Exhaust gas purification catalyst, method for producing same, and method for purifying exhaust gas using said exhaust gas purification catalyst

Abstract

The present disclosure provides a means whereby it is possible to further improve the purification performance (in particular, the purification performance at low temperatures) of CO included in exhaust gas. Disclosed is an exhaust gas purification catalyst comprising a lower catalyst layer and an upper catalyst layer having a predetermined composition, wherein the thickness of the lower catalyst layer is disposed so that the gas inflow side is relatively thick compared to the gas outflow side.

Description

Exhaust gas purification catalyst, its manufacturing method, and exhaust gas purification method using said exhaust gas purification catalyst

[0001] The present invention relates to an exhaust gas purifying catalyst, a method for producing the same, and a method for purifying exhaust gas using the exhaust gas purifying catalyst.The present invention is suitable for purifying exhaust gas emitted from diesel engines.

[0002] Many technologies have been proposed for treating exhaust gases emitted from internal combustion engines. For example, in treating exhaust gases from diesel engines, CO (carbon monoxide), HC (hydrocarbon), PM (particulate matter), NOx (nitrogen oxides), etc. are purified (reduced) by treating the exhaust gases with a combination of catalysts and devices. Here, CO and HC are generally purified by being oxidized mainly by a DOC (diesel oxidation catalyst).

[0003] A known problem with DOCs is that they are unable to exhibit sufficient oxidation performance due to their low catalytic activity at low temperatures immediately after engine start. To address this problem, Japanese Patent Laid-Open Publication No. 2011-220123 discloses an exhaust gas purification catalyst in which the catalyst layer comprises a lower precious metal coating layer in which a precious metal is supported on a carrier substrate, and an upper HC (hydrocarbon) adsorbent layer in which a precious metal is supported on a carrier substrate, the upper HC adsorbent layer having a thickness of 30 to 80 μm. According to this publication, this configuration makes it possible to obtain an exhaust gas purification catalyst that exhibits high CO oxidation activity in a temperature range that includes a low temperature range below 100°C.

[0004] However, as exhaust gas regulations become more stringent, catalysts with higher exhaust gas purification performance are required, and technology to improve exhaust gas purification performance at low temperatures, where catalyst activity is low, has been particularly desired.

[0005] The present invention has been made in view of the above circumstances, and aims to provide a means capable of further improving the purification performance (particularly the purification performance at low temperatures) of CO contained in exhaust gas.

[0006] The present inventors have conducted extensive research to solve the above-mentioned problems, and as a result have found that the above-mentioned problems can be solved by disposing an exhaust gas purifying catalyst including a lower catalyst layer and an upper catalyst layer having a predetermined composition so that the thickness of the lower catalyst layer is relatively thicker on the gas inlet side than on the gas outlet side, thereby completing the present invention.

[0007] That is, an exhaust gas purification catalyst according to one embodiment of the present invention comprises a refractory three-dimensional structure having partition walls extending from a gas inlet end face along a gas outlet end face and defining a plurality of gas flow paths penetrating from the gas inlet end face to the gas outlet end face, a lower catalyst layer formed on the partition walls and containing substantially no zeolite and alumina, platinum, and palladium, and an upper catalyst layer formed on the lower catalyst layer and positioned as an outermost layer and containing 20 g / L or more of zeolite, platinum, and palladium. The lower catalyst layer has a thickness change point in a range from the gas inlet end face to 20.0% but less than 90.0% of the length of the partition walls from the gas inlet end face to the gas outlet end face, and the lower catalyst layer from the gas inlet end face to the change point has an average thickness T １ is the average thickness T of the lower catalyst layer from the change point to the gas outlet end surface. ２ is characterized by being larger than

[0008] FIG. 1 is a front cross-sectional view that schematically shows a part of an exhaust gas purifying catalyst according to one embodiment of the present invention.

[0009] Hereinafter, embodiments of the present invention will be described, but the technical scope of the present invention should be defined based on the claims and is not limited to the following embodiments. In this specification, the numerical range "A to B" means "A or more and B or less." Furthermore, "A and / or B" means "either A or B" or "both A and B." Furthermore, various physical properties in this specification refer to values ​​measured by the methods described in the examples below, unless otherwise specified.

[0010] <Exhaust Gas Purification Catalyst> An exhaust gas purification catalyst (hereinafter also simply referred to as "catalyst") according to one embodiment of the present invention comprises a refractory three-dimensional structure having partition walls extending from a gas inlet end face along a gas outlet end face and defining a plurality of gas flow paths penetrating from the gas inlet end face to the gas outlet end face, a lower catalyst layer formed on the partition walls and containing substantially no zeolite and alumina, platinum, and palladium, and an upper catalyst layer formed on the lower catalyst layer and positioned as the outermost layer and containing 20 g / L or more of zeolite, platinum, and palladium. The lower catalyst layer has a thickness change point in a range from the gas inlet end face to 90.0% or more of the length of the partition walls from the gas inlet end face to the gas outlet end face, and the average thickness T of the lower catalyst layer from the gas inlet end face to the change point is １ is the average thickness T of the lower catalyst layer from the change point to the gas outlet end surface. ２ According to the exhaust gas purifying catalyst of this embodiment, the purification performance of CO contained in exhaust gas (particularly, purification performance at low temperatures) can be further improved.

[0011] The inventors' investigations have revealed that in an exhaust gas purification catalyst having a lower catalytic layer and an upper catalytic layer with a predetermined composition, CO purification performance can be further improved by arranging the lower catalytic layer so that it is thicker on the gas inlet side than on the gas outlet side. While the mechanism by which this effect is achieved is not entirely clear, the following mechanism is presumed. First, the catalyst according to the present invention is characterized in that the lower catalytic layer is substantially free of zeolite, while the upper catalytic layer contains 20 g / L or more of zeolite (i.e., zeolite is concentrated in the upper catalytic layer). This configuration allows for efficient adsorption of HC in the exhaust gas in the upper catalytic layer, while allowing efficient oxidation reactions (particularly CO oxidation reactions) catalyzed by platinum and palladium in the lower catalytic layer. Second, the catalyst according to the present invention is characterized in that the lower catalytic layer on the gas inlet side is thicker than the lower catalytic layer on the gas outlet side. This configuration allows for greater platinum and palladium content in the gas inlet-side lower catalytic layer compared to the gas outlet-side lower catalytic layer, while maintaining high dispersion. This allows the oxidation reaction (particularly the oxidation reaction of CO) on the gas inlet side to proceed more efficiently. As a result, it becomes possible to further improve the CO purification performance at low temperatures compared to conventional techniques. Note that the above mechanism is based solely on speculation, and whether it is correct or incorrect does not affect the technical scope of the present invention.

[0012] The overall structure of the exhaust gas purifying catalyst according to this embodiment will be described in more detail below with reference to the accompanying drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and duplicated explanations will be omitted. Furthermore, the dimensional proportions in the drawings are exaggerated for the sake of explanation, and may differ from the actual proportions.

[0013] FIG. 1 is a front cross-sectional view schematically illustrating a portion of an exhaust gas purification catalyst according to one embodiment of the present invention. This embodiment corresponds to catalyst A in Example 1, which will be described later. As shown in FIG. 1 , the exhaust gas purification catalyst 1 includes a refractory three-dimensional structure 10, a lower catalyst layer 20, and an upper catalyst layer 30. The refractory three-dimensional structure 10 extends from a gas inlet end face 10 a along a gas outlet end face 10 b, and includes partition walls that define multiple gas flow paths that penetrate from the gas inlet end face 10 a to the gas outlet end face 10 b. In FIG. 1 , the length of the partition walls from the gas inlet end face 10 a to the gas outlet end face 10 b is 90 mm.

