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 Moessbauer spectroscopy. The destruction of the layered structure took place at about 300 deg. 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 deg. C, the formation of the MgO and MgFe2O4 spinel phases is observed. 57Fe Moessbauer spectroscopy was employed to monitor the Fe chemical environment for the samples annealed at different temperatures (100-900 deg. C). In situ XRD experiments revealed that the HTs start an interlayer contraction at about 180 deg. 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