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Langmuir 1993,9, 2612-2617
Surface Characterization of Microporous Solids with He Adsorption and Small Angle X-ray Scattering N. Setoyama,? M. Ruike,t T. Kasu,t T. Suzuki,? and K. Kaneko'9t Department of Chemistry, Faculty of Science, Chiba University, 1-33 Yayoi, Inage, Chiba 263,Japan, and Fundamental Research Laboratories, Osaka Gas Co., Torishima, Konohana, Osaka 554, Japan Received October 5, 1992. In Final Form: February 5, 199P The adsorption isotherms of He at 4.2 K and N2 at 77 K on pitch-based activated carbon fiber (ACF) and ACF treated at 1473K in Ar (ACF-1473)were determined. Also their small angle X-ray scattering (SAXS) spectra were measured in air. The amount of He adsorption expressed in terms of the adsorbed volume using Steele's theoreticaldensityof the adsorbed He layer was compared with that of N2 adsorption. The He adsorption isotherm had much steeper uptake below PIP0 = 0.02; the amount of He adsorption below PIP0 = 0.02 was greater than that of Nz adsorption by more than 50%; heat-treatment of ACF increased the difference between He and N2 adsorption at low pressure. Both He and N2 adsorption isothermswere analyzed by the Gaussian distribution. The NZadsorption provided completely different peak positions of the micropore size distribution of ACF and ACF-1473,while He adsorption led to slight difference in the mean micropore width. The presence of necked micropores in ACF-1473was evidenced by the distinct adsorption behaviors of He and N2. The distribution of the Guinier gyration radius was detdrmined from SAXS data and the mean Guinier gyration radius was obtained from the Maxwellian distribution. The mean gyrationradius and the width of the slit-shapedscattering entity by the thickness plot of the SAXS data gave an average geometry of micropores. The slit width from the thickness plot was very close to that from molecular adsorption. The heat treatment did not change the micropore geometry of ACF from the SAXS analysis. The difference between molecular adsorption and SAXS results on the heat treatment was attributed to the necked pore structure.
Introduction Further essential development of adsorption science requires an understanding both of molecular aggregate structures on the surface or in pores and of the surface structure of the partially crystalline microporous solids such as activated carbons. Activated carbon is mainly composed of less-crystalline micrographites.13 Activated carbon is a representative of the less-crystalline microporous solids, which has great micropore volume and excellent adsorption properties. Activated carbons have been widely used in technologies; advanced technology is necessaryfor more accuratecontrol of adsorptionprocesses by activated carbons. Especially elucidation of the lesscrystalline structures of activated carbons should be helpful to advance the technology! Although the less-crystalline structure of activated carbons is not well characterized, activated carbons of large micropore volume appear to give rise to a predominant micropore filling in terms of slit-shaped micropores. The micropore structures of activated carbons have been studied by molecular adsorption, mainly, Nz adsorption at 77 K.m The advantage of the as analysis for Nz adsorption isotherms has been proposed by Sing et al.9JO The authors showed that high-resolution ag analysis is + Chiba University. t Osaka Gas Co.
* Abstract published in Advance ACS Abstracto, August 15,1993.
(1) Franklin, R. Z. Acta Crystallogr. 1961,26, 743. (2) Suzuki, T.; Kaneko, K. Carbon 1989,26, 743. (3) Suzuki, T.; Kaneko, K. Chem. Phys. Lett. 1992,191,569. (4) Sing, K. S. W. In Adsorptive Separation;Suzuki, M., Ed.; Institute of Industrial Science, University of Tokyo: Tokyo, 1991; p 161. (5) Marsh, H. Carbon 1987,26,49. (6) McEnaney, B. Carbon 1988,26,267. (7) Sing, K. S. W. Carbon 1989,27,5. (8) Rodriguez-Reinoso, F.;Molina-Sabio, M.; Munecas, M. A. J. Phys. Chem. 1992,96, 2707. (9) Gregg, S. J.; Sing, K. S. W. In Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: London, 1982; Chapter 2. (10) Atkinson, D.; McLeod, A. I.; Sing, K. S.W. J. Chim. Phys. 1984, 81, 791.
