CO 2 activation of olive stones carbonized under different experimental conditions

320 Letters to the Editor / Carbon 39 (2001) 287 – 324 axis, but the outer part of the nanotubes is coated with disordered carbon (Fig. 1). This low-organised carbon is responsible for high TSA and ASA. After annealing MWNTs-9008C at 24008C, both ASA and TSA values decrease strongly. TEM observations show many closed tips and straight and continuous aromatic layers after thermal annealing at 24008C (Fig. 2). Hence, the decrease of the ASA / TSA ratio is related to the noticeable change of the surface structure consisting mainly of extended basal planes. These results have been compared to those obtained for the sample denominated MWNTs-6008C which has the particularity of exhibiting a similar morphology to MWNTs-7008C, but is free of the amorphous carbon coating and has almost all tips closed (Fig. 3). ASA and TSA values are much lower for MWNTs-6008C than for the other as-prepared samples (Table 1). The obtained value of ASA tends to confirm that the high ASA value of MWNTs-7008C may be attributed to the presence of the amorphous carbon on the surface of the nanotubes. Moreover, it also appears that opening / closure of the central canal noticeably contributes to the adsorptive properties of nanotubes. Consequently, the ASA / TSA ratio is lower for MWNTs-6008C than for MWNTs-7008C but the value is close to the one measured for MWNTs-9008C. Sample MWNTs-6008C was also annealed at 28008C and the results were compared to those obtained for MWNTs9008C annealed at 24008C (Table 1). It can be seen that the effect of heat treatment does not really influence the total surface area which decreases slightly from 130 m 2 / g to 103 m 2 / g after annealing. This is not surprising since it has been previously mentioned that the majority of the tips are already closed before annealing. On the contrary, the ASA value decreases significantly which may be attributed to a higher structural order due to the heat treatment above 20008C. Consequently, the value of the ASA / TSA ratio is the lowest among all the series of materials. The results obtained for the different MWNTs demonstrate that the ASA / TSA ratio takes into account the orientation of the aromatic carbon layers (presence of defects) and the microtextural evolutions such as tip closure and formation of more continuous layers. Carbon nanotubes were characterized through the measurement of their active site area and total surface area. A satisfactory correlation has been found between the surface properties of the nanotubes and their microtexture showing the relevance of classical surface characterisation for these carbon materials. References [1] Iijima S. Nature 1991;354:56–8. [2] Ebbesen TW. Annu Rev Mater Sci 1994;24:235–64. ´ [3] Frackowiak E, Gautier S, Gaucher H, Bonnamy S, Beguin F. Carbon 1999;37:61–9. ´´ ´ [4] Frackowiak E, Metenier K, Bertagna V, Beguin F. Appl Phys Lett, submitted. [5] Iijima S, Ichihashi T. Nature 1993;363:603–5. [6] Endo M, Takeuchi K, Kobori K, Takahashi K, Kroto H, Sarkar A. Carbon 1995;33:873–80. ´ [7] Jose-Yacaman M, Miki-Yoshida M, Rendon L, Santiesteban JG. Appl Phys Lett 1993;62:202–4. ´ [8] Hamwi A, Alvergnat H, Bonnamy S, Beguin F. Carbon 1997;35:723–8. [9] Colomer JF, Piedigrosso P, Willems I, Journet C, Bernier P, Van tendeloo G, Fonseca A, Nagy JB. J Chem Soc Faraday Trans 1998;94:3753–8. [10] Ehrburger P, Dallies-Labourdette E, Lahaye J. In: Lahaye J, Ehrburger P, editors, Fundamental issues in control of carbon gasification reactivity, Kluwer Academic, 1991, pp. 461–82. [11] Laine NR, Vastola FJ, Walker Jr. PL. J Phys Chem 1963;67:2030–4. CO 2 activation of olive stones carbonized under pressure ´ ´ M.A. Rodrıguez-Valero, M. Martınez-Escandell, M. Molina-Sabio, * ´ F. Rodrıguez-Reinoso ´ ´ , Universidad de Alicante, Apartado 99, E-3080 Alicante, Spain Departamento de Quımica Inorganica Received 11 September 2000; accepted 19 September 2000 Keywords: A. Activated carbon; B. Activation; Carbonization; D. Porosity Physical (or thermal) activation is one of the methods used for the manufacture of activated carbon. Precursors such as brown coal, peat, wood and, in general, materials *Corresponding