Langmuir 1993,9, 3259-3263
3259
Titration for Basal Plane versus Edge Plane Surface on Graphitic Carbons by Adsorption S.G . Chen and R.T.Yang’ Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, New York 14260 Received June 7, 1993. In Final Form: September 7,1993@
By using three model graphitic carbon materials, a simple technique for measuring the basal plane surface area and edge plane surface area of graphitic carbon materials is proposed. The technique involves adsorption of COZ,which has a high selectivity toward edge plane sites, at 1atm and room temperature. It can also be used to measure active sites for carbon gasification reactions because the edge surface area is responsible for gasification whereas the basal plane has only a negligible contribution. The proposed method could be extended to other crystalline materials and nongraphitic carbon materials. For the purpose of understanding the different adsorption abilities of SO2 and COz on different faces of graphite, a semiempirical molecular orbital (MO)calculation was made. The results of MO calculations show that it can be used to qualitatively predict the adsorption capacity.
Introduction It has been well demonstrated by the use of electron microscopy and optical microscopy (see, for example, refs 1-3) that vacancies and the edge sites on carbon surfaces are active sites and that only these sites contribute to gasification reactions; the defect-free basal plane has only negligible reactivity and thus makes no or little contribution to the gasification reactions. Like the importance of active sites to the activity of the catalyst in heterogeneous catalysis, determination of the active sites or active surface area of carbon materials is of critical importance to the elucidation of the mechanisms as well as the kinetics of the gasification reactions. It is therefore important to have an experimental technique by which one can determine the active sites or active surface area of carbon, and indeed the search for such a technique has long been a subject of research (e.g., refs 4-6). Since the ground-breaking work of Laine et al.,’ voluminous research articles have been devoted to this subject. In the past three decades, many experimental approaches have been proposed and some significant progress toward the characterization of active sites (or active surface area) on the carbon surface has been made. To mention a few, some groups have used transient kinetics (TK)&I4and
* To whom correspondence should be addressed. Abstract published in Advance ACSAbstracts, October 15,1993. (1)Thomas, J. M. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Ed.; Dekker: New York, 1965;Vol. 1, p 287. (2)Yang, R. T. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Ed.; Dekker: New York, 19&1; Vol. 19,p 163. (3)Baker, R. T. K. In Carbon and Coal Gasification Science and Technology; Figuereido, J. L., Moulijn, J. A,, E&.; NATO AS1 Series E; Martinue Nijhoff: Hingham, MA, 1986, No. 105, p 231. (4)Boehm, H. P. In Aduances in Catalysis; Eley, D. D., Selwood, P. W., Weisz, P. B. Eda.;Academic Press: New York, 1966;Vol. 16,p 179. (5)Puri, R.R.In Chemistry andPhysics of Carbon; Walker, P. L., Jr., Ed.; Dekker: New York, 1970; Vol. 6,p 191. (6)W e e r , P. L.,Jr.; Taylor, R. L.; Ranish, J. M. Carbon 1991,29,411. (7)Lame, N.R.;Vastola, F. J.; Walker, P. L., Jr. J . Chem. Phys. 1963, 67,2030. (8)Freund, H. Fuel 1986,65,63. (9)Lizzio, A. A.; Jiang, H.; Radovic, L. R. Carbon 1990,28,7. (10)Radovic, L. R.;Jiang, H.; Lizzio, A. A. Energy Fuels 1991,668. (11)Adschiri, T.; Nozaki, T.; Furusawa, T.; Zhu, Z.AIChEJ. 1991,37, 897. (12)Kapteijn, F.; Meijer, R.; Moulijn, J. A. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1991,36 (3),906. (13)Meijer, R. Kinetics and Mechanism of the Alkali-Catalyzed Gasification of Carbon. Ph.D. Dissertation, University of Amsterdam, The Netherlands, 1992. (14)Kapteijn, F.; Meijer, R.; Moulijn, J. A. Energy Fuels 1992,6,494.
