The effect of iron on the crystalline phases formed upon thermal decomposition of Mg-Al-Fe hydrotalcites

Layered double hydroxides (,DH) containing Mg-II, Fe-III, and Al-III in the brucite-like layers and interlayer carbonate (with a constant M-II/M-III ratio but varying Al-III/Fe-III ratios) have been prepared and characterised by Xray diffraction, thermal analysis, FT-IR and UV-VIS/diffuse reflectance spectroscopies, temperature-programmed reduction and specific surface area assessment through low temperature adsorption of Nz. An Mg,AI-LDH, but with intercalated hexacyanoferrate (III), has been also prepared and characterised, in which simultaneous formation of the carbonate analogue did not occur. Thermal decomposition in air at 450 and 750 degrees C leads to MgO and poorly crystallised MgFe2O4 spinel (crystallinity increasing with the iron content), while for the hexacyano-containing sample, crystallization only is observed after calcination at 900 degrees C. This different behaviour has been related to the initial location of the iron ions.