JPH08266900A - Exhaust gas purifying catalyst and its production - Google Patents

Abstract

PURPOSE: To remove nitrogen oxide at high efficiency from an oxygen-containing exhaust gas by using a catalyst prepared by carrying Pt, Sr, a rare earth metal and Fe on an inorganic carrier. CONSTITUTION: A γ-Al2 O3 >=1mm to <Zmm in particle diameter is impregnated with a Ce nitrate aq. solution, dried at about 100 deg.C for 2hr and fired at about 600 deg.C for 2hr. Succeedingly, the γ-Al2 O3 is impregnated with strontium nitrate aq. solution, dried and fired by the same way, further is impregnated with a dinitrodiammine-Pt nitrate solution, dried and fired by the same way and finally is impregnated with a Fe nitrate aq. solution, dried and fired by the same way. As a result, the catalyst containing 1.6wt.% Pt, 30wt.% Sr, 12wt.% Ce and 5wt.% Fe per the γ-Al2 O3 of 100 is obtained. In this case, a honeycomb like catalyst 3 for med by coating a cordierite or metal-made honeycomb catalyst base body 2 with the catalyst power 3 can be produced. Then nitrogen oxide in a lean burnt exhaust gas is removed at high efficiency.

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 method for purifying combustion exhaust gas containing oxygen, and more particularly to a catalyst for effectively reducing and purifying nitrogen oxides and a method for producing the same.

[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.

Currently, 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 = 14.7: the 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, in recent years, regarding the automobile, the air-fuel ratio is set to the theoretical air-fuel ratio (14.7) from the viewpoint of reducing the fuel consumption rate.
) The above lean burn engine is being developed, and its spread is expected. However, in a lean burn engine, oxygen (at least 0.5% or more) is contained in the exhaust gas as compared with the stoichiometric air-fuel ratio, so in the currently used three-way catalyst, oxidation of hydrocarbons and carbon monoxide mainly progresses, and nitrogen oxidation occurs. It is not possible to effectively reduce things.

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 exhaust gas has been attracting attention. Currently, as such a catalyst, a catalyst in which copper is supported on zeolite (Japanese Patent Laid-Open No. 21914/1992)
No. 1, Gazette of the 69th Catalytic Discussion Group, 3F 108 (19
92)) and a catalyst containing cobalt, rare earth, copper and / or rhodium in zeolite (JP-A-4-219147) and a catalyst composed of barium oxide, lanthanum oxide and platinum (JP-A-5-261287). Has been done.

[0008]

However, the above catalyst and the method for producing the same have the following problems.

[0009] A catalyst in which copper is supported on zeolite
-151706) shows high performance in a relatively low temperature range due to the unique adsorption property of copper for nitrogen oxides, but has a narrow temperature range (temperature window) at which a catalytic action is exhibited.
Therefore, studies on expanding the temperature window have been advanced, and for example, a catalyst containing cobalt, rare earths, copper, and rhodium in zeolite (Japanese Patent Laid-Open No. 4-219147) has been proposed. However, the zeolite-based catalyst has a problem that the catalytic activity decreases when a large amount of water coexists (the 69th Catalysis Debate Meeting, Proceedings 3F108 (1992)). Although intensive investigations have been made on this problem as well, it has not been successful so far to significantly improve the hydrothermal durability of zeolite as a carrier. In order to purify exhaust gas from an internal combustion engine catalyst that operates with high fuel efficiency, such as for lean burn automobiles, high nitrogen oxide purification performance and high durability in a wide temperature range must be provided.

On the other hand, a catalyst composed of barium oxide, lanthanum oxide and platinum (JP-A-5-261287) occludes nitrogen oxides during lean operation of an automobile and theoretical air fuel consumption (oxygen concentration of 0.5% or less). It is released during operation and reduced. This catalyst was obtained by a clever idea, but it does not reduce the demand for a catalyst for reducing and purifying nitrogen oxides in the presence of oxygen.

An object of the present invention is to provide a catalyst for purifying combustion exhaust gas containing oxygen and a method for producing the same, and particularly a catalyst for effectively reducing and purifying nitrogen oxides and a method for producing the same.

[0012]

Means for Solving the Problems As a result of intensive studies made by the present inventor on a catalyst capable of achieving the above object, a catalyst in which Pt, Sr, a rare earth metal and Fe are supported on an inorganic carrier is effective. I found that there is.

