Surface compression of light adsorbates inside microporous PFA-derived carbons

Surface compression is a phenomenon in which strong solid-fluid interactions compress the adsorbate molecules on a surface to a point that lateral repulsion forces appear. In this work, this phenomenon was studied for high pressure adsorption of a collection of light gases on polyfurfuryl alcohol (PFA)-derived carbons. The carbons were mostly microporous with mean pore sizes ranging from 5 to 8 Å. Surface and lateral interactions were decoupled by transforming the adsorption isotherms into Ono-Kondo coordinates. This analysis revealed substantial loss of kinetic energy of hydrogen inside the carbon micropores. Inelastic Neutron Scattering (INS) indicated that there was indeed a hindrance to the rotational motion of H₂ molecules inside the microporous carbon. Extensive analysis using heavier gases showed the observed change in the nature of the lateral forces during adsorption process. Adsorption began with attractive lateral forces; then as the pressure increased and even at low pressures repulsive forces emerged and as the adsorption proceeded the repulsive forces became more dominant. These effects were more pronounced inside small micropores. We believe that these lateral forces can also contribute to the drop in energetics of adsorption with gas loading as seen in heat of adsorption profiles.