Carbonization of PAN grafted uniform crosslinked polystyrene microspheres

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Carbonization of PAN grafted uniform crosslinked polystyrene microspheres Eran Partouche, Shlomo Margel* Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel

A R T I C L E I N F O

A B S T R A C T

Article history:

Micrometer-sized polystyrene/poly(styrene-divinylbenzene) and polystyrene/polydivinyl-

Received 16 September 2007

benzene composite particles of narrow size distribution were formed by a single-step swell-

Accepted 9 February 2008

ing process of uniform polystyrene template particles with emulsion droplets of dibutyl

Available online 20 February 2008

phthalate containing benzoyl peroxide and divinylbenzene in the presence or absence of styrene, followed by polymerization of the monomer(s) within the swollen template particles at 70 °C. Porous poly(styrene-divinylbenzene) and polydivinylbenzene uniform microspheres were formed by dissolution of the polystyrene part of the former composite particles. Hydroperoxide conjugated microspheres were formed by ozonolysis of the former porous microspheres. Uniform poly(styrene-divinylbenzene)/PAN and polydivinylbenzene/PAN core/shell microspheres were prepared by room temperature redox graft polymerization of AN onto the hydroperoxide conjugated particles. Uniform carbon microspheres were prepared by carbonization of the core/shell particles at 800 and 1100 °C under dynamic N2 atmosphere. On the other hand, a similar treatment of the core particles only resulted in destruction of the particle shape. Carbon microspheres of increasing surface area (up to ca. 1000 m2/g) were prepared by activation of the former carbon microspheres with CO2 at 850 °C. The influence of the carbonization temperature of the core/shell particles and the activation time of the carbon particles on the carbon yield and surface area has been elucidated. Ó 2008 Elsevier Ltd. All rights reserved.

1.

Introduction

Porous microspheres of narrow size distribution have attracted much attention among the academic and industrial scientific communities due to their broad range of potential applications, e.g., adsorbents for high-pressure liquid chromatography, hydrogen storage, inks and catalysis [1–4]. Dispersion polymerization is the common method for preparing non-porous uniform micrometer-sized particles in a single-step [5–8]. However, the particles formed by this method possess a relatively small surface area and their properties, e.g., porosity, surface morphology and functionality, can hardly be manipulated [5,8]. Furthermore, uniform parti-

cles of a diameter larger than 5 lm usually cannot be prepared by dispersion polymerization. These limitations have been overcome by several swelling methods of template particles (usually polystyrene (PS) formed by emulsion or dispersion polymerization) with appropriate monomers and initiators, e.g., multi-step swelling [9–15], dynamic swelling [16,17] and a single-step swelling [18,19], followed by polymerization of the monomer(s) within the swollen template particles. Surface modification of the micrometer-sized particles without changing their bulk properties has also attracted much attention. The reasons for seeking this kind of modification are many, e.g., changing the surface composition and

* Corresponding author: Fax: +972 3 6355208. E-mail address: [email protected] (S. Margel). 0008-6223/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2008.02.006

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wettability properties, improving adhesion, protein and enzyme immobilization, blood compatibility, weathering, protection, etc. [20–24]. In the present article protection of uniform core crosslinked PS microspheres from destruction during the formation of carbon particles at elevated temperatures under inert atmosphere was achieved by coating these particles with polyacrylonitrile (PAN), a well known precursor for carbon formation [25]. PAN-based activated carbon has notable adsorbing ability due to its high surface area, narrow pore size distribution and the nitrogen atom content [26,27]. Previous studies aiming to prepare hollow carbon particles based on core/shell microspheres or nanospheres have already been described [28–31]. The present article, however, describes a new process for the formation of uniform carbon microspheres of high surface area. The formation of carbon particles of high surface area usually requires an activation step. This is accomplished by either physical or chemical means, or a combination [32–37]. In the present work activation of the uniform carbon microspheres previously prepared was performed in carbon dioxide atmosphere at 850 °C for 0.5–6.0 h. The effect of carbonization temperature of the different precursors and the activation time of the carbon microspheres on the carbon yield and surface area was also elucidated in these studies.

2.

Experimental

2.1.

