Catalytic Carbonization of Chlorinated Poly(vinyl chloride) Microfibers

Jul 30, 2013 - A simple approach was reported to prepare carbon microfibers embedded with octahedral Fe3O4 crystals (Fe/CMFs) through catalytic ...
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Catalytic Carbonization of Chlorinated Poly(vinyl chloride) Microfibers into Carbon Microfibers with High Performance in the Photodegradation of Congo Red Kun Yao,†,‡ Jiang Gong,†,‡ Jun Zheng,†,‡ Lu Wang,†,‡ Haiying Tan,†,‡ Guangchun Zhang,†,‡ Yichao Lin,†,‡ Hui Na,§ Xuecheng Chen,† Xin Wen,† and Tao Tang*,† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China ‡ University of the Chinese Academy of Sciences, Beijing 100039, China § Alan G. MacDiarmid Institute, College of Chemistry, Jilin University, Changchun 130012, P.R. China ABSTRACT: A simple approach was reported to prepare carbon microfibers embedded with octahedral Fe3O4 crystals (Fe/CMFs) through catalytic carbonization of chlorinated poly(vinyl chloride) (CPVC) microfibers using Fe3O4 as catalyst at 700 °C. Without Fe3O4, irregular carbon was obtained. However, after adding a proper amount of Fe3O4, carbon microfibers with uniform size were prepared. The influence of Fe3O4 content on the morphology of carbon microfibers was investigated. The resultant Fe/CMFs were used as heterogeneous Fenton catalysts and showed high photodegradation efficiency of Congo red under UV or sunlight irradiation. This indicated that the Fe/CMFs had the potential application in wastewater treatment. More importantly, this method provides a novel way to prepare functional carbon (or carbon-based) materials with various morphologies such as spheres, fibers, and foams.

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INTRODUCTION Carbon materials with various morphologies and microstructures have attracted scientific and technological interest for their existing and potential applications in adsorption and degradation for diverse toxic and harmful compounds,1 nonenzymatic glucose sensor,2 energy storage electrodes in supercapacitor,3−5 electro-analytical application,6−8 and support of catalyst.9 This is due to their novel physic-chemical and mechanical properties. Besides organic small molecule compounds, polymers with carbon backbone can potentially be used as precursors to prepare carbon materials. According to the chemical reactions of the backbone chains occurring during the formation of carbon materials, polymers can be classified into two categories: noncharring polymers and charring polymers.10 The noncharring polymers consist of many commercial plastics such as polypropylene and polyethylene. The charring polymers generally include many synthetic polymers, such as phenol−formaldehyde resin,11 polyacrylonitrile,12 poly(vinyl chloride) (PVC),13 and many biomass.14,15 In the recent decade, many efforts have been attempted to convert charring polymers into carbon microfibers with controlled surface geometries and diameters from electrospun precursors and to further explore their potential applications. For example, Zhou et al. prepared the uniform electrospun polyacrylonitrile (PAN) microfibers with diameter of 330 nm and then converted the PAN precursor microfibers into carbon © 2013 American Chemical Society

microfibers with a diameter range from 200 to 300 nm after the subsequent heat-treatments including stabilization and carbonization.16 Mochida et al. put forward a multistage heattreatment procedure to remove chlorine and control carbonization of PVC to prepare spinnable pitch for activated carbon fibers. They provided a dechlorination method for PVC and used the dechlorination product to prepare the activated carbon fibers whose DeSOx activity was tested primarily.13 In our previous study,10 we put up a method of “catalytic carbonization” to control the carbonization of chlorinated PVC (CPVC, a derivative of PVC) based on our strategy of “combined catalysts”.17−24 We select CPVC as the carbon source since it is a commercial product and has a higher carbon conversion after the dehydrochlorination and cross-linking due to its higher content of chlorine than PVC. Using Fe2O3, Fe(OH)3, or Fe3O4 as catalyst, the morphology of the obtained CPVC carbonization product was similar to that of original commercial CPVC microspheres. This was due to the reason that Fe 2 O 3 , Fe(OH) 3 , or Fe 3 O 4 could accelerate the dehydrochlorination and carbonization of CPVC. With “catalytic carbonization” method, carbon microspheres with octahedral Fe3O4 microcrystals uniformly embedded on the Received: April 29, 2013 Revised: July 29, 2013 Published: July 30, 2013 17016