others have used TPD9J0J”26to measure or characterize the active sites and surface areas on carbons. Most of these techniques are chemical rather than physical in nature,6p2’and some of them are actually in situ, i.e., carried out under gasification conditions. In this work, a selective adsorption technique which makes use of the weak chemical interaction, has been used to measure the edge surface area of graphitic carbons. The Brunauer-Emmett-Teller (BET) method of N2 adsorption has been widely used in the measurement of total surface areas of carbon samples. When the carbon sample has very small pores which are not easily accessible The surfaces to N2, COPcan be used instead of N2.2a31 of a graphitic carbon consist of two types: the basal plane surface and the edge plane surface. Because of the different chemical situations, carbon atoms on these two types of surfaces are different energetically and have very different abilities for interactions with a gas adsorptive. It is thus possible to determine these two types of surfaces of graphite by using a selective adsorptive which interacts differentlywith them. For example, N2 adsorption at very low relative pressures (e.g., 1o-S)does exhibit a selectivity for active (strong) sites on carbons.6 But because the NZ adsorption is physical adsorption and shows very little selectivity, very low relative partial pressures are required.
8
(15)Kelemen, S.R.;Freund, H. Carbon 1985,23,619. (16) Marchon, B.; Carrazza, J.; Heinemann, H.; Somorjai, G. Carbon 1988,26,507. (17)Marchon, B.; Tysoe,W. T.; Carrazza,J.; Heinemann, H.; Somorjai, G.J. Phys. Chem. 1988,92,5744. (18)Kyotani, T.; Zhang, 2.; Hayashi, S.; Tomita. Energy Fuels 1988, 6,136. (19)Lizzio, A. A.; Piotrowski, A.; Radovic, L. R. Fuel 1988,67,1691. (20)Zhang, Z.;Kyotani, T.; Tomita, A. Energy Fuels 1988,6,679. (21)Zhu, 2.; Furusawa, T.; Adschiri, T.; Nozaki, T. Prepr. Pap.-Am. Diu. Fuel Chem. 1989,34(I), 87. Chem. SOC., (22)Hall, P. J.; Calo, J. M.; Teng, H.; Suuberg, E. M.; May, J. A,; Lay, W. D. Prepr. Pap-Am. Chem. SOC.,Diu.Fuel Chem. 1989,34 (l), 112. (23)Du,2.;Sarofim, A. F.; Longwell, J. P. Energy Fuels 1990,4,296. (24)Htittinger, K. L.; Nill, J. S. Carbon 1990,28,457. (25)McEnaney, B. In Fundamental Issues in Control of Carbon Gasification Reactiuity. Lahaye, J., Ehrburger, P., Eds.; NATO AS1 Series, Series E Applied Sciences; Kluwer: Boston, 1991;No. 192. (26)Pan, 2. J.; Yang, R. T. Ind. Eng. Chem. Res. 1992,31, 2675. (27)Radovic, L.R.;Walker, P. L., Jr.; Jenkins, R. G. Fuel 1983,62, 849. (28)Walker, P. L., Jr.; Kini, K. A. Fuel 1965,44,453. (29)Marsh, H.; Siemieniewska, T. Fuel 1965,44,355. (30)Anderson, R.B.; Bayer, J.; Hofer, L. J. Fuel 1965,44,443. (31)Walker, P. L., Jr.; Patel, R. L. Fuel 1970,49, 91.
Q743-7463/93/24Q9-3259$Q4.00/00 1993 American Chemical Society
3260 Langmuir, Vol. 9,No. 11,1993
Chen and Yang
Table I. Experimental and Calculated Results for Three Graphite Samples.
Micro 450 Micro 4023
0.270 1.357
18.90 23.64
2.05 0.50
0.05 0.045
18.01 20.00
0.89 3.64
0.67 1.36
8.946 11.37
0 d = average diameter of the graphite flakes. h = average thickness of the graphite flakes. AI, A2 = basal plane and edge plane surface areas, respectively. r = rate of gasification in 1 atm of 02 at 600 OC. Uptake amounts are at 22 'C and 1 atm.
