JP2002361083A - Exhaust gas purification catalyst for internal combustion engine, method for producing the same, and purification device - Google Patents

Abstract

(57) [Summary] An alkali metal or an alkaline earth metal is used as a NOx supplementary component, and these components melt under the influence of high-temperature exhaust gas to move through a catalyst layer, and cause the NOx in the cogelato base. There is a problem that the catalyst is easily bonded to silica, and the catalytic activity characteristics are deteriorated. An object of the present invention is to provide a catalyst in which the movement of a catalytically active component is suppressed. A cordierite honeycomb, a carrier layer on which a slurry containing activated carbon is coated on the honeycomb, dried and fired to form microcavities (physical barriers) and carry a catalytically active component; There is a feature in the exhaust gas purification device composed of the above. Activated carbon is added to an alumina slurry composed of γ-alumina, nitric acid, an alumina precursor, and purified water, and the slurry to which the activated carbon is added is coated on cordierite honeycomb, dried and fired.
The method is characterized by impregnating and firing a NOx supplement component solution containing a and K as main components, and impregnating and firing a noble metal solution, for example, a dinitrodiammine Pt solution, a dinitrodiammine Pd solution, and a nitric acid Rh solution.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to carbon monoxide (C) contained in gas discharged from an internal combustion engine of an automobile or the like.
The present invention relates to a catalyst for purifying O), hydrocarbons (HC), and nitrogen oxides (NOx), and particularly relates to a catalyst capable of maintaining high purification performance and an exhaust gas purification device using the same.

[0002]

2. Description of the Related Art A lean-burn engine, a DI (Direct-Injection) engine operated with lean combustion, and a diesel engine are operated in a state in which there is more air than fuel.
Exhaust gas contains oxygen in excess. Therefore, in a three-way catalyst generally used for purifying exhaust gas in an automobile gasoline engine, HC and CO contained in exhaust gas from a lean burn engine, a DI engine and a diesel engine can be purified, but NOx is reduced. It cannot be purified.

For this reason, a catalyst (lean NOx catalyst) for purifying NOx in exhaust gas containing oxygen effectively has been developed. A catalyst for once trapping NOx in an adsorbent or the like, and then reducing and purifying the trapped NOx. Is being put to practical use.

[0004] For example, there is JP-A-2000-342967. This publication describes that an alumina carrier and TiO2 are mixed and heat-treated to suppress the reaction between the carrier and the NOx trapping component, thereby suppressing grain growth of the noble metal.

[0005] Also, there is Japanese Patent Application Laid-Open No. 10-358.
This publication describes that the heat resistance of a noble metal is improved by forming a heat-resistant compound composed of platinum, an alkaline earth element, and a Group 3A element.

Further, there is JP-A-7-108172. This publication describes a dual structure of an alumina layer supporting an alkaline earth (first supporting layer) and an alumina layer supporting a noble metal (second supporting layer). Thereby, direct contact between the alkaline earth metal and the noble metal catalyst is avoided, and a decrease in the activity of the noble metal catalyst is prevented. As a result, the movement of the NOx trapping component is suppressed, and the heat resistance is improved.

[0007] However, any of these methods has a disadvantage that the cost is increased due to the complicated production method, and the practicality is reduced.

[0008]

SUMMARY OF THE INVENTION Generally, lean NOx
BACKGROUND ART Automobile exhaust gas purifying catalysts such as catalysts and three-way catalysts mainly comprise a carrier layer on the surface of a substrate having a monolith structure, and a catalytically active component is dispersed in the carrier layer. The term "catalytically active component" as used herein refers to a component that is carried in the pores of the carrier and participates in exhaust gas purification.

The carrier is for dispersing and holding the catalytically active component, and various metal oxides or composite oxides are used. Alumina or a material containing alumina as a main component is relatively frequently used. You. Cordierite is used as the material of the monolith structure. This is because cordierite has excellent thermal shock resistance.

One of the important characteristics required for a catalyst used in an exhaust gas purification device for automobiles is a characteristic that can withstand long-term use, that is, a so-called thermal durability. The catalyst is required to be capable of purifying high-temperature exhaust gas and enduring long-term use while maintaining a high purification rate. Lean N
The NOx trapping component contained in the Ox catalyst is composed mainly of an alkali metal such as Na and K and an alkaline earth metal such as Sr and Ba. However, there is a problem that these components are softened and melted due to their low melting point, the surface area is reduced by grain growth, and the activity is reduced by reacting with a carrier such as alumina.

