Performance enhancement of spray-agglomerated MgO-based sorbents for intermediate-temperature CO2 capture through dual-nitrate promotion

Intermediate-temperature CO 2 capture using MgO-based sorbents has attracted increasing attention because of its low regeneration temperature, high theoretical CO 2 uptake, and potential for integration with industrial waste heat. Molten alkali nitrates have been widely employed to enhance the carbonation performance of MgO. However, the interplay between promoter loading, nitrate composition, structural evolution, and cyclic stability remains insufficiently understood, limiting the rational design of high-performance sorbents. In this work, dual-nitrate-promoted MgO-based sorbents were prepared by spray agglomeration, and the effects of the total nitrate loading and NaNO 3 /KNO 3 molar ratio on CO 2 capture performance, structural characteristics, and cyclic stability were systematically investigated. The results demonstrated that the optimum sorbent was obtained at a nitrate-to-MgO molar ratio of 0.05 with an equimolar NaNO 3 /KNO 3 composition, exhibiting the highest CO 2 adsorption capacity and MgO utilization efficiency, achieving a CO 2 capture capacity of 7.76 mmol CO 2 /g-sorbent in the first cycle. Increasing the total nitrate loading promoted the molten-salt-mediated carbonation reaction but simultaneously impaired pore accessibility at excessive loading levels, indicating that the optimum promoter loading is determined by the balance between chemical promotion and structural preservation. Furthermore, regulating the NaNO 3 /KNO 3 molar ratio significantly influenced the promotional efficiency of the molten nitrate phase, and the superior performance of the equimolar system was attributed to the synergistic regulation of promoter dispersion, active-site accessibility, and mass transport during carbonation. Cyclic adsorption experiments showed that the CO 2 capture capacity decreased from 7.76 to 3.66 mmol CO 2 /g-sorbent after 10 cycles. Combined structural characterization demonstrated that the gradual decline in CO 2 capture capacity originated primarily from progressive particle sintering and structural densification rather than irreversible phase evolution. This work establishes the loading-structure-performance and composition-structure-performance relationships of dual-nitrate-promoted MgO sorbents and provides mechanistic insights for the rational design of regenerable MgO-based sorbents for intermediate-temperature CO 2 capture.