JP2002263450A - Exhaust gas purification device - Google Patents

Abstract

(57) [Problem] To improve the HC purification performance of an exhaust gas purification device at the time of cold start of an engine. SOLUTION: An exhaust gas purification device includes an upstream catalyst 10 disposed in an exhaust manifold 2 and a downstream catalyst 20 disposed in an exhaust pipe 3, and the upstream catalyst 10 is a lower layer mainly composed of platinum. And an upper layer mainly composed of rhodium,
The catalyst layer of the downstream catalyst 20 is composed of an upper layer mainly composed of palladium and a lower layer composed of a hydrocarbon adsorbent such as zeolite.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purification device, and more particularly to an exhaust gas purification device excellent in HC purification performance.

[0002]

2. Related Art Generally, an engine is provided with an exhaust gas purifying device for purifying harmful substances in exhaust gas, but the exhaust gas purifying device cannot sufficiently exhibit purification performance until the catalyst reaches an activation temperature (light-off temperature). Therefore, purification of hydrocarbon (HC) discharged at the time of cold start of the engine becomes a problem.
In order to solve this problem, an exhaust gas purification device provided with a catalyst (hereinafter referred to as a proximity catalyst) which is disposed close to the engine body and can be activated early may be used. The required purification performance cannot be achieved until the temperature is reached.

[0003]

Therefore, it is conceivable to arrange the proximity catalyst closer to the engine body or to reduce the capacity of the proximity catalyst to achieve further early activation. However, according to such a configuration, the temperature of the proximity catalyst becomes excessively high after the engine warm-up is completed, particularly during high-speed and high-load operation, so that thermal deterioration is likely to occur. Further, if the exhaust air-fuel ratio is made rich or a catalyst cooling device is provided to solve this problem, problems such as an increase in fuel consumption and an increase in cost occur. Further, although the purification performance of the proximity catalyst can be improved by increasing the amount of the noble metal added to the proximity catalyst, this type of method is costly and is not practical.

[0004] It is an object of the present invention to provide an exhaust gas purifying apparatus having excellent HC purifying performance especially before reaching the catalyst activation temperature, and furthermore, an HC purifying apparatus and an exhaust gas purifying apparatus having excellent durability. Inexpensively.

[0005]

An exhaust gas purifying apparatus according to the present invention has an upstream three-way catalyst carrying platinum or rhodium and a palladium and hydrocarbon adsorbent provided downstream thereof. And a downstream catalyst. In other words, the present invention uses the precious metals of different types on the upstream three-way catalyst and the downstream catalyst to use the precious metals of both catalysts most efficiently in purifying the exhaust gas. In this case, the type of the noble metal carried by each catalyst is optimized from the viewpoint of ensuring the HC purification performance of both the catalysts before the arrival of the catalyst and reducing the cost associated with the use of the noble metal. Specifically, platinum or rhodium, which adsorbs and oxidizes paraffin HC in exhaust gas, is used as the noble metal carried by the upstream three-way catalyst, and palladium is used as the noble metal carried by the downstream catalyst. In the downstream catalyst, palladium and a hydrocarbon adsorbent have the effect of adsorbing and oxidizing olefin HC, and therefore, the olefin HC contained in the exhaust gas at the time of cold start is mainly purified by the downstream catalyst. become.

As described above, the upstream three-way catalyst and the downstream catalyst as a whole have good H before reaching the activation temperature.
In order to achieve C purification performance, the required HC purification performance can be obtained without disposing the upstream three-way catalyst close to the engine body.
The upstream three-way catalyst can be arranged at an appropriate position from the viewpoint of heat resistance without impairing the purification performance. That is, thermal deterioration of the upstream three-way catalyst is prevented, and the durability of the exhaust gas purification device is improved. Also, in the present invention, the required purification performance can be obtained without adding a large amount of expensive noble metal to the upstream three-way catalyst or the downstream catalyst, so that the total amount of noble metal added can be reduced without impairing the purification performance. Therefore, the cost of the exhaust gas purification device is reduced accordingly.

In the present invention, preferably, the upstream three-way catalyst is composed of a lower layer formed on the outer surface of the carrier and made of one of platinum and rhodium, and an upper layer formed on the outer surface of the lower layer and made of the other of platinum and rhodium. Be composed. According to the findings of the present inventors, a three-way catalyst having a single catalyst layer supporting both platinum and rhodium has insufficient HC purification performance, especially at the time of a cold start. Further, purification performance can be improved by adding palladium to the catalyst layer, but palladium is expensive. In this regard, the exhaust gas purification apparatus of the preferred embodiment uses an upstream three-way catalyst having two catalyst layers, a lower layer made of one of platinum and rhodium and an upper layer made of the other noble metal, together with the downstream catalyst of the present invention. Thus, the same level of HC purification performance as that obtained when the upstream three-way catalyst carrying palladium is used together with the downstream catalyst can be obtained. That is, according to the preferred embodiment, an exhaust gas purification device having excellent HC purification performance is provided at low cost.

