JP5024883B2 - Exhaust gas purification catalyst - Google Patents

Description

The present invention relates to an exhaust gas purification catalyst, and more particularly to an NO _x storage reduction type exhaust gas purification catalyst.

In recent years, global warming due to carbon dioxide (CO ₂ ) has become a problem, and reducing the amount of CO ₂ emission in automobiles is also a problem. Lean combustion engines with excellent fuel efficiency are attracting attention as a measure against global warming because they help reduce CO ₂ emitted from automobiles. In lean combustion, exhaust becomes excessive oxygen, and the conventional three-way catalyst used near the theoretical air-fuel ratio A / F = 14.5 cannot sufficiently purify NO _x . Therefore, NO _x storage reduction catalyst for purifying an exhaust gas as a catalyst having a high _the NO _x purification performance under such conditions have been developed and put into practical use.

Such NO _x storage-and-reduction type exhaust gas purifying catalyst, on the honeycomb substrate such as cordierite, forming a catalyst layer containing platinum (Pt), palladium (Pd), a catalytic metal such as rhodium (Rh) In general, a catalyst layer in which an NO _x holding material containing an element such as an alkali metal, an alkaline earth metal, or a rare earth element is supported is known.

In the purification of exhaust gas by the NO _x storage-reduction type exhaust purifying catalyst, Pt, are NO _x is oxidized by catalyst metals such as Pd, after being occluded in the NO _x retentate as nitrates, momentarily air the NO _x NO _x released from the holding material is reduced and purified by the catalytic metals such as Rh by rich (to generate a rich spike). Therefore, in the conventional NO _x storage-and-reduction type exhaust gas purifying catalyst, typically Pt, and the catalyst metal and the NO _x retaining substance such as Pd and Rh are supported on the same catalyst layer. However, other metals of Rh as the catalyst components, especially the use in the same catalyst layer as Pt, there is a problem that Rh and other metals partially alloyed _the NO _x purification performance of the catalyst is lowered.

On the other hand, in the conventional NO _x storage-and-reduction type catalyst for purifying exhaust gas, by heat exposure during use of the catalyst, the alkali ingredient forming the NO _x holding materials, especially potassium are diffused from the catalyst layer which is carried or eluted in catalyst _the NO _x purification performance may deteriorates.

In Patent Document 1, a first layer including at least a hydrocarbon adsorbent, a second layer including TiO _2, and a third layer including Pt and / or Pd together with a compound of an element having an electronegativity of 0.9 or less and Ce. And a fourth layer containing Pt and / or Pd together with a compound of an element having an electronegativity of 0.9 or less, Ce and Rh, and the thickness of the second layer is first to first. An exhaust gas purifying catalyst characterized in that it is 1/20 or more and 1/5 or less of the total thickness up to the fourth layer is described.

In Patent Document 2, a catalytically active component, a carrier layer for supporting the catalytically active component, and a substrate having a monolith structure that holds the carrier are provided, and an alkali metal is interposed between the carrier layer and the substrate. A dense layer that has a lower porosity than the chemical reactivity of the substrate and the alkali metal and lower than the porosity of the substrate and suppresses the gas phase diffusion transfer of the catalytically active component is provided. An exhaust gas purification catalyst for an internal combustion engine is described.
JP 2004-209403 A JP 2005-238168 A

Patent Document 1 describes that the second layer containing TiO ₂ can prevent the alkali component from being transferred to the hydrocarbon adsorbent layer. However, in Patent Document 1, Rh and Pt and / or Pd coexist in the fourth layer, and the heat of the catalyst during use causes the Rh to alloy with other metals to reduce the activity of the catalyst. There is.

  In Patent Document 2, by providing a dense layer such as titania between the carrier layer and the substrate, the amount of alkali metal supported in the carrier is limited by limiting the gas phase diffusion path and / or solid phase diffusion path of the alkali metal component. It is described that the degradation is minimized. However, Patent Document 2 does not describe or suggest any means for suppressing a decrease in catalyst activity due to alloying of a catalyst metal or the like.

