On the Strength of the Hydrogen-Carbon Interaction as Deduced from Physisorption

We deduce a new value for the potential well depth for the C-H-2 interaction on the basis of experimental validations of isotherms of H-2 and D-2 predicted using independently characterized microstructural parameters. We use two carbons, one an activated carbon fiber whose structure has been recently characterized by us (Nguyen, T. X.; cohaut, N.; Bae, J.-S.; Elhatia, S. K. Langmuir 2008, 24, 7912) using hybrid reverse Monte Carlo simulation (HRMC) and the other the commercial Takeda 3A carbon molecular sieve whose pore size distribution is determined here from the 273 K CO2 adsorption isotherm. The conventional grand canonical Monte Carlo simulation technique incorporating a semiclassical Feynman and Hibbs (FH) potential approximation (FHGCMC) as well as path integral Monte Carlo calculations is employed to determine theoretical adsorption isotherms. It is found that curvature enhances the well depth for the LJ C-H-2 interaction by a factor of 1.134 over that for a flat graphite surface, consistent with our recent study (Nguyen, T. X.; cohaut, N.; Bae, J.-S.; Bhatia, S. K. Langmuir 2008, 24, 7912). A value of the C-C well depth of 37.26 K, used for estimating the C-H-2 well depth in conjunction with the Berthelot rules, with the Steele C-C well depth used for interaction with heavier gases (Ar, CO2 and CH4), leads to excellent agreement with experimental isotherms in all cases.