Role of LiNO₃ and Li₂CO₃ in Nitrate-Modified MgO-Based Sorbents for Intermediate-Temperature CO₂ Capture

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 NO_x evolution demonstrates that MgCO₃ drastically lowers LiNO₃ thermostability, triggering NO_x 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 NO_x-emission risk associated with LiNO₃.