Ex-hydroxide magnesium oxide as a model adsorbent for investigation of micropore filling mechanisms

Abstract

The thermal decomposition of magnesium hydroxide to magnesium oxide has been studied under carefully controlled conditions. Analysis of nitrogen and neopentane adsorption isotherm measurements shows that the micropore structure of the decomposition products is very well defined and consists of slit-shaped pores of width 0.93 ± 0.03 nm from 40% to 90% decomposition. The mean pore width increases, up to ca. 1.8 nm, at higher levels of decomposition. For larger pore sizes two stages of secondary micropore filling by nitrogen (surface coverage followed by enhanced adsorption in the body of the pore) are clearly discernible. The results also show very clearly how the mechanism of micropore filling, and hence the shape of the isotherm, is dependent on the ratio of pore width to molecular diameter (d/σ), and not just on d.