[0014] The lower catalyst layer 20 is formed on the partition wall of the refractory three-dimensional structure 10. The lower catalyst layer 20 is made of platinum (Pt), palladium (Pd), and alumina (Al ２ O ３ ) and lanthana (La) as a rare earth oxide. ２ O ３ ) and has a uniform composition throughout. In the embodiment shown in FIG. 1 , the lower catalyst layer 20 is formed continuously on the partition walls from the gas inlet side end face 10 a to the gas outlet side end face 10 b (the entire surface of the partition walls facing the gas flow path is covered with the lower catalyst layer 20), but depending on the manufacturing method of the lower catalyst layer, there may be a small region on the partition wall where the lower catalyst layer is not formed (for example, up to about 5% of the length of the partition wall). Furthermore, in the embodiment shown in FIG. 1 , the partition wall and the lower catalyst layer 20 are formed so as to be adjacent to each other, but another layer may be disposed between the partition wall and the lower catalyst layer 20.

[0015] In the exhaust gas purifying catalyst 1 shown in Fig. 1, the lower catalyst layer 20 has a thickness change point C at a position 50.0% of the length of the partition wall (45 mm from the gas inlet end surface 10a). The average thickness T of the lower catalyst layer from the gas inlet end surface 10a to the change point C is １ is 147 μm, and the average thickness T of the lower catalyst layer from the change point C to the gas outlet end surface 10 b ２ is 95 μm, and T １ T ２ It is thicker than

[0016] The upper catalytic layer 30 is formed on the lower catalytic layer 20. The upper catalytic layer 30 is composed of platinum (Pt), palladium (Pd), and zeolite. The amount of zeolite supported per 1 L of the refractory three-dimensional structure is 40 g / L. In the embodiment shown in FIG. 1 , the upper catalytic layer 30 is continuously formed from the gas inlet end face 10 a to the gas outlet end face 10 b (the entire surface of the partition wall is covered by the upper catalytic layer 30). However, depending on the manufacturing method of the upper catalytic layer, there may be a small region on the partition wall where the upper catalytic layer is not formed (e.g., about 5% of the length of the partition wall). Furthermore, in the embodiment shown in FIG. 1 , the lower catalytic layer 20 and the upper catalytic layer 30 are formed adjacent to each other, but another layer may be disposed between the lower catalytic layer 20 and the upper catalytic layer 30.

[0017] In the exhaust gas purifying catalyst 1 shown in FIG. 1, the average thickness T ３ and the average thickness T of the upper catalyst layer from the change point C to the gas outlet end surface 10b. ４ and 54 μm and 47 μm (|T ３ -T ４ |÷{(T ３ +T ４ )÷2}×100=13.8(%). The average thickness T of the catalyst layer from the surface of the partition wall to the surface of the upper catalyst layer 30 from the gas inlet end surface 10 a to the change point C is ｉｎ is 202 μm, and the average thickness T of the catalyst layer from the surface of the partition wall to the surface of the upper catalyst layer from the change point C to the gas outlet end face 10 b ｏｕｔ is 142 μm, and T ｉｎ T for ｏｕｔ The proportion is 70.4%.

[0018] Next, each of the components included in the exhaust gas purifying catalyst of this embodiment will be described.

[0019] [Fire-resistant three-dimensional structure] The fire-resistant three-dimensional structure serves as a support for the catalyst layer. There are no particular limitations on the type or size of the fire-resistant three-dimensional structure, and any known structure in the field of exhaust gas purification catalysts can be appropriately adopted. As the fire-resistant three-dimensional structure, a honeycomb support is preferably used. Examples of honeycomb supports include monolith honeycomb support, metal honeycomb support, and plug honeycomb support for particulate filters. The material of the honeycomb support is preferably a heat-resistant metal such as cordierite, silicon carbide, silicon nitride, stainless steel, or an Fe—Cr—Al alloy.

[0020] The honeycomb carrier is manufactured by extrusion molding or by winding sheet-like elements. The shape of the gas passage openings (cell shape) may be any of hexagonal, rectangular, square, triangular, and corrugated. A cell density (number of cells / unit cross-sectional area) of 100 to 1200 cells / square inch (15.5 to 186 cells / square centimeter) is sufficient for use, and preferably 200 to 900 cells / square inch (31 to 139.5 cells / square centimeter).

[0021] The length of the refractory three-dimensional structure along the gas flow path (length of the partition wall) is preferably more than 15 mm and not more than 1000 mm, more preferably 30 mm or more and not more than 500 mm, and even more preferably 50 mm or more and not more than 300 mm.

[0022] [Lower Catalyst Layer] The lower catalyst layer is formed between the partition walls of the three-dimensional structure and the upper catalyst layer, which is the outermost layer. While other layers may be disposed between the partition walls and the lower catalyst layer, it is preferable that the partition walls and the lower catalyst layer are adjacent to each other. The lower catalyst layer is substantially free of zeolite and contains alumina, platinum, and palladium. In this specification, "substantially free" refers to "not containing (0 g / L)" or "if contained, the content per 1 L of the refractory three-dimensional structure is greater than 0 g / L and not more than 5 g / L." In the catalyst according to this embodiment, as described above, by concentrating zeolite in the upper catalyst layer, HC in the exhaust gas is efficiently adsorbed in the upper catalyst layer, and poisoning of the platinum and palladium in the lower catalyst layer by HC is suppressed. This allows oxidation reactions (particularly CO oxidation reactions) catalyzed by platinum and palladium to proceed efficiently in the lower catalyst layer. The content of zeolite in the lower catalyst layer is preferably 0 g / L or more and 3 g / L or less, more preferably 0 g / L or more and 1 g / L or less, and even more preferably 0 g / L (not included).

[0023] Alumina functions as a carrier for platinum and palladium. Specific examples of alumina include γ-alumina, δ-alumina, and θ-alumina, with γ-alumina being preferred.

[0024] Alumina is also known as aluminum oxide (Al ２ O ３ ) and an oxide of another element. In this case, examples of the other element include phosphorus, zirconium, silicon, titanium, and lanthanum. Specific examples of the composite oxide include Al-P composite oxide, Al-Zr-P composite oxide, Al-Zr composite oxide, Al-Si composite oxide, Al-Si-Zr composite oxide, Al-Ti composite oxide, Al-Ti-Zr composite oxide, Al-Si-Ti-Zr composite oxide, and Al-La composite oxide. The aluminum content in the composite oxide is expressed as Al ２ O ３In terms of conversion, it is preferably more than 50% by mass and less than 100% by mass, more preferably 60% by mass or more and 99% by mass or less, even more preferably 70% by mass or more and 97% by mass or less, and particularly preferably 75% by mass or more and 95% by mass or less.

[0025] Alumina (aluminum oxide (Al ２ O ３ The specific surface area of ​​the alumina (or alumina in the case of a composite oxide of the alumina and an oxide of another element, if the composite oxide is in the form of a composite oxide of the alumina and an oxide of another element) can be measured by the BET multipoint method in accordance with ISO 9277:2010. The BET specific surface area of ​​the alumina (BET surface area per 1 g of alumina) is preferably 100 m ２ / g or more 250m ２ / g or less, more preferably 110m ２ / g or more 200m ２ / g or less, and more preferably 120m ２ / g or more 180m ２ / g or less. The BET specific surface area of ​​alumina is 100 m ２ / g or more is preferable because the noble metals (platinum and palladium) supported on the alumina are highly dispersed. ２ When the specific surface area of ​​alumina is 0.1 to 1.0 μm, the BET specific surface area is less likely to decrease when the alumina is exposed to high-temperature exhaust gas, which is preferable.

