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Langmuir 1997, 13, 5802-5804
Lack of a Predominant Adsorption of Water Vapor on Carbon Mesopores Y. Hanzawa and K. Kaneko* Physical Chemistry, Material Science, Graduate School of Science and Technology, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263, Japan Received May 14, 1997. In Final Form: August 12, 1997X The mesoporosity and microporosity of carbon aerogels were controlled by the structure of the polymer precursor and by CO2 activation, respectively. Their adsorption isotherms of nitrogen at 77 K were measured, and the porosities were determined by the high-resolution RS-plot. The ratio of the micropore volume to the mesopore volume was 0.10 to 0.24. The adsorption isotherms of water on the carbon aerogels were also measured at 303 K. All water adsorption isotherms had an adsorption hysteresis near P/P0 ) 0.5. The hysteresis of the nonactivated sample is not marked, but activated samples showed a vertical and noticeable hysteresis. The saturated amounts of water adsorption were not close to the mesopore volume but the micropore volume. Hence, it was concluded that water molecules do not predominantly adsorb on carbon mesopores and that the adsorption hysteresis is not described by the Kelvin equation.
Introduction The water confined in a hydrophobic small space has received considerable attention from chemistry, biology, and geology. Water adsorbed in the carbon nanospace should be an important model of water confined in the hydrophobic environment. However, even water adsorption on activated carbon is not fully understood. The adsorption isotherm of water on activated carbon having micropores has a steep uptake with a clear adsorptiondesorption hysteresis in the medium- or high-relativepressure region. McBain et al. associated this noticeable uptake with the two-dimensional condensation.1 Dubinin et al. introduced a phenomenological model of the cluster formation on the hydrophilic site in order to understand the water adsorption isotherm on carbonaceous materials and threw some doubt on the capillary condensation mechanism.2 The hydrophilic model has been widely applied to the water adsorption isotherm of activated carbon.3-8 Both the McBain’s and Dubinin mechanisms are based on the assumption of the liquid state of water adsorbed in the carbon pore without any structural study. Recently, however, Iiyama et al. applied an in situ X-ray diffraction technique to water confined in carbon micropores, showing that a water molecular assembly has a solid-like structure even at 303 K with the aid of electron radial distribution function analysis.9,10 A French group also reported the same conclusion from the recent diffraction study of water-wetted activated carbon.11 Hence X
Abstract published in Advance ACS Abstracts, October 1, 1997.
(1) McBain, J. W.; Porter, J. L. K.; Sessions, R. F. J. Am. Chem. Soc. 1933, 55, 2294. (2) Dubinin, M. M.; Zawerina, E. D.; Serpinsky, V. V. J. Chem. Soc. 1954, 1760. (3) Freeman, J. J.; Tomlinson, J. B.; Sing, K. S. W.; Theocharis, C. R. Carbon 1995, 33, 795. (4) Mu¨ller, E. A.; Luis, F. R.; Vega, L. F.; Gubbins, K. E. J. Phys. Chem. 1996, 100, 1189. (5) Sing, K. S. W. In Fundamentals of Adsorption; Mersmann, A. B., Scholl S. E., Eds.; Engineering Foundation: New York, 1991; p 69. (6) Kaneko, K.; Kosugi, N.; Kuroda, H. J. Chem. Soc., Faraday Trans. 1 1989, 85, 869. (7) Dacey, J. R.; Evans, M. J. Carbon 1971, 9, 579. (8) Pierce, C.; Smith, R. N.; Wiley, J. W.; Cordes, H. J. Am. Chem. Soc. 1951, 73, 4551. (9) Iiyama, T.; Nishikawa, K.; Otowa, T.; Kaneko, K. J. Phys Chem. 1995, 99, 10075. (10) Iiyama, T.; Nishikawa, K.; Otowa, T.; Suzuki, T.; Kaneko, K. In Fundamentals of Adsorption; LeVan, M. D., Ed.; Kluwer Academic Publishers: Hingham, MA, 1996; p 401. (11) Bellissent-Funel, M.-C.; Sridi-Dorbez, R.; Bosio, L. J. Chem. Phys. 1996, 104, 10023.
S0743-7463(97)00498-8 CCC: $14.00
we need to reconsider the old problem of water adsorption on activated carbon. Carbon aerogels prepared by Pekala et al.12,13 have a monolith form, and their physical properties have been studied by Dresselhaus et al.14,15 Carbon aerogels, derived from pyrolysis of dried organic aerogels, are mainly mesoporous carbonaceous material, having a network structure made of carbon particles.16 Activation of carbon aerogels with CO2 at high temperature introduces a large amount of micropores with the characteristic network structure.17 Thus we can get carbon materials which have a bimodal pore size distribution from micropore to mesopore range. Furthermore, the pore size distributions of micropores and mesopores are quite sharp. Hence, water adsorption on the carbon mesopores can be examined in comparison with water adsorption on the carbon micropores. Experimental Section Resorcinol-formaldehyde (RF) gel was obtained by the addition condensation of resorcinol and formaldehyde with a slight amount of sodium carbonate as basic catalyst after Pekala’s method.12,13,16 The molar ratio of resorcinol (R) to catalyst (C) was R/C ) 200. This RF gel was dried under supercritical conditions with CO2, followed by pyrolysis under N2 flow at 1323 K. The carbon aerogel of the resultant vitreous black monoliths was obtained, which is denoted as CA in this letter. The activation of the carbon aerogel under CO2 flow was carried out at 1173 K.17 The activated carbon aerogels are denoted as a-CA-x. Here x is the burn-off by the activation. The adsorption isotherms of nitrogen and water were measured gravimetrically at 77 and 303 K, respectively, after the sample was evacuated below 1 mPa at 383 K for 2 h.
