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[Display omitted] • Li 3 PO 4 and CaCO 3 -doped, NaNO 3 -promoted MgO showed high CO 2 adsorption performance. • Li 3 PO 4 enhanced the surface adsorption of CO 2 by providing moderate basic sites. • CaCO 3 accelerated the carbonation of bulk MgO via the formation of CaMg(CO 3) 2. • The optimized sorbent showed good cyclic stability under harsh but realistic conditions. Molten nitrate-promoted MgO-based sorbents represent a potential class of materials for CO 2 capture at intermediate temperatures. However, their poor stability under harsh but realistic regeneration conditions (e.g., at high temperatures in pure CO 2) is a challenge for the application of CO 2 capture. In this work, a series of NaNO 3 and/or Li 3 PO 4 promoted MgO and MgO-CaCO 3 sorbents were prepared, characterized, and evaluated. The results showed that the NaNO 3 -Li 3 PO 4 promoted MgO-CaCO 3 sorbents had high CO 2 adsorption performance in terms of rate, capacity and stability, which was attributed to the promoting effect of Li 3 PO 4 and CaCO 3. Li 3 PO 4 can enhance the surface adsorption of CO 2 by providing some extra moderate basic sites, while CaCO 3 can accelerate the carbonation reaction of bulk MgO via the formation of CaMg(CO 3) 2. In addition, Li 3 PO 4 can hinder the sintering of sorbent particles, and consequently improve the stability of the sorbent. The optimized sorbent had a stable CO 2 uptake of 0.12 (or 0.26) g CO 2 /g sorbent when 30% (or pure) CO 2 was used at the adsorption stage in long-term cyclic operation (desorption at 500 °C in pure CO 2 for both cases), showing promising applications in intermediate-temperature CO 2 capture and thermochemical energy storage. [ABSTRACT FROM AUTHOR]

[Display omitted] • Cyclic tests of CO 2 adsorption are conducted at high temperature and high pressure. • MgO-Na 2 CO 3 prepared via ball-milling method exhibits a strong skeleton structure. • Excellent cyclic stability is achieved at the sorption condition of 500 °C, 3 MPa. • Na-doped defect is found to enhance CO 2 adsorption with increased electron transfer. • Na 2 CO 3 is found to benefit CO 2 adsorption through electrostatic interactions. MgO-based mixture is a promising candidate for CO 2 capture due to its high theoritical capture capacity, non-corrosiveness, low cost and wide availability. In this work, the effects of preparation methods and sorption conditions on the characteristics of MgO-Na 2 CO 3 are studied. A stable MgO conversion of 0.32 for CO 2 sorption is obtained during the 30-cycle tests at 400 °C, 2 MPa, where the material exhibits a strong skeleton structure with increased specific surface area when prepared by the ball-milling method. A severe sorption condition of 500 °C, 3 MPa is also tested, where the material shows a stable MgO conversion of 0.3 after 20 cycles. With the increase of Na 2 CO 3 content in the mixture, a higher MgO conversion can be achieved, but the cyclic stability will be impaired. In addition to the experiments, density function theory calculations are conducted to explore the sorption mechanism of MgO-Na 2 CO 3 , where CO 2 adsorptions on the pure, the Na-doped, and the Na 2 CO 3 -MgO co-adsorption surfaces are compared. The defect is found to help induce electrons from the Na and Mg atoms to the CO 2 molecule, thus benefiting the CO 2 adsorption process. The presence of Na 2 CO 3 also benefits the adsorption process of MgO due to its electrostatic interaction with CO 2 molecules. This work can serve as the guideline for the design of MgO-based CO 2 absorbent especially for extreme operating conditions. [ABSTRACT FROM AUTHOR]