IR structural evidence of hydrotalcites derived oxidic forms

Ni–Al hydrotalcite-like compounds with the general formula Ni_1?xAl_x(OH)₂](CO₃)_x/2?mH₂O, where 0.25≤x≤0.66, were synthesised using coprecipitation at a constant pH, and were treated hydrothermally. The structures of the oxidic forms obtained by calcination of the hydrotalcites at 450°C and 900°C, respectively, were investigated using X-ray diffraction and, mainly, IR and UV–VIS spectroscopy. A NiO phase was identified by XRD in all calcined samples; an additional oxidic phase — the nickel spinel, NiAl₂O₄ — developed at 900°C. IR spectroscopy results gave supplementary information about the incipient, local organisation of cations in the interstices of the oxygen atoms lattice. IR spectra were different, depending on the samples' composition. In case of the HT precursors calcined at 450°C a structure like a transition alumina (γ-Al₂O₃) was found as a main oxidic phase in samples with a high Al-content; IR spectra of the samples with a high Ni content evidenced NiO as the main oxidic phase; in case of these latter samples, the formation of an oxidic structure with a spinel-type local order was identified at this temperature. This structure developed to an inverse nickel spinel oxidic phase at 900°C, as shown by the IR absorption bands. The NiO structure in the samples with a high Ni content at 450°C was found also in the oxides obtained by calcination at 900°C. The spinel-type local order was also observed by UV–VIS spectroscopy in case of the HT precursors calcined at 450°C, by the presence of both absorption bands of the tetrahedral and octahedral Ni(II) ions in the Al₂O₃ lattice and of octahedral Ni(II) ions in the NiO lattice. The same absorption bands were found also in the samples calcined at 900°C, proving that the NiAl₂O₄ spinel identified has a partial inverse structure, with the Ni(II) ions both in tetrahedral and octahedral crystalline fields. Our found structural data were in accord with the models proposed in the literature.