sequentially stacked on a three-dimensional structure, wherein the first catalyst layer contains rhodium, the second catalyst layer contains palladium, strontium, and aluminum oxide, wherein the strontium content (converted to strontium oxide) in the second catalyst layer is greater than 0 g / L and less than 15 g / L relative to 1 liter of the three-dimensional structure, and wherein the pore volume distribution of the catalyst for purifying waste gas, as determined by nitrogen adsorption, has a pore volume of pores with a pore size of 2 nm or more and less than 40 nm exceeding 0.26 mL / g.

[0145] 2. The catalyst for purifying waste gas as described in 1. above, wherein the total pore volume of the catalyst for purifying waste gas is 0.45 mL / g or more.

[0146] 3. The catalyst for purifying waste gas according to 1. or 2. above, wherein the alumina comprises macroporous alumina, and the macroporous alumina, in the pore volume distribution determined by nitrogen adsorption method, has a pore volume of pores with a diameter of 2 nm or more and 40 nm or less of 0.60 mL / g or more.

[0147] 4. The catalyst for purifying waste gas according to 3. above, wherein the macroporous alumina is contained in the second catalyst layer at a proportion of 50% by mass or more relative to the total mass of the alumina.

[0148] 5. The catalyst for purifying waste gas according to 3. or 4. above, wherein the macroporous alumina is contained in a proportion of more than 20% by mass relative to the total mass of the second catalyst layer.

[0149] 6. The catalyst for purifying waste gas according to any one of 3 to 5 above, wherein the molar ratio of strontium (strontium oxide equivalent) to the macroporous alumina is 0.01 or more and less than 0.08.

[0150] 7. The catalyst for purifying exhaust gas according to any one of 1. to 6. above, wherein the porosity after heat treatment is 42.0% or more and less than 80.0%.

[0151] 8. The catalyst for purifying exhaust gas according to any one of 1. to 7. above, wherein the content of Group II elements other than strontium in the second catalyst layer is less than 0.1 g / L relative to 1 liter of the three-dimensional structure.

[0152] 9. The catalyst for purifying waste gas according to any one of 1. to 8 above, wherein the first catalyst layer and the second catalyst layer comprise cerium oxide-zirconia composite oxide.

[0153] 10. A method for purifying exhaust gas from a gasoline engine, the method comprising the step of treating the exhaust gas discharged from the gasoline engine using any one of the exhaust gas purification catalysts described in any one of 1. to 9.

[0154] Example

[0155] The effects of the present invention will be illustrated using the following examples and comparative examples. However, the technical scope of the present invention is not limited to the following examples. It should be noted that, unless otherwise specified, the operation in the following examples is carried out at room temperature (25°C). In addition, unless otherwise specified, "%" and "parts" refer to "mass %" and "parts by mass," respectively.

[0156] Example 1

[0157] Using an aqueous solution of rhodium nitrate as the Rh source and lanthanum-containing alumina (containing 1% by mass of La2O3 and V) as the alumina raw material. 2-40nm =0.55mL / g, Total pore volume =0.58mL / g, BET specific surface area =155m² 2 / g, average secondary particle size = 65μm (alumina 1), cerium dioxide-zirconia composite oxide (CeO2:ZrO2:La2O3 = 30:60:10 (mass ratio), BET specific surface area = 68m² 2 / g, average secondary particle size (D50) = 15μm (CZ1), each raw material was weighed according to the mass ratio of Rh:CZ1:alumina 1 as shown in Table 1 after calcination of the first catalyst layer. The weighed raw materials were added to pure water and dispersed. After stirring the dispersion for 1 hour, it was wet-milled in a ball mill at 200 rpm for 30 minutes to prepare slurry a1 (solids mass concentration = 40% by mass, pH = 6).

[0158] Next, the slurry a1 was washed onto a cylindrical cordierite support (diameter = 110 mm, length = 97 mm, 750 compartments / square inch, 3 mils) as a three-dimensional structure, with the slurry loading after firing being the value of the first catalyst layer of catalyst A in Table 1. Then, after drying at 150°C for 15 minutes, it was fired in air at 550°C for 30 minutes, thereby obtaining precursor A1 with a first catalyst layer (lower catalyst layer) formed on the cordierite support.

