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.