Influence of Cu, Co and Ni cations incorporated in brucite-type layers on thermal behaviour of hydrotalcites and reducibility of the derived mixed oxide systems

Thermal decomposition of MMgAlCO3NO3 hydrotalcites (where M is Cu2+, Co2+ or Ni2+) with intended M/Mg (0.10/0.61) and M/Al (0.10/0.29) ratios prepared under the same conditions was studied using high temperature X-ray diffraction (HT-XRD) and thermogravimetry coupling with mass spectrometry (TGA–DTA–MS). Introduction of transition metals to the brucite-like layers of hydrotalcites changed their thermal behaviour. The decomposition of nitrate or carbonate anions was coupled with oxidation from Ni2+ to Ni3+ and Co2+ to Co3+. Further increase of temperature resulted in reduction of these cations. Reduction from Cu2+ to Cu+ was also observed above 700 °C. Stabilisation of carbonate anions by Cu2+ introduced into hydrotalcite matrix was found at temperatures above 600 °C. The thermal treatment of hydrotalcite precursors resulted in the formation of mixed oxide and spinel phases, which were stable after cooling to ambient temperature with exception of CuO phase that appeared during cooling down due to segregation effects. Reducibility of the hydrotalcites calcined at different temperatures was determined by temperature-programmed reduction (TPR) experiments. Specific surface areas of hydrotalcites calcined at 600 °C ranged from 226 to 196 m2/g dropping with increase in the calcination temperatures to values ranging between 138 and 49 m2/g depending on transition metal contents.