JP2003290658A - Catalyst for cleaning exhaust gas and manufacture method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst for cleaning an exhaust gas in which a amount of a noble metal carried is reduced and sintering is inhibited. <P>SOLUTION: The catalyst for cleaning the exhaust gas is characterized in that an oxide carrier is carried with at least one metal particle of W or Mo and the metal particle is carried with the noble metal. The metal particle preferably has a diameter of 1-15 nm and a mass ratio of the noble metal/metal particle is 1-10. The catalyst for cleaning the exhaust gas can be manufactured by reducing/precipitating the noble metal on the at least one metal particle of W or Mo. <P>COPYRIGHT: (C)2004,JPO

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 in which the amount of precious metal carried is reduced and sintering is suppressed.

[0002]

2. Description of the Related Art Exhaust gas emitted from an internal combustion engine such as an automobile engine contains carbon monoxide (CO), hydrocarbon (H
C), nitrogen oxides (NO _x ), etc., and these harmful substances are generally platinum (Pt), palladium (Pd), rhodium.
It is purified by an exhaust gas purification catalyst containing a precious metal such as (Rh) as a catalyst component.

[0003]

However, from the viewpoint of environmental protection, these exhaust gas purifying catalysts are required to have further improved purifying performance, and conventionally, mainly, noble metal as a catalyst component is used. This has been done by increasing the loading amount. However, precious metals are expensive and the amount of mining is limited.

On the other hand, the exhaust gas purifying catalyst generally has a reducing atmosphere in which the temperature repeatedly changes between room temperature and about 1000 ° C., and the concentration of HC and CO is relatively high and the O ₂ concentration is low. It is used under conditions where an oxidizing atmosphere with a relatively low concentration of HC and CO and a high concentration of O ₂ is repeated.

However, the above noble metal catalyst component has a property of causing so-called sintering, in which the catalyst component moves on the carrier to form enlarged particles when exposed to such an atmosphere for a long time. Therefore, there is a problem that the exhaust gas purification performance deteriorates with time.

[0006] By the way, the applicant of the present invention has disclosed in Japanese Patent Laid-Open No. 2001-4.
No. 6870 proposes an exhaust gas purifying catalyst containing a complex oxide containing Pt and W as a catalyst component, and in Japanese Patent Laid-Open No. 11-156193, there is proposed an exhaust gas purifying catalyst carrying various alloy catalysts. Proposing a catalyst.

It is an object of the present invention to provide an exhaust gas purifying catalyst in which the amount of precious metal carried is reduced and sintering is suppressed by the exhaust gas purifying catalyst having a structure completely different from that of the known art. To do.

[0008]

The above-mentioned objects are W or M.
This is achieved by an exhaust gas purifying catalyst characterized in that at least one metal particle of o is supported on an oxide carrier, and a noble metal is supported on the metal particle. That is,
The exhaust gas purifying catalyst of the present invention is a catalyst in which a noble metal is supported in a shell shape around a core of W or Mo metal particles. The reason why the above object is achieved by the exhaust gas purifying catalyst having such a structure is considered as follows.

The harmful substances in the exhaust gas are purified by reaction by contacting the noble metal catalyst component supported on the oxide carrier, so that substantially only the noble metal surface becomes an effective catalyst component. However, it is difficult to support all of the noble metals on the oxide support with atomic level thinness,
Further, the noble metal has a property of changing into particles enlarged by sintering as described above. In these cases,
The core portion of the noble metal particles cannot substantially contribute as a catalyst component.

Here, the oxide carrier and W or Mo have a high bond strength due to the intervening oxygen at their interface,
Even under the above exhaust gas atmosphere, the morphology of the metal particles of W or Mo is not substantially changed. Furthermore, the noble metal and W or Mo also have a high bond strength at their interface, and the noble metal supported on the W or Mo metal particles is
The movement under the exhaust gas atmosphere is suppressed, and therefore sintering is substantially not caused.

