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 . 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 . 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.
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.