On the Modification and Characterization of Chemical Surface

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On the Modification and Characterization of Chemical Surface Properties of Activated Carbon: In the Search of Carbons with Stable Basic Properties J. Angel Mene´ndez,† Jonathan Phillips,‡ Bo Xia,‡ and Ljubisa R. Radovic*,† Fuel Science Program, Department of Materials Science and Engineering and Department of Chemical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 Received March 5, 1996. In Final Form: June 19, 1996X Differences between the surface chemical properties of hydrogen- and nitrogen-treated samples of an activated carbon were quantified using several complementary techniques. Calorimetric studies conducted at 303 K revealed that the sample treated in N2 at 1223 K adsorbs a great deal of oxygen with unusually high differential heats. In fact, both the quantity and the heat of adsorption increased when the treatment temperature was raised from 773 to 1223 K. In contrast, samples treated in H2 adsorbed less and less O2 as the temperature of treatment was raised; after treatment at 1223 K, virtually no O2 adsorption occurred. At the same time the H/C ratio in the H2-treated samples decreased with increasing treatment temperature. Point of zero charge measurements revealed that only H2 treatments at high temperature (>1073 K) create basic (hydrophobic) surfaces which are stable after prolonged air exposure. These findings are consistent with the notion that the removal of oxygen in the form of CO and CO2 during high-temperature N2 treatment leaves unsaturated carbon atoms at crystallite edges; these sites are very active for subsequent oxygen adsorption. In contrast, high-temperature H2 treatment accomplishes three tasks: (a) it also removes surface oxygen; (b) it stabilizes some of the (re)active sites by forming stable C-H bonds; (c) it gasifies the most reactive unsaturated carbon atoms. The relative contributions of these three effects depend on the temperature of H2 treatment. The carbon surface resulting from high-temperature H2 treatment is stable against subsequent O2 adsorption in ambient conditions.

Introduction Activated carbons are known to be excellent and versatile adsorbents and are therefore used to remove a broad spectrum of dissolved organic1-3 and inorganic3-6 species from both gas phase and liquid phase. Activated carbons also represent an increasingly important portion of the catalyst support market.7,8 This great flexibility in the applications of activated carbons arises from the very wide range of not only physical surface properties but also chemical properties of commercially available and/or specifically treated carbon materials. Hence the techniques to modify and characterize the surface chemical properties of activated carbons have been of interest for decades.9-12 Because of increasingly stringent clean air † Fuel Science Program, Department of Materials Science and Engineering. ‡ Department of Chemical Engineering. X Abstract published in Advance ACS Abstracts, August 1, 1996.

(1) Akubuiro, E. C.; Wagner, N. J. Ind. Eng. Chem. Res. 1992, 31, 339. (2) Derylo-Marczewska, A.; Jaroniec, M. Surf. Colloid Sci. 1987, 14, 301. (3) Faust, S. D.; Aly, O. M. Adsorption Processes for Water Treatment; Butterworths: Boston, MA, 1987. (4) Corapcioglu, M. O.; Huang, C. P. Water Res. 1987, 21, 1031. (5) Rubin, A. J.; Mercer, D. L. In Adsorption of Inorganics at SolidLiquid Interfaces; Anderson, M. A., Rubin, A. J., Eds.; Ann Arbor Science: Ann Arbor, MI, 1981; p 295. (6) Taylor, R. M.; Kuennen, R. W. Environ. Prog. 1994, 13, 65. (7) Bird, A. J. In Catalyst Supports and Supported Catalysts; Stiles, A. B., Ed.; Butterworths: Boston, MA, 1987; p 107. (8) Radovic, L. R.; Rodrı´guez-Reinoso, F. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1996; Vol. 25, p 243. (9) Boehm, H. P. Carbon 1994, 32, 759. (10) Puri, B. R. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Ed.; Marcel Dekker: New York, 1970; Vol. 6, p 191. (11) McKee, D. W.; Mimeault, V. J. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Ed.; Marcel Dekker: New York, 1973; Vol. 8, p 151. (12) Leon y Leon, C. A.; Radovic, L. R. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1994; Vol. 24; p 213.

