JP2004018309A - Method for manufacturing carbon nanotube - Google Patents
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- JP2004018309A JP2004018309A JP2002175103A JP2002175103A JP2004018309A JP 2004018309 A JP2004018309 A JP 2004018309A JP 2002175103 A JP2002175103 A JP 2002175103A JP 2002175103 A JP2002175103 A JP 2002175103A JP 2004018309 A JP2004018309 A JP 2004018309A
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【0001】
【発明の属する技術分野】
本発明は、カーボンナノチューブの製造方法に関し、より具体的にはCVD法(化学蒸着法)により、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、レーザー照射法及び熱CVD法が主に用いられている。このうちアーク放電やレーザー照射法によるカーボンナノチューブの合成に際しては、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、その生成量も少なく、また生成物中に黒鉛やアモルファスカーボンが混在し、さらには得られるカーボンナノチューブ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、熱CVD法によるカーボンナノチューブの合成では、炭素源となる炭化水素を熱分解させ、触媒金属を含有させた活性基体上に直接カーボンナノチューブを生成させる。その際、生成したカーボンナノチューブを基体から分離する必要があるが、この分離はきわめて困難であり、このため純度の高いカーボンナノチューブを得ることは難しい。
【0004】
ところで、そのような基体とカーボンナノチューブを分離する技術として、超臨界乾燥により調製した触媒を用いたとする報告がある(Chemical Physics Letters 322、321−326、2000)。これによれば、アルミニウムのアルコキシドに鉄塩及びモリブデン化合物を混合して得たゾルを、31℃、1050psi(≒7.24MPa)という条件で、溶媒(エタノール)を二酸化炭素で置換して乾燥させた金属担持アルミナを基体とした触媒を、炭素含有ガスと接触させることにより合成したカーボンナノチューブが、非常に高度な割合で基体と分離できたと報告されている。
【0005】
この報告では、該溶媒と二酸化炭素の溶媒置換条件を超臨界乾燥と呼んでいるが、この条件は正確には超臨界条件の範囲に入っていない。すなわち、二酸化炭素の超臨界条件は温度31.06℃以上、圧力7.3MPa以上であり、上記31℃、7.24MPaという条件では、二酸化炭素の超臨界条件として不十分である。このように、乾燥条件が不完全な臨界条件で行われているため、乾燥が十分に行われておらず、界面張力による収縮した細孔を持つ基体が得られる等の問題点がある。
【0006】
また、上記のように、アルミニウムのアルコキシドに金属塩及び金属化合物を混合するが、そのような条件では、金属が基体であるアルミナの構造にまで取り込まれてしまい、その構造内にAl−O−FeやAl−O−Moの鎖が生成する。すなわち、基体表面の金属量の制御が困難であり、金属を過剰に加える必要がある等の問題点が生じ得ること、またカーボンナノチューブが高分散の金属表面から成長することを考慮すると、金属がアルミナ等の基体の構造に取り込まれることは好ましくない。さらに、この報告での反応温度は800℃以上を必要としている。
【0007】
【発明が解決しようとする課題】
本発明は、従来技術における以上のような問題を解決するためになされたものであり、触媒として厳密な超臨界条件下の溶媒で処理し乾燥した多孔質体に金属を担持した触媒を用いることにより、より有効に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。
【0008】
【課題を解決するための手段】
