JP4209059B2 - Exhaust gas purification catalyst - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust gas purification catalyst, and more particularly to an exhaust gas purification catalyst excellent in durability and exhaust gas purification performance.
[0002]
[Related background]
A lean combustion engine such as a lean burn engine or an in-cylinder injection engine is operated at 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 characteristics and exhaust gas characteristics. While the lean air-fuel ratio operation is performed, NOx (nitrogen oxide) in the exhaust gas cannot be sufficiently purified by the three-way catalyst, and therefore, equipped with a NOx catalyst that stores NOx in the exhaust gas in an oxidizing atmosphere, It is known that NOx occluded in the catalyst is reduced to N2 (nitrogen) in a reducing atmosphere to reduce the amount of NOx discharged 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]
[Problems to be solved by the invention]
However, if a NOx catalyst to which a storage agent such as potassium is added is kept at a high temperature for a long time, cracks may occur in the catalyst, causing a decrease in durability of the NOx catalyst.
In order to investigate the cause of cracks in the NOx catalyst, the present inventors manufactured a NOx catalyst in which potassium is added as a storage agent to a catalyst layer supported on a honeycomb type cordierite carrier, and this NOx catalyst is equipped. An engine bench test and a running test of a vehicle equipped with this kind of engine were conducted. In the bench test and the actual vehicle running test, the engine and the vehicle were operated under such conditions that the NOx catalyst was exposed to a high temperature of 650 ° C. or more for a considerable time. After the operation, the elemental analysis on the cut surface of the NOx catalyst was performed by the EPMA method (electron probe micro-partial analysis method), and the compound of potassium, magnesium, aluminum, silicon and oxygen in the cordierite (Mg2Al4Si5O18) layer of the catalyst It was confirmed that KMg4Al9Si9O36 and a compound KAlSiO4 of potassium, aluminum, silicon and oxygen were present.
[0004]
According to the above experiment, when the NOx catalyst is exposed to high temperature, potassium added to the catalyst layer (washcoat) penetrates into the cordierite carrier, and the potassium reacts with cordierite in a high temperature atmosphere. It is thought that this compound is formed. Here, it is understood that potassium is easy to penetrate into the cordierite carrier because the potassium compound has high water solubility and low melting point. When a compound having a coefficient of thermal expansion different from that of cordierite is formed in the cordierite carrier, cracks occur in the cordierite carrier as the catalyst temperature changes during and before and after using the catalyst, and the NOx catalyst The strength will decrease.
[0005]
As described above, the NOx catalyst containing potassium or the like as a storage 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 the nitrogen and sulfur components in the exhaust gas, and the NOx occluding capacity is reduced. In this case, it is possible to restore the storage capacity by forming a reducing atmosphere around the NOx catalyst and decomposing nitrate and sulfate, but even if such measures are taken, if the NOx catalyst is used at a high temperature for a long time, Purification performance may be reduced.
[0006]
From the results of the following experiment conducted by the present inventor, one of the causes for the reduction in purification performance is that the occluding agent gradually evaporates and scatters from the NOx catalyst at a high temperature, and a considerable part of the occluding agent in the catalyst disappears. It seems that there is. That is, the present inventor manufactured a NOx catalyst in which a catalyst layer containing potassium as a storage agent is supported on a cordierite carrier, and determined the potassium content in the unused NOx catalyst by the XRF method (fluorescence X-ray spectroscopic analysis method). Next, after using this catalyst at a high temperature for a long time (for example, at 850 ° C. for 32 hours), the potassium content of the catalyst is obtained. Further, the difference in potassium content before and after use is determined by the initial potassium content. The amount of lost potassium was determined by dividing by the rate. As a result, it was found that the amount of potassium disappeared reached several tens to 50% (see FIG. 6).
[0007]
Therefore, 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 disappearance of the occlusion agent.
