JP2901295B2 - Exhaust gas purification catalyst and method of using the same - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an exhaust gas purifying catalyst for removing nitrogen oxides, carbon monoxide and hydrocarbons in exhaust gas discharged from an internal combustion engine such as an automobile engine. More particularly, the present invention relates to a method of purifying an oxygen-excess fuel exhaust gas.

(Prior art) Nitrogen oxides, carbon monoxide and hydrocarbons which are harmful substances in exhaust gas discharged from an internal combustion engine are, for example, Pt, Rh, Pd
Etc. are removed by a three-way catalyst having a carrier supported thereon. However, since exhaust gas contains a large amount of oxygen in exhaust gas of diesel engines, there is no effective catalyst for nitrogen oxide, and exhaust gas purification by the catalyst is not performed.

In recent years, gasoline engines have been required to perform lean combustion for the purpose of lowering fuel consumption and reducing carbon dioxide emissions. However, since the exhaust gas of this lean burn gasoline engine is in an oxygen-excess atmosphere, the conventional three-way catalyst as described above cannot be used, and a method for removing harmful components has not been put to practical use.

As a method for removing nitrogen oxides particularly in such an oxygen-excess exhaust gas, a method of adding a reducing agent such as ammonia, a method of absorbing and removing nitrogen oxides by an alkali, and the like are also known. These methods are not effective methods for use in vehicles that are mobile sources, and their applications are limited.

It is known that a zeolite catalyst obtained by ion-exchanging a transition metal can be used similarly to a conventional three-way catalyst. For example, Japanese Patent Application Laid-Open No. 1-130735 discloses a catalyst capable of selectively reducing nitrogen oxides even in an oxygen-excess exhaust gas containing a trace amount of a reducing agent such as unburned carbon monoxide and hydrocarbon. Proposed.

However, the catalyst according to this conventional proposal has a remarkable deterioration in activity due to long-time use at a high temperature, and there is a point to be further improved in durability, catalytic performance and the like, and it has not yet been put to practical use.

(Problems to be Solved by the Invention) An object of the present invention is to solve the above-mentioned problems of the prior art. Nitrogen oxides are obtained from exhaust gas discharged from an internal combustion engine of an automobile or the like. Another object of the present invention is to provide a catalyst which is resistant to thermal degradation and simultaneously removes carbon monoxide and hydrocarbons, has excellent durability, and has high catalytic activity.

Another object of the present invention is to provide a method for purifying exhaust gas using such a catalyst.

(Means for Solving the Problems) As a result of intensive studies on the above problems, the present inventors have completed the present invention.

That is, the present invention is a zeolite catalyst for removing nitrogen oxides, carbon monoxide and hydrocarbons from an oxygen-excess exhaust gas containing nitrogen oxides, carbon monoxide and hydrocarbons,
An exhaust gas purifying catalyst characterized by being a zeolite having a SiO ₂ / Al ₂ O ₃ molar ratio of at least 15 and containing cobalt and an alkaline earth metal, and a nitrogen oxide, a monoxide, An object of the present invention is to provide a method for removing nitrogen oxides, carbon monoxide and hydrocarbons in exhaust gas, which comprises contacting combustion exhaust gas containing carbon and hydrocarbon.

 Hereinafter, the present invention will be described in detail.

Said zeolite is generally _{xM 2 / n O · Al 2} O 3 · ySiO 2 · zH 2 O ( where n is the valence of the cation, x is a number in the range of 0.8 to 2, y is a number of 2 or more , Z is a number of 0 or more), and the zeolite used in the present invention must have a SiO ₂ / Al ₂ O ₃ molar ratio of 15 or more. . SiO ₂ / Al ₂ O ₃ molar ratio is the upper limit thereof is not particularly limited but, SiO ₂ / Al
_{When the 2} O ₃ molar ratio is less than 15, the heat resistance of the zeolite itself,
Since the durability is low, sufficient heat resistance and durability of the catalyst cannot be obtained. In general, those having a SiO ₂ / Al ₂ O ₃ molar ratio of about 10 to 1000 are used.

