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Thermodynamic Properties of Organometallic Dihydrogen Complexes for Hydrogen Storage Applications

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Thermodynamic Properties of Organometallic Dihydrogen Complexes for Hydrogen Storage Applications View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Caltech Theses and Dissertations Thermodynamic Properties of Organometallic Dihydrogen Complexes for Hydrogen Storage Applications Thesis by David Gregory Abrecht In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy California Institute of Technology Pasadena, California 2013 (Defended December 4, 2012) ii c 2013 David Gregory Abrecht All Rights Reserved iii This work is dedicated to my family... Steel is shaped by the hammer and anvil, but it is the care and patience of the smith that make the shape meaningful. iv Acknowledgements There are many people without whom this document would not exist. First and foremost, I would like to acknowledge my advisor, Brent Fultz, for both his efforts in helping to see this work to completion and for his willingness to take a risk on me, a student from another discipline with an idea that he wanted to follow. Without his support and guidance, this project would neither have gotten off of the ground nor would it have survived the crises of direction, funding, and drive that inevitably plague all research projects. Similarly, I would like to thank all of the current and former members of the Fultz laboratory who have served as mentors, friends, brainstorming collaborators, and sympathetic ears: Channing Ahn, Hongjin Tan, Justin Purewal, Mike Winterrose, Jorge Munoz, Sally Tracy, Tian Lan, Nick Stadie, Max Murialdo, Hillary Smith, Lisa Mauger, Jiao Lin, Dennis Kim, Elsa Callini, Delphine Borivent, and Brandon Keith. The support of this exceptional group of people has been instrumental not only for the training and advice they offered freely but in also helping me maintain the necessary work-life balance to be productive and healthy. I am extremely grateful to the Resnick Sustainability Institute and its staff for their financial, ideological, and personal support of both this project and sustainability science students overall. This group of extraordinary people were not only willing to support student projects in burgeoning fields during difficult and increasingly risk averse economic times, but have done so with the understanding that, in order to produce experienced researchers, you must allow fresh researchers experience. In particular I wish to recognize the sacrifice and long hours put forward by the Institute staff, Harry Atwater, Neil Fromer, Valerie Otten, and Heidi Rusina, to build the Institute into an environment where new ideas can flourish and scientists can be trained and can work effectively toward the v solution to some of our world's toughest challenges. I would like to thank my collaborators Joseph Reiter, Jason Zan, and Phil Wilson at the Jet Propulsion Laboratory for providing access to instruments and for lending their engineering and systems expertise. These men taught me not only what world-class science and engineering looks like, but what otherworldly science and engineering has to look like to be successful, and my work has been and will be only the better for it. Thanks goes to Theo Agapie and the Agapie research group for training and assistance with the chemistry aspects of this work: Madalyn Radlauer, Davide Lionetti, Sibo Lin, Jacob Kanady, Dave Herbert, Guy Eduoard, Justin Henthorn, Kyle Horak, Sandy Suseno, Paul Kelley, Emily Tsui, Josh Wiensch, Steven Chao, Christine Cheng, Nadia Lara, and Eva Nichols. Finally, I would like to thank the instrument and support staff at Caltech, without whom nothing at all would ever get done. In particular, I would like to thank Sonjong Hwang at Caltech's Solid- State NMR Facility, David VanderVelde at Caltech's Liquid NMR Facility, Larry Henling at Caltech's Small Molecule Crystallography Facility, and Carol Garland at Caltech's TEM Facility for training and support during my time here. I am grateful to Mike Vondrus, Mike Roy, Steve Olson, and Rick Gerhart in the Instrument Shops for their help on design, construction and repair of instrumentation. Special thanks go to Avalon Johnson at Caltech's Center for Advanced Computing Research for her assistance with the computational chemistry included in this work, and for management of the computational clusters and software. vi Abstract Hydrogen gas has been lauded as a clean fuel technology for its direct combustion to water vapor and its potential for production from renewable fuel sources. However, as a lightweight gas the low volumetric density of hydrogen makes it inappropriate for use in most fuel applications. Hydrogen storage technologies that promote the compression of the gas to useable volumes are a critical component to further development of a hydrogen