JPH03202157A - Catalyst for purifying exhaust gas - Google Patents

Abstract

PURPOSE:To obtain a NOx removing catalyst having excellent purifying capacity and durability generating no lowering of catalytic activity by supporting copper, alkaline earth metal and rare earth metal on zeolite. CONSTITUTION:As a catalyst removing NOx in exhaust gas, copper, alkaline earth metal such as Mg or Ca and rare earth metal such as La or Ce are supported on zeolite. Whereupon, NO is adsorbed on Cu, and NO and NO2 are adsorbed on alkaline earth metal and rare earth metal, and NO, etc., are immediately subjected to the catalytic reaction with the org. compound in exhaust gas to be reduced to harmless N2. This catalyst has excellent purifying capacity in an oxygen excessive atmosphere held to a wide range temp. region of 800 deg.C or lower and generates no lowering of activity even when it is used for a long time.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention is directed to the treatment of nitrogen oxides (No.

).Related to a catalyst that efficiently removes

(Description of Prior Art) In recent years, nitrogen oxides (N
o, ) and cause air pollution. Therefore, various methods are being considered to remove NOx from the exhaust gas.

Lean burn is also being considered in order to improve the fuel efficiency of automobiles. In this case, the exhaust gas becomes an oxygen-rich atmosphere on the lean side, and in the conventional three-way catalyst in which precious metals are supported on a carrier such as An20s, among the harmful components in the exhaust gas, HC,
Even if Co can be removed by oxidation, NO8 cannot be purified due to the lack of reducing substances in the exhaust gas. Copper (CU) ion-exchanged with zeolite is used as a catalyst to solve this problem.
) catalyst (Japanese Unexamined Patent Publication No. 63-283727).

The basic principle of removing NO in an oxygen-rich atmosphere using this Cu-supported zeolite catalyst is that Cu adsorbs NO, and the adsorbed NOx reacts with reducing unburned hydrocarbons contained in the exhaust gas. The goal is to reduce the amount to N2. Although this catalyst exhibits NOx purification ability at temperatures above 200°C, it has the following problems.

(Problems with conventional technology) Cu-supported zeolite catalysts have extremely excellent properties in terms of initial catalytic activity because Cu has excellent adsorption ability for NOx, but they lack durability, especially durability at high temperatures. There was a problem. Therefore, it has been desired to develop a catalyst that has excellent catalytic activity even after long-term use. The reason for the poor durability of this catalyst is that at high temperatures of about 600°C or higher, copper moves in the zeolite and aggregates, losing its catalytic effect, and the stability of the zeolite structure decreases due to supporting Cu. This is due to the structure breaking down after long-term use. In addition, since the catalyst cannot reduce NO8 at temperatures below 200°C, NOx
There was a problem that it could not be purified. The reason for this is 20
This is because under an excess of oxygen at 0° C. or lower, some NO exists as NO2, and the Cu-supported zeolite catalyst cannot reduce NO2 to N2.

(Objective of the Invention) The present invention has been made to solve the problems of the prior art, and has a purification ability superior to that of conventional catalysts in a wide temperature range of 80°C or less in an oxygen-rich atmosphere. It is an object of the present invention to provide a novel catalyst for removing No. 2, which has excellent durability and does not lose its catalytic activity even when used for a long time.

(Description of the first invention) The first invention is a catalyst for removing nitrogen oxides in exhaust gas in an oxygen-rich atmosphere and in the presence of hydrocarbons, which comprises Cu and alkaline earth metals in zeolite. The present invention relates to an exhaust gas purification catalyst characterized in that it supports one or more kinds of rare earth metals and one or more kinds of rare earth metals.

Since the catalyst according to the present invention supports a composite of Cu, alkaline earth metal, and rare earth metal, in an oxygen-rich atmosphere,
It has a superior NOA removal ability compared to the conventionally known zeolite catalyst supporting Cu. CU has the ability to selectively adsorb No, and is superior to other materials in its No adsorption ability. Furthermore, alkaline earth metals themselves also adsorb No and NO2 and have catalytic activity. In the present invention, a rare earth metal is further added, and this rare earth metal itself adsorbs No and No2 and has No purification activity. It demonstrates performance.

Although the reaction in which the catalyst according to the present invention exerts its excellent effects is not clear, when exhaust gas comes into contact with the catalyst, No.
is adsorbed on Cu, and NO and NO2 are adsorbed on alkaline earth metals and rare earth metals, and this No etc. immediately reacts with organic compounds in the exhaust gas and is reduced to harmless N2.

The reaction formula for this reduction reaction is estimated to be as follows.

u HC+ v N O2-9 w H20+ y C○2+8N2 Moreover, Cu ions are easily reduced to metal Cu at a temperature of 600 to 800°C, move on the zeolite, aggregate, and have a disadvantage that durability is reduced.

