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Journal of Analytical and Applied Pyrolysis 94 (2012) 91–98

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Journal of Analytical and Applied Pyrolysis

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Pyrolysis of waste polypropylene for the synthesis of carbon nanotubes

Neeraj Mishraa,∗, Gobind Dasb, Alberto Ansaldoc, Alessandro Genoveseb, Mario Malerbab,Mauro Poviab, Davide Ricci c, Enzo Di Fabriziob, Ermanno Di Zittid, Madhuri Sharone, Maheshwar Sharone

a Nanotechnology Research center (NTRC), Birla College of Arts Science and Commerce, Kalyan (w) 421301, Thane, Maharashtra, Indiab Nanobiotech Facilities, Italian Institute of Technology, Via Morego 30, I-16163 Genoa, Italyc Soft Material Design Lab., Dept. of Robotics, Brain, and Cognitive Science, Italian Institute of Technology, Via Morego 30, I-16163 Genoa, Italyd Dept. of Biophysical and Electronic Engineering, University di Genova, Via Opera Pia 11a, I-16145 Genoa, Italye N. S. N. Research Centre for Nanotechnology & Bionanotechnology, Ambernath (W), Maharashtra, India

a r t i c l e i n f o

Article history:Received 17 June 2011Accepted 24 November 2011Available online 13 December 2011

Keywords:PolypropylenePyrolysisMWCNTsXRDRaman scatteringSEMTEMHRTEM

a b s t r a c t

Waste polypropylene (PP) is used as precursor for synthesizing multi-walled carbon nanotubes (MWC-NTs) by single stage chemical vapor deposition (CVD) method using nickel as catalyst. The pyrolyticdegradation of PP to MWCNT was achieved by exposing the catalyst and precursor to temperature 600,700and 800 ◦C under argon and hydrogen atmosphere for an hour. The resultant carbon was purified andcharacterized by XRD, Raman scattering, SEM, TEM and HRTEM. All the analysis confirmed the graphiticnature and multi-walled morphology of the CNT. Moreover the MWCNT was found to exhibit high trans-mittance to visible light up to 85% at 550 nm, comparable to that of typical ITO films (90%), suggestingthat MWCNTs can be used for optoelectronic devices.

The present work could be employed for synthesizing CNTs, having wide range of applications, and forenvironment protection as well since waste plastic is being used.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Plastic is a nuisance and destruction for mangrove ecosystemdue to its non-dissociation property in nature [1]. Thin plastic bagshave no recyclable value and are abandoned/disposed by publicdirectly or by packing refuse in it. Being cheap, hawkers, vendors,and shopkeepers use them for packing items. Governments from allover the world have taken a serious note of this plastic menace andeven banned plastic below 50 �m (along with plastic carry bags).It is necessary to have mechanism in place for regulating its use,recycling and disposal instead of banning or stop making of plas-tics to remove the plastics from the environment [2,3]. Recyclingand combustion process make new product which is less valuablecompared to product form by pyrolysis method.

Converting waste plastics into valuable products such as carbonnanotubes (CNTs) could be an important and profitable option forindustry and for environmental protection as well. CNTs, due to itshigh tensile strength, mechanical, electrical, and chemical proper-ties, offer a great potential for different applications spanning from

∗ Corresponding author. Tel.: +91 9322912462; fax: +91 251 2680800.E-mail address: mishraneerajprakash@gmail.com (N. Mishra).

textile [4], electronics [5], microwave absorption [6], actuator [7]to biomedical [8].