Chemicals

The following analytical-grade chemicals were purchased from Aldrich, and were used without further purification: ethanol (HPLC-grade), acetonitrile (99.9%), N,N-dimethylformamide (DMF, 99.9%), dibutyl phthalate (DBP, 99.9%), sodium metabisulfite and benzoyl peroxide (BP, 98%). Pure N2 and CO2 were purchased from Linde. Styrene (S, 99%) and divinylbenzene (DVB, 80%) were passed through an activated alumina (ICN) to remove the inhibitor before use. Acrylonitrile (AN) was distilled before use. Water was purified by passing deionized water through an Elgastat Spectrum reverse osmosis system (Elga Ltd., High Wycombe, UK). Ozone was produced by passing a current of oxygen through a corona discharge (Ozomax Canada) at voltages from 4.5 to 9 kV.

2.2.

Methods

2.2.1.

Synthesis of uniform PS template microspheres

PS template microspheres of 2.2 ± 0.2 lm were prepared according to the literature [5,7,38].

2.2.2. Synthesis of uniform micrometer-sized PS/P(S-DVB) and PS/PDVB composite particles by a single-step swelling process Uniform micrometer-sized PS/P(S-DVB) and PS/PDVB composite particles of size of 5.1 ± 0.4 and 5.0 ± 0.5 lm, respectively, were formed by a single-step swelling of the uniform PS template particles with emulsion droplets of DBP (swelling agent) containing BP (initiator) and DVB (crosslinking monomer) in the absence or presence of styrene (monomer, 1/2 by volume), followed by polymerization of the monomer(s)

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within the swollen template particles at 70 °C, according to the literature [38,39].

2.2.3. Synthesis of uniform micrometer-sized P(S-DVB) and PDVB particles Uniform micrometer-sized P(S-DVB) and PDVB particles were prepared by dissolving the PS part of the PS/P(S-DVB) and PS/PDVB composite particles with DMF. Briefly, 500 mg of the PS/P(S-DVB) or PS/PDVB composite particles dispersed in 30 mL of DMF were shaken at room temperature for ca. 12 h. The dispersed particles were then centrifuged, and the supernatant containing the dissolved PS template polymer was discarded. The crosslinked P(S-DVB) or PDVB particles were then washed by intensive centrifugation cycles with DMF, water and ethanol. The obtained particles were then dried in a vacuum oven.

2.2.4. Synthesis of uniform micrometer-sized oxidized P(SDVB) and PDVB particles Four hundred milligrams of the micrometer-sized P(S-DVB) or PDVB particles dispersed in 20 mL pure water were introduced into a 250 mL round bottom flask. An O3/O2 stream containing ozone output of 4 g/h was bubbled through the dispersed particles at room temperature for 30 min at a flow rate of 1.0 L/min. This oxidation process resulted in the formation of several conjugated oxygen-containing groups onto the P(S-DVB) and PDVB microspheres such as hydroperoxides, ketones and acids. The conjugated hydroperoxides function as initiators in the followed redox graft polymerization process. The oxidized particles were then washed free of excess ozone by intensive centrifugation cycles with water, until the supernatant did not show any indication of free ozone, as measured spectrophotometrically with KI [40]. These measurements indicated that the conjugated hydroperoxide concentration was 0.8 mmol/g particles.

2.2.5. Redox graft polymerization of AN on the oxidized P(S-DVB) and PDVB microspheres In a 15 mL pressure tube (Aldrich), 100 mg of the oxidized P(S-DVB) or PDVB particles were dispersed in 5 mL of acetonitrile containing different concentrations of AN (20 and 40 vol%). For initiating the graft polymerization, 400 lL of an aqueous solution containing 0.4 mmol sodium methabisulfite were added to the former dispersion. The sealed pressure tube was then shaken at room temperature for 22 h. The formed P(S-DVB)/PAN and PDVB/PAN core/shell particles were then washed by intensive centrifugation cycles with DMF, water and ethanol, respectively, and then dried in a vacuum oven. The influence of AN concentration on the PAN grafting yield and the properties of the formed carbon microspheres was also elucidated.

2.2.6. Carbonization of the P(S-DVB)/PAN and PDVB/PAN composite microspheres Eight hundred milligrams of P(S-DVB)/PAN or PDVB/PAN core/shell particles were carbonized at 800 or 1100 °C for 2.5 h. The carbonization was performed under nitrogen atmosphere at a flow rate of 150 mL/min and heating rate of 10 °C/ min.

