JPH08224479A - Production of catalyst for purifying exhaust gas - Google Patents

Abstract

PURPOSE: To produce an exhaust gas purifying catalyst capable of exhibiting each action of a transition metal, a NOx absorbing material, and a catalyst noble metal at the maximum and excellent in the durability of NOx purifying performance. CONSTITUTION: After a transition metal is loaded on a porous carrier, a catalyst noble metal is loaded an finally a NOx absorbing material is loaded. Then, since the reaction between the transition metal and the porous carrier occurs, the surface of the porous carrier is reformed, SOx is hardly stuck and the NOx absorbing material is carried at high dispersion. And trouble such as the elution of the carried metal at the time of carrying each metal does not occurs.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an exhaust gas purifying catalyst for purifying exhaust gas emitted from an internal combustion engine such as an automobile. More specifically, the present invention relates to an exhaust gas with excess oxygen, that is, monoxide contained in the exhaust gas. Nitrogen oxides (NO _x ) in exhaust gas containing oxygen in excess of the amount of oxygen required to completely oxidize reducing components such as carbon (CO), hydrogen (H ₂ ) and hydrocarbon (HC) The present invention relates to a method for producing an exhaust gas-purifying catalyst that can be efficiently reduced and purified.

[0002]

2. Description of the Related Art Conventionally, a three-way catalyst for purifying exhaust gas by oxidizing CO and HC and reducing NO _x has been used as a catalyst for purifying exhaust gas of automobiles. As such a three-way catalyst, for example, a porous carrier layer made of γ-alumina is formed on a heat resistant base material made of cordierite, and platinum (Pt) and rhodium (R) are formed on the porous carrier layer.
A catalyst carrying a precious metal such as h) is widely known. Also known is a three-way catalyst in which ceria (cerium oxide) having an oxygen storage capacity is used in combination to enhance low-temperature activity.

On the other hand, in recent years, from the viewpoint of protecting the global environment, carbon dioxide (CO ₂ ) in exhaust gas discharged from internal combustion engines such as automobiles has become a problem, and as a solution to this problem, so-called lean combustion in which lean combustion is performed in an oxygen excess atmosphere is performed. Burn is promising. In this lean burn, the fuel consumption is reduced, so that the fuel efficiency is improved and the generation of CO ₂ which is the combustion exhaust gas can be suppressed.

On the other hand, the conventional three-way catalyst purifies by simultaneously oxidizing and reducing CO, HC, and NO _x in the exhaust gas burned by the air-fuel ratio stoichiometric mixture. Therefore, it does not show sufficient purification performance for reducing and removing NO _x in the oxygen rich atmosphere of exhaust gas during lean burn. Therefore, it is desired to develop an exhaust gas purifying catalyst and an exhaust gas purifying system that can efficiently purify NO _x even in an oxygen excess atmosphere.

Therefore, the applicant of the present application has previously proposed an exhaust gas purifying catalyst in which an alkaline earth metal and Pt are supported on a porous carrier such as alumina (Japanese Patent Laid-Open No. 5-317652), or lanthanum and Pt on a porous carrier. A supported exhaust gas-purifying catalyst (Japanese Patent Laid-Open No. 5-168860) has been proposed. According to these exhaust gas-purifying catalysts, NO _x is absorbed on the lean side by an oxide of an alkaline earth metal or an oxide of lanthanum (NO _x absorbent), which is released on the stoichiometric or rich side to generate HC or Since it reacts with a reducing component such as CO, a good NO _x purification performance can be obtained even on the lean side.

However, the alkaline earth metal and Pt are aluminum.
In the exhaust gas purifying catalyst carried on the carrier, NO
_xIn some cases, the purification rate was not sufficient. This should be processed
SO existing in exhaust gas_xIs NO such as Ba_xWith absorber
Reacting, for example, Ba is NO _xWithout absorption capacity of BaSO
_FourThis is because it is converted to. This phenomenon is called "sulfur poisoning"
Is known.

Therefore, the applicant of the present application has filed Japanese Patent Application Laid-Open No. 6-2620.
No. 40 discloses N containing an alkali metal or a rare earth metal.
Disclosed is an exhaust gas-purifying catalyst that carries an O _x absorbent and at least one transition metal selected from Fe, Ni, Co, and Mn together with a catalytic noble metal. According to this exhaust gas purifying catalyst, the transition metal and the NO _x absorbent are SO ₂
Reacts with to form complex sulfate. This complex sulfate is N
Since it decomposes at a lower temperature than the single sulfate of the O _x absorbent,
The produced complex sulfate is easily decomposed, and the original function of the NO _x absorbent is restored. Therefore, NO even after endurance
_x High purification rate can be maintained.

In order to produce this exhaust gas-purifying catalyst, as described in the example of JP-A-6-262040, first, a catalytic precious metal is supported on a porous carrier such as alumina, and then NO _x. Soak in a mixed aqueous solution of water-soluble compound of absorber and water-soluble compound of transition metal and dry
By firing, the NO _x absorbent and the transition metal are supported simultaneously.

