Thermal decomposition of synthesised layered double hydroxides based upon Mg/(Fe,Cr) and carbonate

Abstract

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 Fe 3+, Cr 3+ trivalent cation ratio. The resulting materials were characterised by XRD, and TGA/DTG to determine the stability of the hydrotalcites synthesised. Hydrotalcites of formula Mg 6(Cr 0.8,Fe 0.2) 2(OH) 16(CO 3)· xH 2O, Mg 6(Cr 0.6,Fe 0.4) 2(OH) 16(CO 3)· xH 2O, Mg 6(Cr 0.4,Fe 0.6) 2(OH) 16(CO 3)· xH 2O and Mg 6(Cr 0.2,Fe 0.8) 2(OH) 16(CO 3)· xH 2O formed by intercalation with the carbonate anion as a function of Fe 3+, Cr 3+ trivalent cationic ratio show variation in the d-spacing attributed to the size of the cation. The thermal decomposition of stitchtite–pyroaurite solid solution follows the following steps: (a) removal of adsorbed water (<100 °C), (b) elimination of the interlayer structural water (100–150 °C), and (c) the simultaneous dehydroxylation and decarbonation of the hydrotalcite framework (300–400 °C). The effect of replacement of Cr 3+ by Fe 3+ has no effect on the dehydroxylation temperature. The ion current curves provide evidence for the formation of Fe 3+ and Cr 3+ 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, MgFe 2O 4, MgCr 2O 4, and MgFeAlO 4.