Adsorption/desorption studies on nanocrystalline alumina surfaces

Two kinds of nanocrystalline alumina powders, boehmite (gamma-AlOOH, particle size d approximately 10 nm, BET surface area A(BET) = 180-200 m(2) g(-1)) and corundum (alpha-Al(2)O(3), d approximately 400 nm, A(BET) = 7 m(2) g(-1)) were used for comparative investigation by thermogravimetry (TG). The remarkable difference in the dehydration profiles between the two samples gives evidence for a distinct difference in their structures. In the following pyridine adsorption/desorption experiment, gamma-alumina was found to possess much more (20 times) and much stronger acidic sites than corundum. The activation energy of pyridine desorption was obtained from the respective minima in the first derivative of the TG-curves (DTG) at various heating rates (1-20 K min(-1)); the activation energy for pyridine desorption is smaller for gamma-alumina (61.5 kJ mol(-1)) than for corundum (78.8 kJ mol(-1)). Furthermore, the adsorption of water, carbon tetrachloride, and hexane on those alumina specimens provides evidence for the highly hydrophilic nature of their surfaces. The shift of T(max) to higher temperatures upon desorption of water was ascribed to the different adsorption coverage and the different energy required for removal of adsorbed water molecules.