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.