Study on carbon nanotubes and activated carbon hybrids by pyrolysis of coal

Journal Pre-proof Study on mixtures of carbon nanotubes and activated carbon by pyrolysis of coal Xuemei Lv, Tiankai Zhang, Yunhuan Luo, Yongfa Zhang, Ying Wang, Guojie Zhang

PII:

S0165-2370(19)30442-5

DOI:

https://doi.org/10.1016/j.jaap.2019.104717

Reference:

JAAP 104717

To appear in:

Journal of Analytical and Applied Pyrolysis

Received Date:

11 June 2019

Revised Date:

6 October 2019

Accepted Date:

20 October 2019

Please cite this article as: Lv X, Zhang T, Luo Y, Zhang Y, Wang Y, Zhang G, Study on mixtures of carbon nanotubes and activated carbon by pyrolysis of coal, Journal of Analytical and Applied Pyrolysis (2019), doi: https://doi.org/10.1016/j.jaap.2019.104717

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Study on mixtures of carbon nanotubes and activated carbon by pyrolysis of coal Xuemei Lv, Tiankai Zhang, Yunhuan Luo, Yongfa Zhang*, Ying Wang, Guojie Zhang,

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Taiyuan University of Technology, Taiyuan 030024, Shanxi, China

Highlights：

Carbon nanotubes (CNTs) and activated carbon (AC) were simultaneously obtained from

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Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province,

coal.

Pyrolysis temperature and time have important influence on the structure of mixtures.

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Good quality nanotubes were obtained by pyrolysis at 900-950 °C for 45-90 min.

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The intrinsic mineral elements in coal and the added k-based catalysts co-catalyze the

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growth of the CNTs.

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

Abstract:

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Mixtures of carbon nanotubes (CNTs) and activated carbon (AC) were obtained from

bituminous coal via the potassium hydroxide catalytic pyrolysis method. Scanning electron microscopy (SEM), thermogravimetric analysis (TGA), Raman spectroscopy, X-ray Diffraction (XRD), and nitrogen adsorption measurements were used to investigate the characteristics of the samples and the effects of the final pyrolysis temperature and time on the structure of the carbon materials. The results indicated that the optimal range for the pyrolysis

temperature and time for CNT growth were 900–950 °C and 45–90 min, respectively. The diameters of the as-prepared CNTs were in the range of 50–250 nm and their lengths were ~15 μm. During the pyrolysis process, potassium hydroxide not only acted as the catalytic precursor to catalyze the CNT growth but also reacted with carbon to produce abundant micropores. Furthermore, the content of CNTs in the product after demineralization was significantly reduced. It was further confirmed by Inductively Coupled Plasma (ICP) analysis that the content

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of Fe, Co, and Ni decreased after demineralization, indicating that minerals in coal play an

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important role in the growth of coal-based CNTs.

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Keywords: carbon nanotubes; activated carbon; coal; catalytic; pyrolysis 1. Introduction

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Carbon nanotubes (CNTs) are cylindrical seamless hollow tubes made up of one or more layers of curly graphene sheets [1, 2]. They have attracted extensive interest due to their unique

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physical and chemical properties and have been applied in many fields such as energy storage, as catalyst supports, and for adsorption [3-6]. Currently, a large amount of time and energy has been devoted to preparing CNTs from high purity hydrocarbon gases such as methane,

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acetylene, and ethylene [7-9]. However, these source gases are expensive and dangerous; hence, there has been significant interest in the use of relatively low cost and secure feedstock such as coal. Over the past two decades, many researchers have prepared CNTs from coal or coal gas via the arc method, the plasma jet method, and the chemical vapor deposition (CVD) method

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[10-13]. Furthermore, in studies using the arc [14] and plasma jet methods [12], it is believed that the basic components of the CNTs are derived from aromatic fragments generated during discharge, which are directly involved in the formation of CNTs without further cracking. In this regard, Qiu et al. [11, 15, 16] have undertaken detailed research. However, in the CVD method, the carbon source is attributed to carbon-containing gases such as hydrocarbon gases and carbon monoxide in the coal gas. For example, Moothi et al. [17] introduced the output gases from coal pyrolyzed at 400–700 °C into a chemical vapor deposition reactor. It was found

that methane and carbon monoxide were the primary sources of carbon for the synthesis of CNTs within the CVD reactor, and they decreased with increasing pyrolysis temperature. The highest CNT production rate was obtained at a pyrolysis temperature of 400 °C. At this temperature, the pyrolysis gas was composed of 6% CO and 31% CH4 and the outer diameter of the CNTs ranged from 20 to 40 nm. Qiu et al. [18] also reported that CNTs and nanocapsules (CNCs) filled with iron carbide-oxide were prepared from coal gas at 950 °C with ferrocene as

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a catalyst. Despite these meaningful studies, these methods either required more energy or needed gas output from coal pyrolysis, which was not cost-effective and not suitable for large-

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scale CNTs preparation. In addition, during the traditional synthesis of CNTs, studies mainly focused on the catalytic effect of transition metals such as Fe, Ni, and Co and their alloys [19-

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21], which were difficult to remove after CNT synthesis. Therefore, researchers have proposed using alkali and alkaline earth metals [22, 23] to catalyze the growth of CNTs; however, these

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studies have focused on the decomposition of hydrocarbon gases to produce CNTs, which greatly limited the utilization of coal for high value-added products.

