JPH11138021A - Production of catalyst for purifying exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the ability to purify NOx by highly dispersing and carrying a NOx storage material, in a catalyst which carries the NOx storage material. SOLUTION: A coat layer is formed by the use of slurry which contains a basic binder and a carrier powder carrying a NOx storage material. As the use of the basic binder prevents the carried NOx storage material from eluting to the slurry, and prevents particles of the NOx storage material from becoming large by crystallization, the NOx storage material is highly dispersed and carried to improve the ability to store NOx .

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to relates to a method of manufacturing the exhaust gas purifying catalyst for purifying exhaust gas discharged from an internal combustion engine such as an automobile, more particularly, of the NO _x storage reduction carrying _the NO _x storage material The present invention relates to a method for producing an exhaust gas purifying catalyst.

[0002]

2. Description of the Related Art Conventionally, as a catalyst for purifying exhaust gas of automobiles, CO and CO in exhaust gas at a stoichiometric air-fuel ratio (stoichiometric) have been used.
A three-way catalyst that purifies by simultaneously oxidizing HC and reducing NO _x is used. As such a three-way catalyst, for example, a coat layer made of γ-alumina is formed on a heat-resistant substrate made of cordierite or the like, and a noble metal such as platinum (Pt) or rhodium (Rh) is supported on the coat layer. What has been known is widely known.

On the other hand, in recent years, carbon dioxide (CO ₂ ) in exhaust gas discharged from internal combustion engines such as automobiles has become a problem from the viewpoint of protection of the global environment. As a solution, lean combustion in an oxygen-excess atmosphere has been proposed. Burn is promising. In this lean burn, the use of fuel is reduced in order to improve fuel efficiency, and the generation of CO ₂ as combustion exhaust gas can be suppressed.

On the other hand, in the conventional three-way catalyst, when the air-fuel ratio is at the stoichiometric air-fuel ratio (stoichiometric), CO, HC, NO
It is one which simultaneously redox purifies _x, in the excess oxygen atmosphere of the exhaust gas during the lean-burn, NO _x
Does not show sufficient purification performance for the reduction and removal of water. Therefore, development of a catalyst and a purification system capable of purifying NOx even in an oxygen-excess atmosphere has been desired.

Accordingly, the applicant of the present application has previously proposed an exhaust gas purifying catalyst in which an alkaline earth metal such as Ba and Pt are supported on a porous carrier such as alumina (for example, Japanese Patent Application Laid-Open No. Hei 5-317625). Using the exhaust gas-purifying catalyst, by controlling so that the stoichiometric-rich side in a pulsed manner the air-fuel ratio from the lean side, NO _x is occluded in the alkaline earth metal (NO _x storage material) in the lean side, it order to be cleaned reacts with the reducing components such as HC and CO in the stoichiometric-rich side, it is possible to efficiently purify NO _x even in the lean burn.

In order to manufacture such a NO _x storage-reduction type catalyst, a slurry containing a porous oxide such as alumina and a binder is prepared, and this slurry is coated on a cordierite or metal honeycomb substrate. Baking to form a coat layer. Then, the base material having the coat layer is immersed in a solution in which a noble metal compound is dissolved to carry the noble metal, and then immersed in a solution in which the NO _x occluding material is dissolved, thereby carrying the NO _x occluding material. Further, a production method is also known in which a slurry is prepared from a binder powder and a carrier powder in which a NO _x storage material and a noble metal are supported on alumina or the like, and the slurry is coated on a honeycomb substrate and fired.

[0007] NO in the exhaust gas purifying catalyst_xPurification
The reaction oxidizes NO in the exhaust gas to NO _xThe first step
And NO_xNO on storage material_xA second step of storing ozone,
NO_xNO released from the storage material_xThe third of which is reduced over a catalyst
It is known to consist of steps. However
With conventional exhaust gas purifying catalysts, lean atmosphere
As a result, grain growth occurs in Pt, and the above-mentioned
The reactivity of the first and third steps is reduced.
There is a condition.

