Infrared spectroscopic evidence for the structural OH groups of spinel alumina modifications

In the structure of spinel aluminas (γ, η, δ etc.), treated at moderate temperatures, a certain part of cation vacancies are always hydrated. Protons trapped in the octahedral and tetrahedral vacancies give rise to wide bands at about 3500 and 3300 cm−1, respectively. These bands are not affected by CO or pyridine adsorption or by D2O treatment at 300 K. Evacuation at temperatures higher than 750 K results in their disappearance, but after heating the sample in water vapour the structural hydroxyls can be partly restored. Treatment in D2Ogives rise to the corresponding OD bands at 2590 and 2480 cm−1, respectively. Complete saturation of all the cation vacancies by protons in the spinel lattice gives an ideal “protospinel” structure with the formula HAl5O8 or Al2O3·0.2 H2O. The role of protonated vacancies and proton migration in the formation of alumina structure and their influence on the properties of oxides is discussed.