JP2013203639A - Complex oxide powder - Google Patents
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本発明は、少なくとも比表面積が大きい複合酸化物合成粒子からなる複合酸化物粉末およびその製造方法に関する。 The present invention relates to a composite oxide powder comprising composite oxide composite particles having at least a large specific surface area and a method for producing the same.
自動車エンジン等の内燃機関から排出された排ガスは窒素酸化物（ＮＯx）、一酸化炭素（ＣＯ）、炭化水素（ＨＣ）等を含むが、それら物質は大気排出前に排ガス浄化用触媒で酸化または還元されて浄化される。この排ガス浄化用触媒として、白金（Ｐｔ）、ロジウム（Ｒｈ）、パラジウム（Ｐｄ）等の貴金属を金属酸化物担体に担持させた三元触媒が代表的である。 Exhaust gas discharged from internal combustion engines such as automobile engines contains nitrogen oxides (NOx), carbon monoxide (CO), hydrocarbons (HC), etc., but these substances are oxidized or exhausted with an exhaust gas purification catalyst before being discharged into the atmosphere. Reduced and purified. A typical example of the exhaust gas purifying catalyst is a three-way catalyst in which a noble metal such as platinum (Pt), rhodium (Rh), palladium (Pd) is supported on a metal oxide carrier.
金属酸化物担体は、比表面積が大きいことに加えて、排ガス中の酸素濃度の変動を吸収できる酸素吸蔵能（ＯＳＣ：Oxygen Storage Capacity）に優れることが求められる。このような観点から、金属酸化物担体には、表面積が大きなアルミナ（Ａｌ２Ｏ３）、ジルコニア（ＺｒＯ２）、チタニア（ＴｉＯ２）等の他、ＯＳＣに優れるセリア（ＣｅＯ２）が用いられることが多い。セリアは、ＯＳＣに優れるのみならず、担持した貴金属（特に白金）の粒成長（シンタリング）を抑制するともいわれている。このようなセリアを金属酸化物担体に用いた例が下記の特許文献１にある。 In addition to a large specific surface area, the metal oxide support is required to have an excellent oxygen storage capacity (OSC: Oxygen Storage Capacity) capable of absorbing fluctuations in oxygen concentration in the exhaust gas. From such a viewpoint, ceria (CeO 2 ) excellent in OSC is used for the metal oxide support, in addition to alumina (Al 2 O 3 ), zirconia (ZrO 2 ), titania (TiO 2 ) and the like having a large surface area. There are many cases. It is said that ceria not only excels in OSC but also suppresses grain growth (sintering) of supported noble metals (especially platinum). An example in which such ceria is used for a metal oxide support is disclosed in Patent Document 1 below.
特許文献１は、ジルコニア水性ゾルとセリア水性ゾルを１：１で混合した混合ゾルのｐＨをそれぞれの粒子の等電点付近に調整することで凝集を生じさせ、ジルコニアを中心部としセリアを外皮部とした構造をもつ金属酸化物粒子を提案している。しかし、その合成後の金属酸化物粒子は、粒径が５０ｎｍ未満と小さく（［００１４］参照）、その表面におけるＣｅＯ２濃度は高々５６％（ＸＰＳ定量分析値）に過ぎず、ジルコニアの外周囲をセリアが十分に覆っているとは言い難い。また特許文献１に記載された具体的な測定データは、表面に存在するＣｅＯ２の濃度のみであり、比表面積やＯＳＣ等は何ら評価されていない。 In Patent Document 1, aggregation is generated by adjusting the pH of a mixed sol obtained by mixing a zirconia aqueous sol and a ceria aqueous sol in a ratio of 1: 1 to the vicinity of the isoelectric point of each particle, and ceria is crusted with zirconia at the center. We have proposed metal oxide particles with the structure of a part. However, the synthesized metal oxide particles have a particle size as small as less than 50 nm (see [0014]), the CeO 2 concentration on the surface is only 56% (XPS quantitative analysis value), and the outer periphery of zirconia It's hard to say that Ceria covers enough. The specific measurement data described in Patent Document 1 is only the concentration of CeO 2 present on the surface, and no specific surface area, OSC, or the like has been evaluated.
本発明はこのような事情に鑑みて為されたものであり、従来の金属酸化物粒子よりも、少なくとも比表面積が大きい複合酸化物合成粒子からなる複合酸化物粉末およびその製造方法を提供することを目的とする。 The present invention has been made in view of such circumstances, and provides a composite oxide powder comprising composite oxide composite particles having at least a specific surface area larger than that of conventional metal oxide particles and a method for producing the same. With the goal.
本発明者はこの課題を解決すべく鋭意研究し試行錯誤を重ねた結果、酸化ジルコニウムゾルと酸化セリウムゾルの混合ゾルを十分にミリングした後、乾燥、焼成することにより、微細な酸化ジルコニウム粒子が凝集した二次粒子の表面に、微細な酸化セリウム粒子が結合した複合酸化物合成粒子を得ることに成功した。そしてこの複合酸化物合成粒子は、比表面積が大きく、優れた酸素吸蔵能を発揮することもわかった。この成果を発展させることにより、以降に述べる一連の本発明を完成するに至った。 As a result of intensive research and trial and error to solve this problem, the inventor has sufficiently milled a mixed sol of zirconium oxide sol and cerium oxide sol, and then dried and baked to aggregate fine zirconium oxide particles. We succeeded in obtaining composite oxide composite particles with fine cerium oxide particles bound to the surface of the secondary particles. It was also found that this composite oxide composite particle has a large specific surface area and exhibits an excellent oxygen storage capacity. By developing this result, a series of the present invention described below has been completed.
