JP2004182548A - Method of manufacturing carbon nanotube - Google Patents
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【０００１】
【発明の属する技術分野】
本発明は、ＣＶＤ法によりカーボンナノチューブを生成した後、精製処理して高純度のカーボンナノチューブを製造する方法に関するものである。
【０００２】
【従来の技術】
従来、カーボンナノチューブの合成にはアーク放電法、レーザー照射法、熱ＣＶＤ法が主に用いられている。このうちアーク放電法やレーザー照射法においては、排気装置や高電圧大電流電源などの高価かつ危険な装置を必要とするのに加え、カーボンナノチューブの生成量も少ない。また、生成したカーボンナノチューブには、黒鉛やアモルファスカーボンが混在し、合成目的物であるカーボンナノチューブ自体についても径や長さのばらつきが大きい等の問題がある。
【０００３】
また、触媒金属を含有させた基体上で、炭素源となる炭化水素を熱分解させることにより、該基体上に直接カーボンナノチューブを生成させる熱ＣＶＤ法においては、生成カーボンナノチューブを基体から分離する必要がある。しかし、この分離が極めて困難であり、このため熱ＣＶＤ法で高純度のカーボンナノチューブを得るのは難しいという問題がある。
【０００４】
上記のような熱ＣＶＤ法において、基体とカーボンナノチューブを高度に分離可能な製造技術として超臨界乾燥により調製した触媒を用いるとするＪｉｅ Ｌｉｕらの報告がある〔Ｃｈｅｍｉｃａｌ Ｐｈｙｓｉｃｓ Ｌｅｔｔｅｒｓ，３２２（２０００） ｐ．３２１−３２６〕。しかしこの方法は、高圧装置等の高価な装置を必要とし、基体の大量合成が難しいという問題点がある。
【０００５】
同じく、上記のような熱ＣＶＤ法において、酸化マグネシウムを基体として用いた例としてＬｉ Ｑｉｎｇｗｅｎらの報告がある（Ｊｏｕｒｎａｌ Ｍａｔｅｒｉａｌ Ｃｈｅｍｉｓｔｒｙ，２００２，１２，ｐ．１１７９―１１８３）。彼らは酸化マグネシウムに鉄、コバルト、ニッケルのうちの一つとモリブデンを組み合わせた金属を担持させてカーボンナノチューブを合成し、触媒と基体材料を塩酸に溶解することにより精製している。
【０００６】
しかし、上記Ｌｉ Ｑｉｎｇｗｅｎらの報告において、それらいずれの金属を担持した場合にも、精製後には、得られた試料のうちの１０ｗｔ％分の触媒が残存しており、十分な純度のカーボンナノチューブは得られていない。酸化マグネシウム基体に金属を担持させた触媒体を用いる合成法によれば単層あるいは多層のカーボンナノチューブが得られるが、精製処理後でもそのように多量の不純物が含まれているため、これを直ちに各種用途に供することはできず、実用に供し得る高純度のカーボンナノチューブとするにはさらに研究、開発が必要である。
【０００７】
【非特許文献１】Ｃｈｅｍｉｃａｌ Ｐｈｙｓｉｃｓ Ｌｅｔｔｅｒｓ，３２２（２０００） ｐ．３２１−３２６
【非特許文献２】Ｊｏｕｒｎａｌ Ｍａｔｅｒｉａｌ Ｃｈｅｍｉｓｔｒｙ，２００２，１２，ｐ．１１７９―１１８３
【０００８】
【発明が解決しようとする課題】
本発明者らは、上記純度、精製上の問題点を解決するため鋭意実験、検討を続けたところ、純度９８ｗｔ％以上の高純度のカーボンナノチューブを製造し得ることを見い出した。すなわち、本発明は、高比表面積で多孔質酸化マグネシウムからなる基体に触媒金属及び助触媒金属を担持してなる触媒体を用いてカーボンナノチューブを合成した後、特定の精製処理をすることにより、高純度のカーボンナノチューブを製造する方法を提供することを目的とするものである。
【０００９】
【課題を解決するための手段】
本発明は、多孔質酸化マグネシウムからなる基体に対して触媒金属及び助触媒金属を担持してなる触媒体に、反応温度下で炭素源気体を流通させて、カーボンナノチューブを生成した後、生成カーボンナノチューブを含む粗生成物を精製処理して高純度のカーボンナノチューブを製造する方法であって、前記精製処理を、該粗生成物を微細に粉砕した後、酸溶液またはアルカリ溶液と十分に混練し、次いで還流処理することにより基体、触媒金属及び助触媒金属を溶解し、固液分離することにより純度９８ｗｔ％以上のカーボンナノチューブを製造することを特徴とする高純度カーボンナノチューブの製造方法を提供する。
【００１０】
【発明の実施の形態】
本発明は、高純度のカーボンナノチューブを製造する方法であり、まず、カーボンナノチューブを多孔質酸化マグネシウムからなる基体に対して触媒金属及び助触媒金属を担持してなる触媒体に反応温度下で炭素原料気体を流通させて生成する。生成カーボンナノチューブは、基体である酸化マグネシウム、触媒金属及び助触媒金属が含まれた粗生成物として得られるので、これら不純物は精製処理してカーボンナノチューブから除去する必要がある。以下、特に必要な場合を除き、触媒金属及び助触媒金属の両者を合わせて適宜「触媒金属」という。
【００１１】
