Evaluation of the microporosity in activated carbons by n-nonane preadsorption

Evaluation of the Microporosity in Activated Carbons by n-Nonane Preadsorption F. RODRIGUEZ-REINOSO, J. M. MARTIN-MARTINEZ, M. MOLINA-SABIO, R. TORREGROSA, AND J. GARRIDO-SEGOVIA Departamento de Qufmica Inorgdnica, Facultad de Ciencias, Universidad de Alicante, Alicante, Spain Received April 24, 1984; accepted November 28, 1984 The n-nonane preadsorption technique has been applied to two series of activated carbons with a wide range of microporosity. Both series have been prepared from two different agricultural byproducts by carbonization in N2 and activation in a CO2 flow and they cover a 8-82% burn-off range. The results have been compared with the direct adsorption of N2 (77°K) and CO2 (273°K). The preadsorption technique provides very good results and similar to those obtained from the DubininRadushkevich equation only when the microporosity is narrow (when more than 85% of the pore volume is filled with N2 at P/Po = 0.1); for wider microporosity (samples with a burn-off larger than 60%) the technique fails because the n-nonane is removed from the wider micropores when outgassing at room temperature. © 1985AcademicPress,Inc. 1. INTRODUCTION

The n-nonane preadsorption method was proposed by Gregg and Langford (1) to evaluate the micropore volume of a carbon black rendered microporous by controlled oxidation. The excellent results obtained by these authors led us to use their technique to evaluate the micropore volume of activated carbons prepared from agricultural by-products (2) and further reacted with air to large degrees of burn-off, but the results were not as satisfactory because of the wide range of micropore size distributions of the carbons studied. Since the preadsorption method would be a direct measure of the micropore volume it could be used as a test for the validity of equations such as those put forward by the school of Dubinin (3, 4) or subsequent modifications (5, 6), all of them based on the Polanyi theory of adsorption. Marsh and Campbell (7) compared the micropore volumes of two series of activated carbons (from polyfurfuryl alcohol and polyvinylidene chloride) obtained by applying the DubininRadushkevich (DR) equation to the adsorp-

tion of CO2 (273°K) and by the preadsorption of n-nonane. They found that, in general, the DR plots gave values below the n-nonane figure, the differences being frequently very large. In the case of activated carbons prepared from agricultural by-products (olive stones and almond shells) and further reacted with dry air at 623°K it was found (2) that there was a good agreement between the N2 DR results and the preadsorption of n-nonane only when the microporosity was very narrow; otherwise, the DR values were larger than the micropore volume deduced from the preadsorption of n-nonane. These results were interpreted as due to the fact that nnonane was retained (upon evacuating at room temperature) only in narrow micropores whereas the DR value would include the larger micropores. In order to further test the validity of the DR equation and the possible general use of the n-nonane preadsorption techniques to activated carbons, two series of carbons have been prepared, using the same precursors (olive stones and almond shells), in which

315

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0021-9797/85 $3.00 Copyright© 1985by AcademicPress,Inc. All fightsof reproductionin any form reserved.

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RODRIGUEZ-REINOSO

the microporosity has been developed in a rather uniform way as shown by molecular probe adsorption (8), so that the possible limits of application of the n-nonane preadsorption might be more precisely established. II. M A T E R I A L S A N D M E T H O D S

