Transformation of primary siderite during coal catalytic pyrolysis and its effects on the growth of carbon nanotubes

Fuel Processing Technology 198 (2020) 106235

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Research article

Transformation of primary siderite during coal catalytic pyrolysis and its effects on the growth of carbon nanotubes Tiankai Zhang, Qi Wang, Xuemei Lv, Yunhuan Luo, Yongfa Zhang

T

⁎

Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Bituminous coal Catalytic pyrolysis Siderite Carbon nanotubes Growth mechanism

Although the synthesis of carbon nanotubes from coal can efficiently reduce the cost of preparing these materials, the effect of the primary minerals present in coal (e.g., Fe-containing siderite) on the growth of these structures has not been studied in depth. In this study, we used a bituminous coal containing siderite and investigated the changes of the original siderite phase and the growth mechanism of carbon nanotubes during a KOH-catalyzed coal pyrolysis process. The results shown that the primary Fe minerals played an important role in the formation of carbon nanotubes during coal pyrolysis. During the KOH-catalyzed pyrolysis of coal, the Fe present in the raw material migrated from the bulk to the surface of the coal particles, and enriched in some areas of the surface of the coal particle. KOH-catalyzed coal pyrolysis had a significant effect on the formation of micropores and the increase of specific surface area and pore volume in coal. The primary siderite in coal and the catalytic growth of carbon nanotubes, in line with the following mechanism: FeCO3 → α-Fe → Fe3C + Graphite → carbon nanotubes. This study can provide new ideas for the utilization of low-rank coal resources rich in Fe mineral components.

1. Introduction At present, traditional exploitation methods of coal resources such as pyrolysis, gasification, and combustion are challenged due to their environmental impact [1–4]. The development of new applications and technologies for coal resources including electrode materials [5,6], capacitors [7,8], carbon molecular sieves [9], and carbon nanotubes (CNTs) directly [10,11] or indirectly [12,13] prepared from coal raw materials has become important. In addition, the preparation of threedimensional graphene materials from coal tar pitch, which indirectly uses coal as a raw material to prepare high-efficiency electro catalyst carriers, has also become a development direction for clean and highvalue utilization of coal resources [14–16]. Since Pang et al. prepared for the first time CNTs from coal [17], many studies have reported on the effects of the coal rank, the coal type, and the reaction conditions on the growth of CNTs [10,18,19]. The results reveal that the yield of CNTs is directly proportional to the fixed carbon content of the coal material and inversely proportional to its volatile matter content [18,20,21]. Thus, the higher the fixed carbon content of the material, the greater number of carbon atoms or carbon clusters are generated during the arc discharge process, which favors the formation and growth of CNTs. In contrast, hydrogen and other heteroatoms hinder the formation of CNTs by participating in some ⁎

plasma reactions during the reaction. Mineral components have been reported to hinder the growth of CNTs on coal, making it necessary a previous demineralization step [20]. The type of CNTs product obtained depends on the reaction conditions [20]. Awasthi et al. used a bituminous coal from India as raw material and non-catalytically obtained multi-walled CNTs by discharge under an argon atmosphere [22]. The addition of Fe and NieY catalysts resulted in the generation of single-walled CNTs, while using a mixture of hydrogen and argon generated graphene as a product [22]. This shift from multi- to singlewalled CNTs can be explained by the simultaneous evaporation of the Fe and NieY catalysts and carbon in coal at high temperatures, with the particles of the catalysts condensing into nuclei and being saturated with carbon atoms [22]. At temperatures near the eutectic temperature, single-walled CNTs are formed on the catalyst particles. The formation of graphene instead of multi-walled CNTs can be explained by hydrogen saturating the dangling bonds of the edge six-membered carbon ring of the graphene layer. This saturation prevents the graphene layer from curling to form CNTs [22,23]. When used for preparing CNTs, a metal catalyst is usually added to coal. Using bituminous coal as a raw material and a Fe powder catalyst, Awasthi et al. prepared single-walled CNTs with average diameter and length of ca. 1.7 nm and 0.2 μm, respectively [22]. Using anthracite as a raw material and an external Fe powder catalyst, Qiu et al. prepared

Corresponding author. E-mail address: [email protected] (Y. Zhang).

https://doi.org/10.1016/j.fuproc.2019.106235 Received 16 August 2019; Received in revised form 25 September 2019; Accepted 7 October 2019 0378-3820/ © 2019 Elsevier B.V. All rights reserved.

