ARTICLE pubs.acs.org/JPCC
Comparative Characterization of Carbon Adsorbents and Polymer Precursors by Small-Angle X-ray Scattering and Nitrogen Adsorption Methods V. M. Gun’ko,†,‡,* S. T. Meikle,† O. P. Kozynchenko,§ S. R. Tennison,§ F. Ehrburger-Dolle,|| I. Morfin,|| and S. V. Mikhalovsky† †
School of Pharmacy & Biomolecular Sciences, University of Brighton, Brighton BN2 4GJ, U.K. Chuiko Institute of Surface Chemistry, 17 General Naumov Street, 03164 Kiev, Ukraine § MAST Carbon International Ltd., Henley Park, Guildford, Surrey, GU3 2AF, U.K. Laboratoire interdisciplinaire de Physique (LiPhy), UMR 5588 CNRS - UJF, 38402 Saint Martin d'Heres Cedex, France
)
‡
bS Supporting Information ABSTRACT: A variety of activated carbons (ACs) with different burnoff degrees (060%) and polystyrene cross-linked with divinylbenzene (PSDVB) were studied using small-angle X-ray scattering (SAXS) and nitrogen adsorption methods. The ACs demonstrate increased deviation of the pore shape from the model of slit-shaped pores with increasing burnoff degree and parallel increasing contributions of pores in the 0.350 nm range. The pore size distributions (PSDs) calculated using SAXS and density functional theory (DFT) methods have similar shapes but a more detailed picture for broad pores with SAXS PSDs. The PSDs and chord length distributions of ACs and PSDVB adsorbents have certain close features depending on the specific surface area because contributions of narrower pores and thinner pore walls increase with increasing specific surface area practically independent of the material type.
’ INTRODUCTION Activated carbons (ACs) as the most effective adsorbents with high porosity (Vp = 12.5 cm3/g) and large specific surface area (SBET = 10003500 m2/g) are typically used in the particulate form of various sizes in the 11000 μm range.14 The adsorption properties of ACs depend strongly on the aforementioned characteristics and contributions of nanopores (pore half-width x < 1 nm), mesopores (1 < x < 25 nm), and macropores (x > 25 nm), as well as on the number and type of surface functionalities containing H, O, N, P, S, and other noncarbon atoms.57 These textural and structural features influence other properties of the materials used such as catalysts, catalyst carriers, etc.1,8,9 The contributions of pores of different sizes depend on the pore structure of the precursor material, carbonization, and subsequent activation conditions, the type of activating agent (CO2, water vapor, basic and acidic compounds, etc.), and degree of burnoff.14 To produce ACs, different raw natural materials (fruit stones and shells, vegetable fibers, etc.), mesophase pitch, or synthetic polymer microbeads are used. Some synthetic polymers (such as polystyrene, polydivinylbenzene, polystyrene cross-linked with divinylbenzene, phenol-formaldehyde resins, etc.) in the form of porous beads can be used as both adsorbents and precursors of ACs. However, during AC preparation, textural and structural properties of precursors and carbonized materials undergo significant changes. A deeper insight into the features of these changes is of importance from both theoretical and practical points of view. It is also of interest to compare textural r 2011 American Chemical Society
features of ACs composed of structures with condensed aromatic rings in partially crumpled carbon sheets forming clusters and nanoparticles and polymer adsorbents composed of noncondensed aromatic units (polystyrene, divinylbenzene, etc.), which fuse upon carbonization. The most common method used for textural characterization of porous adsorbents is adsorption of different probe compounds with appropriate treatment of the adsorption isotherms.3,10 More detailed information can be obtained by comparing results of adsorption and small-angle X-ray scattering (SAXS) measurements11,13 with comprehensive numerical analysis of the experimental data. Therefore, the aim of this work was a comparative textural characterization of AC and polymer adsorbents using SAXS and nitrogen adsorption data analyzed using various software.
