JP2006110402A - Composite material for easily releasing carbon dioxide and catalyst for cleaning exhaust gas - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a composite material for easily releasing carbon dioxide which has remarkably improved CO shift performance by promoting the release of CO<SB>2</SB>, and to provide a hydrogen enrichment catalyst, an exhaust gas cleaning catalyst, an exhaust gas cleaning catalyst system and an exhaust gas cleaning system in each of which the composite material is used. <P>SOLUTION: The composite material for easily releasing carbon dioxide has such a structure that a phase of CeO<SB>2</SB>is in contact with a phase of an M-Zr compound oxide (wherein M is lanthanide) and easily releases the CO<SB>2</SB>bonded to the phase of CeO<SB>2</SB>. The hydrogen enrichment catalyst is obtained by depositing Pt on this composite material. The exhaust gas cleaning catalyst contains the hydrogen enrichment catalyst. The exhaust gas cleaning catalyst system is composed of the exhaust gas cleaning catalyst and an NOx removal catalyst. The exhaust gas cleaning system is provided with the exhaust gas cleaning catalyst, so that the atmosphere of exhaust gas is varied between a hydrogen-lean atmosphere and a hydrogen-rich atmosphere and the exhaust gas having the varied atmosphere is supplied to the exhaust gas cleaning catalyst. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a carbon dioxide easily desorbing complex, a hydrogen-enriched catalyst, an exhaust gas purifying catalyst, an exhaust gas purifying catalyst system, and an exhaust gas purifying system, and more particularly, promoting carbon dioxide desorption. The present invention relates to an easily desorbable carbon dioxide complex, a hydrogen-enriched catalyst using the same, an exhaust gas purification catalyst, an exhaust gas purification catalyst system, and an exhaust gas purification system.

  In order to comply with exhaust gas regulations that will become stricter in the future, it is necessary to purify exhaust gas in a wider temperature range than before. In particular, considering that the exhaust gas temperature further decreases as the combustion efficiency of the internal combustion engine by lean combustion improves, it is necessary to purify exhaust gas, particularly nitrogen oxide (NOx), even in a lower temperature range.

The NOx purification using the NOx adsorption type catalyst currently used is very effective mainly in a relatively high temperature range of 300 ° C. or higher, but the NOx purification rate is remarkably lowered in a low temperature range below that.
This is because the trapped NOx is difficult to desorb at low temperatures. As a result of examining this point in detail, for example, in an exhaust gas purification system of an automobile, the exhaust gas atmosphere may be temporarily rich for a short time (hereinafter referred to as “rich spike”). Of the generated hydrocarbon (HC), carbon monoxide (CO), and hydrogen (H ₂ ), CO was found to suppress NOx desorption.

Therefore, when CO that suppresses NOx desorption is selectively removed and only H ₂ is used as a reducing agent, NOx desorption is remarkably promoted and NOx purification performance is dramatically improved. I understood. On the other hand, a system has been proposed in which a catalyst that selectively removes CO out of CO and H ₂ generated during rich spikes is arranged on the upstream side of the NOx adsorption catalyst (see, for example, Patent Document 1).

  Examples of the reaction for selectively removing CO include (i) a CO selective oxidation reaction and (ii) a CO shift reaction, and a catalyst in which a noble metal is supported on cerium oxide is effective in promoting such a reaction. (For example, refer to Patent Documents 2 and 3).

On the other hand, as a method for improving the catalyst for promoting the CO shift reaction, for example, adding sulfur to Pt / TiO ₂ has been proposed in order to weaken the adsorption power of Pt and CO for the purpose of releasing CO poisoning. (For example, refer to Patent Document 4).
JP 2001-234737 A JP 2003-024749 A JP 2003-013728 A JP 2002-224570 A

  However, with the CO selective oxidation reaction alone, it is difficult to increase the ratio of hydrogen in the reducing agent because the CO selectivity is not high. Therefore, it is necessary to greatly improve the performance of the CO shift reaction. Further, the conventional catalysts and systems described above have room for improvement because the CO shift performance gradually decreases due to catalyst poisoning.

On the other hand, the present inventors have hydrogen-rich catalyst mainly CeO ₂ practical 300 ° C. vicinity of the temperature, in a rich atmosphere, desorption of CO ₂ combined with CeO ₂ is rate-limiting in the CO shift reaction I got the technical knowledge.

