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The use of physisorption for the characterization of microporous carbons
THE USE OF PHYSISORPTION FOR THE CHARACTERIZATION OF MICROPOROUS CARBONS Department
KENNETH S. W. SING of Chemistry, Brunei University, Uxbridge, Middlesex. UB8 3PH. England
Abstract-Most active carbons are highly microporous (i.e.. their large internal surface is located within pores of width no greater than 2 nm.) Gas adsorption (physisorption) techniques are widely used for the characterization of microporous carbons, but the interpretation of the data is not entirely straightforward. Because of the proximity of the pore walls, the rota1 micropore volume cannot be readily evaluated and it is recommended that it should be expressed in the form of the effective pore volume available to a particular adsorptive. Micropore filling appears to occur in two stages: (1) a primary process in pores of molecular dimensions and (2) a secondary, or cooperative, process in somewhat wider pores. No single adsorptive can be expected to provide a reliable assessment of the micropore size distribution; although nitrogen (at 77 K) is useful for routine measurements, it is recommended that a number of larger probe molecules should be used to characterize the micropore structure. Key Words-Porous
carbon. micropore size, gas adsorption. physisorption.
1. INTRODUCTION f-iow can we most effectively utilize physisorption measurements for the characterization of porous carbons? What principles should govern the choice of adsorptives and experimentai techniques? What are likely to be our future needs? These and other related questions are becoming increasingly important in view of the growing interest in porous carbons as industrial adsorbents and catalyst supports and also because of the rapid developments now taking place in the design and marketing of commercial equipment for adsorption measurements. A characteristic feature of most activated carbons is their large internal surface located within very narrow pores. Part of this internal surface-indeed, often the major part-is confined within pores of width no larger than 2 nm and the pore size usually extends down to molecular dimensions. These adsorbents are thus highly microporous. Any mesoporosity (i.e., pores in the range 2-50 nm) is likely to be rather ill defined (in contrast to many oxide adsorbents). while the extent of the macroporosity (i.e., pores > 50 nm) appears to be mainly due to the nature of the precursor. The adsorbent activity is dependent on the magnitude of the internal surface, the distribution of pore size and shape, and the surface chemistry of the carbon. In recent years, much of the considerable amount of academic interest in porous carbons has been focused on their microporous nature and on the related physisorption mechanisms. There is now little doubt that pores of molecular dimensions (e.g., slit-shaped pores of width < -2 molecular diameters) are filled preferentially at very low pip” and that this process of “micropore filling” is associated with enhanced adsorbent-adsorbate interactions[l]. We must be more cautious about the interpretation of physisorption in the somewhat wider pores (of up to -5 molecular diameters), but it seems likely that this involves a secondary micropore filling process. which
is associated with adsorbate-adsorbate and entropic effectsf2]. As the theory of micropore filling is still at a rudimentary stage of development, it has been necessarv to employ various empiricalf 1.31 or semiempirical[4,.5] procedures to anatyze adsorption isotherm data and to evaluate the micropore capacity (i.e., the amount of an adsorptive required to fill all the micropores in a given adsorbent). To convert this quantity into the micropore volume, it is generally assumed that the pores have been filled by the condensed adsorptive having the same density as the bulk liquid. However, it can readily be shown[6] that the amount of gas required to fill a very narrow pore cannot
provide
a true measure
of the micropore
voi-
ume unless allowance is made for the proximity of the walls in affecting the degree of molecular packing. To overcome this uncertainty, it is therefore recommended that the term effective pore volume should be adopted and, if necessary, linked with the uptake of a particular adsorptive. Various attempts have been made to employ physisorption measurements to determine the micropore size distribution[7-91. For this purpose, it is generally assumed that the location and shape of the adsorption isotherm (sometimes expressed in the form of a “characteristic curve”) is directly related to the pore size distribution[lO]. but the intrinsic adsorbent-adsorbate heterogeneity is seldom taken into account. We are forced to conclude that, at present, there is no reliable procedure available for the computation of the micropore size distribution from a single isotherm.
