JPH09299796A - Catalyst for purification of exhaust gas and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance catalytic activity by using α-Al2 O3 as a carrier and highly dispersing and carrying Rh. SOLUTION: An Rh salt soln. is impregnated into a carrier made of α-Al2 O3 , Rh is allowed to enter into solid soln. in the carrier by heat treatment at 800-1,000 deg.C in an oxidizing atmosphere and fine Rh particles of <=2nm particle diameter are deposited on the surface of the carrier by heat treatment at 700-1,000 deg.C in a reducing atmosphere.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to α-A as a carrier.
The present invention relates to an exhaust gas purifying catalyst in which l ₂ O ₃ is used and rhodium (Rh) as a catalytic precious metal is supported on the carrier. The exhaust gas-purifying catalyst of the present invention can efficiently purify NO _x in exhaust gas.

[0002]

2. Description of the Related Art Conventionally, a three-way catalyst has been used as an exhaust gas purifying catalyst for automobiles, which purifies the exhaust gas by simultaneously oxidizing CO and HC and reducing NO _x . As such a three-way catalyst, for example, a carrier layer made of γ-Al ₂ O ₃ is formed on a heat-resistant honeycomb substrate made of cordierite or the like, and a catalytic precious metal such as platinum (Pt) is carried on the carrier layer. The ones made to be widely known.

Further, in order to produce a conventional exhaust gas purifying catalyst, a carrier having a carrier layer made of γ-Al ₂ O ₃ is immersed in an aqueous solution of Pt salt or Rh salt, pulled up, dried and calcined. Pt or Rh is supported by an adsorption supporting method, or a water absorption supporting method of impregnating a predetermined amount of an aqueous solution of Pt salt or Rh salt, evaporating and drying to support. γ-Al
₂ O ₃ is also referred to as activated alumina, and is widely used as a catalyst carrier because it has a high specific surface area. As the catalytic noble metal, Pt, which has a high activity of oxidizing HC and CO,
Generally, Rh, which has an excellent activity of reducing NO _x by a reducing substance such as HC in exhaust gas, is used together.

However, although the catalyst in which Rh is supported on the γ-Al ₂ O ₃ carrier is excellent in the initial purification activity, Rh is solid-dissolved in the carrier in a high-temperature oxygen-excessive atmosphere, so that the catalyst active site on the surface of the carrier becomes It was revealed that there is a problem that the NO _x purification performance is significantly reduced due to the decrease. Therefore, it was conceived that Rh is supported on a carrier in which Rh does not form a solid solution, and one using zirconia (US Pat.
3189) and those using α-Al ₂ O ₃ (JP-A-55-41894).

Japanese Patent Laid-Open No. 55-41894 discloses Rh.
Is dissolved in γ-Al ₂ O ₃ in a high temperature excess oxygen atmosphere, but if α-Al ₂ O ₃ is used as a carrier, the solid solution is suppressed and the catalytic activity of Rh is maintained for a long time. Has been done.

[0006]

However, the specific surface area of zirconia and α-Al ₂ O ₃ is significantly lower than that of γ-Al ₂ O ₃ . Therefore, it is difficult to support Rh in α-Al ₂ O ₃ in a high dispersion by the conventional adsorption-supporting method or water-absorption supporting method, and therefore, the catalytic activity points are reduced and the catalytic activity is low from the beginning. There is a defect.

The present invention has been made in view of such circumstances, and it is an object of the present invention to enhance catalytic activity by supporting Rh in a highly dispersed manner using α-Al ₂ O ₃ as a carrier.

[0008]

The features of the exhaust gas purifying catalyst of the present invention as set forth in claim 1 for solving the above-mentioned problems are α-
An exhaust gas purifying catalyst comprising a carrier made of Al ₂ O ₃ and Rh carried on the carrier, wherein 50% or more of the carried amount of Rh is carried as fine particles having a particle diameter of 2 nm or less. .

The exhaust gas purifying catalyst according to claim 2 is characterized in that in the exhaust gas purifying catalyst according to claim 1,
The supported amount of Rh is 0.01 to 0.3 g per 120 g of the carrier.
Is to be. Further, the exhaust gas purifying catalyst according to claim 3 is characterized in that, in the exhaust gas purifying catalyst according to claim 1, the amount of Rh supported is 0.15 to 120 g of the carrier.
It is to be 0.3 g.