Our recent ~ 0 r k , 3however, ~ * ~ ~ indicates that C02 is highly selective for bonding with these two surfaces, and because the adsorption is far from being saturated on both surfaces, the coverages are very different. In contrast to C02, SO2 molecules adsorb strongly on both surfaces, and both are near saturation even at a low gas-phase partial pressure of s02.
=
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Experimental Section The apparatus used for measuring the amounts of adsorption and carbon gasification rates was a thermal gravimetric analysis (TGA) system which had the desired sensitivity (0.01mg). The conditions for adsorption were as follows: the partial pressures of carbon dioxide and sulfur dioxide were varied between 0.33 and 1.00atm (pure COz or pure Sod, and the temperatures were 22 and 60OC. Before measuring the uptake, the graphite samples were degassedat 120"Cin He for severalhours in order toremove the physically adsorbed gases. The gasification reactions were carried out at 600 OC in 1 atm of 0 2 . As in the adsorption experiments the carbon samples used for gasification were also subjected to degassing at 120OC. COz and SO2 used in adsorption were of high purity (99.99% minimum), the 02 used was extra dry (99.6% minimum), and all were used without further treatment. Because the reaction rate changed to some extent at both very low and very high burnoff levels, only reaction rates between 10% and 15% burnoff were used. The average particle size (diameter) of the graphite flakes was based on the number average, and at least 200 particles were included for averaging for each sample. The sizes were measured by viewing under a scanning electron microscope (SEM). The samples were diskshaped. The BET surface areas were measured by the standard N2 adsorption method (using a Quantasorb Analyzer) because the carbon samples used in this study were nonporous. In principle, only two graphite samples with well-defined geometry are needed in order to determine the two characteristic constants q1 and qi (see below). In this work, three samples were used. They were SP-1,Micro 450,and Micro 4023. The SP-1 graphite was obtained from Union Carbide, and the Micro 450 and Micro 4023 graphite samples were from Asbury Graphite Mills. All three samples were highly graphitic, of high purities, and were of uniform disk-shaped sizes. The impurities for these three samples were 4 0 0 ppm, 0.95%,and 0.25% ,respectively.
SP-1 I - '
0.0
0.2
0.4
'
I - ,
0.6
I
0.8
'
1.0
Partial pressure of CO, (atm)
Figure 1. Adsorption isotherms of COz on different graphite samples (see Table I).
of SP-1is about 33 pm. It is close to the value measured by Walker et al.,34which was 30 pm. Column 4 is the average thickness of graphite flakes which is calculated by eq 1. It can also be directly measured from the SEM pictures. But as only very few flakes stand on the edge in an upright position, it is difficult to collect a good number of such flakes. Thus, following the work of Walker et al.,34we used the BET total surface area and the average diameter to calculate the thickness h. Assuming that N2 adsorption has no selectivity between the edge plane surface and basal plane surface, and because the samples are graphitic carbons with no pores, then if all the graphite flakes can be taken as disks, the thickness h can be calculated from the geometry by the following equation:
1 h = 0.9091 (A, - 1.8182/d)
Column 1 is the steady-state gasification rates (at 1015%burnoff) of graphite samples at 600 OC and in 1atm of 0 2 . Column 2 is the BET total surface area measured by NZadsorption except for Micro 4023 whose BET total surface area was measured by the manufacturer. Column 3 shows the average diameter of the samples. It is obtained by measuring the sizes of over 200 particles by SEM. The graphite flakes are taken as a disk. The average diameter
Here At and d are the BET total surface area and average disk diameter, respectively. The results are listed in column 4. Knowing the average particle diameter d and thickness h and the density (2.2g/cm3was used in this study) of the samples, the edge plane surface area A2 and basal plane surface area AI can be simply calculated from the geometry of the flakes. These values are listed in columns 5 and 6. Columns 7 and 8 are, respectively, the uptake amounts of C02 and SO2 at 1 atm and 22 OC. The adsorption isotherms of C02 at two different temperatures are shown in Figures 1 and 2. The data at other partial pressures and temperatures are not listed in Table I because they are not used in the calculation of q1 and 92. As shown in Table I, for SP-1the uptake amount of SO2 is almost 20 times as large as that of CO2, indicating that the adsorption of SO2 is much stronger than that of COZ. Calculations show that at 1 atm and 22 "C, each nm2 of the surface of graphite can adsorb about five SO2 molecules. It is nearly monolayer adsorption or may be already in multilayer adsorption.