2. CHOICE
OF ADSORPTIVE
2. I Nitrogen In light of the foregoing comments, it might appear that nitrogen does not have any special role to play in the characterization of porous carbons. This
6
K. .% W. SING
conclusion would be contrary to the recent recommendations of the IUPAC[ll], and therefore it is imperative that we review the arguments for and against the use of nitrogen as a standard adsorptive. First, we recall that the BET (Brunauer-EmmettTeller) nitrogen adsorption method[l2] is now universally accepted[l] as a standard procedure for the characterization of a variety of solid materials (industrial adsorbents, catalysts, pigments, clays, building materials, etc.). It is, of course, recognized that the BET area should not be regarded as the true surface area of microporous solid[ 1,111. However, the BET area is accepted as a useful indication of adsorbent activity and as such is featured widely in the scientific and technical literature. Nitrogen is also the preferred adsorptive for the determination of mesopore size distribution[ 11. The general applicability of this method is partly dependent on the fact that the isotherm path followed by the nitrogen multilayer on a nonporous solid is remarkably insensitive to any change in the chemical nature of the surface[l3]. In this respect, nitrogen is perhaps unique! This “universal” character[l4] of the nitrogen multilayer can be demonstrated by plotting the isotherm in a reduced form (as in Fig. 1) or constructing Frenkel-Halsey-Hill (FHH) by plots[l3]. Thus, the FHH index has been found [13,15] to have the nearly constant value r = 2.70 ? 0.03 for nonporous samples of carbon (graphitized and ungraphitized), silica (hydroxylated and dehydroxylated), alumina, chromia, titania (rutile and anatase), and zirconia, over the range A,,, - 10 to 200 m’ g-l. The standard isotherm in Fig. 1 has been used to construct nitrogen a, plots for a wide range of porous carbons[l6]. In this empirical method[l7], the
amount of gas adsorbed by the porous carbon is plotted against a,, the reduced standard adsorption (placing cl, = 1 at p/p” = 0.4); deviations from linearity are interpreted in terms of the different stages of pore filling and multilayer adsorption on the “external” (i.e., nonmicroporous) surface. The approach has been found to be especially useful in exploring the changes in microporosity and mesoporosity that result through alteration of the conditions of pretreatment and activation of active carbons such as charcoal cloth[l8]. The main disadvantage of nitrogen is that it is somewhat atypical in its molecular size and shape and hence in its micropore filling behavior. Thus, nitrogen has been found[19,20] to give consistently higher values of the effective micropore volume than are obtained by the use of other adsorptives (see, for example, the results in Figs. 2, 3, and 4). There are two possible explanations for these differences: either (1) the narrow width (-0.30 nm) of the nitrogen molecule allows it to squeeze into slit-shaped pores which are inaccessible to other more spheroidal molecules or (2) the packing of the nitrogen molecules in narrow pores does not conform to that in the liquid state. It is likely that both factors contribute to these differences and it follows that we should be especially cautious in making use of nitrogen data for the assessment of micropore volume. 2.2 Hydrocarbons The use of selected hydrocarbon vapors for the study of microporosity is illustrated by the results in Figs. 2, 3, and 4: these isotherms were determined[20] on three typical microporous carbons having different ranges of pore size. The amount of vapor adsorbed is expressed in each case as the vol-
0 Sooty silica q
Elftex120
0 Vulcan 3
1
0.2
0.4
p,po 0.6
0.8
I.0
Fig. 1. Reduced nitrogen isotherms on nonporous carbons and carbon-coated
silica.
Characterization
of microporous carbons
----7 ___
<
$g-G_X1 _; -_
0000
____ 0 omNITROGEN
.’
-PROPANE
/ -7
/-/
o.ISOBUTANE MNEOPENTANE
I
0.2
I
1
0.4
0.6
I
0.8
I
1
0.5
P/P0 Fig. 2. Adsorption
I
1.0
1
I.5
I
20
%
isotherms and corresponding a, plots for Carbosieve. Closed points--desorption.
ume of liquid adsorptive, and the effective micropore volume is obtained by back extrapolation of the corresponding CI,plot. Charcoal cloth JF518 has a broad distribution of micropores and the fact that the hydrocarbon isotherms are almost identical indicates that there is very little difference in the de-
Open points-adsorption.
gree of accessibility with respect to molecular diameters in the range 0.43 to 0.62 nm[21]. On the other hand, Carbosieve has a narrow distribution of pores of molecular dimensions: the extensive lowpressure hysteresis obtained with isobutane and neopentane is due to the slow penetration of these bulky
CI~ Nitrogen w Propane o* Isobutane AANeopentane
0.2
0.6
04
P/P0
Fig. 3. Adsorption
0,8
1.0
05
I.0
1-5
2-o
"c,
isotherms and corresponding a, plots for charcoal cloth JF 144. Open points--adsorption. Closed points-desorption.
K. S. W. SING
o@Isobotane
0.1
0.6
I.0
0.5
P/ PO
I.0
13
2.0
dS
Fig. 4. Adsorption isotherms and corresponding 4 plots for charcoal cloth JF 518. Open pointsadsorption.
Closed points-desorption.
molecules and is consistent with the molecular sieve character of the adsorbent. By applying the a, method and taking account of the two stages of micropore filling, we are able to arrive at a semiquantitative assessment of the micropore size distribution[20]. On the basis of the isotherm data in Figs. 2, 3, and 4, a tentative estimate of the ranges of effective pore size is given in Table 1. 2.3 Carbon dioxide Some carbonaceous adsorbents have been found to give a surprisingly low uptake of nitrogen at 77 K. This is due to an “activated entry” effect[22], which is associated with the slow diffusion of the adsorbate molecules through very narrow pore entrances or constrictions. In such cases, it has been established[U] that the uptake of carbon dioxide (at 195 or 273 K) is likely to be much larger, although the minimum dimensions of the two molecules are not very different (for CO?, 0.28 nm; for N2, 0.30 nm). The most important factor in causing the greater uptake of CO2 is the higher temperature of measurement, thus allowing the molecules to over-