author. Tel.: 134-9-6590-3544; fax: 134-96590-3454. ´ E-mail address: reinoso@ua.es (F. Rodrıguez-Reinoso). containing lignin and cellulose, may be directly activated without previous carbonization, but in most cases a carbonization stage is carried out prior to activation. This approach permits better control of the pyrolysis of the precursor and a subsequent improvement in the yield of the process or in the properties of the char. Carbonization of lignocellulosic materials implies a set of reactions such as 0008-6223 / 01 / $ – see front matter  2001 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 00 )00246-3 Letters to the Editor / Carbon 39 (2001) 287 – 324 321 Table 1 Carbonization yield, density, and gasification rate of the chars Bulk density (g / cm 3 ) Pressure (MPa) Yield (%) 5108C 510–8508C 5108C 8508C 1.0 0.7 0.5 0.1 41.8 41.3 40.9 32.1 84.0 83.9 85.6 84.2 0.63 0.62 0.62 0.57 0.73 0.71 0.71 0.60 depolymerization, cracking, and dehydration of lignin and cellulose, thus leading to a char with a rudimentary pore structure, and other products such as tar and volatile liquids and gases. The proportion of these products, although a function of the chemical composition of the precursor, may be modified to increase the conversion to char by controlling the carbonization process. Some of the variables of carbonization (temperature, heating rate, etc.) have been already described for several lignocellulosic materials [1–3]. The aim of this work is to study the influence of pressure carbonization of olive stones prior to activation, in order to provide a further understanding of the effect of carbonization conditions over both the yield and porous structure of activated carbons. Olive stones, particle size 2.8–3.5 mm, were carbonized in a laboratory pilot plant with a vertical reactor that operates at temperatures up to 6008C and pressures up to 4.5 MPa. Carbonization was carried out up to 5108C and pressures up to 1.0 MPa (supplied by nitrogen), with a heating rate of 18C / min. This operating method increases residence time of volatile matter and tars within the particle, thus facilitating their conversion to solid carbon material. Since at 5108C the carbonization process was not complete, the chars obtained this way were further carbonized from room temperature up to 8508C in a horizontal furnace, with a heating rate of 58C / min and a nitrogen flow of 80 ml / min. All chars were activated at 8008C in a horizontal furnace using a carbon dioxide flow of 80 ml / min, and a heating rate of 58C / min. Weight loss upon activation and bulk density of the chars and activated carbons were monitored. The microporosity of chars and the resulting activated carbons was determined by the application of the Dubinin– Raduskhevich equation to the adsorption data of N 2 (77 K) and CO 2 (273 K) obtained using automatic adsorption systems. The application of this equation to the adsorption of N 2 at 77 K provides the total volume corresponding to micropores (up to 1.8 nm in width), whereas the application to CO 2 at 273 K leads to the volume of narrow micropores (up to about 0.7 nm) [4]. The adsorption of N 2 at 77 K on the four chars was very slow under the standard conditions for gas adsorption measurements, as is typical of chars with a rudimentary narrow microporosity caused by constrictions. d Hg (g / cm 3 ) R (% / h) 1.27 1.24 1.27 1.08 0.62 0.68 0.59 0.55 Table 1 includes the yield obtained for the carbonization under pressure at 5108C, and for the further carbonization process at 8508C. Increasing pressure during carbonization leads to an increase in yield from 27 to 35%, the main differences being found in the first stage up to 5108C. On the other hand, pressures above 0.5 MPa do not increase the yield, thus suggesting that the conversion of the volatile matter and tar to a char takes place at a relatively low pressure. It is well known that in chemical activation the effect of phosphoric acid or zinc chloride is to restrict the formation of tar during carbonization, thus increasing the yield of the process. Studies carried out with lignocellulosic materials indicate that activation with phosphoric acid at 4508C and with zinc chloride at 5008C, both followed by heat treatment at 800–8258C, leads to yields [5,6] similar to those described in Table 1 for the pressure carbonization. It is thus possible to deduce that it will be difficult to reach larger conversions to char since the elimination of oxygen and hydrogen (46 wt.