JP2022114539A - Exhaust purification catalytic device and exhaust purification method using the device - Google Patents

Abstract

To provide an exhaust purification catalytic device, purifying the exhaust including large volume of HC and further inhibiting back-pressure elevation which is one of output decreasing causes of an internal combustion engine.SOLUTION: An exhaust purification catalytic device is characterized in that a honeycomb catalyst disposed in a flow of an exhaust discharged from an internal combustion engine to purify the exhaust is at least arranged at upstream and downstream sides 1 and 2 of the exhaust flow; a material of a honeycomb structure used for the honeycomb catalyst comprises an inorganic oxide; the honeycomb structure has a cell opening part at both ends thereof, and the cell composing that structure shares a cell wall with an adjacent cell to be cumulated; a catalyst layer including a hydrogen carbide adsorbent comprising at least zeolite is covered on a cell wall of the honeycomb structure used for the upstream honeycomb catalyst; and a thickness of a cell wall 12 composing the honeycomb structure used for the downstream honeycomb catalyst is thinner than that of the cell wall 11.SELECTED DRAWING: Figure 1

Description

TECHNICAL FIELD The present invention purifies harmful components and environmentally hazardous substances contained in exhaust gas emitted from an internal combustion engine mounted on an automobile or the like, and particularly relates to an exhaust gas purifying catalyst device that is excellent in the ability to purify hydrocarbon components. .

Exhaust gas is emitted from internal combustion engines such as automobiles that are operated using fossil fuels. Such exhaust gas contains harmful components such as hydrocarbons (HC), nitrogen oxides (NOx), carbon monoxide (CO), particulate matter (PM) containing soot components derived from fossil fuel combustion, and global warming. It contains environmentally hazardous substances that are considered to be the cause of pollution. A method using a honeycomb catalyst is widely used to purify these harmful components and environmentally hazardous substances, and the harmful components and environmentally hazardous substances are purified by the honeycomb catalyst and then discharged into the atmosphere.

Regulatory limits have been set by the administrative agencies of each country for the release of such hazardous substances and substances of concern into the atmosphere. increasing.

Among harmful components contained in exhaust gas, HC is derived from incompletely combusted fuel. It is known that such HC is generated particularly frequently when the temperature of the combustion chamber of the internal combustion engine is low. A typical state in which the temperature of the combustion chamber of the internal combustion engine is low is when the internal combustion engine is started from a cold state.

Not only internal combustion engines but also hybrid (HV) systems that use electric motors are known as power sources for automobiles, and with rising environmental awareness in the market, harmful substances and environmentally hazardous substances in government agencies are expected to be strengthened in the future. Emission regulations are also in place, and it is expected that they will become more popular in the future.

It goes without saying that the amounts of various harmful components and environmentally hazardous substances discharged from automobiles have a correlation with the fuel efficiency of automobiles. In the HV system, when driving a vehicle, not only the electric motor is assisted, but also the electric motor alone is operated to drive the vehicle.

On the other hand, when the vehicle runs only with the electric motor in the HV system, or when the vehicle runs with the assistance of the electric motor, naturally the internal combustion engine is in a non-operating state or a low rotation operating state.

If the automobile is driven only by the electric motor and then the internal combustion engine is operated, there is a concern that a large amount of HC will be emitted, as is the case when the internal combustion engine is cold started. Also, even if the electric motor assists, the amount of fuel used in the internal combustion engine, that is, the amount of fuel burned, will decrease, and in this situation, the temperature of the combustion chamber tends to decrease, and the HC contained in the exhaust gas will decrease. There is concern that the amount will increase. In recent years, the performance of batteries and electric motors has improved, and it is expected that there will be more opportunities to emit HC derived from low-temperature exhaust gas from vehicles equipped with HV systems that will be offered to the market in the future.

In addition, it is expected that the fuel consumption performance required for automobiles will continue to increase, even if it is not based on HV systems. An improvement in fuel efficiency means less fuel used for combustion, and the amount of heat obtained from the combustion of less fuel is naturally smaller. It is.

As described above, catalysts having a hydrocarbon adsorption (HC-trap) function are known as one of techniques for suppressing the emission of HC contained in internal combustion engines into the atmosphere (Patent Documents 1 to 3).

The HC-trap catalyst is combined with another catalyst, a three-way catalyst (TWC) for purifying HC, CO, and NOx in the case of gasoline automobile exhaust gas purification, and is implemented as a catalytic device in the exhaust gas stream. Also, the HC-trap catalyst itself generally contains a catalyst component having a function of oxidizing TWC or HC.

Both the HC-trap catalyst and the TWC mounted on the vehicle are obtained by coating the cell walls of the honeycomb structure with a composition that serves as a catalyst component. In the HC-trap catalyst, it is common to coat a component having an HC-trap function on the cell walls, and then coat a TWC or a catalyst composition layer having an HC oxidation function thereon (Patent Document 1, 3). By providing the catalyst composition layer having HC oxidation function on the component having HC-trap function in this way, when HC is desorbed from HC-trap, the HC component passes through the catalyst composition layer. , and a certain amount of HC components are purified here. After that, the HC components desorbed from the HC-trap catalyst and discharged into the flow of the exhaust gas are further purified by the downstream TWC in the case of a catalyst device in which a TWC is arranged downstream (Patent Document 1).