Accordingly, the present invention provides a novel structure, and an object thereof is to provide an exhaust gas purification catalyst having an excellent catalytic activity for storage of NO _x.

The present invention for solving the above problems is as follows.
(1) A catalyst layer having a multilayer structure containing a metal oxide and a catalyst metal is formed on a substrate, and one layer constituting the catalyst layer is mainly composed of Pt, Pd or a combination thereof as the catalyst metal; Diffusion that further includes at least a NO _x holding substance, and the other layer is mainly composed of Rh as the catalyst metal, and has a higher concentration of acidic oxide between the one layer and the other layer than the other layer the barrier layer is provided, wherein the NO _x held than the other layer the content of substances Many in the one layer, the diffusion barrier layer comprises zirconia, titania, silica or combinations thereof and contains the catalytic metal The catalyst for exhaust gas purification , wherein the one layer is an upper layer on the exhaust gas flow channel side of the catalyst layer, and the other layer is a lower layer on the substrate side of the catalyst layer .
(2) The exhaust gas-purifying catalyst as described in (1) above, wherein the NO _x retention substance contains potassium as a constituent component.

According to the present invention, it is possible to obtain an exhaust gas-purifying catalyst with significantly improved NO _x storage capacity.

The exhaust gas purifying catalyst of the present invention has a multi-layered catalyst layer containing a metal oxide and a catalyst metal on a base material, and one layer constituting the catalyst layer is Pt, Pd or those as the catalyst metal. the combination of mainly, at least NO _x retentate further comprise other layers together mainly composed of Rh as the catalyst metal, acidic oxidation than said other layer between the other layers and the one layer A diffusion barrier layer having a high concentration of substances is provided, and the content of the NO _x holding substance is higher in the one layer than in the other layers.

The present inventors have found that in _the NO _x storage-reduction type exhaust gas purifying catalyst, the catalyst layer on the substrate and multilayer structure, mainly Pt, Pd or combinations thereof in one layer constituting the catalyst layer It has been found that the NO _x occlusion ability of the resulting catalyst can be remarkably improved by supporting the catalyst metal and supporting the catalyst metal mainly composed of Rh in the other layer.

In the purification of exhaust gas by _the NO _x storage reduction type catalyst for purifying an exhaust gas, Pt, are NO _x is oxidized by catalyst metals such as Pd, after being occluded in the NO _x retentate as nitrates, momentarily the rich air-fuel ratio the NO _x NO _x released from the holding material is reduced and purified by the catalytic metals such as Rh by the. Therefore, in order to perform a series of reactions in the storage reduction of such NO _x smooth, conventional, these catalytic metals and NO _x retentate been generally considered to be necessary to carry to some extent close to each other It was. Therefore, as in the exhaust gas purifying catalyst of the present invention, the Rh reducing activity of oxidation activity of the NO _x is high catalyst metal Pt and Pd and NO _x is high catalyst metal separated into separate layers respectively by carrying Te is that very surprising that _the NO _x storage ability of the resulting catalyst for purification of exhaust gas is improved, is surprising.

  Also, it is generally known that when Rh is used in the same catalyst layer as other metals, particularly Pt, Rh and other metals may partially alloy to reduce the activity of the catalyst. Yes. According to the exhaust gas purifying catalyst of the present invention, the catalyst components Pt, Pd, and Rh are separated and supported in separate layers. Therefore, there is a problem of alloying with Rh and other metals as described above. It can be avoided.

Although it is not intended to be bound by any particular theory, it is considered that, for example, NO _x is reduced and purified in the following manner in the exhaust gas purifying catalyst of the present invention. That is, first, NO _x oxidized by Pt or Pd carried on one layer is occluded as nitrate NO _x holding material in the same layer. Then, NO _x released from the NO _x retentate moves from one layer to the other layer by diffusion or the like by the rich spike, where it is reduced and purified by the Rh carried on the other layer. Therefore, even when Pt or Pd having a high oxidation activity and Rh having a high reduction activity are separated and supported in separate layers, it is considered that a series of reactions relating to NO _x storage reduction is not inhibited. Moreover, in the exhaust gas purifying catalyst of the present invention, as described above, alloying such as Rh and Pt is suppressed by such separation and support, so that the catalyst metal having a high oxidation activity and the catalyst metal having a high reduction activity are the same catalyst. It is considered that the NO _x storage capacity and the NO _x purification performance can be remarkably improved as compared with the conventional NO _x storage reduction type exhaust gas purification catalyst supported on the layer.

