-Puel
GasificationAcknowledgment
The Authors are grateful to the director-general of British Coal Utilisation Research Association for permission t o present this paper.
Literature Cited Adama, A. M., and Kramers, W. J., private communication. Arthur, J. R., Nature, 157,732 (1946). iirthur, J. R., Trans. Faradag Soc., 47, 164 (1951). Arthur, J. R., and Bowring, J. R., IND.ENG.CHEM.,43, 528 (1951).
Arthur, J. R., and Bowring, J. R., J . Chem. Soc., 1949, Sl. Baker, H. B., J. Chem. Soc., 1885, 349. Bangham, D. H., and Stafford, J., Ibid., 1925,1085. Crone, H. G., and Bowring, J. R., Symposium on Combustion of Carbon, Nancy (1949). Davis, H., and Hottel, H. C., IND. EKG.CHEM., 26,889 (1934).
Garner, W. E., et al., J. Chem. Soc., 1929, 1123; 1930, 2037; 1932, 129; 1935, 144. Gaydon, -4. G., PTOC. Roy. Sac. (London),A.176,505 (1940). Gaydon, A. G., "Spectroscopy and Combustion Theory,'' p. 79, London, Chapman and Hall, 1948. Hirst, W., and Cannon, C. G., J . S c i . Inatiuments, 20, 129 (1943). Hornbeck, G., "Third Symposium on Combustion. Flame, and Explosion Phenomena," p. 501, Baltimore, WilliaJns and Wilkins, 1949. Jones, R.E., and Townend, D. T. A., J . Soc. Chem. Ind., 68, 197 (1949). Kondratjew, V., 2. Physik, 63,322 (1930). Kondratjew, V., et al., J . Phys. Chem. (U.S.S.R.), 11, 331 (1938). Letort, hl., and Martin, J., Bull. soc. chim. France, 1947, 400. Parker, A. S., and Hottel, H. C., IND.ENG. CHEW,28, 1334 (1936). Strickland-Constable, R. F., Trans. Faraday Soc., 40, 333 (1944). Whittingham, G., Fuel, 19,244 (1950). RECEIVED for review July 31,1951.
ACCEPTED M a r c h 8,1952.
Reactions of Artificial Graphite Kinetics of Oxidation of Artificial Graphite at Temperatures of 425" to 575" C. and Pressures of 0.15 to 9.8 Cm. of Mercury of Oxygen Earl A. Gulbransen and Kenneth F. Andrew WESTINGHOUSE RESEARCH LABORATORIES, EAST PITTSBURGH, PA.
A systematic study has been made of the chemical reaction of p u r e artificial graphite with oxygen at temperatures of 425" to 575" C. a n d at pressures of 0.15 to 9.8 cm. of mercury of oxygen a n d with carbon dioxide at temperatures of 500' to 900' C. a n d 7.6 cm. of mercury of carbon dioxide. T h e naturg of the resulting surface oxides has been investigated. T h e oxidation data are correlated with the fundamental postulates of the activated state theory of chemical reactions on surfaces. Oxidation rate data can b e fitted to the empirical CtZ, where K a n d C a r e constants a n d equation W = K t t is the time. T h e initial rate constant, K, follows a n exponential ratelaw as a function of the temperature, K = Z e - E I R T . A n energy of activation of 36,700 calories per mole is calculated for E. As a function of the pressure K follows the equation K = A BP. T h e effect of pretreatment on the rate of reaction with oxygen has been investigated. T h e formation of a surface oxide with oxygen a t 500 O C.is a gradual a n d not a simultaneous process. Surface roughness as determined by the adsorption of krypton a t liquid nitrogen temperatures can b e correlated to the value calculated from
the extent of the surface oxide formation. Heating to 950" C. increases the surface roughness, as does oxidation at 500' C. Oxidation at room temperature decreases the surface roughness. T h e absolute value for the reaction rate of graphite is determined from surface roughness measurements a n d kinetic data. T h e fundamental postulates of the activated state theory of chemical reactions on surfaces are discussed. Theoretical rate expressions for the oxidation of graphite a r e compared with experimental rates of reaction. Two adsorption processes, immobile adsorption with dissociation and mobile adsorption, are shown to b e possibly the rate-controlling processes for the oxidation of pure graphite. T h e reaction of graphite with carbon dioxide a t 500' C. a n d 7.6 cm. of mercury of carbon dioxide corresponds to the formation of one fortieth or less of a monolayer of surface oxide. Elemental iron greatly accelerates this reaction. Preoxidation is found to change the initial rate of reaction with only a minor effect on the long-term reaction. Results show that the surface oxide that is observed on degassing is not a preliminary step in the reaction.
