JPH09299795A - Catalyst for purification of exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To further improve NOx removing performance by enhancing the oxidation reactivity of NO. SOLUTION: An NO oxidation catalyst is disposed on the upstream side of a flow of exhaust gas and an NOx occlusion catalyst is disposed on the downstream side. Since a part having high NO oxidizing performance is separated from a part having high NOx occluding performance, NO oxidizing ability is improved as compared with that of the conventional catalyst for purification of exhaust gas and the rate of removal of Nox is also increased.

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 catalyst for purifying exhaust gas emitted from an internal combustion engine such as an automobile. More specifically, the present invention relates to exhaust gas with excess oxygen, that is, carbon monoxide (CO) contained in the exhaust gas. ), Hydrogen (H ₂ )
And an exhaust gas purifying catalyst capable of efficiently reducing and purifying nitrogen oxides (NO _x ) in exhaust gas containing oxygen in excess of the oxygen amount necessary for completely oxidizing reducing components such as hydrocarbons (HC). .

[0002]

2. Description of the Related Art Conventionally, as a catalyst for purifying exhaust gas of automobiles, a three-way catalyst has been used which purifies CO and HC in exhaust gas at the stoichiometric air-fuel ratio by simultaneously oxidizing and reducing NO _x . . As such a three-way catalyst, for example, a porous carrier layer made of γ-alumina is formed on a heat-resistant substrate made of cordierite or the like, and platinum (Pt), rhodium (Rh), or the like is formed on the porous carrier layer. What carried the catalyst noble metal is widely known. Further, a three-way catalyst using ceria (cerium oxide) having an oxygen storage ability and having enhanced low-temperature activity is also known.

On the other hand, in recent years, from the viewpoint of protecting the global environment, carbon dioxide (CO ₂ ) in exhaust gas discharged from internal combustion engines such as automobiles has become a problem, and as a solution to this problem, so-called lean combustion in which lean combustion is performed in an oxygen excess atmosphere is performed. Burn is promising. In this lean burn, the use of fuel is reduced to improve fuel efficiency, and the generation of CO ₂ , which is the combustion exhaust gas, can be suppressed.

On the other hand, in the conventional three-way catalyst, when the air-fuel ratio is the stoichiometric air-fuel ratio (stoichiometric), CO, H in the exhaust gas
It purifies by oxidizing and reducing C and NO _x at the same time, and does not show sufficient purification performance for reducing and removing NO _x in an oxygen excess atmosphere of exhaust gas during lean burn. Therefore, it has been desired to develop a catalyst and a purification system that can purify NO _x even in an oxygen excess atmosphere.

Therefore, the applicant of the present application has previously proposed an exhaust gas purifying catalyst in which an alkaline earth metal such as Ba and Pt are supported on a porous carrier such as alumina (for example, Japanese Patent Laid-Open No. 5-317625).
Issue gazette). By using this exhaust gas purifying catalyst to control the air-fuel ratio from the lean side to the stoichiometric or rich side in a pulsed manner, N
O _x is occluded in the alkaline earth metal (NO _x occluding element), it is to be cleaned reacts with the reducing components such as HC and CO in the stoichiometric or rich side, efficiently even NO _x in lean burn Can be purified.

[0006]

The NO _x purification reaction in the exhaust gas purifying catalyst is a first step of oxidizing NO in the exhaust gas to NO _x and a second step of storing NO _x on the catalyst. If, and the NO _x released from occluded NO _x or catalyst known to be composed of a third step of reduction on the catalyst.

However, in the conventional exhaust gas purifying catalyst, the reactivity of the second step and the third step is relatively high, but the reactivity of the first step is low, and the NO
_x Purification performance was limited. The present invention has been made in view of such circumstances, and an object thereof is to further improve the NO _x purification performance by increasing the reactivity in the first step.

[0008]

The exhaust gas-purifying catalyst according to claim 1 for solving the above-mentioned problems is characterized by an NO oxidation catalyst in which at least a noble metal is supported on a porous carrier, and an alkali metal on the porous carrier. It is configured by combining at least one NO _x storage element selected from alkaline earth metals and rare earth metals and a NO _x storage catalyst supporting a noble metal.

