Inelastic and Reactive Scattering Dynamics of Hyperthermal O and O2 on Hot Vitreous Carbon Surfaces

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• 作者：Vanessa J. Murray ; Brooks C. Marshall ; Philip J. Woodburn ; Timothy K. Minton
• 刊名：Journal of Physical Chemistry C
• 出版年：2015
• 出版时间：July 2, 2015
• 年：2015
• 卷：119
• 期：26
• 页码：14780-14796
• 全文大小：980K
• ISSN：1932-7455

文摘

We have undertaken a series of experiments to learn the mechanisms of carbon oxidation over a wide range of temperatures that extend to the conditions encountered during atmospheric re-entry, with a particular interest in understanding how these mechanisms change with temperature. We report here the hyperthermal scattering dynamics of ground-state atomic oxygen, O(³P), and molecular oxygen, O₂(³危_g^{鈥?/sup>), on vitreous carbon surfaces at temperatures from 600 to 2100 K. A molecular beam containing neutral O and O₂ in a mole ratio of 0.93:0.07 was prepared with a nominal velocity of 7760 m s鈥?, corresponding to a translational energy of 481 kJ mol鈥? for atomic oxygen. This beam was directed at a vitreous carbon surface, and angular and translational energy distributions were obtained for inelastically and reactively scattered products with the use of a rotatable mass spectrometer detector. Unreacted oxygen atoms exited the surface through both impulsive scattering and thermal desorption. The preferred scattering process changed from impulsive scattering to thermal desorption as the surface temperature increased. O₂ scattered mainly impulsively from the surface, and its scattering dynamics were essentially unaffected by surface temperature. The predominant reactive product was carbon monoxide (CO). Carbon dioxide (CO₂) was also formed at lower surface temperatures. The flux of the CO product rose with temperature to a maximum at approximately 1500鈥?900 K, depending on heating rate, and then decreased with increasing surface temperature. The CO₂ flux dropped dramatically with increasing surface temperature and was below detectable limits above 1100 K. A minor reactive pathway was identified that produced O₂, presumably through a direct Eley鈥揜ideal reaction of an incident oxygen atom with an O atom residing on the surface. Decreased oxygen surface coverage at higher temperatures was found to limit the reactivity of the surface by inhibiting the production of CO and CO₂ at very high surface temperatures. The observed inelastic and reactive scattering behavior reveals a complex interplay between reactivity and surface temperature.}