The combined use of different approaches in the characterization of microporous carbons

Carbon Vol. 27. No. I. pp. 23-32. Prtntcd in Great Bntain.

t~~tR-~223~RY $MHI+ .w Copyrzght C 1989 Per@non Press plc

I9R9

THE COMBINED USE OF DIFFERENT APPROACHES IN THE CHARACTERIZATION OF MICROPOROUS CARBONS F. RODRIGUEZ-REINOSO, D~partamento

J. GARRIDO, J. M. MARTEN-MARTINEZ, M. MOLINA-SABIO, and R. TORREGROSA de Quimica Inorglnica e Ingenieria Quimica, ~niversidad de Alicante, Alicante. Spain.

Abstract-Adsorption of Nz (77 K). CO, (273 K), and several hydrocarbons (273 or 298 K) and nnonane preadsorption for two series of activated carbons with more-or-less distorted type I isotherms have been analyzed by the Dubinin-Radushkevich (D-R), Langmuir, and a, methods. When the microporosity is narrow and uniform, all theoretical and experimental approaches lead to values of micropore volume that are in good agreement. For carbons with wide micropore size distributions the Langmuir, u, and D-R methods applied to N, (77 K) (if for the latter a straight portion in the D-R plot can be drawn in the 0.05-0.3 range of relative pressure) yield the total micropore volume (as the conventional D-R plots for hydrocarbons) whereas n-nonane preadsorption and CO: give the volume corresponding to narrow micropores, in this way allowing a more complete characterization of the microporosity. Key Words--Porosity,

isotherm analysis, adsorption isotherms.

and E is the characteristic free energy of adsorption for a given system. By combining eqns (1) and (2), the well-known Dubinin-Radushkevich (D-R) equation can be obtained:

1. rNTRODU~ON Physical adsorption on microporous carbons is a very complex process that cannot be described by any single current theory; thus, the micropore volume of some carbons seems to be dependent on the adsorptive and the method used for calculation. Although the IUPAC classification[ l] defines micropores as pores with dimensions less than 2 nm, there is some evidence]21 indicating that two different types of micropores can be distinguished: narrow micropores, which are filled at very low relative pressures (-0.01 for nitrogen adso~tion at 77 K) and wider micropores-supermicropores as they are called by Dubinin[3]-which are filled at larger relative pressures (up to ~0.2). Narrow micropores are filled by a primary filling process involving enhanced adsorbent-adsorbate interactions in very narrow pores and wider micropores are filled by a different secondary process, probably of cooperative type[2]. There is, of course, the additional problem of the mechanism of micropore filling being dependent on the ratio of pore width to molecular diameter rather than on the absolute value of the pore dimension. Dubinin and Radushkevich[4] developed the micropore volume filling theory, based on the Polanyi concept of characteristic curve, to describe the adsorption on micropores. The characteristic curve could be expressed as

V/V,, = exp [-(A/E)L]

V/V,, = exp [ - B( T/p)? log’ (P,,lP)]

where B is the structural constant and 6 a similarity coefficient (6 = 1 for benzene). Equation (3) may be written for plotting as log v = log v,, - zf log’ ~~~~/P) where L) = E(T/@)‘. A more general expression by Dubinin and Astakhov[S]:

(4)

was later developed

V/V,, = exp [ -(A/E)pr]

(5)

which is applicable to a wider range of carbons, and a further modification was introduced by Stoeckli et a/.16,7] based on the summation of the contrit)utions from individual micropore groups. Although ail these equations have been successfully applied to a number of experimental systems, their validity cannot be taken for granted, as shown by several authors[8,9]. Since microporous carbons lead to more-or-less distorted type I isotherms, they could be mathematically described by the well-known Langmuir equation[8]:

(1)

where V is the volume adsorbed at the relative pressure PIP,, and V,, is the volume of the micropores; A is the affinity (the differential free energy of adsorption): A = RT In (HP,,)

(3)

P/V = l/(V,,, . B) + P/V,,,

(6)

from which the monolayer volume (V,,,) can be calculated. In spite of the possible fit of the experimental data, the validity of the Langmuir model for

