An Experimental and Molecular Simulation Study of the Adsorption of

Dec 29, 2010 - K. Vasanth Kumar , Kathrin Preuss , Zheng Xiao Guo , and M. ... Molecular Simulation of CO2 Adsorption in the Presence of Water in ...
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An Experimental and Molecular Simulation Study of the Adsorption of Carbon Dioxide and Methane in Nanoporous Carbons in the Presence of Water Pierre Billemont,† Benoit Coasne,*,‡ and Guy De Weireld*,† †

Thermodynamics Department, Facult e Polytechnique, UMons, Universit e de Mons, 20 place du Parc, 70000 Mons, Belgium, and ‡Institut Charles Gerhardt Montpellier, CNRS (UMR 5253), ENSCM, Universit e Montpellier 2, 8, rue de l’Ecole Normale, 34096 Montpellier, France Received August 4, 2010. Revised Manuscript Received December 6, 2010

The adsorption of carbon dioxide and methane in nanoporous carbons in the presence of water is studied using experiments and molecular simulations. For all amounts of adsorbed water molecules, the adsorption isotherms for carbon dioxide and methane resemble those obtained for pure fluids. The pore filling mechanism does not seem to be affected by the presence of the water molecules. Moreover, the pressure at which the maximum adsorbed amount of methane or carbon dioxide is reached is nearly insensitive to the loading of preadsorbed water molecules. In contrast, the adsorbed amount of methane or carbon dioxide decreases linearly with the number of guest water molecules. Typical molecular configurations obtained using molecular simulation indicate that the water molecules form isolated clusters within the host porous carbon due to the nonfavorable interaction between carbon dioxide or methane and water.

1. Introduction Research efforts in the field of carbon dioxide capture and storage (CCS) have significantly increased in the past years.1,2 Although the exploitation of CCS technologies is probably limited in the next 2 or 3 decades, it may significantly contribute to the reduction of CO2 emissions. Among nanoporous solids that can be used as confining hosts, coals which are mainly composed of carbon are very promising candidates as they are present in a very large amount in sedimentary basins. From an industrial point of view, a conceivable process would be to store supercritical CO2 in unminable coal seams as it might, in the same time, provoke the liberation of methane that is naturally stored in these coal seams. The quantity of natural gas produced from coal beds (coal bed methane or CBM) increases every year, and large commercial projects operate in the United States, China, and Australia. The process is not yet a mature technology as many of its fundamental and practical aspects still need to be understood and developed in order to enhance the recovery process. Laboratory research is crucial in order to assess the adsorption/ desorption mechanism and the storage capacity, swelling, and permeability of the host material to understand and apply the process.3,4 At low to intermediate pressures, most of the natural gas contained in coals is stored as an adsorbed phase in the micropores. It is therefore crucial to understand the adsorption behavior of CO2 and CH4 to predict and determine the phase diagram *To whom correspondence should be addressed: e-mail benoit.coasne@ enscm.fr, Tel þ33 4 67 16 3459, Fax þ33 4 67 16 3470 (B.C.); e-mail guy. [email protected], Tel þ32 65 374 203, Fax þ32 65 374 209 (G.D.W.). (1) IPCC. In Metz, B., Davidson, O., de Coninck, H. C., Loos, M., Meyer, L. A., Eds.; IPCC Special Report on Carbon Dioxide Capture and Storage; Prepared by Working Group III of the Intergovernmental Panel on Climate Change; Cambridge University Press: Cambridge, UK, 2005. (2) Aaron, D.; Tsouris, C. Sep. Sci. Technol. 2005, 40, 321. (3) White, C. M.; Smith, D. H.; Jones, K. L.; Goodman, A. L.; Jikich, S. A.; La Count, R. B.; Du Bose, S. B.; Ozdemir, E.; Morsi, B. I.; Schroeder, K. T. Energy Fuels 2005, 19, 659. (4) Mazzotti, M.; Pini, R.; Storti, G. J. Supercrit. Fluid. 2009, 47, 617.

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of pure CO2, pure CH4, and their mixture confined in porous carbons before modeling and making predictions for practical field applications. When studying such systems, it is also important to take into account the presence of water trapped in the coal seams as it has a significant effect on the CO2 adsorption capacity of coals. Many experiments performed in the laboratory have attempted to reproduce as closely as possible the underground temperature and pressure conditions in the coal seam with both dry coal samples5-14 and moisture-equilibrated samples.7,15,16 In the specific case of coal, Krooss et al.7 investigated the influence of water on the CO2 and CH4 adsorption capacities. The methane sorption capacity of moisture-equilibrated coals was found to be 20-25% lower than that of dry coals. On the other hand, the effect of moisture on CO2 adsorption was found to be more complex as different shapes of the adsorption isotherm were obtained for the dry and moisture-equilibrated coals. Methane measurements reported by Hildenbrand et al.17 on dry and wet materials show a significant reduction up to 50% of the methane adsorption capacity when water is adsorbed. In the past few years, (5) De Gance, A. E.; Morgan, W. D.; Yee, D. Fluid Phase Equilib. 1993, 82, 215. (6) Krooss, B. M.; van Bergen, F.; Gensterblum, Y.; Siemons, N.; Pagnier, H. J. M.; David, P. Int. J. Coal Geol. 2002, 51, 69. (7) Busch, A.; Gensterblum, Y.; Krooss, B. M. Int. J. Coal Geol. 2003, 55, 205. (8) Busch, A.; Gensterblum, Y.; Krooss, B. M.; Littke, R. Int. J. Coal Geol. 2004, 60, 151. (9) Fitzgerald, J. E.; Pan, Z.; Sudibandriyo, M.; Robinson, R. L.; Gasem, K. A. M.; Reeves, S. Fuel 2005, 84, 2351. (10) Bae, J. S.; Bhatia, S. K. Energy Fuels 2006, 20, 2599. (11) Ottiger, S.; Pini, R.; Storti, G.; Mazzotti, M.; Bencini, R.; Quattrocchi, F.; Sardu, G.; Deriu, G. Environ. Prog. 2006, 25, 355. (12) Siemons, N.; Busch, A. Int. J. Coal Geol. 2007, 69, 229. (13) Day, S.; Sakurovs, R.; Weir, S. Int. J. Coal Geol. 2008, 74, 203. (14) Day, S.; Duffy, G.; Sakurovs, R.; Weir, S. Int. J. Greenhouse Gas Control 2008, 2, 342. (15) Ottiger, S.; Pini, R.; Storti, G.; Mazzotti, M. Adsorption 2008, 14, 539. (16) Ottiger, S.; Pini, R.; Storti, G.; Mazzotti, M. Langmuir 2008, 24, 9531. (17) Hildenbrand, A.; Krooss, B. M.; Busch, A.; Gaschnitz, R. Int. J. Coal Geol. 2006, 66, 179. (18) Goodman, A. L.; Busch, A.; Bustin, R. M.; Chikatamarla, L.; Day, S.; Duffy, G. J.; Fitzgerald, J. E.; Gasern, K. A. M.; Gensterblum, Y.; Hartman, C.; Jing, C.; Krooss, B. M.; Mohammed, S.; Pratt, T.; Robinson, R. L.; Romanov, V.; Sakurovs, R.; Schroeder, K.; White, C. M. Energy Fuels 2004, 18, 1175.