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Activated carbon (AC) is an excellent carbonaceous material with abundant pores, and has important applications in fields such as catalyst carriers, adsorption, and energy storage [24-26] because of its high specific surface area (SSA) and pore volume (PV), which mainly depend on

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the micropores. When the PV is in the range of 0.20–0.60 cm3/g, the SSA can reach 400–1000 m2/g or even higher. The preparation methods for AC usually include physical and chemical activation methods. Physical activation often takes advantage of oxidizing gases such as carbon dioxide and water vapor. Chemical activation is performed by adding activators such as KOH, H3PO4, and ZnCl2. Studies have shown that these chemical reagents are conducive to

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dehydration, dehydrogenation, formation of cross-links, and solid skeletons, which may be used as oxidative inhibitors to reduce the volatility and carbon loss of raw materials at high temperatures [27-31]. Among the chemical reagents, potassium hydroxide (KOH) is the most widely used. It is generally believed that the reaction of KOH with carbon starts with a solidsolid reaction and then proceeds with a solid-liquid reaction, mainly by the reduction of potassium and the formation of carbonates and carbon oxides [32]. This is followed by the

formation of complexes with coal and carbonates, which act as active sites to enhance activation during the gasification process [33]. Furthermore, Lillo-Ródenas et al. [34, 35] verified the activation reaction between carbon and KOH as 6 KOH  2C  2 K  3H 2  2 K 2 CO3 via the theoretical feasibility of its thermodynamics and TPD experiments. Coal is an ideal material for the preparation of CNTs and AC, impurities that are present such as H, S, and Fe may promote the growth of CNTs. For example, H can initiate

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graphitization to promote the formation of CNTs [14]; S can form active sites by forming eutectics with the metal catalysts [36]; and metal elements such as Fe can even act as catalysts

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to increase the yield and growth rate of the CNTs [37]. Furthermore, both CNTs and AC have the advantages of a larger surface area and stronger adsorption. The application of CNTs and

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AC mixtures to sewage treatment and as a catalyst carrier has broad prospects [38, 39]. In addition, the CNTs have strong conductivity but their perfect tubular configuration often leads

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to insufficient electrocatalytic activity. The use of AC can improve this defect and improve the catalytic reduction ability of ions in the electrolyte. This allows the CNTs and AC mixtures to

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also be used to prepare supercapacitors and solar cells [40, 41]. However, mixtures of CNTs and AC are currently prepared by mechanical mixing via added binder or ball milling [42], or via vapor deposition of CNTs on AC [43, 44], which is generally difficult to evenly disperse or

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uneconomical. Moreover, research on the simultaneous preparation of CNTs and AC mixtures by pyrolysis using coal as the raw material has rarely been reported. To address the issues stated above, CNTs and AC mixtures were obtained from coal by pyrolysis with KOH and characterized. Furthermore, we investigated the effects of the final pyrolysis temperature and

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time on the structural evolution, the degree of graphitization, and the specific surface area (SSA) of the products. In addition, the catalysis of the intrinsic minerals in coal has also been studied. 2. Experimental 2.1 Materials In the present experiment, bituminous coal obtained from China was used for the formation of CNTs and AC. The proximate and ultimate analyses of the coal samples are shown in Table

1. Demineralized coal was prepared as follows: 400 ml of hydrochloric acid (HCl; 15%) was added to 40 g of raw coal and refluxed in a water bath at 80 °C for 6 h, then filtered and washed with distilled water 2 to 3 times. After that, 400 ml of hydrofluoric acid (HF; 40%) was added and heated in a water bath at a constant temperature of 70 °C for 6 h. It was then filtered and washed with distilled water 2 to 3 times. Finally, the previous hydrochloric acid demineralization process was repeated, washed with distilled water to neutral, and then dried at

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105 °C for 24 h. Table 1 Proximate and Ultimate Analyses of Raw Coal (%) analysis Wad/%[a]

ultimate analysis Wd/%[b]

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Proximate M

VM

FC[c]

C

H

N

S

5.54

3.26

30.89

60.31

73.73

4.43

1.44

0.24

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A

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[a] Air dried basis; [b] Dry basis; [c] Calculated by difference. A: Ash; M: Moisture; VM: Volatile matter; FC: Fixed carbon; C: carbon; H: Hydrogen; N: Nitrogen; S: Sulphur.

