Hydrogen Sorption Characteristics of Ordered Mesoporous Carbons

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Hydrogen Sorption Characteristics of Ordered Mesoporous Carbons: Experimental and Modeling View Point Talla Venkata Rama Mohan,† Sridhar Palla,‡ Balaiah Kuppan,† Niket S. Kaisare,‡ and Parasuraman Selvam*,†,§,∥ National Centre for Catalysis Research and Department of Chemistry, and ‡National Centre for Catalysis Research and Department of Chemical Engineering, Indian Institute of Technology-Madras, Chennai 600 036, India § School of Chemical Engineering and Analytical Science, The University of Manchester, Manchester M13 9PL, United Kingdom ∥ Department of Chemical and Process Engineering, University of Surrey, Guildford, Surrey GU2 7XH, United Kingdom Downloaded via COLUMBIA UNIV on December 2, 2018 at 17:20:49 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

ABSTRACT: Ordered mesoporous carbons NCCR-41, NCCR-56, CMK-3, CMK-1, NCCR-11, and CMK-8 with varying pore size and pore dimensionality are prepared by adopting a nanocasting method employing sucrose as a carbon source and various ordered mesoporous silica templates. Systematic characterization of all the materials is done by various analytical and imaging techniques. Excess hydrogen sorption isotherms at 77 K and room temperature at high pressures are obtained using volumetric technique experiments. Correlation of the hydrogen sorption behavior with textural properties is discussed. Modeling of adsorption isotherms is done by converting excess adsorption to absolute adsorption isotherms using four different approaches. A nonlinear least-squares curve fitting technique is used to simultaneously estimate the volume of the adsorbed phase and absolute adsorption isotherm parameters for the modified Dubinin−Astakhov model.

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INTRODUCTION Over the next decades, the primary concerns of the world are going to be depletion in fossil fuel reserves, increasing rate of global warming, climate change, deterioration of air quality, and the rising energy demand. These societal concerns and future demands have urged scientists to look for a clean energy that is sustainable and renewable. Among the various available alternatives, hydrogen fuel is considered a promising option1 to replace fossil fuels in both stationary power plants as well as motive power in vehicles. However, the main bottleneck for the hydrogen fuel to be commercially viable for transportation sector is its storage and transfer.2 Research efforts on surmounting the storage step to make hydrogen a consistent and viable technology have been pursued and different materials are being studied. Hitherto, none of the available storage concepts or materials studied such as metal hydrides, complex chemical hydrides, metal organic frameworks and carbon nanostructures are able to meet all the stringent specifications and storage capacity set by the US Department of Energy.3−5 Carbon materials such as activated carbons, carbon nanotubes, graphene composites, and nanostructured carbons have generated significant interest during the past decade for hydrogen storage. Hydrogen is stored either by physisorption and compression in nanopores or by metal impregnation wherein storage occurs through hydrogen spillover mechanism.6−8 Among the various carbon materials, ordered mesoporous carbons (OMCs) are considered to be an attractive option due to their versatile features, such as ordered pores along with high surface areas and pore volumes, resistance to acidic and basic environments, and recyclability. Studies of hydrogen storage on © XXXX American Chemical Society

these OMCs focus on cryogenic temperature and moderate pressures. For example, Xia and Mokaya9 have determined the sorption behavior of CMK-3 at 77 K and up to a pressure of 20 bar, while Terrés et al.10 have measured the adsorption capacity up to 60 bar for CMK-1 material. Kim et al.11 have studied the effect of precursor used for the synthesis of ordered mesoporous carbons on hydrogen storage and established that sucrose is a better source. Xia et al.12 have compared the hydrogen sorption characteristics of mesoporous carbons with a few disordered carbons, and analyzed the effect of activating the carbons using physical or chemical activation methods. Hydogen storage properties of activated CMK-3 have been studied by Lee et al.,13 and Choi and Ryoo,14 whereas the influence of metal doping was studied by Giasafaki et al.15 The maximum hydrogen adsorption capacity reported for unactivated CMK-3 was 1.8 wt % at 77 K and 8 bar.15 However, to the best of our knowledge, there are few reports on the effect of pore size and pore dimensionality of the OMCs on hydrogen adsorption at high pressures. The OMC materials, in general, are synthesized by the following two methods: hard-templating and soft-templating methods.16 Although the former method is tedious, it is preferred than the latter as the soft-templating suffers from limited synthesis options with varying pore sizes. In addition, the hardtemplating method yields high quality samples. Hence, in the present study, an attempt is made to synthesize ordered Received: July 19, 2018 Accepted: November 14, 2018

