Synthesis of Nanosize Supported Hydrotalcite-like Compounds CoAl<i>_x</i>(OH)₂₊₂<i>_x</i>(CO₃)<i>_y</i>(NO₃)<i>_x</i>_-₂<i>_y</i><i>·n</i>H₂O on γ-Al₂O₃

Abstract

Nanosize (5−7 nm) supported Co II Al-hydrotalcite-like compounds (Co II Al-HTlcs) on γ-Al 2 O 3 have been synthesized with a coprecipitation method in which the γ-Al 2 O 3 acts as a source of aluminum for coprecipitation and a support for the compound formation. The synthesis parameters identified are initial concentrations of metal and base, aging time, and more importantly, the addition sequence of the metal or base solution. Using multiple characterization methods, the supported Co II Al-HTlcs have been investigated for their structural phase and chemical composition. The chemical formula determined for these samples is CoAl x (OH) 2+2 x (CO 3 ) y (NO 3 ) x - 2 y · n H 2 O/γ-Al 2 O 3, where x = 0.25−0.40, y = 0.02−0.07, and n = 1.3−1.7, and the distance of inter-brucite-like sheets ( d 003 ) is in the range of 7.65−7.79 Å. Because of the formation of nanosize crystallites, decomposition of the supported Co II Al-HTlcs occurs at ≈211−217 °C, which is markedly lower than that of unsupported pure Co II Al-HTlcs. After calcination in air, the supported Co II Al-HTlcs are transformed into cubic spinel oxides Co II Co 2 - x III Al x O 4 /γ-Al 2 O 3 and high catalytic activity for the N 2 O decomposition reaction is observed for these supported spinel catalysts.