Production of Carbon Nanotubes from Polyethylene Pyrolysis Gas and

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PRODUCTION OF CARBON NANOTUBES FROM POLYETHYLENE PYROLYSIS GAS AND EFFECT OF TEMPERATURE NOELIA ARNAIZ, MARIA FRANCISCA GOMEZ-RICO, Ignacio Martin-Gullon, and Rafael Font Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie401688n • Publication Date (Web): 09 Sep 2013 Downloaded from http://pubs.acs.org on September 15, 2013

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Industrial & Engineering Chemistry Research

PRODUCTION OF CARBON NANOTUBES FROM POLYETHYLENE PYROLYSIS GAS AND EFFECT OF TEMPERATURE NOELIA ARNAIZ*; MARIA FRANCISCA GOMEZ-RICO; IGNACIO MARTIN GULLON; RAFAEL FONT Department of Chemical Engineering, University of Alicante. P. O. Box 99, 03080, Alicante, Spain *[email protected] Fax number: +34 965 90 3826

1 ACS Paragon Plus Environment


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Abstract Carbon nanotubes (CNT) were produced by catalytic chemical vapor deposition using, as carbon source, a mixture of hydrocarbons and hydrogen that simulates the effluent gases from pyrolysis of polyethylene (PE). An Fe/Al3O3 catalyst was used in a range of temperatures from 600 ºC to 800 ºC.

Multi-wall carbon nanotubes of 20 nm in diameter and length on the order of microns were obtained. Higher yields were observed at 650 ºC, where no prior catalyst reduction was necessary. TEM, XRD and Raman spectrometry show a higher crystalline quality at 750 ºC, although the balance yield-quality indicates that 650 ºC is a satisfactory temperature for producing CNTs at a reasonable cost, since no extra hydrogen is necessary for the process. In addition to this, the effluent gas from the process can be further used for energy production.

Keywords: carbon nanotubes; pyrolysis stream; waste; polyethylene


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1. Introduction Carbon nanofilaments are crystalline materials composed mainly of carbon, where the basic unit is graphene and whose use and value are increasing due to its extraordinary mechanical and conductive properties. These filaments have applications in the automotive, construction or sports sectors as a filler in polymer nanocomposites because of their high resistance and low weight. They are also used in the fields of catalysis as a metal catalyst support.

The difference between carbon nanotubes (CNTs) and carbon nanofibers (CNFs) is related to the orientation of graphene layers in the filament structure. In CNTs, the graphene layers are rolled into perfect cylinders, whereas in CNFs graphene layers are oriented in another way 1. Among these different structures, CNTs exhibit the best mechanical and conductive properties, and CNFs have the advantage of a lower dispersive energy which allows better functionalization. Its main production method is by catalytic chemical vapor deposition (CVD) 2, 3 where gas-phase molecules of hydrocarbons are decomposed catalytically at high temperatures (between 500 to 1100 ºC) and carbon is deposited on a transition metal catalyst (mostly Fe, Co or Ni) in the elemental state. Depending on the hydrocarbon, metal nature and temperature, different carbon filament structures can grow 4. For example, fishbone nanofibers, where the layers of graphene are sloped, can be generated from the interaction of a nickel catalyst with some hydrocarbon or CO. Multi-wall carbon nanotubes (MWCNTs) are normally grown by the decomposition of acetylene or an aromatic compound over supported Fe catalysts 5.

On the other hand, disposed polymers in municipal solid waste have increased in recent years because their use is extended to all fields of society. The packaging sector remains the highest user of polymers followed by construction, automotive and electrical and electronic equipment. Specifically, polyethylene (PE) is the most used polymer by market share with 29% of the total of plastics 6. One fraction of plastic waste is recovered by recycling, as plastic or into other chemicals, or by energetic valorization

7, 8

. However there is still an important amount that is

landfilled. Comparing the data from Europe in 2005 with those observed in 2010, in the last years recovering has been increased from 10.4 to 14.3 million tonnes and landfilling has been slightly reduced from 11.6 to 10.4 million tonnes. Nevertheless, it is still significant because of the total post-consumer plastic waste was increased by 12.3%, thus it would be interesting if this fraction could be used for some purpose 6, 9.



