Comprehensive investigation of CO2 adsorption on Mg–Al–CO3 LDH-derived mixed metal oxides

Abstract

Layered double hydroxides (LDHs) have been intensively studied for high-temperature CO₂ capture. However, big differences in the CO₂ capture capacity, ranging from 0.28 to 0.6 mmol g⁻¹, have often been reported for the same Mg–Al–CO₃ LDH. Furthermore, how the active Mg–O species that are responsible for CO₂ adsorption are formed is still unclear. In this work, we have performed a comprehensive investigation on the CO₂ adsorption characteristics of Mg–Al–CO₃ LDH-derived mixed metal oxides. Based on these results we proposed the possible adsorption sites and the mechanisms for CO₂ adsorption. Initially, the effects of synthesis method, Mg : Al ratio, pretreatment conditions, adsorption conditions, and thermal stability on the CO₂ adsorption capacity were systematically studied. By carefully examining the structural changes during thermal treatment using X-ray diffraction and solid state NMR, we suggest that the active Mg–O species could be induced either by the substitution of Mg²⁺ by Al³⁺ in the periclase MgO lattice, or by the diffusion of Al³⁺ out of the octahedral brucite layers. This work not only suggests the optimal testing conditions for LDH-derived CO₂ adsorbents, but also provides a clearer understanding of the CO₂ adsorption sites and mechanisms on LDH-derived mixed oxides and sheds light on the synthesis and utilization of LDH-derived high-temperature CO₂ adsorption materials.