Characterization and Adsorption Modeling of Silicon Carbide-Derived

Nov 27, 2008 - Finally, the correlation between compressibility of the Si-CDC samples under high pressure adsorption and their synthesis temperature w...
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Langmuir 2009, 25, 2121-2132

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Characterization and Adsorption Modeling of Silicon Carbide-Derived Carbons T. X. Nguyen, J.-S. Bae, and S. K. Bhatia* DiVision of Chemical Engineering, The UniVersity of Queensland, St. Lucia, Brisbane, QLD 4072, Australia ReceiVed August 22, 2008. ReVised Manuscript ReceiVed NoVember 27, 2008 We present characterization results of silicon carbide-derived carbons (Si-CDCs) prepared from both nanoand micron-sized βSiC particles by oxidation in pure chlorine atmosphere at various synthesis temperatures (600-1000 °C). Subsequently, the adsorption modeling study of simple gases (CH4 and CO2) in these Si-CDC samples for a wide range of pressures and temperatures using our Finite Wall Thickness model [Nguyen, T. X.; Bhatia, S. K. Langmuir 2004, 20, 3532] was also carried out. In general, characterization results showed that the core of Si-CDC particles contains predominantly amorphous material while minor graphitization was also observed on the surface of these particles for all the investigated synthesis temperatures (600-1000 °C). Furthermore, postsynthetic heat treatment at 1000 °C for 3 days, as well as particle size of precursor (βSiC) were shown to have slight impact on the graphitization. In spite of the highly disordered nature of Si-CDC samples, the adsorption modeling results revealed that the Finite Wall Thickness model provides reasonably good prediction of experimental adsorption data of CO2 and CH4 in all the investigated Si-CDC samples at the temperatures of 273 K, 313 K, and 333 K for a wide range of pressure up to 200 bar. Furthermore, the impact of the difference in molecular size and geometry between analysis and probing gases on the prediction of the experimental adsorption isotherm in a disordered carbon using the slit-pore model is also found. Finally, the correlation between compressibility of the Si-CDC samples under high pressure adsorption and their synthesis temperature was deduced from the adsorption modeling.

Introduction Carbide-derived carbon (CDC) has recently emerged as a very promising carbonaceous microporous material for gas mixture separation and energy storage because of its high surface area, well-tunable pore sizes, and very high capacitance.2,3 In practice, CDC is routinely prepared by oxidation of a carbide precursor in pure chlorine atmosphere or its mixture with H2.3,4 Microporosity of CDC is mainly created from extraction of metal out of the crystal structure of the carbide precursor. Consequently, the structure of CDC is not only dependent upon the chemical structure of its carbide precursor but also synthesis conditions as well as the size of carbide particles. In particular, CDC derived from βSiC contains a significant fraction of sp3 C-C bonds inherited from sp3 Si-C bonds, while that derived from TiC contains a significant fraction of sp2 C-C bonds inherited from sp2 Ti-C bonds. Further, silicon CDC (Si-CDC) derived from micron-sized carbide particles is more ordered than that derived from nanosized carbide particles.5 Following the recent international surge in interest in searching for porous materials for energy storage, CDCs have been prepared from several different carbide precursors (e.g., SiC, TiC, VC, ZrC).6-8 Much work on the characterization of CDC using various * To whom correspondence may be addressed. E-mail: [email protected]. (1) Nguyen, T. X.; Bhatia, S. K. Langmuir 2004, 20, 3532. (2) Chmiola, J.; Yushin, G.; Gogotsi, Y.; Portet, C.; Simon, P.; Taberna, P. L. Science 2006, 313, 1760–1763. (3) Gogotsi, Y.; Nikitin, A.; Ye, H.; Zhou, W.; Fischer, J. E.; Yi, B.; Foley, H. C.; Barsoum, M. W. Nat. Mater. 2003, 2, 591–594. (4) Gogotsi, Y.; Welz, S.; Ersoy, D. A.; McNallan, M. J. Nature 2001, 411, 283–286. (5) Dash, R. K. Ph.D. Thesis, Drexel University, Chapter 4, p 102, https:// idea.library.drexel.edu/handle/1860/867, 2006. (6) Yushin, G.; Dash, R.; Jagiello, J.; Fischer, J. E.; Gogotsi, Y. AdV. Funct. Mater. 2006, 16, 2288–2293. (7) Gogotsi, Y.; Dash, R. K.; Yushin, G.; Yildirim, T.; Laudisio, G.; Fischer, J. E. J. Am. Chem. Soc. 2005, 127, 16006–16007. (8) Urbonait, S.; Wachtmeister, S.; Mirguet, C.; Coronel, E.; Zou, W. Y.; Csillag, S.; Svensson, G. Carbon 2007, 45, 2047–2053.

methods such as X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectra, and adsorption has also been carried out.2-8 Among these, the adsorption method has played an important role in searching for the optimum pore size for largest energy storage capacity as well as highest selectivity.3 However, although pore size has been well-recognized as one of decisive factors that dictates adsorption dynamics and equilibrium of adsorbate in porous carbons, pore connectivity has recently been shown to have a severe impact on adsorption behaviors.9,10 In this regard, CDC derived from Ti3SiC2 at 300 °C was shown to have stronger adsorption of small molecules compared with that prepared at higher temperatures. However, this CDC was also experimentally shown to display an accessibility problem with differences in Ar and N2 surface area.3 This accessibility problem, occurring because of the highly disordered nature of the CDC, particularly when prepared at low synthesis temperature, has received little attention in the existing literature. A combination of characterization and adsorption modeling is necessary to obtain a comprehensive understanding of the impact of not only pore structure but also pore topology of CDC materials on adsorption behavior. Such understanding is crucial for synthesizing advanced porous carbons with optimum microstructure for a specific purpose. Although there has been a report on atomistic construction of TiC using the reverse Monte Carlo (RMC) simulation technique,11 adsorption modeling for CDCs, especially Si-CDC, has scarcely been carried out. Therefore, in the current work we present the characterization of Si-CDCs as well as the adsorption modeling study of simple gases (CO2 and CH4) in these Si-CDCs at temperatures of 273, 313, and 333 K for a wide range of pressures up to 200 bar. Si-CDC samples used in this study were prepared (9) Nguyen, T. X.; Bhatia, S. K. J. Phys. Chem. C 2007, 111, 2212–2222. (10) Nguyen, T. X.; Bhatia, S. K. Langmuir 2008, 24, 146–154. (11) Zetterstro¨m, P.; Urbonaite, S.; Lindberg, F.; Delaplane, R. G.; Leis, J.; Svensson, G. J. Phys.: Condens. Matter 2005, 17, 3509–3524.

