Activation mechanisms on potassium hydroxide enhanced microstructures development of coke powder

Journal Pre-proof Activation mechanisms on potassium hydroxide enhanced microstructures development of coke powder

Xiaojing Chen, Huirong Zhang, Yanxia Guo, Yan Cao, Fangqin Cheng PII:

S1004-9541(19)30844-4

DOI:

https://doi.org/10.1016/j.cjche.2019.07.023

Reference:

CJCHE 1556

To appear in:

Chinese Journal of Chemical Engineering

Received date:

14 October 2018

Revised date:

15 July 2019

Accepted date:

31 July 2019

Please cite this article as: X. Chen, H. Zhang, Y. Guo, et al., Activation mechanisms on potassium hydroxide enhanced microstructures development of coke powder, Chinese Journal of Chemical Engineering(2019), https://doi.org/10.1016/j.cjche.2019.07.023

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© 2019 Published by Elsevier.

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Activation Mechanisms on Potassium Hydroxide Enhanced Microstructures Development of Coke Powder

Xiaojing Chen a, Huirong Zhang a, Yanxia Guo a,, Yan Cao b, Fangqin Cheng a,*

State Environmental Protection Key Laboratory on Efficient Resource-utilization

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a

Techniques of Coal Waste, Institute of Resources and Environmental Engineering,

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Shanxi Collaborative Innovation Center of High Value-added Utilization of

Institute for Combustion Science and Environmental Technology (ICSET), Western

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b

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Coal-related Wastes, Shanxi University, Taiyuan 030006, China

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Kentucky University (WKU), Bowling Green, Kentucky 42101, United States

The e-mail addresses of all authors:

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Xiaojing Chen: [email protected] Huirong Zhang: [email protected] Yanxia Guo: [email protected] Yan Cao: [email protected] Fangqin Cheng: [email protected]



Corresponding author. Tel./fax: +86-351-7018553. E-mail addresses: [email protected]; [email protected]

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Abstract Coke powder is expected to be an excellent raw material to produce activated carbon because of its high carbon content. Potassium hydroxide (KOH), as an effective activation agent, was reported to be effective in activating coke powder. However, the microstructures development in the coke powder and its mechanisms when KOH was applied were still unclear. In this study,

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effects of KOH on the microstructure activation of coke powder were investigated using the surface area and pore structure analyzer, scanning electron microscope (SEM) and

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thermogravimetry-differential scanning calorimetry-mass spectrometry (TG-DSC-MS), etc. Results

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revealed that the addition KOH at its lower ratio (mass ratios of KOH and coke powder in a range

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of 0.5 and 1) decreased the specific surface area and average lateral sizes, but sharply increased

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of the specific surface area to 132 m2 g-1 and 355 m2 g-1 and decreased of the space size of

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aromatic crystallites upon the further increase of the KOH addition amounts (ratios of KOH and coke powder in a range of 3 and 7), generating a number of new micropores and mesopores. The

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mechanisms study implied surface reactions between KOH and aliphatic hydrocarbon side chain and other carbon functional groups of the coke powder to destruct aromatic crystallites in one dimension and broaden pores at lower KOH addition. In the activation process, KOH was decomposed to be more active components, which can be rapidly destruct the aromatic layers in spatial scope to form developed porous carbon structures within coke powder at higher KOH addition. Keywords: activated carbon; coke powder; activation; structure; potassium hydroxide

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1. Introduction Coke powder is a kind of by-product of the coking production, and also wastes derived from coking, metallurgical and chemical industries. China is one of the largest coke production and consumption countries. Abandoned coke powder accounts for about 40 million tons annually [1– 3]. Coke powder waste generally has smaller particle sizes averaged less than 5 mm, and thus

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cannot satisfy for the use in regular coking utilizations [1, 4]. Therefore, a large amount of coke powder waste has been stockpiled, causing serious environmental concerns [5].

