Influence of the divalent cation on the thermal activation and reconstruction of hydrotalcite-like compounds

The influence of the divalent cation on the thermal decomposition and subsequent reconstruction of Mg-3-Al, Ni-3-Al, and Mg-2-Ni-Al hydrotalcite-like compounds has been investigated by in situ XRD. Diffraction studies were complemented by ICP-OES, AAS, TEM, N-2 adsorption, FT-IR, TGA, and TPD-MS characterizations. The decomposition mechanism, as for the evolvement of chemical species and phase transitions, is not influenced by the divalent cation in the brucite-like sheets. An intermediate dehydrated layered phase is formed at 423-473 K in all samples, which is transformed into the corresponding mixed oxide at 573-623 K. The characteristic platelet-like morphology in all as-synthesized hydrotalcites is preserved in Mg-Al oxide, while uniform nanoparticles are generated in Ni-Al oxide. Mg-Ni-Al oxide exhibits intermediate morphological features between the binary samples. The divalent cation stipulates the ability of the resulting oxide to recover the original layered structure. In contrast with the facile reconstruction of Mg-Al oxide, no sign of recovery was observed in Ni-Al oxide upon exposure to water vapor at room temperature. In the latter sample, even rehydration of the intermediate layered phase was not fully reversible. Addition of nickel to the binary Mg-Al sample dramatically reduces the ability of the resulting mixed oxide to recover the hydrotalcite structure. This is tentatively attributed to the intimate mixing of the various cations in the oxide phase. Relevant kinetic parameters of the reconstruction process were obtained by fitting the in situ XRD data with the Avrami-Erofe'ev model. The rate coefficient for reconstruction of the ternary Mg-Ni-Al oxide was reduced by 1 order of magnitude with respect to the binary Mg-Al oxide.