Langmuir 1999, 15, 1435-1441
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Nitrogen Adsorption Study of Surface Properties of Graphitized Carbon Blacks Michal Kruk, Zuojiang Li, and Mietek Jaroniec* Department of Chemistry, Kent State University, Kent, Ohio 44240
William R. Betz Supelco, Inc., Supelco Park, Bellefonte, Pennsylvania 16823 Received April 28, 1998. In Final Form: December 7, 1998 Surface and structural properties of commercially available Carbopack graphitized carbon blacks with specific surface areas from 6 to 225 m2/g were studied using nitrogen adsorption in a wide range of pressures. Specific surface areas, total pore volumes, and pore size distributions were evaluated. It was observed that the carbons with lower specific surface areas exhibited a higher degree of graphite crystallinity. Lowpressure adsorption data were carefully examined in order to obtain information about surface heterogeneity of the samples in terms of adsorption potential distributions, high-resolution θ-plots, and adsorption energy distributions. It was shown that the graphitized carbon blacks under study exhibited relatively high surface homogeneity, which increased with an increasing degree of graphite crystallinity. An examination of the θ-plots and adsorption potential distributions allowed us to draw a conclusion that the average adsorption energy decreased with a decrease in the degree of crystallinity. It was suggested that positions of monolayer formation peaks and two-dimensional fluid-solid phase transition peaks on adsorption potential distributions provide information about the degree of crystallinity and surface homogeneity of graphitized carbon blacks.
Introduction It is well-known that nitrogen adsorption at 77 K is a convenient and powerful method to evaluate specific surface areas, pore volumes, and pore size distributions of porous solids.1-5 However, a possibility of analysis of surface properties on the basis of low-pressure nitrogen adsorption data is not fully recognized, despite the fact that many suitable methods of data analysis were developed and carefully examined in the past (see refs 1 and 6 and references therein). The application of the nitrogen adsorption technique for surface characterization has recently become much easier than before because of the commercial availability of adsorption instruments capable of performing measurements starting from relative pressures as low as 10-7. It was recently demonstrated that various kinds of porous materials, such as nongraphitized carbon blacks,7 graphitized carbon blacks,8 and porous inorganic oxides (silica, alumina, titania, and zirconia)9 exhibit markedly different low-pressure nitrogen adsorption properties. Moreover, low-pressure adsorption (1) Jaroniec, M.; Madey, R. Physical Adsorption on Heterogeneous Solids; Elsevier: Amsterdam, The Netherlands, 1988. (2) Rudzinski, W.; Everett, D. Adsorption of Gases on Heterogeneous Solid Surfaces; Academic Press: London, 1991. (3) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1982. (4) 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. (5) Roquerol, J.; Avnir, D.; Fairbridge, C. W.; Everett, D. H.; Hayness, J. H.; Pernicone, N.; Ramsay, J. D. F.; Sing, K. S. W.; Unger, K. K. Pure Appl. Chem. 1994, 66, 1739. (6) Szombathely, M., v.; Brauer, P.; Jaroniec, M. J. Comput. Chem. 1992, 13, 17. (7) Kruk, M.; Jaroniec, M.; Bereznitski, Y. J. Colloid Interface Sci. 1996, 182, 282. (8) Jaroniec, M.; Kruk, M.; Choma, J. In Characterization of Porous Solids IV; McEnaney, B., Mays, T. J., Rouquerol, J., Rodriguez-Reinoso, F., Sing, K. S. W., Unger, K. K., Eds.; Royal Society of Chemistry: London, 1997; p 163. (9) Jaroniec, C. P.; Jaroniec, M.; Kruk, M. J. Chromatogr. A 1998, 797, 93.