[0009]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
Even with the exhaust gas-purifying catalyst produced by the production method described in JP-A-62040, the sulfur poisoning resistance after endurance cannot be said to be sufficient, and the NO _x absorption capacity after endurance is not a little lowered. The present invention has been made in view of such circumstances, and is an exhaust gas purification catalyst that can maximize the effects of the transition metal, the NO _x absorbent, and the catalytic precious metal, and that has excellent durability of NO _x purification performance. Is intended to be manufactured.

[0010]

The method for producing a catalyst for purifying exhaust gas of the present invention which solves the above-mentioned problems is such that the oxygen concentration in the exhaust gas has a stoichiometric ratio required to oxidize components to be oxidized in the exhaust gas. A method for producing an exhaust gas-purifying catalyst for purifying nitrogen oxides in exhaust gas in an excess oxygen excess atmosphere,
It is characterized in that the transition metal is loaded on the porous carrier, then the catalytic noble metal is loaded, and finally the NO _x absorbent is loaded.

[0011]

The manufacturing method of the present invention has a feature in the order of loading each metal, and the transition metal is loaded on the porous carrier first. Then, since a reaction occurs between the transition metal and the porous carrier, it is considered that the surface of the porous carrier is modified by this, SO _x is less likely to adhere, and the catalyst metal is supported in a highly dispersed manner. To be

The catalytic precious metal is then carried on the porous support carrying the transition metal. If the transition metal is loaded after the catalytic noble metal is loaded in the reverse order, a phenomenon occurs in which the loaded catalytic noble metal is covered with the transition metal, the catalytic activity of the catalytic noble metal is lowered, and the oxidation reaction of NO is inhibited. As a result, the NO _x purification performance decreases. However, in the present invention, since the catalytic metal is supported after the transition metal, the above-mentioned problems do not occur.

Then, in the manufacturing method of the present invention, NO
_x Absorbent is carried. If you reverse this order NO _x
When the catalytic noble metal is loaded after the absorbent is loaded, when the porous carrier is immersed in the aqueous solution of the catalytic noble metal, the aqueous solution of the catalytic noble metal is acidic so that the alkaline side NO _x absorbent is eluted into the aqueous solution of the catalytic noble metal. The problem occurs, N
The amount of the O _x absorbent carried decreases. However, in the present invention, the supported catalytic noble metal does not elute into the NO _x absorbent solution, so the supported amount does not change.

Further, since the transition metal is supported first and the surface of the porous carrier is modified, the NO _x absorbent is supported in high dispersion. The exhaust gas purifying catalyst produced by the production method of the present invention, the porous carrier surface is presumed to have been modified, thereby SO _x sulfur poisoning adhesion is prevented is prevented by a transition metal.

The transition metal and the NO _x absorbent are SO _x.
And complex sulfate is formed. This complex sulfate is N
O more decomposed at low temperatures as compared with the _x absorber alone sulfates, also absorption of NO _x material easily decomposed because it is highly dispersed, easily decomposed when the exhaust gas flow of absorption of NO _x material originally of the NO _x Absorbency is restored. Therefore, sulfur poisoning is prevented. Further, since the NO _x absorbent is highly dispersed and supported, its catalytic activity is maximized.

Due to these actions, the exhaust gas-purifying catalyst produced according to the present invention has excellent NO _x purification performance even after endurance.

[0017]

【Example】

[Examples of the invention] The material of the porous carrier is not particularly limited,
Alumina, silica, silica-alumina, titania and the like can be selected and used. Above all, it is particularly preferable to use alumina which is excellent in heat resistance and noble metal dispersibility.
This porous carrier may be used as a carrier substrate or may be used by coating the surface of a monolith carrier substrate.

Transition metals include Ni, Fe, Co and M.
n, Cu, etc. are exemplified. Of these, Ni is particularly preferable. The method of supporting the transition metal is to use a solution of chloride or nitrate of the transition metal to adhere it to a porous carrier using an impregnation method, a spraying method, a slurry mixing method, etc., and dry and calcine it to support it. be able to. The amount of the transition metal supported is optimally in the range of 0.01 to 0.3 mol with respect to 1 liter of the porous carrier, and if it is less than this, the supporting effect cannot be obtained,
Even if the amount is too large, the NO _x purification performance after endurance is greatly reduced.

As the catalyst noble metal, Pt, Rh and Pd are used.
1 type or multiple types of these can be used. The catalyst noble metal loading method is an impregnation method using a chloride or nitrate thereof.
The particles can be attached by using a spraying method, a slurry mixing method, etc., dried and fired to be carried as in the conventional case. The supported amount of the catalytic noble metal is preferably 0.1 to 20 g with respect to 1 liter of the porous carrier for any noble metal.
0.5 to 10 g is particularly preferable. Even if the supported amount of the catalytic noble metal is further increased, the activity is not improved, and the effective utilization cannot be achieved. Further, if the amount of the catalytic noble metal supported is less than this, sufficient activity for practical use cannot be obtained.