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2.2.7.

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Activation of the carbon microspheres

Seventy milligram of the carbon particles made of P(S-DVB)/PAN or PDVB/PAN microspheres at 800 or 1100 °C were activated in CO2 at a flow rate of 140 mL/min at 850 °C for 0.5–6.0 h.

2.3.

Analysis

The diameter and size distribution of the various microspheres were determined by scanning electron microscopy (SEM, JEOL, JSM-840 instrument). The conjugated-hydroperoxide concentration was determined according to Carlsson and Wiles [40]. The grafting yield of PAN on the micrometersized crosslinked P(S-DVB) and PDVB particles was measured according to the following equation: % Grafted PAN ¼ ð% experimental N=% theoretical NÞ  100 ð1Þ The % experimental N, C, H and O were measured by elemental analysis model EA1110, CE Instruments, Thermoquast. The surface area of the various particles was measured by the Brunauer–Emmet–Teller (BET) method [41]. Pore volume and pore size distribution were determined from the nitrogen adsorption isotherms using the Barrett, Joyner and Halenda (BJH) method [42] and by density functional theory (DFT) [43,44] software by Gemini III model 2375, Micrometrics. The thermal behavior of the particles was measured by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The analysis was performed with a TC15 system equipped with TGA, model TG-50 and DSC, model DSC-30, Mettler Toledo. Raman measurements were performed with a Jobin Yvon Micro Raman (model HR800, k = 632.817 nm at 20 mW, slit = 100 lm, hole = 1000 lm, two 30 s scans) using different objectives.

3.

Results and discussion

Uniform crosslinked PS particles of different degrees of crosslinking, P(S-DVB)] and PDVB, were synthesized by a singlestep swelling process of PS template particles with DVB in absence or presence of styrene (1/2 by volume) [18,39]. The P(S-DVB) and PDVB microspheres were then oxidized by bubbling ozone through an aqueous dispersion containing these particles. This treatment commonly results in slight degradation of the polymers [45] and in the formation of several oxygen-containing groups conjugated to the particles, e.g., hydroperoxides [38]. These conjugated hydroperoxides have been used as initiator for redox graft polymerization of AN onto the crosslinked particles at room temperature [46]. The special features of such a graft polymerization are a very short induction period, a relatively low energy of activation, and the relatively low temperature required. The resultant uniform PAN grafted particles were then used as precursors for carbon microspheres formation [29]. The properties of the produced carbon microspheres depend on the PAN content and the carbonization temperature. Uniform activated carbon microspheres (ACM) with tunable surface area were produced by activation of the carbon microspheres in carbon dioxide at 850 °C for different time periods.

3.1. Characterization of the PS, PS/P(S-DVB), PS/PDVB, P(S-DVB) and PDVB microspheres Table 1 illustrates the diameter, size distribution and surface area of the PS, PS/P(S-DVB), PS/PDVB, P(S-DVB) and PDVB microspheres. The calculated surface area of the 2.2 ± 0.2 lm PS template microspheres is similar to that of the measured one: 2.7 and 2.9 m2/g, respectively, indicating the non-porous structure of these particles. On the other hand, Table 1 indicates that the surface areas of the 5.1 ± 0.4 lm PS/P(S-DVB) and 5.0 ± 0.5 lm PS/PDVB composite particles (48.3 and 407.0 m2/g, respectively) are significantly higher than that of the calculated ones (1.2 m2/g for both), indicating the highly porous structure of these composite particles. It should be noted that according to Table 1 the surface area of the PS/ PDVB composite particles is 8.4 times higher than that of the PS/P(S-DVB). This is probably due to the significantly higher DVB monomeric units contents of the PS/PDVB composite particles, as already indicated in previous publications by Kedem and Margel; and, Cheng and coworkers [11,39]. Table 1 also illustrates that the dissolution of the PS part of the former composite particles does not effect the size and size distribution of the particles, but leads to 2.2 and 1.6 - fold increase in the surface area of the P(S-DVB) and PDVB particles, respectively. Figs. 1A–C confirm by SEM images the narrow size distribution of the various particles, the nonporous structure of the PS template microspheres (Fig. 1A) and the porous structures of the P/(S-DVB) and PDVB particles (Figs. 1B and C, respectively). A comparison between the SEM images shown in Figs. 1B and C illustrates easily the higher porosity of the PDVB particles compared to that of the P(SDVB) ones. TGA and DSC studies indicated the higher decomposition temperature of the PDVB microspheres compared to that of P(S-DVB), 464 and 428 °C, respectively. The increased decomposition temperature is most likely due to the increased crosslinking character of the PDVB microspheres compared to that of the P(S-DVB).