Further, the characteristics of the exhaust gas purifying catalyst according to claim 2 which is a more desirable mode for solving the above-mentioned problems,
A NO oxidation catalyst formed by carrying at least a noble metal on a porous support, an alkali metal on a porous support, comprising carrying at least one NO _x storage element and a noble metal selected from alkaline earth metals and rare earth metals NO _x It is configured by combining an occlusion catalyst and a NO _x reduction catalyst in which a precious metal is supported on a porous carrier.

In order to further improve the NO oxidation activity of the NO oxidation catalyst, the NO oxidation catalyst may be further loaded with an acidic element as described in claim 3, or the NO oxidation catalyst as described in claim 4. It is desirable that the porous carrier of (1) be composed of an acidic oxide. In order to further improve _the NO _x storage activity of the NO _x storage catalyst, it is desirable to configure the porous carrier of the NO _x storage catalyst as claimed in claim 5, basic oxides.

In the exhaust gas purifying catalyst according to claim 1, as a method for combining the NO oxidation catalyst and the NO _x storage catalyst, as described in claim 6, a method of arranging them in series in the exhaust gas passage in this order, Alternatively, as described in claim 7, the NO oxidation catalyst and the NO _x storage catalyst may be used as powder catalysts, and both powder catalysts may be mixed and used. In the exhaust gas purifying catalyst according to claim 2, as a method of combining the NO oxidation catalyst, the NO _x storage catalyst and the NO _x reduction catalyst, a method of arranging them in series in the exhaust gas flow passage in this order as described in claim 8. Or a method of using a NO oxidation catalyst, a NO _x storage catalyst and a NO _x reduction catalyst as powder catalysts and mixing three kinds of powder catalysts, or NO as described in claim 10. A catalyst formed by mixing powders of an oxidation catalyst and a NO _x storage catalyst;
A method of arranging a NO _x reduction catalyst in series in the exhaust gas flow channel in this order, and further comprising mixing an NO oxidation catalyst, and a powder of a NO _x storage catalyst and a NO _x reduction catalyst as described in claim 11. A method of arranging the catalyst in series in the exhaust gas passage in this order is desirable.

Furthermore, it may be combination of a plurality of the NO oxidation catalyst and _the NO _x storage catalyst as claimed in claim 12, the NO oxidation catalyst as claimed in claim 13 and _the NO _x storage catalyst and NO _x It is also possible to combine a plurality of reduction catalysts.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION

(1) NO Oxidation Catalyst The NO oxidation catalyst according to claims 1 and 2 is obtained by supporting a noble metal on a porous carrier such as alumina, silica, zirconia, silica-alumina, or zeolite. The noble metal is preferably selected from Pt, Pd and Rh.

The amount of the noble metal supported on the NO oxidation catalyst is preferably 0.1 to 20 g, and particularly preferably 0.5 to 10 g, per liter of the porous carrier. Even if the supported amount of the noble metal is further increased, the oxidation activity is not improved, and its effective use cannot be achieved. On the other hand, if the amount of the noble metal supported is less than this, sufficient activity for practical use cannot be obtained. For NO oxidation catalysts, it is desirable to acidify the porous support. This raises the oxidation state of the noble metal carried. Then, it is assumed that the generated NO _x is released quickly, and that the sintering of the noble metal is suppressed because the NO _x storage element is not supported, and the NO oxidation activity is improved.

In this way, the method for acidifying the porous carrier and
Then, there is a method of supporting an acidic element. Such an acid
As the element, V, Nb, Mo, W, Mn, Ti, Zr
And at least one selected from Si
You. In addition, as a method of acidifying the porous carrier itself,
There is a method of forming a porous carrier from an oxide. like this
ZrO as a suitable acidic oxide_Two, TiO_Two, SiO _Two,
CeO_TwoEtc. are exemplified, and these single oxides or
It is preferable to form the porous carrier from the composite oxide.

