Infrared emission spectroscopic study of the thermal transformation of Mg-, Ni- and Co-hydrotalcite catalysts Original Research Article

The thermal behaviour of synthetic hydrotalcites, Mg6Al2(OH)16CO3·nH2O, Co6Al2(OH)16CO3·nH2O and Ni6Al2(OH)16CO3·nH2O, has been studied by thermogravimetric analysis (TGA/DTA) and infrared emission spectroscopy (IES). The Mg-hydrotalcite shows a weight loss up to 70°C due to physically adsorbed/interlayer water. Between 70°C and 230°C weight is lost due to decarbonisation (DTA endotherm 222°C). The dehydroxylation basically takes place between 230°C and 420°C (DTA endotherm 400°C). The different steps are more overlapping for the Co-hydrotalcite. In contrast, the DTA of Ni-hydrotalcite shows a complex of endothermic maxima between 73°C and 207°C followed by a strong endotherm at 323°C. The Mg-hydrotalcite IES show major changes around 350–400°C indicating the end of the dehydroxylation. In this temperature range `Alʹ–OH bands disappear and some new bands are observed indicating the formation of spinel (MgAl2O4) together with MgO, which is another product formed after dehydroxylation. Similar results are obtained for the Co-hydrotalcite, which shows the formation of CoAl2O4 spinel and/or cobalt-oxide. All bands in the takovite IES at 200°C disappear at 300–350°C, while new bands are formed suggesting the formation of a spinel-like phase rather than of Al2O3. So, IES supports the reaction of Ni-hydrotalcite to a NiAl2O4 spinel phase+NiO and not Al2O3+NiO.