Honeycomb catalyst and method for purifying gasoline automobile exhaust gas using the same

Abstract

To provide a honeycomb catalyst used for purifying a harmful substance and an environmental load substance in an exhaust gas discharged from an internal combustion engine or the like, in which a catalyst layer having high gas diffusion property is formed to improve purification performance.SOLUTION: A honeycomb catalyst includes a cylindrical cell whose end is opened, and a catalyst composition coated on a cell wall of a honeycomb structure integrated by sharing a wall of the cell. The catalyst composition includes inorganic oxide particles having an approximately spherical shape, and inorganic oxide particles having a tabular shape. A method for manufacturing the honeycomb catalyst is also provided.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a honeycomb catalyst having excellent gas diffusion properties, a method for producing the same, and a method for purifying exhaust gases emitted from gasoline-powered automobiles using the honeycomb catalyst. [Background technology]

[0002] Exhaust gases are emitted from internal combustion engines that run on fossil fuels, such as automobiles. These exhaust gases contain harmful substances and environmentally hazardous substances that are believed to cause global warming, such as hydrocarbons (HC), nitrogen oxides (NOx), carbon monoxide (CO), and particulate matter (PM) containing soot components derived from the combustion of fossil fuels. A method using honeycomb catalysts is widely used to purify these harmful substances and environmentally hazardous substances, and they are purified by the honeycomb catalyst before being released into the atmosphere.

[0003] Regulatory limits for the emission of such harmful components and substances of environmental concern into the atmosphere are set by government agencies in each country, and as awareness of environmental issues in the market increases, these regulatory limits are becoming stricter every year.

[0004] Honeycomb structures used as catalysts include flow-through honeycomb structures and wall-flow honeycomb structures. Flow-through honeycomb structures are made of tubular cells with open ends that are integrated together and share the cell walls that make up the cells, and are made of ceramics or metals such as stainless steel. Wall-flow honeycomb structures are made of tubular cells with one end of the opening blocked, and the cells are integrated so that the blocked and open parts alternate on both end faces of the honeycomb structure. Wall-flow honeycomb structures have porous cell walls that allow air to pass through, filtering out PM in exhaust gases and preventing it from being released into the atmosphere.

[0005] PM filtered from exhaust gas accumulates in the wall-flow honeycomb structure, but is then catalyzed, where it is burned and removed by the heat of the exhaust gas and the action of the catalyst, regenerating the catalyzed wall-flow honeycomb structure. The wall-flow honeycomb structure from which PM has been removed can then capture PM again. Wall-flow honeycomb structures like this are made of porous ceramics to achieve both heat resistance and gas permeability. While extremely small PM particles may slip through the porous cell walls and catalyst layer, many of these particles are combusted when they come into contact with the heated catalyst layer, becoming CO2 and being released into the atmosphere.

[0006] Like wall-flow honeycomb catalysts, flow-through honeycomb structures are used with the cell walls catalyzed. Harmful components and environmentally hazardous substances in exhaust gases are purified by coming into contact with the catalytic components in the cell walls and then released into the atmosphere.

[0007] These catalyzed honeycomb structures are placed in the flow of exhaust gas, and the presence of such honeycomb structures creates resistance to the flow of exhaust gas. Resistance to the flow of exhaust gas is also called pressure loss, and in internal combustion engines, it can cause a decrease in power output. The wall-flow type honeycomb structures described above tend to have large pressure losses because they have blocked parts in the cells. The impact of such pressure losses is significant in internal combustion engines that emit exhaust gases with high flow rates, such as the gasoline engines installed in many passenger cars.

[0008] On the other hand, the flow-through type honeycomb structure has low pressure loss because both ends of the cells are open, and it can be said that this honeycomb structure does not cause a significant increase in pressure loss when purifying exhaust gases emitted from internal combustion engines that are required to operate at high revolutions, such as gasoline-powered automobiles.

[0009] Honeycomb catalysts are used to purify exhaust gases emitted from internal combustion engines, and the reactants, with the exception of PM, are gases. With honeycomb catalysts, these gas components are purified by contact with the solid catalyst, so the catalytic reaction can be said to be a gas-phase reaction. In recent years, with emission regulations for harmful components and environmentally hazardous substances becoming increasingly strict, there is a demand for more efficient gas-phase reactions in honeycomb catalysts.

[0010] In order to efficiently promote a catalytic gas-phase reaction, it is necessary for the exhaust gas containing the reactants to come into efficient contact with the active species and promoter components in the catalyst composition. The improvement in purification performance due to such efficient contact with the reactants is achieved by high gas diffusivity. Conversely, a catalyst with low gas diffusivity will have low purification performance (Patent Document 1, Patent Document 2). Furthermore, in layered materials such as the catalyst composition layer described below, gas diffusivity is synonymous with gas permeability (Patent Document 3).

[0011] Typically, the catalyst components used in catalyst compositions are in the form of fine particles. Because most of these particles undergo a crushing process to reduce their size, the catalyst components generally have a nearly spherical shape. In conventional honeycomb catalysts, these roughly spherical catalyst components are formulated and coated in layers on the cell wall surface. However, because the nearly spherical catalyst component particles are coated on the cell wall surface, the particles are closely packed together to form a catalyst layer, resulting in a dense catalyst layer. Furthermore, a densely formed catalyst composition layer has few voids, which is unfavorable for gas diffusion, making it difficult to improve purification performance.

[0012] One way to avoid the formation of such a dense catalyst layer is to increase the amount of voids in the catalyst composition layer. However, simply increasing the voids may reduce the adhesion between the cell wall surface and the catalyst composition layer, raising concerns that the catalyst composition layer may peel off from the cell wall. If the catalyst composition peels off, the amount of catalyst supported on the honeycomb catalyst will decrease, resulting in a decrease in the exhaust gas purification efficiency. Furthermore, the peeled catalyst components may be released into the atmosphere, potentially becoming a new source of environmental pollution.

[0013] Furthermore, when the goal is to improve the exhaust gas permeability of the catalyst composition layer, it is possible to use catalyst components with a large particle size, even if they are approximately spherical. If catalyst components have large particle sizes, the pores formed between the particles will also be large, which may reduce the resistance to gas permeation. However, a large particle size means that the geometric surface area per unit volume of the particle is small. Since the surface of the catalyst component is the main reaction site for catalytic reactions, it is difficult to effectively utilize the catalyst component with particles with a small geometric surface area.

