JP2616799B2 - Automotive exhaust purification system - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an automobile exhaust gas purification apparatus for an automobile engine using a gas mixture having a fuel ratio lower than the stoichiometric air-fuel ratio, that is, a so-called lean burn engine, and particularly to an exhaust gas. The present invention relates to an automobile exhaust gas purification device for purifying nitrogen oxides NOx in a high rate.

(Prior Art) Conventionally, if the air-fuel ratio of a gas mixture taken into an automobile engine is made larger than the stoichiometric air-fuel ratio, the fuel efficiency (energy output per unit fuel amount) is improved up to a certain ratio, and the stoichiometric air-fuel ratio is improved. It is known that the CO concentration in the exhaust gas can be greatly reduced while keeping the torque fluctuation almost constant compared to the case where the fuel ratio is smaller than the fuel ratio, and a mixed gas with a fuel ratio lower than the stoichiometric air-fuel ratio is used. There is a demand for the practical use of an automobile engine, a so-called lean burn engine, and the spread of automobiles equipped with a lean burn engine.

On the other hand, in this lean burn engine, the concentration of NOx in the exhaust reaches a maximum at a predetermined air-fuel ratio, and the air-fuel ratio at which the concentration of NOx in the exhaust reaches a maximum varies slightly depending on weather, operating conditions, etc. Therefore, if the concentration of NOx in the exhaust exceeds the reference value due to the weather, operating conditions, etc. in a lean burn engine, or if it is attempted to keep the agricultural level of NOx in the exhaust to a predetermined low concentration. In addition, it is necessary to provide a precise and complicated control mechanism, and there is also a problem that the drivability of the vehicle is reduced due to the restriction by the control mechanism.

In addition, these problems are lean burn exhaust, that is,
Exhaust gas purifying catalysts have also been proposed that can be eliminated by purifying and reducing NOx in exhaust gas generated when fuel is burned at an air-fuel ratio larger than the stoichiometric air-fuel ratio in a lean air-fuel ratio region. (Japanese Patent Application No. 62-288684).

(Problems to be Solved by the Invention) Certainly, according to the exhaust gas purifying catalyst in which the deolite ion-exchanged with the transition metal is carried on the refractory carrier, the NOx concentration in the lean exhaust gas is reduced to some extent. However, in order to greatly reduce the burden on the control mechanism and the like for maintaining the concentration below a predetermined level, and ultimately to eliminate the burden, purify NOx in lean exhaust at a higher ratio. It is desired.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide an automobile exhaust purification device capable of purifying NOx in lean exhaust at a high rate.

(Means for Solving the Problems) In order to achieve the above object, an automobile exhaust gas purification apparatus according to the present invention comprises a catalyst I for converting paraffin into olefin and a catalyst II for decomposing nitrogen oxide in an exhaust passage of a lean burn engine. And the catalyst I is disposed on the upstream side of the passage of the catalyst II.

Examples of the catalyst I include ZSM-5, ferrierite, and zeolites obtained by modifying these with metals such as Ni and Zn. A method of modifying ZSM-5 with metals such as Ni and Zn, and a method using a metal. For details such as the property of converting modified paraffin to olefin of ZSM-5, see Journal of Catalysis 10
0,322-327 (1986).

For the catalyst II, for example, Cu, Co, Cr, Ni, Fe, M
Zeolite ion-exchanged with a transition metal such as g or Mn, particularly Cu, can be used.

(Action) Although both the paraffin and the olefin are originally contained in the exhaust gas, in the present invention, the paraffin contained in the exhaust gas hydrocarbon is converted into the olefin by the catalyst I, and the ratio of the olefin is increased. Therefore, the catalyst II comes into contact with an exhaust gas containing a hydrocarbon having a higher olefin ratio than before. The decomposition of NOx in exhaust gas by this catalyst II
It is promoted as the concentration of olefin contained in the contacting exhaust gas increases. The reason is presumed as follows.