10.1021/la8027429 CCC: $40.75  2009 American Chemical Society Published on Web 01/05/2009

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in our laboratory from both nano- and micron-sized βSiC particles at various temperatures (600-1000 °C). The characterization task was carried out using helium pycnometry, XRD, and highresolution TEM (HRTEM). In addition, our Finite Wall Thickness (FWT) model,1 utilizing nonlocal density functional theory (NLDFT), was used for characterization using Ar adsorption at 87 K, and for the prediction of experimental adsorption isotherms of CO2 and CH4 in Si-CDC. The FWT model was previously shown to be a good model for the characterization of activated carbons and prediction of the adsorption isotherms of simple gases in these carbons.12,13 The Si-CDCs are generally more amorphous than activated carbons. Therefore, it is also interesting to study the performance of the graphitic single slit-pore models such as the FWT model in terms of prediction of adsorption behaviors of simple gases in the Si-CDCs.

2. Experimental Section In this work, CDCs were synthesized from commercial micronand nanosized βSiC powders provided by Sigma-Aldrich. Chlorination experiments were performed on SiC powders using ultrahigh purity chlorine (BOC Gases, 99.9%) and ultrahigh purity argon (BOC Gases, 99.999%) as reactive and purging gases, respectively. For CDC synthesis, we adopted the CDC preparation procedure described in detail by Nikitin et al.14 In the procedure, the SiC powder was placed in a quartz sample boat and loaded into a horizontal quartz tube furnace. The tube with the quartz sample boat inside was initially purged by argon for 30 min before a heating rate of 30 °C per minute was employed to raise the temperature of the furnace to that desired for chlorination. The argon purge was maintained through the tube furnace during the heating process until the temperature of tube furnace reached the set reaction temperature. The argon purge was stopped and replaced with pure chlorine flow to begin the chlorination step once the desired temperature was attained and stabilized. In the chlorination experiment, pure Cl2 gas flowed over the SiC sample at the rate of 50 mL per minute until the chlorination reaction of SiC was completed. After the completion of the chlorination reaction, the Ar flow was used to cool the sample down to ambient temperature as well as to flush metal chlorides remaining in the sample. The residual chlorine and metal chlorides were captured in sodium hydroxide solution. For convenience, the short name “SiCDC-X-Y-Z” will be used for Si-CDCs throughout the text. In particular, X denotes the size of SiC precursor, while Y depicts the synthesis temperature of the CDC. Z depicts the condition of the post-treatment of CDC sample. For instance, SiCDC-50NM-1000H3 represents a CDC sample prepared from 50 nm SiC particles at a synthesis temperature of 1000 °C and followed by a 3 day heat treatment at the same temperature. For the characterization, the skeletal density of the Si-CDC samples was measured using a helium pycnometer (Micromeritics Accupyc 1330). XRD analysis was done using Cu KR radiation (40 kV, 40 mA, l ) 1.54056 Å), with a step size of 0.02° (2θ) and fixed slit mode. Raman spectroscopy was measured using a Renishaw Ramanscope Raman spectrometer. 1K series He-Cd laser excitation (325 nm) was utilized. The local atomistic structure of Si-CDC samples was analyzed from HRTEM images obtained using a TECHNAI F30GTEM cryo-electron transmission electron microscope), with optimal resolution of 1.5 nm at 300 kV. Pore structure analysis of Si-CDC samples was performed by interpretation of argon adsorption isotherm at 87 K using our FWT model.1 For adsorption measurements, a Micromeritics ASAP2020 volumetric adsorption analyzer was used to obtain the adsorption isotherm data of argon at 87 K and CO2 at 273 K. High-pressure adsorption data of CO2 and CH4 at 313 and 333 K, up to 200 bar pressure, were (12) Nguyen, T. X.; Bhatia, S. K.; Nicholson, D. Langmuir 2005, 21, 3187– 3197. (13) Nguyen, T. X.; Bhatia, S. K. J. Phys. Chem. B 2004, 108, 14032–14042. (14) Nikitin, A.; Gogotsi, Y. Nanostructured carbide-derived carbon. In Encyclopedia of Nanoscience and Nanotechnology; Nalwa, H. S., Ed.; American Scientific Publishers: Valencia, CA, 2003; Vol. 7, pp 553-574.

Nguyen et al. measured using a gravimetric sorption system (Rubotherm Pra¨zisionsmesstechnik GmbH, Bochum, Germany). The detailed description of the high-pressure adsorption measurement procedure is given elsewhere.15

3. Mathematical Modeling As indicated above, we utilized our recently proposed characterization procedure, the FWT model,1 to obtain the pore size distribution (PSD) and pore wall thickness distribution (PWTD) of the Si-CDC samples. Subsequently, these characteristic results (PSD and PWTD) were used to predict the adsorption of CH4 and CO2 in these Si-CDC samples. The FWT model has been previously described in detail elsewhere.1 In brief, the FWT model, based on the slit-pore model, approximates the microstructure of porous carbons in terms of a set of independent slit-like pores, with carbon walls having a finite number of perfect graphene sheets. The general adsorption isotherm (GAI), in terms of excess adsorbed amount, Γex(P), is given as

Γex(P) )

∫ [Fˆ (P, Hin) - Fb]f(Hin)dHin

(1)

where f(Hin) is the PSD of the adsorbent, Hin () Hcc - σc) is the geometrical pore width, with Hcc being the physical pore width, σc is the effective diameter of the carbon atom, and Fb is the bulk density. Fˆ (P, Hin) is the average density in a pore of width Hin at a given pressure P. The average density is expressed16 as ∞

∞

m)1

l)1

Fˆ (P, Hin) ) Σ p(m) Σ p(l)

1 Hin

Hcc

∫ Flm(P, Hin, z)dz

(2)

0

where p(n) is a wall thickness probability distribution, and Flm(P,Hin,z) is the local density profile in a pore of geometrical width Hin, with the left wall having l graphene layers and the right wall having m layers. Here we assume that the thicknesses of the two opposing walls of a pore are uncorrelated, and that the interaction potential between adsorbed molecules in neighboring pores is insignificant in comparison to the fluid-solid potential energy, justified by our recent study.13 The local density profile is determined using Tarazona NLDFT17 with fluid-fluid and solid-fluid potential models described as follows. Potential Models. The interaction potential between fluid molecules is taken as the Lennard-Jones (LJ) 12-6 potential

