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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.

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Portable Document Format (PDF) file https://doi.org/10.1107/S0021889810011805/ks5248sup1.pdf
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