JP2003093886A - Catalyst and method for preparing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a catalyst having an inner catalyst layer 12 containing zeolite and an outer catalyst layer 13 containing a carbonate of a catalyst metal formed on a carrier 11, which carries Ag on the inner catalyst layer 12 without carrying Ag on the outer catalyst 13. SOLUTION: The method comprises steps of supporting zeolite on the carrier 11, immersing the zeolite in an aqueous solution of silver nitrate and then coating slurry containing a catalyst material for forming the outer catalyst layer 13 on the inner catalyst layer 12.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a 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.

It is also known that supporting Ag on zeolite improves its HC adsorption performance.

[0006]

However, according to the research by the present inventor, in order to improve the HC adsorption performance of zeolite,
In some cases, even if g was supported by the impregnation method, its HC adsorption performance did not improve as expected. That is, it is a case in which Ag is impregnated and supported after the inner layer containing the zeolite as described above and the outer layer containing the alkali metal or alkaline earth metal are formed in layers on the carrier. In this case, it is considered that Ag is supported not only on the outer layer but also on the zeolite in the inner layer, but from the viewpoint of HC adsorption performance, etc. It was not supported.

An object of the present invention is to provide a catalyst having a layered structure having such an inner layer and an outer layer, in which A is formed in the inner layer.
The purpose is to reliably support a catalytic metal such as g.

[0008]

Means for Solving the Problems As a result of research to solve the above-mentioned problems, the present inventor has found that the reason why Ag was not supported as expected on the zeolite in the inner layer was that when impregnated with an Ag solution, It was found that the Ag formed a sparingly soluble salt in the outer layer, and completed the present invention.

That is, according to the present invention, an inner catalyst layer containing, on a carrier, a first catalyst metal having a property of forming a sparingly soluble carbonate in water, and an outer side containing a carbonate of a second catalyst metal. A method for producing a catalyst comprising a catalyst layer, the method comprising: a step of supporting a support material for forming the inner catalyst layer on the carrier; and a step of adding the first catalyst metal to the support material after the step A. Step B of impregnating the solution
And step C of coating the slurry containing the catalyst material for forming the outer catalyst layer after step B
It is characterized by having and.

Therefore, although the first catalytic metal has the property of forming a sparingly soluble carbonate, the support material is not brought into contact with the solution of the first catalytic metal with the carbonate of the second catalytic metal. Since it can be impregnated and supported on the substrate, it is possible to avoid the occurrence of carbonate of the first catalytic metal and insufficient impregnation of the support material. In addition, since the first catalytic metal is impregnated and supported by the support material, when the slurry containing the catalytic material for forming the outer catalyst layer is coated, the first catalytic metal is eluted into the slurry and carried on the inner catalyst layer. Can be avoided.

Regarding step C, the second
Even if the above-mentioned coating is carried out in the state that the catalyst metal is supported on the support material, after the support material not carrying the second catalyst metal is coated, the second catalyst metal is applied to the support material by an impregnation method. You may make it carry | support.

When Ag is adopted as the first catalytic metal, this Ag has a property of forming a carbonate which is hardly soluble in water. However, by adopting the method for producing a catalyst according to the present invention, Ag can be efficiently included in the catalyst layer. Then, as a support material for the inner catalyst layer, β
If a zeolite such as type zeolite or MFI is adopted, the catalyst can be used for exhaust gas purification to enhance the HC adsorption performance. The Ag may be impregnated into the support material as an aqueous silver nitrate solution.

As the second catalytic metal, at least one metal selected from alkali metals and alkaline earth metals can be adopted. That is, alkali metals and alkaline earth metals generate carbonates by firing the catalyst in the air atmosphere, but the first catalyst metal is loaded on the support material of the inner catalyst layer before the alkali metals and alkaline earth metals. Therefore, there is no problem that the first catalytic metal forms a carbonate and impregnates insufficiently. Then, if the obtained catalyst is used for purifying exhaust gas, the alkali metal or alkaline earth metal catalyst functions as a NOx adsorbent.

Further, in the step B, the support material may be impregnated with a solution for mixing an aqueous solution of silver nitrate and another solution, for example, an aqueous solution of bismuth acetate.

Further, according to the present invention, the carrier has an inner catalyst layer containing a first catalyst metal having a property of forming a sparingly soluble carbonate in water and an outer catalyst layer containing a carbonate of a second catalyst metal. A catalyst having a catalyst layer formed thereon, wherein a layer containing a support material for the first catalyst metal is formed on the carrier, and the first catalyst metal is impregnated and supported on the layer to form the inner catalyst layer. The outer catalyst layer is formed after the inner catalyst layer is formed.

Therefore, in this catalyst, the first catalytic metal and the second catalytic metal work effectively in the inner catalytic layer and the outer catalytic layer, respectively.