[0026] Alumina (aluminum oxide (Al ２ O ３ The content of alumina (when it is in the form of a composite oxide of alumina) and an oxide of another element) is preferably 10 g / L or more and 300 g / L or less, more preferably 20 g / L or more and 250 g / L or less, and even more preferably 50 g / L or more and 200 g / L or less. If the alumina content is within the above range, the precious metals (platinum and palladium) can be sufficiently dispersed and supported.

[0027] Examples of alumina raw materials include alumina (gamma alumina, delta alumina, theta alumina, etc.) powder, and aluminum chloride, aluminum nitrate (for example, aluminum nitrate nonahydrate), aluminum sulfate, aluminum acetate, aluminum hydroxide, etc., which become alumina upon firing. Among these, alumina (gamma alumina, delta alumina, theta alumina, etc.) powder is preferred.

[0028] In addition, alumina is aluminum oxide (Al ２ O ３ When the compound (II) is contained in the form of a composite oxide of an oxide of another element, examples of the raw material for the oxide of the other element include the following. Examples of raw materials for phosphorus oxide (phosphorus sources) include phosphoric acid, phosphorous acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate. Of these, phosphoric acid is preferred. Examples of raw materials for zirconia (zirconium sources) include zirconium oxynitrate, zirconium oxychloride, zirconium nitrate, basic zirconium sulfate, zirconium carbonate, and zirconium hydroxide. Of these, zirconium nitrate is preferred. Examples of raw materials for silica (silicon sources) include silicon oxide, orthosilicate, metasilicic acid, and silica sol. Examples of raw materials for titania (titanium sources) include inorganic titanium compounds such as titanium tetrachloride and titanium sulfate, titanium oxalate, and tetraisopropyl titanate. Examples of raw materials for lanthana (lanthanum sources) include lanthanum oxide, lanthanum nitrate, lanthanum sulfate, lanthanum carbonate, and lanthanum acetate.

[0029] Platinum and palladium function as catalysts for the oxidation reaction. The platinum content per 1 L of the refractory three-dimensional structure in the lower catalyst layer is preferably 0.05 g / L or more and 5 g / L or less, more preferably 0.1 g / L or more and 4 g / L or less, and even more preferably 0.15 g / L or more and 3 g / L or less. The palladium content per 1 L of the refractory three-dimensional structure in the lower catalyst layer is preferably 0.05 g / L or more and 5 g / L or less, more preferably 0.1 g / L or more and 4 g / L or less, and even more preferably 0.15 g / L or more and 3 g / L or less. The ratio of the platinum content to the palladium content in the lower catalyst layer (Pt / Pd) is preferably 1 / 10 or more and 50 / 1 or less, more preferably 1 / 5 or more and 30 / 1 or less, and even more preferably 1 / 1 or more and 20 / 1 or less. If this ratio is within the above range, the oxidation reaction (particularly the oxidation reaction of CO) can proceed efficiently.

[0030] Examples of the noble metal raw material (noble metal source) include nitrates, acetates, ammonium salts, amine salts, carbonates, etc. of each noble metal. Among these, nitrates (platinum nitrate and palladium nitrate) are preferred.

[0031] The lower catalyst layer optionally contains at least one promoter selected from the group consisting of Group 1 elements, Group 2 elements, and rare earth elements. Specific examples of the promoter include potassium, magnesium, calcium, strontium, barium, and lanthanum. These elements are contained in the catalyst in the form of oxides, sulfates, or carbonates after firing. Among these, the lower catalyst layer preferably contains an oxide of a rare earth element (rare earth oxide), and lanthanum oxide (La ２ O ３ It is more preferable that the lower catalyst layer contains a promoter, which can further improve the efficiency of the oxidation reaction (particularly the oxidation reaction of CO). The promoter (preferably lanthanum oxide (La ２ O ３ The content of )) is preferably 0.5 g / L or more and 20 g / L or less, more preferably 0.8 g / L or more and 10 g / L or less, and even more preferably 1 g / L or more and 5 g / L or less.

[0032] The lower catalyst layer may contain the above-mentioned alumina, platinum, and palladium, an optional promoter, and other components, as long as the effects of the present invention are not significantly impaired. Examples of other components include manganese, iron, cobalt, nickel, and copper. However, the content of other components per 1 L of the refractory three-dimensional structure in the lower catalyst layer is preferably 10 g / L or less, more preferably 5 g / L or less, even more preferably 3 g / L or less, particularly preferably 1 g / L or less, and most preferably 0 g / L (not included).

[0033] The total loading amount of the lower catalyst layer is not particularly limited, but is preferably 10.6 g / L to 335 g / L, more preferably 21 g / L to 271 g / L, and even more preferably 51.3 g / L to 212 g / L. If the total loading amount of the lower catalyst layer is within the above range, it is possible to achieve the desired exhaust gas purification performance while suppressing an excessive increase in pressure loss.

[0034] In the catalyst according to the present embodiment, the lower catalyst layer has a thickness change point in a range from the gas inlet side end face to 20.0% but less than 90.0% of the length of the partition wall from the gas inlet side end face to the gas outlet side end face, and the average thickness T of the lower catalyst layer from the gas inlet side end face to the change point is １ is the average thickness T of the lower catalyst layer from the change point to the gas outlet end face. ２ In this way, by forming the lower catalyst layer on the gas inlet side to be thicker than the lower catalyst layer on the gas outlet side, it is possible to arrange a large amount of platinum and palladium in the lower catalyst layer on the gas inlet side while maintaining high dispersion. This makes it possible to proceed with an oxidation reaction (particularly the oxidation reaction of CO) on the gas inlet side with higher efficiency. In this specification, the position of the change point and the average thickness T １ and T ２ The values ​​of are determined by the methods described in the Examples below.

[0035] The position of the change point must be within a range of more than 20.0% and less than 90.0% of the length of the partition wall from the gas inlet end face. From the viewpoint of further improving the CO purification performance, the range is preferably more than 20.0% and not more than 80.0%, more preferably 30.0% or more and 80.0% or less, even more preferably 30.0% or more and 70.0% or less, particularly preferably 40.0% or more and 70.0% or less, and most preferably 33.0% or more and 67.0% or less.

[0036] Average thickness T １ The value of the average thickness T is preferably 70 μm or more and 300 μm or less, more preferably 100 μm or more and 200 μm or less, and even more preferably 110 μm or more and 170 μm or less. ２ The value of T is preferably 45 μm or more and 190 μm or less, more preferably 65 μm or more and 130 μm or less, and even more preferably 70 μm or more and 110 μm or less. ２ T for １ The ratio (T １ / T ２ ) is preferably 1.1 or more and 2.0 or less, more preferably 1.2 or more and 1.8 or less, and even more preferably 1.3 or more and 1.7 or less.

