Activation of coal tar pitch carbon fibres: Physical activation vs. chemical activation
Introduction
Chemical activation with KOH or NaOH is an effective method to prepare activated carbon materials [1], [2], [3]. Both hydroxides are very effective activating compounds in different carbon materials: coal [1], bituminous [4], anthracite [2], [3], [5], nanotubes [6], etc. Chemical activation presents several advantages and disadvantages compared to physical activation. The main advantages are the higher yield, lower temperature of activation, less activation time and generally, higher development of porosity. Among the disadvantages, the activating agents are more expensive (KOH and NaOH vs. CO2 and H2O) and it is also necessary an additional washing stage. Moreover, these hydroxides are very corrosive. Physical activation with carbon dioxide or steam is the usual procedure to obtain activated carbon fibres (ACF). Chemical activation of carbon fibres by ZnCl2, AlCl3, H3PO4, H3BO3,…, has been reported [7]. However, the studies on chemical activation by KOH and NaOH are scarce [8], [9], [10], [11] and focus on PAN-based carbon fibres as precursors. Then, there are not papers dealing with activation of general purpose carbon fibres. In the present work ACF with high surface area are obtained by physical (CO2) and chemical activation (KOH and NaOH) of commercial carbon fibres from coal tar pitch.
Section snippets
Raw material
Milled carbon fibres from an isotropic coal tar pitch precursor were used (DONACARBO·S-241, Osaka gas Co., Ltd). The mean fibre length and diameter are 0.13 mm and 13 μm respectively. The ash content for the raw material is 0.03 wt%.
Physical activation
A given weight of Donac carbon fibres was placed into a horizontal cylindrical furnace (65 mm i.d.) under flowing nitrogen (100 ml/min). For most activations, approximately 2.5 g of fibres were used; however, some activations used 3.6–5.1 g. The furnace was purged
Chemical activation
ACF prepared by chemical activation (CA) show isotherms of type I (Fig. 1). The knees of the isotherms are wider with increasing the hydroxide/carbon ratio and, therefore, the pore size distributions (PSD) are also wider as can be seen with the DFT PSD (Fig. 2, Fig. 3). Total pore volume also increases with the ratio showing a development of porosity (Table 1). In the case of NaOH and for high ratios (6:1 and 8:1), the adsorption capacity and the proportion of mesopores obtained are
Conclusion
Chemical activation of carbon fibres by hydroxides is more appropriate than physical activation to reach high porosity development, maintaining the fibre morphology and a reasonable yield. In addition it is more selective than physical activation for the materials studied. Pore size distributions of the samples prepared by KOH are narrower than those obtained by NaOH. Nevertheless, NaOH develops more porosity than KOH for the same hydroxide/carbon fibres ratio. Furthermore, the surface of the
Acknowledgments
The authors thank MCYT the financial support (Project MAT 2000-0621). J.A.M.-A. thanks Universidad de Alicante for the Ph.D. Thesis fellowship. B.C.M. would like to thank the National Science Foundation's International Research Fellowship program for the support in this research.
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