Thermal decomposition of the layered double hydroxides of formula Cu₆Al₂(OH)₁₆CO₃ and Zn₆Al₂(OH)₁₆CO₃

Abstract

Zn-Al hydrotalcites and Cu-Al hydrotalcites were synthesised by coprecipitation method and analysed by X-ray diffraction (XRD) and thermal analysis coupled with mass spectroscopy. These methods provide a measure of the thermal stability of the hydrotalcite. The XRD patterns demonstrate similar patterns to that of the reference patterns but present impurities attributed to Zn(OH)₂ and Cu(OH)₂. The analysis shows that the d003 peak for the Zn-Al hydrotalcite gives a spacing in the interlayer of 7.59 Å and the estimation of the particle size by using the Debye-Scherrer equation and the width of the d003 peak is 590 Å. In the case of the Cu-Al hydrotalcite, the d003 spacing is 7.57 Å and the size of the diffracting particles was determined to be 225 Å.

The thermal decomposition steps can be broken down into 4 sections for both of these hydrotalcites. The first step decomposition below 100°C is caused by the dehydration of some water absorbed. The second stage shows two major steps attributed to the dehydroxylation of the hydrotalcite. In the next stage, the gas CO₂ is liberated over a temperature range of 150°C. The last reactions occur over 400°C and involved CO₂ evolution in the decomposition of the compounds produced during the dehydroxylation of the hydrotalcite.