Ultrahigh Vacuum Studies of the Kinetics and Reaction Mechanisms of Ozone with Surface-Bound Fullerenes
Davis, Erin Durke
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Acquiring in depth knowledge of the ozone oxidation of surface-bound fullerenes advances the understanding of fullerene fate in the environment, as well as the reactivity of ozone with carbonaceous nanomaterials. Recent ultrahigh vacuum studies of the reaction of gasphase ozone with surface-bound fullerenes have made it possible to observe the formation and subsequent thermal decomposition of the primary ozonide (PO). As the use of nanomaterials, such as C60, continues to increase, the exposure of these molecules to humans and the environment is of growing concern, especially if they can be chemically altered by common pollutants. These experiments are made possible by combining ultrahigh vacuum surface analysis techniques with precision dosing using a pure O3 gas source. The experimental setup also provides the capability of monitoring surface-bound reactants and products in situ with reflection-absorption IR spectroscopy, while gas-phase products are detected with a mass spectrometer. Our results indicate that ozone adds across a 6/6 bond on the C60 cage, forming an unstable intermediate, the primary ozonide. The observed initial reaction probability for the PO is γ = 4.1 x 10-3. Energies of activation for the formation and decomposition of the PO were obtained via temperature-dependent studies. After formation, the primary ozonide thermally decomposes into the Criegee Intermediate which can rearrange or, upon further exposure to ozone, react with another ozone molecule to form a variety of products such as carbonyls, anhydrides, esters, ethers, and ketenes. Larger fullerenes (C70, C76, C78, and C84) were also exposed to gas-phase ozone, in order to observe the reaction rate for ozonolysis and to propose an initial mechanism for ozone exposure. The results indicate that the structure of the fullerenes has little to no impact on the rate of oxidation via ozone. Lastly, Terbium endohedral were exposed to ozone, in an effort to determine whether ozone was capable of oxidizing both the outer fullerene cage, as well as the Tb atom sequestered inside. The preliminary XPS data suggests ozone oxidizes both within an hour of continuous exposure. Understanding this atmospherically-relevant reaction from both a mechanistic and kinetic standpoint will help predict the environmental fate of fullerenes and their oxides.
- Doctoral Dissertations