Oxidization activated carbon fiber through nitrocellulose combustion

Transcription

1 Colloids and Surfaces A: Physicochem. Eng. Aspects 308 (2007) Oxidization activated carbon fiber through nitrocellulose combustion Wenzhong Shen a,b,, Hui Wang c, Yihong Liu a, Qingjie Guo a, Yuanli Zhang c a State Key Laboratory of Heavy Oil, China University of Petroleum, Dongying, Shandong , PR China b Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy Sciences, Taiyuan, Shanxi , PR China c Institute of Coal Chemistry, Chinese Academy Sciences, Taiyuan, Shanxi , PR China Received 1 March 2007; received in revised form 6 May 2007; accepted 15 May 2007 Available online 18 May 2007 Abstract Activated carbon fiber was oxidized through nitrocellulose combustion below 300 C to introduce oxygen-complex on its surface. The pore structure, elemental composition, chemical functional groups of activated carbon fiber before and after oxidized were investigated. The result showed that the oxygen-complex could be efficiently introduced after nitrocellulose combustion; especially for the imide; and the micropore diameter was slightly wider after oxidization. It provided a simple method to introduce oxygen-complex using nitrocellulose combustion. The activated carbon fiber oxidized by nitrocellulose combustion displayed higher adsorption capacities for ammonia and carbon disulfide than that of the original activated carbon fiber due to the introduced oxygen-complexes Elsevier B.V. All rights reserved. Keywords: Activated carbon fiber; Oxidization; Nitrocellulose combustion; Adsorption 1. Introduction Porous carbons have been widely used as adsorbents, catalyst/catalyst supports, electronic material and energy storage material due to its higher surface area and larger pore volume [1]. Activated carbon fiber (ACF) as a novel porous carbon with various shapes (cloth, paper, monoliths and felt) has narrower pore distribution and the opening micropore locates on the external surface of fiber, so, it displays faster adsorption dynamics than commercial activated carbon because it shortens the molecular diffusion path inside the fiber [2]. ACF is thought as an excellent adsorbent for solvent recovery, water purification, environmental protection and energy storage [3 5]. Both the pore structure and surface chemistry composition of ACF significantly influence its adsorption capacities, catalytic activity or as catalyst supports performance. The heteroatoms on the surface of ACF take significant role on its application. The oxygen containing functional groups show the most important Corresponding author at: State Key Laboratory of Heavy Oil, China University of Petroleum, Dongying, Shandong , PR China. Tel.: ; fax: address: shenwzh2000@yahoo.com (W. Shen). effect due to its determining the surface acid and base property [6 8]. The oxygen atoms on the ACF surface could be introduced through physical and chemical approaches, such as, heat-treatment under air/oxygen [9], impregnated with strong oxidants (concentrated sulfuric acid, nitric acid, hydrogen peroxide) [10], oxygen plasma treatment [11], etc. Nitrocellulose (mainly C 6 H 7 N 3 O 11 ) is easy to combust and widely used in making smokeless gun powder, as a propellant, lacquer, photographic emulsions and medical treatment material. The decomposition of nitrocellulose is a unimolecular process, so, it is uniform and predictable release of energy. It was reported that hierarchical porous silica oxide could be controlled by manipulating the nitrocellulose composition [12]. The nitrogen- and oxygen-containing radicals will be produced when nitrocellulose combustion, these radicals are easy to interact with carbon to form complexes when the nitrocellulose is combusted on the surface of activated carbon fiber. However, there is no report about oxidization activated carbon fiber through nitrocellulose combustion. In this work, nitrocellulose combustion was applied to ACF surfaces oxidization to introduce oxygen-complexes, which enhanced the ACFs adsorption capacities for ammonia and carbon disulfide /$ see front matter 2007 Elsevier B.V. All rights reserved. doi: /j.colsurfa 转载