[0008]
[Means for Solving the Problems]
The present invention relates to an exhaust gas purifying catalyst obtained by adding a NOx storage agent to a catalyst layer. Cordierite A suppression layer made of at least one of silica and zeolite that suppresses the movement of the NOx storage agent is provided between the support and the catalyst layer to suppress the movement of the NOx storage agent in the catalyst.
[0010]
Cordierite When provided between the carrier and the catalyst layer, for example Cordierite When the support is coated with the suppression layer, the penetration of the storage agent added to the catalyst layer into the support is surely prevented by the suppression layer provided in a layer between the catalyst layer and the support. And since the penetration of the storage agent into the carrier is suppressed by the suppression layer, the formation of the compound due to the reaction between the composition component of the material constituting the storage agent and the composition component of the support is suppressed, and this is caused by the formation of this compound. The occurrence of cracks in the supporting carrier and thus the deterioration of the durability of the catalyst are prevented.
[0011]
In general, in a catalyst in which a catalyst layer is supported on a support composed of a large number of cells, the support is used in order to satisfy mechanical, physical or chemical requirements imposed on the catalyst, such as prevention of peeling of the catalyst layer from the support surface. The thickness of the catalyst layer is increased at the corner portion of each cell, and therefore the catalyst layer has a deep layer portion near the corner portion. As described above, the reducing atmosphere is reduced by, for example, enriching the exhaust air / fuel ratio in order to recover the storage performance by decomposing sulfate produced by the reaction between the storage agent in the catalyst layer and the sulfur component in the fuel. However, since the gas diffusion is poor in the deep part of the catalyst layer, the decomposition of the sulfate is difficult, and the particle growth of the sulfate is likely to proceed, and the NOx storage performance is reduced due to the consumption of the storage agent.
[0012]
This point, Cordierite The present invention is formed by forming a suppression layer between the carrier and the catalyst layer. In the light The requirement that the catalyst layer at the support corner should 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 occlusion agent accompanying the growth of sulfate is Consumption is reduced.
[0013]
Good Preferably, the at least one acidic substance is selected in consideration of the reactivity between the acidic substance and the storage agent. For example, if the storage agent is potassium, Mosquito It is preferable to use an acidic oxide or a complex oxide included as an acidic substance.
[0014]
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 storage agent on the catalyst is exhibited by the suppression layer, and the NOx storage effect of the storage agent is excellently achieved.
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, the reducing substance trapped in the suppressing layer by the reducing substance adsorbing ability of the suppressing layer decomposes the sulfate and nitrate in the catalyst layer and the suppressing layer, and the NOx occlusion performance is restored.
[0015]
Preferably, the suppression layer is made of zeolite. According to the catalyst provided with zeolite as the suppression layer, advantages similar to those of the preferred embodiments described above are exhibited. That is, zeolite has a cation exchange ability and molecular sieving action, and is excellent in the ability to fix the storage agent and the ability to adsorb HC. The occlusion agent that moves in the catalyst may be ionized in the presence of high-temperature water vapor, and is fixed as ions by the cation exchange ability of acid sites on the zeolite (see FIG. 5). Zeolite has a three-dimensional network structure and a high specific surface area. Since the occluding agent is highly dispersed on the zeolite having such a structure, the occluding agent is difficult to penetrate into the carrier especially when the zeolite is provided between the catalyst layer and the carrier. Furthermore, zeolite is excellent in HC adsorption ability (more generally, reducing substance adsorption ability). Even when the internal combustion engine is in a lean operation state, a slight amount of HC is contained in the exhaust gas, and decomposition of nitrate and sulfate of the storage agent is promoted by HC adsorbed on the zeolite. That is, even during the lean operation, the suppression layer made of zeolite having HC adsorption ability continuously decomposes the nitrate and sulfate of the occluding agent using the slight amount of HC contained in the exhaust gas, and stores the NOx of the catalyst. Contributes to performance recovery.