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double-walled CNTs with diameters ranging from 1.0 to 5.0 nm and layer spacing of 0.41 nm [24]. Different types of CNTs including multiwalled and bamboo-shaped CNTs were also prepared from a coal raw material and with Fe as a catalyst [25–27]. The Fe catalyst was found to control the type of product obtained depending on its particle size [28]. Thus, small particles resulted in carbon atoms to diffuse mainly on the Fe catalyst particles, thereby selectively generating single-walled CNTs. When the Fe atoms were aggregated in larger particles, the surface diffusion of the carbon atoms was hindered, thereby favoring bulk phase diffusion of the catalyst particles and generating double- or multiwalled CNTs [28]. Some researchers have suggested that Fe may play a role in the preparation of CNTs from coal [21,29]. However, the addition of Fe catalyst is still preferred by relevant researchers to investigate the catalytic effect. The growth of CNTs catalyzed by primary Fe minerals (e.g., siderite) present in coal has not been studied in the literature. Considering that coal often contains Fe mineral components and the important role of Fe in the growth of CNTs, it is necessary to investigate the growth of CNTs catalyzed by primary Fe components of coal. In this paper, it was found for the first time that primary siderite catalyzed the growth of CNTs during KOH-catalyzed pyrolysis of bituminous coal. A large amount of carbon nanotubes were formed in the KOH-catalyzed bituminous coal pyrolysis product, and some CNTs were coated with Fe particles. However, no additional Fe catalysts were added to the raw materials. In order to understand the changes of the primary Fe minerals of coal during its catalytic pyrolysis and catalytic formation mechanism of the CNTs, we employed herein bituminous coal and demineralized bituminous coal as raw materials. We studied the changes underwent by these Fe minerals of coal and the micromorphology, elemental composition, crystal structure of the catalyst particles generating the CNTs, and gave the transformation of the siderite and the mechanism of CNTs growth.

Table 2 Ash composition of RC (wt%) [30]. SiO2

Al2O3

Fe2O3

CaO

MgO

K2 O

Na2O

Others

43.01

21.33

10.53

19.67

1.14

0.98

0.72

2.62

Table 2. Four kinds of constant mineral elements (Si, Al, Fe, and Ca) and two kinds of trace elements (Ni and CO) are shown in Table 3. 2.2. Catalyst loading methods RC, DC, and graphite (G) were used as carriers to load KOH, and the specific process is reported elsewhere [30]. The KOH loaded samples were named as RC-K, DC-K, and G-K, with KOH contents of 37.88, 38.05, and 38.19 wt%, respectively. The G (> 99.95%) and KOH (> 95%) used in the experiments were purchased from Shanghai Aladdin Biochemical Technology Co., LTD. (China). FeCO3 (particle size < 48 μm) was added to the G, G-K, DC, and DCK samples and ground uniformly with an agate mortar. The as-obtained samples were named G-Fe, G-K/Fe, DC-Fe, and DC-K/Fe, respectively. All samples contained 10 wt% of FeCO3. FeCO3 (> 99%) was purchased from Guangdong Wengjiang Chemical Reagent Co., Ltd. (China). The sample treatment methods and the nomenclature used in the experiment are shown in Table 4. 2.3. Experimental method Pyrolysis reaction: the sample (8.0 g) was placed in the intermediate temperature zone of the reaction tube of the horizontal tube furnace, after which the air inside the tube was purged with nitrogen. Then, the sample was heated to 550 °C from room temperature at a heating rate of 10 °C/min and remained at this temperature for 30 min. The sample was subsequently heated to 900 °C at a heating rate of 10 °C/min and remained at this temperature for 90 min. Finally, the sample pyrolysis product was obtained by stopping the heating and cooling the tube furnace to room temperature under nitrogen. The samples used in the experiment were RC, RC-K, DC-K, DC-Fe, DC-K/Fe, G-Fe, G-K/Fe, and FeCO3.

2. Experimental 2.1. Materials The air-dried bituminous coal (Shanxi, China) used herein was ground and sieved below 74 μm to obtain the raw coal sample (RC). 40 g of RC were taken and 400 mL of a hydrochloric acid solution with a concentration of 6 M were added. The mixture was maintained at constant temperature in a water bath (60 °C) for 4 h under stirring, after which the sample was washed repeatedly with deionized water 5–8 times. Then, 500 mL of a 40 wt% hydrofluoric acid solution was added to the remaining sample and maintained at constant temperature in a water bath (60 °C) for 4 h under stirring, after which the sample was repeatedly washed with deionized water 5–8 times. 400 mL of 6 M hydrochloric acid solution were added to the remaining sample and maintained at constant temperature in a water bath (60 °C) for 4 h under stirring. Then, the sample was repeatedly washed with deionized water 5–8 times, filtered, and finally wash with deionized water until the filtrate was neutral. Finally, the as-obtained sample was dried at 100 °C for 4 h to obtain a HCl-HF-HCl demineralized coal (DC) product. The proximate and ultimate analysis of the RC and DC samples are shown in Table 1. The ash analysis of the RC sample is shown in

2.4. Materials characterization The evolution of the functional groups on the samples was analyzed on a German TENSOR27 Fourier transformed infrared spectroscopy (FTIR) device in a scanning range of 400–4000 cm−1 and with a resolution of 4 cm−1. The sample was pre-mixed with KBr and pressed before the measurements. The crystal structure of the sample was obtained using a SHIMADZU LabX X-ray diffraction (XRD) 6000 diffractometer. The Ka excited by the Cu target was used as the X-ray source (λ = 0.154 Å) with tube potential and current of 30 kV and 40 mA, respectively. The scanning range (2θ) was 5–90°, while the scanning speed and the step size were 4°/min and 0.02°, respectively. The surface morphology and elemental distribution of the sample were observed on a HITACHI SU8020 field-emission scanning electron microscopy (SEM) device equipped with a HORIBA EX-350 X-ray energy spectrometer operating at an acceleration voltage of 20 kV. The sample