The work described in Ref. (2) included two series of carbons prepared by reacting two activated carbons (one from almond shells and the other from olive stones and using CO2 as activating agent) with dry air at 623°K; this reaction led to a considerable widening of the microporosity with increasing gasification. For the work described here two series of activated carbons were prepared by activation with carbon dioxide to comparable burn-offs using a relatively low reaction rate to favor a gradual development of the microporosity (9). The raw materials were carbonized in N2 at l l23°K for 2 hr and later activated in CO2 at 1098°K for different periods of time to obtain six samples in each series with a burn-off ranging from 8 to 82%; in all cases the heating rate was 5°K • min -I and the gas flow 70 cm 3. min -~. The nomenclature of the samples will include C (almond shells) or D (olive stones) followed by the percentage burn-off with respect to the carbonized material. For comparative purposes a charcoal cloth, AM/G (supplied by CDE, Porton Down, United Kingdom) with a welldefined microporosity has also been used in this work. The adsorption isotherms were measured in a conventional gravimetric system using silica spring balances (Thermal Syndicate, England) conveniently adapted for the nnonane preadsorption; the experimental procedure described in Ref. (2) can be summarized as follows: after determination of the N2 (77°K) adsorption isotherms the carbons were outgassed (523°K for 4 hr at 1 0 - 6 Torr; 1 Torr = 133 Pa) and n-nonane admitted to the system at 77°K for 30 min, left in contact with the sample for 3 hr at 298°K and later evacuated at room temperature for 12 hr Journal of Colloid and InterfauceScientw, Vol. 106, No. 2, August 1985

ET AL.

(the background pressure of the preadsorbed sample after evacuation w a s 1 0 - 6 Torr); then, the adsorption of N2 (77°K) was carried out in the carbons with the n-nonane preadsorbed. In order to test whether the adsorption of n-nonane has produced any modification on the porous texture of the carbons, the samples with n-nonane were outgassed at 673°K for 5 hr under high vacuum and the adsorption of N: (77°K) was again determined. In all cases this later isotherm was coincident with that obtained for the original carbon before the n-nonane preadsorption. Adsorption equilibrium time was short, specially in samples with moderate to high burn-off but 30 min were allowed for each experimental point; samples C-9 and D-8 required longer periods of time to reach equilibrium (5 days in the case of I)-8); when the n-nonane was preadsorbed the equilibrium was reached very quickly in all cases. The adsorption of CO2 at 273°K was determined in the same experimental system; equilibrium time was short but 30 min were allowed for each experimental point. III. R E S U L T S

Figures 1 and 2 include the N2 adsorption isotherms (77°K), expressed as millimoles per gram, vs relative pressure, for the carbons for both series, and the corresponding N2 adsorption isotherms (77°K) after the preadsorption of n-nonane. The isotherms corresponding to the charcoal cloth have been included in Fig. 2. Since the CO2 adsorption isotherms are similar to our published isotherms for this type of carbon (10, 11), they have not been included for the sake of space. The N2 equivalent surface areas (S) of all carbons have been determined using the BET equation (the range of linearity of the BET plots was from P/Po = 0.03 to P/Po = 0.2) and they can be found in Table I together with the C(BET) values; both the N2 equivalent surface areas determined after the preadsorption of n-nonane (Se) and the equivalent surface areas calculated from the

PREADSORPTION IN ACTIVATED CARBONS Series - C

,a

36 C-82 32 2B C -65

24

2o

i C-53

317

log 2 Po/P = 1.6 for low b u m - o f f samples and from log 2 Po/P = 6.0 to log 2 Po/P = 3.9 for the highest burn-off samples) and from the n-nonane preadsorption method. Figure 3 includes plots of the micropore volumes as a function o f burn-off: (i) VotNz) calculated from the D R equation; (ii) V~, calculated as the vertical separation (at P/Po = 0.6) o f the parallel branches of the N2 adsorption isotherms before and after n-nonane preadsorption; (iii) V,, taken as t h e volume of nnonane retained in the carbon after evacuating at r o o m temperature; and (iv) Vo(co2),

0

D-70

20[b

0.0

0.2

AMIG

0.4

0.6

0.8

i.0

PIP.

FIG. 1. N2 adsorption isotherms (77°K) without (a) and with (b) preadsorbed n-nonane (series C).

amount of n-nonane retained after evacuating at r o o m temperature (Sn) have also been included in Table I. The Am values used for the calculation o f equivalent surface areas were 0.162 n m 2 for N2 at 7 7 ° K (12) and 0.844 n m 2 for n-nonane at 2 9 8 ° K (13). The micropore volume has been calculated by applying the D R equation to the adsorption o f N2 at 77°K (the range of linearity of the D R plots was from log 2 Po/P = 6.0 t o

~

b

3 O

E

12

4 00

O2

0~

0.6 0.a

1~

P/P.