(2) 23

24

F. RODRiGUEZ-REINOSO et al.

physisorption on microporous adsorbents is subject to severe criticisms[lO]. A different approach to the study of the adsorption or microporous carbons would be the use of adsorptive molecules of selected sizes as molecular probes; this would be an unambiguous way of determing the micropore volume available to each molecular dimension. This is, however, a very tedious experimental procedure and cannot be easily applied to relatively large micropores. Alternatively, the preadsorption technique[ll] consists of filling the micropores with large molecules before carrying out the adsorption of, say, nitrogen, on mesopores and external surface, n-nonane being the most common preadsorbate used[8]. This technique has been applied to several types of carbons but it has some limitations-as have most methods-for carbons with a wide micropore size distribution[l2,13]. There is an additional empirical approach for the assessment of microporosity in carbons-the use of the t or ~1, methods[8]-provided an appropriate standard isotherm that is obtained on a nonporous reference adsorbent having a similar surface structure to the carbon under investigation is available. Although this requirement is problematic in carbon adsorbents, the reference material recently proposed by Rodriguez-Reinoso et al.[14] seems to be very useful for activated carbons and similar materials, even though the practical application of the a, method is not straightforward[l5]. The objective of this investigation was to compare all these approaches-using different adsorptiveswhen applied to two series of activated carbons covering a wide range of pore-size distributions. In this way, the usefulness and limitations of the different methods can be evaluated. 2.

EXPERIMENTAL

Two series of activated carbons were used for this investigation. Series C was prepared from almond shells and series D from olive stones, as described elsewhere[ 12,161. The precursors were carbonized in nitrogen at 1123 K for 2 h and subsequently activated on CO* at 1098 K for different periods of time in order to cover a wide range of burnoff (882%). The percentage of burnoff is included in the nomenclature of the samples. The characterization of the carbons was carried out by means of physical adsorption of Nz (77 K), CO2 (273 K), n-butane (273 K), benzene (298 K), 2,2_dimethylbutane (298 K), and iso-octane (2,2,4trimethylpentane) (298 K) in conventional gravimetric adsorption systems using lOOO-torr Baratron pressure transducers. The preadsorption of n-nonane was studied in the same experimental systems. More details can be found elsewhere[l2,17,18]. All carbons were outgassed overnight under high vacuum at 573 K; for some adsorptives, such as CO*, additional outgassing at 473 K and 673 K were tested

for some carbons, but no significant differences in the isotherms were found. 3. RESULTS

AND

DISCUSSION

It was shown in a previous work[l9] that the activation with CO2 of carbonized lignocellulosic materials such as those used here produces activated carbons that are predominantly microporous. For carbons with up to 20% burnoff there is a creation of microporosity that is relatively narrow and uniform but, as the burnoff increases, there is also a widening of the microporosity with an increasing contribution from wide micropores (supermicropores). This evolution is clearly seen in Fig. 1 where the N, (77 K) adsorption isotherms (n, amount adsorbed in mmol/g vs. relative pressure) for carbons of the two series under investigation are included. The porosity of the carbons changes with burnoff and the overall evolution is rather similar for the two precursors; this evolution is also very similar for all other adsorptives used. As stated above, several approaches may be used to analyze the adsorption isotherms. In what follows, such analysis will be presented, indicating what seems to be the advantages and disadvantanges of all methods under investigation. 3.1 D-R equation According to eqn (4), a plot of log n versus log? (P&P) will,allow the calculation of the micropore volume, V,,, of the carbons from the intercept of the linear D-R plot, assuming that the pores are filled with liquid adsorbent. The Nz D-R plots for some typical carbons of both series may be found in Fig. 2, where it is apparent that the form of the plot is clearly dependent on the burnoff of carbon. The DR plots for the more microporous carbons (low burnoff) are linear in almost the whole range of relative pressures covered although the linearity does not extend above PIP,, = 0.2. The range of linearity for carbons with larger burnoff is shorter and decreases with increasing burnoff so that for carbons D-70, D80, C-65, and C-82 the plots show a clear upward deviation. The calculated values of V, (density for liquid nitrogen, 0.808 g cm-j) for carbons of the two series are listed in Table 1. Two V,, values are given for carbons with medium and large burnoff since a second straight portion at higher relative pressures (0.05-0.3) can be drawn (although, of course, the uncertainty in drawing such straight line is much larger than at low relative pressures). The D-R plots for all other adsorptives-an example is given in Fig. 3 for benzene at 298 K-lead to the micropore volumes also listed in Table 1. The general behavior for the carbons of the two series for all adsorptives except CO* (discussed later) may be summarized as follows. 3.1.1. Carbons with low burnoff. The D-R plots for these carbons are linear in almost the whole range of relative pressures and the V,, values may be cal-

