No abstract

α-Alumina results from the complete dehydration of several minerals of the form Al2O3·nH2O. 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 H3 m Al2 - m O3, 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.

The interactions of hydrogen with ceria have been the subject of several experimental studies, but whether hydrogen atoms enter the bulk or are merely chemisorbed on the surfaces is disputed. We combine first-principles calculations with thermodynamic arguments and conclude that the uptake of small amounts of hydrogen by ceria is spontaneous below 665 K. The calculations also predict that hydrogen atoms within the bulk form hydroxyl groups and slightly expand the lattice. The predicted hydrogen content, OH vibrational frequency, and ceria lattice expansion upon hydrogen uptake are consistent with available data.