Dynamic adsorption of ammonia on activated carbons measured by flow microcalorimetry

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1 Applied Catalysis A: General 233 (2002) Dynamic adsorption of ammonia on activated carbons measured by flow microcalorimetry M. Domingo-García a, A.J. Groszek b, F.J. López-Garzón a,, M. Pérez-Mendoza c a Grupo de Investigación en Carbones, Departamento de Química Inorgánica, Facultad de Ciencias, Universidad de Granada, Granada, Spain b Microscal Limited, 79 Southern Row, London W10 5AL, UK c Department of Chemical Engineering, The University of Edinburgh, The King s Buildings, Mayfield Road, Edinburgh EH9 3JL, UK Received 11 June 2001; received in revised form 26 February 2002; accepted 28 February 2002 Abstract Several activated carbons of different textural and chemical surface characteristics have been used to study the ammonia adsorption. The textural characteristics were determined by N 2 and CO 2 adsorption at 77 and 273 K. The chemical surface groups were estimated by thermal programmed desorption followed by mass spectrometry (TPD-MS) and by selective titrations in aqueous solutions. The ammonia adsorption was studied under dynamic conditions from N 2 diluted flow using a flow adsorption microcalorimeter. The ammonia adsorption consists of reversible and irreversible components. The former is assigned to physisorption process while the latter is adsorption on chemical groups. The plots of the differential heats of adsorption versus the cumulative adsorption point to the existence of a wide distribution of acid sites some of which are very strong. However, they are not always easily accessible to NH 3 because constrictions in the pores-network hinder the access which forces to re-arrangement Elsevier Science B.V. All rights reserved. Keywords: Adsorption; Carbon materials; TDP-MS; Microcalorimetry 1. Introduction Catalytically active sites on the surfaces of solids possess different strengths and accessibilities. The latter property is especially important for short contacts between reactives and the catalysts in which a large proportion of their surface resides in micropores having diameters similar to those of the reactives and/or reaction products. In such cases, the reaction kinetics may be strongly diffusion controlled, leading to poor yields of the desired products. These facts are particularly important in the catalytic Corresponding author. Tel.: ; fax: address: flopez@ugr.es (F.J. López-Garzón). behaviour of carbon materials because they usually have a polymodal distribution of pores ranging from less than 1 nm up to hundreds of nanometres [1]. Moreover, carbon materials usually have chemical functionalities which are also relevant in their catalytic and adsorption behaviour. Most of the chemical groups consist of oxygen functionalities [2] which are mainly placed in the interior of the bulk, i.e. in the micropores. In addition, the working conditions of the catalyst invariably involve displacement of the carrier gases by the reactives, and frequently high temperatures and pressures. Characterisation of the active sites under such conditions has been usefully studied by flow methods, in which the experimental conditions can be made closely similar to real catalyst operation X/02/$ see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S X(02)

2 142 M. Domingo-García et al. / Applied Catalysis A: General 233 (2002) conditions. The advantages of these methods were recently discussed by Fadoni and Lucarelli [3]. In the present work, several activated carbons of different textural and chemical characteristics were used to study the adsorption of ammonia. The base probe was adsorbed under dynamic conditions from nitrogen at atmospheric pressure using flow adsorption microcalorimetry methodologies developed by one of the authors [4]. The adsorption of the probe was studied by saturation of the adsorbent with a 5 vol.% NH 3 /N 2 mixture, the heat evolution occurring during the adsorption process being continuously measured with simultaneous monitoring of concentration changes of the probe in the carrier gas leaving the adsorbent. 2. Experimental Three commercial activated carbons were used: BPL, manufactured by Chemviron; CAL from Calgon; and GAe from CECA. In addition, other sample, GAe-ox1, was prepared by oxidation of GAe in aqueous solution of (NH 4 ) 2 S 2 O 8. The preparation of this sample also involved pyrolysis in N 2 flow at 773 K after the oxidation. Further details of this preparation method are given elsewhere [5]. The textural characteristics of the samples were obtained by N 2 and CO 2 adsorption at 77 and 273 K, respectively. The surface area accessible to nitrogen was obtained using the BET equation while that accessible to carbon dioxide was calculated by applying the Dubinin Astakhov equation. Moreover, the CO 2 adsorption data permit the evaluation of the micropore size distributions and the mean value of the micropore width by using the Dubinin Stoeckli equation [6]: dw dl = W 0ML 2 2π exp [ M 2 (L 2 0 ] L2 ) 32 The use of this equation supposes a Gaussian distribution of pores of half-width, L being the pore width, L 0 the pore width at the maximum micropore distribution curve, W the micropore volume of pores of L width, and W 0 is the total micropore volume. The chemical nature of the surface was analysed by thermal programmed desorption followed by mass spectrometry (TPD-MS). The sample (0.1 g) was pre-conditioned in He flow at 393 K for 30 min before the desorption runs. After cooling down at room temperature, it was heated at 20 K/min in He flow up to 1273 K. The desorbed amounts of CO 2, CO, H 2 O and H 2 were monitored with a quadrupole mass spectrometer as a function of the temperature. In addition, the surface chemical nature of some selected samples was studied by chemical titration in aqueous solution following the analytical method proposed by Boehm [7,8]. According to this method, carboxylic groups are titrated with NaHCO 3. The difference between the groups titrated with Na 2 CO 3 and NaHCO 3 is considered to be lactones, and the difference between those titrated with NaOH and Na 2 CO 3 is phenols. The ammonia dynamic adsorption was studied using a Microscal Flow Adsorption Microcalorimeter Mark 4Vms fitted with a thermal conductivity detector. Full details of the experimental procedure have been published recently [4,9,10]. The heats and amounts of adsorption/desorption have been obtained after applying dead volume corrections to the raw results. The arrival of ammonia to the down stream detector, which was a thermal conductivity detector, was accurately determined for each case from blank experiments in which the calorimetric cell was filled with a low surface area solid, such as quartz sand, producing negligibly small heat effects and uptakes of NH 3. In situ electrical calibration was carried out for each experiment. Before the measurements the samples were purged with N 2 at 423 K for 20 h. Then the N 2 flow was exchanged by a 5% NH 3 /N 2 mixture. Heats of adsorption and the corresponding amounts of adsorbed ammonia were measured. The data were collected every second and measured the rates of heat evolution, in microwatts, and the rates of NH 3 uptake or release in nanomoles per second [10]. After saturation of the sample was complete, desorption was performed by switching the mixture to N 2.Inall cases, the amount desorbed and heat produced in the desorption were smaller than those in the adsorption cycle. Therefore, both chemisorption and reversible adsorption takes place during the initial saturation. Following desorption of the reversible component of the initial adsorption, a second adsorption desorption cycle which is reversible, was measured. The heats and amounts of the reversible adsorption are then subtracted from the combined quantities obtained in the initial cycle, the difference representing irreversible adsorption assumed to be chemisorption. Changes

3 M. Domingo-García et al. / Applied Catalysis A: General 233 (2002) Table 1 Textural characteristics Sample S N2 (m 2 /g) S CO2 (m 2 /g) L 0 a (nm) W 0 b (cm 3 /g) CAL BPL GAe GAe-ox a L 0 : mean value of the micropore width. b W 0 : micropore volume. in the differential molar heats of adsorption taking place during the adsorption processes can be calculated by dividing the rates of heat evolution by the corresponding adsorption quantities. 3. Results and discussion The textural characteristics in Table 1 show that the samples have large surface areas and well developed microporosity. The values of these parameters are very close for samples BPL and GAe, and they have been increased for sample GAe-ox1 as a consequence of the chemical treatment with (NH 4 ) 2 S 2 O 8. All samples have larger CO 2 than N 2 surface areas which means that there are constrictions in the micropores which partially hinder the N 2 adsorption at 77 K [11]. These constrictions are more important in percentage in sample GAe-ox1 than in the other samples which means that the treatment with (NH 4 ) 2 S 2 O 8 has developed the surface areas and the microporosity of the parent sample (GAe), although a part of these is hardly reached by the N 2 molecule because of the constrictions. The micropore size distributions obtained by using the Dubinin Stoeckli equation applied to the CO 2 adsorption data are shown in Fig. 1. They are typical Gaussian curves showing the variation of the population of pores of a specific width, L. For each sample, the value of L at the maximum of the curve, L 0,is the mean value of the micropore size distribution. It is seen in Fig. 1 that CAL has the narrowest micropore size distribution, the curves of GAe and BPL are similar and GAe-ox1 has the most open micropore size distribution. The mean value of the micropore size distribution, L 0 (the maximum of the curves) ranges between 1.47 and 1.77 nm, for CAL and GAe-ox1, respectively (Table 1), whereas the values for BPL and GAe are very similar. The CO 2 desorption profiles obtained by TPD-MS are shown in Fig. 2a. The profile of CAL stands out from the rest as it has much larger amounts of CO 2 desorbing groups. In fact, the profile of this sample has a wide band between 450 and 1100 K with a maximum at 590 K and a shoulder near 680 K. The wide band may be attributed to carboxylic groups with different stabilisation energies [12] in such a way that the profile at higher temperature may be assigned to carboxylic groups in very stabilised structures [13] although it may also be attributed to other chemical groups as anhydrides or lactones [14]. The profiles of BPL and GAe are very similar with a maximum at Fig. 1. Micropore size distributions obtained from Dubinin Stoeckli equation. L is the micropore width, dw/dl is the variation of the micropore volume with the micropore width.

