DRFT-IR studies of the surface of gamma-alumina

gamma-Alumina surfaces dehydrated at 450 and 700 degrees C were studied by using the conventional pyridine adsorption-DRFTIR technique. Three types of Lewis acid sites are present on the surface, and in each case pyridine adsorbed on them is concomitantly interacting with their adjacent OH groups. Because only three types of Lewis acid sites are present, only three hydrogen bonding bands were observed upon pyridine adsorption. From correlation of OH bands with hydrogen bonding bands, it is found that weak Lewis acid sites have type II66 or II64 OH groups as their neighbors, medium strong Lewis acid sites have type III OH groups near by, and strong Lewis acid sites have type I-6 OH groups next to them. Dehydration at 700 degrees C generates more types of OH groups that also interact with pyridine. The reactivity of OH groups is not determined by their acidity and space restrictions, but rather by their proximity to Lewis acid sites. We assigned the three types of Lewis acid sites to five-, four-, and three-coordinate Al3+ ions.