[0014] The densification of the catalyst composition layer on the cell wall surface of a honeycomb structure also depends on its manufacturing method. There are two main methods for coating the cell wall surface of a honeycomb structure with a catalyst composition: powder coating and wash coating. The powder coating method is used to catalyze wall-flow honeycomb structures. In summary, the powder coating method involves supplying powdered catalyst components together with an airflow from the open cell end face of the honeycomb structure into the cells, and then using the filtering function of the wall-flow honeycomb structure to discharge only the airflow from the other open cell end face, thereby depositing the powdered catalyst composition on the cell wall surface.

[0015] As described above, the powder coating method utilizes the filtering function of the honeycomb structure to catalyze the catalyst. Therefore, the deposited catalyst composition layer is bulky. While this method is advantageous for forming a catalyst layer with high gas diffusion properties, it cannot be used to catalyze flow-through honeycomb structures, which lack filtering capabilities. Furthermore, powder coating can sometimes be difficult to adhere the catalyst composition to the cell walls, resulting in uneven distribution of the catalyst components along the axial direction of the tubular cells. This uneven distribution of the catalyst components can result in thick and thin catalyst layers being formed along the axial direction of the tubular cells. In the thinner sections, the distance that exhaust gases must pass through the catalyst composition is shortened, shortening the reaction time and reducing purification performance. In the thicker sections, exhaust gas permeability is reduced, resulting in the exhaust gas passing through the thinner catalyst layer, which is more permeable, further reducing purification performance.

[0016] On the other hand, the washcoat method is used to coat the cell walls of a flow-through honeycomb structure with a catalyst composition. The washcoat method involves mixing catalyst components with a liquid medium such as water to form a slurry. A predetermined amount of the composition is supplied to the open end faces of the cells by suction or pressure, and then the slurry is spread over the cell wall surfaces by suction or air blowing. The liquid-mixed slurry is usually coated in a layer on the cell walls as the water is absorbed by the cell walls. During this process, the catalyst components, which are inorganic oxide particles, move toward the cell walls while adhering to each other, resulting in the formation of a dense catalyst layer.

[0017] In this way, in honeycomb catalysts using flow-through honeycomb structures, the catalyst layer that is formed settles while narrowing the gaps between the catalyst component particles that make up the catalyst layer, causing the catalyst composition layer to become denser in the direction of minimizing the space within the catalyst composition layer, resulting in the formation of a dense catalyst composition layer with little space between the catalyst component particles and poor gas diffusion properties. [Prior art documents] [Patent documents]

[0018] [Patent Document 1] Patent No. 6362040 [Patent Document 2] Patent No. 6763555 [Patent Document 3] JP 2012-9353 A Summary of the Invention [Problem to be solved by the invention]

[0019] The present invention aims to form a catalyst layer with high gas diffusion properties in a honeycomb catalyst used to purify harmful substances and environmentally hazardous substances in exhaust gases emitted from internal combustion engines, etc., in order to improve purification performance. [Means for solving the problem]

[0020] As a result of intensive research to solve the above problems, the inventors discovered that the above problems can be solved by combining inorganic oxide particles of a specific shape in the catalyst composition to be coated on the honeycomb catalyst, and thus completed the present invention.

[0021] In other words, the present invention is a honeycomb catalyst in which a catalyst composition is coated on the cell walls of a honeycomb structure in which cylindrical cells with open ends are accumulated and share the cell walls, and the honeycomb catalyst is characterized in that the catalyst composition contains approximately spherical inorganic oxide particles and plate-shaped inorganic oxide particles.

[0022] The present invention also provides a method for manufacturing the above-mentioned honeycomb catalyst, characterized in that the catalyst composition is coated on the cell walls of a honeycomb structure using a washcoat method, and the catalyst composition is slurried by mixing plate-shaped inorganic oxide particles and then mixing with a stirring blade, without performing a contact grinding process between the stirring blades.

[0023] Furthermore, the present invention relates to a method for purifying exhaust gas from a gasoline automobile using the above honeycomb catalyst. [Effects of the Invention]

[0024] According to the present invention, exhaust gases can more easily reach deep into the catalyst composition layer than in conventional honeycomb catalysts, and a catalyst layer with high gas diffusivity can be formed, which is effective in improving the purification performance of honeycomb catalysts.

[0025] Furthermore, according to the present invention, it is possible to efficiently purify high-velocity exhaust gas emitted from a gasoline-powered automobile. [Brief explanation of the drawings]

[0026] [Figure 1] 1 is a diagram schematically illustrating an embodiment of the present invention (A) and a conventional catalyst composition layer (B). [Figure 2] 1 is a diagram showing a honeycomb catalyst manufactured by a washcoat method and a schematic enlarged view of a part of the open end face to show the state (thickness) of the catalyst layer. [Figure 3] 1 is a scanning electron microscope (SEM) image of plate-shaped inorganic oxide particles used in an embodiment of the present invention. [Figure 4] 1A-1C are SEM images of different plate-like inorganic oxide particles that can be used in embodiments of the present invention. [Figure 5] 1 is an SEM image of a catalyst composition layer containing substantially spherical inorganic oxide particles and plate-like inorganic oxide particles, which are components of a catalyst layer of the present invention. [Figure 6] 1 is an SEM image of a conventional catalyst composition layer made of approximately spherical inorganic oxide particles. [Figure 7] FIG. 1 is a schematic diagram showing plugs, which are the smallest units constituting a wall-flow type honeycomb structure, and porous cell partition walls, together with arrows indicating the flow direction of exhaust gas. [Figure 8] 1 shows the coating length of the catalyst composition layer in the catalysts of Examples and Comparative Examples, together with a schematic diagram of the configuration of the catalyst layer. DETAILED DESCRIPTION OF THE INVENTION

[0027] The present invention provides a honeycomb catalyst in which a catalyst composition is coated on cell walls of a honeycomb structure in which cylindrical cells having open ends are accumulated while sharing the cell walls, The honeycomb catalyst (hereinafter referred to as "the honeycomb catalyst of the present invention") is characterized in that the catalyst composition contains approximately spherical inorganic oxide particles and plate-like inorganic oxide particles.

[0028] [Honeycomb structure] The honeycomb structure used in the honeycomb catalyst of the present invention is not particularly limited as long as it is an accumulation of cylindrical cells with open ends that share a common cell wall, and may be a flow-through type or a wall-flow type. Such a honeycomb structure is not particularly limited, and may be appropriately selected from honeycomb structures used by those skilled in the art and available on the market, depending on the state of the exhaust gas to be purified.

[0029] The material of the honeycomb structure is not particularly limited, and may be, for example, an inorganic oxide such as cordierite, silica, alumina, or zeolite extruded into a honeycomb shape, or a metal plate such as stainless steel processed into a continuous wave shape and rolled up into a cylindrical shape to form a honeycomb.