S0743-7463(96)00202-8 CCC: $12.00

and clean water legislation, the interest in the more effective use of these materials, as well as in their better and easier chemical surface characterization,13,14 is on the rise. The preparation of carbons with basic and/or hydrophobic surface properties15-17 is of great interest in a variety of applications. For example, it is known that the adsorption capacity of activated carbons often decreases after long storage in the presence of moisture. This aging effect has been ascribed to a gradual surface oxidation.18,19 Carbons with basic properties are much more resistant to this effect.19-22 It has been reported also that the adsorption affinity of phenolic compounds increases with increasing basicity (hydrophobicity) of the carbon surface and with increasing electron-withdrawing ability of the substituents on the aromatic ring.23-25 The presence of basic surface groups on carbon has also been found to increase the SO2 adsorption capacity of activated carbon.26,27 Finally, treatment with ammonia and/or hydrogen or nitrogen at high temperatures caused a considerable (13) Bandosz, T. J.; Jagiello, J.; Contescu, C.; Schwarz, J. A. Carbon 1993, 31, 1193. (14) Mene´ndez, J. A.; Illa´n-Go´mez, M. J.; Leo´n y Leo´n, C. A.; Radovic, L. R. Carbon 1995, 33, 1655. (15) Leon y Leon, C. A.; Solar, J. M.; Calemma, V.; Radovic, L. R. Carbon 1992, 30, 797. (16) Papirer, E.; Li, S.; Donnet, J.-B. Carbon 1987, 25, 243. (17) Boehm, H. P.; Voll, M. Carbon 1970, 8, 227. (18) Adams, L. B.; Hall, C. R.; Holmes, R. J.; Newton, R. A. Carbon 1988, 26, 451. (19) Barton, S. S.; Evans, M. J.; MacDonald, J. A. F. In 22nd Biennial Conference on Carbon; San Diego, CA, 1995; p 438. (20) Sto¨hr, B.; Boehm, H. P.; Schlo¨gl, R. Carbon 1991, 29, 707. (21) Bansal, R. C.; Vastola, F. J.; Walker, P. L., Jr. Carbon 1974, 12, 355. (22) Verma, S. K.; Walker, Jr., P. L. Carbon 1992, 30, 837. (23) Moreno-Castilla, C.; Rivera-Utrilla, J.; Lo´pez-Ramo´n, M. V.; Carrasco-Marı´n, F. Carbon 1995, 33, 845. (24) Radovic, L. R.; Ume, J. I.; Scaroni, A. W. In Fundamentals of Adsorption; LeVan, M. D., Ed.; Kluwer: Boston, MA, 1996, p 749. (25) Tamon, H.; Okazaki, M. In Fundamentals of Adsorption; Suzuki, M., Ed.; Kodansha: Tokyo, 1993; p 663.