Therefore, by using W or Mo metal particles as a core and supporting the precious metal in the form of fine particles or an extremely thin film thereon, the amount of the precious metal in a volume corresponding to the core is substantially reduced, and Sintering can be suppressed.
FIG. 1 is a model diagram of the exhaust gas purifying catalyst of the present invention. The present invention is not limited to this model diagram.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The exhaust gas purifying catalyst of the present invention comprises:
At least one metal particle of W or Mo is supported on an oxide carrier, and a noble metal is supported on the metal particle. As the oxide carrier, in addition to oxides such as alumina, silica and zirconia, silica-alumina, ceria-zirconia, alumina-ceria-zirconia, ceria-
Examples thereof include complex oxides such as zirconia-yttria and zirconia-calcia, and those composed of fine particles having an average particle diameter of 1 μm or less can be preferably used.

W and / or Mo may be added to such an oxide support.
Carrying ammonium tungstate para pentahydrate
Japanese product 5 (NH_Four)₂・ 12 WO₃・ 5H₂O, metatungsten
Ammonium acid (NH_Four)₆[H₂W₁₂O₄₀], Seven morib
Hexammonium decanoate tetrahydrate (NH_Four)₆Mo₇O_{twenty four}・ 4
H₂O, ammonium metamolybdate (NH_Four)_Four[Mo ₈
O₂₆] A solution of a compound soluble in a solvent such as water, or
Impregnates oxide support powder with a mixed solution of these compounds
And then drying and firing.
Wear.

Alternatively, a fine powder such as tungsten trioxide WO ₃ , molybdenum trioxide MoO ₃ or a mixed fine powder thereof is mixed with a powder of an oxide carrier and then dried.
It can also be supported by firing.

Next, a noble metal is supported on the W and / or Mo-supported oxide carrier. This supporting method is based on the dinitrodiammine platinum complex Pt (NH ₃ ) ₂ (N
O ₂ ) ₂ , chloroplatinic acid H ₂ PtCl ₆ .6H ₂ O, platinum acetylacetonate Pt (acac) ₂ , palladium nitrate Pd
(NO ₃ ) ₂ , palladium chloride PdCl ₂ , rhodium nitrate R
Using soluble noble metal compounds such as h (NO ₃ ) ₃ and rhodium chloride RhCl ₃ .4H ₂ O, the solution of these compounds was adjusted to a suitable pH, impregnated into the oxide carrier powder, and then dried. -Can be performed by firing.

[0016] Preferably, the noble metal is supported by reduction precipitation. More specifically, the above oxide carrier supporting W and / or Mo is dispersed in water to form a slurry,
The above noble metal compound is dissolved in this. Then, add hydrazine N ₂ H ₄ and sodium thiosulfate N to the slurry.
a ₂ S ₂ O ₃ , potassium thiosulfate K ₂ S ₂ O ₃ , ammonium thiosulfate (NH ₄ ) ₂ S ₂ O ₃ , sodium sulfite Na ₂ S
O ₃ , borohydrides such as sodium borohydride NaBH ₄ , etc., reducing agents such as hypophosphite, citrate, formic acid CH ₂ O ₂ and oxalic acid CH ₂ O ₄ , and sodium L-ascorbate C ₆ A noble metal compound is reduced by adding a buffering agent such as H ₇ O ₆ Na or ethylenediaminetetraacetic acid salt to precipitate a noble metal such as Pt in a fine form.

According to such reduction deposition, it is relatively easy to deposit the noble metal in the form of fine particles of an atomic level unit or an extremely thin film on substantially all W and / or Mo. Then, preferably 300 to 70 in the atmosphere.
By heating at a temperature of 0 ° C. for several hours and firing, the precious metal can be firmly supported.

In the catalyst carrier of the present invention thus obtained, the metal particles of W or Mo are preferably 1 to 15
nm, more preferably 1-10 nm. This diameter can be controlled by the type and concentration of the above W or Mo compound, firing conditions, and the like.

The mass ratio of noble metal / W or Mo is preferably 1 to 10, more preferably 2 to 5, and the "metal particles of W or Mo" carried on the oxide carrier means tungsten oxide. And molybdenum oxide metal oxides, and W
Alternatively, it is meant to include both Mo metals.

