Decomposition Pathways of Hydrotalcite-like Compounds Mg_1-xAl_x(OH)₂(NO₃)_x·nH₂O as a Continuous Function of Nitrate Anions

Thermal decomposition pathways of our recently prepared hydrotalcite-like compounds Mg 1 - x Al x (OH) 2 (NO 3 ) x · n H 2 O in x = 0.20−0.34 ( J. Phys. Chem. B 2001, 105, 1743−1749) have been investigated with XRD, DTA, TGA, FTIR, and combined TGA/FTIR techniques. It has been found that, unlike those in carbonated hydrotalcites, the dehydroxylation and decomposition of anions in low nitrate-content ( x ) hydrotalcites are separated, while the two processes in the high x samples are overlapped. In line with our recent structural models, the dehydroxylation process in the samples with high x value can be further differentiated into steps, depending on chemical nature of hydroxyl group and nitrate content. The layered structure of these hydrotalcite compounds becomes thermally more stable when more nitrate ions are intercalated. The depletion of nitrate anions (decomposed into NO 2 and O 2 ) in the low x compounds is a continuous process, whereas that in the high x compounds is a discrete one. At 400 °C, most nitrate anions are still retained in the interlayer space with both D 3 h and C 2 v symmetries, although the dehydroxylation reaction in the low x samples is largely completed. At 500 °C, intercalated nitrate anions are mostly decomposed and the remaining ones are mainly in C 2 v symmetry with a standing configuration between two dehydroxylated brucite-like layers. The nearest distance between two oxygen octahedrons changes from 3.088 to 3.040 Å to 2.99−2.97 Å when the hydrotalcite-like phase is topotactically transformed to a rock-salt-like phase.