on mesoporous silicas with physically coated and/or chemically bonded organic ligands was shown to be strongly dependent on the nature of these groups and their surface coverage.10-12 The latter findings opened new opportunities in the characterization of modified porous oxides, providing the means for identification of certain surface functional groups and qualitative assessment of the surface coverage of immobilized ligands on the basis of adsorption data.10-12 Because of the distinct features of low-pressure nitrogen adsorption on different types of surfaces,7-12 adsorption methods are very promising in studies of many adsorbents, such as various porous carbons. Graphitized carbon blacks appear to be especially interesting because of their surface and structural homogeneity, which manifests itself in the large size of graphite-like domains and their significant stacking height, low amount of functional groups, and imperfections, such as twists, nonaromatic links, and carbon vacancies.13 Because of these structural features, surfaces of highly graphitized carbon blacks exhibit a remarkably narrow range of interaction energies with respect to various adsorbates. This surface homogeneity leads to unique nitrogen adsorption properties, which are reflected by the presence of distinct layering transitions (i.e., formation of consecutive adsorbed layers)3,4 and twodimensional disordered fluid-ordered solid phase transition.14 One can expect that the degree of structural perfection of graphitized carbons (later referred to as the degree of graphitization) has an influence on these (10) Kruk, M.; Jaroniec, M.; Gilpin, R. K.; Zhou, Y. W. Langmuir 1997, 13, 545. (11) Jaroniec, C. P.; Gilpin, R. K.; Jaroniec, M. J. Phys. Chem. B 1997, 101, 6861. (12) Kruk, M.; Jaroniec, M. In Surfaces of Nanoparticles and Porous Materials; Schwarz, J. A., Contescu, C., Eds.; Marcel Dekker: New York, 1998; p 443. (13) Leon y Leon, D. C. A.; Radovic, L. R. Chem. Phys. Carbon 1994, 24, 213. (14) Chung, T. T.; Dash, J. G. Surf. Sci. 1977, 66, 559.
10.1021/la980493+ CCC: $18.00 © 1999 American Chemical Society Published on Web 01/14/1999
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transitions. X-ray diffraction provides useful information about the degree of graphitization,15 so it is interesting and potentially very useful to study a correlation between the former and the low-pressure nitrogen adsorption data. However, it needs to be kept in mind that because of many possible sources of structural and surface heterogeneity of porous carbons, a complete and unambiguous characterization of surfaces of these materials is difficult13 and would require the application of many other experimental techniques, which would be beyond the scope of this preliminary work. It also needs to be noted that the concept of surface heterogeneity and its importance in adsorption studies were extensively described in two recent monographs.1,2 In the current work, low-pressure nitrogen adsorption was used to study surface properties and porous structures of several commercially available graphitized carbon blacks (Carbopack F, C, Y, B, and X) with BET specific surface areas from 6 to 225 m2/g. The main emphasis was placed on the investigation of the relation between adsorption properties and the graphitization degree as well as on the comparison of surface heterogeneities of these samples with one another and with other carbon blacks.7,8 It was shown that surface homogeneity as well as average adsorption energy for Carbopack samples decreased with the decrease in the degree of graphite crystallinity. The latter was shown to be lower for graphitized carbons with higher specific surface areas. The decrease in the surface homogeneity manifested itself in a gradual weakening and/or disappearance of steps corresponding to the second-layer formation and the twodimensional fluid-solid phase transition as well as in a shift of the latter transition and the first-layer formation to higher pressure values. The adsorption potential at which these transitions take place was shown to be related to the degree of graphite crystallinity, providing the way to estimate this quantity on the basis of low-pressure nitrogen adsorption data. Thus, it was concluded that lowpressure adsorption data are suitable for the assessment of surface heterogeneity of graphitized carbon blacks. Experimental Section Materials. Graphitized carbon blacks Carbopack F, C, Y, B, and X are commercial samples from Supelco (Bellefonte, PA). These carbon blacks are used as packings in gas chromatography.16,17 The graphitized carbon black FT-G(2700) and the nongraphitized carbon black BP 280, which were used for comparative purposes, are described in detail elsewhere.7,8,18,19 Measurements. The carbon samples were prepared for the percentage crystallinity measurements by inserting the sample into a deep glass mount and pressing down to fill the cavity, and the intensities were measured over the angular range from 24.5 to 28.5. The samples were subsequently compared with standards prepared by mixing Ceylon flake graphite and amorphous carbon.15 Peak to background measurements were made rather than just peak intensity measurements, and no effects from the particle size were noted. The X-ray diffractometer used to measure the graphite crystallinity was manufactured by Phillips Norelco with a Cu X-ray source (1.54178 Å) at 30 mA and 40 kV. The step size was set at 0.5° with a 1.0 s count time. The angular range was from 5° to 140°. The raw data were collected using DATASCAN (NT-based software) and JADE PC-based software was employed for the data analysis. (15) Aune, F.; Brockner, W.; Oye, H. A. Carbon 1992, 30, 1001. (16) Leboda, R.; Lodyga, A.; Gierak, A. Mater. Chem. Phys. 1997, 51, 216. (17) Ross, P.; Knox, J. H. Adv. Chromatogr. 1997, 37, 121. (18) Kruk, M.; Jaroniec, M.; Gadkaree, K. P. J. Colloid Interface Sci. 1997, 192, 250. (19) Olivier, J. P. J. Porous Mater. 1995, 2, 217.