The present invention has been made in view of the above-described problems of the prior art and further technical knowledge. The object of the present invention is to promote the desorption of CO ₂ and remarkably improve the CO shift performance. An object of the present invention is to provide a carbon dioxide easy-releasing composite, a hydrogen-enriched catalyst, an exhaust gas purification catalyst, an exhaust gas purification catalyst system, and an exhaust gas purification system using the same.

As a result of intensive studies to achieve the above object, the present inventor is able to achieve the above object by bringing a CeO ₂ phase and an M-Zr composite oxide phase (M: lanthanoid element) into contact with each other. The headline and the present invention were completed.

That is, the carbon dioxide readily desorbing complex of the present invention is formed by contacting a CeO ₂ phase and an M-Zr complex oxide phase (M: lanthanoid element), and desorbing CO ₂ bound to the CeO ₂ phase. It is easy.
The hydrogen-enriched catalyst of the present invention is obtained by supporting Pt on the carbon dioxide easy-releasing complex of the present invention.
Furthermore, the exhaust gas purifying catalyst of the present invention comprises the hydrogen-enriched catalyst of the present invention.

The exhaust gas purification catalyst system of the present invention includes the exhaust gas purification catalyst and the NOx purification catalyst of the present invention.
Furthermore, the exhaust gas purification system of the present invention is an exhaust gas purification system comprising the exhaust gas purification catalyst of the present invention, wherein the exhaust gas atmosphere is changed over a lean or rich atmosphere, and the exhaust gas purification system is changed. A system for supplying the exhaust gas to the catalyst for use.

According to the present invention, carbon dioxide that can promote the desorption of CO ₂ and remarkably improve the CO shift performance by bringing the CeO ₂ phase into contact with the M-Zr composite oxide phase (M: lanthanoid element). An easily desorbable composite, a hydrogen-enriched catalyst, an exhaust gas purification catalyst, an exhaust gas purification catalyst system, and an exhaust gas purification system using the same can be provided.

Hereinafter, the carbon dioxide easily detachable complex of the present invention will be described.
As described above, the carbon dioxide easily detachable complex of the present invention is formed by contacting a CeO ₂ phase and an M-Zr composite oxide phase (M: lanthanoid element), and CO ₂ bonded to the CeO ₂ phase. It is easy to detach.
Here, the reaction mechanism in the hydrogen enriched catalyst estimated at present will be described with reference to the drawings.
FIG. 1 is a schematic explanatory view showing a reaction mechanism in a hydrogen-enriched catalyst under various conditions. As shown in the figure, the hydrogen-enriched catalyst has, for example, Pt which is a catalyst noble metal on ceria.
First, (a) in the figure shows a case where a normal reaction cycle is rotating in a rich atmosphere. As shown in (a) (1), CO generated during rich spike is Pt and It is known that the affinity is high and adsorbed. As shown in FIGS. 6A and 6B, CO reacts with ceria in the vicinity of Pt and pulls out oxygen atoms (O) at this time, so that, as shown in FIGS. desorbed as CO _2. As shown in FIGS. 4A and 4C, the ceria partially lacking oxygen atoms (O) adsorbs the water (H ₂ O) generated during the rich spike and pulls out the oxygen atoms (O). the _{H 2} O which is as shown in FIG. (a) (5), eliminated as _{H 2,} the concentration of _{H 2} in the exhaust gas becomes high. It should be noted that the reaction cycle is returned from FIGS. 5A to 5A to 5A.

On the other hand, FIG. 7B shows a case where the normal cycle does not rotate in the rich atmosphere after the rich spike is repeated. As shown in FIG. CO generated during the spike is adsorbed by Pt. As shown in FIGS. 2B and 2B, CO ₂ hardly diffuses into the gas phase, forms a relatively stable carbonate on ceria, and subsequent reaction does not proceed easily.

  Incidentally, FIG. 4C shows the case in a lean atmosphere, but as shown in FIG. 2C, ceria does not participate in the reaction in the lean atmosphere, and FIG. ) To (3), the oxidation reaction proceeds on Pt. It should be noted that the reaction cycle is returned from (c) (3) to (c) (1).