A feature of the method for producing an exhaust gas purifying catalyst of the present invention according to claim 4 which is capable of producing the exhaust gas purifying catalyst is that the carrier comprising α-Al ₂ O ₃ is impregnated with a Rh salt solution to impregnate Rh. Particles obtained by performing an impregnation step of forming a carrier, an oxidation treatment step of heat-treating an Rh-impregnated support at 800 to 1000 ° C. in an oxidizing atmosphere to form an oxidation treated carrier, and a heat treatment of the oxidation treated carrier at 700 to 1000 ° C. in a reducing atmosphere And a reduction treatment step of precipitating fine Rh particles having a diameter of 2 nm or less on the surface of the carrier.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION In the exhaust gas purifying catalyst of the present invention,
At least 50% of the supported amount of Rh is supported on the α-Al ₂ O ₃ carrier as fine particles having a particle diameter of 2 nm or less. Therefore, since Rh is supported in an extremely fine state and in a highly dispersed state, the active points of the catalyst are increased and a high NO _x purification performance is exhibited.

And α-Al ₂ O ₃ has excellent thermal stability,
Moreover, solid solution of Rh hardly occurs. Therefore, even when exposed to a high-temperature oxygen-excess atmosphere during use, Rh does not easily agglomerate, Rh is maintained in a highly dispersed state, and the durability of NO _x purification performance is excellent. When the amount of supported Rh having a particle size of more than 2 nm is 50% or more of the supported amount, the active sites of the catalyst decrease, and the NO _x purification performance deteriorates. The finer the particle diameter is 2 nm or less, the more desirable it is, and the more the fine particles are, the more desirable.

The amount of Rh supported is 0.
01-0.3g is a suitable range. When the amount of Rh carried is less than 0.01 g, sufficient NO _x purification performance cannot be obtained, and even when more than 0.3 g is carried, the NO _x purification performance is saturated and expensive Rh is wasted. Further, when the supported amount exceeds 0.3 g, the number of coarse particles having a particle diameter of 10 nm or more increases.

[0014] Incidentally, if the loading amount of Rh with 0.15~0.3g per carrier 120 g, it is possible to effectively use the most Rh, the particle size of most of the Rh becomes 2nm or less, particularly high _the NO _x purification performance Is obtained. In the method for producing an exhaust gas purifying catalyst of the present invention, first, α-Al
_An Rh-impregnated carrier is manufactured by impregnating a carrier composed of ₂ O ₃ with a Rh salt solution. Rh salts include rhodium nitrate (Rh
(NO ₃ ) ₃ ), rhodium chloride (RhCl ₃ ) and the like are used, and in the case of these salts, they are used as an aqueous solution. An organic solvent such as alcohol may be used depending on the type of Rh salt.

This Rh-impregnated carrier has an oxidation treatment step of 800 to 10 in an oxidizing atmosphere such as air.
It is heat-treated at 00 ° C. and becomes an oxidation-treated carrier. α-Al ₂
O ₃ has a low interaction with Rh and Rh does not easily form a solid solution,
In this oxidation treatment step, Rh is solid-soluted in the impregnated Rh in the depth range of 3 to 5 nm from the surface of the carrier. If the heat treatment temperature in the oxidation treatment step is less than 800 ° C., it becomes difficult for Rh to form a solid solution, and R on the carrier surface in the reduction treatment step in the next step becomes difficult.
Precipitation of h is also difficult, and thus it is difficult to make Rh fine. Further, when the heat treatment temperature exceeds 1000 ° C., the Rh particles become coarse and it becomes difficult to form a solid solution in α-Al ₂ O ₃ .

The heat treatment time in the oxidation treatment step is
1 to 3 hours is sufficient. If the heat treatment time is less than 1 hour, the solid solution of Rh becomes insufficient and it becomes difficult to precipitate Rh on the surface of the carrier in the subsequent reduction treatment step.
It becomes difficult to miniaturize h. Also, if you do it for 3 hours or more, Rh
Diffuses into the α-Al ₂ O ₃ carrier, making it difficult for Rh to precipitate in the subsequent reduction treatment. Also, heat energy is wasted.