本発明は、カーボンナノチューブの製造方法であって、触媒として有機溶媒を用いるゾル−ゲル法(sol−gel technique)により調製されたAl化合物またはSi化合物の湿潤ゲル(同じく調製されたAl化合物及びSi化合物を含む湿潤ゲルである場合を含む)を超臨界条件下の溶媒で処理し乾燥することにより製造した多孔質体に触媒金属を担持した触媒を用い、この触媒を十分な反応温度下で炭素原料気体の流通下におくことを特徴とするカーボンナノチューブの製造方法を提供する。ここで、前記湿潤ゲルの超臨界条件下の溶媒での処理、乾燥は、有機溶媒と超臨界条件下の溶媒が均一相を形成する条件下での超臨界条件で行うことが重要である。
【0009】
【発明の実施の形態】
本発明は、触媒を十分な反応温度下で炭素原料気体の流通下におき、該触媒上にカーボンナノチューブを成長させることによりカーボンナノチューブを製造する方法である。ここで、触媒として、高純度のアルミナ(Al2O3)、シリカ(SiO2)及びアルミナ−シリカ(Al2O3/SiO2)のうちのいずれか一つの多孔質体を担体とし、この担体に触媒金属とその触媒助剤金属を含有させたものを用いる。
【0010】
上記多孔質体の担体は高純度であることが重要である。アルミナの場合、Na、K等のアルカリ金属、硫黄及びシリカ等の不純物の含有量が少ない高純度のアルミナであることが重要である。具体的には、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総含有量が0.05wt%以下、好ましくは0.01wt%以下であることが重要である。本発明においては、この多孔質体を以下の(1)〜(3)の方法で製造する。
【0011】
(1) 有機溶媒で希釈した高純度のAlのアルコキシド、Siのアルコキシド、またはその両者を原料とし、これを加水分解し、前駆体としてのゾルを調製した後、室温で一定期間静置してゲル化させて湿潤ゲルとする。ここで、アルコキシドを構成するアルコキシ基としては式CnH2n+1OH(n=1〜4)で示されるアルコールに対応するアルコキシ基が用いられる。有機溶媒としては、好ましくは式CnH2n+1OH(n=1〜4)で示されるアルコール、すなわちメタノール、エタノール、n−プロピルアルコール、i−プルロピルアルコール、n−ブチルアルコール、i−ブチルアルコール、s−ブチルアルコール、t−ブチルアルコールのうちのいずれか一種が用いられる。この湿潤ゲルには加水分解により生成した水、アルコール類等が含まれ、これらがその細孔及び空隙中に残存している。
【0012】
(2) (1)で得た湿潤ゲルを超臨界条件下の溶媒で処理する。すなわち(1)で得た湿潤ゲルの細孔及び空隙中に残存する該有機溶媒、水、アルコール類等を超臨界条件下の溶媒で置換する。この溶媒としては、該有機溶媒の溶解度が高く、臨界温度の低い溶媒、例えば二酸化炭素やアンモニアなどが用いられるが、好ましく二酸化炭素を用いる。この溶媒を用い、この溶媒の超臨界条件下で該有機溶媒、加水分解で生成した水、アルコール類等を溶解して除去し、乾燥することにより、湿潤ゲル時の、すなわち湿潤ゲルの状態で有する、細孔径及び細孔容積の構造を有する乾燥ゲルを得る。
【0013】
(3) 次に、(2)で得られた乾燥ゲルを焼成する。これにより溶媒、例えば二酸化炭素は気体となり、初期の、すなわち湿潤ゲルの状態、これに続く乾燥ゲルの状態における細孔径及び細孔容積を保った多孔質体が得られる。Alのアルコキシドを原料とするとアルミナ(Al2O3)の多孔質体が得られ、Siのアルコキシドを原料とするとシリカ(SiO2)の多孔質体が得られ、Alのアルコキシド及びSiのアルコキシドの両者の混合物を原料とするとアルミナ−シリカ(Al2O3/SiO2)の多孔質体が得られる。これらの多孔質体は、通常粉体状で得られるが、粉体状のほか、ペレット状、板体状などの各種の形状とすることができる。
【0014】
次に、上記多孔質体(担体)に対して、触媒金属と触媒助剤金属を含有させて触媒を製造する。すなわち、該多孔質体を触媒金属と触媒助剤金属が溶解して含まれる溶液と接触させ、該多孔質体にその触媒金属と触媒助剤金属を含有させる。触媒金属としては、カーボンナノチューブの成長に活性を有する金属、具体的にはPd、Cr、Fe、Co、Ni及びCuから選ばれた少なくとも1種類以上の金属、好ましくはPd、Fe、Co及びNiから選ばれた少なくとも1種類以上の金属が用いられる。また、触媒助剤金属としてはMoが用いられる。