fuel-based economy. A number of solid-state storage systems based on metal and complex hydrides, chemical hy- drides, and high-surface-area materials have been developed for use as storage media for hydrogen fuel. However, these technologies, which are separated broadly into absorption and adsorption com- plexes, display typical binding enthalpies too high or too low for practical applications, respectively. In recent years, new complexes designated as Kubas materials containing unsaturated metal centers and high surface areas have been produced and show binding energies intermediate to absorption and adsorption complexes while maintaining beneficial kinetics and binding capacities, demonstrating great promise as hydrogen storage candidates. This behavior has been attributed to the Kubas bond, which binds H2 chemically as a molecule to an unsaturated metal center based on the Dewar-Chatt- Duncanson molecular binding model. However, the bulk absorption mechanism and thermodynamic behaviors for these interactions remain unknown. Kubas materials have historically been evaluated as physisorption materials for hydrogen storage applications; however, the recent display of absorp- tion plateaus and exact stoichiometric values in isotherm measurements has led to uncertainty in the use of the physisorption model to explain the bulk interaction mechanism, which is necessary to accurately assess thermodynamic properties. The bulk thermodynamic mechanism of hydrogen binding in dihydrogen complexes, which dis- vii play \pure" Kubas bonds, is also poorly understood in the solid state. Identifying the mecha- nism of Kubas bonding can lead to improved thermodynamic characterization of both dihydrogen complexes and new generation hydrogen storage materials. To address this issue, the solid-state 24 complex [Mn(CO)dppe2][BArF ] was synthesized and the thermodynamic behavior and properties 2 24 of the hydrogen absorption reaction to form the dihydrogen complex [Mn(η -H2)dppe2][BArF ] were measured over the temperature range 313K{373K and pressure range 0{600 torr using the Sieverts method. The absorption behavior was accurately described by Langmuir isotherms, and enthalpy and entropy values of ∆H◦ = -52.2 kJ/mol and ∆S◦ = -99.6 J/mol-K for the absorption reaction were obtained from the Langmuir equilibrium constant. The observed binding strength was similar to metal hydrides and other organometallic complexes, despite rapid kinetics suggesting a site-binding mechanism similar to physisorption materials. Electronic structure calculations us- ing the LANL2DZ-ECP basis set were performed for hydrogen absorption over the organometallic + fragments [M(CO)dppe2] (M = Mn, Tc, Re). Langmuir isotherms derived from calculation for absorption onto the manganese fragment successfully simulated both the pressure-composition be- havior and thermodynamic properties obtained from experiment. Results from calculations for the substitution of the metal center reproduced qualitative binding strength trends of 5d > 3d > 4d previously reported for the group 6 metals. 2 Solid-state complexes [FeHdppe2][NTf2] and [FeH(η -H2)dppe2][NTf2] were also synthesized and measured by x-ray diffraction and M¨ossbauerspectroscopy to determine the thermal decomposition pathway. Simulations at the B3LYP/TZV level of theory and experimental M¨ossbauerspectra 2 confirmed the direct thermal decomposition from singlet state [FeH(η -H2)dppe2][NTf2] to triplet state [FeHdppe2][NTf2] under vacuum conditions at 398K. Sieverts analysis showed samples to have extremely rapid H2 uptake rates and significantly slower decomposition rates, with an Arrhenius activation energy of Ea = 32.3 kJ/mol for hydrogen release. Evaluation of the partial quadrupole splitting values from M¨ossbauerspectroscopy revealed a value of Q(H2) = -0.245 mm/s, similar to the values obtained by Morris and Schlaf for similar complexes and significantly different from values obtained for hydrides, indicating an underutilized mechanism for identification of dihydrogen ligands. viii + Langmuir simulations of hydrogen absorption onto the singlet-state complexes [MHdppe2] (M = Fe, Ru, Os) revealed the same 5d > 3d > 4d trend and similar enthalpies to those observed for the group 7 complexes. Singlet-state thermodynamic values from simulation were consistent with experimental observations for Ru and Os, and ruthenium complexes were found to have thermodynamic properties within appropriate ranges for hydrogen storage applications. Simulated thermodynamic values for Fe complexes were found to significantly underestimate experimental behavior, demonstrating the importance of the magnetic spin state of the molecule to hydrogen binding properties. ix Contents Acknowledgements iv Abstract vi 1 Introduction to Hydrogen Storage Technologies 1 1.1 Background . .2 1.2 Evaluation of Thermodynamic Properties . .3 1.3 Properties
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