Pore development of thermosetting phenol resin derived mesoporous carbon through a commercially nanosized template

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1 Materials Science and Engineering A 473 (2008) Pore development of thermosetting phenol resin derived mesoporous carbon through a commercially nanosized template Zhihong Tang a,b, Yan Song a,, Yongming Tian a,b, Lang Liu a, Quangui Guo a a Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan , PR China b Graduate University of Chinese Academy of Sciences, Beijing , PR China Received 26 December 2006; received in revised form 13 March 2007; accepted 23 April 2007 Abstract Mesoporous carbons (MCs) with high specific surface area and pore volume were synthesized from thermosetting phenol resin (TPR) by using commercial nanosized silica particles as template. Based on the results of thermogravimetric analysis, nitrogen adsorption, mercury adsorption and high-resolution transmission electron microscopy (HRTEM), mechanism of the pore formation of MCs was proposed. Silica particles not only participated in the pore formation of MCs but also influenced the thermosetting process of the carbon precursor. The mechanism of pore formation in the MCs may be described as follows: mesopores were introduced by the removal of silica particles; small mesopores were created by the combination of aperture between TPR and silica particles and opened pores in the matrix generated by the release of small molecules in the carbon during carbonization process; macropores were produced by the aggregation of silica particles and the collapse of carbon wall Elsevier B.V. All rights reserved. Keywords: Mesoporous carbon; Nanosized silica particles; Template; Mechanism 1. Introduction Mesoporous carbons (MCs) have attracted much attention because of their potential applications in many fields such as adsorption of large molecules from liquid phase, carbon electrode for supercapacitors, catalyst support due to their high specific surface areas, large mesopore ratio, inert nature in certain rigorous circumstance and easy for regeneration. MCs can be synthesized by selecting raw materials, adjusting carbonization and activation condition, catalytic gasification and template technique [1 7]. The previous three methods can only introduce mesopores in a limited range and cannot prepare MCs with high mesopore ratio and tailored structure. Since Kresge et al. successfully synthesized mesoporous silica through a liquid-crystal template [8], synthesis of MCs utilizing inorganic template has been described widely [2,5 7,9 11]. The method has been proved to be a better technique for synthesis of MCs with high surface areas, large pore volumes and controlled structures. Through impregnation or infiltration of carbon precursors like furfuryl alcohol, phenol resin or sucrose into pores of template framework, porous carbon was obtained by the removal of the Corresponding author. Tel.: ; fax: address: yansong1026@126.com (Y. Song). inorganic template. In such process, synthesis of inorganic template is unavoidable, the preparation procedure is quite complicated and accordingly the product is cost-intensive. Therefore, a simple and economic method by using commercially nanosized silica particles as template to prepare MCs was proposed [2,5]. However, only the feasibility of utilization of silica particles as template to prepare mesoporous carbon was proved, the synthesis parameters were not investigated systematically and the mechanism of the pore development was still unclear. In this paper, MC was synthesized from thermosetting phenol resin (TPR) by using nanosized silica particles as template, the thermosetting process of TPR/silica composite was investigated by thermogravimetric analysis. Based on the results of thermogravimetric analysis, nitrogen adsorption, mercury adsorption and high-resolution transmission electron microscopy (HRTEM) characterization, the mechanism of the pore formation of MCs was proposed. 2. Experimental 2.1. Synthesis of mesoporous carbon Nanosized silica particles (particle size, ca. 10 nm; surface area, 40 m 2 /g and skeletal density measured by Hg adsorption, /$ see front matter 2007 Elsevier B.V. All rights reserved. doi: /j.msea