The carbon dioxide easily detachable complex of the present invention is obtained by bringing the CeO ₂ phase and the M-Zr complex oxide phase into contact so as not to form a relatively stable carbonate on ceria in FIG. for example, a CeO ₂ phase and M-Zr composite oxide phase is contacted with the surface to near the surface of the secondary particles from the viewpoint of preferable contact area is increased, the gas of CO ₂ before the CeO ₂ and CO ₂ are bonded By releasing into the phase, CO ₂ desorption is promoted, and the performance of the oxidation reaction and the CO shift reaction can be improved.

In addition, the carbon dioxide easily detachable composite of the present invention has a core-shell structure in which one of the CeO ₂ phase and the M-Zr composite oxide phase forms a core portion and the other forms a shell portion. Is preferable from the viewpoint of further improving the performance of the CO shift reaction.

Furthermore, it is preferable that the content of the M-Zr composite oxide phase of the carbon dioxide easy-release complex of the present invention increases as the complex moves from the inside to the outside. When such a core-shell structure is formed, the elimination of CO ₂ can be promoted, and the performance of the CO shift reaction can be further improved. As the preferred example in which the contact surface or near the surface of the secondary particles as described above, include those thin shell of M-Zr composite oxide phase are formed a CeO ₂ phase as the core portion It is done.

  Further, in the carbon dioxide readily detachable complex of the present invention, as the lanthanoid element constituting the M-Zr composite oxide phase, the M-Zr composite oxide phase is likely to release oxygen as compared with the ceria phase. Preferably, Ce, Pr, Nd or Sm and any of these elements can be used in combination.

Furthermore, in the carbon dioxide easily detachable composite of the present invention, the M-Zr composite oxide phase has the following general formula (1):
M _x Zr _1-x O ₂ (1)
(M in the formula represents a lanthanoid element, and x represents a number satisfying 0 <x <1). Further, the M-Zr composite oxide is not a mixed oxide of M oxide and Zr oxide but exists as M-Zr composite oxide, and x is set to 0.16 to 0.75. Thus, the oxygen releasing ability is increased, the accumulation of CO ₂ is suppressed, the desorption of CO ₂ is promoted, and the CO shift performance can be further improved.

Next, the hydrogen-enriched catalyst of the present invention will be described.
As described above, the hydrogen-enriched catalyst of the present invention is composed by supporting Pt on the carbon dioxide easily leaving complexes of the present invention, by transmitting and generate H _2, to improve the H 2 _/ CO ratio Is.
Thus, by bringing Pt and the carbon dioxide easily detachable complex close to each other, CO ₂ is easily desorbed as compared with CeO ₂ , and as a result, the CO shift reaction is further promoted.

Next, the exhaust gas purifying catalyst of the present invention will be described.
As described above, the exhaust gas purifying catalyst of the present invention contains the hydrogen-enriched catalyst of the present invention. In addition, as long as the effect of the present invention is not impaired, it may be supported on a high specific surface area substrate such as alumina, and further, for example, a noble metal such as Pd or Rh, or a zirconium-containing alumina that is included when supporting Rh, for example. Other cocatalysts may be added. Thereby, NOx poisoning is suppressed, CO ₂ is easily desorbed, and the exhaust gas purifying catalyst is more excellent in CO shift reaction performance.

Next, the exhaust gas purification catalyst system of the present invention will be described.
As described above, the exhaust gas purification catalyst system of the present invention includes the exhaust gas purification catalyst of the present invention and a NOx purification catalyst. With respect to the exhaust gas purification catalyst and the NOx purification catalyst, it is desirable to dispose the exhaust gas purification catalyst on the upstream side and the NOx purification catalyst on the downstream side in the exhaust gas flow direction in the exhaust gas flow path. Thereby, when the rich spike is inserted, the exhaust gas purification catalyst on the upstream side can increase the H ₂ / CO ratio in the exhaust gas, and as a result, the NOx conversion rate can be improved.

Next, the exhaust gas purification system of the present invention will be described.
As described above, the exhaust gas purification system of the present invention includes the exhaust gas purification catalyst of the present invention, changes the atmosphere of the exhaust gas over a lean or rich atmosphere, and supplies the exhaust gas purification catalyst to the exhaust gas. Supply.
By appropriately changing the exhaust gas atmosphere over a lean or rich atmosphere, that is, by appropriately controlling the rich spike according to the operating conditions, the CO ₂ desorption can be made easier, The performance of the CO shift reaction can be improved. Thereby, fuel consumption can also be improved.
The exhaust gas purification system of the present invention includes a case where the exhaust gas purification catalyst provided is a hydrogen enriched catalyst.

