JPH09239276A - Exhaust gas cleaning catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a catalyst capable of selectively reducing NOx for cleaning to N2 at a high conversion rate even in the presence of a vapor by mixing a zeolite powder bearing tin with a manganese oxide powder. SOLUTION: An exhaust gas cleaning catalyst which is useful when a reducing agent is injected into an exhaust gas to clean NOx by reduction, is prepared by mixing a zeolite powder bearing Sn with Mn2 O3 powder. In this case, zeolite is crystalline aluminosilicate expressed by the formula xM2 /n.Al2 O3 .ySiO2 as is well-known, and should preferably have SiO2 /Al2 O3 molar ratio of 10-200. In addition, the surface pore of the zeolite should be small enough to not allow the penetration of a polycyclic aromatic hydrocarbon compound, measuring 10Å or less. Thus, coke is hardly generated, and the structure breakdown and the catalytic activity deterioration by the blocking of pores are prevented from occurring.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying catalyst for purifying exhaust gas emitted from an automobile or the like, and more specifically to NO _x by injecting a reducing agent such as hydrocarbon into the exhaust gas.
The present invention relates to an exhaust gas-purifying catalyst useful for reducing and purifying exhaust gas.

[0002]

2. Description of the Related Art Monolith catalysts and pellet catalysts have been widely used as exhaust gas purifying catalysts for automobile internal combustion engines. These exhaust gas purifying catalysts have a structure in which a catalyst metal is supported on a catalyst supporting layer such as activated alumina. As the catalyst metal, Pt, Pd, Rh, etc. are used alone or in combination of two or more. It is called a three-way catalyst because it oxidizes HC and CO in the exhaust gas and reduces NO _x to purify these three components at the same time.

By the way, for the purpose of reducing fuel consumption and reducing carbon dioxide gas emission, it is necessary to burn fuel lean in an internal combustion engine of an automobile. In this case, the oxygen concentration in the exhaust gas becomes leaner than the theoretical value (the minimum oxygen concentration required to completely oxidize the unburned components in the exhaust gas), and the three-way catalyst causes NO in the exhaust gas to rise. The drawback is that _x cannot be reduced and removed. Further, in a diesel engine, since a large amount of air is used for combustion, the oxygen concentration in the exhaust gas becomes large, and similarly,
Purification of O _x is difficult.

Therefore, in recent years, zeolite-based catalysts utilizing zeolite have been drawing attention, and the applicants of the present application have proposed some zeolite-based catalysts having excellent NO _x purification performance.
For example, JP-A-1-130735 discloses an exhaust gas purifying catalyst in which a transition metal is ion-exchanged and supported on zeolite. According to this catalyst, an active site of a transition metal exists in the pores of zeolite, and NO _x is adsorbed to the active site to cause a reaction and purification. Copper (Cu) is preferable as the transition metal.

Further, in Japanese Patent Application Laid-Open No. 3-89942,
Disclosed is an exhaust gas purifying catalyst in which a rare earth element and copper (Cu) are supported on zeolite. JP-A-3-2021
Japanese Patent Publication 57 discloses an exhaust gas purifying catalyst in which an alkaline earth metal, a rare earth element, and copper (Cu) are supported on zeolite. Further, in Japanese Patent Laid-Open No. 2-149317, Cu, Mn, Fe, N is added to hydrogen type zeolite.
A zeolite-based catalyst carrying an element selected from i, Co, Rh, Pd, Pt, V, and Mo is disclosed.

Further, a reducing agent such as light oil or propane is injected into the exhaust gas in an oxygen excess atmosphere, and this reducing agent causes NO _x.
Various methods for purifying nitrogen by selectively reducing it to nitrogen (N ₂ ) have been studied. For example, Japanese Examined Patent Publication No. 5-16887 uses a catalyst selected from a proton type zeolite, an alkali metal type zeolite or an acidic metal oxide,
NO _{x in} exhaust gas in an oxygen excess atmosphere in the presence of hydrocarbons
A method of purifying is described.

