JPH1147593A - Production of catalyst for purifying exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce an exhaust gas purifying catalyst certainly developing a high NOx purifying ratio. SOLUTION: A carrier layer is formed from a slurry containing mordenite, alumina and a binder. Subsequently, a part of Pt is supported on alumina of the carrier layer by using a dinitrodiamine platinum nitrate aq. soln. and the remainder of Pt is supported on mordenite of the carrier layer by using a tetraammine platinum hydroxide soln. Thereafter, the carrier layer is treated with sulfuric acid to convert Pt supported alumina to a Pt supported solid ultra-strong acid.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing an exhaust gas purifying catalyst. Exhaust gas purifying catalysts obtained by this method include carbon monoxide (CO) and hydrocarbons (HC).
It is suitable for purifying nitrogen oxides (NOx) in exhaust gas containing oxygen in excess of the amount required to oxidize NOx.

[0002]

2. Description of the Related Art Conventionally, a three-way catalyst for purifying exhaust gas by simultaneously oxidizing CO and HC and reducing NOx has been used as an exhaust gas purifying catalyst for automobiles.
As such a catalyst, for example, a porous support layer made of γ-alumina is formed on a heat-resistant carrier base material such as cordierite, and Pt, Pd, and a precious metal such as Rh supported on the support layer. Widely known.

[0003] However, the purification performance of the above-mentioned conventional catalyst greatly varies depending on the air-fuel ratio (A / F) of the engine. That is, on the lean side, the amount of oxygen in the exhaust gas increases,
Oxidation reaction to purify CO and HC is active, but NO
The reduction reaction for purifying x becomes inactive. Conversely, on the rich side, the amount of oxygen in the exhaust gas decreases, and the oxidation reaction becomes inactive, but the reduction reaction becomes active.

[0004] On the other hand, in the case of driving in an urban area, the vehicle frequently starts and stops, so that the A / F
Changes frequently in the range from near the stoichiometric state to the rich state. In order to respond to such a demand for low fuel consumption during traveling, it is necessary to operate on the lean side, which supplies an air-fuel mixture as much as possible. Therefore, development of a catalyst that can sufficiently purify NOx on the lean side is also desired.

For this reason, an exhaust gas purifying catalyst employing a mordenite having an HC adsorbing ability in a carrier layer has been proposed (JP-A-4-27437). In this catalyst, while the temperature of the exhaust gas is low, the pores of the mordenite adsorb HC in the exhaust gas. Conversely, when the temperature of the exhaust gas rises, the mordenite pores adsorbed H
Release C. For this reason, in this catalyst, as the temperature of the exhaust gas rises, NOx in the exhaust gas can be reduced with HC to improve the NOx purification rate.

[0006]

However, the present inventors have proposed that, besides mordenite, other acidic porous oxides (first porous oxides) such as zeolite can reduce the HC adsorbing ability similarly to mordenite. It was found to have. It has also been found that if the support layer has a solid superacid, cracking occurs when HC is released from the support layer, and NOx is efficiently reduced by the cracked HC. This solid superacid can be obtained by subjecting an amphoteric porous oxide (second porous oxide) to an acid treatment.

For this reason, in a catalyst in which the support layer is made of the acidic first porous oxide and the solid superacid, NOx
It can be expected that the purification rate can be greatly improved and the three-way catalyst performance can be further improved. However, when producing such a catalyst, it has become clear that the NOx purification rate cannot always be improved depending on the production method.

The present invention has been made in view of the above-mentioned conventional situation, and has as its object to provide a manufacturing method capable of manufacturing an exhaust gas purifying catalyst which reliably exhibits a high NOx purifying rate. .

[0009]

According to a first aspect of the present invention, there is provided a method for producing an exhaust gas purifying catalyst, comprising: a carrier substrate; a carrier layer formed on the carrier substrate; and a noble metal carried on the carrier layer. A method for producing an exhaust gas purifying catalyst for purifying exhaust gas by purifying carbon monoxide, hydrocarbons and nitrogen oxides in exhaust gas in an oxygen-excess atmosphere, comprising: And a second porous oxide that is amphoteric and becomes a solid superacid by acid treatment, and a binder, and a supporting layer forming step of forming the supporting layer using a slurry containing the binder;
An acidic noble metal supporting step of supporting a part of the noble metal on the second porous oxide of the supporting layer with an acidic noble metal solution, and removing the remainder of the noble metal on the first porous oxide of the supporting layer with an ammine-based noble metal A step of supporting an ammine-based noble metal supported by a solution, and the second step of supporting the noble metal by performing an acid treatment on the supported layer.
And an acid treatment step of using a porous oxide as the solid superacid.

