IR structural evidence of hydrotalcites derived oxidic forms

Ni-Al hydrotalcite-like compounds with the general formula [Ni1-xAlx(OH)(2) ](CO3)(x/2).mH(2)O, where 0.25 less than or equal to x less than or equal t o 0.66, were synthesised using coprecipitation at a constant pH, and were t reated hydrothermally. The structures of the oxidic forms obtained by calci nation of the hydrotalcites at 450 degrees C and 900 degrees C, respectivel y, were investigated using X-ray diffraction and, mainly, IR and UV-VIS spe ctroscopy. A NiO phase was identified by XRD in all Calcined samples; an ad ditional oxidic phase - the nickel spinel, NiAl2O4 - developed at 900 degre es C. IR spectroscopy results gave supplementary information about the inci pient, 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 degrees C a structure like a transition alumina (gamma-Al2O3) was found as a main oxidic phase in sample s 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, th e formation of an oxidic structure with a spinel-type local order was ident ified at this temperature. This structure developed to an inverse nickel sp inel oxidic phase at 900 degrees C, as shown by the IR absorption bands. Th e NiO structure in the samples with a high Ni content at 450 degrees C was found also in the oxides obtained by calcination at 900 degrees C. The spin el-type local order was also observed by UV-VIS spectroscopy in case of the HT precursors calcined at 450 degrees C, by the presence of both absorptio n 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 degrees 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 stru ctural data were in accord with the models proposed in the literature. (C) 2000 Elsevier Science B.V. All rights reserved.