Langmuir 1991, 7, 109-115
109
Molecular Resolution Analysis of a-FeOOH-Dispersed
Activated Carbon Fibers K. Kaneko Department of Chemistry, Faculty of Science, Chiba University, Chiba 260, Japan Received June 28, 1989.I n Final Form: June 6, 1990 The irregularity of the micropore walls of a-FeOOH-dispersedactivated carbon fibers (a-FeOOH-ACF) was examined by the adsorption of planar, spherical, and zigzag molecules at 303 K and powder X-ray diffraction at room temperature. a-FeOOH-ACF gave more diffuse X-ray diffraction patterns with high background than the original ACF, indicating that the surface of a-FeOOH-ACF has a more amorphous structure than that of the original ACF. a-FeOOH-ACF can adsorb more planar molecules and fewer zigzag molecules than the original ACF. The relationshipsbetween the logarithm of the monolayercapacity from the BET plot and the logarithm of molecular area determined from both molecular weight and liquid density at 303 K were linear, indicative of fractal pore walls. The surface fractal dimension, D,, of the micropore wall was determined from the linear plots; the dispersion of ultrafine a-FeOOH increased the D Evalue. The relationships between the logarithm of the micropore volume from the DR plots and the logarithm of molecular volume were linear; the apparent exponents determined from the plots are greater than the D, values, suggesting slightly heterogeneous micropore size distribution.
Introduction Recently, the adsorption properties of various types of activated carbon fibers (ACFs) have been studied,'S2 revealing that ACF is superior to granular activated carbons (ACs) in adsorption capacity and rate. AC has a wide pore size ranging from micropores to macropores,3 while ACF has a narrower pore size distribution. Recently, ACF has been manufactured from cellulose-, polyacrylonitrile-, phenol resin-, and pitch-based starting materials. This type of ACF can be used as a good model system to study micropore fillinga4#5The interest here is micropore filling by not only vapors but also a super critical gas. Micropore filling by a super critical gas is not a predominant process, because the micropore filling is a limited case of physical adsorption which is effective for vapors.6 As the three-dimensional critical temperature of NO is 180 K, NO gas near room temperature is a supercritical gas. The NO gas fills the micropores of the ACF in the form of the dimer (NO)z near room temperature, as shown from magnetic susceptibility m e a s ~ r e m e n t . ~Modifying ?~ ACF with active a-FeOOH or a-Fe2Oa makes it possible to enhance the NO micropore filling.419 Thus the amount of NO adsorbed on a-Fe203-dispersed ACF reaches 320 mg/g a t 303 K, corresponding to 82% of the micropore volume. Some NO molecules are not directly chemisorbed on the iron oxides on the ACF surface but are sorbed in the micropores. Also, Cu(OH)2 dispersion enhances the micropore filling of NO,l0 and therefore it was proposed that the micropore filling mechanism for the enhancement of NO adsorptivity is chemisorption-assisted. Characterization of the dispersed substances on the ACF and the micropores of the ACF is necessary to determine the (1) Freeman, J. J.; Gimblett, F. R. G.; Sing, K. S. W. Carbon 1989,27,
85.
(2) Sing, K. S. W. Carbon 1989, 27, 27. (3) Juntgen, H. Carbon 1977, 15, 273. (4) Kaneko, K. Langmuir 1987,3, 357. (5) Kaneko, K.; Nakahigashi, Y.; Nagata, K. Carbon 1988,26, 327. (6) Dubinin, M. M. Carbon 1983, 21, 359. (7) Kaneko, K.; Fukuzaki, N.; Ozeki, S. J. Chem. Phys. 1987,87,776.