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Figure 1. SEM images of CPVC-Fe3O4-x and Fe/CMF-x: (a) CPVC-Fe3O4-0, (b,c) Fe/CMF-0, (d) CPVC-Fe3O4-0.5, (e,f) Fe/CMF-0.5, (g) CPVC-Fe3O4-1, (h,i) Fe/CMF-1, (j) CPVC-Fe3O4-2, and (k−l) Fe/CMF-2.

between the disk electrode and the collector. An electric potential of 15 kV was applied between the orifice and the ground at a distance of 20 cm. A copper substrate (4 cm ×4 cm) was used for collecting the CPVC-Fe3O4-x in a controlled environment with a temperature of 15 ± 1 °C and a relative humidity of 56 ± 1%. After electrospinning for a certain period of time, the obtained thick sheet of CPVC-Fe3O4-x was taken down from the copper substrate. Finally, the CPVC-Fe3O4-x was dried under vacuum at 80 °C for 48 h to obtain dry microfibers. The average length of the fibers was at least 10 cm. Preparation of Fe3O4/Carbon Microfibers (Fe/CMFs). Fe/CMFs were prepared according to our previous report10 and designated as Fe/CMF-x, where x represented the content of Fe3O4 in the mixture. Briefly, a crucible containing CPVCFe3O4-x (about 1.5 g) was heated at 700 °C until the flame from the crucible’s upper brim went out (for about 1.5 min). The obtained Fe/CMF-x was cooled to room temperature and weighed. The carbon conversion of CPVC was calculated by dividing the amount of carbon in the residue by that of the carbon element in the sample, and the yield of Fe/CMF-x was calculated by dividing the amount of residue in the crucible by that of the sample. Characterization. Field-emission scanning electron microscope (SEM, XL30ESEM-FEG) was used for morphological characterization of the CPVC-Fe3O4 and Fe/CMFs. The surface element composition of Fe/CMFs was characterized by means of X-ray photoelectron spectroscopy (XPS) carried out on a VG ESCALAB MK II spectrometer using an Al Kα exciting radiation from an X-ray source operated at 10.0 kV and 10 mA. High-resolution spectra of Fe 2p and Cl 2p were analyzed to identify various chemical species. The phase

surface were synthesized by carbonizing commercial CPVC microspheres via a one-pot approach. In the present study, we demonstrated a novel way to synthesize CMFs through “catalytic carbonization” of electrospun CPVC microfibers using Fe3O4 as catalyst. The resultant Fe3O4/carbon microfibers (Fe/CMFs) showed high performance as heterogeneous Fenton catalyst in the photodegradation of Congo red (CR) under UV or sunlight irradiation.

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EXPERIMENTAL SECTION Materials. The commercial CPVC resins from Weifang Jinshan Chemical Co., Ltd., China (trademark CPVC-J700, polymerization degree about 700, chlorine content 63−69 wt %, granular size 100−250 μm) were used. Dimethylformamide (DMF) and tetrahydrofuran (THF) were purchased from Beijing Chemical Co., China. Fe3O4 nanoparticles with an average diameter of 10 nm were prepared according to the previous report.25 Congo red (CR, supplied by Alfa Aesar) was analytical grade quality and used without further purification. Deionized water was used for solution preparation in all experiments. Preparation of Fe3O4/CPVC Microfibers (CPVC-Fe3O4). The Fe3O4/CPVC microfibers were designated as CPVCFe3O4-x, where x represented the content of Fe3O4. A certain quantity of Fe3O4 nanoparticles were dispersed in 10 mL of a mixture solution of DMF and THF (volume ratio 7:3) under gentle stirring for 30 min, and then the mixture was dispersed by ultrasound for 1 h. After that, 2.40 g of CPVC were dissolved in the above mixture under stirring for 12 h. The sample solution was electrostatically drawn from the tip of the needle (0.51 mm spinning gauge) by applying high voltage 17017

