JP2003093885A - Catalyst for cleaning exhaust gas and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance heat resistance of a catalyst for cleaning exhaust gas containing zeolite and an alkali metal or an alkali earth metal. SOLUTION: An inner catalyst layer 12 containing zeolite; an outer catalyst layer 13 containing at least one kind of metal selected from alkali metals and alkali earth metals; and an intermediate catalyst layer 14 are formed on a carrier 11. A content of the metal in the inner catalyst layer 12 is made to less than 1% of a content of the metal in the outer catalyst layer 13. It is avoided that a structure of zeolite is destroyed by the metal.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art A three-way catalyst that simultaneously oxidizes HC (hydrocarbon) and CO (carbon monoxide) and reduces NOx (nitrogen oxide) is known as a catalyst for purifying exhaust gas of an engine such as an automobile. Has been. This is generally a noble metal such as Pd, Pt, or Rh as an active metal, alumina as a support material that stabilizes the active metal and expands the contact area with gas to improve the purification performance, and an oxygen storage material. It is often composed of ceria (co-catalyst) as (OSC). However, this three-way catalyst has poor purification performance when the exhaust gas temperature is low, and the NOx purification performance deteriorates significantly when the engine air-fuel ratio becomes lean.

On the other hand, Japanese Patent Laid-Open No. 2001-11317
According to Japanese Patent Laid-Open No. 3 (1999), after adsorbing HC and NOx exhausted in a cold region immediately after the engine is started and the catalytic metal becomes active, the HC and NOx are released and purified. Have been described. The catalyst is formed by layering a hydrocarbon adsorbent layer containing zeolite and a catalyst metal layer containing a noble metal and a NOx adsorbent on the surface of the carrier, with the former inside. An alkali metal or an alkaline earth metal is used as the NOx adsorbent, and NOx in the catalyst metal layer and the hydrocarbon adsorbent layer.
The content ratio of the adsorbent is 60/40 to 99/1 by weight.

In Japanese Patent Laid-Open No. 11-13462, a hydrocarbon adsorbent layer containing zeolite is formed on the surface of a carrier as a catalyst having a hydrocarbon adsorbent and a NOx adsorbent, and a catalyst metal layer is formed thereon. 1 layer or 2 layers are formed,
It is described that these layers are impregnated with a barium acetate solution and fired. Japanese Unexamined Patent Publication No. 9-79026 discloses that a NOx adsorbent and HC are provided in the middle of an exhaust pipe of an engine.
It is described that a catalyst obtained by coating an adsorbent and a three-way catalyst on the same carrier is arranged.

[0005]

When the zeolite as the HC adsorbent and the alkali metal or alkaline earth metal as the NOx adsorbent are used in combination as described above, according to the research by the present inventors, they are exposed to high temperature. As a result, it was found that the NOx adsorption capacity did not change so much, but the HC adsorption capacity significantly decreased. After researching the cause,
Alkali metals and alkaline earth metals promote the structural destruction of zeolite when exposed to high temperatures.

FIG. 8 shows the specific surface area after heat aging of β-zeolite not supporting NOx adsorbent and β-zeolite supporting various alkali metals or alkaline earth metals as NOx adsorbent. The results are shown below. The thermal aging is to hold the test material at a temperature of 800 ° C. for 24 hours. According to the figure, the β-type zeolite carrying the NOx adsorbent has a considerably smaller specific surface area than the one not carrying the NOx adsorbent.

The object of the present invention is to solve the problem of structural destruction of zeolite by an alkali metal or an alkaline earth metal in an exhaust gas purifying catalyst in which such a zeolite is used in combination with an alkali metal or an alkaline earth metal. Is to improve the heat resistance of.

[0008]

In order to solve such a problem, the present invention adopts a method of separating zeolite and alkali metal or alkaline earth metal on the same carrier. did.

That is, the invention according to claim 1 comprises, on the surface of the carrier, a first layer containing zeolite and a second layer containing at least one metal selected from alkali metals and alkaline earth metals. The exhaust gas purifying catalyst is characterized in that the content of the metal in the first layer is less than 1% of the content of the metal in the second layer.

Therefore, when the catalyst is exposed to a high temperature, the zeolite of the first layer is prevented from being destroyed by the alkali metal or the alkaline earth metal, and the reduction of the specific surface area of the zeolite is prevented. Zeolite and alkali metal or alkaline earth metal can be effectively used for purification of exhaust gas in each layer.

As the above zeolite, β-type zeolite is preferable, but other zeolite such as MFI may be adopted.

According to a second aspect of the present invention, in the exhaust gas purifying catalyst according to the first aspect, the first layer is arranged inside the second layer.

