Journal of Analytical and Applied Pyrolysis 43 (1997) 125%13X. G. de la Puente, J.J. Pis b-*, J.A. Menhdez b, P. Grange a

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1 ELSEVIER Journal of Analytical and Applied Pyrolysis 43 (1997) 125%13X JOURNAL OI ANALYTICAL and APPLIED PYROLYSIS Thermal stability of oxygenated carbons functions in activated G. de la Puente, J.J. Pis b-*, J.A. Menhdez b, P. Grange a a UnirG de Cataivse et Chimie des MatCriaux Diuis&, Universiti Catholique de Louvain. Place Croix du Sud, 2,17, 1348 Louvain-la-Neucr. Belgium h Institute National del Carbdn, Conseja Superior de Invesrigaclonrs Cienr@as, Apartado Oairdo. Spuin Received I8 June 1997: received in revised form 23 July 1997: accepted 23 July 1997 Abstract Activated carbons with various degrees of oxidation were prepared by treatment with concentrated nitric acid at various temperatures. Selective removal of the oxygen complexes by heating under nitrogen flow at 573, 773 and 1073 K was also considered. The samples were characterised through temperature-programmed desorption (TPD), Fourier transformed infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) techniques. FTIR and XPS techniques were found to be very useful for relating the shape of CO, desorption profiles during pyrolysis with the nature of the functional groups desorbed. Three steps in TPD profiles were observed: (i) the desorption of carboxylic acid groups in the form of CO2 in the lower temperature range, K; (ii) the decomposition of lactone groups created during HNO, treatment and, to some extent, originated during heating rearrangements, in the temperature range K; and (iii) the elimination of the more stable ether groups upon treatment at temperatures above 823 K. The combination of these three techniques (TPD, FTIR and XPS) gives a reasonable picture of the surface chemistry of the oxidised activated carbons Elsevier Science B.V. Kqwords: Activated carbon; Nitric acid oxidation; Pyrolysis; Surface oxygen groups: Thermal treatment; TPD; FTIR; XPS * Corresponding author. Tel.: ; fax: /97/$ Elsevier Science B.V. All rights reserved PII SO (97)00060-O

2 126 G. de la Purnte et ~1., J. Anul. Appl. Pyrolysis 43 (1997) Introduction Electrochemical properties [l-3], acid-base behaviour [446], hydrophilic-hydrophobic nature [668] and other properties of activated carbons are known to be strongly influenced by the presence of heteroatoms on the carbon surface and in particular by oxygen. This, in turn, affects their behaviour when used as adsorbents [9,10], catalyst [11,12] and/or catalyst supports [l 11. Thus, carbons with large surface areas are frequently used as supports for heterogeneous metal catalysts and their performance is expected to be strongly influenced by the functional groups present on their surfaces [II]. For this application, the initial metal dispersion, resistance to agglomeration and the catalytic behaviour of the system are all sensitive to the state of the carbon surface. For example, in molybdenum [ 131, heteropoly acids [ 141, palladium [ 151 and platinum [ 161 supported on carbon, modification of the carbon surface chemistry resulted in a significant change of the loading capacity and of the catalytic properties. Consequently, it is of great importance to be able to characterise, as accurately as possible, the nature of the oxygen-containing species present on carbon surfaces. In this sense, transmission infrared spectroscopy, widely used to characterise the surface groups on several oxides, has been applied in the characterisation of various types of carbon and carbonaceous materials [17]. X-ray photoelectron spectroscopy (XPS) has also been used for oxidised carbons, mainly carbon fibers. These two spectroscopic techniques, however, have some limitations. In the case of IRTF various degrees of success have been attained, depending on the carbon system examined, because activated carbons can act as a very effective black body absorbers, thereby markedly diminishing or completely attenuating beam energy throughput. Therefore, these carbons materials must be diluted with an appropriate transparent medium to allow sufficient signal intensity. The information provided by XPS spectra is not specific enough to probe the detailed structure of the oxidised layer, especially in carbons with a low oxygen content. On the other hand, the amount of CO and CO, released during temperature programmed desorption (TPD) experiments allows the estimation of the relative amount of the oxygenated species and gives partial information on their nature as well. In addition, the interpretation of the TPD spectra, however, is not always straightforward and in most cases it needs to be complemented with additional information from other techniques. The aim of this paper is to illustrate the potential of a combination of TPD with Fourier transformed infrared spectroscopy (FTIR) and XPS for the characterisation of the surface chemistry of carbon materials. Particular attention was paid to characteristic temperatures at which CO/CO, are released during TPD, as a tool to identify oxygen-containing surface groups on carbon materials. Discrete peaks of the CO, spectra were related with the type of oxygen functionality that give rise to these peaks by analysing (using IRTF and XPS) different samples of a chemically modified activated carbon which surface groups were selectively removed from the carbon surface by thermal treatments.

