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Physical properties of silver-containing pitch-based activated carbon fibers
PERGAMON
Carbon 37 (1999) 1619–1625
Physical properties of silver-containing pitch-based activated carbon fibers S.K. Ryu a , *, S.Y. Kim a , N. Gallego b , D.D. Edie b a
Dept. of Chemical Engineering, Chungnam National University, Taejon 305 -764, South Korea b Dept. of Chemical Engineering, Clemson University, Clemson SC 29634 -0909, USA Received 15 October 1998; accepted 25 March 1999
Abstract Silver-containing pitch-based carbon fibers were prepared and activated in steam. SEM and TEM were used to investigate the surface morphology and the behavior of the silver particles in fibers. Physical properties such as density, tensile strength, and electrical resistivity were measured. The SEM photos of fibers containing silver (at initial concentrations of 1000 and 10 000 ppm) were similar to those of non-activated carbon fibers at high level burn-off. Silver particles accelerate the activation rate. However, the specific surface areas of silver-containing activated carbon fibers were similar to those of non-silver containing activated carbon fibers. The apparent density and the tensile strength of the 10 000 ppm silvercontaining carbon fibers were 1.677g / cm 3 and 24 kg f / mm 2 , and these decreased to 0.795 g / cm 3 and 6 kg f / mm 2 , respectively, at 69% burn-off. The electrical resistivity of isotropic pitch-based carbon fiber was 97 mV m. By comparison, as the initial silver content of the fiber was increased to 1000 and 10 000 ppm, the resistivity decreased to 69 and 57 mV m, respectively. These resistivities depended on the total pore volume and increased exponentially with increasing specific surface area of fibers. 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Activation; C. BET surface area; D. Mechanical properties; Electrical properties
1. Introduction During the past 10 years, ACFs have attracted a considerable attention because of their high adsorption rates and capacities, their ease of synthesis in various forms, and their wide applicability. Although a high degree of surface activity is characteristic of many forms of carbon, pore size is the unique feature of ACF. These fibers contain micropores, almost exclusively, making them attractive for separating small molecules. Thus, currently ACFs are used in many gas separation, water purification, and solvent recovery processes. However, many toxic materials that should be removed from the environment, are large molecules, ranging from SO x , NOx and phenol, to humine, proteins and various pesticides. For these applications, the ACFs must contain mesopores (pores with larger diameters). ACFs containing mesopores could also prove valuable in other applications, such
*Corresponding author.
catalysis and electronics. Recently several approaches have been examined for controlling the pore size distribution and enhancing the mesopore content of activated carbons and ACFs. Using cobalt as an activation catalyst and silver as an antibacterial agent, Oya et al. [1] prepared an antibacterial phenolic-resin-based activated carbon fiber containing mesopores. Although the cobalt did generate mesopores, it was strongly suppressed by the silver, and it alloyed with the silver to form larger catalyst particles. Ozaki et al. [2] produced mesoporous carbon fibers from a polymer blend. A phenolic resin served as the precursor and polyvinylbutyral as the pyrolyzing polymer. In this case, the activation process was omitted, and pyrolyzing the polyvinylbutyral created the porosity. Although the results of both studies appear promising, many issues must be resolved before mesoporous ACFs can be commercialized. Large-scale production will require that the precursor be readily available, uniform and thermally stable. Also, the selectivity of mesoporous activated carbon fibers must be verified for larger molecules. In this paper, pitch-based carbon fibers containing different concentrations of silver
0008-6223 / 99 / $ – see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 99 )00086-X
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were activated, and their structural and mechanical properties were compared.
2. Experimental A precursor pitch with a softening point of 2408C was produced by reforming a NCB (Naphtha Cracking Bottom, SK oil, Korea) at 3908C for 3 h. Silver nitrate (m.p. 2128C) was directly added to the precursor pitch at initial silver concentrations of 1000 and 10 000 ppm, and then thoroughly mixed. These mixed precursors were melt-spun into fibers using an apparatus described earlier [3]. The diameter of the extrusion capillary was 1.0mm and the melt-spinning temperature was 3008C. Then, the precursor fibers were stabilized at 2908C for 2 h, and carbonized at 10008C for 30 min. The average diameters of the silvercontaining carbon fibers were 23.0 and 22.3 mm. The fibers were activated by steam diluted in nitrogen at 9008C. The carbon fiber and the activated carbon fiber without silver are noted as CF-S0 and ACF-S0. The fibers with an initial silver content of 1000 ppm silver are noted as CF-S1 and ACF-S1, respectively. Finally, the fibers with an initial silver content of 10 000 ppm are noted as CF-S2 and ACF-S2, respectively. It should be noted that the concentration of silver in the activated fibers probably changed during activation process because of consecutive reactions. The fibers were subjected to the mixture of nitrogen and steam for different periods of time, and this produced varying amounts of weight loss. The last two figures in the sample designation give the weight loss of the carbon fibers during activation (or, percent burn off). Thus, the activated fiber formed from the precursor containing 10 000 ppm silver that had a 69% burn-off is designated as ACF-S2-69. SEM and TEM were used to observe the microscopic structures and the distribution of silver particles in the fibers. The final silver content of the fibers was
measured by ICP emission spectroscopy after the fiber samples were ashed and dissolved in nitric acid. The BET specific surface areas of the fiber samples were determined from the adsorption isotherms of N 2 at 77K. The microporous volume was determined from the isotherms using a s -method [4]. The apparent densities of the fiber samples were determined by the liquid displacement method using a mixture of tetrabromoethane and carbon tetrachloride, and the apparent densities of the ACFs were calculated using the measured weight and apparent volume of the fibers [5]. Single-filament testing was employed to determine the tensile strength of fibers (ASTM standard D3544-76). The diameter and the cross-sectional area of each fiber was determined using the mounting and image analysis procedure employed in the previous study [6]. The electrical resistivity of the fiber was measured using a four-point probe technique, similar to that described by Coleman [7].
