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Structure evolution and optimization in the fabrication of PVA-based activated carbon fibers
Journal of Colloid and Interface Science 321 (2008) 96–102 www.elsevier.com/locate/jcis
Structure evolution and optimization in the fabrication of PVA-based activated carbon fibers Shu-Juan Zhang, Hui-Min Feng, Jian-Ping Wang, Han-Qing Yu ∗ Department of Chemistry, University of Science & Technology of China, Hefei 230026, China Received 14 October 2007; accepted 7 January 2008 Available online 7 February 2008
Abstract The structure and composition evolution of polyvinyl alcohol (PVA) fibers during the fabrication of activated carbon fibers (ACF) by a newly developed method were systematically elucidated. The pore structure of the fibers was significantly influenced by the carbonization and activation conditions. The elemental composition and chemical structure evolution of the fibers during the heat treatment processes were evaluated by elemental analysis, Fourier transform infrared spectrophotometry (FTIR), and X-ray photoelectron spectroscopy (XPS). Crystal structure evolution of the fibers during the heat treatment processes was elucidated by X-ray diffraction (XRD) analysis. Based on these understandings, the process conditions were optimized using an L9 (3)4 orthogonal array design matrix. Appropriate process parameters for the fabrication of PVA-ACFs were established as carbonizing the dehydrated fiber at 300 ◦ C for 60 min, and then lifting the temperature to 900 ◦ C with a heating speed of 10 ◦ C/min in an inert atmosphere, thereafter keeping the fiber at 900 ◦ C for 60 min in an oxidizing atmosphere. © 2008 Elsevier Inc. All rights reserved. Keywords: Activated carbon fiber; Fabrication; Optimization; Structure
1. Introduction As a type of effective carbonaceous adsorbents, activated carbon fibers (ACFs) have attracted continuing attention because of their several outstanding merits over conventional activated carbon. ACFs have higher adsorption/desorption rates because of their smaller diffusion limitations, excellent adsorption capacities at low concentrations of adsorbates, and outstanding flexibility in fabricating a wide variety of textile forms, such as cloth, paper, and felt, for versatile applications owing to their good mechanical properties [1]. However, the high cost of ACFs at the present time limits their application as special adsorbents and catalysts or catalyst supports for gas or liquid separations and reactions for environmental pollution control. Therefore, to endow ACFs with more particular functions is essential for their practical development. Both pore structure and surface chemistry are key characters governing the properties of ACFs [2]. Through judicious selection of a precursor and * Corresponding author. Fax: +86 551 3601592.
E-mail address: [email protected] (H.-Q. Yu). 0021-9797/$ – see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2008.01.012
careful control of both carbonization and activation steps, it is possible to tailor ACFs for particular applications [3]. A novel method for the fabrication of ACFs from a low-cost and high-strength precursor, polyvinyl alcohol (PVA) fiber, has been developed in our previous work [4]. Through a combination of preoxidation, dehydration, carbonization, and activation under a certain tension, PVA-based ACFs have been obtained with yields of 8–37%. The fabrication method successfully overcame the drawbacks of the other methods, producing PVAACFs with high yields and good mechanical properties. The main objective of the present work was to elucidate the structure and composition evolution of the fibers during the fabrication process, and consequently to establish a set of optimized process parameters for the fabrication of PVA-ACFs. 2. Experimental 2.1. Materials Continuous filament yarn of wet spinning PVA fiber (RF), purchased from Hunan Xiangwei Co., China, was used as the precursor. The RF has a softening point around 215–
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2.3. Analytical approaches 2.3.1. Pore structure analysis Nitrogen adsorption measurements were performed at −196 ◦ C with a volumetric adsorption analyzer (OMNISORP 100 CX, Coulter, USA) at relative pressures of 10−6 to 1. All samples were degassed for 3 h at 300 ◦ C prior to the vacuum volumetric analyses. The adsorption isotherm results were used for the analysis of surface area and pore structure. 2.3.2. Chemical structure analysis Fourier transform infrared spectrophotometry (FTIR) was employed to characterize the functional groups of fiber samples with a Magna IR 550 spectrometer (Nicolet Instrument Co., USA) using a potassium bromide disc technique. The elemental composition in fibers was determined with an elemental analyzer (Elementar Vario ELIII, Analysensysteme GmbH, Germany). The N, C, and H contents were determined directly, while the O content was calculated from the difference between unity and the percentage sum of N, C, and H contents. The surface components of the fiber samples were analyzed by X-ray photoelectron spectroscopy (XPS) with an ESCALAB MK II spectrometer (VG Instrument Group LTD., UK) using MgKα radiation (E = 1253.6 eV). The intensity of the XPS peak was recorded as counts per second (cps). A nonlinear, Shirley-type baseline and an iterative least-squares fitting algorithm with a Gaussian–Lorentzian sum function were used to deconvolve the XPS peaks. The C1s electron binding energy corresponding to graphitic carbon was set at 284.6 eV and used as a reference to position the other peaks. Fig. 1. Schematic procedure for the fabrication of PVA-ACF.
224 ◦ C. The salt residue from the coagulation bath in RF was about 15%. Diammonium phosphate (DAP) and ammonium sulfate (AS), purchased from Shanghai Chemical Reagent Co., China, were used as dehydrating agents. 2.2. Sample preparation The fabrication procedure for PVA-ACF is shown schematically in Fig. 1. Detailed information has been given in the previous paper [4]. To evaluate the influence of preparation conditions, a set of fibers was denoted as “carbonization time in unit of min–activation temperature in unit of ◦ C–activation time in unit of min,” e.g., 60–900–60, which indicates that the fibers were carbonized in air at 300 ◦ C for 60 min and then activated at 900 ◦ C for 60 min with carbon dioxide as the activating agent. Unless otherwise stated, the heating speed was 10 ◦ C/min, the carbonization time was 60 min, and the ACF reported without a sample code in this work is 60–900–60. CF500 and CF800 were the CF fibers that were heat-treated under nitrogen gas protection to 500 and 800 ◦ C, respectively. Prior to analysis, all fiber specimens were refluxed with acetone for 12 h and were then washed thoroughly with double-distilled water and dried in an oven for 24 h under vacuum at 120 ◦ C.