In addition to the multi-layering of the catalyst layer described above, the present inventors further carry a NO _x holding substance in the one layer, and the other layer is interposed between the one layer and the other layer. also by the concentration of the acidic oxide is provided a high diffusion barrier layer was found that it is possible to further improve _the NO _x storage capacity of the catalyst.

In a conventional NO _x storage-and-reduction type catalyst for purifying exhaust gas, by heat exposure during use of the catalyst, the alkali ingredient forming the NO _x holding materials, especially potassium, but it is diffused or eluted from the catalyst layer carried it may decrease the _the NO _x purification performance of the catalyst Te are generally known. According to the present invention, by providing a diffusion barrier layer between one layer on which the NO _x holding substance is supported and the other layer, the other layer is formed by diffusion or elution of such NO _x holding substance. It is thought that the movement of can be suppressed.

No Without intending to be bound by any particular theory, the storage capacity of the NO _x in the exhaust gas purifying catalyst of the present invention has been improved, the Pt and Pd and Rh as a catalyst component separate each By separating and supporting in layers, alloying of these catalytic metals was suppressed, and by providing the diffusion barrier layer between one layer and the other layer constituting the catalyst layer, It is considered that this is because the movement of the NO _x holding material supported in one layer to the other layer is suppressed, and as a result, the performance of the catalyst is improved.

According to the present invention, the substrate is not particularly limited, but any material generally used in exhaust gas purifying catalysts can be used. Specifically, the substrate is made of, for example, a ceramic material having heat resistance such as cordierite (2MgO · 2Al ₂ O ₃ · 5SiO ₂ ), alumina, zirconia, silicon carbide, or a metal foil such as stainless steel. Metal materials can be used.

  In a base material using a metal material, a wash coat layer is generally coated on a cell portion formed by alternately arranging a flat plate and a corrugated plate made of stainless steel. In this case, due to the surface tension of the washcoat liquid, a larger amount of the washcoat liquid adheres to the vicinity of the contact portion where the flat plate and the corrugated plate are in contact with each other than the other surface portions, resulting in thick coating. Therefore, in a base material using a metal material, the contact area between the coat layer and the exhaust gas is smaller than that of a ceramic material such as cordierite. Moreover, in such a base material, since the coefficient of thermal expansion of the material such as alumina and the metal material constituting the catalyst coat layer is different, the coat layer is peeled off from the base material at a high temperature and the catalyst performance is deteriorated. There is a problem. Therefore, in the present invention, it is particularly preferable to use a ceramic material such as cordierite having excellent heat resistance and a low coefficient of thermal expansion as the base material.

  According to the present invention, the metal oxide coated on one layer and other layers constituting the catalyst layer is not particularly limited, but any porous carrier generally used as a catalyst carrier can be used. . Preferred metal oxides include metal oxides selected from the group consisting of alumina, ceria, zirconia, silica, titania and combinations thereof.

  According to the present invention, a catalyst metal mainly composed of platinum (Pt), palladium (Pd) or a combination thereof is supported on one layer constituting the catalyst layer, and rhodium (Rh) is mainly composed on the other layer. A catalytic metal is supported. In addition, the expression “mainly” used in the present invention means that it accounts for more than 50% of the total mass of the catalyst metal supported on the one layer or the other layer. In addition, one layer and the other layer in the present invention may be any layer as long as they separate and carry a catalyst metal mainly composed of Pt, Pd or a combination thereof and a catalyst metal mainly composed of Rh. One layer may be arranged on the upper layer (exhaust gas flow path side) of the catalyst layer, and the other layer may be arranged on the lower layer (base material side) of the catalyst layer, or vice versa.

According to the present invention, one layer constituting the catalyst layer further includes at least a NO _x holding material.

