JP7262975B2 - Ceria-Zirconia Composite Oxygen Absorption-Desorption Material and Exhaust Gas Purification Catalyst - Google Patents

Description

TECHNICAL FIELD The present invention relates to a ceria-zirconia composite oxide oxygen absorbing/releasing material and an exhaust gas purifying catalyst exhibiting high oxygen absorbing/releasing properties in a low temperature range (200 to 800° C.).

Exhaust gas emitted from internal combustion engines such as automobiles contains harmful components such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides ( _NOx ). , must be released into the atmosphere.

A three-way catalyst is used to purify such harmful components of the exhaust gas. Catalysts in which noble metals such as platinum (Pt), palladium (Pd) and rhodium (Rh) are carried on carriers such as oxides are widely used as the catalysts.

In a three-way catalyst, it is preferable to keep the ratio of fuel and air (air-fuel ratio) constant (to the stoichiometric air-fuel ratio) in order to enhance the function of precious metals. Accordingly, the air-fuel ratio changes greatly. For this reason, feedback control using an oxygen sensor is used to keep the air-fuel ratio (A/F) constant, which fluctuates depending on the operating conditions of the engine, but A/F changes over 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 engine control.

Therefore, it is necessary to finely adjust the atmosphere on the catalyst side. Ceria (cerium oxide CeO ₂ ) has an oxygen storage capacity (hereinafter referred to as “OSC”), and is therefore widely used as a co-catalyst for adjusting the oxygen partial pressure of automobile exhaust gas purification catalysts. This utilizes the redox reaction of Ce ³⁺ /Ce ⁴⁺ . Ceria is generally used as a ceria-zirconia composite oxide in which it is dissolved with zirconia (zirconium oxide ZrO ₂ ) in order to enhance its properties.

The ceria-zirconia-based composite oxide is generally mixed with alumina at a rate of several percent to several tens of percent in a state in which catalyst metals such as Pt, Pd, and Rh are supported, and is applied to the inner wall of a metal honeycomb or a ceramic honeycomb. It is wash-coated (coated) to a thickness of μm to several hundred μm and used as a catalytic converter (honeycomb catalyst structure).

With the tightening of exhaust gas regulations in recent years, catalyst metals and their carrier oxides have been investigated in order to improve the performance of catalysts, such as high activity and long life. In particular, a higher OSC can be obtained under a low temperature range of 200 to 400° C., and there is a high demand for a technique for oxidizing reduced chemical species (eg, carbon monoxide and propylene).

For example, there are the following conventional techniques for improving the performance of a ceria-zirconia composite oxide oxygen absorbing/releasing material, an exhaust gas purifying catalyst, and a honeycomb structure for exhaust gas purifying. Patent Document 1 discloses an oxygen absorption/release material containing a ceria-zirconia composite oxide phase and at least one rare earth oxide phase for the purpose of improving exhaust gas purification performance at low temperatures, wherein the rare earth oxide phase has a structure in which is directly bonded to the ceria-zirconia-based composite oxide phase, and the rare earth content is more than 0 in the rare earth oxide phase / (rare earth oxide + ceria-zirconia-based composite oxide phase) molar ratio Oxygen absorption and release materials are described that are less than or equal to .3. It is described that by doing so, the activity of the noble metal catalyst supported on the ceria-zirconia-based composite oxide can be improved, and the exhaust gas purification performance at low temperatures can be improved. However, no case has been reported that focuses on a grain structure in which a larger OSC can be obtained under a specific low temperature range around 200°C.

Patent Document 1 describes that a large OSC is exhibited from a low temperature around 200 ° C., but the molar ratio of cerium and zirconium is in the range of 0.7 ≤ Zr / (Ce / Zr) ≤ 0.85 and Ce is included. It is described that the amount is low and the solid solution particles are reduced in the temperature range of 1000 to 1300°C. It does not have a grain structure in which two phases coexist as in the present invention, and there is no description of low-temperature OSC development.

In Patent Document 2, the amount of oxygen released in a reducing atmosphere at 100 to 500° C. is 80% of the theoretical limit when a noble metal is supported on a support containing a composite oxide mainly composed of CeO ₂ and ZrO _{2 .} However, there is no description of the amount of oxygen release in the low temperature range of around 200°C. In addition, there is no description regarding the improvement of the OSC amount at low temperature by the particle shape.

In Patent Document 3, a cerium oxide-zirconium oxide-based composite oxide characterized by mixing cerium-zirconium composite oxide derived from a melting method and cerium dioxide derived from a wet method, and cerium derived from a melting method in a cerium-containing solution - A cerium oxide-zirconium oxide _- based composite oxide characterized by dispersing and neutralizing the zirconium composite oxide and then heat-treating ^it . is small.

