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Surface Properties of Porous Carbon Obtained from Polystyrene Sulfonic Acid-Based Organic Salts Deon Hines,† Andrey Bagreev,‡ and Teresa J. Bandosz*,† Department of Chemistry, The City College of New York, and The Graduate School of the City University of New York, New York, New York 10031 Received October 31, 2003. In Final Form: February 2, 2004 Pyrolysis of polysterene sulfonic acid-co-maleic acid salts at 800 °C resulted in formation of new materials consisting of porous carbon and metal species dispersed on the surface. After hydrochloric acid treatment, the metal oxides/salts were removed. Obtained materials were characterized using adsorption of nitrogen, thermogravimetric analysis, Raman spectroscopy, and scanning electron microscopy with energy dispersive analysis of X-rays. The results showed highly developed porous structures in the range of micro- and mesopores. The porous features of new materials resemble those characteristics for carbon foams. The differences in the porous structure are linked to the type of transition metal used for the modification of the initial polymer and the chelation process. Macro- and mesopores are spherical/cylindrical in shape, and they are likely formed when release of pyrolysis gases, such as CO2, NO2, SO2, H2S, and CxHy, occurs. Moreover, reduction of metal, its migration to the surface, and agglomeration contribute to development of porosity. Depending on the reactivity of the metal used for cation exchange (Fe, Co, or Ni) either sulfides (nickel and cobalt) or oxides (cobalt and iron) are formed on the carbon surface.
Introduction The chemistry of carbon material has attracted the attention of a number of researchers in recent years.1 Carbon scientists seem to focus on the possibility of enhancing many of the physical properties of carbon material. Some of these properties include magnetic, electronic, gas adsorption, and microstructure, such as porosity. To achieve these goals, unique methods have been examined with varying degree of success. These include doping organic precursors with elements such as boron and potassium.2,3 A very promising technique of modification, which relies on the arc-discharge principle,4 seems to be limited to rare earth elements and not to the transition metals. Other techniques based on wet chemistry approach5-8 resulted in low yield of the final products. It was found that pyrolysis of certain organic salts containing metal cations leads to carbonaceous materials with a high surface area and well-developed microporosity.9-11 In these salts, metal cations occupy the * To whom correspondence should be addressed. Tel: (212) 6506017. Fax: (212) 650-6107. E-mail:
[email protected]. † The Graduate School of the City University of New York. ‡ Department of Chemistry, The City College of New York. (1) Radovic, L. R.; Rodriguez-Reinoso, F. Carbon Materials in Catalysis. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; M. Dekker: New York, 1997; Vol. 25, pp 243-358. (2) Oka, H,; Inagaki, M.; Kaburagi, Y.; Hishiyama, Y. Solid State Ionics 1991, 121, 157. (3) Carroll, D. L.; Redlich, Ph.; Blase´, X.; Charlier, J. C.; Curran, S.; Ajayan, P. M.; Roth, S.; Ruhle, M. Phys. Rev. Lett. 1998, 81, 2332. (4) Zhou, C.; Kong, J.; Yenilmez, E.; Dai, H. Science 2000, 290, 1552. (5) Guerret-Piecourt, C.; Bouar, Y. L.; Loiseau, A.; Pascard, H. Nature 1994, 372, 761. (6) Inagaki, M.; Okada, Y.; Miura, K.; Konno, H. Carbon 1999, 37, 329. (7) Zhong, Z.; Chen, H.; Tang. S.; Ding, J.; Lin, J.; Tan, K. L. Chem. Phys. Lett. 2000, 330, 41. (8) Goutfer-Wurmser, F.; Konno, H.; Kabuagi, Y.; Oshida, K.; Inagaki, M. Synth. Met. 2002, 118, 33. (9) Putyera, K.; Bandosz, T. J.; Jagiello, J.; Schwarz, J. A. Carbon 1996, 34, 1559. (10) Schwarz, J. A.; Putyera, K.; Jagiello, J.; Bandosz, T. J. U.S. Patent 5,614,460, 1997. (11) Schwarz, J. A.; Putyera, K.; Jagiello, J.; Bandosz, T. J. U.S. Patent 5,837,741, 1998.