3. Results and discussion
3.1. Structural properties SEM photos of silver-containing carbon fibers are shown in Fig. 1. As the photos show, the surface textures of 1000 ppm silver-containing carbon fibers (CF-S1) and their activated counterparts (ACF-S1-63) were similar to the surface texture of the carbon fibers formed from the isotropic pitch that did not contain silver (CF-S0). However, the surface textures of 10 000 ppm silver-containing carbon fibers differed significantly from the surface texture of the isotropic pitch fibers that did not contain silver. There were some chips on the surface of CF-S2 which might be caused by the coalescence of some silver particles. No pores were observed in the fracture surfaces of the silver-containing carbon fibers (CF-S1, CF-S2), a
Fig. 1. SEM photos of silver-containing CFs and ACFs.
S.K. Ryu et al. / Carbon 37 (1999) 1619 – 1625
characteristic feature of isotropic carbon materials [8]. The SEM photos of the activated carbon fibers are similar to those of non-activated carbon fibers, even at high levels of burn-off. Apparently there were immense micropores in the fibers. Some macropores were observed on the fracture surfaces of ACF-S2 at high level burn-off. These pores were developed by the separation of coalesced silver particles during high temperature steam activation. Oya et al. [1] also observed these large pores in the metalcontaining ACFs and explained that the formation of large pores must be related to pore channeling caused by metal particles. Oya reported that the silver concentration increased as burn-off increased. To explain this, he postulated that the silver particles penetrated into the fiber core during burn-off. However, our results show different point of views on pore channeling. As Fig. 2 shows, macropores developed on the surface of sample ACF-S2, and the number of macropores increased with burn-off at 9008C. At 28% burn-off only a few macropores could be detected on the surface of fibers. Most of the macropores were developed at between 40% and 60% burn-off, and the number of macropores on the surface appeared to be constant during this stage of activation. This indicates that the macropores developed around the silver particles by the catalytic activation. Then, the silver particles separate from the fibers during burn-off of the fiber surface film, creating caves which are larger in diameter than the silver particles. If the particles migrated into the fiber core, as proposed by Oya, one would expect the number of macropores on the surface and the macropore volume to increase with increasing burn-off. Silver particles appeared to accelerate the activation rate. However, SEM analyses indicated that the particles did not
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create channels but, instead, separated as the activation proceeded. To better understand the porous texture created during activation of the silver-containing fibers, TEM was employed and adsorption isotherms were measured. TEM photos of CF-S2 and ACF-S2 at 69% burn-off are shown in Fig. 3. Fortunately, the silver concentration of CF-S1 was about 1000 ppm, which means there were no concentration changes during the oxidation and carbonization of as-spun fibers. However 1000 ppm is too small amount to observe their behavior during activation. For CF-S2, silver particles were easily observed. Some particles appear to have coalesced. The pictures of silver particles were same before and after activation. However, only a few particles were found in the fibers after high level burn-off. It is believed that the most of silver particles, sometimes coalesced particles were sited outward of fibers and seemed to be separated from the fibers developing macropores which were larger than silver particle size by the burn-off of external fiber surface. It was difficult to find the tunnels behind the particles that have reported by Oya et al. [1]. This indicates that silver particles didn’t move into the fiber cores but separated from the fiber surface remaining caves with the activation proceeded. The change in silver concentration during burn-off is shown in Fig. 4. If the burn-off process only removed carbon, the silver concentration increase should follow line (1). If the burn-off process removed carbon and silver particles at the same rate, the silver concentration should remain constant and follow line (2). However, the silver concentration measured by an ICP emission spectroscopy was even lower than line (2) at all levels of burn-off. This indicates that the weight loss of silver particles is larger
Fig. 2. Macropores of silver-containing ACF-S2 at various degrees of burn-off.
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Fig. 3. TEM photos of silver-containing CF-S2 and ACF-S2.
than that of carbon. This observation differs from that reported by Oya et al. [9,10]. They reported that the silver content increased with increasing levels of burn-off. Their explanation was that the metal particles penetrated into the fiber core resulting in channeling and forming larger pores during activation [1]. We found that the silver particles in CF-S2 did not migrate to the fiber core; instead, they separated from the fiber surface creating caves during activation. This different effect may be the result of the initial distribution of silver particles in the fibers prior to activation. It appears that the majority of the silver particles were located in the outer regions of the fibers, as shown in Fig. 3. Therefore, the silver particles were lost more quickly than the carbon during burn-off. This would also explain why, as shown in Fig. 5, the larger con-
centration of silver did not the increase of specific surface area and average pore size of the fibers. If the silver particles are located in the outer regions of the fibers and separate at high levels of burn-off, a higher concentration of silver would have little, if any, effect. Fig. 5 shows the relationship between specific surface area and burn-off. The specific surface areas of all three ACFs linearly increased with increasing burn-off (a trend similar to that reported by Kimber et al. [11]). The specific surface area of the 1000 ppm silver-containing ACFs was higher (1920 m 2 / g at 63% burn-off) than that of the non-silver containing ACFs at the same percent burn-off. However, the specific surface area of the 10 000 ppm silver-containing ACFs was somewhat lower than that of non-silver containing ACFs. By comparison, Oya reported
Fig. 4. Silver content of ACF-S2 at various degrees of burn-off.
Fig. 5. Specific surface area of ACFs at various degrees of burn-off.