The zeolite constituting the catalyst of the present invention may be either a natural product or a synthetic product, and the method for producing these zeolites is not particularly limited, but typically, ferrierite, Y, ZSM-5, Zeolites such as ZSM-11, ZSM-12, ZSM-20, and mordenite can be used. These zeolites can be used as the catalyst of the present invention as they are, or after being treated with an ammonium salt, a mineral acid or the like and ion-exchanged to NH ₄ ⁺ type or H type.

It is essential that the zeolite used in the present invention contains cobalt and an alkaline earth metal. The method for containing cobalt and alkaline earth metal is not particularly limited, and ion exchange, impregnation and the like can be used, but ion exchange is most preferred.

The salts used for ion exchange of alkaline earth metals may be water-soluble, and are preferably nitrates and chlorides having high solubility. Be, Mg, Ca, Sr, B as alkaline earth metals
a, Ra can be used.

As the ion exchange method, a general ion exchange method such as charging an alkaline earth metal salt into a slurry of zeolite and stirring, or charging zeolite into an aqueous solution of an alkaline earth metal salt and stirring may be used. In other words, the liquid temperature is preferably 20 to 100 ° C, preferably 40 to 90 ° C.
The concentration of the alkaline earth metal salt in the aqueous solution is 0.01 to 5 mol /
L, preferably 0.1 to 2 mol / L. The solid-liquid ratio between the zeolite and the aqueous solution is not particularly limited, but sufficient stirring may be performed, and the solid content of the slurry is preferably 5 to 50%.

In the ion exchange of cobalt, the salts may be aqueous salts, preferably divalent acetate. In the cobalt ion exchange, the number of exchanges is not particularly limited, and the exchange rate may be high. If the exchange rate is low, the ion exchange may be repeated two or more times. The upper limit of the number of ion exchanges is not particularly defined, but may be 2 to 5 times.

As the ion exchange method, the same method as that for the alkaline earth metal may be used. The concentration of cobalt acetate in the aqueous solution is 0.01
11 mol / L, preferably 0.1-1 mol / L. If the amount is less than 0.01 mol / L, a large amount of solution is required, so that the operability is reduced. On the other hand, if it is more than 1 mol / L, the ion exchange rate will not be improved to match the amount of the supplied reagent.

There are no particular restrictions on the order in which the alkaline earth metal and cobalt are contained, but when they are incorporated by ion exchange, the order of the alkaline earth metal and cobalt is preferred.

As the content of the alkaline earth metal and cobalt, the molar ratio of the alkaline earth metal is 0.1 to 1 times and the cobalt is 0.5 to 1.7 times, preferably the amount of the alkaline earth metal and the amount of cobalt with respect to the molar number of alumina in the zeolite. 1.0 ~
2.5 times. If the amount of the alkaline earth metal is less than 0.1, the effect of improving the durability and catalytic activity may be small.
If it is more than twice, it is difficult to obtain the effect corresponding to the added amount. If the amount of cobalt is less than 0.5 times, it may not be suitable for use as a catalyst. If the amount of cobalt is more than 1.7 times, it is difficult to obtain durability and activity corresponding to the added amount.

Further, an alkaline earth metal or cobalt can be supported by evaporation to dryness and used. The method of evaporation to dryness may be a usual method, such as charging zeolite into an aqueous solution containing an alkaline earth metal or cobalt, and evaporating water as a solvent with a dryer or the like. The concentrations of the alkaline earth metal and the cobalt salt in the aqueous solution are not particularly limited, but the alkaline earth metal or cobalt may be uniformly deposited, and usually 0.01 to 1 mol / L.

The ion-exchanged sample is used as a catalyst after solid-liquid separation, washing and drying. Also, it can be used after firing if necessary.