[( ) ( ) ]

φff(r) ) 4εff

σff r

12

-

σff r

6

(3)

where r is the interparticle distance, σff is the collision diameter, andεff isthewelldepth.FollowingtheWeeks-Chandler-Andersen (WCA) separation,18 the attractive part of this potential is represented as

φatt(|r - r′|) ) φff(|r - r′|) ) -εff

|r - r′| > rm |r - r′| < rm

(4)

where rm ) 21/6 σff. The interaction potential, φwf(z), between a carbon wall and an adsorbate molecule is represented by19,20 (15) Bae, J. -S.; Bhatia, S. K. Energy Fuels 2006, 20, 2599–2607. (16) Bhatia, S. K. Langmuir 1998, 14, 6231–6240. (17) Tarazona, P. Phys. ReV. A 1985, 31, 2672–2679. Tarazona, P. Phys. ReV. A 1985, 32, 3148–3148. Tarazona, P.; Marconi, U. M. B.; Evans, R. Mol. Phys. 1987, 60, 573–595. (18) Weeks, J. D.; Chandler, D.; Anderson, H. C. J. Chem. Phys. 1971, 54, 5237–5247. (19) Steele, W. A. Surf. Sci. 1973, 36, 317–352. (20) Mays, T. J. Simulations of adsorption and the design of activated carbons. In Fundamentals of Adsorption; LeVan, M. D. Ed.; Kluwer Academic Publishers: Boston, 1996; pp 603-610.

Adsorption Modelling of Si-CDCs

[(

n-1

φwf(z, n) ) 2πFcσcf2∈cf Σ

i)0

2 σcf 5 z + i∆

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) ( 10

-

)]

σcf 4 , z + i∆ z>0

(5)

Here z is the (center-to-center) distance between the fluid molecule and the pore wall surface, n is the number of graphene layers in the pore wall, ∆ is the interlayer spacing, and Fc is the number of carbon atoms per unit area in a single graphene layer. Following Steele,19 the parameters ∆ ) 0.335 nm and Fc ) 38.17 atoms · nm-2, corresponding to graphite, are used. The asymmetric external potential profile, φext(z,l,m), for a slit-shaped pore having pore size H is determined from superposition of the potentials of the opposing pore walls

φext(z, l, m) ) φwf(z, l) + φwf(H - z, m)

(6)

where l and m are the arbitrary numbers of graphene layers in the two opposing walls. From eq 1, the PSD and PWTD of a porous carbon can be simultaneously determined by minimization of the difference between theoretical and measured excess adsorbed amounts of Ar adsorption at 87 K. Conversely, if the PSD and PWTD are known, the theoretical excess adsorbed quantity of adsorbate in a carbon can be determined. In particular, for the prediction of experimental high-pressure adsorption data measured using the gravimetric technique, the theoretical excess adsorbed quantity of adsorbate, as shown in our recent work,21 is given as

mfex ) mfa -

mHe a FHe b

Ffb - ∆VmaxFfb

(7)

where mfa is the absolute adsorbed quantity and is directly determined using the FWT model with the assumption of rigidity of pore structure. mHe a is the absolute adsorbed quantity of helium at its corresponding bulk density, FbHe. In our recent work,21 the He ratio mHe a / Fb was shown to be very similar to the pore volume obtained by interpretation of argon adsorption at 87 K using the FWT model. Accordingly, in this work we replaced this ratio with the argon pore volume obtained from interpretation of argon adsorption at 87 K using the FWT model.1 Further, Fbf is the bulk density of analysis gas, and ∆Vmax is defined as the maximum difference between helium and analysis gas external adsorbent 21 f volumes (VHe as ext and Vext, respectively), and is given

∆Vmax ) Vfext(max) - VHe ext

Figure 1. (a) Variation of helium density of SiCDC samples with synthesis temperature. (b) Helium density of post-treated forms of the SiCDC50NM-1000. CDC-H1 and CDC-H5 are heat-treated forms of the SiCDC50NM-1000 carbon at 1000 °C for 1 and 5 days, respectively. CDCGCO2 depicts the CO2 gasified form of SiCDC-50NM-1000 carbon at 800 °C or SiCDC-50NM-1000-GCO2. CDC-GCO2-H1 represents the 1 day heat-treated form of the gasified carbon, SiCDC-50NM-1000-GCO2.

scale, leading its negligible contribution to the external volume of the carbon particle. Thus, eq 7 can be rewritten as

mfex ) Γfex - ∆VmaxFfb

(8)

For a particular porous carbon, it can be deduced that ∆Vmax is dependent upon accessibility of adsorbed gas in the adsorbent particles. Accordingly, ∆Vmax may have a complex temperature dependence. In practice, ∆Vmax can only be determined such that the excess theoretical adsorbed quantity matches the corresponding experimental data for a wide range of high pressure at which the bulk density of adsorbing gas approaches the adsorbed density. An advantage of eq 7 is that it enables one to avoid the uncertainty of determination of the Gibbs dividing surface between the solid and adsorbed phases in a disordered porous carbon. Another advantage of eq 7 that can be easily seen is that the external volume of a carbon particle is insensitive to the precise location of the boundary that separates the particle from the bulk fluid phase, because, for the case of high surface area carbon, the external surface area of the carbon particle is negligible in comparison to the internal surface area. Accordingly, the variation of this boundary for different adsorbates is within the angstrom (21) Nguyen, T. X.; Cohaut, N.; Bae, J.-S.; Bhatia, S. K. Langmuir 2008, 24, 7912–7922.