[0008] More preferably, the lower layer of the upstream three-way catalyst is made of platinum, and the upper layer is made of rhodium. According to experiments by the present inventors, such an exhaust gas purification apparatus using such an upstream three-way catalyst together with a downstream catalyst is excellent in HC purification performance, especially in a cold state. Preferably, the upstream three-way catalyst is arranged close to the engine body. In this case, the upstream three-way catalyst reaches the activation temperature early, and exhibits the HC purification performance early. However, the degree of proximity of the upstream three-way catalyst to the engine body is restricted from the viewpoint of preventing thermal degradation of the catalyst, and the upstream three-way catalyst is placed in an appropriate place under this restriction.

In the present invention, preferably, the downstream catalyst is formed from a lower layer formed on the outer surface of the carrier and comprising one of palladium and a hydrocarbon adsorbent, and a lower layer formed on the outer surface of the lower layer from the other of the palladium and hydrocarbon adsorbent. Upper layer. More preferably, the hydrocarbon adsorbent is a zeolite, with β-zeolite being particularly preferred. Preferably, the upper layer of the downstream catalyst is composed of palladium, and the lower layer is composed of zeolite, preferably β zeolite. Zeolites, particularly β-type zeolites, have excellent adsorption performance for adsorbing HC, and palladium has excellent oxidation performance, particularly, ability to oxidize olefinic HC. For this reason, olefin-based HC generated during cold start of the engine and hardly adsorbed by platinum and rhodium as upstream catalysts is adsorbed by the zeolite of the downstream catalyst, and is released from the zeolite as the exhaust gas temperature rises thereafter. HC can be purified well in the presence of palladium.

[0010] Preferably, the downstream catalyst of the present invention is arranged as an underfloor catalyst. In this case, the requirements for installing the downstream catalyst are relaxed, so that, for example, even if the capacity of the downstream catalyst is increased, it does not hinder the installation. The exhaust gas purification apparatus of the present invention is installed in an intake pipe injection engine operated at a stoichiometric air-fuel ratio or a lean burn engine including a direct injection engine. When installed in an intake pipe engine, the exhaust gas purification device can further include a second three-way catalyst disposed downstream of the downstream catalyst. Further, the exhaust gas purification device provided in the lean burn engine may further include a NOx trap disposed upstream of the downstream catalyst. Preferably, the second three-way catalyst and the NOx trap are arranged adjacent to the downstream catalyst.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an exhaust gas purifying apparatus according to an embodiment of the present invention will be described with reference to the drawings. The exhaust gas purifying apparatus of the present embodiment is provided, for example, in an intake pipe injection type engine operated at a stoichiometric air-fuel ratio, and an exhaust pipe is connected to an exhaust port of the engine body via an exhaust manifold. The downstream half of the exhaust pipe is installed under the floor of a vehicle (not shown).

As shown in FIG. 1, an exhaust purification system according to the present embodiment includes an upstream three-way catalyst 10 disposed in an exhaust manifold 2 of an engine and a downstream catalyst 20 disposed in an exhaust pipe 3. have. That is, the upstream three-way catalyst 10
Is configured as a proximity catalyst arranged close to the exhaust port 1a of the engine body 1, and reaches the activation temperature early. The proximity catalyst 10 has a function of redoxing carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) under a stoichiometric air-fuel ratio. On the other hand, the downstream side catalyst 20 constitutes an underfloor catalyst together with the second three-way catalyst 30 provided adjacent to the downstream side,
It has sufficient capacity for exhaust gas purification. The downstream side catalyst 20 is configured as an HC adsorption catalyst having a function of adsorbing and oxidizing unburned HC.

More specifically, the proximity catalyst 10 has a cordierite carrier of a porous honeycomb (monolith) type composed of a large number of cells, and each cell is formed, for example, in a square shape.
FIG. 2 shows a quarter of one cell. Cordierite carrier 11, for example, alumina source powder, silica source powder and magnesia source powder, alumina, silica, and a mixture of magnesia such that the ratio of magnesia has a cordierite composition, dispersed in water, the solid The honeycomb molded body was formed into a honeycomb shape, and the formed honeycomb body was fired.

