Boehmite-Derived γ-Alumina System. 2. Consideration of Hydrogen and Surface Effects

A boehmite-derived gamma-alumina (γ-Al 2 O 3 ) system was studied using various complementary techniques to examine surface area and pore size, the amount of hydrogen-containing species, the nature of hydrogen bonding environments, and the location of these species. Using small-angle X-ray scattering, the material examined was shown to have a significantly higher surface area than that typically expected for highly crystalline boehmite-derived γ-Al 2 O 3 . This higher surface area was associated with the presence of closed nanopores, the size of which was found to complement observations from transmission electron microscopy. More hydrogen was determined to be in the structure when measured using prompt-gamma activation analysis than indicated by loss on ignition experiments, suggesting that hydrogen-containing species other than water were also present. Neutron vibrational spectroscopy and infrared spectroscopy showed a reduction in signals associated with water and hydroxide species as the calcination temperature increased. Measurements from small-angle X-ray scattering and prompt-gamma activation analysis show that the surface area and the amount of hydrogen present reduce with increasing temperature treatment. This is associated with a reduction in the amount of amorphous material in the structure and an increase in pore and crystallite size. The evidence obtained suggests that the bulk crystalline structure is relatively well-ordered and contains no interstitial hydrogen. Hydrogen is therefore located at the pore surfaces and within amorphous regions, which themselves are located near pore surfaces.