SiO ₂ / Al ₂ O ₃ molar ratio of the exhaust gas purifying catalyst of the present invention is not changed to SiO ₂ / Al ₂ O ₃ molar ratio substantially of zeolite base material used. Further, the crystal structure of the exhaust gas purifying catalyst is not essentially different before and after ion exchange.

The exhaust gas purifying catalyst of the present invention can be used after being mixed with a binder such as a viscous mineral and molded. Alternatively, zeolite can be molded in advance, and the molded body can contain cobalt by ion exchange. Examples of the binder used for forming the zeolite include clay minerals such as kaolin, attapulgite, montmorillonite, bentonite, allophane, and sepiolite, silica sol, and alumina sol. Alternatively, a binderless zeolite molded body directly synthesized without using a binder may be used.

Further, it is also possible to wash coat zeolite on a honeycomb-shaped substrate made of cordierite or metal and use it.

The removal of nitrogen oxides, carbon monoxide and hydrocarbons from the oxygen-excess exhaust gas can be performed by contacting the exhaust gas purification catalyst of the present invention with an oxygen-excess exhaust gas containing nitrogen oxides, carbon monoxide and hydrocarbons. it can. The oxygen-excess exhaust gas targeted by the present invention refers to an exhaust gas containing oxygen in excess of the amount of oxygen necessary to completely oxidize carbon monoxide, hydrocarbons and hydrogen contained in the exhaust gas. Specific examples of such exhaust gas include exhaust gas emitted from an internal combustion engine of an automobile or the like, particularly exhaust gas in a state where the air-fuel ratio is large (so-called lean region).

In addition, the above-mentioned exhaust gas catalyst is applied to the case of exhaust gas containing carbon monoxide, hydrocarbons and hydrogen and not containing excess oxygen,
Its performance does not change at all.

Hereinafter, the present invention will be described in more detail with reference to Examples.
The present invention is not limited to these examples.

(Example) Example 1 <Preparation of Catalyst 1> Ammonium-type ZSM-5 having a SiO ₂ / Al ₂ O ₃ molar ratio of 40, 20 g
Was added to 180 g of an aqueous solution of barium chloride having a concentration of 1.09 mol / L, and the mixture was stirred at 80 ° C. for 16 hours. After solid-liquid separation, wash thoroughly with water,
Subsequently, the mixture was added to 180 g of an aqueous solution of 0.23 mol / L cobalt (II) acetate tetrahydrate and stirred at 80 ° C. for 16 hours. After the slurry was subjected to solid-liquid separation, the same operation was performed by adding the zeolite cake to an aqueous solution of the above composition prepared again. After the solid-liquid separation, the mixture was thoroughly washed with water and dried at 110 ° C. for 10 hours to obtain Catalyst 1. When the barium and cobalt contents of this catalyst were examined by chemical analysis, barium was contained 0.44 times and cobalt was 1.13 times as divalent with respect to ₃ moles of Al ₂ O in the zeolite.

Example 2 <Preparation of Catalyst 2> Ion exchange was performed in the same manner as in Example 1, except that strontium was used as the alkaline earth metal. When this catalyst was used as catalyst 2 and the strontium and cobalt contents of the catalyst were examined by chemical analysis, the strontium content was 0.23 times and the cobalt content was 1.12 times as divalent with respect to ₃ moles of Al ₂ O _{3 in} zeolite. Was.

Example 3 <Preparation of Catalyst 3> Ion exchange was performed in the same manner as in Example 1, except that magnesium was used as the alkaline earth metal. Catalyst 3
The magnesium and cobalt contents of this catalyst were examined by chemical analysis. As a result, it was found that 0.18 times of magnesium and 1.08 times of cobalt were contained as divalent with respect to ₃ moles of Al ₂ O of zeolite.

Example 4 <Preparation of Catalyst 4> Ion exchange was performed in the same manner as in Example 1, except that calcium was used as the alkaline earth metal. This catalyst was designated as catalyst 4, and the calcium and cobalt contents of this catalyst were examined by chemical analysis. As a result, calcium was 0.16 times and cobalt was divalent at 1.16 times the mole number of Al ₂ O _{3 in the} zeolite.
04 times included.

