A spectroscopic study of the effect of the trivalent cation on the thermaldecomposition behaviour of Co-based hydrotalcites

Various correlations between the size of the trivalent cation (Al, Ga, Fe, and Cr) and the structural characteristics and decomposition behavior of Co -based hydrotalcites (Co/M3+=3) have been identified. In situ IR spectra (b y the relative ratio of the amount of carbonate in low and high symmetry) a nd thermogravimetric analyses show that the larger the trivalent cation, th e lower the relative amount of water in the interlayer space of the as-synt hesized material. Removal of interlayer water upon decomposition induces a symmetry loss of the carbonate groups, leading to splitting of the v(3) vib rational mode of the carbonate groups at elevated temperatures. The extent of splitting was found to continuously decrease as a function of increasing cation size, which suggests a decreasing degree of interaction of the carb onate groups with remaining OH groups in the brucite-like layers. However, the stability of the carbonates in the temperature range of 200-250 degrees C shows an opposite trend, and is apparently dependent on other factors, su ch as the stability of the hydroxides and the layered structure itself, as well as direct interactions of carbonate with the metal ions. Dehydroxylati on of the brucite-like layers, followed by complete carbonate decomposition are the final stages leading to the formation of mixed metal oxides.