JPH1080620A - Exhaust gas cleaning device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust gas cleaning device exhibiting high cleaning ability effective for the whole harmful components in a high temp. region near an internal combustion engine while making the best use of the advantage of a conventional NO2 occlusion reduction type ternary catalyst. SOLUTION: This exhaust gas cleaning device is provided with a nitrogen oxide occluding part formed by carrying at least an alkali metal and/or an alkali metallic salt with a noble metal on a porous carrier to occlude a nitrogen oxide in the region of exhaust gas temperature near the internal combustion engine and the ternary catalyst part arranged at the post stage of the nitrogen oxide occluding part to allow the nitrogen oxide released from the nitrogen oxide occluding part to react with carbon monoxide, hydrocarbons and hydrogen contained in the exhaust gas to reduce.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to the harmful nitrogen oxides contained in the exhaust gas discharged from an internal combustion engine such as an automobile (hereinafter referred to as "NO _x"), carbon monoxide (CO) and hydrocarbons The present invention relates to an exhaust gas purifying apparatus for purifying gas (hereinafter referred to as "HC") effectively, and more specifically, to a nitrogen oxide configured to be suitable for being mounted near a high-temperature internal combustion engine. The present invention relates to an occlusion two-stage exhaust gas purification device.

[0002]

2. Description of the Related Art Exhaust gas discharged from an internal combustion engine of an automobile or the like contains NO _x , C
Since harmful substances such as O and HC are contained, regulations on purification of these harmful substances are becoming increasingly strict from the viewpoint of global environmental protection. On the other hand, from the viewpoint of fuel saving, during normal running except when the vehicle is idling or accelerating, the fuel is leaner than the stoichiometric air-fuel mixture (hereinafter referred to as “stoichiometric state”) (hereinafter “lean state”). )), The control for operating the internal combustion engine has been widely performed. Therefore, there is a strong demand for an exhaust gas purifying apparatus that can effectively purify the above-mentioned harmful substances not only in the stoichiometric state but also in such a lean state.

[0003] An exhaust gas purifying apparatus most widely used to purify the above harmful substances from exhaust gas has utilized a three-way catalyst. This three-way catalyst is prepared by loading platinum (Pt), palladium (P
d), carrying a noble metal catalyst such as rhodium (Rh), and has an action of simultaneously purifying NO _x , CO and HC. To be more specific, in the three-way catalyst, while the NO _x is reduced to N ₂ by reaction with CO and HC, since CO and HC are harmless are oxidized to CO ₂ and H ₂ O is there.

[0004] However, in the conventional three-way catalyst, the three components can be appropriately purified only in a very narrow range around a theoretical air-fuel ratio of 14.6, which is called a window. Therefore, efforts have been made to develop a catalyst that can effectively purify the three components even when the ratio deviates from the stoichiometric air-fuel ratio.

[0005] For example, "Toyota Technical Review"
(Winding 44, No.2, 11 May 1994) has proposed _the NO _x storage reduction three-way catalyst. This NO _x storage-reduction type three-way catalyst is obtained by supporting an alkaline earth metal element such as barium (Ba) in addition to a noble metal catalyst such as Pt on an appropriate carrier such as alumina.
While occluded by barium NO _x as nitrate, in a stoichiometric state, is based on the reaction mechanism of reducing the CO and HC to NO _x released from barium in the active point such as Pt. However, in this case, barium as the NO _x storage material has a low storage temperature range (a storage capacity of a certain level or more can be expected at 250 to 350 ° C.).
The mounting position of the O _x storage reduction three-way catalyst is limited under the floor position and the like away from the internal combustion engine.

By the way, automotive catalysts tend to be mounted below the floor at maniverters closer to the internal combustion engine in order to cope with increasingly severe cold emissions. Therefore, _the NO _x storage-reduction type three-way catalyst even higher temperature range (e.g., 300 to 500 ° C.) but _the NO _x storage capacity is required in, in the above NO _x storage-and-reduction type three-way catalyst using barium, this Such demands cannot be fully satisfied.

[0007] alkali metal is known as _the NO _x storage ability is high material at a high temperature region, it is considered to use in place of the barium in the NO _x storage-and-reduction type three-way catalyst. However, the alkali metal element has another problem that the ability to purify HC (oxidizing ability) by a noble metal such as Pt, which is a catalytically active point, is significantly impaired, and cannot provide a sufficient solution.