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Several works have already shown the viability as a carbon source for the production of carbon nanofilaments: polypropylene

10-15

, polystyrene

11

, polyvinyl chloride

10

, PE

10, 16, 17

, etc. The

3

review of Bazargan et al. compiles the most important processes for this purpose, highlighting CVD as the most promising and preferred method for large-scale production. Regarding PE, this review concludes besides that no major difference in product morphologies exist between using pre- or post-consumer PE and no major difference between products from HDPE and LDPE feedstock. The presence of oxygen is also a point of interest because some of the carbon would react with the oxygen and not be readily available to transform into nanoproducts. In contrast, when a combustion step it is incorporated before CNT formation which introduces CO and CO2 into the system, better CNT growth is achieved. The presence of H2 and H2O is also favourable since it preserves active catalysts. Kukovitsky et al.

16

proposed a process producing crooked

CNTs by pyrolysis of PE after passing the subsequent products over a Ni plate in a quartz tube reactor, with a pressure of 4 atm of hydrocarbons and helium gas and studying two temperatures. Yang et al.

10

proposed a method for the synthesis of the aligned carbon nanotubes by pyrolysis

of PE in the presence of ferrocene catalyst and extra hydrogen, at 800 ºC. Zhuo et al. 17 presented a method for producing CNTs from PE by sequential pyrolysis and combustion to generate CO, CO2 and H2O, as well as hydrocarbons and H2, at 750 ºC. The synthesis of MWCNTs was done on meshes of stainless steel type 304. The presence of H2 and H2O and the use of a silicon carbide honeycomb barrier filter have been proven to keep catalysts active, due to the fact that some soot could be deposited on the catalyst but they permit a clean surface all the time. Therefore, these studies indicate that CNTs, which are high added value materials, can be produced from a direct gaseous stream from PE decomposition, which is rich in light hydrocarbons and hydrogen.

In the present work, a synthetic stream simulating the effluent gases from pyrolysis of PE was used to obtain CNTs, which were grown in a simple reactor, without prior reduction of the catalyst nor extra hydrogen added, thus reducing the process cost with respect to other methods proposed in the literature. In addition to this, the influence of temperature was studied to find the best conditions of yield and quality of CNTs. Hence, a simple alternative recycling process is proposed, compatible with energetic valorization, for waste of PE.

2. Materials and methods 2.1. PE decomposition 4 ACS Paragon Plus Environment

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In order to know the composition of the effluent gases from pyrolysis of PE, prior PE decomposition experiments were done. The PE used in this work was powdered low density PE, LDPE Alcudia® PE-019 (melt flow rate of 2 g/min, density 919 kg/m3) from Repsol (Spain). The decomposition experiments were carried out in a laboratory scale horizontal tubular quartz furnace AOX Euroglass 1600 (Figure 1). Around 200 mg of PE was introduced into the furnace in a boat pushed by a magnet system at a controlled rate. The pyrolysis experiments were carried out in a nitrogen atmosphere at a gas flow of 5 mL s-1. Temperature range studied varied from 600 to 800 ºC, based on data collected from previous research works

8, 18

. Special attention was

paid to obtain hydrogen as decomposition product, since it is required to reduce the metal catalyst in situ when growing CNTs.

Figure 1

Gases and volatile compounds generated in the thermal process were collected in a Tedlar® bag and analyzed by gas chromatography. On the one hand, a Shimadzu GC-14A gas chromatograph was used, equipped with a concentric Alltech CTR I column (6 ft x 1/8 in. and 6 ft x 1/4 in. for the inner and outer columns, respectively) and a thermal conductivity detector (TCD), to value mainly CO and CO2. On the other hand, the light hydrocarbons were analyzed in a Shimadzu GC-17A gas chromatograph with a Supelco capillary Alumina-KCl Plot column (30 m x 0.32 mm) in split injection and a flame ionization detector (FID). The identification and the quantification of all the analyzed compounds were carried out with a prior external standard calibration.

