An X-ray Diffraction and Absorption Study of the Phases Formed upon Calcination of Zn−Al−Fe Hydrotalcites

The thermal decomposition of hydrotalcite-like materials containing Zn(II), Fe(III), and Al(III) is studied. One of these samples contains zinc and aluminum in the brucite-like layers, while iron is incorporated as hexacyanoferrate(III) anions in the interlayer space. A second set of samples with Fe/Al atomic ratios in the range 0−1 contains zinc, iron, and aluminum cations in the layer and carbonate as the interlayer anion. When thermal decomposition is started from the latter precursors, X-ray diffraction only detects the formation of ZnO after calcination at 400−600 °C, while X-ray absorption spectra recorded at the Fe and Al−K absorption edges show that a fraction of trivalent cations migrates to tetrahedral holes in this temperature range. After calcination at 800 °C, the highest temperature achieved, two ZnFe y Al 2 - y O 4 spinels segregate, with a distribution of cations essentially normal, Fe/Al atomic ratios being different from the value of the starting material. On the other hand, thermal decomposition of the precursor that incorporates iron as hexacyanoferrate(III) yields already at 600 °C a mixture of ZnO and a single ZnFe y Al 2 - y O 4 spinel phase, whose Fe/Al atomic ratio coincides with the value of the original sample. Results suggest that the pyrolysis of the cyanide ligands at 400 °C leads to a coordination environment for iron highly disordered beyond the first-coordination shell, thus increasing the reactivity of iron and preventing the segregation of cations during the thermal decomposition of this sample.