Production of activated carbon cloth with controlled structure and porosity from a new precursor

Transcription

1 J Porous Mater (27) 14: DOI 1.17/s Production of activated carbon cloth with controlled structure and porosity from a new precursor J. M. Valente Nabais Æ T. Canário Æ P. J. M. Carrott Æ M. M. L. Ribeiro Carrott Received: 24 January 26 / Revised: 25 July 26 / Published online: 26 January 27 Ó Springer Science+Business Media, LLC 27 Abstract The production of micro and mesoporous activated carbon cloth (ACC) from commercial acrylic textile fibres by physical activation with carbon dioxide and the addition of boric acid and sodium hydrogen phosphate as impregnants is reported. The use of sodium hydrogen phosphate leads to samples with greater mesopore volume whereas other ACC production conditions studied mainly result in microporous materials. This work demonstrates that it is possible to produce carbon materials from a commercial acrylic textile cloth with the maintenance of the precursor shape, as shown by scanning electron microscopy (SEM). The materials are formed from layers of aromatic sheets that in a nanoscale resemble graphite and X-ray diffraction studies indicate microcrystallites with dimensions between 5.9 and 7.6 nm for L a (width) and between 2.4 and 2.7 nm for L c (height), corresponding to 7 to 8 parallel graphene layers. Keywords Activated carbon cloth Acrylic fibers Microporous materials Physical activation X-ray diffraction Nitrogen adsorption 1 Introduction A large variety of fibre-based structures can be formed, including long filaments, yarns and woven or knitted J. M. V. Nabais (&) T. Canário P. J. M. Carrott M. M. L. R. Carrott Centro de Química de Évora & Departamento de Química, Universidade de Évora, Rua Romão Ramalho n 59, Évora, Portugal jvn@uevora.pt cloths, which can be used as precursors for activated carbon cloths (ACC) after conventional carbonisation and the same activation methods used for other carbon materials. The precursor used by us in the work reported here is a commercial cloth made from the plane weave of yarns which in turn are made from acrylic fibres. In this sense, ACC can be viewed as a tridimensional organised structure of activated carbon fibres. Activated carbon cloths when compared with more traditional forms of carbon materials, such as powdered or granular activated carbons, have several advantages directly connected to the characteristics of the activated carbon fibres present in their constitution. The most important advantages are lower resistance to flow of liquids and gases, greater flexibility in the material physical shape, higher rate of adsorption/desorption, more uniform porosity and higher surface area. The first patents about ACC were published in the early 7s by Bailey et al. [1] and Economy and Lin [2]. These authors used viscose rayon and phenolic resins, respectively, as precursors, and these can be considered the main raw materials used since then. Other precursors that have been tested with success include polyacrylonitrile [3 5] and Nomex [6] based fabrics. The precursors can be used as received from the manufacturer or, in order to enhance the adsorption capacity or to control other properties, impregnated with chemicals such as ZnCl 2 :AlCl 3 :NH 4 Cl 3% mixture [7, 8], phosphates and borates [9 11], transition metals oxo-complexes [12] and cobalt and iron chlorides [12, 13]. Figure 1 shows the results of a search made on the web resource using the words activated carbon cloth. From the data collected we can notice a growing interest in ACC with more impact in recent years. As shown in Fig. 1, the number of

