JP2006240974A - Praseodymium oxide-zirconium oxide-based composite oxide and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a praseodymium oxide-zirconium oxide based porous material which has a large amount of oxygen release, and can discharge a large quantity of oxygen in a low temperature region, and has a large specific surface area. <P>SOLUTION: The composite oxide containing praseodymium oxide and zirconium oxide is characterized in that it has a total oxygen release amount of 730 μmol/g or more while the oxygen release amount at 200-350°C is 180 μmol/g or more. Preferably, the oxygen release amount at 200-300°C is 38 μmol/g or more. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a praseodymium oxide-zirconium oxide composite oxide and a method for producing the same.

Automotive exhaust gas catalysts are required to be purified in response to abrupt changes ranging from several ppm to several percent of harmful gas (CO, HC, NO _x ) concentration. In this purification system, CeO ₂ and CeO ₂ —ZrO ₂ type co-catalysts having a large oxygen storage capacity (OSC) have been studied, and depending on the preparation method and composition, a co-catalyst having a higher OSC can be made. The possibility is suggested.
In recent years, regarding the exhaust gas purification system, the problem of the promotion of the three-way catalyst has become the response and exhaust gas purification at a low temperature when starting the engine. As one solution to this problem, the catalyst tends to be mounted close to the engine side.
However, since the exhaust gas becomes hot near the engine, the catalyst aggregates precious metals and CeO ₂ due to thermal deterioration, and the performance of the catalyst gradually deteriorates. Therefore, a contrivance for suppressing grain growth of noble metals and CeO ₂ has been considered.
Therefore, the high temperature stability of the cocatalyst has been actively studied. On the other hand, as another solution, a co-catalyst material is provided that can release oxygen to the catalyst at engine start-up so as to accelerate the temperature rise of the catalyst to a “light-off” temperature. It is mentioned. The light-off temperature is a temperature at which the catalyst purifies exhaust gas by 50%. When oxygen is supplied to the catalyst, it generates an exothermic reaction with harmful gases contained in the exhaust gas via the catalyst, resulting in a temperature rise of the catalyst and the catalyst temperature reaching the light-off temperature early. It becomes possible.

In the CeO ₂ —ZrO ₂ powder, the pyrochlore structure (Ce ₂ Zr ₂ O _{7 + y} (y = 0 to 1)) obtained by reducing the t′-type structure is particularly a redox reaction of CeO ₂ (2CeO ₂ (IV ) = Ce ₂ O ₃ (III) + 1 / 2O ₂ ), which has a large OSC value close to its theoretical value (250 mmol-O ₂ (mol-CeO ₂ ) ⁻¹ ). Further, it has been clarified that the κ-type structure (CeZrO ₄ ) obtained by oxidation of the pyrochlore type structure exhibits excellent oxygen storage / release performance, such as releasing oxygen at a low temperature around 600 ° C. However, research on materials that exhibit excellent oxygen storage and release performance, such as releasing oxygen at a lower temperature, is in progress.

Patent Document 1 discloses that a precipitate formed by mixing a solution containing a water-soluble salt of a metal containing praseodymium with a solution containing carbonate ions is filtered and fired, and an oxide containing praseodymium oxide is baked. "Production method" and "Oxide containing praseodymium oxide, characterized by releasing 1 mg / g or more of oxygen in an inert gas of 200 ° C to 350 ° C".
However, in order to satisfy recent strict exhaust gas purification regulations, further improvement in the storage and release characteristics of oxygen is desired.

Patent Document 2 describes a catalyst composition comprising praseodymium, at least one invariant valence group IIIa element, and optionally an intimate mixture of zirconium oxide, with a total of 5 to 50% praseodymium atoms, 0 to "Compositions based on a total of 45% zirconium atoms and 20-95% invariant valence group IIIa element atoms" are described.
However, nothing is described regarding praseodymium oxide-zirconium oxide composite oxide.

Patent Document 3 discloses that “Ln _α Zr _1-α Ox composed of platinum, palladium and rhodium (where 0.4 <α <0.9, x is the amount of oxygen satisfying the valence of each atom, Ln = And a perovskite-type composite oxide represented by at least one selected from the group consisting of La, Pr and Nd).
However, nothing is said about the release of oxygen at low temperatures.

      JP 2001-113168 A       JP 7-503896       JP 2000-279814 A

  The present invention has been made in view of the above-described problems, and the object of the present invention is to release a large amount of oxygen at a low temperature range with a large oxygen release amount, and further to provide a large specific surface area. The object is to provide a praseodymium oxide-zirconium oxide porous body.

