CO 2 Adsorption Properties of Activated Carbon Fibres under Ambient Conditions

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1 621 CO 2 Adsorption Properties of Activated Carbon Fibres under Ambient Conditions Yoshitaka Nakahigashi 1, Hirofumi Kanoh 1,*, Tomonori Ohba 1, Masumi Baba 1, Yoshiyuki Hattori 2, Naoya Inoue 3 and Masafumi Morimoto 3 (1) Department of Chemistry, Graduate School of Science, Chiba University, Chiba, Japan. (2) Department of Applied Chemistry, Faculty of Textile, Shinshu University, Ueda, Japan. (3) Marketing Department, Malvern Japan, Kobe, Japan. (Received 17 August 212; revised form accepted 4 October 212) ABSTRACT: In this study, we investigated the possibilities of using activated carbon fibre (ACF) as a carbon capture and storage (CCS) technology. The CO 2 adsorption isotherms of ACFs with different porosities were systematically examined at 273 and 298 K under ambient pressure conditions. The porosities of the ACFs were characterized by the adsorption of nitrogen at 77 K. We analyzed the adsorption capabilities of three types of ACFs (A5, A1 and A2) having different slit-shaped pore widths, specific surface areas and micropore volumes. Our results reveal that A5 had ultramicropores and achieved a higher adsorption of CO 2 at low relative pressure (<.15) at 273 K. However, A1, which had an average pore width of.9 nm, exhibited the highest adsorption capacity of 195 mg g 1 at a higher pressure of about 1 kpa, which is a relatively high value compared with that of conventional activated carbon. By establishing the temperature dependence of CO 2 adsorptivity and using Dubinin Radushkevich analysis, we characterized the interaction energy between pores and CO 2 molecules. Our results shed light on the fundamental aspects of CO 2 adsorption of ACFs, moving them towards being a viable CCS. 1. INTRODUCTION Carbon capture and storage (CCS) will play an important role in reducing the levels of greenhouse gas emissions in the atmosphere in an effort to avoid permanent and irreversible damage to the ecological system caused by the combustion of fossil fuels. Chemical absorption using amines is currently being used for the separation of CO 2 in industrial processes such as the sweetening of natural gas as well as in the production of hydrogen or ammonia, and is considered to be the most ready-touse technology for post-combustion CO 2 capture. However, this technology in its present state has a number of drawbacks, such as the high energy requirement associated with sorbent regeneration, amine losses due to evaporation, corrosion, and thermal and chemical degradation of the amines in the presence of oxygen (Plaza et al. 21). In contrast to amine scrubbing, adsorption as a separation technology has the potential to reduce the cost of post-combustion CO 2 capture. Zeolite13X is the adsorbent most extensively studied in CO 2 separation processes because of its high selectivity towards the removal of CO 2 (Cavenati et al. 24). However, several studies in the literature have examined the use of activated carbons (Presser et al. 211), which present important advantages over zeolites, such as greater hydrophobicity, significantly lower cost and lower energy requirements for regeneration. *Author to whom all correspondence should be addressed. kanoh@pchem2.s.chiba-u.ac.jp