© 1996 American Chemical Society

Surface Properties of Activated Carbon

increase in catalytic activity of carbon in oxidation reactions involving molecular oxygen.20 Even though this basic character has been associated with the presence of some oxygen-containing surface groups at edges of carbon crystallites (e.g., chromene- and pyrone-type structures,16,17,28 the main contribution to surface basicity is often from oxygen-free Lewis base sites on the basal planes, i.e., within the graphene layers that constitute the carbon crystallites.15 In other words, carbon basicity is associated with the absence of oxygen-containing groups which are predominantly of acidic nature. For this reason, most of the treatments proposed to obtain carbon surfaces with basic properties consist in heating the carbon in different gases in order to remove the oxygencontaining surface groups. Thus, for example, carbons are known to acquire a basic character upon hightemperature heat treatment (typically, above ∼700 °C) in an inert atmosphere and subsequent exposure to air below ∼200 °C; these are commonly referred to as H-carbons.29 Treatment with ammonia20,30,31 and chlorine19,22,32,33 has been shown to be effective for a wide variety of applications. The former, performed typically at 400-900 °C, removes the acidic oxygen-containing functional groups and it may also introduce basic nitrogen-containing (e.g., amine) groups onto the carbon surface. The latter can lead to the formation of covalent bonds with the carbon active sites,22 thus inhibiting carbon reactivity;34 the desorption of these C-Cl groups has been reported10 to occur only at temperatures in excess of 1200 °C. Treatment of activated carbons in Cl2 at 180 °C resulted in modest hydrophobicity decreases;32,33 on the other hand, treatment of a carbon molecular sieve at 450 °C22 resulted in reduced physisorption of both O2 and Ar. In an early study by Walker and co-workers,21 chemisorption of H2 has been suggested as a method to minimize O2 chemisorption and thus maintain the carbon surface in a hydrophobic state. The effectiveness of this approach, at 5.5 MPa H2 and 150 °C for up to 72 h (using a freshly prepared carbon that had not been exposed to air prior to H2 treatment), has been confirmed recently by Verma and Walker in their efforts to produce carbon molecular sieves with stable hydrophobic surfaces.22 This same concept was briefly illustrated in several earlier studies,35,36 but the H2-treated samples were not characterized in any detail. The practical objective of our study is to explore in more detail the conditions under which stable basic carbon surfaces can be obtained. At a more fundamental level, we have used the techniques of elemental analysis, electrophoresis, mass titration, and microcalorimetry to characterize the changes in carbon surface chemistry that take place during different treatments of activated carbon. Electrophoresis and mass titration have previously been shown to be a powerful tandem,12,14 in terms of both their cost effectiveness and complementarity, while microcalorimetry offers a more detailed analysis of the concentration and energetics of adsorbing sites on the carbon surface.37,38 (26) Davini, P. Fuel 1989, 68, 145. (27) Davini, P. Carbon 1990, 28, 565. (28) Papirer, E.; Dentzer, J.; Li, S.; Donnet, J. B. Carbon 1991, 29, 69. (29) Kinoshita, K. Carbon: Electrochemical and Physicochemical Properties; Wiley-Interscience: New York, 1988. (30) Abotsi, G. M. K.; Scaroni, A. W. Carbon 1990, 28, 79. (31) Vinke, P.; van der Eijk, M.; Verbree, M.; Voskamp, A. F.; van Bekkum, H. Carbon 1994, 32, 675. (32) Hall, C. R.; Holmes, R. J. Carbon 1992, 30, 173. (33) Hall, C. R.; Holmes, R. J. Carbon 1993, 31, 881. (34) McKee, D. W.; Spiro, C. L. Carbon 1985, 23, 437. (35) Pierce, C.; Wiley, J. W.; Smith, R. N. J. Phys. Chem. 1949, 53, 669. (36) Stoeckli, H. F.; Kraehenbuehl, F. Carbon 1981, 19, 353.

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These studies have now provided strong experimental support for two additional findings. First, carbon surfaces prepared by high-temperature N2 treatment (which is known to increase carbon basicity by removing surface oxygen) are not stable; they re-adsorb O2 even at room temperature and rapidly become reacidified. Second, surfaces treated in high-temperature H2 are not only basic, but they retain their basic character for long periods of time. A simple model is then proposed to explain the dramatic differences between the surface chemistries of N2- and H2-treated carbons: The more inert character of the latter not only results from the formation of more stable C-H bonds but is also a consequence of hydrogasification of loosely bound, highly (re)active carbon atoms. Experimental Section Sample Preparation. A commercial activated carbon, NORIT C-granular (Nc), was used as the starting material. It is obtained from wood by chemical activation, using phosphoric acid. In order to remove most of the surface functional groups, ca. 4 g of Nc particles, ground to 100 kcal/mol) apparently have not been reported before. While this question is the subject of our continued research, one obvious explanation is that their detection requires the use of both a clean carbon surface and a truly differential microcalorimeter. The contention that both conditions were met in our study is further supported by the results shown in Figures 3 and 8. The anomalous behavior shown in Figure 3 (decreasing “equilibrium” pressure with increasing surface coverage (46) Johnson, J. L. In Chemistry of Coal Utilization (Second Supplementary Volume); Elliott, M. A., Ed.; Wiley: New York, 1981; p 1491. (47) Hurt, R. H.; Sarofim, A. F.; Longwell, J. P. Combust. Flame 1993, 95, 430.