[0016]
In the present invention, various types of zeolite such as MFI type, Y type, X type, mordenite, and ferrierite can be used as the zeolite constituting the suppression layer, but considering the structural relationship with the adsorbed HC species. Therefore, it is preferable to select one that matches the exhaust gas composition.
Moreover, the cation exchange capacity and heat resistance performance of zeolite depend on the compositional components of the zeolite. That is, the cation exchange capacity is inversely proportional to the SiO2 / AlO2 ratio in zeolite, and the heat resistance is proportional to this ratio. Therefore, for example, the heat resistance of the catalyst can be improved by increasing the above ratio as much as possible. Further, by reducing the above ratio, it is possible to reduce the amount of occlusion agent lost during long-time operation of the catalyst at a high temperature and maintain the occlusion performance.
[0017]
Preferably, the suppression layer does not contain a catalytic material such as a noble metal. In this case, the catalytic action by the catalytic substance is not achieved in the suppression layer, and the chemical reaction between the storage agent fixed in the suppression layer and the SOx in the exhaust gas is difficult to occur. As a result, the NOx storage performance of the catalyst is maintained.
In the present invention, preferably, the occlusion agent contains potassium and the carrier is a porous carrier. The addition of potassium improves the NOx storage capacity of the catalyst. Further, the use of the porous carrier reduces the pressure loss of the exhaust gas, and the exhaust gas makes good contact with the catalyst layer, so that the exhaust gas purification is performed well. On the other hand, in the catalyst having a porous carrier, the circulation of the exhaust gas containing high-temperature water vapor is improved and the storage agent is easily moved, evaporated, and scattered, but this is prevented by the suppression layer in the present invention.
[0018]
Preferably, the suppression layer includes 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 element compound having a high molecular weight, or a layer having a high basicity. Is done.
Preferably, the high acidity layer includes 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, such as zeolite. The layer made of an elemental compound having a large molecular weight is made of, for example, a base material having a large molecular weight and a stable base material such as barium sulfate (BaSO4). The layer having a high basicity is made of a basic material such as barium oxide (BaO).
[0019]
According to the above preferred embodiment, it is considered that the penetration of potassium into the porous carrier is suppressed by the following mechanism.
That is, when the suppression layer is composed of a highly acidic layer, it is understood that potassium is consumed before reaching the surface of the porous carrier by reacting with the highly acidic layer. Further, it is understood that potassium is highly dispersed inside the suppression layer composed of a layer having a high specific surface area, and the suppression layer composed of a layer having a small crystal lattice prevents the movement of potassium. In the case of a suppression layer composed of an elemental compound having a large molecular weight, it is considered that the potassium permeation route to the porous carrier is reduced. And in the case of a suppression layer with a high basicity, this layer has the same properties as potassium, so when potassium approaches the suppression layer, it is repelled and the inductivity of potassium to the porous carrier decreases. It is understood as a thing.
[0020]
As described above, the permeation of potassium into the porous carrier is suppressed, and the durability of the exhaust gas purification catalyst is improved. In addition, in the case of a suppression layer that can suppress the permeation without consuming potassium, the NOx purification action of potassium is not reduced due to the consumption of potassium, and the exhaust gas purification performance of the exhaust gas purification catalyst is suitably maintained. The
In order to evaluate the durability of the exhaust gas purifying catalyst of the present invention and the ability to prevent the disappearance of the occluding agent, as an example, a zeolite layer is provided as a suppressing layer between the catalyst layer to which the occluding agent containing potassium is added and the cordierite carrier. A NOx catalyst was manufactured, and the potassium content in the unused NOx catalyst was determined by the XRF method. In addition, the NOx catalyst is mounted on the engine and used for a bench test or an actual vehicle running test. By this, the potassium content of the NOx catalyst used for a long time (for example, 850 ° C for 32 hours) at high temperatures is obtained. The amount of potassium lost was determined by dividing the difference in potassium content by the initial potassium content. As a result, it was found that the loss of potassium, which is several tens to 50% in the catalyst having the catalyst layer supported on the carrier, is suppressed to about tens of% in the catalyst of the present invention. This experimental result (FIG. 6) shows that the disappearance amount of potassium as a storage agent from the catalyst is greatly reduced by the present invention.