[0011] Further, these components are melted under the influence of high-temperature exhaust gas, move in the catalyst layer, and cover the catalyst components such as noble metals, thereby reducing the exposed area.
Catalyst activity is reduced. In addition, these components
It easily binds to the silica in the cordierite substrate, and easily moves into the cordierite substrate during heat endurance, thereby reducing the amount of catalyst in the catalyst layer and greatly reducing the catalyst activity. . This tendency is particularly large for alkali metals such as Na and K.

The present invention has been made in view of the above-mentioned problems of the prior art, and provides a practical catalyst which is realized by minimizing a change in a production process with respect to suppressing migration of an alkali component.

[0013]

The above object can be attained by the following means. A catalyst comprising a carrier for supporting a catalytically active component, and a substrate having a monolith structure holding the carrier, wherein the carrier is provided with a physical barrier for suppressing movement of the catalytically active component to the substrate. Is characterized by

[0014] Further, the barrier for suppressing the movement of the catalytically active component of the carrier is a plurality of microcavities formed in the carrier. Further, the catalyst is characterized in that the catalytically active component of the carrier contains an alkali metal such as Na or K.

Activated carbon is added to an alumina slurry composed of γ-alumina, nitric acid, an alumina precursor and purified water, and the slurry added with the activated carbon is coated on a cordierite honeycomb, dried and calcined to remove Na and K. A production method of impregnating and firing a NOx supplement component solution as a main component and impregnating and firing a noble metal solution, and a manufacturing method of impregnating and firing the noble metal solution with a dinitrodiammine Pt solution, a dinitrodiammine Pd solution, or a nitric acid Rh solution. There is a feature.

Activated carbon is added to an alumina slurry composed of γ-alumina, nitric acid, an alumina precursor and purified water, and the slurry added with the activated carbon is coated on a cordierite honeycomb, dried and calcined to remove Na and K. Impregnated with NOx supplemental component solution and precious metal solution as main components
There is a feature in the manufacturing method of firing.

Another feature of the present invention is an exhaust gas purifying apparatus comprising a cordierite honeycomb and a carrier layer on which a slurry containing activated carbon is coated, dried and calcined to carry a catalytically active component.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, specific examples of the present invention will be described with reference to the drawings. The present invention is intended to improve the reduction in purification performance caused by the movement of a catalytically active component carried on a carrier into a cordierite substrate during heat endurance. Specifically, a large number of minute cavities are created in the alumina carrier layer of the exhaust gas purifying catalyst, and the movement path of the alkali metal component is restricted while maintaining the strength of the carrier layer. Of the reaction,
As a result, by minimizing the decrease in the amount of the alkali metal carried in the carrier layer, it is possible to realize a catalyst and an exhaust purification device having high heat durability.

The present invention can be realized by making use of, for example, the characteristics of a catalyst production method by the following impregnation method. When carrying the alumina slurry on the cordierite substrate, combustible particles such as activated carbon are mixed into the slurry. When this slurry is coated, dried and fired, the activated carbon is burned off, and a number of minute cavities can be created in the alumina carrier layer. Thereafter, by impregnating a catalyst component such as an alkali metal component and a noble metal, a barrier for suppressing the movement of the NOx trapping component in the catalyst layer can be formed. As described above, the catalyst is characterized in that a physical barrier capable of suppressing the movement of the catalytically active component is provided in the carrier layer of the catalyst. The purpose is to restrict the movement route of the active ingredient while maintaining the strength of the carrier layer. This realizes a catalyst capable of maintaining a high purification rate for a long time and a purification device using the catalyst.

FIG. 1 shows a comparative example in which the physical barrier is provided and a case in which no physical barrier is provided.
(A) shows the case of the conventional method, and (B) shows the case of providing a barrier. In this case, a gap (or cavity) 4 is provided. Cordierite as the base 2 and the alumina layer 6 are alumina or a material containing alumina as a main component. NO
x trapping component 8 is composed of alkali metals such as Na and K and S
It is composed mainly of alkaline earth metals such as r and Ba. In the case of (B), the trapped component 8
Can form a state in which movement to the base 2 is difficult. Even if it moves to the base 2 because of the gap, the distance becomes long. Therefore, a high purification rate can be maintained for a long time.

