Structural and microstructural changes during anion exchange of CoAl layered double hydroxides: anin situX-ray powder diffraction study

Anion-exchange processes in cobalt–aluminium layered double hydroxides (LDHs) were studied byin situsynchrotron X-ray powder diffraction (XRPD). The processes investigated were CoAl–CO3→ CoAl–Cl → CoAl–CO3, CoAl–Cl → CoAl–NO3and 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–CO3phase decreases exponentially while that of the CoAl–Cl phase increases exponentially. Energy-dispersive X-ray spectroscopy (EDXS) studies of a partially chloride-exchanged CoAl–CO3LDH sample along within situXRPD data suggested that the individual particles in the CoAl–CO3sample 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–CO3process 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–NO3anion-exchange reaction, the XRPD patterns reveal the existence of two intermediate phases in addition to the initial CoAl–Cl and final CoAl–NO3phases. Thein situdata 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–SO4anion-exchange reaction show that the process is a two-phase transformation involving a sulfate-containing LDH with a 1H polytype structure.