FT-IR STUDY OF THE SURFACE-PROPERTIES OF THE SPINELS NIAL2O4 AND COAL2O4 IN RELATION TO THOSE OF TRANSITIONAL ALUMINAS

The surface chemistry of δ-Al2O3, NiAl2O4, and CoAl2O4 has been investigated comparatively by FT-IR spectroscopy using adsorbed carbon monoxide and pyridine as probe molecules. The surface OHs of the inverse spinel NiAl2O4 are very similar to those of δ-Al2O3 and other transitional aluminas while those of CoAl2O4 are different and resemble those of other normal spinels. Both carbon monoxide and pyridine adsorption provide evidence for the presence of tetrahedral Al3+ ions exposed on the surface of the inverse spinel NiAl2O4. On the normal spinel CoAl2O4 octahedral Al3+ ions predominate with respect to the tetrahedral ones, also present because of partial inversion. Ni2+ and Co2+ ions are also detected on the respective surfaces. Slight modifications to the literature criteria are suggested for the assignments of the IR bands to surface OHs and of adsorbed pyridine on aluminas. © 1991.