- Email: [email protected]
Effect of precursor preoxidation on the structure of phenolic resin-based activated carbon spheres
ARTICLE IN PRESS Journal of Physics and Chemistry of Solids 71 (2010) 453–456
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Effect of precursor preoxidation on the structure of phenolic resin-based activated carbon spheres Yue Liu a,b, Kaixi Li a,n, Guohua Sun a a b
Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China Graduate University of Chinese Academy of Sciences, Beijing 100039, PR China
a r t i c l e in fo
abstract
Article history: Received 9 May 2009 Received in revised form 13 November 2009 Accepted 14 November 2009
Activated carbon spheres with 3D hierarchical porous structure were prepared from phenol– formaldehyde resins with oxidation treatment in air and physical activation in an inert atmosphere. Based on the results of thermogravimetric analysis, infrared spectrometer (IR), scanning electron microscopy (SEM) and nitrogen adsorption/desorption, the effect of preoxidation on the morphology and structure of activated carbon spheres was investigated. The results show that decomposition and crosslinking reactions occur during the preoxidation and the structural changes of precursor generated by the preoxidation lead to differences in the pore structure of activated carbon spheres. The carbon spheres exhibit the unique 3D hierarchical porous structure, high specific surface area of 1897 m2/g and high pore volume of 2.22 cm3/g. & 2009 Elsevier Ltd. All rights reserved.
Keywords: A. Microporous materials C. Electron microscopy C. Infrared spectroscopy D. Microstructure
1. Introduction The hierarchical porous carbons, containing both interconnected macroporous and mesoporous structures, have attracted much attention for potential applications in many fields of science and engineering [1]. These carbon materials have enhanced properties compared with single-sized pore materials due to the textural transport pores providing high connectivity of the narrow pores and allowing deep penetration of adsorbate molecules into the pore network for shorter time [2]. Recently, the hierarchical micro/mesoporous carbons have been prepared by CO2 activation of ordered mesoporous carbon and utilized as the electrode materials for supercapacitor [3]. A hierarchical meso/macroporous carbon monolith was also obtained using Pluronic F127 and SiO2 opal as templates [4]. In addition, a 3D aperiodic hierarchical porous graphitic carbon material has also been reported [5]. All of these porous carbons all exhibited notable electrochemical double-layer performance. However, their preparation process is complicated and most of carbon materials are powders which restrict their applications. In this paper, 3D hierarchical porous activated carbon spheres were prepared by oxidation treatment in air and physical activation in an inert atmosphere. In the process, preoxidation plays crucial role on the development of highly porous structures. The morphology and pore structure of activated carbon spheres
n
Corresponding author. Tel./fax.: + 86 351 4250292. E-mail address: [email protected] (K. Li).
0022-3697/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2009.12.009
were analyzed by the thermogravimetric analysis, infrared spectrometer (IR), scanning electron microscopy (SEM) and standard nitrogen adsorption measurement to reveal the relationship between preoxidation and the structure.
2. Experimental The novolac-type phenol–formaldehyde resins (PF) and hexamethylenetetramine were firstly dissolved in ethanol, and then poured in an aqueous solution of polyvinyl alcohol (PVA) in a high-pressure reactor. The obtained mixture was heated from room temperature to 130 1C at a rate of 5 1C/min and kept at this temperature for 60 min. After cooling naturally to room temperature, the phenolic resin spheres (PFS) were obtained via solid– liquid separation, followed by washing with distilled water to remove the excess polyvinyl alcohol (PVA) and drying at 110 1C for 24 h. PFS were firstly preoxidized at 300 1C for 120 min in the air and denoted as PFS–O. Then, PFS–O were heated up to 850 1C under a nitrogen atmosphere to activate the sample for 60 min under a stream of steam at a flow rate of 20 ml/min, thus activated carbon spheres were obtained. The samples activated from PFS and PFS–O were denoted as ACS and ACS–O, respectively. The weight changes of PFS and PFS–O were determined by thermogravimetric analysis (TGA, STA 409 PC). And the functional groups were analyzed by IR spectroscopy (BioRad FTS-165 USA). Specific surface area and pore structure of the samples were
ARTICLE IN PRESS 454
Y. Liu et al. / Journal of Physics and Chemistry of Solids 71 (2010) 453–456
characterized using nitrogen sorption at 77 K (Micromeritics ASAP-2020). A scanning electron microscopy (SEM, LEO 438 VP) was used to study the morphology of the samples.