On the other hand, Rh is known as a noble metal in which such grain growth hardly occurs in a lean atmosphere.
Oxidizing ability is inferior to Pt. Therefore, it is considered to use Pt and Rh together. However, when Pt and Rh are used in combination, there is a problem that the oxidizing ability of Pt is reduced. Therefore, the first to NO _x by oxidizing NO as the addition amount of Rh increases
The reactivity of the step decreases, and the NO _x
Storage capacity also decreases. The Rh has poor compatibility with _the NO _x storage material, the characteristics of Rh and NO _x when the storage material to coexist _the NO _x storage material and Rh is a problem that can not be sufficiently exhibited.

[0009]

[0005] Therefore the present inventors have found that in such Japanese Patent Application No. Hei 8-152245, a carrier powder carrying the Rh, and a carrier powder carrying Pt and _the NO _x storage material are mixed, and Rh Thus, proposes a catalyst to separate the Pt and NO _x storage material. According to this separation supported catalyst, a problem that the oxidizing ability of Pt is reduced due to the proximity of Rh is prevented.
Furthermore, by close carrying Pt and _the NO _x storage material, a first step of NO in the exhaust gas is oxidized NO _x by Pt, and a second step of absorbing the NO _x in _the NO _x storage material It is performed smoothly.

[0010] Further, Rh has a significantly smaller grain growth in a lean atmosphere than Pt. Therefore, the presence of Rh improves the durability of the ternary activity. In addition, since Rh is supported separately from the NO _x storage material, mutual incompatibility is eliminated, and the performance of the NO _x storage material and Rh is prevented from lowering. Further according to this separation supported catalyst, high _the NO _x reduction amount in the rich pulse as compared with conventional catalysts, it has been found to significantly less sulfur poisoning. This is because the separated supported Rh, hydrogen high reducing power from the HC and of H ₂ O in the exhaust gas is generated, is because this hydrogen contributes greatly to the elimination of the reduction and SO _x of the NO _x .

[0011] The separation supported catalyst is coated with the first support powder supporting Pt and NO _x storage material, and a second support powder carrying Rh, a slurry prepared by mixing a binder to the honeycomb support substrate coated It is manufactured by forming a layer and baking it. But even separating supported catalyst described above, _the NO _x storage capacity of _the NO _x storage material is a problem not expressed as expected. This is because NO
It has been clarified that this is caused by the fact that the particle size of the _x- occluding material is large and is not highly dispersed.

The present invention has been made in view of such circumstances, in catalyst carrying _the NO _x storage material, NO _x
An object of the present invention is to make the occluding material highly dispersed and supported, thereby improving the NO _x purification ability.

[0013]

Means for Solving the Problems The features of the method for producing an exhaust gas purifying catalyst of the present invention which solves the above-mentioned problems include a substrate, a coat layer formed on the surface of the substrate, and a noble metal supported on the coat layer. and the _the NO _x storage material, a process for the preparation of a more becomes a catalyst for purifying exhaust gas, forming a coating layer using a slurry containing the support powder and the basic binder carrying the _the NO _x storage material made of porous oxide Is to do.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have conducted intensive studies from the beginning to investigate the cause of a decrease in the degree of dispersion due to an increase in the particle size of the NO _x occluding material. As a result, it became clear that the following phenomena occurred during the production of the separated supported catalyst. The separation supported catalyst, a first support powder supporting Pt and NO _x storage material First, a second carrier powder carrying the Rh is formed. In this state, the NO _x occluding material is supported in a carbonate state. Next, the first carrier powder and the second carrier powder are slurried together with a binder, coated on a honeycomb substrate, and then dried and fired.

