In situ investigation of the thermal decomposition of Co-Al hydrotalcite in different atmospheres

High temperature X-ray diffraction (HT-XRD), thermal analysis (TGA-DTA), ma ss spectrometry (MS), in situ Fourier transform infrared (FT-IR) spectrosco py, and in situ Raman spectroscopy have been used to characterize the therm al decomposition of Co-Al hydrotalcite, [Co6Al2(OH)(16)](CO3). 4H(2)O, in a ir and inert atmospheres. In the first decomposition step, water is removed from the structure, a process which is complete at 150-200 degreesC. This transition is followed by dehydroxylation and decarbonation, as well as car bonate reorganization in the interlayer space. These processes require high er temperatures under inert atmospheres than in air. The transition tempera tures also depend on the nature of the technique applied (static vs. dynami c operation). An intermediate metastable mixture of phases is identified, w hich contains the dehydrated layered structure and an emerging spinel-like mixed oxide phase. This phase is formed in the region of 150-175 degreesC i n air and was not observed under inert atmospheres. Dehydroxylation leads t o the collapse of the hydrotalcite phase and is complete at 250-300 (air) a nd 350-400 degreesC (inert gas). Carbonate removal is coupled with the dehy droxylation process, although removal of carbonate groups is only complete at 450 (air) and 600 degreesC (inert gas). Thermal treatment in air finally leads to a solid solution of cobalt spinels [Co(Co,Al)(2)O-4]. Mixtures of CoO and CoAl2O4 are formed upon treatment under inert atmospheres. Based o n the analytical results, a simplified structural model for the decompositi on process is presented. The presence of oxidizable Co2+ cations in the oct ahedral sheets and the diffusion of Co3+ to the interlayer space in the deh ydrated layered structure, and the stability of the solid solution of Co-sp inels formed are identified as key factors in the low thermal stability of the hydrotalcite precursor in air.