JP2003245551A - ABSORPTION AND REDUCTION CATALYST FOR REMOVING NOx - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an absorption and reduction catalyst for removing an NO<SB>x</SB>whose the sintering of the catalyst component is prevented and which is stable with time and exhibits a performance for cleaning an NO<SB>x</SB>at a wide temperature range. <P>SOLUTION: The absorption and reduction catalyst for removing NO<SB>x</SB>comprises a first catalyst having a Pt, Pd and NO<SB>x</SB>absorption material supported on an oxide support and a second catalyst having a Pt, Pd and NO<SB>x</SB>absorption material supported on an oxide support. Preferably, the molar ratio of Pd/(Pt+Pd) in the first catalyst is 0.02-0.2 and the NO<SB>x</SB>absorption material comprises at least one metal selected from Li, K and Ba. The first catalyst is preferably prepared by calcining a solution having a Pt compound and a Pd compound dissolved after impregnating into the oxide support followed by supporting the NO<SB>x</SB>absorption material. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a storage-reduction type NO _x purifying catalyst which has a high durability and in which sintering of catalyst components in a high temperature atmosphere is significantly suppressed.

[0002]

2. Description of the Related Art In order to protect the global environment, it is a world-wide practice to reduce the amount of carbon dioxide (CO ₂ ) emitted from internal combustion engines such as automobile engines, and the amount of nitrogen oxides (NO _x ). Has become a problem. To solve these problems, a lean burn engine with high fuel efficiency and a storage reduction type NO _x purification catalyst as a catalyst for purifying exhaust gas thereof have been developed.

This storage reduction type NO _x purification catalyst generally contains a catalyst component such as platinum (Pt) or palladium (Pd), and an alkali metal or alkaline earth metal such as γ-alumina as a NO _x storage material. It is configured to be carried on a carrier. N
O _x is occluded by an alkali metal or alkaline earth metal under lean conditions, to release the occluded NO _x in temporary stoichiometric-rich conditions, the catalytic activity and the temporary reducing atmosphere, such as platinum , The released NO
_Reduce and purify _x .

However, in order for such a catalyst to efficiently promote the purification reaction of exhaust gas, the catalyst component is supported on the carrier in a highly dispersed manner so that the catalyst component comes into contact with the exhaust gas in a large area. And, this highly dispersed loading state is
It needs to be maintained over time in an exhaust gas atmosphere.

Here, the exhaust gas of an automobile engine is
A temperature change is repeated between room temperature and high at about 1000 ° C., and a reducing atmosphere having a relatively high concentration of HC and CO and a low O ₂ concentration and an oxygen having a relatively low concentration of HC and CO
₂ Repeat the oxidizing atmosphere with high concentration.

[0006]

However, when a catalyst component of a noble metal such as platinum is exposed to such an atmosphere for a long period of time, the catalyst component migrates and enlarges particles, especially in a high temperature oxidizing atmosphere. It has the property of forming, so-called sintering. For this reason, the catalyst component cannot maintain a high contact area with the exhaust gas, and there is a problem that the exhaust gas purification performance deteriorates with time.

By the way, the applicant of the present invention has disclosed in Japanese Patent Laid-Open No. 11-3
In Japanese Patent Laid-Open No. 00203, Japanese Unexamined Patent Publication No. 7-251073, etc., an exhaust gas purifying catalyst containing Pt and Pd and suppressing sintering is proposed. The present invention, by catalytic structure which is different from such prior art, sintering of the catalyst component is suppressed, for storage reduction _the NO _x purification to exhibit stable over time with higher _the NO _x purification performance over a wide temperature range The purpose is to provide a catalyst.

[0008]

SUMMARY OF THE INVENTION The above object is to provide a first oxide carrier having Pt, Pd, and a NO _x storage material supported thereon.
This is achieved by a catalyst and a storage reduction type NO _x purification catalyst including a second catalyst in which Pd and a NO _x storage material are supported on an oxide carrier. That is, the catalyst of the present invention is
A storage reduction type NO _x purification catalyst including a first catalyst and a second catalyst, wherein Pt is supported only on the first catalyst, and Pd
Is carried by both the first catalyst and the second catalyst,
Sintering of t is suppressed and the N value is stable and high over time.
It is a catalyst that exhibits O _x purification performance.

