6416
Langmuir 1999, 15, 6416-6422
Temperature-Programmed Desorption of Oxygen Surface Complexes on Acenaphthylene-Derived Chars: Comparison with Oxygen K-Edge XANES Spectroscopy J. A. Turner and K. M. Thomas* Northern Carbon Research Laboratories, Department of Chemistry, Bedson Building, The University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7RU, England Received September 3, 1998. In Final Form: April 7, 1999 The interpretation of temperature-programmed desorption (TPD) data relating to the decomposition of oxygen surface complexes on carbonaceous materials is complex. Oxygen K-edge XANES spectroscopy was used to characterize a series of acenaphthylene-derived chars oxidized by either partial combustion or low-temperature oxygen chemisorption prior to their study by TPD. Evidence for the presence of mobile hydrogen species was provided by the observation that H2 and H2O were evolved at high temperatures during TPD. A constant CO:CO2 ratio for the desorption products evolved during the TPD of the partially combusted chars at desorption temperatures below ∼1000 K was attributed to surface reactions of mobile oxygen surface species controlling the product ratio. This indicates that desorption product ratios may be an unreliable guide to the functional groups undergoing decomposition, especially for chars with high hydrogen contents. Above ∼1000 K, the CO:CO2 ratio was observed to increase sharply. This was attributed to scavenging of C(O) by mobile hydrogen species, although the possibility that different functional groups might have different mobilities was also considered. The TPD profiles of the surface oxygen complexes formed during oxygen chemisorption at 473 K contained additional peaks not present in the profiles of the samples combusted at 773 K. These additional peaks were attributed to the formation of a wider range of acid-like surface complexes during low-temperature oxygen chemisorption than during combustion. This suggests that care must be taken when applying low-temperature chemisorption data to combustion scenarios.
1. Introduction The formation of oxygen surface complexes has long been recognized as an important reaction step in the combustion of carbonaceous materials, as well as in their reaction with other gasifying agents such as CO2 and H2O.1-11 These surface species are also of great importance in determining the interfacial phenomena in carbon fiberreinforced composite materials,12,13 as well as the surface properties and, hence, applications of active carbons.14-19 Laine et al.6,7 observed that oxygen surface complexes only formed on a small fraction of the total carbon surface. (1) Rhead, T. F. E.; Wheeler, R. V. J. Chem. Soc. 1912, 101, 846. (2) Rhead, T. F. E.; Wheeler, R. V. J. Chem. Soc. 1913, 103, 461. (3) Langmuir, I. J. Am. Chem. Soc. 1915, 37, 1139. (4) Shah, M. S. J. Chem. Soc. 1929, 129, 2661. (5) Walker, P. L., Jr.; Rusinko, F. J.; Austin, L. G. In Advances in Catalysis; Eley, D. D., Selwood, P. W., Weisz, P. B., Eds.; Academic Press: New York, 1959; Vol. XI. (6) Laine, N. R.; Vastola, F. J.; Walker, P. L., Jr. J. Phys. Chem. 1963, 67, 2030. (7) Laine, N. R.; Vastola, F. J.; Walker, P. L., Jr. In Proceedings of the 5th Carbon Conference; Pergamon Press: New York 1963; p 211. (8) Puri, B. R. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Ed.; Marcel Dekker: New York, 1970; Vol. 6, pp 191-282. (9) Laine, N. R. Carbon 1991, 29, 729. (10) Walker, P. L., Jr.; Taylor, R. L.; Ranish, J. M. Carbon 1991, 29, 411. (11) Marsh, H.; Radovic, L. R. In Fundamental Issues in Control of Carbon Gasification Reactivity; Lahaye, J., Ehrburger, P., Eds.; Kluwer Academic Publishers: Netherlands, 1991; pp 595-605. (12) Morra, M.; Occhiello, E.; Garbassi, F. Compos. Sci. Technol. 1991, 42, 361. (13) Gardner, S. D.; Singamsetty, C. S. K.; He, G.; Pittman, C. U., Jr. Appl. Spectrosc. 1997, 51, 636. (14) Szymanski, G. S.; Rychlicki, G. Carbon 1991, 29, 489. (15) Bautista-Toledo, I.; Rivera-Utrilla, J.; Ferro-Garcia, M. A.; Moreno-Castilla, C. Carbon 1994, 32, 93. (16) Kisamori, S.; Mochida, I.; Fujitsu, H. Langmuir 1994, 10, 1241. (17) Vidic, R. D.; Tessmer, C. H.; Uranowski, L. J. Carbon 1997, 35, 1349. (18) Leng, C.-C.; Pinto, N. G. Carbon 1997, 35, 1375. (19) Radovic, L. R.; Silva, I. F.; Ume, J. I.; Menendez, J. A.; Leon, Y.; Leon, C. A.; Scaroni, A. W. Carbon 1997, 35, 1339.
