Langmuir 1992,8, 360-365
360
Cross-Sectional Areas of Adsorbed N2, Ar, Kr, and 0 2 on Carbons and Fumed Silicas at Liquid Nitrogen Temperature Ismail M. K. Ismail University of Dayton Research Institute, c/o Phillips LabIRKFC, Edwards Air Force Base, California 93523- 5000 Received November 1,1990. In Final Form: November 26, 1991 Adsorption of Nz, Ar, Kr, and 0 2 at 76.6 K was investigated on Aerosil samples and three different categories of carbons: graphitized, ungraphitized, and activated graphitized substrates. With the BET equation, the appropriate ranges of relative pressure are defined and the corresponding experimental cross-sectional areas, u (nm2/adsorbedspecies), are reported, based on u(N2) = 0.162. Values of u on the carbons were fitted to physical models illustrating possible arrangements of adsorbed species on the surface. With graphitized carbons, ~ ( 0 2=) 0.147 and u(Ar) = 0.138. With ungraphitized or activated carbons, values of u are 0.157 for Ar, 0.214 for Kr (at P / P = 0.02-0.25), and 0.148 for 0 2 . With Aerosils, however,the value of u(Ar) = 0.186, obtained by the calibration, is high and could not be fitted to a model. An alternative approach has been considered. When comparing the present data with those reported in the literature, the recommended new values on Aerosils (and other silicas) are u(Nz) = 0.143,~(02)= 0.147 and u(Ar) = 0.165 nm2/mol. The present results indicate that oxygen could be considered as the standard gas for adsorption on carbons and silicas, provided that it does not change the carbon structure or its surface characteristics when adsorbed at 77 K.
Introduction Gas adsorption has been used for several decades to estimate surface area of materials, to define type of porosity, to compute pore volumes, and to calculate pore size distributions. The most common adsorbates are simple gases such as N2, Kr, Ar, 02, and C02. With the adsorption isotherm of an adsorbate, X, the BrunauerEmmett-Teller (BET) equation' is normally used at relative pressure, PIP", of 0.05-0.35 to estimate the monolayer volume, Vm. Multiplying Vm by the appropriate cross-sectionalarea, u(X),along with other proportionality constants, yields the BET surface area. With graphitized carbons, this procedure has limitations. For example, the adsorbed Kr atoms or N2 molecules near the completion of the first monolayer undergo several phase changes2including a change from commensurate to incommensurate structure, and the isotherms exhibit a kink2+ which may affect the linearity of the BET plot. Since on graphitized carbons, the true area of an adsorbed N2 molecule or Kr atom at 77 K near the monolayer coverage, as determined by other experimental techniques,'-13 is equivalent to the area of three graphitic hexagons, 0.157 nm2, the true surface area is the product of 0.157 and the volume of gas taken by the sample at the beginning of the isotherm kink which represents the true monolayer coverage. Nevertheless, the BET equation is still useful to yield the same answer in a different way.
When the "apparent" BET value of Vm is computed at PIP" of 0.05-0.21, the product of Vm and the "apparent" value of a(Nz), 0.162 nm2/mol,or a(Kr), 0.214 nm2/atom, gives the same "true" surface area.14 This procedure is especiallyconvenient with graphitic carbons whose Nz and Kr isotherms a t 77 K do not have a distinct kink near the completion of mon01ayer.l~ Values of ~(0.2u(Ar), 1, or a(Kr) on other forms of carbons, e.g., ungraphitized and activated carbons as well as coals and chars, and the limits of the BET equation, have not been clearly defined. The literature indicates that u(X) may vary from one substrate to another. The limits of the BET equation could cover a P/P" range as narrow as 0.0001-0.03815or as wide as 0.05-0.35,' and there is a considerable scatter in a(X) values reported for different adsorbates on carbons. For example, the literature cross-sectional areas (nm2/adsorbed species) are 0.1432and 0.23516 for Kr, 0.11117J8to 0.15119 for Ar, and 0.135 and 0.15918*20 for 0 2 . The most popular ones are u(Ar) on graphitized carbons, 0.138, which has been recommended by several investigator^,'^*^^-^^ and a(Kr) = 0.195.25 The 0.138 is commonly used with ungraphitized and microporous carbons,15but as it will be shown here, this value is not applicable to these classes of carbon. The objectivesof this article are to determine the reliable or most reasonable values of 4021, u(Ar), and a(Kr) on different carbon and silica substrates at 77 K, and to report the corresponding BET limits. The substrates examined
(1) Brunauer, S.; Emmett, P. H.; Teller, E. J.Am. Chem. SOC.1938, 60, 309. (2) Thomy, A.; Duval, X.; Regnier, J. Surf. Sci. Rep. 1981, 1 , 1. (3) Thomy, A.; Duval, X. J. Chim.Phys. 1970, 67, 286. (4) Rouquerol, J.; Partyka, S.; Rouquerol, F. J. Chem. SOC.Faraday Trans. 1 1977, 73, 306.