FIG. 1 is a model diagram of such a catalyst, in which Pd particles are present on an oxide carrier to surround Pt particles, and a NO _x storage material is present around them, and a first catalyst,
The second catalyst, in which Pd is present in the oxide carrier and the NO _x storage material is present around it, is present in a mixed state. The reason why sintering is suppressed in such a catalyst and a high NO _x purification performance can be stably exerted over a wide temperature range over time is considered as follows.

Regarding the first catalyst, Pt has a property that sintering is relatively likely to occur in a high temperature oxidizing atmosphere, while Pd has a property that sintering is relatively unlikely to occur. Therefore, the presence of Pd around the Pt particles can have an effect of suppressing Pt movement.

Especially, when the molar ratio of Pd / (Pt + Pd) is 0.02 to 0.2, such a state can be exhibited, and the solution in which the Pt compound and the Pd compound are dissolved is oxidized. Such a state can be expressed in a finely-divided form by impregnating the material carrier with the material and then calcining, whereby high dispersibility of the catalyst component on the carrier and stability of this high dispersion can be obtained. It is considered to be a thing.

On the other hand, in the purification of NO _x by repeating the lean condition and the temporary stoichiometric-rich condition, Pt exerts a NO _x purification action at a relatively high temperature, but most Pt particles are exposed. Therefore, this NO _x purification action is maintained. Therefore, the first catalyst can mainly provide a high NO _x purification action at a relatively high temperature.

Next, regarding the second catalyst, while Pd has a property that sintering is relatively unlikely to occur, at a relatively high temperature, it does not exert so much NO _x purification action due to its oxygen storage capacity. A high NO _x purification action is exhibited at a relatively low temperature. Therefore, the second catalyst is mainly
It is possible to provide a high NO _x purification action at a relatively low temperature.

Therefore, the coexistence of the first catalyst and the second catalyst makes it possible to provide a catalyst exhibiting a high NO _x purification performance over a wide temperature range, and in particular (Pt + Pd) contained in the first catalyst. It is considered that the NO _x purification performance is optimized when the molar amount of Pd contained in the second catalyst with respect to the molar amount of Pd / (Pt + Pd) is 0.9 to 1.0. To be

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The storage reduction type NO _x purification catalyst of the present invention comprises a first catalyst in which Pt, Pd, and a NO _x storage material are supported on an oxide carrier, and Pd on the oxide carrier. ,
And a second catalyst carrying an NO _x storage material.

The oxide carriers of the first catalyst and the second catalyst are oxides such as alumina, silica and zirconia, as well as silica-alumina, zirconia-ceria and alumina-.
Ceria-zirconia, ceria-zirconia-yttria,
A composite oxide such as zirconia-calcia, which is composed of fine particles having an average particle size of 1 μm or less, can be preferably used, and the oxide supports of the first catalyst and the second catalyst are the same kind of oxidation. It can consist of a material or a different kind of oxide.

With regard to the preparation of the first catalyst, such oxides
Supporting Pt and Pd on the carrier is preferably zinito
Rodiammine platinum complex Pt (NH₃)₂(NO₂)₂, Platinum chloride
Acid H ₂PtCl₆・ 6H₂Platinum compounds such as O and para nitrate
Dium Pd (NO₃)₂, Palladium chloride PdCl₂Etc.
Radium compounds are used, and these compounds
Prepare a solution that has been dissolved in an amount that results in a molar ratio of / (Pt + Pd).
And impregnate this solution into the above oxide support, then dry.
It is done by drying and baking. Firing is performed in the atmosphere, for example.
Heating at ambient temperature of 450-650 ° C for several hours
Can be done by

The NO _x storage material supported on the oxide carrier includes lithium (Li), sodium (Na) and potassium.
(K), alkali metal of rubidium (Rb), and magnesium (Mg), calcium (Ca), strontium (S
r) and at least one kind of alkaline earth metal of barium (Ba) are exemplified, and preferably, at least one kind of metal selected from Li, K, and Ba is included.

Preferably, the NO _x storage material is Pt and Pd.
Is carried and then carried. As a method of carrying this, it is possible to prepare a solution of these NO _x storage materials, impregnate this solution into the above oxide carrier, and then dry and calcine.

