JPH06210175A - 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 durability, 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 so that palladium is present at a higher concn. on the perovskite type multiple oxide than that on the heat resistant oxide. When the catalyst is produced, an aq. palladium salt soln. of <=pH4 or >pH10 is carried on the above- perovskite type multiple oxide by impregnation or adsorption, the oxide is dried, calcined and dispersed in water together with the heat resistant oxide and the resulting slurry 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 having an excellent purifying ability for carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx), particularly in a gasoline engine for automobiles, etc. The present invention relates to an exhaust gas purifying catalyst that exhibits purifying activity even under conditions where the exhaust gas temperature is low, 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. In order to improve the heat resistance of the perovskite type composite oxide catalyst, it is possible to coexist with a heat resistant oxide. When palladium is intended to coexist as a noble metal in a mixture of a perovskite type complex oxide and a heat resistant oxide, it may be considered that the mixture of oxides is impregnated with an aqueous solution of a palladium salt and then fired. The perovskite complex oxide has a specific surface area of 5 to
50 m ² / g, whereas it is also 100 m ² / g be large, the specific surface area is used as the heat-resistant oxides such as ceria composite oxide is large, such as 100 to 250 m ² / g. When the mixture of oxides is impregnated with the aqueous solution of palladium salt, palladium is adsorbed on the powder of each oxide at an adsorption rate substantially proportional to the specific surface area. That is, the noble metal for enhancing the NOx purification capacity is adsorbed at a higher concentration on the heat-resistant oxide than on the perovskite-type composite oxide.

In order to improve both properties at low temperature activity and durability, it is desirable that a noble metal such as palladium be adsorbed at a higher concentration on the perovskite type complex oxide. An object of the present invention is to provide an exhaust gas purifying catalyst that exhibits sufficient purifying activity even under conditions of low exhaust gas temperature and is excellent in durability, and a method for producing the same.

[0005]

In the exhaust gas purifying catalyst of the present invention, the general formula Ln _1-x A _x MO ₃ (Ln is a rare earth metal excluding Ce, A is an alkaline earth metal or Ce, and M is a transition metal. In each case, noble metal coexists in the mixture of the perovskite-type complex oxide represented by 0 <x <1) and the heat-resistant oxide, and the noble metal is more than the above heat-resistant oxide. Also exists in a high concentration on the perovskite complex oxide. In a preferred embodiment, the perovskite-type composite oxide has a structure in which a perovskite-type composite oxide having no precious 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. The noble metal is one or more metals selected from the group consisting of Pd, Pt, Ru, Rh, and Ir, and Pd is particularly preferable because it improves low-temperature purification activity and NOx purification activity.

The type of heat-resistant oxide is not particularly limited, and examples thereof include CeO ₂ and (CeZr) O ₂ .
(CeZrY) O ₂ , (CeZrLa) O ₂ , (CeZr
Nd) O _{2 and} other general formulas (CeZrLn) O ₂ (Ln is C
Complex oxides represented by rare earth metals other than e) are preferred. Further, (CeZr) O ₂ is superior to CeO _{2 in the} effect of enhancing the purification activity at high temperature, and (CeZrL)
n) O ₂ is more preferable because it is more effective in enhancing the purifying activity at high temperatures. In addition, for example, cerium oxide as a main component and at least one metal oxide selected from the group consisting of Al, Si, Zr, Th, and rare earth metal elements (Japanese Patent Laid-Open No. 62-56322).
(See Japanese Patent Publication).