’ EXPERIMENTAL SECTION Materials. Porous phenol formaldehyde resin beads of 0.10.5 mm in diameter were carbonized in a CO2 flow to 1073 K at a ramp rate of 3 K/min (sample labeled as C-0).14 Further activation by CO2 at 1183 K at different residence times produced AC with different burnoff from 5% (C-5) to 60% (C-60). Three series of ACs produced from the same Received: February 24, 2011 Revised: April 7, 2011 Published: May 12, 2011 10727
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10728
585 1158
1173
747
756
1029
NC-0 NC-36
NC-36A
C-5A
C-5B
Norit RBX
530
1999
C-60
MN500
1351
C-45
1131
993
MN200
568
C-30
951
1199
1120
826
904
1268
699 1346
2211
1631
1081
611
(m2/g)
(m2/g)
C-0
sample
SSAXS
SBET
1125 1400
1897 1471
517 479
437
979
939 208
1088 761
793
1067 664 1007
996
600
735
610
888 711
702
1055
726
1126
507 1046
1772
1869
602 1142
1729
1205
884
486
(m2/g)
Snano
878
1165
915
525
(m2/g)
SDFT,MNDb
48
7
147
38 363
235
31
22
137
45
119
78 112
590
860
202
250
144
108
81
(m2/g)
Smeso
4
6
5
5 7
0
2
0
0
0
0
0 0
9
13
19
19
3
2
1
(m2/g)
Smacro
0.442
0.442
0.773
0.773 0.773
0.511
0.511
0.414
0.489
0.489
1.047
0.651 1.033
1.969
1.969
1.969
1.969
1.435
1.082
0.651
(cm3/g)
Vp
0.202
0.197
0.413
0.492 0.263
0.308
0.397
0.374
0.272
0.371
0.562
0.285 0.560
0.590
0.417
0.569
0.663
0.593
0.450
0.248
(cm3/g)
Vnano
0.037
0.045
0.153
0.108 0.233
0.190
0.043
0.039
0.214
0.117
0.485
0.366 0.473
1.175
1.224
0.963
0.638
0.687
0.541
0.337
(cm3/g)
Vmeso
0.203
0.200
0.207
0.174 0.277
0.012
0.071
0.001
0.003
0
0
0 0
0.204
0.328
0.437
0.669
0.155
0.092
0.066
(cm3/g)
Vmacro
0.176
0.608
0.170
Cyl
slit
Cyl
slit Cyl
Cyl
0.037 0.413 0.110
slit
0.417
slit
Cyl
0.189 0.059
slit
slit
slit slit
MND
DFT
MND
DFT DFT
MND
DFT
DFT
MND
DFT
DFT
DFT DFT
MND MND
S/C/Vc
DFT
DFT
DFT
DFT
DFT
method
Cyl
Cyl
slit
slit
slit
slit
pore model
0.028
0.040
0.029 0.014
0.264
0.051
0.065
0.561
0.138
0.079
0.075
Δw
polymer
polymer
AC
AC
AC
AC
carbonizate AC
AC
AC
AC
carbonizate
material
a Note. Contributions of nanopores (Snano, Vnano) at pore half-width x < 1 nm, mesopores (Smeso, Vmeso) at 1 < x < 25 nm, and macropores (Smacro, Vmacro) at x > 25 nm; Δw is the relative deviation of the pore shape from the model (slit-shaped, slit; cylindrical, cyl; slit-shaped and cylindrical pores and voids between spherical particles, S/C/V). The Snano, Smeso, and Smacro values were normalized so that Snano þ Smeso þ Smacro = SBET. b Noncorrected S values were obtained by integration of the fS(x) functions. c Relative contributions of slit-shaped and cylindrical pores and voids between nanoparticles are 0.616, 0.302, and 0.082, respectively, for C-60.
a
Table 1. Textural Characteristics of Carbonizates, Activated Carbons (ACs), and Polymer Adsorbents (MN)
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Figure 1. Relationships between (a) SBET and Vp and specific surface area Snano and pore volume Vnano of nanopores at the pore half-width x < 1 nm; (b) Smeso and Vmeso for mesopores (1 < x < 25 nm), Smacro and Vmacro for macropores (x > 25 nm).