【0015】
これらの触媒金属及び触媒助剤金属は可溶性塩、例えば硝酸塩という形で用いられる。これを溶媒に溶解するが、この溶媒としては、触媒金属及び触媒助剤金属を可溶性塩の形で溶解できる物質、例えば水、低級アルコール等の有機物、あるいは水と水溶性有機物との混合物を用いるが、好ましくは水を用いる。この触媒金属及びその触媒助剤金属を溶解して含む溶液において、その金属の濃度はその飽和濃度以下であるが、通常は0.01〜0.05wt%、好ましくは0.005〜0.01wt%である。
【0016】
前記多孔質体を上記金属塩溶液と接触させる、その接触法としては、浸漬法やスプレー法などが適用できるが、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℃までの範囲、好ましくは50〜60℃の範囲である。多孔質体と金属塩溶液との接触により、金属塩溶液は多孔質体に含浸され、該多孔質体に触媒金属と触媒助剤金属が含有される。
【0017】
この触媒金属及び触媒助剤金属を含有する多孔質体において、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、触媒助剤金属としてのMoの含有量は、金属Moとして0.1〜1.5wt%、好ましくは0.3〜0.8wt%である。多孔質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の生成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、金属水酸化物の形であることができる。金属の形とする場合、前記のようにして得られた金属塩含有多孔質体を水素等の還元雰囲気で還元すればよい。また、金属酸化物の形とする場合、前記のようにして得られた金属塩含有多孔質体を焼成すればよい。
【0018】
本発明において、触媒金属及び触媒助剤金属を含有する触媒を製造するに際し、その好ましい態様としては、(1)先ずそれら金属を含有する多孔質体である粉体状アルミナをつくり、次に、この多孔質体を所要形状に成形する方法と、(2)先ずそれら金属を含有する多孔質体である粉体状アルミナをつくり、シリコン(Si)やシリカ(SiO2)等の基板に塗布する方法がある。この場合、粉体状アルミナの比表面積は100〜500cm2/g、好ましくは300〜500cm2/gである。
【0019】
本発明によりカーボンナノチューブを製造するには、以上のようにして得た、アルミナ、シリカまたはシリカ/アルミナからなる多孔質体の表面に前記触媒金属と前記触媒助剤金属を担持した触媒、すなわち、有機溶媒を用いるゾル−ゲル法により調製されたAl化合物またはSi化合物の湿潤ゲル、あるいは両化合物の湿潤ゲルを超臨界条件の溶媒で処理して該有機溶媒を溶解除去し、乾燥して製造した多孔質体に金属を担持した触媒に、炭素原料気体を流通させながら該触媒表面で熱分解させる。炭素原料としては、メタン等の炭化水素、一酸化炭素またはメタノール等のアルコール類が用いられる。
【0020】
この熱分解温度、すなわち反応温度は400〜1200℃の範囲であるが、本発明においては、特に400〜800℃という低い温度でカーボンナノチューブを製造できる。この点は本発明において重要な効果であり大きな利点である。特に単層カーボンナノチューブは700〜900℃の温度で製造可能である。
【0021】
炭素原料気体がメタンガスの場合、その流通速度は、ガス空間速度(GHSV)で2,000〜200,000hr−1、好ましくは5,000〜10,000hr−1である。メタンガスを熱分解する場合、その気体中にはアルゴンガスや水素ガスをキャリアーとして混入することができる。また、メタンガスには、硫化水素やメルカプタン等のイオウ化合物を適量加えることができる。これにより、触媒上に真っ直ぐなカーボンナノチューブを生成させることができる。
【0022】
本発明において、高純度のカーボンナノチューブを製造するには、炭素原料ガス、例えばメタンガスで接触分解させて得た試料、すなわち粗生成物を、酸もしくはアルカリ水溶液に浸して還流する。そして、ここで分解された不純物をフィルタリングにより分離する。上記酸もしくはアルカリ水溶液は、硝酸または水酸化ナトリウム水溶液、もしくはそれら硝酸または水酸化ナトリウムと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望ましい。