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2.2 Sample Preparation

The coal sample was first pulverized to a < 200 mesh grain size. Then the coal-supported KOH was prepared by impregnation. The weight ratio of KOH to coal powder in the mixtures

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was 1:1. After impregnation, the mixtures were filtered and dried overnight at 105 °C and then crushed to remove any grains. Subsequently the mixtures were placed into a stainless steel reaction vessel. The vessel with the mixtures was placed in the furnace and was heated up to the carbonization temperature and maintained at that temperature for 30 minutes. After 30 min

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of reaction time, the mixtures were then heated up to the final pyrolysis temperature (750, 800, 850, 900, and 950 °C; referred to as AT-750, AT-800, AT-850, AT-900, and AT-950, respectively) for 90 min or heated up to 900 °C and kept for 0, 45, 90, and 180 min (referred to as AM-0, AM-45, AM-90, and AM-180, respectively). For investigation of the two factors, the carbonization temperatures were 450 °C and 550 °C. After cooling naturally, the solid mixtures were neutralized with diluted hydrochloric acid (1 mol/L), filtered, and then washed with

distilled water. Finally, the mixtures of CNTs and AC were obtained by drying in an oven at 105 °C for 24 h. In addition, potassium carbonate (K2CO3) and KOH were separately loaded to raw coal and demineralized coal with the same loading (referred to as R-K2CO3 and D-KOH, respectively). Then, raw coal, R-K2CO3, and D-KOH were pyrolyzed in a furnace, and the same pyrolysis conditions were used as for AM-90.

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2.3 Characterization

The morphology of the CNTs and AC mixtures were observed by scanning electron

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microscopy (SEM; TESCAN, Brno–Kohoutovice, Czech) and the nanostructure of the CNTs was further detected by high-resolution transmission electron microscope (HRTEM; JEM-

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2100F, JEOL, Japan). Energy-dispersive spectroscopy (EDS) was used to identify the elements in the grown CNTs. Thermogravimetric analysis and differential scanning calorimetry (TGA-

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DSC; NETZSCH STA449F5, Bavaria, Germany) were performed to determine the thermal stability of the samples. The quality of the CNTs and AC were evaluated via Raman

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spectroscopy measured using a laser Raman spectrophotometer (HR800, HORIBA, Fukuoka, Japan) excited at a laser wavelength of 514.5 nm. X-ray diffraction patterns (XRD; LabX XRD-

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6000, SHIMADZU, Kyoto, Japan ) were obtained with Cu Kα radiation (λ = 0.154 Å) to analyze the order of the graphite structure in the samples. The pore structure parameters of the mixtures were obtained from the N2 gas adsorption isotherm measured with a 3H-2000PSA2 automatic adsorption instrument at 77 K. The total SSA was calculated by the Brunauer-Emmett-Teller (BET) multi-point method and the micropore volume (Vmic) was obtained using t-plot method.

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The total pore volume (Vtot) was determined from the nitrogen adsorption capacity at a relative pressure of 0.99, and the pore size distribution was obtained from the Barrett-Joyner-Halenda (BJH) and Density Functional Theory (DFT) analysis. Inductively Coupled Plasma (ICP) analysis was performed using an Agilent ICP-OES 720 to determine the Fe, Co, and Ni contents of the raw and demineralized coal. 3. Results and discussion

3.1 Effects of final pyrolysis temperature on formation of CNTs and AC from coal Fig. 1 shows the SEM images of the samples pyrolyzed with coal-loaded KOH at 750, 800, 850, 900, and 950 °C, indicating that the samples have different morphologies at different pyrolysis temperatures. From Fig. 1, it can be seen the growth of CNTs was greatly affected by the pyrolysis temperature. When the pyrolysis temperature was below 850 °C, the samples were formed of only AC containing abundant devolatilization holes on the surface of the coke (Fig.

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1a and 1b). However, when the temperature was raised to 850, 900, and 950 °C, CNTs were seen in the samples. At 850 °C, a small amount of short and thick CNTs appeared (Fig. 1c and

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1d). Upon raising the pyrolysis temperature to 900 °C, a large number of straight CNTs with a diameter of 150–250 nm (> 93%) and length of ~15 μm were formed (Fig. 1e and 1f). On further

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increasing the temperature to 950 °C, the diameters of the CNTs were uneven and the diameter distribution increased to 50–250 nm (> 97%; Fig. 1g and 1h). Some tube ends were cracked and

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bifurcation appeared on the tube walls, indicating that continuing to increase the pyrolysis temperature was not favorable for the CNT growth. Thus, it can be seen that the suitable

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pyrolysis temperature for CNT growth was 900–950 °C.

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a

b

d

e

f

h

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Fig. 1. SEM images of (a) AT-750; (b) AT-800; (c), (d) AT-850; (e), (f) AT-900 and (g), (h) AT-950.