3.2. Characterization of the P(S-DVB)/PAN and PDVB/PAN core/shell microspheres P(S-DVB)/PAN and PDVB/PAN core/shell microspheres were formed by redox graft polymerization of two concentrations of AN (20 and 40% by volume) onto the oxidized microspheres, according to the experimental section [38,46]. The PAN content of these core/shell particles was 12 and 30 wt%, respectively, as calculated from elemental N analysis (see experimental part Section 2.2.3.). Tables 1 and 2 illustrate that the ozonolysis of the P(S-DVB) and PDVB particles did not affect their size and size distribution. It is also interesting to see that the mean diameter of the oxidized particles before and after the coating with PAN increased very slightly. This may indicate that most of the AN polymerization occurs within the pores of the P(S-DVB) and PDVB microspheres. Tables 1 and 2 clearly demonstrate the gradual decrease in the surface area as the treatment process progresses. For example, the surface area of the microspheres before ozonolysis, after ozonolysis and after grafting of 40% AN are 107.5, 71.5 and 46.7 m2/g for P(S-DVB) microspheres and

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Table 1 – Diameter, and calculated and measured surface area of the PS, PS/P(S-DVB), P(S-DVB), PS/PDVB and PDVB microspheresa Microspheres

Diameter (lm)

Surface area (m2/g) Calculateda

PS PS/P(S-DVB) P(S-DVB) PS/PDVB PDVB

2.2 ± 0.2 5.1 ± 0.4 5.1 ± 0.4 5.0 ± 0.5 5.0 ± 0.5

2.7 1.2 1.2 1.2 1.2

799

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Measured 2.9 48.3 107.5 407.0 651.5

a Calculated surface area is based on the assumption that the particles are non-porous spheres with density of 1.0 g/cm3. The calculation was performed according to the following equation: S = 6/(D Æ d), wherein S is the surface area (m2/g); D is the diameter (lm); and d is the density (g/cm3) of the particles.

651.5, 544.6 and 247.1 m2/g for PDVB microspheres, respectively. It should be noted that the decrease in the surface area of the particles is dependent on the AN concentration: the higher the concentration the higher the decrease. For example, polymerization on the oxidized PDVB microspheres of 20 and 40% AN results in PDVB/PAN particles of surface area 480.1 and 247.1 m2/g, respectively. Table 2 also shows that the calculated surface area of the various microspheres is always significantly lower than that of the measured ones, indicating their relative porous structure. Fig. 2 shows a pore-size histogram of the PDVB (A), oxidized PDVB (B) and PDVB/PAN (C) microspheres. Fig. 2 demonstrates for the presented microspheres a single population of

Table 2 – Diameter, and calculated, and measured surface area of the oxidized P(S-DVB) and PDVB microspheres, PAN grafted P(S-DVB) and PDVB microspheres and carbonized P(S-DVB)/PAN and PDVB/PAN micospheres Microspheres

Diameter Surface area (m2/g) (lm) Calculateda Measured

Oxidized P(S-DVB) P(S-DVB)/PAN Carbonized P(S-DVB)/PAN Oxidized PDVB PDVB/PAN Carbonized PDVB/PAN

5.1 ± 0.4 5.2 ± 0.5 2.7 ± 0.4 5.0 ± 0.5 5.1 ± 0.5 3.6 ± 0.6

1.2 1.2 2.2 1.2 1.2 1.8

71.5 46.7 38.7 544.6 247.1 28.1

Ozonolysis and PAN grafting (30 wt%) were preformed according to the experimental part. Carbonizations of the P(S-DVB)/PAN and PDVB/PAN particles were accomplished at 800 °C. a Surface area was calculated as described in Table 1.

