Towards Atomic Level Understanding of Transition Alumina Phases and Their Phase Transformations

L. Kovarik, M. Bowden, D. Shi, A. Anderson, Jianzhi Hu, J. Szanyi, J. H. Kwak and C.H.F. Peden, 2015

Image courtesy of Microsc. Microanal.

Abstract

Transition Al2O3 derived from dehydration of Al hydroxides are highly complex materials with a significant degree of inherent structural disorder. The way in which the disorder is manifested for various heat treatment conditions and for various hydroxide precursors is a highly relevant topic in catalysis, with important implications for rationalization of unique surface chemistry and catalytic behavior of these materials [1]. When heat-treated at relatively high temperatures (>900°C), the complexity of the microstructure is generally associated with the polymorphs of δ-Al2O3 and θ-Al2O3, which form in closely inter-growing structures. This has been a main issue in their reliable characterization, and there are currently none or only poorly fitting crystallographic models available. Similarly, the stability of these polymorphs remains poorly understood and actively studied [2]. In this work we address the structural nature of δ-Al2O3 and θ-Al2O3, and the mode of their phase transformations using combination of in-situ and ex-situ imaging and spectroscopy techniques. The current work mainly relies on the use of HAADF Scanning Transmission Electron Microscopy imaging, XRD, high-resolution NMR and DFT calculations. The HAADF STEM observations were performed with a probe corrected FEI Titan 80-300, and the thermal treatment was performed under in-situ heating conditions inside the TEM with Aduro Protochips heating holder at 900-1100 °C.

Impact Statement

The study addresses the structural nature of δ-Al2O3 and θ-Al2O3 and the mode of their phase transformations using combination of in situ and ex situ imaging and spectroscopy techniques.