Activation studies of vapor-grown carbon fibers with supercritical fluids

Carbon 39 (2001) 2143–2150

Activation studies of vapor-grown carbon fibers with supercritical fluids Yuan-Yao Li, Kazuhiro Mochidzuki, Akiyoshi Sakoda*, Motoyuki Suzuki Institute of Industrial Science, University of Tokyo, 7 -22 -1 Roppongi, Minato-ku, Tokyo 106 -8558, Japan Received 23 August 2000; accepted 23 January 2001

Abstract The objective of this research was to investigate the feasibility of activating vapor-grown carbon fibers (VGCFs) with supercritical fluids (SCFs). VGCFs with a mean diameter of 150 nm and a length of a few micrometres were treated using water or carbon dioxide under supercritical conditions for 10 min. Hydrochloric acid and nitric acid were utilized for the oxidation of the VGCFs prior to the treatment of the SCFs. The nitrogen adsorption results at 77 K showed that the BET surface area of the treated VGCFs can be increased from 9 m 2 / g to about 35 m 2 / g. The adsorption of benzene vapor on the VGCFs revealed that the adsorption capacity of benzene was enhanced after the activation treatments. However, in comparison with commercial activated carbon fibers (ACF), the adsorption amount of benzene is still relatively low. Based on the CHN elemental analysis and X-ray photoelectron spectroscopy (XPS), it was found that oxygen compounds were contained on / in the VGCFs after the treatment by oxidation and by SCFs.  2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon fibers; B. Activation; C. Adsorption

1. Introduction The vapor-grown carbon fiber (VGCF), a central carbon nanotube [1] covered by annular layers of pyrolytic carbon, has been recognized as a promising material because of its outstanding thermal conductivity, electrical conductivity and mechanical properties [2–5] in comparison with commercial fibers such as PAN-based or pitch-based carbon fibers. In addition, VGCFs are produced as short fibers and low-cost materials, which has led to their increased interest for application in various industries. Studies have been carried out for their potential applications. For instance, VGCFs can be utilized as reinforcements in advanced composite materials [6–9], high thermal conductivity materials for electronic devices [10] and space thermal management systems [11,12], and electrodes for lithium-ion rechargeable batteries [13–15]. Recently, research attention has focused on the surface treatments of VGCFs for improving the mechanical properties of the VGCF reinforced composites. By adding *Corresponding author. Tel.: 181-3-3402-6231; fax: 181-33408-1486. E-mail address: [email protected] (A. Sakoda).

functional groups such as oxygen, nitrogen or sulfur on / in the VGCFs, the bonding between a matrix and the VGCFs can be enhanced. The activation of VGCFs was also studied to develop a novel adsorbent by creating pores on / in the VGCFs. Many treatments, for instance, the oxidation with air [16], nitric acid, hydrochloric acid, oxygen plasma and carbon dioxide [17], have been investigated followed by characterization using nitrogen adsorption, elemental analysis, Raman spectroscopy [18], atomic force microscopy [19] and X-ray photoelectron spectroscopy (XPS / ESCA) [20]. These investigations found that after the various treatments, a maximum BET surface area of 60 m 2 / g could be obtained from the original VGCFs (11 m 2 / g) [18] while the percent of oxygen in the VGCFs can be increased from 1.1 to about 20% [17]. Supercritical fluids (SCF), substances which have gaslike and liquid-like qualities, possess high diffusion coefficients [21] and low viscosities similar to that of gases, high densities like that of liquids, and excellent dissolving abilities [22,23]. With these unique properties, SCFs have been studied and used in the extraction of pharmaceutical compounds or decaffeination of coffee, the fractionation for oil or lipid products, dyeing, chromatography, chemical or biochemical reactions as media [24], particle formation

0008-6223 / 01 / $ – see front matter  2001 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 01 )00033-1

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by rapid expansion of supercritical solution (RESS) [25], the regeneration of adsorbents [26,27] and so on [28]. The aim of this study is to investigate the feasibility of activating vapor-grown carbon fibers (VGCFs) with supercritical fluids (SCFs). Supercritical water (T c : 3748C, Pc : 220 atm) and carbon dioxide (T c : 318C, Pc : 73 atm) were employed separately as a reactant or a reaction medium in this study. Pretreatments of the VGCFs were carried out by oxidation with hydrochloric acid or nitric acid before the SCF experiments. The treated VGCFs were characterized by various methods such as nitrogen adsorption at 77 K for the BET surface area, scanning electron microscopy (SEM), elemental (CHN) analysis and X-ray photoelectron spectroscopy (XPS) to determine the composition of the VGCFs. Finally, the adsorption ability of the micromolecules in the VGCFs was examined by adsorption of benzene in the vapor phase and phenol dissolved in an aqueous solution.

