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Infrared and gravimetric studies have yielded new information on the surface hydration of 7-alumina. Dry 7-alumina was found to "chemisorb" one molecule of water per [11][12][13][14][15][16] A.2 of surface at 100°, depending on rehydration procedure. Desorption of chemisorbed water on subsequent heating under vacuum was studied at temperatures between 100 and 900°. Characteristic dehydration isotherms showed that a rapid initial weight loss was followed by a slow continuing loss. Because of these characteristics, the extent of surface hydration is governed primarily by the drying temperature. In addition to the three major isolated hydroxyl bands previously noted in infrared spectra of dry alumina (3800, 3744, and 3700 cm.-1), two additional bands are often observed at 3733 and 3780 cm.-1. Changes in the bands were studied as a function of temperature. Rehydration of the surface between 400 and 800°was also studied by direct spectroscopic observation of the hot alumina. Results show that hydroxyl groups, although apparently mobile, persist as such on the surface at high temperatures on distinct types of sites rather than in a completely random state. Readsorption of water cannot alone explain the difficulty in removing residual hydroxyl groups at high temperatures. Free energy of activation for desorption of water apparently increases continuously as the surface concentration of hydroxyl groups decreases. Although "strain" sites are undoubtedly created by dehydration, characterization of such sites as strained Al-O-Al linkages seems inadequate.

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