T
order to avoid limitation of the reaction by transport of the reacting gas to the surface and the reaction products away from the surface, to avoid temperatures where the reduction of carbon dioxide by carbon becomes feasible thermodynamically, to avoid conditions where the slow heterogeneous wall reaction becomes important (28) and t o avoid the tip of the low pressure explosion peninsula of carbon monoxide and oxygen (28). Slthough a number of methods may be used for the study of the reaction kinetics, it occurred to the authors that a sensitive balance operating in a high vacuum system (14, 15) would be particularly appropriate for studying the oxidation kinetics on strip specimens of pure graphite.
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HE primary chemical reaction of graphite with oxygen is of interest from both a scientific and a technical point of view. If the direct chemical reaction can be studied by a suitable choice of material, method, and experimental conditions of temperature, pressure, and pretreatment. fundamental information can be obtained on the kinetics and mechanism of the reaction, including the nature of the primary reaction product. Such information is important from a technical point of view in t h e design and use of equipment and the choice of conditions for burning solid commercial fuels. To study the primary reaction a t normal pressures it is necessary t o study the reaction a t temperatures we11 below 700" C., in 1034
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 5
-Fuel The apparatus is constructed of glass and ceramic ware (14, 16). T o avoid contamination, metal parts, stopcocks, grease, and rubber joints are eliminated. Although some preliminary pretreatments are done outside of the vacuum system, the final treatments of the specimens are made in place. These include degassing of t h e sample and surface oxide removal by heating in high vacua or in purified hydrogen atmospheres. , To avoid secondary reactions, pure samples of artificial graphite are used, t h e preparation, history, analysis, and gas and surface oxide contents of which are accurately known.
Literature Survey Langmuir (24)first used the graphite filament method t o study the primary chemical reaction of graphite with oxygen. These filaments are heated in the atmosphere of the gas, usually a t low pressures. Since this early work a number of other investigators have used methods based on this technique. Strickland-Constable (45, 46) and Sihvonen (43) used a stationary gas atmosphere, while Sihvonen (43),Meyer (5,51, 54,55), and Duval (8) used a circulating source of gas. Langmuir ($4) found that a t 950" C. carbon dioxide, was formed by the impact of the oxygen molecules on the clean carbon surface. Rhead and Wheeler (59) working on charcoals at temperatures near 500" C. found primary formation of both carbon monoxide and carbon dioxide. Lambert (25) studied the combustion of granular graphite and other sources of carbon in the temperature range of 250' t o 600' C. Graphite and diamond formed carbon dioxide in the primary reaction on a clean carbon surface. Letort and coworkers (86-27) recently have studied the reaction kinetics using a flow method. The chemical and crystallographic factors in the combustion of graphite have been reviewed by Riley (40).
Equilibria Calculations Equilibria calculat,ions are made on a number of reactions in order t o determine the thermodynamic feasibility of the several secondary reactions and the stability of the reaction products. This information is of value in the interpretation of the results. The thermodynamic data are taken from a recent work of the Xational Bureau of Standards (36'). Table I shows t h e seven reactions considered, together with the calculated values of logto KRfor the reactions as written, where KR is the equilibrium constant. The temperature range is 25' to 1127" C. Three reactions are of interest in considering the direct oxidation of pure graphite with dry oxygen: the stability of the reac-
tion products carbon dioxide and carbon monoxide to thermal decomposition and the secondary reaction of carbon dioxide with graphite t o form carbon monoxide. The equilibrium constants for these reactions are shown in columns I, 2, and 4 in Table I. The data show that both carbon dioxide and carbon monoxide are stable t o decomposition, whereas the reaction of carbon dioxide with graphite is possible only above 700' C. This reaction will not interfere in oxidation studies below 600' C. Reactions 3, 5, 6, and 7 are of concern if the oxidation is carried out in water vapor plus oxygen or in hydrogen and oxygen atmospheres, Reaction 5 is feasible up t o a temperature of 675" C. and constitutes a possible secondary reaction if hydrogen is present below 675" C. or if water vapor is present above this temperature. Reaction 6 is of importance only when hydrogen is present above 800' C. or wheii water vapor is present below 800' C. Reaction 7 shows methane t o be stable in t h e presence of graphite t o 550" C., while Reaction 3 shows water t o be stable over the temperature range studied.
Table 11. Spectrographic Analyses %
Element
0.01 0.005 0.002
