JPH11276896A - Catalyst for cleaning exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently decompose NOx even after a catalyst is exposed at high temp., in the catalyst provided with Ba, Pt and Rh for cleaning exhaust gas. SOLUTION: A catalyst powder wherein Pt is carried and Rh is not carried on a mixture of alumina and ceria is formed and a catalyst powder wherein Rh is carried ad Pt is not carried on manganese dioxide is formed and these catalyst powders are mixed and a catalyst layer obtd. by wash-coating the carrier with the mixture is impregnated with a Ba soln. It is possible thereby to prevent both noble metals from being made into an alloy when the catalyst is exposed even at high temp. and to make an interaction among Ba, Pt and Ph remarkable.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an exhaust gas purifying catalyst and a method for producing the same.

[0002]

2. Description of the Related Art When an engine is operated at a lean air-fuel ratio with a low fuel concentration, the exhaust gas has a high oxygen concentration,
In a conventional three-way catalyst, NOx (nitrogen oxide) in exhaust gas
Is difficult to decompose and purify.

[0003] As an exhaust gas purifying catalyst effective for purifying NOx in such a case, Japanese Patent Application Laid-Open No. 9-38493 discloses a catalyst comprising a carrier comprising a composite oxide of alumina and at least one of Co and La. An exhaust gas purifying catalyst in which a noble metal and a NOx storage material are sequentially carried is described. This is because at a high temperature, the alumina reacts with the NOx storage material to cause a decrease in the specific surface area of the alumina and blockage of the pores, and sintering of the noble metal occurs. It is said that the thermal degradation could be suppressed in the form.

In this method for producing a catalyst, an alumina layer is formed on a carrier, and impregnated with Co, dried, and calcined to form a layer of a Co-alumina composite oxide. The impregnation → drying and the Rh impregnation → drying are sequentially performed, followed by firing, and then the Ba impregnation → drying → firing is performed. Therefore, in this catalyst, Pt and Rh are supported on one Co-alumina composite oxide particle in close proximity or overlapping.

[0005]

However, as described above, 1
When two kinds of noble metal particles are supported on one base material particle, when the catalyst is exposed to a high temperature, the two noble metal particles are easily alloyed, and the catalyst purification performance is reduced.

Therefore, the present invention provides a type of NOx storage material that stores NOx when the oxygen concentration is high and releases the NOx when the oxygen concentration is low, and has a catalytic action for decomposing NOx. In the case where different catalytic noble metals are provided, it is an object of the present invention to enhance the stability of each noble metal against heat and to make each of them effectively work for decomposing and purifying NOx.

[0007]

That is, one aspect of the invention of the present application (claim 1) is to provide a NOx storage material having an action of storing NOx in exhaust gas and a catalytic action for decomposing the NOx. The present invention relates to a method for producing an exhaust gas purifying catalyst comprising first and second noble metals of different types, wherein the first catalyst is supported on a base material and the second noble metal is not supported. A step of forming a powder, and a step of forming a second catalyst powder in which the second noble metal is supported on the base material and the first noble metal is not supported, and the first catalyst powder and the second catalyst powder are mixed. The NOx storage material is supported by impregnating the mixture with the solution of the NOx storage material.

According to such a method, a catalyst is obtained in which the first noble metal and the second noble metal are supported on different base materials (however, the types may be the same). The alloying of the two noble metals is reduced even if the metal is exposed to high temperatures. Therefore, each precious metal can decompose and purify NOx by interacting with the NOx storage material on the base material on which each noble metal is carried. In addition, even when the two noble metals work together to perform a catalytic action, each can sufficiently fulfill its role.

It is important to note that in such a method, the NOx occluding material is not impregnated on each catalyst powder but is impregnated and supported on the mixture. (1) The NOx storage material is supported so as to straddle both the base material carrying the noble metal and the base material carrying the second noble metal. It is to become.

Therefore, each of the first noble metal and the second noble metal interact with the NOx storage material on one NOx storage material (particle) to contribute to NOx purification,
NOx purification efficiency is likely to increase. In particular, when the first and second precious metals have different roles in NOx purification, the first and second precious metals play a role of each other on one NOx storage material (particle). This is advantageous for purification of NOx using the NOx storage material as an intermediate. Further, for example, even if the sintering of the first noble metal proceeds, the decrease in NOx purification performance due to the sintering can be compensated for by the second noble metal.

When the catalyst obtained by such a method is coated on a carrier, the catalyst layer formed on the carrier is composed of a base material that supports the first noble metal and the NOx storage material and does not support the second noble metal. A mixture of a base material carrying the second noble metal and the NOx storage material and not carrying the first noble metal,
An exhaust gas purifying catalyst is obtained in which a part of the NOx storage material is supported over both the base material carrying the first noble metal and the base material carrying the second noble metal (claim) Invention according to Item 8).

As a base material on which the first and second noble metals are supported, Al ₂ O ₃ , CeO ₂ , Mn, Co,
At least one oxide of Ti and Fe is selected. At this time, the first and second noble metals may be supported on the same type of base material.

