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Carbon nanotubes (CNTs) production from catalytic pyrolysis of waste plastics: The influence of catalyst and reaction pressure
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Review
Carbon nanotubes (CNTs) production from catalytic pyrolysis of waste plastics: The influence of catalyst and reaction pressure Jianqiao Wanga, Boxiong Shena, , Meichen Lana, Dongrui Kanga, Chunfei Wua,b, ⁎
a b
⁎
School of Energy & Environmental Engineering, Hebei University of Technology, Tianjin, China School of Chemistry and Chemical Engineering, Queens University Belfast, Belfast, Northern Ireland, BT7 1NN, United Kingdom
ARTICLE INFO
ABSTRACT
Keywords: Carbon nanotubes (CNTs) Plastic pyrolysis Pressure Polypropylene
The production of carbon nanotubes (CNTs) by catalytic pyrolysis of plastics is an environmentally friendly and promising method of waste treatment and energy/materials production. Three types of metal catalysts (Fe/ cordierite, Ni/cordierite and Ni-Mg/cordierite) were utilized during the catalytic pyrolysis process of polypropylene in this work. Meanwhile, the influence of reaction pressure (0.5–1.25 MPa) on the synthesis of CNTs was investigated. Carbon formation, especially CNTs, through catalytic pyrolysis of plastics has been tested in a fixed bed reactor, and the materials have been analyzed by temperature program oxidation (TPO), scanning electron microscopy (SEM), high resolution transmission electron microscopy (TEM) and Raman spectroscopy. The highest yield around 93 wt.% filamentous carbon was obtained using the Ni-based catalyst. The strong metal-support interaction within the Ni-Mg-based catalyst suppressed CNTs growth and resulted in shorter and irregular cylindrical carbon tubes. The yield of more uniform and thick CNTs increased with the additional of appropriate reaction pressure, especially at 1.0 MPa (198 mg/gPP). However, an excessive reaction pressure weakened CNTs growth and produced shorter length and larger diameters (around 30–50 nm) CNTs. The fraction of CNTs decreased when the reaction pressure was higher than 0.5 MPa.
1. Introduction Due to the characteristics of durable, moldable and inexpensive, plastics have been massively applied in a number of industries promoting the global economic growth effectively. However, the rapid growth of plastic use and the disposal of waste plastics cause a serious challenge for the environmental scrutiny. A total of 8.3 billion metric tons of plastic have been accumulated in 2017 [1]. Management of waste plastic has been challenging, as only 31.1% of waste plastic has been recycled and 27.3% has been landfilled and contaminate the environment of Europe in 2016. Roughly 12 billion tons of plastic waste are in landfills or in the natural environment all over the world by 2050 [1–4]. Thermal treatments of waste plastic for chemical products have been considered as promising methods to deal with waste plastic related environmental problems and exploit the full potential of waste plastics recycling [5,6]. Pyrolysis and gasification have been utilized to convert waste plastic into gases [7,8], liquid [9] and carbon nanotubes (CNTs) [10–12]. CNTs have wild-range applications such as structural materials, electronic and optical devices due to the excellent chemical and physical properties [13]. The synthetic methods of CNTs include
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arc discharge [14], laser vaporization [15] and chemical vapour deposition (CVD) [16]. Carbon-containing chemicals, such as CH4 and C2H2 are normally used as precursors for CNTs using a CVD method [17,18]. Since these small gases can also be obtained from pyrolysis of waste plastics such as polyethylene (PE), polypropylene (PP), and polyethylene terephthalate (PET), the production of CNTs from waste plastics pyrolysis has advantages of resource conservation and environmental protection [19]. Catalyst development is a critical factor to control the production of CNTs. Nickel-based catalysts are recognized as excellent for the production of CNTs due to the excellent ability of CeC and C–H bond cleavage. Yang et al. [20] studied the effect of nickel species using Ni/ Al2O3 catalyst on the production of CNTs and hydrogen. The results showed that smaller nickel nanoparticles containing metallic nickel and higher dispersion of metal on the support contributed to the high quality of CNTs. Wu et al. [21] investigated co-producing high value CNTs and hydrogen with the different metal molar ratios of Ni/Mn/Al catalyst using low-density polyethylene as feedstock. They found that a higher content of Mn promoted the production of carbon products, and around 90 wt.% of the carbons are CNTs. Furthermore, iron, cobalt and copper based catalysts have been prepared to produce CNTs and
Corresponding authors at: School of Chemistry and Chemical Engineering, Queen University Belfast, Northern Ireland, UK. E-mail addresses: [email protected] (B. Shen), [email protected] (C. Wu).
https://doi.org/10.1016/j.cattod.2019.01.058 Received 11 November 2018; Received in revised form 25 December 2018; Accepted 25 January 2019 0920-5861/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Wang, J., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.01.058
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Besides the introduced catalysts, other various factors of the CNTs synthesis during the pyrolysis process of the polymer plastic have been studied. For example, the influences of reaction temperature, Ni content and water injection on the formation of CNTs with a template-based catalyst (Ni/anodic aluminum, AAO) were investigated by Liu et al. [23]. The presence of steam and lower loading of Ni on AAO resulted in the formation of uniform CNTs. Different support materials like quartz, silicon alumina, zeolite and magnesium oxide have been applied, which would affect the growth of CNTs due to the catalyst-substrate interaction [24]. The operational factor of reaction pressure has also been reported affecting the process of polymer pyrolysis. For example, Murata et al. [25] reported the increase of operation pressure (0.1–0.8 MPa) would increase the gas production while decrease the average molecular weight of the gaseous products by affecting the scission of chain-end CeC bonds during the pyrolysis of polyethylene. Furthermore, Lopez et al. [26] and Ismadji et al. [27] studied the pyrolysis of the waste tyres under vacuum (25 and 50 kPa) and concluded that a negative pressure helped to increase gas yield and decrease the yield of single ring (C10) aromatic hydrocarbons. Other literature also pointed out that secondary reactions in the vapour phase were limited resulting from a shorter residence time during vacuum pyrolysis [28–30]. For example, Xiong et al. [31] investigated the synthesis of double-walled carbon nanotubes (DWCNTs) by chemical vapour deposition under different pressures (60–120 Torr). The amount and purity of the DWCNTs were largely affected by reaction pressure and temperature. However, there are few work investigating the influence of reaction pressure on the formation of CNTs from catalytic pyrolysis of plastic wastes. Herein, in this work, different heterogeneous catalysts have been utilized to produce CNTs using a two-stage fixed bed reactor, and different reaction pressures have been investigated. In addition, the morphology and quality of CNTs synthesis under different reaction pressures have been examined.
