© Copyright 1996 American Chemical Society
DECEMBER 25, 1996 VOLUME 12, NUMBER 26
Letters Activated Carbon Aerogels Y. Hanzawa and K. Kaneko* Department of Chemistry, Faculty of Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263, Japan
R. W. Pekala Lawrence Livermore National Laboratory, Livermore, California 94550
M. S. Dresselhaus Department of Electrical Engineering and Computer Science and Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received May 15, 1996. In Final Form: November 4, 1996X Activated carbon aerogels were obtained from the CO2 activation of carbon aerogels. The adsorption isotherms of nitrogen on activated carbon aerogels at 77 K were measured and analyzed by the highresolution RS plot to evaluate their porosities. The RS plot showed an upward deviation from linearity below RS ) 0.5, suggesting that the presence of micropores becomes more predominant with the extent of the activation. Activation increased noticeably the pore volume and the surface area (the maximum value: 2600 m2‚g-1) without change of the basic network structure of primary particles. Activated carbon aerogels had a bimodal pore size distribution of uniform micropores and mesopores.
Introduction Recently-developed activated carbons have rather uniform micropores of great pore volumes. Superhigh surface area carbons obtained by KOH activation1,2 and activated carbon fibers3-5 are representatives of new activated carbons. These new activated carbons have been applied not only to adsorption and separation technologies but also to electrochemical technology. It has been shown that the new activated carbons have characteristic Abstract published in Advance ACS Abstracts, December 15, 1996. X
(1) Otowa, T.; Tanibata, R.; Itoh, M. Gas Separation and Purification 1993, 7, 241. (2) Sosin, K. A.; Quinn, D. F. J. Porous Mater. 1995, 1, 111. (3) Kaneko, K.; Ishii, C.; Ruike, M.; Kuwabara, H. Carbon 1992, 30, 1075. (4) Kaneko, K. In Adsorption on New and Modified Inorganic Sorbents; Dabrowski, A., Tertykh, V. A., Eds.; Elsevier: Amsterdam, 1996; p 573. (5) Oya, A.; Yoshida, S.; Alcaniz-Monge, J.; Linares-Solano, A. Carbon 1995, 33, 1085.
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structures and properties.6-8 Moreover, the coexistence of uniform micropores and mesopores can extend their potential for applications. Carbon aerogels prepared by Pekala et al.9,10 have the monolith form and their physical properties have been studied by Dresselhaus et al.11,12 This carbon aerogel has a network structure of primary carbon particles, providing predominant mesopores. These carbon particles have slight micropores. The network (6) Ishii, C.; Matsumura, Y.; Kaneko, K. J. Phys. Chem. 1995, 99, 5743. (7) Ruike, M.; Kasu, T.; Setoyama, N.; Suzuki, T.; Kaneko, K. J. Phys. Chem. 1994, 98, 9594. (8) Nakayama, A.; Suzuki, K.; Enoki, T.; Ishii, C.; Kaneko, K.; Endo, M.; Shindo, N. Solid State Commun. 1995, 93, 323. (9) Pekala, R. W.; Alviso, C. T. Mater. Res. Soc. Symp. Proc. 1992, 270, 3. (10) Pekala, R. W.; Alviso, C. T.; Kong, F. M.; Hulsey, S. S. J. NonCryst. Solids 1992, 145, 90. (11) Fung, A. W. P.; Wang, Z. H.; Dresselhaus, M. S.; Dresselhaus, G.; Pekala, R. W.; Endo, M. Phys. Rev. B 1994, 49, 17325. (12) Reynold, G. A. M.; Fung, A. W. P.; Wang, Z. H.; Dresselhaus, M. S.; Pekala, R. W. Phys. Rev. B 1994, 50, 18590.
© 1996 American Chemical Society
6168 Langmuir, Vol. 12, No. 26, 1996
Letters
Table 1. Surface Properties of Carbon Aerogels sample
activation time/h
burn-off/%
at/m2‚g-1
Vt/mL‚g-1
CA a-CA-13 a-CA-49 a-CA-72
1 5 7
12.5 48.6 71.6
713 1314 2260 2600
1.301 1.625 2.714 2.634
micropore ami/m2‚g-1 Vmi/mL‚g-1 366 960 1750 1390
0.115 0.296 0.676 0.656
mesopore ams/m2‚g-1 Vms/mL‚g-1 347 354 510 940
1.186 1.329 2.038 1.978
structure and the size of the primary particle can be controlled by the sol-gel chemistry.9,10 If we can donate a uniform microporosity to the mesoporous carbon aerogel without change of the skeletal carbon gel structure, a promising new carbon having a wide variety of functions can be obtained. However, there are few examples of activated carbon having a bimodal distribution in the micropore and mesopore ranges.13 This Letter reports an unusual activated carbon aerogel having a macroscopic block form regardless of the high porosity. Experimental Section Preparation of Activated Carbon Aerogel. The preparation of carbon aerogels was carried out by the Pekala method.9 Resorcinol-formaldehyde (RF) gels were derived from the sol-gel polymerization of resorcinol and formaldehyde with a slight amount of sodium carbonate as a basic catalyst. The molar ratio of resorcinol (R) to catalyst (C) was held at R/C ) 200. The RF aerogels were dried under the supercritical condition with CO2, followed by the carbonization under N2 flow at 1323 K. The resultant vitreous black monoliths are carbon aerogels, which are denoted by CA in this Letter. The apparent density of CA, which was determined from the geometrical volume and weight of the monolith, was of 0.47 g‚cm-3. The activation of carbon aerogels under CO2 flow was carried out at 1173 K, with the following soak cycle: heating from an ambient temperature to 1173 K for 2 h, holding at 1173 K for a specified time, and cooling down to the ambient temperature. Activated carbon aerogels maintained the monolith form which is not fragile. The activated carbon aerogel will be designated a-CA-x in this Letter. Here, x is the burn-off expressed in terms of percentage. Measurement of N2 Adsorption Isotherm. The adsorption isotherm of nitrogen was measured gravimetrically at 77 K with the use of a computer-aided apparatus. The samples were evacuated at 383 K and 1 mPa for 2 h prior to the adsorption measurements. It took 3 h to attain each adsorption or desorption equilibrium upon measuring the hysteresis.