The present invention is an exhaust gas purifying catalyst for purifying nitrogen oxides in exhaust gas containing oxygen discharged from an internal combustion engine, wherein Pt, Sr, a rare earth metal and Fe are provided on an inorganic carrier.
Is carried.

As the inorganic carrier, Al ₂ O ₃ , Ti
Although porous metal oxides such as O ₂ , SiO ₂ , ZrO ₂ , and MgO can be applied, Al ₂ O ₃ has both good nitrogen oxide purification performance and relatively high heat resistance. In addition, Al ₂
O ₃ is classified into α, γ, θ, etc. according to its structure, and any structure is applicable to the present invention, but when the γ type was used, relatively high nitrogen oxide purification performance was obtained.

The present invention provides an exhaust gas purifying catalyst for purifying nitrogen oxides in exhaust gas containing oxygen, characterized in that Pt, Sr, a rare earth metal and Fe are carried on an inorganic carrier, and the inorganic carrier is aluminum oxide. Preferably, γ-alumina is applied.

Further, as the rare earth metal, La, C
Although e, Y, Pr, Nd and the like give good results, Ce is particularly preferable.

The present invention is an exhaust gas purifying catalyst for purifying nitrogen oxides in exhaust gas containing oxygen, characterized in that Pt, Sr, a rare earth metal and Fe are carried on an inorganic carrier, and the rare earth metal is Ce. It is characterized by

The metal compound used for producing the combustion exhaust gas purifying catalyst of the present invention is a nitrate, acetate, or the like of the metal.
Chlorides, sulfates, carbonates, etc., but the present invention is not limited to these types of metal salts.

In producing the catalyst, a rare earth metal is first supported on an inorganic carrier, Sr is subsequently supported, Pt is subsequently supported, and finally Fe is supported. Purification performance can be obtained.

According to the present invention, in producing the catalyst of the present invention, a rare earth metal is first supported on an inorganic support, and then Sr is added.
Is further carried out, Pt is carried subsequently, and finally Fe is carried.

In the above catalyst production, as a method for supporting Pt, Fe, a rare earth metal or Sr, a conventionally known method such as an impregnation method or a wet or dry kneading method can be applied. Further, the catalytically active component can be supported or included in the inorganic carrier by the coprecipitation method, the sol-gel method or the like.

In the production of the catalyst of the present invention, the inorganic carrier is impregnated with a solution of the rare earth compound, the impregnated material of the rare earth compound is calcined at a temperature at which the rare earth compound decomposes, and the solution of the Sr compound is mixed with the rare earth compound. Impregnation of the contained calcined product, calcining of the Sr impregnated product at a temperature at which the Sr compound decomposes, impregnation of a solution of a Pt compound into the rare earth and Sr-containing calcined product, temperature at which the Pt compound decomposes Calcination of the Pt impregnated material with a solution of the Fe compound in the rare earth and Sr and P
Each step of impregnating the t-containing calcined product and calcining the Fe-impregnated product at a temperature at which the Fe compound decomposes can be included.

Further, in the above catalyst, the composition ratio of each supported component is Pt based on 100 parts by weight of the inorganic carrier.
0.1 to 3 parts by weight, rare earth metal 3 to 30 parts by weight, S
When r is 5 to 40 parts by weight and Fe is 0 to 10 parts by weight, good nitrogen oxide purification performance is obtained.

In the present invention, Pt, Sr and C are deposited on an inorganic carrier.
In an exhaust gas purifying catalyst for purifying nitrogen oxides in exhaust gas containing oxygen, which is characterized by carrying e and Fe, 0.1 to 3 parts by weight of Pt and 3 parts of rare earth metal are used with respect to 100 parts by weight of the inorganic carrier. ˜30 parts by weight, Sr in an amount of 5 to 40 parts by weight, Fe in an amount of 0 to 10 parts by weight, and in particular, Fe in an amount of 0 to 10 parts by weight based on 100 parts by weight of the inorganic carrier.

The catalyst according to the present invention can be used in various shapes such as powder, granules, pellets and honeycombs.