《複合酸化物粉末》
（１）本発明の複合酸化物粉末は、第一金属酸化物からなり一次粒径が１５ｎｍ以下である第一基粒子が凝集して二次粒径が２０ｎｍ以上となった二次粒子からなるコア部と、該第一金属酸化物と異なる第二金属酸化物からなり一次粒径が４０ｎｍ以下である第二基粒子が該コア部の外表面に結合してなるシェル部と、で構成された複合酸化物合成粒子からなることを特徴とする。
<Composite oxide powder>
(1) The composite oxide powder of the present invention is composed of secondary particles composed of a first metal oxide and having a primary particle size of 15 nm or less and agglomerated first base particles to a secondary particle size of 20 nm or more. A core portion, and a shell portion formed of a second metal oxide different from the first metal oxide and having a primary particle size of 40 nm or less bonded to the outer surface of the core portion. It is characterized by comprising composite oxide composite particles.
（２）本発明の複合酸化物粉末を構成する複合酸化物合成粒子は、第一金属酸化物からなる微細な第一基粒子が凝集して大きくなった二次粒子の表面に、第二金属酸化物からなる微細な第二基粒子が緻密に結合したコアシェル構造体からなる。第一基粒子および第二基粒子の一次粒径が小さいため、この複合酸化物合成粒子は、従来の金属酸化物粒子よりも大きな比表面積を有する。 (2) The composite oxide composite particles constituting the composite oxide powder of the present invention have a secondary metal on the surface of secondary particles that are agglomerated by the fine first base particles composed of the first metal oxide. It consists of a core-shell structure in which fine second group particles made of an oxide are closely bonded. Since the primary particle diameters of the first base particles and the second base particles are small, the composite oxide composite particles have a specific surface area larger than that of conventional metal oxide particles.
また複合酸化物合成粒子は、第二基粒子が微粒子であるため少量でも、第一基粒子の二次粒子の表面が第二基粒子により十分に被覆された状態となっている。このため本発明によれば、第二基粒子の使用量を抑制しつつ、第二基粒子に特有な機能を十分に発現させ得る。 In addition, since the second base particles are fine particles, the composite oxide composite particles are sufficiently covered with the second base particles on the surfaces of the secondary particles of the first base particles. For this reason, according to this invention, the function peculiar to 2nd base particle can fully be expressed, suppressing the usage-amount of 2nd base particle.
《複合酸化物粉末の製造方法》
本発明の複合酸化物粉末は、その製造方法を問わないが、例えば、次のような本発明の製造方法により得られる。すなわち本発明の複合酸化物粉末は、第一金属酸化物からなる第一コロイド粒子が分散媒中に分散した第一金属酸化物ゾルと、該第一金属酸化物と異なる第二金属酸化物からなる第二コロイド粒子が分散媒中に分散した第二金属酸化物ゾルとを混合した混合ゾルを得る混合工程と、該混合ゾル中の該第一コロイド粒子と該第二コロイド粒子が凝集した凝集粒子を含む凝集物を得る凝集工程と、該凝集物を乾燥させた乾燥物を得る乾燥工程と、該乾燥物を焼成された焼成物を得る焼成工程と、該焼成物を解砕する解砕工程とを備え、前記第一金属酸化物からなるコア部と該コア部の外表面に前記第二金属酸化物が結合してなるシェル部とで構成された複合酸化物合成粒子からなる複合酸化物粉末が得られることを特徴とする複合酸化物粉末の製造方法によって得られる。
<< Production Method of Composite Oxide Powder >>
The production method of the composite oxide powder of the present invention is not limited, but can be obtained, for example, by the production method of the present invention as follows. That is, the composite oxide powder of the present invention comprises a first metal oxide sol in which first colloidal particles made of a first metal oxide are dispersed in a dispersion medium, and a second metal oxide different from the first metal oxide. A mixing step of obtaining a mixed sol in which a second metal oxide sol in which a second colloid particle is dispersed in a dispersion medium is obtained, and agglomeration of the first colloid particle and the second colloid particle in the mixed sol Aggregation step for obtaining an agglomerate containing particles, a drying step for obtaining a dried product obtained by drying the agglomerate, a firing step for obtaining a fired product obtained by firing the dried product, and crushing for crushing the fired product A composite oxidation comprising composite oxide composite particles comprising a core portion made of the first metal oxide and a shell portion formed by bonding the second metal oxide to the outer surface of the core portion. A composite oxide powder characterized in that a product powder is obtained. Obtained by the method.
なお、コロイド粒子は、液体（特に水）に分散している微粒子である。ゾルは、コロイド粒子を液体である分散媒に分散させた分散系であり、コロイド溶液ともいう。分散媒は、水の他、アルコール、アセチルアセトン等の有機分散媒でもよい。 The colloidal particles are fine particles dispersed in a liquid (particularly water). The sol is a dispersion system in which colloidal particles are dispersed in a liquid dispersion medium, and is also referred to as a colloid solution. The dispersion medium may be water or an organic dispersion medium such as alcohol or acetylacetone.
《その他》
特に断らない限り本明細書でいう「ｘ〜ｙ」は下限値ｘおよび上限値ｙを含む。本明細書に記載した具体的な数値または数値範囲に含まれる任意の数値を、新たな下限値または上限値として「ａ〜ｂ」のような数値範囲を任意に新設し得る。
<Others>
Unless otherwise specified, “x to y” in this specification includes a lower limit value x and an upper limit value y. Any numerical value included in the specific numerical values or numerical ranges described in the present specification can be arbitrarily set as a new lower limit value or upper limit value such as “ab”.
発明の実施形態を挙げて本発明をより詳しく説明する。上述した本発明の構成に、本明細書中から選択した一つまたは二つ以上の構成を任意に付加し得る。本明細書で説明する内容は、本発明に係る複合酸化物粉末のみならず、その製造方法にも適宜適用される。製造方法に関する構成は、プロダクトバイプロセスクレームとして理解すれば物に関する構成ともなり得る。いずれの実施形態が最良であるか否かは、対象、要求性能等によって異なる。 The present invention will be described in more detail with reference to embodiments of the invention. One or more configurations selected from the present specification can be arbitrarily added to the configuration of the present invention described above. The contents described in this specification are appropriately applied not only to the composite oxide powder according to the present invention but also to the production method thereof. A configuration related to a manufacturing method can be a configuration related to an object if understood as a product-by-process claim. Which embodiment is the best depends on the target, required performance, and the like.
