The oxidation of three amorphous carbons and an artificial graphite by atomic oxygen has been studied in the temperature range 14 to 350'C. The plasma of the radio-frequency discharge used to produce oxygen atoms was not in contact with the carbon sample. The rate of oxidation, at a fixed temperature, is dependent upon the duration of the experiment. This rate is also temperature dependent and activation energies of 9.9 to 10-8 kcal mole-1 are reported. The reaction is approximately first order with respect to atomic oxygen concentration. The rate of oxidation of the carbons, at constant temperature and concentration of atomic oxygen, is not influenced appreciably by the wide variations shown by the carbons in their chemical composition, internal surface area and poresizes, and degree of crystallinity. At temperatures above 200°C, the measured activation energy decreases and approaches almost zero at 350°C. The several factors, to which this is attributable, are discussed and evidence produced to show that surface-oxide can retard the rate of oxidation. For one carbon, surface-oxide is formed more readily from atomic oxygen than from molecular oxygen. The activation energy of gasification by atomic oxygen, and its relevance to the understanding of the mechanism of the reaction of molecular oxygen with carbon, is discussed.

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Crystalline Ticonderoga graphite, a pyrolytic graphite and two amorphous carbons have been oxidized with atomic oxygen and those changes which occur on the surfaces have been examined by optical and electron microscopy. Such a study assists in the understanding of the nature of " active sites " on carbonaceous surfaces. Unlike the reaction with molecular oxygen, atomic oxygen produced a general background of conical pits on surfaces of crystalline graphite, but not on the pyrolytic graphite, as well as what are considered to be etch pits, associated with defect structures. There was much less structural reaction anisotropy during oxidation by atomic oxygen than by molecular oxygen. The presence of iron on the surface of Ticonderoga graphite promoted recombination of atomic oxygen, so decreasing oxidation rates and caused the formation of hexagonal hillocks. The edges of these hillocks are composed of (IOiO} or zig-zag planes. There is evidence to suggest the presence of pure defect centres on Ticonderoga graphite. Amorphous carbons appear to possess centres of preferential reactivity in that extensive pitting of the surface occurred.

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