posted on 2020-08-31, 22:04authored byJosiah Roberts, Yang Song, Mark Crocker, Chad Risko
Layered
double hydroxides (LDH) demonstrate significant potential
across a range of applications, including as catalysts, delivery vehicles
for pharmaceuticals, environmental remediation, and supercapacitors.
Explaining the mechanism of LDH action at the atomic scale in these
and other applications is challenging, however, due to the difficulty
in precisely defining the bulk and surface structure and chemical
compositions. Here, we focus on the determination of the structure
of lithium–aluminum (Li–Al) LDH, which has shown promise
in the catalytic depolymerization of lignin, both directly as the
catalyst and as a support for gold nanoparticles. While the relative
positions of the Li and Al metals are generally well resolved by X-ray
crystallography, it is the structures of the anionic layers, consisting
of water and carbonate, that are less well established. Combinatorial
analyses of all possible positions and rotations of the water and
carbonate in the three-layered Li-AL LDH polytope reveals that the
phase space is much too large to examine in any reasonable time frame
in a one-by-one structure exploration. To overcome this limitation,
we develop and deploy a genetic algorithm (GA) wherein fitness is
determined by matching a calculated X-ray diffraction (XRD) pattern
for a given structure to the known experimental XRD pattern. The GA
approach results in structures of high fitness that portend the bulk
Li–Al LDH structure. Importantly, the GA approach offers the
potential to determine the structures of other LDH, and more generally
layered materials, which are generally difficult to describe given
the large chemical and structural space to be explored.