Fig. 1. Schematic diagram of Two-stage pyrolysis reactor system. Table 1 Nitrogen adsorption and desorption of the fresh catalysts. Catalyst materials
BET surface area (m2 g−1)
Average pore size (nm)
Pore volume (cm3 g−1)
Ni/cordierite Fe/cordierite Ni-Mg/cordierite
46.51 16.33 1.37
4.94 8.32 49.62
0.0575 0.0340 0.0170
2. Materials and methods 2.1. Materials Recycled polypropylene (PP) (Sinopec co., ltd, China) was the raw material used to undergo the pyrolysis in this work and the particle size was around 0.6 mm. The catalysts for carbon nanotube synthesis were prepared by an impregnation method. Catalyst support (Mg2Al4Si5O18) was loaded with reactive metals (including 10 wt.% iron, 10 wt.% nickel, and 5 wt.% magnesium bimetallic). Fe(NO3)3·9H2O, Ni (NO3)2·6H2O and Mg(NO3)2·6H2O as the precursors were dissolved in 100 mL of ethanol in proportion and then the cordierite was added into the solutions, respectively. The suspension was thoroughly mixed by water bath stirring at 50 °C followed by drying overnight in an oven at 80 °C. Then the catalysts were calcined in air at 750 °C for 3 h with 2 °C min−1 heating rate. The calcined catalysts were sieved to collect particles with sizes between 300 and 500 um for CNTs synthesis. 2.2. Experimental The experiments were carried out using a two-stage reactor shown in Fig. 1. The reactor for the preparation of CNTs was a stainless steel tube (i.d. 35 mm) enclosed with two separate heating furnaces. The temperature of the upper and lower tube were monitored by two different K-type thermocouples, respectively. A pressure gauge was used to monitor the pressure inside the reactor. The amount of the catalyst with 0.5 g was placed in the second stage of the reactor and 1 g of PP was placed in the upper stage reactor. The second stage of the reactor was firstly heated to 750 °C at a heating rate of 40 °C min−1 and remained constant until the end of the experiment. When the second stage of the reactor reached the target temperature, the first stage of the reactor was heated up to 500 °C at the rate of 10 °C min−1 and PP will be
Fig. 2. XRD patterns of Fe, Ni, and Ni-Mg catalysts.
hydrogen, and iron and nickel catalysts gave the largest yield of hydrogen and CNTs resulting from the appropriate metal-support interactions [22]. In addition, bimetallic metals such as Ni-Fe based catalysts have been applied to promote the production of high-value CNTs [19]. Elements of Mo and Mg have also been verified to promote and preserve the activity of Ni catalysts for the synthesis of CNTs with straight and double-helical structures [13]. 2
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2.3. Materials characterization BET surface area and pore diameter of the catalysts were calculated by N2 adsorption and desorption isotherms on an automatic adsorption equipment (ASAP2020, Micromeritics) operating at 77 K. Temperature programmed reduction (TPR) was performed to characterize the fresh catalyst in a thermo-gravimetric analyzer (TGA, SDT Q600). Around 10 mg of fresh catalyst was put in the crucible to be heated up to 1100 °C with a heating rate at 10 °C min−1 in the reduction atmosphere (5% hydrogen and 95% nitrogen). X-ray diffraction (XRD, D8 FOCUS, BRUKER) patterns were also determined to evaluate the crystalline structures of the catalysts and the interaction between the metals and the catalyst supports. A scanning electron microscopy (SEM, MERLIN compact, ZEISS) and high resolution transmission electron microscopy (HRTEM, Tecnai G2 F20, FEI) with accelerating voltage of 10 KV were utilized to observe the morphology of the fresh catalysts and the CNTs on the surfaces of the reacted catalysts. Raman spectroscopy at a wavelength of 532 nm at Raman shifts between 200 and 3500 cm−1 was also carried out to determine the graphitic quality of the produced carbon materials.
Fig. 3. DTG-TPR profiles of the fresh Fe, Ni, Ni-Mg catalysts.
pyrolyzed at this temperature for half an hour. The carrier gas of N2 flowed through the reactor with a flow at 100 mL min−1. CNTs would be synthesized on the surface of the catalysts. Three pressures of 0.5 MPa, 1.0 MPa and 1.25 MPa were used for investigating the influence of reaction pressure on CNTs formation from catalytic pyrolysis of plastics.
3. Results and discussion 3.1. Characterization of fresh catalysts The results of nitrogen adsorption and desorption of the fresh catalysts are shown in Table 1. BET surface areas of the fresh catalysts are
Fig. 4. Temperature program oxidation (TPO) investigation of reacted catalysts: (a) (b) the weight loss and derivate TPO plot and (c) different carbon types deposited on the spent catalyst. 3
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Fig. 5. the SEM morphology investigation of the reacted catalysts: (a) Ni-based catalyst, (b) Ni-Mg catalyst, and (c) Fe-based catalyst.
magnesium silicate and spinel are detected on the surfaces of the catalysts for the Mg-based catalyst, indicating the strong interaction between magnesium and support. The DTG-TPR curves of the three fresh catalysts present in Fig. 3. There are three notable reduction peaks when the Fe based catalyst was reduced. The reduction proceed for the iron oxide follows the path indicated in Eq. (1) [32]: Fe2O3 → Fe3O4 → [FeO] → Fe°
(1)
For the Fe based catalyst, the first peak at 450 °C is related to the conversion of Fe2O3 into Fe3O4, which is subsequently reduced to FeO and Fe illustrated by the broad peak between 500 °C–750 °C [33,34]. Yuan et al. [35] also reported that iron aluminate was reduced at above 850 °C. So the peak around 820 °C is suggested to be ascribed to the reactions between the metal and the support, resulting in a difficult reduction of the metal oxide [19]. There are two significant peaks shown in the Ni-based catalyst DTGTPR curve. The broad reduction peak between 350 °C to 500 °C can be described as [36]: NiO → Niδ+ → Ni° Fig. 6. The TEM morphology of the CNTs on the surface of the used Ni-based catalyst.
