Development of Mesophase Pitch Derived Mesoporous Carbons

Mar 11, 2006 - to introduce mesopores in the activated carbons: (1) catalytic gasification ... Fax: +81-92-5837798. ... using nanostructured silica ma...
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Langmuir 2006, 22, 3791-3797

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Development of Mesophase Pitch Derived Mesoporous Carbons through a Commercially Nanosized Template W. M. Qiao,*,†,‡ Y. Song,† S. H. Hong,† S. Y. Lim,† S.-H. Yoon,† Y. Korai,† and I. Mochida*,† Institute for Materials Chemistry and Engineering, Kyushu UniVersity, Fukuoka 816-8580, Japan, and State Key Laboratory of Chemical Engineering, East China UniVersity of Science and Technology, Shanghai 200237, China ReceiVed September 13, 2005. In Final Form: January 27, 2006 Mesoporous carbons (MCs) with a high surface area (up to 900 m2/g), large pore volume (up to 2.1 cm3/g), high mesopore ratio (94%), and high yield (70%) were successfully prepared from an AR mesophase pitch, using a commercially nanosized silica template. The removal of the template provided some larger mesopores of 25-50 nm (pore I) with a surface area of ca. 300 m2/g, while the successive carbonization opened the closed pores within the carbon body to give smaller mesopores of 2-10 nm (pore II) with a similar surface area. During the carbonization of pitch precursor, the evaporation of volatile components swells the carbon to introduce the second mesopores among the domains and even microdomain units because of their rearrangements and overlappings in the process. The addition of iron salt with the silica template resulted in a remarkable increase of the surface area (ca. 300 m2/g) by introducing mesopores of 3-5 nm. The resultant MCs maintained some graphitizable natures derived from the anisotropic precursor. Their graphitization at 2400 °C provided the graphitic structure with large surface areas (270-460 m2/g) and mesoporosity.

1. Introduction Mesoporous carbons (MCs) have attracted much attention because they are expected to be used in many fields such as catalyst support for fuel cell, carbon electrode for supercapacitor, chromatographic separation, adsorption/desorption for fairly large molecules, desulfurization from petroleum refining for clean energy, and so on.1-8 Three typical methods have been reported to introduce mesopores in the activated carbons: (1) catalytic gasification,9-11 (2) carbonization of polymer aerogel through the sol-gel process,12,13 and (3) templating carbon precursors such as phenolic resin, furfuryl alcohol, sucrose, poly(vinyl chloride), polyacrylonitaile, mesophase pitch (MP), and poly* To whom correspondence should be addressed. Telephone: +81-925837801. Fax: +81-92-5837798. E-mail: [email protected] (W.M.Q.); E-mail: [email protected] (I.M.). †Kyushu University. ‡ East China University of Science and Technology. (1) Kyotani, T. Control of pore structure in carbon. Carbon 2000, 38, 269286. (2) Lee, J.; Yoon, S.; Hyeon, T.; Oh, S. M.; Kim, K. B. Synthesis of new mesoporous carbon and its application to electrochemical double-layer capacitors. Chem. Commun. 1999, 2177-2178. (3) Lee, J.; Han, S.; Hyeon, T. Synthesis of new nanoporous carbon materials using nanostructured silica materials as templates. J. Mater. Chem. 2004, 4, 478486. (4) Davis, M. E. Ordered porous materials for emerging applications. Nature 2002, 417, 813-821. (5) Chai, G.; Yoon, S.; Yu, J.; Choi, J.; Sung, Y. Ordered porous carbons with tunable pore sizes as catalyst supports in direct methanol fuel cell. J. Phys. Chem. B 2004, 108, 7074-7079. (6) Knox, J. H.; Kaur, G. B.; Millward, G. R. Structure and performance of porous graphitic carbon in liquid chromatography. J. Chromatogr. 1986, 352, 3-25. (7) Tamai, H.; Yoshida, T.; Sasaki, M.; Yasuda, H. Dye adsorption on mesoporous activated carbon fiber obtained from pitch containing yttrium complex. Carbon 1999, 37, 983-989. (8) Han, S.; Sohn, K.; Hyeon, T. Fabrication of new nanoporous carbons through silica templates and their application to the adsorption of bulky dyes. Chem. Mater. 2000, 12, 3337-3341. (9) Rodriguez-Reinoso, F. In Fundamental Issues in Control of Carbon Gasification ReactiVity; Lahaye, J., Ehrburgor, P. Eds.; Kluwer Academic: Norwell, MA, 1991; p 533. (10) Marsh, H.; Rand, B. The process of activation of carbons by gasification with CO2. II. The role of catalytic impurities. Carbon 1971, 9, 63-67. (11) Liu, Z. C.; Ling, L. C.; Qiao, W. M.; Liu, L. Preparation of pitch-based spherical activated carbon with developed mesopore by the aid of ferrocene. Carbon 1999, 37, 663-667.

