Thermal decomposition of synthesized layered double hydroxides based upon Mg/(Fe,Cr) and carbonate
Thermal analysis complimented with evolved gas mass spectrometry has been applied to hydrotalcites of the pyroaurite-stitchtite series containing the carbonate anion prepared by co-precipitation and with varying Fe3+,Cr3+ trivalent cation ratio. The resulting materials were characterized by XRD, and TGA/DTG to determine the stability of the hydrotalcites synthesised. Hydrotalcites of formula Mg6(Cr0.8,Fe0.2)2(OH)16(CO3).xH2O, Mg6(Cr0.6,Fe0.4)2(OH)16(CO3).xH2O, Mg6(Cr0.4,Fe0.6)2(OH)16(CO3).xH2O and Mg6(Cr0.2,Fe0.8)2(OH)16(CO3).xH2O formed by intercalation with the carbonate anion as a function of Fe3+,Cr3+ trivalent cationic ratio show variation in the d-spacing attributed to the size of the cation.
The thermal decomposition of stitchtite-pyroaurite solid solution
a) removal of adsorbed water (< 100 degrees Celsius),
b) elimination of the interlayer structural water (100 – 150 degrees Celsius), and
c) the simultaneous dehydroxylation and decarbonation of the hydrotalcite framework (300 – 400 degrees Celsius).
The effect of replacement of Cr3+ by Fe3+ has no effect on the dehydroxylation temperature. The ion current curves provide evidence for the formation of Fe3+ and Cr3+ carbonates during dehydroxylation. Dehydroxylation results in the collapse of the hydrotalcite structure to that of its corresponding metal oxides, and results in the formation of spinels, including MgO, MgFe2O4, MgCr2O4, and MgFeAlO4.
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