(2)
It is indicated that bulky NiO particles were reduced to Ni [12]. According to literature, the pure NiO sample showed a reduction peak around 385 °C [37]. The shift of the peak temperature to higher temperature indicates the presence of metal-support interaction. The second peak at around 550 °C shows a high interaction between NiO and the support [19–24,32]. The TPR results reveal that bulk NiO crystals are the prevailing metal phases in the Ni-based catalyst. Compared to the Ni catalyst, the Ni-Mg bimetallic co-impregnation catalyst is hardly reduced. The high temperature peak around 900 °C for the nickel-magnesium catalyst reduced by hydrogen was likely caused by the reduction of spinel-metal phase. The spinel-type metal phases
46.51, 16.33, and 1.37 m2 g−1 for Ni, Fe and Ni-Mg based catalysts, respectively. The smallest average pore diameter (from BJH method) was obtained for the Ni based catalysts among the three catalysts. The crystalline structure of the fresh catalysts was shown in Fig. 2. Due to the complex composition of the support, different metal oxides, including alumina, silica are identified in Fig. 2. It is clear that Ni-Mg mixed alloy is notable for the catalyst prepared by magnesium and nickel. It is worth noting that there is no magnesium oxide but some 4
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Fig. 7. the TPO investigation of the catalysts spent on different experiment conditions: (a) TGA-TPO, (b) DTG-TPO, (c) the content of the amorphous carbon and filamentous carbon in the reacted catalysts, (d) the ratio of the two kinds of carbon within each catalyst.
(NiAl2O4 and MgAl2O4) formed by nickel and magnesium migrating into the interior of the support are very resistant to hydrogen reduction [38–40]. This is consistent with the XRD results (Fig. 2) that a co-spinel phase is observed. It is worth noting that no reduction peak is detected at low temperature for Ni-Mg alloy.
considered the distinguish temperature point of the oxidation of amorphous and filamentous carbon in this work. Namely, the weight loss between 550 °C and 750 °C was assign to the oxidation of CNTs [20]. The amount of each type of carbons was determined by the calculations based on the weight loss of catalysts oxidation and the results are shown in Fig. 4(c). Almost the same amount of amorphous carbons and filamentous carbons were produced by iron based catalysts. Around 93 wt.% filamentous carbons are obtained using the nickel catalyst. However, the amorphous carbons are dominant in the spent Ni-Mg catalyst. This is likely related to the absence of NiO and MgO particles detected by TPR experiment (Fig. 3), and the strong metal-support reactions also inhibit the production of filamentous carbons. Zahra et al. [41] also concluded the addition of the element magnesium in the catalyst will suppress the formation of filamentous carbons.
3.2. Carbon nanotube production with different catalysts 3.2.1. TPO analysis of the reacted catalysts Temperature programmed oxidation (TPO) of the reacted catalysts were undertaken to illustrate the carbon deposited on the catalysts surface. The large difference in weight loss shown in Fig. 4(a) indicate that the nickel catalyst has the most carbon production among the three catalysts. The increase of the TPO curve for Fe-based catalyst (Fig. 4(a)) is assigned to the oxidation of iron particles within the spent catalyst. Most of the weight loss of iron based and Ni-Mg catalysts are taken placed before 550 °C whilst the peak temperature of the derivate weight loss of the nickel catalyst in Fig. 4(b) is around 600 °C, which are associated with the oxidation of amorphous and filamentous carbons, respectively. The ratio of different carbon forms were calculated based on the TGA experiment according to Eq. (3):
Mi (%) =
Mi × 100 % MTotal – MFresh
3.2.2. SEM and TEM results of the different reacted catalysts Fig. 5 displays SEM results of the reacted catalysts. Filamentous carbons can be clearly observed on the surface of the three catalysts. However, the amount and the morphology of the filamentous carbons are different. It is consistent with the results of the TPO analysis. As can be seen in Fig. 5(a), the SEM image of the spent nickel catalyst displays bamboo type filamentous carbons entangling with catalyst particles. The TEM image (Fig. 6) confirms the observed filamentous carbons are multiwall carbon nanotubes (MWCNTs) with outer diameters between 10 and 30 nm and inner diameters around 5 nm. There are metal particles wrapped in the tubes, indicating the tip growth mechanism of the CNTs [24]. However, when the catalyst prepared including magnesium, the length of CNTs is very short as shown in the Fig. 5(b). The CNT
(3)
Where MFresh and MTotal mean the mass of the unused and reacted catalysts, respectively. The “i” indicates amorphous or filamentous carbon and the Mi represents the mass of each carbon form on the catalyst surface, respectively. It worth noting 550 °C had been 5
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Fig. 8. SEM morphology analysis of reacted catalysts with different experimental pressures: (a) 0.1 MPa, (b) 0.5 MPa, (c) 1.0 MPa and (d) 1.25 MPa.
observed on the reacted Ni-Mg catalyst is about 50 nm diameter, which is a little thicker than that on Ni catalyst. The metal particle size within the catalyst has been proved to be correlation with the CNTs diameter [11,42]. For example, the large diameter of CNTs is related to the larger metal particles in the Ni-Mg catalyst as shown in Fig.2. The strong metal-support interaction is also suggested to suppress the axial growth of CNTs. SEM image of the iron catalyst is shown in Fig. 5(c). Irregular diameters and lengths of tubular carbons deposited on the catalyst. Comparing to the other two catalysts, the weaker metal-support interaction of the iron based catalyst results in metal sintering and larger metal particles after the experiment. The sintering of metal particles in the iron based catalyst is the main reason for a wide range of CNTs diameters (from 10 nm to more than 100 nm) [33].
kinds of carbons, including amorphous and filamentous carbons, are displayed in Fig. 7(c). It is obvious that the amount of carbons (both CNTs and amorphous carbon forms) on the surface of the reacted catalysts under various pressures are all larger than that under normal pressure. The yield of CNTs was maximum 198 mg / gPP with a relative content of 88.78 wt.% when the experimental pressure was 1 MPa. However, it is worth noting that with the increase of reaction pressure, the proportion of CNTs within the produce carbons is reduced. Although the yield of CNTs obtained under the pressure of 0.5 MPa was 164 mg/gPP, the relative fraction of CNTs was the highest (89.52 wt.%) (Fig. 7(d)). It can be concluded that although a high experimental pressure increases the yield of CNTs, an excessive pressure suppresses CNTs synthesis and lead to more amorphous carbon formation.