As a result of diligent research to achieve the above object, the present inventors have added a precipitating agent to a solution of a praseodymium compound and a zirconium compound under specific conditions, co-precipitated, and calcined the precipitate in an oxidizing atmosphere After that, by firing in a non-oxidizing atmosphere and further firing in an oxidizing atmosphere, the amount of released oxygen is surprisingly large, and a large oxygen release is possible in a low temperature range, and a large specific surface area. It has been found that a praseodymium oxide-zirconium oxide porous body having the above can be obtained. Based on this finding, the present invention
(1) A composite oxide containing praseodymium oxide and zirconium oxide, wherein the total oxygen release amount is 730 μmol / g or more, and the oxygen release amount at 200 ° C. or more and 350 ° C. or less is 180 μmol / g or more. A praseodymium oxide-zirconium oxide-based composite oxide characterized.
(2) The praseodymium oxide-zirconium oxide composite oxide according to (1) above, wherein the oxygen release amount at 200 ° C. or more and 300 ° C. or less is 38 μmol / g or more.
(3) The praseodymium-zirconium oxide composite oxide according to (1) or (2) above, wherein the specific surface area after heat treatment at 1000 ° C. is at least 9 m ² / g or more.
(4) The praseodymium oxide-zirconium oxide composite oxide according to (1) to (3) above, wherein the praseodymium oxide content is 70% or more.
(5) A precipitant is added to a solution of a praseodymium compound and a zirconium compound, co-precipitated, the precipitate is fired in an oxidizing atmosphere, then fired in a non-oxidizing atmosphere, and further fired in an oxidizing atmosphere. A process for producing a praseodymium oxide-zirconium oxide composite oxide characterized by
(6) The method for producing a praseodymium oxide-zirconium oxide composite oxide according to (5), wherein the praseodymium oxide content is 65% or more.
Is to provide.

  According to the present invention, it is possible to obtain a praseodymium-zirconium oxide-based porous body excellent in oxygen storage / release capability, capable of releasing large oxygen at a low temperature range, and having a large specific surface area. Therefore, it can be suitably used in this field.

The composite oxide containing praseodymium and zirconium of the present invention has a total oxygen release amount of 730 μmol / g or more (preferably 750 μmol / g or more) and an oxygen release amount of 200 μC or more and 350 or less (180 μmol / g or more). Preferably 200 μmol / g or more).
Furthermore, the oxygen release amount at 200 ° C. or more and 300 or less is preferably 38 μmol / g or more (preferably 40 μmol / g or more).
Since the total oxygen release amount is 730 μmol / g or more, it is possible to release a large amount of oxygen to the catalyst, so that the exhaust gas purification efficiency is improved.
In addition, the oxygen release amount at 200 ° C. or more and 350 ° C. or less is 180 μmol / g or more, and the oxygen release amount at 200 ° C. or more and 300 ° C. or less is 38 μmol / g or more. This has the advantage that the exhaust gas purification efficiency is improved when the engine is started.
The specific surface area of the composite oxide of the present invention is characterized in that it is at least 9 m ² / g (preferably 10 m ² / g) or more even after being subjected to heat treatment at 1000 ° C. for 5 hours. Is preferred. Since the specific surface area is at least 9 m ² / g, the response to release and absorption of oxygen on the oxide surface is improved, so that the exhaust gas purification efficiency is improved.
On the other hand, the praseodymium oxide content in the composite oxide is not particularly limited, but the praseodymium oxide content is 10% or more (preferably 40% or more, more preferably 70% or more, particularly preferably 75 to 95%) is preferable because oxygen can be released.
Although an example of the manufacturing method of the praseodymium oxide-zirconium oxide composite oxide of the present invention is described below, the present invention is not limited by this at all.

First, the praseodymium raw material used in the present invention is not particularly limited, and examples thereof include inorganic acid salts such as nitric acid, carbonic acid, sulfuric acid, and chloride, and organic acid salts such as acetic acid. Less nitric acid or chloride is preferred.
Further, the zirconium raw material is not particularly limited, and examples thereof include inorganic acid salts such as nitric acid, carbonic acid, sulfuric acid, and chloride, and organic acid salts such as acetic acid. Things are preferred.
The purity of the praseodymium raw material and the zirconium raw material is not particularly limited, but is preferably 99.9% by weight or more in purity. Next, the method for dissolving the praseodymium raw material and the zirconium raw material is not particularly limited as long as the raw material is dissolved. Alcohols such as water, methanol, and ethanol can be used as the solvent. The concentration of the solution may be appropriately determined depending on the mixing ratio of the raw materials described above, but is usually about 1 to 25% by weight.