[0017]

As described above, according to the present invention, an inner catalyst layer containing a first catalytic metal having a property of forming a sparingly soluble carbonate and an outer catalyst containing a carbonate of a second catalytic metal. In manufacturing a catalyst having a layer, a step A of supporting a support material for forming the inner catalyst layer on a carrier, and the step of forming the first support material on the support material after the step A.
Since the method includes the step B of impregnating the catalyst metal solution and the step C of coating the slurry containing the catalyst material for forming the outer catalyst layer after the step B, the first catalyst metal is added to the inner catalyst layer. It can be reliably supported on the support material, which is advantageous in obtaining a catalyst having desired performance.

Further, the catalyst according to the present invention has an inner catalyst layer containing a first catalytic metal having a property of forming a sparingly soluble carbonate and an outer catalyst layer containing a carbonate of a second catalytic metal. The inner catalyst layer is formed by impregnating and supporting the first catalyst metal on the support material layer formed on the carrier, and the outer catalyst layer is formed after the inner catalyst layer is formed. The catalyst metal and the second catalyst metal work effectively in the inner catalyst layer and the outer catalyst layer, respectively.

[0019]

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 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 above 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, and 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. To do.

-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 above inner catalyst coat layer.

-Impregnation with Ag and Bi- The inner catalyst coat layer and the intermediate catalyst coat layer are impregnated with a mixed solution of silver nitrate and bismuth acetate, dried and dried.
Baking at 0 ° C. is performed (step B).

-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 is formed by the same method as in forming 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.

In addition, Ba and Sr in the inner catalyst layer 12
The total supported amount of Mg and Mg is less than 1% of the total supported amount of the outer catalyst layer 13.

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

<Evaluation test> The above-mentioned catalysts were subjected to HC purification performance 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 test catalyst was heated to 80 ° C. at the catalyst inlet gas temperature in an N ₂ gas stream, and was kept at that temperature while HC (benzene) was 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 to 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 1, and the space velocity SV was 25000 h ⁻¹ .

[0039]

[Table 1]

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.

Further, a high HC adsorption rate even after aging and a relatively high value even though the HC oxidation rate is low are shown in that the substantially entire amount of impregnated Ag is the inner catalyst layer. It is considered that the carrier is carried by No. 12. That is, since the outer catalyst layer 13 is formed after impregnating and supporting Ag on the inner catalyst layer 12, in other words, the inner catalyst layer 1 is passed through the outer catalyst layer 13.
Since 2 was not impregnated with Ag, Ag could be reliably supported on the inner catalyst layer 12 (this point will be described in detail in the second embodiment below).

Further, since the slurry for the outer catalyst layer is coated with Ag impregnated and supported on the inner catalyst layer 12 and fixed to the zeolite by firing, the Ag of the inner catalyst layer 12 is coated at the time of coating. It is considered that the amount of slag dissolved in the slurry for the outer catalyst layer also decreased.

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 the present embodiment comprises an inner catalyst layer 12 and an outer catalyst layer 13 formed in layers on the surface of a carrier 11. 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 producing 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 Embodiment 1 (step A). The inner catalyst coating layer is impregnated with a mixed solution of silver nitrate and bismuth acetate, dried and baked at 500 ° C. (step B). Then, the catalyst powder C is used to form an outer catalyst coat layer in the same manner as in Embodiment 1 (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 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 example. Next, an outer catalyst coat layer was formed on the inner catalyst coat layer by using the catalyst powder C in the same manner as in the above example. Thereafter, a mixed solution of silver nitrate and bismuth acetate was impregnated from the surface of the outer catalyst coating layer, dried and calcined at 500 ° C. to obtain a comparative catalyst. The amount of each component carried is 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 is 10 g / L, and the impregnated and supported amount of Bi is 0.5 g / L. In addition, B in the inner catalyst layer 12
The total supported amount of a, Sr, and Mg is the outer catalyst layer 13
Of less than 1% of the total supported amount.

<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 the figure, although there is almost no difference in the lean NOx purification rate, the HC adsorption rate and the HC oxidation rate are significantly higher in the embodiment. This is because in the case of the example, substantially all of the impregnated Ag is supported on the zeolite of the inner catalyst layer 12, whereas in the comparative example, the impregnated Ag stops at the outer catalyst layer 13 It is considered that 12 is impregnated with only a small amount of Ag, and the amount of Ag contributing to the improvement of the HC adsorption performance of the zeolite is small.

That is, in the case of the comparative example, the outer catalyst layer 13
Ba, Sr, and Mg are converted into carbonates by firing. When the inner catalyst layer 12 is impregnated with a silver nitrate aqueous solution from the surface of the outer catalyst layer 13, taking Ba as an example,
The next reaction causes silver carbonate, which is hardly soluble in water, to easily remain in the outer catalyst layer 13. Therefore, the inner catalyst layer 12
It is considered that the amount of Ag penetrating into the steel has decreased.