pyrrole. The template can be nanosized particles such as silica nanoporous materials such as mesoporous silica and zeolite, etc.14-28 (12) Li, W.; Lu, A.; Guo, S. Characterization of the microstructures of organic and carbon aerogels based upon mixed cresol-formaldehyde. Carbon 2001, 39, 1989-1994. (13) Tamon, H.; Ishizaka, H.; Araki, T.; Okazaki, M. Control of mesoporous structure of organic and carbon aerogels. Carbon 1998, 36, 1257-1262. (14) Han, S.; Lee, K. T.; Oh, S. M.; Hyeon, T. The effect of silica template structure on the pore structure of mesoporous carbons. Carbon 2003, 41, 10491056. (15) Kawashima, D.; Aihara, T.; Kobayashi, Y.; Kyotani, T.; Tomita, A. Preparation of mesoporous carbon from organic polymer/silica nanocomposite. Chem. Mater. 2000, 12, 3397-3401. (16) Han, S.; Kim, M.; Hyeon, T. Direct fabrication of mesoporous carbons using in-situ polymerized silica gel networks as a template. Carbon 2003, 41, 1525-1532. (17) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism. Nature 1992, 352, 710-711. (18) Ryoo, R.; Joo, S. H.; Jun, S. Synthesis of highly ordered carbon molecular sieves via template-mediated structural transformation. J. Phys. Chem. B 2002, 103, 7743-7746. (19) Jun, S.; Joo, S. H.; Ryoo, R.; Kruk, M.; Jaroniec, M.; Liu, Z. Synthesis of new, nanoporous carbon with hexagonally ordered mesostructure. J. Am. Chem. Soc. 2000, 122, 10712-10713. (20) Kleitz, F.; Choi, S. H.; Ryoo, R. Cubic Ia3d large mesoporous silica: Synthesis and replication to platinum nanowires, carbon nanorods, and carbon nanotubes. Chem. Commun. 2003, 17, 2136-2137. (21) Yang, H.; Shi, Q.; Liu, X.; Xie, S.; Jiang, D.; Zhang, F. et al. Synthesis of ordered mesoporous carbon monoliths with bicontinuous cubic pore structure of Ia3d symmetry. Chem. Commun. 2002, 23, 2842-2843. (22) Lu, A. H.; Schmidt, W.; Spliethoff, B.; Schuth. F. Synthesis of ordered mesoporous carbon with bimodal pore system and high pore volume. AdV. Mater. 2003, 15, 1602-1606. (23) Lu, A. H.; Li, W. C.; Schmidt, W.; Schuth, F. Template synthesis of large pore ordered mesoporous carbon. Microporous Mesoporous Mater. 2005, 80, 117-128. (24) Fuertes, A. B.; Alvarez, S. Graphitic mesoporous carbons synthesised through mesostructured silica templates. Carbon 2004, 42, 3049-3055. (25) Li, Z. J.; Jaroniec, M. Silica gel-templated mesoporous carbons prepared from mesophase pitch and polyacrylonitrile. Carbon 2001, 39, 20802082. (26) Li, Z. J.; Jaroniec, M. Colloidal imprinting: A novel approach to the synthesis of mesoporous carbons. J. Am. Chem. Soc. 2001, 123, 9208-9209. (27) Li, Z. J.; Jaroniec, M. Mesoporous carbons synthesized by imprinting ordered and disordered porous structures of silica particles in mesophase pitch. J. Phys. Chem. B 2004, 108, 824-826. (28) Yang, C.-M.; Weidenthaler, C.; Spliethoff, B.; Mayanna, M.; Schuth, F. Facile template synthesis of ordered mesoporous carbon with polypyroolr as carbon precursor. Chem. Mater. 2005, 17, 355-358.

