Theoretical Study on the Structural Properties and Relative Stability of M(II)−Al Layered Double Hydroxides Based on a Cluster Model

Abstract

The [M2Al(OH2)9(OH)4]3+ clusters (M = divalent cation Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, or Cd2+), which include the basic information of layered double hydroxides (LDHs) lattice structure with the most economical size, have been investigated by density functional theory (DFT) to shed light on the structural properties and relative stability of M(II)-Al binary LDHs layers with a M2+/Al3+ ratio of 2. The geometric parameters (bond distance and bond angle), natural bond orbitals (NBO), stretching vibration frequencies of three-centered bridging OH groups (nu(O3-H)), as well as binding energy of the cluster model were systematically studied. It was found that the geometries and the nu(O3-H) frequency for the calculated clusters are remarkably influenced by the electronic structure of the divalent cations, such as valence electronic configuration, natural bond orbitals, natural charge transfer, and bond order. The calculated binding energies are in good agreement with the relative stability of the experimental results for the corresponding LDHs. The calculation results reveal that the 2Ni-Al cluster shows the highest stability among the open-shelled cation-containing clusters, while the stability of the 2Cu-Al cluster is the weakest; the 2Mg-Al and 2Zn-Al clusters are the most stable ones among the closed-shelled cation-containing clusters. These findings are in high accordance with the experimental results. Therefore, this work provides a detailed understanding of how the electronic structure of cations plays a more significant role in the structural properties and relative stability of the corresponding LDHs layers rather than ionic size.