Fe-CATALYZED CARBON NANOTUBE FORMATION

Pergamon

Carbon Vol. 34, No. 10, pp. 1249%1257,1996 Copyright 0 1996 Elsevier Science Ltd

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Fe-CATALYZED CARBON NANOTUBE FORMATION

K. HERNADI,~ A. FONSECA,~** J. B.NAGY,~ D. BERNAERTS~ and A. A. LUCAY “Institute for Studies in Interface Sciences, Facultds Universitaires Notre-Dame de la Paix,

61 rue de Bruxelles, B-5000, Namur, Belgium bEMAT, University of Antwerp (RUCA), Groenenborgerlaan 171, B-2020, Antwerp, Belgium

(Received 8 February 1996; accepted in revised form 14 May 1996)

Abstract-The catalytic production of carbon nanotubes was investigated using various iron catalysts. Catalyst samples were made by different preparation methods in order to improve both the quality and the quantity of as-prepared carbon nanotubes. The catalysts were tested in the decomposition of different hydrocarbons in the temperature range 650-800°C using either fixed bed flow or fluidized bed reactor. The quality of the products was characterized by means of transmission electron microscopy. By using Fe/silica, the highest activity ever observed in catalytic nanotube formation can be reached. Copyright 0 1996 Elsevier Science Ltd

Key Words-Carbon nanotubes, catalytic synthesis, Fe-catalyst, iron carbide.

1. INTRODUCTION

The recent discovery of fullerenes and hollow gra- phitic tubules of nanometer dimension opened a new chapter in carbon chemistry. Because of their esti- mated chemical and physical properties [l-4], spec- ulations about the possible applications of carbon nanotubes have been published [S-7]. For the synthesis of carbon nanotubes several methods have been proposed. Although the arc-discharge method has been developed for Cm synthesis, the growth of fullerene tubes was observed on its cathode [8-121. Other methods such as plasma decomposition of hydrocarbons [ 13,141 and co-evaporating catalyst during a carbon arc-discharge [ 15-171 improved the technique of nanotube synthesis. Recently, Ivanov et al. [ 18,191 optimized the catalytic synthesis towards the production of turbostratic carbon nanotubes having fullerene-like diameters.

From the early 7Os, the catalytic role of iron and other transition metals in the formation of filamentous carbon has been widely investigated [20-2.51. Although, in catalysis research, much attention was paid to the prevention of deactivation of catalysts, many papers dealt with the mechanism of carbon filament formation too.

Already in the 7Os, the structure and morphology of carbon microstructures formed over iron and nickel foils in pyrolysis of hydrocarbons were studied by Baird et al. [ 261. Boellaard et al. [ 271 also studied the morphology of filamentous carbon formed by carburization over supported Ni and Fe catalysts. Baker et al. investigated the formation of filamentous carbon in the decomposition of acetylene over Ni-Fe surfaces, using various oxide additives [28]. In a subsequent paper [29] they reported that Fe0 has at least one order of magnitude higher activity in

*Author to whom correspondence should be addressed.

1249

filamentous carbon formation than Fe. They also proved that Fe& is not active in filament formation. Recently, the same authors established the effect of CO on the decomposition of ethylene over an iron catalyst [30]. Sacco et al. [31] presumed a growth mechanism and they found that Fe& could act as an active phase. Audier and Coulon suggested a mechanism for the growth of carbon tubes in which the bulk diffusion of the carbon through the metal particles appeared as the rate-limiting step [32]. Alstrup also developed a new model explaining carbon filament growth over supported transition metal catalysts [33]. Very recently, Fonseca et al.

suggested a growth mechanism leading to carbon tubules and tubule connections on a catalyst particle at a molecular level [34].

Since many literature data suggested its high activity in filamentous carbon formation and Co/silica has already been found to be effective in the synthesis of nanotubes [l&19], we started to investigate the catalytic effect of iron in carbon nanotube formation by using pyrolytic decomposition of different hydrocarbons.