According to a second aspect of the present invention, in the method for producing an exhaust gas purifying catalyst according to the first aspect, the second porous oxide is at least one of alumina, titania and zirconia. There is a feature. A heat-resistant material such as cordierite can be used as the carrier substrate. The carrier substrate may have a honeycomb shape or a pellet shape.

As the first porous oxide, commercially available zeolites such as mordenite, ZSM-5, USY, and ferrierite can be used. Since the first porous oxide has the ability to adsorb HC, the pores adsorb HC in the exhaust gas while the temperature of the exhaust gas is low, and when the temperature of the exhaust gas rises, the pores become Releases the adsorbed HC. For this reason, in the catalyst thus obtained, as the temperature of the exhaust gas rises, NOx in the exhaust gas is reduced to H.
By reducing with C, the NOx purification rate can be improved.

As the second porous oxide, alumina, titania and zirconia can be used. In particular, when the second porous oxide is alumina, sintering of the catalyst can be prevented, which is preferable from the viewpoint of heat resistance. The slurry used in the support layer forming step contains the first porous oxide and the second porous oxide, and is prepared together with a binder such as silica and water. Then, the slurry is applied to a carrier substrate and fired to form a carrier layer.

As the acidic noble metal solution used in the acidic noble metal supporting step, an aqueous solution of dinitrodiamine platinum nitrate, an aqueous solution of dinitrodiamine palladium nitrate, an aqueous solution of dinitrodiamine rhodium nitrate, or the like can be employed. If the supporting layer is immersed in an acidic noble metal solution together with the carrier substrate and dried, a predetermined amount of the noble metal can be supported on the second porous oxide of the supporting layer.

As the ammine-based noble metal solution used in the step of supporting an ammine-based noble metal, a tetraammine platinum hydroxide solution, a tetraammine palladium hydroxide solution, a tetraammine rhodium hydroxide solution, or the like can be used. If the carrier layer is immersed in an ammine-based noble metal solution together with the carrier substrate and dried, a predetermined amount of the noble metal can be carried on the first porous oxide of the carrier layer.

As the acid used in the acid treatment step, sulfuric acid, molybdic acid, tungstic acid and the like can be employed.
By this acid treatment, the second porous oxide supporting the noble metal is converted into a solid super-strong acid (strongly acidic substance) supporting a noble metal such as aluminum sulfate, zirconium sulfate, or titanium sulfate supporting the noble metal. When HC is released from the support layer by the solid superacid supporting the noble metal, cracking (cra) occurs.
NOx is efficiently reduced by the cracked HC.

The support layer can support an oxygen storage / release material made of a rare earth element oxide such as ceria. In such a production method, since the noble metal is supported by the acidic noble metal solution on the amphoteric second porous oxide before the acid treatment,
The precious metal loading efficiency of the acidic precious metal solution is maintained.
In addition, the noble metal is supported on the second porous oxide by the acidic noble metal solution, and the noble metal is supported on the first porous oxide by the ammine-based noble metal solution. The amount of the noble metal supported can be adjusted.
Furthermore, since the noble metal is supported on the first porous oxide and the second porous oxide after the formation of the supporting layer, the noble metal does not hide inside the supporting layer that cannot contact the exhaust gas. Also,
Even if the noble metal-supported second porous oxide of the support layer is treated with an acid to form a noble metal-supported solid superacid, the noble metal is not poisoned by the acid at this time.

Therefore, in the exhaust gas purifying catalyst according to this manufacturing method, the NOx purifying rate is surely improved.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described together with Comparative Examples 1 to 3.

[0019]

【Example】

(Supporting Layer Forming Step) First, as the first porous oxide, H
A commercially available NH ₄ ⁺ type mordenite (HSZ660NHA, manufactured by Tosoh Corporation) having C adsorption ability is prepared. Also, as the second porous oxide, an alumina powder (manufactured by WR Grace Co., Ltd .: high heat-resistant alumina) is prepared.