especially effective for correct assessment of the microporosity."J2 However, the Nz molecules are strongly adsorbed a t the entrance of the micropores due to the quadrupolemoment, blocking further adsorption.13J4Such blocking interferes with the correct assessment of the microporosity. C02 adsorption near room temperature has been applied to the pore analysis of activated carbons having necked micropores.14 Also adsorption of organic vapor molecules of different sizes is effective to clarify the complex pore However, the micropores are too small compared with these probe molecules to evaluate the microporosity precisely. An HzO molecule is smaller than an Nz molecule, but the former molecule cannot be adsorbed monomolecularlyon the carbon surface in the low pressure region.1g20 A He atom is the smallest spherical monoatomic molecule and interacts weakly with any solid surface. He adsorption at 4.2 K is a promising method for the accurate assessment of the microporosity, not only of activated carbons but also of microporous solids in general. The authors showed that He adsorption at 4.2 K can evaluate correctly the micropore volume of the activated carbon.21*22 Activated carbon fiber (ACF) materials have more uniform microporous structures and (11) Kaneko, K.; Ishii, C.; Ruike, M.; Kuwabara, H. Carbon 1992,30, 1075. (12) Kaneko, K.; Ishii, C. Colloid Surf. 1992, 67, 203. Faraday (13) Rouquerol, J.; Parytka, S.;Rouquerol, F. J. Chem. SOC., Trans. I 1977, 73,306. (14) Garrido, J.; Linares-Solano, A.; Martin-Martinez, J. M.; MolinaSabio, M.; Rodriguez-Reinoso, F.; Torregrow, R. Langmuir 1987,3, 76. (15) Carrott,P. J. M.;Roberta,R. A.;Sing,K.S. W. In Characterization
of Porous Solids, Unger, K. K., et al., Eds.; Elsevier Science Publisher: Amsterdam, 1988; p 89. (16) Kaneko, K. Langmoir 1991, 7,109. (17) Sato, M.; Sukegawa, T.; Suzuki, T.; Hagiwara, S.;Kaneko, K. Chem. Phys. Lett. 1991,181, 526. (18) Dubinin, M. M.; Serpinsky, V. V. Carbon 1981,19,402. (19) Evans, M. J. B. Carbon 1987,25, 81. (20) Kaneko, K.; Kosugi, N.;Kuroda, H. J. Chem.Soc.,Faraduy Trans. 1 1989, 85, 869.
(21) Kuwabara, H.; Suzuki, T.; Kaneko, K. J. Chem. SOC.,Faraday Trans. 1991,87, 1915.
0743-7463193/2409-2612$04.00/0 0 1993 American Chemical Society
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slightlyorientedmicrographiticstructures.s26 Thus, ACF has simpler structures than ordinary granulated activated carbon. ACF is the best model system of the lesscrystalline microporous solids;there are several works on micropore filling of ACF by supercritical gases.2s28 Recently physical properties of ACF have gathered much attention.2w2 The comparative examination of both N2 and He adsorption should shed light on the micropore structure of activated carbons. However, molecular adsorption cannot determine the geometricalparameters of the slit-shapedmicroporesother than the microporewidth. If we introduce another technique together with molecular adsorption to characterize the micropore structures and/ or micrographitic structures, we may elucidate the structures of even partially crystallinemicroporoussolidsmuch better. Microheterogeneitiessuch as micropores in solid materials lead to small angle X-ray scattering (SAXS) due to the electron density d i f f e r e n ~ e .In ~ ~activated carbon science, the SAXS technique has been applied to the characterization.34 Dubinin and Stoeckli correlated the characteristic adsorption energy of Nz adsorption with the Guinier gyration radius, which has been used for determination of the micropore width of activated carb o n ~ Nishikawa .~~ et d. examined the Guinier gyration D 0 1.0 2.0 radius change with HzO adsorption for ACFs under as controlled humidity; the possibility of micropore and 1. High resolution CY,plota of ACF and ACF-1473for Nz ~ micrographitic structural changes was p r ~ p o s e d .In~ ~ ~ ~Figure adsorption isotherms. situ X-ray diffraction studies on ACFs during adsorption showed a serious decrease in the interlayer spacing of the has a double Dewar-type cryostat, a quartz balance, a quartz micrographite~.