Therefore, since the zeolite of the first layer is covered by the second layer, it is avoided that it is directly exposed to high temperature exhaust gas, which is advantageous for preventing thermal deterioration.

According to a third aspect of the present invention, in the exhaust gas purifying catalyst according to the first aspect, the metal from the second layer to the first layer is provided between the first layer and the second layer. Is provided with an intermediate layer that prevents movement of the.

Therefore, the zeolite of the first layer is protected from the alkali metal or alkaline earth metal of the second layer by the intermediate layer, and the structural destruction or reduction of the specific surface area of the zeolite due to the alkali metal or alkaline earth metal is prevented. To be done.

The invention according to claim 4 is the exhaust gas purifying catalyst according to claim 3, wherein the intermediate layer is
It is characterized by containing an oxide having the properties of a solid acid.

Therefore, even if the alkali metal or alkaline earth metal in the second layer moves toward the first layer, it is captured by the acidic oxide or the amphoteric oxide in the intermediate layer, so that the structure of the zeolite in the first layer is Destruction or reduction in specific surface area is prevented. As the oxide having the property of a solid acid, titania,
Examples thereof include zirconia, alumina, zeolite, silica and ceria.

The invention according to claim 5 is characterized in that the carrier surface is provided with a first layer containing zeolite and a second layer containing at least one metal selected from alkali metals and alkaline earth metals. A method for manufacturing an exhaust gas purifying catalyst, comprising: a step A of coating the zeolite with the carrier to form the first layer; a step B of supporting the metal on a support material; Coating the carrier with the metal-supporting material to form a layer.
Is performed prior to step C above.

Therefore, since the alkali metal or alkaline earth metal is used for the coating of step C while being supported on the support material, it is mixed with the first layer previously formed and adversely affects the zeolite. It has less effect.

According to a sixth aspect of the present invention, in the method for producing an exhaust gas purifying catalyst according to the fifth aspect, in the step B, the supporting material is loaded with the metal and then baked, In step C, the baked metal-supporting support material and the basic binder are mixed to form a slurry, and the second layer is formed by coating the slurry.

Therefore, since the alkali metal or the alkaline earth metal is firmly supported on the support material by the firing in step B, even if the slurry is formed in step C, the amount eluted into the slurry is small. Moreover, since the basic binder is used to form the second layer, the slurry becomes basic, and the amount of the solid base alkali metal or alkaline earth metal eluted into the slurry is further reduced. Therefore, it is less likely that an alkali metal or an alkaline earth metal is mixed into the first layer when the second layer is formed, resulting in destruction of the zeolite.

According to a seventh aspect of the present invention, in the method for producing an exhaust gas purifying catalyst according to the sixth aspect, the basic binder is a zirconia binder.

Since zirconia is a basic oxide and the slurry becomes basic, the elution of alkali metal or alkaline earth metal into the slurry is reduced.

The invention according to claim 8 is the method for producing an exhaust gas purifying catalyst according to claim 5, further comprising a step D for forming an intermediate layer between the steps A and C. It is characterized by being

Therefore, migration of the alkali metal or alkaline earth metal toward the first layer is prevented in the intermediate layer, and the zeolite is protected from the alkali metal or alkaline earth metal.

The invention according to claim 9 is the method for producing an exhaust gas purifying catalyst according to claim 8, wherein in the step D, the intermediate layer is formed by an oxide having the property of a solid acid. Characterize.

Therefore, even if the alkali metal or alkaline earth metal moves toward the first layer, it is captured by the acidic oxide or amphoteric oxide in the intermediate layer, and the zeolite in the first layer is alkali metal or alkaline earth metal. Protected from.

[0028]

As described above, according to the invention of claim 1, the first surface containing zeolite and at least one metal selected from alkali metals and alkaline earth metals are formed on the surface of the carrier. And a second layer containing, wherein the content of the metal in the first layer is less than 1% of the content of the metal in the second layer, destruction of zeolite by alkali metal or alkaline earth metal is avoided. The heat resistance of the catalyst is improved.

According to the invention of claim 2, in the exhaust gas purifying catalyst according to claim 1, the first
Since the layer is arranged inside the second layer, the first layer
Direct exposure of the zeolite in the layer to hot exhaust gas is avoided, which is advantageous for preventing its thermal deterioration.

According to the invention of claim 3, in the exhaust gas purifying catalyst according to claim 1, the first
An intermediate layer is provided between the layer and the second layer to prevent migration of the metal from the second layer to the first layer, thus protecting the zeolite from alkali metals or alkaline earth metals and It is further advantageous for improving heat resistance.

According to the invention of claim 4, in the exhaust gas purifying catalyst of claim 3, since the intermediate layer contains an oxide having the property of a solid acid, the second layer When the alkali metal or alkaline earth metal moves toward the first layer, it stays in the intermediate layer, which is more advantageous for protecting the zeolite by the intermediate layer.

