Synthesis and characterization of hydrotalcites containing Ni(II) and Fe(III) and their calcination products

A layered double hydroxide with the hydrotalcite-like structure containing Mi(II) and Fe(III) cations in the brucite-like layers with the formula [Ni0 .7Fe0.3(OH)(2)](CO3)(0.15).0.94 H2O has been prepared by coprecipitation. O n hydrothermal treatment, a coproduct, identified as a NiFe2O4 spinel, is f ormed. Both samples, as well as the solids obtained from them by calcinatio n at 450 degrees C (where only mixed oxides are present) and 750 degrees C (where crystallization of well defined phases has taken place), have been c haracterized by powder X-ray diffraction (PXRD), X-ray absorption (XAS) and FT-IR spectroscopies, thermal analysis (differential and thermogravimetric ), temperature-programmed reduction, and specific surface area assessment. XAS results show that in both calcined samples all Ni(II) cations are in oc tahedral holes, while Fe(III) cations are equally distributed between octah edral and tetrahedral holes. For the sample calcined at 750 degrees C, coor dination parameters at the first and second shells of Ni(II) and Fe(III) ca tions coincide with those expected for a mixture of NiO and the NiFe2O4 spi nel, in agreement with the detection of both crystalline compounds by PXRD. Although PXRD only detects crystalline NiO in the sample calcined at 450 d egrees C, the presence of Fe(III) cations in tetrahedral holes discards the formation of a Ni(II)-Fe(III) oxide solid solution with a rock salt struct ure, thus indicating that Fe(III) ions are forming an amorphous phase at th is temperature. In this sample, coordination parameters at the second shell of Fe(III) are also different from those expected for a spinel-like struct ure. In agreement with the experimental data, a structure for the cluster p resent in the amorphous phase is proposed. Reducibility of cations in the s ample calcined at 450 degrees C depends on the synthesis method and on the formation of spinel-like nuclei.