cubic framework of the crystallites and organic molecules are localized between these “layers”. If metal cations in an organic precursor are alkali or alkaline earth metals, they are easily washed out from the formed carbon framework, thus producing porous carbon with a low ash content.11 On the other hand, if these cations are transition metals, for instance, nickel, cobalt, or iron, they should remain present in the final product in the form of welldispersed metal clusters with sizes ranging from atomic to a few nanometers in diameter. It is noteworthy that in such transition metal-carbon based materials the metal content can be easily controlled by the cation-exchange process. The transition metal cations present in an organic precursor should influence the porosity of a final product12 and its performance as an adsorbent. The recent study of the materials obtained by pyrolysis of sewage sludge (combination of inorganic oxides and organic phase) revealed their good performance as adsorbents of sulfurcontaining gases.13-16 Moreover, during pyrolysis in the presence of catalytic metals new forms of carbon can be created. At high temperature the organic matter vaporizes, dehydrogenation occurs, and carbon can be deposited back on the surface of an inorganic support with formation of carbon nanotubes or filaments. This may happen due to the presence of highly dispersed catalytically active metals.18-20 The objective of this paper is to study the effect of transition metals used in organic salts on the structural (12) Konno, H,; Matsuura, R.; Yamasaki, M.; Habazaki, H. Synth. Met. 2002, 1215, 167. (13) Bagreev, A.; Bandosz, T. J.; Locke, D. C. Carbon 2001, 39, 1971. (14) Bagreev, A.; Bashkova, S.; Locke, D. C.; Bandosz, T. J. Environ. Sci. Technol. 2001, 35, 1537. (15) Bashkova, S.; Bagreev, A.; Locke, D. C.; Bandosz, T. J. Environ. Sci. Technol. 2001, 35, 3263. (16) Bagreev, A.; Locke, D. C.; Bandosz, T. J. Ind. Eng. Chem. Res. 2001, 40, 3502. (17) Endo, M.; Takeuchi, K.; Kobori, K.; Takahasi, K.; Kroto, H. W.; Sakar, A. Carbon 1995, 33, 873. (18) Fonseca, A.; Hernadi, K.; Piedogrosso, P.; Colmer, J. F. Appl. Phys. A: Mater. Sci. Process. 1998, 67, 11. (19) Li, W. Z.; Xie, S. S.; Qian, L. X.; Chang, B. H.; Zou, B. S.; Zhou, W. Y.; Zhao, R. A.; Wang, G. Science 1996, 274, 1701.
10.1021/la0360613 CCC: $27.50 © 2004 American Chemical Society Published on Web 03/16/2004
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properties of carbonaceous materials. To accomplish our objectives, we carbonized commercially available poly(styrene sulfonic acid-co-maleic acid), sodium salt. To study the effect of metals, sodium was exchanged with either cobalt, nickel, or iron. Park and Jung21 and Putyera et al.9 reported using polystyrene-based material as precursors for the preparation of activated carbon. We assume the microstructure of the precursor to be essentially the polystyrene carbon framework with interspersed chelated metal ions. The maleic moiety provides additional opportunity for chelation with metal ions. The nature of the substrate, enriched with numerous opportunities for bonding and chelating, strongly suggests that metals can be highly dispersed within the carbonaceous matrix. Moreover, the functional groups present along with nitrates from the introduced salts should act as pore formers during their thermal decomposition. Since reduced metal can migrate to the surface during carbonization, the structure is expected to gradually expand leading to foam/nanofoam formation. The pillaring effect of metals during carbonization of polystyrene-divinylbenzene based resin with sulfonic acid groups was observed by Nakagawa et al.22 They found their precursors useful for production of carbon molecular sieves (CMS). Experimental Section Materials. Poly(styrene sulfonic acid-co-maleic acid) salts containing sodium, iron, cobalt, or nickel were used as organic precursors. First the cation exchange was done (using Fe(NO3)3, Co(NO3)2, and Ni(NO3)2) and the samples were heated at 200 °C for 4 h. We