JP2000310114A - Automobile exhaust emission control device, exhaust emission purifying catalyst and hc adsorbent - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently reduce and decompose NOx by providing a first catalyst for adsorbing aromatic hydrocarbon in exhaust emission, and a second catalyst for reducing NOx in exhaust gas by reacting the aromatic hydrocarbon adsorbed by the first catalyst with the NOx. SOLUTION: An exhaust emission purifying catalyst provided in the exhaust passage of a diesel engine comprises a first catalyst (HC adsorbent) 5 having Zr suitable for adsorption of aromatic hydrocarbon, which is supported by zeolite 4 as a base material (support material), and a second catalyst 6 having Pt suitable for reductive purification of NOx in oxygen excess atmosphere, which is similarly supported by the zeolite 4. In the first catalyst 5, a metal element for adsorbing the aromatic hydrocarbon is preferably bonded to the acid point of the zeolite. The metal element suitable for adsorption of the aromatic hydrocarbon is chemically bonded to the acid point, whereby the acid point intensity of this part is enhanced.

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 apparatus, an exhaust gas purifying catalyst and an HC adsorbent used for purifying a combustion exhaust gas of a diesel engine of an automobile or the like.

[0002]

2. Description of the Related Art Japanese Patent No. 2,616,799 discloses an example of an automobile exhaust gas purifying apparatus. That is, in the exhaust passage of the lean burn engine, the catalyst I for converting paraffin to olefin and the catalyst II for decomposing NOx (nitrogen oxide) are placed at the same position, or the catalyst I It is arranged to be on the upstream side. This publication discloses that HC (hydrocarbon) in the exhaust gas is adsorbed at the active site of the catalyst II and causes partial oxidation to become CO to decompose NOx. The partial oxidation of HC adsorbs HC. Olefins have higher adsorptivity than paraffin, and therefore, the catalyst I is provided to enhance the adsorption of HC, zeolite is preferable as catalyst I, and transition is used as catalyst II. It is described that a zeolite supporting a metal by ion exchange is preferable.

[0003]

However, for example, the exhaust gas of a diesel engine not only contains paraffins and olefins, but also contains aromatic hydrocarbons (aromatics) in a larger amount than those. However, this aromatic hydrocarbon does not work sufficiently as a reducing agent for NOx purification. Therefore, with respect to such exhaust gas containing aromatic hydrocarbons, there is a limit in improving NOx purification performance only by providing a catalyst for converting paraffin into olefin. One of the reasons that the aromatic hydrocarbon does not effectively contribute to NOx purification is that it is relatively stable chemically, but the other reason is that it is difficult to adsorb to zeolite or the like.

That is, other HCs such as olefins are easily or chemically adsorbed to the acid sites or pores of the zeolite, and the adsorption state is maintained until the temperature at which the NOx reduction catalyst exhibits an activity of about 250 ° C. The adsorbed HC
Can be used to reduce and purify NOx. However, in the case of aromatic hydrocarbons, such adsorption is unlikely to occur, and even if adsorption occurs, it is desorbed at about 100 ° C. Therefore, aromatic hydrocarbons hardly contribute to NOx purification.

In addition, in an atmosphere having a high oxygen concentration, there is a problem that aromatic hydrocarbons are partially oxidized by a NOx reduction catalyst during desorption to generate CO.
That is, this CO can serve as a reducing agent for NOx purification, but because of the low desorption temperature of aromatic hydrocarbons, even if CO is produced, it is discharged without being used for NOx purification. That is the problem.

As is apparent from the above, an object of the present invention is to effectively utilize aromatic hydrocarbons in exhaust gas for NOx purification. Therefore, aromatic hydrocarbons are adsorbed to a relatively high temperature. An object of the present invention is to develop an HC adsorbent capable of holding the catalyst, and to develop a catalyst capable of efficiently reducing and decomposing NOx by combining the HC adsorbent with a NOx reduction catalyst.

[0007]

In order to solve such problems, the present inventor has found that when a specific metal element is bonded to zeolite, aromatic hydrocarbons in exhaust gas can be adsorbed well. The invention described below has been completed.

[0008] The invention of this application is an automobile exhaust gas purification apparatus for purifying exhaust gas discharged from an engine of an automobile, which is disposed in an exhaust passage of the engine and adsorbs aromatic hydrocarbons in the exhaust gas. And a second catalyst provided in the vicinity of the first catalyst and reducing the NOx by reacting an aromatic hydrocarbon adsorbed by the first catalyst with NOx in the exhaust gas. It is characterized by having.

