JP6923289B2 - Oxygen absorption / release material - Google Patents

Description

The present invention relates to an oxygen absorption / release material composed of a ceria-zirconia-based composite oxide, and more particularly to an oxygen absorption / release material having a high oxygen absorption / release rate, which is used as an auxiliary catalyst in an exhaust gas purification catalyst.

In the three-way catalyst used for exhaust gas purification of gasoline engines, in order to enhance the function of precious metals, it is preferable to keep the ratio of fuel and air (air-fuel ratio) constant (to the theoretical air-fuel ratio), but acceleration, deceleration, The air-fuel ratio changes greatly depending on the driving conditions such as low-speed driving and high-speed driving. For this reason, the air-fuel ratio (A / F), which fluctuates depending on the operating conditions of the engine, is kept constant by feedback control using an oxygen sensor, but the A / F fluctuates with time according to the feedback time. Therefore, it is difficult to maintain the exhaust gas atmosphere at or near the stoichiometric air-fuel ratio only by controlling the engine.

Therefore, it is necessary to fine-tune the atmosphere on the catalyst side. Ceria (cerium oxide CeO ₂ ) has an oxygen absorption / release capacity (Oxygen Storage Capacity, hereinafter simply referred to as "OSC"), and is therefore widely used as an auxiliary catalyst for adjusting the oxygen partial pressure of an automobile exhaust gas purification catalyst. There is. This utilizes the redox reaction of Ce ³⁺ / Ce ^4+. The ceria is generally used as a ceria-zirconia-based composite oxide dissolved in zirconia (zirconium oxide ZrO _{2) in order to enhance its properties.}

The ceria-zirconia-based composite oxide is generally mixed with alumina at a ratio of several% to several tens of percent in a state where a catalytic noble metal is supported, and is washed on the surface of the honeycomb carrier with a thickness of several tens of μm to several hundreds of μm. It is formed on the coat layer and functions as a co-catalyst.

The catalyst wash-coated inside the honeycomb carrier purifies by contact with the flowing exhaust gas, but the total length of the honeycomb carrier is as short as about 10 cm to about 30 cm at most, and the time for purifying the exhaust gas is very short. In order to complete the purification in this short time, it is necessary to complete the oxygen partial pressure adjustment by ceria zirconia in a very short time in milliseconds. Conventionally, ceria-zirconia-based composite oxides have been developed with a focus on maintaining the specific surface area after the heating durability test in order to increase the contact area with the exhaust gas (see Patent Document 1).

For example, Patent Document 2 and Patent Document 3 are developments focusing on the oxygen release rate of ceria-zirconia-based oxides. Patent Document 2 focuses on the oxygen release rate up to 60 seconds after the start of reduction, and Patent Document 3 increases the amount of OSC by combining ceria zirconia having a pyrochloro structure with ceria zirconia having a fluorite-type structure. We are proposing ceria zirconia, which has a large release rate.

On the other hand, there is, for example, Non-Patent Document 1 as an example of measuring the ionic conductivity of a ceria-zirconia-based composite oxide.

Japanese Unexamined Patent Publication No. 2012-180271 Japanese Unexamined Patent Publication No. 2011-121851 International Publication No. 2012/105454 Pamphlet

"Material", Vol60, No. 3, pp. 194-197 (Effect of Ceria Addition on Oxygen Relaxation and Ion Conductivity of Yttria Stabilized Cubic Zirconia), Mar. 2011

As described above, ceria-zirconia-based oxides as oxygen absorption / release materials have been required to have a high OSC value and a large specific surface area to maintain the OSC value in automobile exhaust gas purification catalysts and the like, and developments have been made in response to these values. rice field. However, as electronic control of engines has become widespread and electronic control of fuel injection has become more and more complicated in order to improve fuel efficiency and engine performance, the demand for ceria-zirconia oxides has also changed. Specifically, it is a ceria-zirconia-based oxide having an OSC ability capable of responding to complicated electronic control at high speed.

Therefore, in the development of ceria-zirconia-based oxides as oxygen absorption / release materials so far, in Patent Document 1, the composition of additive elements and the like for obtaining a high OSC (oxygen absorption / release capacity) value and excellent durability is obtained. It was only a study. Since the requirement for OSC rate has not been clarified so far, almost no attempt has been made to improve the OSC rate of ceria-zirconia-based oxides, and even the evaluation method has not been clarified.