Here, nitric acid is conventionally used as the binder.
Acidic alumina sol (alumina sol + aluminum nitrate)
Is generally used and is acidic. So vine
When the powder and the first carrier powder come into contact, NO_xStorage material and nitric acid
Reacts with aluminum, NO _xThe storage material is ion
It elutes in the rally and produces nitrate. This nitric acid
Salts are easy to grow crystals, so when drying and firing the coat layer
The crystal grows and consequently the NO supported on the coat layer_xOcclusion material
And the degree of dispersion decreases.

[0016] Also in _the NO _x storage reduction type catalyst, in advance _the NO _x storage material and loaded with powder and precious metal manufactured by the manufacturing method was slurried to form a coating layer catalyst,
Phenomenon particle size of the as well as _the NO _x storage material becomes dispersed degree is greatly reduced is observed. Therefore, in the method for producing an exhaust gas purifying catalyst of the present invention, a basic binder is used as a binder contained in the slurry. This improves the stability of the NO _x storage material in the slurry, and prevents elution and crystallization. Therefore, the NO _x occluding material is maintained in the initial highly dispersed and supported state, and a high NO _x purifying ability is obtained.

Examples of the basic binder include (basic) zirconia sol, (basic) titania sol, (basic) silica sol, (basic) alumina sol and the like. One or more of these may be used. They can be used in combination. The pH of the slurry is desirably 9 or more. _The NO _x storage material and the pH is less than 9 of the slurry is likely to be crystallized by dissolution in the slurry.

The porous oxide can be selected from alumina, silica, zirconia, silica-alumina, zeolite and the like. One of these may be used, or a plurality of them may be mixed or combined for use.
When producing a separated supported catalyst, use alumina for the first powder and zirconia for the second powder from the viewpoint of heat resistance or because Zr is compatible with Rh. Is preferred. When zirconia is used for the second powder, the steam reforming reaction activity of Rh is significantly improved.

As the NO _x occluding element constituting the NO _x occluding material, at least one element selected from alkali metals, alkaline earth metals and rare earth metals can be used. Examples of the alkali metal include lithium (Li), sodium (Na), potassium (K), and cesium (Cs). Further, the alkaline earth metal refers to a Group 2A element of the periodic table, and includes magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). Examples of the rare earth metal include lanthanum (La), cerium (Ce), praseodymium (Pr) and the like.

[0020]

The present invention will be specifically described below with reference to examples and comparative examples. (Example 1) 480 g of alumina powder ("MI386" manufactured by WR Grace Co., Ltd.) was impregnated with a predetermined amount of a barium acetate aqueous solution having a predetermined concentration, and water was evaporated while stirring. This was pulverized, and calcined at 500 ° C. for 3 hours to carry Ba. Ba
Is 1.6 moles per 480 g of alumina powder.

Next, the Ba-supported alumina powder obtained above was treated with an aqueous solution of ammonium hydrogen carbonate having a concentration of 0.3 mol / L.
The mixture was immersed in a liter, stirred for 15 minutes and filtered. This is 110
C. for 3 hours and pulverized to obtain a Ba / alumina powder. In Ba / alumina powder, Ba becomes barium carbonate and is uniformly supported on alumina. This Ba / alumina powder was impregnated with a predetermined amount of an aqueous solution of dinitrodiammineplatinic nitric acid having a predetermined concentration to carry Pt. The supported amount of Pt is 8.0 g per 480 g of alumina powder. Further, Rh was supported in the same manner using an aqueous solution of rhodium nitrate having a predetermined concentration. The supported amount of Rh is 0.4 g per 480 g of alumina powder. This was filtered, dried at 110 ° C. for 3 hours, pulverized, and fired at 400 ° C. for 2 hours to prepare a first powder.

On the other hand, 480 g of zirconia powder (“RC100” manufactured by Daiichi Kagaku KK) was impregnated with an aqueous solution of rhodium nitrate having a predetermined concentration to carry 2.0 g of Rh. This is filtered at 110 ° C for 3
After drying for a period of time and pulverizing, the mixture was fired at 400 ° C. for 1 hour to prepare a second powder. 340 g of basic zirconia sol (“NSZ-30B” manufactured by Nissan Chemical Co., Ltd., zirconia concentration: 30% by weight), 1000 g of pure water, and 10 g of 28% ammonia water are thoroughly mixed with the total amount of the first powder, the total amount of the second powder, and Then, the slurry was prepared by milling with an attritor. The pH of the slurry is 9.

