JPH0814029A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PURPOSE:To prevent degradation of palladium and platinum supported by supports of a catalyst for exhaust cleaning. CONSTITUTION:An upper stream side catalyst 8 is arranged in an engine exhaust passage and a lower stream side catalyst 11 composed of three way catalyst is arranged in its lower stream side. Palladiuim is supported by only an exhaust gas flowing side end range 8a of catalyst support in the upper stream side catalyst 8 and platinum and rhodium are supported by the rest catalyst support range 8b in the lower stream side of the exhaust gas flowing side end range 8a. And, cerium is supported on catalyst support in the exhaust gas flowing side end range 8a.

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 device for an internal combustion engine.

[0002]

2. Description of the Related Art Conventionally, various combinations of a three-way catalyst and an oxidation catalyst have been arranged in an engine exhaust passage in order to improve exhaust gas purification performance. For example, Japanese Patent Laid-Open No. 4-2
In the internal combustion engine described in Japanese Patent No. 87820, in addition to platinum, an oxidation catalyst containing palladium, which is heat resistant and has a strong oxidizing power, is arranged at the exhaust gas outlet of the engine, and three catalysts are provided in the exhaust passage downstream of the oxidation catalyst. The original catalyst is placed. The three-way catalyst is generally poor in heat resistance, so that the three-way catalyst is arranged in the engine exhaust passage away from the exhaust gas outlet of the engine so that the exhaust gas having a lowered temperature flows into the three-way catalyst. However, if the three-way catalyst is placed away from the exhaust gas outlet of the engine in this way, it takes time for the three-way catalyst to be activated after the engine is started, and therefore the exhaust gas cannot be purified during this time. The problem arises.

Therefore, in the above-mentioned internal combustion engine, an oxidation catalyst is arranged at the outlet of the engine exhaust gas from which high-temperature exhaust gas flows, and this oxidation catalyst is activated immediately after the engine is started to promptly carry out the oxidation reaction of HC after the engine is started. The catalyst is started and the three-way catalyst is activated early by this heat of oxidation reaction. Further, in this internal combustion engine, purification of HC is carried out by an oxidation catalyst, and purification of CO and NOx is carried out by a three-way catalyst so that each catalyst plays a role of a purifying action.

On the other hand, in the internal combustion engine disclosed in Japanese Patent Laid-Open No. 62-136245, an upstream catalyst is arranged at the outlet of the exhaust manifold, and a three-way catalyst is arranged in the exhaust passage downstream of the upstream catalyst. . CO, NO with one three-way catalyst
It is difficult to sufficiently purify x and the like. Therefore, in this internal combustion engine, in addition to palladium having an oxidizing function, platinum and rhodium having a redox function, that is, a ternary function are supported on the carrier of the upstream side catalyst, In addition to the oxidizing function, the side catalyst also has a three-way function.

Further, in the internal combustion engine described in JP-A-4-118053, four catalysts are arranged in series in an engine exhaust passage, and two catalysts in the middle are three-way catalysts containing platinum and rhodium. And palladium is distributed at a high concentration in the exhaust gas inflow side end region of each catalyst carrier. In this internal combustion engine, the oxidation reaction of HC and CO in the exhaust gas flowing into the catalyst is promoted by the palladium, and the heat of the oxidation reaction at this time activates the entire catalyst early.

[0006]

However, palladium is used to oxidize HC, and then NOx is used in a three-way catalyst.
It is known that platinum and rhodium are preferable as the catalyst to be supported on the carrier of the three-way catalyst when the reduction action of is strongly promoted. Further, it is known that the purification efficiency of HC, CO, and NOx by the three-way catalyst becomes highest when the air-fuel ratio of the air-fuel mixture becomes the stoichiometric air-fuel ratio. Therefore, when the three-way catalyst is used, the air-fuel ratio becomes The stoichiometric air-fuel ratio is controlled. Further, it is known that when cerium is carried on the carrier of the three-way catalyst at this time, the purification efficiency of HC, CO, and NOx is further enhanced.