phenol-formaldehyde resin precursor were studied. The first series of ACs was comprised of C-0 (carbonizate), C-30, C-45, and C-60, with burnoff of 0, 30, 45, and 60%, respectively. The second series was comprised of NC-0 (carbonizate of the same precursor but a different batch), AC at 36% burnoff (NC-36), and NC-36 exposed to air in an oven at 300310 °C for 15 h (NC-36A with the textural characteristics similar to those of NC36). The third series of samples was comprised of ACs with a low burnoff (5%) but undergoing oxidation by 20% HNO3 solution at 50, 70, or 90 °C for 2 h. Carbon particle (granule) size was in the 0.10.5 mm range. Commercial cellulose-coated activated carbon Norit RBX (utilized in Adsorba hemoperfusion column, Gambro) and commercial polymeric (polystyrene cross-linked with divinylbenzene) adsorbents Hypersol-Macronet MN100, MN200, MN250, and MN500 (Purolite) were used in comparative textural characterization of AC and polymeric adsorbents using low-temperature nitrogen adsorption and small-angle X-ray scattering (SAXS) methods. Nitrogen Adsorption. The textural characteristics of studied materials were determined using the nitrogen adsorption isotherms (Figure S1 in Supporting Information) recorded at 77.4 K using Micromeritics Gemini or ASAP 2010 adsorption analyzers. The specific surface area (Table 1, SBET) was calculated according to the standard BET method.3 The studied carbons have a significant contribution of nanopores (x < 1 nm) to the total porosity. The BET constant (cBET) value criterion, which is used to judge whether the BET model can be applied for surface area calculation, is satisfied as cBET < 450 for all samples because they are not pure nanoporous materials. The total pore volume Vp was evaluated by converting the volume of nitrogen adsorbed at p/p0 ≈ 0.980.99 (p and p0 denote the equilibrium pressure and the saturation pressure of nitrogen at 77.4 K, respectively) to the volume of liquid nitrogen per gram of adsorbent. The pore size distributions (PSDs) were calculated using the Density Functional Theory (DFT) method15 with a regularization procedure based on the CONTIN algorithm.1620 The differential R PSDs with respect to pore volume (PSDV, fV(x) ∼ dV/dx, R fV(x)dx ∼ Vp) and specific surface area (PSDS, fS(x) ∼ dS/dx, fS(x)dx ∼ S) were shown as incremental ones (IPSD, ΣΦV,i(x) = Vp, ΣΦS,i(x) = S). The differential fS(x) functions were used to calculate the specific surface area (S) and to estimate
the deviation (Δw) of the pore shape from the model of slitshaped or cylindrical pores or a mixture of different pore types Δw = SBET/S 1.18 Additionally, to show a complex pore shape of maximum activated carbon C-60, the model with a mixture of slit-shaped and cylindrical pores and voids between nanoparticles (S/C/V) was applied with self-consistent regularization allowing determination of contributions of pores of different types.21 The method proposed by Nguyen and Do22 and modified for different pore types (MND method)18 was used with different models of pores. Additionally, nonlocal DFT, NLDFT (with the model of slit-shaped/cylindrical (S/C) pores), and quenched solid DFT, QSDFT (slit-shaped pore model) (Quantachrome software, version 2.02), methods were used to compute the PSDs of ACs. Further details of the calculations are given in the Supporting Information (SI). Small Angle X-ray Scattering. SAXS measurements were performed on the French CRG beamline D2AM at the European Synchrotron Radiation Facility (ESRF), Grenoble, France (Figure S2 in SI). The PSD functions were calculated from the SAXS data using the integral equation described in detail elsewhere23 (see SI). The chord length distributions (CLDs), which characterize a geometrical statistical description of a solid phase in adsorbents, were also calculated from the SAXS data24 (see SI). The SAXS method was successfully used for textural characterization of carbon adsorbents,1113 porous polymers and composites,25 glassy carbon,26 carbon foam,27 coal,28 and other materials.23,24
’ RESULTS AND DISCUSSION There are certain common features in the surface areapore volume relationships for the adsorbents studied (Table 1, Figure 1, and Figure S3 in SI). Both the nanopore volume (Vnano at pore half-width x < 1 nm) and surface area (Snano) increase with higher burnoff degree providing about a third of the total pore volume increase (Vp) (Figure 1). The increase in Vnano(Snano) occurs due to a decrease in the pore wall thickness and the size of nanoparticles with an increasing burnoff degree. This assumption is confirmed by an increase in the intensity of the chord length (l) distribution (CLD) at l < 2 nm (Figure 2) with increasing burnoff degree. The CLD is related to solid fraction of the materials. Figure 1 shows that the V(S) functions for all samples have close increments for 10729
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Figure 2. SAXS and N2 DFT pore size distributions for (a) C-0, (b) C-30, (c) C-45, and (d) C-60 (inset with the SAXS PSDs at R = 1.2 108 (curve 1) and 0.001 (2)) and PSDs for these ACs with (e) SAXS and (inset in e) N2 DFT and (f) chord length distributions.
