Synthesis of Highly Ordered Ir-Containing Mesoporous Carbon

Jan 31, 2008 - tems.1 This type of carbon, with high surface areas and large uniform pores ... Graduate School of the Chinese Academy of Sciences. (1)...
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Chem. Mater. 2008, 20, 1881–1888

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Synthesis of Highly Ordered Ir-Containing Mesoporous Carbon Materials by Organic–Organic Self-Assembly Peng Gao,†,‡ Aiqin Wang,† Xiaodong Wang,† and Tao Zhang*,† State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China, and Graduate School of the Chinese Academy of Sciences, Beijing 100049, China ReceiVed September 30, 2007. ReVised Manuscript ReceiVed December 4, 2007

By simply adjusting the molar ratio of resorcinol (R) to formaldehyde (F) and optimizing the aging time of the gel, highly ordered mesoporous carbons (OMCs) have been synthesized under strong acidic conditions through aqueous self-assembly of R/F with triblock copolymer F127, without addition of any promoters. It was found that both the excess amount of R (R/F g 1/2) and the long aging time (96 h) were favorable to the formation of the OMC. This facile route was further extended to the synthesis of highly ordered iridium-containing mesoporous carbons (Ir-OMC) by adding H2IrCl6 to the reaction mixture. The resultant Ir-OMCs were characterized by nitrogen sorption, X-ray diffraction (XRD), and transmission electron microscopy (TEM). The results showed that iridium particles, with sizes of ∼2 nm, were highly dispersed in the carbon matrix, while the ordered mesostructure of carbons remained well. Comparing with the Ir/OMC sample prepared by postimpregnation, such one-pot synthesized Ir-OMC samples had smaller Ir particles sizes and therefore exhibited higher activities and stabilities toward the catalytic decomposition of N2H4.

Introduction Synthesis of ordered mesoporous carbons (OMCs) has attracted growing interest due to their potential applications in separation, catalysis, and energy storage/conversion systems.1 This type of carbon, with high surface areas and large uniform pores, has previously been fabricated by a nanocasting strategy using ordered mesoporous silicas (such as SBA-15)2 or colloidal crystals3 as hard templates. Nevertheless, nanocasting is a very fussy, high-cost, and thus industrially unfeasible method. It is therefore desirable to construct OMC with a facile self-assembly method similar to the preparation of ordered mesoporous silicas, which will be low-cost and suitable for mass production. The first effort toward this goal was made by Dai et al.,4 who successfully synthesized highly ordered porous carbon films by selfassembly of a PS-P4VP/resorcinol-formaldehyde mixture. However, the employment of the expensive block copolymer PS/P4VP still has some limitations for large-scale application. On the basis of this organic–organic self-assembly strategy, a more general and robust method for the synthesis of OMC was further developed by the same group, in which a * To whom correspondence should be addressed: Tel +86-411-84379015; Fax +86 411 8469 1570; e-mail [email protected]. † Dalian Institute of Chemical Physics, Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences.

(1) (a) Yang, Z. X.; Xia, Y. D.; Mokaya, R. J. Am. Chem. Soc. 2007, 129, 1673. (b) Zeng, J. H.; Su, F. B.; Lee, J. Y.; Zhou, W. J.; Zhao, X. S. Carbon 2006, 44, 1713. (c) Chai, G. S.; Yoon, S. B.; Kim, J. H.; Yu, J. S. Chem. Commun. 2004, 23, 2766. (d) Choi, W. C.; Woo, S. I.; Jeon, M. K.; Sohn, J. M.; Kim, M. R.; Jeon, H. J. AdV. Mater. 2005, 17, 446. (2) Joo, S. H.; Choi, S. J.; Oh, I.; Kwak, J.; Liu, Z.; Terasaki, O.; Ryoo, R. Nature (London) 2001, 412, 169. (3) (a) Jiang, P.; Hwang, K. S.; Mittleman, D. M.; Bertone, J. F.; Colvin, V. L. J. Am. Chem. Soc. 1999, 121, 11630. (b) Johnson, S. A.; Ollivier, P. J.; Mallouk, T. E Science 1999, 283, 963. (4) Liang, C. D.; Dai, S. Angew. Chem., Int. Ed. 2004, 43, 5785.