From the viewpoint of the catalyst, the activity increased with increasing pyrolysis temperature to a certain level and then the catalyst was slowly deactivated [45, 46]. At a low temperature, the activity of the catalyst was so low that only a small amount of carbon atoms pass through the catalyst to form a CNT crystal face. On increasing the temperature, the active sites for CNT growth increased with the rising catalyst activity and more carbon atoms were deposited on the active sites; hence, the yield of CNTs was significantly increased [47].

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However, when the temperature was higher than 900 °C, the tube diameter distribution of the CNTs was enlarged and the defects were increased. This may be due to the deterioration of the

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stability and deactivation of the catalyst. Italiano et al. [48] divided the deactivation process into three stages, including a slow deactivation, a fast deactivation, and complete deactivation.

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First, the growth of nanocarbon caused a dilution of the active sites. Then, the active sites of the catalyst were encapsulated by the precipitated carbon. Finally, the catalyst was completely

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deactivated as the CNTs grew on the active catalyst without detaching it from the substrate. From the perspective of the carbon source, coal is a complex mixture of macromolecular

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organic compounds that are composed of polyaromatic ring systems and bridges or chains that connect them. Coal releases a large amount of volatile content, such as hydrocarbons that include alkanes and aromatic species during the carbonization and high temperature pyrolysis

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process [17], which are left in the vessel. Furthermore, the coal was converted into coke. During this process, the bridges or chains were continuously detached, and the heavy molecules depolymerized into free radicals and then rearranged. Previously, we demonstrated that the volatile content in raw coal has a certain influence on the growth of CNTs through comparison with the semi-coke pyrolysis experiment and the CH4 catalytic cracking experiment [49].

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Moreover, as shown in Fig. 1g, the CNT branch continued to grow on the formed CNT tube wall and the diameter of the branch was significantly smaller than the original tube diameter, thereby forming Y-type CNTs. Studies have shown that the formation of Y-type CNTs is related to the participation of five-membered ring and seven-membered ring carbon chains in the reaction, which indirectly proved that carbon chains were involved in the formation of the CNTs

[50]. Therefore, the carbon source for CNTs growth may be derived not only from hydrocarbons but also from the bridges or chains and free radical fragments. The research has shown that the graphitization properties of carbon nanomaterials are directly proportional to their thermal stability [51]. In addition, compared with other forms of carbon, highly crystalline nanofibers and graphene layers have been found to have antioxidant properties [52]. From the TGA curves (Fig. 2a), it was observed that the final oxidation

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temperature of the samples gradually increased with the final pyrolysis temperature up to 900 °C. This indicates that the degree of graphitization of the samples gradually increased.

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However, when the pyrolysis temperature was higher than 900 °C, the final oxidation temperature decreased. This may be ascribed to the increase of defects in the CNTs.

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Furthermore, we can observe from the DSC curves (Fig. 2b) that the sample with a pyrolysis temperature of 900 °C had two distinct exothermic peaks, clearly indicating that the sample

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contained two different types of carbon. This was also consistent with the observations in the SEM images. In addition, the weight loss ratio in the temperature range of the second

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exothermic peak (595–754 °C) was ~ 20%, which could be attributed to the content of CNTs in the sample. However, the sample with a pyrolysis temperature of 850 °C showed a broad exothermic peak. This could be caused by the combination of two exothermic peaks and their

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boundary was not obvious due to the low degree of graphitization. For the product with a pyrolysis temperature of 950 °C, there was almost no second exothermic peak. This was probably because the graphite layers of the CNTs were severely damaged at high temperatures.

Mass %

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80

(a)

AT-750 AT-800 AT-850 AT-900 AT-950

60

40

20

0

18

(b)

AT-750 AT-800 AT-850 AT-900 AT-950

15

DSC/(uV/mg)

100

12 9 6 3

150

300

450

600

Temperature(

)

750

900

0

150

300

450

600

Temperature(

)

750

Fig. 2. (a) TGA and (b) DSC curves of AT-750, AT-800, AT-850, AT-900 and AT-950.

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Raman analysis is commonly used to evaluate the defect qualities of the CNTs and AC [53]. Two characteristic bands were observed in the Raman spectra of all the samples (Fig. 3). The first band was the D-band (1305–1350 cm-1), corresponding to the A1g inactive respiratory vibration mode, which was activated by the amorphous carbon and unorganized graphite. The second band was the G-band (1500–1600 cm-1), which was produced by the splitting of the E2g stretching mode, reflecting the ordered graphite sheet structure. This indicated that the product

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had a certain similarity to the structure of graphite. Generally, the relative intensities of the D and G-bands (ID/IG) were used as an indicator of the quality of the CNTs and AC with graphene.

