Cerium oxide-zirconium oxide composite oxide, method for producing cerium oxide-zirconium oxide composite oxide, and exhaust gas purification catalyst

Abstract

Provided is a cerium oxide-zirconium oxide composite oxide which has a core-shell structure including a core and a shell that covers the core. When the amount of cerium in the entire cerium oxide-zirconium oxide composite oxide is denoted by C1 (wt.%) and the amount of cerium in a surface layer portion of the cerium oxide-zirconium oxide composite oxide as measured by X-ray photoelectron spectroscopy is denoted by C2 (atm%), C1 is 5 wt.% or more and C2 is 2 atm% or less.

Description

Cerium oxide-zirconium oxide-based composite oxide, method for producing cerium oxide-zirconium oxide-based composite oxide, and exhaust gas purification catalyst

[0001] The present invention relates to a cerium oxide-zirconium oxide-based composite oxide, a method for producing a cerium oxide-zirconium oxide-based composite oxide, and an exhaust gas purification catalyst.

[0002] Cerium oxide has Ce ４＋ and Ce ３＋ with a redox potential as low as about 1.6 V, and the reaction of the following formula proceeds reversibly, so it has oxygen storage capacity (hereinafter referred to as OSC), and is used as a co-catalyst or catalyst carrier for three-way catalysts for automobiles.

[0003] CeO ２ ⇔CeO ２－Ｘ +X / 2O ２ (X=0 to 0.5)

[0004] However, it is also well known that the OSC of pure cerium oxide is very low, on the order of X=0.005.

[0005] Therefore, in order to improve this, by solid-dissolving zirconium oxide in cerium oxide, (1) the heat resistance of the specific surface area of cerium oxide is improved, and (2) Zr with a small ionic radius ４＋ inserted into the cerium skeleton to alleviate the volume increase when the above reaction occurs, and many reports have been made on improving OSC and the like.

[0006] Patent Document 1 discloses a cerium oxide-zirconium oxide-based composite oxide in which (1) a cerium-zirconium composite oxide derived from a melting method and (2) cerium dioxide derived from a wet method are mixed. It is also disclosed that the cerium oxide-zirconium oxide-based composite oxide has a large OSC at low temperatures and an appropriate OSC (paragraph

[0016] ).

[0007] International Publication No. 2011 / 108457

[0008] However, the inventors have found that when rhodium (Rh) is supported as a catalyst on a cerium oxide-zirconium oxide composite, the Rh is oxidized by Ce, reducing the catalytic activity, indicating that there is room for improvement.

[0009] The present invention has been made in view of the above-mentioned problems, and its object is to provide a cerium oxide-zirconium oxide composite oxide that has an appropriate OSC and can suppress a decrease in catalytic activity. It also aims to provide a method for producing the cerium oxide-zirconium oxide composite oxide. Furthermore, it aims to provide an exhaust gas purification catalyst having the cerium oxide-zirconium oxide composite oxide.

[0010] The inventors of this invention conducted thorough research to address the above-mentioned problems. As a result, they discovered that the above-mentioned problems could be solved by adopting the following configuration, and thus completed the present invention.

[0011] The present invention provides the following: [1] A cerium oxide-zirconium oxide composite oxide having a core-shell structure comprising a core and a shell covering the core, wherein when the amount of cerium oxide in the entire cerium oxide-zirconium oxide composite oxide measured by X-ray fluorescence analysis is C1 (wt%), and the amount of cerium in the surface layer of the cerium oxide-zirconium oxide composite oxide measured by X-ray photoelectron spectroscopy is C2 (atm%), C1 is 5 wt% or more and C2 is 2 atm% or less.

[0012] According to the above configuration, the amount of cerium C1 in the entire cerium oxide-zirconium oxide composite oxide is 5 wt% or more, so an appropriate OSC can be obtained. Here, C2 being 2 atm% or less means that the amount of cerium in the surface layer of the cerium oxide-zirconium oxide composite oxide is small. Since the C2 of the cerium oxide-zirconium oxide composite oxide is 2 atm% or less and the amount of cerium in the surface layer is small, the catalyst (for example, rhodium) is less likely to be oxidized even when supported. As a result, the decrease in catalytic activity can be suppressed.

[0013] Based on the above, the above configuration makes it possible to have an appropriate OSC and suppress the decrease in catalytic activity.

[0014] Furthermore, the present invention provides the following: [2] The cerium oxide-zirconium oxide composite oxide according to [1], characterized in that the core is composed of a first composite oxide comprising zirconium oxide and cerium oxide, the crystallite size of the first composite oxide is 30 nm or more, the shell is composed of a second composite oxide comprising zirconium oxide and a rare earth oxide other than cerium oxide, the crystallite size of the second composite oxide is less than 20 nm, and the weight ratio of the first composite oxide to the second composite oxide ([first composite oxide]:[second composite oxide]) is 10 to 50:50 to 90.

[0015] If the core contains cerium oxide, it can have a more appropriate OSC. Furthermore, if the crystallite size of the first composite oxide is 30 nm or larger, it can be said that it was manufactured by a melting method, indicating that the melting has progressed sufficiently and that high crystallinity has been obtained. When high crystallinity is obtained, the balance between surface energy and internal energy is biased towards internal energy, which is advantageous for the movement of lattice oxygen. As a result, it can have a more appropriate OSC.

[0016] Furthermore, if the content of the first composite oxide is 10% or more, the amount of usable OSC can be increased. Also, if the content of the first composite oxide is 50% or less, the content of the second composite oxide can be set to 50% or more. When the content of the second composite oxide is 50% or more, the core can be said to be sufficiently covered by the shell, and the exposure of Ce can be said to be small. As a result, oxidation of the catalyst supported by Ce can be more effectively prevented, and the decrease in catalytic activity can be more effectively suppressed. Also, if the content of the second composite oxide is 90% or less, the content of the first composite oxide can be set to 10% or more, and the amount of usable OSC can be increased.

[0017] Further, the present invention provides the following. [3] The specific surface area after heat treatment at 1000° C. for 3 hours in an air atmosphere is 20 m ２ / g or more and 60 m ２ / g or less, wherein the cerium oxide-zirconium oxide-based composite oxide according to [1] or [2] above.

[0018] The specific surface area after heat treatment at 1000° C. for 3 hours in an air atmosphere simulates the state of the specific surface area after long-term use as an exhaust gas purification catalyst. When the specific surface area after heat treatment at 1000° C. for 3 hours in an air atmosphere is 20 m ２ / g or more, it can be said that the heat resistance of the specific surface area is high, and the performance as a catalyst is higher.

[0019] Further, the present invention provides the following. [4] A method for producing the cerium oxide-zirconium oxide-based composite oxide according to any one of [1] to [3] above, comprising: step 1 of obtaining a hydroxide by simultaneously adding a second solution containing a rare earth other than zirconium and cerium and an alkali to a first solution containing a first composite oxide containing zirconium and cerium; and step 2 of heat-treating the hydroxide.

[0020] According to the production method, since the hydroxide is obtained by simultaneously adding the second solution and the alkali to the first solution, a core-shell structure can be suitably obtained.

[0021] Further, the present invention provides the following. [5] An exhaust gas purification catalyst characterized by comprising: the cerium oxide-zirconium oxide-based composite oxide according to any one of [1] to [3] above; and a noble metal supported on the cerium oxide-zirconium oxide-based composite oxide.