4 144 M. Domingo-García et al. / Applied Catalysis A: General 233 (2002) Fig. 2. Desorption profiles obtained by TPD-MS: (a) CO 2 desorption profiles of all the smaples, (b) CO desorption profiles of all the samples, (c) CO 2, CO, H 2 O and H 2 desorption profiles of CAL. 550 K and a shoulder near 700 K which are also assigned to carboxylic groups with different stabilisation energies. The CO 2 maximum at 550 K has been shifted to higher temperature in sample GAe-ox1 and it is smaller than in sample GAe, which is due to this sample being pyrolised at 770 K after the treatment with (NH 4 ) 2 S 2 O 8. However, the CO 2 profile of this sample has a maximum near 1000 K which may be assigned to very stabilised carboxylic groups or to anhydrides or lactones [15]. The CO desorption profiles are shown in Fig. 2b. The profile of CAL has a wide band ranging from 650 up to 1250 K with a maximum near 950 K and a shoulder at 1100 K. There is also a relative maximum at low temperature (550 K) which is coincident with other maxima due to water and CO 2 at the same temperature (Fig. 2c) which may be attributed to desorption and condensations between several adjacent chemical groups such as: phenol phenol, phenol carboxyl or carboxyl carboxyl condensations [16]. This relative maximum for CO desorption at low temperature (550 K) also appears in samples BPL and GAe and it is also coincident with one of water and CO 2 although the largest peaks are in the range between 950 and 1100 K. This at 950 K is more relevant in CAL sample and it is attributed to phenol and hydroquinones, while this at 1100 K is more important in GAe-ox1, GAe and BPL and it is assigned to carbonyl and quinones [17]. TPD-MS experiments have shown the existence of several chemical groups of different stability on the surface of the samples, namely carboxylics, phenolics, lactones, carbonyls, quinones and hydroquinones which mainly desorb as CO 2 and CO. The desorbed amounts of CO 2 and CO, measured by TPD-MS and collected in Table 2, show large differences between CAL and the other samples. The desorbed amounts of CO 2 and CO from CAL are more than two-fold those from BPL and GAe and as a consequence of this the total oxygen content of CAL (Table 2) is much larger than that of the rest of the samples. Typical results obtained in the flow calorimeter for the ammonia adsorption on BPL are collected in Fig. 3, which depicts simultaneous plots of the rate of adsorption versus time (Fig. 3c) and the rate of heat evolution (Fig. 3a). After the adsorption was completed (see Section 2), N 2 was passed to allow Table 2 Amounts of CO and CO 2 desorbed measured by TPD-MS Sample CO ( mol/g) CO 2 ( mol/g) O (%) CAL BPL GAe GAe-ox

5 M. Domingo-García et al. / Applied Catalysis A: General 233 (2002) Fig. 3. Dynamic ammonia adsorption and desorption on BPL activated carbon: (c and a) rates of ammonia adsorption and heat evolution, respectively; (d and b) rates of ammonia desorption and heat evolution, respectively. ammonia desorption. The rates of ammonia desorption and heat evolution are shown in Fig. 3d and b, respectively. It is worth noting that the desorbed amount is much smaller than the adsorbed one and that the evolved heat during the desorption is also significantly smaller. Similar results are obtained for the ammonia adsorption/desorption process on the rest of the samples. This means that the ammonia adsorption consists of two different components: one of these is irreversible and the other is more labile or reversible. It is also seen in Fig. 3 that the uptake is almost negligible at 3000 s, while heat is released up to about 4500 s. This heat evolution, which has been already reported in other experimental systems [4,9,10], after the uptake of ammonia stops is attributed to its diffusion from the low energy sites to higher energy sites having low accessibility. The lack of further uptake of NH 3 may be due to irreversibly adsorbed molecules on the borders of micropores blocking NH 3 towards the end of the adsorption process. The variation of the ammonia adsorption versus time is obtained for all the samples (Fig. 4) from the experimental results of the dynamic measurements. The trend found in Fig. 4 is rather unexpected considering the textural and chemical characteristics of the samples. In fact, the very low adsorption of CAL and the small increase with the time is not supported by the surface areas nor by the micropore volume.