[0007]

However, using a reducing agent
NO_xEven if the catalyst selectively reduces and purifies NO
_xN_TwoThe conversion rate to was not sufficient. Recent research
According to research, it is
We understand that one of the factors is the inhibition of the reaction by water vapor.
ing. In other words, if the atmosphere does not contain water vapor, N
O_xWith a high conversion rate_TwoEven though it can be selectively reduced and purified
On the other hand, the conversion rate decreases significantly in the atmosphere containing water vapor.
It is known that there is a phenomenon of doing.

Since a large amount of water vapor is contained in exhaust gas from automobiles, it is desired to develop a catalyst that can efficiently reduce and purify NO _x even in the presence of water vapor. The present invention has been made in view of such circumstances, and an object thereof is to provide a catalyst capable of selectively reducing and purifying NO _x to N ₂ at a high conversion rate even in the presence of water vapor.

[0009]

The characteristics of the exhaust gas purifying catalyst of the present invention for solving the above-mentioned problems are that a zeolite powder carrying tin (Sn) and manganese oxide (Mn ₂ O ₃ ) powder are mixed. Is to be.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION As is well known, zeolite has the general formula
xM_Two/ N ・ Al_TwoO_Three・ YSiO_TwoCrystallinity
With alumino-silicic acid, the difference between M (n-valent metal), x and y
Therefore, the pore size in the crystal structure is different, and many types
Are commercially available. Also Si⁴⁺Part of Al³⁺Replace with
Therefore, there is a shortage of positive charge, and to compensate for the shortage, Na
⁺, K ⁺Has the property of holding cations such as
Therefore, it has a high cation exchange capacity.

The SiO ₂ / A of the zeolite used in the present invention
The molar ratio of l ₂ O ₃ is preferably 10 to 200. If it is less than 10, the thermal stability at a high temperature of 500 ° C. or higher deteriorates. On the other hand, when it is more than 200, the ion exchange point is decreased, so that the ion exchange is decreased, that is, the catalytic activity is decreased. Since it is estimated that the thermal deterioration is mainly due to the structural change around aluminum, it is preferable to use zeolite with a molar ratio of 20 or more, especially when it is desired to ensure durability at high temperatures. In particular, it is recommended to use ZSM-5 with a molar ratio of 20-200.

[0012] In the hydrogen type zeolite, dealumination from the zeolite easily occurs and the durability is lowered. Therefore, it is preferable to use the Na type. Further, it is desirable that the pores on the surface of zeolite are as small as 10 angstroms or less. By making the pores of a size that does not allow the polycyclic aromatic hydrocarbon compound to enter, it becomes difficult for coke to form, and it is also possible to prevent structural destruction and catalyst activity deterioration due to pore clogging.

The amount of Sn supported on the zeolite is preferably in the range of 0.1 to 3 in molar ratio with respect to the aluminum element in the zeolite. If the supported amount of Sn is less than this, the reaction rate becomes slow, and if the supported amount of Sn is more than this, only hydrocarbons such as propene may be oxidized and the nitrogen addition rate may decrease. As a method for supporting Sn on zeolite, an ion exchange supporting method such as an adsorption method or an impregnation method can be used as in the case of a conventional zeolite-based catalyst.

The Sn-supported zeolite powder preferably has an average particle size of 1 μm or less. If the average particle size is larger than this, the reaction rate becomes slower, and NO _x
Purification rate decreases. The mixing ratio of the Sn-supporting zeolite powder and the manganese oxide powder is preferably in the range of Sn-supporting zeolite / Mn ₂ O ₃ = 0.1 to 10 by weight ratio. If this ratio is smaller than this, the nitrogen addition rate at high temperatures is reduced, and if it is larger than this, the nitrogen addition rate at low temperatures is reduced.

The manganese oxide powder preferably has an average particle size of 1 μm or less. If the average particle size is larger than this, the oxidation of NO to NO ₂ is reduced, and the NO _x purification performance is reduced. The exhaust gas purifying catalyst of the present invention can be used by coating a mixed powder of Sn-supported zeolite powder and manganese oxide on a monolith carrier base material or a metal carrier base material. Further, the mixed powder itself can be molded into a pellet shape or a honeycomb shape and used.