Characterization

of microporous

carbons

25

32

D-80

C-65

c-53 D-52 C-36

0

0.2

0.4

0.6

0.6

c-29

D-34

C -16

D-19

c-9

D-

1.0

PI p.

Fig. 1. N> (77 K) adsorption

0

isotherms

culated without much uncertainty. The V,, values for all adsorptives clearly indicate that carbons C-9 and D-8 (and, to a lesser extent, C-16 and D-19) exhibit molecular sieving toward the largest molecules, specially iso-octane. This is not surprising since, as shown previously[20], the type of carbonized materials used here has constrictions in the micropores that are removed on gasification in CO?: this removal is only partial below 10% burnoff. 3. I .2. Carbons with medium burnoff (40650%:). The D-R plots are linear in a relatively large range of relative pressures (although shorter than in group above) for both N, and any other adsorptives-see Fig. 3 for the case of adsorption of benzene at 298 K on carbons of series D. In this group the V,, values deduced from N, and all hydrocarbons are almost coincident. 3.1.3. Carbons with large burnoff (>50%). The N1 D-R plots clearly show two different linear portions whereas the D-R plots for hydrocarbons are linear up to PIP = 0.2. For carbons C-53 and D-52.

02

04

on carbons of

series C

06 and

0.8

1.0

2

4

6

Fig. 2. D-R plots for NL (77

PI P*

D.

the hydrocarbons give V,, values that are in agreement with the value obtained from the low relative pressure portion of the N, D-R plots. For all other carbons with larger burnoff, the V,, values are in good agreement only if the high pressure portion of the N1 D-R plots are used to calculate V,,. The deviations found for 2,2_dimethylbutane and iso-octane on carbons C-65 and C-82 could be due to a slight molecular sieving effect produced by the opening of new narrow micropores in this material[l6,17]. More complete information may be obtained if the characteristic curves are used instead of the DR plots. The characteristic curves for carbons D-8, D-34. and D-80, belonging to the three groups given above. are included in Fig. 4. The plots for carbon D-8 clearly show the molecular sieving effect exhibited by the narrow microporosity and the curves are ordered according to the minimum dimenstons of the adsorptives. The curves for carbon D-34 are coincident for all adsorptives and consequently the micropore volume is common to all of them. The char-

16 I

0

8

0

K) on some carbons

2

6

of series C and D.

a

F. RODRIGUEZ-REINOSO et al.

26

Table 1. Micropore volume (cm’ g-‘) of activated carbons Dubinin-Radushkevich Langmuir

N2 77 K

a-method NP

Sample

Ip

hp

Benzene 298 K

n-butane 213 K

2,2-DMB 298 K

Iso-octane 298 K

CO? 273 K

N2 77 K

D-8 D-19 D-34 D-52

0.26 0.31 0.39 0.50

0.55

0.21 0.25 0.36 0.49

0.21 0.28 0.38 0.49

0.16 0.25 0.36 0.50

0.03 0.21 0.35 0.50

0.26 0.30 0.36 0.41

D-70 D-80

0.57 0.62

0.67 0.78

0.69 0.75

0.66 0.76

0.66 0.78

0.68 0.76

c-9 C-16 C-29 C-38 c-53 C-65 C-82

0.26 0.32 0.39 0.45 0.54 0.67 0.79

0.49 0.59 0.79 1.09

0.24 0.30 0.37 0.44 0.55 0.74 1.12

0.24 0.31 0.37 0.44 0.59 0.81 1.03

0.20 0.27 0.34 0.37 0.53 0.70 1.07

0.06 0.25 0.41 0.45 0.54 0.57 0.83

curves for the hydrocarbons on carbon D80 are almost coincident but somewhat different from that of N, for which two well-defined straight portions are found (they correspond to the two portions of the D-R plot shown in Fig. 2), that at higher relative pressures lead to the same V. value as hydrocarbons. The characteristic curves for these and other carbons with large burnoff show that although N, (77 K) and benzene (298 K) cover a similar range of (A/P)Z, benzene does not exhibit two different branches as nitrogen does[l7]. The results obtained with CO, (273 K) merit a separate discussion. The CO? D-R plots are relatively linear in the whole range of relative pressures used, as shown in Fig. 5 for some carbons of series D taken as typical acteristic

