JPH0838902A - Exhaust gas purification catalyst and device - Google Patents

Abstract

(57) [Summary] [Objective] To provide an exhaust gas purification method for purifying nitrogen oxides with high efficiency. A nitrogen oxide is removed from an exhaust gas containing oxygen of an internal combustion engine by using an exhaust gas purifying catalyst which is a spinel type oxide containing Ni and Fe. [Effect] According to the present invention, nitrogen oxides can be efficiently purified from exhaust gas in an oxygen excess atmosphere.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalyst and a device for purifying combustion exhaust gas containing oxygen, and more particularly to a catalyst and a device for purifying nitrogen oxides.

[0002]

2. Description of the Related Art Exhaust gas emitted from an internal combustion engine of an automobile or the like contains nitrogen oxides and the like, which are harmful to humans and also cause environmental damage such as acid rain. Therefore, various studies have been made on methods for purifying nitrogen oxides in exhaust gas.

At present, in automobiles, the air-fuel ratio of the engine is set to a stoichiometric value, that is, near the stoichiometric air-fuel ratio (A / F = 14.6: weight ratio of air A to fuel F), and the exhaust gas produced is made of precious metals (rhodium, palladium). , Platinum) is used as a three-way catalyst to reduce nitrogen oxides to nitrogen, and hydrocarbons and carbon monoxide are oxidized to purify them.

By the way, regarding automobiles, in recent years, from the viewpoint of reducing the fuel consumption rate, the air-fuel ratio is changed to the theoretical air-fuel ratio (14.6).
Development of the lean burn engine described above has been promoted and its spread is expected. However, in a lean burn engine, oxygen is contained in the exhaust gas in excess (at least 0.5% or more) compared to the stoichiometric air-fuel ratio, so in the currently used three-way catalyst, only oxidation of hydrocarbons and carbon monoxide proceeds. However, reduction of nitrogen oxides cannot be performed effectively.

On the other hand, diesel engines such as diesel automobiles are conventionally operated at a high air-fuel ratio with excess oxygen. Therefore, the above three-way catalyst cannot be applied, and an effective method for reducing nitrogen oxides cannot be found.

One of the methods for removing nitrogen oxides currently in practical use is the NH ₃ reduction method using a V ₂ O ₃ —TiO ₂ catalyst. This method is characterized in that nitrogen oxides can be removed even if a large amount of oxygen coexists in the exhaust gas. However, this method uses NH ₃ , which is a harmful substance, and requires an NH ₃ supply tank, and thus is difficult to be used for a mobile small internal combustion engine such as an automobile.

Therefore, in recent years, much research has been conducted on a catalyst for purifying nitrogen oxides without using NH ₃ in an oxidizing atmosphere in the presence of excess oxygen. Among them, a method of removing nitrogen oxides by utilizing hydrocarbon and oxygen contained in the exhaust gas has attracted attention.

Currently, as such a catalyst, a catalyst in which copper is supported on zeolite (Japanese Patent Laid-Open No. 4-219141, No. 69)
The 3rd Demonstration Meeting on the 3rd Debriefing of the 3rd catalyst 108 (1992)) and a catalyst containing cobalt, rare earth, copper and / or rhodium in zeolite (JP-A-4-219147) have been reported.

[0009]

However, in the above-mentioned method, a catalyst in which copper is supported on zeolite (JP-A-1-151
No. 706) shows extremely high performance in a low temperature range due to the unique adsorption property of copper to nitrogen oxides, but the temperature range (window) in which a catalytic action is exhibited is narrow.

Therefore, a catalyst for expanding the window, a catalyst containing cobalt, rare earth, copper, and rhodium in zeolite (Japanese Patent Laid-Open No. 4-219147) is proposed,
It has become possible to purify nitrogen oxides in a relatively wide temperature range. In addition, it is known that the catalyst in which copper is supported on zeolite (The 69th Catalytic Discussion Group, Proceedings 3F108 (1992)) has a decreased catalytic activity when a large amount of water coexists. However, the hydrothermal durability of zeolite as a carrier is said to be limited to 700 ° C, and further higher nitrogen oxide purification performance and durability are required for automobile catalysts that require durability of 80 ° C. There is.

[0011]

In order to solve the above-mentioned problems, the present inventor has proposed a catalyst having a wide operating temperature range, a small effect of steam, and a catalyst structure which is difficult to change before and after the reaction. As a result of earnest studies, it was found that a spinel type oxide catalyst containing at least one of Ni and Fe and particularly described by the chemical formula AB ₂ O ₄ can serve this purpose.

The present invention is characterized by being a spinel type composite oxide containing at least one of Ni and Fe.
Alternatively, it is an oxide containing at least one of Ni and Fe and described by the chemical formula AB ₂ O ₄ (A and B are metal elements).