Various zeolites such as MFI-type, FAU-type, *BEA-type, and MSE-type zeolites are known to be effective materials for such HC-trap catalysts and have the HC-trap function (Patent Document 1 , 2).

The HC adsorption-desorption of the HC-trap catalyst is affected by temperature fluctuations in the zeolite. That is, it utilizes the effect that HC is adsorbed by zeolite when the temperature of the HC-trap catalyst is low, and the adsorbed HC is desorbed when the temperature of the HC-trap catalyst is high. Such adsorption/desorption behavior of HC in zeolite is also called thermal swing adsorption (TSA).

The HC desorbed due to the increase in the temperature of the HC-trap catalyst is expected to be purified by the warmed HC-trap catalyst and downstream TWC, but the temperature of the HC-trap catalyst and downstream TWC must be sufficiently high. The HC is not purified and passes through the downstream TWC and is discharged into the atmosphere.

In order to prevent such discharge of HC, it is desirable to quickly raise the temperature of the subsequent TWC. As a method for that purpose, Patent Document 1 proposes increasing the TWC cell density, which is a downstream honeycomb catalyst. By increasing the cell density of the TWC, that is, the number of cells per unit cross-sectional area in the honeycomb structure, the geometrical surface area of the honeycomb structure is increased to increase the area in contact with the exhaust gas after combustion, thereby increasing the TWC downstream. A rapid rise in temperature can be expected.

A TWC with an increased geometric surface area can certainly be expected to raise the temperature, but on the other hand, if the number of cells per unit cross-sectional area increases, it will become a resistance to the exhaust gas flow and cause a problem of an increase in back pressure. . The increase in back pressure is also called pressure loss, and it hinders the flow of exhaust gas after combustion, resulting in insufficient scavenging in the combustion chamber. It also acts as a resistance to the upward movement of the piston in the room, and is considered to be one of the factors that reduce the output of the internal combustion engine.

Japanese Patent Application Laid-Open No. 2003-200049 JP 2004-105821 A JP-A-02-56247

In this way, the exhaust gas emitted in a state where the catalyst is not sufficiently warmed up, such as when the internal combustion engine is running cold, is released into the atmosphere while containing a large amount of HC. In particular, vehicles equipped with HV systems, which have become very popular in recent years due to their low fuel consumption (hereinafter sometimes simply referred to as "HV vehicles"), operate the internal combustion engine itself less even during normal driving, and the temperature rises. Exhaust gases containing a large amount of HC have been discharged from vehicles that often emit low exhaust gases.

In addition, in the conventional catalyst system used to purify such exhaust gas, a honeycomb structure with a high cell density was used as a means of quickly warming the honeycomb catalyst, so the back pressure of the exhaust gas increased, resulting in pressure loss. , leading to a decrease in the output of the internal combustion engine. An internal combustion engine whose output has decreased needs to be operated at a higher rotational speed in order to obtain more output. In that case, the amount of fuel consumed increases, and the amount of emissions of harmful components and environmentally hazardous substances may rather increase.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a technique for purifying exhaust gas containing a large amount of HC and suppressing an increase in back pressure, which is one of the causes of a decrease in the output of an internal combustion engine.

As a result of intensive research conducted by the present inventors in order to solve the above problems, the present inventors have found that a honeycomb catalyst disposed in an exhaust gas stream emitted from a combustion engine for purifying the exhaust gas is provided at least upstream and downstream in the exhaust gas stream. In the exhaust gas purifying catalyst device disposed on the upstream side, the cell walls of the honeycomb structure used for the upstream honeycomb catalyst are coated with a catalyst layer containing at least a hydrocarbon adsorbent made of zeolite, and further, the upstream honeycomb catalyst The inventors have found that the above problems can be solved by, for example, making the cell walls of the honeycomb structure used for the honeycomb catalyst on the downstream side thinner than the cell walls of the honeycomb structure used for the catalyst. completed.

That is, the present invention is an exhaust gas purifying catalyst device arranged in an exhaust gas flow discharged from a combustion engine, wherein honeycomb catalysts for purifying the exhaust gas are arranged at least upstream and downstream in the exhaust gas flow,
The material of the honeycomb structure used in the honeycomb catalyst consists of inorganic oxides,
The honeycomb structure has cell openings at both ends,
The cells constituting the honeycomb structure are accumulated by sharing the cell walls with adjacent cells,
The cell walls of the honeycomb structure used for the upstream honeycomb catalyst are coated with a catalyst layer containing at least a hydrocarbon adsorbent made of zeolite,
Exhaust gas characterized in that the thickness of the cell walls forming the honeycomb structure used in the honeycomb catalyst on the downstream side is thinner than the thickness of the cell walls forming the honeycomb structure used in the honeycomb catalyst on the upstream side. It is a purification catalyst device.

Further, the present invention is a method for purifying exhaust gas, characterized in that the exhaust gas purifying catalytic device is arranged in the flow of exhaust gas discharged from a combustion engine.

According to the exhaust gas purifying catalyst device of the present invention, the thickness of the cell walls constituting the honeycomb structure used for the honeycomb catalyst on the downstream side is covered with the catalyst layer containing the hydrocarbon adsorbent made of zeolite, and the HC- By making the cell walls of the honeycomb structure used in the upstream honeycomb catalyst having a trap function thinner than the cell walls, the downstream honeycomb catalyst can be warmed up more easily than the upstream honeycomb catalyst. The honeycomb catalyst on the downstream side can quickly purify the HC desorbed from the catalyst.