The oxidation-treated carrier in which Rh is solid-solved in the oxidation treatment step is heat-treated at 700 to 1000 ° C. in a reducing atmosphere in the reduction treatment step. As a result, the solid solution Rh becomes fine particles having a particle diameter of 2 nm or less and is deposited on the surface of the carrier, and is highly dispersed and supported. As the reducing atmosphere, for example, a gas containing hydrogen gas in an inert gas is used. When the heat treatment temperature is lower than 700 ° C., it is difficult to precipitate Rh, and when the heat treatment temperature exceeds 1000 ° C., the fine Rh particles precipitated are coarsened.

The heat treatment time in the reduction treatment step is
1 to 3 hours is sufficient. When the heat treatment time is less than 1 hour, the precipitation of Rh is insufficient, and when the heat treatment is carried out for 3 hours or more, the fine Rh particles precipitated are coarsened. Also, heat energy is wasted.

[0019]

EXAMPLES The present invention will be specifically described below with reference to Examples and Comparative Examples. (Example 1) (1) Preparation of α-Al ₂ O ₃ Commercially available γ-Al ₂ O ₃ powder was prepared, and 120 in the air.
By heating at 0 ° C for 10 hours, a specific surface area of 7 m2 /
g of powdered α-Al ₂ O ₃ carrier was prepared. (2) Impregnation Step This α-Al ₂ O ₃ carrier is impregnated with a predetermined amount of a rhodium chloride aqueous solution having a predetermined concentration, the water content is evaporated to dryness, and then dried in air at 120 ° C. for 12 hours, and then at 600 ° C. A Rh-impregnated carrier was prepared by firing at 3 hours. The supported amount of Rh is 0.0 as metal Rh with respect to 120 g of α-Al ₂ O ₃ carrier.
It is 4 g. (3) Heat Treatment Step This Rh-impregnated support was heat-treated at 600 ° C. for 1 hour in nitrogen gas containing 5% by volume of hydrogen. This heat treatment is performed in order to reduce the impregnated and supported Rh compound into metal Rh. (4) Oxidation treatment step Next, an oxidation treatment step in which heat treatment temperatures of 600 ° C., 900 ° C., and 1200 ° C. are adopted, and the Rh-impregnated carrier after the heat treatment step is heat-treated in the atmosphere at each temperature for 1 hour I went.

The proportion of Rh in solid solution was determined by X-ray photoelectron spectroscopy (XPS), and the results are shown in FIG. The use of a γ-Al ₂ O ₃ carrier in place of the α-Al ₂ O ₃ carrier, likewise impregnation process, by measuring the ratio of Rh was dissolved Similarly for having been subjected to the heat treatment step and the oxidizing step, The results are also shown in FIG. From FIG. 1, α-Al ₂ O
_{With 3} carriers, the heat treatment temperature in the oxidation treatment process is 800
When the temperature is lower than ° C, the solid solution ratio of Rh is 80% or less.
Therefore, it is preferable to perform the heat treatment at a temperature of 800 ° C. or higher in the oxidation treatment step.

Therefore, in this example, the heat treatment temperature of the oxidation treatment step was set to 1000 ° C., and the Rh-impregnated support after the heat treatment step was heat-treated at 1000 ° C. for 1 hour in the atmosphere to obtain an oxidation treatment support. (4) Reduction treatment step Next, five levels of heat treatment temperatures were adopted between 400 and 800 ° C., and the above oxidation treatment carrier was added at a temperature of 0.1 atm of nitrogen gas containing 5% by volume of hydrogen gas at each temperature. Then, a reduction treatment step of performing heat treatment for 10 minutes was performed.

The proportion of Rh deposited was determined by X-ray photoelectron spectroscopy (XPS), and the results are shown in FIG. The use of a γ-Al ₂ O ₃ carrier in place of the α-Al ₂ O ₃ carrier, likewise impregnation step, heat treatment step, the ratio of Rh precipitated Similarly for having been subjected to the oxidation treatment process and the reduction treatment process It measured and the result is also shown in FIG. As shown in FIG. 2, α-Al ₂ O ₃ is more γ-Al in the reduction process.
_It can be seen that Rh in solid solution is more likely to precipitate than ₂ O ₃ . Further, in the α-Al ₂ O ₃ carrier, when the heat treatment temperature in the reduction treatment step is less than 700 ° C., the Rh precipitation rate is 50%.
It is as follows. Therefore, in the reduction process, 70
It can be seen that it is preferable to perform the heat treatment at a temperature of 0 ° C. or higher.