According to the present invention, the aromatic hydrocarbon is the first
Since the catalyst is adsorbed, NOx can be reduced and purified on the second catalyst by using the adsorbed aromatic hydrocarbon as a reducing agent. The NOx reduction mechanism in this case is considered as follows. That is, when the catalyst temperature (exhaust gas temperature) of the aromatic hydrocarbon adsorbed on the first catalyst increases,
The highly active aromatic hydrocarbon thus desorbed is partially oxidatively decomposed on the second catalyst near the first catalyst, and the resulting partially oxidized hydrocarbon or CO serves as a reducing agent. It reacts with NOx on the second catalyst, and the NOx is reduced. In addition, since CO is used for NOx reduction purification in this manner, the amount of CO emitted to the atmosphere is reduced.

As is apparent from the above description, it is preferable that the first catalyst and the second catalyst are provided close to each other. That is, it is preferable that the adsorption species constituting the first catalyst and the catalyst species constituting the second catalyst are arranged on the same base material.

[0011] The invention of this application relates to an aromatic hydrocarbon and NO
an exhaust gas purifying catalyst for reducing and purifying NOx by contacting a combustion exhaust gas containing x and a first catalyst for adsorbing aromatic hydrocarbons in the exhaust gas, and a catalyst in the vicinity of the first catalyst. A second catalyst that is disposed and reduces the NOx by reacting the aromatic hydrocarbon adsorbed on the first catalyst with NOx in the exhaust gas.

Also in this case, NOx can be reduced and purified by utilizing aromatic hydrocarbons in the exhaust gas in the same manner as in the above invention.

As the first catalyst, a catalyst obtained by binding a metal element for adsorbing an aromatic hydrocarbon to an acid site of zeolite is preferable. That is, the acid point of zeolite can be a place where HC is adsorbed, but the acid point strength is low, so that aromatic hydrocarbon cannot be adsorbed and held sufficiently. Therefore, a metal element suitable for adsorbing an aromatic hydrocarbon is chemically bonded to the acid site to increase the acid site strength at that portion.

As the above-mentioned zeolite, MFI, FA
U and Y type zeolites, β zeolites and the like can be employed.

As the metal element to be bonded to the acid site of the zeolite, Zr, Ti or Hf is preferable, and Zr is particularly preferable. That is, Zr (IV) bonded to the acid site is
It becomes a strong Lewis acid having polarity, and is considered to have a strong π-electron accepting property. Therefore, it is estimated that aromatic hydrocarbons having conjugated π-electrons are well adsorbed.

The second catalyst is preferably a catalyst in which a catalytic metal is supported on zeolite. This is because the oxygen concentration in the exhaust gas is 4% even in an atmosphere having an excessive oxygen concentration.
This is because even in the above case, it is suitable for purifying NOx efficiently. Further, as such a catalyst metal, a noble metal is preferable, and it is preferable that the noble metal is supported on the zeolite by ion exchange. Further, such noble metals include Pt, Pd, Rh and the like, and among them, Pt is preferable. This is because it is advantageous for improving NOx purification performance.

It is preferable that Zr constituting the first catalyst and the noble metal constituting the second catalyst are supported on the same zeolite. As a result, the adsorbed species Z
This is because r and the noble metal, which is a catalytically active species, have an arrangement relationship close to each other, and the aromatic hydrocarbon adsorbed and desorbed by Zr easily reacts with NOx on the noble metal.

The above exhaust gas purifying catalyst is preferably used for purifying exhaust gas of a diesel engine. This is because the exhaust gas of this diesel engine has a high oxygen concentration and contains a large amount of aromatic hydrocarbons.

The invention of this application relates to an HC adsorbent for adsorbing hydrocarbons in combustion exhaust gas, wherein Zeolite has Zr
Is carried. With such an HC adsorbent, aromatic hydrocarbons can be adsorbed and held at a relatively high temperature.
This is advantageous for reduction purification of Ox. The HC adsorbent is capable of adsorbing aromatic hydrocarbons in an oxygen-excess atmosphere, and is therefore preferably used for adsorbing hydrocarbons in combustion exhaust gas having an oxygen concentration of 4% or more.

