AN AMERICAN CHEMICAL SOCIETY JOURNAL MARCH/APRIL 1988
VOLUME 2, NUMBER 2
0 Copyright 1988 by the American Chemical Society
Art i c 1es XPS Characterization of Glassy-CarbonSurfaces Oxidized by 02,C02, and HN03t S. R. Kelemen* and H. Freund Exxon Research & Engineering Company, Annandale, New Jersey 08801 Received June 10, 1987. Revised Manuscript Received August 5, 1987
XPS and AES were used to characterize glassy-carbon surfaces following exposure to O2and C02. Temperature-programmed decomposition (TPD) was used to monitor the subsequent decomposition chemistry. The ability of O2 and C02 to dissociatively chemisorb depends on the extent of prior oxidation. Initial oxidation on clean surfaces by C02 and O2 produced species having comparable thermal stability and characterized by a 285.8-eV C(1s) peak in the difference spectrum. These species decomposed above 700 "C to produce gaseous CO. Additional exposure to C02up to 700 "C did not increase the oxygen surface coverage. Additional exposure to O2generated additional surface oxygen with a 531.5-eV O(1s) peak and a 285.8-eV C(1s) peak in the difference spectrum. The 285.8-eV C(1s) peak is representative of carbon atoms with a single bond to oxygen, i.e., ether-like species. A smaller C(1s) peak was observed a t 288.8 eV corresponding to carbon having a total of three bonds to oxygen atoms, i.e., carboxylate groups. The decomposition of carboxylate and ether-like species accounts for the production of CO and COz between 400 and 650 "C. Carbonyl functionalities represent a small minority of the total oxygen population present after dissociative O2 adsorption, gasification of carbon by C 0 2 , and combustion of carbon by 02.Oxidation of glassy carbon by HN03 favors the formation of carboxylate groups characterized by a 288.4-eV C(1s) peak and O(1s) peaks a t 531.2 and 532.7 eV. The surfaces oxidized by HNOBproduced a complex low-temperature TPD pattern involving C02, CO, and H 2 0 products. Above 500 "C CO was the dominant product.
Introduction The reactivity of carbon toward gasification by either C02 or H 2 0 as well as combustion by O2can be related to the O2chemisorption properties of the reactant carb0n.l" It is thought that O2 dissociation occurs a t edge carbon atoms that are also involved in the higher temperature reactions. O2adsorption can therefore be used as a relative measure of the active surface area. The conditions for O2 adsorption are often arbitrarily chosen and questions arise as to the proportion of sites actually probed in the adsorption experiment. The amount will be sensitive to the duration of exposure, the temperature, and the condition 'Presented at the Symposium on the Surface Chemistry of Coals, 193rd National Meeting of the American Chemical Society, Denver,, CO, April 5-10, 1987.
of the carbon surface. In addition there is very little known about the nature of the species produced by O2dissociation or the bonding environment of the reactive species in combustion and gasification processes. Our approach to (1)Laine, N.R.Vastola, F. L.; Walker, P. L., Jr. J.Phys. Chem. 1963, 67,2030. ( 2 ) Radovic, L. R.; Walker, P. L.; Jenkins, R. J. Fuel 1983,62, 191. (3)Nayak, R. V.; Jenkins, R. G. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1984,29(2),172. (4)Suuberg, E. M.; Calo, J. M.; Wojkowicy, M. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1986,31(3), 186. (5)Kyotani, T.; Zhang, Z. G.; Hayashi, S.; Tomita, A. Prepr. Pap.Am. Chem. SOC.,Diu. Fuel Chem. 1987,32(1),279. (6)Muhlen, H. J.; van Heek, K. H.; Juntgen, H. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1987,32(1),286. (7)Khan, M. R.Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1987, 32(1),298. (8)Otake, Y.;Jenkins, R. G. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1987,32(1),310.
