Thermal decomposition of a hydrotalcite-containing Cu–Zn–Al precursor: thermal methods combined with an in situ DRIFT study

A Cu–Zn–Al precursor (CZA) was synthesized efficiently by coprecipitation of the corresponding cations with sodium carbonate at constant pH and temperature. The starting precursor contained a mixture of two hydroxycarbonate phases: rosasite and a Cu–Zn hydrotalcite-like phase. The thermal decomposition was studied by conventional thermal methods (TGA, DTA and EGA-MS) as well as by in situ FTIR spectroscopy (DRIFT). Analysis of the CZA precursor showed similar results by both procedures. Dehydration, dehydroxylation and decarbonation of the precursor were analysed in situ by monitoring the hydroxyl and carbonate infrared bands. A Cu–Zn hydrotalcite phase, one of the components of the CZA precursor, was also prepared independently. A detailed FTIR study revealed an interesting effect upon heating this hydrotalcite. At 373–423 K, a carbonate rearrangement in the interlayer space takes place during the loss of interlayer water. Carbonate groups change from their symmetrical coordination with interlayer water molecules to an arrangement involving the OH groups of the octahedral M(OH)m layers. This phenomenon certainly takes place in the CZA material as well but, in this case, it cannot be observed, probably due to the complexity of the material formed by two hydroxycarbonate phases.