As used herein, the term “NO _x retention material” is intended to encompass any material that can retain NO _x in its native form or in another form, such as nitrate. The term “hold” includes “adsorption” or “occlusion”.

According to the present invention, the NO _x holding material includes alkali metals such as lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), beryllium (Be), magnesium (Mg). ), Calcium (Ca), strontium (Sr), alkaline earth metals such as barium (Ba), scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium ( Nd), dysprosium (Dy), ytterbium (Yb) and the like, and combinations of these compounds. Examples of such compounds include carbonates, nitrates and oxides of the above metals.

According to the present invention, the content of the NO _x holding substance is larger in one layer containing the catalytic metal mainly composed of Pt, Pd or a combination thereof than the other layer containing the catalytic metal mainly composed of Rh.

The NO _x retentate that contain more in the same layer as Pt or Pd oxidation activity is high catalyst metal NO _x, absorbing the NO _x retentate Pt and / or NO _x that has been oxidized by Pd immediately Therefore, the NO _x storage capacity of the NO _x storage reduction type exhaust gas purifying catalyst can be remarkably improved.

  According to the present invention, a diffusion barrier layer having a higher concentration of acidic oxide than the other layer is provided between the one layer and the other layer.

The term “acidic oxide” used in the present invention refers to an oxide that is at least acidic with respect to the alkali component constituting the NO _x holding material supported on the one layer. Examples of such oxides include zirconia, titania, silica, and combinations thereof.

By providing a diffusion barrier layer containing these acidic oxides between one layer that constitutes the catalyst layer and the other layer, an alkali component that constitutes the NO _x retention material supported on the one layer, in particular, You can avoid reducing the _the NO _x purification performance of the catalyst from entering or elution potassium in other layers.

Although not intended to be bound by any particular theory, by providing such a diffusion barrier layer, an alkali that attempts to penetrate or elute from one layer to another due to high temperatures during use of the catalyst. It is believed that some of the components react with the acidic oxide of the diffusion barrier layer that exists between these layers. In this case, it is expected that the NO _x holding substance that has reacted with the acidic oxide can no longer contribute to the NO _x storage reduction. However, it is considered that the presence of such a diffusion barrier layer suppresses the movement of other NO _x holding substances from the one layer to the other layer. For this reason, as a whole of the NO _x holding material, a large amount of the NO _x holding material is stably present near Pt or Pd, which is a catalyst metal having high NO _x oxidation activity, in the one layer as compared with the case where no diffusion barrier layer is present. be able to. Therefore, it is considered that it is possible to maintain _the NO _x purification performance of the catalyst at high state by providing the diffusion barrier layer.

  In addition, the expression “the concentration of the acidic oxide is higher than that of the other layer” used in the present invention simply means that the acidic oxide defined above is contained at a higher concentration than the other layer. is there. Therefore, the diffusion barrier layer in the present invention is not limited to the one containing only the acidic oxide defined above, but other oxides such as alumina, ceria and other materials such as metals, etc. Can be included. However, in order to more effectively suppress the migration of the alkali component supported on the one layer to the other layer, the diffusion barrier layer in the present invention is composed only of the acidic oxide defined above, More preferably, no catalyst metal or the like is contained.

Although it does not specifically limit as thickness of a diffusion barrier layer, It is preferable to set it as the range of 10 micrometers-50 micrometers. For example, in the case of a diffusion barrier layer made of zirconia (ZrO ₂ ), if the thickness is less than 10 μm, the effect of suppressing the movement of the NO _x holding material to another layer may not be sufficiently obtained. In addition, when the thickness exceeds 50 μm, the movement of the NO _x holding material is suppressed, but the diffusion of the exhaust gas to other layers is also suppressed, and as a result, the exhaust gas purification performance of the catalyst may be reduced. is there. Furthermore, if the diffusion barrier layer is made thicker, the thickness of the entire catalyst layer is also made thicker, so that the volume of each cell in the substrate is reduced and the exhaust gas ventilation resistance is increased. Therefore, it is preferable that the diffusion barrier layer is sufficiently thick and as thin as possible to suppress migration of the NO _x retention material to the other layers.

