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 Ni(II) and Fe(III) cations in the brucite-like layers with the formula [Ni 0.7 Fe 0.3 (OH) 2 ](CO 3 ) 0.15 ·0.94 H 2 O has been prepared by coprecipitation. On hydrothermal treatment, a coproduct, identified as a NiFe 2 O 4 spinel, is formed. Both samples, as well as the solids obtained from them by calcination at 450 °C (where only mixed oxides are present) and 750 °C (where crystallization of well defined phases has taken place), have been characterized 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 octahedral holes, while Fe(III) cations are equally distributed between octahedral and tetrahedral holes. For the sample calcined at 750 °C, coordination parameters at the first and second shells of Ni(II) and Fe(III) cations coincide with those expected for a mixture of NiO and the NiFe 2 O 4 spinel, in agreement with the detection of both crystalline compounds by PXRD. Although PXRD only detects crystalline NiO in the sample calcined at 450 °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 structure, thus indicating that Fe(III) ions are forming an amorphous phase at this temperature. In this sample, coordination parameters at the second shell of Fe(III) are also different from those expected for a spinel-like structure. In agreement with the experimental data, a structure for the cluster present in the amorphous phase is proposed. Reducibility of cations in the sample calcined at 450 °C depends on the synthesis method and on the formation of spinel-like nuclei.