Table 1 Proximate and ultimate analyses of the RC and DC samples [30]. Samples

RC (raw coal) DC (demineralized coal)

Proximate analyses (wad/%)

Ultimate analyses (wdaf/%)

Moisture

Ash

Volatile matter

C

H

S

N

O⁎

4.65 0.81

6.24 0.35

29.37 30.76

82.73 82.46

4.75 4.05

0.25 0.22

0.78 0.67

11.49 12.60

ad, air dry basis; daf, dry ash free basis; ∗, by difference. 2

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Table 3 Elemental analysis the RC and DC samples. Samples

Si/mg/kg

Al/mg/kg

Fe/mg/kg

Ca/mg/kg

Ni/mg/kg

Co/mg/kg

RC (raw coal) DC (demineralized coal)

12,524.51 638.72

7046.43 125.03

4600.01 228.07

8767.21 449.90

7.14 0.35

6.30 0.31

peak of the aromatic ring structure at 1593 cm−1 [32,33], a deformation vibration absorption peak of -CH3 at 1429 cm−1, a deformation vibration absorption peak of -CH2 at 1375 cm−1, and a deformation vibration band of CeH functional group outside of aromatic structure at 700–900 cm−1 [34,35]. Compared with RC, DC lacked the following bands: a stretching vibration peaks of OeH group of silicate and clay mineral components at 3694 cm−1 [36], the vibrating peaks of groups such as Si–O–Si and Al–O–Al in silicate and aluminate minerals at 900–1200 cm−1 [36], the stretching vibration peaks of SieO and Si–O–Al structures of quartz, silicate, and other mineral components at 400–600 cm−1 [36]. The above results indicated that the HCl-HF-HCl pickling and demineralization processes were effective in removing the mineral components from coal while producing little effect on the organic carbon structure of the aliphatic and aromatic groups of RC [36]. The SEM analysis of the pyrolysis products of RC-K and DC-K is shown in Fig. 2. While the pyrolyzed RC-K sample contained a large number of CNTs, DC-K formed a significantly lower amount of these materials and only in some specific areas. FTIR analysis showed that acid elution was effective in removing mineral components such as silicate and aluminate from coal while maintaining its organic structure nearly intact. Thus, acid elution has been found to not alter the pyrolysis reactivity of coal, and, in some cases, this treatment can even result in higher pyrolysis activities [35,36]. In addition, the loading of KOH in RC-K and DC-K was nearly similar (37.88 and 38.05 wt%, respectively), and the pyrolysis reaction conditions were maintained constant for both samples. Therefore, other factors should be responsible for the lower CNTs content of the pyrolyzed DC-K sample. The main difference between RC and DC lies in the ash content after the demineralization treatment. Thus, the ash content of RC was reduced from 6.24 to 0.35 wt% after this treatment (94.39%). In addition, ICP revealed significant removal (94.9–98.3%) of Si, Al, Fe, and Ca from coal via demineralization, in line with the ash reduction ratio. Therefore, the lower content of mineral components in coal is the main reason explaining the low CNTs content of the DC-K pyrolysis product. This result indicated that the mineral components in coal play an important role in the formation of CNTs.

Table 4 Samples treatment methods and nomenclature. Treatment methods

No treatment

Load KOH

Add FeCO3

Load KOH and add FeCO3

Nomenclature

RC DC G

RC-K DC-K G-K

– DC-Fe G-Fe

– DC-K/Fe G-K/Fe

was previously subjected to a vacuum spray treatment. The structural characterization of the samples were carried out on a FEI Tecnai G2 F20 high-resolution transmission electron microscopy (HR-TEM) device equipped with an EDAX GENESIS energy-dispersive X-ray spectroscopy (EDS) device. The accelerating voltage was 200 kV. The contents of Si, Al, Fe, Ca, Co, and Ni in the samples were determined on an Agilent ICPOES730 inductively coupled plasma (ICP) atomic emission spectrometer. The sample was digested with nitric acid before the measurements. The pore structure parameters of the samples were determined by Beishide 3H-2000PS2 specific surface area and pore size analyzer. The samples were degassed at 150C for 4 h. Adsorption and desorption isotherms were obtained by isothermal static volumetric method at 77.4 K and 0.01–0.99 relative pressure. The BET multipoint method was used to calculate the total specific surface area of the samples. The BET 4V/A method was used to calculate the average pore diameter. The specific surface area and volume of micropores, external specific surface area (mesopores + macropores) and total pore volume were calculated by t-Plot method.