When the catalyst of the present invention is used in a honeycomb form, the honeycomb-shaped catalyst substrate is first coated with an inorganic carrier, and then the rare earth metal, Sr, Pt,
In addition to the method of supporting Fe, first, a rare earth metal is supported on an inorganic carrier, then Sr is supported, and then P is further supported.
t, and finally, a method of coating a catalyst powder obtained by supporting Fe on a cordierite or metal honeycomb-shaped catalyst substrate can be applied, and the latter can obtain higher nitrogen oxide purification performance.

A method for producing a catalyst for purifying nitrogen oxides in an exhaust gas containing oxygen discharged from an internal combustion engine, wherein a rare earth metal is first loaded on the inorganic support, and then Sr is loaded thereon. Then, Pt is loaded, and finally Fe.
The present invention also includes a method for producing a honeycomb-shaped catalyst in which the catalyst powder obtained by supporting the above is coated on a honeycomb catalyst base made of cordierite or metal.

[0028]

[Function] The reaction mechanism of nitrogen oxide reduction purification in the presence of oxygen and hydrocarbons has not been clarified at present. Then, as a result of strenuous research based on pioneering research examples, hypothesis setting, trial-and-error method, etc., combustion with the use of a catalyst carrying rare earth metal, alkaline earth metal, Pt and Fe as an inorganic carrier Exhaust gas 200 ° C to 400
It has been found that it exhibits a high nitrogen oxide purification capability at ° C.

It is known that Sr and Pt form a solid solution at 700 ° C. or lower. Therefore, it is conceivable that Sr and Pt can be dispersed on the carrier while being close to each other to secure a large number of active sites. Alkali metal or alkaline earth metal has the ability to adsorb nitrogen oxides.
t has a purifying ability for nitrogen oxides. It is considered that the nitrogen oxide adsorbed on Sr can be efficiently purified on Pt when Sr and Pt are in a solid solution or in a state close thereto. The reason why Pt is supported on Sr is more effective because Pt, which is an active component for purifying nitrogen oxides, is closer to the gas phase because it becomes easier to contact with hydrocarbon as a reducing agent. To be

The rare earth metal is characterized by improving the heat resistance and durability of the catalyst. Ce, L as rare earth metals
Although there are a, Nd, Sm, Pr, etc., Ce or La was good in combination with SrPt. Especially Ce
The combination with was optimal. Ce exists as oxidized Ce on the inorganic carrier. It is known that Ce oxide has an excellent oxygen storage effect. It is considered that a local oxygen-depleted state can be formed around Pt by including Ce oxide on the carrier and attracting oxygen in the combustion exhaust gas toward the carrier.

Further, it is known that Fe exhibits various catalytic effects as an oxide of Fe ₂ O ₃ , Fe ₃ O _4, etc.
It is also known that stable γ-Fe ₂ O ₃ can be obtained by the interaction between the carrier Al ₂ O ₃ and Fe. γ-Fe
₂ O ₃ is effective for the water gas shift reaction and can generate hydrogen from water and CO. Due to these properties,
It is possible that hydrogen with a strong reducing ability is supplied from water and CO in the combustion exhaust gas.

By each of the above-mentioned actions, after the rare earth metal is contained in the inorganic carrier, the alkaline earth metal and then Pt,
Finally, it is understood that the catalyst containing Fe can efficiently purify nitrogen oxides.

[0033]

EXAMPLES Examples of the present invention will be specifically described below, but the present invention is not limited to these examples and does not deviate from the spirit described in the claims of the present invention. It can be changed within the range.

(Example 1) γ-having a particle size of 1 mm or more and less than 2 mm
Al ₂ O ₃ is impregnated with an aqueous solution of Ce nitrate and the temperature is about 2 ° C at about 2
After drying for an hour, it was baked at about 600 ° C. for 2 hours. Then, it was impregnated with an aqueous strontium nitrate solution and similarly dried and baked. Further, a dinitrodiammine Pt nitric acid solution was similarly impregnated, dried and fired. Finally, an aqueous solution of Fe nitrate was similarly impregnated, dried and fired. From the above, γ-Al ₂ O ₃ 10
0, Pt 1.6 wt%, Sr 30 wt%, Ce
Example catalyst 1 containing 12 wt% and Fe 5 wt%
I got

By the same method, an example catalyst 2 containing 12 wt% of La was obtained by using a nitric acid La solution instead of the Ce nitric acid aqueous solution.

