Construction Strategy for Atomistic Models of Coal Chars Capturing Stacking Diversity and Pore Size Distribution

While there are efficient construction
strategies for crystalline,
amorphous, and polymeric structures, there are few that enable construction
where there are distributions of properties within structures of various
order/disorder. Coal chars are one example and are particularly challenging.
Chemical and physical properties that vary over length scales along
with the distributions of the stacking extent impact the local domain
size and orientation. These properties influence char reactivity.
Similarly, the pore size distribution imparts access to the reactive
surface. Thus, relatively large-scale structures (50 000s of
atoms) are needed with control of the distributions of structural
features and to allow for incorporation of mesoporosity. These challenges
limit the size and availability of coal char models limiting the effectiveness
of atomistic simulations. Here, a highly automated and rapid method
for construction of large-scale char models is demonstrated using
Fringe3D and Vol3D in-house scripts incorporating the high-resolution
transmission electron microscopy (HRTEM)-determined distributions
of stacking extent, alignment, and capturing an assumed bimodal pore
size distribution simultaneously. Two large-scale scaffold char structures
were constructed (currently comprised of stacks omitting cross-links,
heteroatoms, and curvature). One structure (within a 100 × 100
× 100 Å cube) captured distributions of fringe lengths,
their orientations, and stacking extents (data from the literature).
The inherent micropore size presents a peak at 8–10 Å
in diameter. A second char with a larger volume (104 × 104 ×
104 Å) accommodates a similar extent of structural diversity
with mesoporosity (∼22 Å diameters) and additional microporosity
to capture the desired distribution. Here, the accommodation of a
pore size distribution was obtained with assumed 88% microporous and
12% mesoporous volumes. These char models have different pore size
distributions but reasonable (and similar) atomic H/C ratios (∼0.26),
diversity in orientation, stacking, sizes of graphene layers, and
helium densities (∼1.80 g/cm3). Such structures
are expected to allow for exploration of structure and behavior relationships,
such as the contribution of porosity to char reactivity as an independent
parameter.