3.3. The influence of the pressures on the carbon production with Ni/ cordierite catalyst
3.3.2. SEM and TEM results of the reacted catalysts from different reaction pressures As shown in Fig. 8, filamentous carbons are clearly observed from the SEM analysis of the reacted catalysts derived from catalytic pyrolysis of polypropylene under different experimental pressures. The TEM images in Fig. 9 reveals that the prevalent carbons presented are so-called bamboo-shaped MWCNTs [21]. The SEM analysis is also consistent with the TPO results that the amount of CNTs increase as high pressure is applied. With the further increase of experimental pressure, more uniform and thick CNTs are observed. However, much shorter CNTs are shown in Fig. 8(d) in relation to the spent catalyst under pressure of 1.25 MPa, indicating that a higher pressure might suppresses the elongation of CNTs. The TEM morphology of CNTs synthesized under atmosphere is presented in Fig. 7. Thin CNTs with outer diameters about 10–30 nm, 3–5 nm wall thickness and up to micrometer length are clearly observed. In addition, parallel graphene layers with adjacent spacing of 0.34 nm can be observed. This is a typical parameter of MWCNTs
3.3.1. TPO analysis of the reacted catalysts under different pressures The results of TPO experiments and the content of amorphous and filamentous carbons derived from different pressures are illustrated in Fig. 7. As shown in Fig. 7(a), a maximum weight loss of 22.5% occurs in the sample obtained at 1.0 MPa. Even though the carbon deposition decreased when higher pressure of 1.25 MPa was utilized compare with 0.5 MPa and 1.0 MPa, it is higher than that produced at atmospheric pressure. It is demonstrated that the added pressure helps to increase the yield of CNTs during the catalytic pyrolysis of polypropylene. The derivative TPO results, presented as Fig. 7(b), show that most of the formed carbons have been oxidized after 550 °C. It is suggested that most of the carbons formed are filamentous. The peak temperature of the TPO results derived from using four reaction pressures is around 600 °C. It is indicated that there is a small difference of thermal stability and graphitization degree of formed carbons [19]. The yields of the two 6
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Fig. 10. Raman spectra of the reacted catalysts.
atmosphere pressure. This is consistent with the observation of SEM images. Moreover, with the further increase of reaction pressure, the parallelism of the graphene layer in the CNTs is poor compared to that produced under atmospheric pressure. It is suggested that reaction pressure could significantly affect the formation of CNTs from catalytic pyrolysis of plastics. Raman spectroscope is an effective assessment to determine the graphitization of the CNTs on account of its detection of the changes in the polarizability of the carbon atoms. As shown in Fig. 10, three bands can be the obviously observed. The bands at around 1348 cm−1 called D band is assigned as the disordered carbon structure with atoms lattice defect. The G band (at around 1580 cm−1) caused by the in-plane stretching vibration of the C atom sp2 hybridization indicates the graphitic nature of the sample (i.e., crystallinity of the sample, pristine arrangement of atoms) [21,46]. G’ band (at around 2690 cm−1) is derived from the two-phonon, second order scattering process. The G’ band is used as an indication of the purity of CNTs due to the twophoton elastic scattering process [47]. The intensity of the D band normalized to the G band (ID/IG) coupled with the ratios of IG’/IG are used to estimate the defects and crystallinity of CNTs. It is demonstrated that the catalysts under different experimental pressures have an ID/IG ratio between 0.45 to 0.83, and an IG’/IG ratio from 0.18 to 0.54. In general, increasing reaction pressure of carbon nanotube synthesis leads to a high degree of disordered CNTs. This is consistent with that the proportion of the filamentous carbons in the produced carbons is reduced with the further increase of reaction pressure (Fig. 7(d)). 4. Conclusion Three types of metal catalysts (Fe/cordierite, Ni/cordierite and NiMg/cordierite) and different reaction pressures (0.5–1.25 MPa) were utilized to evaluate the synthesis of CNTs from catalytic pyrolysis process of polypropylene. The following conclusions are obtained: 1) There are significant amount of carbons formed on the reacted catalysts, and most of them are MWCNTs. Nickel catalyst produces the largest amount filamentous carbons (around 93 wt.%) than the other catalysts. 2) The strong metal-support reaction suppresses the growth of CNTs and results in the production of short and irregular CNTs when magnesium-based catalyst was used. 3) CNTs yield is increased and the uniformity of CNTs is enhanced when a high pressure is used in the process, especially at 1.0 MPa (198 mg/gPP) 4) Further increase of reaction pressure weakens CNTs growth and results in the formation of shorter and large diameters (around
Fig. 9. Transmission electron microscopy (TEM) of the carbon deposition on the used Ni-based catalysts: (a) 0.5 MPa, (b) 1.0 MPa and (c) 1.25 MPa.
[43–45]. The presence of metal nanoparticles encapsulated inside the nanotubes indicates that the mechanism of CNTs growth at normal pressure is tip growth mode. The outer diameter of the CNTs produced at high pressures (Fig. 9(a)–(c)) is larger than that produced at 7
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30–50 nm) CNTs. 5) The ratio of CNTs was reduced when the reaction pressure is higher than 0.5 MPa.
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Acknowledgements The project was supported by National Natural Science Foundation of China (21706050), Key Project Natural Science Foundation of Tianjin (18JCZDJC39800), the Key Project of Science and Technology of Tianjin (18ZXSZSF00040), The Project of Science and Technology of Tangshan (18130211A), and Tianjin science popularization project (18KPXMSF00080). References [1] R. Geyer, J.R. Jambeck, K.L. Law, Production, use, and fate of all plastics ever made, Sci. Adv. 3 (7) (2017). [2] PlasticsEurope, Plastics - the facts 2014/2015: an analysis of European plastics production, demand and waste data, PlasticsEurope (2015) 1–34. [3] J.R. Jambeck, R. Geyer, C. Wilcox, T.R. Siegler, M. Perryman, A. Andrady, R. Narayan, K.L. Law, Plastic waste inputs from land into the ocean, Science 347 (6223) (2015) 768–771. [4] A.L. Brooks, S.L. Wang, J.R. Jambeck, The Chinese import ban and its impact on global plastic waste trade, Sci. Adv. 4 (6) (2018). [5] M.Y. He, B. Xiao, Z.Q. Hu, S.M. Liu, X.J. Guo, S.Y. Luo, Syngas production from catalytic gasification of waste polyethylene: influence of temperature on gas yield and composition, Int. J. Hydrogen Energy 34 (3) (2009) 1342–1348. [6] A. Demirbas, Pyrolysis of municipal plastic wastes for recovery of gasoline-range hydrocarbons, J. Anal. Appl. Pyrol. 72 (1) (2004) 97–102. [7] D.D. Yao, H.P. Yang, H.P. Chen, P.T. Williams, Investigation of nickel-impregnated zeolite catalysts for hydrogen/syngas production from the catalytic reforming of waste polyethylene, Appl. Catal. B-Environ. 