Abstract
ABSTRACTAlumina nanoparticles were synthesized by thermal dehydration of Al(OH)3 (gibbsite), which shows the sequential formation of AlOOH (bohemite), γ, δ, θ, and α-alumina phases upon heat treatment in the temperature range: 150–1150°C. Alumina nanoparticles were characterized by X-ray diffraction and 27Al MAS-NMR studies to investigate the structural transitions, in particular, the changes in Al—O speciation as a function of heat treatment. γ-alumina is produced by annealing bohemite at 400°C, and transforms into α-alumina via the formation of δ and θ-alumina as the intermediate phases. The formation of θ and α-alumina starts at 1020°C and the concentration of these phases increases upon annealing at 1100°C, while the concentrations of γ and δ-alumina decreases steadily. The heat treatment at 1150°C produces α-phase (84%) and the remnant θ-phase (16%). 27Al MAS-NMR revealed the existence of two types of Al—O speciation i.e. AlO4 and AlO6 structural units whose relative concentration depends upon the transition alumina phases.
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