3. Results and discussion 3.1. Effect of the primary minerals on the preparation of CNTs by coal pyrolysis As revealed by the FTIR qualitative analyses of RC and DC (Fig. 1) and the corresponding FTIR absorption peaks (Table 5), both samples showed the following absorption peaks: a NeH group or an OeH group stretching vibration peak by crystalline water or adsorbed water at 3000–3500 cm−1 [31,32], an asymmetric stretching vibration peak of aliphatic -CH2 at 2916 cm−1, a symmetrical stretching vibration peak of aliphatic -CH3 group at 2848 cm−1 [33], a C]C stretching vibration

3.2. TEM-EDS analysis of the pyrolysis products of RC-K and DC-K The mineral composition of coal is relatively complex and is mainly

2.3

2.06

(a)

(b) 2.04

Absorbance

Absorbance

2.2

2.1

2.0

2.02

2.00 RC DC

1.9

4000

3500

3000

2500

2000

1500

1000

RC DC

1.98

500

Absorbance (cm

4000

3800

3600

600

400

Absorbance (cm-1)

-1

)

Fig. 1. FTIR spectra of RC and DC (a: 4000–400 cm−1 full spectrum, b: 4000–3650 cm−1, and 700–400 cm−1 segmentation spectrum). 3

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Table 5 FT-IR absorption peaks and the corresponding functional group assignment. Wave number (cm−1)

Functional group

Compounds

3000–3700 2916 2848 1593 1429, 1375 900–1200 700–900 400–600

O-H, N-H -CH -CH C=C -CH Si-O-Si, Al-O-Al C-H Si-O, Si-O-Al

O–H or NeH stretching vibrations Stretching vibration of -CH2 Stretching vibration of -CH3 C=C stretching vibrations -CH3, -CH2 deformation vibrations Si-O-Si, Al-O-Al compounded vibrations Out of plane CeH deformation vibrations Si-O stretching vibrations, Si-O-Al compounded stretching vibrations, quartz

Fig. 2. SEM analysis of the pyrolysis products of RC-K (a) and DC-K (b).

(a)

(b)

5000 Element C

(c)

Weight % 75.3 24.7

Fe

4000

Atomic % 93.42 6.58

Counts

3000 Fe C Cu

2000 Fe

1000

Cu Si

Fe

K

Cu

0 0

4

Energy (keV)

8

12

Fig. 3. TEM analysis of the RC-K pyrolysis product (a), amplification of identification area in a (b), EDS analysis of the identification area in b (c).

4

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(a)

(b)

10000

(c)

Element C Fe

Weight % 86.27 13.73

Fe

Cu

Atomic % 96.69 3.31

C

8000

Counts

6000 4000 Fe

2000 Cu Si

Fe

K

Cu

0 0

4

8

12

Energy (keV) Fig. 4. TEM analysis of the DC-K pyrolysis product (a), amplification of identification area in a (b), EDS analysis of the identification area in b (c).

decreased from 4600 to 228 mg/kg (95.04% decrease), close to the ash reduction value (94.39%). This indicated that the content of Fe in DC decreased to the original value of ca. 5%. Despite this significant decrease in the Fe content in DC, Fe3C catalyst particles were found on the CNTs of the pyrolyzed DC-K product (Fig. 4a–c). The CNT coated with the catalyst particles was about 62 nm in diameter, 20 nm in thickness and 800 nm in length. According to the analysis of the pyrolysis product of RC-K, it can be concluded that Fe played an important role in the formation of CNTs during the pyrolysis of RC-K and DC-K.

composed of Fe, Ca, Al, and Si elements and their compounds [5]. Trace elements such as Co and Ni among others may be also present in coal. Among these elements, Fe, Co, and Ni are often used as catalysts for the preparation of CNTs [18]. Therefore, in order to understand the role of these elements in the formation of CNTs, TEM-EDS analyses of the pyrolysis products of RC-K and DC-K were carried out. The results are shown in Figs. 3 and 4. The morphology of CNTs in the pyrolysis product of RC-K is shown in Fig. 3a. Part of the CNTs were coated with catalyst particles. The diameter of the CNT was about 40 nm, the wall thickness was about 13 nm, and the length was about 500 nm (Fig. 3b). The basic composition of the catalyst particles (Fig. 3b) coated in the CNTs is shown in Fig. 3c. These catalyst particles basically contained C and Fe, and minor amounts of Si, K, and Cu. C, Fe, and Si were derived from RC (K resulted from the added KOH and Cu from the support network used during the characterization). As shown in Fig. 3b, the ordered lattice fringe spacing of the catalyst particles was 0.204 nm. This value corresponded to the (220) crystal surface spacing of Fe3C [37,38], indicating that the catalyst particles were well-crystallized in the form of Fe3C. Therefore, we inferred that the Fe present in RC promoted the generation of CNTs during the pyrolysis of RC-K, while Co, Ni, and other elements had no catalytic effect as a result of their low content (Co and Ni ICP contents in RC were 6.30 and 7.14 mg/kg, respectively) [29] or weak catalytic activity compared with Fe [39,40]. Since no additional Fe was added to RC during the experiment, the Fe catalyst particles originated from the primary Fe-containing material. ICP analysis revealed a Fe content in RC of 4600 mg/kg. In order to reduce the effect of Fe, HCl-HF-HCl washing treatments were carried out to demineralize RC. After acid washing, the content of Fe in coal