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DOI: 10.1021/acs.jced.8b00627 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

mesoporous carbon architectures with varying pore sizes and pore dimensionality using the hard-templating (nanocasting) route. These samples were systematically characterized using powder X-ray diffraction (XRD), Brunauer−Emmett−Teller (BET) isotherms, transmission electron microscopy (TEM), and Raman spectroscopy. Thereafter, volumetric hydrogen adsorption experiments and modeling studies on the hydrogen storage behavior at 77 K and room temperature were conducted. The modeling of hydrogen adsorption is useful in analysis and system-level design of hydrogen storage systems.17,18 A porefilling model, modified Dubinin−Astakhov (D-A) isotherm, with six parameters, is used to describe hydrogen adsorption data over the experimental conditions.17 An adsorption isotherm model not only captures the functional dependence of the amount of hydrogen adsorbed on temperature and pressure, it can also predict important parameters such as heat of adsorption. The volumetric technique is used to measure the excess amount of gas adsorbed. But, the thermodynamically relevant quantity is the absolute adsorption.19 Using excess adsorption at high pressures can lead to maxima in adsorption isotherm, negative values of isosteric heat of adsorption, and singularity in the isosteric heat of adsorption that does not have a physical meaning.20 Since the adsorption isotherm models describe the absolute amount adsorbed, conversion of excess adsorption to absolute adsorption is necessary for modeling hydrogen storage. This conversion requires identification of the so-called Gibbs dividing surface, so that absolute adsorption can be calculated more precisely.21 However, several authors debate that it is impossible to directly measure the adsorption volume.20,22 Since the gas adsorption volume cannot be measured experimentally, either constant density (Lewis et al.;23 Mehta and Danner24) or constant volume approximations (Salem et al.;25 Payne et al.26) have been used. In another approach, Richard et al.18 obtained the adsorbed volume as an additional regression parameter, which was simultaneously obtained while fitting parameters for the D-A)model to experimental excess adsorption data. The major approaches will be discussed in methodology section to motivate the choice of the latter approach to simultaneously obtain D−A isotherm parameters and absolute adsorption.

Figure 1. Comparison of different methods for CMK-3 at 77 K for converting excess to absolute adsorption. Note that the three approaches show maxima in na, which results in nonphysical behavior.

of 0.010 and step time of 2 sec. Nitrogen adsorption measurements were carried out on a Micrometrics ASAP 2020 instrument. Raman analysis was carried out using a Bruker multiram FT-Raman spectrometer equipped with 1064 nm Nd:YAG laser excitation source and a liquid nitrogen cooled germanium diode detector. A 2100 JEOL microscope operated at 120 kV was used to carry out TEM studies after coating the carbon on 200 mesh lacey foam coated copper grid. Elemental analysis was done using PerkinElmer 2400 series CHNS/O analyzer. Hydrogen uptake capacity measurements were performed using ultra-high-purity hydrogen (99.9995%) at 77 K and 298 K on a Micrometrics HPVA apparatus after outgassing the samples at 473 K overnight in vacuum. Excess Adsorption Data to Absolute Adsorption. A functional representation of adsorption data using the modified D-A isotherm were developed using high-pressure adsorption data on mesoporous carbons at two different temperatures (298 K and 77 K). Since the model describes absolute adsorption, we first compute it from the experimentally obtained excess adsorption data. The amount of adsorbed gas per unit mass of the adsorbent is related as