The present invention has the effect of preventing this movement and aggregation of Cu by interposing alkaline earth metals and rare earth metals between Cu ions, thereby preventing a decrease in catalytic activity and improving durability.

In addition, NO8 exists as NO2 at temperatures below 200°C under excessive oxygen conditions, and becomes N at temperatures higher than 200°C.

It exists as. This catalyst produces NO at temperatures below 200°C.
Although it does not have the ability as a catalyst to reduce NO2 with HC, it has the ability to adsorb and hold NO2, and when the temperature of the catalyst rises to 200°C or higher and NO2 changes to NO, NO is reduced by HC. It can be reduced to N2 and has NO2 purification ability even at low temperatures below 200°C. Conventional Cu-containing zeolite catalysts have no ability to adsorb NO2 or catalytic ability to reduce NO2 to N2 at temperatures below 200°C, and cannot purify NO2. metals and rare earth metals are N O2
Since it has adsorption ability, it is possible to purify NO at a wide range of temperatures below 800°C.

In addition, strong acid sites, which are ion exchange points in zeolite, are a factor in the formation of coke, which is a combination of many graphites produced by decomposition of hydrocarbons, leading to pore clogging due to coke and even structural destruction of the zeolite. Alkaline earth metals and rare earth metals can eliminate excess strong acid sites that serve as attachment points for hydrocarbons, prevent coke formation, and prevent catalyst deterioration.

(Other inventions of the first invention) An invention that embodies the first invention will be described below.

In the present invention, the zeolite is SiO2 and Azz
It is composed of a tetrahedral network structure of O, and the individual tetrahedral structures are connected to each other through oxygen bridges through their corners, creating a network structure penetrated by passages and cavities.

Exchangeable cations (I-('-, Na+, etc.) are introduced into the negatively charged ion exchange points (strong acid sites) of the lattice.The molar ratio of SiO2/A120a is 10 to 200.
is desirable.

If it is less than 1O, thermal stability will deteriorate at high temperatures of 600°C or higher. Also, when the number exceeds 200, Al1203
Since the amount decreases and the ion exchange point decreases, the amount of ion exchange decreases, that is, the catalytic activity decreases. It is assumed that the main cause of thermal deterioration is structural changes around aluminum, so if you want to ensure durability especially at high temperatures, S i 0
A zeolite with a molar ratio of 2/Al1203 of 20 or more with a low A j220s t is used. Of these, S
iO2/ AI! Z'5M-5, Y or mordenite structures in which the molar ratio of 203 is 20 to 200 are particularly desirable. Further, as the zeolite, it is desirable to have NH, + or H+ attached to strong acid sites, which facilitates ion exchange with Cu or alkaline earth metals, but Na-type zeolites can also be used. Further, it is desirable that the pores on the zeolite surface be as small as 10 Å or less. By making the pores large enough to prevent polycyclic aromatic hydrocarbons from entering, it is difficult to generate liquid coke.
Structural destruction and catalyst activity reduction due to pore clogging can also be prevented.

The amount of Cu supported is 5 to 80 per A1 atom in the zeolite.
% is desirable. If it is less than 5%, a sufficient catalytic effect cannot be obtained. The catalyst performance improves as the amount of support increases, but 80
% or more, Cu tends to move and aggregate, resulting in deterioration, and it also becomes difficult to support alkaline earth metals and rare earth metals.

8- Rare earth metals are used by supporting more than ■ species. Desirable rare earth metals include LaXCe, Nd1YXPr, and Sm. The supported amount is 0.1 to 10 in weight ratio to zeolite.
%. The effect is shown from 0.1% by weight, but 0.3% by weight or more is preferable to obtain a sufficient effect. However, if it exceeds 10%, the number of acid sites necessary for the reaction will decrease, making it difficult for the reaction between No and the hydrocarbon to proceed.

Increasing the amount of rare earth metal supported generally reduces the optimum NOx
Since the temperature at which the catalyst can be purified shifts to higher temperatures, it is necessary to increase the supported amount depending on the catalyst usage conditions.

One or more types of alkaline earth metals are supported and used.

Among them, Mg, Ca, Sr, and Ba are preferable.

The alkaline earth metal functions to eliminate unnecessary acid sites involved in carbon precipitation, which causes a decrease in activity, and to prevent dealumination from zeolite.

The amount of alkaline earth metal supported is preferably 0.05 to 2% by weight relative to the zeolite. In order to obtain a sufficient effect, the content is preferably 0.1% by weight or more. However, even if the amount exceeds 2% by weight, there is no improvement in catalyst activity.

