IR-SPECTROSCOPIC DETECTION OF LEWIS ACID SITES ON AL2O3 USING ADSORBED CO - CORRELATION WITH AL-OH GROUP REMOVAL

CO adsorption at low temperatures has been used to probe Al3+ Lewis acid sites created upon dehydroxylation of Al2O3, using transmission infrared spectroscopy. Al2O3 was dehydroxylated in the temperature range from 475 to 1200 K. There is an approximate linear correlation between the decreasing integrated absorbance of the Al-OH IR features and the increasing integrated absorbance of the adsorbed CO features throughout the full coverage range of hydroxyl groups. Two CO adsorption sites have been identified: the first (nu-CO = 2195 cm-1) appears immediately upon mild dehydroxylation, while a second site (nu-CO = 2213 cm-1) appears after an 800 K dehydroxylation treatment. CO adsorption on the Lewis acid sites formed by dehydroxylation of Al2O3 involves a binding energy of about 21 kJ/mol.