2.2. Production of carbon nanotubes 2.2.1.

Catalyst preparation

The catalyst necessary for the synthesis of nanofilaments was iron supported on alumina, which has been widely used for this kind of processes with CVD

19, 20

obtaining high yields. This

4

catalyst was prepared by the co-precipitation method . Briefly, an aqueous solution of Al(NO3)3·9H2O (1%) was mixed with Fe(NO3)3·9H2O and an aqueous solution of Na2CO3 (1 mol/l) was added until pH 7.0 was achieved. The resulting precipitate was filtered and washed several times with distilled water. The precipitate was then dried in an oven (108ºC, 24 h) and 5 ACS Paragon Plus Environment


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finally calcined in a furnace (450ºC, 8 h) to obtain the Fe2O3/Al2O3. Since the elemental iron is the active form, a previous or in situ reduction is required for the CNT growth. For this reason, hydrogen was necessary together with the hydrocarbon stream resulting from PE decomposition to confirm the presence of the Fe/Al2O3 catalyst needed in the CNTs reactor. The structure of the resultant catalyst was characterized by transmission electron microscopy (TEM) using a JEOL JEM-2010. The amount of iron and aluminum and their dispersion was characterized by X ray fluorescence (XRF) with a Philips Magix Pro and by scanning electron microscopy (SEM) employing electron dispersion spectroscopy (EDS) to obtain a mapping using a Hitachi S3000N equipped with an X-ray detector Bruker XFlash 3001.

2.2.2. MWCNT production A synthetic hydrocarbon mixture (Air Liquid) with the composition corresponding to the optimized conditions of PE decomposition was purchased. Corresponding or extra hydrogen was supplied from a different feedstock. A 50 cm length horizontal quartz reactor and with an inner diameter of 75 mm, controlled by an electric heater, was used to grow CNTs by the substrate method. The substrate method consists of loading the Fe/Al2O3 catalyst precursor on the crucible and passing the appropriate gas (hydrocarbons and hydrogen) during a period of time to grow carbon nanofilaments 1. Different hydrocarbons/hydrogen ratios and several temperatures were studied.

Figure 2

0.1 g of the prepared catalyst was loaded in the reactor and this was heated to the designated temperature under a nitrogen flow. Then, the nitrogen flow was stopped and the mixture of hydrocarbons and hydrogen (reproducing that obtained in PE pyrolysis runs) was added in the desired ratio at 0.3 L/min. After 40 min reaction, the system was cooled down to room temperature in a nitrogen flow.

Two sets of experiments were studied with the synthetic mixture. The first series was carried out at different hydrogen concentrations keeping the temperature constant at 600 ºC. Previously, a single experiment was also done with a prior reduction of the catalyst, in hydrogen atmosphere, at 550 ºC for 1.5 h. Although the purpose was to merely use the hydrogen proportion obtained


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from PE pyrolysis, this study with different proportions was done in order to know the catalyst behavior under different conditions.

A second set of experiments were carried out to analyze the influence of temperature keeping the hydrogen proportion constant. The temperature ranged from 600 to 750 º C and the hydrogen concentration was 22 vol. %. This second set was done to know the best temperature for the process. The highest temperature used was 750 ºC since at higher temperatures, although it was checked that nanofilaments grow, there was also formation of soot.

Table 1. Experiments of nanofilaments growth carried out by CVD and the substrate method. GROWTH

PRIOR

EXP.

REDUCTION

T (ºC)

H2 (vol. %)

600-33%-PR

Yes (550ºC)

600

33

600-33%

No

600

33

600-25%

No

600

25

600-12,5%

No

600

12.5

600-22%

No

600

22

650-22%

No

650

22

700-22%

No

700

22

750-22%

No

750

22

Moreover, another two singular experiments using pure CH4 as carbon source instead of the hydrocarbon mixture at 650 ºC were carried out, in order to check if CH4 yields filaments or behaves as an inert in comparison to olefins.