(14) Ismail, I. M. K. Carbon 1990,28, 423. (15) Fernandez-Colinas, J.; Denoyel, R.; Grillet, Y.; Rouquerol, F.; Rouquerol, J. Langmuir 1989,5, 1205. (16) Deitz, V. R.; Berlin, E. J. Colloid Interface Sci. 1973, 44, 57. (17) Singleton, J. H.; Halsey, G. D., Jr. J.Phys. Chem. 1954,58, 330. (18) McClellan, A. L.; Harnsberger, H. F. J. Colloid Interface Sci. 1967, 23, 577. (19) Smith, W. R.; Ford, D. G. J. Phys. Chem. 1965,69, 3587. (20) Razouk, R. I.; EIGobeily, M. A. J. Phys. Chem. 1950, 54, 1087. (21) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: New York, 1982; p 76. (22) Walker, P. L., Jr.; Foresti, R. J.; Wright, C. C. Ind. Eng. Chem. 1953,45, 1703. (23) Aristov, B. G.; Kiselev, A. V. Russ. J.Phys. Chem. (Engl. Traml.) 1963.37.1359. .- - -, .. , - - -. . (24) Berezkina, Yu. F.; Dubinin, M. M.; Sarakhov, A. I. Akad. Nauk SSSR, Ser. Khim. 1969, 2653; Bull. Akad. Sei. USSR, Diu. Chem. Sci. (Engl. Transl.) 1969, 2495. (25) Beebe, R. A.; Beckwith, J. B.; Honig, J. M. J. Am. Chem. SOC. 1945, 67, 1554.
(5) Grillet, Y.; Rouquerol, F.; Rouquerol, J. J. Colloid Interface Sci. 1979, 70, 239.
( 6 ) Larher, Y. J. Chem. Phys. 1978, 68, 2257. (7) Kjems, J. K.; Passel, L.; Taub, H.; Dash, J. G. Phys. Rev. Lett. 1974, 32, 724. (8) Chinn, M. D.; Fain, S. C., Jr. Phys. Rev. Lett. 1977, 39, 146. (9) Frenkel, D.; McTague, J. P. Annu. Rev. Phys. Chem. 1980,31,491. (10) Diehl, R. D.; Fain, S. C., Jr. Phys. Rev. B: Condens. Matter 1982, 26, 4785. (11) Stephens, P. W.; Heiney, P. A.; Birgeneau, R. J.; Horn, P. M.; Moncton, D. E.; Brown, G. S. Phys. Rev. E Condens. Matter 1984,29, 3512. (12) Fain, S. C., Jr. Carbon 1987, 25, 19. (13) Bojan, M. J.; Steele, W. A. Langmuir 1987, 3, 116.