refer to the initial polymers as PSS-MA-Na, PSSAMA-Fe, PSS-MA-Co, and PSSA-MA-Ni. Then the carbonization was carried out in a horizontal furnace with nitrogen as an inert gas (300 mL/min flow rate). The samples were heated with a rate of 50 °C/min and held at 800 °C (final carbonization temperature) for 40 min. As a next step a Soxhlet washing with distilled water was performed to remove an excess of water-soluble inorganic salts (sodium and excess of transition metal salts). The carbon samples are designated as C-Fe, C-Co, and C-Ni, where the last two letters refer to the transition metal ion used in a precursor. The initial sample is referred to as C. To remove metals the carbonized materials were washed with 18% HCl for 24 h. Then the Soxhlet washing was done with distilled water to remove the excess of water-soluble chlorides and hydrochloric acid. After this treatment, the letter “A” is added to the names of the samples. Methods. Adsorption of Nitrogen. On the materials obtained, sorption of nitrogen at its boiling point was carried out using an ASAP 2010 analyzer (Micromeritics). Before the experiments, the samples were outgassed at 120 °C to constant vacuum (10-4 Torr). From the isotherms, the surface areas (BET method), total pore volumes, Vt (from the last point of isotherm at relative pressure equal to 0.99), total surface area SDFT, volumes of micropores Vmic, and mesopore volume Vmes along with pore size distributions were calculated. The last four quantities were calculated using density functional theory, DFT.24,25 pH. The pH of a carbon sample suspension provides information about the acidity and basicity of the carbon surface. A sample of 0.4 g of dry carbon powder was added to 20 mL of distilled water, and the suspension was stirred overnight to reach equilibrium. Then the pH of suspension was measured. (20) Nikolaev, P.; Bronikowski, M. J.; Bradley, R. K.; Rohmund, F.; Colhert, D. T.; Smith, K. A.; Smalley, R. E. Chem. Phys. Lett. 1999, 313, 91. (21) Park, S. J.; Jung, W. Y. Carbon 2002, 40, 2021. (22) Nakagawa, H.; Watanabe, K.; Harada, K.; Miura, K. Carbon 1999, 37, 1455. (23) Ago, H.; Ohshima, S.; Uchida, K.; Yumura, M. J. Phys. Chem B 2001, 105, 10453. (24) Lastoskie, Ch. M.; Gubbins, K. E.; Quirke, N. J. Phys. Chem. 1993, 97, 4786. Lastoskie, Ch. M.; Gubbins, K. E.; Quirke, N, Langmuir 1993, 9, 2693. (25) Olivier, J. P.; Conklin, W. B. Int. Conf. Surf. Colloid Sci., 7th, Compiegne, France, 1991.
Figure 1. DTG curves in nitrogen for the polymeric precursors. Thermal Analysis. Thermal analysis was carried out using a TA Instrument thermal analyzer. The instrument settings were as follows: heating rate 10 °C/min and a nitrogen atmosphere with 100 mL/min flow rate. For each measurement about 25 mg of a ground carbon sample was used. SEM/EDX. Scanning electron microscopy (SEM) images were obtained at Zeiss-LEO using a LEO 1550 FESEM. SEM images with energy-dispersive X-ray (EDX) analysis were obtained on a LEO 1455 VP SEM with tungsten source with an energy dispersive analysis of X-rays (EDAX) Phoenix-Po EDX analyzer at Lamont Doherty Earth Observatory of Columbia University. The as-received samples were mounted using silver support. XRD. X-ray diffraction (XRD) was performed on a Phillips Analytical X-ray diffraction analyzer with Cu KR as a source of radiation. The analyzed powdered carbon samples are spread as thin layers on a glass slide and evaluated. Raman Spectroscopy. On the acid-washed samples, Raman spectroscopy were carried out using laser radiation of wavelength 457.9 nm from a Coherent Innova 200 argon-ion laser. The excitation power was maintained at 100 mW, and the Raman scattering was collected and dispersed by a Spex 1877, 0.6-m spectrometer. A cooled (-133 °C) Spex Spectrum-1 CCD camera was coupled to the spectrometer and used as the detector. Elemental Analysis. The content of metals in the samples was determined using ICP in Shiva laboratories, Syracuse, NY. Content of carbon, hydrogen, oxygen, and sulfur was determined in Huffman Labs, Boulder, CO.