Furthermore, in the exhaust gas purifying catalyst device of the present invention, the cell walls of the honeycomb structure in the downstream honeycomb catalyst are thinned, so that the total cell opening area in the honeycomb structure can be increased, and the output of the internal combustion engine can be increased. It is also possible to suppress an increase in the back pressure, which is one of the causes of the decrease.

FIG. 1 is a schematic diagram showing an embodiment of the present invention. FIG. 2 is a schematic diagram showing the prior art described in Patent Document 1. As shown in FIG. FIG. 2 is a schematic view enlarging a part of the cell walls of the upstream honeycomb catalyst in FIG. 1; FIG. 2 is a schematic view enlarging a part of the cell walls of the downstream honeycomb catalyst in FIG. 1; FIG. 5 is an enlarged schematic view of part of the cell walls of the upstream and downstream honeycomb catalysts in another embodiment of the present invention.

The exhaust gas purifying catalyst device of the present invention (hereinafter referred to as "the device of the present invention") is disposed in the exhaust gas stream discharged from a combustion engine, and has a honeycomb catalyst for purifying exhaust gas at least upstream and in the exhaust gas stream. In the exhaust gas purifying catalyst device arranged on the downstream side, the material of the honeycomb structure used for the honeycomb catalyst is made of an inorganic oxide, and the honeycomb structure has cell openings at both ends to form a honeycomb structure. The adjacent cells share the cell wall and are accumulated, and the cell wall of the honeycomb structure used for the upstream honeycomb catalyst has a catalyst layer containing at least a hydrocarbon adsorbent made of zeolite. The thickness of the cell walls forming the coated honeycomb structure used for the honeycomb catalyst on the downstream side is thinner than the thickness of the cell walls forming the honeycomb structure used for the honeycomb catalyst on the upstream side. and

[Internal combustion engine, vehicle]
The internal combustion engine in which the device of the present invention is installed is not particularly limited, and can be used for all internal combustion engines that operate by burning fossil fuels containing HC. Such internal combustion engines include gasoline engines and diesel engines. Gasoline engines have a smaller mixture ratio of fuel and air (air-fuel ratio: mass of air/mass of fuel) than diesel engines. A large amount of HC is included in the exhaust gas emitted inside.

In the device of the present invention, even when the temperature of the exhaust gas or the exhaust gas purifying catalyst is low, such as during a cold start, it is possible to quickly raise the temperature of the later-stage honeycomb catalyst, which will also be described later. It is also possible to quickly purify HC desorbed from the HC-trap in the upstream honeycomb catalyst. In particular, in a vehicle such as a hybrid vehicle, which has few opportunities to operate the internal combustion engine even when driven and tends to emit low-temperature exhaust gas containing a large amount of HC, the catalyst in the exhaust gas purifying catalyst device of the present invention does not increase. Warm action works effectively.

Further, in the apparatus of the present invention, since the honeycomb catalyst on the downstream side, which can raise the temperature quickly, has a large opening area in the honeycomb structure as described later, the back pressure does not rise significantly, and the honeycomb catalyst is arranged in the flow of the exhaust gas. A decrease in the output of the internal combustion engine due to this is also suppressed.

[Positions]
The arrangement of the upstream honeycomb catalyst and the downstream honeycomb catalyst that constitute the apparatus of the present invention is not particularly limited, and the upstream honeycomb catalyst and the downstream honeycomb catalyst may be arranged adjacently immediately below the exhaust manifold. Alternatively, the upstream honeycomb catalyst and the downstream honeycomb catalyst may be arranged adjacently (in tandem) under the floor of the vehicle, or the upstream honeycomb catalyst may be arranged directly under the exhaust manifold of the internal combustion engine and the downstream honeycomb catalyst may be arranged under the vehicle floor. good. Further, the upstream honeycomb catalyst in these catalyst arrangements may be arranged directly below the exhaust port of the internal combustion engine to further raise the temperature of the honeycomb catalyst. Although the apparatus of the present invention can accommodate various catalyst arrangements as described above, it is preferable that the honeycomb catalyst on the upstream side and the honeycomb catalyst on the downstream side are arranged in tandem from the viewpoint of effectively utilizing the heat of the exhaust gas. A tandem arrangement in which the upstream honeycomb catalyst is arranged immediately below the exhaust manifold is more preferable, and a tandem arrangement in which the upstream honeycomb catalyst is arranged immediately below the exhaust port is more preferable.

[Honeycomb catalyst]
The apparatus of the present invention has honeycomb catalysts for purifying exhaust gas emitted from an internal combustion engine, which are arranged at least upstream and downstream in the exhaust gas flow. The material of the honeycomb structure used in the honeycomb catalyst is composed of an inorganic oxide, the honeycomb structure has cell openings at both ends, and the cells constituting the honeycomb structure share the cell walls with adjacent cells. It is an accumulation.

[Honeycomb structure: material]
The material of the honeycomb structure used in the honeycomb catalyst is composed of inorganic oxides, and examples thereof include those composed of one or more kinds of inorganic oxides such as cordierite, silica, alumina, silicon carbide, and zeolite. be done. Among them, cordierite is inexpensive, has excellent durability even in a harsh exhaust gas environment, and is the most widely used honeycomb structure material for exhaust gas purification for automobiles, and is also preferable for application to the present invention. It can be said that it is a thing.

