Location of the Spinel Vacancies in γ-Al2O3

A non-spinel model for the structure of gamma-Al2O3, with 25 % of the Al3+ cations at tetrahedral positions, has been the subject of wide interest. However, O-17 NMR measurements and, more recently, Al-27 NMR measurements have shown that there are considerably more Al3+ cations at tetrahedral positions. This means that the Al3+ vacancies in gamma-Al2O3 are not at tetrahedral but at octahedral positions, as in isostructural gamma-Fe2O3 and in accordance with density functional theory predictions. This has consequences with regard to the surface structure of gamma-Al2O3, and thus, for catalysis.