  According to a preferred embodiment of the present invention, the one layer is disposed on the upper layer of the catalyst layer, that is, on the exhaust gas flow path side, and the other layer is disposed on the lower layer of the catalyst layer, that is, on the substrate side.

FIG. 1 is a schematic view showing a cross section of an exhaust gas purifying catalyst according to a preferred embodiment of the present invention. The exhaust gas purifying catalyst 1 of the present invention has a catalyst layer having a three-layer structure comprising a lower layer 3, a diffusion barrier layer 4 and an upper layer 5 on a substrate 2, and Pt 6 and a NO _x holding substance 7 are supported on the upper layer 5. The lower layer 3 has a configuration in which Rh8 is supported.

The NO _x storage-and-reduction type exhaust gas purifying catalyst using the ceramic material such as cordierite as the substrate, NO _x retentate carried on the catalyst layer is eluted at high temperatures combined with silica constituting the cordierite, in some cases a problem that lowering _the NO _x purification performance of the catalyst occurs. However, according to a preferred embodiment of the present invention, even when a ceramic material such as cordierite is used as the base material, the NO _x holding substance supported on the upper layer of the catalyst layer is further below the catalyst layer by the diffusion barrier layer. Can suppress the intrusion into the base material, so that it is possible to avoid the problem that the NO _x holding substance elutes into the cordierite and binds to silica, thereby reducing the NO _x purification performance of the catalyst.

  The exhaust gas purifying catalyst of the present invention having a catalyst layer having a multilayer structure on a base material and providing the diffusion barrier layer between one layer constituting the catalyst layer and the other layer is known to those skilled in the art. It can be produced by any known method. Hereinafter, in order to facilitate understanding, the present invention will be described with respect to a catalyst layer having a multilayer structure, with the layer on the exhaust gas flow channel side as the upper layer and the layer on the substrate side as the lower layer. The one layer and the other layer may be any one of these upper layers and lower layers, and are not limited to any one of the upper layer and the lower layer.

  In the production of the exhaust gas purifying catalyst of the present invention, for example, first, a lower layer containing a metal oxide and a catalytic metal is formed on a base material such as cordierite by a known wash coat method, and then, similarly, a diffusion barrier is formed. A layer and an upper layer are formed. In addition, when forming a lower layer or an upper layer containing a metal oxide and a catalytic metal using a wash coat method, for example, after forming a metal oxide layer by a wash coat method, the obtained metal oxide layer is known. The catalyst metal may be supported by the impregnation method or the like, or wash coating may be performed using a metal oxide powder that has previously supported the catalyst metal by the impregnation method or the like. According to the latter method, the catalyst metal can be more uniformly dispersed and supported in the catalyst layer as compared with the case where the catalyst metal is impregnated and supported after the wash coating.

  The coating amount of the upper layer and the lower layer with respect to the substrate can be arbitrarily determined according to the desired exhaust gas purification performance. For each of the upper layer and the lower layer, for example, 50 to 250 g / substrate-L (1 liter of substrate) It is possible to coat with an amount of hit). Moreover, a catalyst metal can be carry | supported to an upper layer and a lower layer, respectively, for example in the load of 2-10g / base-L.

When one layer in the present invention is used as the lower layer of the catalyst layer, after the lower layer is formed on the substrate, the NO _x holding substance is supported on the lower layer, and one layer in the present invention is defined as the upper layer of the catalyst layer. In this case, after forming the upper layer on the diffusion barrier layer, the NO _x holding material is supported on the upper layer. Carrying Specifically, metal salts constituting the NO _x retentate example, the NO _x retentate by known impregnation method or the like using a solution containing an acetate or nitrate with a predetermined concentration in the lower layer or the upper layer can do. Alternatively, the loading of the NO _x holding material on the lower layer or the upper layer was obtained by impregnating the above solution with the metal oxide constituting the lower layer or the upper layer in advance to carry the NO _x holding material on the metal oxide. This may be done by wash-coating the powder on the substrate or on the diffusion barrier layer. The NO _x holding material can be supported on the lower layer or the upper layer at a loading amount of 0.2 to 1.0 mol / base-L, for example.

  EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited to these Examples.

In this example, exhaust gas in which Pt and NO _x holding material are supported on the upper layer of the catalyst layer, Rh is supported on the lower layer, and a diffusion barrier layer made of zirconia (ZrO ₂ ) is further provided between the upper layer and the lower layer. A purification catalyst was prepared and examined for its NO _x storage capacity.

[Example 1]
First, ZrO ₂ —Al ₂ O ₃ —TiO ₂ composite metal oxide powder (ZrO ₂ : Al ₂ O ₃ : TiO ₂ = 3: 8: 1 (weight ratio)) was impregnated with an aqueous rhodium nitrate solution at 200 ° C. And dried to remove moisture, and calcined in air at 450 ° C. for 2 hours to obtain Rh / ZrO ₂ —Al ₂ O ₃ —TiO ₂ powder A. The powder A had an Rh loading of 0.8 wt%.

  Next, 100 g of the obtained powder A was dispersed in 1580 g of ion-exchanged water, and 320 g of alumina sol as a binder component was added to the ion-exchanged water and stirred to prepare a slurry. Next, the obtained slurry was coated on a cordierite honeycomb substrate (φ130 mm, L155 mm) by a wash coating method, dried at 200 ° C. for 2 hours, and then fired in air at 400 ° C. for 4 hours, on the substrate. A lower layer having a coating amount of 210 g / substrate-L (thickness: 58 μm) was formed. The loading amount of Rh was 0.4 g / base material-L.

Next, a zirconia sol was coated on the obtained lower layer, dried at 200 ° C. for 2 hours, and then fired in air at 400 ° C. for 4 hours. The coating amount was 60 g / base material-L (thickness on the lower layer). A diffusion barrier layer made of zirconia (ZrO ₂ ) having a thickness of 22 μm was formed.

Next, a mixed metal oxide powder of Al ₂ O ₃ —CeO ₂ (Al ₂ O ₃ : CeO ₂ = 70: 30 (weight ratio)) was impregnated with an aqueous solution of dinitrodiammine platinum nitrate and dried at 200 ° C. to obtain moisture. Then, this was calcined in air at 450 ° C. for 2 hours to obtain Pt / Al ₂ O ₃ —CeO ₂ powder B. The amount of Pt supported on this powder B was 4 wt%.

  Next, 100 g of the obtained powder B was dispersed in 1580 g of ion-exchanged water, and 320 g of alumina sol as a binder component was added to the ion-exchanged water and stirred to prepare a slurry. Next, the obtained slurry is coated on the base material on which the lower layer and the diffusion barrier layer are formed by a wash coat method, dried at 200 ° C. for 2 hours, and then baked in air at 400 ° C. for 4 hours to form on the diffusion barrier layer. An upper layer having a coating amount of 60 g / base material-L (thickness: 22 μm) was formed. The supported amount of Pt was 2.0 g / base material-L.

Next, an aqueous solution in which potassium acetate (CH ₃ COOK), barium acetate ((CH ₃ COO) ₂ Ba) and lithium acetate (CH ₃ COOLi) are dissolved in the base material on which the upper layer, the diffusion barrier layer and the lower layer are formed. After impregnation and drying at 200 ° C. for 2 hours, firing in air at 450 ° C. for 2 hours to carry a total of 0.45 mol of K, Ba and Li per liter of the substrate, from the upper layer, diffusion barrier layer and lower layer Thus, an exhaust gas purifying catalyst of the present invention having a three-layered catalyst layer was obtained.

[Comparative Example 1]
100 g of powder A and 100 g of powder B prepared in Example 1 were dispersed in 3160 g of ion exchange water, and 640 g of alumina sol as a binder component was added to this ion exchange water and stirred to prepare a slurry. Next, the obtained slurry was coated on a cordierite honeycomb substrate (φ130 mm, L155 mm) by a wash coating method, dried at 200 ° C. for 2 hours, and then fired in air at 400 ° C. for 4 hours, on the substrate. A catalyst layer having a coating amount of 270 g / substrate-L (thickness: 100 μm) was formed. The loadings of Rh and Pt were 0.4 and 2.0 g / base material-L, respectively.