  The present invention will be described in more detail with reference to examples and comparative examples, but the present invention is not limited to these examples.

Example 1
[Preparation of carbon dioxide detachable complex]
Ion exchange water was added to cerium nitrate hexahydrate until the oxide concentration reached 2% to prepare a metal salt solution. The solution was neutralized with 25% aqueous ammonia (5 times the amount of oxide), and the precipitate was filtered and washed. Pure water is added to the resulting precipitate until an oxide concentration of 5% is obtained, and a slurry is prepared. A cerium nitrate solution and a zirconium nitrate mixture in which the weight ratio of CeO ₂ / ZrO ₂ is 60/40 previously dissolved is mixed. A liquid (200% with respect to the precipitate) was added, and after stirring, neutralized with 25% aqueous ammonia, and the precipitate was filtered and washed. The obtained precipitate was dried at 150 ° C. for 24 hours and calcined at 400 ° C. for 5 hours to obtain Ce _0.16 Zr _0.84 O ₂ / CeO ₂ .

[Production of exhaust gas purification catalyst]
The obtained Ce _0.16 Zr _0.84 O ₂ / CeO ₂ powder is put into a tetraammine Pt hydrochloride aqueous solution. After stirring for 1 hour, drying was performed at 150 ° C. for a whole day and night. Then, 2% Pt / Ce _0.16 Zr _0.84 O ₂ / CeO ₂ powder was obtained by firing in air at 400 ° C. for 2 hours.

In a magnetic ball mill, 2% Pt / Ce _0.16 Zr _0.84 O ₂ / CeO ₂ powder and 9.9% with respect to the total coating amount so as to be 79.9% with respect to the total coating amount. Alumina sol was mixed so that 10.5% γ-alumina was mixed with respect to the total coating amount, 10% nitric acid solution was added, and pulverized for 30 minutes to obtain a slurry.
This slurry is coated on the exhaust gas contact surface of a cordierite monolith support (catalyst capacity: 1.2 L, number of cells: 900 cells / 2.0 mils), excess slurry is removed by an air stream, and after drying, Firing was performed at 400 ° C. for 30 minutes. The exhaust gas purifying catalyst of this example was obtained by coating 477 g / L per unit volume. This was designated as an exhaust gas purifying catalyst 1. The exhaust gas purifying catalyst 1 contains 380 g / L as Pt / Ce _0.16 Zr _0.84 O ₂ / CeO _{2 and} 7.6 g / L as metal Pt.

(Example 2)
[Preparation of carbon dioxide detachable complex]
Separating cerium nitrate hexahydrate and zirconium nitrate dihydrate to a CeO ₂ / ZrO ₂ weight ratio of 60/40, adding ion-exchanged water to an oxide concentration of 2%, and adding a metal salt Was dissolved to obtain a mixed solution. The mixture was neutralized with 25% aqueous ammonia (5 times the amount of oxide), and the precipitate (60C40Z) was filtered and washed. Pure water was added to the resulting precipitate until the oxide concentration reached 5%, and a slurry was prepared. A pre-dissolved cerium nitrate solution (50% for 60C40Z oxide) was added, and after stirring, The mixture was neutralized with 25% aqueous ammonia, and the precipitate was filtered and washed. The obtained precipitate was dried at 150 ° C. for 24 hours and calcined at 400 ° C. for 5 hours to obtain CeO ₂ / Ce _0.75 Zr _0.25 O ₂ .

[Preparation of exhaust gas purification catalyst]
Example 1 except that the obtained CeO ₂ / Ce _0.75 Zr _0.25 O ₂ powder was used instead of the Ce _0.16 Zr _0.84 O ₂ / CeO ₂ powder used in Example 1. The same operation was repeated to obtain an exhaust gas purifying catalyst of this example. This was designated as an exhaust gas purification catalyst 2. The exhaust gas purifying catalyst 2 contains 380 g / L as Pt / CeO ₂ / Ce _0.75 Zr _0.25 O _{2 and} 7.6 g / L as metal Pt.