[0020]

As described above, according to the present invention, the first catalyst for adsorbing the aromatic hydrocarbons in the combustion exhaust gas, and the first catalyst provided near the first catalyst and adsorbed on the first catalyst are provided. Since the second catalyst for reducing the NOx by reacting the aromatic hydrocarbon with the NOx in the exhaust gas is provided, the aromatic hydrocarbon in the exhaust gas is effectively used for NOx reduction purification. This is advantageous for preventing the emission of unpurified NOx and CO.

In particular, those in which Zr is bonded to the acid site of zeolite can adsorb and hold the aromatic hydrocarbon up to a relatively high temperature, which is advantageous for the purification of NOx.

[0022]

Embodiments of the present invention will be described below with reference to the drawings.

<Structure of Exhaust Gas Purifying Catalyst> In FIG. 1, reference numeral 1 denotes a diesel engine, and an exhaust gas purifying catalyst 3 is provided in an exhaust passage of the diesel engine 1. As shown in FIG. 2, the exhaust gas purifying catalyst 3 has a zeolite 4 as a base material (support material) in which Zr suitable for adsorbing aromatic hydrocarbons is supported.
A catalyst (HC adsorbent) 5 and a second catalyst 6 in which the same zeolite 4 carries Pt suitable for reduction and purification of NOx in an oxygen-excess atmosphere (in an exhaust gas having an oxygen concentration of 4% or more). I have. The exhaust gas purifying catalyst 3 is supported on a carrier by a binder and disposed in the exhaust passage 2.

<Example 1> 4 of MFI having a Cayban ratio of 30
g of a 5.0 mM zirconium tetrachloride aqueous solution (400 cc)
And stirred at 40 ° C. for 4 hours. This solution was filtered, and the solid content was washed and dried, and calcined at 400 ° C. for 1 hour (support of Zr on MFI). The obtained powder is P
The mixture was mixed with a tetraammine platinum solution and 40 cc of water so that the t amount was 0.5 wt%, and the mixture was stirred at 90 ° C. for 8 hours (ion exchange treatment of MFI with Pt). This solution was filtered, and the solid content was washed and dried, and calcined at 500 ° C. for 2 hours.

The powder obtained as described above is mixed with a binder (alumina hydrate) and water to form a slurry, which is then coated with a cordierite honeycomb support (capacity 25 mL, cell density 400 cells / square inch (62.0 Cell / c
m ² ), and the cell partition thickness is 6 mil (15.2 × 10 ⁻³ c).
m)), and was dried at 150 ° C. for 1 hour and baked at 540 ° C. for 2 hours.

The amount of impurities in the obtained catalyst is 1% or less. This applies to other catalysts described later.

Example 2 Exhaust gas purification was carried out under the same conditions and method as in Example 1 except that the stirring time of the mixed solution of MFI and the aqueous solution of zirconium tetrachloride was 8 hours in the step of loading Zr on the MFI. A catalyst was prepared.

<Comparative Example> M
Example 1 except that Pt was directly ion-exchanged on FI.
An exhaust gas purifying catalyst not carrying Zr was prepared under the same conditions and method as in Example 1.

<Evaluation of Catalysts> Each of the above catalysts was incorporated into a simulated exhaust gas flow system of a fixed bed normal pressure flow type. 30 when the temperature is raised at the speed of minutes
The NOx purification rate at 0 ° C and the NOx purification rate at 400 ° C were determined.

Simulated exhaust gas 1,3,5-trimethylbenzene; 170 ppm C, NO; 8
00 ppm, CO; 200 ppm, CO ₂ ; 2%, O ₂ ;
10%, the remainder N ₂ SV = 85000 / h - Results - The results are shown in Table 1. According to the table, in the case of each of the catalysts of Example 1 and Example 2, the NOx purification rate was about twice that of the comparative catalyst at both 300 ° C. and 400 ° C., and Zr was supported on the MFI. For example, it can be seen that there is a remarkable effect in improving the NOx purification performance of the exhaust gas containing the aromatic hydrocarbon. This is considered as follows. That is, in the case of the example catalyst, as shown in FIG.
It is considered that aromatic hydrocarbons (benzene is represented as a representative in the figure) are adsorbed on Zr of the first catalyst, which is desorbed after relatively high temperature and reacts with NOx on Pt of the second catalyst. Can be On the other hand, in the case of the comparative example catalyst, as shown in FIG. It is considered that they do not react even if NOx is encountered on the Pt.