At least one of the catalysts can be used to remove nitrogen oxides from the exhaust gas containing oxygen of an internal combustion engine.

The metal salt used for the above catalyst preparation is nitrate,
There are acetates, hydrochlorides, sulfates, carbonates, etc., and the present invention is not limited to these kinds of metal salts.

The catalyst preparation method of the present invention includes any method, such as a method of once converting each metal salt into an oxide and then mixing and firing the mixture, or a method of mixing and firing the metal salts.

When the metal salt is converted to an oxide, it is fired at 500 ° C. or higher and lower than 750 ° C. When obtaining a spinel-type oxide, it is desirable to perform a baking treatment at 900 ° C. or higher and lower than 100 ° C. for 2 hours or longer and shorter than 10 hours.

The catalyst produced as described above may have any shape such as powder, granules, pellets, or honeycomb, and it can be used by supporting it on any porous carrier such as cordierite or metal honeycomb. .

The present catalyst can be applied to gases containing at least oxygen, carbon monoxide, hydrocarbons and nitrogen oxides, and can be used for internal combustion engine exhaust gases such as gasoline engine exhaust gas and diesel engine exhaust gas.

The use conditions of the catalyst of the present invention are as follows: catalyst temperature or exhaust gas temperature: 100 ° C. or higher and 800 ° C.
Below, preferably 200 ℃ or more and 500 ℃ or less Oxygen concentration: It is desirable to use at 0.5% or more and 20% or less, nitrogen oxide concentration and hydrocarbon concentration is 0.5 or more and 10 or less in C / N atomic ratio, Preferably, it is desirable to use 1 to 5 or so.

As described above, the present catalyst can remove nitrogen oxides from exhaust gas from lean burn engines, diesel engines, and the like. The basic method of using the catalyst produced according to the present invention is as shown in FIG. 1, in which the present catalyst is placed in a reactor and exhaust gas is circulated in the reactor.
This is to bring the catalyst and exhaust gas into contact with each other to purify nitrogen oxides and to discharge the exhaust gas out of the reactor. Also, FIG.
As described above, the present catalyst may be arranged between the internal combustion engine and the three-way catalyst. Further, as shown in FIG. 3, when the internal combustion engine is operated under stoichiometric conditions, the exhaust gas may flow only to the three-way catalyst (for example, a bypass is provided). Further, as shown in FIG. 4, the present catalyst component may be contained in the three-way catalyst, or may be supported on the three-way catalyst for use.

FIG. 5 shows a method in which a porous honeycomb is coated with the present catalyst component and then a three-way catalyst component is coated thereon. The top and bottom of the coat may be reversed.

[0022]

Although the reaction mechanism for purifying nitrogen oxides in the presence of excess oxygen and hydrocarbons is not clear, taking NiFe ₂ O ₄ included in the present invention as an example, it is considered that its action is expressed as follows.

The spinel type oxide used in the above invention is represented by the general chemical formula AB ₂ O ₄ (A and B are metallic elements).
It is a substance with high temperature durability of 900 ° C or higher. NiFe
_{The 2} O ₄ spinel oxide is said to be less active than the Ni oxide and the Fe oxide in the complete oxidation activity of propylene. Since hydrocarbons, NOx, and oxygen are reaction substances in the NOx purification reaction, the NOx purification performance deteriorates when only the combustion of hydrocarbons progresses. Therefore, NiFe ₂ O ₄ has a spinel structure to weaken the hydrocarbon combustion activity and NO
x It is considered that the purification performance is maintained.

By the above action, it is considered that the spinel type catalyst containing Ni and Fe has excellent durability and can remove nitrogen oxides efficiently.

[0025]

The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

(Example 1) In atomic ratio, Ni: Fe = 1:
The catalyst was prepared so that 2 was obtained. In the case of the catalyst of Example 1, nitrate was used as a metal salt, and each was added so that the above atomic ratio could be achieved. Distilled water was added to this, and the mixture was kneaded for about 15 minutes with a fryer until the dry mixture changed into a paste. . The paste was dried at about 100 ° C. for about 2 hours and then baked at about 900 ° C. for 10 hours. Then, it is naturally cooled to room temperature. This powder catalyst was pressure-molded to a particle size of 1 mm or more and less than 2 mm to obtain a test catalyst.

"Comparative Example 1" In atomic ratio, Y: Sr: Co =
Comparative catalyst 1 was prepared according to Example 1 so that it was 1/3: 2/3: 1.