《複合酸化物粉末》
（１）基粒子および二次粒子
本発明の複合酸化物粉末は、少なくとも２種以上の金属酸化物からなる基粒子で構成される。少なくともその一種である第一金属酸化物からなる第一基粒子がコア部を構成し、他種の第二金属酸化物からなる第二基粒子がコア部の表面に結合してシェル部を構成する。
<Composite oxide powder>
(1) Base Particles and Secondary Particles The composite oxide powder of the present invention is composed of base particles composed of at least two kinds of metal oxides. The first base particles made of at least one kind of the first metal oxide constitute the core part, and the second base particles made of another kind of the second metal oxide bind to the surface of the core part to form the shell part. To do.
金属酸化物の具体的な組成は問わない。金属酸化物は、例えば、酸化ジルコニウム（ジルコニア）、酸化アルミニウム（アルミナ）、酸化チタン（チタニア）、酸化セリウム（セリア）等である。このうち、第二金属酸化物は、触媒用金属酸化物担体やガラス研磨材等に適した希土類元素の酸化物、特に酸化セリウムが好ましい。また第一金属酸化物は、耐熱性、高強度、酸素イオン導電性等に優れる酸化ジルコニウムが好ましい。特に第一金属酸化物が酸化ジルコニウムであり、第二金属酸化物が酸化セリウムであると、熱安定性が高くＯＳＣ向上に非常に優れた効果を発揮するため好ましい。 The specific composition of a metal oxide is not ask | required. Examples of the metal oxide include zirconium oxide (zirconia), aluminum oxide (alumina), titanium oxide (titania), cerium oxide (ceria), and the like. Among these, the second metal oxide is preferably a rare earth element oxide suitable for a metal oxide carrier for a catalyst, a glass abrasive or the like, particularly cerium oxide. The first metal oxide is preferably zirconium oxide that is excellent in heat resistance, high strength, oxygen ion conductivity, and the like. In particular, it is preferable that the first metal oxide is zirconium oxide and the second metal oxide is cerium oxide because the thermal stability is high and the effect of improving OSC is exhibited.
一次粒径は、いずれの基粒子の場合も、１５ｎｍ以下、１２ｎｍ以下さらには１０ｎｍ以下であると好ましい。一次粒径が過大では、十分な比表面積や酸素吸蔵能等の確保が困難となる。一次粒径の下限値は問わないが、敢えていうと、１ｎｍ以上さらには３ｎｍ以上であると好ましい。なお、本発明に係る一次粒径の特定方法については後述する。 The primary particle diameter is preferably 15 nm or less, 12 nm or less, and further 10 nm or less for any base particle. When the primary particle size is excessive, it is difficult to ensure a sufficient specific surface area and oxygen storage capacity. The lower limit of the primary particle size is not limited, but dare to say that it is preferably 1 nm or more, and more preferably 3 nm or more. In addition, the identification method of the primary particle size which concerns on this invention is mentioned later.
第一基粒子が凝集した二次粒子の粒径（二次粒径）は、２０ｎｍ以上、５０ｎｍ以上さらには６０ｎｍ以上であると好ましい。二次粒径の上限値は、敢えていうと、１７０ｎｍ以下さらには１５０ｎｍ以下であると好ましい。二次粒径が過小では第二金属酸化物とコアシェル構造を取りにくく、第二金属酸化物の使用量をあまり抑制できず、二次粒径が過大では第二金属酸化物に特有な効果の発現が低下し得る。なお、本発明に係る二次粒径の特定方法についても後述する。 The particle size (secondary particle size) of the secondary particles in which the first base particles are aggregated is preferably 20 nm or more, 50 nm or more, and more preferably 60 nm or more. The upper limit value of the secondary particle size is preferably 170 nm or less, more preferably 150 nm or less. If the secondary particle size is too small, it is difficult to form the core-shell structure with the second metal oxide, and the amount of the second metal oxide used cannot be suppressed so much. Expression can be reduced. A method for specifying the secondary particle diameter according to the present invention will also be described later.
第二金属酸化物の使用量を低減しつつ、コアシェル構造からなる好特性の複合酸化物合成粒子を得るには、第二金属酸化物（例えばセリア）に対する一金属酸化物（例えばジルコニア）のモル比が１．５〜９、２〜７さらには３〜５であると好ましい。このモル比が過小では第二金属酸化物の使用量の低減を図れず、過大では機能材である第二金属酸化物が過少となり好特性な複合酸化物合成粒子が得られない。 In order to obtain composite oxide composite particles having a good core-shell structure while reducing the amount of the second metal oxide used, the mole of one metal oxide (for example, zirconia) relative to the second metal oxide (for example, ceria). The ratio is preferably 1.5 to 9, 2 to 7, and more preferably 3 to 5. If this molar ratio is too small, the amount of the second metal oxide used cannot be reduced, and if it is too large, the second metal oxide, which is a functional material, becomes too small to obtain composite oxide composite particles having good characteristics.
（２）特性および構造
複合酸化物合成粒子の比表面積は、周知のＢＥＴ法で測定される。本発明の複合酸化物合成粒子は比表面積が大きく、比表面積は２０ｍ２／ｇ以上さらには２２ｍ２／ｇ以上となり得る。
(2) Characteristics and Structure The specific surface area of the composite oxide composite particle is measured by a well-known BET method. The composite oxide composite particle of the present invention has a large specific surface area, and the specific surface area can be 20 m 2 / g or more, further 22 m 2 / g or more.