different types of pores. This result can be explained by keeping particle integrity of ACs at different burnoff degrees causing pore wall thinning and diminution of nanoparticle sizes in their aggregates forming microparticles (granules). However, the observed scatter is due to the different origin of the adsorbents (carbonizate, ACs, polymers). The scatter for Vmeso (Smeso) is much greater (correlation coefficient is low, R2 = 0.494, for linear approximation of Vmeso vs Smeso) than that for other relationships (R2 = 0.901 for Vp(SBET), 0.926 for Vnano(Snano), and 0.994 for Vmacro(Smacro)). The maximum scatter for mesopores is due to contributions of opposite shifts of the PSD peaks of narrow (toward larger x values) and broad (toward smaller x values) mesopores with increasing specific surface area at higher burnoff (Figure 2e). Within the same series of samples, the relationship between the Vp and SBET or SSAXS values is practically linear (Figure S3, SI). ACs with contributions of nano-, meso-, and macropores relatively similar to the total porosity (such as C-30C-60) can provide much better adsorption kinetics of both low- and high-molecular compounds in comparison with
predominantly nanoporous ACs.29 Therefore, the observed tendencies in the V(S) relationships are of practical importance for ACs applications. The observed scatter in these relationships shows the influence of the adsorbent origin on its textural features. Figure 2 shows SAXS PSDs calculated using the regularization procedure at the regularization parameter16 R = 0.001. However, a decrease of the R value to ≈(1/3) 108 (determined on the basis of F-test and confidence regions16) allows us to obtain a more detailed picture of nanopores (Figure 3a). The SAXSderived PSD of C-60 is in agreement with detailed PSDs calculated using different DFT methods (Figure 3b). Notice that small values of R < 107 result in a low intensity of the PSD peaks of mesopores. Therefore, most SAXS-based PSD calculations were carried out at R = 0.001 (giving the PSD shape similar to the DFT PSD) despite a lower resolution for nanopores. The SAXS-derived PSD peaks at x ≈ 0.21 nm observed for carbonizate and all ACs are incomplete (Figure 3) because maximum q values used for all samples were approximately 10730
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Figure 3. (a) SAXS-derived PSDs for carbons C-0, C-30, C-45, and C-60 calculated at a small value (∼108) of the regularization parameter R and (b) comparison of the SAXS PSD of C-60 with the PSDs calculated with different DFT methods (DFT and QSDFT with a slit-shaped pore model and NLDFT with a slit-shaped/cylindrical pore model).
1.25 Å1 corresponding to spacing ≈0.5 nm (i.e., x ≈ 0.25 nm). However, noise contribution increases at q = 23 Å1 at the boundary between SAXS and wide-angle X-ray scattering and outside the Porod range.12 Therefore, in this q range the PSD calculation becomes questionable, and a smaller boundary value q ≈ 1.25 Å1 was used (Figure S2 in SI). The use of the slitshaped/cylindrical pore model with NLDFT gives the PSD peaks of nanopores shifted in comparison with the DFT, QSDFT (slitshaped pore model), and SAXS PSDs (Figure 3b). The similarity between the latter peaks allows one to assume that the slit-shaped pore model is appropriate to describe nanopores in the ACs studied. According to the CLDs for C-0C-60 (Figure 2f), the pore wall thickness is mainly in the range of 0.52.0 nm. This corresponds to stacks of between two and five carbon sheets with the maximum at ∼0.8 nm corresponding to three-layer stacks (the distance between adjacent sheets is close to 0.4 nm, as it is 0.380.39 nm in graphitic materials). Notice that carbon sheets in ACs can be relatively small (in comparison with those in graphite or graphitized materials) and composed of clusters having a structure similar to crumpled paper sheets.14 On the basis of this data interpretation, it can be assumed that a simple slit-shaped pore model may be inadequate for describing broad pores in the ACs studied, especially with increasing burnoff degree. Therefore, several pore models, slit-shaped (DFT, NLDFT, QSDFT), cylindrical (DFT) or mixed slit-shaped and
Figure 4. SAXS and N2 DFT and NLDFT pore size distributions for (a) NC-0, (b) NC-36, and (c) NC-36A.