Influent and effluent gases of the experiments were collected in Tedlar® bags and analyzed by gas chromatography using a Shimadzu GC-14A gas chromatograph equipped with a Supelco Carbosieve SII (4 m x 1/8") column and a TCD to value mainly hydrogen and the Shimadzu GC-17A gas chromatograph as commented in section 2.1.

The purity and phase structure of the solid products were characterized by X-ray diffraction (XRD) using a Bruker D8-Advance with Göbel mirror operating at 40 kV and 40 mA and by Raman spectroscopy using a Jobin-YvonLabRam spectrometer employing an Ar laser of 514



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nm. The morphologies of the as-prepared products were examined by TEM using a JEOL JEM-2010 as commented in section 2.2.1.

3. Results and discussion 3.1. PE decomposition The major components of the non condensable effluent gas obtained from PE decomposition were the light hydrocarbons ethylene, methane, propylene and ethane, as well as hydrogen. Table 2 shows the gas composition for the experiments at 600 ºC and at 800 ºC, adjusted to 100 vol. % with the major components. The percentage of minor components of the effluent gas represented 20 and 10 vol. % for the experiments at 600ºC and 800 ºC, respectively, and they are not included in the table since the synthetic mixture was purchased only with the major ones. Moreover, the experiment can be observed where the hydrogen concentration was the highest, which corresponded to pyrolysis at 800 ºC. This experiment was selected for the subsequent study of growing nanofilaments from gases obtained in this section.

Table 2. Major components of the product gas from pyrolysis at 600 ºC and 800 ºC. TEMPERATURE (ºC)

600 ºC

800 ºC

Hydrogen (vol. %)

7.37

21.88

Methane

17.98

23.78

9.55

5.78

Ethane

(vol. %) (vol. %)

Ethylene

(vol. %)

42.82

35.72

Propylene

(vol. %)

22.29

12.83

3.2. Production of CNTs 3.2.1.

Catalyst characterization

With respect to the oxides content in the catalyst, the results of XRF confirmed the presence of iron

and aluminum oxides in the structure after preparation with a molar ratio Fe:Al of 1:2.5. TEM exploration (Figure 3a), showed that iron was well dispersed over the Al2O3 support and that the Fe2O3 nanoparticles size distribution, obtained by counting 300 iron nanocrystals, was in a range on the order of 2 nm as depicted as the size distribution histogram (Figure 3b).

Figure 3 8 ACS Paragon Plus Environment

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The iron dispersion can be better monitored by SEM. Figure 4a corresponds to a regular SEM image, and Figures 4b and 4c to the corresponding images including EDS mapping: iron particles in green color and aluminum particles in red color. As can be observed, Al and Fe were well dispersed since the corresponding colors were homogeneously distributed in the whole image. As a consequence, the catalyst can be considered to be satisfactorily synthesized.

Figure 4

3.2.2.

MWCNT production

The carbon reaction yields obtained in the experiments are listed in Table 3. Among the experiments at a fixed temperature of 600 ºC and different hydrogen concentration, the highest conversion was achieved in the run without prior reduction and performed with 25 vol. % of hydrogen (similar value to that obtained at 33 vol. % of hydrogen) Regarding experiments at a fixed hydrogen concentration (22 vol. %, the same as that obtained from pyrolysis of polyethylene) where temperature was varied, the highest conversion was obtained at 650ºC.

Singular experiments with CH4 as the only carbon source indicated that CH4 does not yield carbon nanofilaments at these low temperatures. This is consistent and agrees with the literature, since this compound is highly stable at the working temperatures of this study due to the strong C-H bonds

21, 22

. Consequently, the same behaviour can be assumed for ethane because of its

similar chemical structure of saturated hydrocarbon. Therefore, the carbon filaments in the experiments with the PE synthetic mixture came mainly from ethylene and propylene, and, for this reason, the carbon yield data (shown in Table 3) were all calculated taking into account only these two compounds.