In the same manner as in Example catalyst 1, Example catalyst 3 containing no Fe was obtained.

(Experimental Example 1) A nitrogen oxide purification performance test was conducted on the above-mentioned catalysts under the following conditions. Catalyst 3 cm ³
Was charged into a Pyrex reaction tube. This was heated from the outside in an electric furnace to 150 ° C, and then model gas of exhaust gas containing almost no oxygen (NO: 0.1 vol%, C
₃ H ₆ : 0.05 vol%, CO: 0.6 vol%, O ₂ : 0.6 vo
l%, water vapor 10 vol%, nitrogen balance; stoichiometric model exhaust gas) (10%) while circulating at a space velocity of 60,000 h ^-1
The temperature was raised to 550 ° C. at a rate of ° C./m to carry out the reaction by the temperature raising method of stoichiometric exhaust gas. After cooling to room temperature, heating from outside again to 300 ° C with an electric furnace,
Model gas of exhaust gas containing excess oxygen (NO: 0.06v
_{ol%, C 3 H 6:} 0.04vol%, CO: 0.1vol%, CO
_{2: 10vol%, O 2:} 4vol%, steam 10 vol%, the balance nitrogen; less lean model exhaust gas) space velocity 60,000H ^-1
And the lean exhaust gas was allowed to undergo a steady reaction (300 ° C. constant).

The concentration of nitrogen oxides purified by the above operation was measured by the chemiluminescence method to obtain the NOx purification rate. N
The Ox purification rate was defined by the nitrogen oxide removal rate at the outlet with respect to the catalyst layer inlet, and was calculated by the following equation.

[0039]

[Equation 1]

For the catalysts 1, 2 and 3 of the example, a steady reaction of the lean model exhaust gas (at a constant temperature of 300 ° C.) was performed. The obtained NOx purification rate is shown in Table 1.

[0041]

[Table 1]

(Experimental Example 2) Further, a nitrogen oxide purification performance test was conducted on the above-mentioned catalysts under the following conditions. 3 cm ³ of catalyst was packed in a Pyrex reaction tube. this,
After heating from the outside in an electric furnace to 150 ° C, a model gas of exhaust gas containing almost no oxygen (Stoichi model exhaust gas) is passed at a space velocity of 60,000h ^-1 and 10 ° C / m.
The temperature was raised to 550 ° C. at a rate of 1, and the reaction was carried out by the stoichiometric exhaust gas temperature raising method. Subsequently, the catalyst temperature is set to 150 ° C.
After that, the model gas of the exhaust gas containing excess oxygen (lean model exhaust gas) is flown at a space velocity of 60,000 h ^-1 and is heated to 550 ° C at a speed of 10 ° C / min to raise the temperature of the lean exhaust gas. The reaction by the method was performed.

The method for measuring the nitrogen oxide concentration and the method for calculating the removal rate were the same as in Experimental Example 1.

The obtained results are shown in Table 2.

[0045]

[Table 2]

(Example 2) The type, amount, preparation method, etc. of the active component are the same as those of Example catalyst 1, but only the Fe concentration is 0 to 1.
Example catalysts 3 to 7 were obtained by changing the amount between 5 wt%.

According to Experimental Example 1, NOx of lean exhaust gas
The purification rate was determined and Table 3 was obtained.

[0048]

[Table 3]

Example 3 The catalyst of Example 1 was calcined in air at 700 ° C. or 850 ° C. for 5 hours, and then the purification rate in lean exhaust gas was measured according to Experimental Example 1, and the results shown in Table 4 were obtained.

[0050]

[Table 4]

Example 4 γ-Al ₂ O ₃ having a particle size of 6 μm was impregnated with an aqueous solution of Ce nitrate, dried at about 100 ° C. for about 2 hours, and then baked at about 600 ° C. for 2 hours. Successively, the aqueous solution was impregnated, dried, and fired in the order of Sr nitrate solution, dinitrodiammine Pt solution, and Fe nitrate solution. From the above, with respect to 100 parts by weight of γ-Al ₂ O ₃ , Fe 5 wt
%, Pt 1.6 wt%, Sr 30 wt% and Ce12
A catalyst fine powder containing wt% was obtained.