Patent Document 2 states that the OSC response rate can be improved by adding a specific amount of indium oxide to the ceria-zirconia oxide. However, the use of the hydrogen H ₂ as a reducing agent in the measurement of the OSC response speed in the real exhaust gas conditions not fully determine the required characteristics since those are reduced by carbon monoxide CO. In addition, although the ceria-zirconia oxide is evaluated as it is, in reality, it is used by supporting a noble metal, so it should be judged whether the OSC response speed is improved by the characteristic of supporting the noble metal. Has not been evaluated. Preliminary studies by the present inventors have found that the inclusion of indium oxide does not always improve the OSC response rate under carbon monoxide reduction conditions when a noble metal is supported. Further, in the ceria-zirconia-based oxide used as the OSC material, indium oxide is a special and expensive additive, so a method for improving the OSC response speed without using indium oxide is desired.

In Patent Document 3, it is assumed that the ceria-zirconia-based oxide having a pyrochlorostructure can absorb and release a large amount of oxygen, and the ceria-zirconia-based oxide having a cubic (fluorite structure) has a high oxygen absorption / release rate, and these are combined. Ceria-zirconia oxides are said to exhibit excellent catalytic performance. The evaluation standard of the OSC speed is the OSC characteristic after the atmosphere has settled to stoiciometry after the lean to rich fluctuation, and the OSC speed immediately after the atmosphere change, which is the object of the present invention, is measured. Not enough, and the characteristics have not been evaluated sufficiently.

In Non-Patent Document 1, the internal frictional strength of the ceria-zirconia-based composite oxide is measured to discuss the relationship with the ionic conductivity, but the OSC material is not related to the oxygen absorption / release rate.

The present inventors have found that a ceria-zirconia-based composite oxide having a large amount of oxygen absorption and release does not necessarily have a large OSC rate. Therefore, in recent engine control, the conventional ceria-zirconia-based composite oxide having a large amount of oxygen absorption / release is insufficient, and rather, the ceria-zirconia-based composite oxide having a large OSC rate even if the amount of oxygen absorption / release is not large is insufficient. The reality is that the development of is required.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an oxygen absorbing / releasing material made of a ceria-zirconia-based composite oxide having a high OSC rate.

The present inventors have diligently studied to solve the above problems, and as a result, the ceria-zirconia-based composite oxide having a high oxygen absorption / release rate and a large oxygen absorption / release amount as an oxygen absorption / release material has an ionic conductivity. The present invention was completed by discovering that it is a large ceria-zirconia-based composite oxide.

That is, the gist of the present invention is as follows.

(1) A ceria-zirconia-based composite oxide containing a metal ion having a valence of less than tetravalent, and the molar ratio of cerium to zirconium in the ceria-zirconia-based composite oxide is cerium / (cerium +). Metal ions having a valence of 0.33 or more and 0.83 or less (excluding 0.47) and less than tetravalent in zirconium) are relative to the total cations of the ceria-zirconia-based composite oxide. It is one of Ca ions or Pr ions contained in an amount of 3 mol% or more and 15 mol% or less, and the ionic conductivity of the ceria-zirconia-based composite oxide sintered at 1450 ° C. is 1.0 × 10 at 400 ° C. ^-5 S / cm and not more than 1.0 × ¹⁰ -2 S / cm der less is, the oxygen storage and release rate (OSC speed) Delta] t ₅₀ is not less than 17.0 seconds, and releasing oxygen volume (OSC amount) 300μmol oxygen absorption and release material characterized der Rukoto more -O ₂ / g.

As described above, if the oxygen absorption / release material composed of the ceria-zirconia-based composite oxide having a high OSC speed of the present invention is used for the automobile exhaust gas purification catalyst, it helps to purify the exhaust gas that changes every moment according to the running condition. The purification performance of harmful components of precious metals will be higher than ever.

It is the schematic of the OSC speed measuring apparatus.

