JPH06185342A - Exhaust emission control device - Google Patents

Abstract

PURPOSE:To obtain excellent NOx purification performance from low to high temperature areas by efficiently utilizing HC in exhaust gas for decomposition and purification of NOx. CONSTITUTION:A middle and high temperature activating NOx catalyst 2 is provided such that HC is adsorbed in a low temperature side area, the adsorbed HC is desorbed in a high temperature side area, and NOx is purified in the high temperature side area. A low temperature activating NOx catalyst 3 is arranged on a downstream side of the middle and high activating NOx catalyst for dicomposing and purifying NOx in the low temperature side area where HC desorption can be started by means of the mizzle and high temperature activating NOx catalyst 2, in the presence of HC. The pairs of the catalysts 2 and 3 are alternately arranged in an exhaust passage 1 in the vicinity of each other.

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 for an exhaust system of a lean burn engine of an automobile,
In particular, the present invention relates to a catalyst device that effectively utilizes HC (hydrocarbon) for decomposition and purification of NOx (nitrogen oxide) and has NOx purification ability in a wide temperature range from low temperature to high temperature.

[0002]

2. Description of the Related Art CO (carbon monoxide) and HC oxidation and NO are used as catalysts for automobile exhaust gas purifying devices.
Three-way catalysts that simultaneously perform the reduction of x are generally known. This three-way catalyst is formed by supporting Pt (platinum) and Rh (rhodium) on γ-alumina, for example, and the engine air-fuel ratio (A / F) is the theoretical air-fuel ratio A / F =
A high purification efficiency can be obtained when controlling to the vicinity of 14.7.

By the way, there is a demand to make the air-fuel ratio lean in order to reduce the fuel consumption of the engine. In that case, since the exhaust gas also becomes a lean atmosphere with excess oxygen, the above three-way catalyst cannot oxidize and purify CO and HC, but cannot reduce and purify NOx.

Therefore, in recent years, research on a zeolite catalyst in which a transition metal is supported on the zeolite by ion exchange has been advanced. With this zeolite catalyst, a reducing agent (for example, HC) that coexists even in a lean atmosphere
Thus, NOx can be catalytically decomposed into N ₂ and O ₂ . Therefore, the HC adsorbing member is arranged on the upstream side of the NOx purifying catalyst, and the HC in the HC adsorbing member is increased.
If HC is effectively supplied to the NOx purification catalyst by controlling the adsorption / desorption of NOx, the HC can be contributed to the purification of NOx.

On the other hand, for example, in Japanese Unexamined Patent Publication No. 3-141816, a bypass passage parallel to an HC adsorbing member arranged upstream of an HC purifying catalyst and an exhaust gas flow are described.
A switching valve for switching between the adsorption member side and the bypass passage side is provided, and the adsorption / desorption of HC in the HC adsorption member is controlled by switching the flow path of the exhaust gas according to the exhaust gas temperature. Exhaust gas purification device is described. Therefore, instead of the above HC purifying catalyst, NO
It is considered that the HC can be effectively supplied to the NOx purification catalyst by arranging the x purification catalyst.

[0006]

However, in that case, when the exhaust gas temperature reaches a predetermined temperature, the HC adsorbed on the HC adsorbing member is desorbed almost all at once and supplied to the NOx purifying catalyst. Therefore, H in the NOx purification catalyst
The decomposition and purification of NOx by C is carried out only once in a predetermined temperature range, and HC is decomposed in a wide temperature range.
It is not enough to decompose and purify NOx by utilizing the.

HC is NOx in a wide temperature range.
In order to be supplied to the purification catalyst, in order to finely control the flow path by the temperature sensor and the switching valve,
For example, it is possible if the desorption amount of HC is controlled by adjusting the opening degree of the switching valve. However, there is a problem of space for installing the bypass passage, and a temperature sensor for operating the switching valve according to the exhaust gas temperature is required, so that the cost problem cannot be ignored.

The present invention has been made in view of the above points, and an object thereof is to devise its arrangement by using an HC adsorbing member and a NOx purifying catalyst so as to have a simple structure but a wide temperature range. N utilizing HC in the region
It is to be able to purify Ox, and thus to obtain an excellent NOx purifying ability from low temperature to high temperature.

[0009]

In order to achieve the above object, in the present invention, a NOx purifying catalyst for purifying NOx in the presence of HC is arranged downstream of the HC adsorbing member, and the HC adsorbing member is also provided. And two or more pairs of NOx purification catalysts are arranged alternately.