[0030] A honeycomb structure is an accumulation of tubular cells that share their walls, providing a large geometric surface area within a limited volume. By catalyzing the cell wall surfaces, a honeycomb catalyst with a large surface area can be obtained. As described in the Background Art section, the honeycomb structures commonly available on the market are flow-through and wall-flow honeycomb structures. In catalytic reactions, contact between reactants and the catalyst surface promotes the reaction, so a large surface area can be said to be advantageous for the reaction.

[0031] The catalyst surface contributes most to the reaction, but in a catalyst composition formed in a layer, the catalyst components inside the catalyst composition layer, i.e., on the cell wall side in a honeycomb catalyst, are unlikely to come into contact with the reactants, and it is difficult to say that the catalyst components are effectively utilized in conventional honeycomb catalysts. In the present invention, as will be described later, by imparting high gas diffusivity to the catalyst composition layer, it is possible to effectively utilize the catalyst components inside the catalyst composition layer coated on the cell walls of the honeycomb structure, thereby obtaining a highly reactive honeycomb catalyst, or in the case of an exhaust gas catalyst, a catalyst with high purification performance for harmful substances and substances of concern.

[0032] Honeycomb structures are commercially available in a variety of shapes, including cylindrical and elliptical cylindrical shapes, but the shape used in the present invention is not particularly limited and may be appropriately selected depending on the specifications of the exhaust system in which the honeycomb catalyst of the present invention is used. The specifications of a honeycomb structure are expressed by the number of cells per unit cross-sectional area (cell density) along with the outer size. The size and cell density of the flow-through honeycomb structure used in the present invention are also not particularly limited, and may be appropriately selected depending on the specifications of the exhaust system from among honeycomb structures that have been used as catalysts in the past for purifying exhaust gases. The cell density of such a honeycomb structure is 62 to 124 cells / cm for gasoline passenger cars. 2 (400-800 cells / inch 2 ) is preferred, with 62-93 cells / cm 2 (400-600 cells / inch 2 ) is the most preferred method.

[0033] [Nearly spherical inorganic oxide particles] The roughly spherical inorganic oxide particles used in the honeycomb catalyst of the present invention correspond to many conventionally used catalytic components. As mentioned above, many catalytic components are used in combination in catalytic compositions for purifying automobile exhaust gases. The fact that the present invention can arbitrarily adopt conventionally used catalytic components increases the degree of freedom in catalyst design and also makes it possible to improve purification performance by simply adding plate-shaped inorganic oxide particles to existing catalysts. Examples of inorganic oxides that make up the roughly spherical inorganic oxide particles include alumina, silica, titania, ceria, zirconia, and composite oxides containing at least one of these oxides.

[0034] Such substantially spherical inorganic oxide particles are commercially available in a wide variety of materials, and as mentioned above, they can be said to be materials that have been widely used by those skilled in the art. The method for producing such substantially spherical inorganic oxide particles is not particularly limited, but examples include a method in which the components that form the inorganic oxide particle raw material are mixed and fired, and the fired product is pulverized and classified using a ball mill or the like, and a spray-drying method in which an aqueous solution of the raw material is sprayed and the sprayed aqueous solution particles are directly heated to obtain oxide particles. Inorganic oxide particles obtained in this manner by a pulverization method are polished during the pulverization process, resulting in roughly spherical particles with rounded corners. Furthermore, in the spray-drying method, the sprayed aqueous solution particles become roughly spherical due to surface tension, so that the fired inorganic oxide particles also have a roughly spherical shape.

[0035] Theoretically, it is preferable that the approximately spherical inorganic oxide particles be perfectly spherical in combination with the plate-like inorganic oxide particles. However, the use of perfectly spherical inorganic oxide particles as a catalyst component that can be used in exhaust gas catalysts that are supplied in large quantities to the market would result in a significant increase in costs, and in fact, most catalyst components are not available as perfectly spherical particles, making it practically impossible to use perfectly spherical components as catalyst components. Therefore, in the present invention, conventional catalyst component particles are described as approximately spherical inorganic oxide particles in contrast to plate-like inorganic oxide particles.

[0036] Such substantially spherical inorganic oxide particles may be specified by their aspect ratio. The aspect ratio of the substantially spherical inorganic oxide particles used in the present invention is preferably 2 or less, and more preferably 1.5 or less. With such an aspect ratio, spaces are easily formed by point contact with the plate-like inorganic oxide particles, and many of the catalytic components conventionally used by those skilled in the art can be selected, providing a high degree of freedom in the design of automobile exhaust gas purification catalysts, which require a variety of properties depending on combustion control.

[0037] Although such substantially spherical inorganic oxide particles are sometimes specified by their particle size, the particle size of the substantially spherical inorganic oxide particles in the present invention is not particularly limited and may be appropriately selected from particle sizes conventionally adopted by those skilled in the art. Specifically, particles having a volume-based average particle diameter (D50) of 1 to 100 μm can be adopted as the substantially spherical inorganic oxide particles in the present invention. Furthermore, such substantially spherical inorganic oxide particles may be primary particles, or may be secondary particles formed by aggregation of primary particles.

[0038] [Plate-shaped inorganic oxide particles] The plate-shaped inorganic oxide particles used in the honeycomb catalyst of the present invention are not particularly limited as long as they are plate-shaped. For example, when specified by their width, thickness, and major axis, the width is preferably at least twice the thickness, more preferably at least three times, and is preferably 1 / 10 to 2 times, and more preferably 1 / 5 to 1 time, the major axis of the approximately spherical inorganic oxide particles. The major axis is preferably 5 to 100 μm, more preferably 10 to 50 μm. Examples of inorganic oxides constituting the plate-shaped inorganic oxide particles include silica, titania, zeolite, and plate-shaped inorganic oxide particles composed of two or more natural or synthetic components. Zeolite, in particular, can be synthesized into particles of various shapes and sizes with good reproducibility by manipulating the raw material concentration, raw material components, stirring speed, temperature, etc. during the manufacturing process, making it a preferred material for the plate-shaped inorganic oxide particles of the present invention. The length of the major axis of a plate-like inorganic oxide particle refers to the length of the longest straight line among those having an end point on the periphery of a face constituting a set of parallel flat surfaces, and the width refers to the length of the longest straight line among those having an end point on the periphery of a face constituting a set of parallel flat surfaces and intersecting perpendicularly to the line corresponding to the length of the major axis of the plate-like inorganic oxide particle.

[0039] In the catalyst composition layer, the plate-like inorganic oxide particles can be sandwiched between the contact points between the aforementioned roughly spherical inorganic oxide particles, or can enter the spaces formed between the roughly spherical inorganic oxide particles and contact the roughly spherical inorganic oxide particles to expand the spaces, thereby forming spaces that are effective for gas diffusion. This is schematically shown in Figure 1. In Figure 1, (B) shows the state of the catalyst components and voids in a catalyst composition layer composed of conventional catalyst components, and (A) shows the state of the catalyst components and voids of the present invention.

