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 al uminum in the brucite-like layers, while iron is incorporated as hexacyanof errate(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 cation s in the layer and carbonate as the interlayer anion. When thermal decompos ition is started from the latter precursors, X-ray diffraction only detects the formation of ZnO after calcination at 400-600 degreesC, while X-ray ab sorption spectra recorded at the Fe and AI-K absorption edges show that a f raction of trivalent cations migrates to tetrahedral holes in this temperat ure range. After calcination at 800 degreesC, the highest temperature achie ved, two ZnFeyAl2-yO4 spinels segregate, with a distribution of cations ess entially normal, Fe/Al atomic ratios being different from the value of the starting material. On the other hand, thermal decomposition of the precurso r that incorporates iron as hexacyanoferrate(III) yields already at 600 deg reesC a mixture of ZnO and a single ZnFeyAl2-yO4 spinel phase, whose Fe/Al atomic ratio coincides with the value of the original sample. Results sugge st that the pyrolysis of the cyanide ligands at 400 degreesC leads to a coo rdination environment for iron highly disordered beyond the first-coordinat ion shell, thus increasing the reactivity of iron and preventing the segreg ation of cations during the thermal decomposition of this sample.