Therefore, in this embodiment, the heat treatment temperature in the reduction treatment step is set to 800 ° C., and the oxidation-treated support is heat treated in nitrogen gas containing 5% by volume of hydrogen gas at 800 ° C. for 1 hour to obtain the heat treatment temperature of the present embodiment. An exhaust gas purifying catalyst was obtained. (Example 2) The supported amount of Rh was changed to α-Al ₂ O ₃ carrier 120.
An exhaust gas-purifying catalyst of Example 2 was obtained in the same manner as in Example 1 except that the amount of g was 0.08 g.

(Example 3) The supported amount of Rh was changed to α-Al ₂ O.
An exhaust gas-purifying catalyst of Example 3 was obtained in the same manner as in Example 1 except that the amount of 0.10 g was changed with respect to 120 g of ₃ carriers. (Example 4) The supported amount of Rh was changed to α-Al ₂ O ₃ carrier 120.
An exhaust gas-purifying catalyst of Example 4 was obtained in the same manner as in Example 1 except that 0.20 g was used for g.

(Embodiment 5) The amount of Rh carried is α-Al ₂ O.
₃ except that the 0.30g against carrier 120g in the same manner as in Example 1 to obtain an exhaust gas purifying catalyst of Example 5. (Example 6) The supported amount of Rh was changed to α-Al ₂ O ₃ carrier 120.
An exhaust gas-purifying catalyst of Example 6 was obtained in the same manner as in Example 1 except that 0.50 g was used for g.

Comparative Example 1 An exhaust gas purifying catalyst of Comparative Example 1 was obtained in the same manner as in Example 1 except that the oxidation treatment step and the reduction treatment step were not performed. (Comparative Example 2) The amount of Rh supported was changed to α-Al ₂ O ₃ carrier 120.
An exhaust gas-purifying catalyst of Comparative Example 2 was obtained in the same manner as in Example 1 except that the amount of g was 0.10 g and that the oxidation treatment step and the reduction treatment step were not performed.

(Comparative Example 3) The supported amount of Rh was changed to α-Al ₂ O.
An exhaust gas-purifying catalyst of Comparative Example 3 was obtained in the same manner as in Example 1, except that the amount of 0.30 g per 120 g of ₃ carriers and that the oxidation treatment step and the reduction treatment step were not performed. (Test / Evaluation) Each of the exhaust gas-purifying catalyst powders was compacted into a pellet catalyst having a particle size of 0.5 to 1.0 mm. Then, for each pellet catalyst, the evaluation gas shown in Table 1 was used, the catalyst amount was 1.0 g, and the gas flow rate was 3.3 L / min.
Under the conditions of, the temperature of the incoming gas should be 5 from 100 ℃ to 500 ℃
The NO _x purification rate was measured while raising the temperature at a rate of ° C / min. Then, the respective 50% purification temperatures (initial) were obtained, and the results are shown in Table 3.

[0028]

[Table 1] The electron micrographs of the surfaces of the exhaust gas-purifying catalysts of Example 5 and Comparative Example 3 are shown in FIGS. 3 and 4, respectively. From FIG. 3, in the exhaust gas purifying catalyst of Example 5, the particle size is 1 to 2n.
It is observed that Rh particles of m are deposited on the α-Al ₂ O ₃ surface. However, in Comparative Example 3, it can be seen that the particle size of the Rh particles is coarsened to 5 to 10 nm.

In this way, the particle size of the Rh particles deposited on the α-alumina carrier for each exhaust gas-purifying catalyst was measured by observing with an electron microscope, and the results are shown in Table 3. Next, while alternately flowing the oxidizing gas and the reducing gas shown in Table 2 in a cycle of 10 minutes, each pellet catalyst was treated under the conditions of a catalyst amount of 2.0 g, a gas flow rate of 1.0 L / min, and a gas temperature of 1000 ° C. An endurance test of treating for 5 hours was performed, and then the NO _x purification rate was measured in the same manner as above. Then, the respective 50% purification temperatures (after endurance) were determined, and the results are shown in Table 3.

