JP2001129402A - Catalyst for purifying exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the deterioration of the durability and exhaust gas purifying capacity of a catalyst for purifying exhaust gas by restraining that an absorbent added to a catalyst layer penetrates into a catalyst support and the absorbent vaporizes and scatters from the catalyst. SOLUTION: A zeolite, silica or titania layer 50 is formed between the catalyst layer 30 to which the absorbent is added and the catalyst support 10. The movement of the absorbent from the layer 30 to the support 10 is restrained by the layer 50 even when the catalyst is used over a long time at high temperature.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an exhaust gas purifying catalyst, and more particularly to an exhaust gas purifying catalyst excellent in durability and exhaust gas purifying performance.

[0002]

[Related Background Art] Lean-burn engines such as lean-burn engines and in-cylinder injection engines use a lean air-fuel ratio that is leaner than the stoichiometric air-fuel ratio in a predetermined operating range in order to improve fuel efficiency and exhaust gas characteristics. Be driven. NOx (nitrogen oxide) in exhaust gas during lean air-fuel ratio operation
Is not sufficiently purified by a three-way catalyst. Therefore, a NOx catalyst for storing NOx in exhaust gas in an oxidizing atmosphere is provided, and the NOx stored in the catalyst is reduced to N2 (nitrogen) in a reducing atmosphere. Is known to reduce NOx emissions to the atmosphere. In this type of storage-type lean NOx catalyst, for example, as described in JP-A-9-85093, potassium (K)
Is added as a NOx storage agent to improve the NOx storage performance.

[0003]

However, if a NOx catalyst to which an occluding agent such as potassium is added is kept at a high temperature for a long time, cracks may occur in the catalyst, which causes a decrease in the durability of the NOx catalyst. ing. N
In order to investigate the cause of crack generation in the Ox catalyst,
The present inventors have proposed a NOx formed by adding potassium as a storage agent to a catalyst layer supported on a honeycomb-type cordierite carrier.
A catalyst was manufactured, and a bench test of an engine equipped with the NOx catalyst and a running test of a vehicle equipped with this type of engine were performed. In the bench test and the actual vehicle running test, the engine and the vehicle were operated under the condition that the NOx catalyst was exposed to a high temperature of 650 ° C. or more for a considerable time. After completion of the operation, element analysis on the cut surface of the NOx catalyst was performed by the EPMA method (electron probe micropartial analysis method), and the cordierite (Mg2Al4Si5O18) of the catalyst was analyzed.
It was confirmed that a compound KMg4Al9Si9O36 of potassium, magnesium, aluminum, silicon and oxygen and a compound KAlSiO4 of potassium, aluminum, silicon and oxygen were present in the layer.

According to the above experiment, when the NOx catalyst is exposed to a high temperature, potassium added to the catalyst layer (wash coat) penetrates into the cordierite carrier, and potassium reacts with the cordierite under a high temperature atmosphere. To form the above compound. Here, it is understood that the potassium compound easily penetrates into the cordierite carrier because of its high water solubility and low melting point. Then, when a compound having a different coefficient of thermal expansion from cordierite is formed in the cordierite carrier, cracks are generated in the cordierite carrier with a change in the catalyst temperature during and before and after the use of the catalyst, and the NOx catalyst The strength will be reduced.

[0005] As described above, a NOx catalyst containing potassium or the like as an occluding agent is used in an oxidizing atmosphere. In this oxidizing atmosphere, nitrates and sulfates of the occluding agent are formed by a chemical reaction between the occluding agent and nitrogen or sulfur components in the exhaust gas,
The NOx storage capacity decreases. In this case, the storage capacity can be restored by forming a reducing atmosphere around the NOx catalyst and decomposing nitrates and sulfates. However, even if such measures are taken, if the NOx catalyst is used at a high temperature for a long time, Purification performance may decrease.

From the results of the following experiments conducted by the inventor, one of the causes of the decrease in the purification performance is that the occluding agent gradually evaporates and scatters from the NOx catalyst at a high temperature, and a considerable portion of the occluding agent in the catalyst is removed. Is considered to be disappearing. That is, the present inventor produced a NOx catalyst in which a cordierite carrier supported a catalyst layer containing potassium as a storage agent,
The potassium content in the unused NOx catalyst was determined by the XRF method (X-ray fluorescence spectroscopy), and the potassium content of the catalyst was determined after using the catalyst at a high temperature for a long time (for example, 850 ° C. for 32 hours). The content was determined, and the difference in potassium content before and after use was divided by the initial potassium content to determine the amount of potassium disappeared. As a result, the amount of potassium disappeared was found to range from several tens to 50% (see FIG. 6).