2.3.3. Crystal structure analysis The X-ray diffraction (XRD) patterns of fiber specimens were measured with an X’Pert PRO SUPER X-ray diffractometer (Philips Co., The Netherlands) equipped with a rotation anode using CuKα radiation (λ = 0.154187 nm) as the source. 2.3.4. Liquid phase adsorption analysis PVA-ACFs of about 0.03 g were immersed in 50 ml of 0.1 g/L methylene blue (MB) solution and the mixtures were shaken at 25 ◦ C for 24 h. The concentrations of MB solutions before and after adsorption were determined spectrophotometrically at 665 nm. The quantity of adsorption was calculated from the change of MB concentration. An I2 –KI aqueous solution with an I2 concentration of 0.02 M was prepared. PVA-ACFs of about 0.05 g were immersed in 50 ml of the solution and the mixtures were sealed in iodine flasks and shaken at 25 ◦ C for 24 h in the dark. The amount of adsorbed I2 was obtained by Na2 S2 O3 titration with a starch indicator. 3. Results and discussion 3.1. Pore structure evolution To understand the evolution of pore structures of fibers during the fabrication process, fibers prepared under different con-
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Fig. 2. Nitrogen adsorption isotherms of fibers prepared under various conditions. Left: linear X-axis, right: logarithmic X-axis.
ditions were subjected to the determination of low-temperature nitrogen adsorption. Fig. 2 shows their nitrogen adsorption isotherms. According to the Brunauer–Deming–Deming–Teller (BDDT) classification, the adsorption isotherm of 60–800–0 was of type II, originating from typically nonporous solids, whereas the one of 60–900–60 was of typical type I, which is indicative of the domination of micropores. The adsorption isotherm of 60–800–30 appeared to be of a mixture type of I and II, while that of 30–900–120 was a mixture type of I and IV. The isotherm of type IV originates from mesoporous materials. Therefore, it could be inferred that micropores were gradually formed with the activation process and that some of them were simultaneously enlarged to mesopores. The contribution of mesopores in 30–900–120 was clearly reflected by the upward swing of the isotherm on a logarithmic scale of relative pressure (Fig. 2). The same isotherms expressed on a logarithmic scale of relative pressure show the adsorption behavior at superlow pressures more clearly. There was a significant delay in the adsorption of 60–800–0, whereas the adsorption of 60–900–60 and 30–900–120 increased steeply at low relative pressures. These results demonstrate that the carbonization and activation conditions had a significant effect on the isotherms of PVA-ACFs. Under identical conditions, activating agents had no significant effect on the isotherms of PVA-ACFs [4]. The specific surface areas of PVA-ACFs calculated with the standard BET method [3] are listed in Table 1. The yield of PVA-ACF was an indication of the degree of activation. As the degree of activation increased, the ACF yield decreased with an increase in BET specific surface area (SBET ). The upper limit of the surface area of activated carbon was found to be 2630 m2 /g, which originates from carbon in the form of a graphene sheet structure with a single atom thickness [5]. This structure is brittle, leading to poor mechanical properties. Therefore, the PVA-ACFs with yields of 25–33% were considered to be welldeveloped. The ones with a higher yield, such as 60–800–30, were not activated sufficiently, whereas the ones with a lower yield, e.g., 30–900–120, were over-activated. The pore size distributions (PSDs) of the fibers obtained by means of DA [6] and BJH [7] methods are shown in Fig. 3. Interesting results were found in the PSD plots of the fibers with
Table 1 Surface and porous structure parameters of PVA-ACFs Sample
Yield (%)
SBET (m2 /g)
Hmic (nm)
Hmes (nm)
Vmic (ml/g)
Vt (ml/g)
D
60–800–0 60–800–30 60–900–60 30–900–120
39.6 36.9 25.3 8.7
27 251 995 2128
2.34 1.06 0.71 1.15
46.1 5.56 3.91 3.98
0.014 0.124 0.398 0.761
0.014 0.133 0.424 0.901
– 2.47 2.74 2.47
Note. Hmic and Hmes : modal micropore and mesopore diameter; Vmic : micropore volume; Vt : total pore volume; D: fractal dimension.
Fig. 3. PSD plots of PVA-ACFs prepared under various conditions.
various activation degrees. In a certain range, a lower activation degree results in a larger modal diameter of the micropores. This is attributed to the different relative contributions of micropores and mesopores/macropores in the samples. The direct observation of the fibers via SEM and indirect analysis from nitrogen adsorption isotherms has demonstrated that mesopores and macropores appeared in the preoxidation process, earlier than the formation of micropores, which were developed during the activation process [4]. Therefore, the fiber with a lower activation degree had a statistically larger pore diameter. The
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modal pore diameter of 60–900–60 was 0.71 nm in the micropore range and that of the mesopore range was around 4.0 nm. This structure ensured a high adsorption capacity, attributed to the high adsorption potentials in ultramicropores. Meanwhile, the presence of mesopores/macropores facilitated the transport of adsorbates in adsorption and provided adsorption sites for larger molecules. Therefore, the PVA-ACF with this pore structure was especially suitable for application in liquid adsorption. The fiber obtained with a further increased activation degree, e.g., 30–900–120, showed a larger modal pore diameter than the well-developed one, 60–900–60. However, the modal mesopore diameter of 30–900–120 was still about 4.0 nm, implying that 4.0 nm is a preponderant size in the mesopore development of PVA-ACFs. The fractal theory developed by Mandelbrot provides a systematic approach to the characterization of many natural and engineered systems that have no definite form or regularity [8,9]. A scale-independent parameter named surface fractal dimension (Ds ) is widely used for quantifying the degree of surface irregularity. The values of Ds lie in the range 2–3 [9–11]. A perfectly regular and smooth surface possesses a Ds of 2, while a higher Ds suggests a more irregular and space-filling surface [9]. The fractal version of the classical Frenkel–Halsey– Hill (FHH) equation for multilayer adsorption was employed to evaluate the surface fractal dimension from the single nitrogen adsorption isotherm [10,11] 1 P0 ln θ = − ln ln (1) + C, q P where θ represents the fraction of surface coverage; P and P0 are the equilibrium and saturation pressures of the adsorbate; C is the intercept; and the parameter q is related to the surface dimension of the sample. The determination of surface fractal dimension, Ds , requires knowledge of the adsorption regime. The magnitude of the parameter 1/q distinguishes two possible regimes. If the van der Waals attraction between the solid and adsorbed film, which tends to make the gas/film interface to replicate the surface roughness, is dominant, the Ds value can be calculated with the following equation: 1 . Ds = 3 1 − (2) q Conversely, if the liquid/gas surface tension (capillary force), which tends to move the interface away from the surface to reduce the interface area, is more important, the Ds value can be estimated from the equation below: Ds = 3 −
1 . q
(3)
No reasonable Ds value was obtained for 60–800–0, suggesting that the FHH equation was not appropriate for evaluating the fractal geometry of 60–800–0. Reasonable Ds values were obtained for all the PVA-ACFs using Eq. (3) (listed in Table 1), indicating that the capillary force dominated in the interaction between nitrogen and PVA-ACFs. As shown in Table 1, the Ds value of 60–900–60 was the highest among the
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Fig. 4. FTIR spectra of fibers sampled from different heat treatment stages.
