Synthesis and Carbon Capture Properties of Calcium-based Layered Double Hydroxides

Abstract

Calcium-based layered double hydroxides (LDHs) are a class of materials gaining attention as a tool for decarbonization as global warming continues to intensify. This thesis focuses on the synthesis and CO2 capture behavior of CaAlCl and CaFeCl LDHs, with an emphasis on understanding their response to prolonged direct air capture (DAC) exposure, along with the characteristic changes that come with it. Initial experiments involving exposure inside the carbonation chamber to 20% CO2 and 30% relative humidity for 24 hours demonstrated that both LDHs are capable of incorporating CO2, with CaFeCl showing a higher uptake. Thermogravimetric Analysis (TGA) indicated the formation of carbonate ions, while X-ray diffraction (XRD) revealed that short-term exposure causes reduction in crystallinity but no significant formation of carbonate phases such as calcite. CaFeCl was subsequently chosen for DAC exposure, and over a 42-day exposure period, significant structural and chemical evolution was observed. TGA results showed increasing mass loss along with emergence of multiple peaks in the carbonate decomposition region, suggesting the formation of distinct carbonate environments. Hydroxyls and water molecules were consumed, though water content varied along with ambient humidity conditions. XRD analysis confirmed the formation of calcite and vaterite and LDH peak intensity decreased, meaning the structure degrades over time. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) further supported these findings, with the spectrum for Day 42 showing strong resemblance to the spectrum for calcite and very little similarity with that for Day 0. These results indicate that CO2 is initially incorporated as interlayer carbonate species and progressively evolves into stable calcite and vaterite phases during prolonged exposure. Overall, these results highlight the importance of exposure time in governing CO2 capture mechanisms and show that the LDH itself undergoes a dynamic structural evolution during the process.