複合酸化物合成粒子の構造は、Ｘ線回折（ＸＲＤ）により特定される。例えば、第一金属酸化物が酸化ジルコニウムであり、第二金属酸化物が酸化セリウムである場合、両者を分散、焼成させると、酸化ジルコニウムが原料段階の単斜晶酸化ジルコニウム（ｍ−ＺｒＯ２）から正方晶酸化ジルコニウム（ｔ−ＺｒＯ２）へ相転移することが知られている。この相転移の進行具合つまり正方晶酸化ジルコニウムの生成具合をＸＲＤで観察することにより、酸化ジルコニウムと酸化セリウムの分散状況を把握できる。 The structure of the composite oxide composite particle is specified by X-ray diffraction (XRD). For example, when the first metal oxide is zirconium oxide and the second metal oxide is cerium oxide, when both are dispersed and fired, the zirconium oxide is a monoclinic zirconium oxide (m-ZrO 2 ) in the raw material stage. Is known to undergo phase transition from tetragonal zirconium oxide (t-ZrO 2 ). By observing the progress of this phase transition, that is, the formation of tetragonal zirconium oxide by XRD, it is possible to grasp the dispersion state of zirconium oxide and cerium oxide.
具体的にいうと、複合酸化物合成粒子に特性Ｘ線（ＣｕＫα線／波長λ＝１．５４１８Å）を照射して得られたＸ線回折パターンに基づき、単斜晶ジルコニアの最大ピークの強度Ｉｍ（２θ＝２８．１８９°／θ：入射角、２θ：回折角）と正方晶ジルコニアの最大ピークの強度Ｉｔ（２θ＝２９．９５３°）を特定する。これらからピーク強度比（Ｉｔ／Ｉｍ）を求めることにより、酸化ジルコニウムと酸化セリウムの分散状況の把握が可能となる。このピーク強度比は１．７以上、１．８以上さらには１．９以上であると好ましい。ピーク強度比が過小では酸化ジルコニウムと酸化セリウムの分散が不十分となり、複合酸化物合成粒子の比表面積や酸素吸蔵能等の大幅な向上を図ることは難しい。 Specifically, based on the X-ray diffraction pattern obtained by irradiating the composite oxide composite particles with characteristic X-rays (CuKα rays / wavelength λ = 1.5418４), the intensity Im of the monoclinic zirconia peak is Im. (2θ = 28.189 ° / θ: incidence angle, 2θ: diffraction angle) and tetragonal zirconia maximum peak intensity It (2θ = 29.953 °) are specified. By obtaining the peak intensity ratio (It / Im) from these, it becomes possible to grasp the dispersion state of zirconium oxide and cerium oxide. This peak intensity ratio is preferably 1.7 or more, 1.8 or more, and more preferably 1.9 or more. If the peak intensity ratio is too small, the dispersion of zirconium oxide and cerium oxide becomes insufficient, and it is difficult to significantly improve the specific surface area and oxygen storage capacity of the composite oxide composite particles.
（３）用途
本発明の複合酸化物粉末は、その用途を問わないが、例えば、排ガス浄化用触媒、特にその金属酸化物担体として用いられると好ましい。この他、本発明の複合酸化物粉末はガラス研磨材等としても利用され得る。なお、複合酸化物粉末の用途や仕様に応じて、第一金属酸化物や第二金属酸化物、粒径等は適宜選択されると好ましい。
(3) Uses The composite oxide powder of the present invention may be used for any purpose, but is preferably used, for example, as an exhaust gas purifying catalyst, particularly as its metal oxide support. In addition, the composite oxide powder of the present invention can be used as a glass abrasive or the like. In addition, it is preferable that the first metal oxide, the second metal oxide, the particle size, and the like are appropriately selected according to the use and specifications of the composite oxide powder.
《複合酸化物粉末の製造方法》
本発明の複合酸化物粉末の製造方法は、少なくとも混合工程、凝集工程、乾燥工程、焼成工程および解砕工程を備える。これら各工程について以下に説明する。
<< Production Method of Composite Oxide Powder >>
The method for producing a composite oxide powder of the present invention includes at least a mixing step, an aggregation step, a drying step, a firing step, and a crushing step. Each of these steps will be described below.
（１）混合工程
混合工程は、原料となる少なくとも２種以上の金属酸化物ゾルを混合した混合ゾルを得る工程である。すなわち混合工程では、少なくともその一種である第一金属酸化物ゾルと他種の第二金属酸化物ゾルが混合される。各金属酸化物の種類（組成）については、上述した複合酸化物粉末の場合と同様である。
(1) Mixing process A mixing process is a process of obtaining the mixed sol which mixed the at least 2 or more types of metal oxide sol used as a raw material. That is, in the mixing step, at least one kind of the first metal oxide sol and another kind of the second metal oxide sol are mixed. About the kind (composition) of each metal oxide, it is the same as that of the case of the complex oxide powder mentioned above.
金属酸化物ゾルは、前述したように、コロイド粒子が分散媒中に分散したものであり、このコロイド粒子が複合酸化物合成粒子を構成する基粒子となる。従って基粒子について説明した内容（一次粒径等）はコロイド粒子についても該当する。すなわち、第一コロイド粒子と第二コロイド粒子の一次粒径は共に４０ｎｍ以下であると好ましい。また、混合される各金属酸化物ゾルの割合についても、前述した第一金属酸化物と第二金属酸化物のモル比に関する内容が該当する。 As described above, the metal oxide sol is obtained by dispersing colloidal particles in a dispersion medium, and the colloidal particles serve as base particles constituting the composite oxide synthetic particles. Therefore, the contents (primary particle size etc.) described for the base particles also apply to the colloidal particles. That is, the primary particle diameters of the first colloid particles and the second colloid particles are both preferably 40 nm or less. Moreover, the content regarding the molar ratio of the 1st metal oxide mentioned above and the 2nd metal oxide corresponds also about the ratio of each metal oxide sol mixed.
（２）凝集工程
凝集工程は、混合ゾル中の第一コロイド粒子と第二コロイド粒子を凝集させた凝集粒子を得る工程である。この凝集工程は、具体的にいうと例えば、混合ゾルをメディアを用いてミリングするミリング工程である。メディアはジルコニアボール、シリカボール、アルミナボール等、公知のものを利用できるが、第一金属酸化物と同組成のものを利用すると好ましい。ミリング条件は適宜選択されるが、例えば、５０〜５００ｒｐｍで１０〜１００時間行えばよい。
(2) Aggregation step The aggregation step is a step of obtaining aggregated particles obtained by aggregating the first colloid particles and the second colloid particles in the mixed sol. More specifically, this agglomeration process is, for example, a milling process in which the mixed sol is milled using a medium. As the media, known media such as zirconia balls, silica balls, and alumina balls can be used, but it is preferable to use media having the same composition as the first metal oxide. Milling conditions are selected as appropriate, and may be performed, for example, at 50 to 500 rpm for 10 to 100 hours.