本発明においては、その精製処理において、粗生成物を微細に粉砕した後、酸溶液もしくはアルカリ溶液と混練することが必須である。この混練は、本発明において非常に重要であり、十分に徹底して行うことが重要である。これにより不純物である基体、触媒金属を酸溶液もしくはアルカリ溶液に十分溶解する。次いで還流処理する。還流処理後、固液分離し、分離固体を乾燥することにより純度９８ｗｔ％以上のカーボンナノチューブが得られる。
【００１２】
このように、本発明の高純度カーボンナノチューブの製造方法は、まず（１）カーボンナノチューブを合成する触媒体を製造すること、（２）触媒体を用いてカーボンナノチューブを合成して粗生成物を得ること、次いで（３）粗生成物を精製処理することからなるので、以下これら工程を順次説明する。
【００１３】
〈（１）カーボンナノチューブ合成用触媒体の製造〉
本発明においては、カーボンナノチューブを生成し成長させるために、多孔質酸化マグネシウムからなる基体に対して触媒金属と助触媒金属を含有させてなる触媒体を用いる。多孔質酸化マグネシウムは粉体の形であるのが好ましく、多孔質酸化マグネシウムの粉体は硝酸マグネシウム、炭酸マグネシウム等のマグネシウム塩を熱分解することにより得られる。
【００１４】
多孔質酸化マグネシウムの粉体は高純度である必要があり、特にＮａ、Ｋ等のアルカリ金属、硫黄及びシリカからなる不純物の含有量が０．０５ｗｔ％以下、好ましくは０．０１ｗｔ％以下であることが重要である。本発明で用いる多孔質酸化マグネシウムとはこのような高純度の多孔質酸化マグネシウムを意味する。また、この粉体の比表面積は３０〜１５０ｃｍ２／ｇの範囲である。
【００１５】
上記多孔質酸化マグネシウムの粉体に対して触媒金属と助触媒金属を担持して含有させる。触媒金属としてはＰｄ、Ｃｒ、Ｆｅ、Ｃｏ、Ｎｉ及びＣｕから選ばれた金属のうちの少なくとも１種類以上の金属が用いられる。それら金属のうち特に好ましい金属はＰｄ、Ｆｅ、Ｃｏ及びＮｉのうちの少なくとも１種類以上の金属である。助触媒金属としてはＭｏが用いられ、これを触媒金属とともに用いることでカーボンナノチューブの生成速度を促進させることができる。
【００１６】
多孔質酸化マグネシウムの粉体に対してそれら触媒金属と助触媒金属を担持させる仕方としては、該多孔質酸化マグネシウムの粉体に対して、触媒金属と助触媒金属を含む溶液を接触させる。該溶液としては、好ましくはそれら金属を塩の形で含む溶液が用いられる。それら金属塩は硝酸塩、アセチルアセトン錯塩その他の可溶性塩であるのが好ましい。
【００１７】
溶媒としては、水、あるいは低級アルコール（メタノール、エタノール等）などの有機溶媒、または水と水溶性有機溶媒との混合液が用いられ、特に好ましくは水が用いられる。こうして調製した触媒金属及び助触媒金属を含む溶液において、それら金属塩の濃度は、それら金属塩の飽和溶解濃度以下であるが、通常は各金属塩ごとに０．００５〜０．５ｗｔ％、好ましくは０．００５〜０．１ｗｔ％である。
【００１８】
次いで、触媒金属塩及び助触媒金属塩を含む溶液を多孔質酸化マグネシウムの粉体に接触させて担持させる。その接触法としては、浸漬法やスプレー法、その他適宜の方法が用いられるが、好ましくは浸漬法が用いられる。その接触温度は室温から８０℃の範囲、好ましくは５０〜６０℃の範囲である。多孔質酸化マグネシウムの粉体と触媒金属塩及び助触媒金属塩の溶液との接触により、触媒金属塩及び助触媒金属塩は該粉体に含浸され、担持される。
【００１９】
こうして得られる触媒金属及び助触媒金属を担持し含有する多孔質酸化マグネシウムの粉体において、該粉体に対する触媒金属の含有量は、金属として１〜２０ｗｔ％、好ましくは５〜１０ｗｔ％である。また、該粉体に対する助触媒金属であるＭｏの含有量は、金属として０．１〜１．５ｗｔ％、好ましくは０．３〜０．８ｗｔ％である。
【００２０】
多孔質酸化マグネシウムの粉体に含有されたそれら金属の形は、カーボンナノチューブを生成し、その反応速度を促進させる形であればよく、金属の形のほか、金属酸化物や金属水酸化物の形であることができる。金属の形の場合は、上記のようにして得られた金属塩を含有する多孔質酸化マグネシウムの粉体を水素等の還元雰囲気で還元することにより得られる。また、金属酸化物の形の場合、上記のようにして得られた金属塩を含有する多孔質酸化マグネシウムの粉体を焼成することにより得られる。
【００２１】
触媒体の形状は、触媒金属及び助触媒金属を含有する多孔質酸化マグネシウムの粉体の形でもよく、触媒金属及び助触媒金属を含有する多孔質酸化マグネシウムの粉体を成形した成形体の形でもよく、触媒金属及び助触媒金属を含有する多孔質酸化マグネシウムの粉体をＳｉやＳｉＯ２等の基板に担持させた形でもよい。こうして作製された触媒体には、その表面に触媒金属と助触媒金属が担持されている。
【００２２】
〈（２）カーボンナノチューブの合成〉
以上のようにして製造した触媒体を用いてカーボンナノチューブを合成して粗生成物を製造する。触媒体を電気炉等の反応容器に配置し、これに炭素原料すなわち炭素源を流通させながら、炭素源を触媒体の表面で熱分解させてカーボンナノチューブを生成させる。炭素源としてはメタン、エタン、プロパン等の炭化水素、一酸化炭素、あるいはメタノール、エタノール等の低級アルコール類が用いられる。