【0023】
【実施例】
以下、実施例に基づき本発明をさらに詳しく説明するが、本発明が実施例により限定されないことはもちろんである。
【0024】
〈実施例1〉
(t−C4H9O)3Al(アルミニウムトリターシャリーブトキシド)をi−C3H7OH(イソプロピルアルコール)に添加し、80℃で2時間還流した。その後、室温まで冷却した後、希薄アンモニア水を加え、撹拌しながら加水分解を行ってゾルを調製し、室温で10時間保持した。得られた試料は湿潤ゲルの状態であった。この試料を高圧容器に封入し、二酸化炭素を導入して13.2MPaまで昇圧させた。温度を室温から60℃まで昇温した後、2時間保持した。引き続き、ポンプにより2mL/minの速度で二酸化炭素を3時間送給し、容器内のi−C3H7OH、t−C4H9OH(ターシャリーブチルアルコール)を抽出して、完全に二酸化炭素のみの状態とした。次いで、1atm/minの速度で大気圧まで減圧した。
【0025】
その後、室温まで冷却させ、高圧容器から生成アルミナ粉体を取り出した。この粉体を、硝酸鉄〔Fe(NO3)2・9H2O〕0.2gと酸化モリブデンアセチルアセトナート〔(CH3COCHCOCH3)2MoO2〕0.01gをメタノール35mLに溶解させて得た溶液中に30分間浸した後、3時間、超音波処理により分散させた。この溶液を減圧乾燥した後、電気炉に挿入した。電気炉中、アルゴン雰囲気下で700℃まで昇温した後、メタンとアルゴンの混合ガスを30分間流通させた。触媒表面を、電子顕微鏡(日立製作所社製:S・5000)により観察したところ、触媒表面にはチューブ状の生成物が生成していることがわかった。
【0026】
上記生成物を1mol/Lの硝酸水溶液中で2時間還流した後、濾過し、乾燥した。こうして得られた試料を透過型電子顕微鏡(日本電子社製:JEM・2000FX II)で観察したところ、太さ(直径)がいずれも1nm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いることがわかった。触媒の残存は観察されなかった。こうして、担体であるアルミナや触媒金属である鉄、モリブデンといった不純物が完全に除去された高純度のナノチューブを得ることができた。
【0027】
〈実施例2〉
実施例1で合成した前記アルミナ粉体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〔(CH3COCHCOCH3)2MoO2〕0.01gをメタノールに溶解させて得た溶液中に30分間浸した後、3時間、超音波処理により分散させた。これを減圧乾燥して触媒を得た。この触媒を用いて、800℃で実施例1と同様に、メタンとアルゴンの混合ガスを30分間流通させた。得られた生成物について、実施例1と同様に硝酸処理を行い、乾燥させ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。こうして、担体であるアルミナやPd、Moといった触媒金属が完全に除去された高純度なカーボンナノチューブを得ることができた。
【0028】
【発明の効果】
本発明によれば、湿潤ゲルを超臨界乾燥させた多孔質担体に金属を担持させた触媒を用いることにより、400〜1200℃という反応温度で、非常に簡便なCVD法により、カーボンナノチューブを製造することができる。本発明によれば、特に400〜800℃という低い反応温度でカーボンナノチューブを合成できることから、省エネルギーの点及び装置コストの点でも、非常に有利な方法である。
【0029】
また、本発明によれば、生成した触媒を含むカーボンナノチューブを酸もしくはアルカリ水溶液中で還流することにより、触媒を構成する成分である担体及び触媒金属を容易に除去することができ、高純度のカーボンナノチューブを合成することができる。この製造方法をスケールアップすることにより、カーボンナノチューブの大量合成を可能とすることができる。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing carbon nanotubes, and more specifically, to a method for producing carbon nanotubes by a CVD method (chemical vapor deposition method).