第一コロイド粒子を酸化ジルコニウム粒子、第二コロイド粒子を酸化セリウム粒子とした場合、凝集工程中に酸化ジルコニウム粒子表面に酸化セリウム粒子が凝集した状態となっている。このような状態物を本発明では凝集物という。 When the first colloid particles are zirconium oxide particles and the second colloid particles are cerium oxide particles, the cerium oxide particles are aggregated on the surface of the zirconium oxide particles during the aggregation process. Such a state thing is called an aggregate in this invention.
ちなみに本発明に係る凝集工程により、そのような特有の凝集現象が生じる理由は次のように考えられる。酸化ジルコニウム粒子および酸化セリウム粒子は、それぞれ分散剤によって安定したコロイド状態となっている。ミリングによる凝集工程を行うことで、メディアからのエネルギーによりその安定状態が崩れ、酸化ジルコニウム粒子と酸化セリウム粒子の凝集が進行する。 Incidentally, the reason why such a specific aggregation phenomenon occurs in the aggregation process according to the present invention is considered as follows. Zirconium oxide particles and cerium oxide particles are each in a stable colloidal state by the dispersant. By performing the aggregation process by milling, the stable state is broken by the energy from the media, and the aggregation of the zirconium oxide particles and the cerium oxide particles proceeds.
（３）乾燥工程
乾燥工程は、凝集物を乾燥させて乾燥物を得る工程であり、具体的には、凝集物中に含まれる水等の分散媒を蒸発等させて除去する工程である。乾燥工程は、大気中で自然乾燥させることも考えられるが、例えば、凝集物を８０〜１５０℃さらには１００〜１３０℃に加熱して乾燥させる加熱乾燥工程を行えば効率的である。この乾燥温度が過小では乾燥時間が長くなる。乾燥温度が過大では金属酸化物ゾル中に含まれていた有機物等が硬化、炭化等して、複合酸化物合成粒子の形態に影響し得る。従って加熱乾燥工程を行う場合、乾燥温度は上記の範囲内とするのが好ましい。
(3) Drying step The drying step is a step of drying the aggregate to obtain a dried product, and specifically, a step of removing the dispersion medium such as water contained in the aggregate by evaporating. The drying process may be naturally dried in the atmosphere. For example, it is efficient to perform a heating and drying process in which the aggregate is heated to 80 to 150 ° C. and further to 100 to 130 ° C. for drying. If this drying temperature is too low, the drying time becomes longer. If the drying temperature is excessively high, the organic matter contained in the metal oxide sol may be cured, carbonized, etc., and affect the form of the composite oxide composite particles. Therefore, when performing a heat drying process, it is preferable that drying temperature shall be in said range.
（４）焼成工程
焼成工程は、乾燥物を焼成させて焼成物を得る工程である。乾燥物を急激に焼成させると、金属酸化物粒子が緻密に結合した焼成物が得られ、比表面積や酸素吸蔵能等の低下を招く。そこで焼成工程は、低温焼成工程と、該低温焼成工程後の高温焼成工程とからなると好適である。具体的には、低温焼成工程は、乾燥物を４５０〜７００℃さらには５５０〜６５０℃で焼成させた低温焼成物を得る工程とすると好ましい。また高温焼成工程は、低温焼成物を７５０〜１１５０℃さらには８００〜１０５０℃で焼成させた高温焼成物を得る工程とすると好ましい。なお、低温焼成工程と高温焼成工程は連続的に行えばよい。
(4) Firing step The firing step is a step of obtaining a fired product by firing a dried product. When the dried product is rapidly baked, a baked product in which metal oxide particles are densely bonded is obtained, which causes a decrease in specific surface area, oxygen storage capacity, and the like. Therefore, it is preferable that the firing step includes a low temperature firing step and a high temperature firing step after the low temperature firing step. Specifically, the low-temperature firing step is preferably a step of obtaining a low-temperature fired product obtained by firing the dried product at 450 to 700 ° C, further 550 to 650 ° C. The high-temperature firing step is preferably a step of obtaining a high-temperature fired product obtained by firing the low-temperature fired product at 750 to 1150 ° C, further 800 to 1050 ° C. Note that the low-temperature firing step and the high-temperature firing step may be performed continuously.
（５）解砕工程
解砕工程は、焼成物を解砕して粉末状とする工程である。この解砕工程は各種のミルやミキサー等を用いて行うことができる。こうして得られた複合酸化物粉末は、第一金属酸化物のコロイド粒子（第一基粒子）からなるコア部とコア部の外周囲に第二金属酸化物のコロイド粒子（第二基粒子）が結合してなるシェル部とで構成された複合酸化物合成粒子となっている。ちなみに本発明の複合酸化物粉末は粒径が５〜２０μｍであると好ましい。この粒径はレーザー回折／散乱式粒度分布計で測定して特定される。
(5) Crushing step The crushing step is a step of crushing the fired product to form a powder. This crushing process can be performed using various mills, mixers, and the like. The composite oxide powder thus obtained has a core portion composed of colloidal particles (first base particles) of the first metal oxide and colloidal particles (second base particles) of the second metal oxide around the outer periphery of the core portion. It is a composite oxide composite particle composed of a shell part formed by bonding. Incidentally, the composite oxide powder of the present invention preferably has a particle size of 5 to 20 μm. This particle size is specified by measuring with a laser diffraction / scattering particle size distribution meter.
実施例を挙げて本発明をより具体的に説明する。 The present invention will be described more specifically with reference to examples.