AgNO ₃ + BaCO ₃ → Ag ⁺ + Ba
^{_{2+ + NO 3 - + CO 3}} - 2Ag + + CO 3 - → Ag 2 CO 3 ↓ In addition, such reactions by eluting Ba, Sr and Mg are moved from the outer catalyst layer 13 on the inner catalytic layer 12, the catalyst is a high temperature Promotes structural breakdown of zeolites when exposed to. This is also considered to be the reason why the HC adsorption rate and the HC oxidation rate of the comparative example are lower than those of the examples.

Further, when a mixed solution of silver nitrate and bismuth acetate is subjected to impregnation as in the above embodiment, when this mixed solution comes into contact with the outer catalyst layer 13, the following reaction causes a silver acetate which is hardly soluble in water. Is easily generated and easily stops at the outer catalyst layer 13. This is also considered to be the reason why the amount of Ag penetrating into the inner catalyst layer 12 decreases.

Ag ⁺ + Bi (CH ₃ COO) ₃ → AgC
H ₃ COO ↓ + Bi ³⁺ <Regarding lean burn engine control> The exhaust gas purifying catalyst according to the above embodiment contains a NOx adsorbent such as Ba 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. 6, 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 procedure 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 a succeeding 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 adsorbing amount of the NOx adsorbing material increases, the fuel injection amount is increased and the air-fuel ratio is enriched even when the engine is operating in the lean air-fuel ratio. 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 and the air-fuel ratio is made rich, so that the exhaust gas temperature rises and the oxidation reaction in the catalyst also occurs. 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 upstream portion of the catalyst 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 is a flowchart showing fuel injection control of the engine according to the present invention.

[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

Front page continuation (51) Int.Cl. ⁷ Identification code FI theme code (reference) B01J 37/02 B01D 53/36 104A F01N 3/10 ZAB 102H (72) Inventor Keiji Yamada 3 Shinchi, Fuchu-cho, Aki-gun, Hiroshima Prefecture No. 1 in Mazda Co., Ltd. (72) Inventor Kenji Okamoto No. 3 Shinchi, Fuchu-cho, Aki-gun, Hiroshima Prefecture F-term in Mazda Co., Ltd. (reference) 3G091 AA02 AA17 AA24 AB09 AB10 BA39 GA06 GB01W GB02W GB03W GB05W GB09X GB10X GB17X 4D048 AA06 AA13 AA18 AB05 AB07 BA01X BA02Y BA03X BA11X BA14Y BA15X BA19X BA22X BA30X BA31X BA33X BA34X BA41X BB02 BC01 BD01 CC36 CC46 EA04 4G069 AA03 AA08 BA01B BA07B BB04B BB12C BC01A BC08A BC10B BC12B BC13B BC25A BC25B BC25C BC32A BC32B BC32C BC43B BC71B BC72B BC75B BE08C CA03 CA09 DA06 EA19 EE01 EE06 EE08 FA01 FA02 FA03 FB14 FB15 FB16 FB19 FB20 FB23 FC02 ZA19B

Claims (6)

[Claims] 1. An inner catalyst layer containing a first catalyst metal having a property of forming a sparingly soluble carbonate in water, and an outer catalyst layer containing a carbonate of a second catalyst metal on a carrier. A method for producing a formed catalyst, which comprises a step A of supporting a support material for forming the inner catalyst layer on the carrier, and a solution of the first catalyst metal on the support material after the step A. A method for producing a catalyst comprising: an impregnating step B; and a step C of coating the slurry containing the catalyst material for forming the outer catalyst layer after the step B. 2. The method for producing a catalyst according to claim 1, wherein the first catalytic metal is Ag. 3. The method for producing a catalyst according to claim 1 or 2, wherein the second catalytic metal is at least one metal selected from alkali metals and alkaline earth metals. A method for producing a catalyst, comprising: 4. The method for producing a catalyst according to claim 2, wherein in step B, the support material is impregnated with Ag as an aqueous silver nitrate solution. 5. The method for producing a catalyst according to claim 4, wherein in step B, the support material is impregnated with a liquid mixture of an aqueous silver nitrate solution and an aqueous bismuth acetate solution. Method. 6. An inner catalyst layer containing a first catalyst metal having a property of forming a sparingly soluble carbonate salt in water, and an outer catalyst layer containing a carbonate salt of a second catalyst metal on a carrier. The formed catalyst, wherein a layer containing a support material of the first catalytic metal is formed on the carrier, and the first catalytic metal is impregnated and supported on the layer to form the inner catalytic layer, A catalyst, wherein the outer catalyst layer is formed after the inner catalyst layer is formed.