Hereinafter, specific examples of the present invention will be described, but the present invention is not limited to these examples. Examples of the present invention will be described by taking as an example the case where the carrier is alumina and the monolith base material is cordierite. [Example] 10 wt% of activated carbon was added to an alumina slurry composed of gamma alumina, nitric acid, an alumina precursor and purified water. It was found that the amount of activated carbon added at this time is preferably 5 to 30 wt%. If the added amount of activated carbon is small, the number of cavities generated is reduced, so that the effect is reduced. If the added amount is large, slurry coating becomes difficult, and honeycomb clogging tends to increase. These characteristics are shown in FIG. A represents the relationship between the amount of activated carbon added and the number of cavities. The greater the number of cavities, the more effective the barrier, but there are problems. B represents the ease of coating,
When the addition ratio of activated carbon is increased, coating itself becomes difficult, and even if coated, a new problem of clogging occurs. That is, although the effect as a barrier increases, coating in the manufacturing process becomes difficult.

It has been found that the particle size of the activated carbon is preferably 5 to 20 μm. This tendency is shown in FIG. Characteristic A represents the relationship between the activated carbon particle size and the size of the cavity. The larger the cavity, the more effective the barrier. Also,
If the particle diameters of the activated carbons are uniform, the resulting variation in the size of the cavity is small. FIG. 3 shows that coating becomes more difficult as the particle size increases, as shown by characteristic B. If the particle size is small, the effect is small because the size of the cavity formed is small. However, if the particle size of the activated carbon is large, there are problems such that slurry coating becomes difficult, and even if the coating can be performed, the clogging of the honeycomb tends to increase.

The slurry to which the activated carbon was added was coated on a cordierite honeycomb at an alumina loading of 190 g / L.
(A lump volume), followed by drying and firing to prepare an alumina-coated honeycomb having many fine cavities in the alumina layer. The thus-prepared honeycomb is impregnated with a NOx trapping component solution containing Na and K as main components and calcined, and then a noble metal solution, for example, a dinitrodiammine Pt solution, a dinitrodiammine Pd solution,
Impregnation and baking with a nitric acid Rh solution were performed. The catalyst prepared in such a manufacturing process is herein referred to as an example catalyst (also, the NOx trapping component solution and the noble metal solution may be simultaneously impregnated and fired).

Next, a description will be given of a comparative example catalyst in which characteristics are compared with those of the above example catalyst. Alumina slurry composed of γ-alumina, nitric acid, alumina precursor and purified water is prepared, coated until the amount of alumina carried is 190 g / L, dried and fired, so that no voids are formed in the alumina layer. An alumina-coated honeycomb was prepared.

The thus prepared honeycomb was impregnated with the same amount of the catalytically active component in the same manner as in the above-mentioned catalyst, and calcined to prepare a comparative catalyst. These schematic configuration comparisons are as shown in FIG. In the comparative example catalyst,
It can be seen that the NOx trapping component can easily move, but in the catalyst of the example, the movement of the NOx trapping component is suppressed by the presence of the cavity.

Next, a comparison of characteristics between the comparative catalyst and the example catalyst will be described. The honeycomb catalyst was subjected to a heat durability test at 830 ° C. × 60 hr in an air atmosphere furnace.

The purification performance of the catalyst after the heat durability test was evaluated using a model gas test apparatus. A model gas simulating the stoichiometric combustion exhaust gas of an actual vehicle was passed through the catalyst, and then switched to a model gas simulating lean burn combustion. After switching from the stoichiometric gas to the lean gas, the NOx purification rate of the catalyst one minute later was calculated by the following equation. NOx purification rate (%) = (1− (N at catalyst outlet)
Ox concentration / NOx concentration at catalyst inlet)) × 100

At this time, the gas temperature was varied between 300 and 500 ° C. to evaluate the temperature dependence of the catalyst performance. FIG. 4 shows an example of the evaluation. As is clear from FIG. 4, the catalyst of the example obtained a purification rate characteristic exceeding the NOx purification rate of the comparative catalyst in all temperature ranges of 300 to 500 ° C. The purification rate is improved by about 15% at the maximum. This is the result of suppressing the movement of the alkali metal component to the monolith substrate. As described above, a catalyst having excellent purification rate performance can be prepared by a relatively easy method.

[0029]

According to the present invention, it is possible to prepare a catalyst in which the movement of the catalytically active component is suppressed by a relatively easy method,
A purification device having a high purification rate is obtained.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of the suppression of NOx trapping component transfer according to the present invention.

FIG. 2 is a graph showing the amount of activated carbon added and the number of cavities or ease of coating.

FIG. 3 is a diagram showing the particle size of activated carbon and the size of a cavity or ease of coating.

FIG. 4 is a graph showing the purification characteristics of a catalyst according to the present invention.