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structure of Fe/CMFs was analyzed by X-ray powder diffraction (XRD) using a D8 advance X-ray diffractometer with Cu Kα radiation operating at 40 kV and 200 mA. The vibrational property of Fe/CMFs was characterized by Raman spectroscopy (T6400, excitation-beam wavelength: 514.5 nm). The thermal stability of CPVC-Fe3O4 fibers was measured by thermal gravimetric analysis (TGA) using TA Instruments SDT Q600 under N2 flow at 10 °C/min. The surface area of the Fe/ CMFs was measured by means of the Brunauer−Emmett− Teller (BET) method (ASAP2020, Micromeritics) with nitrogen as the adsorbing gas. The contents of Fe, Cl, and Na elements in Fe/CMFs were measured by inductively coupled plasma-optical omission spectrometer (ICP-OES, iCAP 6000 Series, Thermo Scientific). The contents of C and H elements in Fe/CMFs were measured by means of Elementar (Vario EL, Germany). The content of oxygen in Fe/ CMFs was determined by the difference. Photodegradation of CR Using Fe/CMFs. The resultant Fe/CMFs were used as catalyst for the degradation of CR. The photochemical reactor was made of a 50 mL beaker and equipped with a magnetic stirring bar. A 15 W tubular 254 nm UV lamp (from Spectronics Corporation, USA), was used as UV light source. The initial pH of CR solution was adjusted by 0.5 mol/L HCl solution. In a typical experiment, a solution of 50 mL containing CR (100 mg/L) and designed concentrations of Fe/CMFs and H2O2 was magnetically stirred under UV irradiation. Under sunlight irradiation, the concentration of CR was 50 mg/L, and all experiments were done in the area of direct sunlight on sunny day between 10 a.m. and 2 p.m. in the month of January in Changchun, Jilin province, China. At a given irradiation time intervals, 1.0 mL of the solution was withdrawn by syringe, diluted, filtered through 0.45 μm membrane and immediately analyzed with UV/vis/NIR spectrophotometer (Lambda 900). Degradation efficiency of CR by the Fe/CMFs is calculated by the following expression: degradation efficiency of CR(%) =

Figure 2. Diameters of CPVC-Fe3O4-x and Fe/CMF-x fibers.

(Figure 1f). This was ascribed to that such a low content of Fe3O4 was not enough to catalyze carbonization of CPVC microfibers to quickly form carbon microfibers, since the contact of Fe3O4 with the CPVC was rather limited. After increasing the content of Fe3O4 to 1 wt %, the resultant carbon microfibers were composed of individual microfibers (Figure 1h,i). This indicated that the contact of Fe3O4 with the CPVC in CPVC-Fe3O4-1 was actually enough and accordingly the melting and binding of CPVC fibers could be avoided owing to the effectively promoted effect of dehydrochlorination and carbonization of CPVC by Fe3O4. However, with further increasing the content of Fe3O4, octahedral Fe3O4 nanoparticles were exposed onto the surface of the carbon microfibers. This is possibly because with the increasing content of Fe3O 4 nanoparticles, heat treatment may promote the growth of octahedral Fe3O4 nanoparticles. At high temperature, Fe3O4 nanoparticles first melt, aggregated and grew into the octahedral Fe3O4 crystals. From the nitrogen adsorption− desorption isotherms of different carbon microfibers, the surface area of these carbon microfibers were obtained. The specific surface area (SBET) of irregular carbon from pure CPVC microfibers was 614.1 m2/g. After adding Fe3O4, the SBET of carbon microfibers increased to 689.1 m2/g for Fe/CMF-0.5, 983.6 m2/g for Fe/CMF-1, and 663.2 m2/g for Fe/CMF-2. Table 1 presents the effect of Fe3O4 on the carbonization of CPVC into Fe/CMF-x at 700 °C. The carbon conversion of CPVC without Fe3O4 was 88.2 wt %, indicating that the majority of carbon element in the CPVC microfiber remained after the carbonization. The addition of Fe3O4 further promoted the carbonization of CPVC microfibers into carbon microfibers. With the increase of Fe3O4 content, both the

100%(C0 − C t) C0

where C0 and Ct are the initial and t min concentration of CR (mg/L) in the solution, respectively.

