Hydrogen adsorption on pillar[6]arene: A computational study

A systematical theoretical work on physisorption of H2 molecules by both pristine and Li+-decorated perhydroxylated pillar[6]arene (PPA) in the gas phase has been performed by using density functional theory (DFT) and 0th order symmetry-adapted perturbation theory calculations (SAPT0). A complementary analysis using ab initio molecular dynamics (AIMD), atoms-in-molecules (AIM) and independent gradient model (IGM) methods has also been done. Our calculations have shown that H2 molecules can be adsorbed favorably inside the PPA hollow compared with pristine graphene. However, the obtained magnitudes of adsorption energy (Ead) fall outside the optimal range of energies for hydrogen storage. On the other hand, Li+ decoration has drastically improved both Ead and quantity of adsorbed hydrogen molecules. It has been established that each cation center is capable of uptaking from one to five H2 molecules per Li+, and their Ead are in the range from -3.57 to -5.40 kcal/mol. SAPT0 calculations have indicated the prevalence of London dispersion or induction forces in the cases of adsorption inside PPA hollows and on Li+, respectively. AIM and IGM analyses have exhibited that a noncovalent bonding exists between Li+ and H2, and the adsorbed H2 molecules are undissociated. AIMD simulations allow estimating gravimetric densities (GD) at various temperatures, and the maximal GD have been calculated to be 6.0, 4.8, and 3.8% at T = 77, 233, and 300 K, respectively. Besides this, AIMD calculations validate stability of studied structures and the quantity of hydrogen adsorbed. We believe that some of the considerations derived herein establish the high potential of the studied PPA models for hydrogen storage.

Automated summary

This study systematically investigates the physisorption of H2 molecules by PPA, both pristine and Li+-decorated, in the gas phase. The results show that Li+ decoration significantly enhances the adsorption energy and quantity of adsorbed hydrogen molecules.