% and 6 wt.%, respectively, in the original olive stone) below 5008C is not only through the formation of water, but also through the formation of other carbon compounds [7]. The bulk density of the starting material is 0.7760.01 g / cm 3 . This value decreases when carbonizing up to 5108C (Table 1), but increases at 8508C, thus reflecting both the creation of porosity associated with the large weight loss and the subsequent contraction / densification of the particle. It is to be noted that the increase in yield caused by pressure leads to an increase in both bulk and apparent density (Table 1). When the chars are activated under a flow of carbon dioxide at 8008C the gasification process is similar in all cases (Table 1), although it slightly increases for increasing pressures. Similar results have been described when analyzing the effect of heating rate during the carbonization of olive stones [2]. Flash pyrolysis increased the proportion of gases and volatile matter, with a subsequent decrease in char yield, but this did not seriously affect the gasification rate or the microporosity developed upon activation. Fig. 1 shows an increase of the narrow micropore volume (measured by CO 2 at 273 K) with burn-off for all samples. There is practically no difference between carbons obtained at different pressures, thus indicating that 322 Letters to the Editor / Carbon 39 (2001) 287 – 324 Fig. 1. Evolution of micropore volume as a function of burn-off for activated carbons from olive stone carbonized at: y, 0.1 MPa; h, 0.5 MPa; ^, 0.7 MPa; s, 1.0 MPa. Bold symbols: total microporosity (adsorption of N 2 at 77 K); open symbols: narrow microporosity (adsorption of CO 2 at 273 K). the extra material retained under pressure does not have any effect on the development of microporosity of the char. When analyzing the total microporosity (N 2 at 77 K), the tendency is the same, indicating that the degree of activation (burn-off) is the main parameter governing the development of microporosity during activation of the char with carbon dioxide. The development of microporosity upon activation leads to a decrease in density. Fig. 2 shows that the difference in density caused by pressure carbonization is maintained upon activation. Since the evolution of microporosity is similar in all series, this means that the differences in density of the chars are caused by the different meso- and Fig. 3. Evolution of micropore volume as a function of overall yield for activated carbons from olive stones. Symbols as in Fig. 1. macroporosities, the development of which is similar in all chars during activation. A previous study that analyzed the activation of different lignocellulosic chars showed that the differences in apparent (mercury) density of the chars were due to the meso- and macroporosity developed during carbonization [3]. In conclusion, the main effect of carbonizing under pressure prior to activation under carbon dioxide is to increase the yield and the density of the final activated carbon in respect to the conventional carbonization, with very little effect on the porosity developed during activation. Thus, when the data of micropore volume (Fig. 1) are plotted as a function of the overall yield of the process (Fig. 3), the effect of carbonization with or without pressure is evident. The larger yield and density due to pressure carbonization affords the possibility of extending the activation process to a larger burn-off, thus making it possible to obtain a higher micropore volume while maintaining a reasonable yield and density. However, the extra yield and density achieved may not be enough to compensate for the additional cost of the pressure carbonization system. Acknowledgements Financial support from the CICYT (Project QUI980663) is acknowledged. References Fig. 2. Evolution of bulk density upon activation. Symbols as in Fig. 1. ´ ´ ´ ´ [1] Lopez-Gonzalez JD, Martınez-Vilchez F, Rodrıguez-Reinoso F. Preparation and