JPH06210174A - Catalyst for purification of exhaust gas and its production - Google Patents

Abstract

PURPOSE:To provide a catalyst having satisfactory purification activity even in the condition of a low temp. of exhaust gas and excellent in heat resistance, as well. CONSTITUTION:The objective catalyst has palladium allowed to coexist with a mixture of a perovskite type multiple oxide with a heat resistant oxide. A compsn. represented by a general formula Ce1-ySiyO2 (0<y<1) is used as the heat resistant oxide. When the catalyst is produced, an aq. palladium salt soln. of <=pH4 or >pH10 is carried on the above-mixture by impregnation or adsorption and the mixture is dried and fired.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to an exhaust gas purifying catalyst excellent in carbon monoxide (CO), hydrocarbon (HC) and nitrogen oxide (NOx) purifying ability, particularly in an automobile gasoline engine, etc. The present invention relates to an exhaust gas purifying catalyst that exhibits purifying activity even under low exhaust gas temperature, and a method for producing the same.

[0002]

2. Description of the Related Art As a three-way catalyst for purifying exhaust gas, a noble metal catalyst in which a noble metal such as Pt, Rh, or Pd is supported on an alumina carrier has been put into practical use and widely used. Further, a complex oxide having a perovskite structure composed of a rare earth metal, an alkaline earth metal and a transition metal is CO, H
Practical use is expected as an inexpensive three-way catalyst for purifying C and NOx for exhaust gas (Japanese Patent Laid-Open No. 59-8704).
No. 6, JP-A-60-82138). Although this perovskite type composite oxide has excellent CO and HC purification ability, it is slightly inferior in NOx purification ability and is not sufficient for practical use as a three-way catalyst for automobile exhaust gas. Therefore, NO of the perovskite type complex oxide catalyst
It has also been proposed that a noble metal coexists in order to enhance the x purification capacity (see Japanese Patent Laid-Open No. 1-168343).

[0003]

These catalysts show excellent purifying activity under conditions where the exhaust gas temperature is high, such as when a vehicle is running, but exhibit sufficient purifying activity under conditions where the exhaust gas temperature is low, such as during idling. Not shown. With the tightening of exhaust gas regulations, there is a demand for a catalyst that exhibits sufficient purification activity even under such low exhaust gas temperature conditions. It is an object of the present invention to provide an exhaust gas purifying catalyst that exhibits sufficient purifying activity even under conditions where the exhaust gas temperature is low and that has excellent heat resistance, and a method for producing the same.

[0004]

The exhaust gas-purifying catalyst of the present invention has a general formula Ln _1-x A _x MO ₃ (Ln is one or more rare earth metal except Ce, and A is an alkali except Sr. One or more kinds of earth metals and Ce, M is one or more kinds of transition metals and noble metals, 0 <x <1)
And a perovskite complex oxide represented by the general formula Ce
_A noble metal is allowed to coexist in a composite oxide or solid solution composed of a heat resistant oxide represented by _1-y Si _y O ₂ (0 <y <1).

In a preferred embodiment, the perovskite-type composite oxide has a structure in which a perovskite-type composite oxide containing noble metal as a core has a perovskite-type composite oxide formed around it as a solid solution. When the noble metal is present in excess, the noble metal which has not been dissolved completely is dispersed as a metal or an oxide in the form of fine particles. Precious metal is P
It is one or more metals selected from the group consisting of d, Pt, Ru, Rh and Ir, and Pd is particularly preferable because it improves low-temperature purification activity and NOx purification activity.

It is known that the composition represented by the general formula Ce _1-y Si _y O ₂ (0 <y <1) is an oxide having a stable specific surface area even at high temperature (Japanese Patent Laid-Open No. 62-62). See Japanese Patent No. 56322). In the present invention, this composition is used as a heat-resistant oxide in order to increase the heat resistance of the perovskite-type composite oxide.

In one embodiment of the production method of the present invention, the general formula L
n _1-x A _x MO ₃ (Ln is one or more rare earth metals other than Ce, A is an alkaline earth metal other than Sr and Ce
1 or 2 or more of them, M is 1 or 2 or more of a transition metal and a noble metal, a perovskite complex oxide represented by 0 <x <1), and a general formula Ce _1-y Si _y O
₂ (0 <y <1) A complex oxide or a solid solution consisting of a heat-resistant oxide is impregnated or adsorbed with a noble metal salt aqueous solution adjusted to pH 4 or below, and dried. It is baked later. When prepared by adjusting the pH to 4 or less, the perovskite-type composite oxide is an oxide in which the perovskite-type composite oxide surrounds the perovskite-type composite oxide in which the noble metal is solid-soluted.