Example 5 <Preparation of Catalyst 5> Ammonium-type ZSM-5 having a SiO ₂ / Al ₂ O ₃ molar ratio of 40, 20 g
Was added to 180 g of an aqueous solution of cobalt (II) acetate tetrahydrate having a concentration of 1.23 mol / L, and the mixture was stirred at 80 ° C. for 16 hours. After the slurry was subjected to solid-liquid separation, the same operation was performed by adding the zeolite cake to an aqueous solution of the above composition prepared again. Continue with concentration 1.
Injected into 180 g of an aqueous solution of 09 mol / L barium chloride,
Stirred for 6 hours. After solid-liquid separation, the mixture was sufficiently washed with water, and dried at 110 ° C. for 10 hours. When barium and cobalt contents of this catalyst were examined by chemical analysis, barium was contained 0.58 times and cobalt was contained as divalent 1.22 times with respect to ₃ moles of Al ₂ O _{3 of} zeolite.

Example 6 <Preparation of Catalyst 6> Ammonium-type ZSM-5 having a SiO ₂ / Al ₂ O ₃ molar ratio of 40, 20 g
Was added to 180 g of an aqueous solution of cobalt (II) acetate tetrahydrate having a concentration of 0.23 mol / L, and the mixture was stirred at 80 ° C. for 16 hours. After the slurry was subjected to solid-liquid separation, the same operation was performed by adding the zeolite cake to an aqueous solution of the above composition prepared again. After solid-liquid separation,
After sufficiently washing with water and drying at 110 ° C. for 10 hours, the cobalt content of the zeolite was determined by chemical analysis. As a result, it was found that the zeolite contained 1.40 times as cobalt divalent with respect to ₃ moles of Al ₂ O of the zeolite. Further, 20 g of the zeolite was charged into 29 ml of a 0.05 mol / L barium nitrate aqueous solution containing a barium amount equivalent to 1 wt% as metal barium, and then heated at 85 ° C. for 10 hours, followed by 11 hours.
Evaporation to dryness was performed by drying at 0 ° C. for 10 hours. This catalyst was designated as catalyst 6.

Comparative Example 1 <Preparation of Comparative Catalyst 1> Ammonium-type ZSM-5 having a SiO ₂ / Al ₂ O ₃ molar ratio of 40, 20 g
Was added to 180 g of an aqueous solution of cobalt (II) acetate tetrahydrate having a concentration of 0.23 mol / L, and the mixture was stirred at 80 ° C. for 16 hours. After the slurry was subjected to solid-liquid separation, the same operation was performed by adding the zeolite cake to an aqueous solution of the above composition prepared again. After solid-liquid separation,
After sufficiently washing with water and drying at 110 ° C. for 10 hours, this catalyst was used as Comparative Catalyst 1. When the cobalt content of this catalyst was examined by chemical analysis, it was found to be contained 1.39 times as cobalt divalent with respect to ₃ moles of Al ₂ O _{3 of} Zeorato.

Comparative Example 2 <Preparation of Comparative Catalyst 2> Ammonium-type ZSM-5 having a SiO ₂ / Al ₂ O ₃ molar ratio of 40, 20 g
Was added to an aqueous solution of copper (II) acetate hydrate having a concentration of 0.1 mol / L weighed so as to be twice the number of moles of alumina contained therein, and 2.5% ammonia water was added immediately. The pH of the aqueous solution was adjusted to 10.5 with stirring at room temperature for 16 hours. After the solid-liquid separation, the catalyst was sufficiently washed with water and dried at 110 ° C. for 10 hours. When the copper content of this catalyst was examined by chemical analysis, it was found to be contained 1.04 times as copper divalent with respect to ₃ moles of Al ₂ O in the zeolite.