10.1021/la052494p CCC: $33.50 © 2006 American Chemical Society Published on Web 03/11/2006

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Table 1. Properties of MP, MCs, and Their Graphitized Ones elemental analysis MPb P0 P5 P5Fe P5-G P5Fe-G a

C

H

94.84

5.16

87.77 85.46 99.70 99.64

1.42 1.38 0.11 0.10

O

H/C

0

0.65

10.81 13.16 0.19 0.26

0.19 0.19 0.01 0.01

ash (wt %)

d002 (nm)

Lc (nm)

R (I1350/I1580)

∆νa (cm-1)

0.07 0.06 0.08 0.06

0.343 0.360 0.358 0.344 0.343

3.00 1.67 1.40 1.25 2.36 1.96

0.833 0.825 0.808 1.050 1.060

110 100 52 44

∆ν, the width at the half-maximum of the 1580 cm-1 band in Raman spectra. b MP, softening point/220 °C; soluble part in pyridine/50%.

Han et al. reported a preparation of MCs with a mesopore ratio of 50-80%, where the pore connectivity was improved, by using spherical silica sol and elongated silica as templates in the resorcinol-formaldehyde gel as a carbon precursor.14 Kawashima et al. prepared MCs through a sol-gel process using tetraethyl orthosilicate as a silica precursor and furfuryl alcohol as a carbon source.15 Because Kresege et al. reported a synthesis of mesoporous silica through the sol-gel polymerization of the silica precursor by using a self-assembling surfactant as a template,17 carbons with uniform mesopores were synthesized by using mesoporous silica as a template in the carbon precursors.2,18-23 In most cases, the particular synthesis of the template is necessary for MCs. The process is definitely complicated, increasing the production cost of MCs. Thus, more simple and practical methods should be developed. MP, exhibiting excellent graphitizability, high carbonization yield, microstructures composed of domains and even microdomains, is recognized as a unique anisotropic carbon precursor to develop carbon materials with high performances. Li et al. synthesized MCs from MP or its soluble component in quinoline using silica gel and colloidal silica as templates.25-27 The obtained carbons exhibited high mesoporosity with a surface area of 200-600 m2/g. So-called imprinting of the template in the pitch introduced the mesopores. The swelling of the carbon precursor may introduce additional mesopores.29 Zhao et al. proposed a simple melt impregnation to synthesize MCs with a surface area of 390 m2/g, where they applied the synthetic pitch as a carbon precursor and mesoporous silica (SBA-15) as a template.30 The synthetic pitch with a softening point of 104 °C appears as an intermediate product to develop MP by the successive heat treatment. Such a pitch should have a high fluidity to impregnate easily into the silica template. However, the carbonization of the pitch impregnated into the silica without oxidative stabilization may swell the pitch to form some bubbles. They ignored the effect on porous structures in the carbonized product. Recently, Dai et al. functionalized the surface of the MCs using diazoniums, which were in-situ-generated and reacted with the carbon surface of the MCs.31 Arylic reagents carrying the reactive substitute on the 4 poisiton (Ar-R, R ) chlorine, ester, and alkyl) may react with the hexagonal edge of the graphene. Graphitized carbons indicate their promising application in liquid chromatography because the graphitization eliminated all heteroatoms.6 Fuertes et al. synthesized graphitized mesoporous carbons (GMCs) through PVC-impregnated with mesostructured silica.24 Li et al. graphitized MCs from colloidal imprinting.32,33 However, the surface area of the GMCs is very limited. (29) Wang, Y. G.; Korai, Y.; Mochida, I.; Nagayama, K.; Hatano, H.; Fukuda, N. Modification of synthetic mesophase pitch with iron oxide, Fe2O3. Carbon 2001, 39, 1627-1634. (30) Yang, H. F.; Yan, Y.; Liu, Y.; Zhang, F. Q.; Zhang, R. Y.; Meng, Y. et al. A simple melt impregnation method to synthesized ordered mesoporous carbon carbon and carbon nanofiber bundles with graphitized structure from pitches. J. Phys. Chem. B 2004, 108, 17320-17328.