With such a catalyst, the above-described action is obtained, and high NOx purification characteristics are maintained even after the catalyst is exposed to a high temperature.

The base material for supporting the first noble metal and the base material for supporting the second noble metal can be of different types (the inventions of claims 2, 9 and 10). That is, it is effective to increase the thermal stability of the base material itself to improve the heat resistance of the catalyst, but the base material having high thermal stability often has a small specific surface area. In general, when two types of noble metals are used, one of them can often be reduced in amount in comparison with the other. In such a case, the first precious metal having a large specific surface area carries a large amount of the first noble metal per unit base material amount, and the second specific base material having a small specific surface area but having high thermal stability per unit base material amount. If the second precious metal having a small amount of support is supported, the sintering of the first base material may occur and a part of the first noble metal may be lost (even if the first precious metal is partially buried in the base material). Since the required amount of the second noble metal is ensured on the second base material, it is possible to prevent a large decrease in catalytic performance (the invention according to claim 11).

That is, if not only the first noble metal but also a small amount of the second noble metal is supported on the base material having a large specific surface area and low heat resistance, the amount of the second noble metal becomes extremely large due to the sintering of the base material. And the subsequent catalyst performance is greatly reduced, but this can be prevented.

For example, according to the study of the present inventors, Pt is effective in purifying NOx when the oxygen concentration in exhaust gas is high, but the NOx purification rate decreases when the oxygen concentration becomes low. However, when a small amount of Rh is used together with Pt, a decrease in the NOx purification rate is suppressed. Therefore, it is preferable to employ such Pt as the first noble metal and Rh as the second noble metal (the invention according to claim 4 and the invention according to claim 13).

That is, in such a catalyst, when the oxygen concentration in the exhaust gas is high (air-fuel ratio lean), Pt
Oxidizes NO in the exhaust gas to oxidized NO ₂ , making it easier to be stored in the NOx storage material, and reduces the oxygen concentration in the exhaust gas (stoichiometric or air-fuel ratio rich) to release NOx released by the NOx storage material Then, each of Pt and Rh reduces and purifies this NOx.

In this case, when the oxygen concentration in the exhaust gas is low, the NOx reduction and purification performance is considered to be higher for Rh than for Pt. Therefore, one NOx storage material particle is Pt
When the oxygen concentration in the exhaust gas is high, in a case where the oxygen concentration in the exhaust gas is high in a state in which both the base material carrying Rh and the base material carrying Rh To P
NOx oxidized by t and carried on the NOx storage material
When the NOx is released from the NOx storage material, it is reduced and purified mainly by the action of Rh.
Will be efficiently purified.

In the above combination of Pt and Rh, Pt is supported on a first base material having a large specific surface area.
A small amount of Rh may be supported on the second base material having a small specific surface area and a high thermal stability.

As the first base material, there is alumina, and CeO ₂ can be used in combination with alumina.
Of course, it is possible to use only CeO ₂ as the first base material. On the other hand, Mn, Co, T
Preferably, it is an oxide of at least one metal selected from i and Fe (the invention according to claim 5).

As the alumina, γ-alumina is preferred. Since CeO ₂ has an oxygen storage capacity (O ₂ storage effect), when the engine is operated near the stoichiometric air-fuel ratio (λ = 1), the catalyst functions as a three-way catalyst,
This is effective in simultaneously and efficiently purifying not only NOx but also HC (hydrocarbon) and CO (carbon monoxide). On the other hand, oxides such as Mn have high thermal stability and are effective not only for preventing sintering of the first base material but also for oxidizing NO in exhaust gas to NO _2. This is because the storage performance of the NOx storage material is thereby improved.

In the step of impregnating the mixture with the solution of the NOx storage material, the mixture is impregnated with the solution of the first noble metal together with the solution of the NOx storage material to additionally carry the first noble metal. Is preferable (the invention according to claim 3). That is, a part of the first noble metal is previously supported on the base material, and the remaining part is supported by the impregnation step. Thereby, the heat resistance of the catalyst can be improved, and the NO at the time of fresh catalyst (at the beginning of use of the catalyst) and at the time of stoichiometry (λ = 1)
x Purification performance is increased.

That is, the heat resistance is improved because a part of the first noble metal is previously supported on the base material, so that the dispersibility of the noble metal becomes high and the noble metal itself when exposed to high temperature is improved. This is because sintering and sintering between the noble metal and the second noble metal can be prevented. Further, the NOx purification performance is improved because the remaining portion of the first noble metal is disposed close to the second noble metal together with the NOx occluding material, and the interaction between the first noble metal, the second noble metal and the NOx occluding material becomes more remarkable. It is because it becomes.

The NOx storage material is preferably at least one metal selected from alkali metals such as Na, alkaline earth metals such as Ba and Sr, and rare earth metals such as La. The invention according to claim 6). In particular, Ba is excellent in NOx storage properties. In addition, M
Among n, Co, Ti and Fe, Mn and Co are NO.
It is more effective by decomposing and purifying x.

