Atomistic Insight into the Structural Differences of the η- and γ-Al₂O₃ and the Related Catalytically Active Sites

Abstract: Transition aluminas, particularly η- and γ-Al₂O₃, are widely used in catalysis due to their high surface area and tunable acidity. Although both phases are commonly described as defective spinels, their exact crystal structures and the origins of their distinct surface properties remain debated. The transformation pathways from oxyhydroxide precursors, bayerite for η-Al₂O₃ and boehmite for γ-Al₂O₃, strongly influence their microstructure and, consequently, their surface atomistic arrangements, which ultimately govern their surface acidity. Atomic-resolution ADF-STEM imaging shows that γ-Al₂O₃ can be viewed as an assembly of η-phase domains interconnected by antiphase domain boundaries (APBs) along the (100) planes. These APBs consist of alternating O–Al6–O and O–Al6–O–Al4 atomic rows. Furthermore, the (100) surface of γ-Al₂O₃ structurally resembles its APBs, however, incorporates pentacoordinated aluminum Al5 instead of octahedrally coordinated Al6 (see Figure). Tilt-series TEM analyisis shows that γ-Al₂O₃ predominantly exposes (111) and (100) facets, whereas η-Al₂O₃ is mainly terminated by (111) facets and nanosteps. This distinct surface termination explains the different acidity of the two phases: γ-Al₂O₃ exhibits predominantly Lewis acidity due to undercoordinated Al sites on its (100) surfaces, while η-Al₂O₃, being exclusively (111)-terminated, presents oxygen-rich surfaces associated with dominant Brønsted acidity.