[0037] The lower catalyst layer is formed by applying a slurry for the lower catalyst layer containing the raw materials of the above components onto the partition walls of the fire-resistant three-dimensional structure, followed by drying and firing. At this time, by repeating the cycle of applying, drying and / or firing the slurry for the lower catalyst layer at least twice (preferably twice), the position of the change point and the average thickness T １ and T ２can be controlled. As an example, as in Example 1 described later, a method can be mentioned in which the entire partition wall (100% of the total length) is wash-coated with a slurry for the lower catalyst layer, and after drying (or after drying and firing), the lower catalyst layer is wash-coated with the slurry for the lower catalyst layer up to a desired length (50.0% of the length in Example 1) from the gas inlet side end face, followed by drying and firing to form the lower catalyst layer. According to this method, the average thickness T ２ The position of the change point can be controlled by adjusting the length of the wash coat (the area where the slurry for the lower catalyst layer is applied) during the second wash coat. The average thickness T １ can be controlled. As another example, as in Example 11 described later, a method can be mentioned in which a slurry for the lower catalyst layer is wash-coated from the gas inlet side end face to a desired length (50.0% length in Example 11), dried and fired, and then the slurry for the lower catalyst layer is wash-coated over the entire partition wall (100% of the total length), dried and fired, to form a lower catalyst layer. According to this method, the position of the change point can be controlled by adjusting the length of the wash coat (the range over which the slurry for the lower catalyst layer is applied) in the first wash coat. The average thickness T ２ The average thickness T can be controlled by adjusting the coating amount of the slurry for the lower catalyst layer in the first and second wash coating. １ According to these methods, the position of the change point and the average thickness T １ and T ２ However, as can be seen from a comparison between Examples 1 and 11, the same effect can be achieved if the catalyst has a predetermined structure, so it goes without saying that the lower catalyst layer may be formed by a method other than the above.

[0038] The drying and calcination steps for forming the lower catalyst layer are not particularly limited. In the drying step, the slurry is dried at a temperature of preferably 50°C or higher and 300°C or lower, more preferably 80°C or higher and 200°C or lower, for preferably 2 minutes to 20 hours, more preferably 5 minutes to 10 hours. In the calcination step, the slurry dried in the drying step is calcined in air at a temperature of preferably 300°C or higher and 1200°C or lower, more preferably 400°C or higher and 700°C or lower, for preferably 2 minutes to 10 hours, more preferably 5 minutes to 5 hours. The drying and calcination steps may be performed separately, or may be performed in a single heat treatment without separating the drying and calcination steps.

[0039] As described above, when the lower catalyst layer is formed by repeating the cycle of applying, drying, and firing the lower catalyst layer slurry at least twice (preferably twice), it is preferable that the components contained in the lower catalyst layer slurry in each cycle, other than the dispersion medium (e.g., water, a lower alcohol such as ethanol or 2-propanol, or an organic alkaline aqueous solution), have substantially the same composition. Furthermore, it is preferable that the resulting lower catalyst layer also have substantially the same composition as a whole. This facilitates control of the catalytic activity of the lower catalyst layer and enables the lower catalyst layer to be endowed with high CO oxidation performance. Here, "having substantially the same composition" means that the types of each component are identical and that the value of {(maximum concentration - minimum concentration) / minimum concentration} × 100 for each component is within 10%. This value is preferably within 5%, more preferably within 3%, even more preferably within 1%, and particularly preferably 0% (having the exact same composition). The composition of the lower catalyst layer can be confirmed by inductively coupled plasma (ICP) emission spectroscopy or X-ray fluorescence (XRF) analysis.

[0040] [Upper catalytic layer] The upper catalytic layer is formed on the lower catalytic layer and is located as the outermost layer. Although other layers may be disposed between the lower catalytic layer and the upper catalytic layer, it is preferable that the lower catalytic layer and the upper catalytic layer are adjacent to each other. The upper catalytic layer contains 20 g / L or more of zeolite, platinum, and palladium.

[0041] Zeolite (hydrated aluminosilicate) has the function of adsorbing HC in exhaust gas. In the catalyst according to this embodiment, by disposing a predetermined amount of zeolite in the upper catalyst layer, adhesion (poisoning) of HC to the precious metals (platinum and palladium) contained in the lower catalyst layer is suppressed, thereby further improving CO purification performance (particularly purification performance at low temperatures).

[0042] The type of zeolite is not particularly limited, and may be either natural or synthetic. Specific examples include A-type, X-type, Y-type, L-type, beta-type (BEA-type), ZSM-type, CHA-type, ferrierite-type, Linde-type, faujasite-type, MCM-22-type, and mordenite-type. Of these, the beta-type (BEA-type) is preferred.

[0043] The specific surface area of ​​the zeolite can be measured by the BET multipoint method in accordance with ISO 9277:2010. The BET specific surface area of ​​the zeolite (BET surface area per 1 g of zeolite) is preferably 320 m ２ / g or more 830m ２ / g or less, and more preferably 400m ２ / g or more 700m ２ When the BET specific surface area is within the above range, HC in exhaust gas can be sufficiently adsorbed.

[0044] The shape of the zeolite is not particularly limited and may be any shape, such as granular, fine particle, powder, cylindrical, conical, prismatic, cubic, pyramidal, or irregular shape, but is preferably granular, fine particle, or powder, and more preferably powder.

[0045] The content of zeolite per 1 L of the refractory three-dimensional structure in the upper catalyst layer is essentially 20 g / L or more, preferably 20 g / L to 70 g / L, more preferably 30 g / L to 70 g / L, and even more preferably 40 g / L to 70 g / L. If the content of zeolite is within the above range, HC in the exhaust gas can be sufficiently adsorbed.

[0046] Platinum and palladium function as catalysts for the oxidation reaction. The platinum content per 1 L of the refractory three-dimensional structure in the upper catalyst layer is preferably 0.01 g / L or more and 2 g / L or less, more preferably 0.015 g / L or more and 1.8 g / L or less, and even more preferably 0.02 g / L or more and 1.5 g / L or less. The palladium content per 1 L of the refractory three-dimensional structure in the upper catalyst layer is preferably 0.01 g / L or more and 2 g / L or less, more preferably 0.015 g / L or more and 1.8 g / L or less, and even more preferably 0.02 g / L or more and 1.5 g / L or less. The ratio of the platinum to palladium content in the upper catalyst layer (Pt / Pd) is preferably 1 / 10 or more and 50 / 1 or less, more preferably 1 / 5 or more and 30 / 1 or less, and even more preferably 1 / 1 or more and 20 / 1 or less. If the ratio is within the above range, the oxidation reaction can proceed efficiently. The noble metal raw material (noble metal source) used in the upper catalyst layer is the same as that explained in the section about the lower catalyst layer, and therefore a detailed explanation thereof will be omitted here.

[0047] The upper catalyst layer optionally contains alumina. The inclusion of alumina improves the dispersion of precious metals (platinum and palladium), further promoting the CO oxidation reaction in the upper catalyst layer. The alumina and its raw materials contained in the upper catalyst layer are the same as those described in the lower catalyst layer section, and therefore detailed description is omitted here. The alumina content per 1 L of the refractory three-dimensional structure in the upper catalyst layer is preferably 0 g / L or more and 50 g / L or less, more preferably 2 g / L or more and 30 g / L or less, and even more preferably 5 g / L or more and 20 g / L or less.

[0048] The upper catalyst layer may contain the above-mentioned zeolite, platinum, and palladium, as well as optional alumina, and other components other than those described above, as long as the effects of the present invention are not significantly impaired. Examples of other components include manganese, iron, cobalt, nickel, and copper. However, the content of other components per 1 L of the refractory three-dimensional structure in the upper catalyst layer is preferably 10 g / L or less, more preferably 5 g / L or less, even more preferably 3 g / L or less, particularly preferably 1 g / L or less, and most preferably 0 g / L (not included).

[0049] The total loading amount of the upper catalyst layer is not particularly limited, but is preferably 20.02 g / L or more, more preferably 20.03 g / L to 103.6 g / L, and even more preferably 35.04 g / L to 93 g / L. If the total loading amount is within the above range, it is possible to achieve the desired exhaust gas purification performance while suppressing an excessive increase in pressure loss.

[0050] In the catalyst according to this embodiment, the lower catalyst layer and the upper catalyst layer are adjacent to each other, and the average thickness T of the upper catalyst layer from the gas inlet end face to the change point is ３ and the average thickness T of the upper catalyst layer from the change point to the gas outlet end surface. ４ It is preferable that the average thickness T ３ and T ４ The values ​​of T are determined by the methods described in the Examples below. ３ and T ４ and are "substantially the same" means that |T ３ -T ４ |÷{(T ３ +T ４)÷2}×100 is within 20%. By adopting such a configuration, as shown in FIG. 1, the surface of the upper catalyst layer (outermost layer) conforms to the shape of the lower catalyst layer, and the thickness of the entire catalyst layer can be thicker on the gas inlet side and thinner on the gas outlet side. This causes turbulence and contraction in the gas flow, further improving the diffusion of exhaust gas into the catalyst layer. Therefore, the CO purification performance (particularly purification performance at low temperatures) can be further improved. However, the above value is preferably within 10%, more preferably within 5%, even more preferably within 3%, and particularly preferably within 1%, and is preferably 0% (T ３ and T ４ and are the same).

[0051] Average thickness T ３ and average thickness T ４ The values ​​of are each independently preferably 20 μm or more and 150 μm or less, more preferably 45 μm or more and 120 μm or less, and even more preferably 60 μm or more and 100 μm or less.

[0052] In the catalyst according to this embodiment, the average thickness T of the catalyst layer from the surface of the partition wall to the surface of the upper catalyst layer from the gas inlet end face to the change point ｉｎ The average thickness T of the catalyst layer from the surface of the partition wall to the surface of the upper catalyst layer from the change point to the gas outlet side end face ｏｕｔ It is preferable that the ratio of the carbon dioxide gas is 50% or more and 90% or less. The ratio is more preferably 65% ​​or more and 87% or less, and even more preferably 70% or more and 80% or less. By adopting such a configuration, it is possible to generate turbulent flow and contracted flow while suppressing pressure loss, thereby further improving the diffusion of exhaust gas into the catalyst layer. As a result, it is possible to further improve the CO purification performance (particularly the purification performance at low temperatures).

[0053] The upper catalyst layer is formed by applying a slurry for the upper catalyst layer containing the above-mentioned raw materials onto the lower catalyst layer, followed by drying and firing. At this time, the slurry for the upper catalyst layer is applied to the entire partition wall (100% of the total length), thereby achieving an average thickness T ３ and T ４The drying and calcination steps for forming the upper catalyst layer are the same as those for the lower catalyst layer, and therefore detailed description thereof will be omitted here.

[0054] The configuration of the exhaust gas purifying catalyst according to this embodiment has been described above, and methods for forming the lower catalyst layer and the upper catalyst layer have also been described. That is, according to another embodiment of the present invention, there is provided a method for producing an exhaust gas purifying catalyst, the method comprising: applying a lower catalyst layer slurry containing alumina, a platinum source, and a palladium source onto the partition walls of the refractory three-dimensional structure, drying and firing the slurry to form the lower catalyst layer; and applying an upper catalyst layer slurry containing a zeolite, a platinum source, and a palladium source onto the lower catalyst layer, drying and firing the slurry to form the upper catalyst layer.

[0055] <Method for purifying exhaust gas> According to yet another aspect of the present invention, there is provided a method for purifying exhaust gas, which comprises contacting the above-mentioned exhaust gas purification catalyst with exhaust gas emitted from an internal combustion engine. Examples of the internal combustion engine include a diesel engine, a diesel hybrid engine, an engine that uses natural gas or the like as fuel, and a gasoline engine. Of these, a diesel engine is preferred.

[0056] The gas hourly space velocity (GHSV) of the exhaust gas is not particularly limited, but is preferably 1,000 h －１ Over 500,000h －１ Less than 5,000 hours, more preferably －１ Over 200,000 hours －１ The linear velocity of the gas is also not particularly limited, but is preferably 0.1 m / sec or more and 8.5 m / sec or less, and more preferably 0.2 m / sec or more and 6.7 m / sec or less.

[0057] According to the present invention, it is possible to improve the CO purification performance at low temperatures even after high-temperature endurance. Here, high-temperature endurance is performed by exposing the catalyst to an atmosphere of preferably 650°C or higher and 1200°C or lower for 5 hours or longer and 500 hours or shorter, more preferably 700°C or higher and 900°C or lower for 10 hours or longer and 100 hours or shorter.

[0058] To measure the CO conversion rate (oxidation rate) of exhaust gas using a diesel engine, it is preferable to use an evaluation mode for exhaust gas regulations, such as the NEDC (New European Driving Cycle) mode, JC08 mode, WLTC, FTP75, FTP1199, NRTC, or NRSC mode. For example, when evaluation is performed in the NEDC mode, it is performed in accordance with United Nations Economic Commission for Europe, Addendum 82: Regulation No. 83.

[0059] According to the present invention, excellent purification performance (particularly CO purification performance) can be achieved for low-temperature exhaust gas. Here, low temperature is preferably 150°C or higher but lower than 250°C, and more preferably 175°C or higher but lower than 200°C. In this specification, "exhaust gas temperature" means the temperature of the exhaust gas at the catalyst inlet. Here, "catalyst inlet" refers to the portion of an exhaust pipe in which an exhaust gas purification catalyst is installed, extending from the catalyst end face on the exhaust gas inlet side toward the internal combustion engine by 10 cm, and also refers to the central portion of the exhaust pipe in the longitudinal (axial) direction.

[0060] Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments and can be modified in various ways within the scope of the claims. Furthermore, the embodiments described in this specification can be combined in any manner to form other embodiments.

[0061] The following embodiments are also included within the scope of the present invention: a catalyst for purifying exhaust gas as set forth in claim 1 having the features of claim 2; a catalyst for purifying exhaust gas as set forth in claim 1 or 2 having the features of claim 3; a catalyst for purifying exhaust gas as set forth in any one of claims 1 to 3 having the features of claim 4; a catalyst for purifying exhaust gas as set forth in any one of claims 1 to 4 having the features of claim 5; a catalyst for purifying exhaust gas as set forth in any one of claims 1 to 5 having the features of claim 6; a catalyst for purifying exhaust gas as set forth in any one of claims 1 to 6 having the features of claim 7; and a catalyst for purifying exhaust gas as set forth in any one of claims 1 to 7 having the features of claim 8.

[0062] The present invention will be described in more detail below with reference to examples. However, the technical scope of the present invention is not limited to the following examples. Unless otherwise specified, each operation was carried out under the conditions of room temperature (25°C) and relative humidity of 40% RH or more and 50% RH or less.

[0063] <Examples of Preparation of Exhaust Gas Purifying Catalyst> [Example 1] (Formation of Lower Catalyst Layer) (1) Platinum nitrate (platinum source), palladium nitrate (palladium source), powdered alumina (alumina raw material, BET specific surface area: 140 m) ２ / g) and lanthanum acetate (lanthana raw material) were used as raw materials for the Pt:Pd:Al ２ O ３ :La ２ O ３ The raw materials were weighed out so that the mass ratio was 1.02:0.34:96.67:1.97. The weighed raw materials were added to deionized water and stirred for 30 minutes, and then nitric acid was added to adjust the pH to 5, thereby obtaining a mixed dispersion liquid a1. The mixed dispersion liquid a1 was wet-pulverized in a ball mill at a rotation speed of 200 rpm for 30 minutes, thereby obtaining a slurry 1 (solid concentration 29% by mass).