According to the study of the present inventor, NOx
It has been found that NOx purification performance is improved by forming a catalyst layer having a storage noble metal and a catalyst noble metal that decomposes NOx, and then forming a zeolite layer carrying a noble metal thereon. The invention according to the method for manufacturing such an exhaust gas purifying catalyst (the invention according to claim 7) is as follows.

That is, it comprises a step of forming a first catalyst powder in which Pt is supported on a mixture of alumina and CeO _2, and a step of forming at least one metal selected from Mn, Co, Ti and Fe. Forming a second catalyst powder in which Rh is supported on the oxide;
and forming a third catalyst powder carrying h. and forming an inner layer made of a mixture of the first catalyst powder and the second catalyst powder on a carrier, and forming the third layer on the inner layer. After forming the outer layer made of the third catalyst powder, the NOx storage material is made of at least one metal selected from alkali metals, alkaline earth metals and rare earth metals and has a function of storing NOx in exhaust gas. By impregnating the inner layer and the outer layer with a solution, the NOx occluding material is supported on both the inner and outer layers. At this time, N
By impregnating the mixed solution of the solution of the Ox storage material and the solution of the noble metal, the dispersibility of the noble metal and the NOx storage material can be improved.

Thus, an exhaust gas purifying catalyst in which the inner catalyst layer and the outer catalyst layer are formed in layers on the carrier (the invention according to claim 14) is obtained. That is, the inner catalyst layer includes a catalyst component in which Pt and a NOx storage material are supported on a mixture of alumina and CeO ₂ and Rh is not supported,
An oxide of at least one metal selected from Mn, Co, Ti and Fe has a mixture of Rh and a catalyst component in which NOx storage material is supported and Pt is not supported. On the other hand, the outer catalyst layer is made of Pt
And a catalyst component on which Rh and the NOx storage material are carried.

In such a catalyst, the noble metal supported on the zeolite in the outer catalyst layer contains NO in the exhaust gas.
This activates x and HC to enhance NO purification performance. That is, NO in the exhaust gas becomes NO ₂ and NO
x It becomes easy to be occluded by the occlusion material, and HC becomes easily energetically reactive due to partial oxidation and cracking.
The reduced decomposition reaction of NOx by the activated HC easily proceeds. Of course, in the inner catalyst layer, Pt and Rh are separately supported on the alumina side and the metal oxide side such as Mn, so that the above-described effects can be obtained, and even after the catalyst is exposed to a high temperature. High NOx purification performance is maintained.

As is apparent from the above description, it is effective to use an oxide such as Mn as the second base material.
It is considered that this is because NO in the exhaust gas is converted to NO ₂ , so that the catalyst layer formed on the carrier carries the first noble metal on the first base material such as alumina and carries the second noble metal. Exhaust gas purification catalyst comprising a mixture of a catalyst powder that does not carry the second noble metal and the NOx storage material on an oxide having an action of oxidizing NO to NO ₂ and does not carry the first noble metal ( The invention according to claim 12) is characterized in that even after exposure to a high temperature, N
It is effective to decompose Ox efficiently.

[0030]

As described above, according to the invention of this application, a step of forming a base material that supports a first noble metal and does not support a second noble metal, and a step of forming a base material that supports the second noble metal. 1
Forming a base material that does not carry a noble metal,
Since the base material supporting the first noble metal and the base material supporting the second noble metal were mixed, and the mixture was impregnated with the solution of the NOx storage material, the NOx storage material was supported. Even when the catalyst is exposed to a high temperature, the alloying of the two noble metals is reduced, and each noble metal can decompose and purify NOx by interacting with the NOx storage material on the base material on which each noble metal is supported.

A part of the NOx occluding material is supported so as to straddle both the base material carrying the first noble metal and the base material carrying the second noble metal. Since the shape crosses over both the first precious metal and the second precious metal, the first precious metal and the second precious metal interact with the NOx storage material on one NOx storage material (particles) to purify NOx. , Which is advantageous for the purification of NOx using the NOx storage material as an intermediate.

If the base material for supporting the first noble metal and the base material for supporting the second noble metal are of different types,
By supporting a noble metal suitable for each base material, it becomes easy to utilize the characteristics of the base material in improving the heat resistance of the catalyst and the NOx purification rate. For example, if a relatively large amount of Pt is supported on alumina and a relatively small amount of Rh is supported on a metal oxide such as Mn, the metal can be highly dispersed and supported on Pt and Rh. The oxide can improve the storage performance of the NOx storage material, and furthermore, Mn
Since the metal oxides such as have little sintering problem, the thermal stability of Rh supported on the metal oxides will eventually increase, and even if the catalyst is exposed to a high temperature, Rh will be NO.
This effectively contributes to the purification of x and improves NOx purification performance.