Nevertheless, influent and effluent gases from the experiments of growing nanofilaments were analyzed and the carbon mass balance was closed with only 14% of error. This point is of singular importance due to the fact that it can be considered that the mass balance was closed in this process. 9 ACS Paragon Plus Environment


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Table 3. Carbon yields in the experiments. CARBON REACTION YIELD EXP.

vol. % of carbona

g of carbon/g of catalystb

600-33%-PR

7.98

6.63

600-33%

9.82

8.15

600-25%

10.37

9.31

600-12,5%

7.10

7.57

600-22%

6.13

5.95

650-22%

15.82

15.27

700-22%

11.91

10.78

750-22%

6.04

6.04

a

% with respect to carbon volume in ethylene and propylene fed (carbon in CNTs/carbon in feed hydrocarbon stream) b grams of carbon with respect to grams of catalyst input (carbon in CNTs/catalyst input)

With respect to CNT characterization, the morphologies of the nanofilaments observed by TEM were MWCNTs from 4 to 13 layers (Figure 5a), with lengths typically on the order of microns (Figure 5b) and 20 nm in diameter. Figures 5 and 6 show TEM micrographs of two samples produced at fixed temperature and varying the hydrogen ratio. It can be pointed out that no significant differences are observed among them. Figure 5c makes it possible to observe an iron catalyst particle at the end of a nanotube and how it grows from this particle.

Figure 5

Figure 6

Figures 7 and 8 exhibit TEM images of two representative experiments at temperatures of 650ºC and 750ºC respectively. As can be observed, nanotubes present more evident cristallinity at 750ºC. The walls are straighter and reveal fewer defects.


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Figure 7

Figure 8

Figure 9 shows the XRD patterns of the samples obtained, which reveal a sharp peak corresponding to highly graphitic material at 2θ around 26°. For the set of experiments carried out at 600 ºC and different hydrogen concentration, XRD plots show that the values of full width-half maximum (FWHM) of this peak 002 were similar in all of them, indicative of similar number of layers (Lc) and interspace, d002. Thus, hydrogen concentration has little influence on crystallinity of resulting filaments at 600ºC.

Figure 9

In contrast, in the experiments with increasing temperature and fixed hydrogen percentage this peak 002 was sharper, the distance d002 had steady values around 0.344-0.36 nm (as expected in MWCNT due to glide defects between adjacent cylinders

23

), and the parameter Lc increased

(Table 4). Accordingly this revealed an improvement of the crystalline degree when temperature increased. Therefore, the nanotubes with the best properties were, in agreement to TEM, those obtained at 750 ºC and 22 vol. % of hydrogen. Although the yield was higher in the experiment at 650 ºC, the quality of nanotubes obtained at 750 ºC is satisfactory for many applications.

Table 4. Distance d002 and parameter Lc of the experiments at 22 vol. % of hydrogen. EXP. 600-22% 650-22% 700-22% 750-22%

d002 (nm) 0.341 0.344 0.348 0.343

Lc 002 (A) 32.415 31.565 34.313 43.933



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Characterization by Raman spectroscopy was presented in Figure 10. In the experiments carried out at 600 ºC, no great differences were observed in the spectra with the variation of hydrogen, thus indicating that hydrogen ratio has no influence on crystallinity, which agrees with XRD results. On the other hand, in these experiments at 600 ºC, D-band circa 1300 cm-1, related to crystal defects, was higher than G-band circa 1600 cm-1, corresponding to graphite or crystalline material, for every experiment. Furthermore, 2D band around 2600 cm-1, related also to crystallinity and the presence of parallel graphitic layers, was not especially high. Therefore, these parameters indicated a high concentration of defects.