the other peak is from 10 to 30 nm. And it is very remarkable to note that the adsorbent account and pore volume of ACS–O are very large in comparison with that of ACS from Fig. 1. So, the
3. Results and discussion It can be seen from Table 1 that the effect of precursor preoxidation on the structure of activated carbon spheres is very notable. Compared with the original ACS, the surface area and pore volume of activated carbon spheres obtained by preoxidation increase obviously, which exhibit high BET surface area of 1897 m2/g and high total pore volume of 2.22 cm3/g. Fig. 1 shows N2 adsorption/desorption isotherms and the corresponding BJH pore size distribution curves of ACS and ACS–O. Both carbon materials show a type-IV isotherm with a sharp capillary condensation step at high relative pressures (P/P0 = 0.85–0.96) and an H1-type hysteresis loop, indicating a relatively large pore size. However, a larger hysteresis loop occurs after P/P0 = 0.45 in the adsorption–desorption isotherm of ACS–O, which indicates that ACS–O has more developed mesopores and broader pore size distribution. Moreover, the adsorption/ desorption isotherm of ACS–O gradually increases with the relative pressure from 0.4 to 0.9 comparing with that of ACS, which suggests that the pore size distributions of ACS–O are broader than that of ACS. As shown in Fig. 1b, it appears a bimodal pore size distribution. The first peak ranges from 3 to 4 nm and
Fig. 2. FT-IR spectra of the PFS and the PFS–O.
Table 1 The physical properties of the samples. Sample
SBET (m2/g)
Vtotal (cm3/g)
Vmicro (cm3/g)
Vmeso (cm3/g)
Ratiomeso (%)
¯ (nm) D
ACS ACS–O
1196 1897
0.86 2.22
0.42 0.17
0.44 2.05
51.2 92.3
2.89 4.68
¯ , average pore size SBET: BET surface area; Vtotal: total pore volume, measured at P/P= 0.99; Vmicro: micropore volume; Vmeso: mesopore volume; Ratiomeso = Vmeso/Vtotal; D calculated from the N2-adsorption isotherm (4V/A by BET).
Fig. 1. N2 adsorption/desorption isotherms (a) and BJH pore size distributions (b) of the ACS and ACS–O.
ARTICLE IN PRESS Y. Liu et al. / Journal of Physics and Chemistry of Solids 71 (2010) 453–456
preoxidation treatment of precursor can obviously promote the pore structure development and improve the physical properties of activated carbon spheres. The FT-IR spectra of PFS and PFS–O (Fig. 2) illustrate that there are obvious changes of characteristic bands between 755 and 1750 cm 1, including the the aromatic C–H out-of-plane
455
deformation vibrations (820 and 755 cm 1), the breathing C–O vibrations (between 1000 and 1400 cm 1), the aliphatic deformation vibrations (between 1370 and 1500 cm 1) and the aromatic ring C–C stretching vibrations (1596 cm 1). The intensities of these bands for sample PFS–O decrease significantly, indicating a reduction in the number of the
Fig. 3. SEM images of the the contour and cross section of the samples. (a) PFS, (b) cross section of PFS, (c) PFS–O, (d) cross section of PFS–O.
Fig. 4. SEM images of the the contour and cross section of the samples. (a) ACS, (b) cross section of ACS, (c) ACS–O, (d) cross section of ACS–O.
ARTICLE IN PRESS 456
Y. Liu et al. / Journal of Physics and Chemistry of Solids 71 (2010) 453–456
smaller pores which are on the wall of macropores, as shown by arrows in Fig. 4d. It further confirms the above depiction that preoxidation promotes the development of porosity. In addition, the weight loss during carbonization of the samples was traced by thermogravimetric analysis, and the results are shown in Fig. 5. There is a very significant difference in the weight loss of the PFS and PFS–O, which indicates that preoxidation promotes the crosslinking of phenolic resin spheres and improves the transformation efficiency of the carbon to enhance the thermal stability [8]. It is consisted with the analysis of FT-IR.
