Chemical Physics Letters 367 (2003) 475–481 www.elsevier.com/locate/cplett
SWNTs as catalyst and/or support in the catalytic decomposition of hydrocarbons K. Hernadi
a,b,*
, L. Thi^en-Nga a, E. Ljubovic a, L. Forr o
a
a b
Institute of Physics of Complex Matter, Ec ole Polytechnique Federale de Lausanne, CH-1015 Lausanne, Switzerland Department of Applied and Environmental Chemistry, University of Szeged, H-6720 Szeged, Rerrich B. t er 1., Hungary Received 19 July 2002; in final form 18 October 2002
Abstract Various SWNT samples as either catalyst or catalytic support were used in the chemical vapor deposition (CVD) method for the growth of MWNTs. Catalysts were prepared by the impregnation of SWNT soot with different transition metals. Decomposition of acetylene was investigated at different temperatures (650–720 °C). The quality of both original SWNTs and the newly formed carbon nanostructures was assessed by TEM. A significant difference was found between selectivities of original (known as metallic impurities in SWNT soot) and the posteriorly deposited metallic particles. In the presence of active catalyst SWNT ropes tend to disappear, absorbed into carbon fibers or MWNTs. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Carbon nanotubes are not only formidable structures for electronic and mechanical applications due to their unique electronic properties and high YoungÕs modulus, but they are also interesting as starting materials for making other structures. They are easily manipulated and they lend themselves to diverse chemical creativity. Among the many creative possibilities one can consider the filling of SWNT with C60 [1–3], with the so-called endohedral metallofullerenes [4,5], with metals (like Fe), ferromagnetic alloys (like NiS), semiconductors (like Ge) [6–8] or with other exotic
*
Corresponding author. Fax: +36-62-544-619. E-mail address:
[email protected] (K. Hernadi).
crystal (like metal halides) [9], etc. Within the framework of filling carbon nanotubes, one can also ask the question: whether one can fill nanotubes with nanotubes that is in the large hollow core of a MWNT can one place an SWNT or an SWNT rope, and what would be the right strategy to achieve this? Furthermore, it would be interesting to know if these compounds can co-exist. Do SWNT samples, for example, survive in the environments that are common for catalytic MWNT? Can any evidence for the transformation of the original material be found? It is known that the reaction conditions favourable for the formation of various kinds of carbon nanotubes can be very different [10]. For example, the catalytic growing of SWNT requires significantly higher temperature than is necessary for MWNT formation [11–14].
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 7 3 0 - X
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Although the formation of carbon nanotubes has been widely investigated in the last decade, different preparation techniques were developed for both single-wall and multi-wall carbon nanotube synthesis [15–18]. However, the above questions have not been addressed. Our strategy has been to use SWNTs as a catalytic support in the chemical vapor deposition (CVD) method for the growth of MWNTs. Another goal of our work has been to check contingent catalytic activity and selectivity of the original metal content of various SWNT samples in the decomposition reaction of acetylene. By transmission electron microscopy (TEM) we have followed the growth of MWNT, their structure and hollow core, in order to see whether SWNTs are incorporated within the tubes. The evolution or rather the retrogression of the SWNT material was also followed.
2. Experimental In our experiments we used commercially available SWNT samples of different origin: MSWNT (MER: Materials and Electrochemical Research: 50% SWNT, amorphous carbon and metallic particles), D-SWNT (DEL: Dynamic Enterprises: approx. 20% SWNT with metal and other carbon particles), C-SWNT (CarboLex: approx. 15% SWNT with metal and other carbon particles) and R-SWNT (Rice: 70% of SWNTs and metallic particles). The percentage here denotes volume fraction. For catalyst preparation, 10 mg of SWNT sample was first sonicated in isopropanol. One part of SWNT was used for a blank experiment without added catalyst. To the other portions 10% of a metal (Fe, Co or Ni) was added and sonication was continued until the metal salt (Cobalt(II)-acetate tetrahydrate, Fluka; Ferrichloride anhydrous, Fluka; Nickel(II)-chloride hexahydrate, Fluka) had dissolved. Then the samples were allowed to evaporate until almost dry and used as catalysts in acetylene decomposition with different reaction times (1–30 min). Final drying was accomplished in the quartz boat used for the reaction otherwise the carrier gas would have blown the fluffy material away. Catalytic carbon decomposition was carried out in the
temperature range 650–720 °C with a gas feed of 70 l/h N2 and 20 ml/min acetylene. Following ultrasonic treatment, the quality of both original SWNTs and newly formed carbon nanostructures was assessed by TEM (Hitachi HF2000 high-resolution microscope at 200 kV).