(9)

Γfex

where the term on the right-hand side of eq 9 is defined as Γex on the left-hand side of eq 1

4. Results and Discussion 4.1. Characterization of Si-CDCs. We present here the characterization results of a series of Si-CDCs prepared from both 50 nm and 50 µm SiC particles by chlorination of pure βSiC at various temperatures (600-1000 °C), as mentioned in the Experimental Section. Furthermore, the characterization of specifically prepared post heat-treated forms of the SiCDC-50NM1000 sample at 1000 °C for a period of 1 or a few days, as well as its gasified forms at 1100 °C followed by 1 day heat treatment were also carried out. As discussed earlier, the characterization task involved the investigation of both solid and pore structures. 4.1.1. Solid Structure. Helium Density. Helium density is the so-called skeletal density. Figure 1 depicts a slightly gradual increase in the helium density of Si-CDC carbon samples, prepared from 50 nm SiC particles, with increasing synthesis temperature (600-1000 °C), indicating a pore accessibility problem for helium in the microstructure of the investigated Si-CDC samples. Such

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a pore accessibility problem results in an enhancement of helium adsorbent volume due to incomplete contact of helium to the carbon surface and then decrease in density. This trend is very similar to that observed for activated carbon composites at early stages of gasification.22 A possible explanation is that this trend is due to the highly disordered structure of Si-CDC samples prepared at low synthesis temperature, such as SiCDC-50NM600. Such highly disordered structure is more likely to result in the formation of locally constricted spaces or pore mouths, which prevent helium molecules from deeply penetrating the porous structure, and this in turn lowers helium density. It is interesting to see that, regardless of synthesis temperature, the helium density of these carbons (2.5-2.7 g/cm3) is significantly higher than the true density of graphite, suggesting the presence of a significant fraction of sp3 C-C bonds in the investigated Si-CDC samples. The presence of a high fraction of sp3 C-C bonds in Si-CDC samples may be related to the sp3 hybridized state of carbon atoms in the Si-C bonds of their βSiC precursor. Furthermore, we also observed that, with further heat treatment at 1100 °C for a period of 1 or 3 days (3.2-3.4 g/cm3), the helium density of the SiCDC-50NM-1000 sample dramatically increased in comparison with its virgin carbon (2.7 g/cm3) and approached the true density of diamond (3.5 g/cm3). This indicates predominance of sp3 C-C bonds in the heat-treated Si-CDC samples, as observed for tetrahedral carbon (ta-C). However, this may not necessarily indicate the formation of new sp3 C-C bonds during heat treatment. Instead, there is more likely an increase in pore accessibility to helium due to the more ordered nature of heattreated forms compared with that of their virgin carbons. This hypothesis is supported by the fact that, when the SiCDC-50NM1000 carbon was partially gasified with carbon dioxide at 800 °C (∼23% of carbon conversion), the helium density (3.4 g/cm3) of the gasified form of this carbon, named as Si-CDC50NM-1000-GCO2, significantly increases compared with that of its virgin carbon (2.7 g/cm3). Accordingly, this clearly indicates that the CO2 gasification eliminates local bottle-necks in the virgin carbon, leading to a dramatic increase in helium accessibility of the carbon, and consequently also the increase in helium density of its gasified form. Such increase in gas accessibility to porous carbons by slight CO2 gasification has recently been experimentally reported.23 X-ray Diffraction. Figure 2a,b shows XRD patterns of SiCDC samples prepared at various temperatures (600-1000 °C) for 50 nm and 50 µm SiC particles, respectively. In addition, for comparison, the XRD pattern of the precursor (βSiC) is also plotted. From these figures, it can be seen that, regardless of the size of the SiC particle used for preparation of the investigated Si-CDC samples the 002 peak, which pertains to interlayer spacing between two graphitic sheets, is highly diffusive, suggestive of the highly amorphous nature of these Si-CDC samples. However, the 002 peak of Si-CDC samples prepared from 50 µm βSiC particles is significantly sharper than that of samples prepared from 50 nm SiC particles, indicating an effect of the size of SiC particle on graphitization degree of the investigated Si-CDC samples. This will be clarified in the later section. In addition, Figure 3a depicts XRD patterns of heat-treated forms of the SiCDC-50NM-1000 sample at 1100 °C for periods of 1 and 3 days. From this figure, it is seen that there is growth of a small sharp 002 peak of the heat-treated form, SiCDC-50NM-1000H3, in a period of 3 days, in comparison with that of its virgin (22) Tran, K.; Berkovich, A. J.; Tomsett, A.; Bhatia, S. K. Energy Fuels 2008, 22, 1902–1910. (23) Garrido, J.; Linares-Solano, A.; Martı´n-Martı´nez, J.; Molina-Sabio, M.; Rodrı´guez-Reinoso, F.; Terregrosa, R. J. Chem. Soc., Faraday Trans. 1 1987, 83, 1081–1088.

Nguyen et al.

Figure 2. XRD patterns of Si-CDC samples prepared from (a) 50 nm SiC particles and (b) 50 µm SiC particles.

carbon SiCDC-50NM-1000. This indicates a slight increase in the graphitization after heat treatment. The minor extent of graphitization observed for the heat-treated form, SiCDC-50NM1000-H3, is due to the fact that the heat treatment temperature used (1100 °C) is lower than the experimental as well as theoretically predicted temperature for the occurrence of predominant graphitization in ta-C carbon.24 From Figure 3b the XRD pattern of the CO2 gasified form of the Si-CDC-50NM1000 sample at 800 °C, Si-CDC-50NM-1000-GCO2, indicates a very minor graphitization compared with that of its precursor. However, this subtle extent of graphitization may lead to dramatic increase in pore accessibility for helium in the CO2 gasified form, Si-CDC-50NM-1000-GCO2, and therefore a significant increase in helium density of this carbon, as mentioned above. Similarly, the XRD of the heat-treated form of the Si-CDC50NM-1000-GCO2 sample at 1100 °C for a period of one day, Si-CDC-50NM-100-GCO2-H1, showed only a slight extent of graphitization compared with its precursor. Transmission Electron Microscopy. Figure 4a-d illustrates TEM images of four Si-CDC carbons prepared from 50 nm βSiC particles at various temperatures of 600, 700, 800, and 1000 °C. From the figure, it is clearly observed that there is a predominance of amorphous material in the core of these four investigated Si-CDC samples, while a small amount of graphitized material was observed on the shell of all the investigated Si-CDC particles. Such small graphitic crystallites are associated with very small 002 peaks in XRD patterns, presented in the above section. From Figure 4b, it is interesting to see the presence of small diamondlike crystallites between graphitic regions at the shell of the SiCDC-50NM-700 carbon particle prepared at 700 °C. Figure (24) Marks, N. A.; Cover, M. F.; Kocer, C. Mol. Simul. 2006, 32, 1271–1277.

Adsorption Modelling of Si-CDCs

Figure 3. XRD patterns of post-treated forms of the Si-CDC-50NM1000 carbon. (a) Heat-treated samples at 1000 °C for 1 or 3 days (SiCDC50NM-1000-H1 and Si-CDC-50NM-1000-H3, respectively), and (b) the CO2-gasified form of the Si-CDC-50NM-1000 carbon at 800 °C, Si-CDC-50NM-1000-GCO2, and the heat-treated form of the Si-CDC50NM-1000-GCO2 carbon at 1100 °C for a period of 1 day, Si-CDC50NM-1000-GCO2-H1.