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METHODOLOGY Synthesis and Characterization. All the ordered mesoporous silica (OMS), namely, MCM-41, IITM-56, SBA-15, MCM-48, SBA-11, and KIT-6, were prepared using a similar procedures reported earlier.27−32 These OMSs were used as a hard template, and sucrose as carbon precursor for synthesizing ordered mesoporous carbon using nanocasting method.28,29,33−35 The silica template was impregnated using acidified sucrose solution followed by drying and polymerization. The impregnation step was carried out twice, and the obtained silica polymer composite was pyrolyzed to obtain the silica−carbon composite, which was then treated with dilute solutions of HF for template removal. To ensure that the fluorine is completely washed, the samples are washed with ethanol after HF treatment. The resulting template-free carbon materials synthesized from MCM-41, IITM-56, SBA-15, MCM-48, SBA-11, and KIT-6 templates were designated as NCCR-41, NCCR-56, CMK-3, CMK-1, NCCR-11, and CMK-8 respectively. The materials were characterized using various techniques. Powder X-ray diffraction patterns of the solid catalysts were recorded by a Rigaku Miniflex II diffractometer using Cu Ka (λ = 0.1506 nm) in 2θ range of 0.6 to 5 with a 2θ step size

nex = na − ρg Va

(1)

The experimentally measured dead space volume (e.g., from helium experiments) includes both adsorbate and interstitial void space volumes. Since the two cannot be independently measured, various approaches were used to convert excess adsorption data to absolute adsorption. We term the first approach as the “effective thickness approximation”, given by Payne et al.:26 Va = 2σS

(2)

where σ is molecular diameter of an adsorbate and S is specific surface area. The second method is “liquid density approximation”,23 in which the density of adsorbed gas is assumed equal to the liquid hydrogen density, ij ρg yzz j zz na = nexjjj1 − jj ρliq zzz k {

−1

B

(3) DOI: 10.1021/acs.jced.8b00627 J. Chem. Eng. Data XXXX, XXX, XXX−XXX


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Figure 2. Small-angle XRD of (A) one-dimensional hexagonal OMS; (B) one-dimensional hexagonal OMC. Figure 3. Small-angle XRD patterns of (A) 3D-cubic OMS; (B) 3D-cubic OMC.

The next approximation is a total pore volume approximation, where Va is approximated as open pore volume calculated by nitrogen BET experiments. With this “pore volume approximation”25 we get, na = nex + ρg VBET

Adsorption Isotherms Fitting with Modified DA Model. The modified D-A model is a pore-filling model in the literature used to model hydrogen adsorption on carbon.17 Absolute adsorption is defined as É Ä ijÅÅÅÅ i RT ym ij p yzmÑÑÑÑyz zz lnjj o zz ÑÑzz na = n0 exp jjjjÅÅÅ− jjjj z z jÅÅÅ k α + βT z{ jjk p zz{ ÑÑÑÑzz Ö{ kÇ

(4)

Figure 1 compares the absolute adsorption amounts for CMK-3 at 77 K computed using all these approaches mentioned above. The liquid hydrogen density at its boiling point was chosen as ρliq = 0.07 g cc−1 in the second approch, whereas VBET = 1.08 cm3 gm−1 used as specific volume of adsorbed phase in the third approach. The green dotted line represents the “curve fitting method”, discussed in the next subsection, where the pore volume was calculated by considering it as another parameter of the adsorption isotherm model.36 Absolute adsorption amount reaches a maximum at 15 bar pressure for the liquid density approximation; the effective thickness method also follows a similar trend since the Va calculated in eq 2 is negligible. With the total pore volume approach, absolute adsorption shows a maximum between 20 and 25 bar. Thus, these approaches yield a nonphysical behavior of maxima in absolute adsorption. Consequently, the nonlinear curve-fitting approach, where Va is also a fitting parameter, is used as described presently.

(5)

Nonlinear curve-fitting approach is used to determine the empirical parameters: Limiting adsorption n0, pseudo saturation pressure p0, enthalpy factor α, entropic factor β and surface heterogeneity parameter m. Since the experimental data is in excess adsorption amount, nex is computed using eq (1) and (5), and Va is used as the sixth fitting parameter. The experimental excess adsorption data is obtained at various pressures and two different temperatures (77 K and 298 K). Weighted nonlinear least-squares approach is used to obtain the optimum values of the parameters that minimize the least-squares objective: y ij (i) jj n − nm(i) zzz zz + F = ∑ jjj ex jj max(nex ) zzz z 77K j { k 77K 2

C

∑ 298K

y ij (i) jj nex − nm(i) zzz zz jj z jj jj max(nex ) zzz { k 298K

2

(6)



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(see Figure 3A). We can see from Figure 3B that the X-ray diffraction patterns of NCCR-11 and CMK-8 show wellresolved reflections associated with cubic symmetry, whereas for CMK-1 the symmetry has a change which is well established due to the structural transformation that occurs during the silica framework etching.33 Figure 4 and Figure 5 represent the N2 adsorption−desorption isotherms and pore size distributions of the OMSs, and OMCs.

