JP2020070219A - Ceria-zirconia-based composite oxide oxygen absorption and release material, and exhaust gas purification catalyst - Google Patents

Abstract

To provide a ceria-zirconia-based composite oxide oxygen absorbing and releasing material and an exhaust gas purifying catalyst that exhibit great oxygen-absorbing and oxygen-releasing characteristics in a low temperature range (at 400°C or lower).SOLUTION: A ceria-zirconia-based composite oxide oxygen absorbing and releasing material, and an exhaust gas purifying catalyst using the same material, comprising ceria-zirconia composite oxide particles containing at least one rare earth element having an ionic radius of 80 to 120 pm, the surface of which is modified with an oxide of a rare earth or an alkaline earth metal having an electro-negativity of 0.89 to 1.22 that functions as an adsorption phase for reducing chemical species, the content of the oxide is at least 2 to 20 mass% of the total, the oxygen absorption and emission characteristics (volume of OSC) is 194 to 327 μmol-Ogat 200°C, the rate of utilization of CeOis 45 to 57%, the oxygen absorption and emission characteristics (OSC) is 248 to 361 μmol-Ogat 400°C, and the rate of utilization of CeOis 48 to 72%.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a ceria-zirconia-based composite oxide oxygen absorption / release material and an exhaust gas purification catalyst that exhibit a large oxygen absorption / release property 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 (NO _x ), and therefore is purified by an exhaust gas purification catalyst. , Need to be released to the atmosphere.

  There is a three-way catalyst for purifying such harmful components of exhaust gas. As the catalyst, a catalyst in which a precious metal such as platinum (Pt), palladium (Pd) or rhodium (Rh) is supported on a carrier such as an oxide is widely used.

  In a three-way catalyst, it is preferable to keep the fuel-air ratio (air-fuel ratio) constant (theoretical air-fuel ratio) in order to enhance the function of the noble metal, but in operating conditions that fluctuate in various running conditions. Accordingly, the air-fuel ratio changes greatly. For this reason, the air-fuel ratio (A / F), which fluctuates depending on the engine operating conditions, is kept constant by feedback control using an oxygen sensor, but there is a temporal fluctuation in A / F depending on 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 finely adjust the atmosphere on the catalyst side. Since ceria (cerium oxide CeO ₂ ) has an oxygen storage / release property (Oxygen Storage Capacity, hereinafter referred to as “OSC”), it is widely used as a co-catalyst for adjusting the oxygen partial pressure of a vehicle exhaust gas purification catalyst. This utilizes the redox reaction of Ce ³⁺ / Ce ⁴⁺ . The ceria is generally used as a ceria-zirconia-based composite oxide that is solid-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 ratio of several% to several tens of% in a state where a catalytic metal such as Pt, Pd, Rh, etc. is supported, and the ceria / zirconia-based composite oxide is mixed with the inner wall of a metal honeycomb or a ceramic honeycomb to a dozen or more It is used as a catalytic converter (honeycomb catalyst structure) after being wash-coated (coated) to a thickness of μm to several hundreds of μm.

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

  For example, there are the following conventional techniques related to the performance improvement of the ceria-zirconia-based composite oxide oxygen absorbing / releasing material, the exhaust gas purifying catalyst, and the exhaust gas purifying honeycomb structure. Patent Document 1 discloses an oxygen absorbing / releasing material containing a ceria-zirconia-based composite oxide phase and at least one rare earth oxide phase for the purpose of enhancing exhaust gas purification performance at a low temperature. Has a structure in which it is directly bonded to the ceria-zirconia-based composite oxide phase, and the rare earth content exceeds 0 in a rare earth oxide phase / (rare earth oxide + ceria-zirconia-based composite oxide phase) molar ratio. Oxygen absorption and release materials that are less than or equal to 0.3 are described. 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 is specialized in a particle structure that can obtain a larger OSC in a specific low temperature region near 200 ° C.

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

In Patent Document 2, a carrier containing a noble metal supported on a carrier containing a complex oxide containing CeO ₂ and ZrO ₂ as main components has an oxygen amount released in a reducing atmosphere of 100 to 500 ° C. of 80% of a theoretical limit value. Although the characteristic of being present is described, there is no description of the oxygen release amount in the low temperature region near 200 ° C. Moreover, there is no description about the improvement of the OSC amount at low temperature due to the particle shape.