《試料の製造》
〈試料１〜７〉
（１）原料
酸化ジルコニウムゾル（第一金属酸化物ゾル）と酸化セリウムゾル（第二金属酸化物ゾル）を用意した。各ゾルともに分散媒は水（イオン交換水）と有機アミンからなる溶液である。試料３および試料４に係る酸化セリウムゾルはｐＨ８であったが、それ以外のゾルは全てｐＨ９．５であった。
<Production of sample>
<Samples 1-7>
(1) Raw materials A zirconium oxide sol (first metal oxide sol) and a cerium oxide sol (second metal oxide sol) were prepared. In each sol, the dispersion medium is a solution composed of water (ion exchange water) and an organic amine. The cerium oxide sols related to Sample 3 and Sample 4 had a pH of 8, but all other sols had a pH of 9.5.
酸化ジルコニウムゾル中に分散している酸化ジルコニウム粒子（第一コロイド粒子、第一基粒子）と、酸化セリウムゾル中に分散している酸化セリウム粒子（第二コロイド粒子、第二基粒子）の一次粒径は表１に示した。この一次粒径はＢＥＴ法で求めた比表面積から次式に基づいて計算により算出した。一次粒径（ｎｍ）＝６／（Ｄ×Ｓ）×１０００
ここで、Ｄは粒子の真比重（ｇ／ｃｍ３）、Ｓは比表面積（ｍ２／ｇ）である。
Primary particles of zirconium oxide particles (first colloid particles, first base particles) dispersed in the zirconium oxide sol and cerium oxide particles (second colloid particles, second base particles) dispersed in the cerium oxide sol The diameter is shown in Table 1. The primary particle size was calculated from the specific surface area determined by the BET method based on the following formula. Primary particle size (nm) = 6 / (D × S) × 1000
Here, D is the true specific gravity (g / cm 3 ) of the particles, and S is the specific surface area (m 2 / g).
また酸化ジルコニウム粒子からなる二次粒径の粒径（二次粒径）は、動的光散乱式粒度分布計により測定し、その値を表１に併せて示した。 Further, the secondary particle size (secondary particle size) composed of zirconium oxide particles was measured with a dynamic light scattering particle size distribution meter, and the values are shown in Table 1.
（２）製造
上記の両ゾルを表１に示す混合比（モル比）で混合して混合ゾルを得た（混合工程）。この混合ゾルをセラミックス製のポット（容積：２Ｌ）に入れ、そこへメディア（ジルコニアボール：直径１０ｍｍ）：１ｋｇを加えた。このポットをボールミルにセットして、回転数１７５ｒｐｍで回転させた（凝集工程、ミリング工程）。この操作を行った時間（ミリング時間）は表１に併せて示した。
(2) Production Both sols described above were mixed at a mixing ratio (molar ratio) shown in Table 1 to obtain a mixed sol (mixing step). This mixed sol was put in a ceramic pot (volume: 2 L), and media (zirconia balls: diameter 10 mm): 1 kg was added thereto. This pot was set on a ball mill and rotated at a rotation speed of 175 rpm (aggregation process, milling process). The time for this operation (milling time) is also shown in Table 1.
こうして得られた混合ゾル（凝集物）とメディアを分離し、回収した混合ゾルを１１０℃×２４時間加熱して乾燥させた（加熱乾燥工程）。得られた乾燥物を大気中で軽く解砕して粉末状の乾燥物とした。 The mixed sol (aggregate) thus obtained and the media were separated, and the collected mixed sol was dried by heating at 110 ° C. for 24 hours (heat drying step). The obtained dried product was lightly crushed in the atmosphere to obtain a powdery dried product.
この粉末状の乾燥物を、電気炉で６００℃×３時間加熱して焼成させた（低温焼成工程）。これに引き続いて、得られた低温焼成物を電気炉で１０００℃×３時間加熱して焼成させ、高温焼成物を得た（高温焼成工程）。こうして焼成工程を２段階に分け、焼成を緩やかに進行させた。 This powdery dried product was fired by heating at 600 ° C. for 3 hours in an electric furnace (low temperature firing step). Subsequently, the obtained low-temperature fired product was heated and fired at 1000 ° C. for 3 hours in an electric furnace to obtain a high-temperature fired product (high-temperature firing step). In this way, the firing process was divided into two stages, and the firing proceeded slowly.
得られた高温焼成物をプロペラ式ミキサーで解砕して平均粒径１４μｍの複合酸化物粉末を得た（解砕工程）。 The obtained high-temperature fired product was pulverized with a propeller mixer to obtain a composite oxide powder having an average particle size of 14 μm (pulverization step).
〈試料Ｃ１〉
酸化ジルコニウムゾルを酸化ジルコニウム粉末に替えて、試料３と同様な方法で複合酸化物粉末を製造した。
<Sample C1>
A composite oxide powder was produced in the same manner as Sample 3 by replacing the zirconium oxide sol with a zirconium oxide powder.
〈試料Ｃ２〉
酸化ジルコニウムゾルを水酸化ジルコニウム粉末に替えて、試料３と同様な方法で複合酸化物粉末を製造した。
<Sample C2>
A composite oxide powder was produced in the same manner as Sample 3 by replacing the zirconium oxide sol with zirconium hydroxide powder.
〈試料Ｃ３〉
ミリング工程を省略して、他は試料１と同様な方法で複合酸化物粉末を製造した。
<Sample C3>
A complex oxide powder was produced in the same manner as Sample 1 except that the milling step was omitted.
《測定》
（１）外観
得られた各複合酸化物粉末を透過型電子顕微鏡（ＴＥＭ）で観察した。試料１および試料３に係るＴＥＭ写真を、それぞれ図１Ａおよび図１Ｂ（両図を併せて図１という。）に示した。
<Measurement>
(1) Appearance Each obtained complex oxide powder was observed with a transmission electron microscope (TEM). TEM photographs of Sample 1 and Sample 3 are shown in FIG. 1A and FIG. 1B (referred to as FIG. 1 together).