Patent document 4 describes a CeO ₂ —ZrO ₂ —Al ₂ O ₃ composite oxide in which Al/(Ce+Zr) is in the range of 1/20 to 50/20, and has improved low-temperature activity by increasing the oxygen release rate. is. Furthermore, the low-temperature activity is enhanced by suppressing grain growth of Pt while maintaining the heat resistance of the carrier. However, there is no description that a high OSC amount was obtained by improving the grain structure as in the present invention.

Patent Document 5 describes the amount of OSC in the ceria-zirconia composite oxide, but the ceria utilization rate calculated from the OSC in the low temperature range at 200° C. is as low as 10 to 30%. The ceria utilization rate in the low temperature range of the ceria-zirconia powder developed by the present invention is 48% or more, which is higher than that.

JP-A-2003-275580 JP-A-2003-265958 WO2011/108457 JP-A-2005-104799 JP-A-2004-339025

As described above, in recent years, exhaust gas purifying catalysts have had the problem that the amount of OSC in the low temperature range is low and the purification rate in the low temperature range is low. Therefore, there is a demand for oxygen release capacity in a lower temperature range (200 to 800° C.), and developments corresponding to this are being continued. However, conventional exhaust gas purifying catalysts are not satisfactory in the amount of oxygen released under low-temperature use environments of 400° C. or less, and further improvements are desired.

Therefore, in view of the above circumstances, the present invention aims to provide a ceria-zirconia-based composite oxide oxygen-absorbing/releasing material that can release more oxygen even in a relatively low-temperature environment of 400°C or lower. Make it an issue.

In order to solve the above-mentioned problems, the present inventors have made intensive studies and found that when an adsorbed phase of reducing chemical species is present on the surface layer of the ceria-zirconia composite oxide particles, the direct reduction molecules are generated in the ceria-zirconia composite oxide particles. Adsorption to the surface can be prevented, and formation of an oxygen defect phase in the surface layer can be prevented. As a result, it was found that a large OSC amount of 194 to 361 μmol-O ₂ ·g ⁻¹ was obtained at 400°C or less, and the CeO ₂ utilization rate was 45 to 72% or more, and the present invention was completed. bottom.
That is, the gist of the present invention is as follows.

(1) The ceria-zirconia composite oxide is particles containing at least one rare earth element having an ionic radius of 80 to 120 pm, and the particle surface has an electronegativity of 0.0. A ceria-zirconia composite oxide oxygen storage/release material characterized by being modified with an oxide of 89 to 1.22.

(2) The oxide having an electronegativity of 0.89 to 1.22 is a rare earth or alkaline earth metal oxide, and the content of the oxide is 2 to 20% by mass of the total. The ceria-zirconia-based composite oxide oxygen storage/release material according to (1).

(3) The ceria-zirconia composite oxide contains at least one of the rare earth elements La, Y, and Nd having an ionic radius of 80 to 120 pm, and the ions of the rare earth elements are added to the ceria-zirconia composite oxide. The ceria-zirconia-based composite oxide oxygen storage/release material according to (1) or (2) above, characterized by containing 0.5 mol % or more and less than 22 mol % of the total amount of cations.

(3) The ceria-zirconia composite oxide contains at least one of the rare earth elements La, Y, and Nd having an ionic radius of 80 to 120 pm, and the ions of the rare earth elements are added to the ceria-zirconia composite oxide. The ceria-zirconia-based composite oxide oxygen storage/release material according to (1) or (2) above, characterized by containing 0.5 mol % or more and less than 22 mol % of the total amount of cations.

(4) The above (1) characterized by having an oxygen absorption/desorption characteristic (OSC amount) in a low temperature range of 400° C. of 248 to 361 μmol-O ₂ ·g ⁻¹ and a CeO ₂ utilization rate of 48 to 72%. ) to (3).

(5) The above (1) characterized by having an oxygen absorption/desorption characteristic (OSC amount) in a low temperature range of 200° C. of 194 to 327 μmol-O ₂ ·g ⁻¹ and a CeO ₂ utilization rate of 45 to 57%. ) to (3).

(6) An exhaust gas purifying catalyst comprising a ceria-zirconia composite oxide oxygen storage/release material according to any one of (1) to (5) above and carrying a noble metal thereon.