ACFs with various degrees of activation. The Ds values, in addition to the PSD results, provide further support for the conclusion that 60–900–60 was well developed. 3.2. Chemical structure evolution FTIR analysis is a useful method for comparing qualitatively either vibrating absorption spectra of fibers or relative intensities of the respective bands. Fig. 4 shows the FTIR spectra of the fibers sampled from different heat treatment stages. The precursor fiber, RF, had a strong O–H stretching vibration band at 3440–3420 cm−1 , a C–H stretching vibration band at 1096 cm−1 , a CH2 bending vibration band at 1423 cm−1 , C–H and O–H bending vibration bands at 1326 cm−1 , strong C–O stretching vibration bands at 1144 and 1096 cm−1 , and skeletal vibration bands at 916 and 849 cm−1 . With the process of heat treatment, all the transmission bands were gradually reduced, indicating the loss of O–H and C–O groups and the destruction of the skeletal structure of PVA. In the spectrum of DF, the bands at 1707–1712 and 1624–1652 cm−1 were the strongest ones, and the former was newly formed. The two bands came from the stretching vibration of C=O and that of C=C, respectively, which were an indication of ketones and/or enols [12]. These evolutions confirm the dehydration in preoxidation and dehydration processes. With further heat treatment, the bands were rapidly reduced until few bands were observable. The PVA-ACFs had three weak bands at 3438, 1550, and 1083 cm−1 [4]. The band at 3438 cm−1 was typically from the absorption originating from O–H or N–H and that at 1083 cm−1 was from C–O single bonds, such as those in ethers, phenols, lactones, and hydroxyl groups. The band around 1550 cm−1 has been observed by many researchers and has not been interpreted unequivocally [13,14]. It might be the oxygen double bond conjugated with the carbon basal planes [15].
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Fig. 5. XPS C1s spectra of fibers sampled from different heat treatment stages. Table 2 The contents of elements in fibers evaluated by elemental composition (EC) and XPS analyses Sample RF PF DF CF500 CF800 ACF
wt% of element (EC)
at% of element (XPS)
C
O
N
H
C1s
O1s
N1s
Burn-off (wt%)
49.37 56.20 63.84 80.09 85.14 79.64
41.58 35.56 30.42 14.63 11.91 17.08
0 0 0.89 1.79 0.87 0.99
9.05 8.24 4.85 3.49 2.08 2.29
50.40 69.26 72.75 86.95 85.15 79.64
49.60 28.78 20.49 9.66 13.24 17.73
0 1.95 6.76 3.39 1.61 2.63
0 15.6 37.3 55.9 60.9 73.6
Table 2 lists the contents of elements in the fibers evaluated with elemental analysis. Contrary to the change of O and H contents, the C content steadily increased with heat treatment until activation. The evolution of elemental composition had a good correlation with the burn-off, suggesting that the dominant change in the fibers during heat treatment was the loss of noncarbon elements, O and H, and that the activation treatment endowed the fibers with oxygen-containing groups. XPS determination provides valuable information for different functional groups formed at the fiber surface. The contents of elements obtained from XPS survey spectra are also summarized in Table 2. They were consistent with those obtained from the elemental analysis. Elemental analysis is a kind of determination in the bulk phase, whereas XPS is a surface one with a detection depth within around 4 nm. The consistency of the two kinds of analytical results suggests that the PVA-ACF is essentially a surface solid with over half of the atoms on the surface [16]. To clarify the functional groups and their relative contents, deconvolution procedures were performed on the C1s spectra with a Gaussian–Lorentzian sum function. Fig. 5 shows the C1s spectra of the fibers and their deconvolved peaks. The C1s signals of all the fibers consisted of a major peak at 284.6 eV, which corresponded to nonfunctionalized C, i.e., the contributions of C (Csp2 and Csp3 ) belonging to the carbon skeleton
Table 3 The relative contents of C1s existed in different forms evaluated by XPS Sample RF PF DF CF500 CF800 ACF
at% of element C–C(H)
C–O
77.3 71.7 72.8 81.7 81.1 62.3
22.7 22.0 19.9 16.2 16.3 25.8
C=O
COO
6.3 7.3
8.0
2.0 2.6 3.9
of the material and the contributions of aliphatic Csp3 , which corresponded to hydrocarbons [15]. The relative intensity of the peak at 284.6 eV increased gradually upon heat treatment, but then decreased with the process of activation, implying that some carbon atoms bonded to hydroxyl groups were removed and some of the carbon atoms were functionalized through the creation of carbon–oxygen bonds. More information could be obtained from the satellites of the C1s peak at higher binding energies [17]. The shifts from the main peak ranged from 1.5 eV for carbon atoms singly bonded to oxygen to 4 eV for carbon atoms in carboxyl groups or esters [18]. As illustrated in Fig. 5, a curve fitting with the defined binding energy and peak width show the decrease in the C–O satellite at 286.1 eV and the increase in the C=O satellite at 287.5 eV as a consequence of heat treatment. The decrease in the C1s satellite at around 286.1 eV was attributed to the removal of –OH through dehydration. This is in agreement with the FTIR results. After activation, a new satellite at around 288.6 eV appeared. Meanwhile, the relative intensity of the nonfunctionalized carbon decreased, whereas those at 286.1 and 287.5 eV increased, suggesting the formation of ether/phenol/alcohol (286.1 eV), carbonyl/quinine (287.5 eV), and carboxyl/lactone (288.6 eV) through the functionalization of carbon by means of the creation of carbon–oxygen bonds. The relative contents of these groups are listed in Table 3. The effects of these surface func-
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Fig. 6. XRD patterns of fibers sampled from different heat treatment stages.
tional groups on adsorption processes are our next research destination.
Table 4 Levels and values of process parameters for steam activation experiment Level
Factor A heating speed (◦ C/min)
Factor B carbonization time (min)
Factor C activation temperature (◦ C)
Factor D activation time (min)
1 2 3
2 5 10
0 30 60
800 900 1000
30 60 90
3.3. Crystal structure evolution As shown in Fig. 6, the precursor PVA fiber was highly crystalline. A considerable amount of investigations have demonstrated that the PSDs of ACFs are closely associated with the crystalline characters of the precursors. Highly crystalline precursors, e.g., aramid fibers, lead to microporous ACFs with narrow pore size distributions, whereas low-crystallinity precursors, such as rayon, result in ACFs with a wider pore size distribution [19–22]. The PSD analytical results provide support for this conclusion. Upon heat treatment, as shown in Fig. 6, the crystal structure of fibers completely disappeared until new crystallites were formed after activation. The PVA-ACFs prepared in this work had two broad peaks at 2θ = 22.4–24.0◦ and 43.7–44.2◦ , corresponding to the 002 and 10 reflections of disordered micrographite stacking, respectively [23]. The crystal size decreased with the increase in activation degree [4]. The small crystal size and short-range ordering between crystallites provide the prerequisites for the realization of a high specific surface area [24]. 3.4. Optimization of the fabrication process As found in our previous work [4], the stabilization process was the decisive step in the preparation of PVA-ACFs, which determined the final structures of fibers. In addition, the effects of activating agents on the texture and surface chemistry of PVA-ACF have proven to be insignificant. In the elucidation
of pore structure evolution, both carbonization and activation conditions have been found to be crucial for the final texture of PVA-ACFs. To understand the relative importance of each factor, it is often desirable to run a large number of screening experiments with a conventional “change-one-factor-at-a-time” method. Therefore, an L9 (3)4 orthogonal array design matrix was employed to evaluate the effects of the four factors, i.e., carbonization time, heating speed during activation process, activation temperature, and activation time, on the adsorption capacities and yields of PVA-ACFs. Orthogonal experimental design is an effective mathematical method for multifactor experiments. With it the individual effect of each among several factors can be found with fewer runs than in conventional designs [25]. Taking a suite of steam activation experiments as an example, the levels of the process parameters for orthogonal design are listed in Table 4. The assignment of experiments and the ranges obtained from intuitionistic analysis are summarized in Table 5. Based on the range analysis, carbonization time and activation temperature were judged as the most important factors
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Table 5 Assignment of factors and levels for fabrication of PVA-ACF using L9 (34 ) matrix and the range analysis Trial No.