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Activated carbon (AC) is one of widely used carbon materials [6]. AC is prepared firstly by the

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carbonization of the carbon containing materials at 400–600 oC with the absence of air, and then by the chemical and/or physical activation [7, 8]. The main raw materials of AC can be wood, coal

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and others with high carbon content [9–11]. Coke powder, greater than 90 % in its carbon

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content, can be promising starting material of AC [4], in term of an economic view. Further, only the activation process is required because coke powder has experienced pyrolysis at high

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temperature up to around 950–1050 oC [12]. However, the previous pyrolysis undergoes a high

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temperature procedure, making the reactivity of the obtained coke powder lower, and thus difficult to be activated. This was attributed to the formation of its small interlayer spacing of aromatic crystallites, the large size of the crystallites unit, and dense structures [13, 14]. The KOH activation is an effective method in the preparation of AC [15, 16], making the development of mesopores and micropores and the higher surface area within the AC product [17–19]. Previous studies applied the KOH activation procedure to effectively make AC from coke powder. For example, Zhang et al. [20] produced AC with a surface area of 951 m2 g-1 in a formula of 8:4.7:1 of KOH, coal tar and coke powder under 850 oC for 40 mins. Luo et al. [21] prepared the AC by KOH activating coke powder (4:1 of KOH and Coke powder) at 900 oC for 80 mins. Its adsorption value of methylene blue reached 304.8 mg g-1 and the removal efficiency of Cr6+ in waste water reached 93.2 %. Previous studies focused more on conditions of the activation process or properties of the activated coke powder, but less on the microstructures development

Journal Pre-proof in the coke powder and its mechanisms when KOH was applied, which is critical toward the activation process of coke powder to expand its wide utilizations. In this study, effects of the KOH addition on the microstructures development in coke powder during its the activation process were investigated. The obtained AC products were characterized using the specific surface area and pore structure analyzer, X-ray powder diffractometer (XRD), scanning electron microscope (SEM), high-resolution transmission electron microscopy (TEM), fourier transform infrared spectroscopy (FT-IR) and thermogravimetry-differential scanning

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calorimetry-mass spectrometry (TG-DSC-MS). Mechanisms on the activation of coke powder

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using KOH were explored, providing the theoretical guidance for the further scaled-up coke

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powder derived AC production and its industrial application. 2. Materials and methods

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2.1 Materials

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Coke powder from Xiangrui coking plant in Shanxi of China was used as the raw material. It

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was crushed and sieved to 150–380 μm in particle size. The proximate and ultimate analysis of

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the obtained coke powder is given in Table 1.

Table 1 Proximate and ultimate analysis of the coke powder

Proximate analysis (wt%)

Ultimate analysis (wt%, daf )

FCad

Mad

Vad

Aad

C

H

S

N

O

84.64

0.16

1.80

13.4

96.53

0.28

1.05

0.86

1.28

2.2 The activation method The coke powder was impregnated with the saturated solution of KOH (by weight) at room temperature for 4 hours, and then dried at 110 oC for 20 hours. The mixture was calcined in a tube furnace at 850 oC for 2.5 hours in a nitrogen flow at 200 mL min-1. The calcined sample was

Journal Pre-proof cooled to the room temperature under the N2 atmosphere before it was treated using 6.5 mol L-1 of HCl for 1 hour at the room temperature. The activated sample was finally washed by the hot water, dried for further characterizations. The activated samples were denoted as 0.5KCP, 1KCP, 3KCP and 7KCP, respectively, corresponding to the KOH treatment with ratios of KOH to Coke powder at 0.5, 1, 3 and 7. The coke powder was denoted as CP.

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2.3 Characterizations Nitrogen adsorption isotherms were obtained using a Micromeritics Adsorption Apparatus

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(ASAP2460, Micromeritics, USA) at 77 K to have pore structure information of the samples. The

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BET surface area (SBET) was determined according to the Brunauer-Emmett-Tellerequation

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equation. The total pore volume (Vt) was determined based on the adsorption of liquid nitrogen

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at a relative pressure of 0.98, and the average pore diameter (Dav) was determined according to

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the manufacturer’s software. The microporous surface area (Smic) was calculated by the t-plot

[7].