˚ . The total pore volumes measured at mesoporous of ca. 33 A 0.994 P/P0 were 0.72, 0.62 and 0.23 cm3/g for the PDVB, oxidized PDVB and PDVB/PAN microspheres, respectively. It seems that the significant decrease (ca. 63%) in the total pore volume of the PDVB/PAN microspheres due to the polymerization of AN, occurs mostly within the pores. This was also learned from our previous cross section TEM studies [46]. However, the minor decrease (ca. 14%) in the total pore volume of the oxidized PDVB relative to the PDVB is still unclear. A possible explanation for this decrease is due to the accumulation of the oxygen-containing groups within the mesoporous such as hydroperoxides, ketones and acids, as a consequence of the exposure to ozone [47–49].

Fig. 1 – SEM photomicrographs of the PS template particles (A) and the P(S-DVB) (B) and PDVB (C) particles.

800

dV/dD

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A B C

20

30

40

50

60

spectrum of the PDVB polymer correspond to the ring deformation mode at 640 cm1, ring breathing mode at 1000 cm1, C–C stretches at 1150–1200 cm1, CH2 scissoring at 1446 cm1, ring skeletal stretch at 1609 cm1 and the C@C vinyl mode at 1630 cm1. The vinyl C–H stretches are, quite naturally, in the same frequency range as the aromatic C–H stretches at a range of 3000–3100 cm1 [50,51]. Ozonolysis of these particles leads to increasing concentrations of oxygencontaining groups, as illustrated by the increased carbonyl peak at ca. 1701 cm1 (Fig. 3B). A qualitative indication of the success of the AN graft polymerization was obtained by the peak formation at ca. 2243 cm1 belonging to the C„N stretch (Fig. 3C). Similar qualitative results (Raman) were also obtained for the P(S-DVB) microspheres.

Pore Diameter (A)

Fig. 2 – Pore size distribution of PDVB (A), oxidized PDVB (B) and PDVB/PAN (C) microspheres. Synthesis, ozonolysis and PAN grafting (30 wt%) were performed according to the experimental part.

D-band

1.0 0.8

I

D G-band

0.6 II

0.4 0.2 0.0

C

0.8 0.6

Intensity, A.U.

2243 cm-1

0.4 0.2 0.0

B

0.8 0.6 1701 cm-1

0.4 0.2 0.0 0.8

A

-1

1630 cm 1609 cm-1

0.6 0.4 0.2 0.0 500

1000

1500

2000

2500

3000

3500

4000

Raman Shift, cm-1

Fig. 3 – Raman spectra of the PDVB (A) oxidized PDVB (B), PDVB/PAN (C) and carbon particles made of PDVB/PAN (D-I solid line) and of P(S-DVB)/PAN (D-II dotted line) microspheres. Synthesis, ozonolysis, PAN grafting (30 wt%) and carbonization (800 °C) were performed according to the experimental part.

Fig. 3 depicts the Raman spectra of the PDVB microspheres before ozonolysis (A), after ozonolysis (B) and after PAN grafting (C). Fig. 3A shows the prominent peaks in the Raman

3.3.