[0030]

[Table 2]

[0031]

[Table 3] From Table 3, it can be seen that the exhaust gas-purifying catalysts of Examples 1 to 6 have lower 50% NO _x purification temperatures at the initial stage and after durability than those of Comparative Examples 1 to 3, and are excellent in NO _x purification performance.
This is because the supported Rh particle size is 2 nm in the example.
In contrast to the following, it is considered that this is due to the large difference of 5 to 30 nm in the comparative example.

The degree of decrease in the NO _x purification rate by the durability test was small in both the examples and the comparative examples, and α-Al ₂ O
Both of the effects obtained by using ₃ were excellent, and no defects due to the finer Rh particle size were observed in the examples. In Example 6, since the amount of Rh supported was too large, there were Rh particles with a particle size of 10 nm, and this coarse R
The h particles hardly contribute to the NO _x purification performance and have a problem in cost. Therefore, the amount of Rh supported was 0.
30 g or less is suitable.

[0033]

That is, according to the present invention, α-Al ₂ O
_Since Rh can be carried in a highly dispersed manner while using ₃ carriers, the catalyst has many active sites and can efficiently purify NO _x .

[Brief description of drawings]

FIG. 1 is a graph showing a relationship between a heat treatment temperature and an Rh solid solution ratio in an oxidation treatment step.

FIG. 2 is a graph showing the relationship between the heat treatment temperature and the Rh precipitation rate in the reduction treatment step.

3 is an electron micrograph showing Rh particles of the exhaust gas purifying catalyst of Example 5. FIG.

FIG. 4 is an electron micrograph showing Rh particles of an exhaust gas purifying catalyst of Comparative Example 3.

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[Procedure amendment]

[Submission date] May 24, 1996

[Procedure amendment 1]

[Document name to be amended] Drawing

[Correction target item name] Figure 3

[Correction method] Change

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[Figure 3]

[Procedure amendment 2]

[Document name to be amended] Drawing

[Correction target item name] Fig. 4

[Correction method] Change

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FIG. 4

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Noriyuki Suzuki, Aichi Prefecture, Nagakute-cho, Aichi-gun, Nagakute-cho 1 41, Yokomichi, Toyota Central Research Institute Co., Ltd. (72) Inventor Hideo Sofugawa, Nagakute-cho, Aichi-gun, Aichi 1 in 41 Chuo Yokoido, Central Research Institute, Toyota Central Research Institute Co., Ltd. (72) Inventor Masayuki Fukui Nagakute-cho, Aichi-gun, Aichi 1 in 41, Yokochi Central Research Institute, Toyota Central Research Institute (72) Inventor, Akira Morikawa 1 of 41 Yokomichi, Nagakute, Nagakute-cho, Aichi-gun, Aichi Prefecture Toyota Central Research Institute Co., Ltd.

Claims (4)

[Claims] 1. A catalyst for exhaust gas purification comprising a carrier composed of α-Al ₂ O ₃ and rhodium supported on the carrier, wherein 50% or more of the supported amount of rhodium has a particle diameter of 2 nm or less. An exhaust gas purifying catalyst, which is carried as fine particles. 2. The amount of the rhodium carried is the carrier 120.
The exhaust gas-purifying catalyst according to claim 1, wherein the amount is 0.01 to 0.3 g per g. 3. The amount of rhodium carried is the carrier 120.
The exhaust gas-purifying catalyst according to claim 1, which is 0.15 to 0.3 g per g. 4. An impregnating step of impregnating a carrier made of α-Al ₂ O ₃ with a rhodium salt solution to obtain a Rh-impregnated carrier, and the Rh impregnated carrier.
An oxidation treatment step in which the impregnated carrier is heat-treated at 800 to 1000 ° C. in an oxidizing atmosphere to be an oxidation treated carrier, and the oxidation treated carrier is heat treated at 700 to 1000 ° C. in a reducing atmosphere to form fine particles having a particle diameter of 2 nm or less. A method for producing an exhaust gas-purifying catalyst, which comprises a reduction treatment step of depositing rhodium particles on the surface of the carrier.