6 146 M. Domingo-García et al. / Applied Catalysis A: General 233 (2002) Fig. 4. Plots of ammonia adsorption vs. time derived from the experimental results of the dynamic adsorption. Moreover, the micropore distribution of CAL (Fig. 1) is not so different to those of the other samples to explain the large difference in Fig. 4. The behaviour of CAL can be explained if both textural and chemical surface characteristics are considered. This sample has the narrowest microporosity and the highest amount of oxygen surface groups (Table 2). It is known [2,18,19] that the chemical functionalities are mainly in the most energetic parts of the carbon materials which are the borders and the entrances of the micropores. Therefore, the concentration of chemical functionalities in the micropore borders of this sample should be very high and it is likely that they hinder the access of the ammonia to other chemical groups inside the sample. The behaviour of sample GAe-ox1 is very different to that of CAL as the former adsorbs the largest amounts of ammonia, which clearly increase with time. This is because GAe-ox1 has the widest micropore size distribution (Fig. 1) and smaller amount of chemical functionalities than CAL. Therefore, the access of the adsorbate to the chemical groups in GAe-ox1 is relatively easy, and it adsorbs larger amounts of NH 3 than GAe and BPL do because the oxygen content (chemical functionalites) in the former is larger. The explanation of the trend found for samples GAe and BPL is not evident based on the textural characteristics, as they (surface areas, micropore volumes and L 0 ) are very similar for both samples. However, the chemical groups which desorb as CO in GAe are larger than in BPL (Table 2), which suggests that larger amounts of phenol (which desorb as CO) are accessible to ammonia on GAe. This assumption should be reflected in the heat of adsorption as the acidic character of phenols is smaller than that of carboxyls and lactones. This aspect will be considered later on. The plots of the molar heat of adsorption versus the cumulative adsorption (in mmol/g) give useful information about the ammonia adsorption under dynamic conditions (Fig. 5a and b). It is noticeable the trend found for CAL: at small uptake, the evolution of heat per mole of adsorbed ammonia is not very large ( 71 kj/mol), it reaches a minimum ( 60 kj/mol) as the amount adsorbed increases, but rapidly increases almost asymptotically, i.e. there is a very important evolution of the adsorption heat with a small rise of the uptake. This behaviour is a consequence of the huge amount of chemical groups which hinder the access to the most energetic (acidic) sites. Once the molecules have been adsorbed in the most accessible sites they can re-arrange to more thermodynamically favoured acidic groups. Similar kinetic (re-arrangement) behaviour has been reported for other adsorbate adsorbent systems [4,9,20,21]. In this case, it seems to be related to the existence of a large number of acidic groups which are hardly reached at the early stages of the adsorption process. It is remarkable that heat values as large as 150 kj/mol are evolved in the re-arrangement process. This value is two-fold the initial adsorption value and it is larger or similar to that reported for the ammonia adsorption on several zeolites [4,22 24]. The trend of BPL (Fig. 5a) is very different to this of CAL as ammonia is adsorbed with the evolution of very large heat ( 250 kj/mol) at low uptakes, it decreases to values which are larger, in absolute value, than 50 kj/mol reaching a quasi-plateau and it also

7 M. Domingo-García et al. / Applied Catalysis A: General 233 (2002) Fig. 5. (a and b) Variation of the molar heat of adsorption with the cumulative adsorption of ammonia. has re-arrangement for large values of cumulative adsorption. Except at low values of cumulative adsorption, the behaviour of GAe (Fig. 5b) is almost coincident with this of BPL, which is in agreement with the very similar textural and chemical surface characteristics of both samples. Nevertheless, the large difference at low cumulative adsorption suggests that the most acidic sites of GAe are not easily reached by the adsorbate, which is in agreement with the above statement that an important part of the adsorption on this sample is produced on phenolic groups. Moreover, it can be deduced from Figs. 4 and 5 that GAe has more accessible acidic sites than BPL, but the accessibility to stronger acidic sites is easier in the latter. It is worth noting that GAe-ox1 is the only sample for which the evolution of heat increases, in absolute value, at the early stages of the adsorption and with almost no re-arrangement. This is due to this sample has the widest micropore size distribution, and therefore, the accessibility to the acidic