2 W. Shen et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 308 (2007) Experimental 2.1. Sample preparation Commercially viscose-based activated carbon fiber (designated as ACF-0), which was activated at 850 C with steam, was supplied by Zichuan Carbon Fiber Limited Company, Qinhuangdao (PR China). Firstly, the ACF-0 was dried at 120 C for 2 h to remove the adsorbed steam and organic molecules, then, it was impregnated with nitrocellulose acetone solution and the loading amount was selected as 1 or 5 (wt%) nitrocellulose by varying nitrocellulose concentration at room temperature for 4 h. Finally, the impregnated ACFs were heated to 250 C and 300 C for 30 min in air. For comparison, the ACF-0 was impregnated with acetone and heated to 300 C in air. The samples were labeled as ACF-0-300, ACF-1-300, ACF and ACF according to the nitrocellulose loading amount and heat-treatment temperature, respectively Characterization The nitrogen adsorption isotherms of samples were measured on a Micromeritics ASAP 2000 (Micromeritics Ins. Corp.) at 77.4 K in the range of relative pressure from 10 6 to 1. All samples were out-gassed at 300 C for 2 h before the measurement. The special surface area and pore structure were determined by BET equation and density functional theory (DFT), respectively. Thermal gravimetric analysis (TGA) was operated under air at C with a heating rate of 10 C/min by a TG151 Thermoanalyzer (Cahn). The surface functional groups were investigated by the transmission infrared spectra obtained from a Fourier Transform Spectrophotometer (Nicolet Magna-IR 830 spectrometer). Elemental analysis of sample was analyzed using an Analysensysteme GmbH Elementar Vario EL (Germany). The nitrogen, carbon, hydrogen content was determined directly, while the oxygen content was calculated by difference. The SEM images of samples were observed through a JEOL Electron Microscope (JEOL Electron Microscope, Japan) operated at 25 kv and 15 ma. Before observation, the samples were sputtered with gold for 2 min in order to avoid charging. Table 1 The primary parameters of samples Sample S BET (m 2 /g) V total (cm 3 /g) V micropore (cm 3 /g) V mesopore (cm 3 /g) ACF ACF ACF ACF ACF surface area and pore volume increase with heat-treatment temperature and nitrocellulose loading. The specific surface areas and pore volumes of the ACF-0 and ACF are 892 m 2 /g and 1006 m 2 /g, 0.33 cm 3 /g and 0.38 cm 3 /g, respectively, this indicates that the ACF5-300 has a more developed pore structure. Fig. 1 shows the nitrogen adsorption isotherms and pore size distribution of ACF-0 and ACF The nitrogen adsorption isotherms of samples are Type I according to the BDDT (Brunauer Deming Deming Teller) classification, which illuminates the domination of micropores in the pore structure. The adsorption isotherms give a gradual adsorption increase with relative pressure (P/P 0 ) for ACF-0 and ACF-5-300, which are attributed to the existence of mesopore and macropore. The ACF has wider pore size distribution, especially in nm, than that of ACF-0. This implies that not only the exterior surface but the micropore surface carbon are oxidized during the nitrocellulose combustion process. As a result, the specific surface area, micropore volume and pore diameter of ACFs are slightly increased and widened after nitrocellulose combustion Adsorption examination The adsorption examination was performed in a sealed vessel at 20 C. The ammonia or carbon disulfide was firstly placed into the vessel to reach its saturated vapor, then, the ACF samples were transferred in. After certain adsorption time, the ACF samples were weighted and the difference of weight was the adsorbed ammonia or carbon disulfide, respectively. 3. Results and discussion 3.1. Pore structural properties of samples The pore structural parameters of samples determined by N 2 adsorption isotherms at 77.4 K are listed in Table 1. The special Fig. 1. The nitrogen adsorption isotherms (a) and pore size distribution (b) of ACF-0 and ACF

3 22 W. Shen et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 308 (2007) Fig. 2. The thermal gravimetric curve of ACF-0 (a) and ACF with 5.0 wt% nitrocellulose (b) under air. Fig. 3. The FT-IR spectra of ACFs Thermogravimetric analysis ACF-0 and activated carbon fiber loaded with 5.0 wt% nitrocellulose are investigated by TGA in air. The weight losses with temperature are drawn in Fig. 2. The weight loss of ACF-0 changes in less degree before 500 C, because the burnt of carbon is slow at low temperature; the burnt of carbon is faster after 500 C. The mainly weight loss of activated carbon fiber loaded with 5.0 wt% nitrocellulose takes place in two areas. One ranges from 190 Cto250 C, which is contributed to the combustion of nitrocellulose due to its ignition point around C. In addition, part carbon on the surface of ACFs reacted with oxygen during