The catalyst of Example 1 and the catalyst of Comparative Example 1 were subjected to a nitrogen oxide purification performance test under the following conditions. Catalyst 3 cm ³
Was charged into a Pyrex reaction tube. This was heated from outside by an electric furnace to 150 ° C., and then a reaction gas was circulated to raise the temperature to 600 ° C. at a rate of 10 ° C./m to carry out a reaction. The reaction gas, as lean model exhaust gas, NO: 0.06%, C ₃ H ₆ : 0.04%, CO: 0.0.
1%, CO ₂ : 10%, O ₂ : 4%, water vapor 10%, balance nitrogen, NO: 0 as stoichiometric model exhaust gas.
_{1%, C 3 H 6:} 0.05%, CO: 0.6%, O 2: 0.6
%, Steam 10%, balance nitrogen: space velocity 20,000h
^It was distributed so that it would be ^-1 . The concentration of nitrogen oxides purified by the above operation was measured by chemical spectroscopy. Table 1
Shows the purification rate at the catalyst layer inlet temperature of 200 to 500 ° C. and the catalyst layer inlet temperature at that time. The catalyst of Example 1 showed a higher purification rate than the catalyst of Comparative Example 1 in the entire temperature range. Here, the purification rate is the removal rate of nitrogen oxides at the outlet with respect to the catalyst layer inlet, and was calculated according to the following equation.

[0029]

[Equation 1]

[0030]

[Table 1]

As a result of performing powder X-ray diffraction measurement before and after the reaction, almost no structural change was observed in Example catalyst 1, but in Comparative example catalyst 1, Y ₂ O ₃ and Co ₃ O ₄ were observed.
Was partially observed.

(Example 2) In a cordierite honeycomb,
Example catalyst 1 was washed with 100 g / l of wash coat to 700
After calcination at 2 ° C. for 2 hours, 100 g / l washcoat of the three-way catalyst component was carried out and calcination at 900 ° C. for 2 hours to prepare Example catalyst 2.

The honeycomb volume of Example catalyst 2 was set to 6 cm ³ and was filled in a Pyrex reaction tube. Each was heated from the outside by an electric furnace to 150 ° C., and then a reaction gas was circulated to raise the temperature to 600 ° C. at a rate of 10 ° C./m to carry out a reaction. The reaction gas, as a lean model exhaust _{gas, NO: 0.06%, C 3} H 6: 0.04%, CO: 0.
1%, CO ₂ : 10%, O ₂ : 4%, steam 10%, balance nitrogen: NO: 0.1 in the case of stoichiometric model exhaust gas
_{%, C 3 H 6: 0.05} %, CO: 0.6%, O 2: 0.
6%, steam 10%, and the balance nitrogen were circulated at a flow rate of 3000 cc / min. The concentration of nitrogen oxides purified by the above operation was measured by chemical spectroscopy. 250 ° C to 4
The effect at 00 ° C is that the lean exhaust gas has an average nitrogen oxide removal rate of about 45%, and the stoichiometric exhaust gas has an average removal rate of about 1%.
It became 00%.

[0034]

As is apparent from the examples, according to the present invention, a material which has undergone a high temperature heat history of 900 ° C. for 10 hours,
Nitrogen oxides can be efficiently purified from exhaust gas in an oxygen excess atmosphere.

Further, according to the present invention, a catalytic material having a nitrogen oxide purifying ability and a high durability and a nitrogen oxide purifying apparatus using the same can be obtained, which can be used for consumer products such as automobile internal combustion engines and cooking utensils. It is possible to purify nitrogen oxides from a wide range of sources such as the combustion exhaust gas of, and industrial combustion exhaust gas emitted from boilers of factories and thermal power plants.

[Brief description of drawings]

FIG. 1 is an apparatus using a spinel type oxide catalyst.

FIG. 2 is a two-stage device using a spinel type oxide catalyst and a three-way catalyst.

FIG. 3 is an apparatus using a spinel-type oxide catalyst using a bypass and a three-way catalyst.

FIG. 4 is a composite device using a spinel type oxide catalyst and a three-way catalyst.

FIG. 5 shows an example of containing a porous honeycomb.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Spinel type oxide catalyst, 2 ... Three-way catalyst, 3 ... Bypass switching valve, 4 ... Bypass channel, 5 ... Porous honeycomb.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical display location F01N 3/28 301 B (72) Inventor Yasuo Noto 2520 Takaba, Katsuta-shi, Ibaraki Hitachi Ltd. Factory Automotive Equipment Division

Claims (3)

[Claims] 1. An exhaust gas purification catalyst containing oxygen for an internal combustion engine, which is a spinel type composite oxide containing at least one of Ni and Fe. 2. Purification of exhaust gas containing oxygen from an internal combustion engine, characterized by being an oxide containing at least one of Ni and Fe and represented by the chemical formula AB ₂ O ₄ (A and B are metallic elements). catalyst. 3. An apparatus for removing nitrogen oxides from an exhaust gas containing oxygen of an internal combustion engine, which comprises at least one of the catalysts according to claims 1 and 2.