Comparative Example 3 <Preparation of Comparative Catalyst 3> Ammonium-type ZSM-5 having a SiO ₂ / Al ₂ O ₃ molar ratio of 40, 20 g
Was added to 180 g of an aqueous solution of barium chloride having a concentration of 1.09 mol / L, and the mixture was stirred at 80 ° C. for 16 hours. After solid-liquid separation, wash thoroughly with water,
After drying at 110 ° C. for 10 hours, Comparative Catalyst 3 was obtained. When barium content of this catalyst was examined by chemical analysis, barium content was 0.76 times that of zeolite Al ₂ O ₃ moles.

Example 7 <Evaluation of activity of catalyst> The catalysts 1 to 6 prepared in Examples 1 to 6 were crushed after press molding and sized to 12 to 20 mesh, and 0.65 g thereof was filled in a normal pressure fixed bed reaction tube. . A gas having the following composition (hereinafter, referred to as a reaction gas) was circulated at a flow rate of 600 ml / min., The temperature was raised to 500 ° C., and the temperature was maintained for 0.5 hour to perform pretreatment. Then 200 ° C
The temperature was raised to 800 ° C. at a rate of 5 ° C./min. (Reaction 1). It was kept as it was at 800 ° C. for 5 hours, and cooled by changing the flowing gas to nitrogen. The mixture was cooled to room temperature, the reaction gas was used as a flowing gas, the temperature was raised to 200 ° C, and the temperature was maintained for 0.5 hour to perform pretreatment. Thereafter, the temperature was raised to 800 ° C. at a rate of 5 ° C./min. (Reaction 2). Table 1 shows the result of evaluating the durability of the catalyst by the change in the maximum activity value in Reaction 1 and Reaction 2 with NO as the harmful component of nitrogen oxide in the reaction gas. NO
The purification rate is represented by the following equation.

NO conversion rate _{(%) = (NO in -NO} out) / NO in × 100 NO in: fixed bed reaction tube inlet NO concentration NO _out: fixed bed reaction tube outlet NO concentration reactive gas composition NO 700ppm O ₂ 4% CO 1000ppm C ₃ H ₆ 400 ppm H ₂ O 3% N ₂ Balance Comparative Example 4 <Evaluation of Comparative Catalyst Activity> Comparative Catalysts 1 to 3 obtained in Comparative Examples 1 to 3 were used in Example 7
Table 1 shows the results of the evaluation of the activity using the same method as described above.

(Effects of the Invention) From Table 1, it can be seen that the catalyst of the present invention has an initial and reaction gas of 80%.
In addition to the activity after holding at 0 ° C. for 5 hours, there is an effect that the exhaust gas purifying ability in the excess oxygen exhaust gas is higher than that of the comparative catalyst, and that it shows extremely excellent heat resistance and durability.

Therefore, by bringing the catalyst of the present invention into contact with the exhaust gas, an effect is obtained that nitrogen oxides, carbon monoxide and hydrocarbons can be purified even in an excess oxygen state.

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Shinichi Matsumoto 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation (72) Inventor Kazunobu Ishibashi 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation ( 72) Inventor Koji Yokota 41 at Chuo-Cho, Yokomichi, Nagakute-cho, Aichi-gun, Aichi Prefecture Inside Toyota Central R & D Laboratories, Inc. (56) References JP-A-1-130735 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) B01J 29 / 064,29 / 42 B01D 53/86

Claims (2)

(57) [Claims] 1. A zeolite catalyst for removing nitrogen oxides, carbon monoxide and hydrocarbons from an oxygen-excess exhaust gas containing nitrogen oxides, carbon monoxide and hydrocarbons, comprising: SiO ₂ / Al
An exhaust gas purifying catalyst, which is a zeolite having a ₂ O ₃ molar ratio of at least 15 or more and contains cobalt and an alkaline earth metal. 2. The exhaust gas purifying catalyst according to claim 1, wherein a combustion exhaust gas containing nitrogen oxides, carbon monoxide and hydrocarbons is brought into contact with said exhaust gas purifying catalyst. How to remove.