2 182 J Porous Mater (27) 14: Fig. 1 Number of publications about ACC publications increased from 13 to 28 if we compare the years with Taking advantage of their chemical and physical characteristics ACC have been studied for several applications including adsorption of cations [3, 8, 14, 15], anions [16], VOC s [17], pesticides [18], benzoic acid [9] and phenolic compounds [19], in catalysis [16, 2] and in gas separations [6, 21]. Nevertheless, in order to broaden their application area and to increase the worldwide consumption of ACC the development of new materials with specific properties that can be controlled during the production process is still needed. In this sense, the aim of the work reported here is the production of ACC with controlled porosity and structural properties using a new precursor. 2 Experimental 2.1 ACC production The precursor used for the production of ACC was the commercial acrylic textile cloth FISIVON TM V35 1,3dtex, provided by Fisipe (Barreiro, Portugal). According to the manufacturer the fibre, used to produce the cloth, had been polymerised from acrylonitrile (~9 w%) and vinyl acetate (~1 w%) monomers. For the production of the ACC a strip of about 2 13 cm and a horizontal tubular furnace were used. Stabilisation of the precursor was carried out by heating to 3 C at a rate of 1 C min 1 under a constant N 2 flow of 85 cm 3 min 1 and maintaining for 2 h. The cloth was then carbonised by raising the temperature at a rate of 5 C min 1 to 8 C and maintaining at that temperature for 1 h. Activation was carried out by raising the temperature again by 15 C min 1 to 9 C and then switching to a CO 2 flow of 85 cm 3 min 1, maintaining for the appropriate time in order to obtain several burn-offs (indicated in the sample designations after NTA), switching back to the N 2 flow and allowing to cool to below 5 C before removing the ACC from the furnace and storing in a sealed sample flask. The carbonised sample was designed NTC. Similar strips of precursor were impregnated during 1 min with 15 cm 3 of the following solutions: boric acid 3%(w/v) and sodium dihydrogenphosphate dihydrate 1%(w/v). All reagents used were pro analysis from Riedel-de-Haën. The impregnated strips were oven dried overnight before the carbonisation/activation. The impregnated strips and also a non impregnated strip were carbonised by raising the temperature at a rate of 5 C min 1 to 8 C and maintaining at that temperature for 1 h. Activation was carried out by raising the temperature again by 15 C min 1 to 9 C and then switching to a CO 2 flow of 85 cm 3 min 1, maintaining for the appropriate time in order to obtain several burn-offs (indicated in the sample designations after ABA for boric acid and after FA for sodium dihydrogenphosphate) switching back to the N 2 flow and allowing to cool to below 5 C before removing the ACC from the furnace and storing in a sealed sample flask. The carbonised samples were respectively designed by ABC and FC. The sample not impregnated was designed by NTCSE and NTASE respectively for the carbonised and activated samples. Sample designations and principal production conditions can be found in Table ACC characterisation 2.3 Elemental composition Elemental analysis of carbon, hydrogen, sulfur, nitrogen and oxygen was carried out using a Eurovector EuroEA elemental analyser 2.4 Textural characterisation Nitrogen adsorption isotherms at 77 K were determined using a CE Instruments Sorptomatic 199 after outgassing the samples at 4 C to a residual vacuum of mbar. 2.5 SEM Scanning electron micrographs carried out in secondary electron imaging mode were measured at the University of Lisbon (Portugal) using a JEOL model

3 J Porous Mater (27) 14: Table 1 Samples designations and principal production conditions Sample Impregnant Stabilisation step Burn-off/% wt NTC None Yes NTA15 None Yes 15 NTA26 None Yes 26 NTA45 None Yes 45 NTCSE None No NTASE21 None No 21 NTASE33 None No 33 NTASE48 None No 48 FC Sodium dihydrogenphosphate No FA32 Sodium dihydrogenphosphate No 32 FA41 Sodium dihydrogenphosphate No 41 FA71 Sodium dihydrogenphosphate No 71 ABC Boric acid No ABA15 Boric acid No 15 ABA33 Boric acid No 33 ABA43 Boric acid No 43 JSM-52 LV, after plating the samples with gold, using a Zeiss DSM XRD X-ray powder diffraction patterns were determined using a Bruker AXS D8 Advance diffractometer with Cu Ka radiation at a step size of.2 between 5. and Results and discussion 3.1 SEM Four representative SEM micrographs are shown in Fig. 2. The fabric used as precursor can be considered a plain cloth [16], where filling yarn alternates up and under successive warp yarn. In turn, each yarn is composed by a group of acrylic fibres with the composition referred to in Sect These fibres are also visible in Fig. 3 where a close view of sample NTA26 shows the top view of just one fibre. It can be seen in Fig. 2 that the physical structure of the precursor is maintained after the ACC production. Also, if we compare Fig. 2(b) and (c) with (a) we can note that the ACC production process leads to the densification of the yarns with consequent shrinkage of the materials. As can be seen in Fig. 4 when the ACC production is made without impregnants or the stabilisation step the activated carbon material produced looses the cloth structure. This fact occurs because during the thermal treatment involved in the preparation procedure the precursor suffers decomposition and passes through a process of fusion, prevented by the stabilisation step. When the impregnants are used the stabilisation step is unnecessary probably due to the catalytic effect that these impregnants have on the cyclisation and dehydrogenation reaction that occurs during stabilisation. Further details about these reactions can be found in ref [22]. It can be seen from Fig. 2d, that when sodium dihydrogenphosphate is used as impregnant we can observe the formation of small spheres with diameter between 1 and 4 lm that can be found on the external surface of the material and on the openings of the biggest pores found on these materials. The figure also shows a detail of the spheres composed of the elements P, O and Na, as determined by the SEM EDX microelement analysis. This composition is consistent with the formation of Na 4 P 2 O 7, referred in the literature as the product of the thermal degradation of sodium dihydrogenphosphate at 3 9 C [23]. 3.2 Reactivity The carbonisation yield obtained by us was 43 45%w for all samples, which is superior to the values achieved with viscose cloth. In the later case the reported yield values in the literature vary from 13 to 34%w, depending on the conditions used [9, 24 27]. This result is consistent with our previous work concerned with the production of activated carbon fibres from the acrylic fibres utilised to produce the cloth yarns present on the textile cloth used as precursor [28]. The reactivity of samples toward activation with carbon dioxide can be evaluated by the rate of activation (k) determined from a plot of the burn-off as a function of the time of activation, shown in Fig. 5. Considering the activation as a zero order reaction the slope of the linear regions of the representation are equal to k values [28 3].

4 184 J Porous Mater (27) 14: Fig. 2 SEM images of the precursor (a) and of ACC samples: NTA15 (b) ABA15 (c), FA71 (d) Fig. 3 SEM micrograph of sample NTA26 Fig. 4 SEM micrograph of sample NTASE48 The ACC s produced with a stabilisation step prior to the carbonisation appear to have only one activation region with activation rate of 7.3% h 1. In all other samples we can calculate two activation rates corresponding to two distinct regions of activation for low and higher burn-offs. The highest rate of activation is obtained for samples impregnated with sodium dihydrogenphosphate (samples FA). In this case the cut-off point between the first and second activation region is 3% burn-off and from the slopes of the lines we can calculate activation rates of 31.8 and 13.4% h 1, respectively. The activation rate determined for the NTASE samples is 1.4 and 6.8%h 1, respectively for burn-off values inferior and superior to 2%. As would be expected the rate of activation is higher when the activation is performed in samples without the stabilisation step, which modify the chemical structure of the precursor by converting the open polymeric chain into a cyclic structure capable of supporting higher temperatures [22]. The formation of a more resistant structure that resists the carbon dioxide attack can also explain why NTA samples have only one activation region and NTASE two. The samples produced with boric acid impregnation show 8.3 and 4.9% h 1 as activation rates for samples with burn-off less and higher than 3%. Despite the