Cu, alkaline earth metals, and rare earth metals are supported by an ion exchange method or an impregnation method. Both the ion exchange method and the impregnation method are carried out using acetates, nitrates, etc. of Cu, alkaline earth metals, and rare earth metals. In both methods, the order of loading does not matter. You may do both at the same time. Both the ion exchange method and the impregnation method may be commonly used. For example, in the case of the ion exchange method, Na+, H+, etc. introduced into the negatively charged ion exchange points of the zeolite lattice are Cu,
This is done by exchanging rare earth metal or alkaline earth metal ions. Supporting by ion exchange method is carried out by the following steps. An ion exchange process in which zeolite is immersed in an aqueous solution of Cu, rare earth metal, acetate or nitrate of alkaline earth metal for 24 to 48 hours, and a drying process in which the zeolite is heated at 100 to 120°C for approximately IO hours, at a temperature of 500 to 700°C. It consists of a firing process in which the material is held for several hours. Further, in the impregnation method, the salt is immersed in an aqueous solution of the salt for 1 to 2 hours and then dried in the air to be supported. In the ion exchange method, ions of Cu, rare earth metals, and alkaline earth metals are ion-exchanged with cations in zeolite, and the adhesion of Cu, rare earth metals, and alkaline earth metals is strong.

When performing ion exchange, it is easier to perform the exchange in a solution made slightly basic by adding ammonia or the like. Also,
The pH of the solution is preferably in the range of 9-12.

The catalyst supporting Cu, alkaline earth metal, and rare earth metal according to the present invention may have any shape or structure such as powder, pellet, or honeycomb.

In addition, a binder such as alumina sol or silica sol may be added to a powdered catalyst and molded into a predetermined shape, or water may be added to form a slurry and applied onto a refractory substrate such as alumina in the shape of a honeycomb. You can.

The catalyst according to the present invention purifies NO8 in exhaust gas by reacting it with unburned hydrocarbons or oxygen-containing compounds produced by partial combustion.

This hydrocarbon etc. can remain in the exhaust gas, but if the amount of hydrocarbon etc. is insufficient than the amount required for the reaction to occur, hydrocarbons may be added to the exhaust gas from outside. It is better.

(Example) The present invention will be explained in detail below.

Example 2 A catalyst according to the present invention was prepared, and the catalyst was evaluated for purification activity against No using a model gas in a lean state with excess oxygen. Similar activity evaluations were also conducted for comparative catalysts.

Zeolite H-type ZSM 5 (Si02/A1
203=40) powder, Cu+Ba+Y (No.

1), Cu+Mg+Y (No, 2), Cu+Ca
It was immersed for 15 minutes in a mixed aqueous solution of acetates of +Y (No. 3), Cu + Ba + La (La concentration is set to 4 levels. No. 4 to 7), and then dried at 110° C. for 10 hours. after that,
The example catalyst (NO1 to 7) was fired at 500°C for 3 hours in air.
) was obtained. Comparative catalyst No. 1 was prepared by soaking the zeolite in an aqueous solution of Cu acetate (0.1 moj2/V) and subjecting it to ion exchange overnight to support Cu.

C1, and after ion-exchanging Cu, it was washed with water and heated to 110°C.
After drying for 10 hours, a catalyst obtained by supporting Ba by the impregnation method as described above was designated as comparative catalyst No. C2.

Table 1 shows the supported amounts of these catalysts.

The present example catalysts No. 1 to 7 and comparative catalyst No. ClXC2 in the form of pellets were held at 800° C. for 3 to 45 hours in a model gas simulating automobile exhaust gas shown in Table 2. Thereafter, the No. 1 purification rate at 400°C was measured in the model gas shown in Table 2, and the results are shown in Table 3. It can be seen that catalysts Nos. 1 to 7 of this example have significantly better durability than the comparative catalysts.

Example 2 The durability of the catalyst according to the present invention was evaluated considering city driving.

500 g of powder of catalyst No. 4 and No. C1, C2 shown in Table 1 and 70 g of silica sol (10 wt% SS102)
Mix and stir 0 and 100g of pure water, and adjust the pH with ammonia water.
was adjusted to lO ~ 11 to obtain a coating slurry,
The slurry was coated at 120 g/12 on a 0.71 carbon fiber honeycomb carrier.

The carrier coated with catalyst No. 4 was used as catalyst No.
8.No.

Comparison of CI-coated carriers with catalyst No., C3, No., C
The carrier coated with No. 2 was designated as comparative catalyst No. C4.

The initial activity and durability of these catalysts were evaluated using an actual engine.

Durability test conditions Example catalyst No. 8, comparative catalyst No. C3, C4, vehicle weight l equipped with a 1600cc lean burn engine
It is attached to the engine exhaust system of a 10-ton car with a pattern that simulates city driving where the maximum gas temperature is around 800℃.
I ran 00km and 30000km.

NO, purification rate measurement Average air-fuel ratio of the engine: 22, entering gas temperature: 400°C
No, the purification rate was measured. The results obtained are shown in Table 4.

Claims (1)

[Claims] A catalyst for removing nitrogen oxides from exhaust gas in an oxygen-rich atmosphere in the presence of hydrocarbons, which comprises zeolite, copper, one or more alkaline earth metals, and one or more rare earth metals. An exhaust gas purifying catalyst characterized by supporting.