Interfacial Interactions Govern the Mechanisms of CO2 Absorption and Desorption on A2CO3-Promoted MgO (A = Na, K, Rb, and Cs) Absorbents

In this study, we investigated the detailed mechanisms of CO2 absorption and desorption on A(2)CO(3)-promoted MgO absorbents (A = Na, K, Rb, and Cs). We analyzed the materials formed at various stages of the absorbents during CO2 absorption and desorption by thermal decomposition-gas chromatography/mass spectroscopy, in situ IR spectroscopy, solid-state magic-angle spinning NMR spectroscopy on C-13, Na-23, Mg-25, and K-39 nuclei, diffuse reflectance UV-vis spectroscopy, and in situ X-ray diffraction as well as the conventional thermogravimetric analysis, ex situ X-ray diffraction, and elemental distribution mapping by energy-dispersive X-ray analysis using scanning electron microscopy. The absorption of CO2 of the absorbents occurs in two steps. The first step is a fast process involving basic sites on the MgO surface formed by the interaction between A(2)CO(3) and MgO. Because the basicity of these sites depends on the size of A ion, the kinetics and capacity of CO2 absorption and the desorption properties of this process are strongly dependent on the nature of A. Since basic sites are formed at the interface between A(2)CO(3) and MgO, the observation of this first step depends on the method of sample preparation among other factors, explaining the failure of observing this step in the previous studies on similar absorbents. The second step is a slower process during which the double carbonate phase between Mg and A is formed. The diffusion of solid-state materials required to form the double carbonate phases explains the slow kinetics of this step. The phase stability of the double carbonates also influences the kinetics and capacity of CO2 absorption in the second step. We also find that, against the conventional belief, a physical mixture between A(2)CO(3) and MgO undergoes chemical changes in addition to dehydration and dehydroxylation during the pretreatment step of thermogravimetric analysis, which, in fact, is a necessary step for the first step process to occur.