2 154 Z. Tang et al. / Materials Science and Engineering A 473 (2008) g/cm 3 ) were ultrasonically dispersed in ethanol solution of TPR. The mixture of SiO 2 and TPR was obtained after the solvent evaporation. The mixture (SiO 2 /TPR) was stabilized in air at 120 C for 5 h, followed by heat-treated in nitrogen at 700 C for 1 h to obtain SiO 2 /C composite. The composite was then washed by excessive NaOH solution to remove the silica particles, followed by washing with distilled water to neutral and drying at 110 C for 48 h to obtain MC. The samples before and after washing with NaOH solution were labeled as SW-n and S-n respectively, in which n represented the mass ratio of silica to TPR Characterization Nitrogen adsorption desorption isotherms were performed at 77 K on a Micromeritics ASAP-2000 volumetric adsorption system. The specific surface area was calculated from the adsorption data in the relative pressure interval from 0.05 to 0.35 using the Brunauer Emmett Teller (BET) method. The pore size distribution curve was gained from desorption branch by using Barrett Joyner Halenda (BJH) method. The total pore volume (V total ) was calculated at the relative pressure of The micropore volume (V micro ) was determined by t-plot model, and the mesopore volume (V meso ) was calculated by the difference of V total and V micro. The other method to characterize the pore size distribution (corresponding to m) of as-prepared carbons was examined by a mercury porosimetry (Micrometeritics Autopore IV9500, USA). It is based on the pressure applied to effect penetration of the mercury into the pores. The variation of the external applied pressure causes changes in the intruded volume, which is in turn related to the pore-size distribution. Weight changes of the TPR and composite (the mass ratio of TPR to silica was 5:4) were determined by thermogravimetric analysis (TGA, STA 409 PC). The sample was heated up to 900 C at the rate of 5 C/min in nitrogen. The size of silica particle was observed by means of transmission electron microscopy (TEM). TEM measurement was conducted using Hitachi H The sample was prepared by dispersing the products in ethanol with an ultrasonic bath for 15 min, then a few drops of the resulting suspension were placed on a copper grid. The morphology of S-3 was observed by using high resolution TEM (JEM-2010), the sample was treated with the same process as described above. Fig. 1. TG curves of the samples. (a) Thermosetting resin and (b) thermosetting SiO 2 /TPR composite. 3. Results 3.1. TG measurement of the thermosetting samples Fig. 1 shows the TG curves of TPR and SiO 2 /TPR composite. The sharp weight loss for the resin appeared from 300 to 600 C, and for the composite appeared from 300 to 700 C. The weight loss of the resin and composite reached 38.4 and 47.9%, respectively, when they were heated to 900 C, which indicated that the addition of silica particles resulted in the increase of weight loss of the carbon precursor of approximately 10% Pore structure of the samples BET surface area, pore volume and other parameters of all samples are listed in Table 1. BET surface areas of S-0 and SW-3 were 52 and 131 m 2 /g, respectively, which showed that the addition of silica particles improved the surface area even if they were not removed. Fig. 2 displays the nitrogen adsorption desorption isotherms and BJH pore size distribution curves of S-3 and SW- 3, respectively. There were sharp pore distributions at about 10 nm and a relative weak peak at about 2.5 nm for S-3 and 27 nm for SW-3, respectively (Fig. 2b). The N 2 adsorption amount (Fig. 2a), surface area and pore volume (Table 1) of S-3 were much higher than that of SW-3, which implied that the removal of silica particles improved surface area and pore volume of MCs greatly. Table 1 Properties of MCs and the corresponding composite Sample name SiO 2 /TPR (g/g) V SiO2 /MC a (cm 3 /g) S BET (m 2 /g) V total (cm 3 /g) V micro (cm 3 /g) Ratio meso b (%) S S S S S S SW a V SiO2 /MC, the volume of SiO 2 used/(mass of MCs). b Ratio meso =(V total V micro )/V total.

3 Z. Tang et al. / Materials Science and Engineering A 473 (2008) Fig. 2. Adsorption desorption isotherms (a) and BJH pore size distributions (b) of MCs (SW-3 and S-3). Nitrogen adsorption desorption isotherms and BJH pore distributions of samples (S-1, S-3 and S-5) prepared by adding different amount of silica particles are shown in Fig. 3. Isotherms of all the MCs belonged to type-iv, which revealed the formation of mesopores after the removal of silica particles. There were sharp pore distributions at about 10 nm for these three samples as shown in Fig. 3b. The morphology images of S-3 are shown in Fig. 4. It can be seen clearly that S-3 exhibited meso- Fig. 3. Adsorption desorption isotherms (a) and BJH pore size distributions (b) of MCs (S-1, S-3 and S-5). pores at about nm, which were consistent with nitrogen adsorption desorption isotherms and BJH pore size distributions of MCs presented in Fig. 3. From Table 1 it can be seen that the simple addition of silica particles improved the BET surface area, total pore volume and mesopore ratio of TPR-based carbon greatly, from 52 m 2 /g, 0.04 cm 3 /g and 75% to 1067 m 2 /g, 2.99 cm 3 /g and 95%, respectively. With the increase of addition amount of silica particles, BET surface areas and total pore vol- Fig. 4. HRTEM images of S-3.