A 1.3-liter cordierite honeycomb substrate is immersed in the slurry, pulled up, blown off excess slurry, dried at 250 ° C., and baked at 600 ° C. for at least 20 minutes to form a coat layer. The catalyst of Example 1 was used. The coating layer is formed in an amount of 322.2 g per liter of the honeycomb substrate, and the honeycomb substrate 1
The amount of alumina contained in the first powder was 120 g and the amount of zirconia in the second powder was 120 g per L.

Example 2 The same Ba / alumina powder as in Example 1 was impregnated with a predetermined amount of a dinitrodiammineplatinum nitric acid aqueous solution having a predetermined concentration to carry Pt. The supported amount of Pt is 6.4 g per 480 g of alumina powder. Further, Rh was supported in the same manner using an aqueous solution of rhodium nitrate having a predetermined concentration. Rh
Is 0.32 g per 480 g of alumina powder. This is filtered, dried at 110 ° C for 3 hours, pulverized, and then dried at 400 ° C for 2 hours.
The first powder was prepared by baking for an hour.

Example 1 except that this first powder was used.
A slurry was prepared in the same manner as described above. The pH of the slurry is 9. Using this slurry, a coat layer was formed in the same manner as in Example 1. The coat layer is 321.1g per liter of honeycomb substrate
The first powder contains 120 g of alumina of the first powder and 120 g of zirconia of the second powder per 1 L of the honeycomb substrate.

Further, Pt and Rh were carried in the same manner as in Example 1 by using the honeycomb substrate having the coat layer formed thereon. Pt
Was supported by 0.4 g per liter of the honeycomb substrate, and Rh was supported by 0.02 g per liter of the honeycomb substrate. Thus, the catalyst of Example 2 was prepared. (Example 3) A first powder was prepared in the same manner as in Example 1 except that 4.0 g of Pt was supported and 0.2 g of Rh was supported. A slurry was prepared in the same manner as in Example 1 except that this first powder was used. The pH of the slurry is 9.

Using this slurry, a coat layer was formed in the same manner as in Example 1. Coat layer per 1L of honeycomb substrate
320.5 g were formed, and the amount of alumina of the first powder was 120 g and the amount of zirconia of the second powder was 12 per 1 L of the honeycomb substrate.
0g is included. Furthermore, the honeycomb substrate on which the coating layer was formed was immersed in an aqueous solution of dinitrodiammineplatinum in nitric acid and an aqueous solution of rhodium nitrate, pulled up and blown off excess droplets, dried at 250 ° C, and dried at 400 ° C for 1 hour. Fired. 1.0 g of Pt is supported per 1 L of honeycomb substrate, and Rh
Was supported in an amount of 0.05 g per liter of the honeycomb substrate. Thus, the catalyst of Example 3 was prepared.

(Example 4) 1.6 g of Pt is supported, and 0.08 g of Rh is loaded.
A first powder was prepared in the same manner as in Example 1 except that it was supported. A slurry was prepared in the same manner as in Example 1 except that this first powder was used. The pH of the slurry is 9. Using this slurry, a coat layer was formed in the same manner as in Example 1. The coating layer is 319.
7 g are formed, and the amount of alumina of the first powder is 120 g and the amount of zirconia of the second powder is 120 g per liter of the honeycomb substrate.

Further, the honeycomb substrate on which the coating layer was formed was immersed in an aqueous solution of dinitrodiammineplatinum in nitric acid and an aqueous solution of rhodium nitrate, and was lifted up to blow off excess droplets. For 1 hour. 1.6 g of Pt was carried per 1 L of honeycomb substrate, and Rh was carried at 0.104 g per 1 L of honeycomb substrate. Thus, the catalyst of Example 4 was prepared.