In the exhaust gas purifying catalyst of the present invention, the oxidation reaction of NO to NO ₂ is promoted by the manganese oxide powder. And is reduced and purified by NO2 _2, Sn supported zeolite contained in NO ₂ and advance in the flue gas formed by reaction with a reducing agent such as a hydrocarbon. In the exhaust gas purifying catalyst of the present invention, the presence of water vapor not only hinders the above reaction, but rather acts to promote the reaction, so that an extremely high NO _x purification performance is obtained. The reason for this has not been clarified yet, but it is presumed that the effect that the oxidation reaction activity of NO by the manganese oxide powder does not decrease even if water vapor is present may be one factor.

[0017]

EXAMPLES The present invention will be specifically described below with reference to Examples and Comparative Examples. (Example 1) Na-type zeolite powder ("Na-ZSM-
5 ", molar ratio SiO ₂ / Al ₂ O ₃ = 23.8) is immersed in distilled water, and the mixture is allowed to stand still overnight while bubbling nitrogen gas. This removes oxygen in the zeolite. Next, the zeolite is taken out in a nitrogen stream, and while bubbling nitrogen gas, it is immersed in a 1.3 wt% SnCl ₂ aqueous solution and left standing overnight. This was dried for 24 hours to obtain Sn-supporting zeolite powder in which Sn was ion-exchanged and supported.
This Sn-supported zeolite powder has an average particle size of 1 μm, and as a result of elemental analysis, the molar ratio of Sn to Al (Sn /
Al) is 0.5 and the ion exchange level is 100%.

Next, the Sn-supported zeolite powder and the manganese oxide (Mn ₂ O ₃ ) powder having an average particle size of 0.3 μm are physically mixed at a weight ratio of 1: 1 and the mixture is prepared by a known method. The catalyst of Example 1 was prepared by pelletizing. (Test) Using a fixed bed flow type catalytic reactor, the model gases having the compositions shown in Table 1 were passed through, and the conversion rates of NO to N ₂ with and without water vapor were measured. The obtained results are graphed and shown in FIG.
In the measurement, the amount of catalyst used was 0.5 g and the space velocity SV was 10,000 h-1. Then, the catalyst was held in helium at 500 ° C. for 2 hours, and then the conversion rate at each temperature was measured while gradually lowering the temperature.

[0019]

[Table 1] Comparative Example 1 The catalyst of Comparative Example 1 was prepared by pelletizing only the Sn-supported zeolite powder prepared in Example 1. A test was conducted in the same manner as in Example 1, and the results are shown in FIG.

Comparative Example 2 The catalyst of Comparative Example 2 was prepared by pelletizing only the manganese oxide powder used in Example 1. A test was conducted in the same manner as in Example 1, and the results are shown in FIG. (Evaluation) From FIG. 1, in Comparative Example 2 of manganese oxide alone,
Almost no N ₂ is produced regardless of the presence or absence of water vapor, and NO
No reduction purification is performed. Further, in the case of the Sn-supported zeolite alone of Comparative Example 1, the conversion rate is higher in the presence of water vapor, but the conversion rate is at most about 20%. However, in Example 1 in which Sn-supported zeolite and manganese oxide were used together, a very high conversion rate of up to about 80% was shown in the presence of water vapor, and a higher conversion rate was obtained in the presence of water vapor.

That is, it is clear that the catalyst of Example 1 has not only the inhibition of the reaction by water vapor but also the reaction is further promoted by the presence of water vapor and is extremely excellent in the reduction and purification performance of NO. It is clear that the synergistic effect is obtained by mixing manganese oxide. In the above test, propene (C ₃ H ₆ ) in the model gas is consumed by the NO reduction reaction and may be oxidized by oxygen (O ₂ ) in the model gas. Therefore, the selectivity of the reaction of propene can be known by measuring the conversion rate of propene to CO _x and calculating the proportion of CO _x produced by the reduction of NO.

