JP2001132440A - Exhaust emission control device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To maintain a purifying rate in a low temperature area high likewise as in a conventional way and effectively utilize HC for the reduction of NOx even if a rich spike has been input shallow. SOLUTION: In an exhaust emission control device comprising a three-way catalyst 2 and an NOx storage reduction type catalyst 3 arranged upstream and downstream respectively, in the case where exhaust emission under the condition that a lean atmosphere whose mole ratio of oxidized components to the reduced components in the exhaust emission in over 14.6 and a rich atmosphere whose mole ratio of the oxidized components are alternately repeated flows into the catalyst 2, the time while the catalyst 2 holds is limited to one second or less. In this case, this time is a from the time point when the atmosphere of the exhaust emission becomes stoichiometric atmosphere after passing through the catalyst 2 becomes stoichiometric atmosphere from a rich atmosphere until is becomes the lean atmosphere of 50% of the maximum lean atmosphere. The oxidation of HC at the time of rich spike by the catalyst 2 can be controlled, leading to the effective utilization of the reduction of NOx.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to relates to exhaust gas purifying apparatus of an internal combustion engine, more particularly to an exhaust gas purifying apparatus using a three-way catalyst and NO _x storage-and-reduction type catalyst.

[0002]

In recent years, global warming has become a problem due to CO _2, reducing the emissions of CO ₂ has become an issue. In automobiles, reducing the amount of CO _{2 in} exhaust gas has become an issue, and lean burn engines have been developed that burn lean fuel in an oxygen-rich atmosphere. According to this lean burn engine, the fuel consumption is improved, so that the emission of CO ₂ can be suppressed.

[0003] When purifying harmful components in exhaust gas from a lean burn engine, because of an oxygen-excess atmosphere,
Reduction purification of NO _x is difficult. Therefore JP-A-5-317652, carrying the _the NO _x storage material selected from alkali metals, alkaline earth metals and rare earth elements with a noble metal
NO _x storage-and-reduction type exhaust gas purifying catalyst.
Using this NO _x storage-and-reduction type catalyst, by controlling the mixture composition as a stoichiometric-rich atmosphere in a pulsed manner during the lean atmosphere, to proceed efficiently and reduction of HC and CO oxidation and NO _x And high purification performance can be obtained.

[0004] That NO _x becomes oxidized is NO in the exhaust gas in a lean atmosphere, because it is occluded in _the NO _x storage material NO _x
Emission is suppressed. When the controlled to the stoichiometric-rich atmosphere in a pulsed manner, NO _x from _the NO _x storage material is released, because it is reduced by reaction with reducing components such as HC present in the exhaust gas, NO _x emissions Is suppressed. Therefore it is possible to suppress the emission of the NO _x in all the atmosphere of the lean-rich.

[0005] Controlling the air-fuel mixture so as to provide a stoichiometric to rich atmosphere in a pulsed manner is called a rich spike, and the degree to which the rich atmosphere is created by the rich spike is expressed as deep or shallow. In other words, to make a rich atmosphere with a rich spike is referred to as "introducing a rich spike deeply", and to make a mild stoichiometric to rich atmosphere is referred to as "introducing a shallow rich spike".

By the way, in a low temperature range such as at the time of starting,
Since the noble metal supported on the NO _x storage reduction type catalyst is at or below the activation temperature, no oxidation-reduction reaction occurs and only the NO _x
There is a problem that HC is also emitted. Therefore,
The 5-195755 discloses a three-way catalyst is disposed immediately below the engine upstream of the NO _x storage reduction catalyst, the exhaust gas purifying apparatus used in a system to introduce the rich spike is disclosed in the middle of lean atmosphere ing. According to such an exhaust gas purifying apparatus, the three-way catalyst for the heated early to be disposed immediately below the engine becomes an active temperature, the NO ₂ by oxidizing NO with oxidizes HC. Therefore, the emission of HC in the low temperature range can be suppressed, and NO ₂ is stored in the NO _x storage reduction catalyst on the downstream side, so that the emission is suppressed.

[0007]

The three-way catalyst exhibits the highest activity in an atmosphere of exhaust gas burned near the stoichiometric air-fuel ratio (stoichiometric). Therefore, in order to maintain the exhaust gas atmosphere in the vicinity of the stoichiometry, the three-way catalyst usually contains a substance having an oxygen storage / release capability such as ceria. As the noble metal supported on the three-way catalyst, Pd having excellent durability in a lean atmosphere is often used.