A slurry is prepared from 150 parts of the mordenite, 75 parts of alumina powder, 295 parts of pure water, and 80 parts of silica sol having a solid content of 35%. Thereafter, a honeycomb-shaped carrier substrate made of cordierite (capacity: 1.7 liters) is prepared and immersed in a slurry. After that, it is pulled up and the excess slurry is blown off, and 10
After drying at 0 ° C. for 2 hours, baking is performed at 500 ° C. for 2 hours. Thus, a carrier layer is formed on the carrier substrate. The carrying amount of the carrying layer was 240 g per liter of the carrier substrate. (Acid Noble Metal Loading Step) Next, an aqueous solution of dinitrodiamine platinum nitrate (manufactured by Tanaka Kikinzoku Co., Ltd .: 8P, pH = 1 to 3) was prepared as an acidic noble metal solution to support a part of Pt, and supported on this. The carrier substrate on which the layer has been formed is dipped.
After pulling it up and blowing off the excess aqueous solution,
After drying at 2 ° C for 2 hours, baking is performed at 250 ° C for 2 hours.
In this way, Pt is supported on the alumina powder before the acid treatment by the aqueous solution of dinitrodiamine platinum nitrate, so that the efficiency of supporting Pt by the aqueous solution of dinitrodiamine platinum nitrate is maintained. (Ammine-based noble metal supporting step) Thereafter, as an ammine-based noble metal solution for supporting the rest of Pt, a tetraammine platinum hydroxide solution (AT10, p.
H = 8 to 10) is prepared, and the carrier substrate on which the carrier layer is formed is immersed therein. After pulling it up and blowing off the excess aqueous solution, it is dried at 100 ° C. for 2 hours and then baked at 250 ° C. for 2 hours. Thus, Pt by the tetraammine platinum hydroxide solution is supported on the mordenite of the supporting layer.

During the acidic noble metal supporting step and the ammine-based noble metal supporting step, Pt is supported on alumina powder by an aqueous solution of dinitrodiamine platinum nitrate and Pt is supported on mordenite by a tetraammine platinum hydroxide solution.
The mordenite and the alumina powder can control the amount of Pt selectively carried. The supported amount of Pt is 2 g per liter of the carrier substrate.

Further, since Pt is supported on the mordenite and alumina powder after the formation of the support layer, Pt does not hide inside the support layer which cannot contact the exhaust gas. (Acid treatment step) Then, 1N sulfuric acid is prepared, and a carrier substrate having a carrier layer carrying Pt is immersed for one hour. Then, it is baked at 600 ° C. for 3 hours. By this acid treatment,
The Pt-supported alumina powder is Pt-supported aluminum sulfate, which is a Pt-supported solid superacid (strongly acidic substance). During this time, Pt is not poisoned by sulfuric acid.

Thus, the catalyst of the embodiment is obtained. In the above embodiment, the ammine-based noble metal supporting step is performed after the acidic noble metal supporting step. However, the acidic noble metal supporting step may be performed after the ammine-based noble metal supporting step.

[0024]

Comparative Example 1 First, alumina powder of the same kind as in the example was prepared, and 100 g of this alumina powder was put into 1 liter of 1N sulfuric acid and stirred for 1 hour. Thereafter, the mixture is filtered and baked at 600 ° C. for 3 hours. By this acid treatment, the alumina powder is converted into an aluminum sulfate powder as a solid superacid (strongly acidic substance).

[0025] 150 parts of mordenite of the same kind as in the example,
A slurry is prepared from 75 parts of aluminum sulfate powder, 295 parts of pure water, and 80 parts of silica sol having a solid content of 35%. Then, similarly to the embodiment, a carrier layer is formed on the carrier base material. Thereafter, the carrier layer together with the carrier substrate is immersed in an aqueous solution of dinitrodiamine platinum nitrate to carry Pt on aluminum sulfate in the carrier layer.

Next, the carrier layer together with the carrier substrate is immersed in a tetraammine platinum hydroxide solution to carry Pt on the mordenite of the carrier layer. Thus, the catalyst of Comparative Example 1 is obtained.
However, in such a production method, when Pt is supported on aluminum sulfate, which is a solid superacid, the aluminum sulfate is already acidic. This is because the SO ₄ ^2-of sulfuric acid used for the acid treatment is coordinated to Al, pulls the electrons towards the SO ₄ ^2-. The same applies when titania or zirconia is used.
For this reason, in the obtained catalyst of Comparative Example 1, the loading efficiency of Pt on aluminum sulfate was reduced, and the expected N
Ox purification rate cannot be obtained.