PIG. 2. N2 adsorption isotherms (77°K) without (a) and with (b) preadsorbed n-nonane (series D and charcoal cloth AM/G). Journal of Colloid and Interface Science, Vol. 106, No. 2, August 1985

318

RODRIGUEZ-RE1NOSO ET AL. TABLE I C Values and Equivalent Surface Areas (m2.g t) of Carbons

Sample

C

S

S®

S.

SJS

Sample

C

S

S~

Sa

S./S

C-9 C-16 C-30 C-53 C-65 C-82

3160 1792 1426 761 708 587

661 848 1022 1383 1714 2014

17 26 43 78 222 897

567 755 914 1276 1592 1798

0.86 0.89 0.89 0.92 0.93 0.89

D-8 D-19 D-34 D-52 D-70 D-80

2805 1908 1495 1347 1201 1115

647 797 989 1271 1426 1525

22 38 50 83 224 376

464 696 885 1373 1349 1423

0.72 0.87 0.89 0.92 0.95 0.93

AM/G

1725

1122

83

926

0.83

calculated from the D R equation (the range of linearity of the D R plots was from log 2 Po/P = 10.4 to log 2 Po/P = 3.6 for low b u m off samples and from log 2 Po/P = 3.6 to log 2 Po/P = 2.4 for the highest burn-off samples). The liquid densities used were 0.808 g . c m -3 for N 2 at 77°K, 0.717 g . c m -3 for n-

nonane at 298°K, and 1.10 g - c m -3 for CO2 at 273°K. IV. DISCUSSION

All N 2 adsorption isotherms for the different carbons are of type I, specially for samples with low burn-off; upon gasification in CO2 the a m o u n t adsorbed increases and there is also a gradual opening of the knee of the © / isotherm and a gradual increase in the slope Series - C / of the linear branch. In other words, there is / 0.6 an increase in micropore volume and a parallel widening of the microporosity with increasing activation. However, it is important (14 to note that there are no very noticeable changes in the knee and linear branch of the © ~ ( N 2) isotherms up to carbons C-30 and D-34 ~ 0.2 e~ "7 • , v.(co,2) meaning that up to these percentages of • Vn burn-off the creation of new microporosity ~: oo predominates over the pore widening; for Series- D higher burn-off there is a more noticeable /" contribution of micropore widening to prob Q6 /FO ~/ duce larger micropores and mesopores as denoted by the more rounded knee and steeper linear branch. This is the predicted behavior according to Dubinin (14) since he claims that for a burn-off below 50% the 0.2 microporous texture prevails whereas the proportion of larger pores is more important for burn-off above 50%. oo ;o 40 6; ~o ,.=,o-o. These arguments can be quantified by determining the percentages of pore volume FIG. 3. Evolution of micropore volume with burn-off." (with respect to P/Po = 0.95) filled with N2 (©) N2 at 77°K, (~) n-nonane preadsor0tion, (O) CO2 at 273°K, (e) n-nonane retained at room temperature. at different low relative pressures since we i