Specifically, in the invention of claim 1, an HC adsorbing member for adsorbing HC in the exhaust gas in the low temperature side region and desorbing the adsorbed HC in the high temperature side region, A NOx purification catalyst that is arranged on the downstream side of the HC adsorbing member and purifies NOx in the exhaust gas in a temperature range including the temperature at which the HC adsorbing member starts to separate HC and in the presence of HC. The pairs are alternately placed close to each other and 2
Provide more than one pair.

As the HC adsorbing member, for example, a metal-containing silicate such as zeolite, which is a crystalline porous body having microscopic pores, is suitable, and a transition metal is supported on the metal-containing silicate by ion exchange or the like. Is preferred. When zeolite is used, hydrogen-type zeolite is preferable in terms of heat resistance. As the zeolite, A type, Y type, ZSM-5, ferrierite,
Mordenite and the like can be used. As transition metals to be supported on such zeolite, Co, Ni,
Mo-Co, Cu-Mg, Cu, etc. can be used. Also,
For metal-containing silicates other than zeolite, for example, 2
It is also possible to use a composite metal-containing silicate having a skeleton-constituting element of crystals of one or more metals, a non-alumino metal-containing silicate containing no Al, and the like. As the skeleton-constituting metal element, Sc, Y, La, Ce, Nd in 3A group,
Tb, Ti and Zr in the 4A group, V and Nb in the 5A group, Cr, Mo and W in the 6A group, 7
Mn, Tc, and Re in Group A, and Fe and C in Group 8
o, Ni, Rh, Pd, Pt, and Cu in the 1B group
However, Zn is used in the 2B group, and Al and G are used in the 3B group.
a, In, and in the 4B group, Ge and Sn are 5B.
Sb and Bi can be adopted for each group. The ratio between the skeleton-constituting element (Me) and Si is Si / M
e> 5 is preferred.

On the other hand, as the NOx purification catalyst, H
Any material that decomposes and purifies NOx in the presence of C may be used. For example, a transition metal such as a noble metal may be supported on a high specific surface area oxide such as γ-alumina, MgO, SiO ₂ , or the metal-containing silicate described above. It can be used for things.

The HC adsorbing member and the NOx purifying catalyst can be used while being supported on their respective carriers or processed into a pellet type shape or the like. When the carrier is supported, a cordierite honeycomb carrier is suitable as the carrier.

According to a second aspect of the present invention, in the above-mentioned first aspect of the present invention, the NOx purification catalyst contains a noble metal. Pt and Ir are preferable as the noble metal,
In addition to these, Rh, Pd, Ru, Os, Ag, and Au can be used alone or in combination.

According to the invention of claim 3, the invention according to claim 1 or 2
In the invention of claim 1, the HC adsorbing member is replaced with NOx
Has a high-temperature active NOx purification capacity for purifying
Moreover, it is assumed that the maximum activation temperature is higher than the NOx purification catalyst. The higher temperature side than the NOx purification catalyst means, for example, 350 to 600 when the NOx purification catalyst purifies NOx at a catalyst temperature of 250 to 400 ° C.
It suffices that the NOx is purified at the catalyst temperature of ° C.

[0016]

According to the invention of claim 1, when the exhaust gas temperature is low and the temperature of each HC adsorbing member is in the low temperature range,
HC in the exhaust gas is adsorbed by each HC adsorbing member, and when the temperature of the exhaust gas rises and the temperature of each HC adsorbing member reaches the temperature range on the high temperature side, each HC adsorbing member desorbs the adsorbed HC. I will let you. At this time, since each HC adsorbing member is sequentially arranged from the upstream side to the downstream side in the exhaust system, and the NOx purifying catalyst is arranged between each HC adsorbing member, the respective HC adsorbing members are arranged between the respective temperatures. Differences occur. That is, the temperature of the HC adsorbing member on the upstream side is the highest, and the temperatures of the other HC adsorbing members are lower on the downstream side. As a result, first, the upstream HC adsorbing member reaches the high temperature side temperature range to desorb HC, and the following HC adsorbing members reach the high temperature side temperature range later than that, and The HC is desorbed later than the above.
The HC desorbed from each HC adsorbing member is supplied to the NOx purifying catalyst located on the downstream side of the HC adsorbing member. In this NOx purification catalyst, the HC itself is purified by combustion and contributes to decomposition and purification of NOx in the exhaust gas in a temperature range including a temperature at which the HC adsorbing member starts to release HC. As described above, since the desorption of HC in each HC adsorbing member is sequentially performed in different temperature ranges of the exhaust gas, HC contributes to the purification of NOx in a wide temperature range including the above-mentioned temperature ranges.