[0022] According to the exhaust gas purification catalyst, since it comprises the cerium oxide-zirconium oxide-based composite oxide, it has appropriate OSC, suppresses oxidation of the catalyst caused by contact between Ce and the catalyst, and can suppress a decrease in catalytic activity.

[0023] According to the present invention, it is possible to provide a cerium oxide-zirconium oxide composite oxide that has an appropriate OSC and can suppress the decrease in catalytic activity. Furthermore, it is possible to provide a method for producing the cerium oxide-zirconium oxide composite oxide. Furthermore, it is possible to provide an exhaust gas purification catalyst having the cerium oxide-zirconium oxide composite oxide.

[0024] This is an SEM image of the cerium oxide-zirconium oxide composite oxide obtained in Example 1. This is an SEM image of the cerium oxide-zirconium oxide composite oxide obtained in Comparative Example 1.

[0025] Embodiments of the present invention will be described below. However, the present invention is not limited to these embodiments. In this specification, zirconia (zirconium oxide) is a general term and includes impurity metal compounds, including hafnia, in an amount of 10% by mass or less. In this specification, the expressions "contains" and "includes" include the concepts of "contains," "includes," "substantially consists of," and "consists only of."

[0026] The maximum and minimum values ​​of the content of each component shown below are, independently of the content of other components, the preferred maximum and minimum values ​​of the present invention. Similarly, the maximum and minimum values ​​of the various parameters (measured values, etc.) shown below are, independently of the content (composition) of each component, the preferred maximum and minimum values ​​of the present invention.

[0027] [Cerium Oxide-Zirconium Oxide Composite Oxides] An example of a cerium oxide-zirconium oxide composite oxide according to this embodiment will be described below. However, the cerium oxide-zirconium oxide composite oxide of the present invention is not limited to the following examples.

[0028] The cerium oxide-zirconium oxide composite oxide according to this embodiment has a core-shell structure having a core and a shell covering the core. When the amount of cerium in the entire cerium oxide-zirconium oxide composite oxide measured by X-ray fluorescence analysis is defined as C1 (wt%), and the amount of cerium in the surface layer of the cerium oxide-zirconium oxide composite oxide measured by X-ray photoelectron spectroscopy is defined as C2 (atm%), then C1 is 5 wt% or more, and C2 is 2 atm% or less.

[0029] As described above, in the cerium oxide-zirconium oxide composite oxide according to this embodiment, when the amount of cerium in the entire cerium oxide-zirconium oxide composite oxide measured by X-ray fluorescence analysis is C1 (wt%), and the amount of cerium in the surface layer of the cerium oxide-zirconium oxide composite oxide measured by X-ray photoelectron spectroscopy is C2 (atm%), the amount of cerium C1 in the entire cerium oxide-zirconium oxide composite oxide is 5 wt% or more. Since C1 is 5 wt% or more, it is possible to have an appropriate OSC.

[0030] The C1 is preferably 6 wt% or more. The C1 is preferably 25 wt% or less, more preferably 24 wt% or less. When the C1 is 25 wt% or less, the formation of a core-shell structure can prevent the C2 from exceeding 2 atm%. The C1 is preferably 5 wt% or more and 25 wt% or less, more preferably 6 wt% or more and 24 wt% or less.

[0031] The cerium oxide-zirconium oxide composite oxide has a C2 content of 2 atm% or less. Here, a C2 content of 2 atm% or less means that the amount of cerium in the surface layer of the cerium oxide-zirconium oxide composite oxide is small. Because the C2 content of the cerium oxide-zirconium oxide composite oxide is 2 atm% or less and the amount of cerium in the surface layer is small, the catalyst is less likely to be oxidized even when a catalyst is supported on it. As a result, the decrease in catalytic activity can be suppressed.

[0032] The C2 is preferably 1.9 atm% or less. The smaller the C2, the better, but for example, it can be 0 atm% or more, 0.1 atm% or more, etc. The C2 is preferably 0.1 atm% or more and 2 atm% or less, more preferably 0 atm% or more and 1.9 atm% or less.

[0033] The cerium oxide-zirconium oxide composite oxide has a core-shell structure comprising a core and a shell covering the core.

[0034] In this specification, a core-shell structure refers to a state in which the shell completely covers the core, and a state in which the shell covers the core but a portion of the core is exposed. The state in which the shell completely covers the core means that the C2 (atm%) is 0 atm%. The state in which the shell covers the core but a portion of the core is exposed means that the C2 (atm%) is greater than 0 atm% and 2 atm% or less.

[0035] Preferably, the core is composed of a first composite oxide comprising zirconium oxide and cerium oxide. If the core contains cerium oxide, a more suitable OSC can be obtained. The core may be composed only of the first composite oxide, or it may contain elements (oxides) other than the first composite oxide.

[0036] The content of elements (oxides) other than the first composite oxide is preferably 0% by mass or more and 20% by mass or less relative to the entire core (when the entire core is considered to be 100% by mass).

[0037] [First composite oxide] The first composite oxide comprises zirconium oxide and cerium oxide. The first composite oxide may consist only of zirconium oxide and cerium oxide. The first composite oxide may consist of zirconium oxide, cerium oxide, and rare earth oxides other than cerium oxide. The first composite oxide may consist of zirconium oxide, cerium oxide, rare earth oxides other than cerium oxide, and oxides of other elements (hereinafter referred to as "first other element oxides"). Examples of the first other element oxides include: A) Transition metal oxides (excluding oxides of rare earth elements and noble metal elements) B) Alkaline earth metal oxides C) Platinum group metals. The rare earth elements are Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Examples of the transition metals include Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Nb, Mo, Ta, W, and Ag. Examples of the alkaline earth metals include Mg, Ca, Sr, and Ba. Examples of the platinum group metals include Rh, Pd, Pt, and Ir.

[0038] The zirconium oxide content relative to the entire first composite oxide is preferably 45 mol% to 55 mol%. When the zirconium oxide content relative to the entire first composite oxide is 45 mol% to 55 mol%, suitable heat resistance can be obtained.

[0039] The zirconium oxide content relative to the entire first composite oxide is more preferably 46 mol% or more, and even more preferably 47 mol% or more. The zirconium oxide content relative to the entire first composite oxide is more preferably 54 mol% or less, and even more preferably 53 mol% or less. The zirconium oxide content relative to the entire first composite oxide is more preferably 46 mol% or more and 54 mol% or less, and even more preferably 47 mol% or more and 53 mol% or less.

[0040] The content of cerium oxide relative to the entire first composite oxide is preferably 45 mol% to 55 mol%. When the content of cerium oxide relative to the entire first composite oxide is 45 mol% to 55 mol%, a more appropriate OSC can be obtained.

[0041] The content of cerium oxide relative to the entire first composite oxide is more preferably 46 mol% or more, and even more preferably 47 mol% or more. The content of cerium oxide relative to the entire first composite oxide is more preferably 53 mol% or less, and even more preferably 54 mol% or less. The content of cerium oxide relative to the entire first composite oxide is more preferably 46 mol% or more and 54 mol% or less, and even more preferably 47 mol% or more and 53 mol% or less.