【００２３】
炭素源の熱分解温度、すなわち反応温度は４００℃〜１２００℃である。本発明においては４００℃〜８００℃という低い温度においてもカーボンナノチューブを製造することができる。このように低い温度においてもカーボンナノチューブを製造できることから設備コスト、エネルギー消費を少なくできるので、実用上非常に有用である。単層カーボンナノチューブは７００℃〜９００℃の温度で製造することができる。
【００２４】
炭素源がメタンの場合、その流通速度は、ガス空間速度（ＧＨＳＶ）で２０００〜２０００００ｈｒ−１、好ましくは５０００〜１００００ｈｒ−１である。メタンを熱分解する場合、その気体中にはアルゴンや水素をキャリアとして混入することができる。また、メタンに硫化水素やメルカプタン等の硫黄化合物を適量加えることにより、触媒体上に真っ直ぐなカーボンナノチューブを形成することができる。
【００２５】
〈（３）粗生成物の精製処理〉
カーボンナノチューブは触媒体の表面から成長して生成する。このため、カーボンナノチューブは触媒体、すなわち基体、触媒金属（すなわち触媒金属及び助触媒金属）を含む粗生成物として得られるので、カーボンナノチューブを触媒体から離脱させ、精製処理する必要がある。その精製処理として、本発明においては、まず粗生成物を粉砕して微細化する。粉砕は粗生成物が十分微細化するように行う。粉砕後、必要に応じて例えば２００メッシュ以下というようにふるいがけしてもよい。
【００２６】
次いで、上記微細粉砕物を酸溶液もしくはアルカリ溶液と混練する。酸水溶液またはアルカリ水溶液としては、硝酸や塩酸の水溶液または水酸化ナトリウム水溶液、もしくはそれらのアセトンまたは過酸化水素水との混合溶液であることが好ましい。
【００２７】
上記混練処理は、本発明において非常に重要な処理であり、十分に徹底して行う必要がある。本混練処理は、上記微細粉砕物が少量であれば乳鉢等で行ってもよいが、上記微細粉砕物が多量の場合には例えば湿式混練ミル、連続スクリュー型混練機、高速流動型ミキサー等により上記微細粉砕物と酸溶液もしくはアルカリ溶液の混合物に圧縮、せん断、摩擦作用を徹底して与えて行う。次いで、混練液を沸点下で（沸騰状態で）還流処理する。これにより不純物である基体、触媒金属を酸溶液もしくはアルカリ溶液に十分溶解させ、カーボンナノチューブの精製効率を劇的に向上させることができる。
【００２８】
また、上記還流処理に際して、その前に上記混練液を濾過し、固形物を再び酸水溶液またはアルカリ水溶液に浸し、これを還流処理することにより、精製効率をより確実にすることができる。次いで、還流処理済みの溶液を濾過して固形物を分離することにより、純度９８ｗｔ％以上の高純度のカーボンナノチューブを得ることができる。
【００２９】
【実施例】
以下、実施例を基にして本発明をさらに詳しく説明するが、本発明が実施例に限定されないことはもちろんである。各実施例及び比較例において、基体としては、Ｎａ、Ｋ等のアルカリ金属、硫黄及びシリカからなる不純物の含有量が０．０１ｗｔ％の多孔質酸化マグネシウムの粉体を用いた。
【００３０】
《実施例１》
比表面積が１３０ｍ２／ｇの多孔質酸化マグネシウムの粉体１０．０ｇを、硝酸鉄〔Ｆｅ（ＮＯ３）２・９Ｈ２Ｏ〕２．２ｇと酸化モリブデンアセチルアセトナート〔（ＣＨ３ＣＯＣＨＣＯＣＨ３）２ＭｏＯ２〕０．１２ｇをメタノール３００ｍｌに溶解させて得た溶液中に３０分間浸し、さらに３時間、超音波処理により分散させた。次いでこの溶液を減圧乾燥し、乾燥物すなわち粉体形の触媒体を得た。
【００３１】
上記乾燥物（＝触媒体）をアルミナ製の船形るつぼに入れ、電気炉中に配置した。電気炉の反応管は内径３ｃｍ、長さ１ｍの石英管で、長さ方向の中央部６００ｍｍが加熱領域であり、その加熱領域の中央部５００ｍｍに乾燥物を配置した。電気炉をアルゴン雰囲気下で８００℃まで昇温させた後、メタンとアルゴンの混合ガスを３０分間流通させた。
【００３２】
得られた生成物を走査型電子顕微鏡（ＳＥＭ、日立製作所社製 Ｓ−５０００、以下同じ）により観察したところ、触媒体表面にカーボンナノチューブが生成していることがわかった。図１はその上面から撮影した写真を図面化したものである。図１のとおり、カーボンナノチューブとともに、数多くの微細な触媒粒子が観察される。
【００３３】
粗生成物試料、すなわち触媒体を含むカーボンナノチューブの一部を、空気中で８００℃まで昇温させながら重量変化を熱天秤で測定した。図４にその重量変化を示している。図４中未精製試料として示すとおり、４００℃程度までで全重量の２４ｗｔ％が消失しているが、それ以降、重量低減は殆どみられない。重量低減分はカーボンナノチューブの消失分に相当するが、試料中に基体、触媒金属が７６ｗｔ％残存していることがわかる。
【００３４】
〈精製処理〉
