Influenceof Structural Heterogeneity on Diffusionof CH4 and CO2 in Silicon Carbide-Derived NanoporousCarbon

摘要

We investigate the influence of structural heterogeneity on the transport properties of simple gases in a Hybrid Reverse Monte Carlo (HRMC) constructed model of silicon carbide-derived carbon (SiC-DC). The energy landscape of the system is determined based on free energy analysis of the atomistic model. The overall energy barriers of the system for different gases are computed along with important properties, such as Henry constant and differential enthalpy of adsorption at infinite dilution, and indicate hydrophobicity of the SiC-DC structure and its affinity for CO2 and CH4 adsorption. We also study the effect of molecular geometry, pore structure and energy heterogeneity considering different hopping scenarios for diffusion of CO2 and CH4 through ultramicropores using the Nudged Elastic Band (NEB) method. It is shown that the energy barrier of a hopping molecule is very sensitive to the shape of the pore entry. We provide evidence for the influence of structural heterogeneity on self-diffusivity of methane and carbon dioxide using moleculardynamics simulation, based on a maximum in the variation of self-diffusivitywith loading. A comparison of the MD simulation results with self-diffusivitiesfrom quasi-elastic neutron scattering (QENS) measurements and, withmacroscopic uptake-based low-density transport coefficients, revealsthe existence of internal barriers not captured in MD simulation andQENS experiments. Nevertheless, the simulation and macroscopic uptake-baseddiffusion coefficients agree within a factor of 2–3, indicatingthat our HRMC model structure captures most of the important energybarriers affecting the transport of CH4 in the nanostructureof SiC-DC.