According to the second aspect of the present invention, the noble metal supported on the NOx purification catalyst promotes radicalization of HC, so that the HC is brought into a state more suitable for NOx decomposition and purification. As a result, as described above, the HC adsorbing member and the NOx purification catalyst are alternately superposed on each other, so that the NO
x Even if the total capacity of the purification catalyst decreases, NO
x A large decrease in the purification rate can be suppressed.

According to the third aspect of the invention, the HC adsorbing member is NO.
When the exhaust gas has a relatively high temperature, NOx in the exhaust gas is mainly purified by the HC adsorbing member. As a result, NOx in the exhaust gas
Is purified mainly by the NOx purification catalyst on the low temperature side and mainly by the HC adsorbing member on the high temperature side.

[0019]

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

(Embodiment 1) In Embodiment 1, as shown in FIG. 1, the HC adsorbing members are provided at three positions of an exhaust passage 1 which is an exhaust system of an automobile engine: an upstream position, a middle position and a downstream position. Medium-high temperature active NOx catalyst 2 (hereinafter referred to as medium-high temperature catalyst), and N located downstream of the medium-high temperature catalyst 2.
Low temperature active NOx catalyst 3 (hereinafter referred to as Ox purification catalyst)
3 pairs are referred to as “low temperature catalyst” and are alternately arranged close to each other.

The medium-high temperature catalyst 2 comprises a cordierite honeycomb carrier carrying a catalyst material having HC adsorbing ability and medium-high temperature active NOx purifying ability. This catalytic material is
While the HC in the exhaust gas is adsorbed in the temperature range lower than about 250 ° C, the HC is desorbed in the temperature range higher than about 250 ° C, and in the temperature range higher than about 300 ° C. This time, it has the property of decomposing and purifying NOx in the exhaust gas. In the above catalyst material, pentasil-type ZMS-5 as a metal-containing silicate is loaded with Co as an active species by ion exchange at an ion exchange rate of 105%, and Pt is added thereto in an amount of 0.2 g per liter of a carrier. The above honeycomb carrier is wash-coated with an alumina binder. This honeycomb carrier has a capacity of 400 cells / inch ² , and its capacity is 0.2 liter.

On the other hand, the low-temperature catalyst 3 comprises a honeycomb carrier having the same cross-sectional shape as that of the carrier, and a catalyst material having a low-temperature active NOx purifying ability supported thereon. This catalytic material has a temperature range from about 150 ° C to about 400 °,
In addition, it has the property of decomposing and purifying NOx in the presence of HC. The catalyst material is a H-type zeolite having a Cavan ratio of 70 and Pt, Ir and Rh as Pt: I as active species.
r: Rh = 30: 3: 1 and 6 g per 1 liter of the carrier, and the medium-high temperature catalyst 3
It is wash-coated with an alumina binder in the same manner as in.

For comparison with the above catalysts 2 and 3, a Cu zeolite catalyst as a medium-high temperature catalyst was used in Comparative Example 1.
As a comparative example 2, a Pt-Rh alumina catalyst as a low-temperature catalyst was used, and the Cu zeolite catalyst and a Pt-Rh alumina catalyst located on the downstream side of the catalyst were used as 1.
A pair of comparative example 3 was produced. The Cu zeolite catalyst uses the same zeolite and carrier as the medium-high temperature catalyst 2 and Cu is ion-exchanged into the zeolite to obtain 1
The catalyst material supported at a Cu ion exchange rate of 40% is supported on a carrier. Further, the Pt-Rh alumina catalyst uses the same carrier as the low temperature catalyst 3, and Pt and Rh are added to γ-alumina having a high specific surface area.
The catalyst material is supported on the carrier at a ratio of 5: 1 and at a ratio of 1.6 g per liter of the carrier. The active temperature ranges and the catalytic abilities of the middle-high temperature catalyst and the low-temperature catalyst are set to be substantially the same as those of the middle-high temperature catalyst 2 and the low-temperature catalyst 3 in this embodiment when viewed as a single body.