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method, and the pore size distribution (PSD) was calculated by the density functional theory (DFT)

The CP and their activated samples were investigated using X-ray Diffractometer (XRD, D2-Phaser, Bruker, Germany) (a Cu Kα radiation, 30 kV, 10 mA, and an advanced linear detector). The samples were scanned in a 2θ range from 10 ° to 80 ° with a 0.01 ° step and a speed at 0.1 s step-1. Several critical crystallite structural parameters of the samples, such as the interlayer spacing (d002), the average lateral size (La), stacking height (Lc) and the number of aromatic layers (N) can be calculated based on following equations [22]: d002 = λ⁄(2 sin θ002 )

(1)

La =1.84λ/(B100 cos θ100 )

(2)

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Lc =0.89λ/(B002 cos θ002 )

(3)

N=Lc /d002

(4)

where λ is the wavelength of the radiation used (1.54184Å); B002 and B100 are the widths of the (002) and (100) phases, respectively, at 50 % height of phase peaks; θ002 and θ100 are the corresponding scattering angles or peak positions.

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The morphology structures were examined using Scanning Electron Microscope (SEM, JSM-7001F, Jeol, Japan) and High-resolution Transmission Electron Microscopy (TEM, JEM-2010,

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Jeol, Japan) [23].

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The surface chemical properties of the samples were investigated by Fourier Transform

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Infrared Spectroscopy (FT-IR, 1730, Perkin-Elmer, USA). FT-IR spectra of samples were conducted

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at room temperature using potassium bromide/sample pellet technique at the ratio of 300 to 1.

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The spectra were acquired in the range of 4000–400 cm-1 wavenumber with 2 cm-1 resolution. For all spectra, a linear baseline correction was used.

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The mass changes and occurred reactions of the activation process were characterized using a thermal analyzer (Setsys Evolution, Setaram, France) coupled to a mass spectrometer (Omnistar GSD320, Pfeiffer, Germany). For each test, approximately 10 mg sample was loaded in a platinum crucible with a blank platinum crucible as the reference. The temperature ramp was performed from the ambient temperature to 1100 °C at a heating rate of 10 oC min-1 under a nitrogen flow rate of 100 mL min-1. The evolved gases during the thermal process were analyzed in the attached mass spectrometer with the multiple ion detection method (MID). 3. Results and discussion 3.1 Effects of the KOH activation on structures of CP

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3.1.1 Pore structure analysis Investigation on effects of the KOH activation on the pore structures of CP, such as the pore size distribution, the pore volume and the specific surface area, was conducted via the BET

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analysis using the various amount of KOH (Fig. 1 (a) and (b)).

Fig. 1 Adsorption/desorption isotherms of N2 at 77 K (a) and pore-size distribution (b) at the various mass ratios of KOH to CP The N2 adsorption capacities of the 0.5KCP and 1KCP were slightly lower than that of the CP.

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The increase of the amount of KOH，in 3KCP and 7KCP, significantly improved their N2 adsorption capacities. According to classification of IUPAC, the N2 adsorption isotherms of the CP, 0.5KCP and 1KCP were close to that of type Ⅱ, presenting the adsorption property of macroporous and non-porous solid. The N2 adsorption isotherms of 3KCP and 7KCP were close to that of type I at low relative pressures and type IV at high relative pressures, presenting the adsorption property

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of micropores and mesopores, respectively [24]. As shown in Fig. 1 (b), the corresponding pore-size distribution of the CP was mainly micropores and mesopores (1–3 nm) within 100 nm.

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The pore sizes of 0.5KCP and 1KCP were mesopores (2–10 nm) within 100 nm. Results in both Fig.

