JP2004345890A - Porous composite oxide and method for producing the same - Google Patents

Abstract

An object of the present invention is to provide a composite metal oxide having sufficient mesopores, particularly 2 nm or more, even after firing at high temperature.
A mesopore comprising a secondary particle aggregate which is an aggregate of primary particles of a composite oxide containing two or more metal elements, and having a pore diameter of 2 to 100 nm between the secondary particles. After firing at 600 ° C. for 5 hours in an oxygen atmosphere, the ratio of the pores between the secondary particles having a diameter of 10 nm or more in the mesopores is 10% or more of the total mesopore volume. Is to be. This composite oxide is formed by suppressing aggregation of primary particles so as not to form large secondary particles.
[Selection diagram] Fig. 1

Description

【００５８】
一般に、ｗ／ｏ型マイクロエマルションは、界面活性剤をオイルに溶解した液に水を添加し、攪拌するインジェクション法によって形成する。このようなことから、水と界面活性剤のモル比：ｗ／ｓ値（Ｗ値）を増加するとは、一般に添加する水の量を増加することにあたる。すなわち、図４の３相マップでは、１の方向に移動することになる。このような条件では、表１に示したように、水滴径の増加はわずかであり、水滴間距離が大幅に減少する。このような場合には一次粒子の凝集は大きく、二次粒子は大きくなってしまう。さらに水を増加すると、中間領域に入らずに、２相分離領域に入る場合が多い。ｗ／ｏ型でもｏ／ｗ型でもない中間領域にするには、あらかじめ界面活性剤濃度を増加させておく必要がある。
【００５９】
次に、図４において３の方向に移動する場合、すなわちオイルを増加する場合は、水滴径は大きくなり、水滴間距離も大幅に増加する。この場合は大量のオイルを添加してもｗ／ｏ型のマイクロエマルションは壊れることは少なく、水滴の大きさの増加、水滴間距離の増加も自由に変化できるので制御性がよい。但し、これでも水滴径と水滴間距離を独立に制御するという観点からは十分ではないので、図５に示すように、ベクトルを少しずらすことによって水滴径と水滴間距離を独立に制御することが可能になる。
【００６０】
すなわち、図５に示すように、オイルを多くしかつ界面活性剤濃度を高めることによって４の方向に移動させると、水滴径は変化せず、水滴間距離のみを増大させることができる。また、水相中のイオンの濃度を変えずに溶液量を多くする、すなわち溶液の希薄化によって５の方向に移動させた場合は、水滴径をより大きくし、水滴間距離の変化は比較的少ない。但し、こちらの領域は、界面活性剤に対して水、オイルとも増加させるため、その領域は狭く、マップの位置によって状態の変化が敏感すぎるため、精度よい調整が必要となる。
【００６１】
本発明において、大きな二次粒子を形成しないようにするためには、逆ミセルの水滴径を大きくすることなく、水滴間距離のみを増加させることが必要である。上記の結果から明らかなように、水及び界面活性剤に対してオイルの比を高めると、水滴径と水滴間距離の両方を同時に増加する。具体的には、オイルを水に対して体積比で２倍以上にし、界面活性剤に対して体積比で５倍以上増加させればよい。
【００６２】
上記のように、有機相と水相を接触させて加水分解反応を行うと、一般的に水酸化物（前駆体）が生成する。本発明によれば、いずれにしても、生成物を乾燥後、焼成して複合酸化物を製造する。生成物の分離、乾燥方法は従来どおりでよい。焼成条件も従来と同様でよく、焼成の温度、焼成雰囲気などは、特定の複合酸化物の種類に応じて選択すればよい。しかし、一般的にいって、従来と比べてより低温で焼成できる。予め金属元素が均一に分散しているため金属元素を固体中で拡散させるエネルギーが少なくてよいためと考えられる。
【００６３】
本発明の多孔質複合酸化物の製造方法の第二の態様は、有機溶媒中において界面活性剤が形成する逆ミセルの内部の水相において第１の金属元素のイオンを含む水溶液と第２以降の金属元素のイオンを含む水溶液を混合し反応させて、第１の金属と第２以降の金属元素を含む化合物を沈殿させ、これを加水分解し、重縮合させて複合酸化物の前駆体の一次粒子を形成し、この一次粒子を含む系において一次粒子を凝集させて二次粒子を形成し、さらにこの二次粒子を凝集させることを含み、この加水分解時に上記逆ミセル間の距離を十分に保ち、二次粒子間に十分な細孔を形成するように、逆ミセル外の有機相の体積を逆ミセル内の水相の体積及び界面活性剤の体積に対し大きくすることを特徴とする。
【００６４】
この第二の態様の方法において、使用する金属元素、界面活性剤、有機溶媒は第一の態様の方法と共通する。この第二の態様の方法では、第１の金属元素及び第２以降の金属元素をともにイオンとして逆ミセル内の水相中に存在させ、この水相内でいわゆる共沈法によって複合酸化物を形成する。
【００６５】
具体的には、例えば、第１の金属元素の塩の溶液と第２の金属元素の塩の溶液を用意し、第１の態様の方法におけるように、界面活性剤を有機溶媒に溶解し、ここに第１の金属元素の塩の溶液及び第２の金属元素の溶液を添加して、それぞれの金属元素イオンを含む２種の逆ミセルを形成する。あるいは、１つの逆ミセル内に２種以上の金属元素イオンを含ませる。そしてこれとは別に、沈殿剤を含む水溶液を用いて、同様に沈殿剤を水相に含む逆ミセルを用意する。沈殿剤としては、従来の共沈法において用いられている沈殿剤を用いることができ、例えば金属塩を中和することのできるアンモニア、炭酸アンモニウム、水酸化ナトリウム、水酸化カリウム、炭酸ナトリウム等のアルカリ、アルコール等が例示される。これらのうち、後の焼成時に揮発するアンモニア、炭酸アンモニウムが特に好ましく、またこのアルカリ溶液のｐＨは９以上であることが好ましい。
【００６６】
こうして製造した、第１の金属のイオンを含む逆ミセル、第２の金属のイオンを含む逆ミセル、又は第１の金属のイオンと第２の金属のイオンを共に含む逆ミセル、及び沈殿剤を含む逆ミセル、さらには必要により、第３以降の他の金属のイオンを含む逆ミセルを有機溶媒内で混合し、逆ミセルの水相内で反応させ、次いで第一の態様の方法と同様にして熟成させることにより複合酸化物の前駆体を形成させる。この際、各逆ミセルの大きさ及び逆ミセル間の距離を、第１の態様の方法と同様にして調整することにより、得られる複合酸化物において、一次粒子の凝集を抑制し、形成する二次粒子の大きさを制限し、二次粒子間にメゾ細孔を有する多孔質複合酸化物を得ることができる。
【００６７】
この第二の態様の方法によれば、第一の態様の方法と比較して、得られる複合酸化物において２種の金属元素の分散性は若干低いが、一次粒子の凝集を第一の態様の方法と同様に制御することができ、同様の細孔を有する複合酸化物を得ることができる。
【００６８】
本発明の多孔質複合酸化物の製造方法の第三の態様は、第１の金属元素のイオンを含む水溶液と第２以降の金属元素のイオンを含む水溶液を混合し反応させて、第１の金属と第２以降の金属元素を含む化合物を沈殿させ、これを加水分解し、重縮合させて複合酸化物の前駆体の一次粒子を形成し、この一次粒子を含む系において一次粒子を凝集させて二次粒子を形成し、さらにこの二次粒子を凝集させることを含み、第１の金属元素のイオンを含む水溶液と第２以降の金属元素のイオンを含む水溶液の混合物中の合計の金属元素イオン濃度を０．３ｍｏｌ／Ｌ以下として化合物を沈殿させ、その後この沈殿を含む溶液を濃縮し二次粒子を凝集させることを特徴とする。
【００６９】
この第三の態様の方法では、まず、第二の態様の方法と同様に、第１の金属の塩の水溶液と第２の金属の塩の水溶液を用意し、これらを混合し、沈殿剤を添加して反応させ、複合酸化物の前駆体を形成するものであるが、これら２種の金属の塩の溶液を混合する際に、それぞれの水溶液を希薄溶液として混合する。その金属塩溶液の濃度は、０．１５ｍｏｌ／Ｌ以下、好ましくは０．１ｍｏｌ／Ｌ以下とする。このような希薄溶液中で混合することにより、形成される一次粒子の衝突を抑制し、大きな二次粒子を形成しないように凝集を抑制することができる。反応後、一次粒子の沈殿を含む溶液を、例えばノズルからスプレーすることにより、又は浸透膜を通すことにより濃縮し、凝集を進ませ、結果としてメゾ細孔を有する複合酸化物を得ることができる。