i

i

i

Journal of Colloid and Interface Science, Vol. 106, No. 2, August 1985

PREADSORPTION IN ACTIVATED CARBONS are considering basically the microporosity. Table II includes such percentages for P/Po = 0.01 and P/Po = 0.1. Except in the two carbons with the largest burn-off (C-82 and D-80) the two percentages are almost equal for carbons of the two series with comparable bum-off. This indicates that the microporosity is being developed upon activation in similar fashion for the two precursors and, consequently, it is basically a function of burn-off. The percentages given in Table I1 decrease with increasing activation, slowly at the first stages but more drastically for burn-off above 50% with the subsequent more extended widening of microporosity (see columns for no.i/n0.95 - no.01/no.95 in Table II). In the case of the charcoal cloth the N2 isotherm is also of type I (Fig. 2) and the data of Table II show that its low-range porosity is very similar to that of carbons C-53 and D-52. Since these activated carbons are essentially microporous the use of surface area does not have physical meaning and it is much better to refer to pore volume. However, since the concept of surface area is still in widespread use, the term is retained in this discussion but, following Barrer (15), it should be regarded as a monolayer equivalent area. The N2 BET equivalent surface areas (S) given in Table I show that the differences between the carbons of the two series, for comparable degrees of activation, are not large (always in favor of series C) up to 5253% burn-off but they become important for

319

larger burn-off. In both series the equivalent surface area continuously increases with activation reaching extremely high values for the more activated samples. There is also a general decrease in the C(BET) values upon activation, those of series D being, in general, larger; this indicates a progressive widening of the microporosity in both series, the contribution of the microporosity being slightly more important in carbons of series D. The charcoal cloth exhibits equivalent surface area and C(BET) values comparable to carbons with medium burn-off of any of the two series. Before quantifying the n-nonane preadsorption results is important to know whether the porous texture of the carbons is somewhat modified by the introduction of n-nonane at 77°K since this could affect the real micropore volume. For this reason the adsorption of N2 at 77°K was repeated after outgassing the carbons with the n-nonane preadsorbed at 673°K for 5 hr; in all cases this isotherm was exactly coincident with the isotherm obtained with the original carbons and hence the n-nonane has not induced any structural change in the carbons. Consequently, the isotherms corresponding to the application of the preadsorption technique shown in Figs. lb and 2b are "real." In all cases the adsorption of N2 on the carbons with the nnonane preadsorbed is much lower than in the original carbons (Figs. la and 2a) but it increases with activation. For carbons with

TABLE II Percentages of Pore Volume Filled at DifferentRelative Pressures Sample

noot/no.gs

no.i/no.gs

C-9 C-16 C-30 C-53 C-65 C-82

84 81 78 71 63 52

93 93 89 87 83 73

rt0.t

n0.0t

//0.95

no.95

9 11 11 16 20 21

r/O.l

//o.ol

no.95

no.gs

Sample

no.odno.9~

no.dno.gs

D-8 D-19 D-34 D-52 D-70 D-80

85 80 77 72 64 60

94 90 88 87 84 81

10 11 15 20 21

AM/G

73

88

15

9

Journal of Colloid and Interface Science, Vol. 106, No. 2, August 1985

320

RODR1GUEZ-REINOSO ET AL.

up to 52-53% burn-off the isotherms of Figs. l b and 2b do not show an important development of both the knee and the amount adsorbed indicating that practically all the microporosity has been occupied by the nnonane; however, for larger burn-off there is a considerable development of the amount adsorbed and the knee of the isotherm showing the effect of some microporosity that is not being occupied by the n-nonane; this is not surprising considering the wider micropore size distribution of these carbons deduced from the results of Table II. In fact, the N2 external surface areas, Se, deduced from the isotherms of Figs. lb and 2b corresponding to mesoporosity and external surface if the micropores were occupied by nnonane, confirm this point: they slowly increase in the first stages of the activation and are below 100 m 2 . g -1 for up to 52-53% burn-off; but for larger burn-off Se considerably increases reaching values of up to 897 m 2" g-i for carbon C-82. It is clear then that in the later carbons a relatively large proportion of micropores are not occupied by nnonane. In fact, the D R plots for the highest burn-off samples of the two series become curved (even for P/Po > 0.01) showing the existence of large micropores (supermicropores). The charcoal cloth has a behavior similar to carbons of the two series with medium burn-off. On the other hand, the amount of nnonane retained by the carbons after evacuation at room temperature has been used to calculate the corresponding equivalent surface areas, Sn, taking a molecular area of 0.844 n m 2 (13). The results can be found in Table 1 where it is shown that the Sn values increase with increasing burn-off, the S~ values always being lower than the corresponding S values. However, the S d S ratios (Table I) exhibit a maximum for carbons C-65 and D-70 decreasing for the largest burn-offs. This trend can be explained in terms of the accesibility o f the n-nonane molecule to the micropores; thus, in carbon D-8, with very narrow microJournal of Colloid and Interface Science, Vol. 106, No. 2, August 1985