However, when such a three-way catalyst is used in the above-mentioned exhaust gas purifying apparatus, a large amount of HC present in the exhaust gas at the time of injection of the rich spike is oxidized by oxygen released from ceria or the like, and the HC amount decreases. NOx reduction activity in the NO _x storage-and-reduction type on the catalyst is lowered. If the amount of supported Pd is increased in order to improve the oxidation activity in a low temperature range, HC is further oxidized, and the NO _x reduction efficiency is further reduced.

Further, if the rich spike is deeply introduced,
Although the NO _x reduction ability increases with an increase in the amount of HC, there is a problem that fuel efficiency deteriorates.

The present invention has been made in view of such circumstances, and maintains the purification rate in a low temperature region as high as before, and effectively reduces HC even when a rich spike is introduced shallowly. _The purpose is to provide an exhaust gas purification device that can be used for _x reduction.

[0011]

Means for Solving the Problems The characteristics of the exhaust gas purifying apparatus of the present invention for solving the above-mentioned problems, on the upstream side of the exhaust gas flow is arranged a three-way catalyst, NO _x storage reduction catalyst downstream of the three-way catalyst Is characterized in that the stoichiometric holding time of the three-way catalyst is set to 1 second or less.

Here, the stoichiometric retention time is defined as the exhaust gas under the condition that the lean atmosphere in which the molar ratio of the oxidizing component to the reducing component in the exhaust gas exceeds 14.6 and the rich atmosphere in which the molar ratio is 14.6 or less are alternately repeated. When flowing into the source catalyst, it means the time from when the atmosphere of the exhaust gas after passing through the three-way catalyst changes from a rich atmosphere to a stoichiometric atmosphere to a lean atmosphere of 50% of the maximum lean atmosphere.

It is desirable that the three-way catalyst does not contain an oxygen storage / release material. The three-way catalyst does not contain Pd.
It can be configured to include Rh and Pt.

[0014]

DETAILED DESCRIPTION OF THE INVENTION For example, FIG. 3 (a) shows an exhaust gas in which a lean atmosphere in which the molar ratio of the oxidizing component to the reducing component is 15.5 and a rich atmosphere in which the molar ratio is 13.5 are alternately repeated for 5 minutes each. ) Is represented as a square wave. When the exhaust gas under such conditions is supplied to the three-way catalyst, the atmosphere of the exhaust gas emitted from the three-way catalyst is as shown in FIG.
become that way.

That is, first, when the exhaust gas in the lean atmosphere flows into the three-way catalyst, oxygen is occluded by the oxygen storage / release material or the like, so that the atmosphere of the exhaust gas leaving the three-way catalyst becomes stoichiometric, and the oxygen storage / release material or the like As the performance approaches saturation, the atmosphere of the exhaust gas gradually approaches the lean maximum from stoichiometry. Next, when the exhaust gas in the rich atmosphere flows into the three-way catalyst, the oxygen stored in the oxygen storage and release material is released, so the atmosphere of the exhaust gas exiting the three-way catalyst becomes stoichiometric. It gradually approaches the rich maximum. When the exhaust gas in the lean atmosphere flows into the three-way catalyst again, oxygen is stored again in the oxygen storage / release material and the like, so that the atmosphere of the exhaust gas exiting the three-way catalyst gradually approaches the lean maximum value from the stoichiometric state.

That is, after the atmosphere is switched from the rich atmosphere to the lean atmosphere, exhaust gas in the stoichiometric atmosphere is discharged from the three-way catalyst for a while, while the exhaust gas is discharged on the three-way catalyst.
It is considered that the oxidation reaction of HC and CO is occurring, and the longer this time is, the less HC flows into the NO _x storage reduction catalyst.

Therefore, the present inventors have proposed a method in which the three-way catalyst is provided on the downstream side.
In an exhaust gas purifying apparatus provided with a NO _x storage-reduction catalyst, the relationship between the time in the stoichiometric atmosphere and the NO _x purification rate was studied. As a result, when the exhaust gas under the condition that the lean atmosphere in which the molar ratio of the oxidizing component to the reducing component in the exhaust gas exceeds 14.6 and the rich atmosphere in which the molar ratio is 14.6 or less alternately flow into the three-way catalyst, When the time from the time when the atmosphere of the exhaust gas after passing through the source catalyst changes from the rich atmosphere to the stoichiometric atmosphere to the lean atmosphere of 50% of the maximum lean atmosphere (stoichiometric holding time) is 1 second or less, NO _x It has been found that the purification rate is extremely high, and the present invention has been completed.