Figure 4. N2 adsorption−desorption isotherms of (A) one-dimensional hexagonal OMS; (B) 1D-hexagonal OMC. Inset: Pore size distribution.

where n(i)ex is the excess adsorption value obtained from experiments and n(i)m is the model prediction at the same conditions. Since the range of adsorbed hydrogen amounts is very different at 77 K and 298 K, the weighting factor is the reciprocal of the maximum amount adsorbed at that particular temperature. The nonlinear least-squares solver, lsqnonlin, in the MATLAB optimization toolbox is used to compute the parameters.

Figure 5. N2 adsorption−desorption isotherms of (A) three-dimensional cubic OMS; (B) 3D-cubic OMC. Inset: Pore size distribution.

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All OMSs show type-IV isotherm with H1 hysteresis loop (see Figure 4A and Figure 5A), indicating the mesoporous structure, whereas all OMCs show type IV isotherms with H2 hysteresis (see Figure 4B and Figure 5B), indicating that both the structures are ordered and exhibit narrow pore-size distributions. The relative pressure for the onset of capillary condensation on these materials is systematically shifted to the higher P/P0, in accordance with their pore diameter. The difference in the hysteresis type among silica and carbon is because all the carbons are inverse replicas of the parent silica templates. The pore size distributions of the carbons reveal that they are well replicated using the parent silica hard templates. On the other hand, CMK-1 does not show such replication, it should be noted here that the pore diameter should have been equal to wall thickness of MCM-48 if the carbon synthesis had followed simply a geometrical replication process. The difference in wall thickness and pore diameter demonstrates that

RESULTS AND DISCUSSION Figure 2A,B represents the X-ray diffraction patterns of all onedimensional hexagonal silica and carbons synthesized, respectively. It can be seen from Figure 2A that MCM-41, IITM-56, and SBA-15 exhibit characteristic reflections, indicating the formation of highly ordered hexagonal mesoporous structure. While going from CMK-3 to NCCR-41 via NCCR-56 we observe that the reflections shift toward higher two theta, suggesting a decrease in the pore size. The textural and structural parameters of all synthesized ordered mesoporous silica and ordered mesoporous carbons are tabulated in Table-1 and Table-2, respectively. The XRD patterns indicate that the structure remained intact for the mesoporous carbon materials even after the parent silica template removal. MCM-48, SBA-11, and KIT-6 exhibit characteristic reflections indicating the formation of highly ordered 3D-cubic mesoporous structure D



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there was a structural transformation which is also seen from the change in the X-ray diffraction pattern of CMK-1 from that of MCM-48.33 It can be seen from Table 1 and Table 2 that all Table 1. Structural and Textural Properties of Various Ordered Mesoporous Silicas XRD sample

a0a

(nm)

BET b

t (nm)

SBET (m2 g−1)

Vp (cm3 g−1)

TEM DBJH (nm)

One-Dimensional Hexagonal Ordered Mesoporous Silicas MCM-41 4.2 1.7 966 0.86 2.5 IITM-56 6.0 2.2 758 0.89 3.8 SBA-15 10.1 3.7 576 0.96 6.4 Three-Dimensional Cubic Ordered Mesoporous Silicas MCM-48 8.1 1.8 1091 1.03 2.8 SBA-11 13.7 3.9 671 0.69 1.9 KIT-6 20.0 3.2 785 1.07 6.8

DTEM (nm) 2.6 3.6 6.5 2.7 2.1 6.7

For hexagonal system, a0 = 2d100/√3; for cubic system, a0 = d × (h2 + l2 + k2)1/2; SBET = surface area calculated using BET equation; DBJH = pore size determined using BJH method. DTEM = pore size determined using TEM. bWall thickness. For hexagonal system: t = a0 − DBJH. For cubic system: t = (0.23a0 − 0.5DBJH). a

Figure 6. HRTEM images of different 1D-hexagonal silica and carbons: (a) MCM-41, (b) IITM-56, (c) SBA-15, (d) NCCR-41, (e) NCCR-56, (f) CMK-3.