[0042] When the first composite oxide contains rare earth oxides other than cerium oxide, the content of the rare earth oxides other than cerium oxide relative to the entire first composite oxide is preferably 0 mol% or more and 10 mol% or less. When the content of the rare earth oxides other than cerium oxide relative to the entire first composite oxide is 0 mol% or more and 10 mol% or less, the effects of adding the rare earth oxides other than cerium oxide can be obtained while maintaining suitable heat resistance and appropriate OSC.

[0043] The content of rare earth oxides other than cerium oxide in the total first composite oxide is more preferably 1 mol% or more, and even more preferably 2 mol% or more. The content of rare earth oxides other than cerium oxide in the total first composite oxide is more preferably 9 mol% or less, and even more preferably 8 mol% or less. The content of rare earth oxides other than cerium oxide in the total first composite oxide is more preferably 1 mol% or more and 9 mol% or less, and even more preferably 2 mol% or more and 8 mol% or less.

[0044] If the first composite oxide contains an oxide of the first other element, the content of the oxide of the first other element relative to the entire first composite oxide is preferably 0 mol% or more and 10 mol% or less.

[0045] The content of the oxide of the first other element relative to the entire first composite oxide is more preferably 1 mol% or more, and even more preferably 2 mol% or more. The content of the oxide of the first other element relative to the entire first composite oxide is more preferably 9 mol% or less, and even more preferably 8 mol% or less. The content of the oxide of the first other element relative to the entire first composite oxide is more preferably 1 mol% or more and 9 mol% or less, and even more preferably 2 mol% or more and 8 mol% or less.

[0046] The molar ratio (zirconium oxide / cerium oxide) of zirconium oxide to cerium oxide in the first composite oxide is preferably 45 / 55 or more and 55 / 45 or less. When the molar ratio (zirconium oxide / cerium oxide) of zirconium oxide to cerium oxide in the first composite oxide is 45 / 55 or more and 55 / 45 or less, more suitable heat resistance and more appropriate OSC can be obtained. The molar ratio (zirconium oxide / cerium oxide) is more preferably 46 / 54 or more, and even more preferably 47 / 53 or more. The molar ratio (zirconium oxide / cerium oxide) is more preferably 54 / 46 or less, and even more preferably 53 / 47 or less. The molar ratio (zirconium oxide / cerium oxide) is more preferably 46 / 54 or more and 54 / 46 or less, and even more preferably 47 / 53 or more and 53 / 47 or less.

[0047] The crystallite size of the first composite oxide is preferably 30 nm or larger. When the crystallite size of the first composite oxide is 30 nm or larger, it can be said that it was manufactured by a melting method, that the melting has progressed sufficiently, and that high crystallinity has been obtained. When high crystallinity is obtained, the balance between surface energy and internal energy is biased towards internal energy, which is advantageous for the movement of lattice oxygen. As a result, a more appropriate OSC can be obtained.

[0048] The crystallite size of the first composite oxide is more preferably 33 nm or more, and even more preferably 35 nm or more. The crystallite size of the first composite oxide is more preferably 100 nm or less, and even more preferably 90 nm or less. When the crystallite size of the first composite oxide is 100 nm or less, it can have a suitable OSC. The crystallite size of the first composite oxide is more preferably 33 nm or more and 100 nm or less, and even more preferably 35 nm or more and 90 nm or less.

[0049] Furthermore, if the core, i.e., the first composite oxide, is manufactured by a melting method, it is usually composed of a single particle and cannot be composed of aggregates of multiple primary particles. When the first composite oxide is composed of a single particle, the specific surface area becomes smaller, and the frequency of contact with the noble metal decreases. As a result, the decrease in catalytic activity can be further suppressed.

[0050] Preferably, the shell is composed of a second composite oxide comprising zirconium oxide and a rare earth oxide other than cerium oxide. If the shell contains zirconium oxide and a rare earth oxide other than cerium oxide, and contains little to no cerium oxide, the decrease in catalytic activity can be further suppressed. The shell may consist only of the second composite oxide, or it may contain elements (oxides) other than the second composite oxide.

[0051] [Second Composite Oxide] The second composite oxide comprises zirconium oxide and rare earth oxides other than cerium oxide. The second composite oxide may consist only of zirconium oxide and rare earth oxides other than cerium oxide, or it may consist of zirconium oxide, rare earth oxides other than cerium oxide, and oxides of other elements (hereinafter referred to as "second oxides of other elements"). Examples of the second oxides of other elements include: A) Transition metal oxides (excluding oxides of rare earth elements and noble metal elements) B) Alkaline earth metal oxides C) Platinum group metals. The rare earth elements are Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Examples of the transition metals include Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Nb, Mo, Ta, W, and Ag. Examples of the alkaline earth metals include Mg, Ca, Sr, and Ba. Examples of the platinum group metals include Rh, Pd, Pt, and Ir.

[0052] The second composite oxide preferably does not contain cerium oxide. While it is preferable that the second composite oxide does not contain cerium oxide, it may contain a small amount. If the second composite oxide contains cerium oxide, the cerium oxide content relative to the entire second composite oxide is preferably 5% by mass or less, more preferably 4% by mass or less. If the shell does not contain cerium oxide, or if the cerium oxide content is 2% by mass or less, the decrease in catalytic activity can be further suppressed.

[0053] The zirconium oxide content relative to the entire second composite oxide is preferably 50 mol% to 90 mol%. When the zirconium oxide content relative to the entire second composite oxide is 50 mol% to 90 mol%, suitable heat resistance can be obtained.

[0054] The zirconium oxide content relative to the entire second composite oxide is more preferably 51 mol% or more, and even more preferably 52 mol% or more. The zirconium oxide content relative to the entire second composite oxide is more preferably 89 mol% or less, and even more preferably 88 mol% or less. The zirconium oxide content relative to the entire second composite oxide is more preferably 51 mol% or more and 89 mol% or less, and even more preferably 52 mol% or more and 88 mol% or less.

[0055] The content of rare earth oxides other than cerium oxide in the total second composite oxide is preferably 10 mol% to 50 mol%. When the content of rare earth oxides other than cerium oxide in the total second composite oxide is 10 mol% to 50 mol%, suitable heat resistance can be obtained.

[0056] The content of rare earth oxides other than cerium oxide in the entire second composite oxide is more preferably 11 mol% or more. The content of rare earth oxides other than cerium oxide in the entire second composite oxide is more preferably 49 mol% or less. The content of rare earth oxides other than cerium oxide in the entire second composite oxide is more preferably 11 mol% or more and 49 mol% or less.

[0057] If the second composite oxide contains an oxide of the second other element, the content of the oxide of the second other element relative to the entire second composite oxide is preferably 0 mol% or more and 5 mol% or less.

[0058] The content of the oxide of the second other element relative to the entire second composite oxide is more preferably 1 mol% or more, and even more preferably 2 mol% or more. The content of the oxide of the second other element relative to the entire second composite oxide is more preferably 4 mol% or less. The content of the oxide of the second other element relative to the entire second composite oxide is more preferably 1 mol% or more and 4 mol% or less.

[0059] The molar ratio of zirconium oxide to rare earth oxides other than cerium oxide in the second composite oxide (zirconium oxide / rare earth oxides other than cerium oxide) is preferably 50 / 50 or more and 90 / 10 or less. The molar ratio (zirconium oxide / rare earth oxides other than cerium oxide) is more preferably 51 / 49 or more, and even more preferably 52 / 48 or more. The molar ratio (zirconium oxide / rare earth oxides other than cerium oxide) is more preferably 89 / 11 or less, and even more preferably 88 / 12 or less. The molar ratio (zirconium oxide / rare earth oxides other than cerium oxide) is more preferably 51 / 49 or more and 89 / 11 or less, and even more preferably 52 / 48 or more and 88 / 12 or less.