N2

. VW

V,,

71 K

0.25 0.32 0.40 0.54

0.26 0.31 0.38 0.51

0.16 0.24 0.31 0.41

0.25 0.31 0.39 0.53

0.48 0.51

0.68 0.79

0.52

0.47 0.50

8::

0.25 0.30 0.39 0.43 0.51 0.56 0.58

0.27 0.33 0.43 0.50 0.59 0.69 0.99

0.26 0.32 0.39 0.47 0.54 0.62 0.59

0.19 0.27 0.32 0.38 0.45 0.56 0.63

0.27 0.33 0.40 0.49 0.58 0.76 1.08

examples. Consequently, the extrapolation to calculate V, appears to be less ambiguous for carbons with large burnoff than in the case of N2. When the V, values deduced from the CO, D-R plots are compared with those of Nz and hydrocarbons (see Table l), it is found that at low to medium burnoff the values are very similar. The micropores are narrow enough so that the adsorption of N2 and CO, (either pore filling by both adsorbates or filling by N2 and surface coverage by COJ will give the same micropore volume, the D-R plots being linear over a wide range of relative pressures for N2 and CO,. As the burnoff increases, the N, D-R plots are not as linear as those for CO?; the average microporosity has been widened by activation and the V,, value calculated from N2 is larger than the V,, obtained from COZ--

1.0 -

aa-

D-80

-0 c

D-52

g 06-

oj-kz-----=-. D-19

I 0

I

I

I

I

1

2

3

4 log2 ( S/P)

Fig. 3. D-R plots for benzene (298 K) on some carbons of series D.

Characterization of microporous carbons (A 10)*(kJ.m-'I* -1.0

0

50

150

100

- 2.0

D-8

-3-o

-4-o 0 > c - 1.0

I

’

’

0

using an additional lower-range pressure transducer and data obtained at 90 K. There is a common characteristic curve for carbon D-19; the linearity extends over a wide range of A2 and the intercept for both adsorptives and the three temperatures used leads to a common value of V,. Although the curve is apparently similar for carbon D-52, it is seen that CO, and NZ give the same value of V,, only if lowpressure data for the latter are available; if only the N, data obtained for PIP, > 5.10-j are used the intercept would give a value of V,, larger than the intercept of the CO, data. This phenomenon is enhanced in carbon D-80 for which the difference in micropore volume measured by N? (77 K) and CO? (273 K) is even larger as shown in Table 1. The deviation of N1 experimental points below the extrapolation of CO1 (273 K) data is due to the restricted diffusion of N2 into very narrow micropores at very low adsorption temperature (the larger deviation corresponds to 77 K for which the restricted diffusion would be more important than at 90 K)[ IS]. Figure 6 illustrate clearly that N1 and CO? data are sampling quite different ranges of the microporefilling process. The analysis of many series of activated carbons prepared from these and similar precursors[21,22] has shown that when the microporosity is wideburnoff above around 40 to 50%-the use of only relative pressures above 0.01 for Nz at 77 K (the range covered in most experimental systems used in the characterization of activated carbons) would lead