The concept of “active surface area” (ASA) was introduced by direct analogy with catalytic reactions to represent the portion of the total available surface area involved in surface complex formation. The ASA of outgassed samples of Graphon (a highly graphitized carbon black) was measured by the uptake of oxygen gas at 573 K, making the assumption that a surface complex consisted of a single oxygen atom chemisorbed onto a carbon atom on the edge of a graphene layer (which they calculated to have an area of 8.3 × 10-20 m2). At this temperature, the oxygen was chemisorbed on the carbon surface with negligible combustion occurring, as determined by the evolution of the product gases CO and CO2. Values of ASA determined in this way were subsequently found by many workers to be extremely useful in correlating carbon structural factors with combustion (or, more generally, gasification) kinetics.10,11,20-25 Thermal desorption methods for studying oxygen surface complexes can broadly be grouped into two categories: linear temperature-programmed desorption (linear TPD) experiments26-33 and isothermal desorption (also (20) Lizzio, A. A.; Jiang, H.; Radovic, L. R. Carbon 1990, 28, 7. (21) Ahmed, S.; Back, M. H. Carbon 1985, 23, 513. (22) Hoffman, W. P. Carbon 1991, 29, 769. (23) Marsh, H.; Diez, M. A.; Kuo, K. In Fundamental Issues in Control of Carbon Gasification Reactivity; Lahaye, J., Ehrburger, P., Eds.; Kluwer Academic Publishers: Netherlands, 1991; pp 205-220. (24) Lahaye, J.; Dentzer, J.; Soulard, P.; Ehrburger, P. In Fundamental Issues in Control of Carbon Gasification Reactivity; Lahaye, J., Ehrburger, P., Eds.; Kluwer Academic Publishers: Netherlands, 1991. (25) Essenhigh, R. H. In Chemistry of Coal Utilisation; Elliot, M. A., Ed.; Wiley: New York, 1981; Vol. 2nd Suppl.; pp 1153-1312. (26) Phillips, R.; Vastola, F. J.; Walker, P. L., Jr. Carbon 1970, 8, 197. (27) Bansal, R. C.; Vastola, F. J.; Walker, P. L., Jr. Carbon 1970, 8, 443. (28) Marchon, B.; Carrazza, J.; Heinemann, H.; Somorjai, G. A. Carbon 1988, 26, 507.