Accordingly, an object of the present invention is to provide an exhaust gas purifying catalyst capable of greatly reducing the degree of deterioration of exhaust gas purifying performance due to loss of an occluding agent.

[0008]

SUMMARY OF THE INVENTION The present invention provides an exhaust gas purifying catalyst in which an occluding agent is added to a catalyst layer, which is provided with a suppressing layer made of a material for suppressing the movement of the occluding agent. It is characterized in that movement is suppressed. In the present invention, when the exhaust gas purifying catalyst is exposed to a high temperature, the movement of the occluding agent that moves in the catalyst is suppressed by the suppression layer. The suppression layer has a layered structure, and the formation of such a suppression layer greatly contributes to suppression of the movement of the occluding agent, and the occluding agent evaporates from the catalyst and disappears due to the scattering, and further, the exhaust gas purification performance of the catalyst. Is prevented from decreasing.

In the present invention, preferably, the suppression layer is made of a material having an acid point. In this case, it is understood that the occluding agent that moves in the catalyst is captured and fixed at the acid sites of the constituent material of the suppression layer, and the movement of the occluding agent in the catalyst can be effectively suppressed. In the present invention,
Preferably, the suppression layer is formed between the carrier and the catalyst layer and / or on at least one of the outer surfaces of the catalyst layer.

When provided between the carrier and the catalyst layer, for example, when the carrier is coated with a suppression layer, the penetration of the occluding agent added to the catalyst layer into the carrier causes a layered structure between the catalyst layer and the carrier. Thus, it is reliably prevented by the suppression layer provided. Then, since the intrusion of the occluding agent into the carrier is suppressed by the suppression layer, the formation of a compound due to the reaction between the composition component of the material constituting the occluding agent and the composition component of the carrier is suppressed, and the formation of the compound is caused by The generation of cracks in the carrier and the reduction in the durability of the catalyst are prevented. In addition, when the suppression layer is formed on the outer surface of the catalyst layer, the evaporation and scattering of the occluding agent from the catalyst layer,
It is reliably prevented by the suppression layer provided in a layer on the outer surface of the catalyst layer. The catalyst of the present invention may have a plurality of catalyst layers. In this case, one or more suppression layers are arranged at appropriate positions in the catalyst according to the arrangement of the catalyst layers.

In general, a catalyst having a catalyst layer carried on a carrier composed of a large number of cells satisfies the mechanical, physical or chemical requirements imposed on the catalyst, such as prevention of peeling of the catalyst layer from the surface of the carrier. Therefore, the thickness of the catalyst layer is increased at the corners of each cell of the carrier, and accordingly, the catalyst layer has a deep portion near the corners. As described above, the reducing atmosphere is reduced by, for example, enriching the exhaust air-fuel ratio to recover the storage performance by decomposing the sulfate generated by the reaction between the storage agent in the catalyst layer and the sulfur component in the fuel. However, in the deep part of the catalyst layer, the gas diffusion is poor, so that the decomposition of the sulfate is difficult, and the particle growth of the sulfate easily proceeds.
Due to the consumption of the occluding agent, the NOx occluding performance deteriorates.

In this regard, in the preferred embodiment of the present invention in which the suppression layer is formed between the carrier and the catalyst layer or on the outer surface of the catalyst layer, the requirement that the catalyst layer at the corner of the carrier be thickened is relaxed.
The thickness of the catalyst layer can be made uniform as a whole, gas diffusion in the catalyst layer is promoted, and the consumption of the occluding agent accompanying the growth of sulfate decreases. In the present invention, preferably, the suppression layer is
It is composed of at least one material selected from the group consisting of a composite oxide containing an acidic substance. Each of the complex oxides containing an acidic substance contains at least one acidic substance selected from the group consisting of transition elements of Group IV, V and VI and typical elements of Group IV, V and VI.

[0013] In this preferred embodiment, the acidic oxide or composite oxide having a high storage capacity for the storage agent and a high thermal stability prevents the exhaust gas purifying performance of the catalyst from deteriorating and improves the durability. For example, the composite oxide is composed of a combination of oxides that exhibit acid sites. More preferably, the at least one acidic substance is selected in consideration of the reactivity between the acidic substance and the occluding agent. For example, when the storage agent is potassium, it is preferable to use an acidic oxide or a composite oxide containing silica or tungsten as an acidic substance.