2 622 Yoshitaka Nakahigashi et al./adsorption Science & Technology Vol. 3 No CO 2 adsorption has been used to evaluate narrower micropores in microporous materials (Rodríguez-Reinoso et al. 1989; Lozano-Castelló et al. 24). However, studies assessing the applicability of activated carbons as a CO 2 adsorbent under ambient conditions began only relatively recently as evidenced by the small number of relevant references in the literature. The CO 2 adsorption capabilities of carbide-derived carbons with different pore sizes have been examined in detail (Presser et al. 211), and results of various studies have shown carbon molecular sieves to exhibit high adsorptivity towards the removal of CO 2 (Wahby et al. 21; Silvestre-Albero et al. 211). Activated carbon fibres (ACFs) are so widely used and commercialized that their physicochemical properties have relatively been well defined. Studying the adsorptivity of ACFs towards the removal of CO 2 will promote the use of activated carbons as a CCS technology. In the present study, we systematically examined CO 2 adsorption isotherms of different ACFs at 273 and 298 K under ambient pressure to gather fundamental data useful for evaluating them as a CCS technology. 2. EXPERIMENTAL SETUP Pitch-based ACFs, A5, A1 and A2, were supplied from AD ALL Co. Ltd. and were used as adsorbents. The ACFs were pre-treated at 423 K under vacuum for 2 hours before performing the adsorption experiments. Adsorption isotherms of N 2 at 77 K were measured using AUTOSORB-1-MP (Quantachrome Instruments), while adsorption isotherms of CO 2 at 273 and 298 K were measured using AUTOSORB-iQ (Quantachrome Instruments) and BELSORP Mini (BEL Inc., Japan). 3. RESULTS AND DISCUSSION The porosities of the three different types of ACFs were characterized through N 2 and CO 2 adsorption isotherms at 77 K and 273 K, respectively. Figure 1 shows the N 2 adsorption isotherms at 77 K. The three isotherms are of type I, indicating the presence of micropores in the ACFs. These results indicate that A5 and A1 have narrower micropores than A2, as appreciable uptakes were observed at very low relative pressures (<1 5 ) in their isotherms. The isotherms were analyzed using α s plots (Kaneko 1994). Table 1 summarizes the pore parameters. The average pore width for each sample was calculated by assuming a regular slit-shaped pore for the ACFs. The CO 2 adsorption isotherms at 273 K also reflect the porosities of each sample. Higher adsorption was observed for A5 at P/P <.15 (Figure 2), which arises from the narrower pore width of A5 and agrees well with the known order of pore width, i.e. A5 < A1 < A2. However, under higher relative pressure, the amount of CO 2 adsorbed on A1 was greater than that on A5 because of its greater pore volume, which plays an important role in determining the saturation capacity. The CO 2 adsorptivity of A1 and A2 tended to increase above P/P >.3 because of their greater micropore volume, although the CO 2 adsorption of A5 was nearly saturated at that pressure. The volume of CO 2 adsorbed (195 mg g 1 ) on A1 at 273 K and 1 kpa is a relatively large amount compared with the amounts adsorbed by conventional activated carbons (Blanco López et al. 2; Lozano-Castelló et al. 24; Ravikovitch and Neimark 25; Sun et al. 27), although carbide-derived carbons and carbon molecular sieves exhibit even higher adsorptivities towards CO 2 (Wahby et al. 21; Presser et al. 211; Silvestre-Albero et al. 211).

3 CO 2 Adsorption Properties of ACFs under Ambient Conditions 623 Adsorbed amount of N 2 /mg g P/P P/P Figure 1. N 2 adsorption isotherms of ACFs: circles, A5; triangles, A1; squares, A2. (a) Linear plot, (b) semi-log plot. TABLE 1. Pore Parameters of Activated Carbon Fibres Obtained from N 2 Adsorption Isotherms at 77 K Pore width (nm) Specific surface area (m 2 g 1 ) Micropore volume (cm 3 g 1 ) A A A Adsorbed amount of CO 2 /mg g P/P P/P Figure 2. CO 2 adsorption isotherms of ACFs at 273 K: circles, A5; triangles, A1; squares, A2. (a) Linear plot, (b) semilog plot. The temperature dependence of ACFs towards the adsorption of CO 2 was also examined, whose relative isotherms at 273 K and 298 K are shown in Figure 3. The three isotherms at 298 K have shapes similar to those collected at 273 K. However, the amounts of adsorbed CO 2 are lower than those at 273 K because of physical adsorption. In particular, the isotherm of A5 at 298 K shows a

4 624 Yoshitaka Nakahigashi et al./adsorption Science & Technology Vol. 3 No Adsorbed amount of CO 2 /mg g P/kPa Figure 3. CO 2 adsorption isotherms of activated carbon fibres: circles, A5; triangles, A1; squares, A2. Filled symbols: 273 K, Open symbols: 298 K. marked decrease in the amount of adsorbed CO 2, suggesting the greater temperature dependence. By applying the Clausius Clapeyron equation to these adsorption data, the isosteric heat (q st ) values of CO 2 adsorption of the three ACFs were obtained (Figure 4). As expected from the temperature dependence observed in the data in Figure 3 and the adsorption potential related to 5 4 q st /KJ mol H sub H vap Adsorbed amount/mg g 1 12 Figure 4. Isosteric heat (q st ) of CO 2 on activated carbon fibres: circles, A5; triangles, A1; squares, A2. Dashed lines represent enthalpy changes of sublimation ( H sub ) and vapourization ( H vap ) of CO 2, respectively.