Results and Discussion The differential thermogravimetry (DTG) curves for the organic precursors with various metal salts are presented in Figure 1. Peaks represent normalized weight losses in various temperature ranges. For all samples four common peaks are present. They are with maxima at about 110, 200-400, 450, and 720 °C. In the analysis of their origin, we have to remember that in all cases the same polymer is present along with sodium chloride (initial) or sodium nitrate (after cation exchange). Taking this into account, we assign the first peak to the removal of physically adsorbed water. The peaks present between 200 and 400 °C are likely related to the decomposition of nitrates and carboxylic groups of maleic acid. In this process the
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agglomerates of oxides should be formed. The peaks differ in their position due to various thermal stabilities of nitrates (iron nitrate is least stable, Td ) 125 °C).26 A peak at 450 °C, present on DTG curves for all samples, represents decomposition of polystyrene and sulfonic acid salts.27-28 The analysis of the off gases released during this process showed a significant emission of hydrogen sulfide and hydrocarbons (ethylene and/or ethane). Hydrogen sulfide is likely the product of SO2 reduction. The release of SO2 and water in this temperature range from decomposition of metal sulfonate groups of polystrene sulfonic acid-based resins was also found by Nakagawa et al.22 They concluded that the stability of metal sulfonate groups depends on the type of metal. Released in the decomposition process, metals (sodium and either iron, nickel, or cobalt) form corresponding sulfides and sulfates. Then the aromatization and polycondensation of benzenelike rings occur with releasing of a small amount of ethane and hydrogen. Sodium sulfate decomposes to sodium, SO2, and H2S at 720 °C as a result of reducing atmosphere. Both of those gases were detected at that temperature range. Formation of H2S is a result of decomposition of sodium sulfides and sulfates accompanied by reduction of hydrogen released during carbon polycondensation. While water and SO2 act as pore formers for carbon, sulfates of transition metals, if formed, are reduced to sulfides with further heating. Their decomposition temperature is higher than 1000 °C.26 The peak at 830 °C seen on the curve for the sodium form likely represents decomposition of sodium carbonate.26 We have to remember here that our process of carbonization in the furnace was stopped at 800 °C, so sulfates are expected not to be totally reduced. On the surface of “as-received” materials taken form the furnace large agglomerates of oxides from decomposition of nitrates should be present along with well-dispersed sulfates and sulfides. Those metal oxides, except Fe2O3, are difficult to reduce with carbon. When the temperature reaches 600 °C, Fe2O3 is reduced to FeO and, as a next step, to Fe(0), which is manifested by a broad peak with maximum at 680 °C.26,29 This process overlaps with the formation of very stable iron sulfide. Moreover, when the materials were exposed to oxygen, a very strong exothermic effect was noticed. We attribute this to either oxidation of well-dispersed metals and sulfides or chemisorption of oxygen on the carbon matrix and formation of functional groups at the edges of carbon crystallites.30 The DTG curves for samples after carbonization and washing with water are presented in Figure 2A. As expected, they reveal the differences in the species present on the materials surfaces. As indicated above, the first peak represents removal of physically adsorbed water. The common feature for all samples (from the point of view of the position and intensity) is the peaks between 200 and 550 °C. On the basis of the strong exothermic effect upon exposure to air and the acidic pH of carbons after HCl washing (Table 1), we assign those peaks to decomposition of oxygen-containing functional groups. As indicated in the literature, carboxylic acids decompose to water and CO2 at this temperature range.31,32 An intense peak at 350 °C for the iron-modified sample is a result of (26) Handbook of Chemistry and Physics, 67th ed.; West, R. C., Ed.; CRC Press: Boca Raton, FL, 1986. (27) Siggia, S.; Whitlock, L. R. Anal. Chem. 1970, 42, 1719. (28) van Loon, W. M. G. M.; Boon, J. J.; de Groot, B. Environ. Sci. Technol. 1993, 27, 2387. (29) Thorne, P. C. L.; Roberts, E. R. Fritz Ephraim Inorganic Chemistry; Interscience: New York, 1946. (30) Leon y Leon, C. A.; Radovic, L. R. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; M. Dekker: New York, 1992; Vol. 24, pp 213-310.