the combustion process and the yield of ACF decreased. Nitrogen- and oxygen-containing radicals derived from nitrocellulose combustion interact with the surface carbon to form new surface functional groups. The other is between 500 C and 850 C, only the carbon was burnt off in this area. The weight loss gradually decreased between 300 C and 500 C, which was contributed to the surface carbon reacted with oxygen. The reaction is slower due to the lower temperature; this agrees with ACF surface oxidization by air [9]. It indicates that the nitrocellulose is completely combusted and the ACF with higher yield at 300 C Elemental analysis Elemental analysis data of rayon-based ACFs by nitrocellulose treatment are given in Table 2. Compared with the original ACF-0, the carbon contents of ACFs that are oxidized by nitrocellulose combustion are decreased in different degree; while the contents of heteroatoms are significantly increased, Table 2 Elemental composition of heat-treatment samples under study (the oxygen content was calculated by difference) Samples Elemental composition (%) C H N O ACF ACF ACF ACF ACF the oxygen content is doubled after nitrocellulose combustion at 250 C. The higher nitrocellulose loading is, the less carbon content is, but the higher heteroatoms contents are. Nitrogenand oxygen-containing radicals that will be produced during nitrocellulose combustion are easy combined with the surface carbons, so, the nitrogen and oxygen contents of ACFs increase after nitrocellulose combustion Surface properties The FT-IR spectra of ACFs are drawn in Fig. 3. The asymmetry of peak ranging from 3200 cm 1 to 3600 cm 1 might indicate strong hydroxyl interaction and the presence of overlapping N H bands. The intensity of the bands is significantly elevated with nitrocellulose amount and the treatment temperature. This implied that the OH and N H groups increase on the ACFs surface. Two weak bands at 2929 cm 1 and 2850 cm 1, which account for CH 2 group, are observed after nitrocellulose treatment. The strong peak at 1635 cm 1 should be the overlapping of N H in-plane bending and the conjugated C O groups [13]. The two correlative peaks at 1413 cm 1 and 1502 cm 1 indicate the C O belonged to the carbonate groups [13]. Moreover, there are a number of overlapping bands indicating, for example, C O C, C O N or >C N between 1400 cm 1 and 1000 cm 1. A peak at 1247 cm 1 for ACF was the C O stretches of phenolic hydroxyls due to no carboxylic acid groups appearing at cm 1 [10]. This spectra suggest that ACFs contains oxygen-containing functionality as primarily consisting of phenolic hydroxyls, imide and ethers through nitrocellulose combustion The SEM morgraphology of samples The SEM images of ACFs before and after nitrocellulose combustion are shown in Fig. 4. The surface of ACF-0 looks relatively smooth; the spotted on the surface is the adsorbed dirt. There are amount of spotted on the surface of ACF-5-250, which is the nitrocellulose residue. This indicates that nitrocellulose was not completely combusted at 250 C. When the temperature rise to 300 C, the surface of ACF is as smooth as ACF-0, this suggests that the whole nitrocellulose is combusted. Compared the SEM images, the ACFs surface was not destroyed after nitrocellulose combustion. According to the pore struc-

4 W. Shen et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 308 (2007) Fig. 4. The SEM images of ACF-0 (a), ACF (b) and ACF (c). ture, elemental analysis and SEM images, it proves that it is a simple method to introduce oxygen and nitrogen atoms to the ACF surface through nitrocellulose combustion at 300 C and no damaged to ACF surface The adsorption of ammonia and carbon disulfide The surface oxygen-complexes of ACF including carboxyl and phenolic hydroxyl groups have an acidic nature, and acidic adsorbents strongly interact with basic adsorbates due to the Lewis acid base interaction [14,15]. The adsorption curves of ammonia and carbon disulfide on ACFs are drawn in Fig. 5. The ACF-0 showed the least adsorption capacity for ammonia and carbon disulfide, which is near to 0.16 g/g and 0.22 g/g, respectively. The ACF oxidized with nitrocellulose combustion shows higher adsorption equilibrium capacities for ammonia and carbon disulfide; both the loaded nitrocellulose amount and heat-treatment temperature affect the adsorption equilibrium capacity of ACF for ammonia and carbon disulfide. The higher oxygen content is; the higher equilibrium adsorption capacity is. Especially, the ACF displays the highest adsorption capacity for ammonia and carbon disulfide,