commercially available triblock copolymer F127 was used as a structure-directing agent and a mixture of phloroglucinol and formaldehyde as a carbon precursor.5 Independently, Zhao et al. synthesized a family of OMCs with tunable pore structures, including 2-D hexagonal, 3-D bicontinuous, bodycentered cubic, and lamellar symmetries by adjusting the reaction conditions.6–9 It is noted that the type of phenolic resin monomer has a great influence on the formation of OMC. For example, Dai et al. found that phloroglucinol which contains three hydroxyl groups was the best precursor for the synthesis of OMC, while either resorcinol or phenol could not yield an ordered carbon structure under acidic conditions.5 Differently, Zhao et al. always used phenol/formaldehyde as the carbon precursor, and their synthesis was performed under basic conditions.6–9 Although resorcinol-formaldehyde (RF) was the most frequently used organic monomer for the synthesis of carbon gels from the organic sol–gel process,10–13 there has been little success on the synthesis of OMC by using (5) Liang, C. D.; Dai, S. J. Am. Chem. Soc. 2006, 128, 5316. (6) Meng, Y.; Gu, D.; Zhang, F. Q.; Shi, Y. F.; Yang, H. F.; Li, Z.; Yu, C. Z.; Zhao, D. Y. Angew. Chem., Int. Ed. 2005, 44, 7053. (7) Zhang, F. Q.; Meng, Y.; Gu, D.; Yan, Y.; Yu, C. Z.; Tu, B.; Zhao, D. Y. J. Am. Chem. Soc. 2005, 127, 13508. (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) Huang, Y.; Cai, H. Q.; Yu, T.; Zhang, F. Q.; Zhang, F.; Meng, Y.; Gu, D.; Wan, Y.; Sun, X. L.; Tu, B.; Zhao, D. Y. Angew. Chem., Int. Ed. 2007, 46, 1089. (10) Pekala, R. W. J. Mater. Sci. 1989, 24, 3221. (11) Saliger, R.; Bock, V.; Petricevic, R.; Tillotson, T.; Geis, S.; Fricke, J. J. Non-Cryst. Solids 1997, 221, 144. (12) Fung, A. W. P.; Wang, Z. H.; Lu, K.; Dresselhaus, M. S.; Pekala, R. W. J. Mater. Res. 1993, 8, 1875. (13) Han, S. J.; Hyeon, T. Chem. Commun. 1999, 1955.

10.1021/cm702815e CCC: $40.75  2008 American Chemical Society Published on Web 01/31/2008

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Figure 1. N2 adsorption–desorption isotherms of the OMC samples.

RF as the carbon precursor. Tanaka et al.14 made the first attempt to assemble this RF sol with the triblock copolymer F127 to synthesize OMC, but they found that only with the assistance of triethyl orthoacetate (EOA) could the OMC be yielded under acidic conditions. In their very recent report,15 they found that a mixture of resorcinol and phloroglucinol, rather than either of them, could produce continuous OMC films. In addition, Liu et al.16 reported the synthesis of OMC from F108/RF composite in basic media. On the basis of these previous reports, it is believed that the synthesis of OMC with RF as the carbon precursor, especially under acidic conditions, needs to be further explored for a better understanding of the organic–organic self-assembly process. On the other hand, since mesoporous carbons can offer great advantages over the microporous ones in mass transfer and provide better accessibility to reactant molecules when they are used as catalyst supports, it would be interesting to incorporate active metal species into the mesoporous carbons during the synthesis, while retaining the ordered mesostructure. Compared with conventional postsynthesis techniques such as impregnation,17–19 adsorption,20 or ion exchange,21 introducing metal component during the synthesis of carbons possesses its unique advantages: the metal components can be highly dispersed throughout the carbon matrix, and they are very stable against sintering even with a high-temperature treatment.22,23 Actually, in the synthesis of carbon gels via the RF sol–gel process, metal component could be readily incorporated into the carbon matrix by adding a soluble metal salt to the initial mixture.24–26After gelation and carboniza(14) Tanaka, S.; Nishiyama, N.; Egashira, Y.; Ueyama, K. Chem. Commun. 2005, 2125. (15) Tanaka, S.; Katayama, Y.; Tate, M. P.; Hillhouse, H. W.; Miyake, Y. J. Mater. Chem. 2007, 17, 3639. (16) Liu, C. Y.; Li, L. X.; Song, H. H.; Chen, X. H. Chem. Commun. 2007, 757. (17) Raghuveer, V.; Manthiram, A. J. Electrochem. Soc. 2005, A1504. (18) Ding, J.; Chan, K.; Ren, J.; Xiao, F. Electrochim. Acta 2005, 50, 3131. (19) Su, F.; Zeng, J.; Bao, X.; Yu, Y.; Lee, J. Y.; Zhao, X. S. Chem. Mater. 2005, 17, 3960. (20) Ubago-Pérez, R.; Carrasco-Marin, F.; Moreno-Castilla, C. Appl. Catal., A 2004, 275, 119. (21) Lordi, Y.; Yao, N.; Wei, J. Chem. Mater. 2001, 13, 733. (22) Lu, A. H.; Li, W. C.; Hou, Z. S.; Schüth, F. Chem. Commun. 2007, 1038. (23) Liu, S. H.; Lu, R. F.; Huang, S. J.; Lo, A. Y.; Chien, S. H.; Liu, S. B. Chem. Commun. 2006, 3435. (24) Baumann, T. F.; Fox, G. A.; Satcher, J. H.; Yoshizawa, N.; Fu, R. W.; Dresselhaus, M. S. Langmuir 2002, 18, 7073.

