Ayoola, Henry
(2021)
Unraveling the Crystal Structure of Gamma-Alumina through Correlated Experiments and Simulations of a Model System.
Doctoral Dissertation, University of Pittsburgh.
(Unpublished)
Abstract
Gamma-alumina (γ-Al2O3) is a metastable alumina phase possessing useful properties, such as inherently high surface area and acidic surface sites. As a result, it is an important material for heterogeneous catalysis, with applications in oil refining, vehicle exhaust catalytic conversion, and the oxidation of methane—a major greenhouse gas. However, despite the widespread use and study of γ-Al2O3, its precise atomic structure is still not fully understood. This has led to conflicting predictions of γ-Al2O3 properties and resulting catalytic behavior. A major contributor to the uncertainty in the γ-Al2O3 structure has been the use of commercial samples for structural studies. Commercially available γ-Al2O3 is chemically and morphologically inhomogeneous which leads to inconsistent characterization results. The overall aim of my research was to contribute towards full resolution of the γ-Al2O3 structure by unambiguously resolving the space group and Al atom arrangement. In contrast to previous studies, I utilized chemically pure, morphologically well-defined, highly crystalline “model” γ-Al2O3 which I synthesized by controlled thermal oxidation of NiAl. I then studied the atomic structure of the model γ-Al2O3 using synergistic experimental and simulated selected-area electron diffraction (SAED) and high-resolution electron energy-loss spectroscopy (EELS). My results revealed that the cubic spinel-based model was the most accurate γ-Al2O3 structural model. Additionally, I resolved the Al vacancy distribution within the cubic spinel-based model using quantitative analysis of reflection intensities in the correlated experimental and simulated SAED, revealing that 50-80% of vacancies are found on tetrahedral Al sites. Experimental high-resolution cryo-EELS correlated with EELS simulations performed using FEFF (a real-space multiple scattering code) confirmed the presence of vacancies primarily on tetrahedral sites. The synergistic approach of using a model γ-Al2O3 combined with the characterization methodology used in this project could be applied to other structurally complex materials.
In addition, this research has also advanced knowledge on the mechanisms of electron beam damage in γ-Al2O3 and their suppression, and on the interfacial bonding of Pt on γ-Al2O3 (111). Ultimately, a more accurate approach to modeling γ-Al2O3 structure as revealed in this project will accelerate catalyst design and discovery of catalytic applications for such a promising catalyst material as γ-Al2O3.
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Details
| Item Type: |
University of Pittsburgh ETD
|
| Status: |
Unpublished |
| Creators/Authors: |
|
| ETD Committee: |
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| Date: |
26 January 2021 |
| Date Type: |
Publication |
| Defense Date: |
5 October 2020 |
| Approval Date: |
26 January 2021 |
| Submission Date: |
24 July 2020 |
| Access Restriction: |
No restriction; Release the ETD for access worldwide immediately. |
| Number of Pages: |
258 |
| Institution: |
University of Pittsburgh |
| Schools and Programs: |
Swanson School of Engineering > Chemical Engineering |
| Degree: |
PhD - Doctor of Philosophy |
| Thesis Type: |
Doctoral Dissertation |
| Refereed: |
Yes |
| Uncontrolled Keywords: |
Gamma-alumina
Platinum
EELS
SAED
TEM
STEM
Vacancies
Crystal structure |
| Date Deposited: |
26 Jan 2021 21:20 |
| Last Modified: |
26 Jan 2021 21:20 |
| URI: |
http://d-scholarship.pitt.edu/id/eprint/39442 |
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