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RESULTS AND DISCUSSION Morphology of CPVC-Fe3O4-x and Fe/CMFs. To investigate the effect of Fe3O4 on the morphology of CPVC carbonized products, SEM observations for CPVC-Fe3O4-x and Fe/CMFs were conducted. After carbonization of CPVC microfibers (Figure 1a) without Fe3O4, irregular carbon was formed with a large number of broken holes inside (Figure 1b,c). Without Fe3O4, CPVC microfibers may first melt and bind together, and then the bound CPVC microfibers are gradually carbonized, resulting in the formation of irregular carbon lump. Similar result was observed in our previous report and a piece of “carbon lump” was formed after the carbonization of CPVC microspheres without Fe2O3.10 However, as shown in Figure 1e, h, and k, after adding some amount of Fe3O4 into CPVC microfibers, carbon microfibers, a total different morphology of carbon product from CPVC microfibers without Fe3O4, were obtained by catalyzing carbonization of CPVC microfibers. The diameters of carbon microfibers with different Fe3O4 content were all slightly larger than that of raw CPVC microfibers (Figure 2). In the case of Fe/CMF-0.5, some microfibers were bonded with each other

Table 1. Effects of Fe3O4 on the Carbonization of CPVC into Fe/CMF-x at 700 °C Fe/CMFs Fe/CMF-0 Fe/CMF0.5 Fe/CMF-1 Fe/CMF-2

Fe3O4 (g/100 g CPVC)

carbon conversion (wt %)a

yield (wt %)b

0.0 0.5

88.2 91.3

27.5 28.3

1.0 2.0

95.3 97.9

29.4 29.9

a

Calculated by the amount of carbon in the residue divided by that of carbon in the sample. bCalculated by the amount of residue divided by that of sample. 17018

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carbon conversion of CPVC microfibers and the yield of Fe/ CMF-x increased. The highest carbon conversion and yield of Fe/CMF-x was 97.9 and 29.9 wt %, respectively. The high carbon conversion and simple preparation procedure demonstrated that Fe/CMFs could be largely synthesized through “catalytic carbonization” of CPVC microfibers by Fe3O4. As a result, the “catalytic carbonization” method showed advantages than other previous reports in the preparation of carbon materials including carbon microfibers and mesoporous carbon. For example, Zhu et al. synthesized mesoporous carbon from PVC by carbonization and NaOH activation procedures with the total yield of about 25 wt %.26 Mochida et al. used a multistage heat-treatment procedure to remove chlorine and control PVC carbonization to prepare spinnable pitch for activated carbon fibers, but the carbon conversion was always no more than 75 wt %.13,27,28 Phase Structure of Fe/CMF-x. Figure 3 displays the XRD patterns of Fe3O4, CPVC, and Fe/CMF-x. After the carbon-

Figure 4. Raman spectra of Fe/CMF-x.

Table 2. Composition of Fe/CMFs Fe/CMFs Fe/CMF0.5 Fe/CMF-1 Fe/CMF-2

Ca (wt %)

Ha (wt %)

Ob (wt %)

Fec (wt %)

Clc (wt %)

Nac (wt %)

84.46

1.96

11.35

0.98

0.33

0.92

82.15 77.00

1.32 2.01

13.24 15.26

2.27 4.23

0.46 0.95

0.56 0.55

a c

Calculated by elemental analysis. Calculated by ICP-OES.

b

Calculated by the difference.