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tion, the metal phase is uniformly distributed throughout the carbon gel and can therefore be used in different reactions. Previously, we prepared Cu-carbon and Co-carbon xerogels by such one-pot synthesis method and found that these metal–carbon xerogels exhibited superior performance in NO reduction than the metal-doped carbon gels prepared by a postimpregnation method.27 Although much success has been achieved in the incorporation of metal component into the carbon matrix by such one-pot synthesis, the resulting carbon gels often have an uncontrollable pore structure due to the influence of the metal nature on the pore generation. Moreover, in most cases, only microporous carbons with the metal component imbedded into the carbon matrix were yielded,28,29 which largely limited their applications in catalysis. Therefore, if the organic–organic self-assembly induced by the triblock copolymer can be used to synthesize OMC with the metal component incorporation, the resultant metal-OMC composite will have promising catalytic performance due to its accessible mesopores and highly dispersed metallic active sites. To achieve this, the synthesis has to be carried out in acidic media so that the metal precursor can be compatible with the RF sol instead of precipitating as the metal hydroxide. In the present work, we investigated the synthesis of OMC under acidic conditions with RF as the carbon precursor and triblock copolymer Pluronic F127 as the structure-directing agent. To obtain a well-ordered structure, two important factors were considered: aging time of the gel and the molar ratio of R to F. After optimizing the synthesis conditions, we then introduced iridium into the OMC in the synthesis just by adding H2IrCl6 into the RF/F127 reaction mixture. With this simple one-pot procedure, well-dispersed and highly stable Ir nanopaticles were incorporated into the ordered mesoporous carbon framework. The Ir-containing mesoporous carbon (Ir-OMC) from this novel approach exhibited excellent performance in the catalytic decomposition of hydrazine (N2H4). Experimental Section Synthesis. To synthesize OMC under acidic conditions with RF as the carbon precursor, 1.65 g (0.015 mol) of resorcinol was dissolved in a solution composed of 2.5 g of F127 (EO106PO70EO106, MW ) 12 600, purchased from J&K Chemica) and 20 g of ethanol/ water (1/1 vol %) under stirring. When a light brown solution was formed, 0.2 g of HCl (37 wt %) was added as a catalyst. After stirring for 2 h, 2.5 g (0.030 mol, R/F ) 1/2) of formaldehyde (37 wt %) was dropped into the above solution. Followed by an additional hour of stirring, the mixture was kept standing until it turned cloudy and began to separate into two layers. This twophase mixture was further kept aging for 24 or 96 h. Subsequently, the upper layer was discarded while the lower polymer-rich phase (25) Fu, R. W.; Lin, Y. M.; Rabin, O.; Dresselhaus, G.; Dresselhaus, M. S.; Satcher, J. H.; Baumann, T. F. J. Non-Cryst. Solids 2003, 317, 247. (26) Maldonado-Hodar, F. J.; Ferro-Garcıa, M. A.; Rivera-Utrilla, J.; Moreno-Castilla, C. Carbon 1999, 37, 1199. (27) Liu, Z.; Wang, A. Q.; Wang, X. D.; Zhang, T. Carbon 2006, 44, 2345. (28) Moreno-Castilla, C.; Maldonado-Hódar, F. J.; Rivera-Utrilla, J.; Rodríguez-Castellón, E. Appl. Catal., A 1999, 183, 345. (29) Rojas-Cervantes, M. L.; Alonso, L.; Díaz-Terán, J.; López-Peinado, A. J.; Martín-Aranda, R. M.; Gómez-Serrano, V. Carbon 2004, 42, 1575.