3 G. c/r Iu Puentr et cd. J. And. Appl. Pwol~.vi.c 4.1 (1997) l Experimental 2.1. Oxidution and therm& treutments An activated carbon supplied by Merck (18-35 mesh ASTM, Merck-9631 ) (sample referred to as KO) was used in this study. This carbon was oxidised with HNO, in order to incorporate oxygen surface complexes. The procedure was as follows: 200 cm3 of concentrated nitric acid was heated in a glass flask equipped with a stirrer. After reaching the desired temperature (298, 333 and 363 K). 20 g of activated carbon was added. The reactions were stopped after 3 h and the carbons were washed with distilled water until neutral ph indication of the effluent water and dried overnight under nitrogen atmosphere at 283 K. These samples will be referred to as N298, N333 and N363, respectively. Other sample (NR) was oxidised at refhtx, the reaction was conducted in a three-necked flask equipped with a stirrer and a condenser at 383 K for 3 h. Thermal treatments were performed by placing the oxidised carbon sample NR in a tubular furnace under a helium flow of 60 cm3 min and heating it at the desired temperature (573, 773 and 1073 K) for 1 h. Samples will be referred to as NR-573, NR-773 and NR-1073, respectively. These treatments were carried out in order to selectively eliminate the oxygenated groups, previously created by the treatment with HNO, and to identify the chemical functionalities either remaining in or removed from the carbon surface Churucterisution qf surjbce oxygen groups The apparatus used to carry out the TPD experiments consists basically of a flow reactor coupled to a gas chromatograph and a mass spectrometer (GC-MS. Hewlett-Packard G1800A GCD System) for the analysis of the gases evolved during thermolysis. In these experiments, 200 mg of sample were heated at 10 K min up to 1073 K, while He was flowed through with a flow rate of 60 cm min. FTIR spectra were recorded using a Brucker FT 88 spectrometer. The samples were diluted in KBr to a O.l /;, [carbon/(carbon + KBr)] content. Corrections were made with respect to the mass of carbonaceous material in the pellets. The spectrum of the diluent itself was obtained and subtracted from those of the mixture. For the XPS analysis, a Surface Science Instruments SSX-100, Model 206. spectrometer with a monochromatic AlKa source ( ev), operating at 10 kv and 15 ma was used. The spectrometer was interfaced to a Hewlett-Packard 9000/310 computer for data acquisition and treatment. The powdered samples were pressed on stainless steel capsules which were mounted on top of the specimen holder. The samples were degassed at 5.10 torr previous to their introduction in the analysis chamber. During the analysis the pressure did not exceed 5. IO tort-. The lines Cls, Ols, Nls and S3p were investigated. In all cases, the binding energy of the Cls line was f 0.2 ev. No modification in the position of this line at

