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 hydrother mally heated. The thermal decomposition of the compounds was followed by DT A 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 o f 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 com pounds with 0.4 less than or equal to x less than or equal to 0.66. Only Ni O was identified at 450 degrees C calcination temperatures of HT-like precu rsors, 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 90 0 degrees C with temperature-programmed reduction (TPR) experiments; a NiO- Al2O3 structure with a spinel-type local order and NiO were found in all sa mples at 450 degrees C, oxidic forms that were thermally stable up to 900 d egrees C. These results on the sample reducibility were found to be in agre ement with some models proposed in the literature for the oxidic forms deri ved from HT-like structures and also with our own IR and UV-Vis spectroscop ic structural data. (C) 2000 Elsevier Science Ltd. All rights reserved.