That is, in the exhaust gas purifying apparatus of the present invention, the stoichiometric holding time of the three-way catalyst is set to 1 second or less, and the device has little or no oxygen storage / release capability. Therefore, since almost no oxygen is released during the rich spike, many HCs generated during the rich spike
Leaves a considerable amount even after passing through the three-way catalyst, and its HC
There flowing into the NO _x storage-and-reduction type catalyst on the downstream side. And NO _x
HC that has flowed into the storage-reduction catalyst is consumed in the reduction of released NO _x from _the NO _x storage material. As a result, even if the rich spike is shallow, the NO _x purification performance is improved. When stoichiometric retention time of the three-way catalyst is greater than 1 second, NO _x purification rate is rapidly lowered.

To make the stoichiometric holding time less than 1 second,
This can be achieved by reducing the amount of the oxygen storage / release material contained in the three-way catalyst or by not including the oxygen storage / release material. In other words, the stoichiometric retention time can be said to be an index of the oxygen storage / release capacity of the three-way catalyst. Ceria (CeO ₂ ) is a typical oxygen storage / release material, but rare earth metal oxides such as PrO ₄ , NiO, Fe ₂ O ₃ , CuO
And transition metal oxides such as Mn ₂ O ₅ are also exemplified. By using a three-way catalyst not containing or slightly containing these, the stoichiometric retention time can be made 1 second or less.

In the exhaust gas purifying apparatus of the present invention, it is desirable that the three-way catalyst does not contain Pd but contains Rh and Pt.
Since Pd has oxygen adsorption capacity, HC in a rich atmosphere
High activity to oxidize. When using a three-way catalyst comprising Therefore Pd, there is a case where if it contains oxygen storage material in the same manner as _the NO _x purification rate becomes low. Meanwhile Pt and Rh, since oxidation activity in rich atmosphere is lower than Pd, oxidation of HC is suppressed in the rich atmosphere, reducing the efficiency of the NO _x can be improved as compared with Pd. Pt and Rh have a higher oxidation activity in a lean atmosphere than Pd and a lower activation temperature. Therefore it is possible to efficiently oxidized purifying HC and CO in the lean atmosphere in the low temperature region, and also improves storage efficiency of the NO _x by oxidation of NO.

As the three-way catalyst in the exhaust gas purifying apparatus of the present invention, a three-way catalyst composed of a carrier and a noble metal carried on the carrier can be used. As the carrier, a porous oxide such as alumina (Al ₂ O ₃ ), silica (SiO ₂ ), zirconia (ZrO ₂ ), or titania (TiO ₂ ) can be used. As described above, it is preferable to use at least one of Pt and Rh as the noble metal, but it is particularly preferable to use both Pt and Rh. The presence of Rh can suppress Pt grain growth in a high-temperature lean atmosphere,
Heat resistance is improved.

However, when Pt and Rh are co-supported,
There is a problem that the oxidation activity of Pt is inhibited by Rh. Therefore, a carrier powder carrying Pt and a carrier powder carrying Rh are mixed and used, or a three-layer structure in which one of Rh or Pt is carried in the upper layer and the other of Rh or Pt is carried in the lower layer. It is preferable to use a raw catalyst. As a result, Pt and Rh are in a state of being carried close to each other in a separated state,
While suppressing the grain growth of Pt, a decrease in the oxidation activity is prevented, and the oxidation activity is improved. In the case of a two-layer structure, it is preferable that the lower layer carries Pt and the upper layer carries Rh.

The supported amount of the noble metal is 0.1
A range of 〜10% by weight is preferred. If the supported amount is less than this, sufficient purification activity cannot be obtained, and even if the supported amount is more than this, the purification activity is saturated and excess noble metal is wasted.

The NO _x storage-reduction type catalyst in the exhaust gas purifying apparatus of the present invention is selected from a porous oxide carrier, a noble metal supported on the porous oxide carrier, an alkali metal, an alkaline earth metal and a rare earth element. it can be used a porous oxide support supported on the _the NO _x storage material, those similar to the prior art, which is composed of.

As the porous oxide carrier used for the NO _x storage reduction catalyst, alumina, silica, silica-alumina, zirconia, titania, zeolite and the like can be used. One of these may be used, or a plurality of them may be mixed or combined for use. Among them, it is preferable to use γ-alumina having high activity. Note _the NO _x storage reduction catalyst porous oxide carrier used may be a porous oxide support and the same kind of three-way catalyst, may use different ones.