cylindrical pores (S/C, NLDFT), and voids between spherical particles (S/C/V, MND) using two different experimental methods (nitrogen adsorption and SAXS) were applied here for textural characterization of the materials. The overall shape of pores in ACs is complex because several types of pores can be found.13 The narrowest pores of the slitshaped type can be inner pores in carbon nanoparticles.13 The structure of sheets in these nanoparticles can be nonplanar14,30 due to textural traces of a polymer precursor and since activation temperature was lower than the temperature of graphitization. Therefore, nanopores can have a nonideal slit-shaped structure.30 Carbon nanoparticles of a random structure (roughly described as spherical13) aggregate, forming visible granules. Pores in these aggregates are not slit-shaped and can include motifs of voids between spherical particles, slit-shaped, wedge-shaped, cylindrical, and spherical pores. To estimate the deviation of the model of pores from the real pore shape, a criterion Δw (Table 1), based on the ratio of the 10731
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Figure 6. SAXS PSDs for initial and oxidized ACs by HNO3 solution at 50, 70, or 90 °C for 2 h: (a) C-5A and (b) C-5B.
Figure 5. SAXS and N2 MND (slit or S/C/V model of pores) or DFT (slit) and NLDFT (S/C) pore size distributions for (a) C-5A, (b) C-5B, and (c) Norit RBX.
SBET and DFT or MND S values as pore shape independent and dependent, respectively, can be used: the larger the Δw value the greater the deviation.18 There is a clear tendency of an increase in the Δw value for the model of slit-shaped pores with increasing burnoff degree of ACs, i.e., time of heating in the CO2 atmosphere. The use of a mixed model with slit-shaped and cylindrical pores with NLDFT gives smaller deviation, e.g., for C-60 Δw = 0.092. The model of cylindrical pores with DFT or MND methods also gives smaller deviations (Table 1). The QSDFT with the slit-shaped pore model gives Δw = 0.217 and 0.077 for C-45 and C-60, respectively; i.e., it overestimates the specific surface area in comparison with the SBET value. These deviations from the ideal models are due to nonuniformity of pores in ACs that is well seen in high-resolution TEM images.30 Calculations of the specific surface area using the SAXS data (SSAXS) can overestimate the SBET values11,12 (Table 1). However, for some samples, these values are close. The reasons for this difference were discussed previously and include the inaccessibility of a fraction of pores (closed pores, very narrow
pores) for nitrogen molecules, where there is no such limitation for X-rays penetrating into all pores.11,12 The DFT PSDs (slit-shaped pore model) and SAXS PSDs are in good agreement (Figures 2 and 3). Despite certain differences in their shapes, the positions of the main peaks are close. The SAXS PSDs were calculated with the regularization parameter R = 0.001, whereas R = 0.01 was used as a typical value providing smooth and well-resolved PSDs in the DFT calculations. At x > 40 nm there are two SAXS PSD peaks, but the DFT PSD has only one peak because the smaller the R value, the larger the number of peaks. The character of changes in the PSDs of nanopores and macropores with increasing burnoff degree is simpler than changes in the PSDs of mesopores (Figure 2). This difference is reflected in different scatters and values of the correlation coefficient of a linear approximation V vs S (Figure 1 and Figure S3, SI). Both the SAXS and DFT PSDs clearly demonstrate the opposite trends for narrower (2 < x < 7 nm) and broader (7 < x < 15 nm) mesopores (Figure 2e). A similar effect is observed in the second series of ACs (Figure 4). However, in this case, the SAXS PSDs demonstrate macropores at x > 50 nm which are absent in the DFT and NLDFT PSDs because nitrogen fills broad macropores at x > 50 nm weakly and incompletely even at p/p0 ≈ 0.999. A similar difference in the PSDs of macropores is observed for other ACs studied (Figure 5, Table 1), which demonstrate the DFT PSDs mainly at x < 10 nm. On the contrary, the SAXS PSDs do not give a detailed picture of nanopores at x < 1 nm at the regularization parameter R = 0.001 since only one relatively broad peak of nanopores is observed for different samples (Figures 2 and 47). 10732
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Figure 9. (a) SAXS PSDs for AC and polymer adsorbent pairs (C-0 and MN500, NC-36 and MN200) with close specific surface area SBET (Table 1) and (b) DFT (N2) PSDs for MN500 and porous resin beads (SBET = 103 m2/g, Vp = 0.571 cm3/g) used for AC preparation.