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Table 1. Textural Properties of the OMC Samples sample

R/F

aging time (h)

SBET (m2/g)

VPa (cm3/g)

Vmicro (cm3/g)

Vmeso (cm3/g)

Dp (nm)

wall thicknessb (nm)

OMC-1 OMC-2 OMC-3 OMC-4

1/1 1/2 1/3 1/2

96 96 96 24

734 751 674 781

0.66 0.65 0.52 0.72

0.16 0.16 0.15 0.17

0.56 0.55 0.42 0.60

4.5 4.2 3.8 4.6

6.9 5.7

a

7.5

N2 adsorption volume at P/P0 ) 0.998. Wall thickness was calculated as thickness ) a0 - DP, where a0 ) 2d(100)/3. b

0.2 g of quartz sand and was placed in the quartz reactor. A feed gas containing 3 vol % N2H4 in Ar was allowed to pass through the reactor at a rate of 85 mL min-1. N2H4 conversion was used to evaluate the catalytic activity.

Results and Discussion

Figure 2. Low-angle XRD patterns of the OMC samples.

was stirred overnight until a sticky monolith was formed. Finally, the monolith was cured at 85 °C for 48 h and carbonized under a N2 atmosphere at 800 °C for 3 h at a ramping rate of 1 °C/min. In some controlled experiments, R/F molar ratio was varied from 1/1 to 1/3. The synthesis of the Ir-OMC was the same as that of the OMC, except that 0.6 g of H2IrCl6 (44 wt %) was added together with 0.2 g of HCl (37 wt %). For comparison, an Ir/OMC sample was prepared by impregnating the OMC sample with an aqueous solution of H2IrCl6, followed by drying at 85 °C for 48 h and pyrolyzing at 800 °C for 3 h under a nitrogen atmosphere. Characterization. Nitrogen adsorption–desorption isotherms were obtained on a Micromeritics ASAP 2010 apparatus at -196 °C. Prior to the measurements, the samples were degassed at 250 °C for 4 h. The Brunauer–Emmett–Teller (BET) method was used to calculate the specific surface areas. The pore size distributions and the mesopore volumes (Vmeso) were derived from the desorption branches of the isotherms using the Barrett-Joyner-Halen (BJH) method.The total pore volumes, Vp, were estimated from the adsoption branches at a relative pressure (P/P0) of 0.998. The micropore volume, Vmicro, was determined according to the t plot method. The t values were calculated as a function of the relative pressure using the de Bore equation, t ) [13.9900/(0.0340 - log(p/ p0))]0.500. Vmicro was obtained using the equation Vm ) 0.001547YINT, where YINT represents the Y intercept in the t plot. Powder X-ray diffraction (XRD) patterns were recorded with a PANalytical X’Pert-Pro powder X-ray diffractometer using Cu KR radiation. Transmission electron microscopy (TEM) was conducted on a JEOL 2000 EX electronic microscope with an accelerating voltage of 120 kV, while high-resolution transmission electron microscopy (HRTEM) was performed on a Tecnai G2 F30 S-Twin transmission electron microscope operating at 300 kV. The Ir loading amount was determined by a thermogravimetric analysis using a Setaram Stesys16/18 thermoanalyzer, on which 10 mg of the Ir-OMC sample was heated from 25 to 900 °C under air with a ramping rate of 2 °C/min. Activity Tests. Catalytic performance of the Ir-OMC as well as that of the Ir/OMC was evaluated in the catalytic decomposition of hydrazine using a continuous flow fixed-bed reactor system at atmospheric pressure. 50 mg of a catalyst sample was diluted with