That is, in an internal combustion engine, the air-fuel ratio is usually controlled to the stoichiometric air-fuel ratio by feedback-controlling the air-fuel ratio based on the output signal of the air-fuel ratio sensor provided in the exhaust passage. In this case, the air-fuel ratio actually fluctuates alternately between the lean side and the rich side with the stoichiometric air-fuel ratio as the boundary. Cerium takes in oxygen in the exhaust gas to purify NOx when the air-fuel ratio becomes lean, and releases the taken-in oxygen to purify unburned HC and CO when the air-fuel ratio becomes rich. The storage action of oxygen by cerium promotes the purification action of HC, CO, and NOx. Therefore, if cerium is added, H
The purification efficiency of C, CO, and NOx will be improved.

By the way, it is difficult to sufficiently purify NOx particularly with only one three-way catalyst provided on the downstream side. Therefore, in addition to palladium, which promotes the oxidation reaction, is added to the carrier of the catalyst provided on the upstream side. In particular, it is preferable to support platinum and rhodium, which have a strong reducing property for NOx. Therefore, as described above, JP-A-62-1
In the internal combustion engine described in Japanese Patent No. 136245, palladium, platinum and rhodium are carried on the carrier of the upstream side catalyst. However, in this internal combustion engine, palladium, platinum, and rhodium are dispersed throughout the catalyst, and when palladium, platinum, and rhodium are dispersed throughout the catalyst in this way, there arises a problem that platinum is particularly deteriorated.

That is, the upstream side catalyst is exposed to high temperature exhaust gas.
Therefore, the temperature becomes high. It also uses a three-way catalyst.
The air-fuel ratio is alternately made lean and rich in order to
Exhaust gas flowing into the upstream side catalyst is periodically in excess of oxygen.
It becomes a state. However, platinum Pt has excess oxygen at high temperature.
And platinum oxide PtO₂And then platinum oxide PtO ₂
Comrades coalesce with each other and grow into large particles.
I will Catalysts due to the decrease in surface area with larger particles
The function is deteriorated and thus platinum is deteriorated. Immediately
However, if platinum is dispersed in the upstream catalyst, platinum will deteriorate.
Therefore, the redox power of platinum and rhodium is
Will be weakened.

On the other hand, in the internal combustion engine described in Japanese Patent Laid-Open No. 4-287820, palladium and platinum are dispersed in the carrier for the oxidation catalyst provided on the upstream side. Even in this internal combustion engine, the temperature of the oxidation catalyst becomes high, and the exhaust gas flowing into the oxidation catalyst periodically becomes excessive in oxygen, so that platinum is deteriorated. In addition, Japanese Patent Laid-Open No. 4-1180
In the internal combustion engine disclosed in Japanese Patent No. 53, palladium is concentrated and carried on the exhaust gas inflow side end region of the catalyst carrier, but platinum is dispersed throughout the catalyst carrier. Since the oxygen in the exhaust gas is used for the oxidizing action of palladium, the amount of oxygen in the exhaust gas flowing into the catalyst region downstream of the end region of the exhaust gas inflow side is small, thus Platinum does not grow due to insufficient oxygen, and thus platinum does not deteriorate. However, sufficient oxygen exists around platinum in the exhaust gas inflow side end region, which causes a problem that platinum in the exhaust gas inflow side end region deteriorates.

Further, in this internal combustion engine, not only platinum but also palladium is deteriorated. That is, unlike platinum, palladium becomes stable palladium oxide PdO when the oxygen in the exhaust gas is excessive, and is reduced when oxygen in the exhaust gas is reduced to become an unstable metal element of palladium Pd. When oxygen in the exhaust gas becomes excessive periodically, palladium Pd becomes palladium oxide PdO each time, so grain growth does not occur, but if the oxygen in the exhaust gas remains low, the metallic elements of palladium Pd will unite. As a result, grains grow and the palladium deteriorates.

That is, in this internal combustion engine, cerium is carried in addition to platinum and rhodium on the catalyst carrier in the exhaust gas inflow side end region. As described above, this cerium has a storage function of oxygen, and takes in oxygen when oxygen becomes excessive, and releases oxygen when oxygen runs out. The released oxygen is immediately used for the oxidation reaction of HC and CO. Therefore, if cerium is present, the exhaust gas inflow side end region will continue to have a small amount of oxygen in the exhaust gas,
As a result, palladium grows and the palladium deteriorates. As described above, in this internal combustion engine, not only platinum but also palladium is deteriorated.