上記触媒体を含むカーボンナノチューブからなる試料を微細に粉砕し、次いで２００メッシュ以下にふるいがけした。この試料を３ｍｏｌ／ｌの硝酸水溶液に入れ、乳鉢で十分に混錬した。混練後、濾過し、再び３ｍｏｌ／ｌの硝酸水液中に浸し、１時間沸点下で還流した。次いで、還流液を濾過し、固形物を乾燥して精製試料を得た。得られた試料を走査型電子顕微鏡（ＳＥＭ）及び透過型電子顕微鏡（ＴＥＭ、日本電子社製 ＪＥＭ−２０００ＦＸ ＩＩ）で観察した。図２はＳＥＭ写真、図３はＴＥＭ写真である。図２〜３のとおり、太さ（直径）がいずれも１〜２ｎｍ程度の単層カーボンナノチューブがバンドル状になっており、また、触媒の残存はごく僅かしかないことがわかる。こうして単層カーボンナノチューブが７００ｍｇ得られた。
【００３５】
また、上記乾燥固形物、すなわち精製試料を空気中で８００℃まで昇温させながら重量変化を熱天秤で測定した。図４にその重量変化を示している。図４中精製試料として示すとおり、試料中の９９ｗｔ％が消失した。重量低減分は単層カーボンナノチューブの消失分に相当するが、試料中に基体、触媒金属が１ｗｔ％しか残存していなかったことがわかる。こうして、基体である酸化マグネシウムや触媒金属である鉄、助触媒金属であるモリブデンがほぼ完全に除去された高純度の単層カーボンナノチューブを得ることができた。
【００３６】
《実施例２》
比表面積が３０ｍ２／ｇの多孔質酸化マグネシウムの粉体１０．０ｇを基体として用いた以外は、実施例１と同様にして、Ｆｅ及びＭｏを担持し、触媒体表面にチューブを成長させ、精製処理を行った。これにより単層カーボンナノチューブが７００ｍｇ得られた。また、この試料を空気中で８００℃まで昇温させながら重量変化を熱天秤で測定したところ、試料から９９ｗｔ％が消失した。重量低減分は単層カーボンナノチューブの消失分に相当する。こうして、基体である酸化マグネシウム、触媒金属である鉄及び助触媒金属であるモリブデンがほぼ完全に除去された高純度の単層カーボンナノチューブを得ることができた。
【００３７】
《実施例３》
実施例１で用いたのと同じ多孔質酸化マグネシウムの粉体１．０ｇを硝酸パラジウム〔Ｐｄ（ＮＯ３）２〕１．２ｇと酸化モリブデンアセチルアセトナート０．０１ｇをメタノールに溶解させて得た溶液中に３０分間浸し、さらに３時間超音波処理により分散させた。これを減圧乾燥し、乾燥物（＝触媒体）を用いて４００℃で実施例１と同様にしてメタンとＡｒの混合ガスを３０分間流通させた。粗生成物を、実施例１と同様にして、粉砕し、ふるいがけをした後、精製処理を行い、濾過、乾燥させた。
【００３８】
得られた試料を透過型電子顕微鏡で観察したところ、太さ（直径）が２０〜３０ｎｍ程度の多層カーボンナノチューブが生成していることがわかった。また、基体、触媒金属の残存は全く確認できなかった。こうして、不純物が完全に除去された高純度の多層カーボンナノチューブを得ることができた。
【００３９】
《比較例１》
実施例１と同様にして触媒体表面に単層カーボンナノチューブを成長させた。得られた粗生成物試料を３ｍｏｌ／ｌの硝酸水液中に浸し、１時間還流処理した後、濾過し、固形分を乾燥させた。ここでは混練処理は行っていない。この試料を空気中で８００℃まで昇温させながら重量変化を熱天秤で測定したところ、試料から９０ｗｔ％が消失した。重量低減分は単層カーボンナノチューブの消失分に相当するが、生成物試料中に基体、触媒金属が１０ｗｔ％残存しており、高純度の単層カーボンナノチューブを得ることができなかった。
【００４０】
〈比較例２〉
比表面積が８ｍ２／ｇの多孔質酸化マグネシウムの粉体１０．０ｇを基体とし、実施例１と同様にして、Ｆｅ及びＭｏを担持し、触媒体表面にチューブを成長させ、精製処理を行った。これにより、単層カーボンナノチューブが１０ｍｇ得られた。得られた単層カーボンナノチューブは高純度であったが、そのように単層カーボンナノチューブは生成量は僅かに１０ｍｇであり、実施例１と比較して極端に低下した。
【００４１】
【発明の効果】
本発明によれば、触媒体及び合成カーボンナノチューブを含む粗生成物を粉砕して微細化した後、酸溶液もしくはアルカリ溶液と十分に混錬し、その溶液を還流処理することにより、高純度のカーボンナノチューブを効率よく製造することができる。本発明の製造方法は、スケールアップが容易であり、これにより多層カーボンナノチューブ及び単層カーボンナノチューブを省エネルギーかつ低コストで大量に製造することが可能となる。
【図面の簡単な説明】
【図１】実施例１で得られた粉砕前の試料のＳＥＭ写真を図面化して示す図
【図２】実施例１で得られた精製処理後の試料のＳＥＭ写真を図面化して示す図
【図３】実施例１で得られた精製処理後の試料のＴＥＭ写真を図面化して示す図
【図４】実施例１で得られた試料（本発明による精製処理前及び本発明による精製処理後の試料）を空気中で８００℃まで昇温させながら重量変化を測定した結果を示す図[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing high-purity carbon nanotubes by producing carbon nanotubes by a CVD method and then performing a purification treatment.