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1 (a) and Fig. 1 (b) revealed there was no formation of micropores at the lower levels of the KOH

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addition. The increase of KOH addition, the pore sizes of 3KCP and 7KCP shifted to 0.2–5 nm,

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presenting the formation of the new micropores and mesopores.

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Table 2 lists the corresponding pore structure parameters of CP and obtained activated CP. Compared with CP, the specific surface area, microporous surface area and the pore volume of

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0.5KCP and 1KCP decreased, while those of 3KCP and 7KCP increased. This implied the collapsed pore structures in 0.5KCP and 1KCP and the new pores formation in 3KCP and 7KCP. In addition, the average pore diameters of 0.5KCP and 1KCP rose, but declined for 3KCP and 7KCP in comparison to with that of CP. It indicated that KOH may react mainly with the surface functional groups and the defective carbon of both CP and the primary pores at the lower amount (<1), which mainly played a role of broadening pores or collapsing pores [25]. With the increase of KOH addition, enough amount of KOH (>3) can react with the aromatic carbon of CP to generate a large amount of microporous which decreased the average pore diameters of the obtained activated CP [26].

Journal Pre-proof Table 2 Pore structural parameters of the CP samples before and after activated by various amounts of KOH SBET (m2 g-1)

Smic (m2 g-1)

Vt (cm3 g-1)

Dav (nm)

CP

16

7

0.015

3.618

0.5KCP

4

-

0.007

6.840

1KCP

8

-

0.013

6.214

3KCP

132

68

0.083

2.505

7KCP

355

169

0.195

2.195

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3.1.2 Carbon structure analysis

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Samples

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The XRD characterization was investigated on carbon structures of CP before and after

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activated using KOH for further revealing effects of the KOH activation on structures of CP.

Fig. 2 XRD spectra of CP samples before and after activated by KOH As shown in Fig. 2, two diffraction peaks were identified at 2θ of about 26.3 ° and 43.4 ° for both the original CP and those activated by KOH, representing the 002 phase and 100 phase of graphite crystal, respectively [22]. It was generally accepted that the sharper the 002 phase and the stronger and closer to 26.6 ° of the carbon sample was, the higher the degree of the

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graphitization was [27]. It was found that the KOH activation obviously decreased the intensity of its 002 phase. At the ratio of KOH and CP was larger than 3, the 002 phase shifted to the left and the area of this peak increased and the left side of the peak was raised, indicating the formation of disordered structures of the amorphous carbon [28, 29]. There was no significant variation of the 100 phase to be observed. Results evidenced the destruction of the graphite-like crystal

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structures of the CP during the KOH activation. The increase of the addition amount of KOH enhanced the destruction of the graphite-like crystal structures and the formation of the

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disordered amorphous carbon structures.

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The XRD patterns of obtained activated CP showed the clear formation of the asymmetric (002)

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band at around 26 ° as the amount of the KOH increased, likely suggesting the existence of

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another band (γ) on its left side. The (γ) band around 20 ° can be attributed to the saturated

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carbon structures, such as aliphatic side chains attached to the edge of carbon crystallites [29, 30]. The diffraction peaks of the 002 phase at 15–34 ° and the 100 phase at 38–50 ° were further

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peak-fitted using the PeakFit V4.1 software [29], and fitting results are shown in Fig. 3.

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Fig. 3 Curve-fitting of the 2θ peaks at 15–34 ° (a) and 38–50 ° (b) for the CP; 15–34 ° (c) and 38– 50 ° (d) for the 7KCP It was found that the intensity of its 002 phase and 100 phase declined obviously at the ratio of

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KOH and CP was 7 compared with CP, the area of γ peak increased and the area of 002 peak decreased, indicating the destruction of the aromatic crystallites structures in CP and the formation of disordered structures of the amorphous carbon. Fitting results can be used to calculate parameters of aromatic crystallites structures addressing the interlayer spacing (d002), the average lateral sizes (La), the stacking height (Lc) and the number of aromatic layers (N), which