In the present study, a commercially available nanosized silica and Fe(NO3)3 salt (as a catalytic gasification agent) were added in a pyridine-soluble component of MP in the pyridine solvent. The mixture was simply stabilized, carbonized, and then washed successively by KOH and HCl to remove silica and iron to synthesize MCs from the anisotropic pitch. The porosity and graphitization extent of as-prepared MCs were investigated using nitrogen adsorption, X-ray diffraction analysis (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The as-prepared MCs were graphitized further at 2400 °C to develop graphitic carbons with a high surface area and mesoporous structure. 2. Experimental Section 2.1. Raw Materials. AR MP (provided by Mitsubishi Gas Chemical Company) was used as a carbon precursor, whose properties are listed in Table 1. A commercially available silica (Aldrich) was employed as a template (particle size, 25-50 nm; surface area, 340 m2/g; and pore volume, 0.55 cm3/g). Fe(NO3)3 was used as a catalytic gasification agent and a supplement template. 2.2. Preparation of MCs. (a) Silica Addition. The MP pyridinesoluble component was mixed with the template SiO2 in the pyridine solvent. After the solvent was evaporated, the obtained mixture (SiO2/ MP) was stabilized in air at 300 °C and carbonized at 750 °C in Ar into a MC/silica composite. The composite product was washed with 3 M KOH to remove the template to provide the MCs. (b) Fe(NO3)3/Silica Addition. Fe(NO3)3 (10%, silica base) was impregnated on the SiO2 in the solvent. Fe(NO3)3/SiO2 was poured into MP to be vigorously mixed. The mixture of Fe(NO3)3/SiO2/MP was pretreated by the above-mentioned process to obtain a MC/ silica composite containing iron. Finally, the composite was washed with 3 M KOH to remove the silica template and then with 10% HCl to remove iron to prepare MCs. 2.3. Graphitization of MCs. MCs were graphitized at 2400 °C with a heating rate of 10 °C/min under a flow of Ar into GMCs. 2.4. Characterization of MCs and Their Graphitized Ones. The surface areas of MCs and their graphitized ones were measured by nitrogen adsorption isotherms during the relative pressure ranges from 0.05 to 0.35 (Sorptic 1990). Pore-size distributions of MCs and corresponding GMCs were calculated from the desorption profile by the Bopp-Jancso-Heinzinger (BJH) method. The total pore volume (Vtotal) was calculated at P/P0 ) 0.99. The micropore volume (Vmicro) was calculated using the Rs plot. The mesopore volume (Vmeso) was calculated as the difference between Vtotal and Vmicro. The graphitic extents of MCs and GMCs were collected with an X-ray diffractometer (Rigaku Geigerflex, 40 kV, 30 mA, Cu KR1 target) to calculate the crystalline parameters (interspacing, d002; dimension (31) Li, Z. J.; Dai, S. Surface functionalization and pore size manipulation for carbons of ordered structure. Chem. Mater. 2005, 17, 1717-1721. (32) Li, Z. J.; Jaroniec, M. High surface area graphitized carbon with uniform mesopores synthesized by a colloidal imprinting method. Chem. Commun. 2002, 1346-1347. (33) Yoon, S. B.; Chai, G. S.; Kang, S. K.; Yu, J. S.; Gierszal, K. P.; Jaroniec, M. Graphitizaed pitch-based carbons with ordered nanopores synthesized by using colloidal crystals as templates. J. Am. Chem. Soc. 2005, 127, 4188-4189.

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Figure 1. Adsorption-desorption isotherm and pore-size distribution of MCs (P5 and P5Fe) and their graphitized ones (P5-G and P5Fe-G). Table 2. Properties of Fe(NO3)3/Silica/MP-Derived MCs and Their Graphitized Ones sample P0 P2.5 P5 P7.5 P10 P5Fe P5-G P5Fe-G a

2 2 2 2 2 2 2 2

MP/SiO2/Fe(NO3)3 (g/g/%)

yield (wt %)

SBET (m2/g)

Vtotal (cm3/g)

Vmicro (cm3/g)

ratiomesoa (%)

0 2.5 5 7.5 10 5 5 5

71.5 71.0 70.3 71.2 70.2 66.4

5 423 608 636 598 908 275 460

1.00 1.29 1.46 1.25 2.16 0.71 1.50

0.06 0.07 0.08 0.06 0.12 0.03 0.10

94.0 94.6 94.5 95.2 94.4 95.7 93.3

0 0 0 0 0 10 0 10

Ratiomeso ) (Vtotal - Vmicro)/Vtotal.

of crystal, Lc) according to the JSPS procedure.34 Prior to the measurement, the powdered sample was mixed with the standard silicon (200 mesh, 99.99%) at the ratio of 98:2 sample/silicon (wt/wt) for MC or 90:10 (wt/wt) for GMC. Raman spectra (JASCO, NMS-2000B) of MCs and GMCs were also applied to examine the degree of graphitization. The excitation source was a 514.5 nm Ar-ion laser line. Morphologies of MCs were observed under SEM (JSM-6700F) and TEM (JEM-2100F).