4 156 Z. Tang et al. / Materials Science and Engineering A 473 (2008) Fig. 5. Mercury adsorption curves (a) and pore size distributions (b) of S-1 and S-3. umes of MCs increased firstly and then decreased to some extent. Fig. 5 shows the mercury adsorption curves and pore size distribution calculated from mercury adsorption. It can be found that S-3 had more macropores and sharper pore size distribution at about 300 nm and 20 m than S Discussion It is believed that the pore structure of MC obtained by template method was the replica of the template, and the pore size of MC relied mainly on the dimension of the template. When the nanosized particles were empolyed as template, pore size of the MC would depend on the diameter of the particles. The size of silica particle used in the experiment was about 10 nm, so the abundant pores at about 10 nm observed in Figs. 3 and 4 should be attributed to the removal of silica particles. A lot of closed pores (mainly mesopores from 2 to 10 nm) were produced around silica particles when the resin was carbonized because of the evolution of small molecules and rearrangement of nanosized microdomains composed of the stacking of cluster units [2]. After the removal of the silica, such closed mesopores will be opened to the surface, which can be observed from HRTEM. As shown in Fig. 1, the weight loss of the SiO 2 /TPR composite was higher than that of the pure resin. The cross-linking process of the composite became more difficult because of the addition of silica particles, which will lead to the release of more small molecules during the carbonization process, as a result, small mesopores formed. The more the particles added, the more difficulty for the composite to cross-link, the more small mesopores appeared. The different expansion coefficient of TPR and silica particles in the carbonization process possibly produced some aperture between TPR and silica, which could introduce some mesopores in the composite, The pores with the pore size of about 27 nm of SW-3 (shown in Fig. 2) indicated that small parts of the large mesopores probably created by the combination of the aperture and the mesopores produced by the release of small molecules. Macropores in the MCs shown in Fig. 5 might be produced either by the clustering of silica particles, or by the collapse of carbon walls when excessive silica was added. Excessive silica particles made carbon precursor inadequate to form pore walls, and even if pore walls were formed, they were too weak to support pores during the silica removing [6]. In a word, the mechanism of pore formation in the MCs may be described as follows: mesopores were introduced by the removal of silica particles, the aperture between resin and silica particles, the release of small molecules in the carbon and the opened pores in the matrix; while the clustering of silica particles and the collapse of carbon wall resulted in the appearance of macropores. It was noticeable that the volume of silica added to the precursor was much higher than the total pore volume caculated by nitrogen adsorption of MCs (Table 1), which suggested that pores introduced by template were at the expense of wasting of part of the template. The waste of template was inevitable even for the smallest addition of silica particles, and the more the silica particles added, the more the templates would be wasted. The reason might be as follows: some of the silica particles would be adhered to the surface of the composite, and some would aggregate seriously, which were not effective in the formation of mesopores. When nanosized particles were selected as template, the aggregation was inevitable because of the high surface tension, high surface energy and surface electronic effect of the nanosized particles. The aggregation phenomenon might be improved by modifying the surface property of the silica particles. 5. Conclusions Mesoporous carbons (MCs) with high specific surface areas and pore volumes were synthesized from thermosetting phenol resin by using commercial nanosized silica particles as template. The cross-linking process of the composite became more difficult for the addition of silica particles, which would lead to more small molecules release during the carbonization process. The mechanism of pore formation in the MCs may be described as follows: the mesopores were introduced by the removal of silica particles, the split between the resin and silica particles, the release of small molecules in the carbon and the opened pores in the matrix and the aggregation of silica particles and the collapse of carbon wall resulted in the appearance of macropores.

5 Z. Tang et al. / Materials Science and Engineering A 473 (2008) Acknowledgements The authors acknowledge the financial support of Fund of National Nature Science Foundation of China ( ), Fund of ICCCAS (Institute of Coal Chemistry, Chinese Academy of Sciences) for Youth and Fund of Key Laboratory in Shanxi Province. References [1] L.H. Wang, M. Fujita, M. Inagaki, Electrochim. Acta 51 (2006) [2] W.M. Qiao, Y. Song, S.H. Hong, S.Y. Lim, S.-H. Yoon, Y. Korai, I. Mochida, Langumuir 22 (2006) [3] C.L. Liu, Q.G. Guo, J.L. Shi, New Carbon Mater. 19 (2004) 124. [4] Z. Guo, G.S. Zhu, B. Gao, D.L. Zhang, G. Tian, Y. Chen, W.W. Zhang, S.L. Qiu, Carbon 43 (2005) [5] P. Kim, H. Kim, J.B. Joo, W. Kim, I.K. Song, J. Yi, J. Power Sources 145 (2005) 139. [6] K. Bohme, W.D. Einicke, O. Klepel, Carbon 43 (2005) [7] P.X. Hou, T. Yamazaki, H. Orikasa, T. Kyotani, Carbon 43 (2005) [8] C.T. Kresege, M.E. Lenowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 352 (1992) 710. [9] J.E. Hampsey, Q.Y. Hu, Z.W. Wu, L. Rice, J.B. Pang, Y.F. Lu, Carbon 43 (2005) [10] Y.H. Chou, C.S. Cundy, A.A. Garforth, V.L. Zholobenko, Microporous Mesoporous Mater. 89 (2006) 78. [11] A. Garsuch, R.R. Sattler, S. Witt, O. Klepel, Microporous Mesoporous Mater. 89 (2006)