Preparation of Activated Ordered Mesoporous Carbons with a ...

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Nov 11, 2007 - tention due to their promising use as adsorbents, in chromato- graphic packing, as ... structure of the template SBA-15,18 ordered mesoporous carbons with a channel .... Chemical Society. Published on Web 02/29/2008 ... wt %, the number of smaller micropores with a PSD centered at 0.7 nm slightly ...
Langmuir 2008, 24, 2967-2969

2967

Preparation of Activated Ordered Mesoporous Carbons with a Channel Structure Dingcai Wu,† Yeru Liang,† Xiaoqing Yang,† Chong Zou,† Zhenghui Li,† Guifen Lv,† Xianhua Zeng,† and Ruowen Fu*,†,‡ Materials Science Institute, PCFM Laboratory, School of Chemistry and Chemical Engnieering, and Institute of Optoelectronic and Functional Composite Materials, Sun Yat-sen UniVersity, Guangzhou 510275, P. R. China ReceiVed NoVember 11, 2007. In Final Form: February 15, 2008 Activated ordered mesoporous carbons with a channel structure (AOMCs-CS) were successfully prepared by imposing CO2 activation on ordered mesopore carbon C-FDU-15. It is found that the continuous carbon framework of the precursor C-FDU-15 plays an important role in keeping the order structure of the resulting AOMCs-CS. The mild activation (e.g., 31 wt % burnoff) does not impair the order degree. After that, the order degree gradually decreases with further increasing burnoff. However, the basic hexagonal mesostructure of C-FDU-15 can still be found in the AOMCs-CS when the burnoff is up to 73 wt %, although many carbon walls are punched and thus many larger mesopores and marcropores are generated. With increasing burnoff, the surface area and volume of micropores increase first and then decrease, and the surface area and volume of mesopores continuously increase. The highest measured Brunaruer-Emmett-Teller (BET) surface area, micropore volume, and total pore volume of the AOMCsCS reach 2004 m2/g, 0.50 cm3/g, and 1.22 cm3/g, respectively.

Nowadays, many novel mesoporous carbon materials including carbon aerogels,1,2 wormholelike mesoporous carbons,3 and ordered mesoporous carbons (e.g., CMK-3,4 CMK-5,5 COU-1,6 C-FDU-15,7,8 and Fiber-C-g9) have attracted considerable attention due to their promising use as adsorbents, in chromatographic packing, as electrode materials for supercapacitors and rechargeable batteries, as advanced catalyst supports, as thermal insulators, and in a variety of other applications.5,10-13 Many efforts have been made to provide insight into how to control and tailor the particle size and shape, the pore structure, and the carbon surface of the mesoporous carbon materials for a specific application. As one of such efforts, activation has proved to be very effective in extending their potential for applications by donating microporosity to the carbon frameworks without change of the skeletal carbon structure. For example, activated carbon aerogels were first prepared by Kaneko and co-workers14 and have proved to have better properties, for example, electrochemical property15 and adsorption property,16 than those of their * To whom correspondence should be addressed. Telephone: +86020-84114527. Fax: +86-020-84115112. E-mail: [email protected]. † Materials Science Institute, PCFM Laboratory, School of Chemistry and Chemical Engineering. ‡ Institute of Optoelectronic and Functional Composite Materials. (1) Wu, D. C.; Fu, R. W.; Dresselhaus, M. S.; Dresselhaus, G. Carbon 2006, 44, 675. (2) Wu, D. C.; Fu, R. W.; Zhang, S. T.; Dresselhaus, M. S.; Dresselhaus, G. Carbon 2004, 42, 2033. (3) Han, B. H.; Zhou, W. Z.; Sayari, A. J. Am. Chem. Soc. 2003, 125, 3444. (4) Jun, S.; Joo, S. H.; Ryoo, R.; Kruk, M.; Jaroniec, M.; Liu, Z.; Obsuna, T.; Terasaki, O. J. Am. Chem. Soc. 2000, 122, 10712. (5) Joo, S. H.; Choi, S. J.; Oh, I.; Kwak, J.; Liu, Z.; Terasaki, O.; Ryoo, R. Nature 2001, 412, 169. (6) Tanaka, S.; Nishiyama, N.; Egashira, Y.; Ueyama, K. Chem. Commun. 2005, 2125. (7) Meng, Y.; Gu, D.; Zhang, F. Q.; Shi, Y. F.; Yang, H. F.; Li, Z.; Yu, C. Z.; Tu, B.; Zhao, D. Y. Angew. Chem., Int. Ed. 2005, 44, 7053. (8) Meng, Y.; Gu, D.; Zhang, F. Q.; Shi, Y. F.; Cheng, L.; Feng, D.; Wu, Z. X.; Chen, Z. X.; Wan, Y.; Stein, A.; Zhao, D. Y. Chem. Mater. 2006, 18, 4447. (9) Liang, C. D.; Dai, S. J. Am. Chem. Soc. 2006, 128, 5316. (10) Suda, H.; Haraya, K. Chem. Commun. 1997, 93. (11) Rodrı´guez-Reinoso, F. Carbon 1998, 36, 159. (12) Dresselhaus, M. S. Annu. ReV. Mater. Sci. 1997, 27, 1. (13) Moreno-Castilla, C.; Maldonado-Ho´dar, F. J. Carbon 2005, 43, 455. (14) Hanzawa, Y.; Kaneko, K.; Pekala, R. W.; Dresselhaus, M. S. Langmuir 1996, 12, 6167.

