On the Characterization of Chemical Surface

Journal of Colloid and Interface Science 248, 116–122 (2002) doi:10.1006/jcis.2001.8207, available online at http://www.idealibrary.com on

On the Characterization of Chemical Surface Groups of Carbon Materials M. Domingo-Garc´ıa,∗ F. J. López Garzón,∗,1 and M. J. Pérez-Mendoza† ∗ Departamento de Qu´ımica Inorgánica (Grupo de Investigación en Carbones), Facultad de Ciencias, 18071 Granada, Spain; and †Department of Chemical Engineering, The University of Edinburgh, The King’s Buildings, Mayfield Road, Edinburgh EH9 3JL, United Kingdom Received June 21, 2001; accepted December 27, 2001; published online February 21, 2002

permits the titratrion of a wide range of acidic groups because of its Brønsted and Lewis basic character. The latter allows the diffusion of ammonia into small micropores containing chemical groups. The other basic molecular probes are larger than NH3 . Therefore, it is more difficult for them to reach the small micropores, which means that that the acid chemical groups may be partially titrated. The aim of this paper is to characterize the chemical surface groups of several carbon materials by different experimental methods and to contrast the results of these measurements. For this purpose, FTIR has been used for a qualitative analysis and the quantitative evaluation has been carried out by titrations in aqueous solutions (Boehm’s method), thermal programmed desorption connected to mass spectrometry (TPD-MS), and ammonia adsorption measured by TG.

This paper deals with the use of several methods to characterize the chemical surface groups of carbon materials. Several samples embracing a wide range of acidic and basic characteristics have been used for this purpose. The quantitative determinations have been carried out by selective titrations in aqueous solutions, thermal programmed desorption connected to mass spectrometry (TPD-MS), and ammonia adsorption–desorption, measured by thermogravimetry (TG). The results show some inconsistencies between the different experimental methods. These mainly arise because chemical transformations can be produced during the experiments. Moreover, the textural characteristics of the carbon materials and the existence of π electrons on the graphitic planes are important factors to be considered in the discussion of the results. C 2002 Elsevier Science (USA) Key Words: carbon materials; surface functionalities; characterization.

EXPERIMENTAL INTRODUCTION

The properties of carbon materials as adsorbents, catalysts, and catalyst supports depend on the textural characteristics and on the chemical surface functionalities. The former are described by the surface area and porosity, and the latter mainly consist of the surface oxygen containing chemical groups, which can be acidic, basic, or neutral (1). The nature of the chemical surface groups is currently determined by Fourier-transformed infrared spectroscopy (FTIR, DRIFT) and X-ray photoelectron spectroscopy (XPS). The quantitative determination of them can also be carried out by selective titrations in aqueous solutions at room temperature following the method proposed by Boehm (2, 3). The results obtained by selective titrations can be contrasted by other experimental methods. Among them, thermogravimetry (TG), thermal programmed desorption (TPD), and, very frequently, calorimetric measurements are used (4– 11). These techniques use several probes, namely ammonia, pyridine, 2,6-dimethyl pyridine, or isopropyl amine, to determine the acidic groups. Ammonia is a very convenient probe for characterizing the acidic chemical surface groups of carbon materials because its basic character and its small size. The former 1 To whom correspondence should be addressed. Fax: 34-958-248526. E-mail: [email protected].

0021-9797/02 $35.00

C 2002 Elsevier Science (USA)

All rights reserved.