[0013]

In order to solve the above problems, according to the present invention, an exhaust gas purifying catalyst is arranged in the exhaust passage of an engine, and an output signal of an air-fuel ratio sensor provided in the exhaust passage. In an internal combustion engine in which the air-fuel ratio is maintained substantially at the stoichiometric air-fuel ratio based on the above, palladium is supported only on the exhaust gas inflow side end region of the catalyst carrier and the remaining catalyst on the downstream side of the exhaust gas inflow side end region. Platinum and rhodium are supported only in the carrier region, and cerium is not supported on the catalyst carrier in the exhaust gas inflow side end region.

Further, according to the present invention, in order to solve the above problems, in the first invention, the catalyst carrier is made not to carry cerium at all over the whole. According to the present invention, in order to solve the above-mentioned problems, in the first invention, a three-way catalyst containing cerium is arranged in the exhaust passage downstream of the catalyst. According to the present invention, in order to solve the above problems, in the first invention, the amount of palladium carried on the catalyst carrier in the exhaust gas inflow side end region is 1. It is considered to be 0 grams or more.

[0015]

In the first aspect of the invention, HC is mainly oxidized by the palladium carried on the catalyst carrier in the exhaust gas inflow side end region. Since cerium is not supported on the catalyst carrier in the exhaust gas inflow side end region, oxygen in the exhaust gas flowing in the exhaust gas inflow side end region becomes periodically excessive,
Therefore, palladium is unlikely to deteriorate. Further, platinum and rhodium are not supported on the catalyst carrier in the exhaust gas inflow side end region, so that even if the oxygen in the exhaust gas flowing in the exhaust gas inflow side end region becomes periodically excessive. The problem of platinum deterioration does not occur. Since the excess oxygen in the exhaust gas is used for the oxidation reaction by palladium, the amount of oxygen in the exhaust gas flowing into the remaining catalyst carrier region downstream of the end region of the exhaust gas inflow side is small, and thus is supported in this catalyst carrier region. Platinum is hard to deteriorate.

In the second aspect of the invention, cerium is not carried on the remaining catalyst carrier region downstream of the exhaust gas inflow side end region. In the third invention, a three-way catalyst having an enhanced redox capacity by containing cerium is arranged downstream of the catalyst described in the first invention. In the fourth aspect, the amount of palladium carried on the catalyst carrier in the end region of the exhaust gas inflow side is set to 1.0 g or more per liter of exhaust amount of one cylinder in order to ensure a good oxidation reaction.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, 1 is an engine body, 2 is an intake manifold branch pipe, 3 is a fuel injection valve attached to each intake manifold branch pipe, 4 is an exhaust manifold, and 5 is an assembly of an exhaust manifold 4. An air-fuel ratio sensor arranged in the section, 6 is an upstream catalytic converter which is connected to the outlet of the exhaust manifold 4 via a short exhaust pipe 7 and has an upstream catalyst 8 built in, and 9 is an exhaust pipe 10. Upstream catalytic converter 6
2 shows downstream catalytic converters each of which is connected to the outlet of the internal combustion engine and has the downstream catalyst 11 built therein. The upstream catalytic converter 6 is arranged near the outlet of the exhaust manifold 4 so that the high temperature exhaust gas flows into the upstream catalyst 8.
On the other hand, the downstream side catalytic converter 9 is mounted under the floor of the vehicle body so that the exhaust gas having a lowered temperature flows into the downstream side catalyst 11.

The air-fuel ratio sensor 5 generates an output signal indicating whether the air-fuel ratio is lean or rich, and this output signal is input to the control device 12. The control device 12 controls the fuel injection amount from the fuel injection valve 3 based on this output signal,
When the air-fuel ratio sensor 5 is generating an output signal indicating that the air-fuel ratio is lean, the fuel injection amount is gradually increased, and the air-fuel ratio sensor 5 generates an output signal indicating that the air-fuel ratio is rich. The fuel injection amount is gradually decreased while the engine is running. As a result, the air-fuel ratio alternates between lean and rich with the stoichiometric air-fuel ratio as a boundary, and thus the air-fuel ratio is maintained at approximately the stoichiometric air-fuel ratio.

