JPH0352644A - Catalyst for purifying exhaust gas - Google Patents

Abstract

PURPOSE:To obtain a catalyst which can efficiently removes NOx in exhaust gas in an environment of excessive oxygen by allowing zeolite to carry alkaline earth metal. CONSTITUTION:A catalyst which efficiently reduces and removes NOx in exhaust gas in as environment of excessive oxygen with presence of hydrocarbon can be obtained by allowing zeolite to carry alkaline earth metal such as Ca, etc. By this method, the alkaline earth metal is introduced to an ion-exchange point to eliminate excess strong acid points where hydrocarbon deposites, and therefore, production of cokes and deterioration of catalyst can be prevented. The obtd. catalyst shows no decrease in the activity after used for a long time in an atmosphere of excessive oxygen in a wide temp. range below about 800 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a catalyst that efficiently removes nitrogen oxides from exhaust gas discharged from internal combustion engines such as automobiles, nitric acid manufacturing plants, etc.

(Description of Prior Art) In recent years, exhaust gases emitted from internal combustion engines such as automobiles, nitric acid manufacturing factories, etc. contain nitrogen oxides (NOx), which are harmful substances.
x) and cause air pollution. Therefore, removal of nitrogen oxides from this exhaust gas is being studied in various fields.

Lean burn is also being considered in order to improve the fuel efficiency of automobiles. In this case, the air-fuel ratio becomes a lean oxygen-rich atmosphere,
Conventionally used three-way catalysts in which noble metals are supported on carriers such as A120, remove HC and C among the harmful components in exhaust gas.
Even if NOx can be removed by oxidation, NOx cannot be purified because there is no reducing substance in the exhaust gas. As a catalyst that solves this problem, there is a Cu catalyst ion-exchanged with zeolite (Japanese Patent Application Laid-Open No. 63-283-727).

The basic principle of NOx removal in an oxygen-rich atmosphere using this Cu-supported zeolite catalyst is to reduce NOx to N2 by reducing unburned hydrocarbons contained in exhaust gas. Although this catalyst exhibits NOx purification ability at temperatures above 200°C, it has the following problems.

(Problems with Prior Art) Cu-supported zeolite catalysts have problems with durability, particularly durability at high temperatures, and there has been a desire to develop a catalyst with even greater durability. The reason for poor durability is that copper moves and aggregates in the zeolite at temperatures above 600°C, reducing its catalytic effect, and the stability of the zeolite structure decreases, causing the structure to partially change after long-term use. This is because they are easily destroyed. Further, there is a problem that the ability to reduce and remove NOx is low at temperatures below 200°C. The reason for this is that part of Noe exists as NO2 under an excess of oxygen at 200° C. or lower, and it is difficult for the Cu-supported zeolite catalyst to reduce NO2 to N2.

(Object of the Invention) The present invention was made to solve the problems of the prior art described above, and the catalyst activity decreases even when used for a long time in an oxygen-rich atmosphere and in a wide temperature range of 800°C or less. An object of the present invention is to provide a novel catalyst for removing NOx, which has excellent durability.

(Description of the first invention) The first invention is a catalyst for reducing and removing NOx in exhaust gas in an oxygen-rich atmosphere in the presence of hydrocarbons, etc. The present invention relates to an exhaust gas purifying catalyst characterized in that it supports at least one species.

By having the above-mentioned structure, the catalyst according to the present invention can efficiently reduce NOx, that is, NO2, NO
can be removed. NO2 or No in the exhaust gas is adsorbed onto the alkaline earth metal. This catalyst does not directly decompose these NO2 etc. into nitrogen and oxygen, but instead uses lower organic compounds such as trace amounts of unburned hydrocarbons contained in the exhaust gas or oxygen-containing organic compounds produced by partial combustion. NO2 etc. are reduced and removed using the following formula.