10.1021/la981165c CCC: $18.00 © 1999 American Chemical Society Published on Web 07/10/1999
Desorption of Oxygen Surface Complexes
referred to as step TPD) experiments.34-37 A comparison of the two types of experiment was carried out by Tremblay et al.,38 who concluded that while step TPD could probably be used to distinguish between the desorption of different functional groups, the technique would not have sufficient resolution to detect surface heterogeneity. As a result the TPD technique used mainly nowadays to study the desorption (or, more correctly, decomposition) of oxygen surface complexes is linear TPD and this technique was used in this study. Although TPD is a powerful technique for determining decomposition temperatures in simple systems, studies of oxygen surface complexes on carbonaceous materials are complicated by the fact that the oxygen is not lost by simple desorption but by the decomposition of the surface to produce gaseous carbon oxides. Consequently, the interpretation of TPD data relating to the decomposition of oxygen surface complexes on carbonaceous materials represents a formidable challenge. X-ray absorption nearedge structure (XANES) spectroscopy has been shown to be a very useful technique for the identification of nitrogen,39-43 sulfur,44-47 and oxygen48 functional groups in carbonaceous materials. In this paper, oxygen K-edge XANES spectroscopy is used to provide an insight into the processes involved in the decomposition of oxygen surface complexes on a series of acenaphthylene-derived chars during TPD. 2. Experimental Section 2.1. Materials Used. Acenaphthylene was supplied by the Aldrich Chemical Co. (minimum assay 95%). All gases were supplied by BOC Group plc. The gases used were of the following grades: Pureshield argon (99.998%), oxygen (99.5%), nitrogen (oxygen-free, 99.998%), and helium (Grade N4.5/A 99.995%). 2.2. Carbonization of Acenaphthylene. Acenaphthylene was carbonized in a silica combustion boat which was heated in a horizontal tube furnace at 1 K min-1 to 873 K in flowing (∼10 (29) Marchon, B.; Tysoe, W. T.; Carrazza, J.; Heinemann, H.; Somorjai, G. A. J. Phys. Chem. 1988, 92, 5744. (30) Calo, J. M.; Hall, P. J. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1989, 34, 71. (31) Lizzio, A. A.; Radovic, L. R. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1989, 34, 102. (32) Du, Z.; Sarofim, A. F.; Longwell, J. P. Energy Fuels 1990, 4, 296. (33) Zhuang, Q.; Kyotani, T.; Tomita, A. Energy Fuels 1995, 9, 630. (34) Tucker, B. G.; Mulcahy, M. F. R. Trans. Faraday Soc. 1969, 65, 274. (35) Dollimore, J.; Freedman, C. M.; Harrison, B. H. In 3rd Conference on Industrial Carbons and Graphite; Society of Chemical Industry: London, 1970; p 250. (36) Barton, S. S.; Harrison, B. H.; Dollimore, J. Trans. Faraday Soc. 1973, 69, 1039. (37) Calo, J. M. In Fundamental Issues in Control of Carbon Gasification Reactivity; Lahaye, J., Ehrburger, P., Eds.; Kluwer Academic Publishers: Netherlands, 1991; pp 369-376. (38) Tremblay, G.; Vastola, F. J.; Walker, P. L., Jr. Carbon 1978, 16, 35. (39) Mullins, O. C.; Mitra-Kirtley, S.; van Elp, J.; Cramer, S. P. Appl. Spectrosc. 1993, 47, 1268. (40) Mitra-Kirtley, S.; Mullins, O. C.; van Elp, J.; George, S. J.; Chen, J.; Cramer, S. P. J. Am. Chem. Soc. 1993, 115, 252. (41) Mitra-Kirtley, S.; Mullins, O. C.; van Elp, J.; Branthaver, J. F.; Cramer, S. P. Energy Fuels 1993, 7, 1128. (42) Mitra-Kirtley, S.; Mullins, O. C.; van Elp, J.; Cramer, S. P. Fuel 1993, 72, 133. (43) Zhu, Q.; Money, S. L.; Russell, A. E.; Thomas, K. M. Langmuir 1997, 13, 2149. (44) George, G. N.; Gorbaty, M. L. J. Am. Chem. Soc. 1989, 111, 3182. (45) Keleman, S. R.; George, G. N.; Gorbaty, M. L. Fuel 1990, 69, 939. (46) Gorbaty, M. L.; George, G. N.; Keleman, S. R. Fuel 1990, 69, 945. (47) George, G. N.; Gorbaty, M. L.; Keleman, S. R.; Sansone, M. Energy Fuels 1991, 5, 93. (48) Turner, J. A.; Thomas, K. M.; Russell, A. E. Carbon 1997, 35, 983.