The exhaust gas purifying catalyst of the present invention thus prepared can be used for purifying exhaust gas of an internal combustion engine by coating it on a monolith substrate or pelletizing it. Hereinafter, the present invention will be described more specifically with reference to Examples.

[0021]

EXAMPLES Example 1 CeO ₂ --ZrO ₂ powder as an oxide carrier was prepared as follows.
(Molar ratio of Ce / Zr = 1/1, specific surface area 70 m ² / g)
Four types of exhaust gas purifying catalysts of the present invention were prepared in which W particles were supported on Pt and Pt was supported on the W particles. C
eO ₂ -ZrO ₂ after the powder is impregnated with an aqueous solution of ammonium tungstate para pentahydrate, 50 in the atmosphere
It was subjected to calcination at 0 ° C. for 2 hours, and W particles were supported on the CeO ₂ —ZrO ₂ powder.

This W / CeO ₂ -ZrO ₂ powder was dispersed in 2000 g of ion-exchanged water at 60 ° C. in an amount shown in Table 1, and 0.20% by mass of sodium borohydride was added to the obtained slurry. Concentration, sodium ascorbate 2.
A concentration of 00% by mass and chloroplatinic acid were sequentially added in the amounts shown in Table 1, and the mixture was placed under gentle stirring for 24 hours to reduce and precipitate Pt from H ₂ PtCl ₆ .

After this reduction precipitation, the slurry was filtered and washed, dried at 120 ° C. for 2 hours in the atmosphere, and further heat-treated at 500 ° C. for 2 hours in the atmosphere. By this method, four kinds of exhaust gas purifying catalysts having the compositions shown in Table 1 were obtained.

[0024]

[Table 1]

Example 2 The CeO of the oxide support is prepared as follows.₂-ZrO₂Powder
Mo particles are supported on, and Pt is supported on the Mo particles.
Prepared four kinds of exhaust gas purifying catalysts of the present invention
It was CeO₂-ZrO₂6 Ammonium heptamolybdate in powder
After impregnating with an aqueous solution of um tetrahydrate, in an air atmosphere
CeO provided by firing at 500 ° C x 2 hours₂-ZrO ₂powder
Finally, Mo particles were supported.

This Mo / CeO ₂ -ZrO ₂ powder is shown in Table 2.
In the amount shown in (2), 2000 g of ion-exchanged water at 60 ° C. was dispersed, and the obtained slurry had a sodium borohydride concentration of 0.20% by mass and sodium ascorbate of 2.
A concentration of 00% by mass and chloroplatinic acid were sequentially added in the amounts shown in Table 2, and the mixture was placed under gentle stirring for 24 hours to reduce and precipitate Pt from H ₂ PtCl ₆ .

After this reduction precipitation, the slurry was filtered and washed, dried at 120 ° C. for 2 hours in the atmosphere, and further heat-treated at 500 ° C. for 2 hours in the atmosphere. By this method, four kinds of exhaust gas purifying catalysts having the compositions shown in Table 2 were obtained.

[0028]

[Table 2]

Comparative Example 1 CeO ₂ -ZrO ₂ powder was added to dinitrodiammine platinum complex P
It is impregnated with an aqueous solution of t (NH ₃ ) ₂ (NO ₂ ) ₂ and then exposed to 120
Dry at ℃ × 2 hours, and further in the air at 500 ℃ × 2
The Pt was supported on the CeO ₂ —ZrO ₂ powder by heat treatment for a period of time. In this method, the impregnation amount of the dinitrodiammine platinum complex aqueous solution was changed to obtain Pt / CeO ₂ -ZrO ₂
Mass ratio of 0.5 / 95.5, 1/99, 2/98, 3
Four types of exhaust gas purification catalysts of / 97 were obtained.

-Evaluation of Catalyst Performance- Each of the catalysts of the above Examples and Comparative Examples was compressed and crushed to form a pellet-shaped catalyst having a diameter of about 1 to 3 mm, and then 800 ° C x 3 in an air atmosphere. It was subjected to a time endurance treatment.