0887-0624/88/2502-0111$01.50/0 0 1988 American Chemical Society
112 Energy & Fuels, Vol. 2, No. 2, 1988
help answer these questions has been to study glassy carbon, which is a relatively pure form of carbon that showed properties with respect to O2 adsorption similar to those of the edge surface of graphite.+'l Our results suggested that a large fraction of the exposed glassy-carbon surface is made up of edge carbon atoms. We used AES and XPS to directly monitor the surface oxygen concentration, TPD to monitor thermal decomposition products, and XPS C(1s) and O(1s) lines shapes to provide some insight into the surface bonding environment. X P S has been used to characterize the surfaces of a wide variety of different carbons that include glassy carbon,12-14 graphite,15J6and carbon fibers.1621 Information has been gathered about the degree of surface oxidation and the type of oxygen complexes formed after various oxidative t r e a t m e ~ ~ t s . ' ~ In - ~general ~ J ~ ~ a distinction about the kind of surface oxide is made based on the C(1s)emission, which occurs at binding energies higher than those from the main C(1s)line. Extensive XPS studies of organic molecules and polymeric material^^^-^' have demonstrated that the magnitude of the C(1s)peak shift to higher binding energy is related to the number of carbon oxygen bonds and is in the range of 1.5 f 0.4 eV per bond. The result is significant because common functionalities can be grouped according to their C(1s)shift. The guidance from these simple additivity rules has been employed in the interpretation of results from carbon surfaces. For example 1.5-eV shifts are associated with alcohols and ethers, -3.0-eV shifts with carbonyl groups, and -4.5-eV shifts with acids and esters.12-14~17~29 The O(1s) peak is also sensitive to the chemical environment, but interpretation in terms of oxygen functionality is complicated by larger relaxation effects.25 We have previously used AES in a comparative study of the O2and COzoxidation of glassy-carbon surfa~es.~ As expected O2 has a much higher reactivity. This was associated with a more facile gaseous dissociation step at high oxygen coverages, which generated lower energy CO formation sites. The available XPS results indicated that C02 and O2produced similar oxygen functionalities, which we interpreted to be carbonyls; however, a definitive identification based on the C(1s)peak was not made. The object
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(9) Kelemen, S. R.; Freund, H. Carbon 1985,23, 723. (10) Kelemen, S. R.; Freund, H. Carbon 1985,23, 619. (11) Kelemen, S. R.; Freund, H.; Mims, C. A. J . Vac. Sci. Technol., A 1984,2, 987. (12) Miller, C. W.; Harweik, D. H.; Kuwana, T. Recent Aduances in Analytic Spectroscopy; Pergamon: Oxford, England, 1982; pp 233-247. (13) Miller, C. W.; Harweik, D. H.; Kuwana, T. Anal. Chem. 1981,52, 2319. (14) Kamau, G. N.; Willis, W. S.; Rusling, J. F. Anal. Chem. 1985,57, 545. (15) Schlogl, R.; Boehm, H. P. Carbon 1983,21, 345. (16) Evans, S.; Thomas, J. M. h o c . R. SOC.London, A 1977,353,103. (17) Ishitani, A. Carbon 1981,19, 269. (18) Takahagi, T.; Ishitani, A. Carbon 1984, 22, 43. (19) Proctor, A,; Sherwood, P. M. A. J. Electron Spectrosc. Relat. Phenom. 1982,27, 39. (20) Kozlowski, C.; Sherwood, P. M. A. J. Chem. SOC.,Faraday Tram. 1 1984,80, 2099. (21) Kozlowski, C.; Sherwood, P. M. A. J. Chem. Soc., Faraday Tram. 1 1985,81, 2745. (22) Kozlowski, C.; Sherwood, P. M. A. Carbon 1986,24, 357. (23) Clark, D. T.; Thomas, H. R. J.Polym. Sci., Pol.ym. Chem. Ed. 1976, 14, 1671. (24) Clark, D. T. Adu. Polym. Sci. 1977,24, 125. (25) Clark, D. T. J . Polym. Sci., Polym. Chem. Ed. 1978, 16, 791. (26) Clark, D. T.; Cromarty, B. J.; Dilks, A. J . Polym. Sci., Polym. Chem. Ed. 1978,16, 3173. (27) Clark, D. T.; Dilks, A. J. Polym. Sci., Polym. Chem. Ed. 1979,17, 957. (28) Kelemen, S. R.; Mims, C. A. Surf. Sci. 1983, 133, 71. (29) Daniels, J.; Festenberg, C. V.; Raether, H.; Zeppenfeled, K. Springer Tracts Mod. Phys. 1970, 54, 78.
Kelemen and Freund of the present work is to obtain high-resolution C(1s)and O(ls) spectra produced under well-defined oxidation conditions. The spectra should lead to functional group identification. This information coupled with TPD results provides a good basis for further mechanistic understanding of carbon oxidation and gasification processes.