Level of factor
Virtual parameter
Response
A
B
C
D
A
B
C
D
Yield (%)
QMB (mg/g)
QI2 (mg/g)
1 2 3 4 5 6 7 8 9
1 1 1 2 2 2 3 3 3
1 2 3 1 2 3 1 2 3
1 2 3 2 3 1 3 1 2
1 2 3 3 1 2 2 3 1
2 2 2 5 5 5 10 10 10
0 30 60 0 30 60 0 30 60
800 900 1000 900 1000 800 1000 800 900
30 60 90 90 30 60 60 90 30
35.1 27.0 10.9 27.0 22.9 24.3 25.2 26.9 20.0
5 287 334 28 113 70 159 90 207
806 1746 1952 1057 1176 1112 1530 1098 1230
Range Yield QMB QI2
0.7 138 386
10.7 139 300
9.1 147 547
4.4 64 392
determining the yields of the final ACFs, whereas activation temperature was the key factor governing the adsorption capacities of ACFs for MB and I2 . Since the molecular size of MB is larger than that of I2 , the MB adsorption capacity (QMB ) is indicative of larger pores, and that of I2 (QI2 ) indicates the volume of micropores. A high ACF yield generally corresponds to a low adsorption capacity. Thus, a compromise among adsorption capacity, yield, and energy consumption was needed for optimization. From the range values in Table 5, it was confirmed that the heating speed had little effect on either ACF yields or adsorption capacities. For the sake of saving preparation time, a heating speed of 10 ◦ C/min was selected. With a balanced consideration for ACF yields and adsorption capacities, appropriate carbonization and activation parameters for the fabrication of PVA-ACF were established as carbonizing the dehydrated fiber at 300 ◦ C for 60 min, then lifting the temperature to 900 ◦ C in an inert atmosphere, and thereafter dwelling at 900 ◦ C for 60 min in an oxidizing atmosphere. 4. Conclusions The pore structure, chemical structure, and crystal structure evolutions during the fabrication of PVA-ACFs by the newly developed method were systematically elucidated. Both carbonization and activation conditions were found to have significant effects on the development of the pore structure. Based on these understandings, the process parameters for the fabrication of PVA-ACF were optimized by using an L9 (3)4 orthogonal array design matrix. Appropriate carbonization and activation parameters were established. The combination of both physical texture and chemical structure of porous carbonaceous adsorbents governs the adsorption of adsorbates on them. The former controls the physical access and the contact between an adsorbate molecule and a surface for adsorption, whereas the chemical interactions between adsorbate molecules and the adsorbent surface regulate the overall extent of adsorption once the adsorption sites are accessed. The elucidation of the structure evolution in the preparation of PVA-ACFs might help us to untangle the complex web of interactions and explore the roles of physical factors
and chemical interactions involved in the adsorption of various adsorbates by those PVA-ACFs. Acknowledgment The authors thank the Ministry of Science and Technology of China for financial support through the Hi-Tech Research and Development Program (863 Project, No. 2004AA649300). References [1] M. Suzuki, Carbon 32 (1994) 577. [2] S.J. Park, K.D. Kim, J. Colloid Interface Sci. 212 (1999) 186. [3] J. Choma, M.W. Burakiewicz, M. Jaroniec, Z. Li, J. Klinik, J. Colloid Interface Sci. 212 (1999) 438. [4] S.J. Zhang, H.Q. Yu, H.M. Feng, Carbon 44 (2006) 2059. [5] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1982. [6] M.M. Dubinin, V.A. Astakhov, Adv. Chem. Ser. 102 (1971) 69. [7] E.P. Barret, L.G. Joyner, P.P. Hallenda, J. Am. Chem. Soc. 73 (1951) 373. [8] B.B. Mandelbrot, The Fractal Geometry of Nature, W.H. Freeman and Co., New York, 1983. [9] Q.F. Hou, X.C. Lu, X.D. Liu, B.X. Hu, J.Q. Cui, J. Shen, Surf. Coat. Technol. 190 (2005) 394. [10] P. Pfeifer, J. Wu, M.W. Cole, J. Krim, Phys. Rev. Lett. 62 (1989) 1997. [11] P. Pfeifer, M.W. Cole, New J. Chem. 14 (1990) 221. [12] S.J. Zhang, H.Q. Yu, Water Res. 38 (2004) 309. [13] C.L. Mangun, K.R. Benak, M.A. Daley, J. Economy, Chem. Mater. 11 (1999) 3476. [14] Z.Y. Ryu, H.Q. Rong, J.T. Zheng, M.Z. Wang, B.J. Zhang, Carbon 40 (2002) 1144. [15] G. de la Puente, J.J. Pis, J.A. Menéndez, P. Grange, J. Anal. Appl. Pyrol. 43 (1997) 125. [16] K. Kaneko, Solid State Phys. 27 (1992) 1. [17] J.B. Tomlinson, J.J. Freeman, K.S.W. Sing, Carbon 33 (1995) 789. [18] T.T.P. Cheung, J. Appl. Phys. 53 (1982) 6857. [19] J.B. Tomlinson, J.J. Freeman, K.S.W. Sing, Carbon 33 (1995) 789. [20] J.J. Freeman, J.B. Tomlinson, K.S.W. Sing, Carbon 33 (1995) 795. [21] F. Stoeckli, T.A. Centeno, A.B. Fuertes, J. Muñiz, Carbon 34 (1996) 1201. [22] M.C. Blanco López, S.M. Villar-Rodil, A. Martínez-Alonso, J.M.D. Tascón, Micropor. Mesopor. Mater. 41 (2000) 319. [23] M.A. Short, P.L. Walker Jr., Carbon 1 (1963) 3. [24] K. Kaneko, C. Ishii, M. Ruike, H. Kuwabara, Carbon 30 (1992) 1075. [25] G.E.P. Box, W.G. Humter, J.S. Hunter, Statistics for Experiments, John Wiley & Sons, New York, 1978.