According to the present invention, a ceria-zirconia-based composite oxide exhibiting a large OSC amount of 194 to 361 μmol-O ₂ ·g ⁻¹ in a low temperature range of 200 to 800°C, particularly in a low temperature range of 400°C or less, absorbs oxygen. It can be an emissive material. That is, in the ceria-zirconia-based composite oxide oxygen absorption-release material of the present invention, 45 to 72% of the theoretical OSC amount is expressed, the oxygen deficiency concentrated phase expression is suppressed in the ceria-zirconia surface layer in a reducing atmosphere, and the ceria-zirconia Oxygen can be released from the entire particle, and an OSC amount closer to the theoretical oxygen capacity can be expressed in a low temperature range of 400°C.
In addition, if the oxygen absorbing/releasing material composed of the ceria-zirconia-based composite oxide is used for automobile exhaust gas purification catalysts, it has a remarkable effect of being able to satisfactorily purify exhaust gases even in a low temperature range of 400° C. or less.

FIG. 1 is a particle image diagram in which a ceria-zirconia-based composite oxide is used as a base material (mother phase), and the surface layer is modified with an adsorption phase. FIG. 2 is a diagram showing ionic radii of cationic species used in a ceria-zirconia composite oxide. FIG. 3(a) is a conceptual diagram showing the cause of the difference in the OSC amount between the Pd-supported product and the Pd-unsupported product, and FIG. 3(b) is a conceptual diagram showing an example of the Pd-supported product. is. Fig. 4(a) is a conceptual diagram showing how particles with an adsorbed phase develop the amount of low-temperature OSC. Fig. 4(a) shows a substrate (mother phase) of ceria-zirconia composite oxide (CZ) modified with _Nd2O3 as an adsorbed phase _. 4(b) is a micrograph of a ceria-zirconia-based composite oxide oxygen storage/release material, and FIG. 4(b) is for explaining that particles with an adsorbed phase (Nd ₂ O ₃ ) can express a larger amount of low-temperature OSC. It is a conceptual diagram. Temperature dependence of OSC amount. FIG. 10 is a diagram showing the results and comparison of ceria-zirconia particles modified with a reduced molecule adsorption phase exhibiting a large amount of OSC in a low temperature range of 200°C.

The present invention will be described in detail below.

The oxygen absorbing/releasing material of the present invention is a ceria-zirconia composite oxide containing at least one rare earth element ion in the ceria-zirconia composite oxide. It is possible to improve the oxygen absorption/release characteristics, durability, heat resistance, etc. of the product. Then, by using a ceria-zirconia-based composite oxide as a base material (mother phase) and forming a particle structure in which two phases in which a reducing chemical species adsorption phase is modified on the particle surface layer coexist, ceria・Oxygen can be released from the entire zirconia particles, and an OSC amount closer to the theoretical oxygen capacity can be expressed.

First, the reason why the amount of OSC is expressed will be described with reference to FIG.

FIG. 3 is a conceptual diagram showing the cause of the difference in OSC expression between Pd-supported and non-supported products. FIG. 3(a) shows an example of Pd-supported product, and FIG. It is a conceptual diagram.
As shown in FIG. 3( a ), when Pd is not supported as a catalytic precious metal (PGM) on ceria-zirconia composite oxide (CZ) as an oxygen absorbing/releasing material, the CZ base material (mother phase ), when many reducing molecules such as hydrogen 3 (H ₂ ) flow onto the particle surface of 1, only CeO ₂ in the very surface layer of the CZ substrate 1 becomes CeO _1.5 (CeO ₂ ⇒ _{CeO 1.5} + O _{0 .5} ), and oxygen can be extracted to form water 4 (H ₂ O). However, only oxygen in the surface layer is selectively extracted, and as a result, an oxygen defect layer 5 appears on the particle surface of the CZ base material (mother phase) 1, and oxygen in the center of the particle moves to the surface layer. I can't. As a result, the O ^2- ions in the oxide are depleted, and especially in the low temperature range, the ionic conductivity is low, so the oxygen (O ^2- ions) supplied from the center of the particle is small, and the amount of oxygen that can be extracted is relatively small. becomes low, and the reduction reaction of O ²⁻ +H ⁺ ⇒H ₂ O+e− ⁻ does not proceed. As a result, a large amount of oxygen defect layer 5 is formed on the grain surface layer, and a high OSC amount cannot be obtained.

On the other hand, as shown in FIG. 3(b), when Pd is supported as a catalytic precious metal (PGM) on the oxygen absorbing/releasing material, when hydrogen 3 (H ₂ ), which is a reducing molecule, flows, On the Pd catalyst 6, the oxygen (O) 7 from the CZ base material (mother phase) 1 causes the O ²⁻ +H ⁺ ⇒H ₂ O+e ⁻ reaction to proceed smoothly, and the particles of the CZ base material (mother phase) 1 H ₂ of the reducing molecule 3 is not adsorbed on the surface, and a large number of oxygen defect forming sites are less likely to appear on the surface layer of the particle. Therefore, the co-catalyst supporting the Pd catalyst tends to develop a larger amount of OSC than the co-catalyst not supporting the Pd catalyst.