[0040] In the conventional catalyst (B), the catalyst component particles 3 constituting the catalyst composition layer are approximately spherical, so the catalyst component particles are in close contact with each other and the space formed by the catalyst component particles is narrow, which is unfavorable for gas diffusion.

[0041] In contrast, in the catalyst (A) of the present invention, the plate-like inorganic oxide particles 2 are sandwiched between the approximately spherical inorganic oxide particles 3, thereby widening the spaces between the approximately spherical inorganic oxide particles, thereby widening the space 1 in the catalyst composition layer and improving gas diffusibility.

[0042] [Concerns about plate-shaped inorganic oxide particles being too thin] In this way, in the present invention, the plate-like inorganic oxide particles push apart the gaps between the approximately spherical inorganic oxide particles, thereby forming spaces that are advantageous for gas diffusion. However, if the plate-like inorganic oxide particles are extremely narrow, that is, very thin and needle-like, and have a short major axis, they may simply enter the spaces formed by contact between the approximately spherical inorganic oxide particles without coming into contact with the approximately spherical inorganic oxide particles, and the spaces may end up being filled with very thin needle-like particles, which may actually reduce the spaces formed by contact between the approximately spherical inorganic oxide particles.

[0043] [Concerns about plate-shaped inorganic oxide particles being too large] Furthermore, if the plate-like inorganic oxide particles are too large, the distance between catalyst component particles other than the plate-like inorganic oxide particles may become too large, which may actually reduce the catalytic activity.

[0044] The catalyst composition used in the honeycomb catalyst of the present invention contains the above-mentioned approximately spherical inorganic oxide particles and plate-like inorganic oxide particles, and their content is not particularly limited as long as they are present together in the same composition, but for example, the mass ratio per unit volume of the honeycomb structure in the catalyst layer (plate-like inorganic oxide particles / approximately spherical inorganic oxide particles) is preferably 5 / 100 to 40 / 100, and more preferably 10 / 100 to 25 / 100. Needless to say, if the content of the plate-like inorganic oxide particles is low, it may be difficult to achieve the effects of the present invention. However, if the content is too high, the distance between the approximately spherical inorganic oxide particles may become too wide, making it difficult to achieve the interaction in a catalyst that requires the interaction of catalytic components with different functions, such as TWC, which will be described later.

[0045] Honeycomb catalysts containing inorganic oxide particles, such as those of the present invention, have long been widely used, with various types available on the market. Honeycomb catalysts, particularly those used for purifying gasoline-fueled automobile exhaust, are also known as three-way catalysts (TWCs). Because a single catalyst composition purifies CO, HC, and NOx, the main harmful and environmentally hazardous components in exhaust gas, they are manufactured by appropriately blending catalyst components with different functions. CO and HC are converted to carbon dioxide (CO2) and water, respectively, through oxidation, while NOx is converted to nitrogen (N2) and water through a reduction reaction using HC, a reducing component in exhaust gas. Suitable catalyst components are then selected for each of these oxidation and reduction reactions. In addition, there is an oxygen concentration that is suitable for such oxidation and reduction reactions, and in order to adjust this oxygen concentration, oxygen storage components (OSC: Oxygen storage components) that have the ability to store and release oxygen, and NOx storage components (LNT: Lean NOx traps) that temporarily store NOx in an atmosphere with low levels of reducing components such as HC (often in exhaust gases in a lean combustion state) are also used.

[0046] In this way, the catalytic components contained in TWC each have their own individual functions, but at the same time, their mutual effects influence each other to purify harmful components and substances of concern overall. If the plate-like inorganic oxide particles are too large and the spacing (distance between component particles) is too large, there is a risk that this interaction will be hindered.

[0047] Furthermore, if the plate-like inorganic oxide particles are too large, the amount of void space in the catalyst composition layer becomes too large, and in catalysts mounted on vehicles for purifying automobile exhaust gases, the catalyst composition layer may be prone to peeling off from the cell wall surface due to vibrations during driving or pressure from the exhaust gas flow. In flow-through honeycomb catalysts where the catalyst composition layer has peeled off, the exhaust gas purification performance may be reduced.

[0048] The plate-like inorganic oxide particles of the present invention also include particles having a square pillar shape. Such square pillar particles have a width-to-thickness ratio [thickness / width] of 1 / 2 to 1. Although the width and thickness are similarly constituted by including flat surfaces, the length of the major axis of the particle is preferably about 3 to 20 times, and more preferably about 4 to 10 times, the thickness or width.

[0049] Although columnar inorganic oxide particles can be expected to form spaces that can improve gas diffusion, similar to plate-like inorganic oxide particles, depending on their shape, if the particle size is too large or the content in the catalyst composition is too high, the distance between the catalyst component particles may become too large, inhibiting the interaction between the catalyst component particles, or increasing the thickness of the catalyst layer may reduce the opening cross-sectional area of ​​the cells, resulting in an increase in back pressure in the exhaust gas. Therefore, in the present invention, it can be said that the use of plate-like inorganic oxide particles is preferable to that of columnar inorganic oxide particles.

[0050] [Plate-shaped inorganic oxide particles: Plane that makes up the plate] The plate-like inorganic oxide particles of the present invention preferably have at least one pair of parallel flat surfaces. Such flat surfaces allow for point contact between the approximately spherical inorganic oxide particles and the plate-like inorganic oxide particles, enabling the formation of larger spaces and improved gas diffusion. Furthermore, macro-sized recesses or through-holes may be formed on the flat surfaces.

[0051] The plane of the plate-like inorganic oxide particles of the present invention does not refer to a theoretically perfect plane, but may be a plane that is a component of the geometric shape of an actually existing inorganic oxide particle. Specifically, such a plane may be a plane that constitutes the crystalline shape of an inorganic substance, and examples thereof include particles composed of planes derived from crystal faces, such as single crystals of various zeolites.

[0052] [Plate-shaped inorganic oxide particles: crystalline primary particles] Such plate-like inorganic oxide particles are preferably crystalline primary particles. As mentioned above, inorganic oxide crystals often have a particle shape with a plane as a geometrical element. Furthermore, crystalline primary particles often have superior durability compared to secondary particles of the same size, and are more likely to maintain their particle shape even when used in an environment exposed to high temperature and high humidity, such as an automobile exhaust gas purification catalyst, making it easier to maintain the space with excellent gas diffusion properties in the present invention.