The ceria-zirconia-based composite oxide of the present invention having a high oxygen absorption / release rate (sometimes referred to as OSC rate) has a higher OSC rate as the ionic conductivity increases, and an oxygen absorption / release amount (referred to as OSC amount). If it is used as an automobile exhaust gas purification catalyst, it can help purify exhaust gas that changes from moment to moment depending on the running condition.

When the ionic conductivity is low, oxygen in the ceria-zirconia-based composite oxide is difficult to move, so that the OSC rate is low and the harmful component purification performance is low.

The ceria-zirconia-based composite oxide of the present invention refers to metal ions (Ca, Sc, Sr, Y, Ba, La, Pr, Nd, Sm, Pr, Yb, etc.) having a valence of less than tetravalent cerium or zirconium. ) Is a ceria-zirconia-based composite oxide partially substituted. Examples of the substituted metal element that are particularly effective include Sc, Y, La, Nd, Pr and the like. Examples of the substitution effect of each element include improvement of ionic conductivity for Sc, Y and the like, and improvement of heat resistance for La, Nd, Pr and the like. The amount of substitution is not particularly limited, but is preferably 20 mol% or less with respect to the total cation of the ceria-zirconia oxide, and 3 mol (mol)% or more and 20 mol% or less in order to obtain a larger substitution effect. It is preferably 3 mol% or more and 15 mol% or less.

The material having a high oxygen absorption / release rate (ceria / zirconia-based composite oxide) according to the present invention has an ionic conductivity of a sintered body measured by an AC impedance method of 1.0 × ^10-5 S / cm or more at 400 ° C. Is desirable. The upper limit of the ionic conductivity is not particularly limited, but is preferably 1.0 × ^10-2 S / cm or less. When the ionic conductivity is high, the movement of oxygen in the particles is likely to occur, and the purification performance is improved. When the ionic conductivity is less than 1.0 × ^10-5 S / cm, the movement of oxygen in the particles is unlikely to occur, and the purification performance is poor. A third component is added to increase the ionic conductivity, but the amount of Ce ions that absorb and release oxygen decreases by the amount of the increased third component, so the ionic conductivity is 1.0 x ^10-2 S / cm. Preparation of higher materials is difficult. Therefore, a metal ion having a valence of less than tetravalent was contained, and the ionic conductivity was set to 1.0 × ^10-5 S / cm or more at 400 ° C.

Further, in the material having a high oxygen absorption / release rate according to the present invention, it is desirable that the molar ratio of cerium to zirconium of the ceria-zirconia-based composite oxide is 0.83 or less in terms of cerium / (cerium + zirconium). Desirably, it is 0.66 or less. When cerium / (cerium + zirconium) exceeds 0.83, the amount of cerium is large and the valence fluctuation of cerium is less likely to occur, which is not desirable for improving the oxygen absorption / release rate. If the molar ratio of cerium / (cerium + zirconium) exceeds 0, the OSC rate can be improved, but in order to obtain a greater effect, it is preferably 0.004 or more, more preferably 0.20. That is all.

The ceria-zirconia-based composite oxide of the present invention can obtain a high oxygen absorption / release rate, but from the viewpoint of obtaining a faster oxygen absorption / release rate, cerium / (cerium + zirconium) is less than 0.33. It is more preferable to have it. In this case, it is desirable that metal ions having an ionic radius smaller than ^{Ce 4+ are contained.} By blending metal ions having a small ionic radius, especially Sc and Y, oxygen defects are formed between atoms, and elements with a small ionic radius do not inhibit the movement of oxygen, so that the movement of oxygen is likely to occur, and as a result, the movement of oxygen is likely to occur. It has the effect of increasing the OSC speed. In order to increase the OSC speed and improve the harmful component purification performance of the noble metal, _{it is desirable that the OSC speed Δt 50} is 17.0 seconds or more. It is desirable that the OSC speed Δt ₅₀ is high, and the upper limit is not particularly limited, but 30.0 seconds or less is realistic.

