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

Abstract

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 (by the relative ratio of the amount of carbonate in low and high symmetry) and thermogravimetric analyses show that the larger the trivalent cation, the lower the relative amount of water in the interlayer space of the as-synthesized material. Removal of interlayer water upon decomposition induces a symmetry loss of the carbonate groups, leading to splitting of the ν3 vibrational 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 carbonate groups with remaining OH groups in the brucite-like layers. However, the stability of the carbonates in the temperature range of 200–250 °C shows an opposite trend, and is apparently dependent on other factors, such as the stability of the hydroxides and the layered structure itself, as well as direct interactions of carbonate with the metal ions. Dehydroxylation of the brucite-like layers, followed by complete carbonate decomposition are the final stages leading to the formation of mixed metal oxides.