Molten salt-promoted MgO-based CO2 adsorbents: Selective adsorption on polycrystalline surfaces

Molten salt-doped MgO adsorbent is considered one of the most promising CO2 adsorbents in the field. In this work, MgO-based adsorbents were prepared by one-step calcination using MgCl2·6H2O as magnesium source. The CO2 adsorption performance of MgO-based adsorbents was investigated via different methods. Results showed that the maximum CO2 adsorption capacity of MgO doped by LiNO3-NaNO3-KNO3 was 57.1% at the CO2 concentration of 80% and 350 ℃, and the MgO-based adsorbents showed good regeneration. The nanosheet structure of the MgO-based adsorbents decreased with the increase in the number of cycles, whereas the crystal structures of MgO and alkali metal nitrates did not change because of multiple decarbonization. DFT computation revealed selective adsorption of CO2 on different crystal faces of MgO. The (200) crystal face of molten salt-doped MgO did not have CO2 trap ability. In addition, the doped nitrate did not directly participate in the reaction but reduced the adsorption energy of MgO carbonation. The adsorption energies of the MgO (220) and (222) crystal faces after doping with nitrate were reduced to −2.07 and −3.26 eV, respectively. The overall energy level of adsorption decreased as the number of resonance peaks and the stability of the structure increased. This study explains why MgO currently fails to reach the theoretical adsorption capacity and reveals the underlying mechanism of molten salts.