5 J Porous Mater (27) 14: Burn-off /%w Activation time /h Fig. 5 Burn-off as a function of activation time NTA NTASE significantly different k values observed for FA and ABA samples the cut-off point between the activation regions is the same in both cases. It is well known that during carbonisation amorphous carbon is produced, which can block the pores already formed and promote the creation of surface deposits. The activation mechanism for samples with two regions involves first the amorphous, or less organised, carbon burn-out due to its high reactivity and a second step related to the carbon dioxide attack on the graphene sheets at the edges or other defect sites, which can lead to rearrangements into a more ordered structure. This activation mechanism is believed to be the mechanism of formation of other carbon materials, such as activated carbon fibres, as would be reasonable to expect. The observation here of lower values for the activation rate in the second region is consistent with the mechanism proposed. The sodium dihydrogenphosphate may act as a catalyst for the reaction between carbon dioxide and the material structure resulting in greater activation rates and also, as shown in Sect. 14, on different porosity development. The activation rates published for the production of activated carbon fibres (ACF) from acrylic fibres utilised to produce the cloth yarns present in the ACC precursor, using the same procedure as for the NTA sample, are 2.2 and 5.5% h 1 respectively for the first and second activation regions [28]. We can note that the rate obtained for the ACC is much smaller than the value mentioned for the first activation region for FA ABA ACF. This fact can be attributed to the higher fibre density in the cloth which can lead to a lower diffusivity of the activation agent into the precursor used. Nevertheless, in the ACF production the two activation regions only separate at 4% burn-off. Hence, bearing in mind that the highest burn-off for ACC is 45%w, we can suppose that for more activated ACC another region may appear. 3.3 Microcrystallite dimensions Representative XRD patterns are given in Fig. 6. In all cases the two common peaks for carbon materials due to reflections from the (2) and (1) planes are clearly visible. The interplanar spacing, d 2, can be evaluated by the application of Bragg s Law to the position of the (2) peak. Estimates of the mean microcrystallite dimensions can be obtained by application of the Debye Scherrer equation. When applied to carbon materials, the equation takes the forms [28]: L c ¼ :9k=b cos h 2 L a ¼ 1:94k=b cos h 1 ð1þ ð2þ where b is equal to the peak width at half height corrected for instrumental broadening, L c and L a are estimates for the height and width of the microcrystallites. Units rbitrary A FA71 (2) FC FA32 (1) θ /º Fig. 6 Representative XRD patterns for ACC samples

6 186 J Porous Mater (27) 14: Also the mean number of layer planes in the microcrystallites (N p ) can be estimate using the ratio L c /d 2. The results of characterisation by XRD measurements are shown in Table 2. First we must note that L c and L a are not exactly equal to the height and width of the microcrystallites because the aromatic layers are not totally parallel to each other, which can lead to interferences in the X-ray beam, but can be used as convenient estimates of these quantities [31]. The true microcrystallite size is likely to be slightly greater than the calculated values. With activation we can note a small increase in L a and a decrease in L c and N p. Although d 2 remains between the limits of.345 ±.5 nm for all samples the results in Table 2 suggest that there is a slight increase with burn-off and that d 2 tends to vary in the order ABA > NTASE > FA > NTA. The d 2 values obtained here are smaller that the values reported by us for ACF produced from essentially the same acrylic fibres utilised here to produce the cloth yarns (.36 ±.3 nm) [28], which can be due to a more compact structure of the cloth in