According to the invention of claim 5, in order to form the first layer containing zeolite, the step A of coating the zeolite with the carrier and the support material selected from alkali metal and alkaline earth metal are selected. Since the method comprises the step B of supporting at least one kind of metal and the step C of coating the metal supporting material on the first layer of the carrier to form the second layer,
The amount of alkali metal or alkaline earth metal mixed into the first layer when forming the layer is reduced.

According to the invention of claim 6, in the method for producing an exhaust gas purifying catalyst according to claim 5, in the step B, the support material is supported on the metal and then calcined. In the step C, the calcined metal-supporting material and the basic binder are mixed to form a slurry, and the second layer is formed by coating the slurry. Therefore, the slurry of alkali metal or alkaline earth metal is used. The amount of elution into the first layer is reduced, which is more advantageous in preventing alkali metal or alkaline earth metal from being mixed into the first layer.

According to the invention of claim 7, in the method for producing an exhaust gas purifying catalyst according to claim 6, since the basic binder is a zirconia binder, an alkali metal or alkaline earth metal is used. It is advantageous in preventing elution into the slurry.

According to the invention of claim 8, in the method for producing an exhaust gas purifying catalyst according to claim 5, step D of forming an intermediate layer is performed between step A and step C. Since it is provided, it is advantageous in protecting the zeolite from alkali metals or alkaline earth metals.

According to the ninth aspect of the invention, in the method for producing an exhaust gas purifying catalyst according to the eighth aspect, in the step D, the intermediate layer is formed of an oxide having a solid acid property. Therefore, it is further advantageous in protecting the zeolite from alkali metals or alkaline earth metals.

[0037]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings.

In FIG. 1, 1 is a spark ignition type engine of an automobile, 2 is its cylinder, 3 is a fuel injection valve for directly injecting fuel into a combustion chamber 4, 5 is an ignition plug, 6 is an intake passage, 7 Is an exhaust passage. An exhaust gas purifying catalyst 8 is arranged in the exhaust passage 7.

[First Embodiment] <Structure of Exhaust Gas Purifying Catalyst> Exhaust Gas Purifying Catalyst 8
As shown in FIG. 2, the inner catalyst layer 1 is formed on the surface of the carrier 11.
2, the outer catalyst layer 13, and the intermediate catalyst layer 14 disposed between the two catalyst layers 12 and 13 are formed in layers.

The carrier 11 is a honeycomb carrier made of cordierite. The inner catalyst layer 12 contains zeolite and a binder, and further contains Ag and Bi. Outer catalyst layer 1
3 is formed by a binder and a catalyst component in which Pt, Rh, Ba, Sr and Mg are supported on alumina and ceria which are support materials. Intermediate catalyst layer 1
No. 4 contains a catalyst component in which Pd is supported on alumina and ceria material as a support material, a binder, and further contains Ag and Bi.

The zeolite functions as an HC adsorbent, and β-type zeolite having a Cavan ratio of 300 is used. The component in which Pt and Rh are supported on the alumina and ceria materials works as a three-way catalyst. Above Ba, S
r and Mg have a high exhaust gas oxygen concentration (for example, 4%).
As described above, when the air-fuel ratio is A / F> 16), N in the exhaust gas is
N that adsorbs Ox and releases the adsorbed NOx when the oxygen concentration of the exhaust gas decreases (for example, below 2%)
Acts as an Ox adsorbent. Activated alumina powder (γ-alumina) is used as the alumina. The ceria material is an oxide containing a ceria component, and in this example, C
e · Pr complex oxide (Ce _0.9 Pr _0.1 O ₂ ) is used. This ceria material acts as an oxygen storage material (OSC). An alumina binder is used as a binder for the inner catalyst layer 12 and the intermediate catalyst layer 14, and a basic binder (zirconia) is used as a binder for the outer catalyst layer 13.

Therefore, the exhaust gas purifying catalyst 8 is
It can be said that it is a lean NOx adsorption catalyst having a C trap function.

<Manufacturing Method of Exhaust Gas Purifying Catalyst 8> Next, a manufacturing method of the exhaust gas purifying catalyst 8 will be described.

-Formation of Catalyst Powder- Catalyst powder A is formed by mixing activated alumina powder, the above ceria material, palladium nitrate and water, drying and firing at 500 ° C.

Catalyst powder B is formed by mixing activated alumina powder, the above ceria material, barium acetate, strontium acetate, magnesium acetate, dinitrodiamine platinum nitrate, rhodium nitrate and water, and drying and firing at 500 ° C. (Step B).