S.K. Ryu et al. / Carbon 37 (1999) 1619 – 1625
much lower specific surface areas (670–830 m 2 / g for silver / cobalt-containing ACFs [1], 600–840 m 2 / g for cobalt-containing ACFs [12], and 540–740 m 2 / g for silver-containing ACFs [9]). Evidently, cobalt and silver did not favor micropore development, but instead suppressed the development of BET surface area during catalytic steam activation. This is opposite to what might be expected because metals normally catalyze the activation process. Oya et al. [10] also reported specific surface areas for the silver-containing ACF similar to that found in our study at higher levels of burn-off (approximately 2000 m 2 / g). Apparently silver particles do have a catalytic effect, and the increase of surface area depends on the activation conditions. Perhaps by changing the activation conditions, ACFs with even larger surface areas can be obtained. The average pore diameter of non-silver containing ACF ˚ and the average pore diameters of (ACF-S0-60) was 9.7 A both silver-containing ACFs (ACF-S1-63 and ACF-S2-69) ˚ in spite of the different initial silver conwere 18.8 A, centration. Apparently, increasing the silver content of the carbon fibers from 1000 to 10 000 ppm did not increase either the specific surface area or the average pore size. This indicated that the volume fractions of meso- and macropores are similar in ACF-S1-63 and ACF-S2-69. Tests showed that the meso- and macropore volume fraction of the non-silver containing ACF was 4.2%. By comparison, the volume fractions of meso- and macropores were 13.6% for ACF-S1-63 and 14% for ACF-S2-69, respectively. Of course, the volume fraction of macropores could be negligible and the increase in average pore diameter could result from the development of mesopores in the silver-containing fibers. Nevertheless, the smaller specific surface area of ACF-S2 compared to that of ACF-S1 at similar levels of burn-off does seem rather puzzling. Sample ACF-S2 has a higher initial concentration of silver particles than sample ACF-S1; therefore, one might expect it to develop more macropores. Oya et al. [12] also reported that increasing the metal concentration did not increase the specific surface area of the activated fiber. Their explanation was that, as the cobalt concentration in the fiber increases, the fine particles coalesce more easily. The result would be fewer active particles.
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Fig. 6. Tensile strength of ACFs at various degrees of burn-off.
[11] for carbon fibers that did not contain silver; however, they observed a linear decrease in strength. Oya et al. [9] reported that the tensile strength of their silver-containing ACF was the same at 23% burn-off and at 67% burn-off (60 Mpa). They found that the tensile strength did not decrease with increasing specific surface area (540–740 m 2 / g) and silver concentration (0.29–0.63 wt%). This is surprising, given the large increase in specific surface area and the high level of burn-off reported. The change in apparent density with increasing burn-off is plotted in Fig. 7. The densities of CF-S0, CF-S1 and CF-S2 were 1.65, 1.652, and 1.677 g / cm 3 , respectively, as determined by the liquid displacement method. The apparent densities of the ACFs were calculated using the measured cross-sectional areas and estimated lengths of 1 g samples. This approach has been shown to agree closely with more direct measurements of density [5]. The density of non-silver containing CF decreased rapidly at low levels of burn-off and then more slowly as burn-off increased. The convex downward trend differs from that reported by Kimber et al. [11]. They found that the apparent density of
3.2. Physical properties The effect of activation on the final tensile strength of the ACFs is shown in Fig. 6. The tensile strength of carbon fibers decreased significantly (at all levels of activation) as the initial silver-content of the fibers increased. The tensile strength of carbon fibers with an initial silver content of 10 000 ppm (1wt%) was about one third that of the non-silver containing carbon fibers. The tensile strength of all ACFs decreased rapidly during the initial stages of burn-off and decreased more slowly as the burn-off increased. Similar results were reported by Kimber et al.
Fig. 7. Apparent density of ACFs at various degrees of burn-off.
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the ACF decreased linearly with increasing levels of burnoff. However, Kimber reported that the initial density decreased by one-half at 64% burn-off, which was very similar to our result. The apparent density of ACF produced in the current work is rather high, considering the large pore volume within the fiber. The ACFs that did not contain silver particles had lower apparent densities than the silver-containing ACFs at burn-off levels of less than 70. Perhaps the silver particles contributed to the high initial density, and these particles did not separate during the early stage of burn-off. At 70% burn-off the densities of silver-containing ACFs were similar to that of the non-silver containing ACF-S0. Then, the apparent density of the silver-containing ACFs decreases rapidly at burn-off levels above 80%. However, the apparent densities of the 1000 ppm and the 10 000 ppm silver-containing ACFs were similar at equivalent levels of burn-off, in spite of their different initial concentrations of silver and their different specific surface areas. Perhaps the higher concentration of silver created more mesopores, reducing the density of ACF-S2 to that of ACF-S1. This also would indicate that few silver particles remained in the ACF-S1 and ACF-S2 samples at high levels of burn-off. Evidently, the silver particles didn’t migrate to the fiber core, but instead separated from the fiber surface. The change in diameter of the CFs with respect to burn-off is plotted in Fig. 8. The diameter of non-silver containing carbon fibers increased by about 3.2% during the early stage of burn-off and then slowly decreased after about 30% burn-off. This swelling of isotropic pitch-based carbon fibers during the initial stage of activation was also observed in the previous work [5]. Similar trends were reported during the steam activation of phenolic resinbased carbon fibers by Gondy and Ehrburger [13]. However, we found that the swelling of the pitch-based carbon fibers occurs during the initial stage of activation, while Gondy and Ehrburger reported that swelling of phenolic resin-based carbon fibers occurred in the middle stage of
Fig. 8. Diameter of ACFs at various degrees of burn-off.
