Preparation of Mesoporous Carbon Materials Through

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Materials and Interfaces

Preparation of Mesoporous Carbon Materials Through Mechanochemical Reaction of Calcium Carbide and Transition Metal Chlorides Ke Zhang, Shengjie Tao, Xuebing Xu, Hong Meng, Yingzhou Lu, and Chunxi Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00323 • Publication Date (Web): 11 Apr 2018 Downloaded from http://pubs.acs.org on April 11, 2018

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Industrial & Engineering Chemistry Research

Preparation of Mesoporous Carbon Materials Through Mechanochemical Reaction of Calcium Carbide and Transition Metal Chlorides Ke Zhang1,2,4,Shengjie Tao1,2,Xuebing Xu1,2,Hong Meng2,Yingzhou Lu2,Chunxi Li1,2,3,* 1

State Key Lab. of Chem. Resource Eng.; 2College of Chem. Eng.; 3 Beijing Key Lab. of Energy Environmental Catalysis, Beijing Univ. of Chem. Tech., Beijing 100029, China; 4 Jiangsu Vilory Adv. Mat. Tech. Co., Ltd., Xuzhou Jiangsu, China

ABSTARCT: Three carbon materials (CM) were prepared for the first time by the mechanochemical reaction of calcium carbide (CaC2) and a transition metal chloride (MCln: FeCl3, ZnCl2 or CuCl). Their composition and structure were characterized, and their electrochemical performance was evaluated by cyclic voltammetry, constant current charge and discharge, cycle life and AC impedance, respectively. The reactions may occur through a series reaction, i.e. metathesis reaction between CaC2 and MCln and self-redox of the resulting metal carbides, forming mesoporous CMs with layered structure and specific area of (160~360 m2.g-1). The CM made from ZnCl2 shows good electrochemical performance and higher specific capacity of 84 F·g-1. Further, CMs with better electrochemical performance may be prepared by using other metal halides. This study validates the effectiveness of mechanochemistry for the activation of CaC2, provides a novel method for CMs synthesis at ambient temperature and mild conditions, and might inspire the development of CaC2 chemistry. Keywords: Calcium carbide; carbon material; mesoporous material; ball mill;

*

Corresponding author. Tel/Fax 86-10-64410308. E-mail: [email protected]. 1

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mechanochemistry; mechanical activation 1. INTRODUCTION Porous carbon material(CM) is widely used as sorbent1,2 and electrode material 3,4 for its high surface area, good stability and electrical conductivity. It is generally prepared through pyrolysis of various polymers or chemical vapor deposition(CVD) of small hydrocarbons, which requires complex facilities, high capital investment and production cost. In these processes, high temperature is inevitable for the dehydrogenation or dehydration of the precursors,5 and thus few CMs are prepared at ambient temperature. In order to develop a novel method for the production of CMs under mild conditions, it is necessary to use the precursors without hydrogen. In this regard, metal carbides might be ideal candidates for their high carbon content and free of hydrogen, and the key is to remove the metals efficiently. In 1983, Frederie and Douglas prepared ultra-hard CMs by using A14C3 and Be2C as carbon sources through their reaction with halogenated hydrocarbons.6 Dash et al.7 made porous CM by etching of metal species from titanium carbide in a chlorine environment. Yushin et al.8 made carbon nanotubes by reacting Ti3SiC2 with chlorine gas. Lust et al prepared some CMs by using α-Tungsten Carbide,9 vanadium carbide10 and SiC11 as carbide precursors. All the above CMs were prepared at high temperature (400-1200 ℃) due to the high stability and low reactivity of the carbides. Besides, these metal carbides are expensive and not commercially available, thus they are not viable for the massive production of porous CMs. In contrast, calcium carbide (CaC2) is a cheap commodity chemical with production capacity of 3.8×107 y-1 in 2016 in 2


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China.12 In addition, calcium carbide has higher reactivity than other carbides, which make it more suitable as a feedstock for the production of carbide derived carbon(CDC) under milder conditions. For example, Xie et al. prepared some CDCs by reacting CaC2 with CH2Cl2 (CHCl3 or CCl4)13 or oxalic acid14 in an autoclave at 250-300℃. Dai et al.15 produced CDC by chlorination of CaC2 with Cl2 in a tubular reactor at atmospheric pressure and 100-600 ℃. Osetzky16 made skeleton CDC by reacting CaC2 with MgCl2 at 600-1275℃. All these CDCs are prepared via thermochemical reaction of CaC2 with different reactants, and some reactions are violent and explosive. Therefore, their reaction temperature and pressure is not controllable, and thus not viable for large scale production. In view of the high reactivity of sodium acetylene (NaC2H), CaC2, as its homologue, should also has a comparable reactivity. However, its actual reactivity is very low except to water, which may be attributed to its high lattice energy and insolubility.17 In fact, it is the high lattice energy that stabilizes the molecular reactivity of CaC2, and results in the insolubility and non-availability of the C22anions, and accordingly the limited overall reactivity. In order to liberate the intrinsic reactivity of CaC2, it is necessary to break the lattice structure, increase the accessibility of the C22- anions, and enhance its effective collision with other reactants. This can be achieved easily in a ball mill reactor. In fact, mechnochemistry has been recognized as a greener process for the treatment of persistent organic pollutants(POPs) and many heterogeneous reactions with solid reactants.18-20 In light of this recognition, Li et al.21 studied the mechanochemical reaction of CaC2 with 3