In the previously reported work about the synthesis of OMC,5–9,14–16 the R/F ratio was usually fixed at a certain value, and its effect on the structure ordering has not been discussed. In the present work, we for the first time found that the variation in the R/F ratio exerted a remarkable influence on the resulting structure of the carbon material. Figure 1 presents the nitrogen adsorption–desorption isotherms and the pore size distributions of the four OMC samples. It can be seen that they all show typical type IV isotherms with a sharp capillary condensation step at P/P0 ) 0.4–0.8 and a H1-type hysteresis loop, indicating the mesoporous structures of the materials. Meanwhile, the large volume adsorbed at relative pressures from 0 to 0.1 suggests the presence of micropores. The pore size distributions are very narrow, centering at around 4 nm for the OMC-3 and around 5 nm for the other three samples. From the textural parameters listed in Table 1, one can see that the OMC-1 has very similar textural properties with the OMC-2, characterized by a large surface area of 730-750 m2/g, a micropore volume of 0.16 m3/g, and a mesopore volume of 0.56 m3/g. Apparently, the OMC samples mainly contain the mesopores. Compared with OMC-1 and OMC-2, the OMC-3 which was obtained with R/F ) 1/3 has a lower surface area and a smaller pore volume. On the other hand, the sample OMC-4, which was synthesized with the same R/F ratio as the OMC-2 but with a shorter aging time (24 h), exhibits a higher surface area and a larger pore volume. Clearly, both the R/F ratio and the aging time had great influences on the textural properties of the OMC materials. The structures of the four OMC samples were examined with XRD. As shown in Figure 2, the OMC-3 shows a hardly discernible peak at 2θ ) 0.9964°, implying its disordered structure. In contrast, an intense diffraction peak at 2θ range of 0.5-1° is observed on the other three samples, which can be indexed as (100) reflection of a hexagonal mesostructure. In particular, the OMC-1 which was obtained with R/F ) 1/1 presents a very sharp and narrow diffraction peak at 2θ ) 0.8731°, indicating its well-ordered mesostructure. With a decrease of the R/F ratio from 1/1 to 1/3, the peak intensity decreased (OMC-3 < OMC-2 < OMC-1), and the peak position shifted to a higher 2θ degree. The d values of the OMC-1 and OMC-2 are 10.1 and 9.7 nm, respectively. The increased d value of the OMC-1 means a decreased contraction degree during the carbonization, which can possibly be attributed to the more robust mesoporous structure of the polymers resulting from the excess R (R/F ) 1/1). From

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Figure 3. TEM images of OMC-1 (a), OMC-2 (b), OMC-3 (c), and OMC-4 (d).

Table 1, one can see that the OMC-1 has thicker walls (the wall thickness is about 6.9 nm) than the OMC-2. On the other hand, the OMC-4 gives a broader diffraction line as compared to the OMC-2, implying its less ordered structure. Therefore, a longer aging time and an excess amount of R in the reaction mixture were favorable to the formation of the OMC. The mesostructures of the OMC samples are visualized with TEM. As shown in Figure 3a, parallel channels with a d spacing of 10 nm are clearly observed on the OMC-1, which is in consistent with the XRD results. For the OMC-2 and OMC-4, one can also observe the orderly arranged pore structures (Figure 3b,d), although the regularity is not as good as that of the OMC-1. In contrast, only wormhole-like disordered structure can be seen on the OMC-3 (Figure 3c). It is reminiscent of Tanaka’s work14 where the molar ratio of R/F was actually around 1/3. Therefore, it is not surprising that they only obtained a disordered structure without the use of triethyl orthoacetate (EOA). In their case, EOA presumably played a similar role to the excess R. From the above results, one can see that mesoporous carbon materials with ordered hexagonal pore structures have been fabricated under acidic conditions via carbonization of coassembled F127/RF composite. In this synthesis, the phase separation of the polymer-rich phase from the ethanol/water solvent is a key step. Catalyzed by HCl acid, R and F polymerized slowly to ethanol-soluble resol oligomers with low polymerization degree. The resols with abundant hy-

Figure 4. N2 adsorption–desorption isotherms of the Ir-OMC samples.

droxyl groups could bond with the PEO segments of the selfassembled triblock copolymers through hydrogen bonding. Since only the hydroxyl groups in the R could interact with the triblock copolymer, the excess amount of R would lead to a stronger interaction between them, thus driving the selfassembly process between the RF polymer and the F127. When the linear resols further polymerized with each other to achieve relatively large molecular weight, a glue-like polymer product began to separate from the solvent. The longer the aging time, the more solvent would be removed from the polymer phase owing to the formation of larger oligomers with lower solubility and the solvent extraction

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Table 2. Textural Properties of the Ir-OMC Samples sample

R/F

aging time (h)

SBET (m2/g)

VPa (cm3/g)

Vmicro (cm3/g)

Vmeso (cm3/g)

Dp (nm)

wall thicknessb (nm)

Ir-OMC-1 Ir-OMC-2 Ir-OMC-3 Ir-OMC-4

1/1 1/2 1/3 1/2

96 96 96 24

763 714 684 696

0.66 0.68 0.53 0.83

0.18 0.17 0.17 0.19

0.55 0.56 0.41 0.68

4.4 5.2 3.9 8.6

7.6 6.7

a

N2 adsorption volume at P/P0 ) 0.998. b Wall thickness was calculated as thickness ) a0 - DP, where a0 ) 2d(100)/3.