4 128 G. de la Puente et al. /J. Anal. Appl. Pyrolysis 43 (1997) the beginning and after the analysis was observed. A non-linear, Shirley-type [18], baseline and an iterative least-squares fitting algorithm were used to decompose the peaks, the curves being taken as 85% Gaussian and 15% Lorenzian. Surface atomic concentration ratios were calculated as the ratio of the corresponding peak areas, corrected with theoretical sensitivity factors based on Scofield s photoionisation cross sections [19]. 3. Results and discussion 3.1. Temperature programmed desorption Linear TPD experiments were used to characterise the surface oxygen groups of the different activated carbons prepared (i.e. the degree of oxidation-the total amount of CO and CO, evolved-and the nature and thermal stability of the oxygen complexes formed). In general, in TPD experiments, H,O, CO, and CO evolution is observed. This evolution results from the decomposition of the different oxygen-containing groups present on the carbon surface. In this sense, CO, appears at lower temperatures than CO and has been attributed to the decomposition of carboxylic, anhydride [20] and/or lactone [21] groups: acidic groups where C is bonded to two oxygen atoms. On the other hand, CO desorption takes place at higher temperatures from the decomposition of different types of surface oxygen groups, i.e. phenol, carbonyl, quinone, ether and pyrene groups: weakly acidic, neutral and basic groups, respectively, where C is bonded to one 0 atom [22,23]. Cyclic anhydrides of carboxylic acids produce CO, and CO in thermal decomposition. The comparison of TPD spectra of carbons KO, N298, N333, N363 and NR, presented in Fig. 1, show the effect of the modification of carbon surface chemistry by the HNO, treatment. Though the shape of all TPD spectra is similar (CO, evolution below 1073 K and CO evolution above 673 K) the quantity of the evolved gases increases with increasing treatment temperature, showing a temperature dependence of the HNO, treatment on the total amount of oxygen complexes. It can be observed that CO, is released through the whole temperature range with marked steps (Fig. 1). For all the samples, the CO, evolution increases quickly to reach a maximum at z 523 K and drops at z 623 K (zone I). A second maximum is found at z 723 K (zone II). Groups that are eliminated at temperatures above 823 K produce mainly CO and some CO, (zone III). This shape of the CO, profiles presumably is characteristic of the nature of the functional groups desorbed. In order to check the reproducibility of the experiments, TPD spectra were recorded at various heating rates (5, 10 and 15 K min ~ ). Upon comparing the shape of the CO, profile at different heating rates, it was found that the curves for each heating rate presented the same shape with the maxima at the same temperatures. Thus, the gas evolution data are essentially free of secondary reactions (i.e. CO + C(0) = Cr+ CO, [24]) and characteristic of the functional groups desorbed PO1.

5 G. de iu Puente et al. ij. Anal. Appl. Pyro1ysi.c 43 (1997) The most temperature sensitive groups are carboxylic acid groups [25]. The acidic groups localised on polyaromatic rings are susceptible to mesomerisation and rearrangements. On TPD experiments, it was also observed that H,O desorbed up to temperatures around 673 K (not shown); as samples were dried before experiments, the amount of H,O is felt to be originated from H-bonded H,O associated with acidic oxygen complexes and from condensation of any adjacent phenolic and/or carboxylic acid groups present on the samples [20]. This suggests that, in addition to the elimination of oxygen surface groups. there was also a partial transformation of these groups. During heat treatments, suitable carboxylic and phenolic groups can be transformed into anhydride or lactone groups by dehydration reactions. These groups, more stable than the carboxylic ones, do not decompose at low ( z 573 K) temperatures and release CO, and CO at higher temperature ( x 723 K). In this sense, it is proposed that the decomposition of stable anhydride or lactone groups, created on the carbon surface during HNO, treatment and, to some extent. formed during heating rearrangements, are responsible for the second CO, peak observed at,y 723 K. 3.5 Id t NR A 363 CO? desqtion loo Temperature (K) IO 2.0 IO I.5 IO Id loo0 II00 Temperature (K) Fig. I Temperature programmed desorption (TPD) profiles of the activated at various temperatures. carbons oxidised in HNO:

6 130 G. de la Puente et al.,/j. Anal. Appl. Pyrolysis 43 (1997) uou Wave number cm- Fig. 2. FTIR spectra of the activated carbons oxidised in HNO, at various temperatures FTIR spectroscopy Numerous FTIR spectroscopic studies have been conducted on various forms of carbon and band assignments have been made based on model compounds and expectations of the types of functional groups present, mainly through identification by chemical methods [17, Over the years, there has been frequent disagree- ments regarding band assignments and the fact that these bands can be quite broad has not helped this situation. Nevertheless, a consensus in the assignment of band frequencies to different functional groups is possible [30]. The FTIR spectra of carbons KO, N298, N333, N363 and NR are presented in Fig. 2 and show the effect of the modification of carbon surface by the HNO, treatment. Upon oxidation with nitric acid, an increase of the band between 1000 and 1300 cm ~ is first observed, which is attributed to C-O single bonds, such as those in ethers, phenols and hydroxyl groups. These results indicate that these groups are formed by the first step of the nitric acid oxidation involving reactive and relatively labile sites on the non-aromatic edges of the basal planes. It is suggested that an oxidised layer is mainly formed by ether-like bonds crosslinking the aromatic substrate. In samples obtained by treatment above temperatures of 333 K, a new peak centred at z 1720 cm _ appears, assigned to stretching vibration of carboxyl groups on the edges of the layer planes [31,32] or conjugated carbonyl groups (CL0 double bonds in carboxylic acid and lactone groups). This band begins to develop in the spectrum of the sample N333 and its intensity is stronger in the spectra of the samples treated at higher temperatures. It was not observed in any band of the range cm ~, indicating that anhydrides are not formed-or in very low quantity-during nitric acid oxidation of this activated carbon.