Experimental Section The glassy-carbon starting material was obtained as plates from Atomergic Chemetals. Samples were made in the form of powders and chips. Both powders and chips were used in TPD experiments while chips were used exclusively in XPS and AES experiments. 60-80-mesh powders were produced by grinding and sieving glassy-carbon plates. Powders were found to have 5 m2/g specific surface area based on Kr chemisorption measurements. The powders were out-gassed to 900 "C prior to oxidation. O2and COz oxidation were done in a Dupont Model 951 thermogravimetric analyzer. In these experiments, 50 mg of material was the normal sample size. The glassy-carbon chips were cut from plates with a diamond saw to 1 cm X 1 cm X 1 mm dimensions. The chips were successively polished with Buehler Carbiment silicon carbide paper up to 600 grit number. The samples were further polished with Buehler Alumina paste down to 0.05 pm size. The chips were ultrasonically washed with deionized water and then outgassed to -1300 "C under UHV (ultrahigh vacuum) prior to use. Following this treatment the residual surface sulfur, nitrogen, and oxygen concentrations were below 0.3 atom % based on XPS analysis. Silicon and alumina were below the detection limit of -0.05 atom %. The specific surface area of the chips were 0.3 m2/g based on Kr chemisorption measurements. Oxidative treatments could be given to samples in the TGA apparatus as with powdered samples. The chips were especially suited for use in an atmospheric pressure/UHV sample introduction system. Samples could be given reactive treatment up to 700 OC in 1 atm of gas and returned into a UHV apparatus for surface analysis without exposure to air. Glassy carbon chips, as well as powdered samples that received oxidative treatments in the TGA apparatus and were cooled to room temperature in the reactant gas mixture, could be transferred in atmosphere to respective holders for T P D experimentation. Brief exposure to air at room temperature did not alter reactivity patterns observed in subsequent T P D experiments. Oxidation by nitric acid was accomplished by boiling the samples in HNOB under reflux conditions. The TPD apparatus used with powdered samples was specially constructed as an appendage to a UHV spectroscopy chamber that housed a n Extranuclear quadrupole mass spectrometer interfaced to a P D P 11data acquisition system. This arrangement provided the capability to follow up to 11masses during a T P D experiment. The T P D unit has a base pressure of 5 X Torr pumped separately by a Balzers 300 L/s turbomolecular pump. The 5-10-mg samples were accommodated in a ceramic vessel 9 mm long X 3 mm in diameter. A chromel-alumel thermocouple was inserted into the sample bed. The sample holder was resistively heated by tantalum elements, and the sample temperature was controlled by a Micristar controller. X P S spectra were obtained from a Vacuum Generators ESCALAB equipped with a 150' spherical sector analyzer. The base pressure in the analysis chamber was less than 1 X Torr. Nonmonochromatic Mg Ka radiation was the excitation source. The X-ray source was operated at 300 W (20 mA, 15 kV). Glassy-carbon and graphite samples did not experience charging problems during data acquisition. The binding energy of freshly cleaved Union Carbide XYA monochromator grade graphite was 284.4 eV with respect to the Fermi level. All N(E) spectra were obtained a t 0.9-eV resolution. The signal from the C(1s) peak of graphite corresponded to 35000 counts/s. Typical counting times yielded spectrum with lo6 maximum counts. For presentation convenience each XPS N(E) and O(1s) and C(ls) spectrum is represented in the figures as a line rather than individual data points. The scatter is on the order of the line width. Each XPS difference spectrum represents smoothed data. The scatter is represented by an error bar in each set of curves. Data acquisition and manipulation was by means of the VGS 2000 software package using multiscan averaging. AES measurements were made in a
Energy &Fuels, Vol. 2, No. 2, 1988 113
X P S Characterization of Glassy-Carbon Surfaces I
Giarq Carbon 0, Oxidized
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Figure 1. (A)O(1s) spectrum of glassy carbon after oxidation with O2 at 300 and 700 "C. (B) O(1s) spectrum of glassy carbon after oxidation with COPat 300 and 700 "C.
separate UHV chamber by using a PHI double-passcylindrical mirror analyzer.
Results Carbon Oxidized by O2and COz. Clean glassy-carbon samples were oxidized in O2 and C 0 2 and characterized with XPS. Figure 1 shows the O(1s) signal on the same intensity scale. Each sample was oxidized for 300 s under specified conditions and cooled in the reactant gas. Negligible loss of carbon to gaseous products occurred at 300 "C. The 700 "C oxidations in O2and C 0 2 correspond to 20% and
