Theoretical and experimental study on the topotactic transformation mechanism of Zn-Al layered double hydroxides

Abstract

Layered double hydroxides (LDHs) are a class of minerals with nanoscale two-dimensional layered structure. The topotactic transformation of LDHs under calcination and the reconstruction behavior (memory effect) of the calcined LDHs in aqueous solution play a key role in the fabrication of high dispersed metal nanocatalysts. The thermal topotactic transformation mechanism of ZnAl-layered double hydroxides (LDHs) and memory effect of LDHs were investigated by density functional theory based molecular dynamics (MD) simulation, combined with thermogravimetric/differential thermal analysis (TG-DTA). TG-DTA results reveal that the LDH phase undergoes two key endothermic events at 273 and 800 °C. The results show that the anion in LDHs decomposes to CO2 and H2O via a monodentate intermediate at 273 °C, and H2O releases prior to CO2. After the decomposition of interlayer CO32− at 273 °C, the distribution of metal cations in LDH matrix changed heavily along both LDH(001) facet and the c -axis direction perpendicular to the (001) facet. The dehydrated products cannot reconstruct back to the hydroxide phase, showing no memory effect. At 800 °C, a complete collapse of layered structure occurs, resulting in a totally disordered cation distribution and plenty of holes in the final product. This work will be helpful for the design and preparation of highly dispersed nanocatalysts derived from LDHs precursors.