In one embodiment of the production method of the present invention, the general formula L
n _1-x A _x MO ₃ (Ln is a rare earth metal except Ce, A is S
Alkaline earth metals other than r or Ce and M are transition metals,
In each case, one or more kinds of perovskite-type composite oxides represented by 0 <x <1) were impregnated or adsorbed with an aqueous solution of a noble metal salt having a pH of 4 or less, dried, and calcined. Then, it is dispersed in water together with the heat resistant oxide to form a slurry, which is then dried and fired. 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 precious metal salt aqueous solution is 4 or less, Pd is used as the water-soluble precious metal salt.
Chlorides such as Cl ₂ , PtCl ₂ , RuCl ₃ .3H ₂ O,
Pd (NO ₃ ) ₂ , Ru (NO ₃ ) ₃ , Rh (NO ₃ ) ₃ and other nitrates, Pd (NO ₂ ) ₂ (NH ₃ ) ₂ , Pt (NO ₂ ) ₂ (NH ₃ ) ₂ and other dinitro It is preferable that the aqueous solution exhibits strong acidity such as a diamine salt.

In another embodiment of the production method of the present invention, the perovskite-type composite oxide represented by the general formula Ln _1-x A _x MO ₃ (where A contains Sr) has a pH of more than 10. The precious metal salt aqueous solution thus prepared is supported by impregnation or adsorption, dried and calcined, and then dispersed in water together with the heat resistant oxide to form a slurry, which is then dried and calcined. In the case of the production method in which the pH of the aqueous solution of a noble metal salt is made higher than 10, a basic aqueous solution such as tetraamine palladium dichloride Pd (NH ₃ ) ₄ Cl ₂ or tetraamine palladium hydrochloride Pd (NH ₃ ) ₄ (OH) _{2 is used.} Ammonia water or an acid is added to the solution to prepare a pH> 10, or a chloride such as PdCl ₂ , PtCl ₂ , RuCl ₃ .3H ₂ O, Pd (NO ₃ ) ₂ or Ru (NO _{3) is used.} ) ₃ , Rh (NO ₃ ) ₃
Nitrate such as Pd (NO ₂ ) ₂ (NH ₃ ) ₂ , Pt (N
Ammonia water is added to an acidic aqueous solution of a dinitrodiamine salt such as O ₂ ) ₂ (NH ₃ ) ₂ to prepare a pH of> 10.

[0009]

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. In addition, since it can be used at a high temperature of 900 ° C. or higher including a heat-resistant oxide, and a noble metal effective for purifying NOx is present in a higher concentration on the perovskite-type composite oxide than on the heat-resistant oxide. It becomes a durable catalyst.

[0010]

【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 : Production of heat-resistant oxide The heat-resistant oxide used as a co-catalyst is a commercially available cerium oxide powder having a high specific surface area (CeO ₂ specific surface area 170 m ² / g, purity 99.9% / TREO (all rare earths). Oxide)) 111.9
g, and zirconium oxynitrate (ZrO (N
O ₃ ) ₂ ) aqueous solution (liquid specific gravity 1.51, _{2 in} the liquid in terms of ZrO 2
5.0% by weight) 147.9 g, and yttrium nitrate (Y (NO ₃ ) ₃ ) aqueous solution (liquid specific gravity 1.62, Y ₂ O in the liquid).
₍ 21.7% by weight in terms of ₃ ) is added 26.0 g,
It was dried in the air at 110 ° C. for 10 hours while being well stirred and mixed. Thereafter, conducted for 3 hours at 600 ° C. in _{air, (Ce 0. 65 Zr 0} . 30 Y 0. 05) about 1 O ₂ composite oxide
Obtained 50 g.

Procedure C : Containing Pd in Perovskite Powder 25 parts by weight of a palladium nitrate solution (Pd concentration 4.4% by weight) and 50 parts by weight of pure water were mixed so that the Pd content was 1.1 parts by weight, The pH of the liquid was adjusted to 1.8. Procedure prepared in _{A (La 0. 8 Ce 0} . 2) (Co 0. 4 Fe 0. 6) well were added and stirred deionized water 20 parts by weight of O ₃ perovskite-type composite oxide powder 98.9 parts by weight After that, impregnate with 75 parts by weight of the above Pd salt aqueous solution adjusted to pH 1.8 and sufficiently stir it to
Hold at 0 ° C for 30 minutes. Then, continue stirring 1
After drying at 20 ° C. for 12 hours and baking in air at 600 ° C. for 3 hours, it was ground in an agate mortar and passed through a 180 μm mesh. Pd added to 98.9 parts by weight of the perovskite powder corresponds to 1.1 parts by weight in terms of metal content.