That is, the reaction mechanism of NOx purification is generated by the partial oxidation of hydrocarbons (HC) in the exhaust gas on the catalyst surface, and the CO adsorbed on the catalyst surface is expressed by the following formula: Decomposes NOx according to

On the other hand, partial oxidation of the HC, i.e. CO ^- for the generation of involved adsorption of the HC on the catalyst active sites, the higher the adsorption of the HC occurs partial oxidation easily. Here, the olefin has a double bond, and the π electron of this double bond has a much higher adsorptivity to the metal ion component of the catalyst II, for example, Cu ⁺ , compared to the σ electron of the single bond. For this reason, olefins have higher HC adsorbability than paraffin. Therefore, olefins are more likely to undergo partial oxidation than paraffins.

On the other hand, paraffin is a metal ion component such as Cu ⁺
Adsorbability to catalyst II is low, oxygen is strongly adsorbed to catalyst II, and the ratio of complete oxidation increases.

(Examples) Hereinafter, the present invention will be described specifically with reference to Examples,
The present invention is not limited to the following examples.

The pH was adjusted to 10-12 by the NH ₃ added to an aqueous solution containing Example 1 0.05 mol / copper acetate and 0.01 mol / nickel acetate. A monolith made of cordierite coated with ZSM-5 zeolite by a wash coat method is immersed in the obtained aqueous solution, and the mixture is stirred for about 1 to 24 hours to perform ion exchange. And fired at 500 to 700 ° C. for 1 to 24 hours while passing air through a firing furnace to obtain a monolithic catalyst A.

A monolithic catalyst B was obtained in the same manner as described above, except that nickel acetate was not used.

Each of the catalytic converters loaded with the obtained monolithic catalysts A and B was attached to the exhaust port of an automobile engine, and the engines were operated in a lean state with an air-fuel ratio of 15 or more to compare the amount of NOx in the exhaust. When A was loaded, the amount of NOx in the exhaust gas was smaller.

Further, the amount of copper acetate and nickel acetate contained in the aqueous solution when producing the monolithic catalyst A was changed, and the relationship between these amounts and the amount of NOx in the exhaust gas was examined. ~ 5 mol /, nickel acetate is 0.05 ~ 1 mol /
In the case of the above range, good results are obtained in reducing the amount of NOx in the exhaust gas.Furthermore, when the ferrierite zeolite is used in place of the ZSM-5 zeolite, the same effect as in the monolithic catalyst A is obtained. Good results were obtained in reducing the amount of NOx.

Example 2 NH ₃ was added to an aqueous solution containing 0.05 mol / copper acetate and an aqueous solution containing 0.05 mol / nickel acetate to adjust the pH to 1 respectively.
Adjusted to 0-12. Then, about half of the catalyst disposed on the exhaust upstream side as the catalyst of the cordierite monolith wash-coated with ZSM-5 zeolite contains the nickel acetate and is immersed in a pH-adjusted aqueous solution to perform ion exchange.
About half of the other half was immersed in an aqueous solution containing the above-mentioned copper acetate and adjusted for pH to carry out ion exchange, and thereafter a monolith catalyst C was obtained in the same manner as in Example 1.

The obtained catalytic converter loaded with the monolithic catalyst C was attached to the exhaust port of an automobile engine, the engine was operated under the same conditions as in Example 1, and the catalytic converter loaded with the monolithic catalyst B was attached to the exhaust port of the automobile engine. When the NOx amount in the exhaust gas was compared with that in the case of the attachment, the NOx amount in the exhaust gas was smaller when the monolithic catalyst C was loaded.

(Effects of the Invention) As described above, according to the present invention, the purification rate of NOx contained in exhaust gas of a lean burn engine is improved.

Therefore, a control mechanism such as an air-fuel ratio for maintaining NOx in lean exhaust at a predetermined low concentration can be simplified or omitted, leading to a reduction in cost of a vehicle equipped with a lean burn engine. In addition, control conditions such as the air-fuel ratio are relaxed, so that fuel efficiency can be improved without being substantially restricted by these control conditions, and drivability is also improved.

Furthermore, the purification rate of HC and CO in the exhaust gas is also improved, so that the oxidation catalyst for purifying the exhaust gas can be omitted.

Claims (1)

(57) [Claims] 1. A catalyst I for converting paraffin to olefin and a nitrogen oxide NOx in an exhaust passage of a lean burn engine.
At the same position as the catalyst II or the catalyst I
A vehicle exhaust purification device, which is disposed on the upstream side of the passage of II.