116

Carbon materials of different origins and characteristics were used: C0 and C20, obtained from almond shells, and a commercial activated carbon GAe, which was used as starting material for the preparation of three activated carbons. C0 is a char obtained by pyrolisis at 1223 K in N2 flow, and C20 was prepared by activation of C0 with CO2 at 1223 K. The commercial activated carbon GAe was demineralized and this was the starting material for the preparation of three new oxidized activated carbons: GAe-ox1, GAe-ox2, and GAe-1%O2 P. GAe samples appended −ox1 and −ox2 were prepared by treatments in aqueous solution of (NH4 )2 S2 O8 and H2 O2 , respectively, and further pyrolisis at 773 K in N2 flow. The sample appended −1%O2 P was obtained by treatment of GAe with oxygen plasma. Further details of the preparation methods are given elsewhere (12). The textural characteristics of the samples were determined by N2 and CO2 adsorption at 77 and 273 K, respectively, which determines the surface areas accessible to both molecules. In addition, the CO2 adsorption data have been used to determine the mean value of the micropore size distribution, L 0 , using the Dubinin–Stoeckli equation (12): M 2 L 20 − L 2 dW W0 M L = . √ exp dL 32 2 2π

[1]

117

CHEMICAL SURFACE GROUPS OF CARBON MATERIALS

In this equation W is the micropore volume of those pores with a value of L in width and W0 is the total micropore volume. L 0 is the mean value of the micropore size distribution, i.e., the width at the maximum of the micropore distribution curve, supposedly a Gaussian of half-width . It is necessary to know the value of W0 and to obtain L 0 . The former is obtained from the wellknown Dubinin–Astakhov equation which is also used to determine the n parameter which permits the calculation of (12). Wafers of KBr containing about 0.5% of sample were used to obtain the transmission FTIR spectra in the range 400– 4000 cm−1 . They were dried at 398 K overnight before the spectra were recorded. The titrations in aqueous solution of the chemical surface groups were carried out following the method proposed by Boehm (2, 3). This method uses bases of different strength to titrate acidic groups so that carboxylic groups are titrated with NaHCO3 , the difference between the groups titrated with Na2 CO3 and NaHCO3 is considered to be lactones, and the difference between those titrated with NaOH and Na2 CO3 is attributed to phenols. The amount of basic groups is titrated using HCl solutions. The evolution of the chemical surface groups with the temperature was followed by TPD-MS. Around 0.10 g of sample was used for this purpose. They were placed into a quartz cell and conditioned at 398 K in He flow. Then the temperature was increased to 1273 K in He flow (60 cm3 /min) at a heating rate of 20 K/min. The evolved amounts of CO2 , CO, and H2 O were monitored as a function of the temperature by a mass spectrometer. The ammonia adsorption–desorption process was studied using a (TG) system. The sample (around 50 mg) was heated in N2 flow (75 cm3 /min) up to 573 K and it was kept at this temperature until no weight loss was detected. Then it was cooled down in N2 flow at room temperature, and after that the temperature was risen up to 323 K. Once this temperature was reached, a N2 + NH3 mixture containing 25% in NH3 was flowed to allow the ammonia adsorption–desorption equilibrium to be reached, which was considered when no weight increase was detected. The mixture was changed by the N2 flow (75 cm3 /min) to permit ammonia desorption, keeping the temperature at 323 K. When no weight loss was detected the temperature was increased at 5 K/min up to 573 K and the weight loss was monitored.

TABLE 1 Textural Characteristics Sample

SN2 (m2 /g)

SCO2 (m2 /g)

Lo (nm)

Wo (cm3 /g)

GAe GAe-ox1 GAe-ox2 GAe-1%O2 P C0 C20

1026 1129 1196 1017 412 771

1185 1403 1304 1167 575 977

1.61 1.77 1.84 1.61 0.98 1.23

0.449 0.532 0.495 0.443 0.218 0.371

which means that the constrictions are more important in the micropore structures of the former. There are also differences in the chemical surface groups of both series as it can be deduced from the bands of the FTIR spectra (Fig. 1). The intensity of the bands of C0 and C20 is clearly lower than those of the selected samples of GAe series, which suggests that the amounts of chemical surface groups in samples of C series are smaller than those of GAe series. The summary of the main bands and the assignments (Table 2), which are based on the literature (14–17), points to a large variety of chemical surface groups. The results of the chemical surface groups titrations are collected in Table 3. Phenol groups are most abundant in sample GAe and they have been increased in samples GAe-ox1 and GAe-ox2 as a consequence of the oxidation treatments with (NH4 )2 S2 O8 and H2 O2 , respectively. Moreover, lactonic groups have also been increased by the oxidation treatments but the amounts of carboxylic groups are only a bit larger than in sample GAe. This can be a consequence of the pyrolisis at 773 K after the oxidation treatments which partially eliminates the carboxylic groups. It also may be due to the partial condensation of caboxyls and phenols which produces lactonic groups during the oxidation treatments (18). O