comparison with the fibres used directly as materials precursors. The ACC also shows higher values for L c, L a and N p when compared with the ACF previously studied. These results can be explained by the smaller activation rate observed for ACC, as already mentioned in Sect. 14, which leads to a more ordered layer structure and hence larger microcrystallite dimensions. The NTASE and ABA samples show the greatest decrease in the microcrystallite size when compared with the NTA samples, which is coherent with the higher activation rate reported in Sect. 14. Noteworthy is the case of the FA samples because, despite having the highest activation rate, they show a smaller reduction in L c and N p. This fact can be due to the presence of Na 4 P 2 O 7 spheres that constitute very Table 2 Results of characterisation by XRD measurements Sample d 2 (nm) L a (nm) L c (nm) N p NTC NTA NTA NTCSE NTASE NTASE FC FA FA ABC ABA ABA reactive sites for activation but without significantly altering the ordering of the graphene layers. In the literature it is very rare to find XRD data for ACC, but even so, we can compare our results with previously reported data for commercial and laboratory samples of ACC prepared from viscose rayon [7, 32 34]. All the results are of the same order of magnitude with the d 2 values being similar. In respect to the microcrystallite dimensions we can say that our ACC present bigger L a values and similar or bigger L c values. 3.4 Elemental analysis During the activation process the percentage of carbon increases and the percentage of the heteroatoms nitrogen and hydrogen decrease, as can be seen in Table 3. The oxygen content shows a more irregular pattern because during the activation process some oxygen is eliminated and some oxygen is introduced in the new functional groups formed by the reaction with carbon dioxide. We must note the relatively high quantity of nitrogen, ca. 7 16%w, present in the ACC samples and which could have tremendous importance in several promising areas of application, such as catalysis. For details about the chemical transformation that occurs from the precursor to the activated carbon fibre produced from the acrylic fibres utilised to produce the cloth yarns present in the ACC precursor please see reference [22]. It is also useful to make a comparison between samples by considering the atomic ratio N/C defined by Table 3 Elemental composition of ACC samples Sample N (% w) C (% w) H (% w) O (% w) Precursor NTC NTA NTA NTA NTCSE NTASE NTASE NTASE FC FA FA FA ABC ABA ABA ABA Note: no evidence for the presence of sulfur was found

7 J Porous Mater (27) 14: Eq. 3 and the results of this comparison are shown in Fig. 7. The curve for the samples produced with sodium dihydrogenphosphate is diferent to the others as, for equal burn-off, the N/C ratio is always higher. This demonstrates that, in fact, the use of this impregnant leads to the formation of ACC samples with significantly different characteristics when compared with the ACC produced by the other methods tested. This difference can also be found using the ratio H/C. With increasing degree of activation the level of nitrogen uptake also increases and in all cases the knee of the isotherm becomes more rounded, indicating an increase in the mean pore width. For samples with low burn-off values the plateau is reached for relative pressures inferior to.5 whereas for higher values of burn-off this is only achieved at relative pressures superior to.2. Y/C ¼ð%Y/M mðyþ Þ=ð%C/M mðcþ Þ ð3þ where Y/C is the atomic ratio for the element Y, %Y and %C the mass percentages (obtained by elemental analysis) of the element Y and carbon, respectively, and M m(y) and M m(c) the molar masses for the element Y and carbon. If we sum the mass percentages in Table 3 for all elements we can see that for samples NTA, NTASE and ABA the sum is equal to 98 11%w, whereas for sample FA it is only 93 95%w probably due to the presence of the inorganic deposit of Na 4 P 2 O 7. n ads /mmolg FA71 FA41 FA32 ABA4 ABA3 ABA1 3.5 Textural properties 2 The nitrogen adsorption isotherms are shown in