[0060] The crystallite size of the second composite oxide is preferably less than 20 nm.

[0061] The crystallite size of the second composite oxide is more preferably 18 nm or less, and even more preferably 15 nm or less. The crystallite size of the second composite oxide is more preferably 1 nm or more, and even more preferably 2 nm or more. The crystallite size of the second composite oxide is more preferably 1 nm or more and 18 nm or less, and even more preferably 2 nm or more and 15 nm or less.

[0062] Furthermore, the second composite oxide has one or more monoclinic, tetragonal, or cubic structures and does not have a pyrochlore structure.

[0063] It is preferable that the weight ratio of the first composite oxide to the second composite oxide ([first composite oxide]:[second composite oxide]) is 10 to 50:50 to 90. If the content of the first composite oxide is 10% or more, the amount of usable OSC can be increased. Also, if the content of the first composite oxide is 50% or less, the content of the second composite oxide can be set to 50% or more. If the content of the second composite oxide is 50% or more, the core can be said to be sufficiently covered by the shell, and the exposure of Ce can be said to be small. As a result, oxidation of the catalyst supported by Ce can be more effectively prevented, and the decrease in catalytic activity can be more effectively suppressed. Also, if the content of the second composite oxide is 90% or less, the content of the first composite oxide can be set to 10% or more, and the amount of usable OSC can be increased.

[0064] The weight ratio ([first composite oxide]:[second composite oxide]) is preferably 8 to 40:60 to 92, more preferably 10 to 35:65 to 90.

[0065] <Specific surface area (specific surface area before heat treatment)> The cerium oxide-zirconium oxide composite oxide has a specific surface area of ​​30 m². ２ / g or more 100m ２ It is preferable that the amount is less than or equal to / g. The specific surface area (specific surface area before heat treatment) is 30m². ２ If the value is above / g, it is easier to increase the specific surface area after heat treatment.

[0066] The specific surface area (specific surface area before heat treatment) is more preferably 35 m². ２ It is 1 / g or more. The specific surface area (specific surface area before heat treatment) is preferably large, for example, 90 m². ２ The specific surface area (specific surface area before heat treatment) is more preferably 35 m². ２ / g or more 90m ２ It is less than or equal to / g.

[0067] <Specific surface area after heat treatment at 1000°C for 3 hours in an atmospheric atmosphere> The cerium oxide-zirconium oxide composite oxide has a specific surface area of ​​30 m² after heat treatment at 1000°C for 3 hours in an atmospheric atmosphere. ２ / g or more 60m ２ It is preferable that the amount is less than or equal to / g. The specific surface area after heat treatment at 1000°C for 3 hours in an atmospheric environment is 30 m². ２ A value of 1 / g or higher indicates high heat resistance of the specific surface area, suggesting superior catalytic performance.

[0068] The specific surface area after heat treatment at 1000°C for 3 hours in the aforementioned atmospheric environment is more preferably 35 m². ２ / g or more, more preferably 40m ２ It is 1 / g or more. The specific surface area after heat treatment at 1000°C for 3 hours in the aforementioned atmospheric environment is preferably large, for example, 55 m². ２ The amount is less than or equal to / g. The specific surface area after heat treatment at 1000°C for 3 hours in the aforementioned atmospheric environment is more preferably 35 m². ２ / g or more 60m ２ / g or less, more preferably 40m ２ / g or more 55m ２ It is less than or equal to / g.

[0069] <Specific surface area after heat treatment at 1100°C for 3 hours in an atmospheric atmosphere> The cerium oxide-zirconium oxide composite oxide has a specific surface area of ​​10 m² after heat treatment at 1100°C for 3 hours in an atmospheric atmosphere. ２ / g or more 40m ２ It is preferable that the amount is less than or equal to / g. The specific surface area after heat treatment at 1100°C for 3 hours in an atmospheric environment is 10 m². ２ When the value is greater than or equal to 1g, the specific surface area has higher heat resistance, and the catalytic performance is even higher.

[0070] The specific surface area after heat treatment at 1100°C for 3 hours in the aforementioned atmospheric environment is more preferably 11 m². ２ / g or more, more preferably 12m ２ It is 1 / g or more. The specific surface area after heat treatment at 1100°C for 3 hours in the aforementioned atmospheric environment is preferably large, for example, 39 m². ２ / g or less, 38m２ The amount is less than or equal to / g. The specific surface area after heat treatment at 1100°C for 3 hours in the aforementioned atmospheric environment is more preferably 11m². ２ / g or more 39m ２ / g or less, more preferably 12m ２ / g or more 38m ２ It is less than or equal to / g.

[0071] <Particle size D ５０ > Particle size D of the cerium oxide-zirconium oxide composite oxide ５０ The particle size D is preferably 1 μm or more and 20 μm or less. ５０ If the particle size D is 1 μm or larger, it is possible to suppress an increase in slurry viscosity and peeling of the coating layer. ５０ If the particle size is 20 μm or less, the particles are appropriately small, which helps to suppress the time required for grinding.

[0072] The particle size D ５０ The particle size D is more preferably 2 μm or larger, and even more preferably 3 μm or larger. ５０ The particle size D is more preferably 19 μm or less, and even more preferably 18 μm or less. ５０ The particle size is more preferably 2 μm to 19 μm, and even more preferably 3 μm to 18 μm.

[0073] <OSC> The OSC of the cerium oxide-zirconium oxide composite oxide is preferably 10 μmol / g or more. When the OSC is 10 μmol / g or more, it can be more suitably used as an OSC material for automotive catalyst applications.

[0074] The OSC (600°C) is more preferably 15 μmol / g or more, and even more preferably 20 μmol / g or more. The OSC (600°C) is preferable as it is higher, for example, 500 μmol / g or less, 400 μmol / g or less, etc. The OSC (600°C) is more preferably 15 μmol / g or more and 500 μmol / g or less, even more preferably 20 μmol / g or more and 450 μmol / g or less, and particularly preferably 20 μmol / g or more and 400 μmol / g or less.

[0075] The zirconium oxide content relative to the entire cerium oxide-zirconium oxide composite oxide is preferably 50% by mass or more and 90% by mass or less. When the zirconium oxide content relative to the entire cerium oxide-zirconium oxide composite oxide is 50% by mass or more and 90% by mass or less, suitable heat resistance can be obtained.

[0076] The zirconium oxide content relative to the entire cerium oxide-zirconium oxide composite oxide is more preferably 51% by mass or more, and even more preferably 52% by mass or more. The zirconium oxide content relative to the entire cerium oxide-zirconium oxide composite oxide is more preferably 89% by mass or less, and even more preferably 88% by mass or less. The zirconium oxide content relative to the entire cerium oxide-zirconium oxide composite oxide is more preferably 51% by mass or more and 89% by mass or less, and even more preferably 52% by mass or more and 88% by mass or less.

[0077] The content of cerium oxide in the total cerium oxide-zirconium oxide composite oxide is preferably 5% by mass or more and 30% by mass or less. When the content of cerium oxide in the total cerium oxide-zirconium oxide composite oxide is 5% by mass or more and 30% by mass or less, a more appropriate OSC can be obtained.