OH

O

C

OH

C

C

C

C

C

C

C

C

C

C

The total amounts of chemical surface groups, acidic and basic, in sample GAe-1%O2 P are the highest. The samples of C C0

RESULTS AND DISCUSSION

C20

Transmitance (Arbitrary Units)

Some of the textural and FTIR data have been already reported (12), although they are included in this paper to help the discussion. The textural characteristics in Table 1 show that the samples of GAe series have larger values of surface areas and micropore volumes than samples of C series. Moreover, the mean micropore sizes of GAe-series samples are also larger than those of C series, which means that the former have more open micropore networks. In addition, the differences in surface areas measured by CO2 and N2 (SCO2 > SN2 ) suggest that there are micropore constrictions (12, 13) hindering the N2 adsorption. The differences are larger, in percentage, in C series than in GAe series,

O

C

−H2O

GAe GAe-ox1

4000

3600

3200

2800

2400

2000

1600

1200

800

ν (cm-1)

FIG. 1.

FTIR spectra of C0, C20, GAe, and GAe-ox1.

400

´ AND PEREZ-MENDOZA ´ DOMINGO-GARCÍA, GARZON,

118

TABLE 2 Assignments of FTIR Bands Wavenumber (cm−1 )

Assignment

900–740 3030 1628; 1580

C–H (out-of-plane bending) C–H (stretching) C=C (stretching)

series have very small amounts of acidic groups, in agreement with the low intensity of the FTIR bands in Fig. 1, and basic groups are the only relevant, which is related to the preparation method (19, 20). The TPD profiles of GAe and C series are collected in Figs. 2 and 3, respectively. The discussion of the profiles is based on the literature (21–25). The CO2 profile of sample GAe has a maximum at 550 K and a shoulder near 700 K. The former may be produced by carboxylic groups and the latter by anhydrides and lactonic groups, or more likely by structurally stabilized carboxylic groups (18, 26). The profiles of samples GAe–ox1 and GAe–ox2 also have a maximum at 550 K but it is clearly smaller than this of GAe. Moreover the profiles of both samples have a maximum at 1000 K (not present in the profile of sample GAe), which is probably produced by the desorption of anhydrides and lactonic groups. The CO2 desorption profile of sample GAe-1%O2 P is similar to that of GAe. The total amounts of CO2 evolved, which have been determined from the TPD profiles, are collected in Table 4. The TPD results (Fig. 2a and Table 4) and the data in Table 3 of samples GAe, GAe-ox1, and GAe-ox2 seem to be contradictory because:

Phenolic groups Phenolic groups Phenolic groups Aliphatic structures Unsat. ketone and/or diketone Carbonates and carboxylic structures (oxygen surface groups) Highly stable cyclic ethers containing COCOC groups or ether bridges between rings Aliphatic structures; ether Aromatic structures Unsaturated aliphatic structures

14, 16 14, 15, 24 14, 15, 24 14, 15, 24 14, 16 14 14, 15, 24 14, 16 15, 24 14, 16

(ii) the total amounts of CO2 evolved from GAe-ox1 and GAe-ox2 are smaller than this evolved from GAe (Table 4) but the amounts of carboxyls and lactones (Table 3) are larger in the former. 0.8

a GAe

0.6 F (µmol/g.s)

O–H (stretching) O–H (bending) C–OH (stretching) C–H (stretching) C=O (stretching) Composed Composed