227 (2018) 477–487. [8] C.F. Wu, P.T. Williams, Pyrolysis-gasification of post-consumer municipal solid plastic waste for hydrogen production, Int. J. Hydrogen Energy 35 (3) (2010) 949–957. [9] P.T. Williams, E.A. Williams, Fluidised bed pyrolysis of low density polyethylene to produce petrochemical feedstock, J. Anal. Appl. Pyrol. 51 (1–2) (1999) 107–126. [10] G.Q. Diao, H. Li, H. Liang, I. Ivanenko, T. Dontsova, I. Astrelin, CVD synthesis of multi-walled carbon nanotubes onto different catalysts at low temperature, Nano 13 (4) (2018). [11] V. Jourdain, C. Bichara, Current understanding of the growth of carbon nanotubes in catalytic chemical vapour deposition, Carbon 58 (2013) 2–39. [12] C.F. Wu, M.A. Nahil, N. Miskolczi, J. Huang, P.T. Williams, Production and application of carbon nanotubes, as a co-product of hydrogen from the pyrolysis-catalytic reforming of waste plastic, Process Saf. Environ. Protect. 103 (2016) 107–114. [13] R.J. Song, Q. Ji, Synthesis of carbon nanotubes from polypropylene in the presence of Ni/Mo/MgO catalysts via combustion, Chem. Lett. 40 (10) (2011) 1110–1112. [14] T.W. Ebbesen, P.M.A.J. Nature, Large-scale synthesis of carbon nanotubes, Nature 358 (6383) (1992) 220–222. [15] A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, Y.H. Lee, S.G. Kim, A.G. Rinzler, D.T. Colbert, E. Gustavo, D. Tománek, J.E. Fischer, R.E. Smalley, N. Series, N. Jul, A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y.H. Lee, S.G. Kim, A.G. Rinzler, D.T. Colbert, G.E. Scuseria, D. Tomanek, J.E. Fischer, R.E. Smalley, Crystalline ropes of metallic carbon nanotubes, Science 273 (2007) 483–487. [16] Z.F. Ren, Z.P. Huang, J.W. Xu, J.H. Wang, P. Bush, M.P. Siegal, P.N. Provencio, Synthesis of large arrays of well-aligned carbon nanotubes on glass, Science 282 (5391) (1998) 1105–1107. [17] E.S. Snow, F.K. Perkins, E.J. Houser, S.C. Badescu, T.L. Reinecke, Chemical detection with a single-walled carbon nanotube capacitor, Science 307 (5717) (2005) 1942–1945. [18] K.A. Shah, B.A. Tali, Synthesis of carbon nanotubes by catalytic chemical vapour deposition: a review on carbon sources, catalysts and substrates, Mater. Sci. Semicond. Process. 41 (2016) 67–82. [19] D.D. Yao, C.F. Wu, H.P. Yang, Y.S. Zhang, M.A. Nahil, Y.Q. Chen, P.T. Williams, H.P. Chen, Co-production of hydrogen and carbon nanotubes from catalytic pyrolysis of waste plastics on Ni-Fe bimetallic catalyst, Energy Convers. Manage. 148 (2017) 692–700. [20] R.-X. Yang, K.-H. Chuang, M.-Y. Wey, Effects of nickel species on Ni/Al2O3 catalysts in CNTs and hydrogen production by waste plastics gasification: bench-scale and pilot-scale tests, Energy Fuel 29 (2015).
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Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Review
Carbon nanotubes (CNTs) production from catalytic pyrolysis of waste plastics: The influence of catalyst and reaction pressure Jianqiao Wanga, Boxiong Shena, , Meichen Lana, Dongrui Kanga, Chunfei Wua,b, ⁎
a b
⁎
School of Energy & Environmental Engineering, Hebei University of Technology, Tianjin, China School of Chemistry and Chemical Engineering, Queens University Belfast, Belfast, Northern Ireland, BT7 1NN, United Kingdom
ARTICLE INFO
ABSTRACT
Keywords: Carbon nanotubes (CNTs) Plastic pyrolysis Pressure Polypropylene
The production of carbon nanotubes (CNTs) by catalytic pyrolysis of plastics is an environmentally friendly and promising method of waste treatment and energy/materials production. Three types of metal catalysts (Fe/ cordierite, Ni/cordierite and Ni-Mg/cordierite) were utilized during the catalytic pyrolysis process of polypropylene in this work. Meanwhile, the influence of reaction pressure (0.5–1.25 MPa) on the synthesis of CNTs was investigated. Carbon formation, especially CNTs, through catalytic pyrolysis of plastics has been tested in a fixed bed reactor, and the materials have been analyzed by temperature program oxidation (TPO), scanning electron microscopy (SEM), high resolution transmission electron microscopy (TEM) and Raman spectroscopy. The highest yield around 93 wt.% filamentous carbon was obtained using the Ni-based catalyst. The strong metal-support interaction within the Ni-Mg-based catalyst suppressed CNTs growth and resulted in shorter and irregular cylindrical carbon tubes. The yield of more uniform and thick CNTs increased with the additional of appropriate reaction pressure, especially at 1.0 MPa (198 mg/gPP). However, an excessive reaction pressure weakened CNTs growth and produced shorter length and larger diameters (around 30–50 nm) CNTs. The fraction of CNTs decreased when the reaction pressure was higher than 0.5 MPa.
1. Introduction Due to the characteristics of durable, moldable and inexpensive, plastics have been massively applied in a number of industries promoting the global economic growth effectively. However, the rapid growth of plastic use and the disposal of waste plastics cause a serious challenge for the environmental scrutiny. A total of 8.3 billion metric tons of plastic have been accumulated in 2017 [1]. Management of waste plastic has been challenging, as only 31.1% of waste plastic has been recycled and 27.3% has been landfilled and contaminate the environment of Europe in 2016. Roughly 12 billion tons of plastic waste are in landfills or in the natural environment all over the world by 2050 [1–4]. Thermal treatments of waste plastic for chemical products have been considered as promising methods to deal with waste plastic related environmental problems and exploit the full potential of waste plastics recycling [5,6]. Pyrolysis and gasification have been utilized to convert waste plastic into gases [7,8], liquid [9] and carbon nanotubes (CNTs) [10–12]. CNTs have wild-range applications such as structural materials, electronic and optical devices due to the excellent chemical and physical properties [13]. The synthetic methods of CNTs include
⁎
arc discharge [14], laser vaporization [15] and chemical vapour deposition (CVD) [16]. Carbon-containing chemicals, such as CH4 and C2H2 are normally used as precursors for CNTs using a CVD method [17,18]. Since these small gases can also be obtained from pyrolysis of waste plastics such as polyethylene (PE), polypropylene (PP), and polyethylene terephthalate (PET), the production of CNTs from waste plastics pyrolysis has advantages of resource conservation and environmental protection [19]. Catalyst development is a critical factor to control the production of CNTs. Nickel-based catalysts are recognized as excellent for the production of CNTs due to the excellent ability of CeC and C–H bond cleavage. Yang et al. [20] studied the effect of nickel species using Ni/ Al2O3 catalyst on the production of CNTs and hydrogen. The results showed that smaller nickel nanoparticles containing metallic nickel and higher dispersion of metal on the support contributed to the high quality of CNTs. Wu et al. [21] investigated co-producing high value CNTs and hydrogen with the different metal molar ratios of Ni/Mn/Al catalyst using low-density polyethylene as feedstock. They found that a higher content of Mn promoted the production of carbon products, and around 90 wt.% of the carbons are CNTs. Furthermore, iron, cobalt and copper based catalysts have been prepared to produce CNTs and
Corresponding authors at: School of Chemistry and Chemical Engineering, Queen University Belfast, Northern Ireland, UK. E-mail addresses: [email protected] (B. Shen), [email protected] (C. Wu).