(8)Kaneko, K.; Fukuzaki, N.; Suzuki, T.; Kakei, K.; Ozeki, S. Langmuir 1989, 5, 960. (9) Kaneko, K. In Characterization of Porous Solids; Unger, K. K. et al., Eds.; Elsevier: Amsterdam, 1988; p 183. (10) Kaneko, K.; Ozeki, S.; Inouye, K. Colloid Polym. Sci. 1987,265, 1018.
mechanism of the chemisorption-assisted micropore filling process. In previous investigations, X-ray photoelectron spectroscopy" and extended X-ray absorption near edge structures9J1J2 were used for the characterization of the dispersed substances. Avnir et al. have applied fractal geometry developed by Mandelbrot to chemical processes on solid surfaces. Their molecular fractal analysis is a useful tool for examining quantitatively the surface irregularity of solids.l*16 The relationship between the amount of adsorption and the size of adsorptive molecules gives the surface fractal dimension D,. The D Efor high surface materials is in the range 2-3; a D, near 3 implies that the adsorbate molecules are of a scale similar to the roughness of the surface. Furthermore, the adsorption on a fractal surface has been studied;17J8the D Eaffects the isotherm for multilayer adsorption as suggested by Fripiat et al.18 The pore size distribution from 0.1 nm to micrometers for porous materials is related to the DE.14.21 However, the surface fractal analysis cannot be directly applied to a microporous system which exhibits a molecular sieving effect. Nevertheless, even with microporous systems, molecular adsorption with a series of molecules of various sizes can provide valuable information on the structure and size of micropores and the micropore wall, whether the surface is fractal or not. In this work, we measured adsorption isotherms of organic molecules which have different molecular structures and sizes on a-Fe00H-dispersedACF and applied the surface fractal analysis to discuss the dispersed state of iron oxides and the micropore structures. (11) Kaneko, K.; Ohta, T.; Ozeki, S.; Kosugi, N.; Kuroda, H. Appl. Surf. Sci. 1988, 33/34, 355. (12) Kaneko, K.;Koswi, N.;Kuroda,H. J. Chem. Soc.,Faraday Trans. 1 1989,85, 869. (13) Avnir, D.; Farin, D.; Pfeifer, P. J. Chem. Phys. 1983, 79, 3666. (14) Avnir, D.;Farin, D.; Pfeifer, P. Nature 1984,308, 261. (15) Van Damme, H.; Levitz, P.;Bergaya, F.; Alcover, J. F.; Gaitineau, L.; Fripiat, J. J. J. Chem. Phys. 1986, 85, 616.
(16)Farin, D.; Avnir, D. In The Fractal Approach to Heterogeneous Chemistry: Surfaces, Colloids, Polymers; Avnir, D., Ed.; Wiley: Chicheater, 1989; p 271. (17) Cole, M. W.; Holter, N. S.; Pfeifer, P. Phys. Reo. B 1986,33,8806. (18) Fripiat, J. J.; Gatineau, L.; Van Damme, H. Langmuir 1986, 2, 562. (19) Ng! S . P . ; Fairbridge, C.; Kaye, B. H. Langmuir 1987, 3, 340. (20) Fairbridge, C.; Palmer, A. D.;Ng, S. H.; Furimsky, E. Fuel 1987, 66, 689. (21) Friesen, W. I.; Mikula, R. J. J. Colloid Interface Sci. 1987,120, 263.
0743-7463/91/2407-0109$02.5~/0 0 1991 American Chemical Society
110 Langmuir, Vol, 7, No. 1, 1991
Kaneko 6 00
Y
01
E.
LOO
2
e4
d-A C F
T I
(002)
\
3
200
0
0
1.0
0.5
9 Figure 2. Adsorption isotherms of Nz on a-FeOOH-dispersed activated carbon fibers at 77 K. ?/
10
50
30 2 i 3
70
Table I. Micropore Parameters from Nitrogen Adsorption
(deg.,Cuk)
surface area, m2/g
Figure 1. X-ray diffraction patterns.