Reference

GAe-ox1 GAe-ox2

0.4

GAe-1%O2P 0.2 0.0 250

450

650

850

1050

1250

T(K) 0.8 F (µmol/g.s)

3444 1154 1000–1220 2966; 2918; 2854 1692 1591; 1440; 1382; 1285; 1073; 1019 1119; 1073; 1154; 1285

Group

b

GAe

0.6

GAe-ox1 GAe-ox2

0.4

GAe-1%O2P

0.2 0.0

(i) the CO2 desorption peak at 550 K of samples GAeox1 and GAe-ox2 is smaller than this of the sample GAe although the former have larger amounts of carboxylic groups;

250

450

650

850

1050

1250

T(K)

c

6

GAe

F (µmol/g.s)

GAe-ox1

TABLE 3 Chemical Surface Groups Content (µmol g−1 ) Sample

Carboxyls

Lactones

Phenols

Basic


155 175 180 338 43 43

95 191 219 126 0 0

395 503 499 702 39 31

32 82 24 377 489 774

GAe-ox2

4

GAe-1%O2P

2

0 250

450

650

850

1050

1250

T(K)

FIG. 2.

TPD profiles of GAe series: (a) CO2 , (b) H2 O, and (c) CO.

119


a

0.8

(Table 1). The second one is that samples GAe-ox1 and GAeox2 have a high concentration of anhydride which evolve as CO2 and CO at high temperatures (Figs. 2a and 2c). Nevertheless, the anhydride can be hydrolyzed in aqueous solution rendering two carboxylic groups which will result in an overestimation of the acidic groups in relation to that expected in base of TPD experiments.

C0

F (µmol/g.s)

C20 0.6 0.4 0.2 0.0 250

450

650

850

1050

1250

T(K)

O

F (µmol/g.s)

0.8

OHHO C

C

C

C

C

C

C

C

C

C0

O C

+H2O

b

0.6

C C

C

C20

0.4 0.2 0.0 250

450

650

850

1050

1250

T(K)

c

6

F (µmol/g.s)

O

O

O C

4 C0 C20 2

0 250

450

650

850

1050

1250

T(K)

FIG. 3.

TPD profiles of C0 and C20: (a) CO2 , (b) H2 O, and (c) CO.

The CO2 desorption profile of GAe-1%O2 P is similar to this of GAe although the total amount of CO2 evolved from the former (514 µmol/g, Table 4) is larger than that from the latter (405 µmol/g). Nevertheless the amounts of carboxylic and lactonic groups (Table 3) are much larger for GAe-1%O2 P than for GAe, which support that the titration of the chemical groups in GAe is only partial because of its narrow microporosity. The sample GAe-1%O2 P is less affected by the partial titration because the plasma treatment affects only the external surface (12, 17). Moreover, it is known that one of the most important effect of plasma treatments is to increase the surface energy, in such a way that the surfaces are easily wetted (27), which facilitates the complete titration of the chemical groups in sample GAe-1%O2 P. The maximum at 550 K for samples GAe and GAe-1%O2 P is coincident with others at the same temperature in the H2 O desorption profile (Fig. 2b), which suggests the condensation of carboxylic and phenolic groups producing CO2 and H2 O. O

OH C

The first contradiction can be attributed to the pyrolisis in N2 flow of samples GAe-ox1 and GAe-ox2 at 773 K after the oxidation treatments (see Experimental). Nevertheless, the GAe sample was not pyrolized at this temperature and then it maintains the carboxylic groups which evolve as CO2 at relatively low temperature. The second contradiction can be explained considering two aspects. The first one is that the titration of chemical groups of sample GAe has been partially hindered because of its narrow microporosity, as this sample has the smallest value of L o TABLE 4 TPD Desorbed Amounts (µmol g−1 ) Sample