A known precipitation method can be used as a method for generating a precipitate from the mixed solution. That is, a base may be added to the mixed solution to generate a precipitate.
The base is not particularly limited, and examples thereof include sodium hydroxide, potassium hydroxide, sodium carbonate, ammonium carbonate, ammonia and the like, but it is usually preferable to use ammonia. These bases are usually preferably added as a solution. The concentration of the aqueous solution is not particularly limited, but is usually about 5 to 30% by weight.

  A known recovery method can be used for the precipitate generated by the addition of the base. That is, it may be recovered by performing solidification and liquid separation by filtration, washing with water and the like.

  You may oxidize the powder obtained by the said method by heating in oxidizing atmosphere or air | atmosphere as needed. The heating temperature is not particularly limited, and is usually about 200 to 1000 ° C., preferably about 400 to 700 ° C. The heating time is not particularly limited, and can usually be about 1 to 10 hours after reaching the set temperature.

  The powder obtained by the above method can be pulverized. Although it does not specifically limit about grinding | pulverization, It can grind | pulverize by crushing methods, such as a pin mill, a planetary mill, a ball mill, or a jet mill.

Subsequently, the composite oxide is fired in a non-oxidizing atmosphere, that is, an inert or reducing atmosphere (preferably a reducing atmosphere).
The heat reduction treatment method is not particularly limited, but after loading the complex oxide in a vacuum heating furnace and reducing the pressure, introducing a reducing gas exemplified by hydrogen, carbon monoxide, etc., in a reducing atmosphere, Preferably, the temperature may be about 600 ° C to 1200 ° C. The heating time is not particularly limited, and can usually be about 0.5 to 10 hours after reaching the set temperature. At this time, the concentration of the reducing gas is preferably 1% or more. Under the present circumstances, the reducing agent illustrated by carbon powder etc. can also be used instead of reducing gas.
By performing this treatment, there is an effect of increasing the amount of released oxygen, increasing the amount of released oxygen at a low temperature, and having a large specific surface area.
Although there is no clear conclusion about this mechanism at present, it is presumed to be due to the following mechanism. Since praseodymium oxide is a non-stoichiometric oxide, the valence of praseodymium is in a state where trivalent and tetravalent are mixed. By subjecting this to heat reduction treatment, it is considered that the valence of praseodymium is made uniform to be trivalent.
It is considered that this homogenization is large and enables oxygen release at a low temperature. Moreover, it is thought that the cohesiveness between particles is suppressed by heat reduction treatment. Therefore, it is considered possible to have a large specific surface area.

Then, after reduction heating, it can oxidize by heating in oxidizing atmosphere or air | atmosphere as needed. The heating temperature is not particularly limited as long as the composite oxide is oxidized, and is usually about 200 to 1000 ° C., preferably about 400 to 800 ° C. The heating time is not particularly limited, and can usually be about 1 to 5 hours after reaching the set temperature.
The praseodymium oxide content in the produced composite oxide is not particularly limited, but the praseodymium oxide content is 10% or more (preferably 40% or more, more preferably 65% or more). It is preferable that the effect of firing in a non-oxidizing atmosphere (preferably a reducing atmosphere) becomes clear.

The oxygen release start temperature and release amount of the praseodymium oxide-zirconium oxide composite oxide produced according to the present invention can be determined by using a temperature reduction method (BEL JAPAN INC., MULTITASK TPR). it can.
Specifically, 0.3 g of powder was heated to 350 ° C. and kept in high-purity oxygen gas for 60 minutes for sufficient oxidation. Next, in a 5% carbon monoxide-argon gas stream (100 sccm), heating is performed from 100 ° C. to 900 ° C. at a rate of temperature increase of 10 ° C./min, and the amount of carbon dioxide produced during this time is measured with a quadrupole mass spectrometer. Measurements were made continuously to obtain a carbon dioxide production curve with increasing temperature.
From the obtained carbon dioxide production curve and its area, the total oxygen release amount, the oxygen release amount from 200 ° C. to 350 ° C. and from 200 ° C. to 300 ° C. can be determined.
The specific surface area was measured by the N ₂ adsorption method (Shimadzu Corporation, Flowsorb-II 2300 Type).

  Examples and comparative examples are shown below to further clarify the features of the present invention. In addition, this invention is not limited to the aspect of these Examples.

[Example 1]
Praseodymium nitrate (containing 9.3 g as praseodymium oxide) was dispersed in 1000 g of ion-exchanged water, and an aqueous zirconium nitrate solution (containing 0.7 g as zirconium oxide) was added to prepare a mixed solution. This was added to 300 g of 5 wt% aqueous ammonia to produce praseodymium hydroxide. Then, after suction filtration, it was washed with 500 g of pure water.
This was calcined in the atmosphere at 700 ° C. for 3 hours to obtain an oxide. Next, this was pulverized to a particle size of 10 μm or less, then reduced in a hydrogen gas (99.9 wt%) stream at 1000 ° C. for 5 hours, and then oxidized in air at 800 ° C. for 3 hours to obtain praseodymium. An oxide containing was obtained.