In particular, various methods have been developed for the syn-thesis of CNTs such as arc discharge [9], pyrolysis [10], laser ablation[11], plasma assisted deposition [12] and thermal chemical vapordeposition [13]. Among them, chemical vapor deposition (CVD) iscurrently the most widely used, because it requires relatively low-cost equipment and is capable of producing comparatively largeamounts of CNTs in a short time. Catalytic decomposition of hydro-carbons is a promising means to produce CNTs on a large scale [14].Synthesis of CNTs through thermal decomposition of polymer (orplastic derivatives such as polyethylene (PE), polypropylene (PP))and using catalysts has also been reported [15–19]. Kong and Zhang[17] and Zhang et al. [19] has used PE and MA-PP as precursorfor synthesis of CNTs in which they have mixed PE and PP andferrocene/Ni as catalyst in the stainless steel autoclave heated at700 ◦C for 100 min. Jiang et al. [18] has used combustion methodfor synthesis of CNTs using Ni compound/organic-modified mont-morillonite (OMMT) and polypropylene as a source. Multi-walledcarbon nanotubes (MWCNTs) were efficiently synthesized by cat-alytic combustion of PP using Nickel compounds as catalysts in thepresence of OMMT at 630–830 ◦C by Tang et al. [20]. Mass pro-duction of MWCNTs by means of pyrolysis of virgin or recycled

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polyolefins was described by Arena et al. [21]. In an earlier effortby Zhou et al. [22], two step processes combustion and pyrolysiswere used for synthesis of carbon nanomaterials using catalyst. Inour case, all the catalytic reaction occurs in single stage CVD sys-tem in hydrogen and Ar gas ambient. Consequently, the residualgases such as aliphatic hydrocarbons were predominantly gener-ated without any pollutant.

In the present work, we have made an attempt to exploit wasteplastic as potential precursor for synthesis of CNTs using CVD.The synthesis of MWCNTs from waste PP plastics using Ni asa catalyst in CVD chamber at pyrolysis temperature 600–800 ◦Cis reported. The samples were analyzed by means of Ramanspectroscopy, scanning electron microscopy (SEM), transmissionelectron microscopy (TEM), high resolution transmission electronmicroscopy (HRTEM) and optical absorption spectroscopy. In addi-tion, gas chromatography–mass spectrometry (GC–MS) was alsoperformed to understand the type of the residual gas coming outduring the whole process. Furthermore, we also investigated thevariation in Raman spectra peak intensity by varying the laserwavelength in order to reveal the purity of sample.

2. Experimental

2.1. Sample preparation

Waste polypropylene, collected from Brihanmumbai MunicipalCorporation (BMC) garbage disposal center, was used in the presentwork. The samples were washed, air-dried and shredded into smallpieces. 10 g of shredded PP and 1 mg of nickel (Ni) catalyst were keptinto quartz boat separately. Both were placed inside a long stain-less tube of a furnace (Fig. 1). Ni catalyst was prepared from nickelnitrate by urea decomposition method [23]. Thermal treatmentusing electrical-heated furnace was performed under controlledH2 (10 sccm) and Ar (90 sccm) gas atmosphere at fixed tempera-ture with 1 h dwell time in the temperature range between 600 and800 ◦C in order to catalyze the pyrolysis reaction. The ramp tem-perature of 30 ◦C/min was used during this whole reaction processuntil the required temperature (600, 700 and 800 ◦C, in our case)was achieved. The furnace temperature was allowed to cool downnaturally once the reaction time was over in the atmosphere of100 sccm Ar gas.

2.2. Process mechanism

Precursor molecule such as PP decomposes during interactionwith the catalyst particle surface. This leads to the diffusion of car-bon onto the surface of catalyst or through the catalyst particle[24,25]. When the catalyst gets supersaturated with carbon par-ticle, growth/nucleation of CNTs begins with the precipitation ofcarbon on surface, exposed to reactant gases. It was reported that