2. Experimental The VGCFs shown in Fig. 1 and used for the experiments were from the Showa Denko Co., Japan. They are |150 nm in diameter and a few microns in length. The supercritical fluids apparatus shown in Fig. 2 was from the Taiatsu Techno Co., Japan. It consists of a supercritical salt bath, model TSC-B600 (18 l), with an operating temperature range from 300 to 6008C, a digital temperature controller, a thermalcouple, a stirrer and a 65.9-ml hastelloy reactor, model TSC-006, which allows a maximum temperature of 4508C and a maximum pressure of 450 kgf / cm 2 . A pressure transducer was mounted in the reactor for monitoring the pressure change during the experiments and a valve was used for collecting substances in the gas phase after the experiments. Hydrochloric acid (HCl, 36%) and nitric acid (HNO 3 , 61%) were utilized for the oxida-

Fig. 1. Vapor-grown carbon fibers.

Fig. 2. Supercritical fluid apparatus.

tion treatment of the VGCFs prior to the SCF experiments. Distilled water and dry ice (density: 1.56 g / cm 3 at 2798C, 1 atm) were used as the sources of SCFs. Table 1 lists the treatment conditions of the VGCFs. Samples, VGCF1 and VGCF2, were treated by the SCFs while samples VGCF3, VGCF4 and VGCF5, were oxidized using strong acids followed by the SCF treatments. The supercritical fluid experiments were carried out by immersing the reactor containing the VGCFs and a precalculated amount of H 2 O or dry ice into the supercritical salt bath for 10 min, at a temperature of about 3808C. After the experiment, the reactor was moved from the salt bath to a water bath for the purpose of quenching. Apart from the reacted VGCFs, residual substances in gas and liquid phases were then collected and analyzed by a mass spectrometer and HPLC, respectively. Oxidation of the VGCFs started with soaking 1 g of the VGCFs in 50 ml HCl or HNO 3 at room temperature (258C) for 4 days before drying the VGCFs at about 258C. Various techniques were used to characterize the VGCFs. The BET surface area of VGCFs was determined by a nitrogen adsorption isotherm at 77 K using a Bellsorp36 from the Bellsorp Co., Japan. Scanning electron microscopy was utilized to observe the surface features of the VGCFs. Elemental analysis and XPS were employed for examining the composite of the VGCFs in the bulk and surface regions, respectively. The adsorption of the benzene vapor was performed at 258C using a

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Table 1 Treatment of VGCFs Sample number

VGCF0 a VGCF1 VGCF2 VGCF3 VGCF4 VGCF5 a

Oxidative treatment (pretreatment) material – – – HNO 3 HCl HNO 3

Supercritical fluid treatment VGCFs (g)

SCF

Weight of fluid (g)

Temp. (8C)

Pressure (kgf / m 2 )

Time (min)

– 1 1 0.4 0.4 0.3

– H2O CO 2 H2O H2O CO 2

– 25 19.19 25 25 18.44

– 380 380 380 380 380

– 258 236 298 259 317

– 10 10 10 10 10

VGCF0, original VGCFs.

Bellsorp18 to obtain the adsorption isotherms. The adsorption of phenol was carried out by mixing about 0.05 g VGCFs and a prepared 50-ml aqueous solution containing 10 ppm phenol in a flask and then placing the flask on a rotary shaker at a speed of 180 rev. / min for a period of 3 days. The adsorption capacity was analyzed by a UV– visible spectrophotometer (model UV-1600, Shimadzu Co., Japan). As a comparison, activated carbon fibers (ACF no.: A-15, Osaka Gas Ltd., Japan) were also used for the phenol and benzene adsorptions. The BET surface area of the ACFs was found to be 1579 m 2 / g by nitrogen adsorption at 77 K.

3. Experimental results and discussion

3.1. Supercritical fluid experiments Fig. 3 shows the temporal profiles of pressure and temperature in the reactor in the supercritical bath for the experiments. As can be seen, the pressure rapidly increased to the supercritical pressure of the fluids and then slowly

increased before finishing the experiments in 10 min. The bold lines in the figure show the period of the treatments by the supercritical fluids for the VGCFs. For the experiments with carbon dioxide (VGCF2 and VGCF5), the supercritical status was reached within 1 min while it took 4.5 to 6 min for the experiments with H 2 O (VGCF1, VGCF3 and VGCF4). The substances collected from the gas phase were analyzed after the SCF experiments. It was found that in the experiments with HNO 3 (VGCF3 and VGCF5), yellowish-brown gases were generated, which were identified as NO (mainly) and NO 2 by the mass spectrometer. We believed that the gases were derived from HNO 3 , which might have reacted with the VGCFs or / and the SCFs. Also, the substances collected from the liquid phase were analyzed by HPLC. As expected, Cl 2 and NO 2 3 were found from the experiments with HCl (VGCF4) and with HNO 3 (VGCF3), respectively. After the experiments, corrosion problems with the reactor were found when in the presence of HNO 3 or HCl in supercritical water. The alloy reactor cannot withstand these corrosive conditions so that it was oxidized and released metal ions (VGCF4) into the liquid phase as well as some impurities

Fig. 3. Temporal profiles of pressure and temperature of supercritical fluid experiments.

