(72b) Molecular Dynamics Simulation of Simple Gases in Various Porous Structure Models

We present results of transport diffusion of simple gases, Ar, N2, CH4, CF4, in the reconstructed model of a saccharose-based carbon obtained in our recent work using the hybrid reverse Monte Carlo technique [1]. The results are compared with those for a slit-pore model of the same carbon. The sphere approximated model is employed to represent these fluid molecules, and the dynamic quantities are determined using equilibrium molecular dynamic (EMD) simulations. We also propose a new algorithm based on cluster analysis to generate the initial configuration for the MD simulations free from the insertion of fluid particles in disconnected spaces of the carbon. The proposed technique enables us to detect kinetically closed pores in the disordered structure of saccharose char. Such closed pores, different from physical close pores, are formed at low temperature due to roughness of their surfaces. Such roughness leads to formation of local energetic minimum regions adjacent to the pore mouth, that prevents movement of adsorbate molecules from one local minimum region inside the pore to other outside the pore through the pore mouth or vice versa due to low kinetic energy of the molecules at low temperature. The detailed algorithm and results will be presented at the conference, as also the results of the EMD simulations. Further, the results show that the disordered surfaces of the saccharose char lead to diffuse reflection of colliding molecules, as opposed to nearly specular reflection for ordered carbon surfaces that are relatively smooth such as those of carbon nanotubes. The calculated diffusion coefficients in the saccharose char or in slit-pores with diffuse reflection are consistent with the range of values reported from experimental data fits, confirming the validity of the diffuse reflection model in activated carbons and other carbons that present heterogeneous and disordered surfaces.

[1] Nguyen T. X., Bhatia S. K., Jain S. K., Gubbins K. E., Structure of saccharose-based carbon and transport of confined fluids: hybrid reverse Monte Carlo reconstruction and simulation studies, Mol. Simul. (accepted).