I+--=-OD-34

-2.0

27

- 1.0 D-80

:,:;.-I-

Fig. 4. Characteristic curves for carbons D-8, D-34, and D-52. 0: N? (77 K): LJ: benzene (298 K); El: n-butane (273 K); x: 2.2-DMB (298 K); V: iso-octane (298 K). even larger values are obtained if the straight portion of the N, D-R plot at higher relative pressures is used for extrapolation. It seems that NZ is filling also the wider micropores-although by a different mechanism as shown by the sudden change in the slope of the D-R plot-whereas CO? is either filling only narrow micropores or being adsorbed by a surface coverage mechanism[ 18,211. These results seem to indicate that N2 and CO?, when adsorbed at 77 and 273 K, respectively, do not measure the same type of microporosity in highly activated carbons, the difference increasing with burnoff. It should be remembered here that all adsorption isotherms have been determined using a lOOO-torr pressure transducer so that the range of relative pressure covered is 5.10-’ to 1.0 for Nz (77 K) and 5.W’ to 3.10-* for CO? (273 K). In order to understand the differences, Fig. 6 includes the characteristic curves of N, and CO2 for carbons D19, D-52, and D-80 taken as typical examples; in order to cover a wider range at A2 (f3 has not been considered since it is very similar for NZ and CO,) the N1 data were supplemented with data obtained

I-

D-52

0.5 c

o-

s

\ -05

-

x . 3 \

\

D-80 0.5 -

o-

-0.5

__j_?___

.

\o

t 0

2

4

6

6

’

\

‘) \

b\

10

log2w./P)

Fig. 5. D-R plots for CO, (273 K) on carbons D-19, D52. and D-80.

F.

28

RODRiGUEZ-REINOSO

A2 (kJlmol)* 0

200

100

D-19

curves for carbons D-19, D-52, and D-80. 0. 0: N: (77 K); 0 : N2 (90 K); a: CO: (273 K).

Fig. 6. Characteristics

to values of V, larger than those of CO, at 273 K. Of course, even larger values of V, are obtained if the straight portion at higher relative pressures of the N, D-R plots is used. When the adsorption data of Nz, CO?, and hydrocarbons are compared, one may conclude that for carbons with low burnoff the D-R equation will give the same V,, value (provided that the microporosity is accessible to all molecules), whereas for carbons with large burnoff, hydrocarbons and N? (if the higher relative pressure portion of the D-R plot is used) are measuring the total micropore volume (including supermicropores) whereas CO1 would only measure the narrow microporosity. This means that although the D-R equation has severe limitations when applied to carbons with a wide micropore distribution-imposed by the heterogeneity of the pore system and, consequently, by the adsorptive and the relative pressure range usedit can be used advantageously if a convenient range of adsorptives and adsorption temperatures are selected.

et d.

3.2 Langmuir equation Although the adsorption process as it occurs within the carbons is not the adsorption process on which the Langmuir (or BET (Brunauer-Emmett-Teller)) equation is modeled, the plot of Pin against P normaly yields a straight line of slope l/n,, where n,” is the equivalent monolayer capacity. As Fig. 7 shows, the N2 Langmuir plots for carbons of series D have two straight portions; the slopes of the two portions are the more different the higher the burnoff of the carbon (as for the D-R plots of Fig. 2), and at the same time, there is a simultaneous displacement toward lower pressures of the intersection of the two linear portions. The n,, values for Nz adsorption (taken as the micropore volume of the carbons after the appropriate conversion to liquid) listed in Table 1, which are calculated from the straight portion in the relative pressure range 0.03 to 0.3, show a remarkable agreement with the V, values obtained from the corresponding D-R plots (using either the straight D-R plot obtained for carbons with low burn-off or the straight portion at higher relative pressures for carbons with large burnoff). Thus, the results for the two series of carbons described here show that although the Langmuir model does not describe the adsorption process in heterogeneous microporous solids the more-or-less distorted type I isotherms fit the Langmuir equation in the 0.03 to 0.3 relative pressure range (corresponding to the knee of the isotherm) leading to V,, values that agree with those obtained from the D-R equation as applied here. 3.3 Preadsorption of n-nonane The preadsorption of n-nonane (NP) was developed as an effective way of filling the micropores of an adsorbent while leaving the mesopores and external surface available for the adsorption of nitrogen. This method has been applied to a variety of porous solids and previous results[l2,13] have shown that it works for carbons with relatively narrow microporosity. This technique has been applied to the two series of carbons under investigation following the experimental procedure described elsewhere[12,13]. Typical examples of N, (77 K) isotherms obtained on carbons with preadsorbed n-nonane at 77 K and outgassed overnight at room temperature are given in Fig. 8; the micropore volume of the carbons (VNp) has been calculated as the vertical separation (taken, arbitrarily, at P/P, = 0.6) of the parallel branches of the N, adsorption isotherms before and after n-nonane preadsorption. The calculated values are listed in Table 1 together with the volume of n-nonane (V,,) retained in the carbon after outgassing at room temperature (prior to Nz adsorption). The (VNp) values for all carbons except those with larger burnoff (C-65, C-82, D-70, and D80) are in very good agreement with the NZ D-R low-pressure straight portion values, and this is remarkable considering that they have been obtained from such different methods; as discussed above, the same similarity applies to hydrocarbons when no mo-