Structure evolution and optimization in the fabrication of PVA-based activated carbon fibers Shu-Juan Zhang, Hui-Min Feng, Jian-Ping Wang, Han-Qing Yu ∗ Department of Chemistry, University of Science & Technology of China, Hefei 230026, China Received 14 October 2007; accepted 7 January 2008 Available online 7 February 2008
Abstract The structure and composition evolution of polyvinyl alcohol (PVA) fibers during the fabrication of activated carbon fibers (ACF) by a newly developed method were systematically elucidated. The pore structure of the fibers was significantly influenced by the carbonization and activation conditions. The elemental composition and chemical structure evolution of the fibers during the heat treatment processes were evaluated by elemental analysis, Fourier transform infrared spectrophotometry (FTIR), and X-ray photoelectron spectroscopy (XPS). Crystal structure evolution of the fibers during the heat treatment processes was elucidated by X-ray diffraction (XRD) analysis. Based on these understandings, the process conditions were optimized using an L9 (3)4 orthogonal array design matrix. Appropriate process parameters for the fabrication of PVA-ACFs were established as carbonizing the dehydrated fiber at 300 ◦ C for 60 min, and then lifting the temperature to 900 ◦ C with a heating speed of 10 ◦ C/min in an inert atmosphere, thereafter keeping the fiber at 900 ◦ C for 60 min in an oxidizing atmosphere. © 2008 Elsevier Inc. All rights reserved. Keywords: Activated carbon fiber; Fabrication; Optimization; Structure
1. Introduction As a type of effective carbonaceous adsorbents, activated carbon fibers (ACFs) have attracted continuing attention because of their several outstanding merits over conventional activated carbon. ACFs have higher adsorption/desorption rates because of their smaller diffusion limitations, excellent adsorption capacities at low concentrations of adsorbates, and outstanding flexibility in fabricating a wide variety of textile forms, such as cloth, paper, and felt, for versatile applications owing to their good mechanical properties [1]. However, the high cost of ACFs at the present time limits their application as special adsorbents and catalysts or catalyst supports for gas or liquid separations and reactions for environmental pollution control. Therefore, to endow ACFs with more particular functions is essential for their practical development. Both pore structure and surface chemistry are key characters governing the properties of ACFs [2]. Through judicious selection of a precursor and * Corresponding author. Fax: +86 551 3601592.
E-mail address: [email protected] (H.-Q. Yu). 0021-9797/$ – see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2008.01.012
careful control of both carbonization and activation steps, it is possible to tailor ACFs for particular applications [3]. A novel method for the fabrication of ACFs from a low-cost and high-strength precursor, polyvinyl alcohol (PVA) fiber, has been developed in our previous work [4]. Through a combination of preoxidation, dehydration, carbonization, and activation under a certain tension, PVA-based ACFs have been obtained with yields of 8–37%. The fabrication method successfully overcame the drawbacks of the other methods, producing PVAACFs with high yields and good mechanical properties. The main objective of the present work was to elucidate the structure and composition evolution of the fibers during the fabrication process, and consequently to establish a set of optimized process parameters for the fabrication of PVA-ACFs. 2. Experimental 2.1. Materials Continuous filament yarn of wet spinning PVA fiber (RF), purchased from Hunan Xiangwei Co., China, was used as the precursor. The RF has a softening point around 215–
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2.3. Analytical approaches 2.3.1. Pore structure analysis Nitrogen adsorption measurements were performed at −196 ◦ C with a volumetric adsorption analyzer (OMNISORP 100 CX, Coulter, USA) at relative pressures of 10−6 to 1. All samples were degassed for 3 h at 300 ◦ C prior to the vacuum volumetric analyses. The adsorption isotherm results were used for the analysis of surface area and pore structure. 2.3.2. Chemical structure analysis Fourier transform infrared spectrophotometry (FTIR) was employed to characterize the functional groups of fiber samples with a Magna IR 550 spectrometer (Nicolet Instrument Co., USA) using a potassium bromide disc technique. The elemental composition in fibers was determined with an elemental analyzer (Elementar Vario ELIII, Analysensysteme GmbH, Germany). The N, C, and H contents were determined directly, while the O content was calculated from the difference between unity and the percentage sum of N, C, and H contents. The surface components of the fiber samples were analyzed by X-ray photoelectron spectroscopy (XPS) with an ESCALAB MK II spectrometer (VG Instrument Group LTD., UK) using MgKα radiation (E = 1253.6 eV). The intensity of the XPS peak was recorded as counts per second (cps). A nonlinear, Shirley-type baseline and an iterative least-squares fitting algorithm with a Gaussian–Lorentzian sum function were used to deconvolve the XPS peaks. The C1s electron binding energy corresponding to graphitic carbon was set at 284.6 eV and used as a reference to position the other peaks. Fig. 1. Schematic procedure for the fabrication of PVA-ACF.
224 ◦ C. The salt residue from the coagulation bath in RF was about 15%. Diammonium phosphate (DAP) and ammonium sulfate (AS), purchased from Shanghai Chemical Reagent Co., China, were used as dehydrating agents. 2.2. Sample preparation The fabrication procedure for PVA-ACF is shown schematically in Fig. 1. Detailed information has been given in the previous paper [4]. To evaluate the influence of preparation conditions, a set of fibers was denoted as “carbonization time in unit of min–activation temperature in unit of ◦ C–activation time in unit of min,” e.g., 60–900–60, which indicates that the fibers were carbonized in air at 300 ◦ C for 60 min and then activated at 900 ◦ C for 60 min with carbon dioxide as the activating agent. Unless otherwise stated, the heating speed was 10 ◦ C/min, the carbonization time was 60 min, and the ACF reported without a sample code in this work is 60–900–60. CF500 and CF800 were the CF fibers that were heat-treated under nitrogen gas protection to 500 and 800 ◦ C, respectively. Prior to analysis, all fiber specimens were refluxed with acetone for 12 h and were then washed thoroughly with double-distilled water and dried in an oven for 24 h under vacuum at 120 ◦ C.