5a depicts the TEM image of the sample heat-treated at 1100 °C for a period of 5 days, named as SiCDC-50NM-1000-H5. From this figure, a more abundant amount of graphitic sheets on the shell of the heat-treated form, SiCDC-50NM-1000-H5, compared with that of its virgin carbon, SiCDC-50NM-1000, is evident, while amorphous structure is predominant in the core of the heat-treated carbon. Such graphitization observed on the surface of Si-CDC particles was also reported in the literature.25 The favorable graphitization on the shell of Si-CDC particles can be understood as a breakdown of the symmetry of the sp3 hybridized state of carbon atoms on the surface of Si-CDC particles, which leads to the favorable formation of sp2. In contrast, the sp3 hybridized state of carbon atoms inside the core of Si-CDC particles can be preserved because these carbon atoms are able to form a tetrahedral-like structure with their neighboring carbon atoms once Si is removed from its sp3 Si-C bond by chlorination. Figure 5b depicts the TEM image of the gasified form of the SiCDC-50NM-1000 carbon, named as SiCDC-50NM-1000GCO2 in the above subsection. From this figure, in spite of more prevalent graphitization observed for SiCDC-50NM-1000-GCO2 in comparison with that of its virgin carbon, SiCDC-50NM1000, predominance of amorphous material was again observed for the former carbon. (25) Welz, S.; McNallan, M. J.; Gogotsi, Y. J. Mater. Process. Technol. 2006, 179, 11–22.

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Raman Spectra. Figure 6a depicts Raman spectra of Si-CDC samples (SiCDC-50NM-600, SiCDC-50NM-800, and SiCDC50NM-1000), while its inset illustrates the ID/IG ratio versus synthesis temperatures. ID/IG was determined as the peak height ratio between the D and G bands of Raman spectra in Figure 6a. Farrari and Robertson26 showed a correlation between the ID/IG ratio and the average radius of a graphitic sheet, La. In particular, for La less than 20 Å, the ID/IG ratio increases with increasing La, while for La greater than 20 Å, the ID/IG ratio decreases with increasing La. Thus, the former trend is relevant for the case of the investigated Si-CDC. Accordingly, from the inset of Figure 6a, it can be seen that the ID/IG ratio decreases with increasing synthesis temperature, indicating an increase in the order of microstructure of Si-CDC samples with synthesis temperature. This is consistent with the XRD data and TEM images of these Si-CDC carbons, discussed above. Similarly, Figure 6b depicts Raman spectra of the SiCDC-50NM-1000 sample and their heattreated forms at 1000 °C for 1 and 3 days, SiCDC-50NM-1000H1 and SiCDC-50NM-1000-H3, respectively. From Figure 6b, it can be seen that there is a shift of the G peak to smaller frequency after a heat treatment period of 1-3 days, while the ID/IG ratio increases to 0.39 from 0.36 after a 3 h heat treatment. Such increase in the ID/IG ratios with increasing heat treatment time is not necessarily associated with decrease in ordering. Instead, for the fairly high ordered SiCDC-50NM-1000 and its heattreated forms (SiCDC-50NM-H1 and SiCDC-50NM-1000-H3), the increase in the ID/IG ratio with longer heat-treatment is associated with higher graphitization, as shown by Ferrari and Robertson.26 This is consistent with an increase of the 002 peak in XRD pattern observed for the former carbon compared with that of its virgin carbon. 4.1.2. Pore Structure. Figure 7 depicts the PSD of Si-CDC samples prepared from nanosized SiC particles at three synthesis temperatures (600, 800, and 1000 °C), SiCDC-50NM-600, SiCDC-50NM-800, and SiCDC-50NM-1000, respectively. These PSD results were determined by interpretation of Ar adsorption at 87 K using NLDFT with the FWT model.1 From this figure, it can be observed that the first peak of the PSD of the SiCDC-50NM-1000 sample is significantly higher than that of the Si-CDC-50NM-600 sample, while the remaining peaks of the PSD of these three carbon samples are very similar. Such increase in microporosity with increasing synthesis temperature is due to increase in pore accessibility of adsorbing gases, particularly argon, as mentioned for the case of helium density in subsection 4.1.1. The fast reaction of nanosized SiC particles with chlorine leads to the formation of a very disordered structure, especially at low temperatures such as 600 °C at which diffusion of carbon atoms is very slow. Such highly disordered nature of SiCDC50NM-600 leads to the formation of highly constricted pore mouths in their microstructure, which gives rise to the problem of pore accessibility to Ar adsorbate molecules at 87 K.9,10 This is very consistent with the increase in helium density of Si-CDC samples with increasing synthetic temperature, as mentioned in subsection 4.1.1. Consequently, the higher graphitization degree of the Si-CDC-50NM-1000 sample compared to that of SiCDC50NM-600 explains the higher pore accessibility of Ar in the former carbon, which is manifested as an increase in the microporosity of Si-CDC samples with synthesis temperature. 4.2. Adsorption Modeling. In this section, we discuss the prediction of adsorption isotherms of CO2 and CH4 in three SiCDCs prepared from 50 nm SiC particles (SiCDC-50NM-600, SiCDC-50NM-800, and SiCDC-50NM-1000) for a wide range of pressures using the FWT model. As mentioned in the above (26) Ferrari, A. C.; Robertson, J. Phys. ReV. B 2000, 61, 14095–14107.

