Comprehending the Thermal Decomposition and Reconstruction Process of Sol−Gel MgAl Layered Double Hydroxides

The evolution of MgAl layered double hydroxides (LDHs), prepared by the sol−gel method, throughout the synthesis−crystallization−calcination−reconstruction process is thoroughly studied and compared to a coprecipitated analogue. X-ray diffraction and transmission electron microscopy provide useful information about crystallinity and morphology of the samples. It was found that crystal sizes do not necessarily decrease after calcination−reconstruction. Furthermore, aluminum coordination is studied by 27Al MAS nuclear magnetic resonance and by X-ray photoelectron spectroscopy (XPS), revealing significant differences in the relative populations of tetrahedral aluminum in the surface and the bulk of the calcined sol−gel LDHs. Magnesium and oxygen structural environments are also studied by XPS. A possible correlation between crystal sizes of calcined LDHs and the amount of O2− surface species is discussed. Differences in the structural environment of magnesium in the as-synthesized and the reconstructed LDH are presented and analyzed.