Kruk et al. Nitrogen adsorption isotherms were measured at 77 K on a static volumetric adsorption analyzer ASAP 2010 from Micromeritics Inc. (Norcross, GA). This automatic instrument is equipped with 1000, 10, and 1 Torr transducers and allows for an accurate measurement of nitrogen adsorption data at very low relative pressure starting from 10-6-10-7. The mass of a given graphitized carbon used for the measurements was chosen depending on its specific surface area and was between 4 g (for Carbopack F) and 0.3 g (for Carbopack X). Before the measurements, the samples were degassed under vacuum for 2 h at 473 K. Usually, 50 or more data points were collected below the relative pressure of 0.01 using the incremental dose mode. Incremental doses were initially set to be ≈1/100 of the BET monolayer capacity, and after acquisition of 5-10 first points, they were increased usually by the factor of 2. The minimum equilibration times used were between 12 and 18 min and the maximum equilibration time was 45 min/point. Because of fast equilibration, the times required to equilibrate particular points were usually much shorter than the maximum equilibration time limit and it took about 20 h to collect 50 or more data points below the relative pressure of 0.01. Repeated measurements with different incremental doses (i.e., different times of low-pressure runs) provided essentially identical results, which clearly demonstrated that the data acquired were well-equilibrated. Adsorption data at higher relative pressures were collected for preset values of relative pressures (about 100 points/run). The overall time needed to acquire a nitrogen adsorption isotherm in the entire range of pressures was about 45 h (from 31 to more than 60 h depending on a particular sample). Free volumes of the tubes with the samples were determined by helium measurements after the adsorption runs. The weight loss profiles were recorded using a high-resolution TGA 2950 thermogravimetric analyzer (TA Instruments, New Castle, DE). Calculations. The degree of graphite crystallinity of the graphitized carbon blacks under study were determined as described elsewhere.15 The specific surface area was calculated from adsorption data in the relative pressure range of 0.04-0.20 using the standard Brunauer-Emmett-Teller (BET) method.1-5 The total pore volume was estimated from the amount adsorbed at the relative pressure of about 0.985 and provides the sum of the volume of pores of width below ≈150 nm and the volume corresponding to the amount of nitrogen adsorbed on the walls of pores, in which capillary condensation does not take place at pressures below 0.985 p/p0 (pore size above ≈150 nm). The latter volume may constitute an important contribution to the total pore volume determined for most of the samples under study because of their macroporous character. Pore size distributions (PSDs) were calculated using the DFT software from Micromeritics (Norcross, GA).20 Adsorption potential distributions, Xr(A),1 were obtained by the numerical differentiation of relative adsorption isotherms with respect to the adsorption potential A: Xr ) -(dθ(A)/dA) ) (1/vm) (dv(A)/dA). In the latter equation, θ(A) ) v(A)/vm denotes the relative adsorption defined as the amount adsorbed, v(A), divided by the BET monolayer capacity, vm (expressed in the same units as the adsorbed amount). The adsorption potential, A, is defined to be equal to the change in the Gibbs free energy of adsorption with a minus sign: A ) RT ln(po/p), where R is the gas constant, po is the saturation vapor pressure, and p is the equilibrium vapor pressure. Surface properties of the samples under study were analyzed using the high-resolution θ-plot method,21,22 which is one of the comparative methods to analyze adsorption data.3 In the θ-plot method, the relative adsorption on the material under study is plotted as a function of the relative adsorption on a suitable reference solid. θ-plot curves can be analyzed in order to obtain information about microporosity and surface properties of porous solids.21,22 A graphitized carbon black of the highest surface homogeneity, that is, Carbopack F, was employed as a reference adsorbent. Adsorption energy distributions were calculated from submonolayer adsorption data using the INTEG program6 based on (20) Olivier, J. P.; Conkin, W. B.; Szombathely, M. v. Stud. Surf. Sci. Catal. 1994, 87, 81. (21) Jaroniec, M.; Kaneko, K. Langmuir 1997, 13, 6589. (22) Jaroniec, M.; Madey, R.; Choma, J.; McEnaney, B.; Mays, T. J. Carbon 1989, 27, 77.