Further, the ceria-zirconia-based composite oxide of the present invention has a cerium / (cerium + zirconium) of 0.33 or more and 0.83 or less, more preferably 0.83 or less, particularly from the viewpoint that a larger oxygen absorption / release amount can be obtained. It is 0.33 or more and 0.66 or less. In this case, it is desirable that metal ions having an ionic radius larger than that of ^{Ce 4+ ions, particularly La, Pr, and Nd, are contained.} By blending metal ions with a large ionic radius, the crystal lattice of the composite oxide expands, so that the ionic radius changes easily with the change in valence from Ce ^{4+ to Ce 3+} , which has the effect of increasing the OSC amount of oxygen. Yes, the amount of OSC is preferably 300 μmol-O ₂ / g or more. A large amount of OSC is preferable, and the upper limit is not particularly limited, but 420 μmol-O ₂ / g is realistic.

The OSC speed measured in the present invention is measured as follows. Explaining with reference to FIG. 1, which shows a schematic view of an OSC speed measuring device, first, measurement is performed in a state where nothing is filled in the reaction tube 1 (Blank test). That is, the reaction tube 1 is heated in the electric furnace 3 under _{the O 2 distribution from the O 2} cylinder 2 (3 mass% O ₂ , N ₂ balance) to raise the temperature to 400 ° C. The temperature inside the reaction tube 1 is measured by a thermocouple 4. The _{inside of the reaction tube 1 is purged with N 2 from the N 2} cylinder 5, and CO is circulated from the CO cylinder 6 (1.5 mass% CO, N _{2 balance).} The flow rates of O ₂ , CO, and N ₂ are measured by the flow rate indicator 7. _{O 2} and CO discharged from the reactor 1 are measured by a total of 8 _{O 2 and a total of 9 CO, respectively. The time (t 50 (Blank)} ) when the CO concentration becomes half of the distributed CO concentration is calculated. Next, measurement is carried out in a state where the reaction layer 10 is filled with a ceria-zirconia-based composite oxide powder in which the reaction tube 1 is impregnated with 0.5 mass% of Pd and supported (Sample measurement). The temperature of the reaction tube 1 is raised to 400 ° C. under _{O 2 circulation.} Purge with N ₂ and distribute CO. _{The time (t 50 (Sample)} ) when the CO concentration becomes half of the distributed CO concentration is calculated, and the _{OSC speed Δt 50} is calculated by the following formula.

From the equation of _{Δt 50 = t 50 (Blank)} −t _{50 (Sample)} , the OSC velocity Δt ₅₀ is calculated. The _{ceria-zirconia-based composite oxide having Δt 50} of 17.0 seconds or more, that is, an ionic conductivity of 1 × ^10-5 S / cm or more has excellent oxygen absorption / release characteristics, a high OSC rate, and purification of harmful components of precious metals. Performance can be improved.

Hereinafter, the present invention will be specifically described with reference to Examples (Invention Examples), Reference Examples, and Comparative Examples, but the present invention is not limited thereto.

Ion conductivity is measured as follows. The sample (ceria-zirconia-based composite oxide) is calcined at 1200 ° C. and pulverized with a wet ball mill. PVA as a binder is added in an amount of 1 mass% to 500 g of a sample, and ^{a pressure of 1 ton / m 2} is applied to prepare pellets. It is then sintered at 1450 ° C. and the pellets are coated with silver paste. Pellets coated with silver paste were measured by JIS R1661 AC impedance.

The OSC speed is measured by the measurement method described above, and the OSC amount is measured as follows. The alumina pan is filled with about 10 mg of a sample (ceria-zirconia-based composite oxide) and set in a thermogravimetric analyzer. The sample is reduced at 400 ° C. for 1 hour under _{5% H 2 / Ar circulation.} Then, 100% O ₂ is circulated at 400 ° C. for 10 minutes for oxidation treatment. The weight change before and after the oxidation treatment was calculated as the OSC amount of the sample.

( Reference example 1)
1 l of a solution obtained by mixing pure water with a cerium chloride solution, a zirconium oxychloride solution, an yttrium chloride solution, and a molar ratio of CeO ₂ : ZrO ₂ : Y ₂ O ₃ = 25: 63: 12, 0.4 mol / l. Obtained (liter). 15 g of ammonium peroxodisulfate was added to the obtained mixed solution, and the mixture was heated to 95 ° C. with stirring to obtain a cerium-zirconium composite sulfate. The obtained sulfate slurry was cooled to 60 ° C. and then neutralized by adding aqueous ammonia to obtain a slurry containing a hydroxide. The obtained hydroxide slurry was subjected to a filtration-washing operation four times to obtain a cerium-zirconium composite hydroxide cake. The obtained composite hydroxide cake was dried at 120 ° C. to obtain a composite hydroxide powder, which was packed in a crucible and baked at 700 ° C. for 3 hours in an electric furnace to obtain a ceria-zirconia-based composite oxide powder. rice field.