3. Results 3.1. MCs from Silica/MP and Fe(NO3)3/Silica/MP Mixtures. Figure 1 showed typical adsorption-desorption isotherms and pore-size distributions of as-prepared MCs. Tables 1 and 2 summarized the properties of MCs. The surface area and pore volume of MCs increased with an increase of the ratio of silica/MP from 2.5:2 to 7.5:2 (wt/wt). A higher ratio such as 10:2 (P10) resulted in the decrease of both the surface area and pore volume. An excessive addition of silica may suffer a poor dispersion of nanoparticles in the pitch through their aggregation. Without the addition of silica, the pitch appeared to give a pore structure inaccessible for nitrogen at 77 K (P0) after carbonization. The addition of silica provided the highest (34) Japan Society for the Promotion of Science (117th committee). On the measurement of lattice parameters and unit cell dimension of artificial graphite (in Japanese). Tanso 1963, 36, 25-34.

surface area of 636 m2/g and the largest pore volume of 1.46 cm3/g (P7.5) in the carbonized product. The MP exhibited a high carbonization yield of ca. 70%. MCs (P5) showed high oxygen content (10%) because of oxidative stabilization. The oxygenrich surface is expected to provide a hydrophilic property. Low ash in the product, as shown in Table 1, demonstrates a nearly complete removal of silica. As shown in Figure 1, MCs such as P5 gave a type-IV adsorption-desorption isotherm according to IUPAC classification, exhibiting a definite hysteresis loop. Such a loop has been reported to correspond to the capillary condensation of nitrogen molecules in the mesopores, indicating typically mesoporous structures in the carbonized product. The adsorption and desorption curves are noted to separate individually at a range of relative pressures higher than 0.95, indicating the possible existence of some large mesopores and even macropores. Poresize distributions calculated according to the BJH method shown in Figure 1b are consistent with such observations. The addition of iron salt increased the adsorbed amount of nitrogen (at 77 K) on the resultant MCs (P5Fe) as observed in Figure 1a. Such an additive improved the surface area and pore volume of MCs, up to 908 m2/g and 2.16 cm3/g, respectively. Although P5Fe showed a similar pore-size distribution to that of P5, the former carbon appears to carry more mesopores of 3-10 nm than the latter one.

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Figure 3. XRD pattern of carbonized product of the Fe(NO3)3/ SiO2/MP mixture. (Ratio of MP/SiO2, 2:5; Fe(NO3)3 amount: 10%).

Figure 2. XRD patterns of MCs (P5 and P5Fe) and their graphitized ones (P5-G and P5Fe-G).

3.2. Graphitization of MCs at 2400 °C. Figure 1 also showed adsorption-desorption isotherms and pore-size distributions of GMCs. GMCs also showed a similar type-IV adsorptiondesorption isotherm and multimodal pore-size distribution as shown in parts b and d of Figure 1. The graphitization of MCs at 2400 °C markedly reduced the surface areas of P5 and P5Fe to 275 and 460 m2/g and pore volumes of P5 and P5Fe to 0.71 and 1.50 cm3/g, respectively, attributed to the densification by the graphitization. The shrinkage only slightly decreased the pore size as shown in parts b and d of Figure 1 in comparison with that of the corresponding carbonized product. 3.3. Graphitic Parameters of MCs and Their Graphitized Forms. Figure 2 showed XRD patterns of MCs and their graphitized forms. Without a template, P0 exhibited a symmetrical peak of (002) reflection, indicating that the direct carbonization without any additive is beneficial to develop graphitic layers. The interspacing of layers and dimension of the graphite unit in P0 are 0.343 and 1.67 nm, respectively. The addition of a template of silica slightly decreased the crystallinity of P5 as indicated by its broader 002 peak with a small shift to low angle as observed in Figure 2a. The addition of iron salt improved the crystallinity of P5Fe compared with P5. The interspacings of layers are 0.360 nm for P5 and 0.358 nm for P5Fe as calculated from Figure 2a. Figure 3 showed an XRD pattern of the carbonized Fe(NO3)3/ silica/MP mixture. The peaks for metallic iron were obviously observed at 2θ ) 45° and 65° as shown in Figure 5. Iron nitrate