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have been summarized in Table 3. Table 3 Derived structure parameters extracted from the curve-fitting

d002(Å)

La(Å)

Lc(Å)

N

CP

3.456

46.068

28.486

8.24

0.5KCP

3.458

42.165

27.155

7.85

1KCP

3.466

41.953

20.730

5.98

3KCP

3.489

40.401

18.787

5.38

7KCP

3.513

36.973

17.331

4.93

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Samples

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The d002 value ranged from 3.456 to 3.513 Å. The values of La and Lc ranged from 46.068 to 36.973 Å and 28.486 to 17.331 Å, respectively. The N value of the crystallite structures ranged from 8.24 to 4.93. The rise of KOH addition increased the interlayer spacing (d002), but decreased the average lateral sizes (La), the stacking height (Lc) and the number of aromatic layers (N). This implied that the KOH activation directly correlated with the destruction of the crystallites structures in CP by expanding the interlayer spacing and reducing the size of aromatic crystallites. Compared with CP, the variation in La of 0.5KCP was larger and those in La, Lc, N and d002 of 1KCP, 3KCP and 7KCP were obvious. This implied that the lower KOH amount likely destructed the

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aromatic crystallites in the one dimension and later expanded to the spatial level at a higher KOH amount.

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3.1.3 Morphology analysis

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Fig. 4 SEM (a) and TEM (b) of CP, 0.5KCP and 7KCP

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The SEM micrographs of CP before and after activation are presented in Fig. 4 (a). It was found that a certain number of the cylindrical pores were distributed on the surface of CP. The surface of 0.5KCP became rough, its pore size developed larger and some pores were damaged and disappeared. The surface of 7KCP became even rougher and produced a large number of new pores. It suggested that KOH reacted mainly with the defective carbons on surfaces of both the CP and the primary pores at a lower amount of KOH. As a result, the pore sizes were expanded, making the original micropores and mesopores of the CP change into the mesopores or macropores or the collapsed irregular pores. The SEM investigation supported results on the pore structures and crystal structures discussed in 3.1.1 and 3.1.2. Similarly, the increased amount of

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KOH enhanced its etching effect on the aromatic crystallite of CP, making the formation of a large amount of new pores. Fig. 4 (b) shows the TEM profiles at the edge of CP before and after KOH activation. It showed that the edge of the CP was clear and the interior was layered. Each layer oriented at a certain degree, forming a laminated layer containing multiple layers. Each laminated layer stacked

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regularly, similar to the parallel layer structures of graphite. The edge of 0.5KCP became blurred, but the orderly alignment of interior laminated layer was still kept. The blurring and clutter at the

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edge of 7KCP became stronger. The distortion and deformation among the approximate parallel

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layers appeared, resulting in the random orientation and irregular distribution of the layers and

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formation of disordered structures. This was consistent with results of BET and XRD. Therefore,

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KOH was responsible for etching of defective carbons on surfaces of both the CP and its primary

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pores, resulting in the decreased sizes of the aromatic crystallites (La) and the blurred edge at a lower amount of KOH. The increased amounts of KOH can further moved this effect inside the

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interior of the aromatic crystallites and destroy more aromatic crystallites. This can explain the increased interlayer spacing (d002), and the significantly reduced aromatic layer stacking heights (Lc) and the number of aromatic layers (N) (Table 3). 3.1.4 Surface chemistry analysis FTIR spectroscopy was used to determine the changes in the surface functional groups of the CP during the activation treatments by KOH.