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two interlaboratory comparison studies of high-pressure CO2 sorption were performed for five Argonne Premium coals covering a large maturity range from 0.25% to 1.68% vitrinite reflectance.18,19 The adsorption isotherms were obtained by different research groups using different classical techniques (manometric, volumetric, gravimetric methods). Large deviations, which were observed for both dry and moisture-equilibrated samples, were attributed to the different moisture contents. A well-characterized activated carbon sample, Filtrasorb 400 (F400), was then selected for a more recent interlab comparison.20 The choice of an activated carbon was motivated by the fact that it allows avoiding residual moisture, swelling effect, and absorption in the matrix. Consequently, the activated carbon appears as an ideal candidate to investigate the origin of the differences observed in the sorption capacities and the effect of water on adsorption. From a molecular simulation point of view, many studies have been reported on the adsorption and confinement of CO2 in clays,21 mica,22 zeolites,23,24 nanoporous silicas,25-28 metalorganic frameworks or zeolitic imidazolate frameworks,29-36 and porous carbons.37-43 In the specific case of porous carbons, Tenney and Lastoskie39 investigated the influence of surface heterogeneity on the adsorption of CO2 in activated carbons and coal. These authors found that the amount of adsorbed CO2 increases with an increasing surface concentration of oxygen atoms and that the adsorption on disordered surfaces is enhanced as compared to that on homogeneous graphitic surfaces. Later, Jia et al.40 performed Monte Carlo simulations to investigate the behavior of carbon dioxide and nitrogen mixtures within carbon membranes. These authors found that the separation mechanism (19) Goodman, A. L.; Busch, A.; Bustin, R. M.; Chikatamarla, L.; Day, S.; Duffy, G. J.; Fitzgerald, J. E.; Gasern, K. A. M.; Gensterblum, Y.; Hartman, C.; Jing, C.; Krooss, B. M.; Mohammed, S.; Pratt, T.; Robinson, R. L.; Romanov, V.; Sakurovs, R.; Schroeder, K.; White, C. M. Int. J. Coal Geol. 2007, 72, 153. (20) Gensterblum, Y.; van Hemert, P.; Billemont, P.; Busch, A.; Charriere, D.; Li, D.; Krooss, B. M.; De Weireld, G.; Prinz, D.; Wolf, K. H. A. A. Carbon 2009, 47, 2958. (21) Yang, X.; Zhang, C. Chem. Phys. Lett. 1990, 407, 427. (22) Cole, D. R.; Chialvo, A. A.; Rother, G.; Vlcek, L.; Cummings, P. T. Philos. Mag. 2010, 90, 2339. (23) Deroche, I.; Gaberova, L.; Maurin, G.; Castro, M.; Wright, P. A.; Llewellyn, P. L. J. Phys. Chem. C 2008, 112, 5048. (24) Sant, M.; Leyssale, J. M.; Papadopoulos, G. K.; Theodorou, D. N. J. Phys. Chem. B 2009, 113, 13761. (25) Leyssale, J. M.; Papadopoulos, G. K.; Theodorou, D. N. J. Phys. Chem. B 2006, 110, 22742. (26) Zhuo, S.; Huang, Y.; Hu, J.; Liu, H.; Hu, Y.; Jiang, J. J. Phys. Chem. C 2008, 112, 11295. (27) Qin, Y.; Yang, X.; Zhu, Y.; Ping, J. J. Phys. Chem. C 2008, 112, 12815. (28) Belmabkhout, Y.; Sayari, A. Chem. Eng. Sci. 2009, 64, 3729. (29) Ramsahye, N. A.; Maurin, G.; Bourrelly, S.; Llewellyn, P. L.; Devic, T.; Serre, C.; Loiseau, T.; Ferey, G. Adsorption 2007, 13, 461. (30) Babarao, R.; Jiang, J. Langmuir 2008, 24, 6270. (31) Krishna, R.; van Baten, J. M. Langmuir 2010, 26, 3981. (32) Xu, Q.; Liu, D.; Yang, Q.; Zhong, C.; Mi, J. J. Mater. Chem. 2010, 20, 706. (33) Liu, B.; Smit, B. J. Phys. Chem. C 2010, 114, 8515. (34) Perez-Pellitero, J.; Amrouche, H.; Siperstein, F. R.; Pirngruber, G.; NietoDraghi, C.; Chaplais, G.; Simon-Masseron, A.; Bazer-Bachi, D.; Peralta, D.; Bats, N. Chem.;Eur. J. 2010, 16, 1560. (35) Mu, B.; Schoenecker, P. M.; Walton, K. S. J. Phys. Chem. C 2010, 114, 6464. (36) Torrisi, A.; Mellot-Draznieks, C.; Bell, R. G. J. Chem. Phys. 2010, 132, 044705. (37) Sedigh, M. G.; Onstot, W. J.; Xu, L.; Peng, W. L.; Tsotsis, T. T.; Sahimi, M. J. Phys. Chem. A 1998, 102, 8580. (38) Tsotsis, T. T.; Patel, H.; Najafi, B. F.; Racherla, D.; Knackstedt, M. A.; Sahimi, M. Ind. Eng. Chem. Res. 2004, 43, 2887. (39) Tenney, C. M.; Lastoskie, C. M. Environ. Prog. 2006, 25, 343. (40) Jia, Y.; Wanga, M.; Wua, L.; Gao, C. Sep. Sci. Technol. 2007, 42, 3681. (41) Wang, F. Y.; Zhu, Z. H.; Massarotto, P.; Rudolph, V. AIChE J. 2007, 53, 1028. (42) Levesque, D.; Lamari, D. F. Mol. Phys. 2009, 107, 591. (43) Lim, Y. I.; Bhatia, S. K.; Nguyen, T. X.; Nicholson, D. J. Membr. Sci. 2010, 355, 186.

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depends on the loading and diffusion of carbon dioxide and nitrogen and that carbon dioxide tends to be adsorbed more strongly on the surface of the membrane. The isosteric heat of carbon dioxide adsorption on activated carbon was estimated by Levesque and Lamari by means of grand canonical Monte Carlo simulations.42 The maximum in the isosteric heat curve is reached when the pore width is large enough to allow cooperative effects between the adsorbed molecules. Finally, Lim et al.43 considered several models for CO2 molecules and compared their adsorption in carbon slit pores. The permeability obtained in this molecular simulation study was found to exceed by 3 orders of magnitude that obtained using macroscopic measurements. As suggested by the authors, the simulated permeability provides an upper limit of the experimental permeability (i.e., an upper bound that does not account for the diffusion resistance at the membrane surface and disorder of the pore network). Among the works mentioned above, many authors also considered the coadsorption of mixtures of CO2 with methane or nitrogen in carbon nanopores. On the other hand, molecular simulation of the adsorption of CO2 or CH4 in carbon pores in the presence of water has not been considered in the literature. In this paper, we report both experiments and molecular simulations on the adsorption of CO2 and CH4 in porous carbons in the presence of water. In the case of CO2, two temperatures are considered (300 and 318 K) to examine the adsorption of subcritical and supercritical CO2 (the critical temperature of CO2 is 304.2 K). Both the experiments and molecular simulations consist of determining the adsorption isotherms and isosteric heat of adsorption curves for pure CO2 and pure CH4 and for CO2/H2O and CH4/H2O mixtures in a nanoporous carbon. The experiments were performed on a Filtrasorb 400 activated carbon of Calgon Carbon Corp. with a nanometric pore size while the molecular simulations are carried out for a slit graphite nanopore of a width of H = 1.4 nm. The experimental sample was selected as a first step toward considering real coal samples; it consists of a more simple system with a chemical composition and micropore structure similar to those of natural coal. It is also resistant to high temperatures, which facilitates the removal of moisture and allows reaching more easily a well-defined initial condition (one of the main sources of the discrepancies observed in the interlaboratory comparisons). The remainder of this paper is organized as follows. Section 2 presents the details of the experimental and molecular simulation techniques. Section 3 reports the adsorption isotherms and isosteric heat of adsorption curves for pure CO2 and CH4 and for the same gases in the presence of water in the nanoporous carbons. Both the experimental and molecular simulation data are discussed. Section 4 discusses what can be learnt from the comparison between the experimental and simulation results.