In the embodiment shown in FIG. 1, both the upstream side catalyst 8 and the downstream side catalyst 11 are formed of a monolith catalyst, and the upstream side catalyst 8 has a smaller volume than the downstream side catalyst 11. The upstream side catalyst 8 is divided into an exhaust gas inflow side end region 8a and a remaining catalyst region 8b on the downstream side of the exhaust gas inflow side end region 8a, each of which has a different supported catalyst. The catalyst carrier in the end region 8a carries an oxidation catalyst, and the catalyst carrier in the remaining catalyst region 8b carries a redox catalyst, that is, a three-way catalyst.

In the embodiment shown in FIG. 1, the catalyst carrier in the exhaust gas inflow side end region 8a carries palladium Pd which is uniformly dispersed in the radial direction and the axial direction, and the remaining catalyst region 8b is supported. On the catalyst carrier, platinum Pt and rhodium Rh are uniformly dispersed and supported in the radial direction and the axial direction. Further, no platinum Pt or rhodium Rh is supported on the catalyst carrier in the exhaust gas inflow side end region 8a, and no palladium Pd is supported on the catalyst carrier in the remaining catalyst region 8b. Further, in the embodiment shown in FIG. 1, the catalyst carrier in the exhaust gas inflow side end region 8a and the remaining catalyst carrier 8b are not loaded with cerium Ce at all. In addition, in the embodiment shown in FIG. 1, cerium Ce can be supported on the catalyst carrier of only the remaining catalyst region 8b, but from the viewpoint of ease of manufacturing the upstream side catalyst 8, all catalysts of the upstream side catalyst 8 are Cerium Ce as a carrier
Is preferably not supported.

On the other hand, the downstream catalyst 11 is a three-way catalyst. Platinum Pt, which is a redox catalyst and has a strong reducing power especially for NOx, is used as a catalyst carrier of the downstream side catalyst 11.
And rhodium Rh are supported, and cerium Ce is added to the downstream side catalyst 11 in order to enhance the redox capacity.
Is carried. While the engine is operating, the air-fuel ratio is maintained substantially at the stoichiometric air-fuel ratio while alternately swinging to the lean side and the rich side based on the output signal of the air-fuel ratio sensor 5. At this time, most of unburned HC and part of CO contained in the exhaust gas are oxidized by the palladium Pd carried on the catalyst carrier in the exhaust gas inflow side end region 8a of the upstream catalyst 8. As described above, since the catalyst carrier in the exhaust gas inflow side end region 8a does not carry cerium Ce, the oxygen in the exhaust gas flowing in the exhaust gas inflow side end region 8a becomes periodically excessive. As a result, palladium Pd becomes periodically stable palladium oxide PdO, that is, since palladium Pd does not continuously exist in the state of the metal simple substance for a long time, the metal simple substance of palladium Pd grows grains. Therefore, the palladium Pd is less likely to deteriorate.

Further, as described above, platinum Pt and rhodium Rh are not supported on the catalyst carrier in the exhaust gas inflow side end region 8a. Therefore, there is no problem of deterioration of platinum Pt in the exhaust gas inflow side end region 8a. The exhaust gas that has passed through the exhaust gas inflow side end region 8a flows into the remaining catalyst region 8b, and the oxidizing action of CO and the reducing action of NOx are performed in the remaining catalyst region 8b. The temperature of the exhaust gas flowing into the remaining catalyst region 8b is considerably high. Therefore, if a large amount of oxygen is present in the exhaust gas at this time, platinum Pt becomes platinum oxide PtO ₂ and grain growth occurs. However, since the oxygen in the exhaust gas is used by the oxidation reaction of the palladium Pd in the exhaust gas inflow side end region 8a, the amount of oxygen in the exhaust gas flowing into the remaining catalyst region 8b is small, so that platinum Pt is platinum oxide. It does not grow into PtO ₂ and grow grains. Thus, platinum Pt is less likely to deteriorate.

Next, the exhaust gas flows into the downstream side catalyst 11, CO is oxidized in the downstream side catalyst 11 and NO
x is reduced. By the time the exhaust gas flows into the downstream side catalyst 11, the temperature of the exhaust gas is lowered and the amount of oxygen in the exhaust gas flowing into the downstream side catalyst 11 is small. Therefore, the platinum Pt supported on the catalyst carrier of the downstream side catalyst 11 becomes platinum oxide PtO ₂ and grain growth does not occur.