3.3. SEM-mapping analysis of RC, RC pyrolysis product, and RC-K pyrolysis product In order to determine the distribution of Fe in coal before and after the non-catalytic and KOH-catalyzed pyrolysis processes, SEM-mapping analyses of the RC, RC pyrolysis product (without any catalyst), and RCK pyrolysis products were carried out (Fig. 5). C was the main component of RC, RC pyrolysis product, and RC-K pyrolysis product and the three samples showed nearly similar values (82.73, 87.84, and 85.24 wt%, respectively [30]). In addition, the distribution of carbon in the RC (Fig. 5b), RC pyrolysis product (Fig. 5e), and RC-K pyrolysis product (Fig. 5h) was relatively uniform. Therefore, based on this distribution, we compared the Fe/C atomic ratio for the three samples to study the evolution of the Fe content on the surface of samples before and after the pyrolysis processes. Comparing the Femapping analyses of RC (Fig. 5c), RC pyrolysis product (Fig. 5f), and RC-K pyrolysis product (Fig. 5i), Fe was evenly distributed on RC and the pyrolysis product of RC, although with low contents, with Fe/C atomic ratios of 0.0113 and 0.0142, respectively, according to the 5

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Fig. 5. SEM-mapping analyses of RC, RC pyrolysis product, and RC-K pyrolysis product (a: RC, b: C in a, c: Fe in a, d: RC pyrolysis product, e: C in d, f: Fe in d, g: RC-K pyrolysis product, h: C in g, i: Fe in g).

process.

mapping analysis. The RC-K pyrolysis product showed significantly larger Fe contents (Fe/C = 0.0834). The distribution of Fe element in the region of CNTs existed is more concentrated, which may be due to the inclusion of Fe particles in CNTs. The Fe embedded in the coal particles remained non-volatile during the pyrolysis of coal [41,42]. Thus, Fe was not released from the coal particles accompanying CH4, CO2, and other components generated by pyrolysis of small molecular functional groups such as -CH3 and –COOH [13]. Therefore, the migration of Fe from bulk to the surface of the coal particles was caused by external factors. During the pyrolysis process, small molecular functional groups such as -CH3 and -COOH on RC were gradually converted into gaseous substances such as CH4 and CO2 and were released from the coal particles. As a result, the Fe particles previously protected by these small molecules functional group were exposed on the surface of the coal particles. Comparing the Fe/C atomic ratios of RC (0.0113) and the RC pyrolysis product (0.0142), it can be seen that the pyrolysis of the functional groups had little effect on the exposure of Fe. In the pyrolysis process, the added KOH can react with carbon in coal at high temperatures to form K-containing components such as K2CO3, K2O, and K [43,44], which have the ability to etch the surface and internal macromolecular carbon structure of the coal particles [45–47]. Comparing the Fe/C atomic ratio of the RC pyrolysis product (0.0142) and the RC-K pyrolysis product (0.0834), we observed that the etching of the K-containing compounds on the coal particles have a significant effect on the exposure of Fe. Therefore, it is considered that the etching process of the K-containing compounds can mostly explain the exposure of Fe on the coal particles, while the pyrolysis of the functional groups can have also a certain effect on the

3.4. BET analysis of RC, RC pyrolysis product and RC-K pyrolysis product In order to understand the effect of KOH on the specific surface area and pore structure of coal samples during pyrolysis, BET analysis of RC, RC pyrolysis product and RC-K pyrolysis product was carried out. The results are shown in Fig. 6. As shown in Fig. 6a, the adsorption and desorption isotherms of RC and RC pyrolysis product were relatively flat in the relative pressure range of 0–0.4, indicating that the number of micropores in the two samples was low. The hysteresis loops appeared in the range of relative pressure between 0.4 and 0.99 for both samples. According to the classification of International Union of Pure and Applied Chemistry (IUPAC), the hysteresis loops were H3 type, which indicated that the internal pore of the two samples was mainly a slit pore formed by flaky particles [48]. The adsorption and desorption isotherms of RC-K pyrolysis product rose rapidly within the relative pressure range of 0–0.1, which indicated that there were a lot of microporous structures in the sample. The adsorption and desorption hysteresis loop of the sample was of type H4, which indicates that there were narrow slit-like pores in the sample [48]. In addition, the maximum volume adsorbed of RC-K pyrolysis product was 315.85 mL/g, which was much higher than that of RC 18.89 mL/g and RC pyrolysis product 9.64 mL/g. As can be seen from Fig. 6b, the RC had a specific surface area of 8.09 m2/g, which was mainly composed of mesopores and macroporous specific surface areas. The specific surface area of the RC pyrolysis product was 1.93 m2/g, and the specific surface area of the micropores 6

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100 5

0

Specific surface area (m2/g)