0743-7463/92/2408-0360$03.00/0
0 1992 American Chemical Society
Letters
Langmuir, Vol. 8,No. 2,1992 361 Table I. Cross-Sectional Areas of Oxygen and Argon on Different Carbons and Silicas at 76.6 K sample
nitrogen BET area: m2/g
O(Ar),
U(O2),
nm2/mol BET limit nm2/atom BET limit* Ungraphitized Carbons (UGC) 7.32 0.145 0.02-0.28 0.161 0.02-0.28 N990 8.82 0.149 0.02-0.28 0.158 0.02-0.25 ST MT 25.55 0.148 0.02-0.28 N762 0.159 0.02-0.25 37.02 0.145 0.03-0.27 0.160 0.02-0.28 N550 73.2 0.143 0.02-0.28 0.158 0.02-0.25 v3 80.4 0.145 0.02-0.28 0.159 0.02-0.28 N330 average ~ ( 0 2=)0.146 average u(Ar) = 0.159 Graphitized Carbons (GC) 2.01c 0.145 SP-1 0.01-0.10 0.136 0.01-0.1 59.3c 0.148 V3G 0.01-0.10 0.137 0.01-0.1 74.1c 0.148 0.01-0.10 0.138 0.01-0.1 GCC 83.3' 0.148 Graphon 0.01-0.10 0.139 0.01-0.1 average ~ ( 0 20.147 ) average u(Ar) = 0.138 Activated Graphitized Carbons (AGC) GCC, 9.7% BO 114.2 0.148 0.001-0.10 0.153 0-0.1 GCC, 19.8% BO 120.2 0.147 0.01-0.10 0.154 0-0.1 GCC, 30.0% BO 119.2 0.149 0.01-0.10 0.156 0-0.1 GCC, 50.0% BO 113.8 0.149 0.01-0.10 0.158 04.1 average ~ ( 0 2 =) 0.148 average u(Ar) = 0.155 Fumed Silica Aerosil-50 73.3 0.163 0.05-0.30 0.183 0.04-0.3 Aerosil-150 131.6 0.168 0.05-0.30 0.189 0.04-0.3 Aerosil-200 184.3 0.166 0.05-0.30 0.187 0.04-0.3 average 4 0 2 ) = 0.166 average a(Ar) = 0.186 Based on u(N2) = 0.162 nm2/mol.* Using PO of supercooled Ar. Based on a(N2) = 0.157 nmZ/mol.
here have been used extensively in this laboratory as candidates for deposition of pyrolytic carbon26and silica27 on their surfaces. At some point during the investigation, there was a need to know precisely the a(X) of these adsorbates, especially when the surface characteristics change, for example,from non-graphitic to graphitic after the deposition of pyrolytic carbon,26or from graphitic or non-graphitic to a silica-coated surface after the deposition of Si02.27
Experimental Section The adsorption isotherms of N2, 0 2 , Ar, and Kr at 76.6 K were obtained by a Digisorb2600 volumetric apparatus manufactured by the Micromeritics Instrument Co., Atlanta, GA. The samples were initially outgassed at 573 K for 24 h and then flushed with He. This process was repeated twice at 3 h evacuation with intermediate adsorption runs of N2 at 76.6 K. By the end of this treatment, the surface of substrates, especially the ungraphitized and activated carbons, became stable and clean; they gave reproducible results. For each carbon, the same sample was used for the adsorption of Nz,02,or Ar at random. However, to obtain the Kr isotherms, separate smaller samples (50-80 mg) were examined. The number of experimental data points on each isotherm varied between 40 and 85; they covered a relative pressure range of 0.0014.95, depending on the adsorbate/ adsorbent system. Since the time taken for a single isotherm was 20 h to 8 days, the saturation pressure of adsorbate was updated every 6 h. Data analyses were performed using, when appropriate, the saturation pressure of liquid Nz, liquid 02,solid Kr, or supercooled Ar. It is admitted that since the adsorption temperature (76.6 K)is lower than the triple point of Ar (83.8 K), the saturation pressure of solid rather than supercooled Ar should have been used in the analysis. However, there are three main reasons for adopting the supercooled state of Ar. First, since the development of the BET theory, most investigators have used the supercooled state;1J5~22-24~2*33 a direct comparison (26) Ismail, I. M. K.; Rose, M. M.; Mahowald, M. A. Carbon 1991,29, 575. (27) Hoffman, W. P.; Phan, H. T. In Fundamental Issues in Control of Carbon Gasification Reactiuity; Lahaye and Ehrburger,Eds.; Kluwer Academic Publishing: The Netherlands, 1990. (28) Pace, E. L.; Siebert, A. R. J. Phys. Chem. 1960,64, 961. (29) Brown, C. E.; Hall, P. G. Trans. Faraday SOC.1971,67, 3558.
