Abstract

High temperature X-ray diffraction (HT-XRD), thermal analysis (TGA-DTA), mass spectrometry (MS), in situ Fourier transform infrared (FT-IR) spectroscopy, and in situ Raman spectroscopy have been used to characterize the thermal decomposition of Co–Al hydrotalcite, [Co₆Al₂(OH)₁₆](CO₃)·4H₂O, in air and inert atmospheres. In the first decomposition step, water is removed from the structure, a process which is complete at 150–200 °C. This transition is followed by dehydroxylation and decarbonation, as well as carbonate reorganization in the interlayer space. These processes require higher temperatures under inert atmospheres than in air. The transition temperatures also depend on the nature of the technique applied (static vs. dynamic operation). An intermediate metastable mixture of phases is identified, which contains the dehydrated layered structure and an emerging spinel-like mixed oxide phase. This phase is formed in the region of 150–175 °C in air and was not observed under inert atmospheres. Dehydroxylation leads to the collapse of the hydrotalcite phase and is complete at 250–300 (air) and 350–400 °C (inert gas). Carbonate removal is coupled with the dehydroxylation process, although removal of carbonate groups is only complete at 450 (air) and 600 °C (inert gas). Thermal treatment in air finally leads to a solid solution of cobalt spinels [Co(Co,Al)₂O₄]. Mixtures of CoO and CoAl₂O₄ are formed upon treatment under inert atmospheres. Based on the analytical results, a simplified structural model for the decomposition process is presented. The presence of oxidizable Co²⁺ cations in the octahedral sheets and the diffusion of Co³⁺ to the interlayer space in the dehydrated layered structure, and the stability of the solid solution of Co-spinels formed are identified as key factors in the low thermal stability of the hydrotalcite precursor in air.