However, there is a problem that some of the reduced molecules that have not reacted are adsorbed on the particle surface layer and form local oxygen defect sites. It can be inferred that if the formation of this oxygen defect site is suppressed, an even greater amount of OSC can be expressed even in a low temperature range.

Therefore, in the present invention, in order to suppress the formation of local oxygen defect sites and obtain a larger amount of OSC in a low temperature range, ceria and A two-phase coexistent particle structure modified with rare-earth metal oxides and alkaline-earth metal oxides was designed as an adsorption phase 2 that collects reducing chemical species on the base material (base material) 1 of a zirconia-based composite oxide. . By adopting this two-phase coexisting grain structure, a ceria-zirconia-based composite oxide oxygen storage/release material capable of obtaining a large amount of OSC (CeO ₂ utilization rate) at a lower temperature range, for example, 200° C., could be realized.

That is, in order to obtain a larger amount of OSC, ceria-zirconia oxide having an adsorbed phase on the base material can be used. A low electronegativity metallic element was used for this adsorption phase. The metal element is a rare earth or alkaline earth metal element, and the metal element having an electronegativity in the range of 0.89 to 1.22 is modified as an oxide on the particle surface of the ceria-zirconia composite oxide, and it is used as an adsorption phase. It can be used as an oxygen storage/release material by functioning as Furthermore, by carrying catalytic precious metals (PGM) on the particle surface, an exhaust gas purifying catalyst can be realized.

The catalyst noble metals (PGM) to be supported include noble metals such as Au, Ag, Pt, Pd, Rh, Ir, Ru, Os, etc. Ag, Pt, Pd, and Rh are more preferred. The noble metal is carried on the ceria-zirconia composite oxide oxygen absorption-release material, and is dispersed on the surface of the ceria-zirconia composite oxide oxygen absorption-release material. The dispersed noble metal is preferably present as a metal or oxide and has a size of 10 nm or less. The amount of noble metal supported (=noble metal/(noble metal + ceria-zirconia composite oxide oxygen absorption/release material)) is not particularly limited, but in order to sufficiently exhibit the catalytic function, it should be 0.01% by mass or more and 10% by mass. % or less, more preferably 0.1 mass % or more and 10 mass % or less.

Then, the exhaust gas purifying honeycomb structure is prepared by mixing the above exhaust gas purifying catalyst with alumina at a ratio of several percent to several tens of percent as usual, and forming a thickness of ten and several μm to several hundred μm on the inner wall of the metal or ceramic honeycomb. It is constructed by coating with

With reference to the conceptual diagram of FIG. 4, low-temperature OSC can be realized by forming a two-phase coexistent particle structure in which the CZ base material (mother phase) is modified with an adsorption phase that traps reducing chemical species. FIG. 4(a) is a micrograph of a ceria-zirconia-based composite oxide oxygen storage/release material in which a ceria-zirconia-based composite oxide (CZ) base (mother phase) is modified with Nd ₂ O ₃ as an adsorption phase. , FIG. 4(b) is a conceptual diagram for explaining that particles with an adsorbed phase (Nd ₂ O ₃ ) can develop a larger low-temperature OSC.

As shown in FIG. 4(b), when H ₂ (reduced molecule) 3, which is a reducing chemical species (explained with H ₂ as an example), flows, H ₂ is transferred to supported Pd(1) or Pd Since the O ²⁻ +H ⁺ ⇒H ₂ O+e ⁻ reaction proceeds smoothly on the Pd6 catalyst in (2) and water (H ₂ O) 4 is produced, a large number of oxygen defect formation sites do not appear on the particle surface layer. However, the reduced molecule (H ₂ ) 3 that could not react with Pd(1) or Pd(2) causes local oxygen defect sites. The reduced molecules 3 can be captured by the Nd ₂ O ₃ phase 8 that is the adsorption phase, and can be prevented from being adsorbed to the CZ substrate (mother phase) 1 . Therefore, in the CZ grains with the Nd ₂ O ₃ phase 8 attached as the adsorption phase, it is possible to prevent the appearance of local oxygen defect sites on the grain surface.

In this way, by modifying the particle surface of the ceria-zirconia composite oxide with an adsorbed phase, the appearance of the oxygen defect phase is suppressed, oxygen is supplied to the Pd active sites from the entire particle, and ceria can be used even in the low temperature range of 400 ° C. or less. The rate increased (45 to 72% ceria utilization rate), making it possible to express a large amount of OSC.