Surface Properties of Graphitized Carbon Blacks
Figure 1. Weight loss curves for selected graphitized carbon blacks and a nongraphitized carbon black BP 280.
Figure 2. Weight loss derivatives for selected graphitized carbon blacks and a nongraphitized carbon black BP 280. the regularization method. The Fowler-Guggenheim equation was used as a local adsorption isotherm and patchwise topography of adsorption sites was assumed, as described in our previous work.7 A detailed discussion of the selection of local adsorption models and parameters used in calculation procedures can be found elsewhere.23
Results and Discussion Weight loss profiles and weight loss derivatives for selected Carbopack samples are shown in Figures 1 and 2, respectively. For comparative purposes, data for a nongraphitized carbon black BP 280 were also included. It can be seen that the graphitized carbons exhibited very small weight loss up to 750-1000 K and the subsequent weight decrease was much more pronounced. The nongraphitized carbon exhibited a gradual weight loss at temperatures below 800 K, followed by a rapid decline at higher temperatures. These findings indicate higher thermal stability of graphitized carbon blacks in comparison to nongraphitized carbon blacks. BET specific surface areas, total pore volumes, and degrees of graphite crystallinity of Carbopack samples under study are listed in Table 1. It can be seen that samples with lower surface areas exhibit higher degrees of crystallinity. Nitrogen adsorption isotherms for the graphitized carbon samples are shown in Figures 3 and 4. With the increase in the BET specific surface area, adsorption-desorption hysteresis loops gradually developed and consequently the type of adsorption isotherms changed from II to IV (according to the IUPAC classification4,5). Adsorption-desorption isotherms for Carbopack (23) Heuchel, M.; Jaroniec, M.; Gilpin, R. K.; Brauer, P.; Szombathely, M. v. Langmuir 1993, 9, 2537.
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F and C, which have the surface areas below 10 m2/g, are essentially reversible, which suggests that adsorption proceeded via multilayer formation rather than capillary condensation. This in turn leads to a conclusion that Carbopack F and C are essentially macroporous. It should be noted that the IUPAC classification of pores was used in the current work and pores with widths below 2 nm, between 2 and 50 nm, and above 50 nm are referred to as micropores, mesopores, and macropores, respectively. For Carbopack Y and B, there were noticeable hysteresis loops, but as they were observed at high relative pressures (above ≈0.9) and the adsorption isotherms steeply increased at pressure close to the saturation vapor pressure, the samples appeared to be mostly macroporous. On the other hand, Carbopack X exhibited a well-pronounced adsorption-desorption hysteresis loop and the isotherm gradually leveled as the saturation vapor pressure was approached, which indicates a mesoporous character of this material. As can be seen in Figure 5, the pore size distribution of Carbopack X is broad and most of the pores are in the range from 3 to 30 nm. Since the other Carbopack samples under study are mostly macroporous, their pore size distributions cannot be reliably estimated from nitrogen adsorption data. Low-pressure relative adsorption curves for the graphitized carbon blacks under study are shown in Figure 6. For comparative purposes, data for a nongraphitized carbon black BP 280 were also included. It can be seen that the Carbopack samples exhibited pronounced steps of the formation of the first adsorbed layer at relative pressures between ≈10-4 and 10-3, but their steepness decreased and their position shifted to higher pressures with the decrease in the graphite crystallinity. The decrease in the steepness of these steps can be explained as an effect of the decrease in the surface homogeneity,1,24 whereas the shift to higher pressures is likely to be due to the decrease in the average adsorption energy.25,26 The monolayer formation step was also noticeable on the relative adsorption curve for the nongraphitized sample, but it was much broader and appeared at higher pressures than those for the graphitized samples. Relative adsorption curves for graphitized carbons of a high degree of graphite crystallinity (Carbopack F and C) exhibited clear steps associated with the transition between a two-dimensional disordered fluid and two-dimensional ordered solid phase. These steps were much weaker for samples with higher surface areas (Carbopack Y and B) and not detectable for the sample with the highest surface area (Carbopack X). Our previous work8,27 showed that adsorption potential distributions (APDs) provide a lot of insight into surface and structural properties of carbonaceous adsorbents. For instance, there are significant differences between APDs of graphitized and nongraphitized carbon blacks.8 Therefore, a detailed analysis of APD curves of the samples under study was performed. Shown in Figure 7 is APD for a highly graphitized carbon black (Carbopack F) compared with APD for nongraphitized carbon blacks (Cabot BP 280). It can be seen that the former exhibited three distinct peaks at adsorption potential values of about 5.5, 3, and 0.7 kJ/mol, which can be attributed to the first-layer formation, two-dimensional fluid-solid transition and the second-layer formation, respectively.8 In contrast, the APD (24) Ecke, R. E.; Dash, J. G.; Puff, R. D. Phys. Rev. B 1982, 26, 1288. (25) Lastoskie, C.; Gubbins, K. E.; Quirke, N. Langmuir 1993, 9, 2693. (26) Olivier, J. P. In Fundamentals of Adsorption; LeVan, M. D., Ed.; Kluwer: Boston, 1996; p 699. (27) Li, Z.; Kruk, M.; Jaroniec, M.; Ryu, S.-K. J. Colloid Interface Sci. 1998, 204, 151.