However, in the experiments where temperature increased until 750 ºC with 22 vol. % of hydrogen, the tendency changed. G-band was increasingly higher than D-band, the G/D intensity ratio (IG/ID) increased from 0.89 to 1.45 as can be observed in Table 5. Furthermore, in Figure 10, it is seen how 2D-band increased as well and among all of them, experiment at 700 ºC is which contains the fewest disordered structure and the most parallel graphitic layers although quite similar to experiment at 750ºC. This indicates an important improvement of the crystalline degree with temperature, so the nanotubes with the best quality were, agreed to TEM and XRD, those obtained at 750 ºC, which is in accordance with the study of Yi et al. 24. They deduced that higher temperatures reduced defects and increased the degree of graphitization after treating single wall CNTs in a furnace at different temperatures.

Figure 10

Table 5. Raman spectroscopy measurements of the experiments at 22 vol. % of hydrogen. EXP. 600-22% 650-22% 700-22% 750-22%

IG/ID 0.89 1.01 1.22 1.45

IG/I2D 3.22 1.69 1.62 2.02

ID/I2D 4.17 1.45 1.33 1.39

Lastly, thermogravimetric analysis (TGA) has been used as an additional tool to examine the cristallinity of the nanotubes produced. Figure 11 shows the results obtained. The curves corresponding to the experiments where the hydrogen proportion was varied (Figure 11a) are 12 ACS Paragon Plus Environment

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very similar, thus indicating that this parameter has no important influence on crystallinity, agreeing again with Raman spectroscopy and XRD. There is only a small difference in the curve corresponding to the highest H2 proportion, which shows an oxidation peak at a slightly lower temperature than the rest of experiments.

With respect to the curves resulting from experiments carried out at different temperatures (Figure 11b), there is a significant difference between the run at 600 ºC and the others, showing the peak at a lower temperature, whereas the experiments at 650 ºC, 700 ºC and 750 ºC show a slight tendency of increasing the oxidation temperature when increasing the temperature of the growth experiment, but with no significant differences between them. According to the literature 25, 26

, an increase in the oxidation temperature means an improvement in cristallinity, which in

this case is more clearly observed when increasing the growth temperature from 600 ºC to higher temperatures.

Figure 11

Summarizing, best MWCNTs are obtained at 750ºC but higher yield is achieved at 650 ºC. To have the best cristallinity when growing CNT with olefins a medium temperature is necessary for adequate kinetics and diffusion through the metal particle that produces a well condensed crystal matter, but at the same time a low temperature is necessary for both hydrocarbon and catalyst stability. At high temperatures, the iron nanoparticles tendency is to sinterize, which makes the catalyst inactive. At 750ºC, the olefins are stable and do not decompose into soot, the catalytic reaction mechanism through iron produced higher crystalline quality, and iron particles have sinterized to some extent, which might explain the reason for a lower yield at 750ºC with respect to 650ºC. 4. Conclusions Synthesis of MWCNTs was carried out on an iron catalyst supported on alumina by the CVD method in a simple reactor with a gaseous stream similar to that obtained from PE pyrolysis. The effluent gas from PE pyrolysis has enough hydrogen and hydrocarbons (specifically ethylene and propylene) to reduce the transition metal catalyst and grow CNTs. The most crystalline CNTs corresponded to the highest temperature and the highest yield was obtained at 650 ºC. Furthermore, after this treatment the exhaust gas could be used for other purposes since only some ethylene and propylene are consumed. Therefore, the proposed process is sustainable and economical with respect to hydrogen, since it is not necessary to add more than that provided by 13 ACS Paragon Plus Environment


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the pyrolysis of PE. Benefit is obtained from a waste with a simple and environmentally friendly method.

Acknowledgments

CTQ 2008-05520 project of the Spanish Ministry of Science and Innovation and Prometeo 2009/043/FEDER.