[0159] Aluminum isopropoxide was used as the raw material for preparing the alumina carrier. Two solutions were prepared: one containing 100 ml of isopropoxide and 200 ml of pure water, and the other containing 100 ml of pure water. 100 g of powdered aluminum isopropoxide was taken into a 500 ml beaker and hydrolyzed using the two different concentrations of alcohol solutions. The mixture was vigorously stirred at 43°C for 15 minutes. The resulting hydrogel was then vacuum-dried and shaped at room temperature (25°C). After drying at 120°C for 2 hours, it was calcined at 550°C for 2 hours to obtain alumina 2 precursor. Next, the alumina 2 precursor was added to an aqueous solution obtained by adding lanthanum nitrate to 500 ml of pure water, and the mixture was stirred to obtain a suspension slurry. After evaporating and solidifying the slurry, the product was calcined at 600°C for 3 hours to obtain macroporous alumina (alumina 2) containing lanthanum. The alumina 2 obtained above contains 1% by mass of lanthanum in the form of La2O3, V 2-40nm The concentration is 0.98 mL / g, the total pore volume is 1.1 mL / g, and the BET specific surface area is 165 m². 2 / g, with an average secondary particle size of 37μm.

[0160] Next, the aqueous solution of palladium nitrate as the Pd source, the cerium oxide-zirconia composite oxide (CeO2:ZrO2:La2O3 = 47:47:6 (mass ratio) and the BET specific surface area = 67 m² were weighed. 2 The raw materials, including CZ2 (with an average secondary particle size (D50) of 5.5 μm), strontium hydroxide octahydrate as a strontium source, and alumina 2 as an alumina raw material, were weighed such that the mass ratio of Pd:CZ2:SrO:alumina 2 after calcination of the second catalyst layer was equal to the values ​​in Table 1. The weighed raw materials were added to pure water and dispersed. After stirring the dispersion for 1 hour, it was wet-milled at 200 rpm for 30 minutes using a ball mill to prepare slurry a2 (solids content mass concentration = 40% by mass, pH = 7).

[0161] Next, the slurry a2 is washed onto the precursor A1 with a loading amount after calcination that is the value of the second catalyst layer of catalyst A in Table 1. Then, after drying at 150°C for 15 minutes, it is calcined in air at 550°C for 30 minutes, thereby forming a second catalyst layer (upper catalyst layer) (catalyst A) on the first catalyst layer (lower catalyst layer) of the precursor A1.

[0162] Example 2

[0163] The mass ratio of alumina 1 to alumina 2 in the second catalyst layer becomes the value of catalyst B in Table 1 after the second catalyst layer is calcined. Otherwise, catalyst B is prepared in the same manner as in Example 1.

[0164] Example 3

[0165] The SrO content (loading) in the second catalyst layer becomes the value of catalyst C in Table 1 after the second catalyst layer is calcined. Otherwise, catalyst C is prepared in the same manner as in Example 1.

[0166] Example 4

[0167] The SrO content (loading) in the second catalyst layer becomes the value of catalyst D in Table 1 after the second catalyst layer is calcined. Otherwise, catalyst D is prepared in the same manner as in Example 1.

[0168] Example 5

[0169] As the strontium source in the second catalyst layer, strontium sulfate (SrSO4) was used instead of strontium hydroxide, and the content (loading) of SrSO4 became the value of catalyst E in Table 1 after calcination of the second catalyst layer. Otherwise, catalyst E was prepared in the same manner as in Example 1.

[0170] Comparative Example 1

[0171] Catalyst F was prepared in the same manner as in Example 1, except that alumina 1 was used instead of alumina 2 in the second catalyst layer.

[0172] Comparative Example 2

[0173] The content (loading) of SrO in the second catalyst layer was made to be the value of catalyst G in Table 1 after calcination of the second catalyst layer (i.e., the second catalyst layer was formed without adding strontium hydroxide octahydrate). Otherwise, catalyst G was prepared in the same manner as in Example 1.

[0174] Comparative Example 3

[0175] The SrO content (loading) in the second catalyst layer becomes the value of catalyst H in Table 1 after the second catalyst layer is calcined. Otherwise, catalyst H is prepared in the same manner as in Example 1.

[0176] Comparative Example 4

[0177] Barium sulfate (BaSO4) was used to replace strontium hydroxide in the second catalyst layer, and the content (loading) of BaSO4 after calcination of the second catalyst layer became the value of catalyst I in Table 1. Otherwise, catalyst I was prepared in the same manner as catalyst A.

[0178] That is, slurry a1 was prepared in the same manner as in Example 1. Then, precursor A1 was obtained in the same manner as in Example 1.

[0179] Weigh out palladium nitrate aqueous solution, CZ2, barium sulfate (BaSO4), and alumina 2 such that the mass ratio of Pd:CZ2:BaSO4:alumina 2 after calcination of the second catalyst layer is as shown in Table 1. Add the weighed raw materials to pure water and disperse them. After stirring the dispersion for 1 hour, wet-mill it using a ball mill at 200 rpm for 30 minutes to prepare slurry I2 (solids mass concentration = 40% by mass, pH = 7).