2. Methods 2.1. Experimental Methods. 2.1.1. General Description of the Apparatus. The adsorption isotherms were measured with an in-house-built apparatus around a Rubotherm high-pressure magnetic suspension balance marketed.44,45 The high-pressure part of the magnetic suspension balance is exposed to the sorptive, H2O, CO2, or CH4, at constant temperature and increasing pressure. The excess sorption is determined from the apparent mass change Δm = mmeasured(T,p) m0sample of the sample recorded during this procedure, where (44) Dreisbach, F.; Staudt, R.; Tomalla, M.; Keller, J. U. Proceedings of the 5th international conference on fundamentals of adsorption, Asilomar, CA, May 1995; Kluwer Academic Publishers: Boston, MA, 1996; pp 259-68. (45) Dreisbach, F.; Seif, R.; Losch, H. W. J. Therm. Anal. Calorim. 2003, 71, 73.

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Article Table 1. Composition and Analytical Data for the Activated Carbon Sample F400 Used in This Work20

%

C

O

N

S

H

moisture

fixed carbon

vol matter

ash

F400 sample

89.55 ( 0.22

5.77 ( 0.01

0.25 ( 0.04

0.77 ( 0.01

0.21 ( 0.02

1.52 ( 0.17

91.06 ( 0.28

1.32 ( 0.03

6.10 ( 0.11

m0sample

is the original sample mass. This apparent sample mass change is corrected for a buoyancy term based on the skeletal volume V0sample of the sample. The determination of the skeletal density or volume is performed with helium, which is assumed to be nonadsorbing.46 The excess adsorbed mass is then given by mexcess = Δm þ V0sampleFgas(T,p). The gas phase density Fgas is determined using an appropriate equation of state (EOS). The helium density needed to estimate V0sample is determined by a modified Benedict-Webb-Rubin EOS.47 The water vapor density is estimated using the data by Wagner and Pruss48 while those of carbon dioxide and methane are estimated using the EOS by Span and Wagner49 and the EOS by Setzmann and Wagner,50 respectively. Experimental adsorption data are usually reported in g of adsorbate per g of adsorbent. However, in order to be consistent with the units used in the molecular simulations, experimental adsorbed amounts in the present study are reported in g of adsorbate per cm3 (i.e., the mass of the adsorbate divided by the pore volume of the sample in cm3). The setup has been described in detail by De Weireld et al.51,52 The most important technical features are as follows. The weight changes are measured with an accuracy of 10 μg. The magnetic system consists of an electromagnet linked to the balance and a permanent magnet at the top of the suspension system with the crucible containing the sorbent. The suspension system is housed in a high-pressure adsorption chamber allowing for experiments at high temperatures (243-393 K), high pressure (vacuum to 15 MPa), and corrosive conditions. The pressure is measured with three different pressure sensors: MKS Baratron 621B with a resolution of 1.3 Pa for secondary vacuum to 133.3 kPa, MKS Baratron 621B with a resolution of 32.5 Pa from 133.3 kPa to 3.333 MPa, and a Tecsis-Series P3382 pressure sensor with internal diaphragma for a maximum pressure of 16 MPa with an accuracy of 0.1% of the full scale. The temperature of the gas phase, which is needed for the determination of the density, is measured with a high-precision (class A) Pt100 RTD. The setup is placed in a thermostatic chamber (Weiss Technik, KWP450/70) ensuring constant temperature during experiments. This homogeneous temperature field avoids condensation of subcritical gases. The a priori uncertainty of the excess sorption measurements is estimated at 5% of the maximum sorption. 2.1.2. Sample and Sample Preparation. FILTRASORB 400 (F 400) activated carbon of Calgon Carbon Corp. was kindly supplied by Chemviron Carbon GmbH, Germany. Analytical data for the F400 are reported in Table 1.20 The sample exhibits a bimodal pore size distribution with peaks located at 0.8 and 1.3-1.4 nm. The activated carbon was dried at 473 K for 24 h prior to performing the experiments. 2.1.3. Moisture Equilibration of the Samples. The recommended method to carry out moisture equilibration of samples is the “Standard Test Method for Equilibrium Moisture of Coal at 96 to 97 Percent Relative Humidity and 30°C” (ASTM D 1412-07). This modified ASTM procedure has been used by some authors working on this topic.53,54 In this method, the crushed sample is immersed and agitated in water for 30 min. In a second (46) Sircar, S. Ind. Eng. Chem. Res. 1999, 38, 3670. (47) Mc Carthy, R. D.; Arp, V. D. Adv. Cryog. Eng. 1990, 35, 1465. (48) Wagner, W.; Pruss, A. J. Phys. Chem. Ref. Data 2002, 31, 387. (49) Span, R.; Wagner, W. A. J. Phys. Chem. Ref. Data 1996, 25, 1509. (50) Setzmann, U.; Wagner, W. A. J. Phys. Chem. Ref. Data 1991, 20, 1061. (51) De Weireld, G.; Frere, M.; Jadot, R. Meas. Sci. Technol. 1999, 10, 117. (52) Belmabkhout, Y.; Frere, M.; De Weireld, G. Meas. Sci. Technol. 2004, 15, 848. (53) Krooss, B. M.; van Bergen, F.; Gensterblum, Y.; Siemons, N.; Pagnier, H. J. M.; David, P. Int. J. Coal Geol. 2002, 51, 69. (54) Fitzgerald, J. E.; Robinson, R. L., Jr.; Gasem, K. A. M. Langmuir 2006, 22, 9610.