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Figure 4. TEM image of the Si-CDC samples prepared from 50 nm SiC particles at various temperatures (600-1000 °C).

sections, these investigated Si-CDC samples are predominantly amorphous with increasing order toward SiCDC-50NM-1000. It is of interest to use a graphitic slit-pore model such as the FWT model to predict adsorption behaviors in these Si-CDC carbons, in order to investigate the suitability of this model for porous carbons covering a wide range of disorder. For the sake of clarity adsorption modeling, results are presented in order ranging from low to high pressure adsorption. 4.2.1. Prediction of Subatmospheric Adsorption Isotherm of Carbon Dioxide at 273 K. Figure 8a-c depict comparison of excess theoretical adsorption isotherms of CO2 at 273 K in SiCDC50NM-600, SiCDC-50NM-800, and SiCDC-50NM-1000 carbons, determined using the FWT model and the Infinite Wall Thickness (IWT) model, with corresponding experimental data. The excess theoretical adsorption isotherm at low pressure was computed using eq 1. The microstructure of all the investigated Si-CDC samples is assumed to be rigid under this low pressure condition. For the calculation of local adsorption isotherm in a single pore using NLDFT, LJ CO2-CO2 interaction parameters (collision and hard sphere diameters, σf and σh, respectively, and well depth, εf/kB) are tabulated in Table 1. LJ carbon-carbon interaction parameters are taken to be similar to those for graphite: 19 σc ) 3.4 Å and εc/kB ) 28 K. The LJ fluid-carbon interaction parameters (σcf and εcf/kB) are determined using the generalized Lorentz-Berthelot combining rule,

1 φcf ) (φc + φf) 2

(10)

cf ) (1 - kcf)(εcεf)1⁄2

(11)

where kcf is a binary interaction parameter, considered as an adjustable parameter representing wall-fluid interaction contributions in addition to the pure wall-fluid dispersion interaction. For the predictions of the CO2 isotherms using eq 1, characterization results (PSD and PWTD) of the investigated Si-CDC samples were obtained from interpretation of argon adsorption at 87 K using either the FWT model or the IWT model. In addition, the binary interaction parameter value of kcf ) 0 is used for these predictions. It can be generally observed that both FWT and IWT models provide reasonably good agreement between the excess theoretical and corresponding experimental adsorption isotherms for the three investigated SiCDC samples, indicating good approximation of the slit-pore model for even very disordered carbons such as the Si-CDC samples investigated. 4.2.2. Prediction of Experimental High-Pressure Adsorption Data. It is commonly observed that the experimental high pressure excess adsorption isotherm shows a maximum at moderate pressures (around 6-10 MPa). A similar trend is also observed for the high pressure adsorption isotherms of CO2 and CH4 in the SiCDC samples, as presented in Figure 9a-c. In particular, the excess adsorption isotherm significantly increases in the low

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Figure 6. Raman spectra of (a) the Si-CDC samples prepared from 50 nm SiC particles at various temperatures (600-1000 °C), and (b) the heat-treated forms of the SiCDC-50NM-1000 carbon at 1100 °C for 1 and 3 days (SiCDC-50NM-1000-H1 and SiCDC-50NM-1000-H3, respectively). The inset of panel a illustrates ID/IG ratio against synthesis temperature. Figure 5. TEM image of post heat-treated forms of the SiCDC-50NM1000 virgin carbon. (a) The heat-treated form at 1000 °C for 5 days, and (b) the CO2 gasified form at 800 °C for 90 min (∼23% carbon conversion).

pressure range (left adsorption branch) because of the predominance of the adsorption contribution and decreases in the higher pressure range (right adsorption branch) due to predominance of the bulk contribution in this region. Accordingly, it can be seen that the right adsorption branch is strongly dependent on the pore volume of the porous solid, but not the left branch because of the high bulk density in the high-pressure range, especially at low temperature. In this regard, it can be observed from Figure 9a,c that, although experimental high pressure excess adsorbed quantity of CO2 in the 1000 °C SiCDC sample is higher than that in the 600 °C sample for the low pressure range due to predominant contribution of adsorption, the excess adsorbed quantity for the former sample is lower than that for the latter, in the high pressure range where there is a greater increase in bulk density compared to the adsorbed density. Consequently, the larger pore volume of the 1000 °C sample compared to the 600 °C sample leads to higher excess adsorbed quantity of the former sample in the low pressure region, but lower excess adsorption in the high pressure region where the excess adsorbed quantity increases with increasing pressure. However, the pore volume used for the estimation of the theoretical excess adsorbed quantity is physically ill-defined for a disordered porous carbon at the atomic level because of the roughness of its carbon surface. Furthermore, the adsorbent volume of porous carbon can undergo

Figure 7. PSD of the Si-CDC samples prepared at various temperatures (600-1000 °C), obtained by the interpretation of Ar adsorption at 87 K using the FWT model.

swelling and shrinking under high pressure adsorption, whereas the theoretical excess adsorbed quantity is normally estimated with the assumption of rigidity of the model carbon structure. Consequently, these issues lead to a significant deviation of the theoretical excess adsorption isotherm from the experimental one in the high-pressure range. In order to predict experimental high-pressure adsorption data presented in the current work, the excess theoretical high pressure adsorption data were computed

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Figure 8. Comparison between predicted adsorption isotherms of CO2 at 273 K in the three Si-CDC samples, using FWT and IWT models (solid and dashed lines, respectively), and corresponding experimental data. Table 1. LJ Parameters Used with NLDFT Calculations and GCMC Simulations, for the Investigated Gases Used in This Work gas

σff (Å)

εff/kB (Å) σcf (Å) NLDFT

CH4 CO2 N2

3.6177 3.472 3.572

146.91 221.98 93.98

CO2

3.720

236.10

3.509 3.436 3.494

εsf/kB (Å)

source

64.14 78.84 53.22

10 10 28

81.30

21

GCMC 3.56

using eq 9, which considers possible deformation of adsorbent particle under the high-pressure adsorption condition of adsorbing gas, as described in section 3 and elsewhere.21 Such deformation is represented by ∆Vmax, which is unknown. Accordingly, for a particular porous carbon ∆Vmax was determined by matching the excess theoretical and corresponding experimental adsorption isotherms, as performed in our recent work.21

Figure 9. Comparison between high-pressure adsorption isotherms of CO2 in Si-CDC samples at 313 and 333 K (solid and dashed lines, respectively) predicted by the FWT model and corresponding experimental data.