Results and Discussion Adsorption isotherms of nitrogen are shown in Figure 1. Adsorption isotherms of nitrogen were of type-IV and (12) Pekala, R. W.; Alviso, C. T. Mater. Res. Soc. Symp. Proc. 1992, 270, 3. (13) Pekala, R. W.; Alviso, C. T.; Kong, F. M.; Hulsey, S. S. J. NonCryst. Solids 1992, 145, 90. (14) Fung, A. W. P.; Wang, Z. H.; Dresselhaus, M. S.; Dresselhaus, G.; Pekala, R. W.; Endo, M. Phys. Rev. B 1994, 49, 17325. (15) Reynold, G. A. M.; Fung, A. W. P.; Wang, Z. H.; Dresselhaus, M. S.; Pekala, R. W. Phys. Rev. B 1994, 50, 18590. (16) Hanzawa, Y.; Yoshizawa, N.; Pekala, R. W.; Dresselhaus, M. S.; Kaneko, K. Adsorption, to be submitted. (17) Hanzawa, Y.; Kaneko, K.; Pekala, R. W.; Dresselhaus, M. S. Langmuir 1996, 12, 6167.
© 1997 American Chemical Society
Letters
Langmuir, Vol. 13, No. 22, 1997 5803 Table 1. Pore Structure of Carbon Aerogels micropore
mesopore
sample
at/m2‚g-1
Vt/mL‚g-1
ami/m2‚g-1
Vmi/mL‚g-1
ams/m2‚g-1
Vms/mL‚g-1
Vmi/Vms
VH2O/mL‚g-1
CA a-CA-41 a-CA-54
713 1828 1944
1.30 2.34 2.61
366 1308 1260
0.11 0.45 0.46
347 520 684
1.19 1.89 2.15
0.097 0.24 0.21
0.13 0.38 0.54
Figure 1. Adsorption isotherms of nitrogen on carbon aerogels at 77 K: (a) CA; (b) a-CA-41; (c) a-CA-54. Solid symbols denote desorption. Table 2. Pore Sizes and Condensation Pressures (P/P0)c Evaluated from the Kelvin Equation (P/P0)cmia sample
wmi/nm
CA a-CA-41 a-CA-54
0.63 0.68 0.73
N2
H2O
0.37 0.89 (0.20) 0.40 0.90 (0.22) 0.43 0.91 (0.25)
(P/P0)cmsa Rms/nm 6.8 7.3 6.3
N2
H2O
0.91 0.99 (0.86) 0.92 0.99 (0.87) 0.91 0.99 (0.85)
a The figures in parentheses denote the estimated values for the perfect wetting of θ ) 0°.
have characteristic adsorption-desorption hysteresis.18,19 It should be noted that all adsorption branches are almost vertical, suggesting that the mesopores are uniform and that the network structure of carbon particles is still preserved even after activation. The fundamental shape of the hysteresis loop is type-H1. The type-H1 shape is given by the agglomerates or compacts of uniform spherical particles having narrow pore size distribution.19 Strictly speaking, activation makes the adsorption rise a little gradual. Hence, activation develops not only micropores but also mesopores. The uptake of the amount adsorbed remarkably increases with the progression of activation due to development of micropores. The adsorption isotherms were analyzed by the subtracting pore effect (SPE) method20,21 using the high-resolution RS-plot.19 The pore structures of these carbon aerogels were determined separately, as shown in Table 1. The total surface area increases up to 1940 m2‚g-1 by activation. The total pore volume also increases with the progress of activation. The surface area and pore volume of micropores and mesopores were separately determined, as shown in Table 1. The volume ratio of micropores to mesopores for CA is 0.097, and it increases up to 0.24 by activation. Both the average slit width wmi of micropores and the average pore radius Rms of mesopores are listed in Table 2. The wmi value increases from 0.63 to 0.73 nm with the activation. The activation introduces micropores into carbon particles and (18) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603. (19) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: London, 1982. (20) Kaneko, K.; Ishii, C. Colloid Surf. 1992, 67, 203. (21) 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.
