Adsorption of Molecular Hydrogen on Lithium–Phosphorus Double-Helices

Abstract

The possible interaction of the unprecedented but recently predicted inorganic double-helices made up of lithium and phosphorous (LinPn; n = 7–9) with dihydrogen (H2) molecules is explored via density functional theory-based computations. Because of the large amount of Li → P electron transfer, the Li chain carries a high positive charge, which can be utilized to interact with quite less-reactive elements such as H2. Despite low polarizability of the target species to be bound, these double-helices are found to interact with H2 molecules, having binding energies within a range of 1.7–3.2 kcal/mol per H2 molecule. Further, the periodic calculation with the LiP helix reveals that each Li center binds with two H2 molecules with an average binding energy of 2.5 kcal/mol per H2, and this leads to a 9.6 wt % of H2 uptake. The interactions in Li···H2 are mainly originating from both orbital and electrostatic contributions as reflected in the energy decomposition analysis. However, a global minimum search for H2@Li7P7 by a modified kick algorithm reveals that the lowest energy isomer is a significantly distorted structure from a helix, and having two P–H bonds. Therefore, chemisorption should be preferable over the interaction in molecular form. However, for that purpose, the rupture of the H–H bond in the H2 molecule is essential, which needs at least an activation energy barrier of 14.9 kcal/mol to overcome. Given the fact that the H2 storage in Li-decorated clusters would only be achieved at low temperature, the chemisorption is not likely to take place. Further, their interaction with noble gases (Ar–Rn) is also studied herein. Moreover, an inspection of their band gap structures indicates that the LiP helix could exhibit wide band gap semiconducting properties with a direct band gap value of 2.64 eV.