Figure 5. Low-angle XRD patterns of the Ir-OMC samples (inset: wideangle XRD patterns).

effect. Since the residual solvent among the cross-linked PEO segments may result in defects on the resultant pore walls, a long aging time would be favorable to the formation of an ordered pore structure, as demonstrated in the above results. When iridium was introduced into the OMC during the synthesis, what changes happened to the OMC structure? Figure 4 shows the nitrogen sorption isotherms of the four Ir-OMC samples, and Table 2 lists the corresponding textural parameters. Very similar to the case without iridium, all the isotherms are typical for mesoporous structure, and the textural parameters, including specific surface areas, pore volumes, and pore sizes, do not change significantly as compared to the OMC samples. The only remarkable difference is the pore size distribution of the Ir-OMC-4, which is much broader than that of the corresponding OMC-4. Figure 5 illustrates the low-angle and wide-angle XRD patterns of the Ir-OMC samples. In the low-angle range, one intense diffraction peak can be observed on both the Ir-OMC-1 and the Ir-OMC-2 samples, indicating their ordered mesostructures. In contrast, there is not any discernible peak identified either on the Ir-OMC-3 or on the IrOMC-4. This is very different from the case without iridium addition. Moreover, the peak intensity of the Ir-OMC-1 was stronger than that of the Ir-OMC-2, indicating that the former possessed a more ordered structure. The cell parameter (a0) of the Ir-OMC-2 was calculated to be 12.2 nm, which was larger than that of the OMC-2 (10.9 nm), suggesting that the Ir incorporation into the matrix may suppress the framework shrinkage during the pyrolysis. In the wide-angle region (inset), only the two broad XRD peaks positioned at 21.7° and 43.6° can be identified on the Ir-OMC-1, which is characteristic of amorphous carbon. No any peak of iridium species is observed on this sample. Remarkably different from the Ir-OMC-1, the other three Ir-OMC samples present

three diffraction lines centered at 40.6°, 47.3°, and 69.1°, which can be respectively indexed as Ir (111), Ir (200), and Ir (220) reflections (JCPDS 06-0598). In particular, these diffraction peaks are very sharp on the sample Ir-OMC-3, whereas they are very broad on both the Ir-OMC-2 and IrOMC-4. Thus, according to the broadening of the XRD peaks, we can roughly estimate the particle size of the iridium in the OMC matrix follows the order of Ir-OMC-1 < IrOMC-2 ≈ Ir-OMC-4 , Ir-OMC-3. Clearly, the excess amount of R in the reaction mixture not only favored the formation of highly ordered mesostructure but also stabilized the iridium particles in a highly dispersed state. Figure 6 presents the TEM images of the Ir-OMC samples. In good agreement with the low-angle XRD patterns, the well-ordered mesostructures can be clearly observed on both the Ir-OMC-1 and the Ir-OMC-2, which is in contrast with the completely disordered structures of the Ir-OMC-3 and the Ir-OMC-4. By carefully examining the TEM images of the Ir-OMC-1 and the Ir-OMC-2, we can clearly see that the Ir particles with sizes of 2-3 nm are highly dispersed in the channels or on the walls of the OMC. Moreover, the visible metal particles seem to be more in their amount on the Ir-OMC-2 than those on the Ir-OMC-1, implying that the Ir-OMC-1 contains a large number of Ir particles which are too small to be visible with the TEM. The HRTEM image (not shown) of the Ir-OMC-2 further confirms the metallic nature of the iridium, with interplanar spacing of ∼2.2 Å. For the sample Ir-OMC-4, in spite of its disordered structure, the Ir particles could still be highly dispersed into the carbon matrix, with sizes of 3-5 nm. However, for the Ir-OMC-3, very large iridium particles with sizes of 20-30 nm could be clearly observed along with the disordered structure of carbon. Carbon materials have been widely used as catalyst supports owing to their large surface areas and rich porosities. However, the inert surface of the carbon material often makes it difficult to generate a strong interaction with the active metal species on it. Thereby, the metal particles on the carbon support tend to sinter or aggregate and grow larger as a result of a high-temperature treatment. In this respect, preparing highly dispersed metal nanoparticles on the carbon materials still remains a significant challenge in catalysis. Especially, when the carbon support possesses an ordered mesoporous structure, it will offer great advantages over the microporous ones in mass transfer and provide better accessibility to reactant molecules. For example, Lu et al. prepared molecular-level dispersed Pd clusters in the walls of ordered mesoporous carbons and found that this Pd-OMC catalyst exhibited a high selectivity for the oxidation of alcohols to aldehydes.22 Liu et al. synthesized highly stable Pt particles of 2-3 nm on the pore walls of the ordered mesoporous carbons.23 In the two cases, a multistep nanocasting method