Experimental Section Materials. Cellulose-based ACF and the ACF preoxidized in 6 M HNO, at 373 K for 1h were used. a-FeOOH particles were dispersed on ACF (a-ACF) and preoxidized ACF (a-ox-ACF)in the same way as the method reported earlier.', The amounts of Fe on a-ACF and a-ox-ACF were 8.6 and 4.8 w t % , respectively. X-ray Diffraction. The change in the X-ray diffraction patterns of the ACF with dispersion of a-FeOOH was examined with an X-ray diffractometer (Rigaku Denki 2028). The radiation was nickel-filtered Cu K a (0.154 18 nm) operated at 35 kV and 10 mA. Diffraction patterns were obtained by reflection from the flat surface of the pellets compressed from the ground ACF. Adsorption. Nitrogen adsorption isotherms of these samples were determined gravimetrically at 77 K. Adsorption isotherms of benzene, cyclohexane, n-hexane, pyridine, CHC13, CHZClz, CCld, and n-nonane on the samples were measured gravimetrically at 303 K. The samples were evacuated at 373 K and 10-3 Pa for 15 h prior to the adsorption experiment. All organic solvents were of reagent grade. Results and Discussion Change i n Crystallinity of ACF with a-FeOOH Dispersion. X-ray diffraction of the ACF samples gave diffuse patterns, as shown in Figure 1. Two broad peaks near 28 = 26O and 28 = 43O correspond to reflections from (002) planes and overlapping reflections from (100) and (101) planes, respectively.= The (002) peaks of a-ACF and a-ox-ACF are much broader than that of ACF. The crystallite size L, of the graphite-like structure in samples was calculated by the Scherrer formula from the (002) peak width. The shape factor of 0.9 for an ordinary threedimensional lattice23was used. The calculated L, value is 1.2 nm for ACF, 1.0 nm for a-ACF, and 1.1nm for a-oxACF. The backgrounds due to the noncrystalline scattering for a-ACF and a-ox-ACF are higher than that for ACF. The height of the background reflects the crystallinity of partly crystallized amorphous solid (Ohlberg and StricklerhS6 If the diffraction patterns of a-ACF as a standard amorphous solid are chosen, the relative crystallinity from the background height a t 28 = 60' can be obtained as follows: ACF, 76%;a-ox-ACF, 38%;a-ACF, 0%. There(22) Masters, K. J.; McEnaney,B. In Churacterization of Porous Solids; Gregg, S . J., Sing, K. S.W., Stoeckli, H. F.,Eds.;SOC.Chem. Ind.: London, 1979; p 79. (23) Akamatsu, H.; Inokuchi, H.; Takahashi, H.; Mataunaga, Y. Bull. Chem. SOC.Jpn. 1956,29, 384. (24) Ohlberg, S. M.; Strickler, D. W. J . Am. Ceram. SOC.1962,45,170.
at
ACF 1400 a-ACF 1470 a - o x - ~ ~ ~a70
aeXt 5 5 11
E,,,
pore width,
mL/g
kJ/mol
10-1 nm
0.61
14 14
8f1 9f2 9i2
WO, 0.62 0.36
19
fore, the dispersion treatment of a-FeOOH decreases the crystallite size and crystallinity of the ACF; the micropore surface of a-ACF and a-ox-ACF should be rougher than that of the ACF. HN03 selectively oxidizes and removes amorphous parts of ACF, since a-ox-ACFis more crystalline than a-ACF. There is a possibility that the precursor particles of a-FeOOH are randomly included between graphitic basal planes in the case of a-ACF. Micropore S t r u c t u r e f r o m N2 Adsorption. N2 adsorption isotherms of ACF, a-ACF, and a-ox-ACF are shown in Figure 2. These adsorption isotherms are almost the same as those already p ~ b l i s h e d .The ~ adsorption isotherms are of type I, which is characteristic of microporous solids. t-plots and DR plots were applied to the N2 adsorption isotherms. For the construction of t-plots, the standard thickness of N2 adsorbed on nonporous carbon blacks25was used because IUPAC26recommends that one should use the standard thickness value of N2 on the same substance rather than on the substance having the same c-value of the BET equation. All t-plots pass through the origin and then bend near 0.4 nm to become nearly parallel to the abscissa; the t-plots indicate no presence of mesopores. The specific surface area at and external surface area aextfrom the t-plots were determined (Table I). The aeXtvalues of ACF or a-ACF are less than 1.5 96 of the at. The pore width determined from micropore analysis by use of t-plots is also summarized in Table I. The at value is almost identical with the specific surface area determined from the BET plot. DR plots expressed by eq l 2 7 t B for N2 adsorption isotherms gave good linear relationships
W = Woexp(-t2/E2); E = PE, (1) where W is the amount of adsorption, W Othe micropore volume, t = R T In Po/P the adsorption potential, POthe saturated vapor pressure, Eo the characteristic adsorption energy, and /3 an affinity coefficient. W Oand Eo were obtained from the DR equation by use of B = 0.33 for N:! after D ~ b i n i nalso, ; ~ ~ W Oand EO are shown in Table I. (25) Rodriguez-Reinoso, F.; Martin-Martinez, J. M.; Prado-Burguete, C.; MacEnaney, B. J. Phys. Chem. 1987,91,515. (26) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska,T. Pure Appl. Chem. 1985,57,603. (27) Dubinin, M. M. In Chemistry and Physics of Carbon; Walker, P. L. J., Ed.; Marcel Dekker: New York, 1966; p 51. (28) Dubinin, M. M.; Stoeckli, H. F.J . Colloid Interface Sci. 1980,75, 34.