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(12) Liu, J.; Jiang, Z.; Yu, H.; Tang, T. Catalytic pyrolysis of polypropylene to synthesize carbon nanotubes and hydrogen through a two-stage process. Polymer Degradation and Stability. 2011, 96, 1711-1719. (13) Tang, T.; Chen, X.; Meng, X.; Chen, H.; Ding, Y. Synthesis of multiwalled carbon nanotubes by catalytic combustion of polypropylene. Angewandte Chemie - International Edition. 2005, 44, 1517-1520. (14) Jiang, Z.; Song, R.; Bi, W.; Lu, J.; Tang, T. Polypropylene as a carbon source for the synthesis of multi-walled carbon nanotubes via catalytic combustion. Carbon. 2007, 45, 449-458. (15) Song, R.; Jiang, Z.; Bi, W.; Cheng, W.; Lu, J.; Huang, B.; Tang, T. The Combined Catalytic Action of Solid Acids with Nickel for the Transformation of Polypropylene into Carbon Nanotubes by Pyrolysis. Chemistry – A European Journal. 2007, 13, 3234-3240. (16) Kukovitsky, E. F.; L'Vov, S. G.; Sainov, N. A.; Shustov, V. A.; Chernozatonskii, L. A. Correlation between metal catalyst particle size and carbon nanotube growth. Chemical Physics Letters. 2002, 355, 497-503. (17) Zhuo, C.; Hall, B.; Richter, H.; Levendis, Y. Synthesis of carbon nanotubes by sequential pyrolysis and combustion of polyethylene. Carbon. 2010, 48, 4024-4034. (18) Conesa, J. A.; Marcilla, A.; Font, R. Kinetic model of the pyrolysis of polyethylene in a fluidized bed reactor. Journal of Analytical and Applied Pyrolysis. 1994, 30, 101-120. (19) Louis, B.; Gulino, G.; Vieira, R.; Amadou, J.; Dintzer, T.; Galvagno, S.; Centi, G.; Ledoux, M. J.; Pham-Huu, C. High yield synthesis of multi-walled carbon nanotubes by catalytic decomposition of ethane over iron supported on alumina catalyst. Catalysis Today. 2005, 102–103, 23-28. (20) Philippe, R.; Caussat, B.; Falqui, A.; Kihn, Y.; Kalck, P.; Bordère, S.; Plee, D.; Gaillard, P.; Bernard, D.; Serp, P. An original growth mode of MWCNTs on alumina supported iron catalysts. Journal of Catalysis. 2009, 263, 345-358. (21) Gong, J.; Liu, J.; Wan, D.; Chen, X.; Wen, X.; Mijowska, E.; Jiang, Z.; Wang, Y.; Tang, T. Catalytic carbonization of polypropylene by the combined catalysis of activated carbon with Ni2O3 into carbon nanotubes and its mechanism. Applied Catalysis A: General. 2012, 449, 112-120.


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(22) Gong, J.; Liu, J.; Jiang, Z.; Wen, X.; Chen, X.; Mijowska, E.; Wang, Y.; Tang, T. Effect of the added amount of organically-modified montmorillonite on the catalytic carbonization of polypropylene into cup-stacked carbon nanotubes. Chemical Engineering Journal. 2013, 225, 798-808. (23) Takuya, H.; Mildred, D.; Yoong Ahm, K.; Mauricio, T.; Morinobu, E. Carbon Nanotubes and Other Carbon Materials; CRC Press: 2008. (24) Yi, B.; Rajagopalan, R.; Burket, C. L.; Foley, H. C.; Liu, X.; Eklund, P. C. High temperature rearrangement of disordered nanoporous carbon at the interface with single wall carbon nanotubes. Carbon. 2009, 47, 2303-2309. (25) Gong, J.; Liu, J.; Ma, L.; Wen, X.; Chen, X.; Wan, D.; Yu, H.; Jiang, Z.; Borowiak-Palen, E.; Tang, T. Effect of Cl/Ni molar ratio on the catalytic conversion of polypropylene into Cu–Ni/C composites and their application in catalyzing “Click” reaction. Applied Catalysis B: Environmental. 2012, 117–118, 185-193. (26) Gong, J.; Yao, K.; Liu, J.; Wen, X.; Chen, X.; Jiang, Z.; Mijowska, E.; Tang, T. Catalytic conversion of linear low density polyethylene into carbon nanomaterials under the combined catalysis of Ni2O3 and poly(vinyl chloride). Chemical Engineering Journal. 2013, 215–216, 339-347.