[0053] Furthermore, crystalline primary particles of inorganic oxide do not have extreme flexibility and are difficult to deform even when mixed with approximately spherical inorganic oxide particles to form a catalyst composition layer, which makes it easier to form spaces with excellent gas diffusion properties in the present invention.

[0054] While there are no particular limitations on the crystalline primary particles suitable as the plate-shaped inorganic oxide particles used in the honeycomb catalyst of the present invention, the inventors have been able to achieve the high gas diffusivity that is the effect of the present invention by using MFI zeolites with a high silica-to-alumina molar ratio (SAR: Silica-alumina ratio). Among such MFI zeolites, those with an SAR of 900 or higher are known as Silicalite-1, which has high heat resistance and is less susceptible to dealumination due to its low aluminum content, and also has excellent hydrothermal durability. However, MFI zeolites with an SAR of 200 or higher are expected to have high durability without losing their crystallinity, even when used as materials for automotive catalysts.

[0055] [Gas diffusion] The gas diffusion properties of pores that contribute to improving the purification performance of the catalyst composition layer are difficult to achieve with pore shapes that only have good gas permeability, such as simple cylindrical shapes. However, the pores formed by mixing plate-like inorganic oxide particles and approximately spherical inorganic oxide particles, as in the present invention, are, in principle, random pores. In a catalyst composition layer with such random pores, exhaust gas can diffuse through the catalyst composition layer while coming into contact with many catalyst component particles, thereby improving the purification performance of harmful substances and environmentally hazardous substances in the exhaust gas.

[0056] The importance of improving gas diffusion and the purification performance of exhaust gas catalysts by forming such pores has been recognized and has been studied for some time, as shown in Patent No. 6364118. The method described in Patent No. 6364118 uses a fibrous organic material to form the pores, and the fibrous organic material is burned off in the firing step of the catalyst manufacturing process to form the desired pores.

[0057] However, because the oxygen surrounding the organic matter is consumed during this burnoff, the catalyst composition is heated to a high temperature and in a reducing atmosphere. Precious metals such as platinum, palladium, and rhodium are used as the main active species in catalysts, but in this high-temperature reducing atmosphere, the precious metals become metallic rather than oxides. At high temperatures, metallic metal components bond together (sinter), reducing their dispersibility in the catalyst composition. This reduction in the dispersibility of the metal components is also manifested as a decrease in the surface area (MSA) per unit weight of the precious metal. Because catalytic reactions are primarily promoted on the surface of the active species, this reduction in surface area leads to a decrease in catalytic activity.

[0058] Furthermore, a decrease in MSA is an industrially important issue in catalysts that use expensive precious metals as active species, such as exhaust gas purification catalysts. In other words, when attempting to obtain a predetermined MSA for the precious metals incorporated into the catalyst composition, it is necessary to incorporate a large amount of the precious metal, which increases the price of the exhaust gas catalyst supplied to the market. Since the honeycomb catalyst of the present invention does not undergo a heating and reducing atmosphere such as combustion to form the voids, there is no risk of a decrease in the MSA of these precious metals, even if the catalyst composition contains expensive precious metals such as platinum, palladium, and rhodium.

[0059] [Thickness of catalyst layer] The honeycomb catalyst of the present invention can realize the formation of spaces with excellent gas diffusion properties, which is advantageous for gas-phase reactions. Therefore, it can be applied to the formation of various catalyst composition layers, and is expected to be particularly effective in forming a catalyst composition layer thickly coated on the cell walls.

[0060] Generally, when a catalyst composition is coated in a layer on the cell walls of a honeycomb structure, the thicker the coating layer, the smaller the cross-sectional area of ​​the openings through which reactive gases such as exhaust gases can pass, resulting in an increase in back pressure in the exhaust gases emitted from an internal combustion engine. Therefore, the catalyst composition layer is formed using as little catalyst as possible to ensure the cross-sectional area of ​​the openings of the cells after the catalyst composition layer is formed.

[0061] On the other hand, when coating the cell wall surfaces of a honeycomb structure with a catalyst composition, it is not rare that thick and thin catalyst composition layers are formed in a cross section of a single cell due to manufacturing process issues. Such uneven thickness of the catalyst composition layer is likely to occur when the catalyst composition is coated using the washcoating method described below.

[0062] The washcoat method involves supplying a predetermined amount of slurried catalyst composition from the open end face of a honeycomb structure, and spreading the catalyst composition slurry in the axial direction over the cell wall surface of the honeycomb structure by blowing air from the slurry supply open end face or by suction from the opposite open end face.

[0063] Here, since the catalyst composition is slurried, the catalyst composition slurry tends to form a meniscus at the corners of the cells, forming a thicker catalyst composition layer L1 than the thin catalyst composition layer L2 at the center of the cell wall, as shown in Figure 2. Figure 2 is an enlarged view schematically showing the thickness of the catalyst composition layer in a honeycomb catalyst. Such a meniscus, which tends to form a thick catalyst composition layer, is likely to be formed in honeycomb structures consisting of cells with acute corners, such as cells with triangular or square cross-sectional openings. Of these, square cells are the most popular cell shape for honeycomb structures today due to their mechanical strength and low manufacturing costs. In other words, it can be said that the honeycomb catalysts most commonly supplied on the market have thick catalyst composition layers formed at the corners of the cells.

[0064] In a gas-phase reaction in a solid catalyst such as an exhaust gas purification catalyst, reactants are converted into products by coming into contact with the catalyst surface, and the same is true for a catalyst composition layer coated on the cell wall surface. However, as described above, the catalyst composition layer has been conventionally composed of inorganic oxide particles as the catalyst component, and therefore has spaces formed by the gaps between the particles. When exhaust gas enters this space, the catalytic reaction is promoted on the surfaces of the catalyst component particles that form the spaces.

[0065] The inner space surface of such a catalyst composition layer is relatively easily utilized for catalytic reactions in areas where the catalyst composition layer is thin. However, as the catalyst layer becomes thicker, exhaust gases have difficulty reaching the catalyst components deep inside, near the cell walls. Therefore, in a catalyst composition layer formed thick on the cell walls, the catalyst components are partially ineffectively utilized. The catalyst composition layer thickened by the influence of the meniscus at the corners of the triangular and square cells described above is precisely the area that is difficult to effectively utilize.

[0066] The catalyst composition used in the honeycomb catalyst of the present invention has an excellent ability to form spaces effective for gas diffusion, so that the catalyst components deep inside the cell wall side of such a thickly coated catalyst composition layer can be effectively utilized for catalytic reactions, thereby improving the exhaust gas purification performance of the honeycomb catalyst. Such improvement in exhaust gas purification performance is particularly effective in triangular and square cells, and since square cells are the most commonly used cell cross-sectional shape on the market, it can be said that the present invention can provide an industrially extremely effective honeycomb catalyst.

