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Investigating the Reactivity of Size-Selected Metal Oxide Clusters for Organophosphorus Nerve Agent Decomposition

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Investigating the Reactivity of Size-Selected Metal Oxide Clusters for Organophosphorus Nerve Agent Decomposition INVESTIGATING THE REACTIVITY OF SIZE-SELECTED METAL OXIDE CLUSTERS FOR ORGANOPHOSPHORUS NERVE AGENT DECOMPOSITION By Nicolas G. Blando A dissertation submitted to Johns Hopkins University in conformity with the requirements of the degree of Doctor of Philosophy. Baltimore, Maryland August 2020 © 2020 Nicolas G. Blando All Rights Reserved Abstract Metal and metal oxide nanostructures of varying size and composition are currently the industry standard for protection against chemical warfare agents (CWAs), yet there is limited understanding of the effects that the size and composition of the nanostructures have on the reaction mechanisms. Ultra-small, size-selected metal oxide clusters offer the opportunity of a nearly limitless number of unstudied candidate species as well as a chance to understand reactivity trends that may drive the discovery of more engineered catalysts. Clusters are small aggregates of atoms, <1000, which often display intriguing properties in that size range due quantum confinement, undercoordination and other emergent properties. In this work, the history and development of molecular beam techniques that support the study of size-selected clusters in-vacuo are discussed, leading up to the construction of a novel beamline apparatus designed for the synthesis and deposition of size-selected clusters for reactivity studies. Using these techniques, size selected manganese oxide, niobium oxide, tungsten oxide, copper molybdenum oxide, and copper zirconium oxide clusters were each studied for their ability to adsorb and decompose the organophosphorus nerve agent simulant dimethyl methylphosphonate. Advisor: Dr. Kit H. Bowen Committee: Dr. Howard Fairbrother Dr. Lan Cheng ii Acknowledgments I have been amazingly lucky to have the mentors and support that has allowed me, and even pushed me, to make it to this point. First, I’d like to thank my PhD advisor, Dr. Kit Bowen, who has given me every opportunity to succeed and become a better scientist and experimentalist. I recognize that my path to Johns Hopkins and to becoming a successful experimentalist was largely because he saw potential in me, and I could never thank him enough for that opportunity. Besides the important laboratory experience and mentorship he offered, I’d like to thank him for the opportunity to attend large conferences around the US and world, where experiences I had with collaborators and peers will be cherished forever. Second, I’d like to thank my graduate research committee members, Dr. Howard Fairbrother, Dr. Lan Cheng, and Dr. Art Bragg, for taking the time to be part of my graduate career. I’m particularly grateful to Dr. Fairbrother as well as his students for their support over the years. Next I would like to thank the members of my “sub-group”, the surface team, who have I have spent the last 3 years getting to know. I will forever be indebted to Dr. Zachary Hicks, our original sub-group leader, for the potential he saw in me and his decision to bring me into his project. His never-ending curiosity and drive were a wonderful example for a young scientist. Learning from and getting to know Linjie Wang has been one of the high points of my graduate career. She is a thoughtful and caring person, but as a scientist her power of deduction is without equal, and it has been a privilege to learn from and with her these last few years. Similarly, learning from Mike Denchy and watching him grow into an adept scientist the last couple years has been a privilege, his unwavering determination to fully understand the system or project at hand will continue to serve him well in life. In my short time with Ben Bilik, I have been impressed by his curiosity, and know that he will be iii extremely successful if he can apply that in the Bowen Lab, and similarly, Lucas Hansen’s theoretical prowess will make him a strong experimentalist. Throughout my 6 years in the Bowen lab, there is no one I have been closer to than Dr. Sandra Ciborowski. Her companionship since the very first day of graduate classes and the years of friendship and support since have been invaluable. She is a brilliant scientist and adept problem solver that I know will continue to impress others. Similarly, Mary Marshall was one of the first graduate students I worked closely with, and it has been a great experience to watch her grow into an adept experimentalist, whose advise I often seek, for both science and life questions. It has been gratifying to watch Zhaoguo Zhu become an extremely talented and well-rounded scientist, his personality will continue to brighten rooms wherever he goes. Rachel Harris is an extremely determined scientist and engineer, I have no doubt her tenacity will take her far after she is done at Johns Hopkins. My undergraduate research advisor Kris Hagel is responsible for introducing me to experimental physics, and I will always be grateful that he started that ball rolling. His enthusiasm and ingenuity set a high bar for any scientist. I’d like to thank the chemistry building manager, Boris Steinberg, who has been a great resource for work problems and life advice, and does an enormous amount of work behind the scenes to facilitate our research. My brother Robert for believing in me when no one, including myself, did. I’d like to thank my partner Allie for the endless adventures and unwavering support she has offered. And finally, my parents, without whom I would certainly not be here today. They provided me all the opportunity and skills that I could ever need, but it was their support along the way that I cherish most. iv Table of Contents Abstract .................................................................................................................. ii Acknowledgments ................................................................................................ iii List of Figures ..................................................................................................... viii I Atoms to Clusters ....................................................................................... 1 II Catalysis ..................................................................................................... 7 III Molecular Beams and Mass Spectrometry, a Historical Perspective ....... 11 III.1 Mass Spectrometry ............................................................................... 13 III.1.1 Magnetic Sector Mass Spectrometer ............................................. 15 III.1.2 Quadrupole Mass Spectrometer ..................................................... 18 IV Ion Sources for Cluster Beams ................................................................. 24 IV.1 Condensation and Cooling ................................................................... 25 IV.1.1 Supersonic Expansion ................................................................... 26 IV.2 Pulsed Arc Cluster Ionization Source (PACIS) ................................... 30 IV.3 Laser Vaporization Source ................................................................... 31 IV.3.1 Laser Vaporization Rod Source ..................................................... 33 IV.3.2 Laser Vaporization Disk Source .................................................... 37 IV.4 DC Magnetron Sputtering Source ........................................................ 39 V Ion Transport System ............................................................................... 45 V.1 Ion Guides ............................................................................................ 46 V.2 Electrostatic Optics .............................................................................. 52 V.2.1 Electrostatic Aperture ..................................................................... 52 v V.2.2 Electrostatic Quadrupole Bender .................................................... 55 V.2.3 Thin Plate Einzel Lens .................................................................... 58 VI Vacuum chambers and pumping system .................................................. 60 VI.1 Vacuum theory ..................................................................................... 60 VI.1.1 Differential Pumping ..................................................................... 63 VI.1.2 Vacuum Pumps .............................................................................. 63 VI.2 Cluster Deposition Pumping System ................................................... 67 VII Deposition and analysis system ............................................................... 74 VII.1 Deposition chamber ............................................................................ 74 VII.2 Analysis Techniques and Instrumentation .......................................... 76 VII.2.1 Residual Gas Analyzer ................................................................. 76 VII.2.2 Temperature Programmed Reaction/Desorption Spectroscopy ... 78 VII.3 Photoelectron Spectroscopy for Surface Analysis .............................. 82 VII.3.1 Electron Kinetic Energy Analyzer ............................................... 86 VII.3.2 Low Energy Electron Diffraction Spectroscopy .......................... 89 VII.3.3 Scanning Tunneling Microscope .................................................. 92 VIII Preliminary results of DMMP decomposition on novel metal oxide cluster compositions 95 VIII.1 Brief History of Chemical Weapons .................................................. 95 VIII.2 Development of Nerve Agents .......................................................... 96 VIII.2.1 Organophosphorus
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