JP3296848B2 - Exhaust gas purification catalyst and method for producing the same - Google Patents

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 having an excellent initial purification rate and stable heat resistance, and a method for producing the same.

[0002]

2. Description of the Related Art C as an exhaust gas purifying catalyst for an engine
A three-way catalyst that simultaneously oxidizes O and HC and reduces NOx is used. It is known that this three-way catalyst has high purification efficiency when the air-fuel ratio of the engine is around 14.7, which is the stoichiometric air-fuel ratio. Have been.

On the other hand, lean burn engines have been put into practical use in order to comply with fuel regulations for engines.
Since the exhaust gas in such a lean burn system has a high concentration of oxygen, the three-way catalyst cannot effectively remove NOx.

Therefore, an active species such as Cu is supported on a metal-containing silicate as a catalyst capable of decomposing and purifying NOx in exhaust gas into N ₂ and O ₂ under an oxygen-rich atmosphere. So-called zeolite catalysts hold promise. Further, with respect to the above-mentioned zeolite catalyst, various measures such as selection of active species have been studied in order to always provide an exhaust gas purifying catalyst having excellent NOx removing ability in response to various states of exhaust gas.

For example, Japanese Patent Laid-Open Publication No. Hei 3-202157 proposes a technique for efficiently removing NOx in an oxygen-rich atmosphere. The technology disclosed in the above-mentioned publication is to provide a catalyst in which zeolite supports Cu and at least one kind of alkaline earth metal and one or more kinds of rare earth metals, thereby obtaining excellent NOx in a temperature range of 800 ° C. or less. An object of the present invention is to obtain a catalyst having a purifying ability and durability that can be used for a long time.

[0006]

SUMMARY OF THE INVENTION However, NOx
Cu ion-exchanged zeolite catalysts, which are considered to be useful catalysts for effective removal of
When NOx is purified in a high-oxygen atmosphere by installing it in an actual vehicle equipped with a lean-burn engine despite exhibiting a NOx purification rate exceeding 0%, the amount of NOx between the model gas in the laboratory and the exhaust gas of the actual vehicle is high. It is inevitable that the NOx purification rate decreases due to the various condition differences.

Further, in such a Cu ion exchanged zeolite catalyst, the temperature at which the NOx purifying activity is exhibited is 350 to 40.
Since the temperature is as high as 0 ° C., there is a problem that the integrated NOx purification value is reduced in an atmosphere having a high oxygen concentration.

Further, when the engine is operated at a low load, the catalyst temperature in the low-temperature region is required to be improved due to a decrease in the temperature of the catalyst inlet gas of the exhaust gas. What has an excellent purification rate is needed.

Further, it has been recognized that a catalyst in which Cu and a metal of a rare earth element are supported on a metal-containing silicate, for example, as described in JP-A-3-202157, has improved heat resistance. However, there is a problem when the purification rate in a low-temperature region such as 300 ° C. or lower during the purification of NOx is poor.

In view of the above, it is an object of the present invention to provide an exhaust gas purifying catalyst having excellent initial activity and high thermal stability when purifying NOx in an atmosphere containing high concentration of oxygen in exhaust gas.

[0011]

Means and Action for Solving the Problems A platinum group element (R
u, Rh, Pd, Os, Ir, and Pt) are known as active species to improve the low-temperature activity in the exhaust gas purifying catalyst. In the case where the catalyst is supported on the metal-containing silicate in the embodiment, the earnest research on the behavior of each active species of the platinum group element contributing to the improvement of the NOx purification characteristics of each catalyst has been advanced.

As a result, the catalyst in which Pt and Ir are supported on the metal-containing silicate by dryness, for example, has a synergistic effect between the supported Pt and Ir in the NOx purification reaction in an oxygen-rich atmosphere. It was confirmed that a large contribution was made and a high NOx purification rate was obtained. On the other hand, Pt
It was also confirmed that, when supported on a metal-containing silicate by the impregnation method, the initial activity was not so high, but was thermally stable and exhibited relatively excellent heat resistance.

