Thermal decomposition and structural reconstruction effect on Mg-Fe-based hydrotalcite compounds

The thermal decomposition and structural reconstruction of Mg-Fe-based hydrotalcites (HT) have been studied through thermogravimetric analyses, X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy and Mössbauer spectroscopy. The destruction of the layered structure took place at about 300°C. The broad peaks observed in the X-ray diffractograms suggest that the resultant oxides constitute a solid solution. For samples treated at temperatures higher than 500°C, the formation of the MgO and MgFe₂O₄ spinel phases is observed. ⁵⁷Fe Mössbauer spectroscopy was employed to monitor the Fe chemical environment for the samples annealed at different temperatures (100-900°C). In situ XRD experiments revealed that the HTs start an interlayer contraction at about 180°C. This phenomenon is identified as being due to a grafting process for which the interlamellar anions attach to the layers through a covalent bond. The reconstruction of the HTs was also investigated and its efficiency depends on the thermal annealing temperature and the Mg/Fe ratio. The structure of the reconstructed samples was found to be exactly the same as the parent structure.