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step, the flask containing the immersed sample is placed in a bath at a constant temperature of 30 °C for 3 h. Then, after removing the excess water, the sample is transferred into a small vacuumtype desiccator which hosts a saturated solution of potassium sulfate at 30 °C. After an equilibration time that lasts from 48 to 60 h, the sample is transferred into the adsorption cell. The main disadvantage of the latter method is the lack of reproducibility which is due to the fact that the relative humidity can change (evaporation and condensation might occur) and that the sample can be contaminated upon transfer into the adsorption cell. In the specific case of CO2 adsorption on moisture-equilibrated coals, an interlaboratory comparison between six laboratories has shown the difficulty of preparing initial samples with the same water content.19 To overcome this problem, we developed a procedure that allows preparing moisture-equilibrated coals by equilibrating the sample in situ (i.e., the equilibration is performed in the adsorption cell). In this method, a cylinder containing pure water is placed in the adsorption chamber which contains the sample. The temperature is maintained at the temperature at which the CO2 or CH4 adsorption experiment has to be measured (so that the pressure inside the adsorption cell is the saturating vapor pressure of water at the temperature of the experiment). Then, moisture-equilibrated samples with different amounts of preadsorbed water are obtained by decreasing the pressure in the adsorption cell (after isolating the latter from the cylinder filled with water). The amount of preadsorbed water corresponds to the mass gained by the sample upon moisture equilibration, mads water. Once moisture-equilibrated samples have been obtained using the method described above, CO2 or CH4 adsorption experiments are performed according to the classical procedure; i.e., a quantity of gas is introduced in the adsorption cell, and after an equilibration time, the mass gained by the sample is recorded as a function of the final pressure. Assuming that preadsorbed water remains constant during the experiment, the CO2 or CH4 adsorbed mass at a given pressure and temperature is determined by Δm = mmeasured(T,p) - (m0sample þ mads water). The latter assumption is supported by molecular simulations which show that the amount of preadsorbed water remains constant upon adsorbing CO2 or CH4 at high pressure (see below for a detailed discussion on this issue). On the other hand, from an experimental point of view, the validity of this assumption remains to be demonstrated. Further study, including the determination of the composition of the gas phase in the adsorption cell, is required to clarify this issue. In order to account for the volume occupied by water in the buoyancy effect correction, helium measurements were carried out on samples with different amounts of preadsorbed water. The excess mass of adsorbed CO2 or CH4 at a given temperature and pressure is given by mexcess = Δm þ (V0sample þ Vads water)Fgas(T,p), where Vads water is the volume occupied by adsorbed water. In calculating the density Fgas(T, p) of the bulk phase, water was not taken into account as its partial pressure is negligeable compared to the high pressure of the gas. It should be noted that our excess adsorption isotherms do not strictly follow the classical definition given by Sircar in ref 55. In contrast, in the present work, we adopt the point of view proposed by Siemons et al. in ref 12 for enhanced coal bed methane. In this method, sorption measurements are often done on coals as they are received, i.e., already containing water. As a result, the sample volumes are determined with water while the sample mass is that of the dry ash. The choice of the method by Siemons et al. was motivated by the fact that the effect of preadsorbed water appears more clearly on the adsorption isotherms if the buoyancy effect on water is corrected; in (55) Sircar, S. AIChE J. 2001, 47(5), 1169.

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Table 2. Lennard-Jones Interaction Parameters, and Partial Charges for Water, Carbon Dioxide, and Methane molecule water O-O H-H O-H H-O-H carbon dioxide C-C O-O C-O O-C-O methane CH4-CH4

ε (K) 78.21

σ (nm) 0.3167

q (e)

l (nm)

θ (deg)

U AB ¼

-0.82 þ0.41 109.47

0.2757 0.3033 0.2892

þ0.6512 -0.3256 0.1149 180.0

148.1

0.381

particular, it allows seeing the decrease in gas sorption capacity only due to the occupation of some pores by water without considering the buoyancy effect (which is not specific to water). Moreover, this correction for buoyancy of water does not imply that water is considered as a part of the solid adsorbent; the reference sample mass used in our calculations is the mass of the dry sample. In order to estimate the error on the adsorption isotherms in the presence of preadsorbed water, Figure S1 in the Supporting Information compares the adsorption isotherms obtained using the method by Sircar and those obtained using the methods by Siemons et al. For both methane and carbon dioxide, the two methods lead to very similar adsorption isotherms (although, as expected, the method used in the present work slightly overestimates the sorption capacity of gas in presence of preadsorbed water).

2.2. Molecular Simulation Methods. 2.2.1. Water, Carbon Dioxide, and Methane Models. The single point charge (SPC) model by Berendsen et al. was used for water in this work as it reasonably reproduces the structure and thermodynamics of bulk liquid water at ambient temperature.56 In this model, water is represented as a rigid molecule: the hydrogen atoms are at a distance of 0.1 nm from the oxygen atom, and the HOH angle is 109.47°. The oxygen atom is the center of a Lennard-Jones interaction potential with the interaction parameters reported in Table 2. In addition, the atoms in the water molecule carry the following partial charges: -0.82e for the oxygen and þ0.41e for each of the hydrogen atoms. The rigid model by Harris and Yung was used in this work to describe the carbon dioxide molecule.57 The carbon-oxygen distance is 0.1149 nm. Each of the three atoms is a Lennard-Jones site which also carries a partial charge. The interaction parameters and partial charges as well as the geometry of the carbon dioxide molecule are shown in Table 2. In our simulations, the methane molecule is simply described as a Lennard-Jones sphere with the parameters shown in Table 2. 2.2.2. Carbon Nanopore. The nanoporous carbon used in this work is a slit pore having a constant width H = 1.4 nm. The pore surface is modeled as three graphene sheets that are separated by a distance of 0.335 nm, which corresponds to the interlayer distance in graphite.58 Only three graphene layers are considered as the interaction between the adsorbed molecules and the pore surface becomes negligible beyond the third sheet. The cross-section area of the pore bounded by parallel walls was chosen as 6.4 nm  6.4 nm, which corresponds to ∼20σ  20σ (σ = 0.3167 nm is roughly the size of the water molecule taken as the Lennard-Jones parameter of the water-water interaction). In our simulations, the carbon atom in the graphene layers is described as a Lennard-Jones sphere with the following parameters: σ = 0.34 nm and ε = 28 K. The interaction energy UAB (56) Berendsen, H. J. C.; Postma, J. P. M.; Gunsteren, W. F.; Hermans, J. In Intermolecular Forces; Pullman, B., Ed.; Reidel: Dordrecht, 1981. (57) Harris, J. G.; Jung, K. H. J. Phys. Chem. 1995, 99, 12021. (58) Coasne, B.; Jain, S. K.; Gubbins, K. E. Mol. Phys. 2006, 104, 3491.

1018 DOI: 10.1021/la103107t

2

B X X qA i qj i

0.1 28.129 80.507 47.588

for a set of two adsorbed molecules A and B (A, B = H2O, CO2, or CH4) is given by

j

4πε0 rAB ij

4 þ 4εAB ij

σ AB ij rAB ij

!12 -

!6 3 5 ð1Þ rAB ij

σ AB ij

P where the two first symbols indicate that the interaction is summed over the sites i of the A molecule and the sites j of the B molecule, respectively. The first term in eq 1 is the b Coulombic interaction (qA i and qi are the charges of the sites i AB and j, while rij is the distance between the two sites). The Coulomb interaction in the present work was computed using the Ewald summation technique.59,60 The parameters for the Ewald sum were R = 0.1 A˚-1 and kmax = 5. The second and third terms in eq 1 are the repulsion/dispersion interactions between the two sites, and εAB and σAB are the corresponding ij ij Lennard-Jones energy and size interaction parameters. The unlike Lennard-Jones interaction parameters in eq 1 have been determined from the like parameters using the Lorentz-Berthelot combining rules. As mentioned above, the Np carbon atoms forming the nanopore are described as neutral Lennard-Jones spheres so that the interaction energy UAW between an adsorbed molecule A and the nanopore is simply given by the sum of the Lennard-Jones interactions between the sites i of molecule A and the carbon atoms:

U

AW

¼

Np XX i

j¼1

2 4 4εAC i

σ AC i rAC ij

!12

σAC i - AC rij

!6 3 5

ð2Þ

Again, the unlike interaction parameters, εAB and σAB ij ij , are determined from the like parameters using the Lorentz-Berthelot combining rules. 2.2.3. Grand Canonical Monte Carlo (GCMC). We performed GCMC simulations of adsorption of carbon dioxide and methane at 300 or 318 K in the atomistic model of carbon nanopores. The GCMC technique is a stochastic method that simulates a system having a constant volume V (the pore with the adsorbed phase), in equilibrium with an infinite reservoir of particles imposing its chemical potential μΑ for each species (A = CO2 and CH4) and its temperature T. The absolute adsorption/ desorption isotherm is given by the ensemble average of the number of each adsorbate molecule as a function of the fugacities fA of the reservoir (the latter are determined from the chemical potential μ). The adsorption of pure carbon dioxide and methane was modeled by carrying out GCMC simulations. A different approach was used to simulate the adsorption of carbon dioxide and methane in the presence of water; we fixed certain numbers of water molecules within the pore, and we simulated the adsorption isotherm of carbon dioxide or methane within the pore using the GCMC algorithm. In this approach, the number of adsorbed water molecules is fixed, but these molecules are allowed to move in order to equilibrate the whole system. In order to check whether it is reasonable to assume that preadsorbed water remains in the pore upon CO2 or CH4 adsorption, we conducted the following test. Starting with an initial configuration of the slit carbon nanopore filled with water, we simulated the adsorption of CO2 or CH4 at a very large fugacity of 32 MPa (the largest fugacity considered below in our simulations of CO2 and CH4 adsorption). In these simulations, the amount of adsorbed water is allowed to vary as water is treated in the grand canonical ensemble with a fugacity of ∼10 kPa. No water desorption was observed even when such a large CO2 or CH4 fugacity is considered. This result shows that it is reasonable to assume that preadsorbed water (59) Allen, M. P.; Tildesley, D. J. Computer Simulation of Liquids; Oxford: Clarendon, 1987. (60) Frenkel, D.; Smit, B. Understanding Molecular Simulation: From Algorithms to Applications, 2nd ed.; Academic Press: London, 2002.

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Figure 1. (top) Experimental adsorption isotherm for CO2 at T = 300 K (circles) and 318 K (triangles) on Filtrasorb F400 activated carbon. The inset shows the same data plotted as a function of the density of free CO2 surrounding the sample Fext. (bottom) Experimental isosteric heat of adsorption as a function of the density of CO2 on the Filtrasorb F400 activated carbon. remains in the pore upon CO2 or CH4 adsorption. On the other hand, it must be noted that, starting with an empty pore, we found that CO2 and CH4 fill the carbon slit nanopore while only a few water molecules get adsorbed; this shows that the desorption of preadsorbed water involves a large free energy barrier (the state consisting of large amounts of preadsorbed water coexisting in the pore with CO2 or CH4 is metastable).

3. Results and Discussion 3.1. Experiments. 3.1.1. Adsorption of Pure Carbon Dioxide and Methane. Figure 1 shows the excess adsorption isotherm for carbon dioxide at 300 and 318 K on the activated carbon F400. The data are plotted as a function of the fugacity of the external gas phase. We note that the fugacity is not close to the pressure of the external vapor phase because carbon dioxide is below but close to its critical point at 300 K and supercritical at 318 K. For each temperature, we also show the same data plotted as a function of the density Fext of the CO2 phase surrounding the sample. Under supercritical conditions (T = 318 K), the adsorption isotherm for CO2 measured on the activated carbon F 400 increases until a maximum adsorbed amount of 0.94 g/cm3 is reached at a fugacity of 4 MPa (pressure of about 5 MPa). Then, the excess adsorption isotherm decreases with an inflection point around a fugacity of 6.1 MPa (corresponding to a pressure of 10 MPa). The latter drop is due to a sharp increase of the CO2 density at the critical point, leading to an increase of the buoyancy effect on the adsorbed phase. The maximum excess adsorbed amount is reached at a CO2 gas phase density of around Fext = 0.1 g/cm3 (we recall that Fext is the density of the CO2 phase surrounding the sample). Beyond a CO2 density of Fext = 0.25 g/m3, the excess adsorption isotherm decreases linearly. The intercept of the extrapolated linear part of the excess adsorption isotherm with the density axis provides an estimate of the absorbed phase density. At this point, the densities of the fluid phase and the adsorbed phase are identical; i.e., the two phases cannot be discriminated any longer. The density of the adsorbed CO2 phase was estimated by extrapolation of the excess sorption Langmuir 2011, 27(3), 1015–1024

vs density plots in the density range Fext > 0.25 g/cm3. The density value obtained by this procedure is Fads = 0.956 g/cm3. As will be seen below, this value is in fair agreement (15% larger) with that obtained in the molecular simulations. We believe that the difference of ∼15% is due to the fact that the activated carbon sample does not have a homogeneous pore size distribution and possesses some microporosity of a diameter of about 0.8 nm. In addition, other uncertainties regarding the thickness, density, and composition of carbon walls might explain the difference observed between the experimental and molecular simulation data. At 300 K, i.e., below but close to the critical temperature of CO2, the excess adsorption isotherm for CO2 increases up to a maximum value of 1.014 g/cm3 at a fugacity of 3.8 MPa (corresponding to a pressure of about 5 MPa). Then, the excess adsorption isotherm decreases abruptly around a fugacity of 4.6 MPa (corresponding to a pressure of 6.86 MPa). The latter value corresponds to the saturated vapor fugacity at the experimental temperature of 300 K. The maximum excess adsorbed amount is reached at a CO2 gas phase density of around 0.1 g/cm3. Beyond a CO2 density of Fext = 0.75 g/cm3, the excess adsorption isotherm decreases linearly with increasing Fext. Figure 1 also shows the isosteric heat of adsorption curve as a function of the excess adsorbed amount for CO2 in the activated carbon F400. The latter curve has been obtained from five adsorption isotherms between 300 and 318 K using the isosteric method. In this method, the isosteric heat of adsorption Qst = -ΔH is estimated at a given adsorbed amount Fads using the Clausius-Clapeyron equation:   ∂ ln P Qst ðFads Þ ¼ - ΔH ¼ kB T 2 ð3Þ ∂T Fads At low pore filling (below Fext = 0.35 g/cm3), the isosteric heat of adsorption curve is constant at about 23 kJ/mol. The fact that Qst varies only little with the adsorbed amount in the low pore filling range reveals the homogeneity of the porous carbon. As the pore filling increases above Fads = 0.2-0.3 g/cm3, Qst increases as the loading increases due to the non-negligible fluid/ fluid contribution to the adsorption enthalpy when the pores are filled with CO2. Figure 2 shows the adsorption isotherm for methane at 300 K on the activated carbon F400. In contrast to the results for CO2, the adsorption isotherm for methane is continuous without any sharp and discontinuous change in the adsorbed amount. The adsorbed amount increases up to a maximum value of 0.214 g/cm3 at a fugacity of 5.6 MPa (corresponding to a pressure of 6.2 MPa). Then, the excess adsorption isotherm decreases very slowly with increasing the fugacity. The buoyancy effect on the adsorbed phase is low due to the weak pressure dependence of the CH4 fluid density compared to CO2 at the same temperature and pressure. The maximum excess adsorbed amount is reached at a CH4 fluid density of around 0.05 g/cm3. Beyond the latter CH4 density, the excess adsorption isotherm decreases linearly with increasing the density of the CH4 fluid phase. As in the case of CO2, the intercept of the extrapolated linear part of the excess adsorption isotherm with the density axis provides an estimate of the absorbed phase density. The density obtained by this procedure is 0.24 g/cm3, which is in reasonable agreement with the value obtained in the molecular simulations below. Figure 2 also shows the isosteric heat of adsorption curve as a function of the adsorbed amount for methane on the activated carbon F400. The isosteric heat of adsorption curve is constant at about 15 kJ/mol (as was done for CO2, the isosteric heat of methane adsorption Qst was estimated at a given adsorbed amount Fads using the Clausius-Clapeyron equation). Again, the DOI: 10.1021/la103107t