when the concentration of carbon source such as polypropyleneis too high, there is an over-abundance of decomposition and notenough precipitation resulting in amorphous carbon formation.Therefore, H2 gas is essential for growth because it maintain theactivity of the Catalyst. According to Baker et al. [25], tempera-ture gradient (TG) must be created with vapor–liquid–solid (VLS)model while studying the growth mechanism of carbon nanotubes.Decomposition of PP takes place at around 400 ◦C and dissolution ofcarbon takes either over the surface of catalyst which is an exother-mic reaction. After some time due to carbon accumulation on tothe surface of catalyst, precipitation of C atoms takes place form-ing nucleating sites for proper growth of CNT which is purely anendothermic reaction. TG arises between the hot and cold regionsduring growth of CNTs, first is hot region where carbon atoms dis-solves onto catalyst and the cold region, where carbon atom getsprecipitated after diffusion of carbon on to the catalyst. These tworegions of TG are significant for growth of CNTs. Initially TG providesheat flow through the particles that can support the transport of dis-solved carbon atom from hot to cold region. Finally, the depositionof carbon atom over the catalyst leads to saturation and formationof precipitate. In this process cold region needs a lower carbon con-centration compared to hot region. Due to hot region, continuousdissolution of carbon atom takes place over the surface of cata-lyst and that leads to large surface area for decomposition of morecarbon atom since continuous deposition of carbon atom on to cata-lyst particles leads to catalyst poisoning. Therefore hot region helpsto prevent catalyst poisoning and formation of amorphous carbonwhile cold region preferentially leads to the formation of CNTs, thuspreventing the growth of graphene sheets.

2.3. Characterization techniques

Raman measurements were carried out using Jobin-YvonLabram spectrometer. The laser excitation wavelength was 632.8,532 and 488 nm with a spectral resolution of <1.5 cm−1. All mea-surements were performed using a 50× objective and with poweraround 0.5 mW on the sample. The accumulation time was fixed to50 s. Raman spectra were firstly baseline corrected with 3rd orderpolynomial and, thereafter, have been normalized to the max of thepeak intensity.

The X-rays diffraction (XRD) pattern was recorded using Rigakusmatlab 9 kW (Japan) D/max-�A X-ray diffractometer with graphitemonochromatized Cu K� radiation (� = 1.54178 A).

Purified MWCNTs (15 mg) was dispersed in 40 ml of aqueoussodium dodecyl sulfate (SDS) surfactant (1 wt%) for 2 h of high-shear mixing. The mixture of CNT and SDS was then sonicatedfor 15 min. After sonication, samples were centrifuged at speedof 4000 rpm for 4 h. The solvent was then carefully decanted,leaving micelle-suspended nanotubes solutions at a typical massconcentration of 20–25 mg/l as our MWCNTs sample [26]. Decanted

Fig. 1. Schematic diagram of unit used for synthesizing CNT by CVD method. In this diagram, a) gas cylinder, b) bubbler, c) flow meter, d) stainless steel, e) furnace, f) boatcontaining precursor, and g) boat containing catalyst.

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solution was used for the optical measurement using UV–vis–NIRabsorption spectrophotometer (model: Carry 5000).

Scanning electron microscope (SEM) measurements were per-formed using JEOL JSM-7500FA microscope, equipped with a coldfield emission gun. Samples pyrolyzed at various temperatureswere dispersed in the ethanol and sonicated for 10 min in soni-cator bath, and then a drop of solution was deposited on a siliconplatelet (3 × 3 mm) for imaging.

The samples for transmission electron microscope (TEM) anal-yses were prepared by dropping a dilute solution of ethanol andcarbon nanotubes powder on carbon coated copper grids andallowing the solvent to evaporate at room temperature. Conven-tional TEM images were recorded using JEOL JEM-1011 microscopeworking at an acceleration voltage of 100 kV for all the sam-ples. To understand the deeper intricacies of structure, HRTEMobservations were acquired using a used JEOL microscopy model:JEM-2200 FS, equipped with a field emission electron gun workingat an acceleration voltage of 200 keV, a column Omega filter and aCEOS spherical aberration corrector for objective lens which allowsreaching sub-angstrom spatial resolution of 0.9 A.