CO2

CO

H2 O


405 387 387 514 230 203

1535 2833 2951 1738 417 317

345 232 242 407 274 272

C

C

OH C C

C C

C

C

C

C

+ CO2 + H2O

This means that an important amount of phenolic and carboxylic groups can condense during the TPD experiments because the relatively low stability due to the high concentration on the surface. It is known that the higher the concentration of the chemical groups the lower their stability (18). The CO desorption profiles (Fig. 2c) have a maximum at 1100 K for all the samples and these of GAe-ox1 and GAeox2 also have a shoulder near this (∼975 K) which coincides with other maximum in the CO2 desorption profile (Fig. 2a). This could suggest the existence of anhydride groups (22), although it also can be attributed to phenols, ethers, carbonyls, or quinones (24). The total amount of CO desorbed from samples GAe-ox1 and GAe-ox2 (Table 4) is much larger than these from the other samples, which is in agreement with the high amount of phenolic groups of these samples determined by selective titrations (Table 3). Nevertheless, the sample GAe-1%O2 P has the largest amount of phenolic groups but the CO desorption profile is similar to that of GAe, which has the lowest content


TABLE 5 Values of Ammonia Adsorption (µmol g−1 ) Sample

Sat. at 323 K

IA at 323 K

IA at 573 K


2371 2614 2429 2537 1870 2890

583 755 662 605 281 438

211 335 285 251 59 98

be difficult to explain the amounts of CO which appear in the TPD profiles of Figure 2c. The results of the ammonia adsorption are collected in Table 5. The values of the total adsorption at 323 K are collected in the second column of this table. It has been described in the experimental section that once the adsorption equilibrium at 323 K was reached N2 was flowed at the same temperature to allow ammonia desorption. The initial weight was not reached under this condition which means that not all the ammonia adsorbed at 323 K can be desorbed at the same temperature; i.e., there is an amount of ammonia (from now on denoted IA), which is “irreversibly” adsorbed at 323 K. The IA values at 323 K are collected in Table 5, which also contains the values of the IA at a temperature as high as 573 K. Therefore, the ammonia adsorption at 323 K consists of two components: reversible (RA) and irreversible (IA) adsorption. The plot of the saturation values at 323 K versus the micropore volumes (Fig. 5a) shows a moderate 5

a

4 3 2 1 0

3.5

0.2

0.3

0.4

0.5

0.6

3

Wo (cm /g)

2.8

5 CO2+CO

2.1

1.4 2

CO2

0.7

R = 0.9165

RA (µmol.g-1.10-3)

Oxygen compound desorbed (µmol.g-1.10-3)

on phenolic groups. This behavior has been explained above on the one hand as a result of the restrictions in GAe which partially hinder the titration of the chemical groups, an on the other because the plasma treatment affects only the external surface of sample GAe-1%O2 P. The TPD profiles of samples C0 and C20 are collected in Fig. 3. The amounts of CO2 and CO evolved are much lower than in GAe-series samples, in agreement with the negligible amounts of acidic groups found in these samples (Table 3). Nevertheless, the data in Table 3 show that samples C0 and C20 have important amounts of basic groups, which have been described as chromene, γ -pyrone, or quinones structures (3). If these basic structures were present in C0 and C20, it should be expected that the TPD profiles show larger amounts, particularly of CO (28). Therefore, it can be concluded that the basic character of samples C0 and C20 is mainly due to π electrons on graphytic planes which can produce donor–acceptor interactions (3, 29–32). The comparison of the results of the surface chemical groups obtained by selective titrations and by the TPD experiments is depicted in Fig. 4. This shows two different plots: (i) the total oxygen compounds (CO2 and CO) desorbed in TPD vs the total acidity (i.e., carboxylic, lactonic, and phenolic) and (ii) the CO2 desorbed vs the total acidity. No clear relationship exists between the total oxygen desorbed compounds (CO2 + CO) and the total acidity because not all the oxygen functionalities are acidic. However, there is a linear relationship between the amount of CO2 evolved and the total acidity determined by titrations, which means that the chemical surface groups which evolve as CO2 are responsible for the acidic character of these samples (26). It is necessary to take into account that the amounts of phenol groups are included in the values of the total acidity in Fig. 4, which suggests that most of the titrated phenol groups evolve in TPD experiments as CO2 . This is in agreement with the above statement of the condensation between carboxylic and phenolic groups in TPD experiments. Moreover, this supports that the titration of the chemical groups is only partial; otherwise it would