Figure 2. Adsorption isotherms of water on carbon aerogels at 303 K: (a) CA; (b) a-CA-41; (c) a-CA-54. Solid symbols denote desorption.
simultaneously makes micropores greater. The Rms of a-CA-41 is the greatest (7.28 nm). In case of a-CA-54, some mesopores are developed to be destroyed, and thereby the Rms of a-CA-54 is smaller than that of a-CA-41. Here, all pore volumes were obtained using liquid nitrogen density (0.808 g‚mL-1). However, Aukett et al.22 proposed the adsorbed nitrogen density of 0.93 g‚mL-1 from the molecular simulation. The solid density of nitrogen at 20.7 K is 1.026 g‚mL-1. Hence, the absolute quantities of the pore volume are not the above values. The errors should be about 10% at best. The water adsorption behavior is completely different from that of the nitrogen adsorption. The adsorption isotherms of water on carbon aerogels at 303 K are shown in Figure 2. The water adsorption isotherms of a-CA-41 and a-CA-54 are of type-V, having a noticeable uptake near P/P0 ) 0.5 as well as microporous carbons, and they have an explicit hysteresis. The adsorption hysteresis of a-CA samples has vertical adsorption and desorption branches. On the other hand, CA has a unique water adsorption isotherm different from others. The adsorption uptake near P/P0 ) 0.5 is gradual, and the adsorption hysteresis is not clear. The hysteresis loop is also different from others. The similar water adsorption isotherm on the low burn-off carbon was reported recently by Freeman et al.3 In these authors’ group, a similar result was obtained for a low burn-off activated carbon fiber.23 Hence, this behavior must be associated with the pore structure of the low burn-off activated carbon. Thus, the water adsorption mechanism on CA should be different from that on a-CA. The saturated amount of water adsorption, VH2O, was determined by the extrapolation of the isotherm to P/P0 ) 1.0, which is collected in Table 1. It is noteworthy that VH2O is much smaller than the total pore volume from nitrogen adsorption. On the contrary, VH2O is close to the micropore volume regardless of the presence of plenty of mesopores. The condensation pressure of the adsorption isotherm is evaluated by the Kelvin equation, when the capillary condensation occurs. The goodness of the Kelvin equation (22) Aukett, P. N.; Quirke, N.; Riddiford, S.; Tennison, S. R. Carbon 1992, 30, 913. (23) Kaneko, K.; Hanzawa, Y.; Iiyama, T.; Kanda, T.; Suzuki, T. In Adstracts of Pacific Basin Workshop on Adsorption Science and Technology; Chiba University: Chiba, Japan, 1997; p 2.
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is evidenced in the case of the nitrogen adsorption isotherm. The condensation relative pressure (P/P0)c,N calculated from the Kelvin equation is shown in Table 2 for nitrogen adsorption. The (P/P0)c,N values for mesopores are 0.91-0.92, agreeing well with the observed values. Therefore, the capillary condensation theory can describe the nitrogen adsorption isotherm very nicely. We estimated the condensation relative pressure (P/P0)c,W for water using the Kelvin equation. The liquid water cannot wet the carbon pore-wall. Hence we must take into account the contact θ for evaluation of (P/P0)c,W (θ ) 86° in the literature24 ). All calculated (P/P0)c,W values are almost 1, being far from the observed rising pressures. Even if we assume θ ) 0°, the obtained (P/P0)c,W value is 0.85-0.86, which is still far from the observed one. The routine application of the Kelvin equation to the micropores leads to the apparent condensation pressure (P/ P0)c,Wmi. The (P/P0)c,Wmi values are far from the observed rising pressures, too. Thus, the Kelvin equation cannot interpret the steep adsorption rising of the water adsorption isotherm. The agreement of VH2O and Vmi also indicates inapplicability of the Kelvin equation to analysis of the water adsorption isotherm. Accordingly water vapor does not predominantly adsorb on the activated carbon by the capillary condensation mechanism. The observed steep adsorption of water at the medium P/P0 should be (24) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; Wiley Interscience Publication: New York, 1990.
Letters
associated with the special filling mechanism of water molecules. Iiyama et al. showed the solid-like structure of the water molecular assembly in the carbon micropore with in situ X-ray diffraction.9,10 The clusters of water molecules should be associated with each other to form the solid-like structure with the progress of filling. There is a possibility that the micropore size of CA is fit for the unit size of the water molecular cluster, giving rise to no marked hysteresis. It is noteworthy that CA and a-CA41 show completely different water adsorption isotherms irrespective of the only slight difference of the slit width (0.63 and 0.68 nm). This critical change in the water adsorption of hysteresis should be caused by the cluster size of water molecules. So far we do not completely understand the reason for the difference of the adsorption hysteresis with the pore width. However, we can conclude that the steep adsorption uptake of water vapor on activated carbon at the middle P/P0 is not ascribed to the capillary condensation and that there is a critical micropore size for giving the explicit adsorption hysteresis. Acknowledgment. This work was partially supported by a Grant-in-Aid for Scientific Research on Priority Areas (Carbon Alloy: No. 08355027) by the Ministry of Education, Science and Culture, Japanese Government, and a grant from Tokyo OHKA Foundation for the Promotion of Science and Technology. LA970498R
