The distribution of reactive Ni²⁺ in 2D Mg_2−xNi_xAl-LDH nanohybrid materials determined by solid state ²⁷Al MAS NMR spectroscopy

Abstract

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.

Synthesis and Thermal Degradation of MAl4(OH)12SO4·3H2O with M = Co2+, Ni2+, Cu2+, and Zn2+

“…The 27 Al MAS NMR spectra are shown in Figure (Figure S8 for ZnAl 4 -LDH comparison), and each has two resonance regions, which must contain a total of four δ iso ( 27 Al) resonances from the four crystallographic inequivalent Al sites, c.f., Figure a and Table . , The least and most negatively shifted regions are assigned to (Al3, Al4) and (Al1, Al2) as they have 1 and 2 paramagnetic neighbors (Figure ), respectively. This assignment is based on additive hyperfine shifts, as previously observed for paramagnetic hydrotalcite CoAl-, NiAl-, and MgNiAl-LDH. , Moreover, the Al1 and Al2 resonances for both NiAl 4 - and CoAl 4 -LDH have significantly faster relaxation, which reinforces the assignment. Furthermore, a 27 Al MAS NMR spectrum of chalcoalumite (CuAl 4 -LDH mineral specimen) was recorded, which showed similar features to the spectrum of CuAl 4 -LDH by visual comparison (Figures c and S9), again confirming the assignment.…”

Reevesite with Ordered Intralayer Atomic Arrangement as an Optimized Nickel‐Iron Oxygen Evolution Electrocatalyst

“…Besides the above‐mentioned strategies, the control of intralayer metal atom arrangement, especially for the multi‐metallic compound nanosheets, is an effective way to enhance the intrinsic electrocatalytic activity [27–28] . Similar synergistic effects have also been reported in NiFe LDH.…”

Long-Range and Short-Range Structures of Multimetallic Layered Double Hydroxides

“…normal wall ZrO 2 rotors at room temperature. Owing to the paramagnetic feature of Ni and Co in LDH phases, a spin-echo sequence was employed instead of a classic single-pulse sequence …”