Carbon Dioxide. For NLDFT calculations of high-pressure single-pore adsorption isotherms, all the LJ interaction parameters were taken to be the same as those used for calculations of low pressure isotherms. The binary interaction parameter, kcf ) 0.09367, used for the NLDFT calculations is taken from our previous work.12 Figure 9a-c depicts a comparison of excess theoretical high-pressure adsorbed quantities of CO2 at 313 and 333 K in SiCDC-50NM-600, SiCDC-50NM-800, and SiCDC50NM-1000 carbons using the FWT model (solid and dashed lines, respectively) with corresponding experimental data (filled circles and squares, respectively). It can be seen that the FWT model provides equally good prediction of experimental high pressure adsorption isotherms of CO2 at 313 and 333 K in the three investigated Si-CDC samples. This shows that the difference in adsorption behaviors of CO2 in these three carbons at supercritical conditions is much lesser pronounced than that at subcritical conditions, because of weaker solid-fluid interaction in the former condition. From the results presented above, it can

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be seen that the binary interaction parameter, kcf ) 0.09367, used for good prediction of experimental supercritical high pressure adsorption of CO2, is slightly higher than that used for good prediction of experimental subcritical subatmospheric adsorption of this species in the three investigated SiCDC samples, as presented in subsection 4.2.1. The former was also observed for other carbons such as BPL, PCB, and Norit R1 extra, shown in our previous works. In contrast, as previously shown in our recent work,21 the use of an atomistic carbon model of activated carbon fiber ACF15, obtained by hybrid reverse Monte Carlo (HRMC) simulation in our recent work, with kcf ) 0, provided excellent agreement between CO2 adsorption isotherms predicted by grand canonical Monte Carlo (GCMC) simulations and the corresponding experimental data in this carbon at both subcritical and supercritical conditions, using a three-center model of CO2. Consequently, the use of kcf ) 0.0963 for good prediction of the supercritical CO2 adsorption isotherm may be be associated with the one-center model of CO2 used in NLDFT calculations, or the use of the slit-pore model to approximate the internal structure of the carbon. As presented in our previous work,12 for individual slit-shaped carbon pores, the use of the binary interaction parameter value kcf ) 0 provided very good agreement between the CO2 adsorption isotherm calculated by NLDFT using a one-center molecular model and that predicted by GCMC simulations using a threecenter model. Similarly, we also found that the use of the atomistic carbon model of ACF 15, obtained by HRMC simulation in our recent work,21 with kcf ) 0, provided good agreement between CO2 adsorption isotherms predicted by GCMC simulations using one- and three-center models of CO2 for both subcritical and supercritical conditions, as shown in Figure 10a,b. The above evidence suggests that the use of the slit-pore model to represent the internal structure of the actual carbon may result in different values of the binary interaction parameter used in NLDFT calculations for subcritical and supercritical CO2 adsorption isotherms. In order to confirm this, in Figure 10a,b we compare predictions of an experimental supercritical CO2 adsorption isotherm in activated carbon fiber ACF15 by GCMC simulations using the atomistic structural model of this carbon, obtained by HRMC simulation and presented in our recent work,12 with that predicted by the FWT model using characteristic results (PSD and PWTD) of the ACF15 obtained from interpretation of an experimental argon adsorption isotherm at 87 K in this carbon.21 For NLDFT calculations and GCMC simulations of subcritical and supercritical CO2 adsorption isotherms, the one-center molecular model of CO2 and kcf ) 0 was used. The GCMC simulation method is described in detail elsewhere.21 LJ parameters of CO2 used in the GCMC simulations are given in Table 1. Equation 9 was also utilized to correct the GCMC simulated adsorption isotherm for swelling and shrinking to obtain the theoretical excess adsorption isotherm. We note here a typographical error in the value of σsf for this one-center model of CO2 given in our recent work.21 From these figures, it can been seen that, while good agreement is obtained between the subcritical CO2 adsorption isotherm calculated by the FWT model and that predicted by GCMC simulations using the atomistic carbon model, as shown in Figure 10a, the supercritical CO2 adsorption isotherm predicted by the FWT model deviates above that predicted by GCMC simulations using the atomistic carbon model, as illustrated in Figure 10b. The latter confirms that the value of kcf ) 0.0936 used in NLDFT calculations for good prediction of the experimental supercritical CO2 adsorption isotherm results from the utilization of the slit-pore model to approximate the internal structure of a real carbon. In particular,

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Figure 10. Comparison of CO2 adsorption isotherms in ACF15 carbon predicted by the FWT model (solid line) with those predicted by GCMC simulations using an atomistic structural model of this carbon with a one-center model of CO2 (dashed line) and three-center model of CO2 (symbols). (a) At 273 K and (b) at 310 K.

for disordered carbon whose pore shape is not strictly that of a slit, there is a significant difference in densities of the closely packed adsorbed phase of argon at 87 K and of CO2 in high pressure due to their different molecular geometry.27 This leads to a difference in resultant micropore size distribution of a carbon probed by a spherical molecule such as argon from that probed by a linear molecule such as CO2 using the slit-pore model. Consequently, the use of characterization results of a disordered carbon obtained from the experimental adsorption of a spherical molecule such as argon using the slit-shaped pore model does not guarantee accurate prediction of high pressure adsorption of a nonspherical molecule such as CO2, whose molecular geometry is very different from that of the probing gas in the same carbon, unless the pore morphology of this carbon is slit-like. Methane. The LJ CH4-CH4 interaction parameters used in NLDFT calculations are given in Table 1. Figure 11a-c depicts a comparison of excess theoretical high pressure adsorbed quantities of CH4 at 313 and 333 K in the Si-CDC-50NM-600, Si-CDC-50NM-800, and Si-CDC-50NM-1000 samples (solid and dashed lines) with corresponding experimental data (filled circles and squares). Although the FWT model predicts equally well the experimental adsorption data of CH4 at 313 and 333 K (27) Bhatia, S. K.; Tran, K.; Nguyen, T. X.; Nicholson, D. Langmuir 2004, 20, 9612–9620. (28) Lastoskie, C.; Gubbins, K. E.; Quirke, N. J. Phys. Chem. 1993, 97, 4786– 4796.

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Figure 12. Prediction of experimental CH4 adsorption isotherms (symbols) in (a) the SiCDC-50NM-1000 sample at 313 K, and (b) activated carbon fiber ACF15 at 310 K by the FWT model using characterization results (PSD and PWTD) obtained from experimental adsorption isotherm of Ar at 87 K (solid line) and at 77 K (dashed line).