The catalyst fine powder was wash-coated (coating amount: 100 g / l-honeycomb) on a cordierite honeycomb (400 cells / in ² ) to obtain a catalyst of Example 8.

With respect to 6 cc of Example catalyst 8, the NOx purification rate of lean model exhaust gas was measured by the same method and conditions as in Experimental Example 1 (steady reaction at a constant temperature of 300 ° C.) to obtain a NOx purification rate of 28%.

Example 5 A composition similar to that of Example catalyst 1,
Only the order of loading the active components was changed by the preparation method to obtain the following comparative catalyst.

The active ingredients are Ce, Pt, Sr, and F
Comparative Example catalyst 1 was obtained by incorporating γ-Al ₂ O ₃ in the order of e.

Similarly, Comparative Example catalyst 2 was obtained by incorporating Ce, Fe, Pt, and Sr in γ-Al ₂ O ₃ in this order.

Similarly, Comparative Example catalyst 3 was obtained by incorporating Ce, Fe, Sr, and Pt in γ-Al ₂ O ₃ in this order.

According to Experimental Example 1, NOx of lean exhaust gas
The purification rate was obtained and the results shown in Table 5 were obtained.

[0059]

[Table 5]

[0060]

As is apparent from the examples, according to the present invention, nitrogen oxides can be purified from exhaust gas containing oxygen with high efficiency.

According to the present invention, it becomes possible to highly efficiently purify nitrogen oxides in lean combustion exhaust gas, and combustion exhaust gas from an internal combustion engine such as an automobile engine or combustion exhaust gas from a consumer product such as a cookware, It becomes possible to efficiently purify nitrogen oxides in a wide range of exhaust gas such as combustion exhaust gas emitted from boilers of factories and thermal power plants.

[Brief description of drawings]

FIG. 1 is a schematic diagram of an exhaust gas purification catalyst according to an embodiment of the present invention.

[Explanation of symbols]

 1 ... Honeycomb catalyst, 2 ... Honeycomb part, 3 ... Catalyst part.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroshi Hanaoka 7-1-1 Omika-cho, Hitachi-shi, Ibaraki Hitachi Ltd. Hitachi Research Laboratory (72) Inventor Keiro Ishikawa 7-chome, Omika-cho, Hitachi-shi, Ibaraki No. 1-1 Hitachi Ltd. Hitachi Research Laboratory (72) Inventor Shigeru Shodohata No. 1-1 Omika-cho, Hitachi City, Ibaraki Prefecture Hitachi Ltd. Hitachi Research Laboratory (72) Inventor Yuichi Kitahara Hitachinaka City, Ibaraki Prefecture 2520 Takaba, Takara, Ltd. Within the Automotive Equipment Division, Hitachi, Ltd.

Claims (6)

[Claims] 1. An exhaust gas purifying catalyst for purifying nitrogen oxides in exhaust gas discharged from an internal combustion engine, comprising oxygen characterized in that Pt, Sr, a rare earth metal and Fe are carried on an inorganic carrier. Exhaust gas purification catalyst. 2. The exhaust gas purifying catalyst according to claim 1, wherein the inorganic carrier is aluminum oxide. 3. The catalyst according to claim 1, wherein the rare earth metal is C.
An exhaust gas purification catalyst characterized by being e. 4. The catalyst according to claim 1, wherein the inorganic carrier 10 is used.
An exhaust gas purification catalyst containing oxygen, characterized in that Fe is contained in an amount of 0 to 10 parts by weight with respect to 0 parts by weight. 5. A method for producing a catalyst for purifying nitrogen oxides in exhaust gas containing oxygen discharged from an internal combustion engine, which comprises first supporting a rare earth metal on an inorganic carrier, and then carrying out Sr.
Is carried out, Pt is carried subsequently, and Fe is further carried, and a method for producing an exhaust gas purifying catalyst, comprising: 6. A method for producing a catalyst for purifying nitrogen oxides in an exhaust gas containing oxygen discharged from an internal combustion engine, which comprises coating a catalyst on a catalyst substrate such as a cordierite honeycomb to provide for exhaust purification. First, a catalyst powder obtained by first supporting a rare earth metal, subsequently supporting Sr, further supporting Pt, and further supporting Fe on the inorganic carrier is coated on the catalyst substrate. A method for producing an exhaust gas purifying catalyst, comprising: