Role of LiNO ₃ and Li ₂ CO ₃ in Nitrate-Modified MgO-Based Sorbents for Intermediate-Temperature CO ₂ Capture

Alkali metal nitrates (e.g., LiNO 3, NaNO 3, KNO 3 ) are usually employed to increase the CO 2 adsorption capacity of MgO at intermediate temperatures (200–350 °C). Among them, LiNO 3 is widely added because it effectively lowers the melting point of eutectic nitrate mixtures, but its true influence on CO 2 capture remains disputed. Herein, this study investigates the individual effects of lithium nitrate (LiNO 3 ) and its transformation product, lithium carbonate (Li 2 CO 3 ), on MgO carbonation from both kinetic and thermodynamic aspects. Results indicate that Li + from LiNO 3 induces lattice modification in MgO/MgCO 3 structures, predominantly at the liquid–solid interface and near-surface regions, slowing late-stage diffusion and elevating the MgO/MgCO 3 equilibrium CO 2 pressure. In contrast, by acting as a nucleation seed and stabilizer, Li 2 CO 3 outperformed the corresponding LiNO 3 -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 NO x evolution demonstrates that MgCO 3 drastically lowers LiNO 3 thermostability, triggering NO x emissions well below typical regeneration temperature (∼400 °C), leading to our conclusion that LiNO 3 -rich sorbents cannot avoid gradual transformation to Li 2 CO 3 during operation. Overall, the work reconciles conflicting literature by showing that Li 2 CO 3 rather than LiNO 3 drives performance gains. Therefore, we recommend substituting Li 2 CO 3 for LiNO 3 in molten salt-modified MgO to achieve durable, high-capacity, and environmentally friendly sorbents without the thermodynamic penalty and NO x -emission risk associated with LiNO 3 .