[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, when synthesizing carbon nanotubes by arc discharge or laser irradiation, expensive and dangerous equipment such as an exhaust device and a high-voltage large-current power supply are required, and the amount of the generated carbon nanotubes is small. Graphite and amorphous carbon are mixed, and the resulting carbon nanotubes also have problems such as large variations in diameter and length.
[0003]
In the synthesis of carbon nanotubes by a thermal CVD method, hydrocarbons serving as a carbon source are thermally decomposed to form carbon nanotubes directly on an active substrate containing a catalytic metal. At this time, it is necessary to separate the generated carbon nanotubes from the substrate, but this separation is extremely difficult, and thus it is difficult to obtain high-purity carbon nanotubes.
[0004]
By the way, there is a report that a catalyst prepared by supercritical drying is used as a technique for separating the carbon nanotube from the substrate (Chemical Physics Letters 322, 321-326, 2000). According to this, a sol obtained by mixing an iron salt and a molybdenum compound with an alkoxide of aluminum is dried at 31 ° C. under a condition of 1050 psi (≒ 7.24 MPa) by replacing a solvent (ethanol) with carbon dioxide. It is reported that carbon nanotubes synthesized by contacting a metal-supported alumina-based catalyst with a carbon-containing gas could be separated from the substrate at a very high rate.
[0005]
In this report, the solvent replacement condition between the solvent and carbon dioxide is called supercritical drying, but this condition is not exactly within the range of supercritical conditions. That is, the supercritical condition of carbon dioxide is a temperature of 31.06 ° C. or more and a pressure of 7.3 MPa or more, and the condition of 31 ° C. and 7.24 MPa is insufficient as the supercritical condition of carbon dioxide. As described above, since the drying conditions are performed under imperfect critical conditions, the drying is not sufficiently performed, and there is a problem that a substrate having pores contracted due to interfacial tension is obtained.
[0006]
In addition, as described above, a metal salt and a metal compound are mixed with an alkoxide of aluminum. However, under such conditions, the metal is incorporated into the structure of alumina as a base, and Al-O- is included in the structure. A chain of Fe or Al-O-Mo is generated. That is, considering that it is difficult to control the amount of metal on the surface of the substrate and that there is a need to add an excessive amount of metal, and that carbon nanotubes grow from a highly dispersed metal surface, It is not preferable to be incorporated into the structure of the substrate such as alumina. Furthermore, the reaction temperature in this report requires 800 ° C. or higher.
[0007]
[Problems to be solved by the invention]
The present invention has been made in order to solve the above-described problems in the prior art, and uses a catalyst in which a metal is supported on a porous body that has been treated with a solvent under strict supercritical conditions and dried as a catalyst. Accordingly, an object of the present invention is to provide a method for producing carbon nanotubes more effectively.
[0008]
[Means for Solving the Problems]
The present invention relates to a method for producing carbon nanotubes, comprising a wet gel of an Al compound or a Si compound prepared by a sol-gel technique using an organic solvent as a catalyst (an Al compound and a Si compound prepared similarly). (Including a wet gel containing a compound) with a solvent under supercritical conditions, followed by drying, using a catalyst in which a catalyst metal is supported on a porous body produced under a sufficient reaction temperature. Disclosed is a method for producing carbon nanotubes, which is carried out under the flow of a raw material gas. Here, it is important that the treatment and drying of the wet gel with a solvent under supercritical conditions are performed under supercritical conditions under conditions in which the organic solvent and the solvent under supercritical conditions form a homogeneous phase.
[0009]
BEST MODE FOR CARRYING OUT THE INVENTION
The present invention is a method for producing carbon nanotubes by placing a catalyst under a flow of a carbon source gas at a sufficient reaction temperature and growing the carbon nanotubes on the catalyst. Here, as a catalyst, a porous body of any one of high-purity alumina (Al 2 O 3 ), silica (SiO 2 ) and alumina-silica (Al 2 O 3 / SiO 2 ) is used as a carrier, and A carrier containing a catalyst metal and a catalyst auxiliary metal is used.