[0033]

DESCRIPTION OF THE PREFERRED EMBODIMENTS <Structure of Catalyst> FIG. 1 shows a structure of an exhaust gas purifying catalyst C according to the present invention.
The catalyst C is disposed in an exhaust passage (not shown) for discharging exhaust gas of a lean combustion engine for a vehicle, and HC, CO, NOx, etc. in the exhaust gas during a stoichiometric air-fuel ratio combustion operation. Of air pollutants, and also effectively purifies NOx during lean combustion operation. That is, the catalyst C has a lean NOx purifying action, the oxygen concentration in the lean atmosphere is, for example, 4 to 5% to 20%, and the air-fuel ratio is A / F = 18 or more.

The catalyst C is a honeycomb-shaped carrier 1 made of cordierite, which is a carrier material having excellent heat resistance.
On the support 1, an inner catalyst layer 2 (lower catalyst layer) on the side close to the surface of the support 1, and an outer catalyst layer 3 (upper side) on the side distant from the surface of the support 1 thereon. (Catalyst layer).

The inner catalyst layer 2 is made of a first precious metal (for example, a mixture of alumina and CeO ₂ ) and a first noble metal (for example, P
t) and a catalyst component carrying the NOx storage material, and a second precious metal (for example, an oxide of at least one metal selected from Mn, Co, Ti, and Fe) on a second noble metal (for example, Rh) and a catalyst component carrying a NOx storage material. However, the first base material has a second base material.
Noble metal is not supported, and the first base metal is not supported on the second base material. In addition, a part of the NOx storage material is supported so as to straddle both the first base material and the second base material, and is thus supported so as to straddle both the first and second noble metals. On the other hand, the outer catalyst layer 3 includes a third base material (for example, zeolite) and a noble metal (for example, Pt and Rh) and NOx.
It has a catalyst component in which an occluding material is carried.

The first to third base materials can carry other noble metals such as Ir and Pd in addition to the above noble metals. As the NOx storage material, Ba is mainly used, but another alkaline earth metal, or an alkali metal or a rare earth metal may be used, or two or more metals selected from them may be used in combination. Can be. Ceria can be used as the CeO ₂ component, but a composite oxide of cerium and zirconium can also be used from the viewpoint of improving heat resistance. The impurities in each of the catalyst layers 2 and 3 are set to 1% or less.

<Production method of catalyst> The basic production method of the catalyst C is as follows.

The first catalyst powder is formed by supporting the first precious metal on the first base material by a drying method or the like. Also,
The second catalyst powder is formed by supporting the second precious metal on the second base material by a drying method or the like. Further, a third catalyst powder is formed by supporting a noble metal on the third base material.

The first catalyst powder, the second catalyst powder, the binder, and water are mixed to form a slurry, and the slurry is wash-coated on the carrier 1, dried and calcined.
An inner coat layer is formed.

Next, the third catalyst powder, the binder and water are mixed to form a slurry. The slurry is wash-coated on the carrier 1 having the inner coating layer, and dried and fired to form the inner coating. An outer coat layer is formed over the layer.

Thereafter, the inner layer and the outer layer are impregnated with the solution of the NOx occluding material, and dried and fired, whereby the NOx occluding material is supported on both the inner and outer layers.

<Evaluation of Catalyst> Each catalyst of the following Examples and Comparative Examples was prepared, and the catalyst performance was comparatively evaluated.

-Example 1-Formation of Pt catalyst powder (first catalyst powder) γ-alumina and CeO ₂ -ZrO ₂ composite oxide were mixed as follows:
The mixture is mixed at a weight ratio of 1 and this mixture and a dinitrodiamine platinum solution are mixed at a weight ratio of mixture: Pt (metal amount) = 150: 1, and spray-dried by a spray-drying method. The Pt catalyst powder was formed by performing drying and baking. Drying was performed at a temperature of 100 to 200 ° C. for 1 hour, and baking was performed at a temperature of 500 to 600 ° C. for 2 hours. These drying conditions and firing conditions are the same for "drying" and "firing" in the following description.

Formation of Rh catalyst powder (second catalyst powder) MnO ₂ (manganese dioxide) and rhodium nitrate solution were mixed with M
The mixture was mixed so that the weight ratio of nO ₂ : Rh (metal amount) = 120: 1, and then dried, dried, and calcined in the same manner as in the case of the Pt catalyst powder to form a Rh catalyst powder.

Pt-Rh / MFI catalyst powder (third catalyst powder)
Formation of dinitrodiamine platinum solution and rhodium nitrate solution
The mixture was mixed so that the weight ratio of t: Rh = 75: 1, and this was mixed with an MFI-type zeolite (SiO ₂ / Al ₂ O ₃ = 8).
In combination with 0), Pt-Rh / MFI catalyst powder was formed by performing drying, drying and calcining in the same manner as in the case of the Pt catalyst powder.