[0180] Next, the slurry I2 is washed onto the precursor A1 with a loading amount that is the value of the second catalyst layer of catalyst I in Table 1 after calcination. Then, after drying at 150°C for 15 minutes, it is calcined in air at 550°C for 30 minutes, thereby forming a second catalyst layer (upper catalyst layer) (catalyst I) on the first catalyst layer (lower catalyst layer) of the precursor A1.

[0181] Comparative Example 5

[0182] The mass ratio of alumina 1 to alumina 2 in the second catalyst layer becomes the value of catalyst J in Table 1 after the second catalyst layer is calcined. Otherwise, catalyst J is prepared in the same manner as in Comparative Example 4.

[0183] Comparative Example 6

[0184] Catalyst K was prepared in the same manner as in Comparative Example 4, except that alumina 1 was used instead of alumina 2 in the second catalyst layer.

[0185] Comparative Example 7

[0186] Slurry a1 and slurry a2 were prepared in the same manner as in Example 1.

[0187] Slurry a2 was washed onto a cylindrical cordierite support (diameter = 110 mm, length = 97 mm, 750 compartments / square inch, 3 mils) as a three-dimensional structure, with the loading amount after firing being the value of catalyst L in Table 1 after the first catalyst layer was fired. Then, after drying at 150°C for 15 minutes, it was fired in air at 550°C for 30 minutes, thereby obtaining precursor L1 with the first catalyst layer (lower catalyst layer) formed on the cordierite support.

[0188] Next, slurry a1 is washed onto precursor L1 with a loading amount equal to the value of the second catalyst layer of catalyst L in Table 1 after calcination. Then, after drying at 150°C for 15 minutes, it is calcined in air at 550°C for 30 minutes, thereby forming a second catalyst layer (upper catalyst layer) (catalyst L) on the first catalyst layer (lower catalyst layer) of precursor L1. It should be noted that catalyst L has a structure opposite to the first and second catalyst layers of catalyst A (i.e., the first catalyst layer of catalyst L is the second catalyst layer of catalyst A, and the second catalyst layer of catalyst L is the first catalyst layer of catalyst A).

[0189] Comparative Example 8

[0190] Slurry a1 was prepared in the same manner as in Example 1. Slurry I2 was also prepared in the same manner as in Comparative Example 4.

[0191] Slurry I2 was washed onto a cylindrical cordierite support (diameter = 110 mm, length = 97 mm, 750 compartments / square inch, 3 mils) as a three-dimensional structure, with the slurry loading after firing being the value of the first catalyst layer of catalyst M in Table 1. Then, after drying at 150°C for 15 minutes, it was fired in air at 550°C for 30 minutes, thereby obtaining precursor M1 with a first catalyst layer (lower catalyst layer) formed on the cordierite support.

[0192] Next, slurry a1 is washed onto precursor M1 with a loading amount equal to the value of the second catalyst layer of catalyst M in Table 1 after calcination. Then, after drying at 150°C for 15 minutes, it is calcined in air at 550°C for 30 minutes, thereby forming a second catalyst layer (upper catalyst layer) (catalyst M) on the first catalyst layer (lower catalyst layer) of precursor M1. It should be noted that catalyst M has a structure opposite to the first and second catalyst layers of catalyst I (i.e., the first catalyst layer of catalyst M is the second catalyst layer of catalyst I, and the second catalyst layer of catalyst M is the first catalyst layer of catalyst I).

[0193] The catalyst compositions of catalysts A-E from Examples 1-5 and catalysts F-M from Comparative Examples 1-8 are shown in Table 1 below. The content of Group II elements (including strontium) in the first catalyst layer of catalysts A-E from Examples 1-5 is 0 g / L relative to 1 liter of three-dimensional structure, and the content of Group II elements other than strontium in the second catalyst layer is 0 g / L relative to 1 liter of three-dimensional structure. It should be noted that in Table 1 below, "Alumina 2 content (%)" represents the percentage (%) of the mass of alumina 2 relative to the total mass of alumina in each catalyst (the combined mass of alumina 1 and 2). "AEM / Pd (molar ratio)" represents the molar ratio of alkaline earth metal (Sr or Ba) to Pd in ​​each catalyst. "AEM / Alumina 2 (molar ratio)" represents the molar ratio of alkaline earth metal (Sr or Ba) to alumina 2 in each catalyst. "Alumina 2 value" represents the product of alumina 2 content and AEM / alumina 2 (molar ratio) (=(alumina 2 content)×(AEM / alumina 2 (molar ratio))).

[0194] In addition, the pore volume (mL / g) of catalysts A to E of Examples 1 to 5 and catalysts F to M of Comparative Examples 1 to 8 before and after heat treatment for pores with a pore size of 2 nm or more and 40 nm or less (V in Table 2 below) 2-40nm (mL / g)”), total pore volume (mL / g), and the ratio (%) of pore volume of pores with a pore size of 2nm or larger and 40nm or smaller to the total pore volume (in Table 2 below) 2-40nm Rate (%), and pore retention rate [= (V after heat treatment)] 2-40nm )×100 / (V before heat treatment) 2-40nm The results are shown in Table 2 below.