2.3.3. Crystal structure analysis The X-ray diffraction (XRD) patterns of fiber specimens were measured with an X’Pert PRO SUPER X-ray diffractometer (Philips Co., The Netherlands) equipped with a rotation anode using CuKα radiation (λ = 0.154187 nm) as the source. 2.3.4. Liquid phase adsorption analysis PVA-ACFs of about 0.03 g were immersed in 50 ml of 0.1 g/L methylene blue (MB) solution and the mixtures were shaken at 25 ◦ C for 24 h. The concentrations of MB solutions before and after adsorption were determined spectrophotometrically at 665 nm. The quantity of adsorption was calculated from the change of MB concentration. An I2 –KI aqueous solution with an I2 concentration of 0.02 M was prepared. PVA-ACFs of about 0.05 g were immersed in 50 ml of the solution and the mixtures were sealed in iodine flasks and shaken at 25 ◦ C for 24 h in the dark. The amount of adsorbed I2 was obtained by Na2 S2 O3 titration with a starch indicator. 3. Results and discussion 3.1. Pore structure evolution To understand the evolution of pore structures of fibers during the fabrication process, fibers prepared under different con-
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Fig. 2. Nitrogen adsorption isotherms of fibers prepared under various conditions. Left: linear X-axis, right: logarithmic X-axis.
ditions were subjected to the determination of low-temperature nitrogen adsorption. Fig. 2 shows their nitrogen adsorption isotherms. According to the Brunauer–Deming–Deming–Teller (BDDT) classification, the adsorption isotherm of 60–800–0 was of type II, originating from typically nonporous solids, whereas the one of 60–900–60 was of typical type I, which is indicative of the domination of micropores. The adsorption isotherm of 60–800–30 appeared to be of a mixture type of I and II, while that of 30–900–120 was a mixture type of I and IV. The isotherm of type IV originates from mesoporous materials. Therefore, it could be inferred that micropores were gradually formed with the activation process and that some of them were simultaneously enlarged to mesopores. The contribution of mesopores in 30–900–120 was clearly reflected by the upward swing of the isotherm on a logarithmic scale of relative pressure (Fig. 2). The same isotherms expressed on a logarithmic scale of relative pressure show the adsorption behavior at superlow pressures more clearly. There was a significant delay in the adsorption of 60–800–0, whereas the adsorption of 60–900–60 and 30–900–120 increased steeply at low relative pressures. These results demonstrate that the carbonization and activation conditions had a significant effect on the isotherms of PVA-ACFs. Under identical conditions, activating agents had no significant effect on the isotherms of PVA-ACFs [4]. The specific surface areas of PVA-ACFs calculated with the standard BET method [3] are listed in Table 1. The yield of PVA-ACF was an indication of the degree of activation. As the degree of activation increased, the ACF yield decreased with an increase in BET specific surface area (SBET ). The upper limit of the surface area of activated carbon was found to be 2630 m2 /g, which originates from carbon in the form of a graphene sheet structure with a single atom thickness [5]. This structure is brittle, leading to poor mechanical properties. Therefore, the PVA-ACFs with yields of 25–33% were considered to be welldeveloped. The ones with a higher yield, such as 60–800–30, were not activated sufficiently, whereas the ones with a lower yield, e.g., 30–900–120, were over-activated. The pore size distributions (PSDs) of the fibers obtained by means of DA [6] and BJH [7] methods are shown in Fig. 3. Interesting results were found in the PSD plots of the fibers with
Table 1 Surface and porous structure parameters of PVA-ACFs Sample
Yield (%)
SBET (m2 /g)
Hmic (nm)
Hmes (nm)
Vmic (ml/g)
Vt (ml/g)
D
60–800–0 60–800–30 60–900–60 30–900–120
39.6 36.9 25.3 8.7
27 251 995 2128
2.34 1.06 0.71 1.15
46.1 5.56 3.91 3.98
0.014 0.124 0.398 0.761
0.014 0.133 0.424 0.901
– 2.47 2.74 2.47
Note. Hmic and Hmes : modal micropore and mesopore diameter; Vmic : micropore volume; Vt : total pore volume; D: fractal dimension.
Fig. 3. PSD plots of PVA-ACFs prepared under various conditions.
various activation degrees. In a certain range, a lower activation degree results in a larger modal diameter of the micropores. This is attributed to the different relative contributions of micropores and mesopores/macropores in the samples. The direct observation of the fibers via SEM and indirect analysis from nitrogen adsorption isotherms has demonstrated that mesopores and macropores appeared in the preoxidation process, earlier than the formation of micropores, which were developed during the activation process [4]. Therefore, the fiber with a lower activation degree had a statistically larger pore diameter. The
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modal pore diameter of 60–900–60 was 0.71 nm in the micropore range and that of the mesopore range was around 4.0 nm. This structure ensured a high adsorption capacity, attributed to the high adsorption potentials in ultramicropores. Meanwhile, the presence of mesopores/macropores facilitated the transport of adsorbates in adsorption and provided adsorption sites for larger molecules. Therefore, the PVA-ACF with this pore structure was especially suitable for application in liquid adsorption. The fiber obtained with a further increased activation degree, e.g., 30–900–120, showed a larger modal pore diameter than the well-developed one, 60–900–60. However, the modal mesopore diameter of 30–900–120 was still about 4.0 nm, implying that 4.0 nm is a preponderant size in the mesopore development of PVA-ACFs. The fractal theory developed by Mandelbrot provides a systematic approach to the characterization of many natural and engineered systems that have no definite form or regularity [8,9]. A scale-independent parameter named surface fractal dimension (Ds ) is widely used for quantifying the degree of surface irregularity. The values of Ds lie in the range 2–3 [9–11]. A perfectly regular and smooth surface possesses a Ds of 2, while a higher Ds suggests a more irregular and space-filling surface [9]. The fractal version of the classical Frenkel–Halsey– Hill (FHH) equation for multilayer adsorption was employed to evaluate the surface fractal dimension from the single nitrogen adsorption isotherm [10,11] 1 P0 ln θ = − ln ln (1) + C, q P where θ represents the fraction of surface coverage; P and P0 are the equilibrium and saturation pressures of the adsorbate; C is the intercept; and the parameter q is related to the surface dimension of the sample. The determination of surface fractal dimension, Ds , requires knowledge of the adsorption regime. The magnitude of the parameter 1/q distinguishes two possible regimes. If the van der Waals attraction between the solid and adsorbed film, which tends to make the gas/film interface to replicate the surface roughness, is dominant, the Ds value can be calculated with the following equation: 1 . Ds = 3 1 − (2) q Conversely, if the liquid/gas surface tension (capillary force), which tends to move the interface away from the surface to reduce the interface area, is more important, the Ds value can be estimated from the equation below: Ds = 3 −
1 . q
(3)
No reasonable Ds value was obtained for 60–800–0, suggesting that the FHH equation was not appropriate for evaluating the fractal geometry of 60–800–0. Reasonable Ds values were obtained for all the PVA-ACFs using Eq. (3) (listed in Table 1), indicating that the capillary force dominated in the interaction between nitrogen and PVA-ACFs. As shown in Table 1, the Ds value of 60–900–60 was the highest among the
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Fig. 4. FTIR spectra of fibers sampled from different heat treatment stages.