[Explanation of symbols]

2: substrate 4; cavity (gap) 6; alumina layer 8; N
Ox trapping component

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F01N 3/28 301 B01D 53/36 104A (72) Inventor Yuichi Kitahara 2520 No. Daiba Takaba, Hitachinaka City, Ibaraki Pref. Hitachi, Ltd. Automotive Equipment Group (72) Inventor Osamu Kuroda 2520, Oaza Takaba, Hitachinaka-shi, Ibaraki Co., Ltd. Hitachi, Ltd. Automotive Equipment Group (72) Inventor Takeshi Inoue 2477 Takaba, Hitachinaka-shi, Ibaraki Stock Exchange Inside Hitachi Car Engineering Co., Ltd. (72) Inventor Ryota Doi 2477 Takaba, Hitachinaka-shi, Ibaraki Prefecture Co., Ltd. Inside Hitachi Car Engineering Co., Ltd. In Group (72) Inventor Koji Okude 7-2-1, Omika-cho, Hitachi City, Ibaraki Prefecture Inside the Electric Power and Electricity Research Laboratory, Hitachi, Ltd. (72) Inventor Hidehiro Iizuka 7-2-1, Omika-machi, Hitachi City, Ibaraki Prefecture Electric Power Company, Ltd. Inside the Electric Development Laboratory (72) Inventor Toshio Yamashita 7-2-1, Omika-cho, Hitachi City, Ibaraki Prefecture Inside Electric Power & Electric Development Laboratory, Hitachi, Ltd. (72) Inventor Masato Kaneda 7-2, Omika-cho, Hitachi City, Ibaraki Prefecture No. 1 F-term in Hitachi, Ltd. Electric Power & Electric Development Laboratory 3G091 AA02 AB04 BA07 BA14 FC02 GA06 GA16 GB02W GB05W GB10X 4D048 AA06 AA13 AA18 AB05 BA03X BA05X BA10X BA14X BA30X BA31X BA33X BA41X BB02 A03A04 BA01B BA08A BA08B BA13A BA13B BA26C BC01A BC02A BC02B BC03A BC03B BC69A BC71B BC71C BC72B BC72C BC75B BC75C BE44C BE47C CA03 CA09 EA19 EC09X FA03 FB06 FB14 FB15 FB16 FB19 FC02

Claims (7)

[Claims] 1. A catalyst for purifying exhaust gas discharged from an internal combustion engine, comprising: a carrier for supporting a catalytically active component; and a base having a monolith structure for holding the carrier, wherein the carrier is An exhaust gas purifying catalyst for an internal combustion engine, comprising a physical barrier for suppressing movement of a catalytically active component to a substrate. 2. The method according to claim 1, wherein:
An exhaust gas purifying catalyst for an internal combustion engine, wherein a barrier for suppressing the movement of the catalytically active component is a plurality of minute cavities generated in a carrier. 3. The exhaust gas purifying catalyst for an internal combustion engine according to claim 2, wherein the catalytically active component of the carrier contains an alkali metal such as Na or K. 4. A method for producing a catalyst for purifying exhaust gas discharged from an internal combustion engine, wherein activated carbon is added to an alumina slurry comprising γ-alumina, nitric acid, an alumina precursor and purified water, and the slurry to which the activated carbon is added is added. Coated on cordierite honeycomb, dried and fired, Na,
A method for producing an exhaust gas purifying catalyst for an internal combustion engine, comprising impregnating and firing a NOx supplementary component solution containing K as a main component, and impregnating and firing a noble metal solution. 5. The method according to claim 4, wherein said noble metal solution is a dinitrodiamine Pt solution, a dinitrodiamine Pd solution or a Rh nitrate solution. 6. A method for producing a catalyst for purifying exhaust gas discharged from an internal combustion engine, comprising: adding an activated carbon to an alumina slurry comprising γ-alumina, nitric acid, an alumina precursor and purified water; Coated on cordierite honeycomb, dried and fired, Na,
A method for producing an exhaust gas purifying catalyst for an internal combustion engine, comprising impregnating a NOx supplementary component solution containing K as a main component and a noble metal solution, followed by firing. 7. An apparatus for purifying exhaust gas discharged from an internal combustion engine, comprising coating a cordierite honeycomb and a slurry to which activated carbon is added on the honeycomb, drying and coating the honeycomb.
An exhaust gas purifying apparatus for an internal combustion engine, comprising: a carrier layer that is calcined and carries a catalytically active component.