Examples of the noble metal used for the NO _x storage-reduction catalyst include Pt, Rh, Pd, and Ir. Among them, Pt having high activity is particularly preferable. The amount of the noble metal supported is preferably 0.1 to 10 g per liter of the porous oxide carrier. If the amount is less than this, the purification activity is insufficient, and if the amount is more than this, the effect is saturated and the cost increases.

The NO _x storage material used in the NO _x storage reduction catalyst is at least one selected from alkali metals, alkaline earth metals and rare earth elements. Examples of the alkali metal include lithium, sodium, potassium, and cesium. The alkaline earth metal refers to a Group 2A element in the periodic table, and examples thereof include barium, beryllium, magnesium, calcium, and strontium. Also, rare earth elements include scandium, yttrium, lanthanum,
Cerium, praseodymium, neodymium, dysprosium,
Ytterbium is exemplified.

The amount of the NO _x storage material carried by the NO _x storage reduction catalyst is 0.01 to 1 per liter of the porous oxide carrier.
Desirably, it is in the molar range. If the supported amount is smaller than this range, the NO _x adsorption amount is reduced, so that the NO _x purification ability is reduced. If the supported amount is larger than this range, the noble metal is covered with the NO _x storage material and the activity is reduced.

The three-way catalyst is disposed upstream of _the NO _x storage reduction catalyst in the exhaust gas passage, it is desirable to place directly under the engine. The arrangement interval between the three-way catalyst and NO _x storage-and-reduction type catalyst is not particularly limited, and may be placed adjacent, may be arranged at a predetermined distance. The form one of the three-way catalyst to a portion of the predetermined length from one end face of the carrier substrate, to form a NO _x storage-and-reduction type catalyst to rest, for a three-way catalyst so as to face the exhaust gas flow May be arranged.

[0030]

The present invention will be specifically described below with reference to examples and comparative examples.

(Embodiment 1) FIG. 1 schematically shows an exhaust gas purifying apparatus according to an embodiment of the present invention. This exhaust gas purification device
A three way catalyst 2 disposed on the upstream side of the exhaust gas flow path extending from the engine 1 is composed of the three-way catalyst 2 from _the NO _x storage reduction catalyst 3 which is disposed in the exhaust gas line downstream.

As shown in FIG. 2, the three-way catalyst 2 includes a honeycomb substrate 20 made of cordierite and a lower layer 21 made of γ-Al ₂ O ₃ formed on the surface of the honeycomb substrate 20 and carrying Pt. When,
An upper layer 22 made of θ-Al ₂ O ₃ and formed on the surface of the lower layer 21 and carrying Rh thereon.

The honeycomb substrate 20 has a capacity of 1.3 liters, and the lower layer 21 has a capacity of 160 g per liter of the honeycomb substrate 20.
The Pt is formed at a rate of 1.5
g is carried. The upper layer 22 is formed with 40 g per liter of the honeycomb substrate 20, and 0.3 g of Rh is supported per liter of the honeycomb substrate 20.

The NO _x storage-reduction catalyst 3 comprises a cordierite honeycomb substrate and a coating layer formed on the surface of the honeycomb substrate and composed of γ-Al ₂ O ₃ , TiO ₂ and ZrO _2. , a noble metal made of the coating layer Pt and Rh, Ba, and the _the NO _x storage material consisting of K and Li are carried.

The honeycomb substrate has a capacity of 2.0 liters,
The coating layer is formed in an amount of 250 g per liter of the honeycomb substrate. The breakdown of the coating layer is as follows: 100 g of γ-Al ₂ O ₃ , 100 g of TiO _{2 and} 50 g of ZrO ₂ per liter of the honeycomb substrate.
g. Pt is per liter of honeycomb substrate
2.0g supported, Rh is per liter of honeycomb substrate
0.5 g is carried. And 0.2 mol of Ba, 0.1 mol of K and 0.1 mol of Li are supported per liter of the honeycomb substrate.

An endurance test was conducted in which a gasoline direct injection engine having a displacement of 2 L was used as the engine 1 and only the three-way catalyst 2 was placed and operated at an inlet gas temperature of 800 ° C. for 5 hours.
Next, only the NO _x storage reduction catalyst 3 was placed, and the incoming gas temperature
An endurance test was performed at 700 ° C. for 50 hours.