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Figure 6. TEM images of the Ir-OMC-1 (a), Ir-OMC-2 (b), Ir-OMC-3 (c), and Ir-OMC-4 (d). The inset in (b) is the STEM image of the Ir-OMC-2 where bright dots represent Ir particles.

was used, which involved the synthesis of SBA-15 as a hard template, filling with the carbon precursor and the metal precursor, carbonization, and the final removal of SBA-15 by HF. Therefore, this nanocasting method was difficult to adapt to large-scale production although very highly dispersed and thermally stable metal particles were obtained on the OMC support. Compared with the tedious nanocasting procedure, our present organic–organic self-assembly strategy provided a facile route to the synthesis of highly dispersed metal nanoparticles incorporated into the ordered mesoporous carbons. In this synthesis, when H2IrCl6 was added into the reaction mixture containing RF and F127, H2IrCl6 was supposed to mainly interact with the phenolic resin as a catalyst for cross-linking between R and F, like HCl did. However, the presence of H2IrCl6 might disturb the selfassembly process and thus influence the final ordering of the mesostructure. As shown in Figures 5 and 6, when the gel mixture was aged for 24 h, the resultant Ir-containing carbon exhibited a disordered structure, whereas an ordered mesostructure could be formed without the presence of Ir under the same aging condition. We speculate that with the prolonging of the aging time there were more Ir species moving from the polymer-rich phase to the water/ethanol phase. As a result, the amount of Ir incorporated into the phenolic resin framework would become less, thus imposing a less impact on the self-assembly process. TG analysis

showed that the Ir loading amount of the Ir-OMC-4 and IrOMC-2 was 7.7 and 2.7 wt %, respectively, although the Ir content in the starting reaction mixture was the same (corresponding to the theoretical Ir loading of 8.6 wt %). This result corroborated our above speculation and revealed that our present method had some limitations in incorporating a large amount of iridium. Additionally, we must emphasize that even with a short aging time (24 h), the resultant iridium particles were still highly dispersed in the disordered carbon matrix. However, different from the impact of the aging time, when the R/F value decreased to 1/3, i.e., excess F was present in the reaction mixture, very large Ir particles were obtained, along with the formation of the disordered structure. One possible reason was that, under the presence of excess F, most of the H2IrCl6 was reduced to Ir particles prior to its interaction with the phenolic resin. During the subsequent harsh carbonization process, these preformed Ir particles seriously aggregated or sintered each other. As a result, very large particles were produced onto the carbon matrix. However, when the R/F ratio was equal to or greater than the stoichiometric value (1/2), the F will preferably react with R with the cocatalysis by H2IrCl6 and HCl. With the further polymerization between R and F, H2IrCl6 was probably imbedded into the polymers. During the subsequent carbonization, H2IrCl6 was reduced to metallic Ir by the emitted reducing gases such as CO and H2 or even reduced by carbon itself under a N2 atmosphere. Thus, the Ir particles,

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Figure 7. Low-angle XRD pattern (left) and TEM image (right) of the Ir/OMC-2 sample (inset of left figure: wide-angle XRD pattern).

together with the phenolic resin, constitute the walls of the final mesoporous carbon, while the surfactant F127 forms the pores after the pyrolysis. The STEM image of the IrOMC-2 (inset in Figure 6b) clearly demonstrated that most of the Ir particles (bright dots) are present in the walls of the OMC rather than in the pores. The high dispersity and the high thermal stability of the Ir particles should result from its imbedding into the carbon matrix. To demonstrate the advantage of the one-pot synthesis of Ir-OMC in which Ir was introduced into the OMC structure during the synthesis process, we prepared Ir/OMC-2 by impregnating the preformed OMC-2 with H2IrCl6. For comparison with the Ir-OMC-2, the Ir loadings were controlled almost the same (3.0 wt % for the Ir/OMC-2 and 2.7 wt % for the Ir-OMC-2). Figure 7 shows the XRD pattern and TEM image of the Ir/OMC-2. Doping of the iridium by postimpregnation did not destruct the ordered structure of the OMC-2, as indicated by one intense XRD peak in the low-angle range and by the ordered channels seen on the TEM image. However, the iridium particles are not uniform, from ∼5 to ∼30 nm. The enlarged particles should be caused by the sintering/aggregation occurring during the heat treatment, while the small particles are probably due to the confinement of the carbon channels. Clearly, the postimpregnation method led to an uncontrollable and thus polydispersed particle size distribution. The remarkably different particle size would lead to a difference in the catalytic performance. In the present work, we chose hydrazine (N2H4) decomposition as a probe reaction to evaluate the catalytic performance of the Ir-OMC samples. The catalytic decomposition of hydrazine has been widely used in attitude control of spacecrafts,30 and the commercial catalyst is Ir/Al2O3. Compared with γ-Al2O3, the OMC possesses the advantage of large surface areas, well-ordered mesopores, hydrophobic surfaces, and monolithic shapes, which would make it an interesting alternative support for (30) (a) Cheng, R. H.; Shu, Y. Y.; Li, L.; Zheng, M. Y.; Wang, X. D.; Wang, A. Q.; Zhang, T. Appl. Catal., A 2007, 316, 160. (b) Chen, X. W.; Zhang, T.; Zheng, M. Y.; Wu, Z. L.; Wu, W. C.; Li, C. J. Catal. 2004, 224, 473. (c) Chen, X. W.; Zhang, T.; Xia, L. G.; Li, T.; Zheng, M. Y.; Wu, Z. L.; Wang, X. D.; Wei, Z. B.; Xin, Q.; Li, C. Catal. Lett. 2002, 79, 21. (d) Chen, X.; Zhang, T.; Ying, P. L.; Zheng, M. Y.; Wu, W. C.; Xia, L. G.; Li, T.; Wang, X. D.; Li, C. Chem. Commun. 2002, 288. (e) Zheng, M. Y.; Chen, X. W.; Cheng, R. H.; Li, N.; Sun, J.; Wang, X. D.; Zhang, T. Catal. Commun. 2006, 7, 187.