Preferably, the suppression layer is made of a material that does not inhibit the reactivity between NOx and the storage agent. In this case, the effect of suppressing the movement of the occluding agent on the catalyst is exhibited by the suppressing layer, and the NOx occluding effect of the occluding agent is favorably exhibited.
Alternatively, the suppression layer is made of a material that adsorbs a reducing substance (for example, a reducing gas such as HC). In this case, sulfates and nitrates in the catalyst layer and the suppression layer are decomposed by the reducing substance trapped in the suppression layer by the reducing substance adsorption capacity of the suppression layer, and the NOx storage performance is restored.

Preferably, the constrain layer comprises a zeolite. According to the catalyst provided with zeolite as the suppression layer, the same advantages as those of the preferred embodiment described above are exhibited. That is, zeolite has a cation exchange ability and a molecular sieving effect, and is excellent in the ability to fix an occluding agent and the ability to adsorb HC. The storage agent that moves in the catalyst may be ionized in the presence of high-temperature steam, and is fixed as ions by the cation exchange ability of the acid sites on the zeolite (see FIG. 5). In addition, zeolite has a three-dimensional network structure and has a high specific surface area. Since the occluding agent is highly dispersed on the zeolite having such a structure,
In particular, when the zeolite is provided between the catalyst layer and the carrier, the occluding agent hardly permeates into the carrier. In addition, zeolite is H
Excellent in C adsorption capacity (more generally, reduced substance adsorption capacity).
Even when the internal combustion engine is in a lean operation state, the exhaust gas contains a small amount of HC, and the HC adsorbed on the zeolite promotes the decomposition of the nitrate and sulfate of the occluding agent. In other words, even during the lean operation, the suppression layer made of zeolite having the ability to adsorb HC can suppress the slight amount of H contained in the exhaust gas.
Utilizing C, nitrates and sulfates of the storage agent are continuously decomposed to contribute to recovery of the NOx storage performance of the catalyst.

In the present invention, various types of zeolites such as MFI type, Y type, X type, mordenite, and ferrierite can be used as the zeolite constituting the suppression layer. In consideration of the above, it is preferable to select a material that matches the exhaust gas composition. Further, the cation exchange ability and heat resistance of the zeolite depend on the composition of the zeolite. That is, the cation exchange capacity is inversely proportional to the SiO2 / AlO2 ratio in the zeolite, and the heat resistance is proportional to this ratio. Therefore, for example, by increasing the above ratio as much as possible, the heat resistance of the catalyst can be improved. Further, by reducing the above ratio, the amount of the occluding agent lost due to long-term operation of the catalyst at a high temperature can be reduced to maintain the occluding performance.

[0017] Preferably, the constrain layer does not contain a catalytic substance, such as a noble metal. In this case, the catalytic action of the catalytic substance is not exerted in the suppression layer, and a chemical reaction between the storage agent fixed to the suppression layer and SOx in the exhaust gas is unlikely to occur. And the catalyst N
Ox storage performance is maintained. In the present invention, preferably, the occluding agent contains potassium, and the carrier comprises a porous carrier. The addition of potassium improves the NOx storage capacity of the catalyst. In addition, the use of the porous carrier lowers the pressure loss of the exhaust gas and allows the exhaust gas to come into good contact with the catalyst layer, thereby purifying the exhaust gas satisfactorily. On the other hand, in the catalyst having the porous carrier, the flow of the exhaust gas containing high-temperature steam is improved, and the movement, evaporation, and scattering of the occluding agent are easily caused. In the present invention, this is prevented by the suppression layer.

Preferably, the suppression layer is one of a layer having a high acidity, a layer having a high specific surface area, a layer having a small crystal lattice, a layer made of an elemental compound having a high molecular weight, and a layer having a high basicity. It is composed of one. Preferably, the high acidity layer contains an acidic material having high reactivity with potassium of the catalyst layer, for example, silicon oxide. The layer having a high specific surface area includes a material having a high specific surface area, for example, zeolite. The layer made of an element compound having a large molecular weight is formed of, for example, a base material having a large molecular weight and a stable material such as barium sulfate (BaSO4)
Consists of The layer having a high basicity is made of a base material such as barium oxide (BaO).