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Figure 2. (top) Experimental adsorption isotherm for methane at 300 K on the Filtrasorb F400 activated carbon. The inset shows the same data plotted as a function of the density of free CH4 surrounding the sample Fext. (bottom) Experimental isosteric heat of adsorption as a function of the adsorbed amount for CH4 on the Filtrasorb F400 activated carbon.

fact that Qst varies only little with the adsorbed amount in the low pore filling range reveals the homogeneity of the porous carbon. As the pore filling increases above Fads = 0.1 g/cm3, Qst increases slowly up to 19 kJ with as the loading increases due to the nonnegligible fluid/fluid contribution to the adsorption enthalpy when the pore is filled with CH4. 3.1.2. Adsorption of Carbon Dioxide and Methane in the Presence of Water. We now discuss the adsorption of carbon dioxide and methane on the activated carbon F400 when different amounts of water are already adsorbed. A certain number of water molecules was first adsorbed on the activated carbon. Then, the adsorption isotherm of carbon dioxide or methane within the sample was measured. Figure 3 shows the adsorption isotherms for carbon dioxide at 300 K on the activated carbon F400 in the presence of 0.104 and 0.31 g/cm3 of adsorbed water. The shape of the adsorption isotherms resembles closely that obtained for pure carbon dioxide. Because of the presence of the water molecules, the maximum adsorbed amount in the activated carbon decreases as the number of preadsorbed water molecules increases, in agreement with the simulated data below. Our data also show that the pore filling mechanism is not modified by the presence of these water molecules as it occurs over a similar pressure range. In particular, the pressure at which the maximum adsorbed amount is reached is nearly insensitive to the presence of adsorbed water molecules. Figure 3 also shows the adsorption isotherms for methane at 300 K in activated carbon in the presence of adsorbed water (0.116 and 0.31 g/cm3). As in the case of carbon dioxide, the pore filling mechanism for methane is not affected by the presence of water molecules; the adsorbed amount increases until a maximum value is reached at a fugacity of 5.6 MPa (pressure of 6.2 MPa). Again, the maximum adsorbed amount in the activated carbon decreases as the number of preadsorbed water molecules increases. 3.2. Molecular Simulation. 3.2.1. Adsorption of Pure Carbon Dioxide and Methane. We first discuss the adsorption isotherm of pure carbon dioxide and methane in a slit carbon nanopore of a width H =1.4 nm. Adsorption of the two gases was studied at ambient temperature T = 300 K. In the case of CO2, we also studied the adsorption at 318 K when the fluid is 1020 DOI: 10.1021/la103107t

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Figure 3. (top) Adsorption isotherm for carbon dioxide at 300 K on on the Filtrasorb F400 activated carbon with different amounts of preadsorbed water: (circles) 0.104 g/cm3 (H2O), and (triangles) 0.31 g/cm3 (H2O). The black solid line shows the carbon dioxide adsorption isotherm for the water-free sample. (bottom) Adsorption isotherm for methane at 300 K on the Filtrasorb F400 activated carbon with different amounts of preadsorbed water: (circles) 0.116 g/cm3 (H2O) and (triangles) 0.31 g/cm3 (H2O). The black solid line shows the methane adsorption isotherm for the water-free sample.

Figure 4. (top) Simulated adsorption isotherm for carbon dioxide at different temperatures in a slit carbon nanopore of a width H = 1.4 nm: (circles) T = 300 K and (triangles) T = 318 K. Adsorbed amounts have been converted into the corresponding densities of the confined fluid (defined as the mass of the adsorbed molecules divided by the pore volume). (bottom) Isosteric heat of adsorption as a function of the density of carbon dioxide confined in the slit carbon nanopore of a width H = 1.4 nm: (circles) T = 300 K and (triangles) T = 318 K.

supercritical. Figure 4 shows the adsorption isotherm for carbon dioxide at 300 and 318 K. Adsorbed amounts have been converted into the corresponding densities of the confined fluid. We note that the fugacity is not close to the pressure of the free vapor because carbon dioxide at 300 and 318 K is close to and above its critical point Tc = 304.2 K, respectively. The shape of our simulated adsorption isotherms for CO2 at 300 and 318 K is also Langmuir 2011, 27(3), 1015–1024

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Figure 5. Molecular configurations for carbon dioxide adsorbed at 300 K in a slit carbon nanopore of a width H = 1.4 nm: (top) f = 0.10 MPa and (bottom) f = 2.0 MPa. Gray spheres are the carbon atoms of the slit nanopore. Purple and red spheres are the carbon and oxygen atoms of the carbon dioxide molecules, respectively.

consistent with the results obtained by Lim et al. in a recent study of CO2 adsorption in slit carbon nanopores.43 We first discuss the results obtained for T = 300 K. Figure 5 shows molecular configurations for carbon dioxide adsorbed at this temperature in the slit carbon nanopore at low and high fugacities (f = 0.10 and 2.0 MPa). The adsorbed amount increases in a linear way as the fugacity increases. At a fugacity f ∼ 1.5 MPa, the pore is filled and the adsorbed amount increases slowly as the fugacity increases further. We note that filling occurs below the bulk saturation vapor pressure P0 = 55.2 bar. This result is consistent with the molecular simulations reported by Tenney and Latoskie,39 who found that filling of carbon pores in the size range between 1.3 and 2.4 nm with CO2 at 273 K occurs at pressures much below the saturation vapor pressure. The simulated density of adsorbed/condensed CO2 after pore filling at T = 300 K, 0.80 g/cm3, is in fair agreement with the experimental data obtained in the present work. Again, we believe that the difference of ∼15% is due to the fact that the model considered in the molecular simulations does not account for the microporosity ∼0.8 nm of the experimental sample. Other parameters such as the pore wall thickness, composition, and density need to be considered in the molecular simulations (see refs 61 and 62 for a discussion on the effect of the carbon wall thickness on adsorption and the accessibility of fluids in disordered porous materials). The different molecular configurations shown in Figure 5 suggest that, even at low pressures, layering of the confined fluid is observed and that the carbon pore with H = 1.4 nm accommodates at most three layers. The CO2 adsorption at 318 K is very similar to what is observed at 300 K; the pore filling is reversible and continuous. This result shows that the fluid state (subcritical versus critical) does not play a crucial role on the type of the adsorption isotherm. This result is due to the fact that, at both temperatures, the confined fluid is above its capillary critical temperature, Tcc. The latter, which is defined as the temperature above which capillary condensation is suppressed (61) Nguyen, T. X.; Bhatia, S. K. Langmuir 2004, 20, 3532. (62) Nguyen, T. X.; Bhatia, S. K. J. Phys. Chem. C 2007, 111, 2212.