Figure 1. Adsorption isotherms of nitrogen at 77 K on activated carbon aerogels: O, CA; 0, a-CA-13; 4, a-CA-49; ), a-CA-72. Solid symbols denote desorption.
Results and Discussion The N2 adsorption isotherms are shown in Figure 1 for various values of the activation time at 1173 K in a stream of CO2. The longer the activation time, the greater the amount of adsorption. In particular, the uptake at relatively low pressure (P/P0) noticeably increases with increasing activation time. The adsorption isotherms have a characteristic adsorption hysteresis. The hysteresis loops of non-activated carbon aerogel (CA) and a-CA-13 are of type H1, coinciding with the network structure of agglomerates of uniform spherical particles. Activation for longer times tends to change the loop shape from type H1 to H2, suggesting the decrease of the primary particle size and a partial opening of the network structure. However, even such an activation treatment does not change the basic network structure, and the activated (13) Ghosal, R.; Kaul, D.; Boes, U.; Sanders, D.; Smith, D. M.; Maskara, A. Mater. Res. Soc. Symp. Proc. 1995, 371, 413.
Figure 2. High-resolution RS plots for the adsorption isotherms of nitrogen on activated carbon aerogels at 77 K: (a) CA; (b) a-CA-13; (c) a-CA-49; (d) a-CA-72.
carbon aerogel in a monolith form has both micropores and mesopores. Both microporosity and mesoporosity were separately evaluated using the subtracting pore effect14,15 (SPE) method for the high-resolution RS plots.16 The RS plots based on the standard nitrogen adsorption isotherm of a nonporous carbon are shown in Figure 2. Over the whole range, the characteristic development of porosity by an activation process is supported by the change of the RS plots. The separate determination (14) Kaneko, K.; Ishii, C. Colloid Surf. 1992, 67, 203. (15) Kaneko, K.; Ishii, C.; Rybolt, T. In Characterization of Porous Solids III; Rouquerol, J., Rodriguez-Reinoso, F., Sing, K. S. W., Unger, K. K., Eds.; Elsevier: Amsterdam, 1994; p 583. (16) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: London, 1982; Chapters 2 and 4.
Letters
method of the microporosity and mesoporosity is shown in Figure 2 using solid and broken lines. The solid line is passing through the origin, whose slope gives the total surface area. The intercept and slope of the broken line provide the micropore volume and the mesopore surface area including the macropore surface area, respectively. Here the specific surface area can be given by 57.4 × (slope) for the unit system of the amount of adsorption in mmol‚g-1. Below RS ) 0.5, the longer the activation time, the greater the upward deviation from the linearity of the solid straight line passing though the origin. This indicates the development of micropores. In the range of RS ) 0.7-1.5, the linear region of the RS plot on the broken line becomes narrower, and simultaneously the slope of this line increases with the activation time. As the activation proceeds, the micropores at the surface of the primary particle widen and become smaller mesopores; as a consequence of this, there is a slight change of the hysteresis shape in low relative pressure side. The RS plots show an explicit upward swing corresponding to the capillary condensation above RS ) 1.5. Here, it should be noted that the RS of the upward swing due to the capillary condensation is far from that of the upward swing due to cooperative adsorption in the wider micropore. Only on the RS plot of a-CA-72 can a straight line be drawn for RS > 1.9 (dot-dash line in Figure 2d). Hence, the open surface area of 273 m2‚g-1 and a total pore volume of 2.634 mL‚g-1 were determined from the slope and intercept of the line in the high RS range, respectively. However, other samples do not have such an apparent linear region in the RS plot above RS > 1.9. This is because the intense activation gives rise to both of the partial cleavage of the cross-linking structure and the decrease in the size of primary particles, and the surface area of macropores can be determined. In the almost plateau region in Figure 1, N2 adsorption increases
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by a small amount with increasing relative pressure after the capillary condensation is completed. This is indicative of a small macropore surface area. Accordingly, the total pore volume was approximated by the saturation amount, and the macropore surface area of CA, a-CA-13, and a-CA49 was neglected. With these approximations, the mesoporosity and microporosity of the carbon aerogels could be separately determined. The micropore volume Vmi, the micropore surface area ami, mesopore volume Vms, the mesopore surface area ams, and the total surface area at are collected in Table 1. As the activation process progresses, the total surface area at remarkably increases up to 2600 m2‚g-1. Both ami and Vmi increase with the activation time until 5 h and then become almost saturated over 5 h. On the other hand, ams and Vms increase as increasing the activation time until 5 h. This behavior arises from the development of mesopores from micropores upon the intense activation due to the partial cleavage of the network structure of primary particles. Conclusion The nitrogen adsorption isotherm of nonactivated carbon aerogel and slightly activated carbon aerogel shows a representative hysteresis of type H1, which stems from the network structure of primary carbon particles. Even the intense activation does not break the basic network structure, keeping the monolith form, and it donates the predominant microporosity to the primary particles. Acknowledgment. This work was partially supported by a Grant-in-Aid for Scientific Research on Priority Areas (Carbon Alloy) by the Ministry of Education, Science and Culture of Japan. LA960481T
