An X-ray diffraction and absorption study of the phases formed upon calcination off 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 degreesC, while X-ray absorption spectra recorded at the Fe and AI-K absorption edges show that a fraction of trivalent cations migrates to tetrahedral holes in this temperature range. After calcination at 800 degreesC, 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 degreesC 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 degreesC 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.