between their work and the present study would only be possible with the same assumption. Second, most of the currently available automated adsorption instruments are using PO of supercooled Ar in their operating software; it is convenient for the users to simply change the value of a(Ar) at the end of calculations (whenever needed) instead of changing the entire isotherm and computations from supercooledto solid Ar. Third, whether PO of solid or supercooledAr is chosen for the calibration, the correspondingproduct of u(Ar) and V,yields the same surface area regardless of the chosen value of a(Ar) and the actual state of Ar in the adsorbed layer. The samples examined and their nitrogen BET surface areas are listed in the first and second columns of Table I. The ranges of PIP" were 0.05-0.21 for graphitized carbons and 0.024.28 for the other materials. The table has four groups of materiale: the first includes six ungraphitized carbon blacks, UGC, the second includes four graphitized carbons, GC (three carbon blacks and natural SP-1graphite), the third includes four activated graphitized Columbian carbon (GCC),AGC, and the fourth has three Aerosils (fumed silica supplied by Degussa Co.). The AGC samples were prepared by oxidation of the original GCC in air at 773 K to different levels of burn-off (BO). This process has developedmicroporosity.For example,a GCC samplewith 19.8% BO has ita micropore surface area accounting for 39.1% of the total BET area.%
Results The third column lists values of a(O2) and the corresponding P/P"ranges for the BET best fit. The range is 0.014.1 for GC and AGC samples and is higher for the other materials, 0.02-0.28 for UGC and 0.05-0.30for Aerosils. The limits of best fit were based on the highest correlation coefficient computed with each r u n at different P / P ranges. Changing these limits has little effect on monolayer volumes. For example, when the upper PIP" limit was extended from 0.1 to 0.3,V , of GC increased by 0.5-1.0% in 02 but decreased by 1-4% in Ar. This means (30) Payne, D. A.; Sing,K. S. W.; Turk, D. H. J. Colloid Interface Sci.
1973, 43, 287.
(31) Rouquerol, J.; Rouquerol, F.; Peres, C.; Grillet, Y.;Boudellal, M. Characterization of Porous Solids: Societyof Chemical Industrv: Letchwork, U.K., 1979; p 107. (32)Ali, S.; McEnaney, B. J. Colloid. Interface Sci. 1984, 107, 355. (33) McEnaney, B. Carbon 1987,25, 69.
Letters
362 Langmuir, Vol. 8, No. 2, 1992 Table 11. Apparent Cross-Sectional Area of Kra on Carbons at 76.6 K Using Po of Solid Kr relative pressure range sample
0.02-0.1
0.02-0.2
- 1 ,23
b-
Average value
0.024.3
ungraphitized carbons
N990
ST MT N762 N550 v3 N330 average u(Kr) graphitized carbonsb Aerosil-50, -150,-2W6
0.200 0.190 0.191 0.194 0.200 0.201 0.196
NIA N/A
0.209 0.199 0.203 0.203 0.210 0.210 0.206 0.2146 0.2706
0.226 0.211 0.222 0.223 0.224 0.222 0.221
NIA NIA
' U
x
.I9
-
X
I I I
-
I I
,
a Values of u(Kr)are expressed in nmzlatom. 6 From ref 14between relative pressures of 0.05 and 0.21.