[0013] It has also been confirmed that it is not preferable that Rh is supported on the metal-containing silicate in a state in which it appears on the surface of the catalyst, since Rh itself has not very good heat resistance.

The present invention relates to Pt, Ir and R as described above.
In order to achieve the above object, the invention according to claim 1 is directed to a catalyst in which active species are supported on a metal-containing silicate in a low temperature range exhaust gas. In order to improve the purification rate and improve the thermal stability, a high purification rate in a low-temperature region specific to the noble metal by selecting the active species from the noble metals and supporting the noble metal on the metal-containing silicate. The purpose is to achieve both thermal stability.

Specifically, a solution taken by the invention of claim 1 is directed to an exhaust gas purification catalyst in which active species are supported on a metal-containing silicate in a layered manner. Are provided with Pt active species and Rh active species.

Further, according to the present invention, Rh is supported on the metal-containing silicate in the lower layer of the layered active species, and Pt and Ir are supported on the upper layer of the active species in close proximity to each other. It is intended to produce an exhaust gas purifying catalyst efficiently.

More specifically, a solution taken by the invention of claim 2 is a Rh supporting step of supporting Rh on the metal-containing silicate, and wash-coating the Rh-supporting metal-containing silicate obtained in the Rh supporting step on a catalyst carrier. Thereafter, Pt and Ir are simultaneously impregnated and supported on a catalyst intermediate made of the Rh-supported metal-containing silicate obtained in the wash coating step, and Pt and I are dried and fired.
r supporting step.

The invention according to claim 3 is characterized in that the active species is P
It is intended to carry Rh having a lower heat resistance than t and Ir in a good dispersion state on the lower layer of the layer,
Specifically, in the invention of claim 2, the Rh supporting step is configured such that Rh is supported on the metal-containing silicate by drying a mixture of the metal-containing silicate powder and the Rh-containing solution. Things.

With such a configuration, the above R
Since h is supported on the layered lower layer, Rh, which is relatively weak to heat alone, is prevented from being directly exposed to high heat due to HC combustion or the like on the catalyst surface.
x It is possible to improve the temperature dependency of the purification reaction and improve the thermal stability. Further, since Pt and Ir are supported in close proximity to each other on the layered upper layer, the concentration on the catalyst surface becomes high, and Ir alone having a relatively small NOx purifying ability has a synergistic effect on Pt. The above Pt
An active site is formed in a portion where Ir and Ir are carried in close proximity. As a result, Pt and Ir are carried close to each other, so that the initial purification rate in a low temperature range can be improved.

Further, in the manufacturing method according to the present invention,
First, Rh is supported on a metal-containing silicate, preferably by drying, and then wash-coated on a catalyst support to form a catalyst intermediate.
Since Pt and Ir are simultaneously impregnated and supported by the mixed solution in which t and Ir coexist, Rh is provided in the lower layer and Pt and Ir are provided in the upper layer.

Further, since Pt and Ir are simultaneously impregnated and supported on the above-mentioned catalyst intermediate by a mixed solution coexisting with each other, Pt and Ir can be supported in close proximity.

The metal-containing silicate on which the active species is supported is preferably an aluminosilicate (zeolite) using Al as the metal forming the skeleton of the crystal. If necessary, Ga may be used instead of Al or together with Al. , Ce,
A metal-containing silicate using a metal such as Mn or Tb as a skeleton forming material can also be applied. A-type, X-type, Y-type, mordenite, ZSM
-5 is preferred.

As the metal-containing silicate, Na-type ZSM-5 in which the cationic species is Na is preferably used, and in addition, an H-type metal-containing silicate in which the cationic species is H ⁺ can also be used. .

[0024]

Embodiments of the present invention will be described below.

The metal-containing silicate is Na-type ZSM-5.
(Caban ratio = 30) was used. Rh was dried and solidified on the metal-containing silicate so as to be 0.75 g in equivalent in 1 liter of the catalyst, and then wash-coated on the catalyst carrier. Then, the Rh-supported metal-containing silicate was wash-coated on the catalyst carrier and the catalyst intermediate was converted to P in 1 liter of the catalyst in terms of conversion.
Pt and Ir were simultaneously impregnated and supported such that t was 2.25 g and Ir was 0.75 g, thereby obtaining an example catalyst of the exhaust gas purifying catalyst according to the present invention.