~~~ Recent progress in the SAXS equipsampleholder,a cathetometer,two precisionpressuretransducers ment should give new insights on the structures of less(Baratrongauges, 10and 1000Torrranges),a pressure-measuring crystalline microporous solids; a fundamental and syselectronic device, and a grease-free stainless-steel vacuum line. tematic study on activated carbons using the advanced The He equipment can maintain the temperature of the sample SAXS technique is indispensable to understand the at 4.2 K for 8 h; all He adsorption isotherms were measured within 8 h.21 Pressures 0.02
of the micropores, increasing the heterogeneities in micropore structures. Micropore Size Distribution. The adsorption isotherms can be approximately expressed by the DubininRadushkevich (DR) equation. However, most of the DR plots for high-resolution N2 adsorption isotherms are composed of more than two parts. Such bending arises mainly from the heterogeneity in the micropore size.23 The He molecule can access narrower micropores than the N2 molecule; the micropore size distribution by He adsorption should be different from that by N2 adsorption. We separated the narrow micropores from the wide ones by using the two-term DR equation:41
W = Wn+ W , = W , exp[-(A/j3E,,J21 + W,, exp[-(A/j3E,,l21 (2) Here, A is the adsorption potential which is expressed by A = RT ln(Po/P) at the relative pressure P/Po. W is the amount adsorbed per unit mass of the adsorbent. Wnand W, are the adsorbed amounts in the narrow and wide micropores at P/Po, respectively. WO, and WO, are the volumes of narrow and wide micropores, respectively. EO, and E,, are the characteristic adsorption energies of adsorption in narrow and wide micropores, respectively, and fl is the affinity coefficient. Table I1 collects WO, and WO, from both He and N2 adsorption. The WO,from He adsorption includes more error than that from N2 due to the high thermal conduction effect of the He gas in the high-pressure region, although WO,is negligibly small compared to the WO, value from He adsorption. We determined the size distribution of the predominant narrow micropores by the simplest assumption of a Gaussian distribution, although the micropore size distribution has been calculated from the N2 adsorption isotherm in various ways+2*43The peak positions of the micropore size distributions are shown in Table 11. The determination method is described below. The micropore size distribution of the Gaussian-type can be expressed by the following equation after D ~ b i n i n : ~ ~
Here xo is the mean micropore half width and 6 is the disperson of the distribution. The 6 was chosen from the best fit. The /3 value of He is necessary for transformation of the characteristic energy into the pore width through the Dubinin-Stoeckli (DS)-like relationships under the assumption that the DS-like relationship holds for He adsorption. The j3 of He was determined from data of both pitch-based ACF having the widest micropores (1.5 nm by N2) and cellulose-based ACF which have almost the same micropore volume by He as that by N2. Figure 5 shows the micropore size distributions of ACF and ACF1473 from the N2 adsorption. The pore size distribution of ACF is sharper and greater than that of ACF-1473 by a factor of 2. The peak position of ACF markedly shifts to greater value after heat treatment. This result is indicative of a remarkable molecular sieve effect and the (41) Dubinin, M. M. Carbon 1985,23,373. (42) Jaroniec, M.; Madey, R. J. Phys. Chem. 1989,93, 5225. (43) Seaton, N. A.; Walton,3. P. R. B.; Quirke, N. Carbon 1989,27, 853.
Wor(Nz), mL E-' 2xon(He), nm 0.037 0.088
0.76 1.2
ACF
1.0
2 x d N z ) , nm 0.95 1.5
'
Micropore width 2x I nm
Figure 5. Micropore size distributions of ACF and ACF-1473 by NZadsorption. 1.5
1
Micropore width 2 x I nm
Figure 6. Micropore size distributions of ACF and ACF-1473 by He adsorption.