Figure 4. Raman spectra of MCs (P5 and P5Fe) and their graphitized ones (P5-G and P5Fe-G).

is known to decompose around 300 °C to give iron oxide, which is reduced by carbon into metallic iron around 750 °C. Iron catalyzes the graphitization of P5Fe. GMCs exhibited a sharp 002 peak as shown in Figure 2b. The peaks at ca. 26.0° (002), 43.6° (101), 53.8° (004), and 79.1° (110) indicate graphitic structures of the products. The values of d002 were 0.344 nm for P5-G and 0.343 nm for P5Fe-G, respectively. Lc values were 2.36 nm for P5-G and 1.96 nm for P5Fe-G, respectively. Figure 4 showed Raman spectra of MCs and their graphitized forms. MCs (P5 and P5Fe) exhibited a similar Raman spectrum, where two sharp peaks (1350 cm-1 for G band and 1580 cm-1 for D band) and two broad ones (2500-3100 and 3100-3300 cm-1) were observed. A lower intensity ratio (R value) of the 1350-1580 cm-1 peaks and a smaller width at the half-maximum of the 1580 cm-1 peak (∆ν value) indicate a high degree of graphitization.35 The intensity ratios were calculated as 0.825 for P5 and 0.808 for P5Fe, and ∆ν values were 110 cm-1 for P5 and 100 cm-1 for P5Fe. Such values indicate that iron may catalytically graphitize the present MCs to some extent. (35) The Carbon Society of Japan. Newest experimental technologies (analysis) for carbon materials. Sipec Realize 2001, 88-98.

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Figure 5. SEM photographs of P5 (A and B) and P5Fe (C and D).

Figure 6. TEM photographs of P5 (E and F) and P5Fe (G and H).

GMCs (P5-G and P5Fe-G) clearly exhibited six sharp peaks as observed in Figure 4b, attributed to the D band at 1350 cm-1, G band at 1580 cm-1 with a shoulder peak of the D′ band at 1620 cm-1, 2D′′ band at 2446 cm-1, 2D or G′ band at 2680 cm-1,33,35 D plus G band at 2935 cm-1, and 2D′ band at 3225 cm-1. The graphitization at 2400 °C very markedly decreased ∆ν values of both carbons. Although P5-G and P5Fe-G showed similar R

values about 1.05, higher than those of their corresponding MCs, their ∆ν values were very different, calculated to be 52 cm-1 for P5 and 44 cm-1 for P5Fe. P5Fe-G exhibited a higher degree of graphitization than P5-G. 3.4. Morphologies of MCs. Figures 5 and 6 showed SEM and TEM images of MCs (P5 and P5Fe), respectively. Both P5 and P5Fe exhibited typical mesoporous structures, in-

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cluding small mesopores of 3-10 nm (pore II), large mesopores of 25-50 nm (pore I), and some macropores as observed in the SEM images (Figure 5). Moreover, P5Fe showed more small mesopores than P5. TEM images of MCs (Figure 6) suggest clearly their porous structures. Especially, an addition of iron increased the amount of pores with a more uniform pore size.

