Structural and kinetic analysis of CO2 sorption on NaNO2-promoted MgO at moderate temperatures.

• A NaNO 2 -promoted MgO for CO 2 capture was prepared and characterized. • Doped NaNO 2 induced significant quantities of hydroxide sites and carbonate species. • These desirable features facilitated surface chemisorption processes. • The surface chemisorption process was not dependent on temperature. Alkali metal nitrate-/nitrite-promoted MgO sorbents are promising candidates for intermediate-temperature (200–500 °C) CO 2 capture. However, the structure-performance relationship and kinetic characteristics of NaNO 2 -promoted MgO remain unclear. Here the effects of physical-chemical properties on the CO 2 sorption performance of NaNO 2 -promoted MgO and the sorption kinetics were comprehensively studied to elucidate the detailed role of NaNO 2. Samples were characterized by X-ray diffraction, scanning electron microscopy, N 2 adsorption, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The sorption kinetics were obtained by isothermal thermogravimetry and elucidated using a double exponential model. Compared with pure MgO and NaNO 3 -promoted MgO, NaNO 2 -modified MgO had a lower initial sorption temperature and a unique bimodal sorption characteristic. Characterization results revealed that such bimodal sorption was due to the presence of double promoters (mixture of NaNO 2 and NaNO 3) which implies that some of the nitrite was oxidized to nitrate during the preparation process. Deposition of double promoters further reduced the amounts of hydroxide and carbonate species for pure MgO while still preserving more hydroxide and carbonate species on the surface as compared with NaNO 3 -promoted MgO. The kinetics analysis demonstrated that the double exponential model can describe the sorption process well for both NaNO 3 - and NaNO 2 -promoted MgO, suggesting that the entire sorption occurs as a double process (surface chemisorption and product layer diffusion). Significant differences were seen from NaNO 3 -promoted MgO, and the surface chemisorption process of NaNO 2 -promoted MgO was independent of temperature, which suggests that an increased presence of hydroxide and carbonate species provides more active sites for greatly facilitating surface chemisorption. [ABSTRACT FROM AUTHOR]