CMF-x (x = 0.5, 1.0, and 2.0) were 0.98, 2.27, and 4.23 wt %, respectively, indicating that the iron loading on the surface of Fe/CMFs could be controlled simply by adjusting the loading of Fe3O4 into CPVC microfibers. XPS measurements were used to further characterize the surface element compositions of Fe/CMFs. As shown in Figure 5, the surface of carbon microfibers without Fe3O4 contained

Figure 3. XRD patterns of Fe3O4, CPVC, and Fe/CMFs.

ization of CPVC microfibers without Fe3O4, a wide and weak graphite diffraction peak around 25° was observed, indicating the low degree of graphitization in the Fe/CMF-0. At the same time, the diffraction peaks of sodium chloride (NaCl), i.e. 27.3° (111), 31.7° (200), 45.4° (220), 56.5° (222), and 75.3° (420) appeared in Fe/CMF-0. The existence of NaCl resulted from the reaction of sodium compound in raw CPVC material with HCl from the decomposition of CPVC. For comparison, after carbonization of CPVC-Fe3O4-1 microfibers, the diffraction peaks of Fe3O4, i.e. 30.2° (220), 35.5° (311), 43.2° (400), 54.1° (422), 57.1° (511), and 62.7° (440) appeared.29 Figure 4 displays the Raman spectra of the Fe/CMF-x. The peak at about 1580 cm−1 (G band) corresponds to an E2g mode of hexagonal graphite and is related to the vibration of sp2bonded carbon atoms in a graphite layer, and the D band at about 1350 cm−1 is associated with vibration of carbon atoms with dangling bonds in the plane terminations of disordered graphite or glassy carbons. The ID/IG ratio was in the range of 2.9 to 3.2, indicating the predominantly amorphous/disordered nature of the Fe/CMF-x, which was consistent of the results of XRD. Composition Analysis of Fe/CMFs. The chemical element composition of Fe/CMFs was measured by ICPOES and elemental analysis (Table 2). The results indicated that Fe/CMFs consisted of carbon, hydrogen, chlorine, iron, sodium, and oxygen elements. The sodium element came from the original CPVC microfibers. The contents of iron in Fe/

Figure 5. XPS spectra of Fe/CMF-x.

mainly C and O with a trace amount of Na and Cl elements, while the surface of Fe/CMF-x (x = 0.5, 1, and 2) contained mainly C, O, Fe, and Cl with a trace amount of Na. The spectrum of Fe 2p of Fe/CMF-x (Figure 6a) indicated the existence of doublet Fe 2p3/2 and Fe 2p1/2 with binding energies of about 711 and 724 eV, respectively. The absence of the satellite peak situated at about 719 eV, which was a major characteristic of Fe3+ in γ-Fe2O3,30,31 clearly suggesting the existence of Fe3O4 on the surface of Fe/CMF-x. Figure 6b displays the high-resolution XPS spectra of Cl 2p. Two 17019

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bond together. After the further carbonization, irregular carbon was formed. In contrast, after adding Fe3O4 into CPVC microfibers, Fe3O4 could significantly accelerate the carbonization on the surface of CPVC microfibers. This was speculated to happen even before CPVC microfibers surface melted, which effectively prevented the CPVC microfibers from binding together and favored the formation of individual carbon microfibers. At the same time, Fe3O4 reacted with HCl to form FeCl3/FeCl2. The yielded FeCl3 is a strong Lewis acid and favors the dehydrochlorination of CPVC into polyene and the subsequent cross-linking of polyene to form char. A part of FeCl3 decomposed into FeCl2 at high temperature. In order to study whether FeCl3 could effectively catalyze the dehydrochlorination and carbonization of CPVC fibers, CPVC-FeCl3-x (x = 0.5, 1, and 2) fibers were prepared and carbonized under the same experimental conditions. Strikingly, only irregular carbon rather than carbon fibers were obtained. Hence, it is difficult to explain the promoted dehydrochlorination and carbonization of CPVC using Fe3O4 as catalyst by only deeming FeCl3 to be the active substance under our experimental conditions, although FeCl3 was widely believed to be the active substance to promote the dehydrochlorination of PVC or CPVC.32 To confirm the role of Fe3O4 in the formation of Fe/CMF-x fibers, TGA measurements were conducted for CPVC and CPVC-Fe3O4-x fibers under N2 atmosphere at 10 °C/min (Figure 8). T1, T5, and Tmax

Figure 6. High-resolution XPS spectra of Fe/CMF-x: (a) Fe 2p and (b) Cl 2p.

