CaO-MgO Mixed Oxides from Natural Dolomite for Stable CO 2 Adsorption at High Temperatures: Structural Evolution, Adsorption Performance, and Carbonation Mechanism

CaO-based materials exhibit promising CO2 adsorption performance at high temperatures; however, they gradually deactivate over multiple cycles. To effectively improve adsorption stability, MgO with a high Tammann temperature is introduced to stabilize CaO, using natural dolomite and calcite as magnesium and calcium sources. Compared with CaO-MgO from the direct calcination of dolomite, the mixed oxides prepared via the citrate sol-gel method (denoted as Ca10Mgx-850) show a better adsorption capacity and cyclic stability. For Ca10Mg10-850, Ca10Mg7.5-850, and Ca10Mg5-850, the CO2 adsorption capacity at the 20th cycle remained close to the initial capacity. The Ca10Mg2.5-850 with a high ratio of Ca/Mg shows the best initial adsorption capacity (0.62 g/g), decreasing to 82.3% after the 20th adsorption cycle. Kinetic analysis reveals that CO2 adsorption on Ca10Mgx-850 is limited by CO2 diffusion rather than the carbonation reaction. The combustion of citrate gel releases abundant heat and gas, which facilitates the collapse of mesopores and the formation of macropores. The macroporous structure provides room for volume expansion during the carbonation of CaO, while the well-dispersed MgO particles act as a physical barrier to prevent the sintering of CaO. Moreover, MgO decreases the adsorption energy of CO2 and enhances the electron density in the adsorption configuration at energies far from the Fermi level. The carbonation product on CaO-MgO heterojunctions is more stable than that on a single CaO slab. This study provides a promising dolomite-derived adsorbent for the high-temperature capture of CO2 and elucidates the carbonation mechanism of Ca-MgO mixed oxides.