Dynamics of Graphite Oxidation at High Temperature

Interactions of ground-state atomic and molecular oxygen, O­(3P) and O2(3Σg –), with a highly oriented pyrolytic graphite surface were investigated for a broad range of surface temperatures from 1100 K to approximately 2300 K. A molecular beam composed of 89% O atoms and 11% O2, with average translational energies of 472.1 and 944.4 kJ mol–1, respectively, was directed at the surface with an incidence angle, θi, of 45°. Angle- and velocity-resolved distributions were collected for nonreactively and reactively scattered products with the use of a rotatable mass spectrometer detector. Four scattered products were observed: O, O2, CO, and CO2. O atoms that exited the surface without reacting exhibited both impulsive scattering (IS) and thermal desorption (TD) components. The primary reaction product observed was carbon monoxide (CO). Carbon dioxide (CO2) was measured only with surface temperatures below 1400 K, and O2 was attributed to IS of O2 that was present in the incident beam. Although there is evidence for either Eley–Rideal or hot atom reactions, CO and CO2 were primarily formed by Langmuir–Hinshelwood (LH) reactions. However, the flux angular distributions of the LH products were significantly narrower than a cosine distribution, and the final energies were much higher than those predicted by the Maxwell–Boltzmann distribution characterized by the surface temperature. These observations indicate that CO and CO2 that were produced by LH reactions desorb from the surface over a barrier. The desorption barrier of CO was determined by using the principle of detailed balance (where the desorption and adsorption barriers are equal) and was found to increase from 121 ± 5 kJ mol–1 at 1100 K to 155 ± 7 kJ mol–1 at 1300 K. As the surface temperature increased, the fluxes of CO and CO2 produced by LH mechanisms decreased. Simultaneously, the flux of O atoms that scattered via the TD channel increased, which reduced the surface oxygen coverage at higher temperatures. The combination of reduced O-atom surface coverage and increased desorption barriers for CO suppresses the reactivity of the surface at high temperatures.