Structural and thermal properties of Co-Cu-Fe hydrotalcite-like compounds

The structural properties and thermal decomposition processes of Co-Cu-Fe ternary hydrotalcites (HT) have been studied through X-ray diffraction, thermogravimetric measurements, Fourier-transform infrared and Moessbauer spectroscopies. 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 show the presence of Cu(OH)2 and CuO. The decomposition process was investigated by in situ X-ray, in situ Moessbauer and FTIR experiments. By increasing the temperature from 25 deg. C up to 180 deg. C we observed that the structural disorder increases. This effect has been likely attributed to the Co2+->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 deg. C. 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 deg. C 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