Further, in the exhaust gas purifying catalyst device of the present invention, as described later, the cell walls of the honeycomb structure used for the honeycomb catalyst on the downstream side are made thinner than the cell walls of the honeycomb structure used for the honeycomb catalyst on the upstream side. By doing so, the temperature of the post-stage honeycomb catalyst can be raised quickly, so the effect is exhibited even when the honeycomb structures on the upstream side and the downstream side are made of materials having the same specific heat. As described above, cordierite is the preferred material for the honeycomb structure used for purifying the exhaust gas of internal combustion engines such as automobiles, and is the most widely used one. , and it can be said that the honeycomb structure of the device of the present invention is particularly effective when it is made of such a homogeneous material having a similar specific heat.

[Honeycomb structure: structure]
The structure of the honeycomb structure used in the honeycomb catalyst is not particularly limited, and can be appropriately selected from the structures of honeycomb structures conventionally available on the market according to the purpose for which the catalyst is used. can be adopted.

There are roughly two types of honeycomb structures on the market and conventionally used, one is a flow-through type honeycomb structure and the other is a wall-flow type honeycomb structure. Each of them is an accumulation of tubular flow passages extending from one end face to the other end face in the flow direction of the exhaust gas, and adjacent tubular flow passages (cells) share the wall.

Of these, in the flow-through type honeycomb structure, both ends of each tubular cell are open, and the exhaust gas flows in from one open end and is discharged from the other open end. Here, when the walls of the cells are coated with the catalyst composition, the exhaust gas passes through the cells while contacting the catalyst composition, and during this time the environmentally hazardous substances and harmful components in the exhaust gas are purified.

One wall-flow honeycomb structure has a cell wall made of a porous body through which exhaust gas can pass. There is only one opening. That is, the cells that are plugged at the inlet-side end face of the exhaust gas are open at the outlet-side end face, and the cells that are plugged at the inlet-side end face are open at the outlet-side end face. Now it's the other way around. When such a wall-flow honeycomb structure is coated with a catalyst composition slurry by a wash coating method, which will be described later, the catalyst composition coats the walls made of the porous body, impregnates the walls, or both. coated with Exhaust gas passes through the surface and inside of the walls of the cells, during which particulate components such as soot are filtered out. If substances and harmful constituents are also purified, they are also purified during passage of the exhaust gas.

In the apparatus of the present invention, the cell walls of the honeycomb structure used for the honeycomb catalyst on the downstream side must be thin. A honeycomb used in a honeycomb catalyst on the downstream side is a flow-through type honeycomb structure that does not require such a function and makes it easier to make the cell walls thinner than a wall-flow type honeycomb structure that requires a predetermined thickness. It is preferable as a structure. In addition, one of the effects of the present invention, that the back pressure is difficult to increase, is achieved by having thin cell walls. A flow-through honeycomb structure is preferred. In addition, since the wall flow type honeycomb structure has a filter function, this filter function itself tends to become a flow resistance for the flow of exhaust gas. It can be said that a honeycomb structure is preferable.

[Honeycomb structure: cell wall thickness]
The thickness of the cell walls of the honeycomb structure used for the honeycomb catalyst on the downstream side used in the apparatus of the present invention is relatively thinner than the cell walls of the honeycomb structure used for the honeycomb catalyst on the upstream side. If so, the effect can be exhibited.

Generally, the thickness of the cell walls of honeycomb structures is sometimes expressed as milliinch "mils". In the apparatus of the present invention, the thickness of the cell wall of the honeycomb structure used for the honeycomb catalysts on the upstream and downstream sides is in the range of 0.025 to 0.3 mm (1 to 12 mil), 0.05 to 0.2 mm. A range of (2 to 8 mils) is preferred, and a range of 0.05 to 0.125 mm (2 to 5 mils) is more preferred.

Within such a range of cell wall thickness, the thickness of the cell walls of the honeycomb structure used for the honeycomb catalyst on the downstream side is 2/ of the thickness of the cell walls of the honeycomb structure used for the honeycomb catalyst on the upstream side. It is preferably 3 or less, more preferably ⅓ or less. If the difference in cell wall thickness between the upstream honeycomb catalyst and the downstream honeycomb catalyst is too small, the temperature of the downstream honeycomb catalyst cannot be appropriately increased when HC is desorbed from the upstream honeycomb catalyst. Therefore, the honeycomb catalyst on the downstream side may not be able to sufficiently purify HC. The honeycomb structure is a molded body made of inorganic oxides, and the honeycomb structure itself does not have activity as a catalyst, and does not have a heat generation function due to an oxidation reaction like the TWC catalyst layer described later. An important factor in the side honeycomb catalyst is how effectively the heat supplied from the outside can be used to raise its own temperature.

In addition, since the cell walls of the honeycomb structure constituting the upstream honeycomb catalyst used in the apparatus of the present invention are thicker than the cell walls of the honeycomb structure constituting the downstream honeycomb catalyst, the upstream honeycomb catalyst is It can be said that it becomes difficult to raise the temperature, and the start of desorption of adsorbed HC from the zeolite, which will be described later, is delayed. It can be said that it will be easier to be taken.

