Study of Thermal Stability of Hydrotalcite and Carbon Dioxide Adsorption Behavior on Hydrotalcite-Derived Mixed Oxides Using Atomistic Simulations

Hydrotalcites (HTlcs) or layered double hydroxides (LDHs) have been used in a wide range of applications such as catalysis, electrochemical sensors, wastewater treatment, and carbon dioxide (CO2) capture. In the current study, molecular dynamics simulation was employed to investigate carbon dioxide adsorption behavior on amorphous layered double oxides (LDOs) derived from LDHs at elevated temperatures. The thermal stability of LDHs was first examined by heating the sample up to T = 1700 K. Radial distribution functions confirmed the structural evolution upon heating and the obtained structures were in good agreement with experiments, where periclase was confirmed to be the stable phase in the recrystallized mixed oxides above T = 1200 K. Further, CO2 adsorption was studied as a function of amorphous HTlc-derived oxide composition, where static and dynamic atomistic measures have been employed to characterize the CO2 adsorption behavior. The simulation results showed that the CO2 dynamic residence time on LDH-derived LDOs was sensitive to the Mg/Al molar ratio and the average amount of residence time of CO2 on the surface of LDOs reached maximum when the Mg/Al molar ratio was equal to 3.0. Meanwhile, the activation energy for diffusion also showed local maximum when the Mg/Al molar ratio was 3.0, suggesting that this particular ratio of Mg/Al mixed oxides possessed the highest CO2 adsorption capacity. This is consistent with experimental results. Examination of the binding between CO2 and mixed oxides revealed that both magnesium and oxygen in amorphous LDOs contributed to CO2 adsorption. Further analysis suggested that the interaction between Mg–O and O(LDO)–C were the most important interactions for the physisorption of CO2 on amorphous surface and different CO2 adsorption behavior on different Mg/Al molar ratio surfaces was directly related to their amorphous local structure.