10

Micropore Mesopore + macropore Total pore

8

600 6 400

4

200

2

0 0.0

0.2

0.4

0.6

0.8

0

1.0

RC

RC pyrolysis product

Relative pressure (P/P0)

0.02

0.5

0.4

0.3

0.2 0.01 0.1

0.00

RC

30

Pore volume (mL/g)

Micropore Mesopore + Macropore Total pore

0 RC-K pyrolysis product

Samples

0.0 RC pyrolysis product RC-K pyrolysis product

Average pore diameter (nm)

(c) 0.03

Pore volume (mL/g)

800

Specific surface area (m2/g)

15

RC-K pyrolysis product RC RC pyrolysis product

200

(b) Volume adsorbed (mL/g)

Volume adsorbed (mL/g)

20

(a)

300

(d)

Average pore diameter

25 20 15 10 5 0

RC

RC pyrolysis product RC-K pyrolysis product

Samples

Samples Fig. 6. BET analysis of RC, RC pyrolysis product and RC-K pyrolysis product (a) adsorption and desorption isotherms, (b) specific surface area, (c) pore volume, (d) average pore diameter.

was 0.55 m2/g. The specific surface area of the RC-K pyrolysis product was 846.88 m2/g, and the specific surface area of the micropores was 803.74 m2/g. According to Fig. 6c that the RC pore volume was 0.0292 mL/g, which was composed of mesopores volume and macroporous volume. The pore volume of the RC pyrolysis product was 0.0149 mL/g, of which the micropores accounted for 2.08%. The pore volume of the RC-K pyrolysis product was 0.4886 mL/g, which was 16.73 times of the RC, and the proportion of micropores was 85.15%. It can be seen from Fig. 6d that the average pore diameter of RC, RC pyrolysis product and RC-K pyrolysis product were 14.43 nm, 30.80 nm and 2.30 nm, respectively. The above results indicated that the addition of KOH promoted the formation of internal pores in the coal sample, especially the formation of microporous structures, resulting in a significant increase in the specific surface area and pore volume of the coal sample compared to the direct pyrolysis of the raw coal (without the addition of any catalyst). The formation of this result was mainly due to the etching effect of KOH on the coal sample, that was, the reaction of KOH with the C element in the coal sample [43,44], thereby destroying the original macromolecular carbon structure in the coal sample [45–47]. It was precisely because of the etching action of KOH that the Fe element hidden inside the coal sample was exposed to the surface of the coal sample, providing a necessary catalyst for the formation and growth of the CNTs.

many kinds of minerals such as kaolin, calcite, and other impurities. To avoid the influence of other minerals, pure FeCO3 was added to DC instead of siderite simulating the siderite phase of RC, and to explore the evolution of siderite during the coal catalytic pyrolysis. As shown in Fig. 7a, the XRD patterns of the FeCO3 samples contained the peaks characteristics of the siderite FeCO3 phase (PDF: 29-0696). In the experiment, we added a large amount of FeCO3 (10 wt%) to improve the accuracy of the XRD analysis. In addition, high-purity graphite was used to simulate the reaction of the macromolecular aromatic carbon structure of coal under the same conditions. Also, graphite was added to explain the effect of the macromolecular aromatic carbon structure of coal on the reduction of Fe [6]. Figs. 7 and 8 show the XRD and SEM analysis of the samples, respectively. As shown in Fig. 7a and b, FeCO3 was transformed into Fe3O4 by pyrolysis. Under the same conditions, co-pyrolysis of FeCO3 with DC or graphite generated α-Fe (Fig. 7c and d), indicating that both DC and graphite showed high reducing activity on the oxidized Fe. The carbon atoms of the graphite crystal structure served as reducing sites. In the case of DC, gases such as CO and H2 released during the pyrolysis of coal [49,50] as well as the carbon atoms of the macromolecular aromatic carbon structure were the reducing agents. As shown in Fig. 7d and e, Fe was in the same form in both the G-K/ Fe and G-Fe pyrolysis products (α-Fe). No CNTs were observed in the GK/Fe pyrolysis product (Fig. 8a), although significant etching was observed (Fig. 8b, compared with Fig. 8e showing the flat surface of graphite). This result indicated that KOH can etch graphite at high temperatures, although the carbon atoms generated upon graphite etching were not converted into CNTs. In addition, the addition of KOH had no significant effect on the reduction of FeCO3 to α-Fe by graphite. When DC was used instead of graphite, Fe was as Fe3C in the pyrolysis

3.5. Evolution of the siderite FeCO3 phase during coal pyrolysis As previously reported by our group, Fe in mostly present in RC as siderite [30]. However, due to the low content of Fe in coal (4600 mg/ kg), it is difficult to track the conversion of Fe. In addition, coal contains 7

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400

4000

(a)

ͪ

(b)

ͪ FeCO3

ƺ Fe 3O4

ƺ

300

Intensity (cps)

Intensity (cps)