[0078] The content of cerium oxide in relation to the entire cerium oxide-zirconium oxide composite oxide is more preferably 6% by mass or more, and even more preferably 7% by mass or more. The content of cerium oxide in relation to the entire cerium oxide-zirconium oxide composite oxide is more preferably 29% by mass or less, and even more preferably 28% by mass or less. The content of cerium oxide in relation to the entire cerium oxide-zirconium oxide composite oxide is more preferably 6% by mass or more and 29% by mass or less, and even more preferably 7% by mass or more and 28% by mass or less.

[0079] The content of rare earth oxides other than cerium oxide in relation to the entire cerium oxide-zirconium oxide composite oxide is preferably 5% by mass or more and 40% by mass or less. When the content of rare earth oxides other than cerium oxide in relation to the entire cerium oxide-zirconium oxide composite oxide is 5% by mass or more and 40% by mass or less, suitable heat resistance can be obtained.

[0080] The content of rare earth oxides other than cerium oxide in relation to the entire cerium oxide-zirconium oxide composite oxide is more preferably 6% by mass or more, and even more preferably 7% by mass or more. The content of rare earth oxides other than cerium oxide in relation to the entire cerium oxide-zirconium oxide composite oxide is more preferably 39% by mass or less, and even more preferably 38% by mass or less. The content of rare earth oxides other than cerium oxide in relation to the entire cerium oxide-zirconium oxide composite oxide is more preferably 6% by mass or more and 39% by mass or less, and even more preferably 7% by mass or more and 38% by mass or less.

[0081] [Method for Producing the First Composite Oxide] An example of a method for producing the first composite oxide is described below. However, the method for producing the first composite oxide is not limited to the following example.

[0082] The first composite oxide is obtained by mixing a material containing cerium oxide (also called a cerium oxide raw material) and a material containing zirconium oxide (also called a zirconium oxide raw material) in a predetermined ratio, melting the raw material mixture at a temperature above its melting point, cooling the resulting molten material to form an ingot, then, if desired, crushing the ingot into a powder, subsequently removing the strain within the powder crystals under heating, and then further crushing it to a finer degree.

[0083] The cerium oxide raw material is not particularly limited, but cerium oxide is preferred. The cerium oxide may also be an oxide obtained from nitrates, carbonates, sulfates, acetates, chlorides, bromides, etc.

[0084] The zirconium oxide raw material is not particularly limited, but zirconium element materials containing zirconium oxide, such as baddeleyite, desilicate zirconia, and zirconium oxide, are preferred. The zirconium oxide may also be an oxide obtained from nitrates, carbonates, sulfates, acetates, chlorides, bromides, etc. Furthermore, the cerium raw material and the zirconium raw material may be a mixture of these raw materials or a composite oxide. The purity of the cerium raw material and the zirconium raw material is not particularly limited, but a purity of 99.9% or higher is preferred.

[0085] The cerium oxide raw material and the zirconium oxide raw material are mixed in a predetermined ratio and charged into a melting apparatus. The composition is determined by the molar ratio of zirconium atoms to cerium atoms in the composite oxide (zirconium / cerium), preferably in the range of 5 / 95 to 70 / 30, more preferably 20 / 80 to 65 / 35, and particularly preferably 40 / 60 to 60 / 40. By using this range, the desired oxygen absorption and release performance and heat resistance can be obtained.

[0086] Subsequently, the raw material mixture is melted in the apparatus. The melting method is not particularly limited as long as at least one of the raw material mixtures is melted, and examples include arc type and high-frequency thermal plasma type. Among these, the general electrofusion method, that is, the melting method using an arc-type electric furnace, can be preferably used.

[0087] When employing a melting method using an arc-type electric furnace, the mixture ratio of cerium oxide and zirconium oxide raw materials varies, but a predetermined amount of coke is added to the mixed cerium and zirconium raw materials as needed to promote initial current flow. Subsequently, for example, the secondary voltage is set to 70-100V and the average load power to 80-100kW, and the mixture is heated to a temperature of 2400°C or higher. After the raw materials reach a molten state, they are held for 0.5 to 3 hours to ensure uniform melting. The heating temperature should be 2000°C or higher, but it is preferable to be above the melting point of the raw materials, particularly 2600-2800°C. The holding time in the molten state is preferably 1 to 2 hours. The atmosphere during melting is not particularly limited and can be air, nitrogen, argon, or an inert gas such as helium. The pressure is not particularly limited and can be atmospheric pressure, pressurized pressure, or reduced pressure, but it can usually be carried out under atmospheric pressure.

[0088] After melting is complete, the electric furnace is covered with a carbon lid and slowly cooled for 20 to 30 hours to obtain an ingot. The method of cooling the molten material is not particularly limited, but usually it is removed from the melting apparatus and allowed to cool in the air to 100°C or below, preferably 50°C or below, and especially preferably to room temperature. This makes it possible to obtain an ingot of the first composite oxide in which the cerium oxide raw material and zirconium oxide raw material are homogeneous.

[0089] The ingots after melting are crushed. There are no particular limitations on the crushing method of the ingots, but they can be crushed using crushers such as jaw crushers and roll crushers. Considering handling in subsequent processes, it is preferable to crush the ingots until they become powder of 3 mm or less, or even 1 mm or less, and then classify them.

[0090] The obtained powder can be separated from impurities by magnetic separation, and then, if desired, placed in an electric furnace or the like to remove suboxides from the melting process and distortions within the crystals due to supercooling by oxidation heat treatment. The conditions for oxidation heat treatment are not particularly limited as long as the ingot or powder is oxidized, but are usually 100 to 1000°C, preferably 600 to 800°C. The heat treatment time is not particularly limited, but can be 1 to 5 hours, preferably 1 to 3 hours.

[0091] The powder obtained by the above method can be further pulverized according to the application. While there are no particular limitations on the pulverization process, it can be performed for 5 to 30 minutes using a pulverizer such as a planetary mill, ball mill, or jet mill. This pulverization preferably sets the average particle size of the first composite oxide to 0.2 to 10 μm, more preferably 1.0 to 5.0 μm. Although the detailed reason is unclear, it is thought that pulverization increases the surface area of ​​the composite oxide, enabling greater oxygen release at lower temperatures.

[0092] The first composite oxide may further contain one or more rare earth oxides (excluding cerium oxide). In this case, one or more rare earth oxides (excluding cerium oxide) can be added to the raw material mixture.

[0093] An example of the method for producing the first composite oxide has been described above.

[0094] [Method for Producing the Second Composite Oxide] An example of a method for producing the second composite oxide is described below. However, the method for producing the second composite oxide is not limited to the following example.

[0095] The second composite oxide can be obtained as a solution containing the second composite oxide by mixing a raw material salt containing zirconium and a raw material salt containing a rare earth element other than cerium in a solvent (for example, water).

[0096] Any raw material salt containing zirconium can be used, as long as it supplies zirconium ions. Examples include zirconium oxynitrate, zirconium oxychloride, zirconium nitrate, and at least one of these hydrates. Among these, zirconium oxychloride is preferred from a cost standpoint.