[0027]

COMPARATIVE EXAMPLE 2 The production method of Comparative Example 1 was slightly modified so that aluminum sulfate and mordenite, which are solid superacids constituting a supporting layer only with a tetraammineplatinum hydroxide solution, were mixed with Pt.
Carry. Thus, the catalyst of Comparative Example 2 is obtained.

However, in this production method, since a single tetraammine platinum hydroxide solution is used, the amount of Pt supported on the solid superacid and the mordenite cannot be adjusted selectively.
Again, the expected NOx purification rate cannot be obtained.

[0029]

Comparative Example 3 First, a mordenite of the same type as that of the example was prepared, and this was immersed in a tetraammineplatinum hydroxide solution.
A mordenite supporting t is obtained. Further, the same type of alumina powder as that of the example was prepared, and this was immersed in an aqueous solution of dinitrodiamine platinum nitrate to obtain an alumina powder carrying Pt. 100 g of this Pt-supported alumina powder is introduced into 1 liter of 1N sulfuric acid and stirred for 1 hour. After that, it is filtered and 6
Bake at 00 ° C for 3 hours. By this acid treatment, the Pt-supported alumina powder becomes Pt-supported aluminum sulfate powder, which is a Pt-supported solid superacid (strongly acidic substance).

Then, these are formed into a slurry together with a binder, and this slurry is applied to a carrier substrate and fired.
Thus, a catalyst of Comparative Example 3 is obtained. With this catalyst, a large amount of Pt is easily hidden inside the carrier layer that cannot contact the exhaust gas, and the expected NOx purification rate cannot be obtained even with this catalyst.

[0031]

[Evaluation 1] The catalysts obtained in Examples and Comparative Example 3 were mounted on a converter and subjected to an evaluation test after endurance at 600 ° C. × 50 hours used in a 2400 cc in-line 4-cylinder diesel engine. This evaluation is based on 300-1200p
added before the catalyst in the range of pmC, and set the rotation speed to 1200 rpm.
This was the case where the load was continuously changed while the value of m was constant, and the incoming gas temperature was changed from 150 to 450 ° C., and the NOx purification rate (%) was measured based on the temperature rising characteristics and the temperature falling characteristics. The results are shown in FIG.

FIG. 1 shows that the catalyst of the embodiment can exhibit a higher NOx purification rate in both the temperature raising characteristics and the temperature lowering characteristics than the catalyst of Comparative Example 3. Thus, the catalyst of the example exhibits a high NOx purification rate when purifying exhaust gas in an oxygen-excess atmosphere. This makes it possible to achieve the demand for low fuel consumption and low pollution.

[0033]

[Evaluation 2] The catalyst obtained in the example was mounted on a converter and used for a 2400 cc in-line 4-cylinder diesel engine. Then, the incoming gas temperature is changed from 150 to 450 ° C., and the NOx purification rate (%) is evaluated for the temperature rising characteristics and the temperature falling characteristics. The results are shown in FIG.

FIG. 2 shows that the catalyst of Example 1 had better temperature rising characteristics than temperature decreasing characteristics.

[Brief description of the drawings]

FIG. 1 is a graph showing the NOx purification rates of a catalyst of Example and Comparative Example 3 when the temperature is increased and decreased.

FIG. 2 is a graph showing a relationship between an incoming gas temperature and a NOx purification rate according to the catalyst of Example.

Claims (2)

[Claims] 1. A carrier substrate, a carrier layer formed on the carrier substrate, and a noble metal carried on the carrier layer, wherein carbon monoxide and hydrocarbon in exhaust gas in an oxygen-excess atmosphere. And a method for producing an exhaust gas purifying catalyst for purifying exhaust gas by purifying nitrogen oxides, comprising: an acidic first porous oxide; and an amphoteric second porous acid which becomes a solid superacid by acid treatment. Oxide and a binder,
A supporting layer forming step of forming the supporting layer with a slurry containing: an acidic noble metal supporting step of supporting a part of the noble metal on the second porous oxide of the supporting layer with an acidic noble metal solution; An ammine-based noble metal supporting step of supporting the remainder of the noble metal on the first porous oxide with an ammine-based noble metal solution; and acid-treating the supporting layer to form the second porous oxide supporting the noble metal on the solid. A method for producing an exhaust gas purifying catalyst, comprising: an acid treatment step of forming a super strong acid. 2. The method for producing an exhaust gas purifying catalyst according to claim 1, wherein the second porous oxide is at least one of alumina, titania and zirconia.