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

Abstract

Layered double hydroxides (LDH) 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 X-ray diffraction, thermal analysis, FT-IR and UV-VIS/diffuse reflectance spectroscopies, temperature-programmed reduction and specific surface area assessment through low temperature adsorption of N ₂ . An Mg,Al-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°C leads to MgO and poorly crystallised MgFe ₂ O ₄ spinel (crystallinity increasing with the iron content), while for the hexacyano-containing sample, crystallization only is observed after calcination at 900°C. This different behaviour has been related to the initial location of the iron ions.