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Table 1. BET Surface Area and Total Pore Volume of the Graphitized Carbon Samples
sample
BET specific surface area (m2/g)
total pore volume (cm3/g)
Carbopack F Carbopack C Carbopack Y Carbopack B Carbopack X
6.2 7.9 33 90 225
0.02 0.04 0.21 0.85 0.48
% graphite crystallinity
adsorption potential of monolayer formation (kJ/mol)
adsorption potential of two-dimensional fluid-solid phase transition (kJ/mol)
95 90 75 20 2
5.55 5.55 5.44 5.30 5.11
3.12 3.06 2.92 2.84
Figure 5. Pore size distribution for the Carbopack X graphitized carbon black. Figure 3. Nitrogen adsorption isotherms for Carbopack F and C graphitized carbon blacks.
Figure 6. Low-pressure relative adsorption data for a series of Carbopack graphitized carbon blacks and the nongraphitized carbon black BP 280.
Figure 4. Nitrogen adsorption isotherms for Carbopack Y, B, and X graphitized carbon blacks.
for the nongraphitized carbon is rather featureless. Namely, there was only one noticeable peak related to the monolayer formation, and there were no peaks associated with the two-dimensional fluid-solid phase transition and second-layer formation. Moreover, the monolayer formation peak was very broad and was shifted to lower values of the adsorption potential (about 4.8 kJ/mol in comparison to 5.5 kJ/mol for Carbopack F). Therefore, one can expect that the shape and position of peaks on the adsorption potential distribution is related to the degree of graphitization of a given porous carbon. A detailed examination of the adsorption data for the Carbopack samples under study strongly suggests that it is actually the case. Shown in Figure 8 are the first-layer-formation peaks for the graphitized samples under study and for the nongraphitized carbon black. It can be seen that for the Carbopack carbon blacks the height of these peaks decreased and their position shifted to lower values of the
adsorption potential in the order of decreasing degrees of graphite crystallinity. Likewise, the position of the peak associated with a two-dimensional fluid-solid phase transition shifted to lower adsorption potential values and the peak became broader and weaker for the samples of lower crystallinity (Figure 9). For Carbopack B with a graphite crystallinity of 20%, there is still a weakly pronounced broad peak, whereas for Carbopack X (2% crystallinity) there is no evidence of the two-dimensional fluid-solid transition (data not shown). A similar pattern is observed for the second-layer-formation data shown in Figure 10. The Carbopack samples of higher crystallinity exhibited distinct peaks, which evolved into a shoulder (for Carbopack B) and eventually disappeared for the graphitized material with the lowest degree of graphite crystallinity. Moreover, the position of the second-layerformation peak appeared to shift gradually toward lower values of the adsorption potential, as the crystallinity decreased. In comparison to the graphitized samples, the nongraphitized carbon black exhibited properties somewhat similar to the graphitized material of the lowest crystallinity (Carbopack X), but the latter had a much
Surface Properties of Graphitized Carbon Blacks
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Figure 7. Comparison of adsorption potential distributions for the graphitized (Carbopack F) and the nongraphitized (BP 280) carbon blacks.
Figure 10. Second-layer-formation region on the adsorption potential distributions for the graphitized carbon blacks under study and the nongraphitized carbon black.
Figure 8. First-layer-formation peaks on adsorption potential distributions for the graphitized carbon blacks under study and the nongraphitized carbon black.