[0021]
In addition, after the NOx catalyst according to the present invention was subjected to a bench test or an actual vehicle running test, elemental analysis was performed on the cut surface by the EPMA method. As a result, the abundance of potassium, magnesium, aluminum, silicon and oxygen compounds and potassium, aluminum, silicon and oxygen compounds in the cordierite carrier is a catalyst formed simply by forming a catalyst layer on the surface of the cordierite carrier. It was found that it was considerably less than in the case of.
[0022]
This experimental result shows that the penetration of potassium into the cordierite support (more generally a porous support) is prevented by the suppression layer. In fact, the exhaust gas purifying catalyst of the present invention is highly durable even when it is used for a long time at high temperature, because the formation of a compound due to potassium permeation is prevented and cracks are unlikely to occur in the porous carrier.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an exhaust gas purification catalyst according to a first embodiment of the present invention will be described.
The exhaust gas purification catalyst of this embodiment is configured as a NOx catalyst having a honeycomb (monolith) type cordierite carrier composed of a large number of cells. FIG. 1 shows a part of one cell of the cordierite carrier, and the cell of the cordierite carrier 10 is formed in a square shape, for example. The surface of the cordierite carrier 10 is coated with a silica layer 20, and the catalyst layer 30 is supported on the surface of the silica layer 20. Further, potassium (K) and barium (Ba) are added to the catalyst layer 30 as NOx storage agents. The silica layer 20 functions as a suppression layer that suppresses the penetration of potassium into the cordierite carrier 10 (more generally, a porous carrier).
[0024]
The cordierite carrier 10 is, for example, a mixture of an alumina source powder, a silica source powder, and a magnesia source powder mixed so that the proportions of alumina, silica, and magnesia are cordierite composition. The portion 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, and the cordierite carrier 10 is immersed in this aqueous solution. The aqueous solution of the salt of the silicon compound is absorbed by the surface of the cordierite carrier 10 or the surface layer by the water absorption of the cordierite carrier 10. Thereafter, the cordierite carrier 10 is dried to evaporate water, and a salt of the silicon compound is adsorbed on the surface or surface layer of the cordierite carrier 10. Next, when the cordierite carrier 10 is heated, the salt of the silicon compound is decomposed, and a 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.
[0025]
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 to confirm in advance the relationship between the aqueous solution concentration and the coating state by conducting elemental analysis by the EPMA method or the like on the surface layer portion of the cordierite carrier. Thus, by obtaining the optimum concentration of the aqueous solution of the salt of the silicon compound in advance, it is possible to obtain an optimum covering state that guarantees the adhesion between the cordierite carrier and the catalyst layer and the potassium permeation inhibiting effect of the silica layer.
[0026]
The catalyst layer 30 is formed on the surface of the silica layer 20 as follows, for example. First, a slurry is prepared that includes a powder mainly composed of a noble metal such as platinum, an alkali metal such as potassium, and an alkaline earth metal such as barium. Next, the cordierite carrier 10 on which the silica layer 20 has been formed is immersed in the slurry, and this is dried and fired.
[0027]
As described above, a NOx catalyst obtained by coating the cordierite carrier 10 with the catalyst layer 30 through the silica layer 20 is obtained. As conventionally known, this NOx catalyst is accommodated in a case through a buffer material, for example, and is disposed in an exhaust pipe of a lean combustion internal combustion engine.