For the analysis of gases, the gases liberated during pyrolysiswere collected in a gas collector. For the collection of the gases,one end of the gas collector was attached to CVD system andother end was kept open so that gases coming out from the sys-tem are flushed into the gas collector to make gas collector freefrom other gases. Now the other end of gas collector was closed tocollect the gases coming out from the system. After some time,both the end of gas collector was closed and gas collector wasremoved from the system. This gas sample was taken for analy-sis. Syringe was inserted into gas sampler for collecting the gas.Then syringe was inserted into GC–MS instrument for analysis. Gaschromatography using a GCD-HP 1800A Chromatograph with a(15 mm × 0.25 mm × 0.25 �m) glass column packed with 80–100mesh n-octanes, Porasil C and with a flame ionization detector(FID). Data interpretation was carried out using a NIST data analysissystem. The total weight of gases produced was calculated. In gaschromatography helium gas at the flow rate 1.5 ml/min was usedas a carrier gas with an injection temperature of 250 ◦C. The oventemperature was programmed from 40 ◦C (2 min hold) to 140 ◦C(10 min hold) with heating ramp rate of 8 ◦C/min. Data scan rate ofthe sample was kept at 0.6 s/scan with mass scan range of 10–425m/z. Electron energy of mass spectrometer was 70 eV and the ionsource and coupling temperatures were 230 and 300 ◦C, respec-tively. The ion mass spectra derived were automatically comparedto spectral libraries. Standard solutions were analyzed to verify theidentity of the peaks by retention time and provide quantitativeanalysis.

3. Results and discussion

3.1. Raman scattering

Raman scattering is a very simple spectroscopic technique, yetpowerful tool for characterization of MWCNTs. Raman spectra from

Fig. 2. Typical resonance Raman spectrum of MWCNTs synthesized at differenttemperatures (600–800 ◦C) using a standard grating 600 lines/mm and 633 nm laser.

MWCNTs, grown from waste PP at different temperatures, usingthe excitation wavelength 633 nm are shown in Fig. 2. Variouscharacteristics Raman bands associated to the CNTs centered ataround 1325, 1570, 2650 and a shoulder at around 2900 cm−1 areobserved. The sharp and intense Raman G-band peak at 1567 cm−1

with respect to the weak D-band peak near 1322 cm−1 reflectsthe high purity of the MWCNTs [27]. With the increase in tem-perature, a corresponding increase in intensity of G-peak and atthe same time, decrease in the intensity of D-band is observed.The G′ band for MWCNTs, which is the overtones of D band at2650 cm−1, exhibits strong sp2 CNTs feature. It is a 2nd order twophonon process but, intriguingly, is clearly observable in the Ramanspectra. The peak position of different vibrational bands remainsunchanged but a remarkable variation in peak intensity of G bandis observed for the sample produced with the pyrolysis temper-ature 800 ◦C. The peak position and the ratio of Raman bands forMWCNTs by varying the temperature are summarized in Table 1. Itis clear from the table that the sample, grown at 800 ◦C, shows highpurity as the G band increases on increasing the pyrolysis tempera-ture. The intensity ratio (IG/ID) reaches to 1.74 compared to sampleprepared at 600 ◦C and 700 ◦C. MWCNTs, synthesized at higher tem-perature, have smooth surface and less defect (other carbonaceousmaterials, disorder carbon, etc.). MWCNTs, synthesized at lowertemperatures, show higher impurities, defects, and disorder com-ponents within the sample. The increase in temperature leads tothe decrease in defects, disorder and amorphous carbon and so theimproved purity of CNTs sample.

Furthermore, the measurements were also performed for MWC-NTs samples by varying the excitation wavelength (488, 532, and633 nm). It exhibits very little laser excitation frequency depen-dence. Fig. 3 shows the Raman spectra for MWCNTs sample, grownat 800 ◦C, by varying the excitation frequency. Raman spectra showvery little shift in peak position of different vibrational bands. Suchvariations in spectral parameters by varying the laser frequencyare also observed by Ouyang et al. [28]. The variation in peak

Table 1Raman shift of the MWCNTs prepared at different temperatures and measured using different laser excitation energy.