  Next, the base material having the above catalyst layer was impregnated with an aqueous solution in which potassium acetate, barium acetate and lithium acetate were dissolved in the same manner as in Example 1, and the total amount of K, Ba and Li was 0. 1 per liter of the base material. An exhaust gas purifying catalyst having a catalyst layer with a single layer structure was obtained by loading 45 mol.

[Comparative Example 2]
Exhaust gas purification catalyst having a two-layered catalyst layer composed of an upper layer and a lower layer was obtained in the same manner as in Example 1 except that a diffusion barrier layer composed of zirconia (ZrO ₂ ) was not formed.

[Evaluation of catalyst]
The exhaust gas purification catalysts prepared above were evaluated for their NO _x storage capacity. The test was performed by supplying exhaust gas from a 3.5 liter gasoline engine to each exhaust gas purification catalyst at a rate of 70 g / s and measuring the NO _x amount in the exhaust gas on the catalyst outlet side. Since the test beginning is NO _x in the exhaust gas is occluded by the catalyst, NO _x amount in the exhaust gas at the catalyst outlet is zero. However, NO _x amount in the exhaust gas at the catalyst outlet increases as the _the NO _x storage amount approaches saturation occlusion amount of the catalyst. The amount of NO _x catalyst outlet side measures the time to reach 30% of the amount of NO _x catalyst inlet side, evaluating the treated exhaust gas amount so far as the NO _x 70% storage capacity of each exhaust gas purifying catalyst did. The result is shown in FIG.

FIG. 2 is a graph showing the NO _x 70% storage capacity of the exhaust gas purifying catalysts of Examples and Comparative Examples. In FIG. 2, as 100 the NO _x 70% storage ability of the exhaust gas purifying catalyst of Comparative Example 1 shows the NO _x 70% storage ability of the exhaust gas purifying catalyst of Example 1 and Comparative Example 2.

As can be seen from FIG. 2, even when no diffusion barrier layer is provided, Pt and Rh, which are catalyst metals, are applied to the upper layer and the lower layer, respectively, with respect to the exhaust gas purification catalyst of Comparative Example 1 having a single-layer structure catalyst layer. In the exhaust gas purifying catalyst of Comparative Example 2 having a two-layered catalyst layer coated separately, the NO _x storage capacity was improved about 4 times. However, the exhaust gas purification catalyst of the present invention having a three-layered catalyst layer in which a diffusion barrier layer made of zirconia is further provided on the exhaust gas purification catalyst of Comparative Example 2 is more NO than the exhaust gas purification catalyst of Comparative Example 2. _x occlusion ability improved 1.9 times.

It is a schematic diagram which shows the cross section of the catalyst for exhaust gas purification by the preferable embodiment of this invention. Is a graph showing the NO _x 70% storage ability for each exhaust gas purifying catalysts of Examples and Comparative Examples.

Explanation of symbols

1 Exhaust gas purification catalyst 2 Base material 3 Lower layer 4 Diffusion barrier layer 5 Upper layer 6 Pt
7 NO _x retention material 8 Rh

Claims (2)

A catalyst layer having a multilayer structure containing a metal oxide and a catalyst metal is formed on a substrate, and one layer constituting the catalyst layer is mainly composed of Pt, Pd or a combination thereof as the catalyst metal, and at least NO _x A diffusion barrier layer further comprising a retentive material, wherein the other layer is mainly composed of Rh as the catalyst metal, and has a higher concentration of acidic oxide than the other layer between the one layer and the other layer. provided, wherein the NO _x held than the other layer the content of substances Many in the one layer, the diffusion barrier layer contains no zirconia, titania, comprises silica or a combination thereof and the catalyst metal, the One catalyst is an upper layer on the exhaust gas flow path side of the catalyst layer, and the other layer is a lower layer on the base material side of the catalyst layer . NO _x The exhaust gas-purifying catalyst according to claim 1, wherein the holding substance contains potassium as a constituent component.

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