Mg-Ion Inversion in MgO@Al₂O₃ Oxides: The Origin of Basic Sites

Abstract

Although MgO-Al2O3 is well known as having a spinel structure, the inversion of which occurs by exchange of the trivalent (Al3+) and divalent (Mg2+) cations, little analytical study of the degree of inversion has been carried out. This study concerns a simple methodology to identify the inversion by solid-state NMR spectroscopy, whereby its correlation with the CO2 capture capacity of MgO-rich MgO@MgO-Al2O3 spinel structures is verified. Through Al-27 and Mg-25 NMR spectroscopy, temperature-programmed CO2 desorption, and thermogravimetric analysis, higher inversion is found to occur at low Mg/Al ratios and the inversion is found to decrease as the Mg/Al ratio increases. Moreover, the degree of inversion correlates with CO2 sorption, which is associated with the medium-strength basic sites induced by formation of the unsaturated O2- species. These results will open new pathways to exploit defects in complex oxides beyond spinels and their derivatives for desired applications. This demonstration of MgO-Al2O3 for CO2 sorption can contribute to the design of future CO2 sorbents.