Figure 11. Relations between (i) the degree of graphite crystallinity and the adsorption potential of the first-layer formation, and (ii) the degree of graphite crystallinity and the adsorption potential of the two-dimensional fluid-solid phase transition.
Figure 9. Two-dimensional disordered fluid/two-dimensional ordered solid phase transition peaks on selected graphitized carbon blacks under study.
sharper monolayer formation peak with the maximum at a higher value of the adsorption potential. The observed changes in adsorption potential distribution curves for the graphitized carbon blacks indicated that their surface homogeneity decreased with the decrease in the degree of graphite crystallinity. It should be noted that the Carbopack samples with higher surface areas exhibit lower crystallinity (see Table 1), which suggests that it might be difficult to obtain highly graphitized carbons with large surface areas.
The results presented above show that the adsorption potential distributions provide a significant amount of information about surface homogeneity, which in turn is expected to be related to the degree of graphitization of porous carbons. It appears that for very homogeneous graphitized samples (for example, Carbopack F and C), the position and height of the peak of a two-dimensional fluid-solid phase transition as well as the position and shape of the peak (or shoulder) of the second-layer formation provide information suitable for the assessment of the degree of graphite crystallinity. Shown in Figure 11 is the relation between the degree of graphite crystallinity and the position of the peak of a two-dimensional fluid-solid-phase transition, which can be used to qualitatively assess the graphitization of highly homogeneous graphitized carbons. In the case of graphitized or nongraphitized carbons with a more pronounced surface heterogeneity, one can estimate the degree of crystallinity from the position of the first-layer-formation step on adsorption potential distributions (see Figure 11). Thus, the positions and heights of the three peaks (first-layer formation, two-dimensional fluid-solid transition, and second-layer formation) provide complementary information, which may be used to estimate the surface homogeneity and graphitization degree of porous carbons. These preliminary results are very promising, but future studies will be required to verify whether these findings reflect general properties of porous carbons or merely features of the materials under study. It also needs to be noted
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Table 2. Nitrogen Standard Reduced Adsorption Data for Carbopack F Graphitized Carbon Black (the Adsorbed Amounts Were Divided by the Adsorbed Amount at a Relative Pressure of 0.4, i.e., 2.658 cm3 STP/g) p/p0
Rs
p/p0
Rs
p/p0
Rs
p/p0
Rs
4.13 × 10-6 9.01 × 10-6 1.39 × 10-5 1.87 × 10-5 2.33 × 10-5 2.78 × 10-5 3.22 × 10-5 3.65 × 10-5 4.07 × 10-5 4.84 × 10-5 5.58 × 10-5 6.29 × 10-5 6.96 × 10-5 7.61 × 10-5 8.24 × 10-5 8.85 × 10-5 9.44 × 10-5 1.00 × 10-4 1.06 × 10-4 1.11 × 10-4 1.17 × 10-4 1.22 × 10-4 1.28 × 10-4 1.33 × 10-4 1.39 × 10-4 1.46 × 10-4 1.52 × 10-4 1.57 × 10-4 1.63 × 10-4 1.69 × 10-4 1.75 × 10-4 1.82 × 10-4 1.88 × 10-4
0.0041 0.0083 0.0124 0.0166 0.0207 0.0248 0.0290 0.0332 0.0373 0.0453 0.0532 0.0612 0.0692 0.0772 0.0851 0.0931 0.101 0.109 0.117 0.125 0.133 0.141 0.149 0.157 0.165 0.173 0.181 0.189 0.197 0.205 0.213 0.221 0.229
1.95 × 10-4 2.02 × 10-4 2.09 × 10-4 2.17 × 10-4 2.25 × 10-4 2.34 × 10-4 2.43 × 10-4 2.53 × 10-4 2.63 × 10-4 2.74 × 10-4 2.86 × 10-4 3.00 × 10-4 3.15 × 10-4 3.31 × 10-4 3.50 × 10-4 3.71 × 10-4 3.94 × 10-4 4.21 × 10-4 4.49 × 10-4 4.82 × 10-4 5.22 × 10-4 5.67 × 10-4 6.20 × 10-4 6.82 × 10-4 7.54 × 10-4 8.85 × 10-4 1.05 × 10-3 1.26 × 10-3 1.53 × 10-3 1.90 × 10-3 2.34 × 10-3 2.88 × 10-3 3.51 × 10-3
0.237 0.245 0.253 0.261 0.269 0.276 0.284 0.292 0.299 0.307 0.315 0.323 0.331 0.339 0.347 0.354 0.362 0.369 0.377 0.385 0.392 0.400 0.407 0.414 0.422 0.432 0.442 0.451 0.460 0.469 0.477 0.484 0.491