porosity, n-nonane cannot fill all the micropores, but with increasing activation such accesibility increases and for the largest degrees of burn-off the contrary effect will apply: some of the micropores are too large to retain the n-nonane after outgassing at room temperature. As mentioned above, since we are dealing with essentially microporous carbons (Table II), the use of pore volume has a more clear physical meaning than surface area and, consequently, we will focus now on the micropore volume of the samples. The more commonly used empirical equation for the determination of the micropore volume is the D R and it has been used here to calculate the values plotted in Fig. 3. In any of the two series V0(N2 ) increases linearly with activation although the values are somewhat lower for carbons of series D, specially for larger burnoff; Fig. 3 also shows the evolution of the micropore volume deduced from the n-nonane preadsorption technique (V~). The excellent agreement of V0~N2) and V~ in most samples is relatively surprising considering that they have been obtained from such different methods; on the other hand, the increasing differences bewteen VOtN2)and V~ seen in Fig. 3 for the two carbons with larger burn-off of each series is a confirmation of the assumption advanced in a previous paper (2) about the limitations of the n-nonane preadsorption technique when applied to activated carbons. With increasing activation and subsequent increase in micropore sizes the n-nonane is not retained after evacuation at room temperature in the larger micropores and, consequently the VD value will be lower than Vo~2), the difference increasing with the proportion of larger micropores. The data of Table II, concerning the pore volume filled with N2 at P/Po = 0.01 and P/Po = 0.1, indicate that when less than around 85% of the pore volume is filled below P/Po = 0.1 the preadsorption technique yields too low V'0 values; similar percentages were found for two series of activated carbons reacted in

PREADSORPTION IN ACTIVATED CARBONS

air at 623°K (2, 16). The charcoal cloth exhibits a behavior similar to medium burnoff carbons (C-53 and D-52). Figure 3 also includes the plots of Vn (volume of n-nonane retained after evacuation at room temperature) as a function of burn-off. As in the case of V0(N2) and V~ the plots are linear and only the points corresponding to the carbons with larger burn-off of each series (C-82, D-70, and D-80) drop below the straight line. It is important to note that the slope of the two plots for each series are very similar but Vn is always lower than V0tN2)(or V'0). The lower values of V~ could possibly be due to the different packing of n-nonane compared to nitrogen or to steric effects of n-nonane (1). However, it could also be that n-nonane does not fill all the micropores and is just blocking the entrance of some of them; this would not affect the V'0 value but would lead to a too low Vn value; in fact, Vn/V'o increases from 0.72 for C-9, to 0.82 for C-30, to 0.86 for C-65, being 1.07 ( ~ 1.0) for C-82. This analysis fits in well with the change in curvature of the knee of the isotherms and the fall in the C(BET) values. It is also important to note that the slopes of the plots for the two series are very similar and this seems to be an indication of the fact that the micropore volume is essentially a function of the burn-off and independent of the precursor under the experimental conditions used. In the case of the charcoal cloth, VO(N2)and V~ are also very similar (0.46 and 0.44 cm3.g -~, respectively) and larger than V~ (0.35 cm3-g -1) and consequently the behavior is, again, similar to medium burn-off carbons of any of the two series. The adsorption o f C O 2 at 273°K can help to better understand the plots of Fig. 3 for V0~N2~, lro and Vn. As it is well known the low relative pressure-(<0.03)covered in the adsorption of CO2 at 273°K has led to some authors (7) to consider this adsorbate as ideal to determine the micropore volume in microporous carbon adsorbents; on the other