polychloro-hydrocarbons at room temperature, and obtained CMs with rich alkynyl groups, high specific area and good electrochemical properties. The resulting acetylenic carbon materials (ACM) show excellent adsorption of mercury from wastewater due to the specific and strong interaction between C≡C group and mercury.22 Meanwhile, CaC2 has proved to be one of the most effective reagents for the mechenochemical treatment of POPs due to its very strong Lewis basicity and reactivity, achieving a resource utilization of POPs and yielding valuable ACM.23 It is known that metathesis reaction may take place between CaC2 and a transitional metal chloride (MCln) under appropriate conditions. The driving force of the reaction arises from the strong affinity between Mn+ and C22- in light of the Lewis hard and soft acid-base theory. For example, MC2 (M=Fe, Co, Ni) can be prepared by the reaction of CaC2 with MCl2 in acetonitrile in the temperature range of 100~200 o

C.24 Further, some metal carbides can be decomposed into metals and carbons

through self-redox reaction due to the oxidizability of metal ions and reducibility of acetylenic anions. For example, the dry acetylides of copper and silver are explosive under slight shock, yielding CMs with high specific area (~1600 m2.g-1) and excellent electrochemical performance.25,26 If these two processes are combined together, a unified CM production process may be developed. Based on this consideration, the mechanochemical reaction of CaC2 with transition metal chlorides (FeCl3, ZnCl2, or CuCl) was studied in a planetary ball mill at ambient temperature. This study has yet to be reported until now. The composition, morphology and structure of the resulting CMs were characterized and their potential applications in electrochemistry were 4


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explored. This study proposed a new process for the facile preparation of porous CMs. The as-prepared CMs may be used as electrode materials or as raw materials for further chemical modification or activation, since the CMs with heteroatom (N, B, O)-doping and/or appropriate structure and higher specific area usually show better electrochemical performance.27-32 2. EXPERIMENTAL 2.1 Reagents and Equipment All the chemicals are AR grade reagents unless otherwise specified, and used as received without further purification. CaC2 (74 wt%) was purchased from Tianjin Fuchen Chem. and ground into 100 mesh particles before use. Anhydrous FeCl3, ZnCl2 and CuCl were bought from Tianjin Bodi Chem. Com. Nitric acid, anhydrous ethanol, and KOH were bought from Beijing Chem. Works (China). The main equipment used include planetary ball mill (QXQM-1, Changsha Tian-chuang, China), high-speed pulverizer (model KC-04, Beijing Tonghe Sci. Tech., China), electronic balance (AR2130, Ohaus, USA), and electrochemical workstations (model CHI660E, Shanghai Chenhua Instr. Co., China). 2.2 Mechanochemical Reaction of CaC2 and Transition Metal Chlorides The reaction of CaC2 and transition metal chlorides (MCln) was carried out in a planetary ball mill at ambient temperature with stoichiometric ratio of the reactants. Bulk CaC2 was firstly grounded 5 min in a high-speed pulverizer, and then sieved to get 100 mesh powders for use. For illustration, 11.396 g of CaC2 powder (~8.547 g pure CaC2), 18.173 g of ZnCl2, and 183 stainless steel balls (2×φ15, 4×φ12, 24×φ10, 5



43×φ8, 110×φ5 in mm) were added to the reactor, sealed and vacuumed by a water pump, and then fixed on the support of the ball mill. The ball mill conditions were as follows: rotating speed 450 rpm, stop 10 min for every 30 min milling, accumulative milling 3 h. After reaction, the product mixture was treated with dilute nitric acid to remove all the residual reactants and metals, washed with distilled water until pH=7, and finally vacuum dried the filter cake at 120 ℃ for 10 h to obtain CDC materials. For clarity, the resultant CMs by using FeCl3, ZnCl2, and CuCl are denoted as CDC-Fe, CDC-Zn, and CDC-Cu, respectively. 2.3 Characterization of the Carbon Materials The carbon content of the CMs was analyzed by elemental analyzer (vario EL cube, Germany), and the whole composition was analyzed by X-ray energy-dispersive spectroscopy (EDS, 250Xi, Thermo Fisher Sci., China). The structure and morphology of the CMs were observed using scanning electron microscope (SEM, S-4700 Hitachi, Japan) and high-resolution transmission electron microscope (TEM, JEM-3010). The structure of the CMs was analyzed using Raman spectrometer (Renishaw, UK) and X-ray diffractometer (XRD, Bruker D8, Germany). The specific surface area and pore size distribution of the CMs were analyzed by ASAP2020 surface area meter (Micrometritics, USA). 2.4 The Test of Electrochemical Performance The electrochemical performance of the CMs was tested in a three-electrode system using Hg/HgO as the reference electrode, 1 cm×1 cm Ni sheet as the auxiliary