Laser Raman bands of CNTs

Temperature

600 ◦C 700 ◦C 800 ◦C

G D IG/ID G′ G D IG/ID G′ G D IG/ID G′

488 nm 1583 1364 1.73 2713 1581 1363 1.86 2714 1583 1363 2.90 2715532 nm 1583 1347 0.70 2685 1576 1345 0.68 2684 1579 1350 1.74 2695633 nm 1571 1322 0.63 2644 1569 1323 1.20 2647 1565 1321 1.74 2641

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Fig. 3. Raman spectrum of MWCNTs prepared at 800 ◦C using Ni as a catalyst andmeasured at different laser excitation wavelengths viz. 488, 532 and 633 nm.

position and peak intensity ratio by changing the excitation laserwavelength is listed in Table 1. These results suggest that MWCNTsynthesized at 800 ◦C is very pure and is less contaminated withany other impurities.

3.2. X-rays diffraction (XRD) analysis

XRD pattern was recorded for purified CNT hybrid using Ni as acatalyst and waste polypropylene as a precursor, pyrolyzed at fixedtemperature 600, 700 and 800 ◦C in the range of 20 to 85◦ 2�, shownin Fig. 4a–c. “Powder cell” software was used for the quantitativeanalysis. Two XRD pattern for samples pyrolyzed at 600 and 700 ◦Cshow that the MWCNTs show different kind of impurities. Iron-nickel oxide (JCPDS 01-074-6507) was found to possess as a majorcontamination of sample pyrolyzed at 600 ◦C. In addition few otherunidentified peaks were also observed. The presence of magnetite(JCPDS 01-088-0866) and NiFe (JCPDS 01-071-6116) as impurity inthe MWCNT sample was found in abundance. With the increase oftemperature, reaching to 800 ◦C, the impurity contribution reducesdramatically. Fig. 4c shows the presence of graphite (JCPDS 00-056-0159) and Ni (JCPDS 00-004-0850). The strongest diffraction peakat around 26◦, 42◦, 55◦ and 77◦ (related to (0 0 2), (1 0 0), (0 0 4), and(1 1 0) reflections of graphite, respectively) altogether indicate thepresence of graphitic structure. Instead, the weak intensity bandsin XRD spectra at 44◦ are related to the metallic Ni (catalyst used),respectively [29]. “Powder cell” software was used for the quanti-tative analysis. Data analysis revealed the Percentages of 11.7% asNi and 88.3% as graphite.

3.3. SEM analysis

Fig. 5 shows the SEM image of MWCNTs growth in the presenceof Ni catalyst at fixed temperature 600, 700 and 800 ◦C for 1 h in theatmosphere of Ar and H2. Images shown in Fig. 5a and b illustratehigh degree of impurity and irregularities which were found to bedisappeared when is annealed at 800 ◦C (see Fig. 5c). The averagediameter of tube CNTs is 20 nm and the length is of few tens to hun-dred of microns [30]. It could be clearly seen that after purificationalso there is no damage to CNTs but some catalyst are present inthe agglomerated form.

3.4. TEM analysis

Fig. 6 shows conventional TEM images of purified MWCNTs syn-thesized from waste PP using Ni as a catalyst. In order to removeamorphous carbon, the sample was first oxidized at 500 ◦C andto remove the catalysts, the oxidized powder was immersed in

Fig. 4. XRD patterns of the purified MWCNTs synthesized at different temperaturesin the range of 600–800 ◦C. The strongest diffraction peaks were observed at 26◦

in 2� corresponds to (0 0 2) graphite reflection at 3.348 A. a) XRD patterns of thepurified MWCNTs synthesized at 600 ◦C using Ni as a catalyst in the atmosphereof Ar and H2 gas, b) XRD patterns of the purified MWCNTs synthesized at 700 ◦Cusing Ni as a catalyst in the atmosphere of Ar and H2 gas and c) XRD patterns of thepurified MWCNTs synthesized at 800 ◦C using Ni as a catalyst in the atmosphere ofAr and H2 gas.