[0097] As a raw material salt containing rare earth elements, any salt that supplies ions corresponding to the rare earth element is acceptable. For example, when the rare earth element is La, at least one of the following can be used: lanthanum nitrate, lanthanum chloride, lanthanum acetate, or their hydrates. When the rare earth element is Nd, at least one of the following can be used: neodymium nitrate, neodymium chloride, neodymium acetate, or their hydrates. When the rare earth element is Pr, at least one of the following can be used: praseodymium nitrate, praseodymium chloride, praseodymium acetate, or their hydrates. When the rare earth element is Y, at least one of the following can be used: yttrium nitrate, yttrium chloride, yttrium acetate, or their hydrates.

[0098] The concentration of the zirconium salt is not particularly limited, but it is generally preferable to use 5 to 250 g of zirconium oxide, and more preferably 20 to 200 g, per 1000 g of solvent.

[0099] The concentration of the rare earth element salt is not particularly limited, but generally it is preferable to use 5 to 250 g, and particularly 20 to 200 g, of the rare earth element oxide per 1000 g of solvent.

[0100] The solvent for preparing the aqueous solution can be appropriately selected depending on the type of raw material salt, but it is generally preferable to use water (pure water, deionized water, etc.; the same applies hereinafter).

[0101] An example of a method for producing the second composite oxide described above has been explained.

[0102] The following describes an example of a cerium oxide-zirconium oxide composite oxide. However, cerium oxide-zirconium oxide composite oxides are not limited to the examples below.

[0103] [Method for producing cerium oxide-zirconium oxide composite oxide] The method for producing cerium oxide-zirconium oxide composite oxide according to this embodiment comprises: Step 1, to obtain a hydroxide by simultaneously adding a second solution containing zirconium and rare earth elements other than cerium, and an alkali to a first solution containing a first composite oxide containing zirconium and cerium; and Step 2, to heat-treat the hydroxide.

[0104] The first solution is obtained by adding the first composite oxide to water.

[0105] As described above, the second solution can be obtained as a solution containing a second composite oxide by mixing a raw material salt containing zirconium and a raw material salt containing a rare earth element other than cerium in a solvent (for example, water).

[0106] In the first step, the second solution and an alkali are added simultaneously to the first solution to obtain a hydroxide. By adding the second solution and an alkali simultaneously to the first solution, a suitable core-shell structure can be obtained.

[0107] When adding the second solution to the first solution, it is preferable to add it in such a way that the weight ratio of the first composite oxide to the second composite oxide ([first composite oxide]:[second composite oxide]) is 10 to 50:50 to 90.

[0108] Examples of the aforementioned alkalis include alkali hydroxides such as sodium hydroxide and potassium hydroxide, and aqueous ammonia. Among these, aqueous ammonia is preferred from the viewpoint of having fewer impurities.

[0109] The addition of the alkali is carried out until the pH is preferably within the range of 8 to 11, more preferably within the range of 9 to 11. This preferably yields a precursor (precipitate, hydroxide) of the cerium oxide-zirconium oxide complex oxide.

[0110] Next, the hydroxide is heat-treated (step 2).

[0111] The temperature of the heat treatment is preferably 400°C or higher, and more preferably 500°C or higher. The temperature of the heat treatment is preferably 1100°C or lower, and more preferably 1000°C or lower. The temperature of the heat treatment is preferably 400°C or higher and 1100°C or lower, and more preferably 500°C or higher and 1000°C or lower.

[0112] The duration of the heat treatment is preferably 1 hour or more, and more preferably 2 hours or more. The duration of the heat treatment is preferably 20 hours or less, and more preferably 19 hours or less. The duration of the heat treatment is preferably 1 hour or more and 20 hours or less, and more preferably 2 hours or more and 19 hours or less.

[0113] The atmosphere for the heat treatment is not particularly limited and may be air, an inert gas (e.g., nitrogen), etc.

[0114] The aforementioned heat treatment can be carried out, for example, in an electric furnace using an alumina casing.

[0115] As described above, a cerium oxide-zirconium oxide composite oxide can be obtained.

[0116] The cerium oxide-zirconium oxide composite oxide can be used as an exhaust gas purification catalyst by supporting a precious metal on it. That is, the exhaust gas purification catalyst comprises a cerium oxide-zirconium oxide composite oxide and a precious metal supported on the cerium oxide-zirconium oxide composite oxide.

[0117] As for the method of supporting the noble metal on the cerium oxide-zirconium oxide composite oxide, a conventionally known method may be employed.

[0118] Examples of the aforementioned precious metals include those commonly used as catalysts for exhaust gas purification, such as rhodium, palladium, and platinum.

[0119] The exhaust gas purification catalyst, having the cerium oxide-zirconium oxide composite oxide, has an appropriate OSC and can suppress a decrease in catalytic activity.

[0120] The present invention will be described in detail below with reference to examples, but the present invention is not limited to the following examples unless it exceeds the gist of the invention. The cerium oxide-zirconium oxide composite oxides in the examples and comparative examples contain hafnium oxide as an unavoidable impurity at a concentration of 1.3 to 2.5% by mass relative to the zirconium oxide (calculated by the following formula (X)). <Formula (X)> ([Mass of hafnium oxide] / ([Mass of zirconium oxide] + [Mass of hafnium oxide])) × 100 (%)

[0121] [Preparation of Cerium Oxide-Zirconium Oxide Composite Oxides] (Example 1) <Preparation of the First Composite Oxide> Using high-purity zirconium oxide (99.9% purity) as the raw material for Zr and high-purity cerium oxide (99.9% purity) as the raw material for Ce, a powder of cerium-zirconium composite oxide derived by the melting method (powder of the first composite oxide) was produced according to the following procedure. First, to prepare 10 kg of powder, high-purity zirconium oxide (4.2 kg) and high-purity cerium oxide (5.8 kg) were separated and mixed, and melted at over 2200°C using an arc-type electric furnace with a secondary voltage of 85 V, average load power of 99.5 kW, energizing time of 1 hour 50 minutes, and total energy of 182 kWh. In addition, 500 g of coke was used to promote the initial energizing. After the melting was completed, the electric furnace was covered with a carbon lid and slowly cooled in the air for 24 hours to obtain an ingot. The obtained ingots were crushed to a thickness of 3 mm or less using a jaw crusher and a roll crusher, and then powder of 1 mm or less was collected using a sieve. Next, in order to remove suboxides from the melting process and distortions in the crystals due to supercooling, the ingots were calcined in an electric furnace in air at 800°C for 3 hours, and then crushed with a jet mill to obtain the first composite oxide powder.

[0122] <Preparation of the second complex oxide mixed solution> Zirconium nitrate solution (ZrO ２ Conversion: 88.0 g) and lanthanum nitrate solution (La ２ O ３ Conversion: 12.0 g) and yttrium nitrate solution (Y ２ O ３ (Converted to 30.0 g) was mixed to prepare 1300 g of a mixed solution with a concentration of 10 wt% (total concentration of zirconium, lanthanum, and yttrium).

[0123] <Preparation of Cerium Oxide-Zirconium Oxide Composite Oxide> 70 g of the powder of the first composite oxide prepared above was added to 700 ml of pure water to prepare a slurry. While stirring the obtained slurry at 300 rpm, 1300 g of the mixed solution of the second composite oxide prepared above and 650 g of 25% aqueous ammonia were added simultaneously over 180 minutes. The mixture was then kept for 60 minutes. Subsequently, the mixture was filtered and washed with water to obtain a hydroxide. The obtained hydroxide was calcined in air at 700°C for 5 hours to obtain the cerium oxide-zirconium oxide composite oxide according to Example 1.