A two-phase coexisting particle structure in which the substrate (mother phase) of the ceria-zirconia composite oxide (containing at least one rare earth element) of the present invention is modified with a rare earth or alkaline earth metal oxide (adsorption phase) , the base material (mother phase) is ceria-zirconia composite oxide particles containing at least one rare earth element having an ionic radius of 80 to 120 pm, and the oxide phase of the rare earth element in the base material is ceria・It has a structure that is directly bonded to the zirconia-based composite oxide phase, and the rare earth content is more than 0 and 0.3 or less in the rare earth oxide phase/(rare earth oxide + ceria/zirconia-based composite oxide phase) molar ratio. is preferably

Also, the adsorption phase that scavenges the reducing species utilized oxides of low electronegativity elements. That is, oxides of rare earth or alkaline earth metals are used. The rare earth or alkaline earth metal oxide modifies the surface of the ceria-zirconia composite oxide particles, and the metal element of the rare earth metal ion or alkaline earth metal ion used in the adsorption phase has an electronegativity of 0.89 to 0.89. The range of 1.22 is used, and the surface of the ceria-zirconia composite oxide is modified with a rare earth or alkaline earth metal oxide.

Examples of rare earth oxides include Y ₂ O ₃ , Nd ₂ O ₃ , CeO ₂ , La ₂ O ₃ , Gd ₂ O ₃ , Pr ₆ O ₁₁ and composite oxides thereof such as Ce _1-x Nd _x O _{2- 0.5x} or the like can be used. CaO, SrO and BaO can be used as oxides of alkaline earth metals.

Table 1 shows the electronegativity of rare earth metal ions and alkaline earth metal ions and the electronegativity evaluation (○: excellent, Δ: good, ×: poor).

When a metallic element having an electronegativity of less than 0.89 is used, the electronegativity becomes large and attracts P and S, poisoning the oxide and easily degrading it. On the other hand, if the electronegativity exceeds 1.22, the electron-attracting power becomes weaker, and the effect may be diminished.

Therefore, the present invention used rare earth or alkaline earth metal ions with electronegativities in the range of 0.89 to 1.22.

The negative charge of the oxygen atoms contained in the metal oxide used in the adsorption phase increases as the difference in electronegativity between the oxygen atoms and the atoms adjacent to the oxygen atoms increases. As this negative charge increases, the electrostatic interaction between oxygen atoms and metal ions contained in the oxide of the adsorption phase becomes stronger, attracting chemical species such as reduced molecules and acting as an adsorption phase.

When the adsorbed phase exists on the surface layer of the ceria-zirconia composite oxide phase, it is possible to prevent the direct reduction molecules from being adsorbed on the surface of the ceria-zirconia composite oxide particles, thereby preventing the formation of the oxygen defect phase on the surface layer. be able to.

In addition, the present invention is not limited by the above conjecture.

Therefore, in the ceria-zirconia-based composite oxide oxygen storage/release material of the present invention, it is important that the ceria-zirconia-based composite oxide is modified with the aforementioned adsorption phase. It is important that the adsorption phase uses a metal element with low electronegativity and that the ceria-zirconia composite oxide contains a rare earth element with an ionic radius of 80-120 pm. A mere ceria-zirconia composite oxide cannot provide a large amount of OSC (200° C.-OSC amount of 290 μmol-O ₂ ·g ⁻¹ or more) in a low temperature range.

The ceria-zirconia-based composite oxide oxygen absorption-release material of the present invention is a ceria-zirconia-based composite oxide in which La, Y, and Nd having an ionic radius of 80 to 120 pm are used in the ceria-zirconia-based composite oxide. wherein at least one rare earth element ion is contained in the ceria-zirconia composite oxide. If the ionic radius is less than 80 pm, the crystal lattice becomes small, which may hinder the oxidation-reduction reaction of ceria. On the other hand, if a metal ion with an ionic radius exceeding 120 pm is used, there is a risk that it will not form a solid solution with the ceria-zirconia composite oxide. FIG. 2 shows the ionic radius/pm of each metal of the rare earth and alkaline earth metal cationic species used in the ceria-zirconia composite oxide. Ions suitable for use are the trivalent rare earth metal ions Y ³⁺ , Nd ³⁺ , La ³⁺ , the tetravalent rare earth metal ions Zr ⁴⁺ , Ce ⁴⁺ and the trivalent cerium metal ions Ce ³⁺ .

In the ceria-zirconia composite oxide, the low electronegativity oxide is a rare earth or alkaline earth metal oxide, and the content of the modified oxide is 2 to 20% by mass of the total. A ceria-zirconia composite oxide characterized by

If the content of the modified oxide is less than 2% by mass of the total, the adsorption effect is weakened, and local oxygen defect sites appear, resulting in a large amount of OSC (194-361 μmol −O ₂ ·g ⁻¹ and CeO ₂ utilization of 45-72%) expression cannot be expected. On the other hand, if it exceeds 20% by mass, the theoretical amount of OSC per unit weight decreases, and it is difficult to confirm an improvement in the amount of OSC.