Example 5 Using the first powder and the second powder prepared in the same manner as in Example 1, a coat layer was formed in the same manner as in Example 1. The coating layer is 31 per liter of the honeycomb substrate.
9.3 g were formed, and the amount of alumina of the first powder was 120 g and the amount of zirconia of the second powder was 120 g per liter of the honeycomb substrate.
include.

Further, the honeycomb substrate on which the coating layer was formed was immersed in an aqueous solution of dinitrodiammineplatinum in nitric acid and an aqueous solution of rhodium nitrate, pulled up and sprayed off excess droplets, dried at 250 ° C., and dried at 250 ° C. For 1 hour. 2.0 g of Pt was supported per 1 L of honeycomb substrate, and 0.1 g of Rh was supported per 1 L of honeycomb substrate. Thus, the catalyst of Example 5 was prepared.

(Example 6) Alumina powder ("MI386" WR)
A predetermined amount of a barium acetate aqueous solution having a predetermined concentration was impregnated into a mixed powder of 480 g of Grace Co., Ltd.) and 120 g of titania powder (“ST21” manufactured by Ishihara Sangyo Co., Ltd.), and the water was evaporated while stirring. After crushing, bake at 500 ℃ for 3 hours
Ba was loaded. The amount of Ba supported was 1.6 per 600 g of the mixed powder.
Is a mole.

Next, the Ba-supported alumina / titania powder obtained above was immersed in 6 liters of a 0.3 mol / L aqueous solution of ammonium hydrogen carbonate, stirred for 15 minutes, and filtered.
This was dried at 110 ° C. for 3 hours and pulverized to obtain a Ba / alumina / titania powder. In the Ba / alumina / titania powder, Ba becomes barium carbonate and is uniformly supported on the alumina / titania.

The Ba / alumina / titania powder was impregnated with a predetermined amount of an aqueous solution of dinitrodiammineplatinic nitric acid having a predetermined concentration to carry Pt. The supported amount of Pt is 4.0 g per 600 g of the mixed powder. Further, Rh was supported in the same manner using an aqueous solution of rhodium nitrate having a predetermined concentration. Rh loading is mixed powder
0.2 g per 600 g. This was filtered, dried at 110 ° C. for 3 hours, pulverized, and fired at 400 ° C. for 2 hours to prepare a first powder.

On the other hand, 384 g of zirconia powder (“RC100” manufactured by Daiichi Rare Element Co., Ltd.) was impregnated with an aqueous solution of rhodium nitrate having a predetermined concentration to carry 2.0 g of Rh. This is filtered at 110 ° C for 3
After drying for a period of time and pulverizing, the mixture was fired at 400 ° C. for 1 hour to prepare a second powder. Using the first powder and the second powder, a slurry was prepared in the same manner as in Example 1. The pH of the slurry is 9
It is. Then, a coat layer was formed in the same manner as in Example 1. The coating layer is formed in an amount of 320.5 g per liter of the honeycomb substrate, and the amount of alumina and titania of the first powder is 144 g and the amount of zirconia of the second powder is 96 g per liter of the honeycomb substrate.
include.

Further, the honeycomb substrate on which the coating layer was formed was immersed in an aqueous solution of dinitrodiammineplatinum in nitric acid and an aqueous solution of rhodium nitrate, pulled up and blown off extra droplets. For 1 hour. 1.0 g of Pt was supported per 1 L of honeycomb substrate, and 0.05 g of Rh was supported per 1 L of honeycomb substrate. Thus, the catalyst of Example 6 was prepared.

Example 7 The catalyst obtained in Example 6 was further immersed in an aqueous solution of potassium nitrate having a predetermined concentration and an aqueous solution of lithium nitrate having a predetermined concentration. C. and fired at 500.degree. C. for 1 hour. K and Li are each per 1L of honeycomb substrate
0.1 mol was carried. Thus, the catalyst of Example 7 was prepared.

