Hydrogen adsorption in Si-LTA and LTA-4A zeolites: A Gibbs Ensemble Monte Carlo simulation study

Abstract

Storing a targeted amount of hydrogen is one of the most challenging tasks. Several porous materials are being tested for the physisorption of hydrogen. We perform, NPT-Gibbs Ensemble Monte Carlo Simulation of hydrogen storage inside two zeolites: Si-LTA and LTA-4A. Classical three-site Silvera-Goldman potential is used to model hydrogen molecules. Gravimetric densities, radial distribution functions, adsorption enthalpies and specific regions for hydrogen are analyzed in detail in this study. Although number of hydrogen molecules stored in LTA-4A is more compared to Si-LTA, their gravimetric densities are comparable. At 77 K and 200 bar, almost 3 wt% of hydrogen can be stored in these zeolites. Introduction of sodium ions enhance the number of hydrogen molecules adsorbed in LTA-4A. Because of the presence of sodium ions at the windows of the cages, it can store these extra hydrogens inside it. It is also observed that for both these zeolites, preferred adsorption sites are always α-cages compared to the channels or β-cages. Replacement of silicon atoms with aluminium does not have any change on the uptake, but the presence of sodium counter ions increase the uptake. At low loading, enthalpies of adsorption are enhanced by 2 kJ/mol, because of the counter ions. Even though hydrogen uptake is less in these zeolites, we find important aspects of hydrogen adsorption in general which will help to devise porous materials with higher storing capabilities.