To carry the NO _x absorbent, a solution of its chloride, nitrate or the like is used to adhere it by using an impregnation method, a spraying method, a slurry mixing method, etc., and it is dried and baked to carry it. Can be made. As the NO _x absorbent, at least one selected from alkali metals, alkaline earth metals and rare earth elements can be used. Examples of the alkali metal include lithium, sodium, potassium, rubidium, cesium and francium. Also, the alkaline earth metal means an element of Group 2A of the periodic table, barium,
Examples include beryllium, magnesium, calcium, and strontium. Examples of rare earth elements include scandium, yttrium, lanthanum, cerium, praseodymium, and neodymium.

The content of the NO _x absorbent is preferably in the range of 0.05 to 1.0 mol with respect to 100 g of the material of the porous carrier. When the content is less than 0.05 mol, the NO _x absorption capacity is small and the NO _x purification performance is lowered, and even when it exceeds 1.0 mol, the NO _x absorption capacity is saturated and at the same time HC
However, problems such as an increase in the emission of [Examples] Hereinafter, specific examples will be described. The "parts" mentioned below mean "parts by weight". (Example 1) First, alumina powder ("MI-386" W
(Made by R Grace Co., Ltd.), impregnated with a predetermined amount of nickel nitrate aqueous solution of a predetermined concentration, dried at 250 ° C., and then dried at 500 ° C. for 1 hour.
Ni was supported by firing for a time. The amount of Ni supported is 0.1 mol per 120 g of alumina powder.

Next, a predetermined amount of a dinitrodiammine platinum aqueous solution having a predetermined concentration was impregnated into Ni-supported alumina powder, dried at 250 ° C., and baked at 500 ° C. for 1 hour to support Pt. The supported amount of Pt is 2.0 g per 120 g of alumina powder. Then, the alumina powder carrying Ni and Pt was impregnated with a predetermined amount of an aqueous sodium nitrate solution having a predetermined concentration, dried at 250 ° C., and then baked at 500 ° C. for 1 hour to support Na. The amount of supported Na is 120 alumina powder.
It is 0.3 mol per gram.

The resulting supported alumina powder was pelletized by a conventional method to obtain a pellet catalyst of Example 1. (Comparative Example 1) The same as Example 1 except that Pt was loaded first, Ni was loaded next, and Na was loaded last. (Comparative Example 2) The same as Example 1 except that Pt was loaded first, Na was loaded next, and Ni was loaded last. (Comparative Example 3) The same as Example 1 except that Na was loaded first, Ni was loaded next, and Pt was loaded last. (Comparative Example 4) The same as Example 1 except that Na was loaded first, Pt was loaded next, and Ni was loaded last. (Comparative Example 5) The same as Example 1 except that Ni was loaded first, Na was loaded next, and Pt was loaded last. (Comparative Example 6) The same as Example 1 except that Na was first supported and then Pt was then supported without supporting Ni. (Comparative Example 7) The same as Example 1 except that Pt was first loaded without Ni, and then Na was loaded. (Test) Each of the obtained pellet catalysts was placed in a catalytic converter, and lean exhaust gas (A / F = 20) containing 300 ppm of SO ₂ was passed for 4 minutes, and then SO ₂ was added to 3
A durability test was conducted in which flowing of rich exhaust gas (A / F = 14) containing 00 ppm for 1 minute was repeated for 2 hours. The temperature of the gas entering the catalyst of each exhaust gas is 500 ° C., and the space velocity (SV) is 100,000 / Hr.

With respect to each of the catalysts after the endurance, the NO _x absorption amount (the number of moles) was measured when the model exhaust gas corresponding to A / F = 20 was allowed to flow at the inlet gas temperature of 300 ° C. for 30 minutes. The absorption of NO _x amount is divided by the moles of the NO _x absorbent present in the catalyst, it calculates the absorption of NO _x amount of the NO _x absorbent per mole, and the results are shown in Table 1.

[0025]

[Table 1] (Evaluation) From Table 1, it is clear that a significantly high NO _x absorption amount can be obtained even after the endurance only when each metal is supported in the order of Example 1.

From the comparison between Example 1 and Comparative Example 5 and the comparison between Comparative Example 6 and Comparative Example 7, it is understood that it is preferable to support Na before Pt. And Example 1 and Comparative Example 7
From the comparison, it is clear that the NO _x absorption amount is greatly increased by supporting Ni first.

[0027]

According to the method for producing an exhaust gas purifying catalyst of the present invention, the NOx can be reduced without increasing man-hours and raw material cost by simply changing the loading order of the same type and the same amount of supported metal as the conventional method.
_x It is possible to stably manufacture an exhaust gas purifying catalyst having excellent durability of purification performance.

Claims (1)

[Claims] 1. A catalyst for purifying exhaust gas, which purifies nitrogen oxides in exhaust gas in an oxygen-excess atmosphere in which the oxygen concentration in the exhaust gas exceeds the stoichiometric ratio required to oxidize components to be oxidized in the exhaust gas. A method for producing an exhaust gas-purifying catalyst, which comprises supporting a transition metal on a porous carrier, then supporting a catalytic noble metal, and finally supporting an NO _x absorbent.