It is therefore an object of the present invention, while taking advantage of the conventional NO _x storage-and-reduction type three-way catalyst, an effective purification capability for the three harmful components even at high temperature region in the vicinity of the internal combustion engine An object of the present invention is to provide an exhaust gas purification device that can be used.

[0009]

In order to solve the above-mentioned problems, the present invention provides an exhaust gas purifying apparatus for purifying nitrogen oxides, carbon monoxide and hydrocarbons contained in exhaust gas of an internal combustion engine. A nitrogen oxide occlusion at a preceding stage in which at least an alkali metal element and / or an alkali metal salt is contained or mixed with a noble metal in a porous carrier in order to occlude nitrogen oxides in an exhaust gas temperature range in the vicinity of the internal combustion engine. And the nitrogen oxides released from the nitrogen oxide storage unit are disposed downstream of the nitrogen oxide storage unit so as to react with carbon monoxide, hydrocarbons and hydrogen contained in the exhaust gas for reduction. And a three-way catalyst section.

As described above, according to the present invention, the nitrogen oxide storage unit (first stage) specializing in the storage of NO _x and the three-way catalyst unit (second stage) acting as a known three-way catalyst are clearly defined. And placed in tandem. The barium in a conventional NO _x storage-and-reduction type three-way catalyst causes problems when replacing the alkali metal element, and are the same part to exert portion and the three-way catalyst function of _the NO _x storage, precious metal catalysts This is because the alkali metal element hinders the reaction when HC is oxidized. Therefore, in the present invention, the nitrogen oxide storage unit dedicated to storing NO _x is separated from the three-way catalyst unit, and the alkali metal contained in the nitrogen oxide storage unit (including alkali metal salts; the same applies hereinafter). Did not hinder the reaction in the three-way catalyst section.

According to the structure of the present invention, in the lean state,
A nitrogen oxide storage unit of the preceding stage, NO _x contained in the exhaust gas to be occluded in the alkali metal element as nitrates, does not unpurified of the NO _x is released to the outside.
Moreover, since the alkali metal element exerts sufficient _the NO _x storage ability at high temperature region in the vicinity of an internal combustion engine, a problem configured as maniverter the exhaust gas purifying apparatus of the present invention does not occur. Note that CO and HC contained in the exhaust gas in the lean state are oxidized and purified by excess oxygen in the subsequent three-way catalyst section.

On the other hand, when the state changes from the lean state to the stoichiometric state (or a rich state with a slight excess of fuel), the NO _x occluded in the preceding stage is released (this restores the NO _x occlusion ability of the alkali metal element). , And is reduced by CO and HC in the three-way catalyst section in the latter stage. As a result, N
O _x is purified as N ₂ , while CO and HC
Is oxidized to CO ₂ and H ₂ O and purified.

[0013] In the present invention, as the alkali metal element used in the nitrogen oxide storage portion of the front, can be cited potassium (K) and cesium (Cs), and it allows the loading amount is sufficient _the NO _x storage It is preferred to be in the range of 0.1 to 1.0 (mol / l carrier) in consideration of economy, and a known method can be adopted as a method for supporting the carrier. In addition, these alkali metal elements may be supported alone on the porous carrier, or may be supported by blending. Further, as long as an alkali metal element is used, an alkaline earth metal such as barium (Ba) may coexist. This can be clearly understood with reference to FIG.

That is, FIG. 1 shows that a porous carrier having a weight ratio of alumina carrier / ceria carrier (hereinafter, simply referred to as alumina / ceria ratio) of 4/1 is Pt2.5 (g / l carrier) -Rh0. 17 is a graph showing the relationship between the NO _x storage capacity and the temperature when a predetermined amount of an alkali metal element (single or a mixture) is further supported on the substrate, and the deterioration condition is air. And left at 800 ° C. for 5 hours. For comparison, FIG.
The O _x storage capacity is also shown. The alumina-based carrier referred to here is mainly Al ₂ O having a γ-type or δ-type crystal structure.
_{As mentioned above} , a material in which a part of Al is replaced with La for improving heat resistance can be widely used. The ceria of the ceria-based carrier referred to herein is mainly CeO ₂ having a fluorite-type crystal structure.
Can be widely used in which a part of is replaced with Zr, Si, Y or the like.