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Figure 6. (top) Simulated adsorption isotherm for methane at 300 K in a slit carbon nanopore of a width H = 1.4 nm. Adsorbed amounts have been converted into the corresponding densities of the confined fluid. (bottom) Simulated isosteric heat of adsorption as a function of the density of methane confined in the slit carbon nanopore of a width H = 1.4 nm.

(the adsorption isotherm becomes reversible and continuous), is such that (Tc - Tcc)/Tc ∼ σ/H, where H is the pore size and σ is the size of the confined molecule.63-65 We also show in Figure 4 the isosteric heat of adsorption at 300 and 318 K for carbon dioxide in the slit carbon nanopore as a function of the density of the confined fluid (defined as the mass of the adsorbed molecules divided by the porous volume of the slit pore). The isosteric heat of adsorption curves for the two temperatures are very similar. The isosteric heat of adsorption ranges around 16 kJ/mol and increases only sligthly with loading prior to pore filling. This reveals the high degree of surface homogeneity of the porous carbon. On the other hand, the isosteric heat of adsorption above capillary condensation/pore filling ∼28 kJ/mol is much larger due to the non-negligible fluid/fluid contribution to the enthalpy of adsorption when the pore is filled with CO2. These results are fully consistent with the experimental observations above. Figure 6 shows the adsorption isotherm for methane at 300 K in the slit carbon nanopore of a width H = 1.4 nm. Again, adsorbed amounts have been converted into the corresponding densities of the confined fluid by dividing the mass of the adsorbed molecules by the porous volume of the slit pore. Figure 7 shows molecular configurations for methane adsorbed in the slit carbon nanopore at low and high fugacities (f = 1 MPa and f = 24 MPa). As in the case of carbon dioxide, the adsorption isotherm for methane is of type I in the IUPAC classification as the filling of the pore is continuous and reversible without any sharp and discontinuous increase in the adsorbed amount. The latter type is characteristic of adsorption in very small nanochannels (such as zeolites). The simulated density of CH4 obtained after pore filling, 0.22 g/cm3, is in good agreement with the experimental data obtained in the present work (0.24 g/cm3). As in the case of carbon (63) Coasne, B.; Gubbins, K. E.; Pellenq, R. J. M. Adsorption 2005, 11, 289. (64) Pellenq, R. J. M.; Coasne, B.; Denoyel, R.; Puibasset, J. Stud. Surf. Sci. Catal. 2006, 160, 667. (65) Pellenq, R. J. M.; Coasne, B.; Denoyel, R. O.; Coussy, O. Langmuir 2009, 25, 1393.

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Figure 7. Molecular configurations for methane adsorbed at 300 K in a slit carbon nanopore of a width H = 1.4 nm: (top) f = 1 MPa and (bottom) f = 24 MPa. Gray spheres are the carbon atoms of the slit nanopore while cyan spheres are methane molecules.

dioxide, the molecular configurations show layering of confined methane with the carbon pore with H = 1.4 nm accommodating at most three layers. Such a significant layering has been already reported in the case of pure fluids or mixtures composed of Lennard-Jones spheres.66-68 We also show in Figure 6 the isosteric heat of adsorption for methane in the slit carbon nanopore as a function of the density of confined fluid. The isosteric heat of adsorption increases in a monotonous way with the adsorbed amount from 12.5 kJ/mol at low loading up to 18 kJ/mol at full loading. Again, such a behavior is consistent with the high degree of surface homogeneity of the porous carbon; the porous carbon/fluid contribution to the isosteric heat is nearly constant while the fluid/fluid contribution increases continuously with loading so that the totat isosteric heat of adsorption increases with increasing loading. 3.2.2. Adsorption of Carbon Dioxide and Methane in the Presence of Water. We now discuss the adsorption of carbon dioxide and methane in the 1.4 nm slit carbon nanopore with different amounts of water are already adsorbed (0.005, 0.052, and 0.26 g/cm3). We fixed a certain number of water molecules within the pore, and we simulated the adsorption isotherm of carbon dioxide or methane within the pore using the GCMC algorithm. In this approach, the number of adsorbed water molecules is fixed, but these molecules are allowed to move in order to equilibrate the whole ensemble. Figure 8 shows the adsorption isotherms for carbon dioxide at 300 K in the slit carbon nanopore in the presence of adsorbed water. Again, adsorbed amounts have been converted to the corresponding densities of the confined fluid. When small amounts of water are already adsorbed (0.005 and 0.052 g/cm3), the adsorption isotherms for carbon dioxide closely resemble that obtained for pure carbon dioxide. In particular, the pore filling mechanism is not affected by the presence of water molecules. This result is in (66) Coasne, B.; Czwartos, J.; Gubbins, K. E.; Hung, F. R.; SliwinskaBartkowiak, M. Mol. Phys. 2004, 102, 2149. (67) Czwartos, J.; Coasne, B.; Gubbins, K. E.; Hung, F. R.; SliwinskaBartkowiak, M. Mol. Phys. 2005, 103, 3103. (68) Coasne, B.; Czwartos, J.; Gubbins, K. E.; Hung, F. R.; SliwinskaBartkowiak, M. Adsorption 2005, 11, 301.

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Figure 8. (top) Simulated adsorption isotherm for carbon dioxide at 300 K in a slit carbon nanopore of a width H = 1.4 nm with different amounts of preadsorbed water: (squares) 0.005 g/cm3 (H2O), (triangles) 0.052 g/cm3 (H2O), and (circles) 0.26 g/cm3 (H2O). Adsorbed amounts have been converted into the corresponding densities of the confined fluid (defined as the mass of the adsorbed molecules divided by the pore volume). The black solid line shows the carbon dioxide adsorption isotherm when no water molecule is adsorbed. (bottom) Adsorption isotherm for methane at 300 K in a slit carbon nanopore of a width H = 1.4 nm with different amounts of preadsorbed water: (squares) 0.005 g/cm3 (H2O), (triangles) 0.052 g/cm3 (H2O), and (circles) 0.26 g/cm3 (H2O). Adsorbed amounts have been converted into the corresponding densities of the confined fluid. The black solid line shows the carbon dioxide adsorption isotherm when no water molecule is adsorbed.