Ammonia adsorption (µmol.g-1.10-3)

120

b

4 3 2 1 0

0 0

0.4

0.8

1.2

Total acidity (µmol.g-1.10-3)

FIG. 4. The total acidity versus the amounts of CO2 and the total oxygen (CO2 + CO) content.

0.5

1

1.5

2

Lo (nm)

FIG. 5. Ammonia adsorption. (a) Saturation at 323 K versus the micropore volumes; (b) reversible adsorption versus the mean micropore size.

121


(i) The IA is produced in both chemical surface groups and small micropores. (ii) The titration in aqueous solution of the acidic chemical surface groups is only partial. The first hypothesis which suggest that IA is partially produced in micropores is supported by reported data (35, 36) for several adsorbate–adsorbent systems at high temperatures. It occurs when the adsorption is produced in pores of dimension very close to the molecular size, in such a way that the heat of adsorption is much larger (two or threefold) than that expected for the adsorption produced on a flat surface. Morevoer, the desorption from these pores is frequently hindered if there are constrictions

IA (µmol.g-1.10-3)

2

R = 0.6909

a

1.0 0.8 0.5 0.3 0.0 0

0.5

1

1.5

Acidic groups (µmol.g-1.10-3)

1.0 IA (µmol.g-1.10-3)

increase in the adsorption with Wo . The difference between the saturation values and IA at 323 K (second and third columns of Table 5) is the amount of ammonia which has been desorbed after N2 was flowed: i.e., it is the RA. Most of the adsorbed amount is reversible in all cases, in such a way that the RA values are larger than 75% of the total adsorption. RA should be related to the textural characteristics of the samples as it can be considered a physical process produced in micropores. The plot (Fig. 5b) of RA versus the mean micropore size, L o (Table 1), shows almost no variation of RA as L o increases. This is because the ammonia molecular size (0.36 nm) is much smaller than the mean pore size of the samples and therefore an increase in the pore dimension (larger than 0.98 nm) scarcely influences the reversible adsorbed amount. Significant amounts of IA appear at a temperature as high as 573 K. This is larger than 36% of the total IA in samples of GAe series, and it represents around 21% in samples C0 and C20. Similar IA at different temperatures and under dynamic conditions has been reported not only for ammonia adsorption but also for other polar molecules on zeolites (9, 33–35). The data in literature usually relate IA to acidic chemical surface groups (33–35). This is supported by the data of samples GAeox1 and GAe-ox2 for which an increase in IA with respect to GAe is observed (Table 5). This increase is produced because the amount of acidic groups in these samples (Tables 3 and 4) is larger than in sample GAe. For the same reason, an increase even larger in IA for sample GAe-1%O2 P should be expected (Tables 3 and 4). However, the values of IA for GAe and GAe-1%O2 P are very close. To explain this fact it is necessary to take into account that, before the ammonia adsorption, the samples were preconditioned at 573 K. The TPD profiles (Fig. 2) of GAe and GAe-1%O2 P at temperatures higher than this are very similar (i.e., the amounts and nature of the remaining chemical surface groups after the treatment at 573 K are very similar) which support the close values of IA for the two samples. Therefore, it seems that IA is related to chemical surface groups, although this statement is not so evident if the data of C0 and C20 in Table 5 are considered because they suggest that ammonia is also irreversibly adsorbed on both samples in spite of their negligible amounts of acidic chemical surface groups (Table 3). This behavior can be explained by two different hypotheses:

b

0.8 R2 = 0.9351 0.5 0.3 0.0 0.5

1

1.5

2

Lo (nm)