ACFs with various degrees of activation. The Ds values, in addition to the PSD results, provide further support for the conclusion that 60–900–60 was well developed. 3.2. Chemical structure evolution FTIR analysis is a useful method for comparing qualitatively either vibrating absorption spectra of fibers or relative intensities of the respective bands. Fig. 4 shows the FTIR spectra of the fibers sampled from different heat treatment stages. The precursor fiber, RF, had a strong O–H stretching vibration band at 3440–3420 cm−1 , a C–H stretching vibration band at 1096 cm−1 , a CH2 bending vibration band at 1423 cm−1 , C–H and O–H bending vibration bands at 1326 cm−1 , strong C–O stretching vibration bands at 1144 and 1096 cm−1 , and skeletal vibration bands at 916 and 849 cm−1 . With the process of heat treatment, all the transmission bands were gradually reduced, indicating the loss of O–H and C–O groups and the destruction of the skeletal structure of PVA. In the spectrum of DF, the bands at 1707–1712 and 1624–1652 cm−1 were the strongest ones, and the former was newly formed. The two bands came from the stretching vibration of C=O and that of C=C, respectively, which were an indication of ketones and/or enols [12]. These evolutions confirm the dehydration in preoxidation and dehydration processes. With further heat treatment, the bands were rapidly reduced until few bands were observable. The PVA-ACFs had three weak bands at 3438, 1550, and 1083 cm−1 [4]. The band at 3438 cm−1 was typically from the absorption originating from O–H or N–H and that at 1083 cm−1 was from C–O single bonds, such as those in ethers, phenols, lactones, and hydroxyl groups. The band around 1550 cm−1 has been observed by many researchers and has not been interpreted unequivocally [13,14]. It might be the oxygen double bond conjugated with the carbon basal planes [15].
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Fig. 5. XPS C1s spectra of fibers sampled from different heat treatment stages. Table 2 The contents of elements in fibers evaluated by elemental composition (EC) and XPS analyses Sample RF PF DF CF500 CF800 ACF
wt% of element (EC)
at% of element (XPS)
C
O
N
H
C1s
O1s
N1s
Burn-off (wt%)
49.37 56.20 63.84 80.09 85.14 79.64
41.58 35.56 30.42 14.63 11.91 17.08
0 0 0.89 1.79 0.87 0.99
9.05 8.24 4.85 3.49 2.08 2.29
50.40 69.26 72.75 86.95 85.15 79.64
49.60 28.78 20.49 9.66 13.24 17.73
0 1.95 6.76 3.39 1.61 2.63
0 15.6 37.3 55.9 60.9 73.6
Table 2 lists the contents of elements in the fibers evaluated with elemental analysis. Contrary to the change of O and H contents, the C content steadily increased with heat treatment until activation. The evolution of elemental composition had a good correlation with the burn-off, suggesting that the dominant change in the fibers during heat treatment was the loss of noncarbon elements, O and H, and that the activation treatment endowed the fibers with oxygen-containing groups. XPS determination provides valuable information for different functional groups formed at the fiber surface. The contents of elements obtained from XPS survey spectra are also summarized in Table 2. They were consistent with those obtained from the elemental analysis. Elemental analysis is a kind of determination in the bulk phase, whereas XPS is a surface one with a detection depth within around 4 nm. The consistency of the two kinds of analytical results suggests that the PVA-ACF is essentially a surface solid with over half of the atoms on the surface [16]. To clarify the functional groups and their relative contents, deconvolution procedures were performed on the C1s spectra with a Gaussian–Lorentzian sum function. Fig. 5 shows the C1s spectra of the fibers and their deconvolved peaks. The C1s signals of all the fibers consisted of a major peak at 284.6 eV, which corresponded to nonfunctionalized C, i.e., the contributions of C (Csp2 and Csp3 ) belonging to the carbon skeleton
Table 3 The relative contents of C1s existed in different forms evaluated by XPS Sample RF PF DF CF500 CF800 ACF
at% of element C–C(H)
C–O
77.3 71.7 72.8 81.7 81.1 62.3
22.7 22.0 19.9 16.2 16.3 25.8
C=O
COO
6.3 7.3
8.0
2.0 2.6 3.9
of the material and the contributions of aliphatic Csp3 , which corresponded to hydrocarbons [15]. The relative intensity of the peak at 284.6 eV increased gradually upon heat treatment, but then decreased with the process of activation, implying that some carbon atoms bonded to hydroxyl groups were removed and some of the carbon atoms were functionalized through the creation of carbon–oxygen bonds. More information could be obtained from the satellites of the C1s peak at higher binding energies [17]. The shifts from the main peak ranged from 1.5 eV for carbon atoms singly bonded to oxygen to 4 eV for carbon atoms in carboxyl groups or esters [18]. As illustrated in Fig. 5, a curve fitting with the defined binding energy and peak width show the decrease in the C–O satellite at 286.1 eV and the increase in the C=O satellite at 287.5 eV as a consequence of heat treatment. The decrease in the C1s satellite at around 286.1 eV was attributed to the removal of –OH through dehydration. This is in agreement with the FTIR results. After activation, a new satellite at around 288.6 eV appeared. Meanwhile, the relative intensity of the nonfunctionalized carbon decreased, whereas those at 286.1 and 287.5 eV increased, suggesting the formation of ether/phenol/alcohol (286.1 eV), carbonyl/quinine (287.5 eV), and carboxyl/lactone (288.6 eV) through the functionalization of carbon by means of the creation of carbon–oxygen bonds. The relative contents of these groups are listed in Table 3. The effects of these surface func-
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Fig. 6. XRD patterns of fibers sampled from different heat treatment stages.
tional groups on adsorption processes are our next research destination.
Table 4 Levels and values of process parameters for steam activation experiment Level
Factor A heating speed (◦ C/min)
Factor B carbonization time (min)
Factor C activation temperature (◦ C)
Factor D activation time (min)
1 2 3
2 5 10
0 30 60
800 900 1000
30 60 90
3.3. Crystal structure evolution As shown in Fig. 6, the precursor PVA fiber was highly crystalline. A considerable amount of investigations have demonstrated that the PSDs of ACFs are closely associated with the crystalline characters of the precursors. Highly crystalline precursors, e.g., aramid fibers, lead to microporous ACFs with narrow pore size distributions, whereas low-crystallinity precursors, such as rayon, result in ACFs with a wider pore size distribution [19–22]. The PSD analytical results provide support for this conclusion. Upon heat treatment, as shown in Fig. 6, the crystal structure of fibers completely disappeared until new crystallites were formed after activation. The PVA-ACFs prepared in this work had two broad peaks at 2θ = 22.4–24.0◦ and 43.7–44.2◦ , corresponding to the 002 and 10 reflections of disordered micrographite stacking, respectively [23]. The crystal size decreased with the increase in activation degree [4]. The small crystal size and short-range ordering between crystallites provide the prerequisites for the realization of a high specific surface area [24]. 3.4. Optimization of the fabrication process As found in our previous work [4], the stabilization process was the decisive step in the preparation of PVA-ACFs, which determined the final structures of fibers. In addition, the effects of activating agents on the texture and surface chemistry of PVA-ACF have proven to be insignificant. In the elucidation
of pore structure evolution, both carbonization and activation conditions have been found to be crucial for the final texture of PVA-ACFs. To understand the relative importance of each factor, it is often desirable to run a large number of screening experiments with a conventional “change-one-factor-at-a-time” method. Therefore, an L9 (3)4 orthogonal array design matrix was employed to evaluate the effects of the four factors, i.e., carbonization time, heating speed during activation process, activation temperature, and activation time, on the adsorption capacities and yields of PVA-ACFs. Orthogonal experimental design is an effective mathematical method for multifactor experiments. With it the individual effect of each among several factors can be found with fewer runs than in conventional designs [25]. Taking a suite of steam activation experiments as an example, the levels of the process parameters for orthogonal design are listed in Table 4. The assignment of experiments and the ranges obtained from intuitionistic analysis are summarized in Table 5. Based on the range analysis, carbonization time and activation temperature were judged as the most important factors
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Table 5 Assignment of factors and levels for fabrication of PVA-ACF using L9 (34 ) matrix and the range analysis Trial No.
