The potential energy curve and Langmuir isotherm of hydrogen adsorption by a truncated carbon sphere

The potential energy curve for the adsorption of a hydrogen molecule by a truncated hollow sphere consisting of carbon atoms was evaluated by using the Lennard–Jones function for the pair interaction between a carbon atom and a hydrogen molecule. The sphere surface was regarded as a continuum with a uniform density identical to that of a graphite layer. The lower limit of the potential was found to be −200 meV when the sphere had a radius of 3.4 Å and no opening. By increasing the radius of the opening to 2.9 Å, the energy barrier for an incoming molecule disappeared and the lower limit increased to −150 meV, which is three times as deep as that observed for a graphite surface. The Langmuir isotherm for the truncated sphere of this size was evaluated based on the eigenvalues of the potential curve. We found that the pressure yielding a half occupancy was <50 bar at a temperature below 250 K. This indicates that a carbon pore with this shape and size can store a hydrogen molecule under mild conditions.