u H C + v N O 2 −1→wHto
10yCO2 +zN2 In addition, alkaline earth metal ions, such as Ca0, are C
It exists on the surface of the catalyst in the form of aO, sharing it with oxygen, and has the property of being difficult to be reduced to Ca. Therefore, the catalyst according to the present invention exhibits extremely excellent durability even at high temperatures of 600° C. to 800° C. without reducing and migrating agglomeration and reducing catalyst activity. In addition, strong acid sites, which are ion exchange points in zeolite, are a factor in the formation of coke in which a large number of graphites bonded together are generated by decomposition of hydrocarbons, leading to pore clogging by coke and structural destruction of the zeolite. By introducing the alkaline earth metal into the ion exchange point, it can eliminate excess strong acid sites that serve as attachment points for hydrocarbons, prevent the formation of coke, and prevent deterioration of the catalyst. In addition, NOx is 20% under oxygen excess.
At temperatures below 0°C, it exists as NO2, and at temperatures above 200°C, it exists as No. Although this catalyst does not have the ability to reduce NO2 with HC at temperatures below 200°C, it does have the ability to adsorb and retain NO2.
When the temperature of the catalyst rises to 200℃ or more and NO2 changes to No, No can be reduced to N2 by HC, and 200℃
It has NO2 purification ability even at low temperatures below ℃. Conventional C
When u-containing zeolite catalyst is below 200℃, NO2
The catalyst according to the present invention has a small catalytic ability to adsorb NO2 and a catalytic ability to reduce NO2 to N2, and its NO2 purification ability is weak.
Purification of NOx is possible over a wide range of temperatures below 0°C.

(Description of the second invention) Hereinafter, an invention embodying the first invention (hereinafter referred to as the second invention)
Explain.

In the second invention, zeolite is composed of a tetrahedral network structure of Sing and Al20s, the individual tetrahedral structures are connected to each other through oxygen bridges through their corners, and the zeolite is a 3-dimensional structure with passages and cavities passing through it. It creates the original network structure. Ion exchange points with negative charges in the lattice (
Exchangeable cations (H+, Na+, etc.) are introduced into the strong acid sites. The molar ratio of S io2 /Af20a is desirably 1 to 200. If the amount is less than 1, thermal stability will deteriorate at high temperatures of 600° C. or higher. Moreover, when it exceeds 200, the amount of Aj2zOs decreases and the number of ion exchange points decreases, resulting in a decrease in the amount of ion exchange, that is, a decrease in catalytic activity. 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, the molar ratio of Sin2/Al203 should be changed to A.
A zeolite with a low amount of jll203 of 20 or more is used. Further, it is desirable that the pores on the zeolite surface be as small as about 4 to 10 pores. If the pores are about the same size as a benzene ring, coke is less likely to form, and structure destruction and catalyst activity reduction due to pore clogging can be prevented.

Among these zeolites, S i 02 /AI! 2
Particularly desirable are ZSM-5, Y, or mordenite structures in which the molar ratio of 03 is 30 to 50.

These have a large ion exchange capacity and are thermally stable.

This is also because it has an appropriate acid site and acid amount for the reduction reaction of NOx by HC.

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

Among them, M g SC a XS r −, B a
is preferred.

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 wt%. In order to obtain a sufficient effect, the content is preferably 0.1'wt% or more. However, even if the amount exceeds 2 wt%, there is no improvement in catalyst activity. The alkaline earth metal is usually supported by an ion exchange method or an impregnation method. Ion exchange involves introducing N into the negatively charged ion exchange points of the zeolite lattice.
This is done by exchanging a+, H+, etc. with 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 day and night in an aqueous solution of alkaline earth metal acetate or nitrate, and
Drying step of heating at 120° C. for about 10 hours, 500-700
It consists of a firing step held at a temperature of 0°C 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, alkaline earth metal ions are ion exchanged with cations in zeolite, and alkaline earth metals have strong adhesion.

The alkaline earth metal-supported catalyst according to the second invention may have any shape or structure, such as powder, pellet, or nickel.

In addition, a binder such as alumina sol or silica sol may be added to a powdered catalyst and molded into a specified 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. It may also be used.

The catalyst according to the second invention purifies N O x in exhaust gas by reacting it with 02 to C8 lower organic compounds such as unburned hydrocarbons or oxygen-containing organic compounds produced by partial combustion. It is.

This hydrocarbon, etc. may remain in the exhaust gas, but if the amount of hydrocarbon, etc. is insufficient than the amount necessary for the reaction to occur, add hydrocarbon, etc. from the outside to the exhaust gas. It's good to do that.

(Example) Examples of the present invention will be described below.

Example 1 A catalyst according to the present invention was prepared, and its NO purification activity was evaluated using a model gas in a lean state with excess oxygen. Similar activity evaluations were also conducted for comparative catalysts.

This example: Preparation of medium (No. 1) ZSM-5 structure zeolite (SiO2/AAiO, molar ratio 40) was dissolved in a 0.02M calcium acetate aqueous solution for 3 hours.
Ion exchange was performed by holding at 0°C for one day and night. The ion exchange rate in this case was 43%. After ion exchange 1
After heating to 10℃ for 10 hours and drying, further heating to 500℃
Zeolite catalyst No. 3 was calcined for 3 hours to support calcium. I got 1.

The amount of calcium supported is 0.73 weight (w) based on the total amount of catalyst.
t)%.