Molecular Resolution Analysis of FeOOH-ACF
Langmuir, Vol. 7, No. 1, 1991 111
d-ACF
n
n
d-ACF,
e
w
,-d
.
Ill
'""1 I ' O 0I 0
0
0.5
1.0
P/p.
Figure 3. Adsorption isotherms of benzene on a-FeOOHdispersed activated carbon fibers at 303 K.
Adsorption Isotherms of Organic Molecules. The micropores of the ACF are made of micro-graphite-like layers which are randomly connected to each other. The X-ray diffraction data described above indicate the presence of a noncrystalline region, which probably combinesthe graphite-like crystallites. It is presumed that the average micropore surfaces are not flat but more complex. Planar, spherical, and zigzag-shaped molecules which can enter micropores were used as adsorptives to investigate the fractal structure of the micropore surface of ACF. Planar Molecules. Figure 3 shows the adsorption isotherms of benzene at 303 K. The isotherms of benzene resemble those of Nz; the amount of adsorption increases steeply with the relative pressure in the lower relative pressure of 0.1. This feature indicates that benzene molecules also fill micropores. The amount of benzene adsorbed on a-ACF is much greater than that on ACF by 100 mg/g, although the amount of NOadsorption is comparable. The adsorption isotherms of cyclohexane, where the molecules have a roughly planar structure without a-electrons, are also type I. The amount of cyclohexane adsorbed on a-ACF is greater than that on ACF by 40 mg/g. The adsorption behaviors of a-ACF and ACF for benzene and cyclohexane are clearly different despite their similar molecular sizes. a-ACF has a slightly larger amount of pyridine adsorption than ACF, as is the case with cyclohexane adsorption. The uptake of pyridine on a-ox-ACF is much less than that on a-ACF by about 200 mg/g. Nearly Spherical Molecules. T h e molecular symmetries of CC4, CHCl3, and CH2Cl2 are Td,C3", and C2", respectively. The CCl4 molecules can be regarded as spherical molecules, and the exchange of C1 with H reduces the symmetry. All adsorption isotherms of the alkyl chlorides are of type I. Figure 4 shows the adsorption isotherms of CCl4 at 303 K. The order of amount of CC14 adsorption is similar to that of benzene adsorption. The amount of adsorption of a-ACF reached 1g/g of adsorbent at a relative pressure of 0.8 PIPO. The amount of CHC13 on ACF is comparable to that on a-ox-ACFin spite of their distinct difference in pore volumes determined by N2; the dispersion of a-FeOOH on the preoxidized ACF markedly enhances the adsorptivity of CHCl3. In the case of CHzClz
0
0.5
0
1.0
P/
e
Figure 4. Adsorption isotherms of CC4 on a-FeOOH-dispersed activated carbon fibers at 303 K. P
I
ACF
I
Ln
72
a , - . A
rp
d-OX-ACF
01
0
I .o
0.5 p/
e
Figure 5. Adsorption isotherms of hexane on a-FeOOHdispersed activated carbon fibers at 303 K. adsorption, the reverse in the amount of adsorption on a-ACF and ACF was observed; the adsorption on ACF is slightly greater than that on a-ACF. Zigzag Molecules. The adsorption isotherms of alkane molecules are type I, indicating that hexane and nonane molecules are adsorbed through a micropore filling process. The adsorption characteristics of a-ACF and ACF for planar and zigzag molecules, however, are quite different. Figure 5 shows adsorption isotherms of hexane-a Cs-chain molecule. In this case, ACF can adsorb 60 mg/g more hexane molecules than a-ACF. This tendency is more evident in the adsorption of nonane-a Cs-chain molecule. The nonane adsorption of the ACF is much greater (125 g/g) than that of a-ACF. The difference between the adsorption of a-ACF and a-ox-ACFfor nonane is not so marked as that for N2 adsorption. Dubinin-Radushkevich (DR) Analysis for Organic Vapor Adsorption. As the type of adsorption isotherms of these organic molecules indicates the presence of micropore filling processes, the DR equation was applied to these isotherms to obtain micropore parameters for each molecule. Figure 6 shows the DR plots for benzene