According to this NOx catalyst, NOx in the exhaust gas is occluded in the form of nitrate under the action of the catalyst species dispersed in the catalyst layer 30 during engine operation at a lean air-fuel ratio. Further, during engine operation at a rich air-fuel ratio, nitrate is decomposed, and the stored NOx is reduced to nitrogen and released from the NOx catalyst into the atmosphere.
[0028]
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, in the conventional NOx catalyst formed by coating a cordierite carrier with a catalyst layer to which potassium is added, potassium moves into the cordierite carrier and reacts with silicon in the carrier as described above. As a result, a compound is produced, and a crack occurs in the cordierite carrier, impairing the durability of the NOx catalyst. On the other hand, in the NOx catalyst of the present embodiment, according to elemental analysis by the EPMA method, potassium added to the catalyst layer 30 and the cordierite carrier 10 even when the NOx catalyst is used at a high temperature for a long time. It became clear that the production | generation of a compound with the silica component of was suppressed. The reason is considered that the movement of potassium from the catalyst layer 30 to the cordierite carrier 10 is prevented by the silica layer 20. In this way, since a compound having a coefficient of thermal expansion different from that of the cordierite carrier 10 is not generated in the cordierite carrier 10, occurrence of cracks in the cordierite carrier 10 due to the generation of the compound is prevented.
[0029]
Hereinafter, an exhaust gas purification catalyst according to a second embodiment of the present invention will be described.
As shown in FIG. 3, the exhaust gas purification catalyst of the present embodiment includes a titania layer 40 mainly composed of titanium dioxide (TiO 2) instead of the silica layer 20 as compared with that of the first embodiment. The point formed as is different. The other points are the same as those of the first embodiment, and the exhaust gas purification catalyst can be manufactured by a method substantially similar to that of the first embodiment.
[0030]
Even in the exhaust gas purification 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 elemental analysis by the EPMA method, when used at a high temperature for a long time Further, it was found that the penetration of potassium added to the catalyst layer 30 into the cordierite carrier 10 was prevented. In this way, since the permeation of potassium is prevented, this exhaust gas purification catalyst is rich in durability. Further, in the NOx catalyst according to the present embodiment, the loss of potassium from the catalyst layer 30 is reduced. This reason is considered to be the same as that of the silica layer.
[0031]
Hereinafter, an exhaust gas purification catalyst according to a third embodiment of the present invention will be described.
As shown in FIG. 4, the exhaust gas purification catalyst of the present embodiment is different from that of the first embodiment in that a zeolite layer 50 is formed as a suppression layer instead of the silica layer 20. The other points are the same as those of the first embodiment, and the exhaust gas purification catalyst can be manufactured by a method substantially similar to that of the first embodiment.
[0032]
In forming the zeolite layer 50 on the cordierite carrier 10, the zeolite constituents may be dispersed in the aqueous dispersant as in the first embodiment, but may be dispersed in the organic dispersant. good. In addition, a dispersion in water (sol) or a charged dispersion solution (colloid) of hydrates such as silica and alumina can also be used as an adhesive.
[0033]
In the catalyst of the present embodiment provided with the zeolite layer 50 as a suppression layer, the zeolite layer 50 has an acid site having a cation exchange ability and is excellent in the ability to fix the storage agent (potassium in the present embodiment). The occlusion agent moving in the catalyst may be in an ionized state in the presence of high-temperature water vapor. As schematically shown in FIG. 5, the occlusion 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 capacity. The zeolite layer 50 has a three-dimensional network structure and a high specific surface area. Since potassium is highly dispersed on the zeolite having such a structure, it is difficult for potassium to enter the cordierite carrier 10. Furthermore, the zeolite layer 50 is excellent in the 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, a slight amount of HC is contained in the exhaust gas, and decomposition of potassium nitrate and sulfate is promoted by HC adsorbed on the zeolite layer 50 having HC adsorption ability. That is, even during the lean operation, the zeolite layer 50 continuously decomposes nitrates and sulfates using a small amount of HC contained in the exhaust gas, thereby contributing to the recovery of the NOx occlusion performance of the catalyst.