characteristic peaks of Cl 2p3/2 and Cl 2p1/2 were observed at 200.3 and 198.4 eV, respectively. The former peak confirmed the existence of C−Cl bond and the latter was ascribed to FeCl2 or FeCl3, resulting from the reaction of Fe3O4 with the released HCl from the decomposition of CPVC. Possible Mechanism about the Formation of Fe/ CMFs. Based on the above results, a possible mechanism was put forward to explain the formation processes of irregular carbon without Fe3O4 and Fe/CMFs under the catalysis of Fe3O4 through the carbonization of CPVC microfibers at 700 °C (Figure 7). Without Fe3O4, CPVC microfibers first melt and

Figure 8. TGA (a) and DTG (b) curves of CPVC-Fe3O4-x under N2 flow at 10 °C/min. The inset images show the magnification of the part marked by the dashed cycles.

represented the temperature at which 1 wt %, 5 wt % and maximum weight loss rate occurred, respectively. Table 3 summarizes the effect of Fe3O4 on degradation temperatures of CPVC according to TGA results (Figure 8). The maximum weight loss temperature of CPVC was 300.8 °C and the second degradation stage of CPVC was not obvious. After adding

Figure 7. Scheme of the formation mechanism of CFs. 17020

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carbonization of CPVC microfibers resulted in more content of Fe3O4 included in the Fe/CMFs. Thus more active sites for the decomposition of H2O2 into hydroxyl radicals were formed during the early degradation stage of CR, which promoted the degradation of CR. Degradation of CR Using Fe/CMFs under Sunlight Irradiation. It is very attractive for the utilization of sunlight irradiation to promote the heterogeneous Fenton reaction, since it not only sharply decreases the expense of operation but also provides a possible approach to utilize sunlight as about 50% solar energy in visible light region. A preliminary study showed that the degradation efficiency of CR under sunlight irradiation was not as high as that under UV irradiation, so the Fe/CMF dosage was increased to 1.5 g/L and the initial CR concentration was reduced to 50 mg/L. Figure 10 displays the

Table 3. Degradation Temperatures of CPVC-Fe3O4-x According to TGA Results (Figure 8) CPVC-Fe3O4-x

T1a (°C)

T5b (°C)

Tmaxc (°C)

CPVC-Fe3O4-0 CPVC-Fe3O4-0.5 CPVC-Fe3O4-1 CPVC-Fe3O4-2

251.1 233.1 226.9 221.6

273.5 266.7 264.9 262.7

300.8 299.8 298.6 296.9

a

Represents the temperature at 1 wt % weight loss rate occurred. Represents the temperature at 5 wt % weight loss rate occurred. c Represents the temperature at which maximum weight loss rate occurred. b

Fe3O4 (0.5−2.0 g/100 g CPVC), the first degradation stage of CPVC was accelerated. T1, T5, and Tmax of CPVC decreased with the increasing added amount of Fe3O4. For example, after adding 0.5 (g/100 g CPVC) Fe3O4, T1, T5, and Tmax of CPVC decreased by 18.0, 6.8, and 1.0 °C, respectively. With further increasing the content of Fe3O4 to 2.0 (g/100 g CPVC), T1, T5, and Tmax of CPVC decreased by 29.5, 10.8, and 3.9 °C, respectively. Based on the above results, it could be concluded that Fe3O4 could promote the dehydrochlorination of CPVC into polyene, which favored the formation of individual carbon fibers. Finally, Fe/CMFs with minor FeCl2 and/or FeCl3 uniformly dispersed on the surface were obtained by this simple method through Fe3O4 catalyzing the fast degradation and carbonization of CPVC microfibers. Degradation of Congo Red (CR) Using Fe/CMFs under UV Irradiation. To study the potential application of the Fe/ CMF-x in wastewater treatment, Fe/CMF-x (x = 0, 0.5, 1, and 2) were evaluated as heterogeneous Fenton catalysts for the degradation of CR.33,34 As shown in Figure 9, the degradation