[0067] [Catalyst manufacturing method] The honeycomb catalyst of the present invention is a mixture of spherical inorganic oxide particles and plate-shaped inorganic oxide particles coated on the cell walls. While the manufacturing method is not particularly limited, the washcoating method has long been used to coat catalyst compositions on honeycomb structures, and is therefore a suitable method for manufacturing the honeycomb catalyst of the present invention. The washcoating method facilitates control of the adhesion of the catalyst layer to the cell walls and the coating length of the catalyst layer on the honeycomb structure, allowing for precise catalyst design. Furthermore, controlling the length of the catalyst layer makes it possible to coat specific regions (zones) of the honeycomb structure with multiple catalyst compositions, thereby increasing the diversity of catalyst design and becoming a mainstream specification for exhaust gas catalysts in recent years.

[0068] The washcoat method involves supplying a predetermined amount of a catalyst composition, which is made by mixing a liquid medium such as water with inorganic oxide particles as the catalyst component, to the open end faces of the cells of a honeycomb structure, and then spreading the catalyst composition slurry over a predetermined length on the cell wall surface by blowing air through the open end faces of the cells to which the slurry has been supplied, or by suction from the open end face opposite to the open end face of the cell to which the catalyst composition slurry has been supplied.

[0069] When manufacturing the honeycomb catalyst of the present invention, if a washcoat method is used, the catalyst composition is used in the form of a slurry. Slurrying the catalyst composition requires a mixing process of the catalyst components, and conventionally, grinding and mixing methods such as ball mills have been used in the manufacture of many catalyst composition slurries. Grinding and mixing methods such as ball mills can also grind clustered catalyst component particles, making them a very effective method for homogenizing the catalyst composition. However, the plate-like inorganic oxide particles used in the catalyst composition of the present invention must maintain their shape. Using grinding and mixing methods such as ball mills can destroy the shape, which can make it difficult to form the spaces necessary for improving gas diffusion.

[0070] Therefore, when producing the catalyst composition slurry used in the production of the honeycomb catalyst of the present invention, a grinding / mixing means such as a ball mill is not used. Instead, at least the plate-shaped inorganic oxide particles are mixed under non-destructive conditions after mixing. Examples of non-destructive mixing methods include mixing using agitator blades to prevent the plate-shaped inorganic oxide particles from being crushed due to contact between the stirring elements, mixing methods using ultrasonic waves, bubbling methods, and combinations of these methods. Among these, methods that prevent the plate-shaped inorganic oxide particles from being crushed due to contact between the stirring elements are preferred. By using such non-destructive mixing means, the plate-shaped inorganic oxide particles are more likely to maintain their shape, which makes it easier to form spaces with excellent gas diffusion properties in the catalyst layer of the honeycomb catalyst of the present invention. Therefore, it is preferable to mix the plate-shaped inorganic oxide particles in the final stage of the production process of the catalyst composition slurry.

[0071] The honeycomb structure having the catalyst composition slurry spread on the cell walls as described above is dried and fired to become the honeycomb catalyst of the present invention that is supplied to the market. The conditions for drying and firing are not particularly limited.

[0072] [Preferred Use] The honeycomb catalyst of the present invention has excellent gas diffusion properties, and is therefore expected to be effective in purifying high-velocity exhaust gases. High-velocity exhaust gases have a short contact time with the catalyst components, which means that the catalytic reaction does not take place for a sufficient amount of time, resulting in insufficient purification of harmful components and environmentally hazardous substances. This problem is particularly pronounced in catalyst composition layers with low gas diffusion properties, since the catalytic reaction is primarily promoted on the catalyst surface, as described above.

[0073] However, since the honeycomb catalyst of the present invention has a catalyst composition layer with excellent gas diffusion properties, the surface of the catalyst particles can be utilized for reactions deep inside the catalyst composition layer in the cell wall direction, and excellent purification performance can be expected even in purifying high-flow-rate exhaust gases. One example of a source of such high-flow-rate exhaust gases is an automobile equipped with a gasoline engine. Gasoline engines have a lower compression ratio than diesel engines, which reduces the load on the piston movement in the combustion chamber and makes them easier to operate at high revolutions, resulting in a high exhaust gas flow rate. The honeycomb catalyst of the present invention is expected to be effective in purifying high-flow-rate exhaust gases emitted from such gasoline engines. [Example]

[0074] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples in any way.

[0075] Manufacturing Example 1 [Catalyst composition slurry 1: Approximately spherical inorganic oxide particles only] The following components and water were mixed using a stirrer equipped with a stirring blade so as not to generate fine particles, to prepare a catalyst composition slurry 1 for TWC. Palladium nitrate aqueous solution (Pd metal equivalent): 0.96 parts by weight γ-alumina: 16.8 parts by weight Rhodium nitrate aqueous solution (Rh metal equivalent): 0.16 parts by weight Cerium-zirconium composite oxide: 11.1 parts by weight (CeO 2 / ZrO 2 (equivalent composition ratio 3 / 7)

[0076] [Nearly spherical inorganic oxide particles] The particle sizes D50 and D90 of the catalyst composition slurry 1 thus obtained were measured, and found to be D50 of 9 μm and D90 of 26 μm. The particle sizes were measured by laser diffraction. The components in catalyst composition slurry 1 that are expressed as particle sizes in this manner are γ-alumina and cerium-zirconium composite oxide, both of which correspond to the approximately spherical inorganic oxide particles of the present invention.

[0077] [Honeycomb structure 1] Honeycomb type: Wall flow honeycomb structure Material: Cordierite Shape: Cylindrical Size: Diameter 118.4mm, Height 127mm Cell shape: Square Cell density: 46.5 cel / cm 2 (300cel / inch 2 ) Cell partition thickness: 0.2 mm (8 mils) Porosity: 63% Average pore diameter (D50): 16 μm

[0078] The porosity and average pore diameter of the honeycomb structure used in the examples of the present invention were determined from values ​​measured by mercury intrusion porosimetry at an intrusion pressure of 400 MPa.

[0079] In such a wall-flow honeycomb structure, as shown in the schematic diagram of Figure 7, exhaust gas flowing in from the open end of a cell is blocked from flowing in the axial direction by plugs, shown as plugs in Figure 7. The exhaust gas blocked from flowing in the axial direction permeates the porous cell walls, enters cells adjacent to the cell into which the exhaust gas flowed, and flows out from the open end of the honeycomb structure on the opposite side from the side into which the exhaust gas flowed. At this time, PM is filtered out of the exhaust gas by the filtering action of the cell walls, and harmful components and environmentally hazardous substances in the exhaust gas are purified by the catalyst composition layer formed on the surface of the cell walls.