Formation of Inner Coat Layer The above Pt catalyst powder, Rh catalyst powder, and alumina binder were
Mixing was performed so that the weight ratio was 0: 2: 5, and ion-exchanged water was added thereto to prepare a slurry.
A carrier having a honeycomb structure (420 g / L; this g / L means the weight per 1 L of the carrier; the same applies hereinafter) is immersed in the slurry, pulled up, and the excess slurry is blown off. 342 g coat
/ L was wash-coated with the slurry. Next, this was dried and fired to form an inner coat layer.

Formation of Outer Coat Layer Pt-Rh / MFI catalyst powder and alumina binder were mixed in 5:
The mixture was mixed so as to have a weight ratio of 1 and ion-exchanged water was added thereto to prepare a slurry. The slurry was applied to the carrier on which the inner coat layer was formed so that the coated amount after drying was 24 g / L. Then, the outer coat layer was formed by subjecting to a wash coat, followed by drying and baking.

Impregnation of Ba Next, both the inner and outer coat layers of the carrier were impregnated with barium acetate so that the amount of Ba became 30 g / L, followed by drying and calcination, whereby the catalyst of Example 1 was used. Obtained.

Accordingly, the structure of this catalyst is as shown in Table 1. The feature of the catalyst of Example 1 is that Pt and Rh are separately supported on different base materials in the inner catalyst layer, and the base material of Rh is Mn oxide.

The inner catalyst layer of this catalyst has γ-
It has 150 g / L of alumina, 150 g / L of CeO ₂ -ZrO ₂ composite oxide and 12 g / L of manganese oxide, 30 g / L of binder, 2 g / L of Pt for noble metal, R
h is 0.1 g / L. In the outer catalyst layer of this catalyst, the base material is 20 g / L of MFI, the binder is 4 g / L, and the precious metal is 0.5 g / L of Pt and 0.006 g / L of Rh. Ba is contained as a NOx storage material in an amount of 30 g / L in accordance with the inner and outer catalyst layers.

[0051]

[Table 1]

Example 2 A catalyst of Example 2 was obtained in the same manner as in Example 1 except that cobalt oxide (Co ₂ O ₃ ) was used instead of manganese oxide (see Table 1).

Example 3 Formation of Pt Catalyst Powder (First Catalyst Powder) γ-Alumina, CeO ₂ -ZrO ₂ Composite Oxide and MnO
₂ and 25: 25: 2 in a weight ratio of 25: 25: 2.
This mixture and a dinitrodiamineplatinum solution are mixed with:
Pt (metal amount) was mixed so as to have a weight ratio of 75: 1, and dried, dried and calcined in the same manner as above to form a Pt catalyst powder.

Formation of Rh catalyst powder (second catalyst powder) γ-alumina, CeO ₂ -ZrO ₂ composite oxide and MnO
₂ and 25: 25: 2 in a weight ratio of 25: 25: 2.
This mixture was mixed with rhodium nitrate so that the weight ratio of mixture: Rh (metal amount) = 1500: 1 was obtained.
By performing the same drying, drying and baking as described above, Rh
A catalyst powder formed.

Pt-Rh / MFI catalyst powder (third catalyst powder)
Formation of Pt-Rh / M by the same material and method as in the previous example.
An FI catalyst powder was formed.

Formation of Inner Coat Layer The above Pt catalyst powder, Rh catalyst powder, and alumina binder were mixed at a weight ratio of 5: 5: 1, and a slurry was prepared by adding ion-exchanged water thereto. .
This slurry was wash-coated on the same carrier as in Example 1 so that the coating amount after drying was 330 g / L, and dried and fired to form an inner coating layer.

Formation of Outer Coat Layer An outer coat layer having a coating amount after drying of 24 g / L was formed by the same material and method as in the previous example.

Ba impregnation and loading Barium acetate was impregnated and supported on both the inner and outer coating layers of the above carrier so that the Ba content was 30 g / L in the same manner as in the previous example.

Therefore, the catalyst of Example 3 obtained is different from the catalyst of Example 1 in that the base material of Rh in the inner catalyst layer is not Mn oxide but the same base material as Pt. The base material of Pt and the base material of Rh are each a mixture of γ-alumina, CeO ₂ —ZrO ₂ composite oxide and MnO ₂ (see Table 1), and the other conditions are the same as in Example 1.

Example 4 Formation of Pt Catalyst Powder (First Catalyst Powder) Gamma-alumina and CeO ₂ —ZrO ₂ composite oxide were mixed in a ratio of 1:
1 and the mixture was mixed with a dinitrodiamine platinum solution to obtain a mixture: Pt (metal amount) = 7.
The Pt catalyst powder was formed by mixing at a weight ratio of 5: 1 and performing the same drying, drying and calcining as before.

Formation of Rh Catalyst Powder (Second Catalyst Powder) Gamma-alumina and CeO ₂ —ZrO ₂ composite oxide
1 and the mixture is mixed with rhodium nitrate at a weight ratio of mixture: Rh (amount of metal) = 1500: 1. By baking, a Rh catalyst powder was formed.