The catalyst layer 12 on the cordierite carrier 11 is 2
In the present embodiment, the lower layer 13 of the catalyst layer is made of a noble metal mainly containing platinum (Pt), and the upper layer 14 is made of a noble metal mainly containing rhodium (Rh). The catalyst layer 12 is formed, for example, as follows. First, a slurry containing platinum as a main component as a noble metal is prepared, and the cordierite carrier 11 is immersed in the slurry, and dried and fired to form a lower layer 13 containing platinum as a main component on the surface of the cordierite carrier 11. Is done. Next, a slurry containing rhodium as a main component as a noble metal is prepared, and the cordierite carrier 11 is immersed in the slurry, dried and fired.

FIG. 3 shows a quarter of one cell formed in the downstream catalyst 20. This downstream side catalyst 20 also has a cordierite carrier 21 similarly to the proximity catalyst 10,
The catalyst layer 22 on the cordierite carrier 21 is constituted by a two-layer coat. In the present embodiment, the lower layer 23 of the catalyst layer 22 is made of a noble metal mainly containing palladium (Pd), and the upper layer 24 is made of HC mainly containing β-type zeolite.
It is composed of an adsorbent.

The catalyst layer 22 is formed, for example, as follows. First, a slurry containing β-type zeolite as a main component as an HC adsorbent is prepared, and a cordierite carrier is immersed.
A lower layer 23 mainly composed of β-type zeolite is formed on the surface. Next, a slurry containing palladium as a main component as a noble metal is prepared, and the cordierite carrier 21 is immersed in the slurry, dried and fired.
4 are formed.

Here, the lower layer 23 of the downstream catalyst 20 is formed.
The zeolite β will be further described with reference to FIG.
As described above, β-type zeolite is a type of FER-type or ZSM-type zeolite.
The zeolite pore size is larger than
Large amount of HC adsorption during operation. That is, β-type zeolite
Is excellent in HC adsorption performance. Also, as shown in FIG.
The amount of HC adsorbed on zeolite at engine startup is
SiO in light _Two/ Al_TwoO_ThreeRatio of 40 to 10
It becomes maximum in the area of 0%. On the other hand, S in zeolite
iO_Two/ Al_TwoO_ThreeThe lower the ratio, the higher the heat resistance of the zeolite
Is reduced. Therefore, in the present embodiment, SiO 2_Two/ Al_Two
O_ThreeThe ratio is set to, for example, 100%. Also, in FIG.
As shown in FIG.
The amount of C adsorbed is β zeo per liter of downstream catalyst capacity.
The maximum light weight is between 100 and 150 grams.
You. In the present embodiment, with the use of the exhaust gas purification device,
Considering that the adsorption amount of
The amount of light is set to, for example, 150 grams / liter.
You.

Hereinafter, the operation of the exhaust gas purifying apparatus having the above configuration will be described. When the engine is started cold, hydrocarbons (H
Exhaust gas containing C) flows into the proximity catalyst (upstream three-way catalyst) 10 disposed in the exhaust manifold 2, and further, the downstream catalyst 20 and the second three-way catalyst disposed in the exhaust pipe 3. 3
Flows into zero. Generally, paraffin-based HC and a large amount of olefin-based HC (unsaturated HC)
According to the exhaust gas purification apparatus of the present embodiment, the paraffin-based HC is a proximity catalyst (upstream three-way catalyst).
10 is selectively adsorbed and oxidized by platinum and rhodium constituting the lower layer 13 and the upper layer 14, respectively, and the olefin-based HC is transferred to the lower layer 23 and the upper layer 2 of the downstream catalyst 20.
4 is selectively adsorbed and oxidized by the palladium and β-type zeolite constituting each.

The proximity catalyst 10 has platinum and rhodium separated and supported on the lower layer 13 and the upper layer 14, and has the same H level as the catalyst supporting palladium and rhodium.
In particular, the platinum in the lower layer 13 of the proximity catalyst 10 satisfactorily adsorbs and oxidizes the paraffin-based HC in the exhaust before reaching the activation temperature.
As described above, the proximity catalyst 10 is disposed at an appropriate position in the exhaust manifold 2 from the viewpoint of preventing thermal degradation, so that even in a high-load / high-speed operation region where the exhaust gas temperature rises after the engine warm-up is completed. Does not cause deterioration.