[0080] Comparison example 1 [Honeycomb catalyst 1: Wall-flow type honeycomb catalyst, consisting of only roughly spherical inorganic oxide particles] A predetermined amount of catalyst composition slurry 1 was supplied to a wall-flow honeycomb structure from the exhaust gas inlet end, followed by air blowing from the exhaust gas inlet end at a controlled pressure and time to spread the slurried catalyst composition slurry 1 to a predetermined length on the honeycomb structure. Next, a predetermined amount of catalyst composition slurry 1 was supplied to the exhaust gas outlet end, followed by air blowing from the exhaust gas outlet end at a controlled pressure and time to spread the catalyst composition slurry 1 to a predetermined length on the honeycomb structure. In this manner, each cell was coated on the open end using a washcoat method. After drying, the honeycomb structure was fired at 550°C for 1 hour in an air atmosphere to obtain the honeycomb catalyst of Comparative Example 1. The catalyst layer configuration and coating length for Comparative Example 1 are shown in Figure 8 along with a schematic diagram. The catalyst composition loading in Zone 1 shown in Figure 8 was 13 g / L, and the catalyst composition loading in Zone 2 was 32 g / L.

[0081] This honeycomb catalyst 1 was cut in the axial direction of the cells, and the catalyst composition layer that appeared on the cell walls was observed using an SEM. The SEM image is shown in Figure 6. Although the presence of coarse particles is noticeable in Figure 6, all particles that can be confirmed in the image are approximately spherical, and no particles with an aspect ratio exceeding 2 were confirmed.

[0082] Manufacturing example 2 [Plate-shaped inorganic oxide particles A] Zeolite was synthesized as plate-like inorganic oxide particles as follows. First, 29.9 g of ammonium fluoride (Fujifilm Wako Pure Chemical Industries, Ltd.) was dissolved in a solution prepared by mixing 271.4 g of 40% aqueous tetrapropylammonium hydroxide solution (Seichem Co., Ltd.) and 3,266.0 g of water. 287.5 g of Nipsil ER (Tosoh Silica Corporation) was added to this solution and the mixture was stirred overnight. The composition of the mixture was as follows: The numerical values ​​of each component in this mixture represent the molar ratio of the amount of substance when the amount of substance of SiO2 is taken as 1.

[0083] 1 SiO2 0.001 Al2O3 0.121 TPAOH (TPA: tetrapropylammonium cation) 0.182 (NH4 + +NH3) 0.182 F - 43.74 H2O

[0084] Next, this raw material composition (mixture) was placed in a 5,000cc stainless steel autoclave and heated from room temperature to 175°C over 3 hours while stirring at 300 rpm. The temperature was then maintained at 175°C for 45 hours while stirring. The hydrothermally treated products were all dried at 105°C, crushed, and calcined at 600°C for 5 hours to obtain a new product. Powder X-ray diffraction analysis (XRD) confirmed that the product was a single-phase MFI-type zeolite. Furthermore, composition analysis using X-ray fluorescence (XRF) revealed an SAR of 952. This confirmed that the inorganic oxide particles were Silicalite-1, a highly durable material.

[0085] Furthermore, when the size of the inorganic oxide particles obtained in this manner was observed by SEM, it was confirmed that they were plate-shaped with uniform particle size. The SEM image is shown in Figure 3. Furthermore, visual observation of a particle in a 50 μm square in Figure 3 revealed that the particle was plate-shaped with at least one set of parallel flat surfaces, with a major axis length of 18 μm, a width of 3.5 μm, and a thickness of 1.6 μm.

[0086] Manufacturing Example 3 [Plate-shaped inorganic oxide particles B] 0.4 g of acidic ammonium fluoride (Fujifilm Wako Pure Chemical Industries, Ltd.) was dissolved in a solution made by mixing 7.1 g of 40 wt% tetrapropylammonium hydroxide aqueous solution (Seichem Co., Ltd.) and 4.7 g of water. The entire amount of this solution was impregnated into 23.3 g of CP-7104 (SAR: 275, Zeolyst Co., Ltd.) zeolite beta seed crystals, and the container was left sealed overnight. The composition of the synthesis composition was as follows: The numerical values ​​of each component in this mixture represent the molar ratio of the amount of substance when the amount of substance of SiO2 is taken as 1.

[0087] 1 SiO2 0.036 Al2O3 0.040 TPAOH (TPA: tetrapropylammonium cation) 0.020 (NH4 + +NH3) 0.040 F - 1.848 H2O

[0088] Next, this raw material composition (mixture) was placed in a 100 cc stainless steel sealed pressure vessel with a Teflon (registered trademark) inner cylinder and allowed to stand at 140°C for 14 days. The products after this hydrothermal treatment were all dried at 105°C, pulverized, and then calcined at 600°C to obtain a product. Powder X-ray diffraction analysis of the product thus obtained confirmed that the product was a single phase of MFI zeolite, and composition analysis by X-ray fluorescence (XRF) revealed that the SAR was as high as 278, the same as that of the seed crystals.

[0089] The shape of the MFI zeolite powder obtained in this manner was confirmed by SEM. The image is shown in Figure 4. SEM observation confirmed that the zeolite was plate-like with at least one set of parallel planes, and that it also contained macro-sized pores and through-holes, which are thought to be advantageous for improving gas diffusion.

[0090] The major axis length, width, and thickness of this plate-like MFI zeolite were confirmed from SEM images in the same manner as for the plate-like inorganic oxide particles A. The major axis length was approximately 6 μm at most, including particles of smaller length generated by pulverization, and the width was 3 μm at most, including particles of smaller width generated during crushing, but the thickness of the main particles, including the protruding portions, was approximately 0.5 μm. It goes without saying that in the case of powders containing such small particles, the small particles may be removed as necessary using a dry classification device or the like before use as the catalyst component of the present invention.

[0091] Manufacturing Example 4 [Catalyst composition slurry 2: Catalyst composition slurry 1 + plate-like inorganic oxide particles] To catalyst composition slurry 1, part of the γ-alumina was replaced with plate-shaped inorganic oxide particles A, and plate-shaped inorganic oxide particles A were added in an amount such that the loading amount in honeycomb structure 1 was 13 g / L in zone 1 (1.7 g / L of plate-shaped inorganic oxide particles A) and 32 g / L in zone 2 (4.2 g / L of plate-shaped inorganic oxide particles A).The mixture was mixed using a stirring blade so as not to be crushed, and catalyst composition slurry 2 was prepared.

[0092] Example 1 [Honeycomb catalyst 2: Wall-flow type honeycomb catalyst: Contains plate-shaped inorganic oxide particles and approximately spherical inorganic oxide particles] Using this catalyst composition slurry 2, a honeycomb catalyst 2 was prepared in the same manner as the honeycomb catalyst 1.