2. EXPERIMENTAL

Carbon nanotubes were synthesized in the catalytic decomposition of acetylene, ethylene and propylene in the temperature range 650-800°C over supported Fe catalysts. Catalysts were made by different methods (impregnation, ion-adsorption precipitation) and by using various supports. For the preparation of the catalysts, basic iron acetate (Fe(OH)(CH,COO),, ICN Pharmaceuticals, Inc.) was used except for one sample where FeCI, (RPL, UCB) was applied as precursor. The following materials were used as catalyst support: graphite flakes (natural, 99.5%; Johnson Matthey GmbH), silica

1250

Support

K. HERNADI et al.

Table 1. Formation of carbon deposit over different iron catalysts

Graphite Y ZSM-5 Silica Silica Silica Silica

Method impr. impr. impr. impr. ion-ads. ion-ads. (pH = 7) ion-ads. (FeCl,) Carbon yield (%) 3.4 41.6 21.3 29.5 116.6 160.0 184.0

30 minutes of synthesis at 700°C.

(“SILICA GEL 60”, Merck, particle size: 15-40 pm), NaY (Union Carbide) and ZSM-5 (NESTE, Finland). For the impregnation method, the amount of iron acetate was set to 2.5 wt% metal/support. The concen- tration of the initial solution was chosen five times higher for the ion-adsorption precipitation method. The Fe/graphite sample was treated at 1200°C for 5 hours in Hz atmosphere. The other samples were calcined at 450°C for 4.5 hours. In the case of certain samples prepared by ion-adsorption precipitation, the pH of the solution was set to 7 before filtration. A more detailed description of catalyst preparation is given elsewhere [ 35,361.

Reactions were carried out-on a catalyst amount of approx. 30 mg-in two different types of reactor: fixed bed flow reactor and fluidized bed reactor. The fixed bed flow reactor is composed of a quartz boat containing the catalyst placed in an horizontal quartz tube (14 mm in diameter), while the fluidized bed reactor is constituted of a vertical quartz tube (6.4 mm in diameter) containing a sintered glass in the middle supporting the catalyst. In the latter reactor the gas flow enters from the bottom and is adapted so as to make the catalyst floating. The gas feeds applied during the reactions were the following: 8 ml/min acetylene (or other hydrocarbon) (Alphagas, 99.5% purity) in nitrogen flow of 75 ml/min in the fixed bed flow reactor, and the same amount of acetylene in nitrogen flow of 30 ml/min in the fluidized bed reactor.

For calculation of carbon yield (deposited carbon during the reaction), the following formula was used:

carbon yield (%) = (m,,, - m,,,)/m,,, 100,

where meat is the initial amount of the catalyst (before reaction) and mtot is the total weight of the product after reaction.

The nature of the carbon deposit on the catalyst surface was characterized by transmission electron microscopy (Philips CM 20 and JEOL 200 CX, 200 kV). For the sample preparation a Rh-Cu grid and the glue technique were used [36].

3. RESULTS AND DISCUSSION

3.1 Catalytic role of iron carbide According to literature data [ 27,30,37739], trans-

ition metal carbide was considered to be responsible for deposition of carbon and filament formation. Because of the previous investigations just referred to, unsupported Fe& and iron carbide produced on the surface of different supports such as graphite

flakes and silica, were tested in the catalytic decomposition of acetylene. For the preparation of iron carbide, several methods are suitable. Like the majority of metals, iron reacts with carbon at high temperatures (below its melting point) in a reducing atmosphere [40]. This observation was applied for the preparation of catalyst Fe/graphite. Production of carbides can also be carried out in the reduction of oxides by carbon-containing gases. This method is mainly used to obtain the carbides of iron, nickel and cobalt [40]. A sample of Fe/silica was prepared in this way.