In Patent Document 3, a cerium-zirconium composite oxide derived from a melting method and a cerium dioxide derived from a wet method are mixed, and a cerium oxide-zirconium oxide-based composite oxide, and cerium derived from a melting method in a cerium-containing solution. -The cerium oxide-zirconium oxide-based composite oxide is characterized in that the zirconium composite oxide is dispersed, neutralized, and then heat-treated, but it has a very OSC amount of 15 µmol-O ₂ · g ^-1 at 600 ° C. Is small.

In Patent Document 4, Al / (Ce + Zr) is a CeO ₂ —ZrO ₂ —Al ₂ O ₃ composite oxide having an Al / (Ce + Zr) range of 1/20 to 50/20, and has an improved oxygen release rate and improved low temperature activity. Is. Furthermore, the low temperature activity is enhanced by suppressing the Pt grain growth 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 OSC amount of the ceria-zirconia composite oxide, but the ceria utilization rate calculated from the OSC in the low temperature region at 200 ° C. is as low as 10 to 30%. The utilization rate of ceria in the low temperature region of the ceria-zirconia powder developed by the present invention is 48% or more, which is higher than that.

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

  As described above, in recent years, the exhaust gas purifying catalyst has a 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, oxygen releasing ability in a lower temperature range (200 to 800 ° C.) is required, and development corresponding to it is being continued. However, the conventional exhaust gas purifying catalysts have not been satisfactory in the amount of oxygen released under a low temperature use environment of 400 ° C. or lower, and further improvement is desired.

  Therefore, in view of the above circumstances, the present invention provides a ceria-zirconia-based composite oxide oxygen absorption / release material capable of releasing a larger amount of oxygen even in a use environment under a relatively low temperature environment of a low temperature range of 400 ° C. or lower. It is an issue.

In order to solve the above problems, the inventors of the present invention have diligently studied, and when an adsorbing phase of a reducing chemical species is present in the surface layer of the ceria-zirconia-based composite oxide particles, the direct reducing molecule causes the ceria-zirconia-based composite oxide particles. Adsorption on the surface can be prevented, and formation of oxygen defect phase in the surface layer can be prevented. As a result, it was found that a large OSC amount of 400 ° C. or lower at 194 to 361 μmol-O ₂ · g ⁻¹ was obtained, and the CeO ₂ utilization rate was 45 to 72% or more, thereby completing the present invention. did.
That is, the gist of the present invention is as follows.

  (1) The ceria-zirconia mixed oxide is a particle containing at least one kind of rare earth element having an ionic radius of 80 to 120 pm, and the particle surface has an electronegativity of 0. A ceria-zirconia-based 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 an oxide of a rare earth or alkaline earth metal, and the content of the oxide is 2 to 20% by mass of the whole. The ceria-zirconia-based composite oxide oxygen storage / release material according to (1) above.

  (3) The ceria-zirconia-based composite oxide contains at least one rare earth element of La, Y, and Nd having an ionic radius of 80 to 120 pm, and the rare-earth element ions are contained in the ceria-zirconia-based composite oxide. The ceria-zirconia-based composite oxide oxygen storage / release material according to (1) or (2) above, which is contained in a molar ratio of 0.5 mol% or more and less than 22 mol% with respect to the total amount of cations.

  (3) The ceria-zirconia-based composite oxide contains at least one rare earth element of La, Y, and Nd having an ionic radius of 80 to 120 pm, and the rare-earth element ions are contained in the ceria-zirconia-based composite oxide. The ceria-zirconia-based composite oxide oxygen storage / release material according to (1) or (2) above, which is contained in a molar ratio of 0.5 mol% or more and less than 22 mol% with respect to the total amount of cations.

(4) The oxygen absorption / desorption characteristics (OSC amount) in the low temperature region of 400 ° C. are 248 to 361 μmol-O ₂ · g ⁻¹ , and the CeO ₂ utilization rate is 48 to 72%. ) To (3), a ceria-zirconia-based composite oxide oxygen storage / release material.

(5) The oxygen absorption / desorption characteristics (OSC amount) in a low temperature region of 200 ° C. are 194 to 327 μmol-O ₂ · g ⁻¹ , and the CeO ₂ utilization rate is 45 to 57%. ) To (3), a ceria-zirconia-based composite oxide oxygen storage / release material.