Figs. 8 and 9. It can be seen that the isotherms are all type I with very low slope in the multilayer region indicating a low external surface area. For the FA samples the isotherms present a slightly higher slope indicating that the external area is higher, as can also be observed in Table 4. All the isotherms are reversible with the exception of that determined on FA71, which approaches type H2 hysteresis according to the classification of IUPAC [35, 36] p/p o Fig. 8 Nitrogen adsorption (open symbols)/desorption (closed symbols) isotherms determined at 77 K on ABA and FA 1 8 NTASE4 NTA4 N/C,2,18,16,14,12,1,8,6,4,2 NTA NTASE FA ABA Burn-off / %w Fig. 7 Activated carbon cloth N/C atomic ratio n ads /mmolg NTASE3 NTA2 NTASE2 1 NTA1 NTC p/p o Fig. 9 Nitrogen adsorption/desorption isotherms determined at 77 K on NTA and NTASE

8 188 J Porous Mater (27) 14: Table 4 Textural characteristics of ACC samples Sample S BET (m 2 g 1 ) a s Method DR Method V S (cm 3 g 1 ) S ext (m 2 g 1 ) V o (cm 3 g 1 ) E o (kj mol 1 ) NTC NTA NTA NTA NTASE NTASE NTASE FA FA FA ABA ABA ABA The nitrogen isotherm for sample FA71 is quite different to the other isotherms. Besides the existence of a hysteresis loop the isotherm shows, after an initial part where the small micropores are being filled, a very smooth and almost linear approach to the plateau. The isotherm shape indicates the existence of a broader pore size distribution that goes from small micropores to small mesopores. It is also interesting to note that the only carbonised sample that shows nitrogen adsorption at 77 K is NTC, which is the only product made with the stabilisation step prior to the carbonisation. Noteworthy is also the fact that in spite of the loss of the cloth structure in the NTASE samples, they still develop porosity, as can be seen in Fig. 9 and Table 4. The isotherms were analysed by means of the a s method, using previously published standard data for the adsorption of N 2 on carbon materials [37] andby the Brunauer Emett Teller (BET) and Dubinin Radushkevich (DR) methods. The a s plots, not shown here, reveal good linearity in the linear range and, from the slope and intercept, the values of external surface area and total pore volume, given in Table 4, were calculated. The values of the apparent BET area and of the micropore volume and characteristic energy calculated from the intercept and slope of each DR plot can also be seen in Table 4. The different porosity development for samples FA is clearly seen in Figs. 1 and 11 where the pore volumes, V s and V o, and apparent BET area, S BET, are plotted as a function of burn-off. For all samples, with the exception of the FA samples, the porosity development is very similar with a linear increase of both the apparent BET area and the micropore volume as the sample burn-off increases. For the FA samples, V s, V o and S BET are smaller than the values obtained for other samples with similar burn-offs. The general trend shows a different behaviour with an initially faster increase after which there is a stabilisation of the values. V s and V o are almost equal for samples with low burn-off, less than 26%w, indicating the presence of only small micropores. On the other hand, for samples with higher burn-off the values of V s become greater than V o, which indicates materials with a broader pore size distribution and the existence of pores with larger mean pore widths. The pores enlargement motivated by the use of impregnants is consistent with the work reported on the literature [1, 3, 38]. 4 Conclusions The results presented show that commercial acrylic textile cloth can be used to produce ACC with V o,v s /cm 3 g NTA NTASE ABA Burn-off /%w Fig. 1 Variation of V s (open symbols) and V o (closed symbols) with burn-off FA