Kaneko
112 Langmuir, Vol. 7,No. 1, 1991 6.5 [
I
substance
\ -
Table 11. Pore Volume for Organic Molecules
1
benzene cyclohexane pyridine
cc14
(I-ACF
CHC13 CHzClz hexane nonane
mg/g mL/g ACF a-ACF a-ox-ACF ACF a-ACF a-ox-ACF 475 340 555 900 567 780 425 483
585 370 590 992 757 750 365 355
365 235 315 590 455 473 293 265
0.55 0.44 0.57 0.57 0.39 0.60 0.65 0.68
0.67 0.48 0.61 0.63 0.52 0.57 0.56 0.50
0.42 0.30 0.35 0.38 0.31 0.36 0.45 0.37
6.0
I
\
ACF
The isosteric heat of adsorption at the coverage of l / e can be estimated from29 Qst
5.5 I 0
I
5
I
10
I J 15
In2p,/p
Figure 6. DR plots for benzene adsorption. 6.5
1
a i Figure 7. DR plots for pyridine adsorption.
adsorption isotherms. All plots are almost linear and parallel to each other; the characteristic adsorption energy for benzene for each sample is not so different from each other. Similar behavior was observed for CCld, CH2C12, cyclohexane, hexane, and nonane. In contrast, pyridine and CHCl3 have different DR plots. In particular, three DR plots for pyridine on ACF, a-ACF, and a-ox-ACFcross each other, as shown in Figure 7; EOfor a-ox-ACF is much greater than those for ACF and a-ACF. In the case of CHCl3 adsorption, the DR plots for ACF and a-ox-ACF cross. The interaction between basic pyridine or acidic CHCl3 and the a-FeOOH-dispersed ACF surfaces is likely to be stronger than van der Waals interaction. These molecules do not fit the yardstick molecule for the fract a l analysis. T h e micropore volume W Oand t h e characteristic adsorption energy Eo for each molecule were determined from the DR plot. Values for organic molecules were estimated from the ratio of the molar volume of each gas compared to that of benzene.27
=
m v+E
(2)
where AHvis the heat of vaporization for each molecule. /3, EO,WO, and qst for each organic molecule are listed in Tables I1 and 111. Fractal and Defective Structure of Micropore Walls. The adsorption properties of organic molecules having various structures and sizes are described above. The good linear DR plots confirm a process of micropore filling for these organic vapors in micropores of the ACF samples, as occurs with NZat 77 K. As the micropore filling of the condensable vapors is mainly governed by van der Waals forces, the orientation and conformation of an adsorptive molecule to the surface structure (geometrical factor) is t h e most important factor. Molecular fractal analysis is a powerful approach to estimate this geometrical factor in adsorption quantitatively. Data on molecular area determined by adsorption are not necessarily enough to construct the plot in this Molecular sizes can be derived from molecular kinetic data, van der Waals constants, and solid and liquid densities.33 As the state of the molecules in micropores is assumed to be quasi-liquid, the average molecular area from the molecular weight and liquid density at 303 K34 was used in the calculations. The cross-sectional area of an adsorbed Nz molecule was taken as 0.162 nm2. The molecular area of benzene is 0.34nm2,being nearly equal to the value 0.31 nm2 for a randomly oriented benzene molecule.35 Typical BET plots are shown in