[0002]
[Prior art]
Conventionally, an arc discharge method, a laser irradiation method, and a thermal CVD method have been mainly used for synthesizing carbon nanotubes. Among them, the arc discharge method and the laser irradiation method require expensive and dangerous devices such as an exhaust device and a high-voltage large-current power supply, and also generate a small amount of carbon nanotubes. In addition, graphite and amorphous carbon are mixed in the generated carbon nanotube, and the carbon nanotube itself, which is a synthesis target, also has problems such as large variations in diameter and length.
[0003]
In a thermal CVD method in which hydrocarbons serving as a carbon source are thermally decomposed on a substrate containing a catalytic metal to directly generate carbon nanotubes on the substrate, it is necessary to separate the generated carbon nanotubes from the substrate. There is. However, this separation is extremely difficult, and therefore, it is difficult to obtain high-purity carbon nanotubes by the thermal CVD method.
[0004]
In the above-mentioned thermal CVD method, there is a report by Jie Liu et al. That a catalyst prepared by supercritical drying is used as a production technique capable of highly separating a substrate and carbon nanotubes [Chemical Physics Letters, 322 (2000) p. . 321-326]. However, this method has a problem that an expensive device such as a high-pressure device is required, and mass synthesis of the substrate is difficult.
[0005]
Similarly, there is a report by Li Qingwen et al. (Journal Material Chemistry, 2002, 12, p. 1179-1183) as an example of using magnesium oxide as a substrate in the thermal CVD method as described above. They support a metal that combines molybdenum with one of iron, cobalt, and nickel on magnesium oxide to synthesize carbon nanotubes, and purify the catalyst and base material by dissolving it in hydrochloric acid.
[0006]
However, in the report of Li Qingwen et al., Even when any of these metals is supported, after purification, 10 wt% of the catalyst in the obtained sample remains, and carbon nanotubes of sufficient purity cannot be obtained. Not obtained. According to the synthesis method using a catalyst body in which a metal is supported on a magnesium oxide substrate, single-walled or multi-walled carbon nanotubes can be obtained. It cannot be used for various purposes, and further research and development are required to obtain high-purity carbon nanotubes that can be used practically.
[0007]
[Non-Patent Document 1] Chemical Physics Letters, 322 (2000) p. 321-326
[Non-Patent Document 2] Journal Material Chemistry, 2002, 12, p. 1179-1183
[0008]
[Problems to be solved by the invention]
The present inventors have conducted intensive experiments and studies in order to solve the above-mentioned problems of purity and purification, and have found that high-purity carbon nanotubes having a purity of 98 wt% or more can be produced. In other words, the present invention provides a specific purification treatment after synthesizing carbon nanotubes using a catalyst body comprising a catalyst metal and a promoter metal supported on a porous magnesium oxide substrate having a high specific surface area, It is an object of the present invention to provide a method for producing high-purity carbon nanotubes.
[0009]
[Means for Solving the Problems]
The present invention provides a catalyst body that supports a catalyst metal and a co-catalyst metal on a substrate made of porous magnesium oxide. A method for producing a high-purity carbon nanotube by purifying a crude product containing nanotubes, wherein the purifying treatment is carried out by finely pulverizing the crude product, and then sufficiently kneading with an acid solution or an alkali solution. And then subjecting the substrate, the catalyst metal and the cocatalyst metal to a reflux treatment to dissolve the solid, and then to perform solid-liquid separation to produce a carbon nanotube having a purity of 98 wt% or more. .
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
The present invention is a method for producing high-purity carbon nanotubes. First, carbon nanotubes are formed on a catalyst body comprising a porous magnesium oxide substrate and a catalyst metal and a co-catalyst metal supported thereon at a reaction temperature. It is generated by flowing the raw material gas. The resulting carbon nanotube is obtained as a crude product containing magnesium oxide, a catalyst metal, and a co-catalyst metal as a base, and therefore, it is necessary to remove these impurities from the carbon nanotube by a purification treatment. Hereinafter, unless otherwise required, both the catalyst metal and the promoter metal are collectively referred to as “catalyst metal” as appropriate.
