Molten salt-driven basic site evolution in brucite-derived MgO for high-capacity and durable intermediate temperatures CO2 capture

MgO was obtained from the calcination of naturally occurring brucite (Mg(OH)₂) and the NaNO₃ was used to promote carbon capture at 300 °C under 50 % CO₂. As the mass ratio of NaNO₃/MgO is 22 %, the CO₂ adsorption capacity achieves to 438.2 mg/g initially and remains 75 % at the 30th cycle. The molten NaNO₃ fills into the pores of MgO and facilitates the crystallite growth, resulting in the specific surface area from 114 m²/g to 12 m²/g. However, due to the dissociation of Mg²⁺-O²⁻ bonds by molten NaNO₃, the medium basic sites increase to 8.21 mmol/g, enhancing of CO₂ adsorption. The initial adsorption in first 10 mins is fast and the subsequent slow adsorption accounts for over 90 % of the total CO₂ adsorption. In situ characterization confirms the contribution of NaNO₃ in the carbonation, which provides O²⁻ by itself and dissolves the MgO. Precipitates of MgCO₃ grow into particles in melting NaNO₃ rather than impermeable layers, so the carbonation level of MgO improves to 40 %. The decrease of 25 % adsorption capacity is ascribed to residual carbonates after desorption at 450 °C. DFT calculations demonstrate that the adsorption energy decreases with the loading of NaNO₃ on MgO, as the valence band maximum gets closer to the Fermi level. After the carbonation of NaNO₃-modified MgO, the π-bonding electronic state of CO₂ and the p-band center of the substrate moves away from the Fermi level, indicating a stable configuration. This study provides a promising brucite-derived adsorbent for medium-temperature CO₂ capture.