7 G. de la Puentr et al. J. Anal. Appl. Pyrol~,si.c 43 (1097) lil Upon the formation of the 1720 cm band, an increase in the absorption at 1580 cm is found. This band is generally ascribed to C-C double bonds made infrared-active by binding of oxygen near one end [33]. The presence of the oxygen double bond. conjugated with the carbon basal planes. results in a stronger adsorption at 1580 cm ~. The FTIR spectra confirm the formation of carbonyl, lactone, ether and hydroxyl groups during the oxidation process. Results obtained using this technique have allowed us to establish some hypotheses about the type of surface oxygen groups present in the activated carbons, but cannot conclusively establish their chemical structures. The discussion about TPD spectra in the previous section shows the usefulness of this technique in the analysis of the surface chemistry of the carbon samples. In this way, a thorough study of the TPD profiles reflects the degree of surface oxidation and somehow discriminate the nature of oxygen complexes. In order to identify the nature of the oxygenated species in the various steps observed in TPD experiments. an analysis of the activated carbons after selective removal of the created chemical functionalities was carried out. For this purpose. in addition to the carbon treated at various temperatures, activated carbon oxidised in nitric acid at reflux temperature (NR) was chosen because it has the highest quantity of oxygen functional groups. Separate samples prepared from the activated carbon NR by treatment in He at and 1073 K (NR-573. NR-773 and NR-1073 respectively). were characterised by FTIR and XPS. These treatment temperatures were 2 50 K above those corresponding to the maximum desorption rate of each observed peak in the TPD spectra. Sample NR-573 shows the effect of the elimination of H,O and the more unstable CO,-type complexes. Heat treatment up to 773 K produces the selective elimination of most of the acidic groups of the carbon surface (CO,-type complexes). The CO-type groups that decompose at T < 773 Ii are also eliminated. but most of the CO-type complexes remain on the surface. By means of the spectra UOU Wove number cm- Fig. 3. FTIR spectra of the activated carbons resulting from thermal treatments in He at various temperatures.

8 132 G. de la Puente et al. /J. Anal. Appl. Pyrolysis 43 (1997) 12S- 138 f,o (4 Fig. 4. Possible rearrangements and mesomerisation during heating treatment at 573 K: formation of lactones from aromatic carboxylic acids with neighbouring hydroxyl groups (a), lactols from aromatic carboxylic acids (b) and carbonyl groups or ether bridges between rings (c). recorded for sample NR-1073, the effect of the elimination of all the CO,-evolving complexes as well as most of the CO-type functional groups can be assessed. The FTIR spectra of carbons NR, NR-573, NR-773 and NR-1073 are displayed in Fig. 3 and show the effect of the modification of carbon surface chemistry by thermal treatments. The main feature observed is the progressive decrease in the 1720 cm - band, indicating that C=O groups from carboxylic acids and lactones are desorbed. Comparing the spectra of NR and NR-573 samples, the main features observed are: (i) A decrease in the 1720 cm- band, assigned to C=O groups in carboxylic acids and lactones and in the peak at around 1200 cm-, which is attributed to C-OH single bonds, indicating that carboxylic acids are the most temperature sensitive and are desorbed under heat treatment at 573 K. (ii) A decrease in the absorption at 1580 cm ~, indicating a reduction of the oxygen double bond conjugated with the carbon basal planes. In the band between 1000 and 1300 cm -, the absorption at around 1060 cm -_ remains unchanged and even increases slightly. This band is attributed to C-O single bonds, such as those in ethers and lactones [34]. These results indicate that these groups remain on the activated carbon surface after treatment at 573 K and that they are to some extent formed by rearrangements of the acidic groups localised on polyaromatic rings. The mechanisms of formation of these groups are presented in Fig. 4: (a) formation of lactones from aromatic carboxylic acids with neighbouring hydroxyl groups; (b) lactols from aromatic carboxylic acids; and (c) carbonyl groups or ether bridges between rings.