Characterization

of microporous carbons

29

D-8

D-34

D-52

0

200

400

600

800

P( torr)

Fig. 7. Langmuir plots for NL (77 K) on some carbons of series D

14

0.2

0.4

0.6

08

1.0

PI e

Fig. 8. N2 (77 K) adsorption isotherms on carbons of series C with n-nonane preadsorbed

30

F. RODRIGUEZ-REINOSOet al.

lecular sieving is exhibited by carbons. For the four carbons with larger burnoff, the difference between the D-R and NP methods increases with increasing burnoff as advanced in a previous paper[l2]; with increasing activation and subsequent increase in micropore size, the n-nonane is not retained after outgassing at room temperature in the larger micropores and, consequently, the micropore volume obtained will be lower than the one obtained from the D-R plot, the difference increasing with the proportion of larger micropores. The V,, values are lower than V,, values, and this could be taken as indicative of n-nonane not filling all the micropores but just blocking the entrance of some of them; this would not affect the V,, values but would lead to a too low value of V,,. It is also interesting to compare the micropore volumes deduced from the adsorption of CO, and those obtained from the NP method; V,(CO,) and V,, are almost coincident for carbons with low and very large burnoff but the CO2 value becomes lower than V,, value for all other carbons; on the other hand, the V,, values are lower than Vo(C02) at low and medium burnoff, but they are very similar for carbons with large burnoff. This effect might be ex-

O.OlQl

plained in terms of the different accessibility to the microporosity by molecules of CO, and n-nonane on account of their different minimum molecular dimensions. When the micropores are very narrow the n-nonane molecules block the pore entrances and do not fill them completely, whereas the CO, molecules are able to enter and fill the whole narrow microporosity. The widening of the micropores produced by the increasing activation leads to an appreach of V,, toward the V,(CO,) and, in turn, to V,,.

On the other hand, the n-nonane preadsorption results, as those of adsorption of CO, (273 K), help to understand the effect of activation on the porosity of the carbonized precursors. As Table 1 shows, both techniques indicate that there is a continuous increase in the volume of narrow micropores up to about 65% burnoff, but this increase is very slight for larger burnoff. However, since N, and hydrocarbons results show a continuous increase in micropore volume, this means that at very large burnoff, the widening of narrow micropores to give wider micropores or even mesopores is more important than the development of narrow micropores. We will return back to this point later.

0.5

0.9

P/p,

35

_- -

O0 O0

30

c-62

~ooT3vQ+--

-00

0 0

25

-? 20 WI 2 5 C

15

10

5

0 0

0.5

1.0

1.5

2.0

Fig. 9. a-Plots for N2 (77 K) on carbons of series C.