[0034]
The zeolite layer 50 of the present embodiment does not contain a catalytic material such as a noble metal such as Pt. Therefore, the catalytic action due to Pt or the like is not achieved in the zeolite layer 50, and the potassium fixed to the zeolite layer 50 and the SOx in the exhaust gas. Therefore, the consumption of the storage agent accompanying such a chemical reaction is reduced, and the NOx storage performance of the catalyst is maintained.
[0035]
The zeolite layer 50 can be configured by using various types of zeolite such as MFI type, Y type, X type, mordenite, and ferrierite. At this time, in consideration of the structural relationship with the adsorbed HC species, a type of zeolite that matches the exhaust gas composition is selected.
Further, the cation exchange capacity of zeolite is inversely proportional to the SiO2 / AlO2 ratio in zeolite, and its heat resistance is proportional to this ratio. In the present embodiment, the above ratio is made as large as possible in order to improve heat resistance. In addition, by preparing the zeolite composition component so as to reduce the SiO2 / AlO2 ratio, it is possible to increase the ability of the zeolite to capture the occluding agent. In this case, the amount of the occluding agent lost by prolonged use at high temperatures is reduced. .
[0036]
In order to evaluate the durability of the exhaust gas purifying catalyst of this embodiment formed by forming the zeolite layer 50 between the cordierite carrier 10 and the catalyst layer 30 and the ability to prevent the storage agent from disappearing, a storage agent containing potassium was added. A NOx catalyst in which zeolite was provided as a suppression layer between the catalyst layer and the cordierite support was manufactured, and the potassium content in the unused NOx catalyst was determined by the XRF method. In addition, the NOx catalyst is mounted on the engine and used for a bench test or an actual vehicle running test. By this, the potassium content of the NOx catalyst used for a long time (for example, 850 ° C for 32 hours) at high temperatures is obtained. The amount of potassium lost was determined by dividing the difference in potassium content by the initial potassium content.
[0037]
FIG. 6 shows the experimental results of the catalyst of the present embodiment provided with the zeolite layer 50, the unmeasured catalyst having the catalyst layer supported on the carrier, the catalyst of the first embodiment provided with the silica layer 20, and the titania layer 40. It shows with the experimental result about the catalyst of provided 2nd Embodiment.
As shown in FIG. 6, the amount of potassium lost in the untreated catalyst ranges from several tens to 50%, whereas in the catalyst of this embodiment, the amount of potassium lost is suppressed to about several tens of percent. I understood. This experimental result shows that the loss | disappearance amount from the catalyst of potassium which is a storage agent is reduced significantly. The amount of potassium lost in the catalysts of the first and second embodiments was about 20% by number.
[0038]
Further, as in the case of the first and second examples, the catalyst of this embodiment was subjected to a bench test and an actual vehicle running test, and then elemental analysis was performed on the cut surface by the EPMA method. As a result, it was found that the potassium added to the catalyst layer 30 is prevented from penetrating into the cordierite carrier 10 even when used at a high temperature for a long time.
The present invention is not limited to the above embodiment, and can be variously modified.
[0039]
For example, in the above-described embodiment, the honeycomb type cordierite carrier 10 is used as a carrier, but the present invention is also applicable to an exhaust gas purification catalyst provided with a carrier made of a material other than cordierite. When a metal carrier is used, penetration of the occluding agent into the carrier poses no problem, but an effect of preventing the occluding agent from scattering is obtained, and a reduction in the exhaust gas purification performance of the catalyst is prevented. Further, when the honeycomb type cordierite carrier is used, the cordierite carrier cell is not limited to a rectangular shape, and may be, for example, a triangular shape or a hexagonal shape.