In the case of the production method in which the pH of the aqueous solution of the noble metal salt is 4 or less, the water-soluble noble metal salt is PdCl ₂ , P
Chlorides such as tCl ₂ and RuCl ₃ 3H ₂ O, Pd (N
O ₃ ) ₂ , Ru (NO ₃ ) ₃ , Rh (NO ₃ ) ₃ and other nitrates, P
A dinitrodiamine salt such as d (NO ₂ ) ₂ (NH ₃ ) ₂ or Pt (NO ₂ ) ₂ (NH ₃ ) ₂ is preferably one whose aqueous solution exhibits strong acidity.

In another embodiment of the production method of the present invention, a perovskite type complex oxide represented by the general formula Ln _1-x A _x MO ₃ (A in this case includes Sr) and the general formula Ce _1-y Si _y
A complex oxide or solid solution consisting of a heat-resistant oxide represented by O ₂ is impregnated or adsorbed with an aqueous solution of a noble metal salt prepared to have a pH of more than 10, dried and baked.

In the case of the production method in which the pH of the aqueous solution of the noble metal salt is made higher than 10, tetraamine palladium dichloride Pd (NH ₃ ) ₄ Cl ₂ and tetraamine palladium hydrochloride Pd (NH ₃ ) ₄ (OH) ₂ are used. Ammonia water or acid is added to a basic aqueous solution so as to be adjusted to pH> 10, or a chloride such as PdCl ₂ , PtCl ₂ , RuCl ₃ .3H ₂ O, Pd (NO ₃ ) ₂ or Ru is used. _{(NO 3) 3, Rh (} N
Nitrate such as O ₃ ) ₃ or Pd (NO ₂ ) ₂ (NH ₃ ) ₂ , Pt
Ammonia water is added to an acidic aqueous solution of a dinitrodiamine salt such as (NO ₂ ) ₂ (NH ₃ ) ₂ to prepare a pH> 10 for use.

[0011]

The catalyst of the present invention exhibits excellent purifying activity against HC, CO, and NOx even under a low exhaust gas temperature of about 100 to 200 ° C. during idling. Further, as a heat-resistant oxide, a general formula Ce _1-y Si _y O ₂ (0 <
Since it contains the composition represented by y <1), 900 ° C.
The catalyst has high heat resistance and can be used even at the above high temperatures.

[0012]

【Example】

(Example 1) Step A:. Perovskite-type composite oxide crystalline powder manufacturing method perovskite-type composite oxide _{(.. La 0 8 Ce 0} 2) (Co 0 4
Fe _{0. 6)} the O ₃ powder preparation methods will be described. Lanthanum nitrate 103.9g, cerium nitrate 26.1g, cobalt nitrate 3
An aqueous solution prepared by dissolving 4.9 g and iron nitrate 72.7 g in pure water.
I prepared 3 liters. Next, 0.5 liter of an aqueous solution in which 50 g of sodium carbonate was dissolved was prepared as a neutralizing coprecipitant. The neutralizing coprecipitant was added dropwise to the above aqueous solution to obtain a coprecipitate. The coprecipitate was thoroughly washed with water, filtered, and dried under vacuum. This is baked at 600 ° C for 3 hours in the air and then crushed,
Thereafter, subjected to calcination for 3 hours in air at 800 ° C., it was further pulverized to prepare a _{(La 0. 8 Ce 0.} 2) (Co 0. 4 Fe 0. 6) O 3 powder.

Procedure B : Preparation of heat-resistant oxide 20 parts by weight of pure water was added to 97.5 parts by weight of ceric oxide CeO _{2 to} prepare an aqueous solution of tetraethylammonium silicate (SiO ₂ / (C ₂ O ₅ ) ₄ N +). A molar ratio equal to 1 and a SiO ₂ concentration of 116 g / liter) were weighed out to 2.5 parts by weight of SiO ₂ and stirred thoroughly. After that, it was dried at 120 ° C. for 12 hours, calcined at 350 ° C. for 3 hours, crushed with an agate mortar, passed through a 180 μm mesh, and C containing 2.5 wt% of SiO ₂ was added.
An eO ₂ powder was obtained.

[0014] Step C: slurry coating _{(carrying) (.. La 0 8 Ce} 0 2) (.. Co 0 4 Fe 0 6) O 3 perovskite-type composite oxide powder 50 parts by weight, prepared in Step B SiO
50 parts by weight of _2- containing CeO ₂ powder, ceria sol (solid content 10
50% by weight (solid content: 5 parts by weight), zirconia sol (solid content: 30% by weight): 3.3 parts by weight (solid content: 1 part by weight), so that the total solid content becomes 50% by weight. 58.7 parts by weight of pure water was added and pulverized with a ball mill for 12 hours while mixing to obtain a slurry. After allowing the slurry to flow into the cordierite honeycomb, the excess slurry was blown off with an air stream to coat it uniformly. The slurry-coated honeycomb was dried at 120 ° C. for 12 hours and fired in air at 600 ° C. for 3 hours to obtain a honeycomb sample.