[0037] Each catalyst after the durability test was arranged as shown in FIG. 1, and operated at EC mode was measured HC, and purification rate of CO and NO _x, respectively. Table 1 shows the results.

(Test Example) The lower layer 21 of the three-way catalyst 2 in Example 1 was composed of a mixture of γ-Al ₂ O ₃ and CeO ₂ ,
Six types of three-way catalysts were prepared by varying the amount of O ₂ added. Each three-way catalyst was mounted in the exhaust gas flow path of a 3 L gasoline direct injection engine, and the gas atmosphere of the three-way catalyst was changed so that the molar ratio of the oxidizing component to the reducing component (A / F) Lean atmosphere of 15.5 and A / F of 1
The outgas from each three-way catalyst was analyzed under the condition that the rich atmosphere of 3.5 was alternately repeated for 5 seconds. The result of this analysis was graphed as shown in FIG. 3B, and the time from when the rich atmosphere was changed to the stoichiometric atmosphere to when the lean atmosphere became 50% of the maximum lean atmosphere was determined and defined as the stoichiometric holding time.

Further, the same NO _x storage-reduction type catalyst as in the first embodiment is arranged downstream of the three-way catalyst, and the engine is rotated at 2 rpm.
While operating at 000 rpm, NO for each three-way catalyst
The NO _x purification rates when passing through the _x storage reduction catalyst were measured. FIG. 4 shows the relationship between the stoichiometric holding time of each three-way catalyst and the measured NO _x purification rate.

FIG. 4 shows that when the stoichiometric holding time exceeds 1 second, the NO _x purification rate sharply decreases, and it is clear that the stoichiometric holding time is preferably 1 second or less.

The stoichiometric retention time of the three-way catalyst 2 used in Example 1 was 0.2 seconds.

(Example 2) 5 g of Pt per liter of the honeycomb substrate was further carried in a range of 20 mm from the upstream end face of the same three-way catalyst as in Example 1. Except that this was used as a three-way catalyst, an exhaust gas purifying apparatus was constructed in the same manner as in Example 1, and the purification rate after the durability test was measured in the same manner.
Table 1 shows the results. The stoichiometric retention time measured in the same manner as in the test example of the three-way catalyst 2 was 0.2 seconds.

(Example 3) On the same honeycomb substrate 20 as in Example 1, a coating layer made of γ-Al ₂ O ₃
200 g per liter, Pt and Rh on the coat layer
1.5 g per liter of honeycomb substrate
An exhaust gas purifying apparatus was constructed in the same manner as in Example 1 except that a three-way catalyst carrying 0.3 g was used, and the purification rate after the durability test was measured in the same manner. Table 1 shows the results. The stoichiometric retention time measured in the same manner as in the test example of the three-way catalyst 2 is
0.2 seconds.

Example 4 The same three-way catalyst as in Example 1 was further loaded with 10 g of Pd per liter of the honeycomb substrate in a range of 20 mm from the upstream end face. Except that this was used as a three-way catalyst, an exhaust gas purifying apparatus was constructed in the same manner as in Example 1, and the purification rate after the durability test was measured in the same manner.
Table 1 shows the results. The stoichiometric retention time measured in the same manner as in the test example of the three-way catalyst 2 was 0.7 seconds.

(Example 5) A three-way catalyst comprising 5.0 g of Pd per liter of a honeycomb substrate in the lower layer 21 and 0.5 g of Rh per liter of the honeycomb substrate in the upper layer 22 was used. Except for this, an exhaust gas purifying apparatus was constructed in the same manner as in Example 1, and the purification rate after the durability test was measured in the same manner.
Table 1 shows the results. The stoichiometric retention time measured in the same manner as in the test example of the three-way catalyst 2 was 0.9 seconds.

Example 6 Except that a three-way catalyst comprising 120 g of the lower layer 21 per liter of the honeycomb base material and carrying 3.0 g of Pd per liter of the honeycomb base material on the lower layer 21 was used. In the same manner as in Example 1, an exhaust gas purification device was constructed, and the purification rate after the durability test was measured in the same manner. Table 1 shows the results. The stoichiometric retention time measured in the same manner as in the test example of the three-way catalyst 2 was 0.9 seconds.