5 24 W. Shen et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 308 (2007) Acknowledgements This work was supported by Natural Science Foundation of Shandong Province (grant no. Y2006F09) and Science Foundation of Key Laboratory of Carbon Materials (Chinese Academy Sciences) (grant no. KFJJ0503). References Fig. 5. The adsorption of ammonia (a) and carbon disulfide (b) on ACFs. ACF-0 ( ), ACF ( ), ACF ( ), ACF ( ) and ACF ( ). which is more twice than that of ACF-0. The effect of nitrocellulose combustion on the pore volume of ACF is poor, so, the adsorption capacity is mainly determined by the surface functional groups. Ammonia is a relatively low boiling point gas, the pore size and pore volume of ACF has very little effect on its adsorption, ACFs with very high oxygen contents tended to have higher adsorption capacities for ammonia on the basis of the direct interaction of the enhanced polar and acid groups, such as, carboxylic acid and phenolic hydroxyl functional groups with the basic contaminant [10,16]. In the case of carbon disulfide adsorption, the interaction force of carbon disulfide and oxygen is easy formed, so, the adsorption capacity of ACF is improved when the oxygencontaining function groups increased. 4. Conclusion The oxygen-complex could be introduced on the surface of activated carbon fiber by nitrocellulose combustion below 300 C; it is a simple and controllable method. The pore diameter is slightly widened and surface is not damaged during nitrocellulose combustion process. The introduced oxygen-complexes mainly contain phenolic hydroxyls, imide and ethers, which enhance the adsorption capacities for ammonia and carbon disulfide molecules at ambient temperature. [1] L.R. Radovic, F. Rodriguez-Reinoso, in: Thrower (Ed.), Chemistry and Physics of Carbon, vol. 25, Dekker, New York, 1997, p [2] M. Isao, K. Yozo, S. Masuaki, K. Shizuo, H. Tomohiro, S. Yorimasa, Removal of SOx and NOx over activated carbon fibers, Carbon 38 (2000) [3] J. Yang, P. Juan, Z.M. Shen, R. Guo, J.P. Jia, H.J. Fang, Y.L. Wang, Removal of carbon disulfide (CS 2 ) from water via adsorption on active carbon fiber (ACF), Carbon 44 (2006) [4] M.Ä. Fontecha-Camara, M.V. Lopez-Ramon, M.A.Ä. Lvarez-Merino, C. Moreno-Castilla, Temperature dependence of herbicide adsorption from aqueous solutions on activated carbon fiber and cloth, Langmuir 22 (2006) [5] Y.S. Lee, Y.H. Kim, J.S. Hong, J.K. Suh, G.J. Cho, The adsorption properties of surface modified activated carbon fibers for hydrogen storages, Catal. Today 120 (2007) [6] B.K. Kim, S.K. Ryu, B.J. Kim, S.J. Park, Adsorption behavior of propylamine on activated carbon fiber surfaces as induced by oxygen functional complexes, J. Colloid Interf. Sci. 302 (2006) [7] H. Oda, A. Yamashita, S. Minoura, M. Okamoto, T. Morimoto, Modification of the oxygen-containing functional group on activated carbon fiber in electrodes of an electric double-layer capacitor, J. Power Sources 158 (2006) [8] Y. El-Sayed, T.J. Bandosz, Role of surface oxygen groups in incorporation of nitrogen to activated carbons via ethylmethylamine adsorption, Langmuir 21 (2005) [9] Z.M. Wang, N. Yamashita, Z.X. Wang, K. Hoshinoo, H. Kanoh, Air oxidation effects on microporosity, surface property, and CH 4 adsorptivity of pitch-based activated carbon fibers, J. Colloid Interf. Sci. 276 (2004) [10] C.L. Mangun, K.R. Benak, M.A. Daley, J. Economy, Oxidation of activated carbon fibers: effect on pore size, surface chemistry, and adsorption properties, Chem. Mater. 11 (1999) [11] S.J. Park, B.J. Kim, Influence of oxygen plasma treatment on hydrogen chloride removal of activated carbon fibers, J. Colloid Interf. Sci. 275 (2004) [12] X.S. Li, G.E. Fryxell, C.M. Wang, J. Young, Templating mesoporous hierarchies in silica thin films using the thermal degradation of cellulose nitrate, Micropor. Mesopor. Mater. 99 (2007) [13] S. Biniak, G. Szymanski, J. Siedlewski, A. Swiatkowski, The characterization of activated carbons with oxygen and nitrogen surface groups, Carbon 35 (1997) [14] Y. El-Sayed, T.J. Bandosz, H. Wullens, P. Lodewyckx, Adsorption of ethylmethylamine vapor by activated carbon filters, Ind. Eng. Chem. Res. 45 (2006) [15] S.J. Park, S.Y. Jin, Effect of ozone treatment on ammonia removal of activated carbons, J. Colloid Interf. Sci. 286 (2005) [16] S.J. Park, B.J. Kim, Ammonia removal of activated carbon fibers produced by oxyfluorination, J. Colloid Interf. Sci. 291 (2005)