Procedure D : Slurry coating (support) 50 parts by weight of the Pd-containing perovskite powder obtained in Procedure A, 50 parts by weight of the heat-resistant oxide powder produced in Procedure B,
50 parts by weight of ceria sol (solid content 10 wt%) (5 parts by weight of solid content), and zirconia sol (solid content 30 wt%)
%) To 3.3 parts by weight (1 part by weight of solid content), and 58.7 parts by weight of pure water so that the total solid content is 50% by weight, and mixed while pulverizing for 12 hours in a ball mill. A slurry was obtained. After allowing the slurry to flow into a cordierite honeycomb, the excess slurry was blown off with an air stream to uniformly coat the slurry. 1 honeycomb after slurry coating
It was dried at 20 ° C. for 12 hours and fired in air at 600 ° C. for 3 hours to obtain a honeycomb sample.

Procedure E : Re-addition of Pd Furthermore, 12.5 parts by weight of the same palladium nitrate solution (Pd concentration 4.4 wt%) used in Procedure C was mixed with 50 parts by weight of pure water to adjust the pH of the solution. Prepared to 1.8. The honeycomb-shaped sample was immersed in the whole amount of this solution, kept at 40 ° C. for 2 hours to adsorb Pd, dried at 120 ° C. for 12 hours, and then baked in air at 600 ° C. for 3 hours. 1
A catalyst sample of was obtained.

The analysis result of this catalyst sample by EPMA (electron beam microanalyzer) is shown in the photograph of FIG. In the photograph, the image is an image by scanning electron microscope (SEM), black large particles are perovskite type complex oxides,
The size is about 10 μm. The white small particles are refractory oxide particles and have a size of about 3 μm. A line that traverses the center in the horizontal direction is a reference line indicating the position where line analysis was performed, and the waveform represents the Pd concentration on the analysis line.
According to the result of this photograph, the Pd concentration in the outer peripheral portion of the perovskite type complex oxide particle is higher than that of the other.

Example 2 The palladium nitrate solution used in the procedure E of Example 1 was changed to tetraammine palladium nitrate (Pd concentration 4.6 wt%), and 50 parts by weight of aqueous ammonia was added to adjust the pH to 13.0. A catalyst sample of Example 2 was obtained in the same manner as in Example 1 except for the above.

(Example 3) Operations similar to those of steps A to C of example 1 except that the palladium nitrate solution used in step C of example 1 was changed to tetraamminepalladium nitrate (Pd concentration 4.6 wt%). Thus, a powder was obtained in which Pd was added in an amount corresponding to 1.1 parts by weight in terms of metal content with respect to 98.9 parts by weight of the perovskite type complex oxide powder. Then, by the same operation as in Example 1, a catalyst sample of Example 3 was obtained.

Example 4 The palladium nitrate solution used in the procedure C of Example 1 and the palladium nitrate solution used in the procedure E were both changed to tetraammine palladium nitrate (Pd concentration 4.6 wt%), and the other examples were used. A catalyst sample of Example 4 was obtained in the same manner as in 1.

Example 5 In the procedure C of Example 1,
Example 1 except that 50 parts by weight of the palladium nitrate solution was weighed so that the Pd content was 2.2 parts by weight and the procedure E was omitted.
A catalyst sample of Example 5 was obtained by the same operation as above.

Example 6 In the procedure C of Example 1,
The palladium nitrate solution was replaced with tetraammine palladium nitrate (Pd concentration 4.6 wt%), 47.8 parts by weight was weighed so that the Pd content was 2.2 parts by weight, and 50 parts by weight of aqueous ammonia was added to obtain pH = 13. A catalyst sample of Example 6 was obtained by the same procedure as in Example 1 except that the procedure was omitted and the procedure E was omitted.