(32)Kikkinides, E. S.; Yang, R. T. Ind. Eng. Chem. Res. 1991, 30, 1981. ( 3 3 ) Kikkinides, E. S.; Yang, R. T. Znd. Eng. Chem. Res., in press.
(34) Walker, P. L., Jr.; Austin, L. G.; Tietjen, J. J. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Ed.; Marcel Dekker: New York, 1965; Vol. 1, p 327.
Results and Discussion It is desirable to have samples with a wide range of edge surface areas. As will be shown later, the edge surface areas of the three samples used here vary in the range 0.054-3.64m2/g,and the correspondingBET total surface areas are in the range 1.95-23.64m2/g. The experimental and some of the calculated results are summarized in Table 1.
Basal versus Edge Plane Surface on Graphitic Carbons 1.or
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Langmuir, Vol. 9, No. 11, 1993 3261
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.-v O
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.
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Partial pressure of CO2(atm)
Total surface area(m2/g-esrbon)
Figure 2. Adsorption isotherms of COZon different graphite samples (see Table I).
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Figure 4. Relationship between gasification rate (in 1 atm of 02 at 600 O C , expressed in (g/g)/h) and the total surface area. IS, , , I , , . I I
fi
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2
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Figure 3. Correlation between uptake amounta at 22 O C and 1 atm and total surface area.
Figure 6. Relationship between gasification rate (in 1 of atm 02 at 600 O C , expressed in (g/g)h) and the edge surface area.
Figure 3 is the plot of the uptake amount vs BET total surface area for C02 and SO2 at 1 atm and 22 OC. It is interestingto note that the uptake of COz does not correlate well with the BET total surface area whereas that of SO2 does. This is a result of the fact that under these conditions C02 shows a high selectivity for the edge plane over the basal plane and because the adsorption on both surfaces is far below being saturated and, consequently, the C02 coverage on the edge plane surface is much larger than that on the basal plane, Although the edge plane surface area is small as compared with the basal plane surface area, neither is negligible. For S02, however, the result is very different. It shows almost no selectivity and is strongly adsorbed on both surfaces (the amount adsorbed on the basal plane is about equal to that on the edge plane; both are about 0.5 mg/m2). As aresult, its uptake amount is almost a linear function of the BET surface area. The above results provide the basis for selecting C02 as a probe molecule but not S02. The same plots for other partial pressures are similar to Figures 1and 2 and are not shown here. Figure 4 shows the relationship between the C02 gasification rate and total surface area for these three samples. It is seen that the gasification rate does not correlate well with the BET total surface area of the graphite. As the gasification rate is proportional to the active site density, the above results show that the graphite samples have various amounts of basal plane surface area which contribute little to the gasification rate. Figure 5 shows the correlation between the gasification rate and the edge surface area. In contrast to the BET total surface area, the edge plane surface area correlates well with the gasification rates, which indicates that the
edge surfacearea is responsiblefor the gasificationreaction. This is in agreement with the conclusion from earlier studies.s From the uptake amount of Con, we can calculate different strengths of adsorption of C02 on the two different planes by the following equations:
A, + A , = A ,
(3)
where q1 is the amount adsorbed per unit area of the basal plane, q 2 is that on the edge plane, qt is the total amount adsorbed on 1g of sample, and AI, A2, and At are the basal plane surface area, edge plane surface area, and BET total surface area per gram of carbon. Using the data listed in Table I and least-squares regression, we obtained the following values of q1 and q 2 for C02 adsorption on these two different surfaces at 22 *C and 1 atm. basal plane: q1 = 0.0254 mg/m2
(4)
edge plane: q2 = 0.233 mg/m2
(5)