4. Conclusions
Fig. 5. TG curves of the PFS and the PFS–O.
IR-observable groups. Meanwhile a new brand (1740 cm 1) appears which can be attributed to tetra-substituted benzene ring. All of these FT-IR results show that the preoxidation decreases some groups such as hydroxymethyl and promotes the crosslinking of PFS [6,7]. The morphology of the samples is displayed in Fig. 3. The PFS has a good sphericity and smooth surface (Fig. 3a), and it exists abundant air bubbles in the inner of PFS (Fig. 3b), which were formed from the volatilization of solvent during the balling and curing process of PF. The structure is disadvantageous to the activation of PFS, especially the smooth surface of PFS which influences the penetration of activated reagent and prevents the development of porous texture of ACS. By oxidation at 300 1C for 2 h, SEM of the surface and cross section of PFS–O clearly show the produce of macropores (Fig. 3c) and the formation of abundant pores (Fig. 3d), which provide the interconnection of the pores and allow deep penetration of activated reagent for further activation of PFS to promote the development of pore structure. Scanning electron micrograph images of activated spheres are shown in Fig. 4. After activation, both activated carbon spheres are still as round and regular as the original resin spheres, but the outer surface of ACS–O is rougher, even very large cracks are formed, as shown in Fig. 4c. The cross section of ACS–O (Fig. 4d) shows 3D interconnected macroporous structure and presents a striking contrast to that of ACS (Fig. 4b). The porosity increases evidently. And macropores are penetrated each other by the
Activated carbon spheres with 3D hierarchical porous texture have been successfully prepared from the cheap commercial phenolic resins as carbon precursor by the preoxidation and activation. In the process, preoxidation had crucial effect on the structure of ACS–O, which created new pores on the surface of PFS, facilitated the further activation of PFS–O and eventually resulted in the formation of 3D hierarchical porous texture. Meanwhile, preoxidation promoted the curing of PFS and improved the thermal stability of activated carbon spheres. The obtained ACS–O have a high specific surface area of 1897 m2/g, large total pore volumes of 2.22 cm3/g and high mesoporous ratio of 92.3%, which will be promising in some applications such as carbon electrode for supercapacitors. References [1] G.J. Lee, S.I. Pyun, The effect of pore structures on fractal characteristics of meso/macroporous carbons synthesised using silica template, Carbon 43 (2005) 1804–1808. [2] V.M. Gun’ko, O.P. Kozynchenko, V.V. Turov, S.R. Tennison, V.I. Zarko, Y.M. Nychiporuk, T.V. Kulik, B.B. Palyanytsya, V.D. Osovskii, Y.G. Ptushinskii, A.V. Turov, Structural and adsorption studies of activated carbons derived from porous phenolic resins, Colloids Surf. A 317 (2008) 377–387. [3] K.S. Xia, Q.M. Gao, J.H. Jiang, J. Hu, Hierarchical porous carbons with controlled micropores and mesopores for supercapacitor electrode materials, Carbon 46 (2008) 1718–1726. [4] Y. Zhao, M.B. Zheng, J.M. Cao, X.F. Ke, J.S. Liu, Y.P. Chen, T. Tao, Easy synthesis of ordered meso/macroporous carbon monolith for use as electrode in electrochemical capacitors, Mater. Lett. 62 (2008) 548–551. [5] D.W. Wang, F. Li, M. Liu, G.Q. Lu, H.M. Cheng, 3D Aperiodic hierarchical porous graphitic carbon material for high-rate electrochemical capacitive energy storage, Angew. Chem. Int. Ed. 47 (2008) 373–376. [6] Y. Cochen, Z. Aizenshtat, Investigation of pyrolytically produced condensates of phenol–formaldehyde resins, in relation to their structure and decomposition mechanism, J. Anal. Appl. Pyrolysis 22 (1992) 153–178. [7] Y.F. Chen, Z.Q. Chen, S.Y. Xiao, H.B. Liu, A novel thermal degradation mechanism of phenol–formaldehyde type resins, Thermochim. Acta 476 (2008) 39–43. [8] S.R. Tennision, Phenolic-resin-derived activated carbons, Appl. Catal. A: Gen. 173 (1998) 289–311.