【００７０】
【実施例】
実施例１
内容積１５リットルのビーカーにシクロヘキサン８．６リットル、ポリエチレン（ｎ＝５）ノニルフェニルエーテル３５０ｇを入れ、硝酸セリウム４３ｇと蒸留水１２０ｍＬよりなる水溶液を加え攪拌した。室温下でマグネチックスターラーを用い攪拌して逆ミセル（油中水型マイクロエマルション、水滴実測直径１７ｎｍ）を作成した。これとは別に、ジルコニウムブトキシド５０ｇをシクロヘキサン０．８リットルに溶解させたジルコニウムアルコキシド溶液を作成し、これを上記マイクロエマルションに加えた。この際のシクロヘキサン（有機相）に対する水（水相）の体積比は７８である。室温下においてこの混合物をよく攪拌すると、ただちにビーカー内が白黄色に曇り、コロイド粒子（二次粒子、粒径１０ｎｍ程度）が生成した。
【００７１】
次に、コロイドの凝集を調節するためにアンモニア水でｐＨを８に調整した。
さらに攪拌を約１時間続け熟成を行った。母液を炉別し、得られた沈殿をエタノールで３回洗浄し、８０℃で一夜乾燥後、大気中６００℃、９００℃及び１０００℃で５時間焼成して、セリウムとジルコニウムを含む複合酸化物（セリアジルコニア）を得た。複合酸化物のＣｅ／Ｚｒモル比は１／１であった。
【００７２】
実施例２
基本的に実施例１と同様にしてＳｒＺｒＯ_３複合酸化物を製造した。すなわち、内容積１５リットルのビーカーにシクロヘキサン８．６リットル、ポリエチレン（ｎ＝５）ノニルフェニルエーテル３５０ｇを入れ、硝酸ストロンチウム３７ｇと蒸留水１００ｍＬよりなる水溶液を加え攪拌した。室温下でマグネチックスターラーを用い攪拌して逆ミセル（油中水型マイクロエマルション、水滴実測直径１５ｎｍ）を作成した。これとは別に、ジルコニウムブトキシド６８ｇをシクロヘキサン０．８リットルに溶解させたジルコニウムアルコキシド溶液を作成し、これを上記マイクロエマルションに加えた。このときの逆ミセル間距離は約２２ｎｍであった。室温下においてこの混合物をよく攪拌すると、ただちにビーカー内が白黄色に曇り、コロイド粒子（粒径１０ｎｍ程度）が生成した。
【００７３】
次に、コロイドの凝集を調節するためにアンモニア水でｐＨを１０．２に調整した。さらに攪拌を約１時間続け熟成を行った。母液を炉別し、得られた沈殿をエタノールで３回洗浄し、８０℃で一夜乾燥後、大気中６００℃、９００℃及び１０００℃で５時間焼成して、ストロンチウムとジルコニウムを含む複合酸化物（ストロンチウムジルコニア）を得た。得られた粉末をラマン散乱法、Ｘ線回折法等で分析し、結晶形態がペロブスカイトであることを確認した。
【００７４】
実施例３
内容積１５リットルのビーカーにシクロヘキサン８．６リットル、ポリエチレン（ｎ＝５）ノニルフェニルエーテル３５０ｇを入れ、硝酸セリウム４２．５ｇ、硝酸ジルコニウム３０．１ｇ及び蒸留水１２０ｍＬよりなる水溶液を加え攪拌した。室温下でマグネチックスターラーを用い攪拌して逆ミセルを作成した。次にこの逆ミセルを、アンモニアを水相に含む逆ミセル溶液に滴下し、攪拌を続け、コロイド粒子を形成した。続いて、実施例１と同様にしてコロイドを凝集させ、セリウムとジルコニウムを含む複合酸化物（セリアジルコニア）を得た。
【００７５】
実施例４
内容積１５リットルのビーカーに、それぞれ濃度０．０２５ｍｏｌ／Ｌとなるように硝酸セリウム及び硝酸ジルコニウムを溶解した水溶液１０リットルを加え、硝酸イオンに対して同量のアンモニア水を滴下し、中和してコロイド粒子を形成した。続いて、このコロイド粒子を含む溶液を乾燥空気中に噴霧して、溶液量が半分になるようにした。その後、この溶液にアンモニアを滴下してｐＨを約８に調整することにより凝集体を得た。これをろ過し、８０℃で一晩乾燥させ、規定濃度で焼成し、セリウムとジルコニウムを含む複合酸化物（セリアジルコニア）を得た。
【００７６】
比較例１
２リットルの蒸留水に硝酸セリウムと硝酸ジルコニウムをそれぞれ０．５ｍｏｌ／Ｌとなるように添加し、溶解させた。この水溶液にアンモニア水を滴下し、ｐＨを９に調整し、沈殿を得た。これをろ過し、乾燥後、６００℃で焼成した。
【００７７】
比較例２
２リットルの蒸留水に非イオン系界面活性剤（ポリオキシエチレンオクチルフェニルエーテル）を泡が立たない程度に添加した。その後、これに硝酸セリウムと硝酸ジルコニウムをそれぞれ０．５ｍｏｌ／Ｌとなるように添加し、溶解させた。この水溶液にアンモニア水を滴下し、ｐＨを９に調整し、沈殿を得た。これを水洗、ろ過し、乾燥後、６００℃で焼成した。
【００７８】
性能評価
得られた多孔質複合酸化物について、６００℃、９００℃及び１０００℃での焼成後の細孔容積を、液体窒素温度下、窒素吸着により測定した。その結果を図６に示す。
また、得られた多孔質複合酸化物について、６００℃及び９００℃での焼成後の細孔分布を同様に窒素吸着により測定した。その結果を図７に示す。
【００７９】
図６及び７に示す結果から明らかなように、従来の複合金属酸化物では高温で焼成すると細孔容積が大きく低下し、また細孔分布はそのピークが高い側にシフトするものの、ピークの値が低下し、細孔容積が低下したことを示している、これに対して、本発明の複合金属酸化物では高温で焼成しても細孔容積が低下せず、細孔分布もピーク値が高い方にシフトするのみで、そのピーク高さ及び細孔容積はほとんど変化しなかった。
【００８０】
セリアジルコニアはＺｒ格子にＣｅが高分散に置換した場合に、酸素吸蔵性能が高いことが知られている（Ｃａｔａｌ． Ｔｏｄａｙ， ７４， ２２５−２３４ （２００２）， Ｙ．Ｎａｇａｉら）。そこで酸素吸蔵量を測定することで置換量を予測することができる。特に低温（３００℃以下）における酸素吸蔵量は顕著にその影響を受ける。ここで酸素吸蔵量はＣｅの酸化、還元で表すことができる。即ち、
【化１】

【００８１】
上記実施例１並びに比較例１及び２で得られたセリアジルコニアの酸素吸蔵能を、酸素パルス吸着法を用いて評価した。得られた酸素吸蔵能からセリウムの利用率を計算し、ここでＣｅ（ＩＶ）Ｏ_２→Ｃｅ（ＩＩＩ）Ｏ_１．５＋１／２Ｏ_２の場合をＣｅ利用効率１００％とし、結果を図８に示す。
【００８２】
図８に示されているように、本発明によれば、高温焼成後でも大きなＣｅの利用効率が得られており、これは複合水酸化物合成時からＺｒとＣｅの高分散が達成されていることを示す。このように本発明により製造したセリアジルコニアはこれまでにない低温活性の高い触媒原料として利用できることが判明した。
【００８３】
さらに、本発明の複合金属酸化物は、特にディーゼル排気ガスの浄化触媒として有用である。ディーゼルでは、ＮＯｘを還元するために排気系にＨＣ源として軽油を添加している。ところが、この軽油成分は排気系で気化しても分子量が大きいため、従来の金属複合酸化物中には十分に拡散することができず、この拡散が律速となり、反応が進まない。ところが、本発明の複合金属酸化物では、２ｎｍ以上のメゾ細孔を十分に有しているため、軽油成分も内部に拡散させることができ、十分な浄化を達成することができる。
【００８４】
【発明の効果】
本発明によれば、高温で焼成しても十分な細孔容積を有する複合金属酸化物が提供され、排気ガス浄化用触媒として有用である。
【図面の簡単な説明】
【図１】本発明の複合金属酸化物の構成を示す図である。
【図２】従来の複合金属酸化物の構成を示す図である。
【図３】本発明の方法における逆ミセルの大きさとその間の距離を示す模式図である。
【図４】マイクロエマルションにおける水、界面活性剤及びオイルの関係を示す３相マップである。
【図５】図４の一部の拡大図である。
【図６】各種のセリウム−ジルコニウム複合酸化物の焼成時の細孔容積の変化を示すグラフである。
【図７】各種のセリウム−ジルコニウム複合酸化物の６００℃と９００℃での焼成後の細孔分布の変化を示すグラフである。
【図８】各種のセリウム−ジルコニウム複合酸化物の酸素吸蔵量の変化を示すグラフである。
【符号の説明】
１…一次粒子
２…二次粒子
３…メゾ細孔
４…逆ミセル
５…界面活性剤
６…水相[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a composite oxide and a method for producing the same. In particular, the present invention relates to a porous composite oxide having pores of a sufficient size even after firing at a high temperature, and a method for producing the same.
