Alkali metal nitrates (e.g., LiNO3, NaNO3, KNO3) are usually employed to increase the CO2 adsorption capacity of MgO at intermediate temperatures (200-350 °C). Among them, LiNO3 is widely added because it effectively lowers the melting point of eutectic nitrate mixtures, but its true influence on CO2 capture remains disputed. Herein, this study investigates the individual effects of lithium nitrate (LiNO3) and its transformation product, lithium carbonate (Li2CO3), on MgO carbonation from both kinetic and thermodynamic aspects. Results indicate that Li+ from LiNO3 induces lattice modification in MgO/MgCO3 structures, predominantly at the liquid-solid interface and near-surface regions, slowing late-stage diffusion and elevating the MgO/MgCO3 equilibrium CO2 pressure. In contrast, by acting as a nucleation seed and stabilizer, Li2CO3 outperformed the corresponding LiNO3-containing counterpart in isothermal MgO carbonation tests by up to 203% and sustained the highest MgO conversion among the carbonate-modified systems examined at the end of our cycling test. Furthermore, mass-spectrometric tracking of NOx evolution demonstrates that MgCO3 drastically lowers LiNO3 thermostability, triggering NOx emissions well below typical regeneration temperature (∼400 °C), leading to our conclusion that LiNO3-rich sorbents cannot avoid gradual transformation to Li2CO3 during operation. Overall, the work reconciles conflicting literature by showing that Li2CO3 rather than LiNO3 drives performance gains. Therefore, we recommend substituting Li2CO3 for LiNO3 in molten salt-modified MgO to achieve durable, high-capacity, and environmentally friendly sorbents without the thermodynamic penalty and NOx-emission risk associated with LiNO3.
Liu et al. (Sat,) studied this question.
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