-Formation of Inner Catalyst Coat Layer- Zeolite and an alumina binder (hydrated alumina) are mixed, water is added thereto, and the mixture is stirred with a disperser to obtain a slurry. Immerse the honeycomb carrier 11 in this slurry,
By repeating the operation of pulling up and removing the excess slurry by air blow, a predetermined amount of the slurry is coated on the carrier. Thereafter, the inner catalyst coat layer is formed by drying and baking at 500 ° C. (step A).

-Formation of Intermediate Catalyst Coat Layer- Using the above catalyst powder A instead of the above zeolite, an intermediate catalyst coat layer is formed by the same method as the formation of the above inner catalyst coat layer (step D).

-Impregnation of Ag and Bi- The inner catalyst coating layer and the intermediate catalyst coating layer were impregnated with a mixed solution of silver nitrate and bismuth acetate, dried and dried.
Bake at 0 ° C.

-Formation of outer catalyst coat layer- Using the catalyst powder B in place of the zeolite and zirconia binder in place of the alumina binder, the outer catalyst coat layer was formed in the same manner as in the formation of the inner catalyst coat layer. Forming (step C).

Example An exhaust gas purifying catalyst was prepared by the above-described manufacturing method, in which the amount of each component carried in the inner catalyst layer 12, the outer catalyst layer 13 and the intermediate catalyst layer 14 was as follows. Inner catalyst layer 12; β-type zeolite = 100 g / L Outer catalyst layer 13; Pt = 3.5 g / L, Rh = 0.3 g
/ L, Ba = 30 g / L, Sr = 10 g / L, Mg = 1
0 g / L, alumina = 100 g / L, ceria material = 100
g / L Intermediate catalyst layer 14; Pd = 2.0 g / L, alumina = 30
g / L, ceria material = 10 g / L The impregnated and supported amount of Ag is 10 g / L, and the impregnated and supported amount of Bi is 0.5 g / L.

Further, Ba and Sr in the inner catalyst layer 12 are
The total supported amount of Mg and Mg is less than 1% of the total supported amount of the outer catalyst layer 13. This point was confirmed by the following experiments A and B.

The impurity in each catalyst layer is 1
% Or less. This point is the same in other Examples and Comparative Examples described later.

-Experiment A- 10 g of the above catalyst powder B and 3.8 of the zirconia binder were used.
50 g of water and 50 g of water were weighed and mixed, and the elution amount of Ba, Sr, and Mg from the catalyst powder B was quantitatively analyzed. The results are shown in Table 1. According to the same table, the total elution amount of Ba, Sr, and Mg combined is 2.4% of the total supported amount of the catalyst powder B, which is extremely small. In the catalyst powder B, Ba, Sr, and Mg are converted into carbonates by firing, but since the zirconia binder is basic and the mixed solution becomes basic, it is difficult for carbonates such as Ba to elute. . On the other hand, in the case of the alumina binder, the acetic acid component contained therein causes the solution when mixed with the catalyst powder B and water to be acidic, and carbonates such as Ba are easily eluted.

[0054]

[Table 1]

-Experiment B- A honeycomb having a capacity of 25 cc was immersed in water and pulled up, and the water was dropped by air blow to determine the water absorption amount of the honeycomb. In addition, after supporting 100 g / L of β-type zeolite on a honeycomb having a capacity of 25 cc, the same water absorption treatment is performed,
The water absorption of the zeolite-supported honeycomb was determined. The results are shown in Table 2.

[0056]

[Table 2]

From the results shown in Table 2, the water absorption of the honeycomb was 2.0.
The water absorption of β-zeolite at 0.4 g is 0.4 g (2.4
g-2.0g = 0.4g). That is, the ratio of water absorption is zeolite / honeycomb = 1/5. Therefore, the total supported amount of Ba, Sr, and Mg on the β-type zeolite is 0.48% (2.4% × 1/5) of the total supported amount of the catalyst powder B.
= 0.48%).

Therefore, Ba and S in the inner catalyst layer 12 are
The total supported amount of r and Mg is less than 1% of the total supported amount of the outer catalyst layer 13.

<Evaluation Test> The above-mentioned catalysts were subjected to HC purification performances at the time of freshness and after aging at 800 ° C. for 24 hours in the air atmosphere.
That is, the HC adsorption rate, the HC oxidation rate (the rate of oxidation of adsorbed HC), the HC purification rate (HC adsorption rate × HC oxidation rate), and the lean NOx purification rate were examined by a rig test.

In the HC purification performance evaluation mode, the temperature of the catalyst gas at the catalyst inlet is raised to 80 ° C. in the N ₂ stream and the temperature is maintained at that temperature while HC (benzene) is maintained.
Is 1500 ppmC, NO is 100 ppm, and O ₂ is 1.
HC, NO and O ₂ gases were introduced into the N ₂ gas for 2 minutes so as to be 0%, after which only the introduction of HC gas was stopped and the catalyst inlet gas temperature was changed from 80 ° C. to 400 ° C.
The temperature is raised at a rate of ° C / min. The space velocity SV was set to 25000 h ^-1 .

