Theoretical study on the topotactic transformation and memory effect of M (II) M (III)-layered double hydroxides

Abstract

The topotactic transformation mechanism and memory effect of NiAl- and MgFe- layered double hydroxides (LDHs) are investigated by density functional theory (DFT)-based molecular simulation under their two key thermal decomposition temperatures (365, 800 °C for NiAl-LDHs, and 380, 800 °C for MgFe-LDHs). The results show that at the first temperature, the interlayer carbonate in both LDHs decompose to CO₂ and H₂O via a monodentate intermediate. During the dehydroxylation of the layers, for both LDHs the metal cations maintain their original distribution within the LDH (0 0 1) facet, while migrating substantially along the c-axis direction, and the layered structure of MgFe-LDHs is destroyed earlier than those of NiAl-LDH. Meanwhile, MgFe-LDHs can keep the memory effect longer than NiAl-LDHs, and the memory effect will disappear when the four-coordinated metal cations increased. At 800 °C, the layered structure of NiAl-LDHs is slightly destroyed, while a complete collapse of layered structure occurs in MgFe-LDHs. These results agree well with the experimental findings. This work will be helpful for the design and preparation of nanocatalysts derived from LDHs precursors.

Keywords:

• Layered double hydroxides
• topotactic transformation
• memory effect
• molecular dynamics

Acknowledgements

We acknowledge the National Supercomputing Center in Shenzhen and the ‘Chemical Grid Project’ of Beijing University of Chemical Technology for providing the computational resources.