Like the first catalyst, the second catalyst is preferably palladium nitrate Pd (NO ₃ ) ₂ or palladium chloride Pd.
A palladium compound such as Cl ₂ is used to prepare a solution in which these compounds are dissolved in a predetermined amount, and the above oxide carrier is impregnated with this solution, and then dried and calcined to support Pd, Further, preferably Li, K,
And at least one NOx storage material selected from Ba.

The first catalyst and the second catalyst thus prepared
The catalyst is, for example, wet-mixed with a ball mill or the like using water as a medium and then dried, or the powdery first catalyst and the second catalyst are dry-mixed with a Henschel mixer or the like, and the storage reduction type of the present invention is used. A NO _x purification catalyst can be obtained.

Further, storage reduction _the NO _x purification catalyst of the present invention, when coated on conventional honeycomb substrate, carrying of the NO _x storage material of the first catalyst and the second catalyst, _the NO _x storage material Before being loaded, Pt and Pd loaded first catalyst and second
After the catalyst is coated on the honeycomb base material, the coating layer on the honeycomb base material is impregnated with the solution of the NO _x storage material, and then dried and fired. Hereinafter, the present invention will be described more specifically with reference to Examples.

[0023]

Example 1 An aqueous solution in which a dinitrodiammine platinum complex and palladium nitrate were dissolved in γ-alumina powder (specific surface area: about 180 m ² / g) (Pt concentration 4.61% by mass, Pd concentration 4.33% by mass). 5.75 g of P per 120 g of γ-alumina powder
t and 0.15 g of Pd, and after drying,
Firing was performed for 1 hour in an air atmosphere at 300 ° C. Separately, γ-
An aqueous solution of palladium nitrate (Pd concentration 4.
33% by mass) to impregnate 120 g of γ-alumina powder with 3.25 g of Pd, and after drying, 40
Firing was performed for 1 hour in an atmosphere of 0 ° C.

These PtPd / γ-alumina powder and P
d / γ-alumina powder was mixed in a mass ratio of 1: 1 and further mixed with 100 g of alumina sol (Nissan Chemical A-200) and 2
Ion-exchanged water (00 cc) was added and mixed in a ball mill for 2 hours to prepare a slurry. The obtained slurry is wash-coated on a cordierite honeycomb substrate having a diameter of 30 mm and a length of 50 mm, dried at 120 ° C. for 2 hours, and then fired at 500 ° C. for 1 hour in an air atmosphere to obtain a honeycomb substrate 1. 5.75 g Pt and 3.4 g P per liter
carried d.

Next, the coating layer is made to absorb an aqueous solution in which barium acetate, potassium acetate and lithium acetate are dissolved, dried and then baked in an air atmosphere at 500 ° C. for 1 hour to give 0 per 1 liter of honeycomb substrate. .2 mol B
The catalyst of the present invention was prepared by carrying a, 0.1 mol of K, and 0.1 mol of Li.

Comparative Example 1 An aqueous solution (Pt concentration 4.61% by mass, Pd concentration 4.33% by mass) in which dinitrodiammine platinum complex and palladium nitrate were dissolved in γ-alumina powder was used per 120 g of γ-alumina powder. It was impregnated with 5.75 g of Pt and 0.15 g of Pd, dried, and then baked in an air atmosphere at 400 ° C. for 1 hour.

100 parts of this PtPd / γ-alumina powder
g of alumina sol and 200 cc of ion-exchanged water were added and mixed in a ball mill for 2 hours to prepare a slurry. Then, the honeycomb substrate was wash-coated with the obtained slurry in the same manner as in Example 1, and then, further in Example 1.
In the same manner as above, 0.2 mol of Ba, 0.1 mol of K, and 0.1 mol of Li are carried per liter of the honeycomb substrate,
A comparative catalyst was prepared.

Comparative Example 2 Aqueous solution in which palladium nitrate was dissolved in γ-alumina powder
Γ-alumina powder 1 using (Pd concentration 4.33% by mass)
It was impregnated with Pd of 3.25 g per 20 g, dried, and then calcined for 1 hour in an air atmosphere at 400 ° C.

To this Pd / γ-alumina powder, 100 g of alumina sol and 200 cc of ion-exchanged water were added and mixed in a ball mill for 2 hours to prepare a slurry. Then, in the same manner as in Example 1, the resulting slurry was wash-coated on the honeycomb substrate, dried and fired, and then, in the same manner as in Example 1, per 1 liter of the honeycomb substrate.
A catalyst of Comparative Example was prepared by supporting 2 mol of Ba, 0.1 mol of K and 0.1 mol of Li.