（２）Ｘ線回折（ＸＲＤ）
各試料から任意に抽出した複合酸化物合成粒子について、その構造解析を粉末Ｘ線回折装置（株式会社リガク製Ｍｕｌｔｉ Ｆｌｅｘ）を用いて行った。用いた特性Ｘ線はＣｕＫα線（波長λ＝１．５４１８Å）である。各試料について得られたＸ線回折パターンから、単斜晶酸化ジルコニウム（ｍ−ＺｒＯ２）の最大ピーク値Ｉｍ（２θ＝２８．１８９°）と正方晶酸化ジルコニウム（ｔ−ＺｒＯ２）の最大ピーク値Ｉｔ（２θ＝２９．９５３°）との比（ピーク強度比：Ｉｔ／Ｉｍ）を求めた。その結果を表１に併せて示した。なお、試料１、試料２および試料Ｃ３に係るＸ線回折パターンを図２に例示した。
(2) X-ray diffraction (XRD)
Structural analysis of the composite oxide composite particles arbitrarily extracted from each sample was performed using a powder X-ray diffractometer (Multi Flex, manufactured by Rigaku Corporation). The characteristic X-ray used is a CuKα ray (wavelength λ = 1.5418Å). From the X-ray diffraction patterns obtained for each sample, the maximum peak value Im (2θ = 28.189 °) of monoclinic zirconium oxide (m-ZrO 2 ) and the maximum peak of tetragonal zirconium oxide (t-ZrO 2 ) The ratio (peak intensity ratio: It / Im) with the value It (2θ = 29.953 °) was determined. The results are also shown in Table 1. In addition, the X-ray-diffraction pattern which concerns on the sample 1, the sample 2, and the sample C3 was illustrated in FIG.
（３）比表面積
各試料に係る複合酸化物合成粒子の比表面積をＢＥＴ法により測定した。この測定は、各試料から任意に抽出した約１ｇの粉末について、ＢＥＴ比表面積測定装置（株式会社島津製作所製トライスター）を用いて行った。その結果を表１に併せて示した。
(3) Specific surface area The specific surface area of the composite oxide composite particles according to each sample was measured by the BET method. This measurement was performed on about 1 g of powder arbitrarily extracted from each sample by using a BET specific surface area measuring device (Tristar manufactured by Shimadzu Corporation). The results are also shown in Table 1.
（４）酸素吸蔵能
各試料に係る複合酸化物合成粒子の酸素貯蔵能（ＯＳＣ）を次のようにして求めた。先ず、各試料に係る複合酸化物粉末を圧縮成形したペレットを作製し、これを供試材（約０．１ｇ）とした。この供試材を耐久試験装置に充填し、水素ガスとアルゴンガスの混合ガスからなる雰囲気中で、８００℃まで昇温した。これにより供試材の還元処理を行った。引き続き、６００℃まで冷却したアルゴンガス雰囲気へ酸素を導入して、供試材へ酸素を吸収させた。このときの酸素の吸収量に基づいて各試料のＯＳＣを算出した。その結果を表１に併せて示した。
(4) Oxygen storage capacity The oxygen storage capacity (OSC) of the composite oxide composite particles according to each sample was determined as follows. First, pellets obtained by compression-molding the composite oxide powder according to each sample were produced and used as test materials (about 0.1 g). The specimen was filled in an endurance test apparatus, and the temperature was raised to 800 ° C. in an atmosphere composed of a mixed gas of hydrogen gas and argon gas. Thereby, the reduction treatment of the test material was performed. Subsequently, oxygen was introduced into an argon gas atmosphere cooled to 600 ° C., and oxygen was absorbed into the test material. Based on the amount of oxygen absorbed at this time, the OSC of each sample was calculated. The results are also shown in Table 1.
《評価》
（１）粒形態
図１からわかるように、本実施例に係る複合酸化物合成粒子は、酸化ジルコニウム粒子からなる二次粒子（コア部）の外周囲に、酸化セリウム粒子（シェル部）がほぼ均一的に薄く結合した状態となっている。
<Evaluation>
(1) Grain shape As can be seen from FIG. 1, the composite oxide composite particles according to the present example are substantially composed of cerium oxide particles (shell parts) on the outer periphery of secondary particles (core parts) made of zirconium oxide particles. It is in a state of being uniformly thinly bonded.
この酸化ジルコニウム粒子に係る二次粒径は８０〜１３０ｎｍ程度であり、その外周囲にある酸化セリウム粒子は３〜１０ｎｍ程度であることもわかる。 It can also be seen that the secondary particle size of the zirconium oxide particles is about 80 to 130 nm, and the outer cerium oxide particles are about 3 to 10 nm.
また表１からわかるように、試料１〜５のピーク強度比は、いずれも１．７以上さらには１．８以上である。このピーク強度比は、試料６、試料７、試料Ｃ１および試料Ｃ３のピーク強度比より十分に大きい。このことからも、試料１〜５に係る複合酸化物合成粒子は、酸化ジルコニウム粒子と酸化セリウム粒子が単に均一に混在した状態ではなく、酸化セリウム粒子が酸化ジルコニウムからなる二次粒子の外周囲に適度に分散している状態にあることがわかる。なお、試料Ｃ２に係る酸化ジルコニウム粒子の一次粒子は非常に微粒子であるため、酸化ジルコニウム粒子と酸化セリウム粒子が均一に分散し、ピーク強度比が大きくなったと考えられる。 Further, as can be seen from Table 1, the peak intensity ratios of Samples 1 to 5 are all 1.7 or more, further 1.8 or more. This peak intensity ratio is sufficiently larger than the peak intensity ratios of Sample 6, Sample 7, Sample C1, and Sample C3. Also from this, the composite oxide composite particles according to Samples 1 to 5 are not in a state where the zirconium oxide particles and the cerium oxide particles are simply mixed together, but around the secondary particles where the cerium oxide particles are made of zirconium oxide. It turns out that it exists in the state disperse | distributed moderately. In addition, since the primary particles of the zirconium oxide particles according to the sample C2 are very fine particles, it is considered that the zirconium oxide particles and the cerium oxide particles are uniformly dispersed and the peak intensity ratio is increased.