FIG. 6. IA versus (a) acid chemical surface groups content and (b) mean micropore size.

in the micropore network as in the case of samples C0 and C20. The existence of constrictions in these samples has been stated because of the differences between the N2 and CO2 surface areas (SCO2 > SN2 , Table 1) (13). The second hypothesis means that samples C0 and C20 have acidic chemical groups which have not been titrated due to the narrow microporosity (Table 1). These restrictions arise from constrictions which hinder the access of the alkali to the chemical groups (19). However, this second hypothesis seems to be less plausible in this case because TPD profiles in Fig. 3 reveal the negligible amounts of acidic chemical groups in the two samples. Therefore the first hypothesis suggesting that IA can be produced in chemical surface groups and in small micropores of dimension close to the molecular size seems to be plausible. This is also supported by the plots in Figs. 6a and 6b. The first one shows a poor lineal relationship (regression coefficient 0.6909) between IA and the amount of acidic groups, which strongly suggests that there is also a textural factor influencing IA. This can be seen in Fig. 6b, which shows a good fit (regression coefficient 0.9305) between IA and the mean micropore size, L o . This plot clearly correlates both parameters in terms of the accessibility of ammonia to the adsorption sites: chemical groups or micropores of dimension close to molecular size. The extrapolation of the straight line of Fig. 6b to IA = 0 renders a value of L o = 0.38 nm, which is very close to the ammonia molecular dimension (0.36 nm) which suggests that this dimension is the smallest to allow the ammonia adsorption.


122

CONCLUSIONS

This paper points out the contradictions in the evaluation of chemical surface groups of carbon materials by different experimental methods which have to be considered. These arise from different reasons depending on the technique. Among them, chemical transformations (mainly condensations) may occur during TPD measurements rendering unexpected amounts of CO and CO2 . In relation to chemical titration, it is noticeable that the basic character of the carbon surfaces may be produced by π electrons on graphytic planes. Two different factors, constrictions in microporosity, which can hinder the access of the titration probe, and hydrolysis of the chemical groups, may result in misestimated values of the total surface chemical groups. The ammonia adsorption can also lead to unexpected results as it is a mixed process produced on chemical surface groups and in pores. ACKNOWLEDGMENTS This work has been supported by DGYCIT under Project PB97-0831 and by MCYT under Project BQU2001-2936-CO2-01. M.P.M. acknowledges the financial support of MECD.

REFERENCES 1. Kinoshita, K. “Carbon Electrochemical and Physicochemical Properties,” Wiley, New York, 1988. 2. Boehm, H. P., Adv. Catal. 16, 179 (1966). 3. Boehm, H. P., Carbon 32, 759 (1994). 4. Eigenmann, F., Maciejewski, M., and Baiker, A., Thermochim. Acta 359, 131 (2000). 5. Meziani, M. J., Zajac, J., Jones, D. J., Partyka, S., Roziere, J., and Auroux, A., Langmuir 16, 2262 (2000). 6. Auroux, A., Gervasine, A., and Guimon, C., J. Phys. Chem. 103, 7195 (1999). 7. Decourty, B., Occelli, M. L., and Auroux, A., Thermochim. Acta 312, 27 (1998). 8. Babitz, S. M., Williams, B. A., Kuehne, M. A., Kung, H. H., and Miller, J. T., Thermochim. Acta 312, 17 (1998). 9. Anurov, S. A., Colloid J. 61, 131 (1999). 10. Morillo, E., Pérez-Rodriguez, J. L., Real, C., and Sánchez-Soto, P. J., J. Therm. Anal. 44, 313 (1995). 11. Shen, J. Y., Kobe, J. M., Chen, Y., and Dumesic, J. A., Langmuir 10, 3902 (1994).

