Synthesis of CoIICoIII 2-xAlxO4-Al 2O3 nanocomposites via decomposition of CoII 0.73CoIII 0.27(OH) 2.00(NO3)0.23(CO3) 0.02·0.5H2O in a sol-gel-derived γ-Al2O3 matrix

Abstract

Nanocomposite materials of CoIICoIII 2-xAlxO4-Al 2O3 (0 ≤ × < 1.63) have been prepared from biphasic xerogels that contain the cobalt hydrotalcite-like compound CoII 0.73CoIII 0.27(OH) 2.00(NO3)0.23(CO3) 0.02·0.5H2O and sol-gel-derived Al2O3. With XRD/FTIR/XPS/DSC/TGA methods, an investigation has been carried out to understand the formation processes of the composites and chemical reactivity of the embedded cobalt hydrotalcite-like compound with alumina gel matrixes at various heating temperatures. It has been found that cubic spinels of Co3O4 and CoIICoIII 2-xAlxO4 are generated sequentially inside alumina matrixes upon decomposition of the hydrotalcite-like compound in static air. The average spinel crystallite size is in the range of 9-13 nm at 350°C to 31-38 nm at 800°C, whereas the specific surface area of the composites is reduced gradually from 277-363 m2 g-1 at 350°C to 184-224 m2 g-1 at 800°C. On one hand, the catalytic effect of cobalt on combustion reactions of organics trapped within the amorphous alumina gel matrixes is elucidated by varying the cobalt content and synthesis conditions. On the other hand, the retardation effect of γ-Al2O3 matrixes on the growth of the crystallite is also revealed. It has been indicated that the content of aluminum (x) in CoIICoIII 2-xAlxO4-Al 2O3 is indeed a function of the heating temperature. After thermal decomposition of the cobalt hydrotalcite-like compound, the Co3O4 phase starts to form at a temperature as low as 200°C and is fully developed at 350°C. At 500-800°C, the formed Co3O4 is converted to the CoIICoIII 2-xAlxO4 phase due to the reaction between the included phase and alumina matrixes.