[0002]
[Prior art]
A composite oxide is an oxide in the form of a compound of two or more metal oxides, and has no oxoacid ion as a structural unit. One of the important uses of this composite oxide is as a catalyst and a catalyst carrier, and in particular, an exhaust gas purifying catalyst for an internal combustion engine is known.
[0003]
For example, it has been proposed to use a cerium-zirconium composite oxide as a carrier for a three-way catalyst (for example, see Patent Document 1). The three-way catalyst purifies by simultaneously oxidizing hydrocarbons (HC) and carbon monoxide (CO) and reducing nitrogen oxides (NOx) in the exhaust gas of the internal combustion engine. Cerium oxide having oxygen storage / release capability (OSC) for storing oxygen in an oxidizing atmosphere and releasing oxygen in a reducing atmosphere is added. However, when the three-way catalyst containing the catalyst metal and cerium oxide is used at a high temperature of 800 ° C. or more, the OSC tends to decrease due to the crystal growth of cerium oxide. In order to maintain high OSC, zirconium oxide is added to cerium oxide to form a cerium-zirconium composite oxide.
[0004]
Further, it has been proposed to use a composite oxide composed of two or more of alumina, zirconia, titania, iron oxide, ceria, and magnesia as a carrier for a NOx storage reduction catalyst (for example, see Patent Document 2).
[0005]
[Patent Document 1]
JP-A-2002-220228 (pages 2 to 4)
[Patent Document 2]
JP 2001-170487 A (pages 2 to 4)
[0006]
[Problems to be solved by the invention]
As a method of producing a composite oxide powder, generally, a powder co-firing method of mixing and firing a powder of each of the precursors such as metal oxides or carbonates and hydroxides, an aqueous solution of a plurality of metal inorganic salts A coprecipitation method in which an alkali is added to neutralize to form a colloidal dispersion of an oxide or a hydroxide, and an alkoxide method in which water is added to a plurality of metal alkoxides dissolved in an organic solvent to hydrolyze the same. Has been.
[0007]
In the powder simultaneous firing method, there is a limit to which the powder can be made finer, and firing at a high temperature is necessary to obtain a composite oxide from the powder. At high temperature firing, grains grow and the surface area decreases. In practice, it is difficult to obtain a composite oxide fine powder having a high surface area and being completely uniform at the atomic level. The coprecipitation method utilizes a neutralization precipitation reaction of a plurality of inorganic ions in an aqueous solution, and the particle size of the formed colloidal particles is fine. Colloid particles tend to be particles of a single metal oxide or metal hydroxide, and do not produce a uniformly mixed composite oxide at the atomic level. The conventional alkoxide method utilizes the hydrolysis of a plurality of metal alkoxides in an organic solvent.However, the stability and the rate of the hydrolysis reaction are different depending on the type of the metal alkoxide. There is a priority to produce a product, and again, it does not produce a uniformly mixed composite oxide at the atomic level.
[0008]
In addition, although the conventional method controls the pores of the obtained composite oxide, it generally has a pore distribution peaking at small pores of about 2 to 7 nm. In addition, conventional composite oxides are composed of aggregates of primary particles and have small pores between the primary particles, so that when sintering proceeds at a high temperature, the sintering between the temporary particles proceeds. As a result, the entire crystal shrinks, and the pore volume becomes extremely small.
[0009]
In the above Patent Document 1, the pores of the carrier are 2 to 100 nm, but basically only increase the pore volume between the primary particles, and as a result, sintering between the primary particles proceeds, The crystal as a whole will shrink and the pore volume will decrease.
[0010]
Therefore, in the present invention, the metal constituting the composite oxide is uniformly distributed, and even if fired at a high temperature, the pores, particularly the volume of the mesopores having a pore diameter of 10 nm or more are reduced, and the performance is reduced. The purpose of the present invention is to provide a composite oxide.
[0011]
[Means for Solving the Problems]
According to a first aspect of the present invention, there is provided a composite oxide containing two or more metal elements, which comprises an aggregate of secondary particles which is an aggregate of primary particles. In the porous composite oxide having mesopores having a pore diameter of 2 to 100 nm, after firing at 600 ° C. for 5 hours in an oxygen atmosphere, the pores between the secondary particles having a diameter of 10 nm or more among the mesopores are removed. The proportion is such that it is at least 10% of the total mesopore volume.
[0012]
According to a second aspect, in the first aspect, the primary particles have a particle size of 3 to 15 nm, and the secondary particles have a particle size of 30 to 100 nm.
[0013]
In a third aspect based on the first aspect, the metal element comprises two kinds of cerium and zirconium.
[0014]
According to a fourth aspect, in the first method for producing a porous composite oxide, a solution in which a compound of a first metal element that forms a hydroxide by hydrolysis is dissolved in an organic solvent; An emulsion containing ions of the second and subsequent metal elements is mixed with an aqueous phase inside the reverse micelle formed by the surfactant, and the compound of the first metal element is hydrolyzed at the interface of the reverse micelle and the second and subsequent metal elements are mixed. To form primary particles of the precursor of the composite oxide by polycondensation, aggregating the primary particles in a system containing the primary particles to form secondary particles, and further forming the secondary particles Including the aggregation, the distance between the reverse micelles during the hydrolysis is sufficiently maintained, and the volume of the organic phase outside the reverse micelles is reduced so that pores of a sufficient size are formed between the secondary particles. Water in It is made larger to the volume of the volume and surface active agents.
[0015]
In a fifth aspect, in the fourth aspect, the volume of the organic phase outside the reverse micelle is twice or more the volume of the aqueous phase inside the reverse micelle.
[0016]
In a sixth aspect, in the fourth aspect, the volume of the organic phase outside the reverse micelles is at least five times the volume of the surfactant.
