Natural Double Layered Hydroxides: Structure, Chemistry, and Information Storage Capacity

摘要

Layered double hydroxides (LDHs) constitute an important group of materials with many applications ranging from catalysis and absorption to carriers for drug delivery, DNA intercalation and carbon dioxide sequestration (Rives 2001; Duan and Evans 2006). The structures of LDHs are based upon double brucite-like hydroxide layers [M_n~(2+)M_m~(3+)(OH)_(2(m+n))]~(m+), where M~(2+) = Mg~(2+), Fe~(2+), Mn~(2+), Zn~(2+), etc.; M~(3+) = Al~(3+), Fe~(3+), Cr~(3+), Mn~(3+), etc. The positive charge of the layer is compensated by interlayer species that may consist of anions (CO_3~(2-), Cl~-, SO_4~(2-), etc.) or both anions and cations (Na~+, Ca~(2+), Sr~(2+), etc.). Structural features of LDHs such as cation ordering, charge distribution and polytypism have an immediate influence upon their properties and have been under extensive experimental and theoretical investigations recently. In particular, Mg-Al cation order is important for catalytic activity of MgAl LDHs correlated with the numbers of Al~(3+) sites at the closest distance from an Al~(3+) site (Kim et al. 2003). Different distribution of Al in a Mg hydroxide matrix also results in different charge distribution in the interlayer, which is critically important for intercalation reactions.