[Honeycomb structure: cell density]
Factors that determine the structure of the honeycomb structure include the material, cell wall thickness, and cell density. The cell density is represented by the number of cells per unit area of a cross section perpendicular to the axis of the honeycomb structure. The cell density of the honeycomb structure constituting the downstream honeycomb catalyst used in the exhaust gas purifying catalyst device of the present invention is not particularly limited. It can be appropriately selected from structures. A specific example of such a cell density is preferably 62 to 124 cells/cm ² (400 to 800 cells/inch ² ), more preferably 62 to 93 cells/cm ² (400 to 600 cells/inch ² ). preferable.

In addition, since the apparatus of the present invention does not cause a significant increase in back pressure, the honeycomb structure used for the upstream honeycomb catalyst and the cells of the honeycomb structure used for the downstream honeycomb catalyst It can be said that it is preferable to select each in the range of 62 to 93 cells/cm ² (400 to 600 cells/inch ² ) because it is more effective when there is no great difference in density.

In this way, specifications that do not have a large difference in the cell density of the honeycomb structures on the upstream side and the downstream side include, for example, when the cell densities of the honeycomb structures on the upstream side and the downstream side are the same, or when the honeycomb catalyst on the downstream side This includes the case where the cell density of is smaller than that on the upstream side. According to such specifications, it is possible to more effectively suppress an increase in back pressure caused by the honeycomb structure on the downstream side.

[Upstream honeycomb catalyst: hydrocarbon adsorbent]
In the apparatus of the present invention, the cell walls of the honeycomb structure used for the honeycomb catalyst on the upstream side are coated with a catalyst layer containing at least a hydrocarbon adsorbent made of zeolite. As the zeolite that serves as such a hydrocarbon adsorbent (hereinafter sometimes referred to as "HC-trap material"), there is no problem if the zeolite that has been conventionally used by those skilled in the art is appropriately used. Select and use from relatively large pore zeolites such as MFI type, FAU type (USY etc.), *BEA type, MSE type (MCM-68 etc.) as listed in 2. is preferred.

HC components emitted from internal combustion engines are derived from the combustion of fossil fuels such as gasoline and light oil, and their molecular sizes are diverse. HC components with large molecular sizes such as are also included. Since the HC-trap material is required to adsorb HC components of such various molecular sizes, it is desirable that the zeolite employed also has large pores. Moreover, such a zeolite having large pores may be used as it is, but it may be one in which a transition metal such as iron or copper is supported by a technique such as ion exchange. Such HC-trap materials may be used in combination with zeolites with small pores such as CHA, AEI, and AFX in addition to zeolites with large pores as described above.

[Upstream honeycomb catalyst: layer structure]
Since the cell walls of the honeycomb structure used for the honeycomb catalyst on the upstream side of the apparatus of the present invention are coated with a catalyst layer containing at least the HC-trap material, it is mentioned above that the HC-trap material is contained. In addition, in order to purify HC desorbed from the HC-trap material contained in the upstream honeycomb catalyst, a catalyst component that does not contain zeolite, which is an HC-trap material, may be included. .

When a catalyst component not containing zeolite is used together with the HC-trap material for the honeycomb catalyst on the upstream side of the apparatus of the present invention, the HC-trap material may be mixed with the catalyst component, but the cell walls of the honeycomb structure After coating the catalyst layer containing the HC-trap material thereon, it is desirable to coat the catalyst layer containing the zeolite-free catalyst component thereon.

The zeolite-free catalyst component used in the upstream honeycomb catalyst is appropriately selected from compositions used for purifying exhaust gas emitted from conventional internal combustion engines. When used for purifying exhaust gas discharged from a fuel cell, a noble metal catalyst containing a noble metal component supported on inorganic oxide particles may be used. Preferably, the noble metal catalyst comprises a three-way catalyst, also referred to as TWC. The composition of the TWC is adjusted in various details according to each individual specification, but its basic composition is a carrier of inorganic oxide particles containing one or more of alumina, zirconia, ceria, titania, etc., and platinum , palladium, rhodium, etc., and combined with an oxygen storage method-emission material represented by ceria particles. The interaction of these catalyst components in the TWC not only purifies HC by oxidation, but also purifies NOx, CO, and the like.

As a preferred embodiment of the honeycomb catalyst on the upstream side of the apparatus of the present invention, the cell walls of the honeycomb structure are coated with a catalyst layer containing an HC-trap material (hereinafter sometimes referred to as "HC-trap catalyst layer"). It is desirable to coat thereon a noble metal catalyst layer (hereinafter sometimes referred to as "TWC catalyst layer") containing a noble metal component (catalyst component not containing zeolite) supported on inorganic oxide particles. As described above, the TWC composition consists of particulate catalyst components, and by coating the surface of the HC-trap catalyst layer with such particulate components, the particles are desorbed from the HC-trap catalyst layer. Some of the HC can be cleaned up with a TWC catalyst bed. In addition, since the surface of the HC-trap catalyst layer is covered with the TWC catalyst layer, it is expected that the HC desorbed from the HC-trap catalyst layer will be delayed from being returned to the flow of the exhaust gas. During the delay time thus generated, the temperature rise of the post-stage honeycomb catalyst is accelerated, and HC conversion becomes possible more efficiently.