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Fig. 5 FTIR spectrum of CP, 0.5KCP and 7KCP

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Fig. 5 displays the FTIR spectra of CP before and after the KOH activation by different amounts

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of KOH. CP presented the complicated vibration peaks, majorly including those of aliphatic C–H and −CH, the aromatic C=O, the aliphatic ether and alcohol C–O, and the aromatic C=C. The

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strongest bands around 3400 cm-1 should be ascribed to the existence of free and intermolecular

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bonded hydroxyl groups [31]. The peaks at around 2922 cm-1 and 2859 cm−1 represented the

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stretching and bending vibrations of aliphatic C–H, and the deformation of aliphatic −CH2 and −CH3 appeared at 1375 and 1455 cm-1 [32]. A weak absorption peak at 1726 cm-1 was due to the stretching vibration of the aromatic carbonyl/carboxyl C=O groups, and the aliphatic ether C–O and alcohol C–O stretching at around 1035 cm-1 and 1091 cm-1 were also associated with oxygen groups [33, 34]. The aromatic C=C ring stretching appeared at 1630 cm−1 [34]. The as-received and the activated CP showed the similar transmittance positions although the intensities were different. The changes of 0.5KCP both in the peak positions and intensities were unapparent. But almost all the peak intensities of 7KCP decreased significantly. It suggested that the activation at a higher KOH amount could react with the side chains, the surface functional groups and the aromatic carbon of the CP, and therefore played the important role of activating CP.

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3.2 Process analysis of CP activation by KOH Fig. 6 shows the thermal behavior of the CP samples before and after the KOH activation in TG-DSC in a temperature ramp from the room temperature to 1100 oC, integrating the off gas

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analysis by an on-line mass spectrometry (MS).

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Fig. 6 TG-DSC-MS analyses of CP (a), the KOH activation with ratio of KOH to CP at 0.5 (b) and the KOH activation with ratio of KOH to CP at 7 (c)

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It was found that the mass loss of the CP was very small (3 %) until 1100 oC, and the major

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mass loss range occurred below 200 oC accompanied by a small endothermic peak in its DSC

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curve. The coupled MS results implied that the only mass loss of CP was attributed to the

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moisture desorption below 200 oC, indicating the stability of CP. The TG-DSC results of CP at the KOH addition of 0.5, shown in Fig.6 (b), revealed that the mass

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loss of the mixture was also small (only 8 %). Further integrating MS results, it was found moisture was released up to 300 oC, partially attributing to the volatilization of the absorbed water and the oxidization of hydrogen in the activation process. Carbon monoxide (CO) released in a temperature range of 600–900 oC, mainly attributing to reactions between KOH and carbon in CP [35]. Fig.6 (c) shows the TG-DSC-MS profile of the CP mixed with KOH at 7. The total mass loss was greater than 91 %, and there were three distinct mass loss peaks. The mass loss below 400 oC was about 17 %, with the release of much moisture and less H2. Its mass loss at the temperature range of 650–850 oC was about 20 %, similarly releasing H2 and H2O. Its most obvious mass loss

Journal Pre-proof was 52 %, appearing above 850 oC with the release of H2 and CO. The coupled DSC curve revealed several endothermic peaks at the corresponding temperature ranges, indicating that the higher amount of KOH addition and the higher temperature significantly enhanced reactions between KOH and CP. 3.3 Mechanisms of CP activated by KOH

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Previous studies studied on the likelihood of reaction routes of the KOH activation of carbon derived from many raw carbon materials. It showed that KOH would firstly react with the surface

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functional groups and the side chains in the macromolecular structures of coal and petroleum

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coke at the lower temperatures, and then react directly with aromatic carbon at the higher

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temperatures [25, 26, 36]. CP is also carbonaceous structures and mainly contains carbon,

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volatile and ash, similar to coal and petroleum coke. It can be rationally speculated that the

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reaction path of CP activated by KOH may be similar to those of coal and petroleum coke. However, our practices revealed the activation reactions between KOH and CP was much slower

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than those of coal and petroleum coke, because of more graphite-like compact structures and less surface functional groups and side chains in CP [13, 14]. Overall analysis, involving BET, XRD, SEM, TEM and TG-DSC-MS and also referenced literatures [35–39], revealed likely mechanisms on the KOH activation of CP as follows: (1) Below 200 oC, the physical dehydration of the CP and KOH mixture occurred. (2) Within 200–400 oC, KOH reacted with various functional groups and aliphatic hydrocarbon side chain in CP, to release H2O and H2. Active components, such as K2O, K2CO3, were likely formed [36, 37]. The relevant reactions were as follows: KOH+-OH→-O- K+ +H2 O(g)