Pt-Rh / MFI catalyst powder (third catalyst powder)
Formation of Pt-Rh / M by the same material and method as in the previous example.
An FI catalyst powder was formed.

Formation of Inner Coat Layer The above Pt catalyst powder, Rh catalyst powder, and alumina binder were mixed at a weight ratio of 5: 5: 1, and a slurry was prepared by adding ion-exchanged water thereto. .
This slurry was wash-coated on the same carrier as in Example 1 so that the coating amount after drying was 330 g / L, and dried and fired to form an inner coating layer.

Formation of Outer Coat Layer An outer coat layer having a coating amount of 24 g / L after drying was formed by the same material and the same method as in the previous examples.

Ba impregnation and loading Barium acetate was impregnated and supported on both the inner and outer coating layers of the above carrier so that the Ba content was 30 g / L in the same manner as in the previous example.

Therefore, the catalyst of Example 4 obtained is different from the catalyst of Example 1 in that the base material of Rh in the inner catalyst layer is not Mn oxide but the same base material as Pt ( Others are the same as in Example 1.

Comparative Example Formation of Inner Coat Layer Gamma-alumina, CeO ₂ —ZrO ₂ composite oxide, and alumina binder were mixed at a weight ratio of 5: 5: 1, and ion-exchanged water was added thereto to obtain a slurry. Was prepared, and this was coated on the same carrier as in Example 1 with a coating amount of 330 g / L after drying.
Then, the inner coat layer was formed by wash-coating and drying and firing.

Impregnation of Pt and Rh on inner coat layer Dinitrodiamine platinum solution and rhodium nitrate solution
The mixture was mixed at a weight ratio of t: Rh = 20: 1, impregnated into the inner coat layer so as to be 2.1 g / L, and dried and fired.

Formation of Outer Coat Layer Next, with the same material and method as in the embodiment, the coat amount after drying was
An outer coat layer of 4 g / L was formed.

Ba Impregnation Carrier Barium acetate was impregnated into the inner and outer coat layers by the same method as in the previous example so that the amount of Ba became 30 g / L, and then dried and fired.

The main difference between the catalyst of Comparative Example 1 obtained as described above and the catalyst of Example 1 is that Pt and R
and h are not separated and supported, and manganese oxide and cobalt oxide are not provided (see Table 1).

-Evaluation Method- After subjecting the test catalyst to a thermal aging treatment at 900 ° C. for 24 hours in the air, the catalyst is attached to a fixed-bed flow reaction evaluation apparatus. At first, the simulated exhaust gas having the lean air-fuel ratio is supplied to the catalyst until the NOx purification rate is stabilized. Next, the gas composition is switched to flow the simulated exhaust gas rich in the air-fuel ratio, and thereafter the gas composition is switched again to the lean air-fuel ratio, and the NOx purification rate is measured for 130 seconds from the switching point to evaluate the lean NOx purification performance. . The catalyst temperature and the simulated exhaust gas temperature were 350 ° C., the gas composition was as shown in Table 2, and the space velocity SV was 25000 h ⁻¹ .

[0073]

[Table 2]

The results are shown in FIG. Example 4 and Comparative Example differ depending on whether Pt and Rh of the inner catalyst layer are separated and supported on the base material, and the difference between the NOx purification rates of the two is recognized as the effect of the separation and support. That is, since Pt and Rh in Example 4 are separated and supported, alloying by thermal aging treatment is suppressed, and each contributes effectively to NOx purification even after thermal aging treatment. Conceivable.

The third embodiment and the fourth embodiment are different from each other in that the Pt of the inner catalyst layer
It differs depending on whether or not each of the base material of Rh and the base material of Rh contains a Mn oxide. The difference between the NOx purification rates of the two is recognized as the effect of adding the Mn oxide. That is, in the third embodiment,
The sintering of alumina is suppressed by the presence of the Mn oxide, and NO in the exhaust gas is easily oxidized to NO ₂ and easily absorbed by Ba.
It is considered that the Ox purification rate was higher than in Example 4.

The third embodiment and the first embodiment are different from each other in that the Pt of the inner catalyst layer
Whether each base material of Rh and Rh is of the same type,
It depends on whether or not it is supported on the n-oxide.
The difference in x purification ratio is recognized as an effect of supporting Rh on Mn oxide. That is, since the Mn oxide is difficult to sinter, unlike the third embodiment, most of the Rh supported on the Mn oxide is not buried in the Mn oxide even by the thermal aging treatment. It is considered that the state of being carried on the surface is maintained, and this effectively contributes to NOx purification.

The second embodiment differs from the first embodiment in that a Co oxide is used instead of a Mn oxide, but shows a high NOx purification rate as in the first embodiment. Therefore, it is recognized that this Co oxide also has the same function as the Mn oxide.
The same result as in Example 2 can be obtained when Co ₃ O ₄ is used as the Co oxide.

