Structural evolution of layered double hydroxides during thermal decomposition and reconstruction: an in situ Raman study

Layered double hydroxides (LDHs) are anionic clays with tunable composition, high ion-exchange capacity, and excellent thermal and chemical stability, making them attractive for applications in catalysis, environmental remediation, adsorption, and energy storage. A key feature of LDHs is the memory effect, whereby layered double oxides (LDOs) formed by calcination can reconstruct the original layered structure upon hydration. This work investigated the thermal decomposition and reconstruction of four LDH systems with different cation compositions: MgAl, MgAlNi, MgAlFe, and MgFe. LDH precursors were synthesized by controlled co-precipitation and calcined at 650 °C to obtain the corresponding LDOs. Structural changes before and after calcination were characterized by X-ray diffraction (XRD) and in situ Raman spectroscopy. XRD confirmed the formation of carbonate-intercalated LDHs and their conversion into mixed metal oxides after calcination. Raman spectroscopy revealed dehydration, dehydroxylation, carbonate decomposition, and collapse of the layered structure during heating. Among the investigated compositions, MgAlNi exhibited the highest thermal stability. The memory effect was evaluated under different hydration conditions. Neither dry nor humidified CO₂ induced detectable reconstruction, whereas direct contact with liquid water promoted recovery of the layered structure. MgAl showed the strongest reconstruction, followed by MgAlFe. MgAlNi required greater water availability and additional heating, while MgFe showed no detectable reconstruction, indicating that replacing Al with Fe strongly suppresses the memory effect. These results demonstrate that the reconstruction behaviour of LDH-derived oxides is strongly governed by cation composition and hydration conditions, highlighting the value of in situ Raman spectroscopy for monitoring structural transformations in LDH materials.