3000

2000

ͪ

1000

ͪ ͪͪ

0

20

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Fig. 7. XRD analysis of the samples: a: pure FeCO3, b: FeCO3 pyrolysis product, c: DC-Fe pyrolysis product, d: G-Fe pyrolysis product, e: G-K/Fe pyrolysis product, and f: DC-K/Fe pyrolysis product. The graphs embedded in the graphs d and e are respectively enlarged results in the range of 2θ=40-90°. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

of DC-K/Fe, Fe shifted from α-Fe to Fe3C by via adsorption and fusion of the surrounding carbon atoms or carbon clusters by the α-Fe particles [37]. These carbon atoms or clusters originated from the KOH added. KOH was mainly converted to K2CO3 at high temperatures (Fig. 7f), with minor amounts of K2O and K potentially being formed, as reported elsewhere [43,44]. These forms of K showed strong cracking and etching effects [45–47]. They can crack carbon-containing substances such as CH4, C2H6, and etch the aromatic carbon structure of coal to produce carbon atoms or clusters. These carbon atoms or clusters were absorbed and incorporated into the α-Fe particles to form Fe3C particles. With the continuous integration of carbon atoms or clusters, the carbon atoms in these Fe3C particles gradually reached super-saturation. Then, the carbon atoms or clusters precipitated on the surface of the Fe3C particles in the form of CNTs. When KOH was used as the only catalyst, it catalyzed the cracking of methane to CNTs [30]. However, in the formation of coal-based CNTs, K-containing compounds such as K2CO3 and K2O formed from

product of DC-K/Fe (Fig. 7f), with a large number of CNTs being present in this sample (Fig. 8c). In contrast, Fe was as α-Fe in the pyrolysis product of DC-Fe (Fig. 7c), with no CNTs being formed on this sample (Fig. 8d). The above analysis indicated that the existence of Fe3C may be related to the generation of CNTs. At the same time, the carbon source of CNTs were mostly small molecules such as CH4, C2H6, and CO produced during coal pyrolysis [13]. The carbon produced by the etching of the aromatic carbon structure of coal macromolecules by K-containing compounds may be involved in the formation of CNTs, although it was not the main carbon source for the growing of these materials. The presence of Fe3C in the pyrolysis product of DC-K/Fe was consistent with the TEM-EDS analysis. In the pyrolysis of DC-Fe, the Fe of FeCO3 was transformed into α-Fe instead of Fe3C, indicating that α-Fe is stable at high temperatures and did not show any catalytic cracking effect on carbonaceous substances such as CH4 and C2H6 produced upon pyrolysis. In the pyrolysis product 8

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Fig. 8. SEM analysis of the samples: a: G-K /Fe pyrolysis product, b: amplification of identification area in a, c: DC-K /Fe pyrolysis product, d: DC-Fe pyrolysis product, and e: G.

KOH mainly served as cracking sites for carbonaceous materials such as CH4 and C2H6. Also, these compounds etched the macromolecular aromatic carbon structure to form carbon atoms or carbon clusters. These species resulting from the etching process served as a carbon source for the conversion of α-Fe into Fe3C and for the growth of CNTs catalyzed by Fe3C. In the case of coal samples with high siderite content, the Fe3C resulting from siderite was the main catalyst promoting the growth of CNTs. The fraction of CNTs catalyzed by K species (e.g., K2CO3, K2O, etc.) was low.

3.6. Analysis of the mechanism for CNTs growth catalyzed by primary siderite in coal As shown in the FeeC phase diagram (Fig. 9) at 900 °C (highlighted with a the red line), as the carbon atoms or carbon clusters continuously merge into α-Fe particles, the α-Fe phase is gradually converted into αFe + γ-Fe, γ-Fe, γ-Fe + Fe3C, and finally Fe3C + Graphite phases. The

Fig. 9. FeeC equilibrium phase diagram (obtained from [50,52,53]).

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Fig. 10. TEM analysis of the nanocapsules of RC-K pyrolysis product: a: catalyst particles with small diameter and b: catalyst particles with large diameter.

Fig. 11. Schematic diagram of the growth process of CNTs catalyzed by the primary siderite phase of coal. (Yellow, blue, and red dots correspond to FeCO3, α-Fe, and Fe3C, respectively. I, II, and III in e represent catalyst particles with small, medium, and large particle sizes, respectively). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

surface of the catalyst particles to form carbon shells simultaneously, deactivating the catalyst. Finally, the catalyst with small particle size was present in the form of nanocapsules, and these species cannot catalyze the formation of CNTs. The catalyst particles with large diameter have low surface energy and therefore these particles are not suitable to adsorb carbon atoms or carbon clusters, resulting in a low internal carbon content and insufficient precipitation to form CNTs. Once the reaction was completed or the temperature was lowered, the carbon atoms inside the catalyst particles gradually precipitated to form a carbon shell [50]. As a result, the catalyst was deactivated and eventually formed nanocapsules. The catalyst particles of medium size have a moderate ability to adsorb carbon atoms or clusters. The integration and precipitation of carbon atoms or clusters on the surface of these catalyst particles can therefore reach equilibrium, and finally CNTs are formed on the surface of the catalyst particles [55,56]. In summary, the transformation of the primary siderite of coal and the growth mechanism of CNTs catalyzed by this coal followed the mechanism: FeCO3 → α-Fe → Fe3C + Graphite → CNTs. The specific process is shown in Fig. 11. First, the siderite embedded in the coal particles was reduced to α-Fe by both the macromolecular aromatic structure carbon of coal and the gaseous species (e.g., H2, CO, etc.)

