Multimodal Approach for Rationalization and Quantification of Structural Disorder in Transition Al 2 O 3

摘要： Transition aluminas form a basis of important catalytic and catalytic support materials due to their unique surface acidity, high temperature structural stability, and their viability for synthesis as high surface area materials. The origin of transition aluminas attractive properties has been extensively studied in the past 50+ years, but despite this effort, there are number of structural, electronic and surface properties that remain poorly understood. The poor understanding stems mostly from the inadequate crystallographic description of transition alumina polymorphs of γ-Al2O3, δ-Al2O3, θ-Al2O3 that evolve as a continuum of metastable structures. This work focuses on rationalization and quantification of structural disorder in Boehmite derived transition aluminas by using a suite of complementary imaging, spectroscopy and quantum calculation techniques. Based on aberration corrected STEM HAADF imaging, it will be shown that the individual polymorphs of δ?Al2O3 and θ-Al2O3 accommodate a significant degree of structural disorder, which leads to loss of crystallographic periodicity. The complexity of this disorder can be rationalized as an atomic scale intergrowth of closely related crystallographic variants. As a part of this work, we present a crystallographic approach based on real space interpretation of projected atomic potential that enabled us to unambiguously derive Al3+ coordination in δ-Al2O3 and θ-Al2O3 from a series of low-index STEM HAADF images. This work lead to a full crystallographic description of the crystallographic variants belonging to δ-Al2O3 and θ-Al2O3 family. To quantify the heterogeneous microstructure of transition aluminas on statistically relevant bases, we employed novel approaches for characterization of structural disorder using XRD and NMR techniques. XRD is one of the most employed methods for crystallographic bulk characterization, but historically it has not been successfully used on transition alumina. It will be shown that the crystallographic polymorphs and their structural disorder can be quantified with the use of recursive algorithms as implemented in DIFFAX and TOPAS XRD simulation packages. To further validate the quantification of structural disordered in transition Al2O3, we also employed complementary NMR spectroscopy, combined with DFT NMR simulation methods. As a part of this talk, we also discuss results from DFT energetic calculations explaining how energetic degeneracy between the number of crystallographic variants leads to structural disorder in transition aluminas.