On the other hand, in the downstream side catalyst 20, before reaching the catalyst activation temperature, the olefinic HC becomes β β in the lower layer 23.
Adsorbed by the zeolite, and then the olefinic HC is desorbed from the β-zeolite as the exhaust gas temperature rises.
Although released, the olefinic HC is well purified in the presence of palladium, which forms the upper layer 24 of the downstream catalyst and has excellent oxidation ability. From this point, it is desirable to configure the downstream catalyst 20 so that the HC desorption temperature from the β-type zeolite matches the activation temperature of the downstream catalyst 20. Then, after the completion of the warm-up of the engine, good exhaust gas purification is performed with the support of the second three-way catalyst 30.

As described above, in the exhaust gas purifying apparatus of the present embodiment, the proximity catalyst 10 and the downstream catalyst 20 carry different kinds of noble metals, and both the catalysts 10 and 20 are set to the activation temperature. Even before the arrival, they have the effect of adsorbing and oxidizing paraffin-based HC and olefin-based HC, respectively, and exhibit good HC purification performance as a whole. Moreover,
Since the required purification performance can be obtained without adding a large amount of expensive noble metal to both catalysts 10 and 20, the exhaust purification device is inexpensive.

[Examples 1 to 4 and Comparative Examples 1 and 2]
In order to evaluate the HC purification performance of the present invention, the exhaust purification devices according to Examples 1 to 4 belonging to the present invention and the exhaust purification devices according to Comparative Examples 1 and 2 to be compared with the exhaust purification devices were manufactured. The HC purification efficiency of each exhaust gas purification device after warm-up and after warm-up was measured.

Table 1 shows the configurations of the exhaust gas purifying apparatuses according to Examples 1 to 4 and Comparative Examples 1 and 2, and the measured values of the purification efficiency. In Table 1, the value of the purification efficiency is a relative value when the purification efficiency of Comparative Example 1 is 1.0.

[0024]

[Table 1]

Exhaust gas purifying apparatuses according to Examples 1 to 4 and Comparative Examples 1 and 2 will be further described. These exhaust gas purifying apparatuses do not have the second three-way catalyst shown in FIG. The capacities of the catalyst 10 and the downstream catalyst 20 were 1 liter and 1.3 liter, respectively, and the total amount of the noble metal carried on each catalyst was 3 g per liter of the catalyst capacity. The ratio Pt: Rh between the amount of platinum carried and the amount of rhodium carried was 5: 1, and the ratio Pd: Rh between the amount of palladium carried and the amount of rhodium carried was 20: 1.

As is evident from Table 1, the exhaust catalyst devices of Examples 1 to 4 were lower than those of Comparative Examples 1 and 2 in the cold state.
The purification efficiency is particularly excellent, and the HC purification efficiency after completion of warm-up is equal to or higher than that. Regarding the HC purification efficiency after the completion of warm-up, the difference between Examples 3 and 4 and Comparative Examples 1 and 2 is not numerically large, but it is difficult to improve the HC purification efficiency and there is a slight difference. However, it is a significant difference considering that it has a large practical significance.

Further, in Embodiments 1 and 3, HC in the cold state
Although the purification efficiencies are the same, the catalyst layer of the upstream three-way catalyst is formed by a two-layer coat composed of platinum and rhodium, as compared with the first embodiment in which the catalyst layer of the three-way catalyst is formed by a single layer mixture of palladium and rhodium. The configured third embodiment is inexpensive (the relationship between the second embodiment and the fourth embodiment is the same). Also,
The third embodiment is superior to the first embodiment in HC purification efficiency after completion of warm-up.

A comparison between Example 3 and Example 4 shows that when the catalyst layer of the upstream three-way catalyst 10 is formed of a two-layer coat of platinum and rhodium, the upper layer 1
It can be understood that the rhodium 4 can improve the HC purification efficiency in the cold state more than the platinum. The description of the embodiment is finished above, but the present invention is not limited to this embodiment.

For example, in the above embodiment, the case where the present invention is applied to an exhaust gas purifying apparatus for an intake pipe injection type engine has been described. However, the present invention is also applicable to a lean burn engine including a direct injection type engine. . FIG. 7 exemplifies an exhaust gas purification device provided in a direct injection type engine. The device includes an upstream three-way catalyst 10 disposed in an exhaust manifold 2 and a downstream catalyst disposed in an exhaust pipe 3. The catalyst includes a catalyst (HC trap) 20 and a NOx storage catalyst 40 disposed adjacent to the upstream side of the downstream side catalyst 20. Also in the exhaust gas purification apparatus of FIG. 7, the upstream three-way catalyst 10 and the downstream catalyst 20
Thereby, a good HC purifying action is exhibited particularly at the time of cold start of the engine.

