Hydrogen and the structure of the transition aluminas

ex-Alumina results from the complete dehydration of several minerals of the form Al2O3. nH(2)O The "transition" aluminas, gamma-alumina, eta-alumina, and delta-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 avai lable experimental data, show that the spinel aluminas exist over a range o f hydrogen content captured by the empirical formula H3mAl2-O-m(3), with di fferent greek-letter phases corresponding to different distributions of the Aluminum (Al) ions on the two cation sublattices. Calculations of densitie s and vibrational frequencies of bulk OH bonds are in excellent agreement w ith available data. The theory reconciles seemingly inconsistent data and r eveals a remarkable property of the spinel aluminas: They are "reactive spo nges" in that they can store and release water in a reactive way. This chem ical activity offers a basis for understanding long-standing puzzles in the behavior of aluminas in catalytic systems.