Langmuir, Vol. 15, No. 19, 1999 6417 cm3 min-1) argon gas and held at temperature for a dwell time of 1 h prior to cooling to room temperature. The resulting char was ground and sieved to a particle size fraction 38-75 µm. 2.3. Partial Combustion of Carbonized Acenaphthylene. Samples of the acenaphthylene char were combusted in air to various extents of burnoff using a Stanton Redcroft model STA 780 thermogravimetric analyzer (TGA). The procedure involved heating a char sample to 773 K in flowing (50 cm3 min-1) nitrogen at 50 K min-1, and then switching the gas stream to air until the desired burnoff was obtained. The gas was subsequently changed back to nitrogen, and the sample cooled to room temperature. The samples were stored under argon prior to their study by TPD or oxygen K-edge XANES. 2.4. Oxygen Chemisorption. The acenaphthylene char sample was cleaned and outgassed prior to chemisorption by heating at 20 K min-1 in vacuo to a final heat treatment temperature of 1073 K, with a dwell time of 15 min. A relatively low cleaning temperature was chosen to prevent excessive thermal annealing. Chemisorption was carried out for 50 h at a temperature of 473 K and an oxygen partial pressure of 200 mbar in a Hiden Analytical Ltd. model IGA-003 “Intelligent Gravimetric Analyzer” (IGA). 2.5. Elemental Analysis. The elemental analyses for the carbons studied were determined by Butterworth Laboratories Ltd., Middlesex, England. 2.6. Temperature-Programmed Desorption. Temperature-programmed desorption experiments were carried out using the Hiden Analytical model IGA-003 coupled to a Hiden Analytical HAL II/300 quadrupole mass spectrometer.49 Samples of ca. 50 mg were heated at a rate of 10 K min-1 to a nominal (furnace) temperature of 1223 K in a quartz reactor vessel. The maximum temperature was limited by the design of the furnace and reactor vessel. Flowing (50 cm3 min-1 at 1 bar) helium was used to flush the evolved gases into the mass spectrometer. The mass spectrometer was set up to monitor ions at the following massto-charge ratios (m/z): 1, 2, 4, 14, 18, 28, 32, and 44 (m/z 14 was recorded to check for nitrogen contributions to the m/z 28 peak). The partial pressures recorded were corrected for variations in the sensitivity of the mass spectrometer using correction factors supplied by the manufacturer. The reactor vessel was evacuated to 99.99%) platinum plates (supplied by Goodfellow Metals Ltd.). The surface of the plates was roughened with 25 µm diamond polishing compound prior to cleaning with carbon tetrachloride. The (49) Benham, M. J.; Ross, D. K. Z. Phys. Chem. 1989, 25, 163. (50) Hollas, J. M. Modern Spectroscopy, 3rd ed.; Wiley: Chichester, England, 1996. (51) Surman, M.; Cragg-Hine, I.; Singh, J.; Bowler, B. J.; Padmore, H. A.; Norman, D.; Johnson, A. L.; Walter, W. K.; King, D. A.; Davis, R.; Purcell, K. G.; Thornton, G. Rev. Sci. Instrum. 1992, 63, 4349.
6418 Langmuir, Vol. 15, No. 19, 1999
Turner and Thomas
Figure 1. Oxygen K-edge XANES spectra of the acenaphthylene chars and selected oxygen functional group reference compounds. * From Turner et al.39 Table 1. CHN Analyses of the Unoxidized Char and after Combustion to X ) 0.5 at 773 K elemental analysis (% m/m) sample
C
H
N
oxygen by difference
(H/C) × 102
unox. char X ) 0.5, 773 K
96.72 85.76
3.03 2.70