In the above embodiment, the lower layer 13 and the upper layer 14 of the upstream three-way catalyst 10 are made of platinum and rhodium, respectively. However, as shown in Example 4, even if the constituent materials of both layers are reversed. Further, as shown in Embodiments 1 and 2, palladium and rhodium may be mixed in the catalyst layer of the upstream three-way catalyst 10. In addition, in the embodiment, the lower layer 23 and the upper layer 24 of the downstream catalyst 20 are respectively composed of palladium and β-type zeolite. However, as shown in Example 2, the constituent materials of both layers may be reversed. In place of β-type zeolite, other zeolites such as FER type and ZSM type may be used. Also,
It is not essential that the downstream catalyst 20 be configured in the form of an underfloor catalyst.

In the above embodiment, the honeycomb type cordierite carrier 10 is used as a porous carrier. However, the present invention is applicable to an exhaust gas purifying catalyst having a carrier made of a material other than cordierite. . When a honeycomb-type cordierite carrier is used, the cells of the cordierite carrier are not limited to square cells, and may be, for example, triangular or hexagonal cells.

[0032]

The exhaust gas purifying apparatus of the present invention includes an upstream three-way catalyst supporting platinum or rhodium and a downstream catalyst supporting palladium and a hydrocarbon adsorbent, and thus the total amount of noble metal carried on both catalysts. Therefore, even when the temperature reaches the catalyst activation temperature, both catalysts exhibit a good HC purifying action as a whole, and it is possible to provide an inexpensive exhaust gas purifying apparatus having excellent HC purifying performance. Further, it is not necessary to dispose the upstream three-way catalyst too close to the engine body, and the durability of the exhaust gas purifying device and the catalyst is improved.

In the exhaust gas purifying apparatus described in the embodiment, the catalyst layer of the upstream three-way catalyst is composed of a lower layer made of one of platinum and rhodium and an upper layer made of the other noble metal, and a catalyst layer of the downstream catalyst. Is composed of a lower layer composed of one of palladium and a hydrocarbon adsorbent and an upper layer composed of the other, and zeolite is used as the hydrocarbon adsorbent, so that good HC purification performance can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic view showing an exhaust gas purification device according to an embodiment of the present invention.

FIG. 2 is a partially enlarged sectional view showing a quarter of one cell of the upstream three-way catalyst shown in FIG.

FIG. 3 is a partially enlarged cross-sectional view showing a quarter of one cell of the downstream catalyst shown in FIG. 1;

FIG. 4 shows HC by a downstream catalyst when the engine is started.
It is a figure which shows the relationship between the adsorption amount and the kind of zeolite which comprises a downstream side catalyst.

FIG. 5 shows HC by a downstream catalyst at the time of engine start.
Adsorption amount and SiO ₂ in zeolite constituting downstream catalyst
Is a diagram showing the relationship between / Al ₂ O ₃ ratio.

FIG. 6 shows HC by a downstream catalyst at the time of engine start.
It is a figure which shows the relationship between the amount of adsorption and the amount of zeolite added to a downstream catalyst.

FIG. 7 is a schematic diagram showing an exhaust gas purification device according to a modification of the present invention, which is provided in a direct injection engine.

[Explanation of symbols]

 Reference Signs List 10 upstream three-way catalyst (proximity catalyst) 11, 21 carrier 12, 22 catalyst layer 13, 23 lower layer 14, 24 upper layer 20 downstream catalyst

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F01N 3/24 B01D 53/36 103B (72) Inventor Takayuki Onodera 5-33-8 Shiba 5-chome, Minato-ku, Tokyo Mitsubishi Motors Corporation (72) Inventor Kenji Morimoto 5-33-8 Shiba, Minato-ku, Tokyo Mitsubishi Motors Corporation F-term (reference) 3G091 AA17 AB02 AB03 AB10 BA14 BA15 BA19 FA02 FB11 GA06 GB05W GB06W GB07W GB09Y GB17X HA12 4D048 AA06 AA13 AA18 AB05 BA10X BA11X BA30X BA31X BA33X BB02 BB16 CC32 CC36 CC47 EA04 4G066 AA16B AA20B AA22B AA28D BA07 CA28 CA35 CA51 DA02 FA33 4G069 AA02 BC13 BC07 BC07 BC ZA19A ZA19B ZF05A ZF05B

Claims (1)

[Claims] 1. An exhaust gas comprising: an upstream three-way catalyst supporting platinum or rhodium; and a downstream catalyst provided downstream of the upstream three-way catalyst and supporting palladium and a hydrocarbon adsorbent. Purification device.