Structural and microstructural changes during anion exchange of CoAl layered double hydroxides: an in situ X-ray powder diffraction study

Anion-exchange processes in cobalt-aluminium layered double hydroxides (LDHs) were studied by in situ synchrotron X-ray powder diffraction (XRPD). The processes investigated were CoAl-CO3 CoAl-Cl CoAl-CO3, CoAl-Cl CoAl-NO3 and CoAl-CO3 CoAl-SO4. The XRPD data show that the CoAl-CO3 CoAl-Cl process is a two-phase transformation, where the amount of the CoAl-CO3 phase decreases exponentially while that of the CoAl-Cl phase increases exponentially.

Energy-dispersive X-ray spectroscopy (EDXS) studies of a partially chloride-exchanged CoAl-CO3 LDH sample along with in situ XRPD data suggested that the individual particles in the CoAl-CO3 sample are generally anion-exchanged with chloride one at a time. In contrast with the CoAl-CO3 CoAl-Cl transformation, the XRPD data show that the reverse CoAl-Cl CoAl-CO3 process is a one-phase transformation.

Rietveld refinements indicate that the occupancy factors of the carbon and oxygen sites of the carbonate group increase, while that of the chloride site decreases. In the CoAl-Cl CoAl-NO3 anion-exchange reaction, the XRPD patterns reveal the existence of two intermediate phases in addition to the initial CoAl-Cl and final CoAl-NO3 phases.

The in situ data indicate that one of these intermediates is a mixed nitrate- and chloride-based LDH phase, where the disorder decreases as the nitrate content increases. The XRPD data of the partial CoAl-CO3 CoAl-SO4 anion-exchange reaction show that the process is a two-phase transformation involving a sulfate-containing LDH with a 1H polytype structure.