321

hand, the adsorption of CO2 has some limitations because of its high quadrupole moment and its sensitivity to surface polarity (12). Nevertheless CO2 is very commonly used in the characterization of carbon adsorbents and in this context its application in this work is useful. The micropore volumes (V0~co2)) obtained from the adsorption of CO2 at 273°K for all the carbons described here have been plotted in Fig. 3 to facilitate the comparison. Again, the data fit straight lines except for carbons with very high burn-off, those in which the n-nonane values, V~ and Vn, also dropped below the corresponding straight lines. The straight lines fitting the micropore volumes obtained from the adsorption of CO2 for each series have lower slopes than the other two plots of Fig. 3. Thus, V0~N2)and V0~co2) are very similar for carbons C-9 and D-8, with low burn-off, but V~co2) becomes lower for the other carbons, the difference being larger the higher the burn-off o f the carbon. Lamond and Marsh (17) found a similar trend when comparing V~N~ and V0(co2) for an activated series of carbons prepared from polyfurfuryl alcohol. On the other hand, the V~co2) plot crosses the Vn plot at around 50% burn-off and, consequently, the V~co2) values become lower than Vn for carbons C82, D-70, and D-80, which are the samples in which both V'0 and Vn fail to fit the straight lines. The comparison of the micropore volumes calculated from the different methods (V0~N~), Vb, Vn, V0~co2)) and their evolution with burn-off may be summarized as follows: (i) At low burn-off in both series the adsorption of N2 and CO2 involves volume filling rather than surface coverage, because the micropores are narrow enough to produce enhancement of the adsorption potential, consequently the values of V0tN2)and V~co2) are close to one another. At higher bum-off N2 continues to adsorb by micropore filling, but in the case of CO2 the dominant mechanism is surface coverage, because the pores Journal of Colloid and InterfaceSc/ence, Vol. 106,No. 2, August19~5

322

RODRIGUEZ-REINOSO ET AL.

are wider and the relative pressure of C O 2 is low (<0.03). Consequently the V~co:) values fall below the V0tN2) values as burn-off increases. Lamond and Marsh (17) have proposed a similar mechanism to explain their results. (ii) The 1I, values are lower than V0tco2) only up to medium burn-off, but become larger at higher burn-offs. This effect can be explained in terms of the different accessibility to the microporosity by the molecules o f CO2 and n-nonane on account of the difference in their minimum dimensions (CO2 < n-nonane). When the micropores are very narrow the n-nonane molecules block the pore entrances and do not fill them completely, whereas the CO2 molecules are able to enter and fill up the whole microporosity. The widening of the micropores produced by the increasing activation leads to an approach of 1I, toward V0tco2), and around 50-60% burn-off both values are very similar; this can be taken as an indication that the percentage of small micropores with dimensions between those of the CO2 and n-nonane molecules decreases as burn-off increases, leading to a microporosity equally accessible to both molecules. A further test of the effect of micropore widening on the n-nonane preadsorption technique is to subtract the N2 isotherm obtained after the preadsorption of n-nonane from the N 2 isotherm corresponding to the original carbon. This artifact would then be "equivalent" to determining the N2 isotherms corresponding to the micropores occupied by n-nonane. Such isotherms have been included in Fig. 4 for the two series of carbons; there is a continuous increase of the amount of N2 filling the micropores up to carbons C-65 and D-52 besides a gradual opening of the isotherm's knee because of the widening of the microporosity. The plots for carbons C82, D-70, and D-80 are a confirmation of the discussion carried out above about the plots o f Fig. 3 for the same samples; thus, for instance, the plot of carbon C-82 has a very wide knee and the amount adsorbed is Journal of Colloid and Interface Science, VoL 106, No. 2, August1985