Example 8 384 g of barium-stabilized zirconia powder (containing 1 mol% of Ba) was impregnated with an aqueous solution of rhodium nitrate having a predetermined concentration to carry 2.0 g of Rh. This was filtered, dried at 110 ° C. for 3 hours, pulverized, and fired at 400 ° C. for 1 hour to prepare a second powder. The first prepared in Example 6
A slurry was prepared in the same manner as in Example 1 using the powder and the second powder. The pH of the slurry is 9. Then, a coat layer was formed in the same manner as in Example 1. The coating layer is formed in an amount of 320.5 g per liter of the honeycomb substrate, and the amount of alumina + titania of the first powder per liter of the honeycomb substrate is 144
g, zirconia content of the second powder is 96 g.

Next, the honeycomb substrate on which the coating layer was formed was immersed in an aqueous solution of dinitrodiammineplatinic acid in nitric acid and an aqueous solution of rhodium nitrate, pulled up and blown off, and dried at 250 ° C. Calcination was carried out at ℃ for 1 hour. 1.0 g of Pt was supported per 1 L of honeycomb substrate, and 0.05 g of Rh was supported per 1 L of honeycomb substrate. This was immersed in an aqueous solution of potassium nitrate having a predetermined concentration and an aqueous solution of lithium nitrate having a predetermined concentration, pulled up and blown off extra droplets, dried at 250 ° C., and baked at 500 ° C. for 1 hour. K and
Li was supported in an amount of 0.1 mol per 1 L of the honeycomb substrate. Thus, the catalyst of Example 8 was prepared.

Comparative Example 1 A comparative example was performed in the same manner as in Example 1 except that a nitric acid-based acidic binder (“Catapal D” manufactured by Vista Chemical Co., Ltd.) was used instead of the basic zirconia sol during the preparation of the slurry. The catalyst of Example 1 was prepared.
The pH of the slurry is as follows. (Comparative Examples 2 to 8) In the same manner as in Examples 2 to 8, except that a nitric acid-based acidic binder ("Catapal D" manufactured by Vista Chemical Co., Ltd.) was used instead of the basic zirconia sol during the preparation of the slurry. The catalysts of Comparative Examples 2 to 8 were prepared. The pH of each slurry was 4.

<Evaluation Test> Each of the obtained catalysts was used for 1.
Installed in the exhaust system of an 8L lean burn engine,
A durability test equivalent to 10,000 km was performed. Thereafter, in the same exhaust system, the amount of stored NO _x at the time of the lean gas inlet temperature of 400 ° C.
_x reduction amount, and to measure the NO _x emissions in 10-15 mode operation. Table 1 shows the results.

[0042]

[Table 1] From Table 1, catalysts of Examples 1-8 of the present invention is improved greatly _the NO _x purification performance compared to Comparative Example 1-8 in which the corresponding, which is in place of the acidic binder during slurry preparation basic It is clear that the effect is obtained by using the binder.

[0043]

According to the That method of manufacturing the exhaust gas purifying catalyst of the present invention, can be highly dispersed and supported the _the NO _x storage material for coarsening of particles due to crystallization of the NO _x storage material is prevented, It is possible to stably and easily produce a catalyst having excellent NO _x purification ability.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kiyotaka Hayashi 7800, Chihama, Oto-cho, Ogasa-gun, Shizuoka Prefecture Inside (72) Inventor Katsuyuki Murata 7800, Chihama, Oto-cho, Ogasa-gun, Shizuoka Prefecture Cataler Industry Inside the corporation

Claims (1)

[Claims] And 1. A substrate, a coating layer formed on the substrate surface, and a noble metal and NO _x storage material supported on the coat layer, a process for the preparation of a more comprising an exhaust gas purifying catalyst, porous the method of manufacturing the exhaust gas purifying catalyst, characterized by using a slurry containing a carrier powder carrying the _the NO _x storage material consists quality oxide and a basic binder to form the coating layer.