[0195] [Table 1]

[0196]

[0197] [Table 2]

[0198]

[0199] [Exhaust Gas Purification Capacity Evaluation Test: Ignition (LO) Test]

[0200] For catalysts A to E of Examples 1 to 5 and catalysts F to M of Comparative Examples 1 to 8, the exhaust gas purification capability was evaluated through the following ignition (LO) test. Specifically, each catalyst (0.92L) was placed 25cm downstream of the exhaust port of a 3.0-liter MPI engine. A heat treatment (durability treatment) was performed at 1000°C (catalyst inlet temperature) for 50 hours, with an A / F ratio of 14.6, an amplitude of ±0.5, and a frequency of 1.0Hz. It should be noted that the catalyst inlet temperature was measured 1cm above the catalyst end face.

[0201] Next, with A / F = 14.6, amplitude = ±0.5, and frequency = 1.0 Hz, the exhaust gas, heated from 150°C to 500°C at a rate of 20°C / min, was fed into each catalyst. Samples were taken from the gas exiting the catalyst at this point, and the purification rates for CO, HC, and NOx were calculated. The temperature at which the purification rate of each gas reached 50% was set as T50 (°C). The results are shown in Table 3 below. Figure 1 A lower T50 indicates a higher ignition rate (higher catalytic performance) for each gas.

[0202] [Table 3]

[0203] Table 3

[0204]

[0205] From Table 3 above and Figure 1 It can be seen that the catalysts A to E in Examples 1 to 5 have significantly lower T50 values ​​than the catalysts F to M in Comparative Examples 1 to 8 for each gas (CO, HC, NOx), and therefore have significantly higher catalytic performance.

[0206] This application is based on Japanese Patent Application No. 2021-184009 filed on November 11, 2021, the disclosure of which is included herein and incorporated as a whole.

Claims

1. A catalyst for purifying waste gas, comprising a first catalyst layer and a second catalyst layer sequentially stacked on a three-dimensional structure. The first catalyst layer contains rhodium. The second catalyst layer comprises palladium, strontium, and aluminum oxide, wherein the strontium content in the second catalyst layer exceeds 0 g / L and is less than 15 g / L relative to 1 liter of the three-dimensional structure, wherein, The strontium content is calculated using strontium oxide. The catalyst for waste gas purification, as determined by nitrogen adsorption, has a pore volume exceeding 0.26 mL / g for pores with a diameter of 2 nm or larger and 40 nm or smaller. The alumina comprises macroporous alumina, wherein, in the pore volume distribution determined by nitrogen adsorption, the pore volume of pores with a diameter of 2 nm or larger and 40 nm or smaller is 0.60 mL / g or larger. The molar ratio of strontium to macroporous alumina is greater than 0.01 and less than 0.80, wherein the molar ratio of strontium is expressed in strontium oxide. The product of the proportion of macroporous alumina in all alumina in the second catalyst layer and the molar ratio of strontium to macroporous alumina is 12.0 to 16.5, wherein the molar ratio of strontium is converted to strontium oxide.

2. The catalyst for purifying waste gas according to claim 1, wherein, The total pore volume of the catalyst used for waste gas purification is above 0.45 mL / g.

3. The catalyst for purifying waste gas according to claim 1, wherein, The molar ratio of strontium to macroporous alumina is greater than 0.03 and less than 0.50, wherein the molar ratio of strontium is converted to strontium oxide.

4. The catalyst for purifying waste gas according to claim 1, wherein, The macroporous alumina is contained in the second catalyst layer at a proportion of 50% by mass or more relative to the total mass of the alumina.

5. The catalyst for purifying waste gas according to claim 1, wherein, The macroporous alumina is contained in a proportion of more than 20% by mass relative to the total mass of the second catalyst layer.

6. The catalyst for purifying waste gas according to claim 1, wherein, The porosity after heat treatment is above 42.0% and less than 80.0%.

7. The catalyst for purifying waste gas according to claim 1, wherein, The content of Group II elements other than strontium in the second catalyst layer is less than 0.1 g / L relative to 1 liter of the three-dimensional structure.

8. The catalyst for purifying waste gas according to claim 1, wherein, The first catalyst layer and the second catalyst layer contain cerium oxide-zirconia composite oxide.

9. A method for purifying exhaust gas from a gasoline engine, the method comprising the step of treating the exhaust gas discharged from the gasoline engine using the exhaust gas purification catalyst according to any one of claims 1 to 8.

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