[0047] consist of (Comparative Example 1) The lower layer 21 γ-Al ₂ O ₃ and CeO ₂ Prefecture, per liter of γ-Al ₂ O ₃ and CeO ₂ of the honeycomb base material is formed so as to 80g, respectively . Then, 5.0 g of Pd was added to the lower layer 21 per liter of the honeycomb substrate.
An exhaust gas purifying apparatus was configured in the same manner as in Example 1, except that a three-way catalyst was used, which supported 0.5 g of Rh of the upper layer 22 per liter of the honeycomb substrate as in Example 1. Similarly, the purification rate after the durability test was measured. Table 1 shows the results
Shown in The stoichiometric retention time measured in the same manner as in the test example of the three-way catalyst 2 was 1.1 seconds.

(Comparative Example 2) The lower layer 21 was prepared so that γ-Al ₂ O ₃ was 40 g and CeO ₂ was 120 g per liter of the honeycomb substrate.
An exhaust gas purifying apparatus was constructed in the same manner as in Comparative Example 1 except that the sample was formed, and the purification rate after the durability test was measured in the same manner as in Example 1. Table 1 shows the results. The stoichiometric retention time measured in the same manner as in the test example of the three-way catalyst 2 was 2.0 seconds.

(Evaluation)

[0050]

[Table 1]

[0051] From Table 1, according to the exhaust gas purifying apparatus of each embodiment, it is understood that it is possible to purify a balanced HC, and CO and NO _x at each high purification rate. This is apparently due to the effect of reducing the stoichiometric retention time of the used three-way catalyst to 1 second or less.

[0052]

According to the exhaust gas purifying apparatus of the Effects of the Invention] The present invention, even when the shallow put the rich spike can be utilized for the reduction of effective NO _x and HC, NO _x without deteriorating the fuel economy Can be efficiently purified.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a configuration of an exhaust gas purifying apparatus of the present invention.

FIG. 2 is an explanatory sectional view showing a configuration of a three-way catalyst used in one embodiment of the present invention.

FIG. 3 is an explanatory diagram of a method for measuring a stoichiometric retention time.

FIG. 4 is a graph showing a relationship between a stoichiometric retention time and a NO _x purification rate.

[Explanation of symbols]

1: engine 2: three-way catalyst 3: NO _x storage reduction catalyst 20: honeycomb substrate 21: lower layer 22: upper layer

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (reference) B01J 23/58 ZAB F01N 3/08 A F01N 3/08 3/10 A 3/10 3/20 H 3/20 3 / 24 E 3/24 U F02D 41/04 305A F02D 41/04 305 B01D 53/36 101B 104A F-term (reference) 3G091 AA02 AA12 AA13 AA17 AA24 AA28 AB03 AB06 AB09 BA03 BA14 BA15 BA19 BA39 CB02 DA01 DA02 DA03 DB10 FA03 FA04 FA12 FA13 FB02 FB10 FB11 FB12 FC07 GA06 GA19 GA20 GB01W GB01X GB02W GB02X GB03W GB03X GB04W GB04X GB05W GB06W GB10X GB16X GB17X HA08 HA18 HA47 3G301 HA04 HA15 JA25 JA26 LB04 MA01 NE13 NE03 BA03 A03 A03 A03 A03 A03 A03 A03 BA30Y BA31X BA31Y BA33X BA33Y BA41Y BB02 CC31 4G066 AA13B AA16B AA28B BA05 BA07 CA28 DA02 GA01 GA06 4G069 AA03 AA15 BA01B BA04B BA05B BA13B BC70A BC70B BC72A BC72B BC75A BC75B CA03 CA08 CA09 EA18

Claims (3)

[Claims] Claims: 1. A three-way catalyst is arranged upstream of an exhaust gas stream,
In the exhaust gas purifying apparatus formed by arranging a NO _x storage-and-reduction type catalyst on the downstream side of the three-way catalyst, the exhaust gas purifying apparatus is characterized in that the stoichiometric retention time of the three-way catalyst and less than one second. Here, the stoichiometric holding time means that the three-way catalyst has a condition in which a lean atmosphere in which the molar ratio of the oxidizing component to the reducing component in the exhaust gas exceeds 14.6 and a rich atmosphere in which the molar ratio is 14.6 or less are alternately repeated, respectively. When flowing, the atmosphere of the exhaust gas after passing through the three-way catalyst,
This is the time from when the rich atmosphere changes to the stoichiometric atmosphere until the atmosphere becomes 50% of the maximum lean atmosphere. 2. The exhaust gas purifying apparatus according to claim 1, wherein the three-way catalyst does not contain an oxygen storage / release material. 3. The exhaust gas purifying apparatus according to claim 1, wherein the three-way catalyst does not contain Pd but contains Rh and Pt.