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Effect of precursor preoxidation on the structure of phenolic resin-based activated carbon spheres Yue Liu a,b, Kaixi Li a,n, Guohua Sun a a b
Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China Graduate University of Chinese Academy of Sciences, Beijing 100039, PR China
a r t i c l e in fo
abstract
Article history: Received 9 May 2009 Received in revised form 13 November 2009 Accepted 14 November 2009
Activated carbon spheres with 3D hierarchical porous structure were prepared from phenol– formaldehyde resins with oxidation treatment in air and physical activation in an inert atmosphere. Based on the results of thermogravimetric analysis, infrared spectrometer (IR), scanning electron microscopy (SEM) and nitrogen adsorption/desorption, the effect of preoxidation on the morphology and structure of activated carbon spheres was investigated. The results show that decomposition and crosslinking reactions occur during the preoxidation and the structural changes of precursor generated by the preoxidation lead to differences in the pore structure of activated carbon spheres. The carbon spheres exhibit the unique 3D hierarchical porous structure, high specific surface area of 1897 m2/g and high pore volume of 2.22 cm3/g. & 2009 Elsevier Ltd. All rights reserved.
Keywords: A. Microporous materials C. Electron microscopy C. Infrared spectroscopy D. Microstructure
1. Introduction The hierarchical porous carbons, containing both interconnected macroporous and mesoporous structures, have attracted much attention for potential applications in many fields of science and engineering [1]. These carbon materials have enhanced properties compared with single-sized pore materials due to the textural transport pores providing high connectivity of the narrow pores and allowing deep penetration of adsorbate molecules into the pore network for shorter time [2]. Recently, the hierarchical micro/mesoporous carbons have been prepared by CO2 activation of ordered mesoporous carbon and utilized as the electrode materials for supercapacitor [3]. A hierarchical meso/macroporous carbon monolith was also obtained using Pluronic F127 and SiO2 opal as templates [4]. In addition, a 3D aperiodic hierarchical porous graphitic carbon material has also been reported [5]. All of these porous carbons all exhibited notable electrochemical double-layer performance. However, their preparation process is complicated and most of carbon materials are powders which restrict their applications. In this paper, 3D hierarchical porous activated carbon spheres were prepared by oxidation treatment in air and physical activation in an inert atmosphere. In the process, preoxidation plays crucial role on the development of highly porous structures. The morphology and pore structure of activated carbon spheres
n
Corresponding author. Tel./fax.: + 86 351 4250292. E-mail address: [email protected] (K. Li).
0022-3697/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2009.12.009
were analyzed by the thermogravimetric analysis, infrared spectrometer (IR), scanning electron microscopy (SEM) and standard nitrogen adsorption measurement to reveal the relationship between preoxidation and the structure.
2. Experimental The novolac-type phenol–formaldehyde resins (PF) and hexamethylenetetramine were firstly dissolved in ethanol, and then poured in an aqueous solution of polyvinyl alcohol (PVA) in a high-pressure reactor. The obtained mixture was heated from room temperature to 130 1C at a rate of 5 1C/min and kept at this temperature for 60 min. After cooling naturally to room temperature, the phenolic resin spheres (PFS) were obtained via solid– liquid separation, followed by washing with distilled water to remove the excess polyvinyl alcohol (PVA) and drying at 110 1C for 24 h. PFS were firstly preoxidized at 300 1C for 120 min in the air and denoted as PFS–O. Then, PFS–O were heated up to 850 1C under a nitrogen atmosphere to activate the sample for 60 min under a stream of steam at a flow rate of 20 ml/min, thus activated carbon spheres were obtained. The samples activated from PFS and PFS–O were denoted as ACS and ACS–O, respectively. The weight changes of PFS and PFS–O were determined by thermogravimetric analysis (TGA, STA 409 PC). And the functional groups were analyzed by IR spectroscopy (BioRad FTS-165 USA). Specific surface area and pore structure of the samples were
ARTICLE IN PRESS 454
Y. Liu et al. / Journal of Physics and Chemistry of Solids 71 (2010) 453–456
characterized using nitrogen sorption at 77 K (Micromeritics ASAP-2020). A scanning electron microscopy (SEM, LEO 438 VP) was used to study the morphology of the samples.