Figure 8. The linearity is enough to estimate the monolayer capacities for various gases, which are shown in Table IV. However, the concept of monolayer capacity is ambiguous in micropore filling, except for bilayer micropore filling;2s the monolayer capacity of the monolayer-sizedmicropores will be underestimated, while that of three- or four-layer-sized micropores is overestimated due to the cooperative effect. The DR analyses of the detailed N2 adsorption isotherms of ACF samples will yield the amount of Nz adsorbed by the bilayer micropore filling, which is nearly equal to the monolayer capacity determined by the t-plots; the BET plot for (29) Kawazoe, K.; Astakhov, V. A.; Kawai, T.; Eguchi, Y. Kagaku Kogaku 1971,35, 1006. (30) McClellan,A. L.; Harnsberger, H. F. J . Colloid Interface Sci. 1967, 23,577. (31) Nay, M. A.; Morrison, J. L.Can. J. Res. 1949, B27,205. (32) Gregg, S. J.; Sing, K. S. W. In Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: New York, 1982; p 81. (33) Kennard, E. H. In Kinetic Theory of Cases; McGraw-Hill: New York, 1938; Chapter 4. (34) Handbook of Chemistry and Physics, 54th ed.; CRC-Exprees:New York, 1973-1974; p F-3. (35) Gregg, S. J.; Sing, K. S. W. In Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: New York, 1982; p 81. (36) Kakei, K.; Ozeki, S.;Suzuki, T.; Kaneko, K. J . Chem. SOC.,Faraday Trans. 1990,86,371.
Langmuir, Vol. 7,No.I, 1991 113
Molecular Resolution Analysis of FeOOH-ACF
Table 111. Affinity Coefficient j9 Estimated from Molecular Volume, Characteristic Adsorption Energy &, and Isosteric Heat of Adsorption qat at the Coverage of l/e EO,kJ/mol qa, kJ/mol substance ACF a-ACF a-ox-ACF ACF CY-ACF CY-OX-ACF B benzene 15 17 17 49 51 51 1.00 cyclohexane pyridine
cc4
CHCla
18 8.9 17 15 20 15 8.5
17 9.4 18 22 20 16 7.7
16 19 15 19 21 14 9.3
54 48 50 36 46 53 61
d-O X- A C F
54 49 52 42 46 55 59
52 58 49 40 47 52 62
1.21 0.90 1.09 0.92 0.72 1.47 2.01
3 c
m
1 0
E E
Y
$
-C
2
1
2.5
3.0
3.5
4.0
I n w(Io-*nm2)
Figure 9. BET surface area as a function of molecular area for ACF. From left to right: N2, CH2C12, CHCls or pyridine, benzene, CC&, cyclohexane, hexane, and nonane. 0. I
0.2 p/
0.3
e
3-
Figure 8. BET plots for benzene adsorption isotherms. Table IV. Monolayer Capacity in mmol/g ACF a-ACF CY-OX-ACF
substance
N2 benzene cyclohexane pyridine
cclr
CHCla CH2Clz
hexane
nonane
17.6 5.33 3.28 5.77 4.33 3.53 8.29 4.55 2.91
17.9 5.64 3.77 5.73 5.20 5.00 6.63 3.48 2.51
7.21 3.69 2.06 3.46 2.71 3.25 4.40 2.78 2.06
the N2 adsorption isotherm below the relative pressure of 0.1 provides the monolayer capacity almost similar to that by the t-plot within 10%. Although we did not analyze the organic vapor adsorption data from both t-plots and BET plots, we must use the BET monolayer capacities for organic vapors as approximate values, when the organic molecules can be adsorbed as bilayers in the micropores. Figure 10 shows the relationships between the logarithm of the monolayer equivalent capacity in mmol/g from the BET plot and log u for ACF. An approximately linear relationship is observed; the linearity indicates that the micropore wall of ACF can be represented as a fractal surface: Figure 10 shows such relationships for a-ACF and a-ox-ACF. Linear relations are also observed in this case. These linear relationships indicate that almost all the organic vapor molecules used in this work were adsorbed as a monolayer on each pore wall; namely, they are filled as bilayers in micropores of these ACF samples, as expected from the pore widths and molecular sizes.