hydrochloric acid (HCl) at room temperature for 12 h [31]. TheMWCNTs were collected with a filter, and rinsed with de-ionizedwater and ethanol to remove HCl and then dried in vacuum. It wasfound that initially yield of materials is 20% and after purificationyield only decreases by 1% due to removal of some catalyst par-ticles (final yield after purification is 19%). Then, the sample wassonicated in ethanol and a drop of solution was deposited and letit evaporated on the copper micro-grid for TEM. Ni particles and

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Fig. 5. SEM image of the MWCNTs synthesized at different temperatures in therange of 600–800 ◦C for dwell time of 1 h using Ni nanoparticles that catalyzed theformation of the CNTs. a) SEM image of the purified MWCNTs synthesized at 600 ◦Cusing Ni as a catalyst in the atmosphere of Ar and H2 gas, b) SEM image of the purifiedMWCNTs synthesized at 700 ◦C using Ni as a catalyst in the atmosphere of Ar andH2 gas and c) SEM image of the purified MWCNTs synthesized at 800 ◦C using Ni asa catalyst in the atmosphere of Ar and H2 gas.

Fig. 6. TEM images of the carbon nanotubes obtained from the pyrolysis wastepolypropylene (PP) and Ni nanoparticles at different temperatures in the range of600–800 ◦C. a) TEM image of the purified MWCNTs synthesized at 600 ◦C using Ni asa catalyst in the atmosphere of Ar and H2 gas, b) TEM image of the purified MWCNTssynthesized at 700 ◦C using Ni as a catalyst in the atmosphere of Ar and H2 gas andc) TEM image of the purified MWCNTs synthesized at 600 ◦C using Ni as a catalystin the atmosphere of Ar and H2 gas.

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Fig. 7. HRTEM images of CNTs synthesized at 800 ◦C by single stage CVD (pyrolysis) method using Ni as a catalyst. MWCNTs with diameter 5–10 nm (A), and the presence ofNi nanoparticles within the MWCNTs (B).

bundles of MWCNTs are found (see in Fig. 6). Fig. 6 shows theTEM images of samples pyrolyzed at fixed temperatures; 600, 700and 800 ◦C. In the figure, it is clearly observed the amount of Nicatalyst reducing with the increase in pyrolysis temperature. Even-tually, at 700 ◦C, the amount of Ni is found in very low abundance.The estimated diameter of MWCNT is in the range of 10–25 nm.HRTEM analysis of the CNT (Fig. 7) shows that it contains curly typegraphene with its diameter in the range of 25 nm (Fig. 7A). Thoughtit is difficult to count the number of layers, but approximately itcontains about 20 layers. Fig. 7B shows the presence of Ni metal inthe center of the tube.

3.5. Optical properties of CNTs

Transmittance spectra of powder MWCNTs in suspension (in1 wt% SDS) were measured with light normally incident on thecuvette using SDS (1 wt%) solution as a blank for reference. Thetransmittance spectra of MWCNTs within the spectrum range from200 to 1400 nm shows a monotonic increase in the visible regionand becomes relatively flat (Fig. 8) in the near-infrared region [32].The dispersed MWCNTs exhibit sufficiently high transmittance to

Fig. 8. Transmittance spectra of purified MWCNTs prepared at 800 ◦C using Ni as acatalyst.

visible light up to 85% at 550 nm. This suggests that thin film ofMWCNT may be used as a transparent electrode for some spe-cific applications like transparent electrode provided the materialshows good electrical conductivity. Experiments are being con-ducted in this direction to find out the most suitable thickness andtemperature of its preparation.

Fig. 9. GC–MS of gases coming out during pyrolysis at 800 ◦C in the presence of Nicatalyst in the atmosphere of Ar and H2, shows (a) total ion chromatogram result-ing from pyrolyzing PP at 800 ◦C for 60 min in the presence of Ni catalyst in theatmosphere of Ar and H2 gas and (b). Mass spectra of peaks between 16.3 and17.5 min.