precursor carbon aerogels. Recently, the activation of CMK-3 has also been reported by Gao and co-workers, and the as-prepared activated CMK-3 had an obviously better H2 sorption capacity than the precursor CMK-3.17 In addition, it has been shown that the ordered mesoporous structure of activated CMK-3 gradually became disordered and then changed into a thoroughly disordered wormlike pore structure when the activation time was prolonged.17 Unlike ordered mesoporous carbon CMK-3 constructed with an ordered array of carbon nanorods or nanowires with a reverse structure of the template SBA-15,18 ordered mesoporous carbons with a channel structure (AOMCs-CS) such as COU-1, C-FDU15, and Fiber-C-g have an ordered hexagonal mesostructure which is the same as that of SBA-15. As a result of such a structural difference, C-FDU-15 has been proven in the research of Zhao and co-workers to have a higher mechanical stability than CMK3, likely due to its continuous carbon framework.8 Similarly, we believe that such a continuous carbon framework will help to keep the basic 2-D hexagonal mesostructure when imposing an intense activation on the ordered mesoporous carbons. In this study, C-FDU-15 was prepared and then activated. The microstructure change of the resulting AOMCs-CS during the activation treatment was investigated in detail by X-ray diffraction (XRD), transmission electron microscopy (TEM), and N2 adsorption. The XRD pattern for the precursor C-FDU-15 in Figure 1 shows an intense diffraction peak (10) and two resolved diffraction peaks (11) and (20), indicating that the as-prepared C-FDU-15 has a 2-D hexagonal mesostructure.8 Furthermore, ordered straight channels can be visibly observed in the TEM images of C-FDU15 (See Figure 2A). These results show that C-FDU-15 prepared here is a typical ordered mesoporous carbon with a channel structure (OMC-CS). When imposing 3 h of CO2 activation treatment on C-FDU-15, the resulting AOMC-CS, that is, AOMC(15) Zhu, Y. D.; Hu, H. Q.; Li, W. C.; Zhang, X. Y. J. Power Sources 2006, 162, 738. (16) Wu, D. C.; Sun, Z. Q.; Fu, R. W. J. Appl. Polym. Sci. 2006, 99, 2263. (17) Xia, K. S.; Gao, Q. M.; Wu, C. D.; Song, S. Q.; Ruan, M. L. Carbon 2007, 45, 1989. (18) Zhao, D. Y.; Feng, J. L.; Huo, Q. S.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548.

10.1021/la703511z CCC: $40.75 © 2008 American Chemical Society Published on Web 02/29/2008

2968 Langmuir, Vol. 24, No. 7, 2008

Figure 1. XRD patterns of the precursor C-FDU-15 and related AOMCs-CS with various burnoffs.

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Figure 3. N2 adsorption-desorption isotherms of C-FDU-15 and related activated samples.

Figure 4. DFT pore size distributions of FDU-15 and related activated samples. Figure 2. TEM images of typical unactivated and activated samples: (A) C-FDU-15; (B) AOMC-CS31; (C) AOMC-CS65; and (D) AOMC-CS73.

CS31, has a burnoff of 31 wt %. It can be seen that the XRD pattern (Figure 1) and the TEM image (Figure 2) of AOMCCS31 are very similar to those of its precursor, indicating that such a burnoff hardly destroys the order degree of the resulting AOMC-CS. When increasing the burnoff from 31 to 65 wt % by prolonging the activation time, more and more carbon walls are punched and thus many neighboring channels are interconnected (see Figure 2B and C). In addition, the carbon frameworks slightly shrink and the unit cell parameter (a) correspondingly decreases (see Table 1). Furthermore, as the burnoff continuously increases to 73 wt %, the punching action from activation becomes very intense and partial carbon frameworks thoroughly collapse, resulting in the formation of larger mesopores and macropores (see Figure 2D). The existence of these large pores in the AOMCCS73 can also be supported by its N2 adsorption-desorption isotherm, in which a great increase of the adsorption amount at relative pressures above 0.9 occurs (see Figure 3). As a result,

