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.
Does the Chemical Activity of an Interstitial Hydrogen at Surface of Solids Arise from Pauli Repulsion ?