Carbonization of the P(S-DVB) and PDVB microspheres

It is well known that PAN can function as an efficient precursor for carbonaceous materials [25]. Fig 3D demonstrates by Raman spectroscopy that carbonization of the PDVB/PAN and P(S-DVB)/PAN microspheres at 800 °C resulted in concealing of the polymers typical peaks and formation of new two significant bands, labeled D for disorder carbon mode and G for graphite vibration mode, respectively, which are dominant in the Raman spectrum of disordered carbon. Fig. 3D also compares between the carbon particles made of PDVB/PAN (I) and those made of P(S-DVB)/PAN microspheres (II). The Raman spectra of the carbon particles made of PDVB/PAN microspheres exhibits a D-band near 1326 cm1 and a G-band at 1588 cm1. On the other hand, the D-band for the carbon particles made of P(S-DVB)/PAN microspheres appears upwards at 1340 cm1 while the G-band is widened but remains near 1588 cm1. The phenomenon of shifting upwards and broadening in the D-band implies an increased level of disorder and a decrease in the graphitic domain size [52]. Table 1 shows that the size distribution is retained and that the mean diameter of the carbon particles made of P(SDVB)/PAN and PDVB/PAN microspheres decreases from 5.2 ± 0.5 to 2.7 ± 0.4 lm and from 5.1 ± 0.5 to 3.6 ± 0.6 lm, respectively. The constant size distribution and the decrease in diameter is also illustrated by the SEM pictures of the carbon microspheres (Figs. 4A and B) made of P(S-DVB)/PAN and PDVB/PAN particles (Fig. 1B and C), respectively. The relative lower decrease in the diameter of the carbon particles made of PDVB/PAN (32%), compared to that made of P(S-DVB)/PAN microspheres (47%), can be explained by the increase in rigid domains (due to the higher crosslinking degree) of the PDVB relative to P(S-DVB). Fig. 4C and D demonstrate by SEM images that carbonization at 800 °C of the P(S-DVB) and PDVB core microspheres resulted in destruction of their spherical shape: scaly fine carbon films and rigid block of carbon grinded by mortar and pestle, respectively, both with metallic luster. It seems that the PAN grafted shell protects the microspheres from structure destruction and that the spherical shape remains, as demonstrated by Fig. 4A and B for the carbon particles made of P(S-DVB)/PAN and PDVB/PAN microspheres, respectively. The reason for this phenomenon is probably due to separation (achieved via the PAN coating) between the core particles and hence prevents their decomposition.

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It is important to mention that carbonization at 1100 °C exhibited similar results.

3.4.

Activation of the carbon microspheres

ACMs were obtained by activation of the carbon particles made of P(S-DVB)/PAN and PDVB/PAN microspheres with carbon dioxide at 850 °C for different time periods. Fig. 5 shows the variation in the burn-off as function of activation time of the carbon microspheres prepared at 800 (A) and 1100 °C (B). The wt% loss was determined from the change in the weight of the particles before and after activation. Fig. 5 demonstrates a general trend of increased weight loss during activation time for the carbon particles prepared at 800 and 1100 °C from both P(S-DVB)/PAN and PDVB/PAN microspheres. Fig. 5 illustrates clearly that carbonization at 800 °C of the P(S-DVB)/PAN and PDVB/PAN microspheres yielded a larger weight loss during activation than at 1100 °C. For example, the burn-off of the carbon particles made of P(S-DVB)/PAN microspheres, for activation time of 0.5, 1.0 and 2.0 h, resulted in wt% loss of 36.5%, 69.1%, and 98.0% for carbonization at 800 °C and 8.6%, 9.4% and 16.7% for carbonization at 1100 °C, respectively. We assume that the difference in the rate of the weight loss is due to the looser structure of the carbon microspheres which carbonized at the lower temperature [53]. This is in agreement with the results of Lee and coworkers [54]. On the other hand, opposite behavior was reported by Sun and coworkers in their studies of PAN-based activated carbon hollow fibers [55]. Fig. 5B illustrates a similar behavior in the burn-off versus time of the

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two carbon types made of P(S-DVB)/PAN and PDVB/PAN microspheres carbonized at 1100 °C. By contrast, the burnoff rate of the carbon particles made of P(S-DVB)/PAN microspheres at 800 °C (Fig 5A) is significantly higher than that of the carbon particles made of PDVB/PAN microspheres prepared at the same carbonization temperature. This significant difference in the burn-off rate can also be explained by the looser structure of the carbon particles made of P(S-DVB)/ PAN microspheres relative to those made of PDVB/PAN. When carbon microspheres are heated in carbon dioxide, the carbon composition in the microspheres reacted with the carbon dioxide to produce evolved carbon monoxide [56], which leads to weight loss accompanied by an increase in surface area. Fig. 6 illustrates the variation in the surface area as a function of activation time of the carbon microspheres made of P(S-DVB)/PAN and PDVB/PAN particles prepared at 800 (A) and 1100 °C (B). Fig. 6 shows increase in the surface area with the activation time for all the particle types. Fig. 6 also illustrates that for both carbonization temperatures the surface area of the ACM made of PDVB/PAN microspheres containing 30 wt% grafted PAN, rapidly increased in the first 0.5 h and then gradually increased with continued activation. This phenomenon may indicate that there are two different mechanisms for the reaction of the carbon dioxide with the carbon particles: first, a rapid reaction with the carbon made of the grafted PAN and then a slow reaction with the rigid core carbon made of the PDVB. The highest values of surface area measured in this case were 984 and 588 m2/g for carbonization temperatures of 800 and 1100 °C, respectively. The

Fig. 4 – SEM photomicrographs of the carbon products made of P(S-DVB)/PAN (A), PDVB/PAN (B), P(S-DVB) (C) and PDVB (D) microspheres. Synthesis, ozonolysis, PAN grafting (30 wt%) and carbonization (800 °C) were performed according to the experimental part.