qualitative agreement with the work by Day et al.13 in which it was found that the same adsorption model can be used to fit the experimental data under all conditions (different moisture contents). Moreover, the pressure at which pore filling is reached is not shifted with respect to that for pure carbon dioxide. The adsorption isotherm obtained when a large amount of water is adsorbed (0.26 g/cm3) also resembles that obtained for pure carbon dioxide. Nevertheless, in this case, the adsorbed amount at low fugacity increases more rapidly with fugacity than for pure carbon dioxide. This result is due to the non-negligible water/CO2 interaction that enhances the adsorption capacity of the porous carbon by increasing the average adsorption enthalpy at a given loading. This is in contrast to the experimental data which suggest that, even for large amounts of adsorbed water, the CO2 adsorption is not enhanced by the presence of the adsorbed water molecules. However, we note that given the pore disorder of the experimental sample (pore size distribution and complex pore shape), the effect of water adsorbed on the adsorption of guest molecules is necessarily attenuated. Further studies including molecular simulations for a sample with a pore size distribution and morphological disorder are needed to clarify this issue. Our simulated data show that the maximum adsorbed amount of carbon dioxide taken at a fugacity of ∼2 MPa decreases linearly with the number of adsorbed water molecules in the nanopore (Figure 9). It should be emphasized that the effect of preadsorbed water on the adsorption of CO2 is more marked in the simulated data than in the experimental data. As mentioned above, we believe that quantitative differences between the experimental and simulated data are due to the fact that the model considered in the simulations does not account for all of the features of the Langmuir 2011, 27(3), 1015–1024

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Figure 9. Maximum simulated adsorbed amount of carbon dioxide (closed symbols) and methane (open symbols) at 300 K in a slit carbon nanopore as a function of the amount of preadsorbed water. The maximum adsorbed amounts of carbon dioxide and methane are taken at a fugacity of ∼2 and 24 MPa, respectively.

Figure 11. (top) Molecular configurations for methane adsorbed at 300 K in a slit carbon nanopore of a width H = 1.4 nm with 0.052 g/cm3 of preadsorbed water. Gray spheres are the carbon atoms of the slit nanopore while cyan spheres are methane molecules. Red and white spheres are the oxygen and hydrogen atoms of the water molecules. These molecular configurations were taken at full loading of the pore. (bottom) Same as left after removing the methane molecules for the sake of clarity.

Figure 10. (top) Molecular configurations for carbon dioxide adsorbed at 300 K in a slit carbon nanopore of a width H = 1.4 nm the carbon atoms of the slit nanopore. Purple and red spheres are the carbon and oxygen atoms of the carbon dioxide molecules. Red and white spheres are the oxygen and hydrogen atoms of the water molecules. These molecular configurations were taken at full loading of the pore. (bottom) Same as left after removing the carbon dioxide molecules for the sake of clarity.

experimental sample (microporosity, pore wall thickness, composition, and density). We show in Figure 10 a molecular configuration for carbon dioxide in the slit carbon nanopore with 0.052 g/cm3 of preadsorbed water. As in the case of the adsorption of pure carbon dioxide, significant layering of carbon dioxide in the presence of water is observed. Close inspection of the molecular configuration after removing the carbon dioxide molecules (Figure 10) reveals that the water molecules form isolated clusters due to the nonfavorable carbon dioxide/water interaction (i.e., hydrophobic interaction). We note that the possible formation of water clusters upon the adsorption of CO2 in wet coals has been suggested in the literature.13 Figure 8 also shows the adsorption isotherms for methane at 300 K in the slit carbon nanopore in the presence of adsorbed water. Again, adsorbed amounts have been converted into the corresponding densities of confined methane. For all amounts of adsorbed water (0.005, 0.052, and 0.26 g/cm3), the adsorption isotherms for methane resemble that obtained for pure methane. Langmuir 2011, 27(3), 1015–1024

The pore filling mechanism is not affected by the presence of water molecules; the adsorbed amount increases in a continuous way with increasing fugacity. As in the case of carbon dioxide, the maximum adsorbed amount of methane taken at a fugacity of 25 MPa decreases linearly with the number of guest water molecules (Figure 9). We also show in Figure 11 a molecular configuration for methane in the slit carbon nanopore when 0.052 g/cm3 of water is already adsorbed. Again, as in the case of the adsorption of pure methane, significant layering of confined methane is observed in the presence of water. Close inspection of the molecular configuration after removing the methane molecules (Figure 11) reveals that the water molecules form isolated clusters due to the nonfavorable methane/water interaction. We note that this effect is more marked for methane than for CO2 due to the fact that the methane/water interaction is weaker than the CO2/water interaction (in the latter case, there is a nonnegligible Coulomb interaction that arises from the partial charges carried by the atoms of each molecule).

4. Discussion and Conclusion In this paper, we report experiments and molecular simulations of the adsorption of CO2 and CH4 in nanoporous carbons in the presence of water. Both the experimental and simulated data show that filling of the activated carbon by carbon dioxide and methane occurs in a continuous and reversible way. Moreover, for both fluids, the maximum adsorbed amount predicted in the molecular simulation is in fair agreement with the experimental values. The experimental and molecular simulation data for the isosteric heat of adsorption curves are in good agreement; the isosteric heat of adsorption increases in a monotonous way with the adsorbed amount. The latter behavior is consistent with the relative high surface homogeneity of the porous carbon; the porous carbon/fluid contribution to the isosteric heat is nearly constant while the fluid/ fluid contribution increases continuously with loading so that the total isosteric heat of adsorption increases with increasing loading. DOI: 10.1021/la103107t

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Whatever the amount of preadsorbed water molecules, both the experimental and molecular the adsorption isotherms for carbon dioxide and methane resemble those obtained for the pure fluids. The pore filling mechanism does not seem to be affected by the presence of the water molecules. Moreover, the pressure at which the maximum adsorbed amount of methane or carbon dioxide is reached is nearly insensitive to the loading of preadsorbed water molecules. In contrast, due to the presence of the water, the adsorbed amount of carbon dioxide or methane decreases linearly with the number of guest water molecules. As a result, for large amounts of adsorbed water molecules, the change in the adsorption isotherm corresponding to pore filling is attenuated as a non-negligible part of the porosity is already occupied by water and, therefore, cannot be filled by carbon dioxide or methane. Typical molecular configurations obtained using molecular simulation indicate that the water molecules form isolated clusters within the host porous carbon. The latter result is due to the nonfavorable interaction between carbon dioxide or

methane and water. The presence of other fluids with CO2-philic or CH4-philic interactions would probably act in a different way by favoring, for instance, the condensation of these two gases. In future work, we plan to consider porous carbons (such as coal) which exhibit pores with significant surface and morphological disorders and heterogeneities. Realistic numerical models of such disordered porous carbons, which are available in the literature, could be used to address the effect of the surface and morphological disorders on the adsorption/condensation of methane and carbon dioxide.69,70

(69) Pikunic, J.; Clinard, C.; Cohaut, N.; Gubbins, K. E.; Guet, J. M.; Pellenq, R. J. M.; Rannou, I.; Rouzaud, J. N. Langmuir 2003, 19, 8565. (70) Brochard, L.; Vandamme, M.; Coussy, O.; Pellenq, R. J. M. Langmuir, to be submitted.

Supporting Information Available: Figure S1. This material is available free of charge via the Internet at http:// pubs.acs.org.

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Acknowledgment. This project was funded in part by the French National Research Agency ANR (research project “SIMONANOMEM” ANR-07-NANO-055-04). Calculations were performed using supercomputers at the Institut de Developpement et des Ressources en Informatique Scientifique (IDRIS, CNRS, grant no. 96223).

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