The Microstructure of γ-Alumina

Though γ-Al2O3 has played a central role in heterogeneous catalysis for more than two centuries, its microstructure continues to be debated. Specifically, the positions of Al3+ cations within the crystal lattice have been discussed extensively in the literature. Many authors uphold that the cations primarily occupy spinel sites, while others endorse the occupation of non-spinel sites. The other main point of dispute is whether the structure contains interstitial hydrogen, with some authors supporting a partially hydrated model and others claiming that the structure must be completely dehydrated. The use of different structural models directly affects the predicted geometry of γ-Al2O3 at the surface, which in turn has significant implications for its catalytic utility. A comparison of theoretical data to experimental infrared (IR), X-ray diffraction (XRD), and selected area electron diffraction (SAED) evidence suggests that γ-Al2O3 features cations primarily in spinel positions, while IR and nuclear magnetic resonance (NMR) data indicate that interstitial hydrogen is present within the bulk structure.