Specifically, an aqueous solution of rhodium nitrate corresponding to 0.09 g of Rh was added to 15 g of Na-type ZSM-5 powder, and the mixture was heated and stirred at about 60 ° C.
Rh was dried and supported on the -5 powder. This Rh dry solid supported N
A-type ZSM-5 was added with hydrated alumina as a binder to form a slurry, and was wash-coated on a catalyst carrier (cordrite honeycomb carrier (400 cells per square inch)), and then 150 ° C. × 2 hours in the atmosphere. And dried at 500 ° C. for 2 hours in the atmosphere to obtain a catalyst intermediate.

On the other hand, 0.0973 g of divalent platinum ammine crystals (Pt concentration = 5) corresponding to 0.05625 g of Pt
7.79%) and 0.0875 g of iridium trichloride, equivalent to 0.01875 g of Ir, are dissolved in a mixed solution of water and ethanol to obtain a mixed solution of a Pt component and an Ir component. The total volume was adjusted to 3.5 cc.

The catalyst intermediate obtained by wash-coating the Rh-dried Na-type ZSM-5 was impregnated with 3.5 cc of a mixed solution of a Pt component and an Ir component, and the mixed solution was sufficiently mixed with the catalyst material. After drying at 150 ° C. for 2 hours in the air, and then 500 ° C. in the air
The firing was performed for 2 hours.

By the above operation, 0.75 g of Rh is supported by dryness in 1 liter of the catalyst on a reduced basis.
In the present invention, 5 g of Pt and 0.75 g of Ir are supported by simultaneous impregnation, and the Rh is supported on a lower layer of the support layer, and the Pt and Ir are supported on an upper layer of the support layer. Exhaust gas purifying catalysts in Examples are prepared.

Further, various catalysts carrying the same active species as the exhaust gas purifying catalyst according to the present invention were prepared, and Comparative Examples 1 to 10 were prepared.

That is, the metal-containing silicate is Na-type Z
Using SM-5 (Caban ratio = 30), the metal-containing silicate was converted to 0.75 g of Rh per liter of catalyst.
And then wash-coated on the catalyst carrier, followed by impregnating the wash-coated catalyst intermediate by impregnation with Pt and / or Ir or Pt and / or Ir. The catalysts of Comparative Examples 1 to 5 were prepared by supporting them. The supported amounts of Pt and Ir when these catalysts were 1 liter in Comparative Examples 1 to 5 and the method of supporting Pt and Ir on the catalyst intermediate were as shown in Table 1.

[0032]

[Table 1]

The metal-containing silicate is Na-type ZSM.
-5 (Cyban ratio = 30), the same amount of each active species as in the examples was supported in various amounts on one liter of the catalyst in terms of the metal-containing silicate, and this was washed onto the catalyst carrier. The catalysts of Comparative Examples 6 to 10 were prepared only by coating. The supported amounts of Pt, Ir, and Rh when the catalyst was 1 liter and the method of supporting the metal-containing silicate in Comparative Examples 6 to 10 are as shown in Table 2.

[0034]

[Table 2]

The catalysts of the present invention prepared in the above Examples and the catalysts of Comparative Examples 1 to 10 were used as samples, and NOx
The purification rate was measured, and the results are shown in Table 3.

The above NOx purification rate was determined for each catalyst sample.
The maximum NOx purification rate and the catalyst inlet gas temperature when the maximum NOx purification rate was expressed were measured for the initial activity expressed at the time of freshness and the activity after endurance after 800 ° C. × 8 hours. The measurement was conducted by using a model gas in a high oxygen concentration atmosphere corresponding to a lean burn of A / F = 22, flowing the sample through the catalyst at SV = 55000 h ⁻¹ , and at the same time until the temperature at the catalyst inlet gas reached 450 ° C. 30 ° C /
This is based on a method of measuring while heating at a rate of minutes.

