JPH0653228B2 - Complex oxide catalyst - Google Patents

Description

TECHNICAL FIELD The present invention relates to carbon monoxide (CO) and hydrocarbons (CxH) discharged from domestic combustors, automobiles or various industrial furnaces.
y) such as incompletely burned material and unburned material
The present invention relates to an oxidation catalyst that converts into harmless substances such as O ₂ ) and water (H ₂ O).

2. Description of the Related Art Conventionally, most of the catalysts used for this type of application utilize precious metals such as platinum (Pt), palladium (Pd), and rhodium (Rh). However, noble metal-based catalysts cause sintering and deterioration of activity when used at high temperature for a long time,
Moreover, it is very expensive in terms of cost. Independent transition metal oxides such as cobalt (Co), nickel (Ni), and manganese (Mn) also have some activity, but at high temperatures, they form oxides with low oxidative activity, and carrier components such as alumina. A spinel is formed between and to progress in the direction of deterioration.

Perovskite composite oxides of rare earths such as La and transition metal oxides such as Ni and Co are well known as catalysts having both high activity and heat resistance, but their activity is better than that of precious metal catalysts. It can not be said.

Problems to be Solved by the Invention In order to solve the above-mentioned drawbacks of the perovskite composite oxide catalyst, cobalt, which is the active site of the perovskite composite oxide, must be efficiently exposed on the catalyst surface. La Co O ₃ (although the same behavior occurs in a complex oxide in which Ce ⁺⁴ or Sr ⁺² is introduced into a part of La ⁺³ ), the perovskite-based macroscopic particles are prepared by mixing the starting materials La and Co in an equimolar ratio. La Co
Although a perovskite structure of O ₃ is formed, the very surface phase of the catalyst has a higher La concentration than Co and has a property that Co does not easily appear on the surface. Therefore, the present invention is intended to solve this point.

Means for Solving Problems To solve the above problems, La or L of A site
Compared to the total amount of a + Ce or La + Sr, C of B site
By preparing a catalyst in which the amount of o is increased slightly, a catalyst having an overwhelmingly high activity can be obtained as compared with a catalyst having an equivalent ratio.

Action By slightly increasing the B site Co content of the La ₃ Co ₃ O ₃ based perovskite composite oxide (1.01 to 1.06),
The composition of the surface of the perovskite complex oxide should be the original La C
The equivalent ratio of La: Co becomes close to 1: 1 like o ₃
The activity is improved. If too much B-site Co is added, free Co oxide, which cannot form a complex oxide, will be present on the catalyst surface, and when the temperature is raised, it becomes poorly active CoO, which worsens the activity of the catalyst. Tend to do.

EXAMPLES Examples according to the present invention will be described below.

(1) Prepare an aqueous solution of lanthanum acetate and cobalt acetate so that the molar ratio of each metal is 1: 1.00 to 1.06, dry with a rotary evaporator, calcine at 400 ° C, and further in air at 850 ° C for 5 hours. The powder was fired in to produce fine powder of perovskite complex oxide. The chemical reaction of the produced fine powder is as shown in FIG. 1, and a normal flow reactor (normal pressure, reaction gas composition: CO1% / Air balance, flow rate 300 cm ³
/ Min) was used. The catalyst amount was 0.18 g, and after diluting with quartz sand, the total amount was 2.4 cm ³ . As the reaction tube, a quartz tube having an inner diameter of 10 mm was used. The gas produced was analyzed by gas chromatography. Note that by introducing a Ce or Sr in a part of La (La0.9Ce0.1Co O _3, La0.8Sr
The results of the same catalyst powder preparation method for 0.2CoO ₃ ) are also shown in FIG. As can be seen from the results shown in FIG. 1, the activity can be greatly increased by slightly increasing the molar ratio of B site compared to A site. In particular, the effect is most remarkable when the amount of B site is 1.02 to 1.03.

Next, examples of the powder carrying method (2) and the direct carrying method (3) carried on a practical cordierite honeycomb carrier are shown.

(2) Prepare an aqueous solution of lanthanum nitrate and cobalt nitrate such that the molar ratio of each metal is 1: 1.00 to 1.06, and add them to a large excess of aqueous sodium hydroxide solution with some hydrogen peroxide solution. Mix and stir to form and precipitate a hydroxide. The precipitate is washed thoroughly with water to remove the alkali content,
Firing at 50 ° C. for 5 hours in air to produce perovskite composite oxide fine powder. Further, it is washed with water, mixed with an appropriate amount of colloidal alumina, and sufficiently kneaded with a special mill to prepare a catalyst fine powder slurry. The catalyst fine powder slurry is diluted with water, and the cordierite honeycomb is impregnated therein. When water is absorbed by the fine pores existing in the cordierite honeycomb, the catalyst fine powder is attracted to the surface of the cordierite honeycomb, and the catalyst powder is fixed together with the alumina sol at that location. A perovskite composite oxide catalyst body is prepared by firing a honeycomb having a catalyst fixed thereto at 800 ° C. for 5 minutes or more in air.
FIG. 2 shows the conversion rate of this catalyst in propane (C ₃ H ₈ ). The test apparatus was a normal flow reactor (normal pressure, reaction gas composition; C ₃ H ₈ 1% / Air balance, flow rate 30).
0 cm ³ / min) was used. The catalyst has a diameter of 20 mm and a thickness of 1
It was set to 0 mm. A quartz tube having an inner diameter of 20 mm was used as the reaction tube.
The gas produced was analyzed by gas chromatography. As can be seen from the results shown in Fig. 2, the effect is most remarkable when the amount of B site is 1.03.