the other peak is from 10 to 30 nm. And it is very remarkable to note that the adsorbent account and pore volume of ACS–O are very large in comparison with that of ACS from Fig. 1. So, the
3. Results and discussion It can be seen from Table 1 that the effect of precursor preoxidation on the structure of activated carbon spheres is very notable. Compared with the original ACS, the surface area and pore volume of activated carbon spheres obtained by preoxidation increase obviously, which exhibit high BET surface area of 1897 m2/g and high total pore volume of 2.22 cm3/g. Fig. 1 shows N2 adsorption/desorption isotherms and the corresponding BJH pore size distribution curves of ACS and ACS–O. Both carbon materials show a type-IV isotherm with a sharp capillary condensation step at high relative pressures (P/P0 = 0.85–0.96) and an H1-type hysteresis loop, indicating a relatively large pore size. However, a larger hysteresis loop occurs after P/P0 = 0.45 in the adsorption–desorption isotherm of ACS–O, which indicates that ACS–O has more developed mesopores and broader pore size distribution. Moreover, the adsorption/ desorption isotherm of ACS–O gradually increases with the relative pressure from 0.4 to 0.9 comparing with that of ACS, which suggests that the pore size distributions of ACS–O are broader than that of ACS. As shown in Fig. 1b, it appears a bimodal pore size distribution. The first peak ranges from 3 to 4 nm and
Fig. 2. FT-IR spectra of the PFS and the PFS–O.
Table 1 The physical properties of the samples. Sample
SBET (m2/g)
Vtotal (cm3/g)
Vmicro (cm3/g)
Vmeso (cm3/g)
Ratiomeso (%)
¯ (nm) D
ACS ACS–O
1196 1897
0.86 2.22
0.42 0.17
0.44 2.05
51.2 92.3
2.89 4.68
¯ , average pore size SBET: BET surface area; Vtotal: total pore volume, measured at P/P= 0.99; Vmicro: micropore volume; Vmeso: mesopore volume; Ratiomeso = Vmeso/Vtotal; D calculated from the N2-adsorption isotherm (4V/A by BET).
Fig. 1. N2 adsorption/desorption isotherms (a) and BJH pore size distributions (b) of the ACS and ACS–O.
ARTICLE IN PRESS Y. Liu et al. / Journal of Physics and Chemistry of Solids 71 (2010) 453–456
preoxidation treatment of precursor can obviously promote the pore structure development and improve the physical properties of activated carbon spheres. The FT-IR spectra of PFS and PFS–O (Fig. 2) illustrate that there are obvious changes of characteristic bands between 755 and 1750 cm 1, including the the aromatic C–H out-of-plane
455
deformation vibrations (820 and 755 cm 1), the breathing C–O vibrations (between 1000 and 1400 cm 1), the aliphatic deformation vibrations (between 1370 and 1500 cm 1) and the aromatic ring C–C stretching vibrations (1596 cm 1). The intensities of these bands for sample PFS–O decrease significantly, indicating a reduction in the number of the
Fig. 3. SEM images of the the contour and cross section of the samples. (a) PFS, (b) cross section of PFS, (c) PFS–O, (d) cross section of PFS–O.
Fig. 4. SEM images of the the contour and cross section of the samples. (a) ACS, (b) cross section of ACS, (c) ACS–O, (d) cross section of ACS–O.
ARTICLE IN PRESS 456
Y. Liu et al. / Journal of Physics and Chemistry of Solids 71 (2010) 453–456
smaller pores which are on the wall of macropores, as shown by arrows in Fig. 4d. It further confirms the above depiction that preoxidation promotes the development of porosity. In addition, the weight loss during carbonization of the samples was traced by thermogravimetric analysis, and the results are shown in Fig. 5. There is a very significant difference in the weight loss of the PFS and PFS–O, which indicates that preoxidation promotes the crosslinking of phenolic resin spheres and improves the transformation efficiency of the carbon to enhance the thermal stability [8]. It is consisted with the analysis of FT-IR.