When the ionic conductivity of the obtained powder was measured by the above-mentioned method, it was 1.30 × 10 ^-3 (S / cm).

Further, Pd was impregnated and carried at a ratio of 0.5 mass% to the obtained powder, and the OSC rate (Δt ₅₀ ) and the amount of OSC were measured. As a result, the OSC rate of Δt ₅₀ was 29.0 seconds and the OSC. The result was that the amount was 303 μmol-O _{2 / g.}

( Reference example 2)
As shown in Table 1, the ceria-zirconia-based composite oxide powder is the same as in Reference Example _{1 except that the compounding composition is CeO 2} : ZrO ₂ : Sc ₂ O ₃ = 25: 68.7: 6.3. Got

When the ionic conductivity, OSC velocity, and OSC amount of the obtained powder were measured in the same manner as in Reference Example ^{1, 8.95 × 10-5} (S / cm), Δt ₅₀ was 18.5 seconds, and the OSC amount was 300 μmol. It was −O ₂ / g.

( Reference example 3)
As shown in Table 1, the ceria-zirconia-based composite is the same as in Reference Example _{1 except that the compounding composition is CeO 2} : ZrO ₂ : Y ₂ O ₃ : La ₂ O ₃ = 34: 52: 8: 6. Oxide powder was obtained.

When the ionic conductivity, OSC rate, and OSC amount of the obtained powder were measured in the same manner as in Reference Example ^{1, 3.07 × 10-5} (S / cm), Δt ₅₀ was 20.4 seconds, and the OSC amount was 327 μmol. It was −O ₂ / g.

( Reference example 4)
As shown in Table 1, the composition is the same as that of Reference Example 1 _{except that the compounding composition is CeO 2} : ZrO ₂ : Y ₂ O ₃ : Pr ₂ O ₃ = 32.6: 57: 6.2: 4.1. To obtain a ceria-zirconia-based composite oxide powder.

When the ionic conductivity, OSC rate, and OSC amount of the obtained powder were measured in the same manner as in Reference Example ^{1, they were 7.43 × 10-5} S / cm or more, Δt ₅₀ was 17.8 seconds, and the OSC amount was 315 μmol-. It was O ₂ / g.

(Example 1 )
As shown in Table 1, the ceria-zirconia-based composite oxide powder is the same as in Reference Example _{1 except that the compounding composition is CeO 2} : ZrO ₂ : Pr ₂ O ₃ = 75: 15.6: 9.4. Got

When the ionic conductivity, OSC velocity, and OSC amount of the obtained powder were measured in the same manner as in Reference Example ^{1, they were 8.31 × 10-5} S / cm or more, Δt ₅₀ was 18.2 seconds, and the OSC amount was 337 μmol-. It was O ₂ / g.

(Example 2 )
As shown in Table 1, the ceria-zirconia-based composite oxide powder was prepared in the same manner as in Reference Example _{1 except that the compounding composition was CeO 2} : ZrO ₂ : CaO = 37.5: 53.1: 9.4. Obtained.

When the ionic conductivity, OSC rate, and OSC amount of the obtained powder were measured in the same manner as in Reference Example ^{1, they were 2.41 × 10-5} S / cm or more, Δt ₅₀ was 20,2 seconds, and the OSC amount was 350 μmol-. It was O ₂ / g.

(Reference example 5 )
As shown in Table 1, the composition is different from that of Reference Example 1 _{except that the compounding composition is CeO 2} : ZrO ₂ : Y ₂ O ₃ : Nd ₂ O ₃ = 42.2: 47.1: 6.4: 4.3. Similarly, a ceria-zirconia-based composite oxide powder was obtained.