[0064] (2) Slurry 1 was wash-coated onto a 1.46 L cylindrical cordierite carrier having a diameter of 143.8 mm and a length of 90 mm (gas passage shape (cell shape): square, number of cells: 400 cells per square inch of cross-sectional area (1 inch = 25.4 mm)), over a length (90 mm) that was 100% of the total length of the cordierite carrier, so that the amount of support after firing was 98.81 g (67.68 g / L) per total volume of the cordierite carrier. Next, the resultant was dried at 150°C for 20 minutes and then fired in air at 500°C for 1 hour, thereby obtaining a precursor 1 in which a lower catalyst layer 1 was formed on the cordierite carrier.

[0065] (3) Platinum nitrate (platinum source), palladium nitrate (palladium source), powdered alumina (alumina raw material, BET specific surface area: 140 m ２ / g), and lanthanum acetate (lanthana raw material) were used as raw materials for the Pt:Pd:Al ２ O ３ :La ２ O ３The raw materials were weighed out so that the mass ratio was 1.02:0.34:96.67:1.97. The weighed raw materials were added to deionized water and stirred for 30 minutes, and then nitric acid was added to adjust the pH to 5, thereby obtaining a mixed dispersion liquid a1. The mixed dispersion liquid a1 was wet-pulverized in a ball mill at a rotation speed of 200 rpm for 30 minutes, thereby obtaining a slurry 2 (solid concentration 18% by mass).

[0066] (4) Next, the slurry 2 was wash-coated onto the precursor 1 from the gas inlet end face to 50% of the total length (45 mm) of the cordierite carrier so that the amount of zeolite supported after calcination was 24.70 g (16.92 g / L) per total volume of the cordierite carrier. The resulting mixture was then dried at 150°C for 20 minutes and calcined in air at 500°C for 1 hour to obtain a precursor 2 in which a lower catalyst layer 2 was formed on a lower catalyst layer 1. The zeolite content in the lower catalyst layers (lower catalyst layer 1 and lower catalyst layer 2) was 0 g / L.

[0067] (Formation of Upper Catalyst Layer) (5) Platinum nitrate (platinum source), palladium nitrate (palladium source), powdered alumina (alumina raw material, BET specific surface area: 140 m ２ / g), and powdered beta zeolite (zeolite raw material, silica / alumina (molar ratio) = 35 to 40, BET specific surface area: 582 m ２ / g) of each raw material was added to the Pt:Pd:Al ２ O ３ The raw materials were weighed out so that the mass ratio of the raw materials to the zeolite was 0.30:0.10:19.92:79.68. The weighed raw materials were added to deionized water and stirred for 30 minutes, and then nitric acid was added to adjust the pH to 5, thereby obtaining a mixed dispersion a2. The mixed dispersion a2 was wet-pulverized in a ball mill at a rotation speed of 200 rpm for 30 minutes, thereby obtaining a slurry 3 (solid concentration 18% by mass).

[0068] (6) Slurry 3 was wash-coated onto precursor 2 over a length (90 mm) that was 100% of the total length of the cordierite carrier, such that the amount of zeolite supported after calcination was 73.29 g (50.20 g / L) per total volume of the cordierite carrier. The resulting mixture was then dried at 150°C for 20 minutes and calcined in air at 500°C for 1 hour to obtain catalyst A of this example, in which an upper catalyst layer was formed on lower catalyst layer 1 and lower catalyst layer 2. The zeolite content in the upper catalyst layer was 40 g / L.

[0069] [Example 2] In the above (Formation of the lower catalyst layer) (3), the solid content concentration of the slurry 2 was changed to 25 mass%. In the above (Formation of the lower catalyst layer) (4), the slurry 2 was wash-coated onto the precursor 1 up to a length (20 mm) that was 22.2% of the total length from the gas inlet end face of the cordierite carrier so that the amount supported after firing was 24.70 g (16.92 g / L) per total volume of the cordierite carrier. Except for these, a catalyst B of this example was obtained in the same manner as in Example 1.

[0070] [Example 3] In the above (Formation of the lower catalyst layer) (3), the solid content concentration of the slurry 2 was changed to 21 mass%. In the above (Formation of the lower catalyst layer) (4), the slurry 2 was wash-coated onto the precursor 1 up to a length (30 mm) that was 33.3% of the total length from the gas inlet end face of the cordierite carrier so that the amount supported after firing was 24.70 g (16.92 g / L) per total volume of the cordierite carrier. Except for these, a catalyst C of this example was obtained in the same manner as in Example 1.

[0071] [Example 4] In the above (Formation of the lower catalyst layer) (3), the solid content concentration of the slurry 2 was changed to 16 mass%. In the above (Formation of the lower catalyst layer) (4), the slurry 2 was wash-coated onto the precursor 1 up to a length (60 mm) that was 66.7% of the total length from the gas inlet end face of the cordierite carrier so that the amount supported after firing was 24.70 g (16.92 g / L) per total volume of the cordierite carrier. Except for these, a catalyst D of this example was obtained in the same manner as in Example 1.

[0072] [Example 5] In the above (Formation of the lower catalyst layer) (3), the solid content concentration of the slurry 2 was changed to 15 mass%. In the above (Formation of the lower catalyst layer) (4), the slurry 2 was wash-coated onto the precursor 1 up to a length (70 mm) that was 77.8% of the total length from the gas inlet end face of the cordierite carrier so that the amount supported after firing was 24.70 g (16.92 g / L) per total volume of the cordierite carrier. Except for these, a catalyst E of this example was obtained in the same manner as in Example 1.

[0073] [Example 6] In the above (Formation of the lower catalyst layer) (3), the solid content concentration of the slurry 2 was changed to 14 mass%. In the above (Formation of the lower catalyst layer) (4), the slurry 2 was wash-coated onto the precursor 1 up to a length (80 mm) that was 88.9% of the total length from the gas inlet end face of the cordierite carrier so that the amount supported after firing was 24.70 g (16.92 g / L) per total volume of the cordierite carrier. Except for these, catalyst F of this example was obtained in the same manner as in Example 1.

[0074] [Comparative Example 1] In the above (Formation of the lower catalyst layer) (3), the solid content concentration of the slurry 2 was changed to 12 mass%. In the above (Formation of the lower catalyst layer) (4), the slurry 2 was wash-coated onto the precursor 1 from the gas inlet end face of the cordierite carrier to a length (90 mm) that was 100% of the total length, so that the amount supported after firing was 24.70 g (16.92 g / L) per total volume of the cordierite carrier. Except for these, a catalyst G of this comparative example was obtained in the same manner as in Example 1.

[0075] [Example 7] In the above (Formation of upper catalyst layer) (5), the raw materials of platinum nitrate, palladium nitrate, powdered alumina, and powdered beta zeolite were each converted into a Pt:Pd:Al ２ O ３The raw materials were weighed so that the mass ratio of the zeolite to the precursor was 0.50:0.17:33.11:66.23, and slurry 3 (solid content concentration 15 mass%) was prepared using these raw materials. In the above (Formation of upper catalyst layer) (6), slurry 3 was wash-coated onto precursor 2 over a length (90 mm) that was 100% of the total length of the cordierite carrier so that the amount of the zeolite supported after firing was 44.09 g (30.2 g / L) per total volume of the cordierite carrier. Aside from these, catalyst H of this example was obtained in the same manner as in Example 1. The zeolite content in the upper catalyst layer was 20 g / L.

[0076] [Example 8] In the above (Formation of upper catalyst layer) (5), the raw materials of platinum nitrate, palladium nitrate, powdered alumina, and powdered beta zeolite were each mixed to form a Pt:Pd:Al ２ O ３ The raw materials were weighed so that the mass ratio of the zeolite to the precursor was 0.37:0.12:24.88:74.63, and slurry 3 (solid content concentration 17% by mass) was prepared using these raw materials. In the above (Formation of upper catalyst layer) (6), slurry 3 was wash-coated onto precursor 2 over a length (90 mm) that was 100% of the total length of the cordierite carrier, so that the amount of the zeolite supported after firing was 58.69 g (40.2 g / L) per total volume of the cordierite carrier. Aside from these, catalyst I of this example was obtained in the same manner as in Example 1. The zeolite content in the upper catalyst layer was 30 g / L.