Regarding the above-mentioned present invention example and comparative examples 1 to 3,
Using an engine with a displacement of 1.3 liter, the inlet gas temperature was changed by changing the front and rear positions of each catalyst while the engine load was constant, and the NOx purification rate of each was measured. The result is shown in FIG. In all cases, the total catalyst capacity is the same.

Comparing the inventive example with the comparative example 3, the comparative example 3 has one peak on the low temperature side, whereas the inventive example shows three peaks. The reason for this is that in Comparative Example 3, the HC adsorbed to the medium-high temperature catalyst is released only once when the medium-high temperature catalyst reaches a predetermined temperature, and the desorbed HC is supplied to the low-temperature catalyst. This is because the supply of HC by the three medium and high temperature catalysts 2 is performed a total of three times in different inlet gas temperature regions in the present invention example, whereas it is performed only once.

More specifically, in the example of the present invention,
When the exhaust gas inlet temperature is low, each of the middle and high temperature catalysts 2 becomes H
Adsorb C respectively. As exhaust gas temperature rises,
First, the medium-high temperature catalyst 2 at the upstream position reaches a predetermined temperature and desorbs HC. This HC is supplied to the low temperature catalyst 3 at the upstream position and burned to purify itself and contribute to NOx decomposition and purification. This forms the first peak. Then, when the exhaust gas temperature further rises, the middle-high temperature catalyst 2 reaches a predetermined temperature this time, desorbs HC, and contributes to the decomposition and purification of NOx in the low-temperature catalyst 3 in the middle flow position. This forms a second peak.
Finally, when the medium-high temperature catalyst 3 at the downstream position reaches a predetermined temperature, H
C is desorbed, the low temperature catalyst 3 at the downstream position burns the HC and decomposes and purifies NOx, and a third peak is formed. In this way, the medium-high temperature catalysts 2 sequentially supply the respective HCs to the low-temperature catalyst 3 in the temperature range in which the exhaust gas inlet temperatures are different, so that the HCs are burned and purified in the respective temperature ranges, whereby NOx Contributes to decomposition and purification.

Further, the above three peaks are formed substantially continuously, but the combustion of HC in each low temperature catalyst 3 promotes the temperature rise of the intermediate high temperature catalyst 2 and the low temperature catalyst 3 located on the downstream side. As a result, the drop between peaks is small and a substantially stable purification rate is maintained.

Compared to Comparative Example 2 of the present invention, the first peak on the low temperature side of the present invention is lower than the peak of Comparative Example 2 because the total capacity of the low temperature catalyst 3 of the present invention is Comparative Example. Two
The reason is that the purification rate of the present invention example is not as low as the peak of Comparative Example 3 even though it is half of that of the low temperature catalyst.
The supported amount of Ir and Rh) is larger than that of the low temperature catalyst of Comparative Example 2, and the noble metal promotes radicalization of HC,
It is considered that this is because the state becomes more suitable for the decomposition and purification of Ox, and as a result, even if the total capacity of the NOx purification catalyst decreases, a large decrease in the NOx purification rate is suppressed.

The reason why the purification rate on the high temperature side of the present invention example as a whole is low when the present invention example is compared with the comparative example 1 is for the same reason as in the case of the above low temperature catalyst, but in this case, the comparative example 3 It is because the purification rate of each medium-high temperature catalyst 3 at the midstream position and the downstream position was delayed by the laminated structure rather than the purification rate being lower, rather, the purification characteristics of the example of the present invention were shifted to the high temperature side, that is, the whole. As a result, it is considered that the NOx purification capacity has substantially expanded to the high temperature side. Incidentally, when two pairs of the medium-high temperature catalyst 2 and the low-temperature catalyst 3 were used, the purification rate at the same inlet gas temperature increased, and instead, the third peak on the low temperature side was not observed.

(Embodiment 2) In Embodiment 2, a medium-high temperature active NOx catalyst as an HC adsorbing member and a medium-high temperature catalyst are provided at two positions, an upstream position and a downstream position of an exhaust passage which is an exhaust system of an automobile engine. Two pairs of low temperature active NOx catalysts as NOx purification catalysts located on the downstream side are alternately arranged close to each other.

The medium-high temperature catalyst uses the same carrier and zeolite as those used in the medium-high temperature catalyst 2 of Example 1, and a catalyst material obtained by supporting Cu-Mg as an active species on the carrier by ion exchange. It is carried. On the other hand, the low temperature catalyst has substantially the same structure as the low temperature catalyst 3 of the above embodiment, except that Pt, Ir and Rh are supported on zeolite at a ratio of 3 g per liter of the carrier to form a catalyst material. is there.