2.5

Characterization

of microporous carbons

3.4 The a, method The CX, plots for carbons of series C (Fig. 9), taken as typical examples since those for carbons of series D are very similar, were constructed with the adsorption data of the new reference material[l4] taken as standard; the amount of nitrogen adsorbed was plotted against (Y,,the reduced standard adsorption at the corresponding relative pressure (with (Y, = 1 at P/P,, = 0.4). The micropore volume is calculated from the intercept of the extrapolated multilayer region. The corresponding values have been listed in Table 1. The CL,plots for carbons with low burnoff are typical of primary micropore filling in pores of molecular dimensions[2]. The micropore volume obtained from the extrapolated multilayer region is in fairly good agreement with the D-R values; as mentioned above, the D-R plots for these carbons are linear in a very large range of relative pressures. The CY, plots for carbons with large burnoff show an upward swing at about a, = 0.57 (P/P,, = 0.02) that could perhaps be indicative of a secondary micropore filling of wider micropores; this upward swing seems to be coexisting with the large upward deviation of the D-R plots discussed previously. Because of this, the micropore volume deduced from the (Y,plots is in agreement only with the V,, calculated from the higher relative pressure straight portion of the D-R plot. These results confirm the points discussed above for the application of the D-R equation to the adsorption of CO:. Nz, and hydrocarbons. The D-R plots for Nz and hydrocarbons or Langmuir for Nz can provide a measure of the total micropore volume (primary and secondary filling) although the D-R plots for NZ are not unambiguous for calculating V,, because of the upward deviation at high relative pressures; this uncertainty is not found in the (Y,plots for these carbons with high burnoff, and consequently this method could be more precise in this type of highly activated carbons. However, this is true[ 151 only if the N2 adsorption isotherm is moreor-less distorted type I, clearly approaching a limiting value of n (amount adsorbed) as P/P,, + 1. On the other hand, CO1 at 273 K, because of the much lower relative pressure range covered, would only measure the narrow microporosity; very low pressure data for Nz at 77 K. provided there are no problems of restricted diffusion, would also give a measure of this narrow microporosity (primary micropore filling). These experimental findings may be used then to evaluate the microporosity of the two series of carbons more completely. The volume of narrow micropores (V, corresponding to the primary pore filling process) is given directly by the adsorption of CO? (273 K)-and in many samples by the n-nonane preadsorption technique-whereas the total volume of micropores (including narrow and wide micropores) is provided by the use of the a, plot of the NZ (77 K) results. In this way, the difference will yield the volume of wide micropores, V, correspondCAR 27:1-C

31

Table 2. Volume (cm’ g’) for the different micropore filling processes SamdIe

V,!

VP

VT

D-S D-19 D-34 D-52 D-70 D-80

0.25 0.31 0.39 0.53 0.64 0.76

0.26 0.30 0.36 0.41 0.48 0.51

0 0.01 0.03 0. I2 0.16 0.25

c-9 C-16 C-29 C-38 C-53 C-65 C-82

0.27 0.33 0.40 0.49 0.58 0.76 1.08

0.25 0.30 0.39 0.43 0.51 0.56 0.58

0.02 0.03 0.01 0.06 0.07 0.20 0.50

ing to the secondary pore filling process. As shown in Table 2, the volume of supermicropores is almost nil for carbons with low burnoff but is increasingly more important as activation proceeds, especially for carbons with burnoff above 70%. A similar type of argument has been used by us to develop the socalled carbon dioxide substraction method[U] to evalute the nonmicroporous surface area of activated carbons. 4. CONCLUSIONS

The results presented here and their analysis indicate that there is no single adsorptive or theoretical approach that provides complete information on the micropore structure of activated carbons. The results described here for activated carbons exhibiting moreor-less distorted type I isotherm may be summarised as follows. 1. When the microporosity is narrow and uniform-but accessible to the adsorptives-all experimental and theoretical approaches lead to micropore volumes which are in very good agreement. 2. For highly activated carbons-with a wide micropore size distribution-the adsorption of hydrocarbons is useful in providing the total micropore volume (including primary and secondary filling processes) and the results are comparable with those deduced from the Langmuir and CX, plots constructed from the Nz (77 K) adsorption isotherms. In this type of highly activated carbons, the D-R equation generally underestimates the micropore volume for Nz at 77 K, although if a 0.05 to 0.3 relative pressure range straight portion can be drawn in the D-R plot the micropore volume is in fairly good agreement with the (Y,method. On the other hand, the adsorption of CO1 (273 K) and n-nonane preadsorption (although with the limitations indicated above) are restricted to narrow micropores. 3. All this means that for a simple evaluation of activated carbons one would use CO2 (273 K) to determine the volume of narrow micropores (pri-

E RODR~UEZ-REINOSO et al.

32

mary filling process) and N2 (77 K)-applying the a, method with our recently proposed isotherm as standard-to calculate the total micrpore volume. In this way the volume of wider micropores (supermicropores), which are filled by the secondary filling process, can be easily evaluated. Acknowledgments-This

C.A.I.C.Y.T.

work was supported (project No. 996/84).

by the

REFERENCES 1. IUPAC Manual of Symbols and Terminology, Appendix 2, Part 1, Colloid and Surface Chemistry, Pure Appl. Chem. 52, 2201 (1982).