4.22 × 10-3 5.01 × 10-3 5.85 × 10-3 6.73 × 10-3 7.67 × 10-3 8.09 × 10-3 8.72 × 10-3 9.50 × 10-3 0.0104 0.0125 0.0150 0.0176 0.0201 0.0253 0.0301 0.0402 0.0501 0.0604 0.0702 0.0804 0.0903 0.100 0.120 0.140 0.160 0.180 0.200 0.224 0.249 0.273 0.297 0.322 0.351
0.497 0.502 0.507 0.511 0.519 0.528 0.535 0.540 0.544 0.549 0.553 0.555 0.558 0.561 0.564 0.569 0.574 0.579 0.584 0.589 0.594 0.599 0.609 0.621 0.635 0.653 0.671 0.703 0.739 0.782 0.829 0.878 0.923
0.375 0.400 0.425 0.450 0.475 0.500 0.525 0.550 0.575 0.600 0.625 0.650 0.675 0.700 0.725 0.750 0.774 0.799 0.819 0.839 0.859 0.878 0.899 0.911 0.920 0.929 0.939 0.948 0.958 0.967 0.973 0.985
0.967 1.000 1.030 1.057 1.082 1.107 1.133 1.158 1.186 1.219 1.254 1.295 1.341 1.393 1.449 1.510 1.574 1.644 1.708 1.781 1.867 1.973 2.12 2.22 2.32 2.44 2.60 2.81 3.12 3.51 3.98 5.59
that a meaningful examination of the surface properties of graphitized carbons by means of nitrogen adsorption appears to require adsorption equipment capable of performing measurements at relative pressures down to 10-4 or lower in a reproducible manner. This is especially important for samples which do not exhibit pronounced steps of two-dimensional fluid-solid transition and secondlayer formation, in which case the first-layer-formation step provides primary information about surface homogeneity. In addition, one needs to keep in mind that a sufficient number of adsorption isotherm points need to be acquired to allow for a meaningful data analysis. The high-resolution θ-plot method was also employed to study surface properties of the Carbopack samples. The graphitized carbon with the highest graphite crystallinity (Carbopack F) was chosen as a reference adsorbent. The reference nitrogen adsorption data for Carbopack F are listed in Table 2 in the form of the standard reduced adsorption Rs as a function of the relative pressure. The obtained isotherm points cover the range from 10-6 to 0.98 and can be used for a comparative plot analysis of porous carbons of various degrees of graphitization. It should be noted that the reference nitrogen adsorption isotherm for a nongraphitized carbon black (BP280) was recently reported.18 As was described in detail elsewhere,21 the shape of θ-plot curves provides information about surface heterogeneity, average adsorption energy, and microporosity of porous materials. In the case when the sample under study and the macroporous reference adsorbent exhibit the same surface properties and the former does not have micropores, the submonolayer part of the θ-plot is linear and passes through (0,0) and (1,1) points of the θ-plot graph. Deviations from linearity reflect differences in surface homogeneity or average adsorption energy or may arise from microporosity of the material under study. As
Figure 12. High-resolution θ-plots for Carbopack graphitized carbons (calculated using Carbopack F as a reference adsorbent).
can be seen in Figure 12, the deviations from the linear behavior increase in the sequence of increasing surface area (Carbopack C, Y, B, and X). Since Carbopack C, Y, and B samples are essentially macroporous and exhibit very low adsorption at relative pressures below 10-4, they are not expected to have any micropores. Consequently, the shape of their θ-plots should reflect differences in energetic heterogeneity as well as average adsorption energy between these samples and the reference material (i.e., Carbopack F). It can be seen that the θ-plot for Carbopack C exhibits only minor deviations from linearity, indicating similarity of surface properties between this sample and the reference. However, θ-plots for Carbopacks Y and B show appreciable deviations from linearity, which revealed higher surface heterogeneity of the materials. Moreover, the comparative plot data suggest that average adsorption energies for Carbopack Y and B are smaller than the average adsorption energy for Carbopack F. The
Surface Properties of Graphitized Carbon Blacks
Figure 13. High-resolution θ-plots for a graphitized carbon black Sterling FT-G(2700) and a nongraphitized carbon black BP 280 (calculated using Carbopack F as a reference adsorbent).