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A low ID/IG ratio indicated better graphitization and a perfect structure. According to Fig. 3, when the temperature was lower than 900 °C, the ID/IG ratios were all ~ 0.9, indicating that the

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degree of graphitization was low. Then, at above 900 °C, 2D-bands (~ 2700 cm-1) generated by second-order two-phonon scattering near the K point of the Brillouin zone appeared in the

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samples, which was an indicator of long-range order. Moreover, the ID/IG ratio decreased significantly from 850 °C to 900 °C (0.22 at 900 °C). This indicated a higher degree

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of graphitization and a more perfect structure for the sample at 900 °C. Therefore, the degree of rearrangement and crystallization was higher than etching under these conditions [54]. However, the ID/IG ratio increased on increasing the temperature to 950 °C (0.74 at 950 °C),

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which may be due to further etching of the CNTs by KOH. In addition, it is worth noting that the G-bands of AT-900 and AT-950 samples were red shifted by 5–10 cm-1 from the 1582 cm1

band of the highly oriented pyrolytic graphite (HOPG) to a lower wavenumber. Hiura et al.

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[55] attributed this phenomenon to the spherical curvature of graphite sheets in CNTs.

AT-850 AT-900

AT-750 AT-800

AT-950

G-band D-band

1000

1500

2000

2500

-1

3000

Raman Shift (cm )

Fig. 3. Raman spectra of AT-750, AT-800, AT-850, AT-900 and AT-950.

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Intensity (a.u.)

2D-band

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Fig. 4 illustrates the XRD patterns of AT-750, AT-800, AT-850, AT-900, and AT-950. The results show that all the washed samples mainly had two peaks of carbon (C), which

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appeared at ~ 26.6° (002) and 44.7° (101). The peak at 44.7° can be attributed to turbostratic carbon [56] and the ratio between these two peaks (I101/I002) is usually used as an indicator of

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the order of the graphite structure in the carbon material. In our analysis, the ratios were 1.17, 1.09, 0.48, 0.15, and 0.66 for samples at 750, 800, 850, 900, and 950 °C, respectively.

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Therefore, when the temperature was raised from 750 to 900 °C, the order of the graphene sheets gradually increased. However, further raising the temperature to 950 °C resulted in a slight decrease in the order. AT-750 AT-800

AT-950

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AT-850 AT-900

20

40

60 2θ (deg)

80

100

Fig. 4. XRD patterns of AT-750, AT-800, AT-850, AT-900 and AT-950. The adsorption isotherms and pore size distributions of the products are shown in Fig. 5 and the calculated values for the pore structure parameters are summarized in Table 2. As seen

from the Fig. 5, the curves can be mainly divided into three regions: first, when P/P0 < 0.1, the adsorption of the micropores was significantly enhanced, which was mainly attributed to monolayer adsorption. Due to the adsorption potential, the micropore had a strong ability to capture the adsorptive molecules, which meant that it was rapidly filled by the adsorptive molecules in a very short time. Then, it entered the multi-molecule adsorption layer stage. As the relative pressure gradually increased, the adsorption amount also slowly increased and the

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adsorption platform was gradually formed. Finally, at P/P0 = 0.5, a hysteresis loop was formed due to capillary condensation [57], which mainly occurred on adsorbents with intermediate

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pores. Moreover, according to the IUPAC classification of the hysteresis loops, the hysteresis loop of the mixtures can be classified as H4 type, indicating that the products were porous

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materials with narrow slit micropores and mesopores. As more intuitively shown in Fig. 5b and

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5c, the samples mainly possessed micropores of 0.4–0.8 nm and mesopores of 2–4.7 nm.

Table 2 Characteristics of the pore structure of AT-750, AT-800, AT-850, AT-900 and AT-950. νtot/cm3·

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SBET

AT-750

77.57

603

/m2·g-1

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sample

Yield[a]/%

g-1

νmic/cm3·g-1

Micropore content[b]

DAve/nm

0.30

0.24

0.80

2.01

AT-800

77.36

710

0.34

0.28

0.82

1.93

AT-850

75.73

934

0.46

0.37

0.80

1.97

AT-900

75.44

1183

0.61

0.48

0.79

2.04

AT-950

74.54

1178

0.60

0.47

0.78

2.05

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[a] The yield represents the mass after pyrolysis of the raw coal loaded with KOH divided by the mass before pyrolysis. [b] The micropore content represents the micropore volume divided by the total volume.

400

(a)

(b)

0.30 0.24

300

AT-750 AT-800 AT-850 AT-900 AT-950

3

0.18

8 d V /d w (cm 3 /g  n m )

AT-750 AT-800 AT-850 AT-900 AT-950

dV/dw (cm /gnm)

3 Volume Adsorbed (cm /g)

500

0.12

200

AT-750 AT-800 AT-850 AT-900 AT-950

6 4 2 0 0.4

0.06

100

(c)

0.8 1.2 1.6 Pore diameter (nm)

2.0

0.00 0.0

0.2

0.4

0.6

Relative Pressure (P/P0)

0.8

1.0

0

4

8

12

Pore diameter (nm)

16

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0

Fig. 5. (a) N2 adsorption isotherms; (b) BJH mesopore size distributions; (c) DFT micropore

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size distributions of products at different final pyrolysis temperature.