According to the above preferred embodiment, it is considered that permeation of potassium into the porous carrier is suppressed by the following mechanism. That is, when the suppression layer is composed of a layer of high acidity, it is understood that potassium reacts with the layer of high acidity and is consumed before reaching the surface of the porous carrier. Further, it is understood that potassium is highly dispersed inside the suppression layer composed of a layer having a high specific surface area, and that the suppression layer composed of a layer having a small crystal lattice blocks the movement of potassium. In the case of the suppression layer made of an elemental compound having a large molecular weight, it is considered that the number of paths for permeating potassium into the porous carrier is reduced. Then, in the case of a high basicity suppression layer, since this layer has the same properties as potassium, when potassium approaches the suppression layer, it is repelled and the inducibility of potassium to the porous carrier is reduced. It is understood.

As described above, permeation of potassium into the porous carrier is suppressed, and the durability of the exhaust gas purifying catalyst is improved. Further, in the case of a suppression layer capable of suppressing the permeation without consuming potassium, the potassium NO
x The exhaust gas purifying performance of the exhaust gas purifying catalyst is preferably maintained without lowering the x purifying action. In order to evaluate the durability and the occluding agent disappearance preventing ability of the exhaust gas purifying catalyst of the present invention, zeolite is provided as a suppressing layer between a catalyst layer to which an occluding agent containing potassium is added and a cordierite carrier as an example. Make NOx catalyst and use unused NO
The potassium content in the x catalyst was determined by the XRF method. Further, the NOx catalyst is mounted on the engine and subjected to a bench test and an actual vehicle running test, whereby a high temperature is maintained for a long time (for example, 85
The potassium content of the NOx catalyst used over a period of 32 hours at 0 ° C.) was determined, and the difference between the potassium content before and after use was divided by the initial potassium content to obtain the potassium loss. As a result, in a catalyst in which a catalyst layer is supported on a carrier, the amount of potassium loss ranging from tens to 50% is as follows.
It was found that with the catalyst of the present invention, it was suppressed to about 10% or more. The results of this experiment (FIG. 6) show that the loss of potassium as the storage agent from the catalyst is significantly reduced by the present invention.

Further, after the NOx catalyst according to the present invention has been subjected to a bench test or an actual vehicle running test, the cut surface
Elemental analysis was performed by the PMA method. As a result, the amount of the potassium, magnesium, aluminum, silicon and oxygen compounds and the potassium, aluminum, silicon and oxygen compounds present in the cordierite carrier is determined by the catalyst formed by simply forming a catalyst layer on the surface of the cordierite carrier. It was recognized that the number was considerably smaller than in the case of.

The results of this experiment show that the penetration of potassium into the cordierite carrier (more generally a porous carrier) is prevented by the suppression layer. In fact, even when the catalyst for purifying exhaust gas of the present invention is used at a high temperature for a long time, generation of compounds due to permeation of potassium is prevented, cracks are less likely to be generated in the porous carrier, and the durability is high.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an exhaust gas purifying catalyst according to a first embodiment of the present invention will be described. The exhaust gas purifying catalyst of the present embodiment is a honeycomb (monolith) composed of a large number of cells.
It is configured as a NOx catalyst having a cordierite carrier of the type. FIG. 1 shows a part of one cell of the cordierite carrier, and the cells of the cordierite carrier 10 are formed in, for example, a square shape. A silica layer 20 is coated on the surface of the cordierite carrier 10, and a catalyst layer 30 is supported on the surface of the silica layer 20. And in the catalyst layer 30,
Potassium (K) and barium (Ba) are added as NOx storage agents. The silica layer 20 functions as a suppression layer that suppresses permeation of potassium into the cordierite carrier 10 (more generally, a porous carrier).

The cordierite carrier 10 is prepared, for example, by mixing alumina source powder, silica source powder, and magnesia source powder in such a manner that the alumina, silica and magnesia ratios are in a cordierite composition, and dispersing the mixture in water. The solid content is formed into a honeycomb shape, and this honeycomb formed body is fired. The silica layer 20 is formed on the surface of the cordierite carrier 10 as follows, for example.
First, a water-soluble salt of a silicon compound is diluted with water to prepare an aqueous solution having a predetermined concentration.
Immerse. The aqueous solution of the salt of the silicon compound is absorbed into the surface or the surface layer of the cordierite carrier 10 by the water absorption of the cordierite carrier 10. Then, the cordierite carrier 10
Is dried to evaporate the water, and the salt of the silicon compound is adsorbed on the surface or the surface layer of the cordierite carrier 10. Next, when the cordierite carrier 10 is heated, the salt of the silicon compound is decomposed, and the silica layer 20 is formed on the surface of the cordierite carrier 10. That is, the cordierite carrier 10 is covered with the silica layer 20.

