(444c) Understanding Adsorption and Diffusion In Disordered Nanoporous Carbons

Disordered nanoporous carbons (DNCs) are highly heterogeneous materials that are synthesized from carbonaceous precursors such as woods, coconut shells, coals, polymers and crystalline carbides.  Due to their predominately micro- and meso-porous features and large specific surface areas, they have shown exceptional performance in numerous technologies aimed at emissions reduction, environmental remediation and energy storage. However, despite their ubiquitous nature, accurately characterizing the disordered nanostructures of these materials is notoriously challenging.  As a result, the potential of DNCs has not been fully realized since the usual structure-function paradigm cannot be presently applied to formulate rational design principles to guide their synthesis and application.

Atomistic simulation methods provide complementary information to experimental studies in the ongoing effort to understand the relationship between the nanostructure of DNCs and the thermodynamic and dynamic behaviors of guest phases confined within their pores.  Using realistic models for DNCs (e.g., Palmer et al., Carbon, 47, 2904 (2009); Carbon, 48,1116 (2010)), we have applied several simulation methods to achieve an improved understanding of how the structural features impact the adsorption and diffusion of small gas molecules.  In particular, we have used a combination of grand canonical Monte Carlo and molecular dynamics simulations to investigate the impact that the compliancy of the carbon framework has on the observed isotherms of methane over a wide range of bulk conditions.   By examining adsorption in both rigid and compliant models, we have established that there are measurable changes in the adsorbent structure for bulk pressures in excess of a few hundred bars. These changes are manifested in both the pore size distributions and simulated scattering profiles of the DNC models.   We also find the magnitudes of these changes are intimately tied to the local atomic structure of the adsorbent, which has been found experimentally to be linked with synthesis temperature (Palmer et al., Carbon, 48,1116 (2010)). Finally, we investigate the impact that these structural changes have on the dynamic behavior of the confined methane molecules by calculating the self- and transport-diffusion coefficients.