Indirect synthesis of Al2O3 via radiation- or photochemical formation of its hydrated precursors

Abstract γ -, θ - and α -modifications of aluminium oxide (alumina) were successfully prepared by calcination of precursor solid phase obtained by irradiation of clear aqueous solutions by UV light or electron beam. For the precipitate to form, formate anion must be present in the solution in sufficient concentration. According to X-ray diffraction, the precipitate was found to consist of γ -AlOOH or a mixture of γ - and α -Al(OH) 3 , when aluminium chloride or aluminium nitrate was used, respectively. The addition of hydrogen peroxide as a OH radical source and sensitizer markedly improved the efficiency of the preparation. Some hints for the apparently very complex mechanism involved were listed and discussed. Calcination of the dried precipitate at 500–800 °C produced highly porous γ -alumina with high specific surface area ( ca . 150 m 2  g −1 ). Mixture of γ - and θ -transition aluminas was obtained at 1000 °C and pure, stable corundum α -Al 2 O 3 formed at 1200 °C. Samples were further investigated by means of scanning electron microscopy and specific surface area or porosity measurement. According to N 2 adsorption isotherm, the precipitate contains mostly mesopores with average pore size 7 nm with specific surface area of ca . 100 m 2  g −1 . Possible applications of the material as sorbent or catalyst as well as a pure matrix for thermoluminescence dosimetry were briefly contemplated. Strong light-induced thermoluminescence was observed in both γ - and α -Al 2 O 3 samples at 300–400 °C, whereas in the absence of light, only an inexpressive glow peak at 179 °C was observed.