-Evaluation of NO _x purification performance- Each of the above-obtained catalysts was subjected to heat treatment in an air atmosphere at 850 ° C for 2 hours, and then the following model gas was used at 300 ° C.
The NO _x purification rate at 400 ° C. and 500 ° C. was measured. The results are summarized in FIG. Model gas composition: 500ppm NO + 2000ppm HC + 0.6% CO
+ 10% CO ₂ + 0.3% O ₂ + 5% H ₂ O (remainder: N ₂ )

-Particle size of catalyst component-The same as in Example 1 except that the honeycomb substrate was not used,
The amounts of catalyst components Pt and Pd shown in Table 1 and the NO _x storage materials Ba, K, and Li were carried on the γ-alumina powder, and the respective γ-alumina powders were heated at 750 ° C. in the atmosphere.
Inductively coupled plasma emission spectrometry after firing for 5 hours
The composition of Pt and Pd was analyzed by (ICP), and the particle size of the catalyst component was measured by powder X-ray diffraction (XRD). The results are summarized in FIG. The particle diameter shown in FIG. 3 is shown as a relative value based on the particle diameter of Pt alone as the catalyst component.

[0032] - Results from - from _the NO _x purification rate shown in FIG. 2, Comparative Example 1, but _the NO _x purification rate of the low temperature side is relatively low, _the NO _x purification rate of the high temperature side is relatively high, Comparative Example The result of No. 2 is that the NO _x purification rate on the low temperature side is relatively high, but the NO _x purification rate on the high temperature side is relatively low, whereas in Example 1, the NO _x purification rate is high over the entire temperature range.
_x Indicates the purification rate. This can be achieved by including a first catalyst and a second catalyst, _the NO _x purification performance of the catalyst is expressed in the present invention as the sum of both of the NO _x purifying performance, thereby high _the NO _x purification rate over temperature Is understood to indicate that

Further, from the results shown in FIG. 3, it can be seen that the molar ratio Pd / (Pt + Pd) of the first catalyst has a specific range in which the growth of the particle diameter of the catalyst component is suppressed. Therefore, it is considered that the NO _x purification performance of the first catalyst is more effectively maintained within this range.

[0034]

EFFECTS OF THE INVENTION It is possible to provide a storage reduction type NO _x purification catalyst that suppresses sintering of catalyst components and stably exhibits high NO _x purification performance over a wide temperature range.

[0035]

[Table 1]

[Brief description of drawings]

FIG. 1 is a model diagram showing a possible form of a catalyst of the present invention.

FIG. 2 is a graph comparing NO _x purification rates.

FIG. 3 is a graph showing particle diameters of catalyst components after heat treatment.

Continued front page    F-term (reference) 4D048 AA06 AB02 BA10X BA14X                       BA15X BA30X BA31X BA41X                       BB02 EA04                 4G069 AA03 AA08 BA01A BA01B                       BA13A BA13B BB02A BB02B                       BB04A BC03A BC03B BC04A                       BC04B BC13A BC13B BC72A                       BC72B BC75A BC75B CA02                       CA03 CA08 CA13 EA19 FA02                       FB19 FC08

Claims (5)

[Claims] 1. Pt, Pd, and NO _x on an oxide support.
A storage-reduction type NO _x purification catalyst comprising a first catalyst supporting an storage material and a second catalyst supporting Pd and an NO _x storage material on an oxide carrier. 2. The storage reduction type NO _x purification catalyst according to claim 1, wherein the molar ratio of Pd / (Pt + Pd) in the first catalyst is 0.02 to 0.2. 3. The molar amount of Pd contained in the second catalyst relative to the molar amount of (Pt + Pd) contained in the first catalyst is Pd /
The storage reduction type NO _x purification catalyst according to claim 1 or 2, wherein the molar ratio of (Pt + Pd) is 0.9 to 1.0. 4. The NO _x storage material contains at least one metal selected from Li, K, and Ba.
The storage reduction-type NO _x purification catalyst according to any one of 1. 5. An oxide carrier is impregnated with a solution in which a Pt compound and a Pd compound are dissolved, and then baked, and then NO _x.
A method for producing a first catalyst included in the storage reduction type NO _x purification catalyst according to any one of claims 1 to 4, wherein an adsorption material is carried.