In addition, purification of HC and CO in the exhaust gas in the TWC catalyst layer is an oxidation reaction using oxygen in the exhaust gas, and thus heat is generated. On the other hand, in a honeycomb structure made of an inorganic oxide such as cordierite, the cell walls themselves hardly contribute to the reaction. Not only HC, but also catalytic reactions in exhaust gas purification are promoted at a correspondingly high temperature, so oxidation of HC and CO in the TWC catalyst layer promotes heat generation of the honeycomb catalyst itself, and further improvement of the purification rate can be expected. . Since the TWC catalyst layer is desirably formed on the outermost surface in direct contact with the exhaust gas in order to promote the heat generation of the catalyst, the HC-trap catalyst layer is formed on the cell wall of the honeycomb structure on the upstream side. and the TWC catalyst layer are preferably coated in this order.

[Downstream honeycomb catalyst]
In the honeycomb catalyst on the downstream side of the apparatus of the present invention, if the thickness of the cell walls constituting the honeycomb structure used therein is thinner than the thickness of the cell walls constituting the honeycomb structure used in the honeycomb catalyst on the upstream side, good.

A catalyst layer is coated on the cell walls of the honeycomb structure used for the downstream honeycomb catalyst. Examples of such a catalyst layer include a catalyst layer containing no zeolite. The zeolite-free catalyst layer preferably has an oxidation function for purifying HC released from the honeycomb catalyst on the upstream side. It is desirable to include a noble metal with an oxidizing function. In addition, if the internal combustion engine in which the device of the present invention is installed is mounted in a gasoline automobile, it is desirable to reduce and purify NOx in addition to the oxidation of HC. The catalyst composition is desirably TWC, which uses a combination of platinum, palladium, and Rh, like the upstream honeycomb catalyst.

Note that the honeycomb catalyst on the downstream side may contain an HC-trap material made of zeolite like the honeycomb catalyst on the upstream side. It is desirable to include zeolite in an amount less than the amount of zeolite per volume. If the honeycomb catalyst on the downstream side contains a large amount of HC-trap material, even if HC is adsorbed on the HC-trap material of the honeycomb catalyst on the downstream side, the honeycomb catalyst itself on the downstream side will release HC. If the temperature is not sufficiently raised for oxidation, or if HC is desorbed while the temperature is rising, there is a risk that the HC will be discharged into the atmosphere as it is without being purified. In order to eliminate such concerns, it is conceivable to arrange another catalyst behind the honeycomb catalyst on the downstream side, or to electrically heat the honeycomb catalyst on the downstream side. In the case of automobiles, the increase in cost is reflected in the price of the vehicle, reducing its commercial value in the market.

When the honeycomb catalyst on the downstream side contains the HC-trap material, the HC-trap material and the above-described zeolite-free catalyst, preferably a catalyst having an oxidation function, more preferably TWC, are mixed to form a honeycomb structure as a catalyst composition layer. However, the cell walls of the honeycomb catalyst on the downstream side are coated with an HC-trap catalyst layer having a lower loading amount than the honeycomb catalyst on the upstream side, and zeolite is coated thereon. A non-containing catalyst layer, preferably a catalyst layer having an oxidation function, more preferably a TWC catalyst layer may be coated. By constructing such a layer structure, the same effect as that of the honeycomb catalyst on the upstream side can be expected.

Those skilled in the art specify the amounts of catalyst components in such upstream honeycomb catalysts and downstream honeycomb catalysts as amounts [g/L] per unit volume of the honeycomb structure. That is, it can be specified by this amount per unit volume [g/L] that the amount of HC-trap material such as zeolite contained in the downstream honeycomb catalyst is smaller than that in the upstream honeycomb catalyst. When the honeycomb catalyst on the downstream side contains the HC-trap material, the amount of the HC-trap material per unit volume of the honeycomb structure is preferably 1/2 or less of the amount of the HC-trap material contained in the honeycomb catalyst on the upstream side. It is more preferably 3 or less.

Moreover, the amount of the catalyst component may be specified as the thickness of the catalyst layer, in addition to the amount per unit volume [g/L] as described above. When the thickness is specified as the thickness of the catalyst layer, it can be specified as the average value of the thickness of the catalyst layer coated on the cell walls observed from the cross section in the direction perpendicular to the axis of the honeycomb structure. Such an average value of the thickness of the catalyst layer is the average value of the thick and thin portions of the catalyst layer. It may be an average value with the wall central portion.

[Other catalyst layers]
The preferred configurations of the catalyst layers in the upstream honeycomb catalyst and the downstream honeycomb catalyst in the apparatus of the present invention are as described above. Configurable. As the catalyst layer having such other functions, for example, in order to improve the adhesion between the HC-trap catalyst layer and the cell wall in the honeycomb catalyst on the upstream side, Specifications include providing a coating layer called a base coat layer and laminating two or more TWC catalyst layers having different compositions in order to control the activity of the TWC catalyst layer. Such multi-layering of the base coat layer and TWC is the same in the structure of the catalyst layer in the honeycomb catalyst on the downstream side.

[Manufacturing method of honeycomb catalyst]
As described above, both the upstream honeycomb catalyst and the downstream honeycomb catalyst used in the apparatus of the present invention have the catalyst layer coated on the cell walls of the honeycomb structure. The method of coating the catalyst composition layer on the honeycomb catalyst used in the device of the present invention is not particularly limited, and the coating can be performed by a wash coating method widely practiced by those skilled in the art.