(Example 3)
[Production of exhaust gas purification catalyst system]
The exhaust gas purifying catalyst 1 produced in Example 1 is arranged on the upstream side of the exhaust gas flow path, and the NOx purification catalyst shown below is arranged on the downstream side to obtain the exhaust gas purifying catalyst system of this example. It was.
Activated alumina was impregnated with dinitrodiammine Pt solution, dried, and then fired in air at 400 ° C. for 1 hour to obtain Pt-supported alumina powder (powder A). The Pt concentration of this powder was 3.0%. Next, an activated alumina powder was impregnated with an aqueous Rh nitrate solution, dried and then fired in air at 400 ° C. for 1 hour to obtain an Rh-supported alumina powder (B powder). The Rh concentration of this powder was 2.0%.
576 g of powder A, 86 g of powder B, 238 g of activated alumina powder, and 900 g of water were placed in a magnetic ball mill and mixed and ground to obtain a slurry. This slurry was attached to a cordierite monolith support (catalyst capacity: 1.7 L, number of cells: 400 cells), excess slurry in the cells was removed by air flow, and the slurry was dried at 130 ° C. The catalyst was fired for a period of time to obtain a coat layer 200 g / L catalyst.
This was impregnated with an aqueous Ba solution, dried, and calcined in air at 400 ° C. for 1 hour to obtain a NOx purification catalyst having a coat layer of 250 g / L.

(Comparative Example 1)
The same operation as in Example 1 was repeated except that CeO ₂ (manufactured by Daiichi Rare Element Co., Ltd.) powder was used instead of Ce _0.16 Zr _0.84 O ₂ / CeO ₂ powder used in Example 1. Thus, an exhaust gas purifying catalyst of this example was obtained. This catalyst contains 380 g / L as Pt / CeO _{2 and} 7.6 g / L as metal Pt. Table 1 shows the specifications of the exhaust gas purifying catalyst in each example.

Here, FIG. 2 shows X of the Ce _0.16 Zr _0.84 O ₂ / CeO ₂ powder used in Example 1 and the CeO ₂ / Ce _0.75 Zr _0.25 O ₂ powder used in Example 2. It is a graph which shows the result by a line diffraction analysis (XRD). From this graph, it can be seen that each crystal structure exists.
3 shows (a) Ce _0.16 Zr _0.84 O ₂ / CeO ₂ powder used in Example 1 and (b) CeO ₂ / Ce _0.75 Zr _0.25 used in Example 2. O ₂ powder scanning electron microscope to observe the surface state of the particles (SEM) photographs.
Further, FIG. 4 is an SEM photograph in which the internal state of the particles was observed by slicing the particles of Ce _0.16 Zr _0.84 O ₂ / CeO ₂ powder used in Example 1 and observing the cross section.
From the results of XRD and SEM photographs, it is considered that the CeO ₂ phase and the M-Zr composite oxide phase in the carbon dioxide easy-release complex are in contact at the particle level.

FIG. 5 is a graph showing the weight ratio of Ce / Zr by energy dispersive X-ray fluorescence analysis (EDX) at measurement positions 1 to 5 indicated by arrows in FIG. As the weight ratio of Ce is higher among the CeO ₂ and _Ce 0.16 _{Zr 0.84} O _2, indicating that the content of CeO ₂ is large. According to this result, it is considered that there are many CeO ₂ components inside and there are many Ce _0.16 Zr _0.84 O ₂ components as they move to the surface.
6 to 10 are graphs showing weight ratio data and atomic ratio data at measurement positions 1 to 5 indicated by arrows in FIG. 4, respectively.

[Performance evaluation]
The exhaust gas purifying catalyst of each example was subjected to rapid durability under the following conditions, and then the model gas was evaluated to measure the oxidation amount and the CO shift rate. The obtained results are also shown in Table 1.

(Endurance conditions)
-Engine displacement 3000cc
・ Fuel Gasoline (Product name: Nisseki Dash)
・ Durable temperature 750 ℃
・ Durability 30 hours

(Model gas evaluation conditions)
-Reaction gas composition (rich gas 1) CO: 1 vol%
(Rich gas _{2) CO: 1vol%, H} 2: 1vol%, H 2 O: 3vol%
(Lean _{gas) CO: 0.1vol%, H 2} O: 3vol%
_{NOx: 0.1vol%, O 2:} 3vol%
・ Reaction temperature 300 ℃
・ Space velocity 72000h ^-1

Here, the oxidation amount will be described. FIG. 11 is an explanatory diagram showing the definition of the oxidation amount. The amount of oxidation refers to the amount of CO ₂ produced when the above (rich gas 1) is flowed. In this case, since there are no oxygen atoms in the gas phase, the reaction amount of oxygen atoms in the oxygen storage material (OSC). It corresponds to.
The CO shift rate means the CO conversion rate when the above (rich gas 2) is made to flow and becomes almost constant. That is, the following equation (2)
CO shift rate (%) = ((inlet CO concentration) − (outlet CO concentration)) / (inlet CO concentration) × 100 (2).