The HC adsorption rate was calculated based on the catalyst inlet HC concentration and the catalyst outlet HC concentration for the above two minutes. H
The C oxidation rate was calculated based on the amount of HC adsorbed on the catalyst in the above 2 minutes and the concentration of HC at the catalyst outlet at the time of the temperature rise.

Further, a simulated exhaust gas having a lean air-fuel ratio (gas composition A) is flown through the catalyst for 60 seconds, and then a gas composition is switched to flow a simulated exhaust gas rich in air-fuel ratio (gas composition B) for 60 seconds. After repeating the cycle of 5 times, the gas composition was switched to the air-fuel ratio lean (gas composition A), the NOx purification rate for 60 seconds from this switching time was measured, and this was taken as the lean NOx purification rate. The catalyst temperature and the simulated exhaust gas temperature were 350 ° C., the gas composition was as shown in Table 3, and the space velocity SV was 25000 h ⁻¹ .

[0063]

[Table 3]

The results are shown in FIG. There is little decrease in the HC adsorption rate due to aging. On the other hand, although the HC oxidation rate is lowered due to aging, it is not so large. The lean NOx purification rate is slightly lowered due to aging. Since the decrease in the HC adsorption rate is small,
It can be seen that the β-type zeolite in the inner catalyst layer is hardly destroyed by aging, that is, its specific surface area is hardly reduced.

Bi impregnated in the intermediate catalyst layer 14 is Pd.
It exists as the atom closest to the atom, and Pd and Ag react with each other to form a Pd-Ag alloy, that is, Pd.
It is possible to prevent the low temperature activity for HC purification from decreasing, or to reduce the amount of Ag effective for improving the HC adsorption performance.

[Embodiment 2] As shown in FIG. 4, the exhaust gas purifying catalyst 8 of this embodiment comprises a carrier 11 having an inner catalyst layer 12 and an outer catalyst layer 13 formed in layers. The carrier 11 and the inner catalyst layer 12 are the same as those of the first embodiment. The outer catalyst layer 13 is made of Pt, Rh, Pd, Ba, S.
It is formed by a binder and a catalyst component in which r and Mg are supported on alumina and ceria which are support materials. That is, unlike the first embodiment, there is no intermediate catalyst layer, and Pd is contained in the outer catalyst layer 13. Other configurations are the same as those in the first embodiment.

Next, a method for manufacturing the exhaust gas purifying catalyst 8 will be described.

Activated alumina powder, the above ceria material, barium acetate, strontium acetate, magnesium acetate, dinitrodiamine platinum nitrate, rhodium nitrate, palladium nitrate and water are mixed, dried and calcined at 500 ° C. to obtain the catalyst powder. Form C.

The inner catalyst coat layer is formed in the same manner as in the first embodiment. The inner catalyst coating layer is impregnated with a mixed solution of silver nitrate and bismuth acetate, dried and baked at 500 ° C. Thereafter, the catalyst powder C is used to form the outer catalyst coat layer in the same manner as in the first embodiment.

Example An exhaust gas purifying catalyst was prepared by the above-described manufacturing method, in which the amount of each component carried in the inner catalyst layer 12 and the outer catalyst layer 13 was as follows. Inner catalyst layer 12; β-type zeolite = 100 g / L Outer catalyst layer 13; Pt = 3.5 g / L, Rh = 0.3 g
/ L, Pd = 2.0 g / L, Ba = 30 g / L, Sr =
10 g / L, Mg = 10 g / L, alumina = 150 g /
L, ceria material = 150 g / L The impregnated and supported amount of Ag in the inner catalyst layer 12 is 10 g /
The impregnated and supported amount of L and Bi is 0.5 g / L. The total supported amount of Ba, Sr, and Mg in the inner catalyst layer 12 is less than 1% of the total supported amount of the outer catalyst layer 13.