[0010]
It is important that the porous carrier be of high purity. In the case of alumina, it is important that the content of impurities such as alkali metals such as Na and K, sulfur, and silica be small and high-purity alumina. Specifically, it is important that the total content of impurities composed of alkali metals such as Na and K, sulfur, and silica be 0.05 wt% or less, preferably 0.01 wt% or less. In the present invention, this porous body is manufactured by the following methods (1) to (3).
[0011]
(1) High-purity Al alkoxide and / or Si alkoxide diluted with an organic solvent are used as raw materials, hydrolyzed to prepare a sol as a precursor, and then left at room temperature for a certain period of time to prepare a sol. Gel to a wet gel. Here, the alkoxy group constituting the alkoxide alkoxy groups corresponding to the alcohol of formula C n H 2n + 1 OH ( n = 1~4) are used. As the organic solvent, preferably an alcohol of the formula C n H 2n + 1 OH ( n = 1~4), i.e. methanol, ethanol, n- propyl alcohol, i- pluronic pills alcohol, n- butyl alcohol, i- butyl Any one of alcohol, s-butyl alcohol, and t-butyl alcohol is used. The wet gel contains water, alcohols, and the like generated by hydrolysis, and these remain in the pores and voids.
[0012]
(2) The wet gel obtained in (1) is treated with a solvent under supercritical conditions. That is, the organic solvent, water, alcohols and the like remaining in the pores and voids of the wet gel obtained in (1) are replaced with a solvent under supercritical conditions. As this solvent, a solvent having a high solubility of the organic solvent and a low critical temperature, for example, carbon dioxide or ammonia is used, but carbon dioxide is preferably used. Using this solvent, the organic solvent, water produced by hydrolysis, alcohols, etc. are dissolved and removed under supercritical conditions of this solvent, and dried to obtain a wet gel, that is, a wet gel state. A dried gel having a structure of pore diameter and pore volume is obtained.
[0013]
(3) Next, the dried gel obtained in (2) is fired. As a result, the solvent, for example, carbon dioxide becomes a gas, and a porous body is obtained in which the pore diameter and the pore volume in the initial state, that is, in the state of the wet gel, and subsequently in the state of the dry gel are maintained. When an Al alkoxide is used as a raw material, a porous body of alumina (Al 2 O 3 ) is obtained. When a Si alkoxide is used as a raw material, a porous body of silica (SiO 2 ) is obtained. When a mixture of both is used as a raw material, a porous body of alumina-silica (Al 2 O 3 / SiO 2 ) is obtained. These porous bodies are usually obtained in a powder form, but may be in various forms such as a pellet form and a plate form in addition to the powder form.
[0014]
Next, a catalyst is produced by adding a catalyst metal and a catalyst auxiliary metal to the porous body (carrier). That is, the porous body is brought into contact with a solution containing the catalyst metal and the catalyst auxiliary metal dissolved therein, and the porous body contains the catalyst metal and the catalyst auxiliary metal. As the catalytic metal, a metal having an activity for growing carbon nanotubes, specifically, at least one or more metals selected from Pd, Cr, Fe, Co, Ni, and Cu, preferably Pd, Fe, Co, and Ni At least one kind of metal selected from the following is used. In addition, Mo is used as the catalyst auxiliary metal.
[0015]
These catalyst metals and catalyst auxiliary metals are used in the form of soluble salts, for example, nitrates. This is dissolved in a solvent, and as the solvent, a substance capable of dissolving the catalyst metal and the catalyst auxiliary metal in the form of a soluble salt, for example, water, an organic substance such as a lower alcohol, or a mixture of water and a water-soluble organic substance is used. However, water is preferably used. In the solution containing the catalyst metal and the catalyst auxiliary metal dissolved therein, the concentration of the metal is not more than the saturation concentration, but is usually 0.01 to 0.05 wt%, preferably 0.005 to 0.01 wt%. %.