2. P. J. M. Carrott, R. A. Roberts, and K. S. W. Sing, Carbon 25, 59 (1987).

3. M. M. Dubinin, In Characterization of Porous Solids (Edited by S. J. Gregg, K. S. W. Sing, and H. F. Stoeckli), pp. l-11. Society of Chemical Industry, London (1979). 4. M. M. Dubinin and L. V. Radushkevich, Zhur. fiz. &him. 23,469

(1949).

5. M. M. Dubinin and V. A. Astakhov, Adv. Chem. Ser. 102, 69 (1971). 6. H. F. Stoeckli, J. Ph. Houriet, A. Perret, and J. Huber, In Characterization of Porous Solids (Edited by S. J. Gregg, K. S. W. Sing, and H. F. Stoeckli), pp. 31-39. Society of Chemical Industry, London (1979). 7. M. M. Dubinin and H. F. Stoeckli, J. Colloid Interface Sci. 75, 34 (1980).

8. S. J. Gregg and K. S. W. Sing, Adsorption, Surface Area and Porosity, 2nd ed. Academic Press, London (1982). 9. B. McEnaney and K. J. Masters, Thermochimica Acta 82, 81 (1984).

10. J. Koresh, J. Colloid Interface Sci. 88, 398 (1982).

11. S. J. Gregg and J. F. Langford, Trans. Faraday Sot. 65, 1394 (1969). 12. E Rodriguez-Reinoso, J. M. Martin-Martinez, M. Molina-Sabio, R. Torregrosa, and J. Garrido-Segovia, J. Colloid Interface Sci. 106, 315 (1985). 13. A. Linares-Solano, J. D. L6pez-Gonzfilez, J. M. Martin-Martinez, and F. Rodriguez-Reinoso, Adsorption Sci. Technol. 1, 123 (1984). 14. F. Rodriguez-Reinoso; J. M: Martin-Martinez, C. PradoBurguete, and B. McEnaney, J. Phys. Chem. 91,515 (1987). 15. j. M: Martin-Martinez, M. Molina-Sabio, E Rodriguez-Reinoso, and R. Torregrosa, Fuel, in press. 16. J. Garrido, J. M. Martin-Martinez, M. Molina-Sabio, F. Rodriguez-Reinoso, and R. Torregrosa, Carbon 24, 469 (1986). 17. J. G&rid& A. Linares-Solano,

J. M. Martin-Martinez, M. Molina-Sabio. F.. Rodrfeuez-Reinoso. and R. Torregrosa, J. Chem. $0;. Fara&y Trans. 1,83,1081(1987). 18. J. Garrido, A. Linares-Solano, J. M. Martin-Martinez, M. Molina-Sabio, F. Rodrfguez-Reinoso, and R. Torregrosa, Langmuir 3, 76 (1987). 19. F. Rodriguez-Reinoso, A. Linares-Solano, M. MolinaSabio, and J. D. L6pez-Gonz&lez, Adsorption Sci. Technol. 1, 211 (1984). 20. F. Rodriguez-Reinoso, J. D. L6pez-GonzBlez, and C. Berenguer-Melero, Carbon 20,513 (1982); Carbon 22, 13 (1984).

21. F. Rodriguez-Reinoso, In Carbon and Coal Gasification (Edited by J. L. Figueiredo and J. A. Moulijn), pp. 601-644. Martinus Nijhoff, Dordrecht, Netherland (1986). 22. F. Rodriguez-Reinoso, J. M. Martin-Martinez, M. Molina-Sabio, I. PCrez-Lled6, and C. Prado-Burguete, Carbon 23, 19 (1985).

23. C. Salinas-Martinez Rodriguez-Reinoso,

de Lecea, A. Linares-Solano, F. and A. Sepfilveda-Escribano, In Characterization of Porous Solids (Edited by K. K. Unger, J. Rouquerol, K. S. W. Sing and H. Kral), pp. 173-182, Elsevier, Amsterdam (1988).