the materials except CMK-1 are successfully replicated from their parent silica matrices. Figures 6 and 7 illustrate the TEM images of prepared ordered mesoporous silica and ordered mesoporous carbons and the calculated pore sizes are given in Table 1 and Table 2. The presence of highly ordered pore channels confirms the periodicity of the mesoporous structure. The pore size calculated from the TEM image is in good agreement with the pore size derived from N2 sorption isotherms and is long-range ordered. The carbon particles are not hollow which indicates that the formation of a carbon structure occurred uniformly throughout the entire volume of silica template. Thus, these materials are well ordered and well replicated. Figure 8 presents the Raman spectrum of all the prepared carbons materials. Two distinct features at ∼1290 cm−1 and ∼1595 cm−1 are clearly seen in all the carbons which can further be deconvoluted into five peaks D1, D2, D3, D4, and G-bands.37 The G-band is associated with the E2g vibration mode of crystalline graphite whereas the D1-band is attributed to the A1g mode disordered graphite structure. The other band, D2, is assigned to E2g symmetry of the surface graphene layers, whereas D3 and D4 are assigned to amorphous carbon and A1g

symmetry of the polyene structures, respectively. The relative intensity ratio (IG/ID) and the in-plane crystallite size (La) values are tabulated in Table 2. These values indicate that prepared carbon materials are composed of small graphene sheets with a small degree of graphitization. 3.2. Hydrogen Storage and Modified D-A Model. The excess hydrogen adsorption measured for the one-dimensional and three-dimensional carbons are shown in Figure 9 and Figure 10, respectively, at temperatures of 77 K and 298 K. Hydrogen sorption studies of all the materials at 298 K showed a linear rise up to 50 bar; for 77 K an initial rise followed by a fall in adsorption capacity, beyond a maximum, was observed. At 77 K, among the hexagonal OMCs, NCCR-41 shows the highest hydrogen uptake followed by CMK-3 and NCCR-56; at 298 K all the three carbons showed similar adsorption capacities with negligible difference. From Figure 9, we can say that the lower pore size material (NCCR-41) is favored for hydrogen adsorption. The high uptake of CMK-3 over NCCR56 may be due to the presence of a greater number of micropores in CMK-3 than that of NCCR-56 (see Table 2). The trend in the adsorption capacities is in accordance with

Table 2. Structural and Textural Properties of Various Ordered Mesoporous Carbons XRD sample

BET

TEM

a0a(nm) tb(nm) SBET(m2 g−1) Smicro(m2 g−1) Vp(cm3 g−1) DBJH(nm) DTEM(nm)

NCCR-41 NCCR-56 CMK-3

3.4 5.0 9.5

1.5 2.1 5.7

1079 1036 1022

CMK-1 NCCR-11 CMK-8

8.3 12.2 20.7

1.7 2.4 6.6

787 963 1168

Elemental (wt %) C

H

One-Dimensional Hexagonal Ordered Mesoporous Carbons 0.80 1.8 1.7 89.8 0.63 1.07 2.9 2.3 94.5 0.72 1.15 3.8 4.0 93.2 0.57 Three-Dimensional Cubic Ordered Mesoporous Carbons 52 0.51 2.4 4.1 91.6 0.92 83 0.90 3.7 3.4 92.5 0.75 106 1.22 3.8 3.8 89.1 0.96 37 20 217

N

Raman G-band(cm−1) D-band(cm−1) ID/IGc La(nm)

0.48 0.16 0.24

1596 1598 1596

1293 1288 1293

2.66 2.79 2.65

115 110 116

0.79 0.32 0.73

1599 1595 1593

1292 1290 1296

2.63 3.10 2.92

117 99 105

For hexagonal system, a0 = 2d100/√3; for cubic system, a0 = d × (h2 + l2 + k2)1/2. SBET = surface area calculated using BET equation; Smicro = micropore surface area calculated from t-plot; DBJH = pore size determined using BJH method. DTEM = pore size determined using TEM. bWall thickness for hexagonal system: t = a0 − DBJH. For cubic system: t = (0.23a0 − 0.5DBJH). cThe integrated intensity ratio (ID/IG) of the D- and G- bands is used to quantify the in-plane crystallite sizes. a

E



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Figure 9. Experimental hydrogen excess adsorption isotherm measurements (symbols) and modified D-A isotherm fits (lines) at 77 K and at 298 K at various pressures for 1D-hexagonal carbons.