9 J Porous Mater (27) 14: Acknowledgments The authors are grateful to Angel Ortiz (Univ. Extremadura, Spain) for the SEM EDX microanalytical elemental maps and to FISIPE-Fibras Sintéticas de Portugal S.A. (Portugal) for the provision of samples. 6 S BET /m 2 g Burn-off /%w Fig. 11 Variation of S BET with burn-off controlled pore structure depending on the preparation conditions. The linear relationship between sample burn-off and activation time found in our case indicates that the activation follows a zero order reaction. The activation mechanism is based on one or two steps where firstly the less well organised carbon, produced during the carbonisation, is burnt out and secondly the carbon dioxide reacts with the structure of the pseudographitic layers. The activation rate for the second step is smaller than for the first region. The use of sodium dihydrogenphosphate as impregnant produces samples with quite different properties to the other samples obtained due to the formation of inorganic deposits that lead to an increase in the activation rate and the porosity development. The microcrystallites have larger dimensions than activated carbon fibres produced using the same procedure, mainly the crystallite height and width and also the mean number of layer planes. Despite the fact that the samples produced have interesting physical properties and mechanical strength the flexibility of the ACC samples need to be improved by adjusting the temperature program used during the ACC production in order to obtain less brittle samples. We also prove that the formation of ACC from commercial acrylic textile cloth is only achieved if a stabilisation step at temperature below the degradation temperature is introduced into the production process prior to carbonisation of the precursor. Alternatively ACC can also be made using inorganic impregnants that prevent fusion of the precursor during the production process. NTA NTASE ABA FA References 1. A. Bailey, P. Arthur, P. Maggs, British Patent 131, 11 (1971) 2. J. Economy, R.Y. Lin, US Patent 3769, 144 (1973) 3. J.R. Rangel-Mendez, M. Streat, Water Res. 36, 1244 (22) 4. M. Wu, Q. Zha, J. Qiu, Y. Guo, H. Shang, A. Yuan, Carbon 42, 25 (24) 5. S. You, Y. Park, C. Park, Carbon 38, 1453 (2) 6. S. Villar-Rodil, R. Navarrete, R. Denoyel, A. Albiniak, J. Paredes, A. Martinez-Alonso, J.M.D. Tascón, Microporous Mesoporous Mater. 77, 19 (25) 7. K. Gurudatt, V.S. Tripathi, Carbon 36, 1371 (1998) 8. B.M. Babic, S.K. Milonjic, M.J. Polovina, S. Cupric, B.V. Kaludjerovic, Carbon 4, 119 (22) 9. E. Ayranci, N. Hoda, E. Bayram, J. Colloid Interface Sci. 284, 83 (25) 1. J.J. Freeman, F.G. Gimblett, R.A. Roberts, K.S. Sing, Carbon 4, 559 (1987) 11. N.M. Osmond, Adsorpt. Sci. Technol. 18(6), 529 (2) 12. J.J. Freeman, F.G. Gimblett, K.S. Sing, Carbon 27(1), 85 (1989) 13. A.W. Morawsky, K. Kalucki, M. Nakashima, M. Inagaki, Carbon 32(8), 1457 (1994) 14. K. Kadirvelu, C. Faur-Brasquet, P. Le Cloirec, Langmuir 16, 844 (2) 15. C. Faur-Brasquet, K. Kadirvelu, P. Le Cloirec, Carbon 4, 2387 (22) 16. Y. Matatov-Meytal, M. Sheintuch, Appl. Catal. A-Gen. 231, 1 (22) 17. T. Mays, in: Active Carbon Fibers, Carbon Materials for Advanced Technologies, ed. by T.D. Burchell, (Pergamon, Oxford, 1999) 18. E. Ayranci, N. Hoda, Chemosphere 6(11), 16 (25) 19. E. Ayranci, O. Duman, J. Hazard. Mater. B124, 125 (25) 2. U.M.Meytal, Catal. Today 12, 121 (25) 21. Golden T.C., Golden C.M.A., Zwilling D.P., US Patent 6, 565, J.M. Valente Nabais, P.J.M. Carrott, M.M.L. Ribeiro Carrott, Mater. Chem. Phys. 93(1),1 (25) 23. W. Büchner, R. Schliebs, G. Winter, K.H. Büchel, Industrial Inorganic Chemistry, (VCH, New York, 1989) 24. A.C. Pastor, F. Rodríguez-Reinoso, H. Marsh, M.A. Martínez, Carbon 37, 1275 (1999) 25. F. Rodríguez-Reinoso, A.C. Pastor, H. Marsh, M.A. Martínez, Carbon 38, 379 (2) 26. A. Capon, F.A. Maggs, G.A. Robins, J. Phys. D Appl. Phys. 13, 897 (198) 27. P.J. Carrott, J.J. Freeman, Carbon 29(4/5), 499 (1991) 28. P.J.M. Carrott, J.M.V. Nabais, M.M.L. Ribeiro Carrott, J.A. Pajares, Carbon 39, 1543 (21) 29. J.J. Freeman, F.G. Gimblett, Carbon 25(4), 565 (1987) 3. J.J. Freeman, F.G. Gimblett, R.A. Roberts, K.S. Sing, Carbon 26, 7 (1988) 31. A. Oberlin, S. Bonnamy, K. Lafdi in: Structure and Texture of Carbon Fibers, Carbon Fibers, ed. by J.-B. Donnet, T.K. Wang, S. Rebouillat, J.C.M. Peng, 3rd ed. (Marcel Dekker, New York, 1998)

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