The NO oxidation catalyst may form pellets by itself, or a coat layer made of a porous carrier on the surface of a honeycomb-shaped or pellet-shaped heat-resistant substrate formed of cordierite or metal. It is also possible to form noble metal and support the noble metal or acidic element on the coat layer. (2) _the NO _x storage catalyst _the NO _x storage catalyst, alumina, silica, zirconia, silica - alumina, a porous carrier such as zeolite, at least one NO _x storage selected from alkali metals, alkaline earth metals and rare earth metals It carries an element and a noble metal. The noble metals include Pt, Pd and Rh.
It is preferable to select from

The amount of the noble metal supported on the NO _x storage catalyst is preferably 0.1 to 20 g, and particularly preferably 0.5 to 10 g, per liter of the porous carrier. Even if the supported amount of the noble metal is further increased, the NO _x storage activity is not improved, and its effective use cannot be achieved. On the other hand, if the amount of the noble metal supported is less than this, sufficient activity for practical use cannot be obtained. NO
_{x As} an alkali metal that is an occlusion element, lithium (L
i), sodium (Na), potassium (K), and cesium (Cs). Further, the alkaline earth metal means an element of Group 2A of the periodic table, and magnesium (Mg), calcium (Ca), strontium (Sr), barium (B
a). Examples of rare earth metals include lanthanum (La), cerium (Ce) and praseodymium (Pr).

The porous carrier of the NO _x storage catalyst is preferably composed of a basic oxide. By doing so, NO _x is easily attracted, and the NO _x storage activity is further improved. As such a basic oxide,
Examples thereof include MgO, La ₂ O ₃ , Al ₂ O ₃ and Y ₂ O _3, and it is preferable to form the porous carrier from a single oxide or a composite oxide of these.

This NO_xThe storage catalyst is itself pelletized.
May be formed, cordierite, metal, etc.
Made of honeycomb-shaped or pellet-shaped
Form a coat layer consisting of a porous carrier on the surface of the thermal substrate,
Noble metal and NO in the coat layer _xCarrying an occlusion element
It can also be. (3) NO_xReduction catalyst NO according to claim 2_xThe reduction catalyst is alumina, silica,
Porous zirconia, silica-alumina, zeolite, etc.
A noble metal is supported on a porous carrier. With this precious metal
It is preferable to select from Pt, Pd and Rh. Especially
N_TwoRh is preferable for improving the selectivity.

The amount of the noble metal supported on the NO _x reduction catalyst is preferably 0.1 to 20 g, and particularly preferably 0.5 to 10 g, per liter of the porous carrier. Even if the supported amount of the noble metal is further increased, the NO _x reducing activity is not improved, and its effective use cannot be achieved. On the other hand, if the amount of the noble metal supported is less than this, sufficient activity for practical use cannot be obtained. The NO _x reduction catalyst may form pellets by itself, or a coat layer made of a porous carrier may be formed on the surface of a honeycomb-shaped or pellet-shaped heat-resistant substrate formed of cordierite or metal. However, a noble metal may be supported on the coat layer. (4) As a combination of forming the exhaust gas purifying catalyst as set forth in the example combination NO oxidation catalyst and _the NO _x storage catalyst with NO oxidation catalyst and _the NO _x storage catalyst in claim 1, and mixing both catalyst as powder shape There is a method of coating it on the surface of a honeycomb-shaped or pellet-shaped heat-resistant substrate formed of cordierite or metal.

It is also possible to use both catalysts formed in a honeycomb shape or a pellet shape in series in the exhaust gas passage. In this case, the NO oxidation catalyst is arranged on the upstream side of the exhaust gas passage, and the NO _x storage catalyst is arranged on the downstream side. If the NO oxidation catalyst is A and the NO _x storage catalyst is B, this arrangement example is expressed as AB. Further, as described in claim 12, a plurality of catalysts can be combined in series like A-B-A-B -... (5) Example of combination of NO oxidation catalyst, NO _x storage catalyst and NO _x reduction catalyst As a combination of the NO oxidation catalyst, NO _x storage catalyst and NO _x reduction catalyst to form the exhaust gas purifying catalyst according to claim 2, There are the following combination methods.

First, there is a method in which three types of catalysts are mixed in the form of powder, and the mixture is used by coating the surface of a heat-resistant base material in the shape of a honeycomb or pellet formed of cordierite or metal. It is also possible to use the respective catalysts formed in a honeycomb shape or a pellet shape, arranged in series in the exhaust gas passage. When the NO oxidation catalyst is A, the NO _x storage catalyst is B, and the NO _x reduction catalyst is C, in this case it is expressed as ABC, and in this order from the upstream side to the downstream side of the exhaust gas passage. Deploy. Moreover, as described in Claim 13, ABCA-
May be combined in series a plurality of catalyst in this order as B-C- ···, A-B -A-B- NO oxidation catalyst A as · · · -C and the NO _x storage catalytic B Alternatively, the NO _x reduction catalyst C may be arranged at the end.