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

According to the present invention, a ceria-zirconia-based complex oxide oxygen absorption showing a large OSC amount of 194 to 361 μmol-O ₂ · g ⁻¹ in a low temperature region of 200 to 800 ° C., particularly in a low temperature region of 400 ° C. or lower. It can be a release material. That is, in the ceria-zirconia-based composite oxide oxygen absorbing / releasing material of the present invention, 45 to 72% of the theoretical OSC amount is expressed, the oxygen-deficient concentrated phase expression is suppressed in the ceria-zirconia surface layer in the reducing atmosphere, and the ceria-zirconia It became possible to release oxygen from the entire particles, and it became possible to express the OSC amount closer to the theoretical oxygen capacity in the low temperature region of 400 ° C.
In addition, when the oxygen storage / release material composed of the ceria / zirconia-based composite oxide is used for the automobile exhaust gas purification catalyst, the exhaust gas can be satisfactorily purified even in a low temperature range of 400 ° C. or lower.

FIG. 3 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 thereof is modified with an adsorption phase. It is a figure which shows the ionic radius of the cation species used for a ceria-zirconia-based composite oxide. FIG. 3A is a conceptual diagram showing the cause of the difference in the OSC amount between the Pd-supported product and the unsupported product, FIG. 3A is a conceptual diagram showing an example of the Pd-supported product, and FIG. Is. 4A is a conceptual diagram in which particles having an adsorbed phase develop a low-temperature OSC amount. FIG. 4A shows a base material (mother phase) of a ceria-zirconia-based composite oxide (CZ) modified with Nd ₂ O ₃ as an adsorbed phase. FIG. 4 (b) is a photomicrograph of the ceria-zirconia-based composite oxide oxygen storage / release material, and FIG. 4 (b) illustrates that the particles with the adsorbed phase (Nd ₂ O ₃ ) can express a larger low-temperature OSC amount. It is a conceptual diagram. Temperature dependence of OSC amount. It is a figure which shows the result and comparison which showed the large OSC amount in the low temperature area | region of 200 degreeC with the ceria zirconia particle which modified the reducing molecule adsorption phase.

  Hereinafter, the present invention will be described in detail.

  The oxygen absorbing / releasing material of the present invention is a ceria-zirconia composite oxide in which at least one rare earth element ion is contained in the ceria-zirconia composite oxide, and by containing the rare earth element ion, 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 (matrix), and having a particle structure in which two phases modified with an adsorption phase of a reducing chemical species coexist on the surface layer of the particles, ceria is obtained.・ Oxygen can be released from the entire zirconia particles, and the OSC amount can be expressed closer to the theoretical oxygen capacity.

  First, the reason for the expression of the OSC amount 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) showing an example of Pd-supported product, and FIG. 3 (b) showing an example of Pd-supported product. It is a conceptual diagram.
As shown in FIG. 3 (a), when Pd as the catalytic noble metal (PGM) is not supported on the ceria-zirconia-based composite oxide (CZ) as the oxygen absorbing / releasing material, the CZ base material (mother phase) When a large number of reducing molecules such as hydrogen 3 (H ₂ ) flow on the surface of the particles 1), only CeO _{2 at the} very surface layer of the particles of the CZ substrate 1 is CeO _1.5 (CeO ₂ ⇒CeO _1.5 + O _{0). .5} ), oxygen can be taken out and used as water 4 (H ₂ O). However, only oxygen in the surface layer is selectively taken out, and as a result, the oxygen defect layer 5 appears on the particle surface of the CZ base material (mother phase) 1, and oxygen in the central portion of the particle moves to the surface layer. I can't do it. As a result, the O ²⁻ ion in the oxide is depleted, and its ionic conductivity is low and poor especially in the low temperature region, so the amount of oxygen (O ²⁻ ion) supplied from the central part of the particle is small and the amount of oxygen that can be extracted relatively. 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 surface layer of the particle, and a high OSC amount cannot be obtained.