[0077] [Example 9] In the above (Formation of upper catalyst layer) (5), the raw materials of platinum nitrate, palladium nitrate, powdered alumina, and powdered beta zeolite were each converted into a Pt:Pd:Al ２ O ３The raw materials were weighed so that the mass ratio of zeolite to zeolite was 0.25:0.08:16.61:83.06, and slurry 3 (solid content concentration 20 mass%) was prepared using these raw materials. In the above (Formation of upper catalyst layer) (6), slurry 3 was wash-coated onto precursor 2 over a length (90 mm) that was 100% of the total length of the cordierite carrier, so that the amount of zeolite loaded after firing was 87.89 g (60.2 g / L) per total volume of the cordierite carrier. Aside from these, catalyst J of this example was obtained in the same manner as in Example 1. The zeolite content in the upper catalyst layer was 50 g / L.

[0078] [Example 10] In the above (Formation of upper catalyst layer) (5), the raw materials of platinum nitrate, palladium nitrate, powdered alumina, and powdered beta zeolite were each mixed to form a Pt:Pd:Al ２ O ３ The raw materials were weighed so that the mass ratio of the zeolite to the precursor was 0.21:0.07:14.25:85.47, and slurry 3 (solid content concentration 22% by mass) was prepared using these raw materials. In the above (Formation of upper catalyst layer) (6), slurry 3 was wash-coated onto precursor 2 over a length (90 mm) that was 100% of the total length of the cordierite carrier, so that the amount of the zeolite supported after firing was 102.49 g (70.2 g / L) per total volume of the cordierite carrier. Aside from these, catalyst K of this example was obtained in the same manner as in Example 1. The zeolite content in the upper catalyst layer was 60 g / L.

[0079] [Example 11] In the above (Formation of upper catalyst layer) (5), the raw materials of platinum nitrate, palladium nitrate, powdered alumina, and powdered beta zeolite were each converted into a Pt:Pd:Al ２ O ３The raw materials were weighed so that the mass ratio of zeolite to zeolite was 0.19:0.06:12.47:87.28, and slurry 3 (solid content concentration 23 mass%) was prepared using these raw materials. In the above (Formation of upper catalyst layer) (6), slurry 3 was wash-coated onto precursor 2 over a length (90 mm) that was 100% of the total length of the cordierite carrier so that the amount of zeolite loaded after firing was 117.09 g (80.2 g / L) per total volume of the cordierite carrier. Aside from these, catalyst L of this example was obtained in the same manner as in Example 1. The zeolite content in the upper catalyst layer was 70 g / L.

[0080] Comparative Example 2 In the above (Formation of the upper catalyst layer) (5), the raw materials of platinum nitrate, palladium nitrate, and powdered alumina were each converted into a Pt:Pd:Al ２ O ３ The raw materials were weighed out so that the mass ratio of the above was 0.30:0.10:99.60, and a slurry 2 (solid content concentration 22 mass%) was prepared using these raw materials. Except for this, a catalyst M of this comparative example was obtained in the same manner as in Example 1. The content of zeolite in the upper catalyst layer was 0 g / L.

[0081] [Example 12] In the above (Formation of the lower catalyst layer) (1), the solid content concentration of Slurry 1 was changed to 23% by mass. In the above (Formation of the lower catalyst layer) (2), Slurry 1 was wash-coated onto the cordierite carrier from the gas inlet end face to a length of 44 mm, which was 49% of the total length, so that the loading amount after firing was 24.70 g (16.92 g / L) per total volume of the cordierite carrier. In the above (Formation of the lower catalyst layer) (3), the solid content concentration of Slurry 2 was changed to 27% by mass. In the above (Formation of the lower catalyst layer) (4), Slurry 2 was wash-coated onto the precursor 1 to a length of 90 mm, which was 100% of the total length of the cordierite carrier, so that the loading amount after firing was 98.81 g (67.68 g / L) per total volume of the cordierite carrier. Aside from these, catalyst N of this example was obtained in the same manner as in Example 1. The content of zeolite in the upper catalyst layer was 40 g / L.

[0082] <Changing point C, average thickness T １ ~T ４ , Tｉｎ , T ｏｕｔ Measurement of the change point C> First, each catalyst A to N was cut using sandpaper so as to pass through the center of gravity and the corner of the gas passage port and parallel to the gas flow path, exposing the cross section of the catalyst layer. The cross section of the catalyst layer was then observed using a scanning electron microscope (SEM). From the obtained observation image, the change point C was determined using the following method. Below, an explanation will be given using a front cross section diagrammatically illustrating a part of an exhaust gas purification catalyst according to one embodiment of the present invention shown in FIG. 1 . As shown in FIG. 1 , the catalyst 1 is formed by stacking a lower catalyst layer 20 and an upper catalyst layer 30 on partition walls of a refractory three-dimensional structure 10. In the observation image, as shown in FIG. 1 , line X (here, corresponding to the boundary line between the partition wall and the lower catalyst layer 20) constituting the surface of the partition wall, line Y (here, corresponding to the boundary line between the lower catalyst layer 20 and the upper catalyst layer 30) constituting the surface of the lower catalyst layer 20 facing the upper catalyst layer 30, and line Z (here, corresponding to the boundary line between the lower catalyst layer 20 and the upper catalyst layer 30) constituting the outermost surface of the catalyst layer (the surface of the upper catalyst layer 30 not facing the lower catalyst layer 20) were confirmed. The distance between the line X and the line Y was measured at 10 μm intervals from the gas inlet end surface 10 a in a region excluding a 3 mm region from the gas inlet end surface 10 a. The distance T between the line X and the line Y at the point m on the line X was ｍ and the distance T between the line X and the line Y at point m+1, which is 10 μm away from point m toward the gas outflow side. ｍ＋１ The first point where the difference between and is 5 μm or more is the change start point C １ The distance between the line X and the line Y was measured at intervals of 10 μm from the gas outlet end surface 10 b in a region excluding a 3 mm region from the gas outlet end surface 10 b. The distance T between the line X and the line Y at point n on the line X was ｎ and the distance T between the line X and the line Y at point n+1, which is 10 μm away from point n toward the gas inlet side. ｎ＋１ The first point where the difference between and is 5 μm or more is the change end point C ２ Then, the change start point C １ and the change end point C ２ When a perpendicular line is drawn from the midpoint of the line X to the line X, the foot of the perpendicular line (the intersection of the perpendicular line and the line X) is defined as the change point C.

[0083] Next, the thickness of the lower catalyst layer 20 (corresponding to the distance between line Y and line X in this case) was measured every 10 μm from the gas inlet end face 10 a to the gas outlet end face 10 b, excluding a region of 3 mm from each of the gas inlet end face 10 a and the gas outlet end face 10 b. The arithmetic mean value of the thickness at each point from the gas inlet end face 10 a to the change point C was calculated as the average thickness T of the lower catalyst layer. １ Similarly, the arithmetic mean value of the thickness at each point from the change point C to the gas outlet side end surface 10b was taken as the average thickness T ２ It was decided.

[0084] The thickness of the upper catalyst layer 30 (corresponding to the distance between line Z and line Y in this case) was measured every 10 μm from the gas inlet end face 10 a to the gas outlet end face 10 b, excluding 3 mm regions from each of the gas inlet end face 10 a and the gas outlet end face 10 b. The arithmetic mean value of the thicknesses at each point from the gas inlet end face 10 a to the change point C was calculated as the average thickness T of the upper catalyst layer. ３ Similarly, the arithmetic mean value of the thickness at each point from the change point C to the gas outlet end surface 10b was taken as the average thickness T ４ It was decided.

