Morphology and Surface Properties of Boehmite (γ-AlOOH): A Density Functional Theory Study

The aluminum oxihydroxide boehmite (γ-AlOOH) is the topotactic precursor of γ-alumina, widely used as a support of refining catalysts. We use ab initio calculations and molecular dynamics (MD) to study water–boehmite interfacial properties, which are believed to play a key role during the industrial synthesis of boehmite in aqueous solution. For four relevant crystallographic planes—(010), (100), (001), and (101)—we develop a theoretical approach for calculating surface energies and interfacial energies that cannot be determined experimentally. From these values, the morphology of boehmite nanosized particles is constructed either in vacuum or in water, highlighting the strong effect of the solvent. It is clearly shown that during MD water molecules react on the surfaces by molecular adsorption and dissociative chemisorption, producing surface hydroxyl groups. A detailed analysis of the local structure before and after water adsorption is furnished. Particularly, four main surface hydroxyl groups are identified by their stretching vibrational frequencies: μ2-AlVI (νOH=3676 cm−1), μ1-AlVI (νOH=3712 cm−1), μ1-AlV (νOH=3741 cm−1), and μ1-AlIV (νOH=3819 cm−1). This analysis of surface hydroxyl groups gives us some new insights for the understanding of experimental IR band assignment and Brønsted acidity by ammonia adsorption. The localization of basic and acid Brønsted sites on the boehmite nanosized particles is resolved. An interpretation of these results in the light of γ-alumina is attempted.