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Figure 2. DTG curves in nitrogen for carbonized materials before (A) and after (B) acid treatment. Table 1. pH and Metal Content in Transition Metal Modified Samples
sample
pH after water washing
pH after HCl treatment
initial metal content (%)
metal content after HCl treatment (%)
C C-Co C-Ni C-Fe
7.21 7.99 7.78 6.08
5.50 4.95 5.22 5.70
9.0 10.7 8.3
0.05 0.27 1.00
overlapping of the decomposition of those groups with decomposition of iron oxides (F2O3‚xH2O) or hydroxides present on the surface after reexposure of the iron clusters to oxygen from air.26 The peaks over 600 °C, since they are not revealed on the curve for the C sample, represent reduction of nickel and cobalt sulfates. DTG curves done on the samples after acid washing support our hypothesis about the species present on the surface (Figure 2B). After that treatment the peaks over 600 °C disappear for C-CoA and C-NiA due to dissolution of sulfates. Peaks representing decomposition of oxygen groups are smaller. This is likely the result of “cleaning” of the carbon surface and the removal of water-soluble small molecule organic compounds containing oxygen. It is interesting that for the nickel-modified sample a new peak at about 350 °C is revealed. That peak might (31) Molina-Sabio, M.; Munecas-Vidal, M. A.; Rodriguez-Reinoso, F. In Characterization of Porous Solids II; Rodriguez-Reinoso, F., Rouquerol, J., Sing, K. S. W., Unger, K. K., Eds.; Elsevier: Amsterdam, 1991; pp 329-339. (32) Papirer, E.; Dentzer,; Li, S.; Donnet, J. B. Carbon 1991, 29, 69.
Surface Properties of Porous Carbon
Langmuir, Vol. 20, No. 8, 2004 3391 Table 2. Yield of Materials on Various Stages of Their Preparation
sample
yield after carbonizationa (%)
yield after water washingb (%)
final yield (after HCl treatment)c (%)
C C-Co C-Ni C-Fe
37.5 41.6 40.6 43.0
10.9 17.6 16.8 16.4
10.8 13.8 7.5 13.9
a Y ) 100 - [((W - W )/W ) × 100], where W is the weight of C i C i i the initial material, WC is the weight of the material after carbonization. b Yw ) 100 - [((Wi - Ww)/Wi) × 100], where Wi is the weight of the initial material, Ww is the weight of the material after carbonization and water washing. c Yf ) 100 - [((Wi - WHCl)/Wi) × 100], where WHCl is the weight of material after carbonization and HCl washing.
Figure 3. XRD pattern for samples before and after acid washing.
represent decomposition of nickel hydroxide26 formed during contact with air and not totally removed by washing. It overlaps with the peak from decomposition of oxygen-containing groups. On the surface of the C-FeA sample, iron oxides seem to be still present and a new peak appears between 300 and 500 °C. This peak might be related to iron hydroxides and chlorides33 formed when the sample was exposed to HCl and hydrolysis of chlorides occurred. Support for the results described above are changes in the content of transition metals present in samples before and after acid washing. They are summarized in Table 1. For all carbonized samples without acid treatment, the content of metals is similar and ranges between 8 and 11%. Washing with HCl significantly decreases cobalt and nickel content, while the content of iron is still about 1%. During this process hydrogen sulfide rotten egg odor could be easily detected due to the reaction of HCl with sulfides. Iron chlorides, even when formed from iron oxides, hydrolyze in water and form insoluble hydroxides,33 which causes their total removal by use of this method very difficult. The much higher content of nickel than cobalt after washing is consistent with a new peak on the DTG curve (Figure 2B), which we assigned to decomposition of nickel hydroxide.26 The X-ray diffraction patterns for materials derived from nitrates are shown in Figure 3. For carbonized initial polymer, C, a broad diffraction peak linked to the graphitic structure along with two sharp diffraction peaks at about 2Θ equal to 44 and 54° are noticed. The broad range of the first diffraction peak represents the amorphous structure of the carbon material. Since the diffraction peak at 2Θ equal to 54° was not present in the initial polymer and it disappears after water washing, we link it to the (33) Baes, C. F., Jr.; Mesmer, R. E. In Hydrolysis of Cations; Wiley: New York, 1976; pp 211-215.
