Abstract
We demonstrate the importance of London dispersion forces in defining the adsorption capacity within expanded graphite, a simple model of the more complex experimental geometries of activated carbon, using a combination of the non-local correlation functional of Dion et al. paired with a recent exchange functional of Cooper (vdW-DFC09x) and a classical continuum model. Our results indicate that longer ranged interactions due to dispersion forces increase the volume over which molecules interact with a porous medium. This significantly enhances the adsorption density within a material, and explains recent experimental work showing that the densification of H2 in carbon nanopores is sensitive to the pore size. Remarkably, our slit pore geometries give adsorption densities of 3 wt. % at 298 K and 20 MPa which correlates well with experimental values for 9 ˚A pores – a value that could not be predicted using (local density approximation) LDA calculations. In its entirety, this work presents a powerful approach for assessing molecular uptake in porous media and may have serious impacts on efforts to optimize the properties of these materials.