Various specifications of the washcoat method have been studied by those skilled in the art, and applications (Japanese Patent Application Publication No. 2003-506211 and Japanese Patent Application Publication No. 2011-529788) have also been implemented. A predetermined amount of the catalyst composition slurry is supplied from the open end face, and the catalyst composition slurry is spread over a predetermined length toward the other open end face of the honeycomb structure by means of air pressure or the like, or the excess catalyst is It removes the composition slurry.

In the wash-coating method, zone coating may be applied in which different catalyst compositions are separately applied to portions of the honeycomb structure. In any case, the desirable layer structures of the upstream honeycomb catalyst and the downstream honeycomb catalyst are as described above.

In addition, when the HT-trap catalyst layer made of zeolite is provided in the lower layer of the honeycomb catalyst on the upstream side, the HC-trap catalyst layer does not have a portion directly exposed to the flow of the exhaust gas, and the TWC catalyst layer has an oxidation function. It is desirable to be completely covered with a layer of catalyst composition such as. If there is a portion of the HC-trap catalyst layer that is directly exposed to the flow of exhaust gas, the effect of delaying the desorption of HC from the front-stage honeycomb catalyst and the effect of purifying HC desorbed from the HC-trap material may not be obtained or may be reduced. I have something to do.

The apparatus of the present invention described above can purify the exhaust gas by placing it in the exhaust gas stream discharged from the combustion engine.

[Embodiment 1]
Examples of embodiments of the present invention are described in detail below. FIG. 1 is a schematic diagram showing an embodiment of the device of the present invention, omitting a catalyst layer. In FIG. 1, the honeycomb catalyst is arranged behind the upstream honeycomb catalyst, i.e., the downstream honeycomb catalyst in the downstream side with respect to the flow of the exhaust gas. It is thinner than the cell walls of the honeycomb structure used for the honeycomb catalyst.

As mentioned above, the material of the honeycomb structure in the embodiment of FIG. 1 is not particularly limited. As described above, cordierite is desirable because of its high durability, and is desirable for both the upstream honeycomb catalyst and the downstream honeycomb catalyst of the present embodiment.

As shown in FIG. 1, the thickness of the cell walls of the honeycomb structure in the present embodiment is thinner in the honeycomb catalyst on the downstream side than in the honeycomb catalyst on the upstream side. As a result, the cell walls in the honeycomb catalyst on the downstream side can warm up quickly, and the HC-trap material contained in the honeycomb catalyst on the downstream side is desorbed by the action of the catalytic component having an oxidation function coated on the honeycomb catalyst on the downstream side. HC purification can be facilitated.

FIG. 3 is an enlarged view of the cell wall portion 11 of the honeycomb catalyst on the upstream side in FIG. 1, which is an embodiment of the present invention. The cell wall 12 of the honeycomb catalyst on the upstream side is thicker than the cell wall 22 of the honeycomb catalyst on the downstream side, which will be described later, so that it is difficult to warm up even when receiving the heat of the exhaust gas. In addition, in FIG. 3, the catalyst composition layers omitted in the drawing are schematically shown here, the HC-trap catalyst layer 14 on the cell wall 12, and the TWC on the HC-trap catalyst layer 14 A catalyst layer 13 is coated.

With the structure of the catalyst composition layer shown in FIG. 3, when the HC adsorbed by the HC-trap catalyst layer is desorbed, it passes through the TWC layer, which is the upper layer. , the geometry of the desorbed HC is purified, the amount of HC that must be purified by the downstream honeycomb catalyst can be reduced, and the overall amount of HC emitted to the atmosphere can also be reduced.

Also, by covering the HC-trap catalyst layer 14 with the TWC layer 13 as shown in FIG. can be delayed to gain time for the downstream honeycomb catalyst to warm up. In addition, since the TWC layer 13 becomes an obstacle, even when the HC-trap catalyst layer 14 warms up, a large amount of HC is not desorbed at once and released into the exhaust gas. Since HC is supplied to the downstream honeycomb catalyst 2, the reaction time becomes longer, the HC conversion activity of the downstream honeycomb catalyst 2 can be improved, and the amount of HC finally discharged into the atmosphere. can also be reduced.

FIG. 4 is an enlarged view of the cell wall portion 21 of the downstream honeycomb catalyst in FIG. 1, which is an embodiment of the present invention. The cell walls 22 of the honeycomb structure in the downstream honeycomb catalyst 2 are thinner than the cell walls 12 in the upstream honeycomb catalyst 1 . As a result, the cell walls 22 can be quickly warmed up by receiving the heat of the exhaust gas.

In this embodiment, the surface of the cell wall 22 is coated with the TWC catalyst layer 23, and the HC desorbed from the HC-trap material layer of the upstream honeycomb catalyst 1 is effectively removed by the downstream honeycomb catalyst 2 that warms up quickly. Purification becomes possible, and the amount of HC emitted into the atmosphere can be reduced. In addition, since the downstream honeycomb catalyst in the embodiment has a thin cell wall, the sum of the opening areas of the cells in the honeycomb structure is large. A rise in pressure can be made small.

[Embodiment 2]
FIG. 5 shows another embodiment of the present invention, and is an enlarged partial view 11 of the cell wall of the honeycomb catalyst on the upstream side in FIG. 1 and an enlarged view 21 of the cell wall of the honeycomb catalyst on the downstream side. In this embodiment, the HC-trap catalyst layer is coated on the cell walls of both the upstream and downstream honeycomb catalysts. The honeycomb catalyst on the upstream side is the same as in Embodiment 1, but the amount of HC-trap material in the HC-trap catalyst layer 24 in the honeycomb catalyst on the downstream side is the same as that of the HC-trap catalyst layer in the honeycomb catalyst on the upstream side. less than the amount of material. As a result, even when the downstream honeycomb catalyst is not sufficiently heated, pressure loss and a large amount of HC are not desorbed. It becomes possible to raise the temperature of the HC-trap catalyst layer 23 on the side.

