Thermal behaviour of hydrotalcite-like compounds: study of the resulting oxidic forms

Hydrotalcite (HT)-like compounds of 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 to 0.66, have been synthesised by coprecipitation at constant pH and were hydrothermally heated. The thermal decomposition of the compounds was followed by DTA and TGA and the resulting products at different temperatures were studied by X-ray and IR spectroscopy techniques. In every case, there was a loss of interlayer water in a first stage, followed by the complete decomposition of the product at around 450-500 degrees C, depending on the hydrotalcite composition. A slightly different decomposition behaviour was found for compounds with 0.4 less than or equal to x less than or equal to 0.66. Only NiO was identified at 450 degrees C calcination temperatures of HT-like precursors, whereas the corresponding spinel, NiAl2O4, was formed by interaction between NiO and Al2O3, at 900 degrees C. New information was evidenced by combining isothermal pre-reduction of oxidic systems obtained at 450 and 900 degrees C with temperature-programmed reduction (TPR) experiments; a NiO-Al2O3 structure with a spinel-type local order and NiO were found in all samples at 450 degrees C, oxidic forms that were thermally stable up to 900 degrees C. These results on the sample reducibility were found to be in agreement with some models proposed in the literature for the oxidic forms derived from HT-like structures and also with our own IR and UV-Vis spectroscopic structural data. (C) 2000 Elsevier Science Ltd. All rights reserved.