[0017]
In a seventh aspect, in the fourth aspect, the diameter of the aqueous phase in the reverse micelle is 5 nm, and the distance between the reverse micelles is 20 nm or more.
[0018]
In an eighth aspect, in the first method for producing a porous composite oxide, the aqueous solution containing the ion of the first metal element and the aqueous solution containing the ion of the first metal element in the aqueous phase inside the reverse micelle formed by the surfactant in the organic solvent are mixed with An aqueous solution containing ions of the subsequent metal elements is mixed and reacted to precipitate a compound containing the first metal and the second and subsequent metal elements, which is hydrolyzed and polycondensed to form a precursor of the composite oxide. Forming primary particles, agglomerating the primary particles in a system containing the primary particles to form secondary particles, and further including aggregating the secondary particles.During the hydrolysis, the distance between the reverse micelles is reduced. The volume of the organic phase outside the reverse micelles is increased with respect to the volume of the aqueous phase and the volume of the surfactant in the reverse micelles, so as to maintain sufficient volume and form sufficient pores between the secondary particles.
[0019]
In a ninth aspect, in the eighth aspect, the volume of the organic phase outside the reverse micelle is twice or more the volume of the aqueous phase inside the reverse micelle.
[0020]
In a tenth aspect, in the eighth aspect, the volume of the organic phase outside the reverse micelle is at least five times the volume of the surfactant.
[0021]
According to an eleventh aspect, in the eighth aspect, the diameter of the aqueous phase in the reverse micelle is 5 nm or more, and the distance between the reverse micelles is 20 nm or more.
[0022]
In a twelfth aspect, in the first method for producing a porous composite oxide, an aqueous solution containing ions of the first metal element and an aqueous solution containing ions of the second and subsequent metal elements are mixed and reacted. Precipitates the compound containing the metal and the second and subsequent metal elements, hydrolyzes and polycondensates to form primary particles of the precursor of the composite oxide, and aggregates the primary particles in the system containing the primary particles. Forming secondary particles, and further aggregating the secondary particles, wherein the total amount of metal in a mixture of an aqueous solution containing ions of the first metal element and an aqueous solution containing ions of the second and subsequent metal elements The compound is precipitated at an element ion concentration of 0.3 mol / L or less, and then the solution containing the precipitate is concentrated to aggregate the secondary particles.
[0023]
BEST MODE FOR CARRYING OUT THE INVENTION
As shown in FIG. 1, the porous composite oxide of the present invention is formed by aggregating secondary particles 2 having a particle size of about 100 nm, which are aggregates of primary particles 1 of a composite oxide having a particle size of about 5 to 15 nm. In addition to having pores between the primary particles 1, the secondary particles 2 have mesopores 3 having a diameter of 10 to 100 nm. In addition, even after firing at a high temperature of 600 ° C., the porous composite oxide of the present invention has almost no change in the pore volume as compared with before firing, and the pores having a diameter of 10 nm or more have a total pore volume of 10%. Accounting for more than%. On the other hand, as shown in FIG. 2, the conventional composite oxide is composed of secondary particles 2 which are aggregates of primary particles 1 but is composed of one large secondary particle as a whole. No pores are formed between the primary particles, and only pores exist between the primary particles. When this aggregate is fired at a high temperature, the pore volume decreases and the whole shrinks. In this specification, the pore diameter is a value measured by a nitrogen adsorption method at a liquid nitrogen temperature. The pore volume distribution is a value obtained from the nitrogen adsorption amount with respect to the nitrogen introduction pressure by the BJH method which is a general calculation method.
[0024]
The reason why the pore volume does not decrease even when the composite oxide of the present invention is fired at a high temperature is considered as follows. That is, unlike the conventional composite oxide, the composite oxide of the present invention has pores between the secondary particles as shown in FIG. 1, so that the bonding state between the primary particles is strong and weak. However, there is. Where the pores are large, the bond is weak and the cracks easily spread from there during firing. On the other hand, since the sintering proceeds at a high bonding point due to the high-temperature firing, the small-diameter pores are crushed, and the large-diameter pores increase. As a result, the pore volume does not change much as a whole, but changes to one having a large pore diameter. On the other hand, in the conventional composite oxide, since the bonding between the primary particles is in a substantially uniform state, the sintering proceeds at a stretch by high-temperature sintering. This means that the volume of secondary particles, which are aggregates of primary particles, and the pore volume are reduced. As described above, in the composite oxide of the present invention, since there are portions having different bonding forces between the primary particles, a decrease in volume due to sintering between the primary particles due to high-temperature firing is suppressed.
[0025]
The type of the porous composite oxide in the present invention is not particularly limited, and may be any composite oxide containing at least a first metal element and a second metal element. Complex oxide systems are known in many textbooks and handbooks, and oxidize many metal elements such as alumina, zirconia, ceria, silica, iron oxide, manganese oxide, chromium oxide, and yttrium oxide to form metal oxides. In most cases, the composite oxide can be formed by adding a second or later metal element. It is known per se what elements form a composite oxide. The present invention can be applied to all the composite oxides as long as a hydrolyzable raw material or an inorganic metal salt raw material is present.
[0026]
An example of such a composite oxide is a cerium-zirconium composite oxide. This composite oxide is composed of zirconium oxide Zr ₂ It has a crystal structure of O, and a part of zirconium in this crystal structure is substituted by cerium. Conventionally, cerium oxide was supported on a carrier together with a catalyst metal. When a catalyst was used at a high temperature, the cerium oxide crystal growth reduced the oxygen storage / release capacity (OSC) of cerium oxide. By using it as an oxide, a decrease in OSC can be suppressed even when used at a high temperature. Furthermore, the composite oxide of the present invention has sufficiently large pores even after firing at a high temperature, and is capable of diffusing large molecular weight HC such as HC in diesel exhaust gas into the carrier. And can be purified by exerting OSC.
[0027]
Another example is a composite oxide of a rare earth metal such as lanthanum and zirconium. When part of zirconium in the crystal structure of zirconium oxide is replaced with lanthanum, zirconium is tetravalent and lanthanum is trivalent, so that an oxygen defect in which oxygen does not exist in the crystal lattice is formed. When an alkali metal is added to this composite oxide, electrons are donated to oxygen vacancies. The electron donated oxygen deficiency has a very strong basicity, and thus the electron donated oxygen deficiency constitutes a strong base point. At such a strong base point, nitric oxide NO in the exhaust gas is captured, and as a result, a large amount of nitric oxide is adsorbed in the composite oxide. That is, this composite oxide has a NOx storage effect, and can be used as a NOx storage reduction catalyst. The lanthanum-zirconium composite oxide has pores of a sufficiently large size even after firing at a high temperature, so that the exhaust gas can be quickly diffused, and the exhaust gas can be purified more efficiently.
[0028]
Next, a method for producing the porous composite oxide of the present invention will be described. Conventional composite oxides are composed of secondary particles which are aggregates of primary particles, but the composite oxide of the present invention firstly forms relatively small secondary particles having a size of about 100 nm once from the primary particles. A plurality is formed, and then the secondary particles are formed by agglomeration. That is, after the primary particles are formed, if the concentration of the primary particles is high in the reaction system, the primary particles aggregate as a whole and form large secondary particles, and therefore, in the present invention, as shown in FIG. After forming the primary particles, when aggregating the primary particles, a physical space is formed between the primary particles to suppress the collision between the primary particles, to suppress the primary particles from aggregating as a whole, and first to about 100 nm. Are formed, and by agglomerating them, pores are formed between the secondary particles.
[0029]
The first aspect of the method for producing a porous composite oxide according to the present invention comprises: a solution in which a compound of a first metal element that hydrolyzes to form a hydroxide is dissolved in an organic solvent; An emulsion containing ions of the second and subsequent metal elements is mixed with the aqueous phase inside the reverse micelle formed by the agent, and at the interface of the reverse micelle, the compound of the first metal element is hydrolyzed, and the second and subsequent metals are hydrolyzed. The element is taken in and polycondensed to form primary particles of the precursor of the composite oxide, and in the system containing the primary particles, the primary particles are aggregated to form secondary particles, and further the secondary particles are aggregated. During the hydrolysis, the distance between the reverse micelles is sufficiently maintained, and the volume of the organic phase outside the reverse micelles is reduced within the reverse micelles so as to form pores of a sufficient size between the secondary particles. Aqueous phase volume and surfactant Characterized by large relative volume.