Therefore, in the present invention, the content of modified oxides is set to 2 to 20 mass % of the total.

In the present invention, an index of ceria utilization rate is used in order to indicate the efficiency of expression of the OSC amount of the ceria-zirconia-based composite oxide. When all the ceria (CeO ₂ ) contained in the ceria-zirconia composite oxide is reduced to trivalent ceria (CeO _1.5 ), the ceria utilization rate is expressed as 100%, and the OSC amount at that time reaches the theoretical capacity. ( _CeO2 → _CeO1.5 + _0.25O2 ).

The ceria-zirconia composite oxide oxygen absorbing/releasing material that exhibits a large OSC in a low temperature environment of 200 to 400° C. according to the present invention is characterized by an OSC amount of 200° C. and an OSC amount of 194 μmol-O ₂ ·g ⁻¹ . Above, preferably 194 to 327 μmol-O ₂ ·g ⁻¹ , a CeO ₂ utilization rate of 45 to 57%, preferably 47 to 55%, and a 400°C-OSC amount of 248 μmol-O ₂ ·g ⁻¹ or more. , preferably 248-361 μmol-O ₂ ·g ^-1 and a CeO ₂ utilization of 48-72%, preferably 48-61%.

In the ceria-zirconia composite oxide, the CeO ₂ (ceria) utilization rate is 45% or more in the low temperature range of 200 ° C., the OSC amount is 194 μmol-O ₂ · g ^-1 or more, and the CeO ₂ The phenomenon that the utilization rate is 48% or more and the OSC amount is 248 μmol-O ₂ ·g ⁻¹ or more has not been completely clarified, but rare earth element ions are ceria-zirconia composite oxidation 0.5 mol% or more and less than 22 mol%, preferably 2.5 mol% or more and 20 mol% or less, and the modified oxide content (modification amount) is 2 to 20 mol% of the total amount of cations in the product. % by mass.

As a method for producing the ceria-zirconia composite oxide oxygen absorber of the present invention, a generally known method for producing a ceria-zirconia composite oxide can be employed.

In the present invention, the amount of OSC in the low temperature range was measured using ASAP (model number: ASAP2020) manufactured by Shimadzu Corporation. 0.5% by mass of Pd was supported on the ceria-zirconia composite oxide described above, and the sample was calcined at 600° C. for 1 hour to support the Pd catalyst, which was used as a sample for measuring the amount of OSC.

A sample for OSC amount measurement that had been pre-dried (at least 40 minutes at 200°C under reduced pressure) prior to ASAP measurement was measured by ASAP.

Two bundles of 0.1 g of glass wool were packed at the bottom of an ASAP-dedicated glass tube, and 0.15 g of the aforementioned ceria-zirconia composite oxide supporting Pd was packed thereon. placed. The role of the glass wool is to prevent the aforementioned ceria-zirconia powder from blowing off and scattering when the pressure is reduced.

Attach the ASAP dedicated glass tube with such settings to the ASAP, 200 ° C. x 10 minutes (reduced pressure), then 200 ° C. x 10 minutes (pure hydrogen flow), and hold again in a reduced pressure state (hold for 10 minutes and leak After carrying out a check), pure oxygen was supplied to a dedicated glass tube at a fixed amount and at a constant pressure, and the amount of oxygen absorbed was determined from the pressure fluctuation, and the amount of OSC (μmol-O ₂ ·g ⁻¹ ) was calculated.

A method for calculating the OSC theoretical capacity (μmol-O ₂ ·g ^-1 ) and the Ce utilization rate (%) will be described.

The theoretical OSC amount (%) is given by the following formula (1) because it is caused by the valence change of ceria.

4CeO ₂ ⇔ 2Ce ₂ O ₃ + O ₂ (1)

According to the formula (1), 1 mol of oxygen is released with respect to 4 mol of Ce. The theoretical capacity of OSC (μmol-O ₂ ·g ⁻¹ ) released from 1 g of CZ is given by the following formula (2).

C _Ce ÷ ( _MCe + 2M _O ) ÷ 4 ÷ 100 (2)

Here, the symbols of the above formula (2) will be explained.
C _Ce : Ce content in the sample (mass%), M _Ce : Ce atomic weight (140.116 g/mol), _MO : O atomic weight (15.999 g/mol)

The Ce utilization rate (%) is obtained by dividing the actually measured OSC capacity (μmol-O ₂ ·g ^-1 ) by the OSC amount (μmol-O ₂ ·g ^-1 ) calculated by the above formula (2), and expressed as a percentage. The obtained value was defined as the Ce utilization rate (%). Putting them together gives the following formula (3).