5 CO 2 Adsorption Properties of ACFs under Ambient Conditions 625 pore width, A5 indicated the highest q st value > 4 kj mol 1. The q st value for A1 was smaller than that for A5 and similar to H of sublimation of CO 2 and that for A2 was lower than H of the vapourization of CO 2. The dependence of q st on the amount of CO 2 adsorbed is modest and nearly constant, although the data for A5 are not as tight and those for A1 gradually decrease with increasing CO 2 adsorption. The desorptivity is also an important consideration for practical use. A5 exhibited perfect reversibility as indicated by its desorption isotherm, even though its q st value is > 4 kj mol 1. Some kinds of zeolite require large amounts of energy to regenerate and desorb CO 2 molecules because the interactions are so strong (Chue et al. 1995; Silvestre-Albero et al. 211), but ACFs require only a decrease in CO 2 pressure at room temperature. To compare microporosity of the ACFs, the Dubinin Radushkevich (DR) equation [ln(w/w ) = -(RT/βE ) 2 [ln(p /P)] 2 ] was applied to the isotherm data, where W, W, β and E are the amounts of adsorbed CO 2, the saturation amount, the affinity coefficient and the characteristic energy, respectively. Figure 5 shows the DR plots for N 2 adsorption at 77 K and CO 2 adsorption at 273 K. The curves for A5 and A1 in the N 2 adsorption isotherms at 77 K exhibit better linearity in the range of [ln(p /P)] 2 than A2, which possesses a smaller amount of narrower micropores and a larger amount of wider pores. In contrast, the curves for all three ACFs show good linearity towards adsorption of CO 2 at 273 K. Using the linear portion from 2 to 5 of [ln(p /P)] 2 for each curve, W and βe were evaluated, the results of which are summarized in Table 2. The βe value reflects the strength of the interaction potential between pores and molecules and the value was In (W N2 /mg g 1 ) In (W CO2 /mg g 1 ) [In (P /P)] [In (P /P)] 2 Figure 5. Dubinin Radushkevich plots of N 2 (a) and CO 2 (b) adsorption: circles, A5; triangles, A1; squares, A2. TABLE 2. Dubinin Radushkevich Parameters of Activated Carbon Filters Obtained from N 2 and CO 2 Adsorption Isotherms N 2 A5 A1 A2 W /mg g βe /kj mol CO 2 A5 A1 A2 W /mg g βe /kj mol

6 626 Yoshitaka Nakahigashi et al./adsorption Science & Technology Vol. 3 No found to decrease (A5 > A1 > A2) with increasing pore width in both adsorption isotherms. The βe of A5 for CO 2 adsorption was remarkably large, leading to q st at φ = e 1 = 28 kj mol 1, where φ is fractional filling. However, the W value for N 2 adsorption for the three ACFs was different from that for CO 2 adsorption. Because A5 has more ultramicropores and A2 contains relatively larger pores, W of A5 in CO 2 adsorption was larger than that of A2 while the opposite is true for N 2 adsorption at 77 K. Thus, the CO 2 adsorptivity of commercial ACFs can be understood from the viewpoint of pore parameters such as pore width. 4. CONCLUSIONS In this study, the CO 2 adsorptivity of commercial pitch-based ACFs with different pore widths were examined under ambient conditions. Under very low CO 2 pressure, the ACF featuring ultramicropores exhibited high adsorptivity while the sample with an average pore width of.9 nm exhibited the highest adsorptivity (195 mg g 1 ) at 1 kpa. The interaction between the pores and CO 2 molecules was elucidated by analyzing the temperature dependence of adsorptivity and the related DR plots. It is likely that certain mixtures of these ACFs will exhibit optimum adsorptivity of CO 2 for each individual adsorption system and set of conditions. Our results provide useful information about the practical use of commercial ACFs as a CCS technology. ACKNOWLEDGEMENT This work was partially supported by a Grant-in-Aid for Fundamental Scientific Research (B) (Grant No ) by the Japan Society for the Promotion of Science. REFERENCES Blanco López, M. C., Martínez-Alonso, A. and Tascón, J.M.D. (2) Carbon 38, Cavenati, S., Grande, C.A. and Rodrigues, A.E. (24) J. Chem. Eng. Data 49, 195. Chue, K. T., Kim, J. N., Yoo, Y. J., Cho, S. H. and Yang R. T. (1995) Ind. Eng. Chem. Res. 34, 591. Kaneko, K. (1994) J. Membrane Sci. 96, 59. Lozano-Castelló, D., Cazorla-Amorós, D. and Linares-Solano, A. (24) Carbon 42, Plaza, M. G., García, S., Rubiera, F., Pis, J.J. and Pevida, C. (21) Chem. Eng. J. 163, 41. Presser, V., McDonough, J., Yeon, S.-H. and Gogotsi, Y. (211) Energy Environ. Sci. 4, 359. Ravikovitch, P. I. and Neimark, A.V. (25) Adsorption 11, 265. Rodríguez-Reinoso, F., Garrido, J., Martin-Martínez, J. M., Molina-Sabio, M. and Torregrosa, R. (1989) Carbon 27, 23. Silvestre-Albero, J., Wahby, A., Sepúlveda-Escribano, A., Martínez-Escandell, M., Kaneko K., Rodríguez- Reinoso, F. (211) Chem. Commun. 47, 684. Sun, Y., Wang, Y., Zhang, Y., Zhou Y. and Zhou, L. (27) Chem. Phys. Lett. 437, 14. Wahby, A., Ramos-Fernández, J., Martínez-Escandell, M., Sepúlveda-Escribano, A., Silvestre-Albero, J., Rodríguez-Reinoso, F. (21) ChemSusChem. 3, 974.