<Another Example>-Example 5- In the method of Example 1, half of the Pt amount of 2 g / L is supported on the first base material of the first catalyst powder, and the remaining half is In the Ba impregnating and supporting step, it was supported by impregnation together with Ba. That is, a barium acetate solution and a dinitrodiamine platinum solution were mixed, impregnated into inner and outer coat layers, and dried and fired. Others are the same as the first embodiment.

-Example 6- In the method of Example 4, half of the 2 g / L of Pt is supported on the first base material of the first catalyst powder, and the other half is Ba-impregnated in the Ba impregnation-supporting step. And supported by impregnation. That is, a barium acetate solution and a dinitrodiamine platinum solution were mixed, impregnated into inner and outer coat layers, and dried and fired. Others are the same as the fourth embodiment.

Therefore, the lean NOx purification rate of the fifth and sixth embodiments was measured by the above-described evaluation method, and the fifth and sixth embodiments, the first and second embodiments were measured.
And about the comparative example, the NOx purification rate in the stoichiometry after the heat aging treatment was measured. The results are shown in Table 3.

[0081]

[Table 3]

Example 5 differs from Example 1 in the method of supporting Pt, in which half the amount of Pt was supported by impregnation together with Ba.
However, the stoichiometric NOx purification rate is higher than in the first and second embodiments. This is the same as Example 5
In the case of Pt, the dispersibility of Pt was increased, and sintering due to the heat aging treatment was suppressed.
This is considered to be due to the fact that Pt, Rh, and Ba were more prominently interacted with each other, and were arranged close to Rh together with a.

In the sixth embodiment, the method of supporting Pt is different from that of the fourth embodiment in that half of Pt is supported by impregnation together with Ba. However, the lean NOx purification rate and the stoichiometric NOx purification rate are different from those of the fourth embodiment. It is higher than 4.

Examples 7 and 8 A catalyst of Example 7 was obtained in the same manner as in Example 1 except that titanium oxide (TiO ₂ ) was used instead of manganese oxide. A catalyst of Example 8 was obtained in the same manner as in Example 1, except that iron oxide (Fe ₂ O ₃ ) was used instead of manganese oxide. The amounts of titanium oxide and iron oxide in these catalysts were determined in Example 1.
Is the same as the amount of manganese oxide.

The lean NO.
When the purification rate was measured by the same evaluation method, the purification rate was 65.5% in Example 7 and 58.50 in Example 8.
8%. Therefore, it can be seen that similar effects can be obtained even with TiO ₂ or Fe ₂ O ₃ , although to a lesser extent, as compared with Mn oxides or Co oxides.

<Usefulness of Manganese Oxide, Cobalt Oxide, etc. as Base Material> Catalyst powder in which Rh is supported on Al ₂ O ₃ ,
Catalyst powder in which Rh is supported on MnO ₂ , and Rh in Co ₃
A catalyst powder supported on O ₄ was prepared, and Rh
And the particle size of Rh after the heat aging treatment were measured. The results are as shown in Table 4.

[0087]

[Table 4]

According to the table, R supported on Al ₂ O ₃
h has a small particle size, but is sintered and coarsened by thermal aging treatment, whereas MnO ₂ and Co ₃
Rh supported on O ₄ hardly sinters. This indicates that the use of MnO ₂ or Co ₃ O ₄ as the base material ensures the dispersibility of the noble metal and is advantageous for NOx purification.

From the above, the usefulness of the present invention is apparent, and this is not limited to the two-layer coated catalyst as in the above embodiment, but can be similarly applied to a single-layer coated catalyst or a pellet type catalyst.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a catalyst structure according to an embodiment of the present invention.

FIG. 2 is a graph showing the NOx purification rates of an example catalyst and a comparative example catalyst.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Carrier 2 Inner catalyst layer 3 Outer catalyst layer C Exhaust gas purification catalyst

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI B01J 29/44 B01J 23/64 104A (72) Inventor Makoto Kyogoku 3-1, Fuchu-cho, Shinchu, Aki-gun, Hiroshima Prefecture Mazda Corporation ( 72) Inventor Kenji Okamoto 3-1, Shinchi, Fuchu-cho, Aki-gun, Hiroshima Mazda Co., Ltd.

Claims (14)