https://doi.org/10.1016/j.cattod.2019.01.058 Received 11 November 2018; Received in revised form 25 December 2018; Accepted 25 January 2019 0920-5861/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Wang, J., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.01.058
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Besides the introduced catalysts, other various factors of the CNTs synthesis during the pyrolysis process of the polymer plastic have been studied. For example, the influences of reaction temperature, Ni content and water injection on the formation of CNTs with a template-based catalyst (Ni/anodic aluminum, AAO) were investigated by Liu et al. [23]. The presence of steam and lower loading of Ni on AAO resulted in the formation of uniform CNTs. Different support materials like quartz, silicon alumina, zeolite and magnesium oxide have been applied, which would affect the growth of CNTs due to the catalyst-substrate interaction [24]. The operational factor of reaction pressure has also been reported affecting the process of polymer pyrolysis. For example, Murata et al. [25] reported the increase of operation pressure (0.1–0.8 MPa) would increase the gas production while decrease the average molecular weight of the gaseous products by affecting the scission of chain-end CeC bonds during the pyrolysis of polyethylene. Furthermore, Lopez et al. [26] and Ismadji et al. [27] studied the pyrolysis of the waste tyres under vacuum (25 and 50 kPa) and concluded that a negative pressure helped to increase gas yield and decrease the yield of single ring (C10) aromatic hydrocarbons. Other literature also pointed out that secondary reactions in the vapour phase were limited resulting from a shorter residence time during vacuum pyrolysis [28–30]. For example, Xiong et al. [31] investigated the synthesis of double-walled carbon nanotubes (DWCNTs) by chemical vapour deposition under different pressures (60–120 Torr). The amount and purity of the DWCNTs were largely affected by reaction pressure and temperature. However, there are few work investigating the influence of reaction pressure on the formation of CNTs from catalytic pyrolysis of plastic wastes. Herein, in this work, different heterogeneous catalysts have been utilized to produce CNTs using a two-stage fixed bed reactor, and different reaction pressures have been investigated. In addition, the morphology and quality of CNTs synthesis under different reaction pressures have been examined.
Fig. 1. Schematic diagram of Two-stage pyrolysis reactor system. Table 1 Nitrogen adsorption and desorption of the fresh catalysts. Catalyst materials
BET surface area (m2 g−1)
Average pore size (nm)
Pore volume (cm3 g−1)
Ni/cordierite Fe/cordierite Ni-Mg/cordierite
46.51 16.33 1.37
4.94 8.32 49.62
0.0575 0.0340 0.0170
2. Materials and methods 2.1. Materials Recycled polypropylene (PP) (Sinopec co., ltd, China) was the raw material used to undergo the pyrolysis in this work and the particle size was around 0.6 mm. The catalysts for carbon nanotube synthesis were prepared by an impregnation method. Catalyst support (Mg2Al4Si5O18) was loaded with reactive metals (including 10 wt.% iron, 10 wt.% nickel, and 5 wt.% magnesium bimetallic). Fe(NO3)3·9H2O, Ni (NO3)2·6H2O and Mg(NO3)2·6H2O as the precursors were dissolved in 100 mL of ethanol in proportion and then the cordierite was added into the solutions, respectively. The suspension was thoroughly mixed by water bath stirring at 50 °C followed by drying overnight in an oven at 80 °C. Then the catalysts were calcined in air at 750 °C for 3 h with 2 °C min−1 heating rate. The calcined catalysts were sieved to collect particles with sizes between 300 and 500 um for CNTs synthesis. 2.2. Experimental The experiments were carried out using a two-stage reactor shown in Fig. 1. The reactor for the preparation of CNTs was a stainless steel tube (i.d. 35 mm) enclosed with two separate heating furnaces. The temperature of the upper and lower tube were monitored by two different K-type thermocouples, respectively. A pressure gauge was used to monitor the pressure inside the reactor. The amount of the catalyst with 0.5 g was placed in the second stage of the reactor and 1 g of PP was placed in the upper stage reactor. The second stage of the reactor was firstly heated to 750 °C at a heating rate of 40 °C min−1 and remained constant until the end of the experiment. When the second stage of the reactor reached the target temperature, the first stage of the reactor was heated up to 500 °C at the rate of 10 °C min−1 and PP will be
Fig. 2. XRD patterns of Fe, Ni, and Ni-Mg catalysts.
hydrogen, and iron and nickel catalysts gave the largest yield of hydrogen and CNTs resulting from the appropriate metal-support interactions [22]. In addition, bimetallic metals such as Ni-Fe based catalysts have been applied to promote the production of high-value CNTs [19]. Elements of Mo and Mg have also been verified to promote and preserve the activity of Ni catalysts for the synthesis of CNTs with straight and double-helical structures [13]. 2
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2.3. Materials characterization BET surface area and pore diameter of the catalysts were calculated by N2 adsorption and desorption isotherms on an automatic adsorption equipment (ASAP2020, Micromeritics) operating at 77 K. Temperature programmed reduction (TPR) was performed to characterize the fresh catalyst in a thermo-gravimetric analyzer (TGA, SDT Q600). Around 10 mg of fresh catalyst was put in the crucible to be heated up to 1100 °C with a heating rate at 10 °C min−1 in the reduction atmosphere (5% hydrogen and 95% nitrogen). X-ray diffraction (XRD, D8 FOCUS, BRUKER) patterns were also determined to evaluate the crystalline structures of the catalysts and the interaction between the metals and the catalyst supports. A scanning electron microscopy (SEM, MERLIN compact, ZEISS) and high resolution transmission electron microscopy (HRTEM, Tecnai G2 F20, FEI) with accelerating voltage of 10 KV were utilized to observe the morphology of the fresh catalysts and the CNTs on the surfaces of the reacted catalysts. Raman spectroscopy at a wavelength of 532 nm at Raman shifts between 200 and 3500 cm−1 was also carried out to determine the graphitic quality of the produced carbon materials.
Fig. 3. DTG-TPR profiles of the fresh Fe, Ni, Ni-Mg catalysts.
pyrolyzed at this temperature for half an hour. The carrier gas of N2 flowed through the reactor with a flow at 100 mL min−1. CNTs would be synthesized on the surface of the catalysts. Three pressures of 0.5 MPa, 1.0 MPa and 1.25 MPa were used for investigating the influence of reaction pressure on CNTs formation from catalytic pyrolysis of plastics.