3. Results Here we interpret the quality of the initial SWNT, their behaviour under CVD conditions, as well as the carbon deposit grown over transition metal impregnated SWNT samples under CVD conditions. These results are summarized in Table 1. 3.1. Characterization of raw materials TEM observations showed that along with the single-wall carbon nanotubes, the original MSWNT sample contained large amounts of amorphous carbon and metallic particles. The latter were not covered with graphitic shells which would make them Ôeasy to accessÕ during the catalytic reaction. The D-SWNT samples contained about 20% amorphous carbon and highly dispersed metallic particles in more or less well-graphitized shells. The C-SWNT samples contained SWNT in the lowest proportion, the amount of amorphous carbon and (often ÔbareÕ) metallic fragments were significant. The R-SWNT was a purified sample with lots of ropes, however, TEM showed some catalytic particles in graphitic shells in rather fuzzy images that may have been caused by small amounts of adhered amorphous carbon. 3.2. Characterization of carbon nanostructure grown over the raw SWNT samples The raw M-SWNT soot proved to be a weekly active catalyst. The MWNT was limited to regions where exposed metallic particles were present. As is shown in Fig. 1a, multi-wall carbon nanotubes formed that are not well graphitized and represented only a small portion of the carbon nanostructures in this system. Catalytic behaviour of raw D-SWNT was similar to that of the M-SWNT sample.
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Table 1 TEM characterization of samples: the quality of the initial SWNT samples is shown in the first row; their behaviour under CVD conditions are summarized in the second row; third row: characterization of carbon deposits grown over transition metal impregnated SWNT samples under CVD conditions
Raw material
Raw material as catalyst
Impregnated material as catalyst
M-SWNT
D-SWNT
C-SWNT
R-SWNT
Approx. 50% SWNTs; amorphous carbon; some Ôeasy to accessÕ metallic particles Low activity, little carbon deposit: mainly not well graphitized multi-wall carbon nanotubes Only few SW ropes left; fibers (approx. diameter of 0.1 lm) and more or less graphitized MWNTs formed
Approx. 20% SWNTs; about 20% amorphous carbon; highly dispersed metallic particles in more or less well-graphitized shells Small amount of carbon deposit, which contains some tubular structure; the walls are not graphitized
Approx. 15% SWNTs; amorphous carbon; often ÔbareÕ metallic fragments
Approx. 70% SWNTs mainly in ropes; some amorphous carbon; metallic particles in perfectly graphitized shells
Lot of amorphous carbon, thin fibers, etc.; ÔbareÕ metallic particles seemed to act as active, but not selective catalyst Very few SMNT left; quite different carbon structures depending on the metal impregnated
Very few fibers and cone-like structures; SWNT bundles remained mostly untouched
Amorphous carbon, fibers and quite a lot MWNT; no SWNT were detected; formation of Ôhairy coreÕ typical in Fe-impregnated samles
Mainly MWNT and SWNT; small amount of fibers and amorphous carbon; no correlation between original SWNT and growing MWNT
Fig. 1. Carbon nanostructures grown over raw SWNT samples: (a) not well graphitized multi-wall carbon nanotubes formed on the surface of raw M-SWNT sample under CVD conditions; (b) R-SWNT bundles remained mostly untouched under the above-mentioned reaction conditions.
C-SWNT without added metal contained large amounts of amorphous carbon, thin fibers, etc. since in that sample we had a huge amount of ÔoriginalÕ catalyst accessible for acetylene molecules. Its decomposition resulted in non-selective carbon deposition. Catalytic activity of the raw R-SWNT in the decomposition of acetylene was also investigated.