4. Conclusions
Fig. 5. TG curves of the PFS and the PFS–O.
IR-observable groups. Meanwhile a new brand (1740 cm 1) appears which can be attributed to tetra-substituted benzene ring. All of these FT-IR results show that the preoxidation decreases some groups such as hydroxymethyl and promotes the crosslinking of PFS [6,7]. The morphology of the samples is displayed in Fig. 3. The PFS has a good sphericity and smooth surface (Fig. 3a), and it exists abundant air bubbles in the inner of PFS (Fig. 3b), which were formed from the volatilization of solvent during the balling and curing process of PF. The structure is disadvantageous to the activation of PFS, especially the smooth surface of PFS which influences the penetration of activated reagent and prevents the development of porous texture of ACS. By oxidation at 300 1C for 2 h, SEM of the surface and cross section of PFS–O clearly show the produce of macropores (Fig. 3c) and the formation of abundant pores (Fig. 3d), which provide the interconnection of the pores and allow deep penetration of activated reagent for further activation of PFS to promote the development of pore structure. Scanning electron micrograph images of activated spheres are shown in Fig. 4. After activation, both activated carbon spheres are still as round and regular as the original resin spheres, but the outer surface of ACS–O is rougher, even very large cracks are formed, as shown in Fig. 4c. The cross section of ACS–O (Fig. 4d) shows 3D interconnected macroporous structure and presents a striking contrast to that of ACS (Fig. 4b). The porosity increases evidently. And macropores are penetrated each other by the
Activated carbon spheres with 3D hierarchical porous texture have been successfully prepared from the cheap commercial phenolic resins as carbon precursor by the preoxidation and activation. In the process, preoxidation had crucial effect on the structure of ACS–O, which created new pores on the surface of PFS, facilitated the further activation of PFS–O and eventually resulted in the formation of 3D hierarchical porous texture. Meanwhile, preoxidation promoted the curing of PFS and improved the thermal stability of activated carbon spheres. The obtained ACS–O have a high specific surface area of 1897 m2/g, large total pore volumes of 2.22 cm3/g and high mesoporous ratio of 92.3%, which will be promising in some applications such as carbon electrode for supercapacitors. References [1] G.J. Lee, S.I. Pyun, The effect of pore structures on fractal characteristics of meso/macroporous carbons synthesised using silica template, Carbon 43 (2005) 1804–1808. [2] V.M. Gun’ko, O.P. Kozynchenko, V.V. Turov, S.R. Tennison, V.I. Zarko, Y.M. Nychiporuk, T.V. Kulik, B.B. Palyanytsya, V.D. Osovskii, Y.G. Ptushinskii, A.V. Turov, Structural and adsorption studies of activated carbons derived from porous phenolic resins, Colloids Surf. A 317 (2008) 377–387. [3] K.S. Xia, Q.M. Gao, J.H. Jiang, J. Hu, Hierarchical porous carbons with controlled micropores and mesopores for supercapacitor electrode materials, Carbon 46 (2008) 1718–1726. [4] Y. Zhao, M.B. Zheng, J.M. Cao, X.F. Ke, J.S. Liu, Y.P. Chen, T. Tao, Easy synthesis of ordered meso/macroporous carbon monolith for use as electrode in electrochemical capacitors, Mater. Lett. 62 (2008) 548–551. [5] D.W. Wang, F. Li, M. Liu, G.Q. Lu, H.M. Cheng, 3D Aperiodic hierarchical porous graphitic carbon material for high-rate electrochemical capacitive energy storage, Angew. Chem. Int. Ed. 47 (2008) 373–376. [6] Y. Cochen, Z. Aizenshtat, Investigation of pyrolytically produced condensates of phenol–formaldehyde resins, in relation to their structure and decomposition mechanism, J. Anal. Appl. Pyrolysis 22 (1992) 153–178. [7] Y.F. Chen, Z.Q. Chen, S.Y. Xiao, H.B. Liu, A novel thermal degradation mechanism of phenol–formaldehyde type resins, Thermochim. Acta 476 (2008) 39–43. [8] S.R. Tennision, Phenolic-resin-derived activated carbons, Appl. Catal. A: Gen. 173 (1998) 289–311.