Mechanisms of absorption and desorption of CO₂ by molten NaNO₃-promoted MgO

In order to realize carbon capture and sequestration (CCS), a technology proposed to circumvent the global warming problem while maintaining the present level of economic activity, the development of efficient carbon-capturing agents is of prime importance. In addition to the prevailing amine-based agents that operate at temperatures lower than 200°C, agents that can operate at higher temperatures are being considered to reduce the cost of CCS. For the mid-temperature (200-500°C) operation, alkali nitrate-promoted MgO is a promising candidate; whose detailed reaction mechanisms are not yet fully understood, however. In the present study, we have performed a comprehensive investigation on the mechanisms of CO₂ absorption and desorption of NaNO₃-promoted MgO. Highly efficient CO₂ absorbents were obtained by decomposing Mg₅(CO₃)₄(OH)₂·4H₂O with NaNO₃ intimately mixed with it. Our collective data, including isothermal CO₂ uptake curves, MgO solubility in molten NaNO₃, and observations on the reaction of MgO wafers with CO₂, indicate that the absorption takes place in the molten NaNO₃ medium in which both CO₂ and MgO are dissolved. MgCO₃ is formed inside the molten promoter through the nucleation and growth steps. The decomposition of MgCO₃ back to MgO, that is desorption of CO₂, is also facilitated by molten NaNO₃, which we attribute to the decreased relative stability of MgCO₃ with respect to MgO when in contact with molten NaNO₃. The relative affinity of molten nitrate to MgO and MgCO₃ was estimated by measuring the 'contact angles' of nitrate on them. Implications of our findings for the real applications of alkali nitrate-promoted MgO absorbents with numerous repeated cycles of absorption and desorption of CO₂ are discussed.