activation. Gondy and Ehrburger did not observe swelling when their pitch-based carbon fibers were subjected to steam activation, and they could not detect swelling when their phenolic resin-based carbon fibers were subjected to CO 2 activation. They postulated that a transient uptake of oxygen might occur during the CO 2 activation of isotropic pitch-based carbon fibers, and perhaps this does not occur during steam activation. We suspect that the reaction gases are merely produced faster than they can escape during the early stage of activation and this causes fiber swelling. The diameter of silver-containing carbon fibers decreased from the very beginning of activation, but the diameter then became relatively constant. This indicates that the silver particles accelerate the burn-off of the surrounding carbon, creating macropores and allowing the product gases to exit more easily. However, the constant fiber diameter at high levels of burn-off may also indicate that many micropores are being created in the fibers. The fiber diameter decreased rapidly above 80% burn-off. Fig. 9 shows the relationship between the electrical resistivity and the specific surface area of activated silvercontaining carbon fibers and activated carbon fibers that did not contain silver. The initial electrical resistivity of isotropic pitch-based CF that did not contain silver was 97 mV m (the standard deviation for the 16 samples tested was 10.6). This electrical resistivity is 3.2 times higher than the 30 mV m reported for Ashland isotropic pitchbased carbon fibers [14] and 4.4 times higher than the 22 mV m reported for Torayca T-300 PAN-based carbon fibers [15]. The electrical resistivity of the initial fibers (prior to activation) was decreased to 69 and 57 mV m, by addition of silver at levels of 1000 and 10 000 ppm, respectively. This was because of the high electrical conductivity of silver. These resistivities increased exponentially as the specific surface areas of fibers increased. This rapid increase seems logical because the electrical resistivity depends not only on specific surface area but
Fig. 9. Electrical resistivity of ACFs with respect to specific surface area.
S.K. Ryu et al. / Carbon 37 (1999) 1619 – 1625
also on the porosity of the fibers. The micropore size becomes slightly larger with increasing levels of burn-off. In the early stage of steam activation, only narrow micropores are developed, resulting in high specific surface areas. As burn-off increases some micropores coalesce, creating wider micropores or mesopores that reduce the electrical conductivity. However, Kimber et al. [11] reported that the electrical conductivity rapidly decreased at relatively small levels of weight loss and then slowly decreased at higher levels of burn-off. One explanation for this different trend might be that their pore size distribution was not the same as ours. Their ACFs could have contained more pores than ours at high levels of burn-off because their increase of specific surface area with burn-off was very similar to ours. The silver-containing ACFs exhibited lower resistivities than the nonsilver containing ACFs. On the other hand, the resistivities of both silver-containing ACFs were similar at equal specific surface areas in spite of the difference in silver concentration. However, if one plots the electrical resistivity versus burn-off, ACF-S1 has a higher resistivity than ACF-S2 at the same burn-off because the sample has higher specific surface area. We have observed that different carbon fibers require a different level of burn-off to obtain the same specific surface area. Thus, the porosity of ACF-S1 might be different from that of ACF-S2 at the same specific surface area. The similar electrical resistivities of both silver-containing ACFs at the equivalent specific surface areas indicates that either total pore volume is a more important factor than the pore size distribution or that both ACFs have similar microporosities. Actually, the average pore sizes of both silver-containing ACFs were very similar at the equivalent levels of burn-off, in spite of the difference in the initial silver concentration.
4. Conclusions Silver particles in pitch-based carbon fibers accelerated the rate of activation, created macropores, and increased the average pore diameter. The silver particles did not migrate toward the fiber core; rather they separated from the fiber surface, some coalesced particles created caves with increasing the activation. Prior to activation, it appeared that the majority of silver particles were located in the outer regions of the fibers. Because of this, the loss
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of silver particles was greater than the loss of carbon during activation. If the precursor pitch were prepared well and the distribution of silver particles were more homogeneous, one would expect that the specific surface area and the average pore diameter of the activated carbon fiber would increase as the silver concentration of the initial fiber increased. Increasing the initial concentration of silver decreased the final tensile strength of carbon fiber but had little affect on the apparent density during steam activation. Even though, the electrical resistivities of carbon fibers increased exponentially as their specific surfaces increased, the initial concentration of silver appeared to have a minor affect. Based on these results, metal-containing carbon fibers can be activated, yielding carbon adsorbents with controlled pore volumes, adequate physical properties, and acceptable electrical resistivities.
References [1] Oya A, Yoshida S, Alcaniz-Monge J, Linares-Solano A. Carbon 1996;34(1):53–7. [2] Ozaki J, Endo N, Ohizumi W et al. Carbon 1997;35(7):1031–3. [3] Ryu SK, Ko KR, Rhee BS, Ehrburger P. In: Extended abstracts, 21st biennial conf. on carbon. SU of New York, Buffalo (New York. USA), American Carbon Society, 1993, pp. 623–4. [4] Rodriguez-Reinoso F, Martin-Martinez JM, Prado-Burguete B, McEnaney B. J Phys Chem 1987;91:515–6. [5] Ryu SK, Ko KR, Gondy D, Ehrburger P. Carbon 1996;34(6):808–10. [6] Edie DD, Fain CC, Robinson KE, Harper AM, Rogers DK. Carbon 1993;31(6):941–9. [7] Coleman LB. Rev Sci Inst 1975;46:1125–9. [8] Yoshida A. Tanso 1990;141:56–69. [9] Oya A, Wakahara T, Yoshida S. Carbon 1993;31(8):1243–7. [10] Oya A, Yoshida S, Abe Y, Iizuka T, Makiyama N. Carbon 1993;31(1):71–3. [11] Kimber GM, Vego A, Rantell TD. In: Extended abstracts, 23rd biennial conf. on carbon. Penn State U, University Park (Pennsylvania USA), American Carbon Society, 1997, pp. 454–5. [12] Oya A, Yoshida S, Alcaniz-Monge J, Linares-Solano A. Carbon 1995;33(8):1085–90. [13] Gondy D, Ehrburger P. Carbon 1997;35:1745–9. [14] Frysz CA, Shui AP, Chung DDL. Carbon 1997;35(7):893– 916. [15] Wang XJ, Chung DDL. Carbon 1997;35(5):706–9.