[0037]

[Table 3]

According to the results shown in Table 3, the catalysts of the examples according to the present invention showed a superior NOx purification rate in the initial activity and the activity after endurance as compared with the catalysts of the other comparative examples, and showed a stable heat resistance. It can be seen that the property has been improved.

Further, the catalyst of the example according to the present invention has a slightly lower initial activity than that of the catalyst of Comparative Example 9, but its expression temperature is low, and it is excellent in the low temperature range of exhaust gas. It shows that it has catalytic activity.
The reason why the initial activity is slightly lowered is that the noble metal species of Pt and Ir supported by the impregnation method are slightly inferior in dispersibility, so that an overconcentration part may be generated in the surface noble metal concentration. It is presumed that a portion where the expression of the initial activity was weak occurred.

Further, even when Pt and Ir are impregnated and supported on the catalyst intermediate obtained by wash-coating the Rh-containing metal-containing silicate, as shown in Comparative Examples 4 and 5, When Ir and Ir are impregnated successively and not simultaneously, the above Pt and Ir have an excellent catalytic activity because it is difficult to be in a close support state, which is a condition for sufficiently exhibiting a desired function as a catalyst. Is not obtained.

When observing the catalytic activities of the catalysts of Comparative Examples 1 to 3, the active sites in the catalyst are formed in a portion where Pt and Ir are supported in close proximity or at least coexist. Obviously. This is because the catalysts of Comparative Examples 6 and 8 to 10 carrying Pt and Ir or Pt, Ir, and Rh are not thermally stable but have improved initial activity. Therefore, its relevance is recognized.

[0042]

As described above, according to the exhaust gas purifying catalyst according to the first aspect of the present invention, when the active species is carried on the metal-containing silicate, Rh is contained in the lower layer of Pt and Ir.
Are provided in the upper layer, respectively, so that Rh improves the thermal stability of the catalyst without being directly exposed to high heat on the surface of the catalyst, and Pt and Ir are supported in close proximity to each other so that active sites are formed on the upper layer of the catalyst. Is formed, so that the initial activity of the catalyst can be improved and the heat resistance can be improved.

According to the method for producing an exhaust gas purifying catalyst according to the second and third aspects of the present invention, the metal-containing silicate carrying Rh by dry-drying is optionally coated on the catalyst carrier by drying. Since a catalyst intermediate is formed and the catalyst intermediate is simultaneously impregnated with Pt and Ir by the mixed solution, the Rh is supported on the lower layer of the catalyst, and the Pt and Ir are supported on the upper layer of the catalyst in proximity to the upper layer of the catalyst. The exhaust gas purifying catalyst according to the first aspect of the present invention can be reliably manufactured.

Continuation of the front page (72) Inventor Hideharu Iwakuni 3-1, Shinchi, Fuchu-cho, Aki-gun, Hiroshima Mazda Co., Ltd. (56) References JP-A-4-59049 (JP, A) JP-A-3-262541 ( JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B01J 21/00-37/36 B01D 53/86

Claims (3)

(57) [Claims] An exhaust gas purifying catalyst comprising an active species supported on a metal-containing silicate in a layered manner, wherein a Rh active species is provided in a lower layer and a Pt active species and Ir are provided in an upper layer.
An exhaust gas purifying catalyst comprising an active species. 2. a Rh-supporting step of supporting Rh on the metal-containing silicate; a wash-coating step of wash-coating the Rh-supported metal-containing silicate obtained in the Rh-supporting step on a catalyst carrier, followed by drying and firing; A Pt and Ir supporting step of simultaneously impregnating and supporting Pt and Ir on a catalyst intermediate made of a Rh-supported metal-containing silicate obtained in a wash coat step, followed by drying and firing. Method for producing catalyst. 3. The exhaust gas according to claim 2, wherein the Rh supporting step comprises supporting the Rh on the metal-containing silicate by drying a mixture of the metal-containing silicate powder and the Rh-containing solution. A method for producing a gas purification catalyst.