Infrared study of some surface properties of boehmite (γ-AlO2H)

Abstract

Two specimens of microcrystalline boehmite (γ-AlO₂H) with rather different particle size and crystallinity have been examined by FTIR spectroscopy, and the surface behaviour has been found to depend very little (if at all) on the degree of crystallinity. The vacuum thermal activation of the material occurs in two steps: the first step involves the reversible elimination of coordinated water molecules (with no change in surface structure and/or coordination), whereas the second involves surface dehydroxylation together with a substantial and nonreversible structural/coordinative modification of the surface. Both processes occur at temperatures lower than that of the phase transition γ-AlO₂H →γ-Al₂O₃, i.e. on a material still unmodified in its bulk features. IR investigation of the adsorption of CO, CO₂ and pyridine yields adequate information on the surface properties of both the unmodified and the surface-modified material. At the surface of the ‘virgin’ boehmite there are already some coordinatively unsaturated Al atoms with (quasi-) tetrahedral coordination, and a larger number of coordinatively unsaturated Al atoms with octahedral coordination. The latter centres are active towards CO₂ adsorption (yielding various types of carbonate-like species and an end-on linear CO₂ complex) and towards pyridine, whereas they are inactive towards CO at 300 K and partly active at 78 K. The probe molecules used can monitor, though with a different degree of promptness, the early structural/coordinative transformations occurring, upon vacuum activation, at the surface of boehmite unmodified in its bulk features.