Investigation on the Mechanisms of Mg(OH)2 Dehydration and MgO Hydration by Near-Infrared Spectroscopy

The mechanism for the dehydration of Mg(OH)2 and hydration of MgO was investigated by near-infrared (NIR) spectroscopy. Mg(OH)2 showed three absorption bands in the NIR region at 7339, 7306, and 7157 cm–1, due to the different hydroxyl groups (7339 cm–1: OH(e) and OH(c) located on the edge and corner of the Mg(OH)2 sheet, 7306 cm–1: OH(t) located on the terrace of Mg(OH)2 sheet, 7157 cm–1: OH(i) located on the interlayer of Mg(OH)2 sheets). During the phase transition from Mg(OH)2 to MgO at approximately 400 °C, the interlayer OH(i) of Mg(OH)2 disappeared by a dehydration reaction and the OH(t) on the (001) plane of Mg(OH)2 was converted to the hydrogen-bonded hydroxyl groups on the (111) plane of MgO (7309 cm–1). Furthermore, when MgO was in contact with water vapor at 85 and 50 °C (RH 85%), the isolated hydroxyl groups of the MgO surface (7360 cm–1) were easily hydrated to form hydrogen-bonded ones (7309 cm–1). When a monolayer Mg(OH)2 sheet was formed on the MgO surface, OH(e), OH(c), and OH(t) were observed at 7345 and 7309 cm–1. As the hydration reaction proceeded, the intermediate state like stacking of several Mg(OH)2 sheets, which are related to the decrease in the OH(t) and the spontaneous increase in the interlayer OH(i), was experimentally confirmed.