[0021] (Example 7) perovskite-type composite oxide used in Example _{4 (La 0. 8 Ce 0} . 2) (Co 0. 4 Fe 0. 6) O
₃ powder _{(La 0. 8 Sr 0.} 2) (Co 0. 4 Fe 0. 6) instead of the O ₃ powder, others to obtain a catalyst sample of Example 7 in the same manner as in Example 4.

[0022] (Example 8) perovskite-type composite oxide used in Example _{6 (La 0. 8 Ce 0} . 2) (Co 0. 4 Fe 0. 6) O
₃ powder _{(La 0. 8 Sr 0.} 2) (Co 0. 4 Fe 0. 6) instead of the O ₃ powder, others to obtain a catalyst sample of Example 8 in the same manner as in Example 6.

[0023] (Example 9) perovskite-type composite oxide used in Example _{5 (La 0. 8 Ce 0} . 2) (Co 0. 4 Fe 0. 6) O
₃ powder instead of _{La 0. 8 Ce 0. 2} CoO 3 powder, others to obtain a catalyst sample of Example 9 in the same manner as in Example 5.

(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.

[0025]

[Table 1]

[0026]

[Table 2]

The measurement of the catalytic activity and the durability test were conducted 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 were switched every one second. The space velocity (SV) of the gas flow through the catalyst is 30,
000 / hour.

Rich gas Lean gas CO 2.6% 0.7% HC (concentration of 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.

[Brief description of drawings]

FIG. 1 is a micrograph showing the metal composition and palladium distribution of a catalyst of an example.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Ichiro Takahashi 3000 Yamanoue, Ryuo Town, Gamo-gun, Shiga Prefecture Daihatsu Industry Co., Ltd. Shiga Technical Center

Claims (7)

[Claims] _1. A compound represented by the general formula Ln _1-x A _x MO ₃ (Ln is a rare earth metal other than Ce, A is an alkaline earth metal or Ce, and M is a transition metal. x <1)
A noble metal coexists in a mixture of a perovskite type complex oxide and a heat resistant oxide, and the noble metal is present in a higher concentration on the perovskite type complex oxide than on the heat resistant oxide. An exhaust gas purifying catalyst characterized by: 2. The perovskite-type composite oxide has a structure in which a perovskite-type composite oxide containing noble metal is formed around a core of the perovskite-type composite oxide containing no precious metal as a nucleus. Exhaust gas purifying catalyst described. 3. The exhaust gas-purifying catalyst according to claim 1, wherein the noble metal or the oxide of the noble metal is dispersed in a fine particle state. 4. The noble metal is Pd, Pt, Ru, Rh and I.
The exhaust gas-purifying catalyst according to claim 1, which is one or more metals selected from the group consisting of r. 5. The exhaust gas purifying catalyst according to claim 4, wherein the noble metal is Pd. 6. A compound represented by the general formula Ln _1-x A _x MO ₃ (Ln is a rare earth metal except Ce, A is an alkaline earth metal except Sr or Ce and M are transition metals, and each is one or more kinds. , 0
In the perovskite type complex oxide represented by <x <1),
A noble metal salt aqueous solution having a pH adjusted to 4 or less is carried by impregnation or adsorption, dried and calcined, and then dispersed in water together with a heat resistant oxide to form a slurry, which is then dried and calcined. Exhaust gas purification catalyst manufacturing method. 7. A compound represented by the general formula Ln _1-x A _x MO ₃ (Ln is a rare earth metal except Ce, A is an alkaline earth metal or Ce, and M is a transition metal. x <1)
The perovskite-type composite oxide represented by
It is characterized in that a precious metal salt aqueous solution prepared to be larger is supported by impregnation or adsorption, dried and calcined, and then dispersed in water together with a heat-resistant oxide to form a slurry, which is then dried and calcined. Exhaust gas purification catalyst manufacturing method.