[0016]
The porous body is brought into contact with the metal salt solution. As a contact method, an immersion method, a spray method, or the like can be applied, but an immersion method is preferably used. The contact temperature is in the range from room temperature to 80 ° C, preferably in the range from 50 to 60 ° C. By contact between the porous body and the metal salt solution, the metal salt solution is impregnated into the porous body, and the porous body contains a catalyst metal and a catalyst auxiliary metal.
[0017]
In the porous body containing the catalyst metal and the catalyst auxiliary metal, the content of the catalyst metal is 1 to 20% by weight, preferably 5 to 10% by weight as the metal. Further, the content of Mo as a catalyst aid metal is 0.1 to 1.5 wt%, preferably 0.3 to 0.8 wt%, as metal Mo. The form of the metal contained in the porous body may be any form that promotes the formation of carbon nanotubes, and may be in the form of a metal oxide, a metal hydroxide or the like in addition to the form of the metal. In the case of a metal form, the metal salt-containing porous body obtained as described above may be reduced in a reducing atmosphere such as hydrogen. In the case of forming a metal oxide, the metal salt-containing porous body obtained as described above may be fired.
[0018]
In the present invention, in producing a catalyst containing a catalyst metal and a catalyst auxiliary metal, preferred embodiments thereof are as follows: (1) First, a powdery alumina which is a porous body containing those metals is produced, A method of forming the porous body into a required shape, and (2) first, a powdery alumina which is a porous body containing the metal is prepared and applied to a substrate such as silicon (Si) or silica (SiO 2 ). There is a way. In this case, the specific surface area of the powdery alumina is 100 to 500 cm 2 / g, preferably 300 to 500 cm 2 / g.
[0019]
In order to produce carbon nanotubes according to the present invention, the catalyst obtained by carrying the catalyst metal and the catalyst auxiliary metal on the surface of a porous body made of alumina, silica or silica / alumina obtained as described above, A wet gel of an Al compound or a Si compound prepared by a sol-gel method using an organic solvent, or a wet gel of both compounds is treated with a solvent under supercritical conditions to dissolve and remove the organic solvent, and then dried to manufacture. The carbon material gas is allowed to thermally decompose on the surface of the catalyst while flowing the carbon raw material gas through the catalyst supporting the metal on the porous body. As the carbon raw material, hydrocarbons such as methane, carbon monoxide, or alcohols such as methanol are used.
[0020]
The pyrolysis temperature, that is, the reaction temperature is in the range of 400 to 1200 ° C. In the present invention, carbon nanotubes can be produced particularly at a low temperature of 400 to 800 ° C. This is an important effect and a great advantage in the present invention. In particular, single-walled carbon nanotubes can be manufactured at a temperature of 700 to 900C.
[0021]
When the carbon raw material gas is methane gas, its flow velocity is 2,000 to 200,000 hr -1 , preferably 5,000 to 10,000 hr -1 in gas hourly space velocity (GHSV). When methane gas is thermally decomposed, argon gas or hydrogen gas can be mixed as a carrier into the gas. Further, an appropriate amount of a sulfur compound such as hydrogen sulfide or mercaptan can be added to the methane gas. Thus, straight carbon nanotubes can be generated on the catalyst.
[0022]
In the present invention, in order to produce high-purity carbon nanotubes, a sample obtained by catalytic decomposition with a carbon raw material gas, for example, methane gas, that is, a crude product is immersed in an acid or alkali aqueous solution and refluxed. Then, the impurities decomposed here are separated by filtering. The acid or alkali aqueous solution is desirably a nitric acid or sodium hydroxide aqueous solution, or a mixed solution of the nitric acid or sodium hydroxide and acetone or hydrogen peroxide.
[0023]
【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.