the order degree of the resulting AOMCs-CS, for example, AOMC-CS65 and AOMC-CS73, gradually becomes lower when further increasing the burnoff from 31 to 73 wt % (see Figures 1 and 2). However, it should be noted that the formation of these interconnected channels and large pores by the activation treatment will help to meet the needs of rapid mass transport for many applications such as separation, catalysis, and energy storage. Furthermore, the TEM images in Figure 2 vividly show that, even at a very high burnoff level, for example, 73 wt %, the basic ordered mesostructure of the precursor can still be found in the resulting products, for example, AOMC-CS73. This is consistent with the results from the XRD patterns and N2 adsorptiondesorption isotherms. In Figure 1, the diffraction peak (10) still remains resolved, although the diffraction peaks (11) and (20) have disappeared. And, in Figure 3, a distinct hysteresis loop in the medium range of relative pressure is still found in AOMCCS73 and its shape is very similar to those of others, most likely implying the existence of a mesopore structure of the precursor. Kaneko and co-workers have also reported that an intense activation treatment with 72 wt % burnoff does not change the basic network structure of carbon aerogels composed of a continuous carbon nanoframework, although a partial opening

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Langmuir, Vol. 24, No. 7, 2008 2969 Table 1. Pore Structure Parameters of the Precursor C-FDU-15 and Related AOMCs-CS with Various Burnoffs

sample

burnoff (wt %)

a (nm)

SBET (m2/g)

Sext (m2/g)

Smic (m2/g)

Vt (cm3/g)

Vmes (cm3/g)

Vmic (cm3/g)

Dmes (nm)

C-FDU-15 AOMC-CS31 AOMC-CS65 AOMC-CS73

0a 31 65 73

8.2 8.0 7.4 7.6

612 1222 2004 1696

249 425 907 858

362 797 1096 838

0.34 0.67 1.10 1.22

0.14 0.42 0.49 0.78

0.17 0.37 0.50 0.38

3.8 4.0 3.9 6.5

a

0 wt % burnoff represents no activation treatment imposed on the precursor C-FDU-15.

of the network structure occurs.14 In contrast, Gao and co-workers have found that, upon intense activation, the basic ordered mesostructure of ordered mesoporous carbon CMK-3 will be thoroughly destroyed.17 Therefore, we suppose that the continuous carbon skeletal structure in the precursor C-FDU-15 plays a very important role in preserving the basic ordered mesostructure of related activated samples with high burnoffs, for example, 73 wt %. According to the density functional theory (DFT) results (see Figure 4, and Figure S1 and Table S1 in the Supporting Information), it can be concluded that, with increasing the burnoff to 31 wt %, substantial new micropores are generated and meanwhile some larger micropores with a pore size distribution (PSD) centered at 1.3 nm widen and become smaller mesopores (ca. 2-6 nm in diameter). As the burnoff further increases to 65 wt %, the number of smaller micropores with a PSD centered at 0.7 nm slightly decreases and the number of the larger micropores and the smaller mesopores continuously increases. As a result of the above pore structure change, with increasing the burnoff to 31 wt % or 65 wt %, the corresponding BET surface area and total volume of the AOMCs-CS samples greatly increase from the original 612 m2/g and 0.34 cm3/g to 1222 m2/g and 0.67 cm3/g or to 2004 m2/g and 1.10 cm3/g, respectively. Subsequently, when the burnoff continues to increase and reaches 73 wt %, the amount of micropores and smaller mesopores decreases, and a few large pores are generated. Figure 4 shows that these generated large pores mainly distribute in the range of 10-100 nm and have a maximum at ∼37 nm. Correspondingly, the average mesopore diameter of the AOMC-CS73 is 6.5 nm,

larger than that of C-FDU-15 and related activated samples with lower burnoffs. Therefore, with further increasing the burnoff to 73 wt %, the corresponding total volume continuously increases to 1.22 cm3/g, but the corresponding BET surface area decreases to 1696 m2/g (see Table 1). In conclusion, AOMCs-CS have been successfully prepared. The order degree of AOMCs-CS is basically unvaried in the burnoff range below 31 wt % and subsequently gradually becomes lower with further increasing the burnoff to 73 wt %. As highlighted here, due to the existence of a continuous carbon skeletal wall, even such an intense activation with 73 wt % burnoff does not thoroughly destroy the ordered mesostructure. With increasing the burnoff, the amount of micropores first increases and then decreases, and the amount of mesopores continuously increases. As a result, the typical AOMCs-CS have great BET surface areas (e.g., 2004 m2/g) and large micropore and mesopore volumes (e.g., 0.50 and 0.78 cm3/g, respectively). Acknowledgment. This research was supported by the Project of NNSFC (50472029, 50632040), the Scientific Foundation of Guangzhou (2007Z2-D2041), and the Scientific Research Foundation for Youth Scholars of Sun Yat-sen University (1131211). Ms. Dan Jiang and Mr. Chaolun Liang (Instrumental Analysis & Research Center, Sun Yat-sen University) are acknowledged for help with TEM measurements. Supporting Information Available: Experimental section and DFT cumulative pore volume parameters. This material is available free of charge via the Internet at http://pubs.acs.org. LA703511Z