[0011]
In the present invention, in the purification treatment, it is essential that the crude product is finely pulverized and then kneaded with an acid solution or an alkali solution. This kneading is very important in the present invention, and it is important to carry out thoroughness. Thereby, the substrate and the catalyst metal, which are impurities, are sufficiently dissolved in the acid solution or the alkali solution. Next, a reflux treatment is performed. After the reflux treatment, solid-liquid separation is performed, and the separated solid is dried to obtain carbon nanotubes having a purity of 98 wt% or more.
[0012]
As described above, according to the method for producing a high-purity carbon nanotube of the present invention, first, (1) producing a catalyst for synthesizing carbon nanotubes, and (2) synthesizing carbon nanotubes using the catalyst to produce a crude product Obtaining and then (3) purifying the crude product, these steps will be sequentially described below.
[0013]
<(1) Production of catalyst for carbon nanotube synthesis>
In the present invention, in order to generate and grow carbon nanotubes, a catalyst body containing a catalyst metal and a promoter metal with respect to a substrate made of porous magnesium oxide is used. The porous magnesium oxide is preferably in the form of a powder, and the powder of the porous magnesium oxide is obtained by thermally decomposing a magnesium salt such as magnesium nitrate or magnesium carbonate.
[0014]
The powder of the porous magnesium oxide needs to be of high purity, and particularly contains 0.05 wt% or less, preferably 0.01 wt% or less of impurities composed of alkali metals such as Na and K, sulfur and silica. This is very important. The porous magnesium oxide used in the present invention means such a high-purity porous magnesium oxide. The specific surface area of this powder is in the range of 30 to 150 cm 2 / g.
[0015]
A catalyst metal and a promoter metal are supported and contained in the porous magnesium oxide powder. As the catalyst metal, at least one metal selected from metals selected from Pd, Cr, Fe, Co, Ni, and Cu is used. Among these metals, particularly preferred metals are at least one of Pd, Fe, Co and Ni. Mo is used as the co-catalyst metal, and by using this together with the catalyst metal, the generation rate of carbon nanotubes can be accelerated.
[0016]
As a method of supporting the catalyst metal and the promoter metal on the porous magnesium oxide powder, a solution containing the catalyst metal and the promoter metal is brought into contact with the porous magnesium oxide powder. As the solution, a solution containing these metals in the form of a salt is preferably used. The metal salts are preferably nitrates, acetylacetone complex salts and other soluble salts.
[0017]
As the solvent, water, an organic solvent such as a lower alcohol (methanol, ethanol, or the like), or a mixed solution of water and a water-soluble organic solvent is used, and water is particularly preferably used. In the solution containing the catalyst metal and the co-catalyst metal thus prepared, the concentration of the metal salt is not more than the saturated dissolution concentration of the metal salt, but is usually 0.005 to 0.5 wt% for each metal salt, preferably. Is 0.005 to 0.1 wt%.
[0018]
Next, a solution containing the catalyst metal salt and the co-catalyst metal salt is brought into contact with the porous magnesium oxide powder to be supported. As the contact method, an immersion method, a spray method, or any other appropriate method is used, but the immersion method is preferably used. The contact temperature ranges from room temperature to 80 ° C, preferably from 50 to 60 ° C. By contacting the porous magnesium oxide powder with a solution of the catalyst metal salt and the promoter metal salt, the catalyst metal salt and the promoter metal salt are impregnated into the powder and supported.
[0019]
In the thus obtained porous magnesium oxide powder supporting and containing the catalyst metal and the cocatalyst metal, the content of the catalyst metal relative to the powder is 1 to 20% by weight, preferably 5 to 10% by weight as the metal. The content of Mo as a promoter metal in the powder is 0.1 to 1.5 wt%, preferably 0.3 to 0.8 wt%, as a metal.
[0020]
The form of these metals contained in the porous magnesium oxide powder may be any form that generates carbon nanotubes and accelerates the reaction rate.In addition to metal forms, metal oxides and metal hydroxides may be used. Could be in shape. In the case of the metal form, it can be obtained by reducing the porous magnesium oxide powder containing the metal salt obtained as described above in a reducing atmosphere such as hydrogen. In the case of a metal oxide, it can be obtained by firing the porous magnesium oxide powder containing the metal salt obtained as described above.
[0021]
The shape of the catalyst body may be in the form of a porous magnesium oxide powder containing a catalyst metal and a co-catalyst metal, or may be in the form of a compact formed by molding a porous magnesium oxide powder containing a catalyst metal and a co-catalyst metal. Alternatively, a form in which a porous magnesium oxide powder containing a catalyst metal and a promoter metal is supported on a substrate such as Si or SiO 2 may be used. The catalyst body thus produced has a catalyst metal and a promoter metal supported on its surface.
[0022]
<(2) Synthesis of carbon nanotube>
Using the catalyst body manufactured as described above, carbon nanotubes are synthesized to produce a crude product. The catalyst body is placed in a reaction vessel such as an electric furnace, and while the carbon material, that is, the carbon source is passed through the catalyst body, the carbon source is thermally decomposed on the surface of the catalyst body to generate carbon nanotubes. As the carbon source, hydrocarbons such as methane, ethane, and propane, carbon monoxide, and lower alcohols such as methanol and ethanol are used.