The appropriate concentration of the aqueous solution of the salt of the silicon compound used for forming the silica layer 20 varies mainly depending on the water absorption characteristics of the cordierite carrier 10. Therefore, it is preferable that the relationship between the concentration of the aqueous solution and the coating state be confirmed in advance by performing elemental analysis on the surface layer portion of the cordierite carrier by the EPMA method or the like. By previously determining the optimum concentration of the aqueous solution of the salt of the silicon compound in this manner, it is possible to obtain an optimum coating state that ensures the adhesion between the cordierite carrier and the catalyst layer and the potassium permeation suppression effect of the silica layer.

The catalyst layer 30 is formed, for example, as follows.
It is formed on the surface of the silica layer 20. First, a slurry containing a powder mainly containing a noble metal such as platinum, an alkali metal such as potassium, and an alkaline earth metal such as barium is prepared. Next, the cordierite carrier 10 on which the silica layer 20 has been formed is immersed in the above-mentioned slurry, dried and fired.

As described above, a NOx catalyst obtained by coating the cordierite carrier 10 with the catalyst layer 30 via the silica layer 20 is obtained. As is conventionally known, this NOx
The catalyst is housed in a case via, for example, a buffer material, and is arranged in an exhaust pipe of the lean burn internal combustion engine. According to this NOx catalyst, NOx in exhaust gas is stored in the form of nitrate under the action of the catalyst species dispersed in the catalyst layer 30 during operation of the engine at a lean air-fuel ratio. In addition, during operation of the engine at a rich air-fuel ratio, nitrate is decomposed, and the stored NOx is reduced to nitrogen and released from the NOx catalyst to the atmosphere.

When an internal combustion engine equipped with such a NOx catalyst is operated for a long time, the NOx catalyst is exposed to a high temperature for a long time. In this case, a conventional NOx formed by coating a cordierite carrier with a catalyst layer to which potassium is added is used.
In the catalyst, as described above, potassium moves into the cordierite carrier and reacts with silicon or the like in the carrier to form a compound.
The durability of the Ox catalyst will be impaired. On the contrary,
In the NOx catalyst according to the present embodiment, according to the elemental analysis by the EPMA method, even when the NOx catalyst is used at a high temperature for a long time, the potassium added to the catalyst layer 30 and the silica component of the cordierite carrier 10 are mixed. It was revealed that the formation of the compound was suppressed. The reason is that the catalyst layer 3
It is considered that the transfer of potassium from 0 to the cordierite carrier 10 is prevented by the silica layer 20. As described above, since a compound having a different coefficient of thermal expansion from that of the cordierite carrier 10 is not generated in the cordierite carrier 10, the occurrence of cracks in the cordierite carrier 10 due to the formation of the compound is prevented.

Hereinafter, an exhaust gas purifying catalyst according to a second embodiment of the present invention will be described. As shown in FIG. 3, the exhaust gas purifying catalyst of the present embodiment is different from that of the first embodiment in that
The difference is that instead of the silica layer 20, a titania layer 40 containing titanium dioxide (TiO2) as a main component is formed as a suppression layer. Other points are the same as those of the first embodiment, and this exhaust gas purifying catalyst can be manufactured by substantially the same method as that of the first embodiment.

In the exhaust gas purifying catalyst of the present embodiment in which the titania layer 40 is formed between the cordierite carrier 10 and the catalyst layer 30, according to the elemental analysis by the EPMA method, the exhaust It was found that even when used, potassium added to the catalyst layer 30 was prevented from penetrating into the cordierite carrier 10. As described above, since the permeation of potassium is prevented, the exhaust gas purifying catalyst has high durability. In the NOx catalyst according to the present embodiment, the loss of potassium from the catalyst layer 30 is reduced. The reason is considered to be the same as that of the silica layer.

Hereinafter, an exhaust gas purifying catalyst according to a third embodiment of the present invention will be described. As shown in FIG. 4, the exhaust gas purifying catalyst of the present embodiment is different from that of the first embodiment in that
The difference is that the zeolite layer 50 is formed as a suppression layer instead of the silica layer 20. Other points are the same as those of the first embodiment, and this exhaust gas purifying catalyst can be manufactured by substantially the same method as that of the first embodiment.

In forming the zeolite layer 50 on the cordierite carrier 10, the zeolite constituents may be dispersed in an aqueous dispersant as in the case of the first embodiment. You may do it. In addition, an aqueous dispersion (sol) of a hydrate such as silica or alumina or a charged dispersion solution (colloid) can also be used as the adhesive.

