Hydrogen and the Structure of the Transition Aluminas

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.

Surface acid–base properties and hydration/dehydration mechanisms of aluminum (hydr)oxides

“…However, their structures and chemical surface compositions are still not well understood. While the major view is that the γ -alumina has a defect spinel structure, there have been reports that the Al 2 O 3 ·nH 2 O (0 < n <∼ 0.6) form for transition alumina [37]. The α-alumina is the end-structure of transition aluminas formed at high temperature, usually known as "corundum," which exists in nature.…”

Production of γ-Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; from Kaolin

“…In addition, the formation of γ-alumina was approved by FTIR spectrum (Figure 3). Reference [20] showed that γ-alumina is known to have a spinel structure which exists over a range of hydrogen content captured by the empirical formula H 3m Al 2-m O 3 . According to this result, the IR spectra of Figure 3 obtained in our experiment could be readily explained.…”

Dual Nanoparticle/Substrate Control of Catalytic Dehydrogenation

“…In addition, it has recently been established that there are major disparities in catalyst behavior when different alumina polytypes are used as supports. Among the so-called transition aluminas, all of which are porous and have a cubic spinel structure, [10,11] chromia on g-alumina is a highly active catalyst with a lifetime of several years, whereas the activity of chromia on c-alumina degrades within weeks. [12,13] Differences between the surfaces of these two polytypes are well-known and have been attributed to different reconstructions, controlled by the different distributions of bulk Al vacancies.…”