Carbon 37 (1999) 1619–1625
Physical properties of silver-containing pitch-based activated carbon fibers S.K. Ryu a , *, S.Y. Kim a , N. Gallego b , D.D. Edie b a
Dept. of Chemical Engineering, Chungnam National University, Taejon 305 -764, South Korea b Dept. of Chemical Engineering, Clemson University, Clemson SC 29634 -0909, USA Received 15 October 1998; accepted 25 March 1999
Abstract Silver-containing pitch-based carbon fibers were prepared and activated in steam. SEM and TEM were used to investigate the surface morphology and the behavior of the silver particles in fibers. Physical properties such as density, tensile strength, and electrical resistivity were measured. The SEM photos of fibers containing silver (at initial concentrations of 1000 and 10 000 ppm) were similar to those of non-activated carbon fibers at high level burn-off. Silver particles accelerate the activation rate. However, the specific surface areas of silver-containing activated carbon fibers were similar to those of non-silver containing activated carbon fibers. The apparent density and the tensile strength of the 10 000 ppm silvercontaining carbon fibers were 1.677g / cm 3 and 24 kg f / mm 2 , and these decreased to 0.795 g / cm 3 and 6 kg f / mm 2 , respectively, at 69% burn-off. The electrical resistivity of isotropic pitch-based carbon fiber was 97 mV m. By comparison, as the initial silver content of the fiber was increased to 1000 and 10 000 ppm, the resistivity decreased to 69 and 57 mV m, respectively. These resistivities depended on the total pore volume and increased exponentially with increasing specific surface area of fibers. 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Activation; C. BET surface area; D. Mechanical properties; Electrical properties
1. Introduction During the past 10 years, ACFs have attracted a considerable attention because of their high adsorption rates and capacities, their ease of synthesis in various forms, and their wide applicability. Although a high degree of surface activity is characteristic of many forms of carbon, pore size is the unique feature of ACF. These fibers contain micropores, almost exclusively, making them attractive for separating small molecules. Thus, currently ACFs are used in many gas separation, water purification, and solvent recovery processes. However, many toxic materials that should be removed from the environment, are large molecules, ranging from SO x , NOx and phenol, to humine, proteins and various pesticides. For these applications, the ACFs must contain mesopores (pores with larger diameters). ACFs containing mesopores could also prove valuable in other applications, such
*Corresponding author.
catalysis and electronics. Recently several approaches have been examined for controlling the pore size distribution and enhancing the mesopore content of activated carbons and ACFs. Using cobalt as an activation catalyst and silver as an antibacterial agent, Oya et al. [1] prepared an antibacterial phenolic-resin-based activated carbon fiber containing mesopores. Although the cobalt did generate mesopores, it was strongly suppressed by the silver, and it alloyed with the silver to form larger catalyst particles. Ozaki et al. [2] produced mesoporous carbon fibers from a polymer blend. A phenolic resin served as the precursor and polyvinylbutyral as the pyrolyzing polymer. In this case, the activation process was omitted, and pyrolyzing the polyvinylbutyral created the porosity. Although the results of both studies appear promising, many issues must be resolved before mesoporous ACFs can be commercialized. Large-scale production will require that the precursor be readily available, uniform and thermally stable. Also, the selectivity of mesoporous activated carbon fibers must be verified for larger molecules. In this paper, pitch-based carbon fibers containing different concentrations of silver
0008-6223 / 99 / $ – see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 99 )00086-X
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S.K. Ryu et al. / Carbon 37 (1999) 1619 – 1625
were activated, and their structural and mechanical properties were compared.
2. Experimental A precursor pitch with a softening point of 2408C was produced by reforming a NCB (Naphtha Cracking Bottom, SK oil, Korea) at 3908C for 3 h. Silver nitrate (m.p. 2128C) was directly added to the precursor pitch at initial silver concentrations of 1000 and 10 000 ppm, and then thoroughly mixed. These mixed precursors were melt-spun into fibers using an apparatus described earlier [3]. The diameter of the extrusion capillary was 1.0mm and the melt-spinning temperature was 3008C. Then, the precursor fibers were stabilized at 2908C for 2 h, and carbonized at 10008C for 30 min. The average diameters of the silvercontaining carbon fibers were 23.0 and 22.3 mm. The fibers were activated by steam diluted in nitrogen at 9008C. The carbon fiber and the activated carbon fiber without silver are noted as CF-S0 and ACF-S0. The fibers with an initial silver content of 1000 ppm silver are noted as CF-S1 and ACF-S1, respectively. Finally, the fibers with an initial silver content of 10 000 ppm are noted as CF-S2 and ACF-S2, respectively. It should be noted that the concentration of silver in the activated fibers probably changed during activation process because of consecutive reactions. The fibers were subjected to the mixture of nitrogen and steam for different periods of time, and this produced varying amounts of weight loss. The last two figures in the sample designation give the weight loss of the carbon fibers during activation (or, percent burn off). Thus, the activated fiber formed from the precursor containing 10 000 ppm silver that had a 69% burn-off is designated as ACF-S2-69. SEM and TEM were used to observe the microscopic structures and the distribution of silver particles in the fibers. The final silver content of the fibers was
measured by ICP emission spectroscopy after the fiber samples were ashed and dissolved in nitric acid. The BET specific surface areas of the fiber samples were determined from the adsorption isotherms of N 2 at 77K. The microporous volume was determined from the isotherms using a s -method [4]. The apparent densities of the fiber samples were determined by the liquid displacement method using a mixture of tetrabromoethane and carbon tetrachloride, and the apparent densities of the ACFs were calculated using the measured weight and apparent volume of the fibers [5]. Single-filament testing was employed to determine the tensile strength of fibers (ASTM standard D3544-76). The diameter and the cross-sectional area of each fiber was determined using the mounting and image analysis procedure employed in the previous study [6]. The electrical resistivity of the fiber was measured using a four-point probe technique, similar to that described by Coleman [7].