(3) The cordierite honeycomb carrier having a large number of fine pores therein is impregnated with a solution of ceric nitrate (cerium nitrate hexahydrate 750 g / 1H ₂ O) for 30 minutes or more, and the adhering water is removed. Shake off, dry well, calcine at 400 ° C, and calcine at 900 ° C for 1 hour in air. Next, this carrier is impregnated for 30 minutes or more in a prepared solution of an aqueous solution of lanthanum nitrate and cobalt nitrate in which the molar ratio of each metal is 1: 1.00 to 1.06, and the adhered water is shaken off and well dried. After calcining at 800C, it is calcined in air at 800C for 2 hours to prepare a perovskite composite oxide catalyst body. The propane (C ₃ H ₈ ) of this catalyst is shown in FIG.
Shows the conversion rate in. The test equipment and test method are the same as in case (2), and are omitted. As can be seen from the results in Fig. 3, the effect is most remarkable when the amount of B site is 1.04.

FIGS. 4 and 5 show model diagrams of the perovskite powder-supported catalyst body and the perovskite direct-supported catalyst body. In FIGS. 4 and 5, many small pores 2 are present in the cordierite carrier 1. In the case of powder carrying, catalyst fine powder (perovskite complex oxide fine powder) on the carrier surface 3
4 is attached by porous alumina 5 formed when the alumina sol is fired. In case of powder loading, pore 2
There is no catalyst fine powder 4 therein.

In the case of direct loading, the exposed surface of the cordierite material including the wall surface 6 in the pores 2 is covered with a protective film 7 made of CeO ₂ or ZrO ₂ . Further, the catalyst direct compound 8 is formed on the protective film 7.

Even from the above-mentioned examples, the effect is large when the A site: B site of the perovskite composite oxide catalyst is 1: 1.01 to 1.06, and the B site is 1.02 to 1.04 among them, despite the difference in the production method. At that time, the effect is remarkable.

As the cause, the results of the composition of the surface phase of the catalyst powder [measured by X-ray photoelectron spectroscopy (XPS)] and the composition of the catalyst powder bulk [measured by fluorescent X-ray analysis (XRFS)] shown in FIG. 6 are shown. When La at the A site and Co at the B site have an equivalent ratio, Co in the bulk is almost the same as the calculated value, whereas the Co amount in the catalyst surface phase is extremely small. That is Co
Has a property of being hard to appear on the surface. When the Co amount is increased little by little, the Co of the surface phase increases accordingly, and the Co amount 1.
In the vicinity of 03, the theoretical value is taken, and the catalytic activity takes the highest value.
It is considered that when the amount of Co further increases, free Co oxide exists on the catalyst surface, and when heated to a high temperature, it becomes CoO and the catalytic activity is rather lowered.

Effects of the Invention The effects of the present invention are as listed below.

(1) By only slightly increasing the proportion of cobalt,
A catalyst body having an overwhelmingly high activity can be obtained.

(2) The perovskite complex oxide catalyst body using cobalt at the B site can be used for this method regardless of the preparation of the catalyst body, and has a high activity utilization as an industrial preparation method.

(3) Since no noble metal is used, high activity comparable to that of a noble metal catalyst can be expected, and a catalyst body having high heat resistance can be obtained.

[Brief description of drawings]

FIG. 1 is a characteristic diagram showing the effect of excess Co in the CO oxidation reaction (250 ° C.) of a perovskite composite oxide catalyst, and FIG. 2 is a C ₃ H ₈ of a perovskite powder-supported catalyst body.
FIG. 3 is a diagram showing the conversion rate, FIG. 3 is a diagram showing the C ₃ H ₈ conversion rate of the perovskite direct supported catalyst, FIG. 4 is a perovskite powder supported catalyst model diagram, FIG. 5 is a perovskite direct supported catalyst model diagram, and FIG. The figure is a characteristic diagram showing the catalyst composition and the composition in the bulk of the perovskite complex oxide catalyst (La CoO ₃ ) when cobalt is excessive. 1 ... Cordierite carrier, 2 ... Pore, 4 ... Catalyst fine powder, 7 ... Protective film, 8 ... Catalyst direct compound.

Claims (4)

[Claims] 1. A perovskite complex oxide having an ABO ₃ structure, wherein A is lanthanum (La), B is cobalt (Co), and the molar ratio of A: B is 1: 1.01 to 1.06. A composite oxide catalyst supported on a ceramic honeycomb carrier such as cordierite. 2. La Co O ₃ (Co increase 1.01 to 1.06)
2. The complex oxide catalyst body according to claim 1, wherein the surface of the ceramic honeycomb carrier is coated with a fine powder of perovskite complex oxide having the structure described above together with a supporting aid such as alumina sol. 3. A ceramic honeycomb carrier having a large number of pores is impregnated with an aqueous solution of cerium salt such as cerium nitrate and cerium acetate and then calcined on the calcined stabilized carrier to form La 2 Co 2 O 3. ₃ (Co increase 1.01〜
The composite oxide catalyst body according to claim 1, which is prepared by impregnating a mixed solution of nitrates from which the composite oxide of 1.06) is obtained, drying and firing. 4. Cerium (Ce) or strontium (S) in a part of lanthanum (La) at the A site of ABO _3.
The composite oxide catalyst body according to claim 1, 2, or 3 wherein r) is introduced.