Mg-Ion Inversion in MgO@MgO-Al₂ O₃ Oxides: The Origin of Basic Sites

Although MgO-Al₂ O₃ 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-Al₂ O₃ spinel structures is verified. Through ²⁷ Al and ²⁵ Mg 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 O^2- 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-Al₂ O₃ for CO₂ sorption can contribute to the design of future CO₂ sorbents.