On the other hand, as shown in FIG. 3B, when Pd as a catalytic noble metal (PGM) is supported on the oxygen absorbing / releasing material, when hydrogen 3 (H ₂ ) that is a reducing molecule flows, O ² + H ⁺ ⇒H ₂ O + e ^- reaction proceeds smoothly by oxygen (O) 7 from the CZ base material (matrix phase) 1 on the Pd catalyst 6, and particles of the CZ base material (matrix phase) 1 H ₂ of the reducing molecule 3 is not adsorbed on the surface, and it is difficult for a large amount of oxygen defect forming sites to appear on the surface layer of the particle. Therefore, the cocatalyst carrying the Pd catalyst is more likely to exhibit a larger OSC amount than the case without the Pd catalyst being carried.

  However, there is a problem that some of the unreacted reducing molecules are adsorbed on the surface layer of the particles to form local oxygen defect sites. It can be inferred that if the formation of the oxygen deficiency site is suppressed, a larger OSC amount 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 OSC amount in a low temperature region, as shown in the particle image diagram of the ceria-zirconia-based composite oxide in FIG. We designed a two-phase coexisting particle structure modified with rare earth metal oxides or alkaline earth metal oxides as an adsorbing phase 2 for collecting reducing chemical species on a base material (base material) 1 of a zirconia-based composite oxide. .. By adopting this two-phase coexisting particle structure, it was possible to realize a ceria-zirconia-based composite oxide oxygen storage / release material that can obtain a large OSC amount (CeO ₂ utilization rate) at a lower temperature range, for example, 200 ° C.

  That is, in order to obtain a larger amount of OSC, it can be realized with the ceria-zirconia oxide in a state where the adsorbed phase is present on the base material. A metal element having a low electronegativity 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 the adsorbed phase is used. An oxygen storage / release material can be obtained by functioning as. Furthermore, an exhaust gas purifying catalyst can be realized by supporting a catalytic noble metal (PGM) on the surface of the particles.

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

  In the exhaust gas purifying honeycomb structure, the exhaust gas purifying catalyst is mixed with alumina in a ratio of several% to several tens% according to a conventional method to form a metal or ceramic honeycomb inner wall with a thickness of ten dozen μm to several hundred μm. It is composed by coating.

It will be described with reference to the conceptual diagram of FIG. 4 that low-temperature OSC can be realized by using a two-phase coexisting particle structure modified with an adsorption phase that collects a reducing chemical species on a CZ base material (mother phase). FIG. 4A is a micrograph of a ceria-zirconia-based composite oxide oxygen-absorbing / releasing material obtained by modifying a base material (mother phase) of a ceria-zirconia-based composite oxide (CZ) with Nd ₂ O ₃ as an adsorption phase. FIG. 4B is a conceptual diagram for explaining that particles having an adsorbed phase (Nd ₂ O ₃ ) can express a larger low temperature OSC.

As shown in FIG. 4 (b), when H ₂ (reducing molecule) 3, which is a reducing chemical species (explaining H ₂ as an example), flows, H ₂ is loaded onto Pd (1) or Pd. On the Pd6 catalyst of (2), the O ^{2 −} + H ⁺ ⇒H ₂ O + e ⁻ reaction proceeds smoothly and water (H ₂ O) 4 is generated, so that a large amount of oxygen deficiency formation site does not appear in the particle surface layer. However, the reducing molecule (H ₂ ) 3 that cannot react with Pd (1) or Pd (2) causes a local oxygen defect site. The reduced molecule 3 can be prevented from being captured by the Nd ₂ O ₃ phase 8 which is an adsorption phase and adsorbed on the CZ base material (mother phase) 1. Therefore, in the CZ particles having the Nd ₂ O ₃ phase 8 as the adsorption phase, it is possible to prevent the occurrence of local oxygen defect sites on the particle surface.

  Thus, by modifying the particle surface of the ceria-zirconia composite oxide with an adsorption phase, the oxygen deficiency phase development is suppressed, oxygen is supplied from the entire particle to Pd active points, and ceria is utilized even in a low temperature region of 400 ° C or lower. The rate was increased (ceria utilization rate 45 to 72%), and it became possible to express a large amount of OSC.

  Two-phase coexisting particle structure obtained by modifying the base material (mother phase) of the ceria-zirconia-based composite oxide (containing at least one kind of rare earth element) of the present invention with a rare earth or alkaline earth metal oxide (adsorption phase) In, the base material (mother phase) is ceria-zirconia-based composite oxide particles containing at least one kind of 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. -Has a structure 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 preferred.