6


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electrode, and 6 mol.L-1 KOH solution as the electrolyte. The working electrode was prepared as follows: CM and polytetrafluoroethylene (PTFE) were mixed at a mass ratio of 8:1, dispersed uniformly with ethanol, and then pressed on one end of the foam Ni strip. The working electrode area was about 1 cm × 1cm and vacuum dried in a rotatory drier. The cyclic voltammetry curve, constant current charge and discharge, cycle life and AC impedance were measured in an electrochemical workstation. The voltage test range was -1 to 0 V, the frequency of the AC impedance was 10-2-105 Hz, and the test temperature was maintained at (25±2) ℃. The specific capacitance Cm (F.g-1) of the electrode was calculated using the constant current charge and discharge curve with Eqn. (1).

(1) where I is discharge current(A), t is discharge time (s), ∆V is discharge voltage drop (V), m is the mass of the carbon electrode material (g). The specific capacitance of the electrode was also calculated using the cyclic voltammetry curve with Eqn. (2). Cm =

∫ IdV

(2)

2mv∆V

where I is response current(A),

is the numerical integration of the CV

curve, m is the mass of the carbon electrode material (g), ν is the scan rate(V.s-1), and ∆V is the voltage window range applied(V). The energy density (E in Wh.kg-1) and power density (P in W.kg-1) of the CMs were calculated using the constant current charge and discharge curve with Eqn. (3) 7



and (4), respectively.

(3)

(4)

3. RESULTS AND DISCUSSION 3.1 Exploration of Reaction Conditions Taking the reaction of CaC2 and ZnCl2 as an example, the effect of milling speed and time on the yield of CM was investigated. As shown in Table S1 (Supporting Information), at a higher milling speed of 600 rpm, the carbon yields were all above unity irrespective the milling time (1h, 3h, 5h). Similarly, at a longer milling time of 3 h, the carbon yields were also above unity when the milling rate exceeds 300 rpm. However, no reaction occurs at the milling speed of 150 rpm. This indicates that the mechanical energy intensity played an important role, and there exists a threshold value, beyond which the reaction can take place easily.33 Therefore, a compromise condition is chosen for the following experiments, namely, ball milling 3h at 450 rpm. Under this condition, the yield of CMs from the reaction of CaC2 with FeCl3, ZnCl2, and CuCl is close to 1, as shown in Table 1. Here, the CM yield is defined as the ratio of the carbon amount received versus the theoretical one when the carbon element in CaC2 is completely converted to CM product. The results show that CaC2 is highly activated under ball milling conditions and its reaction can proceed efficiently for the preparation of CDC materials at mild conditions.

Table 1. Reaction Results of CaC2 and Transition Metal Chlorides via 3h Milling 8


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at 450 rpm CMs

Metal chloride

CM yield(%)

Carbon content (%)

CDC-Fe

FeCl3

101

77.7

CDC-Zn

ZnCl2

106

73.2

CDC-Cu

CuCl

99

71.2

3.2 Composition and Structure of the CMs Elemental composition of the CMs The carbon content of the CMs was measured by elemental analyzer, and other element were analyzed by EDS energy spectrum. The results are presented in Figure 1 and Table S2 (Supporting Information). It is found that the carbon content was in the range of (72~80 wt%), the total amount of Mg, Al, Si, Ca and S from the impurity calcium carbide ranges from 4 % to 5 wt%, other impurities from MCln account for 2.1~7.5 wt%, and the oxygen content covers a wider range of (8.3~21.9 wt%). It is presumed that some oxygen is originated from the partial oxidation of the nanosized CMs in the milling and post treatment processes, and the remaining oxygen may be ascribed to the adsorption of moisture and air on the CMs.34 C Al Cl Cr

O Si Ca Zn

Mg S Fe Cu

CDC-Fe CDC-Zn

0

CDC-Cu 0

1

2

3

4

5

Energy/K ev

Figure 1. EDS spectra of the CMs 9


6


Morphology of the CMs The morphology of the CMs was analyzed by SEM and TEM, and presented in Figure 2. Obviously, the CMs are composed of aggregates of nanoparticles with varying primary particle size. Among them, CDC-Fe shows the highest degree of agglomeration and has some hard aggregates, while CDC-Zn shows looser structure with primary particle size of approximately 20-30 nm. On the surface of CDC-Fe, some tiny balls are visible with diameter of about 100 nm. This may be associated with the formation of Fe-C alloy, and such iron specie is hard to be removed by dilute acid. In contrast, the metal content in other CMs is quite low (

Preparation of Mesoporous Carbon Materials Through

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