As shown in Table 2, the sample pyrolyzed at 750 °C had the lowest SSA, which was

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mainly related to the dense structure of coal. As the temperature increased to 900 °C, the SSA of the sample increased 0.96 times to 1183 m2/g. As the temperature continued to rise, the SSA

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decreased slightly. However, the change in the average pore size of the samples showed an opposite trend and dropped to a minimum of 1.93 nm at 800 °C. With increasing pyrolysis

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temperature, the catalyst decarbonized at the active site and the metal K (boiling point 762 °C) steam shuttled through the microcrystalline layers to force the sheets to separate. This promoted

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the distortion of the layers and accelerated the formation of micropores, resulting in an increase in SSA. During this process, the average pore size had the lowest value, which can be attributed to the original pores closing or shrinking because of the surface stress during the thermal expansion of coal. Subsequently, as the pyrolysis reaction gradually penetrated into the interior of the carbon material, a part of the pores were interconnected and some pores collapsed. This

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led to a gradual increase of the average pore size and a small decrease in the SSA. In addition, the fluctuation range of the total pore volume of the samples was 0.30–0.61 ml/g. The proportion of the micropore volume was ~ 80% of the total pore volume. Moreover, the yield of the product decreased slightly with increasing pyrolysis temperature. 3.2 Effects of final pyrolysis time on the formation of AC and CNTs from coal

Fig. 6 presents SEM images of the samples pyrolyzed at 900 °C for 0, 45, 90, and 180 min. It can be seen from the Fig. 6a and 6b that no CNTs were formed when the pyrolysis time at 900 °C was 0 min, revealing that the growth of CNTs occurred in the high temperature stage of coal pyrolysis. As the pyrolysis time was extended to 45 min, a large number of CNTs with diameters ranging from 150–250 nm (> 98%; Fig. 6c and 6d) were formed. In addition, when the pyrolysis time reached 90 min, the surface of the CNTs became smoother, the tube

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diameters were more uniform, and their distribution was narrower; almost all the diameters were within 50–150 nm (Fig. 6e and 6f). However, on further prolonging the pyrolysis time to

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180 min, it was found that the diameters of the resulting CNTs were uneven. The distribution was broader and the number of CNTs seemed to decrease (Fig. 6g and 6h). Consequently, 45–

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a

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90 min were selected as the optimum pyrolysis time range for CNTs grown at 900 °C.

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Fig. 6. SEM images of (a), (b) AM-0; (c), (d) AM-45; (e), (f) AM-90 and (g), (h) AM-180. The observed results may be attributed to the fact that when the pyrolysis time was shorter (45 min), the carbon source generated by pyrolysis was diverse. This resulted in the formation of CNTs with different morphologies and uneven diameters. With the extension of the pyrolysis

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time, the composition and content of the carbon source changed continuously and the morphology of the CNTs changed accordingly. When the pyrolysis time was longer (180 min), a part of the catalyst was deactivated because of carbon coating. This resulted in a significant reduction in the number of active sites for CNTs formation. To further reveal the influence of the pyrolysis time on the samples, the XRD patterns for samples pyrolyzed at 900 °C for 0–180 min are shown in Fig.7. It can be seen that when the pyrolysis time was 0 min, there was a broad peak at 26° (2θ) and the ratio of I101/I002 was 1.42,

showing that a low temperature (< 900 °C) was unfavorable for graphitization. On extending the pyrolysis time to 45 min, the broad peak became narrower, the intensity increased significantly, and the ratio of I101/I002 was reduced to 0.16. On further extending the pyrolysis time, the width and intensity of the two peaks were not significantly changed and the I101/I002 ratios of the samples pyrolyzed at 900 °C for 90 and 180 min increased slightly to 0.19 and 0.20, respectively. Therefore, the sample pyrolyzed at 900 °C for 45 min exhibited highly

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ordered graphene sheets, and further prolonging the pyrolysis time had less of an effect on the order of the graphite structure. In addition, an unwashed sample was also measured by XRD

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(Fig. 7). From Fig. 7, it can be seen that the unwashed sample showed more complex peaks and the peaks of carbon became less obvious. The results show that these peaks can be assigned to

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potassium compounds, including K2CO3, K2O, KOH, and graphite-potassium intercalation compounds (K-GICs) [58]. The appearance of KOH could be attributed to the hydrolysis of

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K2CO3 or K2O during the measurement. The K-GICs were formed because K, which was generated during the pyrolysis process, provided an electron to the p-electron network of

◆ K2CO3

▼ KOH

● K2O ▲K-GICs ▽ C ◆ ◆

▼ ▲◆●

▽

▲

AM-0 AM-45 AM-90 AM-180 AM-90,unwashed

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◆

▽

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carbon.