2-
l -
2.5
I 3.5
I 3.0
I 4
In w (10-2nm2 )
Figure 10. BET surface area as a function of molecular area for a-ACFand CY-ox-ACF. The order of adsorptives is the same as in Figure 9. Consequently, the molecular fractal analysis can be applied to the data. The surface molecular fractal dimension D, value was determined from eq 3 (ACF, 2.3 f 0.2; a-ACF, 2.4 i 0.1; a-ox-ACF, 2.1 f 0.2): a
0:
,,-D.P
(3) The scattering of the data in Figures 9 and 10 is probably caused by both the deviation from the bilayer micropore filling and the specific interaction between an adsorptive and the pore surface; the data for benzene and CCld may deviate because of the former, while the deviation of the data for acidic CH3C1could be attributed to the latter. 8
Kaneko
114 Langmuir, Vol. 7,No. I, 1991
3t-
0 d-ACF A d-Ox-ACF
2-
1 -
O
\
+ 5
L
In v
6
( 0 . 1 ~ ~ ) ~
Figure 11. Micropore volume as a function of molecular volume for ACF. The order of adsorptives is the same as in Figure 9. Although the value of the monolayer capacity is uncertain, important information on the structure of the micropore wall can be determined from the D,value. The D, values are rather small compared with those reported on granular activated carbons;'3J4 the micropore walls of ACF samples are more graphitic. In particular, a-ox-ACFhas an ordered two-dimensional micropore wall like graphite, because its D, is nearly equal to 2. The preoxidation with HNO3 should remove amorphous regions and leave only micrographitic layers. This conclusion agrees with X-ray diffraction results. It is now largely accepted that micropore volume, rather than surface area, per se, is a more valid quantity for the characterization of microporous systems. Establishment of the relationship between the micropore structure and the molecular volumes should, therefore, be preferable. The relationship between the logarithm of the micropore volume WO and molecular volume u calculated from molecular weight and liquid density is shown in Figures 11and 12, where good linearity is observed. CHC13 deviates from the linear relationship probably because of its strong acidity. The above relationship can be described by the following equation:
wo
0:
(4)
u+
where y is an exponent. The y values are as follows: ACF, 2.4 f 0.05; a-ACF, 3.0 f 0.05; and a-ox-ACF, 2.5 f 0.2. All y values are greater than the D,values determined by eq 2. On the other hand, the monolayer capacity in mmol/g can also be expressed a,
a
~
-
~
a
f
~
(5)
Consequently, when a, coincides with WO,y must be identical with D,. WOis not equal to a, in the micropore filling, except for bilayer filling. The difference between y and D, provides valuable information on the micropore structure of the ACF samples. If the volume of pores where molecules do not fill as bilayers is expressed by AW, eq 6 is obtained:
W, = a, + AW (6) When AW is positive, molecules are filled not only as bilayers but also as triple layers and/or more multilayers. In the case of a negative AW, part of the adsorbed
Figure 12. Micropore volume as a function of molecular volume for a-ACF and a-ox-ACF. The order of adsorptives is the same as in Figure 9.