[0024]
<Example 1>
Was added (t-C 4 H 9 O ) 3 Al (aluminum tri-tertiary butoxide) in i-C 3 H 7 OH (isopropyl alcohol) and refluxed for 2 hours at 80 ° C.. Then, after cooling to room temperature, diluted ammonia water was added, and hydrolysis was performed with stirring to prepare a sol, which was kept at room temperature for 10 hours. The obtained sample was in a wet gel state. This sample was sealed in a high-pressure container, and the pressure was increased to 13.2 MPa by introducing carbon dioxide. After the temperature was raised from room temperature to 60 ° C., it was maintained for 2 hours. Subsequently, carbon dioxide was fed by a pump at a rate of 2 mL / min for 3 hours, and i-C 3 H 7 OH and t-C 4 H 9 OH (tertiary butyl alcohol) in the container were completely extracted. It was in a state of only carbon dioxide. Next, the pressure was reduced to atmospheric pressure at a rate of 1 atm / min.
[0025]
Then, it was cooled to room temperature, and the produced alumina powder was taken out of the high-pressure vessel. This powder, iron nitrate [Fe (NO 3) 2 · 9H 2 O ] 0.2g molybdenum oxide acetylacetonate and [(CH 3 COCHCOCH 3) 2 MoO 2 ] 0.01g dissolved in methanol 35mL obtained After immersion in the solution for 30 minutes, it was dispersed by sonication for 3 hours. After this solution was dried under reduced pressure, it was inserted into an electric furnace. After the temperature was raised to 700 ° C. in an electric furnace under an argon atmosphere, a mixed gas of methane and argon was passed for 30 minutes. When the surface of the catalyst was observed with an electron microscope (S.5000, manufactured by Hitachi, Ltd.), it was found that a tubular product was formed on the surface of the catalyst.
[0026]
The above product was refluxed in a 1 mol / L aqueous nitric acid solution for 2 hours, then filtered and dried. Observation of the sample thus obtained with a transmission electron microscope (JEM 2000FX II, manufactured by JEOL Ltd.) revealed that single-walled carbon nanotubes each having a thickness (diameter) of about 1 nm were bundled. all right. No residual catalyst was observed. Thus, a high-purity nanotube from which impurities such as alumina as a carrier and iron and molybdenum as catalyst metals were completely removed was obtained.
[0027]
<Example 2>
1.0 g of the alumina powder synthesized in Example 1 was combined with 1.2 g of palladium nitrate [Pd (NO 3 ) 2 ] and 0.01 g of molybdenum acetylacetonate [(CH 3 COCHCOCH 3 ) 2 MoO 2 ] in methanol. Immersed in a solution obtained by dissolving in water for 30 minutes, and then dispersed by ultrasonication for 3 hours. This was dried under reduced pressure to obtain a catalyst. Using this catalyst, a mixed gas of methane and argon was passed at 800 ° C. for 30 minutes in the same manner as in Example 1. The obtained product was subjected to nitric acid treatment in the same manner as in Example 1, and the dried sample was observed with a transmission electron microscope. As a result, multi-walled carbon nanotubes having a thickness (diameter) of about 20 to 30 nm were generated. I knew it was there. Thus, high-purity carbon nanotubes from which the catalytic metals such as alumina, Pd, and Mo as carriers were completely removed were obtained.
[0028]
【The invention's effect】
According to the present invention, carbon nanotubes are produced by a very simple CVD method at a reaction temperature of 400 to 1200 ° C. by using a catalyst in which a metal is supported on a porous carrier obtained by supercritically drying a wet gel. can do. According to the present invention, carbon nanotubes can be synthesized particularly at a low reaction temperature of 400 to 800 ° C., which is a very advantageous method in terms of energy saving and apparatus cost.
[0029]
According to the present invention, the carbon nanotubes containing the generated catalyst are refluxed in an acid or alkali aqueous solution, so that the carrier and the catalyst metal, which are components of the catalyst, can be easily removed, and a high-purity carbon nanotube can be obtained. Carbon nanotubes can be synthesized. By scaling up this production method, mass synthesis of carbon nanotubes can be made possible.
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