that by extending the BET limits, the computed value of ~ ( 0 2is ) lowered by 1% from 0.147 and 0.145 nm2/molecule, different size distribution and, hence, yield a slightly but a(Ar) is raised by as much as 4% from 0.138 to 0.143 different V , than the corresponding larger samples used for GC or from 0.155 to 0.162 nm2/atom for AGC. The for N2, 02,and Ar analysis. Earlier tests revealed that important point in the table is that the average value of changing the sample caused V , to fluctuate by fl-4%. 4 0 2 ) on the three categories of carbons is the same; 0.147 Based on these two reasons, the maximum error in f 0,001 nm2/mol. On Aerosils, 4 0 2 ) is higher, 0.166 nm2/ estimating u(Kr), from one sample to another, could be as mol. high as 8 % ;this explains the scatter in a(Kr)values shown With argon (fourth column), the average value of a(Ar) in Figure 1and listed in Table 11. In spite of this scatter, on silica, 0.186 nm2/atom, is also higher than on all other the average values of a(Kr) can be used for other Kr/ carbons. However, a(Ar) is influenced by the type of ungraphitized carbon systems at 77 K to estimate the carbon surface under consideration, 0.138 f 0.002 nm2/ surface area. First, the BET analysis is performed at any atom on GC, and averages to 0.157 f 0.002 nm2/atom on selected relative pressure range to yield an "apparentn UGC and AGC. Unlike 02,the packing density of Ar is V,. Then, with the corresponding "apparent" value of dependent on surface characteristics. The value of 0.138 a(Kr) taken from the figure, the true surface area can be nm2/atom is in excellent agreement with values that have computed. However, since with GC the apparent value been reported by previous i n v e s t i g a t o r ~ ~ Jand ~ - ~ ~ - ~of ~ ~cr(Kr) ~ ~ is0.214 nm2/atom,14the same value may arbitrarily observed experimentally using neutron ~ c a t t e r i n g .It~ is ~ be recommended (for the sake of simplicity) for UGC and interesting to note that the value of 0.157 nm2/atom on AGC, provided that the BET analyses are executed at a UGC and AGC happened to be exactly equivalent to the P/PO range of 0.02-0.25, as illustrated with the dotted line area of three graphitic hexagons. This fortuity does not in Figure 1. As mentioned earlier, this analysis is based necessarily mean the adsorbed Ar atoms are in registry on PO of solid Kr. with the carbon surface. With Kr adsorption on UGC, the BET plots were never Discussion linear at any PlPO range; instead, they showed a curvature through the entire range, 0.01-0.35. Thus, for the Kr/ u(X) on Graphitized Carbons. It is now established, UGC system, the BET theory deviates. The plots were from neutron diffraction,"^^^ X-ray ~ c a t t e r i n gand , ~ low~~~ slightly convexto the pressure axis; as the relative pressure energy electron diffraction experiments38-39 that on graphincreased, the slope increased but the intercept decreased. itized carbons, a(N2) = a(Kr) = area of three graphitic Since the increase in slope exceeded the decrease in hexagons = 0.157 nm2/adsorbed species. In oxygen, the intercept, the net effect was a lowering in the apparent monolayer density varies with coverage and adsorption value of V, with increasing Kr relative pressure (Table t e m p e r a t ~ r e . For ~ ~ oxygen the 2-D phase diagram on 11). The calculated "apparent" value of a(Kr) on each graphitized carbons is complicated, it has 9 monolayer carbon increased by extending the range from 0.02-0.1 to phases and 6 triple points. At 10-50 K, the density is 0.02-0.2 and finally to 0.02-0.3. Dependence of the 0.065-0.085 molecule/A2,which corresponds to a(Od values "apparent" average a(Kr) on PIP" is displayed in Figure of 0.154-0.118 nm2/molecule.40 From the computer sim1,which can be utilized to select the proper value of a(Kr) ulation at 30-70 K,4l a condensed layer is formed first on according to the upper relative pressure range chosen for carbons with a density of 0.074molecule/A2which can be the BET analysis. compressed to 0.112 molecule/A2. A t liquid nitrogen temIt is noted in the table that with each column, a(Kr) is perature, the density may be taken as 0.074 molecule/A2, scattered around an average value. The scatter is attributed to two main reasons: weight of the samples and (35) Eckert, J.; Ellenson, W . D.; Hastings, J. B.; Passel, L. Phys. Rev. inconsistency of the carbon batch. First, to perform the Lett. 1979, 43, 1329. (36)Horn, P. M.; Birgeneau, R. J.; Heiney, P.; Hammonds, E. M. Phys. Kr analysis in a reasonable length of time, the weight of Rev. Lett. 1978, 41,96i. the samples whose surface areas are high had to be reduced (37)Specht, E. D.; Sutton, M.;Birgeneau, R. J.; Moncton, D. E.; Hom, to 50-80 mg. A fluctuation of f l mg in sample weight P. M.Phys. Reo. €3: Condens. Matter 1984,30, 1589. yields as high as 2 7% error in V,. Second, each lot of the (38) Diehl, R. D.; Samuel, M. F.; Fain, S. C., Jr. Phys. Reu. Lett. 1982, 48, 177. UGC has some range of particle size distribution. The (39)Diehl, R. D.; Fain, S. C., Jr. J. Phys. Chem. 1982, 77, 5065. small samples used for Kr analysis could have a slightly (40) Toney, M. F.; Fain, S. C., Jr. Phys. Reu. E Condens.Matter 1987, (34)Taub, H.; Carneiro, K.; Kjems, J. K.; Passell, L.; McTague, J. P. Phys. Reu. E S o l d State 1977,16, 4551.