Over unsupported Fe&, no carbon deposit was observed either by weighing or by electron microscopy. A Fe/graphite catalyst sample showed very low activity; the carbon yield was found to be 3.4%. According to the electron microscopic observations, carbon nanotubes and fibers formed on the surface as illustrated in Fig. 1. However, some regions contain carbon fibers hollow in the middle, but their structure is not well turbostratic, and their appearance is “tortuous” as can be seen even on the low magnification image. Mostly fibers and not turbostratic tubes with different diameter can be seen on the image. Both the quality of the nanotubes and the quantity of the carbon yield (49.5%) were much higher in the case of Fe/silica. Further investigations proved that previous reduction of iron oxide is unnecessary (as was supposed by Baker et al. [29]); acetylene is able to reduce the catalyst to the required extent under reaction conditions. On the probable analogy with Co/silica [35], the most active catalyst is only reduced to about 20%, so iron carbide alone cannot be responsible for the activity. This is in accordance with our results obtained over unsupported Fe&. To sum up, it is sufficient to form well-dispersed iron oxide on the surface of the support from which catalytic centers active in carbon nanotube formation can develop during the decomposition of acetylene.

3.2 Catalyst support andpreparation method After developing the necessary pretreatment of the

catalyst, different supports and preparation methods were used in order to find catalyst samples which are more effective in carbon nanotube formation. It follows from the foregoing that graphite flakes did not prove to be a good catalyst support so investigations were continued with silica and different types of zeolites. As different preparation methods, ion- adsorption precipitation on silica gel, ion-exchange on zeolites and impregnation on both supports (using Fe acetate or FeCl, as starting material) were applied.

Fe-catalyzed carbon nanotube formation 1251

Fig. 1. Carbon nanostructures formed in the decomposition of acetylene at 700°C over Fe/graphite catalyst.

Zeolite-supported samples prepared by ion- exchange were found to be inactive in the formation of carbon nanotubes, while the samples made by impregnation showed higher activity. The Fe/Y catalyst gave better results than Fe/ZSM-5 as it can be seen from data summarized in Table 1. The quality of carbon nanotubes formed over Fe/Y prepared by impregnation is illustrated in Fig. 2. Using Fe/ZSM-5, not only the decrease of the activity (about half that of Fe/Y), but amorphous carbon and carbon fibers were also observed. The structure of the zeolite support may modify the effectiveness of the catalyst

500 nm I I

particles and this can explain the differences observed between Fe/Y and Fe/ZSM-5 samples.

Applying silica support led to even better results. Although Fe/silica prepared by the impregnation method showed some inhomogeneity in the quality of carbon deposit (besides carbon nanotubes it also contained some amorphous carbon and thick tubes having diameters larger than 50 nm) and produced a carbon yield of lower value (Table l), using the ion- adsorption precipitation method the properties of the catalysts improved remarkably. While the value of the carbon yield increased above lOO%, the tubes

Fig. 2. Carbon nanotubes formed in the decomposition of acetylene at 700°C over Fe/Y catalyst.

1252 K. HERNADI et al.

became homogeneous in diameter, and less amorphous carbon was observed. According to the electron microscopic observations, every catalyst particle is covered by carbon nanotubes of regular diameter. The average value of the outer diameter is approxi- mately lo-20 nm and that of the inner diameter is 5-8 nm. Carbon nanotubes produced on Fe/silica (ion-adsorption precipitation) can be seen in Fig. 3. As can be seen in high resolution images (Fig. 3(b)), the structure of their inner wall is turbostratic [lS] for several layers-up to approx. one half of the tube wall-and that of the outer wall is mainly amorphous carbon. According to these images, thicker tubes also exist and their inner wall structure is turbostratic, too. The macroscopic appearance of this material can be described as follows: the product has a “spongy” texture and it is very fluffy and light, easy to charge electrostatically. The tubes stick to

glass surfaces very strongly. After simply removing the catalyst bed, a fairly large amount (a few mgs) of almost pure carbon nanotube could be collected by scratching the bottom of the quartz boat. It can be seen in Fig. 4, that it contains only a small amount of silica support and is free of amorphous carbon pieces. A catalyst sample which was neutralized during preparation showed even higher activity, keep- ing the same good quality described before and, contrary to other iron catalysts, formation of spiral nanotubes could be observed on its surface as illustrated in Figs 5 and 6. The formation of coiled nanotubes is clearly enhanced in the catalyst formed using a pH = 9 initial solution (Fig. 5). The existence of these helices has already been described [ 18,34-361 using supported Co/silica samples. Although Boehm [ 371 reported a frequent formation of coiled nanotubes on iron, most of our Fe catalysts

Fig. 3. Carbon nanotubes formed in the decomposition of acetylene at 700°C over Fe/silica (ion-adsorption precipitation) catalyst. (a) Low magnification; (b) high resolution.