come the energy barrier at the pore entrance. As has been pointed out recently by Rodriguez-Reinoso and his co-workers[24], the use of both N, and CO, can provide useful information concerning the micropore size distribution and the degree of access to the pore structure. It must be kept in mind, however, that a complicating factor in the case of CO, is its high quandrupole moment, which means that its isotherm is very sensitive to the presence of ions or polar groups on the surface. 2.4 Water vapor A distinctive property of any activated carbon is its low affinity for water vapor[l]. This “hydrophobic character” is obviously of great importance if the adsorbent is to be used for water treatment or the removal of noxious gases in the presence of moisture. However, although the overall adsorbent-adsorbate interaction is relatively weak, there are always some sites on the carbon surface that can interact more strongly (i.e., specifically) with water molecules. Adsorbate-adsorbate interactions (H bonding) also come into play as the vapor pressure is increased: clustering of HZ0 molecules takes place
Table 1. Effective micropore volumes and size range of typical microporous carbons
Carbon
BET-N, area (m’ g-l)
Effective micropore volume (C,H,) (cm3g-l)
Micropore range and distribution (nm)
Carbosieve
1179
0.36
JF 144 JF 518
1236 1793
0.51 0.90
0.3-0.7 Narrow (ca. 45% < 0.6 nm) 0.3-2 Fairly narrow 0.3-3 Broad
Characterization
of microporous carbons
instrumentation and data handling, one might be led to believe that this commercial equipment could provide a comprehensive and reliable means of characterizing adsorbents of all types. However, rapid developments now taking place in the application of new porous carbons for such purposes as water purification, gas separation, and respiratory protection have made it necessary to design new techniques (e.g., high-pressure measurements) for exploratory research and the acquisition of chemical engineering data. Although gravimetric methods are already used for adsorption isotherm studies (e.g., water vapor and organic vapor measurements), there is still considerable scope for the application of automated gravimetric equipment for adsorptionidesorption rate measurements. The rate of desorption is obviously of great importance for the recovery of adsorbed material. It is well known that physisorbed molecules are more difficult to remove from microporous materials than from adsorbents containing wider pores, but very few comparative studies have been made with well-characterized microporous carbons. The results of some preliminary measurements on nonane desorption, shown in Fig. 5. were obtained(281 with Carbosieve and charcoal cloth JF
on the most active regions of the surface and micropore filling begins to occur[25,26]. It is at this stage that competition between the noxious vapor and water becomes a problem. If the carbon is to retain its adsorbent activity, it is thus essential that the steep, micropore filling, part of the water isotherm should be delayed until a high p/p” is attained (at say, in excess of 80% RH). It is already known[27] that the exact shape of the water isotherm is highly dependent on the surface properties of carbon, but more work is required to elucidate the different effects produced by changes in surface treatment and pore structure.
3. OPERATIONAL
PROCEDURES
Over the past few years, great advances have been made in the development of commercial equipment for gas adsorption measurements. This type of fully automated equipment is inevitably expensive and is primarily designed for the routine determination of the surface area and pore size distribution of industrial powders and porous solids. Emphasis is placed on the rate of sample throughout, ease of operation, and the user-friendly nature of the software! In view of the degree of sophistication now available in the
CARBOSIEVECSI
I
l---Scale
I
Change
9
4
I
JF518
Fig. 5. Rates of adsorption and desorption of nonane vapor. (A) Carbon exposed to nonane vapor at 25°C; (B) equilibrium uptake of nonane; (C) start desorption-rotary pumping; (D) diffusion pumping; (E) start heating (2” min) to 250°C; (F) desorption complete.
10
K. S. W. SING
Table 2. Values of rate constant. k, and activation energy, ED, for desorption of neopentane[30]
Carbon A B C
Temperature (“C)
k
ED
(min-’ ?)
(kJ mol-‘)
20 60 20 60 20 60
0.131 0.245 0.134 0.313 0.279 0.511
17.7
in the case of JF 518 the corresponding amount is -47%. There seems little doubt that the strong retention of nonane by Carbosieve is associated with the high heat of primary micropore filling[2,29], whereas with JF 518 the more weakly held nonane is removed at 25°C from the wider micropores. These results are thus consistent with the concept of two stages of micropore filling. In another study[30] of the desorption kinetics of neopentane, it has been possible to derive the values of the activation energy, ED, given in Table 2 for three representative samples of charcoal cloth (A, B, and C), which had similar pore structures to Carbosieve, JF 144, and JF 518, respectively. The values of rate constant, k, were obtained by analysis of the desorption kinetic data as determined over the low pressure range of -5 X lo-’ to -1.5 X 10-j torr.
15.4 11.9
518. Both carbons exhibit a similar fast rate of adbut as expected from the isotherms in Figs. 2 and 4, their desorption behavior is quite different. It is evident that -93% of the adsorbed nonane is retained on outgassing the Carbosieve at 2X, but
sorption,
Open symbols = adsorption Closed symbols
= desorption
(a)
0.4
P/P0
O6
0.8
I.0
(b)
Fig. 6. Nitrogen isotherms on two microporous carbons: (a) adsorption per unit volume. (b) adsorption per unit mass.
Characterization of microporous carbons As expected, the highest value of ED was obtained with sample A and the lowest value with sample C-again confirming the importance of micropore size distribution in controlling the desorption kinetics. A vast amount of academic research has been undertaken on adsorption by porous carbons, but unfortunately it is impossible to check the reliability of much of the published data. Many measurements have been made on ill-defined materials and often no indication has been given of the reproducibility and accuracy of the data. In an effort to improve the situation. the IUPAC has issued[ 1l] a set of recommendations on reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity. It is to be hoped that this guidance will lead also to greater conformity in the procedures used for measuring and reporting adsorption isotherm data. In the graphical presentation of an adsorption isotherm the usual. and generally recommended, practice is to plot the amount adsorbed by unit mass of the outgassed adsorbent against pip”. From the technological standpoint, however. a more important consideration may be the amount adsorbed per unit volume of the adsorbent. The effect of making this change is illustrated in Fig. 6: it is evident that for some purposes full advantage cannot be taken of the very high adsorption capacity of the carbon AX21[16]. Thus, the low bulk density of many porous carbons imposes a severe limitation in their potential applications (e.g., for gas storage or respiratory protection).