FIG. 12 is an explanatory diagram showing the definition of the CO shift rate.
The reason why this CO conversion rate may be regarded as being due to the CO shift reaction is explained based on the drawings. In FIG. 11, no CO conversion occurs in the vicinity of 20 to 30 seconds, whereas in FIG. The conversion of CO occurs due to the presence of ₂ O. This can be explained as a CO shift reaction (CO + H ₂ O → CO ₂ + H ₂ ) progressing. The curve indicated by the dotted line in FIG. 12 corresponds to the oxidized portion in FIG.

From Table 1, the exhaust gas purifying catalysts of Examples 1 and 2 belonging to the scope of the present invention have a larger oxidation amount and the CO shift rate than the exhaust gas purifying catalyst of Comparative Example 1 outside the present invention. It turns out that it is high and excellent.
In addition, it can be seen that CO consumption is greater than H ₂ consumption, that is, H ₂ transmission characteristics are large and excellent. That is, it can be seen that the exhaust gas purifying catalyst within the scope of the present invention has a higher H ₂ permeability than the exhaust gas purifying catalyst of Comparative Example 1 outside the present invention.

It is typical explanatory drawing which shows the reaction mechanism in the hydrogen enrichment catalyst estimated at this time. It is a graph which shows the result by XRD of the powder used for Example 1 or 2. 2 is an SEM photograph observing the surface state of powder particles used in Example 1 or 2. 2 is an SEM photograph observing an internal state of powder particles used in Example 1. FIG. It is the weight ratio of Ce / Zr in each measurement position of FIG. It is a graph which shows the result by EDX in the measurement position 1 of FIG. It is a graph which shows the result by EDX in the measurement position 2 of FIG. It is a graph which shows the result by EDX in the measurement position 3 of FIG. It is a graph which shows the result by EDX in the measurement position 4 of FIG. It is a graph which shows the result by EDX in the measurement position 5 of FIG. It is explanatory drawing which shows the definition of oxidation amount. It is explanatory drawing which shows the definition of CO shift rate.

Claims (10)

The CeO ₂ phase and the M-Zr composite oxide phase (M: lanthanoid element) are in contact with each other,
A carbon dioxide readily desorbing complex characterized in that CO ₂ bound to the CeO ₂ phase is easily desorbed. _2. The carbon dioxide easily detachable composite according to claim 1, wherein one of the CeO ₂ phase and the M-Zr composite oxide phase has a core-shell structure in which a core part is formed and the other forms a shell part. body.   It is a carbon dioxide easily detachable composite according to claim 1 or 2, wherein the content of the M-Zr composite oxide phase increases as it moves from the inside to the outside of the composite. Carbon dioxide readily detachable complex.   4. The carbon dioxide easy desorption according to claim 1, wherein the lanthanoid element is at least one element selected from the group consisting of Ce, Pr, Nd, and Sm. Sex complex. The M-Zr composite oxide has the following general formula (1)
M _x Zr _1-x O ₂ (1)
(Wherein M represents a lanthanoid element and x represents a number satisfying 0 <x <1). The dioxide according to any one of claims 1 to 4, Carbon detachable complex.   X in said Formula (1) satisfies 0.16 <= x <= 0.75, The carbon dioxide easy desorption complex of Claim 5 characterized by the above-mentioned.   A hydrogen-enriched catalyst comprising Pt supported on the carbon dioxide easy-releasing complex according to any one of claims 1 to 6.   An exhaust gas purifying catalyst comprising the hydrogen-enriched catalyst according to claim 7.   An exhaust gas purification catalyst system comprising the exhaust gas purification catalyst according to claim 8 and a NOx purification catalyst. An exhaust gas purification system comprising the exhaust gas purification catalyst according to claim 8, wherein the exhaust gas atmosphere is changed over a lean or rich atmosphere, and the exhaust gas is supplied to the exhaust gas purification catalyst. Exhaust gas purification system.

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