A study of interaction potentials for H2 adsorption in Single Walled Nano Tubes: a possible way to more realistic predictions.

A comparative analysis of interaction potentials, classified according to the parametrization method, namely Lorentz-Berthelot rules, semi-empirical or ab initio calculations, found their energy depths to scale, respectively, to ca 30K, ca 40K, and ca 60K. We draw the Potential Energy Surfaces (PESs) for a hydrogen probe molecule inside a Carbon Nano-Tube (CNT): it is shown that the adsorption energy increases with the hard radius of the interaction potential and decreases as the CNT pore enlarges. This is valid just for low-medium pressures, when hydrogen repulsions are negligible. If not, adsorption is driven by H2-H2 hard radius despite all other parameters. Monte Carlo (MC) simulations, following the Gibbs Ensemble (GE) in high density conditions, confirm that the thermodynamic equilibrium of an order-disorder phase transition show no changes throughout any of the studied potentials. We also analyse, in the Grand Canonical (GC) ensemble, the geometric and structural characteristics of square lattice bundles of Single Walled Nano Tubes (SWNTs) with regard to their influence on adsorption storage. To do so, we develop a method for independently simulate inner or outer adsorption in infinitely long nanotube lattice systems. Our results suggest a pressure range for convenient H2 storage and enlighten the influence of CNT size on adsorption performance. In addition, larger CNTs are capable to host further hydrogen layers, but only at very high pressures.