Here, symbols for the above formula (3) will be explained.
A: OSC amount of sample (μmol-O ₂ /%), C _Ce : Ce content in sample (mass%)

The effects of the present invention will be described in detail below based on examples and comparative examples of the present invention. However, the present invention is not limited to these examples.

[Example 1]
First step Solution 1 where cerium chloride solution, zirconium oxychloride solution, lanthanum chloride solution and pure water are mixed so that the cation ratio is Ce:Zr:La = 34.0:60.7:4.6 mol/L. (liter) was obtained. 15 g of ammonium peroxodisulfate was added to the resulting mixed solution, and the mixture was heated to 95° C. with stirring to obtain a cerium-zirconium composite sulfate. After the obtained sulfate slurry was cooled to 60° C., it was neutralized by adding aqueous ammonia to obtain a slurry containing hydroxide. The resulting slurry was filtered and washed four times, dried at 120° C., fired at 700° C. and pulverized in a mortar to obtain a ceria-zirconia composite oxide containing rare earth element ions (Ce _0.34 Zr _0.607 La _0.046 O _1.977 ).

In the second step, after pulverizing the powder obtained in the first step, an aqueous slurry is prepared from the ceria-zirconia oxide, and the Nd salt solution is prepared so that the surface modifier after firing is 2% by mass. was added, and the pH of the solution was changed to precipitate hydroxide on the oxide surface. After removing contaminant ions, the particles were heated and sintered to precipitate neodymium oxide (Nd ₂ O ₃ ) on the particle surface.

Pd was supported on particles obtained by depositing neodymium oxide (Nd ₂ O ₃ ) on the surface of the ceria-zirconia composite oxide. When the amount of OSC was measured by ASAP, 278 μmol-O ₂ ·g ^-1 was observed as a measured value at 200°C in the low temperature range. The CeO ₂ utilization rate at 200°C was 47%, and a relatively large amount of OSC was confirmed in the low temperature range (200°C). In addition, 312 μmol-O ₂ ·g ⁻¹ was observed as a measured value at 400°C in the low temperature range. The CeO ₂ utilization rate at 400° C. was 52%, and a relatively large amount of OSC was confirmed in the low temperature range (200 to 400° C.).

[Examples 2 to 4]
This is the same as the ceria-zirconia composite oxide shown in the first stage of Example 1, except that the type of rare earth oxide that becomes the adsorbed phase with 2% by mass of the surface modifier is changed. In both cases, a large amount of OSC was confirmed, with a ceria utilization rate of 45 to 48% at a low temperature range of 200°C and a ceria utilization rate of 48 to 54% at a low temperature range of 400°C.

[Examples 5 and 6]
The composition of the substrate (mother phase) shown in Example 1 was changed. In both cases, a large amount of OSC with a ceria utilization rate of around 50% was confirmed at low temperatures of 200°C and 400°C.

[Examples 7 to 16]
The ceria-zirconia composite oxide shown in the first stage of Example 1 was modified by increasing the amount of surface modifiers to 5% by mass and changing the type of rare earth oxide or alkaline earth metal oxide used as the adsorption phase. In both cases, a large amount of OSC was confirmed, with a ceria utilization rate of 51 to 57% at a low temperature range of 200°C and a ceria utilization rate of 57 to 63% at a low temperature range of 400°C.

[Examples 17 and 18]
In Examples 17 and 18, the amount of modification of the composite oxides having the compositions of Examples 5 and 6 with rare earth oxide (Nd ₂ O ₃ ) in the adsorbed phase, which is a surface modifier, was increased from 2% to 5%. For example. In both cases, a large amount of OSC with a ceria utilization rate of 50% or more was confirmed at the low temperature range of 200°C and 400°C.

[Examples 19 to 24]
Examples 19 to 24 are examples in which the ceria-zirconia composite oxide having the composition of each of Examples 1 to 6 was modified by increasing the amount of modification of the rare earth oxide in the adsorption phase, which is the surface modifier, to 10%. In both cases, a large amount of OSC with a ceria utilization rate of 47% or more was confirmed at 200° C. and 400° C. in the low temperature range.

[Examples 25-30]
Examples 25 and 30 are examples in which the composite oxides having the compositions of Examples 19 and 24 were modified with 20% of the rare earth oxide in the adsorbed phase as a surface modifier. In both cases, a large amount of OSC with a ceria utilization rate of 46% or more was confirmed at 200° C. and 400° C. in the low temperature range.