REFERENCES
1. S. J. Gregg Area
and K. S. W. Sing,
England (1985). 3. M. M. Dubinin. In
Progress
in Surface
Surface
London J. Rouof Ad-
X0-98. of Pore
Haynes
Bristol.
and Membrane
(Edited by D. A. Cadenhead). Vol. 9. pp. l-70. Academic Press. New York (1975). H. F. Stoeckli, J. Ph. Houriet. A. Perret. and U. Huber. In Characrerisarion of Porous Solids (Ecdited by S. J. Gregg. K. S. W. Sing, and H. F. Stoeckli), pp. 31-39. Society of Chemical Industry. London (1979). P. J. M. Carrott and K. S. W. Sing, In IUP.4C_Sfm posium on Charactertation of Porous Solids (Ldlted by K. K. Unger. J. Rouquerol, K. S. W. Sing and H. Kral). pp. 77-X7. Elsevier (198X). R. Sh.’ Mikhail. S. Brunauer. and E. E. Boder. J. Colloid Interface Sci. 26. 45 ( IYhX). 0. Kadlec. in Characterisation of Porous Solids (Edited by S. J. Gregg. K. S. W. Sing. and H. F. Stoeckli). pp. 13-20. Societv of Chemical Industry. London (1979). S. Sircar. Carbon 25, 39 (19X7). H. Marsh. Curbon 25. 49 (1987). K. S. W. Sing. D. H. Everett. R. A. W. Haul. L. Moscou. R. A. Pierotti. J. Rouquerol, and T. Siemieniewska. Pure Appl. Chem. 57. 603 (1985). S. Brunauer. P. H. Emmett, and E. Teller. J. .4mer. Chem. Sot. 60. 309 (1938). P. J. M. Carrott. A.’ I. icLeod, and K. S. W. Sing, In Adsorprion ar rhe Gas-Solid and Liquid-Solid Interface (Edited by J. Rouquerol and K. S. W. Sing). pp. 403-410. Elsevier, Amsterdam (1982). J. H. de Boer. B. G. Linsen. and Th. J. Osinga. J.
Science
5.
6
I x
9 10 11 12 13
RECOMMENDATIONS
It is now evident that the pore size of certain active carbons can be controlled to within much closer limits than was formerly thought possible. Further efforts should be made to prepare and characterise carbons having pores of uniform size and shape. Additional physisorption measurements are also required on a range of well-defined nonporous carbons (both graphitized and ungraphitized). It is to be hoped that in future only those physisorption studies conducted on well-characterized carbons will be accepted for publication in the scientific literature. No single adsorptive can be expected to provide a reliable assessment of the micropore size distribution. It is suggested that nitrogen (at 77 K) be used for routine adsorption studies of porous carbons but that the limitations of the method are kept in mind. Whenever possible, larger probe molecules should also be employed and the measurements made over a wide range of temperature and pressure. In this manner, it should be possible to throw further light on the mechanisms of pore filling. It is recommended that more attention should be given to the kinetics of adsorption and desorption and to fluid transport through carbon membranes and molecular sieves.
Press,
(1982). 2. D. Atkinson. P. J. M. Carrott. Y. Grillet. querol. and K. S. W. Sing. In Fundamentals soration (Edited bv A. I. Liauis). DD. En&neerin~ Foundatibn. New Yori (iSX?j. 3. K. S. W. Sing. In Principles and Applications Structural Chumcterization (Edited by J. M. and P. Rossi-Doria). pp. 1-l 1. Arrowsmith.
4. 643 (1965).
P. J. M. Carrott. AND
Adsorption.
and Porosir_v, 2nd ed. Academic
Catalysis 4. CONCLUSIONS
11
Langmuir
R. A. Roberts.
and K. S. W. Sing.
4. 740 (1988).
P. J. M. Carrott.
R. A. Roberts.
and K. S. W. Sing,
Carbon
25. 59 (1987). K. S. W. Sing. In Surface
18 19
Area Dererminarion (Edited by D. H. Everett and R. H. Ottewill). pp. 1!5-34. Butterworths. London (1970). J. J. Freeman, F. G. R. Gimblett. R. A. Roberts. and K. S. W. Sing, Carbon 25, 559 (1987). A. R. Roberts, K. S. W. Sing. and V. Tripathy. Langmuir,
3, 331 (1987).
20 P. J. M. Carrott.
R. A. Roberts
and K. S. W. Sing. In
IUPA C Symposium on Characterization of Porous Solid.5 (Edited by K. K. Unger. J. Rouquerol, K. S. W.
21 22 23 24 25 26 27
Sing and H. Kral). pp. X9-100. Elsevier. Amsterdam (1988). D. W. Breck. Zeolire Molecular Sieves. p. 637. ‘Wiley. New York (1973). F. A. P. Maggs. Research 6. S13 (1953). H. Marsh and W. F. K. Wynne-Jones. Carbon 1. 281 (1963). J. Garrido. A. Linares-Solano, J. M. Martin-Martinez. M. Molina-Sabio. F. Rodriguez-Reinoso and R.. Torregrosa, Langmuir 3. 76 (1987). M. M. Duhinin. Carbon 18. 355 (1980) H. F. Stoeckli. F. Kraehenbuehl, and D. Morel. Carbon 21. 589 (1983). P. I_. Walker and J. Janov. J. Colloid Interface Sci 28. 499 (196X).
28 R. A. Roberts. Ph.D. thesis, Brunel University 29 P. J. M. Carrott and K. S. W. Sing. Chem &
(1988). Ind.
360
(19X6).
