A new explanation for the synergistic promoted mechanism of nitrate and nitrite anion species for the high performance MgO adsorbent

The mechanism of CO₂ capture on the nitrate and nitrite anion species synergistic promoted MgO has been rarely studied and still not fully understood. Herein we report a systematic investigation on the CO₂ adsorption performance of molten salts-promoted MgO adsorbent to deeply reveal the synergistic promoted mechanism of nitrate and nitrite. A series of MgO-based solid adsorbents were synthesized and applied for CO₂ capture using NaNO₂/KNO₃ as promoters. The addition of mixed nitrate and nitrite promoters greatly enhances the adsorption capacity of MgO, exhibiting CO₂ capture up to 10.25 mmol·g⁻¹ under optimal conditions. The MgO-based adsorbent in the form of nanosheets has faster adsorption kinetics and better stability, and the possible reaction mechanism of MgO modified by nitrate and nitrite was proposed. Firstly, NO₃^– interacts with the lattice oxygen (O₁^2-) of MgO to form NO₂^– and O₂^2–. Secondly, Mg²⁺ interacts with the free NO₂^– to form [Mg···NO₂]⁺ and further overcomes the energy barrier to generate [Mg²⁺···O^2–] ion pairs. O₂^2– is not stable and prone to side reaction. However, NO₂^– can be converted into O₂^–, and O₂^– is further generated to O₂^2–, facilitating regeneration of NO₃^–. The introduction of molten salt can significantly reduce the reaction energy barrier, and the energy barrier required to generate [Mg²⁺···O^2–] ion pairs is reduced from 6.86 eV to 3.50 eV. Adsorption kinetics studies reveal that the nano-flake MgO facilitates the full contact with the promoter and has faster reaction kinetics in the surface reaction control stage. These insights into the synergistic promoted mechanism of nitrate and nitrite anion species is expected to provide guidance for the further design of the high performance MgO-based CO₂ adsorbent.