<Comparative Example> The same inner catalyst coat layer was formed on the same carrier as in the above-mentioned example, and Ag and Bi were similarly impregnated and supported. Next, a slurry prepared by mixing activated alumina powder, the above ceria material, alumina binder and water is prepared, coated on the inner catalyst coating layer, dried and calcined at 500 ° C. to coat the outer catalyst. Layers were formed. Next, both the inner and outer catalyst coat layers are impregnated with a mixed aqueous solution of barium acetate, strontium acetate, magnesium acetate, dinitrodiamine platinum nitrate, rhodium nitrate and palladium nitrate, dried and baked at 500 ° C. A comparative catalyst was obtained. The amount of each component carried is as follows. Inner catalyst layer 12; β-type zeolite = 100 g / L Outer catalyst layer 13; alumina = 150 g / L, ceria material =
150 g / L The impregnated and supported amount of Ag of the inner catalyst layer 12 was 10 g / L, Bi
The impregnated and supported amount of is 0.5 g / L. Further, each noble metal and each NO in which the inner catalyst layer 12 and the outer catalyst layer 13 are combined.
The loading amount of the x adsorbent is Pt = 3.5 g / L and Rh = 0.
3 g / L, Pd = 2.0 g / L, Ba = 30 g / L, S
r = 10 g / L and Mg = 10 g / L.

<Comparative Test> For both the catalysts of the above-mentioned Examples and Comparative Examples, the HC purification performance after aging and the lean NOx purification performance were examined by the same method as in the first embodiment. Results are shown in FIG. According to this figure, although there is almost no difference in the lean NOx purification rate, the HC adsorption rate is much higher in the embodiment. This is considered to be because, in the case of the example, the structural destruction of the zeolite of the inner catalyst layer 12 or the decrease of the specific surface area was small even after aging.

The HC oxidation rate is also significantly higher in the example. In the case of the comparative example, it is considered that noble metals such as Pt are impregnated and supported on the zeolite, but the sintering causes the destruction of the zeolite. That is, in the case of the embodiment, the precious metal is the outer catalyst layer 1.
It is considered that it is supported on the alumina and ceria material of No. 3, and there is no problem of activity reduction due to structural destruction of zeolite.

<Effect of NOx adsorbent loading on specific surface area of zeolite> FIG. 6 shows M for 100 g of β-type zeolite.
The BET specific surface area after aging was measured and compared with the sample carrying 0.2 mol of each of g, Ca, Sr, and Ba and the sample carrying 0.05 mol of each.

From the figure, Mg, Ca, Sr, Ba (NO
When the amount of x adsorbent) loaded on the zeolite is large, the structural destruction of the zeolite progresses due to aging, and its specific surface area decreases. Therefore, as in the present invention, the amount of the NO x storage material loaded on the zeolite can be increased. It can be seen that it is effective to reduce the HC purification performance.

<Regarding Lean Burn Engine Control> The exhaust gas purifying catalyst according to the above-described embodiment uses NO such as Ba.
It contains x adsorbent and adsorbs NOx in the exhaust gas when the air-fuel ratio is lean. Therefore, when the NOx adsorption amount becomes large, it is necessary to make the air-fuel ratio rich (to reduce the oxygen concentration of the exhaust gas) and release it for reduction purification. Further, the NOx adsorbent has a problem of so-called sulfur poisoning in which the NOx adsorbent loses its function by adsorbing sulfur contained in the exhaust gas in the form of sulfur oxide to form a salt. Therefore, in that case, it is necessary to regenerate by raising the temperature of the catalyst. Hereinafter, fuel injection control for NOx release and regeneration from sulfur poisoning will be described.

As shown in FIG. 7, in step S1 after the start, various data such as the intake air amount of the engine, the engine speed, the accelerator opening, and the catalyst temperature are input. Then, in step S2, an appropriate target air-fuel ratio is set according to the operating state of the engine from various data,
The basic fuel injection amount Qpb is set.

In the subsequent step S3, NO of the catalyst concerned
The x adsorption amount NOe is estimated, and in the subsequent step S4, the SOx (sulfur oxide) adsorption amount SOe of the catalyst is estimated. When the air-fuel ratio is lean, refer to the map created by previously obtaining the NOx adsorption amount and the SOx adsorption amount in each engine operating state (engine speed and engine load) by experiment, and from the engine operating state at that time to the NOx adsorption amount And SOx adsorption amount are calculated, and the NOx adsorption amount NOe and the SOx adsorption amount SOe are estimated by accumulating these. When the air-fuel ratio is rich (excess air ratio λ ≦ 1), the smaller the λ and the longer the rich time is, the larger the subtraction constant is, the smaller the NOx adsorption amount NOe and the SOx adsorption amount SOe are reduced.

In the subsequent step S5, the NOx adsorption amount N
It is determined whether or not Oe is larger than a predetermined threshold value NOo (NOx adsorption amount that is considered to have increased to the extent that NOx release control should be performed (NOx adsorption capacity decreases)). When NOe> NOo, the routine proceeds to step S6, where it is determined whether or not the previous value of the counter TN is zero. This counter T
N corresponds to the time during which the fuel injection amount increase control is being performed.