It was found that SWNT bundles remained mostly untouched under the above-mentioned reaction conditions. Probably due to the lower amount of original impurities, more SW ropes remained than in the previous samples. TEM observations illustrated that besides some fibers and cone-like structures, SWNTs survived even higher reaction temperatures and longer reaction time (Fig. 1b).
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3.3. Characterization of impregnated SWNT materials For impregnation we used iron, cobalt, or nickel which have been found to be extremely active in catalytic MWNT synthesis. As a general observation, after CVD very few SW ropes were observed in these samples. Otherwise, both the quality and the quantity of the newly formed carbon nanostructures showed a surprising variety as a function of SWNT support, reaction temperature or transition metal. Using a C-SWNT sample as a catalyst support, a few SWNT remained and mainly MWNT formed in the acetylene decomposition at 720 °C. However, the quality of the carbon structures was
quite different from the raw C-SWNT and varied significantly depending on the catalyst. Fig. 2a shows tubular structures with surprisingly wide cores with quite thin walls grown on Fe/C-SWNT. According to theoretical calculations, these kinds of structures may show outstanding capacity for hydrogen storage [19]. Over M-SWNT support, mainly fibers of approximate a diameter of 0.1 lm formed together with some more or less graphitized MWNT. Only few SW ropes remained. D-SWNT-supported samples produced amorphous carbon, fibers, and large amounts of MWNT. In these samples SWNT could not be observed, probably they tend to disappear, absorbed in fibers. This fact might explain the for-
Fig. 2. Carbon nanostructures grown over impregnated SWNT samples: (a) TEM image of wide core carbon nanotubes with quite thin walls grown on Fe/C-SWNT; (b) HRTEM images of unique structure of MWNT with Ôhairy coreÕ which is more typical of iron catalyst over D-SWNT sample; (c) Co/R-SWNT: SW bundles, original encapsulated metal particles, MWNT containing catalyst inside.
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mation of MWNT with a Ôhairy coreÕ typical of iron catalyst as is indicated in Fig. 2b. Testing R-SWNT as a catalyst support has led to the following observations: although some fibers and amorphous carbon was found, mainly SWNT and MWNT appeared in an overwhelming amount. It is interesting to remark that no correlation was observed between original SWNT and grown MWNT. In other words SWNT did not seem to determine the growth direction of MWNT. A characteristic image of a sample with SW bundles, original encapsulated metal particles, MWNT containing a catalyst inside can be seen in Fig. 2c. As far as the effect of reaction temperature was concerned, only R-SWNT showed a clear tendency. All others gave such complex products with very different carbon components that it was im-
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possible to really assess the results. In general much more SWNT remained changed and almost no well-graphitized MWNT was observed at the bottom of the temperature range that we studied. Regarding the R-SWNT samples, mainly SWNT and but also some non-graphitized, cone-like fiber was found after acetylene pyrolysis at 620 °C. Investigating the effect of reaction time, we met with similar difficulties as mentioned above. As an overall impression, no formation of carbon deposit was observed in the first few minutes. Comparing the structure of carbon material formed after 10 and 30 min, we found larger quantities of original SWNT and significantly shorter MWNT with shorter reaction time. Albeit there was no striking difference in the catalytic effect of the various transition metals
Fig. 3. HRTEM images about catalytic effect of the various transition metals: (a) Carbon deposit over Co/C-SWNT: well-graphitized MWNT with very little amorphous carbon on its external surface; (b) Carbon deposit over Fe/M-SWNT: well-graphitized MWNT with traces of the slowly vanishing single-wall tubes, which are shown by the arrow; (c) Carbon deposit over Ni/M-SWNT: perfectly straight, arc-discharge-like MWNT which has not yet been observed in any CVD samples.
