Estudo de compostos lamelares contendo Fe usando Espectroscopia Mössbauer de 57Fe e técnicas complementares

Abstract

The structural and thermal decomposition properties of Mg-Fe and Co-Cu-Fe hy-drotalcites (HT) have been studied through thermogravimetric analysis, X ray powder diffraction (XRD), Fourier transform infrared spectroscopy and 57Fe MÄossbauer spectroscopy. In the Mg-Fe system, the destruction of the layered structure took place at about 300 oC. The broad peaks observed in the X ray diffractograms suggests that the resultant oxides constitute a solid solution. For samples treated at temperatures higher than 500 oC the formation of the MgO and MgFe2O4 formation of the MgO and MgFe2O4 spinel phases is observed. 57Fe MÄossbauer spectroscopy was employed to monitor the Fe chemical environment for the samples annealed at different temperatures (100-900 oC). In situ XRD experiments revealed that the HTs start an interlayer contraction at about 180 oC. This phenomenon is identifed as being due to a grafting process for which the interlamellar anions attach to the layers through a covalent bond. The reconstruction of the Mg-Fe HTs was also investigated and its e±ciency depends on the thermal annealing temperature and on the Mg/Fe ratio. The structure of the reconstructed samples was found to be exactly the same as the parent structure. The in situ 57Fe MÄossbauer experiments were performed in the 100-500 oC temperature range con¯rm an increasing structural disorder in this temperature range. The quadrupolar splitting indicates that the maximum disorder occurs at 300 oC. Regarding the Co-Cu-Fe ternary system we have observed that due to the strong Jahn-Teller e®ect the Cu-Fe layered system is stabilized only in the presence of Co2+. At low Co2+ contents, additional phases are segregated in the solids. X ray patterns di®raction The structural and thermal decomposition properties of Mg-Fe and Co-Cu-Fe hydrotalcites (HT) have been studied through thermogravimetric analysis, X ray powder diffraction (XRD), Fourier transform infrared spectroscopy and 57Fe Mossbauer spectroscopy. In the Mg-Fe system, the destruction of the layered structure took place at about 300 oC. The broad peaks observed in the X ray diffractograms suggests that the resultant oxides constitute a solid solution. For samples treated at temperatures higher than 500 oC the formation of the MgO and MgFe2O4 formation of the MgO and MgFe2O4 spinel phases is observed. 57Fe Mossbauer spectroscopy was employed to monitor the Fe chemical environment for the samples annealed at different temperatures (100-900 oC). In situ XRD experiments revealed that the HTs start an interlayer contraction at about 180 oC. 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 Mg-Fe HTs was also investigated and its efficiency depends on the thermal annealing temperature and on the Mg/Fe ratio. The structure of the reconstructed samples was found to be exactly the same as the parent structure. The in situ 57Fe Mossbauer experiments were performed in the 100-500 oC temperature range confirm an increasing structural disorder in this temperature range. The quadrupolar splitting indicates that the maximum disorder occurs at 300 oC. Regarding the Co-Cu-Fe ternary system we have observed that due to the strong Jahn-Teller effect the Cu-Fe layered system is stabilized only in the presence of Co2+. At low Co2+ contents, additional phases are segregated in the solids. X ray patterns diffraction show the presence of Cu(OH)2 and CuO. The decomposition process was investigated by in situ X ray, in situ Mossbauer and FTIR experiments. By increasing the temperature from 25 oC up to 180 oC we observed that the structural disorder increases. This effect has been likely attributed to the Co 2+ to Co3+ oxidation since thermal decomposition was carried out under static air atmosphere. Part of the Co3+ cations could migrate to the interlayer region, thus forming a metastable compound that still has a layered structure. Collapse of the layered structure was observed at about 200 oC. By further increasing the temperature the system becomes more crystalline and the formation of Co3O4 is observed in the X ray patterns. In Cu-rich HT, some of the carbonate anions are released at temperatures higher than 550 oC and this phenomenon is attributed to the formation of a carbonate-rich phase. The specific surface area data present its highest values in the temperature range where the collapse of the layered structure takes place.