Simulation of Gas Diffusion–Adsorption Coupling in Adsorbents for Optimizing Pore Structures

The diffusion and adsorption of gases in porous media are closely related to pore structures. The specific surface area and pore size distribution are commonly used parameters to characterize the adsorbent performance, but these static parameters can hardly describe the complex dynamic coupling of diffusion and adsorption in the pores, which restricts the improvement of the adsorbent performance. In this work, molecular dynamics simulations with hard-sphere/pseudo-particle modeling are conducted to investigate the coupling between diffusion and adsorption of nitrogen in activated carbon with different porosities and mean pore sizes. The results show that the diffusion coefficient increases with an increase in the pore size when the porosity remains unchanged. It is found that in the adsorbent with relatively small pore sizes, the gas molecules (diameter σHS) are mostly adsorbed at the outer shell of the adsorbent particle and hinder subsequent penetration of other molecules, which leads to a small adsorption amount and a low adsorption rate. Such a hindering effect gradually weakens as the mean pore size increases. It is also found that when the porosity (<0.6) remains unchanged, as the pore size increases, both the saturated adsorption amount and average adsorption rate increase first and then decrease, reaching their maximum at about 3.5σHS and 4.6σHS, respectively. When the mean pore size remains unchanged, an increase in porosity can significantly improve the average adsorption rate. These findings indicate that regulating the mean pore size between 3.5σHS and 4.6σHS and increasing the porosity, can obtain better adsorption performance, with both the saturated adsorption amount and adsorption rate being relatively high. This work is expected to be helpful for understanding diffusion and adsorption at the atomic scale, as well as for the design and optimization of high-performance porous adsorbents.