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CARBON

100

Carbon particles made of PDVB/PAN microspheres Carbon particles made of P(S-DVB)/PAN microspheres

4 6 ( 2 0 0 8 ) 7 9 6 –8 0 5

1200

B

Carbon particles made of PDVB/PAN microspheres Carbon particles made of P(S-DVB)/PAN microspheres

1000

80 800 60 600

20 0 100

A

80 60

Surface Area, m2/g

Burn-Off, %

40

400 200

B

0 1000 800 600

40

400

20

200

0

0 0

1

2

3

4

5

6

Time, h

Fig. 5 – Burn-off versus activation time of the carbon particles made of P(S-DVB)/PAN and PDVB/PAN microspheres at 800 (A) and 1100 °C (B). Synthesis, PAN grafting (30 wt%), carbonization and activation were performed according to the experimental part.

change in the surface area versus time of the ACM made of P(S-DVB)/PAN microspheres at 1100 °C (Fig. 6B) indicates a similar trend to that shown for ACM made of PDVB/PAN. The surface area gradually increased within the first 2.0 h and then decreased more slowly with increasing activation. The difference in the activation rates can also be explained as described for the carbon particles made of the PDVB/PAN microspheres. The highest surface area measured in this case was ca. 500 m2/g (Fig. 6B). Fig. 6A shows that the increase in the surface area versus time of the ACM made of P(S-DVB)/ PAN microspheres at 800 °C is significantly higher than that made of PDVB/PAN microspheres. This figure illustrates maximum surface area of 1040 m2/g after activation period of 1.0 h. It should be noted that after activation time of ca. 2.0 h, more than 98.0 wt% of the ACM was lost. Figs. 5 and 6 also illustrate that at 800 °C the burn-off and the surface area increase rates of carbon particles made of P(S-DVB)/PAN are higher than that of PDVB/PAN while an opposite behavior is demonstrated at 1100 °C. The explanation to this opposite behavior is not yet clear. The measured surface areas of the PDVB/PAN microspheres before and after carbonization at 800 °C were 247 and 28 m2/g, respectively (see Table 2). Activation of these carbon particles increased the surface area dramatically e.g., thirty times higher after activation of 6.0 h. Therefore, we chose to examine whether higher initial surface area eventually leads to remarkably higher surface area of the resultant ACM. For this purpose a comparison in the surface area versus time of ACM made of PDVB/PAN containing 12 and 30 wt% PAN was performed. The PDVB/PAN containing 12 wt% PAN particles possesses a surface area of 480 m2/g while that containing 30 wt% PAN possesses a surface area of 247 m2/g. Fig. 6A demonstrates by a dotted line, the varia-

A 0

1

2

3

4

5

6

Time, h

Fig. 6 – Surface area versus activation time of the carbon particles made of P(S-DVB)/PAN and PDVB/PAN microspheres at 800 (A) and 1100 °C (B). Solid and dotted lines represent core/shell particles containing 30 and 12 wt% grafted PAN, respectively. Synthesis, carbonization and activation were performed according to the experimental part.