Figure 11. Comparison between high pressure adsorption isotherms of CH4 in Si-CDC samples at 313 and 333 K (solid and dashed lines, respectively) predicted by the FWT model, and corresponding experimental data.

in all the Si-CDC samples, its slight overprediction can be observed for these Si-CDC samples. Accordingly, the latter may be indicative of a difference in pore accessibility between Ar at 87 K and CH4 at 313K-333 K to these investigated Si-CDC samples. In order to clarify this issue, we compared excess adsorption isotherms of CH4 in the Si-CDC-50NM-1000 sample predicted by the FWT model using characterization results (PSD and PWTD) extracted from experimental Ar adsorption at 87 K and experimental N2 adsorption at 77 K in this carbon, as illustrated in Figure 12a. From this Figure, it can be seen that the adsorption isotherms of CH4 predicted by the FWT model using the characterization results (PSD and PWTD) extracted from experimental adsorption isotherms of Ar at 87 K and N2 at 77 K are identical. This suggests that there is no difference in the pore accessibility of Ar at 87 K, N2 at 77 K, and CH4 at 313 K in the Si-CDC-50NM-1000 sample. Similarly, the slight overprediction of experimental CH4 adsorption isotherms by the FWT model using the characterization

results obtained from experimental adsorption isotherm of Ar at 87 K or N2 at 77 K is also encountered for the case of activated carbon fiber ACF15, as shown in Figure 12b. However, as previously shown in our recent work, while no pore accessibility problem has been found for Ar, N2, and CH4, the use of the atomistic carbon model of the ACF15 carbon obtained by HRMC simulation accurately predicted experimental adsorption isotherms of simple gases such as CO2 and CH4 whose molecular dimensions are very different from that of argon. Accordingly, it suggests that the slight overprediction of the adsorption isotherm of a larger molecule like CH4 using PSD and PWTD obtained from smaller molecules such as Ar and N2 may arise from the difference in excluded pore volume between these species. This can be seen from the fact that the PSD of the ACF15 carbon probed by the spherical approximation using its HRMC atomistic structural model, as presented in Figure 13 and in our recent work,21 contains a fraction of small pores whose size permits accommodation of argon but not methane, although the latter molecule is able to transport through this highly constricted region at supercritical temperatures. This generally leads to a higher excluded volume of CH4 than that of Ar. Deformation of Solid Particles under High-Pressure Adsorption. From successful prediction of the experimental adsorption data of CO2 and CH4 in the above section, the value of ∆Vmax obtained for all three investigated Si-CDC samples was found to be negative. This indicates that there is compression of Si-CDC particles during high-pressure adsorption. Furthermore, an interesting observation was that ∆Vmax is insensitive to temperature

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Figure 13. PSD of the HRMC atomistic model of activated carbon fiber ACF15 obtained by spherical approximation as taken from our recent work.21

Figure 15. Conceptual scheme for describing reduction in the external volume of carbon adsorbent due to compression of closed pore spaces. Incompressible parts comprise an incompressible solid phase and pore spaces accessible to both helium and adsorbing gas. The compressible part is pore space accessible to helium but not to adsorbing gas.

Figure 14. Variation of maximum volume compression of the Si-CDC samples with their synthesis temperature, for adsorption of CO2 and CH4.

but dependent upon adsorbate size as well as synthesis temperature of the investigated Si-CDC samples. Figure 14 illustrates the variation of the absolute value of ∆Vmax with synthesis temperature of the investigated Si-CDC carbons for the two adsorbates, CO2 and CH4. From this figure, an interesting trend is noted that ∆Vmax persistently decreases with increase in synthesis temperature, while the compressibility of Si-CDC particles is higher in CH4 atmosphere than in CO2 or helium (used as a reference in eq 8). The exception can be observed for the case of SiCDC50NM-1000 carbon whose compressibilities in both CH4 and CO2 are similar. This can be related to the compression of closed pores under high-pressure adsorption, as schematically illustrated in Figure 15. Such compression leads to reduction in the external volume of the carbon particle and, consequently, the apparent enhancement of the experimental adsorbed quantity measured using the gravimetric technique due to weaker buoyancy force. In particular, there exist locally constricted pore mouths in highly disordered carbons such as the three investigated Si-CDC samples. Such highly constricted pore mouths give rise to significant reduction in the accessibility with increase of molecular size of the adsorbing gas, i.e., the accessibility of He > CO2 > CH4. Thus, the pores, which accommodate He but not CO2 and CH4, are compressed under high pressure. This leads to reduction in the external volume of the carbon particle in the high-pressure CO2 and CH4 adsorption environment, and therefore to the negative sign of ∆Vmax. This reduction in shrinkage of the external volume of carbon particle during high-pressure adsorption, ∆Vmax,

with increase in ordering of the carbon (i.e., increase in synthesis temperature) is evident in Figure 14. At the high-pressure adsorption condition, the pore mouths can be slightly compressed, leading to restriction for CH4 adsorption but not for CO2 adsorption. This slight compression may block large pores under high pressure. These blocked large pores are highly compressed, while the excess adsorbed amount in these pores is small at high-pressure adsorption conditions. This leads to larger reduction in the external volume of the carbon particle during high-pressure CH4 adsorption compared to that in high-pressure CO2 adsorption, as seen in Figure 14. Further, the relative difference in the external volume of carbon particle becomes less pronounced as the degree of order of the carbon increases, such as for SiCDC-50NM-1000. This was also observed for activated carbon fiber ACF15 in our recent work.21

4. Conclusions This study has characterized the microstructure of Si-CDC samples, investigating both solid structure and pore structure, using different characterization techniques comprising helium density, XRD, Raman spectra, TEM, and adsorption. The solid structure analysis showed the predominance of amorphous material in the core of the Si-CDC particles, while minor graphitization was observed on the surface of these Si-CDC particles. Furthermore, the graphitization degree of the Si-CDC samples is influenced by the particle size of their precursor, βSiC, as well as heat treatment time. The degree of ordering of the Si-CDC samples increases with synthesis temperature. Finally, the pore structure analysis showed correlation between development of microporosity and synthesis temperature. The adsorption modeling task conducted in this work indicates that the FWT model provides slightly better prediction of

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experimental low pressure adsorption data of CO2 at 273 K in more ordered Si-CDC carbons (Si-CDC-50NM-800 and Si-CDC50NM-1000) than the IWT model. Furthermore, the FWT model was found to predict the experimental high-pressure adsorption data of CH4 and CO2 in the three Si-CDC samples up to 200 bar equally well at the two temperatures of 313 and 333 K. The difference in molecular geometry and dimension between analysis and probing gases is seen to impact the quality of prediction of experimental adsorption isotherms of the analysis gas in a disordered carbon by the slit-pore. Furthermore, in this work, the

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compressibility of the Si-CDCs under CO2 and CH4 atmosphere during high pressure adsorption has been found to be wellcorrelated with the accessibility of these gases in the investigated Si-CDC samples. Acknowledgment. This research was supported by a grant from the Australian Research Council, under the Discovery Scheme. LA8027429