3. Results and discussion
3.1. Structural properties SEM photos of silver-containing carbon fibers are shown in Fig. 1. As the photos show, the surface textures of 1000 ppm silver-containing carbon fibers (CF-S1) and their activated counterparts (ACF-S1-63) were similar to the surface texture of the carbon fibers formed from the isotropic pitch that did not contain silver (CF-S0). However, the surface textures of 10 000 ppm silver-containing carbon fibers differed significantly from the surface texture of the isotropic pitch fibers that did not contain silver. There were some chips on the surface of CF-S2 which might be caused by the coalescence of some silver particles. No pores were observed in the fracture surfaces of the silver-containing carbon fibers (CF-S1, CF-S2), a
Fig. 1. SEM photos of silver-containing CFs and ACFs.
S.K. Ryu et al. / Carbon 37 (1999) 1619 – 1625
characteristic feature of isotropic carbon materials [8]. The SEM photos of the activated carbon fibers are similar to those of non-activated carbon fibers, even at high levels of burn-off. Apparently there were immense micropores in the fibers. Some macropores were observed on the fracture surfaces of ACF-S2 at high level burn-off. These pores were developed by the separation of coalesced silver particles during high temperature steam activation. Oya et al. [1] also observed these large pores in the metalcontaining ACFs and explained that the formation of large pores must be related to pore channeling caused by metal particles. Oya reported that the silver concentration increased as burn-off increased. To explain this, he postulated that the silver particles penetrated into the fiber core during burn-off. However, our results show different point of views on pore channeling. As Fig. 2 shows, macropores developed on the surface of sample ACF-S2, and the number of macropores increased with burn-off at 9008C. At 28% burn-off only a few macropores could be detected on the surface of fibers. Most of the macropores were developed at between 40% and 60% burn-off, and the number of macropores on the surface appeared to be constant during this stage of activation. This indicates that the macropores developed around the silver particles by the catalytic activation. Then, the silver particles separate from the fibers during burn-off of the fiber surface film, creating caves which are larger in diameter than the silver particles. If the particles migrated into the fiber core, as proposed by Oya, one would expect the number of macropores on the surface and the macropore volume to increase with increasing burn-off. Silver particles appeared to accelerate the activation rate. However, SEM analyses indicated that the particles did not
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create channels but, instead, separated as the activation proceeded. To better understand the porous texture created during activation of the silver-containing fibers, TEM was employed and adsorption isotherms were measured. TEM photos of CF-S2 and ACF-S2 at 69% burn-off are shown in Fig. 3. Fortunately, the silver concentration of CF-S1 was about 1000 ppm, which means there were no concentration changes during the oxidation and carbonization of as-spun fibers. However 1000 ppm is too small amount to observe their behavior during activation. For CF-S2, silver particles were easily observed. Some particles appear to have coalesced. The pictures of silver particles were same before and after activation. However, only a few particles were found in the fibers after high level burn-off. It is believed that the most of silver particles, sometimes coalesced particles were sited outward of fibers and seemed to be separated from the fibers developing macropores which were larger than silver particle size by the burn-off of external fiber surface. It was difficult to find the tunnels behind the particles that have reported by Oya et al. [1]. This indicates that silver particles didn’t move into the fiber cores but separated from the fiber surface remaining caves with the activation proceeded. The change in silver concentration during burn-off is shown in Fig. 4. If the burn-off process only removed carbon, the silver concentration increase should follow line (1). If the burn-off process removed carbon and silver particles at the same rate, the silver concentration should remain constant and follow line (2). However, the silver concentration measured by an ICP emission spectroscopy was even lower than line (2) at all levels of burn-off. This indicates that the weight loss of silver particles is larger
Fig. 2. Macropores of silver-containing ACF-S2 at various degrees of burn-off.
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Fig. 3. TEM photos of silver-containing CF-S2 and ACF-S2.
than that of carbon. This observation differs from that reported by Oya et al. [9,10]. They reported that the silver content increased with increasing levels of burn-off. Their explanation was that the metal particles penetrated into the fiber core resulting in channeling and forming larger pores during activation [1]. We found that the silver particles in CF-S2 did not migrate to the fiber core; instead, they separated from the fiber surface creating caves during activation. This different effect may be the result of the initial distribution of silver particles in the fibers prior to activation. It appears that the majority of the silver particles were located in the outer regions of the fibers, as shown in Fig. 3. Therefore, the silver particles were lost more quickly than the carbon during burn-off. This would also explain why, as shown in Fig. 5, the larger con-
centration of silver did not the increase of specific surface area and average pore size of the fibers. If the silver particles are located in the outer regions of the fibers and separate at high levels of burn-off, a higher concentration of silver would have little, if any, effect. Fig. 5 shows the relationship between specific surface area and burn-off. The specific surface areas of all three ACFs linearly increased with increasing burn-off (a trend similar to that reported by Kimber et al. [11]). The specific surface area of the 1000 ppm silver-containing ACFs was higher (1920 m 2 / g at 63% burn-off) than that of the non-silver containing ACFs at the same percent burn-off. However, the specific surface area of the 10 000 ppm silver-containing ACFs was somewhat lower than that of non-silver containing ACFs. By comparison, Oya reported
Fig. 4. Silver content of ACF-S2 at various degrees of burn-off.
Fig. 5. Specific surface area of ACFs at various degrees of burn-off.