9 G. dr la Puente et al. J. Anal. Appl. Pyrolysis 43 (I 997) Table I Activated carbons surface composition obtained from XPS analyses Sample C ( %I at) 0 ( %5 at) N (!h at) S ( X at) KO N N N NR NR NR NR (iii) There is no evidence of any band in the range cm --, indicating that anhydrides are not formed by carboxylic acid rearrangements during heat treatment. The spectrum of sample NR-773 shows a decrease of the bands at 1720, 1580 and looo cm -, confirming that groups such as lactones that yield COz plus CO are eliminated in this range of temperatures. Spectrum of sample NR-1073 shows the almost complete elimination of oxygen functional groups as a consequence of heat treatment X-my photoelectron spectroscopy The surface composition obtained by XPS analysis for the different activated carbons are displayed in Table 1. The O/C intensity ratio is low for the original activated carbon and increases with the oxidation temperature. The small amount of sulphur present in the original activated carbon is progressively eliminated by the nitric acid treatment. On the other hand, even when samples were repeatedly washed, some nitrogen remained chemisorbed on the surface of activated carbons. The Cls signal of all the samples consists of a mayor peak at ev, that corresponds to non functionalised C, i.e. the contributions of C (Csp and Csp3) belonging to the carbon skeleton of the material and the contributions of aliphatic Csp that correspond to hydrocarbons. The obtained spectra are represented in Fig. 5 for the oxidation series. It can be observed that the intensity of this peak decreases with the increasing treatment temperature in HNO,, indicating that some of the carbon atoms were functionalised by means of the creation of carbon-oxygen bonds. Upon heat treatment, the relative amount of non-functionalised carbon gradually increases, indicating that the more unstable carbon bonded to oxygen was desorbed. The degree of organisation of the carbon material may be evaluated from the asymmetry of Cl s peak. More information can be gained from satellites of the Cls peak at higher binding energies [ The shifts from the main peak range from 1.7 ev for carbon atoms singly bonded to oxygen to 4 ev for carbon atoms in carboxyl groups or

10 134 G. de la Purnte et ul. /J. Anal. Appl. Pyrolysis 43 (1997) Binding Energy (ev) Fig. 5. XPS spectra (Cls level) of the activated carbons oxidised in HNO, at various temperatures. esters [38]. The satellite at z 291 ev can be ascribed to the n electrons delocalised in the aromatic network of carbon [34,39,40]. Curve fitting with defined BE and peak width for the C=O and C-O group satellites show the increase in oxygen functional groups as a consequence of nitric acid treatment. The increase in the Cls satellite at around ev is especially noticeable, indicating the formation of a great amount of functional groups with GO double bonds, in agreement with FTIR results. The 01s signals lie between 529 and 536 ev. A multitude of oxygen functional groups have their BE in this region. Due to the high electronegativity of oxygen, the 01s signal is not very sensitive to the way in which the oxygen is bound. Fig. 6 gives a view of the effect of the selective removal of the different surface oxygen groups upon heat treatment. We found, essentially, two different binding energies for oxygen singly and doubly bound to carbon [25,41]. The peak near 531 ev BE has been attributed to oxygen doubly bound to carbon and the component at higher BE, 533 ev, is assigned to oxygen linked to carbon by single bonds-e.g. phenol and ether groups--[40,42]. The component having binding energies situated around 535 ev is usually associated with adsorbed water molecules and the component centred at 537 ev corresponds essentially to occluded CO or COz.

11 G. de la Puenie et al. /J. And. Appl. P~W~I.SIJ 43 (1997) The signal of NR clearly consists of contributions of GO and C-O groups. Upon increasing treatment temperature under inert atmosphere, two features of the 01s change. First, the BE corresponding to maximum intensity shifts slightly to a higher value. Secondly, the 01s signal is almost split into two peaks after treatment at 773 K. In between, there is a gradual change. The shift of the 01s signal to higher BEs indicates a decrease in functional groups of the carboxylic acid type and an increase in the relative concentration of groups with carbon-oxygen single NR - N363 N :.5 Binding Energy (ev) Fig. 6. XPS spectra (01s level) of samples resulting from selective removal of oxygen functions bq thermal treatments in He at various temperatures.

