The distribution of reactive Ni 2+ in 2D Mg 2−x Ni x Al-LDH nanohybrid materials determined by solid state 27 Al MAS NMR spectroscopy
Résumé
Layered double hydroxides (LDHs), especially (doped) with transition metals, as well as nanohybrid and 2D materials derived from these structures, are interesting materials due to their catalytic and electrochemical properties. Their reactivity is determined by the atomic level distribution of the transition metal in the LDH cation layer, which is essential to control the design of LDHs with optimized properties. However, low crystallinity, absence of long range order, and/or isoelectronic ions often prevent atomic level structural characterization. A series of poorly crystalline Mg2−xNixAl-NO3 LDH materials were investigated by ultrafast 27Al MAS NMR spectroscopy to determine the distribution of Ni2+ in these as well as possible superstructures and their miscibility gaps. Four Ni2Al-LDH samples with interlayer distances ranging from 7.6 to 17.5 Å were prepared to assess the contribution of inter- and intralayer magnetic interactions. The effects of the Ni2+ content and the atomic level distribution of Ni2+ were probed by ultrafast 27Al MAS NMR spectroscopy: the Al distribution can be modeled using a binomial distribution and neither a superstructure was identified for the MgNiAl-LDH sample nor a miscibility gap. The 27Al isotropic shift, δiso(27Al), is a very sensitive probe for a number of neighboring Ni2+ in the first metal ion sphere, but to a smaller degree it is also affected by the intercalated anion (interlayer distance). These results were used for detailed characterization of an exfoliated (2D)–restacked Mg1.83Ni0.17Al-LDH nanohybrid material and a Mg1.83Ni0.17Al-LDH-alginate nanohybrid material, in which 27Al MAS NMR showed how the structure and partial dissolution of the LDHs were retained. In contrast, both powder X-ray diffraction and vibrational spectroscopies (IR and Raman) reflected only the overall change in sample composition.