30 K. S. W. Sing and V. Tripathi.
Chem & Ind. 210 (1986).
KENNETH S. W. SING of Chemistry, Brunei University, Uxbridge, Middlesex. UB8 3PH. England
Abstract-Most active carbons are highly microporous (i.e.. their large internal surface is located within pores of width no greater than 2 nm.) Gas adsorption (physisorption) techniques are widely used for the characterization of microporous carbons, but the interpretation of the data is not entirely straightforward. Because of the proximity of the pore walls, the rota1 micropore volume cannot be readily evaluated and it is recommended that it should be expressed in the form of the effective pore volume available to a particular adsorptive. Micropore filling appears to occur in two stages: (1) a primary process in pores of molecular dimensions and (2) a secondary, or cooperative, process in somewhat wider pores. No single adsorptive can be expected to provide a reliable assessment of the micropore size distribution; although nitrogen (at 77 K) is useful for routine measurements, it is recommended that a number of larger probe molecules should be used to characterize the micropore structure. Key Words-Porous
carbon. micropore size, gas adsorption. physisorption.
1. INTRODUCTION f-iow can we most effectively utilize physisorption measurements for the characterization of porous carbons? What principles should govern the choice of adsorptives and experimentai techniques? What are likely to be our future needs? These and other related questions are becoming increasingly important in view of the growing interest in porous carbons as industrial adsorbents and catalyst supports and also because of the rapid developments now taking place in the design and marketing of commercial equipment for adsorption measurements. A characteristic feature of most activated carbons is their large internal surface located within very narrow pores. Part of this internal surface-indeed, often the major part-is confined within pores of width no larger than 2 nm and the pore size usually extends down to molecular dimensions. These adsorbents are thus highly microporous. Any mesoporosity (i.e., pores in the range 2-50 nm) is likely to be rather ill defined (in contrast to many oxide adsorbents). while the extent of the macroporosity (i.e., pores > 50 nm) appears to be mainly due to the nature of the precursor. The adsorbent activity is dependent on the magnitude of the internal surface, the distribution of pore size and shape, and the surface chemistry of the carbon. In recent years, much of the considerable amount of academic interest in porous carbons has been focused on their microporous nature and on the related physisorption mechanisms. There is now little doubt that pores of molecular dimensions (e.g., slit-shaped pores of width < -2 molecular diameters) are filled preferentially at very low pip” and that this process of “micropore filling” is associated with enhanced adsorbent-adsorbate interactions[l]. We must be more cautious about the interpretation of physisorption in the somewhat wider pores (of up to -5 molecular diameters), but it seems likely that this involves a secondary micropore filling process. which
is associated with adsorbate-adsorbate and entropic effectsf2]. As the theory of micropore filling is still at a rudimentary stage of development, it has been necessarv to employ various empiricalf 1.31 or semiempirical[4,.5] procedures to anatyze adsorption isotherm data and to evaluate the micropore capacity (i.e., the amount of an adsorptive required to fill all the micropores in a given adsorbent). To convert this quantity into the micropore volume, it is generally assumed that the pores have been filled by the condensed adsorptive having the same density as the bulk liquid. However, it can readily be shown[6] that the amount of gas required to fill a very narrow pore cannot
provide
a true measure
of the micropore
voi-
ume unless allowance is made for the proximity of the walls in affecting the degree of molecular packing. To overcome this uncertainty, it is therefore recommended that the term effective pore volume should be adopted and, if necessary, linked with the uptake of a particular adsorptive. Various attempts have been made to employ physisorption measurements to determine the micropore size distribution[7-91. For this purpose, it is generally assumed that the location and shape of the adsorption isotherm (sometimes expressed in the form of a “characteristic curve”) is directly related to the pore size distribution[lO]. but the intrinsic adsorbent-adsorbate heterogeneity is seldom taken into account. We are forced to conclude that, at present, there is no reliable procedure available for the computation of the micropore size distribution from a single isotherm.