Fig. 4. Almost pure carbon nanotubes collected by scratching the bottom of quartz boat (reactor)

Fig. 5. Carbon nanotubes formed in the decomposition of acetylene at 700°C over Fe/silica (ion-adsorption precipitation using pH = 9 as initial solution, but the pH was close to 7 before filtration) catalyst.

produced carbon nanotubes which were not regularly bent and thus no helices are present.

For the catalyst preparation, iron acetate was used in order to obtain the oxide without any anionic contamination after calcination. In spite of literature data from chemical handbooks, it was not completely soluble in any solvent so small crystals of the salt could adhere to the outer surface of the support, and can result in the formation of thicker tubes in some regions. Though their amount is not too high we tried to avoid this side-effect by using FeCl, as a

starting material. In the catalytic decomposition of acetylene this sample showed the highest activity ever observed in nanotube formation (see Table 1). Their quality is even better and they are not crumbled at all (the overall aspect being similar to that of Fig. 6). The turbostratic structure of the wall is extended through the tube diameter and much less amorphous carbon was found on the outer surface of the nanotubes as is illustrated in Fig. 7. Almost no thicker tubes and fibers were observed in this sample. From this experiment, it can be established that Cl- has


Fig. 6. Carbon nanotubes formed in the decomposition of acetylene at 700°C over Fe/silica (ion-adsorption precipitation and neutralized before filtration) catalyst.

Fig. 7. High resolution image of a carbon nanotube formed in the decomposition of acetylene at 700°C over Fe/silica prepared from FeCl, solution (ion-adsorption precipitation

and neutralized before filtration) catalyst.

no negative effect on either the activity of the catalyst or the quality of carbon nanotubes.

3.3 Reaction time The catalytic behavior of Fe/silica prepared by the

ion-exchange method was studied as a function of reaction time. In Table 2 carbon yield data are summarized. The amount of carbon deposit increases with time. After a reaction time of 1 minute, the macroscopic appearance of the sample did not indi-

Table 2. Carbon yield as a function of reaction time over Fe/silica

Reaction time (min) 1 5 10 30

Carbon yield (%) 4.8 35.3 57.1 116.6

Iron acetate (ion-adsorption precipitation) at 700°C

cate the presence of carbon nanotubes since it was not black but gray. Nevertheless, electron microscopic observation verified the formation of nanotubes even after 1 minute but only a few particles are covered by them. After 5 minutes all of the particles are overlaid by the nanotubes. It is interesting to remark that formation of amorphous carbon on the catalyst surface could not be observed even after 30 minutes. At longer reaction times (> 1 hour), deposition of soot begins on the catalyst surface and on the outer surface of the tubes.

3.4 DiSferent reactants In order to cbmpare the reactivity of acetylene

with other hydrocarbons, catalytic decomposition of methane, ethylene and propylene have been checked at different reaction temperatures (Table 3). In the decomposition of ethylene and propylene at 7OO”C, the carbon yield over Fe/silica (ion-adsorption precipitation) is much lower than in that of acetylene, and the quality of carbon nanotubes was much poorer so that the formation of amorphous carbon and thick tubes became significant, and the nanotubes were

Table 3. Carbon yield as a function of reactant and reaction temperature over Fe/silica

Carbon yield (%)

Reactant 700°C 750°C 800°C

Acetylene 116.6 _ _

Ethylene 34.0 60.1 _

Propylene 32.8 38.8 Methane 0 0 *0

Iron acetate (ion-adsorption precipitation) 30 min reaction.