[0030]
A compound of a first metal element that hydrolyzes to form a hydroxide is referred to herein as a first metal compound. However, the metal constituting the first metal compound is not a metal in a narrow sense, but means an element M that can form an MOM bond.
[0031]
As the first metal compound, a metal compound generally used in a so-called sol-gel method can be used. Examples thereof include metal alkoxides, acetylacetone metal complexes, metal carboxylate, metal inorganic compounds (eg, nitrates, oxychlorides, chlorides, etc.).
[0032]
The metal element M that forms the metal alkoxide includes elements from Group 1 to Group 14, Group 16 includes sulfur, selenium, tellurium, and Group 15 includes phosphorus, arsenic, antimony, and bismuth. It is said that some lanthanoid elements do not form alkoxides. For example, silicon alkoxides and germanium alkoxides are also called metal alkoxides. As the metal alkoxide, various metal alkoxides are commercially available and the production method is also known, so that it is easy to obtain.
[0033]
Metal alkoxide M (OR) _n (Where M is a metal and R is an alkyl group such as methyl, ethyl, propyl, butyl, etc.) is also known, and formally, M (OR) _n + NH ₂ O → M (OH) _n + NROH, then M (OH) _n → MO _{n / 2} + N / 2H ₂ It is represented by O.
[0034]
Acetylacetone metal complex (CH ₃ COCH ₂ COCH ₃ ) _n A hydrolysis reaction of M (where M is a metal) is also known, and (CH ₃ COCH ₂ COCH ₃ ) _n M + nROH → nCH ₃ COCH ₂ C (OH) CH ₃ + M (OH) _n , Then M (OH) _n → MO _{n / 2} + N / 2H ₂ It is represented by O.
[0035]
As the acetylacetone metal complex, various metal complexes are commercially available and the production method is also known, so that it is easy to obtain. Typically, there are aluminum acetonate, barium acetonate, lanthanum acetonate, platinum acetonate and the like, and there are more types than alkoxides.
[0036]
Organometallic compounds such as metal alkoxides and acetylacetone metal complexes can be easily dissolved by selecting an appropriate solvent from alcohols, polar organic solvents, hydrocarbon solvents and the like. As the solvent of the present invention, it is preferable to use a hydrophobic (oil-based) organic solvent that can be separated from an aqueous phase and two phases.
[0037]
Examples of the organic solvent include hydrocarbons such as cyclohexane and benzene, straight-chain alcohols such as hexanol, and ketones such as acetone. Selection criteria for the organic solvent include the solubility of the surfactant, the size of the region in which the microemulsion is formed (the molar ratio of water / surfactant is large), and the like.
[0038]
It is known that when water is added to the organic phase in which the compound of the first metal element that is hydrolyzed to form a hydroxide is dissolved, the hydrolysis reaction of the organometallic compound starts and proceeds. . Generally, water is added to the organic phase in which the first metal compound is dissolved and stirred to obtain a metal hydroxide.
[0039]
In the present invention, a water-in-oil emulsion containing ions of the second and subsequent metal elements is formed in an aqueous phase inside a reverse micelle in which an aqueous phase is finely dispersed with a surfactant in an organic phase, The solution of the first metal compound is added to this emulsion, and the mixture is stirred and mixed, so as to react with the ions of the second and subsequent metal elements in the aqueous phase surrounded by the surfactant in the reversed micelle, Perform disassembly. In this method, it is considered that a large number of reverse micelles serve as reaction nuclei or that the surfactant stabilizes the generated hydroxide fine particles, whereby fine product particles are obtained.
[0040]
In the hydrolysis reaction as described above, by dissolving a plurality of hydrolyzable metal compounds in an organic phase, when brought into contact with water, the plurality of metal compounds are hydrolyzed to form a plurality of metals. It is also known that hydroxides are formed simultaneously.
[0041]
In the present invention, one of the hydrolyzable metal compounds (compound containing the first element) is made to exist in the organic phase, and when the organic phase contacts the aqueous phase, the second metal element, Further, the third and subsequent metal elements are present as ions in the aqueous phase in the reverse micelle.
[0042]
For the presence of ions in the aqueous phase, water-soluble metal salts, particularly inorganic salts such as nitrates and chlorides, and organic salts such as acetates, lactates and oxalates can be used. The ion of the second element present in the aqueous phase may be a complex ion containing the second element in addition to the simple metal ion. The same applies to ions of the third and subsequent elements.
[0043]
When the organic phase and the aqueous phase are brought into contact with each other, the organometallic compound in the organic phase comes into contact with water to cause a hydrolysis reaction to form a first metal hydroxide or oxide. According to the invention, it has been found that ions of the metal present in the aqueous phase are incorporated into the hydroxide (or oxide) of the first metal, which is a hydrolysis product. This phenomenon is not conventionally known. The reason why ions in the aqueous phase are taken into the hydroxide without performing a special sedimentation operation is not fully understood. However, when the first metal compound is an alkoxide as an example, the alkoxide is hydrolyzed. When the second metal ion in the aqueous phase induces the alkoxide, the hydrolysis proceeds, or the hydrolyzed fine hydroxide of the alkoxide captures a predetermined amount of the metal ion in the aqueous phase and aggregates. It is thought to go.
[0044]
According to the present invention, in particular, in the novel production method, the second metal element present in the aqueous phase is contained in the hydroxide obtained by hydrolyzing the compound of the first metal element in the organic phase. Although ions are taken in, a hydroxide in which the first metal element and the second and subsequent metal elements in the obtained hydroxide are very uniformly dispersed can be obtained, and the uniformity can be obtained by the conventional alkoxide method, That is, it has been found that it can be remarkably superior to the case where a plurality of metal alkoxides are present in the organic phase. Even at a relatively low firing temperature, a composite oxide (solid solution) in which the first metal element and the second metal element of the fired composite oxide were ideally mixed at the atomic level was obtained. This has not been achieved by the conventional metal alkoxide method. In the conventional metal alkoxide method, since the stability differs depending on the type of the metal alkoxide, only a non-uniform product is obtained between the first metal element and the second metal element.
[0045]
The relative ratio of the first metal element and the second metal element in the composite oxide obtained according to the present invention is the ratio of the amount of the first metal element in the organic phase to the amount of the second metal element in the aqueous phase. Can be adjusted.
[0046]
In the present invention, the reaction system is preferably a water-in-oil emulsion system or a microemulsion system. In this case, first, the microemulsion diameter is extremely small, from several nm to several tens of nm, and the interface between the organic phase and the aqueous phase is extremely wide (8000 m when the diameter is 10 nm). ² / Liter), the second is that the aqueous phase is shelled, and the homogenization effect is due to the fact that each contains only a small amount of metal ions (approximately 100). Conceivable.
[0047]
In this sense, the diameter of the aqueous phase of the reverse micelle in the microemulsion is 2 to 20 nm, preferably 2 to 15 nm, more preferably 2 to 10 nm.
[0048]
Methods for forming water-in-oil emulsion systems or microemulsion systems are known. As the organic phase medium, those similar to the above organic solvents such as hydrocarbons such as cyclohexane and benzene, linear alcohols such as hexanol, and ketones such as acetone can be used. Surfactants that can be used in the present invention include a variety of nonionic surfactants, anionic surfactants, and cationic surfactants, and can be used in combination with an organic phase component according to the application. it can.