In the catalyst of the present embodiment provided with the zeolite layer 50 as a suppressing layer, the zeolite layer 50 has an acid point having a cation exchange ability, and has an excellent ability to fix an occluding agent (potassium in the present embodiment). The occluding agent moving in the catalyst may be in an ionized state in the presence of high-temperature steam, and as schematically shown in FIG. 5, the occluding agent, for example, potassium is a cation at an acid point on the zeolite layer 50. It is fixed as an ion by the exchange ability. The zeolite layer 50 has a three-dimensional network structure and has a high specific surface area. Since potassium is highly dispersed on the zeolite having such a structure, it is difficult for potassium to penetrate into the cordierite carrier 10. Further, the zeolite layer 50 has an excellent ability to adsorb a reducing substance (for example, a reducing gas such as HC). Even when the internal combustion engine is in a lean operation state, slight H
Decomposition of potassium nitrate and sulfate is promoted by HC adsorbed on the zeolite layer 50 which contains C and has HC adsorption ability. That is, even during lean operation,
The zeolite layer 50 continuously decomposes nitrates and sulfates by using a small amount of HC contained in the exhaust gas to form a catalyst N.
It contributes to recovery of Ox storage performance.

The zeolite layer 50 of the present embodiment does not contain a catalytic substance, for example, a noble metal such as Pt. Therefore, the catalytic action of Pt or the like is not exerted in the zeolite layer 50, and the potassium fixed to the zeolite layer 50 and the exhaust gas SO in
Since the chemical reaction of x is less likely to occur, the consumption of the occluding agent due to such a chemical reaction is reduced, and the NOx storage performance of the catalyst is maintained.

The zeolite layer 50 is made of MFI type, Y type, X
It can be configured using various types of zeolites, such as mold, mordenite, ferrierite, and the like. At this time, adsorption H
A zeolite of a type suitable for the exhaust gas composition is selected in consideration of the structural relationship with the C type. The cation exchange capacity of the zeolite is inversely proportional to the SiO2 / AlO2 ratio of the zeolite, and its heat resistance is proportional to this ratio. In the present embodiment, the above ratio is made as large as possible to improve heat resistance. By preparing the zeolite composition component so that the SiO 2 / AlO 2 ratio becomes small, it is possible to increase the storage capacity of the zeolite for the storage agent.
In this case, the loss amount of the occluding agent due to long-time use at a high temperature is reduced.

The zeolite layer 50 is coated with the cordierite carrier 10
In order to evaluate the durability and the ability of the exhaust gas purifying catalyst of the present embodiment formed between the catalyst layer 30 and the catalyst layer 30 to prevent the disappearance of the occluding agent, the catalyst layer containing a occluding agent containing potassium and the cordierite carrier were evaluated. A NOx catalyst in which zeolite was provided as a suppressive layer in between was produced, and the potassium content in an unused NOx catalyst was determined by the XRF method. In addition, the NOx catalyst is mounted on the engine and used for bench tests and actual vehicle running tests.
Thus, the potassium content of the NOx catalyst used over a long period of time at high temperature (for example, 850 ° C. for 32 hours) is determined, and the difference between the potassium content before and after use is divided by the initial potassium content to obtain the potassium loss. Asked.

FIG. 6 shows the experimental results of the catalyst of this embodiment provided with the zeolite layer 50, the untreated catalyst having the catalyst layer supported on the carrier, the catalyst of the first embodiment provided with the silica layer 20, and the titania. This is shown together with the experimental results for the catalyst of the second embodiment in which the layer 40 is provided. As shown in FIG. 6, the amount of potassium lost by the untreated catalyst ranges from several tens to 50%, whereas the amount of potassium lost by the catalyst of the present embodiment is suppressed to about ten and several percent. I understood. The results of this experiment indicate that the amount of potassium that is an occluding agent from the catalyst is greatly reduced. The amount of potassium lost by the catalysts of the first and second embodiments was about 20% or more.

As in the first and second embodiments,
After subjecting the catalyst of the present embodiment to a bench test and an actual vehicle running test, the cut surface was subjected to elemental analysis by the EPMA method.
As a result, the potassium cordierite carrier 1 added to the catalyst layer 30 even when used at a high temperature for a long time.
It was found that permeation into 0 was prevented. The present invention
The present invention is not limited to the above embodiment, and can be variously modified.