12. Domingo-Garc´ıa, M., López-Garzón, F. J., and Pérez-Mendoza, M., J. Colloid Interface Sci. 222, 233 (2000). 13. Rodr´ıguez-Reinoso, F., and Linares-Solano, A., in “Chemistry and Physics of Carbon” (P. A. Thrower, Ed.), Vol. 21, p. 1, Dekker, New York, 1989. 14. Zawadzki, J., in “Chemistry and Physics of Carbon” (P. A. Thrower, Ed.), Vol. 21, p. 147, Dekker, New York, 1989. 15. Fanning, P. E., and Vannice, M. A., Carbon 31, 721 (1993). 16. Shindo, A., and Izumino, K., Carbon 32, 1233 (1994). 17. Pérez-Mendoza, M., Domingo-Garc´ıa, M., and López-Garzón, F. J., Carbon 37, 1463 (1999). 18. Moreno-Castilla, C., Carrasco-Mar´ın, F., and Mueden, A., Carbon 35, 1619 (1997). 19. Rodr´ıguez-Reinoso, F., in “Introduction to Carbon Technologies” (H. Marsh, E. A. Heintz, and F. Rodr´ıguez-Reinoso, Eds.), p. 35, Secretariado de Publicaciones, University of Alicante, Spain, 1997. 20. Swiatkowski, A., in “Adsorption and Its Application in Industry and Environmental Protection” (A. Dabrowski, Ed.), Vol. II, p. 69, Elsevier, Amsterdam, 1999. 21. Domingo-Garc´ıa, M., López-Garzón, F. J., Moreno-Castilla, C., and Pyda, M., J. Colloid Interface Sci. 176, 128 (1995). 22. Moreno-Castilla, C., Ferro-Garc´ıa, M. A., Joly, J. P., Bautista-Toledo, I., Carrasco-Mar´ın, F., and Rivera-Utrilla, J., Langmuir 11, 4386 (1995). 23. Biniak, S., Szymanski, G., Siedlewski, J., and Swiatkowski, A., Carbon 37, 1379 (1999). 24. Figueiredo, J. L., Pereira, M. F. R., Freitas, M. M. A., and Orfao, J. J. M., Carbon 37, 1379 (1999). 25. Haydar, S., Moreno-Castilla, C., Ferro-Garc´ıa, M. A., Carrasco-Mar´ın, F., Rivera-Utrilla, J., Perrard, A., and Joly, J. P., Carbon 38, 1297 (2000). 26. Otake, Y., and Jenkins, R. G., Carbon 31, 109 (1993). 27. Liston, E. M., J. Adhesion 30, 199 (1989). 28. Fritz, O. W., and Huttinger, K. J., Carbon 31, 923 (1993). 29. León y León, C. A., Solar, J. M., Calemna, V., and Radovic, L., Carbon 30, 787 (1992). 30. López-Ramón, M. V., Stoeckli, F., Moreno-Castilla, C., and CarrascoMar´ın, C., Carbon 37, 1215 (1999). 31. Papirer, E., Dentzer, J., Li, S., and Donnet, J.-B., Carbon 29, 69 (1991). 32. Contescu, A., Vass, M., Contescu, C., Putyera, K., and Schwarz, J. A., Carbon 36, 247 (1998). 33. Gillisdhamers, I., Cornelissens, I., Vraken, K. C., Vandervoort, P., Vansant, E. F., and Daelemans, F., J. Chem. Soc. Faraday Trans. 88, 723 (1992). 34. Groszek, A. J., and Aharoni, C., Langmuir 15, 5956 (1999). 35. Brown, D. R., and Groszek, A. J., Langmuir 16, 4207 (2000). 36. López-Garzón, F. J., and Domingo-Garc´ıa, M., in “Adsorption on New and Modified Inorganic Sorbents” (A. Dabrowski and V. A. Terthyk, Eds.), p. 517, Elsevier, Amsterdam, 1996. 37. Derouanne, E. G., and Chang, C. D., Microporous Mesoporous Mater. 35, 425 (2000).