[0049]
Examples of the nonionic surfactant include polyoxyethylene nonyl phenyl ether represented by polyoxyethylene (n = 5) nonyl phenyl ether and polyoxy ethylene represented by polyoxyethylene (n = 10) octyl phenyl ether. Polyoxyethylene alkyl ether surfactants represented by ethylene octyl phenyl ether, polyoxyethylene (n = 7) cetyl ether, polyoxyethylene sorbitan surfactants represented by polyoxyethylene sorbitan triolate, etc. Can be used.
[0050]
As the anionic surfactant, sodium di-2-ethylenehexylsulfosuccinate can be used, and as the cationic surfactant, cetyltrimethylammonium chloride, cetyltrimethylammonium bromide, or the like can be used.
[0051]
A water-in-oil emulsion system or a microemulsion system is preferable, but the production method of the present invention can be carried out also in an oil-in-water emulsion system.
[0052]
In the present invention, when producing a composite oxide of three or more elements, the third and subsequent elements are present in the aqueous phase in the reverse micelle. This is because if a plurality of hydrolyzable metal compounds are present in the organic phase, the stability of the hydrolyzable metal compounds in the organic phase is different, resulting in a non-uniform product. Of course, the first metal element and the second metal element need to be uniform, but if uniformity is not important between the first metal element and the third metal element, the third element May be present in the organic phase.
[0053]
The reverse micelle containing the ions of the second metal element is prepared by dissolving the surfactant in the organic phase medium, adding an aqueous solution containing the ions of the second metal element to the medium, and stirring the solution. Can be formed.
[0054]
In this manner, the solution of the first metal compound is brought into contact with the reverse micelles containing the ions of the second metal element in the aqueous phase, and the precursor is mixed by hydrolysis to form a precursor of the composite oxide containing the first metal element and the second metal element. After forming the primary particles, the system containing the primary particles is aged at a predetermined temperature (30 ° C. to 80 ° C.) for a predetermined time (2 hours). In this aging step, the primary particles aggregate to form secondary particles. At this time, rather than agglomerating all the primary particles to form large secondary particles, a relatively small secondary particle is formed once, and then a sufficiently large pore is formed between the secondary particles. The hydrolysis and ripening are performed while maintaining a sufficient distance between the reverse micelles so that the secondary particles aggregate with each other.
[0055]
Specifically, as shown in FIG. 3, a reverse micelle 5 formed by a surfactant 4 having a lipophilic group directed outward and a hydrophilic group directed inward is formed, and a second aqueous phase 6 is formed in an internal aqueous phase 6. It contains ions of metal elements. Here, the diameter d of the water droplet ₁ Is at least 5 nm, preferably about 10 nm, and the distance d between water droplets is ₂ Is set to 20 nm or more. Reverse micelles always move and diffuse due to Brownian motion, but in the present invention, the volume of the organic phase with respect to the aqueous phase is increased, and the distance between water droplets, that is, the distance between reverse micelles is sufficiently maintained. The aggregation of the primary particles is suppressed so that the particles do not aggregate to form large secondary particles, and small secondary particles having a size of about 100 nm are formed.
[0056]
In a microemulsion containing reverse micelles, the size of the water droplets and the distance between the water droplets of the reverse micelles are determined by three factors: the amount of water, the amount of oil, and the amount of surfactant. FIG. 4 shows the use area of water, oil and surfactant in the microemulsion. Table 1 below shows an example of the effect of the water droplet diameter and the distance between the water droplets of the reverse micelles in the three-phase map of FIG.
[0057]
[Table 1]

[0058]
Generally, a w / o-type microemulsion is formed by an injection method in which water is added to a solution in which a surfactant is dissolved in oil and the mixture is stirred. For this reason, increasing the molar ratio of water to surfactant: w / s value (W value) generally means increasing the amount of water to be added. That is, in the three-phase map shown in FIG. 4, it moves in one direction. Under such conditions, as shown in Table 1, the increase in water droplet diameter is slight, and the distance between water droplets is greatly reduced. In such a case, the aggregation of the primary particles is large, and the secondary particles are large. When water is further increased, the water often enters the two-phase separation region without entering the intermediate region. In order to obtain an intermediate region that is neither w / o type nor o / w type, it is necessary to increase the surfactant concentration in advance.
[0059]
Next, in the case of moving in the direction 3 in FIG. 4, that is, when increasing the amount of oil, the diameter of the water droplet increases, and the distance between the water droplets also greatly increases. In this case, even if a large amount of oil is added, the w / o-type microemulsion is hardly broken and the size of water droplets and the distance between water droplets can be freely changed, so that the controllability is good. However, this is not enough from the viewpoint of independently controlling the water droplet diameter and the distance between water droplets. Therefore, as shown in FIG. 5, it is possible to independently control the water droplet diameter and the water droplet distance by slightly shifting the vector. Will be possible.
[0060]
That is, as shown in FIG. 5, when the oil is moved in the direction 4 by increasing the amount of the oil and increasing the concentration of the surfactant, the diameter of the water droplet does not change, and only the distance between the water droplets can be increased. In addition, when the amount of the solution is increased without changing the concentration of ions in the aqueous phase, that is, when the solution is moved in the direction of 5 by dilution, the diameter of the water droplets is increased, and the change in the distance between the water droplets is relatively small. Few. However, since this region increases both water and oil with respect to the surfactant, the region is narrow, and the change in the state is too sensitive depending on the position of the map, so that accurate adjustment is required.
[0061]
In the present invention, in order to prevent the formation of large secondary particles, it is necessary to increase only the distance between water droplets without increasing the diameter of the water droplets of the reverse micelles. As is evident from the above results, increasing the ratio of oil to water and surfactant increases both the droplet size and the distance between droplets simultaneously. Specifically, the volume ratio of the oil to water may be twice or more, and the volume ratio of the oil to the surfactant may be five times or more.
[0062]
As described above, when a hydrolysis reaction is performed by bringing an organic phase and an aqueous phase into contact with each other, a hydroxide (precursor) is generally generated. According to the present invention, in any case, the product is dried and then calcined to produce a composite oxide. The method of separating and drying the product may be the same as conventional. The firing conditions may be the same as in the past, and the firing temperature, firing atmosphere, and the like may be selected according to the type of the specific composite oxide. However, generally speaking, it can be fired at a lower temperature than before. It is considered that the energy required to diffuse the metal element in the solid is small because the metal element is uniformly dispersed in advance.
[0063]
The second aspect of the method for producing a porous composite oxide according to the present invention comprises an aqueous solution containing ions of the first metal element in an aqueous phase inside a reverse micelle formed by a surfactant in an organic solvent. An aqueous solution containing the metal element ion is mixed and reacted to precipitate a compound containing the first metal and the second and subsequent metal elements, which is hydrolyzed and polycondensed to form a precursor of the composite oxide. Forming primary particles, aggregating the primary particles in a system containing the primary particles to form secondary particles, further including aggregating the secondary particles, and sufficient distance between the reverse micelles during the hydrolysis. The volume of the organic phase outside the reverse micelles is made larger than the volume of the aqueous phase and the volume of the surfactant inside the reverse micelles so as to form sufficient pores between the secondary particles. .
[0064]
In the method of the second embodiment, the metal elements, surfactants, and organic solvents used are the same as those of the method of the first embodiment. In the method of the second aspect, the first metal element and the second and subsequent metal elements are both present as ions in the aqueous phase in the reverse micelle, and the composite oxide is formed in the aqueous phase by a so-called coprecipitation method. Form.
[0065]
Specifically, for example, a solution of a salt of a first metal element and a solution of a salt of a second metal element are prepared, and a surfactant is dissolved in an organic solvent as in the method of the first embodiment. Here, a solution of the salt of the first metal element and a solution of the second metal element are added to form two kinds of reverse micelles containing respective metal element ions. Alternatively, two or more metal element ions are contained in one reverse micelle. Separately, reverse micelles containing the precipitant in the aqueous phase are prepared using an aqueous solution containing the precipitant. As the precipitant, a precipitant used in a conventional coprecipitation method can be used.For example, ammonia, ammonium carbonate, sodium hydroxide, potassium hydroxide, sodium carbonate, and the like that can neutralize a metal salt can be used. Examples thereof include alkali and alcohol. Of these, ammonia and ammonium carbonate, which volatilize during the subsequent baking, are particularly preferred, and the pH of the alkaline solution is preferably 9 or more.