Although not shown, the catalyst layers of the honeycomb catalyst on the upstream side and the honeycomb catalyst on the downstream side are composed of a mixture of a catalyst component containing a noble metal such as TWC and a catalyst component having an HC-trap function such as zeolite. However, from the viewpoint of carrying out the present invention, even with a catalyst having such a mixed composition, the amount of HC-trap material contained in the catalyst layer coated on the honeycomb catalyst on the downstream side Needless to say, it is desirable that the amount of the HC-trap material contained in the catalyst layer coated on the honeycomb catalyst on the upstream side is smaller.

[Comparative mode]
FIG. 2 shows the conventional catalyst device shown in Patent Document 1 as a comparative form. In the honeycomb structure 2′ used for the downstream honeycomb catalyst, the cell walls are not made thinner than the downstream honeycomb catalyst 2 of the embodiment, and only the cell density is increased. It has become.

2 is the same as the upstream honeycomb catalyst 1 of the embodiment, and the TWC catalyst layer coated on the downstream honeycomb catalyst is the downstream honeycomb catalyst of the embodiment. When the catalyst is the same as the TWC catalyst layer 23 or the HC-trap catalyst layer 24, in the catalyst device in which only the cell density is increased as in the comparative embodiment, the opening area of the cells in the honeycomb structure of the honeycomb catalyst on the downstream side is reduced. The total sum becomes small, which not only delays the temperature rise of the honeycomb structure in the honeycomb catalyst on the downstream side, but also causes an increase in the back pressure in the exhaust gas, which causes a decrease in the output of the internal combustion engine.

1 Upstream honeycomb catalyst 2 Downstream honeycomb catalyst 2′ Downstream honeycomb catalyst 11 with only increased cell density Part of cell wall 12 in upstream honeycomb catalyst Cell wall of honeycomb structure in downstream honeycomb catalyst 13 TWC catalyst layer 14 HC-trap catalyst layer 21 Part of cell wall 22 in honeycomb catalyst on downstream side Cell wall 23 of honeycomb structure in honeycomb catalyst on downstream side TWC catalyst layer 24 HC-trap catalyst layer on downstream side

Claims (9)

An exhaust gas purifying catalyst device arranged in an exhaust gas flow discharged from a combustion engine, wherein honeycomb catalysts for purifying the exhaust gas are arranged at least upstream and downstream in the exhaust gas flow,
The material of the honeycomb structure used in the honeycomb catalyst consists of inorganic oxides,
The honeycomb structure has cell openings at both ends,
The cells constituting the honeycomb structure are accumulated by sharing the cell walls with adjacent cells,
The cell walls of the honeycomb structure used for the upstream honeycomb catalyst are coated with a catalyst layer containing at least a hydrocarbon adsorbent made of zeolite,
Exhaust gas characterized in that the thickness of the cell walls forming the honeycomb structure used in the honeycomb catalyst on the downstream side is thinner than the thickness of the cell walls forming the honeycomb structure used in the honeycomb catalyst on the upstream side. Purification catalytic device. The cell walls of the honeycomb structure used for the upstream honeycomb catalyst are coated with a catalyst layer containing a hydrocarbon adsorbent made of zeolite and a catalyst layer not containing zeolite in this order. 2. The exhaust gas purifying catalyst device according to 1. 3. The exhaust gas purifying catalyst device according to claim 1, wherein the cell walls of the honeycomb structure used for the honeycomb catalyst on the downstream side are coated with a catalyst layer containing no zeolite. On the cell walls of the honeycomb structure used for the honeycomb catalyst on the downstream side, there is a catalyst layer containing zeolite in an amount less than the amount of zeolite per unit volume of the honeycomb structure used for the honeycomb catalyst on the upstream side. 3. The exhaust gas purifying catalyst device according to claim 1 or 2, which is coated. A catalyst layer containing zeolite in an amount smaller than the amount of zeolite per unit volume of the honeycomb structure used in the upstream honeycomb catalyst is provided on the cell walls of the honeycomb structure used in the downstream honeycomb catalyst. 3. The exhaust gas purifying catalyst device according to claim 1 or 2, wherein the catalyst layers not containing zeolite are coated in this order. The exhaust gas purification according to any one of claims 2, 3 and 5, wherein the catalyst layers not containing zeolite in the honeycomb catalysts on the upstream and downstream sides are noble metal catalyst layers containing a noble metal component supported on inorganic oxide particles. Catalytic device. The exhaust gas purifying catalyst device according to any one of claims 1 to 6, wherein the honeycomb structure of the honeycomb catalyst is a flow-through type honeycomb structure in which cells having both ends opened are integrated. The exhaust gas purifying catalytic device according to any one of claims 1 to 7, wherein the zeolite is MFI type, FAU type, *BEA type, or MSE type. A method for purifying exhaust gas, characterized in that the exhaust gas purifying catalyst device according to any one of claims 1 to 8 is arranged in an exhaust gas stream discharged from a combustion engine.

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