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3.6. Gas chromatography–mass spectrometry (GC–MS)

The gas produced during the pyrolysis was highly inflammablewith non-sooty flame. GC–MS analysis show the chromatogram ofthe gas (Fig. 9a) derived from the pyrolysis of waste polypropy-lene at 800 ◦C. At retention time 16.7 min, it shows maximum peakheight, centered at 149 m/z, indicating the presence of diene withC11H20 hydrocarbon with losing 3 hydrogen atoms (Fig. 9b). Gasproducts of catalytic pyrolysis mainly consisted of methane, ethaneand propane with small volume of C5H12 saturated hydrocarbons(43 m/z). As the temperature of pyrolysis was increased, in mostcases, there was an increase in the concentration of the individ-ual gases, reflecting the increase in overall gas yield. Concentrationof the carbon number species in the gas remained fairly constant.However, analysis of the gas showed that the concentration ofaliphatic species above C3 was greatly reduced. It was also foundthat for the gas, the diene to alkane ratio increased for each carbonnumber [33,34]. The gas must be containing lower carbon num-ber hydrocarbon because pyrolysis of PP with respect to catalystis expected to occur through the carbocation mechanism. Duringpyrolysis, PP degrades and its vapors react with Ni catalyst withmoderate temperature of 800 ◦C accelerated the cracking of lighthydrocarbon fragments, but inhibited the aromatization of naph-thalene by activating hydrogen to react with aromatics was wellexplained by Wddin and co-workers [35,36]. Nickel is commonlyused for the hydrogenation of aromatics to aliphatic compounds;that will be one of the reasons for the inhibition of aromatiza-tion. Hence no polycyclic aromatic hydrocarbon (PAHs) molecules,such as naphthalene, phenanthrene and anthracene which arecarcinogenic and mutagenic in nature, are found in GC–MSmeasurements.

According to Sojak et al. [37] and Kaminsky and Nun [38], cat-alytic cracking of PP gives some solid, semi-solid and gases, mostlycontaining branched alkenes, alkanes and alkadienes in a broadrange of carbon atoms number as main decomposition productsof polypropylene. For the present work we have used PP for thepyrolysis, which does not contain any halogen, aromatic compoundor sulfur. Hence it is not possible to produce any toxic gas. More-over, CO gas also cannot be produced because pyrolysis was donein absence of oxygen (i.e. in presence of Ar + H2).

4. Conclusions

The novelty of this work is to synthesize CNTs using the wasteplastic material with high purity. It is observed that pyrolysis ofplastic at 800 ◦C gives the purest form of CNT. This conclusionconfirmed by Raman scattering, HRTEM, SEM and XRD studies.GC–MS suggests that the residual gases during the formation ofCNT from waste plastic are ethane, butane, methane, etc. whichcould be further utilized for the fuel purpose. To investigate thehostile nature of the gas coming from the reactions,GC–MS wasused as potential tool for characterization. Results indicate absenceof PAHs and other noxious pollutants. The synthesis of this mate-rial opens a new research path for not only to synthesize a highquality MWCNTs which can be applied in different fields but alsoto remove the waste plastic from the environment, making theambient clean. Further application can be considered for trans-parent optoelectronic devices as it shows very high transmittance.Thus this work not only provides a means to dispose waste poly-thene of thickness less than 50 �m (as plastic of this thicknesshas created the maximum environmental problem) but to pro-duce MWCNT at a very low cost. Most of the applications of CNTare being shelves mainly because the cost of production of thismaterial is very high. Our research could give the answer to thisproblem.

Acknowledgments

One of the authors, Neeraj Mishra, is grateful to Italian Instituteof Technology, Genoa, Italy for providing facilities for sample char-acterization. He is also thankful to UGC to provide grant to carryout this work.

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