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 su rface cationic sites on the defective spinel-type sesquioxides gamma-A l2O3, theta-Al2O3, and gamma-Fe2O3, on the spinels MgAl2O4, MgFe2O4, M gCr2O4, ZnAl2O4, ZnFe2O4, and ZnCr2O4, as well as on the corundum-type sesquioxides alpha-Al2O3, alpha-Fe2O3, and alpha-Cr2O3, have been com pared. Some Co2+ and Ni2+ spinel-type compounds have also been conside red. An extension of the criteria previously proposed for the identifi cation of the surface sites on aluminum-based materials to ferrites an d chromites is suggested. The OH stretchings of surface hydroxy groups are indicative of the nature and the coordination of the cations to w hich they are bonded. Pyridine species bonded to tetrahedrally- and oc tahedrally-coordinated trivalent cations are well distinguishable on a luminates, while the distiction is more difficult for ferrites.