A novel calcination-quenching synthesis of octahedral MgO crystals with highly exposed (111) facets for enhanced adsorption performance

Abstract

The improvement of the CO₂ adsorption capacity of MgO is necessary for carbon capture. The (111) facet of MgO has garnered considerable attention recently due to its distinctive atomic arrangement of oxygen and magnesium. Theoretically, the (111) facet exposes a high density of low-coordination oxygen anions on MgO, thereby providing abundant active sites for CO₂ adsorption. However, the controlled synthesis of MgO crystals with highly exposed (111) facets remains a significant challenge. In this study, a facile calcination-quenching method was developed to synthesize octahedral MgO crystals with preferentially exposed (111) facets. The effects of calcination temperature, calcination time, and cooling rate on the morphological evolution of MgO crystals were systematically investigated by using anhydrous magnesium carbonate (MgCO₃) as the precursor. X-ray diffraction (XRD) results indicated that the octahedral MgO crystals preferentially expose (111) facets compared to other morphologies. Thermogravimetric analysis (TGA) reveals that the octahedral MgO exhibits superior CO₂ adsorption capacity of 2.15 mmol/g at 50 °C. This value represents enhancements of 525.0 %, 161.5 %, and 144.8 % compared to commercial MgO, flat-surfaced MgO, and spherical MgO, respectively. The fundamental reasons for the superior CO₂ adsorption capability of the MgO (111) facet were elucidated through in situ DRIFTS, CO₂-TPD, and DFT calculations. The presence of unique low-coordinated oxygen anions on the MgO (111) facet results in enhanced Lewis basicity, thereby exhibiting a stronger binding affinity towards CO₂ compared to other facets of MgO. This study provides new insights for synthesizing MgO crystals with highly (111) facets and enhanced CO₂ capture capabilities.