3. Results and discussion 3.1. Characterization of fresh catalysts The results of nitrogen adsorption and desorption of the fresh catalysts are shown in Table 1. BET surface areas of the fresh catalysts are
Fig. 4. Temperature program oxidation (TPO) investigation of reacted catalysts: (a) (b) the weight loss and derivate TPO plot and (c) different carbon types deposited on the spent catalyst. 3
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Fig. 5. the SEM morphology investigation of the reacted catalysts: (a) Ni-based catalyst, (b) Ni-Mg catalyst, and (c) Fe-based catalyst.
magnesium silicate and spinel are detected on the surfaces of the catalysts for the Mg-based catalyst, indicating the strong interaction between magnesium and support. The DTG-TPR curves of the three fresh catalysts present in Fig. 3. There are three notable reduction peaks when the Fe based catalyst was reduced. The reduction proceed for the iron oxide follows the path indicated in Eq. (1) [32]: Fe2O3 → Fe3O4 → [FeO] → Fe°
(1)
For the Fe based catalyst, the first peak at 450 °C is related to the conversion of Fe2O3 into Fe3O4, which is subsequently reduced to FeO and Fe illustrated by the broad peak between 500 °C–750 °C [33,34]. Yuan et al. [35] also reported that iron aluminate was reduced at above 850 °C. So the peak around 820 °C is suggested to be ascribed to the reactions between the metal and the support, resulting in a difficult reduction of the metal oxide [19]. There are two significant peaks shown in the Ni-based catalyst DTGTPR curve. The broad reduction peak between 350 °C to 500 °C can be described as [36]: NiO → Niδ+ → Ni° Fig. 6. The TEM morphology of the CNTs on the surface of the used Ni-based catalyst.
(2)
It is indicated that bulky NiO particles were reduced to Ni [12]. According to literature, the pure NiO sample showed a reduction peak around 385 °C [37]. The shift of the peak temperature to higher temperature indicates the presence of metal-support interaction. The second peak at around 550 °C shows a high interaction between NiO and the support [19–24,32]. The TPR results reveal that bulk NiO crystals are the prevailing metal phases in the Ni-based catalyst. Compared to the Ni catalyst, the Ni-Mg bimetallic co-impregnation catalyst is hardly reduced. The high temperature peak around 900 °C for the nickel-magnesium catalyst reduced by hydrogen was likely caused by the reduction of spinel-metal phase. The spinel-type metal phases
46.51, 16.33, and 1.37 m2 g−1 for Ni, Fe and Ni-Mg based catalysts, respectively. The smallest average pore diameter (from BJH method) was obtained for the Ni based catalysts among the three catalysts. The crystalline structure of the fresh catalysts was shown in Fig. 2. Due to the complex composition of the support, different metal oxides, including alumina, silica are identified in Fig. 2. It is clear that Ni-Mg mixed alloy is notable for the catalyst prepared by magnesium and nickel. It is worth noting that there is no magnesium oxide but some 4
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Fig. 7. the TPO investigation of the catalysts spent on different experiment conditions: (a) TGA-TPO, (b) DTG-TPO, (c) the content of the amorphous carbon and filamentous carbon in the reacted catalysts, (d) the ratio of the two kinds of carbon within each catalyst.
(NiAl2O4 and MgAl2O4) formed by nickel and magnesium migrating into the interior of the support are very resistant to hydrogen reduction [38–40]. This is consistent with the XRD results (Fig. 2) that a co-spinel phase is observed. It is worth noting that no reduction peak is detected at low temperature for Ni-Mg alloy.
considered the distinguish temperature point of the oxidation of amorphous and filamentous carbon in this work. Namely, the weight loss between 550 °C and 750 °C was assign to the oxidation of CNTs [20]. The amount of each type of carbons was determined by the calculations based on the weight loss of catalysts oxidation and the results are shown in Fig. 4(c). Almost the same amount of amorphous carbons and filamentous carbons were produced by iron based catalysts. Around 93 wt.% filamentous carbons are obtained using the nickel catalyst. However, the amorphous carbons are dominant in the spent Ni-Mg catalyst. This is likely related to the absence of NiO and MgO particles detected by TPR experiment (Fig. 3), and the strong metal-support reactions also inhibit the production of filamentous carbons. Zahra et al. [41] also concluded the addition of the element magnesium in the catalyst will suppress the formation of filamentous carbons.
3.2. Carbon nanotube production with different catalysts 3.2.1. TPO analysis of the reacted catalysts Temperature programmed oxidation (TPO) of the reacted catalysts were undertaken to illustrate the carbon deposited on the catalysts surface. The large difference in weight loss shown in Fig. 4(a) indicate that the nickel catalyst has the most carbon production among the three catalysts. The increase of the TPO curve for Fe-based catalyst (Fig. 4(a)) is assigned to the oxidation of iron particles within the spent catalyst. Most of the weight loss of iron based and Ni-Mg catalysts are taken placed before 550 °C whilst the peak temperature of the derivate weight loss of the nickel catalyst in Fig. 4(b) is around 600 °C, which are associated with the oxidation of amorphous and filamentous carbons, respectively. The ratio of different carbon forms were calculated based on the TGA experiment according to Eq. (3):
Mi (%) =
Mi × 100 % MTotal – MFresh
3.2.2. SEM and TEM results of the different reacted catalysts Fig. 5 displays SEM results of the reacted catalysts. Filamentous carbons can be clearly observed on the surface of the three catalysts. However, the amount and the morphology of the filamentous carbons are different. It is consistent with the results of the TPO analysis. As can be seen in Fig. 5(a), the SEM image of the spent nickel catalyst displays bamboo type filamentous carbons entangling with catalyst particles. The TEM image (Fig. 6) confirms the observed filamentous carbons are multiwall carbon nanotubes (MWCNTs) with outer diameters between 10 and 30 nm and inner diameters around 5 nm. There are metal particles wrapped in the tubes, indicating the tip growth mechanism of the CNTs [24]. However, when the catalyst prepared including magnesium, the length of CNTs is very short as shown in the Fig. 5(b). The CNT
(3)
Where MFresh and MTotal mean the mass of the unused and reacted catalysts, respectively. The “i” indicates amorphous or filamentous carbon and the Mi represents the mass of each carbon form on the catalyst surface, respectively. It worth noting 550 °C had been 5
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Fig. 8. SEM morphology analysis of reacted catalysts with different experimental pressures: (a) 0.1 MPa, (b) 0.5 MPa, (c) 1.0 MPa and (d) 1.25 MPa.