  In addition, an oxide of an element having a low electronegativity was used as the adsorption phase for collecting the reducing chemical species. That is, an oxide of rare earth or alkaline earth metal is 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 The range of 1.22 is used, and the surface of the ceria-zirconia-based composite oxide is modified with an oxide of a rare earth or alkaline earth metal.

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

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

  When a metal element having an electronegativity of less than 0.89 is used, the electronegativity becomes large and attracts P and S, and the oxide is poisoned and easily deteriorates. On the other hand, when the electronegativity exceeds 1.22, the force of attracting electrons becomes small and the effect may be diminished.

  Therefore, in the present invention, rare earth or alkaline earth metal ions having an electronegativity in the range of 0.89 to 1.22 were used.

  The negative charge of oxygen atoms contained in the metal oxide used in the adsorption phase increases as the difference in electronegativity between the oxygen atoms and atoms adjacent to the oxygen atoms increases. When this negative charge becomes large, the electrostatic interaction that acts between the oxygen atoms and the metal ions contained in the oxide of the adsorption phase becomes strong, so that chemical species such as reducing molecules are attracted to act as an adsorption phase.

  When the adsorbed phase is present on the surface layer of the ceria-zirconia-based composite oxide phase, it is possible to prevent direct reducing molecules from adsorbing on the surface of the ceria-zirconia-based composite oxide particle and prevent the formation of an oxygen defect phase in the surface layer. be able to.

  The present invention is not limited to the above speculation.

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

The ceria-zirconia-based composite oxide oxygen absorption / release material of the present invention is a ceria-zirconia-based composite oxide, wherein La, Y, Nd having an ionic radius of 80 to 120 pm is present in the ceria-zirconia-based composite oxide. The oxygen storage / release material is characterized in that at least one rare earth element is contained therein, and at least one rare earth element ion is contained in the ceria-zirconia composite oxide. When the ionic radius is less than 80 pm, the crystal lattice becomes small, which may hinder the redox reaction of ceria. On the other hand, when a metal ion having an ionic radius of more than 120 pm is used, there is a risk that it will not form a solid solution with the ceria-zirconia composite oxide, so it was decided to use a rare earth element having an ionic radius of 80 to 120 pm. FIG. 2 shows the ionic radius / pm of each metal of the rare earth and alkaline earth metal cation species used for the ceria-zirconia-based composite oxide. Suitable ions 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 ion Ce ³⁺ .

  In the ceria-zirconia-based 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 whole. Is a ceria-zirconia-based composite oxide.

If the content of the modified oxide is less than 2% by mass of the whole, the adsorption effect is weakened, and local oxygen vacancy sites are developed to generate a large OSC amount in a low temperature region such as 200 to 400 ° C. (194 to 361 μmol). -O ₂ · g ⁻¹ and CeO ₂ utilization rate is 45 to 72%) expression cannot be expected. On the other hand, if it exceeds 20% by mass, the theoretical OSC amount per unit weight decreases, and it is difficult to confirm the improvement of the OSC amount.

  Therefore, in the present invention, the content of the modified oxide is set to 2 to 20% by mass of the whole.

In the present invention, the index of ceria utilization is used in order to show 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-based 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. (CeO ₂ → CeO _1.5 + 0.25O ₂ ).

A feature of the oxygen absorbing / releasing material of the ceria-zirconia composite oxide that exhibits a large OSC under a low temperature environment of 200 to 400 ° C. of the present invention is that the OSC amount is 200 ° C.-OSC amount is 194 μmol-O ₂ · g ^−1. As described above, preferably 194 to 327 μmol-O ₂ · g ⁻¹ , and CeO ₂ utilization is 45 to 57%, preferably 47 to 55%, and 400 ° C.-OSC amount is 248 μmol-O ₂ · g ⁻¹ or more. The oxygen absorbing / releasing material preferably has a 248 to 361 μmol-O ₂ · g ⁻¹ and a CeO ₂ utilization rate of 48 to 72%, preferably 48 to 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 (ceria) is low in the low temperature range of 400 ° C. _{2 The} phenomenon in which the utilization rate is 48% or more and the OSC amount is 248 μmol-O ₂ · g ⁻¹ or more is not completely understood, but rare earth element ions are ceria-zirconia-based composite oxidation. 0.5 mol% or more and less than 22 mol%, preferably 2.5 mol% or more and 20 mol% or less with respect to the total amount of cations in the product, and the content (modification amount) of the modified oxide is 2 to 20% of the total. It is% by mass.

