Interaction of water with the surface of magnesium oxide

Abstract

The adsorption isotherm of H₂O has been measured on three kinds of MgO, and the bulk rehydration has been found to occur at a moderate pressure range of H₂O, which exhibits a break in the isotherm. Infrared studies show that the surface hydration gives an isolated absorption peak at 3730 cm^–1 due to free OH and a broad band centred at 3550 cm^–1 due to hydrogen-bonded OH, whereas the bulk hydration exhibits a new absorption peak at 3695 cm^–1. On evacuation, the bulk OH can be dehydrated at temperatures lower than those of the surface free OH. The exchange reaction of all kinds of OH with D₂O was found to be slow. The initial bulk hydration necessitates a higher vapour pressure of ca. 4–6 Torr, but 3.5 Torr D₂O is sufficient for the deuteration of bulk OH once formed. The successive exposure of the sample, having been subjected to the initial bulk hydration with H₂O, to 10 Torr D₂O causes an additional bulk hydration with D₂O after a semi-complete surface deuteration, and gives rise to an increase in the peak intensities of lower-frequency bands in the OH stretching region.