As can be seen from FIG. 1, in the case of Ba alone, the NO _x storage capacity peaks at about 300 ° C., and at high temperatures exceeding 400 ° C., the NO _x storage capacity sharply decreases. On the other hand, when K or Cs is supported alone or as a mixture, NO in a temperature range exceeding 400 ° C.
_{x The} storage capacity is higher than Ba, and N in a high temperature range
It turns out that it is suitable for O _x storage. In particular, the K-Cs mixture is in the place the peak of _the NO _x storage capacity of about 450 ° C., in which superior stands out _the NO _x storage in a high temperature range.

Further, NO _x of K-Cs mixture at high temperatures
It is better stand out occlusion is as described above, even in this case, the change in _the NO _x storage capacity by the mixing ratio of the alkali metal components is observed. This is illustrated in FIG.
3 and FIG.

That is, FIG. 2 shows the case where the same carrier and noble metal catalyst as those of FIG.
A graph showing the case of changing the mixing ratio of -Cs mixture _the NO _x storage capacity of (the alkali metal components including the case of zero) in relation to the temperature, K a graph of FIG. 3 FIG. 2
Is a graph in which the mixing ratio is plotted again for each temperature on the horizontal axis. First, referring to FIG. 2, in the K—Cs mixture, the peak of the NO _x storage capacity is at a temperature exceeding 400 ° C. regardless of the mixing ratio, and NO in a high temperature region.
It can be seen that the _x storage capacity is superior to the case where each of these alkali metal components is used alone. Also, according to FIG.
The higher the mixture ratio of Cs in the s mixture, the higher the N
O _x Despite obtain a storage capacity, also found a surprising tendency rather _the NO _x storage ability in a high temperature range is reduced in the case of the Cs alone. On the other hand, according to FIG.
At a high temperature of ° C., the lower the mixture ratio of K (the higher the mixture ratio of Cs), the higher the NO _x storage ability (however, up to Cs / K = 3/1 in molar ratio), and the temperature is 400 ° C. Then, the NO _x storage capacity did not change much depending on the mixing ratio.
At 50 ° C., on the contrary, it can be seen that each of the alkali metal components alone has a higher NO _x storage capacity. Taken together, K
The mixture ratio of the -Cs mixture should be appropriately determined according to the temperature of the exhaust gas. However, in general, the temperature is often higher than 400 ° C. in the vicinity of the internal combustion engine. Can be concluded to be advantageous.

According to the present invention, the function of the oxidation catalyst in the nitrogen oxide storage section at the preceding stage is to oxidize NO _x (normally NO) so that the alkali metal element can be stored as nitrate. The type is not particularly limited as long as the object can be achieved, but it is generally preferable that Pt alone or a small amount of Rh be blended with Pt and supported on the porous carrier. In this case, in order to achieve the initial purpose, Pt should be 1 to 8 (g / l carrier).
It is preferable to carry Rh on 0.1 to 1.6 (g / l carrier), and a known method can be adopted.

On the other hand, as the carrier on which the above-mentioned oxidation catalyst and alkali metal element are to be supported, it is preferable to use a mixture of an alumina-based and a ceria-based carrier. However, the form of the carrier is not particularly limited. As long as the three-way catalyst carrier has a specific surface area, it may be porous. Also,
These carriers may be formed into pellets or coated on a monolith made of cordierite, a heat-resistant metal, or the like. However, it has been found that the alumina / ceria ratio has an effect on the NO _x storage capacity, which is shown in FIGS.

That is, FIG. 4 shows the NO _x storage capacity when K is used as an alkali metal element and the same noble metal catalyst as in FIG. 1 is used and supported on two kinds of porous supports having different alumina / ceria ratios. FIG. 5 is a graph showing the relationship with temperature. FIG. 5 shows a case where Cs is used as an alkali metal element and the same noble metal catalyst as that of FIG. 1 is used and supported on three kinds of porous carriers having different alumina / ceria ratios. 5 is a graph showing the relationship between NO _x storage ability and temperature in Example 1. As can be seen from these graphs, for any K and Cs, alumina - is _the NO _x storage ability person in the case of high ratio of alumina in the ceria catalysts tend to be higher, in particular alumina / ceria ratio 4 / 1
Advantageously.