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used, some interesting features were found in the course of TEM investigations. Co catalyst deposited on any SWNT support supplied MWNT with best graphitization with a quite standard quality. Fig. 3a illustrates MWNT grown on Co particle together with very little amorphous carbon on the external surface originating from homogeneous decomposition of acetylene at 720 °C. As has already been mentioned in connection with Fig. 2b, iron catalyst tended to grow MWNT with a Ôhairy coreÕ, not observed using conventional catalyst supports [20]. Fig. 3b, with possible traces of single-wall tubes might, suggests that SWNT coming from the support has a role in the formation of new carbon nanostructures. Nickel provided a rather contradictory picture of catalytic selectivity. Depending on the support, a wide variety of different carbon particle types grew. When using the M-SWNT sample as the support something unique was observed. Ni/M-SWNT produced perfectly straight, arc-discharge-like MWNT (Fig. 3c), something never observed before at this reaction temperature, under CVD conditions.
4. Discussion Acetylene decomposition was investigated using different commercial SWNT samples as the catalyst support. Original samples contained metallic particles in differing amounts. They can act as a catalyst in pyrolysis too, depending on its oxidation state and on coverage by graphitic layers. Testing raw materials in CVD gave the following result: while R-SWNT alone was almost absolutely inactive, C-SWNT produced a lot of amorphous carbon and thick fibers. In the former sample the amount of metal is very low and particles are covered by graphitic layers and not accessible by hydrocarbon molecules. In C-SWNT there is 50% of the original catalyst, probably in a metallic or carbidic form that are not covered by perfect, unassailable graphite layers. This explains the activity of raw C-SWNT and the quality of carbon deposit. Depositing additional transition metal on the surface, a completely new catalytic system develops. The ionic form of transition metal deposited
on the surface of catalyst support oxides at the temperature of the reaction. As a result of the acetylene reactant this oxide is partially reduced at the beginning of the reaction but never resulted in metallic or carbidic compounds [21]. In this way two different kinds of catalytically active centres exist simultaneously next to each other. Of course, the activity of raw catalyst, if any is present, is not suppressed. This can explain the many kinds of carbon nanostructures produced in the reaction over impregnated SWNT materials. While metallic or carbidic catalysts help the formation of conelike fibers [22], the oxidic form of transition metals catalyzes reaction that generally produce MWNT. This mixed feature is less typical of R-SWNT supported impregnated samples. Since in the latter system original metallic particles are inaccessible for reactant hydrocarbon molecules, oxidic catalysis dominates and offers the possibility of controlled amorphous free MWNT synthesis. Investigating the effect of reaction time it was found that after an induction period of a few minutes, the length of MWNT and carbon fibers is lengthening in time without growing in diameter. On the other hand, the estimated amount of untouched SWNT decreased with increasing reaction time. Reactions at different reaction temperatures proved that 650 °C is not high enough to make SWNT react under CVD conditions. Disappearance of SWNT absorbed into different carbon nanostructures is obvious at 720 °C. Comparing raw and impregnated support containing metallic and oxidic particles, respectively, oxide catalysts seem to be more active in transformation of SWNT into MW structures. Studying the morphology of those Ôhairy coreÕ structures may give an explanation of how this comes to pass. Various SWNT samples as either independent catalysts or supports provided a large variety of newly formed carbon nanostructures (amorphous carbon, thin fibers, graphitized MWNT, etc.). For deeply understanding the formation of carbon fibers or perfectly straight, arc-discharge-like MWNT over Ni/M-SWNT, there is still a lot to do. The possibility of the production of perfectly graphitized MWNTs in a simple process under relatively mild CVD conditions is of great importance.
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5. Conclusions Applying SWNT samples as catalyst and/or support in the catalytic decomposition of acetylene, SWNT ropes proved to be stable even at 720 °C in the absence of catalyst. Carbon deposit formation from acetylene can take place over either raw metallic (not perfectly closed by graphitic shells) or transition metal oxide particles deposited later onto the surface, however, a big difference was found in the selectivity of the reaction. While metallic catalyst particles tended to form cone-like carbon fibers, impregnated SWNT samples were selective in the formation of nanotubes having graphitic structure. Disappearing of SWNT absorbed in different carbon nanostructures also occurs in the latter system.
Acknowledgements Authors thank the European Commission (RTN Program, NANOCOMP network, RTN 1-1999-00013) and the Hungarian Ministry of Education (FKFP 0643/2000) for financing this investigation.
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