Level of factor
Virtual parameter
Response
A
B
C
D
A
B
C
D
Yield (%)
QMB (mg/g)
QI2 (mg/g)
1 2 3 4 5 6 7 8 9
1 1 1 2 2 2 3 3 3
1 2 3 1 2 3 1 2 3
1 2 3 2 3 1 3 1 2
1 2 3 3 1 2 2 3 1
2 2 2 5 5 5 10 10 10
0 30 60 0 30 60 0 30 60
800 900 1000 900 1000 800 1000 800 900
30 60 90 90 30 60 60 90 30
35.1 27.0 10.9 27.0 22.9 24.3 25.2 26.9 20.0
5 287 334 28 113 70 159 90 207
806 1746 1952 1057 1176 1112 1530 1098 1230
Range Yield QMB QI2
0.7 138 386
10.7 139 300
9.1 147 547
4.4 64 392
determining the yields of the final ACFs, whereas activation temperature was the key factor governing the adsorption capacities of ACFs for MB and I2 . Since the molecular size of MB is larger than that of I2 , the MB adsorption capacity (QMB ) is indicative of larger pores, and that of I2 (QI2 ) indicates the volume of micropores. A high ACF yield generally corresponds to a low adsorption capacity. Thus, a compromise among adsorption capacity, yield, and energy consumption was needed for optimization. From the range values in Table 5, it was confirmed that the heating speed had little effect on either ACF yields or adsorption capacities. For the sake of saving preparation time, a heating speed of 10 ◦ C/min was selected. With a balanced consideration for ACF yields and adsorption capacities, appropriate carbonization and activation parameters for the fabrication of PVA-ACF were established as carbonizing the dehydrated fiber at 300 ◦ C for 60 min, then lifting the temperature to 900 ◦ C in an inert atmosphere, and thereafter dwelling at 900 ◦ C for 60 min in an oxidizing atmosphere. 4. Conclusions The pore structure, chemical structure, and crystal structure evolutions during the fabrication of PVA-ACFs by the newly developed method were systematically elucidated. Both carbonization and activation conditions were found to have significant effects on the development of the pore structure. Based on these understandings, the process parameters for the fabrication of PVA-ACF were optimized by using an L9 (3)4 orthogonal array design matrix. Appropriate carbonization and activation parameters were established. The combination of both physical texture and chemical structure of porous carbonaceous adsorbents governs the adsorption of adsorbates on them. The former controls the physical access and the contact between an adsorbate molecule and a surface for adsorption, whereas the chemical interactions between adsorbate molecules and the adsorbent surface regulate the overall extent of adsorption once the adsorption sites are accessed. The elucidation of the structure evolution in the preparation of PVA-ACFs might help us to untangle the complex web of interactions and explore the roles of physical factors
and chemical interactions involved in the adsorption of various adsorbates by those PVA-ACFs. Acknowledgment The authors thank the Ministry of Science and Technology of China for financial support through the Hi-Tech Research and Development Program (863 Project, No. 2004AA649300). References [1] M. Suzuki, Carbon 32 (1994) 577. [2] S.J. Park, K.D. Kim, J. Colloid Interface Sci. 212 (1999) 186. [3] J. Choma, M.W. Burakiewicz, M. Jaroniec, Z. Li, J. Klinik, J. Colloid Interface Sci. 212 (1999) 438. [4] S.J. Zhang, H.Q. Yu, H.M. Feng, Carbon 44 (2006) 2059. [5] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1982. [6] M.M. Dubinin, V.A. Astakhov, Adv. Chem. Ser. 102 (1971) 69. [7] E.P. Barret, L.G. Joyner, P.P. Hallenda, J. Am. Chem. Soc. 73 (1951) 373. [8] B.B. Mandelbrot, The Fractal Geometry of Nature, W.H. Freeman and Co., New York, 1983. [9] Q.F. Hou, X.C. Lu, X.D. Liu, B.X. Hu, J.Q. Cui, J. Shen, Surf. Coat. Technol. 190 (2005) 394. [10] P. Pfeifer, J. Wu, M.W. Cole, J. Krim, Phys. Rev. Lett. 62 (1989) 1997. [11] P. Pfeifer, M.W. Cole, New J. Chem. 14 (1990) 221. [12] S.J. Zhang, H.Q. Yu, Water Res. 38 (2004) 309. [13] C.L. Mangun, K.R. Benak, M.A. Daley, J. Economy, Chem. Mater. 11 (1999) 3476. [14] Z.Y. Ryu, H.Q. Rong, J.T. Zheng, M.Z. Wang, B.J. Zhang, Carbon 40 (2002) 1144. [15] G. de la Puente, J.J. Pis, J.A. Menéndez, P. Grange, J. Anal. Appl. Pyrol. 43 (1997) 125. [16] K. Kaneko, Solid State Phys. 27 (1992) 1. [17] J.B. Tomlinson, J.J. Freeman, K.S.W. Sing, Carbon 33 (1995) 789. [18] T.T.P. Cheung, J. Appl. Phys. 53 (1982) 6857. [19] J.B. Tomlinson, J.J. Freeman, K.S.W. Sing, Carbon 33 (1995) 789. [20] J.J. Freeman, J.B. Tomlinson, K.S.W. Sing, Carbon 33 (1995) 795. [21] F. Stoeckli, T.A. Centeno, A.B. Fuertes, J. Muñiz, Carbon 34 (1996) 1201. [22] M.C. Blanco López, S.M. Villar-Rodil, A. Martínez-Alonso, J.M.D. Tascón, Micropor. Mesopor. Mater. 41 (2000) 319. [23] M.A. Short, P.L. Walker Jr., Carbon 1 (1963) 3. [24] K. Kaneko, C. Ishii, M. Ruike, H. Kuwabara, Carbon 30 (1992) 1075. [25] G.E.P. Box, W.G. Humter, J.S. Hunter, Statistics for Experiments, John Wiley & Sons, New York, 1978.