2. CHOICE
OF ADSORPTIVE
2. I Nitrogen In light of the foregoing comments, it might appear that nitrogen does not have any special role to play in the characterization of porous carbons. This
6
K. .% W. SING
conclusion would be contrary to the recent recommendations of the IUPAC[ll], and therefore it is imperative that we review the arguments for and against the use of nitrogen as a standard adsorptive. First, we recall that the BET (Brunauer-EmmettTeller) nitrogen adsorption method[l2] is now universally accepted[l] as a standard procedure for the characterization of a variety of solid materials (industrial adsorbents, catalysts, pigments, clays, building materials, etc.). It is, of course, recognized that the BET area should not be regarded as the true surface area of microporous solid[ 1,111. However, the BET area is accepted as a useful indication of adsorbent activity and as such is featured widely in the scientific and technical literature. Nitrogen is also the preferred adsorptive for the determination of mesopore size distribution[ 11. The general applicability of this method is partly dependent on the fact that the isotherm path followed by the nitrogen multilayer on a nonporous solid is remarkably insensitive to any change in the chemical nature of the surface[l3]. In this respect, nitrogen is perhaps unique! This “universal” character[l4] of the nitrogen multilayer can be demonstrated by plotting the isotherm in a reduced form (as in Fig. 1) or constructing Frenkel-Halsey-Hill (FHH) by plots[l3]. Thus, the FHH index has been found [13,15] to have the nearly constant value r = 2.70 ? 0.03 for nonporous samples of carbon (graphitized and ungraphitized), silica (hydroxylated and dehydroxylated), alumina, chromia, titania (rutile and anatase), and zirconia, over the range A,,, - 10 to 200 m’ g-l. The standard isotherm in Fig. 1 has been used to construct nitrogen a, plots for a wide range of porous carbons[l6]. In this empirical method[l7], the
amount of gas adsorbed by the porous carbon is plotted against a,, the reduced standard adsorption (placing cl, = 1 at p/p” = 0.4); deviations from linearity are interpreted in terms of the different stages of pore filling and multilayer adsorption on the “external” (i.e., nonmicroporous) surface. The approach has been found to be especially useful in exploring the changes in microporosity and mesoporosity that result through alteration of the conditions of pretreatment and activation of active carbons such as charcoal cloth[l8]. The main disadvantage of nitrogen is that it is somewhat atypical in its molecular size and shape and hence in its micropore filling behavior. Thus, nitrogen has been found[19,20] to give consistently higher values of the effective micropore volume than are obtained by the use of other adsorptives (see, for example, the results in Figs. 2, 3, and 4). There are two possible explanations for these differences: either (1) the narrow width (-0.30 nm) of the nitrogen molecule allows it to squeeze into slit-shaped pores which are inaccessible to other more spheroidal molecules or (2) the packing of the nitrogen molecules in narrow pores does not conform to that in the liquid state. It is likely that both factors contribute to these differences and it follows that we should be especially cautious in making use of nitrogen data for the assessment of micropore volume. 2.2 Hydrocarbons The use of selected hydrocarbon vapors for the study of microporosity is illustrated by the results in Figs. 2, 3, and 4: these isotherms were determined[20] on three typical microporous carbons having different ranges of pore size. The amount of vapor adsorbed is expressed in each case as the vol-
0 Sooty silica q
Elftex120
0 Vulcan 3
1
0.2
0.4
p,po 0.6
0.8
I.0
Fig. 1. Reduced nitrogen isotherms on nonporous carbons and carbon-coated
silica.
Characterization
of microporous carbons
----7 ___
<
$g-G_X1 _; -_
0000
____ 0 omNITROGEN
.’
-PROPANE
/ -7
/-/
o.ISOBUTANE MNEOPENTANE
I
0.2
I
1
0.4
0.6
I
0.8
I
1
0.5
P/P0 Fig. 2. Adsorption
I
1.0
1
I.5
I
20
%
isotherms and corresponding a, plots for Carbosieve. Closed points--desorption.
ume of liquid adsorptive, and the effective micropore volume is obtained by back extrapolation of the corresponding CI,plot. Charcoal cloth JF518 has a broad distribution of micropores and the fact that the hydrocarbon isotherms are almost identical indicates that there is very little difference in the de-
Open points-adsorption.
gree of accessibility with respect to molecular diameters in the range 0.43 to 0.62 nm[21]. On the other hand, Carbosieve has a narrow distribution of pores of molecular dimensions: the extensive lowpressure hysteresis obtained with isobutane and neopentane is due to the slow penetration of these bulky
CI~ Nitrogen w Propane o* Isobutane AANeopentane
0.2
0.6
04
P/P0
Fig. 3. Adsorption
0,8
1.0
05
I.0
1-5
2-o
"c,
isotherms and corresponding a, plots for charcoal cloth JF 144. Open points--adsorption. Closed points-desorption.
K. S. W. SING
o@Isobotane
0.1
0.6
I.0
0.5
P/ PO
I.0
13
2.0
dS
Fig. 4. Adsorption isotherms and corresponding 4 plots for charcoal cloth JF 518. Open pointsadsorption.
Closed points-desorption.
molecules and is consistent with the molecular sieve character of the adsorbent. By applying the a, method and taking account of the two stages of micropore filling, we are able to arrive at a semiquantitative assessment of the micropore size distribution[20]. On the basis of the isotherm data in Figs. 2, 3, and 4, a tentative estimate of the ranges of effective pore size is given in Table 1. 2.3 Carbon dioxide Some carbonaceous adsorbents have been found to give a surprisingly low uptake of nitrogen at 77 K. This is due to an “activated entry” effect[22], which is associated with the slow diffusion of the adsorbate molecules through very narrow pore entrances or constrictions. In such cases, it has been established[U] that the uptake of carbon dioxide (at 195 or 273 K) is likely to be much larger, although the minimum dimensions of the two molecules are not very different (for CO?, 0.28 nm; for N2, 0.30 nm). The most important factor in causing the greater uptake of CO2 is the higher temperature of measurement, thus allowing the molecules to over-
