Unraveling the Crystal Structure of Gamma-Alumina Through Correlated Experiments and Simulations of a Model System
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Title Page Unraveling the Crystal Structure of Gamma-Alumina through Correlated Experiments and Simulations of a Model System by Henry Ayoola B. Sc. Chemical Engineering, University of Maryland Baltimore County, 2013 Submitted to the Graduate Faculty of the Swanson School of Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy University of Pittsburgh 2020 Committee Membership Page UNIVERSITY OF PITTSBURGH SWANSON SCHOOL OF ENGINEERING This dissertation was presented by Henry Ayoola It was defended on October 5, 2020 and approved by Ioannis Bourmpakis, Ph.D., Associate Professor Department of Chemical and Petroleum Engineering James R. McKone, Ph. D., Assistant Professor Department of Chemical and Petroleum Engineering Dissertation Co-Director: Wissam A. Saidi, Ph. D., Associate Professor Department of Mechanical Engineering and Materials Science Dissertation Director: Judith C. Yang, Ph. D., Professor Department of Chemical and Petroleum Engineering ii Copyright © by Henry Ayoola 2020 iii Abstract Unraveling the Crystal Structure of Gamma-Alumina through Correlated Experiments and Simulations of a Model System Henry O. Ayoola, PhD University of Pittsburgh, 2020 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 iv 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. v Table of Contents Preface ....................................................................................................................................... xxvi 1.0 Introduction ............................................................................................................................. 1 1.1 Background and Significance ........................................................................................ 1 1.2 Heterogeneous Catalysis ................................................................................................ 3 1.3 Heterogeneous Catalyst Design ..................................................................................... 4 1.3.1 Rational Catalyst Design Approach ...................................................................5 1.3.2 Model System-Based Studies of Heterogeneous Catalysts ...............................6 1.4 Gamma-Alumina: An Important Heterogeneous Catalyst Material ........................ 8 1.4.1 What is Gamma-Alumina (γ-Al2O3)? .................................................................9 1.4.2 Gamma-Alumina Atomic Structure: The Debate Thus Far ..........................12 1.4.3 γ-Al2O3 Structure Models ..................................................................................14 1.4.3.1 Monoclinic Spinel-Based Model ........................................................... 18 1.4.3.2 Tetragonal Nonspinel Model ................................................................ 18 1.4.3.3 Monoclinic Nonspinel Model ................................................................ 19 1.4.3.4 Differences between γ-Al2O3 Models .................................................... 20 1.4.4 Reasons for Difficulty in γ-Al2O3 Structure Determination ...........................21 1.4.5 Importance of Ascertaining the Correct Structure of γ-Al2O3 ......................22 1.5 Platinum/Gamma-Alumina Catalyst .......................................................................... 23 1.6 Overview ........................................................................................................................ 25 2.0 Objectives............................................................................................................................... 27 3.0 Research Design and Methods ............................................................................................. 29 vi 3.1 Research Design ............................................................................................................ 29 3.2 Sample Preparation ...................................................................................................... 30 3.2.1 Single-Crystal γ-Al2O3 (111) Synthesis .............................................................30 3.2.2 Polycrystalline γ-Al2O3 .......................................................................................31 3.2.3 Deposition of Pt Nanoparticles on Model γ-Al2O3 ...........................................31 3.3 Experimental Characterization Tools ........................................................................ 32 3.3.1 X-ray Diffraction (XRD) ....................................................................................32 3.3.2 Surface Characterization Methods: Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) ..............................................................34 3.3.3 Focused Ion Beam (FIB) ....................................................................................37 3.3.3.1 Cross-Sectional TEM Sample Preparation ......................................... 38 3.3.4 Transmission Electron Microscopy (TEM) and Scanning TEM (STEM) ....40 3.3.5 Selected-Area Electron Diffraction (SAED) ....................................................45 3.3.6 Electron Energy-Loss Spectroscopy (EELS) and Energy-Loss Near-Edge Structure (ELNES) .............................................................................................47 3.4 Computational Tools .................................................................................................... 50 3.4.1 Density Functional Theory (DFT).....................................................................50 3.4.2 EELS Simulation ................................................................................................51 3.4.3 Electron Diffraction Simulation ........................................................................53 4.0 Characterization of the Model γ-Al2O3 ............................................................................... 54 4.1 Single-Crystal γ-Al2O3 Thin Film Properties: Surface Orientation, Surface Roughness, Thickness, and Composition ................................................................. 54 4.2 Polycrystalline γ-Al2O3 ................................................................................................. 56 vii 4.3 Model γ-Al2O3 Cross-Sectional TEM/STEM Samples .............................................. 62 5.0 Electron Beam Damage in γ-Al2O3 and Suppression using Cryo-STEM/EELS ............. 64 5.1 Preamble ........................................................................................................................ 64 5.2 Study-Specific Methods ................................................................................................ 66 5.2.1 Sample Preparation and Characterization ......................................................66 5.2.2 Computational Details .......................................................................................66 5.3 Results and Discussion ................................................................................................. 68 5.4 Outcomes ....................................................................................................................... 85 6.0 Evaluating the