Figure 7. HRTEM images of different 3D-cubic silica and carbons: (a) MCM-48, (b) SBA-11, (c) KIT-6, (d) CMK-1, (e) NCCR-11, (f) CMK-8.

Figure 10. Experimental hydrogen excess adsorption isotherms measurement (symbols) and modified D-A isotherm fits (lines) at 77 K and at 298 K at various pressures for 3D-cubic carbons.

materials NCCR-41 shows the highest excess adsorption with an adsorption capacity of 0.29 wt % at 50 bar at room temperature and 2.0 wt % at 77 K and 8 bar pressure. The nonlinear least-squares method that minimizes the performance function of eq 6 was used to compute the modified D-A model parameters for each of the six adsorbents. The quality of the fit is good for the entire range of pressures and the goodness of the fit (R2) is reported in Table 3. A small deviation is observed after 20 bar due to the fall in excess adsorption at 77 K. Total gas phase in the porous material held constant during the modeling, and this is used to relate absolute and excess adsorption. The specific volume of adsorbed phase obtained for the 1D ordered mesoporous carbons are approximately 0.0023 m3 kg−1 and the saturation pressure is 44 MPa for the three adsorbents. The characteristic energy (ε), taken as a function of the enthalpic factor (α), and entropic factor (β), provided significantly better fit for hydrogen. The heterogeneous parameter (m) varies very slightly for all carbons. The 3D ordered mesoporous carbons NCCR-11 and CMK-8 gave nearly the same specific volume of adsorbed phase 0.0029 m3 kg−1 and characteristic energies. Because of the difference in the specific surface area and pore volume of CMK-1 from other carbons, it showed dissimilar D-A parameters The absolute adsorption capacities follow similar trends as that of the excess adsorption, and the model parameters reflect the qualitative trends of the textural properties. Specifically, nmax is highest for NCCR-41 followed by CMK-3, and NCCR-56

Figure 8. Raman spectra of various OMCs.

the pore volume and pore size of the materials. On the other hand, for 3D cubic carbons both at 77 K and 298 K CMK-8 and NCCR-11 show almost similar behavior, while the adsorption capacities for CMK-1 are low. These results are in accordance with the decrease in surface area. Among all F



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Table 3. Parameters for Modified D-A Isotherm for Mesoporous Carbon Materials adsorbent

error (R2)

NCCR-41 NCCR-56 CMK-3

0.9973 0.9997 0.9945

NCCR-11 CMK-8 CMK-1

0.9994 0.9996 0.9978

nmax (mol/kg)

α(J/mol)

β (J/mol.K)

m

Va (m3/kg)

44.0 43.9 44.0

1.11 1.21 1.23

0.0021 0.0023 0.0024

32.6 26.6 49.9

1.13 1.05 1.53

0.0028 0.0030 0.0017

po (MPa)

One-Dimensional (Hexagonal) Ordered Mesoporous Carbons 35.0 2205 3.06 28.2 2343 4.26 30.0 2244 4.84 Three-Dimensional (Cubic) Ordered Mesoporous Carbons 30.8 2151 3.46 31.3 2104 2.33 14.5 1750 10.71

observed. Such a maximum is not observed in the absolute adsorption data at room temperature as the gas adsorption is governed by Henry’s law at this temperature. An analytical expression for the isosteric heat of adsorption can be derived from the Clausius−Clapeyron relation. For the modified D-A isotherm, Richard et al. and Kloutse et al. obtained the following expression:18,38

has a similar trend as that of surface area, and the specific volume of adsorbed phase corroborates with the pore volumes. The D-A parameters, specific volume of adsorbed phase, and saturation pressures are different among the 1D and 3D ordered mesoporous carbons, while the remaining parameters are similar due to their dependence on the surface properties, which are close as reported in the material synthesis. This welldefined model that parametrized at two extreme temperatures would be useful in industrial application to reduce time-consuming experiments for hydrogen adsorption in 1D and 3D ordered mesoporous carbons. The absolute adsorption values, computed using the D-A adsorption are shown in Figure 11 and Figure 12

ij n yz ΔH = α m lnjjj max zzz j na z k {

(7)

Figure 13 and Figure 14 represent the heats of adsorption as a function of pressure, at 77 K and 298 K, respectively. In order

Figure 11. Model predictions of absolute adsorption at 77 K and at 298 K for one-dimensional hexagonal carbons. Figure 13. Heat of adsorption calculated at 77 K and at 298 K for one-dimensional hexagonal carbons.