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Figure 8. Mass spectrometric analysis of gases evolved upon in situ exposure of sample N950 to O2 at room temperature (5% O2 in He at 1 atm).

at very low equilibrium pressures) is an artifact which is a consequence of the existence of highly reactive sites on the surface of incompletely stabilized carbons (e.g., N950, N500, H800, and N950/H650, but not H875 and H950). When first microdoses of O2 are introduced in the calorimeter at room temperature, CO is produced by gasification of the highly reactive sites of types I and/or II which are thus responsible for the very high heats of adsorption observed. This is illustrated in Figure 8, obtained in a separate experiment. Sample N950 was prepared in a quartz reactor connected to a quadrupole mass spectrometer; after cooling to room temperature, it was contacted with 50 cm3/min He containing 5% O2. The production of CO, i.e., 2 mol of gas produced for every mole of gas consumed (2C* + O2 ) 2CO), is indeed consistent with the anomalous low-coverage isotherm behavior shown in Figure 3. An implicit aspect of the model presented in Figure 7 is that certain active sites (e.g., type III) will not adsorb oxygen at ambient conditions. This is consistent with the calorimetric observations (Figures 3-5) and with the finding that the PZC of sample H950 is very high and changes little even after months of air exposure (Figure 6). Moreover, this aspect of the model is consistent with the voluminous literature on the titration of carbon actives sites.48 It has been shown on many occasions that the amount of O2 adsorbed on high-surface-area carbons increases with increasing temperature and that O2 will not adsorb on some active sites until a temperature of 573 K is reached. Finally, it is important to note that the proposed model is also consistent with the measured properties of all hydrogen-treated samples. As the temperature of hydrogen treatment increases, the quantity of adsorbed oxygen decreases (Figures 3 and 4); the initial heat of adsorption also decreases (Figure 4). This is in agreement with well-known linear free energy correlations: as the temperature of hydrogen treatment is lowered, more and more of the active sites of progressively higher energy (e.g., types I and II) “survive”. Indeed, the weight lossswhich is taken to correlate with the extent of hydrogasificationsincreases as the treatment temperature increases. The model is also consistent with the chemical analyses which do not show an increase in H/C ratio following hydrogen treatment at increasing temperatures. A temptingly simple alternative model, according to which hydrogen atoms “passivate” all the surface sites by forming C-H bonds, can thus be discarded. (48) Lizzio, A. A.; Jiang, H.; Radovic, L. R. Carbon 1990, 28, 7.

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At intermediate temperatures (which will depend on carbon reactivity), there appears to exist a trade-off. On one hand, the relatively high H/C ratios (e.g., 0.203 for sample H650) suggest that many reactive sites have indeed been passivated (by C-H bond formation). On the other hand, the weight loss, PZC, and microcalorimetry data show that many reactive sites remain on the surface (e.g., from the decomposition of relatively unstable carboxyl groups) which are not hydrogasified and are thus susceptible to O2 readsorption. Conclusions It has been shown that the techniques of electrophoresis, mass titration, and O2 adsorption microcalorimetry are very useful in providing complementary information regarding the modification of surface chemistry of activated carbons. Using these techniques, it was confirmed that high-temperature treatment in an inert environment is effective in removing oxygen-containing surface groups, but it leaves a surface containing very reactive carbon sites capable of readsorbing oxygen and hence reacidifying, in room-temperature air. Hydrogen treatment at increasing temperatures also removes oxygen, but leaves an increasingly stable “basic” surface nearly free of active sites capable of adsorbing O2 at ambient temperature. A model explaining these differences, both between N2 and H2 treatment and (perhaps more importantly) between H2-treated samples at different temperatures, was presented. It can be summarized as follows. The increase

Mene´ ndez et al.

in the severity of H2 treatment is beneficial not because carbon interaction with hydrogen creates basic surface sites but because of (a) increasing effectiveness of hydrogen in removing acidic oxygen functional groups, (b) stabilization of free carbon sites against subsequent oxidation, and (c) decreasing concentration of active sites. Indeed, a large increase in PZC was observed as the temperature is increased from 650 to 950 °C. Furthermore, the increase in the PZC-IEP difference with increasing temperature suggests that the internal surfaces are protected from reoxidation much more effectively than the external surface. Most important from a practical standpoint is the finding that a very stable basic carbon surface is produced by heat treatment in H2 at 950 °C. When lower temperatures are used in H2 treatment, a less basic carbon is produced but it possesses a more uniform distribution of potential-determining surface groups. Acknowledgment. This study was made possible by financial support from the Carbon Research Center (Penn State) and the U.S. Department of Energy (Grant DEFG22-95PC95225), as well as by a postdoctoral grant to J.A.M. (Spanish Scientific Council). The assistance of Juan Alcan˜iz, in the textural characterization of the carbons, and fruitful discussions with Marı´a J. Illa´nGo´mez (both at the University of Alicante, Spain) are also gratefully acknowledged. LA9602022