[0023]
The thermal decomposition temperature of the carbon source, that is, the reaction temperature is from 400C to 1200C. In the present invention, carbon nanotubes can be manufactured even at a low temperature of 400 ° C. to 800 ° C. Since carbon nanotubes can be produced even at such a low temperature, equipment costs and energy consumption can be reduced, so that they are very useful in practice. Single-walled carbon nanotubes can be manufactured at temperatures between 700C and 900C.
[0024]
If the carbon source is methane, the flow rate, 2000～200000Hr -1 at a gas hourly space velocity (GHSV), preferably 5000~10000hr -1. When methane is thermally decomposed, argon or hydrogen can be mixed as a carrier into the gas. In addition, by adding an appropriate amount of a sulfur compound such as hydrogen sulfide or mercaptan to methane, straight carbon nanotubes can be formed on the catalyst.
[0025]
<(3) Purification treatment of crude product>
The carbon nanotubes grow and grow from the surface of the catalyst. For this reason, the carbon nanotube is obtained as a crude product containing a catalyst, that is, a substrate, and a catalyst metal (that is, a catalyst metal and a co-catalyst metal). Therefore, it is necessary to separate the carbon nanotube from the catalyst and perform a purification treatment. As the purification treatment, in the present invention, first, a crude product is pulverized and refined. The pulverization is performed so that the crude product is sufficiently fine. After pulverization, if necessary, it may be sieved to, for example, 200 mesh or less.
[0026]
Next, the finely pulverized product is kneaded with an acid solution or an alkali solution. The aqueous acid solution or aqueous alkali solution is preferably an aqueous solution of nitric acid or hydrochloric acid, an aqueous solution of sodium hydroxide, or a mixed solution thereof with acetone or a hydrogen peroxide solution.
[0027]
The kneading process is a very important process in the present invention and needs to be thoroughly performed. This kneading treatment may be performed in a mortar or the like if the finely pulverized material is a small amount, but when the finely pulverized material is a large amount, for example, a wet kneading mill, a continuous screw kneader, a high-speed fluid mixer, or the like. The mixture of the finely pulverized material and the acid solution or the alkali solution is subjected to thorough compression, shear and frictional action. Next, the kneaded liquid is subjected to a reflux treatment at a boiling point (in a boiling state). As a result, the substrate and the catalyst metal, which are impurities, can be sufficiently dissolved in an acid solution or an alkali solution, and the purification efficiency of the carbon nanotube can be dramatically improved.
[0028]
Further, prior to the reflux treatment, the kneading liquid is filtered, the solid is immersed again in an acid aqueous solution or an alkaline aqueous solution, and this is subjected to a reflux treatment, whereby the purification efficiency can be further ensured. Next, a high-purity carbon nanotube having a purity of 98 wt% or more can be obtained by filtering the solution subjected to the reflux treatment and separating a solid substance.
[0029]
【Example】
Hereinafter, the present invention will be described in more detail with reference to Examples, but it is needless to say that the present invention is not limited to Examples. In each of the examples and comparative examples, a porous magnesium oxide powder containing 0.01 wt% of impurities composed of alkali metals such as Na and K, sulfur and silica was used as a substrate.
[0030]
<< Example 1 >>
The powder 10.0g of porous magnesium oxide having a specific surface area of 130m 2 / g, iron nitrate [Fe (NO 3) 2 · 9H 2 O ] 2.2g molybdenum oxide acetylacetonate [(CH 3 COCHCOCH 3) [ 2MoO 2 ] 0.12 g was immersed in a solution obtained by dissolving in methanol (300 ml) for 30 minutes, and further dispersed by ultrasonic treatment for 3 hours. Next, this solution was dried under reduced pressure to obtain a dried product, that is, a powdery catalyst.
[0031]
The dried product (= catalyst) was placed in a boat crucible made of alumina and placed in an electric furnace. The reaction tube of the electric furnace was a quartz tube having an inner diameter of 3 cm and a length of 1 m. A heating area was 600 mm at the center in the longitudinal direction, and a dried product was placed at 500 mm at the center of the heating area. After the temperature of the electric furnace was raised to 800 ° C. in an argon atmosphere, a mixed gas of methane and argon was passed for 30 minutes.
[0032]
Observation of the obtained product with a scanning electron microscope (SEM, S-5000 manufactured by Hitachi, Ltd .; the same applies hereinafter) revealed that carbon nanotubes were formed on the surface of the catalyst body. FIG. 1 is a drawing of a photograph taken from the upper surface. As shown in FIG. 1, many fine catalyst particles are observed together with the carbon nanotubes.
[0033]
A change in weight of the crude product sample, that is, a part of the carbon nanotubes containing the catalyst, was measured with a thermobalance while heating to 800 ° C. in the air. FIG. 4 shows the change in weight. As shown as an unpurified sample in FIG. 4, 24 wt% of the total weight has disappeared up to about 400 ° C., but thereafter, the weight has hardly decreased. Although the weight reduction corresponds to the disappearance of the carbon nanotubes, it can be seen that the substrate and the catalyst metal remain in the sample at 76 wt%.