groups is almost the same in all the cumulative adsorption range except in the early stages for which the less acidic groups are the most accessible. For this reason, the amount of adsorbed ammonia on this sample is the largest, but the molar heat of adsorption is almost the same in the whole range. This behaviour also supports that the re-arrangement is produced because there are restrictions to the access to the acidic sites at the early stages of adsorption and they are only reached after the surface concentration of NH 3 exceeds 60 70% of the total adsorption. The summary of the ammonia adsorption parameters is collected in Table 3. This contains the total ammonia adsorption and the amount desorbed after N 2 is flowed (reversible adsorption), being the difference between the two values the irreversible adsorption. Table 3 also contains the thermodynamic parameters of the ammonia adsorption and desorption processes. The comparison of the values in Table 2 and the values of ammonia adsorbed in Table 3 suggests that most of the chemical groups (Table 2) are not accessible to the adsorbate, assuming that one ammonia molecule is adsorbed per chemical surface group. Indeed, not all the chemical functionalites which desorb as CO 2 and CO have to be acidic and, therefore, this could be the Table 3 Ammonia adsorption parameters Sample Amount of adsorption ( mol/g) Amount of desorption ( mol/g) Integral heat of adsorption (J/g) Integral heat of desorption (J/g) Molar heat of total adsorption (kj/mol) CAL BPL GAe GAe-ox Molar heat of irreversible adsorption (kj/mol)

8 148 M. Domingo-García et al. / Applied Catalysis A: General 233 (2002) reason which explains the differences between the values in Tables 2 and 3. This possibility has been checked in two selected samples (GAe and GAe-ox1) whose acidic groups have been determined by selective titrations using bases of different strengths, following the method proposed by Boehm [7,8]. The values (in eq/g) obtained for GAe and GAe-ox1 are: carboxyls, 155 and 175; lactones, 95 and 191; phenols, 395 and 503. These data mean that the total acidity of these samples is larger than the acidic groups seen by the ammonia molecule at 150 C which supports the view that most of the acidic groups are not reached by the adsorbate in the presence of N 2 as carrier and the absence of water. These data together with those in Fig. 4 are relevant because they allow us to explain the behaviour of carbon materials as catalysts. In particular, in the metylamines synthesis [25] the acidic groups are the active centres for the production of these chemicals which is carried out by the ammonia adsorption on the acidic surface groups. The evaluation of these acidic chemical groups is normally carried out by ammonia adsorption under static conditions [19,22 24] which are not very similar to the real experimental conditions in which catalysts are used. This is probably the reason why in many cases, the behaviour of a carbon material as catalyst is hardly explained based on the amount of acidic chemical groups, because an important amount of them are not accessible to the reactives. The fact that most of the acidic groups are not reached by ammonia is even more evident considering that, as commented earlier (Table 3), not all the ammonia is irreversibly adsorbed. It is commonly accepted that the irreversible adsorption occurs on chemical groups, whereas the reversible one is a physical process. The extent of the irreversible adsorption depends on the samples and in the particular case of CAL, it only amounts to 50% of the total. Therefore, these data support the view that most of the acidic chemical groups of the samples are not accessible to the adsorbate. This means that the textural parameters of the activated carbons can be determinant in the adsorption (and catalytic [25]) behaviour of the samples. To show this relationship, the plot between the percentage of irreversible adsorption and the mean size of the micropore distribution, L 0, is depicted in Fig. 6. This shows that the irreversible adsorption is related to the textural characteristics in terms of the accessibility of the adsorbate to the most active sites, i.e. the possibility of reaching the chemical groups in pores is larger as the micropore distribution is more open. The smallest value (in absolute terms) of the integral heat of adsorption (Table 3) is produced by CAL as a consequence of the low amount of adsorbed ammonia. In all the samples, the absolute value of the integral heat of desorption is significantly smaller than the integral heat of adsorption which indicates that the reversible adsorption is a physical process. It should be noted that the absolute values of the combined Fig. 6. Percentage of irreversible adsorption vs. the mean value of the micropore size.