In these examples, as the adsorbed phase which is a surface modifier, rare earth oxides of Nd, Y, Ce, La, Gd and Pr, composite oxides of Ce and Nd, or Ca, Sr and Ba An example using each alkaline earth metal oxide of is shown. These adsorbed phases can be formed by using a salt solution of each rare earth or alkaline earth metal, similar to the second stage process for forming Na ₂ O ₃ in Example 1. For example, when alkaline earth metal Ca is used for the adsorption phase in Example 14, as the second step, after pulverizing the powder obtained in the first step, an aqueous slurry is prepared with a ceria-zirconia oxide. After that, a Ca salt solution is added so that the surface modifier after firing is 5% by mass, and the pH of the solution is adjusted with ammonium carbonate to precipitate carbonate on the oxide surface and modify the particle surface. After this subsequent firing step, a CaO-modified ceria-zirconia particle surface is obtained.

[Comparative Examples 1 to 5]
In Comparative Examples 1 to 5, in the first-stage ceria-zirconia composite oxide oxygen absorption-release material shown in Example 1, the type and content of rare earth element ions contained were different, and the surface was modified with an oxide. This is an example of the amount of OSC when there is no OSC. That is, Comparative Example 1 is an example in which the surface of the ceria-zirconia oxide having the composition corresponding to Examples 1 to 4, 7 to 16, 19 to 22, and 25 to 28 was not modified with an oxide. Comparative Example 2 is an example in which the surface of the ceria-zirconia-based oxide having the composition corresponding to Examples 5, 17, 23, and 29 was not modified with an oxide, and Comparative Example 4 is an example in which the oxide modification was not performed. , 30 in which the surface was not modified with an oxide.
In the comparative examples, in the low temperature range (200° C. and 400° C.), no large amount of OSC was observed as confirmed in the examples.

The results of these Examples and Comparative Examples are summarized in Table 2, and a representative example is shown in FIG.

As shown in Tables 3 and 4, Examples 1-4, 7-16, 19-22, 25-28 and Examples 5, 17, 23, 29 and Examples 5, 17, 23, 29 are substrates ( The composition of the base material) was unified, and the oxide species used in the adsorption phase was changed, and an example was shown in which the effect was verified. The purpose of this is to verify the effect of the adsorbed phase. In the examples, the oxide species and the amount of modification used in the adsorbed phase were changed, but in all cases the ceria utilization rate was 45% or more in the low temperature range (200 to 400 ° C). A large amount of OSC was confirmed.
In the comparative example, no adsorbed phase was added, and the result of changing the base material (base material) was shown, but a large amount of OSC was not observed.

1 CZ base material (mother phase)
2 Adsorption phase (rare earth metal oxides, alkaline earth metal oxides)
_3H2
4H2O
5 Oxygen defect layer
6 Pd
7O
_{8 Nd2O3}

Claims (7)

The ceria-zirconia composite oxide is a particle containing at least one rare earth element having an ionic radius of 80 to 120 pm, and the particle surface serves as an adsorption phase for reducing chemical species and has an electronegativity of 0.89 to 1. A ceria-zirconia composite oxide modified with an oxide of an element of .22 , wherein the oxide of an element having an electronegativity of 0.89 to 1.22 is an oxide of an alkaline earth metal. Oxygen absorbing/releasing material. 2. The ceria-zirconia composite oxide oxygen-releasing material according to claim 1, wherein the content of cerium ions in said ceria-zirconia composite oxide is 0.34 or less in molar ratio. 3. The oxygen-absorbing ceria-zirconia-based composite oxide according to claim 1 , wherein the content of the oxide having an electronegativity of 0.89 to 1.22 is 2 to 20% by mass of the total. emission material. The ceria-zirconia-based composite oxide contains at least one of rare earth elements La, Y, and Nd having an ionic radius of 80 to 120 pm, and the ions of the rare-earth elements are added to the total amount of cations in the ceria-zirconia-based composite oxide. 4. The ceria-zirconia-based composite oxide oxygen storage/release material according to any one of claims 1 to 3, characterized by containing 0.5 mol % or more and less than 22 mol % in molar ratio. Oxygen storage and release characteristics (OSC amount) in a low temperature range of 400° C. are 248 to 361 μmol-O ₂ ·g ⁻¹ and CeO ₂ utilization is 48 to 72 %. The ceria-zirconia composite oxide oxygen storage/release material according to any one of the above. Oxygen storage and release characteristics (OSC amount) in a low temperature range of 200° C. are 194 to 327 μmol-O ₂ ·g ⁻¹ and CeO ₂ utilization is 45 to 57 %. The ceria-zirconia composite oxide oxygen storage/release material according to any one of the above. An exhaust gas purifying catalyst comprising the ceria-zirconia composite oxide oxygen storage/release material according to any one of claims 1 to 6 and a noble metal supported thereon.

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