mass of C/Fe in RC is relatively high (the mass ratio of C/Fe in RC was ca. 179.85). On the other hand, compared with the growth control step of CNTs (the diffusion of carbon atoms inside the catalyst particles [51]), the incorporation of carbon atoms or carbon clusters into the αFe phase is faster. Therefore, during coal pyrolysis, an intermediate Fe phase containing α-Fe + γ-Fe, γ-Fe, γ-Fe + Fe3C had a short existence time. It can be approximated that the siderite in coal was reduced to form α-Fe, which was directly transformed to the Fe3C + graphite state. Due to the etching of the coal particles by the K-containing compounds, the Fe3C catalyst particles embedded inside the coal particles were continuously exposed. Fe3C nanoparticles have high fluidity due to their low melting point [50,53,54]. Therefore, the adjacent Fe3C will fuse to form catalyst particles with different particle sizes. CNTs were generated on the surface of Fe3C catalyst particles with medium size, while particles small or large in size eventually exist in the form of nanocapsules (Fig. 10a and b) [21]. The results were in good agreement with previous statistical results of the diameter distribution of CNTs [30]. The final morphology of the catalyst particles depended on the particle diameters, and this can be explained as follows. Surface diffusion was the main transfer mechanism of carbon atoms in small size catalysts [28], and a large number of carbon atoms precipitated on the

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released by the coal pyrolysis (Fig. 11a → b). Secondly, at high temperatures, the K-containing compounds (e.g., K2CO3, K2O, and K) catalyzed the cracking of CH4, C2H6, etc. and etched the macromolecular aromatic carbon structure of coal, generating carbon atoms or carbon clusters which were subsequently adsorbed and integrated into α-Fe to form Fe3C (Fig. 11b → c). Thirdly, the K-containing compounds etched the coal particles and exposed Fe3C (Fig. 11c → d), and the adjacent Fe3C fused with each other to form catalyst particles of different sizes (Fig. 11d → e). Finally, the carbon atoms or carbon clusters gradually reached the super-saturation state inside the Fe3C particles with medium size and precipitated as CNTs on their surface. In contrast, the carbon atoms or clusters precipitated on the surface of Fe3C particles with smaller or larger sizes in the form of carbon shells (Fig. 11e → f).

[5] [6]

[7]

[8]

[9] [10] [11]

4. Conclusions

[12]

The HCl-HF-HCl pickling and demineralization processes can effectively reduce the mineral content in the coal, and had little effect on the organic carbon structure of the aliphatic and aromatic groups of the raw coal. In the catalytic pyrolysis of coal, the primary Fe minerals in coal played an important role in the formation of CNTs. During the KOH-catalyzed pyrolysis of coal, the Fe present in the raw material migrated from the bulk to the surface of the coal particles, and enriched in some areas of the surface of the coal particle. KOH-catalyzed coal pyrolysis had a significant effect on the formation of micropores in coal and the increase of specific surface area and pore volume. The specific surface area of pyrolysis products was increased from 8.09 m2/g of raw coal to 846.88 m2/g, and the pore volume was increased to16 times of raw coal, and the average pore size decreased from 14.43 nm of raw coal to 2.30 nm. In the KOH-catalyzed pyrolysis of coal, the transformation of the primary siderite and the catalytic growth of CNTs were in line with the mechanism: FeCO3 → α-Fe → Fe3C + Graphite → CNTs. Firstly, the siderite embedded in coal particles was reduced to α-Fe by both the macromolecular aromatic carbon in coal and the gaseous species (e.g., H2, CO) produced by coal pyrolysis. Secondly, at high temperatures, K-containing compounds (e.g., K2CO3, K2O, and K) catalyzed the cracking of CH4, C2H6 and etched aromatic carbon structure of coal macromolecule to produce carbon atoms or carbon clusters. These species were adsorbed and integrated into α-Fe, generating Fe3C as a result. Thirdly, the Fe3C particles exposed by the K-containing compounds etched coal particles, and the exposed Fe3C particles sintered to form Fe3C catalyst particles of different sizes. Finally, CNTs were generated on the surface of the Fe3C particles with medium particle size, while the catalyst particles with small or large size finally existed in the form of nanocapsules.

[13] [14]

[15]

[16]

[17] [18] [19] [20] [21] [22]

[23] [24] [25] [26]

[27] [28]

Declaration of competing interest The authors declare that we have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

[29] [30]

Acknowledgements

[31]

The work was supported by the National Natural Science Foundation of China (Grant Numbers 51774211).

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