Figure 8. N2H4 conversion with the time-on-stream on the three Ir-OMC samples as well as on the Ir/OMC-2 sample.

iridium used in hydrazine decomposition. Figure 8 compares the catalytic performances of the three Ir-OMC samples and that of the Ir/OMC-2 in a microreactor. The Ir loadings for the Ir-OMC-1, Ir-OMC-2, and Ir-OMC-3 are 2.6, 2.7, and 5.5 wt %, respectively, while that of the Ir/OMC-2 is 3.0 wt %. As expected, both the Ir-OMC-1 and the Ir-OMC-2 exhibited higher catalytic stabilities than the Ir-OMC-3 and the Ir/OMC-2. N2H4 conversions on the former two Ir-OMC samples attained 100% even at 30 °C, and no any decay was observed over a 300 min run, demonstrating their high activities and stabilities. In contrast, the N2H4 conversion on the Ir/OMC-2 under the same reaction conditions dropped rapidly with the reaction time, decreasing to ∼80% at 160 min. The Ir-OMC-3 showed a slightly better performance than the Ir/OMC-2; N2H4 conversion maintained 100% in the initial 200 min, and then it gradually decreased to ∼80% at 300 min. Clearly, the catalytic activity and stability correlated well with the particle size of iridium. The very small Ir particles in the Ir-OMC-1 and Ir-OMC-2 yielded a highly catalytic activity and stability, whereas the large Ir particles in the Ir/OMC-2 resulted in a poor stability. The slightly better stability of the Ir-OMC-3 than that of the IrOMC/2 may due to the higher Ir loading of the Ir-OMC-3. In addition, it should be stressed that the catalytic behavior in a microreactor may be very different from that in a thruster because the hydrazine decomposition is a strongly exothermic reaction, and the maximum bed temperature during the reaction in the thruster could reach as high as 800 °C. For the real application in a thruster, the highly dispersed and

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sintering-resistant iridium particles will be very important for the long-term stability of the catalyst. From this point of view, our present Ir-OMC catalyst, possessing the masstransfer beneficial mesopores and the highly dispersed iridium particles, will exhibit promising performance for the hydrazine decomposition in a thruster. This work is ongoing.

Gao et al.

the formation of a well-ordered structure. More interestingly, iridium could be incorporated into the carbon matrix during the synthesis, with highly dispersed state and highly thermal stability, without lowering the ordered structure of the carbon. The Ir-OMC catalysts exhibited superior performance for the hydrazine decomposition than the Ir/OMC one, demonstrating the advantages of the one-pot synthesis process.

Conclusion Highly ordered mesoporous carbons have been synthesized in an acidic environment with RF as the carbon precursor and F127 as the structure-directing agent. It was found that the excess amount of R (R/F ) 1/1) in the reaction mixture and a long aging time (96 h) of the gel were favorable to

Acknowledgment. Supporting of this research by the National Science Foundation of China (NSFC) for Distinguished Young Scholars (No. 20325620) as well as other two NSFC grants (20673116, 20773124) is gratefully acknowledged. CM702815E