This result shows that the adsorption of C02 at 1atm and 22 O C on the edge plane surfaceis about 9 times greater than that on the basal plane surface. It is a reasonable result because the carbon atoms on the edge plane have a free sp2 orbital, i.e., they are not chemically saturated. With the values of q1 and q 2 and by using eqs 2 and 3, one can determine the edge and basal plane surface areas of graphitic carbons by measuring only the uptake qt of C02 at 22 OC and 1 atm and the BET surface area At of
3262 Langmuir, Vol. 9,No. 11, 1993
Chen and Yang n
the same sample. The final equation is as follows: A, =
-
qt - QlA, - qt - 0.02544, 0.208 qz - 41
n
(6)
Here the units for qt are mg/g, and for At they are m2/g. Theoretically, the uptake amount of any adsorbate can be used to calculate the correspondingvalues of q1 and qz. But the purpose of determining q1 and qz is to use these values from well-defined samples to characterize the basal plane surface area A1 and edge plane surface area A:!of other sampleswhose geometries are unknown, and further to use the edge plane surface area to calculate the gasification rates. From eq 6, we fiid that the denominator is qz - 91. When qz is close to q1, any experimental errors in qz and q1 will be enlarged and transferred to A1 and Az. In order to gain sufficient accuracy of A1 and Az, it is necessary to use a selective adsorbate. Moreover, the adsorption conditions also have effects on the relative values of q1 and q2. The conditions of 1atm and room temperature are convenient. We found that COZ is a highly selective adsorbate and the uptakes of COZat 1atm and 22 "C yield satisfactory results. It should be noted that the above q1 and qz values are determined from graphite samples and may only be suitable for the graphitic carbon. For semigraphitic and nongraphitic carbon samples, the graphitic crystallites are smaller than those of the samples used here and the surfaces become more heterogeneous. These carbons will have a distribution of adsorption energies. It is, however, likely that both the edge sites and basal plane sites have their own distribution in adsorption energies, but because of the large difference between two types of surfaces, the two different distributions may not overlap or overlap little with each other in spite of the heterogeneity. It follows that the method proposed here may still be valid for the nongraphitic carbons. Extension of this method to nongraphitic carbons and even coal chars is in progress in our laboratory. It is interesting to compare the strengths of adsorption for CO2 and SOZ. From the uptakes listed in Table I, it can be seen that the adsorption of SO2 is much stronger than that of COZon the basal plane and is even stronger than that of COz on the edge plane. The qualitative reason is the interaction of lone pair electrons of SO2 with the a electrons of graphite.33 SemiempiricalMolecular OrbitalCalculations. In order to understand the reasons for different uptakes of the two gases and adsorption abilities of the two different planes, molecular orbital (MO) theory is used. Molecular orbital theory has been proven to be of utility in our understanding of chemical bonding. It has been used to study the chemisorptionof a variety of species on graphite surface^.^^,^^^^ However, the MO theory is rarely used for studying physical adsorption. Here we use MO theory to simply compare the relative adsorption strengths of the same species on two types of substrates and the relative adsorption strengths of different species on the same substrate. Varieties of programs and different methods of semiempirical MO calculation are available. Here we used the INDO (intermediate neglect of differential overlap) method which is modified from CNDO (complete neglect of differential overlap). The computer program was written (35)Bennett, A. L.;McCarroll, B.; Messmer, R. P. Phys. Rev. E 1971, 3,1397. (36)Hayns, M. R. Theor. Chim. Acta 1976,39,61. (37)Chen, J. P.; Yang, R. T. Surf. Sci. 1989,216,481. (38)Pan, Z.J.; Yang, R. T. J. Catal. 1990,123,206.
n H
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Figure 6. Two graphite substrates used in MO calculations, showing the equilibrium position from geometry optimization (see text for further explanations of the geometries).