  The oxygen release amount and specific surface area of the powder were evaluated. The results are shown in Table 1.

[Example 2]
Praseodymium nitrate (containing 7.6 g as praseodymium oxide) was dispersed in 1000 g of ion-exchanged water, and an aqueous zirconium nitrate solution (containing 2.4 g as zirconium oxide) was further added to prepare a mixed solution.
Thereafter, a composite oxide containing praseodymium and zirconium was obtained in the same manner as in Example 1. The obtained powder was evaluated for oxygen release amount and specific surface area in the same manner as in Example 1. The results are shown in Table 1.

[Example 3]
Praseodymium nitrate (containing 6.7 g as praseodymium oxide) was dispersed in 1000 g of ion-exchanged water, and an aqueous zirconium nitrate solution (containing 3.3 g as zirconium oxide) was further added to prepare a mixed solution.
Thereafter, a composite oxide containing praseodymium and zirconium was obtained in the same manner as in Example 1. The obtained powder was evaluated for oxygen release amount and specific surface area in the same manner as in Example 1. The results are shown in Table 1.

[Comparative Example 1]
An oxide was prepared in the same manner as in Example 1 except that the reduction firing was not performed. The obtained powder was evaluated for oxygen release amount and specific surface area in the same manner as in Example 1. Results are shown in Table 1.

[Comparative Example 2]
An oxide was prepared in the same manner as in Example 2 except that the reduction firing was not performed. The obtained powder was evaluated for oxygen release amount and specific surface area in the same manner as in Example 2. The results are shown in Table 1.

[Comparative Example 3]
An oxide was prepared in the same manner as in Example 3 except that the reduction firing was not performed. The obtained powder was evaluated for oxygen release amount and specific surface area in the same manner as in Example 3. The results are shown in Table 1.

From Table 1, the total oxygen release amount in Comparative Example 1 _725μmol-O 2 / g, Comparative Example 2 _644μmol-O 2 / g, whereas a _357μmol-O 2 / g in Comparative Example 3, performed It can be seen that Example 1 is 839 μmol-O ₂ / g, Example 2 is 795 μmol-O ₂ / g, and Example 3 is 491 μmol-O ₂ / g.
Oxygen release amount at 200 ° C. or higher 350 ° C. or less, in Comparative Example 1 _178μmol-O 2 / g, Comparative Example 2 _141μmol-O 2 / g, whereas a _17μmol-O 2 / g in Comparative Example 3 In Example 1, it was 312 μmol-O ₂ / g, in Example 2, 208 μmol-O ₂ / g, and in Example 3, it was large oxygen release in a low temperature range of 30 μmol-O ₂ / g.

Further, from Table 1, the specific surface area at 1000 ° C. is 8.5 m ² / g in Comparative Example 1, 6.1 m ² / g in Comparative Example ² , and 2.2 m ² / g in Comparative Example 3. Example 1 showed a large value of 15.5 m ² / g, Example 2 10.1 m ² / g, and Example 3 3.5 m ² / g.

The praseodymium oxide-zirconium oxide-based composite oxide of the present invention has a large oxygen release amount, can release a large amount of oxygen at a low temperature range, and has a large specific surface area. It will be appreciated that it is useful as a purification promoter or support.

Claims (6)

  A composite oxide containing praseodymium oxide and zirconium oxide, wherein a total oxygen release amount is 730 μmol / g or more, and an oxygen release amount at 200 ° C. or more and 350 ° C. or less is 180 μmol / g or more. Praseodymium oxide-zirconium oxide composite oxide.   2. The praseodymium oxide-zirconium oxide composite oxide according to claim 1, wherein an oxygen release amount at 200 ° C. or more and 300 ° C. or less is 38 μmol / g or more. 3. The praseodymium oxide-zirconium oxide composite oxide according to claim 1, wherein the specific surface area after heat treatment at 1000 ° C. is at least 9 m ² / g or more.   The praseodymium oxide-zirconium oxide composite oxide according to claim 1, wherein the praseodymium oxide content is 70% or more.   A precipitant is added to a solution of a praseodymium compound and a zirconium compound, and the precipitate is coprecipitated. A method for producing a praseodymium oxide-zirconium oxide composite oxide.   6. The method for producing a praseodymium oxide-zirconium oxide composite oxide according to claim 5, wherein the praseodymium oxide content is 65% or more.