Study of the Thermal Stability of Hydrotalcite and Carbon Dioxide Capture Capacity of Hydrotalcite-derived Mixed Oxides using Molecular Dynamics Simulation

Abstract

Hydrotalcites (HTlcs) or layered double hydroxides (LDHs) are used in a wide range of applications such as catalysis, electrochemical sensors, wastewater treatment and carbon dioxide (CO2) capture. In this study, molecular dynamics simulation was employed to investigate carbon dioxide adsorption in amorphous layered double oxides (LDOs) derived from LDHs at elevated temperatures. The thermal stability of LDH was first examined by heating the sample up to T = 1700 K. Radial distribution functions confirmed the structural evolution upon heating and was in excellent agreement with experiments, where periclase was the main observed phase in the XRD patterns of the recrystallized mixed oxides above T = 1200 K. Further, CO2 adsorption capacity was studied as a function of amorphous HTlc-derived oxide composition, where static and dynamic atomistic measures have been employed to characterize CO2 capture capacity. We found that the CO2 dynamic residence time on LDH-derived LDOs was sensitive to the Mg/Al molar ratio and CO2 capture capacity reached maximum when the Mg/Al molar ratio was equal to 3.0. Meanwhile, the activation energy for diffusion also shows local maximum when the Mg/Al molar ratio is 3.0, suggesting that this particular ratio of Mg-Al mixed oxides possesses the highest CO2 adsorption capacity and that it is consistent with experimental results. Examination of the binding between CO2 and mixed oxides suggests that both magnesium and oxygen in amorphous LDOs contribute to CO2 adsorption. Moreover, the effect of Mg-O and O (LDO)-C interaction are the most significant and the highest CO2 adsorption capacity was observed in the system with the most Mg-O bindings and O (LDO)-C bindings.