When the previous value of the counter TN is zero, the process proceeds to step S7 to set the counter threshold value TNo, and then proceeds to step S8 to increment the counter TN.
When the previous value of TN is not zero, the process directly proceeds from step S6 to S8 to increment the counter TN. The threshold value TNo is set to a small value in a temperature range where NOx is easily released (for example, 200 to 400 ° C.) and a large value in other temperature ranges based on the current catalyst temperature. Threshold TNo
May be set within a range of about 0.5 to 5 seconds in real time.

In the following step S9, it is determined whether or not the counter TN is larger than the threshold value TNo. Counter T
If N is equal to or less than the threshold value TNo, the routine proceeds to step S10, where the rich fuel injection amount Qpλ is given as the fuel injection amount Qpb, and this is set as the fuel injection amount Qp, and fuel injection is executed at a predetermined crank timing ( Steps S11, S
12). In step S9, counter TN> threshold TN
If it is o, the process proceeds to step S13, the counter TN and the estimated value NOe of the NOx adsorption amount are reset, and the fuel injection is executed at the basic fuel injection amount Qpb.

On the other hand, when the estimated value NOe of the NOx adsorption amount is equal to or less than the threshold value NO in step S5, step S5
The process proceeds to step 14 to determine whether the counter TN is being counted. If counting is in progress, the process proceeds to step S8 to continue the NOx release control. If not counting, step S1
In step 5, it is determined whether the estimated value SOe of the SOx adsorption amount is larger than a predetermined threshold value SOo (SOx adsorption amount that is considered to have been poisoned by sulfur enough to perform SOx release control). When SOe is equal to or lower than SOo, the routine proceeds to step S11, where fuel injection is executed with the basic fuel injection amount Qpb (steps S11 and S12).

When the estimated value SOe of the SOx adsorption amount is larger than SOo, the routine proceeds to step S16, where it is determined whether or not the previous value of the counter TS is zero. This counter TS corresponds to the time during which the fuel injection amount increase control is being performed. If the previous value of TS is zero, the process proceeds to step S17 to set the counter threshold value TSo, and then the process proceeds to step S18 to increment the counter TS. If the previous value of TS is not zero, the process directly proceeds from step S16 to S18. The counter TS is incremented. Threshold TSo
Is a predetermined value (for example, 400
C.) or higher, the higher the catalyst temperature, the smaller the value is set. The threshold value TSo may be set within a range of about 1 to 10 minutes in real time.

In the following step S19, the counter TS
Is greater than the threshold TSo. If the counter TS is larger than the threshold value TSo, step S10.
And the fuel injection amount Qpb for rich is set as the fuel injection amount Qpb.
λ is given, and this is set as the fuel injection amount Qp, and fuel injection is executed when a predetermined crank timing is reached (step S1).
1, S12). In step S19, counter TS>
If it is the threshold value TSo, the process proceeds to step S20, the counter TS, the estimated value SOe of the SOx adsorption amount and the estimated value NOe of the NOx adsorption amount are reset, and the fuel injection is executed at the basic fuel injection amount Qpb (steps S11 and S12). .

As described above, when the NOx adsorption amount of the NOx adsorbent increases, the fuel injection amount is increased and the air-fuel ratio is enriched even when the engine is in the lean air-fuel ratio operating state. Since the oxygen concentration of the exhaust gas decreases, NOx is released from the NOx adsorbent. The released NOx is reduced and purified by the noble metal catalyst in the outer catalyst layer 13.

Further, when the degree of sulfur poisoning of the NOx adsorbent becomes high, the fuel injection amount is also increased to enrich the air-fuel ratio, thereby raising the exhaust gas temperature and oxidizing the catalyst. Is activated and the catalyst temperature rises, so SOx is desorbed from the NOx adsorbent. Therefore, the NOx adsorption capacity of the NOx adsorbent can be regenerated.

When sulfur is poisoned, the air-fuel ratio may be made rich, and the exhaust gas temperature may be further increased by retarding the ignition timing. Further, the secondary air may be supplied to the catalyst upstream portion to cause the catalyst to rise. It is also possible to promote the oxidation reaction in step 1 to further raise the temperature of the catalyst.

[Brief description of drawings]

FIG. 1 is a diagram showing an exhaust gas purifying apparatus for an engine according to the present invention.

FIG. 2 is a cross-sectional view showing the layer structure of the exhaust gas purifying catalyst according to the first embodiment of the present invention.

FIG. 3 is a graph showing the HC adsorption rate, the HC oxidation rate, the total HC purification rate, and the NOx purification rate at the time of freshness and after aging in the example of the first embodiment.

FIG. 4 is a sectional view showing a layer structure of an exhaust gas purifying catalyst according to a second embodiment of the present invention.

FIG. 5 shows the HC adsorption rate after aging, the HC oxidation rate, the total HC purification rate of the example and the comparative example of the second embodiment,
A graph showing the NOx purification rate.

