Hydrogen and the Structure of the Transition Aluminas

Abstract

α-Alumina results from the complete dehydration of several minerals of the form Al 2 O 3 · n H 2 O. The “transition” aluminas, γ-alumina, η-alumina, and δ-alumina are known to have a spinel structure but the possibility that they contain hydrogen (H) has been the subject of debate. We present a series of density-functional theory calculations which, together with available experimental data, show that the spinel aluminas exist over a range of hydrogen content captured by the empirical formula H 3 m Al 2 - m O 3, with different greek-letter phases corresponding to different distributions of the Aluminum (Al) ions on the two cation sublattices. Calculations of densities and vibrational frequencies of bulk OH bonds are in excellent agreement with available data. The theory reconciles seemingly inconsistent data and reveals a remarkable property of the spinel aluminas: They are “reactive sponges” in that they can store and release water in a reactive way. This chemical activity offers a basis for understanding long-standing puzzles in the behavior of aluminas in catalytic systems.