Figure 7. SAXS, DFT, NLDFT, and MND PSDs for: (a) MN200, (b) MN500, and (c) SAXS PSDs for polymer adsorbents at specific surface area SSAXS = 1022 (MN100), 1199 (MN200), 796 (MN250), and 951 m2/g (MN500).
Figure 8. Chord length distributions for PSDVB adsorbents.
Air treatment of AC NC-36 at 300310 °C for 15 h (Table 1, sample NC-36A, Figure 4c) or surface oxidation of C-5A and C-5B by HNO3 solution at 5090 °C for 2 h (Figure 6) has little effect on the textural characteristics of the studied ACs. The
textural stability of activated carbon samples in spite of their chemical treatment is of importance for practical applications of ACs in aggressive media or at elevated temperatures. Purolite Hypersol-Macronet series of adsorbents based on polystyrene cross-linked with divinylbenzene, PSDVB, was studied here in comparison with ACs. The polymeric adsorbents have a less ordered internal structure of particles than ACs (Table 1, Figure 7). Similar polymers were used as precursors of ACs.31 The inner porosity (x < 50 nm) of small PSDVB particles demonstrates nearly the same regularities in the characteristics (Figure 7, Table 1) as ACs studied (Figures 26). However, the textural porosity (voids between neighboring particles at x > 50 nm) (Figure 7c) has a rather random character in PSDVB adsorbents in comparison with pores at 5 < x < 50 nm. For the AC pore walls, small chord lengths (mainly l < 2 nm) are characteristic (Figure 2f), and their contribution increases at higher burnoff degree. This feature indicates that the AC walls are thin with a rough surface, and they become thinner and rougher with increasing activation. A more regular shape of AC CLDs (Figure 2f) in comparison with MN CLDs (Figure 8) is due to discrete numbers of carbon sheets in the stacks. MN polymer adsorbents have broader CLD peaks than AC CLD. These assumptions are in agreement with the analysis of the textural features of ACs and MN based on the PSDs (Figures 2 and 7), relative changes in nano- and mesoporosity (Table 1, Figure 1), and increased deviations of the pore shapes from the models as a measure of the surface roughness (Table 1, Δw). The CLD shapes of the AC series are similar because the structure of ACs with increasing burnoff degree does not undergo significant 10733
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The Journal of Physical Chemistry C changes. This is confirmed by the similarity of the PSDs of ACs at different burnoff degrees and produced from the same carbonizate (Figures 2 and 3a). In the case of polymeric adsorbents, there is a similar picture for the CLDs (Figure 8) depending on the specific surface area value. However, a decrease in the specific surface area for MN500 (Table 1) or MN250 (SSAXS = 796 m2/g) in comparison with MN200 (Table 1, SSAXS) shows larger changes in the CLD shape (Figure 8) than that for ACs (Figure 2f). This can be explained by the less ordered structure of PSDVB polymers compared to ACs. The maximum difference in the SBET and SSAXS values for MN500 can be due to certain differences in the characteristics of the same commercial Hypersol-Macronet resin but from two sets used in the SAXS and adsorption measurements as well as due to the difference in the accessibility of pores for the N2 molecules and X-rays. Comparison of the PSDs of carbons, PSDVB, and phenol formaldehyde resin beads (Figure 9) calculated using SAXS and nitrogen adsorption data, as well as the PSDs for other samples studied (Figures 27), shows that there are at least three types of pores in these adsorbents. The first one corresponds to nanopores (x < 1 nm) and narrow mesopores (1 < x < 2 nm), which can be attributed to the inner pores in nanoparticles forming aggregates.13,32 These pores can be modeled by slit-shaped pores. The second type of pores is broad mesopores and narrow macropores at 510 < x < 3050 nm corresponding to mainly pores (voids between nanoparticles) in aggregates of nanoparticles. These textural pores are characteristic for ACs.13,19,20 Similar pores are present in nano-oxides composed with nonporous nanoparticles forming aggregates.33 These pores can be modeled by cylindrical pores and voids between nanoparticles. The third type of pores is broad macropores at x > 50 nm which can be studied using SAXS (or mercury porosimetry) but not by the nitrogen adsorption method. Nitrogen cannot fill these broad macropores even at p/p0 ≈ 0.999 because the adsorption potential is very low, far from the pore walls.3,10 These pores can be considered as voids between nanoparticles and their aggregates. Traces of all the types of pores remain after carbonization and activation of ACs even at a high degree of burnoff (60% here or 8688% previously13). This aspect is of importance for practical application of ACs and polymeric adsorbents because the presence of pores of the aforementioned three types plays an important role in both equilibrium and kinetic adsorption of gases, vapors, ions, liquids, and macromolecules.14,10 The formation of a certain pore structure in a porous aromatic polymer precursor can provide an expected structure of pores in a carbonizate and then in AC. The difference in the pore structure of these three types of materials increases with increasing burnoff degree (i.e., the Vp and SBET values).