[0066]
The reverse micelle containing the first metal ion, the reverse micelle containing the second metal ion, or the reverse micelle containing both the first metal ion and the second metal ion, and the precipitant thus produced, The reverse micelles containing, if necessary, the reverse micelles containing ions of the third and subsequent other metals are mixed in an organic solvent, reacted in the aqueous phase of the reverse micelles, and then treated in the same manner as in the first embodiment. To form a precursor of the composite oxide. At this time, by adjusting the size of each reverse micelle and the distance between the reverse micelles in the same manner as in the method of the first embodiment, in the obtained composite oxide, the aggregation of the primary particles is suppressed, and By limiting the size of the secondary particles, a porous composite oxide having mesopores between secondary particles can be obtained.
[0067]
According to the method of the second aspect, the dispersibility of the two metal elements in the obtained composite oxide is slightly lower than that of the method of the first aspect, but aggregation of the primary particles is reduced in the first aspect. Can be controlled in the same manner as in the above method, and a composite oxide having similar pores can be obtained.
[0068]
In a third aspect of the method for producing a porous composite oxide according to the present invention, an aqueous solution containing ions of the first metal element and an aqueous solution containing ions of the second and subsequent metal elements are mixed and reacted. Precipitate the metal and the compound containing the second and subsequent metal elements, hydrolyze and polycondensate to form primary particles of the precursor of the composite oxide, and aggregate the primary particles in a system containing the primary particles. Forming secondary particles, and further aggregating the secondary particles, wherein the total amount of metal elements in a mixture of an aqueous solution containing ions of the first metal element and an aqueous solution containing ions of the second and subsequent metal elements A compound is precipitated with an ion concentration of 0.3 mol / L or less, and then a solution containing the precipitate is concentrated to aggregate secondary particles.
[0069]
In the method of the third embodiment, first, similarly to the method of the second embodiment, an aqueous solution of a salt of a first metal and an aqueous solution of a salt of a second metal are prepared, and these are mixed to form a precipitant. The mixture is added and reacted to form a precursor of the composite oxide. When the solutions of the salts of these two metals are mixed, each aqueous solution is mixed as a dilute solution. The concentration of the metal salt solution is 0.15 mol / L or less, preferably 0.1 mol / L or less. By mixing in such a dilute solution, collision of the formed primary particles can be suppressed, and aggregation can be suppressed so as not to form large secondary particles. After the reaction, the solution containing the precipitate of the primary particles is concentrated by, for example, spraying from a nozzle or by passing through a permeable membrane to promote aggregation, and as a result, a composite oxide having mesopores can be obtained. .
[0070]
【Example】
Example 1
8.6 liters of cyclohexane and 350 g of polyethylene (n = 5) nonylphenyl ether were put into a beaker having an internal volume of 15 liters, and an aqueous solution consisting of 43 g of cerium nitrate and 120 mL of distilled water was added and stirred. Reverse micelles (water-in-oil microemulsion, measured water droplet diameter 17 nm) were prepared by stirring at room temperature using a magnetic stirrer. Separately, a zirconium alkoxide solution was prepared by dissolving 50 g of zirconium butoxide in 0.8 liter of cyclohexane, and added to the microemulsion. At this time, the volume ratio of water (aqueous phase) to cyclohexane (organic phase) is 78. When this mixture was stirred well at room temperature, the inside of the beaker immediately became cloudy to yellowish yellow, and colloidal particles (secondary particles, particle size of about 10 nm) were generated.
[0071]
Next, the pH was adjusted to 8 with aqueous ammonia to control the aggregation of the colloid.
Stirring was continued for about 1 hour to effect ripening. The mother liquor was separated by filtration, and the obtained precipitate was washed three times with ethanol, dried at 80 ° C. overnight, and calcined at 600 ° C., 900 ° C. and 1000 ° C. for 5 hours in the air to obtain a composite oxide containing cerium and zirconium. (Ceria zirconia) was obtained. The Ce / Zr molar ratio of the composite oxide was 1/1.
[0072]
Example 2
SrZrO is basically performed in the same manner as in Example 1. ₃ A composite oxide was produced. That is, 8.6 liters of cyclohexane and 350 g of polyethylene (n = 5) nonylphenyl ether were placed in a beaker having a capacity of 15 liters, and an aqueous solution consisting of 37 g of strontium nitrate and 100 mL of distilled water was added and stirred. Reverse micelles (water-in-oil type microemulsion, measured water droplet diameter 15 nm) were stirred at room temperature using a magnetic stirrer. Separately, a zirconium alkoxide solution was prepared by dissolving 68 g of zirconium butoxide in 0.8 liter of cyclohexane, and added to the microemulsion. The distance between the reverse micelles at this time was about 22 nm. When this mixture was thoroughly stirred at room temperature, the inside of the beaker immediately became cloudy to white yellow, and colloid particles (particle diameter: about 10 nm) were generated.
[0073]
Next, the pH was adjusted to 10.2 with aqueous ammonia to control the aggregation of the colloid. Stirring was continued for about 1 hour to effect ripening. The mother liquor was separated by filtration, and the obtained precipitate was washed three times with ethanol, dried at 80 ° C. overnight, and calcined at 600 ° C., 900 ° C. and 1000 ° C. for 5 hours in the air to obtain a composite oxide containing strontium and zirconium. (Strontium zirconia) was obtained. The obtained powder was analyzed by a Raman scattering method, an X-ray diffraction method or the like, and it was confirmed that the crystal form was perovskite.
[0074]
Example 3
8.6 liters of cyclohexane and 350 g of polyethylene (n = 5) nonylphenyl ether were put into a beaker having an internal volume of 15 liters, and an aqueous solution consisting of 42.5 g of cerium nitrate, 30.1 g of zirconium nitrate and 120 mL of distilled water was added and stirred. Reverse micelles were prepared by stirring at room temperature using a magnetic stirrer. Next, this reverse micelle was dropped into a reverse micelle solution containing ammonia in an aqueous phase, and stirring was continued to form colloid particles. Subsequently, the colloid was aggregated in the same manner as in Example 1 to obtain a composite oxide containing cerium and zirconium (ceria zirconia).
[0075]
Example 4
To a beaker having an internal volume of 15 liters, 10 liters of an aqueous solution in which cerium nitrate and zirconium nitrate are dissolved to a concentration of 0.025 mol / L are added, and the same amount of aqueous ammonia is added dropwise to nitrate ions to neutralize the solution. To form colloidal particles. Subsequently, the solution containing the colloidal particles was sprayed into dry air so that the amount of the solution was reduced to half. Thereafter, ammonia was added dropwise to this solution to adjust the pH to about 8, whereby an aggregate was obtained. This was filtered, dried at 80 ° C. overnight, and calcined at a specified concentration to obtain a composite oxide containing cerium and zirconium (ceria zirconia).
[0076]
Comparative Example 1
Cerium nitrate and zirconium nitrate were added to 2 liters of distilled water at a concentration of 0.5 mol / L and dissolved. Aqueous ammonia was added dropwise to the aqueous solution to adjust the pH to 9, and a precipitate was obtained. This was filtered, dried and calcined at 600 ° C.
[0077]
Comparative Example 2
A nonionic surfactant (polyoxyethylene octyl phenyl ether) was added to 2 liters of distilled water to such an extent that no foam was formed. Thereafter, cerium nitrate and zirconium nitrate were added to each and dissolved at a concentration of 0.5 mol / L. Aqueous ammonia was added dropwise to the aqueous solution to adjust the pH to 9, and a precipitate was obtained. This was washed with water, filtered, dried and then fired at 600 ° C.