AC F
d-ACF
Figure 13. Possible models of the micropores of ACF samples. Open circles represent the molecules adsorbed as monolayers on the micropore wall. Solid circles represent the molecules between molecules adsorbed as monolayers. molecules are filled monolayerly. Positive AW leads to y > D,, whereas negative AW gives y < D,. In this work, the WOvalues, except for N2 adsorption, are greater than the a, values. The positive value of y - D, (ACF, 0.1; a-ACF, 0.6; and a-ox-ACF, 0.4) indicates heterogeneity in the micropore size. a-FeOOH-dispersed ACF has a micropore distribution with a tail extending to larger size; the tail in a-ACF is more significant than that in a-oxACF. Although data on N2 adsorption alone cannot make clear the difference between the micropore structures of the original ACF and a-FeOOH-modified ACF, the molecular resolution analysis with a series of adsorptives is effective for characterization of the ACF samples. Although we cannot give a precise description of the micropore structure, the models shown in Figure 13 are tentative micropore structures of the ACF samples based on the D, and y - D, values. The curves in Figure 13 represent cross sections of the micropores. The micropore of a-ox-ACF should be close to an ideal slit of the flat wall, because its surface fractal dimension is 2.1. However, there are some defects on the wall where three
Molecular Resolution Analysis of FeOOH-ACF
Langmuir, Vol. 7, No. 1, 1991 115 A . m
~ox-ACF
0 1 4.5
-
3.58'c\t
I 5.0
1 5.5
I n v (0.1nm13
Figure 14. Relationship between isosteric heat of adsorption and molecular volume. From left to right: CH2C12, CHCla or pyridine, benzene, CCld, cyclohexane, and nonane.
and/or four adsorbed layers can be formed because of the positive y - D,. As the fractal dimension of the Koch curve is about 1.3, perpendicular extension of the Kochlike curve gives a surface of D,= 2.3, which can be a model for ACF and a-ACF. In ACF, almost all molecules fill as bilayers in micropores, while the opposite micropore walls are partially expanded to adsorb three and/or four adsorbed layers in the micropores of a-ACF. So far we have no definite information on the structural entity of the defects on the micropore wall, although the unit size of the defect is presumed to be approximately the size of the benzene ring. It is difficult to draw a more definitive model of the micropore structures of the ACF samples. The molecular resolution analysis can be applied to the adsorption energy. Figure 14 shows the relationships between In qatand In u. ACF and a-ACF give good linear relationships, although the points due to CHCl3 and pyridine deviate outstandingly. The data for a-ox-ACF are scattered around the lines of ACF and a-ACF. The
slopes of the lines for ACF and a-ACF are 0.30 and 0.28, respectively. Accordingly, qat is approximately proportional to u1/3,suggesting that qst is related to the average intermoleculardistance. The molecular volume is the most important factor governingthe isosteric heat of adsorption in microporefilling. However, the meaning of the exponent determined from the qst vs u relation is not understood. Recent in situ X-ray showed that adsorption of water changes the interplanar distance of the graphitic layer of the ACF samples and that micropores swell with water adsorption. There is a possibility, therefore, that the structures of the micropore and the graphitic layer may themselves change on adsorption and be dependent on the type of each adsorptive.
Conclusions As most organic molecules used in this work can be adsorbed as bilayers in the micropores of ACF samples, the specific surface area of the micropore walls could be estimated and analyzed by the molecular fractal approach. The surface fractal dimensions of the ACF samples determined from the linear relationships between monolayer capacity and molecular area were 2.1-2.4. The difference was associated with the different structure of the micropore wall, which agrees well with the data obtained by X-ray diffraction. Furthermore, the relationship between micropore volume and molecular volume provides valuable information on heterogeneity in the micropore size distribution of the ACF; dispersion of ultrafine a-FeOOH brought about slight increase in the larger micropores.
Acknowledgment. I acknowledge the Ministry of Education for the Grant in Aid for Fundamental Scientific Research. I am indebted to Prof. D. Avnir for helpful comments and to Dr. J. D. F. Ramsay for help in preparation of the manuscript. I also thank Dr. T. Suzuki for the X-ray diffraction measurements. Registry No. 0-FeOOH, 20344-49-4;N2, 7727-37-9; CHC13, 67-66-3; CH2C12, 75-09-2; CC14, 56-23-5; benzene, 71-43-2; cyclohexane, 110-82-7; n-hexane, 110-54-3; pyridine, 110-86-1; n-nonane, 111-84-2. (37) Suzuki, T.; Kaneko, K. Carbon 1988,26,743. (38) Kaneko, K.; Fujiwara, Y.; Nishikawa, K. J.Colloid Interface Sci. 1989,127, 298.