The NO oxidation catalyst and the NO _x storage catalyst are mixed in the form of powder, and the mixture is coated on the surface of a honeycomb-shaped or pellet-shaped heat-resistant base material made of cordierite, metal, etc. It is also possible to arrange it on the upstream side of the passage and arrange the NO _x reduction catalyst C on the downstream side. Further, the NO _x storage catalyst and the NO _x reduction catalyst are mixed in the form of powder, and the mixture is coated on the surface of a honeycomb-shaped or pellet-shaped heat-resistant base material formed of cordierite, metal, etc. The NO oxidation catalyst A on the upstream side.
May be arranged. (6) Action of the exhaust gas purifying catalyst according to claim 1 In the exhaust gas purifying catalyst according to claim 1, NO in the exhaust gas is oxidized by the NO oxidation catalyst in the lean atmosphere to generate NO.
_x . At this time, if the NO oxidation catalyst is on the acidic side, the oxidation state of the noble metal, which is the active center, becomes high, and the generated NO _x is rapidly released, so that the NO oxidation activity is improved.

Since the NO oxidation catalyst does not support the NO _x storage element, problems such as sintering of precious metals are prevented and the durability is excellent. Generated NO
_x is stored in the NO _x storage element of the NO _x storage catalyst.
Then, when the atmosphere becomes rich, it is subjected to the catalytic action of the adjacently supported precious metal, reacts with reducing components such as HC and CO in the exhaust gas, and is reduced and purified. When the generated NO _x is stored, if the porous carrier of the NO _x storage catalyst is basic, NO _x is more easily attracted and the NO _x storage activity is improved, so the NO _x purification rate is also improved.

As described above, the portion having high NO oxidation performance and NO
_xSince the parts with high storage performance are separated,
Noble metal and NO for carrier_xA conventional type that closely supports the storage element
Compared to exhaust gas purifying catalysts, NO oxidation ability is improved, and NO_x
The purification rate is also improved. (7) Action of the exhaust gas purifying catalyst according to claim 2 The exhaust gas purifying catalyst according to claim 2 is the exhaust gas purifying catalyst according to claim 1.
NO in addition to chemical catalyst_xThe reduction catalyst is placed separately
ing. Therefore, the exhaust gas purifying catalyst according to claim 1,
In addition to the action, NO in rich atmosphere_xReduction catalyst
NO in exhaust gas _xReduce and purify. Also NO_xStorage catalyst
NO released without being reduced by_xNO _xReturn with reduction catalyst
Can be original, NO_xThe purification rate is further improved.

That is, the portion having high NO oxidation performance and NO
_{Since the} part with high _x storage performance and the part with high NO _x reduction performance are separated, the NO oxidation ability and NO _x reduction are higher than those of conventional exhaust gas purification catalysts in which precious metals and NO _x storage elements are closely supported on an alumina carrier. The performance is improved and the NO _x purification rate is improved.

[0027]

EXAMPLES The present invention will be specifically described below with reference to Examples and Comparative Examples. (Preparation of basic catalyst) Alumina, titania, and silica were used as carriers, and each carrier was immersed in a predetermined amount of a dinitrodiammineplatinum aqueous solution having a predetermined concentration, and 5
Stir for hours. Evaporate it to dryness and let it dry in air at 300
The powder catalysts 1 to 3 shown in Table 1 were prepared by firing at 3 ° C. for 3 hours.

The powder catalyst 1 was immersed in a predetermined amount of a barium acetate aqueous solution having a predetermined concentration and stirred for 5 hours. This was evaporated to dryness and calcined in the air at 300 ° C. for 3 hours to prepare powder catalyst 4 shown in Table 1. Also, powder catalyst 3
Was immersed in a predetermined amount of an ammonium tungstate aqueous solution having a predetermined concentration and stirred for 5 hours. Evaporate it to dryness,
The powder catalyst 5 shown in Table 1 was prepared by firing in air at 300 ° C. for 3 hours.