presence of sodium. The sharp diffraction peak at 44° is still present after washing. At this stage we are not able to interpret this diffraction peak. A similar pattern of changes is noticed for transition metal derived samples from which C-Co based materials look the “cleanest” whereas C-Ni based materials appear to be the most contaminated by metal oxides and salts. In fact this is in agreement with the results of elemental analyses. The common feature of all samples is that sharp peak at 44° and the amorphous carbon peak at about 2Θ equal to 26°. In the case of C-Ni, many sharp diffraction peaks are revealed. They represent acid-soluble nickel sulfides or oxides. The surface becomes gradually cleaner after applying water and acid washing. A similar pattern is noticed for iron. However, the sample looks more homogeneous from the chemistry point of view. The iron oxides seem to be well dispersed, so their crystallographic pattern is difficult to detect. Nevertheless the results suggest that Fe3O4 and elemental iron are likely present in the sample. The yield of materials after various steps of carbonization also indicates the “history” of formation and removal of various inorganic species (Table 2). An approximate 40% yield for materials after carbonization decreased to about 17% (11% for sodium form) after water washing which suggests that more than 23% of the mass consists of water-soluble salts containing sodium (sufides, sulfates, carbonates) and sulfates (nickel, cobalt, or iron). Acid washing further decreased the yield for the transition metal modified samples, removing a few percent of weight in sulfides and oxides. The values of the pH for suspensions of the materials are in agreement with the above-described changes in the chemistry. The significant decrease in the values after water washing is the result of the presence of strongly acidic oxygen-containing groups on the carbon surface.30,34 Those species, likely carboxylic groups, were introduced when carbon was exposed to air and the strong exothermic effect was noticed. When inorganic species are present, the pH is a few units higher for all samples but the ironbased one. Taking into account the acidic character of the carbonaceous surface, that “neutralization” effect was likely caused by the presence of basic species such as oxides/hydroxides. The low pH in the case of C-Fe is the result of the acidic character of a hydrolyzed iron species.33,35 The basic pH of the C sample must be the result of the presence of traces of sodium which could neutralize (34) Bandosz, T. J.; Jagiello, J.; Schwarz, J. A.; Krzyzanowski, A. Langmuir 1996, 12, 6480. (35) Bandosz, T. J.; Cheng, K. J. Colloid Interface Sci. 1997, 191, 456.
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Figure 4. Nitrogen adsorption isotherms for initial carbons (A) and acid-washed counterparts (B). Table 3. Structural Parameters Calculated from Nitrogen Adsorption Isotherms sample
SBET (m2/g)
SDFT (m2/g)
Vt (cm3/g)
Vmic (cm3/g)
Vmes (cm3/g)
Vmic/Vt
C C-Fe C-Co C-Ni CA C-FeA C-CoA C-NiA
1378 658 861 798 1348 709 996 913
1117 668 734 629 1125 576 818 800
0.93 0.57 0.79 0.67 0.89 0.58 0.89 0.74
0.46 0.23 0.25 0.23 0.47 0.20 0.28 0.30
0.14 0.12 0.29 0.11 0.14 0.09 0.24 0.13
0.49 0.40 0.32 0.34 0.53 0.34 0.31 0.41
carboxylic groups on the surface. Washing with HCl removed those traces of Na ions (about 0.1%) and left an almost clean carbon surface. Figure 4 shows the nitrogen adsorption isotherms along with the desorption brand. For all samples but the ironbased one, similar hysteresis loops are present. The exception in the case of C-Fe is the result of different shapes of mesopores. The highest uptake was found for the material derived from the initial, unmodified sample. This sample also seems to be the most microporous. The shapes of the isotherms obtained for samples modified with transition metals reveal the mixed, micro-/mesoporous structure. After acid washing the uptake increased slightly, especially for cobalt- and nickel-modified materials. This is expected since about a few percent of nonporous “bias” was dissolved from the surface, and besides the “dilution effect”, the new pores might be open for the nitrogen molecule.
Figure 5. Pore size distributions for initial carbons (A) and acid-washed counterparts (B).
The structural parameters of carbons calculated from nitrogen adsorption isotherms are collected in Table 3. Analysis of the data indicates differences in the porosity of materials, which are likely governed by the effect of metal cation. It is noteworthy that adsorbents obtained from nickel and cobalt salts have larger surface areas and higher volumes of micropores than the C-FeA sample. The degree of microporosity (ratio of Vmic/Vt) is around 32% for all transition metal containing samples. These differences are likely the results of the differences in the mechanism of carbonization. The most microporous appears to be the sample obtained from the sodium form of material where the chelation does not require multiple covalent bonds and thus material is less “ compact”. The pore size distributions calculated using DFT and a slit shape pore model commonly used for activated carbons24,25 are presented in Figure 5. As expected based on the shape of isotherms, the sample obtained from the initial polymer is very microporous with the majority of pores smaller than 30 Å and high contribution of ultramicropores (