S.K. Ryu et al. / Carbon 37 (1999) 1619 – 1625
much lower specific surface areas (670–830 m 2 / g for silver / cobalt-containing ACFs [1], 600–840 m 2 / g for cobalt-containing ACFs [12], and 540–740 m 2 / g for silver-containing ACFs [9]). Evidently, cobalt and silver did not favor micropore development, but instead suppressed the development of BET surface area during catalytic steam activation. This is opposite to what might be expected because metals normally catalyze the activation process. Oya et al. [10] also reported specific surface areas for the silver-containing ACF similar to that found in our study at higher levels of burn-off (approximately 2000 m 2 / g). Apparently silver particles do have a catalytic effect, and the increase of surface area depends on the activation conditions. Perhaps by changing the activation conditions, ACFs with even larger surface areas can be obtained. The average pore diameter of non-silver containing ACF ˚ and the average pore diameters of (ACF-S0-60) was 9.7 A both silver-containing ACFs (ACF-S1-63 and ACF-S2-69) ˚ in spite of the different initial silver conwere 18.8 A, centration. Apparently, increasing the silver content of the carbon fibers from 1000 to 10 000 ppm did not increase either the specific surface area or the average pore size. This indicated that the volume fractions of meso- and macropores are similar in ACF-S1-63 and ACF-S2-69. Tests showed that the meso- and macropore volume fraction of the non-silver containing ACF was 4.2%. By comparison, the volume fractions of meso- and macropores were 13.6% for ACF-S1-63 and 14% for ACF-S2-69, respectively. Of course, the volume fraction of macropores could be negligible and the increase in average pore diameter could result from the development of mesopores in the silver-containing fibers. Nevertheless, the smaller specific surface area of ACF-S2 compared to that of ACF-S1 at similar levels of burn-off does seem rather puzzling. Sample ACF-S2 has a higher initial concentration of silver particles than sample ACF-S1; therefore, one might expect it to develop more macropores. Oya et al. [12] also reported that increasing the metal concentration did not increase the specific surface area of the activated fiber. Their explanation was that, as the cobalt concentration in the fiber increases, the fine particles coalesce more easily. The result would be fewer active particles.
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Fig. 6. Tensile strength of ACFs at various degrees of burn-off.
[11] for carbon fibers that did not contain silver; however, they observed a linear decrease in strength. Oya et al. [9] reported that the tensile strength of their silver-containing ACF was the same at 23% burn-off and at 67% burn-off (60 Mpa). They found that the tensile strength did not decrease with increasing specific surface area (540–740 m 2 / g) and silver concentration (0.29–0.63 wt%). This is surprising, given the large increase in specific surface area and the high level of burn-off reported. The change in apparent density with increasing burn-off is plotted in Fig. 7. The densities of CF-S0, CF-S1 and CF-S2 were 1.65, 1.652, and 1.677 g / cm 3 , respectively, as determined by the liquid displacement method. The apparent densities of the ACFs were calculated using the measured cross-sectional areas and estimated lengths of 1 g samples. This approach has been shown to agree closely with more direct measurements of density [5]. The density of non-silver containing CF decreased rapidly at low levels of burn-off and then more slowly as burn-off increased. The convex downward trend differs from that reported by Kimber et al. [11]. They found that the apparent density of
3.2. Physical properties The effect of activation on the final tensile strength of the ACFs is shown in Fig. 6. The tensile strength of carbon fibers decreased significantly (at all levels of activation) as the initial silver-content of the fibers increased. The tensile strength of carbon fibers with an initial silver content of 10 000 ppm (1wt%) was about one third that of the non-silver containing carbon fibers. The tensile strength of all ACFs decreased rapidly during the initial stages of burn-off and decreased more slowly as the burn-off increased. Similar results were reported by Kimber et al.
Fig. 7. Apparent density of ACFs at various degrees of burn-off.
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the ACF decreased linearly with increasing levels of burnoff. However, Kimber reported that the initial density decreased by one-half at 64% burn-off, which was very similar to our result. The apparent density of ACF produced in the current work is rather high, considering the large pore volume within the fiber. The ACFs that did not contain silver particles had lower apparent densities than the silver-containing ACFs at burn-off levels of less than 70. Perhaps the silver particles contributed to the high initial density, and these particles did not separate during the early stage of burn-off. At 70% burn-off the densities of silver-containing ACFs were similar to that of the non-silver containing ACF-S0. Then, the apparent density of the silver-containing ACFs decreases rapidly at burn-off levels above 80%. However, the apparent densities of the 1000 ppm and the 10 000 ppm silver-containing ACFs were similar at equivalent levels of burn-off, in spite of their different initial concentrations of silver and their different specific surface areas. Perhaps the higher concentration of silver created more mesopores, reducing the density of ACF-S2 to that of ACF-S1. This also would indicate that few silver particles remained in the ACF-S1 and ACF-S2 samples at high levels of burn-off. Evidently, the silver particles didn’t migrate to the fiber core, but instead separated from the fiber surface. The change in diameter of the CFs with respect to burn-off is plotted in Fig. 8. The diameter of non-silver containing carbon fibers increased by about 3.2% during the early stage of burn-off and then slowly decreased after about 30% burn-off. This swelling of isotropic pitch-based carbon fibers during the initial stage of activation was also observed in the previous work [5]. Similar trends were reported during the steam activation of phenolic resinbased carbon fibers by Gondy and Ehrburger [13]. However, we found that the swelling of the pitch-based carbon fibers occurs during the initial stage of activation, while Gondy and Ehrburger reported that swelling of phenolic resin-based carbon fibers occurred in the middle stage of
Fig. 8. Diameter of ACFs at various degrees of burn-off.