[0124] (Example 2) The powder of the first composite oxide was obtained in the same manner as in Example 1. <Preparation of the mixed solution of the second composite oxide> Zirconium nitrate solution (ZrO ２ (Conversion: 154g) and praseodymium nitrate solution (Pr ６ O １１ (Converted to 26 g) was mixed to prepare 1800 g of a mixed solution with a concentration of 10 wt% (total concentration of zirconium and praseodymium). <Preparation of cerium oxide-zirconium oxide composite oxide> 20 g of the powder of the first composite oxide produced above was added to 200 ml of pure water to prepare a slurry. While stirring the obtained slurry at 300 rpm, 1800 g of the mixed solution of the second composite oxide produced above and 900 g of 25% ammonia water were added simultaneously over 180 minutes. The mixture was then kept for 60 minutes. Subsequently, the hydroxide was obtained by filtration and washing with water. The obtained hydroxide was calcined in air at 700°C for 5 hours to obtain the cerium oxide-zirconium oxide composite oxide according to Example 2.

[0125] (Example 3) The powder of the first composite oxide was obtained in the same manner as in Example 1. <Preparation of the mixed solution of the second composite oxide> Zirconium nitrate solution (ZrO ２ Conversion: 108g) and yttrium nitrate solution (Y ２ O ３(Converted to 52 g) was mixed to prepare 1600 g of a 10 wt% mixed solution. <Preparation of cerium oxide-zirconium oxide composite oxide> 40 g of the powder of the first composite oxide produced above was added to 400 ml of pure water to prepare a slurry. While stirring the obtained slurry at 300 rpm, 1600 g of the mixed solution of the second composite oxide produced above and 800 g of 25% ammonia water were added simultaneously over 180 minutes. The mixture was then kept for 60 minutes. Subsequently, the hydroxide was obtained by filtration and washing with water. The obtained hydroxide was calcined in air at 700°C for 5 hours to obtain the cerium oxide-zirconium oxide composite oxide according to Example 2.

[0126] (Comparative Example 1) A powder of the first composite oxide was obtained in the same manner as in Example 1. <Preparation of a mixed solution of the second composite oxide> Zirconium nitrate solution (ZrO ２ Conversion: 108g) and yttrium nitrate solution (Y ２ O ３ (Converted to 52g) was mixed to prepare 1600g of a 10 wt% mixed solution. <Preparation of cerium oxide-zirconium oxide composite oxide> 1600g of the composite oxide mixed solution and 800g of 25% ammonia water were added simultaneously over 180 minutes. The mixture was then kept for 60 minutes. Subsequently, hydroxide was obtained by filtration and washing with water. The obtained hydroxide was calcined in air at 700°C for 5 hours. The first composite oxide and the second composite oxide were mixed to obtain the cerium oxide-zirconium oxide composite oxide according to Comparative Example 1.

[0127] (Comparative Example 2) The powder of the first composite oxide was obtained in the same manner as in Example 1. <Preparation of the mixed solution of the second composite oxide> Zirconium nitrate solution (ZrO ２ Conversion: 42.0 g), lanthanum nitrate solution (La ２ O ３ Conversion: 6.0 g) and yttrium nitrate solution (Y ２ O ３(Converted to 12.0 g) was mixed to prepare 600 g of a 10 wt% mixed solution. <Preparation of cerium oxide-zirconium oxide composite oxide> 140 g of the powder of the first composite oxide produced above was added to 1400 ml of pure water to prepare a slurry. While stirring the obtained slurry at 300 rpm, 600 g of the mixed solution of the second composite oxide produced above and 300 g of 25% ammonia water were added simultaneously over 180 minutes. The mixture was then kept for 60 minutes. Subsequently, the hydroxide was obtained by filtration and washing with water. The obtained hydroxide was calcined in air at 700°C for 5 hours to obtain the cerium oxide-zirconium oxide composite oxide according to Example 1.

[0128] [Composition Measurement of Cerium Oxide-Zirconium Oxide Composite Oxides] The composition of cerium oxide-zirconium oxide composite oxides was analyzed using X-ray fluorescence (XRF) analysis ("ZSX Primus II" manufactured by Rigaku Corporation). The results are shown in the "Composition" column of Table 1. Note that in the "Composition" column of Table 1, the value of CeO ２ The value of " is the "amount of cerium C1 in the entire cerium oxide-zirconium oxide composite oxide" as explained above. In addition, the "Shell Composition" column in Table 1 shows the content of each oxide constituting the shell, when the entire shell (the entire second composite oxide) is set to 100% by mass. This content is a value calculated from the charge ratio during the production of the cerium oxide-zirconium oxide composite oxide. In addition, the "Core Ratio" column in Table 1 shows the content ratio of the core (content ratio of the first composite oxide) when the entire cerium oxide-zirconium oxide composite oxide is set to 100% by mass. This content ratio is a value calculated from the charge ratio during the production of the cerium oxide-zirconium oxide composite oxide.

[0129] [Measurement of Crystallite Size] The crystallite sizes of the first and second composite oxides obtained during the manufacturing process of the cerium oxide-zirconium oxide composite oxides in the examples and comparative examples were measured. Specifically, they were determined by X-ray diffraction analysis (XRD) using an automated X-ray diffractometer. A Cu target was used as the X-ray source, and measurements were taken in the range of 20° ≤ 2θ ≤ 60°. Based on the peak values ​​at 2θ = 28.6° and 29.3°, the crystallite size was calculated using the Sheller method. The results are shown in Table 1.

[0130] [Measurement of Cerium Amount (C2) in the Surface Layer of Cerium Oxide-Zirconium Oxide Composite Oxides by X-ray Photoelectron Spectroscopy] The amount of cerium in the surface layer of cerium oxide-zirconium oxide composite oxides of the examples and comparative examples was measured using an X-ray photoelectron spectrometer (Shimadzu Corporation, product name: AXIS-NOVA). The measurement conditions were as follows: <Measurement Conditions> X-ray output: Al monochromator 100W Beam diameter: 400 μm ２ Beam output: 10kV-10mA Beam irradiation time: 100ms per point Step scan: 100meV Pass energy: 40eV The results are shown in the "Surface Ce amount" column of Table 1.

[0131] [Specific Surface Area Measurement (Specific Surface Area Measurement Before Heat Treatment)] The specific surface area of ​​the cerium oxide-zirconium oxide composite oxides of the examples and comparative examples was measured using the BET method with a specific surface area meter ("Macsorb," manufactured by Mountec). The results are shown in the "Fresh" column of Table 1.

[0132] [Measurement of specific surface area after heat treatment at 1000°C for 3 hours in an atmospheric atmosphere] The cerium oxide-zirconium oxide composite oxides of the examples and comparative examples were heat-treated at 1000°C for 3 hours in an atmospheric atmosphere. The specific surface area after heat treatment at 1000°C for 3 hours in an atmospheric atmosphere was measured using the BET method with a specific surface area meter ("Macsorb," manufactured by Mountec). The results are shown in the "1000°C" column of Table 1.