Therefore, for the catalyst of Example 1, the conversion rate of NO to N ₂ and the conversion rate of propene to CO _x when the partial pressure of water vapor in the model gas was changed were measured.
The ratio (selectivity) of CO _x produced by the reduction of is calculated and shown in FIG. The inlet gas temperature is 350 ° C., and the steam partial pressure is 0 to 6%. Four levels are selected, and the other conditions are the same as in the above test. The selectivity was calculated by the following formula. Selectivity (%) = 100 × (number of oxygen atoms consumed for oxidation of propene to NO) / (total number of oxygen atoms consumed for oxidation of propene) From FIG. ₂ , the conversion rate of NO to N ₂ is about It rapidly increases due to the presence of about 1% water vapor, and then gradually increases due to an increase in the water vapor partial pressure. On the other hand, the conversion rate of propene to CO _x is as high as about 90% even without steam, but it gradually increases due to an increase in steam partial pressure. However, the selectivity shows a behavior similar to that of the conversion rate of NO into N ₂ after rapidly increasing in the presence of about 1% of steam and then gradually increasing due to the increase of the partial pressure of steam. In other words, it was found that the reaction between propene and NO was promoted by the presence of water vapor, which proves that the improvement in the conversion rate of NO to N ₂ due to the presence of water vapor is due to the reaction between propene and NO. ing.

Even when the test is repeated with and without steam, the steam shows a substantially constant conversion rate according to the presence or absence of steam without being affected by the immediately preceding state, so that steam chemically changes the catalyst. It is unlikely that the water vapor is directly involved in the conversion reaction of NO to N ₂ . (Comparative Example 3) A Ce-supported zeolite powder carrying Ce instead of Sn was prepared in the same manner as in Example 1 except that the Ce compound aqueous solution was used instead of the SnCl ₂ aqueous solution. Then, in the same manner as in Example 1, the Ce-supported zeolite powder and the manganese oxide powder were mixed in a weight ratio of 1: 1,
Similarly, pelletizing was performed to prepare a catalyst of Comparative Example 3.

(Comparative Example 4) In the same manner as in Example 1 except that Na-type zeolite powder not supporting Sn was used,
A Na-supported zeolite powder and a manganese oxide powder were mixed in a weight ratio of 1: 1 and pelletized in the same manner to prepare a catalyst of Comparative Example 4. (Test / Evaluation) The catalysts of Comparative Example 3 and Comparative Example 4 were tested in the same manner as in Example 1, and the maximum conversion rates were measured respectively in the absence and presence of water vapor. Table 2 shows the results.

[0025]

[Table 2] From Table 2, the catalysts of Comparative Example 3 and Comparative Example 4 show a high conversion rate in the absence of water vapor, but the conversion rate is greatly reduced in the presence of water vapor. However, Example 1 shows a high conversion in the presence of water vapor,
It can be seen that the combination of supported zeolite and manganese oxide shows extremely unique performance.

[0026]

[Effects of the Invention] That is, the exhaust gas purifying catalyst of the present invention not only does not inhibit the reaction due to water vapor, but also exhibits extremely high NO _x purification performance in the presence of water vapor. Therefore, it is actually extremely useful for purifying automobile exhaust gas containing a large amount of water vapor, and is extremely excellent in practical NO _x purification performance.

[Brief description of drawings]

FIG. 1 is a graph showing conversion rates from NO to N ₂ of exhaust gas purifying catalysts of Examples and Comparative Examples of the present invention.

FIG. 2 is a graph showing the conversion rate of NO to N ₂ and the C of propene in the exhaust gas purifying catalyst according to the example of the present invention with respect to the partial pressure change of water vapor.
The conversion and selectivity of the change to the O _x is a graph showing.

Claims (1)

[Claims] 1. An exhaust gas purifying catalyst comprising a mixture of tin (Sn) -supported zeolite powder and manganese oxide (Mn ₂ O ₃ ) powder.