36, 1248. (41) Bhethanabotla, V. R.;Steele, W.A.Phys.Reu.B: Condens.Matter 1990,41, 9480.
Letters
Langmuir,Vol. 8, No. 2, 1992 363 Table 111. ComDarison between Theoretical and Exmrimental Cross-sectional Areas ~~
theoretical
~
~
~
0.153
GCC 0.162
experimental a(X);nm2/species UGCd AGCe silicas 0.162 0.162 0.162
0.138 0.128
0.135
0.138
0.159
0.155
0.186
1.348
Kr (liquid) Kr (solid)
0.150 0.143
0.155
0.18w 0.21d
0.214
N/A NIA
0.234 0.270
1.300 1.262
02 (liquid)
0.136 0.121
0.136
0.148
0.146
0.148
0.166
1.121
adsorbate NZ(liquid) Nz (solid)
dtda
o
0.162 0.138
Ar (liquid) Ar (solid)
02 (solid)
w
~~
silicas/GC
1.OOO
a d t d = 1.091 ( M / O N ) ~nm2/adsorbed /~: mecies. d t 2 ) = 0.06354(TJPc)2/3; nm2/adsorbedspecies. Graphitized carbons. Ungraphitized carbons. e Activated graphitized carbons. f From ref 14.-
which yields ~ ( 0 2 = ) 0.135 nm2/mo1ecule.4l This value is slightly lower than the "apparent" value of 0.147nm2/mol listed in Table I. The discrepancy between the two values (-8%) indicates that the BET equation is not perfectly adequate to define the true values of Vm for 02 adsorption on GC. In the case of Ar, there is some controversy in the literature. The reported values of a(Ar) in nm2/atom are 0.132 from volumetric adsorption measurements: 0.125 from adsorption calorimetric measurements: and 0.134 from neutron ~ c a t t e r i n g . ~ From ~ the recent work of DAmico et al.42at 44-50 K and 0.81 monolayer coverage, the computed value is 0.19. It is possible that with increasing adsorption temperature to 77 K and at Vm = 1, a(Ar) decreases to the neighborhood of -0.14 nm2/ atom. Apart from D'Amico's work, there is an overall good agreement between the experimental BET value obtained here (0.138nm2/atom) and most of the other values either reported in the literature on GC or computed theoretically using the bulk properties of supercooled Ar. Therefore, the BET model is presumably adequate enough to define a true value of V, for Ar on GC. u(X) on Ungraphitized a n d Activated Graphitized Carbons. This class of carbons has a heterogeneous surface composed of basal planes and active sites whose population is 5-15 % of the totalsurface depending on the sample and its level of BO. From Table I, each average value of ~ ( 0 2 or ) a(Ar) on the two classes of carbon is practically the same regardless of the extent of surface heterogeneity. The average values are 0.147 nm2/02 molecule and 0.157f 0.002nm2/Ar atom. This finding is somewhat surprising because the active surface area of the UGC and, hence, the degree of heterogeneity were found to increase with increasing the starting surface area of the carbon.26 It is possible that the contribution of active surface area to the overall isotherm takes place at the early stages of adsorption, i.e., at very low P/P"values (