[0040]
In the first embodiment, the suppression layer is configured by the silica layer 20 mainly composed of silicon dioxide, and in the second embodiment, the suppression layer is configured by the titania layer 40 mainly composed of titanium dioxide, and the third embodiment. Then, although the suppression layer was comprised by the zeolite layer 50, the constituent material of a suppression layer is not limited to silicon dioxide, titanium dioxide, or zeolite.
[0041]
That is, instead of silicon dioxide, the suppression layer can be composed of a highly acidic layer using another acidic material. Moreover, it can replace with titanium dioxide and can comprise a suppression layer with the layer with high basicity which has base materials, such as alkali metals, such as barium (Ba), and barium oxide (BaO) as a main component. Furthermore, a layer with a high specific surface area containing a material with a high specific surface area such as zeolite as a main component, a stable base material with a high molecular weight, such as a layer made of an elemental compound with a high molecular weight containing barium sulfate as a main component, or a small crystal lattice The suppression layer may be constituted by the layer.
[0042]
More broadly, 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 nitrogen oxides with the above-mentioned storage agent, and a material that adsorbs a reducing substance The suppression layer can be formed from a material including one or more materials selected from the group consisting of: an acidic substance, a transition element of Group IV, Group V and Group VI, and a group IV, Group V and Group VI It may contain one or more materials selected from typical elements.
[0043]
Moreover, in the said embodiment, although one suppression layer 20, 40, or 50 was formed in the outer surface of the support | carrier 10 between the support | carrier 10 and the catalyst layer 30, the formation number and formation site | part of a suppression layer are not limited to this. For example, one suppression layer can be formed on the outer surface of the catalyst layer 30. Further, in the case of a catalyst having a plurality of catalyst layers, one or two or more, depending on the formation mode of the catalyst layer, between the support and the catalyst layer and in the catalyst layer or at least one place on the outer surface of the catalyst layer. The suppression layer can be appropriately formed.
[0044]
【The invention's effect】
The exhaust gas purifying catalyst according to the invention of claim 1 is Cordierite Since the suppression layer made of at least one of silica and zeolite is provided between the support and the catalyst layer to suppress the movement of the NOx storage agent in the catalyst, the NOx storage agent added to the catalyst layer Cordierite The intrusion into the carrier can be reliably prevented by the layered suppression layer, and the durability of the catalyst can be improved and the exhaust gas purification performance can be prevented from lowering.
[0046]
In the invention according to claim 2, the NOx occlusion ability is improved by potassium. Can be it can.
[Brief description of the drawings]
FIG. 1 is a partially enlarged sectional view showing a quarter portion 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 cross-sectional view showing a quarter portion 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 part of one shell of an exhaust gas purifying catalyst according to a third embodiment of the present invention.
FIG. 5 is a schematic diagram showing a potassium fixing action by cation exchange ability of zeolite constituting the suppression layer of the catalyst shown in FIG.
6 is a diagram showing potassium content after a long time use of the catalyst shown in FIG. 4 at a high temperature, together with an unmeasured catalyst, the catalyst shown in FIG. 1 and the catalyst shown in FIG. .
7 is a diagram showing NOx purification efficiency after using the catalyst shown in FIG. 4 for a long time at a high temperature, together with the unmeasured catalyst, the catalyst shown in FIG. 1 and the catalyst shown in FIG. .
[Explanation of symbols]
10 Cordierite carrier
20 Silica layer
30 catalyst layer
40 Titania layer
50 Zeolite layer

Claims (2)

In an exhaust gas purifying catalyst comprising a cordierite carrier and a catalyst layer, wherein at least one selected from the group consisting of alkali metals and alkaline earth metals is added as a NOx storage agent to the catalyst layer,
An exhaust gas purifying catalyst characterized in that a suppression layer made of at least one of silica and zeolite is provided between the cordierite carrier and the catalyst layer to suppress the movement of the NOx storage agent in the catalyst. The exhaust gas purifying catalyst according to claim 1, wherein the NOx storage agent is potassium.

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