collapse of micropores. But, the He adsorption leads to a considerably broad micropore size distribution. Figure 6 shows the micropore size distribution of both samples from the He adsorption. The pore volumes by He adsorption are greater than those by N2 adsorption; the distribution from He is broader than that from N2. The peak position difference between both distribution curves from He is smaller than that from N2. This may be caused by the presence of necked micropores in ACF-1473, which He atoms can move into, but N2 molecules cannot. Here, a small tailing near the origin of the micropore size distribution by He arises from the Gaussian approximation; it should be ignored. The comparison of the micropore size distributions by N2 and He adsorption is shown in Figures 7 and& In the case of ACF, the micropore size distribution from He is broader and shifts to smaller value compared to that from N2. The micropore volume by He adsorption is greater than that by N2 adsorption. This indicates that both wide and narrow micropores can be accessed more correctlyby He adsorption. The smaller He atoms can be adsorbed more completely on the rough surface. This should be associatedwith the surfacefractal structure. The difference in the micropore size distributions of ACF-1473 by He and N2 adsorption is similar to that of ACF but more significant. The micropore size distribution of ACF-1473 by N2 adsorption is almost included in that by He adsorption. The micropore volume
Setoyama et al.
2616 Langmuir, Vol. 9, No.10,1993 1.5
r
1 .
.'
I
.
.
.'. . .
He,'
1.0
10.0
.
X
?
E
. . 0.5
I
0 ' 0
...'
. . . .
.;;
.,..;..,. .
,
;...
.,
.. ..
....
I
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1
2
Micropore width 2x nm
Figure 7. Micropore size distributions of ACF by He and Nz
adsorption.
3 ....'. N, .,
:
....... a.
.
Micropore width 2x1 nm
Figure 8. Micropore size distributions of ACF-1473 by He and Nz adsorption.
by He adsorption is much larger than that by N2 adsorption. Possibly N2 adsorption just probes large micropores accessible to N2 molecules, while the necked micropores remain inaccessible to N2. Also the fractal structure of the micropore wall must be more important in this case.18J7,44,46 We did not examine the quantum effect of He in micropores yet, although the translational energy of a He atom should be quantized. The Micropore Geometry by Small Angle X-ray Scattering. If there are N scattering entities having z electrons without correlation, the small angle X-ray scattering intensity I(s) can be approximated by the Guinier equation at s 0.s3
-
I ( s ) = I(0) exp(-Rts2/3), I(0) * Nz2 (4) Here R, is the gyration radius of the particle and the Guinier approximationis valid for sR, < 1. The concept of the gyration radius is valid for any shape of the particle; we can estimate the particle shape through the gyration radius and other physical information. In ordinary SAXS analysis of activated carbons, the scattering has been assumed to come from the pores.% However, Nishikawa et al. showed that not only pores but also micrographites can cause the scattering in the simulation study of SAXS of ACF sampleses7 This result suggesta that sizes of (44)Avnir, D.;Farin, D. In The Fractal Approach to Heterogeneous Chamietry; Avnir, D.,Ed.;Wiley: New York, 1989; Chapter 4. (45) Kaneko, K.; Sato, M.; Suzuki, T.; Fujiwara, Y.; Nirhikawa, K.; Jaroniec, M.J. Chem Soc., Faraday Trans. 1991,87,179.
Figure 9. Guinier plot of the smallangle X-ray scatteringin the smallest s region for ACF-1473.
micropores and micrographites are similar to each other. Figure 9 shows the Guinier plot of the SAXS data of ACF1473. The Guinier plot is not linear, but there seems to be a linear range which corresponds to the micropore size; the linear plot satisfies the requirement sR, < 1. This Guinier plot provides R, of 10A for ACF-1473. However, we cannot choose a single Guinier linear region from the nonlinear relationshipwhich indicatesthe sizedistribution of scattering particles. Shull and Roeas& presumed the Maxwellian distribution of R, for such a nonlinear relationship. The size distribution,M(R,), having the gyration radius R, is expressed by eq 5 (5)
Here n and ro are parameters, and MOis the total mass of scattering particles. I' denotes the gamma function. In this approximation, the scattering intensity is dearibed by eq 6 log I @ ) = constant - 2
The computer fitting determines n and ro; the size distributionM(R,) is obtained, as shown in Figure 10.The characteristics of the distribution do not change with heat treatment; the size distribution of ACF is slightly broader than that of ACF-1473. The mean R, value, R, was determined from the first-order moment using the distribution. The R, value (15 A for ACF-1473) is severely different from R, (10 A) by the above single line approximation; the introduction of the Maxwellian distribution to the Guinier analysis is very useful. The detailed analytical procedure of SAXS data will be described in another paper.& The R,! value provides an average geometrical size of the ripd body. If we assume that the scattering entity is the slit-shaped pore, R, can be expressed by the following equation (46)Shull, C. G.;I", L. C . J. Appl. Phys. 1947,18, 296.