[0093] This honeycomb catalyst 2 was cut in the axial direction of the cells, and the catalyst composition layer that appeared on the cell walls was observed using an SEM. The SEM image is shown in Figure 5. In Figure 5, both roughly spherical particles and plate-like particles were observed, with the plate-like particles pushing apart the roughly spherical particles.

[0094] Test Example 1 [Gas diffusion evaluation: Gas permeability] To verify gas diffusion, honeycomb catalyst 1 and honeycomb catalyst 2 were installed in a pressure loss measuring device (manufactured by Tsukubarika Seiki Co., Ltd.), and room temperature air was introduced into the installed exhaust gas purification catalyst. 3 The differential pressure between the air inlet and outlet sides of the honeycomb catalyst was measured when the air flow rate reached 1 / min. This value is also called pressure loss, and a smaller pressure loss indicates better gas permeability, i.e., a catalyst composition layer with excellent gas diffusion properties was formed. The results are shown in Table 1.

[0095] [Table 1]

[0096] The results in Table 1 show that the catalyst of Example 1, in which a catalyst composition layer containing approximately spherical inorganic oxide particles and plate-like inorganic oxide particles was formed, had excellent gas diffusivity.

[0097] Test Example 2 [Purification performance evaluation] In order to evaluate the exhaust gas purification performance of honeycomb catalysts with such excellent gas diffusion properties, the following front-stage honeycomb catalyst was placed in front of honeycomb catalyst 1 and honeycomb catalyst 2 to recreate conditions close to those experienced in a vehicle, and the exhaust gas purification performance was evaluated under the following conditions. The results are shown in Table 2. In the table, CO represents the amount of carbon monoxide emitted behind the catalyst, THC similarly represents the total amount of hydrocarbon components, and NOx similarly represents the total amount of nitrogen oxides.

[0098] [Pre-stage honeycomb catalyst: honeycomb structure] Honeycomb type: Flow-through honeycomb structure ·Diameter: 118.4 mm Length: 56 mm Cell density: 600 cel / inch 2 (93 cel / cm 2 ) Cell wall thickness: 3.5 mil (0.09 mm) Volume: 616 cc

[0099] [Pre-stage honeycomb catalyst: catalytic component loading] · Manufacturing method: Wash coat method Firing conditions: 550°C, 1 hour, air atmosphere Palladium (Pd metal equivalent): 6 g / L Rhodium (Rh metal equivalent): 1 g / L γ-alumina: 45 g / L Cerium-zirconium composite oxide (CeO2 / ZrO2 equivalent composition ratio 3 / 7): 90 g / L Barium sulfate: 10 g / L

[0100] [Rating Engine] Supercharged direct fuel injection gasoline engine: 1.5L displacement

[0101] [Evaluation conditions: catalyst layout] The catalyst unit of Example 2 was formed by placing the front-stage honeycomb catalyst upstream of the exhaust gas flow in the exhaust pipe of an automobile equipped with the evaluation engine, and honeycomb catalyst 2 (Example 1) downstream of it, and the catalyst unit of Comparative Example 2 was formed by placing honeycomb catalyst 1 (Comparative Example 1) instead of honeycomb catalyst 2.

[0102] [Evaluation conditions: Evaluation mode] The thus-configured Example 2 and Comparative Example 2 were operated under WLTC mode (World Wide Harmonized Exhaust Gas Test Mode) operating conditions, and the amount of CO, THC, and NOx emitted downstream of the catalyst was determined using a MEXA-ONE (trade name) manufactured by Horiba, Ltd. The results are shown in Table 2.

[0103] [Table 2]

[0104] The results in Table 2 demonstrate that the catalyst unit using the catalyst of Example 1 reduced CO, THC, and NOx emissions. These results demonstrate that the catalyst of the present invention achieves excellent gas diffusion, resulting in a honeycomb catalyst with excellent exhaust gas purification performance. Furthermore, the plate-shaped inorganic oxide particles in the examples of the present invention are zeolite, and a precious metal such as palladium is used in the same composition as the active species of the catalyst. It has been pointed out that the mixed use or contact use of such zeolite and a precious metal such as palladium may result in the poisoning of the precious metal by the zeolite, resulting in a decrease in catalytic activity (see

[0004] of JP 2019-513078 A and

[0003] of JP 07-096183 A). However, it is surprising that the examples of the present application demonstrate improved catalytic activity despite the mixed use of zeolite and a precious metal. [Explanation of symbols]

[0105] 1. Space between catalyst particles (space) 2. Plate-shaped inorganic oxide particles (plate-shaped particles) 3. Nearly spherical inorganic oxide particles (nearly spherical particles) 4 Honeycomb catalyst 41 Cell opening end face of honeycomb catalyst 42 Honeycomb catalyst cell wall 43 Catalyst composition layer 44 Cell Space L1: Thickness of the catalyst composition layer at the corner of the cell L2: Thickness of the catalyst composition layer at the center (thin part) of the cell wall

Claims

1. A honeycomb catalyst in which a catalyst composition is coated on cell walls of a honeycomb structure in which cylindrical cells having open ends are accumulated while sharing the cell walls, The catalyst composition contains substantially spherical inorganic oxide particles and plate-like inorganic oxide particles, The plate-like inorganic oxide particles are zeolite. A honeycomb catalyst for purifying exhaust gas from gasoline automobiles, characterized by:

2. The honeycomb catalyst for purifying gasoline automobile exhaust gases according to claim 1, wherein the width of the plate-shaped inorganic oxide particles is at least twice the thickness, the length of the major axis is at least 1 time the width, and is 1 / 10 to 2 times the major axis of the approximately spherical inorganic oxide particles.

3. 3. A honeycomb catalyst for purifying exhaust gas from gasoline automobiles according to claim 1, wherein the plate-like inorganic oxide particles are crystalline primary particles.

4. A honeycomb catalyst for purifying gasoline automobile exhaust gases according to any one of claims 1 to 3, wherein the opening shape of the cells constituting the honeycomb structure is square, the thickness of the layer of catalyst composition coated on the cell wall surface is greater at the corners of the cells than at the center of the cell walls, and the mass ratio of plate-shaped inorganic oxide particles to approximately spherical inorganic oxide particles per unit volume in the layer of catalyst composition (plate-shaped inorganic oxide particles / approximately spherical inorganic oxide particles) is 5 / 100 to 40 / 100.

5. A method for purifying exhaust gas from a gasoline automobile, which comprises using the honeycomb catalyst for purifying exhaust gas from a gasoline automobile according to any one of claims 1 to 4.

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