Use of methanol as a probe to study the adsorption sites of different MgO samples

Abstract

Methanol adsorption on activated MgO has been studied by gravimetry and FT-IR spectroscopy. It gives rise to three kinds of species: 1. i) one reversibly adsorbed at room temperature (species H) coordinated to Mg 2+ sites but also strongly H-bonded to unsaturated O 2− sites; 2. ii) two methoxy species characterized by a ν(O-C) band at 1115 cm −1 (species I) and 1090 cm − (species II) resulting both from the dissociation in methanol of the O-H bond as shown by use of CH 3 18OH., Considering that faces of the (100) type are preferentially exposed, species H and I are postulated to be formed on regular faces, while species II formation involves the presence of defects, such as Mg 2+ cations in a low coordination state. Extension of the study to five MgO samples, the surface of which varies from 12 m 2 g −1 (smoke) to 565 m 2 g −1 (ex-carbonate), shows that the proportion of defects increases with the surface area, reaching more than 25 % of the total amount of methanol adsorption sites in the case of high surface area samples.