“crumbled”, which showed that the organization of the wall was not good. Increasing reaction temperature favored the developing of well-turbostratic structures and at the same time the amount of soot diminished to a great extent. For example, the carbon yield was found to be 60.1% at 750°C in the reaction of ethylene (Table 3). Carbon nanotubes formed in the catalytic decomposition of ethylene are shown in Fig. 8. Fe/silica was almost inactive in the decomposition of methane even at 800°C. At this reaction temperature no carbon deposit could be weighed, but a very small amount of amorphous soot has been observed by electron microscopy.

In addition to experiments carried out in the fixed bed flow reactor, a fluidized bed reactor was also applied in the production of carbon nanotubes over Fe/silica catalyst. Following literature data [41], its advantages in both material and heat transport inspired us to try this reactor, too. For the sake of comparison, the flow of acetylene and the linear gas speed was set to the same values in both reactors. In the decomposition of acetylene, the quality of carbon nanotubes was investigated at different reaction times. As a result of the identical experimental parameters, values of carbon yields were about the same in both reactors. The real difference was observed in the quality of the product. In the fixed bed flow reactor, formation of amorphous carbon begins at 1 hour and increases at higher reaction times. Appearance of soot on the tubes ensues somewhat earlier in the fluidized bed. While in the first case the homogeneous decomposition of acetylene [42] leads to deposition of soot on the outer surface of the tubes, in the second case additional amorphous carbon from the inner pores

of silica can be released, too, due to the mechanical action in the gas flow [43]. This supposition is confirmed by the observations of “broken” tubes in the system which can be seen in Fig. 9. (However, strong sonication can have a similar effect on the samples [43].) In the same image, the presence of amorphous carbon can also be observed which is characteristic only for the fluidized bed reactor using this catalyst. Since the diameter of the well-turbostratic tubes depends mainly on the dispersion of the catalyst, only their length can grow in time [ 341. Thickening of the tubes results by soot deposition only. This can be seen in the electron microscopic images, too. Breaking the nanotubes (probably at those places where the structure of their wall was not well organized) and cutting up the silica support to smaller pieces occurs in the fluidized bed reactor and the latter phenomenon causes soot liberation from the inside pores. It means that in the fluidized bed reaction not only the homogeneous decomposition of acetylene results in amorphous carbon deposits on the product but their higher amount can be explained by the soot originating from the silica support.

3.5 Fluidized bed reactor

4. CONCLUSIONS

Summarizing the result obtained in the catalytic synthesis of carbon nanotubes over supported Fe catalysts it can be stated that Fe/silica (ion-adsorption precipitation) is able to produce nanotubes with high activity and selectivity. For the possible use of carbon nanotubes, their preparation in large quantity under mild reaction conditions can be of decisive impor- tance. According to the structure of the as-made product, on the basis of their high surface area, it

.._^^_“. )_ 5o(l nm

Fig. 8. Carbon nanotubes formed in the decomposition of ethylene at 750°C over Fe/silica catalyst.


Fig. 9. “Broken” nanotubes together with some amorphous carbon formed in the fluidized bed reactor

could be applied, for example, as catalyst support [44] or as the stationary phase in chromatography.

As far as purification of carbon nanotubes is con-

cerned, despite the lower formation of carbon deposit (Table 1) on Fe/zeolite Y (impregnation), when com- pared to Fe/silica (ion-adsorption precipitation), the isolation of nanotubes is easier when the support is zeolite [36]. In fact, zeolite catalysts being soluble in aqueous HF, the isolation of nanotubes with some amorphous carbon from Co/zeolite Y has already been reported with 76% yield [ 361. The same purification procedure will be applied to the nanotubes produced on Fe/zeolite Y (impregnation).

Acknowledgements-The authors acknowledge the Wallonia Region and the Belgian National Fund for Scientific Research (FNRS, Brussels) for financial support. K. Hernadi is grateful for the fellowship W 015751 (Human Resources Development Project, 3313 HU, Young Scientists Support Program, OTKA). This text presents research results of the Belgian Programme on Inter University Poles of Attraction initiated by the Belgian State, Prime Minister’s Office of Science Policy Programming. The scientific responsibility is assumed by the authors. Thanks are expressed for the good photographs to D. Van Acker.

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