SURFACE SITES ON SPINEL-TYPE AND CORUNDUM-TYPE METAL-OXIDE POWDERS

The surface sites on isostructural metal oxides containing Al3+, Fe3+, and Cr3+ have been investigated by IR spectroscopy. The IR spectra of surface hydroxy groups and of pyridine coordinated on Lewis acidic surface cationic sites on the defective spinel-type sesquioxides gamma-Al2O3, theta-Al2O3, and gamma-Fe2O3, on the spinels MgAl2O4, MgFe2O4, MgCr2O4, ZnAl2O4, ZnFe2O4, and ZnCr2O4, as well as on the corundum-type sesquioxides alpha-Al2O3, alpha-Fe2O3, and alpha-Cr2O3, have been compared. Some Co2+ and Ni2+ spinel-type compounds have also been considered. An extension of the criteria previously proposed for the identification of the surface sites on aluminum-based materials to ferrites and chromites is suggested. The OH stretchings of surface hydroxy groups are indicative of the nature and the coordination of the cations to which they are bonded. Pyridine species bonded to tetrahedrally- and octahedrally-coordinated trivalent cations are well distinguishable on aluminates, while the distiction is more difficult for ferrites.