[Claims] 1. An exhaust gas purifying apparatus comprising: a NOx storage material having a function of storing NOx in exhaust gas; and first and second different noble metals having different catalytic functions for decomposing the NOx. A method for producing a catalyst, comprising: forming a first catalyst powder in which a first noble metal is supported on a base material and no second noble metal is supported; and wherein the second noble metal is supported on the base material and Forming a second catalyst powder that has not been subjected to mixing, mixing the first catalyst powder and the second catalyst powder, and impregnating the mixture with a solution of the NOx storage material to support the NOx storage material. A method for producing an exhaust gas purifying catalyst. 2. An exhaust gas purifying apparatus comprising: a NOx occluding material having a function of occluding NOx in exhaust gas, and first and second different noble metals having different catalytic functions for decomposing the NOx. A method for producing a catalyst, comprising: forming a first catalyst powder in which a first precious metal is supported on a first base material and no second precious metal is supported; and a second base material different from the first base material. Forming a second catalyst powder in which a second noble metal is supported on the material and the first noble metal is not supported, wherein the first catalyst powder and the second catalyst powder are mixed, and the NOx storage material is added to the mixture. A method for producing an exhaust gas purifying catalyst, wherein the NOx storage material is supported by impregnating the NOx storage material. 3. The method for producing an exhaust gas purifying catalyst according to claim 1, wherein the step of impregnating the mixture with the solution of the NOx storage material comprises: An exhaust gas purifying catalyst, wherein a NOx storage material is supported on the mixture by impregnating the mixture with a first noble metal solution, and the first noble metal is additionally supported. 4. The method for producing an exhaust gas purifying catalyst according to claim 1, wherein the first noble metal is Pt, and the second noble metal is Rh. A method for producing a catalyst for purifying exhaust gas. 5. The method for manufacturing an exhaust gas purifying catalyst according to claim 2, wherein the first base material is at least one of alumina and CeO ₂ , and the second base material is Mn. A method for producing an exhaust gas purifying catalyst, comprising an oxide of at least one metal selected from the group consisting of Co, Ti, and Fe. 6. The method for producing an exhaust gas purifying catalyst according to claim 1, wherein the NOx occluding material is selected from the group consisting of an alkali metal, an alkaline earth metal, and a rare earth metal. A method for producing an exhaust gas purifying catalyst, comprising at least one selected metal. 7. A step of supporting Pt on a mixture of alumina and CeO ₂ to form a first catalyst powder which does not support Rh, and at least one of Mn, Co, Ti and Fe. Rh supported on metal oxide and Pt
Forming a second catalyst powder not supporting Pt and Rh on a zeolite, and forming a third catalyst powder obtained by supporting Pt and Rh on zeolite, wherein the first catalyst powder and the second catalyst are formed on a carrier. Forming an inner layer made of a mixture with powder, forming an outer layer made of the third catalyst powder on the inner layer, and then forming at least one selected from an alkali metal, an alkaline earth metal, and a rare earth metal. By impregnating the inner layer and the outer layer with a solution of a NOx storage material made of a kind of metal and having a function of storing NOx in exhaust gas, the NO.
A method for producing an exhaust gas purifying catalyst, wherein x storage material is supported on both the inner and outer layers. 8. A catalyst layer formed on a carrier, comprising: a NOx storage material having a function of storing NOx in exhaust gas;
different kinds of first catalysts having a catalytic action for decomposing x
And a second noble metal, wherein the catalyst layer supports a first noble metal and a NOx storage material and a base material that does not support a second noble metal, and a second noble metal and NO
The NOx storage material is composed of a mixture of a base material that supports the x storage material and does not support the first noble metal, and a part of the NOx storage material is a mixture of the base material that supports the first noble metal and the base material that supports the second noble metal. An exhaust gas purifying catalyst supported on both sides. 9. A catalyst layer formed on a carrier, comprising: a NOx storage material having a function of storing NOx in exhaust gas;
different kinds of first catalysts having a catalytic action for decomposing x
And a second noble metal, wherein the catalyst layer supports a first noble metal and a NOx storage material, and a first base material that does not support a second noble metal. Different from the base material, it carries the second precious metal and NOx storage material and
An exhaust gas purifying catalyst comprising a mixture with a second base material that does not support a noble metal. 10. The exhaust gas purifying catalyst according to claim 9, wherein a part of the NOx storage material is a first base material carrying the first noble metal and a second base material carrying the second noble metal. An exhaust gas purifying catalyst characterized by being supported on both base materials. 11. The exhaust gas purifying catalyst according to claim 9, wherein the second base material has a smaller specific surface area than the first base material.
Exhaust gas, wherein the amount of the second precious metal supported on the second base material per unit base material amount is smaller than the supported amount of the first noble metal per unit base material amount on the first base material. Purification catalyst. 12. The exhaust gas purifying catalyst according to claim 9, wherein the second base material is an oxide having an action of oxidizing NO in the exhaust gas. Catalyst. 13. The exhaust gas purifying catalyst according to claim 8, wherein the first noble metal is Pt, and the second noble metal is Rh. Exhaust gas purification catalyst. 14. An exhaust gas purifying catalyst comprising: an inner catalyst layer and an outer catalyst layer formed on a carrier in a layered form; and a NOx storage material having a function of storing NOx in exhaust gas, wherein the inner catalyst layer comprises: The layer is composed of a mixture of alumina and CeO ₂
The catalyst component where t and the NOx storage material are supported and Rh is not supported, and the oxide of at least one metal selected from Mn, Co, Ti and Fe supports Rh and the NOx storage material. An exhaust gas purifying catalyst, comprising: a catalyst component on which Pt is not supported; and wherein the outer catalyst layer includes a catalyst component on which Pt, Rh, and a NOx storage material are supported on zeolite.