latter conclusion is based on the fact that, for these samples, the intersection points of θ-plot curves with the line passing through the origin and the (1,1) point of the graph are below θ ) 0.5. It was shown that the θ-plot is expected to cross this line at θ ) 0.5, if average adsorption energies for the analyzed sample and reference material are equal and the former does not have micropores.21 In the case when the intersection point is not located at θ ) 0.5, the average adsorption energies are expected to be different, being lower for the sample under study than for the reference solid when θ < 0.5 and higher when θ > 0.5.21 The deviations of the θ-plot for Carbopack X from linearity are more pronounced than those for the other Carbopack samples, but in this case they might be caused not only by the surface heterogeneity but also by the presence of a small amount of micropores. This is indicated by the fact that, in contrast to other graphitized carbons under study, Carbopack X adsorbed an appreciable amount of nitrogen at relative pressures below 10-4 (see Figure 5). This resulted in an appearance of a distinct peak at the adsorption potential distribution for Carbopack X at about 8.5 kJ/mol. The nongraphitized carbon black BP 280 also exhibited appreciable low-pressure adsorption, but such a behavior appeared to result from high surface heterogeneity, since the monolayer formation peak for this sample is very broad and there is no additional peak in the region of high-adsorption potential values (see Figure 8). It was also interesting to compare adsorption data for the graphitized carbon blacks under study with those for selected carbon blacks studied before. It can be seen in Figure 13 that a highly graphitized carbon black Sterling FT-G(2700)8,19 exhibited very similar surface properties to our reference adsorbent (Carbopack F). In contrast, a nongraphitized carbon black BP 2807,8,18 showed significantly different adsorption behavior, which manifested itself in nonlinearity of its θ-plot. Adsorption energy distributions (AEDs) for the graphitized carbon blacks under study are shown in Figure 14. All samples exhibited prominent peaks at 11 kJ/mol and their heights decreased with a decrease in the degree of graphite crystallinity. Since the Carbopack samples with lower surface areas (i.e., F, C, and Y) exhibit clearly pronounced steps corresponding to the monolayer formation and the two-dimensional fluid-solid-phase transition, one can expect that all the adsorbed molecules in the first layer experience essentially the same gas-solid interac-
Langmuir, Vol. 15, No. 4, 1999 1441
Figure 14. Adsorption energy distributions for graphitized carbon blacks under study (Carbopack F, C, Y, B, and X).
tions. Consequently, the AEDs for these carbons should exhibit single sharp peaks. However, in addition to the main peaks at 11 kJ/mol, one can notice additional smaller peaks at 8 kJ/mol on the AEDs. Thus, the presence of these small peaks is most likely an artifact caused by inaccuracy of the local adsorption model and/or the calculation procedure. In the case of Carbopack X, the presence of an additional peak at about 14 kJ/mol can be noticed. Since this peak is present only on the AED for this particular sample, it is likely to be related to the presence of micropores, which was already indicated by the shape of relative adsorption and adsorption potential distribution curves. All the Carbopack samples under study exhibit much narrower adsorption energy distributions than nongraphitized carbon blacks studied in our previous work.7 It should be noted that the positions of peaks on AEDs are dependent to some extent on the local adsorption isotherm and parameters used in calculations23 and thus may be somewhat inaccurate. Conclusions The graphitized carbon blacks under study were essentially macroporous with the exception of the material with the highest surface area, which was mesoporous and seemed to exhibit detectable microporosity. The samples with higher surface areas exhibit lower degrees of graphite crystallinity. The surface homogeneity of the samples decreased with a decrease in crystallinity. It was shown that surface homogeneity of the materials under study can be compared by analyzing their adsorption potential distributions. For highly homogeneous samples, there are distinct peaks associated with the two-dimensional fluidsolid phase transition and the second-layer formation and their position as well as height can be related to the degree of crystallinity. In the case of more heterogeneous materials, their properties can be compared on the basis of the position and height of the first-layer-formation peak on adsorption potential distributions. Thus, the analysis of peaks on the adsorption potential distribution provides a possibility of evaluation of the graphitization degree. The examination of low-pressure adsorption data allowed us to draw a conclusion that the average adsorption energy for the Carbopack samples decreased with an increase in their specific surface area. LA980493+