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20

40

60 2θ (deg)

80

100

Fig. 7. XRD patterns of AM-0, AM-45, AM-90, AM-180 and unwashed sample of AM-90. The effect of total pyrolysis time (0–180 min) on the SSA and the average pore diameter

of the samples is shown in Fig. 8 and Table 3, and the effect was similar to that of the pyrolysis temperature. When the pyrolysis time was increased to 45 min, the SSA of the sample was greatly increased by 399 m2/g but the average pore diameter was reduced. This may be due to

the appearance of CNTs, which have a large SSA and a higher length-diameter ratio. Then the SSA increased to the maximum value (1192 m2/g) at a pyrolysis time of 90 min, which increased by 0.6 times compared with that of non-pyrolysis at 900 °C. However, the average pore size of this process also gradually increased, indicating that the pore-expansion and the pore-increased reactions occurred simultaneously. When the pyrolysis time was over 90 min, part of the pore channels collapsed and pore-expansion reactions occurred, resulting in a further

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increase of the average pore size and a slightly decreased SSA. However, the average pore size was still in the mesopore category due to the small amount of catalyst and the loss of etching

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effects of some catalysts coated in CNTs. The samples also possessed micropores of 0.4–0.8 nm and mesopores of 2–4.7 nm. In addition, according to table 3, when the pyrolysis time was

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> 45 min, the yield of the product was maintained at 77%.

Table 3 Characteristics of pore structure for samples with different pyrolysis time /m2·g-1

SBET

AM-0

80.74

728

AM-45

77.75

1127

AM-90

77.02

AM-180

76.97

νtot/cm3·

Micropore

νmic/cm3·g-1

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sample

Yield[a]/%

g-1

DAve/nm

0.31

0.77

2.19

0.56

0.48

0.84

2.00

1192

0.63

0.50

0.79

2.12

1134

0.62

0.49

0.78

2.20

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0.40

content[b]

[a] The yield represents the mass after pyrolysis of the raw coal loaded with KOH divided by the mass before pyrolysis. [b] The micropore content represents the micropore volume divided by the total volume.

300 250 200 150

0.0

0.2

(b)

AM-0 AM-45 AM-90 AM-180

0.4

0.6

0.8

Relative Pressure (P/P0)

1.0

6.0

(c)

AT-0 AT-45 AT-90 AT-180

4.5 3.0

0.24

3

350

0.32

dV /dw (cm 3 /g  nm )

(a) dV/dD (cm /g nm ）

400

AM-0 AM-45 AM-90 AM-180

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Volume Adsorbed (cm3/g）

450

1.5

0.16

0.0 0.4

0.08 0.00

2

4

0.8 1.2 1.6 Pore diameter (nm)

6

Pore Diameter (nm)

8

2.0

10

Fig. 8. (a) N2 adsorption isotherms versus relative pressure; (b) BJH mesopore size distributions; (c) DFT micropore size distributions of products at different pyrolysis time. 3.3 Formation of CNTs and AC from coal For the formation of CNTs, we proposed a co-catalytic mechanism of "autocatalytic growth of CNTs" and "catalytic growth of the catalyst" in a previous report [49]. According to the relative strength of the "autocatalytic growth of CNTs" and "catalytic growth of the

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catalyst", the formation mechanism of the nanohorn and carbon nanocapsules can be explained. In addition, CNTs with defective structures such as bamboo-like and a dented pipe wall-like

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CNTs appeared. This was caused by the stacking of the inner graphite because the growth rate of the inner graphite was higher than that of the outer layer under the action of the surface

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tension of the catalyst particles.

The growth of CNTs using KOH as the catalytic precursor was confirmed; however, it is

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known from Fig. 7 that KOH was unstable during the pyrolysis process and was converted to K2CO3. Therefore, we used K2CO3 instead of KOH for the pyrolysis experiments and found

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that the product contained a large amount of CNTs, as shown in Fig. 9. This indicated that K2CO3 is an active material for catalyzing the growth of CNTs. In addition, some inorganic

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mineral elements such as Fe, Co, and Ni in coal also had a certain catalytic effect on the formation of CNTs. To further confirm which elements were responsible for the growth of the CNTs, HRTEM and EDS investigation of the AM-90 was employed (Fig. 9) before washing. It can be observed from the Fig. 9a that a small amount of black material was filled in the CNTs. We carried out EDS on the black material in the CNTs and found that its main catalytic

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components were Fe, Co, and K. The presence of the Cu peaks may be assigned to the copper grid, which was used as a specimen support during TEM. Therefore, it can be concluded that Fe, Co, and K catalyze the growth of this part of the CNTs. The remaining CNTs that were not filled with the black material may mainly grow through autocatalysis of CNTs.

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Fig. 9. SEM images of raw coal pyrolysis with K2CO3 C O Al Si S K Fe Co

900

1 600

300

Atomic % 90.3 1.1 0.1 0.1 0.1 0.4 7.4 0.6

O

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0 0.00

(b)

Fe

re

Fe

weight % 68.9 1.1 0.1 0.2 0.2 1.0 26.3 2.1

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(a)

-p

200 C Element

Cu Co Fe

K

2.00

4.00

6.00

Cu 8.00

10.00 keV

Fig. 10. (a) HRTEM images of CNTs in AM-90; (b) EDS of point 1.

