Development of Novel Method to Investigate Unique Facets of Gamma-Alumina and Uncover the Connection between Surface Area and Particle Size

Natalie M. Stuart

doi:10.17918/00001790

Presented herein are three unique demonstrations of computational modelling and its utility in the fields of theoretical chemistry and materials science. Simulations that iteratively solve the Kohn-Sham equations can be utilized to study the stability and movement of internal and external chemical bonds for materials and molecules alike. This methodology is reviewed and shown explicitly for a simple system in Chapter 2. Presented first in this thesis is a literature review of the catalytically relevant, metastable metal oxide, gamma-alumina. The material features an intrinsically defect crystal structure, resulting in high specific surface areas. These qualities lend themselves well to utility as a catalyst support, most notably for precious metals in automotive catalytic converter systems. Gamma-Alumina is difficult to study experimentally due to its incredibly small particle size, orders of magnitude too miniscule to be studied by methods such as single crystal X-ray diffraction. Thus, gamma-alumina's defect crystal structure has been the subject of sustained debate. Though gamma-alumina has been used for centuries, optimal usage of the material cannot be achieved until all its properties are fully understood. Herein is a comparison of documentation, both experimental and computational, on the material's interior (bulk) and exterior (surface) structure. Accurately determining the energy of a solid surface provides crucial information about the material's potential utility for adsorption and catalysis. The standard method of calculating surface energy suffers critical weaknesses when applied to materials that expose atomically different terminations (asymmetric slabs) due to the inaccurate assumption that the two terminations exhibit exactly the same energy. Cleavage of catalytically-relevant gamma-alumina surfaces often results in asymmetric slab models, thus the energies of these surfaces could be generated with higher levels of accuracy. In this thesis, a novel method is presented that expresses a slab's total energy in terms of the energetic contributions of the top (A) and bottom (B) surfaces in both relaxed and frozen states. Total energies for different combinations of these conditions are obtained through a series of density-functional-theory calculations alternately optimizing different parts of the slab model. The equations are then solved for the individual surface energy contributions. This novel surface energy tool was then used to investigate the possible physical constraint on gamma-alumina's particle size. Particles of gamma-alumina were approximated as both cubes and rhombohedrons, which are reasonable assumptions based on the material's atomic structure. Utilization of the novel method presented in Chapter 3 of this thesis allowed for the accurate calculation of surface energies for all terminations ((110)a, (110)b, (101)a, (101)b, (011)a, and (011)b) of the approximated gamma-alumina particles. This information was incorporated into a function of total energy vs edge length to determine the critical edge length at which the formation of two particles becomes energetically favored over continued growth of a single large particle. It has thus been determined that gamma-alumina's defects are energetically preferred to occupy surface layers, to such an extent that it may physically restrain the material's particle size. The final project details the role of computational modeling to answer questions that experiments cannot. After polymerized o-aminophenol, which contains useful electric and optical properties for utility in energy storage devices, was generated via plasma treatment, computational modeling was utilized to glean more information about the polymer's structure. Infrared spectra for several different isomers were generated and combined to demonstrate that the polymer structure was much more likely to be exhibiting at least two geometric arrangements rather than just one structure. In conclusion, the works described in this thesis demonstrate the vast advantages of computational modeling and specific cases in which theory and computation have been successfully applied to answer questions that were unanswerable via experiment.

Development of Novel Method to Investigate Unique Facets of Gamma-Alumina and Uncover the Connection between Surface Area and Particle Size

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Development of Novel Method to Investigate Unique Facets of Gamma-Alumina and Uncover the Connection between Surface Area and Particle Size

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