tions in the surface area as a function of activation time of the carbon microspheres made of PDVB/PAN containing 12 wt% PAN. This figure indeed illustrates a higher initial surface area of the carbon particles made of PDVB/PAN particles containing 12 wt% PAN than that containing 30 wt% PAN, 346 and 28 m2/g, respectively. However, it was rather surprising to realize that this difference in the surface area significantly decreases with increasing activation. For example, before and after activation times of 0.5, 2.0, and 6.0 h, the observed differences in the surface areas are 318, 138, 96 and 82 m2/g, respectively. These results may indicate that the surface area which developed during the activation process is not linearly proportional to the surface area before activation. BJH and DFT studies indicate the presence of mesopores and micropores within the carbon particles made of 30 and 12 wt% PAN. However, prolonged activation resulted in vanishing of the mesopores. In addition, the total pore volume increased with increased activation as expected. For example, after activation times of 1.0, 2.0, and 6.0 h, the total pore volume was 0.26, 0.31, and 0.51 mL/g for 12 wt% PAN and 0.52, 0.59, and 1.17 for 30 wt% PAN, respectively. Table 3 illustrates, by elemental analysis the degradation of the carbon particles and the formation of oxygen-containing groups due to activation of the carbon particles made of P(S-DVB)/PAN and PDVB/PAN microspheres. Table 3 shows that the nitrogen content of the ACM decreases with increasing activation. This finding indicates that the carbon microspheres eliminate nitrogen during activation. Careful examination of the rates of nitrogen decrease revealed that the elimination rates of nitrogen belonging to the carbon particles made of P(S-DVB)/PAN microspheres is

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Table 3 – Effect of activation time in the % N, C, H and O of the carbon particles made of P(S-DVB)/PAN and PDVB/ PAN microspheresa Activation time (h)

N

C

H

74.8 70.0 59.7

1.8 1.6 2.1

13.0 20.1 31.8

Carbon particles made of PDVB/PAN 0.0 5.8 0.5 5.4 1.0 4.4 2.0 4.1 6.0 3.1

82.8 80.2 78.4 76.1 65.7

1.3 2.2 1.5 1.8 1.7

7.1 12.2 15.7 18.0 29.5

a Ozonolysis, PAN grafting (30 wt%), carbonization (800 °C) and activation were preformed according to the experimental part.

higher than those made of PDVB/PAN. This Table also indicates that the carbon content of these carbon particles also decreases with activation time. This decrease is due to the reaction of the carbon dioxide with the carbon atoms at the crystal edge or non-regular part of the carbon microspheres which evolved carbon monoxide [56]. In addition Table 3 demonstrates an increase in the oxygen content during activation for both carbon particle types, which corresponds to the oxidation process [53]. Table 3 also indicates no significant changes in the hydrogen content during the activation of both carbon particles. Fig. 7 shows the Raman spectra of the ACM made of PDVB/ PAN microspheres by CO2 for different time periods: 0.5 (A) 2.0 (B) and 6.0 h (C). Fig. 7 demonstrates increase in both intensities: D and G bands, with increased activation. This finding indicates that prolonged activation leads to formation of new carbon basal planes which tends to increase the carbon content in the microspheres. However, at the same time the aggressive atmosphere of the carbon dioxide chars the carbon

Intensity, A.U.

C

B

A

1000

1500

2000

2500

Raman Shift, cm

3000

3500

Summary and conclusions

O

Carbon particles made of P(S-DVB)/PAN 0.0 10.4 0.5 8.3 1.0 6.4

500

microspheres and leads eventually to a decrease in the carbon content as shown in Table 3.

4.

wt%

803

4000

-1

Fig. 7 – Raman spectra of the ACM made of PDVB/PAN microspheres by CO2 for different time periods: 0.5 (A) 2.0 (B) and 6.0 h (C). Synthesis, ozonolysis, PAN grafting (30 wt%) and carbonization (800 °C) were performed according to the experimental part.

This study demonstrates a simple and convenient method for coating uniform P(S-DVB) and PDVB microspheres with PAN, by ozonolysis of the particles followed by redox graft polymerization of AN. The ozone treatment promotes the formation of conjugated hydroperoxides among other oxygen-containing groups which accumulated within the pores and decreased their volume. The redox graft polymerization of AN on the uniform P(S-DVB) and PDVB microspheres was performed at room temperature and initiated by the conjugated hydroperoxides/ NaHSO3 redox pair. The grafted PAN coating protected the crosslinked core microspheres from destruction during the formation of carbon particles at elevated temperatures under inert atmosphere. The properties of the resulting uniform carbon microspheres with tunable surface area, formed by activation with carbon dioxide for different time periods, depends on the PAN content and the rigidity of these microspheres.

Acknowledgements These studies were partially supported by a Minerva Grant (Microscale and Nanoscale Particles and Films) and by the Israeli Ministry of Commerce and Industry (NFM Consortium on Nanoparticles for Industrial and Applications). The authors would like to thank Dr. Nina Perkas for her help in the surface area measurements.

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