come the energy barrier at the pore entrance. As has been pointed out recently by Rodriguez-Reinoso and his co-workers[24], the use of both N, and CO, can provide useful information concerning the micropore size distribution and the degree of access to the pore structure. It must be kept in mind, however, that a complicating factor in the case of CO, is its high quandrupole moment, which means that its isotherm is very sensitive to the presence of ions or polar groups on the surface. 2.4 Water vapor A distinctive property of any activated carbon is its low affinity for water vapor[l]. This “hydrophobic character” is obviously of great importance if the adsorbent is to be used for water treatment or the removal of noxious gases in the presence of moisture. However, although the overall adsorbent-adsorbate interaction is relatively weak, there are always some sites on the carbon surface that can interact more strongly (i.e., specifically) with water molecules. Adsorbate-adsorbate interactions (H bonding) also come into play as the vapor pressure is increased: clustering of HZ0 molecules takes place
Table 1. Effective micropore volumes and size range of typical microporous carbons
Carbon
BET-N, area (m’ g-l)
Effective micropore volume (C,H,) (cm3g-l)
Micropore range and distribution (nm)
Carbosieve
1179
0.36
JF 144 JF 518
1236 1793
0.51 0.90
0.3-0.7 Narrow (ca. 45% < 0.6 nm) 0.3-2 Fairly narrow 0.3-3 Broad
Characterization
of microporous carbons
instrumentation and data handling, one might be led to believe that this commercial equipment could provide a comprehensive and reliable means of characterizing adsorbents of all types. However, rapid developments now taking place in the application of new porous carbons for such purposes as water purification, gas separation, and respiratory protection have made it necessary to design new techniques (e.g., high-pressure measurements) for exploratory research and the acquisition of chemical engineering data. Although gravimetric methods are already used for adsorption isotherm studies (e.g., water vapor and organic vapor measurements), there is still considerable scope for the application of automated gravimetric equipment for adsorptionidesorption rate measurements. The rate of desorption is obviously of great importance for the recovery of adsorbed material. It is well known that physisorbed molecules are more difficult to remove from microporous materials than from adsorbents containing wider pores, but very few comparative studies have been made with well-characterized microporous carbons. The results of some preliminary measurements on nonane desorption, shown in Fig. 5. were obtained(281 with Carbosieve and charcoal cloth JF
on the most active regions of the surface and micropore filling begins to occur[25,26]. It is at this stage that competition between the noxious vapor and water becomes a problem. If the carbon is to retain its adsorbent activity, it is thus essential that the steep, micropore filling, part of the water isotherm should be delayed until a high p/p” is attained (at say, in excess of 80% RH). It is already known[27] that the exact shape of the water isotherm is highly dependent on the surface properties of carbon, but more work is required to elucidate the different effects produced by changes in surface treatment and pore structure.
3. OPERATIONAL
PROCEDURES
Over the past few years, great advances have been made in the development of commercial equipment for gas adsorption measurements. This type of fully automated equipment is inevitably expensive and is primarily designed for the routine determination of the surface area and pore size distribution of industrial powders and porous solids. Emphasis is placed on the rate of sample throughout, ease of operation, and the user-friendly nature of the software! In view of the degree of sophistication now available in the
CARBOSIEVECSI
I
l---Scale
I
Change
9
4
I
JF518
Fig. 5. Rates of adsorption and desorption of nonane vapor. (A) Carbon exposed to nonane vapor at 25°C; (B) equilibrium uptake of nonane; (C) start desorption-rotary pumping; (D) diffusion pumping; (E) start heating (2” min) to 250°C; (F) desorption complete.
10
K. S. W. SING
Table 2. Values of rate constant. k, and activation energy, ED, for desorption of neopentane[30]
Carbon A B C
Temperature (“C)
k
ED
(min-’ ?)
(kJ mol-‘)
20 60 20 60 20 60
0.131 0.245 0.134 0.313 0.279 0.511
17.7
in the case of JF 518 the corresponding amount is -47%. There seems little doubt that the strong retention of nonane by Carbosieve is associated with the high heat of primary micropore filling[2,29], whereas with JF 518 the more weakly held nonane is removed at 25°C from the wider micropores. These results are thus consistent with the concept of two stages of micropore filling. In another study[30] of the desorption kinetics of neopentane, it has been possible to derive the values of the activation energy, ED, given in Table 2 for three representative samples of charcoal cloth (A, B, and C), which had similar pore structures to Carbosieve, JF 144, and JF 518, respectively. The values of rate constant, k, were obtained by analysis of the desorption kinetic data as determined over the low pressure range of -5 X lo-’ to -1.5 X 10-j torr.
15.4 11.9
518. Both carbons exhibit a similar fast rate of adbut as expected from the isotherms in Figs. 2 and 4, their desorption behavior is quite different. It is evident that -93% of the adsorbed nonane is retained on outgassing the Carbosieve at 2X, but
sorption,
Open symbols = adsorption Closed symbols
= desorption
(a)
0.4
P/P0
O6
0.8
I.0
(b)
Fig. 6. Nitrogen isotherms on two microporous carbons: (a) adsorption per unit volume. (b) adsorption per unit mass.
Characterization of microporous carbons As expected, the highest value of ED was obtained with sample A and the lowest value with sample C-again confirming the importance of micropore size distribution in controlling the desorption kinetics. A vast amount of academic research has been undertaken on adsorption by porous carbons, but unfortunately it is impossible to check the reliability of much of the published data. Many measurements have been made on ill-defined materials and often no indication has been given of the reproducibility and accuracy of the data. In an effort to improve the situation. the IUPAC has issued[ 1l] a set of recommendations on reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity. It is to be hoped that this guidance will lead also to greater conformity in the procedures used for measuring and reporting adsorption isotherm data. In the graphical presentation of an adsorption isotherm the usual. and generally recommended, practice is to plot the amount adsorbed by unit mass of the outgassed adsorbent against pip”. From the technological standpoint, however. a more important consideration may be the amount adsorbed per unit volume of the adsorbent. The effect of making this change is illustrated in Fig. 6: it is evident that for some purposes full advantage cannot be taken of the very high adsorption capacity of the carbon AX21[16]. Thus, the low bulk density of many porous carbons imposes a severe limitation in their potential applications (e.g., for gas storage or respiratory protection).
