129Xe NMR spectroscopy of paramagnetic Mg2-Ni Al layered double hydroxides

Layered double hydroxides (LDHs) are promising materials for ion exchange, catalytic and electrochemical applications. Typically, the materials are composed of agglomerates of LDH platelets with complex mesoporous and macroporous networks within and between the particles. These networks control the diffusion of substrates and thereby influence the kinetics of, e.g. , anion exchange and catalytic properties. Here, we use 129 Xe NMR spectroscopy, field-emission scanning electron microscopy and N 2 sorption to study porous structures and gas transport in Mg 2- x Ni x Al layered double hydroxides. N 2 adsorption analysis shows that mesopore and macropore surface area and pore volume increase with increasing Ni content ( x ). 129 Xe NMR spectra indicate that the mean pore diameter decreases from 28 ± 5 to 16 ± 2 nm and the effective heat of adsorption increases from 8.7 ± 0.4 to 9.4 ± 0.2 kJ mol -1 , when x increases from 2/3 to 5/3. 129 Xe exchange NMR spectroscopy reveals that the exchange rate of Xe between mesopore and macropore sites for the x = 0 sample is high, about 4700 s -1 . The 129 Xe T 1 relaxation time appears to be a very sensitive probe for paramagnetic Ni, whereas the T 2 relaxation time predominantly reflects exchange dynamics. Overall, the 129 Xe NMR analysis, complemented with other methods, provides detailed information about the properties of LDHs on length scale from atomic to bulk thereby improving our understanding of LDH properties and performance. • 129 Xe NMR provides insight into the porous structures of LDH materials • Pore size decreases and heat of adsorption increases with Ni content in MgNiAl-LDH • The exchange rate of Xe between mesopore and macropore sites is high, ∼4700 s -1 • T 1 relaxation reflects better the paramagnetism of the LDHs than the chemical shift