Adsorption of Water Vapour by Microporous Magnesium Oxide

Abstract

Abstract Water vapour adsorption isotherms have been determined on a number of samples of microporous magnesium oxide obtained by controlled decomposition of the hydroxide. Analysis of the results indicates that the microcrystallite restructuring, which occurs after the reaction front has reached the centre of the crystals, involves 2 stages. The first stage corresponds to decomposition levels 90% and final heat treatment temperatures 350°C. The process involves the approach of adjacent microcrystallites with the loss of one micropore and a concomitant increase in the width of another. The second stage corresponds to decomposition levels 98% and final heat treatment temperatures 600°C. This process involves rearrangements of surface ions, leading to a sintering of micropore entrances. In order to effectively reverse the decomposition process, a certain amount of control has to be exercised over the conditions of rehydroxylation. In particular, it is necessary to prevent pore blockage from occurring during the early stages of rehydroxylation, and to create conditions which will reverse the second stage of microcrystallite restructuring, thereby opening up any closed microporosity which exists in the crystals. The results also indicate that, in contrast to other samples of MgO usually studied in the literature, the exposed surfaces are constituted principally of {111} MgO planes.