REFERENCES
1. S. J. Gregg Area
and K. S. W. Sing,
England (1985). 3. M. M. Dubinin. In
Progress
in Surface
Surface
London J. Rouof Ad-
X0-98. of Pore
Haynes
Bristol.
and Membrane
(Edited by D. A. Cadenhead). Vol. 9. pp. l-70. Academic Press. New York (1975). H. F. Stoeckli, J. Ph. Houriet. A. Perret. and U. Huber. In Characrerisarion of Porous Solids (Ecdited by S. J. Gregg. K. S. W. Sing, and H. F. Stoeckli), pp. 31-39. Society of Chemical Industry. London (1979). P. J. M. Carrott and K. S. W. Sing, In IUP.4C_Sfm posium on Charactertation of Porous Solids (Ldlted by K. K. Unger. J. Rouquerol, K. S. W. Sing and H. Kral). pp. 77-X7. Elsevier (198X). R. Sh.’ Mikhail. S. Brunauer. and E. E. Boder. J. Colloid Interface Sci. 26. 45 ( IYhX). 0. Kadlec. in Characterisation of Porous Solids (Edited by S. J. Gregg. K. S. W. Sing. and H. F. Stoeckli). pp. 13-20. Societv of Chemical Industry. London (1979). S. Sircar. Carbon 25, 39 (19X7). H. Marsh. Curbon 25. 49 (1987). K. S. W. Sing. D. H. Everett. R. A. W. Haul. L. Moscou. R. A. Pierotti. J. Rouquerol, and T. Siemieniewska. Pure Appl. Chem. 57. 603 (1985). S. Brunauer. P. H. Emmett, and E. Teller. J. .4mer. Chem. Sot. 60. 309 (1938). P. J. M. Carrott. A.’ I. icLeod, and K. S. W. Sing, In Adsorprion ar rhe Gas-Solid and Liquid-Solid Interface (Edited by J. Rouquerol and K. S. W. Sing). pp. 403-410. Elsevier, Amsterdam (1982). J. H. de Boer. B. G. Linsen. and Th. J. Osinga. J.
Science
5.
6
I x
9 10 11 12 13
RECOMMENDATIONS
It is now evident that the pore size of certain active carbons can be controlled to within much closer limits than was formerly thought possible. Further efforts should be made to prepare and characterise carbons having pores of uniform size and shape. Additional physisorption measurements are also required on a range of well-defined nonporous carbons (both graphitized and ungraphitized). It is to be hoped that in future only those physisorption studies conducted on well-characterized carbons will be accepted for publication in the scientific literature. No single adsorptive can be expected to provide a reliable assessment of the micropore size distribution. It is suggested that nitrogen (at 77 K) be used for routine adsorption studies of porous carbons but that the limitations of the method are kept in mind. Whenever possible, larger probe molecules should also be employed and the measurements made over a wide range of temperature and pressure. In this manner, it should be possible to throw further light on the mechanisms of pore filling. It is recommended that more attention should be given to the kinetics of adsorption and desorption and to fluid transport through carbon membranes and molecular sieves.
Press,
(1982). 2. D. Atkinson. P. J. M. Carrott. Y. Grillet. querol. and K. S. W. Sing. In Fundamentals soration (Edited bv A. I. Liauis). DD. En&neerin~ Foundatibn. New Yori (iSX?j. 3. K. S. W. Sing. In Principles and Applications Structural Chumcterization (Edited by J. M. and P. Rossi-Doria). pp. 1-l 1. Arrowsmith.
4. 643 (1965).
P. J. M. Carrott. AND
Adsorption.
and Porosir_v, 2nd ed. Academic
Catalysis 4. CONCLUSIONS
11
Langmuir
R. A. Roberts.
and K. S. W. Sing.
4. 740 (1988).
P. J. M. Carrott.
R. A. Roberts.
and K. S. W. Sing,
Carbon
25. 59 (1987). K. S. W. Sing. In Surface
18 19
Area Dererminarion (Edited by D. H. Everett and R. H. Ottewill). pp. 1!5-34. Butterworths. London (1970). J. J. Freeman, F. G. R. Gimblett. R. A. Roberts. and K. S. W. Sing, Carbon 25, 559 (1987). A. R. Roberts, K. S. W. Sing. and V. Tripathy. Langmuir,
3, 331 (1987).
20 P. J. M. Carrott.
R. A. Roberts
and K. S. W. Sing. In
IUPA C Symposium on Characterization of Porous Solid.5 (Edited by K. K. Unger. J. Rouquerol, K. S. W.
21 22 23 24 25 26 27
Sing and H. Kral). pp. X9-100. Elsevier. Amsterdam (1988). D. W. Breck. Zeolire Molecular Sieves. p. 637. ‘Wiley. New York (1973). F. A. P. Maggs. Research 6. S13 (1953). H. Marsh and W. F. K. Wynne-Jones. Carbon 1. 281 (1963). J. Garrido. A. Linares-Solano, J. M. Martin-Martinez. M. Molina-Sabio. F. Rodriguez-Reinoso and R.. Torregrosa, Langmuir 3. 76 (1987). M. M. Duhinin. Carbon 18. 355 (1980) H. F. Stoeckli. F. Kraehenbuehl, and D. Morel. Carbon 21. 589 (1983). P. I_. Walker and J. Janov. J. Colloid Interface Sci 28. 499 (196X).
28 R. A. Roberts. Ph.D. thesis, Brunel University 29 P. J. M. Carrott and K. S. W. Sing. Chem &
(1988). Ind.
360
(19X6).
30 K. S. W. Sing and V. Tripathi.
Chem & Ind. 210 (1986).