Further, the second embodiment has a lower NO than the first embodiment.
x Purification rate is high. This is considered to be due to the fact that, in Example 2, Zr was widely dispersed and bonded to the acid sites of MFI due to a longer stirring time, that is, Zr was highly dispersed and supported.

[0032]

[Table 1]

In Examples 1 and 2 above, an aqueous solution of zirconium tetrachloride was used as the Zr solution. However, in order to avoid the possibility that chlorine in this compound becomes a catalyst poison, an MFI having a Cibaban ratio of 30 was used instead. 4 g of zirconium n-
Proboxoxide solution in propanol (5.0 mM) 400
You may make it mix with cc.

<Regarding Aromatic Hydrocarbon Adsorption of the First Catalyst (HC Adsorbent)> Measurement of Adsorption Rate A zirconium n-propoxide propanol solution was used as the Zr solution, and the MFI described in Example 1 was used. Zr-supported MFI (Pt) prepared according to the Zr-supporting process
No support. ) An aromatic hydrocarbon adsorption test was performed on the powder, that is, the first catalyst (HC adsorbent) and the MFI powder not supporting Zr (conventional example). The test method is the same as the measurement of the NOx purification rate described above, in which the test material is incorporated into a simulated exhaust gas circulation device of a fixed bed normal pressure flow type, and the same simulated exhaust gas is flown as before.
That is, the adsorption rate is measured when the temperature is held for 5 minutes at each of 00 ° C, 250 ° C, 300 ° C, and 400 ° C.

The results are shown in Table 2. According to the table, at any gas temperature, the adsorption rate of the first catalyst is higher than that of the conventional example, and the difference between the adsorption rates of the first catalyst and the conventional example increases as the temperature increases. is there. This means that the first catalyst adsorbs and holds aromatic hydrocarbons to a relatively high temperature, and the second catalyst reduces and purifies NOx by a relatively high temperature (for example, 200 ° C. to 25 ° C.).
(About 0 ° C.), it can be seen that the use of the first catalyst is advantageous for NOx purification.

[0036]

[Table 2]

-Measurement of calorific value- Since the above-mentioned selective reduction of NOx depends on the oxidation of the aromatic hydrocarbon on the catalyst, the temperature of the catalyst in contact with the aromatic hydrocarbon is increased. The adsorptivity of aromatic hydrocarbons to the catalyst was evaluated based on the calorific value.

That is, the test method is as follows.
For a sample obtained by mixing 2 μL of 1,3,5-trimethylbenzene, a sample temperature of 10 ° C. /
This is a TG / DTA method in which the temperature is raised to 500 ° C. at a rate of minute and the calorific value during the temperature rise is measured as a differential temperature change. The test catalysts are Pt / Zr / MFI in which Pt and Zr are supported on the MFI of Example 1 and Pt / MFI in which Pt is supported on MFI of the Comparative Example.

The results are shown in FIG. According to the figure, in the case of Pt / Zr / MFI of the example, a peak appears around 250 ° C. and the peak is large, whereas in the case of Pt / MFI of the comparative example catalyst, the peak is small before reaching 200 ° C. Only peaks appear. In the figure,
Since the exothermic reaction appears in the + direction (upward direction) of the vertical axis, it can be said that the larger the peak area in the + direction, the larger the amount of heat generation.

Trimethylbenzene evaporates as the temperature rises. However, the stronger the interaction (adsorption) with the acid sites of the catalyst, the larger the amount of trimethylbenzene remaining without evaporating. Thereafter, the trimethylbenzene was desorbed from the acid sites and activated. It is considered that trimethylbenzene generates a large amount of heat when oxidized on Pt. Therefore, the fact that the example catalyst generates a large amount of heat as described above means that Zr increases the acid point strength and facilitates adsorption of trimethylbenzene. This is consistent with the adsorption rate data in Table 2.

As a blank test, the analysis was performed for only 1,3,5-trimethylbenzene (without catalyst). The results are shown in FIG. According to the figure, no peak appeared, and the reason why the peak appeared in the example catalyst in FIG. 4 was not the result of the combustion of the trimethylbenzene alone, that is, this trimethylbenzene was adsorbed by Zr and desorbed. It turns out that it is the result of having burned away on Pt.