(5)

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KOH+-COOH→-COO- K+ +H2 O(g)

(6)

4KOH+-CH2 - →K2 CO3 +K2 O+3H2 (g)

(7)

(3) At 400–650 oC, KOH eventually melted and decomposed, generating more active components of K2CO3 and K2O. This made reactions switched to those new active potassium components and CP and the etching of carbon can be more fast and sufficient. Furthermore, the

generate metal K [26, 38, 39]. The relevant reactions are:

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2KOH→K2 O+H2 O(g)

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produced K2CO3 and K2O would continue reacting with the aliphatic hydrocarbon side chain to

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K2 CO3 +-CH2 - →K2 O+2CO(g)+H2 (g)

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K2 O+-CH2 - →2K+CO(g)+H2 (g)

(8) (9) (10)

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(4) Above 650 oC, active components of K2CO3 and K2O can react with the aromatic carbon,

K2 O+H2 →2K+H2 O(g)

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-C- +H2 O→CO(g)+H2 (g)

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destroying the aromatic crystallites and gradually forming enriched porous structures [28, 35]. (11) (12)

K2 O+-C- →2K+CO(g)

(13)

1⁄2K2 CO3 +-C- →K+ 3⁄2CO(g)

(14)

It was noticed that the boiling point of the generated metal potassium is 759 oC [39], above which the gaseous metal potassium likely penetrated into layers in the carbon matrix microcrystalline. Thus, the aromatic layers can be opened by gaseous potassium and became distorting and deforming to be the new porous and more active sites where other potassium species, such as K2O and others, can follow in to more vigorously react with carbon and generate more micropores. The mechanisms of the KOH activation of CP can be illustrated in Fig. 7.

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Fig. 7 The process of CP activation by different KOH amounts

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When the amount of KOH was less, the formed active components such as K2O, K2CO3 and

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gaseous metal K in the activation process were limited. It mainly reacted with the surface

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functional groups and the defective carbon of both CP and the primary pores. As a result, KOH played a role of broadening pores and decreased the average lateral sizes of the aromatic

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crystallites. With the increase of KOH addition, enough amounts of potassium active components can react with the aromatic carbon of CP for the generation of rich micropores, gases and surface defects.

4. Conclusions The addition KOH at its lower ratio (mass ratios of KOH and CP in a range of 0.5 and 1) destructed pores to decrease the specific surface area to 4 m2 g-1 and 8 m2 g-1, but sharply increased of the specific surface area to 132 m2 g-1 and 355 m2 g-1 upon the further increase of the KOH addition amounts (ratios of KOH and coke powder in a range of 3 and 7), generating a

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number of new micropores and mesopores. In addition, the lower KOH amounts only destructed the aromatic crystallites in the one dimension, and the higher KOH amount enhanced this destructive effects and made expansion in the spatial scope. The lower amount of KOH reacted mainly with the surface functional groups and the defective carbon of both CP and the primary pores to broaden the pores and destruct the aromatic

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crystallites in the one dimension. The higher amount of KOH induced vigorous reactions. Generating a plenty of gaseous metal K, which penetrated into layers of the carbon matrix and

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opened the aromatic layers to be distorted and deformed. This made more active sites,

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facilitating the active components, such as K2CO3 and K2O, further reacted with aromatic carbon

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to form developed porous carbon structures within coke powder.

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Acknowledgements

The authors greatly thank the financial supports provided by National Key R&D Plan

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(2016YFE0131100, 2017YFB0603101) and the Program for Sanjin Scholars of Shanxi Province and the Talent Training Program of Shanxi Joint Postgraduate Training Base (2016JD07). Compliance with ethical standards

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