When the ionic conductivity, OSC rate, and OSC amount of the obtained powder were measured in the same manner as in Reference Example ^{1, they were 2.94 × 10-5} S / cm or more, Δt ₅₀ was 17.4 seconds, and the OSC amount was 400 μmol-. It was O ₂ / g.

(Comparative Example 1)
As shown in Table 1, the ceria-zirconia-based composite oxidation is the same as in Reference Example _{1 except that the compounding composition is CeO 2} : ZrO ₂ : La ₂ O ₃ = 12.5: 81.2: 6.3. A product powder was obtained.

When the ionic conductivity, OSC velocity, and OSC amount of the obtained powder were measured in the same manner as in Reference Example ^{1, 8.06 × 10-6} (S / cm), Δt ₅₀ was 16.4 seconds, and the OSC amount was 210 μmol. It was −O ₂ / g.

(Comparative Example 2)
As shown in Table 1, the ceria-zirconia-based composite oxide powder is the same as in Reference Example _{1 except that the compounding composition is CeO 2} : ZrO ₂ : Y ₂ O ₃ = 75: 18.7: 6.3. Got

When the ionic conductivity, OSC velocity, and OSC amount of the obtained powder were measured in the same manner as in Reference Example ^{1, 3.65 × 10-6} (S / cm), Δt ₅₀ was 12.5 seconds, and the OSC amount was 194 μmol. It was −O ₂ / g.

(Comparative Example 3)
As shown in Table 1, the ceria-zirconia-based composite oxide powder is the same as in Reference Example _{1 except that the compounding composition is CeO 2} : ZrO ₂ : Y ₂ O ₃ = 50: 40.6: 9.4. Got

When the ionic conductivity, OSC velocity, and OSC amount of the obtained powder were measured in the same manner as in Reference Example ^{1, 2.41 × 10-6} (S / cm), Δt ₅₀ was 14.3 seconds, and the OSC amount was 202 μmol. It was −O ₂ / g.

From Table 1, the ceria-zirconia-based composite oxides obtained in Examples 1 and 2 contain metal ions having a valence of less than tetravalent, and have an ionic conductivity of 1.0 × ^10-5 S at 400 ° C. It was an oxygen absorption / release material of / cm or more, and satisfied the characteristics of excellent OSC speed and OSC amount.

On the other hand, the ionic conductivity of the obtained ceria-zirconia composite oxide in Comparative Example 1, 2 and 3 is less than ^{1.0 × 10 -5 S / cm,} OSC speed, OSC amount indicates a lower value The performance of the oxygen absorbing / releasing material was inferior to that of the examples.
As described above, according to the present invention, it was confirmed that a ceria-zirconia-based composite oxide having a high OSC rate and a large amount of OSC was obtained.

If the material having a high oxygen absorption / release rate of the present invention is used as an auxiliary catalyst for an automobile exhaust gas purification catalyst, it helps to purify the exhaust gas that changes every moment according to the running condition, and improves the harmful component purification performance of precious metals more than ever. Can be planned.

1 Reaction tube 2 O ₂ Cylinder 3 Electric furnace 4 Thermocouple 5 N ₂ Cylinder 6 CO Cylinder 7 Flow indicator 8 O ₂ Total 9 CO Total 10 Reaction layer

Claims (1)

A ceria-zirconia-based composite oxide oxygen absorbing / releasing material containing a metal ion having a valence of less than tetravalent, and the molar ratio of cerium to zirconium in the ceria-zirconia-based composite oxide is cerium / (cerium). + Zirconium) is 0.33 or more and 0.83 or less (excluding 0.47), and the metal ion having a valence of less than the tetravalent is used as the total cation of the ceria-zirconia-based composite oxide. On the other hand, it is one of Ca ion or Pr ion contained in an amount of 3 mol% or more and 15 mol% or less, and the ionic conductivity of the ceria-zirconia-based composite oxide sintered at 1450 ° C. is 1.0 × at 400 ° C. 10 ^-5 S / cm or more and 1.0 × ¹⁰ -2 S / cm der less is, the oxygen storage and release rate (OSC speed) Delta] t ₅₀ is not less than 17.0 seconds, and releasing oxygen volume (OSC amount) oxygen absorption and release material characterized der Rukoto more _300μmol-O 2 / g.

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