Further, the powder catalyst 1 and the powder catalyst 4 were mixed in the same amount to prepare a mixed powder to prepare a powder catalyst 6. Each of these powder catalysts 1 to 6 was pelletized by a conventional method to prepare each pellet catalyst 1 to 6. Finally, the pellet catalysts 1 to 6 were treated in a hydrogen stream at 500 ° C. for 3 hours and used as the following exhaust gas purifying catalyst.

[0030]

[Table 1] (Example 1) As shown in FIG. 1, the above pellet catalyst 1
Was placed on the upstream side of the exhaust gas flow path, and the pellet catalyst 4 was placed on the downstream side to obtain the catalyst of this example.

Then, the rich model gas and the lean model gas shown in Table 2 are respectively introduced at the gas temperature levels of 300.
The transient NO _x purification rate was measured when the mixture was flown at 2 liters / min alternately every 2 minutes at three levels of ℃, 400 ℃ and 500 ℃. This is taken as the initial purification rate, and the results are shown in Table 4.

[0032]

[Table 2] Further, the durability model gas shown in Table 3 was switched between rich for 1 minute and lean for 4 minutes while the incoming gas temperature was 900 ° C.
An endurance test was conducted for a long time. Then, the transient NO _x purification rate was measured in the same manner as described above, and the results are shown in Table 4 as the purification rate after endurance.

[0033]

[Table 3] (Example 2) The pellet catalyst 2 was arranged on the upstream side of the exhaust gas flow path, and the pellet catalyst 4 was arranged on the downstream side to obtain the catalyst of this example. Then, the transient NO _x purification rate was measured in the same manner as in Example 1, and the results are shown in Table 4.

(Example 3) The pellet catalyst 3 was placed on the upstream side of the exhaust gas flow path, and the pellet catalyst 4 was placed on the downstream side to obtain the catalyst of this example. Then, the transient NO _x purification rate was measured in the same manner as in Example 1, and the results are shown in Table 4. (Example 4) The pellet catalyst 5 was arranged on the upstream side of the exhaust gas flow path, and the pellet catalyst 4 was arranged on the downstream side to obtain the catalyst of this example. Then, the transient NO _x purification rate was measured in the same manner as in Example 1, and the results are shown in Table 4.

(Embodiment 5) The pellet catalyst 6 was placed in the exhaust gas passage to obtain the catalyst of this embodiment. Then, the transient NO _x purification rate was measured in the same manner as in Example 1, and the results are shown in Table 4. (Example 6) As shown in FIG. 2, a pellet catalyst 1, a pellet catalyst 4, and a pellet catalyst 1 were arranged in this order from the upstream side to the downstream side to obtain a catalyst of this example. Then, the transient NO _x purification rate was measured in the same manner as in Example 1, and the results are shown in Table 4.

(Comparative Example 1) The pellet catalyst 4 was placed in the exhaust gas passage to obtain the catalyst of Comparative Example 1. Then, the transient NO _x purification rate was measured in the same manner as in Example 1, and the results are shown in Table 4. (Comparative Example 2) The pellet catalyst 4 was placed on the upstream side of the exhaust gas flow path, and the pellet catalyst 1 was placed on the downstream side to obtain the catalyst of Comparative Example 2. Then, the transient NO _x purification rate was measured in the same manner as in Example 1, and the results are shown in Table 4.

Comparative Example 3 Three pellet catalysts 1 were arranged from the upstream side to the downstream side to obtain a catalyst of Comparative Example 3.
Then, the transient NO _x purification rate was measured in the same manner as in Example 1, and the results are shown in Table 4.

[0038]

[Table 4] (Evaluation) In the catalysts of Examples 1 to 4, the NO _x purification activity was improved both in the initial stage and after the durability as compared with Comparative Example 1 and Comparative Example 2. This was because the NO oxidation catalyst was arranged on the upstream side and It is clear that this is the effect of arranging the pellet catalyst 4, which is the NO _x storage catalyst, on the side. The catalyst of Example 5 also has a higher NO _x purification activity than Comparative Examples 1 and 2.
That is, even when the powder catalyst 1 which is the NO oxidation catalyst and the powder catalyst 4 which is the NO _x storage catalyst are mixed and used, the same performance as that of the catalyst of Example 1 is exhibited.