For comparison, each of the above-mentioned catalysts comprises a comparative example 1'comprising only a medium-high temperature catalyst, a comparative example 2'comprising only a low-temperature catalyst, and a pair of medium-high temperature catalyst and low-temperature catalyst. Comparative Example 3 ′ and Comparative Example 3 ′ were produced and tested for the HC purification rate and the NOx purification rate in the FTP mode. The results are shown in Table 1. In all cases, the total catalyst capacity is the same.

[0033]

[Table 1] In Comparative Examples 1'and 2 ', the NOx purification rate is lower than the HC purification rate. It is considered that this is because HC does not sufficiently contribute to the purification of NOx. Actually,
In Comparative Example 3 ', although the HC purification rate is about the same, NO
The x purification rate is improving by 5 to 15%.

On the other hand, in the example of the present invention, the NOx purification rate was further improved by 12% as compared with the comparative example 3,
The HC purification rate also shows an increase of 3% as compared with Comparative Example 3 '. The reason for this is that by arranging two pairs of middle and high temperature catalysts and low temperature catalysts, it is possible to reduce HC in the purification of NOx.
It is considered that this is because the contribution ratio of H.sub.2 was increased, and as a result, combustion purification of HC was also promoted.

[0035]

As described above, according to the first aspect of the present invention, by arranging two or more pairs of the HC adsorbing member and the NOx purifying catalyst located on the downstream side thereof alternately, each H
Since HC can be desorbed to the C adsorbing member in different inlet temperature regions of the exhaust gas and sequentially supplied to the NOx purification catalyst, the HC can be effectively utilized in a wide inlet temperature region to NO.
The purification performance for x can be enhanced, and the purification performance for HC and NOx can be improved in a wide temperature range from low temperature to high temperature. Further, HC can be effectively used for purifying NOx by the structure and arrangement of the catalyst, and a component for switching the bypass passage for the HC adsorbing member and the exhaust gas flow passage according to the temperature of the exhaust gas is unnecessary. Therefore, it is advantageous in terms of the installation space of the exhaust gas purification device and the cost of parts.

According to the second aspect of the present invention, since the NOx purification catalyst is loaded with the noble metal, it is possible to suppress the reduction of the NOx purification rate by the NOx purification catalyst without increasing the total catalyst capacity. Therefore, it is further advantageous in terms of installation space.

According to the third aspect of the present invention, since the HC adsorbing member is provided with the NOx purifying ability on the higher temperature side than the NOx purifying catalyst, not only the NOx purifying catalyst on the lower temperature side can be purified. Since NOx purification can be performed on the high temperature side by the HC adsorption member, NOx purification performance can be exhibited in a wide temperature range from low temperature to high temperature.

[Brief description of drawings]

FIG. 1 is a principle diagram showing an exhaust gas purification device according to a first embodiment of the present invention.

FIG. 2 is a graph showing NOx purification characteristics in various purification devices.

[Explanation of symbols]

 1 Exhaust passage (exhaust system) 2 Medium high temperature active NOx catalyst (HC adsorbing member) 3 Low temperature active NOx catalyst (NOx purification catalyst)

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Makoto Kyogoku 3-1, Shinchi, Fuchu-cho, Aki-gun, Hiroshima Prefecture Mazda Motor Corporation

Claims (3)

[Claims] 1. An HC adsorbing member that adsorbs HC in exhaust gas in a low temperature side region and desorbs the adsorbed HC in a high temperature side region, and is arranged downstream of the HC adsorbing member. , A pair of NOx purification catalysts for purifying NOx in the exhaust gas in a temperature range including a temperature at which HC desorption starts in the HC adsorbing member and in the presence of HC are alternately adjacent and two pairs An exhaust gas purification device characterized in that it is provided as described above. 2. The exhaust gas purifying apparatus according to claim 1, wherein the NOx purifying catalyst contains a noble metal. 3. The exhaust gas purifying apparatus according to claim 1 or 2, wherein the HC adsorbing member has a high-temperature active NOx purifying ability for purifying NOx in the exhaust gas, and its maximum activation temperature is NO.
An exhaust gas purifying device, which is located on a higher temperature side than the x purifying catalyst.