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LEGEND OF TABLES Table 1. Experiments of nanofilaments growth carried out by CVD and the substrate method. Table 2. Major components of the product gas from pyrolysis at 600 ºC and 800 ºC. Table 3. Carbon yields in the experiments. Table 4. Distance d002 and parameter Lc of the experiments at 22 vol. % of hydrogen. Table 5. Raman spectroscopy measurements of the experiments at 22 vol. % of hydrogen.

LEGEND OF FIGURES Figure 1. Diagram of the reactor for pyrolysis and gasification. Figure 2. Diagram of the reactor used for the CNT synthesis. Figure 3. (a) TEM image of the catalyst showing Fe2O3 nanoparticles over support and (b) the diameter distribution histogram. Figure 4. SEM images of Fe catalyst showing (a) a common image of SEM, (b) the iron particles in green color and (c) the aluminum particles in red color. Figure 5. Different TEM images of MWCNTs of the experiment at 600 ºC and 33 vol. % of hydrogen showing (a) CNTs from 4 to 13 layers, (b) CNTs with lengths on the order of microns and (c) an iron particle at the end of a CNT. Figure 6. Different TEM images of MWCNTs of the experiment at 600 ºC and 12,5 vol. % of hydrogen showing CNTs from 4 to 13 layers and 20 nm in diameters. Figure 7. Different TEM images of MWCNTs of the experiment at 650 ºC and 22 vol. % of hydrogen. Figure 8. Different TEM images of MWCNTs of the experiment at 750 ºC and 22 vol. % of hydrogen showing the walls straighter and fewer defects than at 650 ºC. Figure 9. XRD spectrums of MWCNT products. Figure 10. Raman spectrums of MWCNT products. Figure 11. DTG curves of MWCNT products. (a) With variation of hydrogen concentration. (b) With variation of growth temperature.


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Figure 1. Diagram of the reactor for pyrolysis and gasification. 9x3mm (600 x 600 DPI)

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Figure 2. Diagram of the reactor used for the CNT synthesis. 19x9mm (600 x 600 DPI)


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Figure 3. (a) TEM image of the catalyst showing Fe2O3 nanoparticles over support and (b) the diameter distribution histogram. 68x29mm (300 x 300 DPI)



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Figure 4. SEM images of Fe catalyst showing (a) a common image of SEM, (b) the iron particles in green color and (c) the aluminum particles in red color. 285x82mm (300 x 300 DPI)


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Figure 5. Different TEM images of MWCNTs of the experiment at 600 ºC and 33 vol. % of hydrogen showing (a) CNTs from 4 to 13 layers, (b) CNTs with lengths on the order of microns and (c) an iron particle at the end of a CNT. 38x12mm (300 x 300 DPI)



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Figure 6. Different TEM images of MWCNTs of the experiment at 600 ºC and 12,5 vol. % of hydrogen showing CNTs from 4 to 13 layers and 20 nm in diameters. 40x18mm (300 x 300 DPI)


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Figure 7. Different TEM images of MWCNTs of the experiment at 650 ºC and 22 vol. % of hydrogen. 38x12mm (300 x 300 DPI)



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Figure 8. Different TEM images of MWCNTs of the experiment at 750 ºC and 22 vol. % of hydrogen showing the walls straighter and fewer defects than at 650 ºC. 39x13mm (300 x 300 DPI)


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Figure 9. XRD spectrums of MWCNT products. 51x59mm (300 x 300 DPI)



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Figure 10. Raman spectrums of MWCNT products. 61x59mm (300 x 300 DPI)


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Figure 11. DTG curves of MWCNT products. (a) With variation of hydrogen concentration. (b) With variation of growth temperature. 308x123mm (300 x 300 DPI)


Production of Carbon Nanotubes from Polyethylene Pyrolysis Gas and

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