Explanation of the observed dearth of three-coordinated Al on γ-alumina surfaces

NMR data have revealed that gamma-alumina surfaces do not contain any three-coordinated Al atoms, despite the fact that their presence is expected on the basis of the bulk structure of the material. We report density-functional calculations of gamma-alumina (110) surfaces that reveal a massive spontaneous reconstruction of the ideally terminated bulk. The three-coordinated Al atoms initially in the surface layer drop into vacant octahedral interstices in the second layer where they become six-coordinated. The predicted reconstruction is also fully consistent with IR studies of hydroxylated gamma-alumina surfaces and reveals the physical origin of results obtained by earlier computational simulations.