[0078]
Performance evaluation
The pore volume of the obtained porous composite oxide after firing at 600 ° C., 900 ° C., and 1000 ° C. was measured at a liquid nitrogen temperature by nitrogen adsorption. FIG. 6 shows the result.
The pore distribution of the obtained porous composite oxide after firing at 600 ° C. and 900 ° C. was similarly measured by nitrogen adsorption. FIG. 7 shows the result.
[0079]
As is clear from the results shown in FIGS. 6 and 7, in the case of the conventional composite metal oxide, when fired at a high temperature, the pore volume is greatly reduced, and the pore distribution is shifted to a higher side, but the peak value is shifted. Shows that the pore volume has decreased, whereas the composite metal oxide of the present invention does not decrease in pore volume even when calcined at a high temperature, and the pore distribution has a peak value. With only a shift to the higher side, the peak height and pore volume hardly changed.
[0080]
It is known that ceria zirconia has a high oxygen storage performance when Ce is substituted with a high dispersion in the Zr lattice (Catal. Today, 74, 225-234 (2002), Y. Nagai et al.). Therefore, the replacement amount can be predicted by measuring the oxygen storage amount. In particular, the oxygen storage amount at a low temperature (300 ° C. or lower) is significantly affected. Here, the oxygen storage amount can be expressed by oxidation and reduction of Ce. That is,
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[0081]
The oxygen storage capacity of the ceria zirconia obtained in Example 1 and Comparative Examples 1 and 2 was evaluated using an oxygen pulse adsorption method. The utilization rate of cerium was calculated from the obtained oxygen storage capacity, and Ce (IV) O was calculated. ₂ → Ce (III) O _1.5 + 1 / 2O ₂ In the case of (1), Ce utilization efficiency is set to 100%, and the results are shown in FIG.
[0082]
As shown in FIG. 8, according to the present invention, high Ce utilization efficiency was obtained even after high-temperature firing, and this was because Zr and Ce were highly dispersed from the time of composite hydroxide synthesis. To indicate that Thus, it has been found that ceria zirconia produced according to the present invention can be used as a catalyst raw material having an unprecedented low-temperature activity.
[0083]
Further, the composite metal oxide of the present invention is particularly useful as a catalyst for purifying diesel exhaust gas. In diesel, light oil is added as an HC source to an exhaust system in order to reduce NOx. However, even if this gas oil component is vaporized in the exhaust system, it has a large molecular weight, so that it cannot be sufficiently diffused into the conventional metal composite oxide, and the diffusion is rate-limiting, and the reaction does not proceed. However, since the composite metal oxide of the present invention has sufficient mesopores of 2 nm or more, the light oil component can be diffused inside, and sufficient purification can be achieved.
[0084]
【The invention's effect】
According to the present invention, a composite metal oxide having a sufficient pore volume even when calcined at a high temperature is provided, and is useful as an exhaust gas purifying catalyst.
[Brief description of the drawings]
FIG. 1 is a diagram showing a configuration of a composite metal oxide of the present invention.
FIG. 2 is a view showing a configuration of a conventional composite metal oxide.
FIG. 3 is a schematic diagram showing the size of a reverse micelle and a distance between the reverse micelles in the method of the present invention.
FIG. 4 is a three-phase map showing the relationship between water, surfactant and oil in the microemulsion.
FIG. 5 is an enlarged view of a part of FIG. 4;
FIG. 6 is a graph showing changes in pore volume during firing of various cerium-zirconium composite oxides.
FIG. 7 is a graph showing a change in pore distribution of various cerium-zirconium composite oxides after firing at 600 ° C. and 900 ° C.
FIG. 8 is a graph showing changes in oxygen storage amounts of various cerium-zirconium composite oxides.
[Explanation of symbols]
1: Primary particles
2: Secondary particles
3: Mesopore
4: Reverse micelle
5 Surfactant
6 ... Aqueous phase

Claims (12)

A porous composite comprising an aggregate of secondary particles, which is an aggregate of primary particles of a composite oxide containing two or more metal elements, and having mesopores having a pore diameter of 2 to 100 nm between the secondary particles An oxide, wherein after firing at 600 ° C. for 5 hours in an oxygen atmosphere, the ratio of the pores among the secondary particles having a diameter of 10 nm or more in the mesopores is 10% or more of the total mesopore volume. A porous composite oxide characterized by the above-mentioned. The porous composite oxide according to claim 1, wherein the primary particles have a particle size of 3 to 15 nm, and the secondary particles have a particle size of 30 to 100 nm. The porous composite oxide according to claim 1, wherein the metal element is composed of two kinds of cerium and zirconium. It is a manufacturing method of the porous composite oxide of Claim 1, Comprising:
A solution in which a compound of a first metal element that forms a hydroxide by hydrolysis is dissolved in an organic solvent, and a second or later metal is added to an aqueous phase inside a reverse micelle where a surfactant is formed in the organic solvent. Mix with an emulsion containing elemental ions,
At the interface of the reverse micelles, the compound of the first metal element is hydrolyzed, and the second and subsequent metal elements are incorporated,
Polycondensed to form primary particles of the precursor of the composite oxide,
In the system containing the primary particles, the primary particles are aggregated to form secondary particles,
Further including aggregating the secondary particles,
During the hydrolysis, the distance between the reverse micelles is sufficiently maintained, and the volume of the organic phase outside the reverse micelles is reduced to the volume of the aqueous phase within the reverse micelles so as to form pores of a sufficient size between the secondary particles. And a method for producing a porous composite oxide, which is increased with respect to the volume of the surfactant. 5. The method according to claim 4, wherein the volume of the organic phase outside the reverse micelle is at least twice the volume of the aqueous phase inside the reverse micelle. 5. The method according to claim 4, wherein the volume of the organic phase outside the reverse micelle is at least 5 times the volume of the surfactant. The method according to claim 4, wherein the diameter of the aqueous phase in the reverse micelle is 5 nm or more, and the distance between the reverse micelles is 20 nm or more. It is a manufacturing method of the porous composite oxide of Claim 1, Comprising:
In an aqueous phase inside a reverse micelle formed by a surfactant in an organic solvent, an aqueous solution containing ions of the first metal element and an aqueous solution containing ions of the second and subsequent metal elements are mixed and reacted to form a first solution. A compound containing a metal and a second or later metal element,
This is hydrolyzed and polycondensed to form primary particles of a composite oxide precursor,
In the system containing the primary particles, the primary particles are aggregated to form secondary particles,
Further including aggregating the secondary particles,
During the hydrolysis, the distance between the reverse micelles is sufficiently maintained, and the volume of the organic phase outside the reverse micelles is reduced to the volume of the aqueous phase within the reverse micelles so as to form pores of a sufficient size between the secondary particles. And a method for producing a porous composite oxide, which is increased with respect to the volume of the surfactant. 9. The method according to claim 8, wherein the volume of the organic phase outside the reverse micelle is at least twice the volume of the aqueous phase inside the reverse micelle. 9. The method according to claim 8, wherein the volume of the organic phase outside the reverse micelle is at least 5 times the volume of the surfactant. 9. The method according to claim 8, wherein the diameter of the aqueous phase in the reverse micelle is 5 nm or more, and the distance between the reverse micelles is 20 nm or more. It is a manufacturing method of the porous composite oxide of Claim 1, Comprising:
An aqueous solution containing ions of the first metal element and an aqueous solution containing ions of the second and subsequent metal elements are mixed and reacted to precipitate a compound containing the first metal element and the second and subsequent metal elements,
This is hydrolyzed and polycondensed to form primary particles of a composite oxide precursor,
In the system containing the primary particles, the primary particles are aggregated to form secondary particles,
Further including aggregating the secondary particles,
A compound is precipitated by setting the total concentration of metal element ions in a mixture of an aqueous solution containing ions of the first metal element and an aqueous solution containing ions of the second and subsequent metal elements to 0.3 mol / L or less, and then including this precipitation A method for producing a porous composite oxide, comprising concentrating a solution and aggregating secondary particles.

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