For example, in the above embodiment, the honeycomb type cordierite carrier 10 is used as the carrier.
The present invention is also applicable to an exhaust gas purifying catalyst provided with a carrier made of a material other than cordierite. When a metal carrier is used, the penetration of the occluding agent into the carrier is not a problem, but the effect of preventing scattering of the occluding agent is obtained, and a decrease in the exhaust gas purification performance of the catalyst is prevented. When a honeycomb-type cordierite carrier is used, the cells of the cordierite carrier are not limited to square cells, and may be, for example, triangular or hexagonal cells.

In the first embodiment, the suppression layer is constituted by the silica layer 20 containing silicon dioxide as a main component, and in the second embodiment, the suppression layer is constituted by a titania layer 40 containing titanium dioxide as a main component. In the third embodiment, the suppression layer is formed by the zeolite layer 50, but the constituent material of the suppression layer is not limited to silicon dioxide, titanium dioxide, or zeolite.

That is, it is possible to use another acidic material instead of silicon dioxide to form the suppression layer by a layer having a high acidity. Barium (B) is used instead of titanium dioxide.
The suppression layer can be constituted by a layer having a high basicity and containing a base material such as an alkali metal such as a) or barium oxide (BaO) as a main component. Furthermore, a layer having a high specific surface area containing a material having a high specific surface area such as zeolite as a main component,
The suppression layer may be composed of a layer made of a stable base material having a large molecular weight, for example, a high molecular weight elemental compound containing barium sulfate as a main component, or a layer having a small crystal lattice.

In a broader sense, in the present invention, an acidic oxide containing an acidic substance, a composite oxide containing an acidic substance, a material that does not inhibit the reactivity of the nitrogen oxide with the above-mentioned occluding agent, and a reducing substance are used. The constraining layer can be formed from a material that includes one or more materials selected from the group consisting of adsorbing materials, wherein the acidic material is a group IV, V, or VI transition element and
It may include one or more materials selected from group V, group V, and group VI typical elements.

In the above embodiment, one suppression layer 2
Although 0, 40, or 50 is formed on the outer surface of the carrier 10 between the carrier 10 and the catalyst layer 30, the number and locations of the suppression layers are not limited thereto. For example, one suppression layer can be formed on the outer surface of the catalyst layer 30. In the case of a catalyst having a plurality of catalyst layers, one or two or more catalysts may be provided between the carrier and the catalyst layer and in the catalyst layer or at least one place on the outer surface of the catalyst layer, depending on the formation mode of the catalyst layer. The suppression layer can be appropriately formed.

[0044]

In the exhaust gas purifying catalyst according to the first aspect of the present invention, since the movement of the occluding agent in the catalyst is suppressed by providing the suppressing layer in the catalyst, the occluding agent is evaporated and scattered. It is possible to greatly reduce the disappearance of the agent and, consequently, the deterioration of the exhaust gas purification performance of the catalyst. According to the second aspect of the present invention, since the suppression layer is formed between the carrier and the catalyst layer and in at least one place in the catalyst layer or on the outer surface of the catalyst layer, the occlusion agent added to the catalyst layer is introduced into the carrier. Infiltration and evaporation and scattering of the occluding agent from the catalyst layer can be reliably prevented by the layered suppressing layer, and the durability of the catalyst can be improved and the exhaust gas purification performance can be prevented from lowering.

According to the third aspect of the present invention, an acidic oxide or a composite oxide containing at least one acidic substance selected from the group IV, group V and group VI transition elements and typical elements; Since the suppression layer is composed of at least one material selected from the group consisting of a material that does not inhibit the reactivity with the occluding agent and a material that adsorbs the reducing substance, the acidic layer is rich in occluding agent fixing ability and heat stability. Oxides and composite oxides can prevent the catalyst from deteriorating exhaust gas purification performance and improve durability, and use a material that does not hinder the reactivity between nitrogen oxides and the occluding agent to prevent the occluding agent from occluding and prevent occlusion. It is possible to purify the exhaust gas by the NOx storage action of the agent, and to recover the exhaust gas purification ability by regenerating the occluding agent by using the reducing substance captured by the material having the reducing substance adsorption ability.

According to the fourth aspect of the present invention, since the suppression layer is made of zeolite, the movement of the occluding agent in the catalyst is effectively prevented by the cation exchange ability of the zeolite, and the occluding agent is evaporated and scattered by the occluding agent. And the deterioration of exhaust gas purification performance can be prevented. According to the fifth aspect of the present invention, while improving the storage capacity by potassium and the exhaust gas purification performance by the porous carrier, the penetration, evaporation, and scattering of the storage agent, which is likely to occur when the porous carrier is used, into the carrier. Can be effectively prevented by the suppression layer.