Procedure D : Addition of Pd 25 parts by weight of a palladium nitrate solution (Pd concentration: 4.4 wt%) and 1000 parts by weight of pure water were mixed so that the Pd content was 1.1 parts by weight, and the pH of the solution was adjusted. Prepared to 1.8. The honeycomb-shaped sample prepared in procedure C was dipped in the prepared palladium nitrate solution, kept at 40 ° C. for 2 hours to adsorb Pd, and then dried at 120 ° C. for 12 hours, and at 600 ° C. in air. The catalyst sample of Example 1 was obtained by firing for 3 hours.

Example 2 The tetraethylammonium silicate aqueous solution used in the procedure B of Example 1 was converted into CeO ₂ 99.
5 parts by weight of SiO ₂ was weighed to be 0.5 parts by weight, and otherwise the same operation as in Example 1 was performed.
₂ to obtain a CeO ₂ powder containing 0.5 wt%. Using the obtained CeO ₂ powder, a catalyst sample of Example 2 was obtained in the same manner as in Example 1.

Comparative Example A Pt—Rh / Al ₂ O ₃ catalyst, which is a catalyst for automobiles that has already been put into practical use, was used as a catalyst sample of a comparative example. The Pt-Rh content was 0.43 parts by weight. The compositions of Examples and Comparative Examples are shown in Table 1, and the measurement results of the respective catalytic activities are shown in Table 2.

[0018]

[Table 1]

[0019]

[Table 2]

The measurement of the catalytic activity and the durability test were carried out as follows. Measurement of catalytic activity Each sample supported on a honeycomb-shaped (cell number 400 / inch ² ) cordierite carrier (diameter 30 mm, length 50 mm) was measured for activity with the following model gas. The gas temperature is indicated by the gas temperature at the entrance to the catalyst, and the temperature is raised from room temperature,
The temperature at which each of NO, CO, and HC (C ₃ H ₆ + C ₃ H ₈ ) has dropped to 50% of the initial concentration is defined as the 50% purification temperature.

Further, the rich gas and the lean gas are switched every one second. Space velocity of the gas flow through the catalyst (S
V) was set at 30,000 / hour.

Rich gas Lean gas CO 2.6% 0.7% HC (concentration as C ₁ ) 0.19% 0.19% H ₂ 0.87% 0.23% CO ₂ 8% 8% NO 0.17% 0.17% O ₂ 0.65% 1.8% H ₂ O 10% 10% N ₂ balance balance

Durability test 9 times by switching the rich gas and the lean gas every 5 seconds.
A cycle of 30 minutes at 00 ° C. and 30 minutes at 750 ° C. was repeated 15 times to perform a durability test. After the durability test, the catalyst activity was measured by the above method. As is clear from the results in Table 2, in each of the examples, 5 was obtained at the initial stage and after the endurance.
The 0% purification temperature is low and is high in the comparative example.

Claims (4)

[Claims] _1. A general formula Ln _1-x A _x MO ₃ (Ln is one or more rare earth metals except Ce, and A is one or two alkaline earth metals except Sr and Ce. Above, M
Is one or more of transition metals and noble metals, 0 <
A noble metal coexists in a complex oxide or solid solution composed of a perovskite-type complex oxide represented by x <1) and a heat-resistant oxide represented by the general formula Ce _1-y Si _y O ₂ (0 <y <1). An exhaust gas purifying catalyst characterized by 2. The exhaust gas-purifying catalyst according to claim 1, wherein the perovskite-type composite oxide has a perovskite-type composite oxide as a nucleus and a perovskite-type composite oxide in which a noble metal is formed as a solid solution is formed around the nucleus. . 3. The general formula Ln _1-x A _x MO ₃ (Ln is one or more rare earth metals except Ce, and A is one or two alkaline earth metals except Sr and Ce. Above, M
Is one or more of transition metals and noble metals, 0 <
x has a pH of a complex oxide or solid solution composed of a perovskite-type complex oxide represented by x <1) and a heat-resistant oxide represented by the general formula Ce _1-y Si _y O ₂ (0 <y <1). A method for producing an exhaust gas-purifying catalyst, comprising impregnating or adsorbing a noble metal using an aqueous solution of a noble metal salt adjusted to 4 or less, supporting the noble metal thereon, and calcining after drying. 4. A compound represented by the general formula Ln _1-x A _x MO ₃ (Ln is one or more kinds of rare earth metals except Ce, and A is one or two kinds of alkaline earth metals except Sr and Ce. Above, M
Is one or more of transition metals and noble metals, 0 <
x has a pH of a complex oxide or solid solution composed of a perovskite-type complex oxide represented by x <1) and a heat-resistant oxide represented by the general formula Ce _1-y Si _y O ₂ (0 <y <1). 10
A method for producing an exhaust gas-purifying catalyst, which comprises impregnating or adsorbing a noble metal using an aqueous solution of a noble metal salt adjusted to a larger degree, supporting the noble metal, and calcining after drying.