Series ~ C

20 C-65 C -82 C-53

16

C-30 12 C-16 C-9

A

"T K3~

v

E E Series-D

20

D- 70 D-80 D-52 AMIG D-34

16 12

D-20 D-8

(~0

0.2

0/~

0.6

0.8

1.0

PI Po

FIG. 4. N 2 isotherms corresponding to micropores covered by n-nonane.

even lower than the corresponding to carbon C-65. The linear branches for all isotherms are, of course, parallel if n-nonane is only occupying micropores. Such linear branches are not exactly parallel to the relative pressure axis probably because the n-nonane occupying the micropores will act as external surface toward the N2 adsorbed on it. V. CONCLUSIONS

The results given in this work, which confirm the predictions advanced in a previous paper (2), have shown the validity of the use of the n-nonane preadsorption technique for

PREADSORPTION IN ACTIVATED CARBONS

the evaluation of the micropore volume in activated carbons provided the microporosity is narrow (when more than 85% of the pore volume is filled with N 2 at relative pressures below 0.1). For samples with a widened microporosity the technique fails because the n-nonane is removed from the larger micropores upon outgassing at room temperature (prior to the adsorption of N2 at 77°K) as seen in the plots of Fig. 4 and from the too high values of external surface area given in Table I. On the other hand, the amount of nnonane retained by the micropores in activated carbons, if converted in equivalent surface area by using a molecular area for nnonane of 0.844 n m 2, yield results which are comparable to the BET N2 equivalent surface areas, only if n-nonane is accessible to all the micropores and is retained after outgassing at room temperature (i.e., if there are not large micropores). ACKNOWLEDGMENTS This research was supported by the CAICYT (Project No. 795/81). REFERENCES 1. Gregg, S. J., and Langford, J. F., Trans. Faraday Soc. 65, 1394 (1969).

323

2. Linares-Solano, A., L6pez-Gonz~lez, J. D., MartinMartlnez, J. M., and Rodrlguez-Reinoso, F., Adsorpt. Sci. Technol. 1, 123 (1984). 3. Dubinin, M. M., Carbon 20, 195 (1982). 4. Dubinin, M. M., Carbon 21, 359 (1983). 5. Stoeckli, H. F., J. Colloid Interface Sci. 59, 184 (1977). 6. John, P. T., and Nagpal, K. C., Carbon 16, 359 (1978). 7. Marsh, H., and Campbell, H. G., Carbon 9, 489 (1971). 8. L6pez-Gon~lez, J. D., Martln-Martlnez, J. M., M6ndez, A., Molina, M., and Rodrlguez-Reinoso, F., "Proceedings, 16th Biennial Conference on Carbon," p. 307. San Diego, 1983. 9. Rodrlguez-Reinoso, F., L6pez-Gonz~ilez, J. D., and Berenguer, C., Carbon 22, 13 (1984). 10. Linares-Solano, A., L6pez-Gonz~ilez, J. D., MolinaSabio, M., and Rodfiguez-Reinoso, F., J. Chem. Technol. Biotechnol. 30, 65 (1980). I1. L6pez-Gonz~lez, J. D., Martlnez-Vilchez, F., and Rodrlguez-Reinoso, F., Carbon 18, 413 (1980). 12. Gregg, S. J., and Sing, K. S. W., "Adsorption Surface Area and Porosity." Academic Press, New York/ London, 1982. 13. Gregg, S. J., and Stock, R., Trans. Faraday Soc. 53, 1355 (1957). 14. Smisek, M., and Cerny, S., "Active Carbon." Elsevier, Amsterdam, 1970. 15. Barrer, R. M., "Surface Area Determination," p. 90. Butterworth, London, 1970. 16. Rodrlguez-Reinoso, F., Linares-Solano, A., MartinMartlnez, J. M., and L6pez-Gonzfilez, J. D., Carbon 22, 123 (1984). 17. Lamond, T. G., and Marsh, H., Carbon 1, 281 (1964); 1, 293 (1964).

Journal of Colloid and InterfaceScience, Vol. 106,No. 2, August1985