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

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

  Prior to ASAP measurement, a sample for OSC amount measurement preliminarily dried (holding at 200 ° C. for 40 minutes or more under reduced pressure) was measured by ASAP.

  Glass wool: Two bundles of 0.1 g were packed at the bottom of the glass tube for ASAP, 0.15 g of the above-mentioned ceria-zirconia composite oxide loaded with Pd was packed on top of the bundle, and 0.1 g of glass wool was further packed from above. placed. The role of glass wool is to prevent the above-mentioned ceria-zirconia powder from being blown off and released during depressurization.

The glass tube for ASAP with such settings is attached to the ASAP, and after 200 ° C. × 10 minutes (decompression), 200 ° C. × 10 minutes (pure hydrogen flow), and then held in a decompressed state again (holding for 10 minutes to leak). After performing the check), pure oxygen having a constant amount and a constant pressure was sent to a dedicated glass tube, the oxygen absorption amount was calculated by the pressure fluctuation, and the OSC amount (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 represented by the following formula (1) because it is brought about by the fluctuation of the valence of ceria.

4CeO ₂ ⇔ 2Ce ₂ O ₃ + O ₂ (1)

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

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

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

The Ce utilization rate (%) is divided into the observed 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 value obtained was defined as the Ce utilization rate (%). These can be summarized as the following formula (3).

Here, the symbols for the above formula (3) will be described.
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]
Solution 1 in which the first stage cerium chloride solution, zirconium oxychloride solution, lanthanum chloride solution and pure water are mixed to obtain a cation ratio of Ce: Zr: La = 34.0: 60.7: 4.6 mol / L 1. (Liter) was obtained. 15 g of ammonium peroxodisulfate was added to the obtained mixed solution and heated to 95 ° C. with stirring to obtain a cerium / zirconium composite sulfate. After cooling the obtained sulfate slurry to 60 ° C., ammonia water was added for neutralization to obtain a slurry containing hydroxide. The obtained slurry was subjected to filtration-washing operation 4 times, dried at 120 ° C., calcined at 700 ° C., mortar crushed, and ceria / zirconia-based composite oxide containing rare earth element ions (Ce _0.34 Zr _0.607 La). _0.046 O _1.977 ) was obtained.

As a second step, after crushing the powder obtained in the first step, an aqueous slurry was prepared with a ceria-zirconia-based oxide, and the Nd salt solution was prepared so that the surface modification product after firing was adjusted to 2% by mass. Was added to the solution to change the pH of the solution to precipitate hydroxide on the oxide surface, to remove contaminating ions, and then heated and baked to deposit neodymium oxide (Nd ₂ O ₃ ) on the particle surface.

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

[Examples 2 to 4]
This is a modification of the ceria-zirconia composite oxide shown in the first step of Example 1 except that the type of rare earth oxide that forms the adsorbed phase of 2% by mass of the surface modification product. In each case, a large OSC amount was confirmed such that the ceria utilization ratio was 45 to 48% at 200 ° C in the low temperature region, and the ceria utilization ratio was 48 to 54% at 400 ° C in the low temperature region.

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

[Examples 7 to 16]
The type of the rare earth oxide or the alkaline earth metal oxide which is the adsorbed phase in which the surface modification product is increased to 5 mass% is changed from the ceria-zirconia composite oxide shown in the first step of Example 1. In each case, a large amount of OSC having a ceria utilization rate of 51 to 57% at 200 ° C. in the low temperature range and a ceria utilization rate of 57 to 63% at 400 ° C. in the low temperature range was confirmed.

[Examples 17 and 18]
In Examples 17 and 18, the composite oxides having the compositions of Examples 5 and 6, respectively, were prepared by increasing the modification amount of the rare earth oxide (Nd ₂ O ₃ ) in the adsorption phase, which is a surface modification product, from 2% to 5%. Here is an example. In each case, a large amount of OSC having a ceria utilization rate of 50% or more was confirmed at 200 ° C. and 400 ° C. in the low temperature range.