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such as metal oxides into the solid phase (VGCF3 and VGCF4). However, no significant corrosion was observed with the supercritical carbon dioxide (VGCF5). Corrosion in SCFs has been discussed in the literature. Marr and Gamse [28] noted that the chlorine ion causes severe corrosion in supercritical water and, therefore, corrosive supercritical fluids should be used with caution. Alumina reactors were suggested for use instead of alloy reactors [29]. The study found that ceramic reactors are superior in corrosion resistance compared to the conventional reactors.

3.2. Characterizations of SCF-treated VGCFs Fig. 4 shows the VGCF adsorption isotherms of nitrogen at 77 K. As can be seen, all the isotherms had a similar pattern, which suggest that a similar adsorption behavior was performed for the original VGCFs and the treated VGCFs. In addition, isotherms of VGCF3 and VGCF5 were found to be very close. A maximum amount of adsorbed nitrogen is about 100 cc / g. The same case is also applied to the isotherms of VGCF1 and VGCF2 which have an adsorption amount of 30 cc / g nitrogen. These isotherm curves were utilized for the calculation of the BET surface area [30]. Fig. 5 lists the BET surface areas of the VGCFs. Approximately 35 m 2 / g can be obtained as the maximum surface area from the VGCF3, which increased 26 m 2 / g from the original VGCFs. As anticipated, VGCF3 and VGCF5 have an equivalent BET surface area due to their similar isotherm curves. The results also show that the pretreatment of HNO 3 followed by the treatment of the SCFs have a positive effect on creating pores in the VGCFs. In contrast, experiments without the oxidation of the VGCFs seemed to have only a minor effect and not much difference in terms of surface area was found between the treatment of supercritical water (VGCF1) and CO 2 (VGCF2). Figs. 6 and 7 show the pore size dis-

Fig. 5. BET surface area of the VGCFs.

tributions of the VGCFs analyzed by the D–H method [31]. It was found that mesopores were mainly created in the VGCF3 and VGCF5 (Fig. 6) while the pore size was slightly modified in the range of 1 to 5 nm in the VGCF1, VGCF2 and VGCF4 (Fig. 7). These results suggested that there are no significantly created micropores and the treatments could only roughen the surface of the VGCFs. The BET surface area and pore size distribution are two important indexes for adsorbents. Activated carbon fibers (ACFs) normally possess a high surface area, which might be more than 1000 m 2 / g [32]. When compared with the ACFs, the activated VGCFs by the current treatments did not have sufficient micropores and mesopores. This might be because of the difficulty in oxidizing the graphite-like material which is the main component of the VGCFs. Therefore, advanced treatments for activation of the VGCF are needed for micromolecule adsorption. Elemental analysis and XPS were carried out for examining the composition of the VGCFs in the bulk and surface regions, respectively. The quantitative analysis of the VGCFs by

Fig. 4. Adsorption isotherms of nitrogen (77 K) for the VGCFs.

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Fig. 6. Pore size distributions of VGCF0, VGCF3 and VGCF5.

Fig. 7. Pore size distributions of VGCF0, VGCF1, VGCF2 and VGCF4.

elemental analysis is shown in Table 2. The original VGCF (VGCF0) has a high purity of carbon-containing material, which consists of 97.3% carbon, 0.08% hydrogen and 0.22% nitrogen by weight. The contaminants (2.38%) were believed to be catalyst particles which were encapsulated in the tip of the VGCFs [33,34]. All the treated VGCFs had no significant change in terms of hydrogen and nitrogen but, in VGCF3 and 4 and 5, other elements

such as oxygen in the VGCFs or contaminants from the corrosion of the reactor increased. The XPS results in Table 3 reveal that in the surface region of the VGCFs, oxygen was found in VGCF1, VGCF3, VGCF4 and VGCF5. A maximum oxygen composition of 22.5% was obtained from VGCF3. However, other elements such as Ni, Cr and Mo were also detected in the same sample, which suggests that a certain proportion of oxygen was

Table 2 Element analysis of VGCFs Element

VGCF0

VGCF1

VGCF2

VGCF3

VGCF4

VGCF5

C (%) H (%) N (%) Others (%)