（２）比表面積と酸素吸蔵能
上述した複合酸化物合成粒子の粒形態が、比表面積または酸素吸蔵能に反映されていることが表１からわかる。すなわち、試料１〜５はいずれも、比表面積が２０ｍ２／ｇ以上であり、ＯＳＣも５ｍｌ／ｇ以上であり、共に非常に高い値となっている。
(2) Specific surface area and oxygen storage capacity It can be seen from Table 1 that the particle shape of the composite oxide composite particles described above is reflected in the specific surface area or oxygen storage capacity. That is, all of Samples 1 to 5 have a specific surface area of 20 m 2 / g or more and OSC of 5 ml / g or more, both of which are very high values.
（３）混合比（モル比）
表１に示した試料１〜５と試料６〜７の比較から次のことがわかる。ＺｒＯ２／ＣｅＯ２の混合比が試料６のように過大になると、複合酸化物合成粒子の比表面積は十分に大きくなるものの、その酸素吸蔵能は小さくなる。これは複合酸化物合成粒子を構成する機能材であるＣｅＯ２（第二金属酸化物）が相対的に過少になったためと考えられる。逆に、その混合比が試料７のように過小になっても、酸素吸蔵能は十分に大きくなるが、希少なＣｅＯ２の使用量が相対的に増え、その使用量を十分に低減できなくなる。また、焼結性の高いＣｅＯ２の存在量が増加するため、比表面積が低下する。
(3) Mixing ratio (molar ratio)
From the comparison of Samples 1 to 5 and Samples 6 to 7 shown in Table 1, the following can be understood. When the mixing ratio of ZrO 2 / CeO 2 becomes excessive as in sample 6, the specific surface area of the composite oxide composite particles becomes sufficiently large, but the oxygen storage capacity becomes small. This is presumably because CeO 2 (second metal oxide), which is a functional material constituting the composite oxide composite particles, was relatively insufficient. On the other hand, even if the mixing ratio becomes too small as in sample 7, the oxygen storage capacity is sufficiently large, but the amount of rare CeO 2 used is relatively increased, and the amount used cannot be sufficiently reduced. . Moreover, since the abundance of CeO 2 having high sinterability increases, the specific surface area decreases.
（４）試料Ｃ１〜Ｃ３
これに対して試料Ｃ１のように、酸化ジルコニウムゾルを用いずに、一次粒径が比較的大きな酸化ジルコニウム粉末を用いた場合、ピーク強度比、比表面積およびＯＳＣのいずれも非常に小さくなっている。試料Ｃ１に係る合成粒子は、酸化ジルコニウム粒子と酸化セリウム粒子の分散が進行しておらず、試料１〜５のような好適なコアシェル構造体にはなっていないと考えられる。
(4) Samples C1 to C3
On the other hand, when a zirconium oxide powder having a relatively large primary particle size is used without using a zirconium oxide sol as in sample C1, all of the peak intensity ratio, specific surface area, and OSC are very small. . It is considered that the synthetic particles according to Sample C1 do not have a suitable core-shell structure like Samples 1 to 5 because the dispersion of zirconium oxide particles and cerium oxide particles does not proceed.
また試料Ｃ２のように、酸化ジルコニウムゾルを用いずに、一次粒径が十分に小さい水酸化ジルコニウム粉末を用いた場合、ピーク強度比は非常に高いが、比表面積およびＯＳＣは共に非常に小さい。試料Ｃ２に係る合成粒子は、水酸化ジルコニウム粒子と酸化セリウム粒子の分散は十分に進行しているが、水酸化ジルコニウムから酸化ジルコニウムへの変化の過程で焼結が進行し、比表面積が低下したと考えられる。 Further, when a zirconium hydroxide powder having a sufficiently small primary particle size is used without using a zirconium oxide sol as in sample C2, the peak intensity ratio is very high, but both the specific surface area and OSC are very small. In the synthetic particles according to Sample C2, the dispersion of zirconium hydroxide particles and cerium oxide particles has progressed sufficiently, but sintering progressed in the process of changing from zirconium hydroxide to zirconium oxide, and the specific surface area decreased. it is conceivable that.
さらに試料Ｃ３のように、試料１と同様に酸化ジルコニウムゾルと酸化セリウムゾルを用いた場合でも、凝集工程（ミリング工程）を行わない場合、比表面積は大きいものの、ピーク強度比およびＯＳＣは共に非常に小さい。試料Ｃ３に係る合成粒子は、酸化ジルコニウムゾルと酸化セリウムゾルの安定状態がそれぞれ保たれたままとなっており、コアとなる酸化ジルコニウム粒子表面に酸化セリウム粒子が結合できず、やはり、試料１〜５のような好適なコアシェル構造体にはなっていないと考えられる。 Further, as in sample C3, even when zirconium oxide sol and cerium oxide sol were used as in sample 1, when the aggregation step (milling step) was not performed, the specific surface area was large, but both the peak intensity ratio and OSC were very high. small. In the synthetic particles according to the sample C3, the stable states of the zirconium oxide sol and the cerium oxide sol are maintained, and the cerium oxide particles cannot be bonded to the surface of the core zirconium oxide particles. It is considered that such a suitable core-shell structure is not obtained.
Claims (6)
該第一金属酸化物と異なる第二金属酸化物からなり一次粒径が４０ｎｍ以下である第二基粒子が該コア部の外表面に結合してなるシェル部と、
で構成された複合酸化物合成粒子からなることを特徴とする複合酸化物粉末。 A core portion composed of secondary particles composed of a first metal oxide and having a primary particle diameter of 15 nm or less aggregated and a secondary particle diameter of 20 nm or more;
A shell part formed by binding a second base particle comprising a second metal oxide different from the first metal oxide and having a primary particle diameter of 40 nm or less to the outer surface of the core part;
A composite oxide powder comprising composite oxide composite particles comprising:
前記第二金属酸化物は酸化セリウムである請求項１〜３のいずれかに記載の複合酸化物粉末。 The first metal oxide is zirconium oxide;
The composite oxide powder according to claim 1, wherein the second metal oxide is cerium oxide.
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