Figure 12. Model predictions of absolute adsorption at 77 K and at 298 K for three-dimensional cubic carbons.

for the two temperatures. At 77 K the absolute adsorption levels off at high pressure, where the density of hydrogen gas (ρg) at supercritical conditions keeps increasing with pressure. Since the excess adsorption is a difference between absolute adsorption and ρgVa, a maximum in excess adsorption data is

Figure 14. Heat of adsorption calculated at 77 K and at 298 K for three-dimensional cubic carbons.

to compute these values, the equilibrium amount of hydrogen adsorbed (i.e., na) at the respective temperature and pressure is G



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first computed using eq 5 using the model parameters from Table 3. Then, this value is substituted in eq 7 to obtain the corresponding value of the isosteric heat of adsorption. Mesoporous carbons used in this work are heterogeneous materials so the surface energies are not uniform. As more and more adsorbate molecules get into the pores both adsorbate−adsorbate and adsorbate−adsorbent interactions turn into repulsions. The high-pressure region attraction and repulsion of the gas−gas and gas−solid interactions determine the trend of the isosteric heat. For larger pore materials, differential heat of adsorption first shows maximum due to saturation of surface binding, and then a minimum as the intermolecular repulsion becomes significant and eventually increases with pressure because of the surface repulsion.39 Initially, the higher surface energy site attracts the hydrogen molecules first, and it releases a large amount of energy during adsorption. Later gas molecules go to the lower energy site and hence the heat of adsorption decreases while adsorption proceeds. At 77 K, the interaction between gas molecules is high and the molecules have a less adsorbent surface, while this is reverse at room temperature, because the heat of adsorption is high at temperatures over 77 K.

ACKNOWLEDGMENTS

This article is dedicated to Professor Akira Miyamoto on the occasion of his 70th birthday. The authors thank Dr. N.V. Krishna and Dr. P.R. Murthy for the initial work; Professor B. Viswanathan for the encouragement and support.

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FUNDING This work is supported by MNRE (No. 103/140/2008-NT). The authors thank DST for funding NCCR, IIT-Madras. One of the authors (TVRM) would like to thank CSIR for financial support in the form of SRF.

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SUMMARY Several one-dimensional hexagonal (NCCR-41, NCCR-56, and CMK-3) and three-dimensional cubic (CMK-1, NCCR-11, and CMK-8) ordered mesoporous carbons with varying pore sizes are successfully synthesized using different OMS templates, by the nanocasting method. The various characterization techniques employed show that the resultant carbons are structurally ordered and uniform infiltration of carbon is achieved within the silica template. Volumetric hydrogen uptake experiments were conducted at room and cryogenic temperatures and various pressures. It is seen that hexagonal materials are better as hydrogen sorption materials when compared with their cubic counterparts. The experimental excess adsorption was converted to absolute adsorption by defining a specific volume of adsorbed phase (Va) as a constant, and the DubininAstakhov (D-A) micropore volume filling model was applied to model hydrogen adsorption on OMCs over a wide range of pressures and supercritical temperatures. The excess isotherms modeled with the DA model showed good fit for supercritical temperatures 77 K and 298 K. The difference in adsorption volume for 1D and 3D OMCs were observed in the D-A parameters. An expression for the heat of adsorption used was developed from the D-A model. Differential enthalpy of adsorption as a function of pressure was calculated in the range between ∼2 and ∼9 kJ mol−1. In conclusion, to achieve the commercial storage capacity for hydrogen, pure mesoporous carbons either have to be combined with activation methods or have to prepare composites so that a large amount of micropores is present along with mesopores that will facilitate both the transport of hydrogen throughout the material and its storage.

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Parasuraman Selvam: 0000-0001-7807-2985 Notes

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Hydrogen Sorption Characteristics of Ordered Mesoporous Carbons

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