12 136 G. de la Puente et al. /J. Anal. Appl. Pyrolysis 43 (1997) Table 2 Relative concentration of oxygen functionalities obtained from XPS 01s peak deconvolution Sample Binding energy (ev) Relative concentration (!/I,) NR NR NR NR bond, such as phenols and ethers. This confirms the above mentioned difference of stability upon heat treatment of the different oxygen functional groups created on activated carbon surface. Upon heat treatment, when comparing sample NR with NR-573, a very important decrease in C=O peak was observed, corroborating that these groups are the most temperature sensitive ones, which are desorbed first upon treatment in an inert atmosphere. After higher temperature treatments, NR-573 to NR-1073, there was a progressive decrease in the C=O peak, in agreement with FTIR results. The relative amounts of the oxygen-containing functional groups assigned to the XPS peaks are displayed in Table 2. The decrease in the relative amount of the component at 531 ev in NR-573 confirms the desorption of the species that contain mainly oxygen doubly bound to carbon upon heat treatment at 573 K. Carboxylic acid groups are desorbed as CO, in the lower temperature range, with the maximum rate occurring at 533 K, giving rise to the first CO, peak of TPD spectra. The spectra of NR-773 is of special interest because it reflects the selective removal of oxygen-containing functional groups: it has its maximum at BE = ev, while the peak at 531 ev appears just as a mere shoulder. These signals are assigned as the C-O and GO peaks, respectively. This result confirms that, after treatment at 773 K in He, as shown in TPD spectra, most of the C-O groups in carboxylic acid or lactone type groups have been desorbed. The remaining functions are mainly the more stable C-O, phenol or ether groups. The oxygen here will be eliminated only upon treatment at temperatures above 800 K, mainly in the form of CO and some CO, (the observed small third TPD peak). The XPS spectra of sample NR-1073 shows the effect of the removal of most of these oxygenated functional groups as a consequence of heat treatment in He at 1073 K. 4. Conclusions Surface chemistry of an activated carbon was modified by nitric acid oxidation and by thermal treatment in inert atmosphere. The formation and development of oxygenated functional groups during the oxidation process and their thermal

13 G. de la Puente et al. /J..4nal. Appl. Pyrolysis 43 (1997) stability by the selective removal of these groups, were investigated. The combination of heat treatment experiments with the use of TPD, FTIR and XPS techniques has proved to be a powerful method for the characterisation of the surface chemistry of oxidised activated carbons. The total amount of oxygen complexes incorporated to activated carbons as a consequence of nitric acid oxidation depends on treatment temperature. The information that can be attained from TPD experiments is limited to the determination of the degree of surface oxidation and the classification of the oxygen-containing species into either CO,- or CO-releasing groups at different temperatures. However, when TPD spectra are interpreted using complementary techniques, such as FTIR and/or XPS, a great deal of information can be obtained from them. In fact, spectroscopic techniques confirmed the formation of carbonyl, lactone, ether and hydroxyl groups during the oxidation process. On the other hand, the presence of anhydrides was not observed. The shape of the CO, profiles of the TPD spectra is characteristic of the nature of the functional groups desorbed and independent of the experimental conditions used. The FTIR and XPS analyses of the activated carbon after selective removal of the oxygenated functionalities showed that: (i) carboxylic acid groups are desorbed in the form of CO, in the lower temperature range, K; (ii) the decomposition of lactone groups created during HNO, treatment and to some extent, originated during heating rearrangements, takes place in the temperature range K; (iii) the more stable phenol or ether groups are only eliminated upon treatment at temperatures above 823 K. The consistency of the observations with the different techniques used in the data analysis approach shows the usefulness of characteristic temperatures of CO2 TPD profiles in the assessment and identification of oxygen surface functional groups in activated carbons. Acknowledgements G. de la Puente thanks the Fundacion para el Foment0 en Asturias de la Investigation Cientifica Aplicada y la Tecnologia (FICYT) for a postdoctoral fellowship. References [I] K. Kinoshita. Carbon: Electrochemical and Physicochemical Properties. Wiley-Interscience. New York, [2] M.O. Corapcioglu, C.P. Huang, Carbon 25 (1987) 569. [3] J.A. MenCndez, M.J. Illin-G6mez, C.A. Le6n y Le6n, L.R. Radovic, Carbon 33 (1995) [4] H.P. Boehm, M. Voll, Carbon 8 (1970) 227. [5] E. Papirer, S. Li, J.B. Donnet, Carbon 25 (1987) 243. [6] J.A. MenCndez, J. Phillips, B. Xia, L.R. Radovic, Langmuir 12 (1996) [7] S.S. Barton, M.J.B. Evans, J.A.F. MacDonal, Langmuir IO (1994) 4404.

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