4. Discussion The development of mesopores in the carbons can be explained by the template effect, opening of closed pores, and catalytic gasification. In general, MCs prepared through the template method are expected to exhibit a surface area similar to that of the template when it is removed, while the pore structures (pore volume and pore size) should depend upon the dimension of the template particles. P5 exhibited a lot of mesopores with a size of 25-40 nm as shown in Figures 1 and 5-7, named as pore I. The size of pore I is consistent with that of silica nanoparticles. A number of mesopores with 3-10 nm (named as pore II) as well as a small amount of micropores (0.06-0.12 cm3/g) and macropores (>50 nm) are also included in the same carbon. The mesopores smaller than 10 nm (pore II) are introduced through the opening of the closed pores in the carbon as shown in Figure 7. The swelling property of the MP without stabilization at the carbonization was used to synthesize carbon foams, which exhibited macroporous structures with 0.1-0.4 mm in size.36-38 It demonstrated that pore structures can be developed during the carbonization of the pitch. In contrast, oxidative stabilization of the pitch cross-links large molecules in the pitch to suppress the swelling properties and to improve the carbonization yield. Enough stabilization is necessary to eliminate the defects, voids, and pores because of the swellings, developing the denser carbon. However, an intermediate extent of the stabilization may provide mesoporous structures through controlled swelling. The sufficiently stabilized pitch may provide small pores, which are inaccessible for nitrogen molecules. Such a carbon is essentially nonporous, with its surface area being as low as only 5 m2/g. In contrast, the stabilized pitch with a template gives mesopores with a high surface area and large pore volume. A lot of closed pores (mainly mesopores from 2 to 10 nm) are produced around silica particles at the carbonization of the pitch because of the evolution of small molecules and rearrangements of nanosized microdomains composed of the stacking of cluster units.39 After the removal of the silica, such closed mesopores are opened to the surface to provide a larger surface area and larger pore volume. Such mesopores form networked mesoporous channels as observed under SEM and TEM of the present MCs. In a previous paper, Fe(NO3)3 was reported to be an effective additive to modify porous structures of activated carbons through its catalytic gasification activity.40 The addition of the iron salt was confirmed to increase markedly the surface area of the present MCs. Surface areas and pore volumes of the carbonized mixture of Fe(NO3)3/silica/MP and MCs prepared through different (36) Watanabe, F.; Mochida, I. Structural and thermal characteristics of highly graphitizable AR-foam. Tanso 2004, 212, 99-105. (37) Cao, M.; Zhang, S.; Wang, Y. G. Influence of preparation conditions on the pore structure of carbon foam. New Carbon Mater. 2005, 20, 134-138. (38) Qiu, J. S.; Ling, P.; Liu, G. S.; Zhou Y. Effect of iron nitrate on the preparation of carbon foams from mesophase pitch. New Carbon Mater. 2005, 20, 193-197. (39) Mochida, I.; Yoon, S. H.; Lim, S. Y.; Hong, S. H. Progress and effectiveness of structural models of carbons. Tanso 2004, 215, 274-284. (40) Qiao, W. M.; Song, Y.; Yoon, S. H.; Mochida, I. Modification of commercial activated carbon through gasification by impregnated metal salts to develop mesoporous structures. New Carbon Mater. 2005, 20, 198-204.

Figure 7. Microstructures of P5 and P5Fe. Table 3. Effect of Washing Methods on Surface Area and Pore Volumea

carbonized form 10% HCl-washed form 3 M KOH-washed form 3 M KOH-washed plus 10% HCl-washed forms a

SBET (m2/g)

Vtotal (cm3/g)

232 236 891 908

0.59 0.60 1.85 2.16

MP/SiO2/Fe(NO3)3 ) 2 g/5 g/10%.

washing processes are summarized in Table 3. The acid could not effectively remove silica and iron particles as indicated by similar surface areas and pore volumes before and after the washing. In contrast, KOH washing provided a high surface area and large pore volume. The combination of first KOH and second

Mesophase Pitch DeriVed Mesoporous Carbons

HCl washings further increased both the surface area and pore volume, indicating that iron particles were removed by such a successive washing. Because the former particles are closely supported on the surface of the latter particles. The iron salt particles are believed to gasify the carbon around them, introducing some pores on the wall of the mesopores. XRD and Raman spectroscopy indicated that MCs maintained the graphitic structures, which originated from the layered structure in the MP. It should be noted that the dissolution of MP in the solution largely lowered the graphitic nature of the carbonized product through the lowered order of the layered stackings in the MP. The addition of silica slightly enlarged the d002 value and lowered the layered stackings, 0.36 and 1.4 nm (P5), respectively. The addition of iron salt slightly improved the crystallinity of the carbonized product as indicated by the fact that P5Fe exhibited someway improved graphitic parameters, while the graphitization of MCs remarkably improved graphitic parameters as a decreased d002 value of 0.343-0.344 nm, enlarged Lc of 1.96-2.36 nm, and decreased R value of 44-52, respectively. Although the graphitization of mesoprous carbons decreases their surface areas and pore volumes, the present graphitized form still shows a surprising surface area of 460 m2/g and pore volume of 1.5 cm3/g, which are much higher than the reported one (the reported maximum surface area was