When the exhaust gas purifying catalyst of the above embodiment is used, it is combined with an aromatic hydrocarbon decomposing catalyst, and this aromatic hydrocarbon decomposing catalyst is placed upstream of the exhaust gas purifying catalyst (exhaust gas flow). (Upstream side). As such a decomposition catalyst, a catalyst in which at least one of Fe, Co, and Mn is supported on zeolite, particularly a catalyst in which the catalyst is supported by an ion exchange method, is suitable. A noble metal for reduction can be supported, and Pt is particularly preferably supported by an ion exchange method.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an exhaust gas purifying apparatus for a diesel engine according to an embodiment of the present invention.

FIG. 2 is a schematic diagram showing a catalyst configuration and operation of the exhaust gas purification device.

FIG. 3 is a schematic diagram showing a conventional catalyst configuration and operation.

FIG. 4 is a TG / DTA diagram of a sample in which 1,3,5-trimethylbenzene is mixed into each of the catalyst of the example and the catalyst of the comparative example.

FIG. 5: TG / DTA of 1,3,5-trimethylbenzene alone
FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Diesel engine 2 Exhaust passage 3 Exhaust gas purification catalyst 4 Zeolite 5 First catalyst (HC adsorbent) 6 Second catalyst

Continued on the front page (72) Inventor Satoshi Ichikawa 3-1 Shinchi, Fuchu-cho, Aki-gun, Hiroshima Mazda Co., Ltd. (72) Inventor Hiroshi Murakami 3-1 Shinchi, Fuchu-cho, Aki-gun, Hiroshima Mazda Co., Ltd. (72 ) Inventor Yasuhito Watanabe 3-1 Shinchi, Fuchu-cho, Aki-gun, Hiroshima Prefecture F-term (reference) 3G091 AA02 AA18 AB05 AB10 BA14 BA15 BA19 BA39 FB10 GA06 GA16 GB01X GB01Y GB05W GB06W GB07W GB09X GB09Y GB10X GB17X HA08 HA18

Claims (11)

[Claims] 1. An automobile exhaust gas purifying apparatus for purifying exhaust gas discharged from an engine of an automobile, wherein the first catalyst is disposed in an exhaust passage of the engine and adsorbs aromatic hydrocarbons in the exhaust gas. And a second catalyst provided in the vicinity of the first catalyst and reducing the NOx by reacting the aromatic hydrocarbon adsorbed by the first catalyst with NOx in the exhaust gas. Exhaust gas purification device. 2. An exhaust gas purifying catalyst for reducing and purifying NOx by contacting a combustion exhaust gas containing an aromatic hydrocarbon and NOx, wherein the first catalyst adsorbs the aromatic hydrocarbon in the exhaust gas. A catalyst, and a second catalyst disposed in the vicinity of the first catalyst and reducing the NOx by reacting the aromatic hydrocarbon adsorbed by the first catalyst with NOx in the exhaust gas. Exhaust gas purification catalyst. 3. The exhaust gas purifying catalyst according to claim 2, wherein the first catalyst has a metal element for adsorbing an aromatic hydrocarbon bonded to an acid site of zeolite. Exhaust gas purification catalyst. 4. The exhaust gas purifying catalyst according to claim 3, wherein the metal element of the first catalyst is Zr. 5. The exhaust gas purifying catalyst according to claim 2, wherein the second catalyst has a catalytic metal supported on zeolite. 6. The exhaust gas purifying catalyst according to claim 5, wherein the catalytic metal is a noble metal, and is supported on the zeolite by ion exchange. 7. The exhaust gas purifying catalyst according to claim 5, wherein the noble metal is Pt. 8. The exhaust gas purifying catalyst according to claim 4, wherein the Zr constituting the first catalyst and the noble metal constituting the second catalyst are supported on the same zeolite. Exhaust gas purification catalyst. 9. The exhaust gas purifying catalyst according to claim 2, wherein the exhaust gas purifying catalyst is used for purifying exhaust gas of a diesel engine. 10. Zrite supported on zeolite,
HC adsorbent that adsorbs hydrocarbons in combustion exhaust gas. 11. The HC adsorbent according to claim 9, which is used for adsorbing hydrocarbons in a combustion exhaust gas having an oxygen concentration of 4% or more.