Download Thermal Behaviour of Phosphate Intercalated Mg/Al

Thermal Behaviour of Phosphate Intercalated Mg/Al-layered Double Hydroxide
Studied by In-situ X-ray Diffraction and X-ray Absorption Spectroscopy
Akihiro Shimamura 1, Linus Perander 2, James Metson 2
1
Department of Chemical Science and Technology, Graduate School of Advanced
Technology and Science, The University of Tokushima, 2-1 Minamijosanjima,
Tokushima 770-8506, Japan
2
Chemistry Department, The University of Auckland, 23 Symonds Street, 1142
Auckland, New Zealand
Mg/Al Layered Double Hydroxides (LDH) are a promising group of materials that provide a
low cost route to selectively remove or release anionic species through ion exchange. For
example, LDHs can be used to selectively remove or release phosphates or agrochemicals
from polluted waterways or potentially as controlled drug release systems. An important
property, that also makes them such promising materials for a variety of applications, is that
they may be recycled.
These materials undergo structural changes, even at relatively low temperatures (333-473K),
which modify their interlayer spacing and impact on their adsorption capacity and selectivity.
The structural changes upon thermal treatment are due to the formation of a meta-stable
phase through the removal of interlayer water molecules. This meta-phase can revert back to
the original phase by re-hydration/adsorption of moisture from the atmosphere,
accommodated by an increase in the basal plane spacing.
A combination of synchrotron based X-ray powder diffraction and X-ray absorption
spectroscopy has allowed us to investigate the thermal behaviour of these materials in detail.
The in-situ capabilities and short measurement times offered by SXRD avoid problems with
the re-adsorption of water into the interlayer structure during the measurements and the high
intensity and resolution allowed the subtle structural changes to be accurately observed. The
complimentary information obtainable by XANES (X-ray Absorption Near Edge Structure
spectroscopy) was used to demonstrate that the structural changes are also accompanied by
changes in the chemical environment and the way the intercalated anion interacts with the
layer structure. It was observed that both the aluminium and magnesium hydroxides change
to a poorly crystalline oxide forms during thermal treatment and that these oxides revert back
to the hydroxides upon rehydration.