How to determine IR molar absorption coefficients of co-adsorbed species? Application to methanol adsorption for quantification of MgO basic sites.

Methanol adsorption on MgO samples with different morphologies is studied by infrared (IR) spectroscopy coupled to volumetry or thermogravimetry measurements to probe qualitatively and quantitatively the acid-base paired sites. The molar absorption coefficients of ν(OC) vibration of non-dissociated methanol, type I and type II methoxy species are determined by analyzing data obtained under specific adsorption conditions (ε(ND)=2.5 cm μmol(-1); ε(I)=ε(II)=6.1 cm μmol(-1)). Thanks to these results, the amounts of different adsorbed methanol species are evaluated. These various species are formed on surface sites presenting different topologies. Hence, the IR method is the only one allowing us to both discriminate and quantify the defects on the MgO surfaces, in terms of concentrations of convex and concave defective zones. This study reveals that sol-gel preparation leads to a MgO surface presenting a greater amount of concave defective zones than precipitated MgO.