The role of Na decoration on the hydrogen adsorption on coronene: A combined experimental and computational study

Functionalizing carbon-based materials with metal atoms has been proposed as an effective method to enhance the adsorption of hydrogen molecules on these substrates, thereby developing new useful systems for hydrogen storage. In this work, we investigate both experimentally and theoretically the role of sodium atom decoration on hydrogen attachment to coronene, a polycyclic aromatic hydrocarbon considered the smallest prototype of graphene. In the experiments, multiply charged helium nanodroplets are produced and exposed to sodium, coronene and hydrogen vapors, leading to the formation, within equal conditions, of clusters of hydrogen molecules surrounding bare and sodium-decorated protonated coronene, [H-coronene]+ and [H-coronene-Na]+, respectively, whose abundances are accurately measured via mass spectrometry. These clusters are studied computationally by means of quantum Monte Carlo methods, using analytical representations of the H2-substrate interaction potentials based on high-level electronic structure calculations. It is found that the number of H2 molecules attached to both [H-coronene]+ and [H-coronene-Na]+ decreases as the evaporation pressure in the H2 chamber increases; however, the Na-decorated support retains a considerably larger number of molecules than the undecorated one, which is related to the higher evaporation energies of H2 molecules attached to the decorated support. In addition, most of the anomalies observed in the distributions of ion abundances vs. the number of hydrogen molecules have been identified in the theory as particularly stable clusters. For the Na-decorated substrate, it is found that clusters formed by four H2 molecules surrounding Na are very stable and that with the addition of two more molecules, the alkali atom becomes “solvated”.