Table 11. Results of Molecular Orbital Calculations for Total Energy Changes from the gas Phase and Equilibrium Position edge plane b a d plane energy equilibrium energy equilibrium change distance change distance adsorbate (kcal/mol) (A) (kcaVmo1) (A)
coz so2
163.3 203.2
1.504 1.713
27.1 148.8
2.01 1.69
by Rinaldi et al.39and was obtained from the Quantum ChemistryProgram Exchange, Indiana University. It was used without any modification. Electron-electron and nuclewnuclear interactions are included in MO calculations. This program contained geometry optimization to achieve the lowest total energy. In this work only a few parameters which are strongly affected by the interaction between adsorbate molecules and the substrate are optimized. In order to minimize the edge effects of the substrates all edge sites are filled by a hydrogen atom except the edge sites under study (see Figure 6B,right). Several different geometric arrangements of SO2 and COz with the graphite are tried, but only the favorite structures are shown in Figure 6. Table I1 summarizes the results of these calculations. It is clear from Table I1 that the COZadsorption on the edge plane is much stronger than that on the basal plane. For the basal plane the preferred structure is for COz and SO2 approaching the basal plane in a parallel orientation, as shown in Figure 6A, left. For the edge plane, the linear C02 molecule is perpendicular to the basal plane, with the carbon atom in COZforming a bond to the edge carbon atom. SO2 also approaches the edge carbon in an orientation perpendicular to the basal plane, with the S in front to bond to the edge carbon. The equilibrium structures on the edge plane are illustrated in Figure 6B. The results of molecular orbital calculations are summarized in Table I1and Figure 7. The total energy change from the gas phase to the equilibrium position (i.e., adsorbed state) is indicative of the adsorption strength. It is seen that, for COz, the total energy change on the edge plane is 6 times that on the basal plane. For SOZ,the change is only slightly stronger on the edge plane. This result is in agreement with the experimental fact that COZ (39)Rinaldi, D.; Hoggan, P. E.; Cartier, A. QCPE Program No. 684; Quantum Chemistry Program Exchange, Department of Chemistry, Indiana University, Bloomington, IN, 1990.
Basal versus Edge Plane Surface on Graphitic Carbons 150
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generally shorter than the actual values for physical adsorption. Also, temperatures and pressures are not taken into consideration in MO calculations. Although one needs to be cautious in discussion of the MO results, the results obtained in this work provide the correct qualitative features of the experimental facta and are thus encouraging. Graphite is chosen for this study. The approach of using physical adsorption or weak chemisorption to titrate different crystalline faces should be applicable to other crystalline materials.
02/Basal plane 02iEdge plane
0
1
2
3
4
5
6
~ i s t v l c e(A)
Figure 7. Potential energy as a function of internuclear distance from molecular orbital calculations. The geometries are shown in Figure 6.
has a strong preference for the edge plane whereas SO2 adsorbs on both planes with almost equal strengths. It should be noted that the absolute values of the energy change from the MO calculation are much higher than the actual bond energies or heats of adsorption. However, the relative values are generally correct, and are valuable for comparison purposes. The equilibrium positions calculated from the MO theory, i.e., about 1.5 A, are
Conclusion By using graphitic carbon as the model samples and C02 a~the probe molecule, a simple adsorption technique is used for characterizing the edge plane surface of graphitic carbons. This method may also be suitable for characterizing nongraphitic carbon samples and other crystalline materials. Molecular orbital theory has been used to interpret the different physical adsorption strengths of the same adsorbate on different crystal faces and different adsorbates on the same crystal face. The results of the MO calculation are encouraging. Acknowledgment. This research was supported by the NSF under Grant CTS-9120452.