[0034]
<Purification treatment>
A sample composed of the carbon nanotubes containing the catalyst was finely pulverized and then sieved to 200 mesh or less. This sample was placed in a 3 mol / l nitric acid aqueous solution, and sufficiently kneaded in a mortar. After kneading, the mixture was filtered, immersed again in a 3 mol / l aqueous nitric acid solution, and refluxed at the boiling point for 1 hour. Then, the reflux liquid was filtered and the solid was dried to obtain a purified sample. The obtained sample was observed with a scanning electron microscope (SEM) and a transmission electron microscope (TEM, JEM-2000FX II manufactured by JEOL Ltd.). FIG. 2 is an SEM photograph, and FIG. 3 is a TEM photograph. As shown in FIGS. 2 and 3, it can be seen that the single-walled carbon nanotubes having a thickness (diameter) of about 1 to 2 nm are all in a bundle, and the catalyst remains very little. Thus, 700 mg of a single-walled carbon nanotube was obtained.
[0035]
In addition, while the temperature of the dried solid, that is, the purified sample was raised to 800 ° C. in the air, the change in weight was measured with a thermobalance. FIG. 4 shows the change in weight. As shown as a purified sample in FIG. 4, 99 wt% of the sample disappeared. Although the weight reduction corresponds to the loss of the single-walled carbon nanotube, it can be seen that only 1 wt% of the substrate and the catalyst metal remained in the sample. Thus, high-purity single-walled carbon nanotubes from which magnesium oxide as a base, iron as a catalyst metal, and molybdenum as a promoter metal were almost completely removed were obtained.
[0036]
<< Example 2 >>
Except that 10.0 g of porous magnesium oxide powder having a specific surface area of 30 m 2 / g was used as the substrate, Fe and Mo were supported, and a tube was grown on the surface of the catalyst body in the same manner as in Example 1. Purification was performed. As a result, 700 mg of single-walled carbon nanotubes were obtained. When the sample was heated to 800 ° C. in air and the change in weight was measured with a thermobalance, 99 wt% of the sample disappeared. The weight reduction corresponds to the loss of the single-walled carbon nanotube. Thus, a high-purity single-walled carbon nanotube from which magnesium oxide as a base, iron as a catalyst metal, and molybdenum as a promoter metal were almost completely removed was obtained.
[0037]
<< Example 3 >>
1.0 g of the same porous magnesium oxide powder used in Example 1 was obtained by dissolving 1.2 g of palladium nitrate [Pd (NO 3 ) 2 ] and 0.01 g of molybdenum acetylacetonate in methanol. It was immersed in the solution for 30 minutes and dispersed by sonication for further 3 hours. This was dried under reduced pressure, and a mixed gas of methane and Ar was passed for 30 minutes at 400 ° C. using the dried product (= catalyst) in the same manner as in Example 1. The crude product was pulverized and sieved in the same manner as in Example 1, then purified, filtered and dried.
[0038]
Observation of the obtained sample with a transmission electron microscope revealed that multi-walled carbon nanotubes having a thickness (diameter) of about 20 to 30 nm were generated. In addition, no residual substrate and catalyst metal could be confirmed. Thus, a high-purity multi-walled carbon nanotube from which impurities were completely removed was obtained.
[0039]
<< Comparative Example 1 >>
Single-walled carbon nanotubes were grown on the surface of the catalyst body in the same manner as in Example 1. The obtained crude product sample was immersed in a 3 mol / l aqueous solution of nitric acid, refluxed for 1 hour, filtered, and the solid content was dried. Here, the kneading process is not performed. When this sample was heated to 800 ° C. in air and the change in weight was measured by a thermobalance, 90 wt% disappeared from the sample. The weight reduction corresponds to the disappearance of the single-walled carbon nanotubes, but the base material and the catalyst metal remained in the product sample at 10 wt%, and high-purity single-walled carbon nanotubes could not be obtained.
[0040]
<Comparative Example 2>
In the same manner as in Example 1, Fe and Mo were supported, a tube was grown on the surface of the catalyst body, and a purification treatment was performed using 10.0 g of porous magnesium oxide powder having a specific surface area of 8 m 2 / g as a base. Was. Thereby, 10 mg of single-walled carbon nanotubes were obtained. Although the obtained single-walled carbon nanotubes were of high purity, the amount of such single-walled carbon nanotubes produced was only 10 mg, which was extremely lower than that of Example 1.
[0041]
【The invention's effect】
According to the present invention, a crude product containing a catalyst and a synthetic carbon nanotube is pulverized and refined, then sufficiently kneaded with an acid solution or an alkali solution, and the solution is subjected to a reflux treatment to obtain a high-purity product Carbon nanotubes can be manufactured efficiently. The production method of the present invention can be easily scaled up, thereby enabling multi-walled carbon nanotubes and single-walled carbon nanotubes to be mass-produced at low cost with energy saving.
[Brief description of the drawings]
FIG. 1 is a drawing showing an SEM photograph of a sample before pulverization obtained in Example 1; FIG. 2 is a drawing showing a SEM photograph of a sample after purification treatment obtained in Example 1; FIG. 3 is a drawing showing a TEM photograph of a sample after purification treatment obtained in Example 1; FIG. 4 is a diagram showing a sample obtained in Example 1 (before purification treatment according to the present invention and after purification treatment according to the present invention); FIG. 7 shows the results of measuring the weight change while raising the temperature of the sample in air to 800 ° C.
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