Location of the Spinel Vacancies in γ-Al₂ O₃

A non-spinel model for the structure of γ-Al₂ O₃ , with 25 % of the Al³⁺ cations at tetrahedral positions, has been the subject of wide interest. However, ¹⁷ O NMR measurements and, more recently, ²⁷ Al NMR measurements have shown that there are considerably more Al³⁺ cations at tetrahedral positions. This means that the Al³⁺ vacancies in γ-Al₂ O₃ are not at tetrahedral but at octahedral positions, as in isostructural γ-Fe₂ O₃ and in accordance with density functional theory predictions. This has consequences with regard to the surface structure of γ-Al₂ O₃ , and thus, for catalysis.