Aluminium substitution in iron(II–III)-layered double hydroxides: Formation and cationic order

The formation and the modifications of the structural properties of an aluminium-substituted iron(II–III)-layered double hydroxide (LDH) of formula Fe4IIFe(2-6y)IIIAl6yIII (OH)12 SO4, 8H2O are followed by pH titration curves, Mössbauer spectroscopy and high-resolution X-ray powder diffraction using synchrotron radiation. Rietveld refinements allow to build a structural model for hydroxysulphate green rust, GR(SO42−), i.e. y=0, in which a bilayer of sulphate anions points to the Fe3+ species. A cationic order is proposed to occur in both GR(SO42−) and aluminium-substituted hydroxysulphate green rust when y<0.08. Variation of the cell parameters and a sharp decrease in average crystal size and anisotropy are detected for an aluminium content as low as y=0.01. The formation of Al-GR(SO42−) is preceded by the successive precipitation of FeIII and AlIII (oxy)hydroxides. Adsorption of more soluble AlIII species onto the initially formed ferric oxyhydroxide may be responsible for this slowdown of crystal growth. Therefore, the insertion of low aluminium amount (y∼0.01) could be an interesting way for increasing the surface reactivity of iron(II–III) LDH that maintains constant the quantity of the reactive FeII species of the material.

(a) Crystallographical structure of sulphated green rust: SO42− point to the Fe3+ cations (red) that form an ordered array with the Fe2+ cations (green). (b) Width and asymmetry of the synchrotron XRD peaks increase rapidly when some Al3+ species substitute the Fe3+ cations; z is molar ratio Al3+/Fe3+.Figure optionsDownload as PowerPoint slide