characterization of active carbons from olive stones. Carbon 1980;18:413–8. Letters to the Editor / Carbon 39 (2001) 287 – 324 ´ ´ ´ AN, Marcilla [2] Gonzalez MT, Rodrıguez-Reinoso F, Garcıa A. CO 2 activation of olive stones carbonized under different experimental conditions. Carbon 1997;35(1):159–62. ´ ´ ´ [3] Gonzalez JC, Gonzalez MT, Molina-Sabio M, Rodrıguez´ Reinoso F, Sepulveda-Escribano A. Porosity of activated carbons prepared from different lignocellulosic materials. Carbon 1995;33(8):1175–7. ´ ´ ´ [4] Rodrıguez-Reinoso F, Garrido J, Martın-Martınez JM, Molina Sabio M, Torregrosa R. The combined use of different approaches in the characterization of microporous carbons. Carbon 1989;27(1):23–32. 323 ´ ´ [5] Molina-Sabio M, Rodrıguez-Reinoso F, Caturla F, Selles MJ. Development of porosity in combined phosphoric acid– carbon dioxide activation. Carbon 1996;34(4):457–62. ´ [6] Caturla F, Molina-Sabio M, Rodrıguez-Reinoso F. Preparation of activated carbon by chemical activation with ZnCl 2 . Carbon 1991;29(7):999–1007. [7] Jagtoyen M, Derbyshire F. Activated carbons from yellow poplar and white oak by H 3 PO 4 activation. Carbon 1998;36(7–8):1085–97. Apparent fractal dimensions obtained from small-angle scattering of carbon materials W. Ruland* University of Marburg, Department of Chemistry and Center for Materials Science, D-35032 Marburg, Germany Received 25 August 2000; accepted 25 September 2000 Keywords: A. Porous carbon; C. X-ray scattering; D. Microporosity In many recent articles on the small-angle scattering (SAS) of carbon materials (see e.g. Rieker et al. [1,2] and Diduszko et al. [3]), the results have been interpreted in terms of fractal structures. The basis for this interpretation is the appearance of non-integer exponents in the scattering intensity I as a function of s 5 (2sinu ) /l or q 5 2p s (Bragg angle u, wavelength l). According to the scattering theory of fractals (see, e.g. Schmidt [4]), a dependence of the type I(s) ~ s 2 n is expected in certain ranges of s, where n 5 D is valid in the case of mass fractals and n 5 6 2 DS in the case of surface fractals. The ranges of possible values for the fractal dimensions D and DS are 1 , D , 3 and 2 , DS , 3 which means that 1 , n , 3 is valid in the case of mass fractals and 3 , n , 4 in the case of surface fractals. n 5 4 corresponds to Porod’s law [5–7] and is representative for smooth surfaces with sharp density transitions (DS 5 2). If the SAS is measured with a Kratky camera (‘smeared’ intensity J), the corresponding relationship is J(s) ~ s 2 m where m 5 n 2 1 in the approximation of an infinite slit length. In earlier papers [8–11] on the SAXS of carbon fibers, non-graphitizable carbons and glass-like carbons, it was *Fax: 149-6420-822-804. E-mail address: ruland@mailer.uni-marburg.de (W. Ruland). shown that the total intensity decreases more slowly than J(s) ~ s 23 in all cases studied. Fairly straight lines were obtained in log J–log s plots from which non-integer exponents of m could be determined. Furthermore, the deviations of m from Porod’s law ( m 5 3) were decreasing with increasing heat-treatment temperatures (HTT) of the samples. For example, in the case of PAN-base carbon fibers [10], m 5 1.85 was obtained for HTT512508C and m 5 2.58 for HTT527008C. If the theory of fractals had been applied to these results, the existence of mass fractals with D 5 2.85 in the case of the lower HTT and surface fractals with DS 5 2.42 in the case of the higher HTT could have been claimed to exist in the samples studied. Such an interpretation is, however, incorrect since it neglects the contribution of a special kind of scattering which is produced by the imperfect stacking of graphenes. This is due to the fluctuation of the interlayer spacing (1D density fluctuations) and has been discussed in detail in connection with general deviations from Porod’s law [12]. In the range of the validity of Porod’s law, this component of the SAS is represented by relationships of the type If l (s) ~ s 22 and Jf l (s) ~ s 21 respectively. The total intensity is given by I(s) 5 Ip (s) 1 If l (s) and 0008-6223 / 01 / $ – see front matter  2001 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 00 )00256-6