In the present invention, in the stoichiometric state or the rich state, the three-way catalyst section constituting the second stage reacts NO _x released from the first-stage nitrogen oxide storage section with CO and HC to convert these three components. At the same time, it is used for purification, and a known three-way catalyst can be used as it is (however, it is not publicly known to use it as a subsequent stage of the nitrogen oxide storage unit). In particular, P having a high oxidizing ability for HC
It is desirable to use d-based and Pd-Rh-based three-way catalysts.

A known exhaust gas purifying apparatus having a two-stage structure is disclosed in Japanese Patent Application Laid-Open No. 7-008755. More specifically, the exhaust gas purifying apparatus disclosed in the publication discloses a catalyst such as a noble metal catalyst such as Pt or Rh and an alkali metal element such as K or Cs (or a rare earth metal or alkali) as a first catalyst. Earth metal) and a noble metal catalyst such as Pd or Rh as a second catalyst on a carrier such as zeolite. The surface is similar to the two-stage exhaust gas purifying apparatus of the present invention. It seems to be. However, they are fundamentally different for the following reasons.

That is, in the exhaust gas purifying apparatus described in the above publication, the first catalyst generates N in a lean state.
Reducing the O _x into N ₂ and N ₂ O with a reducing gas such as HC is performed mainly, in the second catalyst, N ₂ O released from the first catalyst is decomposed into N ₂ and O ₂ Because On the other hand, in the exhaust gas purifying apparatus of the present invention, the nitrogen oxide occlusion section in the first stage specially stores NO _x generated in a lean state, and the three-way catalyst section in the second stage has a stoichiometric state (or rich state). the NO _x released state) from the nitrogen oxide storage portion of the front is to purify a three-way catalyst function. Such an operational difference is caused by the fact that the exhaust gas purifying apparatus described in the above publication is located at a relatively low temperature (for example, 250 ° C.) away from the internal combustion engine, for example, at a position 1.6 m from the exhaust manifold. This is because the exhaust gas purifying apparatus of the present invention is arranged in a relatively high temperature region near the internal combustion engine, while being arranged at a downstream position (see item [0035] of the publication). Therefore, although both employ a two-stage configuration, they are based on completely different operating principles and are basically different.

[0024]

Next, examples of the present invention will be described together with comparative examples.

[0025]

Example 1 (Preparation of nitrogen oxide storage part) First, 80 g of an alumina carrier and 20 g of a ceria carrier were added so that the alumina / ceria ratio was 4/1, and further, an alumina sol (alumina content: 10% by weight) 50 g and 80 g of water are added and mixed with stirring.
A carrier slurry was prepared. The obtained carrier slurry is
It was coated on a cordierite honeycomb molded body having a capacity of 80 cm ³ , dried, and fired at 600 ° C. for 1 hour. The amount of the carrier adhered after firing was 55 g. Next, Pt is 5.0
(G / l carrier), the coated honeycomb molded body was mixed with a mixture of dinitrodiamine platinum nitric acid aqueous solution and rhodium nitrate aqueous solution prepared to obtain a loading of 0.1 (g / l carrier). It was immersed, dried, and fired at 300 ° C. for 1 hour to prepare a noble metal-supported honeycomb molded body. Furthermore, K
Is immersed in an aqueous solution of potassium nitrate prepared so as to have a content of 0.5 (mol / l carrier), dried and calcined at 300 ° C. for 1 hour to form a nitrogen oxide storage part. Catalyst was obtained.

(Preparation of Three-Way Catalyst Part) First, 30 g of a ceria-based carrier, 45 g of an alumina sol (alumina content: 10% by weight) and 70 g of water were added to 60 g of an alumina-based carrier and mixed with stirring to prepare a carrier slurry. The obtained carrier slurry was coated on a cordierite honeycomb molded body having a capacity of 180 cm ³ , dried, and fired at 600 ° C. for 1 hour.
The amount of the carrier adhered after firing was 35 g. Next, a coated aqueous solution of palladium nitrate and rhodium nitrate prepared so as to obtain a carrying amount of Pd of 5.0 (g / l carrier) and Rh of 0.3 (g / l carrier) was coated. The honeycomb formed body was immersed, dried, and fired at 300 ° C. for 1 hour to obtain a three-way catalyst portion having a desired noble metal loading.

Next, the nitrogen oxide storage unit and the three-way catalyst unit are tandemly arranged in the exhaust manifold of the automobile to constitute an exhaust gas purifying apparatus (maniverter).
It was measured purification rate of the NO _x and HC during mode actual vehicle. The results are shown in Table 1 below together with the catalyst specifications. Here, the reason why the purification rate of CO was not measured is that the problem is the removal of NO _x and HC.

Prior to the measurement of the purification rate during the actual vehicle running in the 10.15 mode, in order to confirm the durability of the catalyst of the exhaust gas purifying apparatus, stoichiometric gas at a temperature of 800 ° C. was flowed for one minute, and at a temperature of 700 ° C. The flow of lean gas for 4 minutes was alternately repeated for 20 hours. The purification rates obtained in Table 1 represent the results after such a catalyst endurance load.

[0029]

Example 2 K was supported at a ratio of 0.37 (mol / l carrier) and Cs was supported at a ratio of 0.13 (mol / l carrier) as alkali metal elements in the nitrogen oxide storage unit in the former stage. except that was subjected to the same procedure as in example 1, NO _x and H at 10.15 mode actual vehicle
The purification rate of C was measured. The results are shown in Table 1 below.

[0030]

Example 3 K was supported at a rate of 0.25 (mol / l carrier) and Cs was supported at a rate of 0.25 (mol / l carrier) as alkali metal elements in the nitrogen oxide storage section at the preceding stage. except that was subjected to the same procedure as in example 1, NO _x and H at 10.15 mode actual vehicle
The purification rate of C was measured. The results are shown in Table 1 below.

[0031]

Embodiment 4 As the alkali metal elements in the nitrogen oxide storage unit at the preceding stage, K was supported at a rate of 0.13 (mol / l carrier) and Cs was supported at a rate of 0.37 (mol / l carrier). except that was subjected to the same procedure as in example 1, NO _x and H at 10.15 mode actual vehicle
The purification rate of C was measured. The results are shown in Table 1 below.

[0032]

Example 5 The same operation as in Example 1 was carried out except that Cs was supported at a ratio of 0.5 (mol / l carrier) as an alkali metal element in the nitrogen oxide storage section at the preceding stage. NO _x and H at .15 mode actual vehicle
The purification rate of C was measured. The results are shown in Table 1 below.

[0033]

Example 6 The alumina / ceria ratio of carrier in the nitrogen oxide storage unit of the preceding stage except that the 2/1, the same operation as in Example 3, NO _x and at 10.15 mode actual vehicle The purification rate of HC was measured. The results are shown in Table 1 below.

[0034]

Example 7 The same operation as in Example 3 was carried out except that only alumina was used as the carrier in the nitrogen oxide storage section at the preceding stage, and NO _x and H during traveling in the 10.15 mode actual vehicle.
The purification rate of C was measured. The results are shown in Table 1 below.

[0035]

Embodiment 8 P as the noble metal in the latter three-way catalyst section
t is 2.5 (g / l carrier) and Rh is 0.5 (g / l carrier).
The same operation as in Example 3 was carried out, except that the carrier was carried at the ratio of (/ l carrier), and the NO _x and HC purification rates during running in the 10.15 mode actual vehicle were measured. The results are shown in Table 1 below.

[0036]

Comparative Example 1 The same operation as in Example 1 was performed, except that Ba was supported at a ratio of 0.5 (mol / l carrier) in place of the alkali metal element in the nitrogen oxide storage section in the preceding stage. NO _x and H during actual vehicle running in 10.15 mode
The purification rate of C was measured. The results are shown in Table 1 below.

The comparative example 1 is different from the conventional NO _x storage-reduction type exhaust gas purifying apparatus in that the nitrogen oxide storage section and the three-way catalyst section are divided and arranged in tandem. I have.

[0038]

Comparative Example 2 Cordierite in Nitrogen Oxide Storage Unit
480cm capacity of honeycomb molded body ^ThreeAnd later
The same operation as in Comparative Example 1 except that the three-way catalyst part of
NO in 10.15 mode actual vehicle running_x
And the purification rate of HC were measured. The results are shown in Table 1 below.
Show.

The comparative example 2 corresponds to a conventional NO _x storage-reduction type exhaust gas purifying apparatus.

[0040]

Comparative Example 3 Ba in Comparative Example 2 in the nitrogen oxide storage unit
Is replaced by K at a rate of 0.25 (mol / l carrier),
The same operation as in the comparative example was performed, except that Cs was supported at a rate of 0.25 (mol / l carrier), respectively.
The NO _x and HC purification rates during running in a 0.15 mode actual vehicle were measured. The results are shown in Table 1 below.

[0041]

COMPARATIVE EXAMPLE 4 The same operation as in Comparative Example 1 was carried out except that the nitrogen oxide storage portion at the former stage in Comparative Example 1 was eliminated and the capacity of the carrier at the latter stage was increased to 480 cm ^3.
The NO _x and HC purification rates during the five-mode actual vehicle running were measured. The results are shown in Table 1 below.

Note that Comparative Example 4 corresponds to a conventional general three-way catalyst.

[0043]

[Table 1]

[0044]

Effect of the Invention As can be seen from Table 1, the purification rate obtained in Examples 1-8 of the present invention is obviously for one or both of the NO _x and HC than those of Comparative Examples 1 to 4 An improvement is observed (in the case of only one improvement, no undue decrease in the purification rate of the other is observed). In particular, Comparative Example 1
Is a nitrogen oxide storage portion Ba alone, even in combination with a three-way catalyst unit in the subsequent stage, it indicates that not provide adequate _the NO _x purification in internal combustion engine near as maniverter, Comparative Example 3 K with excellent NO _x storage capacity at high temperatures
This shows that even if Cs or Cs is used in the nitrogen oxide storage unit, sufficient purification cannot be obtained unless a three-way catalyst unit is provided in the subsequent stage, and the technical significance of the present invention is clear.

Therefore, according to the present invention, harmful substances contained in exhaust gas can be effectively purified even in the vicinity of an internal combustion engine which is in a high temperature range, and is particularly suitable for constructing a maniverter in a light vehicle or the like. This has the effect.

[Brief description of the drawings]

FIG. 1 is a graph showing the NO _x storage capacity of various substances.

FIG. 2 is a graph showing the relationship between the blend ratio of K—Cs as a NO _x storage substance and the NO _x storage capacity.

FIG. 3 is a graph obtained by re-plotting the graph of FIG. 2 so that the horizontal axis indicates the ratio of K.

FIG. 4 is a graph showing how NO _x was determined when the alumina / ceria ratio was changed for an alumina-ceria carrier as a carrier in a nitrogen oxide storage unit when K was supported.
It is a graph which shows whether an occlusion ability changes.

FIG. 5 is a carrier in the nitrogen oxide storage unit.
Alumina-ceria carrier with Cs supported
How is NO changed when the alumina / ceria ratio is changed
_xIt is a graph which shows whether an occlusion ability changes.

Claims (10)

[Claims] 1. An exhaust gas purifying apparatus for purifying nitrogen oxides, carbon monoxide and hydrocarbons contained in exhaust gas of an internal combustion engine, wherein the nitrogen oxide is oxidized in an exhaust gas temperature range near the internal combustion engine. Nitrogen oxide storage part at the preceding stage in which at least an alkali metal element and / or an alkali metal salt is contained or mixed in a porous carrier together with a noble metal to occlude substances, and nitrogen oxides released from this nitrogen oxide storage part A three-way catalyst unit disposed downstream of the nitrogen oxide storage unit so as to react with and reduce carbon monoxide, hydrocarbons and hydrogen contained in the exhaust gas. Purification device. 2. The exhaust gas purifying apparatus according to claim 1, wherein the alkali metal element is potassium. 3. The exhaust gas purifying apparatus according to claim 1, wherein the alkali metal element is cesium. 4. The exhaust gas purifying apparatus according to claim 1, wherein potassium and cesium are mixed and used as the alkali metal element. 5. The exhaust gas purifying apparatus according to claim 5, wherein a larger amount of cesium is mixed than potassium among the alkali metal elements. 6. The exhaust gas purifying apparatus according to claim 1, wherein the porous carrier is an alumina-based carrier. 7. The exhaust gas purifying apparatus according to claim 1, wherein the porous carrier in the nitrogen oxide storage unit is a mixture of an alumina carrier and a ceria carrier. 8. The exhaust gas purifying apparatus according to claim 7, wherein the weight ratio of alumina / ceria in the porous carrier is 4/1. 9. The exhaust gas purifying apparatus according to claim 1, wherein the noble metal in the nitrogen oxide storage unit includes platinum and a small amount of rhodium. 10. The noble metal used in the latter three-way catalyst section is P
The exhaust gas purifying apparatus according to any one of claims 1 to 9, comprising d or Pd-Rh.