activation. Gondy and Ehrburger did not observe swelling when their pitch-based carbon fibers were subjected to steam activation, and they could not detect swelling when their phenolic resin-based carbon fibers were subjected to CO 2 activation. They postulated that a transient uptake of oxygen might occur during the CO 2 activation of isotropic pitch-based carbon fibers, and perhaps this does not occur during steam activation. We suspect that the reaction gases are merely produced faster than they can escape during the early stage of activation and this causes fiber swelling. The diameter of silver-containing carbon fibers decreased from the very beginning of activation, but the diameter then became relatively constant. This indicates that the silver particles accelerate the burn-off of the surrounding carbon, creating macropores and allowing the product gases to exit more easily. However, the constant fiber diameter at high levels of burn-off may also indicate that many micropores are being created in the fibers. The fiber diameter decreased rapidly above 80% burn-off. Fig. 9 shows the relationship between the electrical resistivity and the specific surface area of activated silvercontaining carbon fibers and activated carbon fibers that did not contain silver. The initial electrical resistivity of isotropic pitch-based CF that did not contain silver was 97 mV m (the standard deviation for the 16 samples tested was 10.6). This electrical resistivity is 3.2 times higher than the 30 mV m reported for Ashland isotropic pitchbased carbon fibers [14] and 4.4 times higher than the 22 mV m reported for Torayca T-300 PAN-based carbon fibers [15]. The electrical resistivity of the initial fibers (prior to activation) was decreased to 69 and 57 mV m, by addition of silver at levels of 1000 and 10 000 ppm, respectively. This was because of the high electrical conductivity of silver. These resistivities increased exponentially as the specific surface areas of fibers increased. This rapid increase seems logical because the electrical resistivity depends not only on specific surface area but
Fig. 9. Electrical resistivity of ACFs with respect to specific surface area.
S.K. Ryu et al. / Carbon 37 (1999) 1619 – 1625
also on the porosity of the fibers. The micropore size becomes slightly larger with increasing levels of burn-off. In the early stage of steam activation, only narrow micropores are developed, resulting in high specific surface areas. As burn-off increases some micropores coalesce, creating wider micropores or mesopores that reduce the electrical conductivity. However, Kimber et al. [11] reported that the electrical conductivity rapidly decreased at relatively small levels of weight loss and then slowly decreased at higher levels of burn-off. One explanation for this different trend might be that their pore size distribution was not the same as ours. Their ACFs could have contained more pores than ours at high levels of burn-off because their increase of specific surface area with burn-off was very similar to ours. The silver-containing ACFs exhibited lower resistivities than the nonsilver containing ACFs. On the other hand, the resistivities of both silver-containing ACFs were similar at equal specific surface areas in spite of the difference in silver concentration. However, if one plots the electrical resistivity versus burn-off, ACF-S1 has a higher resistivity than ACF-S2 at the same burn-off because the sample has higher specific surface area. We have observed that different carbon fibers require a different level of burn-off to obtain the same specific surface area. Thus, the porosity of ACF-S1 might be different from that of ACF-S2 at the same specific surface area. The similar electrical resistivities of both silver-containing ACFs at the equivalent specific surface areas indicates that either total pore volume is a more important factor than the pore size distribution or that both ACFs have similar microporosities. Actually, the average pore sizes of both silver-containing ACFs were very similar at the equivalent levels of burn-off, in spite of the difference in the initial silver concentration.
4. Conclusions Silver particles in pitch-based carbon fibers accelerated the rate of activation, created macropores, and increased the average pore diameter. The silver particles did not migrate toward the fiber core; rather they separated from the fiber surface, some coalesced particles created caves with increasing the activation. Prior to activation, it appeared that the majority of silver particles were located in the outer regions of the fibers. Because of this, the loss
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of silver particles was greater than the loss of carbon during activation. If the precursor pitch were prepared well and the distribution of silver particles were more homogeneous, one would expect that the specific surface area and the average pore diameter of the activated carbon fiber would increase as the silver concentration of the initial fiber increased. Increasing the initial concentration of silver decreased the final tensile strength of carbon fiber but had little affect on the apparent density during steam activation. Even though, the electrical resistivities of carbon fibers increased exponentially as their specific surfaces increased, the initial concentration of silver appeared to have a minor affect. Based on these results, metal-containing carbon fibers can be activated, yielding carbon adsorbents with controlled pore volumes, adequate physical properties, and acceptable electrical resistivities.
References [1] Oya A, Yoshida S, Alcaniz-Monge J, Linares-Solano A. Carbon 1996;34(1):53–7. [2] Ozaki J, Endo N, Ohizumi W et al. Carbon 1997;35(7):1031–3. [3] Ryu SK, Ko KR, Rhee BS, Ehrburger P. In: Extended abstracts, 21st biennial conf. on carbon. SU of New York, Buffalo (New York. USA), American Carbon Society, 1993, pp. 623–4. [4] Rodriguez-Reinoso F, Martin-Martinez JM, Prado-Burguete B, McEnaney B. J Phys Chem 1987;91:515–6. [5] Ryu SK, Ko KR, Gondy D, Ehrburger P. Carbon 1996;34(6):808–10. [6] Edie DD, Fain CC, Robinson KE, Harper AM, Rogers DK. Carbon 1993;31(6):941–9. [7] Coleman LB. Rev Sci Inst 1975;46:1125–9. [8] Yoshida A. Tanso 1990;141:56–69. [9] Oya A, Wakahara T, Yoshida S. Carbon 1993;31(8):1243–7. [10] Oya A, Yoshida S, Abe Y, Iizuka T, Makiyama N. Carbon 1993;31(1):71–3. [11] Kimber GM, Vego A, Rantell TD. In: Extended abstracts, 23rd biennial conf. on carbon. Penn State U, University Park (Pennsylvania USA), American Carbon Society, 1997, pp. 454–5. [12] Oya A, Yoshida S, Alcaniz-Monge J, Linares-Solano A. Carbon 1995;33(8):1085–90. [13] Gondy D, Ehrburger P. Carbon 1997;35:1745–9. [14] Frysz CA, Shui AP, Chung DDL. Carbon 1997;35(7):893– 916. [15] Wang XJ, Chung DDL. Carbon 1997;35(5):706–9.