Enhanced surface activity in nanocrystalline alumina as studied by neutron activation analysis, X-ray photoelectron and infrared spectroscopy

Nanocrystalline alumina was synthesized by the gas phase condensation using the DC are plasma technique. Spherical particles ranging in diameter between 20 and 80 nm were obtained. The stoichiometric composition of the powder was investigated by X-ray photoelectron spectroscopy (XPS) and the technique of neutron activation analysis after calcination at different temperatures. Enhanced surface activity in the form of an increase in oxygen content was observed to be maximum in the powder calcined around 800 degrees C. This behaviour is attributed to differences in microchemical stoichiometry, i.e. presence of active centers which are most likely localised in crystallographically defective configurations thereby leading to chemisorptive hydroxyl incorporation at these local sites. This is viewed against the process of microstructural transformation operative at higher temperatures. The stretching modes in the infrared spectrum due to the non-hydrogen bonded hydroxyl groups confirmed the surface activity of this nanocrystalline alumina. Thermogravimetric analysis was used to study the behaviour of nanocrystalline alumina vis-a-vis coarse grain powder, during the process of calcination. (C) 1997 Elsevier Science S.A.