9 M. Domingo-García et al. / Applied Catalysis A: General 233 (2002) molar heats of irreversible and reversible adsorption are at least three times larger than the heat of NH 3 liquefaction (for GAe-ox1) and six times larger for the carbons CAL and BPL. The difference in the two types of adsorption can be analysed with the values of the molar heat of total and irreversible adsorption. The thermodynamics of ammonia adsorption on CAL and BPL seem to be similar to one another and different to those for GAe and GAe-ox1 if the molar heat of total adsorption is considered. But this is not so evident if the molar heat of irreversible adsorption is taken into account, as the irreversible adsorption is more exothermic in CAL, although on sample BPL is also very exothermic. The high value in CAL exists mainly in the final stages of adsorption when re-arrangement is produced, whereas in BPL it is due to the large amount of heat evolved at low cumulative adsorption. For this reason, the heat of irreversible adsorption in BPL is higher than that for GAe. As stated earlier, the acid groups in BPL are more energetic (probably carboxyls) than those in GAe (probably phenols), but the access to these sites in the former is less favoured. GAe-ox1 has the less energetic sites for irreversible adsorption which, according to the TPD-MS and selective titration data, suggests that an important amount of the adsorption is probably produced on phenolic groups. It is worth noting that the difference between the molar heat of total and irreversible adsorption is only 3 kj/mol in GAe-ox1, suggesting that the reversible adsorption is dominated by dispersion forces. This difference amounts to 45 kj/mol for CAL, and 23 and 22 kj/mol for BPL and GAe, respectively. The former is nearly two-fold the ammonia liquefaction heat, which suggests that ammonia is adsorbed into micropores having dimensions close to the molecular size [26 31] or in more than one site [32] by dipole dipole interactions. The reversible adsorption heats for BPL and GAe are close to the ammonia liquefaction heat and account for dipole dipole interactions or for adsorption into micropores. 4. Conclusions This paper points out the importance of the accessibility to the chemical groups on activated carbons in the understanding of their behaviour in adsorption and catalysis. The mere evaluation of the chemical functionalities and surface areas in not always enough to deduce appropriate conclusions. This is particularly important for activated carbons with constrictions in the micropore network which hinder the access of the adsorbate. Nevertheless, the behaviour of this kind of materials under these experimental conditions can be deeply analysed by using flow adsorption microcalorimetry as it is a very reliable technique which allows one to obtain adsorption and thermodynamic data under experimental conditions close to circumstances operating in industrial practice. In this respect, the plots of the differential heats of displacement of N 2 by NH 3 against cumulative adsorption provide a unique method of assessment of adsorbents and catalysts. It has also been shown that the activated carbons used in this work possess a wide distribution of acid sites some of which exceed in strength those existing on acidic zeolites and pillared clays. Nevertheless, they are not always easily accessible to NH 3 adsorption due to constrictions in the micropores which hinder the access of the probe. As a consequence of this, kinetic re-arrangements are produced. Therefore, not only a large number of oxygen groups is desirable in activated carbons, but also the appropriate textural characteristics which allow the access of the probe. In all cases, the ammonia adsorption consists on two components: reversible and irreversible. The former one is produced by the adsorption on chemical groups while the latter is a physical process produced into pores or by dipole dipole interactions. Acknowledgements M.P.M. acknowledges the financial support of the Ministerio de Educación Ciencia y Deporte. The support of DGYCIT under projects PB and BQU CO2-01 is also acknowledged. References [1] R.C. Bansal, J.-B. Donnet, Active Carbon, Marcel Dekker, New York, [2] F. Rodriguez-Reinoso, in: H. Marsh, E.A. Heintz, F. Rodriguez-Reinoso (Eds.), Introduction to Carbon Technologies, Secretariado de Publicaciones, Universidad de Alicante, 1997.

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