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Ultrahigh Vacuum Studies of the Kinetics and Reaction Mechanisms of Ozone with Surface-Bound Fullerenes Erin Durke Davis

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Ultrahigh Vacuum Studies of the Kinetics and Reaction Mechanisms of Ozone with Surface-Bound Fullerenes Erin Durke Davis Ultrahigh Vacuum Studies of the Kinetics and Reaction Mechanisms of Ozone with Surface-Bound Fullerenes Erin Durke Davis Dissertation submitted to the faculty of the Virginia Polytechnic Institute and State University In partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY In Chemistry John R. Morris, Chair Karen J. Brewer Gary L. Long Diego Troya Sungsool Wi October 10, 2011 Blacksburg, Virginia Keywords: C60, ozone, fullerenes, ultrahigh vaccum, endohedrals, ozonolysis Copyright 2011, Erin D. Davis Ultrahigh Vacuum Studies of the Kinetics and Reaction Mechanisms of Ozone with Surface-Bound Fullerenes Erin Durke Davis (ABSTRACT) 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 gas- phase 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 an 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 = 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. For Mom iii Acknowledgments This achievement was nothing short of a group effort. Without the support of my family, friends, and colleagues, this never would have been possible. First, I’d like to thank my husband, Eric. Your constant encouragement, support, and appreciation have made me a better scientist and, more importantly, a better person. I would not be who I am today without you, and for that, I am forever grateful. I would also like to thank my mom, dad, and brother. Mom – to this day, you continue to push me, expect nothing but the best from me, and always remind me to never give up on my goals. Dad – your support and understanding has been endless. Thank you for always providing some much needed perspective. Bryce – you always make me smile. Everyday I’m thankful for such an amazing brother, but as I’ve aged, I’m even more appreciative of our friendship. To my fellow Morris group members: “Fair warning! During graduate school, you may meet heavy resistance!” The quest for my degree has been nothing short of a momentous challenge, but I can honestly say, I would not have made it to the end without each and every one of you. Larry – thank you for teaching me all you could. I enjoyed my graduate research immensely, and that is largely in part to you. Leslie, Jessica, and Will – thank you for engaging discussions, challenging questions, and a little bit of fun on the side. I’m so glad that we went through this experience together and I wish you all the best of luck in the future. Alec, Monica, and Yafen – my ozone team! I, literally, would not have been able to make it through without you! Your help was invaluable and I hope you all enjoyed working together as much as I did. I’d also like to thank Josh Uzarski, Amanda Bolger, Dimitar Panayotov, Tommy Rockhold, Wesley Gordon, and Steve Burrows. My research would not have been possible without the help of Tom Wertalik, the most talented glass blower I know; an important person to know when you work with ozone! Thank you to Dave Simmons and Darrell Link from the Engineering machine shop for all those last minute parts. I’d also like to thank Josh Layfield and Tim Fuhrer for their help with numerous calculations. A big thank you to my committee for all your help and advice over the last few years. Finally, I am forever grateful to my advisor, John Morris. John – your support, patience, and understanding throughout my years in your group made all the difference. I thank you for helping me become the scientist I am today. I can honestly say, I am more passionate about, not only chemistry, but learning and teaching than I have ever been. As difficult as this journey was, it only makes it that much more rewarding now that I’ve reached the end. Thank you for holding your students to such high standards and always pushing me forward. iv Table of Contents List of Figures .................................................................................................................. viii List of Tables ................................................................................................................. xviii Index of Acronyms .......................................................................................................... xix Chapter 1 ..............................................................................................................................1 Introduction and Motivation ................................................................................................1 Thesis Statement ..................................................................................................................1 1.1.1 Background ..................................................................................................1 1.1.1 Carbon Nanomaterials in the Troposphere ..................................................2 1.1.2 Fullerenes .....................................................................................................6 1.1.2.1 The History of Fullerenes ............................................................................6 1.1.2.2 Fullerene Structure .......................................................................................7 1.1.2.3 Fullerene Properties .....................................................................................9 1.1.3 Ozone .........................................................................................................12 1.1.3.1 Ozone’s History .........................................................................................12 1.1.3.2 Ozone’s Properties .....................................................................................12 1.1.3.3 Reactions of Ozone and Organics ..............................................................14 1.1.3.4 Reactions of Ozone with Organics at the Gas-Surface Interface ...............19 1.2 Reactivity of Ozone with C60 .....................................................................22 1.2.1 Reactions of Ozone with C60 in Solution ...................................................22 1.2.2 Reactions of Ozone with C60 in Solid Phase ..............................................26 1.2.3 Computational Studies of Reactions of Ozone with C60 ............................26 1.3 Ultra High Vacuum Experiments ...............................................................28 1.4 Summary ....................................................................................................29 Chapter 2 ............................................................................................................................31 Instrumental Modifications and Experimental Approach ..................................................31 2.1 Vacuum Considerations .............................................................................31 2.2 Experimental Approach .............................................................................32 2.2.1 Ozone Generation, Purification, and Storage ............................................32 2.2.2 Sample Preparation ....................................................................................38 2.2.3 Main Chamber and Analytical Instrumentation .........................................42 2.2.4 Reactant Calibration...................................................................................53 2.2.5 Electronic Structure Calculations ..............................................................56 2.3 Summary ....................................................................................................56 Chapter 3 ............................................................................................................................57
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