observed on the reacted Ni-Mg catalyst is about 50 nm diameter, which is a little thicker than that on Ni catalyst. The metal particle size within the catalyst has been proved to be correlation with the CNTs diameter [11,42]. For example, the large diameter of CNTs is related to the larger metal particles in the Ni-Mg catalyst as shown in Fig.2. The strong metal-support interaction is also suggested to suppress the axial growth of CNTs. SEM image of the iron catalyst is shown in Fig. 5(c). Irregular diameters and lengths of tubular carbons deposited on the catalyst. Comparing to the other two catalysts, the weaker metal-support interaction of the iron based catalyst results in metal sintering and larger metal particles after the experiment. The sintering of metal particles in the iron based catalyst is the main reason for a wide range of CNTs diameters (from 10 nm to more than 100 nm) [33].
kinds of carbons, including amorphous and filamentous carbons, are displayed in Fig. 7(c). It is obvious that the amount of carbons (both CNTs and amorphous carbon forms) on the surface of the reacted catalysts under various pressures are all larger than that under normal pressure. The yield of CNTs was maximum 198 mg / gPP with a relative content of 88.78 wt.% when the experimental pressure was 1 MPa. However, it is worth noting that with the increase of reaction pressure, the proportion of CNTs within the produce carbons is reduced. Although the yield of CNTs obtained under the pressure of 0.5 MPa was 164 mg/gPP, the relative fraction of CNTs was the highest (89.52 wt.%) (Fig. 7(d)). It can be concluded that although a high experimental pressure increases the yield of CNTs, an excessive pressure suppresses CNTs synthesis and lead to more amorphous carbon formation.
3.3. The influence of the pressures on the carbon production with Ni/ cordierite catalyst
3.3.2. SEM and TEM results of the reacted catalysts from different reaction pressures As shown in Fig. 8, filamentous carbons are clearly observed from the SEM analysis of the reacted catalysts derived from catalytic pyrolysis of polypropylene under different experimental pressures. The TEM images in Fig. 9 reveals that the prevalent carbons presented are so-called bamboo-shaped MWCNTs [21]. The SEM analysis is also consistent with the TPO results that the amount of CNTs increase as high pressure is applied. With the further increase of experimental pressure, more uniform and thick CNTs are observed. However, much shorter CNTs are shown in Fig. 8(d) in relation to the spent catalyst under pressure of 1.25 MPa, indicating that a higher pressure might suppresses the elongation of CNTs. The TEM morphology of CNTs synthesized under atmosphere is presented in Fig. 7. Thin CNTs with outer diameters about 10–30 nm, 3–5 nm wall thickness and up to micrometer length are clearly observed. In addition, parallel graphene layers with adjacent spacing of 0.34 nm can be observed. This is a typical parameter of MWCNTs
3.3.1. TPO analysis of the reacted catalysts under different pressures The results of TPO experiments and the content of amorphous and filamentous carbons derived from different pressures are illustrated in Fig. 7. As shown in Fig. 7(a), a maximum weight loss of 22.5% occurs in the sample obtained at 1.0 MPa. Even though the carbon deposition decreased when higher pressure of 1.25 MPa was utilized compare with 0.5 MPa and 1.0 MPa, it is higher than that produced at atmospheric pressure. It is demonstrated that the added pressure helps to increase the yield of CNTs during the catalytic pyrolysis of polypropylene. The derivative TPO results, presented as Fig. 7(b), show that most of the formed carbons have been oxidized after 550 °C. It is suggested that most of the carbons formed are filamentous. The peak temperature of the TPO results derived from using four reaction pressures is around 600 °C. It is indicated that there is a small difference of thermal stability and graphitization degree of formed carbons [19]. The yields of the two 6
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Fig. 10. Raman spectra of the reacted catalysts.
atmosphere pressure. This is consistent with the observation of SEM images. Moreover, with the further increase of reaction pressure, the parallelism of the graphene layer in the CNTs is poor compared to that produced under atmospheric pressure. It is suggested that reaction pressure could significantly affect the formation of CNTs from catalytic pyrolysis of plastics. Raman spectroscope is an effective assessment to determine the graphitization of the CNTs on account of its detection of the changes in the polarizability of the carbon atoms. As shown in Fig. 10, three bands can be the obviously observed. The bands at around 1348 cm−1 called D band is assigned as the disordered carbon structure with atoms lattice defect. The G band (at around 1580 cm−1) caused by the in-plane stretching vibration of the C atom sp2 hybridization indicates the graphitic nature of the sample (i.e., crystallinity of the sample, pristine arrangement of atoms) [21,46]. G’ band (at around 2690 cm−1) is derived from the two-phonon, second order scattering process. The G’ band is used as an indication of the purity of CNTs due to the twophoton elastic scattering process [47]. The intensity of the D band normalized to the G band (ID/IG) coupled with the ratios of IG’/IG are used to estimate the defects and crystallinity of CNTs. It is demonstrated that the catalysts under different experimental pressures have an ID/IG ratio between 0.45 to 0.83, and an IG’/IG ratio from 0.18 to 0.54. In general, increasing reaction pressure of carbon nanotube synthesis leads to a high degree of disordered CNTs. This is consistent with that the proportion of the filamentous carbons in the produced carbons is reduced with the further increase of reaction pressure (Fig. 7(d)). 4. Conclusion Three types of metal catalysts (Fe/cordierite, Ni/cordierite and NiMg/cordierite) and different reaction pressures (0.5–1.25 MPa) were utilized to evaluate the synthesis of CNTs from catalytic pyrolysis process of polypropylene. The following conclusions are obtained: 1) There are significant amount of carbons formed on the reacted catalysts, and most of them are MWCNTs. Nickel catalyst produces the largest amount filamentous carbons (around 93 wt.%) than the other catalysts. 2) The strong metal-support reaction suppresses the growth of CNTs and results in the production of short and irregular CNTs when magnesium-based catalyst was used. 3) CNTs yield is increased and the uniformity of CNTs is enhanced when a high pressure is used in the process, especially at 1.0 MPa (198 mg/gPP) 4) Further increase of reaction pressure weakens CNTs growth and results in the formation of shorter and large diameters (around
Fig. 9. Transmission electron microscopy (TEM) of the carbon deposition on the used Ni-based catalysts: (a) 0.5 MPa, (b) 1.0 MPa and (c) 1.25 MPa.
[43–45]. The presence of metal nanoparticles encapsulated inside the nanotubes indicates that the mechanism of CNTs growth at normal pressure is tip growth mode. The outer diameter of the CNTs produced at high pressures (Fig. 9(a)–(c)) is larger than that produced at 7
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30–50 nm) CNTs. 5) The ratio of CNTs was reduced when the reaction pressure is higher than 0.5 MPa.
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