Surface Characterization of Microporous Solids
Langmuir, Vol. 9,No.10,1993 2617 36A
Q
ACF-1473
I(s) s2 = It(0)exp(-Rts2) (8) Here, Zt(0) is a constant. The linear ln[I(s)s21vs s2 plot leads to the Rtvalue. Figure 11shows the thickness plot of ACF-1473. As the plot has good linearity, the shape of the micropore should be close to slit. The Rt, that is, the pore width w. determined by the thickness plot was 0.7 nm for ACF and 0.8 nm for ACF-1473, respectively; this
Figure 12. Micropore geometry of ACF samples. w. is very closed to the pore width from molecular adsorption, w, (0.76 nm for ACF and 1.20 nm for ACF1473). The w. value is an average of open and closed micropores, whereas w, is the pore width of open pores. In the case of ACF, almost all micropores are open, then both widths from molecular adsorption and SAXScoincide with each other. On the other hand, ACF-1473has closed micropores of small width and there is slight discrepancy between w, and w. values. If we assume 1 = d , we can determine three-dimensionalgeometryof microporesfrom the SAXS data using R, and w. values. Figure 12 shows the micropore geometry of ACF and ACF-1473 estimated by the SAXS analysis. The average micropore geometry is assumed to be slit and it does not almost change with heat-treatment. It should be noted that detailed SAXS analysis can determine not only the micropore width but also the stereoscopic geometry, although we cannot separately determine d and 1. The X-ray diffraction" and SAXS data indicated that the micrographiticand micropore structures are preserved during heat-treatment. In contrast to these data, molecular adsorption behaviors suggest a distinct change in the micropore structures. This contradiction arises from the sensitivity of the entrance structure of open micropores; Nz adsorption depends sensitively on the entrancegeometry of the micropores, while SAXS is insensitive to the local structures. In the real ACF systems, the heattreatment increases the heterogeneity of the micropore size distribution, that is, creation of both necked microporesaccessibleto only a He atom and wider micropores due to the rearrangementof the micrographites,and partial collapse of the micropores. Also closed micropores should be produced by heat-treatment to give rise to slight difference of size distributions by molecular adsorption and SAXS. He adsorption at 4.2 K showed that the micropore analysis only by NZadsorption at 77 K is insufficient and gives a misleadingconclusion. The detailed SAXSanalysis provides a reasonable stereoscopicmicropore structure of disordered solids. The combined characterization of He adsorptionat 4.2 K and SAXS is in particular indispensable to correlate the observed adsorption properties with micropore structures of less-crystallinesolids. In this work, we have presumed that the scattering entity is a micropore according to comparison of the SAXS data with X-ray diffraction ones.
(47) Ruike, M.; Kasu, T.; Suzuki, T.; Kaneko, K. In preparation. (48)P o d , G. $Small Angle X-Ray Scattering; Glatter, O., Kratky, O., EMS.; Academic Prees: London, 1982; Chapter 2. (49) Mateuoka, H. In Lecture Note of the 12th Summer Symposium on Colloid and Surface Chemiutry; Chemical Society of Japan: Hakone, 1990; p 63.
Acknowledgment. We acknowledge the Ministry of Education for the Grant in Aid for Fundamental Scientific Research. Mr. C. Ishii measured the X-ray diffraction of samples. We are indebted to Dr. Nishikawa and Dr. Matsuoka for helpful suggestions on the S A X S studies.
Figure 10. Guinier gyration radius distributions for ACF and ACF-1473 from nonlinear Guinier plots.
Figure 11. Thickness plot of the small angle X-ray scatbring in the middle range for ACF-1473.
w 2+ 1'
+
d2 (7) 12 Here, w ,1, and d are the micropore width, the horizontal length of the cross section of the slit, and the depth of the pore, respectively. Analysis of the scattering at angles higher than the Guinier region provides the information on the shape of the scattering entity. If the scattering entity can be approximated by the slit, the following thickness plot leads to the gyration radius of thickness, Rt, of the lit.^'?^
R',
=