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

[Examples 25 to 30]
Examples 25 to 30 are examples in which the modified amount of the rare earth oxide in the adsorption phase, which is the surface modifier, was increased to 20% in the composite oxides having the compositions of Examples 19 to 24, respectively. In each case, a large amount of OSC having 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 modification product, each rare earth oxide of Nd, Y, Ce, La, Gd, and Pr and a complex oxide of Ce and Nd, or Ca, Sr, and Ba are used. The example using each of the alkaline earth metal oxides of was shown. The adsorption phase can be formed by using a salt solution of each rare earth or alkaline earth metal as in the case of the second step of forming Na ₂ O ₃ of Example 1. For example, when Ca of alkaline earth metal is used for the adsorption phase of Example 14, as a second step, after crushing the powder obtained in the first step, an aqueous slurry is prepared with ceria-zirconia-based oxide. After that, a Ca salt solution adjusted so that the surface modification product after calcination is 5% by mass is added, and the pH of the solution is modified by precipitating a carbonate salt on the oxide surface with ammonium carbonate to modify the particle surface. After the subsequent firing step, a CaO-modified ceria-zirconia particle surface is formed.

[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 types and contents of rare earth element ions contained were different, and the surface was oxide modified. It is an example of the amount of OSC when there is not. That is, Comparative Example 1 is an example in which no oxide modification was performed on the ceria-zirconia-based oxide surface having a composition corresponding to Examples 1 to 4, 7 to 16, 19 to 22, and 25 to 28, Comparative Example 2 is an example in which no oxide modification was performed on the surface of the ceria-zirconia-based oxide having a composition corresponding to Examples 5, 17, 23 and 29, and Comparative Example 4 is Examples 6 and 24. , 30 is a case where no oxide modification was performed on the surface of the ceria-zirconia-based oxide having a composition corresponding to 30.
In the comparative examples, in the low temperature range (200 ° C. and 400 ° C.), no large OSC amount as found in the examples was found.

  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 to 4, 7 to 16, 19 to 22, 25 to 28 and Examples 5, 17, 23 and 29 and Examples 5, 17, 23 and 29 are base materials ( An example was shown in which the composition of the base material) was unified, the oxide species used in the adsorption phase was changed, and the effect was verified. The purpose of this is to verify the effect of the adsorbed phase. In the examples, the oxide species and modification amount 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, the adsorbed phase was not applied and the results were obtained when the base material (base material) was changed, but a large OSC amount was not found.

1 CZ base material (mother phase)
2 Adsorption phase (rare earth metal oxide, alkaline earth metal oxide)
3 H ₂
4 H ₂ O
5 Oxygen defect layer
6 Pd
7 O
8 Nd ₂ O ₃

Claims (6)

  The ceria-zirconia composite oxide is a particle containing at least one kind of rare earth element having an ionic radius of 80 to 120 pm, and the surface of the particle has an electronegativity of 0.89 to 1 as an adsorption phase of a reducing chemical species. A ceria-zirconia-based composite oxide oxygen storage / release material characterized by being modified with an oxide of 0.22.   The oxide having an electronegativity of 0.89 to 1.22 is an oxide of a rare earth or alkaline earth metal, and the content of the oxide is 2 to 20% by mass of the whole. The ceria-zirconia-based composite oxide oxygen storage / release material according to claim 1.   The ceria-zirconia-based composite oxide contains at least one kind of rare earth elements of 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. On the other hand, the ceria-zirconia-based composite oxide oxygen storage / release material according to claim 1 or 2, characterized by containing 0.5 mol% or more and less than 22 mol% in a molar ratio. The oxygen storage / release characteristics (OSC amount) in a low temperature region of 400 ° C. is 248 to 361 μmol-O ₂ · g ⁻¹ , and the CeO ₂ utilization rate is 48 to 72%. The ceria-zirconia-based composite oxide oxygen storage / release material according to any one of the above. The oxygen storage / release property (OSC amount) in a low temperature region of 200 ° C. is 194 to 327 μmol-O ₂ · g ⁻¹ , and the CeO ₂ utilization rate is 45 to 57%. The ceria-zirconia-based composite oxide oxygen storage / release material according to any one of the above. An exhaust gas purifying catalyst, comprising a ceria-zirconia-based composite oxide oxygen storage / release material according to any one of claims 1 to 5, which carries a noble metal.