Abstract
Aluminum hydrate dehydration interfaces were studied using a van der Waals density functional. The interface configurations investigated here as a first exploration of possible interface geometries, were all found to have a reasonable probability of occurring. From gibbsite/boehmite and boehmite/γ-Al2O3 interface simulation cells, the formation of dehydration-related defects during relaxation was observed. H transfer between hydroxyl groups, and separation of hydroxyl groups and H atoms from the lattice, resulted in the formation of chemisorbed H2O and OH2 groups in gibbsite; in boehmite, the formation of OH2 groups and interstitial H was observed. All interfaces show a transfer of small amounts of charge across the interface. Accumulation of charge in spaces interstitial to the lattice was found to play a role in the dehydration process as well. The present study shows the potential of interface studies for elucidating dehydration pathways at the atomic scale, and offers various starting-points for follow-up studies.
Highlights
Alumina (Al2O3) is a ceramic material of high industrial and tech nological importance
Besides the thermodynamically stable corundum form (α-Al2O3), many metastable polymorphs exist. These so-called transition aluminas are stable at ambient conditions, and are desig nated as χ, κ, θ, γ, δ, η, and ρ-Al2O3 [1,2,3]. α-Al2O3 is mainly seen as a structural, optical, and electronic material. γ-Al2O3 is the most investigated transition alumina, and has a most prominent application as catalyst and catalytic support
The PBE calculations resulted in an over estimation of the bulk phase lattice parameters, which is a known feature of generalized gradient approximation (GGA) functionals
Summary
Alumina (Al2O3) is a ceramic material of high industrial and tech nological importance. Besides the thermodynamically stable corundum form (α-Al2O3), many metastable polymorphs exist. These so-called transition aluminas are stable at ambient conditions, and are desig nated as χ-, κ-, θ-, γ-, δ-, η-, and ρ-Al2O3 (see Fig. 1) [1,2,3]. There are seven different known aluminum hydroxides: the four trihydroxide polymorphs gibbsite (γ-Al(OH)3), bayerite (α-Al(OH)3), nordstrandite, and doyleite; the two oxyhydroxides boehmite (γ-AlOOH), and diaspore (α-AlOOH); and the most dehydrated species akdalaite, known as tohdite (5Al2O3⋅H2O or 2Al5O7(OH)) [1,2,3]. Gibbsite and boehmite, which are used as raw materials to produce alumina and a wide variety of other industrial products, are the aluminum hydroxides of highest practical value. The use of boehmite nanosheets for improving membrane-based effluent treatment technologies, for which they hold great promise in terms of environ mental sustainability, is an example of a most promising application of a nanoscale aluminum hydroxide: Zavabeti et al obtained boehmite nanosheets through a green and scalable synthesis method, and demonstrated that these can be used to fabricate membrane filters that show an excellent separation of heavy metal ions and oils from aqueous solutions at extraordinary flux [9]
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