Comparison Using zeolite with the same structure and composition as in Preparation Example 1 of (No. C1), 0.
A zeolite catalyst NαC1 carrying copper was obtained under the same conditions as in Example 1 except that a 1M aqueous copper acetate solution was used. The amount of copper supported is 1.5% of the total amount of catalyst. It was 9 wt%.

Catalyst No. of this example mentioned above. The NOx purification rate of Comparative Catalyst N (LCI powder) was measured in a model gas atmosphere in a lean state with excess oxygen (air-fuel ratio (A/F) l 8 phase, etc.), and the initial performance of each catalyst was determined. Next, a heat treatment was performed at 700°C for 5 hours in the model gas atmosphere to evaluate durability.The composition of the model gas was NO: 670 ppm,
HC: 1 180ppm, H2: 330ppm,
Co: lo00ppm, CO2: lO%, 02:
4%, H20:3%fNt: balance.

Table 1 shows the NOx purification rate of each catalyst at the initial stage and after the durability test. The No purification rate was expressed as the amount of No purified per l metal atom per l minute (not/min/atom).

It can be seen that the catalyst according to this example has the same initial performance as the comparative catalyst, but is significantly superior in durability at 700°C.

Example 2 In this example, activity evaluation for actual engine exhaust gas was performed.

This implementation No. 2 and comparative touch NG. Preparation of C2 60 parts of Ca-supported ZSM-5 structure zeolite produced in Example 1, silica sol 7 with a silica content of 20 wt%
0 parts, 8 parts of a commercially available aqueous solution of 40 wt% aluminum nitrate, and 130 parts of pure water were mixed and stirred. A washcoat slurry was prepared. A 0.7 L (107 mmφ, 78 mm L) monolithic cordierite carrier having 400 channels per 1 m cross-sectional area was immersed in this washcoat slurry and then pulled out. Next, the excess slurry in the channel of the monolithic carrier was blown away with compressed air and dried, and then calcined at 500°C for 3 hours to obtain catalyst No.
It was set as 2. The amount of calcium supported is 2 g per 1 liter of catalyst volume (0.43 wt % of calcium supported). For comparison, Catalyst N (LC2) was prepared by wash-coating a slurry containing Cu-supported ZSM-5 structure zeolite in the same way as Catalyst N (L2).

The following day, washcoat monolith 02 was applied to catalyst Nα2 of this example and comparison catalyst N from the exhaust manifold of the engine.
The catalytic activity was evaluated by installing the catalytic converter in an exhaust pipe separated by m. Engine operating conditions are: Engine: 2000CC (EFI),
Rotation speed: 1600 rpm, manifold negative pressure: 4 4
0 mmHg, air-fuel ratio (A/F): 20, exhaust gas inflow temperature to the catalytic converter is 400°C, NOx,
The concentrations of HC and Co are 500 and 2000.900 pp, respectively.
It is m. Table 2 shows the purification rate of each batch after operating for about 12 hours under these conditions.

As is clear from Table 2, it can be seen that the catalyst according to this example can maintain a high purification rate over a long period of time, especially for No, compared to the comparative catalyst.

Example 3 Using the same lean model gas with excess oxygen as in Example 1, the No purification characteristics of a catalyst with Mg supported on zeolite were evaluated.

ZSM-5 structure zeolite (St.2/Al20, molar ratio 40) of Shii catalyst (N(13)) was dissolved in 0.02M magnesium acetate solution.
The mixture was kept at 0°C overnight to perform ion exchange. The ion exchange rate in this case was 30%. 110 after ion exchange
After heating and drying at ℃ for 10 hours, it was further calcined at 500℃ for 3 hours to support magnesium and obtain catalyst N (L3).The amount of magnesium supported was 0.26 weight (
wt)%.

The comparative catalyst was Comparative Catalyst No. prepared in Example 1. C1 was used. Furthermore, the purification activity evaluation method and comparison method are the same as in Example 1.

The results are shown in Table 3.

It can be seen that the initial performance of the catalyst according to this example is equivalent to that of the comparative catalyst, but the durability performance at 700°C is significantly superior.

Claims (2)

[Claims] (1) Remove nitrogen oxides from exhaust gas under an oxygen-rich atmosphere.
1. A catalyst for exhaust gas purification, which is a catalyst for removing lower organic compounds such as hydrocarbons or oxygen-containing organic compounds in the presence of the organic compounds, and is characterized by having one or more alkaline earth metals supported on zeolite. (2) The exhaust gas purifying catalyst according to claim 1, wherein the alkaline earth metal is Ca, Sr, Mg, or Ba.