[0085] Furthermore, the thickness of the catalyst layer (here, corresponding to the distance between line Z and line X) was measured every 10 μm from the gas inlet end face 10 a to the gas outlet end face 10 b, excluding a region of 3 mm from each of the gas inlet end face 10 a and the gas outlet end face 10 b. The arithmetic mean value of the thickness at each point from the gas inlet end face 10 a to the change point C was calculated as the average thickness T of the catalyst layer. ｉｎ Similarly, the arithmetic mean value of the thickness at each point from the change point C to the gas outlet end surface 10b was taken as the average thickness T ｏｕｔ It was decided.

[0086] These measured values ​​for catalysts A to N are shown in Table 1 below. Note that the value for the change point C is expressed as a percentage of the distance from the gas inlet end face to the change point C relative to the length of the partition wall from the gas inlet end face to the gas outlet end face. Note that the value for catalyst N was the same as that for catalyst A.

[0087]

[0088] <Durability Test> The catalysts A to N were subjected to durability treatment in air at 700° C. for 50 hours using an electric furnace.

[0089] <Evaluation of CO Purification Performance> After the durability test, catalysts A to N and comparative catalyst A (catalyst A with the gas inlet and gas outlet sides reversed) were evaluated for CO purification performance by the following method. Specifically, a test piece 90 mm long and 24 mm in diameter (0.041 L) was cut out from the gas inlet end face to the gas outlet end face of each catalyst. ２ : 10% by volume, CO: 1500 ppm by volume, C ３ H ６ : 50 ppm by volume C1, C ２ H ４ : 150 ppm by volume C1, decene: 600 ppm by volume C1, NO: 30 ppm by volume, N ２ : the remainder (89.99 volume%)) at a space velocity (SV) of 58,600 h. －１ While the mixed gas (O ２ : 10% by volume, N ２ :The remainder (90 volume%)) at a space velocity (SV) of 58,600 h. －１ While the catalyst inlet temperature was maintained at 100°C, the reaction gas was allowed to flow through the test piece at a space velocity (SV) of 58,600 h －１ The reaction gas was then passed through the test piece at a space velocity (SV) of 58,600 h －１ While the gas was flowing through the test piece, the catalyst inlet temperature was increased from 100°C at a rate of 20°C / min. When the temperature reached 195°C, the gas discharged from the catalyst outlet was sampled, and the CO conversion rate was calculated according to the following formula 1.

[0090]

[0091] The results for catalysts A to G are shown in Table 2 below. The results for catalysts A and catalysts H to M are shown in Table 3 below. Note that the results for catalyst N were the same as those for catalyst A.

[0092]

[0093] As shown in Table 2, the present invention can further improve CO purification performance at low temperatures (195°C). Note that, since the loading amounts in the lower catalyst layer and the upper catalyst layer are the same for catalysts A to G, the difference in CO purification performance is thought to be due solely to the difference in the thickness of the lower catalyst layer between the gas inlet side and the gas outlet side.

[0094] Comparing catalysts A to F, it can be seen that the closer the location of the change point is to the 50% position from the gas inlet end face, the more the CO purification performance improves. Note that, because catalysts A to F have the same amount of loading in the lower catalyst layer 2, it is thought that the difference in CO purification performance depends solely on the distance from the gas inlet side to the change point (i.e., the length of the thick portion of the lower catalyst layer).

[0095]

[0096] As shown in Table 3, by supporting a large amount of zeolite in the upper catalyst layer, it is possible to further improve the CO purification performance at low temperatures (195°C). This is thought to be because, as the amount of zeolite in the upper catalyst layer increases, it becomes possible to adsorb more HC (hydrocarbons), which suppresses the adhesion (poisoning) of HC to the precious metals (platinum and palladium) contained in the lower catalyst layer, making it less likely that the CO oxidation reaction will be inhibited.

[0097] This application is based on Japanese Patent Application No. 2023-216942, filed on December 22, 2023, the disclosure of which is hereby incorporated by reference in its entirety.

[0098] REFERENCE SIGNS LIST 1 exhaust gas purifying catalyst, 10 fire-resistant three-dimensional structure, 10a gas inlet end face, 10b gas outlet end face, 20 lower catalyst layer, 30 upper catalyst layer, C change point, C １ Change starting point, C ２ Change end point, t Tangent, T １ , T ２ , T ３ , T ４ , T ｉｎ , Tｏｕｔ Average thickness, X, Y, Z lines.

Claims

1. A fire-resistant three-dimensional structure having partition walls extending from a gas inlet end face along a gas outlet end face and defining a plurality of gas flow paths penetrating from the gas inlet end face to the gas outlet end face; a lower catalyst layer formed on the partition walls, containing substantially no zeolite and containing alumina, platinum and palladium; and an upper catalyst layer formed on the lower catalyst layer and positioned as an outermost layer, containing 20 g / L or more of zeolite, platinum and palladium, wherein the lower catalyst layer has a thickness change point in a range from the gas inlet end face to 90.0% of the length of the partition walls from the gas inlet end face to the gas outlet end face, and an average thickness T of the lower catalyst layer from the gas inlet end face to the change point １ is the average thickness T of the lower catalyst layer from the change point to the gas outlet end surface. ２ A catalyst for purifying exhaust gases that is larger than the 2. The lower catalyst layer and the upper catalyst layer are adjacent to each other, and the average thickness T of the upper catalyst layer from the gas inlet end surface to the change point is ３ and the average thickness T of the upper catalyst layer from the change point to the gas outlet end surface. ４ 2. The exhaust gas purifying catalyst according to claim 1, wherein said first and second carbon atoms are substantially the same.

3. The exhaust gas purifying catalyst according to claim 1, wherein the entire lower catalyst layer has substantially the same composition.

4. The exhaust gas purifying catalyst according to claim 1, wherein the content of zeolite contained in the upper catalyst layer is 40 g / L or more and 70 g / L or less.

5. A catalyst for purifying exhaust gas as described in claim 1, wherein the change point is located in a range of 33.0% to 67.0% from the gas inlet end face with respect to the length of the partition wall from the gas inlet end face to the gas outlet end face.

6. The exhaust gas purifying catalyst according to claim 1, wherein the lower catalyst layer further contains a rare earth oxide.

7. The average thickness T of the catalyst layer from the surface of the partition wall to the surface of the upper catalyst layer from the gas inlet end face to the change point ｉｎ The average thickness T of the catalyst layer from the surface of the partition wall to the surface of the upper catalyst layer from the change point to the gas outlet end surface ｏｕｔ 2. The exhaust gas purifying catalyst according to claim 1, wherein the ratio of 8. The exhaust gas purifying catalyst according to claim 1, wherein the partition wall and the lower catalyst layer are adjacent to each other.

9. A method for producing an exhaust gas purifying catalyst according to any one of claims 1 to 8, comprising: applying a slurry for a lower catalyst layer, the slurry containing alumina, a platinum source and a palladium source, onto the partition wall of the refractory three-dimensional structure, followed by drying and firing to form the lower catalyst layer; and applying a slurry for an upper catalyst layer, the slurry containing zeolite, a platinum source and a palladium source, onto the lower catalyst layer, followed by drying and firing to form the upper catalyst layer.

10. A method for purifying exhaust gas, comprising contacting the exhaust gas purifying catalyst according to any one of claims 1 to 8 with exhaust gas discharged from an internal combustion engine.