2.0 g of each of the catalysts after the endurance treatment,
It is installed inside the reaction tube of an exhaust gas purification performance evaluation device for a laboratory, a model gas (corresponding to stoichiometric exhaust gas) having the composition shown below is circulated, and the catalyst bed temperature is lowered at a rate of 10 ° C / min. It was measured 50% purification temperature (T50) of C ₃ H _6. The flow rate of this model gas is 6.0 l / min / 0.0
The amount of supported metal was 15 g (Pt + W or Mo). _{0.20% CO + 667ppmC 3 H 6} + 0.4% O 2 ( remainder N
₂ ) The T50 of each catalyst thus measured is shown in FIG.

From the results shown in FIG. 2, Pt / W or Mo
Of Pt on W particles and each catalyst of Example 2 on which Pt was supported on Mo particles, and Pt on CeO ₂ —ZrO ₂ at a mass ratio of 7/3. Comparing T50 of each catalyst of Comparative Example 1 directly supporting
When the carried amount of (Pt + W), the carried amount of (Pt + Mo) of Example 2, and the carried amount of Pt of Comparative Example 1 are the same, Example 1 and Example 2 show slightly higher purification performance than Comparative Example 1. (T50
Is slightly lower). Therefore, it can be seen that even if the amount of Pt corresponding to the amount of W or Mo supported is reduced, the same or higher purification performance can be obtained.

-Observation of Morphology of Each Catalyst- For each catalyst of the above-mentioned Examples and Comparative Examples, morphological observation by a transmission electron microscope (TEM) and energy dispersive X-ray spectroscopy (EDX) before and after durability treatment. Elemental analysis was carried out in the spot region of the electron microscope image by.

Before the durability treatment, it was observed from the TEM image that each of the catalysts of Example 1 had metal particles having a diameter of 2 to 10 nm. Also, Pt has a resolution of about 1
Although it was not observed as a single Pt particle even in EDX of nm, Pt and W were detected at the same time from each metal particle, and the concentration of Pt increases toward the edge. Was determined to exist only in.

Similarly, in each of the catalysts of Example 2 before endurance treatment, it was observed that the Mo particles had a diameter of 2 to 10 nm, and it was judged that Pt was present only on the Mo particles. Was done. Further, in each of the catalysts of Examples 1 and 2 after the durability treatment, no significant change was observed in the existing forms of W or Mo and Pt before the durability treatment.

On the other hand, each catalyst of Comparative Example 1 was not observed as a single Pt particle even in the EDX having a resolution of about 1 nm before the endurance treatment, and could be entirely present on the CeO ₂ -ZrO ₂ particle. Although observed, Pt was observed to grow into particles having a diameter of about 20 to 30 nm after the durability treatment.

[0037]

EFFECT OF THE INVENTION It is possible to provide an exhaust gas purifying catalyst in which the amount of noble metal carried is reduced and sintering is suppressed.

[Brief description of drawings]

FIG. 1 is a model diagram of an exhaust gas purifying catalyst of the present invention.

FIG. 2 is a graph comparing 50% purification temperatures of C ₃ H ₆ after endurance treatment at 800 ° C. for 3 hours in an air atmosphere.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) B01J 37/16 B01D 53/36 104Z F01N 3/10 ZAB F term (reference) 3G091 AA02 AB01 BA39 GB01X GB05W GB10X 4D048 AA13 AA18 BA08X BA19X BA26X BA27X BA30X BA31Y BA32Y BA33Y BA34Y BA41X BB01 BB17 4G069 AA03 AA08 BA05B BB02A BB02B BB04A BB04B BC32A BC33A BC45A EC4519 CA45BCAB19CA15BCA19B14BCAB14CA14BBC19B14CA14A

Claims (4)

[Claims] 1. An exhaust gas purifying catalyst, wherein at least one metal particle of W or Mo is supported on an oxide carrier, and a noble metal is supported on the metal particle. 2. The exhaust gas purifying catalyst according to claim 1, wherein the metal particles have a diameter of 1 to 15 nm. 3. The noble metal / metal particle mass ratio is 1
The exhaust gas purifying catalyst according to claim 1 or 2, which is 10 to 10. 4. The exhaust gas purification according to claim 1, wherein the noble metal is reduced and deposited on the metal particles of at least one of W and Mo supported on the oxide carrier. For producing catalyst for automobile.