On the other hand, since the upstream side catalyst 8 is arranged near the outlet of the exhaust manifold 4, high temperature exhaust gas flows into the upstream side catalyst 8 immediately after the engine is started, so that the upstream side catalyst 8 is after the engine is started. Activate quickly. Upstream catalyst 8
Is activated, an oxidation reaction is started in the exhaust gas inflow side end region 8a, and the catalyst in the remaining catalyst region 8b and the downstream side catalyst 11 are promptly activated by the reaction heat of this oxidation reaction.

FIG. 2 shows the temperature T of the upstream catalyst 8 when the purification rate of unburned HC by the palladium Pd carried on the catalyst carrier in the exhaust gas inflow side end region 8a of the upstream catalyst 8 reaches 50%. Shows the relationship between the amount of palladium Pd supported and Q. The amount P of palladium Pd carried is expressed in grams (g / L) per liter of displacement of one cylinder. As the temperature T of the upstream catalyst 8 shown in FIG. 2 is lower, the unburned HC can be purified more quickly after the engine is started. Therefore, the lower the temperature of the upstream catalyst 8 shown in FIG. 2, the more preferable it is. In the embodiment according to the present invention, the amount of palladium Pd carried is set to 1.0 g or more per liter of displacement of one cylinder.

In the embodiment shown in FIG. 1, the amount of platinum Pt supported on the catalyst carrier of the upstream side catalyst 8 is the same as that of the catalyst 1
It is about 1.5 grams per liter, and the amount of rhodium Rh supported on the catalyst carrier of the upstream side catalyst 8 is about 0.3 grams per liter of the catalyst. Further, the amounts of platinum Pt and rhodium Rh supported on the catalyst carrier of the downstream side catalyst 11 are about 1.0 gram and 0.2 gram per liter of the catalyst, respectively, which are supported on the catalyst carrier of the downstream side catalyst 11. The amount of cerium Ce contained is 0.3 to 0.45 mol per liter of catalyst.

FIG. 3 shows a case where the present invention is applied to a V-type engine. In FIG. 3, the same components as those in FIG. 1 are designated by the same reference numerals. In the embodiment shown in FIG. 3, an exhaust manifold 4 is attached to each bank 12 and 13 of the engine body, and each exhaust manifold 4 is connected to a separate upstream side catalytic converter 6 via an exhaust pipe 7. . An upstream catalyst 8 similar to the upstream catalyst 8 shown in FIG. 1 is arranged in each upstream converter 6. Each upstream catalytic converter 6 is connected to a common exhaust pipe 10 via a corresponding exhaust pipe 10 ′, and this exhaust pipe 10 is connected to a downstream catalytic converter 9. In the downstream catalytic converter 9, FIG.
A downstream side catalyst 11 similar to the downstream side catalyst 11 shown in FIG.

[0028]

EFFECTS OF THE INVENTION Unburned HC and CO in exhaust gas are prevented while preventing deterioration of palladium and platinum supported on a catalyst carrier.
And NOx can be satisfactorily purified.

[Brief description of drawings]

FIG. 1 is an overall view of an internal combustion engine.

FIG. 2 is a diagram showing a relationship between a catalyst temperature T and a palladium loading amount Q when the purification rate of unburned HC reaches 50%.

FIG. 3 is an overall view showing another embodiment of an internal combustion engine.

[Explanation of symbols]

 4 ... Exhaust manifold 8 ... Upstream catalyst 8a ... Exhaust gas inflow end region 11 ... Downstream catalyst

Claims (4)

[Claims] 1. An internal combustion engine in which an exhaust gas purifying catalyst is arranged in an exhaust passage of an engine, and an air-fuel ratio is maintained substantially at a stoichiometric air-fuel ratio based on an output signal of an air-fuel ratio sensor provided in the exhaust passage. In the above method, palladium is supported only on the exhaust gas inflow side end region of the catalyst carrier, and platinum and rhodium are supported only on the remaining catalyst carrier region on the downstream side of the exhaust gas inflow side end region. An exhaust gas purifying device for an internal combustion engine in which cerium is not supported on the catalyst carrier in the end region. 2. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein no cerium is supported on the entire catalyst carrier. 3. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein a three-way catalyst containing cerium is arranged in the exhaust passage downstream of the catalyst. 4. The exhaust gas of an internal combustion engine according to claim 1, wherein the amount of palladium carried on the catalyst carrier in the exhaust gas inflow side end region is 1.0 g or more per 1 liter of displacement of one cylinder. Purification device.