’ CONCLUSION Investigations of textural properties of different activated carbons with 560% burnoff, carbonizate (0% burnoff), porous phenol-formaldehyde resin precursor of activated carbon, and polystyrene cross-linked with divinylbenzene adsorbents using SAXS and nitrogen adsorption methods show certain regularities in the relationships between specific surface area and volume of pores (total or partial for different pore types) and pore size distribution. The maximum scatter in these V vs S relationships is observed for mesopores because of the different origin of samples and the opposite trends of the displacement of the PSD peaks observed at the boundary values of the mesopore sizes.
ARTICLE
The ACs studied demonstrate increased deviation of the pore shape from the slit-shaped model with increasing burnoff degree. The PSDs calculated using SAXS and N2 DFT methods have similar shapes with certain differences due to the regularization procedure and the limitations of the experimental methods used. For all adsorbents studied, three types of pores are observed at the pore half-width x < 2 nm, x < 50 nm, and x > 50 nm. An increase in the burnoff changes relative contribution of different pore types because contributions of nanopores at x = 0.51 nm and narrow mesopores at x = 13 nm increase, whereas the PSD peaks of broad mesopores and macropores shift toward narrower pore size. However, highly activated AC at 86% burnoff is characterized by a low PSD intensity of these broad pores.13 Therefore, ACs studied here at 4560% burnoff degree can be optimal for their applications as adsorbents because of a good balance between narrow pores (as adsorption places for relatively small molecules of adsorbates), mesopores and macropores (adsorption places for large molecules of adsorbates, e.g., biotoxins), and transport pores (broad macropores) for all types of adsorbates.
’ ASSOCIATED CONTENT
bS
Supporting Information. Experimental details, calculation methods, and additional figures relating to adsorption and SAXS measurements and analysis. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The work was supported in part by grants under the Seventh Framework Programme of the European Commission, Marie Curie International Research Staff Exchange Scheme (grant no. 230790), and Marie-Curie Industry-Academia Partnerships and Pathways (IAPP) (MONACO-EXTRA project No 218242). The authors are grateful to the ESRF, Grenoble, for access to the French CRG beamline D2AM, and they acknowledge the help of its technical staff, Jean-Franc- ois Berar, Nathalie Boudet, Bernard Caillot, and Stephan Arnaud. ’ REFERENCES (1) Bansal, R. C.; Donnet, J. B.; Stoeckli, F. Active Carbon; Marcel Dekker: New York, 1988. (2) Smisek, M.; Cerny, S. Active Carbon; Elsevier: Amsterdam, 1970. (3) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: San Diego, 1982. (4) Marsh, H.; Heintz, E. A.; Rodriguez-Reinoso, F., Eds. Introduction to Carbon Technologies; University of Alicante, 1997. (5) Juarez-Galan, J. M.; Silvestre-Albero, A.; Silvestre-Albero, J.; Rodríguez-Reinoso, F. Microporous Mesoporous Mater. 2009, 117, 519–521. (6) Lenghaus, K.; GuangHua Qiao, G.; Solomon, D. H.; Gomez, C.; Rodríguez-Reinoso, F.; Sepulveda-Escribano, A. Carbon 2002, 40, 743–749. (7) Molina-Sabio, M.; Rodríguez-Reinoso, F. Colloids Surf. A 2004, 241, 15–25. (8) Rodríguez-Reinoso, F. Carbon 1998, 36, 159–175. (9) Ahumada, E.; Lizama, H.; Orellana, F.; Suarez, C.; Huidobro, A.; Sepulveda-Escribano, A.; Rodrguez-Reinoso, F. Carbon 2002, 40, 2827–2834. 10734
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