97.32 0.08 0.22 2.38

97.61 0.01 0.18 2.20

97.53 0.01 0.15 2.31

64.78 0.26 0.23 34.73

71.46 0.70 0.15 27.69

92.59 0.01 0.09 7.31

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VGCF0

VGCF1

VGCF2

VGCF3

VGCF4

VGCF5

C (%) H (%) N (%) O (%) Others (%)

100.0 0.0 0.0 0.0 0.0

97.2 0.0 0.0 2.8 0.0

100.0 0.0 0.0 0.0 0.0

66.1 0.0 0.0 22.5 11.4

88.7 0.0 0.0 7.5 3.8

98.1 0.0 0.0 1.7 0.2

from the metal oxide substances. These impurities are surely derived from the corrosion of the reactor. As a result, we think that only a small amount of oxygen functional groups were grafted onto the surface of the VGCFs. This could be found in the XPS results of VGCF1 which had no evidence of corrosion.

3.3. Adsorption of benzene and phenol in the VGCFs The isotherms of benzene adsorption in the VGCFs and ACF at 258C are shown in Fig. 8. VGCF3 and VGCF5 exhibited a higher adsorbed volume of benzene, which is |20.5 cc / g at the relative pressure of 90 Torr whilst VGCF0, VGCF1, VGCF2 and VGCF4 possessed a similar adsorption capacity, which is roughly 5.0 cc / g at the relative pressure of 90 Torr. The results revealed that the adsorption capacity of benzene in the VGCFs can be enhanced after the activation treatment, especially by treatments of HNO 3 followed by SCF treatments of H 2 O or CO 2 . Furthermore, the results suggested that the amount of adsorbed benzene increased as the surface area of the VGCF increased. However, in comparison with the ACFs, the ACFs possess a relatively large capacity of benzene (173.5 cc / g at 90 Torr) due to their high surface area. Fig. 9 shows the adsorbed amount of phenol in the VGCFs and

ACFs in a 10 ppm phenol aqueous solution. As expected, there is a fairly low adsorbed amount of phenol in the VGCFs. This is because the VGCFs not only have a relatively low surface area for the adsorption of phenol but also have a high hydrophobic property, which is not appropriate for the adsorption of hydrophilic substances in an aqueous / hydrophilic solution. This evidence can be seen from the floating / suspended VGCFs in the solution after the adsorption experiments while the ACFs were soaked in the solution. The benzene and phenol adsorption results have shown that the VGCFs might not be suitable adsorbents for the adsorption of micromolecules under the current treatments because of insufficient micropores and low internal surface area. However, the study can be extended to the adsorption of macromolecules, which do not require a high microporosity. Adsorption studies of macromolecules such as proteins, bacteria or polymers have increased in recent decades [35–37] in the field of biotechnology, food processing and water treatments. The VGCF could be used as an adsorbent for macromolecules because VGCFs are thin and fine carbon fibers which possess a high external surface area. In addition, the properties of the VGCFs such as good hydrophobicity and high electrical and thermal conductivities [2–5] could assist the physical and chemical

Fig. 8. Adsorption isotherms of benzene vapor at 258C in the VGCFs and ACF.

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Fig. 9. Adsorption capacity of phenol in the VGCFs and ACF.

adsorptions of the macromolecules. This study should be carried out in the future.

4. Conclusions Supercritical water and carbon dioxide have been used to study the activation of VGCFs. VGCFs with a mean diameter of 150 nm and a length of a few micrometres were employed for the oxidation using hydrochloric acid or nitric acid and the treatment of SCFs. The results have shown that, for the range of the experimental conditions studied, a certain amount of mesopores was created in the treated VGCFs which have a maximum BET surface area of 35 m 2 / g. The adsorption of benzene vapor revealed that the adsorption capacity increased with increasing surface area of the VGCFs while the adsorption results of phenol showed that there is not much difference in all the VGCFs. Oxygen was detected in the treated VGCFs which suggested that the fibers contained a small amount of oxygen functional groups. However, highly porous VGCFs grafting a large amount of functional groups could not be produced without further treatments. In addition, the SCF experiments found that the alloy reactor was corroded due to the utilization of hydrochloric acid and nitric acid. All corrosive supercritical fluids should be used with caution.

Acknowledgements The authors would like to thank Mr. Takao Fujii and Mr. Tatsuro Tsuru at the Suzuki, Sakoda & Sakai Laboratory in the IIS for their technical assistance and discussions of the nitrogen, benzene and phenol adsorption experiments. Ms. Kaoru Higashi at the Oda Laboratory in the IIS is also appreciated for the XPS experiments. The Showa

Denko Co., Japan, is acknowledged for providing the VGCFs.

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