According to the sixth aspect of the present invention, any one of a layer having a high acidity, a layer having a high specific surface area, a layer having a small crystal lattice, a layer made of an elemental compound having a high molecular weight, and a layer having a high basicity is used. Since the suppression layer is constituted by one, the movement of the occluding agent can be surely prevented. In particular, when the suppression layer is composed of a layer having a high basicity, the movement of the occluding agent can be prevented without losing the occluding agent from the catalyst layer, and the purifying action of the occluding agent can be maintained.

[Brief description of the drawings]

FIG. 1 is a partially enlarged sectional view showing a quarter of one shell of an exhaust gas purifying catalyst according to a first embodiment of the present invention.

FIG. 2 is a schematic diagram for explaining a formation state of a silica layer inside pores of a cordierite carrier.

FIG. 3 is a partially enlarged sectional view showing a quarter of one shell of an exhaust gas purifying catalyst according to a second embodiment of the present invention.

FIG. 4 is a partially enlarged sectional view showing a quarter of one shell of an exhaust gas purifying catalyst according to a third embodiment of the present invention.

FIG. 5 is a schematic view showing a potassium fixing action by a cation exchange ability of a zeolite constituting a suppression layer of the catalyst shown in FIG. 4;

6 is a diagram showing the potassium content after long-term use of the catalyst shown in FIG. 4 at a high temperature together with the untreated catalyst, the catalyst shown in FIG. 1, and the catalyst shown in FIG. .

FIG. 7 is a diagram showing the NOx purification efficiency after using the catalyst shown in FIG. 4 at a high temperature for a long time, together with the untreated catalyst, the catalyst shown in FIG. 1, and the catalyst shown in FIG. 3; .

[Explanation of symbols]

 Reference Signs List 10 cordierite carrier 20 silica layer 30 catalyst layer 40 titania layer 50 zeolite layer

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Keisuke Tashiro 5-33-8 Shiba, Minato-ku, Tokyo Inside Mitsubishi Motors Corporation (72) Inventor Uniform Iwachimichi 5-33-8 Shiba, Minato-ku, Tokyo Mitsubishi Motors Corporation (72) Inventor Tetsuya Watanabe 5-33-8 Shiba, Minato-ku, Tokyo Mitsubishi Motors Corporation F-term (reference) 4D048 AA06 AB01 BA03X BA03Y BA06X BA06Y BA07X BA07Y BA09X BA09Y BA11X BA11Y BA14X BA14Y BA15X BA15Y BA30X BB02 BB03 BC04 CC36 CC44 EA04 4G069 AA03 AA08 BA02B BA04B BA07A BA07B BA13B BA50 BC01A BC03A BC03B BC08A BC13B BC20A BC24A BC29A BC49A BC53A BC57A CA03 CA13 DA06 EA19 FA06

Claims (6)

[Claims] 1. An exhaust gas purifying catalyst comprising a carrier and a catalyst layer, wherein at least one selected from the group consisting of an alkali metal and an alkaline earth metal is added to the catalyst layer as an occluding agent. A catalyst for purifying exhaust gas, wherein a movement of the occluding agent in the catalyst is suppressed by providing a suppression layer on the catalyst. 2. The exhaust gas according to claim 1, wherein the suppression layer is formed between the carrier and the catalyst layer and at least one position in the catalyst layer or on the outer surface of the catalyst layer. Purification catalyst. 3. An acidic oxide containing at least one acidic substance selected from a group IV, group V, and group VI transition element and a group IV, group V, and group VI element. A composite oxide containing at least one acidic substance, a material that does not inhibit the reactivity of the nitrogen oxide and the storage agent, and one or more materials selected from the group consisting of a material that adsorbs a reducing substance The exhaust gas purifying catalyst according to claim 1, wherein the catalyst is purified. 4. The exhaust gas purifying catalyst according to claim 1, wherein the suppression layer is made of zeolite. 5. The exhaust gas purifying catalyst according to claim 1, wherein the storage agent contains potassium, and the carrier is a porous carrier. 6. The suppression layer is one of a layer having a high acidity, a layer having a high specific surface area, a layer having a small crystal lattice, a layer made of an elemental compound having a high molecular weight, and a layer having a high basicity. The exhaust gas purifying catalyst according to claim 1, wherein the catalyst is constituted by: