JPH11101125A - Emission control device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To satisfactorily purify an emission without emitting adsorbed NOx from leans NOx catalyst, by locating ternary catalyst for oxidizing HC and CO in the emission around a theoretical air-fuel ratio in an exhaust passage of an engine, and by locating the leans NOx catalyst for adsorbing NOx in the emission on the lean side of a theoretical air-fuel ratio and emitting adsorbed NOx around a theoretical air-fuel ratio, downstream of the ternary catalyst. SOLUTION: Lean NOx catalyst 10 carries barium. Further, ternary catalyst 4 carries noble metal in which platinum and rhodium are contained in ceria, having a weight ratio between platinum and rhodium which is set as Pt/Rh>3. The CO purifying rate is set to be lower than that the HC purifying rate around a theoretical air-fuel ratio of the catalyst 4. An emission in which CO is rich, as a reluctant, is fed to the lean NOx catalyst 10 from the ternary catalyst 4 around a theoretical air-fuel ratio, thereby it is possible to promote emission and purification of NOx in the leans NOx catalyst 10 with the use of CO in the emission.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying apparatus, and more particularly to a technical field of an exhaust gas purifying apparatus in which an upstream oxidation catalyst and a downstream NOx adsorption catalyst are disposed in an exhaust passage of an engine.

[0002]

2. Description of the Related Art In recent years, attention has been paid to a lean-burn engine having a low fuel consumption rate as an automobile engine. In this engine, fuel is burned in an oxygen-excess atmosphere with a lean air-fuel ratio. A large amount of NOx is generated, and an exhaust gas purifying device for purifying this NOx is required.

[0003] As this kind of exhaust gas purifying device, conventionally,
As disclosed in JP-A-6-336916, a three-way catalyst is provided upstream of an exhaust passage of an engine, and platinum and barium are carried in an exhaust passage downstream of the three-way catalyst. There has been proposed a structure in which NOx absorbents for adsorbing NOx in the respective components are arranged.

[0004]

By the way, using the catalyst arrangement structure of the above-mentioned conventional exhaust gas purifying apparatus, an oxidation catalyst for oxidizing HC and CO is provided upstream of the exhaust passage.
When NOx adsorbing catalysts for adsorbing NOx are arranged on the downstream side, and when the air-fuel ratio is leaner than the stoichiometric air-fuel ratio, NOx adsorbing catalyst on the downstream side NOx adsorbing catalyst is used.
Then, when the air-fuel ratio becomes close to the stoichiometric air-fuel ratio, HC and CO in the exhaust gas are oxidized by the oxidation catalyst, and N2 adsorbed on the NOx adsorption catalyst is adsorbed.
By purifying and releasing Ox, not only HC and CO in the exhaust gas but also NOx in a lean state can be satisfactorily purified.

However, in that case, actually, N
It is difficult to release the adsorbed NOx from the Ox purification catalyst while purifying the NOx, and a solution is needed.

[0006] The present invention has been made in view of the above-mentioned point, and an object of the present invention is to improve the configuration of an oxidation catalyst disposed on the upstream side of an exhaust passage as described above, so that NOx adsorption on the downstream side is improved. It is an object of the present invention to ensure that NOx is released from the catalyst.

[0007]

In order to achieve the above object, according to the present invention, the purification rate of CO in the upstream oxidation catalyst is made lower than that of HC, and CO is used as a reducing agent in the downstream oxidation catalyst. Through the NOx adsorption catalyst, and the CO
The release or purification of the adsorbed NOx of the adsorption catalyst is promoted.

More specifically, according to the first aspect of the present invention, an oxidation catalyst disposed in the exhaust passage of the engine to oxidize HC and CO in exhaust gas at an air-fuel ratio near the stoichiometric air-fuel ratio, A NOx adsorption catalyst that is disposed in the exhaust passage on the downstream side and adsorbs NOx in the exhaust gas at an air-fuel ratio leaner than the stoichiometric air-fuel ratio and releases the adsorbed NOx at an air-fuel ratio near the stoichiometric air-fuel ratio is used. It is assumed that the exhaust gas purifying apparatus provided is a target and the CO purification rate near the stoichiometric air-fuel ratio of the oxidation catalyst is set lower than the HC purification rate.

With the above configuration, when the air-fuel ratio is leaner than the stoichiometric air-fuel ratio, NOx in the exhaust gas is adsorbed by the NOx adsorption catalyst arranged downstream of the exhaust passage of the engine. Thereafter, when the air-fuel ratio becomes close to the stoichiometric air-fuel ratio, HC and CO in the exhaust gas are oxidized by the oxidation catalyst disposed on the upstream side of the exhaust passage, and the NOx adsorbed by the NOx adsorption catalyst is also absorbed. Is purified and released. This makes it possible to purify HC and CO in the exhaust gas, and in particular, NOx generated in a lean state.

In this case, since the CO purification rate of the oxidation catalyst near the stoichiometric air-fuel ratio is set lower than the HC purification rate, when the oxidation catalyst oxidizes HC and CO near the stoichiometric air-fuel ratio, Due to the decrease in the CO purification rate, exhaust gas rich in CO as a reducing agent is supplied from the oxidation catalyst to the NOx adsorption catalyst on the downstream side. CO in this exhaust gas
Thus, for example, in a NOx adsorption catalyst, a reaction in which NOx is released as NO ₂ is released, and the NO ₂ is further reduced to N ₂ and O _2.
2 which results in NOx
NOx release and purification at the adsorption catalyst can be effectively promoted.

Contrary to the above, when HC in the oxidation catalyst is used as a reducing agent and its purification rate is set lower than CO, and HC-rich exhaust gas is supplied to the NOx adsorption catalyst, Does not directly react with the adsorbed NOx in the NOx adsorbing catalyst, the purification and release of the adsorbed NOx are insufficient, and a large NOx purification rate cannot be expected.

The NOx adsorbing catalyst tends to adsorb SOx (sulfur oxide) in the exhaust gas lean and tends to reduce the adsorbed amount of NOx. However, as described above, the air-fuel ratio is close to the stoichiometric air-fuel ratio. Alternatively, if the amount of CO is large on the rich side, the SOx can be released at a relatively low temperature, and thus the SOx poisoning of the NOx adsorption catalyst can be prevented.

According to the second aspect of the present invention, the air-fuel ratio at which the CO purification rate of the oxidation catalyst increases to a predetermined value or more as the air-fuel ratio increases toward the lean side increases the air-fuel ratio at which the HC purification rate increases to the predetermined value or more. Set leaner than fuel ratio. This makes it possible to realize a configuration in which the CO purification rate in the vicinity of the stoichiometric air-fuel ratio is set lower than the HC purification rate in the oxidation catalyst.

According to the third aspect of the present invention, the oxidation catalyst has a noble metal supported on ceria. Thus, a catalyst structure for making the CO purification rate of the oxidation catalyst lower than the HC purification rate can be easily obtained.

In the invention of claim 4, platinum and rhodium are supported on the oxidation catalyst as noble metals, and the weight ratio of platinum and rhodium is set to Pt / Rh <3 (smaller than 3).
Also in this case, a catalyst structure for making the CO purification rate of the oxidation catalyst lower than the HC purification rate can be easily obtained. In this case, if the weight ratio of platinum and rhodium is Pt / Rh ≧ 3 (3 or more), the effect of lowering the CO purification rate cannot be obtained, so that Pt / Rh <3.

According to a fifth aspect of the present invention, the NOx adsorbing catalyst comprises at least one of an alkali metal and an alkaline earth metal.
It is assumed that a certain metal is supported. In the invention of claim 6, at least one of the alkali metals or alkaline earth metals is barium. Thus, a NOx adsorption catalyst that adsorbs NOx at a lean air-fuel ratio and easily reacts with CO from the oxidation catalyst in the vicinity of the stoichiometric air-fuel ratio can be easily obtained.

[0017]

FIG. 4 shows an exhaust gas purifying apparatus according to an embodiment of the present invention, wherein 1 is a direct injection lean combustion engine for directly supplying fuel to a combustion chamber (not shown) in a cylinder, for example. Reference numeral 2 denotes an exhaust passage for discharging exhaust gas in the combustion chamber. In the exhaust passage 2, a three-way catalyst 4 and a lean NOx catalyst 10 located downstream of the three-way catalyst 4 are arranged. By the two catalysts 4 and 10, HC, CO, NOx and the like in the exhaust gas during the stoichiometric air-fuel ratio combustion operation of the engine 1 (for example, at the time of acceleration or at the time of NOx emission control performed every predetermined period during steady operation). In addition to purifying air pollutants, it also effectively purifies NOx during lean combustion operation (for example, during idling or constant speed operation). The oxygen concentration in the lean atmosphere is 4 to 5% to 20%, and the air-fuel ratio is used under the condition of A / F = 18 or more.

Specifically, the lean NOx catalyst 10
Functions as a NOx adsorption catalyst that adsorbs NOx in exhaust gas at an air-fuel ratio leaner than the stoichiometric air-fuel ratio and purifies and releases the adsorbed NOx at an air-fuel ratio near the stoichiometric air-fuel ratio. As shown in FIG. 3, the lean NOx catalyst 10 includes a honeycomb carrier 11 made of cordierite, and an inner catalyst layer 12 (base coat) is formed on the carrier 11.
And the outer catalyst layer 13 (overcoat) thereon
Layer of catalyst layer is coated.

The inner catalyst layer 12 carries, for example, platinum Pt as a noble metal as a catalyst metal and barium Ba as a NOx absorbent as a support material using alumina and ceria as porous materials.

On the other hand, in the outer catalyst layer 13, platinum, rhodium and barium as noble metals are supported as a support material of zeolite which is a porous material.

Incidentally, the impurity is set to 1% or less. Further, sodium Na, potassium K, strontium Sr, calcium Ca, or the like may be used instead of barium, or two or three of barium and barium may be used in combination. In short, it is sufficient if at least one kind of metal among alkali metals or alkaline earth metals is used.

The support material of the inner catalyst layer 12 may be zeolite instead of alumina and ceria, and the support material of the outer catalyst layer 13 may be alumina or ceria instead of zeolite. And two or three of the zeolites may be combined.

On the other hand, the upstream three-way catalyst 4 is
It functions as an oxidation catalyst that oxidizes HC and CO in exhaust gas at an air-fuel ratio near the stoichiometric air-fuel ratio (note that HC and CO are also purified at an air-fuel ratio leaner than the stoichiometric air-fuel ratio). As shown in FIG. 1, an inner catalyst layer 6 (base coat) is formed on a honeycomb-shaped carrier 5 made of, for example, cordierite.
And the outer catalyst layer 7 (overcoat). The inner catalyst layer 6 supports palladium Pd using, for example, alumina and ceria as support materials.

On the other hand, noble metals platinum and rhodium are supported on the outer catalyst layer 7 using ceria as a support material. The weight ratio of platinum and rhodium is Pt / Rh <3.
(Smaller than 3), and it is preferable that Pt / Rh = 3/2 to 1/5. That is, if the weight ratio of platinum and rhodium is Pt / Rh ≧ 3 (3 or more), the effect of lowering the CO purification rate cannot be obtained, so that Pt / Rh <3. Further, when Pt / Rh = 0, that is, when only platinum is not supported and only rhodium is used, deterioration starts at the stage of firing the catalyst layers 6 and 7 at the time of manufacturing the three-way catalyst 4, which is not preferable. Further, barium of about 3 g / L may be added to the entire inner and outer catalyst layers 6 and 7 of the three-way catalyst 4 in order to improve heat resistance.

Since platinum and rhodium are supported by ceria in the outer catalyst layer 7 and the weight ratio of platinum and rhodium is set to Pt / Rh <3,
As shown in FIG. 1, the air-fuel ratio A1 at which the CO purification rate of the three-way catalyst 4 increases to a predetermined value (for example, 80%) or more as the air-fuel ratio increases toward the lean side, is such that the HC purification rate is the predetermined value. (80%) and leaner than the air-fuel ratio A2 (A
1> A2), so that the CO purification rate of the three-way catalyst 4 near the stoichiometric air-fuel ratio is set lower than the HC purification rate. That is, for example, when the purification rate of HC is 80
% Is A / F = 14.6 (= A2), the air-fuel ratio at which the CO purification rate is 80% or more is A
A / F = 14.62 greater than /F=14.6 (= A
In order to satisfy 1), 8% of CO for 80% purification rate of HC
The 0% purification rate is set to A / F = 0.02 or more on the lean side in the air-fuel ratio. Note that the upper limit of the air-fuel ratio at which the CO purification rate is equal to or higher than a predetermined value is set so that the CO purification rate does not significantly decrease. However, A / F does not exceed 14.75.

When the above catalysts 4 and 10 are manufactured, for example, the inner catalyst layers 6 and 12 are preferably formed by an impregnation method, and the outer catalyst layers 7 and 13 are preferably used by a spray dry method. That is, in the three-way catalyst 4, the binder and
A slurry is prepared by mixing alumina and ceria powder supporting palladium as a noble metal, and this slurry is wash-coated on a carrier 5 and dried and fired,
The inner catalyst layer 6 is formed on the carrier 5. The spray drying method is a method also called a spray drying method, in which a slurry is prepared by mixing a powder of ceria, a platinum solution as a complex and a rhodium solution and water, and spraying the slurry in a heated atmosphere. Dry and bake to produce overcoat powder. Then, the overcoat powder is mixed with a binder to prepare a slurry. The slurry is wash-coated on the carrier 5 from above the inner catalyst layer 6, dried and fired to form the outer catalyst layer 7. .

On the other hand, in the lean NOx catalyst 10, a slurry is prepared by mixing a binder, alumina and ceria powder which do not carry a noble metal, and this slurry is wash-coated on a carrier 11 and dried and calcined.
An inner catalyst layer 12 is formed thereon. Further, a slurry is prepared by mixing zeolite powder, a rhodium solution as a complex, and water, and this slurry is sprayed into a heating atmosphere and dried and fired to obtain an overcoat powder (spray drying method). Then, a slurry is prepared by mixing the overcoat powder with a binder, and the slurry is wash-coated on the support 11 from above the inner catalyst layer 12 and dried and fired. Then, after coating the two catalyst layers 12 and 13 in this manner, each of the catalyst layers 12 and 13 is impregnated with a mixed solution of platinum and barium to carry platinum and barium. Thereafter, drying and baking are performed.

Therefore, in the exhaust gas purifying apparatus of the above-described embodiment, when the engine 1 performs lean combustion operation, for example, during idling operation or constant speed operation, and the air-fuel ratio is leaner than the stoichiometric air-fuel ratio. In addition, CO and HC in the exhaust gas are oxidized and purified by the three-way catalyst 4 arranged on the upstream side of the exhaust passage 2, and the NO in the exhaust gas is reduced by the lean NOx catalyst 10 arranged on the downstream side.
x is adsorbed.

Thereafter, when the stoichiometric air-fuel ratio combustion operation is performed, for example, during the acceleration operation of the engine 1 and the air-fuel ratio becomes close to the stoichiometric air-fuel ratio, the HC in the exhaust gas is similarly reduced by the upstream three-way catalyst 4. CO is oxidized and NOx is reduced at the same time.

The NOx adsorbed on the lean NOx catalyst 10 is released while being purified. At that time, the three-way catalyst 4 has a CO purification rate near the stoichiometric air-fuel ratio set lower than the HC purification rate.
When the CO is oxidized, the CO-rich exhaust gas serving as a reducing agent is supplied from the three-way catalyst 4 to the downstream lean NOx catalyst 10 by setting the reduction of the CO purification rate. The CO in the exhaust gas causes NOx in the lean NOx catalyst 10
React with but which is released as NO _2, the reaction and arises that NO ₂ is further decomposed into N ₂ and O _2. Specifically, for example, in the lean NOx catalyst 10, NO ₃ as NOx is deposited on the surface of the barium particles by barium nitrate B.
Assuming that the barium nitrate is adsorbed as a (NO ₃ ) ₂ , the barium nitrate is replaced by the supply of CO and is decomposed into barium carbonate Ba (CO ₃ ) and nitrogen dioxide NO ₂ under an oxygen atmosphere. Is released from the lean NOx catalyst 10. Then, this nitrogen dioxide is further decomposed into N ₂ and O ₂ under CO, whereby the release and purification of NOx in the lean NOx catalyst 10 can be effectively promoted. Therefore, HC and C in the exhaust gas
O and, in particular, NOx generated in a lean state can be purified.

In this embodiment, the three-way catalyst 4
Is a catalyst in which platinum and rhodium are supported on ceria, and the weight ratio of platinum and rhodium is P
Since t / Rh <3, C in the three-way catalyst 4 as described above
A catalyst structure for making the O purification rate lower than the HC purification rate can be easily obtained.

In the above embodiment, both the three-way catalyst 4 and the lean NOx catalyst 10 are of a two-layer coat, but one or both of them may be of a single-layer coat.

[0033]

Next, a specific embodiment will be described.

NOx Purification Performance (Example) A palladium compound, alumina, ceria, and an alumina binder were prepared as a three-way catalyst, and mixed with ion-exchanged water to prepare a slurry. Next, this slurry was wash-coated on a cordierite carrier having a honeycomb structure of 6 mils 400 cells,
It was dried at a temperature of 50 ° C. and calcined at a temperature of 300 to 600 ° C. for 2 to 4 hours to form an inner catalyst layer.

On the other hand, a slurry is prepared by mixing a platinum dinitrodiamine solution, a rhodium nitrate solution and ceria with water, and this slurry is spray-dried by a spray-dry method, and then calcined at 200 to 600 ° C. for 1 to 24 hours. To obtain an overcoat powder. Then, after mixing this overcoat powder with an alumina binder, a slurry is prepared by adding ion-exchanged water, the slurry is wash-coated on a carrier, dried at a temperature of 150 ° C, and dried at a temperature of 300 to 600 ° C. By firing at a temperature for 1 to 4 hours, an outer catalyst layer was formed on the inner catalyst layer.

In this example, the amount of noble metal was 3.15 g / L per liter of the carrier, and the weight ratio of platinum, palladium and rhodium was Pt / Pd / Rh = 1/17/3.

(Comparative Example) In the configuration of the example, the weight ratio of platinum, palladium and rhodium in the catalyst layer was Pt / Pd /
The case where Rh = 3/26/1 (ordinary trimetal three-way catalyst) was used as a comparative example. Other configurations are the same as those of the embodiment.

The three-way catalysts of the above embodiment and the comparative example are arranged in the exhaust passage of a direct injection reciprocating engine with a displacement of 2000 cc and upstream of the lean NOx catalyst. A / F
F = 40, and a stoichiometric air-fuel ratio during acceleration, and a predetermined mode in which fuel is cut during deceleration. The HC and CO purification rates after passing through the three-way catalyst and the lean NOx catalyst after passing through the lean NOx catalyst. The NOx purification rates in all modes and lean air-fuel ratios were measured. Table 1 shows the results.
Table 2 shows the composition of the exhaust gas at the stoichiometric air-fuel ratio.

[0039]

[Table 1]

[0040]

[Table 2]

From Table 1, according to the embodiment of the present invention,
Compared with the comparative example, the HC purification rate after passing through the three-way catalyst is the same, but the CO purification rate is decreased. The decrease in the CO purification rate causes the NOx purification rate, particularly, the N2 concentration by the lean NOx catalyst.
It can be seen that the Ox purification rate has increased.

Regarding the weight ratio of platinum and rhodium in the three-way catalyst In the three-way catalyst having the same structure as in the above embodiment, the weight ratio of platinum and rhodium in the outer catalyst layer was changed, and the NOx purification rate, HC purification rate and When the change in the air-fuel ratio width (A / F window) when the CO purification rate reached the same predetermined value or more was measured, the results shown in FIG. 5 were obtained. As is apparent from FIG. 5, the weight ratio of platinum and rhodium is Pt / R
When h <3, the reduction of the CO purification rate with respect to the HC purification rate becomes clear, and particularly, when Pt / Rh = 3/2 to 1/5, a remarkable effect is obtained.

Regarding the range of each purification rate of CO and HC in the three-way catalyst The same three-way catalyst as in the above embodiment (the total amount of noble metal is 3.1
5 g / L, the weight ratio of platinum, palladium and rhodium is Pt / Pd / Rh = 1/17/3, and barium is added to improve heat resistance). After heat aging with air for a period of time, the air-fuel ratio is maintained at A / F = 14.7 ± 0.2 and H
The air-fuel ratio at which both the C purification rate and the CO purification rate were 80% was examined. The inlet temperature of the oxidation catalyst is 400 ° C. As a result, the air-fuel ratio at which the HC purification rate becomes 80% is A / F <1.
In contrast to 4.6, the air-fuel ratio at which the CO purification rate was 80% was A / F> 14.6. In the case where barium was not added, the air / fuel ratio at which the CO purification rate was 80% was A / A.
F> 14.65.

After the three-way catalyst was thermally aged at 1000 ° C. for 24 hours with air, the same HC purification rate and C
When the air-fuel ratio at which the O purification rate becomes 80% was examined, the air-fuel ratio at which the HC purification rate became 80% was A / F <14.
In contrast to 4, the air / fuel ratio at which the CO purification rate was 80% was A / F> 14.5.

From this, if the air-fuel ratio at which the HC purification rate is 80% or more is A / F = 14.6, the air-fuel ratio at which the CO purification rate is 80% or more is A / F = 14. H such that A / F = 14.62 larger than .6, for example.
It can be seen that the 80% purification rate of CO may be set to the air / fuel ratio, for example, A / F = 0.02 or more on the lean side with respect to the 80% purification rate of C.

[0046]

As described above, according to the first aspect of the present invention, the oxidation catalyst for oxidizing HC and CO in the exhaust gas near the stoichiometric air-fuel ratio is provided upstream of the exhaust passage of the engine, and the oxidation catalyst is provided downstream thereof. In the exhaust gas on the lean side of the stoichiometric air-fuel ratio
NOx adsorption catalysts that adsorb Ox and purify and release adsorbed NOx near the stoichiometric air-fuel ratio are arranged, and the CO purification rate of the oxidation catalyst near the stoichiometric air-fuel ratio is set lower than the HC purification rate. According to the second aspect of the present invention, the air-fuel ratio at which the CO purification rate of the oxidation catalyst increases to a predetermined value or more as the air-fuel ratio increases toward the lean side is determined by the air-fuel ratio at which the HC purification rate increases to a predetermined value or more. Was also set on the lean side. According to these inventions, the CO-rich exhaust gas is supplied from the oxidation catalyst to the NOx adsorption catalyst near the stoichiometric air-fuel ratio, and the CO in the exhaust gas promotes the release and purification of NOx by the NOx adsorption catalyst, Moreover, SOx poisoning of the NOx adsorption catalyst can be prevented.

In the invention of claim 3, the noble metal is supported on ceria as the oxidation catalyst. Further, in the invention of claim 4, platinum and rhodium are supported on the oxidation catalyst, and the weight ratio is set to Pt / Rh <3. According to these inventions, a catalyst structure for making the CO purification rate of the oxidation catalyst lower than the HC purification rate can be easily obtained.

According to the fifth aspect of the present invention, the NOx adsorption catalyst comprises:
It was assumed to carry at least one kind of alkali metal or alkaline earth metal. In the invention of claim 6, the metal is barium. According to these inventions, it is possible to easily obtain a NOx adsorption catalyst that adsorbs NOx at a lean air-fuel ratio and easily reacts with CO from an oxidation catalyst near a stoichiometric air-fuel ratio.

[Brief description of the drawings]

FIG. 1 is a graph showing HC, CO and NOx purification characteristics of a three-way catalyst near a stoichiometric air-fuel ratio in an embodiment of the present invention.

FIG. 2 is a sectional view showing a main part of a three-way catalyst.

FIG. 3 is a sectional view showing a main part of a lean NOx catalyst.

FIG. 4 shows a three-way catalyst and lean N in an exhaust passage of an engine.
FIG. 3 is a diagram illustrating an arrangement configuration of an Ox catalyst.

FIG. 5 is a diagram showing a change in the A / F window width when the weight ratio of platinum and rhodium in the three-way catalyst is changed.

[Explanation of symbols]

 Reference Signs List 1 engine 2 exhaust passage 4 three-way catalyst (oxidation catalyst) 10 lean NOx catalyst (NOx adsorption catalyst)

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Makoto Kyogoku 3-1 Shinchi, Fuchu-cho, Aki-gun, Hiroshima Mazda Co., Ltd. (72) Kenji Okamoto 3-1 Shinchi, Fuchu-cho, Aki-gun, Hiroshima Mazda Co., Ltd. Inside

Claims (6)

[Claims] An oxidation catalyst disposed in an exhaust passage of an engine for oxidizing HC and CO in exhaust gas at an air-fuel ratio near a stoichiometric air-fuel ratio, and disposed in an exhaust passage downstream of the oxidation catalyst. An exhaust gas purifying apparatus comprising: a NOx adsorption catalyst that adsorbs NOx in exhaust gas at an air-fuel ratio leaner than the air-fuel ratio and releases the adsorbed NOx at an air-fuel ratio near the stoichiometric air-fuel ratio. An exhaust gas purification device, wherein the CO purification rate near the stoichiometric air-fuel ratio of the oxidation catalyst is set lower than the HC purification rate. 2. The exhaust gas purifying apparatus according to claim 1, wherein the HC purification rate is such that the CO purification rate of the oxidation catalyst increases to a predetermined value or more as the air-fuel ratio increases toward the lean side. An exhaust gas purifying device characterized in that the exhaust gas purifying device is set leaner than the air-fuel ratio increasing as described above. 3. The exhaust gas purifying apparatus according to claim 1, wherein the oxidation catalyst comprises a noble metal supported on ceria. 4. The exhaust gas purifying apparatus according to claim 1, wherein platinum and rhodium are supported on the oxidation catalyst, and the weight ratio of platinum and rhodium is Pt / Rh <3. Purification device. 5. The exhaust gas purifying apparatus according to claim 1, wherein the NOx adsorption catalyst carries at least one kind of alkali metal or alkaline earth metal. Purification device. 6. The exhaust gas purifying apparatus according to claim 5, wherein at least one kind of alkali metal or alkaline earth metal is barium.