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Furthermore, in order to eliminate the role of the intrinsic mineral elements in the coal, we pyrolyzed the raw coal directly and found that there were no CNTs in the product, indicating that the intrinsic minerals in the coal did not play a catalytic role alone. Subsequently, we carried out a HCl-HF-HCl three-stage demineralization treatment to reduce the ash content in the coal to 0.27%, the detailed steps are described in the materials. In addition, we performed

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quantitative analysis of the Fe, Co, and Ni in raw coal and demineralized coal by ICP, as shown in Table 4. It was found that the raw coal contained higher Fe, which accounted for ~ 4600 mg/kg, while the contents of Ni and Co were relatively small. After demineralization, the Fe content in the coal decreased significantly, while the content of Co and Ni decreased slightly. Then, demineralized coal was used as a raw material to load with KOH for pyrolysis. The results show that the CNT content in the product was significantly reduced at the same loading, as

shown in Fig. 11. It can be seen that the intrinsic mineral elements in the coal also play a vital role in the growth of coal-based CNTs. Therefore, it was further confirmed that in the growth process of coal-based CNTs, the intrinsic mineral elements in the coal and the added k-based catalysts co-catalyze the growth of the CNTs. Table 4 Ultimate analysis of raw coal and demineralized coal Co

Ni

4600 53.9

6.3 3.0

7.1 6.1

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b

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d

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c

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a

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Raw Coal (mg/kg) Demineralized Coal (mg/kg)

Fe

Fig. 11. SEM images of (a), (b) raw coal pyrolysis directly; (c), (d) demineralized coal pyrolysis with KOH.

The formation of AC can be mainly attributed to the corrasion of KOH. In this process, the carbon skeleton was etched by redox reactions (Eqs. (1)–(4)) between KOH and carbon, which resulted in the formation of a large number of micropores. Furthermore, the metal potassium generated during the pyrolysis process was effectively embedded in the microcrystalline layer of the carbon matrix. Then, additional micropores were formed by

of

removing the potassium and potash from the layers by washing [32, 59].

6 KOH  2C  2 K  3H 2  2 K 2 CO3

ro

(1)

(2)

K 2 CO3  2C  2 K  3CO

(3)

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4 KOH  CH 2  K 2 CO3  K 2 O  3H 2

K 2 O  C  2 K  CO

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(4)

Therefore, KOH affected the growth of the CNTs and AC by these two ways: 1) KOH

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acted as the catalytic precursor to catalyze the growth of CNTs; 2) KOH allowed the crosslinked structure of coal to be more easily destroyed, so that it was easier to release the active substances such as the carbonaceous gas and aromatic compounds [60]. Then, these materials

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produced CNTs by the catalysis of Fe, Co, or K in coal. Meanwhile, the KOH reacted with carbon through redox reactions to form AC with abundant micropores. 4. Conclusion

In summary, mixtures of CNTs and AC were successful obtained via the KOH catalytic pyrolysis method from bituminous coal. The optimal ranges for the growth temperature and

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time of the mixtures were 900–950 °C and 45–90 minutes, respectively. The diameter of CNTs generated at 900 °C were in the range of 150–250 nm (> 93%), and further raising the pyrolysis temperature to 950 °C resulted in an increase of the distribution to 50–250 nm (> 97%). The degree of graphitization, SSA, and PV of the samples gradually increased up to 900 °C and then decreased. The sample pyrolyzed at 900 °C for 45 min exhibited highly ordered graphene sheets. Although further prolonging the pyrolysis time (90 min) had a reduced effect on the

order of the graphite structure and the SSA of the mixtures, the surface of the CNTs was smoother and the distribution of the diameters was narrower (50–150 nm). Based on the HRTEM and EDS analysis, the elements Fe, Co, and K were responsible for the formation of CNTs. KOH affected the growth of the CNTs and AC in these two ways: on the one hand, KOH acted as the precursor to catalyze the growth of CNTs and its conversion product, K2CO3, was the active substance; on the other hand, KOH reacted with the carbon

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skeleton by a series of oxidation-reduction reactions to form AC with abundant micropores and promoted the release of carbon sources. These carbon sources were then used as sources for the

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also play a vital role in the growth of coal-based CNT

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growth of CNTs. In addition, the intrinsic mineral elements such as Fe, Co, and Ni in the coal

Conflict of interest

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Neither the entire paper nor any part of its content has been published or has been accepted elsewhere. It is not being submitted to any other journal. All authors have seen the manuscript

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and approved to submit to your journal. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work

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submitted.

Acknowledgments

The work was supported by the National Natural Science Foundation of China (grant

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numbers 51774211).

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