[0133] [Measurement of specific surface area after heat treatment at 1100°C for 3 hours in an atmospheric atmosphere] The cerium oxide-zirconium oxide composite oxides of the examples and comparative examples were heat-treated at 1100°C for 3 hours in an atmospheric atmosphere. The specific surface area after heat treatment at 1100°C for 3 hours in an atmospheric atmosphere was measured using the BET method with a specific surface area meter ("Macsorb," manufactured by Mountec). The results are shown in the "1100°C" column of Table 1.

[0134] [Particle size D ５０ [Measurement] 0.15 g of the cerium oxide-zirconium oxide composite oxide (powder) of the example and comparative example and 40 ml of a 0.2% sodium hexametaphosphate aqueous solution were placed in a 50 ml beaker, dispersed for 5 minutes using a tabletop ultrasonic cleaner "W-113" (manufactured by Honda Electronics Co., Ltd.), and then measured using a laser diffraction particle size distribution analyzer ("SALD-2300" manufactured by Shimadzu Corporation). The results are shown in Table 1 as "Particle size D after ultrasonic dispersion". ５０ This will be shown in the " " column.

[0135] [Exhaust Gas Purification Performance Evaluation] <Preparation of Catalyst> Rhodium nitrate, the cerium oxide-zirconium oxide composite oxides of the examples and comparative examples prepared above, and alumina binder were weighed so that Rh:cerium oxide-zirconium oxide composite oxide:alumina binder was 0.5:99.5:10 (mass ratio). The cerium oxide-zirconium oxide composite oxide was dispersed in pure water (three times the mass of the cerium oxide-zirconium oxide composite oxide), and then 25% aqueous ammonia (equal to Rh) was added. Then, rhodium nitrate was added and stirred for 30 minutes. Next, the alumina binder was added, the pH was adjusted to around 6.5 with nitric acid, and then wet grinding was performed in a ball mill for 14 hours to obtain a slurry. The slurry was wash-coated onto a cylindrical cordierite carrier with 600 cells, a wall thickness of 4 mils, a diameter of 24 mm, and a length of 30 mm, so that the load after calcination was 110.1 g / L. This was dried at 150°C for 30 minutes, then calcined at 550°C for 30 minutes to obtain a catalyst for measurement.

[0136] <Heat Resistance Treatment> The measurement catalyst manufactured above was subjected to heat resistance treatment under the following conditions: Temperature: 1050°C Time: 14 hours Atmosphere: Rich gas (CO: 3%, C ３ H６ : 1.2%, H ２ : 1.2%, H ２ O: 10%, remainder: N ２ ), Lean gas (O ２ : 6%, H ２ O: 10%, remainder N ２ ), N ２ gas (H ２ O: 10%, remainder N ２ ) to Rich gas → N ２ The gas is switched between gas, then Lean gas, then N2 gas, every 30 seconds.

[0137] <Exhaust Gas Purification Performance Evaluation> The exhaust gas purification performance was evaluated using simulated gas with the heat-treated catalyst as the exhaust gas purification catalyst. NO: 500 ppm, C ３ H ６ : 960 ppm (methane equivalent), C ３ H ８ : 240 ppm (methane equivalent), H ２ : 0.17%, CO ２ : 14.0%, H ２ O: 10.0%, N ２ Under balanced conditions, CO: 0.51–1.29%, O ２ The catalyst was oscillated at 1 Hz in the range of 0.28 to 1.05%, and the temperature of the catalyst inlet (1.0 cm from the catalyst inlet end face) was raised from 100°C to 550°C at a rate of 20°C per minute while flowing at a flow rate of 32 L / min. The gas discharged from the catalyst outlet was analyzed, and the purification rates of CO, HC, and NOx were calculated. The temperature at which each purification rate reached 30% was defined as T30. The results are shown in the "T30" column of Table 1. Examples 1 to 3 have a core-shell structure, and because the amount of cerium in the surface layer is small, the decrease in catalytic activity is suppressed. On the other hand, Comparative Examples 1 and 2 do not have a core-shell structure, so the catalyst (Rh) is oxidized, and the activity is reduced.

[0138] [Oxygen Storage Capacity (OSC (600°C))] First, 0.50 g of the cerium oxide-zirconium oxide-based composite oxide (powder) of Examples and Comparative Examples was subjected to reduction treatment at 600°C for 10 minutes in a 5% hydrogen-argon gas flow. Next, pulsed injection of oxygen gas (0.5 ml per 0.1 seconds) was performed, and the oxygen gas concentration at the outlet was detected by gas chromatography (Shimadzu Co., LTD., GC-8A). That is, at 600°C, 5% H ２ gas was used to cause oxygen desorption, the amount of oxygen stored (consumed) by injection of oxygen pulses was measured, and the oxygen consumption at 600°C was defined as OSC (600°C). OSC is expressed in μmol-O ２ / g, that is, oxygen storage capacity per unit weight of the powder. The results are shown in Table 1.

[0139] [SEM Observation] FIG. 1 shows an SEM image of the cerium oxide-zirconium oxide-based composite oxide obtained in Example 1, and FIG. 2 shows an SEM image of Comparative Example 1. As shown in FIG. 1, the cerium oxide-zirconium oxide-based composite oxide of Example 1 is formed into a core-shell structure. On the other hand, as shown in FIG. 2, in the cerium oxide-zirconium oxide-based composite oxide of Comparative Example 1, only the core is confirmed, and no core-shell structure is formed. Although not shown in the figures, in the SEM images of the cerium oxide-zirconium oxide-based composite oxides of other Examples, the state of being formed into a core-shell structure could also be observed similarly to FIG. 1.

[0140]

Claims

1. A cerium oxide-zirconium oxide composite oxide having a core-shell structure comprising a core and a shell covering the core, wherein when the amount of cerium in the entire cerium oxide-zirconium oxide composite oxide measured by X-ray fluorescence analysis is defined as C1 (wt%), and the amount of cerium in the surface layer of the cerium oxide-zirconium oxide composite oxide measured by X-ray photoelectron spectroscopy is defined as C2 (atm%), C1 is 5 wt% or more and C2 is 2 atm% or less.

2. The cerium oxide-zirconium oxide composite oxide according to claim 1, characterized in that the core is composed of a first composite oxide comprising zirconium oxide and cerium oxide, the crystallite size of the first composite oxide being 30 nm or more, the shell is composed of a second composite oxide comprising zirconium oxide and a rare earth oxide other than cerium oxide, the crystallite size of the second composite oxide being less than 20 nm, and the weight ratio of the first composite oxide to the second composite oxide ([first composite oxide]:[second composite oxide]) being 10 to 50:50 to 90.

3. The specific surface area after heat treatment at 1000°C for 3 hours in an atmospheric environment is 20 m². ２ / g or more 60m ２ The cerium oxide-zirconium oxide composite oxide according to claim 1 or 2, characterized in that it is less than or equal to / g.

4. A method for producing a cerium oxide-zirconium oxide composite oxide according to claim 1 or 2, characterized by comprising: step 1, simultaneously adding a second solution containing zirconium and cerium-other rare earth elements, and an alkali to a first solution containing a first composite oxide containing zirconium and cerium to obtain a hydroxide; and step 2, heat-treating the hydroxide.

5. An exhaust gas purification catalyst characterized by comprising a cerium oxide-zirconium oxide composite oxide as described in claim 1 or 2, and a noble metal supported on the cerium oxide-zirconium oxide composite oxide.

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