Figure 10. Degradation of CR using Fe/CMF-x under sunlight irradiation. Experiment conditions: Fe/CMF-x dosage = 1.5 g/L; initial CR concentration = 50 mg/L; initial H2O2 concentration = 50 mM; and pH 3.5.

degradation efficiency of CR under sunlight irradiation. Only slight degradation efficiency of CR (2.5%) was observed after 180 min sunlight irradiation using Fe/CMF-0. For Fe/CMF0.5, 40.1% degradation efficiency of CR was obtained. The degradation efficiency of CR after 180 min sunlight irradiation further increased to 62.8% and 99.9% using Fe/CMF-1 and Fe/ CMF-2 as heterogeneous Fenton catalyst, respectively. Moreover, compared to Fe/CMF-2, Fe/CMF-1 showed similar morphology and had much greater SBET, but it was instead inferior in the performance of CR degradation. These results showed that the catalytic activity of the resultant Fe/CMF-x should come from Fe3O4 instead of the surface of the fiber or pores in the fiber. Overall, the above results suggested that the Fe/CMFs also showed high performance in the degradation of CR under sunlight irradiation. According to previous studies,35,36 the photodegradation mechanism of CR by Fe/CMF-x via heterogeneous photo Fenton process includes two processes: (i) the formation of radicals (such as •OH and HOO•) from the decomposition of H2O2 catalyzed by Fe3O4 under UV or sunlight irradiation and (ii) the degradation of CR by the resultant radicals. Hence irradiation played an important influence on the degradation of CR by Fe/CMF-x. Compared to natural sunlight, UV light is more power and thus the degradation efficiency of CR is higher under UV light. Besides, the high photo degradation efficiencies of the resultant Fe/CMFs are probably attributed to the following factors: (i) the Fe2+ in Fe3O4 plays an important role in the initiation of the Fenton reaction according to the classical

Figure 9. Degradation of CR using Fe/CMF-x under UV irradiation. Experiment conditions: Fe/CMF-x dosage = 1.0 g/L; initial CR concentration = 100 mg/L; initial H2O2 concentration = 50 mM and pH 3.5.

efficiency of CR using Fe/CMF-0 was as low as 2.2%, indicating that the Fe/CMF-0 could not be used as heterogeneous Fenton catalyst for the degradation of CR. Obviously, this was because no any Fe species existed on the surface of carbon piece. For comparison, the degradation efficiency of CR using Fe/CMF0.5 increased up to 41.6% after UV irradiation for 120 min. Furthermore, the degradation efficiency of CR using Fe/CMF1 or Fe/CMF-2 increased to 77.9% and 99.9%, respectively. During the early stage, the degradation rate of CR increased in the increasing order: Fe/CMF-0.5 < Fe/CMF-1 < Fe/CMF-2. This was ascribed to that more amount of Fe3O4 used for the 17021

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Haber−Weiss mechanism and (ii) the octahedral site in the magnetite structure can easily accommodate both Fe2+ and Fe3+, allowing the Fe species to be reversibly oxidized and reduced while keeping the same structure.37−39

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CONCLUSIONS We reported a simple approach to prepare Fe/CMFs with high carbon conversion by catalytic carbonization of CPVC microfibers using Fe3O4 as catalyst. Without Fe3O4, irregular carbon was obtained from CPVC microfibers. However, under the catalysis of Fe3O4, carbon microfibers with Fe3O4 crystals dispersed on the surface were produced. It was due to the reason that Fe3O4 accelerated the carbonization of CPVC microfibers. The resultant Fe/CMFs could be used as heterogeneous Fenton catalyst to catalyze the degradation of Congo red with high degradation efficiency under UV or sunlight irradiation. Hence the obtained Fe/CMFs had the potential application in wastewater treatment. It is expected that materials with any shape, as long as its surface is coated with CPVC-Fe3O4 mixed solution, can be used to prepare carbon or carbon-based materials with various morphologies or microstructures. The related work is being conducted in our laboratory.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 (0)431 85262004. Fax: +86 (0)431 85262827. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (2124079, 50873099, and 20804045) and Polish Foundation (No. 2011/03/D/ST5/06119).

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