FIG. 6 Mg, Ca, Sr and Ba to β-type zeolite
FIG. 6 is a graph showing the relationship between the amount of supported A and the BET specific surface area of the zeolite after aging.

FIG. 7 is a flowchart showing fuel injection control of the engine according to the present invention.

FIG. 8 is a graph showing specific surface areas of β-zeolite carrying various alkali metals or alkaline earth metals and β-zeolites not supporting these metals after aging.

[Explanation of symbols]

1 engine Two cylinder 3 Fuel injection valve 4 Combustion chamber 5 spark plugs 6 Intake passage 7 exhaust passage 8 Exhaust gas purification catalyst 11 Carrier 12 Inner catalyst layer 13 Outer catalyst layer 14 Intermediate catalyst layer

─────────────────────────────────────────────────── ─── Continued Front Page (51) Int.Cl. ⁷ Identification Code FI Theme Coat (Reference) B01J 37/02 F01N 3/10 A F01N 3/08 Z 3/10 3/24 E 3/28 301C 3 / 24 B01D 53/36 104A 3/28 301 ZAB 102H (72) Inventor Keiji Yamada No. 3 Shinchi, Fuchu-cho, Aki-gun, Hiroshima Prefecture Mazda Co., Ltd. (72) Kenji Okamoto No. 3 Shinchi, Fuchu-cho, Aki-gun, Hiroshima Prefecture No. 1 F term in Mazda Motor Corporation (Reference) 3G091 AA02 AA12 AA17 AA24 AA28 AB06 AB09 AB10 BA10 BA11 BA14 BA15 CA22 CB02 CB03 CB05 DA01 DA02 DA04 DB05 DB10 EA01 EA05 EA07 EA18 FB18 GB04 GB04 GB04W04 GB02W04 GB02W04 GB02W02 GB01X02 GB02W02 GB01X GB05W GB06W GB09W GB09X GB09Y GB10W GB10X GB16X GB17X HA18 HA39 HB07 4D048 AA06 AA13 AA18 AB05 AB07 BA01X BA02Y BA03X BA06X BA07X BA08X BA11X BA14Y BA15X BA19X BA22X BA30X BA31X BA33X BA34X BA41X BB02 BC01 BD01 CC36 CC46 EA04 4G069 AA03 AA08 BA01A BA02A BA04A BC07 FA BB04 BCEE BC07 BC07BC BB04 BCEE BC07 BC07B BB04A BC07 BC07B BB04A BC07 BC07B BB04A BC07 BC07B BB04A BC07 BC07B BB04A BC07 BC07B BB04A BC07 BC07B A BCEA BC07B BB04A BC07 BC07B A BCEA BC07A FA03 FB14 FB15 FB19 FB20 FB23 FB30 ZA19B

Claims (9)

[Claims] 1. A carrier surface is provided with a first layer containing zeolite and a second layer containing at least one metal selected from alkali metals and alkaline earth metals, wherein the first layer contains The exhaust gas purifying catalyst, wherein the metal content is less than 1% of the metal content in the second layer. 2. The exhaust gas purifying catalyst according to claim 1, wherein the first layer is arranged inside the second layer. 3. The exhaust gas purifying catalyst according to claim 1, wherein an intermediate layer that prevents movement of the metal from the second layer to the first layer is provided between the first layer and the second layer. An exhaust gas purifying catalyst characterized by being provided with. 4. The exhaust gas purifying catalyst according to claim 3, wherein the intermediate layer contains an oxide having a property of a solid acid. 5. An exhaust gas purifying catalyst comprising, on the surface of a carrier, a first layer containing zeolite and a second layer containing at least one metal selected from alkali metals and alkaline earth metals. In order to form the first layer, a step A of coating the zeolite with the carrier, a step B of supporting the metal on a support material, and a step of forming the second layer. And a step C of coating the metal-supporting material on the carrier, wherein the step A is carried out prior to the step C. 6. The method for producing an exhaust gas purifying catalyst according to claim 5, wherein in the step B, the supporting material is loaded with the metal and then firing is performed, and in the step C, the firing is performed. A method for producing an exhaust gas purifying catalyst, characterized in that the metal-supporting material described above is mixed with a basic binder to form a slurry, and the second layer is formed by coating the slurry. 7. The method for producing an exhaust gas purifying catalyst according to claim 6, wherein the basic binder is a zirconia binder. 8. The method for producing an exhaust gas purifying catalyst according to claim 5, further comprising a step D for forming an intermediate layer between the steps A and C. Exhaust gas purification catalyst manufacturing method. 9. The method for producing an exhaust gas purifying catalyst according to claim 8, wherein in step D, the intermediate layer is formed of an oxide having a property of a solid acid. For producing catalyst for automobile.