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 Title: Quantum Monte Carlo Study of Hydrogen Adsorption on Carbon and Transition Metal Systems Author(s): Beaudet, Todd D. Director of Research: Martin, Richard M. Doctoral Committee Chair(s): Trinkle, Dallas R. Doctoral Committee Member(s): Stack, John D.; Nayfeh, Munir H.; Martin, Richard M. Department / Program: Physics Discipline: Physics Degree Granting Institution: University of Illinois at Urbana-Champaign Degree: Ph.D. Genre: Dissertation Subject(s): hydrogen storage hydrogen titanium ethylene benzene, titanium dihydride Adsorption transition metal Quantum Monte Carlo (QMC) Diffusion Monte Carlo (DMC) Density Functional Theory (DFT) Abstract: This thesis research has successfully carried out a QMC study of hydrogen adsorption on Ti-ethylene molecular systems demonstrating reversible hydrogen adsorption on molecular TiH$_2$C$_2$H$_4$. This system is chosen as representative of larger carbon-transition-metal systems that may be relevant for practical hydrogen storage. To the author's knowledge this is the first study of hydrogen adsorption on transition metal systems by QMC methods. These systems present challenges in terms of a large number of possible molecular structures that are very close in energy, 3d states of transition elements that are difficult to treat, and molecular geometries that can be difficult to determine. Several studies are presented that demonstrate the suitability of QMC methods for this class of problem. A QMC study of hydrogen on benzene, which has already been published, tests the Slater-Jastrow (SJ) trial function against a more highly correlated Geminal trial function. The Slater-Jastrow form used here is shown to perform equivalently in measuring the small energy differences associated with physisorption. A series of tests is conducted on the Ti atom transition energies. QMC SJ results are found to be in excellent agreement with experiment so that significant cost savings can be achieved by using a pseudopotential for Ti. An extensive study on the TiH$_2$ system which is relevant to the final system studied is also presented. There a fixed-node DMC geometry optimization is conducted. It is shown the Perdew-Burke-Ernzerhof (PBE) functional for Density Functional Theory (DFT) is able to give geometries with energies that are within 1.5 mHa of the DMC optimal geometries. Also, it is consistently demonstrated throughout the work that QMC methods with SJ trial functions are only weakly dependent on the single-body theory used to produce the trial function. The primary results related to hydrogen storage are derived from studies on many structures of Ti-ethylene with up to 5 H$_2$ molecules. Ground and excited states are both considered. Formation energies are calculated and comparison is made to other work. It is shown that at least three hydrogen molecules can be adsorbed with energies in the range considered relevant for practical hydrogen storage. Issue Date: 2011-01-21 URI: http://hdl.handle.net/2142/18600 Rights Information: Copyright 2010 by Todd D. Beaudet Date Available in IDEALS: 2011-01-212013-01-22 Date Deposited: 2010-12
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