Further, the catalyst of Example 6 has an improved NO _x purification activity both at the initial stage and after the durability as compared with Comparative Example 3, and also has an improved NO _x purification activity as compared with Example 1. This is, from the upstream side to the downstream side, a pellet catalyst 1 that is an NO oxidation catalyst, a pellet catalyst 4 that is a NO _x storage catalyst, and a pellet catalyst 1 that is also a NO _x reduction catalyst.
It is clear that this is the effect of arranging in this order.

Further, the catalysts of Examples 2 to 4 exhibited a higher NO _x purification rate than that of Example 1, which is the carrier of the pellet catalyst 2 and the pellet catalyst 3 (silica, titania).
It is considered that the acidity is higher than that of the carrier (alumina) of the catalyst 1, and the pellet catalyst 5 carries W which is an acidic element. That is, the high initial activity of the catalyst of the example is considered to be due to the high NO oxidation ability of the upstream catalyst. Further, the high activity of the catalyst of the example after endurance is due to the high NO oxidation ability of the upstream catalyst and the fact that the upstream catalyst does not contain Ba which is a basic substance. It is considered that the deterioration of performance was suppressed because sintering did not occur.

[0041]

In other words, the exhaust gas purifying catalyst of the present invention is excellent in the reaction activity of oxidizing NO to NO _x, and therefore exhibits a higher NO _x purification activity than in the past. It also has excellent durability because sintering of precious metals is unlikely to occur.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a configuration of a catalyst according to one embodiment of the present invention.

FIG. 2 is a structural explanatory view of a catalyst of a sixth embodiment of the present invention.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location F01N 3/28 301 B01D 53/36 102H B01J 23/64 103A

Claims (13)

[Claims] 1. A NO oxidation catalyst comprising a porous carrier carrying at least a noble metal, and a porous carrier carrying at least one NO _x storage element and a noble metal selected from alkali metals, alkaline earth metals and rare earth metals. NO _x
An exhaust gas-purifying catalyst, which is configured by combining with an occlusion catalyst. 2. A NO oxidation catalyst comprising at least a noble metal supported on a porous carrier, and at least one NO _x storage element selected from an alkali metal, an alkaline earth metal and a rare earth metal and a noble metal supported on the porous carrier. NO _x
NO _x containing a storage catalyst and a noble metal supported on a porous carrier
An exhaust gas purifying catalyst, which is configured by combining a reduction catalyst. 3. The exhaust gas purifying catalyst according to claim 1, wherein the NO oxidation catalyst further carries an acidic element. 4. The exhaust gas purifying catalyst according to claim 1, wherein the porous carrier of the NO oxidation catalyst is made of an acidic oxide. 5. The exhaust gas-purifying catalyst according to claim 1 or 2, wherein the porous carrier of the NO _x storage catalyst is made of a basic oxide. 6. The exhaust gas purifying catalyst according to claim 1, wherein the NO oxidation catalyst and the NO _x storage catalyst are arranged in series in the exhaust gas passage in this order. 7. The exhaust gas-purifying catalyst according to claim 1, wherein the NO oxidation catalyst and the NO _x storage catalyst are mixed with each other. 8. The exhaust gas purifying catalyst according to claim 2, wherein the NO oxidation catalyst, the NO _x storage catalyst, and the NO _x reduction catalyst are arranged in series in the exhaust gas passage in this order. 9. The exhaust gas purifying catalyst according to claim 2, wherein the NO oxidation catalyst, the NO _x storage catalyst, and the NO _x reduction catalyst are mixed with each other. 10. A catalyst obtained by mixing powders of the NO oxidation catalyst and the NO _x storage catalyst, and the NO _x reduction catalyst are arranged in series in the exhaust gas passage in this order. The exhaust gas-purifying catalyst according to claim 2. 11. The NO oxidation catalyst and a catalyst obtained by mixing powders of the NO _x storage catalyst and the NO _x reduction catalyst are arranged in series in the exhaust gas passage in this order. The exhaust gas-purifying catalyst according to claim 2. 12. The exhaust gas purifying catalyst according to claim 1, which is configured by combining a plurality of the NO oxidation catalyst and the NO _x storage catalyst. 13. The exhaust gas-purifying catalyst according to claim 2, which is configured by combining a plurality of the NO oxidation catalyst, the NO _x storage catalyst, and the NO _x reduction catalyst.